Water as a Petrologic Catalyst Driving ¹⁸O/¹⁶O Homogenization and Anatexis of the Middle Crust in the Metamorphic Core Complexes of British Columbia

Abstract

In the Thor-Odin and Valhalla metamorphic core complexes, we have documented a remarkable uniformity of mineral δ18O values in the middle continental crust beneath the detachment faults. For example, in the Thor-Odin Complex, throughout an 8 km thick section of metasedimentary rocks and early Tertiary leucogranites in the hanging wall of the Monashee decollement (MD), quartz δ18O = 12.3 ± 0.5% (lσ S.D.) for metapelite (N = 11), 12.0 ± 0.1% for quartzite (N = 2), 12.6 ± 0.6% (N = 4) for < 1 m thick amphibolite layers, and 12.1 ± 0.4% (N = 24) for the concordant leucogranites. No exceptions have been found to this remarkable 18O/16O homogeneity except locally in a couple of thick amphibolites and within a ductile, relatively impermeable, marble-rich section. Similar zones of 18O/16O homogeneity associated with leucogranite genesis are observed throughout the mid-crustal section of the Valhalla Complex and just beneath the MD in the Monashee Complex, the only difference being that those rocks are overall 0.5 to 1.5% lower in δ18O than in the middle crust at Thor-Odin. These zones of pervasive homogenization in 18O/l6O must be a result of exchange with magmatic or metamorphic H2O, and these same volatiles appear to have been responsible for the leucogranite anatexis. A wide range in quartz δ18O from +8 to +16 within and below the MD suggests that this major thrust fault was impermeable to aqueous fluid flow during the partial melting stage; at that time, the basement appears to have been isolated from the mid-crustal metamorphichydrothermal system.