PETROLOGIC CONSTRAINTS ON CORE COMPLEX EMPLACEMENT, VALHALLA COMPLEX, BRITISH COLUMBIA

Migmatitic pelites and amphibolites from the Valhalla metamorphic core complex, southeastern B.C. record a complex retrograde history that includes melt-involving net transfer reactions and partial re-equilibration at lower pressure conditions. Pelites from within and ~1.5 km above the Gwillim Creek shear zone (GCSZ) contain diffusion zoned garnets that indicate a very fast then moderately fast cooling history consistent with juxtaposition of this migmatitic crustal root zone of the Canadian Cordillera against a cold footwall by the shallowly dipping thrust sense GCSZ, followed by exhumation via the Slocan Lake detachment after melt crystallization. Interdiffusivity of Fe+Mg, governed by a retrograde net transfer reaction, was forward modeled with grt radius varying linearly with T. Thrusting onto a cold footwall at ~60Ma caused fast initial cooling and leucogranitic melt crystallization. Cooling may have continued for 5-10 m.y. during fluid activity that resulted in continued zircon rim and monazite crystallization. A difference in the shape of the diffusion profile with proximity to the GCSZ reflects the time scale of thermal conduction when the complex was thrust onto an effective heat sink.

An amphibolite from a structurally higher level between the Valhalla and Passmore dome areas contains the assemblage grt+pl+hbl+bt+cpx+qtz+sulfides. Fe/(Fe+Mg) in grt ranges from 0.72 to 0.87 and Xsps zoning shows a uniform core (~0.03) and a near rim increase (to ~0.09). Xan ranges from ~0.60 in pl cores to ~0.77 in rims with the highest values adjacent to cpx+hbl. Pl and hbl rims around grt and cpx record a retrograde granulite to amphibolite facies transition. Thermobarometric comparisons of grt+hbl, grt+cpx, gt+pl+qtz+cpx, and gt+pl+qtz+hbl equilibria, using grt and pl cores vs. rims, constrain a cooling and decompression P-T path segment consistent with retrograde amphibolite facies re-equilibration. The timing of decompression is still unclear but it may have begun following initial cooling by thrust emplacement and accelerated with exhumation along the Valkyr-Slocan Lake detachment system. 2D thermal modeling suggests that very fast followed by moderately fast cooling can be accommodated by movement up a shallow thrust ramp followed by normal faulting with erosional denudation.