胶北地体新太古代晚期石英闪长质片麻岩成因及其动力学意义OA

The origin of early continental crust is a hot issue in early Precambrian research.Since the intermediate average composition of the continental crust,unraveling the petrogenesis of intermediate rocks and the processes of crust-mantle interaction is key to understand the formation of continental crust and related geodynamic regimes.In this study,the late Neoarchean quartz dioritic gneisses exposed in the Jiaobei terrane of North China Craton were selected as the research object,and the zircon U-Th-Pb-Lu-Hf isotopes and whole-rock major and trace elements were analyzed.Based on these data,comprehensive studies of the spatial and temporal distribution,geochemical characteristics,and crust-mantle interaction were conducted to decipher the origin mechanism of these intermediate magmas.The results show that the late Neoarchean quartz dioritic gneisses of the Jiaobei terrane are mainly exposed in the northeastern part of Qixia,Majiayao,Huangyandi,Shilizhuang,and northern Dachai,with crystallization ages of 2563～2500 Ma.Based on distinct petrographic characteristics and magmatic crystallization ages,quartz dioritic gneisses are further divided into>2.55 Ga and<2.55 Ga groups.The>2.55 Ga quartz dioritic gneisses show hornblende as the major mafic minerals,with depleted zircon εHf(t2)values.They have relatively low SiO2(mostly of 54.59%～59.27%),Mg#(37.59～50.51),and Sr+Ba(357×10-6～1166×10-6),belonging to the low-to medium-K series.Meanwhile,they show moderately fractionated chondrite-normalized rare earth elements(REE)patterns with low(La/Yb)N(3.92～7.12).In comparison,the<2.55 Ga quartz dioritic gneisses contain less volume of mafic minerals that are dominated by biotite,with less depleted zircon εHf(t2)values.These samples exhibit relatively high SiO2 contents(55.48%～63.54%),Mg#(45.43～63.24),and Sr+Ba(866×10-6～3480×10-6),belonging to the medium-to high-K series.In addition,they have strongly fractionated chondrite-normalized REE patterns with high(La/Yb)N(10.84～65.67),which are chemically analogous to those of Archean sanukitoids.Detailed petrogenetic studies reveal that the>2.55 Ga quartz dioritic gneisses were derived from the partial melting of the depleted mantle weakly modified by the metasomatism of basaltic crustal melts.In contrast,the<2.55 Ga quartz dioritic gneisses were formed by the partial melting of a less depleted mantle that was strongly metasomatized by sediment melts.Taken together,the late Neoarchean quartz dioritic gneisses in the Jiaobei terrane record crust-mantle interactions involving the recycling of crust-derived materials and mantle metasomatism,which could mark an important episode of crustal growth that was likely evolved under a plate subduction setting during the late Neoarchean.