the massabesic gneiss complex, new hampshire: a...

THE MASSABESIC GNEISS COMPLEX, NEW HAMPSHIRE: A STUDY OFA PORTION OF THE AVALON TERRANE

MICHAEL J. DORAIS*, ROBERT P. WINTSCH**, and HARRY BECKER***

ABSTRACT. Geochemical data from the 625 Ma Massabesic Gneiss Complex ofsouthern New Hampshire show strong affinities for other Avalonian rocks of southernNew England and suggest continental rifting in the Late Proterozoic. Migmatizedparagneiss, the dominant rock type in the complex, has major and trace elementcompositions that are compatible with graywackes from continental arcs. The para-gneiss also has strong lithologic, metamorphic, and isotopic similarities to the rocks ofthe Hope Valley zone of Connecticut Avalon, suggesting a possible Hope Valley—Massabesic correlation. At 625 Ma, the paragneiss eNd values are similar to Avaloniancrust in other locations of the orogen.

Two types of amphibolite are present in minor amounts in the paragneiss of theMassabesic Gneiss Complex. The first type is a paramphibolite and consists ofcalc-silicate layers in the Massabesic paragneiss, the second type is metaigneous. Majorand trace element abundances reveal that the protoliths of the orthoamphibolitesrange from continental rift alkaline basalts and tholeiites to N-type MORBs. Orthoam-phibolite eNd (625 Ma) values range from 2.4 to 4 as expected of rift-related magmasderived from partial melting of a depleted mantle source and have the same values asIapetus ocean floor rocks of similar age. Orthoamphibolite major and trace elementgeochemical characteristics overlap those of the Middlesex Fells amphibolites of theEsmond-Dedham zone of eastern Massachusetts Avalon, which range from alkaline totransitional basalts erupted in a continental rift setting. The compositions of orthoam-phibolites define a potential magmatic continuum produced by batch partial melting ofthe mantle initiated during continental rifting and proceeded to ocean basin forma-tion.

The inferred continuity of mafic magmatism from the Esmond-Dedham (Mid-dlesex Fells Formation) to the Massabesic Gneiss Complex (and Hope Valley zone)suggests that these zones are not distinct lithotectonic zones but are parts of a singlelandmass. Massachusetts Avalon (Esmond-Dedham) represents the continental sectionof Avalon where the alkaline to transitional magmas of the early rifting stages arepreserved. According to our tectonic reconstruction, the Massabesic Gneiss Complexis the oceanward, continental margin represented by volcanoclastic sediments with theMORBs representing the initiation of ocean basin development. The leading edge ofthis landmass, of which the Massabesic Gneiss Complex is the only observableremnant, collided with Laurentia during the Acadian Orogeny. The inboard, thicker,more continental trailing-edge, that is, platform Avalon (Esmond-Dedham) collidedlater during the Alleghanian Orogeny.

introductionConsiderable progress has been made in understanding the geologic history of

the Appalachian region in New England, primarily aided by new geochronologicaldata that have revealed the complexities of New England geology (Zartman, 1988;Rankin, 1994; Robinson and others, 1998; and references therein). New England isnow thought to consist of several distinct terranes or composite terranes including theRowe-Hawley, Connecticut Valley, Bronson Hill, Central Maine, Merrimack, Putnum-Nashoba, and Avalon lithotectonic zones that have different ages and/or metamorphichistories (fig. 1). From west to east, they were affected increasingly by the Taconic,Acadian, and Alleghanian orogenies respectively (Rankin, 1994).

*Department of Geology, Brigham Young University, Provo, Utah 84602**Department of Geological Sciences, Indiana University, Bloomington, Indiana 47405***Department of Geology, University of Maryland, College Park, Maryland 20742

[American Journal of Science, Vol. 301, September, 2001, P. 657–682]

657

Rocks of the Avalon composite terrane are exposed in southeastern New Englandin Rhode Island and in large areas of southeastern Connecticut and eastern Massachu-setts (fig. 1). Avalon is thought to represent a fragment of North Africa/Amazonia thataccreted to North America and remained part of North America after Mesozoic riftingof Pangea and the opening of the Atlantic Ocean (Schenk, 1971; Rast and others, 1976;Williams, 1978; Williams and Hatcher, 1982; O’Brien and others, 1983; Nance andMurphy, 1994). The terrane is teconically important because the collision of thiscontinental block with the margin of Laurentia is thought to have caused either theearly Devonian Acadian or the Late Paleozoic Alleghanian orogenies (Osberg, 1978;Dallmeyer and others, 1981; Williams and Hatcher, 1983; Wintsch and others, 1992).However, in spite of increasing amounts of geochemical and geochronological data,the specific role of Avalon in these orogenies is still unclear.

Rocks with Avalonian affinities underlie several of the above mentioned allochtho-nous terranes, extending under the cover rocks as far inland as central Massachusetts,New Hampshire, and Maine (Zartman, 1988; Stewart and others, 1993; Tomascak andothers, 1996). The western-most surficial exposures of Avalon in New England arethought to be the Willimantic and Pelham domes in Connecticut and Massachusettsand the Massabesic Gneiss Complex in New Hampshire (Wintsch, 1979; Aleinikoff andothers, 1979; Zartman and Naylor, 1984; Hodgkins, 1985; Wintsch and others, 1990).The correlation of these inliers with Avalon is based on several features. They share acommon lithologic assemblage including metaigneous rocks with Late Proterozoicages as defined by U-Pb zircon crystallization ages. This age is well established insoutheast Avalon and the Willimantic dome at about 620 Ma (Wintsch and Aleinikoff,1987; Zartman and others, 1988; Wayne and others, 1992). In the Pelham dome,Tucker and Robinson (1990) also determined an age of ;615 Ma. Aleinikoff and

Fig. 1. Generalized geologic map of New England (after Zartman, 1988) showing the lithotectoniczones and the location of the Massabesic Gneiss Complex. Other exposures of Avalon terrane rocks occur inthe Willimantic and Pelham domes and the largest exposure is in southeastern New England. Thesoutheastern New England Avalon contains at least two domains, the Hope Valley and the Esmond-Dedham.

658 M.J. Dorais, R.P. Wintsch, and H. Becker—The Massabesic Gneiss Complex,

others (1995) reevaluated earlier studies of the orthogneiss of the Massabesic GneissComplex (Besancon and others, 1977; Aleinikoff and others, 1979) using the ionmicroprobe and determined an age of ;625 Ma. Thus all the orthogneisses have aremarkably common age of ;620 Ma.

The “type” New England Avalon and the three inlying domes also share a commonlate Paleozoic moderate to high grade metamorphism. Evidence for this event comesfrom U-Pb crystallization and overgrowth ages of metamorphic zircon, monazite, andsphene and from 40Ar/39Ar cooling ages of amphiboles and micas. A prograde event iswell documented by the metamorphism of Pennsylvanian sediments and by Ar coolingages (Dallmeyer and Takasu, 1992). This metamorphism persists west to the HoneyHill—Lake Char fault system in eastern Connecticut and Massachusetts where horn-blende cooling ages range from ;275 to 255 (Wintsch and others, 1992). Sphene andhornblende ages of 305 Ma (Getty and Gromet, 1992) and ;280 Ma (Wintsch andothers, 1992) in the Willimantic dome agree well with sphene and hornblende ages of292 Ma (Tucker and Robinson, 1990) and 287 Ma (Spear and Harrison, 1989) in thePelham dome. In the Massabesic Gneiss Complex, a metamorphic event of a similarage is defined by monazite (289 Ma, Aleinikoff and others, 1979; 282 Ma, Eusden andBarreiro, 1988), sphene (276-263 Ma, Eusden and Barreiro, 1988), and hornblende(260-250 Ma, West, 1993; Lux and West, 1993). Only the Massabesic Gneiss Complexhas any evidence of pre-Alleghanian metamorphism. If a 390 Ma zircon in anamphibolite (Aleinikoff and others, 1995) proves to be metamorphic, then theMassabesic Gneiss Complex may have a more complicated (Acadian) metamorphichistory than its sibling Avalonian outliers. Based on the geochronological evidence,there is no question that all these bodies shared a late Proterozoic igneous event as wellas a Late Paleozoic metamorphic event in the earliest Permian.

