Geochemistry of Precordillera serpentinites, western Argentina: evidence for multistage hydrothermal alteration and tectonic implications for the Neoproterozoic?early Paleozoic

FLORENCIA BOEDO; MONICA ESCAYOLA; SOFIA PÉREZ LUJÁN; GRACIELA VUJOVICH; JUAN ARIZA; MAXIMILIANO NAIPAUER

GEOLOGICA ACTA

UNIV BARCELONA

Lugar: Barcelona; Año: 2015 vol. 13 p. 263 - 278

1695-6133

Serpentinites are a powerful tool to evaluate mantle composition and subsequent alteration processes duringtheir 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 CentralAndes. In these regions a Neoproterozoic to Devonian mafic-ultramafic belt composed of serpentinites,metabasaltic dikes/sills, pillow lavas (with an Enriched to Normal Mid-Ocean Ridge Basalts (E- to N-MORB)geochemical signature) and mafic granulites crop out, spatially associated with marine metasedimentaryrocks. The serpentinite bodies consist of lizardite/chrysotile+brucite+magnetite, with scarce pentlandite andanhedral 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 intermediatechemical signature between a mid-ocean ridge and an arc-related ophiolite. Whole-rock Rare Earth Elements(REE) patterns of serpentinites exhibit enriched REE patterns ((La/Yb)CN=13-59) regarding CI chondritewith positive Eu anomalies. These features are the result of an interaction between hydrothermal fluid andserpentinites, in which moderate temperature (350º?400ºC), CO2-rich, mildly basic hydrothermal fluid wasinvolved 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 lowtemperatures(