The complex age of orthogneiss protoliths exemplified by the Eoarchaean Itsaq Gneiss Complex (Greenland): SHRIMP and old rocks

Field studies integrated with cathodoluminescence petrography and SHRIMP U–Pb dating of zircons from >150 orthogneisses and metatonalites from the Eoarchaean Itsaq Gneiss Complex (southern West Greenland) shows that only a minority contain ≥3840 Ma zircons, whereas the majority carry only younger ones. Rocks containing ≥3840 Ma zircons vary from very rare single-phase metatonalites to more common complexly banded tonalitic migmatites. The former metatonalites have simple oscillatory-zoned ≥3840 Ma zircon with limited recrystallisation and overgrowth, whereas the more common migmatites have much more complicated zircon populations with both ≥3840 Ma and 3650–3600 Ma oscillatory-zoned zircon, more extensive recrystallisation and widespread complex core-rim multiple growth relationships.With only 100–160 ppm Zr in the tonalites and likely melt generation temperatures of >1000 °C, the experimentally determined zircon solubility–melt composition relationships established by other workers shows that the precursor melts to the Itsaq Complex tonalites were strongly undersaturated in zircon, thus any entrained xenocrystic zircon would have been rapidly dissolved. Therefore, the ≥3840 Ma oscillatory-zoned zircons crystallised out of tonalitic melt and gives magmatic age of the rock in which they occur.With an established igneous age of ≥3840 Ma established from such relationships, we interpret the correlated variation between the field nature of these rocks and their zircon petrography/age structure as due to superimposition onto ≥3840 Ma tonalite protoliths of variable amounts of heterogeneous strain, heterogeneous distribution of melt patches formed during in situ anatexis at up to ∼800 °C, plus granitic veining. This explains why geologically simple metatonalites have simple zircon populations, whereas complex orthogneisses have complex zircons. The large amount of integrated field, geochemical and zircon data rule out an alternative interpretation, that the ≥3840 Ma zircons represent an igneous xenocrystic component, present in younger rocks to varying degrees. If this were true, then the structurally simple (less reworked) rocks should still display complex zircon populations.Gneisses with ≥3840 Ma zircon are commonest on Akilia and neighbouring islands, in Itilleq fjord (∼65 km east Akilia) and on the north of Ivisaartoq (∼150 km northeast of Akilia). These include from Itilleq a 3891 ± 6 Ma gneissic tonalite (with minor neosome)—which is currently the oldest rock recognised in the Itsaq Gneiss Complex. Overall, the ≥3840 Ma tonalites are a widespread and unevenly distributed in the Itsaq Gneiss Complex, and they are a volumetrically minor component compared with ∼3800, 3750 and 3700 Ma tonalite generations.Using the subset of our data covering Itilleq and the neighbouring fjords, migmatite samples with ≥3800 Ma igneous zircon are mutually exclusive from migmatite samples with ∼3700 Ma igneous zircon. This suggests that prior to an amalgamation event followed by 3660–3600 Ma high-grade metamorphism, ≥3840 Ma tonalites might have resided in a terrane discrete from ∼3700 Ma tonalites. This is in accord with interpretation of the non-migmatised part of the Complex in the Isua area, where a terrane of ∼3800 Ma tonalites with a minor associated ≥3840 Ma component and a terrane with ∼3700 Ma tonalites were tectonically juxtaposed at ∼3660 Ma.