crustal structure of the western part of the southern granulite terrain of indian peninsular shield derived from gravity data abstract the southern granulite terrain sgt is composed of high grade granulite domain occurring to the south of dharwar craton dc the structural units of sgt show a marked change in the structural trend from the dominant north south in dc to east west trend in sgt and primarily consist of different crustal blocks divided by major shear zones the bouguer anomaly map prepared based on nearly 3900 gravity observations shows that the anomalies are predominantly negative and vary between 125 mgal and 22 mgal the trends of the anomalies follow structural grain of the terrain and exhibit considerable variations within the charnockite bodies two dimensional wavelength filtering as well as zero free air based zfb analysis of the geoid corrected bouguer anomaly map of the region is found to be very useful in preparing regional gravity anomaly map and inversion of this map gave rise to crustal thicknesses of 37 44 km in the sgt crustal density structure along four regional gravity profiles cutting across major shear zones lineaments plateaus and other important geological structures bring out the following structural information the bavali shear zone extending at least up to 10 km depth is manifested as a plane separating two contrasting upper crustal blocks on both sides and the gravity high north of it reveals the presence of a high density mass at the base of the crust below coorg the steepness of the moyar and bhavani shears on either side of nilgiri plateau indicates uplift of the plateau due to block faulting with a high density mass at the crustal base the bhavani shear zone is manifested as a steep southerly dipping plane extending to deeper levels along which alkaline and granite rocks intruded into the top crustal layer the gravity high over palghat gap is due to the upwarping of moho by 1 2 km with the presence of a high density mass at intermediate crustal levels the gravity low in periyar plateau is due to the granite emplacement mid crustal interface and the thicker crust the feeble gravity signature across the achankovil shear characterized by sharp velocity contrast indicates that the shear is not a superficial structure but a crustal scale zone of deformation reaching up to mid crustal level

geobarometry geothermometry and late archean geotherms from the granulite facies terrain of south india geobarometry and geothermometry of garnet bearing chamockitic and metapelitic assemblages from the high grade terrain from south india indicate three fields of equilibration high pressure granulites p 8 3 pm 1 0 kb t 760 pm 40 circ c are restricted to the charnockites and mafic granulites of the north slopes of the nilgiris these place a minimum constraint of 30 km on the maximum thickness of late archean crust medium pressure granulites p 6 4 pm 1 0 kb t 735 pm 40 circ c characterize the charnockites of the central nilgiris and the madras granulites to the east low pressure granulites p 5 0 pm 1 0 kb t 700 pm 20 circ c characterize the charnockites and metapelites south of the bhavani shear zone although migmatites from the kodaikanal massif may reflect somewhat higher temperatures all three granulite fields lie on a metamorphic geotherm piezothermic array which is strongly convex to the temperature axis and characteristic of convective heat transfer

structure and tectonics of the achankovil shear zone southern india an integrated approach to resolve the kinematics of the controversial achankovil shear zone aksz has been attempted involving remote sensing data shaded relief topo maps ground details of lithology and mesoscopic structures an excellent correlation of structural trends exists on all scales of observation the aksz is distinctly defined by nw se trending foliation fabrics with steep dips to southwest the adjacent madurai block and trivandrum block show contrasting lithological and structural characteristics as shown in structural cross sections the mesoscopic structural studies reveal the presence of sub horizontal stretching lineations asymmetric structures like s c fabrics porphyroclasts s shaped folds and shear bands confirming the strike slip component of shear along aksz the deformation undergone by the aksz could be described in terms of an initial dextral deformation d1 reactivated and superimposed by sinistral kinematics d2 which is also supported by megascopic structural interpretation of remote sensing data the megascopic structural interpretation of aksz displays en echelon pattern of lineaments with right overstepping arrangement which can be interpreted as an evidence of the latest sinistral transpressional deformation

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

large scale thrusting in the lower crustal basement of sri lanka abstract the granulite facies highland complex of central and southwestern sri lanka was thrusted onto the amphibolite facies eastern vijayan complex along a deep crustal subhorizontal thrust surface the transport direction is marked by a n s to nnw sse oriented stretching lineation during thrusting folds with axes parallel to the stretching lineation formed sense of shear criteria are scarce in a few localities top to s movement is indicated the approximately n s oriented stretching direction is a constant element through the entire deformational history following thrusting the thrust contact was bent into gentle to open flexure slip folds which usually did not generate a new cleavage these folds vary in wavelength between several metres and kilometres in general their fold envelope is subhorizontal and therefore the thrust must underlie large parts of the highland complex klippen south of the highland complex like the kataragama klippe prove that the thrust plane is subhorizontal nearly up to the south coast i e the thrust plane can be traced for more than 200 km parallel to the transport direction after cooling to upper amphibolite facies conditions thrusting occurred during retrograde metamorphism of the highland complex it is younger than the formation of 577 14 ma old migmatites holzl et al 1991 pegmatites intruded after the formation of the thrust and are found in the highland complex and e vijayan they were probably derived from late to post tectonic granitoids which are common in the e vijayan these pegmatites are not or just marginally deformed and cross cut all other structures further exhumation of the lower crustal rocks was not accompanied by tectonic events all deformation fabrics were thoroughly annealed during subsequent slow cooling

