Upper Cambrian to Middle Ordovician mafic volcanic rocks of the Donjek assemblage comprise the oldest exposed units of the Alexander terrane in the Saint Elias Mountains of northwestern Canada. In this study, we use the geochemical and geological characteristics of these rocks to decipher their tectonic setting, petrogenetic history, and relationship to the early Paleozoic Descon arc system of the Alexander terrane in southeastern Alaska. Donjek assemblage volcanic rocks are subdivided into three geochemical types: transitional basalt (type I), light rare earthenriched island-arc tholeiite to calc-alkaline basalt (type II), and enriched mid-ocean ridge basalt to ocean-island basalt (type III). Simple petrogenetic models illustrate that the basalts were generated by the decompressional partial melting of enriched asthenospheric mantle and variably mixed with depleted mantle and subduction-related components. Analogous geochemical signatures for modern Sumisu Rift and Okinawa Trough lavas imply that the Donjek assemblage basalts erupted during the rifting of the Descon arc. This model provides a new comparative framework for terranes of Siberian, Baltican, and Caledonian affinity in the North American Cordillera and, in particular, suggests a paleogeographic connection to rift-related magmatism in the Seward Peninsula region of the Arctic Alaska-Chukotka terrane.

No abstract

Short‐lived, high‐volume magmatic events or flare‐ups in Cordilleran‐style accretionary systems are presumably triggered by the rapid underthrusting of melt‐fertile lithosphere beneath a continental arc during extreme retroarc shortening. New zircon U‐Pb age and trace element geochemical studies of the Coast Mountains batholith were conducted to test this hypothesis and investigate cross‐orogen linkages between the Coast Mountains arc system and adjacent retroarc elements of the Canadian Cordillera. Late Jurassic (155–147 Ma) granitoids of the Saint Elias plutonic suite in southwestern Yukon were emplaced during a widespread magmatic event and correspond to an intrusive rate of ~350 km2/Myr, analogous to the scale of 160–150 Ma flare‐up activity in the Sierra Nevada batholith. The timing of Late Jurassic high‐volume magmatism was coincident with forearc and intraarc deformation events along the length of the Coast Mountains arc from Alaska to British Columbia. Whole‐rock and zircon rare earth element geochemical results from the Saint Elias plutonic suite confirm that continental lithosphere was a key source component for Late Jurassic granitoids, which strengthens the implied relationship between high‐volume arc magmatism and crustal recycling. Well‐documented episodes of late Middle to early Late Jurassic hinterland thrusting and metamorphism in the Intermontane and Omineca belts of the Canadian Cordillera preceded this high‐volume event and therefore support the hypothesis that retroarc shortening was dynamically linked to flare‐up activity. Late Jurassic magmatism was followed by a 140–125 Ma lull in most of the Coast Mountains batholith, which may be linked to ridge subduction, lithospheric delamination, mantle cooling, or plate reorganization.

No abstract