Radiogenic and stable isotopes of mid-Miocene silicic volcanism in eastern Oregon: Evidence for variable and high Sr / low δ18O domains west of the terrane-cratonic lithosphere transition
Abstract
Widespread mid-Miocene rhyolite volcanism of eastern Oregon mostly coeval with flood basalts of the Columbia River Basalt Province allows for mapping crustal domains using radiogenic and stable isotopes. Rhyolites are thought to be derived in large part by partial melting of the crust and thus yield direct information on the composition of the crust. Silicic volcanism is expressed in the form of numerous domes and tuffs exposed over a wide area (~300 km in N-S dimension and ~100 km in E-W dimension) west of the craton boundary, which runs parallel but mostly east of the Oregon-Idaho state border as delineated by geophysical characteristics and isotopic transitions. Here, we mainly focus on initial 87Sr/86Sr ratios and δ18O obtained from mid-Miocene silicic volcanic centers in eastern Oregon. Our data, in combination with data from the literature, indicate variable 87Sr/86Sr mostly along longitudinal sections, yet more similar ratios in latitudinal directions. Except for rare examples on the west side, dispersion of 87Sr/86Sr ratios among both silicic and basaltic rocks occurs eastward of 118.6°W. For example, rhyolites in the Owyhee region between 117.10°W and 117.25°W retain 87Sr/86Sr ratios ranging from 0.70413 to 0.70566. The most radiogenic Sri ratio of 0.70787 in our study is obtained on a plagioclase separate from Buchanan Dome complex located near the western boundary of our study area. Feldspar separates and fresh groundmass of samples from adjacent centers yield similar 87Sr/86Sr ratios. δ18O values for feldspars range from below 2‰ to above 9‰. In addition, there is a crude trend of rhyolites having lower δ18O and more radiogenic 87Sr/86Sr ratios. With one exception, all samples with 87Sr/86Sr above 0.7050 are depleted in 18O (δ18O <5.5‰), while rhyolites with 87Sr/86Sr below 0.7045 are enriched in 18O (δ18O >6‰). The most depleted oxygen ratios (<2‰) come from rhyolites ~80 km west of the cratonic margin reflecting remelting or assimilation of hydrothermally altered crust. Yet, some compositionally similar rhyolites also retain higher δ18O ratios around 8‰, indicating involvement of different crustal domains. In general, our more radiogenic Sr data are inconsistent with the long known 0.706 and 0.704 (87Sr/86Sr) boundaries. Our preferred model is that more radiogenic crustal domains exist west of the cratonic boundary and correlate with more radiogenic crust of select accreted terrane domains. Alternatively, the crustal transition may be not as sharp as proposed. Furthermore, low δ18O of rhyolite flows in our study clearly cannot be explained by remelting of Cretaceous plutons of the Idaho Batholith and appear irreconcilable with remelting of altered silicic rocks at centers of multiple, confocal caldera cycles- both processes have been proposed to explain low δ18O of rhyolites of the Snake River Plain-Yellowstone area.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFM.V53B2802J
- Keywords:
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- 8400 VOLCANOLOGY;
- 1037 GEOCHEMISTRY Magma genesis and partial melting;
- 1040 GEOCHEMISTRY Radiogenic isotope geochemistry;
- 1041 GEOCHEMISTRY Stable isotope geochemistry