breakup and early seafloor spreading between india and antarctica summary we present a tectonic interpretation of the breakup and early seafloor spreading between india and antarctica based on improved coverage of potential field and seismic data off the east antarctic margin between the gunnerus ridge and the bruce rise we have identified a series of ene trending mesozoic magnetic anomalies from chron m9o 130 2 ma to m2o 124 1 ma in the enderby basin and m9o to m4o 126 7 ma in the princess elizabeth trough and davis sea basin indicating that india antarctica and india australia breakups were roughly contemporaneous we present evidence for an abandoned spreading centre south of the elan bank microcontinent the estimated timing of its extinction corresponds to the early surface expression of the kerguelen plume at the southern kerguelen plateau around 120 ma we observe an increase in spreading rate from west to east between chron m9 and m4 38 54 mm yr 1 along the antarctic margin and suggest the tectono magmatic segmentation of oceanic crust has been influenced by inherited crustal structure the kinematics of gondwanaland breakup and the proximity to the kerguelen hotspot a high amplitude e w oriented magnetic lineation named the mac robertson coast anomaly mca coinciding with a landwards step down in basement observed in seismic reflection data is tentatively interpreted as the boundary between continental transitional zone and oceanic crust the exposure of lower crustal rocks along the coast suggests that this margin formed in a metamorphic core complex extension mode with a high strength ratio between upper and lower crust which typically occurs above anomalously hot mantle together with the existence of the mca zone this observation suggests that a mantle temperature anomaly predated the early surface outpouring steady state magmatic production of the kerguelen lip an alternative model suggests that the northward ridge jump was limited to the elan bank region whereas seafloor spreading continued in the west enderby basin and its sri lankan conjugate margin in this case the mca magnetic anomaly could be interpreted as the southern arm of a ridge propagator that stopped around 120 ma

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

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

gravity magnetic and structural patterns at the deep crustal plate boundary zone between west and east gondwana in sri lanka abstract detailed gravity and magnetic field measurements were carried out in 1989 and 1990 to investigate the eastern boundary between the late archaean granulite complex of the highland hc and the neoproterozoic amphibolite grade complex of the vijayan vc along four profiles based on the density determination of 320 rock samples the average density of hc rocks is 2 78 g cm3 and of vc rocks 2 65 g cm3 all 2d models of the residual bouguer gravity anomalies reveal the hc rocks to be plate wedges with a thickness of about 1 3 km overlying the vc rocks within the border zone between hc and vc a new rock unit with an average density of 2 80 g cm3 could be defined mainly by geological less by geophysical data this rock unit consists of migmatized former charnockitic hornblende biotite garnet gneisses and is called here mahiyangana rocks and wellawaya rocks respectively after the local villages from the viewpoint of the isotopes they should be vc rocks but in fact they represent an independent rock wedge between hc and vc field observations show that these three rock units are separated by shear zones the interpretation of lineation and foliation data indicates that the rock piles were overthrusted in a n s direction like nappes similar to ramp and flat tectonics later a so called f5 folding phase caused the curved trend of the former n s striking boundaries the hc assumed to be a passive continental margin of west gondwana collided in the lower crustal realm with the vc and the mahiyangana rocks and wellawaya rocks a metavolcanic sequence of an active continental margin probably as part of east gondwana during the pan african event

discovery of a neoproterozoic basin in the prydz belt in east antarctica and its implications for gondwana assembly and ultrahigh temperature metamorphism abstract detrital zircon u pb age data from ultrahigh temperature metasedimentary rocks 950 c and 10 12 kbar in the rauer group and a granulite grade metasedimentary rock 850 c and 7 8 kbar in the bolingen islands in prydz bay east antarctica indicate the existence of a neoproterozoic ediacaran cover sequence that was deposited on basement ranging in age between approximately 2800 ma and 940 ma the neoproterozoic succession has a maximum depositional age of approximately 600 ma with derivation from igneous and metamorphic sources of archaean palaeoproterozoic and neoproterozoic age the minimum depositional age is constrained to be approximately 575 ma based on existing th u pb monazite metamorphic ages from the uht metasedimentary rocks the identification of late neoproterozoic protoliths to the uht granulites in the rauer group demonstrate that ultrahigh temperature metamorphism in prydz bay must be ediacaran to cambrian in age the timing of basin development ca 600 575 ma is coincident with the amalgamation of eastern gondwana the palaeogeography of the prydz bay region during the amalgamation of gondwana is uncertain but in our proposed reconstruction we suggest that inherited zircons were derived from indo antarctic crust ocean closure is inferred to have occurred along a suture zone to the southeast of prydz bay and the prince charles mountains our palaeogeographic model invites further work on the possibility of a shared history between india and a large portion of east antarctica prior to the final amalgamation of eastern gondwana during the ediacaran cambrian

