Description:
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.