Abstract

Basaltic andesite, andesite, dacite, and rhyodacite of the Paliza Canyon Formation (9.1 to 8.5 m.y.) and the Bearhead Rhyolite (7.1 to 6.5 m.y.) belong to a high-K, calc-alkaline association in the southern Jemez Mountains, New Mexico. The majority of the rocks of the Paliza Canyon Formation contain augite, hypersthene, and plagioclase phenocrysts. Olivine is an important phenocryst in the basaltic andesite, but it is absent in the other rock types. In the rhyodacite, hornblende is a major phenocryst at the expense of the pyroxenes. Characteristically, a very restricted range in the Fe/Mg composition is found for the pyroxenes (augite from 0.33–0.29) and hornblende (0.41–0.47) throughout the whole range of rock types, from basaltic andesite (∼56% SiO2) to rhyodacite (67% SiO2). On the other hand, the Bearhead Rhyolite lacks pyroxene, and it has two feldspars (sanidine and plagioclase) and bipyramidal quartz and biotite. Its SiO2 content ranges between 76.5% and 77.3%. The Bearhead Rhyolite lies on the two-feldspar surface, or below it, in the sanidine field; the Paliza Canyon rocks plot within the plagioclase volume of the granite system. Both major- and trace-element trends in the Paliza Canyon Formation do not project to the Bearhead Rhyolite composition. Major- and trace-element modeling suggests that the sequence from basaltic andesite to rhyodacite in the Paliza Canyon Formation can be formed by simple fractional crystallization. The Bearhead Rhyolite cannot be formed from the Paliza Canyon rhyodacite by this process, however. Removal of the observed phenocrysts found in the rhyodacite would form a rhyolite enriched in Ba, Hf, Th, U, and light rare earth elements (LREE) and depleted in V and Sc; the Bearhead Rhyolite, however, is depleted in the former group and enriched in the latter. These conclusions are supported by initial 87Sr/86Sr ratios: three samples of Bearhead Rhyolite have initial ratios of 0.7056, 0.7073, and 0.7076; two samples of Paliza Canyon basaltic andesite, on the other hand, have ratios of 0.7041 and 0.7044. The most likely source for the Bearhead Rhyolite is lower crustal material which may have been fused partially from the heat provided by the andesitic magmas of the Paliza Canyon Formation.

First Page Preview

First page PDF preview
You do not currently have access to this article.