云南西盟小马撒喜山期花岗岩体的岩石学特征及岩石成因

ON THE CHARACTERISTICS OF PETROLOGY AND PETROGENESIS OF HIMALAYAN STAGE GRANITE FROM XIAOMASA, XIMENG COUNTY, YUNNAN PROVINCE, CHINA

云南省西部西盟小马撒花岗岩体出露于西盟隆起大黑山复背斜西翼,与上元古界西盟群变质岩呈侵入接触。黑云母Rb-Sr年龄为16.7Ma,Rb-Sr全岩等时线年龄为50.5Ma,属喜山期。熔融包体均—温度750—968℃,气液包体均—温度255—405℃。^(87)Sr/^(86)Sr初始比值为0.7121—0.7183、δ^(18)O值为+9.70—+10.24‰。小马撒花岗岩为陆壳部分熔融的花岗岩浆高侵位结晶的产物,属东南亚含锡S型花岗岩之北延部分。

The granite body from Xiaomasa, Ximeng County, Yunnan province, China, is situated in the northeast verge of Burma Platform, Ximeng rise of 'Three River'(Jinsha River,Lancang River and Nujiang River)fold system.In this area, exposed strata are metamorphic rocks of Upper Proterozoic Ximeng group, they are micaschist, marble, granulite and migmatite. Xiaomasa granite body presents a irregular small stock, The stock is about 3 km long, and 2 km wide, with an area of about 3.7km^2. The major rock body consists of biotite monzonitic granite and muscovite-biotite monzonitic granite. The major mineral composition of the rock are orthoclass(St=0.1-0.75), microcline(St=0.1-0.9, △=0.24-1), anorthose(An=12-30, St=0.32-0.91), quartz, biotite and muscovite, with accessory mineral association: ilmenite, castelmaudite, zircon, apatite, monazite, schorl and garnet et al. According to the results of chemical analyses of 14 samples, average SiO_2 70.82, Al_2O_3 13.86, total iron(Fe_2O_3+FeO) 3.38, MgO 0.10-0.65,CAO 1.05—1.44,total alkali(Na_2O+K_2O)8.07-9.1,Al_2O_3/CaO+Na_2O+K_2O is 1.33-1.40. The differentiated index(DI) of the rock body is 86.6-91.2. The consolidation index(ST) is 1.92-9.76, Mg/Ti is 0.58. These parameters indicate that granite body could be tin-bearing granite. The total content of rare earth element(ΣREE) in the rocks is 59.489-139.353 ppm, with an average value of 90.37 ppm. LREE/HREE is 24.33-50.33(average 36.25). Europium anormal index(σ Eu) is 0.149-0.45. These data suggest relative rich LREE, but lower Σ REE in this rock body. The homogeneous temperatures of the melting inclusion in the rocks are 750°-968℃, those for fluid inclusion are 255°-405℃. The initial ^(87)Sr/^(86)Sr ratio is 0.7121—0.07183, the whole-rock has δ^(18)O-values between +9.70—+10.24‰, whole-rock Rb-Sr isocnron age is 50.5 Ma. Therefore, the definitive time is Himalayan stage. The type of the rock is S-type granite(Chappell and White, 1974). The characteristics of petrology and geochemistry from Xiaomasa granite suggest the petrogenesis that could be related to an Eocene subduetion zone in this region.