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Origin of igneous rocks associated with Mélanges of the Pacific Rim Complex, western Vancouver Island, Canada
Author(s) -
Brandon Mark T.
Publication year - 1989
Publication title -
tectonics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.465
H-Index - 134
eISSN - 1944-9194
pISSN - 0278-7407
DOI - 10.1029/tc008i006p01115
Subject(s) - geology , pillow lava , igneous rock , terrane , oceanic crust , subduction , paleontology , basalt , mesozoic , volcanic rock , geochemistry , crust , volcano , tectonics , structural basin
The Pacific Rim Complex (PRC) has previously been interpreted as a late Mesozoic subduction complex that formed along the western margin of Vancouver Island. This paper examines a specific aspect of this interpretation: that igneous rocks within the PRC are fault slices derived from the footwall or hanging wall of a subduction thrust. The footwall would have been an oceanic plate, and the hanging wall, Wrangellia, a large coherent terrane that underlies most of Vancouver Island. New mapping has shown that the PRC comprises a lower Mesozoic volcanic unit, herein named the Ucluth Formation, and a superjacent sequence, more than 2 km thick, of Lower Cretaceous sediment‐rich mélanges. Within the mélanges are large blocks of volcanic and plutonic rocks, most of which were derived from the underlying Ucluth Formation. A minor fraction of these blocks cannot be assigned to Ucluth; they consist of Upper Jurassic pillow basalt and rare ultramafite. Fossil ages and chemical data indicate that the bulk of the igneous rocks in the PRC, as represented by the Ucluth Formation and blocks derived from the Ucluth, could not have come from Wrangellia nor from a subducting oceanic plate. The blocks of Upper Jurassic pillow basalt do represent fragments of oceanic crust, but field relations indicate that they are not fault slices. The mudstone matrix surrounding the blocks contains interbeds of “green tuff” which are shown to be scree deposits derived from the Ucluth and the Upper Jurassic pillow basalts. My interpretation is that the PRC mélanges formed by surficial mass wasting, and not by faulting along a subduction thrust. Furthermore, regional geologic relations indicate that the present location of the PRC, outboard of Wrangellia, is a result of strike slip faulting during the latest Cretaceous or early Tertiary. This event postdates the formational age of the mélanges by at least 45 m.y. An appendix of chemical analyses, locality descriptions, sample petrography, analytical methods, and estimated accuracy is available with the entire article on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, DC 20009. Document T89‐004; $2.50. Payment must accompany order.