Geology of the Hazelton Volcanic Belt in British Columbia: Implications for the Early to Middle Jurassic evolution of Stikinia

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The Hazelton Group is an Early to Middle Jurassic volcanic and sedimentary succession that was deposited on the Stikine Terrane (Stikinia) of the Canadian Cordillera. The group is distributed across the entire width of Stikinia, a distance of about 300 km. Prior to post‐Hazelton contractional deformation, the width across which volcanism had occurred was at least 450 km. A review of chronologic and stratigraphic information corroborates an earlier suggestion that the Hazelton Group formed as a pair of coeval, partly subaerial volcanic chains separated by a subsiding, mainly sedimentary marine basin (Hazelton Trough). The trough developed in response to extension of Stikinia during the latest Triassic and/or earliest Jurassic, and remained the locus of moderate extension during deposition of the Hazelton Group. Volcanic rocks of the group are diverse, comprising subaerially and subaqueously deposited lavas and volcaniclastic rocks of mafic to felsic composition. Calc‐alkaline to tholeiitic characteristics, and depletion of high‐field‐strength elements relative to alkali and alkaline earth elements indicate a subduction‐related origin. The degree of alkalinity (medium‐ to high‐K) is fairly consistent throughout the group. The original width across which volcanism occurred is two to five times greater than widths of modern volcanic arcs. The anomalous width cannot be accounted for by time transgression of a single volcanic arc, because the two volcanic chains were coeval. Extremely low‐angle subduction of a single oceanic plate is also rejected as an explanation because alkalinity does not differ significantly between the two volcanic chains. Genesis of the Hazelton Group above a single subduction zone is therefore considered unlikely. We propose a model in which a pair of oceanic plates were subducted beneath Stikinia from opposite sides, in a manner analagous to the tectonic regime of the Philippine archipelago. Subduction angles of the two downgoing plates are postulated to be 30° to a depth of 150 km, a configuration consistent with a postulated forearc width of about 175 km on each side. Stikinia is thereby considered to have been an 800‐km‐wide microplate on which two island arcs developed. The Hazelton Trough was a back‐arc region common to both volcanic arcs. During deposition of the Hazelton Group, Stikinia was not part of the Intermontane Superterrane, an amalgam composed of more inboard terranes. Rather, it belonged to an oceanic microplate that accreted to the inboard terranes in late Early Jurassic to early Middle Jurassic time, during which Hazelton volcanism waned and ended.