Late Paleozoic Tectonic-magmatic Evolution Of The Eastern Jiamusi Massif

together with their high Th/U ratios,suggests a magmatic origin.Zircon LA-ICP-MS U-Pb dating result indicates that three approximately NNE-oriented magmatic rock belts mainly distributes within the eastern margin of the Jiamusi Massif,i.e.,the late Carboniferous-middle Permian?305-259 Ma?acid intrusive rock belt,the early-middle Permian?290-274 Ma?basic intrusive rock belt,and the early-middle Permian?290-267Ma?volcanic belt,all of which are sub-parallel to the Yuejinshan accretionary complex belt.The late Paleozoic acid intrusive rocks in the eastern margin of the Jiamusi Massif consist of alkali feldspar granite,syenogranite,monzogranite,granodiorite,graphic granite and granite porphyry.Based on their petrological and geochemical features,these granites can be divided into two groups as follows:?1?weakly peraluminous high-K calc-alkaline I-type granitoids:they have high SiO2?64.98–72.90 wt.%?and K2O?3.08–4.70 wt.%?contents,and are enriched in large ion lithophile elements?LILEs;e.g.,Rb,Th,and U?and light rare earth elements?LREEs?with negative Nb,Ta,and Ti anomalies,which collectively indicate affinities of subduction-related magmas;?2?aluminous A-type granites:they are characterized by high alkalis?Na₂O+K₂O=8.22–8.90 wt.%?,low MgO?0.04–0.09 wt.%?and P2O5?0.01–0.04 wt.%?contents,high Zr and Nb contents,and high10,000×Ga/Al ratios.All the magmatic zircons in these two groups of granitoids have great variations of?_Hf?t??-5.6⁺8.5?and two-stage Hf model ages(T_DM2)of 0.8–1.7 Ga,suggesting that their precursor magmas originated from a heterogeneous source that dominantly involved juvenile crustal components,coupled with the addition of ancient crustal material.In addition,the aluminous A-type granite magmas were probably derived by low degree partial melting of a felsic crustal source in relative shallow levels,whereas the other I-type granitic magmas probably resulted from partial melting of a mafic lower crustal source.The late Paleozoic basic intrusive rocks from the eastern margin of the Jiamusi Massif comprise mainly?hornblende?gabbro,and subordinate plagioclasite and plagiogranite.The gabbroic rocks belong to sub-alkaline tholeiitic basalt series rocks,are enriched in LREEs and LILEs?e.g.,Rb,Ba,Sr?,depleted in HFSEs?e.g.,Nb,Ta,Zr,Hf?,and exhibit positive zircon?_Hf?t?values?+3.1⁺13.8?.In addition,they have low Sr/Y in the spinel facies mantle,whereas the source of the E-MORB was a DMM source significantly enriched by OIB-type components.In contrast,the alkaline basalts were generated from an enriched OIB-type mantle source in the garnet facies and continued melting to spinel facies mantle depths.Therefore,the mafic volcanic rocks of the Yuejinshan Complex,located above the oceanic plate of the Paleo-Pacific Ocean,were most likely derived from chemically heterogeneous mantle sources during back-arc basin spreading and plume-related volcanism.These data,together with previously reported data and the nature of various rock types along the eastern part of the Jiamusi Massif,suggest that the Late Paleozoic intrusive rocks were formed in an active continental margin setting,probably as a result of the westwards subduction of the Paleo-Pacific oceanic lithosphere.Possible causative tectono-magmatic processes include:?1?at 305–290 Ma,an oceanic slab was subducted to the west beneath the Jiamusi Massif,generating continental arc-type magmatism through the underplating of the basaltic magma;?2?at 290–260 Ma,the subducted oceanic slab broke off,resulting extension allowed the formation of A-type granites and bimodal magmatism and eventually the subduction was stopped.The westwards subduction of the Paleo-Pacific oceanic lithosphere during the late Carboniferous to middle Permian resulted in the back-arc lavas,seamounts,and other oceanic fragments accreting onto the eastern Jiamusi Massif,forming the Yuejinshan Complex.