Geochemistry of mafic dikes and sills from the lower McCoy Mountains Formation, La Paz County, western Arizona

Mafic dikes and sills intrude sediments of the Late Jurassic (Early Cretaceous?) lower McCoy Mountains Formation in western Arizona. Some units show evidence for intrusion into wet, unconsolidated sediments, indicating that intrusion was approximately contemporaneous with sedimentation. Some of the units also show evidence for subaerial crystallization, and may in fact be lava flows. Attempts to directly date these rocks have been hampered by metamorphic recrystallization and by our inability, despite repeated attempts, to extract zircons from them. Five units from the Granite Wash Mountains, and two from the Plomosa Mountains, were sampled for geochemical and isotopic study in order to place some further constraints on their tectonic setting. Major element, trace element, and isotopic data reveal a suite of high-AI basaltic to andesitic rocks that are sub-alkaline to moderately alkaline and slightly to moderately enriched in light rare-earth elements (LREE), and have epsilon Nd values ranging from +5 to -6. Two gabbroic sills from the Granite Wash Mountains have epsilon Nd of +5, indicating they were derived from depleted mantle sources. The geochemical and isotopic variation in the suite can be explained either as a product of heterogeneous mantle sources or variable interaction of depleted basalts with continental crust. These rocks appear to be transitional in composition and tectonic setting between rift-related basalts of the Bisbee basin and Jurassic diorites intruded during crustal extension in the eastern Mojave desert region.Documents in the AZGS Document Repository collection are made available by the Arizona Geological Survey (AZGS) and the University Libraries at the University of Arizona. For more information about items in this collection, please contact azgs-info@email.arizona.edu