Evidence for a deep crustal hot zone beneath the Diamante Caldera-Maipo volcanic complex, Southern Volcanic Zone,

DANA DREW; TIMOTHY MURRAY; PATRICIA SRUOGA; MAUREEN FEINEMAN

San Francisco

Congreso; 2010 Fall Meeting AGU; 2010

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Subduction zones at convergent continental margins are dynamic environments that control the long-term evolution and interaction of the crust and residual mantle. The Southern Volcanic Zone (SVZ) of the Andes formed as a result of volcanic activity and uplift due to the eastern subduction of the Nazca Plate beneath the South American Plate. Maipo and neighboring volcanoes in the northern SVZ are unique in that the continental crust is exceptionally thick (~50 km), causing the mantle-derived magma to stall and interact with the crust at multiple levels prior to eruption. Maipo is an andesite/dacite stratovolcano that lies within the Diamante Caldera, which formed approximately 450 Ka during an explosive eruption that produced 350 km3 of rhyolitic ignimbrite. Following post-caldera reactivation Maipo has undergone a complex evolution, first erupting 86 thousand years ago and experiencing seven eruptive events extending to historic times. The Maipo lavas represent a unique geochemical evolution resulting from fractional crystallization, crustal assimilation, and magma mixing in the lower and upper crust. By analyzing trace element compositions, major element compositions and 87Sr/86Sr ratios in sixteen samples, we have begun to constrain the complex geochemical processes that formed this volcano and contribute to the differentiation of Andean continental crust. The major element analysis of the samples reflects the extent of differentiation resulting in dacite to andesite volcanic rock, and was used to distinguish between the seven eruptive events. The trace elements and strontium isotope ratios reflect the composition of the source rock, the extent of crustal assimilation, and the crystallization of minerals from the resulting mantle derived magma. The SiO2 weight percent (ranging from 54.3 to 71.1%) and 87Sr/86Sr  ratios (0.7048 to 0.7057) show a linear correlation nearly identical to that reported by Hildreth and Moorbath (1988, CMP 98, 455-489) for nearby Cerro Marmolejo, suggesting a ubiquitous crustal component in the region. High Sr/Y (up to 37) in the most primitive basaltic andesites more specifically indicates fractionation by garnet in the lower crust. Using Assimilation and Fractional Crystallization modeling with end members taken from Hildreth and Moorbath (1988), the suite of volcanic samples demonstrate 2 to 10% assimilation of radiogenic crust. Evidence for assimilation and fractional crystallization in the lower crust, combined with petrological evidence for further crystallization and magma mixing in an upper crustal magma chamber, suggests that the northern SVZ lies above a deep crustal hot zone (Annen et al. 2006, J Pet 47, 505-539), in which primitive mantle-derived magmas differentiate into intermediate arc magmas, leaving behind a dense refractory restite.  This restite may later delaminate and be recycled into the continental lithospheric mantle, contributing to crust/mantle evolution.