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Geology of Vedder Mountain, near Chilliwack, B.C. McMillan, William John

Abstract

Vedder Mountain can be divided into three units: the eastern sediments, the crystalline rocks and the western sediments. Both eastern and western sediments are essentially unmetamorphosed whereas the crystalline rocks include both medium grade metamorphic rocks and saussuritized dioritic intrusive rocks. The crystalline rocks are bounded by steep southeast dipping faults. White mica-amphibole and garnetiferous white mica-amphibole schists and gneisses, amphibolite, epidote amphibolite and garnet-sphene-white mica schists comprise the metamorphic rocks. The mineral assemblages are typical of the almandite-amphibolite facies of Turner and Verhoogen (1960). Foliated diorites intrude (?) the metamorphic rocks. Basic contact zones, lighter colored diorite dikes, amphibole-feldspar pegmatites and small quartz diorite bodies are thought to represent various phases of differentiation of a parent magma. Pervasive saussuritization characterizes these rocks. In structural succession, the eastern sediments are comprised of chert, granitic and volcanic pebble and cobble conglomerates with plagioclase volcanic arenite interbeds; plagioclase volcanic arenite with conglomerate interbeds near the base of the unit and argillite interbeds near the top; and micro-volcanic arenite with interbeds of plagioclase volcanic arenite, argillite, chert and siliceous argillite with scattered, impure limestone pods. In structural succession, the western sediments consist of argillite; micro-volcanic arenite; chert lenticule arenite and volcanic chert arenite breccia which contain a band of impure, cherty limestone; argillite and chert. Vulcanism produced dacite porphyries which structurally underlie the sediments. The crystalline rocks comprise a tabular body believed to have been emplaced by faulting. Small, ellipsoidal serpentinite bodies lie along the southeast bounding fault of the crystalline slice. During emplacement of the crystalline slice, it appears that the sediments were pushed aside in what has been referred to as phase I deformation. Folding in the argillaceous units was "similar" in nature but in the more competent units it was "concentric." The eastern sediments comprise a synform with near horizontal northeast trending fold axis and steep southeast dipping axial plane. The western sediments comprise a steep, southeast dipping homocline.

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