2002 Denver Annual Meeting (October 27-30, 2002)

Paper No. 14
Presentation Time: 11:30 AM


HINKE, Hans J., LACKEY, Jade Star and VALLEY, John W., Department of Geology and Geophysics, Univ of Wisconsin, Madison, WI 53706, hjhinke@wisc.edu

Values of d18O in whole rock, quartz, garnet, and zircon of the late Cretaceous Dinkey Dome granite and related pendant rocks in the central Sierra Nevada Batholith allow the reconstruction of the pluton’s magmatic evolution.  Coexisting minerals garnet and zircon were analyzed because their slow oxygen diffusion preserves magmatic d18O values.  Dinkey Dome is a peraluminous, two mica, aluminosilicate-bearing granite with Al/(Ca+Na+K) = 1.02 (west side) to 1.07 (east side).  Average garnet compositions are Alm 72.4, Sps 19.5, Pyp 5.8, and Grs 2.3 on the west and Alm 78.6, Sps 19.1, Pyp 0.9, and Grs 1.4 on the east.  Zircon inclusions in garnet indicate some zircons crystallized prior to garnet and record earlier magmatic conditions.

The range of d18O values is: (whole rock) 9.65-10.40‰, (quartz) 10.60-11.32‰, (zircon) 7.00-7.78‰, (garnet) 6.75-7.39‰, and (andalusite) 8.4-8.5‰.  Surrounding Kings Sequence metasedimentary rocks of the Dinkey Creek pendant have d18O (whole rock) of 9.5-11.6‰.   Zircon, quartz and whole rock d18O values are variable but unimodal across the pluton.  On the other hand, garnet d18O values show a distinct bimodal chemistry, changing across a pendant within the pluton.  Garnet major element chemistry closely reflects that of the granite. 

High d18O (zircon) indicates a high d18O input to the granite (possibly metasedimentary rocks), whereas garnet on the east side of the pluton shows contamination by low d18O material after zircon crystallization (possibly hydrothermally altered volcanic rocks).  The shift in d18O of garnet and constant value of d18O in zircon suggests a low d18O source must have been assimilated between the times of crystallization of these two minerals.  No low d18O source material is exposed in the Dinkey Creek Pendant, suggesting contamination in the deep crust rather than at present crustal levels.