超高压变质作用过程中的流体岩石相互作用——中国大陆科学钻探工程主孔(0～2050m)岩心的氧同位素证据  

The fluid-rock interaction of UHP metamorphism:Oxygen isotopic study of the core rocks from the main hole (0～2050 m) of the Chinese Continental Scientific Drilling Project

作　　者：张泽明[1] 肖益林 Jochen HOEFS 高勇军 

机构地区：[1]中国地质科学院地质研究所,北京100037 [2]德国哥廷根大学地学中心

出　　处：《岩石矿物学杂志》2004年第4期289-297,共9页Acta Petrologica et Mineralogica

基　　金：国家重点基础研究发展规划项目 (973计划 ) (2 0 0 3CB716 5 0 1) ;国家自然科学基金重大项目 (4 0 399142 )

摘　　要：中国大陆科学钻探工程主孔位于苏鲁超高压变质带南部的东海地区。该钻孔 0～ 2 0 5 0m井段钻遇的岩石主要是榴辉岩、正片麻岩、副片麻岩和石榴石橄榄岩。激光原位氧同位素研究表明 ,变质矿物的δ18O值具有显著的不均一性 ,从 - 6 .8‰变化至 + 7.5‰。空间上看 ,从近地表至 10 0 0m深度δ18O值逐渐降低 ,向更深处又逐渐升高。位于钻孔 110 0～ 16 0 0m深度之间的正片麻岩是新元古代花岗岩体 ,该岩体的侵入为表壳岩与低氧同位素大气降水之间的水岩交换作用提供了热源 ,并导致岩体附近的围岩发生了最强烈的水岩蚀变 ,因此具有极低的氧同位素值。研究还显示 ,在厘米尺度内 ,大多数超高压变质矿物之间具有平衡的氧同位素组成 ,所获得的超高压峰期变质温度为 70 0～ 882℃ ,与矿物之间的主元素交换温度计获得的结果基本相同。超高压变形与变质分异、高压退变质作用及其伴生的局部流体活动可能是造成超高压变质矿物氧同位素不平衡的主要原因。研究进一步证明 ,超高压变质岩原岩的异常低氧同位素特征在大陆板块的俯冲与折返过程中得以较好保存 ,在超高压变质前期、同期和后期均没有大规模的透入性流体活动发生。与围岩呈构造接触关系的石榴石橄榄岩具有正常地幔岩的氧同位素成分 。The main hole of the Chinese Continental Scientific Drilling Project (CCSD) with a target depth of (5000) m is located in the southern segment of the Sulu ultra-high pressure (UHP) metamorphic belt in central eastern China. Nearly continuous core samples from the depth interval of 0～2050 m are UHP metamorphic rocks consisting of eclogite, paragneiss, orthogneiss (granitic gneiss) and garnet peridotite. In-situ laser oxygen isotope analyses of UHP minerals from drill cores indicate that the δ^(18)O values ranging from -6.8 ‰ to (+7.5‰) gradually decrease from the surface to the depth of 1100 m and then increase with the increasing depth. Late Proterozoic granitic intrusion (orthogneiss) occurring at the depth interval of 1100～1600 m must have been a heat source for the fluid-rock interaction between the supracrustal rock and the low oxygen isotopic meteoric water. Therefore, an extremely intense interaction between the granitic body and the country rock at the contact zone produced the lowest oxygen isotopic value in adjacent eclogite. Most UHP minerals show equilibrium oxygen isotopic compositions at a centimeter-scale. Peak-UHP metamorphic temperatures are estimated at 700～882℃ by the oxygen isotopic thermometer. Deformation and different degrees of recrystallization during syn-UHP metamorphism and post-UHP retrogression as well as related local fluid interaction may be the main factors responsible for oxygen isotopic disequilibrium between UHP and retrograde minerals. Studies also demonstrate that the extremely low-oxygen isotopic compositions of UHP minerals were preserved during the subduction and exhumation of the continental plate, and large-scale fluid flow did not occur during the pre-, syn-, and post-UHP metamorphism. Garnet-peridotites nearly 80 m thick are in tectonic contact with supracrustal rocks and have oxygen isotopic values of normal mantle rocks. This suggests that, in contrast to the deeply subducted supracrustal rocks, the ultramafic rocks are a tectonic slice derived from the ma

关 键 词：超高压变质作用 氧同位素 流体-岩石相互作用 岩心 中国大陆科学钻探工程 

分 类 号：P588.3[天文地球—岩石学] P597.2[天文地球—地质学]