江南造山带西段桂北龙胜地区金车辉长岩矿物化学研究及其地质意义

Mineralogy and mineral chemistry of the Jinche gabbro in the Longsheng area of northern Guangxi in the western segment of the Jiangnan Orogen and its geological significance

桂北龙胜地区位于江南造山带西段,本文利用电子探针对桂北龙胜地区金车辉长岩单斜辉石和斜长石进行了详细的矿物学和矿物化学研究,研究表明单斜辉石的成分为Wo27~28En47~57Fs15~22,属于普通辉石,斜长石的成分为An1.35~9.05Ab90.34~97.45Or0.36~1.05,为钠长石。单斜辉石温压估算结果显示,其形成温度为1 250~1 350℃,该温度基本与软流圈地幔温度(1 280~1 350℃)相当,其形成压力为1.31~2.25 GPa,对应深度为43.2~74.3 km。单斜辉石成分特征指示金车辉长岩为板内拉斑玄武岩,结合区域地质特征,推测金车辉长岩是裂谷构造背景下软流圈地幔物质上涌并发生减压熔融的产物。

The Longsheng area in northern Guangxi lies in the western segment of the Jiangnan Orogen. In this paper, the authors determined the chemical compositions of the clinopyroxene and plagioclase in the Jinche gabbro by electron microprobe analysis. It turns out that the clinopyroxene is augite with Wo27 - 28En47 - 57Fs15 - 22, and the plagiolcase is albite with An1. 35 - 9. 05Ab90. 34 - 97. 45Or0. 36 - 1. 05 in composition. The crystallization temperature and pressure of clinopyroxene were estimated by the clinopyroxene-melt equilibrium and clinopyroxene isotherm, which yielded crystallization temperature, pressures and corresponding formation depth of 1 250 - 1 350℃, 1. 31 - 2. 25 GPa and43. 2 - 74. 3 km, respectively. The temperature is consistent with that of the asthenosphere lithospheric mantle（ 1 280 - 1 350℃）, and under that depth, the decompression melting of the asthenosphere lithospheric mantle can occur. Therefore, it is inferred that the Jinche gabbro might have originated from the asthenosphere lithospheric mantle. In addition, the chemical composition of the clinopyroxene indicates that the Jinche gabbro belongs to the tholeiitic series formed in an intraplate tectonic setting. In conclusion, the authors hold that the Jinche gabbro was formed by partial melting of the upwelling asthenosphere mantle due to rifting.