Upper crustal overprinting of lower crustal processes at Maipo Volcano (34˚ 10' S), Southern Volcanic Zone.

MAUREEN FEINEMAN; DANA DREW; TIMOTHY MURRAY; PATRICIA SRUOGA

Praga

Simposio; Goldschmidt Conference 2011; 2011

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Diamante Caldera – Maipo Volcanic Complex is situated in the northernmost part of the Andean Southern Volcanic Zone (SVZ), upon ~50 km thick continental crust.Receiver function data suggest magma stalling at the base of this thickened crust in the northern SVZ [1], which is consistent with along-arc trace element variations that show increasing evidence for equilibration with garnet in basalts andbasaltic andesites erupted to the northern end of the SVZ [2].In a suite of basaltic andesite to dacite lavas sampled at Maipo, we observe clear evidence of assimilation andfractional crystallzation (AFC) processes in the upper crust,overprinted upon the lower crustal, presumably garnet­equilibrated trace element signature. The basaltic andesites are characterized by Sr/Y = 35-40, Eu/Eu* >0.85, 87Sr/86Sr = 0.7045-0.7050, and 144Nd/143Nd = ~0.5126. These chemical characteristics reflect a significant lower crustal input relativeto basalts and basaltic andesites from the central and southern portions of the SVZ, which sit upon much thinner crust and are characterized by Sr/Y = 15-30, Eu/Eu* ´ 1, 87Sr/86Sr = ~0.7040, and 144Nd/143Nd = ~0.5129 [3]. The Maipo data show continuous geochemical trends from the basaltic andesites tothe dacites, which are characterized by Sr/Y = 14-24, Eu/Eu* ´ 0.50-0.75, 87Sr/86Sr = 0.7054-0.7057, and 144Nd/143Nd = ~0.5125. These dacites are consistent with differentiation from the basaltic andesites by AFC with a plagioclase-rich crystallizing assemblage in an upper crustal magma chamber.Constant Gd/Yb (2.5-3.1) in the lavas may reflect a buffering effect of amphibole during differentiation, although it is rarely observed in the phenocryst assemblage. The high Sr/Y in the more primitive magmas, which suggests garnet fractionationat depth, has been erased in the more evolved magmas. Thus we note that the absence of high Sr/Y in intermediate arc magmas does not necessarily indicate that the magma did notinteract with garnet at depth.