GSA Annual Meeting in Phoenix, Arizona, USA - 2019

Paper No. 39-3
Presentation Time: 9:00 AM-5:30 PM


JOHNSON, Bradford J., Arizona Geological Survey, University of Arizona, 1955 E 6th St, Tucson, AZ 85721

Field relationships and macroscopic textures provide clues to emplacement modes of mafic microgranular enclave swarms in a composite Miocene pluton. The Mount Perkins pluton in the Black Mountains of northwestern Arizona consists of layered mafic cumulates at the base, younger intermediate and commingled mafic and silicic intrusions in the middle, and granitic rocks in the upper part. Medium to fine-grained biotite granodiorite to monzogranite in the middle of the pluton hosts swarms of mafic enclaves, which constitute 5-50% of the rock volume in most swarms and up to 80% in some. Most of the enclaves are a few cm to 1 m across, and they range up to several meters long. Most are microgranular diorite or quartz diorite. Composite enclaves are locally present, themselves containing enclaves with variations in texture and color index. Some swarms contain autoliths of gabbro and quartz diorite derived from older parts of the pluton, and xenoliths of gneiss derived from Paleoproterozoic country rocks.

The mafic microgranular enclaves typically have lobate or crenulate, sharp, chilled margins, notably darker and finer-grained than their interiors. They clearly formed as mafic magma was emplaced into granitic magma that was only partially crystallized. In one well-exposed swarm composed predominantly of large enclaves of this type, the shapes and distribution of the enclaves indicate that they were emplaced as a dike and were not transported far after breaking apart.

Many enclaves in another swarm contain abundant globular quartzofeldspathic aggregates, including some that resemble amygdules and others that, in the field, appear texturally and compositionally identical to leucocratic host granite and may be magmatic ocelli. Perhaps the latter crystallized from residual silicic magma that was incorporated into, and incompletely mixed with, the mafic magma at depth. The ocelli are individual and coalesced spheroids ranging from a few mm to a few cm across, concentrated in parallel bands near chilled margins of enclaves. The enclaves (and the bands of ocelli) are randomly oriented in this swarm. The mafic magma in this case must have decompressed and cooled enough for vesiculation and/or unmixing to occur before being dismembered and transported by the granitic host magma as enclaves in a turbulent suspension.