archean paleoproterozoic transition the indian perspective abstract india has perhaps the richest paleoarchean to paleoproterozoic crustal components on earth miller et al 2018 notwithstanding this much of the indian subcontinent remains unexplored compared to other ancient cratons in north america australia brazil and africa there are five archean cratonic nuclei in india the aravalli bundelkhand singhbhum bastar and the dharwar with well preserved proterozoic supracrustal sequences this paper critically reviews the paleoarchean to paleoproterozoic supracrustal record of the indian subcontinent with special emphasis on the archean paleoproterozoic transition which is generally placed at 2 5 ga based on the emplacement age of the great dyke of zimbabwe in general the archean paleoproterozoic transition in indian cratonic blocks is associated with high continental freeboard condition unlike north america south africa and western australia the indian cratonic blocks except bastar are devoid of paleoproterozoic glacial diamictite mazumder et al 2015 however similar to the pongola supergroup of south africa there are evidences of neoarchean glaciation in dharwar ojakangas et al 2014 in contrast to the global scenario of extensive development of bif across the archean paleoproterozoic transition the indian paleoproterozoic successions are devoid of banded iron formation bif except the 1 85 ga old bif in bundelkhand in contrast the majority of the indian bifs are of paleoarchean or neoarchean age thus the geodynamic and supracrustal data collected from five indian archean cratonic blocks do not correspond uniquely and precisely to a global change in tectonic style onset of neoarchean global magmatism and rifting or lithostratigraphic and environmental changes suggested across the archean paleoproterozoic transition van kranendonk et al 2012 therefore a revision and redefinition of the archean proterozoic boundary is outstanding this article is part of a special issue entitled archean earth processes edited by rajat mazumder and patrick eriksson

new age constraints for the proterozoic aravalli delhi successions of india and their implications abstract proterozoic sedimentary successions of india are important archives of both the tectonic history of the indian subcontinent and the geochemical evolution of earth surface processes however the lack of firm age constraints on many of these stratigraphic units limits their current utility here we present new detrital zircon age data from strata of the southern aravalli delhi orogenic belt adob and the rajasthan vindhyan successions the alwar group of the southern delhi supergroup yielded a large population of 1 2 ga detrital zircon grains which refutes the 1 9 1 7 ga age assertion for this unit detrital zircon age distributions from the southern alwar group differ strongly from the alwar group of the north delhi belt demonstrating miscorrelation between these two regions the jhamarkotra formation of the lower aravalli group contains a large population of 1 9 1 7 ga detrital zircon grains therefore the unit cannot be 2 1 ga as traditionally assumed age distributions of the aravalli and delhi supergroups are similar to those of the lower and upper vindhyan successions and we postulate contiguous sediment sources for both regions with strata of the tectonically deformed adob representing the distal margin equivalents of the vindhyan successions additionally a late paleoproterozoic age for the jhamarkotra formation nullifies the hypothesis that the markedly positive carbonate 13c values in this unit are linked to the 2 3 2 0 ga lomagundi jatuli positive isotope excursion the potential of a large late paleoproterozoic ca 1 7 ga positive 13c excursion contrasts with the long held view of a prolonged period of carbon isotope stasis during the so called boring billion

geochemistry and petrogenesis of mafic magmatic rocks of the jharol belt india geodynamic implication abstract the belt consisting of deep water meta sediments of the aravalli supergroup hosts numerous mafic and ultramafic rocks occurring at two different structural levels these are the basal bagdunda volcanics and the gopir magmatic rocks represented by gopir dikes and flows associated with the ultramafic rocks along the kaliguman lineament the bagdunda and gopir mafic volcanic rocks are sub alkaline varying in composition from high mg tholeiite to basaltic komatiite they are predominantly lree depleted but some flat ree patterns are also observed bulk geochemical data especially the incompatible trace elements discount the possibility of crustal contamination gopir dikes are highly enriched in terms of their ree ce sm n and ce yb n and other incompatible trace elements and closely resemble the basal aravalli volcanics their ratio rock ratio pm and normalized multi element abundance patterns depict their overall enriched nature compared to n morb and primitive mantle the gopir and bagdunda volcanics reflect trace element characteristics transitional between e morb and oib gopir dikes show remarkable similarity with continental tholeiite and continental flood basalts cfbs with negative nb p and ti anomalies and low nb ce ratio unlike the volcanics of both the suites petrogenetic modeling based on the compositionally corrected mg and fe abundances for gopir dikes gopir and bagdunda volcanics indicates a their derivation from non pyrolitic sources b their derivation from sources that were variably enriched in fe mg ratios with large variation in their fe contents and c olivine was a major phase to fractionate followed by a lesser amount of clinopyroxene in the case of gopir volcanics whereas in bagdunda volcanics a combination of olivine and clinopyroxene fractionation is suggested based on the geochemical data supported by field evidence we propose a geodynamic model for the development of the jharol belt in which we suggest that the basement rock i e the banded gneissic complex bgc started rifting probably under influence of a mantle plume during the late archaean early proterozoic period during the opening of the basin magma derived from asthenospheric mantle reached the surface contemporaneously with sedimentation the first phase of volcanism is represented by bagdunda volcanics with continuous rifting the crust became highly attenuated and facilitated asthenospheric upwelling causing high degrees of melting indicated by a large volume of mafic and ultramafic rocks during the second phase of magmatism gopir volcanics that occurred at later stages of jharol sedimentation coeval melting of the sub continental lithosphere under the adjoining bgc craton probably caused the emplacement of dikes along with the gopir volcanics at this stage of progressive rifting an oceanic crust very similar to that in marginal basins developed in the jharol belt

depositional environments of unfossiliferous sediments of the jodhpur group western india abstract the jodhpur group of rajasthan western india the lowest stratigraphic unit of the marwar supergroup upper proterozoic to lower cambrian consists of unfossiliferous mainly horizontal bedded sandstones and shales deposited in an intracratonic basin six lithofacies have been recognized and interpreted in terms of environmental facies as braided fluvial facies low energy beach facies with tidal effects during depositional regression storm influenced beach facies during depositional regression low energy beach facies during depositional transgression lagoonal facies and shelf facies the most common paleogeographic scenario was of braided streams flowing to the sea streams flowed west northwest and north of the pipar road horst block situated in the southern part of the area of study low energy or storm influenced beaches developed along the shoreline beyond which was a wide shelf zone the braided fluvial lithofacies exhibits characteristics intermediate between those of sediments deposited by the modern braided south saskatchewan and platte rivers

detection of natural weathering of upper mcalester coal and woodford shale oklahoma u s a abstract natural weathering refers to the natural oxidation of organic and inorganic matter in rocks at the surface and in the shallow subsurface the objective of this study is to apply routine geochemical analyses including organic petrography to coal and shale samples extending from the weathered to unaltered zones to illustrate the effects of weathering on organic matter in coal and shale some significant observations from the upper mcalester coal and the woodford shale are 1 a sulfate index si using sulfur form analysis data that reveals the transformation of pyrite to sulfate minerals is a very sensitive weathering indicator it increased in weathered samples 2 rock eval pyrolysis data such as hydrogen index hi and oxygen index oi are very sensitive to weathering oi hi increased in weathered samples 3 vitrinite reflectance decreased in weathered samples and 4 petrographic signs of weathering include microfractures pitted surface dark reaction rims and high relief sulfate index and petrographic signs of weathering should be used on surface and shallow subsurface rock samples as an indication of the extent of weathering the validity of other analyses can then be evaluated accordingly

influence of palaeoweathering on trace metal concentrations and environmental proxies in black shales the mineralogical and chemical compositions of lower carboniferous tournaisian marine black shale from the kowala quarry the holy cross mountains poland were investigated this study focuses on disturbances in palaeoenvironmental proxies caused by palaeoweathering which progressively changed the major and trace element abundances palaeomagnetic investigations reveal that the devonian carboniferous succession was weathered during the permian triassic by the infiltration of oxidizing fluids related to karstification following post variscan exhumation the weathering process led to vermiculitization of chlorite partial dissolution of calcite and replacement of pyrite by hematite and goethite moreover the concentrations of some trace metals including co cu pb mo ni as and u significantly decreased consequently some elemental abundance ratios that are used as environmental proxies including u th ni co and v cr were altered elements that are bound to iron sulphides e g mo appear to be especially prone to mobilization by even a lightly weathered black shale the documented weathering including changes in elemental concentrations can potentially create misinterpretations of the original palaeoenvironmental conditions in addition the palaeoweathering of the studied samples appears to have substantially changed the carbon oxygen nitrogen and molybdenum stable isotope values the nitrogen and molybdenum stable isotope ratios in particular appear to be most sensitive to the effects of weathering and therefore are good indicators of palaeo weathering processes the major cause of these changes is the decay of organic matter and pyrite for the organic carbon stable isotopes ratios the main factor that controlls this process appears to be the preferential degradation of labile organic matter a combination of the total organic carbon toc total sulphur ts content mo concentration and stable isotope compositions seems to be the most useful for identify palaeo weathering our results suggest that reductions in ts and mo in tandem with diminished mo stable isotope values in the absence of obvious changes to the toc content provide the most compelling evidence of palaeo weathering

