High-aluminum orthopyroxene megacrysts (HAOM) in the Ahvenisto complex, SE Finland, and the polybaric crystallization of massif-type anorthosites
Abstract The occurrence of high-aluminum orthopyroxene megacrysts (HAOMs) in several massif-type Proterozoic anorthosite complexes has been used as evidence of their polybaric crystallization. Here, we report such petrographic and geochemical (XRF and EMPA) evidence from HAOMs discovered in the 1.64...
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Springer Science and Business Media LLC
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crspringernat:10.1007/s00410-019-1648-5 2023-05-15T16:13:10+02:00 High-aluminum orthopyroxene megacrysts (HAOM) in the Ahvenisto complex, SE Finland, and the polybaric crystallization of massif-type anorthosites Heinonen, Aku Kivisaari, Heli Michallik, Radoslaw M. 2019 http://dx.doi.org/10.1007/s00410-019-1648-5 http://link.springer.com/content/pdf/10.1007/s00410-019-1648-5.pdf http://link.springer.com/article/10.1007/s00410-019-1648-5/fulltext.html en eng Springer Science and Business Media LLC https://creativecommons.org/licenses/by/4.0 https://creativecommons.org/licenses/by/4.0 CC-BY Contributions to Mineralogy and Petrology volume 175, issue 1 ISSN 0010-7999 1432-0967 Geochemistry and Petrology Geophysics journal-article 2019 crspringernat https://doi.org/10.1007/s00410-019-1648-5 2022-01-04T11:20:12Z Abstract The occurrence of high-aluminum orthopyroxene megacrysts (HAOMs) in several massif-type Proterozoic anorthosite complexes has been used as evidence of their polybaric crystallization. Here, we report such petrographic and geochemical (XRF and EMPA) evidence from HAOMs discovered in the 1.64 Ga Ahvenisto rapakivi granite—massif-type anorthosite complex in southeastern Finland. Two different types of HAOMs were recognized: type 1 HAOMs are individual, euhedral-to-subhedral crystals, and up to 15 cm in diameter, and type 2 HAOMs occur in pegmatitic pockets closely associated with megacrystic (up to 30 cm long) plagioclase. The type 1 megacrysts in particular are surrounded by complex corona structures composed of plagioclase, low-Al orthopyroxene, iddingsite (after olivine), and sulfides. Orthopyroxene crystallization pressure estimates based on an Al-in-Opx geobarometer reveal a three-stage compositional evolution in both textural HAOM types. The Al content decreases significantly from the core regions of the HAOM (4.4–7.6 wt% Al 2 O 3 ), through the rims (1.3–3.6 wt%), into the host rock (0.5–1.5 wt%). Enstatite compositions overlap, but are generally higher in the cores (En ~60–70 ) and rims (En ~50–70 ) of the HAOMs than in the host rock (En ~45–60 ) orthopyroxenes. The highest recorded Al abundances in the HAOM cores correspond to crystallization pressures of up to ~ 1.1 GPa (~ 34 km depth), whereas the HAOM rims have crystallized at lower pressures (max. ~ 0.5 GPa, 20 km depth). The highest pressure estimates for the host rock orthopyroxene were ~ 0.2 GPa (< 7 km depth). These observations confirm the polybaric magmatic evolution of the Ahvenisto anorthosites and suggest that the entire 1.65–1.55 Ga Fennoscandian rapakivi suite was emplaced at a relatively shallow level (< 7 km depth) in the upper crust. Global comparison to similar rock types reveals remarkable similarities in the petrogenetic processes controlling HAOM composition and evolution of anorthosite parental magmas. Article in Journal/Newspaper Fennoscandian Springer Nature (via Crossref) Contributions to Mineralogy and Petrology 175 1 |
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English |
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Geochemistry and Petrology Geophysics |
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Geochemistry and Petrology Geophysics Heinonen, Aku Kivisaari, Heli Michallik, Radoslaw M. High-aluminum orthopyroxene megacrysts (HAOM) in the Ahvenisto complex, SE Finland, and the polybaric crystallization of massif-type anorthosites |
topic_facet |
Geochemistry and Petrology Geophysics |
