Recognition and significance of c. 800 Ma upper amphibolite to granulite facies metamorphism in metasedimentary rocks from the NW margin of the Yangtze Block

Abstract

The evolutionary history of the NW Yangtze Block is important in interpreting its location and relationship with the Rodinia supercontinent. Although a Neoproterozoic arc-related tectonothermal event is recognized in the Micangshan area of the NW Yangtze Block, its timing and P–T conditions are poorly constrained. We address this issue by focusing on the garnet–biotite gneiss that represents the main lithology of the Huodiya Group on the NW margin of the Yangtze Block. This gneiss is predominantly composed of garnet, biotite, feldspar and quartz. The peak mineral assemblage in the matrix consists of garnet + biotite + quartz + plagioclase + K-feldspar. The retrograde stage is characterized by embayed rims of the garnet and its associated biotite in the matrix. The P–T conditions in the peak and retrograde stages are constrained to 7–8 kbar and c. 710°C and 5–6 kbar and 650–675°C, respectively, and suggest that 4–5 vol.% melt was produced during an upper amphibolite–granulite facies metamorphic event. The first report of monazite U–Pb dating in the Huodiya Group of the Micangshan area yielded a weighted mean 206 Pb/ 238 U age of 802 ± 5 Ma. The studied samples contained detrital igneous zircons with 206 Pb/ 238 U dates >800 Ma, whereas the metamorphic zircons yielded a weighted mean 206 Pb/ 238 U age of 797 ± 9 Ma. Based on the compressional tectonic environment and previous studies, we propose that the Yangtze Block was probably located on the periphery of the Rodinia supercontinent. Supplementary material: The analytical methods are summarized in Supplementary Text File 1. Representative mineral composition and the results of U–Pb isotopic analyses of monazites and zircons are presented in supplementary table 1–7 . The results of trace element concentrations in garnets are presented in Supplementary table 8 . https://doi.org/10.6084/m9.figshare.c.4698356