A Strongly Calc-alkaline Suite in the Midst of the Tholeiitic Columbia River Basalt Province: Implications for Generating the Calc-alkaline Trend Without Subduction Processes
Abstract
The mid-Miocene lavas of the Strawberry Volcanics (SV), distributed over 3,400 km2 in NE Oregon, comprise a diverse volcanic suite, which span the range of compositions from basalt to rhyolite. The volcanic suite is mainly composed of calc-alkaline (CA) basaltic andesite and andesite, yet tholeiitic (TH) lavas of basalt to andesite occur as well. The SV lies in the heart of nearly coeval flood basalts of the Columbia River province of the Pacific Northwest. The unique combination of strongly CA rocks of the SV in a non-subduction setting provide an excellent opportunity to study controls on inducing CA evolution in the midst of a TH province and independent of processes taking places at an active subduction zone. New 40Ar/39Ar ages indicate CA basaltic andesites to andesites of the SV erupted at least from 14.78±0.13 Ma to 12.44±0.12 Ma demonstrating that CA magmatism of the SV was ongoing during the eruptions of the tholeiitic Wanapum Basalt member of the Columbia River Basalt Group (CRBG). This range will likely be extended to even older ages in the future because existent age dates did not include samples from near the base of the SV. Thickness of intermediate lavas flows of the SV range from 15 m to as thin as 2 m and lavas are characterized by mostly phenocryst poor lithologies. When phenocrysts are abundant they are very small suggesting growth late during eruption. Single lava flow sections can include on the order of 30 conformable flows, testifying to a vigorous eruption history. The thickest andesitic sections are located in the glacially carved mountains of the Strawberry Mountain Wilderness (i.e. Strawberry Mountain, High Lake, and Slide Lake) where several vent complexes are exposed, which are delineated by dikes and plugs with finely interlocking plutonic textures, cross-cutting SV lava flows. Dikes generally strike NW-SE. Subtle variations in major and trace element compositions exist between TH and CA lavas of the SV. The CA lavas of the SV are more enriched in highly incompatible LIL while slightly more depleted in HFSE and REE, particularly HREE, relative to TH lavas. Incompatible trace elements range from the mafic to the silicic end as follows [in ppm]: Rb - CA: 11 to 43, TH: 4 to 37; Ba - CA: 550-1124, TH: 408 to 929; Th - CA: 1.2 to 4.0, TH: 1.2 to 3.5; Nb - CA: 6.0 to 16.2, TH: 9.3 to 23.1; Lu - CA: 0.3 to 0.49, TH: 0.3 to 0.63. On the other hand, great similarity among element patterns of CA and TH samples in incompatible element normalization diagrams is consistent with a common mafic component. Our preliminary interpretation is that CA magmas of the SV are generated when basaltic, CRBG-related magmas interact with the continental crust, while TH magmas escaped comparable crustal modifications. The range in compositions from basalt to CA andesite to dacite may thus be attributed to greater amounts of crustal inputs via magma mixing or AFC processes. Trace element concentrations and ratios of SV basalts are largely indistinguishable from those of CRBG lavas and have the greatest commonality with Steens and Imnaha type lavas (e.g. Zr, Ba, Sr, Th, U, Ba/Nb and Ce/Y).
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.V13B2844S
- Keywords:
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- 1033 GEOCHEMISTRY / Intra-plate processes;
- 1036 GEOCHEMISTRY / Magma chamber processes;
- 8415 VOLCANOLOGY / Intra-plate processes;
- 8499 VOLCANOLOGY / General or miscellaneous