Polymetamorphism and zircon preservation in the Itsaq Gneiss Complex, SW Greenland

Our understanding of the geological history of early Archean crust is limited by poor preservation of igneous features in rocks that have experienced multiple metamorphic and deformation events. Thus, regions with the best preserved Eoarchean rocks, as for example, the northern part of the Itsaq Gneiss Complex (IGC) of Greenland, have been the most intensively studied. The IGC underwent metamorphism at ca 3.6 and 2.7 Ga (Nutman & Bennett 2018). The grade of 2.7 Ga metamorphism varies from granulite facies in the southern part of the IGC (Fӕringehavn terrane) to lower amphibolite facies in the north (Isukasia terrane). This study compares the preservation of zircon in rocks from both terranes of the IGC.

Zircon grains from granitic gneisses in the Fӕringehavn terrane have rounded igneous cores with weak oscillatory zoning, surrounded by well-developed light-CL metamorphic rims. The ²⁰⁷Pb/²⁰⁶Pb zircon age obtained by in situ Secondary Ion Mass Spectrometry (SIMS) of these grains is ca 3.64 Ga for the cores, with metamorphic rims recording an age of ca 2.7 Ga. The Isukasia terrane extends either side of the Isua Supracrustal Belt (ISB), rock samples were collected from both the outer (SSE of the ISB) and inner (NNW of the ISB) Isukasia sub-terranes (Nutman & Bennett 2018). Zircon grains from the outer sub-terrane have well preserved igneous morphologies with evidence of metamictisation and fluid alteration but little to no metamorphic rims. The ²⁰⁷Pb/²⁰⁶Pb zircon ages are scattered towards 2.7 Ga, interpreted as the time of metamorphism, with a subgroup at ca 3.79 Ga that is interpreted as a minimum age for magmatic zircon. However, as the samples collected in the vicinity yielded an age of 3.82 Ga (Nutman et al. 1999, Kielman et al. 2018), the age of ca 3.79 Ga may have been disturbed by subsequent events. Zircon grains from the inner sub-terrane of Isukasia have well-preserved igneous cores with oscillatory zoning. Rounding of pyramidal terminations and thin rims are due to metamorphism. The age of crystalization of the protolith as recorded by igneous zircon is ca 3.71 Ga. 

The difference in the degree of the metamorphism at 2.7 Ga is visible in the structures and preservation of zircon grains. In this example, rounded cores and well-developed metamorphic rims characterize granulite facies, whereas well-preserved cores with oscillatory zoning and thin metamorphic rims represent lower amphibolite facies.

This research was funded by NCN grant UMO2019/34/H/ST10/00619 to MAK

References
Kielman, R., Whitehouse, M.,Nemchin, A., & Kemp, A., (2018). A tonalitic analogue to ancient detrical zircon. Chemical Geology, 499, 43-57.
Nutman, A.P. & Bennett, V.C., (2018). The 3.9-3.6 Ga Itsaq Gneiss Complex of Greenland. In: Van Kranendonk, M.J., Bennett, V.C. & Hoffmann, J.E., (Eds.). Earth’s Oldest Rocks (2^nd ed.), Elsevier, 375-399.
Nutman, A.P., Bennett, V.C., Friend, C.R. & Norman, M.D., (1999). Meta-igneous (nongneissic) tonalites and quartz-diorites from an extensive ca. 3800 Ma terrain south of the Isua supracrustal belt, southern West Greenland: constraints on early crust formation. Contrib. Mineral. Petrol. 137, 364–388.