Petrology, Petro-geochemistry And Tectonic Significance Of The Qitianling Granite Body,Hunan Province

Qitianling rock mass is one of the representative large rock bases in the northern margin of the middle part of Nanling,and it is located in the famous Qianlishan-Qitianling tungsten-tin polymetallic deposit area.There are many achievements in the study of Furong super-large tin deposit in the south of Qatianling rock mass,but the magmatic evolution relationship among various units of rock mass is insufficient,and there are still some disputes among scholars of different tectonic backgrounds in South China in the early Yanshan period.In recent 10 or 20 years,with the proposal and perfection of the theory of the in-situ melting and intra-crustal magma convection of granite,a new understanding of the source region and positioning mechanism of granite has been put forward,and a lot of praise has been received.Some scholars have begun to reinterpret some famous rock masses by using this new theoretical system.Therefore,based on the theory of in-situ remelting of granite,this article discusses the crystallization time of different units of granite from the intrusive relationship,and analyzes the differences of different units and the characteristics of magmatic evolution from petrology and petrochemistry,and study on magmatic origin and localization mechanism of Qitianling rock mass.Combined with regional geological data,the Mesozoic tectonic magmatic activity process and tectonic setting in southeastern Hunan are discussed,and the following understandings are obtained:1.The Qitianling rock body can be divided into middle Jurassic Cailing superunit and Furong superunit and sporadic granitic porphyry and fine granitic dyke.The main lithology is monzogranite and syenogranite,and dark minerals are mainly biotite.The early units contain a certain amount of hornblende.The formation age of rock mass is 147~163 Ma,which can be divided into four formation stages,which reflects at least three stages of in-situ remelting.From old to new unit,rock acidity and the magmatic differentiation index increases gradually,and the hornblende content and dark mineral content decrease,which basically reflects the magmatic evolution sequence of intermediate acid and super acid.The characteristics of main and trace elements show that the magmatic evolution degree of Cailing superunit is similar to that of Lijiadong,Wuliqiao and Nanxi units,and increases slightly from the early to the late.Jiangjunzhai and Huangtang Ling and Huitouwan units tend to the late magmatic evolution characteristics.Consistent with the evolution trend of in situ remelting of granite.2.The deep body of the rock body tends to southeastern,resulting in the regular variation of the rock structure,mineral composition and geochemical composition from the periphery to the center and from the northwest to the south-east.The Qitianling rock mass is the hump of the regional paleo-remelting interface,which is controlled by the deep and large fault structure system of Yanling-Lanshan,in which the magmatic top-erosion is especially developed.The magmatic source of the rock mass should be the granitic magma layer beneath it(probably the melting of the Proterozoic basement),partly all the strata of the Paleozoic-Mesozoic era invaded by the rock mass,and themafic microgranular enclave in the rock mass is the remnant of the melting of the surrounding rock.The assimilation and contamination of magma at the edge of the rock mass is stronger than that of the others and denudation depth of the rock mass is not large.3.Petrological and geochemical characteristics show that Qantianling granite belongs to high potassium calc-alkaline metaluminous granite.Mineralogy and the main trace elements are identified as post-orogenic granites,which were formed in the tectonic environment of compression to tension transformation.Since the Middle Jurassic,South China has belonged to the Ancient Pacific Plate tectonic domain,which was caused by the soft flow circle upwelling,local extension and tension caused by the withdrawal subduction of the Ancient Pacific Plate,and the influence of regional crustal warming,resulting in a regional remelting magmatic layer.The upwelling of the soft current circle has the law of gradual migration from inland to coastal with time.The magmatic activity in southeastern Hunan was basically concentrated in the early Yanshanian period.By Cretaceous,the strong magmatic activity had migrated to the narrow coastal zone,resulting in the cooling and gradual consolidation of the inland remelting magmatic layer.4.Qitianling granite rock mass is closely related to tungsten-tin mineralization.Furong super-large tin ore field and Xintianling large-scale tungsten ore field occur on the north and south sides of the rock body,respectively.The metallogenic age is basically consistent with the magmatic age,which is the middle-late Jurassic.Celing superunit has the potential to form rare earth ore,Furong superunit has tungsten,tin,molybdenum,bismuth polymetallic mineralization,possibly magnetite,uranium,copper mineralization,late granitic porphyry and fine granites have niobium-tantalum mineralization,tin mineralization.The possible mineralization location is fracture zone,hydrothermal alteration zone and surrounding rock contact zone.