Metasomatized and hybrid rocks associated with a Palaeoarchaean layered ultramafic intrusion on the Johannesburg Dome, South Africa

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

“…This question represents the biggest conceptual predicament associated with any interpretation that requires that the Ben Strome Complex preceded the TTG magmas. In the Johannesburg Dome, the Zandspruit ultramafic-mafic complex -an Archean layered intrusion that was emplaced into a greenstone belt and subsequently invaded by TTG magmas -preserves evidence of the metavolcanic rocks that it intruded, despite the exposed intrusion and intruded greenstone belt covering a combined area of less than 1 km 2 (Anhaeusser, 2015). Similarly, the granulite-facies Fiskenaesset Complex (Greenland, NAC), which comprises a series of arc-related, intrusive sills, preserves evidence for the extrusive units intruded by those sills (Polat et al, 2009).…”

“…Further, even if dateable minerals are present, the isotopic system of interest is often so disturbed by subsequent overprinting metamorphic events as to render isochron or regression analysis ambiguous and/or associated with unacceptably large errors (e.g., Timms et al, 2006). Some complexes, such as Zandspruit (Kaapvaal Craton; Table 1), are cross-cut by dateable rocks, providing straightforward field relationships and a quantitative minimum age for the formation of the ultramafic-mafic complex (Anhaeusser, 2015).…”

An evaluation of element mobility in the Modderfontein ultramafic complex, Johannesburg: Origin as an Archaean ophiolite fragment or greenstone belt remnant?

“…As spinels are used as a petrogenetic indicator in rocks of various ages and metamorphic grades (Wood 1990, Barnes andRoeder 2001), including in regions that have experienced multiple phases of amphibolite-to granulite-facies metamorphism (Kusky and Jianghai 2010, Szilas et al 2014, 2015, Guice et al 2018b), these data demonstrate that a rigorous assessment of spinel texture and composition should be undertaken prior to invoking any such interpretation. Despite the partial-to complete-alteration of spinel grains and significant element mobility identified using the bulk-rock data (see Sections 6.1.1 and 6.1.2), the Modderfontein Complex records spinel compositions that are interpreted as close to primary and that can therefore be utilised to aid petrogenetic interpretations.…”

“…1; Robb and Meyer 1995, Barton et al 1999, Poujol andAnhaeusser 2001). The trondhjemitic and tonalitic magma invaded, fragmented, metamorphosed and migmatised a pre-existing ultramafic-mafic crust that is now manifest as centimetre-scale xenoliths to kilometre-scale remnants enclosed within the Lanseria Gneiss (e.g., Zandspruit; see Anhaeusser, 2015). The 3.20-3.17 Ga hornblende-biotite tonalite (known as the Linden Gneiss; 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

“…We have also deciphered broad geological environments for their formation and considered how they relate, petrogenetically and temporally, to other ultramafic-mafic rocks in the LGC and broader NAC (Section 6.4). This adds to a number of recent papersfocusing on ultramafic-mafic rocksthat have successfully utilized similar approaches to extract crucial information about the evolution of various Archean cratons (Szilas et al, 2014(Szilas et al, , 2015Anhaeusser, 2015;Grosch and Slama, 2017;Guice et al, 2018aGuice et al, , 2018bGuice et al, , 2019Pinheiro et al, 2021). Such first-order geological constraints are vitally important if we are to successfully apply modern, high-precision isotopic constraints (e.g., Sm-Nd; Re-Os) to ultramafic-mafic bodies and, ultimately, utilize their potential to provide crucial insights into the geodynamic processes responsible for the creation and destruction of Earth's lithosphere during the Archean and Proterozoic Eons.…”