sm nd rb sr and u pb geochronology of a grenville terrane in southern mexico origin and geologic history of the guichicovi complex abstract the guichicovi complex forms the metamorphic basement near the western edge of the maya terrane in southern mexico petrographic field studies show that the guichicovi complex like other high grade metamorphic terranes in southern mexico consists of paragneisses and a igneous suite of rocks metamorphosed under granulite facies conditions the sm nd isotopic results yield depleted mantle model ages of 1 4 1 6 ga for the metaigneous rocks and of 1 5 2 0 ga for the metasediments on a sm nd isochron diagram the isotopic results of the metaigneous suite of rocks define an isochron whose slope corresponds to an age of 1 44 ga this age can be interpreted as the time of a major crust forming event discordant u pb zircon data from a felsic orthogneiss plot on a chord intersecting the concordia at 1231 43 and at 975 35 ma the upper intercept age is interpreted as the time of zircon crystallization hence the age of intrusion of the granitic precursor the lower intercept age reflects the time of peak granulite facies metamorphism concordant u pb isotope ratios of a zircon fraction from a charnockite 991 4 ma gives the time of crystallisation of the charnockite garnet whole rock ages sm nd between 911 and 933 ma and biotite whole rock ages rb sr between 866 and 882 ma document the cooling history of the guichicovi complex age and petrography of the guichicovi complex confirm its connection with other granulite exposures in mexico which comprise the oaxaquia microcontinent the results support the hypothesis that the precambrian crust in mexico together with the colombian andes and the adirondacks originated during the mid proterozoic the grenville orogenic event depressed these terranes into the roots of the orogen our data indicate that the guichicovi complex stayed for 70 ma at a deep crustal level before uplift subsequent breakup of rodinia left oaxaquia including the guichicovi complex together with gondwana further collisions of gondwana and laurentia and repeated transpressional interactions in the paleozoic and early mesozoic led to the present day position of grenville fragments in mexico

identification of inherited radiogenic pb in monazite and its implications for u pb systematics the simplest interpretation of uranium lead mineral ages in igneous rocks embodies the assumption that melting of pre existing rocks leads to homogenization of the lead isotopes thus resetting the radiometric clock zircons often violate this assumption by only partially dissolving and preserving a component of lead inherited from before the melting episode but until now such behaviour has not been observed in monazite here we report u pb data for zircons and monazites from a tertiary granite from the himalayas in which zircons and monazites contain inherited radio genic lead an analysis of a single entirely igneous zircon crystal plots on concordia at 19 5 0 4 myr eight monazite fractions lie on a line from an upper intercept of 471 10 myr to the youngest fraction which plots slightly above concordia at 21 myr the presence of excess 206pb in the youngest monazite fractions indi cates that the monazite data represent a mixing line between young magmatic monazite and an older inherited component rather than a resetting of 471 myr monazites at 20 myr these data suggest a closure temperature for pb in monazite significantly higher than previously published estimates

u pb dating of monazite and its application to geological problems monazite is an underutilized mineral in u pb geochronological studies of crustal rocks it occurs as an accessory mineral in a wide variety of rocks including granite pegmatite felsic volcanic a

the nilgiri enderbites south india nature and age constraints on protolith formation high grade metamorphism and cooling history abstract the trident shaped nilgiri highland massif represents the deepest level exposure 9 10 kbar c 35 km palaeo depth of granulite grade lower crust in southern peninsular india the crustal section is essentially composed of garnetiferous enderbitic granulites with minor kyanite bearing gneisses quartzites and banded magnetite quartzites its deeper northern part is intruded by intermediate and basic magmatic rocks now represented by non garnetiferous enderbites garnetiferous basic granulites gabbroic to anorthositic two pyroxene granulites and pyroxenites the garnetiferous enderbitic granulites are tonalitic in composition displaying a typical calc alkaline trend and exhibit chemical features of both i type as well as s type granitoids they show moderate enrichment of lree la n 35 70 no to weakly negative eu anomalies and fairly constant levels of hree yb n 6 5 13 0 and resemble the ree spectra of tonalitic trondhjemitic granitoids and greywackes of late archaean to palaeoproterozoic age sedimentary protoliths are indicated by negative df values ranging from 1 to 4 al 2 o 3 cao na 2 o k 2 o molar ratios 1 1 high 18 o whole rock values 10 12 which match the values reported for greywackes whole rock isotope data for garnetiferous enderbites define linear arrays in the rb sr and sm nd evolution diagrams rb sr system 2460 81 ma with sr i 0 7030 5 sm nd system 2506 70 ma with nd i 0 509363 60 implying a provenance of the sedimentary precursors that was dominated by felsic volcanic rocks or tonalitic trondhjemitic granitoids of uniform isotopic composition and early proterozoic age the u pb systematics of composite zircon grains with detrital cores and metamorphic overgrowths in accord with the sm nd and rb sr errorchron ages and sm nd garnet whole rock data suggest that formation of the provenance deposition of the sedimentary precursors and subsequent high grade tectono metamorphic processes must have occurred in the short time span between 2600 and 2500 ma based on the regional geological framework the petrological geochemical and isotopic data the nilgiri granulite terrain is interpreted as a metasedimentary allochthonous unit that was thrust onto the dharwar craton during palaeoproterozoic collisional tectonics

zircon xenocrysts from the kambalda volcanics age constraints and direct evidence for older continental crust below the kambalda norseman greenstones abstract the hangingwall basalt at kambalda western australia contains zircons that have been shown by ion microprobe analyses to have very high u and th contents and a wide variety of crystallization ages nearly all of these zircons certainly are xenocrysts a few might relate to intrusive veinlets the age of the youngest xenocrysts 2693 50ma 2 shows that the eruptive age of the basalt cannot exceed 2743 ma this confirms that the apparent sm nd isochron giving 3200 ma 1 2 for kambalda mafic and ultramafic rocks is a mixing line 2 between unrelated components enriched and depleted in light rare earth elements mixing probably occurred at depth by erosion of 3200 3500 ma old felsic crust from the walls of the hwb conduits the zircon xenocryst ages are the first direct evidence for the presence of very old felsic crust in the eastern yilgarn block the latter implies that the kalgoorlie norseman greenstone sequences were formed in a continental rather than an oceanic environment

evidence for post metamorphic metasomatism of high grade orthogneisses from sri lanka abstract here we discuss the post metamorphic metasomatism of high grade orthogneisses by studying granite looking pink coloured microcline bearing rocks exposed around ambagaspitiya sri lanka these rocks are medium to coarse grained and are more or less homogeneous and isotropic textural and petrographic analyses clearly show that these special rocks are neither deformed nor metamorphosed and that they are not any kind of intrusive rocks the present study shows that these rocks have formed through k metasomatism of once intensely deformed and metamorphosed granodiorite tonalite monzodiorite and quartz monzodiorite the modal compositions of most of these metasomatic rocks of ambagaspitiya are very similar to those of syenite quartz syenite monzonite quartz monzonite and quartz monzodiorite all the original metamorphic rocks namely granitic gneiss metagranite metagranodiorite metatonalite metamonzodiorite metaquartz monzodiorite metadiorite basic dikes and metapelites had undergone at least five ductile deformations d 1 to d 5 and had been metamorphosed under upper amphibolite to granulite facies conditions prior to the metasomatism almost all the parent metamorphic rocks had acquired a well developed gneissic foliation s2 and had suffered at least two intense folding events f 3 and f 5 before the metasomatism occurred all the metamorphic and deformational fabrics of affected metamorphic rocks have been completely or partially obliterated by the metasomatism this indicates that the metasomatic process post dates all ductile deformations d 1 d 5 and the regional metamorphism of the parent metamorphic rocks metagranodiorite metatonalite metamonzodiorite and metaquartz monzodiorite have undergone intense metasomatism it is shown that the metasomatism has nucleated along late stage post d 5 shear zones which may form an interconnected network potassium bearing metasomatic fluids derived from a deep seated k rich source may have migrated along these shear zones the fluids which entered the shear zones have pervaded the orthogneisses through foliation planes and along grain boundaries and microcracks in minerals transforming the host gneisses to metasomatic rocks the main metasomatic transformation has taken place through the replacement of metamorphic plagioclase and plagioclase quartz by microcline and through formation of myrmekite further studies are necessary to unravel the nature composition and the source of these late stage k rich fluids in the lower crust

rb sr whole rock mineral isochron ages of plutonic rocks from the wanni complex sri lanka abstract granitoid plutons in different east gondwana fragments give evidence for vigorous felsic magmatism during the pan african period 800 to 500 ma the sri lankan basement which is mainly composed of proterozoic high grade metamorphic rocks was intruded by a few late to post tectonic syenitic and granitic plutons reliable geochronological data for these plutons are few and some of the available data are inconsistent with the ages of the surrounding metamorphic country rocks this report presents five rb sr whole rock mineral isochron wrmi ages and initial sr isotope ratios of three granitoid plutons from the wanni complex sri lanka namely the ambagaspitiya tonigala and kotadeniya granites two samples from the ambagaspitiya granite yielded ages of 520 5 and 502 15 ma with initial sr ratios of 0 7103 0 0003 and 0 7125 0 0009 respectively whereas ages of 467 27 and 497 11 ma and initial sr ratios of 0 7070 0 0004 and 0 7085 0 0008 were determined for two tonigala granite plutons a single sample from the kotadeniya granite gave an age of 533 19 ma and an initial sr ratio of 0 7202 0 0018 these ages are consistent with the geological relations between the granitoids and their country rocks which underwent granulite facies metamorphism between 650 and 550 ma and also with available u pb zircon ages 550 ma the present study gives improved chronological correlation of the felsic magmatism in east gondwana fragments in sri lanka india madagascar antarctica and western australia each of the sri lankan plutons exhibits different initial sr isotope ratios indicating that their magmas were derived from distinct source materials

garnet plasticity in the lower continental crust implications for deformation mechanisms based on microstructures and sem electron channeling pattern analysis abstract 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 2 110 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

rb sr and sm nd geochronology of the eppawala metamorphic rocks and carbonatite wanni complex sri lanka abstract 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

evidence for a fold nappe in the high grade basement of central sri lanka terrane assembly in the pan african lower crust abstract structural analysis in the kadugannawa complex kc in central sri lanka reveals the presence of an early steep gneissic layering s 1 which has been overprinted by a younger event d 2 in the structurally deepest levels d 2 folds become progressively tighter towards the contact with the underlying highland complex hc the distribution of lithologies in the kc the distribution of steep s 1 and subhorizontal s 2 and the strain gradient towards the kc hc contact suggest that the kc is a d 2 fold nappe overlying the hc vergence of asymmetric sheath folds in sections parallel to a strong nnw sse lineation is towards nnw indicating a top to the nnw displacement granulite facies assemblages were formed during and after the final stages of d 2 in the kc d 2 was the main fabric forming event in the hc where meta igneous and metasedimentary gneisses show l s fabrics defined by the peak granulite facies assemblages the sense of shear deduced from asymmetric foliation boudinage is consistent with the top to the nnw displacement in the kc both units have been refolded around large scale upright d 3 folds the nappe model and nd model ages suggest that the kc and the eastern part of the hc may represent distinct lower crustal terranes juxtaposed during pan african high grade high strain events d 2 the contact between them is interpreted as a terrane boundary in a lower crustal environment the top to the nnw movement sense suggests that the kc may have been originally located south of the eastern highland complex this and the similarity in isotope signatures indicate a pre assembly link of the kc the western highland complex and the vijayan complex of eastern sri lanka

