Metamorphic evolution of sillimanite gneiss in the high-pressure terrane of the Western Gneiss Region (Norway)
<p>Sillimanite-bearing gneisses in the Romsdal region of the Western Gneiss Region (south Norway) have been investigated to document the presence, formation, composition and petrological evolution of the sillimanite-bearing assemblages. Sillimanite is found in augen gneiss, as nodular gneiss,...
| Main Authors: | , |
|---|---|
| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2024-03-01
|
| Series: | European Journal of Mineralogy |
| Online Access: | https://ejm.copernicus.org/articles/36/345/2024/ejm-36-345-2024.pdf |
| _version_ | 1827307730757484544 |
|---|---|
| author | A. K. Engvik J. Jakob |
| author_facet | A. K. Engvik J. Jakob |
| author_sort | A. K. Engvik |
| collection | DOAJ |
| description | <p>Sillimanite-bearing gneisses in the Romsdal region of the Western Gneiss Region (south Norway) have been investigated to document the presence, formation, composition and petrological evolution of the sillimanite-bearing assemblages. Sillimanite is found in augen gneiss, as nodular gneiss, and in well-foliated sillimanite–mica gneiss. Lenses and layers of eclogite occur within the gneiss units. The sillimanite-bearing gneisses are heterogranular and dominated by quartz, plagioclase (An<span class="inline-formula"><sub>29–41</sub></span>), K-feldspar and biotite (Mg# <span class="inline-formula">=</span> 0.48–0.58; Ti <span class="inline-formula">=</span> 0.16–0.36 a.p.f.u.), with variable amounts of white mica (Si <span class="inline-formula">=</span> 6.1–6.3). K-feldspar occurs as porphyroclasts in augen gneiss, and garnet constitutes resorbed porphyroblasts. Garnet (Alm<span class="inline-formula"><sub>46–56</sub></span>Sps<span class="inline-formula"><sub>24–36</sub></span>Prp<span class="inline-formula"><sub>10−20</sub></span>Grs<span class="inline-formula"><sub>4–6</sub></span>; Mg# <span class="inline-formula">=</span> 0.22–0.29) shows rimward-decreasing Mg#, together with a smaller grossular decrease and a marked spessartine increase up to Sps<span class="inline-formula"><sub>36</sub></span>. The foliation is defined by crystal-preferred-orientation micas, elongation of shape-preferred-orientation coarse K-feldspar phenocrysts and a modal banding of phases. Sillimanite occurs as coarse orientation-parallel matrix porphyroblasts, as finer grains and as fibrolitic aggregates. Quartz constitutes coarser elongated grains and monomineralic rods. Pseudosection modelling suggests that the peak-metamorphic mineral assemblage of garnet–sillimanite–feldspar–biotite–quartz–ilmenite–liquid equilibrated at temperatures up to 750 °C and pressures of 0.6 GPa. Subsequent retrogression consumed garnet. Mineral replacement and melt crystallization involved sillimanite, white mica, K-feldspar and quartz. The results document a metamorphic retrogression of the sillimanite gneisses in accordance with the presence of remnants of eclogites and high-pressure granulites in this northwestern part of the Western Gneiss Region.</p> |
| first_indexed | 2024-04-24T18:45:41Z |
| format | Article |
| id | doaj.art-c3d05d7bdf214f5288d799eb88702f8a |
| institution | Directory Open Access Journal |
| issn | 0935-1221 1617-4011 |
| language | English |
| last_indexed | 2024-04-24T18:45:41Z |
| publishDate | 2024-03-01 |
| publisher | Copernicus Publications |
| record_format | Article |
| series | European Journal of Mineralogy |
