Abstract:

The Fangshan pluton exposed in Beijing is one of the typical Yanshanian intermediate-acidic calcalkaline intrusions occurred in the eastern part of North China Craton. For better understanding of the emplacement mechanism and the uplift-erosion history of the Fangshan pluton, we use the suitable Al-in-hornblende geobarometer to estimate the pluton’s emplacement pressure, and the hornblende-plagioclase geothermometer to calculate the temperature of crystallization. The results show that the Fangshan pluton crystallized at the temperature of 655.9-733.7 ℃ and the pressure of (0.39-0.28)±0.06  GPa. Based on the p-t conditions of the pluton crystallization, it is proposed 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 during the emplacement is 7-8 km, and the average erosion/uplift rate for the pluton is in range of 2.33-2.67 km/Ma.

Key words: Fangshan pluton, polybaric crystallization, Al-in-hornblende barometer, uplift and erosion