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Granite Basin, Mescal Mountains – a recessive granitic laccolith?

The Granite Basin laccolith is located in the Mescal Mountains of the San Carlos Indian Reservation, 15 miles ENE of Kearny, Arizona. According to Willden (1964), the feldspar-mica porphyry intruded the Naco Limestone about 62 Ma. Locally, the Pennsylvanian Naco strikes northwesterly and dips about 25 to 30 degrees to the southwest. At the southeastern end of the laccolith, the Naco Limestone is nearly vertical and strikes to the northeast. The chilled zone of this igneous intrusion - where it directly contacts Paleozoic rocks - is only a few inches thick; remarkable for an igneous intrusion nearly 2,000 feet thick.

Willden’s description of the porphyry, ‘contains nearly equant plagioclase phenocrysts and prismatic books of biotite set in a light-gray aphanitic groundmass. Small hornblende crystals are commonly present, and a few rounded and embayed quartz phenocrysts can be identified in most specimens of the larger intrusive bodies. Quartz phenocrysts are generally lacking in the small intrusives. Phenocrysts make up from 40 to 70 percent of the rock.’

On aerial photography, Granite Basin sticks out like a sore thumb, or more accurately like the impression of a reclining thumb. The basin trends 301 degrees and is 4.25 miles long and 0.75 miles wide; a geomorphic expression not unlike that of an enormous half pipe – a u-shaped high sided ramp or runway. How to explain this unusual geomorphic expression? Apparently, the porphyry of Granite Basin is more easily eroded than the surrounding Paleozoic rocks. As the porphyry is eroded, the overlying Naco Limestone retreats to the south.

Laccolith - mass of igneous rock, typically lens-shaped, that has been intruded between rock strata causing uplift in the shape of a dome.

For C.R. Willden's 1964 geologic report and geologic map sheets:

Geology of the Christmas quadrangle, Gila and Pinal Counties, Arizona

Posted M. Conway 7/11/2018