Isua Greenstone Belt

The Isua Greenstone Belt is an Archean greenstone belt in southwestern Greenland, aged between 3.7 and 3.8 billion years. The belt contains variably metamorphosed mafic volcanic and sedimentary rocks, and is the largest exposure of Eoarchaean supracrustal rocks on Earth. Due to its age and low metamorphic grade relative to many Eoarchaean rocks, the Isua Greenstone Belt has become a focus for investigations on the emergence of life and the style of tectonics that operated on the early Earth.

Overview The Isua Greenstone Belt, also known as the Isua supracrustal belt since it is composed primarily of supracrustal rocks, is located in southwestern Greenland, in the Isukasia terrane, near the Nuuk capital region. It forms the largest supracrustal enclave in the Itsaq Gneiss Complex, which predominantly comprises 3850 - 3600 million year old (Ma) felsic orthogneisses. The greenstone belt comprises two major sequences of metamorphosed mafic volcanic and sedimentary rocks, which were divided on the basis of zircon uranium-lead dating. These sequences are the 'southern terrane', which has an age of approximately 3800 Ma, and the 'northern terrane', which has an age of approximately 3700 Ma. The younger southern terrane is further subdivided into two subterranes: one predominantly comprising boninite-like metavolcanic rocks, and the other comprising tholeiitic and picritic metavolcanics. The Isua Greenstone Belt is bounded to the West by the Ivinnguit Fault, which divides the Eoarachaean Itsaq Gneiss Complex from younger (Mesoarchaean) rocks of the Akia Terrane. Elsewhere, it is bounded by felsic orthogneisses of the Itsaq Gneiss Complex. These show a similar age division to the supracrustal rocks of the Isua Greenstone Belt itself, with 3800 Ma gneisses to the south of the belt, and 3700 Ma gneisses to the north of the belt.

Scientific methods A large number of geological and geochemical methods have been applied to the rocks of the Isua Greenstone Belt. These include subdivision of the various lithologies and units within the belt using a combination of geological mapping and U-Pb zircon dating, typically using sensitive high-resolution ion microprobe (SHRIMP), analyses; major and trace element chemistry; structural analyses; geothermobarometry and metamorphic modelling using phase diagrams to determine metamorphic conditions; and a wide range of stable, radiogenic, and short-lived isotope systems.

Lithologies

The Isua Greenstone Belt comprises many different lithologies. The most abundant rock types are mafic metavolcanic rocks with a range of compositions from boninite-like to tholeiites and picrites. Though boninitic amphibolites at Isua are often interpreted as evidence for the action of plate tectonics, these are not true boninites and non-plate tectonic models can also account for their formation. Texturally, the mafic metavolcanics include pillow lavas and pillow breccias, which indicate that the lavas erupted subaqueously, and requires the presence of surface water during the Eoarchaean. More felsic volcanic compositions have been observed, but it is not clear whether these represent volcanic or sedimentary rocks, and the only examples of potential andesite are significantly weathered. The mafic volcanic sequences contain abundant meta-ultramafic rocks, including amphibolites, serpentinites, carbonated-peridotites and peridotite. The majority of these are widely accepted to be intrusive in origin, representing ultramafic cumulates. Some peridotite lenses have been interpreted as obducted mantle fragments, and used as evidence to support the operation of plate tectonics during the formation of the Isua Greenstone Belt. However, this interpretation is contested, and some studies suggest that all peridotites at Isua are cumulates, representing shallow level magma chambers and conduits with the volcanic sequences. Metasedimentary rocks include banded iron formation and detrital quartzite, likely representing a metamorphosed siliciclastic sedimentary rock. Although they do not form part of the supracrustal belt itself, the belt is hosted in and in places intruded by tonalite-trondhjemite-granodiorite (TTG) orthogneisses.

Tectonics The tectonic setting in which the Isua Greenstone Belt formed remains contentious. Ideas can be broadly divided into plate tectonic models, in which the belt formed in one of several possible tectonic settings that exist on the modern day Earth, and non-plate tectonic or non-uniformitarian models, in which the Isua Greenstone Belt formed in a tectonic regime that was different to the modern day Earth. Plate tectonic models can be further subdivided into those that argue that the Isua Greenstone Belt or parts of it represent an ophiolite, a sliver of obducted oceanic crust and mantle, and those that argue that the belt represents an accretionary prism, formed in a subduction zone. Non-plate tectonic models generally suggest a heat pipe or mantle plume origin for the belt. This forms part of a much broader debate about when plate tectonics emerged on Earth, and whether the Archaean Earth operated under a fundamentally different tectonic regime.

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