HAROLD WILLIAMS SERIES

Tectonic Evolution of theAdirondack Mountains andGrenville Orogen Inlierswithin the USA

James M. McLelland1, Bruce W. Selleck1, and Marion E. Bickford2

1Department of GeologyColgate UniversityHamilton, NY, USA, 13346Email: jmclelland@citlink.net

2Department of Earth SciencesHeroy Geology LaboratorySyracuse UniversitySyracuse, NY, USA, 13244

SUMMARYRecent investigations in geochronologyand tectonics provide important newinsights into the evolution of theGrenville Orogen in North America.Here, we summarize results of thisresearch in the USA and focus uponca. 1.4–0.98 Ga occurrences extendingfrom the Adirondack Mountains to thesouthern Appalachians and Texas.Recent geochronology (mainly byU/Pb SHRIMP) establishes that these

widely separated regions experiencedsimilar tectonomagmatic events, i.e.,the Elzevirian (ca. 1.25–1.22 Ga),Shawinigan (ca. 1.2–1.14 Ga), andGrenvillian (ca. 1.09–0.98 Ga) oroge-nies and associated plate interactions.Notwithstanding these commonalities,Nd model ages and Pb isotopic map-ping has revealed important differencesthat are best explained by the existenceof contrasting compositions of deepcrustal reservoirs beneath the Adiron-dacks and the southern Appalachians.The isotopic compositions for theAdirondacks lie on the same Pb–Pbarray as those for the GrenvilleProvince, the Granite-RhyoliteProvince and the Grenvillian inliers ofTexas suggesting that they all devel-oped on Laurentian crust. On theother hand, data from the southernAppalachians are similar to those ofthe Sunsas Terrane in Brazil and sug-gest that Amazonian crust with thesePb–Pb characteristics was thrust ontoeastern Laurentia during its Grenvilliancollision with Amazonia and subse-quently transferred to the latter duringthe late Neoproterozoic breakup ofthe supercontinent, Rodinia, and theformation of the Iapetus Ocean. Theca. 1.3–1.0 Ga Grenville Orogen is alsoexposed in the Llano Uplift of Texasand in small inliers in west Texas andnortheast Mexico. The Llano Upliftcontains evidence for a major collisionwith a southern continent at ca.1.15–1.12 Ga (Kalahari Craton?), mag-matic arcs, and back-arc and forelandbasins, all of which are reviewed.

The Grenvillian Orogeny isconsidered to be the culminating tec-tonic event that terminated approxi-

mately 500 m.y. of continental margingrowth along southeastern Laurentiaby accretion, continental margin arcmagmatism, and metamorphism.Accordingly, we briefly review the tec-tonic and magmatic histories of thesePaleoproterozoic and Mesoproterozoicpre-Grenvillian orogens, i.e., Penokean,Yavapai, and Mazatzal as well as theGranite-Rhyolite Province and discusstheir ~5000 km transcontinental span.

SOMMAIREDes recherches récentes engéochronologie et en tectonique révè-lent d’importants faits nouveaux surl’évolution de l’orogénie de Grenvilleen Amérique du Nord. Nous présen-tons ici un sommaire des résultats decet effort de recherche aux USA enmettant l’accent sur les indices datésentre env. 1,4 et 0,98 Ga, à partir desmonts Adirondack jusqu’au sud desAppalaches et au Texas. Des donnéesgéochronologiques récentes (parmicrosonde SHRIMP principalement)indiquent que les roches de ces régionstrès éloignées les unes des autres ontsubies l’effet d’épisodes tectonomag-matiques similaires, par exemple, auxorogenèses de l’Elzévirien (env.1.25–1.22 Ga), de Shawinigan (env.1.2–1.14 Ga), et du Grenvillien (env.1.09–0.98 Ga), ainsi que des interac-tions des plaques associées. Malgré cespoints communs, la chronologie Nd etla cartographie isotopique Pb a révélédes différences importantes qui s’ex-pliquent plus aisément par des compo-sitions contrastées des réservoirs pro-fonds de croûte sous les Adirondackset le sud des Appalaches. Les compo-sitions isotopiques des Adirondackssont de la même gamme Pb-Pb que

Geoscience Canada, v. 40, http://dx.doi.org/10.12789/geocanj.2013.40.022 © 2013 GAC/AGC®

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GEOSCIENCE CANADA Volume 40 2013 319

ceux de la Province de Grenville, de laProvince Granite-rhyolite et des bou-tonnières grenvilliennes du Texas, sug-gérant qu'ils se sont tous développéessur la croûte des Laurentides. Parailleurs, les données des Appalaches dusud sont semblables à celles du terranede Sunsas au Brésil, ce qui incite àpenser que la croûte amazonienne,avec de telles caractéristiques Pb-Pb, aété poussée sur la portion est de Lau-rentia lors de sa collision grenvillienneavec l’Amazonie puis laissée à cettedernière au cours de la rupture dusupercontinent Rodinia vers la fin duNéoprotérozoïque, avec la formationde l'océan Iapetus. L’orogène deGrenville (1,3 à 1,0 Ga env.) est égale-ment exposé dans le soulèvement deLlano au Texas et dans de petites bou-tonnières dans l'ouest du Texas et lenord du Mexique. Le soulèvement deLlano montre des indices d'une colli-sion majeure avec un continent au sud,entre env. 1,15 et 1,12 Ga (craton deKalahari?), des zones d’arcs magma-tiques, d'arrière-arc et de bassin d'a-vant-pays, chacun étant présenté ci-dessous.

L'orogenèse de Grenville estconsidéré comme l'événement tec-tonique culminant qui marqué la find’une période d’environ 500 ma d’ac-croissement de la marge continentale lelong de la bordure sud-est de la Lau-rentie, par accrétion, magmatisme d’arcde marge continentale, et métamor-phisme. C’est pourquoi, nous passonsbrièvement en revue l'histoire tec-tonique et magmatique de ces orogènespré-grenvilliennes paléoprotérozoïqueset mésoprotérozoïques, pénokéenne,de Yavapai, et de Mazatzal ainsi que laProvince de Granite-rhyolite, et discu-tons de son étendue sur env. 5 000 km.

INTRODUCTIONThe Adirondack Mountains constitutethe southernmost contiguous portionof the Mesoproterozoic GrenvilleProvince (Fig. 1) and, as such, are wellsituated to make geologic connectionswith Grenvillian inliers in theAppalachian Mountains to the south-east and with those in the Midconti-nent to the southwest. A summarychronological chart of major Mesopro-terozoic tectonic events in the Adiron-dacks and the Appalachians is present-ed in Figure 2, in which it can be seen

that evidence for many events of thesame age are present in several loca-tions pointing to a shared tectonic evo-lution (McLelland et al. 2010a). Similartiming of events is also noted betweenthe Adirondacks and the Mesoprotero-zoic inliers of Texas. The shared evolu-tion of these terranes is summarized inthis paper along with new develop-ments of the past several years.

Isotope geochemistry andgeochronology have played major rolesin extending our understanding of theentire Grenville Orogen, including theAdirondacks where they have clarifieda geological history that had remainedobscure for decades. The earliest sys-tematic U–Pb zircon dating studieswere due to L.T. Silver (Silver 1969)who used the Adirondacks as a labora-tory in which to advance his pioneer-ing research in multi-grain U–Pb zir-con dating. Subsequent multi-grainU–Pb zircon dating by McLelland et al.(1988) and McLelland and Chiarenzelli(1990) extended Silver’s results and, forthe first time, established a coherentchronology and sequence of events inthe Adirondack Mountains. The resultsdemonstrated that the region wasaffected by three major orogenicevents: the Elzevirian Orogeny(1.25–1.22 Ga), the Shawinigan Oroge-ny (ca. 1.16–1.14 Ga), and the OttawanOrogeny (ca. 1.09–1.03 Ga) as well asthe late- to post-orogenic anorthosite–mangerite–charnockite–granite(AMCG) suite at ca. 1.15 Ga. Thisframework provided boundary condi-tions enabling the unraveling ofAdirondack tectonomagmatic historyand placing the region into a self-con-sistent plate tectonic overview that wepresent here. More recently, McLellandand co-workers (McLelland et al. 2001,2002, 2004; Heumann et al. 2006; Bick-ford et al. 2010; Chiarenzelli et al.2010, 2011) undertook the first datingof Adirondack zircon by U–PbSHRIMP2 techniques and this hasvastly improved understanding of thedetailed geochronology of events inthe region. Both SHRIMP2 and ther-mal ionization mass spectrometry(TIMS) ages are presented in Table 1where the results of each can be com-pared. Similar SHRIMP2 zircongeochronology has been applied to theGrenvillian inliers of the Appalachiansby Tollo et al. (2006, 2010) and

Aleinikoff et al. (2004, 2008) and refer-ences therein. At about the same time,application of a range of other iso-topic studies, e.g. Sm/Nd crustal dat-ing, oxygen isotopes, carbon isotopes,40Ar/39Ar isotope geochemistry, Hf inzircon, and whole rock Pb/Pb investi-gations greatly expanded the database,while modern geothermometry andgeobarometry enabled a deeper under-standing of processes and eventsinvolved in the evolution of theAdirondacks as well as the Grenvillianinliers of the Appalachians and Texas.References to these studies, togetherwith appropriate citations, are made inthe text itself. Prior to this, we brieflysummarize the pre-Grenvillian Pro-terozoic history of a long-lived seriesof continental margin arcs alongsoutheastern Laurentia, i.e., the Paleo-proterozoic Penokean, Yavapai, andMazatzal orogens, together with theMesoproterozoic Granite-RhyoliteProvince. Tectonomagmatic activityalong these arcs set the stage for theculminating Grenvillian Orogeny thatresulted in the creation of the Rodin-ian supercontinent.

REGIONAL SETTING OF THEGRENVILLE OROGEN WITHIN THEUSAThe Grenville Orogen (Fig. 1) repre-sents the youngest of a series of Pale-oproterozoic to Mesoproterozoiccrustal additions to the southeasternLaurentian margin. From oldest toyoungest, on the basis of Nd modelages (TDM), these include: the Penokean(ca. 2.0–1.8 Ga), Yavapai (ca. 1.8–1.7Ga), and Mazatzal (ca. 1.87–1.65 Ga)provinces or orogens, and the Granite-Rhyolite Province (ca. 1.487–1.37 Ga).Together, these terranes have beeninterpreted as long-lived island andcontinental margin arcs that were thesite of accretion and juvenile pluton-ism that resulted in ~600 million yearsof crustal additions to Laurentia andculminated with the 1.3–0.98 Ga Elze-virian, Shawinigan and Grenvillian oro-genies (Karlstrom et al. 2001; Whit-meyer and Karlstrom 2007, and refer-ences therein). Rocks of the Penokean,Yavapai, and Mazatzal provinces aregenerally tectonized by multiple defor-mational events and are commonlyreferred to as orogens or orogenicbelts. The interpretation of these arcs

320

as accretionary is not universallyaccepted. In particular, parts of theYavapai Province have been interpretedby Hill and Bickford (2001), Bickfordand Hill (2007), and Bickford et al.(2008) to consist of olderPenokean–Trans-Hudson crust thatwas remelted during extension.

In our summary, we haverelied heavily on detailed studies byKarlstrom and Humphreys (1998),Karlstrom et al. (2001), and an exten-sive review by Whitmeyer and Karl-strom (2007), as well as references

given therein. We have excluded theMojave Province, or Mojavia, situatedwest of the Yavapai Province from ourdiscussion, because it has recently beeninterpreted by Nelson et al. (2011) tobe underlain by Yavapai crust overlainby a thick layer of Archean metasedi-mentary debris derived from theWyoming Craton (Fig. 1). Yavapai-ageplutons ascending through thismetasedimentary cover experiencedSm–Nd and Pb-isotope contaminationthat make their crustal ages appear tobe older than they actually are. Figure 1

shows the boundary of the YavapaiProvince provided by Karlstrom et al.(2001), but question marks have beeninserted to acknowledge the alternativeinterpretation that the YavapaiProvince could extend all the way toDeath Valley and on through theTransverse Range of California and bebounded on the west by the 87Sr/86Sr =0.706 and the 208Pb/206Pb lines (Fig. 1)that mark the western edge of Precam-brian crust (Wooden et al. 1998).

modified with deskPDF Editor - Get the free PDF Editor trial download at docudesk.commodified with deskPDF Editor - Get the free PDF Editor trial download at docudesk.com

Figure 1. Generalized map depicting major tectonic and geochronological subdivisions in the USA. The Grenville Province isshown in medium gray and its exposed portions are indicated by heavy outlines. Abbreviations: AD = Adirondack Mountains,AGM = Abilene gravity minimum, AR = Arbuckle Mountains, BF = Brevard Fault, CDC = Cerro Del Carrizalillo, CG = CrabOrchard drill core, EGR = Eastern Granite-Rhyolite Province, GFW = Grandfather Mt. Window, HF = Hayesville Fault, KY =Kentucky drill core, LF = Llano Front, LU = Llano Uplift, MHT = Mars Hill Terrane, OAT = Oklahoma-Alabama Transform,OH = Orchard Hill drill core, PMC = Pecos Mafic Complex, RM = Roan Mt., SDC = Sierra Del Cuervo, SFM = St. FrancoisMts., SGR = Southern Granite-Rhyolite Province, SOA = Southern Oklahoma Aulocogen, TFD = Tallulah Falls Dome, TXD= Toxaway Dome, VH =Van Horn. Numbers: 1 = Mount Holly complex and Chester-Athens Dome, 2 = Berkshire Moun-tains, 3 = Hudson-New Jersey Highlands, 4 = Honey Brook Uplands, 5 = Baltimore Gneiss domes, 6 = Shenandoah Massif, 7= Goochland Terrane, 8 = Sauratown Mountains, 9 = French Broad Massif, 10 = Pine Mountain Uplift, 11 = Corbin Hill.

GEOSCIENCE CANADA Volume 40 2013 321

Penokean ProvinceThe Penokean Province (Fig. 1) isunderlain by 1.89–1.80 Ga Paleopro-terozoic crust (Van Schmus 1980) withSm–Nd TDM of ca. 2.0 Ga (Barovich etal. 1989; Holm et al. 2005), and recordsthe transition from a passive to anactive continental margin at ca.2.4–2.23 Ga. It consists dominantly ofdeformed, metamorphosed volcanicand plutonic calcalkaline arc rocksreferred to as the Wisconsin MagmaticTerrane and was accreted to Laurentiaat ca. 1.89–1.83 Ga (Sims and Carter1996). Subduction is interpreted tohave been to the south, since there areno signs of coeval arc magmatismalong the continental margin; howeverthe arc rocks were eventually thrust

over Laurentia, probably along theNiagara Fault (Sims and Carter 1996).Several Archean tracts occur within thePenokean Province and are probablyexotic fragments. Rocks of similar ageand character occur in the MakkovikOrogen at the northeastern terminusof the Grenville Province (Fig. 1).Dickin and McNutt (1989) described awide belt of 2.09-1.72 Ga calcalkalinegranitic gneiss in the eastern GrenvilleProvince that they interpreted as a pos-sible extension of the Penokean arc(Dickin 2000); however, the age couldbe the result of magma contaminationby underlying Archean crust (cf.DeWolf and Mezger 1994).

