Geochemistry And Zircon Shrimp U-Pb Dating Of Daocheng Dongcuo Granite Rock In Sichuan Province

Dongcuo granite pluton distributed along Daocheng and Litang areas.Tectonically, it is located between in Ganzi–Litang Suture Belt and Jinshajiang SutureBelt. Dongcuo granite pluton is one of the largest granite in Yidun island arc. Therock was a huge arc bath olith, and accompanied by a lot of granite typhones. Itintruded into the Upper Triassic, such as Qugasi Formation and Tumugou Formation.Some researches was carried out, however, without, systematic analysis andunderstanding. In this thesis, based on the samples from the trunk route and profile,we studied the geochemistry and zircon SHRIMP U-Pb dating analysis of Dongcuogranite and following conclusions are got:1. Based on the field investigations, contact zone features and petrologicalcharacteristics, the evolution sequence of Dongcuo granite are put forward: thegranodiorite adamellite-porphyriticmonzogranite-K-feldspargranite and lagiogranite.Magma activities experienced four stages.2. Main body of Dongcuo granite pluton is tomonzogranite with a typical granitestructure and composed of plagioclase, feldspar and quartz, the biotite and amphiboleappeared as minor minerals. Feldspar was very self-shaped board or grain particles,meanwhilepolysynthetictwin and zonalstructuredeveloped.Micro-plagioclase was euhedral to subhedral platy particles, and double-crystal lattice developed.3.The geochemical data show the high SiO₂content ranging from60.51%to79.66%and averaged at74.6%; content of Al₂O₃ranged from8.04%to17.32%andaveraged at13.19%; content of K₂O ranged0.031%to3.22%and averaged at3.75%;content of Na₂O ranged from2.63%to5.63%and averaged at3.47%. Differentiationindex DI are in the range of54.5to96.1. The aluminum index（A/NKC） ranged from0.8to1.18. Hung David alkalinity index （K+N）/A are in the range of0.46to 1.01.Mafic index （FM） ranged from32.9to96.6and averaged at81. The Dongcuogranite belongs to calk-alkaline granite which originated in the environment of thecontinent. The Dongcuo granite is depleted of Ba, Nb and Sr. The ΣREE arerelatively low （29.7ppm-257ppm）. HREE ratios are （0.05ppm-1.67ppm） and（La/Yb）N ranged from0.508and15.55. All these data showed Dongcuo granite areenriched in LREE; the δEu values are less than1with obvious negative Eu anomaly（the δEu=0.15-0.38） and Ce anomaly is not obvious, which showed the Dongcuogranite formed in a volcanic arc–syncollision environment.4. CL images of zircons exhibited obvious magma-shaped growth ring,suggesting a magmatic origin. SHRIMPU-P b weighted average age of porphyriticbiotite monzonitic granite is199.0±2.4Ma and the measured point6.1is an inheritedzircon whose core age is958.7±6.4Ma which displayed the native zircon came fromPrecambrian Neoproterozoic; SHRIMP U-Pb weighted average age of medium tocoarse grained biotite granite is197.1±1.7Ma; SHRIMP U-Pb weighted average ageof K-feldspar granite is192.5±3.7Ma. These ages show that the main body ofDongcuo granite main intruded at the early Yanshanian and this is different with theprevious studies which believed that it intruded at Indosinian. As xenoliths and aresidual roof, fine granodiorite should be older than the199Ma, and at least can beinferred that the granodiorite intruded in the late Indosinian.