Article The Mesoarchean Amikoq Layered Complex of SW Greenland: Part 1. Constraints on the P–T evolution from igneous, metasomatic and metamorphic amphiboles Emil Aarestrup 1 , Taus R. C. Jørgensen 2 , Paul E.B. Armitage 3 , Allen P. Nutman 4 , Ole Christiansen 5 and Kristoffer Szilas 1 * 1 Department of Geosciences and Natural Resource Management, University of Copenhagen, Øster Voldgade 10, 1165 København K, Denmark; 2 Mineral Exploration Research Centre, Harquail School of Earth Sciences and Goodman School of Mines, Laurentian University, Sudbury, ON, Canada; 3 Mkango Resources Ltd, 550 Burrard Street, Suite 2900, Vancouver BC, Canada, V6C 0A3; 4 School of Earth, Atmospheric and Life Sciences, University of Wollongong, 2522, NSW, Australia; and 5 Kommune Kujalleq, Anders Olsensvej B 500, 3920 Qaqortoq, Greenland Abstract The metamorphic history of the Mesoarchean Amikoq Layered Complex within the Akia terrane of SW Greenland was characterised by electron microprobe mineral data and detailed petrography on 12 representative samples, integrated with zircon U–Pb geochronology and petrology. The complex intruded into a >3004 Ma supracrustal association now consisting of granoblastic metabasites with subordinate quartz-rich gneiss. Supracrustal host rocks contain a relict high-temperature assemblage of orthopyroxene–clinopyroxene (± pigeonite exsolution lamellae, exsolved at ∼975–1010°C), which is interpreted to pre-date the Amikoq intrusion. Cumulate to granoblastic-textured rocks of the main Amikoq Layered Complex range modally from leuconorite to melanorite, orthopyroxenite to harzburgite/dunite and rare hornblende melagabbro. Observed mineralogy of main complex noritic lithologies is essentially relict igne- ous with orthopyroxene–biotite and hornblende–plagioclase thermometers yielding temperatures of ∼800–1070°C. An anatectic zircon megacryst from a patchy quartzo–feldspathic leucosome hosted in an orthopyroxene-dominated Amikoq rock reflects local anatexis at peak metamorphic P–T conditions and yields an intrusion minimum age of 3004 ± 9 Ma. Field observations indicate local anatexis of orthopyroxene-dominated lithologies, possibly indicating a post-intrusion peak temperature of >900°C. The last preserved stages of retrogression are recorded in paragneiss plagioclase–garnet, biotite–garnet and host rock ilmenite–magnetite pairs (≤3 kbar and ∼380–560°C). The Amikoq Complex intruded a MORB-like crustal section and the former remained relatively undisturbed in terms of modal mineralogy. Preservation of igneous textures and mineralogy are related to an anhydrous, high-grade metamorphic history that essen- tially mimics igneous crystallisation conditions, whereas local high-strain zones acted as fluid pathways resulting in hydrous breakdown of igneous minerals. There is no evidence of equilibration of the intrusion at sub-amphibolite-facies conditions. Keywords: Archean, Amikoq, metamorphism, amphibole composition, amphibolite facies (Received 6 March 2020; accepted 3 September 2020; Accepted Manuscript published online: 10 September 2020; Associate Editor: Thomas Mueller) Introduction Important clues in unravelling early Earth geodynamics reside in the plagioclase-rich ultramafic–mafic layered complexes and asso- ciated supracrustal sequences found in Archean cratons such as the North Atlantic Craton (NAC) of southern Greenland (Szilas et al., 2015; McIntyre et al., 2019). For example, the Fiskenæsset anorthosite complex has been interpreted as arc-related cumu- lates (e.g. Polat et al., 2012). Similarly, the andesitic supracrustal rocks of calc-alkaline affinity could be associated with arc magma- tism (Garde, 1997, 2007; Klausen et al., 2017; Szilas, 2018; Szilas et al., 2012a, 2012b, 2016a, 2017). Conversely, the ultramafic– mafic norite-associated complexes of the Fiskefjord region, Akia terrane (Garde, 1997; Szilas et al., 2015), appear to have fractio- nated from a relatively dry melt formed by high degrees of partial melting pointing to a mantle plume origin (Szilas et al., 2018). The Mesoarchean ultramafic-mafic layered norite-associated Amikoq Layered Complex (ALC) in central Fiskefjord (Fig. 1) represents another piece in the jigsaw puzzle of the early Earth. Before petrological inferences can be extracted from this com- plex with any confidence, it is important to attempt to discern to what degree any igneous characteristics have survived the poly-metamorphic amphibolite–granulite overprint of the region (e.g. Friend and Nutman, 1994; Garde et al., 2000; Kirkland et al., 2018). The origin of the amphibole is especially important as these could, if igneous in origin, be linked to hydrous melts, plausibly of arc-affinity (e.g. Tollan et al., 2012). Indeed, amphibole from the amphibolite–granulite-facies Fiskenæsset *Author for correspondence: Kristoffer Szilas, Email: krsz@ign.ku.dk © The Author(s), 2020. Published by Cambridge University Press on behalf of The Mineralogical Society of Great Britain and Ireland Cite this article: Aarestrup E., Jørgensen T.R.C., Armitage P.E.B., Nutman A.P., Christiansen O. and Szilas K. (2020) The Mesoarchean Amikoq Layered Complex of SW Greenland: Part 1. Constraints on the P–T evolution from igneous, metasomatic and metamorphic amphiboles. Mineralogical Magazine 1–29. https://doi.org/10.1180/ mgm.2020.68 Mineralogical Magazine (2020), 1–29 doi:10.1180/mgm.2020.68