Geodynamic Implications of Synchronous Norite and TTG Formation in the 3 Ga Maniitsoq Norite Belt, West Greenland

Frontiers in Earth Science

Hugo K. H. Olierook¹  Christopher L. Kirkland¹  Carson Kinney²  Chris Yakymchuk²  Julie A. Hollis³  Pedro Waterton⁴  William R. Hyde⁴  Kristoffer Szilas⁴  Jonas Tusch⁵  David Zakharov⁶  Peter C. Lightfoot⁷  Nicholas J. Gardiner⁸ 

[1] Centre for Exploration Targeting, Curtin Node, School of Earth and Planetary Sciences, Curtin University, Perth, WA, Australia;Department of Earth and Environmental Sciences, Centre for Environmental and Information Technology (EIT), University of Waterloo, Waterloo, ON, Canada;Department of Geology, Ministry of Mineral Resources, Government of Greenland, Nuuk, Greenland;Department of Geosciences and Natural Resource Management, University of Copenhagen, Copenhagen, Denmark;Institut für Geologie und Mineralogie, Universität zu Köln, Köln, Germany;Institute of Earth Sciences, University of Lausanne, Lausanne, Switzerland;North American Nickel Inc., Toronto, ON, Canada;School of Earth and Environmental Sciences, Irvine Building, University of St. Andrews, St. Andrews, United Kingdom;

关键词: norite;    crustal contamination;    tonalite-trondhjemite-granodiorite and tonalite-trondhjemite-granodiorite-like gneisses;    Nd isotope;    Hf isotope;    zircon U-Pb dating;   

DOI  :  10.3389/feart.2020.562062

来源: DOAJ

【 摘 要 】

We present new data for the ∼3.0 Ga Maniitsoq Norite Belt of the Akia Terrane, West Greenland, with the aim of understanding its petrogenesis. The Maniitsoq Norite Belt is hosted in regional tonalite-trondhjemite-granodiorite (TTG) and dioritic orthogneisses, intruded by later sheets of TTG and granite pegmatites, and comprises two main rock types: plagioclase-rich “norites” and pyroxene-rich “melanorites”. Both norites and melanorites have high SiO2 contents (52–60 wt% SiO2), high bulk rock Mg# (0.57–0.83), and low TiO2 contents (0.1–0.7 wt%). Their trace element patterns are defined by depleted heavy Rare-Earth elements, highly enriched light Rare-Earth elements, negative anomalies in Nb, Ta, and Ti, and variable anomalies in Zr, Hf, and Eu. New zircon U-Pb geochronology data and previously published ages establish an emplacement age of 3,013 ± 1 Ma for the majority of the Maniitsoq Norite Belt, with magmatism continuing until 3,001 ± 3 Ma. This ∼12 Myr period of norite magmatism is coeval with an ongoing period of TTG production in the Akia Terrane. Norite Belt emplacement was closely followed by high temperature, low pressure granulite-facies metamorphism at ∼800°C and <9 kbar. These conditions imply high temperature gradients (>900°C/GPa) and that the norite magmas were emplaced into thin crust and lithosphere. Compositions of the norites and melanorites can be explained by derivation from a single mafic parental melt (∼13 wt% MgO), with the norites predominantly accumulating plagioclase and the melanorites predominantly accumulating pyroxene. Evidence from field relationships, the presence of xenocrystic zircon, major element compositions and combined trace element and Hf-isotope modelling suggests the norites were contaminated by assimilation of ∼20–30% continental TTG crust. Geochemical and Hf-Nd isotopic constraints indicate that the norite mantle source was depleted, and that this depletion occurred significantly before the emplacement of the norite magmas. Contemporaneous production of both TTGs and norite, their emplacement in thin crust, and the rapid transition to high temperature, low pressure granulite-facies metamorphism is best explained by their formation in an ultra-hot orogeny. Formation of norites in this setting may be restricted to >2.7 Ga, when geothermal gradients were higher on Earth.

【 授权许可】

Unknown