determination and significance of the mn ii zero field splitting zfs interaction in the geochemistry of travertines an analytical approach based on the electron paramagnetic resonance epr spectroscopy of mn ii in travertines has been developed in order to obtain relevant information about the local inhomogeneity of calcite and about the thermodynamic conditions which control the formation of travertine deposits this information is crucial to constrain the precipitation of travertine under different geochemical contexts an empirical correlation between the spectral features and the zero field splitting zfs interaction has been established through numerical simulations of epr spectra the variability of the investigated parameters and the applicability of the method have been tested on several travertines from central italy

rock magnetism and palaeomagnetism of the oddanchatram anorthosite tamil nadu south india rock magnetic and palaeomagnetic data are reported on the proterozoic massif type anorthosite body from oddanchatram on the northern slopes of the kodaikanal palani ranges tamil nadu alternating field demagnetization treatment indicated a very stable direction of magnetization progressive heating revealed a remanence that became unblocked at 580degreesc the demagnetization analysis yields a characteristic component with a mean direction d m 97 3degrees im 28 8degrees k 51 alpha 95 7 2degrees and n 9 sites in addition to this characteristic component specimens from a few sites show the presence of a low coercivity component with a mean palaeomagnetic direction of d 36 7degrees i 29 4degrees k 63 alpha 95 4 4degrees and n 18 the isothermal remanent magnetization acquisition curves indicate magnetite as the main carrier of the stable remanence the direction of the characteristic component corresponds to a pole position at 9 7degreesn 1 7degreese d p 4 4 d m 7 9 the location of this pole is comparable to palaeomagnetic poles reported for the period of 1100 1000 ma for a range of formations from the indian shield the pole for the secondary component 45degreess 23degreese fits well with poles for 550 ma associated with the pan african thermal event that extensively caused granulite grade metamorphism in the southern granulite terrain despite the high temperature 700degreesc that prevailed during the pan african event the remanence magnetization of 1100 1000 ma is preserved in the oddanchatram anorthosite it is inferred that the northern boundary of the terrain affected by the dominant 550 ma granulitic metamorphism lies close to the southern margin of the oddanchatram anorthosite body thus oddanchatram anorthosite escaped the high temperatures metamorphism the weak secondary magnetization found in a few specimens is attributed to hydrothermal activity associated with the intrusion of pink granites immediately south of the oddanchatram anorthosite body a mid proterozoic magnetization age of the oddanchatram anorthosite supports the view that this body was emplaced remobilized during the eastern ghat orogeny similar to1000 ma and this body can be seen as a western extension of the string of massif type anorthosites in the eastern ghat mobile belt

seismically imaged shallow and deep crustal structure and potential field anomalies across the eastern dharwar craton south indian shield possible geodynamical implications abstract the time bound crustal evolution and subsequent deformation of the cuddapah basin nellore schist belt and eastern ghats terrain of eastern dharwar craton which have undergone sustained geodynamic upheavals since almost 2 0 billion years remain enigmatic an attempt is made here to integrate newly available potential field data and other geophysical anomalies with deep seismic structure to examine the generative mechanism of major crustal features associated with this sector our study indicates that the initial extent of the cuddapah basin sedimentation may have been much larger extending by almost 50 60 km west of tadipatri during paleoproterozoic period which subsequently shrank due to massive erosion following thermal uplift caused by sw cuddapah mantle plume below this region crust is still quite warm with moho temperatures exceeding 500 c similarly nallamalai fold belt rocks bounded by two major faults and extremely low gravity may have occupied a large terrain in western cuddapah basin also before their abrasion no geophysical signatures of thrusting are presently seen below this region and thus it could not be an alien terrain either in contrast nellore schist belt is associated with strikingly high positive gravity possibly caused by a conspicuous horst structure and up dipping mafic crustal layers underneath that resulted due to india east antarctica collision after the cessation of prolonged subduction 1 6 0 95 ga further the crustal seismic and gravity signatures would confirm presence of a totally distinct geological terrain east of the cuddapah basin but the trace of eastern ghats belt is all together missing instead all the geophysical signatures point out to presence of a proterozoic sedimentary terrain east of nellore schist belt it is likely that the extent of prorerozoic sedimentation was much larger than thought today in addition presence of a seismically detected gondwana basin over nellore schist belt apart from some recently discovered similar subsurface gondwana occurrences in intracratonic parts would indicate that dharwar craton was rifting even during gondwana period thereby challenging the long held view of cratonic stability

stratigraphy and geochemistry of the balwan limestone vindhyan supergroup india evidence for the bitter springs δ13c anomaly abstract the vindhyan supergroup of india deposited in an intracratonic basin is one of the important proterozoic marine successions of the world that contains some of the most controversial precambrian fossil discoveries despite their importance the chronology of the host strata and global correlation of the events that occurred within the basin remain equivocal here we present results of a detailed geological geochemical and isotopic sr c o study of the balwan limestone the youngest carbonate formation of the supergroup exposed only in the western sector of the basin our results suggest that the vindhyan basin had become a structurally controlled marginal sea towards the end of its existence and that the limestone was deposited in a subtidal environment that had strong depositional currents we find evidence for a strong storm event or a tsunami during its deposition near primary 87sr 86sr of 0 70676 at the top and 207pb 206pb age of 866 90 ma gopalan et al 2013 of this 120 m thick formation suggest its deposition during late tonian 13c stratigraphy reveals the presence of the globally synchronous bitter springs anomaly 12 shift in the formation the first such report from india

unusual lithospheric structure beneath the hyderabad granitic region eastern dharwar craton south india abstract using multiparametric geological and geophysical data the evolutionary history of the lithosphere beneath the late archean early proterozoic hyderabad granitic region hgr of the eastern dharwar craton of south indian shield has been attempted our study reveals that the entire granitic region and the surroundings 200 km 200 km is being neotectonically uplifted possibly due to a major intrusive body situated at sub crustal depth leading to changes in river courses and also erosion of several kilometers 10 km from its upper crustal column beneath this region the moho is elevated 32 33 km the reduced heat flow is of the order of 28 mw m2 and the asthenosphere is located at shallow depths of about 124 km i e far less than 200 400 km depth found in precambrian shield areas of the globe the cratonic mantle lithosphere beneath hgr contains a highly conductive 15 m hydrous and seismically anisotropic metasomatic zone between depth of 90 and 100 km where the estimated temperatures could be in the range of 900 1000 c the surface granitic layer containing unusually high radioactivity 5 25 w m3 appears very thin probably a kilometer or so beneath which the entire crustal column seems to be differentiated and made up of low radioactive granulite facies rocks of acid to intermediate composition persistent episodic thermal reactivations during the last 2 6 gega years and continuous uplifting seems to have played a major role towards making its crustal column unusual

anomalous crustal and lithospheric mantle structure of southern part of the vindhyan basin and its geodynamic implications abstract tectonically active vindhyan intracratonic basin situated in central india forms one of the largest proterozoic sedimentary basins of the world possibility of hydrocarbon occurrences in thick sediments of the southern part of this basin has led to surge in geological and geophysical investigations by various agencies an attempt to synthesize such multiparametric data in an integrated manner has provided a new understanding to the prevailing crustal configuration thermal regime and nature of its geodynamic evolution apparently this region has been subjected to sustained uplift erosion and magmatism followed by crustal extension rifting and subsidence due to episodic thermal interaction of the crust with the hot underlying mantle almost 5 6 km thick sedimentation took place in the deep faulted jabera basin either directly over the bijawar mahakoshal group of mafic rocks or high velocity high density exhumed middle part of the crust detailed gravity observations indicate further extension of the basin probably beyond nsl rift in the south a high heat flow of about 78 mw m2 has also been estimated for this basin which is characterized by extremely high moho temperatures exceeding 1000 c and mantle heat flow 56 mw m2 besides a very thin lithospheric lid of only about 50 km many areas of this terrain are thickly underplated by infused magmas and from some segments granitic gneissic upper crust has either been completely eroded or now only a thin veneer of such rocks exists due to sustained exhumation of deep seated rocks a 5 8 km thick retrogressed metasomatized zone with significantly reduced velocities has also been identified around mid to lower crustal transition

seismic imaging across the eastern ghats belt cuddapah basin collisional zone southern indian shield and possible geodynamic implications abstract the evolutionary nature of the area encompassing cuddapah basin nellore schist belt and the ongole domain of eastern ghats belt south india which contains paleo neoproterozoic imprints of thick sedimentation intense magmatism subduction accretion and possible collision between eastern dharwar craton and east antarctica is still being debated an attempt has been made here to decipher the deep crustal structure and tectonics of this region by reprocessing deep seismic sounding data acquired earlier through the modeling of first arrival refraction and wide angle reflection travel times the derived crustal seismic velocity structure reveals that the intracratonic proterozoic cuddapah basin containing only 4 km thick sediment is bounded by two major faults the moho reaching fault detected on its eastern boundary demarcates cuddapah basin from the nellore schist belt although a deep normal fault is mapped within the nallamalai fold belt but no thrusting signatures are apparent almost all the crustal layers beneath nellore schist belt and the ongole domain of eastern ghats belt show distinct eastward dipping trend conforming to an upthrusted feature suggesting possible presence of a collisional suture besides the areas lying east of cuddapah basin appear to be an accreted orogenic terrain beneath which lower crust has upwarped substantially the entire stretch of the studied region is underplated by unprecedently thick 20 km high velocity 7 0 7 4 km s magma layer above the moho indicating strong crust mantle thermal perturbation and massive subcrustal erosion further an expression of a deep seated mantle thermal anomaly has also been found below the parnapalle region of the sw cuddapah basin beneath which deeper crustal layers have exhumed

