Lu–Hf and Sm–Nd dating of metamorphic garnet: evidence for multiple accretion events during the Caledonian orogeny in Scotland

Abstract

Caledonian orogenesis in Scotland is currently interpreted in terms of a Mid-Ordovician arc–continent collision (Grampian event) followed by the Silurian collision of Laurentia with Baltica (Scandian event). Lu–Hf and Sm–Nd garnet ages of c . 475–460 Ma obtained from prograde garnets in metasedimentary successions and metabasic intrusions within the Northern Highland and Grampian terranes confirm that the Mid-Ordovician Grampian orogenic event was approximately synchronous in the two terranes. Lu–Hf and Sm–Nd ages of c . 450 Ma obtained from prograde garnets within the Moine Nappe of the Northern Highland terrane provide evidence for a hitherto unrecognized Late Ordovician regional metamorphic event. The existing two-stage Grampian–Scandian model for Caledonian orogenesis in northern Scotland is thus an oversimplification, and the new ages imply a more complex structural evolution. The restriction of the Late Ordovician and Silurian events to the Northern Highland terrane reinforces the suggestion that it was far removed from the Grampian terrane until juxtaposition following major end-Caledonian (Devonian) sinistral displacement along the Great Glen Fault. A similar record of Mid- and Late Ordovician metamorphic events within the Laurentian-derived Uppermost Allochthons of Norway has been attributed to episodic accretion significantly prior to Silurian continent–continent collision and closure of the Iapetus Ocean. Supplementary materials: Results of trace element analysis of the garnets by laser ablation inductively coupled plasma mass spectrometry are available at www.geolsoc.org.uk/SUP18583 .

How the Neoproterozoic S-isotope record illuminates the genesis of vein gold systems: an example from the Dalradian Supergroup in Scotland

“…In the Tyndrum area garnet-grade amphibolite-facies metamorphism was reached (Harte 1988). Peak metamorphism is constrained to 473-465 Ma (garnet and whole rock isochron Sm-Nd; Baxter et al 2002; Lu-Hf and Sm-Nd garnet and whole rock isochron; Bird et al 2013) supported by the age range of broadly syn-metamorphic intrusions (Auchlee granite 475 + 12 Ma U -Pb zircon; Insch gabbro 470 + 8 Ma U-Pb zircon; Dempster et al 2002;Oliver et al 2008).…”

Evidence from Rb–Sr mineral ages for multiple orogenic events in the Caledonides of Shetland, Scotland

“…The Caledonides of the North Atlantic region are the result of a series of orogenic events spanning the early Palaeozoic, culminating in the sinistrally oblique collision of three major continental blocks, Laurentia, Baltica and Avalonia, as the Iapetus Ocean closed (Pickering et al 1988;Soper et al 1992;Dewey & Strachan 2003;Bird et al 2013). The timing of tectonothermal events in such collisional orogens can be constrained by the isotopic dating of metamorphic mineral assemblages and igneous intrusions of known structural age.…”

“…Within various sectors of the North Atlantic Caledonides it has therefore been possible to differentiate deformation fabrics and mineral assemblages formed during early phases of Ordovician arc-continent collision from those formed during Silurian continental collision (e.g. Kinny et al 1999Kinny et al , 2003Roberts 2003;Kocks et al 2006;Bird et al 2013).…”

“…1). Following a flip in subduction polarity, renewed deformation and metamorphism at c. 450 Ma along sectors of the Laurentian margin may correspond to terrane accretion (Bird et al 2013) and/or flat-slab subduction (Dewey et al 2015). The culminating sinistrally oblique continental collision of Baltica and Laurentia during the Silurian and early Devonian formed the Himalayan-scale Scandian orogen (Gee 1975).…”

“…This study aims to (1) place constraints on the ages of structures and regional metamorphic events within Shetland,and (2) propose correlations with orogenic events recorded in other parts of the Caledonides. Bird et al 2013). Ages shown in grey are biotite-feldspar ages; all others are white mica-feldspar.…”