A NORWEGIAN MICROCONTINENT? EVIDENCE FROM TRACE-ELEMENT AND ISOTOPIC CHEMISTRY OF CLASTIC METASEDIMENTARY ROCKS

A large swath of the central Norwegian Caleonides is underlain by the Gula nappe complex (GNC), a composite thrust sheet of predominantly clastic metasedimentary strata. The GNC has been interpreted as a microcontinental terrane thrust onto the Baltoscandian margin in events leading up to the Siluro-Devonian continent-continent collision between Baltica and Laurentia. If so, this microcontinent may have originally rifted from either Baltica or Laurentia during breakup of Rodinia, but its origin is largely unconstrained. The GNC is subdivided into a higher-grade core dominated by calcareous psammite (Singsas Fm.), bounded by lower-grade pelite, metasandstone, and conglomerate (Asli Fm.). Absolute depositional ages of the metasedimentary rocks are unknown; however, an Early Ordovician subduction-related volcanic terrane (U-Pb zircon age of 476 +/- 6 Ma from plagiogranite) is juxtaposed with the eastern GNC, and some rocks of the Asli Fm. are intermixed with or overlie arc and back-arc volcanics, or are intruded by boninites of this terrane. Provenance of GNC metasedimentary rocks is explored using trace-element and isotopic geochemistry. Rare-earth element data 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, such as that exposed in the Western Gneiss region of Norway today. A possible scenario would have the GNC rifting from the Baltoscandian margin in the Late Proterozoic, drifting until Early Ordovician subduction of ocean crust produced a juvenile arc on the eastern margin (present-day coordinates), with related deposition of clastic material, and finally rejoining Baltoscandia in the ultimate continent-continent collision. Alternative hypotheses will also be explored.