2004 Denver Annual Meeting (November 7–10, 2004)

Paper No. 12
Presentation Time: 4:40 PM


LACKEY, Jade Star and VALLEY, John, Department of Geology and Geophysics, Univ of Wisconsin, Madison, WI 53706, jadestar@geology.wisc.edu

Titanite is a widespread accessory mineral in granitic (diorite to granite) rocks of the Sierra Nevada Batholith (SNB), California. Primary and secondary titanite (Tt1 & Tt2) commonly coexist in the same rock and can be differentiated based on color, trace element concentration, and crystal size and morphology, as has been done in geochronology studies of Tt (e.g., Frost et al., 2001, Chem.Geo). In the SNB, Tt1 is of magmatic origin and Tt2 formed during subsolidus alteration of biotite, Fe-Ti oxides, or hornblende. Values of d18O(Tt1 & Tt2) across the central SNB reveal the magmatic and alteration histories of the batholith.

The range of d18O(Tt1) values (3.5–7.6‰, n=111) generally track and reflect regional variations in magma source across the central SNB; values of d18O(Tt2) (1.2–7.6‰, n=87) reflect both magma source and superimposed alteration. Comparison of d18O(Tt) to d18O of zircon (Zc), as D(Zc–Tt), provides a measure of isotopic resetting in Tt because d18O(Zc) gives a reliable record of magmatic d18O (Lackey et al., 2003, GSA abst). Average D(Zc–Tt1) values (1.6±0.3‰; 1s.d.) are generally larger than expected for magmatic temperatures (<0.9‰ @ >800°C, King et al., 2001, GCA) and reflect resetting of d18O(Tt1) during cooling of the SNB. Regionally, variations in D(Zc–Tt2) (avg.=2.4±0.6‰; 1s.d.) correlate to pluton depth (Al-in-hornblende barometry, Ague & Brimhall, GSAB, 1988). For example, D(Zc–Tt2) values are uniform in plutons of the Fine Gold Series, the deepest (3–4 kbar; 10–13 km) exposed in the central SNB, but are highly variable in shallow (1–2 kbar; 3-–7 km) plutons like the Mt. Givens pluton & Tuolumne Intrusive Series. During growth of T2, shallow plutons were subject to meteoric water exchange while deeper plutons were internally buffered.

Trace element concentrations, including REE, are higher and more variable in SNB Tt1 than Tt2, which occurs due to sequestration these elements in Tt1 and other magmatic phases (e.g., Frost, ibid). Correlation of increasing D(Zc–Tt2) values to decreasing Ce and Fe concentrations in Tt2 is consistent with lowering of d18O(Tt2) values by progressively more oxidized meteoric fluids. These results show that analysis of d18O(Tt) is can be used to recognize Tt1 from Tt2 and provides a powerful complement to geochronology studies.