pink sapphire from southern kerala s india implications on india madagascar correlation within gondwana assembly abstract we report here the occurrence of pink sapphires in association with a variety of gemstones from the trivandrum granulite block south of the achankovil shear zone in southern india the mineralization is associated with pegmatites or veins emplaced within granulite facies aluminous supracrustals the sapphires show near pure a1 0 composition 98 43 99 48 wt with traces of cr o 0 02 0 12 wt and feo 0 01 0 12 wt the available radiometric age of 513 2 ma for gem quality zircon associated with pink sapphire in the melankode locality confirms that the mineralization is of late pan african age pink sapphires have been widely reported from a number of localities in southern madagascar including betroka illakaka antranondambo and ambossary sapphires of various hues also occur in the ratnapura gem district in the southwestern part of sri lanka the pink sapphire occurrences in southern madagascar and southern kerala provide strong evidence for india madagascar juxtaposition in the gondwana assembly with the ranotsara shear zone in southern madagascar extending into the achankovil shear zone in southern india

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

continental mafic dyke swarms as tectonic indicators an example from the vestfold hills east antarctica abstract the two main types of magma that were emplaced as mafic dyke swarms in the vestfold hills are high mg tholeiite hmt and fe rich tholeiite frt the former magma type is less dense than the vestfold hills felsic crust whereas the latter is more dense at deep to intermediate crustal levels therefore frt dyke emplacement requires a higher deviatoric extensional stress as is reflected in a more constant orientation palaeoproterozoic hmt frt bimodal dyke swarms were emplaced around 2 4 ga and 2240 ma the geometry and orientation of 2240 ma hmt dykes imply a low deviatoric extensional stress field inconsistent with a mantle plume origin for magmatism current petrogenetic models for hmt magmatism suggest generation at active convergent plate boundaries although this is broadly consistent with emplacement of the bimodal dyke swarms subsequent to a crustal thickening event at 2 5 ga the exact mechanisms for melting and the generation of crustal extensional stress are unknown a possible scenario involves the sinking of gravitationally unstable oceanic crust a major mesoproterozoic magmatic extensional event in the vestfold hills started with normal faulting followed by lamprophyric dyke emplacement and by two distinct episodes of frt dyke emplacement at 1380 and 1245 ma a consistent direction of the stress field during this event suggests control of a linear belt of crustal uplift by comparison with other mafic dyke swarms this is interpreted to be due to a sheet of upwelling asthenosphere elongated plume around 1 4 ga rather than a low angle detachment this upwelling led to thermal erosion of the base of the lithosphere that channelled a subsequent magma pulse at 1245 ma erosion during a long period of uplift prior to 1380 ma was followed by sedimentation during thermal subsidence as suggested by segmentation of 1245 ma dykes the lithospheric structure inherited from the mesoproterozoic magmatic extensional event was probably reactivated during two high grade tectono metamorphic events around 1 0 and 0 5 ga the vestfold hills formed part of a relatively stable craton peripheral to the mobile belt that was formed during these events

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

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

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

the breakup of east gondwana assimilating constraints from cretaceous ocean basins around india into a best fit tectonic model published models for the cretaceous sea oor spreading history of east gondwana result in unlikely tectonic scenarios for at least one of the plate boundaries involved and or violate particular constraints from at least one of the associated ocean basins we link east gondwana spreading corridors by integrating magnetic and gravity anomaly data from the enderby basin off east antarctica within a regional plate kinematic framework to identify a conjugate series of east west trending magnetic anomalies m4 to m0 126 7 120 4 ma the mid ocean ridge that separated greater india from australia antarctica propagated from north to south starting at 136ma northwest of australia and reached the southern tip of india at 126 ma sea oor spreading in the enderby basin was abandoned at 115 ma when a ridge jump transferred the elan bank and south kerguelen plateau to the antarctic plate our revised plate kinematic model helps resolve the problem of successive two way strike slip motion between madagascar and india seen in many previously published reconstructions and also suggests that sea oor spreading between them progressed from south to north from 94 to 84 ma this timing is essential for tectonic ow lines to match the curved fracture zones of the wharton and enderby basins as greater india gradually began to unzip from madagascar from 100 ma in our model the 85 east ridge and kerguelen fracture zone formed as conjugate anks of a leaky transform fault following the 100ma spreading reorganization our model also identi es the afanasy nikitin seamounts as products of the conrad rise hotspot

