Dioctahedral mixed K-Na-micas and paragonite in diagenetic to low-temperature metamorphic terrains

Bulk rock chemical, thermodynamic and textural constraints

P. Árkai, Kenneth Livi, Péter Horváth

Research output: Contribution to journalArticle

6 Citations (Scopus)

Abstract

Metamorphic mineral assemblages in low-temperature metaclastic rocks often contain paragonite and/or its precursor metastable phase (mixed K-Na-white mica). Relationships between the bulk rock major element chemistries and the formation of paragonite at seven localities from Central and SE-Europe were studied, comparing the bulk chemical characteristics with mineral assemblage, mineral chemical and metamorphic petrological data. Considerable overlaps between the projection fields of bulk chemistries of the Pg-free and Pg-bearing metaclastic rocks indicate significant differences between the actual (as analyzed) and effective bulk chemical compositions. Where inherited, clastic, inert phases/constituents were excluded, it was found that a decrease in Na/(Na+Al*) and in K/(K+Al*) ratios of rocks favors the formation and occurrence of Pg and its precursor phases (Al* denotes here the atomic quantity of aluminum in feldspars, white micas and "pure" hydrous or anhydrous aluminosilicates). In contrast to earlier suggestions, enrichment in Na and/or an increase in Na/K ratio by themselves do not lead to formation of paragonite. Bulk rock chemistries favorable to formation of paragonite and its precursor phases are characterized by enrichment in Al and depletion in Na, K, Ca (and also, Mg and Fe 2+ ). Such bulk rock chemistries are characteristic of chemically "mature" (strongly weathered) source rocks of the pelites and may also be formed by synand post-sedimentary magmatism-related hydrothermal (leaching) activity. What part of the whole rock is active in determining the effective bulk chemistry was investigated by textural examination of diagenetic and anchizone-grade samples. It is hypothesized that although solid phases act as local sources and sinks, transport of elements such as Na through the grain boundaries have much larger communication distances. Sodium-rich white micas nucleate heterogeneously using existing phyllosilicates as templates and are distributed widely on the thin section scale. The results of modeling by THERMOCALC suggest that paragonite preferably forms at higher pressures in low-T metapelites. The stability fields of Pg-bearing assemblages increase, the Pg-in reaction line is shifted towards lower pressures, while the stability field of the Chl-Ms-Ab-Qtz assemblage decreases and is shifted towards higher temperatures with increasing Al* content and decreasing Na/(Na+Al*) and K/(K+Al*) ratios.

Original languageEnglish
Pages (from-to)283-314
Number of pages32
JournalCentral European Geology
Volume51
Issue number4
DOIs
Publication statusPublished - Dec 1 2008

Fingerprint

paragonite
thermodynamics
rock
mineral
metapelite
phyllosilicate
aluminosilicate
grain boundary
chemical
thin section
mica
source rock
low pressure
magmatism
aluminum
chemical composition
leaching
sodium
communication

Keywords

  • Bulk rock chemistry
  • Low-temperature metamorphism
  • Metapelite
  • Mixed K-Na-white mica
  • Muscovite
  • Paragonite
  • Thermobarometry

ASJC Scopus subject areas

  • Geology

Cite this

Dioctahedral mixed K-Na-micas and paragonite in diagenetic to low-temperature metamorphic terrains : Bulk rock chemical, thermodynamic and textural constraints. / Árkai, P.; Livi, Kenneth; Horváth, Péter.

In: Central European Geology, Vol. 51, No. 4, 01.12.2008, p. 283-314.

