Synopsis

Whole-rock ɛNd data from early Ordovician, Skiddaw Group (English Lake District) sandstones support correlation of the northern (and oldest) part of the group’s outcrop (ɛNd range of −4.1 down to −8.5) with the Manx Group (Isle of Man) (ɛNd range of −4.7 down t o −7.2). Both groups were deposited on the Avalonian margin of the Iapetus Ocean, with constituent sediment derived from an earlier, possibly Precambrian, continental margin and volcanic arc situated further south in the Avalonian–Gondwanan hinterland. The Nd isotope data from the Skiddaw Group sandstones show a trend of increasing provenance maturity with time, possibly the effect of arc unroofing. The trend is diachronous, with relatively mature sediment being deposited in the south of the Lake District during the Arenig (ɛNd range for sandstones is −7 down to −9.3), whilst generally more juvenile sands fed into the north (ɛNd ranges up to −4.1). The mature sands subsequently extended northward and Llanvirn sandstones from the Skiddaw Group show the most consistently mature provenance characteristics (ɛNd range of −7.6 to −8.7). Manx Group data are relatively homogeneous with no clear temporal trends. Mudstone data from the Skiddaw Group divide into two populations separated by the Causey Pike Fault; there is no overlap in mudstone ɛNd across the fault. The more mature mudstones (ɛNd more negative than −8.1) lie to the south of the fault, an unexpected result in that the sequence there contains juvenile volcaniclastic interbeds. The ɛNd data rule out the adjacent parts of Avalonia as a possible sediment provenance. Instead, a more distant, Gondwanan provenance seems likely, with implications for basin geometry and the timing of rifting along the southern margin of the Iapetus Ocean.

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