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

Using zircon in mafic migmatites to disentangle complex high-grade gneiss terrains – Terrane spotting in the Lewisian complex, NW Scotland

“…Moreover, it is likely that not all mafic and ultramafic bodies formed by the same process (e.g. Rollinson and Gravestock, 2012;Guice et al, 2020).…”

“…However, as all of these protolith ages stem from the same locality, and not all mafic and ultramafic bodies necessarily share the same origin (e.g. Guice et al, 2020), further estimates of protolith ages are needed. Isotope dilution analysis of U-Pb in zircons from a mafic gneiss at Badcall (Assynt block) yielded a variety of ages, including 2711 Ma and 2701 Ma, interpreted by the authors to date granulite facies (Badcallian) metamorphism (Corfu et al, 1994).…”

“…All samples were collected from larger, internally coherent bodies of mafic material (e.g. Johnson et al, 2012;Guice et al, 2020, also see section 2.2) rather than the lenticular pods ubiquitous in the grey gneisses (Rollinson and Gravestock, 2012) or the mafic portion of compositionally banded gneisses. The precise petrogenesis and emplacement mechanisms for the protoliths is debated (see section 2.2), but our samples of mafic gneisses are basaltic in composition (whole rock data in Supplementary Material Table S1 and Fig.…”

Origin of ultramafic–mafic bodies on the Isles of Lewis and Harris (Scotland, UK): Constraints on the Archean–Paleoproterozoic evolution of the Lewisian Gneiss Complex, North Atlantic Craton

“…Ultramafic-mafic complexes occur throughout the granulite-facies Central Region, ranging from cmscale pods to km-scale complexes (O'Hara, 1961;Bowes et al, 1964Bowes et al, , 1966Davies, 1974;Rollinson and Windley, 1980;Sills et al, 1982;Rollinson and Gravestock, 2012;Johnson et al, 2016;Guice et al, 2018aGuice et al, , 2018bGuice et al, , 2020. The relative proportions of the ultramafic and mafic rocks vary dramatically between individual complexes, with some containing no ultramafic rocks and others comprising almost entirely these lithologies (Guice, 2019). With some exceptions, the ultramafic portions are generally metapyroxenite-dominated, form the stratigraphic base of individual occurrences and display primary magmatic layering (Sills, 1981;Sills et al, 1982;Guice et al, 2018aGuice et al, , 2020.…”

“…Within suites of co-genetic samples, the relative mobility of trace elements can be tested by plotting individual elements against those considered to be most immobile (e.g., Zr, Y and Yb) and calculating the R 2 value (Fig. 13; e.g., Cann, 1970;Guice, 2019). Given the possibility that the Maaruig and Loch Mhorsgail bodies were derived from a phase of magmatism distinct from Coltraiseal Mor and Beinn a' Chuailean (see Section 6.1), R 2 values are given (and element mobility tested) for these groups separately (Fig.…”