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General geology and petrology of some Precambrian crystalline rocks from the Vijayan Complex of Sri Lanka
Precambrian Research, 19 (1983) 301--315 Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
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G E N E R A L G E O L O G Y AND P E T R O L O G Y OF SOME PRECAMBRIAN C R Y S T A L L I N E ROCKS FROM T H E VIJAYAN COMPLEX OF SRI L A N K A
KAPILA DAHANAYAKE and H.A.H. JAYASENA, Department of Geology, University of Peradeniya, Peradeniya (Sri Lanka) (Received January 29, 1982; revision accepted August 12, 1982)
ABSTRACT Dahanayake, K. and Jayasena, H.A.H., 1983. General geology and petrology of some Precambrian crystalline rocks from the Vijayan Complex of Sri Lanka. Precambrian Res., 19: 301--315. Detailed field studies of the Precambrian Vijayan Complex terrain, Sri Lanka, reveal the occurrence of granites, gneisses and migmatites in association with calc-silicate gneisses, quartzites and dolerites. Microcline-rich granites and gneisses show both sharp and gradational contacts with the adjacent migmatites. Petrological observations favor a magmatic origin for the granites and gneisses. These bodies seem to have intruded into pre-existing metasedimentary rocks which were subsequently subjected to retrograde metamorphism under amphibolite facies conditions.
INTRODUCTION E x c e p t for some coastal outliers of Miocene sediments and a few scattered Jurassic outcrops, the island of Sri Lanka consists dom i nant l y of Precambrian crystalline rocks with imprints of varying degrees and ages of tectonism and metamorphism. Three major units of rocks can be recognized on the basis of lithology, structure and age (Cooray, 1978): (a) Highland Group, (b) Southwestern Gr o u p and (c) Vijayan Complex. The Highland G r oup of rocks, occurring extensively in the central highlands of Sri Lanka, comprises high-grade m e t a m o r p h i c rocks mostly of the hypersthene-granulite facies. T h e y axe quartzites, marble, garnet--sillimanite gneisses, granulites associated with granites, migmatites, hornbl ende--bi ot i t e gneisses, charnockites and pegmatites. These rocks show remarkable consist e n cy of strike with open and overturned folds giving rise to a series of antiform and s y n f o r m structures, and elongate basins and domes which form a north-plunging synclinorium in the center of the island (Vitanage, 1972). Towards the n o r t h the synclinorium axis trends N--NE. The Southwestern Group, belonging to the cordierite-granulite facies, is characterized by cordierite-bearing gneisses, charnockites, relatively thin quartzites and calciphyres associated with granitic gneisses, augen gneisses,
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migmatites and pegmatites. These rocks form a prominent NW.trending isoclinal fold system. The Vijayan Complex occurs on either side of the Highland Group (Fig. 1), extending from the eastern and western coasts towards the foothills of the central highlands. The complex of rocks, for the most part forming two plateau-like, wide peneplained surfaces, consists of gneisses, granites associated with migmatites, quartzites, doleritic intrusions and pegmatites. These rocks, LEGEND
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considered to be of amphibolite facies and of complex structure, are characterized by many intrusions, extensive shearing and flow folding. Plunging antiforms, synforms and domes are commonly encountered but their axes are difficult to trace in many instances. The Vijayan Complex was regarded as the basement upon which the original sediments of the Highland Group were deposited (Adams, 1929; Coates, 1935; Wadia, 1945; Fernando, 1948). However, Vitanage (1959) considered the granitized and migmatitic Vijayan rocks to be younger than the Highland Group. Cooray (1962) proposed that the Highland and Vijayan rocks differed in metamorphic grade and plutonic history. On the basis of some radiometric age data, Crawford and Oliver (1969) postulated that the Vijayan rocks represented former Highland Group rocks, migmatized and granitized under wetter amphibolite facies conditions. This transformation, called Vijayan retrogressive metamorphism, was believed by the same authors to have terminated about 1250 Ma ago. They also recognized two later "events" at about 650 Ma and 450 Ma. Katz (1971) maintained the earlier view that the Vijayan rocks represent the basement upon which the Highland Group were deposited about 3000 Ma ago. The Vijayan rocks were thought by the same author to have undergone polymetamorphism during at least three periods (2000, 1250 and 650--450 Ma), under PT conditions characteristic of the granulite facies. The boundary between the western Vijayan and Highland rocks is not well-defined, but the boundary at the eastern Vijayan is more distinct and has been considered a structural contact/shear zone, or a plate margin (Vitanage, 1959; Katz, 1971; Hatherton et al., 1975; Munasinghe and Dissanayake, 1979). Although generalized geological and geochronological studies have been done on the Vijayan Complex, no systematic detailed geological studies have been undertaken. In this work, the authors attempt to investigate the general geology and petrology, with a view to understanding the complex geological history of the Vijayan rocks, by field and laboratory studies of a typical Precambrian Vijayan Complex terrain in eastern Sri Lanka. METHODS OF STUDY
The aerial photographs of an area covering 2500 km: on the scale of 1:20 000 were studied and structural observations were made. A reconnaissance field mapping program was undertaken and a geological map on the scale of 1:63 360 was prepared. Approximately 200 rock samples, representative of the mapped area, were studied to determine textural features and mineralogy. PHYSIOGRAPHY AND G E N E R A L GEOLOGY OF THE STUDY A R E A
The study area is covered by the topographic sheets of Elahera, Polonnaruwa, Vakaneri and Rukam (1:63 360). Though served by a network of roads
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suitable for vehicles, the area remains for the most part inaccessible due to dense jungle. However, footpaths and some cart tracks do exist, and traverses along these helped to locate an appreciable number of outcrops in the difficult terrain. A multitude of streams and major rivers (Mahaweli Ganga, Maduru Oya and Bodigoda Aru) drain the area, which forms part of a vast peneplain surface, characterized by prominent inselberg-type erosion remnants reaching maximum elevations of 150 m above MSL. Generally, the landscape of the area can be described as undulating topography controlled for the most part by the underlying structure. Low ridges (< 100 m) occur, with elongate isolated high ridges (> 100 m), on a low-lying (< 50 m above MSL) plateau. Observations on the ridges, road cuttings, river beds, isolated outcrops and quarries help to identify three major rock units: (1) microcline granites and gneisses, (2) hornblende--biotite gneisses and (3) augen gneisses and migmatites. These units are associated with amphibolites, calc-silicate gneiss, dolerite intrusions, quartzites and pegmatites. GEOLOGICAL SETTING AND S T R U C T U R A L RELATIONS OF THE ROCK UNITS
The study area (Figs. 1,2) is characterized by northward-plunging synforms, antiforms and domes. The cores of these structures are occupied mainly by microcline granites and gneisses. Such rocks are surrounded, laterally, by augen gneisses and migmatites which pass into extensively occurring hornblende--biotite gneisses. Within these gneisses isolated calc-silicate gneiss and amphibolite bands are observed as well as concordant and cross-cutting doleritic intrusions. The dominant rock type of the area, hornblende--biotite gneiss, is characteristically weU-foliated and forms a layered sequence of alternating, compositionally and texturally distinct units closely associated with migmatites. Compositions ranging from amphibolitic to highly quartzo-feldspathic or granitic, with fine- to coarse-grained textures, are typical of these gneisses. The thickness of bands varies from a few centimeters to several meters. The migmatitic varieties contain discontinuous leucocratic phases in the form of patches, pods, layers and dilational dikes. Locally, very tight, small-scale folding is observed in the gneisses. Layers of amphibolite have been distended to form boudins and rotated blocks. The gneisses are cut by pegmatitic dikes and veins of granitic composition. Comparatively thinner, doleritic intrusions occur as cross-cutting dikes, or as discontinuous layers conformable to local foliation. Their contacts with adjacent rocks are poorly exposed. Microcline granites and gneisses, forming the cores of domes, antiforms and synforms, occur as minor protrusions, prominent inselbergs and elongate massifs on the otherwise gentle peneplain. These features are characterized, locally, by intense weathering and erosion phenomena leaving trains of granitic/gneissic boulders, extending for a few kilometers, parallel to the ridges, The granitic bodies commonly show distinct foliation in fine-grained matrices.
