The geology and petrology of Volcan San Juan (Nayarit, Mexico) and the compositionally zoned Tepic Pumice

Volcan San Juan is the westernmost Quaternary composite volcano in the Mexi can 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 similar to 60 km(3) throu gh eruption of hornblende-bearing andesites, dacites, and rhyodacites. Stag e 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 Pl inian eruption 14,770 a that produced the hypersthene-hornblende-bearing, c ompositionally zoned, rhyodacitic-andesitic Tepic Pumice and led to formati on of an oval-shaped caldera 4 x 1 km in diameter. Tepic, the capital city of Nayarit State, with a population of similar to 250,000, lies at the NE f oot of Volcan San Juan. This location was buried under 2-9 m of rhyodacitic pumice-fall deposits from the Tepic Pumice eruption. Erosion has since rem oved virtually all exposures of Tepic Pumice > 15 km from the caldera. The estimated volume of erupted tephra is 5.6 km(3), equivalent to 1.2 km(3) of magma. modeling of the eruption dynamics indicates that as the eruption ev olved the column height increased from similar to 15 km to similar to 24 km and maximum mass discharge rate increased from similar to 3 x 10(6) kg/s t o similar to 6 x 10(7) kg/s. During stage-3 activity, a hornblende-andesiti c lava dome partially filled the caldera and sent block lava flows down the north slope of thr cone; total erupted volume was similar to 0.55 km(3). C ompared to other composite volcanoes of the western Mexican Volcanic Belt, the products from San Juan have the highest Sr contents and the lowest Rb, Y, Yb, and Lu contents, resulting in high La/Yb and Sr/Y, with low: Rb/Sr. These observations indicate a strong slab-melt component at San Juan, consi stent with its proximity to thr Middle America Trench where young, hot lith osphere is subducting The San Juan andesites, dacites, and rhyodacites also have relatively high Sr-87/Sr-86, Pb-206/Pb-204, Pb-207/Pb-204, and Pb-208 /Pb-204, and low epsilon(Nd) values compared to these other volcanoes, whic h appear to reflect significant crustal contamination of San Juan's magmas. (C) 2000 Elsevier Science B.V. All rights reserved.