precise time and conditions of peak taconian granulite facies metamorphism in the southern appalachian orogen u s a with implications for zircon behavior during crustal melting events abstract the time of peak thermal conditions in the granulite facies zone of the southern appalachian orogen corresponds to the time of formation of garnet bearing leucosomes generated by biotite dehydration melting in garnet sillimanite gneisses leucosomes contain unusually abundant zircon occurring as inclusions in euhedral plagioclase phenocrysts and garnet isotope dilution thermal ionization mass spectrometric analysis of 13 elongate euhedral zircon crystals from the leucosome yields a concordant u pb age of documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr renewcommand sfdefault wncyss renewcommand encodingdefault ot2 normalfont selectfont declaretextfontcommand textcyr cyr pagestyle empty declaremathsizes 10 9

comment on new ar ar ages of southern indian kimberlites and a lamproite and their geochemical evolution by osborne et al precambrian res 189 2011 91 103 abstract osborne et al 2011 precambrian res 189 91 103 report new 40 ar 39 ar age results for two kimberlites muligiripalle pipe 5 and tummatapalle pipe 13 and one lamproite pochampalle from the archaean dharwar craton in southern india previous studies indicate that kimberlites and related rocks are highly susceptible to extraneous argon contamination leading to anomalously old ages although reliable 40 ar 39 ar ages can be obtained from step heating analyses of groundmass phlogopite grains osborne et al 2011 carried out single step 40 ar 39 ar fusion analyses on phlogopite phenocrysts and xenocrysts this approach is not favoured because it does not allow identification of argon loss gain effects thus complicating data interpretation and reducing confidence in the reliability of the reported ages in particular it is suggested that the pochampalle lamproite was emplaced osborne et al 2011 it is recommended that 40 ar 39 ar geochronology of kimberlite and lamproites include step heating analyses of matrix phlogopite grains with anomalous or contentious results verified using other dating methods

bouguer gravity anomalies and occurrence patterns of kimberlite pipes in narayanpet maddur regions andhra pradesh india the narayanpet kimberlite field that lies southwest of hyderabad the capital city of andhra pradesh india hosts a number of kimberlite pipes these pipes appear to be randomly positioned however based on regional geologic structures revealed by bouguer gravity anomalies especially in a regional gravity map their locations form a definite pattern in the narayanpet maddur region regional bouguer gravity contours exhibit some features of geologic interest 1 the eastward convex regional contours show an increase in convexity from the maddur and kotakonda area on the east to narayanpet on the west 2 convexity is maximum in the vicinity of narayanpet where a large number of kimberlite pipes occur nearly parallel to the regional contour and 3 between narayanpet and the maddur kotakonda region kimberlite pipes occur at intersections of three eastward convex concentric zones with four lineaments one trending northeast southwest and the other three nearly east west these linear trends are be

contrasting high and intermediate pressures of metamorphism in the archaean sargur schists of southern india the archaean karnataka craton of southern india contains eastern and western crustal blocks separated by a major thurst in which sargur schists occur as lenses within tonalitic peninsular gneisses the schist complex comprises pelites quartzitic psammites carbonates and calc silicates iron formations and basic rocks and thus provides many mineral assemblages ideal for the calculation of pt conditions with their gneisses the sargur rocks are unconformably overlain by the dharwar greenstone belts and are generally thought to be older than 3 000 my

geological setting and chemistry of kimberlite clan rocks in the dharwar craton india abstract diamond exploration in india over the past decade has led to the discovery of over 80 kimberlite inferred and lamproite related intrusions in three of the four major archean cratons that dominate the subcontinent these intrusions are proterozoic 1 1 ga and are structurally controlled locally at the intersections of faults regionally in a 200 km wide 1000 km long diamond corridor and globally in the reconstructed supercontinent of rodinia the geochemistry of 57 samples from 13 intrusions in the southern dharwar craton of andhra pradesh has been determined by xrf spectrometry the bodies are iron rich with mg 50 70 and are neither archetypal kimberlites nor ideal lamproites this may be the underlying reason that conventional exploration techniques have thus far failed to locate the primary sources of india s historically famous diamonds the two major fields of kimberlite clan rocks kcr in the dharwar craton wajrakur and narayanpet are separated by a nw se trending transcontinental mumbai chennai gravity lineament about 80 of intrusions in wajrakur are diamondiferous but diamonds have not yet been reported in narayanpet the gravity anomaly may mark the boundary of an architectural modification in the keel of the sub continental lithosphere a suggestion that is supported by differences in kimberlite mineralogy chemistry mantle xenoliths structural setting and crustal host rocks

break up of australia india madagascar block opening of the indian ocean and continental accretion in southeast asia with special reference to the characteristics of the peri indian collision zones abstract linear belts of gondwana basins developed in the indian continent since late palaeozoic along favoured sites of precambrian weak zones like cratonic sutures and reactivated mobile belts the tibetan and sibumasu west yunnan continental blocks that were located adjacent to proto himalayan part of the indian continent rifted and drifted from the northern margin of the east gondwanic indo australian continent during late palaeozoic when the said northern margin was under glacial or cool climatic condition and rift drift tectonic setting the indo burma andaman iba sikule lolotoi blocks were also rifted and drifted from the same northern margin during late jurassic this was followed by the break up of the australia india madagascar continental block during the cretaceous the activity was associated with hot spot related volcanism and opening up of the indian ocean the late cretaceous and tertiary phases of opening of the arabian sea succeeded the early cretaceous phase of opening of the bay of bengal part of the indian ocean the palaeo and neo tethyan sutures in tibet yunnan laos thailand and vietnam reveal the complex opening and closing history of the tethys the iba block rotated clockwise from its initial e w orientation because of 90 e and adjacent dextral transcurrent fault movements caused due to faster northward movement of the indian plate relative to that of australia the india tibet terminal collision during early middle eocene initiated himalayan orogenesis and contemporaneously there was foreland basin development that was accompanied with sporadic but laterally extensive continental flood basalt cfb type and related volcanism the paleogene rocks of the himalayan foreland basin are involved in tectonism and are mostly concealed under older rocks the mesozoic early eocene ophiolite terrane on iba does not represent the eastern suture of the indian plate but occurs as klippe on iba caused due to oblique collision between sibumasu and iba during late oligocene post collisional indentation of y shaped indian continent into the asian collage produced himalayan syntaxes clockwise rotation of the sibumasu block which was then sutured to the tibetan and se asian blocks and tectonic extrusion of the indochina block along the ailao shan red river asrr shear zone highly potassic magmatic rocks were emplaced during late palaeogene at the oroclinally flexed marginal parts of the south china continental lithosphere these magmatic bodies were dislocated by the asrr left lateral shear zone soon afterwards petrogenetic and tectonic processes that generated the eocene cfb volcanics at the himalayan foreland basin may have also produced late palaeogene magmatism from outer parts of the namche barwa syntaxis their site specific location and time sequence suggest them to be genetically related to the india asia collision process and indian continent s indentation induced syntaxial buckling deep mantle reaching fractures were apparently produced during india asia terminal collision at the strongly flexed leading brittle edge of the indian continental lithosphere and possibly later in time at the outer oroclinally bent marginal parts of the rigid south china continental lithosphere generating typical magma the subduction zone that developed along the western margin of iba due to oblique convergence between the iba and the indian plate is still active the northern end of iba ultimately collided with the ne prolongation of the indian continent and was accreted to it during mio pliocene the shillong massif was uplifted and overthrust over the bengal basin located over its passive margin to the south whereas the eocene distal shelf sediments of iba were overthrust over the tertiary shelf of the indian continent

smnd pbpb and rbsr geochronology and petrogenesis of the mafic dyke swarm of mahbubnagar south india implications for paleoproterozoic crustal evolution of the eastern dharwar craton abstract the sm nd rb sr and pb pb isotopic systematics of whole rock and separated mineral fractions from four tholeiitic mafic dykes of the mahbubnagar swarm have been investigated this dyke swarm intrudes the granitic basement of the cuddapah basin in the eastern dharwar craton edc the sm nd data on whole rock samples of four gabbros and one dolerite and 15 mineral fractions yielded a linear array with an isochron age of 2173 64 ma and an initial nd value of 0 4 0 3 the sm nd data of a metapyroxenite and its three mineral fractions plot parallel to this array yielding an isochron age of 2189 123 ma with a negative initial nd value of 1 8 1 4 rb sr and pb pb internal isochron ages of three gabbros and a pb pb internal isochron age of a metapyroxenite dyke range between 2031 and 2284 ma and within the limits of experimental error broadly conform to the sm nd isochron age by contrast rb sr internal isochron ages of three other samples range between 1474 and 1952 ma and one sample yields a pb pb internal isochron age of 3446 ma showing the effects of partial isotopic resetting possibly due to hydrothermal alteration these data suggest that mafic dykes of the mahbubnagar swarm were emplaced at c 2170 ma marking an episode of the crustal extension and fracturing which ultimately resulted in the formation of the cuddapah basin different petrological and geochemical variants of mafic dykes seen in the area most probably represent generation and or crystallization of the progressively evolved batches of magma during a relatively short time interval the initial nd values calculated at 2170 ma range from 0 6 to 2 0 and can be explained as reflecting contamination of depleted mantle derived magma by crustal rocks of edc the initial 87 sr 86 sr ratios 0 7027 0 7064 and model 1 values 8 3 8 5 of apparently undisturbed samples are also consistent with this suggestion

