滇西六合地区新生代正长斑岩中深源包体的矿物学特征与成因意义

MINERALOGICAL CHARACTERISTICS AND PETROGENESIS OF DEEP-DERIVED XENOLITHS IN CENOZOIC SYENITE-PORPHYRY IN LIUHE, WESTERN YUNNAN PROVINCE

在云南省鹤庆县六合乡河东村的正长斑岩中产出多种含石榴石的深源包体。文中详细介绍了这些包体的岩相学特征和退变质特征 ,提供了包体中主要造岩矿物的化学成分分析结果。根据包体中的矿物组合及主要造岩矿物的化学成分特征划分出了 5种包体类型 ,并运用单斜辉石地质温压计、角闪石地质压力计和石榴石单斜辉石Fe Mg交换地质温度计计算了包体形成和退变质的温压条件 ,初步确定石榴石透辉岩及石榴辉石岩来源于上地幔 (87～ 95km) ,在快速上升到下地壳时被正长斑岩岩浆捕获 ;石榴石透辉角闪岩、麻粒岩形成于下地壳 (45～ 5 5km ) ,石榴石斜长角闪岩形成于中下地壳 (约 30km) ,为壳源岩石包体。除石榴辉石岩外 ,其余 4种包体均经历了退变质作用。根据它们的显微结构特征、矿物化学特征及温压计算结果 ,划分了两个变质期次 ,构筑了包体形成及退变质作用的 p T轨迹 ,讨论了退变质过程中压力 深度的变化及意义 ,并推断在退变质过程中包体经历了富Na流体的交代作用。

Several types of garnet-bearing deep-derived xenoliths have been discovered in syenite-porphyry in Liuhe Village, Heqing County, Yunnan Province. In this paper, we reported in detail the petrographic characteristics and retrogressive metamorphic phenomena of the xenoliths. The result of electronic probe microanalysis (EPMA) of main rock-forming minerals is also presented. Based on the mineral assemblages of the xenoliths and the chemical compositions of the main rock-forming minerals, the xenoliths are divided into five categories. The pressure and temperature of rock forming and metamorphism are estimated with clinopyroxene geothermobarometer, amphibole geobarometer and garnet-clinnopyroxene Fe-Mg geothermometer. It is concluded that garnet diopsidites originated from the upper mantle (87～95 km in depth) and were captured by syenitic magma as they moved upwards into the lower crust rapidly; garnet-diopside amphibolite and granulite were formed in the lower crust (45～55 km in depth); and garnet -plagioclase amphibolite came from the middle-upper crust (about 30 km in depth). With an exception of garnet diopsidite, the other four types of xenoliths all underwent retrogressive metamorphism. According to their textures, chemical compositions and estimated pressure and temperature, the p-T trajectory of the deep-derived xenoliths is constructed. It is inferred that the retrogressive metamorphism occurred in two stages, and fluid rich in Na played an important role in it. The significance of the change of pressure and depth during the retrogressive metamorphism is also discussed in this paper.