Geochemical Characteristics Of Wangchengguan Granites And Their Implications For Tectonic Structure In Fengqing, Western Yunnan

Lincang granite batholith,located in the western Yunnan,is the largest complex batholith and also an important component of Paleo-Tethys,which has been drawn great attention both at home and abroad.Wangchengguan granite of Fenqing located in the northern section of Lincang granite batholith,overalling north-south trending.East side connected to the Yangtze microcontinent through the Lancang Jiang Fault Zone and west side to the Sibumasu microcontinent through the Shantouhou Fault and Xiqianjie Fault.Based on actual profile measurement,the thesis focus on the research in petrography,mineral chemistry and rock geochemical characteristics of Wangchengguan granites in the northwest of Fengqing,and compares with the Lincang granite batholith, then diseussed the seientific problems such as sources,mechanisms of genesis, deep dynamic processes and tectonic magmatism evolutions.The type of Wangchengguan granite is simple,the main rocks are medium-coarse porphyroid biotite adamellite. Si O2 range from 69.64% to 72.92%,averagely 71.24%; Al2O3 and K2 O content are higher,range 15.11% to 17.54% and 4.44% to 5.67%, averagely 15.98% and 4.90% respectively,showing the characteristics in alkali-rich and potassium.A/CNK ratio is high,ranging 1.33 to 1.53, with average of 1.42,σ varies from 2.05 to 2.23,averagely 2.15,belongs to high-K calc-alkaline and strongly peraluminous series.They are also enriched in LILE(eg.Rb,Th,K),but strongly depleted in HFSE(eg.U,Nb,Ta,Zr,Hf);Ba is obviously lower,and remarkably negative Sr,Ti,P anomaly,which shows the characteristics of continental crust remelting type.The standard curve of chondrite of rare earth element reveals they are rich in LREE and lack of HREE,with higher fractionation between LREE and HREE.δEu range from 0.28 to 0.39,averagely 0.33,that means there had undergone fractional crystallization of plagioclase during magma evolution.Electron microprobe analysis indicates that biotite from Wangchengguan granites is Fe-biotite,which has a higher content of Ti but lower content of AlⅥ;Most plagioclase are albite and most alkali feldspar are orthoclase and microcline;The initial temperature of magma source(the upper limit temperature of granites) is 787-859℃.Two-feldspar thermometer indicates that the lower limit temperature of solid separation is 378 and 365℃;The rock forming pressure ranges from 3.36 to 4.09 kbar at the emplacement depth of 12.7km to 15.2km;The crystallization temperature and oxygen fugacities(logf O2) of the biotite in wangchengguan granites are 508 to 547 and ℃-14.1 to-16.1,indicating that the biotite formed in a relatively higher temperature and lower oxygen fugacity environment.Based on a comprehensive analysis of the tectonic setting and geochemical characteristics,it is concluded that wangchengguan granites are mainly originated from the remelting of continental crust materials, coming from the dehydration melting of Muscovite in pelite, however Lincang granite batholith are coming from the dehydration melting of biotite in greywacke.Harker diagram, primitive mantle-normalized trace element patterns and chondrite-normalized rare-earth element patterns show that both forming in the same tectonic environment simultaneously,The are the products of partially melt in different parts of crust and then undergoing the crystallization differentiation to a certain degree.Magmatic melting temperature of wangchengguan granites is greater than 800℃,indicating that it belongs to high temperature granites and its formation was probably related to outside heat participation. Combined with tectonic setting discrimination diagrams,wangchengguan granites can be classified into the group of post-collisional granites.We consider that wangchegngguan granites was formed during the transferring setting from compression to extension between the Baoshan block and Simao block.Clay rocks had a large-scale partial melting by pressure decreasing and by mantle-derived magma as a heat resource.Paleo-Tethys ocean had occurred subduction at late-Permian.The continent-continent or continent-are collision had developed at early-Triassic.The evolution of post-orogenic phase had occurred in the region since the middle-to-Triassic time.After main collision,because of the effect of delay-warning,acid magrnatism derived from partial melting of thiekened rust at the late collisional orogeny,and then formed a huge Composit batholith,we called it the main body of “Lincang granite batholith”.