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Partial melting in amphibolites in a deep section of the Sveconorwegian Orogen, SW Sweden

Hansen, Edward; Johansson, Leif LU ; Andersson, Jenny; LaBarge, Leah; Harlov, Daniel; Möller, Charlotte LU and Vincent, Stephanie (2015) In Lithos 236. p.27-45
Abstract
Garnet amphibolite metataxites at the Steningekusten Nature Reserve in southwestern Sweden contain tonalitic patches and veins. Whole rock chemistry suggests that the protoliths were mafic igneous rocks with alkaline affinities. Orthopyroxene megacrysts are present in leucosome in parts of these garnet amphibolites but absent in others. Orthopyroxene megacrysts were formed by vapor-absent melting initiated by incongruent melting of biotite followed by the breakdown of hornblende. The net reaction was Bt + Hbl + PI +/- Qtz <-> Opx + Melt + Cpx + Gt. Melting occurred at pressures of approximately 1 GPa and temperatures which probably exceeded 800 degrees C. Pyroxenes are surrounded by hornblende-quartz symplectites, and hornblende in... (More)
Garnet amphibolite metataxites at the Steningekusten Nature Reserve in southwestern Sweden contain tonalitic patches and veins. Whole rock chemistry suggests that the protoliths were mafic igneous rocks with alkaline affinities. Orthopyroxene megacrysts are present in leucosome in parts of these garnet amphibolites but absent in others. Orthopyroxene megacrysts were formed by vapor-absent melting initiated by incongruent melting of biotite followed by the breakdown of hornblende. The net reaction was Bt + Hbl + PI +/- Qtz <-> Opx + Melt + Cpx + Gt. Melting occurred at pressures of approximately 1 GPa and temperatures which probably exceeded 800 degrees C. Pyroxenes are surrounded by hornblende-quartz symplectites, and hornblende in these coronas has distinctly lower concentrations of (Na + K) and Ti than that in adjacent mesosorne. The hornblende rims formed upon cooling and reaction with crystallizing melt This created a barrier to further reaction thus preserving the orthopyroxene megacrysts. Garnet amphibolite metatexites lacking pyroxene megacrysts have features characteristic of vapor-present melting including lack of peritectic phases predicted by vapor-absent melting reactions, larger amounts of leucosome (14 versus 7%), and less distinct melanosomes. The variation in these migmatites reflects open system behavior, either on a regional scale with the migration of aqueous fluids into the amphibolites or on a local scale with the migration of melt within the amphibolites. Zircons from all units have CL-dark core domains that are dated at 1415-1390 Ma. The core zones are cut and overgrown by CL-dark and CL-bright rims that are dated at 975-965 Ma. The zircon rims are thin in the mesosome but are thicker in the leucosome suggesting that they formed during migmatization. New growth of zircon associated with migmatization at ca. 970 Ma corresponds to the timing of crustal scale partial melting in the deep regions of the Sveconorwegian orogen, synchronous with east-west extension and the intrusion of mafic dykes. If partial melting was driven by an influx of aqueous fluids, they were probably derived from a relatively cool source region, which would indicate tectonic juxtaposition of hotter and cooler terranes. (C) 2015 Elsevier B.V. All rights reserved. (Less)
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author
organization
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type
Contribution to journal
publication status
published
subject
keywords
Migmatite, Vapor-present melting, Vapor-absent melting, Sveconorwegian, U-Pb zircon, Amphibolite
in
Lithos
volume
236
pages
27 - 45
publisher
Elsevier
external identifiers
  • wos:000364891500003
  • scopus:84941624578
ISSN
0024-4937
DOI
10.1016/j.lithos.2015.08.010
language
English
LU publication?
yes
id
f5d17d0a-3e69-4c45-8c28-519ca9b2e0dc (old id 8539740)
date added to LUP
2016-01-20 12:11:10
date last changed
2017-07-02 03:18:40
@article{f5d17d0a-3e69-4c45-8c28-519ca9b2e0dc,
  abstract     = {Garnet amphibolite metataxites at the Steningekusten Nature Reserve in southwestern Sweden contain tonalitic patches and veins. Whole rock chemistry suggests that the protoliths were mafic igneous rocks with alkaline affinities. Orthopyroxene megacrysts are present in leucosome in parts of these garnet amphibolites but absent in others. Orthopyroxene megacrysts were formed by vapor-absent melting initiated by incongruent melting of biotite followed by the breakdown of hornblende. The net reaction was Bt + Hbl + PI +/- Qtz &lt;-&gt; Opx + Melt + Cpx + Gt. Melting occurred at pressures of approximately 1 GPa and temperatures which probably exceeded 800 degrees C. Pyroxenes are surrounded by hornblende-quartz symplectites, and hornblende in these coronas has distinctly lower concentrations of (Na + K) and Ti than that in adjacent mesosorne. The hornblende rims formed upon cooling and reaction with crystallizing melt This created a barrier to further reaction thus preserving the orthopyroxene megacrysts. Garnet amphibolite metatexites lacking pyroxene megacrysts have features characteristic of vapor-present melting including lack of peritectic phases predicted by vapor-absent melting reactions, larger amounts of leucosome (14 versus 7%), and less distinct melanosomes. The variation in these migmatites reflects open system behavior, either on a regional scale with the migration of aqueous fluids into the amphibolites or on a local scale with the migration of melt within the amphibolites. Zircons from all units have CL-dark core domains that are dated at 1415-1390 Ma. The core zones are cut and overgrown by CL-dark and CL-bright rims that are dated at 975-965 Ma. The zircon rims are thin in the mesosome but are thicker in the leucosome suggesting that they formed during migmatization. New growth of zircon associated with migmatization at ca. 970 Ma corresponds to the timing of crustal scale partial melting in the deep regions of the Sveconorwegian orogen, synchronous with east-west extension and the intrusion of mafic dykes. If partial melting was driven by an influx of aqueous fluids, they were probably derived from a relatively cool source region, which would indicate tectonic juxtaposition of hotter and cooler terranes. (C) 2015 Elsevier B.V. All rights reserved.},
  author       = {Hansen, Edward and Johansson, Leif and Andersson, Jenny and LaBarge, Leah and Harlov, Daniel and Möller, Charlotte and Vincent, Stephanie},
  issn         = {0024-4937},
  keyword      = {Migmatite,Vapor-present melting,Vapor-absent melting,Sveconorwegian,U-Pb zircon,Amphibolite},
  language     = {eng},
  pages        = {27--45},
  publisher    = {Elsevier},
  series       = {Lithos},
  title        = {Partial melting in amphibolites in a deep section of the Sveconorwegian Orogen, SW Sweden},
  url          = {http://dx.doi.org/10.1016/j.lithos.2015.08.010},
  volume       = {236},
  year         = {2015},
}