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The geology of part of the Shulaps ultramafite, near Jim Creek, southwestern British Columbia Nagel, Joe Jochen

Abstract

The Shulaps ultramafite lies at the eastern edge of the Coast Plutonic Complex approximately 150 miles from Vancouver, B.C. It is one of the largest "alpine-type" peridotites in British Columbia, consisting primarily of harzburgite and subordinate dunite lying in an elongate zone trending northwest. Irregular bodies of gabbro and clinopyroxenite occur on the western side of the ultramafite associated with pillowed volcanic rocks. The ultramafite is bounded on the northeast by the Yalakom fault, a major regional structure, and on the southwest by rocks on the Fergusson Group, an assemblage of chert, clastic and volcanic rocks, probably of Triassic age. This study outlines the distribution of peridotite, gabbro and other rocks in the area just west of Shulaps Peak, a major topographic feature in the area. The peridotite is almost completely serpentinized and pervasively sheared. It is in tectonic contact with all other rock types, and contains isolated inliers, ranging from a few feet to over 500 feet, of gabbro, greenstone, chert and clastic rocks. Within the area mapped, the serpentinite could be termed a tectonic melange. An interesting feature of the serpentinite is the irregular occurrence of olivine porphyroblasts, formed by the reaction serpentine + brucite = olivine. The gabbro is foliated and in places layered, although no cumulate textures were observed. It is in gradational contact with pillowed volcanic rocks and in tectonic contact with serpentinite. The evidence gathered by previous workers and during this study argues strongly for the hypothesis that the Shulaps ultramafite, and perhaps some of the associated rocks, are an allocthonous piece of oceanic crust (ophiolite) which has been emplaced into its present position by plate-tectonic processes. This emplacement probably took place between Middle Triassic and Lower Jurassic time.

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