chemistry of hydrothermal fluids from the 17 s active site on the north fiji basin ridge sw pacific abstract in june 1989 a diving cruise of the french submersible nautile was made along the north fiji basin spreading ridge the objective was the geological and geochemical study of the active spreading axis and of the hydrothermal processes discovered during the kaiyo cruise in 1988 active anhydrite chimneys associated with underlaying sulfide deposits were studied in the central spreading ridge the transparent venting fluid showed significant salinity depletion the extrapolated concentrations of all the chemical species in the pure end member showed a signature similar to that of the cl depleted hydrothermal fluid from the ashes vent site in the juan de fuca ridge these features suggest that the present hydrothermal end member resulted from subcritical phase separation which generated a low salinity vapor phase followed by condensation and mixing with normal seawater in the last part of the plumbing system of the vents

nd sr isotopic composition of granulites and constraints on the evolution of the lower continental crust in a statistical study of granulite facies rocks we have analysed 32 samples with ages ranging from 2 900 to 200 myr for sm nd rb and sr and 143nd 144nd and 87sr 86sr ratios the sm nd ratio is found to be constant compared with the highly variable rb sr ratio using the model age formalism we show that in many cases internal differentiation of continental crust follows external differentiation with a large time interval

volcanic episodicity and a non steady state rift valley along northeast pacific spreading centers evidence from sea marc i sea marc i side looking sonar images and sea beam bathymetry along a 400 km stretch of the juan de fuca ridge crest provide evidence that excessive extrusive volcanism periodically builds a crestal ridge along the axis of seafloor spreading an elongate summit depression esd or rift valley is commonly observed in the spine of this crestal ridge the crestal ridge volcanic landform has a distinctive shape that is recognized in bathymetric contours both along the spreading axis and at least up to 30 km away from the axis the landform has a plan form shape that resembles a side view of an archer s bow with the long dimension of the bow form parallel to the strike of the ridge in cross section the bow form is flat on top and has steep flanks these bow form shapes can be explained by magma that rises into the crust at a discrete center and flows laterally into belts of ridge parallel dikes similar to icelandic fissure eruptions both the variable dimensions of the esd along axis of the juan de fuca ridge and the relationship among volcanic flow morphologies within and beyond the esd suggest the four different segments of the juan de fuca ridge presented in detail here display different stages in a cycle of oceanic crust accretion this cycle includes episodes in which there is 1 extrusive volcanic construction which widen the crestal ridge prior to the collapse of the summit depression 2 collapse within the summit region of the crestal ridge to form an esd during a phase of volcanic inactivity and 3 renewed magmatism in the esd as its floor widens by extension and brittle fracture of the upper crust this episodic model implies that the width of the young seafloor affected by volcanic extrusion or dominated by tectonic stretching varies through time

seafloor spreading in the north fiji basin southwest pacific abstract the seapso leg 3 cruise december 1985 was carried out in the north fiji basin aboard the ifremer r v jean charcot the main purpose of the cruise was to carry out a geophysical and geological study of the central part of the basin between 16 s and 22 s to this end bathymetric profiles with the help of the seabeam multichannel echosounder single channel seismic reflection profiles and magnetic and gravimetric profiles were made moreover geological samples were taken by dredging and piston coring as well as water samples with the help of a multiprobe one of the aims of the cruise was to explore the present day spreading axis of the north fiji basin we were able to show that accretion the rate of which is about 7 cm yr in the north fiji basin has been constrained in a n s direction for 3 ma very recently between 0 6 and 0 7 ma ago the accretion process changed resulting in the creation of a ridge ridge trench triple junction centered at 173 30 e and 16 40 s water samples taken in the vicinity of this triple junction show significant anomalies of methane and manganese which are probably linked to hydrothermal events in its morphological characteristics transversal as well as longitudinal the ridge of the north fiji basin shows striking similarities to the east pacific rise it presents the same differences in level the same sizes of structures and the same particular features overlapping spreading centers offsets etc another aim of the seapso cruise was to study the complex deformation area located immediately to the west of the fiji islands which we interpret as a distensive type deformation in a strike slip system

