Petrogenesis and tectonic implications of early Paleozoic granitic magmatism in the Jiamusi Massif, NE China: Geochronological, geochemical and Hf isotopic evidence

摘要

This paper reports geochronological, whole-rock geochemical and zircon Hf-isotopic data for the early Paleozoic granitic rocks in the Jiamusi Massif in the eastern segment of the Central Asian Orogenic Belt (CAOB), in order to investigate their precise geochronological framework, petrogenesis, sources and tectonic setting. LA-ICP-MS zircon U-Pb age data indicate that the syenogranite, monzogranite and grano-diorite were emplaced during the period of 530-484 Ma. Geochemically, these granitoids have high SiO_2 (64.66-79.17%) and K_2O (3.08-7.33%), low MgO (0.14-2.63%) and CaO (0.37-3.87%), with A/CNK and δEu values of 0.89-1.10 and 0.16-1.77, respectively. These rocks are characterized by enrichment in Rb and Nd, and depletion in Nb, Ta, P and Ti. In addition, in-situ Hf isotopic analyses of zircons from the granitic rocks reveal that they have ε_(Hf)(t)=-5.8 to +2.3, with two-stage Hf model ages (T_(DM2)) varying from 1.3 Ga to 2.4 Ga, indicating that they probably originated from the partial melting of a dominantly "old" Paleo-Mesoproterozoic crustal source. Additionally, these granites have variable major and trace element concentrations. Magmatic zircons from these rocks record consistent homogeneous U-Pb ages but have heterogeneous ε_(Hf)(t) values, reflecting that they resulted from fractional crystallization and crustal assimilation. Based on these geochemical data combined with regional geological investigations, we propose that the early Paleozoic granitic magmatism in the study area occurred in a post-collisional extensional setting, which was probably related to the collapse of a thickened crust.