Columbia River Basalt Group, Saddle Mountains Basalt, Weissenfels Ridge Member
This sample of Twl from a locality ~9 miles southeast of Moscow, ID is a glomeroporphyritic basalt. Olivine within the section occurs within glomerocrysts and as isolated microphenocrysts. Larger olivine grains occur within the glomerocrysts and exhibit skeletal growth textures. The early interpretation of glomerocryst clusters of synneusis, "together-swimming," or, the sticking together of crystals floating in melt persists as a probable mechanism, particularly along crystal domains (Vogt, 1921); (Helz, 1987). The sample contains no glass, or devitrified glass, and is rather composed of clinopyroxene and plagioclase matrix (intergranular texture). Plagioclase phenocrysts are mainly subhedral, with a few anhedral grains that appear resorbed. Opaques appear to be a mix of magnetite and ilmenite based on crystal shape. Approximately 8% of the olivine is altered to iddingsite, in which there is a green form and reddish brown form. In addition to age, the Weissenfels Ridge member, and particularly the Lewiston Orchards basalt is chemically distinct, with low SiO2 and high P2O5 compared to other CRB members (Hooper, 2000).