Secondary siderite-oxide-sulphide and carbonate-andalusite assemblages in cordierite granulites from Sri Lanka: post-granulite facies fluid evolution during uplift

We report here that some of the pelitic rocks from the Wanni and Highland Complexes of Sri Lanka reacted with CO₂-rich fluids to produce a wide range of unusual secondary carbonate-silicate-oxide-sulphide assemblages. These enable the depth, temperature and fluid compositions of CO₂ reactions to be calculated more rigorously than is generally possible for the patches of arrested charnockite that have been described from Sri Lanka. Magnesite-andalusite-quartz has partially replaced primary cordierite, and siderite-rutile replaced ilmenite. Paragenetic sequences involving primary pyrrhotite, ilmenite and magnetite and secondary pyrite-siderite-rutile-magnetite-(hematite) demonstrate the control which carbonate equilibria have upon evolving fluid compositions during cooling. Direct evidence for the role of graphite as a source of CO₂ is found in the Highland Complex where primary graphite partially reacted with silicates to form secondary siderite assemblages. It is proposed that following peak metamorphism, continued uplift along a clockwise P-T-t path was accompanied by a series of devolatilization reactions involving breakdown of graphite and the continuous production of secondary CO₂-rich fluids. The limited extent of disseminated secondary carbonate reflects the small amount of graphite inferred to have been present in the source rocks. These rocks demonstrate that CO₂-rich fluids, as found in disseminated fluid inclusions, need not form during peak granulite metamorphism but may be an inevitable consequence of continued uplift along a clockwise P-T-t path. The arrested charnockite which overprinted some of the hornblende-bearing felsic-intermediate composition rocks in Sri Lanka most likely formed by the same process. Received: 4 May 1994 / Accepted: 25 October 1996     

Cross-twinning in a natural spinel from Sri Lanka

A modified cross-twinning growth mechanism is put forward to explain the anomalous morphology of a spinel multiple-twin from Sri Lanka, flattened crosswise the twin planes. Cross-twinning in spinel was found also in other specimens from Pegu (Myanmar), and the results were published in a previous paper. This particular type of twinning is derived from the combination of cyclic twinning with lamellar twinning, so that these samples may be thought of as partial fivelings (cubic cyclic {111} twins with five components sharing a common <110> pseudo-fivefold axis). In the present paper, the sample from Sri Lanka has been suitably cut with the aim of focusing the study on the cross-twinning region. The transformation matrices that link the orientation states of each couple of twin components have been determined by means of White Beam Synchrotron Radiation Topography. They showed that the specimen is made up of four twin components (A, B, C and D), with three twin planes: and They also showed that the cross-twinned individuals (B and D) actually are not twinned to each other, and that a simple crystallographic relationship holds between them. X-ray diffraction topography by conventional source allowed to image the crossing-region and to determine that the cross-twinned individuals are in contact through a semi-coherent boundary, with twinning dislocations contributing to relieve the coherency strains. Electron probe microanalyses with wave dispersive spectroscopy showed that the chemical composition is almost homogeneous, at least within the spatial resolution limit of this technique. The similar growth features observed in the spinel sample from Sri Lanka and in those from Myanmar are interpreted as growth marks, indicators of a similar origin: in both cases they are found in impure dolomitic marbles. In particular, the specimen from Sri Lanka results from the interaction of thermal and metasomatic effects due to contact metamorphism. An unusual stepped morphology of the face close to the twin boundary, possibly due to corrosion and re-growth processes acted preferentially at a re-entrant corner by metasomatic fluids, is interpreted as indicator of a metasomatic event that succeeded to the crystal growth, the latter occurred by thermal effect.