In this study, we examined the whole-rock geochemical and Nd isotopic character-istics of paragneiss, leucosomes, and amphibolites of the Massabesic Gneiss Complexwith the intent to:

1. Constrain the provenance and tectonic setting of the paragneiss;2. Discriminate between ortho- and paramphibolites to constrain the tectonic

setting of the orthoamphibolites;3. Determine the Nd isotopic compositions of the various Massabesic Gneiss rock

types;4. Compare all these data with those available for Avalonian rocks of southeastern

New England in order to draw a larger-scale picture of the petrogenesis andtectonic history of the Avalon terrane than can be reconstructed from any onezone.

regional geology and geology of the massabesic gneiss complex

The Massabesic Gneiss Complex consists primarily of Late Proterozoic andPermian sillimanite-zone migmatites (Aleinikoff, 1978; Aleinikoff and others, 1979;Lyons and others, 1982) that extend in a northeast-southwest trend across thesouthern portion of New Hampshire (fig. 1). Emerson (1917), Sriramadas (1966),Carnein (1976), and Aleinikoff (1978) documented the variability of the complex,which was given the name Massabesic Gneiss after the exposures surrounding Massabe-sic Lake southeast of Manchester, New Hampshire. To the southeast, the complex isbounded by the Merrimack belt considered to be in gradational contact with theMassabesic Gneiss Complex as a migmatized equivalent of the Berwick Formation(Bothner and others, 1984; Fagan, 1985). More recently, Goldsmith (1991), Pouliot(1994), Larson (1999), and Larson and others (1999) interpreted the contact to be aductile fault. To the northwest, the complex is separated from the rocks of the CentralMaine terrane by a blastomylonite (Armstrong and others, 1999a, b).

659New Hampshire: A study of a portion of the Avalon Terrane

The Massabesic Gneiss is a complex of migmatitic gneisses of variable texture andstructure. Based on zircon morphology and bulk-rock compositions, Aleinikoff (1978)and Aleinikoff and others (1979) concluded that the dominant rock type in thecomplex is paragneiss. In most outcrops, the gneissosity of the paragneiss is defined bya preferred orientation of biotite in alternating layers of biotite-rich and biotite-poor,quartz-plagioclase-K-feldspar gneiss with ;0.5 to 1 cm grain size. This banding isvariably migmatitic with Mehnert’s (1971) schlieren type the most dominant. Thebiotite-rich folia locally contain rare sillimanite, garnet, and muscovite. Leucosomeoccurs as stringers to small pods within the biotite-rich folia (fig. 2A). Some locationscontain pods of leucosome that are sufficiently large for the outcrops to have beencalled orthogneiss (Aleinikoff, 1978).

Within the paragneiss are relatively rare, small bands and lenses of amphibolitewith a foliation parallel to that of the host paragneiss (Bothner and others, 1984;Larson, 1999; Larson and others, 1998). Larson and coworkers interpreted theseamphibolites to be calc-silicates, consisting of amphibole, plagioclase, epidote, clinopy-roxene, 6quartz, 6diopside, 6garnet. Other amphibolites are more massive, occur-ring as blocks or boudins with dimensions of several meters (fig. 2B). Contacts betweenthese massive amphibolites and paragneiss have been sheared, leaving the relativepremetamorphic age relations difficult to determine.

Cutting both paragneiss and leucosome are undeformed two-mica granites andpegmatites. One of these, the Damon Pond granite at Milford, New Hampshire, isAlleghanian (Aleinikoff and others, 1979) and probably represents partial melts fromdeeper in the Massabesic Gneiss Complex. The associated pegmatites probably are alsoPermian in age because of their lack of metamorphic fabric.

analytical methods

Bulk-rock major and selected trace element analyses were conducted by XRFtechniques at Michigan State University. Analyses of additional trace elements wereobtained by INAA at the Phoenix Memorial Laboratory at the University of Michigan.

Nd isotopic compositions and Nd and Sm concentrations were measured usingisotope dilution—thermal ionization mass spectroscopy techniques at the IsotopeGeochemistry Laboratory, University of Maryland. After adding a mixed REE (149Sm,150Nd) spike, the samples were dissolved in HF-HNO3 at 210°C in screw-top Teflonbeakers in Parr bombs for two days. After separation of the REE fraction on a primarycolumn (AG50W-X8), Nd and Sm were separated on AG50W-X4 resin using 0.2 Mmethylactic acid. Blanks were 500 pg for Nd and less than 100 pg for Sm, and blankcorrections are insignificant or small. Isotopic ratios were measured on a SECTOR 54mass spectrometer with multiple collectors operating in the dynamic mode. Measure-ments of the La Jolla Nd standard over the analysis period yielded 143Nd/144Nd 50.511847 6 10 (2s, fractionation corrected to 146Nd/144Nd 5 0.7219). All 143Nd/144Ndratios are corrected to a value of 0.511860 for the La Jolla Nd standard. eNd values werecalculated using 143Nd/144Nd 5 0.512638 and 147Sm/144Nd 5 0.1966 for the present-day bulk silicate earth (Jacobsen and Wasserburg, 1980).

bulk-rock compositionsMajor elements.—Figure 3 illustrates selected major element compositions (table 1)

of rocks of the Massabesic Gneiss Complex. Amphibolite compositions define threefields: massive amphibolites that appear to be boudinaged dikes define two fields thatcontain approx 48 wt percent SiO2 but have different TiO2 and Fe2O3 contents. Smallbands and lenses of amphibolites define a third field at greater SiO2 contents thatrange between 58 and 61 wt percent. With only one sample as an exception, theparagneiss plots at lower Al2O3 and higher MgO, Fe2O3, and TiO2 contents atequivalent SiO2 values compared to the leucosome and granite.


Fig. 2(A) Photograph of typical Massabesic migmatized paragneiss. (B) Photograph of a continental rifttholeiitic amphibolite exposed in road cuts at exit 8 on interstate 93 in Manchester, New Hampshire.Paragneiss to the right of the amphibolite, Permian aged pegmatites cut both orthoamphibolite andparagneiss.


On an AFM diagram (fig. 4A), the amphibolites that are relatively rich in SiO2 plotin the calc-alkaline field. Additional data presented below indicate that these areparamphibolites and not metabasites, hence their position in the AFM diagram has nosignificance except that it shows this group is chemically distinct from the massiveamphibolites. One group of massive amphibolites plots as tholeiitic basalts in the AFMdiagram; the other group plots in the calc-alkaline field. In the FeO/MgO versus SiO2diagram (fig. 4B), both sets of amphibolites plot in the tholeiitic field.

Paragneiss samples are plotted in (Na 1 Ca)/(Na 1 Ca 1 K) versus Si/(Si 1 Al)(atomic proportions) in figure 5 which defines compositional fields for varioussedimentary rocks (Wintsch and Kvale, 1994). Most of the paragneiss samples plot asgraywackes, but one plots as a mudstone. This sample (MG-31) is highly sheared, andmetasomatic loss of plagioclase probably modified its initial composition.

