Making andesite through shallow hybridization of magmas derived from variably enriched lithospheric mantle
We integrate textural and in situ compositional information from plagioclase and clinopyroxene (Cpx) phenocrysts together with groundmass compositions in early Cretaceous andesite dykes within the Sulu belt of China to propose a new petrogenetic model for andesite. Plagioclase phenocrysts are mostly andesine; they are depleted in high field strength elements (HFSE). However, clinopyroxene (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 crystallization above the Moho from magmas derived from enriched lithospheric mantle. The cores of type IA/IB and type II Cpx have normal major- and trace-element compositional variations and similar 87Sr/86Sr ratios to each other and to plagioclase, consistent with fractional crystallization from a common magma (magma 1). The rims of type IA and IB Cpx also have normal major- and trace-element compositional variations, but these are not as evolved as the cores, and the rims have lower 87Sr/86Sr ratios, demonstrating crystallization from an isotopically-distinct magma (magma 2). Based on modelled major and rare earth element compositions of magmas inferred to have been in equilibrium with different Cpx (± plagioclase) domains, the measured groundmass compositions can be reproduced by variable mixing between the two magmas. Our study demonstrates for the first time that andesite magma can be made through fractionation and shallow hybridization of magmas derived from variably enriched lithospheric mantle.