nd isotopic mapping of the sri lanka basement update and additional constraints from sr isotopes abstract the high grade crystalline basement of sri lanka can be subdivided into three provinces distinguished by distinct isotopic model ages this confirms the conclusions of milisenda et al 1988 made on the basis of much more limited data high grade ortho and paragneisses from the highland complex in central sri lanka yield palaeoproterozoic to archaean nd model ages from 2 0 to 3 4 ga and reflect an extended premetamorphic crustal history the wanni complex in the western and northwestern part of the island is characterized by meso to neoproterozoic nd model ages ranging from 1 0 to 2 0 ga the vijayan complex in the southeast is occupied by neoproterozoic i type granitoids with nd model ages from 1 0 to 1 8 ga this distinct crustal province appears to fit best a detached continental margin plutonic terrane the isotopic data preclude a common premetamorphic history of the wanni and vijayan complexes with the ancient highland complex the combined nd sr and pb isotope data together with recent geochronological studies strongly suggest that the distinct crustal age provinces are separated by tectonic discontinuities and that they represent terranes that were accreted just prior to the high grade metamorphic event in neoproterozoic time

constraints on the stratigraphic age of metasedimentary rocks from the larsemann hills east antarctica possible implications for neoproterozoic tectonics in this study we investigate crystallization ages of detrital zircon from paragneiss from the larsemann hills east antarctica using the single zircon pb pb thermal evaporation technique the 207pb 206pb ages of eleven detrital zircons range from 1200 6 ma to 766 12 ma although these ages must be regarded as minimum crystallization ages investigation of the u content and damage state of the internal crystal structure using fission track analysis suggests that they may approach crystallization ages this is confirmed by zircons from a granitic orthogneiss w20401 which show consistent radiogenic pb ratios released during the final evaporation steps the 207pb 206pb age of this orthogneiss is 940 6 ma similar to its whole rock rb sr isochron age of 1024 45 ma ir 0 7241 40 its sm nd model age tdm is about 2 2 ga further sm nd model ages tdm include 2 1 ga for the paragneiss and 1 6 1 7 ga for a 0 55 ga granite the presented field and isotopic chronological data suggest that 1 the 1 ga event represents emplacement of the orthogneiss protolith but does not necessarily involve high grade metamorphism 2 deposition of the paragneiss protoliths occurred during the neoproterozoic which suggests that accretion of the east antarctic craton had not been completed until the early palaeozoic 3 deformation and granulite facies metamorphism in the region can be attributed to the 0 5 ga pan african event

primary carbonate co2 inclusions in sapphirine bearing granulites from central sri lanka high density co2 rich fluid inclusions from a sapphirine bearing granulite hakurutale sri lanka have been studied by microthermometry raman spectrometry and sem analysis based on textural evidence two groups of inclusions can be identified primary negative crystal shaped inclusions group i and pseudo secondary inclusions which experienced a local limited post trapping modification group ii both groups contain magnesite as a daughter mineral occurring in a relatively constant fluid solid inclusion volume ratio volsolid 0 15 total volume co2 densities for group i and ii differ only slightly both groups contain a fluid which was initially trapped at peak metamorphic conditions as a homogeneous co2 mgco3 mixture thermodynamic calculations suggest that such a fluid co2 15 vol mgco3 is stable under granulite facies conditions after trapping magnesite separated upon cooling while the remaining co2 density suffered minor re adjustments a model isochore based on the integration of the magnesite molar volume in the co2 fluid passes about 1 5 2 kbar below peak metamorphic conditions this remaining discrepancy can be explained by the possible role of a small quantity of additional water

zircon ion microprobe dating of high grade rocks in sri lanka the high grade gneisses of sri lanka display spectacular in situ granulitization phenomena similar to those observed in southern india and of current interest for evolutionary models of the lower continental crust the absolute ages of these rocks are poorly constrained and so using the shrimp ion microprobe we have analyzed small spots on zircons from upper amphibolite to granulite grade quartzitic and pelitic metasediments detrital grains from a metaquartzite of the highland group preserve premetamorphic u pb ages of between 3 17 and 2 4 ga and indicate derivation of the sediment from an unidentified archean source terrain the pb loss patterns of these zircons and the other samples suggest severe disturbance at ca 1100 ma ago which we attribute to high grade regional metamorphism two pelitic gneisses contain detrital zircons with ages up to 2 04 ga and also record an approx 1100 ma event that is also apparent from metamorphic rims around old cores and new zircon growth a granite intrusive into

cryogenian 830 ma mafic magmatism and metamorphism in the northern madurai block southern india a magmatic link between sri lanka and madagascar abstract the northern madurai block southern india lies directly south of and partly deformed by the palghat cauvery shear zone system pcss a potential suture of the neoproterozoic mozambique ocean the kadavur gabbro anorthosite complex lies south of the pcss in the northern madurai block and crystallized at 829 14 ma la icpms zircon data in a supra subduction zone setting the complex contains zircon e hf t values of 12 5 to 8 6 that represent palaeoproterozoic t dm model ages 2 3 2 5 ga these broadly agree with a whole rock neodymium t dm model age of 2287 ma oxygen isotope 18 o ratios range from 5 82 and 6 74 the parental magma for the gabbro anorthosites are interpreted to be derived from a juvenile neoproterozoic mantle contaminated by mesoarchaean igneous infra crustal sources the gabbro anorthosites intrude quartzites with dominantly palaeoproterozoic detrital zircons that contain neoarchaean and mesoarchaean hafnium model ages these quartzite zircons contain metamorphic rims that yield an age of 843 23 ma demonstrating the autochthonous nature of the gabbro anorthosite complex later felsic magmatism is recorded by the 766 8 ma crystallisation age of the protolith of a felsic gneiss cryogenian magmatism in the madurai block is interpreted to form part of an extensive arc magmatic province within the southern east african orogen that can be traced from central madagascar through southern india to the wanni complex of sri lanka this province is interpreted to have formed above a south west dipping subduction system as the mozambique ocean was subducted under the neoproterozoic continent azania

mapping sea surface oil slicks using radarsat 2 quad polarization sar image 1 polarimetric sar decomposition parameters average alpha angle and entropy h are estimated for oil slick contaminated sea surfaces and slick free conditions using a radarsat 2 quad polarization sar image the values of h and within oil slick areas are significantly higher than those of the ambient sea surface indicating the dominance of bragg scattering for the slick free ocean and non bragg scattering for the oil slick area in land classification the conformity coefficient is often used to discriminate surface scattering with double bounce or volume scattering based on this rationale we also develop a method using as a logical scalar descriptor to map oil slicks under low to moderate wind conditions the proposed method is assessed using a radarsat 2 quad polarization sar image of oil slicks in the gulf of mexico analysis shows that when is positive the sea surface is slick free whereas is negative for oil slick areas this method provides a simple and effective mapping technique for oil slick detection

geochronological evolution of the moyar bhavani and palghat shear zones of southern india implications for east gondwana correlations abstract the amphibolite facies moyar bhavani and palghat shear zones separate the south indian dharwar craton dc from the nilgiri and madurai high grade crustal provinces we present sm nd and rb sr isotopic whole rock mineral and small slab data for remnant granulites mylonitic gneisses and granitoids that reveal a neoproterozoic tectonothermal imprint in these shear zones sm nd garnet crystallisation ages and rb sr mica cooling ages from reworked rocks suggest that pan african structurally controlled retrogression and successive cooling in the moyar shear zone msz garnet 624 591 ma muscovite 594 ma biotite 604 540 ma predates tectonometamorphism in the bhavani shear zone bsz garnet 552 ma biotite 521 491 ma and palghat shear zone psz garnet 521 ma biotite 488 485 ma a rb sr small slab isochron age of 624 ma indicates that pan african retrogression in the msz also led to isotopic reequilibration of the rb sr system on a cm scale a sm nd garnet crystallisation age of 513 5 ma reflects emplacement of a postdeformative pegmatite in the bsz during the late cambrian a sm nd garnet age of 2355 18 ma obtained from a granulite remnant in the msz records early palaeoproterozoic high grade metamorphism which may be related to the amalgamation of the nilgiri block and dc along the palaeo moyar zone a relict charnockite from the southern bsz yielded a reproducible sm nd garnet age of 1705 11 ma presumably recording late palaeoproterozoic metamorphism neoarchaean and palaeoproterozoic nd model ages 3 0 2 2 ga suggest that the shear zone rocks are mainly reworked material from the adjacent nilgiri and madurai crustal domains nd model ages of granites and tonalites which intruded the msz syntectonically provide evidence for 1 7 ga crust generation unsheared metasediments exposed between the bhavani and palghat high strain zones were subjected to pan african high grade metamorphism as constrained by sm nd age data of garnets grown during coaxial deformation at 610 560 ma sm nd whole rock data from these supracrustal rocks indicate average crustal residence ages of 1 9 ga which implies contributions from an unknown proterozoic crustal source and disproves their previously suggested correlation to the archaean sargur group these model ages provide an upper limit for the juxtaposition of dc and madurai block along the bsz and psz

a layered basic intrusion deformed and metamorphosed in granulite facies of the sri lanka basement a layered basic intrusion has been found in the central granulite belt of the sri lanka continental basement it intruded parallel to bedding before all or early during deformation of neighbouring metasediments deformation affecting metasediments and the intrusion alike includes flattening to c 1 20 of the original thickness and nnw stretching to c 20 times the original length the intrusion is now 170 300 m thick most of the deformation was acquired under granulite facies metamorphism the intrusion was then folded still at high t by a large f4 synform with an axis parallel to str1 and a steep axial plane a steep axial plane cleavage and minor folds are related to this big fold stretching continued along its axis late during formation of this fold a granite intruded mainly following s4 cleavage planes the intrusion shows a homogeneous gabbroic series at the bottom followed upwards by a differentiated and layered series a thin sequence of ultramafic rocks occurs near the middle this indicates multiple melt injection more homogeneous partly biotite bearing amphibolites form the top of the succession magmatic layering is well preserved but no magmatic minerals or grain fabrics have escaped deformation or metamorphism static annealing under granulite facies conditions outlasted all deformation and was accompanied and followed by the beginning of cooling hornblende plagioclase coronas formed round garnets at this stage geochemical work carried out by stosch 1991 on our samples confirms the cumulate nature of the rocks

