Beate.pdf

Mio^Pliocene adakite generation related to at subduction insouthern Ecuador: the Quimsacocha volcanic center

Bernardo Beate a, Michel Monzier b, Richard Spikings c, Joseph Cotten d,Jose Silva e, Erwan Bourdon f;g, Jean-Philippe Eissen f ;g;*

a Facultad de Geolog|a, Minas y Petroleos, Escuela Politecnica Nacional, A.P. 17-01-2759, Quito, Ecuadorb IRD (Institut de Recherche pour le Developpement) ^ UMR 6524, 5 rue Kessler, 63038 Clermont-Ferrand Cedex, France

c Geologisches Institut, ETH Zentrum, Zurich CH-8092, Switzerlandd UMR 6538, Universite de Bretagne Occidentale, P.O. Box 809, 29285 Brest, France

e IAMGOLD Ecuador S.A., Av. 6 de Diciembre 1529 y Baquedano, Quito, Ecuadorf IRD (Institut de Recherche pour le Developpement), A.P. 17-12-857, Quito, Ecuador

g Instituto Geof|sico, Escuela Politecnica Nacional, A.P. 17-01-2759, Quito, Ecuador

Received 25 January 2001; accepted 30 July 2001

Abstract

New geochemical and geochronological data on the Miocene^Pliocene Quimsacocha volcanic center (QVC) have ledto the recognition of adakitic lavas generated by slab melting related to the flat slab subduction in southern Ecuadorand northern Peru. The QVC, located in the presently inactive southern part of the Ecuadorian arc, was built up duringthree distinctive volcanic phases. The first phase generated a basal edifice with mainly andesitic lava flows, while thesecond phase is characterized by the emplacement of cryptodomes, domes and related outflow breccias comprised ofandesites and some dacites. The last phase released rhyolitic ignimbrites associated with the formation of a largecaldera, which was later partly filled by dacitic^rhyolitic domes. Geochemical data for the QVC indicate higher Al2O3,TiO2, Na2O, Zr and Sr contents and lower Fe2O3*, MgO, Y, MREE and HREE abundances, compared to othereruptive rocks of the Plio^Quaternary volcanic front of Ecuador. Such geochemical features, as well as the frequentpresence of an associated epithermal gold deposit, are characteristic of the involvement of slab melts, also known asadakites [1,2], in the generation of these magmas. After a calc-alkaline arc magmatism phase, slab horizontalization ^ inresponse to the subduction of a buoyant oceanic plateau ^ results in increased involvement of a slab melting componentin the magmas produced. However, pristine adakites were generated and emplaced during a relatively short period, asindicated by zircon fission-track ages. Then volcanic activity stopped and a volcanic gap formed. The identification ofthese adakites, their location and age support a model of slab melting associated with flat slab subduction [M.A.Gutscher et al., Geology 28 (2000) 535^538]. 2001 Elsevier Science B.V. All rights reserved.

Keywords: Andes; Ecuador; Miocene^Pliocene; volcanism; slabs; melts; subduction

0012-821X / 01 / $ ^ see front matter 2001 Elsevier Science B.V. All rights reserved.PII: S 0 0 1 2 - 8 2 1 X ( 0 1 ) 0 0 4 6 6 - 6

* Corresponding author. Tel. : +593-2222-5655; Fax: +593-2250-4020.E-mail addresses: [email protected] (B. Beate), [email protected] (M. Monzier),

[email protected] (R. Spikings), [email protected] (J. Cotten), [email protected] (J. Silva),[email protected] (E. Bourdon), [email protected] (J.-P. Eissen).

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Earth and Planetary Science Letters 192 (2001) 561^570

www.elsevier.com/locate/epsl

1. Introduction

Partial melting of a young slab, leaving a garnetamphibolite to eclogite restite assemblage, produ-ces low-K, high-Al (s 15% Al2O3 at 70% SiO2),high-Na and high-Sr adakitic melts, which aredepleted in Y and HREE, resulting in high Sr/Yand La/Yb values [1,2]. Melting experiments onnatural basalts and metabasalts under hydrousconditions over a pressure range compatiblewith slab subduction (1^4 GPa) result in low-Mg liquids with 55^73% SiO2, which are indistin-guishable from pristine (i.e. low-Mg) natural ada-kites [3,4]. However, until very recently, existingmodels restrict partial melting to the subductionof very young (95 Ma old) oceanic crust [1,2,5],to incipient subduction [6], or in a post-collisionalregime, during thermal reequilibration followingcessation of subduction [7,8].

