Geochemistry of Precordillera serpentinites, western Argentina: evidence 1 for multistage 2 hydrothermal alteration and tectonic implications for the Neoproterozoic-Early Paleozoic.

BOEDO, F; ESCAYOLA, M. P.; PEREZ LUJAN SOFIA; VUJOVICH GRACIELA; ARIZA, J P; NAIPAUER, M.

GEOLOGICA ACTA

UNIV BARCELONA

Lugar: Barcelona; Año: 2015 vol. 4 p. 263 - 278

1695-6133

Serpentinites are a powerful tool to evaluate mantle composition and subsequent alteration processes during their tectonic emplacement. Exposures of this type of rocks can be found in the Argentine Precordillera (Cuyania terrane) and Frontal Cordillera, both located in central-western Argentina, within the Central Andes. In these regions are outcrops of a Neoproterozoic to Devonian mafic-ultramafic belt composed of serpentinites, metabasaltic dikes/sills and pillow lavas (with an E- to N-MORB geochemical signature) and mafic granulites, spatially associated with marine metasedimentary rocks. The serpentinite bodies consist of lizardite/chrysotile + brucite + magnetite, with scarce pentlandite and anhedral reddish-brown Cr-spinel (picotite, pleonaste and spinel sensu stricto) as relict magmatic phases. The original peridotites were moderately-depleted harzburgites (ultramafic cumulates) with an intermediate chemical signature between a mid-ocean ridge and an arc30related ophiolite. Whole-rock REE patterns of serpentinites exhibit enriched REE patterns ((La/Yb)CN=13-59) regarding CI chondrite with positive Eu anomalies. These features are the result of an interaction between hydrothermal fluid and serpentinites, in which moderate temperature (350º-400ºC), CO2-rich, mildly basic hydrothermal fluid was involved and was responsible for the addition of Ca, Sr and REE to serpentinites. The presence of listvenites (silica-carbonate rocks) in the serpentinite margins allow us to infer another fluid metasomatism, where low-temperatures (