Metamorphic evolution of Mesozoic microcontinent suture zones in the Tibet region

Abstract

Microcontinent suture zones were generated by continental collision. However, collisional orogens often exhibit a series of metamorphic products with variable mineral assemblages. It is intriguing how crustal rocks at continental margins were metamorphosed at different geothermal gradients. In this study, we present a synthesis of metamorphic pressure (P), temperature (T) and time (t), as well as the P-T-t path, for Mesozoic medium- and high-grade metamorphic rocks in the Tibet region, focusing on fossil suture zones at Kangxiwa, Jinshajiang, Song Ma, Longmuco-Shuanghu, Changning-Menglian, Bangong-Nujiang, and Sumdo. The results show a series of common clockwise P-T-t paths, with metamorphic T/P ratios varying from 180 to 816 °C/GPa primarily in the Triassic and secondarily in the Permian and Jurassic. These thermobaric ratios allow for category of the target metamorphic rocks into low T/P Alpine type blueschist to eclogite facies series, moderate T/P Barrovian type amphibolite to granulite facies series, and high T/P Buchan type amphibolite to granulite facies series. These three types of metamorphic facies series were produced not only at variable geothermal gradients from 5.4 to 24.5 °C/km, but also at variable timescales from 25 to 60 Myr. The Alpine type metamorphism would mainly take place in the Triassic, indicating that the final closure of Paleo-Tethys Ocean is spatiotemporally associated with the opening of Neo-Tethys Ocean. The microcontinent assembly is predominated by the crustal cold subduction for the Alpine type metamorphism rather than the crustal warm collision for the prograde Barrovian type metamorphism. This difference indicates a relative lack of continental hard collision during the microcontinent assembly in the Triassic. Furthermore, the microcontinent suture zones were rarely remobilized in view of the rare Buchan type metamorphism in the Mesozoic. Nevertheless, the increase of metamorphic T/P ratios between these metamorphic rocks suggests the switch of dynamic regime from syn-collisional compression to post-collisional extension in these suture zones. In addition, time intervals for the dynamic switch vary from 25 to 60 Myr in the Mesozoic suture zones, suggesting differential periods of the tectonic transition from lithospheric thickening during the continental collision to lithosphere thinning. In either case, the Mesozoic Tibet region is considerably different in metamorphic evolution from either the Paleozoic ones in the northernmost margin or the Cenozoic one in the southernmost margin.