Petrology and geochemistry of syenites of Sawtooth Mountain, Davis Mountains, Jeff Davis County, Texas

Sawtooth Mountain, located in the central Davis Mountains in Jeff Davis County, Texas, is the erosional remnant of a sill that intruded Tertiary volcanic rocks during Oligocene time. The sill consists predominantly of silica-oversaturated microsyenite which coarsens upwards towards the apparent center of the sill. The microsyenite contains phenocrysts of anorthoclase rimmed by sanidine that are set in a trachytic matrix of sanidine, anorthoclase, quartz, augite, sodic-calcic amphibole, biotite, zircon, and opaque minerals. Late-stage alkali-rich minerals aegerine-augite and richterite and/or magnesio-kataphorite are present.

Results from major element analyses support in situ fractionation as the process controlling evolution of the magma, but trace and rare earth element data do not support this hypothesis. High fluorine contents in the biotites and amphiboles suggest that volatile fluxing was a process that had a significant role in the evolution of the sill. Evidence of a transition from a magmatic system to a hx/drothermal system includes breccia pines, silicif icat ion kaolinization, and "bearded" amphiboles. Deuteric activity has greatly complicated interpretation of primary magmatic events.