SHRIMP zircon U–Pb geochronology, geochemistry and Sr–Nd–Hf isotopic compositions of a mafic dyke swarm in the Qiangtang terrane, northern Tibet and geodynamic implications

Abstract

Regional mafic dyke swarms are commonly emplaced in extensional tectonic settings and are typically considered to be linked with continental break-up. A large number of mafic dykes (areal extent of ~40,000km2) have recently been documented in the middle of the Qiangtang terrane, northern Tibet. Zircon U–Pb isotope analyses using a sensitive high-resolution ion microprobe (SHRIMP) indicate that the dykes were emplaced in the Early Permian (279±2Ma, 283±1Ma, 285±1Ma and 285±3Ma). Whole-rock geochemical data show that the Qiangtang mafic dykes are tholeiitic in composition and exhibit relative enrichment in light rare earth element (LREE) and depletion in Nb, Ta and Ti, resembling continental intra-plate basalts. Whole-rock Sr–Nd and zircon Hf isotopic data suggest that mafic dyke magma was derived from a depleted mantle source (εNd(T)=+5.1 to+7.6 and εHf(T)=+4.9 to+14.8). The chemical and isotopic characteristics make the mafic dykes similar to the Panjal Traps and Permian basalts in the Tethyan Himalayas. The contemporaneous and widespread emplacement of ca. 283Ma continental intra-plate basaltic rocks in the Qiangtang terrane and in the Himalayas is suggestive of a Large Igneous Province (LIP). This LIP is linked to the initial rifting and opening of the Meso-Tethys Ocean during the Permian.