zircon geochronology of anatectic melts and residues from a highgrade pelitic assemblage at ihosy southern madagascar evidence for pan african granulite metamorphism we report u pb and 207 pb 206 pb zircon ages for a granulite facies gneiss assemblage exposed in a large quarry at ihosy southern madagascar the granulites are derived from pelitic to arkosic sediments and attained equilibrium conditions at 650 700 c and 4 5 kbar higher p t conditions of 750 800 c and 6 kbar in the presence of low water activities have led to dehydration melting processes the formation of granitic melts which partly moved away from their source region intruded into upper parts of the metapelitic gneisses as small granitic veins and left behind granulitic garnet cordierite quartz bearing rocks detrital zircons in a sample of metapelite and a sample of quartzofeldspathic gneiss yielded ages between 720 and 1855 ma suggesting a chronologically heterogeneous source region and a depositional age of less than 720 ma for these rocks high grade metamorphism and anatexis are documented by zircon ages between 526 34 and 557 2 ma with a mean age of about 550 ma the broad lithologies metamorphic grades and ages recorded in the ihosy rocks are similar to those in the wanni complex of northwestern sri lanka and in high grade assemblages of southernmost india and support the contention that all these terrains were part of the mozambique belt which formed as a result of collision of east and west gondwana in latest precambrian time

age nd hf isotopes and geochemistry of the vijayan complex of eastern and southern sri lanka a grenville age magmatic arc of unknown derivation abstract the ca 1 0 1 1 ga vijayan complex vc of eastern and southeastern sri lanka is one of three high grade metamorphic terranes making up the basement of the island and is in tectonic contact with the adjacent older highland complex it consists predominantly of granitoid gneisses ranging in composition from diorite to leucogranite with a distinct calc alkaline geochemical signature and is interpreted as a magmatic arc strong ductile deformation has obliterated almost all original intrusive relationships high grade metamorphism during the pan african event at ca 610 520 ma has produced widespread granulite facies assemblages that are now largely retrogressed and were affected by extensive late metamorphic k metasomatism in southern sri lanka the vijayan gneisses are tectonically interlayered with rocks of the highland complex in a so called mixed zone we report zircon ages whole rock nd and hf in zircon isotopic systematics and geochemical data for a large selection of vijayan gneissses in order to better characterize this complex and its tectonic setting the zircon ages are predominantly in the range 1100 1000 ma with a few early neoproterozoic intrusions and identify the vc as a grenville age magmatic arc many zircons experienced minor to significant lead loss at about 580 ma the nd and hf isotopic data confirm a generally primitive origin for most vijayan gneisses but significant variations in both isotopic systems argue for source heterogeneities and the possible involvement of minor amounts of older continental material in their genesis immobile trace and rare earth element distributions favour an origin of the gneiss protoliths through melting at relatively shallow depth and the chemical variation from diorite and tonalite via granodiorite to granite in the vc can be explained by increasing fractionation of plagioclase and biotite accompanied by amphibole and accessory phases the vc may be comparable in some respect to the composition and evolution of the kohistan arc in pakistan the unique composition age and isotopic characteristics of the vc find no convincing counterpart in other fragments of the gondwana supercontinent and its origin therefore remains exotic and enigmatic

archaean cambrian crustal development of east antarctica metamorphic characteristics and tectonic implications abstract the east antarctic shield consists of a variety of archaean and proterozoic cambrian high grade terranes that have distinct crustal histories and were amalgamated at various times in the precambrian cambrian high grade pan african tectonism at 600 500ma is recognized from four distinct belts the dronning maud land lutzow holm bay prydz bay and denman glacier belts these high grade belts juxtapose distinct mesoproterozoic and neoproterozoic crustal provinces maud rayner and wilkes the rauer terrane and have also marginally affected archaean cratonic remnants in the napier complex and southern prince charles mountains the wilkes province experienced its principal tectonothermal events prior to 1130ma and was not affected by the younger events that characterize the maud province 1150 and 1030 990ma the rayner province 990 920ma and the rauer terrane 1030 990ma these differences between the isotopic event records of the basement provinces now separated by the pan african belts require that the older provinces were not formerly parts of a continuous grenville belt as proposed in the sw us east antartic model east antarctica was not a single unified crustal block within either east gondwana or rodinia until the cambrian which is now demonstrated to be the key phase of high grade and ultrahigh temperature uht metamorphism associated with supercontinent assembly the high grade pan african tectonism is characterized by extensive infracrustal melting clock wise p t paths rapid post peak exhumation along isothermal decompression paths to shallow or mid crustal levels by 500 ma and the generation at least locally of uht conditions a significant flux of heat from the mantle into the deep and initially overthickened crust is required to produce these observed metamorphic effects whilst the thermal evolution can be explained by models that invoke the removal of most of the lithospheric mantle following crustal thickening and prior to rapid extension of the remaining crust these one dimensional models are inconsistent with present crustal thicknesses of 25 35km in the pan african domains of the east antarctic shield

rubidium strontium whole rock ages of kataragama and pottuvil charnockites and east vijayan gneiss indication of a 2 ga metamorphism in the highlands of southeast sri lanka highland group granulite facies rocks of the kataragama klippe in southeast sri lanka yield a rb sr whole rock apparent age of 1930 pm 130 ma mswd 39 and a 87 sr 86 sr intercept of 0 715 pm 0 005 indicating a highlandian metamorphism about 2 0 ga ago a charnockitic gneiss at komari near pottuvil east sri lanka gives a rb sr whole rock isochron age of 820 pm 70 ma mswd 0 78 initial 87 sr 86 sr 0 725 pm 0 007 suggesting a metamorphic resetting at about 0 8 ga the rb sr whole rock data of an east vijayan biotite hornblende gneiss fit a reference isochron of 800 ma with a 87 sr 86 sr intercept of 0 705 the low 87 sr 86 sr intercept may be explained by a juvenile addition to the older crust a review of available data from various isotopic dating methods suggests that the highland group supracrustals were deposited 2 5 2 0 ga ago metamorphosed in the granulite facies about 2 0 ga ml ago and disturbed by resetting events about 1 1 ga m2 0 8 ga m3 and

the timescale and mechanism of granulite formation at kurunegala sri lanka incipient charnockite formation at kurunegala in sri lanka is characterized by the growth of orthopyroxene at the expense of amphibole and biotite in an originally homogeneous gneiss mineral equilibria in the charnockite assemblage record pressure temperature p t conditions of 738 60 c and 6 9 1 2 kbar at 17 0 1 2 log fo2 and ah2o 0 18 0 16 wholerock trace element and isotopic measurements show that charnockite formation was accompanied by a systematic depletion of sm rb pb u sr nd with a fractionation of rb sr sm nd and th u ratios and crystallization of the charnockite assemblage at 535 5 ma major element fe mg ca and sm nd equilibration between minerals occurred at 524 9 ma whereas pb and rb sr underwent continued exchange to 501 5 ma and 486 1 ma respectively trace element data for both amphibolite and charnockite minerals show that depletion on a whole rock scale can be accounted for either by changes in mineral modes or trace element abundances within the immediate area of dehydration the fractionation of sm nd on a whole rock scale is controlled by the breakdown of amphibole without the growth of a major new host phase for sm in the charnockite rubidium and sr are dependent on the relative behaviour of biotite plagioclase and alkali feldspar modelling of dehydration melting involving the breakdown of amphibole biotite and alkali feldspar reproduces the observed sm nd and rb sr fractionation and indicates the loss of small melt fractions on a cm scale from the charnockite these observations suggest that partial melting is the most plausible means of effecting both the dehydration and depletion that accompanies charnockite formation

rubidium strontium whole rock ages of kataragama and pottuvil charnockites and east vijayan gneiss indication of a 2 ga metamorphism in the highlands of southeast sri lanka a discussion

annealing radiation damage and the recovery of cathodoluminescence the structural recovery upon heat treatment of a highly metamict actinide rich zircon u 6000 ppm has been studied in detail using a range of techniques including x ray powder diffraction raman spectroscopy shrimp ion probe electron microprobe transmission electron microscopy and cathodoluminescence analysis the structural regeneration of the amorphous starting material depends on random nucleation it starts between 800 and 950 c when amorphous zrsio4 decomposes to form crystalline zro2 and amorphous sio2 at around 1100 c well crystallised zrsio4 grows at the expense of the oxides u has been retained in the newly grown zircon whereas pb was evaporated during the heat treatment this process is in marked opposition to the reconstitution of moderately metamict minerals which experience a gradual recovery controlled by the epitaxial growth at the crystalline amorphous boundaries both of these recovery processes are not the direct inverse of metamictisation the structural regeneration was found to be connected with a significant increase in the emission of cl in all cases annealing heavily damaged zircon and moderately damaged zircon and monazite we observe that the final wellcrystallised annealing products emit more intense cl than their radiation damaged starting minerals although having almost identical elemental composition our observations are taken as evidence that the cl is not only determined by the chemical composition of the sample but is also strongly controlled by structural parameters such as crystallinity or the presence of defect centres

divariant and trivariant reaction line slopes in fmas and cfmas theory and applications the theory of divariant reactions is investigated in some detail and approaches involving simultaneously operating reactions and x mg isopleths are combined using the gibbs method spear et al 1982 a general equation is derived for x mg isopleth slopes in fmas and cfmas which may be applied to any divariant assemblage in these systems isopleth slopes are calculated for a number of divariant fmas and cfmas reactions inferred to have operated in sri lankan granulites giving more rigorous constraints on the local p t path isopleth slopes are also calculated for the trivariant cfmas reaction orthopyroxene plagioclase garnet quartz the resulting dp dt range from 8 to 18 bar k application to sri lankan granulites confirms an earlier hypothesis faulhaber and raith 1991 that a retrograde path with a dp dt of 10 15 bar k may cause garnet growth in charnockites and garnet breakdown in metabasites it is shown here that dp dt depends primarily on the ca contents of garnet and plagioclase large variations in isopleth and reaction line slopes are possible due to the thermodynamic effect of additional components in solid solutions e g albite in plagioclase this has a good potential for the assessment of the exact form of p t paths equations to calculate isopleth slopes could be included in software packages aimed at defining p t estimates enabling geologists to derive realistic p t paths

caledonian ages in variscan rocks rb sr and sm nd isotopic variations in dioritic intrusives from the northwestern bohemian massif west germany abstract dioritic intrusives and granites from the northwestern bohemian massif west germany were analyzed for the chemical and isotopic composition of rb sr and sm nd the marktredwitz intrusive suite and the tirschenreuth mahring sill intrusions yield apparent rb sr whole rock isochron ages of 468 9 ma and 470 33 ma respectively while the reuth erbendorf granodiorites display a scatter with an apparent age of 545 16 ma the rb sr whole rock ages conflict with geological evidence the diorites intruded metamorphic country rocks which experienced an early devonian metamorphism contact relationships between the diorites and porphyritic granites indicate nearly contemporary emplacement the intrusion ages of two porphyritic granites from this area have been dated at 319 ma and 311 ma the diorites display a variation in nd 320 ma ranging from 1 0 to 4 4 whereas the two granites yield homogeneous initial nd values of 3 7 and 6 6 the variations in elemental concentrations and initial isotopic ratios clearly show a mixing trend within the dioritic rock suites the mixing partners are a presumably mantle derived magma and crustal components of varying composition the isotope systematics of the diorites and the concordance of their rb sr whole rock age values with caledonian rock forming events in central europe point to a pre intrusive caledonian event inherited in these rocks we suggest that the diorites were formed by multistage anatectic processes

