Formation of clinopyroxene plus spinel and amphibole plus spinel symplectites in coronitic gabbros from the Sierra de San Luis (Argentina): a key to post-magmatic evolution

The El Arenal metagabbros preserve coronitic shells of orthopyroxene +/- Fe-oxide around olivine, as well as three different types of symplectite consisting of amphibole + spinel, clinopyroxene + spinel and, more rarely, orthopyroxene + spinel. The textural features of the metagabbros can be explained by the breakdown of the olivine + plagioclase pair, producing orthopyroxene coronas and clinopyroxene + spinel symplectites, followed by the formation of amphibole + spinel symplectites, reflecting a decrease in temperature and, possibly, an increase in water activity with respect to the previous stage. The metagabbros underwent a complex P-T history consisting of an igneous stage followed by cooling in granulite, amphibolite and greenschist facies conditions. Although the P-T conditions of emplacement of the igneous protolith are still doubtful, the magmatic assemblage suggests that igneous crystallization occurred at a pressure lower than 6 kbar and at 900-1100 degrees C. Granulitic P-T conditions have been estimated at about 900 degrees C and 7-8 kbar combining conventional thermobarometry and pseudosection analysis. Pseudosection calculation has also shown that the formation of the amphibole + spinel symplectite could have been favoured by an increase in water activity during the amphibolite stage, as the temperature of formation of this symplectite strongly depends on aH(2)O (< 740 degrees C for aH(2)O = 0.5; < 790 degrees C for aH(2)O = 1). Furthermore, but not pervasive, re-equilibration under greenschist facies P-T conditions is documented by retrograde epidote and chlorite. The resulting counterclockwise P-T path consists of progressive, nearly isobaric cooling from the igneous stage down to the granulite, amphibolite and greenschist stage.