Highland Complex Research Articles

スリランカ・ハイランド岩体の最高変成条件-超高温・高圧変成作用の解析-

スリランカ・ハイランド岩体の最高変成条件-超高温・高圧変成作用の解析-

Ultrahigh-temperature Metamorphism (1150 C, 12 kbar) and Multistage Evolution of Mg-, Al-rich Granulites from the Central Highland Complex, Sri Lanka

Mgand Al-rich granulites of the central Highland Complex, Sri Lanka preserve a range of reaction textures indicative of a multistage P–T history following an ultrahigh-temperature metamorphic peak. The granulites contain a near-peak assemblage of sapphirine–garnet–orthopyroxene–sillimanite–quartz–K-feldspar, which was later overprinted by intergrowth, symplectite and corona textures involving orthopyroxene, sapphirine, cordierite and spinel. Biotite-rims, kornerupine and orthopyroxene-rims on biotite are considered to be late assemblages. Thermobarometric calculations yield an estimated P–T of at least 1100 C and 12 kbar for the near-peak metamorphism. Isopleths of Al2O3 in orthopyroxene are consistent with a peak temperature above 1150 C. The P–T path consists of four segments. Initial isobaric cooling after peak metamorphism (Segment A), which produced the garnet–sapphirine–quartz assemblage, was followed by near-isothermal decompression at ultrahigh temperature (Segment B), which produced the multiphase symplectites. Further isobaric cooling (Segment C) resulted in the formation of biotite and kornerupine, and late isothermal decompression (Segment D) formed orthopyroxene rims on biotite. This evolution can be correlated with similar P–T paths elsewhere, but there are not yet sufficient geochronological and structural data available from the Highland Complex to allow the tectonic implications to be fully assessed.

Geochemical evidence for a Neoproterozoic magmatic continental margin in Sri Lanka—relevance for the Rodinia–Gondwana supercontinent cycle

Abstract Zircon ages of Neoproterozoic crustal blocks in Sri Lanka indicate crystallization and metamorphism contemporaneously with the development of the supercontinent Rodinia as well as the early amalgamation of Gondwana, respectively [Asian J. Earth Sci., in press and references therein]. In this context, the geodynamic significance of the ca. 890 Ma mafic Kandy Complex is enigmatic, as it may represent a magmatic arc during the final assembly of Rodinia as well as a rifting-related complex belonging to its dispersal or even early assembly of Gondwana. Here we interpret geochemical and Nd isotopic data of the Kandy Complex as evidence for its former tectonic setting and its role in the development of the Rodinia–Gondwana supercontinent cycle. A representative sample suite of the Kandy Complex comprises amphibolites, metagabbros, and metadiorites. Some of the samples show relicts of primary magmatic layering. Initial eNd values of −2.8 to +1.3 are interpreted as evidence for melting of a heterogeneous, and ultimately depleted mantle, overprinted by subducted old crustal material. Combined chemical and Nd isotopic data show that the crustal signatures were inherited from the mantle rather than by melt contamination at crustal levels. The samples show enrichment of light rare-earth elements, negative Nb-, and positive Pb-anomalies. These characteristics are similar to those found in modern arcs containing a subducted sedimentary component. The evidence suggests that the Kandy Complex represents a deep crustal level of a magmatic arc. Isotopic as well as geochronological data suggest magmatism over a period of ca. 50 Ma [Asian J. Earth Sci. in press] by repeated rejuvenation of an Andean-type continental margin. The Sm–Nd isotopic systematics indicate that the Kandy Complex represents an integral part of the ca. 1.1–0.75 Ga Wanni Complex in NW Sri Lanka, and that it evolved separately from the presently juxtaposed metasedimentary Highland Complex.

Osumilite and spinel+quartz from Sri Lanka: Implications for UHT conditions and retrograde P-T path

We report here the occurrence of osumilite as fine isolated inclusions in garnet porphyroblasts from a granulite facies rock in the southwestern part of the Highland Complex in Sri Lanka. Garnet porphyroblasts in the matrix are partly rimmed by sillimanite-biotite-K-feldspar assemblage as well as cordierite-K-feldspar-quartz assemblage. Spinel coexisting with quartz is also observed in the same sample. The pressure-temperature condition must be above 950°C at around 7 kbar pressure, on the basis of osumilite stability in KFMASH system. The cooling and decompression continued down to 850°C, into the biotite-sillimanite-K-feldspar-quartz stability field. During decompression fO2 must have increased leading to the stability of Zn-rich-spinel with quartz after cordierite. Our findings provide direct evidence for ultrahigh-temperature (UHT) metamorphic conditions in southwestern Highland Complex, and also suggest the existence of a possible UHT zone within the high-grade terrane of Sri Lanka.

