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