field and textural evidence of two migmatization events in the sierras de cordoba argentina abstract in the metamorphic basement of the central eastern sierra de comechingones sierras pampeanas of cordoba argentina two types of migmatites are found tala cruz metatexites and canada del sauce diatexites rafts of deformed metatexites in the diatexites are interpreted as dismembered and partially assimilated remnants of the tala cruz metatexites two migmatization events are recognized the m2 metamorphic peak produced high grade paragenesis and the first anatexis generating the metatexites 810 840 c and 8 5 9 kbar were calculated for m2 the m4 decompression event produced the diatexites by a second melting of the metatexites 725 780 c and 5 5 6 kbar were estimated for m4 a dynamic contractional m3 event in high ductility and high temperature conditions was recognized between the two migmatization events a correlation could be established between the second migmatization event m4 and the event dated as cambrian 520 530 ma in other areas in the sierras de cordoba in which case the first migmatization event m2 would have been older perhaps as old as neoproterozoic the tectonic evolution of the sierra de comechingones would imply the occurrence of extension shortening cycles interpreted as pull push cycles mirroring the subduction and collision of pampia terrane against the western gondwana margin these cycles would have controlled the evolution of the eastern sierras pampeanas during the neoproterozoic early palaeozoic times

the rapid drift of the indian tectonic plate the breakup of the supercontinent gondwanaland into africa antarctica australia and india about 140 million years ago and consequently the opening of the indian ocean is thought to have been caused by heating of the lithosphere from below by a large plume whose relicts are now the marion kerguelen and reunion plumes plate reconstructions based on palaeomagnetic data suggest that the indian plate attained a very high speed 18 20 cm yr 1 during the late cretaceous period subsequent to its breakup from gondwanaland and then slowed to approximately 5 cm yr 1 after the continental collision with asia approximately 50 myr ago the australian and african plates moved comparatively less distance and at much lower speeds of 2 4 cm yr 1 refs 3 5 antarctica remained almost stationary this mobility makes india unique among the fragments of gondwanaland here we propose that when the fragments of gondwanaland were separated by the plume the penetration of their lithospheric roots into the asthenosphere were important in determining their speed we estimated the thickness of the lithospheric plates of the different fragments of gondwanaland around the indian ocean by using the shear wave receiver function technique we found that the fragment of gondwanaland with clearly the thinnest lithosphere is india the lithospheric roots in south africa australia and antarctica are between 180 and 300 km deep whereas the indian lithosphere extends only about 100 km deep we infer that the plume that partitioned gondwanaland may have also melted the lower half of the indian lithosphere thus permitting faster motion due to ridge push or slab pull

origins of yttrium and rare earth element distributions in metamorphic garnet highly variable distributions of yttrium and rare earth elements y rees are documented in pelitic garnet from the picuris mountains new mexico and from passo del sole switzerland and in mafic garnet from the franciscan complex california the wide variety of these y ree zoning patterns and those described previously in other occurrences imply diverse origins linked to differing degrees of mobility of these elements through the intergranular medium during garnet growth in the picuris mountains large early nucleating crystals have radial profiles of y ree dominated by central peaks and annular maxima in patterns that vary systematically with atomic number superimposed on these features are narrow spikes in hrees and mrees located progressively rimward with decreasing atomic number in contrast profiles in small late nucleating crystals contain only broad central maxima for all y rees in garnet from passo del sole y ree zoning varies radically from sample to sample in some rocks crystals of all sizes display only central peaks for all y rees in others profiles exhibit irregular fluctuations in y ree contents that match up with small scale patchy zoning in y and ca x ray maps in the franciscan complex y ree in garnet cores fluctuate unsystematically but mantles and rims display concentric oscillatory zoning for both major elements and y rees our interpretation of the complexity of y ree distributions in metamorphic garnet centres on the concept that these distributions vary primarily in response to the length scales over which these elements can equilibrate during garnet growth very short length scales of equilibration due to low intergranular mobility produce overprint zoning characterized by small scale irregularities higher but still restricted mobility yields diffusion controlled uptake characterized by patterns of central peaks and annular maxima that vary with atomic number and are strongly influenced by t t paths during garnet growth still greater mobility permits progressively greater potentially rock wide equilibration with major and accessory phase assemblages leading to mineralogical controls an unchanging mineral assemblage during garnet growth produces bell shaped profiles resembling those produced by rayleigh fractionation whereas changes in major and or accessory phase assemblages produce profiles with distinct annuli and sharp discontinuities in concentration the very high mobility associated with influxes of y ree bearing fluids can cause these element distributions to be dominated by factors external to the rock yielding profiles characterized by abrupt shifts or oscillations that are not correlated to changes in mineral assemblages

role of hydrothermal activity in uranium mineralisation in palnad sub basin cuddapah basin india abstract unconformity related uranium mineralisation occurs in banganapalle formation of palnad sub basin cuddapah basin several evidences of hydrothermal activity exist in both basement granite and the cover sediments in koppunuru and rallavagu tanda r v tanda uranium prospects of palnad sub basin profuse development of fracture filled veins consisting of epidote quartz chlorite quartz and quartz is observed at various depths above and below unconformity fluid rock interaction during the formation of these veins has resulted in the alteration of feldspars and mafic minerals of granite and arkosic quartzite into a mineral assemblage consisting of various proportion of illite chlorite muscovite and pyrite with the intensity of alterations being highest near to the unconformity pyrite is often associated with illite dominant alteration zone we infer that circulation of basinal brine through basement granite and cover sediments was responsible for mobilising uranium from granite and its precipitation at favourable locations in cover sediments increase in ph of ore fluid due to illitisation and chloritisation of wallrock together with availability of carbonaceous matter and pyrite as reductant have controlled the localisation of uranium mineralisation in banganapalle formation

reaction localization and softening of texturally hardened mylonites in a reactivated fault zone central argentina the tres arboles ductile fault zone in the eastern sierras pampeanas central argentina experienced multiple ductile deformation and faulting events that involved a variety of textural and reaction hardening and softening processes much of the fault zone is characterized by a d2 ultramylonite composed of fine grained biotite plagioclase that lacks a well defined preferred orientation the d2 fabric consists of a strong network of intergrown and interlocking grains that show little textural evidence for dislocation or dissolution creep these ultramylonites contain gneissic rock fragments and porphyroclasts of plagioclase sillimanite and garnet inherited from the gneissic and migmatitic protolith d1 of the hangingwall the assemblage of garnet sillimanite biotite suggests that d1 related fabrics developed under upper amphibolite facies conditions and the persistence of biotite garnet sillimanite plagioclase suggests that the ultramylonite of d2 developed under middle amphibolite facies conditions greenschist facies mylonitic shear bands d3 locally overprint d2 ultramylonites fine grained folia of muscovite chlorite biotite truncate earlier biotite plagioclase textures and coarser grained muscovite partially replaces relic sillimanite grains anorthite content of shear band d3 plagioclase is c an30 distinct from d1 and d2 plagioclase c an35 the anorthite content of d3 plagioclase is consistent with a pervasive grain boundary fluid that facilitated partial replacement of plagioclase by muscovite biotite is partially replaced by muscovite and or chlorite particularly in areas of inferred high strain quartz precipitated in porphyroclast pressure shadows and ribbons that help define the mylonitic fabric all d3 reactions require the introduction of h and or h2o indicating an open system and typically result in a volume decrease syntectonic d3 muscovite quartz chlorite preferentially grew in an orientation favourable for strain localization which produced a strong textural softening strain localization occurred only where reactions progressed with the infiltration of aqueous fluids on a scale of hundreds of micrometre local fracturing and microseismicity may have induced reactivation of the fault zone and the initial introduction of fluids however the predominant greenschist facies deformation d3 along discrete shear bands was primarily a consequence of the localization of replacement reactions in a partially open system

possible magmatic underplating beneath the west coast of india and adjoining dharwar craton imprint from archean crustal evolution to breakup of india and madagascar abstract the shear wave velocity of the crust along a 660 km profile from the west to the east coast of south india is mapped through the joint inversion of receiver functions and rayleigh wave group velocity the profile consisting of 38 broadband seismic stations covers the archean dharwar craton proterozoic cuddapah basin and rifted margin and escarpment the moho is mapped at a depth of 40 km beneath the mid archean western dharwar craton wdc cuddapah basin cb and the west and east coasts formed through the rifting process this is in contrast with a thin 35 km crust beneath the late archean eastern dharwar craton edc along the profile the average thickness of the upper middle and lower crust is 4 km 12 4 km and 24 4 km respectively above the moho we observe a high velocity layer hvl vs 4 km s of variable thickness increasing from 3 1 km beneath the edc to 11 3 km beneath the wdc and the cb and 18 2 km beneath the west coast of india the seismic wave velocity in this layer is greater than typical oceanic lower crust we interpret the high velocity layer as a signature of magmatic underplating due to past tectonic processes its significant thinning beneath the edc may be attributed to crustal delamination or relamination at 2 5 ga these results demonstrate the dual signature of the archean dharwar crust the change in the geochemical character of the crust possibly occurred at the end of archean when komatiite volcanism ceased the unusually thick hvl beneath the west coast of india and the adjoining region may represent underplated material formed due to india madagascar rifting which is supported by the presence of seaward dipping reflectors and a 85 90 ma mafic dyke in the adjoining island

