Metasomatic reaction zones at peridotite-schist interface in the Frontal Cordillera mafic-ultramafic belt, Central Andes, Argentina

GARGIULO, M. FLORENCIA; BJERG, ERNESTO A.; MOGESSIE, ABERRA

Heidelberg

Congreso; LAK 2011 - 22nd International Colloquium on Latin American Earth Sciences; 2011

Ruprecht-Karls-Universität Heidelberg

Metasomatic reaction zones related to chemical interactions at peridotite-schist contacts are documented in ultramafic (UM) bodies cropping out in Río de Las Tunas Mafic-Ultramafic Belt, Central Andes, Mendoza Province, Argentina. These UM bodies belong to an alpine type belt which has been interpreted as a dismembered and metamorphosed ophiolitic section. The mafic unit dated by Sm-Nd gave a neoproterozoic age of 655±76Ma (López de Azarevich et al., 2009). The reaction zones are developed at the interface between metaperidotite bodies and their country metamorphic rocks belonging to the Guarguaráz Complex. These reaction zones are characterized by concentrates of talc close to the UM body, followed by a fibrous amphibole zone and a black-wall of chlorite near the country-rock. The interface between each mineral zone is transitional and the size of the entire reaction zone varies from 10 cm to 1 m wide. This size is proportional to that of the UM bodies. Several monomineralic pods in the area, such as chlorite, actinolite or talc clots may also represent smaller size metasomatized ultramafics. Magnesite pods also occur when carbonate-bearing levels are present in the surrounding metamorphic basement. Mayor element contents in bulk rock and mineral phases crystallized in the reaction zones demonstrate the chemical exchanges occurred at these interfaces (Gargiulo 2010), showing MgO and CaO transfer from the UM body to the country-rock and the transfer of FeO*, SiO2 and Al2O3 from the schist to the UM body. Geobarometric calculations based on amphibole Al content gave P estimations between 3.1 y 1.8 Kbar to the formation of these reaction zones. According to Sanford (1982) we interpret the development of the reaction zones as a consequence of metasomatism produced in regional metamorphic terranes where the sharp chemical potential (or activity) gradient between pod-like UM bodies and the pelitic or quartzo-feldspathic country rocks has produced elements diffusion or infiltration between these rock-types, in order to reach the chemical equilibrium. The P values obtained allow us to assign the formation of the reaction zones to the latest stage of the pre-Carboniferous regional metamorphism occurred in the area.