Petrogenesis Of The Cretaceous Intermediate-basic Bolcanic Rocks In The Eastern Mo He Basin And Its Geological Implications

This theisis deals with a suite of intermediate-basic volcanic rocks of the eastrn Mohe Basin in the norther Great Xing’an Range.Systematic studies have been conducted on petrology,zircon LA-ICP-MS U-Pb geochronology,whole-rock major and trace geochemistry,in order to 1)determine the formation age of the intermediate-basic volcanic rocks,2)analyze their Petrogenesis and orgin of the magma source,and 3)discuss the spatial–temporal feature of the Early Cretaceous igneous rocks and their possible tectonic setting.Acoording to all the exsiting regional data,we preliminarily establise Mesozoic tectonic evolution history for the norther Great Xing’an Range.Zircon LA-ICP-MS U-Pb dating indicates that the intermediate-basic volcanic rocks of the eastrn Mohe Basin formed at a period between 120Ma-118Ma,which,together with the stratigraphical and paleontological data as well as the observed field contact relationships,suggesting they are results of the late Early Cretaceous magamtism.The Early Cretaceous intermediate-basic volcanic rocks are composed of basalts,basaltic andesites,and andesites.Geochemical data shows that these volcanic rocks are enriched in enriched in large ion lithophile elements and light rare earth elements,and depleted in high field strength elements and heavy rare earth elements.These characteristics along with the fact that the published positive zirconε_Hf（t）values of the coeval intermediate-basic magmatic rocks,indicating that the primary magma of the Early Cretaceous intermediate-basic volcanics was derived from the partial melting of a depleted mantle that was modified by slab-derived fluids.Moreover,The decoupling between zircon Hf isotope and trace element characteristics signifies that the lithospheric mantle source was indeed metasomatized（and thus enriched in incompatible elements）just briefly before the formation of the volcanic rocks.On the basis of rock association,geochemical feature,and published regional information,it is proposed that the large scale of Early Cretaceous magmatism within the Mohe Basin of the Erguna Block taken place in a lithospheric extensional setting probably associated with the closure of the Mongol–Okhotsk Ocean.