Geological and geochemical anatomy of Archaean gneisses from the Vestfold Hills and Rauer Group, East Antarctica

Archaean orthogneisses in the Rauer Group and Vestfold Hills have been examined through a combined field, petrological, geochemical and isotopic study in order to document and evaluate regional terrane correlations in the East Prydz Bay region of East Antarctica, and assess models for early Precambrian crustal growth, both regionally and in general terms. Within the Archaean domain in the Rauer Group, two layered ultramafic-mafic associations, the Torkler-Tango and the Scherbinina Layered Complexes (TTLC & SLC), are hosted in ca. 2800 to 3300-3470 Ma Composite Layered Orthogneiss (CLO). The SLC and TTLC are spatially, compositionally and probably temporally separate metaigneous suites. The TTLC is older than ca. 2800 Ma CLO intrusive sheets, and geochemical similarities between the TTLC and ca. 3300 Ma tonalite-trondhjemite-granite components of the CLO imply that there may be a genetic link between these gneiss units, as both have distinctive arc-like geochemical signatures. The SLC is also cross-cut by ca. 2800 Ma CLO components, but SHRIMP U-Pb zircon dating of the complex reveals that it is only marginally older with a crystallization age of 2844 ± 6 Ma. Geochemically, the SLC has very weak arc-like signatures which are attributed to small amounts of crustal contamination (by CLO), and primary geochemical signatures are inferred to have been derived from an E-MORB-like source. A tectonomagmatic setting similar to that deduced for the early Tertiary East Greenland Margin is envisaged as a possible analogue for events at 2844 Ma in the Rauer Group. However, petrogenetic models for the Rauer Group Archaean domain are tentative, because Sr and Nd isotopes indicate that significant open-system behaviour occurred during fluid infiltration associated with a regionally recognised Pan-African (ca. 500 Ma) event. Detailed mapping of intrusive relations that are preserved in Archaean othogneisses in the Vestfold Hills has allowed a reconstruction of the early high-grade geological history of the terrane. A number of spatially, temporally and compositionally distinct intrusions were emplaced throughout the Crooked Lake magmatic episode between D₁-M₁ and D₂-M₂. The duration of this episode has been precisely defined by SHRIMP U-Pb dating axial planar leucosomes to 2496.3 ± 0.7 Ma (D₁-M₁) and 2475.3 ± 0.7 Ma (D₂-M₂). The distribution of Crooked Lake Gneiss (CLG) indicates that regional-scale tectonic repetition of major units was local rather than regional, and the amount of strain associated with the main folding and fabric producing event (D₂) is heterogeneous.