PROVENANCE OF THE LAKESMAN TERRANE: DETRITAL ZIRCON GEOCHRONOLOGY OF THE SKIDDAW GROUP IN THE LAKE DISTRICT OF NORTHERN ENGLAND

The English Lake district is an inlier of Early Paleozoic rocks surrounded by an unconformable Late Paleozoic cover. Rocks range in age from possibly latest Cambrian to Late Silurian. The oldest unit in the region is the Skiddaw Group, which comprises mudstones and turbiditic sandstones. Abundant soft-sediment deformation structures, together with mass flow deposits (olistostromes or debris flows), indicate an unstable tectonic environment. It is overlain by Late Ordovician arc volcanics of the Borradaile Volcanic Group, which are in turn overlain by deep-water sediments and foreland basin turbidites of the Windermere Supergroup, associated with Iapetus closure.

Terrane assignments of the Lake District rocks have varied. In many discussions the Lake District is included in eastern Avalonia. However, the region is distinguished by markedly different stratigraphy from classic areas of Avalonia in central England and Wales. The Menai Straits fault system, a possible terrane boundary, projects beneath late Paleozoic cover to the south of the Lake District. Accordingly, the Lake District rocks, together with similar rocks from the Isle of Man, have been placed by some researchers in a separate Lakesman Terrane, correlated with Appalachian Ganderia.

The Skiddaw group is divided into several slices by probable Acadian thrusts. In the Northern Fells thrust slices, the stratigraphy is relatively simple; basinal mudstones are interleaved with lenticular units of turbiditic sandstone of Early Ordovician age, which were sampled for detrital zircon geochronology. Preliminary results suggest that Late Neoproterozoic sources contributed the largest proportion of grains. Early Mesoproterozoic zircon is next in abundance. Paleoproterozoic grains with ages > 2000 Ma are consistent with sources in the West African or Amazonia cratons of Gondwana. Zircons between 2.5 and 3.0 Ga indicate Archean cratonic sources. These preliminary results confirm a peri-Gondwanan provenance for the Lakesman Terrane and have implications for the relationship between Ganderia and Avalonia in the Early Paleozoic.