Calc-Alkaline magmatism associated with lithospheric extension in the Eocene and Miocene of the Pacific Northwest, U. S. A
- Washington State Univ., Pullman, WA (United States). Dept. of Geology
A basic tenet of igneous petrology is that calc-alkaline suites are created in the subduction process and that extension is associated with alkalic and bimodal suites. Tectonic models of older terranes often use calc-alkaline suites as sure evidence of subduction. Eocene magmatism in the Pacific Northwest and Miocene volcanism associated with the northern limits of the Basin and Range province appear to contradict these tenets and so to raise reservations concerning their use in developing tectonic models. In northeast Washington State, extension in the Eocene is evidenced by gneissic core complexes and grabens. Recent precise dating of structures and magmatism (plutons, dikes and lavas) leaves no doubt that magmatism and extension are products of the same process. South of the Columbia Plateau, in the Miocene, the Powder River volcanic field consists of basalts, andesites, dacites and rhyolites erupted within the La Grande and Baker grabens; the magmatism clearly coincidental with graben development at ca. 14 Ma. Both basalts and more silicic flows have the typical subduction-related trace element signature. In neither province is progression from calc-alkaline to bimodal volcanism apparent. Both the Eocene and Miocene suites are associated with alkalic rocks. The authors suggest that in areas such as the western Cordillera extension causes the partial melting of sources which already carry the subduction-related signature. This could be crust (intermediate to more silicic magmas) or a depleted subcontinental mantle enriched during an earlier subduction event (basalts); that is, that while calc-alkaline rocks are associated with subduction, they may not require contemporaneous subduction, as is usually assumed.
- OSTI ID:
- 5096882
- Report Number(s):
- CONF-9305259--
- Journal Information:
- Geological Society of America, Abstracts with Programs; (United States), Journal Name: Geological Society of America, Abstracts with Programs; (United States) Vol. 25:5; ISSN GAAPBC; ISSN 0016-7592
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
580000* -- Geosciences
CENOZOIC ERA
CHEMICAL COMPOSITION
CHEMISTRY
DEVELOPED COUNTRIES
GEOCHEMISTRY
GEOLOGIC AGES
GEOLOGIC DEPOSITS
GEOLOGIC HISTORY
GEOLOGIC MODELS
GEOLOGY
IGNEOUS ROCKS
MAGMATISM
METAMORPHIC ROCKS
NORTH AMERICA
PETROGENESIS
PETROLOGY
PLATE TECTONICS
ROCKS
SCHISTS
SUBDUCTION ZONES
TECTONICS
TERTIARY PERIOD
USA
WASHINGTON