RAPID PRODUCTION AND EVOLUTION OF LATE ARCHEAN FELSIC CRUST IN THE VESTFOLD BLOCK OF EAST-ANTARCTICA

The aim of this study is to present a definitive chronostratigraphy, based for the first time on U-Pb zircon ages, for the Vestfold Block of East Antarctica. This region consists of four principal rock units, three of which are composed of felsic orthogneisses, and form the subject of this article. These orthognesis units comprise the previously defined Crooked Lake and Mossel gneisses, and a newly proposed felsic unit, the Grace Lake granodiorite. The entire gamut of significant felsic igneous activity, the two intense tectonothermal events which preceded the cratonisation of the Vestfold Block, and significant post-tectonic uplift were all accomplished during an approximately 50 Ma interval at the end of the Archaean. This conclusion is in marked contrast to a previously proposed model that such activity had occurred episodically over several hundred million years. The Mossel gneiss (predominantly tonalitic orthogneiss) has the oldest igneous precursors, with a significant age spread, from 2526 +/- 6 Ma (in the type area) to 2501 +/- 4 Ma. Precursors of the compositionally varied Crooked Lake gneiss (predominantly dioritic, monzodioritic and monzonitic rocks) also show a distinct range of crystallisation ages, from 2501 +/- 4 Ma in the type area, through 2493 +/- 5 Ma to 2484 +/- 6 Ma. Both units appear to show a general northwards migration of igneous activity. The Grace Lake granodiorite, which is defined here on field and isotopic criteria, yields a pooled mean age (from two rocks) of 2487 +/- 6 Ma. An undeformed, post-granulite-facies quartz diorite dyke was emplaced at 2477 +/- 5 Ma. The new isotopic data contain the age of the first major granulite-facies deformation (D1) to have affected the Vestfold Block to lie between 2501 +/- 4 Ma and 2487 +/- 6 Ma, and that of the second granulite-facies deformation (D2) to 2487 +/- 6 Ma. There is evidence of older precursors for some of the late Archaean gneisses. Zircon cores and xenocrysts together with Sm-Nd model ages indicate that the Grace Lake granodiorite was derived from crustal sources at least 2800 Ma old. The presence of inherited zircon in one of two analysed samples of the Mossel gneiss, and a spread of Sm-Nd model ages, implies derivation by the melting of at least two source components, one at least 2700 Ma old. In contrast, the apparent lack of inherited zircon in the Crooked Lake gneiss samples is consistent with derivation of their precursors either exclusively from the mantle, or by a brief two-stage melting process. The Vestfold Block chronology is completely different from that of the adjacent Rauer Islands region, indicating that they were not juxtaposed during Archaean times. Neither do any nearby Archaean terranes appear to have comparable chronostratigraphy with the Vestfold Block. At our present state of knowledge, only the Napier Complex of Enderby Land, and possibly the southern Prince Charles Mountains of MacRobertson Land contain any rocks of similar age. This makes the Vestfold Block a potentially useful element for theoretical reconstruction of Gondwanaland.