Study on Petrogenesis of Zircons from the Danzhu Granodiorite and Its Geological Implications

This paper presents a systematic study on morphology,geochemistry and geochronology of a zircon population from the Danzhu granodiorite located in the southwest region of the Zhejiang province.The analysis results show that there are two types of zircons from the Danzhu granodiorite:magmatic zircon and metamorphic zircon.The former is characterized by euhedral—subhedral and long-prismatic form,incolorness,transparence,presence of magmatic and mineral inclusions,homogeneous internal structure(occasionally with oscillatory zoning)on the section of grains revealed by backscattered electron(BSE)imaging,and analogue pattern of typological indices of zircon for those from the calco-alkaline granodiorite;the latter is characterized mainly by its occurrence as overgrowth on magmatic zircon,but it also occurs very occasionally as anhedral grains,which is yellow-brown,semi-transparent,and homogeneous on the BSE imaging.These two types of zircon differ also in trace element composition,i.e.,the magmatic zircon is significantly richer in the substituent elements like as U,Th and Y than the metamorphic zircon.The zircon U-Pb dating using LA-ICPMS shows that the magmatic zircon and metamorphic zircon were formed in two different periods:at 1875±33 Ma and at 209±12 Ma respectively,demonstrating the objective response for the Zhe-Min(Zhejiang—Fujian)movement and the Indosinian movement in the southwest region of the Zhejiang province.