New Mexico Geological Society Annual Spring Meeting — Abstracts

Bloodgood Canyon Tuff is a quintessential moonstone tuff, i.e., quenched hypersolvus ignimbrite of quartz and iridescent sandine cryptoperthite (moonstone). With associated lavas (e.g. rhyolite of Diablo Range and Jerky Mountains), it plots at the ternary minimum of the Ab-Or-SiO₂-H₂O system and is interpreted as the evolved end member of high-SiO₂ magmas of the Oligocene "ignimbrite flare-up." Certain two-feldspar tuffs and lavas (e.g. Apache Spring Tuff and Fanney Rhyolite of Bursum cauldron, Mogollon Mountains) are interpreted as less-evolved member of the same system. Differing interpretations of field relations result in conflicting petrologic schemes:

(a) All principal moonstone tuffs correlate with Bloodgood, Canyon Tuff. Although extensive and voluminous, it is merely the evolved basal zone of Apache Spring Tuff and is overflow from the Bursum cauldron (1). Less-evolved Fanney Rhyolite is younger, hence there was a reversal in magmaic evolution.

(b) Moonstone tuffs erupted from several centers (e.g. Gila Cliff Dwellings cauldron) and occur in several stratigraphic levels (e.g. Bloodgood Canyon and Railroad Canyon Tuffs separated by Jerky Mountain Rhyolite). All are younger than the Bursum cauldron assemblages; magmatic evolution was undirectional (2,3).
have not been seen to solve dating, and

Apache Spring, Bloodgood Canyon, and Railroad Canyon Tuffs have not been seen in unequivocal superposition. Current attempts to solve problems by mapping, geochemistry (4), radiometric dating, and magnetostratigraphy (5) have revealed additional complications but point toward eventual solutions.