description |
Abstract The occurrence of high-aluminum orthopyroxene megacrysts (HAOMs) in several massif-type Proterozoic anorthosite complexes has been used as evidence of their polybaric crystallization. Here, we report such petrographic and geochemical (XRF and EMPA) evidence from HAOMs discovered in the 1.64 Ga Ahvenisto rapakivi granite—massif-type anorthosite complex in southeastern Finland. Two different types of HAOMs were recognized: type 1 HAOMs are individual, euhedral-to-subhedral crystals, and up to 15 cm in diameter, and type 2 HAOMs occur in pegmatitic pockets closely associated with megacrystic (up to 30 cm long) plagioclase. The type 1 megacrysts in particular are surrounded by complex corona structures composed of plagioclase, low-Al orthopyroxene, iddingsite (after olivine), and sulfides. Orthopyroxene crystallization pressure estimates based on an Al-in-Opx geobarometer reveal a three-stage compositional evolution in both textural HAOM types. The Al content decreases significantly from the core regions of the HAOM (4.4–7.6 wt% Al 2 O 3 ), through the rims (1.3–3.6 wt%), into the host rock (0.5–1.5 wt%). Enstatite compositions overlap, but are generally higher in the cores (En ~60–70 ) and rims (En ~50–70 ) of the HAOMs than in the host rock (En ~45–60 ) orthopyroxenes. The highest recorded Al abundances in the HAOM cores correspond to crystallization pressures of up to ~ 1.1 GPa (~ 34 km depth), whereas the HAOM rims have crystallized at lower pressures (max. ~ 0.5 GPa, 20 km depth). The highest pressure estimates for the host rock orthopyroxene were ~ 0.2 GPa (< 7 km depth). These observations confirm the polybaric magmatic evolution of the Ahvenisto anorthosites and suggest that the entire 1.65–1.55 Ga Fennoscandian rapakivi suite was emplaced at a relatively shallow level (< 7 km depth) in the upper crust. Global comparison to similar rock types reveals remarkable similarities in the petrogenetic processes controlling HAOM composition and evolution of anorthosite parental magmas. |
format |
Article in Journal/Newspaper |
author |
Heinonen, Aku Kivisaari, Heli Michallik, Radoslaw M. |
author_facet |
Heinonen, Aku Kivisaari, Heli Michallik, Radoslaw M. |
author_sort |
Heinonen, Aku |
title |
High-aluminum orthopyroxene megacrysts (HAOM) in the Ahvenisto complex, SE Finland, and the polybaric crystallization of massif-type anorthosites |
title_short |
High-aluminum orthopyroxene megacrysts (HAOM) in the Ahvenisto complex, SE Finland, and the polybaric crystallization of massif-type anorthosites |
title_full |
High-aluminum orthopyroxene megacrysts (HAOM) in the Ahvenisto complex, SE Finland, and the polybaric crystallization of massif-type anorthosites |
title_fullStr |
High-aluminum orthopyroxene megacrysts (HAOM) in the Ahvenisto complex, SE Finland, and the polybaric crystallization of massif-type anorthosites |
title_full_unstemmed |
High-aluminum orthopyroxene megacrysts (HAOM) in the Ahvenisto complex, SE Finland, and the polybaric crystallization of massif-type anorthosites |
title_sort |
high-aluminum orthopyroxene megacrysts (haom) in the ahvenisto complex, se finland, and the polybaric crystallization of massif-type anorthosites |
publisher |
Springer Science and Business Media LLC |
publishDate |
2019 |
url |
http://dx.doi.org/10.1007/s00410-019-1648-5 http://link.springer.com/content/pdf/10.1007/s00410-019-1648-5.pdf http://link.springer.com/article/10.1007/s00410-019-1648-5/fulltext.html |
genre |
Fennoscandian |
genre_facet |
Fennoscandian |
op_source |
Contributions to Mineralogy and Petrology volume 175, issue 1 ISSN 0010-7999 1432-0967 |
op_rights |
https://creativecommons.org/licenses/by/4.0 https://creativecommons.org/licenses/by/4.0 |
op_rightsnorm |
CC-BY |
op_doi |
https://doi.org/10.1007/s00410-019-1648-5 |
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Contributions to Mineralogy and Petrology |
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175 |
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