geophysical investigation of lithosphere structure beneath sri lanka using surface and internal load variations this paper describes a method for interpreting the lateral changes in the geological structure of the lithosphere by iteratively calculating the spatial variation of a bottom to top loading ratio we use forsyth s coherence method based on multi taper spectral analysis on overlapping window areas we discuss the geological findings and their comparison with existing geological interpretations for the sri lankan region the low thickness of the elastic plate in the sri lankan region suggests a high geothermal gradient the lithospheric geological structure beneath the sri lankan region is consistent with the main crustal units of sri lanka which belong to different ancient plates together with interpretations based on metamorphic rock units our findings support the existence of thrust contact in the central highlands of sri lanka

a note on the impact factor of gondwana research with a view towards understanding the evolutionary history of the complex south indian shield several geological and geophysical studies have been carried out recent geophysical studies include magnetotelluric mt deep seismic sounding dss gravity magnetic and deep resistivity soundings drs in the present study mt results along 140 km andiyur turaiyur east west profile is presented the data are subjected to groom bailey decomposition and static shift correction before deriving a 2 d model the 2 d modeling results have shown that the upper crust up to about 15 km towards western part of the profile have exhibited high resistive character of about 40 000 ohm m as compared to the eastern part less than 5 000 ohm m the mid lower crust has shown a decrease in resistivity in western part of the profile the order of resistivity being 2 000 ohm m an anomalous steep conductive feature less than 100 ohm m is observed near sankari at mid lower crustal depths 20 km towards middle part of the profile this feature is spatially correlatable with the well known moyar bhavani shear zone mbsz the features obtained in the present study are consistent with earlier mt studies in this region and correlatable with other geophysical studies dss studies near the study region gave an evidence for differing crustal structure on either side of mbsz variation in geoelectric character along the profile both in the upper crust and mid lower crust indicate a block structure in the sgt with shear zones acting as boundaries the new evidence in the form of distinct geoelectric structure and also variation in seismic structure indicate a continent continent collision zone in this region and plays an important role for the gondwana reconstruction models of south indian shield

40ar 39ar and k ar geochronology of the dykes from the south indian granulite terrain abstract 40ar 39ar and conventional k ar data on mafic dykes and a felsic dyke together with previous data have been used to constrain the ages of the dyke magmatism in the south indian granulite terrain the age of the mainly ene wsw trending agali anaikatti swarm is estimated to be 1980 25 ma with a very limited occurrence of upper cretaceous 80 90 ma dykes in the area the nw se dharmapuri swarm is dated as at least 1800 ma possibly 1855 9 ma both the nw se and the ne sw dyke swarms of tiruvannamalai are dated at 1650 10 ma only a few dykes in north kerala are of middle proterozoic ca 1700 1650 ma age furthermore the 40ar 39ar results of dharmapuri dykes exhibit age spectrum characteristic of partial degassing they provide strong evidence of latest proterozoic overprinting by a thermal event probably related to the pan african orogeny most of the phanerozoic dykes are confined to the coastal region in kerala province the dolerites of central kerala and the majority of north kerala dolerites intruded during the 70 65 ma period whereas a large gabbro and a felsic dyke in central and north kerala respectively are of upper cretaceous age ca 85 ma and the mean 81 1 5 ma age from these dykes may represent cooling age the oldest phanerozoic upper jurassic 144 6 ma dyke intrusions have a restricted occurrence in south kerala very close to the southern tip of india the proterozoic dykes are interpreted as being lateral intrusions from under plating igneous bodies beneath the eastern shield region the geological record of the terrain does not show indications of significant pre magmatic extension and rifting but dyke magmatism may have led to extension rifting and the consequent development of the india antarctica sri lanka proterozoic high grade terrain the data are also interpreted to suggest that the south indian granulite terrain is an early proterozoic tectono metamorphic terrane with strong thermal imprints of neoproterozoic early palaeozoic age it is difficult to reconcile its extensions into proterozoic sri lanka or antarctica where tectono metamorphism is dated to be of neoproterozoic age the late phanerozoic dykes are predominantly related to the deccan trap event and are associated with mantle plume decompressional tectonics that gave rise to ocean floor spreading between the seychelles and the indian continent the few older ca 145 ma dykes in south kerala may be related to the rift tectonics that preceded the separation of australia antarctica and india

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 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

crustal evolution in south india constraints from nd isotopes nd isotope analyses from a range of lithologies including charnockites metasediments and granitoids that comprise the high grade terrain of south india define contrasting age provinces north of the palghat cauvery shear zone the karnataka craton and the granulite blocks of nilgiri and madras are characterized by model nd ages from 3 4 to 2 4 ga with significant crustal growth during the late archean south of the palghat cauvery shear zone a pan african granulite facies event resulted in sm nd fractionation as observed in several incipient charnockite localities model nd age calculations that allow for sm nd fractionation at 550 ma yield ages in the range of 2 9 to 1 3 ga this range compares closely with published model nd ages from the high grade terrain of sri lanka mid proterozoic crustal growth 1 1 ga observed in the vijayan complex of sri lanka may have provided source material for metasediments characterized by young model nd ages in southern peninsular india the palghat cauvery shear z

low temperature replacement of monazite in the ireteba granite southern nevada geochronological implications the ireteba pluton is a relatively homogeneous 64 ma zircon ion probe age two mica granite that was intruded by two 16 ma miocene plutons at depths ranging from 5 to 13 km deeper levels of the ireteba and miocene plutons were ductilely deformed at 15 16 ma at shallow levels remote from the miocene plutons the ireteba granite appears to have experienced little miocene heating and deformation monazites from different portions of the pluton reflect the different histories experienced by the host rock irregularly shaped patchy zones with high huttonite component thsio4 are widespread in monazite at deep levels adjacent to miocene plutons but less common in shallow level rock monazite grains with extensive replacement generally have irregular embayed surfaces in undeformed rocks distant from the miocene plutons monazites are less modified and more nearly euhedral though fine networks of replacement veins are common and irregular rims are evident in some grains secondary monazite from these samples is poorer in huttonite ion probe th pb dating yields 60 65 ma ages for magmatic and some replacement zones in monazite from the shallow samples and veins yield apparent ages as young as mid tertiary monazites from deep samples yield a few 55 65 ma ages for remnant magmatic zones and abundant miocene ages for replacement zones 14 18 ma these data demonstrate extensive miocene replacement of magmatic monazite especially at deep levels near miocene plutons and they suggest an early replacement episode as well both events were probably related to influxes of fluid the first may have been associated with initial solidification of the ireteba pluton and the second with the miocene plutons and or extensional deformation ambient temperatures at the time of replacement indicate that secondary monazite growth occurred at t as low as 400 c or less

