Mafic Volcanism and the Deep Crust in the Central Andes: In Situ Geochemistry and Isotopic Composition of Young, Small-Volume Mafic Eruptions.

Abstract:

We present data on small volume eruptions of basalt and basaltic andesite from within the modern main arc of the Andean Central Volcanic Zone (CVZ). Regional faulting has allowed for small batches of relatively un-differentiated magma (olivine and pyroxene phyric) to reach the surface, recording a petrogenetic history normally obscured for magmas in the region due to thick crust (> 65 km) and a mid-crustal magma body which acts as a density filter. In situ crystal chemistry, melt inclusion chemistry, and single-crystal radiogenic isotopes reveal a much richer history than whole rock measurements. Bulk major and trace elements follow regional arc differentiation trends and are clearly modified by crustal magmatic processes. In contrast, olivine-hosted melt inclusions appear to record multiple distinct magmas, including potential primary melts. Single crystal olivine ⁸⁷Sr/⁸⁶Sr from Cerro Overo maar (0.7041-0.7071) define a broader range than whole rock ⁸⁷Sr/⁸⁶Sr (0.7062-0.7065), indicating preservation of juvenile melt in olivine-hosted melt inclusions which is lost at the whole rock scale. In situ compositional analyses of olivine, pyroxene, spinel, and melt inclusions, along with single crystal radiogenic and stable isotopes provide insight on the composition(s) of mafic magmas being delivered to the lowermost crust and deep crustal processes. Mineral chemistry data collected using EPMA provides critical P-T constraints allowing for the petrogenetic history of potential endmember magmas to be determined and also provides insights into the structure of the central Andean deep crust.