GR zy Gondwana Research zy Evidence for Post-Metamorphic Metasomatism of High-Grade Orthogneisses from Sri Lanka K.V. Wilbert Kehelpannala and N. Prasanna Ratnayake Structural Geology Programnze, Institute of Fundamental Studies, Hantana Road, Kundy, Sri Lanka e-mail: wilbert@ifs.ac.lk (Mrm~iscript received zyxwvutsr JL~IW zyxwvutsrq 11, 1998; accepted November 16, 1998) Abstract Here we discuss the post-metamorphic metasomatism of high-grade orthogneisses by studying granite-looking, pink-coloured microcliiie-bearing rocks exposed around Ambagaspitiya, Sri Lanka. These rocks are medium- to coarse- grained and are more or less homogeneous aitd isotropic. Textural and petrographic analyses clearly show that these special rocks are neither deformed nor metamorphosed and that they are not any kind of intrusive rocks. The present study shows that these rocks have formed through K-metasomatism of once intensely deformed aitd metamorphosed granodiorite, tonalite, monzodiorite and quartz monzodiorite. The modal compositions of most of these metasomatic rocks of Ambagaspitiya are very similar to those of syenite, quartz syenite, monzonite, quartz monzonite aiid quartz monzodiorite. All the original metamorphic rocks zyxwvutsr - namely granitic gneiss, metagranite, metagranodiorite, metatonalite, metamonzodiorite, metaquartz monzodiorite, metadiorite, basic dikes and metapelites - had undergone at least five ductile deformations, D, to D,, and had been metamorphosed under upper ampltibolite to granulite facies conditions prior to the metasomatism. Almost all the parent metamorphic rocks had acquired a well-developed gneissic foliation (S2) aiid had suffered at least two intense folding events (F3 and FJ before tlte metasomatism occurred. All the metamorphic and deformatioiial fabrics of affected metamorphic rocks have beeit completely or partially obliterated by the metasomatism. This indicates that the metasomatic process postdates all ductile deformations (D, - DJ and tlte regional metamorphism. Of the parent metamorphic rocks, metagranodiorite, metatonalite, metamonzodiorite and metaquartz monzodiorite have undergone intense metasomatism. It is shown that the metasomatism has nucleated along iate-stage, post-D, shear zones, which may form an interconnected network. Potassium-bearing metasomatic fluids, derived from a deep-seated K-rich source, may have migrated along these shear zones. The fluids which entered the shear zones have pervaded the orthogneisses through foliation planes and along grain boundaries and microcracks in minerals, transforming the host gneisses to metasomatic rocks. The main metasomatic transformation has taken place through the replacement of metamorphic plagioclase and plagioclase-quartz by microcline and through formation of myrmekite. Further studies are necessary to unravel the nature, composition and tlte source of these late-stage K-rich fluids in the lower crust. Key words: Orthogneisses, shear zones, post-metamorphic fluids, K-metasomatism, myrmekitization. Introduction Pink coloured, microcline-bearing, granite-looking rocks occur in the high-grade gneiss terrain of Sri Lanka, especially in the newly defined Wanni and Vijayan Complexes (Geological Map of Sri Lanka, 1982). Most of these rocks are granitic in composition and are exposed in areas surrounding Kandy, Galagedara, Aranayake, Mawanella, Kegalle, Galigomuwa, Avissawella and Tonigala in the Wanni Complex (Fig. 1). The origin of the pink-coloured granitic rocks of Sri Lanka is controversial at present (Cooray, 1971; Munasinghe and Dissanayake, 1980;Perera, 1983; Voll and Kleinschrodt, 1991; Kehelpannala, 1991). Whatever their origins, they are metamorphosed and moderately to highly deformed rocks. They exhibit a well- developed gneissic foliation and show evidence for at least z two to three folding events (e.g. Cooray, 1971; Perera, 1983; Kehelpannala, 1991). The origin, deformation and metamorphism of the granitoid rocks in the Vijayan Complex are also poorly understood. The specific purpose of this paper is to discuss the origin of the pink-coloured, granite-looking felsic rocks exposed around Ambagaspitiya, z Gondwcinu zyxwvutsrqponmlkj Kesecirclr, zyxwvutsrqp V zyxwvutsrqponm 2, No. 2, zyxwvutsrqponm pp. 167-184. zyxwvutsrqp 0 1999 Inferiiciiionol A.ssocin~ion jiw Gondwanci Research. Jupan. ISSN: 1342-937X