Palaeoproterozoic terrane assembly in the Lewisian Gneiss Complex on the Scottish mainland, south of Gruinard Bay: SHRIMP U–Pb zircon evidence

Abstract

The so-called 'southern region' of the mainland Lewisian Gneiss Complex is shown to be a package of isotopically distinct tectonostratigraphic units, from south to north the Rona, Ialltaig and Gairloch terranes. These were deformed during and following their accretion to the previously assembled Gruinard, Assynt and Rhiconich terranes to the north. SHRIMP U/Pb zircon geochronology has identified the 2730 Ma granulite facies metamorphic signature of the Gruinard Terrane in a tonalitic gneiss from the Tollie antiform, which is in sheared contact with the Palaeoproterozoic Gairloch Terrane (Loch Maree Group and Ard Gneiss) along the Creag Bhan Belt. Current structural interpretations of the region suggest the tectonic contact may be folded over the Carnmore antiform, coinciding with the high-strain zone along the Gruinard Belt. South of the Gairloch Terrane the Archaean gneisses of the Rona Terrane show a quite different pattern to those of the Gruinard Terrane, yielding older zircon protolith ages (ca. 3135-2880 Ma), and no evidence for the 2730 Ma Gruinard granulite facies metamorphism. Instead they show evidence for metamorphism and melting at ca.

High-precision U–Pb dating of complex zircon from the Lewisian Gneiss Complex of Scotland using an incremental CA-ID-TIMS approach

“…Timing of the eclogite event is not directly constrained and timing of the granulite event is disputed. Based on ion-probe U-Pb dating of zircon it has been proposed that the granulites formed at c. 2490-2480 Ma (Love et al, 2010;Whitehouse and Kemp, 2010), but due to the inherent analytical uncertainty of the technique, an earlier c. 2760-2710 Ma event as previously suggested by others (Cohen et al, 1991;Corfu et al, 1994;Zhu et al, 1997) cannot be ruled out. Corfu et al (1994) attributed a c. 2490-2480 Ma age from the Assynt block as having commenced with granulite metamorphism, but with local rehydration leading to partial melting and retrogression.…”

“…The oldest protoliths of the Lewisian Gneiss Complex may span the Paleo-, Meso-and Neo-Archean (Corfu et al, 1998;Friend and Kinny, Gondwana Research xxx (2014) 2001; Love et al, 2010;Goodenough et al, 2013). They have been variably affected by a Neo-Archean granulite event termed the Badcallian (Park, 1970) and two lower grade Paleoproterozoic events termed the Inverian (Evans, 1965) and Laxfordian (Sutton and Watson, 1950).…”

“…One model initially proposed by Friend and Kinny (2001) and later modified by Kinny et al (2005) and Love et al (2010) states that up to eleven distinct terranes, most of which are separated by shear zones, exist in this relatively small geographic region. This argument is primarily based on the interpreted protolith and metamorphic ages in various fault bounded blocks.…”

“…Goodenough et al (2013) One striking feature of our multi-step partial dissolution zircon dataset is the unequivocal presence of both c. 2500 Ma and c. 2700 Ma components, sometimes evident within single grains. Importantly, if the granulite event in the Assynt block is taken as c. 2490-2480 Ma (Love et al, 2010;Whitehouse and Kemp, 2010), this would signify a difference in timing of granulite events affecting the Assynt and Gruinard blocks. Previously, the apparent absence of a c. 2700 Ma age from the Assynt block was one of the key criteria for its designation as separate terrane (Kinny et al, 2005).…”

Was the Precambrian Basement of Western Troms and Lofoten-Vesterålen in Northern Norway Linked to the Lewisian of Scotland? A Comparison of Crustal Components, Tectonic Evolution and Amalgamation History"

“…Deposition of the Loch Maree Group is constrained between 2.0 Ga, the youngest detrital zircons from a metagreywacke in the Gairloch area [112], and 1903 Ma, the age of the syntectonic Ard gneiss intrusion [102,110]. The latter was emplaced during the early stages of deformation associated with amphibolite to granulite facies metamorphism and interpreted to be related to the development of a subhorizontal shear zone net work.…”