COMPARISON OF THE RAINY RIDGE ANALCIME PHONOLITE SILL AND THE CROWSNEST VOLCANICS, ALBERTA, CANADA

A 5-7 m thick analcime phonolite sill occurs in the middle of the Prot erozoic Gateway Formation in southwest Alberta. The sill consists of s anidine, aegirine-augite, magnesian hastingsite, melanite with hydrogr ossular rims, titanite, and minor biotite, apatite, and opaque mineral s. Mineralogical and chemical similarities to the analcime-rich phases of the Cretaceous Crowsnest Formation found some 20 km to the north s uggest a genetic relationship. Major differences are the presence of a mphibole and hydrogrossular, minerals not reported in the Crowsnest Fo rmation. The presence of amphibole as a primary hydrous phase in the R ainy Ridge sill indicates crystallization from a hydrous magma. Microp robe studies indicate a progressive enrichment of sodium in amphiboles and pyroxenes. An apparent difference in chemical composition and alt eration behavior of primary analcime phenocrysts and groundmass analci me is interpreted to reflect crystallization of analcime from a hydrou s melt at depth, followed by rapid transport to a shallow depth, and c rystallization of the groundmass analcime and hydrogrossular rims.