Eoarchean granulite-facies metamorphism in the Itsaq Gneiss Complex, southwest Greenland

Using petrography, in situ garnet Lu–Hf geochronology, garnet rare-earth element (REE) analysis, zircon U–Pb geochronology and phase equilibrium modelling, we provide unambiguous evidence for Eoarchean granulite-facies metamorphism in the northern Itsaq Gneiss Complex (IGC), southwest Greenland. In situ garnet Lu–Hf geochronology from two samples of variably migmatitic metabasic rocks least affected by subsequent (Neoarchean) reworking yield Lu–Hf isochron ages of 3641 ± 62 Ma (MSWD = 1.7, n = 45/67; all age uncertainties at 2σ level) and 3652 ± 69 Ma (MSWD = 1.8, n = 83/84) from garnet with REE patterns typical of single-stage prograde growth. From the same two samples, zircon grains with textures consistent with metamorphic growth give weighted-mean 207Pb/206Pb ages of 3620 ± 8 Ma (MSWD = 1.2, n = 45) and 3630 ± 8 Ma (MSWD = 0.6, n = 44), respectively. Phase equilibrium modelling constrains peak P–T conditions of Eoarchean (3640–3630 Ma) metamorphism to 8.3–9.0 kbar and 730–820 °C. The thermobaric ratios (T/P) of 800–1000 °C/GPa recorded by the investigated samples are considerably higher (warmer) than previously proposed for granulite-facies metamorphism in the northern IGC, and broadly similar to Archean metamorphic P–T data globally, with no evidence for the bimodality in T/P that characterizes younger metamorphism. Either subduction-driven metamorphism (and plate tectonics) did not operate in the Eoarchean, or the Eoarchean lithosphere had a rheology that prohibited exhumation of subducted rocks.