CONICET | Buscador de Institutos y Recursos Humanos

Granulite calc-silicate marbles occur in the Valle Fértil-La Huerta Ranges (30°00'-31°28' S and 67°15'-68°10' W) in San Juan Province, NW Argentina. These Ranges are constituted mainly by granulite gneisses and migmatites with interlayered siliceous marbles of various thicknesses. Tonalitic- granodioritic and gabbroic-dioritic rocks of Lower Paleozoic age are considered to provide the heat source for a regional contact metamorphism at about 6-7 kbar, with temperatures in excess of 860 °C. Based on the calcite-dolomite ratio three types of marbles are documented: Type (1) dolomite free scapolite-wollastonite-grandite-clinopyroxene-quartz-calcite marbles. Several reaction textures can be observed in these marbles; quartz-calcite replacement texture around wollastonite is the result of the CO2 consuming model reaction wollastonite + CO2 = quartz + calcite. Grandite coronas are developed between scapolite-calcite and wollastonite-scapolite. Based on observed reaction textures between scapolite and grandite the possible end-member reaction 1scapolite + 5calcite + 3quartz = 3grossular + 6CO2 is proposed. These textures are consistent with isobaric cooling from ~850 °C to 750 °C at 6.5 kbar. Grandite occurs as single grain aggregates with a patchy andradite-distribution and as reaction rims developed around scapolite and between scapolite-wollastonite. In Type (1) marbles clinopyroxene can be considered as an excess phase, present at any stage of textural equilibrium with the host. The grandite rims are andradite-rich at the grandite-clinopyroxene interface and grossular-rich at the scapolite-grandite interface. Local heterogeneities at the phase boundaries in the fO2 and CO2 are responsible for such chemical zonation, such as the high-andradite grandite developed at high temperatures and fO2. High oxidizing conditions at beginning of cooling are in good agreement with a CO2 dominated fluid, whereas low activities of CO2 become dominant at more reduced fO2 conditions. Type (2) diopside-forsterite-spinel-corundum-calcite marbles containing dolomite exsolution lamellae but no primary dolomite. Inclusions of coarse-grained corundum in spinel are considered to be evidence of the prograde spinel forming decarbonation reaction 1corundum + 1dolomite = 1spinel + 1calcite + 1CO2. Dolomite-clinopyroxene reaction coronas around forsterite are evident for the reaction 1diopside + 3dolomite = 1forsterite + 4calcite + 2CO2. Type (3) serpentinized forsterite-dolomite marbles with some brucite and tremolitic amphibole. Periclase or pseudomorphic brucite after periclase could not be found. A three step fluid evolution is recorded for these marbles based on stable oxygen and carbon isotopy. Type (2) and (3) marbles showing strong depleted δ18O ratios may record the passage of external infiltrated fluids at any point of the rock?s history and were much more susceptible for a pervasive retrograde influx of water-rich fluids depleted in δ18O. Whereas, Type (1) marbles were not largely affected by retrograde hydration reactions (H2O-consuming) only grandite garnet was locally consumed to form epidote-quartz-spinel symplectites. Additionally, stable isotope data from the suggested peak-metamorphic calc-silicate samples, containing the paragenesis wollastonite-scapolite-clinopyroxene and reactant grandite, rule out any pervasive fluid infiltration, out of isotopic equilibrium with the host. The limited extent of back-reactions in Type (1) calc-silicate layers suggests substantial updip expulsion of volatiles attending recrystallization (here decarbonatization) thus, only small amounts of H2O and CO2 were available for hydration and carbonation during uplift. The fluids, once liberated by decarbonation and dehydration during prograde metamorphism escaped from the reaction site and possibly fluid undersaturated conditions where present at high-temperature metamorphism.

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