Geochemical and isotopic compositions of Late Permian metavolcano–sedimentary sequence in the southern Songpan–Ganzi Orogen, eastern Tibetan Plateau

Abstract

AbstractThe Permian metavolcano–sedimentary sequence of the Jianglang and Taka domes in the southern Songpan–Ganzi Orogen is composed of bedded marble and sandwiched metabasalt. This study presents geochemistry, C–O–Nd isotope systematics and zircon U–Pb geochronology data to explore the formation mechanism and tectonic setting of its protolith. The marble samples have high δ13CV-PDB (4.0 ‰ to −0.1 ‰) and δ18OV-SMOW (16.3 ‰ to 13.6 ‰) values, with similar εNd(t) values (−5.3 to −7.2) to Late Permian conodonts. They display seawater-like features, e.g. superchondritic Y/Ho ratios, negative Ce and positive La, Gd and Y anomalies, indicating a marine carbonate protolith in oxygen-rich palaeoseawater. Their pronounced positive Eu anomalies suggest a contribution (5 % to 1 %) of submarine high-T hydrothermal fluids. The metabasalt samples show low SiO2, high Fe2O3T and MgO contents. They have low La/Sm, (Th/Ta)PM and (La/Nb)PM ratios, without correlations between MgO and other major/trace elements. These signatures could be attributed to insignificant fractional crystallization and crustal contamination. Their rare earth elements (REE) patterns, trace element profiles and trace element ratios are comparable to those of mid-ocean ridge basalt (MORB), suggesting a depleted MORB-type mantle reservoir. Melt modelling illustrates low degrees (<10 %) of partial melting in the spinel stability field at a depth of 30–60 km. Moreover, the metabasalt contains 2453.7–258.9 Ma detrital zircons significantly related to the Palaeoproterozoic crystalline basement, Rodinia supercontinent, Caledonian events and c. 260 Ma Emeishan large igneous province. Combined with previous studies, we advocate that the investigated metavolcano–sedimentary suquence was most likely formed during the Late Permian in a mature back-arc basin of the Palaeo-Tethys ocean.