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

u pb geochronology and isotope geochemistry of the archean and proterozoic rocks of north central madagascar abstract new u pb zircon ages and sm nd and rb sr isotopic data are presented for orthogneisses from north central madagascar including ile sainte marie alaotra beforona maevatanana and ambatolampy ambatomarina a migmatite tonalite gneiss from ile sainte marie is dated precisely at documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr renewcommand sfdefault wncyss renewcommand encodingdefault ot2 normalfont selectfont declaretextfontcommand textcyr cyr pagestyle empty declaremathsizes 10 9 7 6 begin document landscape 3187 pm 2 end document ma and has a sm nd model age tdm of 3204 ma thereby establishing a middle archean age for the oldest juvenile gneisses in northeast madagascar dated orthogneisses int

development and structural architecture of the eastern black sea the cenozoic sedimentary fill of the eastern black sea figure 1 is up to 10 km in thickness its bedding is almost horizontal with the the exception of basin flanks the eastern black sea basin is separated from the cenozoic sorokin and tuapse troughs by the buried shatsky and tetyaev ridges which have mesozoic and paleocene eocene sedimentary cover figure 2

calcite and aragonite precipitation from seawater solutions of various salinities precipitation rates and overgrowth compositions abstract the reaction rate and composition of calcite and aragonite overgrowths precipitated from seawater solutions of various salinities i e s 5 15 25 35 44 were determined at 25 c and 10 2 5 atm co2 partial pressure using a constant disequilibrium seeded technique the rate data were fitted to an empirical rate law of the form log r n c or a 1 log k where n is the empirical reaction order and k is the rate constant calcite precipitation rates in seawater solutions do not vary appreciably as a result of salinity variations over the range investigated while those for aragonite show an increase in going from the higher i e s 35 44 to the lower i e s 5 15 25 salinity range this study also confirms previously published findings that above a given saturation state i e c 2 6 aragonite precipitates more rapidly than calcite at 25 c the incorporation of sr2 in aragonite and mg2 in calcite overgrowths are independent of the precipitation rate the partition coefficient of sr2 in aragonite is approximately equal to unity and is unaffected by salinity variations between 5 and 44 however the mg2 partition coefficient in calcite increases with decreasing salinity of the parent seawater solutions possibly as a result of variations in the sulfate content of the solutions and solids the experimental results were discussed in the context of a number of geological environments

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

what sets the size of current ripples water flowing over sand in fluvial and marine settings often results in the formation of current ripples found in modern and ancient deposits on earth and mars ripple stratification records flow directions and fluid properties that are crucial to interpreting sedimentary records despite decades of observations of current ripples there is no universal scaling relation to predict their size or to distinguish them from dunes here we use dimensional analysis and a new data compilation to develop a scaling relation that collapses data for equilibrium wavelengths of ripples forming under unidirectional flows results show that ripples are larger with more viscous fluids coarser grains smaller bed shear stresses and smaller specific gravity of sediment the scaling relation also segregates ripples from dunes highlighting a narrow regime of transitional bedforms that have morphologic properties and sediment transport conditions that overlap with both ripples and dunes our analysis shows that previous absolute size based definitions of ripples and dunes only hold for certain conditions such as water flows transporting siliciclastic grains on earth the new theory allows estimates of ripple sizes in foreign fluids and on other planets including meter scale ripples in methane flows on titan or in viscous brines on mars

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

single zircon ages for granitoid gneisses in the central zone of the limpopo belt southern africa and geodynamic significance abstract ion microprobe evaporation and vapour digestion single zircon emplacement ages for granitoid rocks throughout the central zone cz of the limpopo mobile belt south africa define three broad age groups namely at 3188 3314 2637 2734 and ca 2500 ma respectively all these granitoids including the youngest group display polydeformational features suggesting that much of the tectonic history of the cz occurred in the late archaean and early proterozoic the oldest group of predominantly tonalitic trondhjemitic granodioritic gneisses was found in relatively small domains from the extreme w to the e of the cz and probably represents remnants of a once more widespread early archaean terrain the most abundant granitoid gneisses belong to the 2 6 2 7 ga age group and were emplaced into already ductilely deformed gneisses of the oldest group several of these rocks were derived from intracrustal melting rare metamorphic zircons with ages of 2560 2575 ma reflect a high grade metamorphic event in the cz that has also been postulated by other workers a strongly deformed orthogneiss with a protolith emplacement age of 2510 ma proves beyond doubt that at least part of the structural history in the cz occurred in the early proterozoic an anatectic granite in the extreme w of the cz intruded at 2022 ma and is probably related to the peak of granulite facies metamorphism previously dated at 2027 6 ma in the messina area the second period of granitoid magmatism is broadly contemporaneous with charnockite and enderbite emplacement and granulite facies metamorphism in the northern and southern marginal zones of the limpopo belt broad contemporaneity of extensive granitoid magmatism in all three zones raises doubts about the exotic nature of the cz and suggests a revised evolutionary model for the limpopo belt

a post younger dryas black sea regression identified from sequence stratigraphy correlated to core analysis and dating the north western black sea shelf is the widest continental shelf of the black sea where the danube delta prodelta system includes important records on the black sea water level fluctuations this study integrates newly acquired high resolution single channel seismic reflection profiles and calypso piston cores recovered along a transect extending from the danube delta deep into its deep sea fan the correlated results provide information on the late quaternary architecture of the danube black sea shelf and on the role of global glacio eustatic fluctuations on the building of the danube delta prodelta the sedimentary sequences in the black sea are strongly affected by water level changes for the whole duration of late glacial holocene period the level of the black sea was controlled by the regional climate modifications rather than the global eustatic changes the seismic sequences recognized on a very high resolution seismic profile were dated by two calypso piston cores based on the obtained data during the last glacial maximum the black sea was a land locked lake with the level below 120 m in the course of the deglaciation the lake level rose up to 40 m which is confirmed by occurrence of the prodelta lobes formed after the rise in water level originated from the melt water following the younger dryas 11 000 8500 c 14 bp there occurred a new level lowering to the level of 100 m identified by the forced regression deposits recorded on the romanian shelf this last lowstand previously recognized by a pronounced shoreline with a characteristic beach profile and a belt of coastal dunes is also indicated by the prodelta at 100 m depth all these coastal features as well as the incised anastomosed channel system remained preserved on the shelf resulting from a rapid ultimate transgression starting immediately after 8500 c 14 bp c 2010 elsevier ltd and inqua all rights reserved