hindoli group of rocks in the eastern fringe of the aravalli delhi orogenic belt archean secondary greenstone belt or proterozoic supracrus tals abstract the very low grade metamorphic sequence of volcano sedimentary rocks sandwiched between the platform sediments of the vindhyan supergroup to the east and the banded gneissic complex bgc to the west in the eastern fringe of the aravalli delhi orogenic belt has remained a stratigraphic enigma in the precambrian geology of rajasthan this sequence known earlier as the gwalior series and in contemporary literature as the hindoli group has been considered by several workers as a proterozoic supracrustal unit and by some others as an archean secondary greenstone belt based purely on geological considerations u pb zircon geochronology was conducted to find an answer to this controversy on samples of felsic volcanics conformably intercalated with the hindoli sediments and hence considered contemporaneous with them zircons from a sample of massive rhyodacite gave a concordia age of 1854k7 ma though zircons from a sample of felsic tuff gave a wide range of ages between 3259 1877 ma careful consideration of the nature of the samples and their constituent zircons suggests that the hindoli group rocks represent a low grade proterozoic supracrustal cover sequence in the eastern part of the bhilwara belt broadly synchronous to the aravalli bhilwara sedimentation around 1 8 ga

metamorphism graphite crystallinity and sulfide anatexis of the rampura agucha massive sulfide deposit northwestern india located adjacent to the banded gneissic complex rampura agucha is the only sulfide ore deposit discovered to date within the precambrian basement gneisses of rajasthan the massive zn pb sulfide orebody occurs within graphite biotite sillimanite schist along with garnet biotite sillimanite gneiss calc silicate gneisses amphibolites and garnet bearing leucosomes plagioclase hornblende thermometry in amphibolites yielded a peak metamorphic temperature of 720 780 c whereas temperatures obtained from fe mg exchange between garnet and biotite 580 610 c in the pelites correspond to postpeak resetting thermodynamic considerations of pertinent silicate equilibria coupled with sphalerite geobarometry furnished part of a clockwise p t t path with peak p t of 6 2 kbar and 780 c attained during granulite grade metamorphism of the major zn rich stratiform sedimentary exhalative deposits orebody and its host rocks arsenopyrite composition in the metamorphosed ore yielded a temperature and log f s2 range of 352 c 8 2 to 490 c 4 64 thus indicating its retrograde nature contrary to earlier research on the retrogressed nature of graphite raman spectroscopic studies on graphite in the metamorphosed ore reveal variable degree of preservation of prograde graphite crystals 490 43 c with a maximum at 593 c the main orebody is mineralogically simple sphalerite pyrite pyrrhotite arsenopyrite galena deformed and metamorphosed while the pb ag rich sulfosalt bearing veins and pods that are irregularly distributed within the hanging wall calc silicate gneisses show no evidence of deformation and metamorphism the sulfosalt minerals identified include freibergite boulangerite pyrargyrite stephanite diaphorite mn jamesonite cu free meneghinite and semseyite the last three are reported from agucha for the first time stability relations of cu free meneghinite and semseyite in the pb ag rich ores constrain temperatures at 550 c and 300 c respectively features such as 1 low galena sphalerite interfacial angles 2 presence of multiphase sulfide sulfosalt inclusions 3 microcracks filled with galena pyrargyrite without any hydrothermal alteration and 4 high contents of zn ag and sb in galena indicate partial melting in the pbs fe0 96s zns 1 ag2s cufes2 system which was critical for metamorphic remobilization of the rampura agucha deposit

depositional ages of the stratiform base metal deposits at mount isa and mcarthur river australia based on u pb zircon dating of concordant tuff horizons potassium rich tuff beds occur interbedded with major stratiform pb zn mineralization in shales and dolomitic shales of the mount isa group northwest queensland and the mcarthur group northern territory these deposits are some 600 km apart yet they exhibit many similarities which together with regional correlations suggest that the deposits formed by similar processes and that the successions developed more or less contemporaneously zircon is sparsely distributed as tiny crystal fragments or phenocrysts in the tuffs and its morphology and u pb systematics affirm that it is primarily of igneous origin u pb zircon data from a tuff in the mineralized h y c pyritic shale member of the mcarthur group give an age of the tuff of 1 690 super 29 sub 25 m y and from a tuff in the mineralized urquhart shale of the mount isa group an age of 1 670 super 20 sub 17 m y these results are consistent with all other geologic controls and are regarded as the best estimates of the depositional ages of the groups the ages of the mount isa and mcarthur groups are thus the same to within the present experimental uncertainties essentially confirming geologic synchrony of these important sedimentary successions pooled whole rock data from several of the mount isa tuff marker beds give an updated rb sr age of about 1 480 m y interpreted as indicating that isotopic exchange and regional metamorphism had essentially ceased by this time

lead isotope studies bearing on the genesis of copper orebodies at mount isa queensland significant differences between the lead zinc silver and copper ore bodies unlike the lead zinc silver deposits the copper orebodies appear epigenetic two texturally distinct generations of iron sulfides later relative to the lead zinc mineralization origin of the main copper ore bodies from the basement metamorphic and granitic complexes direct relationship between the regional metamorphism generation of hypersaline fluids and metal extraction modified journal abstract

sulfur and carbon isotope compositions in the stratiform zn pb cu sulfide deposits of the rajpura dariba belt rajasthan nw india a model of ore genesis the sulfur isotope composition of 86 sulfide minerals from the middle proterozoic metamorphosed stratiform sediment hosted zn pb cu sulfide deposits of dariba and sindeswar kalan located within the rajpura dariba belt in rajasthan nw india have been determined in addition 16 carbonaceous and 2 carbonate rock samples from the ore zone have been analyzed for their ctot and corg contents and carbon isotope compositions the sulfur isotope compositions range from 9 1 to 6 7 mean value of 1 9 increasing 34s values stratigraphically upward are observed particularly for pyrite and pyrrhotite suggesting a syngenetic origin for the sulfur no marked lateral isotopic variations or isotopic variation in minerals from successive laminae in banded ore samples occur fractionation of sulfur isotopes between coexisting sulfides suggests that the original isotopic pattern was basically preserved during the amphibolite facies metamorphism suffered by the deposits corg in carbonaceous rocks ranges 0 5 9 3 wt with 13c values between 21 and 31 mean of 25 4 in keeping with the biogenic derivation of the carbon recrystallized dolostones have 13c values close to 14 4 geological evidence and isotopic features are consistant with the following genetic scheme a base metal ores along the belt formed from geothermal emanations carrying h2s produced by the chemical reduction of seawater sulfates and leaching of mafic volcanics in a semiclosed with respect to so4 shallow water rift related basin with high biological activity b pyrite and pyrrhotite formed diagenetically by bacterial reduction of sulfate in pore seawater in a system open to h2s thus bringing about the gradual enrichment of 34s in these minerals stratigraphically upward and c northward in the belt at sindeswar kalan the basin of ore deposition was relatively more open

