MAGMA ENVELOPES, ENCLAVES AND ROGUE CRYSTALS IN THE ATASCOSA LOOKOUT LAVA FLOW: MAGMA COMMUNICATION ACROSS A RANGE OF CRUSTAL LEVELS

The Atascosa Lookout lava flow is a 23.8 Ma trachydacitic flow that caps the rhyolitic to dacitic middle Tertiary volcanic sequence exposed in the Atascosa Mountains of southern Arizona. The flow is remarkable for the variety of disequilibrium features it hosts, including Cr diopside, resorbed and overgrown feldspar phenocrysts, quartz-bearing enclaves, feldspar glomerocrysts, and clusters of plagioclase +/- chromian diopside, magnesian augite, quartz, hornblende, and orthopyroxene crystals. Enclaves are commonly surrounded by an envelope of chilled magma of composition contrasting with both the trachydacitic groundmass and the more siliceous enclaves. Hence, at least three magma compositions are represented in the lava flow. A possible fourth magma composition is trapped in the interstices between crystals in the crystal clusters. The variety of mineral assemblages, mineral compositions, and melt compositions represented in the Atascosa Lookout lava flow indicate that the flow hosts materials that equilibrated across a range of crustal levels. Despite the compositional range of the components of the lava flow, Sr isotopic ratios of all of the materials in the flow are similar (initial ⁸⁷Sr/⁸⁶Sr values range from 0.7076 to 0.7078), indicating that all of the minerals and melts represented in the flow had a broadly common source, and that batches of magma stored in reservoirs across a range of crustal depth are likely to have been contaminated to different degrees by crustal material. The presence of phenocrysts, crystal clusters, and melts that originated at a range of depths in a single lava flow indicates that communication between magma reservoirs was efficient. Explosive volcanism preceding the lava flow may have initiated material exchange between magma reservoirs over a range of crustal levels.