THE PROTEROZOIC DEFORMATION OF THE VESTFOLD HILLS BLOCK, EAST ANTARCTICA - IMPLICATIONS FOR THE TECTONIC DEVELOPMENT OF ADJACENT GRANULITE BELTS

The Archaean Vestfold Block occurs at the margin of the 1000 Ma mobile belt of East Antarctica, and shows a prolonged and very complicated Proterozoic deformational history, in which at least seven extensional and compressional deformational episodes alternated with emplacement of at least fourteen different dyke swarms. These swarms can be grouped into five major suites, each bracketed by predominantly compressional deformational events. The most voluminous dyke suites 4 and 5 were emplaced around 1380 Ma and 1250 Ma, respectively. The former is associated with a progressive sequence of extensional structures that evolved from pseudotachylites to ductile normal shears as heat flow increased in response to extensional processes and dyke emplacement. Similar increased heat flows probably accompanied the emplacement of suite 5. The subsequent 1000 Ma compressional events and associated recrystallisation of earlier structures and dykes at amphibolite facies grade, show a marked intensity increase towards the SW Vestfold Hills, where a second pegmatite suite was associated with rehydration of the granulites. In the Vestfold Hills, for a long period during the Mesoproterozoic the compressional and extensional stages alternated along an E-W axis. In the Rauer Group this direction parallels the strain gradient associated with 1000 Ma events, and is approximately normal to the outcrop trace of the envelope to the complexly folded Archaean-Proterozoic boundary. It suggests that Mesoproterozoic extensional and compressional events, including the formation of the 1000 Ma mobile belt, occurred along similar structural trends. In the NE half of the Rauer Group a large number of mafic dykes truncating older gneiss foliations can be recognized. Going W across the Rauer Group, the number of dyke types decreases and the older dykes are increasingly reworked by the 1000 Ma events. Most of these dykes can be correlated with the multiple dyke suites in the Vestfold Hills, and therefore, some of the granulite events that have been described as part of the 1000 Ma events, but which pre-date the emplacement of most dykes, probably correlate with the extensional events that formed between 1250 and 1400 Ma in the Vestfold Hills. This has major implications for the interpretation of the described decompressional P-T path in the Rauer Group which connects an upper P-T section that corresponds to the pre- to syn-dyke granulite events with a lower P-T section representing post-dyke granulite events, dated at 1000 Ma: Comparison with the Vestfold Block suggests that the path may be the aggregate result of several unrelated events within an extensional-compressional tectonic cycle. The Vestfold Hills Block serves as an example that much information about high-grade mobile belts may be obtained from adjacent 'stable' terrains, and that extreme care is required in interpreting the 'simple' decompressional P-T-t-deformation paths which are generally described in the mobile belt of East Antarctica.