The Geological Records And Geochemical Constraints On The Late Paleozoic Continental Margin Tectonic Evolution Of The Jiamusi-Khanka Block

The Jiamusi-Khanka Block is located at the eastern margin of Eurasia and occupies the key area where the superposition and transition between the Paleo-Asian Ocean and Paleo-Pacific Ocean occurred.Due to the subduction of paleo-oceanic plate,the Jiamusi-Khanka Block has undergone significant geological events,such as the tectonic transition from the passive continental margin to the Andean continental margin and the transition from the Andean continental margin to the western Pacific trench-arc-basin system.However,the specific timing of tectonic transition in the eastern margin of Eurasia is still unclear,and the tectonic setting of the transition remains controversial.In this study,to better understand the timing and tectonic setting of the transition,we report on a new combined study of petrological,geochemical and geochronological data of the volcanic-sedimentary rocks from both east and west margins of the Jiamusi-Khanka Block.The new results will provide key evidence for the timing of the transition from passive to active continental margin in the eastern margin of Eurasia,and then provide further insights into the tectonic setting of the superposition and transition between the Paleo-Asian Ocean and circum Pacific Ocean.The sedimentary cover of Jiamusi-Khanka Block is dominated by the Late Paleozoic-Mesozoic stratigraphic sequence at the eastern margin,which has the feature of transition from the shallow-sea facie to the continental facies.Based on the relationships of the stratigraphic sequence in adjacent areas but lack of accurate geochronology and geochemical study,previous studies divide them into,from bottom to top,Early-Middle Devonian Heitai Formation,Middle Devonian Laotudingzi Formation,Late Devonian Qilikashan Formation,Early Carboniferous Beixing Formation,Late Carboniferous Guangqing Formation,Late Carboniferous-Early Permian Zhenzishan Formation,Early Permian Erlongshan Formation and Early Triassic Nanshuangyashan Formation.In this study,we report on a combined study of zircon U-Pb dating and whole rock geochemical analysis of the sandstones from the Heitai Formation,sandstones from the Zhenzishan Formation and andesites from the Erlongshan Formation.The results show that the Heitai Formation sandstones have the maximum depositional age of Late Devonian(374±3 Ma)and were formed in a passive continental margin setting,the Zhenzishan Formation sandstones have the maximum depositional age of Late Carboniferous(308±3 Ma)and were formed in an active continental margin setting,and two Erlongshan Formation andesite samples have the ages of 284±2 Ma and 286±4 Ma,respectively,and belong to the medium-K calc alkalic series,enriched in the LREEs((La/Yb)_N=5.0-7.1)and fluid-mobile elements(such as Ba,Th,Sr and U),depleted in the HREEs(LREE/HREE=6.4-7.3)and HFSEs(such as Nb,Ta,Zr and Y),and with no obvious Eu anomaly(Eu/Eu*=0.87-1.02),indicative of an island arc setting.All the results above suggest that the tectonic transition from a passive continental margin to an active continental margin at the east Jiamusi-Khanka Block took place during Late Devonian to Late Carboniferous and island arc magmatism occurred in Early Permian.Combined with the abundant previous studies of the island arc magmatic rocks with ages of 274-302 Ma along the eastern margin of Jiamusi-Khanka Block,we identify an Early Permian island arc magmatic rock belt.The stratigraphic units at the western margin of the Jiamusi-Khanka Block are dominated by the Huangsong Group in the Dongning area and Qinglongcun Group in the Wolong area.To better understand the tectonic transition at the western margin,we report on a new combined study of zircon U-Pb dating and whole rock geochemical analysis of the Yanwangdian Formation sandstones and Shuangqiaozi Formation andesites from the Huangsong Group and the biotite plagioclase gneisses within the Qinglongcun Group.The results show that the Yanwangdian Formation sandstones have maximum depositional age of Late Carboniferous(299±3 Ma)and were formed in a passive continental margin setting,two Shuangqiaozi Formation andesites have the ages of 265±4 Ma and 267±3 Ma,respectively,and belong to the medium to low-K calc alkalic series,enriched in the LREEs((La/Yb)_N=1.92-3.03)and fluid-mobile elements(such as Sr and U),depleted in the HREEs(LREE/HREE=2.76-3.77)and HFSEs(such as Nb,Ta and Ce),and with no obvious Eu anomaly(Eu/Eu*=0.89-1.38),indicative of an island arc setting.The biotite plagioclase gneiss samples from the Qinglongcun Group have the protolith ages of 262±3 Ma,262±2 Ma,262±2 Ma and254±3 Ma,respectively,the protoliths belong to the calc alkalic series rhyolite with the features of enrichment in LREEs((La/Yb)_N=4.71-8.83)and fluid-mobile elements(such as Rb,Th and U),depletion in HREEs and HFSEs(such as Nb,Ta and Ce)and obvious negative Eu anomalies(Eu/Eu*=0.48-0.68),indicative of an island arc setting.All the results above suggest that the tectonic transition from a passive continental margin to an active continental margin at the west Jiamusi-Khanka Block occurred during Late Carboniferous to Late Permian and island arc magmatism occurred in Late Permian.Combined with the abundant previous studies of the island arc magmatic rocks with ages of 250-278 Ma along the western margin of Jiamusi-Khanka Block,we identify a Late Permian island arc magmatic rock belt.In combination of the results above with the previous studies on the Nadanhada accretionary complex and Ji-Hei high pressure metamorphic belt,we build a new tectonic model for the tectonic transition and evolution of the paleo-oceanic domains in the eastern Eurasia during the late Paleozoic:(1)during Late Devonian to Late Carboniferous,as a part of the Central Asian Orogenic Belt(CAOB),the Jiamusi-Khanka Block was mainly affected by the subduction/closure of the Paleo-Asian Ocean,and followed by an initial back-arc extension setting in the west Jiamusi-Khanka Block.In the Late Carboniferous,due to the subduction of the Mongolia-Okhotsk Ocean or the Panthalassa(the predecessor of Paleo-Pacific Ocean)which further provided driving force for the back-arc extension,enhanced the back-arc extension in the west margin and resulted in the rifting of Jiamusi-Khanka from the CAOB and the opening of Heilongjiang Ocean.At the same time,the eastern margin of Jiamusi-Khanka Block transferred from a passive continental margin to an active continental margin setting and was followed by the Early Permian large-scale island arc magmatism.(2)During Late Carboniferous to Late Permian,due to the eastward subduction of Heilongjiang Ocean beneath the western margin of Jiamusi-Khanka Block,a tectonic transition from a passive continental margin to an active continental margin setting and Late Permian island arc magmatism occurred along the western margin.(3)During Late Permian to Late Triassic,the subduction of Paleo-Pacific Ocean caused the emplacement of Nadanhada accretionary complex.In the meantime,Heilongjiang Ocean gradually closed and the Jiamusi-Khanka Block rejoined to the CAOB.The combined efforts of the subduction of Paleo-Pacific Ocean and the closure of Heilongjiang Ocean accounted for the final emplacement of Ji-Hei high pressure metamorphic belt,marking the tectonic transition from Paleo-Asian Ocean to the Paleo-Pacific in eastern Eurasia finished.