Provincias magmaticas acidas y evolucion tectonica de Gondwana: Andes chilenos (28-31°S)

Abstract

ABSTRACT. The batholiths of the Andean Cordillera in the Atacama and Coquimbo Regions (28-31°S) reflect tectonic processes associated with the evolving margin of the Gondwana supercontinent. The batholiths consist of the Carbonilerous-Lower Permian Elqui Superunit which records the final assembly of Gondwana, and the Permian-Triassic (Lower Jurassic?) Ingaguas Superunit, which coincides with the period of stability of the supercontinent. These events are tied to those occurring along the Gondwana margin from Peru to Australia. The oldest Elqui unit, Guanta, is composed of calc-alkaline tonalites and granodiorites formed along an active continental margin. These granitoids are intruded by leucocratic peraluminous granitoids (Cochiguas and El Volcan units) derived from melting of variable crustal sources. Some of these granitoids have trace element signatures consistent with a high pressure residual mineralogy suggesting that they formed by melting of a thickened crust. All of the Elqui granitoids are mesozonal and show evidence of contemporaneous and post-emplacement deformation. Their uplift is contemporaneous with a compressional deformation (San Rafael Phase) in the Argentine foreland. The Ingaguas Superunit is an association of epizonal, post-collisional, intrusives which include granitoids derived from deep levels in a garnet-bearing thickened crust (Los Carricitos Unit) and hypersilicic, calc-alkaline to transitional A-type granites, indicating extensive crustal melting of a garnet-poor crust. These granites and the synchronous Pastos Blancos Rhyolites are part of the Choiyoi Magmatic Province which extends for more than 2,500 km along the Central and Southern Andes. Regional considerations suggest that the uplift of the Elqui Superunit and the San Rafael Phase could have resulted from the oblique collision of an allochthonous block to the west in the mid-Permian. This collision could explain crustal thickening and the termination of subduction and block rotations in the Argentine Frontal Cordillera. The removal of the inactive subducted plate would favor decompressional melting, generating large volumes of basalt that could accumulate at the base of the crust and produce the crustal melting that formed the Choiyoi Province. A similar Permian collision has also been suggested in the New England Fold 8elt in Australia. These collisions coincide with the end of the rapid movement of Gondwana relative to the South Pole. In the central part of the Gondwana margin from Patagonia to Antarctica, subduction continued until the Jurassic. 'Thermal blanketing' of the mantle by the stationary supercontinent resulted in accumulation of basaltic magmas at the crust-mantle boundary. The basaltic magmas accumulated below the collage of arc magmatic rocks and exotic blocks accreted in the Paleozoie produced extensive melting forming the Gondwana granite-rhyolite provinces. These events preceded the Jurassic dispersal of the Gondwana supercontinent in this region.