Geochemistry, petrogenesis and tectonic significance of the volcanic rocks of the Las Tortolas Formation, Coastal Cordillera, northern Chile

摘要

The Las Tórtolas Formation extends from 29° to 26° S along the northern chilean coast and has been interpreted as a Carboniferous accretionary prism. This essentially metasedimentary turbiditic formation constituted the late Paleozoic basement of the Upper Triassic-Jurassic magmatism during the inception of the Andean cycle in northern Chile. The geochemical characteristics and field relations studied in the volcanic rocks of the Las Tórtolas Formation, described with detail in this work, mean a key fact to assess the notable paleogeographic changes occurred during the pre-Andean cycle in the western margin of South America.The volcanic domain included in the Las Tórtolas Formation is mainly comprised by basaltic rocks, with minor tuffs and rhyolites, and presents thrust-fault contacts with the mélange facies to the west and the metasedimentary Eastern Series to the east. Together, the Las Tórtolas Formation was conformed by the stacking of the basal highly deformed units of this accretionary complex, represented by the mélange facies, and the volcanic domain that became overlain by the turbiditic sequences at their top.The geochemical characteristics of the Las Tórtolas basaltic rocks involve an enriched mantle source, as a mantle plume. In spite of the N-MORB signature observed in the basal hyaloclastitic basalts, the lava flows, as the metabasites sampled in the mélange facies, yield E-MORB characteristics according to the multi-element and tectonic discrimination diagrams and several immobile element ratios as the Th/Yb, TiO2/Yb and Nb/Yb, among others. Therefore, the origin of the volcanic domain of the Las Tórtolas Formation may be related to a hot mantle plume impinged on the base of the oceanic lithosphere under the tip of the overriding plate, which firstly promoted the partial melting of the depleted surrounding lithosphere (N-MORB hyaloclastites) and continued with the extrusion of lava flows and pyroclastic rocks derived from the enriched source, interlayered with the distal turbiditic succession. The development of the magmatic system along the oceanic crust enabled the formation of magma chambers where small volumes of rhyolitic melts were fractionated. The geochemical evolution of this basaltic magmatism was correlated with the building of volcanic edifices during a first submarine stage (hyaloclastites, pillows and lava flows) and a subsequent sub-aerial explosive phase depicted by sedimentary and rhyolitic breccias.Finally, we explore a tectonic setting defined by a plume overridden by the lithosphere during the relative convergence with the continental margin to explain the formation of the volcanic domain and its accretion to the sedimentary units. Besides, this tectonic configuration may be taken into account as the driving force of the tectonic and magmatic events that characterize the pre-Andean cycle.