A Permian island-arc with a continental basement: the Black Dyke Formation(Nevada), North American Cordillera

In western Nevada, remnants of the Black Dyke Formation record Early Permia n island-are volcanism. This formation is characterized by amphibole-phyric basalt with fragments of amphibole-gabbro and diorite porphyry, clinopyrox ene-phyric basalt and plagioclase-phyric mafic andesite. These igneous rock s exhibit medium to high-g calc-alkaline affinities. Their epsilon (Nd(T)) and epsilon (Sr(T)) ratios range between +5.5 and -1.5, and +4.7 and +22.5, respectively. In northern Sierra Nevada, Early Permian are-rocks are simil ar to those of the Black Dyke Formation, being calc-alkaline with a wide ra nge of epsilon (Nd(T)) ratios (+0.6 to -11.6), lower than those of the Blac k Dyke Formation. These Nd isotopic ratios indicate the presence of a contaminated mantle sou rce. The highest epsilon (Nd(T)) (+5.5) ratio approaches the lower limit of MORE-like source. Modelling indicates that the volcanics derived from a MO RE-like source contaminated by subducted sediments or are basement with con tinental affinity. Such Nd isotopic heterogeneity is observed in oceanic is land-arcs whose source included a large continental component (sediments or crustal basement) such as Sunda or Banda arcs. and in island-arcs built on continental crust such as the New Zealand arc. The Nd isotopic ratios of Early Permian are rocks from northern Sierra Neva da and western Nevada implies the presence of a continental crust basement, including Precambrian crustal material and/or sediments. (C) 2001 Elsevier Science B.V. All rights reserved.