北秦岭松树沟接触变质岩的氧同位素研究

An Oxygen Isotope Study of Contact Metamorphic Rocks from Songshugou in North Qinling

对北秦岭松树沟阿尔卑斯型超镁铁岩体接触变质带的角闪岩和斜长角闪片岩进行了全岩和单矿物氧同位素组成测定。矿物对氧同位素地质测温结果得到，角闪岩为７４５℃～７７０℃，角闪片岩为５００℃～６０５℃，均与已知的岩石学温度计结果相吻合，指示了这些变质岩内部的氧同位素平衡。靠近超镁铁岩体的角闪岩受热触变质作用影响，导致了矿物之间的氧同位素再平衡；远离超镁铁岩的角闪片岩基本未受热触变质作用影响，因此氧同位素温度代表了秦岭群区域变质岩的形成温度。角闪岩和斜长角闪片岩全岩δ１８Ｏ值为８．５～１０．３。

The oxygen isotope composition of whole rock and single minerals has been determined for amphibolite and pliagioclase amphibole schist within the contact metamorphic zone of an Alpine type ultramafic rok in Songshugou, North Qinling. Mineral pair isotope geothermometry yields temperatures of 745℃ to 770℃ for the amphiboite and 500℃ to 605℃ for the plagioclase amphibole schist. The isotopic temperatures are consistent with the results derived from petrological geothermometry, suggesting oxygen isotope equilibrium within the metamorphic rocks studied. The amphibolite is very close to the ultramafic rock and has thus been influenced by thermal contact metamorphism, resulting in oxygen isotope reequilibration among the minerals. The amphibole schist is far away from the ultramafic rock and hence has not undertaken the thermal contact metamorphism. As a result, the isotopic temperatures obtained for the amphibole schist can be used to represent the formation temperature of the regional metamorphic rocks in this region. The whole rock δ 18 O values of the amphibolite and amphibole schist range from 8.5‰ to 10.3‰, indicating that their protolith would probably be of the geochemical origin of igneous rocks.