A New Appraisal of Sri Lankan BB Zircon as a Reference Material for LA‐ICP‐MS U‐Pb Geochronology and Lu‐Hf Isotope Tracing

A potential zircon reference material (BB zircon) for laser ablation‐inductively coupled plasma‐mass spectrometry (LA‐ICP‐MS) U‐Pb geochronology and Hf isotope geochemistry is described. A batch of twenty zircon megacrysts (0.5–1.5 cm³) from Sri Lanka was studied. Within‐grain rare earth element (REE) compositions are largely homogeneous, albeit with some variation seen between fractured and homogeneous domains. Excluding fractured cathodoluminescence bright domains, the variation in U content for all analysed crystals ranged from 227 to 368 μg g^?1 and the average Th/U ratios were between 0.20 and 0.47. The Hf isotope composition (0.56–0.84 g/100 g Hf) is homogeneous within and between the grains – mean ¹⁷⁶Hf/¹⁷⁷Hf of 0.281674 ± 0.000018 (2s). The calculated alpha dose of 0.59 × 10¹⁸ g^?1 for a number of BB grains falls within the trend of previously studied, untreated zircon samples from Sri Lanka. Aliquots of the same crystal (analysed by ID‐TIMS in four different laboratories) gave consistent U‐Pb ages with excellent measurement reproducibility (0.1–0.4% RSD). Interlaboratory assessment (by LA‐ICP‐MS) from individual crystals returned results that are within uncertainty equivalent to the TIMS ages. Finally, we report on within‐ and between‐grain homogeneity of the oxygen isotope systematic of four BB crystals (13.16‰ VSMOW).     

New LA-ICPMS U–Pb ages of detrital zircons from the Highland Complex: insights into late Cryogenian to early Cambrian (ca. 665–535 Ma) linkage between Sri Lanka and India

ABSTRACT

Here we report new LA-ICPMS U–Pb zircon geochronology of ultrahigh temperature (UHT) metasedimentary rocks and associated crystallized melt patches, from the central Highland Complex (HC), Sri Lanka. The detrital zircon ²⁰⁶Pb/²³⁸U age spectra range between 2834 ± 12 and 722 ± 14 Ma, evidencing new and younger depositional ages of sedimentary protoliths than those known so far in the HC. The overgrowth domains of zircons in these UHT granulites yield weighted mean ²⁰⁶Pb/²³⁸U age clusters from 665.5 ± 5.9 to 534 ± 10 Ma, identified as new metamorphic ages of the metasediments in the HC. The zircon ages of crystallized in situ melt patches associated with UHT granulites yield tight clusters of weighted mean ²⁰⁶Pb/²³⁸U ages from 558 ± 1.6 to 534 ± 2.4 Ma. Thus, using our results coupled with recently published geochronological data, we suggest a new geochronological framework for the evolutionary history of the metasedimentary package of the HC. The Neoarchean to Neoproterozoic ages of detrital zircons indicate that the metasedimentary package of the HC has derived from ancient multiple age provenances and deposited during the Neoproterozoic Era. Hence, previously reported upper intercept ages of ca. 2000–1800 Ma from metaigneous rocks should be considered as geochronological evidence for existence of a Palaeoproterozoic igneous basement which possibly served as a platform for the deposition of younger supracrustal rocks, rather than timing of magmatic intrusions into the already deposited ancient sediments, as has been conventionally interpreted. The intense reworking of entire Palaeoproterozoic basement rocks in the Gondwana Supercontinent assembly may have caused sediments of multiple ages and provenances to incorporate within supra-crustal sequences of the HC. Further, our data supports a convincing geochronological correlation between the HC of Sri Lanka and the Trivandrum Block of Southern India, disclosing the Gondwanian linkage between the HC of Sri Lanka and Southern Granulite Terrain of India.     

Thermal gradients in the Sri Lankan granulite terrane: a garnet–biotite thermometric approach

Thermal zoning of the Highland Complex, Sri Lanka has been delineated using the Fe²⁺–Mg distribution coefficient between garnet and biotite from garnet–biotite gneiss samples collected with wide geographical distribution. In order to minimize the potential for retrograde Fe–Mg exchange and maximize the potential for retaining peak equilibrium K_D (garnet–biotite) and temperature, garnet and biotite included within feldspar and quartz without other mineral inclusions have been selected. The calculated results indicate four distinct temperature contours with K_D values varying from 1.84 to 6.38 and temperature varying from 996 to 591 °C. From the present results, it is possible to divide the Highland Complex into two major metamorphic zones: a high‐temperature area in the central region and a low‐temperature area in the south‐western and north‐eastern region. In conjunction with the metamorphic pressure variations estimated from the granulites of the Highland Complex in previous studies, it is shown that the high‐ and low‐temperature areas are complemented by a high‐pressure region towards the eastern side and a low‐pressure region towards the western side of this complex. This thermal dome is interpreted to be an artifact of the different crustal levels exhumed following Pan‐African metamorphism.     

