GEOLOGY OF MEADOW CREEK BASIN, SOUTHERN BLACK MOUNTAINS, ARIZONA: RECORD OF POST-SUPERERUPTION VOLCANISM (REU PROJECT, 2016)

Meadow Creek basin (MCB; southern Black Mountains AZ) is 4 km E of the caldera of the 18.8 Ma Peach Spring Tuff (PST) supereruption (Ferguson et al 2013) and reveals a 3 m.y record of the aftermath of that eruption. Building on previous work in the area (Ransome 1923, Thorson 1971, Liggett & Childs 1982; Ferguson pers. com.), REU students mapped a 15 km²area within MCB and expanded their spatial scope through remote sensing (Schwat et al, Helfrich et al, Thompson et al 2016). Students further investigated the sequence with optical petrography and analysis by XRF, SEM-EDS, and LA-ICPMS.

The MCB map area included: 18.50 Ma Antelope rhyolite lava; Sitgreaves Tuff (high-Si rhyolite, undated); 17.58 Ma Meadow Creek trachyte lava; and diabase intrusions (dike, lopolith), trachyandesite dikes, and capping lava (~14-16 Ma). Antelope lava, rich in magmatic enclaves, may mark continuing mafic input into silicic chambers as postulated for triggering of the PST eruption (Schlaerth et al 2016; Pamukcu et al 2013). Meadow Creek trachyte built a large dome on the N side of the map area; rimmed feldspar megacrysts and a diverse, reacted mineral assemblage, together with previous zircon + whole-rock isotope data (McDowell et al 2016), suggest that this lava documents entrainment of crystal mush in ascending magma (Collins et al 2016). Sitgreaves Tuff mostly comprises high-Si rhyolite, pumice-rich pyroclastic density currents (Wallrich et al 2016). The tuff, up to 200 m thick, was topographically confined by Meadow Creek dome and Antelope lava. The diabase intrusions and capping lava (49-51 wt% SiO₂) are distinct in texture and composition (55-57% SiO₂) from trachyandesite dikes, though they are probably similar in age (Smith et al 2016). Where the dikes intrude Sitgreaves Tuff, tuff is highly indurated and commonly vitrophyric, reflecting welding that resulted from heating above the glass transition (Williams et al 2016).

The MCB sequence reveals distinct changes in magmatism following PST eruption (Claiborne et al 2016), e.g.: (1) Eruptive flux diminished; (2) Rhyolite, rare pre-PST, became dominant; (3) Evidence for open-system processes involving mafic magma increased, but erupted magmas were cooler and more hydrous; (4) in its final stage, magmatism switched from intermediate-to silicic and calc-alkaline to mafic and tholeiitic.