The genesis of granodiorites and dark enclaves from the Kaerqueka deposit in east Kunlun belt: Evidence from zircon U-Pb dating, geochemistry and Sr-Nd-Hf isotopic compositions

Abstract: The porphyritoid dioritic enclaves are widely distributed in granodiorites of the Kaerqueka copper deposit, East Kunlun belt. The enclaves are mostly aligned ovals, with similar mineralogy to the host rocks. However, the modal percentage of hornblende is significantly higher than that of the host rock. It has apparent imbalanced structure with development of needle-like apatite. The granodiorite and enclaves have the same age of 234 Ma, which indicates that they might have been formed by magma mixing. As for geochemical characteristics, the enclaves contain higher values of SiO2 and MgO than the host rock and are hence more mafic. They are also enriched in large LILE and LREE, and are depleted in HFSE. The LaN/YbN ratios of host granodiorite (11.6?19.0) are significantly higher than the ratios of the enclaves (5.0?9.7). The Nb/Ta values of host granodiorite (10.6?11.7) are lower than those of the enclaves (11.6?14.8). They have similar Sr, Nd and Hf isotopic compositions. The granodiorite has εNd(t) values of ?5.3 to ?4.2, (87Sr/86Sr)i values of 0.71110?0.71125, εHf(t) values of ?5.8 to ?1.6, tDM(Hf) values of 1012?1102 Ma. The enclaves have εNd(t) values of ?5.2 to ?5.0, (87Sr/86Sr)i values of 0.71114?0.71171, εHf(t) values of ?6.3 to ?3.4, and tDM(Hf) values of 937?1129 Ma. These data indicate that, at the subduction-collision transition stage of Triassic period, the enriched EMII type lithosphere mantle underwent partial melting and produced mafic magma under the influence of upwelling asthenosphere materials. The partial melting of lower crust materials, which had been underplated by the mantle, formed the felsic magma chamber. The injection of mafic magma into the felsic magma chamber formed enclaves. After mixing of evolved mafic magma with felsic magma, enclaves-bearing homogeneous magma was formed. The crystallization of this homogeneous magma formed granodiorites containing enclaves.