High-temperature Retrogression of Granulite-facies Marbles form the Reynolds Range Group, Central Australia: Phase Equilibria, Isotopic Resetting and Fluid Fluxes

Abstract

Reynolds Range Group rocks underwent granulite-facies metaINTRODUCTION morphism (M2) at ~1·6 Ga (~5 kbar, 750–800°C) and were The uplift history of high-grade metamorphic belts comsubsequently retrogressed in narrow strike-parallel zones at 1·59– monly involves partial re-equilibration of mineral as1·57 Ga. Within these zones, metacarbonates that initially equisemblages to lower temperature and pressure conditions. librated at X2>0·8 during M2 were mineralogically reset This re-equilibration is facilitated by fluid–rock interby the infiltration of water-rich fluids (X2Ζ0·02–0·3) at action, and new mineral assemblages are often best 650–700°C and 3–4 kbar. dO(Carb) values of the retrogressed developed in discrete zones of extensive retrogression metacarbonates were variably reset during fluid infiltration, with the (e.g. Van Reenen, 1986; Cartwright, 1988; Peters & lowest values (10–13‰) suggesting that the fluids that caused Wickham, 1994; Cartwright & Buick, 1995). Petrological retrogression were exsolved from segregated partial melts, themselves studies of retrograde mineral assemblages may provide derived from the underlying granulite-facies metapelites. Minimportant information about the tectonics and hydroeralogical and isotopic resetting were locally accompanied by silica geology of the lower and middle crust. metasomatism. The mineralogically reset marbles record time-inHowever, although there have been many recent quantegrated fluid fluxes of typically ~10–10 m/m, and metasomatic titative studies of metamorphic fluid–rock interaction in rocks record time-integrated fluid fluxes as high as ~10–10 m/ contact aureoles (Cartwright & Valley, 1991; Ferry & m. For upward flow of high-temperature fluids through the marbles Dipple, 1991; Young & Morrison, 1992; Jamveit et al., over a distance of ~200 m in ~18 Ma, the observed mineralogical 1992; Davis & Ferry, 1994) and during prograde regional and isotopic resetting, and metasomatism require intrinsic permetamorphism (Bickle & Baker, 1990a, 1990b; Ferry, meabilities between ~10 and 10 m that vary across strike 1992; Stern et al., 1992; Leger & Ferry, 1993; Cartwright on a centimetre to metre scale, indicating that fluid flow was strongly et al., 1995; Buick & Cartwright, 1996), there have been channelled. relatively few studies of high-temperature retrogression (Vennemann & Stuart-Smith, 1992; Peters & Wickham, 1994; Cartwright & Buick, 1995). Therefore, the origins of the retrograde fluids, and the absolute timing of