西昌—滇中地区前震旦纪细碧-角斑岩系特征、成因及产出环境

The Characteristics, Origin and Tectonic Setting of Presinian Spilite-keratophyre Series in Xichangcentral Yunnan Region

西昌—滇中地区前震旦纪细碧-角斑岩系为早元古代(17—20亿年)产物。主要产出于大红山群和河口群。区域构造上大体分布于安宁河—易门深断裂的两侧,呈零星出露。其化学成分的显著特点是Na_2O含量高(6—8％),而且富铁(Fe_2O_3+FeO>10％),TiO_2的含量亦较高。根据岩石中残留有岩浆岩特有的结构、岩石组合、Sr同位素及^(87)Sr/^(86)Sr比值,认为岩石是由富钠的岩浆直接结晶而成。根据火山-沉积组合和野外标志,岩石是在半深海相环境中喷发和沉积的。

The Presinian spilite-keratophyre series in Xichang-central Yunnan region is confined mostly to the Dahongshan and Hekou groups. In regional tectonics it scatteredly exposed on the both sides of Anninghe-Yimen deep fracture. The spilitekeratophyre series is closely associated with eomagmatic volcanics or hypabyssal intrusions including albite gabbro-diabase, albitophyre and quartz albitophyre.The keratophyre represented by albite grannlite from the jiangyi-Pintan formation of Hekou group has a zircon U-Pb age of 1.725Ga, the migmatic gneiss in the Dibadu formations, which is overlaid by Dahongshan group, yields a Rb-Sr age of 1.706Ga, and the remant migmatic gneiss in Kangding group, a crystalline basement in this region, gives zircon U-Pb ages from 2.061 to 2.451Ga. Threrfore, Dahongshan and Hekou groups would have age from 1.7 to 2Ga. The spilite-keratopbyre series is petrochemically similar to those in the other parties of the world, except for having higher Na_2O (6-8wt％), Fe_2O_3 +FeO and TiO_2 and lower E_2O, MgO and CaO contents. According to the Wright alkalinity coefficient, the spilitekeratophyre in this region should be belonged to the subalkalic rock series.The presence of remained magmatic flowage structure, felsophyric and spherulitic textures in the rocks, absence of replacement remnant of intermediate and basic plagioclase in the albite and consitence of composition of Na_2O in the rocks indicate that the spilite-keratophyre in this region is neither metasomatogenic nor metamorphogenic but rather the magmatogenic rock resulted from the crystallization of intermediate-basic magma enriched in Na and H_2O. The samples from the spilite-keratophyre are plotted to the field A in the logτ-logσ diagram. On the basic of the rock association and some field critera. The spilite-keratophyre should be considered to be an eruptive-sedimentary in a bathyal environment.