The Polymetamorphism of the Little Willow Formation, Wasatch Mountains, Utah

Abstract

The Little Willow Formation, of apparent Middle Precambrian age, crops out along the Wasatch Fault Zone, in the foothills of the Wasatch Mountains, approximately 13 miles south-southeast of Salt Lake City, Utah. This metamorphic complex, exposed at the intersection of the Uinta and Wasatch tectonic trends, is composed of quartzofeldspathic gneisses, pelitic mica schists, amphibolites and a heterogeneous migmatitic unit. The formation is unconformably overlain to the east by quartzites and phyllites of the late Precambrian Big Cottonwood Formation; intruded on the southeast by the Tertiary Little Cottonwood quartz monzonite stock; and is covered and bounded on the remaining sides by Quaternary glacial and alluvial sediments. The migmatite is believed to be part of the Little Willow Formation, not the Big Cottonwood. Both the migmatite and the typical Little Willow are characterized by spaced schistosity, crenulated foliation, similar orientation of the S1 foliation, recrystallized granoblastic textures and moderate-grade metamorphism. The Big Cottonwood displays a minor phyllitic foliation which is subparallel to bedding, minor recrystallization and except for thermal effects close to the stock, a low-grade metamorphic assemblage. Metamorphic phenomena of several regional events are recognizable within Little Willow rocks. Spaced schistosity, crenulated foliation and rotated porphyroblasts of andalusite and garnet are among the evidence for the Middle Precambrian regional metamorphism. During the Laramide Orogeny (late Cretaceous-early Tertiary) the unconformable contact between the Little Willow and Big Cottonwood was apparently the locus of thrust movement. This has resulted in a cataclastic texture in many of the rocks. Intrusion of the Little Cottonwood quartz monzonite stock, 24-31 million years ago, thermally metamorphosed the surrounding rocks. Estimated metamorphic pressures of approximately 4 kb and temperatures in excess of 600°C appear to have allowed preservation of some regional features while initiating some new ones. Characteristics of this thermal event include recrystallization of the earlier cataclastic textures, growth of second-generation andalusite, growth of sillimanite (possibly second generation) and replacement of foliated sillimanite by muscovite. Wasatch normal faulting, which occurred about 20 million years ago, and subsequent erosion are responsible for the uplift and exposure of the Little Willow Formation and apparently formed localized zones of rebrecciated mylonite within the recrystallized rock. Migmatitic structures, foliated sillimanite and evidence for possible anatexis found in the southeastern portion of the Little Willow area are easiest explained by the regional metamorphic event; however some question remains, since their development during the later thermal event is also a possibility.