Tectono-metamorphic evolution of paragneiss in the Chengde metamorphic Complex, northern Paleoproterozoic Trans-North China Orogen

Abstract

Metasedimentary rocks can provide important information for reconstructing tectonic evolution of an orogen. In this study, we report the newly identified metapelite and metagraywacke samples from the Chengde Metamorphic Complex, the Trans-North China Orogen in the North China Craton. The garnet-bearing paragneiss generally occurs as country rocks of mafic granulite and amphibolite, and displays compositional layering with distinct gneissosity. The metapelite mainly consists of garnet, sillimanite, biotite, muscovite, plagioclase, and quartz, while the metagraywacke commonly comprises garnet, plagioclase, K-feldspar, quartz, and biotite/chlorite. Application of conventional geothermobarometers and phase equilibrium modelling constrains diverse peak metamorphic pressure–temperature (P-T) conditions (CD35, ∼800 ℃/10.5 kbar determined by phase equilibrium modelling; CD37, 677−716 ℃/9.6−9.8 kbar determined by geothermobarometers; CD45, 665−691℃/9.1−9.6 kbar and ∼ 745 ℃/11.2 kbar determined by geothermobarometers and phase equilibrium modelling respectively). The metapelite sample CD35 also records two retrograde stages, defining a clockwise P-T path with isothermal decompression (ITD) process. U-Pb dating of zircon and monazite yields metamorphic ages of 1910−1796 Ma, implying the timing of subduction to the lower crust. Combining previous studies in the Chengde Metamorphic Complex, we conclude that this complex is composed of imbricate rocks with different peak metamorphic P-T conditions, metamorphic grades, metamorphic ages, protolith origins, or protolith ages, indicating that these rocks were subducted to different depths in the subduction channel and then exhumed together to shallower crustal levels.