Yavapai Province The next major volcanic and plutonicterrane accreted to Laurentia was thelarge and complex Yavapai Provincewith a crustal TDM age of 2.0–1.8 Ga,slightly older than the 1.8–1.7 Ga juve-nile calcalkaline granite–greenstonebelts, including pillow basalts, tonaliteand granite plutons, and massive sul-phide deposits that dominate the ter-rane and have been interpreted asisland arc complexes (Karlstrom et al.2000; Whitmeyer and Karlstrom 2007).Note, however, that one of us (MEB)has called attention to the bimodalityof Yavapai rocks, the presence of ca.1.85–1.88 Ga xenocrystic zircons, ca.1.85–1.90 Ga Nd model ages, and ca.1.85–1.90 Hf model ages, to argue thatmuch of the Yavapai Province is infact reworked Penokean and Trans-Hudson crust. This argument isstrengthened by the presence of the ca.1.84 Ga Elves Chasm Terrane in theGrand Canyon (Hawkins et al. 1996).The northern margin of the terrane ismarked by the subvertical 1.78–1.75Ga Cheyenne belt shear zone that liesnear the southern terminus of thegreenschist facies Wyoming Craton andcan be geophysically tracked eastwardto its contact with the Penokean Oro-gen. The Yavapai Province basementis so deformed and metamorphosedthat it is currently difficult to resolveplate-tectonic details. Some workers(e.g. Karlstrom and Bowring 1988;Tyson et al. 2002; Jessup et al. 2006)have interpreted geophysical data toindicate a number of accretionary sub-duction zones, analogous to the pres-ent day Banda arc. These workers (c.f.,Whitmeyer and Karlstrom 2007) envi-sioned a picture of almost continuousYavapai-wide subduction and contrac-tional deformation from 1.78 and 1.68Ga. with an orogenic peak at ca.1.71–1.69 Ga. As noted above, one ofus (MEB) believes the history of theYavapai Province is more complexthan this, but accepts that arc accretionwas probably locally important. Meta-morphic rocks exposed at the surfacetoday commonly give evidence of pro-tracted, mid-crustal metamorphism at~350–450 MPa. Lateral temperaturegradients of ~300° C in these rocksbear witness to the major role playedby granitic plutons in advecting heattowards the surface. In southeastern

Figure 2. Chart summarizing major Mesoproterozoic (ca. 1.5–0.9 Ga) tectonic andmagmatic events in eastern Laurentia. Gray Rectangles: 1) Orogenies with the fol-lowing labels: R = Rigolet, OT = Ottawan, S = Shawinigan, and E = Elzevirian. 2)Major igneous events: H= Hawkeye Granite, M = Mitchell Granite, MR = MountRogers. Circles: White circles represent late- to post-tectonic AMCG suites, where-as black circles represent regions with only MCG magmatism. Black Rectangles:represent pre-orogenic arc magmatism in the Elzevirian (E) and Shawinigan (S)orogenies. Abbreviations: ADK = Adirondack Mts., AHT = Adirondack High-lands Terrane, ATH = Athens Dome, BRK = Berkshire Mountains, CGP = Cana-dian Grenville Province, CH = Chester Dome, FBM = French Broad, HH = Hud-son Highlands, MH = Mount Holly complex, NJH = New Jersey Highlands, HB =Honey Brook Uplands, BG = Baltimore Gneiss domes, GOOCH = GoochlandTerrane, H = Hawkeye Granite, SHEN = Shenandoah Massif, WC = Wolf Creek.Data for this time scale is summarized with references in the text of this article andin McLelland et al. 2010a. (After McLelland et al. 2010a).

322TA

BLE1.

SUMMARY

OFISOTO

PICAGES

INTH

EADIRONDACK

SANDGRE

NVILLIANINLIER

SOFTH

EAPP

ALACH

IANS

LOCA

LITY

COMPO

SITION

AGE(M

a)±

REFERE

NCE

T DM(Ga)

REFERE

NCE

ORO

GEN

YADIRONDACK

LOWLA

NDS

Rossie

Antwerpsuite

diorite

gran

odiorite

1205

181,2

Shaw

inigan

Hermon

Granite

gran

itegran

odiorite

1182

83

Shaw

inigan

HydeScho

olGne

iss

gran

ite,ton

alite

1172

s11

41.33

,avof

327

Shaw

inigan

Edwardsville

North

Ham

mon

dAMCG

gran

itesyen

ite11

644

5AMCG

Leucosom

esinmetap

elite

sgran

iticpe

gmatite

1180

1160

(S3)

173

ADIRONDACK

HIGHLA

NDS

DysartMou

ntHollySuite

tona

litegran

odiorite

1301

(t4)

151

1.39

,avof

3Fragmen

t1.4

1.3arc

Elzevirian

pluton

charno

ckite

1250

151

Elzevirian

AMCG

gran

itoidpluton

sman

gerite

togran

ite11

58(t5,S8)

51,6

1.22

avof

627

AMCG

AMCG

anotho

site

suite

anorthosite

togabb

ro11

54(S

12)

61.6

1.33

27AMCG

Haw

keye

suite

gran

ite10

96(m

7)8

11.32

27un

certain

Lyon

Mou

ntainGranite

Atype

leucogranite

1050

(S10

)10

6,7

1.35

27Late

Ottaw

anLyon

Mou

ntainpe

gmatite

gran

iticpe

gmatite

1041

(I6,S9)

97,8

Late

Ottaw

anLeucosom

esinmetap

elite

sgran

iticpe

gmatite

1161

(S5)

89

Late

Ottaw

anmon

azite

score,m

antle

,rim

leucosom

ean

dgran

ite11

76,105

1,10

2617

,5,5

3,30

Shaw

inigan

toLate

Ottaw

anGRE

ENMOUNTA

INS,VER

MONT

DysartMou

ntHollySuite

tona

lite,tron

djhe

mite

1430

1330

t9

101.46

28Fragmen

t1.4

1.3arc

College

Hillpluton

gran

ite12

4412

20t

810

1.47

28Elzevirian

FoliatedShaw

inigan

gran

ites

gran

ite11

72(S

1)7

11Shaw

inigan

Pegm

atite

inau

gengneiss

gran

ite10

366

11Late

Ottaw

anRimson

zircon

sNA

1060

1070

(S4)

11Ottaw

anCa

rdinalBroo

ksuite

rapa

kivigran

iteca.100

010

Rigolet

BERK

SHIREMASSIF,M

ASSACH

USETTS

Tyringha

mgran

iticgneiss

gran

ite,75%

ofba

semen

t11

799

121.46

12Shaw

inigan

Zircon

cores

NA

1070

1050

(S)

12Inhe

ritedfrom

LMG

Alaskite

sills

anddikes

1004

997(S)

1012

Rigolet

HUDSO

NHIGHLA

NDS,NEW

YORK

DysartMou

ntHollySuite

tona

litetron

djhe

mite

1336

(S9)

913

Fragmen

t1.4

1.3Gaarc

CampSm

ithpluton

icrock

aplite

1238

(S1)

713

Elzevirian

Storm

King

Shaw

inigan

pluton

gran

ite11

74(S

1)8

131.46

28Shaw

inigan

Cano

puspluton

mon

zonite

diorite

1142

S1)

813

AMCG

Brew

ster

gran

ite(AMCG

?)ho

rnblen

degran

ite11

34(S

1)7

13AMCG

?Dan

bury

augengran

itemicroclinegran

ite10

45(S

1)11

14Late

Ottaw

anGne

issan

dmigmatite

biotite

gran

ite10

5710

4812

13,14

Late

Ottaw

anCrystalLakegran

itegran

ite10

5814

15Ottaw

anNEW

JERS

EYHIGHLA

NDS

DysartM

t.HollySuite

tona

lite

1366

1282

(S)

1316

Pluton

icgneiss

dacite

gran

odiorite

1254

1248

(S)

816

Fragmen

t1.4

1.3arc

Byram,LakeHop

atcong

pluton

sAtype

gran

ite11

8811

82(S)

1116

Elzevirian

Mou

ntEvegran

iteAtype

gran

ite10

194

16Shaw

inigan

mon

azite

,zirconmetam

orph

icNA

1045

1024

(S)

16Late

Ottaw

nRigolet

BALTIM

ORE

GNEISS

DOMES

Late

postOttaw

nlayeredfelsicgneiss

biotite

quratz

feldspar

1247

(S8)

19Aleinet

al20

04Elzevirian

foliatedbiotite

gran

itebiotite

gran

ite10

75(S

9)15

Ottaw

anzircon

overgrow

thsI

1220

Late

Elzevian

zircon

overgrow

thsII

1160

Shaw

inigan

AMCG

zircon

overgrow

thsIII

1020

Late

Ottaw

anRigolet

GEOSCIENCE CANADA Volume 40 2013 323

California 1.71–1.68 Ma events arereferred to as the Ivanpah Orogenyand in the mid-continent the term‘Central Plains Orogeny’ has beenused. These, and other local names,have been subsumed by the termYavapai Orogeny (Karlstrom andBowring 1988, and Karlstrom et al.2001). Plutons of Yavapai orMazatzal age (ca. 1.7 Ga) occur inthe Killarney magmatic belt situatedwithin the northwestern corner ofthe Grenville Province (Davidson1998; Rivers and Corrigan 2000).

Mazatzal ProvinceThe formation of the YavapaiProvince was followed by the assem-bly of the Mazatzal Province with1.7–1.6 Ga rocks yielding TDM =1.8–1.6 Ga. The terrane includesjuvenile 1.7–1.65 Ga oceanic arcassemblages, including greenstonebelts and 1.66–1.65 Ga calcalkalineplutons consistent with continentalmargin back-arc affinities perhapsrelated to periods of slab rollbackwithin an otherwise continuous con-traction from 1.8–1.6 Ga. TheMazatzal Province has a complex,and apparently, transitional structuralcontact with the Yavapai Provinceand plutons of Mazatzal age occurwell within the Yavapai (e.g. Bickfordet al. 1989). This has been interpret-ed to be the result of collision alonga south-dipping subduction zone inwhich the Yavapai Orogen margin ofLaurentia was partially subductedbeneath the Mazatzal Province at1.68–1.65 Ga. When the subductionzone jammed, the contractionalstrain was accommodated by lowangle back-thrusting away from thecollisional zone in each block at ca.1.65–1.60 Ga. The orogen extendseastward in the subsurface and reap-pears in the Grenville Provincewhere rocks of Mazatzal age andcharacter occur near Georgian Bay(Slagstad et al. 2009) on eastern LakeHuron (Fig. 3) and are spectacularlyrepresented in the 800 km long, ca.1.67–1.65 Ga juvenile (TDM =1.71Ga) Trans-Labrador Batholithemplaced by arc accretion and conti-nental margin arc magmatism(Gower and Krogh 2002). To thesouth of the batholith, the very largeLabradorian Province has a TDM of

TABLE1(con

t.).

SUMMARY

OFISOTO

PICAGES

INTH

EADIRONDACK

SANDGRE

NVILLIANINLIER

SOFTH

EAPP

ALACH

IANS

LOCA

LITY

COMPO

SITION

AGE(M

a)±

REFERE

NCE

TDM(Ga)

REFERE

NCE

ORO

GEN

YSH

ENANDOAHMASSIF

1.62

1.37

23metagranitoids,G

roup

1gran

ite,m

onzonite,diorite

1183

1144

(S12

)12

181.69

avof

821

Fragmen

t1.4

1.3Gaarc

metagranitoids,Group

2gran

ite,m

onzonite,diorite

1143

1111

(S3)

1218

1.57

avof

2Shaw

inigan

AMCG

metagranitoids,Group

3gran

ite,m

onzonitte,diorite

1078

1028

(S14

)8

181.5av

of11

AMCG

Haw

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324

~1.7 Ga (Dickin 2000).

Granite-Rhyolite Province and A-type Mesoproterozoic Granitic Magmatism in the MidcontinentSituated between the Mazatzal and

Grenville orogens, the Granite-RhyoliteProvince (Fig. 1) of the mid-continentis known mainly from exposures in theSt. Francois Mountains (SFM) of Mis-souri and the eastern Arbuckle Moun-tains of Oklahoma. In addition, a large

number of drill holes have penetratedthe province’s Precambrian basementand have been dated by Bickford andVan Schmus (1985). On the basis ofthis dating, and extensive research inthe St. Francois and Arbuckle moun-tains, two major subprovinces can beidentified, the Southern Granite Rhyo-lite (SGR) Subprovince (Fig. 1) and theEastern Granite-Rhyolite (EGR) Sub-province (Fig. 1; Van Schmus et al.1993a, b). The EGR is characterized byaverage crystallization ages of ca. 1.47Ga and the SGR by average crystalliza-tion ages of ca. 1.37 Ga. Basementrocks in both subprovinces are domi-nated by epizonal granite plutons andcoeval rhyolite with very minor basaltand gabbro indicative of limitedbimodality. In the St. Francois Moun-tains the exposed 1.47 Ga basementconsists of undeformed ignimbrite,caldera complexes, and shallow graniticplutons. Exposures of the 1.38–1.37Ga SGR occur mostly in the easternArbuckle Mountains (Fig. 1 [AR]),where granodiorite, granite, andgranitic gneiss have yielded crystalliza-tion ages of ca. 1.39–1.37 Ga. Localsubsurface samples yield crystallizationages of ca. 1.4–1.34 Ga that are closeto the average age of 1.37 Ga in theSGR.

Sm–Nd TDM ages for the base-ment beneath the EGR and the SGRhave been obtained by Van Schmus etal. (1993a, b) and Rohs and VanSchmus (2007). The results define adividing line between rocks to thenorthwest with TDM >1.55 Ga andthose to the south with TDM<1.55 Ga(Fig. 1). Van Schmus et al. (1993a, b)interpreted this line as demarcating thesouthern terminus of Paleoproterozoiccrust and suggested that it is a candi-date for a suture between the ca. 1.55Ga Laurentian margin and anotheraccreted arc to the southeast. TDM val-ues for the St. Francois Mountaingranite plutons are close to their crys-tallization ages providing a strong argu-ment for derivation via melting ofjuvenile, calcalkaline crust shortly afterits accretion (Van Schmus et al. 1993a,b; Menuge et al. 2002; Rohs and VanSchmus 2007; Slagstad et al. 2009). TheTDM values for the samples from theArbuckle Mountains, which lie southof the TDM line, are ca. 1.45–1.54 Ga,i.e., basically the same as those from

��

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Figure 3. Map showing the southwestern segment of the Grenville Province plusthe Adirondacks and Green Mountains. Black = 1.4–1.3 Ga margin arc and south-eastern fragments, A, LD = Algonquin/Lac Dumoin, (1.5–1.4 Ga calcalkalinegneiss complex), AH =Adirondack Highlands, AL = Adirondack Lowlands, B =Bancroft Terrane, BA = Barillia (2.0–1.85 Ga arc), BSZ =- Bancroft Shear Zone,BG = Bondy Gneiss Dome, CCZ = Carthage-Colton Shear Zone, CMB = CentralMetasedimentary Belt, CMBBZ = Central Metasedimentary Belt Boundary Zone,E = Elzevir Terrane, F = Frontenac Terrane, LBZ = Labelle Shear Zone, LTZ =Lac Taureau Shear Zone, M = Mazinaw Terrane, MHC = Mount Holly complex,MK = Mékanic Terrane, ML = Mont Laurier Terrane, MM = Marcy AnorthositeMassif, MO = Morin Anorthosite, MO-B = Montauban-La Bostonnaiss oceanicarc (1.45–1.35 Ga), MT = Morin Terrane, MU = Muskoka Domain (1.5–1.4 Gamargin arc), MSZ = Maberly Shear Zone, P = Parry Sound Domain, PAB =Parautochthonous Belt, PdL = Parc des Laurentides AMCG Suite, QGS = Quebecgneiss segment, RC = Reservoir Cabonga Terrane, S = Sharbot Lake Terrane, SLR= St. Lawrence River, TWZ = Tawachiche shear zone. (Modified from Hanmer etal. 2000).

GEOSCIENCE CANADA Volume 40 2013 325

the St. Francois Mountains indicatingthat the two regions are underlain bycrust of the same age. Although crys-tallization ages for samples from theArbuckle Mountains are ca. 1.39–1.37Ga, it is plausible that renewed mag-matism at ca. 1.4–1.33 Ga gave rise tothe granitic rocks of the low TDM por-tion of the SGR. This is strengthenedby the presence of a second, minorgranitic episode in the SFM, which iscoeval with magmatism in the Arbuck-le Mountains (Bickford and Mose1975; Thomas et al. 1984; Rohs andVan Schmus 2007).

