Titanomagnetite Rims on Hornblende Phenocrysts in Intermediate Subvolcanic Intrusions, Torud–Ahmad Abad Magmatic Belt, Iran: Examination of Devolatilization and Oxidation Processes
Mafic to felsic adakitic intrusive and extrusive rocks in the Torud–Ahmad Abad magmatic belt occur south-southeast of Shahrood (north of the Central Iran Zone). These adakites, in the form of dykes and other hypabyssal igneous bodies, are emplaced into late Neoproterozoic amphibolite and mylonitized...
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2025-01-01
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| Online Access: | https://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/doi/10.2113/2025/lithosphere_2024_227/651710/lithosphere_2024_227.pdf |
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| author | Fazilat Yousefi David R. Lentz Glenn Bark |
| author_facet | Fazilat Yousefi David R. Lentz Glenn Bark |
| author_sort | Fazilat Yousefi |
| collection | DOAJ |
| description | Mafic to felsic adakitic intrusive and extrusive rocks in the Torud–Ahmad Abad magmatic belt occur south-southeast of Shahrood (north of the Central Iran Zone). These adakites, in the form of dykes and other hypabyssal igneous bodies, are emplaced into late Neoproterozoic amphibolite and mylonitized granites and a thick sequence of Paleocene to middle Eocene volcanic and volcano-sedimentary rocks. These high silica and low silica adakites (HSA and LSA) span a range of lithologies including basaltic andesite, andesite, trachyandesite, dacite, trachydacite, and dacite. The adakites are composed mainly of calcic pyroxene, Ca-Na amphibole, and plagioclase phenocrysts, with minor biotite and titanomagnetite. In these intrusions, iron-titanium oxides crystallized late (within the groundmass) or occur as secondary phases. One of the interesting features of these rocks is the opacitization (magnetite rich selvage) of ferromagnesian phenocrysts, such as hornblende, which is a localized replacement reaction. The intensity and color of these opacitized margins depend on the extent of titanomagnetite-magnetite formed. The SEM-EDS analysis results show that most of magnetite formed at temperatures >500°C related to pressure quenching of the melt due to emplacement and differential volcanic degassing. The phenomenon of opacitization is due to decreasing stability of Fe2+ and hydrous ferromagnesian phenocrysts, such as amphiboles, to form less hydrous to anhydrous pseudomorphs (selvage) in the near-surface environment with oxidative reactions linked to pressure quenching and differential devolatilization of H2 from the melt during hypabyssal emplacement. The rapid decrease in pressure during magma ascent causes hornblende instability and thus helps to create these opaque rims (opacitized) on ferromagnesian phases (magnetite and titanomagnetite-rich assemblage). Hornblende breakdown - destabilization increases due to melt degassing - devolatilization [decrease in P(H2O)] during the process of ascent—emplacement with reduction of magmatic total pressure (decompression) and/or melt oxidation due to differential degassing of H2. |
| format | Article |
| id | doaj-art-e867fbb47fbb4497a323a0bab309c848 |
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| publishDate | 2025-01-01 |
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| spelling | doaj-art-e867fbb47fbb4497a323a0bab309c8482025-08-20T02:08:19ZengGeoScienceWorldLithosphere1941-82641947-42532025-01-012025110.2113/2025/lithosphere_2024_227Titanomagnetite Rims on Hornblende Phenocrysts in Intermediate Subvolcanic Intrusions, Torud–Ahmad Abad Magmatic Belt, Iran: Examination of Devolatilization and Oxidation ProcessesFazilat Yousefi0https://orcid.org/0000-0001-6081-790XDavid R. Lentz1https://orcid.org/0000-0002-9562-6211Glenn Bark2Department of Earth Sciences, University of New Brunswick, Fredericton, New Brunswick, E3B 5A3, CanadaDepartment of Earth Sciences, University of New Brunswick, Fredericton, New Brunswick, E3B 5A3, CanadaDepartment of Civil, Environmental and Natural Resources Engineering, Luleå University of Technology, Luleå, 97187, SwedenMafic to felsic adakitic intrusive and extrusive rocks in the Torud–Ahmad Abad