Magma mixing and crust–mantle interaction in the Triassic monzogranites of Bikou Terrane, central China: Constraints from petrology, geochemistry, and zircon U–Pb–Hf isotopic systematics

Abstract

Mafic microgranular enclaves (MMEs) are common in the monzogranites from the Yangba pluton in the Bikou Terrane of central China. Zircon LA-ICP-MS U–Pb dating yields ages of 208.7±0.7Ma and 209.3±0.9Ma for the host monzogranites, and 211.9±0.8Ma for the MMEs, indicating formation from coeval magmas. The field occurrence and textures including spheroidal shapes, transitional contacts, igneous mineral assemblages, acicular apatites, and oscillatory zoning with repeated resorption surfaces in plagioclase, indicate that the MMEs crystallized from mafic magma that was injected into and mingled with the host felsic magma. The host monzogranites are intermediate-felsic, metaluminous, and lack typical peraluminous minerals or alkaline mafic minerals, suggesting their I-type affinity. The monzogranites display depletion in high field strength elements, with Nb and Ta anomalies, and enrichment in large ion lithophile elements and light rare earth elements (LREE), with slightly negative Eu anomalies (Eu/Eu∗=0.72–0.93). The MMEs from the Yangba pluton are intermediate, metaluminous, and have higher K2O, Al2O3, Fe2O3T, MgO, Ni, and Cr contents than the pluton. The MMEs also display much higher total rare earth element (REE) concentrations and REE patterns that are sub-parallel to those of the host rocks, enrichments in Rb, Th, U, K, and LREE, and depletions in Ba, Nb, Ta, and Ti. These features are similar to those of the host rocks, indicating crust–mantle interaction during their petrogenesis.Zircons from the MMEs have highly variable εHf(t) values (−5.5 to +8.7) with corresponding two-stage Hf model ages (TDM2) of 1.14 to 1.42Ga, indicating they were derived from depleted mantle with crustal contamination. In contrast, zircons from host monzogranite show εHf(t) values ranging from −1.7 to +2.7, with TDM2 of 1.13 to 1.21Ga, corresponding to that of the basement rocks in the Bikou Terrane, which indicates that these granitic magmas were probably derived from the Neoproterozoic juvenile lower crust with minor contribution of Mesoproterozoic ancient lower crust.The integrated petrology and elemental and isotopic geochemistry suggest that the MMEs and host monzogranites were generated by the interaction of a granitic magma and a mafic magma in the lower crust. We envisage that Neoproterozoic subcontinental lithosphere mantle-derived mafic magma intruded the lower continental crust during Late Triassic, following the Late Permian to Early Triassic collision between the South China and North China Blocks.