Abstract

TIBALDI, A.M.; OTAMENDI, J.E.  and  DEMICHELIS, A.H.. Metamorphic evolution of the pyroxene granulites associated with the Sol De Mayo and Suya Taco mafic complexes, north of Sierra de Comechingones, Córdoba. Rev. Asoc. Geol. Argent. [online]. 2007, vol.62, n.2, pp.175-186. ISSN 0004-4822.

This study reports the field relationships, petrology, mineral chemistry, and reaction history of garnet + orthopyroxene + cordierite- bearing granulites that occur as small lenses inside mafic igneous rocks. These high-temperature granulites were found in the Sol de Mayo and Suya Taco mafic complexes from the northern sierra de Comechingones. The most notable difference among the granulites of the two mafic complexes is one of the retrieved peak P-T conditions that are higher in Sol de Mayo (P ~ 8 kbar and T ~ 930 ºC) area than in Suya Taco complex (P ~ 6 and T ~ 780 ºC). However, reaction textures document a prograde-retrograde path that is roughly similar for all the studied granulites. Garnet and orthopyroxene formed to a large extent as peritectic products of partial melting, and melt segregation led to a semipelitic granulite residue containing garnet and orthopyroxene. The retrograde evolution is recorded by coronas of orthopyroxene around garnet, being garnet and orthopyroxene separated by plagioclase + quartz. Continued decompression is evident from subsequently formed symplectites of cordierite + sillimanite and cordierite + spinel, both replacing garnet. This succession of reactions attests to a change from quasi-isobaric heating to decompression followed by final cooling. The deduced clockwise P-T history generally reflects rapid heating during entrapment of the granulite precursor by the mafic magmas, and subsequent decompression driven by either ascent of magmas carrying granulitic xenoliths or uplift of the mafic complexes as a whole lithologic package, or a combination of both processes. In any event, these granulites provide reliable evidence for assessing the level of emplacement of the mafic complexes within the supracrustal sequence of sierra de Comechingones.

Keywords : Granulite; Sierras Pampeanas; Pampean; Thermometry; Barometry; Metamorphic evolution.

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