GARNET GEOCHEMISTRY RECORDS THE ACTION OF METAMORPHIC FLUIDS IN ULTRAHIGH-PRESSURE DIORITIC GNEISS FROM THE SULU OROGEN

Fluids play an important role in subduction-zone metamorphism. Therefore, it is important to elucidate the source and composition of subduction-zone fluids, the timing and P-T conditions of fluid action, and the geochemical consequence for fluid-rock interaction in subduction zones. The action of metamorphic fluids can be deduced from geochemical analyses of metamorphic minerals that grow from or are altered by fluids. Garnet has been widely used to constrain the P-T-t path of HP/UHP rocks. Its growth and recrystallization is usually associated with the action of metamorphic fluids. Thus, garnet can potentially record the action of metamorphic fluids during subduction-zone metamorphism.

We have performed a combined study of major and trace elements, water content and oxygen isotope composition for metamorphic garnet in UHP dioritic gneiss from the Sulu orogen, China. The results indicate the occurrence of a core-rim structure of garnet grains with growth of rims over cores via the mechanism of coupled dissolution-reprecipitation. The garnet cores and rims show distinct geochemical compositions. The cores have high spessartine contents, low grossular contents and low d¹⁸O values (~5.0‰). The rims have low spessartine contents, high grossular contents and high d¹⁸O values (~6.1‰). In addition, the cores exhibit low structural hydroxyl contents of 32 to 48 ppm H₂O, whereas the rims show much higher contents of 112 to 171 ppm H₂O. Although the boundary between core and rim is highly irregular in the garnet grains, the zoning in water contents, element and oxygen isotope compositions are all spatially correlated with each other. Taken all the observations together, the geochemical compositions of garnet cores and rims are primarily dictated by two types of metamorphic fluids with distinctive compositions. Mineral-pair O isotope thermometry yields consistent temperatures of ~550 ^oC, and titanite U-Pb dating yields a metamorphic age of ~226 Ma. Thus, the growth of garnet rims is linked to the action of metamorphic fluids during the exhumation of deeply subducted continental crust. Therefore, the compositions of garnet cores and rims are primarily controlled by the composition of attending fluids. As such, garnet provides an excellent mineralogical record of the fluid action during subduction-zone metamorphism.