Island arc tholeiites of Early Silurian, Late Jurassic and Late Cretaceous ages in the El Fuerte region, northwestern Mexico

Abstract

Three distinctive igneous suites crop out in the El Fuerte block, northwestern Mexico. The oldest of these suites is the Realito Gabbro, which intrudes the Middle-Upper Ordovician Río Fuerte Formation. This gabbro yielded an Early Silurian U-Pb zircon age of ca. 430 ± 15 Ma. The Topaco Formation is a greenschist facies metamorphosed volcanosedimentary sequence. Basaltic rocks of this unit yielded a Late Jurassic U-Pb zircon age of 155 ± 3.5 Ma (Kimmeridgian). The Guamuchil Formation mainly consists of basaltic flows overprinted by greenschist facies contact metamorphism. A sample of this unit yielded a Late Cretaceous U-Pb zircon age of 73 ± 1.5 Ma (Campanian). Geochemical signatures of the three suites suggest an island arc tholeiitic environment. The Upper Jurassic Cubampo Granite is a peraluminous subalkaline granite. Age coincidence and similarities in rare earth element patterns and trace elements concentrations suggest that a genetic link exists between the Cubampo Granite and the Topaco Formation. In a regional context, the Realito Gabbro is coeval with Silurian rocks of the Acatlán Complex in southern Mexico, although geochemical data indicate they originated in different tectonic settings. In a larger scale, the Realito Gabbro rocks are coeval with rocks located along the Appalachian chain and northwestern South America. Late Jurassic magmatism in the El Fuerte region may be the southern prolongation of the continental magmatic belt of Sonora and southwestern USA. Late Cretaceous magmatism of the Guamuchil Formation may correlate with the Sonora-Sinaloa belt of intrusive and volcanic rocks, which was emplaced after accretion of the Guerrero terrane. The island arc tectonic setting indicated by geochemistry of the Mesozoic suites in the El Fuerte region differs from the continental arc setting of north-central Sonora and southwestern USA, which is probably due to mantle source differences.