Petrology and lithogeochemistry of Paleozoic alkalic magmatism in the Selwyn Basin, Yukon, Canada

Abstract

Alkalic magmatism occurred in the Selwyn Basin, Yukon, Canada throughout the Paleozoic, concurrent with extension during passive margin sedimentation. To examine magmatism associated with this extension, geochemical data were obtained from several locations in the Selwyn Basin (MacMillan Pass, Anvil District, Keno Hill, and the Misty Creek Embayment). Volcanic rocks from the Anvil District and the Misty Creek Embayment are dominated by alkalic basalts with LREE-enriched geochemical signatures whereas metavolcanic and dike samples from Keno Hill comprise subalkaline basalts with E-MORB signatures. The Early Ordovician Menzie Creek volcanic rocks of the Anvil District display trace element geochemical signatures intermediate between OIB and E-MORB, whereas the Middle-Late Ordovician volcanic rocks from the Misty Creek Embayment have OIB signatures. Differences in the trace element geochemistry of the sample suites are attributed to the degree of partial melting. The Menzie Creek volcanic rocks formed from large volume melts of enriched mantle that diluted incompatible element signatures in the Early Ordovician. Late Ordovician magmatism produced the Misty Creek Embayment samples, where restricted melt volumes of an enriched mantle source resulted in the most enriched samples geochemically. The Keno Hill samples represent the shallowest melting of the analysed samples and may have resulted from melting of heterogenous subcontinental lithospheric mantle. Magmatic zircons from a Keno Hill metavolcanic sample analysed by CA-TIMS resulted in a primary deposition age of c. 296 ± 0.36 Ma (Early Permian), during a period with little magmatism in the Selwyn Basin and representing a previously unknown unit in the area.