Progressive assembly of a massive layer of ignimbrite with a normal-to-reverse compositional zoning:: the Zaragoza ignimbrite of central Mexico

The Zaragoza ignimbrite and two enclosing rhyodacite pumice fall layers were emplaced during the 15 km(3) (DRE), similar to 0.1 Ma Zaragoza eruption from Los Humeros volcanic centre, 180 km east of Mexico City. The ignimbrite comprises several massive flow-units, the largest of which locally exceeds 20 m in thickness and is regionally traceable. It comprises massive lapilli-ash with vertical elutriation pipes, and has a fine-grained inverse-graded base and a pumice concentration zone at the top. It also exhibits an unusual gradational 'double' vertical compositional zonation that is widely traceable. A basal rhyodacitic (67.6-69 wt% SiO2) zone grades up via a mixed zone into a central andesitic (58-62 wt% SiO2) zone, which, in turn, grades up into an upper rhyodacitic (67.6-69 wt% SiO2) zone. Zoning is also defined by vertical variations in lithic clast populations. We infer that pyroclastic fountaining fed initially rhyodacite pumice clasts to a sustained granular fluid-based pyroclastic density current. The composition of the pumice clasts supplied to the current then gradually changed, first to andesite and then back to rhyodacite. Inverse grading at the base of the massive layer may reflect initial waxing flow competence. The pumice concentration at the top of the massive layer is entirely rhyodacitic and was probably deposited during waning stages of the current, when the supply of andesitic pumice clasts had ceased. The return to rhyodacitic composition may have been the result of eruption-conduit modification during collapse of Los Potreros caldera, marked in the ignimbrite by a widespread influx of hydrothermally altered lithic blocks, and/or a decrease in draw-up depth from a compositionally stratified magma chamber as the eruptive mass flux waned. The massive layer of ignimbrite thins locally to less than 2 m, yet it still shows the double zonation. Correlation of the zoning suggests that the thin massive layer is stratigraphically condensed, and aggraded relatively slowly during the same time interval as did the much thicker (<= 50 m) massive layer.