Decompression‐induced growth of albite porphyroblasts, Fleur de Lys Supergroup, western Newfoundland

• Published in: Journal of Metamorphic Geology
• Main Authors: JAMIESON, R. A., O'BEIRNE‐RYAN, A. M.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 1991
• Subjects: Newfoundland
• Online Access: http://dx.doi.org/10.1111/j.1525-1314.1991.tb00537.x; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1111%2Fj.1525-1314.1991.tb00537.x; https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1525-1314.1991.tb00537.x

Abstract Albite porphyroblasts are widely distributed in pelitic and semi‐pelitic schists of the Fleur de Lys Supergroup, western Newfoundland. Textures and mineral assemblages indicate that albite grew during nearly isothermal decompression from P‐T conditions of about 500° C, 9 kbar, to conditions of 550° C, 6.5 kbar. Three compositional varieties of albite‐bearing schists, here termed PMAQ (paragonite‐muscovite‐albite‐quartz), MMAQ (microcline‐muscovite‐albite‐quartz), and PMMQ (paragonite‐muscovite‐margarite‐quartz), can be distinguished on the basis of pre‐porphyroblast mineral assemblages. Analysis of these assemblages in terms of the composition of the coexisting fluid [log a (Na + /H + ) versus log a (K + /H + )] suggests that, as pressure and temperature changed, the stability field of albite expanded at the expense of coexisting matrix phyllosilicates. This promoted growth of albite on pre‐existing or newly formed nuclei. Late oligoclase in PMAQ and PMMQ samples is associated with replacement of matrix garnet by plagioclase + mica ° Chlorite, particularly in strongly sheared samples.