Most of the known Plio^Quaternary, mostlycircum-Pacic adakite occurrences are related tothe subduction of older lithospheres, ranging from10 to 45 Ma. Furthermore, eight of the 10 knownat subduction zones worldwide are linked topresent or recent occurrences of adakitic magmas[9]. As a result of these observations, an addition-al model was proposed to explain the possibilityof partial melting of the subducting oceanic crust,and therefore adakite generation, for the case ofat subduction of moderately old oceanic crust[9]. According to this model, a change in buoy-ancy occurs when overthickened oceanic crust en-ters the subduction zone, forcing the lower platealong a horizontal path for several hundred km atdepths of approximately 80 km. As a result, aftergradual heating during its horizontal trajectory,partial melting of older oceanic crust (up to 50Ma) in the horizontal slab may occur under hy-drous conditions. Therefore, the narrow, calc-al-kaline volcanic arc tends to widen and acquire anadakitic signature. However, if at subductionpersists for a few million years, the residual asthe-nospheric wedge completely disappears (rather byits cooling than its removal) and volcanism ceases,resulting in a volcanic gap [9].

At present within Ecuador, only the southern-most active volcano of the northern volcanic zone(NVZ) of the Andes, El Sangay volcano, sits

above a normal steeply dipping slab composedof crust that is more than 32 Ma old (Fig. 1).The volcano is erupting basaltic to dacitic calc-alkaline lavas, some displaying adakite-like char-acteristics (e.g. low-Y and HREE abundances)which have been tentatively attributed to AFCprocesses occurring at the base of a s 50 kmthick crust [10]. South of El Sangay, the sub-ducted slab has a at conguration beneathsouthern Ecuador and northern Peru. This cong-uration has been attributed to the subduction oftwo buoyant bodies, the Inca Plateau and theNazca ridge [9,11], resulting in the well-known1500 km long Quaternary volcanic gap fromsouth central Peru and southern Ecuador (i.e. be-tween 15S and 2S of latitude). All other activeEcuadorian volcanoes north of Sangay sit above ayounger (6 24 Ma), weakly seismogenic slab, thegeometry of which is poorly constrained between1S and 1N of latitude [12]. The at subductionpresently occurring under central and northernEcuador was suggested to be a response to thesubduction of the Carnegie Ridge since at least2 Myr ago, resulting in arc widening and wide-spread adakitic volcanism [9,12]. Therefore, fol-lowing these authors, adakitic volcanism shouldbe active in Ecuador for only a brief time span(2^3 Myr or less), followed by a volcanic gap.More recently, it has been suggested [13,14] thatthe slab attening process in response to the sub-duction of the Carnegie Ridge is less advancedand that present-day adakitic volcanism is re-stricted to the narrow volcanic front of the arc.Furthermore, they proposed that this volcanism isnot composed of pure slab melts but is derivedfrom hybridized melts that formed by the reactionof variable proportions of pristine adakites withthe thinned, overlying mantle wedge. Finally, theyconclude that hybridized calc-alkaline volcanism(derived from partial melting of a mantle sourcemetasomatized by adakitic melts, as proposed byseveral authors [15,16]) is widespread in the restof the volcanic province and therefore the Ecua-dorian arc only represents the beginning of thesubduction process proposed by Gutscher et al.[9].

This contribution presents new data from theMio^Pliocene Quimsacocha volcanic center

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B. Beate et al. / Earth and Planetary Science Letters 192 (2001) 561^570562

(QVC) in the presently inactive southern part ofthe Ecuadorian arc with the aim of illustratingand discussing the validity of the at slab meltingmodel recently proposed [9].

2. The Quimsacocha Volcanic Center (QVC)

The QVC is located near 302PS^7914PW (Fig.1), above a typical at slab (Fig. 2). It sits on topof a complex Middle to Late Tertiary calc-alka-line volcanic pile, near the crest of the Ecuadorianwestern cordillera. The general succession of thevolcanic series of this area was presented in re-cently published geological maps [17,18] and issummarized in Fig. 3. However, the knowledgeabout these formations remains limited, especiallytheir detailed geochemistry, with the exception ofthe QVC which represents the latest episode ofvolcanic activity in southern Ecuador and wasstudied in greater detail for the present work.

