Petrologic and thermodynamic constraints on the petrogenesis of the Fiskenæsset Anorthosite Complex, SW Greenland: An anhydrous model for Archean anorthosites

Linnebjerg, Benjamin, Christiansen, Anja, Maier, Wolfgang D. ORCID: https://orcid.org/0000-0002-8654-6658 and Szilas, Kristoffer 2025. Petrologic and thermodynamic constraints on the petrogenesis of the Fiskenæsset Anorthosite Complex, SW Greenland: An anhydrous model for Archean anorthosites. Journal of Petrology 10.1093/petrology/egaf027

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Abstract

Here we present a new study of the petrology, geochemistry and thermodynamic modelling of the ⁓2.97 Ga Mesoarchean Fiskenæsset Anorthosite Complex (FAC) in southern West Greenland. Our results provide new constraints on the parental magma and the crystallization history of the complex with implications for the petrogenesis and the geodynamic setting of Archean anorthosites. Detailed logging, petrography, and mineral chemistry of an ⁓80-meter-long drill-core intersecting anorthosite at Majorqap Qâva, shows that the rock is nearly monomineralic and has homogenous plagioclase compositions averaging An87±1 throughout the drill-core. Based on textural relations,, the composition of mineral inclusions and thermodynamic modelling, we argue that the abundant amphiboles are solely metamorphic in origin and formed by the hydration and recrystallization of primary clinopyroxene. Thermodynamic modelling using Rhyolite-MELTS of various proposed parental magmas shows that the petrogenesis of the FAC rocks and similar high-An anorthosite can best be explained by crystallization of an anhydrous high-Al tholeiitic parental melt at shallow pressure (≤3 kbar), which results in early plagioclase saturation with a short interval of essentially plagioclase-only crystallization. Formation of such voluminous and homogenous anorthosites have further required frequent magma replenishment and physical sorting of the cumulates. Flotation of buoyant plagioclase is possible under anhydrous conditions only, and a process supported by previous studies of the FAC, the occurrence of snow-flake and megacrystic rocks (some with negative Eu-anomalies), and the large variation in Mg# of high-An anorthosites. The modelling results further demonstrate that the relatively evolved chromites (Cr# 46-67 and Cr/Fe2+ of 1-1.2) associated with the anorthosites cannot have co-crystallized with the high-An plagioclase. Instead, we propose that the anorthositic-chromitiferous rocks formed via either melt rock dissolution and replacement reactions in noritic-gabbronoritic cumulates, the injection of chromitite slurries into anorthosite mush, or injection of an anorthosite slurry into chromitite. Alternatively the chromitite compositions of the FAC experienced significant modification during metamorphism. The combined results of this study provide a genetic link between the FAC and tholeiites of the spatially associated Bjørnesund Supracrustal Belt, representing a shallow, dry and open subvolcanic system. We propose a new petrogenetic model in which the high-Al parental magma of the FAC derived from a more primitive picritic precursor, which ponded and assimilated mafic Archean crust in the lower to middle crust prior to final emplacement as plagioclase supersaturated melts in the upper crust. The emplacement of high-Al tholeiites resulted in massive anorthosite formation and feed the Bjørnesund Supracrustal Belt with melts. In contradiction with previous research, we argue against a hydrous subduction zone setting of the FAC and Fiskenæsset region around 3 Ga, suggesting simpler alternatively non-uniformitarian settings (e.g. ocean-plateau, rift, stagnant-lid). A similar model may apply for other Archean anorthosites, involving unique petrogenetic conditions of the Archean, facilitating high-degree melting of dry mantle, magmatic ponding, assimilation of mafic crust and generation of high-Al tholeiites.

Item Type:	Article
Date Type:	Published Online
Status:	Published
Schools:	Schools > Earth and Environmental Sciences
Additional Information:	License information from Publisher: LICENSE 1: URL: https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model, Start Date: 2025-03-21
Publisher:	Oxford University Press
ISSN:	0022-3530
Date of Acceptance:	17 March 2025
Last Modified:	01 Apr 2025 13:45
URI:	https://orca.cardiff.ac.uk/id/eprint/177316

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