Late Ordovician-Late Triassic Tectonic Evolution Of Faku Area In The Eastern Segment Of The Northern Margin Of The North China Craton

Massive data have been reported for the Paleozoic–Early Mesozoic magmatic rocks,which are related to subduction–accretion–closure of the Paleo-Asian Ocean?PAO?,to the south of Solonker–Xar Moron–Changchun–Yanji Suture?SXCYS?along the northern margin of the North China Craton?NCC?.However,as the SXCYS is covered eastwards by the Meso-Cenozoic Songliao Basin,there have not been any relevant reports on systematical and entire Paleozoic-early Mesozoic magma evolution of the area.The Faku area,which belongs to the covered area of the Songliao Basin in the eastern segment of the northern margin of the NCC,has suffered from evolution of the PAO Tectonic Domain since the Plaeozoic era and is a key area for studying subduction–closure of the PAO.In this study,taking Paleozoic–Early Mesozoic magma in the Faku area as the object and combining with previous chronological data and zircon U–Pb dating results of this study,Paleozoic–Early Mesozoic magmatism of the study area is divided into six phases:Cambrian–Late Ordovician?500 Ma⁴38 Ma?,Silurian?438 Ma⁴15 Ma?,Early Permian?293 Ma²74 Ma?,Middle–Late Permian?270²55 Ma?,Late Permian–Middle Triassic?255²45 Ma?,and Middle–Late Triassic?242 Ma²15 Ma?.With rock assemblage,geochemistry and Lu–Hf isotope information,their magma sources and tectonic environment were discussed.Combined with regional magmatic rock assemblages,spatial–temporal characteristics and dynamic setting,etc.,the Paleozoic–Early Mesozoic tectonic evolution history of the covered area of the Songliao Basin in the eastern segment of the northern margin of the NCC was discussed.Late Orodovian magmatic activity?458 Ma?is dominated by intermediate volcano eruption,displays Si-poor and alkali-poor signatures,and is of mid-K calc-alkaline–low-K tholeiitic series.Ba and Sr?LILEs?,Th?HFSE?and LREEs are comparatively enriched,while Nb,Ta and Hf?HFSEs?and HREEs are depleted.?_Hf?t?and two-stage model ages range from+3.80⁺12.37 and 727.7¹497.1 Ma,respectively.The initial magma of Late Ordovician metamorphic andesite was sourced from partial melting of lithospheric mantle that had been affected by subducting fluid replacement and formed in an intra-oceanic arc environment,indicating the existence of subduction inside the PAO in the Late Ordovician.Early Silurian magmatic activity?437⁴32 Ma?is dominated by intermediate–acidic magma intrusion,and the lithologies are represented by tonalite and granodiorite and of metaluminous-weakly peraluminous,mid-K calcalkalic–low-K tholeiitic granites;they are relatively enriched in Rb,Ba,K and Sr?LILEs?,Th?HFSE?and LREEs,and depleted in Nb,Ta,P,Ti?HFSEs?and HREEs.The geochemical signatures are consistent with the adakite.?_Hf?t?values and two-stage model ages range form+0.29⁺6.56 and1001.8¹394.1 Ma respectively.These lithologies are resulted from underplating of K-depleted basaltic mantle-sourced magma in the bottom of Meso–Neo proterozoic lower crust from active continental margins.Combined with regional geologic events,it is believed that the orogeny of the eastern segment of the northern margin of the NCC occurred during this period and Early Silurian granitic plutons were possibly formed by arc–continent?arc–arc?collision between the intra-oceanic arc and the NCC during southwards subduction of the PAO.Early Permian magmatic activity?283²76 Ma?is represented by monzonitic granite and syenogranite,which show high Si and alkali signatures and are of weakly to strongly peraluminous,mid-K–high-K calcalkalic A-type granite.The rocks are enriched in Rb and K?LILEs?,Th?HFSE?,and LREEs,while depleted in Nb,Ta,P and Ti?HFSEs?,and HREEs;they are formed in a subduction-related island arc environment;?