Tectonic discrimination diagrams and trace element characteristics of orthoamphibolites.—The Zr versus Ti diagram (fig. 6) shows the two massive amphibolite groups plot inthe low-K tholeiite and the ocean floor basalt fields and at greater Ti and Zr contentsalong the extension of the ocean floor basalt field. In the Ti-Zr-Y diagram (fig. 7A), theamphibolites plot in the field of ocean floor basalts and low-K tholeiites and as withinplate basalts. The Nb-Zr-Y diagram (fig. 7B) shows that several samples plot as N-type

Fig. 3. Bulk-rock Al2O3, MgO, Fe2O3 and TiO2 versus SiO2 diagrams. The amphibolites of theMassabesic Gneiss complex define three compositional fields: the calc-silicate amphibolites (open squares)plot at relatively high SiO2 concentrations (58-61 wt percent) compared to the orthoamphibolites. Theorthoamphibolites define two fields in Fe2O3 and TiO2 versus SiO2 space with the MORB amphibolites(filled squares) being poorer in Fe2O3 and TiO2 at equivalent SiO2 contents compared to the continental riftalkaline and tholeiite amphibolites (asterisks). The leucosomes (filled diamonds) and Permian, two-micagranites (filled circles) contain less MgO, Fe2O3 and TiO2 and more Al2O3 compared to the paragneisses(open diamonds).


MORBs with the remainder plotting in the within plate tholeiite and within platealkaline basalt fields.

Chondrite-normalized REE patterns of the massive amphibolites again show twogroups (fig. 8A). The amphibolites that plot as N-type MORBs in the Nb-Zr-Y diagram

Table 1

Representative bulk-rock analyses, Massabesic Gneiss Complex, New Hampshire

MORB 5 mid oceanic ridge basalt; CRT 5 continental rift tholeiite; PA 5 paramphibolite; PG 5paragneiss; OG 5 orthogneiss; LS 5 leucosome; G 5 granite.


have flat patterns ranging from 15 to 25 times chondrites. The patterns are LREE poorwhich is consistent with MORB compositions (Bryan and others, 1976; Schilling andothers, 1983). With the exception of the positive Rb and K anomalies thought to be the

Fig. 4(A) AFM diagram for rocks of the Massabesic Gneiss complex. (B) FeO/MgO versus SiO2 diagram(after Miyashiro, 1974).


result of metasomatism during metamorphism of the basalts, these samples haverelatively flat patterns in the extended REE diagram (fig. 8B), again suggesting MORBcompositions (Sun and others, 1979). In figure 9, these amphiboles plot at (Ba/La)Nvalues of less than 2 at low (La/Sm)N which is characteristic of MORBs compared tohigher values of island arc tholeiites and calc-alkaline basalts (Kay, 1977; Sun andothers, 1979).

The other group of massive amphibolites has HREE and HFSE abundancescomparable to the MORB amphibolites, but these amphibolites are as rich as 100 to500 times chondrites in LREE abundances (fig. 8A, B). This LREE enrichment, plusthe enrichment of other incompatible elements such as Rb, Ba, and K in the extendedREE diagram, shows these amphibolites to be similar to continental rift alkaline basaltsand tholeiites (BVST, 1981; Dupuy and Dostal, 1984; Bertrand, 1991) as suggested bythe tectonic discrimination diagrams. Another characteristic of continental rift mag-mas that distinguishes them from MORBs is the depletion of Nb and Ta in spiderdia-grams (fig. 8B; Thompson and others, 1983; Thompson and others, 1984; Dupuy andDostal, 1984; Bertrand, 1991). These negative anomalies are often erroneously por-

Fig. 5. Atomic proportions of (Na 1 Ca)/(Na 1 Ca 1 K) versus Si/(Si 1 Al) for paragneiss samples(after Wintsch and Kvale, 1994).


trayed as being exclusive to subduction-related magmas, but they may be present incontinental rift alkaline basalts and tholeiites as well as a result of crustal contamina-tion (Cox and Hawkesworth, 1985; Dupuy and Dostal, 1984).

In summary, there are two groups of massive amphibolites in the MassabesicGneiss Complex, one group with MORB compositions, the other with continental riftalkaline to tholeiitic compositions.

Trace element characteristics of the paramphibolites, paragneiss, and leucosomes.—Primi-tive mantle-normalized incompatible element abundances of the paragneiss and theSiO2-rich amphibolites are shown in figure 10A. Compared to the orthoamphibolitepatterns, these patterns are rich in Rb, Th, and K, with negative Nb, Ta, P, and Tianomalies. The paragneiss is similar to the third group of SiO2-rich amphibolites thatplot in the calc-alkaline field of figure 4A.

Figure 10B shows chondrite-normalized REE patterns for paragneiss samples.LREE are enriched to 130 times chondrites, HREE elemental abundances are relativelyflat at ;20 to 30 times chondrites. All the paragneiss samples display negative Euanomalies.

Leucosome samples are plotted in two tectonic discrimination diagrams (fig. 11;Pearce and Cann, 1973; Pearce and others, 1984). The samples plot in the volcanic arcand syn-collisional granite fields in the Nb versus Y diagram and in the volcanic arcgranitic field in the Rb versus Nb 1 Y diagram.

Fig. 6. Ti versus Zr diagram. Data from the Middlesex Fells complex from Cardoza and others (1990)and the Waterford Complex from Goldsmith (1987).


Discussion.—The presence of amphibolites with compositions of both MORB andcontinental rift magmas in the Massabesic Gneiss Complex suggests the developmentof a single magmatic series in Late Proterozoic magmatism. We suggest that the earlierstages of continental rifting produced the transitional alkaline to continental rifttholeiites whereas continual rifting led to further depletion of the underlying mantle,eventually producing magmas of MORB compositions.

Fig. 7(A) Ti-Zr-Y diagram (After Pearce and Cann, 1973) showing that the MORB amphibolites (filledsquares) plot in the ocean-floor and low-K tholeiite fields. The continental rift alkaline and tholeiiteamphibolites (asterisks) plot in the within-plate basalt field. (B) Nb-Zr-Y diagram showing that the MORBamphibolites (filled squares) plot as N-type MORBs. The second group of orthoamphibolites (asterisks)have within plate tholeiitic and alkaline affinities.


Fig. 8(A) Chondrite-normalized REE patterns for the Massabesic Gneiss Complex MORB amphibolitesand the continental rift alkaline to tholeiitic amphibolites (solid lines). The MORB amphibolites have flatpatterns with depleted LREE abundances (Bryan and others, 1976; Schilling and others, 1983). Thecontinental rift alkaline and tholeiite amphibolites are enriched in LREE, similar to continental rift magmasof other localities (BVSP, 1981; Dupuy and Dostal, 1984) and to the Middlesex Fells amphibolites ofMassachusetts Avalon (gray lines, Cordoza and others, 1990). (B) Extended REE diagram (After Sun andMcDonough, 1989). The MORB amphibolites have flat patterns except for the positive K2O and Rbanomalies (Sun and others, 1979) which appear to have been enriched by metasomatism during metamor-phism. The Massabesic continental rift alkaline and tholeiite amphibolites are enriched in incompatibleelements and are similar to continental rift magmas of other localities (Dupuy and Dostal, 1984; Bertrand,1991) and to the Middlesex Fells amphibolites of Massachusetts Avalon (gray lines, Cordoza and others,1990).

Aleinikoff (1978) and Aleinikoff and others (1979) concluded that the dominantrock type in the Massabesic Gneiss Complex is paragneiss. Our data support thisconclusion and suggest that the paragneiss is distinguished from the leucosomes by thelower Al2O3 and higher MgO, Fe2O3, and TiO2 contents at equivalent SiO2 values (fig.3). Minimum melts can dissolve only limited amounts of MgO, Fe2O3, and TiO2 (Millerand others, 1985), hence the magmas that formed the orthogneisses and granites hadlimits to the solubility of these elements.

