Ca-rich garnets and associated symplectites in mafic peraluminous granulites from the Gföhl Nappe System, Austria
Mafic peraluminous granulites associated with the mantle-derived peridotites of the Dunkelsteiner Wald provide evidence of the tectono-metamorphic evolution of the Gföhl Nappe System, Austria. They contain the primary assemblage garnet + Al-rich clinopyroxene + kyanite. Large Ca- and Mg-rich gar...
| Main Authors: | , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2018-06-01
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| Series: | Solid Earth |
| Online Access: | https://www.solid-earth.net/9/797/2018/se-9-797-2018.pdf |
| Summary: | Mafic peraluminous granulites associated with the mantle-derived peridotites
of the Dunkelsteiner Wald provide evidence of the tectono-metamorphic
evolution of the Gföhl Nappe System, Austria. They contain the primary
assemblage garnet + Al-rich clinopyroxene + kyanite. Large Ca- and
Mg-rich garnets are embedded in a granoblastic matrix of Al-rich
clinopyroxene, Ca-rich plagioclase and minor hornblende. They were
partially replaced by different generations of symplectites: (a) corundum +
sapphirine + spinel + plagioclase formed around kyanite inclusions,
(b) orthopyroxene + spinel + plagioclase ± hornblende formed at their
rims and (c) clinopyroxene + orthopyroxene + spinel + plagioclase ±
hornblende formed within cracks.
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Large garnets show complex compositional structure comprising several
repeatedly occurring garnet types, which are characterized by specific
compositions. The areal extent and the cross-cutting relations observed in
element distribution maps allowed for the derivation of the relative timing
of the formation of the different garnet types. The compositional features of
the garnets indicate post-formational modification by intra-crystalline
diffusion and metasomatic agents.
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The garnet composition isopleths in equilibrium assemblage diagrams are in
line with compositions modification as indicated by the element distribution
maps. They confirm the deviation of composition from equilibrium for all
garnet types. Furthermore, at least the youngest garnet types show evidence of
metasomatic (Fe + Mg) loss affecting their Ca content. Pressure–temperature (<i>P–T</i>) estimates
are based on equilibrium assemblage diagrams that reproduce satisfactorily
the observed mineral assemblages and measured mineral compositions. Criteria
for checking the existence of preserved equilibrium compositions are
suggested. The results call into question the invariability of the assumption
that the Ca content and/or zoning in garnet preserves primary
<i>P–T</i> information from garnet growth in every case.
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Recrystallization and compositional readjustment of the reactive garnet
volume during symplectite formation led to the development of pronounced,
secondary diffusion-induced zoning profiles overprinting the different garnet
types and post-dating the complex garnet compositional structure. The primary
assemblage is stable between 760 and 880 °C and pressures
> 11 kbar. The bulk composition of the crack symplectites is almost
isochemical to the oldest, broken-down garnet type. These symplectites were
formed above 730 °C and pressures between 5 and 7.5 kbar. The rocks
studied underwent more or less isothermal decompression from pressures above
11 to ∼ 6 kbar at temperatures of about 800 °C. Crack and rim
symplectites were formed after decompression during the early stage of
approximately isobaric cooling under conditions of low differential stress.
Due to limited availability of fluids promoting symplectite formation, the
timescale of symplectite formation calculated from secondary diffusion
profiles associated with crack symplectites is shown to be geologically very
short (< 0.5 ka). |
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| ISSN: | 1869-9510 1869-9529 |