Unravelling the metamorphic history of the Isua supracrustal belt: A garnet-based study in West Greenland

Abstract

The Eoarchean Isua supracrustal belt (ISB) in West Greenland comprises one of the oldest rock records, yet its geological evolution and geodynamic setting are still debated. Major order questions involve the timing and number of the tectono-metamorphic events leading to the formation of mineral assemblages and structures observed today. Interpreted cross-cutting relationships and crystallization ages of the Ameralik (mafic) dykes have been used to suggest an Eoarchean metamorphic age for the most pervasive event (e.g., Nutman et al., 2004, JGS 161, 421-430). Recent studies, in contrast, argue that this event is Neoarchean in age, as constrained via garnet (-plagioclase-hornblende) dating (e.g., Eskesen et al. 2023, Geology 51, 1017-1021; Ramírez-Salazar et al., unpublished data). In this study, we present data of eight garnet grains from two metapelitic samples from the eastern limb of the ISB to acquire further information about the metamorphic evolution. Both samples share a similar mineral assemblage, featuring a matrix predominantly composed of porphyroblastic garnet with ±chlorite, biotite, white mica, quartz, and ±plagioclase. The garnets exhibit a diversity of unevenly distributed inclusions dominated by quartz, plagioclase, white mica, chlorite, and ilmenite. Additionally, apatite, ±monazite, ±allanite, ±rutile, and ±zircon, as well as some iron-arsenide and -sulphide minerals, are abundant as inclusions. Textural evidence in combination with major and trace element zoning reveals three distinct garnet core to rim domains referred to as garnet I to garnet III. The porphyroblasts’ inclusion-rich cores (garnet I) are characterized by a bell-shaped spessartine component and heavy rare earth element zoning pattern, along with a relatively flat grossular and pyrope pattern. The garnet annuli (garnet II) are grossular-rich, with a constant or slightly increasing pyrope and spessartine components already at their minimum. Garnet II typically exhibits less inclusions. A drop in grossular together with an increase in pyrope components, and the virtual absence of inclusions mark the third garnet domain (garnet III). Our examination of the chemical, mineralogical, and textural characteristics across the different garnet domains reveals the growth of garnet through distinct mineral reactions under variable metamorphic conditions. These domains may be linked to either distinct metamorphic growth stages within a single event or disparate metamorphic events in the tectono-thermal history of the ISB. To unravel the chronological sequence of garnet growth, we present texturally resolved dating of individual garnet domains using the Sm-Nd isochron method.