ARTÍCULO
Mixed magmatic-phreatomagmatic explosions during the formation of the Joya Honda maar, San Luis PotosÃ, México
Saucedo R, MacÃas JL, Gómez-Villa W, Rivera-OlguÃn E, Ocampo-DÃaz YZE, Torres-Hernández R, Castro-Govea, R, Sánchez-Núñez, JM, Layer P., Carrasco-Núñez,G.
Geological Society, London Special publication No 446, Monogenetic Volcanism, 255-279. ISSN 03058719 online 20414927, 2017.
ABSTRACT:
The Joya Honda maar (JHm) is located in central Mexico, 35 km NNE of the city of San Luis Potosı´. It lies in the Plio-Quaternary alkaline Ventura-Espı´ritu Santo Volcanic Field located in the eastern part of Mesa Central province. The JHm eruption occurred at 311+19 ka (40Ar/39Ar) along a fissure that formed an elliptical crater (c. 1.3 × 0.9 km wide and c. 270 m deep) with a major axis oriented to the ENE–WSW. The eruption generated pyroclastic surge deposits that preferentially extended up to a distance of 7 km to the NW–NE of the crater, with a very limited distribution to the south. At the crater rim, the sequence is 60–80 m thick on the NE–NW wall and 1–15 m thick on the south–SW rim. The JHm sequence is divided into five units with different structures, textures, granulometry and components. The juvenile basanite clasts of these units display differences in vesicularity, density and morphology under scanning electron microscopy. These units correspond to the same number of eruptive phases as follows: Phase 1 occurred as a series of alternating strombolian and phreatomagmatic explosions that dispersed fall deposits and base surges; Phase 2 began with strombolian activity that emplaced basanite scoria with low contents of mantle xenoliths; Phase 3 continued with phreatomagmatic explosions that emplaced wet and dry pyroclastic surges; Phase 4 generated strombolian explosions rich in mantle xenoliths; and Phase 5 produced a violent strombolian phase that dispersed fallouts rich in mantle xenoliths and intermixed with discrete phreatomagmatic explosions that emplaced pyroclastic surges. These eruptive fluctuations during the genesis of JHm are a response to the relative proportions of magma–water interaction through time and complex faulting of the calcareous rocks underneath the volcano. The distribution and textural characteristics of the deposits suggest that simultaneous or alternating vents were active during the eruption, possibly following a fissure. These variations may have been subordinated to factors such as the availability of groundwater, the velocity of magma ascent, the discharge rate and degassing.