page_title: Eoarchean geology
section_title: Acasta Gneiss Complex
hierarchical_section_title: Eoarchean geology / Acasta Gneiss Complex
caption_attribution_description: English: Acasta
context_page_description: Eoarchean geology is the study of the oldest preserved crustal fragments of Earth during the Eoarchean era from 4 to 3.6 billion years ago. Major well-preserved rock units dated Eoarchean are known from three localities, the Isua Greenstone Belt in Southwest Greenland, the Acasta Gneiss in the Slave Craton in Canada, and the Nuvvuagittuq Greenstone Belt in the eastern coast of Hudson Bay in Quebec. From the dating of rocks in these three regions scientists suggest that plate tectonics could go back as early as Eoarchean.
  All 3 regions contain an abundance of Archean felsic volcanic rocks, including tonalite, trondhjemite and granodiorite series rocks, with minor granulite to amphibolite facies gneiss complexes, which means that the original characters of the rocks has been disturbed by at least one ductile deformation at deep crustal conditions.
  Eoarchean geology is important in investigating earth's tectonic history. It is because the earth had just undergone an transformation to the present-day-similar convective mode and lithosphere from a magma ocean in Hadean Eon, to either a protoplate tectonics or an unstable stagnant lithosphere lid at its infant stages.
context_section_description: The Acasta Gneiss Complex is located in the western part of the Slave Province, and is well exposed along the Acasta river. The Acasta Gnessis Complex contains the oldest known felsic rocks on Earth, with ages up to 4.02 Ga but have rocks as young as 2.95 Ga. It is part of the Slave Province which covers an area approximately 190,000 km2.  After the initial documentation of very ancient zircons present in the Acasta River area, a significant scientific debate regarding the true age of these important rocks was born.  Some geologists suggested that all rocks in the Acasta region were highly metamorphosed and altered 3.3 billion years ago, so that their zircon ages were not reflective of the true ages of the rocks.  This debate culminated in a series of papers and comments regarding the discrepancy between zircon age information and whole-rock data.  The age debate has been mostly resolved by further work in the Acasta area by several research groups as well as the general acceptance by the scientific community of using in situ zircon U-Pb to obtain ages from complicated rocks.  Although complicated rocks, with multiple age domains mixed together, do certainly exist in the Acasta region, much simpler rocks are definitively present as well so the entire Complex was clearly not wholesale overprinted by 3.3 Ga metamorphism.  The oldest known rock unit in the Acasta region is a 4.02 Ga tonalitic unit termed the Idiwhaa Tonalitic Gneiss.  
  
  Notably, one xenocrystic zircon core, which was included in a 3.92 Ga gneiss, has been dated to 4.2 Ga, which is the oldest age recorded in the Acasta area.  However, the rock that originally grew this zircon has not been found, and it may not even exist anymore.  The ages of rocks in the Acasta Gneiss Complex have peaks at 3.92-4.02 Ga, 3.75 Ga, 3.6 Ga, and 3.4 Ga which document major crust forming events.
caption: Map of Acasta Gneiss Complex. Adopted and modified from Koshida et al., 2016
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