Re-evaluating ambiguous age relationships in Archean cratons: Implications for the origin of ultramafic-mafic complexes in the Lewisian Gneiss Complex

Abstract

Archean ultramafic-mafic complexes have been the focus of important and often contentious geological and geodynamic interpretations. However, their age relative to the other components of Archean cratons are often poorly-constrained, introducing significant ambiguity when interpreting their origin and geodynamic significance. The Lewisian Gneiss Complex (LGC) of the northwest Scottish mainland -a high-grade, tonalite-trondhjemite-granodiorite (TTG) terrane that forms part of the North Atlantic Craton (NAC) -contains a number of ultramafic-mafic complexes whose origin and geodynamic significance have remained enigmatic since they were first described. Previous studies have interpreted these complexes as representing a wide-range of geological environments, from oceanic crust, to the sagducted remnants of Archean greenstone belts. These interpretations, which are often critically dependent upon the ages of the complexes relative to the surrounding rocks, have disparate implications for Archean geodynamic regimes (in the NAC and globally). Most previous authors have inferred that the ultramafic-mafic complexes of the LGC pre-date the TTG magmas. This fundamental age relationship is re-evaluated in this investigation through re-mapping of the Geodh' nan Sgadan Complex (where tonalitic gneiss reportedly cross-cuts mafic rocks) and new mapping of the 7 km 2 Ben Strome Complex (the largest ultramafic-mafic complex in the LGC), alongside detailed petrography and spinel mineral chemistry. This new study reveals that, despite their close proximity in the LGC (12 km), the Ben Strome and Geodh' nan Sgadan Complexes are petrogenetically unrelated, indicating that the LGC (and thus NAC) records multiple temporally and/or petrogenetically distinct phases of ultramafic-mafic Archean magmatism that has been masked by subsequent high-grade metamorphism. Moreover, field observations and spinel mineral chemistry demonstrate that the Ben Strome Complex represents a layered intrusion that was emplaced into a TTG-dominated crust. Further to representing a significant re-evaluation of theLGC's magmatic evolution, these findings have important implications for the methodologies utilised in deciphering the origin of Archean ultramafic-mafic complexes globally, where material suitable for dating is often unavailable and field relationships are commonly ambiguous.

Assessing the Validity of Negative High Field Strength-Element Anomalies as a Proxy for Archaean Subduction: Evidence from the Ben Strome Complex, NW Scotland

“…The Ben Strome Complex ( Figure 3)-located 13 km SE of the village of Scourie ( Figure 2)-is one of several large ultramafic-mafic complexes to occur within the Central Region of the LGC [45,[76][77][78][79][80][81]. The origin and geodynamic significance of such complexes has remained enigmatic since they were first described [49], with a wide-range of interpretations suggested, including: fragments of a pre-TTG (possibly oceanic) crust that was invaded by TTG magmas [79]; accreted oceanic crust [53]; the sagducted remnants of greenstone belts [76]; or, as proposed most recently, deformed layered intrusion(s) emplaced into TTG gneiss [45]. Moreover, while the ultramafic-mafic rocks of the LGC have traditionally been regarded as sharing a common origin, [79], recent studies have suggested that the complexes may represent petrogenetically and/or temporally distinct phases of ultramafic-mafic Archaean magmatism [45,81].…”

“…have traditionally been regarded as sharing a common origin, [79], recent studies have suggested that the complexes may represent petrogenetically and/or temporally distinct phases of ultramafic-mafic Archaean magmatism [45,81]. [45]).…”

“…have traditionally been regarded as sharing a common origin, [79], recent studies have suggested that the complexes may represent petrogenetically and/or temporally distinct phases of ultramafic-mafic Archaean magmatism [45,81]. [45]). Structural measurements and stereonet plots are detailed in Guice et al [45]; (b-f) Field photographs detailing sample locations at specific outcrops marked on (a).…”

“…Located on the northwest Scottish mainland, the Ben Strome Complex is an ultramafic-mafic complex that covers 7 km 2 and has been subject to multiple phases of high-grade metamorphism. Although the precise age of the Ben Strome Complex is ambiguous, it has recently been interpreted as representing a deformed layered intrusion that was emplaced into tonalite-trondhjemite-granodiorite (TTG) gneiss between 3.0 and 2.7 Ga, with the ultramafic and mafic rocks representing metamorphosed cumulates [45]. In this paper, we present bulk-rock geochemistry, detailed petrography and mineral chemistry for a suite of ultramafic rocks, which, unlike local mafic, intermediate and felsic lithologies, have not experienced partial melting [45][46][47].…”

“…Although the precise age of the Ben Strome Complex is ambiguous, it has recently been interpreted as representing a deformed layered intrusion that was emplaced into tonalite-trondhjemite-granodiorite (TTG) gneiss between 3.0 and 2.7 Ga, with the ultramafic and mafic rocks representing metamorphosed cumulates [45]. In this paper, we present bulk-rock geochemistry, detailed petrography and mineral chemistry for a suite of ultramafic rocks, which, unlike local mafic, intermediate and felsic lithologies, have not experienced partial melting [45][46][47]. After discussing bulk-rock geochemical controls and the origin of the HFSE anomalies in the Ben Strome ultramafic rocks, we outline the implications for geochemical fingerprinting of ultramafic-mafic units in Archaean cratons and element mobility associated with high-grade metamorphism.…”