PRIMARY AND SECONDARY TITANITE d18O VALUES AS RECORDS OF MAGMATIC AND ALTERATION PROCESSES IN THE SIERRA NEVADA BATHOLITH

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.