Abstract

The Trans-Pecos magmatic province of West Texas and southern New Mexico is a more eroded analogue of the Kenya (Gregory) rift in East Africa. Trans-Pecos alkalic rocks are similar to the basalt-phonolite-trachyte-rhyolite assemblage from fissure and multicenter eruptions, and from some central volcanoes, in the Kenya rift. In both provinces, quartz-normative and nepheline-normative mafic rocks occur, providing likely parents for the entire observed range of silica saturation. Thus, no special mechanism is needed for deriving silica-undersaturated and silica-oversaturated rocks from one another, because both groups evolved independently. The Kenya rocks tend to be more mafic and, at the silicic end of their compositional range, more peralkalic than the Trans-Pecos rocks.

Similarities in tectonic style, extent and duration of magmatism, and igneous rock compositions in the two provinces suggest that the alkalic rocks in both regions were generated by differentiation from quartz-and nepheline-normative parents in the upper parts of elongate mafic or ultramafic intrusions, probably diapiric welts that caused doming and normal faulting.

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