The role of water in the magmatic and tectonic evolution of metamorphic core complexes: A stable isotope study of the southern Omineca crystalline belt, British Columbia, Canada

The oxygen isotope data in this study delineate 2 major episodes of water-rock interaction related to the metamorphic, plutonic, and tectonic development of the metamorphic core complexes in the southern Omineca belt. Episode 1 is a Paleocene pre-extensional metamorphic/magmatic-hydrothermal event. The occurrence of isotopically uniform quartz {dollar}rm(deltasp{lcub}18{rcub}O = 12.5 pm 0.5perthous){dollar} and feldspar {dollar}(10.9 pm 0.7perthous){dollar} throughout different rock types indicates that much of a 6-km-thick section of the mid-crustal Selkirk allochthon underwent internally buffered {dollar}rmsp{lcub}18{rcub}O/sp{lcub}16{rcub}O{dollar} homogenization during Paleocene melting and decompression as it moved up the Monashee decollement thrust ramp. Areas of uniform {dollar}rmdeltasp{lcub}18{rcub}O{dollar} are those with the most leucogranite or those subjected to severe anatexis. Only locally, in the most impermeable (or refractory) zones did {dollar}sp{lcub}18{rcub}{dollar}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 {dollar}rmsp{lcub}18{rcub}O/sp{lcub}16{rcub}O{dollar} 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. A model is introduced utilizing water as a petrologic catalyst: fluids evolved during muscovite breakdown and partial melting of pelites produce {dollar}rmsp{lcub}18{rcub}O/sp{lcub}16{rcub}O{dollar} homogenization with only minor influx of external H{dollar}sb2{dollar}O; this is followed by release of magmatic H{dollar}sb2{dollar}O from these melts as they crystallize (triggering further melting of adjacent feldspathic assemblages) during and after the {dollar}sim{dollar}20 km uplift that occurred in the thrusting event that took place just prior to detachment faulting.; Episode 2 is a series of Eocene synextensional meteoric-hydrothermal events affecting the shallow crust along all of the major detachment faults in the region, and along some parts of the Monashee decollement; these effects were locally enhanced by added heat from some synextensional alkaline intrusions (the Coryell plutons). Very large quartz-feldspar {dollar}rmsp{lcub}18{rcub}O/sp{lcub}16{rcub}O{dollar} disequilibrium effects were imprinted upon the rocks during exchange with hot meteoric waters (initial {dollar}rmdeltasp{lcub}18{rcub}Osim{lcub}-{rcub}15){dollar}; the mineral most affected was feldspar {dollar}rm(deltasp{lcub}18{rcub}O{dollar} down to {dollar}-{dollar}5.0). In the Valhalla core complex, the hanging wall rocks above the Slocan Lake fault are sufficiently uniform to allow us to apply open-system kinetic oxygen isotope exchange modeling, thereby placing constraints on the duration (1-3 Ma) and integrated fluid flux {dollar}rm(ge10sp7 cmsp3sb{lcub}Hsb2O{rcub}/cmsp2sb{lcub}rock{rcub}){dollar} for this hydrothermal metamorphism.