Low-pressure anatexis in Famatinian foreland of Argentina, south-western margin of Gondwana: source heat problem

SEBASTIÁN OSVALDO VERDECCHIA; EDGARDO GASPAR BALDO

Mar del Plata, Provincia de Buenos Aires, Argentina

Congreso; Geosur 2010, International Geological Congress on the Southern Hemisphere; 2010

Within the foreland region of the Famatinian belt in central-western Argentina crop out low- to medium-pressure metamorphic complexes, one of them being the Ordovician metasedimentary rocks from La Cébila. This complex was affected by a main tectono-thermal event (M1) characterized by low pressure (2-2.5 kbar) and high thermal gradient (~80º C/km) conditions. In this complex, a well-defined nearly isobaric metamorphic zonation was developed that progrades towards the intrusive contact with syn- to post-tectonic peraluminous granitic bodies (S-type; 760-780º C crystallization temperature). The metamorphic grade increases from white mica-chlorite (~300-400º C; Kübler index values from 0.23 to 0.17 Δº2θ) to cordierite-K-feldspar (714-740º C; Ti-in-biotite thermometry) zones, through the andalusite and sillimanite stability fields, reaching anatexis conditions with the development of a wide migmatitic belt (1-3 km) adjacent to the granites. In the M1 event (~460 Ma), a well-defined secondary foliation was developed with syn-kynematic blastesis of andalusite and K-feldspar porphyroblasts, associated with a strongly compressive episode (D1). Granitic magma by itself, because of its relatively low crystallization temperature, would not be a suitable heat source to explain the widespread melting in the country rocks, thus ruling out contact metamorphism as the single cause of metamorphism.            The paleontological record (brachiopods in quartzites metamorphosed under sillimanite zone) suggests a marine basin in the foreland in the Early to Middle Ordovician (480-460 Ma). High heat flow during the extensional regime would explain the low-pressure metamorphism in shallow crustal levels (<9 km), but it is difficult to produce anatexis without mafic intrusion associated. The integration of sedimentary, metamorphic and igneous information suggests that heat source of the main metamorphic event could be related to a combination of mechanisms: high heat flow crustal regimen related to the extensional period and immediately a plutonic intrusion (S-type granite) related to the compressional regime.