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Circular 113—Major-element variations in the Potrillo, Carrizozo, and McCartys basalt fields

By J. Renault, 1970, 22 pp., 4 tables, 15 figs.

Sixty-two samples of fresh Quaternary basalt from the Potrillo, Carrizozo, and McCartys volcanic fields were analyzed by X-ray fluorescence spectroscopy to determine areal composition variations, characterize their mean compositions, and correlate their compositions with their tectonic settings. The 62 X-ray fluorescence analyses for the nine major oxides in alkaline and subalkaline basalts displayed considerable lateral variation within closely related genetic units. The number of samples required to characterize the mean compositions of the sampling units with 95% confidence was determined. The lower and upper Carrizozo basalts were adequately sampled and require four and seven samples, respectively, to characterize their means; the undifferentiated basalts require more. On the basis of Kuno's differentiation index, the Carrizozo basalts display conspicuous differentiation, and the Potrillo and McCartys basalts are relatively undifferentiated. Titanium oxide concentration is correlated with tectonic setting. Within each volcanic field, titanium-rich basalts erupted in upthrown blocks and titanium-poor basalts of deeply penetrating fractures is probably greater in the upthrown blocks, and the higher titanium concentration may be related to higher pressures of magma generation in the mantle.

The purpose of this study is to determine areal compositional variations in some New Mexico basalts, characterize their mean compositions, and attempt to correlate their compositions with their tectonic settings. Toward this end, 62 samples of fresh Quaternary basalt from the Potrillo, Carrizozo, and McCartys volcanic fields were analyzed by X-ray fluorescence spectroscopy for SiO2, Al2O3, MgO, CaO, Na2O, K2O, TiO2, MnO, and total iron as FeO.

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