pb pb age of earliest megascopic eukaryotic alga bearing rohtas formation vindhyan supergroup india implications for precambrian atmospheric oxygen evolution abstract though considered to be the earliest known well preserved megascopic body fossil of an eukaryotic alga grypania spiralis has not yet been dated directly and reliably in any of its four known locations in the world we report a pb pb isochron age of 1599 48 ma for the grypania bearing rohtas formation of the vindhyan supergroup central india besides being about 500 myears older than the previously inferred age for this formation this pb pb age together with close age constraints on the same fossil bearing horizons in jixian china and montana usa indicates that grypania bloomed on a global scale between 1 6 and 1 5 ga being an obligate aerobe the appearance of a much older grypania within one of the principal bif deposits near michigan usa has been linked to a significant jump in atmospheric oxygen between 2200 and 1900 ma inferred from geochemical evidence the long gap of 400 myears between the older and younger grypania occurrences could be due to reduction of atmospheric oxygen below the grypania survival threshold as a result of extensive bifs between about 1950 and 1850 ma

seismic imaging of the proterozoic cuddapah basin south india and regional geodynamics abstract cuddapah basin has been believed to be one of the largest intra cratonic proterozoic sedimentary basins of india situated in the eastern part of the dharwar craton of the south indian shield which is magmatically infested and contain thick column of sediments its overall sedimentary thickness as well as the nature of crustal evolution has remained enigmatic based on deep seismic sounding and other geological studies in the past it was perceived that this basin may contain as much as 10 12 km thick sediments the results of our present analysis derived up to a depth of 12 km reveals five layered upper crust associated with velocities i 4 50 km s ii 5 20 5 30 km s iii 5 50 5 80 km s iv 5 85 6 00 km s and v 6 40 km s out of which second and third layers correspond to upper and lower cuddapah sediments the results suggest the presence of only 4 0 km thick sediments in the deepest part of the basin below the nallamalai fold belt which has its implications in the developmental history of the basin a thermal driving force was invoked by earlier workers to account for estimated 10 12 km thick sediments however the present estimate of only 4 km basement depth in the cuddapah basin shows that the role of the thermal driving force may be marginal particularly in the deeper eastern cuddapah as isostatic subsidence due to sedimentary accumulation alone is enough to explain the basin depth further a basement sag of about 10 km would have logically needed lateral extension of the order of several hundreds of kilometers however as our present estimate of the sediment thickness gets reduced from earlier 10 km to only 4 km the size of the basin 44 500 km 2 would be in conformity with isostatic subsidence due to sedimentary accumulation hypothesis the structural features derived from present analysis like maximum depth observed near the thrust suture on the basin margin from where it decreases away from it its association of shallow marine sediments the arcuate shape of the basin along with its areal dimension resembles foreland basin between continent continent collisions

deformation pattern in the kurnool and nallarnalai groups in the northeastern part palnad area of the cuddapah basin south india and its implication on rodinia gondwana tectonics abstract the proterozoic basins of india adjoining the eastern ghats granulite belt eggb in eastern and southern india contain both mesproterozoic and neoproterozoic successions the intracratonic set up and contractional deformation fo the neoproterozoc successions in the paland sub basin in the northeastern part of cuddapah basin and similar crustal shortening in contemporaneous successions lying west of the eggb and nellore schist belt nsb are considered in relation to the proposed geodynamic evolution of the the rodinia and gondwana supercontinents tectonic shortening in the palnad sub basin northeast cuddapah partitioned into top to westnorthwest thrust shear flexural folds and cleavage development under overall e w contraction suggests foreland style continental shortening within an intracratonic set up a thrust sheet containing the nallamalai rocks and overlying the kurnool rocks in the northeastern part of palnad sub basin exhibits early tight to isoclinal folds and slaty phylllitic cleavage which can be correlated with early mesoproterozoic deformation structures in the nothern nallamalai fold belt nfb nne ssw trending folds and cleavage affect the kurnool group and overprint earlier structures in the thrust sheet thrusting of the nallamalai rocks and the later structures may have been related to convergence of the eastern ghats terrane and the east dharwar bastar craton during early neoproterozoic greenvillian and or later rejuvenation related to pan african amalgamation of east and west gondwana

water fluxed melting of the continental crust a review abstract water fluxed melting also known as fluid or water present melting is a fundamental process in the differentiation of continents but its importance has been underestimated in the past 20 years during which research efforts focused mostly on dehydration melting reactions involving hydrate phases in the absence of a separate aqueous phase the presence of a free aqueous phase in anatectic terranes influences all major physical and chemical aspects of the melting process from melt volumes viscosity and ability to segregate from rock pores to melt chemical and isotopic composition a review of the literature shows that melting due to the fluxing of aqueous fluids is a widespread process that can take place in diverse tectonic environments active tectono magmatic processes create conditions for the release of aqueous fluids and deformation driven transient high permeability channels capable of fluxing high temperature regions of the crust where they trigger voluminous melting water fluxed melting can be either congruent in regions at the water saturated solidus or incongruent at suprasolidus p t conditions incongruent melting reactions can give rise to peritectic hornblende or to nominally anhydrous minerals such as garnet sillimanite or orthopyroxene in this case the presence of an aqueous phase is indicated by a mismatch between the large melt fraction generated and the much smaller fractions predicted in its absence the relatively small volumes of aqueous fluids compared to that of rocks imply that melting reactions are generally rock buffered fluids tend to move upwards and down temperature however there are cases in which pressure gradients drive fluids up temperature potentially fluxing suprasolidus terranes crustal regions at conditions equivalent to the water saturated solidus represent a natural impediment to the up temperature migration of aqueous fluids because they are consumed in melting reactions in this case continued migration into supra solidus terranes take place through the migration of water rich melts thus melts become the transport agent of water into supra solidus terranes and responsible for water fluxed melting other processes such as the relatively rapid fluid migration through fractures also allow regional aqueous fluids to by pass the water saturated solidus fluid trap and trigger melting above solidus conditions when aqueous fluids or hydrous melts flux rocks at supra solidus conditions they equilibrate with the surroundings through further melting decreasing water activity and giving rise to undersaturated melts it is in these conditions that hornblende or anhydrous peritectic phases are stabilized unlike dehydration melting the melt fraction generated in this case is not limited by the water contained in hydrous minerals but by the volume of water added to the system unlike melting at the water saturated solidus these melts are capable of rising without freezing and do give rise to upper crustal granitic bodies

imprints of a proterozoic tectonothermal anomaly below the 1 1 ga kimberlitic province of southwest cuddapah basin dharwar craton southern india summary a detailed 2 d modelling of the seismic structure and other geological and geophysical signatures across the cuddapah basin of the southern indian shield suggests upwarping of highvelocity and high density layers which are observed close to the surface below the southwestern part of the basin this anomalous feature is constrained by i a strong gravity high anomaly of about 55 mgal ii a 100 km wide high conductivity anomaly resistivity 100 m extending from surface to a minimum depth of 50 km in the mantle lithosphere and iii large scale massive intrusive activity these features are interpreted to be an expression of a thermal anomaly which may have acted like a plume during the proterozoic and could well correspond to a 1 1 ga kimberlitic activity below this region the thin granitic gneissic crust is underlain by well differentiated high velocity layers possibly due to underplating and densification of much of the crust by extruded magma

lithospheric electrical imaging of the deccan trap covered region of western india 1 we present here the results of magnetotelluric mt studies carried out along a 330 km long ne sw trending traverse from daulatabad at the northeastern end to koyna on the southwestern end across the deccan volcanic province dvp in western india the 2 d mt modeling results obtained along this traverse together with those from other mt traverses in this area are examined and integrated with the results from deep seismic sounding 3 d teleseismic tomography and heat flow studies the deep geoelectric section as deduced from mt models point out to a two layered lithospheric mantle structures with an upper very high resistive layer several thousands of ohm m and a bottom relatively conductive one a few tens of ohm m to a few hundred ohm m the lithospheric electrical structure is inferred to be linked to compositional changes associated with a depleted lithospheric upper mantle in the dharwar craton an impressive feature that the study has brought out is the presence of a well defined lithospheric upper mantle conductor in the depth range of 80 120 km limited to the middle segment of the mt traverse from sangole to partur over the dvp the electrical characterization of the region including the presence of upper mantle conductor shows close similarities with that of other similar cratonic regions like the slave and superior cratons in northwestern canada

an overview of the palaeoproterozoic geology of peninsular india and key stratigraphic and tectonic issues abstract the archaean proterozoic rock successions in india have the potential to enrich the global database on precambrian stratigraphic development and to offer valuable clues to global tectonic reconstructions built over four distinct archaean cratonic nuclei the major palaeoproterozoic supracrustal belts cover sequences include the dhanjori group the singhbhum group including the dalma volcanics and the chandil formation in eastern india the cuddapah supergroup and the adjoining nellore schist belt bordering the eastern dharwar craton the aravalli supergroup in the aravalli delhi fold belt in nw india the lower vindhyan semri group and the mahakosal sausar betul belts close to the central indian tectonic zone citz and possibly the dongargarh supergroup in bastar a major erosional unconformity separates the archaean tonalite trondjhemite gneiss basement in these cratons from the overlying volcano sedimentary successions an overview of palaeoproterozoic stratigraphic development in these cratonic blocks is presented to bring out the salient features for global comparison and to highlight issues requiring further attention multiple metamorphic magmatic and deformation events are recorded in the fold belts at the join of the cratons or their margins inviting application of the plate tectonic paradigm however a comprehensive tectonic model for the amalgamation of the indian archaean nuclei is yet to emerge and is crucial for our understanding of palaeoproterozoic supercontinent development

