| The Altun Orogenic Belt is subdivided into the north Altun,the middle Altun,and the south Altun terrane in which granitoids with multiple periods and stages formed by complex tectonic evolution.With the combination of detailed field work as well as petrographic,geochemical,and geochronological investigations,this study integratedly analyzed the rock type of the granitoids in Altun,distinguished the periods of the granitic magmatism in the north,the middle and the south Altun terrane,and therefore proposed the geodynamic mechanism leading to those granitoids.Primary conclusions are as follows:(1)The secondary minerals in the granitoids mostly are biotite and muscovite.The ratio of A/CNK is relatively high(>1.1),indicating that the granitoids are peraluminous.Most granitoids display negative Eu anomaly,and depleted in Ba,Nb,Sr,P and Ti,showing characteristics of S-type granite,and their protoliths primarily are sediments or metasediments.In addition,they should have similar protoliths as revealed by similar REE and trace element patterns.(2)The north Altun terrane show granitic magmatism with 6 periods of 500Ma(early-middle Cambrian),482-474Ma(early Ordovician),467Ma(middle Ordovician),447-443Ma(late Ordovician),437-431Ma(early Silurian),and 404Ma(early Devonian).(3)The middle Altun terrane show granitic magmatism with 5 periods of 522500Ma(early-middle Cambrian),490-470Ma(early Ordovician),458Ma(middle Ordovician),451-445Ma(late Ordovician),432Ma(early Silurian).(4)The south Altun terrane show granitic magmatism with 7 periods of 483475Ma(early Ordovician),456Ma(middle Ordovician),447-443Ma(late Ordovician),434Ma(early Silurian),385Ma(late Devonian),351-337Ma(early Carboniferous),226Ma(late Trias)?(5)The geodynamic process of the north Altun terrane probably is as follows:the north Altun ocean subducted to the south at?500Ma forming the first period(?500Ma)granitoids;the north Altun ocean successively subducted to the south till?467Ma,forming the second period(482?474Ma)and the third period(467Ma)granitoids;the north Altun ocean closed at?450Ma;the Dunhuang block collided with the north Altun terrane at 450?430 Ma,forming the fourth period(447?443Ma)and the fifth period(437?431Ma)granitoids;in the post-collisional exstension stage at 430?400Ma,the sixth period(404Ma)granitoids eventually formed.(6)The geodynamic process of the south Altun terrane probably is as follows:before?455Ma,the Qaidam block might collided with the south Altun terrane,and the lower time limited of the continental-conitnental collision is?520Ma,forming the 483?475Ma and the 456Ma granitoids;by the end of the collision,it turned to the transition period of intraplate extension at 450?430Ma,forming the 447?443Ma and 434Ma granitoids;in the intraplate extension period after 430Ma,the 385Ma and 351?337Ma granitoids therefore formed;the Altun strike-slip fault then formed in the Triassic period which is probably related to the 226Ma granitoids.(7)The geodynamic process of the middle Altun terrane probably is as follows:the middle Altun terrane is within the south and the north Altun terrane,and the middle Altun granitoids might formed as the result of the other two terranes.Before 455Ma,the middle Altun granitoids is lithologically more affiliated to the south Altun granitoids,in the transitional stage at 450?435Ma,the middle Altun granitoids jointly affected by both the south and the north Altun terrane,and after?435Ma,the middle Altun granitoids is lithologically more affiliated to the north Altun granitoids.The 520?458Ma middle Altun granitoids is related to the collisional event in the South Alun terrane before?455Ma,and the 451?445Ma middle Altun granitoids is the result of the joint effect of both the collisional event between the south Altun and Qaidam block and the collisional event between the Dunhuang block and the north Altun terrane.The 432Ma middle Altun granitoids is formed in the post-collisional extension environment in the north Altun terrane. |
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