蛇绿岩套中超基性岩体的岩石组合:蛇纹岩、异剥钙榴岩和蛇绿碳酸岩——以西阿尔卑斯Zermatt-Saas蛇绿岩为例

LITHOLOGICAL ASSOCIATION OF ULTRAMAFIC ROCKS IN OPHIOLITES: SERPENTINITE, RODINGITE AND OPHICARBONATE——AN EXAMPLE FROM ZERMATT-SAAS OF WESTERN ALPS

蛇纹岩、异剥钙榴岩和蛇绿碳酸岩是蛇绿岩套中超基性单元特有的3类岩石组合,该套岩石组合的形成过程复杂,经历了从地幔岩浆结晶分异、洋脊变质作用改造和俯冲-仰冲构造过程,记录了从地幔岩浆侵位到造山带形成、演化的全程信息。蛇纹岩由方辉橄榄岩、二辉橄榄岩和纯橄岩通过水化和氧化过程而形成;异剥钙榴岩由含水石榴石、符山石、绿帘石族矿物、透辉石和绿泥石等含水和含钙的硅酸盐矿物组成,是由基性岩经历钙交代和水化作用而形成;蛇绿碳酸岩则由高度破碎变形的蛇纹岩角砾和碳酸岩基质(方解石、白云石或菱镁矿)共同组成,碳酸钙主要来自海水参与蛇纹岩化过程产生的富钙热液。阿尔卑斯西部的Zermatt-Saas蛇绿岩体中这3种岩石的组合研究表明:蛇纹岩化过程发生在大洋变质时期,超基性岩体在海水的作用下形成蛇纹岩。蛇纹岩化过程中释放出主要来自斜方辉石和单斜辉石的钙,与水共同作用交代超基性岩体中的基性岩脉,从而形成异剥钙榴岩。蛇绿碳酸岩形成于俯冲变质之前或俯冲变质的早期。这3类岩石一经形成,都经历了其后的叠加变质作用,进而表明Zermatt-Saas蛇绿岩经历了大洋变质、与俯冲、折返和抬升有关的高压变质和区域变质、绿片岩相变质和晚期热液变质作用的pT轨迹演化,代表着西阿尔卑斯从洋脊变质作用到俯?

This paper describes the unique lithological association of serpentinite, rodingite and ophicar-bonate (SRO), and their genetic relationships in ophiolites. This rock association previously has not received much attention by researchers in China. Many of these rocks represent ancient oceanic crust, which has experienced seafloor alteration postdated the igneous activities, interaction with migrating melt/fluid before the ophiolite emplacement and a whole orogenic evolution after the emplacement. Study of this lithological association, therefore, could offer some insights into these processes. A brief review of research work done on this rock association worldwide is presented. Serpentinization of ultra-mafic members of ophiolite is resulted from hydration and oxidation of harzburgite, Iherzolite and du-nite. This process involves release of calcium, mainly from primary Ca-pyroxenes. Rodingitization is a process of calcium enrichment in mafic rocks enclosed in serpentinite. Rodingites are mainly composed of H2O-bearing and Ca-bearing silicate minerals such as hydrogarnet, diopside, vesuvianite, epidote group minerals and chlorite. Ophicarbonate rocks consist of highly brecciated and deformed fragments of ser-pentinized ultramafic rocks set in a matrix of carbonate (calcite, dolomite or magnesite). The carbonates were mainly precipitated from seawater-modified hydrothermal Ca-rich fluids involved in serpentiniza-tion. For a better understanding of the SRO association, rocks of Zermatt-Saas serpentinite complex of western Alps are used as examples to describe the evolution of mineral assemblages during an orogenic cycle. A detailed study of SRO association from Zermatt-Saas reveals that this ophiolite of Western Alps has experienced ocean-floor hydration, subduction-related high-pressure overprint, greenschist facies metamorphism and late hydrothermal alteration. Finally, some discussions on shortcomings in the research work of this lithological association and potential fields for further studies are also presented.