Redefinition of Caledonian and Hercynian regional metamorphism in the Baihaba-Chonghu'er district, northwestern Altay, Xinjiang

Abstract：Based on geologic mapping and integrated study in the Baihaba-Chonghuer district, northwestern Altay, Xinjiang, Lower Devonian fossils were collected from the strata that were originally assigned to the Ordovician or Sinian-Cambrian, and Caledonian and Hercynian isotopic geologic ages were acquired in some distinct plutons. Subzones with quartzite-like and thorn-like textures were distinguished in low-grade metamorphic rocks and kyanite, sillimanite, almandine and staurolite zones were recognized from regional dynamic heat-flow metamorphic rocks. The new evidence indicates that the Caledonian and Hercynian phases of regional low-temperature dynamic metamorphism occurred in the Baihaba district of the study area and that two phases of regional dynamic heat-flow metamorphism occurred in the southeastern part of the study area: the Caledonian phase in Hailiutan and the Hercynian phase in Chonghuer, which are superposed along the Kugen fault. This division gives a new answer to the dispute about the metamorphic types and generations in the area that lasted for nearly half century.