The precambrian of Sri Lanka: a historical review

Abstract

Early studies of the geology of Sri Lanka lasted from the beginning of the century until 1935. The modern phase began in the forties, whereas intensive studies of the Precambrian rocks took place in the last ten years. The term “Vijayan Gneiss”, first used for the gneisses around Colombo, was changed in 1947 to “Vijayan Series” for all rocks outside the “Central Khondalite-Charnockite Belt”. The latter was named the “Highland Series” in 1961, and the “Vijayan Series” was subdivided into an “eastern” and a “western” component, and the name was altered to Vijayan Complex in 1978. In 1959 a “Southwestern Group” was recognized, and in 1935 the “Kadugannawa Gneiss” was named. At present there is a general agreement that the following nomenclature should be used by all workers on the Precambrian of Sri Lanka: Highland Complex (HC) (formerly “Highland Series” and “Southwestern Group”); an intimate association of interbanded ortho- and paragneisses including pelitic gneiss, metaquartzite, marble and charnockitic gneiss, which forms the broad belt of rocks extending from northeast to southwest through the central highlands of the island. Outliers of HC rocks (tectonic klippen) are found in the Vijayan Complex at Kataragama and Maligawila. Vijayan Complex (VC) (formerly “Eastern Vijayan Complex”), consisting of migmatites, granitic gneisses, granitoids and scattered metasediments, and occurring in the east and south of the HC. Wanni Complex (WC) (formerly “Western Vijayan Complex”), consisting of migmatites, granitic gneisses, charnockitic gneisses, minor metasediments and granitoids, lying west of the HC. Kadugannawa Complex (KC) (partly the former “Kadugannawa Gneiss”), for the hornblende and biotite-hornblende gneisses within the elongate synformal basins (“Arenas”) around Kandy. In the early sixties it was noted that the “Highland Series” rocks had been subjected to granulite facies metamorphism and the “Vijayan Series” rocks to amphibolite facies metamorphism. Charnockitic rocks in the “Highland Series” showed the characteristics of metamorphic rocks. Recently, near-peak metamorphic conditions in the HC were established at 770–900°C and 5 kbar in the west (WC) to 9 kbar in the east (HC), with a pronounced regional gradient in palaeopressure. Relict inclusions of staurolite and kyanite in garnet of HC pelitic gneisses have been shown to document a late stage of a prograde metamorphic P-T path. A retrograde P-T path, with coronitic assemblages of retrograde mineral phases like andalusite has also been recorded. Outliers of HC rocks in the VC are klippen thrust over the VC. Parts of KC have been interpreted as a metamorphosed layered basic complex. In the late seventies three phases of deformation were recognized in the HC; presently four phases are known to occur. An early view was that the “Highland Series” was younger than the “Vijayan Series”, the former sediments having been laid down on a Vijayan floor. The HC is now known to be older than the VC, the boundary between them being a thrust contact. The exact position and nature of the HC-WC boundary, however, is still to be resolved. Recent geochronological data suggest that: (1) HC sedimentation as well as igneous activity in the form of basic lava flows, basic sills and dykes took place ∼2.0 Ga ago; (2) intrusion of the earliest granitoids into the HC took place 1.8 to 1.94 Ga ago; (3) orthogneisses of the WC were intruded between 0.67 and 1.1 Ga ago; (4) orthogneisses of the VC were intruded 1030 to 1040 Ma ago; and (5) high-grade regional metamorphism of the HC, VC and WC took place between 455 and 610 Ma ago.