Petrogenesis And Tectonic Implications Of Early Paleozoic Granites In The Eastern Jiamusi Massif

The Jiamusi Massif, located between the North China and Siberia cratons, is the keypart of the Central Asian Orogenic Belt (CAOB), also has been regarded as the mostfamous metamorphic terrane in the Xing’an-Mongolian Orogenic belt. As with the wholeCAOB, the Jiamusi Massif underwent complex multi-phase and multi-stage tectonicevolution and yielded different types of granitoids. New zircon U–Pb ages show that thesegranitoids were mostly emplaced during the late Paleozoic instead of the Proterozoic.However, the temporal-spatial distributions, rock association and petrogenesis of the earlyPaleozoic granitoids still remain unknown, a further research is highly required.This paper reports geochronological, whole-rock geochemical and zircon Hf-isotopicdata for the early Paleozoic granitic rocks in the Jiamusi Massif in the eastern segment ofthe CAOB, in order to investigate their precise geochronological framework, petrogenesis,sources and tectonic setting. LA–ICP–MS zircon U–Pb age data indicate that thesegranitoids were emplaced during the period of530~484Ma. Geochemically, the graniteshave SiO2=64.66%~79.17%, K2O=3.08%~7.33%, MgO=0.14%~2.63%, CaO=0.37%~3.87%, with A/CNK and u values of0.89~1.10and0.16~1.77, respectively.These rocks are characterized by enrichment in Rb and Nd, and depletion in Nb, Ta, P andTi. In addition, in-situ Hf isotopic analyses of zircons from the granitic rocks reveal thatthey have εHf(t)=-5.8to+2.3, with two-stage Hf model ages (TDM2) varying from1.3Gato2.4Ga, indicating that they probably originated from the partial melting of a dominantly “old” Paleo-Mesoproterozoic crustal source. Additionally, these granites have variablemajor and trace element concentrations. Magmatic zircons from these rocks recordconsistent homogeneous U–Pb ages but have heterogeneous εHf(t) values, reflecting thatthey resulted from fractional crystallization and crustal assimilation.In combination with the regional geological investigations, it clearly indicates anexistence of large-scale Late Pan-African magmatism in the Jiamusi Massif, which lastedfor~50Ma. The early Paleozoic granites in the study area belong to the post-collisionalgranite of crustal origin. The formation of which was probably related to the collapse of athickened orogenic belt, indicating a post-collisional extensional setting in the JiamusiMassif during the early Paleozoic.