O-18 O-16 HOMOGENIZATION OF THE MIDDLE CRUST DURING ANATEXIS - THE THOR-ODIN METAMORPHIC CORE COMPLEX, BRITISH-COLUMBIA/

The occurrence of isotopically uniform quartz (delta(18)O = 12.5 parts per thousand +/- 0.5 parts per thousand) and feldspar (10.9 parts per thousand +/- 0.7 parts per thousand) throughout different rock types indicates that much of a 6-km-thick section of the midcrustal Selkirk allochthon underwent internally buffered O-18/O-16 homogenization duri ng Paleocene melting and decompression as it moved up the Monashee thr ust ramp, Areas of uniform delta(18)O are those with the most leucogra nite or those subjected to severe anatexis, Only locally, in the most impermeable (or refractory) zones did O-18 exchange among the rocks, l eucogranite melts, and aqueous fluids fail to go to completion (i.e., in the deepest parts of the section, in a marble-rich zone, around;som e thick amphibolites, and in most garnets), Evidence for O-18/O-16 het erogeneity in the protoliths of these rocks is observed in stratigraph ically correlative lower-grade units elsewhere in British Columbia, as well as in garnets that coexist with isotopically homogeneous quartz, in our model, O-18/O-16 homogenization accompanied muscovite dehydrat ion and partial melting of pelites with only minor influx of external H2O, followed by release of magmatic H2O from these melts (triggering further melting of adjacent feldspathic assemblages) as they were upli fted 20 km during thrusting just prior to onset of detachment faulting , Locally, low delta(18)O in feldspar (down to -3.8) and profound quar tz-feldspar O-18/O-16 disequilibrium were imprinted at shallow levels during meteoric-hydrothermal alteration associated with Eocene detachm ent faulting.