The QVC was constructed during three mainphases of volcanic activity:

Phase 1 generated a basal edice, mainly lavic,consisting of plagioclase and clinopyroxene-richacid andesite lava ows (and subordinate daciteows), which are radially distributed around theQVC edice.

Phase 2 is characterized by the emplacement ofcryptodomes and domes along a NE trending lin-eament at the location of the caldera, resulting inextensive volcanic breccia deposits of (plagio-clase+hornblende)-rich+clinopyroxene acid ande-sites and subordinated dacites. The conspicuouspresence of hornblende in these rocks clearly in-dicates an increase in the water content of themagma at this stage.

Finally, phase 3 involved the eruption of rhyo-litic (plagioclase + biotite + quartz + hornblende)ignimbrites, which are only preserved in distal re-gions. The ignimbrites were accompanied by theformation of a large caldera (4.5 km in diameter)and followed by the emplacement of late daciticto rhyolitic domes which partly ll the caldera.Zircon ssion-track analyses for this last phaseof volcanic activity yielded ages of 5.2 0.4 and4.9 0.3 Ma (Mio^Pliocene transition) for the cal-dera ignimbrite and 3.6 0.3 Ma (early mid-Plio-cene) for the late, caldera-lling felsic domes.These new dates were performed at the ssion-

Fig. 1. Location of the QVC. Cocos^Nazca oceanic platefeatures (modied from [25,26]); black arrows indicate theNazca^South America relative motion according to [27]. Theactive NVZ of the Andes is outlined and the Chimborazo(Ch), Tungurahua (Tu) and Sangay (S) volcanoes are also in-dicated. CR = Cocos Ridge; MR = Malpelo Ridge. 50, 100and 150 km depth contours of the Wadati-Benio zone indi-cated as dotted lines after [12].

Fig. 2. W^E seismological section under the QVC. Data:1964^1998 relocated hypocenters between 2 and 4S [28].Light colored hypocenters are not related with the at slabsegment of southern Ecuador and Peru [11,12], but are de-rived from the steep NE dipping Sangay slab [10]. Open tri-angles represent the projection of the location of the Chim-borazo (Ch), Tungurahua (Tu) and Sangay (S) volcanoes.Note that relative to the active arc, the QVC is in forearcposition.

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B. Beate et al. / Earth and Planetary Science Letters 192 (2001) 561^570 563

track laboratory of the ETH in Zurich (Switzer-land) following the methodology described in [19].The calibration factor was obtained against theFish Canyon Tu standard.

2.1. Geochemistry

Fifteen samples from all three phases of theQVC have been analyzed using ICP^AES meth-ods (Table 1). Their compositions range from me-dium-K acid andesites to rhyolites (59^72% SiO2,all data recalculated to 100% LOI-free; Fig. 4).Two samples from phase 1 (GC-150 and QC-249) revealed an anomalous enrichment in REE(except Ce because of its dierent valence) and Y,giving a negative anomaly in Ce relatively to Laand Nd (and in a lesser extent P, Zr and Ti rela-tive to other REE) on spidergrams. This REEmobility is characteristic of low temperature alter-ation [20,21] in a tropical environment.

Therefore only seven analyses from phase 1 willbe used in the following diagrams and in Section

3, along with three samples each from phases 2and 3. Scattering of the data occurs in severaldiagrams although in general all these composi-tions show overall geochemical coherence. Theresults reveal a relatively low abundance of K2Oas well as other highly incompatible elements (i.e.Th), which is typical of the Plio^Quaternary vol-canic front of Ecuador (Figs. 4 and 5). Further-more, the QVC rocks also possess similar CaOcontents and typically low Y abundances relativeto other eruptive rocks within the Plio^Quater-nary volcanic front. However, rocks of the QVCyield higher Al2O3, TiO2, Na2O, Zr and Sr con-tents and lower Fe2O3*, MgO and Sc contents(Figs. 4 and 5), a signature which is characteristicof the involvement of magmas derived from slabmelting [1^4,14].