detrital mineral age radiogenic isotopic stratigraphy and tectonic significance of the cuddapah basin india abstract the cuddapah basin is one of a series of proterozoic basins that overlie the cratons of india that due to limited geochronological and provenance constraints have remained subject to speculation as to their time of deposition sediment source locations and tectonic geodynamic significance here we present 21 new stratigraphically constrained u pb detrital zircon samples from all the main depositional units within the cuddapah basin these data are supported by hf isotopic data from 12 of these samples that also encompass the stratigraphic range and detrital muscovite 40ar 39ar data from a sample of the srisailam formation taken together the data demonstrate that the papaghni and lower chitravati groups were sourced from the dharwar craton in what is interpreted to be a rift basin that evolved into a passive margin the nallamalai group is here constrained to be deposited between 1659 22 ma and 1590 ma it was sourced from the coeval krishna orogen to the east and was deposited in its foreland basin nallamalai group detrital zircon u pb and hf isotope values directly overlap with similar data from the ongole domain metasedimentary rocks depositional age constraints on the srisailam formation are permissive with it being coeval with the nallamalai group and it was possibly deposited within the same basin the kurnool group saw a return to dharwar craton derived provenance and is constrained to being neoproterozoic it may represent deposition in a long wavelength basin forelandward of the tonian eastern ghats orogeny detrital zircons from the gandikota formation which is traditionally considered a part of the chitravati group constrain it to being deposited after 1181 29 ma more than 700 ma after the lower chitravati group it is possible that the gandikota formation is correlative with the kurnool group the new data suggest that the nallamalai group correlates temporally and tectonically with the somanpalli group of the pranhita godavari valley basin which is tightly constrained to being deposited at 1620 ma these syn orogenic foreland basin deposits firmly link the se india proterozoic basins to their orogenic hinterland with their discovery filling a missing link in the tectonic development of the region

a regularized wiener hopf filter for inverting models with magnetic susceptibility abstract for a magnetic target the spatial magnetic signal can be expressed as a convolutional integral over green s function of an assumed model with susceptibility as its parameter a filter can be used to obtain the susceptibility by minimizing the mismatch between observed and the computed magnetic anomalies in this perspective we report the development of an advanced digital filter which is efficient and can be used to map rock susceptibility from the acquired magnetic data to design the new filter we modified the space domain standard wiener hopf filter by imposing two different constraints i the filter energy constraint and ii normalization of the filter coefficients these constraints make it capable to characterize source bodies from their produced magnetic anomalies we assume that the magnetic data are produced by induced magnetization only and interpretation can be as good as the subsurface model our technique is less sensitive to the data noise which makes it efficient in enhancing the interpretation model the modified filter demonstrates its applicability over the synthetic data with additive white gaussian noise in order to check the efficacy and adaptivity of this tool in a more realistic perspective it is also tested on the real magnetic data acquired over a kimberlitic district adjoining to the western margin of the cuddapah basin in india to identify the source bodies from the anomalies our result shows that the modified wiener hopf filter with the constraint for the magnetic data is more stable and efficient than the standard wiener hopf filter

genesis of aluminous and intermediate granulites a case study in the eastern sierras pampeanas argentina abstract we report petrologic and geochemical features of anhydrous granulites from the eastern sierras pampeanas argentina aluminous grt crd bearing granulite is the most abundant rock type in the study region and hosts intermediate opx grt bearing granulites as interlayered lenses both granulite types crystallized under similar p t conditions of t 850 900 c and p c 7 5 kbar the formation of grt crd granulites by subsolidus dehydration conflicts with the absence of k feldspar in them moreover the aluminous granulites have an ree pattern that precludes postulating they derived by close system metamorphism of an extensively weathered protoliths comparing mineral and rock chemistry with experimental results in simple systems acfm akf and afm permits the mineralogical changes and the bulk composition evolution experienced by granulites during melting to be identified this work demonstrates that orthopyroxene is a primary phase in opx grt granulites however never crystallized within aluminous granulites the aluminous granulites have both mineral and chemical compositions matching those expected for a residuum derived from the partial melting of a greywacke like sedimentary precursor compared with the composition of the average continental crust aluminous granulites are depleted in most lithophile trace elements and lree have somewhat low k rb ratios and high contents of elements having high partition coefficients for garnet and or ilmenite hree y cr ni however aluminous granulites have initial sr isotope ratios 0 713 we discuss the possibility that an intense but short lived melting event led to isotopic disequilibrium during melting of biotite bearing protoliths leaving a granulitic residuum with lower initial sr isotope ratios in contrast opx grt granulites have trace element features e g high sr contents and ree patterns with marked positive eu anomalies that do not support the idea that they are simply residues left after biotite dehydration melting and melt extraction rather these rocks display several petrological major and trace element and isotopic features that suggest they formed through the interaction of mafic magmas and aluminous granulites

inversion of calcite twin data paleostress reconstruction and multiphase weak deformation in cratonic interior evidence from the proterozoic cuddapah basin india abstract paleostress orientations from mechanically twinned calcite in carbonate rocks and veins in the neighborhood of large faults were investigated to comment on the nature of weak upper crustal stresses affecting sedimentary successions within the proterozoic cuddapah basin india application of turner s p b t method and spang s numerical dynamic analysis on cuddapah samples provided paleostress orientations comparable to those derived from fault slip inversion results from the neighborhood of e w faults cutting through the paleoproterozoic papaghni and chitravati groups and the neoproterozoic kurnool group in the western cuddapah basin reveal existence of multiple deformation events 1 ne sw 3 in strike slip to extensional regime along with an additional event having nw se 3 for lower cuddapah samples 2 compressional transpressional event with ese wnw or nne ssw 1 mainly from younger kurnool samples integrating results from calcite twin data inversion fault slip analysis and regional geology we propose that late mesoproterozoic crustal extension led to initial opening of the kurnool sub basin subsequently influenced by weak compressional deformation the dynamic analysis of calcite twins thus constrains the stress regimes influencing basin initiation in the southern indian cratonic interior and subsequent basin inversion in relation to craton margin mobile belts and plausible global tectonic events in the proterozoic

the neoproterozoic cratonic successions of peninsular india abstract the peninsular india hosts extensive record of mesoproterozoic and neoproterozoic successions in several mobile belts and cratonic basins the successions provide excellent opportunities for chronostratigraphic classification in tune with the chronometric classification adopted by iugs for inter regional correlation on a global scale major tectono thermal events at 1000 950 ma in the mobile belts correlatable with the grenville orogeny may be considered as the datum for meso neoproterozoic classification in india principles of chronostratigraphic classification however can not be applied yet to the cratonic successions of india because of inadequate radiometric data paucity of biostratigraphic studies and lack of regionally correlatable stratigraphic or palaeoclimatic datum the kimberlite magmatism which affected the peninsular india on a continental scale at about 1100 ma holds the key to the identification of neoproterozoic successions of the cratonic basins thus the stratigraphically confined diamond bearing conglomerates and or the tuffs associated with kimberlites may be considered as the datum to define the base of the neoproterozoic fixed at about 1000 ma accordingly the rewa and bhander groups in the vindhyan basin the kurnool group in the cuddapah basin the jagdalpur formation in the indravati basin and the sullavai group in the pranhita godavari basin are taken to represent the neoproterozoic successions in the peninsular india the chattisgarh group in the central india the lower part of the marwar supergroup in western rajasthan the badami group in the kaladgi basin and the bhima group are the other possible neoproterozoics in the peninsula the closing phase of the mesoproterozoic in all these basins are characterised by stable shelf lithologic associations attesting to high crustal stability the neoproterozoic basins by contrast mark a new phase of rifting and extension and the basin fills exhibit signatures of initial instability which evolved with time into a more stable platformal condition a major episode of sea level rise has been recorded in most of the basins the riftogenic origin and evolution of the basins are comparable with the history of neoproterozoic basins of australia though there is no unequivocal record of glaciation in the indian formations

magnetic mapping of majhgawan diamond pipe of central india magnetic response over kimberlite lamproite pipes can be used to map their horizontal and vertical cross sections a published ground magnetic vertical intensity contour map is used to map the majhgawan kimberlite pipe near panna in central india using barnett s 1976 method we obtained a reasonable match between the computed and observed fields with three separate bodies the model suggests a possible later intrusion into the main pipe geological studies using drill hole data imply a nearly concentric vertical lithological zoning within the pipe and thus support the hypothesis of multiple intrusions indicated by magnetic data the multiple intrusion hypothesis is of interest to exploration because specific intrusions within one composite pipe may differ markedly in the incidence of diamonds the higher incidence of diamonds reported in the central part of the majhgawan pipe as compared to the outer part also supports the possibility of a later intrusion into the main pipe

c o sr and pb isotope systematics of carbonate sequences of the vindhyan supergroup india age diagenesis correlations and implications for global events abstract the vindhyan supergroup of central india records a substantial portion of proterozoic time and contains some of the most disputed precambrian cambrian fossil discoveries despite the significance of these fossil finds many issues including reliable geochronology remained unresolved similarly the geochemical aspects of the sedimentary sequences and their implications to the evolution of proterozoic oceans have been studied only at a reconnaissance level here we report the results of the pb pb dating for the rohtasgarh limestone from the lower vindhyan supergroup giving an age of 1601 130 ma an estimate that is in accord with the recent u pb zircon dating of the lower vindhyan supergroup by rasmussen et al geology 30 2002 103 and ray et al geology 30 2002 131 sr isotope stratigraphy of carbonates from the upper vindhyan supergroup suggests a mid neoproterozoic 750 650 ma age for these formations this in turn argues for a long hiatus between the deposition of the lower and upper vindhyan supergroups our results conflict with the reports of cambrian age small shelly fossils and ediacaran fossils from the upper vindhyan rocks at the same time they suggest that the alleged trace fossils in the lower vindhyan rocks must be of paleoproterozoic age the most common 18 o values for calcitic components from limestone formations of the supergroup range from 10 to 7 similar to paleo neoproterozoic carbonates worldwide the modes of 13 c in the lower and upper vindhyan supergroups are 0 2 and 3 2 respectively again consistent with the worldwide dataset the secular patterns of carbon isotope trends do not support the earlier assumptions that the carbonate sequences at the southern margin correlate with those at the western or northern margins of the vindhyan basin c and sr isotope data provide new calibration points for the seawater evolutionary curves particularly for the paleoproterozoic for strontium isotopes the data from the lower vindhyan supergroup suggest that seawater departed probably less rapidly from mantle values than was previously believed

