Chemistry, age and P-T estimates on dioctahedral K and Na-bearing micas of unconformity-related mineral assemblage of Tandilia belt, Argentina.

MARTÍNEZ J.C; DRISTAS, J.A.; MASSONNE, H.J.; FRISICALE, M.C.

Córdoba

Congreso; XIX Congreso Geológico Argentino; 2014

Asociación Geológica Argentina

The unconformity-related clay mineral assemblages of the most pervasively altered basement of the Tandilia belt were studied in two distant areas (~50 km). The dioctahedral K-white micas show Tschermak´s and celadonitic substitutions and widely variable Na*=Na/(Na+Ca+K) from muscovite to maximum values of Ms55Pg45 and Ms43Pg57 for the Barker-Villa Cacique and San Manuel areas, respectively. SEM images point at locally occurring paragonite/intermediate K-Na micas finely intergrown within K-white micas and pyrophyllite. P-T pseudosections (H2O saturated), worked out with PERPLE_X, for the Na-K-Fe-Mg-Mn-Al-Si-Ti-O-H system of three selected bulk-rocks constraint the minimum temperatures of formation of the observed clay mineral assemblages between 260 - 280 ºC. The predicted mineral assemblages, almost insensitive to pressure changes and with local occurrence of paragonite, include K-white mica + chloritoid + rutile + hematite ± pyrophyllite. Conditions of pressure and temperature of formation estimated by thermodynamic modelling on naturally occurring mineral assemblages with paragonite have been applied hitherto to Alpine and Variscan low-T metamorphic terrains (Árkai et al., 2008). The formation of such dioctahedral phyllosilicates has also been reported from alteration of precursor minerals in hydrothermal environments (Giorgetti et al., 2003). A 40Ar/39Ar step-heating age of 630 ± 30 Ma on a K-white mica rich sample points to a fluid-related thermal event of the diachronic late Neoproterozoic to Cambrian Brasiliano orogenic cycle in accordance with prior ages