Late Archaean granites of the Napier Complex, Enderby Land, Antarctica: A comparison of Rb-Sr, Sm-Nd and U-Pb isotopic systematics in a complex terrain

Abstract

Abstract Late Archaean granites have been identified within the northern part of the Napier Complex of Enderby Land, Antarctica, although intrusives of this age are not common elsewhere in the complex. The oldest intrusive (3070 ± 34 Ma), synorogenic granite at Proclamation Island, was probably derived from melting of felsic crustal rocks of igneous origin. Two younger granites were emplaced at about 2840 Ma and 2481 ± 3 Ma, and the latter has geochemical similarities with other late and post-orogenic intrusives elsewhere in the East Antarctic Shield. Because of their fractionated character, it is difficult to discern whether the granites were derived by melting of sedimentary or igneous protoliths, but a granulite-facies source is probable. TNdCHUR model ages of the three analysed granites suggest that the Napier Complex is the product of at least two temporally discrete episodes of continental crust formation. The granite precursors probably formed at about 3100 Ma, whereas most of the exposed Napier Complex is much older (back to almost 4000 Ma). Two of the granites appear to record a significant hiatus (in one case of about 600 Ma) between the formation of this crustal precursor and final emplacement. The 3100 Ma old episode of crustal formation was roughly synchronous with a widespread, intense, and high-grade tectonothermal event (D1-M1), which produced metamorphic fabrics and mineral assemblages that are widely preserved within the Napier Complex. It is therefore likely that this new crust formation, deformation and metamorphism are all attributable to a single tectonic episode, possibly related to emplacement of magma into the lower crust. The two younger granites were emplaced at about the time of two subsequent and less intense tectonothermal events (D2-M2 and D3-M3). Concordant Rb-Sr total-rock and U-Pb zircon ages, which are interpreted as emplacement ages, have been obtained for two of the intrusions. However, the third exhibits the unusual relationship of a Rb-Sr isochron age older than the U-Pb zircon age. In this case the Rb-Sr age is believed to date magmatic emplacement. The complex interpretation required for the zircon data is forewarned by a non-perfect alignment of analytical points, in contrast to the perfect analytical alignments produced by the isotopically coherent granites. Two of the three granites yield similar and relatively precise lower intercepts with the U-Pb concordia but these do not appear to have direct geological significance. The new isotopic data are combined with earlier results to derive an integrated tectonothermal evolution for the Napier Complex.