THE PRE-ERUPTIVE MAGMATIC CONDITIONS OF THE BLOODGOOD CANYON TUFF: INSIGHTS FROM ALKALI FELDSPAR AND QUARTZ CRYSTAL SIZE DISTRIBUTIONS AND MINERAL CHEMISTRY

The Bloodgood Canyon Tuff (28.05 ±0.05 Ma) is the largest single ignimbrite erupted within the boundaries of the Mogollon Datil Volcanic Field (MDVF) in southern New Mexico. The MDVF erupted 12,000 km³ of rhyolite in 30 ignimbrites from an estimated 15 calderas between 36 and 24 Ma. Ignimbrite flare-up occurred as a result of the relaxation of compressional stress at the end of the Laramide orogeny. The tuff is sourced from the Bursum caldera and covers an area of ~15,000 km² with an estimated volume of ~1300 km³. Stratigraphically and laterally the Bloodgood Canyon tuff is a geochemically variable high silica rhyolite with a consistent mineralogy, but variable crystallinity. Phenocryst phases include alkali feldspar, quartz, biotite and hornblende with minor Fe-Ti oxides and apatite. Groundmass phases include quartz, biotite, zircon, Fe-Ti oxides and apatite. Here we present crystal size distributions (CSD) of alkali feldspar and quartz from 30 thin sections representing stratigraphic section at variable distances from the source to assess the conditions of the pre-climatic magma chamber. We separate pumice hosted crystals from ash-hosted crystals. We combine this information with textural analysis and mineral chemistry data from three sections representative of the stratigraphic column to assess the thermal state of the magma chamber and present a model for the pre-eruptive conditions of the Bloodgood Canyon tuff magma plumbing system.

CSD patterns for the Bloodgood Canyon tuff samples record three crystallization periods and textures of the feldspars suggest thermal disequilibrium in the chamber. The textures along with the concave up pattern of the CSDs suggest injection of hotter mafic magma into a crystal mush early in the eruptive history. Samples at the top and middle of the section trend towards larger crystal sizes and higher percent crystallinity. Quartz CSD patterns are similar to alkali feldspar suggesting co-crystallization of both alkali feldspar and quartz. Mineral chemistry of alkali feldspar show little variation in %Or and FeO. Sr, Ba and Pb trace element compositions are more variable, suggesting that the mixing magma was of similar composition to the mush.