2005 polliand peru

8/3/2019 2005 Polliand Peru

1/12

O R I G I N A L P A P E R

Marc Polliand Urs Schaltegger Martin FrankLluis Fontbote

Formation of intra-arc volcanosedimentary basins in the westernflank of the central Peruvian Andes during Late Cretaceous obliquesubduction: field evidence and constraints from UPb agesand Hf isotopes

Received: 16 October 2003 / Accepted: 11 December 2004/ Published online: 16 February 2005 Springer-Verlag 2005

Abstract During late Early to Late Cretaceous, thePeruvian coastal margin underwent fast and obliquesubduction and was characterized by important arc

plutonism (the Peruvian Coastal Batholith) and forma-tion of volcanosedimentary basins known as the Wes-tern Peruvian Trough (WPT). We present high-precisionUPb ages and initial Hf isotopic compositions of zirconfrom conformable volcanic and crosscutting intrusiverocks within submarine volcanosedimentary strata ofthe WPT hosting the Perubar massive sulfide deposit.Zircons extracted from both the volcanic and intrusiverocks yield concordant UPb ages ranging from67.890.18 Ma to 69.710.18 Ma, indicating that ba-sin subsidence, submarine volcanism and plutonicactivity occurred in close spatial and temporal rela-tionship within the Andean magmatic arc during the

Late Cretaceous. Field observations, satellite imageinterpretation, and plate reconstructions, suggest thatdextral wrenching movements along crustal lineamentswere related to oblique subduction. Wrench tectonics istherefore considered to be the trigger for the formationof the WPT as a series of pull-apart basins and for theemplacement of the Coastal Batholith. The zircon initialHf values of the dated magmatic rocks fall between 5.5and 7.4, and indicate only very subordinate influence ofa sedimentary or continental component. The absenceof inherited cores in the zircons suggest a complete lack

of old basement below the WPT, in agreement withprevious UPb and Sr isotopic data for batholithic rocksemplaced in the WPT area. This is supported by the

presence of a most likely continuous block of dense($3.0 g/cm3) material observed beneath the WPT areaon gravimetric crustal cross sections. We suggest thatthis gravimetric anomaly may correspond to a piece oflithospheric mantle and/or oceanic crust inherited froma possible Late PermianTriassic rifting. Such youngand mafic crust was the most probable source for arcmagmatism in the WPT area.

Keywords Andean magmatism Continental arc UPb dating Hf isotopes Strike-slip basins

Introduction

During late-Early to Late Cretaceous the relative motionbetween the oceanic Farallon (previously Phoenix) andSouth American plates along the central Andean marginwas characterized by a period of high convergence rate(e.g., Jaillard et al. 2000; Larson 1991; Soler and Bon-homme 1990) with a significant oblique component(northerly to northnortheasterly motion) of the sub-ducted lithosphere (e.g., Jaillard 1994; Gordon andJurdy 1986; Duncan and Hargraves 1984) and a mostlikely absolute trenchward motion of the South Ameri-

can plate (e.g., Jaillard and Soler 1996; Jaillard 1994; P.Soler, unpublished PhD thesis) due to the definitiveopening of the South Atlantic ocean at about 110 Ma(e.g., Scotese et al. 1988; Larson and Pitman 1972).Along the coastal margin of Peru, this period of fast andoblique subduction was characterized by important arcmagmatism and formation of volcanosedimentary ba-sins with high average subsidence rates, known as theMesozoic Western Peruvian Trough (WPT; e.g., Cob-bing 1978; Atherton et al. 1983; Fig. 1a). The develop-ment of the WPT was accompanied and followed by

M. Polliand U. Schaltegger (&) L. Fontbote

Department of Mineralogy,University of Geneva, Rue des Marachers 13,1205 Geneve, SwitzerlandE-mail: [email protected]: [email protected].: +41-22-3796638Fax: +41-22-3793210E-mail: [email protected]

U. Schaltegger M. FrankDepartment of Isotope Geology and Mineral Resources,Federal Institute of Technology ETH,Sonneggstrasse 5, 8092 Zurich, SwitzerlandE-mail: [email protected]

Int J Earth Sci (Geol Rundsch) (2005) 94: 231242DOI 10.1007/s00531-005-0464-5


2/12

intrusion of numerous plutons that form the present-day

Peruvian Coastal Batholith (Fig. 1), mostly emplacedbetween about 100 Ma and 30 Ma (e.g., Soler and Ro-tach-Toulhoat 1990a; Mukasa 1986a). Within the vol-canosedimentary basins, several massive sulfide depositswere formed (i.e., Aurora Augusta, Cerro Lindo, MariaTeresa, Palma, Perubar; Fig. 1a, b), amongst which thePerubar deposit has been exploited from 1978 to 2000and is the best studied (Polliand 2005; Polliand andFontbote 2000; Vidal 1987).

The WPT in the central coastal margin of Peru, be-tween about Trujillo and Pisco, has been subdivided into

two major basin units known as the Huarmey and the

Can ete basins (e.g., Atherton et al. 1983; Cobbing 1978;Fig. 1). According to several authors (e.g., Atherton andAguirre 1992; Atherton and Webb 1989; Cobbing et al.1981), both the Huarmey and Can ete basins had theirmain period of subsidence in the Albian, during whichup to 9,000 m of volcanic (mainly basalt flows), volca-niclastic, and subordinate sedimentary rocks weredeposited. These dominantly submarine volcanic andvolcaniclastic successions have been attributed in theliterature to the Casma Group and been described asAlbian to Cenomanian in age (e.g., Atherton and Webb

Fig. 1 a Geologic map of thePeruvian Coastal Batholith andMesozoic Western PeruvianTrough (WPT) with relatedvolcanosedimentary basinshosting massive sulfidedeposits. Volcanosedimentarybasin limits after Vidal (1987),Atherton et al. (1983), andCobbing (1978). The yellowdoted lines encompass the