It has been argued that theGranite-Rhyolite subprovinces are theresult of the development of a conti-nental margin arc with northwestpolarity and with suturing along theTDM 1.55 Ga line (Van Schmus et al.1993a, b; Rivers and Corrigan 2000;Menuge et al. 2002; Rohs and VanSchmus 2007; Slagstad et al. 2009). Thearc is well exposed in the GrenvilleProvince where evidence for ca.1.5–1.35 Ga granitic magmatism spansthe province from Georgian Bay (Fig.3) to its eastern terminus in Labradorand is informally known as the Britt-Pinware belt (Rivers and Corrigan2000). An accreted outboard arc(1.44–1.35 Ga), known as the Mon-tauban-La Bostonnais Terrane (Fig. 3),occurs near the eastern margin of theMorin Terrane. As pointed out byRivers (1997, 2008) and Rivers andCorrigan (2000), the Britt-Pinware beltrepresents a continental margin,Andean-type arc with northwest polari-ty and southward growth. Associatedhigh-grade metamorphism, known asthe Pinwarian Orogeny provides evi-dence for a regionally extensive oro-genic event. The arc can be projectedalong the strike of subsurface geophys-ical trends (Fig. 1) into the easternmargin of the Granite-Rhyolite andLlano provinces. In the latter case, theMontauban-La Bostonaiss arc (Fig. 3)and the Coal Creek arc, located at thesoutheast margin of the Llano Uplift(Fig. 1), are similar in age and type(Rivers and Corrigan 2000). These cor-respondences strengthen the case for acontinental margin arc origin for theGranite-Rhyolite Province. Rivers andCorrigan (2000) and Slagstad et al.(2009) have presented a plate tectonicmodel for the 500 km long, SW-NE

trending Britt-Pinware belt continentalmargin arc. The width of the arc isabout 500 km along a SE-NW linewith northwest polarity. The leadingedge includes the Muskoka domain(Fig. 3) that straddles the folded exten-sion of the TDM 1.55 Ga line that wassubsequently thrust northwest over thecontinental margin. The Muskokadomain is underlain by large quantitiesof calcalkaline ‘gray’ dioritic to gran-odioritic orthogneiss with crystalliza-tion ages of ca. 1.45 Ga and TDM ofca. 1.55–1.6 Ga typical of the south-east Laurentian continental marginarcs. The arc experienced alternatingpulses of contraction and extensionwith the latter resulting in back-arcbasins and basaltic underplating due toperiods of rollback, slab breakoff, ordelamination of lithosphere resultingin ascent of asthenosphere to the baseof the crust. Within the back-arcbasins, partial melting of gray gneissproduced large granitic plutons, manywith A-type geochemistry, as well asminor anorthosite and gabbro. TheGrenville Province plutons are coevalwith, and intrude rocks of similar ageand composition to those in the Gran-ite-Rhyolite Province. They present acompelling case for an arc-related ori-gin that included periods of slab roll-back, delamination, back arc basins,and underplating by mafic magmascausing melting of older fertile lowercrust.

A complication arising fromthis interpretation is the occurrence of1.45–1.37 Ga A-type plutons thatintrude Paleoproterozoic crust as farwest as eastern California (e.g. Goodgeand Veervort 2006). Because of thesimilarity of ages and chemistry ofthese rocks with those of the Granite-Rhyolite Subprovinces, it is widelyassumed that they once also had acover of coeval and chemically similarrhyolite. Accordingly, the causal mecha-nism of the Granite-Rhyolite Provincemight have extended throughout mostof the western part of Laurentia.Goodge and Veervort (2006) presenteda compelling model for widespread A-type magmatism based on Hf-in-zirconresearch. They have observed thatfrom ca. 2.0–1.6 Ga in southern Lau-rentia “prolific crustal growth assembled alarge volume of new lithosphere in a geologi-cally brief period of time.” This fertile

crust underwent post-orogenic con-ductive heating due to blanketing ofmantle heat flow as well as heat advect-ed by mafic magma ponded at thecrust-mantle interface. Melting of thelower crust produced A-type silicicmagmas that intruded to higher levelsand regional extension ensued. Theductility of the deep crust as well asdensity differences inhibited ascent ofthe mafic magmas, which are minorcomponents of the region. Here wecouple this late magmatic extensionalactivity with the early marginal arc his-tory described in the prior paragraph.Accretion at the Laurentian marginhelped to trigger widespread late- topost-orogenic delamination, extension,and A-type magmatism in the hinter-land (Goodge and Veervort 2006). Kayet al. (1998) have described similar tim-ing and events for the vast Late Paleo-zoic to Jurassic granite-rhyolite terranein South America. This resolution hasthe added advantage of removing thesomewhat illusory distinction betweenorogenic and anorogenic magmatism.

GEOLOGY OF THE ADIRONDACKMOUNTAINSFigures 4 and 5 present generalizedgeologic maps of the Adirondacks thatinclude U/Pb SHRIMP2 ages of theprincipal igneous units (ca. 1.35–1.05Ga). Both this section, and the follow-ing one, should be compared to agedata in Table 1. The region is dividedinto the Adirondack Highlands Terrane(AHT; Fig. 4) and Adirondack Low-lands Terrane (ALT; Fig. 4) separatedby the 1–10 km-wide Carthage-ColtonShear Zone (Fig. 4 [CCZ]) that dips~45°NW and exhibits down-dipoblique normal offset at ca. 1050 Mabut also contains evidence of earlier,ca. 1.6 Ga eastward thrusting (Johnsonet al. 2004; Selleck et al. 2005; Baird etal. 2008; Baird and Shrady 2011). TheAHT is underlain principally byorthogneiss that is relatively resistant toerosion, while the Lowlands are under-lain principally by metasedimentaryrocks, notably marble, that are easilyeroded thus accounting for the topo-graphic differences between the twoterranes.

The oldest rocks in theAdirondacks are a suite of 1.35–1.25Ga tonalitic and granitic plutons(McLelland and Chiarenzelli 1990) that

326

occur in the southern and easternAHT (Fig. 4). However, this suite isnot intrusive into the Adirondacks butis interpreted to have been rifted fromthe 1.4–1.3 Ga Laurentian margin arcat ca. 1.3 Ga (Hanmer et al. 2000) andis discussed in a later section. Other-wise, metasedimentary rocks are theoldest units in both Adirondack ter-ranes with depositional ages bracketedby detrital zircon in migmatiticmetapelites and by cross-cutting rela-tionships. In both terranes the detrital

zircon analyses fall into the interval1.3–1.22 Ga (Heumann et al. 2006;Bickford et al. 2008; McLelland et al.2010a). The youngest detrital zirconage reported for the Irving Pondquartzite in the southernmost AHT aswell as the Swede Mountain quartzitein the southeastern AHT (Silver 1969)is 1.3 Ga (Peck et al. 2010). Bothquartzite units contain 2.7 Ga detritalzircon grains, and these are absentfrom any of the metapelite units. Wesuggest that the quartzite units were

deposited near, or on, the Laurentianmargin shortly after ca. 1.3 Ga. In con-trast, the metapelite units are thoughtto have been deposited well off-shorefrom Laurentia and shielded fromArchean zircon sources by the inter-vening Central Metasedimentary Belt(CMB) arc (McLelland et al. 2010a;Peck et al. 2010). The metapelite unitsof the AHT and ALT are similar inappearance, lithology, and chemistry,and it is tempting to interpret them asa single sedimentary horizon. However,

Figure 4. Generalized geological/geochronological map of the Adirondacks Highlands Terrane (AHT) and Adirondack Low-lands Terrane (ALT). Units designated by patterns and initials consist of igneous rocks dated by U–Pb zircon geochronologywith ages indicated in the legend. Units present only in the AHT (HL only) are: ANT = anorthosite, HWK = Hawkeye Granite,LMG = Lyon Mountain Granite. Present in the southern AHT only (southern HL only) are: RMTG = Royal Mountain Tonaliteand Granodiorite. Units present in the ALT only (LL only) are: HSRG = Hyde School Gneiss and Rockport Granite, RDAG =Rossie Diorite and Antwerp Granodiorite, HERM = Hermon Granite. All representatives of the AMCG suite (anorthosite-mangerite-charnockite-granite) are present in the AHT, but only the MCG portion (mangerite-charnockite-granite) is present inthe ALT. Unpatterned areas consist of metasedimentary rocks, glacial cover, and undivided units. The Canada Lake Isocline isrepresented by the bent-finger pattern northeast of Gloversville. A = Antwerp, AF = Ausable Forks, BLSZ – Black Lake ShearZone, CA = Canton, CCZ = Carthage-Colton Shear Zone, GO = Gouverneur, GM = Gore Mountain, IL = Indian Lake, LM= Lyon Mountain, LP = Lake Placid, NH = North Hammond, P= Piseco antiform, OD = Oregon Dome, R = Rossie, RB =Roaring Brook, ScL = Schroon Lake, SM = Snowy Mountain Dome, T = Tahawus, TL = Tupper Lake, W = Wanakena, X =ophiolite complex. E = town of Eddy. (After McLelland et al. 2010a).

GEOSCIENCE CANADA Volume 40 2013 327

Fig

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328

tectonic evidence, discussed below,suggests that they were deposited atthe same time but on opposite flanksof a wide back-arc basin (McLelland etal. 1993, 1994, 1996, 2010a; Chiaren-zelli et al. 2010). Carbonate rocks, nowmetamorphosed to marble, weredeposited on the shallow flanks ofthese basins and are evaporite-bearing.One of these, the Balmat stromatoliticmarble (also referred to as the UpperMarble), hosts sphalerite SEDEXdeposits, and contains detrital zirconranging in age from 1.28 Ga to 1.22Ga with the younger age fixing themaximum age of deposition.

The oldest igneous rocksintruded into the Adirondacks occur inthe ALT and consist of calcalkalinediorites and granitoid bodies referredto as the Antwerp-Rossie Suite dated atca. 1.2–1.17 Ga by U/Pb SHRIMP(McLelland et al. 2001; Chiarenzelli, etal. 2010). These arc-type plutons arewidespread in the ALT (Fig. 4) andcrosscut all Lowlands metasedimentaryunits thus fixing their minimum age.They were closely followed by the cal-calkaline Hermon Granite Gneissemplaced at ca. 1.18–1.17 Ga), andthis, in turn, was followed by graniteand tonalite assigned to the syntectonicca. 1.172 Ga Hyde School Gneiss (Fig.4). The ca. 1.172 Ga Rockport Granitethat outcrops along the St. LawrenceRiver is correlated with the HydeSchool Gneiss but does not includetonalite and is thought to have beenintruded at a somewhat higher level(Wasteneys et al. 1999). Two minimallydeformed plutons of ca. 1.16–1.15 Gaferroan syenite (Fig. 4) have been datedin the western ALT (Fig. 4,Edwardsville, E and Canton, C) andhave been correlated with AMCGrocks in the AHT (McLelland et al.1988). Peck et al. (2011, 2013) haveshown that the geochemical propertiesof the Edwardsville pluton link itssource rocks to the Frontenac Terrane,which may comprise the underlyingdeep crust in the westernmost ALT.Note that none of the ca. 1.2–1.17 Gaplutons of the ALT occur within theAHT suggesting that they were sepa-rate terranes until ca. 1.16 Ga.

In contrast to the ALT, theAHT consists primarily of orthogneissdominated by the ca. 1.15 Gaanorthosite-mangerite-charnockite-

granite (AMCG) suite (Figs. 4, 5). Hfisotopic concentrations of zircon haveestablished that Adirondack igneousrocks of the AMCG suite were derivedfrom enriched mantle or crustalsources (Bickford et al. 2010). The lessvoluminous ca. 1.1 Ga Hawkeye Gran-ite occurs at several localities in theAHT (Fig. 4). The youngest plutonicunit in the AHT, and one of consider-able geologic importance, is the ca.1.05 Ga Lyon Mountain Granite thatrims much of the AHT periphery (Fig.4).

The ALT shows little, if any,metamorphic effects related to the ca.1.09–1.03 Ma Ottawan orogenic phase(Fig. 2) of the ca. 1.09–0.98 GaGrenvillian Orogeny (Rivers 20082012). Hornblende 40Ar/39Ar coolingages as old as 1.1–1.05 Ga demonstratethat the terrane did not experiencetemperatures exceeding ~500° C dur-ing the Ottawan Orogeny (Streepey etal. 2001; Dahl et al. 2004). Titanite agesas old as 1.16 Ga lead to a similar con-clusion. Accordingly, upper amphibo-lite facies assemblages in the ALT mustbe the result of Shawinigan metamor-phism, which caused widespread ana-texis in metapelitic rocks. U /Pb zircondating by SHRIMP2 has fixed the timeof anatexis at 1.18–1.165 Ga. coevalwith regional deformation andemplacement of Hermon Granodioriteand Hyde School Gneiss. High gradeShawinigan metamorphism affectedthe entire AHT resulting in assem-blages above the second sillimanite iso-grad and caused widespread anatexis inmetapelite (Heumann et al. 2006; Bick-ford et al. 2008; McLelland et al.2010a). To a large extent, Shawiniganeffects in the AHT are overprinted bythe intense granulite facies OttawanOrogeny that produced temperaturesof ~750°–850°C and pressures of~6–9 MPa in the AHT (Bohlen et al.1985; Kitchen and Valley 1995; Darlinget al. 2004; Storm and Spear 2005;among others) and reset titanite ages toca. 1.03 Ga and hornblende 40Ar/39Arages to ca. 0.98 Ga to the southeast ofthe Carthage-Colton shear zone(Mezger et al. 1991, 1992; Streepey etal. 2001; Heumann et al. 2006).

Both the AHT and ALT aredominated by several fold sets, the ear-liest of which (F1) comprises intrafo-lial isoclines of Shawinigan age

(Heumann et al. 2006). In the ALT, theearliest map-scale folds are northeast-trending isoclines (F2) with axial planesoverturned to the southeast and com-monly refolded along sub-parallelnortheast axes (F3). The F2 and F3folds of the ALT strongly deformed1.18–1.16 Ga plutons but are crosscutby ca. 1.15 Ga granitoid bodies hencemust be of Shawinigan age (McLellandet al. 2010 a, b). Upright and open totight late northwest- and north-north-east-trending folds (F4, F5) produced avariety of fold interference patternsbut little penetrative fabric. The age ofthe F4 and F5 folds is uncertain, butthey may be the result of extensionalcollapse at ca. 1.05–1.03 Ga (Rivers,pers. comm. 2013). Northeast-trendingshear zones disrupted the fold complexinto panels but did not alter their inter-nal tectonostratigraphy (Chiarenzelli etal. 2011). The absence of Ottawandeformation in the ALT is consistentwith the undeformed state of the ca.1.16 Ga Kingston dikes (Davidson1998) of the Frontenac Terrane (Fig.5) that lies directly west of the BlackLake Shear Zone (Fig. 4).

Within the AHT, the earliestmap-scale folds are very large, gentlyplunging, recumbent isoclines (F2)whose axes trend east–west in thesouthern and central region and north-east–southwest in the northwest AHT(Figs. 4, 5). The ca. 1.15 Ga AMCGsuite and the ca. 1.1 Ga HawkeyeGranite are penetratively deformed andfolded by F2 deformation; hence F2must be of Ottawan age. In contrast,Ottawan F2 isoclines and fabrics arelargely absent from the ca. 1.05 –1.04Ga Lyon Mountain Granite; hence, thegranite must postdate the major com-pressional phase of the OttawanOrogeny (ca. 1.09–1.05 Ga). All ofthese fabrics and folds are crosscut byca. 1.04–1.03 Ga pegmatite dikes pro-viding a minimum age to the time offabric and fold formation. Extremelylarge, upright F3 folds strike east–westacross the AHT for ~200 km andrefolded F2 isoclines form ‘bent index-finger’ outcrop patterns (Figs. 4, 5,southern Highlands). Equally large,upright F4 folds with NNE axes resultin dome and basin patterns where theyintersect F3 folds, e.g. Piseco Lakeanticline (Fig.5 [PL]).

The preceding brief descrip-

GEOSCIENCE CANADA Volume 40 2013 329

tions of the AHT and ALT reflectimportant similarities and differencesin their tectonic evolution, and thesewarrant explanation. This is expandedupon in the following section and platetectonic models are presented thathave broad implications for Mesopro-terozoic North America.

