An evolving magma chamber within extending lithosphere: an integrated geochemical, isotopic and zircon U-Pb geochronological study of the Gushan granite, eastern North China Craton

Abstract

The Gushan granite, located in the Jiaobei terrane of the eastern North China Craton, carries mafic microgranular enclaves (MMEs) and zoned K-feldspar phenocrysts, and is a typical example of magma generation and evolution in an extensional setting. Here we present whole rock major and trace element geochemistry, Sr–Nd isotopes and zircon U–Pb and Hf isotope data on the granite and its enclaves. The granite was emplaced at ∼120 Ma. It is high in SiO2, K2O + Na2O, LILE and LREE, but low in MgO and HFSE. The granite are also marked by high initial 87Sr/86Sr (0.7103) and low εNd(t) (−18.5 to −18.9) and zircon εHf(t) (−16.6 to −22.0) values. These features indicate that the host granite was mainly derived from ancient North China Craton lower crust. Field, petrographic and compositional studies indicate that MMEs are products of mafic–felsic magma interactions in the generation of the granite in the relatively late magmatic history. Available data suggest that enriched lithospheric mantle was a suitable candidate for the mafic magma end-member involved in the magma interaction process. Internal Sr–Nd isotopic variations within K-feldspar phenocrysts are also evaluated to decipher the magmatic history. In spite of a protracted cooling history and evidence of subsolidus alteration, the K-feldspar phenocrysts preserve marked primary Sr–Nd isotopic variations: initial 87Sr/86Sr ratios increase from the cores (0.7100–0.7102) to rims (0.7105–0.7110), whereas εNd(t) values show decrease from the cores (−17.7 to −20.5) to rims (−21.1 to −27.0). The overall isotopic profiles across the K-feldspar phenocrysts are consistent with a continuous magma contamination process. Highly negative Nd isotopic values of some K-feldspar rims suggest that the Precambrian basement rocks in the upper crust are potential candidates for the contaminant. These results indicate that isotopic heterogeneity could be preserved within single crystal from much older, slower cooled plutonic rocks, and those isotopically zoned minerals are effective recorders of open magma system. Based on petrographic, geochemical and isotopic studies, we envisage multiple magmatic pulses, mafic–felsic magma interaction, and continuous assimilation during the formation of the Gushan granite. A model involving lithospheric thinning and mantle–crust interaction in an extensional setting is proposed to account for the generation of the Gushan granite and its enclaves.