Sri Lanka - India Intraplate Tectonics - Precambrian to Present

The tectonic history of Sri Lanka - India can be traced from the Precambrian to the present. On the basis of the geological record, plate tectonic processes have operated for example, the Highland Group of Sri Lanka may have represented a Precambrian plate tectonic suture. Tectonic models of these Precambrian events may be presented by spreading, collision, subduction, shearing or in situ jostling. The recent tectonic history of Sri Lanka and India relates to the evolution of the Indian Ocean since at least the Cretaceous. Although Sri Lanka is considered to be a part of the larger Indo - Australian plate, it may have had a local independent history as a block within the larger crustal unit of India. There is evidence that the separation of Sri Lanka from India was in part controlled by Precambrian structures and a history of translational, rotational and vertical adjustments to the Indian Ocean developmental plate tectonic stresses still operating.     

Seasonal variability of sea surface chlorophyll-a of waters around Sri Lanka

Remotely sensed data on ocean colour of waters surrounding Sri Lanka received from the Coastal Zone Colour Scanner (CZCS) are processed and analyzed. Raw data of 1 km resolution on relatively cloud free days during 1978–1986 are processed to produce sea surface chlorophyll maps within latitudes 4.5N-11N and longitudes 78E-85E, a region in the Indian Ocean surrounding Sri Lanka. The processed data include about 110 single day maps and composite averages for each month and season. The months of July, August and September are omitted in the calculation of averages due to insufficient data. The waters in the Gulf of Mannar and Palk Bay areas show high chlorophyll-a concentrations throughout the year. However, these high values may represent other suspended particles and dissolved organic matter besides chlorophyll-a as this region is shallow (< 100 m). Regions with high chlorophyll concentrations (> 0.5 mg m^-3) along the coast and western ocean region can be seen in the months of October and November, after the southwest monsoon period. As high surface chlorophyll concentrations may indicate high productivity, these regions need extensive measurements of primary production and also continuous monitoring of fish catches, during and after the southwest monsoon. Studies of particle composition in shallow water areas, in particular waters in Palk Bay and Gulf of Mannar, should be carried out in order to elucidate the effect of non-phytogenic.     

Rapid Lateglacial tree population dynamics and ecosystem changes in the eastern Baltic region

A growing body of evidence implies that the concept of 'treeless tundra' in eastern and northern Europe fails to explain the rapidity of Lateglacial and postglacial tree population dynamics of the region, yet the knowledge of the geographic locations and shifting of tree populations is fragmentary. Pollen, stomata and plant macrofossil stratigraphies from Lake Kurjanovas in the poorly studied eastern Baltic region provide improved knowledge of ranges of north‐eastern European trees during the Lateglacial and subsequent plant population responses to the abrupt climatic changes of the Lateglacial/Holocene transition. The results prove the Lateglacial presence of tree populations (Betula, Pinus and Picea) in the eastern Baltic region. Particularly relevant is the stomatal and plant macrofossil evidence showing the local presence of reproductive Picea populations during the Younger Dryas stadial at 12 900–11 700 cal. a BP, occurring along with Dryas octopetala and arctic herbs, indicating semi‐open vegetation. The spread of Pinus–Betula forest at ca. 14 400 cal. a BP, the rise of Picea at ca. 12 800 cal. a BP and the re‐establishment of Pinus–Betula forest at ca. 11 700 cal. a BP within a span of centuries further suggest strikingly rapid, climate‐driven ecosystem changes rather than gradual plant succession on a newly deglaciated land. Copyright © 2009 John Wiley & Sons, Ltd.     

