Retrograde T-t Histories From Pelitic Migmatites Reflect Structural Distance From the Gwillim Creek Shear Zone, Valhalla Complex, British Columbia

Diffusion-zoned garnets from pelitic migmatites from the Valhalla metamorphic core complex, southeastern B.C. record relatively fast cooling rates that vary with distance above the Gwillim Creek shear zone (GCSZ). Fast cooling was caused by thrusting onto a cold footwall, where rocks closest to the fault began at the highest temperatures and conductively cooled at the fastest rate. Pelitic migmatites adjacent to and structurally above the GCSZ contain garnet with core to rim zoning of Fe/(Fe+Mg), controlled mainly by diffusion during progress of the retrograde net transfer reaction (ReNTR) Grt + Kfs + Melt = Bt + Sil + Plg. Where biotite is present in contact with garnet, retrograde Fe/Mg exchange has resulted in additional increased Fe/(Fe+Mg). A strongly foliated pelitic migmatite with pervasive 2-5mm-thick leucosomes from ~1.5 km structurally above the GCSZ contains garnet with Fe/(Fe+Mg) ranging from 0.72 to 0.93 and Xsps zoning with a uniform core (~0.02) and a near rim increase (to ~0.04). Interdiffusion of Fe+Mg was modeled with garnet radius varying linearly with temperature. This finite difference model calculates Fe-Mg interdiffusivity and generates a diffusion profile based on changing garnet rim boundary conditions governed by the ReNTR. Model fits to measured profiles require a 1-5 m.y. period of slow to moderate cooling (5-25°C/ m.y.) followed by a brief period (<1m.y.) of relatively fast cooling (60-500°C/m.y). In contrast, rocks adjacent to the GCSZ show a very short initial slow cooling step (generally <1m.y.), followed by a short period of fast cooling (100-1000°C/m.y.). The difference in the duration of the initial slow cooling steps between the two localities reflects the time scale of thermal conduction when the complex is thrust onto an effective heat sink. The late cooling history (below 600°C), which is poorly constrained by the diffusion profile due to very slow diffusion at these temperatures, is interpreted to represent unloading by displacement on the Slocan Lake normal fault. 2-D thermal modeling of a low angle thrust ramp yields different T-t histories for hanging wall rocks based on their distance from the thrust. Model results show that transport on the order of cm/yr up a 10-20°-dipping thrust fault can produce cooling rates that are consistent with those calculated from garnet diffusion, and that the duration of initial slow cooling increases with distance from the fault.