Abstract:

The tectonic setting of the Triassic Cuyo basin is analyzed on the basis of the sedimentary record, the regional structural framework, and the geochemistry of the Paramillos de Uspallata basalts. The study region is in the western Precordillera of the province of Mendoza, Argentina (69°13'W and 32°29'S). Overlap of Triassic rift basins in southern South America with the Permo-Triassic Choiyoi granite-rhyolite province suggests a genetic relation in an extensional regime. The Choiyoi rocks are interpreted as crustal melts associated with extensive basaltic underplating during a period of relatively little motion of the Gondwana supercontinent. The Cuyo rift basin developed during the last stages of this silicic magmatism along suture zones of terranes accreted during the Paleozoic. At this time, faulting could occur and basalts were able to penetrate the cooling, refractory crust. Stratigraphic relationships and geochronologic and paleomagnetic data indicate a Middle Triassic (≈235 Ma) age for basalts emplaced during the synrift phase of active faulting in the Cuyo basin. Basalts are absent in the Late Triassic sag phase, which is dominated by generalized subsidence related to thermal decay and sedimentary loading. Later, the Middle Jurassic Andean arc developed to the west contemporaneous with generalized rifting associated with the early opening of the South Atlantic. Relatively, unaltered sills near Uspallata are composed of tholeiitic to slightly alkaline within-plate olivine basalts with moderate TiO2 (2 to 2.2) and K2O contents (1.0 to 1.2), FeO/MgO ratios near 1.35, moderately steep rare earth element (REE) patterns (La/Yb = 8 to 9, La = 15.6 to 17.5 ppm), and mantle-like isotopic ratios (εNd = 1.9, 87Sr/86Sr < 0.7040). Relatively low degrees of melting (4 to 5 percent) in the mantle are suggested, consistent with the comparatively narrow width of the Cuyo basin and eruption in the last stages of Choiyoi magmatism. Highly altered mafic rocks with younger K-Ar ages are interpreted as flows whose ages were reset by Tertiary intrusives.