桐柏杂岩中印支期变质岩包体的变质作用

Metamorphism of Indosinian Metamorphic Enclaves in the Tongbai Complex,Central China

桐柏杂岩位于桐柏地区两条印支期高压变质带之间，长期以来被认为是一个前寒武纪变质杂岩体。由于岩石学和年代学研究资料的缺乏，对其构造属性和变质演化过程一直存在着模糊的认识。通过详细的野外地质调查，在桐柏杂岩中发现了含有石榴石的斜长角闪岩和片麻岩包体。对这些包体的变质作用研究发现，其主要矿物虽然受到晚期低温扩散的影响，但变质反应结构并不发育。变质作用温压估算结果显示，这些包体的峰期变质条件达到了高角闪岩相，其T=560-670℃，P=0．57-0．90GPa。年代学资料表明，桐柏杂岩的主体为强烈变形的燕山期花岗岩体，其中变质岩包体的变质时代为印支期。然而，由于在这些变质岩包体中尚未找到高压或超高压变质的证据，它们是印支期高压／超高压岩石的退变产物，还是只经历了中下地壳变质作用影响的中压岩石，尚需今后进一步的探索。

The Tongbai complex is situated between two Indosinian high-pressure （HP） metamorphie belts in the Tongbai area. This complex has long been thought to experience metamorphism during the Precambrian. Owing to the lack of petrological and geochronological data, the nature and metamor- phic process of the Tongbai complex remain unclear. By the detailed geological survey in the field, the garnet-bearing amphibolite and gneiss enclaves were discovered in the Tongbai complex. The studies show that the reaction textures were not developed in such metamorphic enclaves, although some main minerals have been affected by the late low temperature diffusion. Thermobarometry suggests that the peak metamorphism of the rocks reached upper amphibolite facies conditions of 560 - 670 ℃ and 0.57 - 0.90 GPa. Geochronological data indicate that the metamorphism took place during the Indosinian, whereas the hosting granitic gneisses of the Tongbai complex were emplaced during the Early Yanshanian. Since no HP/UHP evidence has been found in the metamorphic enclaves, whether these rocks are retrograde products of the HP/UHP rocks, or only medium-pressure rocks metamorphosed at the mid to lower crustal levels, further research is needed to answer this question.