Retrograde alterations of phyllosilicates in low-grade metapelite

A case study from the Szendro{double acute} Paleozoic, NE-Hungary

P. Árkai, Isabel Abad, Fernando Nieto, Tibor Németh, Péter Horváth, Viktória K. Kis, Katalin Judik, Juan Jiménez-Millán

Research output: Contribution to journalArticle

3 Citations (Scopus)

Abstract

Frequently, at temperatures lower than the metamorphic peak conditions, slates undergo mineral transformations, usually mediated by fluids. We have studied core material of an epizonal slate series (Szendro{double acute} Mountains, NE-Hungary) to reconstruct the post-metamorphic lower-T hydrothermal alterations using petrographic, X-ray diffraction, electron microprobe methods, and transmission electron microscopy. The borehole crosscuts an upper part of the ca. 600-800 m Lower-Carboniferous flysch-like Szendro{double acute} Phyllite Formation. The samples were metamorphosed reaching epizonal conditions with a mineral assemblage characterized by quartz, muscovite, chlorite and albite. Even in the freshest samples, break-up and loosening of the regional metamorphic structure was observed, with cracks parallel to or crossing the cleavage. In the upper part, chlorite and albite are almost absent, while the presence of paragonite, mixed Na-K mica, and mixed-layered minerals with smectitic component are characteristic. Goethite, halloysite, and subordinate kaolinite are present in the most altered sample (13. 0 m in the profile) which may indicate the position of the fissures in which the circulation of the post-metamorphic fluids was the most intense. Muscovite is the only mica from the lower part of the profile and chlorite becomes a significant constituent, whereas paragonite, halloysite, and kaolinite are missing. Discrete smectite is present in all the samples in spite of being incompatible with the prograde evolution of the sequence. The interleaved smectite layers in chlorite and muscovite/chlorite mixed-layers show at the lattice level textural characteristics indicative of a later alteration process. After the metamorphic peak at epizonal conditions, the introduction of hot fluids through the fractures gave rise to the crystallization of Na-K white micas and muscovite/chlorite under anchizonal conditions. In a final stage of the hydrothermal process, the cooling of the circulating fluids favored the formation of halloysite and kaolinite in the areas near to the fissures, smectites in the samples far away from the fractures, and locally, goethite. The Fe2O3 content increasing upwards indicates oxidizing conditions in the late stage of hydrothermal activity and/or eventually, an influence of a younger near-surface weathering.

Original languageEnglish
Pages (from-to)263-282
Number of pages20
JournalSwiss Journal of Geosciences
Volume105
Issue number2
DOIs
Publication statusPublished - Dec 2012

Fingerprint

metapelite
phyllosilicate
chlorite
Paleozoic
muscovite
halloysite
smectite
kaolinite
paragonite
fluid
albite
goethite
fissure
mica
mineral
phyllite
slate
hydrothermal activity
flysch
hydrothermal alteration

Keywords

  • EPMA
  • Halloysite
  • HRTEM
  • Mixed Na-K white mica
  • Muscovite/chlorite mixed-layers
  • Retrograde hydrothermal reactions
  • XRPD

ASJC Scopus subject areas

  • Geology

Cite this

Retrograde alterations of phyllosilicates in low-grade metapelite : A case study from the Szendro{double acute} Paleozoic, NE-Hungary. / Árkai, P.; Abad, Isabel; Nieto, Fernando; Németh, Tibor; Horváth, Péter; Kis, Viktória K.; Judik, Katalin; Jiménez-Millán, Juan.

In: Swiss Journal of Geosciences, Vol. 105, No. 2, 12.2012, p. 263-282.

Research output: Contribution to journalArticle

Árkai, P. ; Abad, Isabel ; Nieto, Fernando ; Németh, Tibor ; Horváth, Péter ; Kis, Viktória K. ; Judik, Katalin ; Jiménez-Millán, Juan. / Retrograde alterations of phyllosilicates in low-grade metapelite : A case study from the Szendro{double acute} Paleozoic, NE-Hungary. In: Swiss Journal of Geosciences. 2012 ; Vol. 105, No. 2. pp. 263-282.
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abstract = "Frequently, at temperatures lower than the metamorphic peak conditions, slates undergo mineral transformations, usually mediated by fluids. We have studied core material of an epizonal slate series (Szendro{double acute} Mountains, NE-Hungary) to reconstruct the post-metamorphic lower-T hydrothermal alterations using petrographic, X-ray diffraction, electron microprobe methods, and transmission electron microscopy. The borehole crosscuts an upper part of the ca. 600-800 m Lower-Carboniferous flysch-like Szendro{double acute} Phyllite Formation. The samples were metamorphosed reaching epizonal conditions with a mineral assemblage characterized by quartz, muscovite, chlorite and albite. Even in the freshest samples, break-up and loosening of the regional metamorphic structure was observed, with cracks parallel to or crossing the cleavage. In the upper part, chlorite and albite are almost absent, while the presence of paragonite, mixed Na-K mica, and mixed-layered minerals with smectitic component are characteristic. Goethite, halloysite, and subordinate kaolinite are present in the most altered sample (13. 0 m in the profile) which may indicate the position of the fissures in which the circulation of the post-metamorphic fluids was the most intense. Muscovite is the only mica from the lower part of the profile and chlorite becomes a significant constituent, whereas paragonite, halloysite, and kaolinite are missing. Discrete smectite is present in all the samples in spite of being incompatible with the prograde evolution of the sequence. The interleaved smectite layers in chlorite and muscovite/chlorite mixed-layers show at the lattice level textural characteristics indicative of a later alteration process. After the metamorphic peak at epizonal conditions, the introduction of hot fluids through the fractures gave rise to the crystallization of Na-K white micas and muscovite/chlorite under anchizonal conditions. In a final stage of the hydrothermal process, the cooling of the circulating fluids favored the formation of halloysite and kaolinite in the areas near to the fissures, smectites in the samples far away from the fractures, and locally, goethite. The Fe2O3 content increasing upwards indicates oxidizing conditions in the late stage of hydrothermal activity and/or eventually, an influence of a younger near-surface weathering.",
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