Page 194 - Caldera Volcanism Analysis, Modelling and Response

The Sierra Madre Occidental, Mexico 169

Figure 16 Composite ignimbrite feeder dike at the western wall of Juchipila graben caldera,
located west of Jalpa town (modi¢ed from Aguirre-D|¤az and Labarthe-Herna Ł ndez, 2003). (a)
General view to the SWshowing the three di¡erent pyroclastic dikes, named as Dike A, Dike
B and Dike C. (b) Sketch of Figure16a indicating the principal features observed in this compo-
site dike. (c) General view of Dike B showing the leached, light-colour dike margins caused
by hydrothermal £uids and shearing of the pyroclastic material along the borders formed dur-
ing dike emplacement. (d) Detail of the vertical gas-escape pipe in Dike B; the gas pipe is paral-
lel to the dike, indicating a vertical emplacement of the pyroclastic ¢ll material. (e) General
view of Dike C showing one of the margins cooked by the Dike B (dark-color zone); Dike C
was severly altered to clay by hydrothermal £uids probably associated to Dike B. Dike C gradu-
ally changes to a less altered rock towards the opposite side (away from Dike B), where it is a
densely, grey ignimbrite-like material with horizontal cooling joints (perpendicular to dike
margins). (f) Intrusive contact of Dike C with the country rock, which consists of £at-lying
continental deposits gravels and sands (red beds) that pre-date the Oligocene ignimbrites of the
area (Modi¢ed from Aguirre-D|¤az and Labarthe-Herna Ł ndez, 2003).