Assimilation, differentiation, and thickening during formation of arc crust in space and time: The Jurassic Bonanza arc, Vancouver Island, Canada

Abstract

Continental arcs and island arcs, eventually accreted to continental margins, are thought to have been the locus of continental growth since at least the Proterozoic Eon. The Jurassic Bonanza arc, part of the Wrangellia terrane on Vancouver Island, British Columbia, exposes the stratigraphy of an island arc emplaced between 203 and 164 Ma on a thick preexisting substrate of noncontinental origin. We measured the bulk major-and traceelement geochemistry, and Rb-Sr and Sm-Nd isotope compositions of 18 plutonic samples to establish if differentiation involved contamination of the Bonanza arc magmas by the pre-Jurassic basement rocks. The 87 Sr/ 86 Sr and 143 Nd/ 144 Nd isotope ratios of the plutonic rocks at 180 Ma vary from 0.70253 to 0.7066 and 0.512594 to 0.512717, respectively. Assimilation-fractional crystallization modeling using trace-element concentrations and Nd and Sr isotope ratios indicates that contamination by a Devonian island arc in the Wrangellia basement is less than 10%. Rare earth element modeling indicates that the observed geochemistry of Bonanza arc rocks represents two lineages, each defined by two stages of fractionation that implicate removal of garnet, varying in modal proportion up to 15%. Garnet-bearing cumulate rocks have not been reported from the Bonanza arc, but their inference is consistent with our crustal thickness estimates from geological mapping and geobarometry, indicating that the arc grew to at least 23 km total thickness. The inference of garnet-bearing cumulate rocks in the Bonanza arc is a previously unsuspected similarity with the coeval Talkeetna arc (Alaska), where garnetbearing cumulate rocks have been described. Geochronological data from the Bonanza arc show a continuum in plutonic ages from 203 to 164 Ma, whereas the volcanic rocks show a bimodal age distribution over the same span of time with modes at 198 and 171 Ma. We argue that the bimodal volcanic age distribution is likely due to sampling or preservation bias. East-west separation of regions of young and old volcanism could be produced by rollback of a west-dipping slab, forearc erosion by an east-dipping slab, or juxtaposition of two arcs along arc-parallel strike-slip faults.

CO2 transport at shallow depths in arc magmas: evidence from unique orbicular dikes in the Jurassic Bonanza arc, Vancouver Island, Canada

“…8a). These unique REE patterns are nearly identical to other 'primitive' dikes identified in previous studies of the Bonanza arc (DeBari et al 1999;Larocque 2008;D'Souza et al 2016).…”

“…Being the latest phase of magmatism, the orbicular dikes preserve the final more mature state of magma-carbonate reactions that occurred within the system. Their composition indicates they are primitive arc melts, similar to others identified within the Bonanza arc (DeBari et al 1999;Larocque 2008;D'Souza et al 2016).…”

“…We first address the textural and chemical distinction of the orbicules from their host melt, followed by the evidence and extent that they assimilated limestone wallrock. Although all rocks of the Bonanza arc suffered prehnite-pumpellyite facies burial metamorphism, previous studies show they preserve primary igneous trends and 87 Sr/ 86 Sr ratios, despite the fact Rb and Sr can be mobile during metamorphism (Andrew et al 1991;D'Souza et al 2016).…”

Resolving the Architecture and Early Evolution of a Forearc Basin (Georgia Basin, Canada) Using Detrital Zircon

“…Excluding MDS Group 1, all samples are dominated by <170 Ma grains (average = 93%) that originated from the associated arc 12,14 . Grains >170 Ma are attributed to the Sicker Group and Bonanza Arc, which form part of the underlying basement 28–32 and the Descon Arc, which is another accreted terrane 33,34 . The main differences between MDS groups are summarized below (Table 1).…”

Constraints on Trenchward Arc Migration and Backarc Magmatism in the North Patagonian Andes in the Context of Nazca Plate Rollback

“…The spatio‐temporal evolution of the magmatic arc can be studied through the geochemical signature of the associated igneous rocks, as can be seen in the change from tholeiitic basalts and basaltic andesites prevalent in immature arcs, to an increasing proportion of intermediate to more silicic calc‐alkaline rocks with the development of continental‐type and thicker crusts (Chapman et al, ; D'’Souza et al, ; Ducea et al, ). Besides, variations in the tectonic setting in which these magmas are created affect the composition and development of different petro‐tectonic associations: crustal thickening may promote higher assimilation of preexisting crust, whereas lithospheric extension can promote the ascent of asthenospheric mantle, resulting in decompression melting and basaltic volcanism (Chapman et al, ; D'Souza et al, ; Menzies et al, ). Even though rarer, arc products become associated with enriched and deeper sources, as enriched mid‐ocean ridge basalt (E‐MORB) or ocean island basalt (OIB) magmatism, which could reflect particular tectonic events such as backarc basin development, ridge collisions, and short‐lived asthenospheric anomalies (e.g., Gao et al, ; Kay et al, , ).…”