P-T conditions and timing of metamorphism of the Yuanmou area, southern Neoproterozoic Kang-Dian Orogenic Belt, southwest China

Abstract

Geological evolution of the Neoproterozoic Kang-Dian Orogenic Belt (KDOB, southwestern China) attracts great debate in the past decades, due to lack of systematic research on metamorphism and petrochronology. In this study, representative samples of metapelite and garnet amphibolite of the Yuanmou area, southern KDOB, were investigated to constrain their metamorphic P-T paths and timing. Metapelitic samples mainly consist of garnet + biotite + muscovite + quartz + zircon ± kyanite ± staurolite without plagioclase, while the amphibolite mainly contains garnet + amphibole + plagioclase + quartz + titanite. Two and three stages of metamorphic mineral assemblages were recognized in the metapelite and amphibolite, respectively. Clockwise metamorphic P-T paths were retrieved from the representative samples by geothermobarometry and phase equilibria modelling. The peak metamorphic P-T conditions of these samples are diverse, ranging from 7.2 to 13.6 kbar and 590–650 ℃, respectively. SHRIMP U-Pb dating of metamorphic zircon of the metapelite yielded different mean 206Pb/238U ages of ∼ 882–767 Ma, while LA-ICP-MS U-Pb dating of titanite of a garnet amphibolite sample yielded age of 834 ± 15 Ma. Combined with previous data, it is suggested that the Yuanmou area records subduction (as early as ∼ 885 Ma) to collision (before ∼ 682 Ma) processes of the southern KDOB, which experienced a prolonged orogenic process in the Neoproterozoic.