Tectono-magmatism And Its Geological Significance In The Beishan Area Of The Southern Part Of The Central Asian Orogenic Belt

The Beishan orogenic belt,belonging to the southern margin of the Central Asian Orogenic Belt,is located at the intersection of the Tarim,Kazakstan and Siberian plates,and composed of multiple island arcs,micro continental,ophiolite and accretionary complex.After a long history of multiple stages of complex subduction-collage,tforming four ophiolite belts.The complex tectonic environment in this area is one of the key areas to analyze the tectonic evolution of the Central Asian Orogenic Belt and the closure time of the Paleo-Asian Ocean.On the basis of field investigation and literature reading,through comprehensive research methods such as petrology,petrogeochemistry,zircon U-Pb chronology and Sr-Nd isotope geochemistry,this paper makes a thorough discussion on the age,genesis and tectonic environment of Paleozoic-Early Mesozoic rock masses,and obtains the following main understandings.The LA-ICP-MS zircon U-Pb ages of the gabbro in ophiolite of Hongliuhe and Yushishan are 525±3.8Ma and 535±3.6Ma,respectively.The basic rocks of the two places have similar geochemical characteristics,i.e.the basic rocks have both the island arc-like and MORB-like geochemical characteristics.The?_Nd?t?samples are high and positive?+2.91⁺12.25?,(⁸⁷Sr/⁸⁶Sr)_i?0.7035⁰.7053?,which indicate that the source area should be the depleted mantle added by the subduction oceanic crust metamorphic dehydration fluid and formed in the pre-arc environment.It is further pointed out that the formation age of the Hongliuhe-Niuquanzi-Xichangjing ophiolite may indicate that the main oceanic basin of the Paleo-Asian Ocean began to subduct southward in the Early Cambrian.The LA-ICP-MS zircon U-Pb age of the Huaniushan metadacite is 454.7±0.87Ma.The Huaniushan metadacite are high-K calc-alkaline to potassic,enriched in Rb,Th,U,K,Pb,and depleted in Nb,Ta,Ti.They display some characteristics of typical volcanic magmatic rocks,including low Yb,Nb/La,Nb/La,Ce/Th,Ba/Th,Nb/La,Hf/Th,high Th/Ta,and variations of Th.It is the product of the southward subduction of the oceanic basin represented by the Hongliuhe-Niuquanzi-Xichangjing ophiolite belt in the Late Ordovician.The LA-ICP-MS zircon U-Pb age of the Xiaoquandong granodiorite is 395.86±4.2 Ma.The Xiaoquandong granodiorites belong to the potassic series,and have high(⁸⁷Sr/⁸⁶Sr)_i and low?_Nd?t?values.They show some characteristics of A1-A2 type transitional granites,including low Sr,high Y,high?Zr+Nb+Ce+Y?and 10000Ga/Al,which indicate a post-collision extension environment.Studies on the Xiaoquandong granodiorites prove that ocean basin represented by Hongliuhe-Niujuanzi-Xichangjing ophiolite belt had been closed before the early Devonian.The LA-ICP-MS zircon U-Pb age of the Tanshanzidong olivne gabbro is 366±2.8 Ma.The Tanshanzidong olivine gabbros are enriched in Rb,Ba,Sr,U,K,and depleted Nb,Ta,Zr,P,Ti.They exhibit depleted whole-rock Sr-Nd isotopic compositions with normal(⁸⁷Sr/⁸⁶Sr)_i and positive?_Nd?t?values.?Nb/La?=0.23⁰.46,?Hf/Th?=0.88³.81.They have low Yb and Ta contents,and Ta/Yb ratios.Geochemical data show that they could be formed by partial melting of the depleted mantle altered by subduction-derivd fluids in an active continental margin.The LA-ICP-MS zircon U-Pb ages of the Huaniushan quartz diorite,Dashantou quartz diorite and Shuangfengshan granodiorite are 362.5±1.1Ma,348.6±1.9Ma and334.8±2.1Ma,respectively.They are high-K calc-alkaline and per-aluminous I-type granites.They are enriched in Rb,K,Nd,Zr and Hf,and depleted Nb,Ta,P,Sm and Ti.Their Nb/La and Hf/Th ratios are closed to the typical island arc magmatic rocks.The(⁸⁷Sr/⁸⁶Sr)_i?0.7060⁰.7095?and?_Nd?t??-4.81⁺0.52?values show the characteristics of crust-mantle interaction.Geochemical data show that these granitoids were formed by partial melting of basaltic rocks which were previously formed by the partial melting of depleted mantle altered by subduction-derivd fluids in the early stage in an active continental margin.It indicates that the back-arc basin represented by the Huiitongshan-Zhangfangshan ophiolite belt in the southern subducted northward in the Late Devonian,and the subduction lasted until the Early Carboniferous.The LA-ICP-MS zircon U-Pb ages of the Huaniushan and Xiaoquan granites are232.4±0.55Ma and 220±3Ma,respectively.They are high-K calc-alkaline and potassic granites.They show some characteristics of A type granites,including high?Zr+Nb+Ce+Y?and 10000Ga/Al,which indicate a post-collision extension environment.The high(⁸⁷Sr/⁸⁶Sr)_i?0.7073⁰.7074?and low?_Nd?t??-1.74^-1.61?values indicate that they are the products of partial melting of lower crustal materials?including newly formed island arc material from early subduction?.In summary,the lithosphere in Beishan area began to extend and crack before 1.0Ga and gradually expanded into the Paleo-Asian Ocean.The Paleo-Asian Ocean represented by Hongliuhe-Niuquanzi ophiolite began to subduct southward beneath the Dunhuang block in the Early Cambrian.The Huaniushan magmatic arc was formed in the south in the Middle Ordovician.At the same time,a northward subduction occurred in the northern part of the Paleo-Asian Ocean,subducted beneath the Mazongshan massif,and formed Baiyunshan-Xieshan-Dongqiyishan and Yaodongnuru-Gongpoquan magmatic arc.During the Silurian,the oceanic basin represented by the Hongliuhe-Niujuanzi-Xichangjing ophiolite belt underwent large-scale two-way subduction,while the Jijitaizi-Xiaohuangshan back-arc basin was formed in the north and the Huitongshan-Zhangfangshan back-arc basin was formed in the south.The oceanic basin represented by the Hongliuhe–Niuquanzi-Xichangjing ophiolite belt were closed at the end of the Late Silurian,and the southern Beishan entered the post-collision stage.After a short post-collision extension,the Huitongshan-Zhangfangshan back-arc basin subducted northward,and the Jijitaizi-Xiaohuangshan back-arc basin were subducted southward,formed a series of volcanic-arc granites in the Late Devonian.The Huitongshan-Zhangfangshan and Jijitaizi-Xiaohuangshan back-arc basin closed at the end of Early Carboniferous.Since then,the whole Beishan area had entered the post-collision stage.