nature of crust beneath sri lanka using teleseismic receiver function abstract sri lanka occupied a unique position in the eastern gondwana assembly surrounded by its gondwana neighbours madagascar india antarctica and africa it comprises four distinct lithologic units the highland complex the wanni complex the vijayan complex and the kadugannawa complex crustal properties crustal thickness and velocities in sri lanka have been estimated by inversion modeling of receiver functions computed using earthquake data from three stations that operated between 2011 and 2018 results reveal 38 km crustal thickness and high average shear wave velocity vs 3 7 3 8 km s beneath sri lanka low average vp vs ratio 1 72 in the highland complex and high vp vs ratio 1 79 in the wanni complex this suggests that the crust beneath sri lanka is felsic to intermediate in nature and the probable rocks are of granite to granodioritic in composition the probable rocks in wanni complex are felsic to mafic granulite in nature the crustal thickness and average vp vs ratio in the highland complex shows similarity with its gondwana neighbours denoting its unique position in gondwana the average crustal shear wave velocity in sri lanka is also found to be high 3 7 km s which is also comparable with its gondwana neighbours

tectonics of the neoproterozoic southern granulite terrain south india geological correlations between south india and adjacent gondwana fragments are hampered by the lack of a coherent structural framework for india s southern granulite terrain sgt this paper is an attempt at delineating its large scale geometry and neoproterozoic kinematics and tectonics the basic regional pattern is that of a high grade gneissic layering produced during several deformation stages grouped as d1 which became refolded during two regionally separated phases of upright folding d2a nnw trending d2b ne trending and reworked in e to ese trending zones of increased strain d3 of these the palghat cauvery shear zone pcsz which marks the boundary between archaean terrains in the north and proterozoic belts in the south may represent a major crustal shear zone whereas the achankovil zone az is better considered as a high strain zone characterized by flattening with a reverse component d1 structures developed close to the peak of pan african high grade metamorphism d2a and d2b occurred on the retrograde path partly at supersolidus and partly at subsolidus conditions d3 high strain zones show subsolidus textures in view of the general clockwise nature with a clear decompressional component of all published p t paths we propose that d2 d3 occurred during partial exhumation of the terrain correlations with madagascar are weakened as the az is no longer considered a major crustal shear zone whereas correlations with sri lanka are strengthened by the presence of a similar syntactical bend possibly produced by an indentor

the rayner complex of east antarctica complex isotopic systematics within a proterozoic mobile belt new isotopic rb sr u pb zircon and sm nd and petrological data are presented for part of an extensive proterozoic mobile belt locally known as the rayner complex in east antarctica much of the belt is the product of mid proterozoic 1800 2000 ma juvenile crustal formation melting of this crust at about 1500 ma ago produced the felsic magmas from which the dominant orthogneisses of this terrain were subsequently derived deformation and transitional granulite amphibolite facies conditions which peaked at 750 50 c and 7 8 kbar 0 7 0 8 gpa produced open to tight folding about e w axes and syn tectonic granitoids about 960 ma ago subsequent felsic magmatism occurred at about 770 ma and not as has been widely advocated at 500 550 ma which appears to have been a time of widespread upper greenschist facies 400 500 c metamorphism localized shearing and faulting sm nd model ages of 1 65 2 18 ga disprove a previously favoured hypothesis that the rayner complex mostly represents reworked archaean rocks from the neighbouring craton napier complex models that involve rehydration of the napier complex are no longer required since the rayner complex was its own source of water two episodes of proterozoic crustal growth are identified the later of which occurred between about 1200 ma and 1000 ma and was relatively minor sedimentation took place only shortly before late proterozoic orogenesis the multiphase history of the rayner complex has resulted in complex isotopic behaviour three temporally discrete episodes of pb loss from zircon have been identified the earliest two of which are responses to the c 960 ma and 540 ma tectonothermal events fluid leaching was operative during the later event for there is a good correlation between degree of isotopic discordance and secondary mineral growth pb loss during the high grade event was probably governed by the same process or by lattice annealing some zircon suites also document recent pb loss most lower concordia intercepts have no direct geological meaning and are explicable as mixed ages produced by incomplete pb loss during two or more secondary events whereas all zircon separates from the orthogneisses produce u pb isotopic alignments zircons from the only analysed paragneiss produce scattered data in part reflecting a range of provenance the 960 ma event was also associated with the growth of a characteristically low u zircon 300 g g in rocks of inferred high zr content there is ubiquitous evidence for the resetting of rb sr total rock isochrons even samples separated by up to 10 km fail to produce igneous crystallization ages minor mineralogical changes produced by the 540 ma upper greenschist facies metamorphism were sufficient to almost completely reset some rb sr isochrons and to produce open system conditions on outcrop scale at least in one location