Influence of nearshore sediment dynamics on the distribution of heavy mineral placer deposits in Sri Lanka

Beach sediments in Sri Lanka contain industrial-grade heavy mineral occurrences. Samples of both offshore and onshore sediments were collected to examine the provenance, mineralogy and geochemical compositions of the heavy mineral occurrences. Coastal morphodynamic changes along the coastline of Sri Lanka were analyzed using the time-series satellite images. These coastal morphodynamic changes were used to identify the prominent directions of monsoon-influenced longshore currents, coastal sediment accretion and depositional trends and their relationships to the provenance of the heavy minerals. Results show the concentrations of detrital ilmenite, zircon, garnet, monazite, and rutile vary in the onshore and offshore sediments. The heavy mineral potential of the northeastern coast is high (average contents of about 45–50% in the Verugal deposit, 70–85% in the Pulmoddai deposit, and 3.5–5.0% in offshore samples stretching from Nilaveli to Kokkilai), compared to sediments in southwest (average content about 10% in onshore sediments and 2% in offshore sediments from the mouth of the Gin River). Therefore, no economic-grade heavy mineral placers were identified in the offshore environments. The high concentrations of heavy minerals in beach sediments and low concentrations in offshore sediments suggest operation of a panning system in the surf zone to form enriched placer deposits. Major and trace element compositions of beach sediments show marked enrichments of TiO₂, Fe₂O₃, La, Ce, Zr, Cr, Nb, Th and V compared to average Upper Continental Crust (UCC) values. Analysis of prominent coastal longshore transport patterns identifies bidirectional sediment transport in the northeast coast of Sri Lanka. In the southwestern coast, two transport directions occur with anti-clockwise transport from Galle to Hambantota, and clockwise transport from Hikkaduwa to Wadduwa. The heavy minerals in the placers were mainly derived from Precambrian metamorphic rocks, and transported to the coast through the river systems of Sri Lanka.     

Crustal structure and rift tectonics across the Cauvery–Palar basin,Eastern Continental Margin of India based on seismic and potential field modelling

The Cauvery–Palar basin is a major peri-cratonic rift basin located along the Eastern Continental Margin of India (ECMI) that had formed during the rift-drift events associated with the breakup of eastern Gondwanaland (mainly India–Sri Lanka–East Antarctica). In the present study, we carry out an integrated analysis of the potential field data across the basin to understand the crustal structure and the associated rift tectonics. The composite-magnetic anomaly map of the basin clearly shows the onshore-to-offshore structural continuity, and presence of several high-low trends related to either intrusive rocks or the faults. The Curie depth estimated from the spectral analysis of offshore magnetic anomaly data gave rise to 23 km in the offshore Cauvery–Palar basin. The 2D gravity and magnetic crustal models indicate several crustal blocks separated by major structures or faults, and the rift-related volcanic intrusive rocks that characterize the basin. The crustal models further reveal that the crust below southeast Indian shield margin is ～36 km thick and thins down to as much as 13–16 km in the Ocean Continent Transition (OCT) region and increases to around 19–21 km towards deep oceanic areas of the basin. The faulted Moho geometry with maximum stretching in the Cauvery basin indicates shearing or low angle rifting at the time of breakup between India–Sri Lanka and the East Antarctica. However, the additional stretching observed in the Cauvery basin region could be ascribed to the subsequent rifting of Sri Lanka from India. The abnormal thinning of crust at the OCT is interpreted as the probable zone of emplaced Proto-Oceanic Crust (POC) rocks during the breakup. The derived crustal structure along with other geophysical data further reiterates sheared nature of the southern part of the ECMI.     

The geochemistry of beryllium and fluorine in the gem fields of Sri Lanka

A study of the abundance of Be in the gem sediments of Sri Lanka shows that Be is found in the range of 1–13 ppm. Be shows an irregular distribution among sediments. It occurs in the silicate form and due to the proximity to the beryllium bearing rocks, namely granites and pegmatites of the Highland and Southwest Groups of Sri Lanka, very little decomposition of the Be-bearing minerals had taken place. This is further aided by the high resistance to weathering of the beryllium minerals, particularly beryl and chrysoberyl.The beryllian granites and pegmatites of the Precambrian of Sri Lanka are presumed to have been formed due to the magmatic activity associated and related to charnockitic rocks abundant in the main gem bearing areas of Sri Lanka.Fluorine is found in the range of 400–2,000 ppm and the F/Be ratios for all the areas studied show a range of 54–441. The analysis of the averages of these ratios do not show any particular anomaly in any of the areas studied. The narrow ranges of the F/Be ratios indicate the similar conditions under which weathering and geochemical transportation had taken place in the gem fields of Sri Lanka.     

