Crystal clusters, feldspar glomerocrysts, and magma envelopes in the Atascosa Lookout lava flow, southern Arizona, USA: Recorders of magmatic events

The Atascosa Lookout trachyandesite lava flow is the youngest and,most comp ositionally primitive unit in the middle Tertiary Atascosa-Tumacacori-Cerro Colorado volcanic complex (ATCC). The flow hosts a variety of objects of c ontrasting origin, including (I) clusters of plagioclase +/- chromian diops ide, magnesian augite, quarts, hornblende, and orthopyroxene; (2) amoeboid- shaped quarts-bearing enclaves; (3) plagioclase crystals with a concentric interior sane of small melt inclusions (dusty plagioclase); (4) plagioclase crystals with cores filled with large melt inclusions (honeycomb plagiocla se); (5) plagioclase glomerocrysts. The groundmass of the trachyandesitic f low is trachydacite. Some crystal clusters, enclaves, and plagioclase glome rocrysts are surrounded by diffuse envelopes of trachydacite higher in K an d Mg and lower in Si than the trachydacitic groundmass of the flow. This en velope,material is interpreted as foreign ma,magma that engulfed these obje cts as it invaded their host magma. Both the crystal clusters and plagiocla se glomerocrysts may be the remains of cumulate crystal layers, disrupted b y influxes of magma into their reservoirs. Crystals in the lava flow, origi nated in at least three distinct magmas and their hybrids. The groundmass o f the lava flow preserves evidence for repeated infusion of envelope magma into the system, These influxes fueled the invasion of crystal clusters, pl agioclase glomerocrysts, enclaves, and swirls of the envelope magma into th e groundmass of the Atascosa Lookout lava flow Despite the compositional an d textural variety apparent in the lava flow, the magmas involved in its de velopment may have been genetically closely related The collection of featu res in the lava flow resulted from the development of compositional layers in the magma, accumulation of crystal-rich horizons, disturbance of the sys tem by repeated magma influx, and minor crustal assimilation.