REGIONAL ASPECTS OF THE GEOLOGIC HISTORY OF THEADIRONDACK MOUNTAINS

Ca. 1.3 Ga Rifting of the ca.1.4–1.3 Ga Continental Margin Arcand Opening of Two Back ArcBasins As noted previously, the oldest rocks inthe Adirondacks are 1.4–1.3 Ga juve-nile tonalite and granodiorite (McLel-land and Chiarenzelli 1990; Daly andMcLelland 1991) exposed in the south-ern and eastern AHT (Fig. 2). The dis-tribution of units of similar age andchemistry in the southwest GrenvilleProvince is depicted in Figure 3, inwhich their area of outcrop is shownin black (Hanmer et al. 2000). Thisregion contains the remnants of anextensive southeast-facing continentalmargin arc (Hanmer et al. 2000; Riversand Corrigan 2000; Karlstrom et al.2001; Rivers 2008; Rivers et al. 2012),which arose as a southeastern succes-sor to an earlier (ca. 1.5–1.45 Ga)Andean arc that extended eastwardinto Labrador and is informally knownas the Britt-Pinware granitoid belt(Rivers and Corrigan 2000). McEach-ern and van Breemen (1993) and Han-mer et al. (2000) proposed that thesoutheastern part of the 1.4–1.3 Gaarc was rifted from Laurentia along anortheast-trending fault zone in thevicinity of the Bancroft Terrane (Figs.3, 6A, B). Rifting was accompanied byintrusions of ca. 1.29–1.25 Ga gabbroand nepheline syenite (Lumbers et al.1990; Pehrsson et al. 1996). On thebasis of the distribution of ca. 1.4–1.3Ga continental margin rocks, it isinferred that the arc was split by intra-arc rifting with the northwestern rem-nants remaining in today’s BancroftTerrane where they comprise theDysart granitoid suite (Easton 1992).To the southeast, rifted arc fragmentswere transferred eastward in a widen-ing marginal basin (Figs. 6A, B, 7A)underlain by oceanic crust as indicated

by the presence of younger volcanicand sedimentary rocks in the CentralMetasedimentary Belt (CMB). Dickinand McNutt (2007) and Dickin et al.(2010) used Nd isotopes to identifydomains in the CMB that are underlainby older basement that they correlatedwith the rifted arc, and which may haveserved as nuclei for Elzevirian island

arcs during the closing stages of themarginal basin(s) that evolved andamalgamated from ca. 1.29 to 1.22 Ga(Fig. 7A). McLelland et al. (1996,2010a) and Chiarenzelli et al. (2011)proposed that two back-arc basinswere formed by the rifting of the con-tinental-margin arc (Figs. 6A, B, 7A),i.e., the Central Metasedimentary Belt

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Figure 6. Schematic panel diagram summarizing the distribution and interaction ofvarious segments of northeastern Laurentia during the interval 1.3-1.05 Ga encom-passing the rifting, opening and closing of the Central Metasedimentary Belt back-arc basin (CMBB), Trans-Adirondack back-arc basin (TAB), and the Ottawan colli-sion with Amazonia. Note that prior to ca. 1.3 Ga subduction had been to thenorthwest beneath the Laurentian margin. Blocky black arrows indicate polarity ofsubduction. Different shades of gray identify important terranes and arcs. Blockywhite arrows represent extension. The crustal block labeled Adirondis is based, inpart, on the model proposed by Gower (1996) for a large E–W crustal fragmentrifted from Laurentia at ca. 1.4–1.3 Ga and reattached by collision at ca. 1.2–1.0Ga. We have amended Gower’s Adirondis by adding a N–S continuation in orderto include Vermont (VT), New York (NY), and New Jersey (NJ) where rifted frag-ments of the Dysart-Mount Holly suite occur. Note that Amazonia is inferred toremain south of the Adirondis region until ca. 1.09 Ga, but its collision with theLaurentian margin farther to the south is inferred to have influenced pre-Ottawantectonics in the Central Metasedimentary Belt (CMB) during the ShawiniganOrogeny. The Pyrites ophiolite complex is represented by the small black region inthe CMB. Abbreviations: AM = Amazonia, AMCG = anorthosite-mangerite-charnockite-granite suite, AT = Atikonak River Anorthosite, CCZ = Carthage-Colton Zone, FGF = future Grenville Front, LSJ = Lac St. Jean Anorthosite, MA= Marcy anorthosite, MO- Morin anorthosite. (After McLelland et al. 2010a).

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Figure 7. Plate tectonic models showing proposed tectonic evolution along a line from the Adirondack Mountains to the Cen-tral Metasedimentary Belt Boundary Zone (CMBBZ). The green areas schematically represent the Dysart-Mount Holly suite.Abbreviations: AHT =Adirondack Highlands Terrane, AHT-GMT = Adirondack Highlands-Green Mountains Terrane, ALT= Adirondack Lowlands Terrane, AMCG = anorthosite-monzonite-charnockite-granite suite, BSZ = Bancroft shear zone, CCZ= Carthage-Colton Shear Zone, CGB = Central Gneiss Belt, CMB = Central Metasedimentary Belt, E = Elzevir Terrane,EASZ = Eastern Adirondack shear zone, F = Frontenac Terrane, HSG = Hyde School Gneiss , MSZ = Maberly Shear Zone,OPH = Pyrite Ophiolite Complex, RLSZ = Robertson Lake Shear Zone, TAB = Trans-Adirondack basin. (Modified afterMcLelland et al. 2010a).

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basin (CMBB) and Trans-Adirondackbasin (TAB). The eastern margin ofthe TAB consisted of the AHT-GreenMountains (i.e., Mount Holly) Terrane(GMT) that contains its own riftedfragments of the 1.4–1.3 Ga continen-tal-margin arc. During closure of theCMBB and TAB, two magmatic islandarcs formed: the CMB or CompositeArc belt (CAB) to the west and theAHT-GMT arc to the east. The latterformed along the western margin ofAdirondis (Figs. 6A, B, 7A), a termoriginally introduced by Gower (1996)who used it to account for crust riftedfrom Laurentia at ca. 1.4–1.3 Ga andreattached by collision at ca.1.2–1.0Ga. We have extended his originalAdirondis to include a ~NE–SWprong on its western margin thatincludes the following terranes: Ver-mont (Mount Holly), AHT, and Hud-son-New Jersey Highlands (Fig. 6A).Also included is the Mauricie Terranelocated just east of the Morin Terrane(Fig. 6E [MO]). The reason for includ-ing these terranes is that all of themcontain juvenile 1.4–1.3 Ga tonaliteand granodiorite rifted from the Lau-rentian margin arc (McLelland andChiarenzelli 1990; Karabinos andAleinikoff 1990; Karabinos et al. 1999;Ratcliffe and Aleinikoff 2001, 2008;Volkert et al. 2010). However, this inno way implies that the bedrock geolo-gy of these regions extends eastwardinto Quebec or Labrador. The conti-nental-margin arc fragments arereferred to as the Dysart-Mount Hollysuite (Rivers and Corrigan 2000). Fig-ure 6A, B show the sequence of eventsdescribed above in map format andFigure 7A shows them in plate tectoniccross sections. Rivers and Corrigan(2000) estimated that the overall widthof the two back-arc basins was similarto that of the Sea of Japan. Volkert etal. (2010) have identified the Trans-Adirondack back-arc basin as far assouthern New Jersey, and Aleinikoff etal. (2008) reported rocks of Elzevirianage as far south as the BaltimoreGneiss domes.

The ALT formed along theeastern, trailing margin of the CMBarc and was built primarily from shelfand rise sediments (Chiarenzelli et al.2011) deposited along the arc’s easternmargin (Figs. 6B, 7A) from ca. 1.3–1.22Ga with the timing based on detrital

zircon ages (Heumann et al. 2006).Most of the rise rocks (sandstones,pelites, and volcaniclastic rocks) weredeposited on oceanic crust of theTrans-Adirondack basin. Upon closureof the basin at ca. 1.21 Ga., slices ofthe oceanic crust were obducted ontothe ALT where they occur as thePyrites ophiolite complex (Figs. 6C,7B) described by Chiarenzelli et al.(2011).

Closure of the Backarc Basins andthe Elzevirian Orogeny (ca.1.25–1.22 Ga)Within the western CMB, the oldest

post-rifting rocks are tholeiite formedwithin primitive outboard arcs at ca.1.285 Ga and then deformed and over-lain by mafic volcanic and volcaniclas-tic rocks (Carr et al. 2000 and refer-ences therein). The entire package wasintruded by ca. 1.27 Ga tonalite plu-tons (e.g. Elzevir Pluton) and scattereddeformation was underway. Farther tothe east, calcalkaline volcanic rockstogether with carbonate rocks, quartz-rich clastic rocks, and volcaniclasticrocks formed from ca. 1.28–1.27 Gaand probably represent amalgamationof individual arcs into a mature arcsystem. By ca. 1.245–1.22 Ga wide-spread metamorphism and tectonismwas followed by gabbroic and graniticstitching plutons that intruded theolder supracrustal sequences with theage of magmatism younging eastward(Carr et al. 2000 and references there-in).

Calcalkaline Elzevirian plutonsdated at ca. 1.25–1.22 Ga are presentwithin the AHT (McLelland et al.2010a), the Mount Holly complex (Rat-cliffe et al. 1991; Ratcliffe andAleinikoff 2001), and Hudson High-lands (Ratcliffe and Aleinikoff 2008).In the Baltimore Gneiss domes calcal-kaline granitoid plutons have beendated at ca. 1.25–1.24 Ga and fall with-in the Elzevirian Orogeny interval(Aleinikoff et al. 2004). These agessuggest that the back-arc basins, islandarc accretion, and the Elzevirian oroge-ny extended at least this far south.

The Early Shawinigan Orogeny,1.21–1.2 GaThe Shawinigan orogeny was originallydefined by Corrigan (1995), Rivers(1997), and Rivers and Corrigan (2000)

on the basis of studies near the smalltown of that name in the Mauricieregion of Quebec. Initially, Corrigan(1995) bracketed the tectonismbetween 1.19 and 1.16 Ga, but thisinterval was subsequently widened to1.19–1.14 Ga. As discussed by McLel-land et al. (2010 a, b), the lower end ofthis age range includes the ca. 1.155Ga anorthosite-mangerite-charnockite-granite (AMCG) magmatism that post-dated the contractional phase of theShawinigan orogeny. In addition,McLelland et al (2010 a, b) have arguedthat, in the Adirondacks, the onset ofthe Shawinigan Orogeny began at ca.1.22 Ga, and similar ages have beenproposed in Canada by Corriveau andVan Breemen (2000) and Wodicka etal. (2004). Accordingly, we break ourdiscussion of Shawinigan events intoearly contractional, late- to post-con-tractional, and AMCG events (Fig. 2).

At ca. 1.22 Ga, arcs formedduring closure of the CMB marginalbasin when it collided with the Ban-croft Terrane passive margin (Hanmer1988; Hanmer and McEachern 1992)on the northwest side of the basin(Figs. 6C, 7B) and its underlying Lau-rentian basement (Central Gneiss Belt,CGB) forming the northwest-vergingCentral Metasedimentary Belt Bound-ary Zone (CMBBZ). The northwest-verging thrusts caused subduction tostep out to the east (Fig. 7B) with anorthwestern polarity beneath theAdirondack Lowlands Terrane (McLel-land et al. 1996, 2010b). Peck et al.(2004) investigated δ18O values in zir-con from the Lowlands and Frontenacterranes and concluded that their ele-vated values (11–12‰) reflect oxygeninput from hydrated oceanic crustcompatible with northwestern polarityduring the early Shawinigan Orogeny.It was at this time that slices of ophio-lite were obducted onto the Lowlandsterrane (Figs. 6C, 7B) as the Pyritesophiolite complex (Chiarenzelli et al.2011).

At ca. 1.21 Ga, calcalkaline arcmagmatism was initiated in the ALTwith emplacement of the Rossie Dior-ite and the Antwerp Granodiorite(Figs. 6C, 7B) above the northwest-dip-ping subduction zone (McLelland et al.2010a; Chiarenzelli et al. 2010). TheAntwerp Granodiorite crosscuts allmetasedimentary units in the ALT thus

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providing a minimum age (>1.21 Ga)for the sedimentary sequence. Equiva-lents of the Rossie-Antwerp suite donot occur in the AHT, which is consis-tent with their having been a separateterrane at this time (Figs. 6C, 7B) aswell as being on the opposite side ofthe subduction zone. A maximum ageof ca. 1.3 Ga for the metasedimentaryrocks is fixed by their oldest detritalzircon grains, and the absence ofArchean zircon is consistent with dep-osition in a basin outboard of the Lau-rentian continent. Although the detritalzircon signature of the metasedimenta-ry sequence in the AHT is somewhatmore complex, similar metapelitedeposited in the TAB at this time con-tain a detrital zircon suite (ca. 1.3–1.22Ga) that fixes the maximum age ofdeposition, and the absence ofArchean zircon indicates a locationoutboard of Laurentia. The minimumage is fixed by crosscutting ca. 1.16 Gaplutons of the AMCG suite.

Main Contractional Phase of theShawinigan Orogeny, 1.2–1.16 Ga By ca. 1.19 Ga early collision-relateddeformation and metamorphism in theALT, as well as the AHT, was under-way (Figs. 6D, 7C). In the ALT theevent was accompanied by emplace-ment of syntectonic plutons of ca.1.18 Ga inequigranular Hermon Gran-ite (Heumann et al. 2006) and ca. 1.17Ga Hyde School Gneiss consisting ofapproximately equal amounts ofleucogranite and tonalite (McLelland etal. 1992; Wasteneys et al. 1999). On theCanadian side of the St. LawrenceRiver, the ca. 1.17 Ga Rockport Gran-ite is interpreted to be a shallow equiv-alent of granitic Hyde School Gneiss.None of these Shawinigan igneousrocks are found in the AHT, consistentwith the fact that the subduction zonehad a westward polarity beneath theALT, and indicating that full-scale colli-sion probably did not take place untilca. 1.17–1.16 Ga (Figs. 6D, 7C). Dur-ing collision, upper amphibolite-faciesmetamorphism in both terranes result-ed in widespread anatexis ofmetapelitic rocks transforming theminto migmatites with leucosomes com-plexly folded by Shawinigan deforma-tion. Igneous zircon from leucosomein metapelite from both the Adiron-dack Highlands and Lowlands terranes

has been dated at ca. 1.18 –1.16 Ga(Heumann et al. 2006).

During the main Shawinigancollision, the ALT was thrust eastwardover the AHT for an unspecified dis-tance (Figs. 6D, 7C). The thrustingtook place along a suture correspon-ding roughly to the present day CCZ(Fig. 7C), and its eastward vergence isrecorded by kinematic indicators in theca. 1.164 Ga Diana Syenite Complex(McLelland et al. 2010a, Baird andShrady 2011). Overthrusting of theALT placed it into the domain of theAdirondack orogenic lid accounting forits preservation of Shawinigan titaniteages (Mezger et al. 1991; McLelland etal. 1996, 2010a), as well as hornblende40Ar/39Ar ages indicating that Ottawantemperatures in the Lowlands did notexceed 500°C (Streepey et al. 2001).Similar arguments can be made for theCMB in general, one of two majorexamples of the Ottawan OrogenicLid proposed by Rivers (2008). Ulti-mately, this lid was juxtaposed with thedeep-to-middle crust during lateOttawan orogenic collapse (Rivers2008, 2012). In the case of the westernAdirondacks, the detachment faultaccommodating the collapse was thenorthwest-dipping CCZ that juxta-posed the Shawinigan amphibolitefacies assemblages of the ALT againstthe granulite facies assemblages of theAHT.

The large-scale F2 and F3folds in the ALT developed during theShawinigan Orogeny (Fig. 7C). Thetime of origin of the upright F4 andF5 folds is uncertain, but they mayhave formed when the ALT was partof the orogenic lid, as seems to be thecase elsewhere in the GrenvilleProvince (Rivers 2012). Within theAHT, evidence for widespread penetra-tive Shawinigan orogenesis and meta-morphism is provided by deformed1.18–1.16 Ga pegmatitic leucosome inmetapelite identical to those in theLowlands (Heumann et al. 2006; Bick-ford et al. 2008) and by stronglydeformed, foliated xenoliths in ca. 1.15Ga plutons (McLelland et al. 2010a).Heumann et al. (2006) employedEMPA Ultrachron dating of monazitein oriented thin sections from an isocli-nally (F2) refolded isocline (F1) todemonstrate that early axial planar foli-ation (S1) formed at ca. 1.18 Ga (i.e.,

Shawinigan Orogeny), whereas thelater (S2) axial planar foliation formedat ca. 1.05 Ga (i.e., Ottawan Orogeny).This result provides confidence inassigning a Shawinigan age to manyminor F1 isoclines. It is worth notingthat, prior to dating leucosome inmetapelite, the full extent and nature ofShawinigan metamorphism in theAHT remained largely camouflaged bythe intense Ottawan overprint. It ispossible that similar considerationsmay help to explain the apparentpaucity of Shawinigan ages in much ofthe eastern Grenville Province; howev-er, we note that Wodicka et al. (2003)reported pluton ages of ca.1.18–1.17Ga in the vicinity of the Havre St.Pierre anorthosite massif.

