Transformaciones texturales, mineralógicas y químicas en metagabros afectados por cizallamiento dúctil, sierra de La Huerta, San Juan, Argentina

CASTRO DE MACHUCA, BRÍGIDA; ARANCIBIA, GLORIA; MORATA, DIEGO; BELMAR, MAURICIO; PONTORIERO, SANDRA; PREVILEY, LORENA

La Plata

Congreso; XVI Congreso Geológico Argentino; 2005

Asociación Geológica Argentina - Universidad Nacional de La Plata

In the La Huerta range, Western Pampean Ranges, the orogenic activity of the Famatinian cycle decreased after the Mid-Ordovician. Final compression was concentrated along discrete and narrow NW-SE to NNW-SSE striking ductile shear zones. A particular outcrop of sheared metagabbro provides an excellent example that shows the entire strain-gradient fabric from undeformed or low-deformed metagabbro (protolith) to mylonite/ultramylonite. This study discusses mineralogical, microstructural and compositional changes experienced by the metagabbro during progressive deformation. The protolith, a granoblastic coarse-grained metagabbro composed of Opx+Cpx+An+Am±Ol and Al-Spl±Fe-Ti oxides, exhibits coronitic and symplectitic textures. Original texture is overprinted by banded enhance material segregation (stripped metagabbro) and grain size-reduction (mylonite/ultramylonite). Dynamic recrystallization of pyroxenes, amphibole and plagioclase point toward upper-amphibolite to lower granulite facies conditions (>650º-700ºC, ~5 Kbar) during mylonitization. Mineralogical and chemical changes between protolith and mylonite are not significant, except for an increase in Fe-Ti oxides and decrease in the abundance of Al-Spl, together with minor losses of MgO and FeO, and gains of alkalies, CaO and H2O. Thus, although strong textural changes occurred during mylonitization, they were not accompanied by notorious mineralogical and/or chemical changes, suggesting that chemical reactions were constrained by the anhydrous nature of the mineral assemblage of the protolith.