Primitive mantle-normalized extended spider-grams of samples from the three phases are notsubparallel but display a progressive variation re-sulting in a closed fan-like arrangement with therhyolite trends showing lower REE abundances

Fig. 3. Simplied geological map of the QVC area modied after [17,18], showing the three successive deposits linked to the for-mation of this volcanic center; (1) QVC phase 3: caldera-lling rhyolitic domes and ows (Pliocene: 3.6 0.3 Ma, new ssion-track age performed at the ssion-track laboratory at the ETH-Zurich, Switzerland. Calibration factor was obtained against theFish Canyon Tu standard); (2) QVC phase 3: rhyolitic ignimbrites (Mio^Pliocene: 5.2 0.4 and 4.9 0.3 Ma, new ssion-trackages); (3) QVC phase 2: cryptodomes, domes and breccia deposits of acid andesites and subordinated dacites (Upper Miocene);(4) QVC phase 1: acid andesite and subordinated dacite ows (Mid-Miocene); (5) Turupamba ignimbrite (Miocene); (6) Turivolcanoclastic formation (Miocene); (7) Ayancay sedimentary formation (Miocene); (8) Sta Isabel andesite (Miocene); (9) Plan-charrumi ignimbrite (Oligocene); (10) Soldados ignimbrite (Oligocene); (11) Saraguro (Eo^Miocene); (12) indeerentiated ande-sites; (13) caldera; (14) fault; (15) village.

EPSL 5970 31-10-01


Tab

le1

Maj

or(w

t%)

and

trac

e(p

pm)

elem

ent

abun

danc

esin

sele

cted

Qui

msa

coch

apr

oduc

ts

Sam

ple

num

ber

QC

-249

QC

-150

QC

-154

QC

-212

QC

-205

QC

-153

QC

-246

QC

-141

QC

-142

QC

-143

QC

-146

QC

-145

QC

-140

QC

-100

QC

-109

Stan

dard

PM

-SL

imit

ofde

tect

ion

Seri

es1

11

11

11

11

22

23

33

SiO

258

.85

60.1

060

.70

61.4

062

.00

62.4

062

.00

63.0

063

.25

58.8

060

.30

60.5

071

.00

71.0

071

.70

47.2

0T

iO2

0.89

0.83

0.83

0.80

0.76

0.74

0.76

0.67

0.67

0.95

0.84

0.83

0.25

0.27

0.26

1.12

Al 2

O3

18.2

018

.00

17.8

017

.65

17.7

517

.45

17.8

017

.55

17.7

018

.35

17.9

017

.75

15.5

515

.85

15.6

017

.38

Fe 2

O3*

6.07

5.35

5.25

5.35

5.20

4.68

5.18

4.33

4.37

5.90

5.60

5.60

1.93

1.90

1.83

10.1

5M

nO0.

080.

060.

060.

060.

050.

050.

060.

050.

050.

070.

070.

070.

030.

020.

020.

16M

gO2.

782.

002.

001.

831.

921.

621.

281.

661.

492.

732.

542.

490.

450.

180.

339.

25C

aO6.

656.

006.

205.

905.

605.

455.

005.

204.

806.

806.

306.

103.

042.

843.

0612

.40

Na 2

O4.

304.

884.

904.

654.

604.

784.

334.

804.

554.

624.

704.

624.

804.

834.

872.

08K

2O

1.00

1.30

1.32

1.22

1.16

1.47

1.28

1.40

1.38

1.06

1.24

1.30

2.03

1.95

1.98

0.14

P2O

50.

210.

240.

240.

220.

190.

210.

200.

200.

190.

230.

220.

220.

090.

090.

090.

04L

OI

0.88

0.93

0.53

0.95

0.77

0.75

1.95

0.64

1.22

0.58

0.44

0.64

0.80

1.01

0.36

Tot

al99

.91

99.6

999

.83

100.

0310

0.00

99.6

099

.84

99.5

099

.67

100.

0910

0.15

100.

1299

.97

99.9

410

0.10

99.9

2R

b16

.522

.022

.022

.023

.025

.022

.528

.528

.514

.520

.021

.049

.544

.046

.01.

0Sr

710

870

885

720

700

830

627

685

630

785

825

800

545

535

545

270

1B

a50

064

067

060

558

074

073

572

574

049

259

562

01

040

105

01

000

150

1Sc

1310

109

119

108

8.2

1313

133

3.5

3.0

34.0

0.1

V18

015

415

515

215

513

014

811

211

419

018

017

831

2833

193

1C

r18

1315

1318

1414

1212

2119

204

54

321

1C

o14

1213

1212

1012

99

1414

12.5

32.

53

491

Ni

65.

56

67

56

54.

58

87.

52

2.5

2.5

123.

01

Y34

.520

.09.

010

.58.

87.

28.

412

.213

.69.

111

.09.

55.

24.

94.

711

.10.

