The Mesoarchean Amikoq Layered Complex of SW Greenland: Part 1. Constraints on theP–Tevolution from igneous, metasomatic and metamorphic amphiboles

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 igneous 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 essentially 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.

The Mesoarchean Amikoq Layered Complex of SW Greenland: Part 2. Geochemical evidence for high-Mg noritic plutonism through crustal assimilation

“…Several geochemical proxies provide clear evidence that the mafic host host rocks (including 'gabbronorites', amphibolites, mafic granulites etc. ; Aarestrup et al, 2020) and the ALC are not comagmatic. Hosting mafic host rocks have lower bulk rock Mg# (50.6-75.5) than undisturbed ALC lithologies (65.0-89.3), a feature also reflected in more evolved mineral compositions (Fig.…”

“…Modal layering on cm-scale has been observed in the field (Aarestrup et al, 2020). Mapping indicates that pyroxenite layers tend to tapper out laterally, i.e.…”

“…S3 in the Supplementary Materials. Minerals from individual samples have homogenous compositions indicative of metamorphic equilibration except for a melanorite from SM with poikilitic and zoned plagioclase of relict igneous origin (Aarestrup et al, 2020 The most abundant lithologies are orthopyroxene and plagioclase dominated and range in modal composition from fine to medium grained hornblende leuconorite to hornblende melanorite and through to coarse hornblende feldspathic orthopyroxenite (Aarestrup et al, 2020).…”

“…The ultramafic sequences: These ultramafic olivine-rich and orthopyroxene-rich elongated, lensoid bodies (Garde, 1997) have modal mineral compositions of peridotites (dunites, harzburgites, orthopyroxene hornblende peridotite) to pyroxenites/hornblendites (olivine orthopyroxenite, olivine orthopyroxene hornblendite) (Aarestrup et al, 2020). However, the most common ultramafic lithologies seem to be (hornblende) dunite-harzburgite-olivine orthopyroxenite-orthopyroxenite.…”

“…As opposed to the noritic parts, most ultramafic samples seem to have been subjected to metamorphic fluids, resulting in formation of chlorite (± amphibole? ), and it is dobious whether igneous textures are preserved (Aarestrup et al, 2020;Szilas et al, 2015aSzilas et al, , 2018.…”