Subduction or sagduction? Ambiguity in constraining the origin of ultramafic–mafic bodies in the Archean crust of NW Scotland

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

“…A suite of ultramafic-mafic complexes, including the Camas nam Buth occurrence at Scouriemore (a site of special scientific interest; SSSI), are most commonly exposed in the northern Central Region and occupy areas between 0.3 and 7.0 km 2 ( Fig. 1; Peach et al, 1907;O'Hara, 1961;Bowes et al, 1964;Davies, 1974;Sills et al, 1982;Rollinson and Gravestock, 2012;Johnson et al, 2012Johnson et al, , 2016.…”

“…may represent Archean suture zone(s) (De Wit et al, 1987;Anhaeusser, 2006a); layered intrusions associated with a range of geodynamic environments (Hoatson and Sun, 2002;Ivanic et al, 2010;Wang et al, 2015;Bagas et al, 2016); subduction-related sills emplaced into oceanic crust (Polat et al, 2009); fragments of arc-related oceanic crust (Szilas et al, 2014); the sagducted remnants of greenstone belts (Johnson et al, 2016); and mantle residues following high degrees of partial melting (Szilas et al, 2017). Some interpretations (e.g., the sagduction hypothesis; Johnson et al, 2016) are compatible with vertical tectonics, while others (e.g., the Archean ophiolites hypothesis; Anhaeusser, 2016a) are compatible with horizontal tectonic models for the Archean Earth. A deeper understanding of how different ultramafic-mafic complexes formed and the means to reliably determine whether or not any of them unambiguously represent Archean oceanic crust is central to answering the question of when plate tectonic processes began to operate on Earth.…”

“…Subsequent identification of a chilled contact at the margin of the complex led to a contrasting (and currently accepted) interpretation, whereby it was intrusive into the Nelshoogte Schist Belt (De Wit et al, 1987). The problem of ambiguous age relationships is exacerbated in high-grade cratonic regions, such as the NAC, where the field relationships may be complicated by long-lived, high-temperature metamorphism and partial melting, rather than primary (igneous) processes and relationships (Nutman et al, 2013;Johnson et al, 2016). Such complications are exemplified by the ultramafic-mafic complexes of the Akilia terrane (western Greenland), where detailed field observations by Whitehouse and Fedo (2003) found no evidence to support the original assumption that they pre-date the volumetrically dominant 3.85 -3.65 Ga tonalitic gneiss.…”

“…As a consequence of uncertain age relationships, the origin(s) of the ultramafic-mafic complexes in the Lewisian Gneiss Complex (LGC) -a fragment of the NAC in northwest Scotland -have been ascribed to a wide-range of geological and geodynamic environments, including: one or more layered intrusion(s) (e.g., Bowes et al, 1964); fragments of a pre-TTG, possibly oceanic, maficultramafic crust (e.g., Sills, 1981); accreted oceanic crust (Park and Tarney, 1987); or the sagducted remnants of Archean greenstone belts (Johnson et al, 2016). In this investigation, we present new detailed geological maps, field descriptions, petrography and mineral chemistry for two ultramaficmafic complexes in the LGC, namely the 7 km 2 Ben Strome Complex and 0.2 km 2 Geodh' nan Sgadan Complex.…”

New constraints on granulite facies metamorphism and melt production in the Lewisian Complex, northwest Scotland

“…While the northern and southern regions expose units recording mainly amphibolite facies assemblages, the central region is primarily comprised of granulite facies rocks (Peach et al, 1907;Sutton & Watson, 1951) and considered to represent relatively deep levels of Archean continental crust (Park & Tarney, 1987). Layered tonalite-trondjhemite-granodiorite (TTG) gneisses dominate the central region, and are intercalated with abundant sheets of metamorphosed mafic to ultramafic units, and relatively rare mica-rich supracrustal rocks (Cartwright & Barnicoat, 1987;Johnson et al, 2016;O'Hara, 1961O'Hara, , 1977O'Hara & Yarwood, 1978;Park & Tarney, 1987;Zirkler, Johnson, White, & Zack, 2012). These mafic and ultramafic bodies, which include metagabbro and pyroxene-rich cumulates, may be up to several 100 m thick and extend for many kilometres in length.…”