Compositionally zoned Cl-rich amphiboles from North Dabie Shan, China: monitor of high-pressure metamorphic fluid/rock interaction processes

Different generations of amphiboles are related to different metamorphic stages of the granulite in Yanzihe, and provide a monitor for fluid/rock interactions and P-T evolution during the high-pressure metamorphism of Dabie Shan. Pressure and temperature estimates suggest that Am-in was formed during prograde metamorphism of ∼10 kbar and 700-800 °C; Am-I was formed under peak metamorphic conditions (∼20 kbar, 800-960 °C), whereas Am-II and Am-III were formed during retrograde metamorphic stages (560-770 °C and 5-7 kbar, and 520-670 °C and <5 kbar, respectively). In contrast to most previous studies, in which the earliest amphiboles to form are typically Cl-poor and later amphiboles become progressively Cl-rich, we show that the earliest amphiboles in the investigated rock are Cl-rich and later formed amphiboles are Cl-poor. The present study also demonstrates that the fluid system of the granulites in North Dabie Shan did not evolve in a simple way: while it behaved as a closed system during prograde and peak metamorphism, after the metamorphic peak it probably acted as an open system in which new fluids were introduced. The varying magnitude of Cl contents in amphiboles stresses the very local fluid control during metamorphism.