3Z

r92

100

104

100

9811

011

011

511

588

100

105

5870

7238

1N

b3.

02.

93.

33.

12.

72.

83.

63.

13.

13.

03.

03.

03.

13.

13.

02.

70.

5L

a19

.421

.016

.014

.513

.017

.013

.316

.515

.811

.516

.514

.013

.513

.514

.02.

80.

5C

e32

.537

.033

.029

.026

.034

.528

.533

.031

.526

.535

.531

.026

.526

.528

.06.

61.

5N

d27

.527

.520

.017

.014

.518

.515

.320

.017

.016

.021

.017

.512

.512

.013

.05.

20.

6Sm

6.00

5.70

3.90

3.65

3.05

3.60

3.35

3.70

3.30

3.15

4.05

3.50

2.30

2.00

2.30

1.79

0.6

Eu

2.10

1.66

1.07

1.04

0.88

1.02

0.94

1.05

0.99

0.99

1.10

1.02

0.61

0.60

0.62

1.06

0.1

Gd

7.50

5.20

2.90

2.90

2.70

2.65

2.75

3.10

2.90

2.70

3.40

3.00

1.70

1.70

1.85

1.99

1.0

Dy

5.20

3.15

1.80

1.85

1.65

1.45

1.60

1.80

1.65

1.80

2.15

1.85

1.00

0.90

0.95

2.04

0.3

Er

2.80

1.75

0.85

0.85

0.75

0.70

0.70

0.95

0.95

0.85

1.00

0.85

0.45

0.40

0.40

1.10

0.5

Yb

2.35

1.42

0.72

0.73

0.71

0.56

0.67

0.77

0.78

0.74

0.86

0.77

0.40

0.38

0.37

1.03

0.1

Th

1.3

1.5

1.5

1.7

1.6

1.8

1.7

1.9

1.7

1.2

1.6

1.8

2.2

2.0

2.1

0.1

0.3

Sr/Y

20.6

43.5

98.3

68.6

79.5

115.

374

.656

.146

.586

.375

.084

.210

4.8

109.

211

6.0

24.3

La/

Yb

8.3

14.8

22.2

19.9

18.3

30.4

19.9

21.4

20.3

15.5

19.2

18.2

33.8

35.5

37.8

2.7

Maj

or,

trac

ean

dR

EE

elem

ents

dete

rmin

edby

ICP

^AE

S(e

xcep

tR

bby

am

eA

ES)

atth

eU

nive

rsit

ede

Bre

tagn

eO

ccid

enta

le,

Bre

st,

Fra

nce,

follo

win

gth

em

eth-

odol

ogy

of[2

1].

The

anal

ysis

ofth

est

anda

rdP

M-S

,an

alyz

edjo

intl

yw

ith

this

set

ofsa

mpl

es,

asw

ell

asth

elim

itof

dete

ctio

nfo

rtr

ace

elem

ents

are

give

n.St

an-

dard

devi

atio

ns(3

sigm

a)ar

e2%

for

maj

orel

emen

tsan

d5%

for

trac

eel

emen

ts.

EPSL 5970 31-10-01


than the acid andesites and dacite (Fig. 4). Onaverage, the La content increases slightly fromacid andesites to dacite, then decreases to rhyo-lites. The Yb abundances follow the same pattern,but the decrease towards rhyolites is more accen-

tuated. Note that sample QC-153 has an unusu-ally low abundance of Y and most REE, espe-cially Gd^Yb, for a dacite. As a consequence,La/Yb ratios, which range from 15 to 22 in acidandesites and dacite (with the exception of sample

Fig. 4. Selected Harker diagrams for QVC rocks (see Table 1); for comparison, about 1400 analyses of Plio^Quaternary Ecua-dorian volcanics from our mainly unpublished data base are shown, separated in the main arc group (dots) and the frontal arcvolcanics showing an adakite-like anity with higher Na2O% and Al2O3% Sr and lower Y contents (area delineated with adashed line).

EPSL 5970 31-10-01


QC-153; La/Yb = 30.4), steadily increase with thesilica content of the lavas, reaching La/Yb = 33^38in rhyolites. All of these values are slightly higherthan those observed for rocks from the Plio^Qua-ternary volcanic front with similar SiO2 contents.

