Major and trace element geochemical analysis of norites in the Hakefjorden Complex to constrain magma source and magma plumbing systems

anglais. Major and trace element data taken from plagioclase crystals from anorthosites and related rock units in the Hakefjorden Complex located off the west coast of Sweden were investigated for the purposes of constraining magma chamber processes and the source of the magma. Although the anorthosite units were completely homogenous in regards to its chemistry, the plagioclase in the associated norite and ilmenite-leuconorite units showed substantial variation. The zoning profiles of the aforementioned plagioclase units showed sudden increases in Sr in the rims of the plagioclase. These patterns are likely to be caused by replenishment of the magma via recharge events. Peaks in An content coincide with patchy disequilibrium textures at the rims of the crystals - these are linked to either decompression via polybaric evolution of the magma chamber, magma recharge events or a combination of both. The peaks in trace element concentrations also coincide with the disequilibrium textures, which provide evidence that the recharge events are linked to the ascent of the magma chamber. The evidence of recharge events and the homogenous nature of the 87Sr/86Sr zoning profiles as indicated by Glommé (2015) suggest that a mantle derived source with crustal contamination is unlikely, and instead favours a lower crustal magma source.

suédois. Jag har analyserat huvud- och spårelementkoncentrationer i plagioklas från en norit-anortositintrusion, Älgön, Hakefjorden vid Sveriges västkust med syfte att fastställa intrusionens magmatiska ursprung, magmans utveckling och dess transport upp genom jordskorpan. Megakrister av plagioklas i anortositen har en homogen fördelning med avseende på huvud- och spårelement medan plagioklas-megakrister i norit och ilmenit-leukonorit uppvisar betydande variation. Koncentrationsprofiler av Sr genom plagioklasmegakrister från norit och ilmenit-leukonorit visar på en ökning i Sr från kärna till kant. Dessa variationer är troligtvis orsakade av påfyllning av magma. Fläckvis varierande Ca-halt i plagioklas sammanfaller med inklusionsrika partier i ytterkanterna av plagioklasmegakrister. Texturen länkas till tryckminskning under kristallisation av magman. Topparna i spårelementskoncentrationer samanfaller också med de inklusionsrika och varierande Ca-halterna, vilket ger belägg för att magman mobiliserats av magmapåfyllningshändelser. Trots att de varierande spårelementskoncentrationerna kan kopplas till påfyllning av ny magma har plagioklas en homogen och relativt radiogen 87Sr/86Sr isotopsammansättning (Glommé 2015), såväl i megakrister som i plagioklaskristaller i matrix. Sammantaget talar det för ett magmaursprung från den djupare delen jordskorpan snarare än ett mantelursprung, där de radiogena Sr-isotopkvoterna skulle förklaras av kontaminering av äldre kisel-rik berggrund.

anglais. A crystal forming in a magma chamber has a lot of parallels to tree rings forming as a tree grows. Just like how a tree ring develops from the centre outwards and reacts to the changing climate as it grows, a crystal might do something very similar. Plagioclase crystals found in rocks often form a profile that varies in both texture and chemistry, the changes in these features will tell a story about how the magma chamber developed. Located just off the west coast of Sweden lies two islands: Älgön and Brattön. These islands are both recognized nature reserves and are visited frequently for their scenery; one attraction of these islands that most visitors might not be aware of is the presence of a peculiar and enigmatic type of rock: anorthosites. These types of rocks have been pondered upon regarding their formation, with many theories being suggested, debunked, and argued over since the early twentieth century. By using microscopes and certain analytical equipment we can observe these rocks very closely to try determine their formation history. The tree ring-esque plagioclase crystals found in these rocks show very distinct features, namely a patchy band around the edge of the crystals which also coincide with some significant changes in their chemistry. These features are likely to be caused by a new batch of magma being injected into the magma chamber, causing changes to the system that the crystal then reacts to. What is the significance of these magma injections? These magma injections have direct implications on what type of source the magma came from and the setting in which it developed in. Previous studies have looked at other chemical signatures of these rocks and noted that it was indicative of a magma source coming from the Earth’s crust rather than the mantle. It was inconclusive however because the argument could be made that the chemistry could still reflect a mantle source if it mixed with the Earth’s crust as the magma chamber developed. This argument however is much less plausible now that magma injections must be considered. These prior studies showed very consistent chemical profiles, and if they were to be explained by mixing with the crust, they should have shown variation in the chemical profiles with each subsequent injection of magma, which was not present. A magma source coming from the Earth’s crust also implies that these rocks formed in a continental collision zone at the time (the type of setting that forms mountains, e.g. the Himalayas). To conclude, the type of rocks that can be found on the islands Älgön and Brattön have been long debated regarding their magma source and tectonic setting. By observing the features in plagioclase crystals found in these rocks we can find evidence for injections of new magma into the chamber as it is developing. The evidence of these injections in tandem with observations from prior studies suggest that the magma source is likely to come from the Earth’s crust. By extension of this, it can also be suggested that these rocks formed in a continental collision zone.