p t x relationships deduced from corona textures in sapphirine spinel quartz assemblages from paderu southern india the sapphirine sa spinel sp quartz qz bearing rocks from paderu occur as lenticular enclaves within the precambrian khondalitecharnockite terrane of southern india in addition these rocks contain orthopyroxene opx sillimanite sill garnet gt cordierite cd biotite potash feldspar kf plagioclase and symplectites of cd kf qz opx the symplectites may have formed from the breakdown of osumilite grain contacts of sapphirine and spinel with quartz are rarely observed and the incompatibility with quartz during later stages is displayed by the development of several types of polymineralic reaction coronas the coronas in the different rock types a b etc are minerals listed from core to rim of corona a 1 sapphirine bearing rock type without spinel sa sill opx sa sill cd sa cd opx a 2 sapphirine and spinel bearing sp sa sill opx qz sp sa sill sp sa opx sp sill opx sp sa sill gt qz sa sill opx sp sa sill opx sa sill opx gt sp sa opx gt sp sa sill gt and b spinel bearing but sapphirine free sp sill opx sp sill gt sp cd commonly the coronas in the rock type a 2 and b also contain ilmeno hematite corundum in the core in association with spinel these rock types also provide textural evidence for later crystallization of cd cd sa and gt qz from opx sill qz and gt sill qz sapphirine is aluminous near 7 mg fe 2 o 9 al fe 3 2 o 3 3sio 2 and contains up to 12 2 wt per cent iron as feo orthopyroxene is also aluminous containing up to 10 4 wt per cent al 2 o 3 sapphirine and spinel have relatively high contents of fe 2 o 3 x mg in the fe mg minerals increases from rock type b to a2 to a1 a sequence of reactions has been deduced from coronas and other reaction textures and from the phase compatibility relations in the feo mgo al 2 o 3 sio 2 h 2 o system the p t x relationships from geothermobarometry and petrogenetic grids viz fe 2 o 3 vs feo and h 2 o vs fe 2 o 3 suggest 1 a retrograde mildly decompressive trajectory from 900 60 c 65 0 7 kb core to 760 50 c 5 0 6 kb rim and 2 the observed mineralogy of the coronas and reactions deduced from them are dependent on the relative feo fe 2 o 3 and h 2 o contents of the rocks feo 3 fe 2 o 3 and h 2 o

thermal gradients in the sri lankan granulite terrane a garnet biotite thermometric approach thermal zoning of the highland complex sri lanka has been delineated using the fe2 mg distribution coefficient between garnet and biotite from garnet biotite gneiss samples collected with wide geographical distribution in order to minimize the potential for retrograde fe mg exchange and maximize the potential for retaining peak equilibrium kd garnet biotite and temperature garnet and biotite included within feldspar and quartz without other mineral inclusions have been selected the calculated results indicate four distinct temperature contours with kd values varying from 1 84 to 6 38 and temperature varying from 996 to 591 c from the present results it is possible to divide the highland complex into two major metamorphic zones a high temperature area in the central region and a low temperature area in the south western and north eastern region in conjunction with the metamorphic pressure variations estimated from the granulites of the highland complex in previous studies it is shown that the high and low temperature areas are complemented by a high pressure region towards the eastern side and a low pressure region towards the western side of this complex this thermal dome is interpreted to be an artifact of the different crustal levels exhumed following pan african metamorphism

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

alkali granites with pan african affinities from kerala s india in the kerala region of the south indian shield alkali granite plutons ranging in age from 550 to 740 ma intrude precambrian charnockites and migmatites all of them are spatially related to regional fault lineaments their geochemistry is similar to pan african post kinematic plutons from the afro arabian shield they are of granitic to adamellitic composition with high sio 2 k 2 o up to 12 and na 2 o and low cao feo and mgo their major element composition together with a low 87 sr 86 sr i 0 703 from one pluton denote a type affinities however local charnockites have such low rb and radiogenic 87 sr that the granites could have formed by crustal melting their overall trace element characteristics are typical of alkaline granites in general but the details show that most resemble more recent volcanic arc granites and one pluton is typical of a within plate setting the presence of co 2 rich fluid inclusions supports the notion that the enrichment of rare earth an

