Tracing Archaean terranes under Greenland's Icecap: U-Th-Pb-Hf isotopic study of zircons from melt-water rivers in the Isua area

The Archaean gneisses of the Nuuk area (southern West Greenland) are partitioned into tectonostratigraphic terranes - blocks of arc-like crust that evolved independently until they coalesced by collisional orogeny. To 'map' terranes to the east under the Inland Ice, sand samples were taken from rivers issuing from the edge of the Icecap; three from the Isua area and one from ~20. km to the south. Bedrock along this part of the ice front consists of ~3820-3600. Ma amphibolite facies rocks. 40. km south of Isua (Kapisilik terrane in the Ivisaartoq area) and also from ~10. km to the north there are Mesoarchaean amphibolite facies gneisses (3070-2950. Ma) exposed at the ice front. In the moraine fields in the Isua area there are erratic blocks of granulite facies gneisses. These were sourced from a hidden terrane to the east under the ice because no such rocks are exposed in the Isua area.The majority of the zircons from the sand samples yielded close to concordant U-Pb ages. Apart from one 2414. Ma grain, all are Archaean. The Isua sands show ~2695, 2710 and 2730. Ma; then 2790, 2805 and 2840. Ma clusters with a few grains back to 2950. Ma and then a complex 3440-3960. Ma spectrum. Less than 1% of the grains have ages between 3000 and 3400. Ma. In the Isua sands, Neoarchaean and late Mesoarchaean zircons form >50% of the population. The sand 20. km to the south shows a similar span of zircon ages, but there is a 2960. Ma peak not seen in the Isua sands.As the Eoarchaean and late Mesoarchaean-Neoarchaean cycles progress, the spread of Th/U and e{open}_Hf values seen in each zircon age population increases. This is interpreted as repeated addition of juvenile material to the crust, but also with increasing amounts of high temperature tectonothermal recycling of older materials within a package. However, there is no isotopic evidence for any contribution of Eoarchaean crust to the later ≥2750Ma Archaean cycle(s), showing evolution in disparate terranes. Furthermore, no evidence for Hadean material beneath the ice is seen in either the U-Pb ages or Hf isotopic compositions of the detrital zircons. On the other hand, 2750-2500Ma metamorphic zircons show a spread of initial e{open}_Hf values of zero to -25, with the most negative indicating zircon growth in Eoarchaean rocks. Overall these results closely mirror those obtained from direct sampling of rocks of the Archaean terranes exposed beyond the Icecap.The zircon data indicate that Mesoarchaean rocks equivalent to the Kapisilik terrane are absent under the ice. Instead, zircons were sourced from a late Mesoarchaean to Neoarchaean terrane affected by high grade metamorphism, whose age spectra perfectly matches the Tasiusarsuaq terrane to the southeast of Nuuk. Guided by the zircon results and aeromagnetic signatures, a terrane map is presented that extrapolates the Tasiusarsuaq terrane northwards under the Inland Ice towards the Tuno terrane in the north of the craton. Implications for different syntheses on the evolution of Archaean crust in southern West Greenland are discussed.