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Spatial and Temporal Variations in Magma Geochemistry Along a NW

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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 MAGMATISM IN TIME AND SPACE: IMPLICATIONS FROM GEOCHEMICAL DATA OF IGNEOUS ROCKS BETWEEN 16° AND 27°S FROM CRETACEOUS 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 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 Todas las contribuciones fueron proporcionados directamente por los autores y su contenido es de su exclusiva responsabilidad. Elevation (m) 6000 73¡W 72¡W 71¡W 13¡S Cuzco 5000 Abancay COCOS PLATE R io PACIFIC V NVZ i 4000 lc a n o PLATE ta Andes SOUTH AMERICA 3000 NAZCA 14¡S PLATE Oroscocha CVZ NAZCA 2000 Quinsachata PLATE Sicuani 1000 SVZ ANTARTIC 75¡W 74¡W PLATE Yauri Cotahuasi 15¡S Puquio 15¡S Coracora Morane FIR Condoroma SAR ANT AND Tuti Colca COR i r a c a Chivay c A u a o i Y Yarihuato R o i Hualca Hualca R Chuquibamba Huarancante l a rr pa a h C Ananto io R HUAM SAB Paquetane Lago Titicaca 16¡S Chachani Puno 16¡S o c i t A i o l i e CHA R Salinas v NIC i il a h r a C a o n i C R o o c i O MIS R UBI o i R s a a an u ih am C S io o R i R PichuPichu a or ilc Rio Vit Camana u Q o i R Arequipa HUAY TICS o g o a o i ic t Rio Ch Maure n a S o i HP TUT R Mollendo o 17¡S b 17¡S m a T i o R Moquegua YUC TIT Miocene Volcanoes KER re Tarata au a R M u i o g e u q o Quaternary Volcanoes M CAS io R a b PERU m u c Shoshonites o TAC L o i R a POM m HUY a S o 18¡S i 18¡S R CAQ a plin a PAR TC acna BOLIVIA io CNE R TAP AJO ZAP CHU CHP Arica Rio LLut TOM GUL R io LAU A CPI z apa LAC ELRN CMA ANO SUP R de io V ito r SUA ACH SUR nes 19¡S 19¡S aro m Ca de io R MAN Name of volcanoes ACM IQM I II 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 = Firura ANO = Co Anocarire Iquique HUAL Uyuni COR = Coropuna MAM = Mamuta AND = Andagua IQM = Isluga IRU ELRS OPA 21¡S HUAM = Huambo LIR = Lirima 21¡S SAB = Volc n Sabancaya POR2 = Porquesa MIN AUC PORU OLA NIC = Nicholson QUIL = Quillacollo PUN CHE CHA = Chachani Hual = Huailla PAL CAR CUEV CEB MIS = Volc n Misti IRU = Irrutupunco CHAN 22¡S HUAY+ HP= Huaynaputina ELR1 = El rojo del Sur AZU 22¡S Tocopilla SPP UBI = Vol n Ubinas OLC = Olca, MIN= Mi o, TICS = Ticsani Opa = Olca-Paroma PUT TUTU = Tutupaca OLC = Volc n Olca COR YUC = Yucamane PMA = Paroma ODT SAI 23¡S TIT = Titiri, AUC = Aucanquilcha LIN 23¡S CAS = Casiri OSC = N salar Carcote TAC = Volc n Tacora Ola = Ollague LAS Salar de Atacama HUY = Huyalas Poru = Porunita Antofagasta CAQ = Caquena PUN = Puntilla 24¡S CNE = Co Negro CHE = Chela 24¡S POM = Pomerape CAR = Carcote NEG PAR = Parinacota PAl = Palpana SOC TAP = Volc n Taapaca CUEV = Las Cuevas LUL CHU = Chucullo CEB = Cebollar 25¡S AJO = Ajoya CHAN = Chanca 25¡S ZAP = Zapahuira Azu = Azufre LTA LAU = Lauca SPP = San - Pedro Poru a CHP = Choquelimpe PUT = Putana COP/TOM= Copaquilla ODT = Ojos del Toro 26¡S LAC = Quebrada Laco COR = C¡ Apagado 26¡S LAU = Lauca SAI = Sairecabur GUL = Gullatire LIC = Licancabur QC-01 = Qui acollo LAS = Lascar ELRN = El rojo del NorteNEG = Negrillar 27¡S CPI = Co Pichican SOC = Socompa 27¡S CMA = Co Margarita LUL = Llullaillaco OJOS DEL SALADO 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-Pleistocene volcanoes and sites of additional volcanic/plutonic samples of Miocene to Cretaceous age. Quaternary volcanics >Miocene volcanics back arc shoshonites 200 160 120 Y / a B 80 40 0 45,00 55,00 65,00 75,00 SiO 2 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 Quaternary volcanics > Miocene volcanics back arc shoshonites 200 160 120 Y / a B 80 40 0 0 200 400 600 800 1000 1200 1400 1600 N-S Distance (Km) projected onto arc profile (Fig. 1) 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.
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