structural relationships and srnd isotope systematics of polymetamorphic granitic gneisses and granitic rocks from central rajasthan india implications for the evolution of the aravalli craton this paper reviews the state of structural and geochronological knowledge of precambrian cratonic rocks in part of central rajasthan india and contributes new observations and isotopic data on the structural features and age of granitic rocks in the area the granitic rocks comprise plutons of two suites bgc and sendra suites which intrude ortho and paragneisses the archean banded gneiss complex bgc and its inliers as well as the proterozoic delhi supergroup nearly all the plutons show a foliation and a lineation which are mostly the result of solid state deformation although a magmatic component of strain is apparent in some of them the intensity of these structural fabrics varies from weak to extremely strong samples of granitic rocks from the bgc suite yield ages from sm nd isotopic data which scatter around a 2 83 ga reference isochron nd t of 4 1 with depleted mantle model ages ranging from 2 6 to 3 0 ga scatter of the data suggests that the bgc is a composite group of gneisses and confirms the antiquity of the bgc within this part of the aravalli delhi orogenic belt rb sr isotopic data of the bgc suite lie on a 1 7 ga reference isochron indicating a thermal event at that time data from plutons of the sendra suite on the other hand show some scatter but define a model ii isochron with slope equivalent to an age of 0 966 250 ga initial 87sr86sr of 0 70390 0 01889 mswd 311 the scatter of data could reflect open system behavior during a subsequent thermal event malani volcanics although independent evidence for such is lacking in the study area ages calculated from our rb sr data are within the error of published results from previous workers structural observations on granitic rocks within the bgc inlier at ajmer suggest that the inlier could represent a remobilized diapiric ridge emplaced into the delhi supergroup during the 1 7 ga thermotectonic event contemporaneously with formation of d1 structures in the wall rock although the radiometric age of the latter structures is not well constrained microtexture mineralogy and field relations of granitic rocks of the sendra suite point to those plutons being emplaced into the delhi supergroup during the 0 966 ga thermotectonic event late syntectonic relative to d2 indicating that d2 is 0 96 1 0 ga d3 represents retrograde deformation synchronous with cooling of the regional thermal framework and local pluton cooling probably between 0 96 and 0 90 ga further geochronological geochemical and structural work on the plutons and wall rock of both suites is necessary to establish a rigorous thermotectonic chronology by which to accurately model the crustal evolution of this part of central rajasthan

evidence of sulfide melting and melt fractionation during amphibolite facies metamorphism of the rajpura dariba polymetallic sulfide ores abstract partial melting of sulfide ores during prograde metamorphism could have been more prevalent than generally accepted however identification of such melting is difficult as sulfide melts do not form glasses and the textures generated on quenching are obliterated due to the tendency of sulfides for ready recrystallization the polymetallic base metal sulfide deposit at rajpura dariba rajasthan india is a typical stratiform ore metamorphosed to the middle amphibolite facies the peak metamorphic temperature of 600 c should have been sufficient to initiate sulfide melting as evident from experimental studies in the zns pbs cu 2 s fes 2 s system further syn metamorphic melting of the original sedex ore was abetted by the high f s2 condition that prevailed as a consequence of barite dissolution a zn fe s melt containing minor pb sb and cu but no ag fractionated from an initial melt in the above system resulting in a residual immiscible sulfosalt bearing pbs melt the final metallic melts represented by formation of dyscrasite ag 3 sb from the sulfosalt bearing melt and breithauptite nisb or ullmannite nisbs from the sulfosalt absent melt were a product of independent fractional crystallization of the immiscible sulfide and pbs sulfosalt melts

provenance and tectonic significance of the palaeoproterozoic metasedimentary successions of central and northern madagascar abstract new detrital zircon u pb age data obtained from various quartzite units of three spatially separated supracrustal packages in central and northern madagascar show that these units were deposited between 1 8 and 0 8 ga and have similar aged provenances the distribution of detrital zircon ages indicates an overwhelming contribution of sources with ages between 2 5 and 1 8 ga possible source rocks with an age of 2 5 ga are present in abundance in the crustal segments antananarivo antongil and masora domains either side of a purported neoproterozoic suture betsimisaraka suture zone recently possible source rocks for the 1 8 ga age peak have been recognised in southern madagascar all three supracrustal successions as well as the archaean blocks onto which they were emplaced are intruded by mid neoproterozoic magmatic suites placing a minimum age on their deposition the similarities in detrital pattern maximum and minimum age of deposition in the three successions lend some support to a model in which all of madagascar s archaean blocks form a coherent crustal entity the greater dharwar craton rather than an amalgamate of disparate crustal blocks brought together only during neoproterozoic convergence however potential source terranes exist outside madagascar and on either side of the neoproterozoic sutures so that a model including a neoproterozoic suture in madagascar cannot be dispelled outright

contrasting geochronological evolution of the rajpura dariba and rampura agucha metamorphosed zn pb deposit aravalli delhi belt india abstract the timing of metamorphism of the rajpura dariba and rampura agucha massive sulfide deposits in the aravalli delhi belt india has been constrained through u th pb chemical dating of monazite from the host rocks of these deposits the rampura agucha deposit was metamorphosed during the neoproterozoic ca 1 0 ga grenville age collisional orogeny metamorphic domains in monazite from the host rocks of the rajpura dariba deposit furnish mesoproterozoic 1621 42 ma ages with no evidence of any neoproterozoic metamorphism a paleoproterozoic 1869 22 ma age cluster from the dariba monazites corroborates the ca 1800 ma published pb isotope model age of sulfide mineralization as the lithotectonic association and pb isotope model age of the agucha deposit is similar to that of the dariba deposit the former should also have been affected by the mesoproterozoic event monazites from agucha preserve little evidence of the mesoproterozoic metamorphism possibly because the neoproterozoic high grade metamorphic event affecting the rocks almost completely re equilibrated pre existing grains thus the rajpura dariba deposit has experienced a single tectonothermal event at ca 1 62 ga while the rampura agucha deposit has undergone two metamorphic events at ca 1 62 ga and 1 0 ga this difference is because the rampura agucha deposit is located close to or within the grenvillian orogenic front in the aravalli delhi belt whereas the rajpura dariba deposit is far away from the grenvillian orogenic front and hence was unaffected by it

storm event beds in a paleoproterozoic rift basin aravalli supergroup rajasthan india abstract storm event beds in the paleoproterozoic riftogenic sedimentary succession of aravalli supergroup are described from a 12 8 m thick sandstone mudstone interbedded unit in zawar area rajasthan india the storm event beds include different primary structural assemblages indicating deposition from waning storm current sequential arrangement of beds with characteristic primary structural assemblages suggests deposition under a transgressive phase and overall retrogradational evolution of the storm succession provides evidence in favour of faster downsagging of the basin floor the pb zn sulphide ore bearing sedimentary succession of zawar records repeated downsagging and exhumation of the basin floor in the frame of continental rift tectonics

the 2 7 2 0 ga volcano sedimentary record of africa india and australia evidence for global and local changes in sea level and continental freeboard abstract the 2 7 2 0 ga volcano sedimentary records of the african indian and australian cratons indicate two broadly defined periods of extensive drowning of the emergent continental areas concomitant with lowered freeboard carbonate banded iron formation bif platforms characterised the first such event at ca 2 6 2 4 ga africa and australia to 2 7 ga india these earlier globally enhanced sea levels are ascribed to increased mid ocean ridge activity possibly related to breakup of a postulated late archaean southern supercontinent alternatively a transition from global scale catastrophic mantle overturn events to the onset of plate tectonics may have occurred in the late archaean nelson 1998 earth planet sci lett 158 109 119 both explanations of increased mid ocean ridge activity are compatible with significant early to middle archaean crustal growth armstrong 1981 phil trans r soc london a 301 443 472 with the emergent high freeboard cratons being subjected to aggressive weathering and erosion enhanced continental crustal growth near the archaean proterozoic boundary mclennan and taylor 1982 j geol 90 347 361 related to the development of significant island arc complexes would have resulted in common lowered freeboard enhanced sea level conditions at the passive margins of the southern cratons the diachronous nature of these earlier transgressions in the various cratons may reflect the effect of local tectonic movements and or the thermal state of the cratons from ca 2 4 2 2 ga cratons that make up the present day continents of india africa and australia had relatively high continental freeboard and lowered sea levels glacigenic deposits are preserved on the kaapvaal africa singhbhum india and pilbara australia cratons the second broadly defined drowning event at ca 2 15 ga was probably due to post glacial climatic amelioration freeboard was reduced by the combination of eustatic rise and the reestablishment of aggressive weathering as warmer palaeoclimates returned in india carbonates were more prominent than the siliciclastic sediments including prominent black shales seen in africa and australia

