滇西保山地块晚三叠世火山岩特征及其构造环境

Characteristics and Tectonic Environment of the Late Triassic Volcanic Rocks in the Baoshan Block of Western Yunnan

滇西保山地块晚三叠世火山岩(牛喝塘组)具双峰式特征,主要岩性为富钠或钾的玄武岩、玄武安山岩和流纹岩,少量安山岩,局部伴有火山角砾熔岩。火山岩的特征是少数岩石含有矿物石英,个别岩石含橄榄石斑晶,多数岩石含辉石,主元素含量以富碱贫磷为特征,稀土元素具有总量高、轻稀土较富集,轻重稀土分馏显著的特点;微量元素强不相容元素普遍富集,Ni、Cr含量低。根据Cullers和Graf (1984)的分类,该期火山岩属陆内构造环境,明显不同于大洋或岛弧环境稀土总量较低和相对富重稀土的特征。同时,由于酸性火山岩大量存在,以及属于碱性和拉斑两个系列的基性火山岩组成的双峰式火成岩套,具有明显的大陆裂谷火成岩的特征,表明该期火山岩形成于大陆裂谷环境。

The Late Triassic volcanic rocks (Niuhetang Formation) had bimodal characteristic in the Baoshan block of western Yunnan, which the main lithology was Basalts rich in sodium or potassium, basaltic andesite, rhyolite, a small amount of andesite, and locally accompanied by volcanic breccia lava. The characteristics of volcanic rock were a few rocks containing mineral quartz, individual rock containing olivine porphyry, most of the rocks containing pyroxene. The main elements content was characterized by rich alkali and poor phosphorus. The rare earth elements that had the characteristics of high total, were rich in light rare earth, and the fractionation of the light rare earth to heavy rare earth was significant. The strong incompatible elements of trace elements were enriching generally, the contents of Ni and Cr were low. According to the classification of Cullers and Graf (1984), we believe that the volcanic rocks of this period belonged to intracontinental tectonic environment. It was obviously different from the characteristics of ocean or island arc environment that had lower total rare earth and relatively rich heavy rare earth. At the same time, because the acidic volcanic rock was abundant, and bimodal igneous suite consisted of basic volcanic rocks that belonged to two series of alkaline and lamellar. The bimodal igneous suite had obvious characteristics of continental rift igneous rocks. It showed that the volcanic rocks of this period formed in continental rift environment.