Mid-Neoproterozoic (ca. 845 Ma) metamorphism of the southwestern Yangtze Block and its tectonic implications.

• Dahongshan Group in southwest Yangtze indicates a medium clockwise P-T path with a near-isothermal decompression. • Monazite indicates the peak metamorphic age is ca. 845 Ma with two possible substages at ca. 849 Ma and ca. 843 Ma. • Medium pressure metamorphism due to re-accretion of outborad terrane(s) to inner Yangtze during protracted oceanic plate subduction. • A major tectonic boundary lied on western margin of Yangtze in early-mid Neoproterozoic and occupied an external location of Rodinia. The evolutionary history of the Yangtze Block plays an increasingly important part in reconstructing Proterozoic paleogeography and associated supercontinent cycles. We document mid-Neoproterozoic amphibolite facies metamorphism related to crustal thickening and possible terrane accretion within an accretionary orogen along the southwestern margin of the Yangtze Block. Kyanite-staurolite bearing schists in the Dahongshan Group record a main deformation stage and a peak metamorphic assemblage Grt rim + Ky + St + Bt + Ms + Rt + Pl ± Ilm. Petrography and phase equilibrium calculations suggest a clockwise P-T path under medium-pressure facies condition. Monazite U-Th-Pb ages from the Dahongshan Group constrain the timing of peak metamorphism to 845 ± 6 Ma (n = 33, MSWD = 1.01), which overlaps the metamorphic climax in the northern segment of the southwestern Yangtze Block. Based on the comprehensive analysis of geochemical, metamorphic and sedimentary data from the Yangtze Block, we consider that the outboard domain occupied by the Dahongshan Group along with the Hekou, Huili, Julin (Yuanmou) groups constitutes a terrane(s) that was separated from the rest of the Yangtze Block by a marginal sea formed at the end of the Mesoproterozoic. Mid-Neoproterozoic re-accretion to the inner Yangtze Block during ongoing convergent plate margin activity resulted in the deformation and metamorphism of these rock units. Similarities in the Neoproterozoic evolution of the Yangtze Block, the Seychelles, and the western margin of Greater India suggest a protracted history of oceanic plate subduction periodically interrupted by coupling across the plate margin driving deformation and metamorphism. [ABSTRACT FROM AUTHOR]