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Geochemical variations within the Roza Member, Wanapum Basalt, Columbia River Basalt Group: Implications for the magmatic processes affecting continental flood basalts

Barton Sawyer Martin, University of Massachusetts Amherst


Plagioclase phyric lavas of the Roza Member cover approximately 40,000 km$\sp2$ in Oregon and Washington. Field relationships in 85 stratigraphic sections and geochemical analysis of 765 samples indicate that the Roza Member consists of 1 to 4 flow units. Systematic variations in Cr, Nb, Zr, P$\sb2$O$\sb5$, and TiO$\sb2$ allow these flow units to be divided into 6 chemically distinct subtypes. Incompatible element abundances decrease while the Cr concentration and plagioclase phenocryst population increase upward through the Roza succession. Most other elements display only minor variations. Stratigraphic relations between the 6 Roza subtypes are complex. The areal distribution of each subtype reflects the interaction of the constructional topography of older Columbia River basalts, regional structure, geomorphology, and location and timing of activity along the 175-km long linear Roza vent system. The distribution of the chemically defined subtypes within the dikes and vents indicate that only short segments of the vent system were active at any given time. Calculations (assuming a conductive cooling regime--an assumption supported by primary intraflow features and cooling-joint measurements), indicate that decades elapsed between successive Roza eruptions. These temporal constraints are supported by a lack of saprolite horizons between Roza cooling units and an absence of large-scale geochemical variation due to magma reservoir processes, and are consistent with estimates of Columbia River basalt recharge rates. The earliest Wanapum eruptions (the Robinette Mountain and Dodge Basalts, Eckler Mountain Member) were distinctly more mafic than the subsequent "main" Wanapum Frenchman Springs-Roza-Priest Rapids sequence (e.g., Rob. Mtn.: Mg' $\approx$ 61, Nb $\approx$ 4ppm, Cr $\approx$ 137ppm; versus Roza: Mg' $\approx$ 39, Nb $\approx$ 15ppm, Cr $\approx$ 33ppm). The main Wanapum succession is enriched in both the large ion lithophile elements (K, Rb, Ba, Th) and radiogenic isotopes relative to the Eckler Mountain lavas. However, similar incompatible element rations for the Wanapum basalts imply lava derivation from comparable mantle sources. Major and trace element modelling suggest that coupling fractional crystallization of OL + PL + CPX + TiMT ($\approx$5:60:34:1) with periodic eruption of $\approx$10 volume percent of a steady state reservoir, minor ($\approx$3%) assimilation of crust similar to the Wallowa Batholith, and recharge by a Robinette Mountain-like parental magma can generate lavas compositionally similar to the Frenchman Springs-Roza-Priest Rapids sequence.

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Recommended Citation

Martin, Barton Sawyer, "Geochemical variations within the Roza Member, Wanapum Basalt, Columbia River Basalt Group: Implications for the magmatic processes affecting continental flood basalts" (1991). Doctoral Dissertations Available from Proquest. AAI9120914.