Petrographical and Geochronological Investigations on a Garnet-Tourmaline Pegmatite from Ringgold Knoll, Oates Coast, Antarctica

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The Ringgold Knoll pegmatite, a late-stage member of the Granite Harbour Intrusives, crosscuts high-grade Wilson gneisses of the Oates Coast. The pegmatite mineral assemblage consists of K-feldspar, plagioclase, quartz, garnet (almandine-spessartine-pyrope), dark tourmaline (schorl-dravite), muscovite, apatite, monazite, zircon, blue Al-rich tourmaline and dumortierite in order of decreasing abundances. Major, minor and rare earth elements are reported for the greater part of the mineral assemblage. The time of pegmatite emplacement is constrained by Rb-Sr and Sm-Nd isochron ages of 492 ± 8 (2σ) Ma and 500 ± 40 (2σ) Ma, respectively. -from Authors

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... Rb-Sr whole-rock dating of migmatitic to anatectic rocks and related intrusives from the Wilson Hills (Adams & Roland, 2002) gave poorly defined ages in the range of about 650 -690 Ma, which were interpreted to indicate a Neoproterozoic pre-Ross-orogenic age of the basement rocks. Formation of a late-tectonic pegmatite at Ringgold Knoll in the central very-highgrade zone of the Wilson Hills (Fig. 2) was dated at about 490 Ma by Rb-Sr whole-rock and U-Pb monazite dating (Schüssler & Henjes-Kunst, 1994;Schüssler et al., 1999). 40 Ar-39 Ar dating of amphibole and mica from igneous and metamorphic rocks of that area gave ages in the ranges of 486 -472 Ma and 476 -466 Ma, respectively ( Schüssler et al., 1999;Schüssler et al., 2004.;Henjes-Kunst, ...
... Age determinations on a pegmatite from the central very-high-grade zone which bears evidence of high-temperature deformation of its magmatic plagioclase (Schüssler & Henjes-Kunst, 1994;Schüssler et al., 1999) constrain the age of peak metamorphism to >490 Ma. This is further supported by the SHRIMP zircon age of 482 ± 4 Ma for the undeformed granitic dyke G8-60.1 from Archer Point. ...
... In these rocks, zircon is expected to crystallize during isothermal decompression or even during subsequent cooling when low values of zircon saturation of the partial melts are reached. The Ross-orogenic zircon ages for the metamorphic rocks G8-58.1 (younger age population), G8-59.1, and G8-57.4 postdate not only the formation age of the pegmatite from Ringgold Knoll (Schüssler & Henjes-Kunst, 1994;Schüssler et al., 1999) and in part also that of granitic dyke G8-60.1 but fall into the ranges of 40 Ar-39 Ar ages of metamorphic and igneous amphibole and mica of that area (Fig. 20). The 40 Ar-39 Ar ages for the central zone in the Wilson Hills area ( Schüssler et al., 1999;Schüssler et al., 2004) are interpreted to date different stages of its Ross-orogenic tectonometamorphic history. ...
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.
... The larger size fractions A and B are interpreted rock isochron age of 492±8 Ma (Schü ssler and to be multiply discordant, and from their data, Henjes-Kunst, 1994). Metamorphism of the couneven older protolith ages can be deduced. ...
... Oates Coast, compared to the eastern and western zones (Schü ssler, 1996). At the moment, no real zircon and by Rb-Sr and Sm-Nd whole rock-mineral isochrons (Schü ssler and Henjes-Kunst, 1994). evidence for either of these two possibilities was recognized. ...
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Within the basement of the northern Wilson Terrane at Oates Coast, a very-high-grade central zone is distinguished from high-grade zones to the east and west. In the central zone, P–T estimates of 8 kbar and 800°C derive from the relic assemblage: (1) Crd+Bt+Sil+Spl+Pl+Qtz for an earlier medium-pressure granulite-facies metamorphism which is also documented by relic assemblages Qtz+Pl+Bt+Opx (±Grt±Cpx). A subsequent low-pressure granulite-facies to upper-amphibolite-facies stage with pervasive migmatization took place at 4–5.5 kbar and minimum 700°C, as derived from mineral reactions and thermodynamic calculations on the assemblages (2) Grt+Crd+Bt+Pl+Qtz and (3) Grt+Bt+Sil+Pl+Qtz±Spl. Decompression at still high temperatures and a clockwise directed P–T–t path are indicated by reactions Bt+Sil+Qtz=Crd+Grt+Kfs+V and Grt+Sil+Qtz+V=Crd.
... The stratigraphic relations between the different units are not constrained for the central and northern part of the WT. Major thrust systems in the northwestern part of the WT (Flöttmann & Kleinschmidt, 1991) separate units with contrasting metamorphic degrees and were activated during final tectonometamorphism of the Ross Orogeny (Schüssler & Henjes-Kunst, 1994;Henjes-Kunst, unpubl. results). ...
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Metasediments in the three early Palaeozoic Ross orogenic terranes in northern Victoria Land and Oates Land (Antarctica) are geochemically classified as immature litharenites to wackes and moderately mature shales. Highly mature lithotypes with Chemical Index of Weathering values of ≥ 95 are typically absent. Geochemical and Rb-Sr and Sm-Nd isotope results indicate that the turbiditic metasediments of the Cambro-Ordovician Robertson Bay Group in the eastern Robertson Bay Terrane represent a very homogeneous series lacking significant compositional variations. Major variations are only found in chemical parameters which reflect differences in degree of chemical weathering of their protoliths and in mechanical sorting of the detritus. Geochemical data, 87Sr/86Sr t=490Ma ratios of 0.7120 - 0.7174, εNd,t=490Ma values of -7.6 to -10.3 and single-stage Nd-model ages of 1.7 - 1.9 Ga are indicative of an origin from a chemically evolved crustal source of on average late Palaeoproterozoic formation age. There is no evidence for significant sedimentary infill from primitive "ophiolitic" sources. Metasediments of the Middle Cambrian Molar Formation (Bowers Terrane) are compositionally strongly heterogeneous. Their major and trace element data and Sm-Nd isotope data (εNd,t=500Ma values of -14.3 to -1.2 and single-stage Nd-model ages of 1.7 - 2.1 Ga) can be explained by mixing of sedimentary input from an evolved crustal source of at least early Palaeoproterozoic formation age and from a primitive basaltic source. The chemical heterogeneity of metasediments from the Wilson Terrane is largely inherited from compositional variations of their precursor rocks as indicated by the Ni vs TiO2 diagram. Single-stage Nd-model ages of 1.6 -2.2 Ga for samples from more western inboard areas of the Wilson Terrane (εNd,t=510Ma -7.0 to -14.3) indicate a relatively high proportion of material derived from a crustal source with on average early Palaeoproterozoic formation age. Metasedimentary series in an eastern, more outboard position (εNd,t=510Ma -5.4 to -10.0; single-stage Nd model ages 1.4 - 1.9) on the contrary document stronger influence of a more primitive source with younger formation ages. The chemical and isotopic characteristics of metasediments from the Bowers and Wilson terranes can be explained by variable contributions from two contrasting sources: a cratonic continental crust similar to the Antarctic Shield exposed in Georg V Land and Terre Adélie some hundred kilometers west of the study area and a primitive basaltic source probably represented by the Cambrian island-arc of the Bowers Terrane. While the data for metasediments of the Robertson Bay Terrane are also compatible with an origin from an Antarctic-Shield-type source, there is no direct evidence from their geochemistry or isotope geochemistry for an island-arc component in these series.
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