Direct evidence for the style and composition of volcanism associated with magmatic centers of the Oligocene Ortiz porphyry belt, New Mexico, has been largely removed by erosion. Careful study of volcaniclastic sediments of the coeval Espinaso Formation provides an indirect record of the composition of erupted materials and mode of eruptive activity associated with the Cerrillos Hills and Ortiz Mountains eruptive centers. Petrographic and major-element chemical analyses of conglomerate clasts from near Cerrillos and at Espinaso Ridge confirm the progression from calc-alkaline to alkaline magmatism, as suggested by earlier work on Ortiz intrusive rocks. Based on modal mineralogy, three distinct stratigraphic intervals are recognized at the Cerrillos Locations. The lowest is dominated by a phenocryst assemblage of hornblende + clinopyroxene + plagioclase; in the second interval, clinopyroxene is almost completely absent. The stratigraphically highest interval contains variable amounts of clinopyroxene with repeated trends from hornblende-rich to clinopyroxene-rich compositions. Similar hornblende-to-clinopyroxene trends can be seen in the lower part of the section at Espinaso Ridge, but these rocks are compositionally distinct from those near Cerrillos. At a stratigraphic break high in the section, interpreted to be the calc-alkaline-to-alkaline transition, hornblende + clinopyroxene gives way to biotite + clinopyroxene.
Major-element data on clasts from Espinaso Ridge suggest that these clasts can be related by fractionation of plagioclase ± hornblende and/or clinopyroxene and are consistent with the hypothesis that Espinaso Ridge material was derived from the Ortiz eruptive center. In contrast, Cerrillos-area clasts generally have less silica than those of Espinaso Ridge, compatible with the observation that the Cerrillos section contains less pyroclastic debris. Cerrillos data show the effects of fractionation by an Fe-Ti phase and, possibly, plagioclase. These rocks were probably derived from the Cerrillos Hills eruptive center. Differences between compositions of volcanics from the two eruptive centers may be related to the depths of emplacement of their subjacent magma chambers. The mineralogy of the reworked pyroclastic material suggests that explosive eruptions originated from a deeper magma body than the ones that provided lavas that are represented by conglomerate clasts.