zircon u pb and galena pb isotope evidence for an approximate 1 0 ga terrane constituting the western margin of the aravalli delhi orogenic belt northwestern india abstract zircon u pb ages of 987 6 4 and 986 3 2 4 ma have been established for rhyolites in the southern and northern parts respectively of the ambaji sendra arc terrane in the western fringe of the aravalli delhi orogenic belt pb isotope data for volcanogenic massive sulphide deposits within this arc terrane define a linear trend which is considered a useful reference palaeoisochron or mixing line at an age of about 990 ma and establish continuity of the terrane between ambaji and sendra these results are in contrast to the traditional and continuing assignment of the ambaji sendra terrane to the 1700 ma delhi supergroup the isotopic composition of galena from an apparently epigenetic occurrence at punagarh hill yields a model age of about 940 ma suggesting that the punagarh group could form part of the same arc sequence the phulad lineament which has been considered by most workers to represent the western boundary of the aravalli delhi orogenic belt appears in terms of this stratigraphic assignment to represent an oblique structure which has not greatly offset the punagarh and ambaji sendra domains within the arc terrane the eastern boundary of the terrane is marked by the sabarmati fault a zircon u pb age of 836 7 5 ma for the siwaya gneissic granite in the southern part of the ambaji sendra belt is in accord with previous age data for felsic erinpura plutons that have intruded the arc sequence a monazite age of 826 5 ma may reflect slow cooling of the siwaya pluton or a younger thermal event a model age of about 820 ma for galena is obtained from the tosham sn cu mineralized zone in haryana state about 280 km north northeast of ajmer which is related to the felsic anorogenic malani volcanism plutonism hence this widespread magmatism found extensively west of the ambaji sendra terrane may have been coeval with erinpura plutonism or followed it very closely the present geochronological data warrant the recognition of the ambaji sendra arc terrane as defined here as a distinct metallogenic province which saw the formation of vms type deposits in the late mesoproterozoic around 1 0 ga the pb isotope compositions of the deposits however do not clearly define the metal sources the lead from danva prospect is most primitive and must have the greatest component from a juvenile mantle source the birantiya khurd deposit on the other hand contains a much greater component of lead from a significantly older crustal source

the protocontinental nature and regional variability of the central metasedimentary belt of the grenville province lead isotope evidence the lead isotopic compositions of sulphide samples mainly galenas from a representative set of mineral deposits within the central metasedimentary belt of the grenville province have been determi

petrology and geochemistry of greywackes from the aravalli supergroup rajasthan india and the tectonic evolution of a proterozoic sedimentary basin the proterozoic aravalli supergroup in south central rajasthan western india is predominantly composed of coarse low grade meta arenites carbonates greywackes interbedded fine grained silty arenites and metapelites resting on an archaean basement complex of gneisses schists and high grade metasediments we have differentiated four lithofacies a b c and d within the greywacke suite each with characteristic qfl composition grain size and major trace and rare earth element concentrations tectonic setting for each facies type could only be discriminated using qmflt plots greywackes of facies a and d were derived from a recycled orogen and facies b was derived from arc related andesitic source rocks a continental block provenance is indicated for the silty arenites of facies c this petrofacies based interpretation shows good agreement with environmental reconstructions based on lithofacies and sedimentary structure analyses the only exception is facies b whose andesitic parentage as deduced from petrofacies analysis could not be demonstrated at its outcrop these provenance discriminations were tested through tectonic setting interpretations based on major trace and rare earth elements the geochemical data broadly confirmed the tectonic interpretations made on the basis of the petrological attributes use of these laboratory based methods in conjunction with field measurements of primary sedimentary structures allow us to discriminate provenance and tectonic setting fairly precisely a proterozoic plate model with subduction to the west has been proposed to explain the sedimentological variation in the aravalli supergroup

structural geometry of uraniferous rocks in the umra area aravalli supergroup india abstract uranium mineralization occurs in palaeoproterozoic metasediments of the aravalli supergroup in the umra area udaipur district rajasthan india the host rocks for mineralization were deposited in a shallow marine shelf type environment over an archaean basement gneiss the banded gneissic complex a reducing environment at the depositional site associated with large scale biogenic activity was responsible for precipitation of uranium bearing minerals our structural analysis shows that the rocks of the mineralized area have been involved in four phases of deformation the carbonaceous phyllite unit near umra shows a mirror image pattern resulting from the superposition of upright f 2 folds with ne sw striking axial planes over early f 1 isoclinal folds the exploration block is located on one limb of the regional f 2 fold

mid proterozoic pb pb ages for some himalayan base metal deposits and comparison to deposits in rajasthan nw india abstract lead isotope ratios from three base metal deposits rangpo gorubathan in the eastern lesser himalaya and bageswar in the lesser himalaya of kumaun have been studied the first two deposits generally stratiform and stratabound occur within pelitic psammopelitic wacke type metasediments which are locally carbonaceous the bageswar deposit is hosted by silicified dolomitic carbonate rocks the pb isotope ratios appear to be homogenous in two and heterogenous in one of the deposits model pb pb ages obtained for the rangpo and gorubathan deposits are 1800 ma and for the bageswar deposit 1550 1700 ma dispersion in isotopic ratios reflect the geochemical heterogeneity particularly with respect to larger variation in the composition of pb isotope ratios in the bageswar deposit may be explained by the participation of pb from heterogenous sources pb isotope characteristics of the himalayan deposits are compared with those of a number of base metal deposits in the delhi aravalli belt in rajasthan nw india ore lead from the deposits of both the himalaya and of rajasthan are enriched in 206pb and 207pb however lead in all cases plots on the evolution lines corresponding to sources with higher u pb ratios but the sources of lead in the himalayan deposits had a lower th u ratio the geologic observations as well as the pb isotope compositions are consistent with a sedimentary diagenetic origin of the himalayan base metal deposits

discussion of geomechanical modeling of the nucleation process of australia s 1989 m5 6 newcastle earthquake by c d klose earth planet sci lett 256 2007 547 553 abstract on the basis of a numerical simulation klose earth planet sci lett 256 547 553 2007 proposed that the m w 5 2 5 6 earthquake that nucleated beneath newcastle new south wales australia at 10 30 am december 28 1989 aest was the result of 200yr of coal mining in the region we argue here that klose s earth planet sci lett 256 547 553 2007 ultimate conclusion is not supported by his own model or by available geological and seismic data in particular the 1989 newcastle earthquake was not anomalous with respect to regional seismic activity

present status of the geochronology of the precambrian rocks of rajasthan abstract age determinations mostly by rb sr analyses of the precambrian rocks of rajasthan by us and by others are summarized and discussed broad periods of acid magmatism at 1 3000 2900 m y 2 2600 2500 m y 3 2000 1900 m y 4 1700 1500 m y and 5 850 750 m y were identified the oldest rocks in the area are the yet undated banded gneisses bgc east of udaipur intruded by the untala granite dated at 2950 m y and hence of mid archean age the basal status hitherto attributed to the berach granite dated at 2600 m y is no longer tenable the radiometric control on the beginning and duration of the overlying aravalli supergroup is not yet satisfactory though a lower limit at 2000 m y is indicated heron s original delhi rocks have recorded two magmatic events widely separate in space and time while the earliest granitic activity at 1600 m y is recorded only in the alwar basin in the northeast the younger activity between 850 750 m y is widespread as shown by the nearly concordant ages of erinpura type granites along the aravalli mountain range and the malani rhyolites in the western plains of the aravalli range

molecular structure of reactive coals during oxidation carbonization and hydrogenation an infrared photoacoustic spectroscopic and optical microscopic study abstract coals from western canada were examined for structural changes occurring during oxidation carbonization and hydrogenation by means of ft ir spectroscopy optical microscopy and chemical analysis the coals are of cretaceous to eocene age ranging in rank from subbituminous c to medium volatile bituminous 0 42 1 47 r o results indicate that the reactivity of the coals during the above mentioned processes depends on rank and petrological composition infrared spectroscopy is shown to be sensitive to the molecular structural changes occurring during these processes and provides a chemical basis for interpreting data from chemical analysis and optical microscopy

the problem of the precambrian basement in rajasthan western india abstract the banded gneissic complex bgc of rajasthan considered to form the basement underlying the precambrian proterozoic aravalli metasediments shows an erosion surface marked by a conglomerate and an angular unconformity with gneissic foliation crossing the metasedimentary bands at only a few places the bgc is a composite gneiss evolved by extensive migmatization of metasedimentary rocks of diverse composition and possibly metaigneous rocks the contact between the bgc and the aravalli rocks is a gently curved surface whereas the gneissic foliation as well as the large scale metasedimentary enclaves within the gneissic complex show all the intricate patterns of super imposed folding traceable in the aravalli rocks this implies that the basement gneisses have been involved in ductile deformation with the aravalli rocks the migmatization being synkinematic with the first deformation in the latter all these apparently conflicting lines of evidence can be resolved if the gneisses as we see them now represent not the original but the mobilized basement with the bgc aravalli boundary representing for a large part a migmatite front rather than the original basement cover interface only at a few places was there a chance of the original basement escaping mobilization and thus little chance of identifying this original interface

