The Xinxing Formation From Zhangguangcai Range Group In Eastern Heilongjiang Province: Rock Association, Geochronology And Tectonic Implications

Zhangguangcai Range Group in eastern Heilongjiang province is distributed in the eastern margins of the Songnen-Zhangguangcai Range Block, which is located in the eastern segement of the Mongolia-Hinggan Orogen Belt and adjacent to the Jiamusi Block separated by the Mudanjiang Fault. The chronology of the Zhangguangcai Range Group has been a controversial issue. The formation time is believed in the Silurian or Neo-proterozoic on the basis of field geology and fossil evidence.The study area is situated between the Songnen-Zhangguangcai Range and Jiamusi blocks and experienced the modification of later tectonic actions. Thus, the issues about the association, geochronology and petrogenesis of the Zhangguangcai Range Group have been not solved. In the present study, detailed studies of petrology, chronology and geochemistry are carried out for the Xinxing Formation from the Zhangguangcailing Group in order to reveal their formation time and tectonic setting as well as the tectonic evolution between the Songnen-Zhangguangcai Range and Jiamusi blocks.1 Rock associations and chronology of the Xinxing FormationPetrographic study indicates that the previously believed biotite-bearing plagioclase gneiss from the Xinxing Formation is mainly composed of tonalite, monzogranite, and granodiorite with weakly gneissic structure, as well as minor biotite quartz schist.Detrital zircons from biotite quartz schist in the Xinxing Formation yield age populations at 284, 352, 385, 413, 443, 483, 517, 540, 581, 651, 1018, 1449, 1824, 2155, 2465, 2543 Ma, implying that the maximum timing of deposition of the Xinxing Formation is 284 Ma(Early Permian). LA-ICP-MS zircon U-Pb dating results indicate that the tonalite, monzogranite, and granodiorite formed in the Late-Ordovician (450±6 Ma, 443±3 Ma), the middle of Silurian (425±3 Ma, 424±2 Ma), and the early permian (281±2 Ma), respectively.2 Geochemistry of granitoids form the Xinxing Formation2.1 Early Paleozoic granitoidsThe Early paleozoic granitoids are composed of the Late-Ordovician tonalite and the middle of Silurian monzogranites.The late-Ordovician tonalites have SiO₂ = 59.65%⁷0.61%, MgO = 0.56% 1.23%, and are characterized by enrichment in light rare earth elements (LREEs) and large ion lithophile elements (LILEs), depletion in heavy REE (HREEs) and high field strength elements(HFSEs, such as Nb, Ta, Ti), and obviously negative Eu anomalies (δEu=0.50⁰.70), similar to the volcanic rocks from island arc and/or active continental margin settings.Compared with the late-Ordovician tonalites, the middle Silurian monzogranites have relatively high SiO₂ (73.51%⁷5.69%) and total alkali contents (Na₂O+K2O=7.18⁸.35%) as well as low MgO (0.17%⁰.27%). The monzogranites have lower REE abundance (∑REE=53.17¹85.14 (×10-6)) and are characterized by enrichment in LREEs and LILEs, depletion in HREEs and HFSEs such as Nb, Ta, Ti, and obviously Eu anomalies (δEu=0.64¹.92), similar chemically to the coeval granitoids in the study area.2.2 Early Permian granodioritesThe early Permian granodiorites belong chemically to the peraluminous and are characterized by enrichment in total alkali. They have SiO₂ = 61.27%⁶3.03%, TiO₂= 0.18% 0.52%, MgO = 0.22% 1.45%, Fe₂O₃ = 5.44% 6.44%. Their REE abundances are∑REE=160.98¹90.50 (×10-6) and are characterized by enrichment in LREEs and LILEs, depletion in HREEs and HFSEs such as Nb, Ta, Ti, and obviously negative Eu anomalie (δEu=0.78⁰.92), similar to the coeval granitoids in the study area.3 Tectonic settings of formation of granitoids from the Xinxing Formation and regional tectonic evolution3.1 The early Paleozoic granitoids Firstly, the early Paleozoic granitoids mainly consist of tonalite and monzogranite association. Chemically, they are similar to igneous rocks from active continental margin setting.Secondly, in the K₂O vs. Na₂O diagram, these early Paleozoic granitoids, including the late-Ordovician tonalites and middle Silurian monzogranites belong to the"I-type"granites.Third, these early Paleozoic granitoids could form under the tectonic setting of the subduction zone in the tectonic setting discrimination diagrams.Therefore, together with Early-Middle Ordovician calc-alkaline volcanic rocks (including basalt, andesite, dacite and rhyolite) in the stduy area, it is suggested that the early Paleozoic granitoids from Xinxing Formation could form under the active continental margin tectonic setting, which could be related to the westward subduction of oceanic plate between the Jiamusi and the Songnen-Zhangguangcai Range blocks. The appearance of A-type granites could represent, together with detrital zircons with ages of about 500Ma, implies that the collision and amalgamation of the Songnen-Zhangguangcai Range and Jiamusi blocks had finished in the end of Silurian.3.2 The Early Permian granodiorites The Early Permian granitoids from the Xinxing formation are granodiorites. Geochemically, they belong to'I-type'granites. Combined with the coeval calc-alkaline vocanic rocks occurred in the eastern margin of the Jiamusi block, we conclude that these early-middle Permian formed under an active continental margin tectonic setting, i.e., the subduction of the Paleo-Asian oceanic plate beneath the Jiamusi block.3.3 Paleozoic tectonic evolution between the Songnen-Zhangguangcai Range and Jiamusi blocksIn Early Paleaozoic, existence of tonalite, monzogranite, together with the coeval calc-alkaline volcanic rocks, indicate that the subduction of the oceanic plate beneath the Songnen-Zhangguangcai Range block (ca 450Ma) as well as collision of the Jiamusi and Songnen-Zhangguangcai Range blocks (ca 425Ma).In the Permian, the occurrence of granodiorites with'I type'granites, together with the coeval calc-alkaline volcanic rocks located in the eastern margin of the Jiamusi Block, implies that the westward subduction of the Paleo-Asian oceanic plate might have happened in the early Permian.