shrimp u pb zircon ages of the fuping complex implications for late archean to paleoproterozoic accretion and assembly of the north china craton the fuping complex is situated in the central part of the north china craton and consists of four major lithological assemblages fuping tonalitic trondhjemitic granodioritic gneisses longquanguan augen gneisses wanzi supracrustal assemblage and nanying granitic gneisses shrimp u pb geochronology combined with u th and cathodoluminescence cl imaging of zircon enables resolution of magmatic and metamorphic events that can be directed towards understanding the late archean to paleoproterozoic history of the fuping complex cl images reveal the coexistence of magmatic and metamorphic zircons in nearly all rock types of the fuping complex the metamorphic zircons occur as either single grains or overgrowth or recrystallization rims surrounding and truncating oscillatory zoned magmatic zircon cores and are all characterized by nebulous zoning or being structureless with extremely high luminescence and very low th contents these features make them distinct from magmatic zircons that are characterized by concentric oscillatory zoning comparatively low luminescence and high th and u contents shrimp u pb analyses on magmatic zircons reveal that the tonalitic trondhjemitic and granodioritic plutons of the fuping gneisses were emplaced at 2523 14 ma 2499 10 ma and 2486 8 ma respectively whereas the monzogranitic and granitic plutons of the longquanguan augen gneisses were intruded respectively at 2510 22 ma and 2507 11 ma prismatic and oscillatory zoned zircons dominate in the pelitic rocks of the wanzi supracrustal assemblage and are interpreted as detritus from igneous source rocks the concordant and discordant u pb ages of 2502 5 ma and 2507 14 ma obtained from two pelitic rock samples indicate these rocks must have been deposited no earlier than 2507 ma ago in addition a zoned zircon grain in one pelitic rock sample has a near concordant age of 2109 5 1 ma which may provide a maximum depositional age for the wanzi supracrustal rocks shrimp results also reveal that granitic magmatism assigned to the nanying granitic gneisses occurred over a protracted interval from 2077 13 ma to 2024 21 ma the nebulously zoned zircon grains and overgrowth recrystallization zircon rims from different rocks yielded similar concordant 207pb 206pb ages in the range 1875 to 1802 ma interpreted as approximating the age of regional metamorphism of the fuping complex timing of primary zircon crystallization and regional metamorphism of the fuping complex is in general agreement with recent u pb zircon ion probe results for the wutai and hengshan complexes that bound the fuping complex to the northwest these areas are characterized by the emplacement of major granitoid bodies at around 2 50 ga to 2 48 ga ago deposition of supracrustal rocks in the paleoproterozoic intrusion of paleoproterozoic granitic bodies at 2 1 to 2 0 ga and regional metamorphism at 1 875 to 1 802 ga these data indicate that the fuping and hengshan complexes do not represent an older crystalline basement to the wutai complex as suggested in previous tectonic models but together with the wutai complex represent elements of a single late archean to paleoproterozoic magmatic arc system that has been subsequently tectonically disrupted and juxtaposed during the collision of the eastern and western north china blocks at 1 85 ga which resulted in the final assembly of the north china craton

pbpb dating of apatite by a stepwise dissolution technique abstract apatites from different geologic settings have been dated by a new sequential dissolution method the procedure is based on stepwise exposure of apatite to weak intermediate acids a pegmatite sample from the kerala khondalite belt in southern india yielded an age of 509 25 ma apatites from an enderbite sample from the northern marginal zone of the limpopo belt in southern africa yielded an age of 1970 26 ma which is in agreement with the high grade metamorphism at around 2 0 ga reported from the triangle shear zone and amphibolite facies metamorphism in the transition zone apatites from an amphibolite facies gneiss from the transition zone south of the nmz yielded an age of 1878 18 ma this age is somewhat younger than a sphene age on the same rock of 1971 17 ma and is believed to reflect the time of retrograde metamorphism in the presence of a fluid phase the sequential dissolution method is a new tool that allows dating of apatites which have relatively unradiogenic pb ratios because the spread can be significantly increased and no correction for common lead is needed this study further demonstrates that apatites from magmatic and high grade metamorphic rocks yield ages very close to those of monazite zircon or garnet unless the rock subsequently interacted with a fluid phase

evolution of the continental crust in the kerala khondalite belt southernmost india evidence from nd isotope mapping u pb and rb sr geochronology in order to decipher crustal genesis and evolution as well as to refine the metamorphic history of the kerala khondalite belt kkb southernmost india we have applied u pb monazite rb sr biotite feldspar dating and nd isotope geochemistry the kkb belonged to an eastern gondwana mobile belt that underwent ultra high temperature metamorphism at pan african times it is dominated by nd model ages ranging between 2 0 and 3 0 ga except at its northernmost boundary the achankovil unit au where middle proterozoic nd model ages were recognised 1 3 1 6 ga the new data show that these ages are not restricted to cordierite bearing metasediments alone but can be found in other lithologies of the achankovil unit as well peak metamorphism occurred between 590 and 550 ma as recorded by u pb dating of monazite from charnockites and garnet biotite gneisses rb sr ages of minerals in pegmatite intrusions record cooling to ca 400 500 c by 490 470 ma based on a systematic regional sampling this contribution confirms and refines former models based on smaller datasets the major issue is that the kerala khondalite belt must have undergone an extremely slow cooling history 3 6 c ma comparable to the one described for sri lanka and madagascar still such ultra high temperatures associated to slow cooling processes cannot be satisfactorily fit into a common tectonic context 2004 published by elsevier b v

ca 820 640 ma sims u pb age signal in the peripheral vijayan complex sri lanka identifying magmatic pulses in the assembly of gondwana abstract sri lanka comprises three roughly north south trending amphibolite to granulite facies lithotectonic complexes from west to east the highland complex the wanni complex and the vijayan complex these terranes were correlated with other east gondwana continental terranes with similar lithologies forming at similar ages the wanni complex and the vijayan complex have been interpreted as volcanic arc terranes brought together by a double sided subduction the highland complex represents the metamorphosed accretionary prism within the suture when the wanni and vijayan complexes were juxtaposing against each other in contrast to the wanni and highland complexes the vijayan complex has yielded only a few geochronological data with satisfactory precision previous studies suggested that the vijayan complex comprises 1100 924 ma granitic gneisses which were metamorphosed during 590 456 ma more recently 772 617 ma mafic intrusions have been identified this study divides the vijayan granitic gneisses and the associated melt products geochemically into a low nb series and a more primitive high nb series our sims u pb zircon data suggested that both series have protolith magmatic ages of 1062 935 ma and metamorphic ages of 580 521 ma which is consistent with previous work however some of the vijayan granitic gneisses and granitic anatectic melt products at the highland vijayan tectonic mixed zone preserve an additional tonian cryogenian 820 630 ma age signal this age signal suggested that felsic magmatism also occurred when mafic granulites were emplaced along the highland vijayan boundary which is broadly coeval with to the bimodal magmatism occurring along the highland wanni boundary this study also suggests that charnockitisation in the vijayan complex occurred at 562 6 ma during the neoproterozoic regional metamorphism the tonian cryogenian signal preserved in the highland vijayan tectonic mixed zone can also be found in the alkaline intrusion hosted by the namuno terrane and the lurio belt in mozambique this indicates a relationship between the vijayan granitic gneisses and the lurio foreland metagranitic basement while the namuno terrane and the lurio belt are correlated with the highland vijayan tectonic mixed zone the ages and the isotope signatures of these granitic bodies further suggest a genetic relationship of these granitic bodies with various magmatic intrusions in east antarctica

multistage graphite precipitation through protracted fluid flow in sheared metagranitoid digana sri lanka evidence from stable isotopes abstract the digana metagranitoid situated within the digana shear zone a critical junction between the highland and wanni complexes of sri lanka hosts a variety of graphite types with structural evidence showing formation during different stages prior to and after deformational events affecting this terrain here we discuss their occurrence association and stable isotopes the carbon isotopes of graphite reported in this study from the sri lankan high grade metamorphic terrain differ significantly from the composition of graphite reported elsewhere in other precambrian terrains the stable isotope composition from random crystals depicts an enriched carbon isotopic composition in the range 1 6 to 8 suggesting precipitation by reduction from co2 rich fluids intracrystalline microscale traverses made on graphite crystals indicate the 13c composition of the rims becoming progressively lighter in 13c in metagranitoid hosted samples with the variation reaching up to 1 5 consistent with the changing fluid regimes induced by decreasing temperature during the graphite growth and shows the preservation of core at higher temperatures a highly enriched carbon isotopic composition 0 6 with a rimward enrichment of 13c observed in migmatites contrary to the values in metagranitoid indicates preferential enrichment of heavy isotopes as a result of the partial melting and dissolution reprecipitation of graphite the oxygen isotopic composition of a quartz calcite vein show values in the range 18 5 to 20 typical for recycled crustal source the tight constraint in the isotopic data from different graphite types studied indicates a homogeneous fluid source for the deposits prolonged fluid flow in this region is envisaged in this study which is related to the multistage deformation of the crustal scale shear zone

the role of fluids in granulite facies metamorphism as deduced from oxygen and carbon isotopic compositions abstract based on stable isotope data of whole rock samples the main fluid rock exchange reactions in the high grade rocks of the crystalline basement of sri lanka is deduced to have occurred prior to granulite facies metamorphism the influx of external fluids during and after granulite facies conditions was only channellized and did not give rise to the formation of interconnected fluid films in rock volumes larger than several cm3 fluid present conditions were thus only manifest for relatively short periods of time much less than 106 y so that the conditions of granulite facies metamorphism of the sri lankan basement rocks in general can be characterized as fluid absent

a tectonic perspective of incipient charnockites in east gondwana abstract examples of spectacular occurrences of charnockite in an arrested stage of formation in crustal segments of east gondwana comprising india sri lanka and antarctica are evaluated to constrain the broad tectonic framework under which the charnockite formed field structures petrologic features and information including fluids trapped within inclusions in minerals suggest that the conversion of upper amphibolite facies gneisses to charnockites was brought about by the local desiccation induced by the infiltration of co2 rich fluids through faults and shears under brittle quasi brittle conditions the stable isotopic composition of these fluids is consistent with a juvenile sublithospheric source available geochronologic data indicate multiple events of the incipient charnockite formation at 2500 1000 and 500 ma in these crustal domains in each case incipient charnockite formation post dates regional metamorphism hence the process of local desiccation is inferred to have operated in cratonized crustal segments the p t t paths characterized by isothermal decompression are consistent with incipient charnockite development during the waning stages of collisional mobile belts when the tectonic regime changed from compressional to extensional crustal distension triggers the release of co2 rich fluids from depth which are transported to mid crustal levels through magmatic conduits a notion supplemented by the occurrence of alkaline intrusives of pan african age degassing of these magmas could have supplied the co2 required for stabilizing the anhydrous assemblages in the charnockites