Research output: Contribution to journalArticle

@article{8271f7db5f224738bbe652737083f262,
title = "Dioctahedral mixed K-Na-micas and paragonite in diagenetic to low-temperature metamorphic terrains: Bulk rock chemical, thermodynamic and textural constraints",
abstract = "Metamorphic mineral assemblages in low-temperature metaclastic rocks often contain paragonite and/or its precursor metastable phase (mixed K-Na-white mica). Relationships between the bulk rock major element chemistries and the formation of paragonite at seven localities from Central and SE-Europe were studied, comparing the bulk chemical characteristics with mineral assemblage, mineral chemical and metamorphic petrological data. Considerable overlaps between the projection fields of bulk chemistries of the Pg-free and Pg-bearing metaclastic rocks indicate significant differences between the actual (as analyzed) and effective bulk chemical compositions. Where inherited, clastic, inert phases/constituents were excluded, it was found that a decrease in Na/(Na+Al*) and in K/(K+Al*) ratios of rocks favors the formation and occurrence of Pg and its precursor phases (Al* denotes here the atomic quantity of aluminum in feldspars, white micas and {"}pure{"} hydrous or anhydrous aluminosilicates). In contrast to earlier suggestions, enrichment in Na and/or an increase in Na/K ratio by themselves do not lead to formation of paragonite. Bulk rock chemistries favorable to formation of paragonite and its precursor phases are characterized by enrichment in Al and depletion in Na, K, Ca (and also, Mg and Fe 2+ ). Such bulk rock chemistries are characteristic of chemically {"}mature{"} (strongly weathered) source rocks of the pelites and may also be formed by synand post-sedimentary magmatism-related hydrothermal (leaching) activity. What part of the whole rock is active in determining the effective bulk chemistry was investigated by textural examination of diagenetic and anchizone-grade samples. It is hypothesized that although solid phases act as local sources and sinks, transport of elements such as Na through the grain boundaries have much larger communication distances. Sodium-rich white micas nucleate heterogeneously using existing phyllosilicates as templates and are distributed widely on the thin section scale. The results of modeling by THERMOCALC suggest that paragonite preferably forms at higher pressures in low-T metapelites. The stability fields of Pg-bearing assemblages increase, the Pg-in reaction line is shifted towards lower pressures, while the stability field of the Chl-Ms-Ab-Qtz assemblage decreases and is shifted towards higher temperatures with increasing Al* content and decreasing Na/(Na+Al*) and K/(K+Al*) ratios.",
keywords = "Bulk rock chemistry, Low-temperature metamorphism, Metapelite, Mixed K-Na-white mica, Muscovite, Paragonite, Thermobarometry",
author = "P. {\'A}rkai and Kenneth Livi and P{\'e}ter Horv{\'a}th",
year = "2008",
month = "12",
day = "1",
doi = "10.1556/CEuGeol.51.2008.4.1",
language = "English",
volume = "51",
pages = "283--314",
journal = "Central European Geology",
issn = "1788-2281",
publisher = "Akademiai Kiado",
number = "4",

}

TY - JOUR

T1 - Dioctahedral mixed K-Na-micas and paragonite in diagenetic to low-temperature metamorphic terrains

T2 - Bulk rock chemical, thermodynamic and textural constraints

AU - Árkai, P.

