Magmatic evolution of the Permo-Carboniferous Oslo rift: U-Pb- and Lu-Hf-in-zircon evidence from the 299-289 Ma Larvik plutonic complex

The Permo-Carboniferous Oslo rift in southern Norway registers substantial late Paleozoic rifting and lithospheric thinning in the southwestern part of the Fennoscandian shield, presumably related to the waning stages of the Variscan orogeny. The Oslo rift comprises a > 500-km-long crustal segment of four polarity off-set half grabens (from the north to the south: Rendalen, Akershus, Vestfold, Skagerrak) characterized by varying magmatic activity during a period of ≤ 65 m.y. The magmatic evolution of the Oslo rift has been divided into five or six phases with the most voluminous igneous (mafic to intermediate) activity having occurred at the Permian-Carboniferous transition at ~300-275 Ma. This resulted in the emplacement of wide-spread basalts, latitic flows ("rhomb porphyries"), and corresponding intrusive rocks. We present new U-Pb- and Lu-Hf-in-zircon isotope data on the ~1000 km2 Larvik plutonic complex in the central part of the Oslo rift (southern flank of the Vestfold graben segment). Structural observations show that the Larvik complex consists of ten successively emplaced semicircular plutons (Rings 1 through 10) with the oldest (Ring 1) in the east and the youngest (Ring 10) in the west. Furthermore, the Larvik complex shows a systematic shift from weakly oversaturated magmatic compositions (quartz monzonite) in the east to undersaturated (nepheline-bearing monzonite and nepheline syenite) compositions in the west. Our LAMS U-Pb-in-zircon data (>900 spots on 23 samples from the ten ring plutons) imply an overall 10-m.y. emplacement history from 298±0.4 Ma (Ring 1) to 289.7±0.5 Ma (Ring 10) for the Larvik complex. Our data further display an overall decrease in emplacement ages from east to west, multiple intrusive events (297 Ma and 289 Ma) for Ring 5, and a comparatively old age (296.5±0.9 Ma) for Ring 9. These data show that the emplacement of the ten ring plutons probably took about twice as long as henceforth believed (5-6 m.y.) and that the locus of the main magmatic phase of the Oslo rift shifted from the eastern flank of the segment to the western flank of it. Our LAMS Lu-Hf-in-zircon data (~700 spots of 23 samples from the ten ring plutons) indicate an overall eHf (at 295 Ma) value of 7.4±3.2 (2-sigma). This value is clearly less unradiogenic than that of the contemporaneous model MORB mantle (eHf +15.4) and corresponds well with one of the previously proposed mantle source components of the Oslo rift (initial eNd value of ~+4). Assuming an ~ eHf +7, eNd +4 mantle component as the prime precursor of the (intermediate) larvikites, it is quite evident that these magmas were not substantially contaminated by unradiogenic crustal materials at the crust-mantle boundary. The individual plutons do not show systematic differences larger than ±2 to 4 eHf units, which further demonstrates that the evolution of the oversaturated and undersaturated magmas was not associated with marked crustal contamination (incorporation of unradiogenic Hf) during emplacement.