Hornblende-dacite pumice in the Tshirege Member of the Bandelier Tuff: Implications for magma chamber and eruptive processes
James A. Stimac
Hornblende-dacitic pumice fragments (HBDP) and their disaggregation products (streaks, wisps and xenocrysts) are common in the Tshirege (upper) Member of the Bandelier Tuff, generally comprising <1 vol% of the unit. HBDP are typically ellipsoidal in shape and have a groundmass of acicular plagioclase and hornblende set in vesicular rhyolitic glass. Coarse-grained minerals were out of equilibrium in HBDP and represent either phenocrysts that grew prior to mixing (mainly plagioclase and pyroxene), or xenocrysts derived from entrained rhyolitic magma (dominantly alkali feldspar and quartz). Groundmass minerals (mainly plagioclase and hornblende) mantle larger crystals and have textures and compositions that indicate rapid, undercooled crystallization from a limited supply of liquid. These features indicate that HBDP represent partially quenched blobs of a hybrid layer formed during addition of intermediate magma to the rhyolitic Bandelier chamber, and thus are equivalent to many quenched mafic inclusions in rhyolites and mafic microgranular enclaves in granites. Textural features of HBDP are due primarily to pre-eruptive processes, not vesiculation within the vent. HBDP are not cogenetic with the rhyolitic components of the Tshirege Member and their chemical variations are due mainly to crystal fractionation and mixing with rhyolitic magma. The presence of HBDP in the earliest erupted material suggests that addition of this magma may have destabilized the Bandelier system and triggered its eruption. This may have occurred as vesicular hybrid magma either plumed buoyantly through silicic magma, or moved more efficiently toward the vent and in the conduit system.
- Stimac, James A., 1996, Hornblende-dacite pumice in the Tshirege Member of the Bandelier Tuff: Implications for magma chamber and eruptive processes, in: The Jemez Mountains Region, Goff, Fraser; Kues, Barry S.; Rogers, Margaret Ann; McFadden, Les D.; Gardner, Jamie N., New Mexico Geological Society, Guidebook, 47th Field Conference, pp. 269-274.