Geochemical Characteristics And Tectonic Significance Of Paleozoic Intrusive Rocks In Tiancang,Beishan Region

The Tiancang intrusive body in the southern belt of Beishan,Gansu Province,is located in the southern part of the Central Asian orogenic belt.Therefore,the study of this area is of great significance for the determination of the subduction closure time limit of the Liuyuanyang（Paleo Asian Ocean）in the southern belt of Beishan,and can play a constraint role in the tectonic evolution of the southern belt of Beishan.In this paper,the Honggoushan adamellite body and Shuangjianshan gabbro diorite body in Tiancang area,east of the southern belt of Beishan,are taken as the research objects,and the research methods of petrology,zircon U-Pb chronology,and rock geochemistry are used to study the petrogenesis,formation environment,and tectonic background of igneous rock,and the main achievements are as follows:（1）The LA-ICP-MS zircon U-Pb age of the Honggoushan highly differentiated I-type monzogranite is 467.2±2.9Ma,indicating a formation age of the Middle Ordovician.The rock has high SiO₂（70.74～77.48%）,high Al₂O₃（12.34～15.71%）,rich potassium and sodium,with almost equal content（K₂O content 4.05～5.35%,Na₂O content 4.00～5.55%,K₂O/Na₂O ratio 0.96～1.04,average 1.00）,low Mg O（0.10～0.16%,Mg^#=17～44）,TiO₂（0.05～0.11%）,P₂O₅（0.01～0.03%）,and Fe₂O₃^T（0.38～1.26%）,A/CNK（0.95～1.03）,10000×Ga/Al values range from 2.50 to 2.88,Zr+Nb+Ce+Y values range from 121.02 to 220.11 ppm,Nb/U ratios range from 5.15to 9.77,and La/Ce ratios range from 0.39 to 0.64;The rock is rich in Pb,while also enriched in Rb,Th,U,Ta,Hf,and depleted in Ba,Nb,Ce,Sm,and Eu.The above characteristics indicate that the rock is a quasi aluminous to weakly peraluminous,high potassium calcium alkaline,and highly differentiated I-type granite.In the tectonic environment discrimination map,it is shown that the volcanic arc has the characteristics of collisional granite,indicating that the rock mass was formed by partial melting and fractional crystallization of mantle derived magma that heated the lower crust under the collisional environment.The discovery of Middle Ordovician granite related to the subduction of the Liuyuan Ocean in the eastern part of the Beishan South Belt for the first time indicates that the Liuyuan Ocean was in a subduction stage during the Middle Ordovician.（2）The Shuangjianshan gabbro diorite has a LA-ICP-MS zircon U-Pb age of 277.1±2.2Ma and was formed in the Early Permian.The rock has moderate SiO₂（50.31～57.12%）,high Al₂O₃（14.59～18.05%）,A/CNK ratio（0.48～0.88）,rich Na₂O and poor K₂O（K₂O content 0.04～1.85%,Na₂O content 0.53～3.49%）,K₂O/Na₂O content 0.07～0.64,high Mg O（3.02～3.37%,Mg^#=42～67）,TiO₂（0.42～1.01%）,P₂O₅（0.03～0.24%）,and Fe₂O₃^T（6.09～8.26%）,and the rock is rich in Pb and U La,losses Ba,Nb,Ce,Pr,and Zr,δEu value（0.71～1.91）,Eu has negative and positive anomalies,Zr/Nb value（12.49～26.50）is close to transitional mantle and enriched mantle,Nb/Ta（11.27～15.23）,and is located in the middle of continental crust ratio.The above characteristics show that the Shuangjianshan gabbro diorite is a high potassium calc alkaline to tholeiitic series.The tectonic environment discrimination map shows post collision and intraplate extensional granite,which is a post collision extensional tectonics tectonic environment in the early Permian.Partial melting and fractional crystallization occurred during the formation of magma.Similar to a large number of previous research results in this area,it shows that Liuyuanyang was closed before the early Permian.（3）On the basis of previous and current research results,the tectonic evolution of the Liuyuan Ocean（Paleo Asian Ocean）in Tiancang area in the Paleozoic is as follows:in the early Ordovician,when the Liuyuan Ocean（Paleo Asian Ocean）subducted below the Huaniushan magmatic arc,volcanic arc magmatism began to occur.During this period,Honggoushan highly differentiated I-type monzonitic granite intrusion（467.2±2.9Ma）,which is dominated by volcanic arc granite and also has the characteristic background of collisional granite.During the Late Carboniferous（approximately 310Ma）,the Liuyuan Ocean underwent subduction and closure,forming the Liuyuan ophiolitic melange.In the early Permian,the lithosphere was delaminated,the nearby mantle asthenosphere was upwelling,and the lithosphere was extensional.At the same time,intermediate basic rock bodies were intruded,forming the early Permian Shuangjianshan gabbro diorite（277.1±2.2 Ma）with post collision and intraplate extension geochemical characteristics.