summary and discussion of p t estimates from garnet pyroxene plagioclase quartz bearing granulite facies rocks from sri lanka abstract in granulite facies rocks of granitic to basic composition from the highland complex hc of sri lanka maximum p t estimates for about 115 locations have been obtained from core compositions of garnet orthopyroxene and plagioclase their distribution shows a decrease in pressure from the east and southeast where pressures exceed 9 kbar towards the west and southwest where pressures are about 5 kbar the accompanying temperature decrease in the order of about 200 c is less pronounced the regional distribution pattern of garnet quartz clinopyroxene reaction textures around orthopyroxene and plagioclase indicates that the lower crustal rocks of the eastern and southeastern parts of the hc have experienced an episode of near isobaric cooling subsequent uplift resulted in the breakdown of garnet and clinopyroxene to orthopyroxene plagioclase this reaction texture is found in all parts of the hc with the exception of the west and southwest where the higher crustal levels are exposed comparisons of the array of calculated maximum p t data with published continental geotherms for normal to elevated mantle heat flows 30 50 70 and 90 10 3 w m 2 at the base of a 36 km crust show that the depth versus temperature gradient is not a simple geothermal one the p t array in which the gradient is steeper than the geotherms would be consistent with magmatic heating this model would suggest a scenario in which intrusions that were generated in the lower crust were emplaced at higher crustal levels and heated up the surrounding rocks however since the number of known slightly pre to synmetamorphic intrusions in sri lanka is very limited other mechanisms of the heat supply seem to be required prior to uplift and exposure of this partial cross section through the lower to upper crust

the pan african event in east antarctica a view from sri lanka and the mozambique belt abstract late pan african 580 550 ma high grade deformation of the sri lankan basement gave rise to a fold and thrust belt the resulting axial planes to upright folds and earlier stretching lineations define a large scale syntaxial bend this syntaxis is attributed to e w shortening and simultaneous sinistral strike slip along a n s trending transcurrent zone located west of sri lanka prior to gondwana break up the resulting geometry is consistent with a corner effect brun and burg 1982 produced by westward indenting of the basement underneath eastern sri lanka during collision in gondwana reconstructions the sinistral strike slip zone fits into the late kinematics of the mozambique belt shackleton 1986 these new structural data constrain the position of sri lanka in gondwana and favour reconstructions in which sri lanka is located near the lutzow holm bay in east antarctica a review of data from adjacent gondwana fragments suggests that the tectonometamorphic evolution of the sri lankan basement is closely related to the interplay between two orogenic belts the mozambique belt and the lutzow rayner belt it is argued that a late event of n s shortening gondwana coordinates in the lutzow rayner belt can be attributed to a pan african tectonometamorphic event in this interpretation the lutzow rayner belt developed in part simultaneously with the mozambique belt reworking an older granulitic basement in the rayner complex

isotopic evidence for chaotic imprint in upper mantle heterogeneity 1 the intrinsic structure of the isotope data set of samples from the mid atlantic ridge and east pacific rise believed to represent the isotopic composition of their mantle source reveals a close relationship between sample spatial distribution and their geochemical features the spatial distribution of their isotopic heterogeneity is self similar on a scale between 5000 and 6000 km about 1 6 of earth s circumference suggesting a self organized structure for the underlying mantle this implies the imprint of chaotic mantle processes induced by mantle flow and likely related to an early phase of highly dynamic behavior of the earth s mantle the size of the identified self organized region reflects the large length scale of upper mantle chemical variability and it is likely frozen since the proterozoic the geochemical heterogeneity of the asthenosphere along the ridges is believed to record a transition in the thermal conditions of the earth s mantle and to be reflected in the l 6 peak expansion of several geophysical observables

oxygen isotope thermometry based on a refined increment method and its application to granulite grade rocks from sri lanka abstract in this paper we first present a revision of the increment method a semi empirical technique which allows the calculation of o isotope fractionation factors between all silicates provided their structure and chemical composition is known for the temperature dependency the experimentally calibrated high pressure mineral calcite fractionation factors of chiba et al 1989 are taken as a basis then this method is applied to granulite facies parageneses from sri lanka including quartz k feldspar sillimanite garnet pyroxenes amphiboles cordierite magnetite and ilmenite it can be shown that o isotope equilibration occurred at granulite facies conditions at about 830 c due to static recrystallization locally retrograde re equilibration occurred also at much lower temperatures due to late d4 in most cases however o isotope disequilibria resulted from retrograde oxygen intracrystalline diffusion it can be proved by model calculations that the o diffusion data for water saturated systems reported in the literature are too high by two orders of magnitude to explain the measured isotopic compositions the fractionation patterns allow the discrimination between open and closed system behaviour the regional distribution of open and closed system patterns corresponds to the frequency of occurrence of late stage migmatization western part and to differences in amount and composition of fluid inclusions

u pb zircon geochronology of the adirondack mountains and implications for their geologic evolution u pb zircon studies of metamorphosed igneous rocks in the adirondack mountains have yielded preliminary ages within the range 1420 990 ma several geochronologically and geochemically distinct episodes of igneous intrusion and at least one pre granulite facies dynamothermal metamorphic event are documented this information is consistent with recent field and geochronological studies throughout the grenville province and suggests that a complex sequence of events occurred in the adirondack mountains prior to the widespread deformation and metamorphism commonly attributed to the approximately 1100 1000 ma ottawan phase of the grenvillian orogenic cycle

upb geochronology of the sri lankan basement abstract u pb ages of zircons and monazites from ortho and paragneisses representing the major crystalline basement units of sri lanka establish a new geochronological framework which is consistent with previously published work on sm nd model ages rb sr whole rock ages and u pb mineral ages in the highland complex three newly dated orthogneisses one previously dated by baur et al 1991 and one dated by kroner and williams 1992 all intruded 1 85 to 1 9 ga ago this widespread intrusion event occurred soon after the end of sediment deposition which is estimated at 1 9 to 2 0 ga using upper intercept ages of zircons from metapelites in the highland complex within the boundaries of this complex established by sm nd model age mapping milisenda et al 1994 only one orthogneiss has been found which is substantially younger than 1 8 ga this is the metatonalite from polonnaruwa which intruded 670 ma ago baur et al 1991 three orthogneisses from the vijayan complex which lies to the east of the highland complex yield similarly consistent crystallization ages of 1023 8 1016 28 and 1033 121 ma in the wanni complex which lies to the west of the highland complex a sillimanite bearing gneiss which is apparently of metasedimentary origin but contains zircons of igneous appearance yields a primary crystallization age of 793 4 ma consistent with a previously dated orthogneiss from kurunegala of 770 ma baur et al 1991 in addition single zircon data by kroner et al 1994 indicate primary crystallization ages of 1 0 to 1 1 ga for orthogneisses from the vavuniya charnockite province which is considered to be part of the wanni complex these results confirm the age contrast between the highland complex on the one hand and the vijayan complex and wanni complex on the other as previously inferred from sm nd model ages milisenda et al 1988 1994 post tectonic granitoids and pegmatites intruded throughout all three complexes 550 ma ago the age of regional high grade metamorphism mostly granulite facies in the highland complex mostly amphibolite facies in the vijayan complex and both granulite and amphibolite facies in the wanni complex is constrained to be less than 670 ma the primary crystallization age of the polonnaruwa metatonalite it is more closely defined by lower intercept zircon ages which include one concordant age of 608 4 ma and one discordant but well defined age of 611 23 ma and by a nearly concordant monazite age of 592 2 ma this indicates that peak metamorphic conditions occurred 610 ma ago numerous other lower intercept ages between 590 and 550 ma and post tectonic intrusion ages of 550 ma set a lower age limit for high grade conditions and deformation the temperature history between 610 and 550 ma is not yet well resolved and it is not clear whether there was a separate thermal event 550 ma ago previously published whole rock rb sr and sm nd isochrons for gneisses from the highland complex yielding ages of 2 ga and greater and previously interpreted to date the time of granulite grade metamorphism must be reinterpreted in the light of the new data presented here as well as other recently published results e g burton and o nions 1990b baur et al 1991 and in view of the accumulated evidence for very limited element mobility during high grade metamorphism these whole rock ages are in fact identical or close to the ages of primary granitoid emplacement rather than the age of granulite facies metamorphism

pb diffusion in zircon abstract chemical diffusion of ti under anhydrous conditions at 1 atm and under fluid present elevated pressure 1 1 1 2 gpa conditions has been measured in natural zircon the source of diffusant for 1 atm experiments was a zro2 tio2 zircon mixture with experiments run in crimped pt capsules diffusion experiments done in the presence of h2o co2 fluid were run in a piston cylinder apparatus using a source of ground tio2 zrsio4 and sio2 with oxalic acid added to produce h2o co2 vapor and partially melt the solid source material yielding an assemblage of rutile zircon melt vapor nuclear reaction analysis nra with the resonant nuclear reaction 48ti p 49v was used to measure diffusion profiles for both sets of experiments the following arrhenius parameters were obtained for ti diffusion normal to cover the temperature range 1350 1550 c at 1 atm d ti 3 33 10 2 exp 754 56 kj mol 1 r t m 2 s 1 ti diffusivities were found to be similar for experiments run under fluid present conditions a fit to all of the data yields the arrhenius relation d 1 34 102 exp 741 46 kj mol 1 rt m2 s 1 these data suggest that zircon should be extremely retentive of ti chemical signatures indicating that the recently developed ti in zircon crystallization geothermometer watson e b harrison t m 2005 zircon thermometer reveals minimum melting conditions on earliest earth science 308 841 844 will be quite robust in preserving temperatures of zircon crystallization titanium diffuses somewhat faster in zircon than larger tetravalent cations u th and hf but considerably more slowly than pb the ree and oxygen hence ti crystallization temperatures may be retained under circumstances when radiometric ages or other types of geochemical information are lost

the sm nd age of kambalda volcanics is 500 ma too old although sm nd isotopic analyses of volcanics from kambalda western australia form a 3 2 ga linear array pb pb analyses of the same suite give an isochron age of 2 73 0 03 ga and a 2 7 ga model age for the associated sulphides we suggest that the 3 2 ga age is incorrect and that the sm nd array results from mixing between depleted mantle and either older continental crust or enriched mantle