The nature of the sedimentary protolith of the paragneiss can be inferred fromthe REE abundances. The chondrite-normalized REE diagram (fig. 10B) includespatterns of representative graywackes from several tectonic settings (Taylor andMcClennan, 1985). Graywackes from fore-arc settings have the lowest REE abun-dances, particularly the LREE. Graywackes shed from Andean-type continental arcs arericher in REE, with LREE abundances ranging from 100 to 130 times chondrites.Graywackes from passive margin settings that are rich in quartz overlap those from

Fig. 9. Chondrite-normalized ratios of La/Sm and Ba/La showing fields for island arc and oceanicbasalts (After BSVP, 1981). Massabesic MORB orthoamphibolites plot at low (Ba/La)N values which isconsistent with the overall MORB signature of these samples.

M.J. Dorais, R.P. Wintsch, and H. Becker 669

Andean-type settings but tend to be slightly more enriched with the LREE concentra-tions reaching 130 times chondrites. The Massabesic Gneiss Complex paragneiss hasLREE abundances that overlap both these LREE-rich graywackes. The relatively lowSiO2 contents of some of the paragneiss samples suggest that an Andean-type setting isprobably a more appropriate source region than a passive continental margin. Addition-ally, the relatively high Ni, Cr, and Sr contents of some of the paragneiss samples (table2) suggest a more primitive volcanic component to the paragneiss that fits thevolcanoclastic origin suggested by Aleinikoff and coworkers (1979) and our data infigure 5.

The conclusion that the dominant gneiss in the Massabesic Gneiss Complex is amigmatized paragneiss is supported by plots of Massabesic Gneiss Complex rocks inthe ACF diagram (fig. 12). The leucosomes plot at high Al2O3 contents, separate fromthe paragneiss that plots as typical clastic sediments (Orville, 1969). The massiveamphibolites plot within the labradorite-clinopyroxene-orthopyroxene and olivinevolume as do basalts. However, Orville (1969) demonstrated that many paramphibo-lites, being mixtures of clastic sediments and/or mudrocks with carbonates, plot in this

Fig. 10(A) Extended REE diagram for the paragneiss and paramphibolites of the Massabesic Gneisscomplex (Normalization constants after Sun and McDonough, 1989). (B) Chondrite-normalized REEpatterns for the paragneisses (solid lines) of the Massabesic Gneiss complex compared to patterns ofgraywackes from fore-arc settings (gray dashed lines), Andean-type settings (gray dotted lines), and passivemargin settings (solid gray lines) (After Taylor and McClennan, 1985).


Fig. 11. Nb versus Y and Rb versus Nb 1 Y diagrams (after Pearce and others, 1984). The leucosomesplot in the volcanic arc field and the syn-collisional field in (A) and in the volcanic arc field in (B).


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same volume. The SiO2-rich amphibolites plot along a tie line between the paragneis-ses and dolomite. The similarity of the SiO2-rich amphibolites in trace elementabundances to known metasedimentary rocks, that is, the paragneiss samples (fig.10A), suggests that the amphibolites also had metasediments as protoliths, hence theseamphibolites are paramphibolites whose major element compositions in the ACFdiagram (fig. 12) probably result from addition of dolomite to typical paragneiss.

The tectonic discrimination diagrams (fig. 11) suggests that the leucosomesoriginated in volcanic arc settings. In actuality, the leucosomes could not haveoriginated in a volcanic arc setting; they clearly occur as migmatites produced in aninferred syn-collisional environment. Their compositions were determined by thechemical signature of the partially melted metasedimentary rocks and not tectonicsetting, and are another indication of graywacke source rocks. Similar interpretationsof tectonic discrimination diagrams are presented by Brown and others (1984), Clarke(1992), and Forster and others (1997).

Thus all three rock types, the paramphibolites, paragneiss, and leucosomes, havecompositions that are compatible with derivation from graywackes in a continentalmargin setting.

Fig. 12. ACF diagram (symbols as in fig. 3). The orthoamphibolites (filled squares and asterisks) plotwithin the Lab—CPX—OPX, Oliv volume. The paramphibolites (open squares) plot within the samevolume as the orthoamphibolites, but their compositions can be explained by the addition of dolomite to thetypical paragneiss as they plot along a line connecting the two endmembers.


neodymium isotopic compositions

Neodymium isotope data are given in table 3. Lacking a precise age, we use the625 Ma age obtained from U-Pb data on zircon for subsequent calculations and initialNd isotopic compositions.

Figure 13A shows eNd versus time for the amphibolites and paragneisses. Both thecontinental rift and MORB samples define a restricted range of eNd values (625 Ma)from 12.4 to 14.0. The positive values indicate a mantle-derived (juvenile) magmawhere the reservoir had been previously depleted such as what one would expect forrift-related magmas. The amphibolites plot well above the range of values displayed byGrenville rocks and are compatible with the Avalon-like crust at this time. Tworepresentative paragneiss samples (MG-1, MG-36) also plot within the Avalon field.These samples are high-grade gneisses, being the dominant rock type of the Massabe-sic Gneiss Complex. In this diagram, one typical paragneiss (MG-28) is anomalous,plotting in the Grenville field.

Figure 13B illustrates fSm/Nd versus eNd for the continental rift samples, MORBsand paragneiss where fSm/Nd reflects the difference of Sm/Nd between sample andCHUR (DePaolo and Wasserburg, 1976). Also plotted are the fields of Iapetus oceanfloor rocks and Avalonian rocks from Fryer and others (1997). At 625 Ma, theMassabesic amphibolites plot within the Iapetus ocean floor rocks with the MORBs atpositive fSm/Nd values as expected (fig. 13B). One sample (MG-10A) plots within theAvalonian field, suggesting that this sample may have assimilated Avalonian crust. Thissample has an eNd (0) of 24.0, also indicating assimilation of old crust. The Massabesicparagneisses, including the anomalous sample MG-28 from the previous diagram, plotwithin the Avalonian field as defined by Barr and Hegner (1992), Whalen and others(1994), and Kerr and others (1995).

correlation of massabesic gneiss complex with avalon of se new englandParagneiss and leucosomes.—The Avalon terrane of southeastern New England is a

composite terrane consisting of several domains that experienced different intensitiesof Alleghanian metamorphism. The southwestern Hope Valley domain of O’Hara andGromet (1985) and a southeastern portion of the Esmond-Dedham domain eachexperienced high grade Alleghanian metamorphism (Murry and others, 1990; Murryand Dallmeyer, 1991), while the central Esmond-Dedham zone has escaped metamor-phism since the late Precambrian (Skehan and Rast, 1990). No Acadian or Taconicmetamorphism has been identified. The Esmond-Dedham zone contains 600 to 650

Table 3

Nd isotopic data for Massabesic Gneiss Complex rocks

*Calculated using the parameters of Goldstein and others, 1984.


Fig. 13(A) Plot of eNd versus time for the Massabesic Gneiss complex orthoamphibolites and paragneiss.The amphibolite samples plot in the Avalon field (Barr and Henger, 1992; Keppie and others, 1997; Fryerand others, 1997; Pe-Piper and Piper, 1998). Two representative samples of paragneiss also plot in the Avalonfield whereas one paragneiss is anomalous, plotting in the Grenville field. (B) Plot of f Sm/Nd versus eNd forMassabesic Gneiss Complex orthoamphibolite and paragneiss calculated at 625 Ma. The continental riftsamples plot in the negative f Sm/Nd and positive eNd field. The two MORB samples (MG-26, 7-13-95-1A) andone continental rift tholeiite (MG-33) plot in the Iapetus ocean floor field (Fryer and others, 1997). Onecontinental rift sample (MG-10A) plots in the Avalon field (after Barr and Hegner, 1992), suggestingpossible assimilation of Avalonian material for this sample. All three paragneiss samples plot in the Avalonfield. DM 5 depleted mantle.


Ma plutons that range in composition from granite to diorite (Kovach and others,1977; Zartman and Naylor, 1984; Thompson and others, 1996). Late Proterozioc maficvolcanic rocks that erupted both prior and subsequent to the Late Proterozoic graniticmagmatism as well as Devonian anorogenic plutons are present in this zone. The ;620Ma leucogneisses of the Hope Valley zone (Hermes and Zartman, 1985) experiencedAlleghanian metamorphism and have minor amounts of mafic rocks. Anorogenicgranites are absent.

