The North American Cordillera is composed of several lithospheric fragments that accreted to North America since the late Devonian. These lithospheric fragments are characterized by oceanic basin or volcanic arc sequences. The Late Paleozoic and Triassic arc sequences were interpreted to belong to a single arc system. In the past ten years, major advances have been made in understanding Permlan arc rocks. The late Paleozoic to Triassic arc sequences of Eastern Klamath and Sierra Nevada (California) are built on oceanic and continental basement, respectively. In western Nevada, upper Paleozoic arc sequences are exposed in two distinct areas: Black Rock Desert and Excelsior Mountains. In the .Black Rock Desert, the Bilk Creek Paleozoic-Triassic arc sequences show strikig slmllarities with the Eastern Klamath and Blue Mountains arc terranes. Magmatism is continuous from mid-Permian until Late Triassic time, and rests conformably on Early Permian carbonates with Tethyan affinities, and derives from a N-MORB type mantle source. In the Excelsior Mountains, the Black Dyke Formation is composed of lavas and pyroclastic breccias overlain conformably by volcaniclastic turbldities. This volcanic actlvity occurs during Early Permian around 276 Ma. This arc magmatism shares similarities with the upper Paleozoic Sierra Nevada arc. In both areas, volcanic arc sequences are characterized by: (i) Early Permian age; (ii) calc-alkaline volcanic rocks; and (iii) low eNd (T) values ranging between -11 to +5.5. In Sierra Nevada, upper Paleozoic arc was built on a lower Paleozoic metamorphose sedimentary sequence the Shoo Fly Complex. This arc was separated from North American margin by an oceanic basin, Golconda Basin. Low eNd(T) values of magmatic rocks of Sierra Nevada and Excelsior Mountains suggest that magmas derive from a mantle source contaminated by an old continental crustal component, probably Proterozoic. During magma ascent and stocking in the arc crust, assimilation of crustal material took place while melts differentiated by crustal fractionation. Upper Paleozoic arc sequences of Sierra Nevada and Excelsior Mountains are folded, and overlain unconformably by Triassic and Jurassic sediments, respectively. These deformations and unconformities are related to the Sonoma Orogeny, reflecting the collision of this arc with the North American margin. After the accretion of this arc, a calc-alkaline to shoshonitic magmatism occurs. during Triasslc time on the North American margin. Magmatic dlverslty reflects probably variations of the nature and/or thickness of the crust of the North American margin, on which built this andean-type Triassic arc
Les séquences magmatiques d'arc du Paléozoïque supérieur et du Trias du Nevada (Etats-Unis d'Amérique) et de Colombie britannique (Canada) : structure, pétrologie et géochimie : implications dans l'évolution géodynamique des Cordillères nord-américaines et les processus d'accrétion continentale .