the precambrian of sri lanka a historical review early studies of the geology of sri lanka lasted from the beginning of the century until 1935 the modern phase began in the forties whereas intensive studies of the precambrian rocks took place in the last ten years the term vijayan gneiss first used for the gneisses around colombo was changed in 1947 to vijayan series for all rocks outside the central khondalite charnockite belt the latter was named the highland series in 1961 and the vijayan series was subdivided into an eastern and a western component and the name was altered to vijayan complex in 1978 in 1959 a southwestern group was recognized and in 1935 the kadugannawa gneiss was named at present there is a general agreement that the following nomenclature should be used by all workers on the precambrian of sri lanka highland complex hc formerly highland series and southwestern group an intimate association of interbanded ortho and paragneisses including pelitic gneiss metaquartzite marble and charnockitic gneiss which forms the broad belt of rocks extending from northeast to southwest through the central highlands of the island outliers of hc rocks tectonic klippen are found in the vijayan complex at kataragama and maligawila vijayan complex vc formerly eastern vijayan complex consisting of migmatites granitic gneisses granitoids and scattered metasediments and occurring in the east and south of the hc wanni complex wc formerly western vijayan complex consisting of migmatites granitic gneisses charnockitic gneisses minor metasediments and granitoids lying west of the hc kadugannawa complex kc partly the former kadugannawa gneiss for the hornblende and biotite hornblende gneisses within the elongate synformal basins arenas around kandy in the early sixties it was noted that the highland series rocks had been subjected to granulite facies metamorphism and the vijayan series rocks to amphibolite facies metamorphism charnockitic rocks in the highland series showed the characteristics of metamorphic rocks recently near peak metamorphic conditions in the hc were established at 770 900 c and 5 kbar in the west wc to 9 kbar in the east hc with a pronounced regional gradient in palaeopressure relict inclusions of staurolite and kyanite in garnet of hc pelitic gneisses have been shown to document a late stage of a prograde metamorphic p t path a retrograde p t path with coronitic assemblages of retrograde mineral phases like andalusite has also been recorded outliers of hc rocks in the vc are klippen thrust over the vc parts of kc have been interpreted as a metamorphosed layered basic complex in the late seventies three phases of deformation were recognized in the hc presently four phases are known to occur an early view was that the highland series was younger than the vijayan series the former sediments having been laid down on a vijayan floor the hc is now known to be older than the vc the boundary between them being a thrust contact the exact position and nature of the hc wc boundary however is still to be resolved recent geochronological data suggest that 1 hc sedimentation as well as igneous activity in the form of basic lava flows basic sills and dykes took place 2 0 ga ago 2 intrusion of the earliest granitoids into the hc took place 1 8 to 1 94 ga ago 3 orthogneisses of the wc were intruded between 0 67 and 1 1 ga ago 4 orthogneisses of the vc were intruded 1030 to 1040 ma ago and 5 high grade regional metamorphism of the hc vc and wc took place between 455 and 610 ma ago

age of metamorphism in the high grade rocks of sri lanka controversy concerning the age of regional granulite grade metamorphism in sri lanka stems from the fact that different isotopic systems apparently record different events published isotopic data show that zircons from igneous and sedimentary protoliths experienced considerable pb loss at 550 ma and that new growth of zircon monazite rutile and garnet between 539 pm 6 and 608 pm 3 ma probably dates near peak metamorphic conditions upper concordia intercept ages between 670 and 1950 ma for magmatic zircons from high grade orthogneisses and charnockites date igneous emplacement and also support the occurrence of granulite metamorphism after 670 ma a granulite of igneous origin from the kataragama klippe in southern sri lanka yielded discordant ion microprobe data that suggest emplacement of the protolith at 1770 1880 ma thus a rb sr whole rock age of 1930 pm 130 ma from the same suite is unlikely to reflect isotopic homogenization during granulite formation and we interpret this and oth

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

anticorrelated rb sr and k ar age discordances leuchtenberg granite ne bavaria germany the leuchtenberg granite oberpfalz ne bavaria displays a continuous differentiation trend ranging from mildy peraluminous coarse grained porphyritic biotite granites bg to strongly peraluminous medium to fine grained garnet bearing muscovite granites gmg the rb sr and k ar age determinations of whole rock and mineral samples from the granite and associated intermediate rocks redwitzites have revealed two divergent age gradients rb sr wholerock dates decrease and initial 87sr 86sr ratios increase for successively more evolved subsets of the granite all bg samples 87rb 86sr 2 16 yield a date of 326 2 ma with a low initial 87sr 86sr ratio of 0 70778 0 00013 1 while all gmg samples 87rb 86sr 70 to 1000 yield a younger date of 317 2 ma with an enhanced initial 87sr 86sr ratio of 0 7146 0 0039 the k ar measurements on biotites and muscovites give closely concordant dates for the gmg 326 323 ma and the southern lobe of the bg 324 320 ma the northern lobe of the bg including the redwitzites shows a well defined trend of decreasing k ar dates from 320 ma to 300 ma towards the northwest critical consideration of both isotope systems leads to the conclusion that the rb sr system of the gmg was disturbed by a later hydrothermal event the ca 326 ma whole rock rb sr date for the bg is not in conflict with any of the k ar mineral dates and is taken as approaching the crystallization age of the leuchtenberg granite the k ar age progression within the northern lobe of the bg indicates that this part either cooled down over a protracted period of some 20 ma or experienced reheating at ca 300 ma the study highlights the potential of combined rb sr and k ar dating in deciphering detailed chronology on the scale of a single igneous intrusion

petrology of granulite facies metapelites of the highland complex sri lanka implications for the metamorphic zonation and the p t path in the pan african granulite facies highland complex of sri lanka the metamorphic grade increases from the central northwestern part towards boththe east and the southeast in metapelites the following prograde changes in metamorphic assemblage are recognized biotite ilmenite garnet or co cordierite sillimanite alkali feldspar quartz in the central northwestern part give way to garnet rutile sillimanite ilmenite or biotite alkali feldspar quartz in the e and se and are replaced by gar garnet cordierite biotite magnetite ilmenite alkali feldspar quartz bearing assemblages in the southwest cordierite coexisting with garnet is restricted to the western highland complex where metamorphic pressures of 5 7 kbar havebeen determined in the eastern highland complex where garnet clinopyroxene quartz is stable in metabasic rocks metamorphic pressures of 8 10 kbar have been obtained from barometers on metapelites phase relations of metapelites as well as geobarometry point to a continuous metamorphic transition between the eastern and the western part of the highland complex a clockwise p t path for the whole highland complex is indicated by the succession kyanite sillimanite andalusite an even earlier stage of the prograde metamorphism documented by sillimanite needles older than kyanite inclusions in garnet hhave been found in two samples of the eastern highland complex this points to a strong pressure increase at amphibolite facies conditions probably due to tectonic thickening of the crust during prograde metamorphism other inclusion assemblages in garnet and reaction textures indicate the breakdown of staurolite to kyanite spinel garnet and subsequent continued garnet growth in the sillimanite field at the expense of biotite sillimanite and quartz during further heating ternary feldspars in metapelites bear evidence of extreme metamorphic temperatures of about 900 c previously only indicated by coexisting pyroxenes in metabasic and charnockitic rocks within a few garnets inclusions of staurolite which are ti rich 1 34 wt tio2 and richer in fe xfe 0 745 than the enclosing garnet have survived the very high grade metamorphism the high grade stage of the prograde metamorphism was accompanied by strong flattening and a general n s stretching which is many metapelites ceased after peak temperatures but proceeded in others tectonic uplift of the whole highland complex occurred only after a phase of near isobaric cooling in the lower crust zircon 610 ma and biotite cooling ages 460 ma holzl et al 1991 as well as retrograde p t paths which passed the andalusite stability field are similar in the eastern and western part of the highland complex this suggests that the lower crustal cross section exposed in the highland complex was already tilted during cooling

behavior of isotopic systematics during deformation and metamorphism a hf nd and sr isotopic study of mylonitized granite a combined compositional and hf nd and sr isotopic study was performed on a suite of samples of progressively deformed granite from a mylonite zone in the harquahala mountains western arizona to evaluate the effects of deformation and metamorphism on the isotopic systematics of typical continental crustal rocks the 1 4 ga harquahala granite was deformed during mesozoic thrusting along the harquahala thrust granite in the resulting 60 m wide shear zone ranges from protomylonite to ultramylonite in most of these mylonites the protolith is not megascopically recognizable and can be discerned only by the progressive transition to undeformed granite isotopic analyses of hf nd and sr from the shear zone document the immobility of the hf and nd isotopic systems relative to that of the sr isotopic system during deformation the rb sr isotopic data show considerable scatter on an isochron plot exhibiting both gains and losses of rb and sr from the whole rock systems in contrast the sm nd and lu hf isotopic systematics are mostly well behaved on isochron diagrams plotting mostly in tight clusters or along 1 4 ga isochrons these results show that while the sr isotopic system in crustal rocks is quite susceptible to later tectonic disturbance both hf and nd isotopic systems can provide reliable model age information in continental crustal terranes even when the rocks have been subjected to low to medium grades of deformation and metamorphism

the origin of high δ18o zircons marbles megacrysts and metamorphism the oxygen isotope ratios 18o of most igneous zircons range from 5 to 8 with 99 of published values from 1345 rocks below 10 metamorphic zircons from quartzite metapelite metabasite and eclogite record 18o values from 5 to 17 with 99 below 15 however zircons with anomalously high 18o up to 23 have been reported in detrital suites source rocks for these unusual zircons have not been identified we report data for zircons from sri lanka and myanmar that constrain a metamorphic petrogenesis for anomalously high 18o in zircon a suite of 28 large detrital zircon megacrysts from mogok myanmar analyzed by laser fluorination yields 18o from 9 4 to 25 5 the u pb standard cz3 a large detrital zircon megacryst from sri lanka yields 18o 15 4 0 1 2 se by ion microprobe a euhedral unzoned zircon in a thin section of sri lanka granulite facies calcite marble yields 18o 19 4 by ion microprobe and confirms a metamorphic petrogenesis of zircon in marble small oxygen isotope fractionations between zircon and most minerals require a high 18o source for the high 18o zircons predicted equilibrium values of 18o calcite zircon 2 3 from 800 to 600 c show that metamorphic zircon crystallizing in a high 18o marble will have high 18o the high 18o zircons 15 from both sri lanka and mogok overlap the values of primary marine carbonates and marbles are known detrital gemstone sources in both localities the high 18o zircons are thus metamorphic the 15 25 zircon values are consistent with a marble origin in a rock dominated system i e low fluid external rock the lower 18o zircon values 9 15 are consistent with an origin in an external fluid dominated system such as skarn derived from marble although many non metasomatized marbles also fall in this range of 18o high 18o 15 and the absence of zoning can thus be used as a tracer to identify a marble source for high 18o detrital zircons this recognition can aid provenance studies in complex metamorphic terranes where age determinations alone may not allow discrimination of coeval source rocks metamorphic zircon megacrysts have not been reported previously and appear to be associated with high grade marble identification of high 18o zircons can also aid geochronology studies that seek to date high grade metamorphic events due to the ability to distinguish metamorphic from detrital zircons in marble

