Petrology of Eocene dikes near Lake Chelan, WA: evidence of mantle and crustal melting during the Challis event

Abstract:

The Eocene was a tectonically and volcanically complex time in the Pacific Northwest, characterized by regional extension, rapid uplift, widespread magmatism, and a high thermal flux. Extension was manifested by core complex formation, normal faulting, and dike emplacement, the latter represented by the 48 Ma Teanaway swarm (TS), 53-46 Ma dikes associated with the Colville Igneous Complex (CIC), the 46.6 Ma Corbaley Canyon swarm (CCS), and a previously unstudied cluster near Lake Chelan (LC) that is the focus of this study.

LC dikes generally strike NW, range up to 8m in width, and are compositionally diverse with a general NE-ward increase in SiO₂ (47-84 wt. % SiO₂) over a distance of ~120 km. All dikes display arc affinities (calc-alkaline, HFSE depletions) but can be divided into two suites: (1) basalt-andesite-dacite-rhyolite (BADR), and (2) adakites (Sr/Y > 40, La/Yb_N > 10, Sr > 350ppm, Y < 18ppm, Yb < 1.8ppm). Sr and Nd isotopic data (⁸⁷Sr/⁸⁶Sr_i = 0.70360 – 0.70530; εNd(t) = +4.45 to -1.92) for the two suites overlap, suggesting both are mixtures of material from depleted mantle and older crustal sources. We suggest LC basalt dikes represent mantle-derived magmas that drove melting of thick arc crust, generating adakite melts from an eclogite lower crust and, at shallower depths, andesite-rhyolite melts from a combination of fractional crystallization and crustal assimilation/melting.

LC dikes share chemical similarities with volcanic rocks of the CIC and CCS including calc-alkaline trends and compositions that range from basalt to rhyolite. The TS dikes however, are distinctly different in chemical composition (tholeiitic, dominantly basaltic andesites), Sr-Nd isotopic composition, and orientation (NW-striking). Ongoing U-Pb dating of LC dikes should help to determine the extent to which changes in dike chemistry and orientation are temporal or spatial or both.