Correlation among lower to upper crustal components in an island arc: the Jurassic Bonanza arc, Vancouver Island, Canada

Abstract

The Westcoast Crystalline Complex (WCC), Island Intrusions, and Bonanza Group of Vancouver Island, Canada, form three different crustal levels of the Early to Middle Jurassic Bonanza island arc. Differential uplift has exposed the plutonic roots and the volcanic carapace of the arc for a strike length of ~500 km, and for another 250 km on the Queen Charlotte Islands. At deeper crustal levels within the arc, influx of mantle-derived magmas was accompanied by metamorphism and melting of Wrangellian basement rocks, yielding the heterogeneous WCC. Upward mobilization and hybridization of magmas to shallower levels in the crust resulted in the batholiths of the Island Intrusions and the lavas and pyroclastic rocks of the Bonanza Group. New U-Pb crystallization ages for plutonic rocks of the arc span an age range of 190.3 ± 1.0 to 168.6 ± 5.3 Ma. Ages of the WCC and western Island Intrusions are indistinguishable and overlap with published fossil and isotopic ages for the Bonanza Group. Younger Middle Jurassic ages for the eastern Island Intrusions overlap with those for plutonic rocks in the southern Coast Belt and Queen Charlotte Islands. All plutonic and volcanic rocks within the arc have overlapping geochemical signatures, supporting their comagmatic origin. All are light rare earth element-enriched with abundances 10-50× chondrites. The most mafic noncumulate gabbroic rocks have compositions typical of island arc basalts, with intermediate values of Al2O3 (16-17 wt.%) and high MgO (7-9 wt.%). More differentiated rocks follow a calc-alkaline trend with concomitant increase in Al2O3 (18-20 wt.%). Their geochemistry indicates varying degrees of mixing with melts of mafic Wrangellian basement.

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

“…Specific details of Rb, Sr, Sm, and Nd separation and analytical procedures can be found in Creaser et al (1997Creaser et al ( , 2004. Whole-rock chemical and isotopic analyses from this study were combined with data from previous work (Larocque, 2008;Larocque and Canil, 2010;Fecova, 2009;Paulson, 2010;DeBari et al, 1999;Andrew et al, 1991;Isachsen, 1987;Samson et al, 1990). The geochronological database that we used was compiled from all available zircon U-Pb and igneous hornblende Ar-Ar ages (Isachsen, 1987;DeBari et al, 1999;Breitsprecher and Mortensen, 2004;Fecova, 2009;Nixon, 2011aNixon, , 2011bNixon, , 2011cNixon, , 2011dCanil et al, 2012).…”

“…In contrast to the Bonanza arc, the basement of the Talkeetna arc is not exposed, and the latter arc may have developed directly on oceanic crust (DeBari and Sleep, 1991). Additionally, garnet-bearing cumulate rocks are present in the Talkeetna arc section but not in the Bonanza arc (DeBari et al, 1999).…”

“…Deltaic and marine conglomerates, sandstones, siltstone, and shale of the Cretaceous Nanaimo Group (Muller, 1977) overlie the Bonanza arc rocks. The Bonanza arc is geochronologically correlative to the Jurassic Talkeetna arc in Alaska (DeBari et al, 1999), but there are some important distinctions. In contrast to the Bonanza arc, the basement of the Talkeetna arc is not exposed, and the latter arc may have developed directly on oceanic crust (DeBari and Sleep, 1991).…”

“…Geobarometry indicates a restricted and generally uniform depth of equilibration of 2-10 km for the Island plutonic suite . The Westcoast complex is composed of hornblendites and gabbroic to granodioritic rocks occasionally in contact with rocks of the Devonian Sicker arc (DeBari et al, 1999). The Westcoast complex shows equilibration depths of 10-17 km using Al-in-hornblende geobarometry, but those results have high uncertainty .…”

“…8C) has no equivalent in the Bonanza arc and likely represents the garnet-bearing cumulate rocks known from the former arcs (DeBari and Coleman, 1989;Jagoutz, 2010) but not in the Bonanza arc. The inference of garnet-bearing cumulate rocks in the petrogenesis of the Bonanza arc is significant because it provides a previously unknown similarity with the coeval Talkeetna arc (DeBari et al, 1999).…”