Abstract:

 A suite of mediumpressure metamorphic rocks composed of amphibolites, metapelites, marbles and intercalated quartzites occur in the Kuanping Group, Northern Tongbai. In this paper we report petrological results on four garnetbearing metapelites from this group, including a staurolitekyanitebearing sample collected from the Huilongsi Formation, and a sillimanitebearing sample taken from the Zuolaozhuang Formation. Garnet from samples in the Huilongsi Formation exhibits a pronounced compositional zoning that resulted from progressive metamorphism, whereas that from samples in the Zuolaozhuang Formation is weakly affected by late lowtemperature diffusion. In such a case, the garnet domains that grow at peak metamorphic stage were chosen to calculate its formation temperatures. Using conventional garnetbiotitemuscoviteplagioclasequartz geothermobarometry, pT conditions were estimated to be 590610 ℃ and 0.951.14 GPa for the Huilongsi Formation, and 630650  ℃ and 0.650.89 GPa for the Zuolaozhuang Formation, in agreement with lower and upper amphibolite facies, respectively. These pT estimates are in accordance with pT regions limited by metamorphic reaction curves. A transformation from kyanite to sillimanite is also indicated in the Zuolaozhuang Formation. The close association of lowerupper amphibolite facies Kuanping Group, lower amphibolite facies Erlangping Group and granulite facies Qinling Group with similar metamorphic ages (440400 Ma) suggests that collisional orogenesis occurred between arc, microcontinent and southern margin of the North China craton during the Silurian to the Early Devonian.

Key words: metapelite, mediumgrade metamorphism, Kuanping Group, Northern Tongbai