quantification of loss of calcite pyrite and organic matter due to weathering of toarcian black shales and effects on kerogen and bitumen characteristics abstract comparison of geochemical data on posidonia shale early toarcian from a shallow unweathered core and from adjacent weathered exposures of the same fades was performed results revealed that quantitatively the most affected petrographic constituent is pyrite and that organic matter and carbonate were also altered by weathering due to this process of weathering the bulk composition of organic matter especially the molecular composition of soluble organic matter and the fluorescence colour of organic particles are changed estimates of weathering rates reveal that the annual release of sulphur and organic carbon from black shales may significantly add to anthropogenic pollution

isotopic composition of phanerozoic ore leads from the swedish segment of the fennoscandian shield 95 analyses of ore lead isotope ratios from 23 phanerozoic ore deposits from the swedish segment of the fennoscandian shield form a marked linear trend on a 207pb 204pb versus 206pb 204pb diagram the line may be interpreted in a two stage model the lead being derived from 1 8 0 15 ga old svecokarelian basement and mineralization occurring at 0 4 0 15 ga the initial composition of the svecokarelian rock lead was similar to the lead in early proterozoic volcanogenic sulfide ores in sweden the large spread in the isotope ratios was caused by a combination of selective leaching of different minerals in the source rocks mixing with less radiogenic caledonian lead and local or regional variations in the u th and pb contents of the basement as a consequence conventional methods of identifying source rocks from lead isotopic data e g mu values th u ratios may not be directly applicable phanerozoic ore lead development in the swedish section of the fennoscandian shield was ensialic that is the ore lead was almost entirely derived from the precambrian basement although this basement does not appear to be anomalously enriched in pb no juvenile or mantle lead was apparently contributed to this section of the crust after the precambrian except for that mechanically transported onto the western edge of the shield by the caledonian nappes however some of europe s largest lead deposits are included in these swedish phanerozoic mineralizations suggesting that it was the nature of the processes involved rather than the richness of the source that determined their formation

origin of late palaeoproterozoic great vindhyan basin of north indian shield geochemical evidence from mafic volcanic rocks abstract on the western and southern margins of the sickle shaped vindhyan basin of north indian shield there are basal vindhyan mafic volcanic rocks referred to as khairmalia volcanics and jungel volcanics respectively these volcanics vary in composition from low ti tholeiite to high ti alkali basalt showing close affinity with continental flood basalts cfb and ocean island basalts oib respectively the parental magmas of khairmalia and jungel alkali basalts were formed by different degrees of partial melting of a garnet lherzolite the magma of khairmalia tholeiites was generated by a relatively higher degrees of partial melting of a garnet spinel lherzolite the geochemical data coupled with available geological and geophysical data favour a rift type origin of this basin which evolved as a peripheral basin showing many similarities with paleogene himalayan foreland basin the existing radiometric age data suggest that the origin of vindhyan basin is linked with aravalli satpura orogeny at about 1800 1600 ma collision occurred along the aravalli delhi fold belt adfb and central indian tectonic zone citz with west and south subduction respectively during this process the subducting lithosphere suffered extensional deformation on its convex side and some pre existing large faults in the already thin leading edge of subducted plate also reactivated and tapped magma generated by decompressional melting of the subcontinental mantle the simultaneous processes such as flexural subsidence reactivation of pre existing faults heating thermal cooling and contraction during volcanism resulted in the formation of curvilinear warp parallel to the emerging mountain front the lower vindhyan volcano sedimentary succession was deformed and exposed to erosion before the deposition of upper vindhyan rocks the orogenic forces were active intermittently throughout the vindhyan sedimentation

proterozoic tectonic evolution and metallogenesis in the aravalli delhi orogenic complex northwestern india abstract the proterozoic aravalli delhi orogenic complex hosts a large number of economically important stratabound base metal sulphide deposits a few w sn deposits associated with granites are also known in the complex based on tectono lithological features the following domains are distinguished the archaean basement consisting of the banded gneissic complex and granites the early proterozoic bhilwara aravalli jharol and north delhi belts and the middle to late proterozoic south delhi belt and the vindhyan basin intra cratonic rifting of the archaean basement commencing 2 2 ga ago resulted in the rock association of the bhilwara belt a thick pile of detritus derived largely from felsic sources and to a minor extent from tholeiitic sills with the geochemical characteristics of ocean floor basalts stratiform zn pb cu sulphide deposits at rajpura dariba rampura agucha and also possibly those at pur banera and jahazpur formed 1 8 0 04 ga ago by convective seawater circulation in zones of crustal extension the metal content of exhalative brines was precipitated in troughs where biologic activity was prolific the shelf sediments of the aravalli belt characterized by dolomites with stromatolitic phosphorites were deposited on a passive continental margin at the rifted western edge of the archaean basement complex the monotonous pelitic pile of the jharol belt represents deep pelagic continental rise sediments the rakhabdev lineament delineates the shelf rise boundary the mafic ultramafic bodies along this lineament represent aravalli oceanic crust which subducted westward and eventually obducted as a consequence of an initial collision of the aravalli continental margin with an incipient arc to the west around 1 5 ga ago in the aravalli belt at zawar strongly radiogenic stratabound pb zn deposits were formed close to the basement in second order basins with biologic activity by hydrothermal solutions convecting through a heterogeneous source the north delhi belt comprises three sedimentation domains the khetri sub basin the alwar sub basin and the lalsot bayana sub basin sedimentation in this belt commenced with shelf carbonates followed by coarse clastic sediments and volcanites finally pelites and semi pelites were deposited in a multi lagoonal shallow water locally evaporitic environment synkinematic granitic intrusives in the north delhi rocks are in the age range of 1 7 1 5 ga the stratabound cu sulphide deposits of the khetri belt and the pyrite pyrrhotite deposits at saladipura with a pb pb model age of 1 8 ga were formed from hydrothermal seawater and partly bacteriogenic sulphur in small shallow rift related basins the south delhi belt with extensive mafic volcanism localized felsic volcanism argillaceous arenaceous carbonate accumulations in successive elongated basins and conspicuous felsic plutonism is interpreted as an island arc the geochemistry of pillowed and massive metavolcanites and an ophiolite assemblage indicate subduction in the western side of the arc part of the rocks of the delhi belt may also have formed in rift related back arc basins as suggested by the ocean floor characteristics of metavolcanic rocks subduction on the western side of the arc ultimately led to its terminal collision with the aravalli continental margin around 1 0 ga involving intense progressive strain and oblique convergence small cu zn deposits were formed within sedimentary intercalations in calc alkaline basalts close to the hypothetical subduction zone extensional tectonics in the back arc basins produced stratiform zn pb cu deposits ambaji deri around 1 1 ga ago by high temperature reduction of seawater convecting through multiple sources sn w mineralizations are apparently related to the malani felsic igneous phase of 735 ma rb sr age

evolution of lower proterozoic epicontinental deposits stromatolite bearing aravalli rocks of udaipur rajasthan india abstract the aravalli rocks 2060 ma old which crop out around udaipur western india comprise a thick sequence of metasediments with stromatolites and basal volcanics resting unconformably over a peneplained basement known as the banded gneissic complex ca 2585 ma old the rocks have undergone a very low grade of metamorphism and display a complex structure resulting from two major and several minor episodes of folding there are two distinctly different facies sequences in the aravalli rocks indicating deep sea and nearshore shelf environments the stratigraphic sequence of the rocks deposited under the shelf environment starts with basic volcanics and tuffs altered to greenschists and quartizites with arkosic conglomerate in the next sequence carbonates predominate in association with orthoquartzites carbonaceous phyllites phyllites and stromatolitic rock phosphate the carbonate sequence passes upward into greywacke phyllite lithic arenite in the distal parts and conglomerate arkose orthoquartzite in proximal areas the youngest sequence comprises orthoquartzite with silty arenite the distribution of different facies particularly that of dolomite with stromatolitic rock phosphate is controlled by sea floor topography suggesting the presence of an epicontinental sea bounded by a landmass to the west and a series of islands and shoals sedimentation in the shelf and epicontinental sea was presumably triggered by development of fault controlled troughs along craton margins terrigenous debris was deposited in newly developed troughs with contemporaneous volcanicity along trough margins with the erasing of the ephemeral relief in the provenance carbonate deposition was initiated the environment encouraged algal growth and formation of stromatolitic rock phosphate carbonaceous phyllites developed in areas of restricted circulation rapid influx of terrigenous detritus with renewed tectonism in the next phase resulted in the deposition of a turbidite sequence of greywacke phyllite and lithic arenite in the deeper parts of the epicontinental sea and conglomerate arkose orthoquartizite in the marginal areas the final phase of sedimentation was presumably under fluvial conditions which marked the completion of epicontinental trough filling the nature of the terrigenous clasts indicates a predominantly granitic source of sediments supply of sediment was mainly from the continent to the east and partly from a landmass to the west the cycle of sedimentation noted in the epicontinental aravalli sea is broadly similar to the model of tectonic stages suggested by krynine 1942