shallow gas transport and reservoirs in the vicinity of deeply rooted mud volcanoes in the central black sea abstract the principal objective of this paper is to understand the gas migration process and the structures of mud volcanoes in the central black sea multichannel seismic lines were acquired across 6 mud volcanoes in the central black sea msu yuzhmorgeologiya malyshev kornev goncharov and vassoevitch based on a high resolution seismic processing we analyzed acoustic anomalies and studied near surface sediment structures of these mud volcanoes four types of pathways for gas and fluid migration and three types of gas reservoirs were recognized a regional bottom simulating reflection bsr seems to be absent in most parts of the study area however a clear bsr was observed in one of the seismic profiles the free gas migrating upwards along these pathways were sealed by gas hydrates or fine grained sediments in the gas hydrate stability zone four seismic units were separated according to a suggested age model and identified seismic facies combining with the possible sedimentary processes of the central black sea we try to reveal the active mechanism of these mud volcanoes these mud volcanoes reveal two to three major active stages these active stages might be related to distinct sea level falls which seem to be one of the main trigger factors of the mud volcano eruptions in the central black sea a structural model for the msu mud volcano was presented with respect to mechanisms of gas migration and the origin of the gas

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

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 late holocene evolution of the black sea a critical view on the so called phanagorian regression abstract throughout its geologic history the black sea experienced major sea level changes accompanied by severe environmental modifications including geomorphologic reshaping the most spectacular changes were driven by the quaternary glaciations and deglaciations that reflect responses to milankovitch cycles of 100 and 20 ky periodicity major sea level changes were also considered for a shorter and more recent cyclicity the concept of the phanagorian re and transgression cycle supposedly with a minimum sea level stand of 5 6 m below its present position in the middle of the 1st millennium bc was established in 1963 by fedorov for the black sea region it was based on archaeological and palaeogeographical research conducted around the ancient greek colonies of the cimmerian bosporus in particular at the name giving site of phanagoria where underwater prospection had revealed the presence of a large number of submerged relics of the classical greek era analyses of sediment cores as well as 14c dated fossil coastal bars in the western and southern parts of taman peninsula show that contemporary coastal bars are related to different sea levels the dissymmetry can reach up to 6 m around 500 bc this and more evidence from drill cores confirms that on taman peninsula many of the apparent sea level changes are tectonically induced the subsidence may have been initiated by the release of gas from mud volcanoes inherited along anticline axes other observations around the black sea confirm that submerged archaeological sites correspond to areas where subsidence has taken places while the so called holocene highstand said to have been located above the present day sea level is associated with uplift areas triggered by the ongoing caucasus orogeny recent oceanographic research carried out in the black sea area shows that since the black sea was reconnected with the mediterranean sea i e 7500 14c bp at the latest both marine water bodies have been in equilibrium this fact and arguments from archaeology history hydrodynamics etc lead us to question the existence of the phanagorian regression it is important to note that none of the sea level curves established for the eastern mediterranean shows a comparable regression transgression cycle of several metres during the 1st millennium bc

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

thermochronological dating of brittle structures in basement rocks a case study from the onshore passive margin of sw sri lanka 1 the brittle structural inventory of southern and southwestern sri lanka has been studied by kinematic mineralogical and thermochronological techniques thermochronological analyses of faults comprise apatite and zircon fission track ft data and apatite u th sm he ages from fault planes and undisturbed host rocks and range between 65 and 230 ma the ages of both settings are undistinguishable for topographic altitudes below 100 m while fault planes from higher elevations are significantly younger than the corresponding host rocks thermal history modeling and qualitative interpretation of the thermochronological data identify at least five episodes of thermal overprint associated with faulting activity occurring at 159 18 144 14 120 10 94 8 and 70 10 ma the kinematic mineralogical and thermochronological data collectively show that sri lanka was subjected to major n s oriented extension subsequent to the gondwana breakup the resulting brittle structures exert primary control on the island s geomorphology especially on the southern escarpment of the sri lankan highlands

further evidence for an early carboniferous 340 ma age of high grade metamorphism in the saxonian granulite complex u pb and pb pb zircon ages for metamorphic zircons from granulites in the saxonian granulite complex are reported using the shrimp ion microprobe conventional multigrain and single gain techniques and the evaporation method this is complemented by a pb pb evaporation age for a post granulite granite emplaced into the schist mantle around the granulites during uplift of the complex we also demonstrate that zircon ages are not reset during high grade metamorphism as commonly argued but have a very high closure temperature and usually preserve the isotopic composition reflecting the time of their formation multifaceted zircons from four granulite samples that probably grew close to the peak of high grade metamorphism yielded identical u pb and pb pb ages of 340 ma which support previously published data and unambiguously show that the granulites formed during a lower carboniferous event and not in the early palaeozoic or precambrian as previously suggested older cores in some of the metamorphic zircons reveal early palaeozoic components at 470 485 ma that we interpret as ages reflecting magmatic crystallization of the granulite precursors one sample suggests an inherited component as old as 1700 ma the post granulite granite has a pb pb evaporation age of 333 1 1 0 ma and the short time interval between granulite metamorphism and granite intrusion implies that uplift crustal extension and cooling of the granulite complex occurred rapidly after peak metamorphic conditions

neoproterozoic amorphous ekanite ca2th0 9u0 1si8o20 from okkampitiya sri lanka a metamict gemstone with excellent lead retention performance a rare gemstone form of ekanite from okkampitiya eastern sri lanka has a chemical composition of ca2th0 9u0 1si8o20 corresponding to that of the tetragonal mineral ekanite the okkampitiya material however has undergone amorphization and is found in a fully metamict state which is ascribed to the long term accumulation of high levels of self irradiation damage 1 38 x 10 20 alpha decay events per gram the ekanite nevertheless does not show any post growth chemical alteration and the radiogenic pb has been retained the okkampitiya ekanite yielded a neoproterozoic pb 207 pb 206 age of 562 1 0 8 ma uncertainty quoted at the 95 confidence level that is concordant within the uncertainties of decay constants the remarkable pb retention performance of ekanite contradicts the common hypothesis that metamictization in minerals results in u pb discordance the exceptional chemical durability of the heavily radiation damaged geological material described here has also implications for materials science research high radiation resistance a key criterion in the search for advanced nuclear waste forms may not be a prerequisite for high aqueous durability of a solid

