New UPb and Ar/Ar isotopic age constraints on the timing of Eocene magmatism, Fort Fraser and Nechako River map areas, central British Columbia
Twenty-three new, precise, Eocene U-Pb and 40 Ar/ 39 Ar age determinations for calc-alkaline volcanic rocks of the Ootsa Lake Group and associated intrusive rocks, widespread in the Nechako Plateau in central British Columbia, constrain the timing of the Eocene magmatism to 53.2-47.6 Ma, with a local duration of as little as 2-3 million years. The new dates show that magmatism in the study area is partly coeval with that responsible for the Babine Igneous Suite - Newman Volcanic Suite (53-50 Ma) to the north, and for the Endako Group (51-45 Ma), which overlies the Ootsa Lake Group; however, locally the three magmatic suites are distinct in age and (or) the strata record a magmatic hiatus of as much as 7.5 million years. The ages generally young from north to south (52-47 Ma) along the western portion of the study area. The Babine Igneous Suite - Newman Volcanic Suite represents the oldest member of this series. However, in the east, the Ootsa Lake Group volcanic rocks are generally older (53-51 Ma). The anomalously older ages may be related to the interaction of magmatism and formation of a nearby and coeval core complex, which ongoing studies show was uplifted at about the same time during the Eocene. Felsic plutonism associated with the Ootsa Lake Group occurred between 50.5 and 47.3 Ma. These plutons were emplaced in an extensional setting along north-northeast-trending faults. The new dates, stratigraphic relationships, and suggested correlations of Eocene strata in the study area with that to the west and north require a revision of the stratigraphic nomenclature for the Ootsa Lake and Endako groups. 696