MID-MIOCENE ICELANDITES OF EASTERN OREGON: MAGMATIC LINEAGE TO MAIN PHASE CRBG WITH CONSTRAINTS ON THEIR CRUSTAL STORAGE SITES

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 component in strongly zoned (rhyolite-icelandite) ignimbrites.

Our compiled data set in combination with published data is 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, and with less than 2.4 wt% TiO₂. Al₂O₃ is less than 16.1 wt% and total alkali content (Na₂O+K₂O) has a maximum value of 6.9 wt%. 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 consistent with tracking some petrogenetic evolution path. Towards less evolved magmas, the two icelandite groups project back to main phase CRBG. The more enriched Group 1 is more consistent with a parent basalt magma like Imnaha Basalt and the less enriched Group 2 with a magma as Picture Gorge Basalt with Grande Ronde basaltic andesite samples (including Hunter Creek and Birch Creek samples of the Malheur Gorge area) in between. This suggests that the majority of these icelandites are the products of continued evolution from magmas of the Grande Ronde Basalt through a variety of processes including fractionation but also mixing with felsic material such as a rhyolite or contamination with country rock.