Geological and geochemical evidence for variable magmatism and tectonics in the southern Canadian Cordillera: Paleozoic to Jurassic suites, Greenwood, southern British Columbia

The Paleozoic and early Mesozoic rocks of the Greenwood mining camp in sout hern British Columbia are a part of the Quesnel terrane in the eastern part of the Intermontane Belt of the Canadian Cordillera. Upper Paleozoic rocks include the Knob Hill Group composed of oceanic tholeiitic basalts (with ( La/Yb)(n) approximate to 0.4-1.2), associated with deep ocean sedimentary r ocks and serpentinites; the Attwood Group that comprises island-arc tholeii tes (with (La/Yb)(n) approximate to 1-4 and positive epsilon (Nd) values), clastic sedimentary rocks and limestones; and a unit of oceanic gabbros wit h (La/Yb)(n) < 0.5. These lithologically defined units occur as tectonicall y emplaced slivers of oceanic crust probably produced during the closure of the Slide Mountain basin during the Permian. They are unconformably overla in by Middle Triassic calc-alkaline volcanic and sedimentary rocks of the B rooklyn Group. The Brooklyn Group volcanic rocks have characteristics of ma ture island-arc rocks, including (La/Yb)(n) approximate to 2.5-4.5 and posi tive epsilon (Nd) values. The Paleozoic rocks are crosscut by a 200 million years old granodioritic intrusion containing zircon with an Early Proteroz oic inheritance age (similar to2.4 Ga). By inference, southern Quesnellia m ay have been well offshore from the ancestral North American margin in the Mississippian, in close proximity to the margin by the Middle Triassic, and contiguous with it by the Early Jurassic. It is suggested that the complex tectonic history of extension and contraction of the southern Canadian Cor dillera during the post Middle Jurassic can be extended in south-central Br itish Columbia as far back as the upper Paleozoic.