geochemistry and geochronology of high grade rocks from the grove mountains east antarctica evidence for an early neoproterozoic basement metamorphosed during a single late neoproterozoic cambrian tectonic cycle abstract the grove mountains of east antarctica are an inland continuation of the prydz belt the high grade metamorphic complex in this area is composed of felsic orthogneisses and mafic granulites with minor paragneisses and calc silicate rocks u pb zircon analyses using shrimp and la icp ms techniques reveal that the protoliths of mafic granulites and felsic orthogneisses were emplaced during a short time interval of ca 920 910 ma mafic granulites can be divided into low and high ti groups they have initial nd values nd t ranging from 0 8 to 1 9 tio 2 is positively correlated with feo t mgo and la nb ratios whereas it shows a negative correlation with nd t values indicating that the petrogenesis of their protoliths involved partial melting of a weakly enriched subcontinental lithospheric mantle and fractional crystallization of the magma accompanied by minor crustal contamination felsic orthogneisses have an affinity of a 2 type granites characterized by enrichment in ree y zr th and ga and high ga al and y nb ratios most of them have nd t values of 0 7 to 3 5 and nd depleted mantle model ages t dm of 1 76 1 65 ga and a few have low nd t values of 10 4 to 10 6 and old t dm of 2 46 2 27 ga reflecting a heterogeneity in their source region their protoliths were probably produced by high temperature partial melting of tonalitic granodioritic rocks triggered by the underplating of mantle derived mafic magma during post orogenic extension u pb analyses also reveal a metamorphic age of ca 2050 ma from detrital zircons in a paragneiss suggesting that the sedimentary materials of the paragneiss may have come from an as yet undiscovered early paleoproterozoic orogen of unknown provenance voluminous mafic felsic intrusives and a small amount of sedimentary rocks constitute an early neoproterozoic basement of the grove mountains subsequently this basement experienced only a single late neoproterozoic cambrian metamorphic cycle at ca 550 535 ma the available data indicate that the prydz belt is a collage of multiple basement terranes and each of them has a distinct tectonic evolution this supports the suggestion that the prydz belt may represent a late neoproterozoic cambrian collisional orogen that resulted in the final phase of the gondwana assembly

constraints on the timing of pan african granulite facies metamorphism in the kerala khondalite belt of southern india shrimp mineral ages and nd isotopic systematics abstract the supracrustal kerala khondalite belt kkb southern india constitutes an important but little understood terrain in the evolution of east gondwana here we report shrimp u pb zircon and titanite ages for a suite of metasedimentary rocks from the kkb metamorphic ages at documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr renewcommand sfdefault wncyss renewcommand encodingdefault ot2 normalfont selectfont declaretextfontcommand textcyr cyr pagestyle empty declaremathsizes 10 9 7 6 begin document landscape 530 2 pm 4 4 end document and documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd us

experimental evidence that ooid size reflects a dynamic equilibrium between rapid precipitation and abrasion rates ooids are enigmatic concentrically coated carbonate sand grains that reflect a fundamental mode of carbonate sedimentation and inorganic product of the carbon cycle trends in their composition and size are thought to record changes in seawater chemistry over earth history substantial debate persists concerning the roles of physical chemical and microbial processes in their growth including whether carbonate precipitation on ooid surfaces is driven by seawater chemistry or microbial activity and what role if any sediment transport and abrasion play to test these ideas we developed an approach to study ooids in the laboratory employing sediment transport stages and seawater chemistry similar to natural environments ooid abrasion and precipitation rates in the experiments were four orders of magnitude faster than radiocarbon net growth rates of natural ooids implying that ooids approach a stable size representing a dynamic equilibrium between precipitation and abrasion results demonstrate that the physical environment is as important as seawater chemistry in controlling ooid growth and more generally that sediment transport plays a significant role in chemical sedimentary systems

compaction of lithic sands experimental results and applications compaction experiments showed that the degree of physical compaction of a lithic sand is related to the amount and type of lithic material present these experiments produced a product that is texturally indistinguishable from natural lithic sandstones generally sedimentary lithic fragments are more ductile than metamorphic lithic fragments volcanic lithic fragments when altered to phyllosilicate minerals by weathering or diagenesis are extremely ductile mineralogy and intragranular microporosity influence ductility ratios based on weight percent of soft ductile minerals clay and micas to hard brittle minerals quartz and feldspar within the lithic fragments provides a good indication of ductility a ratio greater than 1 indicates the lithic material will be h ghly ductile microporous lithic fragments are more ductile than dense lithic fragments physical compaction models were developed based on experimental data from well sorted lithic sandstones for which the percentage of preserved porosities were functions of the effective stress the type of lithic material and the quantity of lithic material these models are derived from laboratory sandstones with at least 25 lithic grains in the following categories moderately ductile metamorphic lithic fragments highly ductile shale lithic fragments and extremely ductile altered volcanic lithic fragments application of the compaction models of the experimental compaction techniques to modern subsurface sand samples or outcrop equivalents can provide valuable estimates of the preserved porosities i e reservoir potential for lithic sandstones that may be exploratory objecti es in frontier basins examples using the models and experimental compaction techniques for exploration are given experimental data showed that precompaction cement and overpressure may be beneficial to porosity preservation an early formed partial cement can retard compaction and preserve porosity by stabilizing the sand pack a later cement does not have this beneficial effect overpressure when developed early retards compaction by reducing the effective stress of course a late developing overpressure is not effective at preserving porosity because compaction is an irreversible process

sources of stream sediments in the granulite terrain of the walawe ganga basin sri lanka indicated by rare earth element geochemistry abstract thirty eight samples of stream sediments draining high grade metamorphic rocks in the walawe ganga river basin sri lanka were analysed for their ree contents together with samples of metamorphic suites from the source region the metamorphic rocks are enriched in light ree lree compared to heavy ree hree and are characterised by high la lu ratios and negative eu anomalies the chondrite normalised patterns for these granulite grade rocks are similar to that of the average post archaean upper crust but they are slightly enriched with la and ce the ree contents of the n sm n and lower la n lu n ratios relative to other fractions the lower la n lu n ratio is related to the depletion of heavy minerals in the

