The Granite Mountains, also called Granite Mountain, is a small mountain range in the Southeast Fairbanks Census Area of the U.S. state of Alaska. The Granite Mountains are southeast of Delta Junction, the nearest city. This mountain range is part of the larger Alaska Range.
Geology
The bulk of the Granite Mountains is composed of light gray, medium to coarse-grained granodiorite and quartz monzonite. They are composed of plagioclase potassium feldspar, quartz, biotite, hornblende, and accessory minerals. The plagioclase is euhedral to subhedral; the potash feldspar is anhedral; and some of the quartz is anhedral, but most is interstitial. The Plagioclase exhibits polysynthetically twins and zonation and the potassium feldspar displays Carlsbad twinning. Biotite and green hornblende occur as inclusions in the feldspars and as independent grains. In the western part of the Granite Mountains, the granodiorite and quartz monzonite are deeply weathered. Further west, they are in intrusive contact with the older Birch Creek Schist. The southerneast quarter of the Granite Mountains is underlain by quartz, mica schist. It can be inferred from regional studies that these granodiorite and quartz monzonite were intruded into older metamorphic rocks during Cretaceous time and contemporaneously with the deformation of the Alaska Range.
On the northeast flank of the Granite Mountains and west of the Gerstle River lie three ridges composed of steeply tilted beds of moderately consolidated pebble, cobble, and boulder conglomerate. Some of the beds contain boulders as large as in diameter and boulders of lithologies not found in modern drainage basins adjacent to the ridges. These beds are suspected of being Pre-Quaternary glacial deposits.