OLDER PRECAMBRIAN ROCKS OF THE DIAMOND BUTTE QUADRANGLE, GILA COUNTY, ARIZONA

Abstract

Rocks intruded by Precambrian granite have been previously described in several districts of central Arizona, but it has not been possible to demonstrate their stratigraphic order. In the Diamond Butte quadrangle five older Precambrian formations can be placed in depositional sequence. The oldest is the Alder Formation, consisting of wacke, slate, quartzite, and conglomerate. It is conformably overlain by the Flying W formation consisting of interbedded basic and acidic volcanic rocks and conglomerate. This is overlain with slight unconformity by the conglomerate, quartzite, and slate of the Houden formation. Above the Houden is the Board Cabin formation composed of porphyritic, pillow, and pyroclastic volcanic rocks, and volcanic sediments. The rhyolites and conglomerates of the Haigler formation complete the sequence. The Haigler is believed to be overlain by still younger rhyolites, and the Alder may be underlain by an older sequence of slate and basic volcanic rocks. This volcanic-sedimentary rock sequence portrays a recurrently unstable marine environment in which a large portion, if not all, of the sediment was derived from contemporary volcanic rocks. Through time the parent magma became enriched in potassium and silicon, and finally engulfed its own extrusive and sedimentary deposit to crystallize at relatively shallow depths as quartz porphyry, granophyre, and granite. Evidence of pre-existing metamorphic or plutonic rocks has not been found, and this depositional-orogenic cycle may represent the initial continental formation in this area.