Page 47 - Caldera Volcanism Analysis, Modelling and Response

22 Fidel Costa

previously erupted post-caldera magmas. In contrast, the youngest flow erupted at
70 ka contains zircons with model ages at 101 ka and indicates a new zircon
crystallisation episode. Combining this age information with the changes of Sr and
Nd isotopes or element ratios indicative of magma evolution, Vazquez and Reid
(2002) proposed that the magmatic system of the Central Plateau member might
have been differentiating for W100 ky.
39
Finally, the 40 Ar/ Ar data reported by Gansecki et al. (1996, 1998) shows the
presence of sanidine and plagioclase xenocrysts (up to 370 My old). Diffusion
calculations indicate maximum times for immersion in magma of a couple of years
or less. Such short times were interpreted as xenocrysts entrapment during
fracturing and conduit propagation (Gansecki et al., 1996).

3.4. Long Valley system
The Long Valley volcanic field is located in east-central California, within and east
of the Sierra Nevada. It is part of an active regional transtensional zone at the Sierra
Nevada-Basin and Range transition (e.g., Bailey et al., 1976; Bailey, 2004; Hildreth,
2004). Hildreth (2004) distinguished six successive foci of silicic volcanism driven
by basaltic intrusion of the deep crust, the location of which has moved repeatedly.
The simplified chronology of events involves (1) a pre-caldera dacite field
3
(2.5–3.5 Ma) and the Glass mountain rhyolites (~100 km ; 0.79–2.2 Ma), (2) caldera
3
collapse and eruption of the Bishop Tuff (600 km ; 0.76 Ma) and (3) a long
3
history of post-caldera volcanism, starting with the Early Rhyolite (ca. 100 km ,
650–760 ka), and followed by series of small volume Moat Rhyolites (101–527 ka).
In younger times, there are two distinct focus of activity at the Mono Craters chain
and Inyo domes (Holocene to ca. 50 ka), and Mono Lakes (0.25–14 ka) (Figure 7).
3
The volume of deposits o650 ka is ca. 7–8 km (Hildreth, 2004). The reader is
referred to Reid (2003) for a summary of residence time data in Long Valley and
also to Simon and Reid (2005) for the most recent data and interpretation.

3.4.1. Pre-caldera magmas: petrological attributes and time scale information
Glass Mountain includes W60 phenocryst poor (o5 wt%) high-silica rhyolite
flows. They comprise an old sequence (2.2–1.3 Ma) of variable compositions and
erupted from discrete magma bodies, and a young sequence (1.2–0.79 Ma) which is
compositionally similar to the most evolved units of the Bishop Tuff and thus
probably issued from the same reservoir (Metz and Mahood, 1985, 1991; Hildreth,
2004). Pre-eruptive conditions for these magmas are ca. 700–7701C, ca. 200 MPa,
and water contents of 2.5–6 wt% (Metz and Mahood, 1991).

3.4.1.1. Residence times of the pre-caldera magmas. Current interest in the
application of geochemical methods to determine the residence times of silicic
magmas probably started with the study of pre-caldera lavas of Glass Mountain and
the publication of Halliday et al. (1989). They obtained Rb–Sr isochrons of ages
from ca. 1 to 2 Ma. These overlapped with eruption ages of the oldest lavas available
at that time (K–Ar; Metz and Mahood, 1985) but also implied residence times of ca.