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GEM2: Geo-mapping for Energy and Minerals
Primitive arc magmatism in northern Stikinia; Margins through time: GAC-MAC 2016; Geological Association of Canada-Mineralogical Association of Canada, Joint Annual Meeting, Programs with Abstracts vol. 39, 2016 p. 64-65, Natural Resources Canada, Contribution Series
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Volumetrically minor, primitive (MgO ~16 wt. % LOI-free), olivinephyric, pi...
Volumetrically minor, primitive (MgO ~16 wt. % LOI-free), olivinephyric, picritic magmas were erupted explosively in the northern Stikine terrane (Stikinia) of the Canadian Cordillera during the Middle to Late Triassic. These pyroclastic picrite deposits occur within the Stuhini Group and comprise upward-fining sequences of normally graded tuff breccia to olivine tuff, capped by rhythmically stratified ash tuff. The tuff is largely altered to serpentine, chlorite and clay, but preserves primary textures, including pseudomorphed olivine-rich lapilli, devitrified scoria, and cuspate bubble-wall glass shards. Isolated flows, breccia, and sills of broadly similar age and geochemical character are also known from the central and southern Quesnellia [1,2]. Comparable magmas are rare in modern arc settings and limited to several spatially and temporally restricted occurrences in the Solomon Islands (~14 wt.% MgO), Vanuatu (15 wt.% MgO), Lesser Antilles (>12.5 wt.% MgO), Japan, and the Aleutian Islands (>16 wt.% MgO). The unusually primitive (high-MgO) chemistry of these occurrences has been attributed to anomalously high geothermal gradients associated with subduction of young, hot oceanic lithosphere and spreading ridges [3, 4] or rapid mantle wedge counter-flow [5]. The Stikinia pyroclastic rocks have high, but variable MgO concentrations (22-38 wt.%) and relatively constant FeOTOT contents (9-10 wt.%). Within-suite geochemical variation reflects variable amounts of olivine accumulation (20-65%) into a parental liquid with MgO content of ~16 wt. %. The estimated composition of accumulated olivine (Fo91), and the Mg-number (Mg/(Mg+Fe2+) of the calculated parental liquids (?0.74), suggests that the pyroclastic picrites formed directly from a primary magma that has undergone minimal fractionation. The high degree of melting required to generate the Middle-Upper Triassic picrites and their inferred high volatile contents suggest that these magmas were potent carriers of metals from the mantle and may have been key in the metallogenic endowment of the Stikinia.
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