Textures, reaction paths and structure in wollastonite-scapolite calc-silicate granulites from the San Carlos Massif, Eastern Pampean Ranges near Córdoba, Argentina

MARTINO, ROBERTO DONATO; GUERESCHI, ALINA BEATRIZ

Brasilia

Simposio; Granulites & Granulites 2006; 2006

Universidad de Brasilia

Several small outcrops (< 100 m2) of wollastonite-scapolite calc-silicate layered granulites are recognized in the migmatitic San Carlos Massif. This massif spread by more than 1500 km2, forming part of the northwestern tract of the Eastern Pampean Ranges near Córdoba (Argentina), coincident with a regional thermal axis of granulitic rocks oriented N 330º and extended by 150 km in lenght and 40 km in width. The calc-silicate granulites exhibit a sequence of reaction textures that allowed to determine their retrograde textural T-XCO2 path. The highest temperature recorded at the M2 metamorphic peak, which coevally develop the S2 main subhorizontal foliation coincident with the layering, is represented by the paragenesis: Wo + Scp + Di + Cal + Ttn ± Qtz. This paragenesis indicates at least 760°C and XCO2 > 0,25 at 6 Kb, based on experimental results on CASV system and pressure estimated from the surrounding metapelitic rocks. The calc-silicates granulites have partially re-equilibrated at lower temperatures, as evidenced by quartz coronas between wollastonite and calcite and Pl-Cal-Qtz symplectitic intergrowths replacing scapolite generated during pre-M3 cooling. Combined decreases of T, P and XCO2 by water-rich fluids infiltration during M3, have later developed garnet coronas around the symplectitic intergrowths. Further cooling and water infiltration have produced epidote from plagioclase in the late stages of M3. The layering of the rocks has controlled the nucleation and growth of garnet and epidote producing two paths: (a) garnet followed by epidote at higher temperature, and (b) epidote alone at lower temperature. Textural evidence shows that the retrograde garnet was not produced by reactions between wollastonite and scapolite at higher temperature, but it was originated at lower temperature and postdates scapolite breakdown to Pl-Cal-Qtz. A great variety of non-cylindrical folding (sheath, tubular, oblique and intrafolial refolded folds), developed mainly in the calc-silicate granulites and marbles during the sin- to postmetamorphic D2 deformation, point out a high non-coaxial constrictional deformation overprinted to S2 foliation. This D2 event, linked to the M2 metamorphic peak with an estimated age of 520 Ma, would represent a period of crustal thickening with thrusting and penetrative high strain. The general movements deduced from stretching lineations and scarce kinematic indicators show top to SW movements. After D2 it is recognized a D3 decompression event, associated to an extensional tectonic producing a weak deformation represented by shear bands perpendicular and parallel to S2. This event would have yield an isostatic exhumation deduced from the decompression textures found in metapelites and marbles. The D2-D3 deformational events and the M2-M3 metamorphic events would be related to a subduction following by a collision, developed on the western margin of Gondwana during the Pampia terrane emplacement onto this margin.