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Coarse-grained massive granites sensu stricto are common. Locally the granites show intense fracturing with as much as five systems of closely spaced joints and the steep dip of the gneiss is a prominent and noteworthy feature, which tends to become less steep towards the migmatitic country rocks, away from the granitic/gneissic intrusion. The contact between the granitic intrusions and adjacent migmatites is sometimes abrupt and sometimes gradational. At abrupt contacts, the marginal parts of the granite intrusions reveal intense shearing phenomena characterized by the complex joint system referred to earlier (Fig. 3). Mylonitic shear zones are also encountered. The granite body and the adjacent migmatitic host rocks both show joints and cracks along which pink silicified material has been intruded. At gradational contacts,
Fig. 3. The sharp contact (indicated by arrows) between migmatite and granite/gneiss. Note shattered nature of granite on the right side. (Maduru Oya proposed dam site, SW map area, Fig. 1.)
essentially basic relict features, in the form of discontinuous schlieren and streaks of differing intermediate composition, are observed (Fig. 4}. Generally, the parallel structure of these relict bodies reflects the trend of the granites/ gneisses. In such instances, the granite body as a whole, the internal structure within it and the associaSed basic border zone, show a parallel orientation, according to the main tectonic direction (Fig. 2), and form markedly elongated topographic features. Calc-gneisses are widely distributed as minor rock units in the layered gneiss complex and two types are observed. The first occurs as discontinuous lenses,
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Fig. 4. The granite/gneiss body and the dark basic schlieric border zone (Madurankerni Kulam, NE map area, Fig. 1). generally concordant with local foliation, although continuous bands do exist. The second t y p e is oblique to the foliation and is of a localized nature. Augen gneisses are scarce and when present they occur at the noses of plunging antiforms and domes, closely associated with granitic/gneissic bodies. Quartzites are rare in the mapped area and are either associated with granitic bodies, or interlayered with hornblende--biotite gneiss. Pegmatites occur as wide, crosscutting bodies, sometimes as thick as 40 m, in the hornblende--biotite gneiss. PETROLOGY OF THE LITHOLOGIC UNITS
Microcline granites and gneisses These are medium- to coarse-grained, pinkish rocks characterized by a massive or gneissic structure, with dominant pink feldspar and quartz associated with accessory biotite and hornblende. Magnetite and apatite are sparsely distributed. In thin sections, the texture is hypidiomorphic granular with a b u n d a n t K-feldspar, quartz and plagioclase. K-feldspars are mostly microclines, sometimes perthites, and oligoclase forms the plagioclase content. According to modal analysis, these rocks are granites or quartz syenites in the classification of Streckeisen (1967) (Fig. 5), which reflects the high potash c o n t e n t of the granitic and gneissic bodies. Microcline occurs as subhedral grains t h a t are xenomorphic against quartz
308
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and plagioclase. The microperthites within the microcline are of flame and string types, restricted to the core of the grain, and patchy and vein type which develop preferentially along the grain boundaries. Sometimes, relict plagioclase grains, riddled with sericite and dotted with quartz grains, are found in the microclines. The orientation of the twinned plagioclase inclusions reflects the grid twinning of the host grains. Plagioclase mainly displays albite twinning, but some are twinned according to the Carlsbad and pericline laws. Zoning is generally absent but a few grains exhibit faint compositional changes. Some plagioclase shows secondary deformation twinning and alteration products such as sericite, muscovite and epidote are found in certain grains and myrmekitic texture is evident in some grains. Well-developed reaction margins have been observed where plagioclase grains are associated with microcline grains (Fig. 6). Quartz is generally present as elongate grains with sutured boundaries. Some grains show undulatory extinction. Orthoclase with Carlsbad twinning is rarely present. Lath-shaped biotite shows faint lepidoblastic texture. Hornblende for the most part is chloritized and shows assimilation by later-formed quartz and plagioclase megabtasts. Rounded and elongate apatite grains are present as well as subangular and euhedral zircon. Euhedral magnetite grains show
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Fig. 6. Micrograph of granite. (Mp) perthitic microcline, (Mr) replacement perthite, (M) microcline, (P) plagioclase, (Q) quartz. alteration to hematite. Sphene, cracked or wedge-shaped, schorlite and cassiterite are found occasionally.
Hornblende--biotite gneisses and migmatites Hornblende--biotite gneisses and associated migmatites are dark gray to black, fine- to coarse-grained, with moderate to strong foliation. Hornblende and biotite are c o m m o n in a dominant quartzo-feldspathic matrix. The ratios of plagioclase/orthoclase and biotite/hornblende seem to decrease adjacent to the granitic intrusions. In thin section, the texture varies from panidiomorphic granoblastic/lepidoblastic to hypidiomorphic granoblastic. Feldspar, quartz, hornblende and biotite are abundant. The latter two minerals show local chloritization. Sphene, apatite, magnetite and zircon occur as accessories. Modal analyses for these rocks indicate that they are granodioritic and tonalitic according to the classification of Strekeisen (1967) (Fig. 5). Plagioclase grains are generally subhedral and are xenomorphic against surrounding grains. Faint to well-developed albite twinning with a high frequency of twin lamellae is observed. Biotite, d o t t e d quartz grains, hornblende and
310 magnetite grains occur as inclusions within plagioclase. Alteration products such as sericite, muscovite and kaolinite are observed mainly within cracks. Myrmekitic texture is generally restricted to plagioclase, with well.developed reaction margins, where these occur in association with microcline grains. Microcline displays faintly developed grid-twinning at the edges, occasional undulose twinning and flame and string type microperthite development, which is generally restricted to the center of microcline grains. Quartz may occur in appreciable amounts, but is generally deficient in rocks with increasing orthoclase content. Anhedral and elongate quartz grains sometimes show well-developed undulose extinction and grain boundaries are sharp and sutured. Some quartz grains develop around amphiboles and muscovite occurs as inclusions in certain grains. Lath-shaped biotite is present, generally giving a faint lepidoblastic texture, although it is randomly dispersed in some rocks. Infiltration by later quartz, microcline and plagioclase, and replacement by chlorite and muscovite are common. Hornblende is present as an accessory or important mineral and shows alteration to biotite and chlorite. Metamicts and overgrowths are common. Calcite is sometimes present as an accessory along grain boundaries of plagioclase. Sericite is generally restricted to the core of plagioclase grains, whereas muscovite is found disseminated in the grains.
