Mineralogical Characteristics And Geochemistry Characteristics Of Qinglingliang Granitoid Pluton And Its Geological Significance

This paper mainly studies mineralogy,trace and REE geochemistry and geochronology of Qinlingliang granitoid pluton,to deal with problems such as,petrogenesis,the source of material and the tectonic environment of this area,and provides some evidences for the study of the genesis of the rapakivi texture granite.The Qinlingliang granitoid pluton is located in the north of Shangdan fault belt of Qinling orogen(central China).The pluton is mainly composed of rapakivi texture granite,mafic microgranular enclave(MME),K-feldspar pegmatite and monzogranite.Through the Laser ablation inductively coupled plasma mass spectrometry(LA-ICP-MS)zircon U–Pb analysis method,the thesis gives the result as follows: rapakivi texture granite 224.2±1.4Ma and 220.1±0.9Ma,MME 224.1±1.4Ma,219.2±1.3Ma and 222.2±1.8Ma,K-feldspar pegmatite 216±1Ma,monzogranite 209.2±1.8Ma.Rapakivi texture granite is acid rock,classified by minerals as monzogranite.The major minerals are K-feldspar,plagioclase,quartz,amphibole and biotite,accessory minerals are magnetite,apatite,sphene,zircon.MME is intermediate rock,classified by minerals as quartz-monzodiorite.The primary minerals are plagioclase,amphibole,biotite,K-feldspar and quartz,accessory minerals are magnetite,apatite,sphene,zircon.Qinlingliang rapakivi texture granite,MME,monzogranite are metaluminous high-K calc-alkalic rocks,and K-feldspar pegmatite is high-K shoshonite rock.Except monzogranite that alkali rich,Mg#=0.5,other is alkali and magnesium rich,Fe-poor.Except Ba,their trace elements patterns are similar,enriched in Th,U,La,Ce,Nd,Zr,Hf,depleted in Ta,Nb,Sr,P,Ti.Except Eu,chondrite-normalized REE patterns are similar,?REE=159.87~371.44?g/g,(La/Yb)N=10.97~21.68.Qinlingliang rapakivi texture granite,MME,K-feldspar pegmatite,monzogranite are I-type granite,but have definite I-A transitional granite in geochemical characteristics.Their source of magma may be similar,and the crust source material is predominant,mixed some mantle magma.The temperature and pressure conditions for rock formation are similar to those of garnet stability.From the mineralogical and geochemical aspects,it is proved that the rapakivi texture granites and enclaves occur in orogenic belts.The tectonic setting may be post-collision environment transformed from compression to extension.The magma is formed in the environment of medium temperature,water-rich and medium oxygen fugacity.Through the study of petrography,mineralogy,magmatic mixing and metasomatism both exist in Qinlingliang rapakivi texture granite and MME,as to which mechanism plays a dominant role on the formation of rapakivi texture,what is the relationship between the two,further studies need to be done.