SEDIMENT PROVENANCE IN THE HELGELAND NAPPE COMPLEX (HNC), NORWEGIAN CALEDONIDES: ND ISOTOPIC DATA AND IMPLICATIONS FOR ORDOVICIAN TECTONIC EVOLUTION

The Helgeland Nappe Complex is part of the Uppermost Allochthon, the farthest outboard of a series of nappes thrust onto Baltica during the Silurian-Devonian Scandian orogeny. The HNC in turn is divided into the Sauren-Torghatten (S-T), Lower, Middle and Upper nappes. The S-T and Middle nappes consist of dismembered ophiolites unconformably overlain by metaconglomerate, pelitic schist, psammite, and minor carbonate that probably formed outboard of Laurentia. Deposition of the supracrustal rocks of the S-T and Middle nappes must postdate emplacement of the ophiolites, one of which is dated at 497 Ma (Dunning and Petersen 1988). At least part of the metasedimentary sequence post-dates 482 Ma based upon the youngest detrital zircon ages in a pelitic schist. Both nappes have been metamorphosed to amphibolite facies at 475-480 Ma (Barnes et al. 2006), constraining deposition to approximately 480 Ma.

In the S-T Nappe, clasts from a basal conglomerate sampled on the islands of Leka and Rødøy have positive initial ?Nd (0 to +7.5). In the Middle Nappe, three conglomerate units have been identified. The basal conglomerate contains gabbro clasts with radiogenic initial Nd isotopic compositions that are indistinguishable from the gabbros upon which they are deposited (?Nd = +6.9 to 7.8). Initial Nd isotope ratios of a greenstone clast and of mafic schists from these lowest stratigraphic levels also are radiogenic. The upper two conglomerate units contain matrix and clasts with less radiogenic Nd isotopic compositions (?Nd = -0.8 to –6.5). Pelitic schists from both the S-T and Middle nappes have the lowest initial ?Nd (-11.8 to –15.5). We interpret the data to indicate that the basal conglomerate received clasts from the underlying oceanic crust, whereas the clast population in the upper conglomerates reflects an influx of cobbles of continental affinity. Some of the finer-metasedimentary rocks, particularly those near the base of the section, contain detritus primarily from oceanic sources, but the pelitic schists with more negative ?Nd are dominated by material from continental sources. These isotopic data document changing sediment provenance reflecting input from new source areas during collision and amalgamation of continental and oceanic fragments prior to emplacement upon Laurentia.