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

F.L. BOEDO ; M.P. ESCAYOLA; S.B. PÉREZ LUJÁN; G.I. VUJOVICH; J.P. ARIZA; M. NAIPAUER

GEOLOGICA ACTA

UNIV BARCELONA

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

1695-6133

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 areoutcrops of a Neoproterozoic to Devonian mafic-ultramafic belt composed of serpentinites, metabasaltic dikes/sills and pillow lavas (with an Enriched to Normal Mid-Ocean Ridge Basalts (E- to N-MORB) geochemicalsignature) and mafic granulites, spatially associated with marine metasedimentary rocks. The serpentinite bodiesconsist 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 moderatelydepletedharzburgites (ultramafic cumulates) with an intermediate chemical signature between a mid-ocean ridgeand an arc-related ophiolite. Whole-rock Rare Earth Elements (REE) patterns of serpentinites exhibit enrichedREE patterns ((La/Yb)CN=13-59) regarding CI chondrite with positive Eu anomalies. These features are the resultof 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 REEto serpentinites. The presence of listvenites (silica-carbonate rocks) in the serpentinite margins allow us to inferanother fluid metasomatism, where low-temperatures (<250ºC), highly-oxidized, highly-acid fluid lead to theprecipitation of silica. The association of these metasomatized serpentinite bodies with neoproterozoic continentalmargin sucessions and MORB magmatism at the suture zone of the Cuyania and Chilenia terranes suggests thedevelopment of an oceanic basin between them during the Neoproterozoic-Early Paleozoic.