The Massabesic Gneiss Complex has strong affinities with the Hope Valley portionof Avalon composite terrane and also with the Pelham dome of Massachusetts. They allshare a common suite of ;620 Ma felsic orthogneisses that experienced Alleghanianmetamorphism, contain relatively minor amounts of mafic rocks, and share an absenceof anorogenic granites. Tucker and Robinson (1990) interpret the Pelham domeparagneisses as immature feldspathic wackes with a quartz-rich continental source thatwere deposited along a rifted continental margin (Rankin, 1994), a similar setting issuggested by this study for the Massabesic Gneiss Complex. The ;625 Ma orthogneis-ses of the Massabesic Gneiss Complex are the same age as plutonic rocks of southeast-ern New England (Aleinikoff and others, 1995; Wintsch and Aleinikoff, 1987). Addition-ally, the Massabesic paragneiss has the same range of eNd (625) as other Avalonianrocks (fig. 13B; Barr and Hegner, 1995) which is distinct from Grenvillian rocks at thattime.

Orthoamphibolites.—The question of how the Massabesic orthoamphibolites relateto other Late Proterozoic amphibolites of the Avalonian terrane of southeastern NewEngland can be addressed by comparing the compositions of the Massabesic orthoam-phibolites with amphibolites of the Middlesex Fells Formation of the Esmond-Dedhamzone of eastern Massachusetts and to amphibolites of the Waterford Complex of theHope Valley zone in Connecticut. The Middlesex Fells complex consists of a bimodalassociation of felsic and mafic volcanic rocks occurring as roof pendants and largeblocks in the Dedham Granite north of Boston, Massachusetts (Cardoza and others,1990). The mafic rocks have experienced low grade contact metamorphism by theDedham Granite. The Waterford complex (Goldsmith, 1987) consists primarily ofgranodioritic rocks interpreted as a candidate for a caldera (Wintsch and others,1990). Amphibolites are present throughout the complex in the upper part of thesection as dikes and flows.

For comparisons of the Massabesic Gneiss Complex amphibolites with those ofAvalon of southeastern New England to have any validity, the amphibolites must be thesame age. Although none were directly dated, a common age is implied. MassabesicGneiss Complex rocks are about 620 Ma which is our best estimate for the age of itsamphibolites. Southern Connecticut amphibolites occur as mafic enclaves in the datedWaterford Complex and as interlayered volcanic rocks in the extrusive cap (Wintschand others, 1990). Thus these may be confidently identified as late Proterozoic. It hasbeen suggested that the Middlesex Fells rocks correlate with 700 to 800 Ma rocks ofNewfoundland (Strong and others, 1978; Strong, 1979; O’Brien and others, 1983).This age difference between the Massabesic amphibolites and the Middlesex Fellscomplex would invalidate any comparisons with the younger orthoamphibolites of theMassabesic Gneiss Complex. However, a sill with Middlesex Fells compositions in-truded the Cambridge Argillite which includes an ash bed which was dated at 642 Ma.(Thompson, personal communication). This provides a maximum age for the Mid-dlesex Fells amphibolite and permits an age correlation with the Waterford and theMassabesic Gneiss Complexes. It is thus reasonable to explore regional variations inbasalt geochemistry.

Cardoza and others (1990) determined that the amphibolites of the MiddlesexFells complex are alkaline to transitional basalts that have signatures of continental rift


environments. These rocks define a continuum of compositions at high Ti concentra-tions on the Zr-Ti diagram (fig. 6), overlapping the range of low-K tholeiites and oceanfloor basalt compositions shown by the Massabesic Gneiss Complex orthoamphibo-lites.

Figure 8A shows the chondrite-normalized REE patterns for the Middlesex Fellsamphibolites, the sill in the Cambridge Argillite and the amphibolites of the Massabe-sic Gneiss Complex. The alkaline basalts of the Middlesex Fells rocks are rich in LREE,containing abundances up to 500 times chondrites. The sill plots at the REE-rich endof the alkaline basalts. The transitional basalts defined by Cardoza and others (1990)contain between 90 and 200 times chondritic abundances of the LREE, overlappingwith the Massabesic continental rift tholeiitic amphibolites. Figure 8B shows theextended REE plots for these rocks. Both the sill and the amphibolites of theMiddlesex Fells complex are rich in incompatible elements. There is a range ofcompositions between the alkaline basalts and the transitional basalts that plot at lowerincompatible element concentrations. The transitional basalts overlap the plots of thecontinental rift tholeiites of the Massabesic Gneiss Complex.

Given that the Massabesic Gneiss Complex has strong correlations with Avalonof southeastern New England, we interpret these similarities in amphibolitecompositions to suggest that the two suites of amphibolites may represent acompositional continuum. A magmatic continuum would suggest that the continen-tal rifting envisioned by Cardoza and coworkers proceeded to ocean basin forma-tion as shown by the MORB compositions of the Massabesic Gneiss Complexamphibolites. This scenario would suggest that the Esmond-Dedham zone ofMassachusetts contains alkaline magmas of the early rifting stages of the inland,continental section of the rift which was followed by eruption of continental riftbasalts. The Massabesic Gneiss Complex would be the continental margin repre-sented by the volcanoclastic sediments, continental rift tholeiites, and the initialformation of adjacent ocean basin represented by the MORBs. Therefore, in spiteof the differences between the Esmond-Dedham and Hope Valley zones as definedby O’Hara and Gromet (1985), a magmatic continuum from the Middlesex Fells tothe Massabesic Gneiss Complex amphibolites suggests that the zones were origi-nally continuous.

Implications.—The suggestion that the Massabesic Gneiss Complex is representa-tive of the oceanward margin of Avalon has a bearing on the nature of crustal materialsinvolved in the Acadian and Alleghanian orogenies. Because Avalon of southeasternNew England lacks evidence of involvement during the Acadian orogeny, one can inferthat the Acadian orogeny resulted from the collision of Laurentia and whatever rockswere outboard of this portion of Avalon. If the Massabesic Gneiss Complex is thetrailing edge of that landmass, then we infer that its continental margin sediments andadjacent oceanic crust collided with North America during the Acadian Orogeny andcould have produced metamorphic zircons at 390 Ma (Aleinikoff and others, 1995).The MORB amphibolites of the Massabesic Gneiss Complex could therefore be the lasttraces of the Avalonian side of the Iapetus ocean basin. The similar eNd (625 Ma) valuesof the Massabesic orthoamphibolites to those of rocks interpreted to represent Iapetusocean floor by Fryer and others (1997) support this interpretation (fig. 13B).

conclusionsWe interpret the Late Proterozoic Massabesic Gneiss Complex to correlate with

the Avalon terrane of southeastern New England and, based on neodymium isotopicdata, with the Avalon terrane of Canada. The Massabesic Gneiss Complex, the Pelhamdome, and the Hope Valley zone share similar ages, high-grade metamorphism andcooling curves (Wintsch and others, 1992), strongly foliated gneisses in similarlithologic packages, and have minor amounts of amphibolites of similar compositions.


The potential continuum from alkaline and continental rift tholeiitic magmas of theEsmond-Dedham zone to continental rift tholeiites and MORB magmas of the HopeValley/Massabesic zone suggests that the two zones were continuous. The AcadianOrogeny may have resulted from subduction of Iapetus ocean basin followed byobduction of arc graywackes, the last traces of which are represented by the MassabesicGneiss Complex.

acknowledgmentsWe thank Mike Brown for his encouragement to proceed on the collaborative

project and Meg Thompson for the analysis of the Middlesex Fells sill. We are verygrateful to John Aleinikoff, Tom Armstrong, and Dave Stewart for reviews of an earlierversion of the manuscript and Dyk Eusden and Jo Laird for journal reviews. Thisresearch was supported by NSF grant EAR-9418203 to Wintsch and EAR-9909410 toWintsch and Dorais.