Amphibolites These are black, or mottled black and white, fine- to medium-grained rocks of massive or gneissic structure. Amphibolite bands are interlayered concordantly within hornblende--biotite gneisses in the form of lenses, pods or rods. These rocks consist of quartz, feldspar, and ferromagnesian minerals such as hornblende, garnet and diopside. Hornblende defines the foliation of the amphibolites. In thin section, these rocks show a general hypidiomorphic granoblastic texture with green hornblende, plagioclase, quartz, and pyroxenes in that order of abundance. Hornblende is in places faintly chloritized and has anhedral form with xenomorphic character against quartz and plagioclase. This mineral exhibits strong pleochroism from yellow to dark brown and has high relief with clear margins which sometimes exhibit a corroded n a c r e . Diopside grains are surrounded by larger hornblende grains. A few prismatic sections of tremolite and actinolite are also observed and they resemble replaced pyroxene grains. Diopside grains associated with magnetite exhibit a sieve-like appearance. Plagioclase grains, of cloudy appearance, are subhedral in form and are xenomorphic against surrounding quartz and ferromagnesian minerals. A majority are untwinned, but when present, the twinning accords with albite, Carlsbad or pericline laws and secondary deformation twinning m a y also occur. A gradual compositional variation is observed in these grains from the edges towards the core.
311 Quartz, when present, occurs as singular, small rounded or elongate grains. These grains, at times anhedral in form, show blurred, sutured boundaries and undulose extinction is c o m m o n .
Calc-silicate gneisses These are gray to dark colored rocks of generally gneissic structure with medium- to coarse-grained textures. A wide variety of minerals are present, though the rock can be monomineralic in places. Plagioclase, scapolite and diopside are dominant with accessory microcline, sphene, quartz, garnet and apatite. Quartz-rich varieties grade into pale-green diopsidic quartzites. In thin section, plagioclases are generally subhedral mostly displaying pericline twinning, but to a lesser extent accords with Carlsbad and albite laws. Plagioclases are generally associated with scapolite and pyroxene. Internal compositional variations, suggestive of zoning, are sometimes present but rarely observed near scapolite grains. Some plagioclase grains show a patchy antiperthitic development and in places, sericitic alteration products are observed along cracks and fractures. Subhedrai scapolite c o m m o n l y occurs as the principal mineral in some calc-silicate gneisses. This mineral shows low relief with numerous irregular cracks. Polygonal granoblastic aggregates o f diopside generally appear around scapolite grains b u t in certain calc-gneiss bands, pyroxenes are enclosed by the scapolite. Microcline is also observed as subhedral grains with prominent crosshatched twinning. Intergranular formation of albite is also noted. Early formed pyroxenes are observed as inclusions. Calcite occasionally occurs along the edges of microcline. Diopside tends to dominate the mineralogy of some gneisses and augite is also encountered. At the edges of pyroxene grains, chlorite and amphiboles occur as replacements. Quartz, when present, is anhedrai in form, and shows undulose extinction, clear unsutured grain boundaries and a well-developed strain effect.
Dolerite intrusions These show wide variations in mineralogy. They are fine- to mediumgrained, gray-colored rocks of massive structure. Faint gneissic structure is sometimes observed. The bodies can be monomineralic, b u t generally clinopyroxenes and plagioclase dominate the mineralogy with accessory hornblende, garnet, epidote, magnetite and olivine. In thin section, diabasic texture is c o m m o n in some dolerites. Welltwinned clinopyroxense with corroded outlines, their margins replaced or altered to chlorite, hornblende and biotite, occur together with plagioclase laths. Plagioclase is subhedral to euhedral with albite, Carlsbad and pericline twinning. Marked or faint compositional variations are present in these
312 grains, suggestive of zoning from the edges towards the core and alteration products such as sericite are common. In meta-dolerites, xenoblastic grains of clinopyroxene and hornblende associated with plagioclase, display a sieve-like texture. Coronas of hornblende or magnetite develop around a central garnet grain in a groundmass of plagioclase. In other instances, rings of amphibole and garnet form coronas around a central diopside mass (Fig. 7).
~mm Fig. 7. Micrograph of doleritebody showing corona development. (D) diopside,(T--A) tremoliteactinolite,(P) plagioclase. DISCUSSION Field and laboratory investigations indicate that the Vijayan Complex of Sri Lanka is characterized by microcline-bearing granites/gneisses which have intruded into a pre-existing terrain of hornblende--biotite gneisses, amphibolites, calc-sflicategneisses and quartzites. This rock suite, indicative of an ancient sedimentary sequence, appears to have undergone W - g r a d e metamorphism, perhaps at the same time (2000 M a ago, Crawford and Oliver, 1969) as the granulite facies Highland Group of central Sri Lanka. In the microcline granite/gneiss bodies, two principal modes of occurrence are noted. (a) Elongate stock-like granite intrusions have sharp contacts with adjacent migmatites and indicate later metasomatic effects.
