The Proterozoic Hustad igneous complex: a low strain enclave with a key to the history of the Western Gneiss Region of Norway

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The Hustad Igneous Complex (HIC) is a concentrically zoned, high K calc-alkaline intrusive complex, with a core of quartz–monzonite and granite surrounded successively by monzodiorite and pyroxenite, preserved in a 10 km2 lens within the strongly ‘caledonised’ Western Gneiss Region (WGR) of Norway. Upper intercept UPb zircon ages of 1654±1 and 1653±2 Ma, obtained from the granite and the monzodiorite, respectively, overlap with a RbSr whole rock age of 1656±49 Ma (I.R.: 0.70193±0.0009, MSWD:6.6) obtained from the felsic members of the complex and a SmNd whole rock age of 1555±180 Ma (I.R.: 0.51067±13, MSWD:3.3). The UPb age of 1654±1 Ma is interpreted to date the time of intrusion. A 50 m thick dolerite dyke that can be followed for almost 4 km across the HIC yields a UPb zircon and baddeleyite age of 1251±3 Ma. The dyke also contains a secondary polycrystalline zircon generation, which together with baddeleyite defines a lower intercept age of 384±9 Ma. The Caledonian events were also marked by the emplacement of a swarm of pegmatites, which crosscut all the other units and provide a UPb zircon age of 401±1 Ma. A RbSr whole-rock age for the pegmatites is older at 452±26 Ma (I.R.: 0.7105, MSWD:4.3) indicating some initial isotopic heterogenity and formation of the pegmatites from material with a long crustal residence time. A concordant UPb monazite age of 390±2 Ma from the pegmatite and the above 384±9 Ma lower intercept age date metamorphism caused by percolating fluids. Post pegmatite deformation transformed the complex along shear zones into gneisses indiscernible from the surrounding gneisses. The HIC is characterised by high mg-numbers (0.39–0.80), high Ni and Cr values, and depletion of Nb and Ta relative to LIL elements. Phenocrysts of ortho- and clinopyroxene have high concentrations of Cr, supporting the Cr-rich nature of the complex. The suite is LREE-enriched, has a fractionated and U- shaped REE pattern (Cen 90–143, Ybn 12–40), eNd values ranging from +0.0 to +2.7, and the felsic members display weak positive Eu anomalies while the pyroxenite has a negative Eu anomaly. The complex bears a geochemical resemblance to the late-tectonic, high-BaSr granite complexes of West Scotland, and particularly to their volcanic counterparts, the Mg-rich Old Red Sandstone andesites. Overall the data suggest generation of the suite in a subduction environment from material derived by melting of a recently enriched, but previously long-term LREE-depleted mantle. The HIC is one of the few well-preserved remnants of the igneous protoliths of the 1686–1640 Ma gneisses that make up the WGR. In fact, the pyroxenite is compositionally identical to orthopyroxene eclogite found as lenses in the surrounding gneisses and forms a likely protolith to this enigmatic rock. The metamorphism had little influence on the RbSr and SmNd systems of the felsic rocks, but it appears to have affected more strongly those in the mafic rocks, probably as a result of the more intense mineralogical transformations and metasomatic loss of Rb and LREE during high-grade metamorphism.