References

Aleinikoff, J. N., ms, 1978, Structure, petrology, and U-Th-Pb geochronology in the Milford (159 quadrangle,New Hampshire: Ph.D. thesis, Dartmouth College, 247 p.

Aleinikoff, J. N., Walter, M., and Fanning, C. M., 1995, U-Pb ages of zircon, monazite, and sphene from rocksof the Massabesic Gneiss complex and Berwick Formation, New Hampshire and Massachusetts:Geological Society of America, Abstracts with Programs, v. 27, p. 26.

Aleinikoff, J. N., Zartman, R. E., and Lyons, J. B., 1979, U-Th-Pb geochronology of the Massabesic Gneissand the granite near Milford, south-central New Hampshire: New evidence for Avalonian basement andTaconic and Alleghenian disturbances in eastern New England: Contributions to Mineralogy andPetrolology, v. 71, p. 1–11.

Armstrong, T. R., Aleinikoff, J. N., and Burton, W. C., 1999a, Structural and geochronologic constraints onthe tectonothermal evolution of the Massabesic Gneiss Complex, southeastern New Hampshire:Geological Society of America Abstracts with Programs, v. 31, p. A2.

Armstrong, T. R., Aleinikoff, J. N., Walsh, G. J., Wintsch, R. P., Kunk, M. J., and Burton, W. C., 1999b, Extentand tectonic significance of Alleghenian deformation and metamorphism in southern New Hampshire:American Geophysical Union 1999 Spriong Meeting, v. 80, p. S362.

Armstrong, T. R., Tracy, R. J., and Hames, W. E., 1992, Contrasting styles of Taconian, eastern Acadian andwestern Acadian metamorphism, central and western New England: Joururnal of Metamorphic Geology,v. 10, p. 415–426.

Barr, S. M., and Hegner, E., 1992, Nd isotopic compositions of felsic igneous rocks in Cape Breton Island,Nova Scotia: Canadian Journal of Earth Sciences, v. 29, p. 650–657.

Basaltic Volcanism Study Project, 1981, Basaltic volcanism on the terrestrial planets: New York, PergamonPress, Inc., 1286 p.

Bertrand, H., 1991, The Mesozoic tholeiitic province of northwest Africa: A volcano-tectonic record of theearly opening of central Atlantic, in Kampunzu, A. B. and Lubala, R. T., editors, Magmatism inExtensional Structural Settings; The Phanerozoic African Plate: Springer-Verlag, p. 147–188.

Besancon, J. R., Gaudette, H. E., and Naylor, R. S., 1977, Age of the Massabesic Gneiss, southern NewHampshire: Geological Society of America, Abstracts with Programs, v. 9, p. 242.

Billings, M. P., 1956, The Geology of New Hampshire; Part II Bedrock Geology: Concord, New HampshireState Planning and Development Committee, 200 p.

Bothner, W. A., Boudette, E. L., Fagan, T. J., Gaudette, H. E., Laird, J., and Olszewski, W. J., 1984, Geologicframework of the Massabesic Anticlinorium and the Merrimack Trough, southwestern New Hampshire,in Hanson, L. S., editor, Geology of the coastal lowlands, Boston, Massachusetts to Kennebunk, Maine:New England Intercollegiate Geologic Conference, 76th Annual Meeting, p. 186–206.

Brown, G. C., Thorpe, R. S., and Webb, P. C., 1984, Geochemical characteristics of granitoids in contrastingarcs and comments on magma sources: Journal of the Geological Society of London, v. 141, p. 413–426.

Bryan, W. B., Thompson, G., Frey, F. A., and Dickey, J. S., 1976, Inferred settings and differentiation inbasalts from the Deep Sea Drilling Project: Journal of Geophysical Research, v. 81, p. 4285–4304.

Cardoza, K. D., Hepburn, J. C., and Hon, R., 1990, Geochemical constraints on the paleotectonic setting oftwo late Proterozoic mafic volcanic suites, Boston-Avalon zone, eastern Massachusetts, in Socci, A. D.,Skehan, J. W., and Smith, G. W., editors, Geology of the Composite Avalon Terrane of Southern NewEngland: Geological Society of America Special Paper., 245, p. 113–131.

Carnein, C. R., ms, 976, Geology of the Suncook 15-minute quadrangle, New Hampshire: Ph.D. dissertation,Ohio State University, Columbus, Ohio. 196 p.

Clarke, D. B., 1992, Granitoid Rocks. Topics in the Earth Sciences 7: London, Chapman and Hall, 283 p.Cox, K. G., and Hawkesworth, C. J., 1985, Geochemical stratigraphy of the Deccan Traps, at Mahabaleshwar,

western Ghats, India, with implications for open system magmatic processes: Journal of Petrology, v. 26,p. 355–377.


Dallmeyer, R. D., Blackwood, R. F., and Odom, A. L., 1981, Age and origin of the Dover fault: Tectonicboundary between the Gander and Avalon zones of the northeastern Newfoundland Appalachians:Canadian Journal of Sciences, v. 8, p. 1431–1442.

Dallmeyer, R. D., and Takasu, A., 1992, 40Ar/39Ar ages of detrital muscovite and whole-rock slate/phyllite,Narragansett Basin, RI-MA, USA: Implications for rejuvenation during very low-grade metamorphism:Contributions to Mineralogy and Petrology, v. 110, p. 515–527.

DePaolo, D. J., and Wasserburg, G. J., 1976, Nd isotopic variations and petrogenetic models: GeophysicalResearch Letters, v. 3, p. 249–252.

Dupuy, C., and Dostal, J., 1984, Trace element geochemistry of continental tholeiites: Earth and PlanetaryScience Letters, v. 67, p. 61–69.

Emerson, B. K., 1917, Geology of Massachusetts and Rhode Island: United States Geological Survey Bulletin,v. 597, 289 p.

Eusden, J. D. Jr., and Barreiro, B., 1988, The timing of peak high-grade metamorphism in central-easternNew England: Atlantic Geology, v. 24, p. 241–255.

Forster, H.-J., Tischendorf, G., and Trumbull, R. B., 1997, An evaluation of the Rb vs. (Y 1 Nb)discrimination diagram to infer tectonic setting of silicic igneous rocks: Lithos, v. 40, p. 261–293.

Fryer, B. J., Greenough, J. D., and Owen, J. V., 1997, Iapetus ocean floor stuffed into a suture zone: xenolithNd isotopic evidence for Dunnage-equivalent basement in central Newfoundland: Canadian Journal ofEarth Sciences, v. 34, p. 1392–1400.

Getty, S. R., and Gromet, L. P., 1992, Geochronological constraints on ductile deformation, crustalextension, and doming about a basement-cover boundary, New England Appalachians: AmericanJournal of Science, v. 292, p. 359–379.

Goldsmith, R., 1987, Geochemistry of the New London area, southeastern Connecticut: United StatesGeological Survey Open File Report, p. 87–328.

–––––– 1991, Stratigraphy of the Nashoba zone, eastern Massachusetts: An enigmatic terrane, in Hatch, N. L.,editor, The bedrock geology of Massachusetts: United States Geological Survey Professional Paper 1366E-J, p. F1–F22.

Goldstein, S. L., O’Nions, R. K., and Hamilton, P. J., 1984, A Sm-Nd isotopic study of atmospheric dusts andparticulates from major river systems: Earth and Planetary Science Letters, v. 70, p. 221–236.

Hall, L. M., and Robinson, P., 1982, Stratigraphic—tectonic subdivisions of southern New England, in St.Julien, P., and Beland, J., editors, Major structural zones and faults of the northern Appalachians:Geological Association of Canada Special Paper 24, p. 15–41.