313 (b) Medium-sized intrusions of granite, with gradational contacts and evidence of metasomatism, are associated with migmatites, basic border zones and augen gneisses. At abrupt contacts, the granite is highly fractured and pinkish silicified material is found within its cracks as well as in the migmatitic host rocks. Petrological studies of granites/gneisses and associated migmatites show the following phenomena: (a) rimming of plagioclase by microcline, (b) replacement of biotite by muscovite and (c) replacement of hornblende by biotite. Such phenomena and field evidence indicate that potash-rich emanations probably initiated a migmatization process associated with alkali metasomatism. At gradational contacts, the schlieric border zone and the structures within the granite/gneiss body itself, imply "synkinematic" formation of granites (Raguin, 1957). The associated augen gneisses vouch for a feldspathization process that perhaps accompanied the granitization. The granites and gneisses of the study area have the mineral assemblage: microcline + plagioclase + quartz +_ orthoclase + ilmenite + apatite -+ biotite + hornblende +- cassiterite. Generally, these rocks show a lepidoblastic texture due to the presence of biotite flakes and prismatic hornblende. This structure conforms to the general foliation of the associated migmatites and may be due to syn-tectonic or post-tectonic deformation. On the basis of texture, much evidence exists favoring a magmatic origin for the microcline-bearing granites and gneisses. Flame and string-type microperthites suggest unmixing and the mesoperthites indicate relatively high magmatic temperatures up to 660°C (Tuttle and Bowen, 1958). The dominant albite twinning and marked absence of antiperthites indicate the magmatic and intrusive nature of these rocks (Tobi, 1962; Mehnhert, 1968). Furthermore, quartz and albite feldspar, which feature the frequently observed myrmekitic texture, support magmatic or deuteric crystallization, or a deuteric stage of granitic intrusion. In some calc-silicate gneisses, scapolite is the dominant mineral with subsidiary plagioclase, where regional metamorphism appears to have transformed the plagioclase of the dolerite bodies into scapolite and in the process destroyed the igneous texture. The oblique nature of some calc-silicate gneisses to the associated granite/gneiss bodies implies, perhaps, the earlier existence of dolerite dikes. Modal analyses of granites and gneisses of the study area, when applied to Streckeisen's classification, indicate the high K-feldspar content of these rocks which suggests deep-seated conditions and intermediate PT values (Didier, 1973). Moreover, the An contents of about 17 and plagioclase with polysynthetic albite--pericline twinning, suggest conditions characteristic of the almandine--amphibolite facies (Turner and Verhoogen, 1962). Calcsilicate gneisses also show dominant pericline twinning and together with the conspicuous absence of wollastonite, reinforces the suggestion of almandine-amphibolite facies PT conditions (Weeks, 1956; Harker and Tuttle, 1958). The petrological studies of some doleritic bodies indicate retrograde metamorphism with corona development produced by the reaction: H20 + Diop-
314
side + Plagioclase -~ Tremolite--actinolite + Garnet (after de Waard, 1964). The presence of deformed twinning and exsolution phenomena in the plagioclase of these dolerite bodies, granites and gneisses suggests a widespread deformation event. CONCLUSIONS
Available Rb/Sr and K/Ar age determinations from various lithological units of Sri Lanka suggest at least four events (Crawford and Oliver, 1969). The same authors postulated that the oldest event occurred more than 2000 Ma ago under granulite facies conditions when the Highland Group of rocks were formed. It is also believed that Vijayan retrogressive metamorphism ended about 1250 Ma ago. However, age data for two hornblende--biotite gneiss samples (Crawford and Oliver, 1969) in our study arev have yielded 1370 and 1396 Ma, suggesting continued tectonic activity in the area up to these times at least. The microcline granite intrusions have not been dated. However, the available field and laboratory evidence suggests the intrusion of granites into a pre-existing granulite facies terrain, accompanied by metasomatic and granitization processes. Retrograde phenomena such as the break-down of hornblende and biotite to chlorite in the granites/gneisses, migmatites and associated gneisses, and the corona development in meta-dolerites show the generally widespread nature of this retrograde metamorphism. Petrological studies indicate that the said "event" took place under PT conditions characteristic of amphibolite facies. On the basis of our studies, the following sequence of events is envisaged; (a) Intrusion of granites into a pre-existing granulite facies terrain. (b) Metasomatism and granitization (c) Emplacement of doleritic intrusions (d) Retrograde metamorphism under amphibolite facies conditions. It is emphasized here that further studies of the area, involving chemical analysis and age determinations, will be undertaken to produce a more realistic geological history for the area. ACKNOWLEDGMENTS
The authors thank Messrs S.M.J. Ranasuriya and P.S.A. Perera for their help and assistance in the field. The services rendered by Mr. Duleep Dantanarayana are gratefully acknowledged. Mrs. Jayanthi Wijesekera and Mr. K. Dunuhappawa are thanked for drafting the figures and typing the manuscript respectively. REFERENCES Adams, F.D., 1929. The geology of Ceylon. Can. J. Res., 1: 425--511. Coates, J.S., 1935. The geology of Ceylon. Ceylon J. Sci. Sect. B, 19: 101--187.
315 Cooray, P.G., 1962. Charnockites and their associated gneisses in the Precambrian of Ceylon. Q. J. Geol. Soc. London., 118: 239--273. Cooray, P.G., 1978. Geology of Sri Lanka. In: P. Nutalaya (Editor), Third Reg. Conf. on Geol. and Miner. Resour. of SE Asia, Bangkok, Thailand, 14--18 Nov., 1978, Asian Inst. Technol., Bangkok, pp. 701--710. Crawford, A.R. and Oliver, R.L., 1969. The Precambrian geochronology of Ceylon. Spec. Publ. Geol. Soc. Aust., 2: 283--306. De Waard, D., 1964. Mineral assemblages and metamorphic subfacies in the granulite facies terrains of the Little Moose Mountain syncline, south central Adirondak Highlands. K. Ned. Akad. Wet. Proc. Amsterdam, B., 67 : 344--362. Didier, J., 1973. Granites and their Enclaves. Elsevier, Amsterdam, 393 pp. Fernando, L.J.D., 1948. The geology and mineral resources of Ceylon. Bull. Imp. Inst. London, 46: 303--325. Harker, R.I. and Tuttle, O.F., 1956. Experimental data on the P c o - - T curve for the reaction Calcite + Quartz, Wollastonite + Carbon dioxide. Am. J. ~ci., 254: 239--256. Hatherton, T., Pattiarachi, D.B. and Ranasinghe, V.V.C., 1975. Gravity map of Sri Lanka. Geol. Surv. Dept. Rep., Sri Lanka, Prof. Paper., 3 : 3 9 pp. Katz, M.B., 1971. The Precambrian metamorphic rocks of Ceylon. Geol. Rundsch., 60: 1523--1549. Mehnert, K.R., 1968. Migmatites and the Origin of Granitic Rocks. Elsevier, Amsterdam, 393 pp. Munasinghe, T. and Dissanayake, C.B., 1979. Is the Highland--Eastern Vijayan boundary in Sri Lanka, a possible mineralized belt? Econ. Geol., 74: 1495--1496. Raguin, E., 1957. Geologie du Granite. Masson, Paris, 2nd Ed., 275 pp. Streckeisen, A.L., 1967. Classification and nomenclature o f igneous rocks. Neues Jahrb. Mineral. 107: 144--160. Tobi, A.C., 1962. Characteristic patterns of plagioclase twins. Nor. Geol. Tidsskr, 42: 264--271. Turner, F.J. and Verhoogen, J., 1962. Igneous and Metamorphic Petrology. McGraw-Hill, New York, NY, 649 pp. Tuttle, O.F. and Bowen, N.L., 1958. Origin of granite in the light of experimental studies in the system NaA1Si3Os--KA1Si3Os--SiO2--H2. Geol. Soc. Am. Mem., 7 5 : 1 5 4 pp. Vitanage, P.W., 1959. Geology of the country around Polonnaruwa. Geol. Surv. Ceylon, Mem. 1 : 7 5 pp. Vitanage, P.W., 1972. Post Precambrian uplifts and regional neotectonic movements in Ceylon. 24th Intern. Geol. Congr. Sec. 3, Montreal, pp. 6 4 2 - 6 5 4 . Wadia, D.N., 1945. The three superposed peneplains of Ceylon. Ceylon Dept. Mineral. Prof. Paper 1: 25--38. Weeks, W.F., 1956. A thermochemical study of equilibrium relations during metamorphism of siliceous carbonate rocks. J. Geol., 64: 245--270.