magmatic belt occur south-southeast of Shahrood (north of the Central Iran Zone). These adakites, in the form of dykes and other hypabyssal igneous bodies, are emplaced into late Neoproterozoic amphibolite and mylonitized granites and a thick sequence of Paleocene to middle Eocene volcanic and volcano-sedimentary rocks. These high silica and low silica adakites (HSA and LSA) span a range of lithologies including basaltic andesite, andesite, trachyandesite, dacite, trachydacite, and dacite. The adakites are composed mainly of calcic pyroxene, Ca-Na amphibole, and plagioclase phenocrysts, with minor biotite and titanomagnetite. In these intrusions, iron-titanium oxides crystallized late (within the groundmass) or occur as secondary phases. One of the interesting features of these rocks is the opacitization (magnetite rich selvage) of ferromagnesian phenocrysts, such as hornblende, which is a localized replacement reaction. The intensity and color of these opacitized margins depend on the extent of titanomagnetite-magnetite formed. The SEM-EDS analysis results show that most of magnetite formed at temperatures >500°C related to pressure quenching of the melt due to emplacement and differential volcanic degassing. The phenomenon of opacitization is due to decreasing stability of Fe2+ and hydrous ferromagnesian phenocrysts, such as amphiboles, to form less hydrous to anhydrous pseudomorphs (selvage) in the near-surface environment with oxidative reactions linked to pressure quenching and differential devolatilization of H2 from the melt during hypabyssal emplacement. The rapid decrease in pressure during magma ascent causes hornblende instability and thus helps to create these opaque rims (opacitized) on ferromagnesian phases (magnetite and titanomagnetite-rich assemblage). Hornblende breakdown - destabilization increases due to melt degassing - devolatilization [decrease in P(H2O)] during the process of ascent—emplacement with reduction of magmatic total pressure (decompression) and/or melt oxidation due to differential degassing of H2.https://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/doi/10.2113/2025/lithosphere_2024_227/651710/lithosphere_2024_227.pdfopacitization, magnetite, amphibole breakdown, decompression, differential devolatilization, volcanic degassing, oxidation |
| spellingShingle | Fazilat Yousefi David R. Lentz Glenn Bark Titanomagnetite Rims on Hornblende Phenocrysts in Intermediate Subvolcanic Intrusions, Torud–Ahmad Abad Magmatic Belt, Iran: Examination of Devolatilization and Oxidation Processes Lithosphere opacitization, magnetite, amphibole breakdown, decompression, differential devolatilization, volcanic degassing, oxidation |
| title | Titanomagnetite Rims on Hornblende Phenocrysts in Intermediate Subvolcanic Intrusions, Torud–Ahmad Abad Magmatic Belt, Iran: Examination of Devolatilization and Oxidation Processes |
| title_full | Titanomagnetite Rims on Hornblende Phenocrysts in Intermediate Subvolcanic Intrusions, Torud–Ahmad Abad Magmatic Belt, Iran: Examination of Devolatilization and Oxidation Processes |
| title_fullStr | Titanomagnetite Rims on Hornblende Phenocrysts in Intermediate Subvolcanic Intrusions, Torud–Ahmad Abad Magmatic Belt, Iran: Examination of Devolatilization and Oxidation Processes |
| title_full_unstemmed | Titanomagnetite Rims on Hornblende Phenocrysts in Intermediate Subvolcanic Intrusions, Torud–Ahmad Abad Magmatic Belt, Iran: Examination of Devolatilization and Oxidation Processes |
| title_short | Titanomagnetite Rims on Hornblende Phenocrysts in Intermediate Subvolcanic Intrusions, Torud–Ahmad Abad Magmatic Belt, Iran: Examination of Devolatilization and Oxidation Processes |
| title_sort | titanomagnetite rims on hornblende phenocrysts in intermediate subvolcanic intrusions torud ahmad abad magmatic belt iran examination of devolatilization and oxidation processes |
| topic | opacitization, magnetite, amphibole breakdown, decompression, differential devolatilization, volcanic degassing, oxidation |
| url | https://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/doi/10.2113/2025/lithosphere_2024_227/651710/lithosphere_2024_227.pdf |
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