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 Permian island-arc volcanism. This formation is characterized by amphibole-phyric basalt with fragments of amphibole-gabbro and diorite porphyry, clinopyroxene-phyric basalt and plagioclase-phyric mafic andesite. These igneous rocks exhibit medium to high-K calc-alkaline affinities. Their εNd(T) and εSr(T) ratios range between +5.5 and −1.5, and +4.7 and +22.5, respectively. In northern Sierra Nevada, Early Permian arc-rocks are similar to those of the Black Dyke Formation, being calc-alkaline with a wide range of εNd(T) ratios (+0.6 to −11.6), lower than those of the Black Dyke Formation. Nd isotopic ratios indicate the presence of a contaminated mantle source. The highest εNd(T) (+5.5) ratio approaches the lower limit of MORB-like source. Modelling indicates that the volcanics derived from a MORB-like source contaminated by subducted sediments or arc basement with continental affinity. Such Nd isotopic heterogeneity is observed in oceanic island-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. isotopic ratios of Early Permian arc rocks from northern Sierra Nevada and western Nevada implies the presence of a continental crust basement, including Precambrian crustal material and/or sediments.