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G E N E R A L G E O L O G Y AND P E T R O L O G Y OF SOME PRECAMBRIAN C R Y S T A L L I N E ROCKS FROM T H E VIJAYAN COMPLEX OF SRI L A N K A
KAPILA DAHANAYAKE and H.A.H. JAYASENA, Department of Geology, University of Peradeniya, Peradeniya (Sri Lanka) (Received January 29, 1982; revision accepted August 12, 1982)
ABSTRACT Dahanayake, K. and Jayasena, H.A.H., 1983. General geology and petrology of some Precambrian crystalline rocks from the Vijayan Complex of Sri Lanka. Precambrian Res., 19: 301--315. Detailed field studies of the Precambrian Vijayan Complex terrain, Sri Lanka, reveal the occurrence of granites, gneisses and migmatites in association with calc-silicate gneisses, quartzites and dolerites. Microcline-rich granites and gneisses show both sharp and gradational contacts with the adjacent migmatites. Petrological observations favor a magmatic origin for the granites and gneisses. These bodies seem to have intruded into pre-existing metasedimentary rocks which were subsequently subjected to retrograde metamorphism under amphibolite facies conditions.
INTRODUCTION E x c e p t for some coastal outliers of Miocene sediments and a few scattered Jurassic outcrops, the island of Sri Lanka consists dom i nant l y of Precambrian crystalline rocks with imprints of varying degrees and ages of tectonism and metamorphism. Three major units of rocks can be recognized on the basis of lithology, structure and age (Cooray, 1978): (a) Highland Group, (b) Southwestern Gr o u p and (c) Vijayan Complex. The Highland G r oup of rocks, occurring extensively in the central highlands of Sri Lanka, comprises high-grade m e t a m o r p h i c rocks mostly of the hypersthene-granulite facies. T h e y axe quartzites, marble, garnet--sillimanite gneisses, granulites associated with granites, migmatites, hornbl ende--bi ot i t e gneisses, charnockites and pegmatites. These rocks show remarkable consist e n cy of strike with open and overturned folds giving rise to a series of antiform and s y n f o r m structures, and elongate basins and domes which form a north-plunging synclinorium in the center of the island (Vitanage, 1972). Towards the n o r t h the synclinorium axis trends N--NE. The Southwestern Group, belonging to the cordierite-granulite facies, is characterized by cordierite-bearing gneisses, charnockites, relatively thin quartzites and calciphyres associated with granitic gneisses, augen gneisses,
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migmatites and pegmatites. These rocks form a prominent NW.trending isoclinal fold system. The Vijayan Complex occurs on either side of the Highland Group (Fig. 1), extending from the eastern and western coasts towards the foothills of the central highlands. The complex of rocks, for the most part forming two plateau-like, wide peneplained surfaces, consists of gneisses, granites associated with migmatites, quartzites, doleritic intrusions and pegmatites. These rocks, LEGEND
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considered to be of amphibolite facies and of complex structure, are characterized by many intrusions, extensive shearing and flow folding. Plunging antiforms, synforms and domes are commonly encountered but their axes are difficult to trace in many instances. The Vijayan Complex was regarded as the basement upon which the original sediments of the Highland Group were deposited (Adams, 1929; Coates, 1935; Wadia, 1945; Fernando, 1948). However, Vitanage (1959) considered the granitized and migmatitic Vijayan rocks to be younger than the Highland Group. Cooray (1962) proposed that the Highland and Vijayan rocks differed in metamorphic grade and plutonic history. On the basis of some radiometric age data, Crawford and Oliver (1969) postulated that the Vijayan rocks represented former Highland Group rocks, migmatized and granitized under wetter amphibolite facies conditions. This transformation, called Vijayan retrogressive metamorphism, was believed by the same authors to have terminated about 1250 Ma ago. They also recognized two later "events" at about 650 Ma and 450 Ma. Katz (1971) maintained the earlier view that the Vijayan rocks represent the basement upon which the Highland Group were deposited about 3000 Ma ago. The Vijayan rocks were thought by the same author to have undergone polymetamorphism during at least three periods (2000, 1250 and 650--450 Ma), under PT conditions characteristic of the granulite facies. The boundary between the western Vijayan and Highland rocks is not well-defined, but the boundary at the eastern Vijayan is more distinct and has been considered a structural contact/shear zone, or a plate margin (Vitanage, 1959; Katz, 1971; Hatherton et al., 1975; Munasinghe and Dissanayake, 1979). Although generalized geological and geochronological studies have been done on the Vijayan Complex, no systematic detailed geological studies have been undertaken. In this work, the authors attempt to investigate the general geology and petrology, with a view to understanding the complex geological history of the Vijayan rocks, by field and laboratory studies of a typical Precambrian Vijayan Complex terrain in eastern Sri Lanka. METHODS OF STUDY
The aerial photographs of an area covering 2500 km: on the scale of 1:20 000 were studied and structural observations were made. A reconnaissance field mapping program was undertaken and a geological map on the scale of 1:63 360 was prepared. Approximately 200 rock samples, representative of the mapped area, were studied to determine textural features and mineralogy. PHYSIOGRAPHY AND G E N E R A L GEOLOGY OF THE STUDY A R E A
The study area is covered by the topographic sheets of Elahera, Polonnaruwa, Vakaneri and Rukam (1:63 360). Though served by a network of roads
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suitable for vehicles, the area remains for the most part inaccessible due to dense jungle. However, footpaths and some cart tracks do exist, and traverses along these helped to locate an appreciable number of outcrops in the difficult terrain. A multitude of streams and major rivers (Mahaweli Ganga, Maduru Oya and Bodigoda Aru) drain the area, which forms part of a vast peneplain surface, characterized by prominent inselberg-type erosion remnants reaching maximum elevations of 150 m above MSL. Generally, the landscape of the area can be described as undulating topography controlled for the most part by the underlying structure. Low ridges (< 100 m) occur, with elongate isolated high ridges (> 100 m), on a low-lying (< 50 m above MSL) plateau. Observations on the ridges, road cuttings, river beds, isolated outcrops and quarries help to identify three major rock units: (1) microcline granites and gneisses, (2) hornblende--biotite gneisses and (3) augen gneisses and migmatites. These units are associated with amphibolites, calc-silicate gneiss, dolerite intrusions, quartzites and pegmatites. GEOLOGICAL SETTING AND S T R U C T U R A L RELATIONS OF THE ROCK UNITS
The study area (Figs. 1,2) is characterized by northward-plunging synforms, antiforms and domes. The cores of these structures are occupied mainly by microcline granites and gneisses. Such rocks are surrounded, laterally, by augen gneisses and migmatites which pass into extensively occurring hornblende--biotite gneisses. Within these gneisses isolated calc-silicate gneiss and amphibolite bands are observed as well as concordant and cross-cutting doleritic intrusions. The dominant rock type of the area, hornblende--biotite gneiss, is characteristically weU-foliated and forms a layered sequence of alternating, compositionally and texturally distinct units closely associated with migmatites. Compositions ranging from amphibolitic to highly quartzo-feldspathic or granitic, with fine- to coarse-grained textures, are typical of these gneisses. The thickness of bands varies from a few centimeters to several meters. The migmatitic varieties contain discontinuous leucocratic phases in the form of patches, pods, layers and dilational dikes. Locally, very tight, small-scale folding is observed in the gneisses. Layers of amphibolite have been distended to form boudins and rotated blocks. The gneisses are cut by pegmatitic dikes and veins of granitic composition. Comparatively thinner, doleritic intrusions occur as cross-cutting dikes, or as discontinuous layers conformable to local foliation. Their contacts with adjacent rocks are poorly exposed. Microcline granites and gneisses, forming the cores of domes, antiforms and synforms, occur as minor protrusions, prominent inselbergs and elongate massifs on the otherwise gentle peneplain. These features are characterized, locally, by intense weathering and erosion phenomena leaving trains of granitic/gneissic boulders, extending for a few kilometers, parallel to the ridges, The granitic bodies commonly show distinct foliation in fine-grained matrices.
