South Mountains Granodiorite. Is part of a composite pluton (along with Telegraph Pass Granite and Dobbins Alaskite, both new). Where undeformed, forms bold spheroidally weathered outcrops of gray (fresh), tan or brown (weathered), medium-grained, equigranular to slightly porphyritic granodiorite with 35 to 45 percent plagioclase, 25 to 35 percent quartz, 15 to 25 percent potassium feldspar, and 5 percent biotite. Where mylonitic, forms ledges and massive outcrops that have rounded tops with serrated edges, is darker than the undeformed type due to smearing or streaking of biotite and to a general reduction in grain size. Several phases occur on borders. Locally includes igneous and metamorphic rocks. Intrudes Precambrian Estrella Gneiss (new). Age is middle Tertiary (about 25 Ma). Report includes geologic map.
Type locality: along Summit road, 500 m northeast of Telegraph Pass, on main ridge of mountains. Named from South Mountains, Maricopa Co., AZ.
Reference localities designated: (1) east flank Dobbins Lookout, in sec. 16, T. 1 S., R. 3 E.; (2) roadcuts in center SW/4 sec. 16, T. 1 S., R. 3 E.; and (3) cuts along first 100 m of Hidden Valley Trail, in SE/4 NW/4 sec. 15, T. 1 S., R. 3 E.

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

South Mountains Granodiorite. Widely distributed in the eastern and southern parts of South Mountains, Maricopa County, Arizona. Is first phase, or oldest intrusion (of 3), of a middle Tertiary pluton. Maximum age is 24.9 +/-3.0 Ma (Miocene or Oligocene).

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