COMPOSITION OF MAFIC FIAMME OF THE RHYOLITIC BUCHANAN TUFF, EASTERN OREGON: UNDERSTANDING MAGMAS THAT DRIVE SILICIC MAGMATISM

The Buchanan Tuff (BT) crops out along the eastern flank of the Harney Basin, eastern Oregon and now has been age dated with a preliminary age of 15.12 ± 0.01 Ma. It is a compositionally zoned rhyolitic ignimbrite with a distinct mafic component that seems to be related to magma activity of the Columbia River Basalt Group (CRBG).

Bulk rock compositions of whole samples and on separated mafic and silicic components as well as microprobe analyses of glasses and minerals reveal the following. Bulk tuff samples are mostly low-silica rhyolite with 3-4 wt.% FeO* while separated mafic and silicic components yield compositions of 60.4 wt.% SiO₂, 8.6 wt.% FeO*, and of 73 wt.% SiO₂, 3.3 wt.% FeO*, respectively. The most mafic glass composition is slightly silica poorer than the separated mafic bulk sample and there is a nearly continuous range of glasses from ~59 to 64 wt.% SiO₂. Silicic glass also indicates a range but analyses cluster at 69, 72, and 75-76 wt.% SiO₂. Some intermediate glasses could be the product of mixing between observed mafic and rhyolitic melts but there are hints that the range observed within mafic glasses is not solely due to mixing but may rather suggest magmatic evolution within the mafic spectrum. Of the ~4% phenocrystic minerals observed, anorthoclase belonging the rhyolite magma is most abundant, followed by Fe-rich clinopyroxene also of rhyolite magma. Least abundant are rare plagioclase and mafic silicates that belong to icelanditic magma.

Similarly, icelandite-rhyolite zoned ignimbrites occur further east and include the 15.5 Ma Wildcat Creek Tuff and the youngest of the Dinner Creek Tuff units. The three units indicate subtle differences in phenocryst and melt composition but all three attest to storage of evolved tholeiitic magmas of likely late CRBG origin as driver for silicic magmatism.