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Coarse-grained massive granites sensu stricto are common. Locally the granites show intense fracturing with as much as five systems of closely spaced joints and the steep dip of the gneiss is a prominent and noteworthy feature, which tends to become less steep towards the migmatitic country rocks, away from the granitic/gneissic intrusion. The contact between the granitic intrusions and adjacent migmatites is sometimes abrupt and sometimes gradational. At abrupt contacts, the marginal parts of the granite intrusions reveal intense shearing phenomena characterized by the complex joint system referred to earlier (Fig. 3). Mylonitic shear zones are also encountered. The granite body and the adjacent migmatitic host rocks both show joints and cracks along which pink silicified material has been intruded. At gradational contacts,
Fig. 3. The sharp contact (indicated by arrows) between migmatite and granite/gneiss. Note shattered nature of granite on the right side. (Maduru Oya proposed dam site, SW map area, Fig. 1.)
essentially basic relict features, in the form of discontinuous schlieren and streaks of differing intermediate composition, are observed (Fig. 4}. Generally, the parallel structure of these relict bodies reflects the trend of the granites/ gneisses. In such instances, the granite body as a whole, the internal structure within it and the associaSed basic border zone, show a parallel orientation, according to the main tectonic direction (Fig. 2), and form markedly elongated topographic features. Calc-gneisses are widely distributed as minor rock units in the layered gneiss complex and two types are observed. The first occurs as discontinuous lenses,
307
Fig. 4. The granite/gneiss body and the dark basic schlieric border zone (Madurankerni Kulam, NE map area, Fig. 1). generally concordant with local foliation, although continuous bands do exist. The second t y p e is oblique to the foliation and is of a localized nature. Augen gneisses are scarce and when present they occur at the noses of plunging antiforms and domes, closely associated with granitic/gneissic bodies. Quartzites are rare in the mapped area and are either associated with granitic bodies, or interlayered with hornblende--biotite gneiss. Pegmatites occur as wide, crosscutting bodies, sometimes as thick as 40 m, in the hornblende--biotite gneiss. PETROLOGY OF THE LITHOLOGIC UNITS
Microcline granites and gneisses These are medium- to coarse-grained, pinkish rocks characterized by a massive or gneissic structure, with dominant pink feldspar and quartz associated with accessory biotite and hornblende. Magnetite and apatite are sparsely distributed. In thin sections, the texture is hypidiomorphic granular with a b u n d a n t K-feldspar, quartz and plagioclase. K-feldspars are mostly microclines, sometimes perthites, and oligoclase forms the plagioclase content. According to modal analysis, these rocks are granites or quartz syenites in the classification of Streckeisen (1967) (Fig. 5), which reflects the high potash c o n t e n t of the granitic and gneissic bodies. Microcline occurs as subhedral grains t h a t are xenomorphic against quartz
308
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and plagioclase. The microperthites within the microcline are of flame and string types, restricted to the core of the grain, and patchy and vein type which develop preferentially along the grain boundaries. Sometimes, relict plagioclase grains, riddled with sericite and dotted with quartz grains, are found in the microclines. The orientation of the twinned plagioclase inclusions reflects the grid twinning of the host grains. Plagioclase mainly displays albite twinning, but some are twinned according to the Carlsbad and pericline laws. Zoning is generally absent but a few grains exhibit faint compositional changes. Some plagioclase shows secondary deformation twinning and alteration products such as sericite, muscovite and epidote are found in certain grains and myrmekitic texture is evident in some grains. Well-developed reaction margins have been observed where plagioclase grains are associated with microcline grains (Fig. 6). Quartz is generally present as elongate grains with sutured boundaries. Some grains show undulatory extinction. Orthoclase with Carlsbad twinning is rarely present. Lath-shaped biotite shows faint lepidoblastic texture. Hornblende for the most part is chloritized and shows assimilation by later-formed quartz and plagioclase megabtasts. Rounded and elongate apatite grains are present as well as subangular and euhedral zircon. Euhedral magnetite grains show
309
Fig. 6. Micrograph of granite. (Mp) perthitic microcline, (Mr) replacement perthite, (M) microcline, (P) plagioclase, (Q) quartz. alteration to hematite. Sphene, cracked or wedge-shaped, schorlite and cassiterite are found occasionally.
Hornblende--biotite gneisses and migmatites Hornblende--biotite gneisses and associated migmatites are dark gray to black, fine- to coarse-grained, with moderate to strong foliation. Hornblende and biotite are c o m m o n in a dominant quartzo-feldspathic matrix. The ratios of plagioclase/orthoclase and biotite/hornblende seem to decrease adjacent to the granitic intrusions. In thin section, the texture varies from panidiomorphic granoblastic/lepidoblastic to hypidiomorphic granoblastic. Feldspar, quartz, hornblende and biotite are abundant. The latter two minerals show local chloritization. Sphene, apatite, magnetite and zircon occur as accessories. Modal analyses for these rocks indicate that they are granodioritic and tonalitic according to the classification of Strekeisen (1967) (Fig. 5). Plagioclase grains are generally subhedral and are xenomorphic against surrounding grains. Faint to well-developed albite twinning with a high frequency of twin lamellae is observed. Biotite, d o t t e d quartz grains, hornblende and
310 magnetite grains occur as inclusions within plagioclase. Alteration products such as sericite, muscovite and kaolinite are observed mainly within cracks. Myrmekitic texture is generally restricted to plagioclase, with well.developed reaction margins, where these occur in association with microcline grains. Microcline displays faintly developed grid-twinning at the edges, occasional undulose twinning and flame and string type microperthite development, which is generally restricted to the center of microcline grains. Quartz may occur in appreciable amounts, but is generally deficient in rocks with increasing orthoclase content. Anhedral and elongate quartz grains sometimes show well-developed undulose extinction and grain boundaries are sharp and sutured. Some quartz grains develop around amphiboles and muscovite occurs as inclusions in certain grains. Lath-shaped biotite is present, generally giving a faint lepidoblastic texture, although it is randomly dispersed in some rocks. Infiltration by later quartz, microcline and plagioclase, and replacement by chlorite and muscovite are common. Hornblende is present as an accessory or important mineral and shows alteration to biotite and chlorite. Metamicts and overgrowths are common. Calcite is sometimes present as an accessory along grain boundaries of plagioclase. Sericite is generally restricted to the core of plagioclase grains, whereas muscovite is found disseminated in the grains.
