Abstract

The late Cenozoic lavas of the Raton-Clayton region of New Mexico can be divided into five groups: (1) the Raton-Clayton lavas, widespread alkali olivine basalts; (2) the hornblende andesites and dacites of the Red Mountain lavas; (3) a separate group of pyroxene andesite from Sierra Grande volcano; (4) a feldspathoidal group of basanites and olivine nephelinites; and (5) the Capulin-type basaltic lavas, which are characterized by abundant “cloudy” plagioclase phenocrysts.

The olivines of the feldspathoidal rocks zone toward calcium enrichment rather than iron enrichment as in more silicic rocks. This trend can be related to the silica activity of the magma and to changes in pressure during crystallization. The pyroxenes in the lavas of this suite are rich in alumina, and those of the feldspathoidal rocks show strong sector zoning. Ilmenite from one basanite contains over 40 mole percent of the geikelite (MgTiO3) component, and other oxides also have high contents of MgO. The distribution of Mg between oxide phases and olivine follows predictions from thermodynamic data, but does not agree with experimental results. The Fe-Ti distribution in coexisting oxide phases indicates oxygen fugacities near FMQ for the Raton-Clayton and Capulin basaltic rocks, and higher values for the feldspathoidal, andesitic, and dacitic rocks. Estimates of partial pressures of water based on the Kudo-Weill feldspar geothermometer range from 5 kb for a basanite, 2 to 3 kb for the andesites and dacites, to near 1 kb for the Capulin magmas.

Fractional crystallization and crustal contamination do not appear to have been significant processes in the origin of these lavas. Partial melting in the lower crust (Red Mountain lavas) and upper mantle under different physical conditions seems to be a more likely explanation of the diversity of rock types displayed in this suite. High water pressures, in particular, may have played a significant role in the genesis of the feldspathoidal and Sierra Grande andesitic rocks.

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