evolution of proterozoic aravalli delhi fold belt in the northwestern indian shield from seismic studies a deep seismic reflection study across the aravalli delhi fold belt situated in the northwestern part of the indian shield has revealed a deep penetrating 25 km wide crustal scale thrust fault dipping reflections from the upper crust to the moho and a divergent reflection fabric paleo subduction zones and island arc signatures are identified from the present study seismic images of the crust reveal tectonics of the region with two distinct episodes of rifting sedimentation collision and suturing corresponding to the aravalli and delhi orogenies plate tectonic processes were responsible for the evolution of the paleoproterozoic aravalli and mesoproterozoic delhi fold belts with the juxtaposition of the bundelkhand craton in the east and the marwar craton in the west a 50 v m electrical conductor extending to a depth of 25 km and a steep gradient gravity anomaly of 70 mgal extending all along the strike of the fold belt with conspicuous lows on either side correlate well with the deepseated dipping reflections sutures inferred from the seismic reflection data the present study suggests that a high velocity thick crust was produced in the proterozoic orogens of the region q 2000 elsevier science b v all rights reserved

tourmaline in proterozoic massive sulfide deposits from rajasthan india we have analyzed the chemical composition and boron isotope composition of tourmaline from tourmalinites granite and a quartz tourmaline vein from the deri ore zone and from a pegmatitic band in the rampura agucha ore body these two proterozoic massive sulfide deposits occur in the aravalli delhi orogenic belt rajasthan northwest india tourmaline from stratiform tourmalinites closely associated with the massive sulfides in the deri deposit have preserved their original chemical compositions despite regional and thermal metamorphism in the area these tourmalines have low fe fe mg ratios 0 19 0 30 mean 0 26 that suggest formation close to the sediment sea water interface the 11b values 15 5 and 16 4 are compatible with boron derived from leaching of argillaceous sediments and or felsic volcanics underlying the original massive sulfide deposit during its formation boron isotope compositions measured in tourmaline from a post ore granite and quartz tourmaline vein in the deri deposit indicate that boron in these tourmalines was derived from the tourmalinites produced during ore formation the boron isotope systematics of a coarse brown tourmaline crystal from a pegmatitic band on the hanging wall contact of the rampura agucha deposit indicate that 45 25 of the boron within the original tourmaline was lost during upper amphibolite facies regional metamorphism

age source and stratigraphic implications of pb isotope data for conformable sediment hosted base metal deposits in the proterozoic aravalli delhi orogenic belt northwestern india abstract stratiform sediment hosted base metal deposits in the aravalli delhi orogenic belt of northwestern india are hosted by the sediment dominated bhilwara aravalli and delhi sequences of proterozoic age pb isotope data for 36 samples from five ore districts provide useful stratigraphic and approximate age information model pb ages using a newly developed proterozoic model are 1800 ma for the rampura agucha rajpura dariba and saladipura deposits 1700 ma for the zawar deposits and 1100 ma for the ambaji and deri deposits mineralization in this orogenic belt thus occurred at three stages in middle to upper proterozoic times the oldest of these reflected by deposits in the bhilwara sequence was of broad regional extent the rampura agucha deposit is thus hosted by highly metamorphosed rocks that belong to the bhilwara belt and not to an older supracrustal component of the archean banded gneissic complex the host rocks to the saladipura deposit are bhilwara equivalent and are not part of the delhi supergroup as previously mapped and therefore stratigraphic assignments in the area must be re examined model ages of 1100 ma for the ambaji deri deposits are the only evidence obtained in this study regarding the age of the delhi supergroup and are in apparent conflict with much older ages indicated by previous studies rocks of such late upper proterozoic age may be restricted to the southwestern segment of the orogenic belt or alternatively thrust stacking and or interfolding of lithologically similar sequences of a wide range of ages throughout the belt may not yet be evident because of the limited age and isotopic data presently available all the data can be readily explained by the extraction of pb from sediments derived from an upper crustal basement the isotopic compositions of the zawar deposits provide the clearest evidence for an ancient u enriched upper crustal source the ambaji deri pb data suggest a mature status of the delhi are when these deposits were formed

effects of weathering on biological marker and aromatic hydrocarbon composition of organic matter in phosphoria shale outcrop gc ms analyses were performed on core samples collected from a shale outcrop of the permian phosphoria formation in utah u s a to study effects of weathering on selected biological marker and aromatic phenanthrene hydrocarbon compounds among the biological markers the most important weathering effects are a decrease in 20s 20r diastereomer ratio of the c sub 29 steranes and loss of low molecular weight triaromatic steroids a decrease in the c sub 19 through c sub 22 tricylcic terpanes occurs relative to the total c sub 19 c sub 26 tricyclic fraction pronounced loss of methyl substituted phenanthrenes occurs relative to phenanthrene no major effect on the overall distribution of pentacyclic terpanes is evident

erroneous coal maturity assessment caused by low temperature oxidation previous work carried out on different outcrop coals from the french massif central revealed abnormally high tmax values which initially were attributed to high temperature oxidising fluids this increase in tmax which was exclusively observed for medium to low volatile bituminous coals rr 1 5 was accompanied by a very clear exponential like decrease of the hydrogen index hi together with an increase in the oxygen index oi with the aim of checking the possibility that this rise in tmax could simply be caused by weathering a few mature coal samples rr 1 4 were experimentally oxidised at rather low temperatures 110 125 and 150 c and for durations of up to 6 5 month then the oxidation residues were subjected to rock eval 6 pyrolysis and to vitrinite reflectance measurements these experiments effectively reproduce the geochemical changes displayed by the naturally altered coals thus supporting the hypothesis that the previously studied natural alteration process could have been ultimately caused by weathering the observed artificial oxidation path could be divided into two stages i a strong decrease in hi and an increase in oi and ii a constant hi value and a slow rise in oi the increase in tmax seems related to the transition between the first and the second stages this behaviour of hi and oi during artificial oxidation suggests that these two rock eval derived indices can be practically used to detect a moderate and even a low oxidation degree finally if one ignores the constancy of the vitrinite reflectance values all along the oxidation path the decrease of hi and the concomitant increase of tmax mimic a thermal evolution pathway

the detection of weathering in coal by petrographic rheologic and chemical methods abstract this study examines petrographic rheologic proximate and ultimate analyses techniques to investigate the effects of natural weathering on coal the applicability of such tests to both the detection of weathering and to the prediction of the properties of a fresh coal based upon analyses of a weathered sample is discussed rheologic parameters and a petrographic staining technique showed the most sensitive response to weathering the latter test is considered the most suitable of these for the detection of weathering as it is independent of prior knowledge of the properties of the fresh coal and of the petrographic composition a petrographic approach is also considered most suitable for estimation of fresh coal characteristics from weathered samples rank may be closely approximated and petrographic type determined microscopically even on highly weathered samples and these two parameters may be used to estimate properties such as volatile matter calorific value and dilatation minor modification should be made to vitrinite reflectance measurements according to the degree of weathering as revealed by selective staining

tectono sedimentary setting of the paleoproterozoic zawar pb zn deposits rajasthan india abstract the paleoproterozoic aravalli basin of rajasthan india hosts a number of pb zn sulphide deposits various ore genetic models for these deposits are available in the literature but none of them are backed by comprehensive basin models reconstructed on the basis of sedimentological analysis in the present work an attempt has been made to propose an actualistic basin model for the aravalli basin which was conducive to pb zn sulphide mineralization the zawar mineralized belt is one of the producing pb zn sulphide belts in the aravalli basin the aravalli sedimentary succession which is developed on the basement rocks in the zawar area is more than 2500 m thick the whole basin fill succession an unconformity bounded single mega sequence has been classified into four facies assemblages the sedimentary facies attributes and their architecture in each facies assemblage advocate in favour of an ensialic rift basin which experienced repeated down sagging and exhumation each foundering event of the rift basin was related to movements along some basin marginal and or intrabasinal faults which are manifestation of crustal stretching such faults were directly responsible for sediment dispersal in the basin in this overall tectono sedimentary frame the ore bearing sediments were deposited as a slope base fan deposit in deep marine environment the granitic basement derived sediments that underlie the mineralized horizons are conceived as a source for metals pb b isotopic signatures in galena and tourmaline and the intrabasinal faults that were active during sedimentation might have acted as conduits for upward transmission of the ore brine

