MULTI-STAGE MIXING FOLLOWING ASSIMILATION-FRACTIONAL CRYSTALLIZATION AT CERRO UTURUNCU, ANDEAN CENTRAL VOLCANIC ZONE, SW BOLIVIA AS REVEALED FROM IN SITU LASER ABLATION ISOTOPIC ANALYSES OF PLAGIOCLASE

Cerro Uturuncu is an andesitic to dacitic composite volcano located in the Andean Central Volcanic Zone, SW Bolivia. We present new major and trace element data and Sr isotopic ratio profiles of plagioclase phenocrysts from Uturuncu lavas and domes. We have identified four plagioclase phenocryst populations representing a complex magmatic plumbing system. These populations are: (1) normally zoned, (2) reverse zoned, (3) oscillatory zoned, and (4) unzoned. Reverse zoned and oscillatory zoned phenocrysts commonly display complex zoning patterns ⁸⁷Sr/⁸⁶Sr ratios (0.7092 - 0.7276), the latter of which are in disequilibrium with whole rock ratios. Consistent core to rim decreases of ⁸⁷Sr/⁸⁶Sr ratios and coincident increases in Sr concentration in plagioclase with maximum ⁸⁷Sr/ ⁸⁶Sr ratios of 0.7139 to 0.7276 (significantly higher on average than those measured in the whole rock, are found in the cores), and minimum ratios of 0.7105 to .7138 are found near the rims. These data demonstrate that Uturuncu magmas, regardless of the nature of mantle or crustal sources, were modified by open system processes operating at crustal depths. One-dimensional diffusion modeling suggest that observed isotopic crystal heterogeneities cannot have existed for more than a few thousand years (0.4-1200 years) at inferred magmatic temperatures (850-1050º C). The chemical and isotopic variability observed in Uturuncu phenocrysts within a single lava flow or dome suggest that although shallow crustal assimilation and magma mixing appear to have had a limited effect on whole-rock chemistry, a complex late-stage petrogenetic history is recorded within the magmatic cargo of crystals and magmatic inclusions.

The isotopic, textural and compositional characters of these phenocrysts suggest that these crystals were inherited from isotopically more evolved crust then periodically recharged by a higher- temperature more mafic magmas. Though no Sr isotopic ratios exist for plagioclase from basement rocks or local ignimbrites, An compositions of plagioclase cores in Uturuncu lavas, domes and magmatic inclusions can be related to the local basement and xenoliths observed in the lavas. Phenocryst outer rims can be related to a combination of disaggregation of the magmatic inclusions during mixing and crystallization in the hybrid melts.