Ganderia–Laurentia collision in the Caledonides of Great Britain and Ireland

Abstract

Analytical procedures and results: U-Pb detrital zircon geochronologyDetrital zircons were analysed at the University of Alberta Radiogenic Isotope facility. Samples were crushed and zircons were separated and concentrated by standard techniques using a Wilfley table, heavy liquids and magnetic separator. The grains were ablated using a New Wave Research UP213 Nd:YAG with aperture imaging system. The wavelength was 213 nm, with a fluence of 3 Jcm -2 , a 4 Hz pulse rate and a spot size of 40 µm. Ablated ions were analysed with a Nuplasma multicollector ICP-MS with plasma power, gas flows, detector configuration and isotope measurements as described by Simonetti et al. (2005). Results from each sample site were recorded in 30 one-second integrations after a settling time of 3 s. Blanks and standards were recorded for the same duration as unknowns.For the Southern Uplands samples, analysed first, separated zircons were picked, avoiding cracked or altered grains, and ~200 grains were mounted in an epoxy mount and polished to a depth required to approximately expose grain centres. Ablation points were selected using a combination of electron backscatter images, reflected, and transmitted light, so as to avoid obvious inclusions, discontinuities and cracks. Grains were analysed in sequences of 10 or 12, preceded and followed by at least two analyses of standards. Results were normalized using the in-house standard LH94-15 with isotopic ratios determined by thermal ionization mass-spectrometry (TIMS) as quoted by Simonetti et al. (2005), yielding a concordant age of 1830 Ma.

The Southern Uplands Terrane in Scotland – a notional controversy revisited

“…However, provenance evidence from the Northern Belt of the Southern Uplands makes this unlikely at the terrane boundary. The derived (in sediment mass flows) shelly faunas in the Kirkcolm Formation have relatively locally, in situ correlatives in the Midland Valley Terrane, albeit in Ireland, whilst detrital mineral grains and zircon dating make a Dalradian provenance likely for all of the sandstones of the Southern Uplands Terrane (Clarkson et al 1992;Hutchinson & Oliver 1998;Mange et al 2005;Waldron et al 2008Waldron et al , 2014 Ireland. There, a high-resolution, airborne conductivity survey proved spectacularly successful in delineating the thin zones of carbonaceous mudstone in the DownLongford extension of the Southern Uplands Terrane, including concealed inliers with no apparent surface exposure (Beamish et al 2010).…”

“…The picture is further complicated by a likely Dalradian provenance for most of the detrital zircons in Southern Uplands sandstones (e.g. Waldron et al 2008Waldron et al , 2014 Midland Valley conglomerates. These were derived from the south but from a provenance that is not represented within the currently exposed Southern Uplands Terrane, a circumstance established by a wide range of evidence and discussed by, amongst others, Bluck (1983Bluck ( , 1984, Syba (1989), Stone & Evans (2001) and Phillips et al (2004).…”

“…Although no Portpatrick Formation sandstones were analysed, Waldron et al (2008) were not convinced of the southerly, Avalonian origin for Neoproterozoic zircons that had been proposed by Phillips et al (2003). Instead, Waldron et al (2008) Setting aside the volcaniclastic components, the zircon data of Waldron et al (2008Waldron et al ( , 2014 was seen as strongly supporting a broadly Dalradian provenance for much of the Southern Uplands 'continental' sediment. This had been favoured by Hutchison & Oliver (1998) …”

“…Further evidence from detrital zircon populations was provided by Waldron et al (2008Waldron et al ( , 2014. In the first of these papers data was presented from five of the lithostratigraphic divisions of the Leadhills Supergroup where, within sandstones from the Kirkcolm, Galdenoch, Glenwhargen and Shinnel and Glenlee formations an abundance of Archaean and Proterozoic zircons were related to potential sources within the Laurentian basement.…”

The deep crustal magnetic structure of Britain

“…(Kimbell & Stone 1995;Phillips et al 2003). See subsequent discussion by Waldron et al (2008Waldron et al ( , 2014 (Allen & Jackson 1985;Pratt et al 1995;Howells & Smith 1997). The age of the Rosslare gneisses is not known, but they have a complex metamorphic history which predates intrusions dated at 650 Ma (Winchester & Max 1982 Bott et al (1972) suggested magnetic Scourian gneiss at shallow depth but this was not supported by a seismic refraction experiment (Hall & Al-Haddad 1979).…”