云南景谷岔河、茂密河火山岩及其构造环境

THE CHARACTERISTICS AND TECTONIC ENVIRONMENT OF VOLCANIC ROCKS IN CHAHE AND MAOMIHE OF JINGGU,YUNNAN PROVINCE

晚三叠世-早侏罗世,岔河、茂密河喷发了一套安山岩-英安岩-流纹岩与粗安岩的杂色火山岩系,火山岩属钙碱性与碱性系列。剖面从下至上,具有4个喷发旋回。按剖面厚度计算,火山岩总厚度岔河为921.72m,茂密河为709.69m。其中岔河火山碎屑岩占39.11%,茂密河占48.6%。主要熔岩普遍为斑状结构,斑晶为斜长石和角闪石,按体积计算,含量最高达55%(安山岩)。具有标准矿物石英分子。稀土总量由64.38突变到277.09,δEu接近1,(La/Yb)N比值由2.51变化到16.89,分配模式右倾斜。微量元素分配模式呈锯齿状,明显亏损Sr,P,Ti元素,w(Zr)/w(Y)和w(Ti)/w(Y)平均值分别大于3.5和小于400,K2O平均质量分数为2.99%。TiO2平均值为0.67%。上述特征充分说明岔河、茂密河的火山岩属大陆弧构造环境。结合火山岩共生的沉积岩有复成分砾岩、砂岩、泥岩、灰岩,研究区当时处于从滨海向浅海过渡的喷发环境。

A suite of volcanic rocks,which consist of andesites-dacites-rhyolites and mugearites, were extruded on the Chahe and Maomihe at the late Triassic period-early Jurassic period. The volcanic rocks are mostly calc-alkalic and alkalic series. Four volcanic cycles have been recognized on the section from the bottom to the top. The thickness of Chahe volcanic rocks is 972.72 meter and Maomihe 709.69 meter.The volcaniclastic rocks in Chahe account for (39.11%) and 48.6% in Maomihe. The structure of the main lava is porphyristic with porphyristic crystals of plagioclase and amphibole which can reach a maxmum amount of 55%. The volcanic rocks have standard quarts. The amount of the lanthanon increases abruptly from 64.38 to (277.09). δEu approaches 1. (La/Yb)_N is changed from 2.51 to 16.89. The distribution model is right oblique in the orientation. The distribution model of the trace element is the sawtooth seriel. The Si, P and Ti elements are evidently rare in comparison with the mantle-derived rocks. The ration of w(Zr)/w(Y) and (w(Ti))/w(Yb) is more than 3.5 and less than 500. w(TiO_2)% 0.67% on the average, w(K_2O) 2.99%.Therefore the volcanic rocks in Chahe and Maomihe belong to the continental arc environment. The sediments have complex conglomerates, sandstones, carbonate rocks and mudstones. The area belongs to the transional eruptive environment from the coastal region to the shallow sea.