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

Abstract

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 ε Nd (T) and ε Sr (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 ε Nd (T) ratios (+0.6 to −11.6), lower than those of the Black Dyke Formation. These Nd isotopic ratios indicate the presence of a contaminated mantle source. The highest ε Nd (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. The Nd 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.