ANATOMY OF A RECHARGE MAGMA: DACITE PUMICE BLOCKS IN THE TSHIREGE MEMBER OF THE BANDELIER TUFF, VALLES CALDERA, NEW MEXICO

The 1.26 Ma Tshirege Member of the Bandelier Tuff is a compositionally zoned rhyolitic tuff with small amounts (typically ~1% by volume) of grey dacitic pumice distributed throughout most flow units, though dacite pumice is not found in the lithic-rich Qbt3 flow unit. Injection of dacite magma into a compositionally zoned rhyolitic magma body may have triggered an explosive eruption that led to the formation of the Valles caldera. The dacite pumice contains 5–20% plagioclase phenocrysts ranging in size from 0.1–5 mm, variable amounts of hornblende phenocrysts which are interpreted to have formed from pressure quenching of dacite against rhyolite magma, and trace amounts of biotite, hornblende, and Fe-Ti oxide pheno- and microphenocrysts. Larger plagioclase crystals in the dacite pumice exhibit strong oscillatory zoning and range from An₆₅ to An₃₀. Trace element zoning in these crystals is consistent with multiple episodes of recharge or crystal transfer between mafic–intermediate magmas prior to injection of the dacite, i.e. the dacite has a complex history pre-dating its injection into the main rhyolitic chamber.

The dacitic pumices can be divided into two distinct groups on the basis of petrography and geochemistry. The first group has lower SiO₂ (~68%), relatively low light to middle REE concentrations, and is found in the early erupted portions of the Tshirege Member including the basal plinian Tsankawi pumice bed, and occasionally higher in the sheet. The second group has higher average SiO₂ (~70%) and light to middle REE concentrations, and has only been found within the later erupted portions of the sequence. Pumices of the second group contain large (~1-2 cm) quartz and heavily resorbed ternary feldspar phenocrysts. The seived ternary feldspars may have been produced by immersion of alkali feldspar, derived from the rhyolite, in hotter, more Ca-rich dacite magma. The second group of dacite pumices therefore appear to record intimate mixing of dacitic and coarsely porphyritic rhyolitic magmas, perhaps in the lower, crystal-rich part of the main rhyolitic magma body.