Page 193 - Caldera Volcanism Analysis, Modelling and Response

168 Gerardo J. Aguirre-Dı ´az et al.

and Aguirre-Dı ´az, 2001). The dike is lithic-rich, with 30–40 vol.% of lithic content.
The lithics reach up to 40 cm in diameter, and are mostly of welded ignimbrite and
propilitised andesite. The pyroclastic fill material of the dike is composed of
pumiceous glass shards and pumice lumps. Next to the dike there is a co-ignimbrite
lithic-lag breccia with the same type of lithics and components than those of the dike,
suggesting that this dike was the feeder of the flat-lying lithic-rich ignimbrite.
Another ignimbrite feeder dike of the Juchipila graben caldera is about 10 km to
the west of Jalpa. This dike is unusual in the sense that it is composed of
three different pyroclastic dikes that were vertically emplaced episodically using the
same conduit (Figure 16). Aguirre-Dı ´az and Labarthe-Herna ´ndez (2003) already
mention this composite dike, which is about 50 m thick and has a general
orientation of N201W and an inclination of 751 to the NE. The three pyroclastic
dikes are named here as Dike A, Dike B, and Dike C (Figure 16). In all three dikes
evidence can be observed for vertical emplacement of pyroclastic material,
including a general vertical arrangement of components, vertical fiamme, and what
appear to be degassing pipes that are also parallel to the vertical fiamme, suggesting
a near surface portion of the observed dike (Figure 16c and d). The pyroclastic dike
leached and cooked the country rock (flat-lying red bed deposits), or one of the
previously emplaced dikes, indicating that hot pyroclastic material filled the conduit
(Figure 16e and f).

4.4. Co-ignimbrite lithic-lag breccias of the graben caldera of Bolan ˜os

The graben of Bolan ˜os is located to the west of the Juchipila graben (Figure 12).
It is a large graben about 110 km long and 20–30 km wide, with a general N–S
orientation. It branches at the north to the Valparaiso (West branch) and Jerez
graben (East branch, Figures 4 and 12), making a total length of about 180 km
for the Bolan ˜os-Jerez graben system. The Bolan ˜os graben is the deepest of all
the grabens in the southern SMO, with a topographic depth of 1,800–2,000 m.
The deepest segment of this graben is about 100 km long and includes the mining
towns of Bolan ˜os and San Martı ´n de Bolan ˜os (Figure 5), between 21.21 and 22.01
Lat. N (Figure 12). Based on geologic evidence, we deduced that this segment of
the graben is a piece-meal graben caldera. The evidence includes the following
observations: co-ignimbrite lithic-lag breccias and post-collapse lava domes, both
aligned along the graben caldera walls, and the chaotic collapse of blocks inside
3
the graben caldera. A large-volume ignimbrite (possibly over 1000 km , volcanic rock
volume), the Alacra ´n ignimbrite, resulted from the graben caldera collapse. This
ignimbrite is one of the thickest and most voluminous observed in the SMO. Near
vent outflows are about 350 m thick. The intra-caldera ignimbrite thickness is
unknown because the available bore-hole data do not reach the bottom of the graben
caldera, but at least 400 m has been measured by the mining companies. However, the
intra-caldera ignimbrite should be much thicker than this minimum value.
We propose that the pyroclastic flows that produced the Alacra ´n ignimbrite
were erupted through fissure vents controlled by the Bolan ˜os graben master faults.
This conclusion was inferred by Scheubel et al. (1988), and was confirmed more
recently by Aguirre-Dı ´az and Labarthe-Herna ´ndez (2003). The most significant