The origin of the pink granites of Sri Lanka — another view

Pink granites are common throughout the Precambrian metasedimentary terrain of Sri Lanka and play a significant role in reconstructing the geologic evolution of this country. Essentially microcline-bearing rocks, they form conformable layers with the associated metasedimentary rocks and are always interlayered and associated with rocks possessing amphibolite-facies mineral assemblages. This association is seen throughout the terrain at all scales. Further, they always tend to fold around the major structures of the terrain and contain conformable layers and enclaves of rare metasediments, and have a gneissic appearance in places. Their field relations, associations, mineralogy, petrography, petrofabrics, chemistry, and also age, suggest that they are metasediments, probably arkoses, which have undergone the same set of metamorphic and deformational events as other Precambrian metamorphic rocks of Sri Lanka. Their present ‘granite-like’ character does not necessarily imply that they are the products of crystallization of magmatic or anatectic melts, or that they have had a replacement origin. Rather, chemical composition has determined their present ‘granite-like’ character, due to granitization by simple isochemical recrystallization — “Treptomorphism” — during highgrade metamorphism, at a P_H2O which was sufficient to retard their grade to amphibolite facies under granulite facies metamorphic conditions, at which the other metasediments of Sri Lanka have metamorphosed.     

Carbon-isotope constraints on fluid advection during contrasting examples of incipient charnockite formation

Abstract Incipient charnockite formation within amphibolite facies gneisses is observed in South India and Sri Lanka both as isolated sheets, associated with brittle fracture, and as patches forming interconnected networks. For each mode of formation, closely spaced drilled samples across charnockite/gneiss boundaries have been obtained and δ¹³C and CO₂ abundances determined from fluid inclusions by stepped-heating mass spectrometry. Isolated sheets of charnockite (c.50 mm wide) within biotite–garnet gneiss at Kalanjur (Kerala, South India) have developed on either side of a fracture zone. Phase equilibria indicate low-pressure charnockite formation at pressures of 3.4 ± 1.0 kbar and temperatures of about 700°C (for X_H2O= 0.2). Fluid inclusions from the charnockite are characterized by δ¹³C values of ?8% and from the gneiss, 2 m from the charnockite, by values of ?15%. The large CO₂ abundances and relatively heavy carbon-isotope signature of the charnockite can be traced into the gneiss over a distance of at least 280 mm from the centre of the charnockite, whereas the reaction front has moved only 30 mm. This suggests that fluid advection has driven the carbon-isotope front through the rock more rapidly than the reaction front. The carbon-front/reaction-front separation at Kalanjur is significantly larger than the value determined from a graphite-bearing incipient charnockite nearby, consistent with the predictions of one-dimensional advection models. Incipient charnockites from Kurunegala (Sri Lanka) have developed as a patchy network within hornblende–biotite gneiss. CO₂ abundances rise to a peak near one limb of the charnockite, and isotopic values vary from δ¹³C of c.?5.5% in the gneiss to ?9.5% in the charnockite. The shift to lighter values in the charnockite can be ascribed to the formation of a CO₂-saturated partial melt in response to influx of an isotopically light carbonic fluid. Thus, incipient charnockites from the high-grade terranes of South India and Sri Lanka reflect a range of mechanisms. At shallower structural levels non-pervasive CO₂ influxed along zones of brittle fracture, possibly associated with the intrusion of charnockitic dykes. At deeper levels, in situ melting occurred under conditions of ductile deformation, leading to the development of patchy charnockites.     

