Mineralogy of the reaction zones in the contact between tonalitic intrusives and ultramafic rocks, Neoproterozoic-lower Paleozoic basement (Alta Gracia, Córdoba)

Abstract

This paper presents new mineralogical data of reaction zones developed in the contact between bodies of tonalitic composition and serpentinized ultramafic rocks, located in the eastern slope of the Sierra Chica, Córdoba. The reaction zones are greenish, brownish to light greyish coloured rocks and 0.1 to 1 meters thick. Three mineralogical zones are distinguished from tonalitic dykes (metatonalites) to ultramafic rocks: (i) magnesio-hornblende-rich zone, (ii) chlorite-rich zone (clinochlore) and (iii) Anthophyllite-rich zone. These reaction zones constitute black-walls and are interpreted as metasomatic rocks that resulted from chemical exchanges through diffusion and infiltration mechanisms between two chemically contrasting rock types (reaction skarns). The formation of these zones evolved from a first stage with high Si and Mg activity, followed by a Ca and Al-rich second stage. In a third stage, magnesio-hornblende and anthophyllite were altered to clinochlore, whereas a last stage involved the formation of vermiculite-rich veinlets in the magnesio-hornblende and clinochlore zones. The first and second stage were developed under amphibolite facies conditions, coincident with those established for the Pampean orogenic cycle in Sierra Chica, whereas the third and fourth stages respond to mineral associations in greenschist facies.