Geochemistry of the Pigroot "mafic" phonolite.
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Project type:
Abstract:
The chemistry of the Pigroot "Mafic" Phonolite is enigmatic, and does not easily fit into standard chemical classifications. Petrographic analysis has noted a pronounced textural heterogeneity on an outcrop scale. Whether this textural heterogeneity results in a chemical variation has not yet been tested due to a lack of geochemical whole rock analyses.
The Pigroot "Mafic" Phonolite contains an extensive xenolith suite. Xenoliths include:
a)Stranraer Group Sediments.
b)Haast Schist Group.
c)Type I: Plagioclase (An50),Augite, Titano-magnetite Metagabbro.
d)Type II: Plagioclase (An60), Ti-Augite, Spinel88 Metagabbro.
e)Lherzolite.
f)Websterite.
These xenolith types have been described and classified. Petrographic and geochemical analyses of these assemblages have been carried out and are presented. The xenoliths are believed to be accidental, captured by the phonolite as it rose rapidly from the upper mantle.
A review of geothermometry for the spinel-lherzolite assemblage, shows that precise temperatures cannot yet be deduced, especially when minor elements are present in appreciable concentrations. Empirical improvements to the published geothermometers are required in order to improve the precision, by the use of thermodynamically valid projections to compensate for the effects of minor elements.
A provisional geotherm for East Otago is tentatively suggested, deduced from pressure-temperature relationships of the xenolith assemblage. It shows that a higher than normal geothermal gradient (>35°C/km) exists. This suggests that a steady state thermal structure has not yet been established beneath eastern Otago.
The Pigroot "Mafic" Phonolite contains an extensive xenolith suite. Xenoliths include:
a)Stranraer Group Sediments.
b)Haast Schist Group.
c)Type I: Plagioclase (An50),Augite, Titano-magnetite Metagabbro.
d)Type II: Plagioclase (An60), Ti-Augite, Spinel88 Metagabbro.
e)Lherzolite.
f)Websterite.
These xenolith types have been described and classified. Petrographic and geochemical analyses of these assemblages have been carried out and are presented. The xenoliths are believed to be accidental, captured by the phonolite as it rose rapidly from the upper mantle.
A review of geothermometry for the spinel-lherzolite assemblage, shows that precise temperatures cannot yet be deduced, especially when minor elements are present in appreciable concentrations. Empirical improvements to the published geothermometers are required in order to improve the precision, by the use of thermodynamically valid projections to compensate for the effects of minor elements.
A provisional geotherm for East Otago is tentatively suggested, deduced from pressure-temperature relationships of the xenolith assemblage. It shows that a higher than normal geothermal gradient (>35°C/km) exists. This suggests that a steady state thermal structure has not yet been established beneath eastern Otago.
Named Localities:
Thesis description:
52 leaves, Diagms, map (folded in pocket); 30 cm.
Department:
OU geology Identifier:
1985Teagle
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OURArchive access level:
Location (WKT, WGS84):
POLYGON ((170.383207361148834 -45.178030754383968,170.422464715100716 -45.179686225756278,170.421318482791662 -45.199094993957964,170.383367692531237 -45.198040705128463,170.383207361148834 -45.178030754383968))
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Citation
Teagle, DAH, “Geochemistry of the Pigroot "mafic" phonolite.,” Otago Geology Theses, accessed May 15, 2025, https://theses.otagogeology.org.nz/items/show/194.