大别造山带超高压变质岩稳定同位素地球化学

STABLE ISOTOPE GEOCHEMISTRY OF ULTRAHIGH PRESSURE METAMORPHIC ROCKS

通过对大别造山带超高压变质岩的氢、氧和碳同位素地球化学研究，发现了显著的稳定同位素异常。结果对扬子板块向华北板块的俯冲提供了如下化学地球动力学制约： (1)北大别榴辉岩和基性麻粒岩与南大别超高压榴辉岩和片麻岩一样，具有较低的δ 18O值，表明它们在板块俯冲前经历了高温地表水热液蚀变，在超高压变质阶段流体以亏损 13C为特征； (2)部分低δ 18O值榴辉岩的共存矿物对之间保存了氧同位素平衡分馏，记录了榴辉岩相变质温度，指示这些榴辉岩在地幔的居留时间较短，折返初期的冷却速率较大； (3)退变质作用过程中存在显著的差异性氢氧同位素交换，早期阶段流体具有内部缓冲性质，而晚期则具有外部来源； (4)东大别大理岩显著富集 13C，反映出其母岩灰岩的沉积环境为封闭的大陆边缘盆地，西大别苏家河大理岩的δ 13C值接近于正常海相碳酸盐，指示其母岩形成环境为宽广开阔的海洋。

A comprehensive study of oxygen,hydrogen and carbon isotope geochemistry has been carried out for ultrahigh pressure metamorphic rocks in the Dabie orogen.A number of stable isotope anomalies have been discovered for the rocks.The results place constraints on the chemical geodynamics of the Yangtze plate subduction beneath the Sino- Korean plate:(1)eclogite and mafic granulite from North Dabie also have low δ 18O values as the ultrahigh pressure eclogite and gneiss in South Dabie,therefore their protoliths experienced hydrothermal alteration by surface water at high temperature before plate subduction,ultrahigh pressure metamorphic fluid is characteristic of 13C- depletion;(2)Oxygen isotope equilibrium has been preserved in part in lowδ 18O eclogites and yields reasonably concordant isotope temperatures,suggesting a short residence time when the eclogites were formed in the mantle and a rapid cooling in the early stage of their exhumation;(3)there are differential isotope exchange in the processes of retrograde metamorphism,the retrograde fluid in the early stage was internally buffered,whereas that in the later stage is of exotic origin;(4)marbles from eastern Dabie are significantly enriched in 13C and thus indicate a sedimentary environment of closed basin between the continental margins,where marbles from western Dabie haveδ 18O values as the normal marine limestone and suggest a sedimentary environment of open sea.