Neoproterozoic Buchan-type Metamorphism of the Northwestern Margin of the Yangtze Craton and Tectonothermal Implications

The tectono-metamorphic mechanism along the margins of the Yangtze Craton in the Neoproterozoic has been a controversy for a long time. The Micangshan terrane is located at the northwestern margin of the Yangtze Craton and is mainly composed of Neoproterozoic metamorphosed volcanic and sedimentary sequences. In this work, two representative garnet-bearing two-mica schist samples and two representative garnet-bearing biotite-plagioclase gneiss samples were performed detailed petrological and geochronological research. Three genera-tions of mineral assemblages include the prograde (M1), peak (M2) and retrograde (M3) stages, respectively. Conventional geothermobarometry and phase equilibria modeling define the respective P-T conditions to be 2.0-3.0 kbar/596-686 degrees C for the M1, 2.6-4.8 kbar/632-693 degrees C for the M2 and 1.3-2.7 kbar/500-550 degrees C for the M3 stages and thus anticlockwise P-T paths were retrieved. The regional peak Buchan-type metamorphism with a higher geothermal gradient of ca. 40-75 degrees C/km was firstly identified in the northwestern margin of the Yangtze Craton. Secondary ion mass spectrometry (SIMS) U-Pb dating of magmatic zircon yielded the formation ages of the protolith of the rocks to be 838-863 Ma. SIMS U-Pb dating of metamorphic zircon and monazite LA-ICP-MS U-Pb dating yielded the metamorphic ages to be 820-794 Ma and 810-761 Ma, respectively, for the prograde (M1) to peak (M2) stage. Biotite 40Ar/39Ar dating yielded similar plateau ages and normal isochron ages of 750-745 Ma and are interpreted to represent the retrograde (M3) stage. Combined with previous studies, we propose that there was a transition of tectonic regime from compressional continental arc environment to extensional back-arc environment for the Micangshan terrane, which may be related to the southwards sub-duction of the South Qinling belt beneath the NW margin of the Yangtze Craton in the Neoproterozoic.