New Mexico Geological Society Annual Spring Meeting — Abstracts

The Organ Needle Quartz Monzonite was the earliest major intrusive phase of the late Oligocene Organ Mountain Bathol ith. An anomalously
mafic facies that initially occurred at the center of the intrusion has been sampled for geochemical and petrographic analysis at approximately 85 meter intervals perpendicular to the intrusive contacts. Preliminary petrographic and field data indicate that the mafic facies has an overall outward concentric zonation from monzodiorite to monzonite, and that it is also completely gradational into the surrounding quartz syenite. Compositional heterogeneities on a scale of centimeters to a few meters across suggest the existence of partial mechanical mixing within the body, The mafic facies probably originated as an upward pulse of relatively undifferentiated magma within a compos; onally zoned magma chamber, that occurred in response to the rapid extrusion of more highly differentiated magma from the top of the chamber.