Making andesite through shallow hybridization of variably enriched lithospheric mantle-derived magmas

Published: 17 August 2022| Version 1 | DOI: 10.17632/55wb9n4r9w.1
Contributors:
Songjie Wang,
Michael Brown

Description

We integrate information from plagioclase and clinopyroxene (Cpx) phenocrysts in post-orogenic andesites within the Sulu belt of China to provide a new perspective on andesite petrogenesis. Plagioclase phenocrysts are mostly andesine; they are depleted in high field strength elements (HFSE). However, Cpx phenocrysts are either reversely-zoned (type I) or homogeneous (type II), with the zoned Cpx divided into subtypes IA and IB. All Cpx has high Mg#, low Na2O and generally low Al2O3, with depletions in HFSE and variably high 87Sr/86Sr ratios, suggesting origination from enriched lithospheric mantle-derived magmas that crystallized above the Moho. The cores of type IA and type II Cpx have normal compositional variations and similar 87Sr/86Sr ratios, consistent with fractional crystallization from a common magma. They have decreasing 87Sr/86Sr ratios with decreasing Mg#, and Cpx with lower 87Sr/86Sr and Mg# overlap the 87Sr/86Sr ratios of plagioclase, indicating crustal contamination for the early-crystallized magma before plagioclase appeared on the liquidus. By comparison, type IB Cpx cores are compositionally distinct from those of type IA and the type II Cpx, indicating crystallization from a different magma. Furthermore, the rims of type IA and IB Cpx have compositions that contrast with the cores, demonstrating crystallization from a third magma. We modelled mixing using plausible proportions of the phenocrysts and the magmas inferred to have been in equilibrium with different Cpx, which produce acceptable compositions of the matrix and the andesite. Therefore, our study demonstrates that orogenic andesites can be made through shallow hybridization of variably enriched lithospheric mantle-derived magmas

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Igneous Petrology

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