Paper No. 2
Presentation Time: 1:45 PM
OXYGEN ISOTOPES IN ZIRCON AND GARNET: A RECORD OF MAGMATIC PROCESSES AND CONTAMINATION IN THE DINKEY DOME PERALUMINOUS GRANITE, SIERRA NEVADA BATHOLITH
QUINTERO, Raiza, Department of Geoscience, University of Wisconsin-Madison, 1215 W. Dayton, Madison, WI 53706, KOZDON, Reinhard, Department of Geoscience, University of Wisconsin-Madison, Madison, WI 53706, KITAJIMA, Kouki, Department of Geoscience, University of Wisconsin-Madison, 1215 W Dayton St, Madison, WI 53706, LACKEY, Jade Star, Geology Department, Pomona College, Claremont, CA 91711 and VALLEY, John W., Department of Geoscience, University of Wisconsin, Madison, WI 53706, rquintero@wisc.edu
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 δ
18O values of bulk zircon concentrates are uniform across the entire pluton (δ
18O = 7.9 ± 0.10 ‰). Values δ
18O of garnet on the west side of the pluton reflect high temperature equilibrium with zircon (Δ
18O(grt-zrc) = -0.06 ± 0.13 ‰), however, on the eastern side, δ
18O values of garnet are lower and not equilibrated with zircon (Δ
18O(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 δ
18O that records changing magma chemistry during interactions with lower d
18O 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.
