PROTOLITH COMPOSITION OF CORDIERITE–GEDRITE BASEMENT ROCKS AND GARNET AMPHIBOLITE OF THE BEARPAW LAKE AREA OF THE THOR–ODIN DOME, MONASHEE COMPLEX, BRITISH COLUMBIA, CANADA

The Thor–Odin dome of the Monashee complex, southeastern Canadian Cordillera, British Columbia, contains basement gneiss of North American tectonic affinity, including a distinctive cordierite–gedrite rock that is restricted in occurrence to the basement rocks and a regionally occurring garnet amphibolite. All of these rocks were penetratively deformed and metamorphosed at upper-amphibolite to lower-granulite facies conditions in the Cordilleran orogeny during the Late Cretaceous to Paleocene. The cordierite–orthoamphibole (dominantly gedrite) unit appears to define a discontinuous marker horizon or horizons within the Paleoproterozoic basement paragneiss. The cordierite–gedrite rocks have a unique bulk-rock composition that is characterized by depletions in the alkali elements and Ca, and enrichments in Al, Mg and Fe. They have flat rare-earth element (REE) patterns, depletions in most low field-strength elements (LFSE), and enrichments in high field-strength elements (HFSE). The measured 87Sr/86Sr values for these rocks vary from 0.74923 to 0.85962, and the eNd(today) values vary from −15.3 to −20.6. The cordierite–gedrite rocks are interpreted as Paleoproterozoic mafic volcanic rocks that were hydrothermally altered, likely prior to metamorphism. Hydrothermal alteration explains their distinctive bulk-rock composition. Interlayered and spatially associated lenses of garnet amphibolite are of uncertain age, although they are likely Precambrian, on the basis of their Nd isotopic composition. The garnet amphibolite rocks are characterized by high Mg and Ti values and depletions in the alkali elements. They have flat REE patterns, depletions in most LFSE and enrichments in HFSE. The garnet amphibolite samples have measured 87Sr/86Sr values that vary from 0.70953 to 0.74319, and eNd(today) values that vary from −0.8 to −7.3. The garnet amphibolite rocks are interpreted as Proterozoic metamorphosed mafic dykes or possibly volcanic rocks that postdated the formation of the protolith of the cordierite–gedrite rocks.