| spelling | doaj.art-c3d05d7bdf214f5288d799eb88702f8a2024-03-27T07:02:08ZengCopernicus PublicationsEuropean Journal of Mineralogy0935-12211617-40112024-03-013634536010.5194/ejm-36-345-2024Metamorphic evolution of sillimanite gneiss in the high-pressure terrane of the Western Gneiss Region (Norway)A. K. EngvikJ. Jakob<p>Sillimanite-bearing gneisses in the Romsdal region of the Western Gneiss Region (south Norway) have been investigated to document the presence, formation, composition and petrological evolution of the sillimanite-bearing assemblages. Sillimanite is found in augen gneiss, as nodular gneiss, and in well-foliated sillimanite–mica gneiss. Lenses and layers of eclogite occur within the gneiss units. The sillimanite-bearing gneisses are heterogranular and dominated by quartz, plagioclase (An<span class="inline-formula"><sub>29–41</sub></span>), K-feldspar and biotite (Mg# <span class="inline-formula">=</span> 0.48–0.58; Ti <span class="inline-formula">=</span> 0.16–0.36 a.p.f.u.), with variable amounts of white mica (Si <span class="inline-formula">=</span> 6.1–6.3). K-feldspar occurs as porphyroclasts in augen gneiss, and garnet constitutes resorbed porphyroblasts. Garnet (Alm<span class="inline-formula"><sub>46–56</sub></span>Sps<span class="inline-formula"><sub>24–36</sub></span>Prp<span class="inline-formula"><sub>10−20</sub></span>Grs<span class="inline-formula"><sub>4–6</sub></span>; Mg# <span class="inline-formula">=</span> 0.22–0.29) shows rimward-decreasing Mg#, together with a smaller grossular decrease and a marked spessartine increase up to Sps<span class="inline-formula"><sub>36</sub></span>. The foliation is defined by crystal-preferred-orientation micas, elongation of shape-preferred-orientation coarse K-feldspar phenocrysts and a modal banding of phases. Sillimanite occurs as coarse orientation-parallel matrix porphyroblasts, as finer grains and as fibrolitic aggregates. Quartz constitutes coarser elongated grains and monomineralic rods. Pseudosection modelling suggests that the peak-metamorphic mineral assemblage of garnet–sillimanite–feldspar–biotite–quartz–ilmenite–liquid equilibrated at temperatures up to 750 °C and pressures of 0.6 GPa. Subsequent retrogression consumed garnet. Mineral replacement and melt crystallization involved sillimanite, white mica, K-feldspar and quartz. The results document a metamorphic retrogression of the sillimanite gneisses in accordance with the presence of remnants of eclogites and high-pressure granulites in this northwestern part of the Western Gneiss Region.</p>https://ejm.copernicus.org/articles/36/345/2024/ejm-36-345-2024.pdf |
| spellingShingle | A. K. Engvik J. Jakob Metamorphic evolution of sillimanite gneiss in the high-pressure terrane of the Western Gneiss Region (Norway) European Journal of Mineralogy |
| title | Metamorphic evolution of sillimanite gneiss in the high-pressure terrane of the Western Gneiss Region (Norway) |
| title_full | Metamorphic evolution of sillimanite gneiss in the high-pressure terrane of the Western Gneiss Region (Norway) |
| title_fullStr | Metamorphic evolution of sillimanite gneiss in the high-pressure terrane of the Western Gneiss Region (Norway) |
| title_full_unstemmed | Metamorphic evolution of sillimanite gneiss in the high-pressure terrane of the Western Gneiss Region (Norway) |
| title_short | Metamorphic evolution of sillimanite gneiss in the high-pressure terrane of the Western Gneiss Region (Norway) |
| title_sort | metamorphic evolution of sillimanite gneiss in the high pressure terrane of the western gneiss region norway |
| url | https://ejm.copernicus.org/articles/36/345/2024/ejm-36-345-2024.pdf |
| work_keys_str_mv | AT akengvik metamorphicevolutionofsillimanitegneissinthehighpressureterraneofthewesterngneissregionnorway AT jjakob metamorphicevolutionofsillimanitegneissinthehighpressureterraneofthewesterngneissregionnorway |