U N I V E R S I D A D D E C O N C E P C I Ó N DEPARTAMENTO DE CIENCIAS DE LA TIERRA 10° CONGRESO GEOLÓGICO CHILENO 2003

CRUSTAL CONTROL ON CENTRAL ANDEAN MAGMATISM IN TIME AND SPACE: IMPLICATIONS FROM GEOCHEMICAL DATA OF IGNEOUS ROCKS BETWEEN 16° AND 27°S FROM CRETACEOUS TO RECENT

TIMES

WÖRNER, G. 1, MAMANI, M. 1, RUPRECHT, P. 1, MERCIER, R., HARTMAN, G. 1, SIMON, K.(1; Y THOURET, JC. 2 1Abt. Geochemie, GZG Universität Göttingen, Goldschmidtstr. 1, 37077 Göttingen, Germany Gwoerner@gwdg.de 2Coordination des Recherches Volcanologiques, Universite Blaise Pascal (Clermont II, 5 rue Kessler, 63000 Clermont-Ferrand, France Miocene and Quaternary volcanism of the Central Andean Volcanic Zone (CVZ: 12°40'S to 26°S) result from the subduction of the oceanic Nazca plate beneath the South American plate. Below the CVZ, the continental crust reaches a thickness of up to 70 km. The upper crust is formed by Precambrian, Paleozoic rocks, sedimentary rocks, marine Mesozoic rocks, covered by continental sediments of the Cretaceous. The potential sources that would have contributed to create the geochemical characteristics of the magmas in the CVZ of the Andes would be: fluids from the subducted oceanic crust, the asthenospheric mantle, the lithospheric mantle, the lower continental crust and the upper crust. Geochemical data obtained on 290 new samples of the Miocene and Quaternary volcanic centers from S. Peru (12°40'S to 18°22'S) and additional 200 samples from Northern Chile S of 22°S were combined with our published data from the area between 18°S to 22°S (Wörner et al., 1992, 1994; Fig. 1). A regional segmentation in the composition of the volcanic centers, a gap in the Quaternary volcanism of the CVZ and the presence of Plio-Quaternary (shoshonitic) volcanism NE behind the arc are important features. We first use Ba/Y ratio and Sr-, Nd-, and Pb-isotope data to characterize these lavas. The Ba/Y ratio is favored over the Sr/Y ratio as an indicator of deep crustal assimilation because the Sr/Y ratio is more susceptible to plagioclase fractionation in

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100020003000400050006000

Elevation (m)