Amphibolites These are black, or mottled black and white, fine- to medium-grained rocks of massive or gneissic structure. Amphibolite bands are interlayered concordantly within hornblende--biotite gneisses in the form of lenses, pods or rods. These rocks consist of quartz, feldspar, and ferromagnesian minerals such as hornblende, garnet and diopside. Hornblende defines the foliation of the amphibolites. In thin section, these rocks show a general hypidiomorphic granoblastic texture with green hornblende, plagioclase, quartz, and pyroxenes in that order of abundance. Hornblende is in places faintly chloritized and has anhedral form with xenomorphic character against quartz and plagioclase. This mineral exhibits strong pleochroism from yellow to dark brown and has high relief with clear margins which sometimes exhibit a corroded n a c r e . Diopside grains are surrounded by larger hornblende grains. A few prismatic sections of tremolite and actinolite are also observed and they resemble replaced pyroxene grains. Diopside grains associated with magnetite exhibit a sieve-like appearance. Plagioclase grains, of cloudy appearance, are subhedral in form and are xenomorphic against surrounding quartz and ferromagnesian minerals. A majority are untwinned, but when present, the twinning accords with albite, Carlsbad or pericline laws and secondary deformation twinning m a y also occur. A gradual compositional variation is observed in these grains from the edges towards the core.
311 Quartz, when present, occurs as singular, small rounded or elongate grains. These grains, at times anhedral in form, show blurred, sutured boundaries and undulose extinction is c o m m o n .
Calc-silicate gneisses These are gray to dark colored rocks of generally gneissic structure with medium- to coarse-grained textures. A wide variety of minerals are present, though the rock can be monomineralic in places. Plagioclase, scapolite and diopside are dominant with accessory microcline, sphene, quartz, garnet and apatite. Quartz-rich varieties grade into pale-green diopsidic quartzites. In thin section, plagioclases are generally subhedral mostly displaying pericline twinning, but to a lesser extent accords with Carlsbad and albite laws. Plagioclases are generally associated with scapolite and pyroxene. Internal compositional variations, suggestive of zoning, are sometimes present but rarely observed near scapolite grains. Some plagioclase grains show a patchy antiperthitic development and in places, sericitic alteration products are observed along cracks and fractures. Subhedrai scapolite c o m m o n l y occurs as the principal mineral in some calc-silicate gneisses. This mineral shows low relief with numerous irregular cracks. Polygonal granoblastic aggregates o f diopside generally appear around scapolite grains b u t in certain calc-gneiss bands, pyroxenes are enclosed by the scapolite. Microcline is also observed as subhedral grains with prominent crosshatched twinning. Intergranular formation of albite is also noted. Early formed pyroxenes are observed as inclusions. Calcite occasionally occurs along the edges of microcline. Diopside tends to dominate the mineralogy of some gneisses and augite is also encountered. At the edges of pyroxene grains, chlorite and amphiboles occur as replacements. Quartz, when present, is anhedrai in form, and shows undulose extinction, clear unsutured grain boundaries and a well-developed strain effect.
Dolerite intrusions These show wide variations in mineralogy. They are fine- to mediumgrained, gray-colored rocks of massive structure. Faint gneissic structure is sometimes observed. The bodies can be monomineralic, b u t generally clinopyroxenes and plagioclase dominate the mineralogy with accessory hornblende, garnet, epidote, magnetite and olivine. In thin section, diabasic texture is c o m m o n in some dolerites. Welltwinned clinopyroxense with corroded outlines, their margins replaced or altered to chlorite, hornblende and biotite, occur together with plagioclase laths. Plagioclase is subhedral to euhedral with albite, Carlsbad and pericline twinning. Marked or faint compositional variations are present in these
312 grains, suggestive of zoning from the edges towards the core and alteration products such as sericite are common. In meta-dolerites, xenoblastic grains of clinopyroxene and hornblende associated with plagioclase, display a sieve-like texture. Coronas of hornblende or magnetite develop around a central garnet grain in a groundmass of plagioclase. In other instances, rings of amphibole and garnet form coronas around a central diopside mass (Fig. 7).
~mm Fig. 7. Micrograph of doleritebody showing corona development. (D) diopside,(T--A) tremoliteactinolite,(P) plagioclase. DISCUSSION Field and laboratory investigations indicate that the Vijayan Complex of Sri Lanka is characterized by microcline-bearing granites/gneisses which have intruded into a pre-existing terrain of hornblende--biotite gneisses, amphibolites, calc-sflicategneisses and quartzites. This rock suite, indicative of an ancient sedimentary sequence, appears to have undergone W - g r a d e metamorphism, perhaps at the same time (2000 M a ago, Crawford and Oliver, 1969) as the granulite facies Highland Group of central Sri Lanka. In the microcline granite/gneiss bodies, two principal modes of occurrence are noted. (a) Elongate stock-like granite intrusions have sharp contacts with adjacent migmatites and indicate later metasomatic effects.
