湖北黄梅乱泥滩花岗岩体的同位素年龄及成因初析

ISOTOPE AGE AND PRIMARY STUDY ON GENESIS OF LUANNITAN GRANITE IN HUANGMEI COUNTY,HUBEI PROVINCE

黄梅乱泥滩花岗岩侵入年龄为124.1Ma,完全冷却暴露地表年龄为80Ma,据岩石化学、稀土元素地球化学、^(87)Sr/^(86)Sr初始比值、∈Nd值、δ^(18)0值等综合分析,均显示该岩体具有“S”型花岗岩类的特征,其原始物质来自地壳。岩浆侵位深度为4.8km,结晶温度为798℃,氧逸度fO_2=1×10^(-11)a,压力P=6.57×10~8Pa岩体侵位时处于张性构造环境的陆内碰撞带。

Luannitan granite is major intrusive body in Meichuan complex intrusive body in Huangmei county, Hubei province. It is located in the southeast edge of Daibie mount, which is considered as a collision belt between Huabei terrain and lower Yangzi terrain. Its cropout area is about 370Km2. The wall rock is amphogneiss of Feihushan formation of Daibieshan group and plagioclase-gneiss of Qijiaoshan formation of Hongan group, which all formed in Proterozoic. Its intrusive age is 124.1 Ma but the age of its perfect cooling and exposing to the ground is 80 Ma. It went through 44 Ma from intrusion to perfect cooling which can be divided into four cooling stages: First stage (124.1-118.3Ma); Second stage (118.3-102.1 Ma); Third stage (102.1-87 Ma) and Fourth stage (87-80 Ma). According to petrochemistry, geochemistry of rare earth elements, Sr and Nd initial ra-tio,18O isotope and ACF diagram, we consider that the granite belongs to 'S' type, that its original material came from crust and it fromed in result of partial melting of the crust. The intrusive depth of magma is about 4.8 km, the temperature of crystallization is 798℃, the fO2 is 1 × 10-11Pa and the pressure is 6.57 × 108Pa. The granite body is intercontinental co-collision granite and its tectonic environment of diagenesis belongs to tension condition.