Most of the compositions unambiguously plotin the adakitic eld (according to [3]) on a Yversus Sr/Y diagram (Fig. 5), with Sr/Y ratiosranging from 46 to 116. Again, sample QC-153yields unusually high Sr/Y and La/Yb ratios (ofrespectively 115 and 30), which are very high for aSiO2 content of 63.1%, close to those expectedfor a rhyolite. But its low Y and Yb contents(respectively 7.2 and 0.56 ppm) are characteristicof adakites. Sample QC-142, another dacite(SiO2 = 64.25%), with a Y content of 13.6 ppmand a Sr/Y ratio of 46.5 is the only sample whichplots at the limit of the calc-alkaline eld.

3. Discussion

The most remarkable common signature withinextrusive rocks of the QVC is a distinct adakiticcharacter, emphasized by their unusually highAl2O3, Na2O and Sr contents, as well as lowK2O, MREE, Y and HREE abundances, relativeto the Ecuadorian volcanic front. In spite of someinconsistencies, it is also clear that both the silicacontent and the adakitic character (Sr/Y vs. Y)greatly increased in the last phase of the QVC(Fig. 5). However, the rhyolite REE patterns areconsistently below those of the acid andesites anddacite, which cannot be accounted for by process-es such as crystal fractionation from a commonparent magma or dierential partial melting froma common source. In addition, the deepening of anegative anomaly in MREE with increasing silicacontent suggests that hornblende played an activerole in the genesis of the series (amphibole retainsHREE but still has a larger preference forMREE).

Only traces or minor amounts of hornblendeare present in the phase 1 products of the QVC.In contrast, hornblende phenocrysts are ubiqui-tous in the phase 2 lavas and are present, butsubordinate to biotite, in those of phase 3. There-fore, upper-crustal hornblende fractionation maypartly explain the observed MREE anomaly.However, this process cannot be responsible forthe presence of this anomaly in almost all QVCcompositions as well as its increasing magnitudein the rhyolites. In addition, any consequent pla-gioclase fractionation or accumulation process(plagioclase is the most abundant phenocryst inall of the rocks of the QVC) is rejected due tothe insignicant Eu anomaly observed in mostREE patterns. Therefore, despite the fact thatupper-crustal processes such as hornblende frac-tionation can, in part, explain some of the unusu-al geochemical characteristics of the QVC prod-ucts, it is clear that they do not provide asatisfactory explanation for the main geochemicaltrends.

Most of these unusual geochemical character-istics can be understood in terms of deep litho-spheric processes. According to [9], at subduc-tion of moderately old slabs can result in partial

Fig. 5. Primitive-normalized extended spidergrams for QVCrocks (normalization values are from [29]). Nb* = Nb wasused for the QVCs data whereas Ta was used for the CB(as the Nb data were not available, [22]).

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melting of oceanic crust under hydrous condi-tions, generating adakitic magmas. Flat slab sub-duction is now occurring beneath the QVC [11],as supported by the presence of the postulatedbuoyant Inca Plateau which began subducting10^12 Myr ago. The evolution of the QVC coin-cides with the timing of subduction of the IncaPlateau, subsequent attening of the slab beneathsouthern Ecuador and northern Peru and the gen-eration of adakite from a moderately old (V32Ma) slab. Extrusive activity within the QVC prob-ably began during Late Miocene, as these forma-tions sit above the Turi volcanoclastic formationassigned to a Mid- to Late Miocene age [17,18].The volcanism became more adakitic and acidicwith time until its extinction at approximately3.6 0.3 Myr ago. Note that the nal magmaticevents in northern Peru took place between 14Ma and 3 Ma with the emplacement of the lastbatholiths in the Cordillera Blanca and their co-eval ignimbrites of the Yungay formation [22].Furthermore, similar geochemical characteristicsbetween the QVC volcanics and some of the Cor-dillera Blanca batholith are observed (Figs. 5 and6). Indeed, its leucogranites, quartz diorites andtonalites have been described as dominantly high-silica rocks (s 70%), Na- and Sr-rich, Y- andHREE-depleted, with many of the characteristicsof oceanic slab melts [22]. Such arguments havebeen used by [9] to reinterpret the origin of thismagmatism to the partial melting of the oceaniccrust at the level of a at slab, as a response to thepresence of the buoyant Inca Plateau, and not to

the partial melting of underplated basalts as pre-viously proposed [22]. Their Mg#s and Cr and Nicontent, similar to and higher as those of theQVCs volcanites, indicate limited assimilationof peridotitic material during the percolation ofthe magma through the mantle wedge.

