Abstract

Ion microprobe U-Pb, δ18O, and Ti depth profiling analyses of natural zircon rims permit unprecedented assessment of the relationship between timing, temperature, and geochemical environment during crystallization and cooling of deep orogenic crust. Zircon from migmatite in a deeply exhumed gneiss dome in the Valhalla metamorphic core complex, southeast British Columbia, Canada, records the timing of melt crystallization and subsequent fluid-rock interaction before the final stages of extension and exhumation. Zircon interiors reveal a weighted mean age of 58 ± 2 Ma, interpreted as the timing of melt crystallization. Depth profiling U-Pb measurements of unpolished zircon rims yield an age of 51 ± 2 Ma over 4 μm. Ti-in-zircon thermometry indicates 650 °C for both rim and interior, confirming that the complex remained at high temperature during zircon crystallization. Previous δ18O measurements suggest that high-temperature fluids with δ18O of ~10‰ pervasively infiltrated the complex. The oxygen zircon results clearly resolve a δ18O contrast between zircon interiors (7.2 ± 0.2‰) and rims (8.4 ± 0.2‰). The lighter interior δ18O values indicate the timing of melt crystallization and equilibration of the rims with melt at 58 Ma ago. Alternatively, δ18O values from the 51 Ma rims match the composition predicted (8.4‰) from fluid interaction based upon previous work. Integrating the age and δ18O results reveals that amphibolite facies fluid-rock interaction persisted until final rapid exhumation of the Valhalla complex. The results further indicate the power of using the ion microprobe to track the timing of fluid infiltration in the crust as well as the effect of fluid infiltration on the subgrain scale.

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