Revista de la Asociación Geológica Argentina
versión On-line ISSN 1851-8249
La Cocha ultramafic bodyis a spinel harzburgite composed of olivine, enstatite and spinel, almost tocompletely hydrated to associations of serpentine minerals. Some hornblenditeand spinel pyroxenites lenses intercalated in the serpentinite also arerecognized. Garnet - sillimanite gneisses associated with pyroxene amphibolitesand forsterite marbles are common country rocks. Forsterite marbles, forsterite- tremolite marbles, hornblendites and non-pyroxene amphibolites lens-likebodies are in an almost central position at the serpentinite body, also ascountry rocks. Finally, tabular and irregular leucocratic intrusives crosscutall mentioned rocks. The main structure of ultramafic body is a low cylindricalrecumbent fold interpreted as a major sheath fold, currently cut off byerosion. The ultramafic body present different degrees of weathering and atleast three events were identified. In the first stage permeating hydrousfluids affects the spinel harzburgite which was partially to totallyserpentinizated. With increasing serpentinization the rock increased in MgO, Al2O3and K2O and it was empoverished in trace elements and REE,especially HREE. Later, a second event represented by a serpentine veinssystems was produced by permeating hydrous fluid too. In a third event, in thesynformal hinge zone where axial plane cleavage develops, accessing silicatefluids produced enrichment in amphibol and Al2O3, Fe2O3(T),CaO, TiO2, REE, Sr, Zr y Hf, and empoverishment in MgO and K2O.
Palabras llave : Geochemistry; Serpentinite; Ultramafic massif; La Cocha; Sierras Chicas; Central Argentina.