Constraining high-grade metamorphism in the Lewisian [abstract only]

The Lewisian complex of NW Scotland is dominantly composed of Archaean tonalitic to granodioritic gneisses, ultramafic bodies and minor metasedimentary components. Although the area is internationally well known and has been much studied for over a century, the recognition and precise timing of some high-grade metamorphic events has proven difficult to ascertain. This is partly due to repeated deformational and metamorphic episodes in the Palaeoproterozoic which overprint and obscure earlier events. We present data from both laser ablation (LA) ICP-MS and an adaptation of a U-Pb chemical abrasion ID-TIMS technique applied to multi-age component zircons from the Assynt (“Central”) block of this region. The new data reveal a previously unrecognised complexity and provide the first unequivocal proof of an Archean to Paleoproterozoic granulite metamorphic event in the Assynt area. LA-ICP-MS U-Pb dating has indicated a ca 2.8 Ga protolith age for a tonalitic gneiss with evidence for a ca. 3.5 Ga xenocrystic component (the oldest discovered in the UK). Non-conventional U-Pb ID-TIMS utilising a combination of high-temperature annealing followed by multi-step incremental dissolution on single grains allows identification of Pb-loss and multi-generational age trajectories on 206Pb/238U- 207Pb/235U plots. A combination of LA-ICP-MS and this non-conventional TIMS work has dated zircon growth at ca 2.7 Ga (“Badcallian”) and 2.5 Ga (“Inverian”) with later Pb-loss occurring at ca 1.9 Ga and ca 1.7 Ga (early and late “Laxfordian” respectively). This TIMS method is unique in that it combines a pseudo-spatial resolution normally associated with an in-situ technique but benefits from the highprecision analysis required to differentiate between these metamorphic events at ca 2.7 and 2.5 Ga. Zircon Hf isotopes indicate that some gneisses from the Assynt area are typical of Archaean continental crust (epsilon Hf ca -1). The tonalite gneisses however have strongly negative epsilon Hf values of -7 to -10 indicating a more complex history of derivation through partial melting of ancient crust with residual garnet as a long-lived control on Hf. Moreover, consistent zircon epsilon Hf values from inherited cores, igneous overgrowths and two separate metamorphic events indicate that the tonalitic gneisses were formed by crustal recycling, rather than new additions to the crust. These events may be summarised as: zircon crystallisation from a magma at ca 3.5 Ga, partial melting and crustal recycling producing the tonalite gneiss protoliths at ca 2.8 Ga, a prolonged lower crustal residence in granulite P-T conditions by ca 2.7 Ga, further metamorphism in amphibolite conditions at ca 2.5 Ga and later deformation associated with punctuated terrane amalgamation events between ca 1.9 Ga and ca 1.7 Ga. The occurrence of a 2.7 Ga metamorphic event preserved in gneisses from Assynt contradicts the assertion of some previous studies that it does not exist in this region and suggests at least some local terrane amalgamation occurred in the Archean.