"Ultrahigh density" carbonic fluids in ultrahigh-temperature crustal metamorphism

Ultrahigh temperature (UHT) granulites in Tonagh Island, Napier Complex, East Antarctica record peak metamorphic pressure-temperature (P-T) conditions of up to 9 kbar and 1100°C. Sapphirine, garnet, orthopyroxene and quartz in these rocks contain very high density fluid inclusions with melting temperatures close to that of pure CO2 (−56.6°C) and homogenization temperatures down to −34.9°C translating into high CO2 densities (up to 1.07 g/cm3). The UHT granulites of Vizianagram in Eastern Ghats Belt, India which were subjected to extreme crustal metamorphism at >1000°C and 8-9 kbar also carry very high density (up to 1.15 g/cm3) pure CO2 inclusions in quartz adjacent to spinel rimmed by various coronas of sillimanite, orthopyroxene and garnet. Garnets in a granulite facies rock from Salem in southern India that equilibrated at peak P-T conditions of 740-800°C and 9-11 kbar carry the highest density (1.17 g/cm3) pure CO2 yet reported from continental crust. This rock also contains very high density CO2 inclusions in plagioclase and quartz. High density (0.998 g/cm3) pure CO2-rich fluid inclusions also occur abundantly within garnets from a mafic granulite in Ampitiya, central Highland Complex, Sri Lanka. The peak P-T conditions of metamorphism of the mafic granulite are around 10.6 kbar and 985°C with subsequent rapid isothermal decompression along a clock-wise path down to 5.5 kbar. In most of the cases above, the representative isochores for the CO2 inclusions either penetrate through or pass very close to the P-T windows defined from mineral phase equilibria indicating that the fluid inclusions were trapped at the time of peak or near post-peak metamorphism. We thus recognize a group of “ultrahigh density” (UHD) CO2-rich fluids that characterize UHT rocks and other granulites formed under extreme crustal metamorphism. These fluids contrast sharply with the lower density (generally <1.0 g/cm3) CO2 inclusions commonly reported from normal granulite facies rocks in various terrains. The presence of “synmetamorphic” UHD CO2 within various minerals is consistent with the low water activities predicted by the mineral assemblages in these rocks. Although the source of the CO2 is equivocal, mantle derived mafic magmas could have provided the heat and volatiles required for crustal metamorphism at extreme conditions displayed by these rocks.

Modeling of retrograde diffusion zoning in garnet: evidence for slow cooling of granulites from the Highland Complex of Sri Lanka

¶Diffusion modeling of zoning profiles in garnet rims from mafic granulites is used to estimate cooling rates in the Proterozoic basement of Sri Lanka, which represents a small, but important fragment of the Gondwana super-continent. Metamorphic peak temperatures and pressures, estimated with two-pyroxene thermometry and garnet–clinopyroxene–plagioclase–quartz (GADS) barometry, yield 875±20 °C and 9.0±0.1 kbar. These peak metamorphic conditions are slightly higher than results obtained by garnet-biotite Fe–Mg exchange thermometry of 820±20 °C. Reset flat zoning profiles were observed in most garnets. Only narrow garnet rims touching biotite exhibit retrograde zoning in terms of Fe and Mg exchange.

Proterozoic crustal evolution of southernmost India and Sri Lanka

Abstract The Southern Granulite Terrain of India comprises four Proterozoic domains (from north to south): the Madurai Block, and the Achankovil, Ponmudi and Nagercoil Units. All but the Achankovil Unit share similarities with the Highland Complex of Sri Lanka: (1) evidence for Palaeoproterozoic crust formation (from U-Pb ages and 2.0–3.0 Ga Nd model ages); and (2) Pan-African ultrahigh-temperature metamorphism between 610 and 550 Ma. More specifically, the Ponmudi Unit and the Madurai Block share lithological and petrological characteristics with the low- P and high- P domains of the Highland Complex, respectively. The Achankovil Unit is correlated with the Wanni Complex of Sri Lanka on the basis of lithologies and Nd model ages (1.0–2.0Ga). These domains and the Vijayan and Kadugannawa Complexes of Sri Lanka represent Late Mesoproterozoic-Early Neoproterozoic crustal domains. Similarities in tectonic style, degree of metamorphism and Neoproterozoic U-Pb ages, suggest a common Pan-African tectonothermal evolution for the lower crustal domains of southern India, Sri Lanka and the Lützow-Holm Bay of Eastern Antarctica. Sri Lanka may have been located at the junction between the Mozambique Belt and a second belt partly involving areas that underwent a Grenvillian high-grade event during the formation of Rodinia. Madagascar shows a similar Pan-African metamorphic overprint, but is located in a different position and lacks the influence of the second belt. Alternative views are discussed.

