大别山双河片麻岩氢氧同位素特征及其变质地球化学意义

Oxygen and Hydrogen Isotope Characteristics of Gneisses Associated with Eclogite at Shuanghein in the Dabie Mountains and Their Implications for Metamorphic Geochemistry

对大别山双河地区与超高压榴辉岩共生的黑云母片麻岩和围岩花岗质片麻岩进行了详细的氢、氧同位素研究。结果表明，这两种片麻岩在氧同位素组成上存在显著差别：黑云母片麻岩与共生的榴辉岩δ１８Ｏ值呈同步变化，全岩δ１８Ｏ值从－４．７‰变化至＋１０．３‰；花岗质片麻岩的δ１８Ｏ值变化范围较小，全岩δ１８Ｏ值为－４．２‰～＋０．８‰。这些片麻岩中不同矿物之间基本达到并保持了氧同位素平衡分馏，黑云母片麻岩中石英－多硅白云母和石英－石榴石矿物对的氧同位素温度分别为４２５℃～６３５℃和６２０℃～７６５℃，花岗质片麻岩中则分别为３９５℃～５４０℃和５６０℃～６２０℃，均与其角闪岩相退变质温度基本吻合。因此这两种片麻岩的实测δ１８Ｏ值代表了其变质原岩的氧同位素组成。两种片麻岩中含羟基矿物的氢同位素组成相似，绿帘石（和黝帘石）的δＤ值均落在－３７‰～－６４‰范围内，黑云母为－７８‰～－１０９‰。黑云母低的δＤ值指示了其中含有大气降水信息。岩石冷却过程中发生的同位素退化交换作用以及变质作用期间矿物反应对氧同位素漂移影响不大，因此黑云母片麻岩和花岗质片麻岩的１８Ｏ亏损是岩石在遭受角闪岩相变质作用之前与流体进行同位素交换作用的结果。应用稳定?

The oxygen and hydrogen isotope compositions of mineral separates have been determined for two types of gneisses—biotite gneiss and granitic gneiss—associated with eclogites from the Shuanghe district in the eastern Dabie Mountains. The results show significant differences in oxygen isotope composition between biotite gneiss and granitic gneiss. The biotite gneiss has a large δ 18 O range of -4 7‰ to +10 3‰ similar to the associated eclogites, while the δ 18 O values of the granitic gneiss vary only from -4 2‰ to +0 8‰. Oxygen isotopic equlibrium, however, has been achieved and preserved among various minerals in these gneisses. Oxygen isotopic geothermometry based on quartz phengite and quartz garnet fractionations yields 425℃￣635℃ and 620℃￣765℃ for biotite gneiss, respectively; and those for granitic gneiss are 395℃￣540℃ and 560℃￣620℃, respectively. These isotopic temperatures are close to the retrograde metamorphic temperatures of amphibolite facies. Thus the protoliths of both gneisses have the δ 18 O values as low as -5‰￣-4‰. Hydrogen isotope ratios were determined for epidote / zoisite (-37‰￣-64‰) and biotite (-78‰￣-109‰) from both gneisses. The low δD values for the micas are consistent with an origin of ancient meteoric water. Because no significant shift in the oxygen isotope compositions of the gneisses was controlled by the retrograde exchange processes of oxygen isotopes between the minerals during cooling of the rocks and by the net transfer reactions between the minerals during metamorphism, the 18 O depletion of both gneisses is interpreted to result from isotopic exchange of fluid with the rocks before amphibolite facies metamorphism. According to application of fluid flow model on stable isotopes and the investigation of the associated eclogites, it is suggested that the correlative 18 O depletion of the biotite gneiss and associated eclogites is due to the infiltration of meteoric water into their precursor rocks prior to the ultrahigh pressure metamorphism. Therefore, the biotite gneiss and associated eclogites experienced the same geodynamic processes were thus part of a single tectonic entity throughout the course of subduction, ultrahigh pressure metamorphism and exhumation. They are foreign relative to the granitic gneiss.