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Spatial and Temporal Variations in Magma Geochemistry Along a NW

Spatial and Temporal Variations in Magma Geochemistry Along a NW

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 IN TIME AND SPACE: IMPLICATIONS FROM GEOCHEMICAL DATA OF IGNEOUS ROCKS BETWEEN 16° AND 27°S FROM TO RECENT TIMES

1 1 1 1 (1 2 WÖRNER, G. , MAMANI, M. , RUPRECHT, P. , MERCIER, R., HARTMAN, G. , SIMON, K. ; Y THOURET, JC.

1Abt. Geochemie, GZG Universität Göttingen, Goldschmidtstr. 1, 37077 Göttingen, Germany [email protected] 2Coordination des Recherches Volcanologiques, Universite Blaise Pascal (Clermont II, 5 rue Kessler, 63000 Clermont-Ferrand, France

Miocene and volcanism of the Central Andean Volcanic Zone (CVZ: 12°40'S to 26°S) result from the of the oceanic beneath the . Below the CVZ, the continental reaches a thickness of up to 70 km. The upper crust is formed by , rocks, sedimentary rocks, marine rocks, covered by continental of the Cretaceous. The potential sources that would have contributed to create the geochemical characteristics of the in the CVZ of the would be: fluids from the subducted , the asthenospheric , the lithospheric mantle, the lower and the upper crust. Geochemical data obtained on 290 new samples of the and Quaternary volcanic centers from S. (12°40'S to 18°22'S) and additional 200 samples from Northern 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- data to characterize these . 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 fractionation in

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CHILE SAR = Sara Sara SUP = Puquintinca LIR YAH = Yarihuato 20¡S SUR = Salar de Surire POR 20¡S ANT = Antapuna ACM = Co Macusa QUIL Salar de FIR = ANO = Co Anocarire Iquique HUAL Uyuni COR = MAM = Mamuta AND = Andagua IQM = IRU ELRS OPA 21¡S HUAM = Huambo LIR = Lirima 21¡S SAB = Volc n POR2 = Porquesa MIN AUC PORU OLA NIC = Nicholson QUIL = Quillacollo PUN CHE CHA = Chachani Hual = Huailla PAL CAR CUEV CEB MIS = Volc n IRU = Irrutupunco CHAN 22¡S HUAY+ HP= ELR1 = El rojo del Sur AZU 22¡S Tocopilla SPP UBI = Vol n OLC = Olca, MIN= Mi o, TICS = Opa = Olca-Paroma PUT TUTU = OLC = Volc n Olca COR YUC = PMA = Paroma ODT SAI 23¡S TIT = Titiri, AUC = LIN 23¡S CAS = OSC = N salar Carcote TAC = Volc n Ola = Ollague LAS HUY = Huyalas Poru = Porunita CAQ = Caquena PUN = Puntilla 24¡S CNE = Co Negro CHE = Chela 24¡S POM = CAR = Carcote NEG PAR = PAl = Palpana SOC TAP = Volc n CUEV = Las Cuevas LUL CHU = Chucullo CEB = Cebollar 25¡S AJO = Ajoya CHAN = Chanca 25¡S ZAP = Zapahuira Azu = Azufre LTA LAU = SPP = San - Pedro Poru a CHP = Choquelimpe PUT = COP/TOM= Copaquilla ODT = Ojos del Toro 26¡S LAC = Quebrada Laco COR = C¡ 26¡S LAU = Lauca SAI = GUL = Gullatire LIC = QC-01 = Qui acollo LAS = ELRN = El rojo del NorteNEG = Negrillar 27¡S CPI = Co Pichican SOC = 27¡S CMA = Co Margarita LUL = ACH = Achecalane LTA = Lastaria SUA = Arintinca 100 km 28¡S 28¡S 75¡W 74¡W 73¡W 72¡W 71¡W 70¡W 69¡W 68¡W Fig. 1: Sampled Plio- volcanoes and sites of additional volcanic/plutonic samples of Miocene to Cretaceous age. Quaternary volcanics >Miocene volcanics back arc shoshonites 200

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Fig. 2: Ba/Y - SiO2 plot showing that the Ba/Y - ratio does not change with differentation. intermediate rocks. Unless 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 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 Quaternary volcanics > Miocene volcanics back arc shoshonites 200

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N-S Distance (Km) projected onto arc profile (Fig. 1) Fig. 3 : Ba/Y for 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 .

We interpret these results as clear evidence for a crustal control on chemistry. Mantle lithospheric source, a subduction, or the tectonic , 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 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 "" (e.g. Gutscher et al., 2000; Beate et al., 2001). However, the setting for the formation of adakites is related to the 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 , 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 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. 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 to . This excludes the role of an already modified mantle source by sediment subduction (e.g. Stern, 1991) or the influence of old enriched continental 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 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 generation related to flat subduction in southern : 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 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 ? Geology 28: 535-538. Mahlburg-Kay S., Mpodozis C., Coira B. (1999) magmatism, tectonism, and 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 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 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.

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