3.0 g/cm3 wedge at 10-kmdepth. Distribution of theCretaceous Peruvian CostalBatholith, Casma Group, andEarly Cretaceous strata afterthe digital version of thegeological map of Peru(INGEMMET 2000). b Mainlineaments and structures basedon Landsat-5 TM image (RedBand 7; Green Band 4; BlueBand 2) interpretation,downloaded from http://www.zulu.ssc.nasa.gov/mrsid/.c Geologic interpretation of thedeep sturctures along the 9S

and 12S gravimetric, crustaland subcrustal cross sections(Couch et al. 1981; Jones 1981)of the continental margin ofPeru

232


3/12

1989; Cobbing et al. 1981). The extensional tectonicsetting during which the Casma Group and relatedvolcanosedimentary basins were formed was essentiallyattributed either to an extensional marginal basin withincipient formation of oceanic crust (e.g., Atherton andAguirre 1992; Atherton 1990; Atherton and Webb 1989;Atherton et al. 1985), or to the extensional tectonic(pull-apart?) subsidence within the volcanic arc (e.g.,Jaillard 1994; P. Soler, unpublished PhD thesis; Soler1991). The overall geologic, lithologic and geometricframework of the WPT and related volcanosedimentarybasins remains, however, poorly known, except in someareas near Huarmey and Culebras (Webb 1976; Myers1980), and more detailed work is necessary to reachmore sound geodynamic conclusions.

We present new UPb age determinations and initialHf isotopic compositions of zircon from conformablevolcanic and crosscutting intrusive rocks within sub-marine volcanosedimentary strata attributed so far tothe Casma Group and hosting the Perubar massivesulfide deposit in the northeastern part of the Can etebasin (50 km east of Lima, 1155S 7634W; Fig. 1).

These new data (1) demonstrate that the volcanosedi-mentary rocks of the Perubar area are much younger(Maastrichtian) than the Albian to Cenomanian age sofar assumed for the Casma Group; (2) allow a betterunderstanding of the temporal relationships betweenbasin subsidence, subaqueous volcanism, hydrothermalmassive sulfide mineralization, and plutonism during theuppermost Cretaceous in this region of the PeruvianAndes, (3) provide evidence for a formation of the WPTduring extensional tectonic subsidence within the vol-canic arc, and (4) constrain the melt sources and thenature of the basement beneath the central Peruviancoastal margin. Finally, this paper proposes a new

hypothesis for the continental growth of the Peruvianactive margin during late Early to Late Cretaceous,when the tangential component between the convergingSouth American and Farallon plates was large.

Geologic setting of the volcanosedimentary rockshosting the Perubar deposit

The Late Cretaceous volcanosedimentary strata thathost the Perubar massive sulfide deposit are located inthe northeastern part of the Can ete basin (Fig. 1a).These strata are truncated to the east and north by

younger intrusions (i.e., the Canchacaylla monzogranitepluton and the Inclinado quartz monzodiorite porphyrystock, Fig. 2) and overlapped to the south by more re-cent continental volcanic deposits (i.e., the CalipuyGroup, Fig. 2). The Canchacaylla monzogranite yieldsan hornblende KAr age of 66.72 Ma (Vidal 1987).The smaller and moderately altered hornblende+pla-gioclase+biotite phenocryst-bearing Inclinado quartzmonzodiorite porphyry (sample number 4 in Fig. 3a),that follows the northsouth-trending Inclinado faultover a distance of 1,400 m (Fig. 2), yields a concordant

zircon UPb age of 67.890.18 Ma (Table 1). The Ri-cardo Palma tonalite, delimiting the western part of thebasin (Fig. 2), is overlain by the volcanosedimentarystrata and most probably constituted the western bed-rock shoulder of the basin (Fig. 2). According to Vidal(1987) and Mukasa (1986a), the Ricardo Palma tonaliteyields a 82.02.3 Ma hornblende KAr age and con-cordant zircon UPb ages of 86.4 and 84.4 Ma, respec-tively. It was emplaced into older magmatic rocks of theCoastal Batholith and Early Cretaceous sedimentarystrata such as those exposed further west of the studyarea along the coast line (Fig. 1a, b). Both the older andyounger intrusions occurring in the Perubar area belongto the Cretaceous Peruvian Coastal Batholith.

The stratigraphy of the studied volcanosedimentarysuccession at Perubar has been divided into four mainlithological units defined, from bottom to top, as theFootwall, Prospective, Hangingwall and Upper Units,respectively. Their lithological description is given inFig. 3a. These volcanic and sedimentary strata wereerupted and deposited in a pull-apart marine basinenvironment (Fig. 3b; Polliand 2005) without impor-

tant periods of interrupted sedimentation, since lessthan 2 Ma separate the end of the Footwall Unit sedi-mentation and the deposition of the Upper Unit(Fig. 3a).

The Hangingwalland Upper Unit deposits (Fig. 3) areinterpreted as the result of a tectonically controlledcaldera collapse, where collapse and magmatic plumbingwere influenced by pre-existing faults (Polliand 2005).The Hangingwall Unit polymictic breccias representdebris avalanche deposits resulting from a gravitationalfault-block subsidence, that occurred prior to a cata-strophic eruption responsible for the deposition of theUpper Unit pyroclastic and/or volcaniclastic debris flow

deposits (Polliand 2005). The latter are inferred to begenetically related to the ignimbrite that outcrops alongthe Inclinado fault (Fig. 2), interpreted as the synvol-canic infill of a major eruptive fissure delineated by thatsame fault (Fig. 3b; Polliand 2005).

The massive sulfide mineralization, situated withinthe Prospective Unit (Fig. 3), was formed at the seafloorduring a period of coeval submarine volcanism includingthe formation of lava domes, hyaloclastites, volcani-clastic sediments, and the sedimentation of impurelimestones (Polliand 2005). The mineralizing hydro-thermal event caused the strong hydrothermal alterationof the host and footwall rocks. A strongly hydrother-

mally altered, devitrified, perlitically and/or hydrother-mally fractured, flow-banded rhyolite, showing relictfeldspar crystals and situated in the upper portion of theLower Unit (sample number 1 in Fig. 3a), and a stronglyhydrothermally altered porphyritic rhyolite with pla-gioclase > quartz > K-feldspar (sample 2 in Fig. 3a)directly overlying the massive sulfide mineralization,yield zircon UPb concordant ages of 69.710.18 Maand 68.920.16 Ma, respectively (Table 1 and Fig. 4).In addition, moderately altered aplites and rhyodaciticdykes with plagioclase > K-feldspar phenocrysts

233


4/12

crosscut the Footwall and Prospective Units and possiblythe whole stratigraphic column, although crosscuttingrelationships with the Hangingwalland Upper Units werenot observable in the field (Fig. 3). One of theserhyodacitic dykes was dated and yields a concordantzircon UPb age of 67.940.16 Ma (Table 1 and sam-

ple number 3 in Fig. 3a; Fig. 4).

