| Located in the central part of the mainland of China,the western end of the Qinling orogenic belt lies in the transitional region of the Qinghai-Tibet Plateau,the Loess Plateau and the Qinling orogenic belt.It has undergone multi-stage and multi-stage tectonic evolution stages,including the Neoproterozoic plate splitting and convergence,the convergence and collision of the North China plate and the Yangtze plate in the early Mesozoic,and the intracontinental tectonic evolution after the Late Mesozoic.During this process,there are many stages of magmatism.The study of intrusive rocks can restrict the formation and evolution of orogenic belts in the study area.This paper chooses several intrusive rock bodies(veins)in Chengxian area of the western Qinling orogenic belt to carry out systematic petrology,petrogeochemistry and zircon U-Pb chronology,and to analyze the source characteristics,petrogenesis and tectonic background of the intrusive rocks in this area.On the basis of previous studies,the age data of four intrusive rocks(veins)in this area have been determined by LA-ICP-MS zircon U-Pb dating method.The age of the South Mass of Zhuanmiaozi is 234.5±4.1 Ma,the age of the North Master of Daqiao is 221.5 ±6.1 Ma,and the age of the Guanyin Mass is 236.1±3.6 Ma.Among them,Daqiaobei dike and Guanyin dike are closely related to gold mineralization.It shows that the formation of gold deposits in Chengxian area is closely related to magmatic activities in the Late Triassic.The Zhuanmiaozi granodiorite has a porphyry-like structure with a Ritman index of 1.39 and a alkalinity rate of 1.81.It is a calc-alkaline rock with high aluminium and rich silicon.Large ion lithophile elements K,Rb,Ba and Th are obviously enriched,while high field strength elements Hf,Zr,Sm,Y and Yb are obviously depleted,while Nb and Ta are slightly enriched.The heavy and light rare earth elements in the rocks are distinct,?Eu=0.61.Mineralogical and geochemical characteristics show that the Zhuanmiaozi granodiorite is S type granite.Based on the regional tectonic background,it is considered that the Zhuanmiaozi intrusion originated from partial melting of the lower crust during the intracontinental uplift stage after plate collision in the Late Triassic.Guanyinyan granite porphyry has porphyry structure,strong weathering in the later stage,rich in silicon,poor in iron,magnesium and calcium.?43=0.26,A.R.=1.49.It belongs to calc-alkaline rock.Rb,Ba and Th were enriched,while Nb,Zr,Hf,Sm,Y and Yb were depleted.Ba,Nb and Zr showed negative anomalies and Ce showed positive anomalies.The total amount of LREE and HREE is obviously neptunium,and ?Eu= 0.73.Europium is a weak deficit.Based on the regional background,this paper considers that the Guanyin granite porphyry belongs to volcanic arc granite and originates from the slow melting of the crust after the subduction and collision of the Late Triassic plate. |
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