Formation of a continental rift in the East Kunlun Orogen: evidence from late Silurian bimodal volcanic rocks

Late Silurian bimodal volcanic rocks in the middle and western parts of the East Kunlun Orogen (EKO), composed mainly of a basaltic mafic endmember and rhyolitic and trachytic felsic endmembers, are indicative of a continental rift origin. Our U – Pb data show that the bimodal volcanic rocks formed at 428–423 Ma. They have an SiO2 gap between 53 and 65 wt.%. The basalts have ocean island basalt (OIB) or mid-ocean ridge basalt (MORB)-like rare earth element distribution patterns, lack negative Nb, Ta, and Ti anomalies, and have generally positive εHf(t) values, suggesting they were derived from the asthenospheric mantle with little crustal contamination. The distinct trace element compositions of basalts in the Kunlunhe and Nuomuhong areas may result from varying degrees of partial melting in the source regions. In contrast, the felsic endmembers yield bulk continental crust-like chemical characteristics (e.g. depletions in Nb, Sr, P, and Ti) and were derived by the partial melting of lower crust. The Silurian bimodal volcanic rocks in the EKO formed in an intraplate extensional environment indicative of a continental rift. The sedimentary environments of late Silurian carbonate strata also indicate a continental rift setting. Combining these data with the voluminous mantle-derived magmatic activity and the formation of Cu – Ni deposits in the EKO, we suggest a continental rift was formed in the EKO during the late Silurian.