nd isotopic evidence for the position of southernmost indian terranes within east gondwana abstract sm nd model ages of orthopyroxene bearing massif charnockites from the cardamom hills massif and adjoining supracrustal rocks from the kerala khondalite belt in southernmost india are used to infer some of the relationships within these rocks and between them and neighboring areas most of these rocks have model ages of 2 1 2 8 ga with most charnockites in the range 2 2 2 6 ga thus 3 0 3 4 ga archean rocks to their north did not contribute material to either suite and the two suites may have been juxtaposed after formation of the supracrustal rocks the similarity of sm nd isotope systems in the two units studied here supports an argument that the massif charnockites were the primary sole source of the detritus incorporated into the supracrustal rocks a cordierite gneiss representative of a relatively minor lithology in the supracrustal belt has a model age of 1 3 ga the protolith of this gneiss not only formed from much younger material than the rest of the belt but also formed significantly after the other metasedimentary rocks the source material of the gneiss protolith may have been located in the wanni and vijayan complexes of sri lanka the overlap of the model ages of rocks in this area and those in the highland complex of sri lanka supports the notion that these two sets of rocks were joined to each other in gondwana they belong to a belt that ran from antarctica through sri lanka and india into madagascar this belt was involved in pan african tectono metamorphism as reflected in the 550 ma age of the last granulite forming event throughout the belt

evidence for isotopic equilibration of smnd whole rock systems in early archaean crust of enderby land antarctica smnd isotopic data indicate that differential ree mobility occurred on a whole rock scale during transitional amphibolite to granulite facies regional metamorphism 700 c 7 kbar in early archaean rocks 3930 ma of the napier complex of enderby land antarctica the degree of mobility is independent of metamorphic grade but correlates directly with development of tectonic fabric whole rock samples with d3 m3 internal fabrics lie along an array corresponding to an age of 2410 100ma whereas samples preserving only earlier fabrics preserve an older albeit imprecisely defined isochron age in contrast to a widely held belief such changes did not require the presence of a large hydrous fluid flux if the mechanism responsible for smnd resetting at this locality where tchur ages range from 1990 ma to 6090 ma is more widespread than is currently recognised isolated smnd model ages particularly in complex terrains should be treated with caution

deformation of a high grade gondwana fragment sri lanka abstract 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

u pb geochronology of zircons from river sediments in sri lanka implications on early archean to late cambrian magmatism and episodic crustal growth geochronology of zircons from river sediments in sri lanka has never been attempted here we present rare earth elements ree and u pb isotopes of 700 zircon grains by laser ablation icp ms technique from sediments of the major rivers in the sri lankan basement rivers of mahaweli kelani kalu walawe maduru oya and maha oya most of the studied zircons display oscillatory zoning and th u 0 1 confirming their igneous origin u pb age distribution of these river sand zircons exhibits two major populations depicting mesoproterozoic to archean similar to 30 1100 3100 ma and neoproterozoic to late cambrian similar to 70 430 1000 ma magmatism the two populations could be further resolved into intensive and episodic magmatic peaks at 480 680 ma 680 1000 ma 1100 1300 ma 1300 1700 ma 1700 1900 ma 2100 2300 ma 2300 2600 ma and 2600 3100 ma these river sediment zircon data are consistent with the principal episodes of global continental crustal growth and the recognized u pb ages of magmatism are mostly correlative with those obtained from basement rocks by previous studies of sri lanka the older zircons found in this study may have mainly originated from reworked recycled ancient palaoproterozoic to archean crust a younger zircon group of 450 700 ma with homogenous core rim zoning texture and th u ratios 0 1 were inferred to be of metamorphic origin indicating their possible crystallization at late neoproterozoic metamorphic events associated with the final assembly of the gondwana supercontinent thus our data imply that the present day continental crust of sri lanka may have predominantly formed and grown in the neoproterozoic while there is evidence that crustal growth began at least in the paleoproterozoic further the mesoproterozoic peaks of the frequency distribution of u pb ages of river sand zircons at similar to 1 6 ga and similar to 1 1 ga might reflect triggering of subduction in a paleo tectonic setting forming the wanni and vijayan dual arc systems respectively

isotopic mapping of age provinces in precambrian high grade terrains sri lanka nd model ages of amphibolite and granulite grade rocks in sri lanka form a simple regional pattern that broadly correlates with mappable geological units and is in effect an isotopic map of the island s basement the granulite grade units of the highland group and southwest group have model ages of 2 2 3 0 ga indicating derivation mainly from late archean sources they are bounded to the east and west by late proterozoic gneisses of the vijayan complex with model ages of 1 1 2 0 ga the isotopic data identify three distinct crustal provinces and are not consistent with earlier suggestions that the vijayan gneisses are retrograde equivalents of the highland granulites sri lanka is not a direct continuation of the archean dharwar craton of southern india identification of vijayan type juvenile crustal terrains in other gondwana fragments may play a key role in determining the precise attachment of southern india sri lanka in eastern gondwana

cosmogenic nuclide evidence for low weathering and denudation in the wet tropical highlands of sri lanka 1 some of the lowest weathering and erosion rates in any mountain range in the world have been measured using cosmogenic nuclides in the steep humid tropical highlands of sri lanka the total preanthropogenic denudation rates were measured in creek sediments and soil samples from unperturbed rain forest sites bedrock from mountain crests and bedrock from inselbergs denudation rates are in the range of 5 30 t km 2 yr 1 2 11 mm ky 1 these rates average denudation over the last 50 250 ky weathering exports in rivers draining the mountainous central highlands show that silicate weathering rates are also low varying from 5 to 20 t km 2 yr 1 today 2 7 mm ky 1 but they represent a significant fraction of the total denudation all these observations run contrary to the conventional geomorphologic and geochemical wisdom that would predict rapid erosion for highlands of high relief temperatures and precipitation we speculate that the high relief in sri lanka represents the remnant of a geomorphic block that was uplifted during rifting at 130 ma or even earlier and that was reduced to the interior of the island by rapid receding of escarpments after continental breakup it is possible that throughout this history hillslopes where not exposing bare bedrock were protected by thick weathered profiles such clay rich layers would inhibit silicate weathering by shielding bedrock from weathering agents in the absence of landscape rejuvenation physical erosion rates are low and fresh mineral surfaces are not being supplied the observation that wet steep tropical highlands can have low rates of rock weathering and erosion has some potentially profound implications for the long term controls of atmospheric co2 budgets high temperature and precipitation which are much invoked though controversial agents for silicate dissolution and co2 drawdown become ineffective in promoting weathering in areas that are not tectonically active

pb loss patterns in zircons from a high grade metamorphic terrain as revealed by different dating methods upb and pbpb ages for igneous and metamorphic zircons from northern sri lanka abstract we report u pb and 207 pb 206 pb ages for multigrain fractions single grains and grain domains in zircons from highgrade igneous and sedimentary rocks of the vavuniya charnockite province vcp in northern sri lanka using the ionmicroprobe shrimp a new vapour digestion technique vdt and evaporation charnockitic enderbitic and mangeritic gneisses are derived from i type tonalitic syenitic granodioritic and granitic precursors for which the u pb systematics of zircons as determined by shrimp suggest crystallization ages between 1000 and 1100 ma many translucent round and multifacetted zircons in these rocks are not of metamorphic but of igneous origin and acquired their shape through partial zircon dissolution and recrystallization the large majority of zircons exhibit severe and variable pb loss some 540 580 ma ago which we ascribe to granulite grade metamorphism almost immediately followed by local retrogression most of the zircons lost pb from the entire grain with no domains remaining that reflect the original pb isotopic composition it was therefore impossible to obtain meaningful 207 pb 206 pb evaporation ages for such grains and this cautions against use of this method in zircons that experienced severe pb loss however zircons formed during high grade metamorphism apparently retain their primary pb isotopic composition and were dated successfully by the evaporation technique yielding ages between 553 26 and 560 30 ma conventional multigrain analysis and small fractions following vdt for two samples although much more precise than the shrimp data yielded only strongly discordant data points with patterns reflecting variable pb loss these data alone would not have made it possible to obtain reliable primary zircon crystallization ages 207 pb 206 pb evaporation ages for detrital zircons from two garnetiferous gneiss samples of sedimentary origin in the centre of the vcp revealed a maximum age of 1329 18 ma whereas a metaquartzite near the southern margin of the vcp contained zircons with 207 pb 206 pb ages between 1857 14 and 2543 14 ma we interpret the latter rock as belonging to the highland complex which occurs adjacent to the vcp our zircon data in combination with nd model ages milisenda et al 1994 suggest the vcp to be part of the wanni complex a distinct crustal domain in northern sri lanka characterized by relatively young ages in comparison with much older ages in the adjacent highland complex

melt producing and melt consuming reactions in the achankovil cordierite gneisses south india migmatitic cordierite gneisses within the achankovil zone az of southern pan african india record melt producing and subsequent melt consuming mineral reactions early mineral assemblages bt sil qtz and bt sil spl deduced from inclusion textures in garnet prophyroblasts break down via successive dehydration melting reactions to high t phase assemblages e g grt crd liq opx liq spl crd liq later back reactions between the restite and the in situ crystallizing melt resulted in thin cordierite coronas separating garnet from the leucosome and partial resorption of garnet to opx crd or crd bt qtz symplectites leucosomes generally display a moderate low strain gneisses to strong high strain gneisses depletion of alkali feldspar attributed to mineral melt back reactions partly controlled by the degree of melt segregation using a kfmash partial petrogenetic grid that includes a melt phase and qualitative pseudosections for microdomains of high and low al si ratios the successive phase assemblages and reaction textures are interpreted in terms of a clockwise p t path culminating at about 6 7 kbar and 900 950 c this p t path is consistent with but more detailed than published results which suggests that taking a melt phase into account is not only a valid but also a useful approach comparing p t data and lithological and isotopic data for the az with adjacent east gondwana fragments suggests the presence of a coherent metasedimentary unit exposed from southern madagascar via south india az and sri lanka wanni complex to the lutzow holm bay in eastern antarctica

isotopic constraints on emplacement age of anorthositic rocks of the marcy masiff adirondack mts new york the igneous emplacement age of anorthositic rocks comprising the marcy massif have been constrained by fixing minimum and maximum ages for their intrusion minimum ages have been fixed by a u pb dating of cores of air abraded zircons from the anorthosite 1113 ma b u pb dating of baddeleyite within the anorthosite 1087 ma c u pb dating of baddeleyite from an olivine metagabbro crosscutting marcy massif anorthosite 1109 ma d u pb zircon dating of jotunitic rocks crosscutting the marcy anorthosite massif 1143 pm 31 ma maximum emplacement ages are based upon a sheets of anorthositic rock crosscutting mangeritic charnockitic gneiss dated by u pb zircon at 1125 pm 10 ma 1134 pm 4 ma and 1135 pm 43 ma and b anorthosite related pyroxenite dikes crosscutting mangerite that contains xenoliths of identical pyroxenite these results constrain emplacement of the marcy anorthosite to 1113 1138 ma with the most probable age in the interval 1125 1135 ma this time of in

