Chapter 3.3 The Itsaq Gneiss Complex of Southern West Greenland and the Construction of Eoarchaean Crust at Convergent Plate Boundaries

Abstract

The ca. 3000 km 2 Itsaq Gneiss Complex of the Nuuk region, southern West Greenland 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 Itsaq Gneiss Complex 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 Itsaq Gneiss Complex 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 convergent plate boundaries, if the subducting oceanic crust is young and thus hotter than average. In the Itsaq Gneiss Complex, 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 Ma construction 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 convergent plate boundaries. The Itsaq Gneiss Complex and all other Eoarchaean complexes, present a strong case that the oldest continental crust was built at ancient convergent plate boundaries by intrusion of tonalites into tectonically intercalated suites of supracrustal rocks (predominantly basalt), gabbros and upper mantle peridotites.