Zircon U-Pb Dating And Tectonic Setting Of Intrusive Rocks In Wulan Along The North Margin Of Qaidam Terrane

Mesozoic intrusive rocks are widely distributed in the Wulan region along the Northern margin of Qiadam terrane, which range in lithology from gabbro, diorite, granodiorite to granite. LA-ICP-MS zircon dating yielded Middle Triassic ages of 246±0.7 Ma,245±0.7Ma,242±0.8 Ma,241±0.9 Ma for emplacement of gabbro, diorite, granodiorite and granite, respectively. Inherited zircons from diorite and granite yield Paleozoic ages of 433 to 464Ma as well as Neoproterozoic ages of 928 to 1043 Ma. These rocks show a wide range of chemical compositions, with SiO2 of 50.61 to 76.55%, total alkali of 2.27 to 8.43%, and rittmann index of 0.54 to 2.16. Except for the gabbros, which have relatively low K2O contents and belong to medium-K calc-alkaline series, the other sample show higher low K2O contents and belong to high-K calc-alkaline series. They are characterized by the arc-like patterns of trace element distribution with positive LILE and LREE anomalies but negative HFSE and HREE anomalies in the spidergram. Zircon εHf(t) for magmatic grains ranges from -6.11 to±1.37, with a decrease trend from gabbro, diorite, granodiorite to granite. Zircon εHf(t) for inherited grains broadly ranges from -10.72～+1.04.The gabbros have high MgO, Cr, Ni and Mg# contents, indicated that they were mainly derived by the partial melting of enriched lithosphere which had been metasomatizd by the fluid from the subducted slab. The diorites may be produced by the fractional crystallization of the gabbro magmas. The granodiorites were generated by magma mixing between the magma from partial melting of the enriched lithosphere mantle and the magma from partial melting of crust rocks. The granites probably resulted from the partial melting of crustal rocks.Intrusive rocks from the Wulan region were formed at an active continental margin environment during in the ocean-to-continent subduction. They have similar major element, trace element, rare-earth element, and Hf isotope compositions to the contemporary magmatic rock in the northern margin of Qaidam terrane, East Kunlun and West Qinling. The intrusive rocks were contemporary with the calc-alkaline magmatism from Buerhanbuda in the East Kunlun orogen, Elashan and Tongren-Zeku area in the West Qinling orogeny. These calc-alkaline magmatism constituted an active continental margin arc formed by the Northern subduction of Paleo-tethys oceanBase on the previous zircon U-Pb ages and our data, the indosinian magmatism at the conjunction part of Qingling, Qianlian and Kunlun orgen can be subdivided into two stage:248～234Ma,220～214Ma. The igneous rocks formed in the early stage result from the northern subduction of the Palaeo-Tethys ocean. However, the magmatic rocks belonging to the late stage were formed under the Postcollisional extension setting.