Abstract:

The Tongbai complex is situated between two Indosinian high-pressure (HP) metamorphic 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 metamorphic 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.

Key words: Tongbai complex, metamorphism, upper amphibolite facies, Indosinian