Page 195 - Caldera Volcanism Analysis, Modelling and Response

170 Gerardo J. Aguirre-Dı ´az et al.

observations for this argument are the near vent deposits related to the Alacra ´n
ignimbrite that occur on both sides of the graben caldera of Bolan ˜os (Figures 17–19).
These deposits include a sequence of phreatomagmatic surges with ballistic lithics
up to 40 cm in diameter (Figures 18c, d and 19f). The surge deposits are lithic-rich
layers interbedded with lithic-poor pumiceous surge deposits and ignimbrites up to
2 m thick (Figures 18b, 19b, c, and d). These ignimbrites include lithic-lag breccias
with lithics of underlying units up to 35 cm in diameter (Figures 18c, d, 19c and d).
Overlying this pyroclastic sequence is the major ignimbrite of Alacra ´n(Figure 18b),
with a co-ignimbrite lithic-lag breccia up to 50 m thick and lithics of underlying
rocks up to 1.5 m in diameter (Figure 18e and f). The pre-ignimbrite lithic-rich
surge sequence and the overlying co-ignimbrite lithic-lag breccias that grade
upward to a major ignimbrite can be observed next to the graben caldera fault
scarps, on both sides of the graben caldera (Figures 17, 18a and 19a), that is, East
and West of the 20–30 km wide depression (Figure 17). The pre-ignimbrite surge
sequence can be interpreted as the decompression phase of the sub-graben caldera
magma chamber previous to the graben caldera collapse, similar to the case of El
Ocote ignimbrite at the Malpaso graben caldera. After the collapse and ignimbrite
emplacement, several rhyolitic lava domes were emplaced along the graben caldera
master faults (Figures 17, 18a and 19e). These domes are thought to represent the

Figure 17 Sketch of a cross-section of the Bolan ì os graben caldera showing the two type
sections for the Alacra Ł n Ignimbrite, the major volcanic event related to the graben caldera
collapse, with co-ignimbrite lag-breccias at both sides of the graben. See text for details.
Photographs of these two sections are shown in Figures 18 and 19.