Petrogenesis of Middle Jurassic to Miocene magmatism within the Nechako plateau, central British Columbia: Insight from petrography, geochemistry, geochronology and tracer isotope studies

Igneous rocks of the Middle Jurassic, Cretaceous, Eocene and Miocene periods dominate the geology of the Nechako plateau in central British Columbia. Study of the geochemistry and tracer isotope values of these rocks provides insight into their different magmatic origins and tectonic settings.; Middle Jurassic plutonism is isotopically primitive and supports emplacement above an active continental arc. Eocene volcanism was regionally continuous between 53 and 47 Ma with episodic related plutonism between 55 and 47 Ma. Eocene volcanism is isotopically primitive (87Sr/86 Sr_i = 0.7040–0.7045; ϵNdi = +0.9 to +4.6) and is interpreted as derived from an enriched upper mantle source. Magmas were variably modified by fractionation and contamination and erupted in a distal arc setting. Parental magmas of isotopically primitive Oligo-Miocene Chilcotin Group basalts are asthenosphere derived. Isotopic results from spine lherzolite xenoliths entrained in Oligo-Miocene lavas indicate enriched components within the upper mantle.