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

ooid cortical stratigraphy reveals common histories of individual co occurring sedimentary grains ooids are a common type of carbonate sand grain that form through a combination of constructive and destructive mechanisms growth via precipitation and diminution via physical abrasion because growth and abrasion obey distinct morphometric rules we developed an approach to quantitatively constrain the history of growth and abrasion of individual ooid grains using the record of evolving particle shape preserved by their cortical layers we designed a model to simulate 10 possible growth abrasion histories for each pair of cortical layer bounding surfaces in an individual ooid estimates for the durations of growth and abrasion of each cortical layer were obtained by identifying the simulated history that best fit the observed particle shape we applied this approach to thin sections of modern lacustrine ooids collected from several locations in the great salt lake gsl ut to assess the spatial and temporal variability of environmental conditions from the perspective of individual grains within a single deposit we found that gsl ooids do not all share the same histories clustering ooid histories by a frechet distance metric revealed commonalities between grains found together locally within a deposit but distinct differences between subpopulations shared among localities across the gsl these results support the tacit view that carbonate sedimentary grains found together in the environment do reflect a common history of sediment transport this general approach to invert ooid cortical stratigraphy can be applied to characterize environmental variability over 1 000 year timescales in both marine and lacustrine ooid grainstones of any geologic age

strain localization and large scale block rotation in the lower continental crust kataragama area sri lanka in sri lanka a suite of rocks is regionally exposed which once formed a part of the middle to lower crust of a collisional orogen with about doubled crustal thickness a subhorizontal to gently west dipping major thrust zone highland basal thrust crosses sri lanka from nne to ssw it separates a granulite facies upper unit the highland complex from an amphibolite facies lower unit the vijayan complex with a displacement of more than 300 km the basal highland complex was migmatized during thrusting rheological contrasts between dry granulites and migmatites were high in the presence of partial melts granulite facies fragments within the migmatites vary from the decimetre to kilometre scale granulites in the area of kataragama formerly interpreted as the kataragama klippe are now identified as a large scale raft within the migmatites which is completely separated from the overriding highland complex during thrusting strain was concentrated in the migmatized basal parts of the highland complex after crystallization of the migmatites thrusting continued and strain was still localized within the migmatites therefore granulite facies parageneses structures and textures are preserved in the granulite rafts the deviation of the granulite facies structures in the kataragama raft from the general trend in the highland complex is interpreted to indicate counterclockwise rotation of the whole raft by about 90 around a subvertical rotation axis

insights from petrography mineralogy and u pb zircon geochronology into the provenance and reservoir potential of cenozoic siliciclastic depositional systems supplying the northern margin of the eastern black sea the eastern black sea is one of the few remaining underexplored hydrocarbon basins in europe reservoir quality is a key risk factor in the basin the analysis of up dip portions of oligocene to pliocene depositional systems exposed around its northern and eastern margins enable the composition and provenance of sandstones within the basin to be predicted two major sources of quartz rich sandstone are present the russian western greater caucasus which became a subaerial sediment source in the oligocene and the east european craton scythian platform these are predicted to form better quality reservoirs at shallow and moderate burial depths than their rock fragment rich counterparts due to lesser amounts of compactional porosity loss oligo miocene sandstones from the russian western greater caucasus were deposited as turbidites in the tuapse trough sandstones from the east european craton scythian platform were largely trapped within the indolo kuban basin north of the greater caucasus they only entered the eastern black sea in large volumes from latest miocene or pliocene time in the region of the kerch taman shelf at the western tip of the greater caucasus however it is possible that a precursor to this system supplied the oligocene to early miocene reservoir sandstones within the subbotina field the dziruli massif in western georgia also generated quartz rich sandstones but these are not thought to have entered the black sea

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

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

quantifying the significance of abrasion and selective transport for downstream fluvial grain size evolution it is well known that pebble diameter systematically decreases downstream in rivers the contribution of abrasion is uncertain in part because 1 diameter is insufficient to characterize pebble mass loss due to abrasion and 2 abrasion rates measured in laboratory experiments cannot be easily extrapolated to the field a recent geometric theory describes abrasion as a curvature dependent process that produces a two phase evolution in phase i initially blocky pebbles round to smooth convex shapes with little reduction in axis dimensions then in phase ii smooth convex pebbles slowly reduce their axis dimensions here we provide strong evidence that two phase abrasion occurs in a natural setting by examining downstream evolution of shape and size of thousands of pebbles over 10 km in a tropical montane stream the geometric theory is verified in this river system using a variety of manual and image based shape parameters providing a generalizable method for quantifying the significance of abrasion phase i occurs over 1 km in upstream bedrock reaches where abrasion is dominant and sediment storage is limited in downstream alluvial reaches where phase ii occurs we observe the expected exponential decline in pebble diameter using a discretized abrasion model the so called box equations with deposition we deduce that abrasion removes more than one third of the mass of a pebble but that size selective sorting dominates downstream changes in pebble diameter overall abrasion is the dominant process in the downstream diminution of pebble mass but not diameter in the studied river with important implications for pebble mobility and the production of fine sediments

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

an extended episode of early mesoproterozoic metamorphic fluid flow in the reynolds range central australia the products of metamorphic fluid flow are preserved in zones within the marbles and metamorphosed semipelites of the upper calcsilicate unit in the granulite portion of the late palaeoproterozoic reynolds range group northern arunta block central australia the zones of retrogression characterized by minerals such as wollastonite grossular and clinohumite local resetting of oxygen isotopic compositions and local major element metasomatism were channelways for water rich fluids derived from granulite facies metapelites u th pb isotopic ages measured by the shrimp ion microprobe on zircon and monazite from a granulite facies semipelite an early semiconcordant aluminous quartz rich fluid flow segregation and a late discordant quartz rich segregation record some of the extended thermal history of the area zircon cores from the semipelite show its likely protolith to be an igneous rock 1812 11 ma old itself derived from a source containing zircon as old as 2 2 ga low th u overgrowths on the zircon grew during granulite facies metamorphism at 1594 6 ma monazite cooled to its blocking temperature at 1576 8 ma zircon cores from the semiconcordant segregation are dominantly 2 3 ga old indicating that the source of the fluids was not the particular metamorphosed semipelite studied two generations of low th u overgrowths on the zircon give indistinguishable ages for the older and younger of 1589 8 and 1582 8 ma respectively the monazite age is the same 1576 12 ma zircon from the late discordant segregation gave 1568 4 ma fluid flow occurred for at least 18 3 ma and ended 26 3 ma after the peak of metamorphism suggesting a very slow cooling rate of 3 c ma 1 the last regional high grade metamorphism in the reynolds range occurred at 1 6 ga not 1 78 ga as previously thought the high grade event at 1 78 ga is a separate event that affected only the basement to the reynolds range group

