Pacific Section AAPG, SPE and SEPM Joint Technical Conference

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Petrology And Geochronology Of Lower Crustal Xenoliths From Joshua Tree National Park


Lower crustal xenoliths were collected from a basaltic dike in Joshua Tree National Park and characterized petrologically in thin section and with a scanning electron microscope to determine the composition and mineral assemblages. Xenoliths containing zircons were dated radiometrically (U-Pb and Pb-Pb) using the Cameca ims 1270 at UCLA. Granitic-type xenoliths contain Proterozoic (1.4 and 1.7 Ga) and Jurassic aged zircons. The ages of the zircons in the xenoliths have strong correlation with xenocrystic zircons in the mid to upper crustal Mesozoic arc related plutons in the region. The mineral assemblages, cumulate textures, similarity of zircon ages to xenocrystic zircons in the plutons and presence of hydrated minerals in the xenoliths suggest they represent the material left behind after partial melting or crystal fractionation formed the magmas that were extracted to form the Mesozoic plutons. There is a surprising absence of any schist xenoliths, though the Orocopia Schist is just a few miles from the host dike, suggesting that the melting of forearc sediments (i.e., those underplated during flat-slab Laramide subduction) may not have been an important process in this convergent margin arc.