西藏泽当地幔橄榄岩中的异常矿物及其指示意义

雅鲁藏布江蛇绿岩带内多个地幔橄榄岩体产有金刚石、碳硅石等异常矿物组合,为了进一步探讨这些异常矿物形成的物理化学条件,在前人已有的研究基础上,对泽当地幔岩体中526 kg的方辉橄榄岩样品开展人工重砂矿物学研究工作。研究表明,同雅鲁藏布江蛇绿岩带内的其他岩体相似,泽当地幔橄榄岩也选出了包含金刚石、碳硅石、锆石等30余种矿物。异常矿物组合指示泽当地幔橄榄岩中存在局部的超高压、极还原环境,可能经历了复杂的演化过程:即古老地壳物质通过深俯冲或者折沉作用,进入地幔甚至是地幔过渡带(410~660 km),随后经历了超高压、极还原环境的改造,在后续的地幔柱或地幔对流作用中从洋中脊上升至浅部环境并返回到地壳中。该过程中地幔橄榄岩中的异常矿物组合记录了岩石的演化信息,因此开展地幔橄榄岩中异常矿物组合的精细矿物学研究,对认识壳-幔物质交换以及深部地幔动力学过程都有重要的研究意义。

Implications of unusual minerals in Zedang mantle peridotite, Tibet

Abstract: The mineral assemblage of such minerals as diamond and moissanite has been discovered in several mantle peridotites in Yarlung Zangbo ophiolite belt. The authors collected 526 kg harzburgite rocks from Zedang mantle peridotite for artificial heavy mineral research so as to further investigate the physical and chemical environment of these unusual minerals. Diamond, moissanite and more than 30 other mineral species were discovered, similar to the result of other mantle peridotite in the Yarlung Zangbo ophiolite belt. These unusual minerals show that high pressure, super-reduction conditions and numerous continental crust materials existed in mantle peridotite, suggesting that ancient crust might have experienced continental deep subduction or lithosphere delamination into mantle or even into mantle transition zone (410-660 km) at the early stage; subsequenty these crustal materials were modified in an environment of high pressure and super-reduction in the deep mantle, and surged upward to shallow crust by mantle plume or mantle convection process. Unusual minerals were trapped within the mantle peridotite in such a recycling process. It is of great significance to understend crust mantle exchange and deep mantle dynamics for comprehensive research on mineralogy of unusual mineral assemblages in mantle peridotite.