Camana

Mollendo

Arequipa

Abancay Cuzco

Sicuani

Yauri

Puno

Moquegua

Tacna

Arica

BOLIVIA

CHILE

PERU

COCOS PLATE

PACIFIC PLATE

NAZCA PLATE

NAZCA PLATE

ANTARTIC PLATE

SOUTH AMERICA

NVZ

CVZ

SVZ

Andes

Iquique

Antofagasta

Tocopilla

Salar de Atacama

Rio

Ati

co

R

ioC

haparral

Rio

Yauca

Rio

Ac

ari

Rio

Ca

ra

veli

Rio

Oco

na

RioCam

an a

Rio

Sih

uas

Rio Vitor

Rio

Q

uilca

Rio

Ch

ili

R

i oTa

mbo

Rio

Moq

uegua

Rio

Locum

ba

Rio

Sam

a

Rio

C

aplina

Rio LLut

RioAzapa

Riode Camarones

Rio de Vitor

Rio Maure Chicoo

Ri o Maure

Rio

San

ti ago

Rio Vilcanota

15¡S

16¡S

17¡S

18¡S

19¡S

73¡W 72¡W 71¡W

14¡S

74¡W75¡W

13¡S

75¡W 74¡W 73¡W 72¡W 71¡W 70¡W 69¡W 68¡W

20¡S

21¡S

15¡S

16¡S

17¡S

18¡S

19¡S

20¡S

21¡S

22¡S

23¡S

24¡S

25¡S

26¡S

27¡S

28¡S

22¡S

23¡S

24¡S

25¡S

26¡S

27¡S

28¡S100 km

NIC

QuinsachataOroscocha

Salar de Uyuni

Lago Titicaca

PAR

SARFIR

ANTCOR

AND

HUAM SAB

CHA MIS UBI

HUAY HP TUT

YUC TIT

KER CAS

TICS

TAPCAQ

CHUGUL

SUPSUASUR

TACPOM

IQM I II

LIR

POR

HUAL

IRUOPA

MIN AUCOLA

SPP

PUTCOR

SAIODTLIN

LAS

OJOS DEL SALADO

NEG

SOC

LUL

LTA

CoracoraPuquio

Yarihuato

Cotahuasi

ChuquibambaHualca Hualca

AnantoHuarancante

Salinas

ChivayTuti Colca

CondoromaMorane

PichuPichu

PaquetaneChachani

Tarata

HUY

CNEZAP

TOMLAU

CMA

MAN

LAC

AJOCHP

ELRNCPI

ACH

ACM

ANO

ELRS

CHEPUN

CHAN

PALCUEV CEB

AZU

QUIL

PORU

CAR

Name of volcanoes

SAR = Sara Sara

YAH = Yarihuato

ANT = Antapuna

FIR = Firura

COR = Coropuna

AND = Andagua

HUAM = Huambo

SAB = Volc n Sabancaya

NIC = Nicholson

CHA = Chachani

MIS = Volc n Misti

HUAY+ HP= Huaynaputina

UBI = Vol n Ubinas

TICS = Ticsani

TUTU = Tutupaca

YUC = Yucamane

TIT = Titiri,

CAS = Casiri

TAC = Volc n Tacora

HUY = Huyalas

CAQ = Caquena

CNE = Co Negro

POM = Pomerape

PAR = Parinacota

TAP = Volc n Taapaca

CHU = Chucullo

AJO = Ajoya

ZAP = Zapahuira

LAU = Lauca

CHP = Choquelimpe

COP/TOM= Copaquilla

LAC = Quebrada Laco

LAU = Lauca

GUL = Gullatire

QC-01 = Qui acollo

ELRN = El rojo del Norte

CPI = Co Pichican

CMA = Co Margarita

ACH = Achecalane

SUA = Arintinca

SUP = Puquintinca

SUR = Salar de Surire

ACM = Co Macusa

ANO = Co Anocarire

MAM = Mamuta

IQM = Isluga

LIR = Lirima

POR2 = Porquesa

QUIL = Quillacollo

Hual = Huailla

IRU = Irrutupunco

ELR1 = El rojo del Sur

OLC = Olca, MIN= Mi o,

Opa = Olca-Paroma

OLC = Volc n Olca

PMA = Paroma

AUC = Aucanquilcha

OSC = N salar Carcote

Ola = Ollague

Poru = Porunita

PUN = Puntilla

CHE = Chela

CAR = Carcote

PAl = Palpana

CUEV = Las Cuevas

CEB = Cebollar

CHAN = Chanca

Azu = Azufre

SPP = San - Pedro Poru a

PUT = Putana

ODT = Ojos del Toro

COR = C¡ Apagado

SAI = Sairecabur

LIC = Licancabur

LAS = Lascar

NEG = Negrillar

SOC = Socompa

LUL = Llullaillaco

LTA = Lastaria

Miocene VolcanoesQuaternary VolcanoesShoshonites

Fig. 1: Sampled Plio-Pleistocene volcanoes and sites of additional volcanic/plutonic samples of Miocene to

Cretaceous age.

0

40

80

120

160

200

45,00 55,00 65,00 75,00

SiO2

Ba/

YQuaternary volcanics >Miocene volcanics back arc shoshonites

Fig. 2: Ba/Y - SiO2 plot showing that the Ba/Y - ratio does not change with differentation.

intermediate rocks. Unless rhyodacite and rhyolitic compositions are concerned, the Ba/Y ratio should be more independent of differentiation than Sr/Y (see Fig. 2). Miocene centers are always lower in incompatible trace element contents (i.e. the Ba/Y ratio) while Quaternary volcanoes show large ranges (e.g. Taapaca) and/or high Ba/Y values (e.g. Huayna Putina). By contrast, the isotopic composition is significantly different e.g. for 206Pb/204Pb in different regions but at any given location is always similar for Miocene and Quaternary rocks. Pb isotopes are dominated by the crustal contribution to the magmas via assimilation and delineate distinct zones with different crustal compositions. Zone 1 around El Misti Volcano near 16,20'°S: 206Pb/204Pb = 17.65 to 17.82. Zone 2 between 15°40'S and 19°S has 206Pb/204Pb of 18.00 to 18,35. A high 206Pb/204Pb ratio of 18,50 to 18,80 is found in two regions: Zone 4 near the Northern termination of the CVZ (14°30'S to 15°40'S) and Zone 5 in the South

