Origin of Late Triassic Granitoids of the Coastal Cordillera of Southern Central Chile (34°–37°S): Multi-Isotopic Evidence of Slab Tearing Effects on Pre-Andean Magmagenesis

Abstract

The Coastal Cordillera of Central Chile (34°–37°S) holds a series of Late Triassic granitoids classically interpreted as early Andean subduction-related magmatism based on their arc-like geochemical signature. Here, we present geochemical, isotopic, and geochronological data and a tomotectonic analysis that challenge this idea indicating a local interruption of the normal subduction process likely associated with a slab-tearing event. Our results suggest that the source of the magmas is related to melting of asthenospheric mantle and crustal rocks of a metasedimentary Paleozoic complex. We suggest that partial melting of these sources was triggered by a slab tear-related asthenosphere upwelling producing high-silica S/I- and S-Type granites of the Constitución and Hualpén areas, and anorogenic A-Type granitoids in Cobquecura area. Also, partial melting of a metasomatized asthenospheric mantle plus continental crust that experienced previous high-temperature hydrothermal alteration would have generated high-silica magmas with low δ18O, high Pb, CHUR-like 87Sr/86Sr, and 143Nd/144Nd ratios that originated La Estrella Granite. Our results offer an alternative explanation for the existence of subtle magmatic arc-like geochemical signatures in the study area and support a segmentation of the active margin during the Late Triassic. The widespread upper-plate magmatic record of slab-tearing, spanning the Coastal Cordillera of Central Chile to the intraplate Neuquén basin in Argentina, and the lower mantle record of a slab gap, detected in ours and previous tomotectonic analyses, make the Late Triassic slab-tearing event in southwestern Pangea the most robustly constrained pre-Cenozoic slab tear process so far.