petrology of rocks formed by interaction of mafic magmas and high grade metamorphic rocks at suya taco area northern sierra de comechingones argentina abstract interaction between mafic magmas and crustal metamorphic rocks at mid crustal levels causes modification of mafic magma composition by assimilation or reaction with crustal materials this petrologic process is studied at the suya taco complex where a suite of mafic magmas interacted with metamorphic rocks at 6 6 1 kbar the suya taco complex consists of close spaced bodies of biotite bearing gabbronorites these magmas intruded into a supracrustal sequence consisting of amphibolite to granulite facies gneisses and migmatites the peak metamorphic mineral assemblage bt qtz pl an 30 grt kfs crd sil of supracrustal rocks crystallised before igneous mafic magmatism interaction between mafic magmas and metamorphic rocks produces two major hybrid lithologies diorites made up of opx bt pl an 38 50 ilm cpx qtz and garnet and orthopyroxene bearing rocks characterised by the coexistence of bt pl an 30 45 grt opx qtz ilm spl kfs the latter lithology may locally appear as a quartz absent and high aluminous mineral assemblage the two hybrid rocks occur interlayered at the contact zone between igneous mafic rocks and high grade gneisses individual layers range from centimeter to meter scales moreover dioritic and orthopyroxene and garnet bearing layers present a spectrum of mineral assemblages and mineral chemistry that spread among those that appear in igneous mafic and metamorphic rocks the work shows that the changes of mineral assemblages and variations of mineral chemistry in the hybrid diorites and orthopyroxene and garnet bearing rocks are the result of interaction processes between mafic magmas and regional metamorphic rocks field relationships petrographic features mineral compositional changes and qualitative analyses of phase relations suggest that rocks formed by interaction between mafic magmas and metamorphic rocks results from 1 assimilation of metamorphic rocks by mafic magmas at variable degrees and 2 ion diffusion among mineral assemblages of contrasting composition the implication of these processes is that ca is transferred from mafic magmas to metamorphic rocks which forces the breakdown of garnet and quartz and induces the crystallisation of plagioclase and orthopyroxene in the resulting hybrid rocks

amphibolite to granulite transition in aluminous greywackes from the sierra de comechingones cordoba argentina we discuss upper amphibolite to granulite facies early palaeozoic metamorphism and partial melting of aluminous greywackes from the sierra de comechingones se sierras pampeanas of central argentina consistent p t estimates obtained from equilibria involving al and ti exchange components in biotite and from more traditional thermobarometric equilibria suggest that peak metamorphism of the exposed section took place at an essentially constant pressure of 7 8 kbar and at temperatures ranging from 650 to 950 c mineral compositions record an initial decompression after peak metamorphism of c 1 5 kbar which was accompanied by a cooling of c 100 c upper amphibolite facies gneisses consist of the assemblage qtz pl bt grt rt ilm the transition to the granulite facies is marked by the simultaneous appearance of the assemblage kfs sil and of migmatitic structures suggesting that the amphibolite to granulite transition in the sierra de comechingones corresponds to the beginning of melting rocks with structural and or chemical manifestations of partial melting range from metatexites to diatexites to melt depleted granulites consisting of the assemblage grt crd pl qtz ilm ath the melting stage overlapped at least partially with decompression as suggested by the occurrence of cordierite in both the migmatites and the residual granulites of two distinct textural types idiomorphic porphyroblasts probably representing peritectic cordierite and garnet rimming coronas metapelitic rocks are unknown in the sierra de comechingones therefore it appears most likely that the al rich residual assemblages found in the migmatites and residual granulites were formed by partial melting of muscovite and sillimanite undersaturated metagreywackes we propose a mechanism for this that relies on the sub solidus stabilization of garnet and the ensuing changes in the octahedral al content of biotite with pressure and temperature

new constraints from zircon monazite and uraninite dating on the commencement of sedimentation in the cuddapah basin india the cuddapah basin in southern india consisting of the palnad srisailam kurnool and papaghni sub basins contains unmetamorphosed and undeformed sediments deposited during a long span of time in the proterozoic in the absence of robust age constraints there is considerable confusion regarding the relative timing of sedimentation in these sub basins in this study u pb isotopic dating of zircon and u th pb total dating of monazite and uraninite from the gritty quartzite that supposedly belongs to the formation banganapalle quartzite have been used to constrain the beginning of sedimentation in the palnad sub basin magmatic and detrital zircons recording an age of 2 53 ga indicate that the sediments were derived from the granitic basement or similar sources and were deposited after 2 53 ga hydrothermally altered zircons both in the basement and the cover provide concordant ages of 2 32 and 2 12 ga and date two major hydrothermal events thus the gritty quartzite must have been deposited sometime between 2 53 and 2 12 ga and represents the earliest sediments in the cuddapah basin monazite and uraninite give a wide spectrum of ages between 2 5 ga and 150 ma which indicates several pulses of hydrothermal activity over a considerable time span both in the basement granite and the overlying quartzite the new age constraints suggest that the gritty quartzite may be stratigraphically equivalent to the gulcheru quartzite that is the oldest unit in the cuddapah basin and that a sedimentary erosional hiatus exists above it

low shear velocities in the sub lithospheric mantle beneath the indian shield 1 ever since its breakup from the gondwanaland 140 myr ago the indian plate was ravaged by four hot spots although the surface manifestations of such deep processes are evident in terms of large igneous provinces like the deccan and the fast drift of the indian plate the modifications to the deep structure remain to be grasped in this study we investigate the mantle transition zone tz structure beneath the indian shield region using 14 000 teleseismic receiver functions from 77 broadband stations sited on diverse geologic terrains the arrival times of the p to s ps conversions from the 410 km discontinuity at most cratonic stations appear to be delayed by 2 s in comparison with the times observed for other precambrian shield regions like africa australia and canada such delays in the conversions from the 410 km discontinuity below the indian shield suggest low shear wave speeds in the lithospheric and sub lithospheric mantles due to higher temperatures together with a thinner high velocity lid that contrasts with a thicker one found beneath most archean cratons a thin transition zone beneath most of the cratonic stations lends support to the enhanced temperatures within the tz itself also a further delay of the tz discontinuities is observed for stations on the southern granulite terrain which was under the influence of the marion plume that is responsible for the separation of madagascar from india although the data do not conclusively show evidence for a 520 km discontinuity an lvl atop the 410 cannot be ruled out beneath certain geological provinces of the indian shield

zircon u pb ages geochemistry and sr nd isotopes of the golshekanan granitoid urumieh dokhtar magmatic arc iran evidence for partial melting of juvenile crust

paleomagnetism of ca 2 3 ga mafic dyke swarms in the northeastern southern granulite terrain india constraints on the position and extent of dharwar craton in the paleoproterozoic abstract the pole positions reported for the mafic dykes occurring in the northeastern southern granulite terrain and easterly trending dykes of dharwar craton are alike while their ages differ by ca 700 ma confounding tectonic reconstruction to resolve the discordance and to constrain the position of the indian continent in the proterozoic paleomagnetic and geochronological studies have been carried out on the mafic dyke swam in the northeastern southern granulite terrain two distinct pole positions are obtained one from the group of tiruvannamalai dykes as 27 7 s 231 5 e dp dm 12 14 with corresponding mean declination dm and inclination im as 125 and 73 8 95 22 7 6 respectively and second from the east coast dykes as 2 32 s 188 2 e dp dm 5 8 with corresponding mean dm and im as 88 9 and 33 8 95 48 7 1 respectively the remanent efficiency of magnetization values between 0 01 and 0 08 suggest that the dykes have been cooled through curie temperature of magnetite which is the carrier of magnetization the baked contact test confirms that the characteristic remanent magnetization is the primary magnetization the sm nd mineral whole rock isochron from one of the tiruvannamalai dykes yielded an age of 2318 60 ma mswd 1 9 this age is the time of intrusion of the tiruvannamalai dykes and is similar within errors to the age of dykes in the dharwar craton thus the contiguity of dharwar craton with the northern block of southern granulite terrain as early as 2 3 ga ago is established the pole position thus obtained corresponds to a high latitudinal position of dharwar craton during early paleoproterozoic

