THE TECTONIC SIGNIFICANCE OF MIXED I- AND S-TYPE MAGMATIC BELTS: LESSONS FROM THE CORDILLERAN OROGEN OF WESTERN NORTH AMERICA

The >500 km wide mid-Cretaceous Omineca Magmatic Belt (OMB), Canadian Cordillera, intrudes accreted terranes and continental margin strata; was synkinematic with spatially associated dextral strike-slip and east-verging thrust faults; youngs from 125 Ma in the west to 90 Ma in the east, and evolved from oxidized magnetite-bearing I-type intrusions to reduced ilmenite-bearing I- and S-type intrusions. Interpreting the OMB as an east-facing arc explains the voluminous I-type, wet magmatism. Orogenic float, consisting of basinal strata transferred from the westwardly subducting lower plate to the upper plate, gave rise to progressive upper plate growth. The result was a magmatic arc that youngs oceanward (to the east) as older arc plutons were displaced away from the trench by the ongoing accretion of orogenic float to the upper plate. The temporal evolution from I- to S-type plutons records the completion of consumption of oceanic lithosphere of the lower plate, and the subsequent entry of a continental margin and its underpinning continental lithosphere into the trench. As shown in geodynamic models (Currie et al., 2007. Geology 35, 1111-1114), subduction of thick passive margin sequences results in detachment of the sedimentary cover sequence from the downgoing plate at a depth of 100 km. The resulting detached plume of wet sediment entered into and contaminated the mantle wedge arc-magma source region, reducing the oxygen fugacity to the point that magnetite was replaced by ilmenite, and giving rise to voluminous S-type magmatism and related Sn-W mineralization. The attempted consumption of buoyant continental lithosphere inevitably led to the termination of subduction. Break off of the subducted slab was recorded at 92 Ma by an intrusive pulse of Au-rich, bimodal metaluminous to alkaline quartz monzonite to syenite, with rare gabbro and lamprophyre. Examples of magmatic suites that evolve from I- to mixed I- and S-type, are syn- to post-kinematic, young in the direction of thrust vergance, and have associated Sn-W mineralization, are numerous and include the Lachlan of Australia, the Hercynian of northern Iberia, the Cape suite of South Africa, and the Cornubian of Cornwall. A common misconception that S-type granites post-date the cessation of subduction is attributable to flawed paleogeographic models and the failure to identify cryptic sutures.