DEPOSITIONAL ENVIRONMENT AND PALEOGEOGRAPHIC DISTRIBUTION OF THE JURASSIC-CRETACEOUS ARC IN THE WESTERN AND NORTHERN GUERRERO TERRANE, MEXICO

The Guerrero terrane of western Mexico contains large volumes of Upper Jurassic-Lower Cretaceous volcanic-sedimentary rocks of arc affinity. The basement of the arc is heterogeneous in composition, formed by previously accreted terranes. Detailed stratigraphic studies in the coastal (Arteaga-Zihuatanejo and Colima) and northern (Zacatecas-Guanajuato) areas indicate that the arc had a complex paleogeography. The stratigraphy of the Arteaga-Zihuatanejo area is made up of basaltic to andesitic lava flows with geochemical compositions similar to present island arcs. They show brecciated and pillowed textures that suggest submarine magmatism. They are interbedded with rudist reefal limestone, volcanic conglomerates (submarine lahars), and other epiclastics. Dinosaur footprints and redbeds indicate periods of subaerial exposure. The abundance of clasts derived from pre-Cretaceous units suggests that its basement was partially exposed during the arc activity. The succession in Arteaga-Zihuatanejo is Late Aptian to Lower Albian in age. The stratigraphy in Colima is Berriasian to Albian in age. It is made up of basaltic to rhyolitic submarine lava flows with arc chemical signatures, ignimbrites and epiclastics, interbedded at the top with thick successions of evaporites, reefal and restricted platform limestone, and some redbeds. The stratigraphy of the arc in Zacatecas-Guanajuato region is made up of basaltic pillowed flows with MORB-IAB and OIB geochemical signatures. They are interbedded with distal volcaniclastic turbidites, black shales, green and black chert that contains radiolarian, few volcanic conglomerates, tuffs and detrital limestone. These lithofacies suggest a deeper marine environment, where fissural basaltic lava flows were interbedded with sediments derived from the volcanic edifices of intermediate composition. Rapid changes in thickness (from up to 4,000 m in Colima to 800 m in Arteaga), differences in lthofacies, and the changes in the geochemical composition of the arc magmas suggest that the arc was under an extensional setting, with associated normal faults that exposed the basement. We interpret this to mean the axis of the arc volcanism was in the Arteaga-Zihuatanejo region. The Colima region might represent an intra-arc basin and the Zacatecas-Guanajuato area a deep marine back-arc basin.