Assembling the laurentian craton: The age of a terrane boundary in the Lewisian Gneiss Complex at Loch Laxford

Category Paleontology and Stratigraphy
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Goodenough, Kathryn۱; Crowley, Quentin۲; Krabbendam, Maarten۱
Holding Date 04 October 2008

Recent work has suggested that the Lewisian Gneiss Complex of North-west Scotland comprises a collage of small-scale Archaean terranes, assembled and variably reworked during the Proterozoic. The Laxford Shear Zone is one possible terrane boundary, separating two crustal blocks with different protolith ages and metamorphic histories. To the south are granulite-facies TTG gneisses, cut by a swarm of Proterozoic dykes (the Scourie Dykes), which have undergone only limited Proterozoic deformation. To the north of the shear zone are amphibolite-facies granodioritic gneisses, also intruded by Proterozoic dykes, but showing extensive post-dyke deformation, migmatisation and metamorphism.
The Laxford Shear Zone itself forms a zone a few kilometres wide, along which early movement occurred before emplacement of the Scourie Dykes (the Inverian event). Later reactivation occurred after dyke emplacement (the Laxfordian event), chiefly along discrete, mappable shears. On the southern margin of the shear zone, within the granulite-facies Assynt terrane, lies a thin but laterally extensive association of strongly deformed amphibolites and garnet-biotite gneisses, interpreted as mafic meta-igneous and metasedimentary rocks. Along the shear zone itself are a variety of granitic sheets, which are syn- to post- Laxfordian deformation. These granitic sheets die out rapidly to the south of the shear zone, whereas to the north they grade into migmatitic gneisses.
If the hypothesis that the Laxford Shear Zone represents a terrane boundary is correct, then the two main questions are: when were the two terranes accreted, and what tectonic processes were operating at that time? To fully answer the first question, we have carried out U-Pb zircon dating, using both spatially constrained LA-ICPMS (zircon) and single crystal TIMS (zircon and titanite) methods. Dating of highly deformed granites and paragneisses within the shear zone provides clear evidence for a metamorphic event at c. 2500 Ma, taken to be the Inverian event. Since all the younger intrusions (Scourie Dykes, granitic sheets) cut across structures formed during this event, initial terrane assembly is considered to have occurred at c. 2500 Ma, possibly representing an important tectonic event within Laurentia.
The age of the younger Laxfordian event is constrained by dates for the granitic sheets. Our dates on these intrusions confirm the existence of two intrusive events, one at c. 1880 Ma and the other at c. 1775 Ma. Thick (up to 50 m) granitic sheets intruded at c. 1880 Ma are focused along the core of the shear zone. Magmatism at 1880 Ma has been recognised across the Laurentian continent, and may be related to a ’superplume’ event at this time, with magmatic activity focused at zones of weakness along craton margins. In contrast, the c. 1775 Ma event at Laxford is associated with more localised reworking, and may be correlated with a previously published c.1740 Ma titanite age for a metamorphic event.