Metamorphic, structural and stratigraphic evolution of the Telkwa Formation/: Zymoetz River area (NTS 103 I/8 and 93L/5), near Terrace, British Columbia
Bibliography: p. 119-128.Late Triassic to Hid-Jurassic Hazelton Group volcanics underlie much of north-central British Columbia and are dominated by Late Triassic to Early Jurassic Telkwa Formation strata. The Telkwa Formation is important (a) from an economic standpoint as a host to precious metal mineralization; and (b) from a tectonic history perspective as a sequence overlapping allocthonous terranes amalgamated in the Late Triassic, and as a monitor of geologic evolution since that time. Telkwa Formation strata rest unconformably atop deformed Permian carbonates, which are among the oldest rocks in the Zymoetz River study area on the southern margin of the Bowser Basin. Subdivision of the Telkwa Formation into a lower intermediate to mafic member, variegated middle member, and an intermediate to felsic and tuffaceous upper member, enables correlation amongst five structural domains which comprise an eastward-dipping homocline. A western intrusive-dominated domain is bounded to the east by west-directed thrust faults involving Permian strata. The remainder of the study area is divided into four domains, primarily by major west-side-down normal faults interpreted to have 2-4km of stratigraphic throw. Limits of a domain along the southern margin of the area are poorly constrained. Deformation within eastern domains is mainly by smaller-scale (<700m) block faulting. Metamorphism of Telkwa Formation rocks generally increases in grade from zeolite to prehnite-pumpellyite facies with depth of burial, but also displays characteristics of contemporaneous and later hydrothermal events. Fluid inclusion composition and thermobarometry is generally consistent with the stability limits of observed authigenic minerals, as well as pressures inferred from the estimated thickness of overlying stratigraphy. Contact metamorphism at the base of the Permian carbonate occurred during Early Eocene intrusion. Garnet-biotite exchange thermometry, wollastonite equilibria, modelled conductive heating, fluid inclusion data, and estimates of the pressure due to overlying strata are all consistent with contact metamorphic P-T conditions of approximately 550°C and Pe20 approaching PFLuro-2Kb. Geothermal gradients within the middle and upper members prior to Eocene intrusion range from 30 to 45°C/km based on fluid inclusion data. Maximum geothermal gradients during and/or following Late Cretaceous to Eocene intrusive events are estimated from laumontite breakdown as between 45 and 85°C/km, but are not of regional extent. A geothermal gradient of 60°C/km is in agreement with local transitions from prehnite-pumpellyite facies to greenschist (epidote-actinolite) facies at the base of the Telkwa Formation. Uplift rates since the early Tertiary based on a 6 to 7km thick succession of Telkwa Formation volcanics averaged approximately 0.1 mm/year