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

single zircon evaporation ages constraining the growth of the archaean aravalli craton northwestern indian shield single grain 207 pb 206 pb zircon ages were determined for granitoids and gneisses which constitute the archaean basement rocks of the aravalli craton of rajasthan northwestern indian shield the protolith ages for two gneisses collected from east of udaipur are 3230 ma and 2887 ma respectively the granitoids display an intrusive relationship with the gneisses and yielded ages ranging between 2666 ma and 2620 ma these ages provide the basis for a geochronological model of evolution of the oldest basement of the aravalli craton

arrested charnockite formation in southern india and sri lanka arrested prograde charnockite formation in quartzofeldspathic gneisses is widespread in the high grade terrains of southern india and sri lanka two major kinds of orthopyroxene producing reactions are recognized breakdown of calcic amphibole by reaction with biotite and quartz in tonalitic granitic gray gneiss produced the regional orthopyroxene isograd manifest in charnockitic mottling and veining of mixed facies exposures as at kabbal karnataka and in the kurunegala district of the sri lanka central highlands chemical and modal analyses of carefully chosen immediately adjacent amphibole gneiss and charnockite pairs show that the orthopyroxene is produced by an open system reaction involving slight losses of cao mgo and feo and gains of sio2 and na2o rb and y are depleted in the charnockite another kind of charnockitization is found in paragneisses throughout the southern high grade area and involves the reaction of biotite and quartz garnet to produce orthopyroxene and k feldspar although charnockite formation along shears and other deformation zones at such localities as ponmudi kerala is highly reminiscent of kabbal close pair analyses are not as suggestive of open system behavior this type of charnockite formation is found in granulite facies areas where no prograde amphibole bearing gneisses exist and connotes a higher grade reaction than that of the orthopyroxene isograd metamorphic conditions of both kabbaltype and ponmudi type localities were 700 800 c and 5 6 kbar lower p h2o in the ponmudi type metamorphism was probably the definitive factor

along strike variations in the composition of sandstones derived from the uplifting western greater caucasus causes and implications for reservoir quality prediction in the eastern black sea oligo miocene outcrops along the southern margin of the western greater caucasus preserve a record of sediments shed from the range into the northern and central parts of the eastern black sea sandstones in the russian western caucasus are significantly more quartz rich than those located farther se in western georgia the latter contain appreciably more mudstone and volcanic rock fragments oligo miocene turbidite systems derived from the russian western caucasus in the tuapse trough and central eastern black sea may therefore form better quality reservoirs at shallow to moderate depths than sediments derived from west georgian volcaniclastic sources in the easternmost part of the basin palynomorph analysis indicates sediment derivation predominantly from jurassic and cretaceous strata in the russian western caucasus and from eocene strata and an increasing proportion of cretaceous strata upsection in western georgia an eocene volcaniclastic source is proposed for the increased rock fragment component in west georgian sandstones eocene volcaniclastic rocks are no longer exposed in the greater caucasus but similar rocks form the inverted fill of the adjara trialet basin farther south in the lesser caucasus the former presence of a northern strand of this basin in the west georgian caucasus is supported by earlier thermochronological work

age and tectonic setting of granitoid gneisses in the eastern desert of egypt and south west sinai strongly deformed and locally migmatized gneisses occur at several places in the southern eastern desert of egypt and in sinai and have variously been interpreted as a basement to pan african 900 to 600 ma supracrustal and intrusive assemblages a suite of grabbroic to granitic gneisses was investigated in the hafafit area which constitutes an i type calc alkaline intrusive assemblage whose chemistry suggests emplacement along an active continental margin and whose granitoid members can be correlated with the so called older granites of egypt 207pb 206pb single zircon evaporation from three samples of the hafafit gneisses yielded protolith emplacement ages between 677 9 and 700 12 ma and document granitoid activity over a period of about 23 ma a migmatitic granitic gneiss from wadi bitan south west of ras banas has a zircon age of 704 8 ma and its protolith was apparently generated during the same intrusive event as the granitoids at hafafit single zircons from a dioritic gneiss from wadi feiran in south west sinai suggest emplacement of the protolith at 796 6 ma and this is comparable with ages for granitoids in north east sinai and southern israel none of the above gneisses is derived from remelting of older continental crust but they are interpreted as reflecting subduction related calc alkaline magmatism during early pan african magmatic arc formation

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

facies specific calcitic and bimineralic ooids from middle and upper cambrian platform carbonates western newfoundland canada abstract oolites in the port au port group consist of intercalated gray oolite deposited on the shallow subtidal mobile fringe of ooid sand shoals and brown oolite which accumulated on intertidal sand flats along the shoal crests gray oolite is composed of finely and coarsely preserved radial ooids that were precipitated and deposited in situ and whose fabric was controlled in large part by suspension versus bedload transport brown oolite is composed of concentric ooids or superficial radial ooids that were either formed and deposited in situ or swept onto the sand flats from the mobile fringe the present fabric of the ooids is the result of protracted burial diagenesis cortices of finely and coarsely preserved radial ooids excluding radial concentric ones and superficial radial ooids were mg calcite originally they changed to calcite still retaining fine fabric and subsequently to a variety of cortical fabrics during shallow burial and altered to blocky fabrics during deep burial concentric ooids and radial concentric ooids were bimineralic originally composed of aragonite now micrite and blocky calcite and mg calcite now radial fibrous calcite primary aragonitic laminae were partially or completely dissolved and crushed during shallow burial resulting voids were filled with blocky calcite during deep burial correlation of original ooid mineralogy fabric and environment of formation indicates that local physical conditions such as turbulence influenced ooid formation in cambrian time this correlation is absent in modern deposits suggesting that more widespread chemical and organic controls now prevail

