Late Jurassic flare‐up of the Coast Mountains arc system, NW Canada, and dynamic linkages across the northern Cordilleran orogen

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.

Along‐Strike Variation in the Magmatic Tempo of the Coast Mountains Batholith, British Columbia, and Implications for Processes Controlling Episodicity in Arcs

“…High MAR events in the southern CMB occur at 161–148, 114–102, 85–70, and 61–48 Ma (Figure ). The Late Jurassic flare‐up coincides with the high MAR event documented along the entirety of the Coast Mountains arc (Beranek et al, ), as well as in the Klamath (Hacker & Ernst, ) and Sierra Nevada (Cecil et al, ; Ducea, ) batholiths to the south. Following this event, a pronounced 140–120‐Ma lull has been described for much of the Canadian margin (Gehrels et al, ) and is hypothesized to result from a change to more oblique sinistral convergence (Engebretson et al, ; Monger et al, ; Müller et al, ; Seton et al, ) or perhaps a mantle cooling event caused by the impingement of a thickened Jurassic arc root on the convecting mantle wedge below (Chin et al, ).…”

“…This event is followed by waning magmatic activity from ca. 140 to 125 Ma, which is also observed along the entire length of the Coast Mountains arc system (Beranek et al, 2017;Gehrels et al, 2009). Despite these similarities, the Jurassic arc is better represented proportionally in the southern Coast Mountains.…”

“…160–145 Ma. A flare‐up at this time has been documented for the entire Coast Mountains arc system from Alaska to the Washington‐Canada border (Beranek et al, ). This event is followed by waning magmatic activity from ca.…”

“…Curves represent the relative probability density of the ages presented. Stippled patterns on the histogram bars at 160–145 Ma are meant to highlight the prevalence of a Cordilleran‐wide flare‐up at that time, documented along the length of the arc from southeast Alaska to the US border (Beranek et al, ).…”

Detrital Zircon Record of a Mesozoic Collisional Forearc Basin in South Central Alaska: The Tectonic Transition From an Oceanic to Continental Arc

“…Direct measurement of the Chitina arc plutons yield 153–150 Ma U‐Pb zircon ages; however, U‐Pb zircon ages of modern river sands draining the Chitina Valley batholith range between 156 and 130 Ma (E. M. Day et al, ; Plafker et al, ; MacKevett, ; Roeske et al, , ; Trop et al, ; Winkler et al, ). Contemporaneous arc‐related plutons from the Saint Elias plutonic suite to the southeast (present coordinates) contain U‐Pb ages on zircon between 155 and 147 Ma (Beranek et al, ). Chisana arc plutons and volcanic rocks yield circa 126–105 Ma radiometric ages (Graham et al, ; Richter et al, ; Snyder & Hart, ).…”