Geochemistry and genesis of granitoid rocks from Southern Sri Lanka

Summary Pyroxene granulites of intermediate to felsic composition comprise a major part of the Precambrian terrain exposed in southern Sri Lanka. Chemical characteristics and field relations reveal that there are two closely associated groups of granitoids both occurring as discrete layers or deformed bodies. Granitic-adamellitic rocks are more common and can be distinguished from tonalitic-trondhjemitic rocks. Granitoids of these two groups and minor metabasalt and metadiorite/meta-andesite have been subjected to high-grade metamorphism and were mapped as charnockite or charnockitic rocks. Rocks of granitic composition are mostly meta-aluminous and, in the AFM diagram, fall in the tholeiitic field. These rocks are mainly A-type granites characterized by high K₂O/Na₂O, FeO/MgO and high Zr, Nb and LREE contents, most belonging to the relatively Ce- and Y-enriched A₂-type. Tonalitic rocks are meta-aluminous to per-aluminous and display a calc-alkaline trend. These rocks are classified as I-type, although some of them have very low K₂O and REE contents.Tonalites and granites are not products of a consanguineous fractionation process. In spite of their close field association, they appear to be polygenetic. The parent magmas of the A-type granites seem to have been differentiated melts derived from felsic continental crust. They were emplaced into normal or attenuated continental crust in a non-Compressive (extensional or trans-tensional) tectonic regime. Tonalites most probably represent volcanic arc granitoids emplaced in a compressive tectonic setting. Present intercalation of these polygenetic granitoids, generated in different tectonic environments, is a result of the complex tectonic activity which prevailed before or during Pali-African (550–610 Ma) granulite facies metamorphism.

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Geochemie und Genese von Granitoiden aus dem süchen Sri Lanka

Zusammenfassung Pyroxengranulite intermediärer bis saurer Zusammensetzung bilden einen Hauptbe standteil des präkambrischen Terrains im südlichen Sri Lanka. Chemische Untersuchungen und Feldbeziehungen ermöglichen die Unterscheidung von zwei räumlich eng assoziierten Gruppen von Granitoiden, die beide lagenförmig oder in Form deformierter Körper auftreten. Granitisch-adamellitische Gesteine sind häufiger und sind von tonalitisch-tronhjemitischen Gesteinen zu unterscheiden. Diese Granitoide und untergeordnet vorkommende Metabasalte und Metadiorite/Metaandesite wurden von hochgradiger Metamorphose erfaßt und als Charnockite bzw. charnokitische Gesteine kartiert. Gesteine granitischer Zusammensetzung sind meist metaluminös und fallen im AFM Diagramm ins Feld der Tholeiite. Es sind A-Typ Granite des an Ce und Y angereicherten A2-Subtyps mit hohem K₂O/Na₂O, FeO/MgO und hohen Gehalten an Zr, Nb und LREE. Tonalitische Gesteine sind met-bis peraluminös und folgen einem kalk-alkalischen Trend. Sie werden als I-Typ Granite klassifiziert, obwohl einige Proben sehr niedrige K₂O und REE Gehalte aufweisen. Die Tonalite und Granite sind nicht die Produkte eines cogenetischen Fraktionie rungsprozesses und sind, trotz ihrer räumlichen Assoziation, polygenetischen Ursprungs. Die Ausgangsmagmen der A-Typ Granite scheinen differentierte Schmelzen saurer krustaler Herkunft zu sein, die in normal dicker oder verdünnter Kruste in einem nicht kompressionellen tektonischen Regime (ex- oder transtensional) intrudierten. Die Tonalite sind wahrscheinlich in einem Kompressionsregime enstandene Granitoide eines Vulkanbogens. Das heutige Nebeneinander dieser polygnetischen, in unterschiedlichen geotektonischen Bereichen gebildeten, Granitoide ist das Ergebnis komplexer tektonischer Aktivität vor oder während der panafrikanischen (550–610 Ma) granulitfaziellen Metamorphose.