UPb and Hf isotopic determinations

Determinations of UPb ages and analysis of initialHf isotopic compositions in zircon were carried out onfour different volcanic, subvolcanic and intrusivelithologies from the Perubar deposit area (see Figs. 3a,4), consisting of: (1) a flow-banded rhyolite situated inthe upper portion of the Lower Unit; (2) a massive

porphyritic rhyolitic lava, directly overlying the massivesulfide orebodies and representing the upper limit ofthe Hangingwall Unit; (3) a porphyritic rhyodaciticdyke, believed to crosscut the whole stratigraphy; (4)the Inclinado quartz monzodiorite porphyry. UPb andHf isotope data are summarized in Tables 1 and 2

(analytical techniques follow those in Schaltegger et al.2002).

Flow-banded rhyolite (MPP657; Fig. 4a)

The zircons are transparent, mostly subhedral to pris-matic, and have a uniform U concentration of 403432 ppm. Three multigrain analysis (sample no. 13)yield reproducible concordant points that define a mean206Pb/238U age of 69.710.18 Ma (Fig. 4). Amongst

Fig. 2 Geologic map of thePerubar deposit area. SeeFig. 3b for cross section.Coordinates are UTM

234


5/12

these three multigrain fractions, Hf isotopic data weredetermined for two of them (1 and 2) and yield uniforminitial Hf of 7.40.2 and 7.00.3.

Porphyritic rhyolitic lava (MPP594; Fig. 4b)

The zircons of this rhyolite are transparent and most of

them have a G-type euhedral morphology (according toPupin 1980). The analysed fractions are U-rich, withrelatively homogeneous concentrations ranging between3,067 ppm and 3,244 ppm. Three multigrain analyses(no. 46) yield well reproducible 206Pb/238U ratios thatdefine a mean age of 68.920.16 Ma. The Hf values forzircon fractions 5 and 6 are 5.73.4 and 7.70.5. TheHf value measured for fraction 4 (i.e., 11.01.3) is be-lieved to represent an analytical outlier, since it stronglydeviates from the other thirteen Hf values centeredaround a value of 6 (Table 2).

Porphyritic rhyodacitic dyke (JG120; Fig. 4c)

The zircon population consists of one type of transpar-ent euhedral grains, which mostly have a G-type mor-phology. About half of the zircons show smalltransparent melt inclusions; they have relatively uniformU concentrations ranging between 332 ppm and593 ppm. Five multigrain fractions (no. 711) were

analyzed and yield similar, reproducible206

Pb/238

U ra-tios defining a concordant age of 67.940.16 Ma. Whenremoving the two analytical points 7 and 8, which yieldsignificantly larger uncertainties due to their highercommon lead concentrations, a mean concordant206Pb/238U age of 67.910.17 Ma can be calculated.Since these two ages are the same within error, the agecalculated on the basis of the five analytical points(67.940.16 Ma) will be preferred. The Hf values ob-tained for all analyzed fractions are relatively uniformand fall between 5.50.3 and 6.90.5.

Fig. 3 a Lithostratigraphiccolumn of the Late Cretaceousvolcanosedimentary stratahosting the Perubar deposit.Numbers 14 delineate datedrock samples (see Table 1).b Longitudinal section acrossthe Perubar VHMS deposit (seeFig. 2 for location). Note thedeeply subsided blocks alongthe steep synsedimentary faults.

This section was constructed onthe basis of drill core,superficial, and undergroundworking informationsprocessed with the Gemcommining software

235


6/12

Table1

U

Pbiso

top

icda

ta

No

.Descr

iption

Weight

(mg

)

Num

ber

ofgra

ins

Concen

tra

tions

Atom

icra

tios

Apparen

tages

(Ma

)

Error

corr

.

U (ppm

)Pbra

d

(ppm

)Pbnonra

d.

(pg

)

Th/Ua

206/204b

206/238c,

d

Error

2s

(%)

207/2

35c

Error

2s

(%)

207/206c,

d

Error

2s

(%)

