Geology, geochemistry and emplacement conditions of the Vega intrusive complex: an example of large-scale crustal anatexis in north-central Norway
The c. 350 km2 Vega intrusive complex is part of the Bindal Batholith and was emplaced at c. 475 Ma into polydeformed supracrustal rocks of the Helgeland Nappe Complex. The intrusive complex is tilted towards the west, exposing asymmetrical zoning. From east to west, the complex is composed of biotite granite, garnet-biotite granite, garnet-bearing muscovite biotite granodiorite and sillimanite-bearing garnet cordierite muscovite biotite granodiorite. In addition, the complex contains small amounts of intrusive migmatite. Granodiorite and intrusive migmatite contain abundant metasedimentary, mafic and ultramafic enclaves. Granodiorite, granite and migmatite are generally peraluminous to strongly peraluminous, calcic to alkalic and magnesian, with initial 87Sr/86Sr ratios of 0.7096–0.7469 and εNd from −7.0 to −11.0. Emplacement of the Vega intrusive complex was coeval with the intrusion of metaluminous dioritic rocks. The intrusive mafic rocks and enclaves in the complex have MORB-like (mid-ocean ridge basalt-like) to calc-alkaline geochemical characteristics. The lack of an isotopic compositional trend between mafic and granitic rocks indicates that magmas did not mix. Instead granitic magmas formed by unmixing of residual phases from crustally derived magmas. Partial melting of supracrustal source rocks may have been related to intra- and underplating of MORB-like magmas into the lower crust during extension.