HT-deformation of garnet: an EBSD study on granulites from Sri Lanka, India and the Ivrea Zone

Garnets from three regional granulite terrains (Sri Lanka, Eastern Ghats, Ivrea Zone) have been analyzed and the results indicate, that garnet can be plastically deformed by dominant dislocation glide/creep with variable dominant slip systems.Garnets from a granulite facies fold hinge in a quartzite of the Highland Complex of Sri Lanka, unequivocally attest to plastic deformation of garnet. Outside the fold hinge the garnets form flat discs elongated parallel to a pronounced stretching lineation and show deformation microstructures like boudinage and pinch and swell structures. In the fold hinge these garnet discs are folded within the quartzitic matrix at temperatures of about 800°C. In spite of the preservation of deformed shapes, intracrystalline deformation microstructures are scarce. The garnets have distinct crystallographic preferred orientations, which can be interpreted by a dominant 1/2〈111〉{110} slip system approaching ‘easy slip’ positions. These are reached rapidly under moderate strains as necessary lattice rotations are small.Similar slip systems were derived for a garnet-bearing quartzite from the Eastern Ghats deformed at temperatures of 750–800°C, but with more influence of initial orientation of garnets. Completely different textures indicative of dominant 〈100〉 [010] slip were found in a restitic garnet–sillimanite granulite from the Ivrea Zone. The variable deformation features and variable slip systems with 1/2〈111〉 and 〈100〉 Burgers vectors prove that garnets bear significant potential for contributing additional information on the HT-deformation of rocks.

The Northwest-Southeast Trending Granite-Gneiss-Migmatite Belt of the Precambrian Basement of Madagascar: Probable Signature of the Collisional Event in the Central-East Part of the East Africa Orogen

Gold occurs in various quantities mainly in the central and southern parts of Sri Lanka. Although gold is recovered mainly from river sediments at present, it has been mined in the past also from a concordant quartz reef in central Sri Lanka and from other types of sediments such as marshy beds surrounding dykes of ultramafic composition in southern Sri Lanka. The sedimentary gold has been interpreted as being derived from the mineralizations of the latest deformational features such as ductile and brittle shears and fractures consisting of quartz veins, minor pegmatites and sulphide mineralizations mainly of pyrite, pyrrhotite and chalcopyrite. The mineralization of gold is exclusively in the Highland Complex rocks of Sri Lanka which represent Archaean-Late Proterozoic metamorphic complexes.In Madagascar gold is present in the Precambrian basement rocks consisting of some greenstone belts and metasediments as a diffused mineral in the rocks, concordant quartz reefs and more recent discordant quartz veins. The concordant quartz veins and diffused gold are related to both Archaean and Proterozoic rock associations and the discordant veins which have been identified as Permo-Triassic are found mainly in the north within the contact zone of Proterozoic rocks and the Phanerozoic cover. Gold is also recovered form ancient and recent alluvial accumulations and lateritic earth. The mineralization of vein deposits is mainly gold-quartz and gold-sulphide associations, the sulphides being galena, chalcopyrite and pyrite.In south India, gold occurs in Archaean metamorphic units comprising of granite-greenstone complexes in the Kolar classic gold field and in granulite terranes in more southerly occurrences such as the historic Wynad fields. In both cases gold occurs in gold-quartz veins which occupy fissures and shears within the Archaean metavolcanics. Gold also occurs in the Attapadi area of Kerala both in sediments and in primary epigenetic gold deposits which are hosted by Precambrian amphibolites and granitic gneisses. The Attapadi lode veins are predominantly structurally controlled and are closely associated with the Proterozoic Bhavani shear zone. Gold is recovered both from the primary and placer deposits and the primary deposits contain gold quartz veins, gold quartz carbonate veins and gold quartz sulphide veins. The sulphide minerals are mainly galena, pyrite, chalcopyrite and arsenopyrite.The rocks in the southern Madagascar, southern India and Sri Lanka belong to granulite terranes that have peak metamorphic imprints and tectonic development during the Pan-African. The emplacement of gold mineralization mainly in Madagascar and south India can be correlated in the Archaean. The gold deposition in the later deformational structures with concordant and discordant gold bearing veins can be correlated also with those in Sri Lanka, as a Pan-African mineralizing event. It is also evident that CO2 rich fluids from a deep seated source has continued from Archaean and were still active during the Proterozoic being responsible for the Pan-African event of gold deposition particularly in the younger structures in these countries.