Partial melting of the slab under hydrous con-ditions, leaving a garnet amphibole restite, ex-plains the marked and constant adakitic charac-teristics of the QVC products (very high Al2O3,Na2O, Sr and low K2O, HFSE, MREE, HREEcontents). Therefore, the trend towards moreacidic compositions at the end of the QVC activ-ity, accompanied by a marked decrease in HREEand the ensuing steepening of the REE patterns,could be related to a decreasing degree of partialmelting. The slab may have cooled by gradualcooling and/or eventual removal of the residualasthenospheric wedge. However, a single partialmelting process cannot explain the inverted pat-terns observed on the REE diagram (higher ratesof partial melting of a MORB source resulting inless steepened REE patterns than lower rates, butwith similar Eu values [13]). Additional fractionalcrystallization processes are required to bring theREE patterns of the acid andesites and daciteabove those of rhyolites. Clinopyroxene and mag-netite fractionation (phase 1) or pyroxene^am-phibole fractionation (phase 2) at upper-crustallevels may account for such a rise.

Thus, phase 3 rhyolites could be considered asthe most pristine adakites whereas most productsof phases 1 and 2 are adakites which have beensubsequently modied by late fractional crystalli-zation processes. Furthermore, based on theirhigher Mg#s and Cr and Ni content, limited as-similation of peridotitic (mantle) material seemsto aect primarily lavas of phase 2, to a lesserextend those of phase 1, and not phase 3.

The presence of adakitic magmatism in the evo-lution of the QVC becomes more important dueto the presence of extensive precaldera epithermalgold mineralizations associated with the emplace-ment of dacitic and rhyolitic domes of phase 3.Indeed, a link between epithermal and porphyrydeposits and adakitic magmatism has alreadybeen underscored by several authors as in thePhilippines [23], and this association has also

Fig. 6. Y versus Sr/Y diagram showing the evolution of theQVC rocks during the three successive phases. Adakite andcalc-alkaline elds after [3], CB = Cordillera Blanca leucog-ranites, quartz diorites and tonalites after [22].

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been statistically demonstrated [24]. This resultsprobably because of the high volatile content ofadakitic magmas that would trigger the formationof a more ecient hydrothermal system, condu-cive to mineralizations. Finally, the combinedgeodynamical and geochemical evolution pro-posed for southern Ecuador and northern Peru,as illustrated by the present study of the QVCvolcanites, does not apply easily to the evolutionof the active Plio^Quaternary volcanic arc ofEcuador. Indeed, the volcanics in this arc, espe-cially in its western cordillera, have adakitic char-acteristics which are, however, not as strong. Thisapplies to the active volcanic front where the ada-kites from Pichincha volcano [13,14] have slightlyweaker adakitic characteristics than those fromthe QVC. It is possible that the ongoing processof slab attening described under most of the arcof central Ecuador, i.e. between latitude of 1Nand 1S [9,12], has just started recently and is notfully established at present.

4. Conclusion

This rst study of the QVC corroborates withthe recently proposed hypothesis of a causal linkbetween slab attening and the ensuing genesis ofadakites [9]. In this model, adakites are generatedduring a relatively short period and progressivelyreplace the usual calc-alkaline arc magmatism,volcanic activity then declines and a volcanicgap is formed.

The data presented here have established suchan evolution for the QVC in southern Ecuador,where typical adakites have been identied (acidandesites to rhyolites characterized by high Al,Na, Sr and low Mg, Sc, Y, HREE contents andassociated epithermal gold mineralizations). Thisadakitic magmatism is a response to the atteningof the slab following the subduction of the morebuoyant Inca Plateau [11], and was followed bythe cessation of the volcanic activity around3.6 0.3 Myr ago. A similar process is observedin the intrusions and volcanism of the CordilleraBlanca of northern Peru whose intrusive activityceased 5.2 0.5 Myr ago [22], slightly before theend of activity of the QVC.

Acknowledgements

B.B. thanks Dr. Richard Spencer, GeneralManager of IAMGOLD Ecuador, for encourag-ing and supporting the writing of this paper,which resulted from the geological mapping ofthe Quimsacocha epithermal gold prospect.Many thanks to M.A. Gutscher for numerous en-thusiastic and stimulating discussions and its care-ful checking of the nal version of this paper. Theauthors thank R.P. Rapp and F.G. Sajona fortheir thoughtful reviews which improved signi-cantly the original manuscript.[AC]

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