Geology and geochronology of the Jurassic magmatic arc in the Magdalena quadrangle, north-central Sonora, Mexico

Abstract

This work reports on the geology and U–Pb LA-ICPMS zircon geochronology of a crustal section that is part of the Jurassic magmatic arc in the Magdalena quadrangle of north-central Sonora, Mexico. This rock succession is variably metamorphosed and strained as it was affected by Late Cretaceous shortening, intruded by early Tertiary granitoids, and further exhumed in the lower plate of the early Miocene Magdalena metamorphic core complex. The older and more extensively exposed Jurassic unit is the >3.5 km thick Sierra Guacomea rhyolite that is composed of massive to poorly bedded rhyolite, bedded quartz-phyric rhyolitic ignimbrite and interbedded ash-fall tuffs and quartz-rich sandstone beds. Three rhyolite samples collected at different localities of its outcrops yielded concordia ages of 175.2 ± 0.9, 171.7 ± 0.6, and 171.4 ± 0.7 Ma. The quartz-phyric Rancho La Víbora, Los Vallecitos, and the Agua Caliente rhyolitic domes that are associated with the Sierra Guacomea rhyolite yield concordia ages of 176 ± 0.8, 174.4 ± 0.9 and 173.1 ± 0.8 Ma, respectively. The Rancho Los Pozos unit composed of interbedded rhyolitic ash-fall tuff and flows, sandstone, siltstone and subordinate limestone beds has an estimated thickness of 600 m and yielded a crystallization concordia age of 170.7 ± 0.6 Ma from a rhyolite bed. The porphyritic El Rincón granite that intrudes into the Sierra Guacomea rhyolite yields crystallization ages of 167.43 ± 0.42 and 164.4 ± 0.7 in samples from different localities. The La Jojoba metasandstone that consists of foliated, quartz-rich to arkosic strata of fluvial origin is at least 900 m thick; detrital zircon grains dated from three sandstone samples yielded dominantly Jurassic ages with peaks at 172, 170, and 163.7 Ma, and a combined maximum depositional age of ca. 163 Ma.The younger plutons are the porphyritic El Nopalito granite that has an interpreted crystallization age of 160.8 ± 0.6 Ma, and the leucocratic, two-mica, garnet-bearing La Cebolla granite that yielded a concordia age of 158.1 ± 1 Ma. These granitic intrusions record the waning magmatic pulses of the arc, in the study quadrangle, but their volcanic equivalents were not identified. Inherited zircon grains in the reported volcanic and plutonic units are only of Jurassic age, except by two Proterozoic zircon grains yielded by the El Nopalito granite. The El Salto granite augen gneiss is a xenolith dated at 1071.9 ± 5 Ma that indicates the presence of Grenvillian basement in the study area.Major- and trace-element geochemical data indicate that the volcanic and plutonic units are silica-rich, mostly high K calc-alkaline to shoshonitic rocks associated with a continental margin arc setting. The plutons are mostly peraluminous, and in conjunction with trace element geochemistry data, they suggest crustal assimilation of magmas emplaced in a probably thickened continental crust. Chondrite-normalized REE patterns and primitive mantle-normalized trace element diagrams also suggest partial melting and fractional crystallization processes.The ages obtained indicate that the arc in the study area developed from ca. 176 to 158 Ma, encompassing a 17 m.y. interval of magmatism and associated sedimentation. Regional correlation and geochronologic published data indicate that the arc crustal section of the Magdalena quadrangle is part of the Jurassic magmatic arc that regionally lasted from ca. 190 to 158 Ma.