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

Inclusion-porphyroblast relationships reveal that the garnets in the metapelites of the Highland Complex (HC) of Sri Lanka have a multiple growth history. Garnets grew during a D2 phase of deformation coinciding with the development of crenulation cleavage and their growth reflects the prograde P-T-t path undergone by the HC. Garnet porphyroblasts in a group of schists enclose kyanite inclusion in the cores and they have a two-stage growth history. Their large inclusion-rich cores grew during early D2 deformation in the kyanite stability field enclosing straight, crenulated, and two sets of trails. Their inclusion-free rims grew during late- to post-D2 deformation in the sillimanite stability field. Straight inclusion trails represent the earliest recognizable fabric (S1) produced by a D1 deformation. Collectively these inclusion trails are the expression of a progressively developed crenulation cleavage, S1-S2. Breakdown of staurolite + muscovite led to the growth of inclusion-rich garnet cores with kyanite and staurolite inclusions. A late- to post-D2 increase of temperature produced sillimanite, and the growth of essentially inclusion-free garnet rims. Porphyroblastic garnets in another group of schists and gneisses have sillimanite inclusion in the cores and they have a protracted growth history spanning sillimanite-kyanite-sillimanite stability fields. Their inclusion-rich cores grew early in D2 deformation enclosing similar trail pattern as in the first group of schists but with sillimanite and fibrolite as the prominent trail-forming minerals. Growth of the outer core area of garnets followed enclosing rare staurolite and kyanite inclusions. Inclusion-free rims grew during late- to post-D2 deformation.