Petrology and Tectonic Setting of A-type John Day Rhyolites, Central Oregon
Abstract
The Oligocene to early Miocene John Day Formation of central and eastern Oregon contains a large assemblage of both ash-flow and air-fall tuffs, which have been assumed to be the products of early Cascade volcanism. However, new mapping and geochemical data suggest that John Day volcanism is not simply the result of westward sweeping subduction-related magmatism. At least three large rhyolitic caldera complexes exposed along the axis of the Blue Mountains represent regionally extensive magmatism that is more similar to the large caldera producing eruptions of the Oligocene “ignimbrite flare-up” in the Basin and Range province. Moreover, reconstruction of the erupted chemical compositions of the altered John Day tuffs reveals alkaline affinities and A-type signatures, which are more commonly associated with extensional tectonics than with volcanic arc settings. The mixed calc-alkaline and alkaline affinities of the Clarno Formation and apparent lack of calc-alkaline rocks in the John Day Formation suggest a fundamental shift in magma generation, while the overlapping lithologies and ages of the Clarno and John Day Formations imply that Clarno activity did not completely end with the start of John Day volcanism. The timing and accretion of the Coastal Range Basalt Province to the North America craton around ~48 Ma likely played a pivotal role in the evolution and development of the volcano-tectonic setting of Oregon, particularly during the transition from Clarno to John Day volcanism. The decreased plate convergence between the Farallon and North American plates between 43 and 28 Ma likely caused slab steepening and lead to the relaxation of horizontal compressive stresses in the crust, thus setting the stage for rapid magma injection into the crust. The subsequent removal of the slab provided a strong thermal pulse that promoted the formation of large shallow magma chambers and the outbreak of widespread extensional volcanism throughout central Oregon during the Eocene and early Miocene.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFM.V11B2267P
- Keywords:
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- 1033 GEOCHEMISTRY / Intra-plate processes;
- 3615 MINERALOGY AND PETROLOGY / Intra-plate processes;
- 8109 TECTONOPHYSICS / Continental tectonics: extensional;
- 8415 VOLCANOLOGY / Intra-plate processes