0

40

80

120

160

200

0 200 400 600 800 1000 1200 1400 1600

N-S Distance (Km) projected onto arc profile (Fig. 1)

Ba/Y

Quaternary volcanics > Miocene volcanics back arc shoshonites

Fig. 3 : Ba/Y for mafic to intermediate rocks along the CVZ (N-S distance). Note, that in a given area, Miocene and

older rocks tend to be lower in Ba/Y while overall the distinction is less clear (see also Fig. 2)

(> 20 °S). Transitions between these regions appear to be abrupt and - judging from previous work - are expected to always directly correlate with the isotopic composition of the underlying continental basement.

We interpret these results as clear evidence for a crustal control on magma chemistry. Mantle lithospheric source, a sediment subduction, or the tectonic erosion, are improbable causes of the observed variations. The variable Ba/Y between Miocene and Modern volcanic rocks is mostly caused by a decrease in Y in the younger rocks. The reason for this Y depletion is garnet in the residue of crustal assimilation after crustal thickening. Previously, the same argument has been made on the basis of Sr/Y and/or La/Yb or Sm/Yb variations in CVZ magmatism through time (Kay et al., 2000). Based on similar findings, many young volcanic rocks in the Andes have been termed "adakites" (e.g. Gutscher et al., 2000; Beate et al., 2001). However, the setting for the formation of adakites is related to the melting of the subducted oceanic crust and a single, simple geochemcial parameter such as Sr/Y is clearly insufficient to make the case for such slab melting (Dorendorf et al, 2000; Mahlburg-Kay et al., 1999; Mahlburg-Kay, 2002; Garrison and Davidson, 2003). A STATISTICAL APPROACH Close to 1000 major and trace element analysis of CVZ samples over more than 1600 km along the arc from Nevado de Sara Sara (S 15°19´; W 73°26´) in the North to Nevados Ojos del Salado (S 27°07´; W 68°33´) in the South are now available based on our regional sampling and permit a statistical approach. This analysis was conducted in a dry run on selected 384 samples including 24 elements. These samples include Quaternary volcanic rocks, Miocene and Pliocene ignimbrites, Miocene volcanics as well as Cretaceous plutonic rocks that formed prior to crustal thickening. Commonly, whole rock geochemical data are used characterize by means of discrimination diagrams of concentrations and elemental ratios of the volcanic products. Our statistical approach is based on a Principal Component Analysis (PCA) and Cluster analysis (Swan and Sandilands, 1995) with the data normalized to 100% H20-free. The Cluster analysis was carried out using the K-means- (empirical classification by calculating the distances inside the n-dimensional space (n = number of elements) and the Ward-Method (agglomerative Method with the aim of a hierarchical structure of similarity). The approach does eliminate the closure problem, and hence spurious correlations may occur. The PCA reduced the 24-dimensional space to a 3-dimensional one, where the elements are only dependant on three different processes occurring during the evolution of the magmas : (1) crustal assimilation at variable depth with or without garnet as residue (> 35 km) and (2) magmatic differentiation. Our Cluster analysis results in a solution of five distinct clusters (K-Means), in which the central young volcanics north of the Pica-Gap, the intrusive rocks, the ignimbrites and the Miocene volcanics are separated. The fifth cluster is a mixture of Quaternary volcanics including mainly south and some north (of the Pica-Gap) samples. The general discrimination between the northern and southern samples is due to a different metamorphic basement (Wörner et al., 1992; Wörner et al., 1994). The Ward-Method leads to a similar result, where the Quaternary volcanics also define their own cluster.

Fig.4 : Results of the cluster analysis The five clusters can be characterized using the three principal components (lower crustal assimilation, garnet in the residue, differentiation) as followed:

