PNEUMATOLYTIC ANDRADITE WITHIN UPPER TSHIREGE MEMBER AT A CENTRAL VENT WITHIN VALLES CALDERA, AND WIDESPREAD GARNET WITHIN UPPER BANDELIER TUFF

Petrographic and microprobe analyses reveal garnet-bearing lithic fragments (GL) within central vent breccia of Tshirege unit Qbt4u, Bandelier Tuff exposed on the resurgent dome of Valles caldera. Nearly half the 27 GL we examined are derived from pre-caldera Permian and Tertiary sandstones, contain conspicuous single grains of interstial garnet and zircon, and are angular clasts that average 0.90 mm² in thin section. More than half of the GL are round, contain no other recognizable minerals within fine-grained matrix except for a single grain of tourmaline, and average 0.12 mm². GL also include one metamorphic rock and one intermediate-composition lava. Trace garnet is ubiquitous in Tshirege within the Valles region. Garnet typically occurs as small single grains (0.0004 to 0.0012 mm²; maximum 0.091 mm²) within tuff matrix, but volumetrically most garnet forms as granular andradite aggregates within GL.

Andradite granules form a necklace a few microns inward from the edge of each fine-grained GL and constitute about 10 volume-% of each lithic. With a single exception, microprobe analyses reveal that all such garnets as well as those occurring within tuff matrix are near end-member andradite. Garnet within the single sandstone GL analyzed by microprobe includes well-formed Mn-rich pyrope and poorly-formed andradite compositionally identical to that described above. We found no broken garnet necklaces demonstrating that andradite did not form prior to eruption but formed in situ after emplacement of tuff. Garnet within intermediate-composition lava corroborates its shallow formation, as do vesicles associated with andradite necklaces.

Andradite typically forms by contact metamorphism of impure calcareous rocks. Necklace-bearing lithics might have originally been fragments of impure Madera limestone, transformed by silica-rich metasomatic fluids. However, not a single carbonate occurs among 638 lithics we have examined within Tshirege. Alternatively, the antecedent lithics may have been clay spherules recognized within Tshirege from the Pajarito Plateau. Thermal alteration of clay could form andradite if the fine-grained matrix were quartz plus feldspar. Regardless of origin, the widespread presence of garnet in Tshirege suggests that in-situ formation of andradite may be common at Valles.