Petrogenesis of McKinney (Snake River) olivine tholeiite in light of rare-earth element and Cr/Ni distributions

摘要

Samples of McKinney Basalt, including a pillow glass, are characterized by light rare-earth element–enriched abundance patterns with small positive Eu anomalies (relative to chondrites) and high Cr/Ni ratios. In particular, the positive Eu anomaly observed in the pillow glass is considered to be characteristic of the McKinney parental magma. These compositional features are apparently inconsistent with derivation of McKinney magma by partial fusion of garnet- or plagioclase-bearing Iherzolite or clinopyroxenite or by crystal fractionation of likely liquidus phases at high or low pressure. Rather, partial melting calculations show that fusion of spinel peridotite or aluminous clinopyroxene peridotite can yield liquids with the rare-earth element patterns and Cr/Ni ratios of McKinney Basalt. An essential feature of the favored models is a relatively large contribution of clinopyroxene to the melt. This result suggests an origin of McKinney magma by fusion of mantle Iherzolite at pressures between the stability fields of plagioclase peridotite (low P) and garnet peridotite or garnet clinopyroxenite (high P), that is, at depths within the continental lithosphere in this region.