Hermes, O. D., and Zartman, R. E., 1985, Late Proterozoic and Devonian plutonic terrane within the Avalonzone of Rhode Island: Geological Society of America Bulletin, v. 96, p. 272–282.

Hodgkins, C. E., ms, 1985, Geochemistry and petrology of the Dry Hill gneiss and related gneisses, Pelhamdome, central Massachusetts. Contribution 48: M.S. thesis, Department of Geology and Geography,University of Massachusetts, 137 p.

Jacobsen, S. B., and Wasserburg, G. J., 1980. Sm-Nd isotopic evolution of chondrites: Earth and PlanetaryScience Letters, v. 50, p. 139–155.

Kay, R. W., 1977, Geochemical constraints on the origin of Aleutian magmas, in Talwani, M. and Pitman,W. C. III, editors, Island Arcs, Deep Sea Trenches and Back-Arc Basins. Maurice Ewing Series 1:Washington, D.C., American Geophysical Union, p. 229–242.

Keppie, J. D., Dostal, J., Murphy, J. B., and Cousens, B. L., 1997, Palaeozoic within-plate volcanic rocks inNova Scotia (Canada) reinterpreted: isotopic constraints on magmatic source and paleocontinentalreconstructions: Geological Magazine, v. 134, p. 425–447.

Kerr, A., Jenner, G. A., and Fryer, B. J., 1995, Sm-Nd isotopic geochemistry of Precambrian to Paleozoicgranitoid suites and the deep-crustal structure of the southeast margin of the Newfoundland Appala-chians: Canadian Journal of Earth Sciences, v. 32, p. 224–245.

Kovach, A., Hurley, P. M., and Fairbairn, H. W., 1977, Rb-Sr whole rock age determinations of the DedhamGranodiorite, eastern Massachusetts: American Journal of Science, v. 277, p. 905–912.

Larson, T. E., ms, 1999, The metamorphic history of the Massabesic Gneiss complex and the BerwickFormation of southeastern New Hampshire: M.S. thesis, University of New Hampshire, Durham, NewHampshire, 117 p.

Larson, T. E., Kerwin, C., Allard, S., Laird, J., and Bothner, W., 1999, The metamorphic and partial tectonichistory of the Massabesic Gneiss Complex and the Berwick formation, southeastern New Hampshire:Transactions of the American Geophysical Union, Spring Meeting, V32B-07.

Larson, T. E., Laird, J., and Bothner, W. A., 1998, Calc-silicate enclaves within the Massabesic GneissComplex, southeastern New Hampshire: Geological Society of America Abstracts with Programs, v. 30,p. 32.

Lux, D. R., and West, D. P. Jr., 1993, New 40Ar/39Ar mica ages from eastern New Hampshire and southernMaine: Implications for the exhumation history of the region: Geological Society of America Abstractswith Programs, v. 25, 2, p. 35.

Lyons, J. B., Bothner, W. A., Moench, R. H., and Thompson, J. B., 1997, Bedrock geologic map of NewHampshire: United States Geological Survey, Map Series.

Lyons, J. B., Boudette, E. L., and Aleinikoff, J. N., 1982, The Avalonian and Gander zones in central easternNew England, in St. Julien, P., and Beland, J., editors, Major structural zones and faults of the northernAppalachians: Geological Association of Canada Special Paper 24, p. 43–65.

Mehnert, K. R., 1971, Migmatites and the origin of granitic rocks. Elsevier, 405 p.Miller, C. F., Watson, E. B., and Rapp, R. P., 1985, Experimental investigation of mafic mineral—felsic liquid

equilibria: Preliminary results and petrogenetic implications: Transactions of the American GeophysicalUnion, EOS, v. 46, p. 1130.


Miyashiro, A., 1974, Volcanic rock series in island arcs and active continental margins: American Journal ofScience, v. 274, p. 321–355.

Murry, D. P., and Dallmeyer, R. D., 1991, Polyphase tectonothermo evolution of basement in southeasternNew England: Evidence from 40Ar/39Ar hornblende ages: Geological Society of America Abstracts withPrograms, v. 23, p. 107.

Murry, D. P., Hermes, O. D., and Dunham, T. S., 1990, The New Bedford area: A preliminary assessment, inSocci, A. D., Skehan, J. W., and Smith, G. W., editors, Geology of the Composite Avalon Terrane ofSouthern New England: Geological Society of America Special Paper 245, p. 155–169.

Nance, R. D., and Murphy, J. B., 1994, Contrasting basement isotopic signatures and the palinspasticrestoration of peripheral orogens: Example from the NeoProterozoic Avalonian-Cadomian belt:Geology, v. 22, p. 617–620.

O’Brien, S. J., Wardel, R. J., and King, A. F., 1983, The Avalon zone; A Pan-African terrane in theAppalachian orogen of Canada: Geological Journal, v. 18, p. 195–222.

O’Hara, K. D., and Gromet, L. P., 1985, Two distinct late Precambrian (Avalonian) terranes in southeasternNew England and their late Paleozoic juxtaposition: American Journal of Science, v. 285, p. 673–709.

Orville, P. M., 1969, A model for metamorphic differentiation origin of thin-layered amphibolites: AmericanJournal of Science, v. 267, p. 64–86.

Osberg, P. H., 1978, Synthesis of the geology of the northeast Appalachians, USA, IGCP Project 27, U.S.A.,Contribution No. 1, Caledonian—Appalachian Orogen of the North Atlantic Region: Geological Surveyof Canada Paper 78–13, p. 137–147.

Pearce, J. A., and Cann, J. R., 1973, Tectonic setting of basic volcanic rocks determined using trace elementanalyses: Earth and Planetary Science Letters, v. 19, p. 290–300.

Pearce, J. A., Harris, N. B. W., and Tindle, A. G., 1984, Trace element discrimination diagrams for thetectonic interpretation of granitic rocks: Journal of Petrology, v. 25, p. 956–983.

Pe-Piper, G., and Piper, D. J. W., 1998, Geochemical evolution of Devonian-Carboniferous igneous rocks ofthe Magdalen basin, eastern Canada: Pb- and Nd-isotopic evidence for mantle and lower crustal sources:Canadian Journal of Earth Sciences, v. 35, p. 201–221.

Pouliot, J. B., 1994, A geophysical and geological investigation along the eastern margin of the MassabesicGneiss Complex, Raymond, N.H.: Geological Society of America Abstracts with Programs, v. 29, p. 83.

Rankin, D. W., 1994, Continental margin of the eastern United States: Past and present, in Speed, R. C.,editor, Phanerozoic Evolution of North American Continent-Ocean Transitions: Geological Society ofAmerica, DNAG Continent-Ocean Transect Volume p. 129–218.

Rankin, D. W., Hall, L. M., Drake, A. A. Jr., Goldsmith, R., Ratcliffe, N. M., and Stanley, R. S., 1989,Proterozoic evolution of the rifted margin of Laurentia, in Hatcher, R. D. Jr., Thomas, W. A., and Viele,G. W., editors, The Appalachian-Ouachita orogen in the United States: Geological Society of America.The Geology of North America, F-2, p. 10–42.

Rast, N., O’Brien, B. H., and Wardle, R. J., 1976, Relationships between Precambrian and lower Paleozoicrocks of the Avalon Platform in New Brunswick, the northeast Appalachians and the British Isles:Tectonophysics, v. 30, p. 315–338.

Robinson, P., Tucker, R. D., Bradley, D., Berry, H. IV, and Osberg, P. H., 1998, Paleozoic orogens in NewEngland, USA: GFF, v. 120, p. 119–148.

Schenk, P. E., 1971, Southeastern Atlantic Canada, northeastern Africa, and continental drift: CanadianJournal of Earth Sciences, v. 8, p. 1218–1251.

