The geodynamic evolution of Mesoarchean anorthosite complexes inferred from the Naajat Kuuat Complex, southern West Greenland

• Published in: Precambrian Research
• Main Authors: Hoffmann, J. Elis, Svahnberg, Henrik, Piazolo, Sandra, Scherstén, Anders, Münker, Carsten
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2012
• Subjects: Ameralik; Greenland; Naajat; Ameralik fjord
• Online Access: https://researchers.mq.edu.au/en/publications/f3481a16-51e4-4784-91ac-5ebe8a156074; https://doi.org/10.1016/j.precamres.2011.12.002; http://www.scopus.com/inward/record.url?scp=84855166597&partnerID=8YFLogxK

Layered anorthosite complexes are typical components of Archean crustal domains. However, the geodynamic settings in which they were emplaced are still discussed as geological relationships are often ambiguous. Here we report major, trace element and high-precision high-field-strength-element (HFSE) data for the recently discovered well preserved Naajat Kuuat Anorthosite Complex from the inner Ameralik fjord region, southern West Greenland. The dataset is complemented by the first combined Hf-Nd isotope analyses for Archean layered anorthosite complexes and U-Pb zircon geochronology. The data contribute to the small database on Archean layered anorthosite complexes and are used to unravel the origin of these complexes and the tectonic regime involved. Fractional crystallisation of olivine, pyroxene, plagioclase and possibly amphibole controls major and trace element variations in the layered intrusion. There are two groups of amphibolites: (1) a group with primitive mantle normalized trace element patterns are similar to those of MORB-like basalts and (2) typical island-arc tholeiites (IAT), apparently indicating an island-arc setting. Lu-Hf regression lines yield an age of 2985 ±59. Ma (MSWD 4) within the error of the Sm-Nd regression age of 2929 ±110. Ma (MSWD 17). The initial e{open}Hf(2985) for the Naajat Kuuat rocks range from +1.6 to +5.8 and the initial e{open}Nd(2985) range from +0.4 to +3.9, either indicating variably depleted mantle sources or variable degrees of crustal contamination. In contrast to most mafic assemblages, ratios of Nb/Ta are highly variable (7.85 to 18.6), reflecting fractionation and accumulation of amphibole, ilmenite and pyroxene. The MORB-like parental liquids have the highest Nb/Ta of ca. 18, consistent with a mantle source overprinted by melt-like components from subducting oceanic crust with high Nb/Ta. The accumulation of plagioclase forming the anorthosites and the primary fractionation of amphibole as well as the occurrence of high-Al basalts within the Naajat Kuuat complex ...