MAGMATIC SOURCE AND SEDIMENT PROVENANCE IN AN ORDOVICIAN SSZ TERRANE, NORWEGIAN CALEDONIDES, AS REVEALED BY ND ISOTOPES AND TRACE-ELEMENT CHEMISTRY

Numerous volcanic terranes of oceanic origin, produced during Paleozoic orogenesis, are now preserved in the upper allochthon of the Scandinavian Caledonides. Whole-rock geochemistry and Nd isotopes of metaigneous and related metasedimentary rocks in the south-central Norwegian Caledonides reveal that elements of several thrust nappes in the upper allochthon comprise an Early Ordovician (~490 Ma) supra-subduction zone terrane that likely formed proximal to Baltica. Boninitic dikes or lavas (Type A) are intercalated with calcareous sediments at the base of the sequence, and are also intermixed with Type B arc tholeiites. Sm/Nd relationships indicate a highly depleted mantle source, while e _Nd(t) values of -5.9 to +2.4 reflect influence of a subduction component. Type B arc tholeiites [e _Nd(t) +6.3 - +7.8] comprise the most abundant rock type. Nd isotopes indicate derivation from a depleted mantle source, with little input from subduction zone or continental sources. Splitting of the arc sequence was followed by extrusion of Type C back-arc basin(?) pillow basalts, hydrothermal vent activity, and deposition of metalliferous sediments. Sm/Nd relationships and e _Nd(t) of +7.3 - +7.6 indicate a highly depleted mantle source for the volcanics. Dikes having Type C chemistry also cross-cut Type B arc tholeiites. Lesser amounts of LREE-enriched magmatic rocks with e _Nd(t) ~+1 (Type D) are intermixed with and intruded into metalliferous sediments. Although their relationship to the other mafics rocks is not entirely clear, Type D magmas can be modeled by mixing between Type B magma and the related sediments.

Absolute depositional ages of the clastic 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. e _Nd(t) of -9.5 to -3.8 indicate variable mixing of juvenile components in the sediment provenance, whereas 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.