313 (b) Medium-sized intrusions of granite, with gradational contacts and evidence of metasomatism, are associated with migmatites, basic border zones and augen gneisses. At abrupt contacts, the granite is highly fractured and pinkish silicified material is found within its cracks as well as in the migmatitic host rocks. Petrological studies of granites/gneisses and associated migmatites show the following phenomena: (a) rimming of plagioclase by microcline, (b) replacement of biotite by muscovite and (c) replacement of hornblende by biotite. Such phenomena and field evidence indicate that potash-rich emanations probably initiated a migmatization process associated with alkali metasomatism. At gradational contacts, the schlieric border zone and the structures within the granite/gneiss body itself, imply "synkinematic" formation of granites (Raguin, 1957). The associated augen gneisses vouch for a feldspathization process that perhaps accompanied the granitization. The granites and gneisses of the study area have the mineral assemblage: microcline + plagioclase + quartz +_ orthoclase + ilmenite + apatite -+ biotite + hornblende +- cassiterite. Generally, these rocks show a lepidoblastic texture due to the presence of biotite flakes and prismatic hornblende. This structure conforms to the general foliation of the associated migmatites and may be due to syn-tectonic or post-tectonic deformation. On the basis of texture, much evidence exists favoring a magmatic origin for the microcline-bearing granites and gneisses. Flame and string-type microperthites suggest unmixing and the mesoperthites indicate relatively high magmatic temperatures up to 660°C (Tuttle and Bowen, 1958). The dominant albite twinning and marked absence of antiperthites indicate the magmatic and intrusive nature of these rocks (Tobi, 1962; Mehnhert, 1968). Furthermore, quartz and albite feldspar, which feature the frequently observed myrmekitic texture, support magmatic or deuteric crystallization, or a deuteric stage of granitic intrusion. In some calc-silicate gneisses, scapolite is the dominant mineral with subsidiary plagioclase, where regional metamorphism appears to have transformed the plagioclase of the dolerite bodies into scapolite and in the process destroyed the igneous texture. The oblique nature of some calc-silicate gneisses to the associated granite/gneiss bodies implies, perhaps, the earlier existence of dolerite dikes. Modal analyses of granites and gneisses of the study area, when applied to Streckeisen's classification, indicate the high K-feldspar content of these rocks which suggests deep-seated conditions and intermediate PT values (Didier, 1973). Moreover, the An contents of about 17 and plagioclase with polysynthetic albite--pericline twinning, suggest conditions characteristic of the almandine--amphibolite facies (Turner and Verhoogen, 1962). Calcsilicate gneisses also show dominant pericline twinning and together with the conspicuous absence of wollastonite, reinforces the suggestion of almandine-amphibolite facies PT conditions (Weeks, 1956; Harker and Tuttle, 1958). The petrological studies of some doleritic bodies indicate retrograde metamorphism with corona development produced by the reaction: H20 + Diop-
314
side + Plagioclase -~ Tremolite--actinolite + Garnet (after de Waard, 1964). The presence of deformed twinning and exsolution phenomena in the plagioclase of these dolerite bodies, granites and gneisses suggests a widespread deformation event. CONCLUSIONS
Available Rb/Sr and K/Ar age determinations from various lithological units of Sri Lanka suggest at least four events (Crawford and Oliver, 1969). The same authors postulated that the oldest event occurred more than 2000 Ma ago under granulite facies conditions when the Highland Group of rocks were formed. It is also believed that Vijayan retrogressive metamorphism ended about 1250 Ma ago. However, age data for two hornblende--biotite gneiss samples (Crawford and Oliver, 1969) in our study arev have yielded 1370 and 1396 Ma, suggesting continued tectonic activity in the area up to these times at least. The microcline granite intrusions have not been dated. However, the available field and laboratory evidence suggests the intrusion of granites into a pre-existing granulite facies terrain, accompanied by metasomatic and granitization processes. Retrograde phenomena such as the break-down of hornblende and biotite to chlorite in the granites/gneisses, migmatites and associated gneisses, and the corona development in meta-dolerites show the generally widespread nature of this retrograde metamorphism. Petrological studies indicate that the said "event" took place under PT conditions characteristic of amphibolite facies. On the basis of our studies, the following sequence of events is envisaged; (a) Intrusion of granites into a pre-existing granulite facies terrain. (b) Metasomatism and granitization (c) Emplacement of doleritic intrusions (d) Retrograde metamorphism under amphibolite facies conditions. It is emphasized here that further studies of the area, involving chemical analysis and age determinations, will be undertaken to produce a more realistic geological history for the area. ACKNOWLEDGMENTS
The authors thank Messrs S.M.J. Ranasuriya and P.S.A. Perera for their help and assistance in the field. The services rendered by Mr. Duleep Dantanarayana are gratefully acknowledged. Mrs. Jayanthi Wijesekera and Mr. K. Dunuhappawa are thanked for drafting the figures and typing the manuscript respectively. REFERENCES Adams, F.D., 1929. The geology of Ceylon. Can. J. Res., 1: 425--511. Coates, J.S., 1935. The geology of Ceylon. Ceylon J. Sci. Sect. B, 19: 101--187.
315 Cooray, P.G., 1962. Charnockites and their associated gneisses in the Precambrian of Ceylon. Q. J. Geol. Soc. London., 118: 239--273. Cooray, P.G., 1978. Geology of Sri Lanka. In: P. Nutalaya (Editor), Third Reg. Conf. on Geol. and Miner. Resour. of SE Asia, Bangkok, Thailand, 14--18 Nov., 1978, Asian Inst. Technol., Bangkok, pp. 701--710. Crawford, A.R. and Oliver, R.L., 1969. The Precambrian geochronology of Ceylon. Spec. Publ. Geol. Soc. Aust., 2: 283--306. De Waard, D., 1964. Mineral assemblages and metamorphic subfacies in the granulite facies terrains of the Little Moose Mountain syncline, south central Adirondak Highlands. K. Ned. Akad. Wet. Proc. Amsterdam, B., 67 : 344--362. Didier, J., 1973. Granites and their Enclaves. Elsevier, Amsterdam, 393 pp. Fernando, L.J.D., 1948. The geology and mineral resources of Ceylon. Bull. Imp. Inst. London, 46: 303--325. Harker, R.I. and Tuttle, O.F., 1956. Experimental data on the P c o - - T curve for the reaction Calcite + Quartz, Wollastonite + Carbon dioxide. Am. J. ~ci., 254: 239--256. Hatherton, T., Pattiarachi, D.B. and Ranasinghe, V.V.C., 1975. Gravity map of Sri Lanka. Geol. Surv. Dept. Rep., Sri Lanka, Prof. Paper., 3 : 3 9 pp. Katz, M.B., 1971. The Precambrian metamorphic rocks of Ceylon. Geol. Rundsch., 60: 1523--1549. Mehnert, K.R., 1968. Migmatites and the Origin of Granitic Rocks. Elsevier, Amsterdam, 393 pp. Munasinghe, T. and Dissanayake, C.B., 1979. Is the Highland--Eastern Vijayan boundary in Sri Lanka, a possible mineralized belt? Econ. Geol., 74: 1495--1496. Raguin, E., 1957. Geologie du Granite. Masson, Paris, 2nd Ed., 275 pp. Streckeisen, A.L., 1967. Classification and nomenclature o f igneous rocks. Neues Jahrb. Mineral. 107: 144--160. Tobi, A.C., 1962. Characteristic patterns of plagioclase twins. Nor. Geol. Tidsskr, 42: 264--271. Turner, F.J. and Verhoogen, J., 1962. Igneous and Metamorphic Petrology. McGraw-Hill, New York, NY, 649 pp. Tuttle, O.F. and Bowen, N.L., 1958. Origin of granite in the light of experimental studies in the system NaA1Si3Os--KA1Si3Os--SiO2--H2. Geol. Soc. Am. Mem., 7 5 : 1 5 4 pp. Vitanage, P.W., 1959. Geology of the country around Polonnaruwa. Geol. Surv. Ceylon, Mem. 1 : 7 5 pp. Vitanage, P.W., 1972. Post Precambrian uplifts and regional neotectonic movements in Ceylon. 24th Intern. Geol. Congr. Sec. 3, Montreal, pp. 6 4 2 - 6 5 4 . Wadia, D.N., 1945. The three superposed peneplains of Ceylon. Ceylon Dept. Mineral. Prof. Paper 1: 25--38. Weeks, W.F., 1956. A thermochemical study of equilibrium relations during metamorphism of siliceous carbonate rocks. J. Geol., 64: 245--270.