The petrology and geochemistry of the Ocate volcanic field, north-central New Mexico

Abstract

The Ocate volcanic field in north-central New Mexico is on the eastern flank of the Sangre de Cristo Mountains. It is a suite of Pliocene-Pleistocene basaltic and intermediate lavas which were erupted in five pulses over a period of ∼7 m.y. (8.3–0.8 m.y. B.P.) during rejuvenation of the Rio Grande Rift. Geochemical and petrographic criteria are used to define five major rock types: alkali olivine basalt (AOB), transitional olivine basalt (TOB), xenocrystic basaltic andesite (XBA), olivine andesites (OA), and dacite. The timing and eruptive volumes of these five lava types define a complex temporal sequence in which the transitional basalts and intermediate rocks are closely associated during the maximum in basaltic activity (4.5–2.0 m.y. B.P.). The alkali olivine basalts have higher incompatible and compatible trace-element abundances and higher normative nepheline (>2%) than do the TOB lavas. The two basalt types apparently were derived from physically discrete or heterogeneous source regions. The AOB lavas could be the products of smaller degrees of melting from similar sources, source heterogeneity, and/or melting at greater depth. The intermediate lavas were generated from basaltic parent magmas by varying degrees of fractionation, assimilation of crustal material, and mixing with silicic melts derived by crustal anatexis. The XBA lavas and dacites are characterized by disequilibrium phase assemblages which include olivine coexisting with biotite and quartz, melt inclusions of K- and Si-rich glass, and a bimodal distribution of plagioclase phenocryst compositions with modes at An60 and An25. The high concentration of Mg, Ni, and Cr in the intermediate lavas is inconsistent with simple fractionation of basaltic parent magmas. The linear trends defined on variation diagrams are consistent with mixing mafic and silicic end members. The lack of continuity between two hybrid types may be a reflection of contrasting subvolcanic environments in which the hybridization took place. The olivine andesites show little petrographic evidence for mixing. Chemical variations within this group define highly nonlinear trends more consistent with extensive crystal fractionation. Calculated liquid lines of descent from available mafic parent magmas (assuming perfect fractional crystallization) do not satisfactorily produce the observed intermediate compositions. If assimilation of a silicic crystal component is paired with crystal fractionation, such that the mass of crustal material is approximately equal to 50% of the fractionated solids, the major- and trace-element characteristics of the olivine andesites can be successfully modeled.