Page 60 - Caldera Volcanism Analysis, Modelling and Response

Residence Times of Silicic Magmas Associated with Calderas 35

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rhyodacite lavas of Steel Bay (2–3 km ; erupted at 27 ka), and thus the residence
times for these pre-caldera lavas vary between 20 and W100 ky. Bacon and
Lowenstern (2005) used zircon dissolution rates to obtain a time of several decades
between zircon entrapment and eruption, and the absence of zircons in the climatic
eruption magma is probably the result of complete zircon dissolution in these hotter
caldera-forming magmas.

3.7.2. Kos plateau tuff
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The caldera associated with this rhyolitic tuff (60 km ) is probably between
the Greek islands of Kos and Nisyros, and has an inferred diameter of ca. 20 km
(Keller 1969; Allen, 2001). The deposits contain numerous granitoid xenoliths
(some with textures indicative of partial melting) of almost the same mineralogy
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as the tuff (Keller 1969; Bachmann et al., 2007a). Single crystal sanidine 40 Ar/ Ar
analysis gave eruption ages at 161.371.1 ka (Smith et al., 1996), although the
40 39
Ar/ Ar ages of plagioclase and quartz are up to 925722 ka, and are thought to
be xenocrysts from earlier events (Smith et al., 2000). In situ dating of zircons from
the tuff and the granitoid xenoliths (Bachmann et al., 2007a) shows that all crystals
have similar age distributions, from that of eruption to ca. 160 ka–340 ka (except for
one point at 500 ka). This gives residence times and zircon crystallization period
of about 180 ky. Since the ages of the tuff and the xenoliths are the same, the latter
are probably pieces of nearly completely crystallised rind of the magma reservoir
which was disrupted during eruption (Bachmann et al., 2007a).

3.7.3. La Pacana caldera
It is located in the Chilean Andes and is the largest described in South America. It
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was the source of two voluminous crystal-rich ignimbrites, the Atana (2,500 km )
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and Toconao (180 km ). They erupted (based on K–Ar data) in a small but not well
resolved time interval, with ages at 3.7770.09 –4.2370.12 Ma, and 4.070.9 –
5.2770.12 Ma, respectively (Lindsay et al., 2001). Schmitt et al. (2002) determined
zircon ages from Toconao a pumice of 4.6570.13 Ma, whereas those from a bulk
ignimbrite sample are much older, at ca. 13 and 470 Ma. The ages for the Atana
zircons are 4.1170.2 Ma, considerably younger than the pumice zircon ages of the
Toconao. Schmitt et al. (2002) calculated average residence times for the two units
of the order of 0.5–0.75 My, the longest residence times calculated so far from ages
obtained by U–Pb on zircon.

4. Interpretation of Residence Times and Integration
with Thermal and Mechanical Constrains

4.1. What do the residence times indicate?
Large ranges of residence times are found within single deposits, between deposits
of single systems, and between systems (Figure 9). The shortest times are those of