Dolomitic marbles and associated calc-silicates, Tandilia Belt, Argentina: geothermobarometry, metamorphic evolution and P-T path.

DELPINO, S.; DRISTAS, J.

JOURNAL OF SOUTH AMERICAN EARTH SCIENCES

Elsevier

Lugar: Amsterdam; Año: 2008 vol. 25 p. 501 - 525

0895-9811

The metamorphic evolution of dolomitic marbles and associated calc-silicate rocks from Punta Tota (NE Tandilia belt, Buenos Aires province, Argentina) has been evaluated through petrographic, geothermobarometric, and fluid inclusion studies. Thin beds of dolomitic marble are intercalated in amphibolites and constitute the upper part of a stratified basement sequence, which starts at the base with garnet migmatites showing a great abundance of pegmatitic segregates, overlain by biotite–garnet gneisses. Peak metamorphic conditions are estimated at 750–800 C and 5–6 kb, followed by near isobaric cooling to about 500–450 C and 5.5–6.5 kb. Anhydrous progressive metamorphic assemblages in both marbles (Fo + Cal + Dol + Cpx + Spl) and adjacent calc-silicate rocks (Cpx + An + Cal + Qtz) strongly retrogressed to hydrous minerals (Tr, Tlc, Grs, Czo, Srp) with decreasing temperatures and increasing water activities. The intense rehydration of the rocks relates to the emplacement of volatile-rich pegmatitic bodies (Qtz + Pl + Kfs + Bt + Grt), which also resulted in the crystallization of clinochlore + phlogopite in the marble and biotite + muscovite in the adjacent calc-silicate rocks. Metamorphic reactions based on textural relations and evaluated on a suitable petrogenetic grid, combined with geothermobarometric results and fluid inclusion isochores, indicate a metamorphic evolution along a counterclockwise P–T path. Two probable geotectonic settings for the determined P–T trajectory are proposed: (1) thinning of the crust and overlying supracrustal basin in an ensialic intraplate tectonic setting and (2) development of a marginal back-arc basin, associated with an oceanic–continental convergent plate margin. In both models, the initial extensional regime is followed by a compressional stage, with overthickening of the basement and supracrustal rocks, during the climax of the Transamazonian cycle at approximately 1800 Ma ago. Continuous convergence and blockage of structures produce transition to transcurrent tectonics (transpression) with a consequent moderate uplift.–T path. Two probable geotectonic settings for the determined P–T trajectory are proposed: (1) thinning of the crust and overlying supracrustal basin in an ensialic intraplate tectonic setting and (2) development of a marginal back-arc basin, associated with an oceanic–continental convergent plate margin. In both models, the initial extensional regime is followed by a compressional stage, with overthickening of the basement and supracrustal rocks, during the climax of the Transamazonian cycle at approximately 1800 Ma ago. Continuous convergence and blockage of structures produce transition to transcurrent tectonics (transpression) with a consequent moderate uplift.