Quantifying Barrovian metamorphism in the Danba Structural Culmination of eastern Tibet

The Danba Structural Culmination is a tectonic window into the late Triassic to early Jurassic Songpan-Garze Fold Belt of eastern Tibet, which exposes an oblique section through a complete Barrovian-type metamorphic sequence. Systematic analysis of a suite of metapelites from this locality has enabled a general study of Barrovian metamorphism, and provided new insights into the early thermotectonic history of the Tibetan plateau. The suite was used to create a detailed petrographic framework, from which four samples ranging from staurolite to sillimanite grade were selected for thermobarometry and geochronology. Pseudosection analysis was applied to calculate P-T path segments and determine peak conditions between staurolite grade at approximate to 5.2kbar and 580 degrees C and sillimanite grade at approximate to 6.0kbar and 670 degrees C. In situ U-Pb monazite geochronology reveals that staurolite-grade conditions were reached at 191.5 +/- 2.4Ma, kyanite-grade conditions were attained at 184.2 +/- 1.5Ma, and sillimanite-grade conditions continued until 179.4 +/- 1.6Ma. Integration of the results has provided constraints on the evolution of metamorphism in the region, including a partial reconstruction of the regional metamorphic field gradient. Several key features of Barrovian metamorphism are documented, including nested P-T paths and a polychronic field gradient. In addition, several atypical features are noted, such as P-T path segments having similar slopes to the metamorphic field gradient, and T-max and P-max being reached simultaneously in some samples. These features are attributed to the effects of slow tectonic burial, which allows for thermal relaxation during compression. While nested, clockwise P-T-t loops provide a useful framework for Barrovian metamorphism, this study shows that the effects of slow burial can telescope this model in P-T space. Finally, the study demonstrates that eastern Tibet experienced a significant phase of crustal thickening during the Mesozoic, reinforcing the notion that the plateau may have a long history of uplift and growth.