Geodynamic evolution of Proterozoic supracrustal rocks from the northern Shackleton Range, East Antarctica

The overall aims of this two-year project were to distinguish different crustal components in the Shackleton Range, East Antarctica, to infer their source, age and tectonic evolution and to document how the individual basement components are related to other terranes of the East Antarctic Craton, and whether they provide evidence for supercontinent formation during the Meso- and/or Neoproterozoic/Cambrian. Towards these aims it was intended to
(1) determine the pressure-temperature conditions of various rock types,
(2) characterize the geochemical signature of (predominantly) mafic igneous rocks via major, minor, trace and rare earth element geochemistry,
(3) ascertain the source region (juvenile mantle, reworked crust, tectonic affinities of metabasic rocks) of felsic and mafic orthogneisses through whole rock isotope and Lu-Hf in-situ zircon analyses and,
(4) establish the timing of various tectonothermal events by combined U-Pb and Lu-Hf zircon isotope analysis.

Our data show that the Shackleton Range is a composite terrane that was affected to various extents by the Palaeoproterozoic Kimban Orogeny, the Mesoproterozoic Grenvillian Orogeny and the late Neoproterozoic/Cambrian Pan-African Orogeny. Three distinct, spatially separated crustal terranes with specific tectonothermal histories, isotope signatures and crustal evolutions could be delineated and are referred to as the Southern, Eastern and Northern Terranes.

Mafic gneisses from the Southern Terrane provide geochemical evidence for a within-plate, probably back-arc origin of their protoliths. A plume-distal ridge origin in an incipient ocean basin is the favoured interpretation for the emplacement site of these rocks at c. 1850 Ma, which, together with a few ocean island basalts, were subsequently incorporated into an accretionary continental arc/supra-subduction zone tectonic setting. Magmatic underplating resulted in partial melting of the lower crust, which caused high-temperature granulite-facies metamorphism in the Southern Terrane at c. 1710-1680 Ma. Mafic and felsic gneisses there are characterised by isotopically depleted, positive Nd and Hf initials and model ages between 2100 and 2000 Ma. They may be explained as juvenile additions to the crust towards the end of the Palaeoproterozoic. These juvenile rocks occur in a narrow, c. 150 km long E-W trending belt, inferred to trace a suture that is associated with a large Palaeoproterozoic accretionary orogenic system. The Southern Terrane contains many features that are similar to the Australo-Antarctic Mawson Continent and may be its furthermost extension into East Antarctica.

The Eastern Terrane is characterised by metagranitoids that formed in a continental volcanic arc setting during a late Mesoproterozoic orogeny at c. 1060 Ma. Subsequently, the rocks experienced high-temperature metamorphism during Pan-African collisional tectonics at 600 Ma. Isotopically depleted zircon grains yielded Hf model ages of 1600-1400 Ma, which are identical to Nd model ages obtained from juvenile metagranitoids. Most likely, these rocks trace the suture related to the amalgamation of the Indo-Antarctic and West Gondwana continental blocks at ~600 Ma. The Eastern Terrane is interpreted as the southernmost extension of the Pan-African Mozambique/Maud Belt in East Antarctica and, based on Hf isotope data, may also represent a link to the Ellsworth-Whitmore Mountains block in West Antarctica and the Namaqua-Natal Province of southern Africa.

Geochemical evidence indicates that the majority of the protoliths of the mafic gneisses in the Northern Terrane formed as oceanic island basalts in a within-plate setting. Subsequently the rocks were incorporated into a subduction zone environment and, finally, accreted to a continental margin during Pan-African collisional tectonics. Felsic gneisses there provide evidence for a within-plate and volcanic arc/collisional origin. Emplacement of granitoids occurred at c. 530 Ma and high-temperature, high-pressure metamorphism took place at 510-500 Ma. Enriched Hf and Nd initials and Palaeoproterozoic model ages for most samples indicate that no juvenile material was added to the crust of the Northern Terrane during the Pan-African Orogeny but recycling of older crust or mixing of crustal components of different age must have occurred. Isotopically depleted mafic gneisses, which are spatially associated with eclogite-facies pyroxenites, yielded late Mesoproterozoic Nd model ages. These rocks occur in a narrow, at least 100 km long, E-W trending belt that separates alkaline ocean island metabasalts and within-plate metagranitoids from volcanic arc metabasalts and volcanic arc/syn-collisional metagranitoids in the Northern Terrane. This belt is interpreted to trace the late Neoproterozoic/early Cambrian Pan-African collisional suture between the Australo-Antarctic and the combined Indo-Antarctic/West Gondwana continental blocks that formed during the final amalgamation of Gondwana.

Scientists

Thomas Will
Hartwig Frimmel
Universität Würzburg

Armin Zeh
Universität Frankfurt/Main

Esther Schmädicke
Universität Erlangen

Research areas

Northern Shackleton Range, East Antarctica

Publications

Schmädicke E, Will TM, 2006. First evidence of eclogite-facies metamorphism in the Shackleton Range, Antarctica: Tracer of a suture between East and West Gondwana? - Geology 34:133-136.

Will TM, Frimmel HE, Zeh A, Le Roux P, Schmädicke E, 2010. Tectonic and crustal evolution of the Shackleton Range, East Antarctica: geochemical and isotope constraints. - Precambrian Research 180:85-112.

Will TM, Zeh A, Gerdes A, Frimmel HE, Millar IL, Schmädicke E, 2009. Palaeoproterozoic to Palaeozoic magmatic and metamorphic events in the Shackleton Range, East Antarctica: Constraints from zircon and monazite dating, and implications for the amalgamation of Gondwana. Precambrian Research 172:25-45.


 Zeh A, Gerdes A, Will TM, Frimmel HE, 2010. Hafnium isotope homogenisation in metasedimentary rocks under amphibolite-facies conditions (>650°C): examples from the Shackleton Range (Antarctica). - Geochimica et Cosmochimica Acta 74:4740-4758.

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Research funding organisation

German Research Foundation

Project numbers: FR 2183-1
Funding period: 2005 – 2007