lead isotopic compositions of the western dharwar craton southern india evidence for distinct middle archean terranes in a late archean craton abstract lead and sr isotopic compositions of whole rocks and feldspar separates from several fielddefined suites of rocks from the western dharwar craton of southern india elucidate a complex history of the area the evolution of the craton earlier than 3 0 ga is difficult to determine because of thermal and metasomatic overprinting at 3 0 ga many whole rocks were completely reset then but feldspar separates preserve some of the earlier history apparently a series of terranes evolved from the mantle over the period 3 4 to 3 0 ga to form the western dharwar craton in each case metamorphism and anatexis of older terranes accompanied extraction of juvenile material from the mantle the oldest gneisses 3 4 ga had radiogenic pb and sr isotopic compositions when they formed and were derived from older crustal protoliths that were plausibly accreted to the crust at or even before 3 8 ga the old gneisses were greatly depleted in rb probably at 3 4 ga when the protoliths of younger magmatic rocks may have separated from the mantle younger gneisses apparently formed from these mafic intermediate protoliths in the age range of 3 2 to 3 0ga the 3 0 ga event in the central part of the craton was accompanied by intrusion of small diapiric trondhjemites formed by melting of older crust and penecontemporaneous metasomatism of country rocks by hydrous fluids carrying u and other lithophilic elements the resultant deep crust was greatly depleted in heat producing elements and the craton has been inactive since 3 0 ga except for production of minor granites from depleted source regions at 2 5 2 6 ga the lack of activity at 2 5 ga contrasts strongly with extensive magmatism and metamorphism in surrounding areas that had not undergone depletion at 3 0 ga lower crustal depletion also has caused present day reduced heat flow to be very low

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

proterozoic to mesozoic east gondwana the juxtaposition of india sri lanka and antarctica jurassic and cambro ordovician paleomagnetic pole positions deduced from rocks of sri lanka and antarctica generally conform when sri lanka is juxtaposed with the coast of lutzow holm bay east antarctica in a manner similar to a current gondwana reassembly the highland and southwest groups mostly proterozoic of sri lanka are correlated with the ongul and skallen groups of east antarctica on the basis of similar lithology structural characteristics structural trend tectonothermal history ranging from early proterozoic to mesozoic and the pressure temperature time path of the main metamorphism some neighboring lithotectonic units are also comparable between the two areas fracture lineament systems of sri lanka and antarctica formed after the latest silurian and during or before the jurassic conform in this reconstruction an improved fit of the 2000 m isobaths of india sri lanka antarctica and continuation of the latest archaean mobile belt from enderby land to peninsular india is obtained by fitting sri lanka into lutzow holm bay and enderby land into the embayment in the east coast of peninsular india this juxtaposition of peninsular india to east antarctica involves about 80 km northward movement and about 30 clock wise rotation of sri lanka relative to india the reassembly thus obtained accords well with data supporting the idea that east gondwana existed from the earliest proterozoic to middle mesozoic although some disruption collision events of limited scale are considered possible

east pacific rise from siqueiros to orozco fracture zones along strike continuity of axial neovolcanic zone and structure and evolution of overlapping spreading centers we have used the sea beam multibeam echo sounding system to survey the east pacific rise epr from 8 20 n to 18 30 n obtaining complete coverage of the epr axial neovolcanic zone and all intervening transform faults here we focus on the epr neovolcanic zone and the definition of overlapping spreading centers ocs s between the orozco and siqueiros transform faults the neovolcanic zone is narrow 0 5 2 0 km and continuous along strike and occurs within and near a strike continuous axial summit graben the neovolcanic zone and summit graben reside along the crest of a volcanic axial high which is 2 10 km wide and which continues uninterrupted along strike for 40 140 km within 20 50 km of an intersection with a transform fault the axial neovolcanic zone narrows and deepens and within only 3 6 km of the intersection the neovolcanic zone turns sharply into the transform valley at seven locations between the siqueiros and orozco transform faults the axial neovolcanic zone is discontinuous and is laterally offset a short distance 1 15 km in contrast with a classic ridge transform ridge plate boundary however offset rise terminations overlap each other by a distance approximately 3 times greater than their offset they curve toward each other and often one merges into the other along strike separating the osc s is an elliptical overlap basin up to 600 m deep whose long axis is subparallel to the adjacent spreading centers the overlap basin is characterized by volcanic constructional edifices a key difference between these offsets and small transform faults in the atlantic is that there are no transform fault structures in the overlap basin which link the offset rise axes a continuous axial depth profile reveals a long wavelength 20 60 km undulation of the rise axis which may be associated with variations in the magmatic budget along strike the lowest points in the axial depth profile occur near transform faults and at osc s suggesting that these features are associated with along strike minima in the magmatic budget we suggest that major phases of volcanic activity propagate episodically along the axis away from centers of magmatic inflation and that osc s develop where magmatic pulses fail to meet due to misalignment of fracture systems sinuous bends in the rise axis or other heterogeneities transform faults fail to develop on fast spreading centers where the lateral offsets are small 15 km because the lithosphere is too thin and weak to maintain a classic rigid plate ridge transform fault pattern the osc geometry is unstable and evolves rapidly and the overlapping rises may propagate at rates faster than the local spreading rate one of the two osc s prevails while the other is abandoned while the locations of osc s may not be fixed relative to the rise axis they may tend to recur near the same place creating scars in older lithosphere which resemble fracture zones but whose fine scale structure is quite different there is excellent agreement between the observed structure of osc s and laboratory studies of osc s using wax models photoelastic studies of overlapping cracks and boundary element modeling of overlapping cracks seismic refraction and reflection near two osc s suggests that large offset osc s have separate magma chambers beneath the offset rise axes while small offset osc s may share a single axial magma chamber three dimensional inversion of magnetic anomalies suggests that significant petrologic anomalies may occur near some osc tips waxing and waning of overlapping magma chambers may give rise to the eruption of highly fractionated feti basalts and associated magnetic anomalies

