The outcrop below is at the base of Camp Ridge in the western Miller Range. It consists primarily of layered quartzofeldspathic gneiss, amphibolite, and aplitic dikes. Structural fabrics in the rocks were imparted during the Ross Orogeny (~500 Ma), based on 40Ar/39Ar and U-Pb data published earlier (Goodge and Dallmeyer, 1992, 1996; Goodge et al., 1993), but we had no clear idea about the original ancestry of these rocks other than some whole-rock Sm-Nd isotopic results published by Borg et al. (1990) that indicated a Precambrian age component.
We separated zircons from a sample of quartzofeldspathic gneiss and analyzed them by SHRIMP U-Pb method at the Australian National University. Images of the zircons in cathodoluminescence are shown here:
We found that some zircons have igneous cores of ~3.1-3.0 Ga age, some metamorphic zones of ~2.995 Ga, both of which are overprinted by mantles giving ages of ~1.73 and ~0.52 Ga. There are also cores of ~1.72 Ga metamorphic zircon overprinted by 500 Ma zircon. All of these ages suggest a long, complicated history of grain growth, dissolution and re-growth over a period of about 2. 5 billion years! Results from this and related samples were published by Goodge and Fanning (1999).