Post-Contractional AMCG Magmatism at the End of theShawinigan Orogeny, ca. 1.16–1.4GaThe AHT is underlain largely by avoluminous AMCG suite that includesthe large Marcy Anorthosite massif(Figs. 4, 7D). Extensive SHRIMP2geochronology has provided evidencethat the granitoid rocks, anorthosite,and associated olivine gabbro arecoeval at 1154 ± 6 Ma (McLelland etal. 2004; Hamilton et al. 2004). AMCGsuites of similar age are present in theCanadian Grenville Province (e.g.Morin, Lac St. Jean, Atikonak; Fig. 6E)as well as southern Norway. The obser-vation that these suites were emplacedduring the terminal stages of contrac-tional orogeny provides an importantboundary condition regarding their ori-gin (Corrigan and Hanmer 1997;McLelland et al. 1996, 2010b). In asmuch as this stage is commonly char-acterized by extensional orogen col-lapse accompanied by leucograniticmagmatism, McLelland et al. (2010b)interpreted the emplacement of theAMCG suite to be the result of delam-ination of the orogenic root followedby ascent of asthenosphere to the baseof the continental crust where itunderwent depressurization-meltingresulting in gabbroic melts that pondedat the crust–mantle interface due todensity controls. As discussed byEmslie (1978) and Emslie et al. (1994),the ponded gabbroic melts precipitatedolivine and pyroxene that sank backinto the mantle and andesine-

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labradorite plagioclase that floated atthese pressures creating a low densitycrystal mush. Advected heat, as well asheat of crystallization, led to wide-spread melting of the calcalkalinelower crust and the production ofAMCG intrusions. In the case of theAHT at ca. 1.155 Ga, the crustal melt-ing appears to have been largely anhy-drous and produced orthopyroxene-bearing rocks. By contrast, similar late-to post-orogenic events at ca. 1.05 Gawere accompanied by large influxes ofhydrothermal fluids that promotedcrustal melting and generatedleucogranite represented by the LyonMountain Granite in the AHT (Fig. 4).In either case, the large-scale anatexisof the crust weakened the orogen,which then underwent extensional col-lapse as AMCG plutons ascended tothe mid crust (Selleck et al. 2005;McLelland et al. 2010a, b; McLellandand Selleck 2011).

It should be emphasized thatthe key to AMCG genesis is to bringasthenosphere to the base of the crustand that there exist a variety of ways todo this. Among these are mantleplumes, back-arc rifting, subductionrollback, ridge subduction and reactiva-tion of crustal-scale shear zones. In thecase of the Adirondacks, geochronolo-gy strongly favours the delaminationmodel presented here.

The Hawkeye Granite Event, ca. 1.1GaThe Hawkeye Granite suite (Fig. 4) isthe least voluminous plutonic suite inthe AHT. Unlike the AMCG granitoidbodies it contains hornblende as themajor mafic mineral and, in manyplaces; Hawkeye Granite stronglycrosscuts AMCG units. Six samplesdated by multigrain TIMS methods(McLelland and Chiarenzelli 1990)yielded ages ranging from 1103 ± 15 to1090 ± 7 Ma with a weighted averageage of 1096 ± 6 Ma. These ages over-lap with the major periods of magma-tism in the Midcontinent Rift (MCR)given by Paces and Miller (1993) andterminate at approximately the sametime as the onset of thrusting that shutoff magmatism in the rift (Cannon1994). We consider that this is notcoincidental but, if a far-field causeand effect exists, it will take moreresearch to establish it. In Figure 7E,

the Hawkeye Granite is shown evolv-ing via gabbro-induced heating in anextensional setting that is consistentwith a far field link to the MCR; how-ever, this represents more speculationthan it does fact.

The Ottawan Phase of the Grenvillian Orogeny, 1.09–1.03 GaThe ca. 1.1 Ga Hawkeye Granite con-tains penetrative foliation and lineationthat must be due to deformation in theOttawan Orogeny that was the finalmajor deformational event in the AHT(Rivers 2008, McLelland et al. 2010a).This conclusion is supported by mon-azite dating that yields robust Ottawanages but only weak and irregular Rigo-let signatures (McLelland et al. 2004;Heumann et al. 2006). We conclude,therefore, that the onset of theOttawan Orogeny postdated 1.1 Ga.On the basis of data from theGrenville Province in Canada, as wellas the timing of thrusting in the MCR,we place the onset of the Ottawanorogenic phase at ca. 1.09 Ga in theAHT. The end of the Ottawan oro-genic phase is bracketed by zirconcrystallization ages of 1.04 Ga forundeformed pegmatite as well as mon-azite ages of ca. 1030 Ma (McLellandet al. 2010a; Lupulescu et al. 2011;McLelland and Selleck 2011; Wong etal. 2012).

With the possible exception oftwo imprecise multigrain zircon ages ofca. 1.08 and 1.07 Ga, no Ottawan plu-tons older than 1.05 Ga are recordedin the AHT (McLelland et al. 2010a).However, monazite ages, and evidencefrom the Grenville Province in Canadademonstrates that large-scale deforma-tion had begun by ca. 1.09 Ga and thebuilding of an extensive, 60–80 kmthick orogenic plateau was underway(Figs. 6F, 7F). Ductile flow at the mid-crustal levels such as represented bythe AHT resulted in large-scale recum-bent F2 isoclines that span the widthof the southern Adirondacks (Figs. 4,5). Modeling indicates that such struc-tures take tens of millions of years todevelop (Jamieson and Beaumont2011). Conversely, the F2 folding andassociated penetrative deformation haslittle, or no, effect on the ca. 1.05 GaLyon Mountain Granite (Figs. 4, 5).This implies that F2 folding must haveterminated by this time thus bracketing

the F2 deformation into the intervalca. 1.09–1.05 Ga. Peak metamorphicmineral assemblages in these rocksrecord temperatures of 750°– 850°Cand pressures of 6–9 MPa consistentwith mid-crustal granulite-facies condi-tions (Bohlen et al. 1985; Florence etal. 1995; Kitchen and Valley 1995; Dar-ling et al. 2004; Storm and Spear 2005).There has been less certainty aboutwhen these granulite-facies P, T condi-tions developed in the AHT, butrobust evidence has recently been pro-vided by mineral assemblages, coupledwith zircon dating, in the famous GoreMountain garnet mine which exposesthe world’s largest garnet crystals (aver-age diameter ~18 cm, largest diameter~1 m; McLelland and Selleck 2012).

Gore Mountain is situated inthe central AHT immediately north ofthe Oregon dome anorthosite massif(Fig. 4[G] and Fig. 5). The deposits areexposed in an open pit mine in coarsealmandine amphibolite that is 50 to100m wide and extends E–W forapproximately 1.5 km and 50–100mwide. The ore body is situated withinca. 1.155 Ga AMCG rocks consistingof a small body of anorthositeenclosed within a large charnockitepluton. A steep border fault of uncer-tain displacement along the southernmargin of the deposit juxtaposes gar-net amphibolite against charnockite tothe south and is associated with injec-tions of ca. 1.05 Ga pegmatites thatare part of the Lyon Mountain Granitesuite. Typical exposures (Fig. 8) displaylarge garnets with coarse, black horn-blende rims or shells set in a muchfiner, gray matrix of plagioclase, horn-blende, and minor orthopyroxene. It iscommon for the garnets to be aligned(Fig. 8A) along directions that we inter-pret to be healed, ductile shears thatare common in the mine (Goldblumand Hill 1992). This is supported bythe occurrence of veins of garnetrimmed by black hornblende along theouter margin of the vein (Fig. 8F).

Gore Mountain megacrysticgarnet crystals show only minimal zon-ing, which is restricted to their margins.Isotopic dating of the garnets has beenaccomplished by Lu/Hf (Connelly2006) and Sm/Nd methods (Basu et al.1989; Mezger et al. 1992). An averageof these results yields a crystallizationage of 1049 ± 5 Ma, which is within

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Figure 8. Typical exposures in the Gore Mountain open pit mine. A) Representative outcrop of Gore Mountain megagarnetamphibolite with 10-15 cm garnet crystals enclosed by rims of black hornblende set in a matrix of gray, finer grained plagio-clase, hornblende, and subordinate orthopyroxene. Note the alignment of garnet megacrysts along diagonal from lower left toupper right. Scale is given by hand in upper left corner of photo. B) Idiomorphic crystal of garnet typical in the Gore Moun-tain occurrence. C) Aligned garnet grains with patches of white plagioclase containing bronze, euhedral crystals of orthopyrox-ene. The assemblage represents the reaction that is typomorphic of the transition from upper garnet amphibolite facies to gran-ulite facies, i.e., Hbl + Grt ! Pl + Opx ± Cpx + V. The boudinaged aspect of the garnet occurrences suggest the reaction wasaccompanied by extensional strain. D) Close-up of reaction in C) that has gone to near completion manifested by deep embay-ment of garnet and nearly complete consumption of hornblende. E) Dikes of ca. 1047 Ma pegmatite crosscutting ca. 1050 Mamegacrystic garnet amphibolite. F) Vein of euhedral garnet crystals bordered by rims of black hornblende suggesting that theassemblage formed by hydrothermal fluids moving along a nearly horizontal ductile shear zone. (Modified from McLelland andSelleck 2011).

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error of the average SHRIMP2 age1050 ± 10 Ma based on 11 samples ofLyon Mountain Granite exclusive ofpegmatite (e.g. McLelland and Selleck2011). The average of 8 SHRIMP2U/Pb zircon ages obtained from latepegmatite intrusions associated withLyon Mountain Granite is 1044 ± 7Ma (McLelland and Selleck 2011).Monazite ages cited earlier indicategrowth of rims at ca. 1050 Ma and vir-tually none younger than ca. 1036 Ma(Wong et al. 2012). Clearly a culmina-tion in Ottawan igneous and metamor-phic activity took place at ca.1050–1035 Ma. Clarification of thisculmination is provided by furtherexamination and discussion of theGore Mountain megagarnets asdescribed below.

Figure 8A shows a typicalview of aligned Gore Mountain gar-nets and their rims. Note, however,that Figure 8C, D show white patchesof plagioclase (An40–60) together withbronze orthopyroxene (En50–60). Thewhite patches embay into the garnetgrains and their rims (McLelland andSelleck 2011). Figure 8D provides aclose-up of a fully developed plagio-clase and euhedral orthopyroxene rimthat embays into the garnet and hasconsumed essentially the entire horn-blende rim. Minor clinopyroxene isalso a product but forms on remnant,or groundmass, hornblende. Clearlythe garnet and hornblende have react-ed to yield coexisting plagioclase andpyroxene as well as a fluid phase. Thisis a well-known reaction and has beenextensively studied (de Waard 1965;Spear 1993, and references therein). Itreflects passage from the upper alman-dine amphibolite facies (~750°C, 6–7MPa) into the granulite facies at tem-peratures at, or slightly above, 800°Cand pressures of 6–9 MPa as reportedfor Adirondack granulite-facies assem-blages (Storm and Spear 2005). Thereaction establishes a prograde, clock-wise path for Ottawan metamorphismand fixes its inception as slightly post-1.05 Ga, given that the ca. 1.05 Gamegagarnet grains had already formed.EMPA Ultrachron dating of nearbymonazite yields an average age of 1046± 14 Ma for thick mantles and 1026 ±5 Ma for thin outer rims (Wong et al.2012). There are no additional over-growths on the monazite grains. These

results demonstrate that peak meta-morphism took place shortly after for-mation of the megagarnet andemplacement of ca. 1.05 Ga LyonMountain Granite and then endedrather abruptly. Our interpretation isthat by ca. 1.05 Ga, the overthickenedOttawan lithosphere had delaminatedand the ascent of asthenosphere to thebase of the crust provided a thermalimpulse that raised temperatures in theAHT to granulite-facies conditions.Simultaneously, the lower and mid-crust were weakened by the high tem-peratures and influx of fluids. Crustalmelting produced the ferroan, A-typeLyon Mountain Granite that ascendedas the weakened orogen underwentrapid collapse (Fig. 7G). The coolupper crust (i.e., ALT orogenic lid) wasdown-faulted into juxtaposition withthe granulite-facies infrastructure pre-served in the AHT. In the west, themajor collapse took place along theCarthage-Colton Shear Zone (CCZ) atca. 1.047 Ga and the Lyon MountainGranite within the fault is bothdeformed by and crosscuts the shearzone (Selleck et al. 2005) documentingthe contemporaneity of the emplace-ment of Lyon Mountain Granite withorogen collapse. Recently, Bonamici etal. (2011) used δ18O zoning profiles intitanite to compute cooling rates in theCCZ, and their results showed thatcooling took place at 6–60 times previ-ous estimates with the best fit being acooling rate of 30–50°C/m.y., a ratevastly in excess of earlier estimates of~2°C/m.y. (Mezger et al. 1991; Streep-ey et al. 2001). It is inferred that thedifferences result from time-averagingof several rapid pulses of down-fault-ing and cooling followed by longerintervals of quiescence. The averagecurve through these punctuated eventsyields much slower cooling rates thanrevealed by the steep collapse intervals.

A record of orogen collapse isalso preserved in the easternmostAdirondacks and exposed in road cutsnear Comstock, NY (Fig. 5[C]). Here,foliation in strongly deformed ca. 1.155Ga augen granite-gneiss dips to theeast-southeast at ~30–40° and containsstrong down-dip ribbon lineation(Wong et al. 2012). Kinematic indica-tors, including spectacular tails on alka-li feldspar megacrysts, document a nor-mal sense of fault displacement.

EMPA Ultrachron dating of monazitesestablishes peak Ottawan metamor-phism at ca. 1.05 Ga with a smallerpulse at ca. 1.03 Ga. Orientation of themonazite grains, and especially the tipsof the grains, indicates down-to-the-east displacement at ca. 1.05–1.03 Ga.Together with the timing of displace-ment along the CCZ, this documentsformation of a symmetrical gneissdome or core complex (Fig. 7G) dur-ing the terminal collapse of theOttawan Orogen (Wong et al. 2011,2012; McLelland et al. 2012). The dif-ference between the two areas is thathanging wall rocks are not exposed inthe eastern AHT suggesting that amaster collapse fault may lie buriedbeneath the Paleozoic cover to the east(McLelland et al. 2011; Wong et al.2012). We suggest that this master faultis an extension of the TawachicheShear Zone (Corrigan and vanBreemen 1997) whose along-strike pro-jection passes just to the east of theAdirondacks in the Champlain Valley(Fig. 3 [TWZ]). This shear zone is sim-ilar in age to the eastern AHT shearzone and drops the 1.3–1.4 Ga Mon-tauban-La Bostonnaiss island arc ter-rane down to the east against theMorin and Mékinac terranes (Fig. 3).McLelland et al. (2010a) have suggest-ed that the Adirondack gneissdome/core complex may extendnorthward through the Morin Terrane(see also Rivers et al. 2012).

TECTONIC EVOLUTION OF THEGRENVILLIAN INLIERS IN THEAPPALACHIANSWhen considering the Grenvillianinliers of the Appalachians, it is impor-tant to keep in mind that these base-ment rocks were part of the Laurent-ian margin that first underwent exten-sion during the opening of Iapetus andthe formation of a passive continentalmargin. This was followed by severalmajor orogenies during which thatmargin was inverted during closure ofthe ocean basin and thrust westwardfor variable and unspecified distances.Accordingly, caution must be exercisedwhen comparing the histories of theMesoproterozoic massifs. As statedpreviously, reference to Table 1 will beuseful in navigating the data presentedbelow.

