Metamorphosed precambrian silicic volcanic rocks in central Arizona

Abstract

Silicic volcanic rocks — dacite, rhyolite, and quartz porphyry — constitute about 35 percent of the Yavapai Supergroup, an older Precambrian sequence in central Arizona. In addition, the series contains about 30 percent pillow and amygdaloidal basalt, 5 percent andesitic rocks, and the remainder is mixed andesitic and silicic-bedded tuffaceous rock. The Yavapai Supergroup is divided into the Ash Creek and Alder Groups, each containing about 20,000 feet of lavas and pyroclastic rocks; no evidence is available to determine the relative ages of the two groups. The pillow basalts, coarsely graded volcanic breccias, and poorly sorted and graded tuffaceous-bedded rocks suggest a marine accumulation in an eugeosyncline. The Yavapai Supergroup is metamorphosed to the greenschist facies. The Ash Creek Group exhibits open folds but is nonfoliated except locally, whereas, the Alder Group is isoclinally folded and dominantly schistose. Twenty-five chemical analyses of the metamorphosed, silicic volcanic rocks reveal varying ratios of sodium and potassium. By comparison with young rhyolitic obsidians, only two rhyolites approach the ratios and total alkali content found in recent obsidians. Five rhyolites contain more sodium and appreciably less potassium, whereas none of the rhyolitic rocks contain more potassium than in young obsidians. High-sodium content is reflected by much albite, and high-potassium content by abundant sericite. Iron-magnesium metasomatism locally has produced much chlorite associated with an increased quartz and decreased albite content. The source of the iron and magnesium may be underlying pillow basalts. Many rhyolitic rocks of Phanerozoic age display variable ratios of Na 2 O and K 2 O caused by hydration of obsidian, hot spring action, and hydrothermal solutions. The redistribution of the alkalis in the Yavapai Supergroup may be caused in part by factors other than regional metamorphism. It is suggested the H 2 O + content, averaging between 5 and 6 percent by volume in these metamorphosed silicic rocks, may be derived in part from interstitial marine water.