ALBERT

Description: Petrological studies of staurolite–garnet–kyanite–biotite schist and garnet–muscovite schist of the Gula Complex, central Norway, provide constraints on metamorphic evolution during Scandian continent–continent collision, burial and exhumation of the Caledonian Upper Allochthon. The biotite schist contains conspicuous porphyroblasts of Fe-rich staurolite, garnet and kyanite, set in a fine-grained, well-foliated matrix of biotite, quartz, minor plagioclase and muscovite. The muscovite schist is fine- to medium-grained with a muscovite–quartz-dominated matrix, including garnet, biotite, minor plagioclase and clinozoisite. Pressure–temperature ( P – T ) modelling based on thermobarometric calculations and construction of P – T pseudo-sections illustrate that metamorphism reached 680 °C with pressures estimated up to 1.01 ± 0.11 GPa. Retrogression and decompression are constrained by secondary mineral reactions: local replacement of kyanite to fibrous sillimanite indicates decompression below 0.7 GPa. Growth of foliation-parallel chlorite reflects cooling below 640 °C and the chlorite formation proceeded during cooling and decompression towards 550 °C and 0.4 GPa. The metamorphism is associated with a strong north–south-trending regional foliation, and retrogression and decompression apparently continued within the same strain regime. The P – T modelling shows that even small variations in whole-rock chemistry and P–T conditions can explain heterogeneity and significant shifts in mineralogy and modal concentration of the index minerals of metapelites.

Description: Larvikite is a peculiar and unique monzonitic rock originating in the Carboniferous–Permian Oslo Rift, SE Norway. The blue iridescence in the feldspar crystals made the rock particularly attractive as ornamental stone, and since the start of industrial scaled production in 1884, the use of larvikite has reached every corner of the global community. With resources for hundreds of years, the region will produce larvikite blocks far into the future. The production of larvikite has changed significantly during the last decades, towards more sustainable production finding new applications and markets for excess rock in the quarries. The significance of larvikite in the global market has also created a wider appreciation of the rock in the Norwegian society, acknowledging the rich history of larvikite production and use.

Description: New U–Pb zircon dating yields a crystallization age of 458±3 Ma for the largely gabbroic Grøndalsfjell Intrusive Complex in the Gjersvik Nappe of the Caledonian Upper Allochthon in Scandinavia. This is identical, within error, to the age of the adjacent Møklevatnet Complex that is dominated by quartz monzodiorite (456±2 Ma), and the two intrusive suites may be regarded as members of a composite intrusion here referred to as the Nesåa Batholith. Mafic members of this calc-alkaline batholith are characterized by slightly positive εNd–εSr values, marked enrichment of the light rare earth elements and high Th/Yb ratios suggestive of a subduction-modified mantle source. The I-type granitoids have similar isotope values and highly fractionated rare earth element patterns, and are interpreted as products from partial melting of garnet-bearing mafic rocks. The Nesåa Batholith intruded a previously deformed, 483 Ma or older, metavolcanic sequence of oceanic arc affinity. The margins of the pluton show evidence for synkinematic emplacement, which is tentatively interpreted in terms of magma ascent controlled by deep-seated shear zones. Further uplift and exhumation of the crystallized plutons was followed by rapid deposition of batholith-derived conglomerates and arkoses in a marginal basin represented by the Limingen Group. The age of the Nesåa Batholith fills the gap in reported ages for Caledonian magmatism, between the Early to Middle Ordovician, oceanic to continental margin type, arc sequences of Laurentian palaeotectonic affinity, and the Late Ordovician–Early Silurian batholith complexes of interpreted Laurentian margin affinity. It is interpreted as an early phase of the more extensive plutonism recorded in the Bindal Batholith of the Uppermost Allochthon to the west. Our model implies that the Early Ordovician oceanic arc sequences of the Gjersvik Nappe were deformed and accreted on to Laurentian margin lithologies prior to Late Ordovician times. This composite crustal assemblage was the source for the voluminous quartz monzodioritic intrusions of the Nesåa Batholith, which formed by partial melting due to ponding of subduction-related mantle derived mafic magmas either within or at the base of the active continental margin.