Abstract
- High- to very-high-grade migmatitic basement rocks of the Wilson
Hills area in northwestern Oates Land (Antarctica) form part of a
low-pressure high-temperature belt located at the western inboard side
of the Ross-orogenic Wilson Terrane. Zircon, and in part monazite, from
four very-high grade migmatites (migmatitic gneisses to diatexites) and
zircon from two undeformed granitic dykes from a central
granulite-facies zone of the basement complex were dated by the SHRIMP
U-Pb method in order to constrain the timing of metamorphic and related
igneous processes and to identify possible age inheritance. Monazite
from two migmatites yielded within error identical ages of 499 ±
10 Ma and 493 ± 9 Ma. Coexisting zircon gave ages of 500
± 4 Ma and 484 ± 5 Ma for a metatexite (two age
populations) and 475 ± 4 Ma for a diatexite. Zircon populations
from a migmatitic gneiss and a posttectonic granitic dyke yielded
well-defined ages of 488 ± 6 Ma and 482 ± 4 Ma,
respectively. There is only minor evidence of age inheritance in
zircons of these four samples. Zircon from two other samples
(metatexite, posttectonic granitic dyke) gave scattered 206Pb-238U
ages. While there is a component similar in age and in low Th/U ratio
to those of the other samples, inherited components with ages up to c. 3 Ga predominate. In the
metatexite, a major detrital contribution from 545 - 680 Ma old source
rocks can be identified. The new age data support the model that
granulite- to high-amphibolite-facies metamorphism and related igneous
processes in basement rocks of northwestern Oates Land were confined to
a relatively short period of time of Late Cambrian to early Ordovican
age. An age of approximately 500 Ma is estimated for the Ross-orogenic
granulite-facies metamorphism from consistent ages of monazite from two
migmatites and of the older zircon age population in one metatexite.
The variably younger zircon ages are interpreted to reflect mineral
formation in the course of the post-granulite-facies metamorphic
evolution, which led to a widespread high-amphibolite-facies
retrogression and in part late-stage formation of ms+bi assemblages in
the basement rocks and which lasted until about 465 Ma. The presence of
inherited zircon components of latest Neoproterozoic to Cambrian age
indicates that the high- to very-grade migmatitic basement in
northwestern Oates Land originated from clastic series of Cambrian age
and, therefore, may well represent the deeper-crustal equivalent of
lower-grade metasedimentary series of the Wilson Terrane.
*Corresponding author (henjes-kunst@bgr.de)