Granite Mountain is composed ofPaleoproterozoicbiotite granodiorite to granite that is known by geologist as the Mint Wash Granodiorite. The Mint Wash Granodiorite consists of medium to coarse-grained, weakly to strongly porphyritic granite with phenocrysts of gray to pink potassium feldspar up to in length and as much as eight percent biotite that is less than in size. This granite grades laterally into a fine grained, equigranular leucogranite with less than two percent biotite that is less than in size. The Mint Wash Granodiorite is almost everywhere undeformed. The porphyritic granite was named the Mint Wash Granodiorite by DeWitt and others for exposures along Mint Wash east of Granite Mountain. In composition, it is an alkali-calcic granodiorite to granite. Zircons from this granite yielded a U-Pb date of 1680 +/- 16 Ma. The Mint Wash Granodiorite comprises the Mint Wash Pluton that has intruded into Paleoproterozoic sedimentary, metasedimentary, and metavolcanic rocks. On the northern end of Granite Mountain, these strata include weakly to moderately metamorphosed mudstone, siltstone, and fine-grained sandstone; basalt lava flows; ferruginous, white and dark laminated silica rock; infrequently laminated, epidote-silica rock; thinly interlayered silica and carbonate; laminated, highly platy, talc and/or pyrophylliteschist. Some of siliceous rock types are suspected of being primary chemical precipitates from Paleoproterozoic ocean water. The south end of the Mint Wash Pluton, has intruded into Paleoproterozoic mafic, metavolcanic rocks that include basalt lava flows and Paleoproterozoic intrusive rocks, hornblende-rich gabbro-norite, gabbro, and gabbro-diorite. These sedimentary, metasedimentary, and metavolcanic rocks were deposited, deformed, and intruded contemporaneously with the Paleoproterozoic schists of the Vishnu Basement Rocks found in the Grand Canyon. The rocks intruded by the Mint Wash Granodiorite are also part of the same tectonostratigraphicterrane, Yavapai tectonic province. The rocks that are part of the Yavapai tectonic province and intruded by the Mint Wash Pluton formed in volcanic island arcs overlying one or more subduction systems, similar to modern volcanic arcs in the Aleutian Islands, the Philippines, and Indonesia. Then, granitic igneous rock, e.g. the Mint Wash Pluton, were later intruded into them when these volcanic arcs collided with and welded to Laurentia and each other ca. 1.7- 1.66 Ga during the Yavapai and Mazatzal orogenies.
Ecology
The biotic communities at Granite Mountain range from montane conifer forest and juniper pinyon woodland, to interior chaparral. Granite Mountain is a nesting site for the peregrine falcon, and the climbing area on the south face is periodically closed to rock climbing, typically from February 1 until July 15 each year.
