DETRITAL RUTILE GEOCHEMISTRY, THERMOMETRY AND GEOCHRONOLOGY AS GUIDES TO PROVENANCE OF JURASSIC-PALEOCENE SANDSTONES OF THE NORWEGIAN SEA

This paper explores the potential for use of rutile geochemistry as a provenance tracer in Jurassic-Paleocene sandstones in hydrocarbon exploration wells from the Norwegian Sea. Previous studies in this area, concentrating on provenance-sensitive heavy-mineral ratios, garnet geochemistry, tourmaline geochemistry, and detrital zircon geochronology, established the presence of five distinct sand types (MN1, MN2a, MN3, MN4, and MN5), sourced from different parts of the Norwegian and Greenland landmasses to the east and west of the basin. Approximately 50 rutile grains from two samples of each of these sand types have been analyzed by laser ablation inductively coupled plasma mass spectrometry. Differences in Cr and Nb contents indicate that there are significant variations in the relative abundance of rutiles derived from metamafic and metapelitic sources, with Norwegian-sourced sandstones (MN1, MN3 and MN5) having higher proportions of metamafic rutile compared with Greenland-sourced sandstones (MN2a and MN4).

Application of single-grain Zr-in-rutile geothermometry illustrates variations in metamorphic grade of the rutile sources. MN1 and MN5 rutiles were mainly derived from lower amphibolite- or eclogite-facies metapelitic rocks of the Caledonian Nappe Domain of mid-Norway, whereas MN3 rutiles were largely sourced from amphibolite- or eclogite-facies rocks of the Western Gneiss Region and adjacent parts of the Caledonian Nappe Domain, where metamafic gneisses and eclogites are widespread. Upper-amphibolite-facies metapelitic rocks, probably the Nathorst Land Group of East Greenland, were largely responsible for MN2a rutile assemblages. MN4 rutiles were mainly derived from granulite-facies metapelitic rocks. SHRIMP U-Pb dating of the granulite-facies rutiles indicates they formed during the Caledonian Orogeny. Their source is probably the Krummedal sequence of East Greenland.

The development of rutile geochemistry as a provenance tracer is especially important given the stability of rutile in both diagenetic and surficial weathering conditions. The technique yields information that can be utilised to reveal the ultimate source-rock lithology and metamorphic facies, even in highly modified sandstones that may have lost most other provenance information.