First Advisor

Martin J. Streck

Term of Graduation

Summer 2021

Date of Publication


Document Type


Degree Name

Master of Science (M.S.) in Geology






Andesite -- Eastern Oregon, Magmas -- Eastern Oregon, Geochemistry



Physical Description

1 online resource (xii, 173 pages)


The La-Grande Owyhee eruptive axis is a mid-Miocene to Pliocene ~300 km long volcanic belt located at the eastern margin of the Columbia River Basalt Group (CRBG) in Oregon. Within this volcanic belt there are numerous vents that have produced a large volume of petrologically diverse rocks ranging from the tholeiitic flood basalts of the CRBG to rhyolitic tuffs of caldera complexes such as the Lake Owyhee Volcanic field. Icelandites, which are iron rich andesites, are known to occur in association with silica saturated to oversaturated tholeiitic basalts as a minor part of continental flood basalt volcanism. This study investigates these icelandites. This includes summarizing their currently known extent, documenting their compositional variations, describing their lithologic and petrographic characteristics, and to petrogenetically relate icelandite magmas to main stage CRBG magmas.

Prior mapping and our work in eastern Oregon have revealed numerous occurrences of mid-Miocene Fe-rich andesite (icelandite) lavas among flood basalts and rhyolites of the Columbia River magmatic province in eastern Oregon from north of Baker City to McDermitt along the Oregon-Nevada state border. Icelandite lavas typically overlie main units of the Columbia River Basalt Group (CRBG) and are intercalated with mid-Miocene rhyolites. This stratigraphic control and select radiometric ages (new and existing) indicate they range in age from >16 Ma (e.g. 16.1) to ~15 Ma suggesting icelandites represent the last pulse of main phase CRBG volcanism. Icelandites are found in proximal mafic pyroclastic deposits, lava flows or as magmatic components in strongly zoned (rhyolite-icelandite) ignimbrites.

Our compiled data set in combination with published data are used to investigate processes involved in formation of these rocks. Icelandites of eastern Oregon range from ~57 to 63 wt% SiO2, with >8 wt% FeO* and as high as 13.4. FeO*/MgO can be as high as 14 and Mg # ranges from 7.1 to 26.1. Although FeO* and FeO*/MgO is characteristically high in all samples, there are subtle differences within icelandites yielding two groups that internally vary with increased silica as discerned mostly by Nb, Ta, Zr, Hf, Th, and rare earth elements. The trace element enriched Group 1 has lower Al2O3 content than the samples of the trace element depleted Group 2. Internally, both groups vary in their TiO2 that decreases with increased SiO2. The two icelandite groups are petrogenetically modeled as being derived from main phase CRBG. The more enriched Group 1 is more consistent with a parent magma similar to the Imnaha Basalt. The compositions of the less enriched Group 2 are consistent with a Picture Gorge Basalt parent magma and Grande Ronde basaltic andesite samples (including Hunter Creek and Birch Creek samples of the Malheur Gorge area) as intermediary compositions. This suggests that the majority of these icelandites are the products of continued magmatic evolution from magmas of the Grande Ronde Basalt through a combination of processes including fractionation and mixing with felsic material such as a rhyolite or contamination with country rock.


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