206/238d)207/235207/206d

MPP657

1

Prism

tosu

broun

d0

.0076

6

432

5.0

5

2.6

0.6

2

876

0.0

1085

0.3

9

0.071

01

0.9

1

0.0

4746

0.6

8

69

.58

69

.65

72

.28

0.7

3

2

Su

bhedra

l

0.0

036

6

419

4.8

6

2.9

0.5

7

372

0.0

1089

0.3

7

0.071

48

1.4

9

0.0

4476

1.3

6

69

.85

70

.11

78

.85

0.4

6

3

Su

bhedra

l

0.0

024

7

403

4.8

8

1.0

0.7

4

684

0.0

1089

0.5

3

0.071

29

1.5

1

0.0

4747

1.4

0

69

.84

69

.92

72

.84

0.3

8

MPP594

4

G-t

ypeseu

h

0.0

011

4

3151

39

.26

0.8

0.9

2

2910

0.0

1076

0.3

9

0.070

17

0.5

1

0.0

4732

0.3

6

68

.96

68

.86

65

.31

0.7

1

5

G-t

ypeseu

hspr

0.0

024

5

3244

40

.93

0.5

0.9

6

11195

0.0

1074

0.3

8

0.070

32

0.4

1

0.0

4748

0.2

0

68

.87

69

.01

73

.60

0.8

7

6

Sma

lla

br

0.0

032

4

3067

39

.60

4.5

1.0

4

1494

0.0

1074

0.3

5

0.070

48

0.5

1

0.0

7458

0.3

4

68

.89

69

.16

78

.43

0.7

4

JG120

7

Cleareu

h

0.0

081

6

332

3.9

8

11

.0

0.7

4

182

0.0

1064

0.7

3

0.070

16

2.3

8

0.0

4783

2.2

7

68

.22

68

.85

90

.93

0.3

0

8

Roun

dclrls

0.0

028

6

403

4.6

3

2.2

0.7

8

357

0.0

1062

1.5

6

0.068

90

2.6

4

0.0

4708

2.9

1

68

.07

67

.66

53

.12

0.1

1

9

Roun

dtoprism

0.0

036

5

593

7.0

7

1.3

0.7

9

1123

0.0

1058

0.4

1

0.069

19

0.8

7

0.0

4742

0.7

5

67

.86

67

.93

70

.56

0.5

1

10

Roun

d

0.0

040

5

465

5.4

0

0.6

0.6

8

2229

0.0

1059

0.4

7

0.069

43

0.6

1

0.0

4759

0.4

6

67

.94

68

.16

75

.84

0.6

7

11

Prism

0.0

044

5

499

5.8

3

1.3

0.7

2

1,1

29

0.0

1059

0.3

4

0.069

12

0.8

7

0.0

4733

0.7

5

67

.92

67

.86

65

.83

0.5

2

MPP338

12

Roun

d

0.0

086

6

238

2.7

3

4.2

0.6

5

344

0.0

1057

0.4

0

0.068

98

1.9

9

0.0

4739

1.8

4

67

.78

67

.73

65

.78

0.4

6

13

spr

0.0

046

5

328

3.9

8

0.7

0.8

7

1378

0.0

1059

0.4

3

0.069

21

0.8

2

0.0

4741

0.7

0

67

.89

67

.95

69

.77

0.5

2

14

Prism

lge

0.0

065

3

103

1.2

3

0.4

0.7

9

3444

0.0

1060

0.4

5

0.069

46

0.6

5

0.0

4753

0.4

9

67

.96

68

.18

76

.15

0.6

6

euheu

hedra

l,clrlsco

lourless,incl

inclusions,prismprisma

tic,sprshort-pri

sma

tic,subhsu

bhedra

l,transp

transparen

t,G

-typezirconsaccord

ing

toPup

in(198

0);ana

lyses

1

6una

bra

ded

,

7

10a

bra

ded

aCa

lcu

latedon

the

basiso

fra

dio

gen

ic208Pb/206Pbra

tios,assum

ingconcordancy

bCorrec

tedfor

fract

iona

tionand

spike

cCorrec

tedfor

fract

iona

tion

(.09

%),sp

ike,

blan

kan

dcommon

lea

d(Staceyan

dKramers

1975)

dCorrec

tedfor

initialThdisequi

librium

,usinganes

tima

tedTh/Ura

tioo

f4for

themel

t

236


7/12

Inclinado quartz monzodiorite porphyry(MPP338; Fig. 4d)

Two zircon populations are present in this rock, the firstconsisting of long prismatic transparent, the second ofround to short-prismatic transparent grains. The short

prismatic zircons have a U content of 238 and 328 ppmand the long prismatic of 103 ppm. Three multigrainanalyses (no. 1214) yield reproducible concordantpoints that define a mean 206Pb/238U age of 67.890.18 Ma. The Hf for the three analyzed fractionsare relatively uniform and range between 5.90.4 and6.60.5.

Tectonic setting

The main structural elements in the Perubar deposit areaare dominantly N- to NNE- and NW-oriented synsedi-

mentary and later reactivated faults (Figs. 2, 5). Thesynsedimentary faults can be recognized by changes ofdepositional facies (Fig. 3b; Polliand 2005). Whenremoving Tertiary dextral normal-slip movements thatoccurred along the main Corte Ladrones and Split faults(Polliand 2005), the synsedimentary faults define a typ-ical rhomb-shaped structure (Fig. 5), suggesting a pull-apart origin for the Perubar basin. At a regional scale,the Perubar deposit is situated between two major NW-oriented crustal lineaments (recognized on a Landsat-5image) extending to the coast and being reflected in the

orientation of the coastline (Fig. 1b). Similar structuresare visible along the Peruvian coast from about 6 to13S (Fig. 1a). The strongly oblique northerly to north-northeasterly motion assumed for the subducting slab(Farallon plate) during Late Cretaceous times (e.g.,Pardo-Casas and Molnar 1987; Gordon and Jurdy 1986;

Duncan and Hargraves 1984), is in agreement withdextral wrenching movements along these NW-orientedcrustal lineaments (Fig. 1b). They were also recognizedand described in the $100 Ma to $63 Ma-old Hauraplutonic complex north of Lima (Bussell 1983). Theorientation and extensional direction of the pull-apartbasin structure evidenced at Perubar are in full agree-ment with the large-scale structures (Figs. 1b, 5) andthus suggest that Late Cretaceous dextral wrench tec-tonics associated with oblique subduction may havegenerated the Perubar strike-slip basin and possibly thewhole WPT.

Discussion of the U/Pb ages and Hf isotope data

Timing of plutonism and basin subsidenceduring the Late Cretaceous

Sedimentation in the Perubar pull-apart basin mostlikely started in the early Maastrichtian, before the69.710.18 Ma dated subaqueous flow-banded rhyolitewas formed (no. 1, Fig. 3). The 1015-Ma older RicardoPalma tonalite (Fig. 2) is inferred to have formed the

Fig. 4 UPb concordiadiagrams for volcanic andintrusive lithologies at Perubar

237


8/12

bedrock and the western shoulder of this subsiding ba-sin. The upper, 540-m thick, volcanosedimentary strata

were deposited between 69.710.18 Ma and67.890.18 Ma, ages that correspond to the flow-ban-ded rhyolite and the Inclinado monzonite porphyry,respectively (no. 1 and 4, Fig. 3), yielding a minimumaverage sedimentation rate of 0.3 mm/year. The massivesulfide mineralization formed within the 0.8-Ma intervalcomprised between the 69.710.18 Ma flow-bandedrhyolite and the 68.920.1 Ma porphyritic rhyoliticlava (no. 1 and 2, Fig. 3). Therefore, the studied volca-nosedimentary strata in this part of the Can ete basincannot be considered as a part of the Casma Group butrepresent a distinct and younger basin sedimentationevent, since they were emplaced in the uppermost Cre-

taceous (Maastrichtian) instead of the mid-Cretaceous(AlbianCenomanian) age previously defined for theCasma Group (Atherton and Webb 1989 and referencestherein).

