Granulite facies corona textures in calcite-dolomite marble from La Huerta range, Province of San Juan, NW-Argentina

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

Meran, Southern Tyrol

Workshop; MITT. ÖSTERR. MINER. GES. 153 (MinPet 2007); 2007

Österreichischen Mineralogischen Gesellschaft

Dolomite bearing and dolomite absent layers can be distinguished, which behaved differently during prograde metamorphism. Mineral assemblages of dolomite absent lithologies contain quartz, scapolite, plagioclase, grossular, wollastonite and clinopyroxene, Calcite is treated as an excess phase in both lithologies. Calcsilicate reaction textures and phase equilibria in the isobaric T-XCO2 space, using the simplified chemical system CASCH [CaO-AI2O3-SiO2-CO2- H2O], at 6 kbar, allows us to postulate a first minimum temperature for the formation of scapolite after the reaction 3anorthite + calcite = meionite at 810°C and variable XCO2. Two distinct reaction textures can be described ; [1] quartz-calcite reaction rims replacing wollastonite formed after the reaction wollastonite + CO2 = quartz + calcite that took place at increasing pressure and slight cooling; [2] grossular reaction rims formed around scapolite after the reaction 3quartz + 5calcite + 1meionite = 3grossular + 6C02 during isobaric heating between 810 to 860°C. An internal increase of the andradite component in these grandite rims lead to higher values of XCO2 and temperature for this reaction. Mineral assemblages in dolomite bearing, MgO-rich, lithologies contain quartz, clinopyroxene, olivine, spinel, corundum and minor amounts of baddeleyite. At least three reaction textures can be distinguished. [1] Clinopyroxene-dolomite coronas engulfing forsterite; [2] quartz-calcite reaction rims around clinopyroxene; [3] formation of spinel on the expense of corundum. Due to the absence of water rich minerals, e. g. amphibole, serpentine minerals dry conditions, in the sense of XCO2 rich fluids, are considered for prograde metamorphism. Hence reaction temperatures are rather high in the T –XCO2 space. Reaction [1] and [2] took place during retrogression; [1] 2forsterite + 4calcite + 2CO2 = 1diopside + 3dolomite at 790°C and XCO2 =0.8; [2] 1diopside + 2C02 = 1dolomite + 2quartz; at 640°C and XCO2 = 0.9; [3] 1dolomite + 1corundum = 1spinel + 1calcite + 1C02 at 730°C and XCO2 = 0. 9. Estimated T-XCO, conditions of about 760°C at 6 kbar for the equilibria zircon + 2magnesite = baddelevite + forsterite + 2C02 agrees with our investigations of low- to medium- grade granulite facies metamorphic peak conditions.