Reactions and textures in grossular–wollastonite–scapolite calc–silicate granulites from Maligawila, Sri Lanka: evidence for high-temperature isobaric cooling in the meta-sediments of the Highland Complex

Grossular–wollastonite–scapolite calc–silicate granulites from Maligawila in the Buttala klippe, which form part of the overthrusted rocks of the Highland Complex of Sri Lanka, preserve a number of spectacular coronas and replacement textures that could be effectively used to infer their P–T–fluid history. These textures include coronas of garnet, garnet–quartz, and garnet–quartz–calcite at the grain boundaries of wollastonite, scapolite, and calcite as well as calcite–plagioclase and calcite–quartz symplectites or finer grains after scapolite and wollastonite respectively. Other textures include a double rind of coronal scapolite and coronal garnet between matrix garnet and calcite. The reactions that produced these coronas and replacement textures, except those involving clinopyroxene, are modelled in the CaO–Al 2O 3–SiO 2–CO 2 system using the reduced activities. Calculated examples of T– X CO2 and P– X CO2 projections indicate that the peak metamorphic temperature of about 900–875 °C at a pressure of 9 kbar and the peak metamorphic fluid composition is constrained to be low in X CO2 (0.1< X CO2<0.30). Interpretation of the textural features on the basis of the partial grids revealed that the calc–silicate granulites underwent high-temperature isobaric cooling, from about 900–875 °C to a temperature below 675 °C, following the peak metamorphism. The late-stage cooling was accompanied by an influx of hydrous fluids. The calc–silicate granulites provide evidence for high-temperature isobaric cooling in the meta-sediments of the Highland Complex, earlier considered by some workers to be confined exclusively to the meta-igneous rocks. The coronal scapolite may have formed under open-system metasomatism.

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.

Gold Occurrences in Madagascar, South India and Sri Lanka: Significance of a Possible Pan-African Event

Gold occurs in various quantities mainly in the central and southern parts of Sri Lanka. Although gold is recovered mainly from river sediments at present, it has been mined in the past also from a concordant quartz reef in central Sri Lanka and from other types of sediments such as marshy beds surrounding dykes of ultramafic composition in southern Sri Lanka. The sedimentary gold has been interpreted as being derived from the mineralizations of the latest deformational features such as ductile and brittle shears and fractures consisting of quartz veins, minor pegmatites and sulphide mineralizations mainly of pyrite, pyrrhotite and chalcopyrite. The mineralization of gold is exclusively in the Highland Complex rocks of Sri Lanka which represent Archaean-Late Proterozoic metamorphic complexes. In Madagascar gold is present in the Precambrian basement rocks consisting of some greenstone belts and metasediments as a diffused mineral in the rocks, concordant quartz reefs and more recent discordant quartz veins. The concordant quartz veins and diffused gold are related to both Archaean and Proterozoic rock associations and the discordant veins which have been identified as Permo-Triassic are found mainly in the north within the contact zone of Proterozoic rocks and the Phanerozoic cover. Gold is also recovered form ancient and recent alluvial accumulations and lateritic earth. The mineralization of vein deposits is mainly gold-quartz and gold-sulphide associations, the sulphides being galena, chalcopyrite and pyrite. In south India, gold occurs in Archaean metamorphic units comprising of granite-greenstone complexes in the Kolar classic gold field and in granulite terranes in more southerly occurrences such as the historic Wynad fields. In both cases gold occurs in gold-quartz veins which occupy fissures and shears within the Archaean metavolcanics. Gold also occurs in the Attapadi area of Kerala both in sediments and in primary epigenetic gold deposits which are hosted by Precambrian amphibolites and granitic gneisses. The Attapadi lode veins are predominantly structurally controlled and are closely associated with the Proterozoic Bhavani shear zone. Gold is recovered both from the primary and placer deposits and the primary deposits contain gold quartz veins, gold quartz carbonate veins and gold quartz sulphide veins. The sulphide minerals are mainly galena, pyrite, chalcopyrite and arsenopyrite. The rocks in the southern Madagascar, southern India and Sri Lanka belong to granulite terranes that have peak metamorphic imprints and tectonic development during the Pan-African. The emplacement of gold mineralization mainly in Madagascar and south India can be correlated in the Archaean. The gold deposition in the later deformational structures with concordant and discordant gold bearing veins can be correlated also with those in Sri Lanka, as a Pan-African mineralizing event. It is also evident that CO 2 rich fluids from a deep seated source has continued from Archaean and were still active during the Proterozoic being responsible for the Pan-African event of gold deposition particularly in the younger structures in these countries.

