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Jurassic and Cretaceous geochronology of the southern Coast Belt, British Columbia, 49° to 51°N

By
Richard M. Friedman
Richard M. Friedman
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Richard L. Armstrong
Richard L. Armstrong
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Published:
January 01, 1995

We report, and tabulate with previous dates, 40 new U-Pb zircon and 14 K-Ar biotite and hornblende dates from the southern Coast Belt of British Columbia. The new dates, mainly from plutons of the Coast Plutonic complex, represent crystallization (U-Pb) and cooling (K-Ar, Rb-Sr) ages. Zircons from most of the rocks are concordant to mildly discordant, the latter commonly indicating minor Pb loss with young lower intercepts. U-Pb data for several rocks also indicate the possibility of inheritance.

Pre–Coast Plutonic complex volcanic rocks (187–167 Ma) correlate with rocks of the Early to Middle Jurassic Bonanza-Island Intrusion arc of Wrangellia. Early Coast Plutonic complex plutons (167–145 Ma) are confined to the western Coast Belt and eastern Insular Belt and tie the western Coast Belt to Wrangellia (Insular Superterrane) by Late Jurassic time. Early Cretaceous (145–112 Ma) Coast Plutonic complex plutons and associated volcanic rocks are also limited to the western Coast Belt and eastern Insular Belt. Widespread Early Cretaceous (ca. 145–130 Ma) uplift is suggested, based on Early Cretaceous (Neocomian) conglomerates exposed across the width of the western Coast Belt. Similar conglomerates in the central Coast Belt are interpreted to represent the earliest stratigraphic tie at this latitude between the Insular and Intermontane superterranes. Mid-Cretaceous igneous rocks (112–90 Ma) extend from Howe Sound in the west to the eastern Coast Belt, and intrude the hanging wall and footwall of the Coast Belt thrust system. They constrain the timing of west-directed thrusting within the Coast Belt thrust system to approximately 91–97 Ma. Their location indicates an approximately 30-km eastward progression of the western limit of major magmatism from ca. 115–100 Ma. Late Cretaceous igneous rocks (90–65 Ma) are located almost exclusively within the central and eastern Coast Belt, in the hanging wall of the Coast Belt thrust system. A belt of 87–84-Ma old plutons provides evidence for an approximately 100-km-eastward shift of the western limit of magmatism between approximately 100 Ma and 90 Ma, coeval with west-directed thrusting along the Coast Belt thrust system.

The distribution of late Early to Late Jurassic and possibly earliest Cretaceous igneous rocks in the southern Coast Belt suggests, but does not prove, that they were generated in an arc specific to Wrangellia (Insular Superterrane). In the context of a regime such as this, one or more intervening subduction zones are required to account for more easterly Jurassic magmatism. By ca. 100 Ma, we envisage a single west-facing arc along the entire length of the North American Cordillera, a regime that continued through the remainder of the Cretaceous period.

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Contents

GSA Special Papers

Jurassic Magmatism and Tectonics of the North American Cordillera

David M. Miller
David M. Miller
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Cathy Busby
Cathy Busby
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Geological Society of America
Volume
299
ISBN print:
9780813722993
Publication date:
January 01, 1995

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