Almandine-spessartine garnets, from the Oxford County pegmatites and the Palermo No. 1 pegmatite, record significant compositional variations according to the degree of evolution of their hosting rock. Garnets from the most fractionated pegmatites (Mt. Mica, Berry-Havey, and Emmons) show the highest Mn, Nb, Ta, Zr, and Hf values, followed by those from the intermediate grade pegmatites (Palermo No. 1) and, finally, garnets from the barren pegmatites show the lowest values (Perham and Stop-35). Iron, Ca, and Mg contents follow an inverse order, with the highest contents in the latter pegmatites. Major element zoning shows increasing Mn values from core to rim in most garnet samples, while trace element zoning is not systematic except for some crystals which show a core to rim depletion for most of these elements. Chondrite normalized HREE (Heavy Rare Earth Elements) spectra show positive slopes for garnets from barren pegmatites, both positive and negative slopes for those associated with the intermediate pegmatite, and negative or flat slopes in garnets from the highly fractionated pegmatites. Ion exchange mechanisms, including Fe2+−1Mn2+1, (Fe2+, Mn2+)−1Si−1Li1P1; and, (Y, Ho3+)2(vac)1(Fe2+, Mn2+)−3, could explain most of the compositional variations observed in these garnets. These compositional variations are the reflection of the composition of the pegmatitic magma (barren pegmatites originate from a more ferromagnesian magma than fractionated pegmatites); and of the coexisting mineral phases competing with garnets to host certain chemical elements, such as biotite, schorl, plagioclase, apatite, Fe-Mn phosphates, Nb-Ta oxides, zircon, xenotime, and monazite.

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<p>The Bergen Arcs, in Norway, consist of several arcuate nappes formed during the Caledonian orogeny 440-420 Ma ago (Bingen et al., 2001; Glodny et al., 2008) when the western margin of Baltica was subducted below Laurentia. This Caledonian orogeny overprinted many of the anorthosites that formed the 930 Ma old (Bingen et al., 2001) granulitic basement. This overprint resulted in both amphibolites and eclogites and have been observed in shear zones within the rocks of the well-studied island of Holsnøy, located on the western margin of the Lindås Nappe. On the adjacent island of Radøy, the Caledonian overprint is associated with amphibolite facies shear zones (Mukai et al., 2014; Moore et al., 2020).</p> <p>In the northern margin of the Bergen Arcs, near the Bergen Arcs Shear Zone, the much less-studied island of Krossøy also exposes the anorthosites from the old granulitic basement and here the Caledonian overprint also resulted only in amphibolite facies metamorphism. The anorthosites in Krossøy are intruded by a series of subparallel mafic granulitic dykes forming the Krossøy dyke swarm, that has never previously been described elsewhere in the Bergen Arcs. The style of deformation in the granulites and the textural evolution in the amphibolite facies overprint are also markedly different from the rocks on Holsnøy and Radøy. The development of ductile Caledonian shear zones may have been facilitated by initial brittle failure of the basement accompanied by fluid infiltration (Jamtveit et al., 2018). Here we investigate the influence of this deformation and fluid infiltration on different features observed on these rocks such as: the occurrence of plagioclase coronas around the garnets on the dykes; the presence of different types of symplectites; the variability of size, deformation and composition observed on the anorthositic feldspars; or the local changes of fluid composition along cm- long fractures. We will show our first analytical results on some of these key features and discuss their relevance in the context of the previous studies of the Bergen Arcs.</p> <p> </p> <p>Bingen, B., David, W. J., & Austrheim, H. (2001). Zircon U-Pb geochronology in the Bergen Arc eclogites and their Protereyoic protoliths, and implications for the pre-Scandian evolution of the Caledonides in western Norway. In <em>GSA Bulletin</em> (Issue 5). https://doi.org/10.1130/0016-7606(2001)113<0640:ZUPGIT>2.0.CO;2</p> <p>Glodny, J., Kühn, A., & Austrheim, H. (2008). Geochronology of fluid-induced eclogite and amphibolite facies metamorphic reactions in a subduction-collision system, Bergen Arcs, Norway. <em>Contributions to Mineralogy and Petrology</em>, <em>156</em>(1), 27–48. https://doi.org/10.1007/s00410-007-0272-y</p> <p>Jamtveit, B., Moulas, E., Andersen, T. B., Austrheim, H., Corfu, F., Petley-Ragan, A., & Schmalholz, S. M. (2018). High Pressure Metamorphism Caused by Fluid Induced Weakening of Deep Continental Crust. <em>Scientific Reports</em>, <em>8</em>(1). https://doi.org/10.1038/s41598-018-35200-1</p> <p>Moore J., Beinlich A., Piazolo S., Austrheim H. & Putnis A.  (2020). Metamorphic differentiation via enhanced dissolution along high permeability zones. <em>Journal of Petrology</em> 61, 10. https://doi.org/10.1093/petrology/egaa096</p> <p>Mukai H., Austrheim H., Putnis CV. & Putnis A. (2014). Textural evolution of plagioclase feldspar across a shear zone: implications for deformation mechanism and rock strength. <em>Journal of Petrology</em>. 55, 1457-1477. https://doi.org/10.1093/petrology/egu030</p>

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