Zircon SHRIMP U-Pb dating and lithogeochemistry of gabbro from the ophiolite in southern Qinghai Province

Abstract：Ophiolites occur as lenses or slices within the Late Triassic metamophic sedimentary rocks in southern Qinghai Province, where no research work has been done before. The gabbro exposed in the Chaggur area is a unit of the ophiolite and belongs to the low-K tholeiitic series. The SHRIMP U- Pb age of zircons from gabrro is (239.8±3.1)Ma, suggesting that the re-spreading time of the Bayanhar ocean was most likely at the early stage of Middle Triassic. The gabrro is characterized by high MgO, Al2O3 and low K2O, TiO2. All rock samples have low REE abundances and show similar REE distribution patterns characterized by slight LREE enrichment with very distinct positive Eu anomalies. (La/Yb)N ratios are 1.10~ 4.40, whereas δEu values are 1.50~ 2.41. The samples are enriched in large ion lithophile elements (LILE) and depleted in Nb, Zr and Ti in the trace spider diagram, implying that the magma source was affected by input of some components from the subduction zone. It is thus considered that lithogeochemistry of the Chaggur gabrro was characterized by intraplate and arc volcanic rocks, and hence the gabrro was formed in a back-arc basin setting and should belong to the western segment of the Garzê-Litang ophiolite mélange.