40Ar-39Ar and Rb-Sr Geochronology

Abstract

This report is the final presentaton of argon-argon and rubidium-strontium geochronological data for samples from the Sierras Septentrionales de Córdoba (Provincia de Córdoba), Sierras de San Luis and Comechingones (Provincias de San Luis and Córdoba), and the Sierras de Chepes and Las Minas (Provincia de La Rioja) areas. The Geoscientific Mapping of the Sierras Pampeanas is a cooperative project between the Australian Geological Survey Organisation (AGSO) and the Dirección Nacional del Servicio Geológico (DNSG) of the Subsecretaría de Minería, funded by the Government ofthe Argentine Republic. As a pilot second generation mapping program, the project aims to update the geoscientific knowledge base, provide a modern framework for resource assessment, and promote exploration and development in the region. The project covers three separate areas totalling 27 000 square kilometres in the southern part of the Sierras Pampeanas, Argentina (Figure 1) where basement Precambrian to Palaeozoic metamorphic and granitic rocks crop out on the eastern margin of the Andean Mobile Bel t. The area, well known for its production of industrial and construction materials also contains numerous metallic deposits. Mineral resources include gold and polymetallic (Ag, Pb, and Zn) vein deposits, Ni-Cu sulfide deposits, tungsten, beryllium, lithium, niobium, tantalum, tin, manganese, and chromium. The three areas were selected to provide key information on their geology and mineral potential through the application of integrated geophysical/geological mapping and metallogenic analysis and to provide a continuous section of the major tectonostratigraphic packages comprising the southern Sierras Pampeanas. As part of the Work Program, geochronological analyses, using U-Pb, Ar-Ar and Rb-Sr techniques of selected samples were undertaken. The aim of the geochronological program is to provide key data to establish the timing of igneous rock crystallisation, major metamorphic/deformation episodes, and mineralising events. The data also allow independent correlation of metamorphic rock packages with numerical age control, assisting geological mapping of the project area, and providing an important database and framework for tectonic interpretation of the Sierras Pampeanas. In total, 41 samples were collected for geochronological analysis, including samples representative of basement rock types found in the three detailed study areas and samples associated with the mineral occurrence mapping program. Ali samples submitted for geochronological analysis were petrographically described and analysed for whole-rock major and trace elements. This report presents 40 Ar-39 Ar data for white mica in eight samples of mineralised and hydrothermally altered rocks; white mica and biotite in five samples of rocks from shear zones; and two white mica samples from pegmatites that intruded synchronous with deformation. Sample location data, petrographic descriptions and whole rock geochemical analyses were reported by Stuart-Smith and others ( 1996), Sims and others ( 1996) and Lyons and others ( l 996), respectively. Brief descriptions of the geological settings of the samples are given in the results for each group of samples, followed by summary comments on 40 Ar-39 Ar ages. Further descriptions of the geology pertaining to the 40 Ar-39 Ar geochronology samples, as well as geological interpretations of the geochronology data in relation to the metallogenic, magmatic and tectonic evolution of the three regio ns studied, are presented in the Economic Geology sections and other parts of the reports by Sims and others ( 1997), Lyons and others (1997) and Pieters and others (1997). The results ofU-Pb geochronological analyses are presented in Camacho and Ireland ( 1996, 1997).