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Abstract:

The major and trace element geochemistry of mafic silicate minerals from alkaline pegmatites within the 10 ring sections of the Larvik plutonic complex, Oslo rift, southern Norway, have been studied to shed light on the complex evolutionary history of the pegmatites. Pyroxene compositions plot within the aegirine-diopside-hedenbergite ternary diagram, trending from the 50-50% Di-Hd tie line towards the 100% Ae endmember. Primary magmatic diopside and hedenbergite with minor aegirine occur primarily within miaskitic to low agpaitic pegmatites, with compositions ranging from Ae3Di45Hd51 to Ae89Di5Hd6, whereas aegirine and aegirine-augite are the primary magmatic clinopyroxenes in agpaitic pegmatites, with compositions ranging from Ae31Di27Hd42 to Ae90Di6Hd4. Secondary clinopyroxene, after amphibole or as a hydrothermal phase within fractures and vugs, is dominantly aegirine, with compositions ranging from Ae59Di16Hd25 to Ae99Di1Hd0. Primary amphibole compositions are dominantly calcic (edenite, ferro-edenite, hastingsite, magnesio-hastingsite, pargasite, and ferro-pargasite), with less common late-stage sodic-calcic amphiboles (katophorite, magnesio-katophorite, ferro-richterite, and taramite); sodic amphiboles (ferro-ferri-nybøite, ferri-nybøite) are only observed in agpaitic pegmatites. All early pegmatite mafic minerals in low agpaitic pegmatites have negative Eu anomalies (Eu/Eu* = 0.16–0.26), positive Ce anomalies, and low Nb/TaN and Y/HoN ratios. Sodic mafic silicates in highly agpaitic pegmatites are HREE-enriched, with Ce/YbN = 0.22–0.58 compared to Ce/YbN > 1 for all other primary mafic phases, are strongly depleted in MREE, and have only slightly negative Eu anomalies (Eu/Eu* = 0.66–0.75). On the basis of major and trace element geochemistry, it can be concluded that miaskitic and alkaline pegmatites in Ring Sections 1–8 within the Larvik complex have been derived from the same parental melt, whereas pegmatites in Ring Sections 9 and 10 have a distinct geochemical signature, suggesting a separate source.<br/>