mineralization and wall rock alteration at the bogala graphite deposit bulathkohupitiya sri lanka the graphite mineralization in the bulathkohupitiya area was studied with an emphasis on the bogala deposit vein type mineralization occurs in the zones of deep seated fractures in supracrustal stratiform metasedimentary rocks resulting from the differential rise of subcrustal migmatites the mineralization at bogala comprises several veins of graphite and a large vein of syenite pegmatite mineralogical and geochemical study of the wall rocks reveals a well zoned alteration halo about the deposit chemical zoning in the altered wall rock and the relict mineral replacement textures indicate deep seated hydrothermal activity formation of vein graphite by decarbonation of carbonates and reduction of co 2 formed during the decarbonation reaction is indicated by the available carbon isotope data chemical zoning in the alteration halo may be a useful aid in the exploitation of known graphite deposits

humite and scapolite bearing assemblages in marbles and calcsilicates of dronning maud land antarctica new data for gondwana reconstructions this study investigates marbles and calcsilicates in central dronning maud land cdml east antarctica the paleogeographic positioning of cdml as part of gondwana is still unclear however rock types mineral assemblages textures and p t conditions observed in this study are remarkably similar to the kerala khondalite belt in india the cdml marbles and calcsilicates experienced a pan african granulite facies metamorphism at c 570 ma and an amphibolite facies retrogression at c 520 ma the highest grade assemblage in marbles is forsterite spinel calcite dolomite in calcsilicates the assemblages are diopside spinel diopside garnet scapolite wollastonite clinopyroxene quartz scapolite anorthite calcite clinopyroxene wollastonite these assemblages constrain the peak metamorphic conditions to 830 20 c 6 8 0 5 kbar and xco2 0 46 during retrogression highly fluoric humite group minerals humite clinohumite chondrodite replaced forsterite and garnet rims formed at the expense of scapolite during reactions with wollastonite calcite or clinopyroxene but without involvement of anorthite metamorphic conditions were about 650 c 4 5 0 7 kbar 0 2 xco2fluid 0 36 and the co existence of garnet clinopyroxene wollastonite and quartz constrains fo2 to fmq 1 5 log units mineral textures indicate a very limited influx of h2o rich fluid during amphibolite facies retrogression and point to significant variations of fluid composition in mm sized areas of the rock gypsum was observed in two samples it probably replaced metamorphic anhydrite which appears to have formed under amphibolite facies conditions the observed extensive anorogenic magmatism anorthosites a type granitoids and the character of metamorphism between 610 and 510 ma suggest that the crustal thermal structure was characterized by a long lived 50 100 ma rise of the crustal geotherm probably caused by magmatic underplating

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

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

metamorphic p t conditions of the kerala south india khondalite belt a granulite facies supracrustal terrain the khondalite belt of southernmost india is an immense section of granulite facies metasediments of probable early proterozoic age dominant rock types are migmatized khondalites garnet sillimanite pm cordierite metapelites acid biotite garnet graphitic gneisses and charnockites quartzofeldspathic gneisses with orthopyroxene throughout much of southern kerala the biotite gneisses are in a state of arrested conversion to charnockite partially decrepitated co 2 inclusions in quartz at the ponmudi mixed biotite gneiss charnockite quarry have significant ch 4 and or n 2 and are texturally late the probable sequence of events in metasediments was 1 migmatization 2 charnockite formation 3 symplectitic cordierite formation and 4 late co 2 entrapment phase equilibrium diagrams of anatexis in aluminous systems and fe mg exchange between garnet and biotite indicate metamorphic temperatures of 650 850 c for the khondalites temperatures based on garnet orthopyroxene

a hierarchical tectonic model of the gulf of mexico and caribbean region abstract a quantitative computer graphic methodology hierarchical tectonic analysis has been used to model the tectonic evolution of the gulf of mexico and caribbean region this method employs a highly structured hierarchical description of the relative motions of pairs of tectonic elements in order to quantify the timing and style of plate interactions the main features described in this model are 1 1 a revised fit of pangea 2 2 the opening of the gulf of mexico in the middle late jurassic 3 3 the formation of a proto caribbean ocean due to the separation of north and south america during the early cretaceous 4 4 the formation of the greater antilles arc along the western margin of the proto caribbean ocean 5 5 the insertion of the farallon plate between north and south america during the middle cretaceous resulting in the northeastward advance of the greater antilles arc and the subduction of proto caribbean oceanic crust 6 6 the complete subduction of the proto caribbean ocean and the collision of the greater antilles arc with the bahamas platform in the late cretaceous 7 7 the truncation of the caribbean section of the farallon plate from its pacific counterpart and the formation of the panama arc latest cretaceous and 8 8 the westward movement of north and south america with respect to the caribbean plate resulting in the opening of the yucatan basin paleocene and subsequently in the evolution of the cayman trough and e w zone of strike slip