The deposition of the studied volcanosedimentary pileand related massive sulfide deposit was quickly followedby the injection of porphyritic rhyodacitic dykes at67.910.17 Ma (no. 3, Fig. 3) and the intrusion of boththe Inclinado porphyry and the Canchacaylla pluton at67.890.18 Ma (no. 4, Fig. 3) and 66.72 Ma (Vidal1987; Fig. 2), respectively. Since these two intrusive rocks

belong to the subduction-related arc magmatism of the

Peruvian Coastal Batholith (e.g., P. Soler, unpublishedPhD thesis; Soler and Bonhomme 1990; Mukasa 1986a),it is likely that the subaqueously erupted volcanic depositsat Perubar represent the effusive counterpart of the plu-tonic arc. Therefore, Late Cretaceous intra-arc strike-slipfaulting, as observed at Perubar (Figs. 3b, 5), was mostprobably responsible for pull-apart basin subsidence andrelated subaqueous volcanism, and provided at the sametime space in the crust for the emplacement of plutons.This interpretation is in agreement with mechanisms re-lated to oblique convergence, the latter being typically

Table 2 Hf isotopic data

Number 176Hf/177Hf(0)

176Hf/177Hf(T)a

2s Hf(T)b

2s T(DM)(Ga)c

MPP6571 0.282937 0.282930 6 7.4 0.2 0.632 0.282927 0.282920 9 7.0 0.3 0.653 MPP5944 0.283041 0.283035 36 (11.0) 1.3 0.415 0.282889 0.282883 94 5.7 3.4 0.736 0.282936 0.282940 14 7.7 0.5 0.61JG1207 0.282883 0.282877 6 5.5 0.3 0.748 0.282887 0.282881 16 5.6 0.6 0.739 0.282923 0.282917 12 6.9 0.5 0.6610 0.282908 0.282902 7 6.3 0.3 0.6911 MPP33812 0.282905 0.282899 10 6.2 0.4 0.6913 0.282895 0.282889 9 5.9 0.4 0.7214 0.282915 0.282909 11 6.6 0.5 0.67

aThe Hf fraction was isolated using Eichrom Ln-spec resin, andmeasured in static mode on a NuPlasma multi-collector ICP-MSusing a MCN-6000 nebulizer for sample introduction. Zircons were

corrected for in situ radiogenic ingrowth from 176 Lu using anestimated 176 Lu/177 Hf of 0.005. The Hf isotopic ratios were cor-rected for mass fractionation using a 179Hf/177Hf value of 0.7325and normalized to 176Hf/177Hf=of 0.282160 of the JMC-475standard (Blichert-Toft et al. 1997). Mean isotopic values are at the95% confidence levelbCalculated with present-day CHUR values of 176Hf/177Hf=0.282772, 176 Lu/177 Hf=0.0332cCalculated with present-day DM values of176Hf/177Hf=0.283252,176 Lu/177 Hf=0.04145 and a crustal 176 Lu/177 Hf=0.017 (Blichert-Toft and Albare` de 1997)

Fig. 5 Model for Late Cretaceous pull-apart origin of the Perubarbasin. The tectonic stress (r1) is inferred from relative plate motionreconstructions between the South American continent and theoceanic Farallon plate (e.g., Gordon and Jurdy 1986; Duncan andHargraves 1984)

238


9/12

resolved into a strongly orthogonal compressional com-ponent at the trench and a strike-slip component ex-pressed by structures in the forearc and arc itself (Fitch1972).

Melt sourcesevidence for lacking Precambrianbasement beneath the WPT

The Hf values determined for the zircon fractions of thefour different analyzed lithologies at Perubar all fallbetween 5.50.3 and 7.70.2 (Table 2). This range ofpositive values indicates only very subordinate contam-ination by a sedimentary or continental componentduring emplacement of both the volcanic and intrusiverocks at Perubar. The absence of inherited Proterozoicages in the zircons measured in both the four datedsamples at Perubar and in the Lima segment of theCoastal Batholith in general (Mukasa 1986a; Fig. 1a)indicates the lack of old basement beneath the WPTarea. This contrasts with the occurrence of inheritedzircons from the Precambrian basement of the Arequipa

craton in plutons from the Arequipa and Toquepalasegments of the Coastal Batholith (Mukasa 1986a;Fig. 1a). Moreover, the Coastal Batholith in the Limasegment displays consistent lead and strontium isotopicratios (206Pb/204Pb: 18.55420.803; 87Sr/86Sr: 0.703070.70516), which infer that the rocks are apparentlyuncontaminated by upper crustal radiogenic compo-nents (e.g., Haeberlin 2004; Macfarlane et al. 1990; Solerand Rotach-Toulhoat 1990a, b; Mukasa 1986b) andthus suggest a lack of old basement below the WPT areabetween Pisco and Chimbote (Fig. 1a). It is supportedby the presence of a most likely continuous block ofdense (3.0 to 3.04 g/cm3) material (oceanic crust and

lithospheric mantle?) beneath the WPT area betweenPisco and Trujillo according to the gravimetric crustalcross sections (Couch et al. 1981; Jones 1981) shown inFig. 1a and c. At the latitude of the 12S profile, weextended the limit of the 3.0 g/cm3 gravity anomaly tothe eastern boundary of the Can ete basin (Fig. 1a), sinceno evidence of significant upper-crustal contaminationcan be identified in the studied volcanic and plutonicrocks at Perubar. This zone, free of old basementunderlying the WPT, would correspond to the leadprovince Ib defined in Macfarlane et al. (1990), the leadof which is believed to have been extracted from anenriched upper mantle reservoir. Similarly, Soler and

Rotach-Toulhoat (1990a) suggested that the homoge-neous strontium isotopic composition ($0.704) of theCoastal Batholith between Pisco and Chimbote is re-lated to an undepleted mantle source (OIB-type or en-riched sub-continental) that was contaminated by fluidsderived from the subducting slab and associated sub-ducted sediments. The range ofHf values between +5.5and +7.7 of the volcanic and intrusive rocks at Perubarare compatible with: (1) such an enriched upper mantlesource reservoir, (2) with a depleted mantle contami-nated by slab-derived fluids carrying Hf (Woodhead

et al. 2001), or alternatively, (3) with a mixture of en-riched mantle and reworked oceanic crust. Hf modelages evidence an average mantle extraction or residenceage of 0.6-0.7 Ga.

