Recording magma ascent: punctuated crystallization during a crustal journey preserved in the Atascosa Lookout lava flow, Atascosa Mountains, Southern Arizona, USA

The 23 Ma trachyandesitic Atascosa Lookout lava flow is the youngest deposit in the Atascosa Mountains of southern Arizona. The flow hosts three melt compositions, including 1) the trachydacitic groundmass, 2) trachydacitic magma envelopes that surround clusters of plagioclase ± chromium diopside, magnesian augite, quartz, hornblende, and orthopyroxene, and are enriched in K and Mg, and depleted in Si relative to the groundmass, and 3) quartz-bearing chilled dacitic magma droplets. It also hosts clusters of crystals including orthopyroxene, diopside (both Cr-rich and Cr-poor), hornblende, plagioclase and quartz. Hornblende occurs in at least three distinct magmas and their hybrids. U-Pb ages of zircon crystals analyzed in situ from contrasting settings within a single thin section of the lava flow range from 21.7+/- 0.5 my to 26.8+/- 0.7 my, recording a minimum of 3.9 my of history of the magmatic complex. Mineral-melt barometry shows orthopyroxene in crystal clusters crystallized at depths greater than 25 km, from the most mafic parent of picro-basaltic composition. Clinopyroxene crystals crystallized at 11.5 - 30 km, and from a more evolved basaltic or trachybasaltic melt. Plagioclase crystallized at a range of crustal depths, 4-23 km, from different melt compositions. These results show that the components of the Atascosa Lookout lava flow crystallized at various levels of the crust and not all in a single magma chamber at one depth as well as from significantly different parental melts. Epsilon Hf values of zircon crystals from different settings in a single thin section are all negative, with the most crust-like being the core of a zircon crystal in a magmatic enclave, and the least crust-like being the core of a zircon crystal in a crystal cluster. The one exception is a zircon crystal with epsilon Hf = 0.7 indicating significant contamination of a partial melt of old lower crust by mantle-derived basalt. The occurrence of zircon crystals of contrasting ages and crystals from different crustal levels and different parentage offers further proof that single magma batches can host materials from different parts of a magmatic system and can be used to interpret a wide range of the temporal and compositional history of a volcanic complex.