ORIENTATION OF EXSOLUTION LAMELLAE AND RODS, AND OPTIMAL PHASE-BOUNDARIES IN ANTIPERTHITE FROM PELITIC GRANULITES, SRI LANKA

The exsolution microtexture of antiperthite grains with bulk compositions Or 17–21 An 33–26 from pelitic granulites from the Highland Complex of Sri Lanka were studied by optical methods. The most common K-feldspar rods are oblique to the c axis by about 5° and obviously nucleated at pre-existing albite-twin boundaries. Where no primary twin boundaries existed, exsolution lamellae of K-feldspar developed. This exsolution resulted in the formation of secondary albite-twin lamellae in the host plagioclase adjacent to the K-feldspar. A large range of orientations of exsolution lamellae has been measured on the universal stage and compared with low lattice-misfit boundaries calculated with the EPLAG program. A best fit is obtained using cell parameters for ordered plagioclase (An 27–37 ) and orthoclase. Nearly all poles to measured lamellae lie between (120) and (211) in the low-misfit belts defined by isolines showing 20% deviation from the lattice-misfit minimum when albite-twin-related cell parameters for the plagioclase host are included. Estimated unit-cell parameters for in situ conditions of exsolution yield insignificantly different optimal phase-boundaries. Results of the optical study and lattice-misfit considerations suggest that initially exsolved lamellae of K-feldspar, during very slow cooling under dry conditions, coarsened and rotated, and Al–Si ordering of the coexisting feldspar phases induced secondary albite-twin lamellae in the plagioclase host.

Garnet plasticity in the lower continental crust: implications for deformation mechanisms based on microstructures and SEM-electron channeling pattern analysis

Elongated garnets which preserved deformational microstructures (boudinage, pinch and swell structures) occur in granulite facies quartzites in the Highland Complex of Sri Lanka. In spite of these microstructures and in contrast with previous reports on garnet plasticity, there are no or only few signs of intracrystalline deformation like subgrains or lattice distortion. This can be explained by annealing and slow static cooling from high temperatures. SEM-electron channeling pattern analysis reveals that the garnets have a significant crystallographic preferred orientation. Aspect ratio/grain-size analysis shows that the deformation mechanism is grain-size sensitive. These features indicate a diffusion assisted dislocation glide mechanism with dominant 1/2 <111>{110} slip system. A comparison of the deformation behavior between garnet, quartz and feldspar shows, that differences in flow strength are low under the high-grade conditions (850±50°C). Garnet is still the phase with the highest flow strength, but (dry) quartz is slightly stronger than feldspar.

Geology of the High Grade Proterozoic Terrains of Sri Lanka, and the Assembly of Gondwana: an Update on Recent Developments

