Petrogenesis And Continental Dynamic Significance Of The Neoproterozoic Granitoids In The Altyn Tagh

The Altyn Tagh Mountain is subdivided into the North Altyn,the Middle Altyn and the South Altyn terrane,by complex tectonic evolution of multiple periods and stages formed.Numerous Neoproterozoic granitoids crop out in the South Altyn continental block,can provide indispensable information on the dynamics of Rodinia supercontinent aggregation during the Neoproterozoic.In this thesis,these granitoids exposed in the South Altyn continental block were elected as the objects,on the basis of detailed field investigation,through petrography,zircon U–Pb chronology,geochemistry,and zircon Hf isotope approaches.We obtained the following conclusions:According to the new age data,combining with previous age date,the Neoproterozoic magmatic activity in the study area is divided into four stages:The first period?999.7⁹97Ma?.The second period?976⁹65Ma?.The third period?957⁹06Ma?has the largest and most widely distributed,which is the peak period of the magmatism during the Neoproterozoic.The fourth period?⁸86Ma?.Combined with the exposed position of granitoids,the Neoproterozoic granitoids exposed in the South Altyn continental block has the trend of decreasing from south to north,from inland to inner edge.The geochemical characteristics of granitoids have high content of SiO₂ and Na₂O+K₂O,Most grantoids are peraluminous,display LREE enrichment and HREE depletion,with negative Eu anomalies,the enrichment of Rb,K,Th,and depleted in Ba,Nb,Ta,P and Ti,showing the characteristics of S-type granite,similar to the syn-collision type granite.Zircon Hf isotope analysis results show:?_Hf?t?values of granitoids are in the range of-8.638⁷.68.The Hf model ages(T_DM2)of granitoids range from 1.36².33Ga,which the peak is 1.7¹.9Ga.These features indicate that the protolith is mainly Paleoproterozoic Mesoproterozoic crust,but in the process of magmatic activities may be affected by the mantle-derived.On the Basis of above study,combines with the geological data onto the adjacent areas,we discuss the characteristics of tectonic evolution of the South Altyn continental block.?I?Early Mesoproterozoic?1.8¹.3Ga?.Influenced by the fragmentation of Columbia supercontinent,the Altyn tectonic zone has several ancient microcontinent,such as Altyn microcontinent,Qilian microcontinent and the Northern Qaidam microcontinent.?II?Late Mesoproterozoic?¹.1Ga?.In the initial convergence stage of Rodinia supercontinent,the microcontinent such as Altyn,Qilian and Northern Qaidam subducted and collided with the Yangtze plate,forming SSZ type ophiolite?1066Ma?.?III?Early Neoproterozoic?1.0⁰.9Ga?.Entering the stage of rodinia supercontinent collision,formed a large number of collisional granitoids in the Altyn-Qilian-Qaidam area.?IV?Late Neoproterozoic?0.85⁰.75Ga?,corresponding to the rifting stage of the Rodinia supercontinent,forms a Pan-Ocean with microcontinents.?V?Early Paleozoic?⁰.5Ga?.The oceanic basin was closed and the microcontinent was deeply subducted.High-pressure and ultra-high pressure metamorphism occurred in the study area.The early rocks were all superimposed by Paleozoic tectonothermal events.