Polygenetic titanite records the composition of metamorphic fluids during the exhumation of ultrahigh-pressure metagranite in the Sulu orogen

摘要

An integrated study of U-Pb ages and trace elements was carried out for titanite and zircon from ultrahigh-pressure (UHP) metagranites in the Sulu orogen, east-central China. The results provide constraints on the composition of metamorphic fluids during the exhumation of deeply subducted continental crust. Titanite has two domain types based on REE patterns and trace element variations, Ttn-I and Ttn-II respectively. These two domains show indistinguishable U-Pb ages of 232 +/- 14 to 220 +/- 8Ma, in general agreement with anatectic zircon U-Pb ages of 223 +/- 4 to 219 +/- 2Ma for the partial melting event during early exhumation. The Ttn-I domains have significantly higher REE, Th, Ta and Sr, and higher Th/U ratios than the Ttn-II domains, indicating that the two domains have grown from metamorphic fluids with different compositions. For the Ttn-I domains, Zr-in-titanite thermometry yields high temperatures of 773-851 degrees C at 2.5GPa, and petrographic observations reveal the presence of melt pseudomorphs. Thus, they are interpreted to have grown from hydrous melts in the early exhumation stage. In contrast, the Ttn-II domains were texturally equilibrated with amphibolite facies minerals such as biotite and plagioclase and contain inclusions of plagioclase and quartz. The Zr-in-titanite thermometry yields lower temperatures of 627-685 degrees C at 1.0GPa. In combination with their REE patterns, they are interpreted to have grown from aqueous solutions at amphibolite facies metamorphic conditions during further exhumation. The differences in Th and Sr contents are prominent between the Ttn-I and Ttn-II domains, signifying the compositional difference between the hydrous melts and aqueous solutions. Therefore, the polygenetic titanite in the UHP metamorphic rocks provides insights into the geochemical property of metamorphic fluids during the continental subduction-zone processes.