川西沙坝麻粒岩原岩特征及变质作用

METAMORPHISM OF THE SHABA GRANULITES IN WESTERN SICHUAN

自从沙坝麻粒岩被发现以来，其形成演化一直颇有争议．它以残留体赋存于康定杂岩下部角闪二辉混合片麻岩中，形成于前寒武纪．通过对沙坝麻粒岩的地球化学特征的研究，认为它的原岩化学成分类似于大陆岛弧钙碱性玄武岩，形成于造山带挤压环境．麻粒岩相变质作用的峰期温度为７５６℃～７６５℃，压力在０．７３～０．８０ＧＰａ．继之之后，又经历了从角闪岩相到绿片岩相的退变质作用，其中，角闪岩相的变质温度为６２３℃～６５２℃，变质峰期压力为０．７０～０．７３ＧＰａ．

The granulite of Shanba is one constituent of the granulite belt located in the western margin of the Yangzi block.The petrogenesis and evolution of the Shaba granulites have been hotly debated since its discovry in 1980.It formed in Precambrian and occurred mainly as relicts in plagiogneiss of the lower Kangding complex.The main paragenesis is intermediary with 32-52 mole%An;the orthopyroxene is classified as hypersthene,the clinopyroxene is augite;the amphibole appears around the orthopyroxene,and is probably a retrograded product of the granulite.The whole rock compositions are plotted in the area of IAT(isand arc tholeithitic)basalt.The granulites have the enrichment of LREE (LREE/HREE=5.71).The geochemical features of rocks and minerals suggest that the source rock of the granulite of Shaba is calc alkaline continent IAT basalt that inrtruded in tectonically compressional setting.After the metamorphism of granulite facies,the Shaba granulite has undergone retrograded metamorphism from amphibolite facies to greenschist facies.The calculation using the two pyroxene geothermometers gave a temperature of 780℃～880℃ which is possibly close to the peak temperature of the granulite facies metamorphism.And also,calculation using the Al in hornblende geobarometer gave a pressure of 0.76 Gpa.It is permitted to suggest that the pressure for the metamorphism of granulite facies may range within the bounds of 0.76～0.8 Gpa,and that the pressure of metamorphism of amphibolite facies may be bounded above 0.76 Gpa.According to the geothermometer of the Ca distribution in coexsting amphibolie and plagioclase,a temperature of 500℃～630℃ is obtained as the temperature of the retrograded metamorphism of amphibolte facies.In this paper is discussed the petrogenesis of the granulite of Shaba.At Shaba, the large scale compression due to the horizontal movement of the Yangzi block resulted in the eruption of middle to basic volcanic magma,which formed as a suite of island arc basalt.Becauce of the continuous movement of the block,these basalts experienced metamorphism up to the granulite facies.