ABSTRACT

The Purcell igneous rocks consist of an extensive accumulation of sills and spatially related altered basaltic flows in the Proterozoic formations of southwestern Alberta and southeastern British Columbia. Stratigraphic studies over the past fifty years indicate a thick succession of fine clastic sediments in the Purcell Mountains which thin markedly toward the east in the Rocky Mountains. Clastic sedimentation was apparently initiated in the west, with carbonate deposition becoming dominant in the upper and eastward extension of the Purcell System.

To the west, the sills are most abundant and thickest in the basal formations of the Purcell System. In the east, most of the sills are found in the Upper Purcell Siyeh formation, with a few thin sills in the lower and higher horizons. The sills studied in the western area are hornblende-quartz diabases. The eastern sills differ in that they contain primary pyroxene.

Two distinct areas of chloritized Purcell lavas of submarine origin are found; one in the northern part of the Lewis thrust block, the other bordering the Rocky Mountain Trench from about latitudes 49° to 50° N. In the western area two main periods of volcanism took place in the Siyeh epoch (G. Hunt 1962). Basaltic lavas in the Lewis thrust block are found through a wide stratigraphic interval, with the thickest accumulation of lava found at the top of the Siyeh formation. Thin, local pyroxene-bearing lavas occur in the Lewis thrust block, but not in the area adjacent to the Rocky Mountain Trench.

Chemically, the western Purcell lavas may be classified as tholeiitic to olivine basalts. The eastern Purcell extrusions belong to the trachybasalt family. Normative olivine is present in both the Purcell extrusive and intrusive phases from the eastern and western areas. Differentiation index diagrams suggest a two-fold grouping of the Purcell igneous rocks, with a tholeiitic basalt and its intrusive quartz diabase equivalents in the west and a more alkaline basalt in the east.