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Oldest RocksWe have already summarized occur-rences of ca. 1.4–1.3 Ga rifted Lau-rentian margin arc fragments of theDysart-Mount Holly suite in theAppalachian inliers from Vermont toNew Jersey and ca. 1.25 Ma Elzevirianintrusions found as far south as theBaltimore Gneiss domes. Figure 1shows the Grenvillian inliers in theAppalachians and Figure 2 summarizestheir SHRIMP2 zircon ages andneodymium model ages (TDM). Recent-ly, Tollo et al. (2010) reported an age ofca. 1.327 Ga for granoblastic gneiss ofintermediate composition (SiO2 =62%) in the Mt. Rogers area of thenorthern French Broad Blue Ridgemassif. These rocks exhibit calcalkalineaffinities on AFM diagrams, are mag-nesian, and plot in the volcanic arcfield on Rb versus (Yb + Ta) discrimi-nation diagrams. They are similar inage and chemistry to the Dysart-Mount Holly tonalite plutons of theAHT, Vermont, and southern New Jer-sey. This is the first recognition ofpossible candidates for the Dysart-Mount Holly suite this far south. Ifthis possibility were borne out, itwould strengthen ties between theGrenvillian inliers of the southernAppalachians and the GrenvilleProvince itself. Further research willhelp to clarify the issue. In the BlueRidge French Broad massif west ofMars Hill (Fig. 1), Berquist (2005)dated similar orthogneiss at ca. 1.38Ga. In the context of the oldest rocksin the Grenvillian inliers, it is impor-tant to note that the ‘ca. 1.8 Gaorthogneiss’ reported from RoanMountain in the Mars Hill Terrane(Carrigan et al. 2003) has been reinter-preted as a granulite-facies metasedi-mentary unit, i.e., Carvers Gap Gran-ulite Gneiss and Cloudland Gneiss(Southworth et al. 2010; Tollo et al.2010; Aleinikoff et al. 2012, 2013). Zir-con in these units is clearly detrital inorigin and ranges in age from 1.68 to1.0 Ga indicating a maximum deposi-tional age ca. 1.0 Ga. These rocks arepart of a narrow belt of paragneissdeposited on Grenvillian basementfrom Virginia to North Carolina andmetamorphosed shortly after ca. 1.0Ga during the Rigolet phase of theGenvillian Orogeny (Aleinikoff et al.2013). The lack of known ca. 1.68 Ga

crust in southeast Laurentia suggeststhat zircon of this age may have beenderived from an exotic source.

Shawinigan and AMCG Age EquivalentsWithin the Mount Holly complex, Ver-mont, and nearby Athens and Chesterdomes, Ratcliffe and Aleinikoff (2001,2008) dated a foliated granite at 1.172Ga, and in Massachusetts 35% of theBerkshire Massif basement is underlainby 1.179 Ga (TDM = 1.46 Ga) Tyring-ham granitic gneiss that intruded allother mapped units, which are general-ly similar to paragneiss units in AHT,Green Mountains, and Hudson High-lands, New York, (Karabinos et al.2003; Ratcliffe and Aleinikoff 2008).In the Hudson Highlands, Ratcliffeand Aleinikoff (2008) obtained an ageof 1.174 Ga for the widespread StormKing hornblende granite (TDM = 1.46Ga, Daly and McLelland 1991) thatwas deformed prior to intrusion by theca. 1.14 and 1.13 Ga Canopus compos-ite pluton (Aleinikoff 2008) composedof biotite monzonite and hypersthene-biotite diorite. The pluton containsdikes of ferrodiorite and ferrogabbroclosely resembling late differentiates ofthe Marcy Anorthosite massif (McLel-land et al. 1994) The Brewster, NewYork, hornblende granite is dated at ca.1.13 Ga (Aleinikoff 2008). The Cano-pus and Brewster plutons crosscut ear-lier gneissic structures and isoclinalfolds thus confirming the Shawiniganage of the structures.

The Byram-Hopatcong horn-blende granite suite of the New JerseyHighlands has been dated at 1.176 Gaby Aleinikoff et al. (2007). Local, smalllayers of anorthositic affinity and thepresence of charnockite, suggest thatsome AMCG magmatism occurred inthe area (Volkert et al. 2010). There isclear evidence for Shawinigan defor-mation from the Green Mountainsthrough the New Jersey Highlands(Ratcliffe and Aleinikoff 2008). In thelatter area, Volkert et al. (2010) attrib-uted the deformation to closure of theeastern arc (Losee arc) of the Trans-Adirondack back-arc basin against Lau-rentia. In their interpretation, this wasfollowed by delamination-related intru-sion of the Byram-Hopatcong suite A-type granite plutons into the deformedcollision zone (Volkert et al. 2010).

Plutons of Shawinigan age (ca.1.18–1.14 Ga) are well represented inthe Shenandoah and French BroadBlue Ridge massifs (Fig. 1). Within theShenandoah massif, there exists arange of ages from 1.183–1.030 Ga(Southworth et al. 2010) with a con-centration of ages into two groups: ca.1.18–1.14 Ga and 1.06–1.03 Ga. Theolder group is designated as MagmaticInterval I (Tollo et al. 2006, 2012), andthe age overlaps with similar rocks inthe Adirondacks, suggesting geologicconnectivity between the Appalachianinliers and Shawinigan-age intrusionsin the Adirondacks. Geochemically, theMagmatic Interval I suite in theShenandoah Massif extends from gra-nodiorite to granite and is subalkaline,tholeiitic, and transitional between per-aluminous and metaluminous (Tollo etal. 2006). These characteristics are notunusual for rocks evolving in a conti-nental margin suite, and almost allsamples plot within the fields of colli-sional granite and volcanic-arc graniteson a Rb versus (Y + Nb) Pearce dis-crimination diagram (Tollo et al. 2006).Most of the Group 1 rocks display A-type chemistry, which is interpreted asreflecting derivation by melting ofdeep crustal calcalkaline precursors(Menuge et al. 2002 and referencestherein). Granitoid rocks of theAdirondack AMCG suite also displayA-type chemistry but are notably lowerin SiO2 than those of the Shenandoahmassif and plot in the within-platefield of the Rb versus (Y + Nb) dia-gram. These differences may reflectdifferences in source compositions,extent of fractionation, etc.

The tectonic setting of Mag-matic Interval I rocks remains some-what enigmatic. Although their similar-ity in age to Shawinigan rocks in theAdirondacks suggests formation withina similar tectonic setting, there is cur-rently little persuasive evidence favour-ing strong Shawinigan deformation/metamorphism in the ShenandoahMassif, although investigations doappear to support some orogenesis atca.1.155–1.144 Ga (Tollo et al. 2006;Southworth et al. 2010).

Within Magmatic Interval 1rocks in the Smoky Mountains at thesouthern end of the western FrenchBroad massif (Fig. 1), Aleinikoff et al.(2007, 2012) have dated granitic leuco-

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some in migmatites at ca. 1.194 Ga andgneissic granitoid rocks at 1.167 Gaboth overlapping with Shawiniganmetamorphism within MagmaticGroup I in the AHT. At Mt. Rogersnear the northern extremity of theFrench Broad massif, Tollo et al.(2006) have dated granitoid rocks at ca.1.174–1.161 Ga. These plutons wereintruded into hot (~750°C) upperamphibolite-facies crust at conditionsreflecting regional Shawinigan meta-morphism. Given these observations,we suggest that it is only a matter oftime before metamorphism of Shaw-inigan age is identified in the Shenan-doah Massif. Lacking from the Shaw-inigan age rocks of the Appalachiansare equivalents of the strongly calcalka-line, subduction-related 1.21–1.17 Gaintrusions of the ALT (i.e., AntwerpRossie suite, Hermon Granite, andHyde School Gneiss), which may implythat, unlike the ALT, the Appalachianmassifs were not situated above a sub-duction zone.

Other Interval I granitic rocksin the southern Appalachian BlueRidge include the Wiley quartzofelds-pathic augen gneiss of the TallulahFalls Dome (Fig. 1) dated by SHRIMPat 1.158 Ga (Carrigan et al. 2003;Hatcher et al. 2004). Within the Tox-away Dome (Fig. 1) orthogneiss yieldages of 1.151 Ga and 1.149 Ga (Carri-gan et al. 2003).

A major difference betweenthe Magmatic Interval 1 granitoidrocks of the northern and southernBlue Ridge massifs and the AMCGsuites of the Adirondacks is theabsence of large masses of anorthositefrom the former. Anorthosite is alsoabsent from Shawinigan-age equiva-lents within the Grenvillian inliers ofthe New England and New JerseyAppalachians. Although small slivers ofanorthosite have been reported in theShenandoah Massif (Pettingill et al.1984; Bartholomew and Lewis 1992),there is a notable absence of signifi-cant occurrences in the Blue Ridgemassifs. This does not mean that themassifs are not related to contempora-neous rocks in the Adirondacks sincesome differences would be expectedwithin any large suite of rocks. Onerather extensive body of Adirondack-type anorthosite occurs withincharnockitic gneiss in the Honey

Brook Uplands inlier (Fig. 1) in Penn-sylvania (Crawford and Hoersch 1984;Tarbert 2007). It has not been directlydated, but Pyle (2004) reported thatmonazite in adjacent charnockite con-strains the anorthosite age to >1070Ma. Sinha et al. (1996) analyzed theanorthosite for lead isotopes andreported that a plot of whole rock206Pb/204Pb versus 207Pb/204Pb forms alinear correlation equivalent to a207Pb/206Pb age of ca. 1.132 Ga, which,given the uncertainties inherent in themethod, is consistent with the 1.155Ga age of Adirondack anorthosite. Ona 207Pb/204Pb versus 206Pb/204Pb plot(Sinha and McLelland 1999) sevenwhole rock samples from HoneyBrook plot very close to the Adiron-dack data (Fig. 9). It is possible thatthis anorthosite was emplaced at ca.1.155 Ga, but SHRIMP II dating ofzircon is required.

Ca. 1.13–1.11 Ga MagmatismThe Appalachian Blue Ridge massifscontain minor plutons of granitic com-position (Magmatic Interval II; Tollo etal. 2006; Southworth et al. 2010) thatare similar in age (i.e., 1.14–1.11 Ga) tothe 1.12–1.1 Ga Hawkeye Granite ofthe Adirondacks. Although MagmaticInterval II (SHRIMP II ages) is signif-

icantly older than the Hawkeye Gran-ite, the ages for the latter wereobtained by multigrain techniques andare probably too young due toOttawan overgrowths on the zircongrains. Six samples of Magmatic Inter-val II rocks have been dated bySHRIMP and are summarized bySouthworth et al. (2010). The tectonicsetting of the Group II granite plutonsremains uncertain. Note that the Mag-matic Group II suite was originallyknown as the Mitchell Granite (Tolloet al. 2006) and is so designated onFigure 2.

Ottawan Magmatism and MetamorphismOttawan orogenesis and magmatismare well represented in almost all ofthe Grenvillian inliers in the Appalachi-ans. The only exceptions are theMount Holly complex of the GreenMountains and the Berkshire Massifwhere evidence is more cryptic. Withrespect to the Mount Holly complex,evidence for Ottawan activity is: 1) anage of 1.036 Ga for migmatitic, peg-matitic augen gneiss (Ratcliffe andAleinikoff 2001, 2008; Aleinikoff et al.2011), 2) four SHRIMP ages of ca.1.06-1.07 Ga zircon rims from olderrocks (Ratcliffe et al. 2011), and 3)

Figure 9. Plot of whole-rock Pb isotope ratios shows the Amazonian affinity ofthe southern and central Appalachians and the distinctly different, and stronglycoherent, data for the Adirondacks, Llano, Labrador, and midcontinent areas.(After Sinha et al. 1996, Sinha and McLelland 1999, Lowey et al. 2003, McLellandet al. 2010a, and Fisher et al. 2011).

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folding and metamorphism of ca.1.149–1.119 Ga augen granite areinterpreted to be the result of theOttawan Orogeny at ca. 1.05 Ga. Thescarcity of Ottawan ages in the MountHolly complex suggests cautionregarding interpretation of the extentof high-grade Ottawan metamorphismin the complex because of its Paleo-zoic overprint. On the other hand, asnoted previously, the determination of‘old’ 40Ar/39Ar ages suggest that it mayform part of the orogenic lid. In theBerkshire Massif, the only evidence forOttawan activity is the common occur-rence of inherited ca. 1.07-1.05 Gacores in the zircons of ca. 1.005 Gaalaskite sills (Karabinos et al. 2003).Although this is only indirect evidencefor local Ottawan orogenesis, it doesstrongly imply the existence of ca. 1.05Ga Lyon Mountain Granite sourcerocks at depth.

Within the Hudson Highlands,Walsh et al. (2004) obtained ages of1.057–1.048 Ga for anatectic leuco-some in migmatites, thus confirmingOttawan high grade metamorphism.Aleinikoff et al. (2012) have dated theCrystal Lake Granite at 1.058 Ga and axenotime-monazite pod in paragneissat 1.036 Ga. In addition, Walsh et al.(2004) dated deformed Danburymegacrystic granite at 1.046 Ga, an ageequivalent to that of the chemicallysimilar Lyon Mountain Granite of theAHT. In the New Jersey Highlands,undeformed trondhjemitic anatectitewithin the ca. 1.3 Ga Losee TonaliticSuite have yielded an Ottawan age of1.030 Ga (Volkert et al. 2010).

The State Farm Gneiss of theGoochland terrane (Fig. 1), has beendated by Sm–Nd (Owens and Samson2004) at 1.046–1.023 Ga and at1.057–1.013 Ga (Owens and Tucker2003) using single grain zircon dating.Aleinikoff et al. (1996) also employedsingle grain zircon dating to obtain acrystallization age of 1.045 Ga for thecoeval Montpelier Anorthosite. Theanorthosite is similar in age and com-position the CRUML-type (Chateau-Richer, St. Urbain, Matawa, Labrieville)AMCG suite of eastern Quebec(Owens et al. 1994). It is also similar tothe ca. 1.045 Ga Roseland anorthositeof the northern Shenandoah Massif(Pettingill et al. 1984; Sinha et al. 1996;Owens and Samson 2004).

In the northern Shenandoahmassif there exists ample evidence forOttawan igneous activity and metamor-phism. Ages for 12 dated samplesrange from 1.063 to 1029 Ga and com-positions from orthopyroxene-bearingleucogranite to monzogranite. Togeth-er these lithologies represent MagmaticInterval III of Tollo et al. (2010) andSouthworth et al. (2010). Metamorphicrims on these and older zircons fallinto the age range of ca. 1.055 - .979Ga, consistent with Ottawan metamor-phism. According to Southworth et al.(2010), Ottawan deformation was lessintense here than elsewhere in theAppalachians. In part, this may be theresult of the late- to-post-tectonicemplacement of the Group III plu-tons, and this is also true in theAdirondacks where the late- to post-tectonic ca. 1.05 Ga Lyon MountainGranite did not experience the earlynappe-forming deformation thataffected Shawinigan and older rocks. Itis also the case for the ca. 1.16–1.15Ga Parc des Laurentide Leucogranite(Fig. 3 [PdL]) in central Quebec (Corri-gan and van Breemen 1996). Accord-ingly, we concur with Tollo et al. (2004)who interpreted the A-typeleucogranitic Magmatic Interval IIIintrusions in the Shenandoah Massifto be compositionally and chronologi-cally similar to Lyon Mountain Graniteof the AHT and to have formed dur-ing ca. 1.05 Ga orogen collapse at theend of the Ottawan Orogeny.

Farther south in the SmokyMountains of the French Broad Mas-sif, weakly foliated granite bodies yield-ed ages of ca. 1.045–1.02 Ga, and inthe Mt. Rogers area at the north end ofthe massif weakly foliated granite bod-ies yielded ages of ca. 1.06 Ga (Tollo etal. 2010). In the nearby GrandfatherMountain Window, the Blowing RockGranitic Gneiss has been dated at ca.1.08 Ga (Carrigan et al. 2003), and inthe Pine Mountain Window high-gradegneiss yields ages of ca. 1.06–1.01 Ga(Steltenpohl et al. 2004).

All of the foregoing observa-tions support the conclusion that theigneous and metamorphic history ofthe AHT is also present in the Grenvil-lian inliers of the northern and south-ern Appalachian massifs and that thisrepresents Grenvillian connectivitythroughout the region. The Shawinigan

Orogeny in the Appalachians is inter-preted to have resulted from the ca. 1.2Ga collisions with Amazonia as itworked its way northward causingtranspressional strain along the easternLaurentian margin and by ca.1.09–0.098 Ga made a direct, north-west verging collision with theGrenville Province (Tohver et al. 2002,2004). In a related way, the Shawiniganclosure of the CMB and TAB mayhave been due, in part, to a far-fieldeffect related to the displacement ofAmazonia along the eastern Laurentianmargin (Figs. 6, 7). This is an attractivemodel; however, and not surprisingly,the final picture will be more complex.We discuss this in the following sec-tions on Nd model ages and Pb-iso-tope arrays.

