Spodumene Formation by Strong In Situ Fractionation of a Lithium Pegmatite in the Norwegian Caledonides

The Scandinavian Caledonides are generally considered an orogenic region with low lithium (Li) abundance, thus explaining the general absence of spodumene-mineralized pegmatites. However, in the Meløy-Glomfjord area of northern Norway several Caledonian Li-enriched pegmatites have been identified. The most evolved among these is the Ågskardet pegmatite, to date the only known spodumene-bearing pegmatite in the Scandinavian Caledonides. The Ågskardet pegmatite was emplaced in the metasediments of the Caledonian Rödingsfjället Nappe Complex, immediately above the Caledonian basal thrust, under high-grade amphibolite-facies conditions. The pegmatite exhibits complex mineralogical-chemical zoning, which is mapped as a wall zone at the contact with the host rocks, followed by intermediate and core zones. Spodumene crystals, ranging from 2 to 8 cm in size, are rare and restricted to randomly distributed quartz-rich domains within the core zone. Fine-grained albite zones containing abundant colored tourmaline occur discordantly across the intermediate and core zones. We aim to understand the pegmatite-internal fractionation that caused Li enrichment in micas, tourmaline, and quartz and formation of spodumene. Major and trace elements in micas, feldspar, tourmaline, and quartz from the different zones of the pegmatite analyzed by electron microprobe and LA-ICP-MS reveal a strong chemical zoning of the pegmatite body. However, the original albite zone melt was likely much richer in Li, as evidenced by the high Li content in mica, which can reach up to 2 wt.% Li. Despite this, spodumene formation did not take place during the final stage of pegmatite development. It is suggested that the crystallization of spodumene was suppressed due to the significant Li consumption by mica and tourmaline, whose crystallization was favored by elevated levels of H2O, F, and B in the albite melt. Bulk rock analyses of pegmatite zones and wall rocks indicate that the local Li enrichment of metasediments facilitated the formation of spodumene-bearing pegmatites in the otherwise Li-poor Scandinavian Caledonides.