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With 11 Figures     

The Indian Ocean tsunami of December 26, 2004: observations in Sri Lanka and Thailand

On December 26, 2004 a great earthquake (M _W 9.3) occurred off the western coast of Sumatra triggering a series of tsunami waves that propagated across the Indian Ocean causing damage and life loss in 12 countries. This paper summarizes the observations of lifeline performance, building damage and its distribution, and the social and economic impact of the tsunami made by the Earthquake Engineering Field Investigation Team (EEFIT) in Thailand and Sri Lanka. EEFIT operates under the umbrella of the UK’s Institution of Structural Engineers. It is observed that good engineering practice can reduce economic losses, but additional measures are required to reduce risk to life.     

Numerical simulation and field survey of inundation due to 2004 Indian Ocean tsunami in Trincomalee,Sri Lanka

This paper outlines the field measurements and numerical modelling carried out to develop a high-resolution tsunami inundation map, as a case study, for the city of Trincomalee on the east coast of Sri Lanka, which was devastated by the 2004 tsunami. We employ the deterministic approach together with numerical simulations based on the probable worst-case scenario to derive the inundation map. Linear and non-linear versions of shallow-water equations have been utilized to simulate tsunami propagation and onshore inundation, respectively. The field data considered in the present paper comprise the extent of inundation, the tsunami heights and the arrival times whilst the model results include the spatial distribution of the flow depth, the peak current speeds and the momentum flux. The computed extent of onshore inundation reproduces the observed overall pattern of inundation in most areas barring the south-eastern part of the city. Further, the model simulations suggest maximum flow depths up to about 2 m in most areas of the city whilst patches of flow depths exceeding 2 m can be seen in a narrow strip along the coastline. The computed current speeds also exceed 3 m/s at some locations adjacent to the shoreline.     

Organic geochemical studies of the Muthurajawela peat deposit of Sri Lanka

Organic constituents present in the Muthurajawela peat deposit of Sri Lanka, have been investigated. Here, we report the variations with depth of sitosterol and sitostane (stigmastane), which were found to be major constituents of extracts and, hence, were selected to study the process of diagenesis.These two compounds were present in anomalous concentrations in the middle horizon of the depth profile.Considering the stratigraphy and sterol concentrations, it can be concluded that the middle horizon resulted from a sudden terrestrial input, which could have been due to a Pleistocene or Quaternary environmental change.     

Botanical and geochemical exploration studies at the seruwila copper-magnetite prospect in Sri Lanka

Geobotanical, biogeochemical and geochemical studies were carried out over the Seruwila copper-magnetite prospect near Trincomalee in Sri Lanka. Soil and plant samples were collected along a 180-m transect across the ore body near a gossanous outcrop at Kollan Kulam. Eighteen elements were determined in plant foliage and soil samples. The data showed that in the soils, the concentrations of the ore elements copper and iron and of the pathfinders Ca, Co, Mg, Mn, Mo, Ni and P, were highly indicative of the position of the ore body as determined by a drilling programme.Plant mapping showed that only two species (Glycosmis mauritiana and Pterospermum canescens) were sufficiently ubiquitous to be of use for biogeochemical prospecting in this area. Neither species had a foliar copper content related to Cu levels in the soils, but G. mauritiana had P levels which related well to the position of the ore body and to the concentrations of Fe, Cu and pathfinders in the soils.Principal components and factor analysis of the biogeochemical and geochemical data showed that most of the variance in the plant-soil system was accounted for by the concentrations of ore elements in the soils. It is concluded that the data will be of use for further geochemical and botanical exploration studies in the region or in other genetically similar ore occurrences in South Asia.     

Monsoon and land use in Sri Lanka

Agricultural land use in Sri Lanka roughly shows a division of the island in two major parts, according to the climatic division into thr Wet and Dry Zones. Therefore, it is the ultimate, most important question for agricultural land use in Sri Lanka whether the seasonal occurrence of a distinct dry season, which is characteristic of the Dry Zone, is also the most delimiting factor for land use or not. It seems true, in fact, that the long SW monsoonal dry season in the Dry zone is the major fact for a large-scale division of Sri Lanka according to the crops under cultivation and the cropping systems. Paddy (rice) and tea are the leading crops in the Dry as well as Wet Zone. They are at the same time the crops with the greatest economic importance for the island: tea for the international market, rice for the national market. Thus, the main attention is drawn on specific climatic effects, in a large- and meso-(local-)scale, upon tea and paddy and their agroclimatic potential of cultivation. Besides this, also some effects of the monsoon climate of Sri Lanka upon animal husbandry, forestry and pests have been studied briefly, added finally by an outlook on the correlation between the monsoon climate and fishery even, as manifested in the phenomenon of migrating fishermen.     

