Metamorphism and chronology of Neoproterozoic granulite facies metasedimentary rock in northwestern Yangtze Block and its tectonic implications

The role of the Yangtze Block in the Rodinia Supercontinent is still debated. The garnet-biotite metatexite in the Micangshan area, northwestern margin of the Yangtze Block, mainly consists of garnet, biotite, plagioclase, K-feldspar, and quartz with minor ilmenite and accessory minerals of zircon and monazite. The prograde-stage is inclusions inside the garnet, including quartz, biotite, plagioclase, and monazite. The peak-stage is characterized by equilibrium garnet, biotite, plagioclase, K-feldspar, quartz, and ilmenite. The retrograde-stage represents the anhedral small flaky biotite and the coexistence of minor anhedral plagioclase and K-feldspar. Combining the P-T estimation from the pseudosection by Perple_X, quartz-in-garnet (QuiG) Raman-barometer, and conventional geothermobarometers, the prograde-stage P-T condition is constrained at 477–555°C, 5.0–6.5 kbar, and the peak and retrograde stages were determined as 800–830°C, 7–8.8 kbar and 603–674°C, 4.5–5.5 kbar, respectively. A clockwise P-T path was constructed. Metamorphic zircons in this rock yield metamorphic ages of 808 ± 5 Ma and 728–746 Ma. Zircons formed at ca. 808 Ma revealed a crystallization temperature of 727 ± 12°C, which is lower than the peak-stage temperature. Thus, zircons were regarded as retrograde products. Inherited zircons in this rock yielded 206Pb/238U apparent dates of 828–939 Ma. These zircons show geochemical characteristics of orogenic or arc-related origins. The 206Pb/238U dates of the monazite span in the range of 788–822 Ma, which suggests two centres at 817 Ma and 803 Ma. As monazite hosted in the garnet, it could record the metamorphic history from the prograde. Thus, 817 Ma is believed to be the time of peak-stage metamorphism, and the 808–788 Ma is interpreted as retrograde time. Based on all of the above, the northwestern margin of the Yangtze Block is suggested to have experienced the collision process, resulting in granulite facies metamorphism at ca. 817 Ma. This study provides additional constraints to decode the role of the Yangtze Block in the Rodinia evolution.