Abstract

The Northern Canadian Cordilleran Miogeocline developed intermittently during the early Paleozoic and hosts alkalic and ultrapotassic volcanic rocks that are spatially restricted in thin beds and lenses and isolated volcanic piles. On the basis of geochemistry and geographic location, these volcanic rocks are subdivided into five main groups. Group I rocks (Porter Puddle and Macmillan rocks) are potassic basanites characterized by high Nb, Ce, and Nb/Y and low Zr/Nb. They are chemically similar to the Mountain Diatreme, indicating a genetic link. Group II rocks (Porter Puddle, Niddery, and Macmillan rocks) are also potassic but have lower abundances of Nb and Ce, higher Zr/Nb, and lower Nb/Y. Group III rocks (Vulcan and Itsi Lakes) are also potassic but are chemically variable, have lower contents of high field strength elements (HFSE) than the groups I and II rocks, and contain elevated Ba contents. Groups I–III are characterized by mica (biotite and phlogopite) phenocrysts, sanidine, augite, and Ba-feldspar, a mineral assemblage typical of ultrapotassic lavas. Group IV (Whale Mountain) alkali basalts are the least enriched in the large ion lithophile elements and have relatively low contents of HFSE compared with Groups I and II basalts. Groups I–III are consistent with partial melting of a previously metasomatized lithospheric mantle that was variably enriched in Ba, Nb, and Ce, whereas the group IV rocks are more typical of asthenospherically derived oceanic island basalt partial melts. The geochemistry of the volcanic rocks is consistent with paleotectonic models of the Selwyn Basin. The Selwyn Basin is a passive continental rift that underwent episodic extension and associated subsidence throughout the lower Paleozoic. Alkalic volcanism, and spatially and temporally associated Ba and base metal mineralization, is concentrated along rift-parallel normal faults, particularly where these faults are offset by transform faults.

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