Petrological, geochemical, isotopic, and geochronological constraints for the Late Devonian?Early Carboniferous magmatism in SW Gondwana (27?32°LS): an example of geodynamic switching

DAHLQUIST J.A.; ALASINO P.H.; BASEI, M; MORALES CÁMERA, M.; MACCHIOLI GRANDE, M.; COSTA CAMPO NETO, M.

INTERNATIONAL JOURNAL OF EARTH SCIENCES

SPRINGER

Año: 2018

1437-3254

We report a study integrating 13 new U?Pb LA-MC-ICP-MS zircon ages and Hf-isotope data from dated magmatic zircons together with complete petrological and whole-rock geochemistry data for the dated granitic rocks. Sample selection wasstrongly based on knowledge reported in previous investigations. Latest Devonian?Early Carboniferous granite samples were collected along a transect of ~ 900 km, from the inner continental region (present-day Eastern Sierras Pampeanas) to the magmatic arc (now Western Sierras Pampeanas and Frontal Cordillera). Based on these data together with ca. 100 publishedwhole-rock geochemical analyses we conclude that Late Devonian?Early Carboniferous magmatism at this latitude represents continuous activity (ranging from 322 to 379 Ma) on the pre-Andean margin of SW Gondwana, although importantwhole-rock and isotopic compositional variations occurred through time and space. Combined whole-rock chemistry and isotope data reveal that peraluminous A-type magmatism started in the intracontinental region during the Late Devonian, with subsequent development of synchronous Carboniferous peraluminous and metaluminous A-type magmatism in the retro-arc region and calc-alkaline magmatism in the western paleomargin. We envisage that magmatic evolution was mainly controlled by episodic fluctuations in the angle of subduction of the oceanic plate (between flat-slab and normal subduction), supporting a geodynamic switching model. Subduction fluctuations were relatively fast (ca. 7 Ma) during the Late Devonian and Early Carboniferous, and the complete magmatic switch-off and switch-on process lasted for ~ 57 Ma. Hf TDM values of zircon (igneous and inherited) from some Carboniferous peraluminous A-type granites in the retro-arc suggest that Gondwana continental lithosphere formed during previous orogenies was partly the source of the Devonian?Carboniferous granitic magmas, thus precluding the generation of the parental magmas from exotic terranes.