Preliminary Monazite Ages of Second Generation Mylonites, Prince Charles Mountains, East Antarctica

Geoffrey T. Nichols1, 2 and Albert Fahey2

1GEMOC, School of Earth Sciences, Macquarie University NSW 2109, AUSTRALIA

2California Institute of Technology, Division of Geological and Planetary Sciences, Mail Code 170-25, Pasadena CA 91125, USA

Three generations of mylonites (MY) associated with crustal thickening and subsequent thinning events have been identified recently in the northern Prince Charles Mountains (nPCM), east Antarctica. First generation mylonites (MY1) predate folding and have been annealed during peak metamorphic conditions; these sub-horizontal structures may represent early thrusts. Second generation mylonites (MY2) developed at ~5 kbar and 700°C, metamorphic conditions that post-date the peak conditions in this region. MY2 preserve pristine mylonitic, to ultra-mylonitic textures, truncate F2-3 folds and consistently display NW-up movement on NE-SW trending, steeply dipping surfaces. These structures may be responsible for rapid uplift in the nPCM soon after peak metamorphic conditions were attained, and thus may account for the shallow predominantly cooling trajectories of P-T paths in this region. Third generation mylonites (MY3) truncate MY2 and are associated with lower grade metamorphic assemblages.

Here we describe new geochronology studies undertaken with an oxygen-sourced Cameca 3f ion-probe at Caltech. Rather than apply mineral separation techniques which inherently erase textural information, we analysed monazite grains directly in thin section. We were thus able to distinguish data from monazites within centimetre wide shears, from monazite ages determined for granoblastic areas of the rock, largely unaffected by MY2 deformation (Figure 1a). Our preliminary data total 19 analyses from a single thin-section of an MY2 shear zone (Nº 77154). The sample from Wall Peak in the south-western nPCM, is a garnet-orthopyroxene-plagioclase-quartz gneiss which underwent mylonitic deformation at ~5 kbar and 700°C.

Th232/Pb208 isotopes were reduced to ages together with separate analyses of monazites of known age measured as a standard. As the measured abundance of Pb204 was very low, no correction was applied for 'common lead'. Figure 1b displays ages of monazites plotted with one sigma errors which are a compound error accounting for possible errors in counting statistics, variability of the standard monazite, and also account for the uncertainty of the age of the standard. Figure 1b displays two groups of data, with the older ages measured from groundmass monazites (with an average of 930±28 Ma), and younger monazites analysed within the MY2 averaging 800±16 Ma. As expected the older groundmass monazites display greater variability; one monazite with an age of ~1100 Ma may represent an inheritance age. The two data groups remain statistically independent to the 2.3 sigma level; thus there is <5% probability that they represent data from the same age. The preliminary data suggest that either the nPCM remained at near peak metamorphic conditions for some 130 Ma before uplift along MY2, or that MY2 are related to a second tectonic episode at 800 Ma.

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