Named for high-grade gneisses exposed in the western South Mountains and the Sierra Estrella, Maricopa Co, AZ in the Southwestern Basin-and-Range region. The central Sierra Estrella is the type locality. At that locality the most abundant lithologies are quartzo-feldspathic gneiss, biotitic gneiss and schist, amphibolite, granitic gneiss, migmatite, and biotite-muscovite schist. A broad definition for the name suggested that would include "many exposures of high-grade metamorphic rocks south and west of Phoenix that are not easily correlated with other Precambrian formations of" AZ. Present also in Buckeye Hills and White Tank Mountains. Forms serrated ridges. Diabasic rocks occur within the formation in several areas; these represent intrusions emplaced after the main deformation. Mineral assemblages indicative of upper amphibolite-facies metamorphism. Most of formation probably derived from intermediate to mafic plutonic or volcanic protoliths. Of Middle Proterozoic (1.7 to 1.6 b.y.) age. Intruded by Komatke Granite (new), South Mountains Granodiorite (new), Telegraph Pass Granite (new), and Dobbins Alaskite (new). Geologic map.

Source: GNU records (USGS DDS-6; Denver GNULEX).

Age modified from Middle Proterozoic to Early Proterozoic, (using the data of Reynolds, 1985) in the Southwestern Basin-and-Range region of Maricopa Co, central AZ.

Source: GNU records (USGS DDS-6; Denver GNULEX).