Implications of a pull-apart originfor the volcanosedimentary basins of the WPT

Several authors (e.g., Atherton and Aguirre 1992; Ath-erton 1990; Atherton and Webb 1989; Atherton et al.1985) consider the WPT as an aborted ocean floorextensional marginal basin that had its main period ofextension and subsidence during the Albian. This modelmainly relies on the interpretation of the gravimetric andseismic data along the 9S, 12S and Pisco crustal crosssections (Fig. 1a, c; Couch et al. 1981; Jones 1981),where the dense 3.0 g/cm3 gravity anomaly is attributedto mantle-derived material that floored and filled up thisAlbian marginal basin. Accordingly, the 2.672.8 g/cm3

blocks underlying the so-called Outer Shelf Higharea(Fig. 1a, c) are interpreted as pieces of rifted continental

crust.However, we have demonstrated that the Albian

marginal basin model cannot be applied to the volca-nosedimentary strata exposed at Perubar, the latter beingMaastrichtian in age and deposited within a pull-apartbasin formed atop of the plutonic arc and related to the Nto NNE motion assumed for the paleo-Pacific slab duringLate Cretaceous times. Since a roughly northward motionof the subducting paleo-Pacific plate beneath the centralPeruvian margin is also assumed for the preceeding Al-bian to Campanian period (e.g., Jaillard 2000; Jaillard1994; Duncan and Hargraves 1984) combined with a highconvergence rate (>10 cm/year; Jaillard and Soler 1996;

Larson 1991), we suggest that wrench tectonics, strike-slipfaulting and subsequent pull-apart basin formation alsooccurred at that time, in line with the proposition ofJaillard (1994) and P. Soler (unpublished PhD thesis,Soler 1991) that the Casma Group-hosting volcanosedi-mentary basins possibly relate to extensional pull-apartsubsidence within the volcanic arc. Thus, the WPT vol-canosedimentary basins might represent distinct intra-arcpull-apart basin generations progressively migratingeastwardwith thearc from Albianto Maastrichtian times.During the AlbianCampanian period, the relative age ofthe subducted lithosphere was rapidly decreasing (e.g.,Jaillard and Soler 1996; Jaillard 1994; Soler and Bonho-

mme 1990), leading most likely to a decrease of the slabdip. This was possibly coupled to an absolute trenchwardmotion of the overriding South American plate (e.g.,Jaillard and Soler 1996; Soler andBonhomme 1990). Bothphenomena could account for the eastward migration ofthe arc.

The inferred intra-arc extensional pull-apart origin forthe WPT volcanosedimentary basins implies a pre-lateEarly Cretaceous origin for the underlying dense 3.0 g/cm3 structure (Fig. 1a, c). This 10 kmthick block of densematerial might represent a piece of lithospheric mantle

239


10/12

and/or oceanic crust inherited from a possible LatePermianTriassic rifting, believed to have developed inthe Eastern Cordillera of Peru and extended into Boliviain the Late TriassicMiddle Jurassic (Sempere et al. 2002and references therein). Several authors (e.g., Haeberlin2004; Ramos and Aleman 2000; Dalziel et al. 1994) sug-gested that this rifting event would have removed thenorthwest prolongation of the Arequipa craton ofsouthern Peru, from which the detached slice or slices arethought to have migrated northwards and possibly rep-resent the allochthonous Oaxaquia Terrane in Mexico.The Outer Shelf High, which, in this case, cannot beconsidered as a piece of rifted continental crust, maytherefore be interpreted as an accretionary complex and/or an accreted oceanic plateau (Nazca ridge?) and/or ac-creted island arc terranes previously formed on the paleo-Pacific plate (Fig. 1c). However, more work needs to bedone prior to reach any firm conclusions on the origin andnature of the present-day deep structures found along thecoastal margin of Peru between $7S and $14S.

The correlation between oblique subduction,basin formation and magmatism

The apparent correlation between plutonism, obliquesubduction and associated strike-slip faulting observed inthe study area suggest that the Cretaceous PeruvianCoastal Batholith might be related to episode(s) of ob-lique subduction, as already observed by Glazner (1991)for major episodes of Mesozoic plutonism in California.According to this author, major episodes of continentalgrowth by addition of plutons to continental margins mayoccur only if there is a significant oblique component atthe trench, the magmatic room problem being solved by

passive emplacement of plutons at releasing bends in thestrike-slip faults and conservation of volume by thrustingat the trench. Therefore, the central Peruvian continentalmargin may have built its own new crust (i.e. the PeruvianCoastal Batholith), by similar processes with magmaoriginating from an enriched mantle source.

Numerous modern intra-arc fault-bounded basinanalogues are observed in strike-slip zones related tooblique subduction, such as the Taupo volcanic zone inthe Kermadec-Tonga intraoceanic arc (Cole and Lewis1981), the Lake Managua pull-apart graben in theNicaraguan Depression (Burkart and Self 1985), basinsalong the central Aleutian Ridge arc platform (Geist and

Scholl 1992), and intra-arc depocenters along the centralSalomon Trough (Ryan and Coleman 1992).

Conclusions

Basin subsidence, submarine volcanism and plutonicactivity occurred in close spatial and temporal rela-tionship within the Andean magmatic arc during theuppermost Cretaceous oblique subduction, indicatingthat wrench tectonics was the trigger of pull-apart basin

formation and magmatism. The example of the Perubarbasin evolution, during which volcanic activity pro-gressively increased to reach a tectonically controlledsubmarine caldera collapse stage that was quickly fol-lowed by the intrusion of porphyries and plutons, fullysupports this interpretation.

