ND AND SR ISOTOPIC DATA ON CONGLOMERATE CLASTS IN THE SAUREN-TORGHATTEN NAPPE, NORTH-CENTRAL NORWAY: A RECORD OF OCEANIC PROVENANCE

The Sauren-Torghatten (S-T) Nappe, the lowermost nappe of the Helgeland Nappe Complex, is composed of oceanic crust fragments and metasedimentary rocks that originated from either Laurentia or a microcontinent. On the islands of Rødøy and Leka, ophiolitic fragments are unconformably overlain by metamorphosed conglomerate and sedimentary sequences. Sm-Nd and Rb-Sr isotopic data from cobbles in the basal conglomerates on Leka and Rødøy constrain the provenance of these cobbles and indicate potential correlations between the two areas.

On Leka, greenschist facies metaconglomerate and finer-grained metasedimentary rocks of the Skei Group unconformably overlie a weathered horizon on the Leka Ophiolite Complex (LOC). Skei Group clasts include gabbro, plagiogranite, limestone, greenstone, marble, turbiditic shale, and sandstone. A U-Pb zircon date of 497±2 Ma from the LOC provides maximum age for deposition (Dunning and Pedersen, 1988). Metamorphism at 475-480 Ma provides an upper age limit for deposition in the S-T Nappe (Barnes et al., 2006). On Rødøy, metasedimentary rocks overlie a fossil weathering zone on the Rødøy Igneous Complex (RIC). The metaconglomerates contain ultramafic, mafic, albitite, marble, and metapelitic clasts.

Initial eNd values of Skei Group gabbro and plagiogranite cobbles (+ 7.0 to + 7.5) overlap values for intrusive rocks of the underlying LOC (+6.9 to +8.0) and indicate these cobbles were locally derived. Initial eNd values of Rødøy marble, albitite, chlorite schist, and gabbroic cobbles (+0.1 to +1.8), however, are distinct from the underlying RIC gabbro (+4.4). Further work is needed to determine whether there is any overlap in isotopic composition of the RIC and overlying cobbles; nevertheless, both the mafic crust and conglomerate clasts from Rødøy have less radiogenic Nd isotopic compositions than do those from Leka. Sr isotopic data corroborate these differences between Leka and Rødøy suggested by the Nd isotopic data. Initial Sr values from Leka conglomerates (0.70603 to 0.70774) are consistently less radiogenic than the values from Rødøy conglomerates (0.70820 to 0.70858). We conclude that although the sequences exposed on Leka and Rødøy may be coeval, the Rødøy sequence was more continent-proximal at the time of ophiolite generation and sediment deposition.