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

Accepted version
Peer-reviewed

Type

Article

Change log

Authors

Taylor, RJM 
Johnson, TE 
Clark, C 
Harrison, RJ 

Abstract

jats:titleAbstract</jats:title> jats:pGeochronological 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.</jats:p>

Description

Keywords

37 Earth Sciences, 3703 Geochemistry, 3705 Geology, 3706 Geophysics

Journal Title

Geology

Conference Name

Journal ISSN

0091-7613
1943-2682

Volume Title

48

Publisher

Geological Society of America

Rights

All rights reserved
Sponsorship
Natural Environment Research Council (NE/P002498/1)
NERC NE/P002498/1