Locational specificities of fertility transition in Sri Lanka

Sri Lanka has one of the lowest fertility rates among poor countries of the world. The fertility decline which began in the 1950s has held steady during the last four decades, despite low levels of economic development. Widespread use of contraception is the primary cause of the fertility decline. However, there is no one single homogenous body of people that can be characterized as a ‘modern’ contracepting population through which new methods of family planning have spatially diffused. There is evidence that the overall fertility decline began even before the establishment of the modern family planning program in Sri Lanka. Our analysis showed four broad regional regimes of fertility transition with considerable social and place-to-place differences in method-specific rates of contraception among them. Non-modern ‘traditional techniques’ of contraception which are widely used are an important part of fertility regulation. Sterilization is the most common method of modern contraception in all regions. Less than a tenth of the protected couples use methods such as the pill and the IUD; moreover, these methods are often discontinued after initial use. Traditional methods are an important part of the contraceptive mix in Sri Lanka, and it would be inadvisable to discourage the use of such practices. This revised version was published online in August 2006 with corrections to the Cover Date.     

Arrested charnockite formation in southern India and Sri Lanka

Arrested prograde charnockite formation in quartzofeldspathic gneisses is widespread in the high-grade terrains of southern India and Sri Lanka. Two major kinds of orthopyroxene-producing reactions are recognized. Breakdown of calcic amphibole by reaction with biotite and quartz in tonalitic/granitic gray gneiss produced the regional orthopyroxene isograd, manifest in charnockitic mottling and veining of mixed-facies exposures, as at Kabbal, Karnataka, and in the Kurunegala District of the Sri Lanka Central Highlands. Chemical and modal analyses of carefully chosen immediately-adjacent amphibole gneiss and charnockite pairs show that the orthopyroxene is produced by an open system reaction involving slight losses of CaO, MgO and FeO and gains of SiO₂ and Na₂O. Rb and Y are depleted in the charnockite. Another kind of charnockitization is found in paragneisses throughout the southern high-grade area, and involves the reaction of biotite and quartz±garnet to produce orthopyroxene and K-feldspar. Although charnockite formation along shears and other deformation zones at such localities as Ponmudi, Kerala is highly reminiscent of Kabbal, close pair analyses are not as suggestive of open-system behavior. This type of charnockite formation is found in granulite facies areas where no prograde amphibole-bearing gneisses exist and connotes a higher-grade reaction than that of the orthopyroxene isograd. Metamorphic conditions of both Kabbaltype and Ponmudi-type localities were 700°–800° C and 5–6 kbar. Lower P(H₂O) in the Ponmudi-type metamorphism was probably the definitive factor.CO₂-rich fluid inclusions in quartz from the Kabbaltype localities support the concept that this type of charnockite formation was driven by influx of CO₂ from some deep-seated source. The open-system behavior and high oxidation states of the metamorphism are in accord with the CO₂-streaming hypothesis. CO₂-rich inclusions in graphitebearing charnockites of the Ponmudi type, however, commonly have low densities and compositions not predictable by vapor-mineral equilibrium calculations. These inclusions may have suffered post-metamorphic H₂ leakage or some systematic contamination.Neither the close-pair analyses nor the fluid inclusions strongly suggest an influx of CO₂ drove charnockite formation of the Ponmudi type. The possibility remains that orthopyroxene and CO₂-rich fluids were produced by reaction of biotite with graphite without intervention of fluids of external origin. Further evidence, such as oxygen isotopes, is necessary to test the CO₂-streaming hypothesis for the Ponmudi-type localities.     

