Structurally elevated segments of a north trending intrarift horst in northern New Mexico expose Late Oligocene/Early Miocene volcanic rocks erupted during the inception of the Rio Grande rift. Postcaldera rocks of the Latir volcanic field on the intrarift horst (Timber Mountian and Brushy Mountian) include two distinct calc-alpaline sequences differing in age by up to 3 m.y. The lower sequence (25 Ma) is dominated by dacite (plag + cpx + opx + Fe-Ti oxides ¿ hbld) but includes basaltic andesite (ol + cpx + plag) and low-silica rhyolite (plag + sanidine + qtz + biotite + Fe-Ti oxides). Dacites contain quenched andesitic micropillows, high-silica rhyolite glass, and sanidine and oligoclase xenocrysts.

Incompatible element concentrations increase with silica content throughout the sequence and are accompanied by concomitant decreases in Sr, Sc, and Cr. The upper sequence (22 Ma) includes basaltic andesite (ol + cpx + plag,) dacite lava flows (plag + hbld + cpx + Fe-Ti oxides + opx + sanidine + sphene ¿ biotite), and a basal lava dome of high-silica rhyolite (qtz + sanidine ¿ biotite ¿ Fe-Ti oxides ¿ sphene ¿ hornblende) overlying remnants of the outflow sheet of the 26-Ma Amalia Tuff. Upper sequence dacites are distinguished from lower sequence dacites by their hydrous mineralogy and lower relative K2O, TiO2, Ar, Y, Nb, Y/Zr, and total rare earth element concentrations for any given silica content. Major and trace element models, to predict fractionation paths from an andesite parent, indicate that suppressed K, Sr, and elevated Mg,Y, and Lu concentrations in the lower sequence dacities resulted from fractionation of phenocyst phases, plus mixing in proportions approaching one to one, with high-silica rhyolite similar in composition to the Amalia Tuff. Low-silica rhyolites may have been derived through fractionation of dacites plus mixing with the high-silica rhyolite. Positive correlations of 87/86Sro with SiO2 are interpreted to reflect the increasing proportions of crustally contaminated high-silica rhyolite in lower sequence, interpreted to reflect the increasing proportions of crustally contaminated high- silica rhyolite in lower sequence, intermediate-composition melts. These differentiation models support a petrogenic link between the mildly peralkaline Amalia Tuff of the caldera-forming phase of the Questa magmatic system and early postcaldera lavas. Subsequent eruption of upper sequence volcanics marks the regeneration of more typical metaluminous magmas characteristic of both precaldera volcanics and postcaldera intrusions.