diapiric trondhjemites of the western dharwar craton southern india diapiric comparatively massive trondhjemites were emplaced in the western part of the dharwar craton of southern india about 3000 ma ago this age coincides with the age of i closure of rb sr systems that now form the youngest isochrons in the predominantly gneissic terrane and ii metasomatic enrichment of the gneisses in u younger events principally about 2500 ma ago are recorded by sparse granites and by deformation of supracrustal sequences without major metamorphism thus the trondhjemites appear to have formed during the last extensive thermochemical event in the craton the trondhjemites are not associated with any mafic rocks and show very little fractionation within and among the various bodies presumably they formed as a single product of partial melting likely source materials for the magmas are rocks of basaltic composition probably amphibolites in the lower crust or along the crust mantle boundary very low zr and high cr contents in the trondhjemites may indicate a slightly ultra

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

geochemistry of high grade supracrustal rocks from the kerala khondalite belt and adjacent massif charnockites south india abstract the kerala khondalite belt kkb comprises a suite of supracrustal rocks of probable proterozoic age that was metamorphosed to the granulite facies the kkb consists of an interlayered sequence of 1 garnet biotite orthopyroxene graphite gneisses 2 khondalites graphite garnet biotite sillimanite cordierite gneisses and 3 cordierite gneisses garnet biotite cordierite orthopyroxene as well as less abundant mafic granulites calc silicates and quartzites petrologic and geochemical data suggest that these rocks were originally arkosic and argillaceous sediments possibly accompanied by felsic volcanic rocks along with minor intercalations of basalts carbonates and quartz rich sandstones the ree patterns of the metasediments are typically lree enriched with large negative eu anomalies indicating a continental provenance the major and trace element compositions of some of the arkosic rocks of the kkb resemble those of the massif charnockites that bound the kkb to the north which suggests that the massif charnockites may have been the source of some of the kkb sediments modern day analogues of the pelite arkose lithologic association observed in the kkb occur in intracratonic rifts and rifted continental margins this association is clearly distinct from the graywacke mafic volcanic association found in the southern indian greenstone belts and many other lower grade archean sedimentary sequences the kkb supracrustals show both lithologic and chemical similarities to many proterozoic supracrustal successions

geology and geochemistry of a 800 m section through young upper oceanic crust in the north fiji basin southwest pacific lagabrielle y auzende j m eissen j p janin m c and cotten j 1994 geology and geochemistry of a 800 m section through young upper oceanic crust in the north fiji basin southwest pacific in j m auzende and t urabe editors north fiji basin starmer french japanese program mar geol 116 113 132 we report the results of geological and structural observations made during one dive of the french submersible nautile during the starmer cruise in 1989 in the vicinity of the 16 40 s triple junction in the north fiji basin this dive provided a spectacular 800 m continuous section of oceanic crust exposed along the northern wall of a 3000 m deep basin at the eastern branch of the triple junction a number of volcanic facies have been encountered including massive and pillowed lavas sheeted dikes and spectacular columnar jointed massive lavas pillowed lavas have been observed only at the base and at the top of the section the ratio between massive and pillowed basalts is high suggesting that the crust in this area grew mostly during a stage of high eruptive rate dikes have been observed in the middle part of the section they probably do not represent the top of a classical dike complex but an isolated set of sheeted dikes possibly the feeders of overlying flows the abundance of vertical tectonic breccias observed during the first part of the dive confirms that the basal part of the wall can be regarded as a major tectonic boundary along which occurred significant vertical and strike slip motions micropaleontological data from sedimentary rocks collected during the dive bring new constraints to the evolution of the triple junction the oceanic crust in the surveyed area is at least as old as 1 9 1 3 ma based on the age of sedimentary rocks collected in talus at the base of the wall and observed at the summit of the section the collected basalts include 1 n type morbs 2 e type morbs and 3 babbs this emphasizes the heterogeneous nature of the mantle beneath the central nfb some portions of the depleted mantle underlying the central nfb triple junction area have recorded contamination from melts or fluids from an ancient subduction zone probably the new hebrides during the opening of the nfb whereas adjacent mantle areas show the influence from an alkali enriched source such alkali enriched characters are reported from recent basalts of the northern nfb south pandora ridge rotuma island

