Geochronology And Petrogenesis Of Granitoids In Yichun-Hegang Area, Heilongjiang Province

The Yichun-Hegang area lies within the Songnen block at the eastern segment of the Xing’anMongolian orogenic belt between the Siberian plate and the North China plate. This area saw theevolution of both the Paleo-Asian Ocean tectonic and the Pacific tectonic. The accuraterecognition of age and origin of the granite in this area can play an important role in discussingthe tectonic evolution of the Northeast of China. Therefore, it’s very significant to discuss indetail on the petrography, geochronology, geochemistry and zircon Hf isotopic characteristics ofthe granite. According to data in this paper, the author determine the ages and stages of the graniteformed in different period, discuss their petrologic and geochemical characteristics, as well astheir the tectonic background.1. Rock association, geochronology and the stages of the the granitic rockThe granite batholith, to the east of the Fengmao Forest Farm, formed at the Late Permianperiod and includes predominantly a series of monzogranite with Phase relationship and differenttexture. The monzonitic granite was intruded by the Early-middle Triassic granodiorite andmonzogranite (244~234Ma), as well as the Late Triassic granite porphyry (210Ma). Granites tothe west of the Fengmao Forest Farm mainly include medium to coarse grained monzogranite andporphyritic biotite monzogranite emplaced in the Late Triassic (211~210Ma).According to the zircon U-Pb dating and existing geochronologic data, the graniticmagmatism in Yichun-Hegang region can be grouped into such three stages as middle-late Permian (266~259Ma), early-middle Triassic (244~231Ma), and late Triassic (222~200Ma).2. Geochemistry of the graniteGeochemical characteristics of the granitic rocks show that all the three stages of middle-latePermian,early-middle Triassic and late Triassic granites belong to the high-K calc-alkaline series.Most samples are met aluminous to slightly per aluminous with A/CNK values of not more than1.1, and the P2O5content decreased as increasing of SiO2content, their (87Sr/86Sr)ivary in a widerange from0.7052to0.71332, suggest that these granites mostly belong to I-type granites, andindividual rocks have chemical characteristics that are transitional between I-and S-type granites.These three stage of granites are of similar trace elements composition and commonly depleted inNb, Ta, Sr, P, Ti. However, they present obviously different REE patterns and Sr, Yb contents. Themiddle-late Permian granitoid belongs to the Himalayan-type granite; the early-middle Triassicgranitoid has higher contents of Sr, lower contents of Yb and belongs to adakite; the Late Triassicgranitoid plots in the Zhejiang and Fujian-Nanling type granites field.3. Petrogenesis and tectonic setting of the graniteThree stage granites have the common or similar magma source. The Lu-Hf isotopic datashow that εHf(t) values of middle-late Permian, early-middle Triassic, and late Triassic granitesrange from-12.93to5.71, from-8.07to3.34, and from-7.74to3.61, respectively. The changesof εHf(t) values (between positive and negative) reflect that there are at least two different magmasource region. That the εNd(t) values change from-7.1to2.1, together the fact that the (87Sr/86Sr)iratios of the late Triassic granite between is from0.7052to0.71332and mostly between-7.1~2.1, suggest the mixture of ancient material and the new basaltic crust. Based on the simulationresults and previous research data, the author concludes that the magma was mainly derived fromnew the basaltic crust, and with contribution of old crustal material, but the mixture proportion ofold material are different in different period of granites.Comprehensive studies indicate that the middle-late Permian granite was related to collisionof the North China plate with the Songnen block. The early-middle Triassic granite was originatedfrom the thickened crust and formed in the orogenic process post-collision of different plates. The late Triassic granite was resulted from continuous collision and extension after closure of thePaleo-Asian Ocean. The geological evolution process of this area, including collision-inducedcrustal thickening and orogeny, subsequent lithospheric extension as well as orogenic beltcollapse can best be interpreted from the three stages of granites.