High-aluminum orthopyroxene megacrysts (HAOM) in the Ahvenisto complex, SE Finland, and the polybaric crystallization of massif-type anorthosites
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 Ahven...
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ftunivhelsihelda:oai:helda.helsinki.fi:10138/308619 2024-01-07T09:43:12+01:00 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 Department of Geosciences and Geography Petrology and Geochemistry 2019-12-19T13:16:04Z 25 application/pdf http://hdl.handle.net/10138/308619 eng eng Springer 10.1007/s00410-019-1648-5 Heinonen , A , Kivisaari , H & Michallik , R 2020 , ' High-aluminum orthopyroxene megacrysts (HAOM) in the Ahvenisto complex, SE Finland, and the polybaric crystallization of massif-type anorthosites ' , Contributions to Mineralogy and Petrology , vol. 175 , no. 1 , 10 . https://doi.org/10.1007/s00410-019-1648-5 ORCID: /0000-0001-6650-4146/work/66368253 49b75717-d806-454f-b271-a48ca4c40178 http://hdl.handle.net/10138/308619 000501221200001 cc_by openAccess info:eu-repo/semantics/openAccess 1171 Geosciences petrology geology mineralogy geochemistry Massif-type anorthosite Orthopyroxene Megacryst HAOM Rapakivi granite PROTEROZOIC ANORTHOSITE PLAGIOCLASE ORIGIN PETROGENESIS EVOLUTION CRUSTAL ZIRCON CONSTRAINTS CHARNOCKITE Article publishedVersion 2019 ftunivhelsihelda 2023-12-14T00:12:14Z 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% Al2O3), 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(similar to 60-70)) and rims (En(similar to 50-70)) of the HAOMs than in the host rock (En(similar to 45-60)) orthopyroxenes. The highest recorded Al abundances in the HAOM cores correspond to crystallization pressures of up to similar to 1.1 GPa (similar to 34 km depth), whereas the HAOM rims have crystallized at lower pressures (max. similar to 0.5 GPa, 20 km depth). The highest pressure estimates for the host rock orthopyroxene were similar to 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 ... Article in Journal/Newspaper Fennoscandian HELDA – University of Helsinki Open Repository Contributions to Mineralogy and Petrology 175 1 |
institution |
Open Polar |
collection |
HELDA – University of Helsinki Open Repository |
op_collection_id |
ftunivhelsihelda |
language |
English |
topic |
1171 Geosciences petrology geology mineralogy geochemistry Massif-type anorthosite Orthopyroxene Megacryst HAOM Rapakivi granite PROTEROZOIC ANORTHOSITE PLAGIOCLASE ORIGIN PETROGENESIS EVOLUTION CRUSTAL ZIRCON CONSTRAINTS CHARNOCKITE |
spellingShingle |
1171 Geosciences petrology geology mineralogy geochemistry Massif-type anorthosite Orthopyroxene Megacryst HAOM Rapakivi granite PROTEROZOIC ANORTHOSITE PLAGIOCLASE ORIGIN PETROGENESIS EVOLUTION CRUSTAL ZIRCON CONSTRAINTS CHARNOCKITE Heinonen, Aku Kivisaari, Heli Michallik, Radoslaw High-aluminum orthopyroxene megacrysts (HAOM) in the Ahvenisto complex, SE Finland, and the polybaric crystallization of massif-type anorthosites |
topic_facet |
1171 Geosciences petrology geology mineralogy geochemistry Massif-type anorthosite Orthopyroxene Megacryst HAOM Rapakivi granite PROTEROZOIC ANORTHOSITE PLAGIOCLASE ORIGIN PETROGENESIS EVOLUTION CRUSTAL ZIRCON CONSTRAINTS CHARNOCKITE |
description |
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% Al2O3), 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(similar to 60-70)) and rims (En(similar to 50-70)) of the HAOMs than in the host rock (En(similar to 45-60)) orthopyroxenes. The highest recorded Al abundances in the HAOM cores correspond to crystallization pressures of up to similar to 1.1 GPa (similar to 34 km depth), whereas the HAOM rims have crystallized at lower pressures (max. similar to 0.5 GPa, 20 km depth). The highest pressure estimates for the host rock orthopyroxene were similar to 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 ... |
author2 |
Department of Geosciences and Geography Petrology and Geochemistry |
format |
Article in Journal/Newspaper |
author |
Heinonen, Aku Kivisaari, Heli Michallik, Radoslaw |
author_facet |
Heinonen, Aku Kivisaari, Heli Michallik, Radoslaw |
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 |
publishDate |
2019 |
url |
http://hdl.handle.net/10138/308619 |
genre |
Fennoscandian |
genre_facet |
Fennoscandian |
op_relation |
10.1007/s00410-019-1648-5 Heinonen , A , Kivisaari , H & Michallik , R 2020 , ' High-aluminum orthopyroxene megacrysts (HAOM) in the Ahvenisto complex, SE Finland, and the polybaric crystallization of massif-type anorthosites ' , Contributions to Mineralogy and Petrology , vol. 175 , no. 1 , 10 . https://doi.org/10.1007/s00410-019-1648-5 ORCID: /0000-0001-6650-4146/work/66368253 49b75717-d806-454f-b271-a48ca4c40178 http://hdl.handle.net/10138/308619 000501221200001 |
op_rights |
cc_by openAccess info:eu-repo/semantics/openAccess |
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Contributions to Mineralogy and Petrology |
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175 |
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