CRUSTAL DIFFERENTIATION PROCESSES AT CERRO UTURUNCU, ANDEAN CENTRAL VOLCANIC ZONE, SW BOLIVIA: INSIGHTS FROM IN SITU SR ISOTOPIC ANALYSES OF PLAGIOCLASE PHENOCRYSTS

Cerro Uturuncu is an andesitic to dacitic composite volcano located in the back-arc of the Andean Central Volcanic Zone, SW Bolivia. We present new major and trace element and Sr isotopic data of plagioclase phenocrysts combined with whole rock Sr isotope ratio (⁸⁷Sr/⁸⁶Sr= 0.71009-0.71653) and oxygen isotope ratios from Uturuncu lavas and domes.

The isotopic, textural and compositional characteristic of plagioclase phenocrysts in Uturuncu magmas suggest that phenocrysts were inherited from isotopically more evolved continental crust, and periodically these magmas were recharged by higher- temperature, lower ⁸⁷Sr/⁸⁶Sr ratio magmas. Ratios of 0.7105-0.7138 are found in rims while ratios of 0.7139-0.7276 are found in the cores reflecting distinct plagioclase phenocryst populations from assimilation of continental crust and mixing with less evolved magmas. These data demonstrate that open-system processes operating at shallow crustal depths modified Uturuncu magmas, regardless of the nature of mantle or crustal sources.

A typical Uturuncu magma (melt plus crystals) only attains its final geochemical identity just before and during eruption. Though plagioclase Sr isotopic data for the central Andean crustal basement is not currently available, it is clear that shallow-level mixing is an important process producing differentiated magmas. This process was only interrupted by the process of quenching due to eruption. The isotopic variability observed in Uturuncu plagioclase phenocrysts within single lava flows suggest shallow crustal assimilation and magma mixing had limited effects on whole rock chemistry, a complex, late-stage petrogenetic history is recorded within the magmatic cargo of crystals and andesite composition magmatic inclusions.