Abstract
Detrital zircons from four samples of upper Dalradian metasedimentary rocks from the Buchan Block in the NE Grampian Highlands of Scotland were analysed by laser ablation inductively coupled plasma mass spectrometry to establish their U–Pb age and trace element composition. The analysed grains (magmatic cores) mainly yield concordant ages ranging from Neoproterozoic to Eoarchaean. Kernel density plots of the data show pronounced peaks in the late Mesoproterozoic, Palaeoproterozoic and Neoarchaean eras. The data are indistinguishable from detrital zircon age spectra from Dalradian rocks elsewhere, an interpretation supported by application of a non-parametric multidimensional scaling algorithm, and are consistent with a Laurentian source. Similar to existing studies from other Dalradian rocks, the age spectra from the Buchan Block reveal an increase in the relative proportion of older detritus with time, suggesting derivation from late Mesoproterozoic (Grenville) then Palaeoproterozoic orogens before widespread exposure and denudation of their Archaean basement rocks. Application of a novel approach to estimate the most likely time of radiogenic-Pb loss indicates that some detrital zircon grains were affected by element mobility around 470–450 Ma as a result of Grampian orogenesis. Supplementary material: Laser ablation inductively coupled plasma mass spectrometry U–Pb and trace element data, a matrix showing results of Kolmogorov–Smirnov (K–S) test, cathodoluminescence imaging of zircons and selected trace element plots are available at http://doi.org/10.6084/m9.figshare.c.3153931
Highlights
IntroductionThe Dalaradian Supergroup of Scotland and Ireland is a thick sequence of Neoproterozoic to lower Palaeozoic rocks that were deposited along the eastern margin of Laurentia, deformed and metamorphosed during the complex Cambrian to Devonian Caledonian orogenic cycle, during the opening and subsequent closure of the Iapetus Ocean (McKerrow et al 2000; Chew and Strachan 2014)
Similar to existing studies from other Dalradian rocks, the age spectra from the Buchan Block reveals an increase in the relative proportion of older detritus with time, suggesting derivation from late Mesoproterozoic (Grenville) Palaeoproterozoic orogens before widespread exposure and denudation of their Archaean basement rocks
The Dalaradian Supergroup of Scotland and Ireland is a thick sequence of Neoproterozoic to lower Palaeozoic rocks that were deposited along the eastern margin of Laurentia, deformed and metamorphosed during the complex Cambrian to Devonian Caledonian orogenic cycle, during the opening and subsequent closure of the Iapetus Ocean (McKerrow et al 2000; Chew and Strachan 2014)
Summary
The Dalaradian Supergroup of Scotland and Ireland is a thick sequence of Neoproterozoic to lower Palaeozoic rocks that were deposited along the eastern margin of Laurentia, deformed and metamorphosed during the complex Cambrian to Devonian Caledonian orogenic cycle, during the opening and subsequent closure of the Iapetus Ocean (McKerrow et al 2000; Chew and Strachan 2014). Of the published detrital zircon age data from (meta)sedimentary rocks from the Dalradian Supergroup (Cawood et al 2003; Banks et al 2007; Chew et al 2009, 2010; McAteer et al 2010b; Strachan et al 2013), there are none from the Buchan Block, a structurally-bound package of upper Dalradian Supergroup rocks in north-east Scotland that exhibits profound differences in its sedimentological, structural, magmatic and metamorphic history when compared to the rest of the Grampian Terrane These differences have led some workers to regard the Buchan Block as an allochthonous crustal terrane comprising pre-Caledonian basement gneisses and cover rocks that were thrust into their current position during the Grampian phase of Caledonian orogenesis (Sturt et al.1977; Ramsay and Sturt 1978)
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