Early Mesozoic Plutonism of the Cordillera de la Costa (34°–37°S), Chile: Constraints on the Onset of the Andean Orogeny

Abstract

New isotopic ages constrain the onset of the Andean Orogeny in central Chile (34°–37°S). They define three pulses of Early Mesozoic magmatism in the Cordillera de la Costa: granitic magmatism at 225–220 Ma, bimodal magmatism at 210–197 Ma, and mafic magmatism at 170–150 Ma. Each pulse has distinct field, petrographic, geochemical, and Sr-Nd-Pb isotopic characteristics and crystallization conditions. Major magmatic sources are the regional Mesozoic subarc-subcontinental mantle and the Paleozoic Chilean continental crust, with the importance of juvenile contributions increasing with time. All three Early Mesozoic magmatic pulses display geochemical signatures that are characteristic of subduction-related magmas of magmatic arcs, such as a Nb-Ta trough and a positive Pb anomaly. The 210–197-Ma magmatism is peculiar for its bimodal nature, the occurrence of transitional A-type granitoids, the presence of fayalite granites, and its association with an episode of crustal extension. A contemporaneous accretionary wedge nevertheless indicates ongoing subduction at that time. The 210–197-Ma magmatic pulse thus was formed in an extensional suprasubduction zone environment. It marks the transition from the Gondwana Orogeny to Andean-style subduction magmatism. We argue that the onset of the Andean Orogeny was related to a reconfiguration of the tectonic conditions of plate convergence during overall near-continuous subduction and arc magmatism since the Paleozoic.