T286 Accessory Minerals as Monitors of Magmatic Processes: New Ideas, Applications, and Pitfalls

Wednesday, 3 November 2010
1:30 PM - 5:30 PM
Mile High Ballroom 1CD

OXYGEN ISOTOPES IN ZIRCON AND GARNET: A RECORD OF MAGMATIC PROCESSES AND CONTAMINATION IN THE DINKEY DOME PERALUMINOUS GRANITE, SIERRA NEVADA BATHOLITH

Paper number: T286-2
Program: Technical Sessions
Time: 1:45 PM – 2:00 PM

Authors

Abstract

The 119 Ma Dinkey Dome peraluminous granite in the central Sierra Nevada Batholith contains magmatic garnet and zircon with bimodal oxygen isotope fractionations that provide a record of magmatic contamination. Laser fluorination δ18O values of bulk zircon concentrates are uniform across the entire pluton (δ18O = 7.9 ± 0.10 ‰). Values δ18O of garnet on the west side of the pluton reflect high temperature equilibrium with zircon (Δ18O(grt-zrc) = -0.06 ± 0.13 ‰), however, on the eastern side, δ18O values of garnet are lower and not equilibrated with zircon (Δ18O(grt-zrc) = -0.60 ± 0.13 ‰, Lackey et al., 2006). These relations suggest that the later-formed garnets, and perhaps earlier zircons, preserve growth zoning of δ18O that records changing magma chemistry during interactions with lower d18O wall rock. In this study, we assess oxygen isotopic zoning in zircon using the WiscSIMS ims-1280 ion microprobe (10 mm spot) to assess the possible role of sedimentary or hydrothermally altered rocks in the generation of peraluminous granitoids in subduction zone batholiths.

Values of δ18O in rims and cores of zircon grains from nine rocks show zoning, confirming that zircon crystallization was synchronous with magma contamination. Furthermore, the evolution from early high δ18O magma towards lower values is confirmed; zircon cores have uniform δ18O values (ave. δ18O = 7.7 ± 0.2 ‰), while rims are lower and more variable in δ18O (ave. δ18O = 6.4 ± 0.4 ‰). Also, the δ18O values of rims from the western part of the pluton are slightly higher (ave. δ18O = 6.6 ± 0.2 %0) than the values obtained for the eastern side (ave. δ18O = 6.1 ± 0.4%0). In order to preserve petrologic context, in situ δ18O measurements of zircon and garnet from thin sections of new Dinkey Dome samples collected in 2010 will be made. This is important because it allows us to assess variability among and within garnets, which may be key for identifying low d18O contaminants that are not identified adjacent to the Dinkey Dome pluton at its present level.