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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