Schilling, J.-G., Zajac, M., Evans, R., Johnson, T., White, W., Devine, J. D., and Kingsley, R., 1983, Petrologicand geochemical variations along the Mid-Atlantic Ridge from 27°N to 73°N: American Journal ofScience, v. 283, p. 510–586.

Skehan, J. W., and Rast, N., 1990, Pre-Mesozoic evolution of Avalon terranes of southern New England, inSkehan, J. W., and Smith, G. W., editors, Geology of the composite Avalon terrane of southern NewEngland: Geological Society of America Special Paper 245, p. 13–53.

Spear, F. S., and Harrison, T. M., 1989, Geochronological studies in central New England I: Evidence forpre-Acadian metamorphism in eastern Vermont: Geology, v. 17, p. 181–184.

Sriramadas, A., 1966, The geology of the Manchester Quadrangle, New Hampshire: New HampshireDepartment of Resources and Economic Development Bulletin, 2, 78 p.

Stewart, D. B., Wright, B. E., Unger, J. D., Phillips, J. D., and Jutchinson, D. R., 1993, Global geosciencetransect 8: Quebec—Maine—Gulf of Maine transect, southeastern Canada and northeastern UnitedStates of America: United States Geological Survey Map I-2329, 17 p.

Strong, D. F., 1979, Proterozoic tectonics of northwestern Gondwanaland: New evidence from easternNewfoundland: Tectonophysics, v. 55, p. 81–101.

Strong, D. F., O’Brien, S. J., Taylor, S. W., Strong, P. G., and Wilton, D. H., 1978, Aborted Proterozoic riftingin eastern Newfoundland: Canadian Journal of Earth Sciences, v. 15, p. 117–131.

Sun, S.-s., and McDonough, W. F., 1989, Chemical and isotopic systematics of oceanic basalts: implicationsfor mantle composition and process, in Saunders, A. D., and Norry, M. J., editors, Magmatism in theOcean Basins: Geological Society (London) Special Publication No. 42, p. 313–345.

Sun, S.-s., Nesbitt, R. W., and Sharaskin, A. Y., 1979, Geochemical characteristics of mid-ocean ridge basalts:Earth and Planetary Science Letters, v. 44, p. 119–138.

Taylor, R. S., and McLennan, S. M., 1985, The continental crust: its composition and evolution: Oxford,Blackwell, 312 p.

Thompson, M. D., Hermes, O. D., Bowring, S. A., Isachsen, C. E., Besancon, J. R., and Kelly, K. L., 1996,Tectonostratigraphic implications of Late Proterozoic U-Pb zircon ages in the Avalon zone of southeast-ern New England, in Nance, R. D. and Thompson, M. D., editors, Avalonian and Related Peri-Gondwanan Terranes of the Circum—North Atlantic: Geological Society of America Special Paper 304,p. 179–191.


Thompson, R. N., Morrison, M. A., Dickin, A. P., and Hendry, G. L., 1983, Continental flood basalts . . . arach-nids rule OK?, in Hawkesworth, C. J. and Norry, M. J., editors, Continental Flood Basalts and MantleXenoliths: Shiva, p. 158–185.

Thompson, R. N., Morrison, M. A., Hendry, G. L., and Parry, S. J., 1984, An assessment of the relative roles ofcrust and mantle in magma genesis: An elemental approach: Philosophical Transactions of the RoyalSociety of London, v. A 310, p. 549–590.

Tomascak, P. B., Krongstad, E. J., and Walker, R. J., 1996, Nature of the crust in Maine, USA: Evidence fromthe Sebago batholith: Contributions to Mineralogy and Petrology, v. 125, p. 45–59.

Tucker, R. D., and Robinson, P., 1990, Age and setting of the Bronson Hill magmatic arc; A re-evaluationbased on U-Pb zircon ages in southeastern New England: Geological Society of America Bulletin, v. 102,p. 1404–1419.

Wayne, D. M., Sinha, A. K., Hewitt, D. A., 1992, Differential response of zircon U-Pb isotopic systematics tometamorphism across a lithologic boundary; an example from the Hope Valley shear zone, southeast-ern Massachusetts, USA: Contributions to Mineralogy and Petrology, v. 109, p. 408–420.

West, D. P. Jr., 1993, The eastern limit of Acadian high grade metamorphism in northern New England:Implications for the location of the “Acadian Suture”: Geological Society of America Abstracts withPrograms, v. 25, 2, p. 89.

Whalen, J. B., Jenner, G. A., Currie, K. L., Barr, S. M., Longstaffe, F. J., and Hegner, E., 1994, Geochemicaland isotopic characteristics of granitoids of the Avalon zone, southern New Brunswick: Possibleevidence for repeated delamination events: Journal of Geology, v. 102, p. 269–282.

Williams, H., 1978, Geological development of the northern Appalachians: Its bearing on the evolution ofthe British Isles, in Bowes, D. R., and Leake, B. E., editors, Crustal evolution in northwestern Britian andadjacent regions: Liverpool, Seal House Press, p. 1–22.

Williams, H., and Hatcher, R. D. Jr., 1982, Suspect terranes and accretionary history of the AppalachianOrogen: Geology, v. 10, p. 530–536.

–––––– 1983, Appalachian suspect terranes, in Hatcher, R. D., Williams, H., and Zietz, I., Jr., editors,Contributions to the Tectonics and Geophysics of Mountain Chains: Geological Society of AmericaBulletin, v. 158, p. 33–53.

Wintsch, R. P., 1979, The Willimantic fault: A ductile fault in eastern Connecticut: American Journal ofScience, v. 279, p. 367–393.

Wintsch, R. P., and Aleinikoff, J. N., 1987, U-Pb isotopic and geologic evidence for late Paleozoic anatexis,deformation, and accretion of the Late Proterozoic Avalon terrane, south-central Connecticut: Ameri-can Journal of Science, v. 287, p. 107–126.

Wintsch, R. P., and Kvale, C. M., 1994, Differential mobility of elements in burial diagenesis of siliciclasticrocks: Journal of Sedimentary Research, v. A64, p. 349–361.

Wintsch, R. P., Sutter, J. F., Kunk, M. J., Aleinikoff, J. N., and Dorais, M. J., 1992, Contrasting P-T-t paths:Thermochronologic evidence for a Late Paleozoic final assembly of the Avalon composite terane in theNew England Appalachians: Tectonics, v. 11, p. 672–689.

Wintsch, R. P., Sutter, J. F., Kunk, M. J., Aleinikoff, J. N., and Boyd, J. L., 1993, Alleghanian assembly ofProterozoic and Paleozoic lithotectonic terranes in south central New England: New constraints fromgeochronology and petrology, in Cheney, J. T. and Hepburn, J. C., editors, Field Trip Guidebook for theNortheastern United States: 1993 Boston: Geological Society of America, v. 1, p. H1–H30.

Wintsch, R. P., Webster, J. R., Bernitz, J. A., and Fout, J. S., 1990, Geochemical and geological criteria for thediscrimination of high-grade gneisses of intrusive and extrusive origin, eastern Connecticut, in Socci,A. D., Skehan, J. W., and Smith, G. W., editors, Geology of the Composite Avalon Terrane of SouthernNew England: Geological Society of America Special Paper 245, p. 187–208.

Zartman, R. E., 1988, Three decades of geochronological studies in the New England Appalachians:Geological Society of America Bulletin, v. 100, p. 1168–1180.

Zartman, R. E., Hermes, O. D., and Pease, M. H. Jr., 1988, Zircon crystallization ages and subsequent isotopicdisturbance events in gneissic rocks of eastern Connecticut and western Rhode Island: AmericanJournal of Science, v. 288, p. 376–402.

Zartman, R. E., and Naylor, R. S., 1984, Structural implications of some radiometric ages of igneous rocks insoutheastern New England: Geological Society of America Bulletin, v. 95, p. 522–539.

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