A layered basic intrusion, deformed and metamorphosed in granulite faciès of the Sri Lanka basement

Abstract

A layered basic intrusion has been found in the Central Granulite Belt of the Sri Lanka continental basement. It intruded parallel to bedding, before all or early during deformation of neighbouring metasediments. Deformation, affecting metasediments and the intrusion alike, includes flattening to c. 1/20 of the original thickness and NNW-stretching to c. 20 times the original length. The intrusion is now 170–300 m thick. Most of the deformation was acquired under granulite facies metamorphism. The intrusion was then folded, still at high T, by a large F4-synform with an axis parallel to str1 and a steep axial plane. A steep axial plane cleavage and minor folds are related to this big fold. Stretching continued along its axis. Late during formation of this fold a granite intruded, mainly following S4 cleavage planes. The intrusion shows a homogeneous gabbroic series at the bottom, followed upwards by a differentiated and layered series. A thin sequence of ultramafic rocks occurs near the middle. This indicates multiple melt-injection. More homogeneous partly biotite-bearing amphibolites form the top of the succession. Magmatic layering is well preserved, but no magmatic minerals or grain fabrics have escaped deformation or metamorphism. Static annealing under granulite facies conditions outlasted all deformation and was accompanied and followed by the beginning of cooling. Hornblende-Plagioclase coronas formed round garnets at this stage. Geochemical work, carried out by STOSCH (1991) on our samples, confirms the cumulate nature of the rocks.