Metamorphism of pelites in NKFMASH - a new petrogenetic grid with implications for the preservation of high-pressure mineral assemblages during exhumation

Journal article

Authors/Editors

Proyer, Alexander

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Publication Details

Author list: Proyer A

Publisher: Wiley

Place: MALDEN

Publication year: 2003

Journal: Journal of Metamorphic Geology (0263-4929)

Journal acronym: J METAMORPH GEOL

Volume number: 21

Issue number: 5

Start page: 493

End page: 509

Number of pages: 17

ISSN: 0263-4929

eISSN: 1525-1314

Languages: English-Great Britain (EN-GB)

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Abstract

A petrogenetic grid for metapelites in the system NKFMASH is presented. The P-T range is investigated in three sections: (1) The high- and ultrahigh-pressure range is discussed in the system NFMASH because phengite is the only stable potassic phase. (2) The transition region is characterised by four NKFMASH-invariant points that separate high-pressure glaucophane-bearing from medium-pressure biotite-bearing metapelites. (3) The medium-pressure range contains the fifth NKFMASH-invariant point. The univariant reactions of this point terminate the stability range of paragonite, which breaks down to form staurolite or kyanite and plagioclase during decompression and/or heating. As the growth of albitic plagioclase by decomposition of paragonite via continuous reactions may be conspicuous already before these staurolite- or kyanite-producing reactions are reached, such albite porphyroblast schists are typical indicators of a former high-pressure metamorphic history.Considering the preservation of high-pressure metapelitic assemblages, those crossing the NKFMASH-transition region during exhumation commonly dehydrate and preservation is unlikely. Three types of metapelites have a fairly good survival potential: (1) low-temperature metapelites (up to c. 540 degreesC) with an exhumation path back into the chlorite + albite stability field, (2) assemblages with chloritoid + glaucophane, and (3) the relatively high-temperature glaucophane + kyanite and jadeite + kyanite bearing parageneses, that are relatively dry at the onset of exhumation. A comparison with data from the literature shows that these rock types are the most abundant in nature.

Keywords

high-pressure, metapelites, NKFMASH, petrogenetic grid, preservation

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