STRATIGRAPHY AND U/PB DETRITAL ZIRCON AGES FROM THE ZIHUATANEJO TERRANE, GUERRERO COMPOSITE TERRANE OF WESTERN MEXICO

The Zihuatanejo terrane is located along the west coast of Mexico, outboard from oceanic and primitive arc assemblages (Guanajuato and Arcelia terranes) all forming part of the Guerrero Composite terrane. The basement of the Zihuatanejo terrane is formed by the Arteaga Complex, which is made up of accreted Triassic ocean-basin-continent slope assemblages that were deformed by Early Jurassic time. The terrane contains granitoids and volcano-sedimentary rocks of Upper Jurassic age, the last are characterized by deformed rhyolitic to andesitic lava flows and volcaniclastics. All are overlain by shallow marine to continental volcano-sedimentary rocks, with island-arc geochemical signatures that range form tholeiite to shoshonite, deposited in alternated subsiding basins and basement highs as indicated by lateral variations in ages and thickness of the succession. Most of the exposed succession is Aptian-Lower Cenomanian in age, but there are some reports of Barremian to Valanginian fossils collected from the volcaniclastic rocks. Preliminary detrital zircon data shows clusters at 90-110, around 130, 160, 260, 500-600, 1,200-1,200 Ma, with few older zircons. The older clusters are similar to those obtained from the turbidites that are part of the Triassic Arteaga Complex. The cluster around 160 suggest that the Jurassic arc was also exposed during most of the Cretaceous. This suggests that parts of the Guerrero arc systems were being uplifted and eroded, while others continued with subsidence and arc volcanism. It suggests that a possible transtensional setting was active during most of the arc activity, up to the end of the Cenomanian-Turonian, when it might have shifted to transpressional. Overlapping the previously deformed arc rocks a Santonian to Maastrichtian new arc (continental arc) developed along the coast.