effects of weathering on nickel and vanadyl porphyrins of a lower toarcian shale of the paris basin the effects of weathering on the organic matter within a narrow sediment interval from surficial to 10 m depth belonging to the lower toarcian shales of the paris basin were studied at a molecular level among the compositional changes observed the most striking feature is a severe breakdown of the metalloporphyrins ni and vo at the surface as evidenced by quantitative data in addition this degradation of the geoporphyrins is accompanied by substantial modification of their composition with preferential degradation of porphyrins with exocyclic rings over polyalkylporphyrins these quantitative and qualitative changes are thought to result from exposure of the sediment to weathering conditions more specifically abiotic oxidation is likely to be the main weathering process responsible for this superficial alteration of the sedimentary porphyrins this is particularly suggested by oxidation experiments carried out on a mixture of sedimentary nickel and vanadyl porphyrins where oxidation of the former complexes occurs at a faster rate thus geoporphyrins may be used as molecular indicators to evaluate the degree of oxidation of weathered geological samples containing this kind of molecule

a mesoproterozoic supercontinent evidence from the indian shield abstract geophysical signatures of late paleoproterozoic collisional sutures fold belts between the deccan protocontinent and bundelkhand craton and between the bundelkhand and mewar cratons of the north indian shield are presented the aravalli and satpura orogenies evolved during these continent continent collision episodes both the orogenic episodes are associated with granulite facies metamorphism and metallogeny serpentinised ultramafic breccia in ophiolitic melange is present in both regions structural discontinuities and oppositely dipping reflection fabrics with high conductivity anomalies are the characteristic features of these collision zones a major gravity high is also observed all along the strike of these fold belts to a distance of 700 1000 km paleoproterozoic 1 8 2 1 ma ages corresponding to compressional activity have also been observed in the southern granulite terrain and other gondwana fragments extensive granites and gneisses of 1900 ma age are observed in the lower himalayas and nanga parbat ranges late paleoproterozoic orogenic activity is also observed in all of the earth s shields intense orogenic activity of this period observed in various widely separated cratonic blocks is related to global plate tectonic processes and supercontinental episodes the available geophysical evidences in various parts of the globe suggest the presence of a supercontinent referred to as columbia during the early mesoproterozoic period

the timing of mineralization in the archean north pilbara terrain western australia mineralizing events in the north pilbara terrain of western australia occurred between 3490 and 2700 ma and include the oldest examples in the world of many ore deposit types these events were pulsed and associated with major volcano plutonic episodes volcanic hosted massive sulfide porphyry cu sn ta pegmatite mafic ultramafic hosted ni cu pge and epithermal deposits and deformation events lode au deposits in many cases the mineralizing events are associated with extension either in rifts pull apart or back arc basins or as the consequence of doming associated with granitoid diapirism although mineralizing events occurred throughout the evolution of the north pilbara terrain the largest deposits are related to the development of the mallina basin which is located between the older east and west pilbara granite greenstone terranes four volcano plutonic and three deformation events occurred in and around the mallina basin between 2950 and 2840 ma mineralization in the east and west pilbara granite greenstone terranes is less intense and occurred over a much longer period compared to other archean granite greenstone terrains the north pilbara terrain is poorly endowed the only known world class deposit in this region is the wodgina ta sn pegmatite deposit but large iron ore deposits are being mined in the northeastern north pilbara terrain the long history of crustal development and slow rate of crustal growth may be responsible for the diversity of mineral deposits and could account for the apparent poor endowment of the north pilbara terrain

fluid chemistry and depositional mechanism of the epigenetic discordant ores of the proterozoic carbonate hosted zawarmala pb zn deposit udaipur district india the proterozoic carbonate hosted zawarmala pb zn deposit has stratiform banded pyrite sphalerite ore as well as discordant sphalerite galena veins and rich massive galena ore shoots the mechanisms of deposition of discordant vein and massive ores have been studied using speciation and solubility calculations based on ore fluid characteristics and physical parameters inferred from petrography fluid inclusion studies and leachate analysis massive galena ore which is the economic ore type cuts across the earlier f2 related vein type ore and is distinct from the vein type ore in terms of mineralogy texture and fluid inclusion characteristics fluid inclusions in quartz associated with the ores show that fluids forming vein type ore are low salinity 4 3 14 7 wt nacl equiv h2o nacl fluids the trapping temperature of vein type ore fluid was estimated to be in the range of 395 to 290 c at a pressure of about 1 450 bars fluids associated with massive galena ore are h2o co2 nacl fluids of lower salinity about 3 4 wt nacl equiv the wide variation in xco2 of fluid inclusions that occur in close proximity to each other within the same sample of massive ore suggests heterogeneous entrapment of the low salinity h2o co2 nacl fluids from the intersection of isochores of local homogeneously entrapped co2 and aqueous biphase fluids of unmixed co2 h2o nacl fluid trapping temperature fluid was estimated to be between 250 and 150 c at pressures of 2 000 to 750 bars despite the low salinity of the ore fluids speciation studies show that chloride complexes are dominant over most of the temperature range of interest in both ore fluids the only bisulfide complex of importance is batchmode documentclass fleqn 10pt legalpaper article usepackage amssymb usepackage amsfonts usepackage amsmath pagestyle empty begin document pb hs 2 0 end document which is dominant below 175 c in fluids associated with massive galena ore data from fluid inclusions and solubility calculations reveal that cooling is the important mechanism of vein type ore deposition in contrast physicochemical data indicate that an increase in reduced sulfur by mixing a regional high metal content low sulfur fluid with a local high sulfur fluid at the site of deposition was the mechanism for massive galena ore deposition the difference in oxidation state and therefore also in metal content of the two fluids is inferred to be a significant factor that caused differences in the modes of deposition of the two ore types and the relatively high value of xco2 0 2033 in fluids associated with massive galena ore

geochemistry zircon ages and whole rock nd isotopic systematics for palaeoproterozoic a type granitoids in the northern part of the delhi belt rajasthan nw india implications for late palaeoproterozoic crustal evolution of the aravalli craton determination of zircon ages as well as geochemical and sm nd isotope systematics of granitoids in the khetri copper belt of the aravalli mountains nw india constrain the late palaeoproterozoic crustal evolution of the aravalli craton the plutons are typical a type within plate granites derived from melts generated in an extensional tectonic environment they display ree and multi element patterns characterized by steep lree enriched and almost flat hree profiles and distinct negative anomalies for sr p and ti initial e nd values range from 1 3 to 6 2 and correspond to crustal sources with mean crustal residence ages of 2 5 to 2 1 ga a lower mafic crustal anatectic origin is envisaged for these granitoids and the heterogeneous e nd t values are inferred to have been acquired from the magma source region zircon pb pb evaporation and u pb ages indicate widespread rift related a type magmatism at 1711 1660 ma in the northern delhi belt and also suggest a discrete older magmatic event at around 1800 ma the emplacement ages of the compositionally distinct a type granitoid plutons and virtually coeval granulite metamorphism and exhumation in another segment of the aravalli mountains further signify that part of the aravalli crust evolved during a widespread extensional event in late palaeoproterozoic time

tectonostratigraphy and base metal mineralization controls aravalli province western india new interpretations from geophysical data analysis analysis and synthesis of multi disciplinary geoscience information from geological literature maps and from digitallyprocessed aeromagnetic and gravity data pertinent to the aravalli province were carried out to address some hitherto unresolved questions about the tectonostratigraphy of this archaean proterozoic metallogenic province based on the magnetic anomalies several tectonic domains were identified these domains bounded by regional scale geophysical lineaments have distinct crustal lithological metamorphic and metallogenic characteristics and correlate broadly with lithostratigraphic belts identified by several earlier workers new interpretations on the tectonostratigraphy and the base metal mineralization controls in the aravalli province are as follows the hindoli sequences in the eastern parts of the province constitute an independent palaeo proterozoic tectonic domain and do not form part of the archaean basement complex the base metal bearing metasedimentary enclaves in the central parts of the province also constitute an independent palaeo proterozoic tectonic domain which is quite distinct from the surrounding basement complex rocks the base metal bearing metavolcano sedimentary sequences in the western parts of the province constitute an independent neo proterozoic tectonic domain the base metal deposits in the province are spatially associated with the regional scale lineaments and with the mafic metavolcanic rocks deduced from the aeromagnetic data the regional scale lineaments which possibly represent proterozoic crustal scale faults are plausible structural controls on the basemetal mineralization in the province the mafic metavolcanic rocks are plausible heat source controls on the sedex and or vmstype base metal mineralizations and are possible metal source controls on the vms type base metal mineralization in the province d 2005 elsevier b v all rights reserved