double saloon door tectonics in the north fiji basin abstract fiji platform rotated counter clockwise from at least 10 2 ma until 1 56 ma while vanuatu arc rotated clockwise from 12 10 ma until the present a revised model incorporating these opposite rotations explains the distribution of magnetic anomalies in the north fiji basin nfb the conjugate margin of southwest fiji platform is southeast vanuatu arc previous models which associate nw oriented anomalies off southwest fiji with similarly oriented anomalies northeast of vanuatu arc are therefore wrong secondly these models propose a nw oriented spreading ridge extending to 19 s 177 5 e almost 500 km southeast of the mapped extension of rifted island arc crust on vanuatu arc this creates an unacceptable overlap with fiji platform in pre rift reconstructions thirdly anomalies off sw fiji which are nw oriented in their present day position were oriented nne during initial breakup rather than aligning with nw oriented anomalies in the western nfb they are matched by nne oriented anomalies off se vanuatu arc with further rotation these conjugate anomaly sets form a fan shape in the southern nfb fourthly previous models which recognise a triple junction only from 3 ma do not explain early wnw ese separation of vanuatu arc and fiji platform required by well documented opposite rotations nfb characteristics which match the double saloon door tectonic model include opposite rotations of island arc terranes backarc seafloor spreading which is both arc parallel and arc perpendicular and rifts propagating north south northeast and northwest features which do not match the double saloon door model include the north fiji fracture zone and the west fiji spreading centre both initiated post 1 5 ma when fiji platform stopped rotating and only one terrane vanuatu arc continued to rotate the nfb developed via double saloon door tectonics from 12 10 ma until 1 5 ma whereas post 1 5 ma opening constitutes single saloon door rotation

recent geodynamic evolution of the north fiji basin southwest pacific a cruise of the r v jean charcot seapso iii december 1985 in the north fiji basin between 16 and 22 s obtained new structural and magnetic data concerning the central part of the basin these have been used for the reinterpretation of the national oceanic and atmospheric administration and u s naval research project aeromagnetic survey 1979 the structural surface ship magnetic and aeromagnetic data allow us to present a sketch of the evolution in four stages of the north fiji basin for the past 10 m y

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

in situ geological and geochemical study of an active hydrothermal site on the north fiji basin ridge abstract in june 1989 a diving cruise by the submersiblenautile was carried out on the north fiji basin spreading ridge the objective of the cruise was a geological and geochemical study of the active spreading axis and associated hydrothermal processes this operation was the third cruise of the french japanese starmer project following the two cruises of the r v kaiyo in 1987 and 1988 six dives along the spreading axis between 16 58 s and 17 00s show that the axial graben consists of alternating n15 trending horsts and grabens extinct hydrothermal sites have been observed all along the graben these consist of fossil chimneys oxide staining and dead shells at 16 59 s an active chimney has been discovered and called the white lady because of its almost exclusively anhydrite composition this chimney expels a peculiar water characterized by low chlorinity and a 285 c maximum temperature in the northern extremity of the n15 axis a wide fossil hydrothermal site has been explored and sampled it is located in an area cut by n15 n140 and n60 trending faults and fissures

pre breakup continental extension in east gondwanaland and the early opening of the eastern indian ocean abstract extension of continental crust by up to 360 km on a north northeast azimuth in the great australian bight occurred prior to the middle cretaceous 96 ma onset of seafloor spreading between australia and antarctica this large amount of continental extension is constrained to the late jurassic 160 ma middle cretaceous interval about a pole estimated to lie about 40 southeastward of southeastern australia using bathymetric data combined with seismic and magnetic determinations of the continent ocean boundaries off australia india and antarctica we determine a revised fit of east gondwanaland prior to continental extension and at various stages thereafter in the first stage from 160 to 132 5 ma m11 india australia rotated from antarctica with continental crustal extension between australia and antarctica and largely transform motion between the coromandel coast margin of india and the kron prins olav kyst margin of antarctica in the second stage from 132 5 to 96 ma india rotated northwestward away from australia antarctica producing m10 and younger magnetic anomalies along the western margin of australia continental extension was greatest between australia and antarctica during this stage the newly determined 132 5 96 ma rotation between india and australia provides an improved fit between the orientation of observed m10 m0 magnetic anomalies and the southward decrease in spreading rate off the western australian margin together with bathymetry the new rotation parameters lead to identification of the abandoned 96 ma spreading ridge along the lost dutchmen and dirck hartog ridges the naturaliste fracture zone is found to lie nearly parallel to the early cretaceous transform direction between india and antarctica suggesting that slow seafloor spreading required between australia and antarctica to accommodate the continental extension further east occurred between naturaliste fracture zone and the m4 m0 anomalies adjacent to the naturaliste plateau the revised fit of india in east gondwanaland reduces the eastward extent of greater india to 95 e but does not change its extent as far north as the cape range fracture zone the new pole determined for the early cretaceous rotation between india and australia antarctica lies within 15 of the southern tip of india significantly closer than in previous determinations thereby reducing the amount of seafloor spreading generated between africa and antarctica by the india antarctica rotation and thus offering a possible solution to the perceived spreading rate problems between m11 and m0 in the somali mozambique and antarctic basins