Nd model ages determined for the high-grade rocks of Sri Lanka delineate three crustal units, viz., the Highland Complex (HC), the Wanni Complex WC), and the Vijayan Complex (VC). The distribution of these three units differs considerably from the three geological divisions demarcated previously on the basis of geological mapping. The centrally located HC comprises mainly granulite grade charnockitic rocks, and metasediments characterized by older Nd model ages (2.0–3.4 Ga). The Highland sedimentary pile was thickened by intermittent granitoid intrusions, most of which are now charnockitic gneiss, and granulites, and basaltic sills, and dikes. All these metaigneous rocks now occur as conformable bands or layers due to intense polyphase deformation. The HC is bounded on the east by the amphibolite grade VC, composed mainly of granitic gneisses, basic gneisses, and migmatites, and they have ‘younger’ Nd model ages (1.1–1.8 Ga). The isotopic, and geochemical characteristics identify the precursors to the Vijayan rocks as I-type calc-alkaline granitoids originated at an ‘arc’-related tectonic environment. Thus, the earlier interpretation that the Vijayan rocks represent reworked HC was rejected. The granulite inliers within the VC, earlier considered as “resisters” to re-working, are now shown as overthrust klippen or rotated rafts of the HC. The WC, demarcated on the basis of Nd model ages (1.1–1.8 Ga) similar to those of the VC, lies west of the HC. It consists mainly of granitic gneisses, charnockitic gneisses, and migmatites, and the metamorphic grade ranges from amphibolite to granulite. Comprehensive geothermobarometric surveys constrain the P-T evolution of the three crustal units, and indicate that both the HC, and WC underwent near isobaric cooling, followed by a decompression with decreasing temperature. Extensive isotopic studies (U-Pb, Pb-Pb, Sm-Nd, Rb-Sr) have established a new geochronological framework for these high-grade rocks of Sri Lanka. The new framework has bracketed the age of high grade metamorphism in the three crustal units at 550–600 Ma. The recent advances in knowledge of the geology of Sri Lanka favour a strong geological correlation of the HC, and the VC of Sri Lanka, respectively, with the Lutzöw-Holm Complex, and the Yatmato-Belgica Complex in the East Antarctica. The geology of the WC suggests a possible correlation with Madagascar, and East Africa. The amalgamation of the three crustal units of Sri Lanka, is apparently related to the two distinct orogenic events that resulted in the assembly of the Gondwana supercontinent.

Sri Lanka–Madagascar Gondwana Linkage: Evidence for a Pan‐African Mineral Belt

Abstract Sri Lanka occupies a unique geologic position in Gondwana. Recent age and isotopic data suggest that the high‐grade basement rocks of Sri Lanka are more closely associated with the southeastern part of Madagascar than with the Archean granulites of southern and eastern parts of India. The occurrences of gem minerals and graphite in the centrally located Highland Complex of Sri Lanka can also be correlated with those of the Kerala Khondalite Belt (KKB) at the southern tip of India and of southeast Madagascar south of the Ranotsara Shear Zone. These geological and mineralogical features indicate the juxtaposition of Sri Lanka with Madagascar and also with the Lutzow‐Holm Bay area in Antarctica. The very close juxtaposition of Sri Lanka with Madagascar suggested here, which has hitherto been given only passing reference, implies that there exists a distinct mineralized belt running from Antarctica through the Highland Complex of Sri Lanka into Madagascar, Mozambique, Tanzania, and farther north. Thi...

Deformation of a High-Grade Gondwana Fragment, Sri Lanka

The Sri Lankan Gondwana fragment is a high-grade gneiss terrain consisting of at least three crustal units, namely the Wanni Complex (WC), the Highland Complex (HC) and the Vijayan Complex (VC), which were juxtaposed during the Pan-African collision. At least six phases of ductile deformation were recognized in the smallest Gondwana fragment, Sri Lanka. There is microscopic and mesoscopic evidence for a first deformation (D 1), which occurred during prograde metamorphism but before the main granulite facies metamorphism. This first deformation is represented by straight to curved and crenulated inclusion trails in garnet porphyroblasts, microfolds in some metapelites and quartzo-feldspathic gneisses and by a S 1 foliation preserved in crenulation folds and some rootless intrafolial folds. D 2 has two main stages, of which the second phase is the most intense and dominant deformation which produced most part of the major composite and transposed compositional layering as well as the prominent stretching lineation (L 2). The first stage of D 2 may have been responsible for the formation of a crenulation cleavage of S 1, as seen in garnet porphyroblasts in some metapelites. The compositional layering (S 2) in metapelites and metasemipelites appears to have formed through the development of a crenulation cleavage. F 2 folds are mesoscopic, isoclinal, rootless and occur as intrafolial folds, and their limbs are highly attenuated. Major part of the deformation during the latter stage of D 2 was non-coaxial. Many minor and large scale recumbent isoclinal folds (F 3) were produced during D 3. Both D 2 and D 3 were coeval with the peak conditions of granulite facies metamorphism. D 4 produced very large, gentle, nearly E-W upright folds probably during the beginning of uplift of the entire high-grade assemblage from deeper crustal levels, still under granulite facies conditions. D 5 is the second strongest deformation and was responsible for the large-scale upright folds (F 5), which control the present map pattern of Sri Lanka. The superimposition of D 5 folds on D 4 folds gave rise to a large-scale type I fold interference pattern of Ramsay (1967). D 6 deformation caused local refolding of the F 5 folds and was accompanied by amphibolite facies conditions during uplift. The deformation represented by the majority of vein-type structures, such as patchy charnockite veins, cordierite-bearing veins and graphite veins clearly post-date D 5. It is argued that the WC, the HC and the VC in Sri Lanka may be three distinctly different Proterozoic crustal units, brought into contact by tectonism in late Proterozoic at two different stages.

