Origin of mafic microgranular enclaves (MMEs) and their host quartz monzonites from the Muchen pluton in Zhejiang Province, Southeast China: Implications for magma mixing and crust–mantle interaction

Abstract

The origin of mafic enclaves in granitoid plutons has long been a matter of debate. In this paper, we present detailed petrographic, mineralogical, geochemical, and SrNdHf isotopic data, UPb zircon dates, and field and petrological observations, for mafic microgranular enclaves (MMEs) and their host quartz monzonites from the Muchen pluton in Southeast China to evaluate their origins. LA–ICP–MS UPb dating of zircon yields crystallisation ages of 112.4±1.2Ma and 112.1±1.0Ma for the MMEs and host quartz monzonites, respectively, indicating their coeval formation during the late Early Cretaceous. Field and petrological observations, such as spheroidal shapes, back-veining, double enclaves, xenocrysts, acicular apatites, and oscillatory zoning with repeated resorption surfaces in plagioclases, suggest that the MMEs are globules of a more mafic magma that was injected into and mingled with the host felsic magma. Geochemically, the host monzonites are intermediate-acidic, metaluminous, alkaline, and K-rich. In contrast, the MMEs are relatively poor in Si and K. The host monzonites are enriched in Rb, Th and U, and depleted in Sr, P, Nb, Ta and Ti, and show moderate to strong europium depletions (δEu=0.12–0.60). These mineralogical and geochemical features classify the quartz monzonites as belonging to the I-type granitoids. The MMEs have broadly similar trace element signatures to those of the host monzonite, but are distinct in having relatively enriched Sr and P, more depleted Zr and Hf, and weak to moderate Eu depletions (δEu=0.43–0.93). Major and trace element data plotted versus isotopic data for the MMEs and the host quartz monzonites yield covariant arrays that result from magma mixing during their petrogenesis. The MMEs and the host quartz monzonites have similar initial 87Sr/86Sr ratios (ISr) of 0.7058–0.7070 and 0.7062–0.7065, respectively, and both have high εNd(t) values (−2.6 to +0.6 for MMEs; −3.2 to −2.4 for quartz monzonites). However, zircons from the MMEs have different εHf(t) values (−0.4 to +6.2) than the host quartz monzonites (−1.0 to +1.8), indicating that the MMEs and host granitoids largely crystallised from different magmas, providing direct evidence for mafic–felsic magma mixing processes. The zircon saturation geothermometer and Al-in-hornblende geobarometer show that the Muchen pluton crystallised at temperatures of 797–851°C and depths of 6–7km. The integrated petrology, and elemental and isotopic compositions suggest that the MMEs and host quartz monzonites were generated by mixing of depleted, mantle-derived, mafic magmas and felsic magmas produced by partial melting of crustal materials in an extensional setting.