The discovery of the Early Triassic gabbro rocks of the Duolong accretionary complexes in southern Qiangtang terrane of Tibet and its geological significance

The outcropped Late Triassic-Jurassic accretionary complexes are located along northern Bangong Co-Nujiang suture zone where a large subduction-type Duolong porphyry copper-gold ore concentration area is developed. Bulk-rock major and trace elements, Sr-Nd isotope and LA-ICP-MS zircon U-Pb data are reported for gabbro rocks sampled from the Duolong accretionary complexes. The 206Pb/238U age of zircon in the gabbro analyzed by LA-ICP-MS is 246.0±1.4Ma, which represents the crystallization age of the magma. Na, Mg, LREE and large ion lithophile elements (Ba, U, La and Sr) are concentrated whereas Ti, K and high-field strength elements (Nb and Ta) are deficient. These characteristics suggest that the geochemistry of the gabbro is similar to that of the island arc type basalt. The isotopic compositions of the Sr and Nd ( (87Sr/86Sr)i=0.707605~0.709778, (143Nd/144Nd)i=0.512515~0.512678, and TDM2=0.54~0.99Ga) suggest that gabbro is developed on the arc in an intraplate environment. It is considered that the end of early Triassic igneous activity probably resulted from the northward subduction of the Bangong Co-Nujiang Ocean crust which interacted with the lithospheric mantle material. The discovery and understand ing provide the northward subduction of the Bangong Co-Nujiang ocean crust in early Triassic with important evidence of magma tism.