Stratigraphy, structure, petrology, geochronology and geochemistry of the Hobson Lake area (Cariboo Mountains, British Columbia) in relation to the tectonic evolution of the southern Canadian Cordillera.

摘要

Detailed field mapping along with petrographic, geochemical and geochronological studies in the Hobson Lake area of the central Cariboo Mountains (Omineca Belt, Canadian Cordillera) has improved the understanding of the deformation and metamorphic history of the area. Three main findings can be drawn from this work. (1) The proposed stratigraphic correlation between the Paleozoic Snowshoe Group of the northwestern Cariboo Mountains and the Mica Creek succession of the central and southeastern Cariboo Mountains is not valid. This conclusion is based on field mapping in the Hobson Lake area and on different Nd isotopic patterns in the two stratigraphic packages. Therefore, a proposed crustal-scale west-verging thrust fault known as the Pleasant Valley Thrust does not exist in the Hobson Lake area. (2) The Hobson Lake Pluton and related granifoid plutons are lens-shaped, I-type plutons that are Middle Jurassic. These plutons cross-cut the regional F₂ fold structures in the country rocks and were subsequently deformed by a later deformation event. The geochemistry of these plutons suggests that they are primitive magmas that have similar isotopic compositions to other arc-related Middle Jurassic plutons of the southern Canadian Cordillera. (3) The Hobson Lake area has undergone three main phases of deformation (D₁, D₂, and D ₃) and two phases of metamorphism (M₁ and M₂). These deformation events all formed southwest-verging fold structures. D ₁, D₂ and M₁ occurred prior to the emplacement of the Hobson Lake pluton and are therefore pre-Middle Jurassic in age and are preserved only at high structural levels in the study area. U-Pb monazite geochronology in the kyanite-staurolite grade schists shows that D₃ deformation and M₂ metamorphism likely occurred in the latest Jurassic to Early Cretaceous. D₃ deformation and M₂ metamorphism are only found at deeper structural levels in the study area. The boundary between the pre-Middle Jurassic structures and Early Cretaceous structures is a gradational metamorphic and structural front that can be mapped in the Hobson Lake area.