Volcanic sequence in Late Triassic--Jurassic siliciclastic and evaporitic rocks from Galeana, NE Mexico

In northeastern Mexico, volcanic rocks interbedded with Late Triassic-Jurassic siliciclastic and evaporitic strata have been linked to magmatic arcs developed in the Pangea western margin during its initial phase of fragmentation. This work provides new petrographie and geochemical data for volcanism included in the El Alamar and Minas Viejas formations outcropping in the Galeana region. Andesitic dykes and sills (n= 10) in the El Alamar redbeds show Si[O.sub.2] = 47.5-59.1% and MgO= 1.2-1.2%, as well as a geochemical affinity to island arc magmas. This work represents the first report of this tectonic setting in the region. Geological and petrographie evidence suggest that this arc system likely developed after ~220 and before--193Ma. Trachy-andesitic and rhyodacitic domes (n= 20) associated with the Minas Viejas gypsum-carbonates sequence show Si[O.sub.2]= 61.8-82.7% and MgO= 0.1-1.0% with a tectonic affinity to continental arc. A rhyodacite sample from this region has been dated by U-Pb in zircon, yielding an age of 149.4 [+ or -] 1.2Ma (n= 21), being the youngest age related to this arc. Finally, we propose a three-step model to explain the tectonic evolution from Late Triassic island arc to Jurassic continental arc system in the northeastern Mexico. KEYWORDS Northeastern Mexico. Island arc. Continental arc. Late Triassic. Jurassic.