Plutonic complexes exposed within the onshore part of the continental Oslo paleorift reveal geometrical relationships which give important clues for the geodynamic evolution of the rift. Field relationships show that the Larvik batholith consists of 10 plutons, which are very similar in appearance and composition, and occupy a very large portion (40 km east - west) of the southern part of the exposed Oslo rift (Petersen, 1978). The easternmost and oldest plutons (1-2) in this batholith are quartz-bearing larvikites, whereas progressively younger larvikite plutons (3-8) were emplaced towards the west. These are approximately silica saturated or contain minor amounts of nepheline. Nepheline syenites are, however, present along the western margin of pluton 6. The last two plutons, the lardalites (nepheline-rich feldspathic rocks), were emplaced in the northwestern part of the batholith. Pegmatites are abundant in all parts of the batholith and the field relationships and the pegmatite mineralogy generally suggest a close genetic relationship to the host plutons. The Larvik batholith was emplaced into a crust composed of Proterozoic gneisses of Sveconorwegian (= Grenvillian age). The nepheline syenite occurring along the western margin of pluton 6 was emplaced into a sequence of lower Paleozoic shales and limestones, and subordinate late Carboniferous nephelinite / basanite lavas.
The various plutons of this composite batholith were choosen for detailed U-Pb isotopic age determination for two reasons: 1. To explore the resolution of TIMS U-Pb isotopic dating of a sequence of petrologically similar plutons forming a single batholith. 2. To set strict time constraints on this part of the rift evolution. Furthermore, the Lu-Hf isotopic compositions of the same zircon and baddeleyite fractions as used for dating, and Pb-isotopes of accessorial galena from the various plutons, have been used to put constraints on the source and evolution of the magmas which evolved to the larvikite - lardalite plutons.
Skeletal zircon were recovered from the quartz-bearing larvikite pluton 2. Abundant baddeleyite and zircon were recovered from nepheline-bearing larvikite plutons 5 and 6, and their associated pegmatites contained large zircons, some of which were skeletal and up to 12 cm long. The lardalite pluton 9 contained abundant zircon and baddeleyite. The skeletal zircons demonstrates that the zircons were crystallized in situ, and the precise U-Pb ages obtained are younging in accordance with field relationships between the plutons: The larvikite pluton 2 was emplaced at 298.6 ± 1.4 Ma, the lardalite pluton 9 at 292.1 ± 0.8 Ma and the other plutons yield ages in between. Thus the entire plutonic complex was emplaced during a period of 5-6 Ma, and is likely to have occurred during one of the most intense tectonic extensional periods in the Oslo paleorift evolution.
The isotopic composition of Hf is similar throughout the complex (eHf generally in the range 6.7-5.5), and only minor variations in the Pb isotope composition of accessorial, primary galena was found for the different. The uniform Hf and Pb isotope signatures suggest that it is unlikely that the quartz- and nepheline-bearing plutons were formed by different degrees of crustal contamination or were derived from different mantle sources. Differences in crystallization conditions, probably involving polybaric differentiation, is more likely.
Petersen, J.S., Geol. Rundschau 67, 330-342 (1978).