GEOCHEMICAL VARIATIONS WITHIN LARGE VOLUME BASALTIC LAVA FLOWS, WANAPUM BASALT FORMATION, COLUMBIA RIVER BASALT GROUP: IMPLICATIONS FOR FLOOD BASALT ERUPTIONS

The Ginkgo and Sand Hollow basalts, Frenchman Springs Member, and the Roza Member are voluminous quartz-normative tholeiitic basalts representing some of the final main CRB sequence eruptions. Multiple flow units are recognized in each. Ginkgo and Roza lavas are abundantly plagioclase phyric, while Sand Hollow lavas are variably plagioclase phyric. All were erupted from NNW-trending vent systems distributed across a >100 km wide region of SE Washington and NE Oregon ~450 km N of the inferred center of the Yellowstone Hotspot.

Detailed geochemical analyses, integrated with fieldwork, allowed Martin (1989) and Self et al (1997) to subdivide the Roza into 5 areally extensive flow units. Cr systematically increases from 22 to 47 ppm in the Roza succession while P₂O₅ and other incompatible elements abundances systematically decrease from the oldest to next to youngest Roza units then increase in the youngest. Integrated studies find that successive Ginkgo and Sand Hollow flow units display significant but non-systematic elemental variations. P₂O₅ in the Sand Hollow ranges from 0.55 to 0.60 wt %; Cr ranges from 30 to 50 ppm. In sections with multiple Sand Hollow flows, the uppermost flow is usually more plagioclase phyric than the underlying flow units. P₂O₅ in the Ginkgo varies between 0.61 and 0.78 wt. %, a greater range than in either the Roza or Sand Hollow; Cr ranges from 9 to 23 ppm. An aphyric zone characterized by higher TiO₂ (3.10 to 3.30 wt % vs. 3.00 to 3.12 wt % in the phyric Ginkgo) and greater incompatible element variation occurs in the upper Ginkgo flow unit in the Pasco Basin.

While all three units were derived from the same source, there appear to be subtle differences in the magma reservoir processes. Martin (1991) modeled the geochemical variations in the 1300 km³ Roza Member as the product of an open magma system undergoing simultaneous fractionation and recharge. The lack of both systematic geochemical variations and sediment between flow units in the >2600 km³ Sand Hollow and1570 km³ Ginkgo basalts suggests that they erupted relatively rapidly from well-mixed magma reservoirs. The localized aphyric zone in the upper Ginkgo may be the product of either in situ crystallization processes or the invasion of the ponded, still mostly molten lava by an aphyric phase erupted from an unexposed segment of the Ginkgo vent system.