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.

The structural evolution of the Arena Gneisses and its bearing on Proterozoic tectonics of Sri Lanka

The Arena Gneisses (AG), which is considered to form an eastern portion of the Wanni Complex (WC), is exposed at five doubly plunging synforms (arenas) in central Sri Lanka, near the boundary between the WC and the Highland Complex (HC). Structural investigation based on detailed field work on two of the arenas classified the following structural sequences from the earliest to the latest: Dn-2, Dn-1, Dn, Dn + 1, and Dn + 2 deformations. The Dn-2 structures include the major compositional banding, pinch-and-swell and boudinaged structures formed by compression normal to the banding. Mesoscopic intrafolial isoclinal to tight folds (Fn-2) were formed synchronologically with Dn-2 deformation. During the Dn-2 stage, the AG thrusted over the HC with top-to-the-southward movement. The asymmetric structures such as asymmetric foliation boudinaged structure, E-W-trending asymmetric folds (Fn-1b) and asymmetric megacrysts, and N-S-stretching lineations were formed by this simple shear deformation. This transport of the AG caused the N-S shortening and produced macroscopic gentle folds (Fn-1a) with mostly E-W-trending upright axial surfaces. During the Dn-1 stage, the eastward movement of the AG took place. This movement was associated with N-S-trending asymmetric folds (Fn-1c) and microfabrics observed in the AG, as well as the eastward vergences of related folds in the HC. N-S-trending upright linear folds (Fn) and related structures such as parasitic folds and axial surface foliations were formed by E-W shortening, which is normal to the axial surface of the macroscopic Fn fold, during the Dn deformational stage. Dextral shear deformation with vertical shear planes took place during Dn + 1 deformation. ENE-WSW- or NE-SE-trending folds (Fn + 1) were developed during Dn + 2 deformation. The southward thrusting of the AG indicates that the WC has displaced over the HC with top-to-the-southward movement. The eastward transportation of the AG and the HC may be due to the collision of the Vijayan Complex (VC) with the HC. Considering the above-mentioned tectonic elements with Sri Lankan geochronological data (e.g. Hölzl et al., 1994), the first thrusting event might have taken place around 700 Ma and the second displacement probably occurred around 570 Ma.

Strain localization and large‐scale block rotation in the lower continental crust, Kataragama area, Sri Lanka

ABSTRACTIn Sri Lanka a suite of rocks is regionally exposed which once formed a part of the middle to lower crust of a collisional orogen with about doubled crustal thickness. A subhorizontal to gently west‐dipping major thrust zone (Highland Basal Thrust) crosses Sri Lanka from NNE to SSW. It separates a granulite facies upper unit, the Highland Complex from an amphibolite facies lower unit, the Vijayan Complex with a displacement of more than 300 km. The basal Highland Complex was migmatized during thrusting. Rheological contrasts between dry granulites and migmatites were high in the presence of partial melts. Granulite facies fragments within the migmatites vary from the decimetre to kilometre scale. Granulites in the area of Kataragama, formerly interpreted as the Kataragama Klippe, are now identified as a large‐scale raft within the migmatites, which is completely separated from the overriding Highland Complex. During thrusting strain was concentrated in the migmatized basal parts of the Highland Complex. After crystallization of the migmatites thrusting continued, and strain was still localized within the migmatites. Therefore, granulite facies parageneses, structures and textures are preserved in the granulite rafts. The deviation of the granulite facies structures in the Kataragama raft from the general trend in the Highland Complex is interpreted to indicate counterclockwise rotation of the whole raft by about 90° around a subvertical rotation axis.