Reappraising the Neoproterozoic ‘East Avalonian’ terranes of southern Great Britain

• Nd- and O-isotopes from Late Neoproterozoic form two distinct terrane groupings.
• Nd- and O-isotopes indicate the predominance of crustal source components.
• Isotopic compositions more closely resemble those of Ganderia in the Appalachians.

Inliers of late Neoproterozoic basement rocks exposed in southern Great Britain, described in summary herein, have been previously attributed to a series of terranes bounded by major crustal lineaments that form a dissected remnant of a single subduction complex. Together these terranes have been termed East Avalonia and interpreted as correlative with West Avalonia of the northern Appalachian Orogen. A review of age and Nd- and O-isotopic data shows an essentially ensialic complex with two broad overlapping divisions: (1) the Monian Composite Terrane of NW Wales and Fenland Terrane of eastern England that are characterised by felsic intrusions at around 615 Ma and have more evolved εNd(t) (− 7.5 to − 2.5), older TDM (1868 Ma to 1269 Ma) and generally more elevated δ18O (up to + 13.7‰); and (2) a more diverse assemblage represented by the Cymru, Wrekin and Charnwood terranes, including more substantive volcano-sedimentary successions and intrusions ranging in age between around 710 Ma and 566 Ma (in which 615 Ma magmatic rocks are not represented), less evolved εNd (t) (− 2.5 to + 3.5), younger TDM (1501–994 Ma) and lower δ18O (+ 6.5‰ to + 10.1‰). Comparison with other Neoproterozoic basement units throughout the northern Appalachian Orogen shows that both divisions from southern Great Britain have closer isotopic affinity with igneous components of Ganderia, which generally have evolved compositions interpreted to reflect an ensialic setting and the input of Gondwanan cratonic source components. They are mostly dissimilar from magmatic rocks of West Avalonia, which typically have less evolved compositions interpreted to reflect the contribution of juvenile Mesoproterozoic source components.

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