Study Of Phase Equilibrium Modeling On KMnFMASH System About Mica-schists Of The Contact Metamorphic Zones In Fangshan, Beijing

The Fangshan pluton exposed in Beijing is one of the Yanshanian intermediate-acidic calc-alkaline intrusions occurred in the eastern part of North China Craton. There are typical contact metamorphic zones in China around the pluton, including sillimanite, garnet-staurolite and andalusite zone in turn. As a great difference exist between the emplacement depth of the pluton and the metamorphic peak pressure, a comprehensive study is carried out on the Fangshan pluton and its contact metamorphic zones. The main new conclusions are the following.The Fangshan pluton crystallized at( 0.39-0.28) ± 0.06 GPa attained by Al-in-hornblende geobarometer and 655.9-733.7 ℃ in the use of hornblende-plagioclase geothermometer. The results suggest that the Fangshan pluton experienced polybaric crystallization during the emplacement. The P-T estimations for emplacement, integrating with the characteristics of rock structure and the zircon U-Pb ages of the lithological units of the Fangshan pluton, suggest that the Fangshan pluton has experienced slow-rate erosion/uplift during the early epoch of emplacement, followed by a rapid erosion/uplift epoch related to the emplacement of the lithologically dominated granodiorite unit. Overall, the P-T conditions estimated by this study suggest that the uplift distance of the Fangshan pluton is 7-8km, and the average erosion/uplift rate for the pluton is in range of 2.33-2.67km/Ma.P-T pseudosection and garnet/biotite isopleths show that the metamorphic peak condition is(0.32-0.42)±0.04 GPa, 580-690℃, up to amphibolite facies about the mica-schists of the contact metamorphic zones in Fangshan based on phase equilibria modeling on KMn FMASH( K2O-Mn O-Fe O-Mg O-Al2O3-Si O2-H2O) system by Thermocalc 3.33. The modeling results also show the contace metamorphic zones underwent clockwise P-T-t paths, which is nearly isobaric heating first, and then isothermal decompressure process. Furthermore, it can be testified that the mineral assemblages of some mica-schists in work area have residual minerals such as kyanite formed by regional metamorphism.Obviously, there is a good coupling relationship between the metamorphic peak condition of the wall rock and the emplacement condition of the Fangshan pluton. It is suggest that the emplacement depth of the Fangshan pluton started at 15-16 km, and ended 8-9km. Therefore, the Fangshan pluton might have formed magmatic diaper on the starting of emplacement, suffered from a rapid uplift and erosion under the regional compressional tectonic setting, and experienced polybaric crystallization during all the emplacement process.