structural constraints on the proterozoic reworking of archaean crust in the rayner complex macrobertson and kemp land coast east antarctica abstract the high grade metamorphic rocks exposed along the macrobertson and east kemp land coasts consist of in the west archaean gneisses reworked during a ductile mid proterozoic deformation and in the east foliated proterozoic charnockite gneiss containing large remnants of gneiss of probable archaean age evidence for three folding events is recognized in the reworked gneisses an early recumbent terrain is deformed by two upright events possessing near vertical axial surfaces the charnockite gneiss preserves evidence of only the final event these three events comprise the rayner structural episode which was a consequence of the ductile thrusting of archaean gneiss westward toward a stable archaean block the napier complex kinematic indicators are consistent with formation of the proterozoic recumbent terrain by dominantly shear deformation dissection of both the rayner and napier complexes by retrograde shear zones and mylonite pseudotachylite zones was a consequence of the northward thrusting of the two complexes during the cambrian

precambrian tectonics and crustal evolution in south india about 3 4 ga ago voluminous calc alkaline magmas represented by the granitoid gneisses of southern india were emplaced into a now poorly preserved non continental crust unstable ensialic basins initiated at about 3 0 ga were filled with volcanic and sedimentary rocks up to about 2 6 ga this basement cover sequence was deformed at the close of the archaean first by northward accretion and thickening of several crustal slabs formerly separated by prisms of stable shelf sediments structures produced by this episode were refolded and dislocated by large n s strike slip shear belts to impart intense steep planar fabrics to large volumes of the crust fluids rich in co possibly derived from sedimentary material driven beneath the crustal slabs by the thickening mechanism purged the deep crust of h o to form granulites upward migration of hot h o rich fluids encouraged partial melting at intermediate crustal levels these late archaean tectonic and thermal events began soon after the close of basin f

the use of cation packing index for characterizing the weathering degree of granitic rocks the weathering state and weatherability of rocks are highly important for engineering geology projects and the use of rocks as building stones the weathering state of rocks can be described by various chemical and petrographical weathering indices to determine weathering state of a rock considerable amount of chemical analyses is required weatherability of rocks depends on not only their mineral composition but also the number of cation replaceable with hydrogen in a mineral in this study the use of the k value cation packing index is defined as the number of cation in a mole for characterizing the degree of weathering of granitic rocks is investigated for the purpose of the study the kurtun granitic rocks are selected as the material of the study the study contains three main stages and these are field investigations laboratory studies and regression analyses during the field studies in situ weathering descriptions and sampling are carried out the samples are subjected to chemical and petrographic analyses and physical and mechanical tests at the final stage of the study a series of regression analyses between the cation packing index and other properties of the samples are performed the results show that the k value can be used as a weathering indicator when compared with the other chemical weathering indices the k value can be used as a weathering index without chemical analyses because it can be determined by mineralogical analyses

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

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

crystallographic fabric development along a folded polycrystalline hematite abstract detailed analyses of microstructures and crystallographic preferred orientations cpos of hematite rocks were conducted in samples of polycrystalline hematite around a tight fold in the quadrilatero ferrifero region southeastern brazil grain size is dominantly very fine in all samples 15 30 m with a slight increase toward the hinge grain aspect ratio increases substantially toward the hinge from an average of 2 in the limb to 9 in the hinge distribution patterns of 0001 axes and poles of planes 10 1 0 11 2 0 and 10 1 4 suggest that intracrystalline slip operate on the basal plane along the direction the distribution of the poles of prismatic planes parallel to the foliation of reference frame indicate that the all the symmetric slip directions of hematite crystals were equally efficient during activation of basal intracrystalline slip increasing aspect ratio is accompanied by cpo intensification toward the hinge where intensification was aided during deformation by bulk rotation of the hematitic plates and possibly by some grain boundary sliding such cpo intensification with the increase of aspect ratio toward the hinge is used to infer that the fold developed by flexural flow

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

combined thrusting and wrenching in the ibero armorican arc a corner effect during continental collision this paper is an attempt to reconcile strain pattern and deformation history in an overall arcuate structure with continental collision processes geological data are first presented for the early history 400 ma of the variscan orogenic belt in france principal stretch trajectories are on a large scale more or less parallel with the trend of the belt they became strongly oblique to this trend near the basal surfaces of the nappes a kinematic model combining two orthogonal shear components is proposed to explain such a strain pattern it is concluded that the observed strain trajectories result from a combination of a southward thrusting and a sinistral wrenching this conclusion is in good agreement with previous conclusions gained by several local microtectonic studies the geodynamic significance of combined thrusting and wrenching at the scale of the ibero armorican arc is discussed and a progressive deformation model is proposed

