The origin(s) and geodynamic significance of Archaean ultramafic-mafic bodies in the mainland Lewisian Gneiss Complex, North Atlantic Craton
Guice, G; McDonald, I; Hughes, H; et al.MacDonald, J; Faithfull, J
Date: 9 March 2020
Article
Journal
Journal of the Geological Society
Publisher
Geological Society
Publisher DOI
Abstract
The geodynamic regime(s) that predominated during the Archaean remains controversial, with the
plethora of competing models largely informed by felsic lithologies. Ultramafic-mafic rocks
displaying distinctive geochemical signatures are formed in a range of Phanerozoic geotectonic
environments. These rocks have high melting points, ...
The geodynamic regime(s) that predominated during the Archaean remains controversial, with the
plethora of competing models largely informed by felsic lithologies. Ultramafic-mafic rocks
displaying distinctive geochemical signatures are formed in a range of Phanerozoic geotectonic
environments. These rocks have high melting points, making them potentially useful tools for
investigating Archaean geodynamic processes in highly metamorphosed regions. We present field
mapping, petrography, traditional bulk-rock geochemistry, and platinum-group element
geochemistry for 12 ultramafic-mafic bodies in the Lewisian Gneiss Complex (LGC), which is a highly
metamorphosed fragment of the North Atlantic Craton in northwest Scotland. Our data indicate that
most of these occurrences are layered intrusions emplaced into the tonalite-trondhjemitegranodiorite (TTG)-dominated crust prior to polyphase metamorphism, representing a significant reevaluation of the LGC’s magmatic evolution. Of the others, two remain ambiguous, but one (Loch an
Daimh Mor) has some geochemical affinity with abyssal/orogenic peridotites and may represent a
fragment of Archaean mantle, although further investigation is required. The ultramafic-mafic
bodies in the LGC thus represent more than one type of event/process. Compared with the TTG host
rocks, these lithologies may preserve evidence of protolith origin(s), with potential to illuminate
tectonic setting(s) and geodynamic regimes of the early Earth.
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