Century scale climate change in the central highlands of Sri Lanka

In this study, an analysis of century scale climate trends in the central highlands of Sri Lanka is presented. Monthly rainfall and temperature records of the period 1869–2006 from five climatological stations were analyzed. The trend is calculated by the least square regression analysis and the significance of the observed trend is estimated using the Mann–Kendall statistic. The results clearly show that there is a statistically significant decrease in annual rainfall in the western slopes of the central highlands. Throughout the last century, the annual reduction of rainfall in Nuwara Eliya which is at an altitude of 1895 m was 5.2 mm/year. The decrease is largely due to the reduction in southwest monsoon rainfall which contributes to 75% of the total reduction. No significant change was observed on the eastern side of the central highlands which receives rainfall predominantly from the northeast monsoons. The mean annual temperature in the mountainous region shows a uniform increasing trend which is in line with the 100-year global temperature increase of 0.8 ± 0.2^°C. Kandy, which is at an altitude of 477 m and closely linked with the rainfall climatology of Nuwara Eliya, showed no significant change in the mean annual temperature. If the current trend continues, in another 100 years, western and eastern slopes of central highlands will receive the same amount of rainfall from the southwest monsoon and the northeast monsoon which will have far reaching consequences for Sri Lanka’s economy and the ecology of the hill country.     

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.     

Supercooled melt inclusions in lower-crustal granulites as a consequence of rapid exhumation by channel flow

Here we report on the unexpected occurrence of felsic (granitic) inclusions with quench textures such as spherulite and dendrite (hereafter referred to as “felsite inclusions”), similar to some volcanic rocks, within garnet in presumably “slowly cooled” lower-crustal granulites of various geologic ages ranging from Early Proterozoic to Middle Paleozoic and wide global distribution (the Limpopo Belt, the Grenville Province, the Lützow-Holm Complex of East Antarctica, the Highland Complex of Sri Lanka, and the southern Bohemian Massif). The well-preserved textures of felsite inclusions are indicative of melts formed by anatexis during high-pressure and high-temperature metamorphism, crystallization under far-from-equilibrium conditions (at > 50 °C undercooling) and subsequent rapid cooling. The occurrence of felsite inclusions in granulites in restricted tectonostratigraphic zones in Sri Lanka, among other examples, may be the first geologic evidence for fast exhumation of lower-crustal rocks to andalusite-stable upper-crustal conditions by channel flow in a continental collision orogen. We hypothesize that granulites ascend episodically along discrete high-strain zones and cool as fast as some felsic magmas. This conclusion sheds new light on the debate regarding the deep crustal processes and necessitates changes to fundamental beliefs about exhumation rates based on rates of plate convergence (1–10 cm/year).     

Reconstruction of the Late Holocene climate and environmental history from North Bolgoda Lake,Sri Lanka,using lipid biomarkers and pollen records

The catastrophic impact and unpredictability of the Indian Ocean Monsoon (IOM) over South Asia are evident from devastating floods, mudslides and droughts in one of the most densely populated regions of the globe. However, our understanding as to how the IOM has varied in the past, as well as its impact on local environments, remains limited. This is particularly the case for Sri Lanka, where erosional landscapes have limited the availability of well-stratified, high-resolution terrestrial archives. Here, we present novel data from an undisturbed sediment core retrieved from the coastal Bolgoda Lake. This includes the presentation of a revised Late Holocene age model as well as an innovative combination of pollen, source-specific biomarkers, and compound-specific stable carbon isotopes of n-alkanes to reconstruct the shifts in precipitation, salinity and vegetation cover. Our record documents variable climate between 3000 years and the present, with arid conditions c. 2334 and 2067 cal a bp. This extreme dry period was preceded and followed by more wet conditions. The high-resolution palaeoenvironmental reconstruction fills a major gap in our knowledge on the ramifications of IOM shifts across South Asia and provides insights during a time of major redistribution of dense human settlements across Sri Lanka.     

