Extending the ancient margin outboard in the Canadian Cordillera: record of Proterozoic crust and Paleocene regional metamorphism in the Nicola horst, southern British Columbia
The Nicola horst exposes plutonic and amphibolite-grade metamorphic rocks and is surrounded by low-grade arc rocks of the Late Triassic Nicola Group. We present new geological mapping and U–Pb, Nd, and metamorphic data for the Nicola horst near Bob Lake, ∼40 km south of Kamloops, British Columbia. The Bob Lake assemblage includes felsic to intermediate metavolcaniclastic rocks, metaconglomerate, schist, phyllite, and other rock types. From U–Pb zircon analysis, a felsic metaporphyry clast in metaconglomerate is 1.04 Ga old. The oldest detrital zircons in metaconglomerate and schist are also near 1 Ga. The Bob Lake assemblage was intruded by 230 Ma tonalite, 219 Ma diorite, and 64 Ma leucogranite and includes 161 Ma felsic porphyry and 157 Ma rhyodacite. Amphibolite-grade metamorphism and ductile flattening and stretching affected all rocks except crosscutting Paleocene granite and granodiorite. The high-grade rocks may be exposed as a result of latest Cretaceous – Eocene extensional ductile flow beneath a thin brittle upper crust. A thickness of ∼20 km of juvenile crust beneath the proposed Quesnel terrane is inconsistent with the evidence of Proterozoic source rocks at surface. We infer that most of the crustal lithosphere in this part of the Intermontane Belt is continental, which does not preclude possibly thick arc rocks in other parts. The local thinness of the Nicola Group, however, is inconsistent with emplacement as a regional allochthon and thus with their inclusion in a Quesnel “terrane.” The Nicola arc succession appears to have been built on the ancient continental margin.