Introduction

Generation and evolution of crust in continental arcs remains a fundamental problem in the Earth sciences. Specifically, the dominant magmas from the mantle in all tectonic settings are basaltic, yet the bulk composition of continental crust is andesitic. The only modern plate tectonic setting where large amounts of andesitic crust are formed is in arcs. Generating large amounts of andesitic crust from a mafic protolith requires the production of a complementary ultramafic, but not peridotitic residue. The fate of this residue raises key questions about crustal evolution. How much residue is produced beneath magmatic arcs, and of what composition? How much of the residue is reprocessed within the crust? Does this residue reside invisibly beneath the Moho in arcs or does it founder into the mantle?

In this project we propose to focus on magmatic addition and crustal recycling during voluminous Late Cretaceous-early Tertiary magmatism along the North American continental margin in British Columbia. This magmatic event, which produced the Coast Plutonic Complex (CPC), is the largest in the Phanerozoic and led to growth and recycling of juvenile continental crust in this region (Figures 1 and 2). The size and relative youth of this batholith complex make it an ideal place to investigate processes of crustal evolution because it has not had a protracted tectonic history. In addition, deep exhumation, erosion and rugged topography provide unmatched access to different crustal levels within the arc.

We propose to examine the lithospheric-scale exchange of material and energy between the mantle and the crust during the formation of a large calc-alkaline batholith. Specifically, we will quantify the amount and composition of ultramafic residues within the arc as a function of time, and we will determine whether they remain within the lithosphere or whether and when they are recycled into the deeper mantle. Our tools will include petrology, geochemistry, active and passive source seismology, geochronology, structural geology, and gravity studies. Petrology, geochemistry, and active-source seismology will determine the bulk-composition of the present lithosphere, providing mass balance constraints on the required ultramafic residues. Active and passive source seismology, gravity, and petrology/geochemistry of younger volcanic rocks will search for the residues in the lithosphere or evidence of their foundering. Structural geology and geochronology will constrain evolution of the arc through time. Integration of our results will allow us to discern the lithospheric scale processes responsible for generation and evolution of one of Earth's largest batholiths.