palaeoproterozoic indian shield in the global continental assembly evidence from the palaeomagnetism of mafic dyke swarms abstract palaeomagnetic study of palaeoproterozoic mafic dykes in the basement along the margins of the cuddapah basin the largest precambrian sedimentary basin in south india is presented in detail for a general discussion of palaeoproterozoic igneous activity in india the results are compared with all earlier published data on mafic dykes in india and are integrated with recently published high precision u pb baddeleyite ages to provide a comprehensive account of palaeoproterozoic igneous activity in india the analysis consolidates palaeomagnetic poles for six age divisions between 2 45 and 1 85 ga with robust statistical criteria our best estimates of overall mean poles from 241 dykes are situated at 1 17 7 n 106 0 e a95 9 0 7n 24 at c 2 45 ga 2 7 1 n 57 2 e a95 4 5 n 69 at c 2 37 ga 3 41 6 s 5 5 e a95 5 1 n 34 at c 2 22 ga 4 4 7 n 343 0 e a95 4 4 n 31 at 2 18 ga 5 49 2 n 332 9 e a95 4 8 n 24 at 1 99 1 89 ga and 6 73 7 n 282 6 e a95 2 9 n 39 at 1 86 ga the data permit us to construct an apparent polar wander path for the indian shield for an 600 ma interval of the palaeoproterozoic eon 2 45 1 85 ga testing and evaluation of continental reconstructions for this interval which are mostly based on geological correlations reveal many inconsistencies between 2 45 and 2 37 ga the indian shield was situated at higher latitudes similar to the yilgarn craton of australia it was subsequently located near the equator at 2 22 2 18 1 99 and 1 86 ga thus an india australia connection is supported during these times but a proposed australia kaapvaal link in zimvaalbara and a dharwar india slave connection in sclavia or a superior zimbabwe india connection in superia are inconsistent with indian data in addition the close palaeomagnetic comparison between the palaeoproterozoic dykes of dharwar bastar bundelkhand cratons in india indicates an age of 2 45 ga for orogenic activity along the central indian tectonic zone hence matching this zone with the 2 0 1 8 ga trans north china orogenic belt or positioning north china adjacent to india or juxtaposition of the indian shield along the western margin of laurentia in the columbia reconstructions is not supported the indian data appear to be in accord with the essential features of the refined protopangaea model and the original ur configuration finally the results are interpreted in terms of four dyke emplacement events in the age range 2 45 2 18 ga linked to short lived 5 10 ma lips developed over mantle plumes the dykes of 1 99 1 89 ga age probably relate to continued long lived igneous activity while the 1 86 ga dykes are relatively fewer in number and may represent waning stage of a large igneous event related to a major mantle plume

rbsr ages of proterozoic kimberlites of india evidence for contemporaneous emplacement abstract rb sr analyses of leached phlogopite macrocrysts from kimberlite pipes 1 2 5 and 7 in south india give concordant ages of 1091 20 1092 15 1093 20 and 1091 10 ma respectively indicating contemporaneous emplacement of these pipes the present results are in contradiction with earlier measurements groundmass mineral assemblages with low rb sr ratios give a tight set of initial sr ratios at 0 7029 2 0 7019 2 0 7029 2 and 0 7030 4 suggesting derivation of the primary kimberlite magma from a relatively uniform and depleted mantle source a similar age and initial sr isotopic ratio of one sample of the majhgawan lamproite from central india imply that the proterozoic kimberlite lamproite activities in india though widely separated in space were nearly contemporaneous

mineralogy of the tk1 and tk4 kimberlites in the timmasamudram cluster wajrakarur kimberlite field india implications for lamproite magmatism in a field of kimberlites and ultramafic lamprophyres abstract a mineralogical study of the hypabyssal facies late cretaceous macrocrystic pulse of tk1 intrusion and the mesoproterozoic aphanitic pulse of tk4 intrusion in the wajrakarur kimberlite field of southern india shows that the rocks contain macrocrysts of forsteritic olivine phenocrysts and microphenocrysts of al na poor diopside and phlogopite set in a groundmass mainly of al na poor diopside and phlogopite other groundmass minerals are spinel perovskite and fluorapatite in tk1 and spinel titanite chlorite calcite and gittinsite in tk4 k richterite and perovskite occur only as inclusions in phlogopite and titanite respectively in tk4 late stage deuteric phases include pyrite and barite in tk1 and strontianite chalcopyrite galena and pentlandite in tk4 diopside microphenocrysts in tk4 exhibit oscillatory zoning with characteristics of diffusion controlled magmatic growth tk1 spinels show magmatic trend 2 that evolves from magnesiochromite and culminates in titaniferous magnetite whereas tk4 spinels are less evolved with magnesiochromite composition only tk1 phlogopites show a simple compositional trend that is typical of lamproite micas while four distinct growth zones are observed in tk4 phlogopites with the following compositional characteristics zone i high cr 2 o 3 and tio 2 and low bao zone ii low cr 2 o 3 zone iii low tio 2 and high bao zone iv low bao forsterite contents and trace element concentrations reveal two xenocrystic core populations and one magmatic rim population for tk1 olivines mineralogically both tk1 and tk4 are classified as diopside phlogopite lamproites rather than archetypal kimberlites the two lamproites are considered to have formed from the same parent magma but crystallised under distinct oxygen fugacity conditions with elevated content of fe 3 in phlogopite spinel and perovskite tk1 appears to have crystallised in a relatively high oxygen fugacity environment multiple growth generations of phlogopite spinel and fluorapatite in tk4 indicate a complex evolutionary history of the magma close spatial and temporal associations of mesoproterozoic kimberlites and lamproites in southern india can possibly be explained by a unifying model which accounts for the generation of diverse magmas from a range of geochemical resevoirs in a continental rift setting

plate margin paleostress variations and intracontinental deformations in the evolution of the cuddapah basin through proterozoic abstract the sedimentary basins are often influenced by adjoining mobile belts as is the case of the proterozoic cuddapah basin in southern india lying in proximity of the eastern ghats granulite belt egb and the southern granulite terrain sgt two major faults gani kalva and kona faults in the western part of the cuddapah basin have been examined in this work to comment on the changes in paleostress conditions affecting the basin evolution punctuated by several stages of extensional and compressional regimes in the time interval 1 9 0 5 ga the tensor solutions were obtained for the mesoscale faults and fault striae using the improved right dihedron method followed by the rotational optimization method tensor delvaux and sperner 2003 combining the paleostress field reconstruction and other structural stratigraphic attributes we propose a possible scenario of punctuated evolution of the cuddapah basin and its inversion the stress regimes are tentatively correlated with multiple stages of basin opening and deformation of basin in fill representing the pre grenvillian through the pan african orogeny the paleostress conditions derived from various stratigraphic horizons provide an opportunity to comment on the changes in tectonic stresses including several stages of extensional and compressive regimes in the cuddapah basin the tensor solutions were obtained for the mesoscale faults and fault striae in the neighborhood of above two fault lines using the improved right dihedron method followed by the rotational optimization method tensor delvaux and sperner 2003 the most prominent stress states recorded around the two fault lines can be classified into those affecting 1 the paleoproterozoic lower cuddapah successions 2 the younger kurnool group and 3 both the groups integrating the stress states of the fault slip analysis obtained from different unconformity bounded rock groups of lower cuddapah and other geologic data we suggest extensional to transtensive pure strike slip regime to be active during the first phase of basin opening and c 1 89 ga mafic igneous activity late compressional to transpressive pure strike slip regimes are well recorded from the younger kurnool group representing late neoproterozoic deformations the tensor solutions from different unconformity bound stratigraphic horizons constrain different states of deformation we relate the extensional to transtensive pure strike slip regimes from lower cuddapah successions to the ongoing basin opening events and the associated palaeoproterozoic igneous activities the late neoproterozoic to early paleozoic compressional to transpressive strike slip regime present the possible effects of compressional activities happening around the cratonic margin the present analysis lends further support to the idea that estimation of paleostress states from fault slip data even in regions of relatively weak deformation in continental interior are important while assessing influence of plate margin stresses on the continental interior

thermal stability and spectroscopic studies of zemkorite a carbonate from the venkatampalle kimberlite of southern india while characterizing the mineralogy of the kimberlite at venkatampalle andhra pradesh india lat 14 56 00 n long 77 22 15 e we found the second occurrence of zemkorite na k 2 ca co 3 2 which was first reported in 1989 in the udachnaya kimberlite in siberia we also report the first fourier transform infrared spectroscopic ftir scanning electron microscopic thermo gravimetric and high temperature heat capacity measurements on this rare carbonate powder x ray diffraction xrd patterns show 24 well resolved diffraction lines all of which can be indexed with a hexagonal cell with a 10 038 5 a and c 12 726 5 a the vibrational spectrum of zemkorite at room temperature exhibits 13 distinct absorption bands in the frequency range 2000 to 400 cm 1 the ir bands of zemkorite indicate a structural similarity with shortite but are quite distinct from other alkali carbonates such as nyerereite butschiite and fairchildite the heat capacity of zemkorite has been measured up to 700 k by differential scanning calorimetric techniques the temperature dependence of the heat capacity of zemkorite was fitted with the polynomial c p 140 2 8 584 10 2 t 2 458 10 6 t 2 the upper thermal stability of zemkorite is 700 k which is similar to shortite zemkorite may have formed during the late stages of kimberlite genesis possibly as a result of metasomatism or by the breakdown of metasomatic natrocarbonatitic minerals or glass that segregated on decompression melting in the upper mantle

geochemistry of mesoproterozoic sedimentary rocks of upper vindhyan group southeastern rajasthan and implications for weathering history composition and tectonic setting of continental crust in the northern part of indian shield abstract the upper vindhyan succession of southeastern rajasthan is divisible into kaimur rewa and bhander groups the major and trace element including rare earth elements data of the upper vindhyan shales and sandstones are investigated to determine the weathering history composition and tectonic setting of mesoproterozoic continental crust cia chemical index of alteration values a cn k plot a al2o3 cn cao na2o k k2o and depletion in u na2o cao sr and ba suggest that the source area experienced moderate to high degree of chemical weathering under warm and humid conditions provenance modeling indicates that the kaimur sandstones are best modeled with a mixture having 40 granitic gneiss 20 tonalite trondhjemite granodiorite ttg 20 mafic enclaves and 20 berach granite of the banded gneissic complex bgc a mixture of 60 granitic gneiss 20 mafic enclaves and 20 berach granite of the bgc can model the rewa and bhander groups it is suggested that the upper vindhyan sedimentation commenced at the time of delhi sausar orogeny at about 1100 1000 ma the orogenic movements uplifted the parts of old continental crust in the bgc terrain creating positive areas which exposed older crustal blocks containing ttg as important component the debris of kaimur sandstone probably derived from these uplifted blocks as indicated by palaeocurrent data the rewa and bhander formations were derived from bundelkhand granitic gneiss complex bggc occurring to the north of the basin and or the chotanagpur granitic gneiss complex cggc of eastern indian shield the derivation of lower and upper groups of vindhyan succession from different source terrains of identical composition suggests that at the time of vindhyan sedimentation the bgc of rajasthan the bggc of central india and the cggc of eastern india had similar lithological composition this implies that well before the origin of the vindhyan basin these discrete terrains evolved as a single unit that existed as a mesoarchaean nucleus in the northern part of the indian shield

