Persistence of melt-bearing Archean lower crust for >200 m.y.—An example from the Lewisian Complex, northwest Scotland

Abstract

Geochronological data from zircon in Archean tonalite–trondhjemite–granodiorite (TTG) gneisses are commonly difficult to interpret. A notable example is the TTG gneisses from the Lewisian Gneiss Complex, northwest Scotland, which have metamorphic zircon ages that define a more-or-less continuous spread through the Neoarchean, with no clear relationship to zircon textures. These data are generally interpreted to record discrete high-grade events at ca. 2.7 Ga and ca. 2.5 Ga, with intermediate ages reflecting variable Pb loss. Although ancient diffusion of Pb is commonly invoked to explain such protracted age spreads, trace-element data in zircon may permit identification of otherwise cryptic magmatic and metamorphic episodes. Although zircons from the TTG gneiss analyzed here show a characteristic spread of Neoarchean ages, they exhibit subtle but key step changes in trace-element compositions that are difficult to ascribe to diffusive resetting, but that are consistent with emplacement of regionally extensive bodies of mafic magma. These data suggest suprasolidus metamorphic temperatures persisted for 200 m.y. or more during the Neoarchean. Such long-lived high-grade metamorphism is supported by data from zircon grains from a nearby monzogranite sheet. These preserve distinctive trace-element compositions consistent with derivation from a mafic source, and they define a well-constrained U-Pb zircon age of ca. 2.6 Ga that is intermediate between the two previously proposed discrete metamorphic episodes. The persistence of melt-bearing lower crust for hundreds of millions of years was probably the norm during the Archean.

Geodynamic Implications of Synchronous Norite and TTG Formation in the 3 Ga Maniitsoq Norite Belt, West Greenland

“…This provides a mechanism for high temperature-low pressure granulite-facies metamorphism that commenced soon after the norite belt emplacement. High crustal temperatures may have also favoured assimilation of continental crust by the norite belt parental magmas, as crustal temperatures may have been close to the melting point of their host TTGs for 10s of Myrs (Perchuk et al, 2018;Taylor et al, 2020;Yakymchuk et al, 2020). This model is consistent with recent suggestions that TTGs in the Maniitsoq region might be derived from relatively low pressure melting of mafic rocks (Yakymchuk et al, 2020), without requiring over-thickening or subduction of mafic crust (Laurent et al, 2020).…”

Seeing through metamorphic overprints in Archean granulites: Combined high-resolution thermometry and phase equilibrium modeling of the Lewisian Complex, Scotland

“…The time of peak Badcallian granulite-facies metamorphism is generally quoted as ~2.7 Ga (e.g., Cartwright and Barnicoat, 1989), but the evidence is still equivocal after decades of study, mostly based on zircon geochronology. Whereas some recent studies (e.g., Crowley et al, 2015) provide evidence for two distinct periods of high-temperature metamorphism at ~2.7 and ~2.49 Ga, others such as Taylor et al (2020) point to an apparent This is the peer-reviewed, final accepted version for American Mineralogist, published by the Mineralogical Society of America. The published version is subject to change.…”

Origin(s) and geodynamic significance of Archean ultramafic–mafic bodies in the mainland Lewisian Gneiss Complex, North Atlantic Craton

“…with recent zircon geochronology conducted by Taylor et al (2020). In this scenario, it likely that some of the adjacent complexes studied here (i.e., Scouriemore and North Scourie Bay; or Ben…”