the expected duration of upward shallowing peritidal carbonate cycles and their terminal hiatuses meter scale accommodation cycles which dominate the stratigraphic record of peritidal carbonate accumulation might provide a time scale of unusually high resolution but the duration of the hiatuses that cap the resulting cyclothems is difficult to determine thousands of rates of peritidal accumulation from many different carbonate sections have been combined according to the length of the time span of measurement to describe a hypothetical average section because all sections are interrupted by hiatuses the average accumulation rate falls progressively with increasing time span and most steeply at the time spans that capture the highest proportion of hiatuses the process of combining rates from many sections cancels local differences in the distribution of hiatuses only hiatuses that have similar spacing and duration in a majority of sections will stand out after compilation the size and position of a pronounced inflection in the combined plot of accumulation rates against time span identifies a dominant cycle period of 100 000 yr of which the terminal hiatus accounts for 80 90 the expected cyclothem is 10 m thick because peritidal accommodation cycles begin and end with the sediment surface at sea level the expected cycle period can be estimated by finding the time span at which accommodation and accumulation rates balance the maximum duration of the hiatus is the time span at which rates of subsidence and sea level fall balance compilations of many empirical measurements were again employed to determine the relationship between time span and the rates of subsidence sea level change and net accommodation this second method confirms the results of the first and can be modified to predict the expected cyclothem in different climatic tectonic and depositional settings because quaternary data dominate short term rates the hypothetical average section best describes a passive margin in an icehouse climate with strong glacio eustatic fluctuations in sea level after modification for an extreme greenhouse climate devoid of ice caps the second method predicts that 20 000 yr accommodation cycles will be preferentially recorded the expected cyclothem is 2 m thick and accounts for about 50 of the cycle period increased subsidence rates whether on active margins in the initial phase of passive margin formation or at the seaward edge of a platform tend to extend the limit of cycle duration increase cyclothem thickness and partition less time to the bounding hiatus increased subsidence however raises the risk that the sediment surface will drown

age constraints on intra formational unconformities in upper jurassic lower cretaceous carbonates in northeast turkey geodynamic and hydrocarbon implications abstract upper jurassic lowermost cretaceous carbonate build ups are imaged on seismic data in the black sea they form important untested hydrocarbon reservoirs that are the focus of active exploration outcrop analogues to these build ups around the black sea contain a series of subaerial exposure surfaces the hiatuses associated with a number of these subaerial exposure surfaces have been dated in a well exposed callovian or upper oxfordian to barremian shallow water inner platform carbonate succession the berdiga formation in the eastern pontides using strontium isotope stratigraphy and foraminiferal biostratigraphy they span the latest kimmeridgian to tithonian or berriasian and the hauterivian to barremian less well constrained but broadly contemporaneous stratigraphic gaps in multiple successions around the black sea provide additional insights and point to a regional driving mechanism the timing of hiatus formation does not correspond to periods of eustatic lowstand it does coincide however with late tithonian to berriasian and hauterivian to early aptian episodes of rifting in the greater caucasus basin located farther to the north thus it is possible that subaerial exposure was caused by rift flank uplift during periods of regional extension uplift due to slab break off is discounted as a control because it post dates rather than pre dates locally developed kimmeridgian magmatism rift flank uplift is likely to have also affected carbonate build ups on the intervening rift shoulders of the eastern black sea the shatskiy ridge and the mid black sea high at outcrop subaerial exposure is often associated with karstification and secondary porosity development similar processes may have occurred in the offshore helping to enhance the reservoir quality of these exploration targets

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

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

whole grain evaporation for 207 pb 206 pb age investigations on single zircons using a double filament thermal ion source a technique has been developed and tested to analyse 207pb 206pb apparent ages by thermal evaporation of radiogenic lead directly from untreated whole zircon grains 0 3 mm the evaporation analyses are performed in the double filament arrangement of a thermal ion mass spectrometer thims the method is a powerful tool to distinguish between different lead components occurring in the same grain because differing activation energies of the competing lead components cause their sequential evaporation from the zircons the evaporation of test samples results in 207pb 206pb apparent ages in good agreement with u pb ages known from literature single zircons from a granite of the marble mountains california yield an age of crystallization of 1 410 30 ma ceylon zircons from heavy mineral bearing gravels yield 560 40 ma as age of crystallization of the pegmatitic gravel sources individuals from a heterogeneous zircon population of a diatexite from the southern schwarzwald sw germany indicate metamorphic zircon formation around 500 ma and the existence of middle proterozoic relics 1 95 0 05 ga

phanerozoic history of the mahanadi region india the mahanadi basin is a nw se stretching gondwana basin perpendicular to the east coast of india the metamorphic basement to the basin mainly comprises rocks of the eastern ghats mobile belt egmb which experienced a complex thermotectonic evolution involving several episodes of deformation and magmatism it can be inferred from 40ar 39ar ages on hornblende 845 ma and biotite 622 and 593 ma from basement rocks that the last high grade metamorphism in the western part of the egmb i e the western khondalite zone occurred during the late grenvillian with any pan african overprint only reaching anchimetamorphic conditions only pseudotachylite intrusions and their related thermal imprint with ages around 515 ma display a direct pan african signature fission track analyses on nine zircon 419 313 ma and 49 apatite 443 119 ma samples indicate a rift related formation of the mahanadi basin as an asymmetrical half graben in two stages during the permo carboniferous and the middle triassic moreover the gondwana breakup in eastern india is signified by dyke intrusions and the thermal overprint of thrusts and faults in the mahanadi region and the onset of ocean floor production in the mahanadi offshore basin at around 120 ma

african linkage of precambrian sri lanka new age and isotopic data show that the high grade basement rocks of sri lanka were not linked to the archaean granulite domain of southern india but experienced their main structural and metamorphic development during the pan african event some 950 to 550 ma ago this occurred when west gondwana and east gondwana collided to form one of the longest collisional structures in the supercontinent the mozambique belt that extends from mozambique to ethiopia and sudan a major tectonic boundary interpreted as a thrust zone divides the highland southwestern complex in the central part of sri lanka from the vijayan complex in the e and se the former is interpreted to represent the remnant of a once extensive passive margin extending west in a gondwana reconstruction via madagasgar to tanzania and mozambique the vijayan complex may have been part of a separate continental margin plutonic assemblage and its collision with the highland southwestern complex marks the final amalgamation of east and west gondwana into a supercontinent some 550 ma ago the sri lankan granulites cannot be correlated with the distinctly older granulites of the eastern ghats belt of india and this suggests that sri lanka was situated close to the se coast of madagascar in a gondwana reconstruction