kinematics of active spreading in the central north fiji basin southwest pacific abstract based on a synthesis of magnetic and bathymetric data we re evaluate the kinematics of the recent opening of the central part of the north fiji basin nfb the westward motion of the pacific plate along the left lateral north fiji fracture zone nffz results in the opening of two n s trending spreading ridges located at 173 30 e and 176 e both ridges show complex features such as propagating rifts ridge jumps and overlapping spreading centres their spreading rates are similar 7 6 to 4 cm yr 1 across the western ridge 5 5 cm yr 1 across the eastern one while the nffz is purely strike slip to the east of the eastern ridge it becomes more complex to the west it changes from transpressional to transtensional then to purely transform and finally joins the western n s ridge in a rrr type triple junction our kinematic analysis shows that most of the left lateral motion along the nffz is transferred to the eastern ridge at the rtf type 176 e triple junction it suggests that the western n s ridge is probably connected to the north with another left lateral transform possibly the south pandora ridge

an example of a recent accretion on the mid atlantic ridge the snake pit neovolcanic ridge mark area 23 22 n abstract the inner rift vailey of the mark area mid atlantic ridge at the kane fracture zone was investigated with the submersible nautile at the latitude of the snake pit hydrothermal field 23 22 n a transect across the axial valley shows a stepwise increase in the sediment thickness away from the axis these steps probably reflect major magmatic and tectonic events in the evolution of the inner floor dives on the summit of the snake pit neovolcanic ridge constrain a generalized geological map and document relations between hydrothermal activity magmatism and faulting the neovolcanic ridge was constructed by the build up of several hundred metres of pillow lava flows its summit is intensely fissured and cut by an axial graben about 100 m wide three hydrothermal sites were recognized within the axial graben the eastern site the most active is structurally related to the eastern fault scarp the central site is located within the graben the western site associated with the western fault scarp is more complex and was constructed by two successive hydrothermal events the new data suggest a model of evolution for the inner floor of the rift valley involving alternating stages of magmatism and volcanism which produce ridges and troughs preserved outside of the axial domain these volcano tectonic cycles have an approximate duration of 100 000 years we discuss variations in the morphology of the mar along strike in the light of this model of evolution

role of microfabrics and magnetic fabrics in the tectonic evolution of the achankovil shear zone south india abstract achankovil shear zone a major crustal discontinuity in south india which extends for about 120 km in a nw se direction separates the khondalite belt from the terrain of massive charnockites in the north progressive mylonitisation and shear fabrics corresponding to a progression in strain as displayed by grain size reduction flattening elongation and dimensional preferred orientation of constituent minerals characterize the shear zone the present work incorporates the results of petrofabric and magnetic susceptibility analyses with a view to interpret the kinematic and microstructural evolution of the shear zone ebsd derived lpo suggests plastic deformation through the activation of intracrystalline slip systems and modifications in the fabric due to the reactivation magnetic fabric data show significant correlation with the field and lpo fabrics reflecting the regional strain kinematics and fabric development the fabrics and kinematic indicators on meso and microscopic scales reveal dominant dextral shear sense with a minor component of sinistral shearing in micro domains due to strain partitioning and or reactivation of the shear zone

