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Plutonic rocks between Hope, B.C. and the 49th parallel Richards, Tom

Abstract

An area of some 400 square miles between Hope, B.C. and the 49th parallel, covering part of the northern Cascades and southern Coast Mountains, was investigated with the purpose of determining the origin and history of the plutonic rocks. Five separate plutonic complexes, which range in age from Late Cretaceous to Miocene, underlie the area. The oldest complex investigated, the Late Cretaceous Spuzzum Intrusions (103-79 M.Y.) emplaced in the catazone-Mesozone, consists of two phases: diorite and tonalite. The older of the two, the diorite, is a zoned intrusion which occupies the central part of the batholith. Sheared sills and stocks belonging to the early Tertiary Yale Intrusions (59-35 M.Y.) comprise the second oldest complex. These bodies crop out in a narrow belt that separates high grade metamorphic rocks of the Custer-Skagit Gneiss from the low grade metamorphic rocks of the Hozameen Group. The Early Oligocene Silver Creek Stock (35 M.Y.) represents the oldest of three epizonal complexes. The epizonal Chilliwack Batholith (29-26 M.Y.) is composed of seven intrusive phases which range in composition from hypersthene-augite diorite to aplitic alaskite. Each of the phases of this batholith appears to have been emplaced in a pulse from an underlying, differentiating magma that was rising through the crust. The youngest complex, the Mount Barr Batholith (21-16 M.Y.) is composed of four intrusive phases, each of which appears also to nave been emplaced in a pulse from an underlying magma. One of the phases of this batholith is in the form of a 3000-foot-thick sill-like body. These three epizonal plutons appear to be related to the north-trending Cascade volcanic-plutonic province, which overlaps the northwest-trending Coast Crystalline Complex, here represented by the Spuzzum and Yale Intrusions.

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