AMS 14C dating of deglacial events in the Irish Sea Basin and other sectors of the British–Irish ice sheet

Sedimentary sequences deposited by the decaying marine margin of the British–Irish Ice Sheet (BIIS) record isostatic depression and successive ice sheet retreat towards centres of ice dispersion. Radiocarbon dating by accelerator mass spectrometry (AMS) of in situ marine microfaunas that are commonly associated with these sequences constrain the timing of glacial and sea level fluctuations during the last deglaciation, enabling us to evaluate the dynamics of the BIIS and its response to North Atlantic climate change. Here we use our radiocarbon-dated stratigraphy to define six major glacial and sea level events since the Last Glacial Maximum. (1) Initial deglaciation may have occurred ⩾18.3 kyr ¹⁴C BP along the northwestern Irish coast, in agreement with a deglacial age of ∼22 ³⁶Cl kyr BP for southwestern Ireland. Ice retreated to inland centres and areas of transverse moraine began to form across the north Irish lowlands. (2) Channels cut into glaciomarine deglacial sediments along the western Irish Sea coast are graded to below present sea level, identifying a fall of relative sea level (RSL) in response to isostatic emergence of the coast. (3) Marine mud that rapidly infilled these channels records an abrupt rise in global sea level of 10–15 m ∼16.7 ¹⁴C kyr BP that flooded the Irish Sea coast and may have triggered deglaciation of a marine-based margin in Donegal Bay. (4) Intertidal boulder pavements in Dundalk Bay indicate that RSL ∼15.0 ¹⁴C kyr BP was similar to present. (5) A major readvance of all sectors of the BIIS occurred between 14 and 15 kyr ¹⁴C BP which overprinted subglacial transverse moraines and delivered a substantial sediment flux to tidewater ice sheet margins. This event, the Killard Point Stadial, indicates that the BIIS participated in Heinrich event 1. (6) Subsequent deposition of marine muds on drumlins 12.7 ¹⁴C kyr BP indicates isostatic depression and attendant high RSL resulting from the Killard Point readvance. These events identify a dynamic BIIS during the last deglaciation, as well as significant changes in RSL that reflect a combination of isostatic loading and eustatic changes in global sea level.     

Precambrian structure and chronology in the Highland Series of Sri Lanka

Field studies in the Kandy district and elsewhere in the Highland Series demonstrate that these granulite facies rocks have been repeatedly deformed. The major folds (D3) postdate one and possibly two phases of isoclinal folding (D1 and D2) during which the main L-S fabric was formed. The main metamorphic peak probably occurred during these early phases, around 2200 Ma ago. The D3 episode appears to have predated the formation of the Vijayan gneisses at 1200 Ma. Hornblende-bearing gneisses mainly restricted to synformal basins are regarded as the result of local partial melting and mobilization during D3.     

Modes of occurrence and provenance of Gemstones of Sri Lanka

The island of Sri Lanka is underlain primarily by Precambrian metamorphic rocks belonging to the Amphibole, Cordierite and Pyroxene facies. Gemstones occur mainly in the Cordierite and Pyroxene facies terrains of the Southwestern and Highland Groups of rocks. Gem mining is carried out on hill tops, river valleys and beds and gemstones are found associated with eluvial, residual and alluvial sediments. The dominant gemstones mined are blue sapphires, rubies and beryl. The sources of gem type minerals are determined to be garnetiferous gneisses, cordierite gneisses, marbles and pegmatites.

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Résumé L'île de Sri Lanka est formée presque entièrement des roches métamorphiques de l'âge precambrien. Celles-ci appartiennent aux faciès granulitiques à cordierite et à pyroxène. Les pierres précieuses se trouvent dans les terrains caractérisés par ces faciès que l'on classe dans les groupes dits Highland et Southwestern. L'exploitation des pierres précieuses se fait aux sommets des collines ainsi que dans les vallées ou dans les lits des rivières et une bonne partie de pierres ainsi exploitées sont des saphirs bleus, rubis et de béryls. Les minéraux qui sont à l'origine des pierres précieuses proviennent des gneiss à grenat ou à cordierite, des marbres ou des pegmatites.

     

Thermal and baric evolution of garnet granulites from Sri Lanka

Abstract Garnet granulites from Sri Lanka preserve textural and chemical evidence for prograde equilibration at temperatures of at least 700–750°C and pressures in the vicinity of 6–8 kbar. Associated strain patterns suggest prograde metamorphism occurred during and immediately following an episode of crustal thickening, with the prograde P–T conditions probably reflecting a combination of the conductive and advective transport of heat at the mid-levels of tectonically thickened crust. The occurrence of prograde wollastonite provides evidence for internally buffered fluid compositions, or fluid absent conditions, during peak metamorphism and precludes pervasive advection of a CO₂-rich fluid. The advective heat component is therefore likely to have been provided by the transport of silicate melt. Intricate symplectitic textures record partial re-equilibration of the garnet granulites to lower pressures (˜ 4–6 kbar) at high temperatures (600–750°C), and testify either to the erosional denudation of the overthick crust prior to significant cooling (i.e. quasi-isothermal decompression) or to a subsequent static heating possibly of early Palaeozoic age (Pan-African). The metamorphic history of the Sri Lankan granulites is compared with high grade terrains in the neighbouring fragments of Gondwana, with the emphasis on similarities with Proterozoic granulites of the East Antarctic craton.     

