Page 54 - Caldera Volcanism Analysis, Modelling and Response

Residence Times of Silicic Magmas Associated with Calderas 29

with the inclusion located in the centre of a 500 mm (radius) sized crystal, it
would take W0.5 My to homogenize 50% of the Ar isotopes. Thus, the presence
of excess 40 Ar in some of the glass inclusion in quartz is consistent with
residence times ca. of 100 ky for the Bishop magma and the example highlights the
need of considering all parameters to retrieve time information from diffusion
models.

3.4.3. Post-Caldera magmas: petrological attributes and time scale information
There is a long history of post-caldera silicic volcanism at Long Valley
proper (e.g., Bailey et al., 1976; Bailey, 2004; Heumann, 1999; Hildreth, 2004;
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Mankinen et al., 1986; Ring, 2000). It started with the ca. 100 km of the
Early Rhyolite (652–751 ka), and was followed by the Moat Rhyolite with
ages of 481–527 ka, the Southeastern rhyolite cluster (329–362 ka), and the west
Moat rhyolites (97–162 ka). Western post-caldera volcanism includes Mammoth
Mountain (57–111ka), mafic scoria cones and lavas (65–160ka), a series of
dacites (27–40 ka) and finally the Mono-Inyo chain (Holocene–20 ka, Wood, 1983;
Miller, 1985). There is a time gap between ca. 160 and 320 ka without volcanic
activity.
Reid et al. (1997) reported zircon model ages (two-point isochrons) for
Deer Mountain ranging from eruption to 110 ky older (mean at 224 ka). For
the South Dead Man dome (erupted at 0.6 ka) ages range from 30 to 278 ka,
3
implying long residence times for such a small volume (0.13 km , Miller 1985). The
similar ages found in the oldest parts of the zircons of both units could mean that
the South Dead Man dome zircons are recycled from Deer Mountain.
Notwithstanding, no deposits have been dated in that range and thus zircon
ages between 200 and 300 ka are probably from crystals recycled from ‘plutonic’
magmas.
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Heumann and Davies (1997) and Heumann et al. (2002) reported 40 Ar/ Ar,
Rb–Sr and U-series disequilibria systematics for several post-caldera lavas. The West
Moat Coulee (referred to by Heumann et al., 2002 as the Low Silica Flow) was
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the largest (ca. 4 km ) and oldest one (ca. 150 ka), and yields 238 U– 230 Th isochrons
that range from eruption age to ca. 200 ka, giving residence times of up to 50 ky
(Table 6 and Figure 8). The data for the younger Deer Mountain and Mammoth
Knolls (both at ca. 105 ka) define Rb–Sr isochrons at ca. 260 ka. This was
interpreted as a feldspar fractionation and magma differentiation event where melts
accumulated at the top of the reservoir where they remained for 150 ky prior to
238 230
eruption. In contrast, the U– Th isochrons from the two domes are different.
Deer Mountain defines ages at ca. 235 ka which overlap with the Rb–Sr ages (when
errors are taken into account) and also agrees with the zircon U–Th series
238 230
disequilibria data of Reid et al. (1997). The Mammoth Knolls U– Th isochron
ages are at ca. 140 ka which is ca. 50 ky younger that the Rb–Sr ages. Heumann
et al. (2002) explained age differences between radiogenic clocks and phases by
considering a cooling history where feldspars and some zircon crystallised earlier
(at a higher temperature) than the allanite which gives younger ages, and inferred
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maximum cooling rates between 7 10 4 and 10 3 Ky .