Page 57 - Caldera Volcanism Analysis, Modelling and Response

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scales are presumably in accord with the Sr/ Sr and Pb/ Pb disequilibria
reported by Wolff et al. (1999) and Wolff and Ramos (2003) between quartz-
bearing glass inclusions, sanidines and the rest of the phases of the Otowi member.
Hervig and Dunbar (1992) also calculated the time for complete homogenisa-
tion of Sr zoning in sanidine to be between 2 and 13 ky. Finally, the results of
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Ar/ Ar experiments on glass inclusion bearing quartz from the Bandelier Tuffs of
Winick et al. (2001) seem to confirm the interpretation of Wolff and co-workers.
Winick et al. (2001) found unrealistically old ages between 11 and 14 Ma
(Table 7) which were interpreted as the result of excess 40 Ar, similar to the Bishop
Tuff case.

3.6. La Garita caldera

La Garita is part of a caldera cluster located in the San Juan volcanic field of
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southern Colorado. The Fish Canyon Tuff (5,000 km ) was erupted from La Garita
and is the largest known pyroclastic eruption on Earth (Lipman, 2000b). The tuff
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was preceded by the pre-caldera Pagosa Peak dacite, a large (200 km ) and poorly
fragmented pyroclastic deposit (Bachmann et al., 2000). A time gap of a few months
occurred between the two eruptions (Bachmann et al., 2007b). The intracaldera
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tuff is overlain by the Nutras Creek dacite lava (o1km ; Table 8). All the deposits
have almost identical geochemistry and very similar mineralogy and textures
suggesting they were erupted from the same reservoir. The Fish Canyon Tuff is
proposed to be the result of rejuvenation of a near solidus upper-crustal intrusive
body of batholithic dimensions (Bachmann and Dungan, 2002; Bachmann et al.,
2002). The pre-eruptive conditions of the Fish Canyon magma were experimen-
tally determined by Johnson and Rutherford (1989) to be 7601C, 240 MPa and
3–3.5 wt% H 2 O, and the liquidus obtained from MELTS (Ghiorso and Sack, 1995)
is ca. 1,0151C. The Fish Canyon Tuff has been used as a test to establish age
concordance between K–Ar and U–Pb isotopic systems, and its sanidine and biotite
are commonly used standards (Table 8).

3.6.1. Time scale information: rejuvenation and remobilization of crystal-rich
mushes
The age the Fish Canyon Tuff has been intensively investigated (e.g., Lanphere and
Baadsgaard, 2001; Schmitz and Bowring, 2001; Daze ´ et al., 2003; Bachmann et al.,
2007b), and is a matter of debate (e.g., Schmitz et al., 2003; Lanphere, 2004).
Almost all minerals have been dated (plagioclase, sanidine, biotite, hornblende,
zircon, titanite) and using different methods (Table 8). In general, the ages obtained
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by Ar/ Ar on, for example, sanidine, are younger than zircons or titanites
dated by U–Pb system by several hundreds of thousands of years. The detailed
geochronological study of Bachmann et al. (2007b) showed that total fusion
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Ar/ Ar ages of all phases almost overlap, with the youngest being those of
sanidine at 28.04 Ma, and those of biotite, plagioclase or hornblende being
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o250 ky older. The sanidine total fusion Ar/ Ar ages of the pre- and post-
caldera deposits are slightly younger (o100 ky) and older (30 ky), respectively,