The Quaternary volcanics north of the Pica-Gap show strongest interaction with lower garnetiferous crust due to the thickened crust. Of special interest is the fact that intermediate rocks rather than the most mafic rocks show most clearly the affect of lower crustal interaction with a garnet signature. Felsic volcanics and ignimbrites also show little crustal assimilation signatures at higher pressures. Evidently, the geochemical signatures that account for lower crustal modification are directly linked to magmatic evolution from mantle wedge basalts to andesites. This excludes the role of an already modified mantle source by sediment subduction (e.g. Stern, 1991) or the influence of old enriched continental lithosphere in magma genesis. Cretaceous intrusive rocks show no evidence of crustal interaction involving garnet, in accordance with a thin crust in Cretaceous times. The ignimbrites form their own cluster mostly due to their larger degree of magmatic differentiation (Fig. 4). In addition, they have distinctly elevated La/Sm ratios. The least differentiated, older Miocene volcanics have consistently flat REE patterns. A fifth cluster (K-Means-Method) represents a "mixed bag" of variable and intermediate samples. There, especially the Quaternary volcanics south of the Pica-Gap are located. THE PCA AND CLUSTER ANALYSIS ALLOWS THE FOLLOWING CONCLUSION : 1) The cluster analysis statistically confirms and underscores the distinct sources and processes that are involved in the genesis of these magmas in space and time, thus supporting the idea that crustal thickening since Miocene and variable crustal components are the main controlling factors. (Wörner et al., 1992; Wörner et al., 1994; McMillan et al. 1993). 2) The strongest geochemical signature of garnet is related to intermediate rather than mafic or more evolved rocks with ages younger than 3 Ma. This excludes an "adakitic" signature and any involvement of slab melts and may provide a means to actually distinguish between slab melts and lower crustal assimilation/melting. 3) Regional differences exist in trace element geochemistry and isotopes of similar age. This supports the notion that not only the Pb-isotopes of magmas are controlled by the age and composition of the Andean crust but also their trace element patterns. 4) We will also present U-series data in combination with other geochemical and isotopic systematics may also help to distinguish garnet signatures caused by slab melting from those of crustal assimilation at high P. REFERENCES Beate, B., Monzier, M., Spikings, R., Cotton, J., Silva, J., Bourdon, E., Eissen, J.-P. (2001) Mio-Pliocene adakite generation related to flat subduction in southern Ecuador: the Quimsacocha volcanic center. Earth Planet. Sci. Lett. 192: 561-570. Davidson, J.P., Harmon, R.S., Wörner, G. (1991) The source of central Andean magmas; some considerations. Geological Society of America Special Paper 265: 233-243. Dorendorf F, Churikova T, Koloskov A, Wörner G (2000) Late Quaternary to Holocene activity at Bakening volcano and surrounding monogenetic centers (Kamchatka): volcanic geology and geochemical evolution. Journal Volcanol. Geoth Res 104 (1-4) : 131-151.

Garrison, J.M, and Davidson, J.P. (2003) Dubious case for slab melting in the Northern volcanic zone of the Andes. Geology 31: 565-568. Gutscher, M.-A., Maury, R., Eissen, J.-P., Bourdon, E. (2000) Can slab melting be caused by flat slab subduction ? Geology 28: 535-538. Mahlburg-Kay S., Mpodozis C., Coira B. (1999) Neogene magmatism, tectonism, and mineral deposits of the Central Andes (22° to 33° S Latitude). In: Skinner B.J. (ed) Geology and Ore Deposits of the Central Andes. Soc Econ Geol Spec Pub 7: 27-59. Mahlburg-Kay S. (2002) Andean adakites from slab melting, crustal thickening, and forearc subduction erosion. 5th International Symposium on Andean Geodynamics 2002, Toulouse, I.R.D. Paris, Abstr. Vol. 405. McMillan, N.J., Davidson, J.P., Wörner, G., Harmon, R.S., Moorbath, S., Lopez-Escobar, L. (1993) Influence of crustal thickening on arc magmatism: Nevados de Payachata volcanic region, northern Chile.Geology 21: 467-470 Stern, C.R. (1991) Role of subduction erosion in the generation of Andean magmas. Geology 19: 79-81 Swan, A.R.H., Sandilands, M. (1995) Introduction to geological data analysis.Cambridge, Mass., Blackwell.p. 1-446 Wörner, G., Moorbath, S., Harmon, R.S. (1992) Andean Cenocoic volcanic centers reflect basement isotopic domains. Geology 20: 1103-1106 Wörner, G., Moorbath, S., Horn, S., Entenmann, J., Harmon, R.S., Davidson, J.P., Lopez-Escobar, L. (1994) Large- and Fine-Scale Geochemical Variations Along the Andean Arc of Northern Chile (17.5° - 22° S) In: Tectonics of the Southern Central Andes. Structure and evolution of an active continental margin (Reutter KJ, Scheuber E, Wigger PJ (eds)) Springer, Berlin.