Rolf-Birger Pedersen

U–Pb dating of detrital zircons for sediment provenance studies—a comparison of laser ablation ICPMS and SIMS techniques

Abstract New developments in U–Pb dating of zircons by laser ablation (LA) ICPMS are described and, for the first time, a direct comparison of detrital zircons dated by LA ICPMS and SIMS methods is presented. True real-time mass bias correction is made by aspirating a Tl/U tracer at the same time as laser ablation. The method is similar to that described in Horn et al. (2000) , except that enriched 233U rather than 235U is used in the tracer solution. Correction for laser-induced Pb/U elemental fractionation is based on a mathematical treatment of time-resolved data that is independent of laser ablation characteristics and does not require external standardisation. Internal corrections for mass bias and elemental fractionation eliminate the effects of variable sample matrix on isotopic ratios and improve the accuracy of U–Pb dating by laser ablation ICPMS. With the proper error propagation, the precision of U–Pb age determinations is only slightly worse than SIMS-based ion probe dating. However, LA ICPMS is capable of much more rapid analysis of the large number of zircons required for sediment provenance studies. There is excellent agreement between concordant laser ablation ICPMS and SIMS analyses of detrital zircons extracted from lower Silurian metasandstone from the Ulven Group (Skarfjell Formation) in the west Norwegian Caledonian nappes. Both LA ICPMS and SIMS U–Pb zircon ages indicate that sedimentary detritus of the Ulven Group was supplied from a terrain containing zircons of Archean, Proterozoic and early Ordovician age.

The use of mantle normalization and metal ratios in discriminating between the effects of partial melting, crystal fractionation and sulphide segregation on Platinum-Group Elements, gold, nickel and copper : examples from Norway

The distribution of noble metals, Ni and Cu in mafic and ultramafic rocks is thought to be controlled by sulphides, chromite, olivine and platinum-group minerals (PGM). One method for presenting noble metal, Ni and Cu data focuses on the sulphide control by recalculating the data to 100% sulphides and presenting the data chondrite normalized. The relative importance of the influence of sulphides, chromite, olivine and PGM on the noble metals, Ni and Cu is examined here using two alternative methods.

A U/Pb age for the Sulitjelma Gabbro, North Norway: further evidence for the development of a Caledonian marginal basin in Ashgill–Llandovery time

The Sulitjelma Gabbro in the central Norwegian Caledonides has been dated with the U/Pb zircon/sphene method to 437±2 Ma. Geological relationships as well as trace-element geochemistry and Nd and Sr isotopic compositions suggest that the gabbro and associated dyke swarms and pillow lava formed during the initial stages of back-arc spreading. The age constrains the timing of juxtaposition of an older island-arc terrane and continental-margin lithologies exposed within the Caledonian nappes. The age of the gabbro is similar to that of the Solund–Stavfjord Ophiolite in west Norway, and confirms that spreading-related magmatism and marginal-basin development took place along the margin of Iapetus in late Ordovician–early Silurian times.

Generation of anorthositic magma by H2O-fluxed anatexis of silica-undersaturated gabbro: an example from the north Norwegian Caledonides

The Skattora migmatite complex in the north Norwegian Caledonides consists of migmatized slightly nepheline-normative metagabbros that are net-veined by numerous (up to 90%) anorthositic and leucodioritic dykes. The average chemical composition of 17 anorthosite dykes is (wt %) 58.4% SiO 2 , 0.2% TiO 2 , 23% Al 2 O 3 , 1.8% FeO t , 0.7% MgO, 6.3% CaO, 7.8% Na 2 O, 0.2% K 2 O. A migmatite leucosome and a dyke have been dated by the U/Pb method on titanite to 456±4 Ma. In low melt fraction areas minor leucosomes are orientated parallel to the foliation. More intense anatexis formed stromatic to schlieric migmatites. The leucosomes are commonly connected to dykes, suggesting that melt segregated and left its source. Dyke thicknesses range from a few centimetres up to several metres. In general, early dykes are parallel to the foliation in the host rock, while the later dykes cut the foliation. Plagioclase (An 20–50 ) is the dominant mineral (85–100%) in the dykes and the leucosome, but 0–15% amphibole is generally present. Field relations, geochemistry and preliminary melting-experiments strongly suggest that the anorthosites originated by H 2 O-fluxed anatexis of the gabbroic host rock.

Contrasting tonalite genesis in the lyngen magmatic complex, north norwegian caledonides

Abstract The Lyngen gabbro (LG), defining the major part of the Lyngen magmatic complex, is characterised by layered gabbros of N-MORB affinity (western suite) and layered gabbronorites, quartz-bearing gabbros and diorites/quartz-diorites of IAT (island-arc tholeiite) to boninitic affinity (eastern suite). The boundary between the eastern and western suites is generally defined by a large-scale ductile shear zone of suboceanic origin, the Rypdalen shear zone (RSZ). Tonalites occur within the RSZ and in the eastern suite of the LG. Variations in field occurrence and chemical composition of the tonalites suggest that they represent two petrologically different groups. Tonalite intrusion (the Vakkas pluton) up to 5 km2 large occur in the eastern suite of the LG, and are characterised by high Y contents (average 26 ppm) and high K 2 O Rb ratios (average 0.062) compared to tonalites on the RSZ. The Vakkas pluton has lightly concave REE (rare earth element) patterns with negative Eu-anomalies, and positive eND-values (+3.7 to +3.9). Geochemical modelling based on the REE and field evidence suggests that these tonalites may have formed by fractional crystallization from a boninitic parental magma. Tonalites related to the RSZ form irregular veins and dikes that net vein the shear zone. They are characterised by low Y contents (average 6 ppm), low K 2 O Rb ratios (average 0.025), and highly variable contents of Na2O, K2O, Sr and Ba, compared to the Vakkas pluton. Tonalites related to the RSZ show substantial variation in the content of the LREEs. They possess low abundances of the HREEs, and absence of, or slightly positive Eu-anomalies. The tonalites have highly variable eND-values (−0.6 to −9.4), probably resulting from enrichment of Nd from an external source. Geochemical modelling suggests that the LREE-rich tonalites formed by H2O-rich partial melting of differentiated products from the eastern suite of the LG. The presence of B in the fluid phase is suggested by the presence of tourmaline-bearing tonalite pegmatites. Thus, the anatectic tonalites of this group could have been formed by water-excess melting of a variety of gabbroic cumulates of the LG. In the LG, LREE-depleted tonalites (eND-values +5.1) also occur, and these are best explained in terms of partial melting of gabbroic cumulates from the transition zone between the eastern and the western suites of the LG.