Abstract

Alkali olivine basalt at Malapai Hill, California, occurs as a late Cenozoic stock that has intruded the Cretaceous White Tank Monzonite. The basalt is chemically and mineralogically similar to other alkaline basalts in the Mojave Desert, although it is devoid of zeolites. The strontium isotopic composition of the basalt (Sr87/Sr86 = 0.7030 ± 0.0006) suggests that it is derived from a mantle that has already experienced one period of partial melting. Olivine-rich Iherzolite nodules in the basalt are high in Mg and low in Si, Al, Ca, Na, and K. The nodules are xenomorphic granular with a tectonite fabric and forsteritic olivine (Fo94–88). The Sr87/Sr86 ratio of the nodules is 0.7043 ± 0.0008. Olivine xenocrysts and corroded xenolith margins suggest that reaction between the nodules and basalt has occurred.

Similar Sr87/Sr86 ratios of Iherzolite nodules and alkali olivine basalt hosts at Malapai Hill are consistent with a related origin for the two rocks. The tectonite fabric and olivine composition of the nodules rule out a cognate origin by fractional crystallization of the basalt. Instead, the nodules are mantle materials that have been depleted in the most easily fusible elements by one or more periods of partial melting. The nodules may represent refractory residue from the area of partial melting or depleted mantle material, unrelated to the basalt but accidentally incorporated into it. Interpretation of the nodules as refractory residue from the site of melting is favored due to the similarity of the nodules and host rocks in strontium isotopic composition, the extreme chemical and petrologic uniformity of the inclusions, and chemical evidence that the nodules could be the end product of more than one partial melting.

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