ABSTRACT

The Big Pine volcanic field encompasses 1000 square kilometers of the northern Owens Valley. It is comprised of 30 basaltic cones and flows emplaced over the past 1.2 million years.

In thin section, Big Pine basalts display marked differences. Some samples contain phenocrysts of plagioclase, resorbed olivine and orthopyroxene and minor nepheline. Others have abundant plagioclase and pyroxene phenocrysts, but sparse olivine and no nepheline. There is also a noticeable difference in phenocryst size. Some basalts are comprised of phenocrysts that exceed 1mm in diameter, while others have much smaller (<0.1mm) phenocrysts. There is no apparent relationship between phenocryst size and mineralogy.

Major element analyses support thin section observations. Two distinct groupings are present. One is generally quartz normative (silica-rich), while the second is nepheline normative (silica-poor). Temporal relationships are difficult to establish, but field observations suggest the older flows were more silica undersaturated. Trace element data is enigmatic. There is a subtle trend for the more incompatible elements to favor silica-rich rocks; however, strontium is clearly anomalous.

The bimodal distribution of silica has been well documented for the Cima basalts. There, it is attributed to systematic variation in melting depth over time. A similar explanation for the Big Pine field is difficult to rationalize. Big Pine basalts were emplaced over a shorter time span and display an inverse relationship, characterized by more recent silica-rich basalts. We conclude Big Pine mineralogy and geochemistry are best explained by partial assimilation of Mesozoic granite as basaltic magma rose upward from the underlying mantle.