^(18)O/^(16)O homogenization of the middle crust during anatexis: The Thor-Odin metamorphic core complex, British Columbia

Abstract

The occurrence of isotopically uniform quartz (δ^(18)O = 12.5‰ ± 0.5‰) and feldspar (10.9‰ ± 0.7‰) throughout different rock types indicates that much of a 6-km-thick section of the mid-crustal Selkirk allochthon underwent internally buffered ^(18)O/^(16)O homogenization during Paleocene melting and decompression as it moved up the Monashee thrust ramp. Areas of uniform δ^(18)O are those with the most leucogranite or those subjected to severe anatexis. Only locally, in the most impermeable (or refractory) zones did ^(18)O exchange among the rocks, leucogranite melts, and aqueous fluids fail to go to completion (i.e., in the deepest parts of the section, in a marble-rich zone, around some thick amphibolites, and in most garnets). Evidence for ^(18)O/^(16)O heterogeneity in the protoliths of these rocks is observed in stratigraphically correlative lower-grade units elsewhere in British Columbia, as well as in garnets that coexist with isotopically homogeneous quartz. In our model, ^(18)O/^(16)O homogenization accompanied muscovite dehydration and partial melting of pelites with only minor influx of external H_2O, followed by release of magmatic H_2O from these melts (triggering further melting of adjacent feldspathic assemblages) as they were uplifted 20 km during thrusting just prior to onset of detachment faulting. Locally, low δ^(18)O in feldspar (down to −3.8) and profound quartz-feldspar ^(18)O/^(16)O disequilibrium were imprinted at shallow levels during meteoric-hydrothermal alteration associated with Eocene detachment faulting.

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