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Journal of Petrology Volume 41 Number 5 Pages 693-716 2000
© Oxford University Press 2000

Crystal Clusters, Feldspar Glomerocrysts, and Magma Envelopes in the Atascosa Lookout Lava Flow, Southern Arizona, USA: Recorders of Magmatic Events

S. J. SEAMAN,*

DEPARTMENT OF GEOSCIENCES, UNIVERSITY OF MASSACHUSETTS, AMHERST, MA 01003, USA

The Atascosa Lookout trachyandesite lava flow is the youngest and most compositionally primitive unit in the middle Tertiary Atascosa–Tumacacori–Cerro Colorado volcanic complex (ATCC). The flow hosts a variety of objects of contrasting origin, including (1) clusters of plagioclase ± chromian diopside, magnesian augite, quartz, hornblende, and orthopyroxene; (2) amoeboid-shaped quartz-bearing enclaves; (3) plagioclase crystals with a concentric interior zone of small melt inclusions (dusty plagioclase); (4) plagioclase crystals with cores filled with large melt inclusions (honeycomb plagioclase); (5) plagioclase glomerocrysts. The groundmass of the trachyandesitic flow is trachydacite. Some crystal clusters, enclaves, and plagioclase glomerocrysts are surrounded by diffuse envelopes of trachydacite higher in K and Mg and lower in Si than the trachydacitic groundmass of the flow. This envelope material is interpreted as foreign magma that engulfed these objects as it invaded their host magma. Both the crystal clusters and plagioclase glomerocrysts may be the remains of cumulate crystal layers, disrupted by influxes of magma into their reservoirs. Crystals in the lava flow originated in at least three distinct magmas and their hybrids. The groundmass of the lava flow preserves evidence for repeated infusion of envelope magma into the system. These influxes fueled the invasion of crystal clusters, plagioclase glomerocrysts, enclaves, and swirls of the envelope magma into the groundmass of the Atascosa Lookout lava flow. Despite the compositional and textural variety apparent in the lava flow, the magmas involved in its development may have been genetically closely related. The collection of features in the lava flow resulted from the development of compositional layers in the magma, accumulation of crystal-rich horizons, disturbance of the system by repeated magma influx, and minor crustal assimilation.

KEY WORDS: crystal clusters; enclaves; glomerocrysts; trachyandesite


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[Abstract] [Full Text] [PDF]


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