CRETACEOUS EVOLUTION OF THE BREAKENRIDGE COMPLEX WITHIN THE SOUTHERN COAST-CASCADE OROGEN: TECTONO-MAGMATIC EVENTS REFLECTING THE GEODYNAMIC SETTING OF THE NORTH AMERICAN CORDILLERA

The Breakenridge pluton was emplaced in relatively shallow crustal levels (< 3kbar) between ca. 107-100 Ma. It was initially buried by orogen-parallel contraction between 100-96 Ma, followed by emplacement of the newly differentiated Snowshoe pluton between 96-94 Ma at structurally higher crustal levels. Orogen-normal compression was the final mechanism for deep burial and attainment of peak metamorphic conditions between 91-86 Ma as constrained by metamorphic monazite and zircon. The peak conditions are constrained by thermobarometry on zoned garnet in a kyanite-bearing migmatitic pelite and three metaplutonic samples. The thermometers (garnet-biotite) and the geobarometers (GMAP, GAPQ, GRIPS and GRAIL) showed temperatures of 500-680°C and pressures of 6.5-10.3 kbar. These P-T-t constraints for peak metamorphic condition are among the first definitive constraints in this part of the southern Coast-Cascade orogen that link in situ age analysis with minerals used for thermobarometry. The in situ metamorphic zircon and monazite U-(Th)-Pb analysis revealed that high grade metamorphism was sub-solidus at ca. 89-87 Ma when monazite appeared, followed by partial melting at 88-86 Ma when the main phase of metamorphic zircon grew. Following orogen-normal contraction and regional folding, oblique right-lateral strike-slip shearing occurred while these rocks were still deep and in the ductile regime.

The newly acquired data are used to propose a model that involves both regional thrusting and magmatic loading. The model consists of three distinct magmatic pulses, characteristic of the Coast-Cascade orogen during the Early to Late Cretaceous (110-80 Ma), which reflect changes in tectonic setting and evolving boundary conditions along the convergent margin of the North American Cordillera. The Breakenridge pluton belongs to the first magma pulse, followed by orogen-parallel displacement. The Snowshoe pluton belongs to the second magma pulse, followed by orogen-normal displacement, folding, and high-grade metamorphism. The third magma pulse is synchronous with the late stages of high-grade metamorphism and early stages of oblique strike-slip movement that is interpreted, in part, to be responsible for rapid exhumation of the study area.