中祁连东段老鸦峡辉长岩地球化学及锆石U-Pb定年研究

Geochemistry and LA-ICP-MS Zircon U-Pb Dating of Laoyaxia Gabbro in the East of Central Qilian

报道了新发现的青海拉脊山北侧老鸦峡辉长岩地球化学和锆石U-Pb定年成果。老鸦峡辉长岩出露面积0.35 km^2左右,由中细粒辉长岩和蚀变辉长岩组成。岩石地球化学测试显示Si O_2含量在47.33%~52.10%之间,Al_2O_3含量介于13.01%~16.74%,Mg O含量为6.41%~9.84%,Ti O_2含量为0.60%~1.44%,岩石具有钙碱性特征。大离子亲石元素Sr、Rb、Ba等富集,高场强元素Nb、Ta、Zr、Hf等亏损。岩石轻稀土元素相对富集,重稀土元素相对亏损,轻重稀土分馏明显特征。采集辉长岩锆石U-Pb测年,获得和谐年龄为447.6±2.2 Ma,NSWD=0.025,为加里东中期晚奥陶世。

This paper reports the geochemical characteristics and zircon U-Pb dating results about Laoyaxia gabbro rock in north side of Lajishan, Qinghai. The gabbro rock bead about 0.35 km2 in area consist of medium fine- grained gabbro rocks and altered gabbro rocks. Rocks have SiO2 contents of 47.33% -52.10% , MgO contents of 6.41% -9.84% , A1203 contents of 13.01% - 16.74% and low TiO2 contents of 0.60% - 1.44%. They belong to Calc-alkalin rock series. The large ion lithophile elements including Sr,Rb,Ba are richment relatively. The high field strength elements including Nb,Ta,Zr,Hf are deficit relativefy. The light earth elements are enrichment and the heavy earth elements are deletion in gabbro rock in which there is obvious fractionation between the both earth elements. Zircon U-Pb dating of gabbro yields a mean 206pb/23SU age of 447.6±2.2 Ma, NSWD =0. 025, which is the late Ordovician in the middle Caledonian.