_Hf?t?values and two-stage model ages range from+7.61⁺12.71 and 489.8⁸10.1Ma respectively,indicating the magma was derived from partial melting of Neoproterozoic–Early Paleozoic juvenile crust.Combined with trace element geochemical signatures,it is believed that Early Permian granites were formed by partial melting of lower crust aroused by underplating of large-scale mantle-sourced magma along crust–mantle boundaries;besides,minor mantle-sourced magma mixed,forming Early Permian parent magma and it mainly documents the retroarc extension aroused by subduction of the PAO towards the NCC.Middle Permian magmatic activity?265²61 Ma?is represented by biotite granodiorite,which is of metaluminous-weakly peraluminous,mid-K calcalkalic–high-K calcalkalic I-type granite;the rock is relatively enriched in Rb,K?LILEs?and LREEs,and depleted in Nb,Ta,P,Ti?HFSEs?and HREEs,formed in a subduction-related island arc environment,and has?_Hf?t?values and two-stage model ages of+8.01⁺11.44 and554.9⁷78.1 Ma respectively,indicating the magma sourced from partial melting of Neoproterozoic–Early Paleozoic juvenile crust.Combined with regional events,it indicates that the Middle Permian plutons were produced by partial melting of lower crust due to underplating of large-scale Mg-Fe depleted mantle-sourced magma along crust-mantle boundaries in a subduction zone environment,and they document subduction of the PAO before collision of the northern margin of the NCC with the blocks to the north.Early Triassic magmatic activity?250 Ma?is represented by granodiorite,displaying Si-depletion signatures and being of strongly peraluminous,mid-K calcalkalic–high-K calcalkalic,highly fractionated I-type granite.The rock,which formed in a subduction-related island arc environment,is relatively enriched in Rb,K?LILEs?and LREEs,and depleted in Nb,Ta,P,Ti?HFSEs?,and HREEs;its?_Hf?t?values and two-stage model ages range from+4.97⁺10.00 and 646.8⁹58.6 Ma respectively,and it is product of underplating of Neoproterozoic depleted basaltic mantle-sourced magma along active continental margin.The rock has consistent geochemical signatures with TTGG?trondhjemite,tonalite,granodiorite and granite?.It is concluded that the Early Triassic intrusions are product of partial melting of PAO crust,which indicates the PAO Plate is still transporting material sources and represents that no delamination–breaking-off between the PAO and the NCC during this period.Middle–Late Triassic magmatic activity?245²27 Ma?is represented by medium-grained monzonitic granite,fine to medium-grained biotite granodiorite and fine-grained biotite hebraic granite;of them,Early Middle Triassic plutons show Si,K enrichment and are of weakly peraluminous,high-K calcalkalic–shoshonite A-type granite;they are enriched in Rb and K?LILEs?,Th?HFSE?and LREEs,and depleted in Nb,Ta,P and Ti?HFSEs?and HREEs;?_Hf?t?values and two-stage model ages range from-2.03⁺6.52 and 1114.5¹882.5 Ma respectively,and it shows that they are product of underplating and contamination of Paleo-proterozoic–Mesoproterozoic lower crust of active continental margins by depleted mantle-sourced magma,indicating a local extensional environment aroused by breaking-off–delamination of the PAO subducting plate after collision of the NCC with the blocks to the north.Late Middle Triassic granitic plutons show Si and K enrichment signatures and are of weakly peraluminous,high-K calcalkalic–shoshonite series;they are enriched in Rb and K?LILEs?,Th?HFSE?and LREEs,and depleted in Nb,Ta,P and Ti?HFSEs?,and HREEs;?_Hf?t?values and two-stage model ages range from-0.42⁺6.43 and 1116.5¹734.0 Ma respectively;these plutons are product of underplating and contamination of Paleoproterozoic–Mesoproterozoic lower crust with thickened active continental margins by depleted mantle-sourced magma;Late Triassic granitic plutons show Si,K,Na enrichment signatures,and are of aluminous–weakly peraluminous,high-K calc-alkaline series;they are enriched in Rb and K?LILEs?and LREEs,and depleted in Th,Nb,P and Ti?HFSEs?and HREEs;?_Hf?t?values and two-stage model ages range from+9.73⁺15.80 and412.3⁶38.7 Ma respectively;these plutons are product of underplating and contamination of Neoproterozoic–Early Devonian juvenile crust by depleted mantle-sourced magma in the bottom.All of the Late Middle Triassic–Late Triassic plutons show high Sr,low Y and Yb,and high Sr/Y and Sr/Yb ratios,which are consistent with the geochemical signatures of adakitic rocks.This suggests that even the PAO had been collided and closed,however,regional post-collisional orogeny still continued to the Late Triassic.