Eocene magmatism in the western Tengchong Block: Implications for crust-mantle interaction associated with the slab rollback of the Neo-Tethys Ocean

Abstract

Granitoids are the major building blocks of Earth’s continental crust, and provide important insights into the evolution of continental crust and related mineral resources. Here we investigate the Eocene composite Lailishan pluton in the western Tengchong Block based on studies of petrology, mineral chemistry, zircon U-Pb-Hf isotopes, whole-rock geochemistry, and Sr-Nd isotope data to understand the petrogenesis and tectonic implications. The pluton is composed of monzogranite, syenogranite and abundant microgranitoid enclaves (ME) in the monzogranite. The zircon U-Pb data show similar rock formation ages in the range of 54–52 Ma. A diabase dyke intruded into the monzogranite at 54 Ma. The geochemical and isotopic signatures (εHf(t) of −11.6 to −3.1 and εNd(t) of −9.4 to −8.8) suggest that both the monzogranite and syenogranite were derived from the partial melting of metabasaltic rocks in the Mesoproterozoic Gaoligong metamorphic basement. The geochemical trends of the syenogranite indicate the significant fractional crystallization of biotite, plagioclase, K-feldspar, apatite, allanite and monazite, which further facilitated the enrichment of Sn in the syenogranite. The typical igneous textures, megacrysts from host granites, compositional zoning of the plagioclase and large variation of εHf(t) from −9.3 to −2.6 of the ME suggest a magma mixing and mingling process, which corresponds well with mass balance calculation using monzogranite and diabase as two endmembers. The mantle-like isotopic characteristics (εHf(t) of −2.4 to 2.0 and εNd(t) of −5.9) and low Th/La, Th/Nb and Nd/Pb ratios of the diabase dyke samples suggest the lithospheric mantle metasomatized by subducted fluids as the magma source. We propose that the slab rollback of the Neo-Tethys Ocean facilitated the interaction between crustal and mantle derived magmas generating a major pulse of Eocene magmatic activity in the Tengchong Block.