Chronology,Geochemistry And Geological Significance Of The Early Carboniferous Granitic Porphyry In Beidashan Alxa

The Central Asian orogenic belt is located between the Siberian and North China plates and the Tarim plate.It was formed by the subduction of the Paleo-Asian Ocean and has undergone a long period of crustal accretion and modification,and has undergone various geological effects such as crustal accretion,post-collision and intra-land orogeny during its formation.Located at the southern margin of the Central Asian orogenic belt,the Alax block has been influenced by the tectonic evolution of the Paleo-Asian Ocean since the Eocene and has developed magmatic activity,which is important for studying the tectonic evolution of the Paleo-Asian Ocean and the Central Asian orogenic belt.Based on detailed field geological work,this paper investigates the magmatic evolution process,genesis type and tectonic background of Beidashan Cijinzi Granite Porphyry in the Alax Block by petrographic,geochemical,zircon chronology and Hf isotope methods,combined with Late Paleozoic magmatic rocks in the Alax Block,to provide a basis for the elucidation of the closure time frame and tectonic evolution of the Paleo-Asian Ocean in the Alax Block.LA-ICP-MS zircon U-Pb dating of the Beidashan Cijingzi granite porphyry shows an intrusion age of 326.3±1.2 Ma,which is Early Carboniferous.Beidashan Cijingzi granite porphyry is rich in Si O₂（75.04%～76.97%）,poor in Fe₂O₃^T（1.62%～1.76%）and Mg O（0.15%～0.24%）;K₂O+Na₂O content is 7.68%～8.24%,belonging to the high potassium-calcium-alkaline series;A/CNK value is 1.0～1.16,belonging to the weak peraluminous series;light rare earth elements are relatively enriched（La _N/Yb _N=2.42～3.60）and Eu is strongly depleted（δEu=0.07～0.19）;meanwhile,Rb,Th,U,Zr and Hf elements are enriched,while Eu,Sr,Ti and P elements are deficient;10⁴Ga/Al is3.81～4.1 and the saturation temperature of zircon is 813℃～828℃,which is an A-type granite;theε_Hf（t）values of zircon are relatively stable,ranging from-3.53 to-1.24,and the two-stage model age(T_DM2)is between 1414 and 1561 Ma.The magma is mainly derived from partial melting of the ancient crust.There are significant differences in the genesis and tectonic setting of Carboniferous and Permian-Early Triassic magmatic rocks in the Alax Block.Most Carboniferous igneous rocks show calc-alkaline-low potassium calc-alkaline and magnesian features,while most Permian-Early Triassic igneous rocks show high potassium calc-alkaline-potassium basalt series and iron features.In terms of whole-rock geochemical trace elements,the Permian-Early Triassic igneous rocks show a higher degree of light and heavy rare earth elements differentiation than the Carboniferous rocks.The mostly Carboniferous igneous rocks of the Alax Block were formed in the tectonic environment of ocean subduction,while the mostly Permian-Early Triassic rocks were formed in the collisional and post-collisional environment,suggesting that the Paleo-Asian Ocean closure event in the Alax Block may have occurred in the Late Carboniferous-Early Permian.Both the Alax Block and the East Tianshan are located on the southern side of the Central Asian orogenic belt.During the Late Paleozoic,a number of porphyry copper ore zones appeared in the East Tianshan,while only a few minerals and a few mineralized sites were found in the Alax Block.By collecting previous research data on these two areas and analyzing the genesis of the Late Paleozoic rocks,we found that the Late Paleozoic rocks of the Alax Block were mainly formed by partial melting of the thickened lower crust,while the genesis of the East Tianshan rocks during the same period was mainly related to the melting of the subducted oceanic crust,and the difference in the genesis of the rocks may be one of the reasons for the obvious differences in the mineralization of the two areas.