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

dynamics of orogenic wedges and the uplift of high pressure metamorphic rocks subduction accretion complexes can be approximated as wedge shaped continua with a rigid buttress behind and a subducting litho spheric slab beneath thick wedges undergoing prograde metamorphism have a negligible long term yield strength and are likely to exhibit a complex nonlinear viscous rheology such a wedge will tend to deform internally until it reaches a stable configuration in which the gravitational forces generated by the wedge geometry balance the traction exerted on its underside by the subducting slab accretion of material at the wedge front will lengthen the wedge and cause it to shorten internally to regain the stable geometry this shortening will be expressed as late out of sequence thrusting backthrusting and folding conversely underplating of sediment or crustal slices will thicken the wedge which may need to extend internally to regain stability extension will cause listric normal faults that may merge downward into zones of ductile extension continued underplating at depth and compensating extension above provides a mechanism for bringing high p low t metamorphic rocks to upper levels in the rear of the wedge where they are commonly observed many major tectonic boundaries in convergent orogens such as the coast range thrust in the franciscan complex major nappe contacts in the alps and the contact between the nevado filabride and higher betic nappe complexes in the betic cordillera show abrupt increases in metamorphic grade downward across them this is consistent with their origin or reactivation as uplift related extensional structures

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

effective elastic thickness along the conjugate passive margins of india madagascar and antarctica a re evaluation using the hermite multitaper bouguer coherence application abstract gondwana correlation studies had rationally positioned the western continental margin of india wcmi against the eastern continental margin of madagascar ecmm and the eastern continental margin of india ecmi against the eastern antarctica continental margin eacm this contribution computes the effective elastic thickness te of the lithospheres of these once conjugated continental margins using the multitaper bouguer coherence method the results reveal significantly low strength values te 2 km in the central segment of the wcmi that correlate with consistently low te values 2 3 km obtained throughout the entire marginal length of the ecmm this result is consistent with the previous te estimates of these margins and confirms the idea that the low te segments in the central part of the wcmi and along the ecmm represents paleo rift inception points of the lithospheric margins that was thermally and mechanically weakened by the combined action of the marion hotspot and lithospheric extension during the rifting the uniformly low te value 2 km along the eacm indicates a mechanically weak lithospheric margin probably due to considerable stretching of the lithosphere considering the fact that this margin remained almost stationary throughout its rift history in contrast the ecmi has comparatively high te variations 5 11 km that lack any correlation with the regional tectonic setting using gravity forward and inversion applications we find a leading order of influence of sediment load on the flexural properties of this marginal lithosphere the study concludes that the thick pile of the bengal fan sediments in the ecmi masks and has erased the signal of the original load induced topography and its gravity effect has biased the long wavelength part of the observed gravity signal the hence uncorrelated flat topography and deep lithospheric flexure together contribute a bias in the flexure modeling which likely accounts a relatively high te estimate

smnd isotopes and ree geochemistry of madras granulites india an introductory statement abstract major and trace elements including ree as well as sm nd isotope analyses of eight granulite samples from the madras area are presented the data provide evidence for the origin of some of the granulites but in other cases demonstrate metasomatic changes in major and trace element contents including lree the data furthermore demonstrate that the analysed granulites are not cogenetic however when plotted in a sm nd isochron diagram the samples yield a linear array which corresponds to an age t 2555 140 ma and nd t 1 1 4 this age of 2555 140 ma could represent the time of granulite facies metamorphism immediately preceded by the formation of the protolith the nd t 1 value may be indicative of a near chondritic mantle source or alternatively of mixing between a depleted mantle and a crustal component

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

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

dehydration and incipient charnockite formation a phase equilibria and fluid inclusion study from south india a 3 m traverse across a charnockite lens in a biotite garnet graphite gneiss quarry from southern india has provided mineral equilibria that indicate charnockite formation occurred under fluid conditions of x h 2 o 0 2 to 0 3 at p 5 5 pm 1 9 kb and t 700 to 750 c dehydration of biotite loss of garnet and growth of orthopyroxene are related to a decrease in x h 2 o in the fluid phase within the charnockite lens isotopic analysis of the fluid entrapped in quartz from the charnockite indicates delta 13 c 8 2 0 3 0 heavier than delta 13 c values from smaller volumes of co 2 entrapped in quartz from the gneiss although the increase in fluid inclusion abundance in the charnockite can be related to oxidation of graphite without influx of co 2 an external source of co 2 is required in order to account for the shift to heavier carbon isotope values observed in both graphite and entrapped co 2 within the charnockite carbon isotope data across the gne