Systematics of xenocrystic contamination: preservation of discrete feldspar populations at McCullough Pass Caldera revealed by 40Ar/39Ar dating

Single crystal 40Ar/39Ar dating of K-feldspars from silicic volcanic rocks containing xenocrysts often yields a spectrum of ages slightly older than those of juvenile sanidine phenocrysts. In contrast, feldspars from thin, low-volume units of the Tertiary (14 Ma) McCullough Pass Tuff define discrete age populations at ∼14 Ma, ∼15 Ma, and ∼1.3 Ga, reflecting the time of eruption, xenocrysts from an older ignimbrite exposed in the caldera wall, and Proterozoic basement K-feldspars, respectively. Conductive cooling and diffusion modelling suggests preservation of such discrete populations is likely only when xenocrystic material is incorporated into the magma very near or at the surface, or is engulfed in thin, rapidly cooled pyroclastic flows during emplacement. Incorporation of xenocrysts into the subvolcanic magma chamber, into thick rhyolite domes or lava flows, or into large, welded ignimbrite sheets will result in partial or total resetting of the K/Ar isotopic system. Similarly, petrographic evidence such as exsolution lamellae may be homogenized under these conditions but not in thin ignimbrites. Extremely low diffusion rates for disordering of the Al–Si tetrahedral siting of basement feldspars suggests that they will retain their ordered structural state given rhyolitic magma temperatures. Thus, even when petrographic and K/Ar isotopic evidence for xenocrystic contamination is obscured, it may be preserved in the form of Al–Si ordering.