大别-苏鲁造山带橄榄岩的流体交代作用

Garnet peridotites in continental collision orogens are one of the most important components for many ultrahigh-pressure (UHP) metamorphic zones worldwide. They record important informations on the composition and evolution of mantle wedge above continental subduction zones, crust-mantle and fluid-rock interactions and chemical geodynamics at convergent plate boundaries. This project will focus on fluid metasomatism of orogenic peridotites in the Dabie-Sulu orogenic belt. The study will be performed on the basis of a careful petrographical study with special attentions to identification of fluid/melt inclusions and multiple solid inclusions. Mineral major-trace elements and radiogenic isotopes as well as water contents and stable isotopes of minerals will be investigated together with the study of metamorphic zirconology (including petrography, trace elementsm, U-Pb, Lu-Hf and O isotopes). The results will be used to determine the storage capacity of water in nominally anhydrous minerals from the garnet peridotites, to quantify the water distribution in the mantle wedge, to identify the mineral growth and recrystallization in response to the fluid metasomatism, to privide constraints on the origin and composition of crustal components in the fluid metasomatism, to determine the timing of the fluid metasomatism with respect to the subduction, peak metamorphism and exhumation of the deeply subducted continental crust, and to provide insights into crust-mantle interaction and fluid-rock interaction in the continental subduction zone.

大陆碰撞造山带石榴橄榄岩作为超高压变质地体的重要组成部分之一，记录了大陆俯冲带之上地幔楔的组成和演化、壳-幔相互作用、流体-岩石相互作用及俯冲带化学地球动力学等丰富信息。本项目以大别-苏鲁造山带石榴橄榄岩为主要研究对象，拟通过系统的岩相学(特别是矿物中熔/流体包裹体和多相固体包裹体的识别)、矿物微区主量、微量元素和同位素、矿物氢氧同位素和水含量分析以及变质锆石学(岩相学、微量元素、U-Pb、Lu-Hf和O同位素)的研究，确定造山带橄榄岩中名义上无水矿物的水储存能力及其控制因素，认识大陆俯冲带地幔楔水分布情况；区分流体交代作用所引起的矿物生长和重结晶，识别造山带橄榄岩中受到流体交代的地壳组分来源、性质和组成，确定流体交代作用与大陆俯冲和折返的时空关系；进而探讨大陆俯冲带壳-幔相互作用和流体-岩石相互作用。

造山带橄榄岩作为大陆碰撞带重要组成部分，为大陆俯冲带壳幔相互作用和流体-岩石相互作用提供了一个重要窗口。本项目以大别-苏鲁和柴北缘造山带橄榄岩为主要研究对象，结合它们的超高压变质围岩，开展了岩相学、全岩元素和同位素、矿物微区主微量元素和同位素、矿物氢氧同位素和水含量以及变质锆石学研究。结果发现地幔楔来源造山带橄榄岩的岩相学、全岩元素和同位素组成以及矿物元素和同位素组成都记录了造山带橄榄岩的地壳交代作用。发现造山带橄榄岩中锆石形成于地壳交代作用，新生长锆石为交代锆石且记录了造山带橄榄岩地壳交代作用的时间，而残留锆石则是由交代介质通过物理搬运进入造山带橄榄岩。发现大陆俯冲带中造山带橄榄岩的地壳交代作用通过流体-橄榄岩反应发生于洋壳俯冲向大陆碰撞转换的各个阶段但主要发生于深俯冲陆壳折返阶段；发现交代流体主要来源于深俯冲大陆地壳的不同部位或不同岩性岩石，但在一些造山带橄榄岩中存在先前俯冲大洋板片来源流体交代作用的证据；发现这些交代流体总体上富集Si-Al-Ca-Na-K, LILE 和LREE并含有一定量 HREE+HFSE，且在大陆俯冲隧道的不同深度表现出不同的性质和组成；发现造山带橄榄岩中存在碳酸盐熔体和含碳流体交代作用；发现大陆俯冲带流体-橄榄岩反应不但通过化学迁移将地壳组分带入造山带橄榄岩还通过物理搬运将锆石等难熔矿物带入造山带橄榄岩；发现地壳交代作用形成的辉石岩具有高的水含量，它们能参与形成高水含量和变化H2O/Ce比值的地幔储库。