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

along axis variations in seafloor spreading in the mark area recent investigations with the manned submersible alvin and the angus deep towed camera sled greatly extended the known range of variations in the style of seafloor spreading along the axis of the mid atlantic ridge five transects of the spreading centre at intervals of10 20 km south of the kane fracture zone at 24 n latitude demonstrate dramatic changes in the style and magnitude of tectonic extension development of the neovolcanic zone expression of hydrothermal venting types oflithologic exposures and morphology of the median valley

a possible triple junction at 14 50 s on the north fiji basin ridge southwest pacific auzende j m grficia mont e bendel v huchon p lafoy y lagabrielle y de alteriis g and tanahashi m 1994 a possible triple junction at 14 50 s on the north fiji basin ridge southwest pacific in j m auzende and t urabe editors north fiji basin sxarner french japanese program mar geol 116 25 35 recent full coverage bathymetric and geophysical surveys yokosuka 90 and 91 carried out in the framework of the starmer japanese french joint project reveal details of the structure of the n160 segment of the north fiji basin ridge despite its intermediate spreading rate this segment shows a slow spreading type morphology with steep 1000 m high walls reaching an axial depth of 4000 m its northern tip at 14 50 s is a complex area involving several plates boundaries this domain can be interpreted either as a complex boundary between two plates or as a new triple junction symmetrical with the 16 50 s triple junction characterizing its southern tip in either case the complexity of the spreading ridge geometry in this area illustrates the instability of the accretionary geometry due to the deformation of the whole north fiji basin between the much larger indo australian and pacific plates

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

carbonic fluid inclusions in south indian granulites evidence for entrapment during charnockite formation field evidence and fluid inclusion studies on south indian incipient charnockites suggest that charnockite formation occurred during a decompressional brittle regime following the peak of metamorphism and regional deformation the most abundant type of inclusions in quartz and garnet grains in these charnockites contain high density carbonic fluids although lower density fluids occur in younger arrays of inclusions discrete fluid inclusion generations optically are observed to decrepitate over well defined temperature intervals and quantitative measurements of co2 abundance released from these inclusions by stepped thermal decrepitation show up to a four fold increase by volume in the incipient charnockites relative to the adjacent gneisses from which they are derived studies based on optical thermometry visual decrepitation and stepped heating inclusion release together indicate that entrapment of carbonic fluids coincided with charnockite formation we confirm that an influx of carbon dioxide rich fluids is associated with the amphibolite granulite transition as recorded by the incipient charnockites the remnants of which are commonly preserved as the earliest generation of high density fluid inclusions

age of younger tonalitic magmatism and granulitic metamorphism in the south indian transition zone krishnagiri area comparison with older peninsular gneisses from the gorur hassan area abstract a major episode of continental crust formation associated with granulite facies metamorphism occurred at 2 55 2 51 ga and was related to accretional processes of juvenile crust dating of tonalitic trondhjemitic granitic gneisses and charnockites from the krishnagiri area of south india indicates that magmatic protoliths are 2550 2530 5 ma as shown by both u pb and 207pb 206pb single zircon methods monazite ages indicate high temperatures of cooling corresponding to conditions close to granulite facies metamorphism at 2510 10 ma these data provide precise time constraints and sr nd isotopes confirm the existence of late tonalitic granodioritic juvenile gneisses at 2550 ma pb single zircon ages from the older peninsular gneisses gorur hassan area are in agreement with some previous sr ages and range between 3200 20 and 3328 10 ma these gneisses were derived from a 3 3 3 5 ga mantle source as indicated from nd isotopes they did not participate significantly in the genesis of the 2 55 ga juvenile magmas all these data together with previous work suggest that the 2 51 ga granulite facies metamorphism occurred near the contact of the ancient peninsular gneisses and the 2 55 2 52 ga juvenile tonalitic trondhjemitic terranes during synaccretional processes subduction mantle plume rb sr biotite ages between 2060 and 2340 ma indicate late cooling probably related to the dextral major east west shearing which displaced the 2 5 ga juvenile terranes toward the west

pan african charnockite formation in kerala south india sm nd mineral ages of gneisses and associated granulites from the ponmudi incipient charnockite locality south india indicate that granulite metamorphism occurred at or shortly after 558 ma proterozoic ages recorded by garnet separates reflect a detrital age or an earlier metamorphic event preserved by inclusions within garnet the age of post metamorphic uplift 440 460 ma is constrained by sr isotope equilibration between biotite and plagioclase since charnockite formation and subsequent uplift north of the palghat cauvery shear zone had terminated by earliest proterozoic time these results confirm two distinct periods of granulite formation in south india and suggest that the palghat cauvery shear zone represents the boundary between two blocks of strongly contrasting geological histories both incipient charnockite formation and subsequent uplift at ponmudi may be correlated with the tectonothermal evolution of the highlands group in sri lanka the similarity between nd and sr model ages for charnockites and gneisses from ponmudi indicates that no significant rb sr fractionation has occurred during the crustal history of these incipient charnockites pb isotopic ratios suggest that th u ratios were fractionated during charnockite formation at about 500 ma in contrast to charnockites found north of the palghat cauvery shear zone fractionation of u pb during the archaean did not occur in the ponmudi granulites