The zircon UPb ages of both volcanic and intrusiverocks at Perubar range between 67.890.18 Ma and69.710.18 Ma, indicating that volcanosedimentaryrocks of at least part of the Casma Group are not Al-bianCenomanian in age but were in fact formed duringMaastrichtian times. The Perubar volcanic-hosted mas-sive sulfide deposit was formed during Maastrichtiantimes, between 69.710.18 Ma and 68.920.16 Ma.Volcanic-hosted massive sulfide (VHMS) deposits in thePeruvian segment of the Andes, such as the Perubardeposit, are generally located along the major NW-ori-ented crustal lineaments, where contemporaneous strike-slip faulting, pull-apart basin opening, and magmaplumbing were most probably acting. These structuresare likely to represent old inherited deep crustal dis-continuities, e.g., transform faults from a Late Permian

Triassic rifting event.The initial Hf values of the magmatic rocks fall be-

tween values of +5.5 and +7.4, and record subordinatecontamination by a low-radiogenic Hf, possibly derivedfrom slab sediments. These values may also represent anenriched mantle source and/or recycled oceanic crust aspossible melt sources. The lack of inherited cores inzircons demonstrates that there is no old basement in-volved in the generation of volcanic and intrusive rocksat Perubar. We think that the data evidence the lack ofold basement under the WPT, such as the PrecambrianArequipa massif in southern Peru (Wasteneys et al.1995). Young and mafic crust is the probable source for

the arc magmatism within the Lima segment of theCoastal Batholith. Voluminous plutonism in an arcenvironment thus requires extension of relatively hotcrust triggering remelting and emplacement in pull-apartstructures. This process is suggested to be equallyimportant in oceanic arcs (such as the Kohistan Arccomplex; Schaltegger et al. 2002) as well as in conti-nental active margin situations such as in the Andes.

Acknowledgements This research was supported by the SwissNational Science Foundation grants FN 200054150.98 and FN2062000.00, and is part of a PhD thesis of the first author at theUniversity of Geneva. We gratefully acknowledge W. Mueller andL. Oldham who instigated this project and transmitted us theirknowledge of the Perubar deposit area, the geologists and staff ofthe Perubar mine for their help during the field work periods,especially R. Egoavil and C. Zumara n, and the geologists fromGlencore Peru, M. Steinmann and S. Bureau, for logistical supportduring the project.

References

Atherton MP (1990) The Coastal Batholith of Peru; the product ofrapid recycling of new crust formed within rifted continentalmargin. Geol J 25:337349

240


11/12

Atherton MP, Aguirre L (1992) Thermal and geotectonic setting ofCretaceous volcanic rocks near Ica, Peru, in relation to Andeancrustal thinning. J S Am Earth Sci 5:4769

Atherton MP, Webb S (1989) Volcanic facies, structure, and geo-chemistry of the marginal basin rocks of central Peru. J S AmEarth Sci 2:241261

Atherton MP, Pitcher WS, Warden V (1983) The Mesozoic mar-ginal basin of central Peru. Nature 305:303306

Atherton MP, Warden V, Sanderson LM (1985) The Mesozoicmarginal basin of Central Peru: a geochemical study of within-plate-edge volcanism. In: Pitcher WS, Atherton MP, Cobbing

EJ, Beckinsale RD (eds) Magmatism at a plate edge; thePeruvian Andes. Blackie, Glasgow, pp 4758

Blichert-Toft J, Albare` de F (1997) The LuHf isotope geochemis-try of chondrites and the evolution of the mantle-crust system.Earth Planet Sci Let 148:243258

Blichert-Toft J, Chauvel C, Albare` de F (1997) Separation of Hfand Lu for high-precision isotope analysis of rock samples bymagnetic sector-multiple collector ICP-MS. Contrib MineralPetrol 127:248260

Burkart B, Self S (1985) Extension and rotation of crustal blocks innorthern Central America and effect on the volcanic arc.Geology 13:2226

Bussell MA (1983) Timing of tectonic and magmatic events in theCentral Andes of Peru. J Geol Soc Lond 140:279286

Cobbing EJ (1978) The Andean Geosyncline in Peru, and its dis-tinction from Alpine geosynclines. J Geol Soc Lond 135:207

218Cobbing EJ, Pitcher WS, Wilson JJ, Baldock JW, Taylor WP,

McCourt W, Snelling NJ (1981) The geology of the WesternCordillera of northern Peru. Overseas Mem Inst Geol Sci, vol 5,pp 143

Cole JW, Lewis KB (1981) Evolution of the Taupo-Hikurangisubduction system. Tectonophysics 72:121

Couch RW, Whitsett RM, Huehn B, Briceno GL (1981) Structuresof the continental margin of Peru and Chile. Geol Soc Am Mem154:703726

Dalziel IWD, Dalla Salda L, Gahagan LM (1994) Paleozoic Laur-entiaGondwana interaction and the origin of the AppalachianAndean mountain system. Geol Soc Am Bull 106:243252

Duncan RA, Hargraves RB (1984) Plate tectonic evolution of theCaribbean region in the mantle reference frame. Geol Soc AmMem 162:8193

Fitch TJ (1972) Plate convergence, transcurrent faults, and internaldeformation adjacent to southeast Asia and the Western Pa-cific. J Geophys Res 77:44324460

Geist EL, Scholl DW (1992) Application of continuum models todeformation of the Aleutian island arc. J Geophys Res 97:49534967

Glazner AF (1991) Plutonism, oblique subduction, and continentalgrowth; an example from the Mesozoic of California. Geology19:784786

Gordon RG, Jurdy DM (1986) Cenozoic global plate motions. JGeophys Res 91:12,38912,406

Haeberlin Y, Moritz M, Fontbote L, Cosca M (2004) Carbonif-erous orogenic gold deposits at Pataz, Eastern Andean Cor-dillera, Peru: geological and structural framework, paragenesis,alteration, and 40Ar/39Ar geochronology. Econ Geol 99:73112