vein graphite deposits geological settings origin and economic significance graphite deposits result from the metamorphism of sedimentary rocks rich in carbonaceous matter or from precipitation from carbon bearing fluids or melts the latter process forms vein deposits which are structurally controlled and usually occur in granulites or igneous rocks the origin of carbon the mechanisms of transport and the factors controlling graphite deposition are discussed in relation to their geological settings carbon in granulite hosted graphite veins derives from sublithospheric sources or from decarbonation reactions of carbonate bearing lithologies and it is transported mainly in co2 rich fluids from which it can precipitate graphite precipitation can occur by cooling water removal by retrograde hydration reactions or reduction when the co2 rich fluid passes through relatively low fo2 rocks in igneous settings carbon is derived from assimilation of crustal materials rich in organic matter which causes immiscibility and the formation of carbon rich fluids or melts carbon in these igneous hosted deposits is transported as co2 and or ch4 and eventually precipitates as graphite by cooling and or by hydration reactions affecting the host rock independently of the geological setting vein graphite is characterized by its high purity and crystallinity which are required for applications in advanced technologies in addition recent discovery of highly crystalline graphite precipitation from carbon bearing fluids at moderate temperatures in vein deposits might provide an alternative method for the manufacture of synthetic graphite suitable for these new applications

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

chronology of early archaean granite greenstone evolution in the barberton mountain land south africa based on precise dating by single zircon evaporation abstract we report precise 207 pb 206 pb single zircon evaporation ages for low grade felsic metavolcanic rocks within the onverwacht and fig tree groups of the barberton greenstone belt bgb south africa and from granitoid plutons bordering the belt dacitic tuffs of the hooggenoeg formation in the upper part of the onverwacht group yield ages between 3445 3 and 3416 5 ma and contain older crustal components represented by a 3504 4 ma old zircon xenocryst fig tree dacitic tuffs and agglomerates have euhedral zircons between 3259 5 and 3225 3 ma in age which we interpret to reflect the time of crystallization a surprisingly complex xenocryst population in one sample documents ages from 3323 4 to 3522 4 ma we suspect that these xenocrysts were inherited during the passage of the felsic melts to the surface from various sources such as greenstones and granitoid rocks now exposed in the form of tonalite trondhjemite plutons along the southern and western margins of the bgb and units predating any of the exposed greenstone or intrusive rocks several of the granitoids along the southern margin of the belt have zircon populations with ages between 3490 and 3440 ma coeval with or slightly older than onverwacht felsic volcanism while the kaap valley pluton along the northwestern margin of the belt is coeval with fig tree dacitic volcanism these results emphasize the comagmatic relationships between greenstone felsic volcanic units and the surrounding plutonic suites some of the volcanic and plutonic units contain zircon xenocrysts older than any exposed rocks these indicate the existence of still older units possibly stratigraphically lower and older portions of the greenstone sequence itself older granitoid intrusive rocks or bodies of older unrelated crustal material our data show that the onverwacht and fig tree felsic units have distinctly different ages and therefore do not represent a single tectonically repeated unit as proposed by others unlike the late archaean abitibi greenstone belt in canada which formed over about 30 ma exposed rocks in the bgb formed over a period of at least 220 ma the complex zircon populations encountered in this study imply that conventional multigrain zircon dating may not accurately identify the time of felsic volcanic activity in ancient greenstones a surprising similarity in rock types tectonic evolution and ages of the bgb in the kaapvaal craton of southern africa and greenstones in the pilbara block of western australia suggests that these two terrains may have been part of a larger crustal unit in early archaean times

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

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

calcitic aragonitic and mixed calcitic aragonitic ooids from the mid proterozoic belt supergroup montana a great variety of ooid types occurs within the siyeh and snowslip formations of the mid proterozoic belt supergroup montana cortical layers are inferred to have been composed either of calcite in a radial concentric or radial with dark rays fabric or aragonite in a radial or concentric fabric the calcitic cortical layers record their original fabrics but the originally aragonitic cortical layers have been replaced by calcite in a range of textures and by quartz and dolomite some formerly aragonitic cortical layers are replaced by calcite spar which contains relics of the original cortical structure others consist of calcite spar without inclusions or columnar calcite which grew radially from the nucleus commonly a calcitic ooid some ooids were wholly composed of calcite others were of aragonite but two phase ooids were common mostly consisting of an inner calcitic part and an outer aragonitic part probable microdolomite inclusions suggest a high mg content of the calcitic cortical layers the depositional environment of these oolites was probably analogous to baffin bay texas where a similar range of ooid types is forming today

active ooid growth driven by sediment transport in a high energy shoal little ambergris cay turks and caicos islands ooids are a common component of carbonate successions of all ages and present significant potential as paleoenvironmental proxies if the mechanisms that control their formation and growth can be understood quantitatively there are a number of hypotheses about the controls on ooid growth each offering different ideas on where and how ooids accrete and what role if any sediment transport and abrasion might play these hypotheses have not been well tested in the field largely due to the inherent challenges of tracking individual grains over long timescales this study presents a detailed field test of ooid growth hypotheses on little ambergris cay in the turks and caicos islands british overseas territories this field site is characterized by westward net sediment transport from waves driven by persistent easterly trade winds this configuration makes it possible to track changes in ooid properties along their transport path as a proxy for changes in time ooid size shape and radiocarbon age were compared along this path to determine in which environments ooids are growing or abrading ooid surface textures petrographic fabrics stable isotope compositions 13 c 18 o and 34 s lipid geochemistry and genetic data were compared to characterize mechanisms of precipitation and degradation and to determine the relative contributions of abiotic e g abiotic precipitation physical abrasion and biologically influenced processes e g biologically mediated precipitation fabric destruction through microbial microboring and micritization to grain size and character a convergence of evidence shows that active ooid growth occurs along the transport path in a high energy shoal environment characterized by frequent suspended load transport median ooid size increases by more than 100 m and bulk radiocarbon ages decrease by 360 yr westward along the 20 km length of the shoal crest lipid and 16s rrna data highlight a spatial disconnect between the environments with the most extensive biofilm colonization and environments with active ooid growth stable isotope compositions are indistinguishable among samples and are consistent with abiotic precipitation of aragonite from seawater westward increases in ooid sphericity and the abundance of well polished ooids illustrate that ooids experience subequal amounts of growth and abrasion in favor of net growth as they are transported along the shoal crest overall these results demonstrate that in the ambergris system the mechanism of ooid growth is dominantly abiotic and the loci of ooid growth is determined by both carbonate saturation and sediment transport mode microbes play a largely destructive rather than constructive role in ooid size and fabric

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