sri lanka indian eastern ghats east antarctica and the australian albany fraser mobile belt gross geometry age relationships and tectonics in precambrian gondwanaland in the reassembly of precambrian gondwanaland many now dispersed precambrian mobile belts appear to be aligned and this geometrical relationship may be significant one example is the alignment of the sri lanka eastern ghats mobile belt of india east antarctica and the albany fraser province of southwest australia sl eg ea af belt although these terrains differ somewhat in detail they have gross similarities in lithology structure and possibly in their age relationships recent dating suggests a common late proterozoic charnockite event the structural and tectonic alignment of this proposed sl eg ea af belt in the precambrian gondwanaland reassembly suggests a once continuous metamorphic terrain geometrically this belt can be fitted to arc swaths of small circles with their center in the middle of antarctica which can be interpreted in a preliminary way as being controlled by transform like plate tectonic processes

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

petrography chemistry and isotopic ages of peninsular gneiss dharwar acid volcanic rocks and the chitradurga granite with special reference to the late archean evolution of the karnataka craton southern india abstract new petrographic major and trace element and isotopic age data are presented for four suites of rocks from the low to medium grade part of the south indian archaean craton in central karnataka two suites are from the peninsular gneiss basement to the late archaean dharwar supergroup the third is a suite of dharwar potassic rhyolites and the fourth is from the chitradurga granite which has intrusive contacts with the dharwar supergroup the chikmagalur granite granodiorite which with its host gneisses forms part of the basement peninsular gneiss to the dharwar supergroup in the bababudan basin yields rb sr and pb pb whole rock isochron dates of 3080 110 ma and 3175 45 ma respectively an initial 87 sr 86 sr ratio of 0 7013 0 0009 and a 1 value 238 u 204 pb of 7 98 host tonalitic gneisses to the chikmagalur granite give an rb sr whole rock isochron date of 3060 160 ma with an initial 87 sr 86 sr ratio of 0 7015 0 0004 and a pb pb whole rock isochron date of 3190 100 ma with a model 1 value of 8 00 a suite of granites and granodioritic tonalitic gneisses chitradurga granitic rocks and gneisses forming part of the peninsular gneiss basement to the dharwar supergroup west of the chitradurga belt yields rb sr and pb pb whole rock isochron dates of 2970 100 ma and 3028 28 ma respectively with an initial 87 sr 86 sr ratio of 0 7035 0 0013 and a model 1 value of 7 62 the suite of dharwar acid volcanic rocks from north of honnali defines a poorly fitted 207 pb 206 pb linear array mswd 15 8 which gives an apparent age of 2565 28 ma and a model 1 value of 7 46 the poor fit of this isochron shows that a closed system was not maintained a conclusion supported by high k abundances which are attributed to a syn or post magmatic ion exchange process the chitradurga granite which intrudes the older rocks of the dharwar supergroup in the chitradurga belt yields a well fitted pb pb whole rock isochron date of 2605 18 ma with a model 1 value of 7 68 the u pb relations in these suites from karnataka are in marked contrast with the commonly severe u depletion and consequently unradiogenic pb isotopic compositions observed in deeply eroded high grade archaean gneiss terranes such as those bordering the north atlantic this contrast indicates that the cratonic rocks in central karnataka represent a relatively high level in the original archaean continental crust

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

morphostructure and magnetic fabric of the northwestern north fiji basin four successive spreading phases are distinguished in the northwestern part of the north fiji basin after an initial ne sw opening a n s spreading phase took place up to the northwesternmost tip of the basin along the south pandora tikopia and 9 30 ridges the n s spreading phase in the northern north fiji basin was followed by an e w opening phase along the central north fiji basin axis a triple junction was probably active during an intermediate stage between the two phases e w spreading underwent a reorganisation that induced the functioning of the 16 40 s triple junction and the development of the e w trending hazel holme extensional zone from the active central spreading axis to the southern tip of the new hebrides back arc troughs active extension also occurs along the e w santa cruz trough which crosscuts the arc platform at the northern end of the n s trending back arc troughs the existence of the back arc troughs is mainly due to the construction of the 400 km long volcanic duff ridge which trapped a piece of the old north fiji basin oceanic crust

