Microstructural features of garnet porphyroblasts in the granulite facies metapelites of the highland complex of Sri Lanka: evidence for a multiple growth history of garnet and early development of crenulation cleavage

V Mathavan

doi:10.4038/jgssl.v21i2.48

Abstract

Journal of the Geological Society of Sri Lanka is a peer-reviewed and open access journal published by the Geological Society of Sri Lanka. It aims to publish the most topical and highest quality papers, summarizing the results of recent research across all sub-disciplines of the Earth Science. Papers are frequently interdisciplinary covering both pure and applied fields of Geology. Contributions often refer to local, regional and/or international studies and emphasize the development of the understanding of fundamental geological processes.

Highlights

Porphyroblasts often preserve key information on deformation and metamorphic evolution of rocks and play an important role in a wide range of metamorphic and structural studies
The present study focuses on porphyroblast-inclusion microstructural features of porphyroblastic garnets in a group of garnet-sillimanite schist and gneiss samples collected from the north-central parts of the Highland Complex (HC)
Microstructural evidence indicates that the growth of cores of the garnet porphyroblasts coincides with the development of a crenulation cleavage formed during D2 deformation

Summary

INTRODUCTION

Porphyroblasts often preserve key information on deformation and metamorphic evolution of rocks and play an important role in a wide range of metamorphic and structural studies (see Johnson, 1999; Johnson et al, 2006). Porphyroblastic garnets in-group B samples vary in size and shape, 3-4 mm subidioblastic to xenoblastic grains are common and some are elongate parallel to the matrix fabric, Sc. Inclusions are abundant in most garnets and as in garnets of group A samples, they often define, a large inclusion-rich cores surrounded by a smaller inclusion-free rims (Figure 5c-d, f). The presence of rare coarse sillimanite inclusion in the rims (Figure 3g) suggests that this growth was at least late in D2, at the same time as the growth of the small garnets containing sillimanite and fibrolite inclusion trails that are parallel to the matrix fabric and the coarsening of matrix. Some garnets in-group B schists preserve S1 fabric as straight trails defined by sillimanite, fibrolite, and quartz, indicating that sillimanite developed early in the host rocks during D1 deformation.

Findings

DISCUSSION

CONCLUSIONS