Abstract

Late Cenozoic lava flows of southwest Utah comprise olivine basalt, with subordinate hypersthene-bearing basalt and andesite. Some basic flows containing normative nepheline may be classified chemically and mineralogically as alkali olivine basalt, others as tholeiitic on the basis of the presence of a reaction relation between olivine and Ca-poor pyroxene. Most of the basic lava flows, however, are transitional between typical alkaline and typical tholeiitic basalt and contain both olivine and hypersthene in the norm. The andesites are mineralogically similar to calc-alkaline andesites but are chemically richer in K2O. Most of the rocks are only mildly alkaline yet are lacking any clear trend of iron enrichment. Sr and Ba are the most abundant trace elements and are strongly enriched in andesite relative to basalt.

Feldspar compositions vary from plagioclase of An70 to potash feldspar of Or70, and olivine shows a considerable compositional range, except in basalt which contains groundmass hypersthene. Clinopyroxene is only weakly zoned and the behavior of its Alz is different according to whether the host rock is chemically alkaline or tholeiitic. Coexisting titano-magnetite and ilmenite in the groundmass of some of the basalts indicate consolidation temperatures in the range of 1040° to 800°C and oxygen fugacities (fo2) between 10−10.3 and 10−13.6.

The basaltic rocks probably originated by partial melting in the mantle, and their diversity is thought to reflect events which took place in the mantle rather than near the Earth's surface. It is unlikely that the andesitic magmas were produced by crystal fractionation of basaltic magma, particularly if this involved removal of plagioclase. Sr87/Sr86 ratios ranging from 0.7037 to 0.7054 place a limit on the possible amount of contamination of basalt by old, radiogenic crustal material. Thus the andesite may be the result of contamination of basaltic magma by young or nonradiogenic crustal material, or it may represent liquid produced by partial melting in the mantle. In either case, material rich in Sr and Ba seems to have been involved.

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