Role of H 2 O in the formation of garnet coronas during near‐isobaric cooling of mafic granulites: the Tasiusarsuaq terrane, southern West Greenland
Abstract The Mesoarchaean Tasiusarsuaq terrane of southern West Greenland consists of Tonalite–trondhjemite–granodiorite gneisses and, locally, polymetamorphic mafic and ultramafic rocks. The terrane experienced medium‐pressure granulite facies conditions during M 1A in the Neoarchean, resulting in...
| Published in: | Journal of Metamorphic Geology |
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| Main Authors: | , , , |
| Format: | Article in Journal/Newspaper |
| Language: | English |
| Published: |
Wiley
2012
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| Subjects: | |
| Online Access: | http://dx.doi.org/10.1111/j.1525-1314.2012.01006.x https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1111%2Fj.1525-1314.2012.01006.x https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1525-1314.2012.01006.x |
| Summary: | Abstract The Mesoarchaean Tasiusarsuaq terrane of southern West Greenland consists of Tonalite–trondhjemite–granodiorite gneisses and, locally, polymetamorphic mafic and ultramafic rocks. The terrane experienced medium‐pressure granulite facies conditions during M 1A in the Neoarchean, resulting in the development of two‐pyroxene melanosome assemblages in mafic granulites containing garnet‐bearing leucosome. Reworking of these rocks during retrogression introduced garnet to the melanosome in the form of overgrowths, coronas and grain necklaces that separate the mafic minerals from plagioclase. NCFMASHTO pseudosection modelling constrains the peak metamorphism during M 1A to ∼850 °C and 7.5 kbar at fluid‐saturated conditions. Following M 1A , the rocks retained their M 1A H 2 O content and became fluid‐undersaturated as they underwent near‐isobaric cooling to ∼700 °C and 6.5–7 kbar, prior to reworking during M 1B . These low H 2 O contents allowed for the formation of garnet overgrowths and coronas during M 1B . The stability of garnet is greatly increased to lower pressure and temperature in fluid‐absent, fluid‐undersaturated mafic rocks, indicating that fluid and melt loss during initial granulite facies metamorphism is essential for the introduction of garnet, and the formation of garnet coronas, during retrogression. The occurrence of garnet coronas is consistent with, but not unique to, near‐isobaric cooling paths. |
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