Constraining the P-T-t history of sapphirine bearing granulites and associated rocks from the 2.9 Ga Fiskenæsset Anorthosite Complex, SW Greenland

One of the best preserved Archean igneous complexes in the world is located in the Fiskenæsset region in SW Greenland and is known as the Fiskenæsset Anorthosite Complex (FAC). The region hosts a variety of high-grade rock types ranging from anorthosites (clino-orthopyroxene / amphibole bearing), metaperidotites, garnetiferous amphibolites, garnetiferous pyroxenites and chromitites. Several studies have interpreted this region to represent a subduction zone setting with hydrous recycling of lithosphere and arc magmatism operating as early as the Mesoarchean, and leading to the formation of the anorthosites (Windley et al., 1973). This interpretations is, however, highly debated. The aim of our project is to constrain the metamorphic history of the FAC post their igneous emplacement, which started as early as the Neoarchean based on metamorphic U-Pb zircon ages (Polat et al. 2010, Keulen et al. 2010). This temporally constrained knowledge of Neoarchean P-T history can be used as fingerprints of the prevalent geodynamic setting in the region and thereby provide insights into the likely formation environment of the FAC, and, more broadly, information on the tectonic processes operating in the Neoarchean.

Here, we report geochemical and petrological results for a group of highly aluminous, sapphirine-bearing amphibolites dominated by orthopyroxene, corundum, phlogopite and anorthitic plagioclase, along with associated garnetiferous amphibolites and granulites from the FAC. These metamorphosed mafic rocks occur as individual bodies and as enclaves within anorthosites. Using an integrated approach of petrography, detailed elemental mapping, geothermobarometry and phase equilibria modelling, we constrained the metamorphic P-T-t history of the terrain. The rocks have been subjected to a multistage metamorphic history with the mafic rocks metamorphosed to amphibolite (M1 metamorphism) at ∼5-7 kbar and ∼700°C  to granulite facies conditions (M2 metamorphism) at ca. ∼11-12 kbar and ∼900°C, forming corundum and eventually sapphirine during retrogression and cooling (M3). A K-rich fluid is further affecting these assemblages during retrogression leading to phlogopitic biotite (formation. Our results further show that the peak metamorphic event at lower crustal depths can be traced back to ~2.63 Ga from in-situ Lu-Hf garnet geochronology. The M3 event appears to be accompanied by metasomatism and a further objective of this study is to constrain the type and composition of the fluid(s) responsible through thermodynamic modelling. This will help to improve our understanding of Archean crustal metamorphic processes and, in particular, the role of element-redistributing fluids in the evolution of cratons.

References:

Windley (1973) Bulletin Grønlands Geologiske Undersøgelse, 106, 1-80

Polat A et al. (2010) Chem. Geol. 277(1-2), 1-20

Keulen A et al. (2010) GEUS Bulletin, 20, 67-70