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.

Taconic Metamorphism Preserved in the Baie Verte Peninsula, Newfoundland Appalachians: Geochronological Evidence for Ophiolite Obduction and Subduction and Exhumation of the Leading Edge of the Laurentian (Humber) Margin During Closure of the Taconic Seaway

“…Regionally, rocks of the Fleur de Lys Supergroup are characterized by the growth of late M 2 albite and garnet (± biotite, chloritoid, kyanite, staurolite and pyroxene) porphyroblasts (Hibbard 1983;Jamieson and Vernon 1987;Jamieson and O'Beirne-Ryan 1991;Stallard 1998;Stallard and Hickey 2002). Porphyroblasts have been interpreted to have formed during a phase of decompression and heating at ca.…”

“…Notwithstanding basement tectonometamorphism (pre-Fleur de Lys Supergroup) of the East Pond Metamorphic Suite that produced migmatite and gneiss (de Wit 1980;Jamieson 1990), the complex and episodic tectonometamorphic evolution of the Fleur de Lys Supergroup (Jamieson and Vernon 1987;Stallard 1998) and Baie Verte oceanic tract can be grouped into four phases of regional deformation and three metamorphic pulses (Bursnall and de Wit 1975;Hibbard 1983;Jamieson 1990;Castonguay et al 2010). Most deformation phases and metamorphic imprints are domainal and fabrics vary in orientation, style and in intensity.…”

Apparent oxygen isotope equilibrium during progressive foliation development and porphyroblast growth in metapelites: implications for stable isotope and conventional thermometry

“…Most regional metamorphic rocks, when examined in detail, exhibit evidence for having experienced complex deformation histories (e.g. Vernon, 1989; Jamieson & Vernon, 1987; Williams, 1994; Stallard, 1998; Bell & Hickey, 1999; Johnson, 1999) that must include at least two periods of foliation development. The assumption of static mineral reaction is clearly untenable.…”

Numerical simulations of spiral‐shaped inclusion trails: can 3D geometry distinguish between end‐member models of spiral formation?

“…Visser & Mancktelow, 1992; Stallard & Hickey, 2001b), crenulation development and microstructural history of deformation (e.g. Stallard, 1998).…”