HR: 08:45h
AN: V21A-02 INVITED [Abstracts]
TI: Zircon Saturation and the Viability of Magma Bodies During Intrusion of the Tuolumne Intrusive Series, Sierra Nevada Batholith, California
AU: * Miller, J
EM: jsmiller@email.sjsu.edu
AF: Dept of Geology, San Jose State Univ, San Jose, CA 95192-0102 United States
AU: Burgess, S
AF: Dept of Geology, San Jose State Univ, San Jose, CA 95192-0102 United States
AU: Miller, C
AF: Earth and Environmental Sciences 117-B, Vanderbilt Univ, Nashville, TN 37235 United States
AU: Miller, R
AF: Dept of Geology, San Jose State Univ, San Jose, CA 95192-0102 United States
AU: Bergantz, G
AF: Earth and Space Sciences, Univ of Washington, Seattle, WA 98195-1310 United States
AB:
The Tuolumne Intrusive Series (TIS) is a prime example of a zoned arc intrusion, and is one of a belt of large (>1000
km2) consanguineous, Cretaceous, zoned intrusions within the Sierra Nevada batholith. Past work linked the
differentiation of the TIS with field petrology and modification of magma in situ in large reservoirs. New
single-zircon geochronology and field studies show that the TIS was assembled from numerous magma inputs from 94-85 Ma,
casting doubt on the existence of a large body of mobile magma during growth and emplacement [1], and calling into question
where the bulk of the chemical differentiation occurred (exposure level or deep source). Zircon saturation temperatures
(Tzrc; [2]) for the TIS are useful for understanding its assembly, particularly in combination with high precision,
single zircon ages. The two largest units of the TIS (Half Dome and Cathedral Peak granodiorites) have low calculated
Tzrc (Half Dome: 710-765°C, mean=740°C; Cathedral Peak: 715-780°C, mean=760°C), which implies
complexities in how zircon ages are used to interpret the construction of the TIS: (1) at T's appropriate for anatexis,
transport, and initial emplacement of granitoid magmas, source-derived zircons would be dissolved; zircons in TIS thus
crystallized during post-emplacement conductive cooling upon reaching Tzrc, or were inherited late; (2) because
Tzrc is not far above the solidus, appreciable age differences (several 105 yr?) might be produced by slow cooling
of a large magma body that was open to heat input; heat added need only have been sufficient to maintain near-eutectic
conditions, perhaps as persistent mush, but with net cooling (qout > qin), such that ages track the
migration of the Tzrc isotherm during solidification; (3) reheating of any mush, or melting of intrusions associated
with earlier inputs to T's >> Tzrc would dissolve older zircons on time scales of 104-105 years (e.g.
[3]); this time scale sets a limit on the longevity of melt-rich, magma bodies formed by remelting of earlier intrusions at a scale that encompasses samples with statistically distinguishable zircon ages. A corollary of 1-3 above is that there may be a correlation between zircon age dispersion and inferred magma body longevity in the TIS. Samples with tight clusters of
concordant, single-crystal weighted mean ages (low MSWD) presumably correspond to areas in TIS where no appreciable heating
(several 100's °C) occurred after cooling through Tzrc. Conversely, samples showing concordant, single-crystal, age dispersion may reflect: (a) reheating/melting of mushy zones or earlier solidified intrusions within the TIS, such that
older zircons were incorporated into new inputs of magma but did not completely dissolve, and/or (b) transient thermal
oscillations about Tzrc in long-lived mushy magma, producing periods of alternating dissolution and growth. Age
progression from older to younger toward the interior would still be preserved even with these complexities. [1] Coleman DS,
Gray W, Glazner AF (2004) Geology 32:433-436; [2] Watson EB, Harrison TM (1983) Earth Planet Sci Lett 64:295-304; [3] Watson EB (1996) Trans Roy Soc Edinburgh: Earth Sci 87:43-56.
DE: 1035 Geochronology
DE: 3640 Igneous petrology
DE: 8439 Physics and chemistry of magma bodies
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2005 Joint Assembly