Rigolet Phase of the GrenvillianOrogenyThe ca. 1.01–0.98 Ga Rigolet phasethat closed out the Grenvillian Oroge-ny is not observed in the ALT and islimited in the AHT to zircon over-growths. A small granitic dike swarmdated at ca. 0.933 Ga (McLelland et al.2001) in the southern AHT is tooyoung to be part of the Rigolet phasebut is noted here for completeness.Magmatism of Rigolet age is well rep-resented in the Grenvillian inliers ofthe New England Appalachians. Rat-cliffe and Aleinikoff (2001) dated theCardinal Brook rapakivi granite suite ofthe Mount Holly complex and Chesterand Athens domes at ca. 1.0 Ga, andKarabinos and Aleinikoff (1990) datedthe granitic Bull Hill Gneiss at ca.0.965 – 0.945 Ga. In the BerkshireMountains, Karabinos et al. (2003)obtained ages of ca. 1.0 - 0.997 Ga foralaskite dikes and sills. In New Jerseythe pristine, undeformed Mt. EveLeucogranite yielded an age of 1.02 Ga(Drake et al. 1991; Gorring et al. 2004;Volkert et al. 2010) that is a borderlinecandidate for a Rigolet intrusion,although an argument for late-Ottawanintrusion could also be made. Withinthe Pine Mountain Uplift of the south-ern Appalachians high-grade graniticgneiss yielded an age of 1.011 Ga(Steltenpohl et al. 2004; Hetheringtonet al. 2006, 2007).

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AGE OF SOURCE ROCKS FORADIRONDACK–APPALACHIANMESOPROTEROZOIC MAGMATISM

Neodymium Model AgesNeodymium model ages, TDM, providea powerful tool for ascertaining thetime at which a given igneous rock, orits precursor, was extracted from themantle. Perhaps not surprisingly, theigneous rocks of most large provincesyield the same TDM, even if individualzircon crystallization ages differ (cf.Daly and McLelland 1991). In recentyears, an increasingly broad databasehas been assembled demonstrating thatterranes with disparate tectonic histo-ries may exhibit similar neodymiummodel ages and, accordingly, were like-ly produced from the same mantlesource. One of the most impressiveassociations extends the length of theGrenville Province from Labrador tothe Adirondacks, the southeasternGranite-Rhyolite provinces, and theGrenvillian rocks of Texas. Along the~5000 km length of this belt, the old-est granitoid plutons have zircon crys-tallization ages of ~1.45–1.35 Ga andTDM ~1.55–1.4 Ga (Daly and McLel-land 1991; McLelland et al. 1996;Menuge et al. 2002; Roller 2004;Dickin and McNutt 2007; Slagstad etal. 2009; Dickin et al. 2010; Fisher et al.2010). Because the crystallization agesof the granitic plutons are within ~100m.y. of their mantle extraction ages,they are considered to be juvenileimplying an arc-related origin (e.g.Bowring et al. 1991). Accordingly, asoutheast-facing continental margin arcsetting has been proposed for the east-ern margin of Laurentia from1.48–1.35 Ga (Rivers and Corrigan2000; Menuge et al. 2002). High-graderemnants of the actual arc are wellexposed (Slagstad et al. 2009) in theMuskoka Domain (Fig. 3). As statedpreviously, this arc is characterized byTDM ages <1.55 Ga. For orthogneisswithin the Adirondacks, εNd versusage plots show that units with crystal-lization ages of ca. 1.155 Ga and ca.1.05 Ga lie on the same Nd evolutiontrajectories as the oldest (ca. 1.35 Ga)juvenile tonalite of the Dysart-MountHolly suite (Daly and McLelland 1991;McLelland et al. 1996).

In eastern Kentucky and cen-tral Tennessee, within ~250 km of the

southern Appalachian Grenvillianinliers (Fig. 1), three drill holes(OH–DC and KY–PU, CO–DC) haveencountered basement rocks, two ofwhich have yielded U/Pb zircon crys-tallization ages of 1.38 Ga (OH) and1.46 Ga (KY) (Van Schmus et al.1993b; Fisher et al. 2010). In addition,the sample KY yielded a TDM age of1.45 Ga (Fisher et al. 2010) and a troc-tolite from a fourth nearby drill-core,CG, produced a TDM of 1.38 Ga (Fish-er et al. 2010). It was thought thatthese rocks comprised part of theSouthern Granite-Rhyolite Province(crystallization ages = 1.4–1.34 Ga) orcorrelate with juvenile plutons intrudedinto parts of the Eastern Granite-Rhy-olite Province (crystallization ages =1.5–1.44 Ga). A question then aroseregarding what relationship the rocksmight have to the basement of thenearby Blue Ridge massifs. In order toaddress this issue, Fisher et al. (2010)determined Sm–Nd concentrations andTDM values for 14 southern FrenchBroad Massif samples and found onlyone value (1.43 Ga) below the1.88–1.55 Ga range of the other 13.These TDM ages, like others in thesouthern Appalachian inliers, are400–500 m.y. older than their magmat-ic crystallization ages. In contrast, theTDM age for drill-hole sample KY is1.45 Ga and for CG is 1.38 Ga, i.e.,normal ages for the Eastern Granite-Rhyolite Province, which lies in thesubsurface within 250 miles of theFrench Broad Massif. On the basis ofthese results, it seems highly unlikelythat the Eastern Granite-RhyoliteProvince forms the basement for thesouthern Appalachian Grenvillianinliers.

Recent studies by Tollo et al.(2006) and Southworth et al. (2010) inthe northern Shenandoah Massif haveproduced 11 TDM ages ranging from1.62 to 1.37 Ga, with 9 of the 11model ages exceeding 1.49 Ga (averageage of 1.51 Ga). The average age over-laps the range of the Eastern Granite-Rhyolite Province (1.55–1.43 Ga), butthe diagnostic 1.5–1.4 Ga arc plutonsof that province have not been recog-nized, making it unlikely that theShenandoah Massif represents anextension of the Eastern Granite-Rhy-olite Province.

It is possible that the

Appalachian results are explicable onthe basis of reworking of basementrocks, but the argument is painfullycontorted and full of special pleadingand speculation – especially given theproximity of the three drill-holes tothe Grenvillian inliers of the southernAppalachians. A path through this con-fusion was provided by Tohver et al.(2004) and Loewy et al. (2003, 2004),who noted the striking similaritiesbetween neodymium model ages in theFrench Broad Massif and those inAmazonia. We discuss this in the nextsection.

Pb Isotope Arrays and BasementMappingSinha et al. (1996) carried out a whole-rock Pb isotope investigation of thesouthern and central Appalachians, andSinha and McLelland (1999) produceda comparable study for the AdirondackMountains. The results are shown inFigure 9. Remarkably, the Pb–Pb arraysfrom the Grenvillian inliers in thesouthern Appalachians and Adiron-dacks data do not overlap. Instead, theAdirondacks plot in the same Pb–Pbspace as those for samples fromLabrador (Loewy et al. 2003, 2004), theGranite-Rhyolite Province (VanSchmus et al. 1993b), and the Llanoand Van Horn uplifts of Texas (Roller2004). In contrast, the Grenvillianinliers of the southern and centralAppalachians form an array that over-laps with that of the Sunsas Orogen ofsouthwest Amazonia (Tohver et al.2004). This result implies that theMesoproterozoic basement in thesouthern and central Appalachians isexotic to Laurentia and probably Ama-zonian in origin. It has been suggestedthat the Sunsas Orogen of westernBrazil was overthrust onto Laurentiaduring collision from ca. 1.2–1.15 Gaand transferred to Laurentia during theNeoproterozoic breakup of the Rodin-ian supercontinent (Tohver et al. 2002,2006; Loewy et al. 2003, 2004; McLel-land et al. 2010a; Fisher et al. 2010).Supporting this model is a recent pale-omagnetic pole for red beds in theAguapei Group (D’Agrella-Filho et al.2008) from which xenotime rims pro-vide a well-constrained 1.15 Ga age fordeposition. This result places thesouthwest Amazonian craton in prox-imity to the southeast margin of Lau-

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rentia at ca. 1.15 Ga and lends supportto the ca. 1.2 Ga collision of Amazo-nia with Laurentia. The collision wastranspressional and sinistral resulting infolding in Laurentia and strike-slipfaulting in Amazonia (Tohver et al.2004). We suggest that the ShawiniganOrogeny recorded in the AppalachianGrenvillian inliers resulted, in part,from the early phases of this collision,while the Grenvillian Orogeny was theproduct of a more head-on collisionwith the northeast-trending southernmargin of Adirondis and the GrenvilleProvince.

The inference of the presenceof exotic crust in the southern andcentral Appalachians implies the exis-tence of a cryptic suture between thatregion and the rest of Laurentia. Thisrealization has re-awakened interest inthe northeast-trending magnetic anom-aly (Fig. 1) referred to as the NewYork–Alabama lineament (King andZeitz 1978). The long-standing inter-pretation of this feature as an ancientsuture is strengthened by the observa-tion that it passes through a narrowtract between the drill holes in Ken-tucky and Tennessee that bear the iso-topic signature of the Eastern Granite-Rhyolite Province and the Grenvillianinliers of the southern and centralAppalachians with Amazonian isotopicsignatures. The exact location is com-plicated by Paleozoic thrusting andrestacking of the basement rocks.Notwithstanding this, the suture possi-bility remains viable and intuitivelyattractive.

THE GRENVILLE OROGEN IN TEXAS

Llano UpliftThe high-grade Mesoproterozoic coreof the Grenville Orogen is exposed inthe large (~9000 km2) Llano Uplift ofcentral Texas (Fig. 1). In addition, sev-eral small uplifts in west Texas andnortheastern Mexico (Fig.1) exposemid-to low P, T assemblages in rockswhere orogeny occurred during the lateMesoproterozoic to early Neoprotero-zoic. To the north, the Grenville Oro-gen is bounded by the Llano Front thatrepresents the northern edge ofGrenvillian deformation (Barnes et al.1999; Rohs and Van Schmus 2007) andappears to coincide with the northernlimit of the Abilene gravity anomaly,

interpreted to reflect a graniticbatholith. Near Van Horn (Fig. 1[VH]), the front is exposed as a narrow(2–5 km.) fold and thrust belt north ofthe overriding Steeruwitz Thrust (Soe-gaard and Callahan 1994; Mosher 1998;Bickford et al. 2011).

The Llano Uplift is underlainby multiply deformed igneous andmetamorphic rocks ranging in agefrom ca. 1.36–1.07 Ga including late-to post-kinematic granitic plutons withcrystallization ages of ca. 1.12–1.07 Ga(Walker 1992; Mosher 1998; Mosher etal. 2008). A wide range of lithologies isrepresented including ophiolite andeclogite, and regional metamorphicgrade was generally at upper amphibo-lite to granulite facies at ca. 1.15–1.12Ga (Mosher 1998). Three major tec-tonic domains have been defined: fromsouth to north and oldest to youngestthey are the Coal Creek, Packsaddle,and Valley Spring domains that extendacross the uplift (Mosher 1998). Theprincipal lithic and tectonic subdivi-sions of the Llano Province aredescribed below, and the account reliesheavily on the research of Mosher(1998), Rohs and Van Schmus (2007),and Mosher et al. (2008).

Coal Creek DomainThe Coal Creek domain is underlain bydioritic to tonalitic plutons structurallyoverlain by thrust sheets of ophioliteconsisting of serpentinized harzbur-gite. Together, the assemblage is inter-preted as an ensimatic arc that wasactive from ca. 1.325 to 1.275 Ga(Roback 1996). Significantly, thedomain exhibits an early magmatic andmetamorphic history not present else-where in the Llano Uplift supportingthe outboard origin of the arc. In addi-tion, Pb and Nd isotopic values for thedomain differ significantly from thatelsewhere in the uplift, and at itsnorthern extremity. The Coal Creekdomain structurally overlies theyounger Packsaddle Formation alongthe Sandy Creek ductile thrust zone(Roback 1996).

Packsaddle DomainThis polydeformed domain is dominat-ed by metamorphosed shallow shelfand slope deposits (e.g. marble,quartzite) with intercalated igneous andvolcaniclastic rocks (Mosher 1998,

2007). The supracrustal rocks record~30 m.y. of deposition on an ancientcontinental shelf along the southernmargin of Laurentia. Igneous layerswithin the sequence have yielded agesbetween ca. 1.25 and 1.24 Ga clearlypost-dating Coal Creek magmatism.One sample near the Sandy Creekthrust yields a 1.274 Ga age, but thiscomes from a complex area near thetectonized contact zone with the CoalCreek domain.

Valley Spring DomainThe Valley Spring domain contains ter-rigenous clastic rocks, but is dominatedby ca. 1.288–1.232 Ga granitic gneissand rhyolitic sheets that may representa continental-margin arc with subduc-tion to the north beneath Laurentia. Asingle sample of the older graniticgneiss (basement?) has yielded an ageof ca. 1.37 Ga. The Valley Springdomain is overthrust by the Packsaddleand Coal Creek domains and showsvariable degrees of deformation thatdecrease in intensity away from thethrust contact. Rivers et al. (2012) havesuggested that the Coal Creek arc maybe a candidate for a southern extensionof the Elzevirian CMB back-arc basinin the Grenville Province.

Orogenic Culmination in the LlanoUpliftOn the basis of existing data, Mosher(1998) and Mosher et al. (2008) sug-gested that north-northeast transportand eclogite-facies metamorphism inthe Llano Uplift was initiated at ca.1.147–1.138 Ga in the west and1.133–1.131 Ga in the east, where itcontinued until ca. 1.12 Ga. In orderto account for these observations,Mosher (1998) and Mosher et al.(2008) invoked a shift in subductionpolarity southward beneath the north-ern margin of a large, outboard south-ern continent with the accreted CoalCreek arc attached to its northern mar-gin by ca. 1.25 Ga. The Llano–south-ern continent collision is inferred tohave begun between 1.15 and 1.14 Gaand to have involved south-directedsubduction of the leading margin ofLaurentia producing eclogite, in addi-tion to that formed at the base of theCoal Creek arc. The subduction alsoled to jamming of the subductionzone, which enhanced uplift,

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retrothrusting, and northeastwardthrusting over the Laurentian margin(Mosher et al. 2008). Soegaard andCallahan (1994) termed these tectonicevents the Llano Orogeny.

The identification of thesouthern continent remains elusive, buta likely candidate is the Kalahari Cra-ton whose Pb–Pb array (Fig. 9) largelyoverlaps that of the Llano-Adirondacktrend (Eglinton and Kerr 1989; Dalzielet al. 2000; Loewy et al. 2003, 2011;Roller 2004; Fisher 2010). Paleomag-netic poles derived by Dalziel (1992)and Weil et al. (1998) are consistentwith this model. We conclude thatorogenesis in the Llano Provincebegan sometime between 1.15 and 1.14Ga and may have been due to collisionwith the Kalahari Craton. Mosher(1998) and Mosher et al. (2008) haveargued cogently that the eastern Lau-rentian collision with Amazonia (dis-cussed in a previous section) could nothave produced the approximatelyorthogonal coeval vergences that existbetween the southern and centralAppalachians and Llano Orogen norcan they explain the orthogonal diver-gence between the eastern Llano andWest Texas vergences. Notwithstandingthe foregoing, the case made for anAmazonian collision (Tohver et al.2002) has its own merits, and shouldnot be dismissed out of hand. Theissue of the ‘collision with a southerncontinent’ deserves further research.

Late- to Post-Tectonic MagmatismThe Llano Uplift is noted for theintrusion (ca. 1.12 –1.07 Ga) of largeferroan, A-type granitic plutons. Theolder (ca. 1.12–1.16 Ga) intrusions areslightly deformed and are consideredto be late-tectonic, but the younger,post-tectonic granite bodies (ca.1.1–1.07 Ga) are circular, undeformed,and have low-pressure contact aure-oles. The region shows evidence ofextension at the time of the late plu-tonism. Mosher et al. (2008) accountedfor these relationships by introducingslab-breakoff and delamination at ca.1.1 –1.07 Ga. Removal of the denselithosphere resulted in ascent of hotasthenosphere to the base of the crust,and these two factors led to buoyancyof the collisional orogen. Extensional,buoyant, and contractional collisionalstresses coexisted for a period of time

resulting in locally variable rifting andfolding/faulting. During this period,influx of asthenospheric heat resultedin increasing melting of continentalcrust of intermediate composition toyield ferroan A-type granite. Thisweakened the lower and middle crustleading ultimately to orogen collapseand the emplacement of late- andpost-tectonic 1.09–1.07 Ga granite plu-tons. Rivers et al. (2012) have suggest-ed that this late extension may reflectthe presence of an orogenic lid analo-gous to that in the Grenville Province.

