OXYGEN ISOTOPE RECORD OF MAGMATIC EVOLUTION: THE GARNET-BEARING DINKEY DOME PLUTON, SIERRA NEVADA

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.