Petrogenesis and geodynamic implications of the Gejiu igneous complex in the western Cathaysia block, South China

Abstract

The Gejiu tin district in western Cathaysia block comprises a series of igneous rocks including equigranular and porphyritic granites, gabbro and nepheline syenite. Systematic SHRIMP or LA-ICP-MS zircon U–Pb analyses of 15 representative samples from various phases of the Gejiu complex yielded Late Cretaceous ages of 78–85Ma. Based on their mineralogical, geochemical and Sr–Nd–Hf isotope characteristics, these rocks are categorized into three groups: felsic rocks, alkaline rocks and mafic rocks. The felsic rock group includes the equigranular and porphyritic granites. Geochemical characteristics include high SiO2 contents, enrichment in Rb, Th, U, Nb, Ta, Nd and Hf and depletion in Ba, K, Sr, P, Eu and Ti compared to primitive mantle. REE patterns feature slight LREE enrichment with pronounced negative Eu anomalies. Geochemical data and Sr-, Nd- and Hf-isotopic compositions indicate that the felsic rocks were probably generated by partial melting of crustal source rocks with a minor input from mantle materials. The mafic rocks (gabbro and mafic microgranular enclaves) have distinct geochemical and isotopic features consistent with derivation from an enriched mantle source, with variable degrees of mixing with crustal-derived magmas. Strontium-, Nd- and Hf-isotopic compositions of the alkaline rocks are similar with those of the mafic rocks, suggesting that they have a similar source. Nevertheless, petrological and geochemical characteristics of these rocks indicate that they experienced extensive crystal fractionation and limited crustal contamination. Based on the emplacement of the gabbro–mafic microgranular enclaves–syenite–granites in the Gejiu district, together with contemporaneous geological events in other parts of the western Cathaysia block, we suggest that a widespread extension-related magmatic episode affected the entire region in the late Cretaceous, possibly as a result of lithospheric thinning, basaltic underplating and associated crustal melting.