West Texas and Northern MexicoSmall exposures of Mesoproterozoicrocks occur near Van Horn, Texas (Fig.1 [VH]), and at Sierra Del Cuervo (Fig.1 [SDC]) and Cerro Del Carrizalillo(Fig. 1 [CDC]) in Mexico. These pro-vide additional insights into the lateevolution of the Grenville orogenalong Laurentia’s southern margin.Among other things, the area containsthe only preserved occurrence of aGrenvillian foreland sedimentary basinin North America, although remnantsof such a feature may be preserved inScotland (Krabbendam et al. 2012;Bonsor et al. 2012).

Near Van Horn (Fig. 1 [VH])four 5–15 km long basement upliftsexpose the gently south-dippingSteeruwitz Thrust that contains, in itsfootwall, the 1.38 to 1.33 Ga polyde-formed (pre-thrust) upper greenschistto amphibolite facies bimodal metavol-canic and immature metasedimentaryrocks of the Carrizo Mountain Groupover polydeformed, but unmetamor-phosed, sedimentary rocks to the north(Soegaard and Callahan 1994). Thelowermost of the underlying units con-sists of stromatolitic, intertidal carbon-ate rocks of the Allamore Formationthat contain thin rhyolitic flows andwelded ash-flow tuffs dated at ca.1.255 Ga and providing the time ofdeposition (Jon McLelland 1996; Bick-ford et al. 2000). This unit is uncon-formably overlain by the TumbledownFormation consisting of sandstonewith rhyolitic clasts and large carbonateblocks derived from the Allamore For-mation. Both the rhyolitic clasts and arhyolitic felsic tuff at the top of theformation yielded an age of ca. 1.243Ga dating the time of deposition.Geochemistry of the bimodal suite and

mafic volcanic rocks suggests that theTumbledown Formation accumulatedin an intracontinental rift or a marginalback-arc basin. Unconformably abovethe Tumbledown Formation is theHazel Formation (~2500 m) consistingof boulder conglomerates and sand-stones derived principally from theunderlying Allamore and TumbledownFormations. In addition, unmetamor-phosed rhyolite and granite bouldersand clasts of unknown provenance arealso present and dated at ca. 1.13–1.12Ga (Roths 1993). Soegaard and Calla-han (1993) interpreted the agglomerat-ic Hazel Formation as formed in aprograding series of coalescing alluvialfans and sand dunes receiving veryimmature debris from a rapidly risingtectonic welt to the south. Tectonictransport of the thrust sheets com-posed of Allamore and TumbledownFormations over the Hazel Formationresulted in the northwest-directed dis-placement of the entire package. Theuppermost thrust sheet in the tectonicpackage consists of Carrizo MountainGroup units transported on theSteeruwitz Thrust, which has beeninterpreted as the culminating tectonicevent in the Van Horn area (Bickfordet al. 2000). However, the possibilitymust be reserved that the forelandthrust belt was produced by out-of-sequence faulting, during which theSteeruwitz thrust preceded the under-lying thrusts in the sedimentary pack-age (Grimes and Copeland 2004).

40Ar/39Ar dating of horn-blende and muscovite within amylonite zone related to the SteeruwitzThrust yielded ages of 1.04 Ga and1.03 Ga, respectively, thus dating latemovements on the fault (Jon McLel-land 1996; Bickford et al. 2000).Grimes and Copeland (2004) produced40Ar/39Ar ages of ca. 1.05–0.99 Ga forhornblende and muscovite in the Car-rizo and Van Horn Mountains. Collec-tively, these ages bracket the HazelOrogeny interpreted as a transpressiveevent that displaced units to the north-northwest (Soegaard and Callahan1994). Mosher (1998) and Mosher etal. (2008) considered the Hazel Oroge-ny to be a late stage, western extensionof the Llano Orogeny, but the age gapof ~90 m.y. suggests that it is bettercorrelated with the 1.01–0.98 Ga Rigo-let Orogeny that also involves low-

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grade foreland folding and thrusting.Five kilometers to the north of theSteeruwitz Thrust, the Allamore andTumbledown Formations are unde-formed and flat-lying. They are re-exposed to the west in the undeformedFranklin Mountains north of El Pasowhere they occur as large roof pen-dants in the ca. 1.12 Ga alkaline RedBluff Granites (Shannon and Barnes1991; Bickford et al. 2000).

The degree of ductile defor-mation and thermal metamorphism ofthe Carrizo Mountain Group increasestoward the south reflecting basin inver-sion causing stacking of thrust sheetsfrom increasing depth. The immaturenature of the sediment in the groupfavours their deposition in a continen-tal rift basin (Rudnick 1983; JonMcLelland 1996; Bickford et al. 2000).After replotting the data of Rudnick(1983) on immobile trace element dis-crimination diagrams of Pearce andCann (1973) and Pearce et al. (1984),Roths (1993) proposed that the CarrizoMountain Group accumulated in a riftsetting, consistent with the composi-tion of dominantly feldspathic, mica-ceous metasandstone units in the cen-tral and southern Carizzo Mountains.

Until recently, the existence ofa continental margin arc had not beendemonstrated in the area. However, therequired arc is now thought to be rep-resented by exposures at Sierra DelCuervo (Fig. 1 [SDC]), Mexico (Blount1993) that consist of metagabbro (ca.1.333 Ga), granite gneiss (1.274 Ga),metatonalite, and metadiorite all ofwhich are metamorphosed to amphi-bolite facies. Geochemical signatures inthese rocks indicate that they are simi-lar to those in magmas generated at thebase of an overthickened continentalor island arc (Gill 1981). Crosscuttingtrondhjemitic dikes have been dated at1.08 Ga (Blount 1993). Approximately100 km to the east, the Cerro Del Car-razilillo (Fig. 1) Mesoproterozoic upliftconsists of polydeformed and mutuallycrosscutting trondhjemite, granite, andtonalite produced during orogenesis.The existence of these two Mexicanexposures provides compelling supportfor an early (1.4–1.3 Ga) back-arc riftbasin along the southern Laurentianmargin from Llano to Van Horn and isconsistent with the culminating LlanoOrogeny caused by collision of the

southern continent at ca. 1.15–1.17 Ga.In contrast, collision with Amazonia atthis time fails to satisfy the directionsof Mesoproterozoic tectonic transportin Texas. In fact, the observation thatin west Texas tectonic transport is tothe north-northwest while in the east itis to the north-northeast suggests anindenter-type collision (Mosher 1998)that Amazonia would not be able toprovide from its position along theeastern margin of Laurentia.It should be noted, that one of us(MEB; Bickford et al. 2000) has sug-gested that the shallow-water deposi-tional characteristics of the AllamoreFormation, the generally low metamor-phic grade of all the West Texas‘Grenvillian’ rocks, and the bimodalnature of the volcanic rock assem-blages, indicate deposition within riftbasins. Further, the occurrence of suchfeatures as ca. 1.17 Ga alkaline syeniteat Pajarito Mountain in New Mexico(Kelley 1968), the 1.1 Ga ApacheGroup (Silver 1978; Wrucke 1989,1993), and the Unkar Group (Elston1993; Larson et al. 1994) in Arizona,and the ca. 1.08 Ga Pahrump Grouprocks in the Death Valley region ofsouthern California (Lanphere et al.1964; Heaman and Grotzinger 1992;Wright and Prave 1993), all of whichhave been interpreted as deposited inrift basins, led Bickford et al. (2000) topropose that a major transcurrent faultsystem extended across southern Lau-rentia that began ca. 1.12 Ga and per-sisted until about 1.08 Ga. It is possi-ble that the continental arc model pro-posed above could be consistent withthe formation of a major transcurrentfault system if the arc–continent colli-sion were transpressional in nature.

It can now be stated withsome confidence that the GrenvilleOrogen along the southern margin ofLaurentia has been reasonably welldefined and extends Mesoproterozoiccontinental margin arcs and Ottawancontinental collisions at least as farsouthwest as the Trans-Pecos regionsof Texas and northeastern Mexico, i.e.,a distance exceeding 5000 km, andrivaling the dimensions of theHimalaya–Alps orogenic belt.

SUMMARYRecent zircon U/Pb SHRIMP2geochronology, together with other

isotope geochemical research, haveprovided a much improved, and morecomplete, understanding of the evolu-tion of the Grenville Orogen in NorthAmerica. This review has focused onthe Grenville Orogen in the USA andhas also presented a brief summary ofthe long-lived Paleoproterozoic toMesoproterozoic continental marginarcs that now underlie the midconti-nent. These arcs were followed by theformation of the Grenville Orogenwhose major tectonic, metamorphic,and magmatic evolution is summarizedbelow.1. The earliest events in the tectono-

magmatic history of the Adiron-dacks took place during the inter-val 1.3–1.25 Ga when two back-arcbasins were formed by rifting of a1.4–1.3 Ga continental margin arcin what is now southwesternOntario. The rifted fragmentsserved as nuclei for the CMB andTAB that evolved into outboardarcs and are preserved in the AHTand Grenvillian inliers in Vermont,the Hudson Highlands, New JerseyHighlands and as far south as theBaltimore Gneiss domes. Recentwork by Tollo et al. (2012) hasdemonstrated that rocks of similarage and chemistry occur near thenorthern margin of the southernBlue Ridge Massif and may repre-sent a southernmost occurrence ofthe rifted continental-margin arc.This possibility warrants furtherresearch. In the Llano Uplift, aswell as in West Texas, northwardsubduction resulted in continentalmargin magmatism, rifting, andformation of back-arc basins at ca.1.4–1.3 Ga (Mosher 1998).

2. Closure of the Grenville Provinceback-arc basins at ca. 1.25–1.22 Garesulted in the Elzevirian Orogenyand calcalkaline magmatism as thearcs amalgamated. By ca. 1.2 Ga,the western basin (CMB) closedout, and the arc collided with east-ern Laurentia along the presentCentral Metasedimentary BeltBoundary Zone giving rise to anearly phase of the ShawiniganOrogeny. This caused subductionto step out to the east and flip to anorthwest polarity beneath theALT that then experienced calcal-kaline arc magmatism from ca.

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1.2–1.19 Ga. The Pyrites ophiolitecomplex was obducted onto theALT at this time. Elzevirian mag-matism has been identified in theGrenvillian inliers of Vermont,New York, and New Jersey. Far-ther south, in the Baltimore Gneissdomes granitoid plutons have beendated at ca. 1.2–1.24 Ga(Aleinikoff et al. 2004), an agerange that coincides with the Elze-virian Orogeny. In the Llano Upliftand West Texas, back arc basinsclosed out during this interval asthe southern continent approachedthe Laurentian margin.

3. Closure of the eastern basin(TAB) resulted in collision of theAHT western margin of Adirondiswith the ALT at the eastern mar-gin of the CMB. This initiated themajor collisional phase of theShawinigan Orogeny (ca.1.19–1.17 Ga) that caused upperamphibolite-facies metamorphismand anatexis in both the ALT andAHT. The ca. 1.18 Ga HermonGranite Gneiss was intruded earlyin the collision and the syntectonicHyde School Gneiss and the Rock-port granite were intruded at ca.1.72 Ga. During the collision, theALT was thrust over the AHT foran unspecified distance andformed an upper crustal orogeniclid. Following the cessation ofmajor contraction, the AMCGsuite was emplaced as stitchingplutons into both the AHT andALT. Rocks of 1.19–1.17 Ga ageare uncommon in the MountHolly complex but are widespreadthroughout the other Grenvillianinliers of the Appalachians empha-sizing the connectivity within thebelt. One major exception is theabsence of anorthosite massifs,although a coarse ca. 1.13 Gaanorthosite in the HoneybrookUplands is a good candidate butrequires accurate zircon dating. Inthe Llano Uplift, the collision withthe southern continent took placefrom ca. 1.15–1.07 Ga while inWest Texas it was closer to1.12–1.03 Ga.

4. At ca 1.1–1.098 Ga, the HawkeyeGranite was intruded into theAHT. A possible correlative in theAppalachian inliers is the ca. 1.12

Ga Mitchell Granite, but the sam-ple base is small and the crystal-lization ages are significantly olderthan the Hawkeye Granite. TheHawkeye Granite zircon ages weredetermined by multigrain analysisand need to be dated by SHRIMPmethods.

5. Collision of Amazonia with north-east Laurentia resulted in the ca.1.09–1.03 Ga Ottawan phase ofthe Grenvillian Orogeny and pro-duced granulite-facies metamor-phism and large fold nappes in theAHT but did not exceed ~500°Cin the ALT orogenic lid.Geochronology and thermo-barometry demonstrate that the P-T-t path was clockwise and thatgranulite-facies conditions did notensue until shortly after ca. 1.05Ga. At that time anatexis of themiddle crust resulted in produc-tion of the Lyon Mountain Gran-ite that advected heat towards thesurface. The thermally weakened,ductile middle crust could nolonger support the broad orogenicplateau and the orogen underwentextensional collapse. The collapsehas been documented along thenormal displacement, west-dippingCarthage-Colton Shear Zone thatseparates the AHT and the ALT(orogenic lid) and has been datedat ca. 1.047 Ga. A similar, but east-dipping, normal-displacement,shear zone in the eastern ALT hasbeen dated at ca. 1.05–1.035 Ga.The Ottawan Orogeny took placethroughout the Grenville inliers ofthe Appalachians where it hasbeen dated at ca. 1.08–1.05 Ga,coeval with its age in the AHT.High-silica, ferroan leucogranitecommonly occur late in the oroge-ny and is not generally penetrative-ly deformed. Similar late- to post-orogenic granitic magmatism ispresent in the Llano Uplift, whereferroan leucogranite was emplacedfrom ca. 1.12–1.07 Ga. The oldermembers of the group exhibit evi-dence of weak deformation,whereas the later plutons are total-ly undeformed and form circularintrusions with low-pressure con-tact aureoles. Both these, and theAdirondack leucogranite, havebeen attributed to delamination of

lithosphere. In the Llano Uplift,continental collision with a south-ern continent is analogous to theca. 1.2 Ga events of the easternGrenville Orogen involving Ama-zonia but occurred from ca.1.15–1.12 Ga.

6. Rigolet tectonomagmatism isrestricted to overgrowths on zir-con and monazite in the Adiron-dacks and southern AppalachianGrenvillian inliers but late dikesand a few small plutons do occurin the New England inliers. InWest Texas, the Hazel Orogenyappears to have taken place at ca.1.03–1.0 Ga and is transitionalfrom late Ottawan to early Rigolet.The development of a low-gradeforeland fold and thrust belt issimilar to the formation of theGrenville Front during the RigoletOrogeny.

The major tectonomagmaticfeatures of the Adirondacks, summa-rized above, can be traced into parts ofthe Canadian Grenville Province, theGrenvillian inliers of the Appalachians,and the Llano Orogen. Precisegeochronology and isotope geochem-istry continue to establish a commontectonic history for much of thisregion but also reveal important differ-ences in such crucial matters as the ori-gin of deep crustal reservoirs, i.e., Ndand Pb isotopic compositions and theproposed transfer of Amazonian crustto the southern Appalachians as well aspossible differences in potential collid-ers (i.e., Amazonia vs. Kalahari).Notwithstanding these differences, thetranscontinental continuity of tectonicand magmatic history is remarkable.During ~500 Ma of Proterozoic earthhistory a series of transcontinentalmarginal arcs existed across most ofNorth America and resulted in enor-mous growth of Laurentia. In addition,the Proterozoic provinces discussed inthis paper can be followed into Scandi-navia, Australia, and Antarctica asdescribed by Karlstrom et al. (2001),among others. The end result was theformation of the supercontinentRodinia regarding which the GrenvilleOrogen has provided, and continues toprovide, significant information.

ACKNOWLEDGEMENTSLarge portions of our research in the

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Adirondacks were supported byNational Science Foundation grantNSF-EAR-0125312, and by grantsfrom the Colgate University ResearchCouncil, the H.O. Whitnall endowmentat Colgate, and the Boyce fund held byColgate Universities Geology Depart-ment. JMM also thanks the USGS forthe support of field work in the east-ern Adirondacks during 1983–1986.We are grateful to Jeff Chiarenzelli,William Peck, Martin Wong, and MikeWilliams for their help and support.Reviews by Toby Rivers, Randy VanSchmus, and Jim Hibbard were excep-tionally helpful and constructive inimproving this contribution in honorof Hank Williams.

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