STRUCTURAL GEOLOGY AND PETROLOGY OF MOUNT GAMSBY, COAST PLUTONIC COMPLEX, BRITISH COLUMBIA

Mount Gamsby is located along the eastern margin of the Coast Plutonic Complex in western British Columbia. It is composed of amphibolite facies gneiss intruded by two distinct plutonic suites and is affected by at least three phases of deformation. The earliest phase of deformation is defined by recumbent folds with shallow ~east-west plunges. These folds are refolded into isoclinal folds which plunge steeply to the southeast and have steeply west dipping axial surfaces. Kinematic analysis shows that this generation of folds formed during conjugate strike slip shearing. Northwest striking shear zones with shallow lineations record sinistral shear, whereas north striking shear zones, also with shallow lineations, deform by dextral strike slip. The mean foliation in the area bisects the shear planes and is axial planar to the steeply dipping folds. This geometry of the shears and foliation has an orthorhombic symmetry and shows that the second generation folds formed during northeast-southwest directed shortening dominated by pure shear. These structures are cut by a series of gently dipping shear zones which record top to the south displacement. The highly localized nature of the shear zones, the development of retrograde mineral assemblages within the shears, and the intense grain size reduction are all consistent with these shears forming during extension. Metamorphic grade at Mount Gamsby is lower than that found in adjacent parts of the Central Gneiss Complex and higher than that found within the Stikine Terrane to the east. This suggests that the rocks at Mount Gamsby may be part of a broad transition zone between the high grade arc core and the low grade Stikine Terrane. If correct, this relationship contrasts with that observed farther north where extensional shearing has juxtaposed granulite facies gneisses with greenschist facies upper crust rocks.