The Metchosin Igneous Complex, a partial ophiolite exposed on southern Vancouver Island, is the most northerly exposure of the Eocene Crescent Terrane. The role of the Crescent Terrane in crustal genesis and Cordilleran tectonics would be affected by its tectonic setting, however that setting is in debate. Analysis of trace element compositions of basalt from the Metchosin Igneous Complex by ICP-MS was used to determine the tectonic setting in which the complex formed. REE and HFSE compositions are transitional between N-MORB and E-MORB and do not suggest a unique tectonic setting. Strong enrichments of Nb and Ta relative to N-MORB are contrary to formation in a subduction zone. In conjunction with existing plate motion data, this makes a rifted-margin origin unlikely. Interaction at a distance between the Yellowstone hot spot and the Kula-Farallon ridge is proposed to satisfy all the geological and geochemical data. Many studies of ophiolites have interpreted high-temperature phases as hydrothermal in origin despite high permeability and low temperatures in sea floor volcanics. Metamorphic assemblages and compositions of metamorphic minerals were used to determine if alteration in the Metchosin Igneous Complex was related to sea floor alteration or obduction. Chlorite geothermometry and amphibole compositions show that peak metamorphic temperatures increase from east to west across the complex. The metamorphic facies increase from prehnite-actinolite and greenschist in the east to amphibolite in the west, corresponding with the temperatures inferred from mineral compositions. The temperature gradient is perpendicular to stratigraphy, whereas hydrothermal patterns are expected to be parallel to stratigraphy. Therefore the pattern of alteration in the Metchosin Igneous Complex is unrelated to sea floor alteration. Metamorphism during obduction has overprinted any hydrothermal alteration patterns. The east-west thermal gradient is attributed to tilting of the complex, either by tectonic forces or by unequal exhumation due to orographic effects.
| Identifer | oai:union.ndltd.org:uvic.ca/oai:dspace.library.uvic.ca:1828/526 |
| Date | 10 April 2008 |
| Creators | Timpa, Sean. |
| Contributors | Canil, Dante|Gillis, Kathryn M. |
| Source Sets | University of Victoria |
| Detected Language | English |
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