Rb - Sr Whole Rock-Mineral Isochron Ages of Plutonic Rocks from the Wanni Complex, Sri Lanka

Granitoid plutons in different East Gondwana fragments give evidence for vigorous felsic magmatism during the Pan-African period (800 to 500 Ma). The Sri Lankan basement, which is mainly composed of Proterozoic high-grade metamorphic rocks, was intruded by a few late- to post-tectonic syenitic and granitic plutons. Reliable geochronological data for these plutons are few, and some of the available data are inconsistent with the ages of the surrounding metamorphic country rocks.This report presents five Rb-Sr whole-rock-mineral isochron (WRMI) ages and initial Sr isotope ratios of three granitoid plutons from the Wanni Complex, Sri Lanka, namely the Ambagaspitiya, Tonigala and Kotadeniya Granites. Two samples from the Ambagaspitiya Granite yielded ages of 520±5 and 502±15 Ma, with initial Sr ratios of 0.7103±0.0003 and 0.7125±0.0009 respectively, whereas ages of 467±27 and 497±11 Ma and initial Sr ratios of 0.7070±0.0004 and 0.7085±0.0008 were determined for two Tonigala Granite plutons. A single sample from the Kotadeniya Granite gave an age of 533±19 Ma and an initial Sr ratio of 0.7202±0.0018.These ages are consistent with the geological relations between the granitoids and their country rocks, which underwent granulite facies metamorphism between 650 and 550 Ma, and also with available U-Pb zircon ages (550 Ma). The present study gives improved chronological correlation of the felsic magmatism in East Gondwana fragments in Sri Lanka, India, Madagascar, Antarctica and Western Australia. Each of the Sri Lankan plutons exhibits different initial Sr isotope ratios, indicating that their magmas were derived from distinct source materials.     

Rb-Sr and Sm-Nd Geochronology of the Eppawala Metamorphic Rocks and Carbonatite,Wanni Complex,Sri Lanka

Petrological studies on the surrounding metamorphic rocks of the Eppawala carbonatite body, Wanni complex, Sri Lanka, revealed that these rocks had been metamorphosed under amphibolite to granulite facies conditions. Garnet-sillimanite-biotite gneiss shows lower range of metamorphic temperature (730–770°C) than the migmatite gneiss (750–780°C) and the pressure varies from 6.6–7.8 kbar to 5.6–6.4 kbar respectively. The metamorphic age of the garnet-sillimanite-biotite gneiss and migmatite gneiss dated 607±23 Ma and 626±16 Ma, respectively for mineral — whole rock isochron in Sm-Nd system. These ages are compatible with the ages of regional high-grade metamorphism occurred 610–550 Ma in the three crustal units in Sri Lanka.Rb-Sr system for biotite, apatite and whole-rock fractions suggests 493±5 Ma for the Eppawala carbonatite body. This age indicates the cooling age of the biotite. The presence of non-crystalline carbonatite matrix and large hexagonal apatite crystals suggests a slow cooling history. Further, low closure temperature of biotite in Rb-Sr system suggests that the intrusion age of carbonatite body should be more than 493 Ma, but non-metamorphosed nature provides evidence that the intrusion age of the carbonatite body should be less than the period of regional metamorphism 610–550 Ma. Therefore, Eppawala carbonatite body has a strong possibility to be a late to post magmatic intrusion. The other late to post magmatic intrusions in the Wanni complex and Highland complex are dated between 580–550 Ma. Therefore, the most probable intrusion age of the Eppawala carbonatite body is suggested to be around 550 Ma.     

Sediment thickness map of the Indian region 79°E–86°E, 2°S–8°S based on satellite gravity interpretation

Satellite free air gravity anomalies over the Indian ocean region 79°E–86°E, 2°S–8°S were obtained from the website http://topex.ucsd.edu and a contour map was compiled. Five profiles of the anomaly have been interpreted in terms of two-dimensional structures in the ocean. Thickness of sediments lying on the oceanic crust determined from the interpretation of gravity profiles were used to compile an isopach map of the region 79°E–86°E, 2°S–8°S. This map in combination with one of the isopach maps compiled by previous workers, provides information regarding the thickness of sediments up to 6° S. According to this map sediment thickness varies from ～600 m over the middle part of the region to ～800 m further south, indicating that thinning of sediments in the middle part of the region is only localized. Information provided by this gravity study may be useful in planning detailed seismological studies to delimit the outer edge of the continental margin of Sri Lanka, defined according to the United Nations Convention of the Law of the Sea (UNCLOS).