PROTEROZOIC CRUSTAL EVOLUTION IN THE PHOENIX MOUNTAINS, ARIZONA

The Phoenix Mountains contain a beautiful record of the evolution of Proterozoic crust. The area contains two distinct Proterozoic sequences, separated by the NNE-trending Squaw Peak fault near the center of the range. West of the fault, the oldest Proterozoic rocks are greenstones and ferruginous quartzite, interpreted to be part of the oceanic Union Hills Group. They are depositionally overlain to the southeast by phyllite and quartzite, correlated with the Alder Group, and this contact represents a clear evolution from oceanic to continental environments, unaccompanied by significant deformation.

East of the fault, the rocks are more continental and include quartzite, phyllite, and rhyolite, repeated by several large, previously unrecognized folds. Rhyolite and quartzite in a north-opening, isoclinal syncline near Squaw Peak have been correlated with the Red Rock Group and Mazatzal Group, respectively. Quartzites and phyllites further to the east are more likely equivalent to the Alder Group.

Granitic intrusions are exposed to the northwest and southeast. The intrusion to the northwest is granodiorite to tonalite, includes a finer grained porphyritic phase, and locally contains steep, northeast-striking mylonitic fabric and shear zones. The granite to the south is less deformed and is coarsely porphyritic.

Proterozoic structures include northeast-striking, steeply southeast-dipping cleavage and at least three large, steeply plunging isoclinal folds. Based on well-preserved cross-beds, quartzite units east of the Squaw Peak fault mostly have their stratigraphic tops to the northwest and are overturned. Units west of the fault also dip southeast, but are upright, except along an isoclinal syncline just west of the fault. The juxtaposition across the Squaw Peak fault of overturned and upright sections, which lack clear stratigraphic ties to each other, implies that the fault has substantial displacement.