Timing of porphyroblast growth in the Fleur de Lys Supergroup, Newfoundland

• Published in: Journal of Metamorphic Geology
• Main Authors: JAMIESON, R. A., VERNON, R. H.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 1987
• Subjects: Newfoundland
• Online Access: http://dx.doi.org/10.1111/j.1525-1314.1987.tb00384.x; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1111%2Fj.1525-1314.1987.tb00384.x; https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1525-1314.1987.tb00384.x

Abstract In the Fleur de Lys Supergroup, western Newfoundland, inclusion trails in garnet and albite porphyroblasts indicate that porphyroblasts overgrew a crenulation foliation, without rotation, probably during the deformation event that produced the crenulations. Further deformation of the matrix resulted in strong re‐orientation and retrograde metamorphism of the matrix foliation, which is consequently highly oblique to the crenulation foliation preserved in the porphyroblasts. The resulting matrix foliation locally preserves relics of the early crenulations, and also has itself been crenulated later in places. Thus the porphyroblasts grew before the later stages of deformation, rather than during the final stage, as had been suggested previously. The new interpretation is consistent with available 40 Ar/ 39 Ar cooling ages which indicate a late Ordovician‐early Silurian metamorphic peak, rather than the Devonian peak suggested by previous workers. The inclusion patterns and microprobe data indicate normal outward growth of garnet porphyroblasts from a central nucleus, rather than as a series of veins as proposed by de Wit (1976a, b). However, the observations presented here support growth of porphyroblasts without rotation, which is implied by the de Wit model.