Geochemical Characteristics And Petrogenesis Of The Granites In Songzishu, Fengqing County, 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.Songzishu granite of Fenqing located in the northern section of Lincang granite batholith.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. There were discussed, based on actual rock mass setion measurement, cientific problems, such as magma sources,rock genesis,deep dynamic processes and evolutions of tectonization and magmatism, to focus on the research in petrography,mineral chemistry and geochemical characteristics of granites, in the northern section of the largest complex batholith, Lincang Granite Batholith that its 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, and also an important component of Paleo-Tethys,drawn great attention both at home and abroad, in Songzishu, Fengqing,Yunnan, in the thesis.The type of Songzishu granite is simple,the main rocks are medium-coarse porphyroid biotite adamellite. SiO2 range from 67.10% to 68.34%,averagely 67.49%; K2 O,Al2O3 and TiO2 content are higher,range 4.21% to 5.40%, 13.68% to15.38% and 0.57% to 0.68%,averagely 4.55%,14.32% and 0.52% respectively,Show the wealthy aluminum and titanium potassium rich characteristics.A/CNK ratio is high,ranging 1.68 to 2.05, with average of 1.83,? averagely1.83,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.?Eu range from 0.47 to 0.68,averagely 0.56,that means there had undergone fractional crystallization of plagioclase during magma evolution.Electron microprobe analysis indicates that biotite from Songzishu granites is Fe-biotite,which has a higher content of Ti but lower content of Al?;Most plagioclase are andesine and oligoclase and most alkali feldspar are orthoclas;The initial temperature of magma source(the upper limit temperature of granites) is 781.24-804.95?.Two-feldspar thermometer indicates that the lower limit temperature of solid separation is 548.23 and 524.53?;The rock forming pressure ranges from 2.74 to 3.41 kbar at the emplacement depth of 10.36 km to 12.13km;The crystallization temperature and oxygen fugacities(logfO2) of the biotite in songzishu granites are 679.46 to 726.26 ? and-9.8 to-11.2,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 songzishu granites are mainly originated from the remelting of continental crust materials,But there's mantle source material contamination, however Lincang granite batholith are coming from the dehydration melting of biotite in greywacke. Primitive mantle-normalized trace element patterns and chondrite-normalized rare-earth element patterns show that both forming in the same tectonic environment simultaneously,There 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 songzishu granites is less than 800?,indicating that it belongs to low temperature granites and its formation was probably related to fluid. Combined with tectonic setting discrimination diagrams,songzishu granites can be classified into the group of post-collisional granites.We consider that songzishu 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.