Mid-Permian rifting in Central China: Record of geochronology, geochemistry and Sr–Nd–Hf isotopes of bimodal magmatism on NE Qinghai–Tibetan Plateau

Abstract

Little has been known regarding ~200-Myr-long geologic history of Central China during the Late Paleozoic between two major continental collisional events marked by ~480–440-Ma North Qinling–North Qaidam and ~230-Ma Dabie–Sulu ultrahigh-pressure (UHP) belts, respectively. The Gonghe basin is juxtaposed with major Chinese continental blocks across several outstanding Tethyan sutures in Central China and may hold answers to many critical questions about the tectonic amalgamation of China. In this paper, geochronological (laser ablation–inductively coupled plasma–mass spectrometry, LA–ICP–MS), mineral chemistry, major and trace element, and Sr–Nd–Hf isotopic data are presented for newly discovered Mid-Permian magmatic rocks in the Gonghe basin on the northeastern Qinghai–Tibetan Plateau. The data indicate that magmatism was active at ~270Ma and shows a bimodal composition including gabbro, diabase and granite. Major oxides and trace elements are obviously correlated with SiO2 between the mafic and felsic rocks. They are both characterized by enrichments in light-rare-earth elements (LREEs) and large-ion-lithophile elements (LILEs), negative Eu anomalies and depletion in high-field-strength elements (HFSEs), together with similar Sr–Nd–Hf isotopic signature, suggesting that they likely shared the same magma chamber, and that the felsic rocks were produced by fractional crystallization of the mafic rocks. The high initial 87Sr/86Sr ratios (0.7061 to 0.7094) and low εNd(t) values (−6.4 to −3.7), as well as Nb–Ta deletions indicate that they were likely derived from enriched mantle metasomatized by subduction-related fluids during the Early Paleozoic. In view of absence of coeval arc-related magmatism in contrast to broad seamount mafic magmatism in the Gonghe basin, the bimodal magmatism is ascribed to an intracontinental rifting event that could have been caused by transtension in Central China since the Mid-Permian, which could have been responsible for the initiation of the Gonghe and Songpan–Ganzi basins.