Dry years and their relationship to crop production in Sri Lanka

Drought is a distinct agroclimatological hazard with far reaching consequences upon crop production. Among them, famines are regarded the most serious effects of climatological drought. Although there is no doubt about the principally valid relationship between drought and production losses it seems more problematic how this relationship can be proved statistically. In this paper annual rainfall data and production figures of the three major crops of Sri Lanka, rice (paddy), tea and rubber, are correlated, but only partially a true relationship between dry years and production losses could be observed. Reason for this may be the moderate degree of annual drought. From the results shown, the question arises how agricultural drought can be defined and how the climatic effects upon crop production can be studied satisfactorily.     

Tidal choking and bed friction in Negombo Lagoon,Sri Lanka

We have studied bed friction and water exchange in a small, almost enclosed, shallow lagoon in Sri Lanka, where both tides and freshwater supply are important for the circulation. Because of the narrow entrance, the lagoon is strongly choked for semidiurnal tidal forcing (the choking coefficient is 0.25). A set of tide gauges positioned on each side of the entrance channel was used to calculate bed friction. Measurements made over three short periods having different freshwater supply to the lagoon indicate an average friction (drag) coefficient C_D=0.0041±0.0002 (using a quadratic friction law). A comparison between observed and calculated velocities in the channel for a variety of velocities, indicates that the quadratic friction law is a good tool for parameterization. High freshwater supply increases the lagoonal sea level substantially and decreases the residence time.     

Factors controlling fluoride contents of groundwater in north-central and northwestern Sri Lanka

Chemical characterization has been made of groundwater bodies at 294 locations in four village districts in north-central and northwestern Sri Lanka, with special focus on fluorine contamination. High fluoride contents in groundwater are becoming a major problem in the dry zone of Sri Lanka, and dental fluorosis and renal failures are widespread. Field measurements of temperature, pH, and electrical conductivity were made during sampling. Chemical analyses of the water samples were later made using atomic absorption spectroscopy, spectrophotometry, and titration. Fluoride concentrations in the study area vary from 0.01 to 4.34 mg/l, and depend on pH and the concentrations of Na, Ca, and HCO₃ ⁻. Basement rocks including hornblende biotite gneiss, biotite gneiss, and granitic gneiss seem to have contributed to the anomalous concentrations of fluoride in the groundwater. Longer residence time in aquifers within fractured crystalline bedrocks may enhance fluoride levels in the groundwater in these areas. In addition, elevated fluoride concentrations in shallow groundwater in intensive agricultural areas appear to be related to the leaching of fluoride from soils due to successive irrigation.     

Test of AMS 14C dating of pollen concentrates using tephrochronology

Previous attempts to radiocarbon date sediments >10 kyr from the high rainfall region of Westland, New Zealand, a critical location for investigation of interhemispheric patterns of climate change, have been problematic. This study, building on recent work by Vandergoes and Prior ( 2003 ), shows that AMS ¹⁴C dating of pollen concentrates has potential to provide more reliable ages than other sediment constituents, including plant macrofossils. The method was applied to sediments from three sites containing the 22.6k ¹⁴C yr Kawakawa Tephra, which provided an independent test of the ¹⁴C ages. Although some minor laboratory contamination was detected in tests on background standards, the modelled relationship between sample mass and measured ¹⁴C content permitted an appropriate correction to be determined. Improved pollen concentrations derived by density separation between 1.4 and 1.2 specific gravity and sieving in the range 10–50 μm provided either older ages than other fractions of the same sample or, where in situ contamination was not evident, equivalent ages. Differences in degree of in situ contamination between depositional environments indicated that, in Westland, lake sites may be less susceptible to contamination by younger carbon than peat sites, where this process may be facilitated by root penetration into underlying sediments. Copyright © 2006 John Wiley & Sons, Ltd.     

