2000-地质学岩石学和带状浮岩.pdfVIP

Journal of Volcanology and Geothermal Research 95 2000 109–156 locatejvolgeores The geology and petrology of Volcan San Juan Nayarit, Mexico ´ ´ and the compositionally zoned Tepic Pumice James F. Luhr Department of Mineral Sciences NHB-119 Smithsonian Institution, Washington, DC 20560, USA Received 14 April 1999; received in revised form 4 August 1999; accepted 4 August 1999 Abstract Volcan San Juan is the westernmost Quaternary composite volcano in the Mexican Volcanic Belt. Its activity is divided ´ into three stages of decreasing eruptive volume. During stage-1 activity, the main cone of San Juan and the adjacent Cerro Alto grew to a combined volume of 60 km3 through eruption of hornblende-bearing andesites, dacites, and rhyodacites. Stage 1 included explosive eruptions of hornblende-dacite and an andesite dated at 33,750 a years ago and 23,010 a. Stage-2 activity involved a major Plinian eruption 14,770 a that produced the hypersthene–hornblende-bearing, composition- ally zoned, rhyodacitic–andesitic Tepic Pumice and led to formation of an oval-shaped caldera 4 1 km in diameter. Tepic, the capital city of Nayarit State, with a population of 250,000, lies at the NE foot of Volcan San Juan. This location was ´ buried under 2–9 m of rhyodacitic pumice-fall deposits from the Tepic Pumice eruption. Erosion has sin