RIS ID
39237
Abstract
The ca. 3000 km2ItsaqGneissComplex of the Nuuk region, southernWestGreenland was the first body of pre-3600 Ma crust discovered. Such ancient gneisses are now also known elsewhere, but in total form only about a millionth of the modern crust. The other 99.9999% of ancient crust was destroyed by melting and erosion over billions of years. Understanding the origin of this oldest crust is hampered by metamorphism (repeatedly) in the amphibolite or granulite facies, and most of it having been strongly deformed. The ItsaqGneissComplex is dominated by polyphase grey gneisses derived from several suites of tonalites, granites and subordinate quartz-diorites and ferro-gabbros that were intruded between 3870 and 3620 Ma. The hosts to these were lesser volumes of supracrustal rocks (amphibolites derived from submarine basalts with some chemical sediments and rarer felsic volcanic rocks), gabbro-anorthosite complexes and rare slivers of >3600 Ma upper mantle peridotite. Tonalite protoliths that dominate the ItsaqGneissComplex resemble younger Archaean TTG (tonalite-trondhjemite-granodiorite) suites in terms of their major and trace element geochemistry. Their Sr and Nd isotopic signatures indicate that they were juvenile additions to the crust. Their bulk compositions, in comparison with other TTG suites and products from melting experiments, show that they formed by partial melting of eclogite facies hydrated mafic rocks with lesser contributions from metasomatised upper mantle. Analogous suites are now generated at convergentplateboundaries, if the subducting oceanic crust is young and thus hotter than average. In the ItsaqGneissComplex, younger (Neoarchaean) strong ductile deformation under amphibolite to granulite facies conditions obliterated much of the evidence of its Eoarchaean tectonic evolution. However, in the north of the Complex around the 35 km long Isua supracrustal belt, maximum metamorphic grade is lower (sub-migmatisation) and superimposed younger (Neoarchaean) deformation is less. Thus tectonic events during the ⩾3600 Maconstruction of the crust can be studied. The belt contains several panels that contain amphibolites derived from submarine basalts, chemical sediments and felsic-intermediate volcanic rocks. These panels are separated by ⩾3600 Ma mylonites and then folded. Nearby smaller supracrustal belts of amphibolites derived from volcanic rocks with chemical sediments are interleaved with upper mantle peridotite and layered gabbros. It is considered that the tectonic intercalation of these unrelated rocks reflects crustal shortening, driven by compression at convergentplateboundaries. The ItsaqGneissComplex and all other Eoarchaean complexes, present a strong case that the oldest continental crust was built at ancient convergentplateboundaries by intrusion of tonalites into tectonically intercalated suites of supracrustal rocks (predominantly basalt), gabbros and upper mantle peridotites.
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Publication Details
Nutman, A. P., Friend, C. R. L., Horie, K. & Hidaka, H. (2007). The Itsaq Gneiss Complex of Southern West Greeland and the Construction of Eoarchaean Crust at Convergent Plate Boundaries. In M. Kranendonk, R. Smithies & V. C. Bennett (Eds.), Earth's oldest rocks (pp. 187-218). Amsterdam: Elsevier BV.