retrograde fluid evolution at the rampura agucha pb zn ag deposit rajasthan india fluid inclusions in mineralized graphite sillimanite mica schist from the rampura agucha pb zn ag deposit rajasthan northwest india have been investigated by microthermometry and raman microspectrometry three different main types of fluid inclusions in quartz can be distinguished 1 gaseous co2 partially mixed with ch4 n2 2 low salinity aqueous inclusions 0 8 eq wt nacl and 3 high salinity aqueous inclusions nacl mgcl2 cacl2 low density co2 rich and low salinity h2o inclusions are contemporaneous and occur together with ch4 n2 inclusions in close association with sulfide mineral inclusions this indicates immiscibility between the gaseous and aqueous phase and participation of these fluids during the deposition or remobilization of the ore which occurred over a wide p 1220 to 200 bar and t 450 to 250 c raman spectra of graphite indicate upper greenschist facies metamorphic conditions although host rocks have been metamorphosed at upper amphibolite facies metamorphic conditions this indicates that graphite re equilibrated with the co2 rich phase during retrograde metamorphism

origin of a giant orebody at navan ireland the navan base metal deposit in ireland formed 360 myr ago1 and is one of the world s largest orebodies with 70 million tonnes of 12 zn pb much of the mineralization was sedimentary exhalative other well known examples of this class of the so called sedex deposits are mount isa in australia sullivan in canada and meggen in germany several models for the formation of such deposits have been suggested here we report the first systematic stratigraphic pb isotopic study of galenas in any giant exhalative zn pb orebody we document evidence that the sources of pb in the navan orebody evolved to produce increased dispersion and less radiogenic averages of lead isotopic compositions this is as expected if the orebody formed from a hydrothermal convection cell which expanded to scavenge metals from progressively deeper parts of the continental crust

positive δ13c excursion in carbonate and organic fractions from the paleoproterozoic aravalli supergroup northwestern india abstract carbon and oxygen isotopic investigations have been carried out on the archean and paleoproterozoic carbonate rocks of the udaipur region in the aravalli mountain belt northwestern india the study has led to the interesting finding of 13 c enrichment in the carbonate rocks of the jhamarkotra formation 13 c carb up to 11 1 v pdb belonging to the 2200 1900 ma paleoproterozoic aravalli supergroup further it is observed that the organic carbon from phosphorite bearing stromatolitic dolomites of the same formation are also enriched in 13 c 13 c org up to 11 1 however it must be emphasized that the 13 c enriched carbonates and the organic fractions are from different stratigraphic levels although in the same formation the former being at a lower stratigraphic level a critical analysis of the field geological petrological as well as the isotopic data indicates that the observed 13 c excursion in the carbonate carbon is not due to local causes such as methanogenesis or evaporitic conditions the shallow water environment of deposition of these carbonates as well as the immediately overlying formations rules out the stratified ocean model for the observed excursion the model involving high sedimentation rates and organic carbon burial has been preferred to explain the 13 c excursion in the carbonates of the jhamarkotra formation the 13 c enrichment in organic fraction occurred in localized regions of the aravalli sedimentary basin where high productivity was supported by phosphorous supply indicating that the diffusion limited pathways of organic carbon fixation were operative in the paleoproterozoic itself the 13 c enriched carbonate rocks of the jhamarkotra formation serve as the indian example for the paleoproterozoic global 13 c excursion

tectonic evolution and base metal mineralisation in the aravalli delhi belt india abstract in the aravalli delhi belt of northwestern india the archaean bhilwara supergroup 3500 2500 ma is overlain by the early proterozoic aravalli supergroup 2500 2000 ma and delhi supergroup 2000 1600 ma this region has been subjected to two proterozoic orogenies the earlier aravalli orogeny 2000 ma and the subsequent delhi orogeny 1600 ma late synkinematic granitic activity saw the emplacement of the bairath khetri granites 1660 1480 ma and these were followed by the erinpura abu idar granites 950 630 ma the lithological units show strong folding on n s to nne ssw axes and cross folding at high angles to these directions the rocks of the aravalli supergroup have generally been metamorphosed in the greenschist facies while those of the delhi supergroup show metamorphism in the greenschist to granulite facies there is widespread base metal mineralisation in the aravalli delhi belt copper mineralisation is recorded in the northern part of the belt within the delhi supergroup and lead zinc copper mineralisation is present in the southern part in rocks of both the delhi and aravalli supergroups the base metal mineralisation is of sedimentary exhalative origin the evolution of the aravalli delhi belt during proterozoic times is largely owing to rift tectonics in an ensialic lithosphere

evidence for the neoproterozoic phulad suture zone and genesis of malani magmatism in the nw india from deep seismic images implications for assembly and breakup of the rodinia abstract deep seismic reflection images across the late mesoproterozoic south delhi fold belt sdfb nw india provide evidence for crustal scale tectonic imbrication and collisional tectonism an andean type margin with eastward subduction of oceanic lithosphere and subsequent collision of volcanic arc with mewar craton is responsible for the evolution of the sdfb contrasting geophysical particularly the deep seismic and gravity models and geological signatures found across the sdfb suggest this as a suture the phulad suture zone psz with its extension into the himalaya post collisional delamination and orogenic collapse are responsible for the equilibrated younger moho and evolution of malani magmatism in the region the present study envisages an evolutionary model for the malani volcanics unambiguously identifying for the first time the sdfb rocks as their basement this model successfully resolves the ambiguity by correlating the marwar terrain with the rodinia assembly rather than later pan african orogeny located further west evolution of the sdfb and malani magmatism are coeval with the rodinia assembly and breakup the south delhi orogeny located between the east and the west gondwana fragments plays an important role for reconstruction of the gondwana

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

provenance of proterozoic basal aravalli mafic volcanic rocks from rajasthan northwestern india nd isotopes evidence for enriched mantle reservoirs abstract komatiites komatiitic basalts and tholeiites from nathdwara rajasthan have been investigated for their nd isotopic characteristics 143nd 144nd vary broadly in the komatiites 0 51131 0 51252 and show a restricted range in komatiitic basalts and tholeiites 0 51120 0 51179 and 0 51141 0 51202 respectively the corresponding epsilon nd nd values indicate an enriched source for these rocks the model ages of komatiitic basalts and tholeiites cluster between 2 2 and 3 0 ga the komatiites show older model ages between 3 5 and 3 8 ga if the correlations found between nd and sm isotopic ratios can be interpreted as isochrones then the komatiites point to the age of 2300 ma and the komatiitic basalts and tholeiites constrain the age of 1800 2000 ma the age of 2300 ma for the komatiite samples is similar to the range estimated for the komatiitic basalt and tholeiite samples 1800 2000 ma considering the errors involved collectively the data suggest an age of 2300 1800 ma for the basal aravalli volcanism the variation of nd isotopic ratios between the komatiites and the associated basalts is too large to reflect analytical bias or open system behavior for sm and nd it may be due to significant mantle heterogeneity in the source region isotopically distinct sources for the komatiites and komatiitic basalts are possible but simple mixing between a depleted mantle component and a crustal component in the manner of magmatic assimilation of wall rock material is inconsistent with the observed isotopic ratios the inference of an enriched mantle component appears inevitable as the alternative model of mixing would have generated more siliceous high magnesian boninitic rocks with much lower fe and ti than are observed this enriched material may have been part of the sub continental lithospheric mantle sclm under the aravalli craton which could have generated the basal aravalli magmatic rocks we suggest that during early middle proterozoic the sclm under cratonic regions consisted of relatively fertile mantle capable of generating komatiites tholeiites and alkalic magmas

u th pb isotope chronology of sulfide ores and rocks in the early proterozoic metavolcanic belt of northern wisconsin lead was probably derived from 3 7 b y old source material about 1 8 to 1 9 b y ago data on metavolcanic and granitic rocks from the belt show that the u th pb systems in most were reset about 1 6 b y ago a time of shearing and retrogressive metamorphism a concordia plot of the whole rock lead data shows that many of the rocks lost lead relative to uranium and thorium during the interval 200 to 400 m y ago similar ores might be sought between the northern wisconsin metavolcanic belt and the upper mississippi valley because the lead lost from the basement there might be available for ore formation in overlying units the lead in the early proterozoic metavolcanic belt of northern wisconsin seems to have been derived from the same mantle parent as the late archean metavolcanic belts of northeastern minnesota and southern ontario modified journal abstract