INTERACTION OF AQUEOUS FLUIDS WITH CALCAREOUS METASEDIMENTS DURING HIGH-T, LOW-P REGIONAL METAMORPHISM IN THE QADDA AREA, SOUTHERN ARABIAN SHIELD

The S.W. Nabitah Mobile Belt, Saudi Arabia, contains a Proterozoic isl and-are complex. In the Qadda area, the metavolcanic-dominated supracr ustal sequence records amphibolite facies regional metamorphism of hig h-T: low-P type. Calcsilicate rocks and aluminous dolomitic marbles wi thin the supracrustal sequence have been studied in detail to refine e stimates of peak metamorphic P-T conditions and assess the role of flu ids during prograde and retrograde metamorphism.Fluid-independent ther mobarometers (including the calcite-dolomite thermometer and P-sensiti ve equilibria involving grossular, wollastonite, anorthite, meionite, quartz and calcite) yield peak P-T conditions of c. 650-660 degrees C, 4 kbar, both higher than previous estimates, giving a revised average thermal gradient of c. 45 degrees C km(-1). The close match between t he peak temperatures implied by calcite-dolomite thermometry and those recorded by univariant devolatilization equilibria suggests that the calcareous rocks were fluid-bearing during late-prograde and peak meta morphic stages. These fluids were essentially binary H2O-CO2 mixtures with low NaCl and HF concentrations. Most were H2O-rich, with X(CO2) b etween 0.02 and 0.2, but values of c. 0.6 are recorded by two samples. High modal abundances of the solid products of decarbonation reaction s (e.g. c. 10-50% wollastonite) in many of the rocks that record low-X (CO2) equilibrium fluids implies infiltration of significant quantitie s of externally derived aqueous fluid during late-prograde metamorphis m, but not enough to exhaust the buffering capacity of the rocks. Calc ulated minimum time-integrated fluid-to-rock ratios of five wollastoni te-bearing calcsilicate rocks range from 0.7+/-0.22 to 1.39+/-0.46 (1 sigma); those of six marbles range from c. 0 to 4+/-1.4. The latter va riation occurs on a metrescale, implying focusing of fluid flow. Diops ide-rich rocks record fluid-to-rock ratios of up to 88+/-48. Penetrati ve wollastonite lineations indicate a temporal link between infiltrati on and distributed ductile deformation. Infiltrating fluids were proba bly derived both from the prograde dehydration of adjacent metabasalts and metatuffs and from crystallization of voluminous pretectonic gran itoid intrusions. In general, fluid-to-rock ratios deduced for the met avolcanic-dominated Qadda area are similar to those recorded by rocks in the metasediment-dominated terrane of N. New England. The occurrenc e of post-tectonic retrograde hydration textures in both carbonate-bea ring and carbonate-free rocks otherwise lacking hydrous minerals testi fies to infiltration of aqueous fluids during retrograde metamorphism in the absence of penetrative deformation. Minimum fluid-to-rock ratio s calculated for secondary grossular reaction rims in some calcsilicat es are c. 0.04. Later patchy hydration of scapolite probably utilized static, pore-filling fluids remaining after the early retrograde infil tration.