Global C cycle perturbations recorded in marbles: a record of Neoproterozoic Earth history within the Dalradian succession of the Shetland Islands, Scotland

The Shetland Islands in Scotland contain a c. 12 kill thick, mostly siliciclastic Neoproterozoic succession that experienced deformation and greenschist- to amphibolite-facies metamorphism during the Ordovician-Silurian Caledonian Orogeny. Correlating these rocks with Neoproterozoic successions elsewhere has remained problematic. However, four marble units punctuate the succession: these are, in stratigraphic order, the Weisdale. Whiteness, Girlsta and Laxfirth Limestones, two of which record hallmark C-isotopic excursions diagnostic of Neoproterozoic time. The Whiteness Limestone is lithologically similar to the 635 Ma Marinoan post-glacial cap carbonate sequence and displays a delta C-13(carbonate) profile in which values decline front a range of 0 to +2 parts per thousand to a nadir of -5 parts per thousand and then rise towards and oscillate around 0 parts per thousand. The Girlsta Limestone is a 700-900 in thick marble unit With values of -9 parts per thousand to -11 parts per thousand and is interpreted as recording the late Neoproterozoic Shuram-Wonoka event. These correlations permit linking the Shetland succession to Neoproterozoic sections elsewhere and also show that rift-related rocks in Shetland post-date the Shuram-Wonoka event (post c. 600 Ma-pre c. 550 Ma) thereby supporting the concept of a diachronous phase of extensional tectonism, from e. 700 Ma in east-central Laurentia to near the close of the Proterozoic in northeastern Laurentia, prior to the opening of the lapetus Ocean.