UHP impure marbles from the Dabie Mountains: Metamorphic evolution and carbon cycling in continental subduction zones

摘要

Impure marbles from ultra-high pressure (UHP) metamorphic belts bear significant information on the metamorphic evolution and carbon cycling during continental subduction and exhumation. In this study, detailed petrological data are presented and a P-T-X(CO2) path is constructed for the impure marbles from the Dabie UHF terrane. Coesite relicts are discovered as inclusions within dolomite from the selected samples, which have a peak assemblage of dolomite, aragonite, garnet, omphacite, phengite, coesite, allanite and rutile. Estimated with the compositions of peak minerals, a P-T condition of 4.05-4.45 GPa at 740-820 degrees C is obtained by conventional geothermobarometry. The modeled fluid compositions have a low X(CO2) (0.01-0.02) at the peak conditions, while the X(CO2) firstly increased during isothermal exhumation and then decreased at later retrogression. The discovery of coesite within dolomite underscores the role of the "pressure vessel" models and highlights the significance of fluid unavailability in preserving coesite in UHP rocks. Neither petrological evidence nor independent peak P-T estimations support the breakdown of dolomite in the studied marbles, which contests recent suggestions. Analysis on the phase relations in the CaO-MgO-SiO2-H2O-CO2 system shows that the bulk rock compositions have a large control on the stable UHP carbonate associations in carbonate-bearing rocks. The low X(CO2) in the peak fluids indicates a weak decarbonation of the impure marbles under sub-arcs. In the last, a large fraction of CO2 is shown to be sequestrated during regional retrogression of clinopyroxene marbles, which has a profound influence and must be considered for the global carbon cycling. (C) 2014 Elsevier B.V. All rights reserved.