Petrography, geochemistry and geochronology of the orogenic magmatism in Rayon: petrological characteristics of the last subduction-related magmas in Sonora, Mexico
Résumé
Neogene magmatism associated to the subduction of the Farallon plate is widely distributed in a NW-SE belt in northwestern Mexico. In Sonora, this magmatism is commonly intercalated with clastic units, and presents calc-alkaline, high-potassium calc-alkaline and shoshonitic affinity. The orogenic magmatism in the region of Rayon is covered by anorogenic pyroclastic deposits related to the proto-Gulf of California and is divided into two stages: early Miocene volcanism (16.51 ± 0.64 Ma) followed by a middle Miocene event (14.88 ± 0.47 Ma). Both rock sequences occur as lavas, domes and volcanic necks, which were affected by a Miocene extensional tectonic regime. Field, petrographic and geochemical characteristics differentiate two sequences: 1) Cerro Prieto volcanic member, composed of vitreous, porphyric, two pyroxenes and olivine andesitic lavas, and 2) Las Agujas Volcanic Complex, consisting of hypabyssal rocks, domes and dacitic lavas with two pyroxenes and amphibole. Geochemically, rocks of both units are calc-alkaline, are enriched in light rare earth elements, and display a pronounced fractionation of the heavy rare earth elements, low Y and Yb concentrations, and high Sr content, which is more prominent in the younger unit. These orogenic magmatic episodes are related to the latest manifestations of the continental arc in Sonora; the adakitic affinity could be related to a particular configuration of the slab in the final stage of the microplate subduction.