GENERATION OF INTERMEDIATE CRUST:VIEW THROUGH THE PLUTONIC WINDOW OF THE JURASSIC BONANZA ARC SECTION, VANCOUVER ISLAND, CANADA

The Jurassic Bonanza arc exposed in oblique section on Vancouver Island, Canada has a structural thickness of at least 15 km and an average bulk composition of basaltic andesite (57% SiO₂, Mg# = 48) similar to bulk continental crust. The most abundant component in this and other arcs is, unremarkably, ‘intermediate plutonic rock’. It is by no means universally known to what extent the addition of new material versus recycling of older material is involved in the generation of crust in arcs. New U-Pb zircon ages for heterogeneous mafic – felsic rocks in the mid-crustal plutonic component of the arc were used to examine the consanguinity of volcanism and plutonism and processes in production of intermediate crust in this arc. The intermediate mid-crustal plutons were emplaced between 197 and 174 Ma, overlapping in time with a more felsic upper crustal (< 10 km depth) plutonic component of the arc. A 12 km thick plutonic arc crust was built as a series of sheets at a rate of ~ 0.003 km³ a^-1 partly into a substrate of pre-Jurassic supracrustal rocks. Deformation focussed in the mid-crust is syn-magmatic, with no obvious relationship between age and differentiation. Our petrological studies of all rock types show intermediate crust of the evolved plutonic section can be explained by Hbl+Plag fractionation; the former as a cryptic intercumulus phase economical in producing evolved liquids by in-situ fractionation (in molar ratio 0.65:1). There is ample field evidence for intermingling and cross-cutting of felsic, intermediate and even mafic magmas at various scales, but no profound Nd-Sr isotopic or field evidence for assimilation and ‘mixing’ of partial melts of country rock. Previously described ‘migmatites’ or melted country rock are re-interpreted as strained (re-intruded) intermediate plutonic rocks. In this sense the Bonanza arc may be an extreme – a ‘clean’ end member example in production of intermediate continental crust in arcs.