大别山东南部高压—超高压变质岩的氢氧同位素地球化学

本文报道了大别山东南部安徽石马地区柯石英榴辉岩组合和黄镇地区石英榴辉岩组合的全岩及单矿物的氢氧同位素特征。石马榴辉岩的δ18O值在1.0‰～7.4‰之间,云母δD值为-76‰～-61‰;黄镇榴辉岩的全岩δ18O值为-1.1‰～-2.1‰,云母δD值为-89‰～-77‰。矿物之间的氢氧同位素分馏既有处于平衡状态者,也有处于不平衡状态者,反映这些岩石除继承变质前母体的18O亏损特征外,还经历了显著的同位素退化交换作用。石马和黄镇榴辉岩的矿物对氧同位素温度均给出两个峰值380～430℃与绿片岩相变质条件一致,600～640℃与高角闪岩相变质条件一致;给不出峰变质温度(750～800℃)是由于岩石经历了退变质流体的强烈改造作用。大别东南部高压-超高压榴辉岩的南部边界为石英榴辉岩与花岗片麻岩的界线,但氧同位素研究指示两者为原位接触关系。

Hydrogen and oxygen isotope geochemistry of high to ultrahigh pressure metamorphic rocks from Southeast Dabie.

The hydrogen and oxygen isotope compositions of single minerals and whole-rock were measured for coesite-bearing eclogite at Shima and quartz-bearing eclogite at Huangzhen in southeastern Dabie. The δ18O values of the eclogites vary from 1.0 to 7.4‰ at Shima but from -2.1 to 1.3‰ at Huangzhen; the δD values of micas from the eclogites are -76 to -61‰ at Shima and -89 to -77‰ at Huangzhen. Hydrogen and oxygen isotope fractionations between the minerals are in both equilibrium and disequilibrium, indicating that the eclogites have not only inherited the 18O-depletion feature of their protoliths, but also experienced strong retrograde alteration during exhumation. Oxygen isotope temperatures of the eclogite mineral-pairs are cumulated at two peak values of 600 to 640℃ and 380 to 430℃, respectively, corresponding to retrograde reequilibration at greenschist-facie and high amphibolite-facies conditions; the peak eclogite-facies temperatures of 750 to 800℃ cannot be retrieved due to the strong retrograde alteration. The high and ultrahigh pressure rocks in southeastern Dabie are geologically separated by the quartz-bearing eclogite and granitic gneiss, but the oxygen isotope analyses indicate an in situ relationship between the two rocks.