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

Abstract

Volcán 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 km 3 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, compositionally 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 Volcán San Juan. This location was buried under 2–9 m of rhyodacitic pumice-fall deposits from the Tepic Pumice eruption. Erosion has since removed 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 evolved the column height increased from ∼15 km to ∼24 km and maximum mass discharge rate increased from ∼3×10 6 kg/s to ∼6×10 7 kg/s. During stage-3 activity, a hornblende-andesitic lava dome partially filled the caldera and sent block lava flows down the north slope of the cone; total erupted volume was ∼0.55 km 3. Compared 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, consistent with its proximity to the Middle America Trench where young, hot lithosphere is subducting. The San Juan andesites, dacites, and rhyodacites also have relatively high 87Sr/ 86Sr, 206Pb/ 204Pb, 207Pb/ 204Pb, and 208Pb/ 204Pb, and low ε Nd values compared to these other volcanoes, which appear to reflect significant crustal contamination of San Juan's magmas.