Mesozoic cooling across the Yidun Arc, central-eastern Tibetan Plateau: A reconnaissance 40Ar/ 39Ar study

Abstract

40Ar/ 39Ar dating of hydrous minerals from igneous and metamorphic rocks was undertaken with the aim of characterizing the thermal history of the Yidun Arc, eastern Tibetan Plateau. Regional metamorphism of Palaeozoic rocks of the western Yidun Arc occurred during the Early Triassic collision with the Qiangtang Block along the Jinsha Jiang Suture. Hornblende, muscovite, and biotite 40Ar/ 39Ar ages from syn-tectonic igneous rocks within this collision zone suggest that the region had cooled to below 300 °C by the Late Triassic. Muscovite samples from mica–schists within this collision zone yield variably discordant age spectra with generally Triassic 40Ar/ 39Ar ages, consistent with cooling of the region to below muscovite closure temperatures by the close of the Triassic. Hornblende and biotite 40Ar/ 39Ar ages from granite and granodiorite plutons in the eastern Yidun Arc suggest post-emplacement cooling of these plutons occurred rapidly, consistent with these granites being high-level intrusions. In sum, these results suggest that by the beginning of the Jurassic the Yidun Arc as a whole had cooled to below 300 °C. Two Cretaceous granites in the central Yidun Arc also experienced rapid cooling, suggesting they too were emplaced at shallow crustal depths. Post-Triassic deformation of the region must have occurred at or below the brittle–ductile transition within the crustal levels now exposed.