GEOCHRONOLOGICAL CONSTRAINTS ON THE METAMORPHIC EVOLUTION AT RUNDVAGSHETTA, EAST ANTARCTICA

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The highest grade rocks in the Lutzow-Holm Complex of East Antarctica are exposed at Rundvagshetta, where peak pressures and temperatures are estimated to have reached ∿10kbar, >900℃ (Y. MOTOYOSHI et al.; The 13th Symposium on Antarctic Geosciences, Programme and Abstracts, Tokyo, Natl Inst. Polar Res., 62,1993). K. SHIRAISHI et al. (J. Geol., 102,47,1994) report U/Pb SHRIMP zircon analyses from a garnet-biotite gneiss. Late Archaean-early Proterozoic ages from zircon cores were interpreted as detrital sedimentary ages, while rim overgrowths with a mean age of 521±9Ma were regarded as the time of peak metamorphism. Further SHRIMP zircon analyses (this study) lend support to this interpretation, but provide additional information. Zircon separates from two metapelitic rocks (sample numbers R-125 and R-12) have been analysed. Both samples contain a primary metamorphic assemblage of garnet+sillimanite+orthopyroxene with secondary cordierite forming at the expense of garnet. Sample R-12 also shows cordierite+sapphirine+spinel symplectites around sillimanite. All zircon grains were imaged by cathodoluminescence prior to ion probe analysis, revealing the presence of growth zones with distinct chemistry. Zircons from R-125 fall into two groups based on internal structure. Type I zircons contain cores which are often broken or corroded. These cores yield late Archaean-early Proterozoic ages and are best interpreted as sedimentary detrital zircon. Overgrowing these cores is a euhedral zone, characterised by high U contents (>1000ppm) and very low Th/U ratios (<0.03). This generation of overgrowth, of probable metamorphic origin, yields a late-Proterozoic age, and has not been distinguished in previous work. A second generation of metamorphic overgrowth forms around the high U band. This second generation zircon has much higher Th/U (typically between 0.5 and 1.0), U contents between 100 and 250ppm, and is comparable in age to the zircon rims dated by K. SHIRAISHI et al. (J. Geol., 102,47,1994). Many grains also show a further narrow rim of zircon which is generally too narrow to analyse, and cannot be distinguished in age from the second generation overgrowths. The second type of zircon in R-125 forms equant, homogeneous grains, often with sector zoning. The chemistry (Th/U and U content) and age of these grains is indistinguishable from the second generation overgrowths in type I zircons. Sample R-12,despite being a similar rock composition to R-125,shows a very different, and much simpler zircon population. All zircons analysed from R-12 are unstructured, unzoned and form a single late Proterozoic-early Cambrian age population, corresponding to the second generation overgrowths in R-125. These grains have a low U content (typically ∿50ppm) and Th/U in the range 0.25-0.5. No detrital cores have been recognized in zircons from R-12. Several questions are raised by these results; 1) why does R-12 lack detrital zircon, and fail to show evidence of the early stage of metamorphic zircon growth seen in R-125? 2) do the two phases of metamorphic zircon growth seen in R-125 represent two distinct thermal/metamorphic events, or 3) are they the result of a single, prolonged P-T cycle, during which zircon growth is episodic and controlled by subtle changes in local chemistry (possibly the result of contemporaneous reactions amongst primary metamorphic assemblages). Answering these questions is central to the interpretation of zircon ages in all high-grade terrains and the subject of ongoing work. Despite the current uncertainty in interpretation of the results presented above, the conclusion that high-grade metamorphic conditions prevailed at Rundvagshetta at around 550Ma is inescapable. When considered in combination with K/Ar and ^<40>Ar/^<39>Ar ages (see G. F. FRASER and McDOUGALL; Proc. NIPR Symp. Antarct. Geosci., 8,137,1995), this age allows us to constrain the rate of cooling following high-grade metamorphism. Biotite from a biotite gneiss at Rundvagshetta yields a K/Ar age of 516±5Ma, and a ^<40>Ar/^<39>Ar age spectrum suggestive of gradual closure to argon diffusion over the period from 525-500Ma. These results suggest that the region cooled below ∿300℃ (closure temperature for argon in biotite) by about 516Ma. Assuming a temperature of ∿900℃ (as estimated by Y. MOTOYOSHI et al. (The 13th Symposium on Antarctic Geosciences, Programme and Abstracts, Tokyo, Natl Inst. Polar Res., 62,1993) for the period of zircon growth at ∿550Ma, the biotite cooling age would suggest an average colling rate of ∿15℃/Ma. For realistic geothermal gradients, cooling of the terrain to <300℃requires considerable uplift from peak metamorphic pressures of ∿10kbars. For example, assuming an upper crustal gradient of 30℃/km implies a maximum pressure constraint of 3-4kbars at ∿516Ma. However, it is not yet clear whether the ∿550Ma period of zircon growth represents peak pressure conditions, or some stage during the decompression history, therefore it is not possible to confidently constrain the rate of exhumation.
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GEOCHRONOLOGICAL CONSTRAINTS ON THE METAMORPHIC EVOLUTION AT RUNDVAGSHETTA, EAST ANTARCTICA

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