ND-ISOTOPE STUDY OF PROVENANCE PATTERNS ACROSS THE BRITISH SECTOR OF THE IAPETUS SUTURE

The Southern Uplands greywacke succession (Scotland) accumulated at th e Laurentian margin of the Iapetus Ocean. It was sequentially incorpor ated into an imbricate, accretionary thrust complex until closure of t he ocean. Thereafter the thrust belt propagated across the suture zone as a foreland thrust belt directed towards the hinterland of Avalonia . A foreland basin migrating ahead of the thrust belt was the depositi onal site for the southernmost Southern Uplands units and the Winderme re Supergroup (English Lake District). A Nd-isotope study has shown th at juvenile ophiolitic detritus was introduced into the oldest, mid-Or dovician, Southern Uplands greywackes before two distinct provenance a reas evolved: one supplying juvenile andesitic detritus in addition to a quartzo-feldspathic component, the other Proterozoic and exclusivel y quartzo-feldspathic. Bimodal composition continued into the early Si lurian but was overlapped from late in the Ordovician by greywackes wi th intermediate Nd-isotope composition. This was not a simple mixing e ffect since the andesitic component is not represented and the necessa ry juvenile component comes from granodioritic and felsitic lithologie s. Intermediate epsilon Nd values are then a consistent feature throug h the Silurian both in the younger strata of the Southern Uplands and in the earliest foreland basin turbidites of the Windermere Supergroup . The transition suggests cessation of volcanicity and erosion of deep er levels of the provenance terrane(s), possibly linked to the evoluti on of the basin system from active margin, accretion-related, to a for eland setting. To the north of the Southern Uplands terrane, beyond th e Southern Upland Fault, a Caradoc to Wenlock turbidite sequence occup ies inliers within the Midland Valley. The older greywackes contain ab undant juvenile ophiolite and plutonic detritus in addition to a quart zo-feldspathic metamorphic component; there are similarities with the most northerly part of the Southern Uplands. From the late Ordovician, epsilon Nd values systematically decline so that early Llandovery Mid land Valley greywackes are exclusively quartzo-feldspathic, derived fr om an ancient source indistinguishable in isotopic terms from that per iodically supplying the Southern Uplands. In general the Llandovery Mi dland Valley provenance was significantly more mature than that contem poraneously supplying the Southern Uplands. Thereafter, the Midland Va lley latest Llandovery and early Wenlock greywackes contain a higher p roportion of a juvenile component, and by the early Wenlock, greywacke s from the Midland Valley, Southern Uplands and Lake District terranes are similar in terms of epsilon Nd. A common provenance seems likely and suggests that by the mid-Silurian all three terranes were in close proximity.