THE PETROGENESIS, ASSEMBLY AND PLUMBING SYSTEM OF A SINGLE FLOOD BASALT ERUPTION

Identification of a single flood basalt eruption unit (or flow field) has enabled the first detailed investigation into the formation of magma bodies that feed flood basalt eruptions. The inflation model for pahoehoe lava provides a physical framework within which samples can be collected within a flow field relative to eruption sequence. This allows insights into sequential magmatic evolution.

Progressive variations within and between lobes of the 2,660 km³ Sand Hollow flow field (Columbia River Province) reveal that evolved tholeiitic lava emplaced earlier in the eruption became progressively more enriched in compatible elements during the later stages, e.g. MgO 3.71 to 4.46 wt%, Fe₂O₃ 14.68 to 15.54 wt%, Cr 38.2 to 49.5 ppm. Initial ¹⁸⁷Os/¹⁸⁸Os ratios show an unprecedented range from 1.569 (first emplaced) to 0.287 (last emplaced) whereas whole rock ⁸⁷Sr/⁸⁶Sr analyses (0.70512 to 0.70522) vary only slightly outside analytical error. Elevated ¹⁸⁷Os/¹⁸⁸Os are coupled to fractionation indices to demonstrate an inverse relationship between the most radiogenic and most evolved samples. The ¹⁸⁷Os/¹⁸⁸Os values are consistent with previous data for the Columbia River Basalts indicating that neither an enriched source nor the sub-continental lithospheric mantle can be the cause of enrichment. AFC calculations require 5 to 10 % assimilated continental crust involving two contaminants with distinct isotopic compositions. Intra-crystal ⁸⁷Sr/⁸⁶Sr analyses (e.g. 0.70524 to 0.70545) reveal plagioclase antecrysts, in disequilibrium with the host melt, that record variable histories in an open magmatic system.

The combined results suggest an established complex plumbing system beneath the Columbia River Flood Basalts during the eruptions of the Wanapum formation. Progressive crustal contamination occurred in an open-system with a flux or recharge of basalt toward the latter stages of the eruption. Contamination may be during lateral transport of magma in a series of conduits cutting unradiogenic Mesozoic oceanic accreted terrane and the suture boundary and erupting through the radiogenic Precambrian North American craton. The antecrysts are derived from an earlier portion of the magmatic history and assembled with the melt immediately prior to eruption.