西藏蛇绿岩地幔橄榄岩中“石榴石”成因的后成合晶及其意义

In recent years, the discovery of abundant diamonds and other ultra-high pressure (UHP) mineral facies (indicative of metamorphism under the Transition Zone condition) in ophiolitic peridotites greatly challenged their traditionally-accepted shallower upper mantle (spinel-facies) origins. These ophiolitic peridotites are intermediate to low- pressure spinel/plagioclase- facies peridotites, while diamonds and other UHPs require much deeper equilibrium depth at least within the stability of high-pressure garnet-facies peridotites. Os-Nd isotopic studies show that these ophiolitic peridotites contain components of ancient lithospheric mantle in varying scales, indicating recycling of ancient lithospheric mantle into the convective upper mantle. In this project, we present systematic petrological, mineralogical and microstructural study of minor pyroxene-spinel symplectites preserved in the Purang and Zedang ophiolitic peridotites (south Tibet) along the Yarlung Zangbo suture, aiming to substantiate and characterize those symplectites that represent the breakdown product of mantle garnet, which may provide a new interpretation for the absence of mantle garnets in these diamondiferous ophiolitic peridotites. Then we try to clarify the intrinsic links between recycling of ancient subcontinental lithospheric mantle and UHP metamorphism of these Yarlung Zangbo ophiolitic peridotites within deeper mantle.

近年来，蛇绿岩地幔橄榄岩中金刚石等超高压矿物相（变质深度达地幔过渡带）的发现对传统的蛇绿岩浅部上地幔（尖晶石相）成因模型提出挑战。通常蛇绿岩地幔橄榄岩为中-低压的尖晶石/斜长石相橄榄岩，而金刚石等超高压矿物相的稳定深度至少在高压的石榴石相橄榄岩稳定区；另外Os-Nd等同位素揭示出这些地幔橄榄岩普遍含有不同规模的古老岩石圈地幔组分，暗示了对流上地幔中再循环古老岩石圈地幔的加入。本申请以西藏雅江缝合带普兰和泽当两个蛇绿岩地幔橄榄岩中保存的少量辉石-尖晶石后成合晶为研究对象，通过系统的岩相学、矿物学和显微组构研究，探讨它们是否为地幔橄榄岩中的石榴石分解产物，这可为解释含金刚石蛇绿岩地幔橄榄岩中石榴石相的缺失提供新视角和新的岩石学标志，并力图阐明雅江带蛇绿岩地幔橄榄岩中古老大陆岩石圈地幔的再循环与深部地幔超高压变质之间的内在联系。

蛇绿岩是构造就位到大陆边缘或造山带内的古大洋岩石圈残片。近年来，在全球多个蛇绿岩发现了金刚石等超高压矿物（>100-300 km），极大挑战了蛇绿岩来源于浅部大洋岩石圈的传统认识，并引发了关于金刚石是否在大洋岩石圈中普遍存在的激烈争论。在项目的资助下，我们对西藏雅江带含金刚石蛇绿岩开展了岩石学、矿物学和Os-Li同位素地球化学工作，通过对地幔橄榄岩中保存的辉石-尖晶石后成合晶体研究，证实它们为早期高压石榴石减压过程中退变产物，提出这些含金刚石蛇绿岩主要来源于深部（~100km）大洋岩石圈，为金刚石的自然成因提供了关键证据。通过对不同橄榄岩的矿物学和Os-Li同位素研究，建立了地幔柱驱动的成熟大洋岩石圈减薄、初始俯冲与含金刚石蛇绿岩剥露就位之间的内在联系，并解释了再循环古老大陆岩石圈地幔物质、金刚石等超高压矿物在蛇绿岩中的普遍出现。