Episodic porphyroblast growth in the Fleur de Lys Supergroup, Newfoundland: timing relative to the sequential development of multiple crenulation cleavages

Abstract

Inclusion trails in garnet and albite porphyroblasts in the Fleur de Lys Supergroup preserve successive generations of microstructures, some of which correlate with equivalent microstructures in the matrix. Microstructure–porphyroblast relationships provide timing constraints on a succession of seven crenulation cleavages (S1–S7) and five stages of porphyroblast growth. Significant destruction and alteration of early fabrics has occurred during the microstructural development of the rock mass. Garnet porphyroblasts grew episodically through four growth stages (G1–G4) and preserve a succession of five fabrics (S1–S5) as inclusion trails. Garnet growth during each of the four growth phases did not occur on all pre‐existing porphyroblasts, resulting in contrasting growth histories between individual garnet porphyroblasts from the same outcrop. Albite porphyroblasts grew during a single stage of growth and have overgrown microstructures continuous with the matrix. The garnet and albite porphyroblast inclusion trails record a succession of crenulation cleavages without any rotation of the porphyroblasts relative to other porphyroblasts in the population.Complex microstructural histories are best resolved by preparing multiple oriented thin sections from a large number of samples of different rock types within the area of study. The succession of matrix foliations must be understood, as it provides the most useful time‐frame against which to measure the relative timing of phases of porphyroblast growth. Comparable microstructures must be identified in different porphyroblasts and in the rock matrix.