on the stability of triple junctions and its relation to episodicity in spreading a convenient representation of triple junctions that involve only ridges r and transform faults f is proposed this representation combines in a simple way information from geographic and velocity spaces the velocity triangle provides the budget of lithospheric surface change which directly results from interactions of the three plates a discussion of the relative positions of the triple junction with respect to the velocity triangle demonstrates that in general there are several triple junction configurations that are compatible with a given triangle this discussion stresses the importance of oblique and asymmetric spreading pairs of triple junction configurations that are of particular interest are rrr rrf and rrr ffr when the triple junction lies outside of the triangle at least one of the ridges will shorten leading to yet another type of potentially unstable configuration the present configurations of the bouvet galapagos and indian ocean triple junctions are reviewed a method is proposed to reconstruct past configurations of the indian ocean junction at the time of anomalies 23 and 28 the main parameters that influence triple junction evolution are the lengths of transform faults the availability of magma and related connectivity of magma chambers and the also related spreading velocities this study confirms an earlier suggestion that activity at constructive plate boundaries occurs in two preferred modes the effusive and tectonic modes corresponding here to rrr and to rrf rff configurations respectively these modes apparently alternate in episodes of typically 1 ma duration the significance of this time constant and consequences for lithospheric mechanics are briefly discussed

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

diffuse back arc deformation in the southwestern pacific earthquake distribution and focal mechanisms from the lau and north fiji back arc basins indicate a diffuse and shear dominated system of deformation at odds with traditional plate tectonic models of lithospheric accretion in back arc basins the entire back arc region between the tonga and new hebrides arcs is more realistically modelled as a giant internally deformed pull apart basin formed along a left step in the transform boundary between the pacific and indo australian plates

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

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

arrested charnockite formation at kottavattam southern india at kottavattam southern kerala india late proterozoic homogeneous leptynitic garnet biotite gneisses of granitic composition have been transformed on a decimetric scale into coarse grained massive charnockite sensu stricto along a set of conjugate fractures transecting the gneissic foliation charnockitization post dates the polyphase deformation regional high grade metamorphism and anatexis and evidently occurred at a late stage of the pan african tectonothermal history geothermobarometric and fluid inclusion data document textural and chemical equilibration of the gneiss and charnockite assemblages at similar p lith t conditions 650 700 degrees c 5 6kbar in the presence of carbonic fluids internally buffered by reaction with graphite and opaque mineral phases x co2 0 7 0 6 x h2o 0 2 0 3 x n2 0 1 logf o2 l7 5 mineralogical zonation indicates that charnockitization of the leptynitic gneiss involved first the breakdown of biotite and oxidation of graphite in narrow outward migrating transition zones adjacent to the gneiss followed by the breakdown of garnet and the neoblastesis of hypersthene in the central charnockite zone compared to the host gneiss the charnockite shows higher concentrations of k na sr ba and zn and lower concentrations of mg fe ti v y zr and the hree with a complementary pattern in the narrow transition zones of biotite breakdown the p lith t x h2o data and chemical zonation patterns indicate charnockitization through subsolidus dehydration reaction in an open system subsequent residence of the carbonic fluids in the charnockite resulted in low grade alteration causing modification of the syn charnockitic elemental distribution patterns and the properties of entrapped fluids we favour an internally controlled process of arrested charnockitization in which during near isothermal uplift the release of carbonic fluids from decrepitating inclusions in the host gneiss into simultaneously developing fracture zones led to a change in the fluid regime from fluid absent in the gneiss to fluid present in the fracture zones and to the development of an initial fluid pressure gradient triggering the dehydration reaction

propagating rifts in the north fiji basin southwest pacific the accretion system in the north fiji marginal basin currently consists of four major differently oriented axes multibeam bathymetry recently acquired over the central north south axis between 18 and 21 s reveals a peculiar tectonic framework fanned patterns of the sea floor topography at the northern and southern tips of this segment are the general shape of a rugby ball these data and the interpretation of magnetic anomalies indicate that this central sector of the ridge has lengthened northward at the expense of the axis aligned n15 e both spreading and propagation rates are about 70 80 mm yr a southward propagation is also envisaged on the basis of the similar morphotectonic framework observed at the southern tip of the north south ridge

a nd isotope investigation of sediments related to crustal development in the british isles the times of original fractionation of the sm and nd component of clastic sediments from a mantle source crustal residence age have been estimated from sm nd model ages calculated relative to a depleted mantle evolution in this way the provenance and evolution of selected precambrian and phanerozoic sediments and metasediments from the british isles have been examined whereas some archaean and early proterozoic sediments have sm nd model ages that are close to their stratigraphic age the phanerozoic sediments analysed have model ages as much as 2 0 ga in excess of their stratigraphic age a more detailed study of lower palaeozoic sediments deposited on the northern margin of the iapetus ocean provides evidence for a marked change of provenance in the ordovician after the deposition of the dalradian supergroup a component with comparatively high143nd 144nd and sm nd ratio presumably basaltic is present in the sediments throughout the accretionary prism crustal residence age estimates average about 1 5 ga for both these lower palaeozoic sediments and modern pelagic clays and collectively fail to provide any evidence for significant continental growth during the phanerozoic