Geology of the Sivorg Terrane, Heimefrontfjella, (East Antarctica), and new U-Pb zircon provenance analyses of metasedimentary rocks.

The Sivorg Terrane is the largest crustal block of the Heimefrontfjella. It consists of a thick supracrustal sequence of metavolcanic and metasedimentary rocks that are intruded by a wide range of predominantly granitic plutonic rocks. The protolith ages of the metavolcanic rocks have been dated at ~1170-1140 Ma and the granitoid intrusions at ~1110-1050 Ma. The best estimate for Grenville-age metamorphism in the Sivorg Terrane is 1090-1060 Ma. Unlike the other two terranes in the Heimefrontfjella, the Sivorg Terrane records intense reworking of Mesoproterozoic rocks during the Late Neoproterozoic- Cambrian East African – Antarctic Orogeny. U-Pb detrital zircon provenance analyses from two samples indicate that at least two age-groups of different supracrustal sequences crop out in the Sivorg Terrane. The older, preorogenic sequence gave youngest detrital ages of ~1140 Ma, which are interpreted as dating the maximum deposition age of the original sediment. These rocks also provide evidence of a Palaeoproterozoic to Archaean foreland. The second sample is dominated by Mesoproterozoic to late Neoproterozoic detrital zircons, with a significant proportion of ages ranging from 1100 to 980 Ma. The youngest ages significantly postdate the Grenville-age metamorphism, so the sediments must have been deposited after or during the Late Mesoproterozoic orogenesis and, as such, might represent remnants of a molasse deposit of the orogen.