INGEMMET (2000) Mapa geolo gico digitalizado del Peru (Escala

1:1000000). Lima, Instituto Geolo gico Minero y Metalurgico,CD-ROM

Jaillard E (1994) Kimmeridgian to Paleocene tectonic and geody-namic evolution of the Peruvian (and Ecuadorian) margin. In:Salfity JA (ed) Cretaceous tectonics of the Andes. Earth evo-lution sciences monograph series. Wiesbaden, Vieweg, pp 101167

Jaillard E, Soler P (1996) Cretaceous to early Paleogene tectonicevolution of the northern Central Andes (018 degrees S) andits relations to geodynamics. Tectonophysics 259:4153

Jaillard E, He rail G, Monfret T, Diaz-Martnez E, Baby P, LavenuA, Dumon JF (2000) Tectonic evolution of the Andes ofEcuador, Peru, Bolivia and northernmost Chile. In: Cordani

UG, Milani EJ, Thomaz Fihlo A, Campos DA (eds) Tectonicevolution of South America. In: 31st international geologicalcongress, Rio de Janeiro, pp 481559

Jones PR (1981) Crustal structures of the Peru continental marginand adjacent Nazca Plate, 9 degrees S latitude. Geol Soc AmMem 154:423443

Larson RL (1991) Latest pulse of Earth; evidence for a Mid-Cre-taceous super plume. Geology 19:547550

Larson RL, Pitman WC (1972) World-wide correlation of Meso-zoic magnetic anomalies, and its implications. Geol Soc AmBull 83:36453661

Macfarlane AW, Marcet P, LeHuray AP, Petersen U (1990) Leadisotope provinces of the Central Andes inferred from ores andcrustal rocks. Econ Geol 85:18571880

Mukasa SB (1986a) Zircon U-Pb ages of super-units in the CoastalBatholith, Peru; implications for magmatic and tectonic pro-cesses. Geol Soc Am Bull 97:241254

Mukasa SB (1986b) Common Pb isotopic compositions of theLima, Arequipa and Toquepala segments in the Coastal Bath-olith, Peru; implications for magmagenesis. Geochim Cosmo-chim Acta 50:771782

Myers JS (1980) Geologia de los cuadrangulos de Huarmey yHuayllapampa; hojas 21-g y 21 h, vol 33. Instituto GeologicoMinero y Metalurgico, Lima, Peru, pp 153

Pardo-Casas F, Molnar P (1987) Relative motion of the Nazca(Farallon) and South American plates since Late Cretaceoustime. Tectonics 6:233248

Polliand M (2005) Genesis, evolution, and tectonic setting of theUpper Cretaceous Perubar Ba-PbZn volcanic-hosted massivesulfide deposit. Terre and Environement, Thesis 3404, Sectiondes Sciences de la Terre, Universite de Gene` ve, (in press)

Polliand M, Fontbote L (2000) The Perubar Ba-Pb-Zn VHMSdeposit, Central Peru. In: Sherlock RL, Logan MAV (eds)VMS deposits of Latin America. Geological Association ofCanada, Mineral Deposits Division, Special Paper, Vancouver2:439446

Pupin JP (1980) Zircon and granite petrology. Contrib MineralPetrol 73:207220

Ramos VA, Aleman A (2000) Tectonic evolution of the Andes. In:Cordani UG, Milani EJ, Thomaz Fihlo A, Campos DA (eds)Tectonic evolution of South America. In: 31st internationalgeological congress, Rio de Janeiro, pp 635685

Ryan HF, Coleman PJ (1992) Composite transform-convergent

plate boundaries; description and discussion. Mar Petrol Geol9:8997

Schaltegger U, Zeilinger G, Frank M, Burg JP (2002) Multiplemantle source during island arc magmatism: UPb and Hfisotopic evidence from the Kohistan arc complex, Pakistan.Terra Nova 14:461468

Scotese CR, Gahagan LM, Larson RL (1988) Plate tectonicreconstructions of the Cretaceous and Cenozoic ocean basins.Tectonophysics 155:2748

Sempere T, Carlier G, Soler P, Fornari M, Carlotto V, Jacay J,Arispe O, Neraudeau D, Cardenas J, Rosas S, Jimenez N (2002)Late PermianMiddle Jurassic lithospheric thinning in Peru andBolivia, and its bearing on Andean-age tectonics. Tectono-physics 345:153181

Soler P (1991) El volcanismo Casma del Peru Central: cuencamarginal abortada o simple arco volcanico?. VII Congreso

Peruano de Geologa, Resumenes Extendidos, Soc Geol Peru,Lima 2, pp 659663

Soler P, Bonhomme MG (1990) Relation of magmatic activity toplate dynamics in central Peru from Late Cretaceous to present.Geol Soc Am Spec Pap 241:173192

Soler P, Rotach-Toulhoat N (1990a) Implications of the time-dependent evolution of Pb- and Sr-isotopic compositions ofCretaceous and Cenozoic granitoids from the coastal regionand the lower Pacific slope of the Andes of central Peru. GeolSoc Am Spec Pap 241:161172

Soler P, Rotach-Toulhoat N (1990b) Sr-Nd isotope compositionsof Cenozoic granitoids along a traverse of the central PeruvianAndes. Geol J 25:351358

241


12/12

Stacey JS, Kramers JD (1975) Approximation of terrestrial leadisotope evolution by a two-stage model. Earth Planet Sci Lett26:207221

Thornburg TM, Kulm LD (1981) Sedimentary basins of the Perucontinental margin; structure, stratigraphy, and Cenozoic tec-tonics from 6 degrees S to 16 degrees S latitude. Geol Soc AmMem 154:393422

Vidal CCE (1987) Kuroko-type deposits in the Middle-Cretaceousmarginal basin of central Peru. Econ Geol 82:14091430

Wasteneys HA, Clark AH, Farrar E, Langridge RJ (1995) Gren-villian granulite-facies metamorphism in the Arequipa Massif,Peru; a Laurentia-Gondwana link. Earth Planet Sci Let 132:6373

Webb SE (1976) The volcanic envelope of the coastal batholith inLima and Ancash, Peru. PhD thesis, University of Liverpool

Woodhead JD, Hergt JM, Davidson P, Eggins SM (2001) Hafniumisotope evidence for conservative element mobility duringsubduction zone processes. Earth Planet Sci Let 192:331346

242

2005 polliand peru

Documents