Hans Årebäck

The nature and origin of an anorthosite associated ilmenite-rich leuconorite, Hakefjorden Complex, south-west Sweden

Abstract Ilmenite-rich leuconorite (IRL) occurs in the Neoproterozoic Hakefjorden Complex (HFC; a norite–anorthosite intrusion) in south-western Sweden. The medium to fine-grained IRL comprises a disc-shaped, subhorizontal unit within the complex with gradual transitions into the host norite. It is characterised by a well-developed subhorizontal magmatic foliation, displayed by the preferred orientation of tabular plagioclase crystals, and it consists of subequal amounts of labradorite and ilmenite with minor interstitial orthopyroxene. The IRL contains a variety of inclusions comprising decimeter- to meter-size anorthosite and norite blocks from the hosting HFC, and millimeter- to decimeter-size magmatic segregations, formed within the IRL-unit, such as Fe–Ti oxide-, fine-grained anorthosite- and fine-scale igneous layered plagioclase–orthopyroxene inclusions. The latter are disc- to sheet-shaped, usually concordant to the foliation and interpreted to represent igneous layers disrupted during deformation in the magmatic state. The deformation also caused the magmatic foliation during simple shear conditions (no lineation is observed), which wraps around all inclusion types. Fractional crystallization starting with deep crustal anorthosite and ultramafic differentiation produced a late stage liquid unusually enriched in Fe and Ti. Such a liquid with plagioclase in suspension is put forward as parental for the IRL. Fe–Ti oxide subsolidus re-equilibration during slow cooling modified the oxide compositions and textural relations indicate that annealing and textural coarsening played an important role in the formation of the densely packed Fe–Ti oxide inclusions.

Ion microprobe discovery of Archaean and Early Proterozoic zircon xenocrysts in southwest Sweden

Abstract A single zircon grain dated by ion probe with a minimum age of 3432±30 Ma was found in a microgranite dyke, which cuts norite of 916±11 Ma Hakefjorden Complex at Älgön on the Swedish west coast north of Gothenburg. Zircons are a rare accessory mineral in this dyke. They could be classified by morphology as orthocrysts and xenocrysts. Data from four orthocrysts show that the dyke originated between 911±14 and 916±11 Ma, and was probably derived from the Hakefjorden Complex contact migmatite. Eight of the dated grains are xenocrysts, probably derived from the Stora Le-Marstrand Formation. They have 207Pb-206Pb minimum ages from c. 1451 to 3432±6 Ma, and the oldest grain has a probable age of 3457±10 Ma, derived from a discordia regression, with 445±38 Ma lower intercept reflecting Phanerozoic lead loss. This grain is the oldest yet dated in Scandinavia. Together with six >1750 Ma zircon grains in both this sample and a related Stora Le-Marstrand-derived sample, it casts doubt on the prevailing model of incremental crustal growth in Scandinavia, southwestwards from an Archaean core in the northeast of the Baltic Shield. These old ages, together with published Sm-Nd data, also contradict the proposed origin of the Stora Le-Marstrand Formation in an isolated oceanic island arc setting. It is more likely that it formed on the eastern (in present-day terms) margin of an Archaean continent, which did not join Baltica till the Sveconorwegian orogenic cycle.

Geology, petrology and alteration geochemistry of the Palaeoproterozoic intrusive hosted Älgträsk Au deposit, Northern Sweden

Abstract The Älgträsk intrusive hosted Au deposit, Skellefte district, northern Sweden, is situated in the oldest, most heterogeneous part of the c. 1.89–1.86 Ga Jörn granitoid complex, which intruded a complex volcano–sedimentary succession in an island arc or continental margin arc environment. The Tallberg porphyry Cu deposit, situated only 3 km west of Älgträsk, is associated with quartz feldspar porphyritic dykes. These dykes are suggested to be genetically related to similar porphyry dykes in Älgträsk and the tonalitic host rock in Tallberg. The granodiorite hosting the Älgträsk Au-deposit does not appear to be genetically related to the tonalite or the porphyry dykes.

Polybaric evolution of the Hakefjorden Complex, southwestern Sweden, deduced from partial dissolution in andesine megacrysts

Abstract Disequilibrium textures in andesine megacrysts, showing re‐sorbed morphologies, of the post‐kinematic Sveconorwegian Hakefjorden Complex provide important information about the emplacement mechanism. These textures consist of fine cellular plagioclase inter‐growths occurring as 0.4–2.5 mm mantles around the megacrysts and as entirely cellular grains. Outside the cellular mantle the megacrysts are covered by a normally zoned rim. The cellular mantles consist of two phases, one relatively Na‐rich (An45) and one relatively Ca‐rich (An59). Compositionally, the Na‐rich phase is similar to the non‐cellular megacryst core (An46) and the Ca‐rich phase is similar to the inner part of the normally zoned rim (An60). This rim grades from An60) to An30 which is about the same as the matrix plagioclases of the surrounding norite. The cellular network of plagioclase intergrowths is interpreted as a result of partial dissolution caused by decompression during the emplacement. The emplacement mechanism can be sum...