geochemical characteristics of kimberlite rock of the anantapur and mahbubnagar districts andhra pradesh south india abstract a comparative study of kimberlite rocks exposed in the anantapur and mahbubnagar districts and the reinterpretation of published data revealed considerable differences between kimberlites of anantapur and mahbubnagar the kimberlites of mahbubnagar show high concentrations of ni cr co zr hf ta th and u and lower abundance of rb sr and ba relative to anantapur the relationships of ni cr zr hf nb ta and th u show that the mahbubnagar kimberlites are distinctly different from the anantapur kimberlites the mineralogical and chemical characteristics of the mahbubnagar kimberlites differ from that of typical kimberlite and lamproite the ce y vs zr nb and zr nb vs la yb plots suggest that the mahbubnagar kimberlites formed by higher degrees of partial melting of the mantle source than that of anantapur kimberlites

mo ni and organic carbon isotope signatures of the mid late mesoproterozoic oxygenation abstract this study performed a redox sensitive trace element rste and organic carbon isotope investigation on the shallow section of 1 2 ga bijaigarh shale kaimur group vindhyan supergroup the results demonstrate that unlike the coeval deeper section in which mo and ni concentrations are several folds higher compared to the upper continental crust ucc the shallow section contains mo and ni equivalent to the upper continental crust ucc this spatially heterogeneous burial of mo and ni is very similar to the modern stratified basins i e black sea and cariaco basin where ni and mo composition of the sediments underlying a shallow oxic water column is comparable to the ucc however the deeper sediments underlying the anoxic euxinic contains several folds higher mo and ni compared to the ucc bulk precipitation of mo in a hydrosphere is mainly controlled by sulphide present in the anoxic euxinic layers of the water column on the other hand ni precipitation is generally controlled by oxygenic primary productivity though the shallow section of the bijaigarh shale is depleted in ni a significant correlation between ni and toc is still relevant and indicates an oxygen rich surface water layer for the respective palaeohydrosphere furthermore 13c org values between 32 0 and 30 4 from the shallow section suggests that the organic carbon exported into the sediments during the bijaigarh shale deposition was primarily derived from oxygenic photosynthesis in the concurrent surface water layers

cambrian to devonian geologic evolution of the sierra de comechingones eastern sierras pampeanas argentina evidence for the development and exhumation of continental crust on the proto pacific margin of gondwana abstract crystalline rocks from the sierra de comechingones eastern sierras pampeanas evolved through three distinct orogenic cycles during the eopalaeozoic 1 the first tectono thermal event named pampean orogeny 550 to 505 ma which peaked in the early cambrian was responsible for extensive metamorphism partial melting juvenile magmatism rapid decompression and persistent tectonic activity large part of the crustal section that was residing at middle levels c 27 km was heated above 800 c during the thermal peak stage of the pampean orogeny decompression of the pampean orogen s core took place at this high temperature the exhumation mechanism that assisted rapid uplifting combined the effects of ongoing tectonic forces with a buoyant instability created by a large amount of anatectic magmas in the middle to lower crust 2 beginning at the early ordovician the famatinian orogeny produced an overall shortening causing pervasive textural reworking of the cambrian metamorphic sequences under a high strain regime by being adjacent to the famatinian magmatic arc the western border of the cambrian crystalline package absorbed imposed deformation along a crustal scale ductile shear zone within this zone the high grade metamorphic rocks were reworked and re hydrated to lower temperature assemblages crystalline rocks currently exposed in the sierra de comechingones show that they crystallised and were exhumed in a setting where tectono thermal activity lasted even though it might have waned until the middle palaeozoic from the latest neoproterozoic c 550 ma until the late devonian c 360 ma tectonic activity was intermittently acting indicating continuous convergence along the proto pacific margin of gondwana

re os depositional age for black shales from the kaimur group upper vindhyan india abstract proterozoic sedimentary deposits from the vindhyan supergroup central india are excellent archives of paleo environmental conditions meaningful interpretation of these geological signatures and their global correlation requires depositional age information for the sediments data on which are limited for the upper vindhyan sediments in this work re os isotopic study of the bijaigarh shale kaimur group from the upper vindhyan has been carried out to infer their depositional age and paleo hydrological conditions the re os isochron for the bijaigarh shale a lagoonal deposit with an open ocean connection provides a depositional age of 1210 52 ma model 3 mswd 69 n 15 2 this age for the kaimur group is younger by 400 ma than the underlying semri group age 1600 ma confirming existence of a long sedimentary hiatus between the two groups the isochron derived initial 187os 188os osi ratio for these shales 0 6 is higher than those reported for mesoproterozoic marine shales 0 3 from other global margins osi ratios of shales could be regulated by regional paleo hydrological conditions osmium mass balance calculations for the restricted basin of the bijaigarh shale show that a minor increase in freshwater influx as low as 2 4 i e salinity change by 0 8 psu to the euxinic deep water of the basin is sufficient to elevate osi ratio from 0 3 marine value to 0 6 osi ratio of the lagoonal shale in contrast to lagoonal shales the osi ratios of marine shales seem to mimic the seawater 187os 188os during their deposition period compilation of marine osi ratios shows coarse synchrony between temporal changes in osi during the proterozoic eon and the atmospheric oxygen level pointing to the dominant role of oxidative continental weathering on paleo seawater 187os 188os ratio

depositional history of the upper vindhyan succession central india time constraints from pb pb isochron ages of its carbonate components abstract age of the upper sedimentary units rewa and bhander groups of the largest proterozoic vindhyan basin in central india has so far been considered to be close to 650 ma late neoproterozoic based on collective proxy dates in the absence of direct isotopic dates a drastic upward revision of this age by more than 300 ma has very recently been proposed from the similarity of the paleomagnetic directions in these units with those of the 1070 ma old majhgawan kimberlite intrusion into their basal parts and the presence of detrital zircons not younger than 1020 ma we report now pb pb isochron ages of 908 72 ma 866 180 ma and 1073 210 ma for the three distinct carbonate horizons bhander balwan and lakheri limestones respectively as independent and direct evidence on the age of the upper vindhyan succession the consistency of the three ages close to 900 ma provides a strong support for the proposed shift on the timing and duration of sedimentation in the upper vindhyan sequence

closure of the clymene ocean and formation of west gondwana in the cambrian evidence from the sierras australes of the southernmost rio de la plata craton argentina abstract the formation of gondwana took place across a series of brasiliano pan african suture zones that record late neoproterozoic to earliest paleozoic collisions between precambrian cratons in south america an internal suture zone marks the disappearance of the clymene ocean that separated the amazon craton from the sao francisco and rio de la plata cratons new geochronological data from the southern end of this massive collision zone in the sierras australes of central eastern argentina document paleoproterozoic crust and suggest an ediacaran age for the oldest sedimentary rocks these two observations extend the known limit of the rio de la plata craton at least 200 km sw of previous estimates new data also confirm the occurrence of late ediacaran to late cambrian magmatism in the sierras australes the age of these hypabyssal to volcanic rocks corresponds to igneous events in the pampean belt along the western margin of the rio de la plata craton although the shallow level magma emplacement in the sierra da ventana study area contrasts with the deeply exhumed rocks of the pampean orogeny type locality these new age data are compared with a broad compilation of geochronological age clymene collision belts to the north the paraguai and araguaia belts the close overlap of the timing of orogenesis indicates the age of clymene ocean closure in its northern reaches in the south the pampean belt was unconfined allowing continued tectonic activity and crustal accretion throughout the paleozoic

carbon oxygen and strontium isotope geochemistry of proterozoic carbonate rocks of the vindhyan basin central india abstract carbon oxygen and strontium isotope compositions of carbonate rocks of the proterozoic vindhyan supergroup central india suggest that they can be correlated with the isotope evolution curves of marine carbonates during the latter proterozoic the carbonate rocks of the lower vindhyan supergroup from eastern son valley and central vindhyan sections show 13c values of 0 v pdb and those from rajasthan section are enriched up to 2 8 in contrast the carbonate rocks of the upper vindhyan succession record both positive and negative shifts in 13c compositions in the central vindhyan section the carbonates exhibit positive 13c values up to 5 7 and those from rajasthan show negative values down to 5 2 the 18o values of most of the carbonate rocks from the vindhyan supergroup show a narrow range between 10 and 5 v pdb and are similar to the best preserved 18o compositions of the proterozoic carbonate rocks in the central vindhyan and eastern son valley sections carbonates from the lower vindhyan exhibit best preserved 87sr 86sr compositions of 0 7059 6 which are lower compared to those from rajasthan 0 7068 4 the carbonates with positive 13c values from upper vindhyan are characterized by lower 87sr 86sr values 0 7068 2 than those with negative 13c values 0 7082 6 a comparison of c and sr isotope data of carbonate rocks of the vindhyan supergroup with isotope evolution curves of the latter proterozoic along with available geochronological data suggest that the lower vindhyan sediments were deposited during the mesoproterozoic eon and those from the upper vindhyan represent a neoproterozoic interval of deposition