GEOCHRONOLOGY AND GEOCHEMESTRY OF JURASSIC VOLCANIC ROCKS IN NORTH-CENTRAL TO NORTHEASTERN MEXICO: IMPLICATIONS FOR THE ORIGIN AND EVOLUTION OF THE NAZAS ARC

Along a belt extending from north central to northeastern Mexico, there are isolated exposures of Jurassic volcanic and volcaniclastic rocks. They are covered by Upper Jurassic and Cretaceous clastic or carbonated successions, and they overlie either Triassic marine and continental sedimentary rocks or Paleozoic metamorphic rocks. The volcanic rocks are mainly rhyolite, rhyodacite, dacite, and andesite or basaltic-andesite; these rocks occur in the form of pyroclastic deposits, lava flows, ash flows tuffs or ignimbrites, breccias, dikes and domes, and a serie of epiclastic deposits. The formal name applied to this volcanic succession, including interbedded conglomerate and sandstone layers in the succession is Nazas Formation, and its type locality is the area of Villa Juárez in northern Durango. Although there have been several studies on these rocks in different localities, petrographic and geochemical data are limited; in addition, isotopic data are insufficiente to determine the tectonic framework, and there is still a great uncertainty about the absolute ages of volcanism. We report field data, petrographic and geochemical data, which indicate these rocks originated in a continental volcanic arc environment. This is inferred from rare earth elements patterns, and other trace elements relations indicative in all the cases of such tectonic regime. In addition, we report new age determinations by the U-Pb method obtained by LA-MC-ICPMS in zircons from volcanic rocks of Santa María del Oro, Villa Juárez and Sierra de Ramírez in Durango, Caopas-Sierra de San Julián in Zacatecas, Aramberri, Nuevo León, Valle de Huizachal, Tamaulipas and Charcas, San Luis Potosí. The results confirm a Jurassic age for these rocks that range from 168 to 193 Ma. The stratigraphic position, geochemical analysis and absolute ages obtained at different new localities, indicate that these rocks are genetically related to subduction that took place during the Lower to Middle Jurassic along the paleo-Pacific margin of Pangea.