Geochemistry, Zircon U-Pb Geochronology And Its Significance Of The Triassic Granites In The Mashan Mountains, Western Yunnan

There are a series of Triassic monzon granites exposed to the eastern margin of the Baoshan block in western Yunnan and the junction of the Changning-Menglian belt-Gengma-Ximeng-Menglian line,forming a granite belt of about 500km,the main rock-including Caojian iron rock,Changning haobadi rock,Yongde Yunling rock,Gengma mountain rock and Menglian Menga rock.Although the scale is far less than the Changning Menglian zone east of the blue sky with snow-Lincang granite belt,but also records the Changning Menglian ocean after the closure of the arc continent collision orogenic process and related mineralization.This paper takes Gengma mountain rock as the research object of study,through detailed geological survey,from rock strata,geochemistry and chronology data to earth mountains of Gengma Granite Emplacement age,genetic type and material sources,provide new insights into the petrogenesis and tectonic setting of the Gengma granite and its relation with the Lincang granite.The Gengma mountain is located in Changning-Menglian rock combination of Gengma Baoshan block on the western side of the passive margin,is composed of Triassic granodiorite and granite and two Paleogene flash granodiorite porphyrite and diorite porphyry-two granite complex granite base.The results show that the triassic granite has three high silicon ?(SiO2)of 65,54%?70.04%,potassium(K2O/Na2O)=1.26?3.58(average 1.94),poor calcium(CaO)=0.86%?3.57%,the low content of alkali(alk=4.04%?7.69%),A/CNK from 0.95 to 1.70,average is 1.173>1.1,with a high potassium calc alkaline peraluminous and strong characteristics of aluminous granite.The rocks are enriched in lile and depleted in high field strength elements,REE is 214.26×10-6?276.06×10-6(average 241.81×10-6),the ratio of LREE/HREE ranged from 7.39 to 10.27 with an average of 8.93,(La/Yb)N was 6.63?13.44,REE fractionation is obvious;(La/Sm)N was 3.51?4.31 light rare earth;?Eu is 0.50?0.68,the average is 0.58,the chondrite normalized REE distribution curve is obviously the right type,and shows moderate negative Eu anomalies.LA-ICP-MS zircon U-Pb dating results show that the crystallization age of the Gengma granite is not concentrated in 225-239 Ma,indicating that the pluton intruded in the middle late three fold.It is shown in the ACF diagram and SiO2-Zr diagram that the Gengma granite is S type granite,and the source rock is rich in feldspar clastic rock.In the structural environment of Nb-Y and Rb-Y+Nd and Hf-Rb/30-Ta×3,the post-collision granite environment is identified.In addition,the geochemical characteristics of the rock,the zircon dating results and the regional geological background,the Gengma mountain triassic granite is the arc(continental)-local collision caused by the local lithography of the lithosphere thinning mantle material.Which is mainly composed of medium and lower crustal material components,accompanied by a small amount of mantle material components involved;the formation of the structural background may be closed for the ancient Tethys ocean and the reverse subduction of the deep oceanic crust in the process of arc-land collision.