sensitive high resolution ion microprobe u pb dating of prograde and retrograde ultrahigh temperature metamorphism as exemplified by sri lankan granulites ultrahigh temperature uht granulites of the central highland complex sri lanka underwent some of the highest known peak temperatures of crustal metamorphism zircon and monazite u pb systems in granulites near kandy the highest grade region 1050 c 0 9 gpa preserve both a record of the timing of prograde and retrograde phases of uht metamorphism and evidence for the ages of older protolith components zircon grains from a quartz saturated granulite containing relics of the peak uht assemblage have remnant detrital cores with dates of ca 2 5 0 83 ga date clusters of ca 1 7 and 1 04 0 83 ga record episodes of zircon growth in the source region of the protolith sediment two generations of overgrowths with contrasting th u record metamorphic zircon growth at 569 5 and 551 7 ma probably in the absence and presence of monazite respectively the age of coexisting metamorphic monazite 547 7 ma is indistinguishable from that of the younger low th u zircon overgrowths zircon from a quartz undersaturated monazite absent uht granulite with a mainly retrograde assemblage is mostly metamorphic 551 5 ma the ca 570 ma zircon overgrowths in the quartz saturated granulite probably record partial melting just before or at the metamorphic peak the ca 550 ma zircon in both rocks and the ca 550 ma monazite in the quartz saturated sample record post peak isothermal decompression a possible model for this pressure temperature time evolution is ultrahot collisional orogeny during the assembly of gondwana locally superheated by basaltic underplating followed by fast extensional exhumation

deformation and metamorphic evolution of a large scale fold in the lower crust the dumbara synform sri lanka abstract in the granulite facies highland complex of sri lanka large scale folds are formed after the peak of metamorphism 800 850 c 8 9kbar at slightly lowered temperatures 700 750 c the folds formed with axes parallel to a marked stretching lineation which is a continuous element throughout the whole deformational sequence in the central parts of the highland complex a lithostratigraphic sequence derived from shelf type sediments can be established numerous laccolithic or sill like intrusions with granitic to basic composition thickened the pile a pronounced foliation and stretching lineation s1 l1 parallel to compositional layering formed during peak metamorphic conditions and continued towards lower temperatures isoclinal folding with axes parallel or normal to this stretching lineation locally complicates the lithological sequence d2 d3 deformation involving grain size sensitive mechanisms led to an equalizing of viscosities between layers of differing composition and thus to very homogeneous deformation during the highest temperature stage the dumbara synform is one of several large scale fourth folds which formed at slightly decreased temperature the fold mechanism changed from the outer deeper level towards the core of the fold stretching parallel to fold axis continued during folding causing small scale extensional shear zones which formed after return to marked viscosity contrasts between compositionally different layers deformation ended in the higher amphibolite facies and was followed by static annealing and slow strain free exhumation

evidence for prograde metamorphic evolution of sri lankan pelitic granulites and implications for the development of continental crust petrological evidence for prograde metamorphic evolution is obtained for pelitic granulites of the newly defined highland complex in sri lanka it includes the occurrences of relict kyanite quartz staurolite corundum kyanite hercynite kyanite and sapphirine kyanite spinel within garnet in the sillimanite quartz rich rocks of special interest is the mineralogy of khondalites graphitic garnet sillimanite k feldspar quartz gneisses the matrix of which is free of hydrous minerals indicating that dehydration reactions have been completed in this kind of rocks however relict hydrous minerals such as al ti rich more than 58 wt al2o3 and more than 1 wt tio2 staurolite and extremely ti rich up to 9 8 wt tio2 biotine occur as inclusions within garnet the local relict mineral assemblages within garnet in khondalites indicate the following continuous staurolite breakdown reactions which may have progressed as temperature increased and pressure decreased staurolite quartz kyanite garnet h2o staurolite corundum kyanite garnet h2o staurolite kyanite hercynite garnet h2o staurolite sillimanite hercynite garnet h2o some of these dehydration reactions are inferred to have also taken place in other widespread pelitic granulites of the highland complex moreover garnet in the highland complex pelitic granulites occasionally preserves growth zoning though modified slightly to extensively by post growth volume diffusion based on the evidence a clockwise prograde p t path of evolution is inferredfor meta sedimentary granulites belonging to the highland complex on the other hand no evidence for prograde metamorphism has so far been obtained in lower grade rocks of the wanni complex to the northwest of the highland complex such a difference between these complexes may be the first petrological feature which supports the new geotectonic division of the sri lankan basement rocks based virtually on the distinctly different neodymium model ages it suggests that these two complexes were amalgamated during the very last part of prograde metamorphism of the highland complex rocks that is during decompression and heating probably by the lower crustal lateral flow after tectonic overthickening and delamination the clockwise prograde p t t path followed by the meta sedimentary granulites of the highland complex is in marked contrast with the near isobaric cooling path for meta igneous granulites suggested by some authors it is proposed that both types of p t t paths albeit for different rock types are possible and resulted from diverse deep crustal processes such as underplating of continental crust by sediments and magmas or near convergent plate boundaries

the origin of the pink granites of sri lanka another view abstract pink granites are common throughout the precambrian metasedimentary terrain of sri lanka and play a significant role in reconstructing the geologic evolution of this country essentially microcline bearing rocks they form conformable layers with the associated metasedimentary rocks and are always interlayered and associated with rocks possessing amphibolite facies mineral assemblages this association is seen throughout the terrain at all scales further they always tend to fold around the major structures of the terrain and contain conformable layers and enclaves of rare metasediments and have a gneissic appearance in places their field relations associations mineralogy petrography petrofabrics chemistry and also age suggest that they are metasediments probably arkoses which have undergone the same set of metamorphic and deformational events as other precambrian metamorphic rocks of sri lanka their present granite like character does not necessarily imply that they are the products of crystallization of magmatic or anatectic melts or that they have had a replacement origin rather chemical composition has determined their present granite like character due to granitization by simple isochemical recrystallization treptomorphism during highgrade metamorphism at a ph2o which was sufficient to retard their grade to amphibolite facies under granulite facies metamorphic conditions at which the other metasediments of sri lanka have metamorphosed

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

u pb isotopic systematics of zircons from prograde and retrograde transition zones in high grade orthogneisses sri lanka we present u pb zircon isotopic data from locally restricted prograde arrested in situ charnockitization and retrograde metamorphic transition zones which are well exposed in proterozoic orthogneisses tectonically interbanded with granulite facies supracrustal rocks of the highland group in sri lanka these granitoid rocks yield apparent ages of 1942 pm 22 ma 770 ma 660 ma and 560 ma all samples show severe pb loss some 550 560 ma ago the main phase of granulite formation could not be dated unambiguously but is bracketed between 660 ma and 550 ma the pervasive pb loss event around 550 560 ma reflects the end of this period of high grade metamorphism and was associated with widespread igneous activity and retrogression this is constrained by the 550 pm 3 ma intrusion age for a post tectonic granite we relate this late phase of thermal activity to crustal uplift of the sri lankan granulites our data unambiguously prove the high grade history of the sri lanka gneisses to be a late

geology of the high grade proterozoic terrains of sri lanka and the assembly of gondwana an update on recent developments abstract 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 lutzow 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

extreme crustal metamorphism during a neoproterozoic event in sri lanka a study of dry mafic granulites abstract garnet clinopyroxene quartz granulites of the central highland complex of sri lanka preserve textural and compositional features indicative of high pressure ultrahigh temperature hp uht crustal metamorphism and multistage retrogression grains of the peak metamorphic assemblage garnet clinopyroxene quartz are commonly separated and embayed by late orthopyroxene plagioclase symplectites however in some domains rare grain to grain associations of the peak assemblages are still preserved thermodynamic modeling in the cao na2o k2o feo mgo al2o3 sio2 system indicates peak metamorphic conditions of 12 5 kbar at 925 c the temperature estimates using garnet and clinopyroxene core compositions are in the range 844 982 c in agreement with the thermodynamic modeling in conclusion the textural geochemical and thermodynamic modeling and thermobarometric data indicate a multistage decompression after hp uht metamorphism u pb zircon laser ablation inductively coupled plasma mass spectrometry

incomplete retention of radiation damage in zircon from sri lanka a suite of 18 zircon gemstones from placers in the highland southwestern complex sri lanka were subjected to a comprehensive study of their radiation damages and ages the investigation included x ray diffraction raman and pl spectroscopy electron microprobe pixe and hrtem analysis as well as u th he and shrimp u th pb age determinations zircon samples described in this study are virtually homogeneous they cover the range from slightly metamict to nearly amorphous generally concordant u th pb ages averaging 555 11 ma were obtained late ordovician zircon u th he ages scattering around 443 9 ma correspond reasonably well with previously determined biotite rb sr ages for rocks from the hswc slightly to moderately metamict zircon has retained the radiogenic he whereas only strongly radiation damaged zircon calculated total fluences exceeding 3 5 10 18 events g has experienced significant he loss when compared to unannealed zircon from other localities sri lanka zircon is about half as metamict as would correspond to complete damage accumulation over a 555 m y lasting self irradiation period suggesting significant annealing of the structural radiation damage insufficient consideration of this has often resulted in significant underestimation of radiation effects in zircon we suggest to estimate effective doses for sri lanka zircon by multiplying total fluences which were calculated using the zircon u th pb age by a correction factor of 0 55 this conversion may be applied to literature data as well because all gem zircon samples from sri lanka this work and previous studies seem to reveal the same general trends of property changes depending on the radiation damage the use of effective doses for sri lanka zircon contributes to more reliable quantitative estimates of radiation effects and makes possible direct comparison between natural and synthetic radiation damaged zircon

fe ti oxide chronometry with application to granulite formation abstract ilmenites occurring in amphibolite and granulite facies orthogneisses in sri lanka yield precise u pb ages coexisting ilmenite magnetite in the granulite facies rocks indicate temperature conditions of 680 60 c and log o 2 17 0 1 2 the u pb age of 498 2 3 3 ma obtained for ilmenite is indistinguishable from the pb pb age of 501 5 ma derived for coexisting silicate minerals and suggests that oxide silicate equilibration has been preserved in contrast fe ti oxides in adjacent amphibolitefacies rocks indicate a temperature of 830 60 c and log o 2 12 7 0 8 and record u pb ages of 1093 7 7 6 ma and 1095 4 4 2 ma these ages are some 600 ma older than those obtained for coexisting silicate phases in the same sample or for both oxide and silicate minerals in the nearby charnockite and relate either to the time of emplacement of the granitic precursor or to some earlier metamorphic episode these results suggest that temperature o2 and fluid conditions estimated from fe ti oxides should be interpreted with extreme caution