Late Paleozoic Magmatism And Its Tectonic Significance In The Beidashan Area Of The Alxa Block

Beidashan area is located in the southwest margin of Alxa block.Its late Paleozoic magmatic activity is comparable with the Yabrai-honggueryulin-noergong structural belt in the East,which may be the West extension of Yabulai-honggueryulin-noergong structural belt.The Yejili basic-ultrabasic intrusion is located in the Beidashan area.The main lithofacies of Yejili north ultrabasic is plagioclase lherzolite,in addition to lherzolite,olivine pyroxenolite and gabbro.The southern basic intrusion is mainly composed of gabbro,followed by olivine-gabbro.In this paper,the field geological survey,petrography,mineralogy and geochemistry were conducted on the Yejili basic-ultrabasic intrusion and granite.Combining with the geological data of the basic rocks and granites in the Beidashan region,I discuss the magmatic evolution,tectonic background and the time of closed and evolution history of the Paleo-Asian Ocean in the Alxa block.Following cognition have been gained in this study:1.The clinopyroxene composition shows that parental magma of the southern basic intrusion from tholeite to calc alkaline.The crystallization temperature of clinopyroxene is 1055.58?1139.26?,the crystallization pressure is 1.12?10.60 kbar,and the corresponding depth is 3.71?34.98 km.With the rise of magma and the decrease of temperature and pressure,the structure of monoclase pyroxene becomes unstable,and pyroxene exsolution appears.At this time,the temperature is 700?950?.The parental magma of the north ultrabasic intrusion is calc-alkaline,the crystallization temperature of clinopyroxene is 800?1000?,the maximum temperature is over 1300?,the crystallization pressure is 7.19?15.05 kbar,and the corresponding depth is 19.54-49.67 km.2.The magma of the Yejili southern basic intrusion originated from the EM ? enriched mantle,which is metasomatism by subduction melt.In the process of magma upwelling and emplacement,it was contaminated by crustal materials.The magma of the Yejili north basic intrusion originated from the lithospheric mantle metasomatized by subducted fluid.In the process of magma upwelling and emplacement,there is little contamination of crustal materials.The isotopic geochemistry shows that the Xiayantan basic intrusion and the Yejili intrusion both originated from EM ? enriched mantle,but the Xiayantan intrusion may have more subduction materials.3.The Yejili granite is weakly peraluminous I-type granitoids of the high potassium calcalkaline series,which are the partial melting products of basaltic rocks rich in biotite under medium pressure.In the process of magma evolution,fractional crystallization is the main controlling action,and some mantle source materials are added.4.Based on the statistics of the age and forming environment of the late Paleozoic magmatic rocks in the Beidashan area,the composition of clinopyroxene in the basic rocks of different ages in the Beidashan area,the study of the magmatic properties of the basic rocks and granites,and the study of the molasse formation of the Amushan formation,it is provided evidences for the closure of the Chaganchulu back arc basin in the early Permian.