Affiliation: College of Arts and Sciences, Department of Geological Sciences
The zoned Organ Needle Pluton is associated with three silicic ignimbrites. Precise 206Pb/238U zircon TIMS ages indicate the pluton was assembled from the top downward, with the most silicic compositions having the oldest ages. The most mafic compositional phase of the pluton, a monzodiorite was emplaced during a discrete episode after the alkali feldspar granite and inequigranular syenite. The tuff of Squaw Mountain and alkali feldspar granite could not have been derived by fractional crystallization and crystal separation from the inequigranular syenite and monzodiorite. These new age relations suggest the isotopic variation within the Organ Needle Pluton initially reflects assimilation of local Precambrian quartz monzonite. As magma emplacement into the region continued, the magma system self buffered, recording a consistent, less-evolved isotopic signature of a single magma source. Furthermore, 206Pb/238U zircon crystallization ages indicate the pluton was emplaced in a younger, separate event from the magmatism that generated the ignimbrites.