Complete hydrothermal re‐equilibration of zircon in the Maniitsoq structure, West Greenland: A 3001 Ma minimum age of impact?

Abstract

Abstract-Zircon in five samples of variably comminuted, melted, and hydrothermally altered orthogneiss from the Maniitsoq structure of southern West Greenland yield a weighted mean 207 Pb/ 206 Pb age of 3000.9 AE 1.9 Ma (ion probe data, n = 37). The age data constitute a rare example of pervasive and nearly complete isotopic resetting of zircon during a regional hydrothermal event. Many zircon grains are homogeneous or display weak flamelike patterns in backscattered electron images. Other grains show complex internal textures, where homogeneous, high-U fronts commonly cut across relict igneous-type oscillatory zonation. Inclusions of quartz, plagioclase, mica, and other Al AE Na AE Ca AE Fe-bearing silicates are very common. In two samples, selective replacement of zircon with baddeleyite occurs along concentric zones with relict igneous zonation, and as specks a few microns large within recrystallized, high-U areas. We interpret the 3000.9 AE 1.9 Ma date as the minimum age of the recently proposed impact structure at Maniitsoq. The great geographical extent and intensity of the hydrothermal event suggest massive invasion of water into the currently exposed crust, implying that the age of the hydrothermal alteration would closely approximate the age of the proposed impact at Maniitsoq. At the western margin of the Taserssuaq tonalite complex, which postdates the Maniitsoq event, a 207 Pb/ 206 Pb mean age of 2994.6 AE 3.4 Ma obtained from zircon has mostly retained igneous-type oscillatory zonation. A subsequent thermal event at approximately 2975 Ma is recorded in several samples by zircon with baddeleyite replacement textures.

Impact controversies: Impact recognition criteria and related issues

“…() recommended a detailed investigation of Maniitsoq zircon grains to investigate the possible presence of impact evidence in the form of planar fractures, shock‐induced granular texture, or twinning in zircon. The numerous zircon images shown in Scherstén and Garde (), however, fail to illustrate any textural evidence of shock deformation. While we explicitly appreciate the high‐quality U‐Pb ages, we must insist that there is still no tangible evidence for impact at Maniitsoq.…”

“…We were surprised to see the cover of the August 2013 issue of Meteoritics & Planetary Science (MAPS) adorned by a field image of a Greenlandic migmatite. The caption refers to the Maniitsoq structure in Greenland and explains that the lithology shown on the cover “is interpreted as due to a crustal‐scale hydrothermal convection cell in a now deeply exhumed Archean impact structure” (Scherstén and Garde ). We found this statement surprising, as the Maniitsoq structure (Garde et al.…”

“…To present such a statement as that related to the MAPS cover—highlighting this structure, in our view gives unnecessary credit to a (as we insist on calling) proposed but unconfirmed impact structure. The article by Scherstén and Garde (), to which this cover refers, contains high‐quality U‐Pb data for zircon, which are interpreted as hydrothermal re‐equilibration of the isotope systems as the result of an impact event. Reimold et al.…”

“…Planar is the key term in planar deformation features , so that the description of “commonly curved, coarsened, and partially annealed” features as “planar features” (p. 1474) by Scherstén and Garde () is not consistent with the published definitions. Their features are not imperfectly preserved, but have never been planar.…”

Apatite: a U-Pb thermochronometer or geochronometer?

“…Specifically, Garde et al (2000) proposed that the AkiaTerrane developed by formation of a c. 3100 Ma magmatic arc over a Paleo-to Mesoarchean crustal core, followed by voluminous emplacement of Mesoarchean TTG gneisses and synchronous low-pressure granulite facies metamorphism and late-tectonic felsic magmatism. However, other models involving extra-terrestrial components have also been proposed for the Akia Terrane Keulen et al, 2014;Scherstén and Garde, 2013) whereby a giant bolide impact at c. 3000 Ma was responsible for a prolonged Mesoarchean thermal reworking, and resulted in a range of melting and deformation features across the region. The Akia Terrane has also been regarded as the crustal link between Archean mid-crust, high-gradeorthogneissamphibolite associations exposed in West Greenland, and the lower-gradegranite-greenstone associations of the Superior Province of eastern Canada (Bridgewater and Schiøtte, 1991;Wilton, 1994).…”