Early Mesozoic Magmatic Activity And Its Geological Significance In The Pangjiahe-Miaogou Area Of Fengxian County,Western Qinling

The structural evolution of the West Qinling during the Indosinian period was generally under the background of subduction and continental collision in the Mianlu Ocean Basin,however,there is still controversy over the initial subduction,full-scale subduction and collision time limit of the Mianlue ocean basin.With the development of these geological processes,huge amounts of magmatic rocks invaded the middle and lower crust of the Western Qinling Mountains or ejected from the surface,forming large-scale magmatic rock belts.This article combines regional geological background and previous research data and achievements,through detailed field investigations and indoor data processing and analysis,comprehensive use of methods such as structural geology,petrology,geochemistry,etc.,the characteristics of the Early Mesozoic magmatism in the PangjiaheMiaogou area of Fengxian County,West Qinling were discussed(Mainly a detailed study of the LAICP-MS zircon U-Pb age,geological and geochemical characteristics of Hejiazhuang-Laoyao rock mass and its tectonic environment)and the structural evolution of the Mianlue belt and the relationship between magmatic rocks and the metallogenesis of the gold deposits in the study area,The following conclusions are drawn:(1)The emplacement time of the granodiorite in Hejiazhuang-Laoyao rock mass in the Western Qinling area is 235-240 Ma,and the invasion occurred in the Early Triassic.The granodiorites from the Hejiazhuang-Laohuyao rock mass are all high-calcium-calcium-alkaline series,showing strong over-alumina characteristics,and belong to type I granite.The source rock is a mixture of the melt produced by the partial melting of the subducting oceanic crust under high-pressure environment and the magma of the high-potassium solution produced by the partial melting of the lower crust contaminated by peridotite due to the infiltration.(2)The dating results,geochemical characteristics and Hf isotopes of the HejiazhuangLaohuyao rock mass show that the rock mass is formed by magmatic activities of different periods in the same source area,the intrusion is located in the tectonic environment where the Early Triassic Yangtze plate subducted towards the Qinling microplate,it is proved that the Yangtze plate and the Qinling microplate are in the subduction stage during the period of 235 ~ 240 Ma.The Mianlue oceanic basin began to reduce subduction in the Early Permian.The expansion ended before the Permian,and began to fully subduct in the early and middle Triassic,and the subduction collision occurred in the middle and late Triassic.(3)The predecessors conducted H-O isotope tests on the Pangjiahe Gold deposit,the Matigou Gold deposit,and the Zuojiazhuang Gold deposit near the Hejiazhuang-Laohuyao rock mass,the analysis shows that the ore-forming hydrothermal fluids of the Matigou Gold deposit,Pangjiahe Gold deposit,and Zuojiazhuang Gold deposit are mainly magmatic hydrothermal fluids.Sulfur isotope tests have been carried out on the pyrite in the Matigou gold deposit,the Santaishan gold deposit and the Wenjiazhuang gold deposit.The results of the study indicate that they have the characteristics of magmatic sulfur sources.The Hejiazhuang-Laohuyao rock mass is closely related to the gold deposit in the study area in time and space.And after discussion,it is believed that there may be an early-Middle Triassic island arc volcanic-intrusive complex zone formed in the deep north of the South Qinling Mountains under the subduction environment of Mianlue.The existence of this deep concealed volcanic zone and its hydrothermal activity indicate that it provided favorable depositralization conditions for thermal energy and ore-bearing fluids in the regional deposits in the Early-Middle Triassic.The Early Mesozoic magmatic activity in the West Qinling area has a close relationship with the depositralization of regional deposits.