The mafic- ultramafic rocks and surrounding basement of the La Huerta and Valle Fertil ranges, San Juan, Argentina

CASTRO DE MACHUCA, BRÍGIDA; MOGESSIE, ABERRA; PREVILEY, LORENA; BJERG, ERNESTO; KOSTADINOFF, JOSÉ; FERRACUTTI, GABRIELA; SCHNEIDER, ISABELLA

Schladming

Workshop; MITT.ÖSTERR.MINER.GES. (MINPET 2005); 2005

Mineralogical Society of Austria

Mafic-ultramafic outcrops in the Valle Fértil-La Huerta ranges form small bodies ranging from ten to a few hundred meters width within the more felsic metaigneous rocks. Field observations suggest a synplutonic origin in the partially crystallized felsic magma. The mafic-ultramafic rocks are mainly massive and coarse-grained and they are commonly cut by narrow mafic dikes (microgabbros) which exhibit chilled margins, suggesting that the mafic rocks were cool at the time of intrusion of the dikes. The dominant rock types are metagabbronorites and olivine-metagabbros. Pyroxenites are dominant among the ultramafic varieties (peridotites, hornblendites and lherzolites). Sometimes these rocks display relic igneous textures (allotriomorphic to hypidiomorphic-granular), in some cases of cumulate type. Occasionally, a rhythmic layering can be observed in the field. However, most of these rocks are strongly recrystallized and primary mafic minerals are almost completely replaced by metamorphic amphibole.The mineralogy comprises Ol (Fo83-75) and Pl (An100-99) considered to be igneous relics, with accesory Ilm and Mag. Abundant coronas of Opx (En82-76Fs18-23Wo0-1)+ Al-rich Spl and Cpx (En47-44Fs6-8Wo50-46) + Al-rich Spl rimming olivine are present. Ca-rich Am (Mg-hastingsite and pargasite) + Al-rich Spl is present as external rim of these coronas. At Ol-Pl boundaries the complete Ol -> Opx ± Spl-> Cpx ± Spl ->Am + Spl ->Pl reaction sequence can be observed, indicating a continuous metamorphic evolution as the result of slow cooling of the mafic-ultramafic bodies. Observed textures suggest reequilibration from lower granulite to upper amphibolite facies conditions during cooling, but preservation of relic magmatic textures demonstrates that the mineral transformations took place under conditions of little or no strain.