HISTORY OF THE WORLD, PART 1: A 3.8-2.7 GA OVERVIEW OF THE AKULLEQ TERRANE, GREENLAND, AND CONTROVERSIES THEREIN

The Akulleq terrane in southwest Greenland potentially shows the entire range of Earth's Archean transitions. 1.1 billion years of Archean tectonism have left these rocks more severely modified than is popularly thought, however, requiring that they be carefully evaluated.

In the northeast is the 3.8-3.7 Ga, predominantly schistose-to-ultramylonitic Isua belt, the world's oldest known volcano-sedimentary section. Main protoliths are basalt (locally as pillow lavas), banded iron formation, dunite and tonalite. Carbonates and supposed volcanoclastics are now interpreted as metasomatic (Rosing et al. 1996). C-isotope data in these and probably correlative rocks further southwest have been interpreted as indicating life, but given high-grade metamorphism and the lack of direct dating, the age of these isotopic ratios is uncertain. Nevertheless, the Isua banded iron formations strongly suggest that life was established by 3.7 Ga.

Broad doming of the Isua belt and enclosing gneisses took place by ~3.5 Ga, in ductile and brittle stages: first as doming of the gneiss fabric, then by emplacement of the radial and concentric ~3.5 Ga Tarssartoq dykes. These mafic dykes, the first known to be intruded into felsic crust, demonstrate stability of the Isua region by that time, although not by development of a lithospheric root (White 2000) or granulitisation/dehydration. Post-Tarssartoq shearing in the western part of the gneiss dome subsequently overturned the western flank of the Isua belt.

The Akulleq terrane was sandwiched between the more extensive 3.2-3.0 Ga Akia and 2.9-2.8 Ga Tasiusarsuaq terranes by 2.7 Ga (Friend et al. 1996). Exposures along the shear zone forming the northern (Akia) terrane boundary demonstrate tectonic interleaving within the Akulleq terrane before final terrane juxtaposition. It is not presently clear how analogous these processes may have been to Phanerozoic processes of accretion due to plate tectonics. How was the relatively small Akulleq terrane constructed, and how were the three terranes accreted? Were subduction processes involved? The answers may shed light on the global ~2.7 Ga cratonisation event.