Petrology, Geochronology And Geochemistry Of Rena-Co Volcanic Rocks Along Southern Margin Of Qiangtang Terrane, Tibetan Plateau

The direction of subduction of Bangong-Nujiang ocean crust has long been a debatein the topic of the geological evolution history of Tibetan Plateau. Priveous studies showthat large amount of Mesozoic igneous activity spreads widely on northern Lhasa terrane,and associated with the model of southward subduction of Bangong-Nujiang ocean crust.However, small magmatic activities with the same age were also discovered on the northside of the suture zone. This thesis presents systematic petrology, geochemistry andisotopic geochronology research of Rena-Co volcanic rocks north of Bangong-Nujiangsuture.The Rena-Co volcanic Rocks are consisted of a series of dacite, olivine basalt andrhyolite, which are lacking in intermediate rocks. Zircon U-Pb isotopic dating shows thatthere were two major period of volcanic activity in Rena-Co area. The dacite crystallizedat the age of ca.154Ma, late Jurassic, and the rhyolite was erupted at the age of ca.111Ma, early Cretaceous, which makes bimodal igneous tectonic assemblage together witholivine basalt. Most of the olivine phenocysts of basalt are chrysolites, intergrow withChromium-enriched spinel, and most of the plagioclase phenocysts of rhyolite haveoscillatory zones. Whole rock geochemistry analysis shows that Rena-Co volcanic rocksare within metaluminous medium-K calc-alkaline seris with enrichment of large lithophileelements such as Rb, Ba, Th and depletion of high field strength elements, such as Nb, Ta,Ti. All the samples shows light rare earthelement-riched feature. The dacite and rhyoliteare adakite-like with high Sr and low Y signature. Tectonic setting discrimination diagramshows that the basalt is within plate type. The dacite samples display depleted whole rockSr-Nd and zircon Hf isotopic feature while the rhyolite and basalt samples shows variousεNd(t) and strongly positive zircon εHf(t) values, which indicates rhyolite has mantle-crustmixed Hf isotopic composition.Rena-Co volcanic rocks are related to the subduction of Bangong-Nujiang oceaniccrust during late Jurassic to early Createous. The dacite which firstly reported in thisthesis is mainly part of the island-arc-magma related to the northward subduction of theocean crust in Jurassic. Associated with the recent research of magma activity of Northern Lhasa terrane, the petrogenesis of~111Ma rhyolite and basalt is possibly due to theupwelling of mantle source materials and the underplating of the newly-formed lowercrust, which are under the frame of the break-off of the southward subductedBangong-Nujiang oceanic crust.