AU - Livi, Kenneth

AU - Horváth, Péter

PY - 2008/12/1

Y1 - 2008/12/1

N2 - Metamorphic mineral assemblages in low-temperature metaclastic rocks often contain paragonite and/or its precursor metastable phase (mixed K-Na-white mica). Relationships between the bulk rock major element chemistries and the formation of paragonite at seven localities from Central and SE-Europe were studied, comparing the bulk chemical characteristics with mineral assemblage, mineral chemical and metamorphic petrological data. Considerable overlaps between the projection fields of bulk chemistries of the Pg-free and Pg-bearing metaclastic rocks indicate significant differences between the actual (as analyzed) and effective bulk chemical compositions. Where inherited, clastic, inert phases/constituents were excluded, it was found that a decrease in Na/(Na+Al*) and in K/(K+Al*) ratios of rocks favors the formation and occurrence of Pg and its precursor phases (Al* denotes here the atomic quantity of aluminum in feldspars, white micas and "pure" hydrous or anhydrous aluminosilicates). In contrast to earlier suggestions, enrichment in Na and/or an increase in Na/K ratio by themselves do not lead to formation of paragonite. Bulk rock chemistries favorable to formation of paragonite and its precursor phases are characterized by enrichment in Al and depletion in Na, K, Ca (and also, Mg and Fe 2+ ). Such bulk rock chemistries are characteristic of chemically "mature" (strongly weathered) source rocks of the pelites and may also be formed by synand post-sedimentary magmatism-related hydrothermal (leaching) activity. What part of the whole rock is active in determining the effective bulk chemistry was investigated by textural examination of diagenetic and anchizone-grade samples. It is hypothesized that although solid phases act as local sources and sinks, transport of elements such as Na through the grain boundaries have much larger communication distances. Sodium-rich white micas nucleate heterogeneously using existing phyllosilicates as templates and are distributed widely on the thin section scale. The results of modeling by THERMOCALC suggest that paragonite preferably forms at higher pressures in low-T metapelites. The stability fields of Pg-bearing assemblages increase, the Pg-in reaction line is shifted towards lower pressures, while the stability field of the Chl-Ms-Ab-Qtz assemblage decreases and is shifted towards higher temperatures with increasing Al* content and decreasing Na/(Na+Al*) and K/(K+Al*) ratios.

AB - Metamorphic mineral assemblages in low-temperature metaclastic rocks often contain paragonite and/or its precursor metastable phase (mixed K-Na-white mica). Relationships between the bulk rock major element chemistries and the formation of paragonite at seven localities from Central and SE-Europe were studied, comparing the bulk chemical characteristics with mineral assemblage, mineral chemical and metamorphic petrological data. Considerable overlaps between the projection fields of bulk chemistries of the Pg-free and Pg-bearing metaclastic rocks indicate significant differences between the actual (as analyzed) and effective bulk chemical compositions. Where inherited, clastic, inert phases/constituents were excluded, it was found that a decrease in Na/(Na+Al*) and in K/(K+Al*) ratios of rocks favors the formation and occurrence of Pg and its precursor phases (Al* denotes here the atomic quantity of aluminum in feldspars, white micas and "pure" hydrous or anhydrous aluminosilicates). In contrast to earlier suggestions, enrichment in Na and/or an increase in Na/K ratio by themselves do not lead to formation of paragonite. Bulk rock chemistries favorable to formation of paragonite and its precursor phases are characterized by enrichment in Al and depletion in Na, K, Ca (and also, Mg and Fe 2+ ). Such bulk rock chemistries are characteristic of chemically "mature" (strongly weathered) source rocks of the pelites and may also be formed by synand post-sedimentary magmatism-related hydrothermal (leaching) activity. What part of the whole rock is active in determining the effective bulk chemistry was investigated by textural examination of diagenetic and anchizone-grade samples. It is hypothesized that although solid phases act as local sources and sinks, transport of elements such as Na through the grain boundaries have much larger communication distances. Sodium-rich white micas nucleate heterogeneously using existing phyllosilicates as templates and are distributed widely on the thin section scale. The results of modeling by THERMOCALC suggest that paragonite preferably forms at higher pressures in low-T metapelites. The stability fields of Pg-bearing assemblages increase, the Pg-in reaction line is shifted towards lower pressures, while the stability field of the Chl-Ms-Ab-Qtz assemblage decreases and is shifted towards higher temperatures with increasing Al* content and decreasing Na/(Na+Al*) and K/(K+Al*) ratios.

KW - Bulk rock chemistry

KW - Low-temperature metamorphism

KW - Metapelite

KW - Mixed K-Na-white mica

KW - Muscovite

KW - Paragonite

KW - Thermobarometry

UR - http://www.scopus.com/inward/record.url?scp=69149084920&partnerID=8YFLogxK

UR - http://www.scopus.com/inward/citedby.url?scp=69149084920&partnerID=8YFLogxK

U2 - 10.1556/CEuGeol.51.2008.4.1

DO - 10.1556/CEuGeol.51.2008.4.1

M3 - Article

VL - 51

SP - 283

EP - 314

JO - Central European Geology

JF - Central European Geology

SN - 1788-2281

IS - 4

ER -