MAFIC PLUTONS IN THE CALEDONIAN BINDAL BATHOLITH, NORWAY

The gabbroic to monzonitic Velfjord plutons were emplaced into the middle crust (6 to 8 kbar pressure) during the Caledonian orogeny. U-Pb (zircon) ages (448 ± 2 Ma) show that these plutons were among the oldest mafic intrusions in the Bindal Batholith. Heat from the plutons caused partial melting of pelitic wall rocks, with consequent formation of contact migmatite and granite. The Velfjord plutons are geochemically and internally distinct. The southern Sausfjellet pluton is calc-alkaline and locally phase layered. Compositional banding, marked by plagioclase-rich layers, is commonly folded and disrupted. The central Akset-Drevli pluton is the oldest of the three. It evolved toward K2O and Fe enrichment and is thus broadly alkaline, but also has shoshonitic features. The northern Hillstadfjellet pluton and associated Aunet pluton represent a complex sequence of emplacement and differentiation. They range from gabbro to monzonite, are predominantly intermediate, and show compositional affinities to the Akset-Drevli pluton. The western Svarthopen pluton is predominantly gabbro and diorite, but locally shows significant hybridization with contact granitic rocks. Initial 87Sr/86Sr ranges from 0.7057 to 0.7101; only the Svarthopen pluton has values > 0.707. It is also the only pluton in which initial 87Sr/86Sr is correlated with SiO2; the data lie on a mixing line with contact granite compositions. eNd ranges from +0.5 to -3.9, and delta18O ranges from +6.8‰ to +9.8‰. Contact granites have initial 87Sr/86Sr from 0.713 to 0.718, eNd from -4.8 to -5.4, and delta18O from +9.5‰ to +15.4‰. Except for Svarthopen, the lack of isotopic variation within plutons suggests that in situ assimilation and mixing were unimportant. Nb, Zr, and Ti depletions plus enrichment of large-ion lithophile elements suggest a subduction-modified mantle source. The overall heterogeneity of parental magmas implies a heterogeneous mantle source, but the distinct ranges in bulk compositions imply deep-seated fractional crystallization and assimilation. The K-rich magmas are the result of this high-P fractionation rather than local assimilation or K-rich mantle sources.