The Cenozoic volcanic complex in the Santa Rosa Range consists of many widely scattered, independent vents, associated with sequences of flows that reach a maximum thickness of 3000 ft. The predominant rock types are basalt and rhyolite-rhyodacite, but intermediate rock types are abundant and widely distributed.
All rocks in the sequence are characterized by a lack of biotite, amphibole, and other hydrous minerals. Olivine is the predominant phenocryst in andesites; olivine and plagioclase are virtually the only phenocrysts in basalts. Xenoliths and xenocrysts are found only in rhyolites, rhyodacites, and dacites.
The most primitive rock type in the sequence is a basalt that resembles mid-oceanic tholeiite chemically, and high-alumina basalt from the Oregon plateaus petrographically. In the northern part of the range, basalts, differentiated from this rock type, approach ferro-gabbroic compositions; in the southern part, the major differences between primitive and differentiated basalt are lower CaO and MgO, and much higher alkali percentages in the latter. Siliceous rocks were produced as end products of differentiation in all parts of the range.
Petrographic and chemical features suggest that titaniferous magnetite was fractionated in larger amounts in the south than in the north. This very likely prevented iron enrichment in the south, but did not lead to early development of a silica enrichment trend. Apparently labradorite was fractionated in amounts large enough to counterbalance the silica enriching effect of fractionating olivine and magnetite, until a very late stage in magmatic history.