ISOTOPIC AND GEOCHEMICAL CONSTRAINTS ON THE ORIGIN AND EVOLUTION OF THE COLUMBIA RIVER BASALT

New major and trace element data on over 70 samples are combined with a wider knowledge of the regional stratigraphy, and of the tectonic evolution of the boundary between the Columbia Plateau and the northern margin of the Basin and Range province, to distinguish three subgroups within the Columbia River Basalt Group (CRBG): the Picture Gorge Basalt; the main sequence of Columbia River flood basalts, here named the Clarkston Basalt; and the Saddle Mountains Basalt. The subgroups are characterized by different incompatible element and Sr-, Nd-, and Pb-isotope ratios, and they are interpreted in terms of different source regions mobilized under different tectonic conditions. The major differences between the subgroups are consistent with partial melting processes in the upper mantle, and it is argued that they reflect previous partial melting episodes which resulted in source regions that were variably enriched and depleted in incompatible elements. The major variations within the Picture Gorge and Clarkston Basalt subgroups include increases in the abundances of large ion lithophile elements (LILE) and increases in the ratios of LILE/high field strength elements (HFSE) which are interpreted as the addition of a lithospheric/subduction-related component.

The Picture Gorge Basalt has a depleted isotopic and chemical signature on which is superimposed an enrichment of LILE to produce a trace element pattern similar to that of other 17-0-Ma basalts erupted south of the Olympic Wallowa Lineament. This pattern is characteristic of volcanism associated with the Basin and Range extensional province, and others have attributed it to a source component derived from an enriched subcontinental lithospheric mantle (SCLM).

Of the Clarkston Basalts, the Imnaha and Grande Ronde Basalts form chemical and isotopic arrays which indicate mixing of components from two distinct source regions. One had high ratios of LILE/HFSE and light rare earth elements (LREE)/HFSE, and as these are not common in oceanic basalts, this component is thought to have been derived from the continental mantle lithosphere. Its isotope ratios are more enriched (older?) than those of the Picture Gorge Basalt, and its Rb/Sr ratios are much higher than those in its source rocks, consistent with preferential mobilization of LILE at the time of magmatism. The second component was derived from an asthenospheric source similar to that of Hawaii basalts and is most obviously attributed to mantle plume activity. Basalts of the Eckler Mountain and Wanapum Formations (smaller, separate formations of the Clarkston Basalt as redefined in this paper) fit this mixing model less well and may represent mixing between more than two components. Flows of the third CRBG subgroup, the Saddle Mountains Basalt, also carry a lithospheric geochemical signature and have long been recognized as having more radiogenic isotopic signatures than the other two subgroups. Thus, Saddle Mountains flows appear to require a lithospheric source enriched in LILE at an even earlier time, and we concur with other workers that the isotopic and chemical evidence implies their derivation from subcontinental lithospheric mantle enriched at approximately 2000 Ma.

Within each subgroup, the chemical effects of partial melting, fractional crystallization, and magma mixing processes can all be distinguished within particular flow sequences. In the Imnaha Basalt variable degrees of partial melting during the generation of the CRBG magmas, and gabbro fractionation within the lower crust, played major roles in their evolution. In the Grande Ronde Basalt fractional crystallization appears to be restricted to < 10%.

The chemical and isotopic data for each CRBG subgroup, and the different sources which those data imply, can be accommodated in a tectonic model which includes the passing of the Yellowstone hotspot south of the center of the CRBG eruption before significant Basin an''d Range extension had moved north of the Brothers Fault zone at 15 Ma.