Lead isotope systematics of galena from pegmatites in the Larvik Plutonic Complex (LPC)

Abstract

The Larvik Plutonic Complex (LPC) is situated in the southernmost parts of the Carboniferous-Permian Oslo Rift, Norway. The alkaline complex is host to several suites of pegmatites ranging from miaskitic to agpaitic, and much is unknown of the origin and source of these pegmatites. Compared to granitic pegmatites, alkaline pegmatites are commonly intraplutonic and cogenetic to the host rock. This makes alkaline pegmatites such as the ones in the LPC important indicators of the evolution of the alkaline complexes as a whole, and towards our understanding of their formation and source. This study aims to utilize Pb-Pb isotope geochemistry on galena crystals from selected pegmatites in the LPC, to investigate possible sources to the pluton. The LPC pegmatite data was modelled using a 295 Ma paleoisochron, which represents the lead isotope composition at the time of formation. The 295 Ma paleoisochron is significantly more radiogenic than the upper mantle, indicating a need of another component to the source. This have been investigated by extrapolating the 295 Ma paleoisochron to the intercept point of the global average juvenile lithosphere, which gave an initial lead composition of ²⁰⁶Pb/²⁰⁴Pb ₌ 16.819, ²⁰⁷Pb/²⁰⁴Pb = 15.482 and ²⁰⁸Pb/²⁰⁴Pb = 36.490. The intersection is at ∼1140 Ma which correlates well with the duration of the Sveconorwegian orogeny that occurred between ∼1290–1000 Ma and gives μ₂^min of 13.880 and μ₂^max of 14.658 which shows that these values are realistic in terms of what can be expected growth of a LILE-enriched, crustal domain in such a system. Galena from hydrothermal ore at Konnerud has similar lead isotope compositions as in the LPC pegmatites, suggesting an almost purely magmatic source, whereas galena in the Kongsberg silver deposit is more radiogenic, suggesting contamination by Cambrian-Ordovician alum shale.