Development of Quartz Ribbons in Felsic Granulites Under Strong Coaxial Deformation in the Highland Complex of Sri Lanka

Abstract

Major lineation and foliation in the metamorphic rocks in the Highland Complex (HC) of Sri Lanka have been interpreted as a result of strong coaxial deformation (D2 ) concurrent with the peak metamorphism (>800°C). Quartz ribbons define the structural fabrics in quart-zofeldspathic granulites (QFG). We carried out microstructural analysis of a QFG sample from the HC. We found that the crosscutting of ribbons by scapolite reaction corona corresponds to the peak metamorphism. This crosscutting relationship indicates that the D2 has taken place earlier than the peak metamorphism. The prolate ellipsoidal shape of ribbons manifests pure shear strain geometry that matches the D2 kinematic framework. The quartz ribbons were formed by the expense of D1 fabric by grain boundary migration during coaxial D2. Hence, rheological implications of felsic granulites concurrent with the peak granulite conditions are misleading. Also, care must be taken when interpreting the tectono-metamorphic history of granulite terrains of Gondwana.