Four types of relationships between rainfall and paddy production in Sri Lanka

Due to the absence of marked thermal differences in Sri Lanka, the paddy-climate relationship has been confined to rainfall alone in this study. Increase in total paddy production is highly attributed by the increase in paddy lands rather than yields, particularly in the drier parts, called Dry Zone. The effective rainfall period for the two principal cropping seasons in Sri Lanka, Maha and Yala, were determined according to the crop calendar of the Districts. The deviation values from the trend lines of 20 years (1961–1980) observations in sown, harvest and yield were well correlated with the deviations from the mean rainfall. The effects of rainfall on paddy production are well pronounced in the Dry Zone during the Maha season and in the Wet Zone during the Yala season. From the statictical analyses, four types of relationship, were observed: (1) Type I is the positive relationship, (2) Type II is the negative relationship, (3) Type III is the no realtionship and (4) Type IV a complex one which has a positive relationship below certain critical rainfall value and no or negative relationship above the critical rainfall value. The combination of Types I and IV shows the areas which need irrigation facilities with proper water management while the combination of the Types II and III indicates the areas which are prone to frequent floods and waterloggings.     

Weathering of Monuments at Jethawanaramaya Complex in North-Central,Sri Lanka

Abstract: Sri Lanka has the richest archaeological sites in Asia. Jethawanarama Complex, one of the valuable sites in the country, is suffering from deterioration due to weathering. Monuments were built mainly from stones (granitic gneiss and marble) and clay bricks. The present study aimed to categorize weathering forms and interpret the recently-developing weathering processes. The growing of lichens on surfaces and the development of saline conditions are the major threats on the survival of monuments other than the typical weathering processes of tropical climates Morinite (NaCa2Al2[PO4)]2[F,OH]5·2H2O) is identified as a weathering product of monuments and is generated from lichens.     

Geochemical and petrological characteristics of Eppawala phosphate deposits,Sri Lanka

The apatite-bearing carbonate rocks at Eppawala, Sri Lanka occur as massive, discontinuous bodies in a Precambrian, high-grade metamorphic terrain, which weather to form economically important phosphate deposits. The ore bodies at Eppawala contain =42% P ₂O ₅, and citric acid solubility of different components varies from 4 to 6%. The parent rocks are mainly made up of calcite, dolomite and apatite, with lesser amounts of ilmenite, magnetite, pyrite, forsterite, phlogopite, enstatite, magnesite, diopside, tremolite and spinel. Most of minerals show an euhedral habit, with a wide range of crystal sizes (from a few millimetres to several decimetres). The Eppawala rocks are characterised by low silica (=0.41%), high phosphorous (=10.58%) and high strontium content (2,960–6,819 ppm). Concentrations of light rare-earth elements in these rocks are comparably higher than those of marbles. The REE fractionation of these rocks is pronounced, and La/Yb ratios vary between 14 and 43. Both apatite and calcite show markedly elevated strontium levels (=0.6%). The d ¹³C _PDB and d ¹⁸O _SMOW values of the carbonates are in the range of –3.4 to –2.2 and 7.7 to 16.4‰ respectively. The euhedral habit, as well as the presence of major quantities of apatite and considerable amounts of iron-bearing minerals suggest that the ore host rock has genetic links to an igneous source rather than to an intensely metamorphosed limestone. The higher light REE contents of the rocks, compared to marbles, also argue against a metamorphic or sedimentary origin. The Sr/Mn and Ce/La ratios in the apatite are ~40 and ~2 respectively, suggesting that they were formed in a carbonatite magma. The markedly increased REE concentrations in the bulk chemistry of the rocks have been shown to be mainly controlled by the content of phosphate minerals. Compared to most carbonatites, the Eppawala rocks are generally depleted in selected trace elements, particularly Ba, Nb, Th, V, U and Zr. This depletion may be due to either a primary infertility of the parent magma with regard to such trace elements, or it is a result of fractional crystallisation during the rock formation. The stable isotope ratios do not plot within the defined "mantle carbonatite box", but still lie within the broader range of carbonatitic rocks. With these data at hand, it can be readily argued that the mode of occurrence, petrography and geochemistry of the Eppawala apatite-bearing carbonates provide conclusive evidence of their carbonatitic origin.