Petrogenesis And Tectonic Significance Of Late Cretaceous Granitic Rocks In The Tongjiang-Fuyuan Area, Heilongjiang Province

We present zircon U-Pb ages, Hf isotope data and whole-rock major and traceelement data for the “late indosinian” granitoids in Tongjiang-Fuyuan area,northeastern Heilongjiang Province. On the basis of detailed field geologicalinvestigation and interior petrography, in combination with region geology, wediscuss the formation time, nature of magma source, tectonic setting of thesegranitoids.These granitoids, outcropping in Jiejinkou, Qindeli and Fuyuan areas, mainlyconsists of biotite and/or amphibole-bearing granodiorite and granodiorite-porphyry.They have similar mineral composition and belong to “I-type granite” on the basis ofthe widespread occurrence of amphibole.The zircons from these selected eight samples are euhedral-subhedral in shape, andcommonly display oscillatory zoning in CL images. These patterns, combined withthe high Th/U ratios ranging mainly from0.31to0.83, suggest that the zircons are ofmagmatic origins. Thus their ages represent the crystallisation timing of these rocks.The dating results indicate that these granitoids formed during the Late Cretaceous(89.5~94.8Ma).Geochemically, these granitoids are metaluminous to weak peraluminous andmiddle to high-K calc-alkaline rocks, and are enriched in SiO2and K2O and depletedin TFe2O3, MgO, CaO and transition elements as a whole, implying that their primarymagma could be derived from partial melting of mafic rocks within crust. Thecontents of major oxides in different samples, comparing with those of SiO2, show theconsistent variations, implying that these granitoids evolved from common primarymagma. They are all enriched in light rare earth elements and large ion lithophile elements (e.g., Rb, K), and depleted in heavy rare earth elements and high fieldstrength elements (e.g., Nb, Ta, and Ti); they all exhibit weakly negative Eu anomalies;and they have Th/U ratios of4.22~9.53, with a average value of6.10, characterizedobviously by the crust source. Combining with zircon εHf(t) values (+4.45~+9.01)and the TDM2ages (583~875Ma), we consider that their source rocks are mafic, andthat their primary magma could be derived from partial melting of Neoproterozoicnewly accreted mafic materials within lower crust.Combined with the coeval igneous rock associations and their spatial distribution,as well as regional geology, the formative tectonic setting of these granitoids has beendiscussed. In summary, we conclude that the Late Cretaceous granitoids, located inTongjiang-Fuyuan area, were emplaced in an active continental margin setting relatedto the subduction of the Paleo-pacific plate beneath the East Asia continent.