ORIGIN OF AN ENIGMATIC METASEDIMENTARY TERRANE IN THE NORWEGIAN CALEDONIDES: CONSTRAINTS FROM TRACE-ELEMENT AND ISOTOPIC GEOCHEMISTRY

The Gula nappe complex (GNC) occupies a central position within the Norwegian Caledonide tectonostratigraphy, comprising an area of over 10,000 km2 of predominantly clastic metasedimentary strata. Recent interpretations place the GNC as a microcontinental terrane thrust onto the Baltoscandian margin in events leading up to the Siluro-Devonian continent-continent collision between Baltica and Laurentia. This microcontinent may have originally rifted from either Baltica or Laurentia during breakup of Rodinia, but its origin is largely unconstrained, as little modern research has been focused on the problem. The GNC is subdivided into the higher-grade Singsås Fm., dominated by calcareous psammite, and the lower-grade Åsli Fm., containing pelitic phyllite and schist, metasandstone, and conglomerate. An extensive supra-subduction zone terrane, formed ~490 Ma, is juxtaposed with the GNC, and rocks of the Åsli Fm. are associated with mafic metavolcanic and plutonic rocks that comprise a back-arc basin to this terrane. Absolute depositional ages of the metasedimentary rocks are uncertain; however, some are intermixed with or intruded by boninites, and some are intimately associated with or overlie arc and back-arc volcanics. Trace-element and isotopic geochemistry is utilized to explore the sediment provenance. REE patterns of the rocks indicate a dominant continental crustal component in the source, whereas Nd(t) of -9.5 to -3.8 indicate variable mixing of juvenile components in the sediment provenance. Depleted mantle model ages ranging from 1.55 to 1.7 Ga are consistent with a major contribution of sediment from crust generated during the Labradorian-Gothian orogenies. Considering Early Ordovician paleogeographic reconstructions of Baltica for the general time of deposition, a likely candidate for this source would be Gothian crust exposed in the Western Gneiss region of Norway.