Fluid rock evolution of high-T calc-silicate marbles: constraints on the P-T-t history of a granulite facies terrain

GALLIEN, FLORIAN; MOGESSIE, ABERRA; BJERG, ERNESTO; DELPINO, SERGIO; CASTRO DE MACHUCA, BRÍGIDA

Oslo

Congreso; 33° International Geological Congress (33IGC); 2008

International Union of Geological Sciences (IUGS)

This study was carried out in the southern part of the Sierra de La Huerta, in the Province San Juan, NW-Argentina. The Sierra de La Huerta belong to the Western Sierras Pampeanas system and is part of a subduction related magmatic arc that experienced high grade granulite facies peak conditions at 860°C and 6-7 kbar in the Late Cambrian and Early Ordovician. The metacarbonate units of the La Huerta Range represent a heterogeneous sequence of layered impure dolomite-, calcite marbles respectively, 10-70 metres thick. High grade metamorphosed granulite-gneisses, migmatites and amphibolites represent the metamorphic basement which accommodates the marble units. The associated magmatic suite comprises tonalitic granodiorites, granites, gabbros - gabbronorites and pegmatites and is considered to be the advective heat source that leads to rather high temperatures at intermediate crustal levels. The marbles comprise a suite of Type (1) dolomite absent scapolite-wollastonite-grandite-clinopyroxene-quartz-calcite marbles; Type (2) diopside-forsterite-spinel-corundum-calcite marbles containing dolomite exolution lamellae and Type (3) serpentinized forsterite-spinel-dolomite marbles. An isobaric cooling path from peak-metamorphic 860°C to 750°C at 6.5 kbar is inferred based on Type (1) scapolite-wollastonite-grandite reaction textures. Mineral-reaction textures and stable carbon, oxygen isotope geochemistry implies a time related four step temperaturefluid/rock evolution that is recognizable in the three different marble types: (1) Infiltration of a pre-peak-metamorphic fluid, depleted in δ18O, affected all three marble types, and caused a depletion of the primary sedimentary oxygen values; (2) At incipient metamorphism, the processes of batch- and Rayleigh devolatilization became more important and contributed to a coupled, slight C-O depletion in the Type(1). The Type (1) marbles were not affected by a later fluid infiltration (water rich retrogression) out of isotopic equilibrium, hence the Type (1) marbles behaved as a closed system and still record the fluid-evolution liberated during prograde heating and high-T isobaric cooling. (3) Due to the low extent of retrograde reactions, such as hydration reactions, water absent conditions were dominant at isobaric cooling, when corona grandite was formed in the Type (1) marbles. (4) Intense hydration reactions of the Type (2) and (3) marbles lead to the formation of serpentinized forsterite, brucite, talc and tremolitic amphibole. Hence they record the latest event of fluid evolution and show strong depletion in the stable oxygen isotope values. The three marble Types of the La Huerta Range record the fluid evolution that occurs before, during and after peak metamorphism.