timing of continental arc type magmatism in northwest india evidence from u pb zircon geochronology charnockites and granodiorites which occur within granulite facies metasediments of the bhilwara supergroup of rajasthan northwest india are cogenetic and exhibit petrologic characteristics indicative of a magmatic derivation zircon u pb data yield a common crystallization age of 1723 14 7 ma these rocks earlier believed to represent oldest archean and deepest crust actually mark a major proterozoic event they are believed to be the products of continental arc magmatism where a basic layer underplating the lower crust led to intrusion of differentiated melts in crustal domains undergoing dry metamorphism

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

late paleoproterozoic magmatism in delhi fold belt nw india and its implication evidence from epma chemical ages of zircons abstract zircons from six granitoid plutons occurring in the northern part of the delhi fold belt nw india have been chemically dated using an electron microprobe most of the zircons occurring in the granites are chronologically unzoned and gave ages around 1 78 1 71 ga but younger ages of 1 1 0 9 ga have also been obtained from the rim of some grains monazite and zircons from the metasediments of delhi supergroup adjacent to the granitoids were also analyzed the older age of 1 9 1 7 ga obtained for monazites could possibly indicate their provenance age detrital zircon grains mostly rounded in the delhi metasediments which gave older ages of 1 9 ga could possibly have been derived from the archaean basement rocks exposed in this aravalli delhi fold belt the geochemical signatures of these granitoids are consistent with an extensional tectonic regime it is quite likely that contrary to the prevailing assumption of the granitoids being intrusive our geochronologic data suggests them to be the basement for sedimentation the new geochronologic information on these granitoids in the northern part of the delhi fold belt and their comparison with other correlative events establishes a widespread thermal event during late paleoproterozoic

pressure temperature fluid evolutionary history of the polymetamorphic sandmata granulite complex northwestern india abstract an assemblage of granulite facies metamorphosed pelitic quartzofeldpathic basic and calc silicate rocks is exposed within amphibolite facies tonalitic gneisses amphibolites and pelitic schists belonging to the banded gneissic complex at sandmata northwestern india the granulite facies rocks are bounded by a ductile shear zone and are intruded by enderbite and metanorite reaction textures geothermobarometric data and phase equilibria considerations show that a the granulite facies rocks were metamorphosed at ca 8 kbar 820 c during an early phase of metamorphism b enderbite and metanorite were emplaced subsequent to this metamorphism c all the rocks including the surrounding amphibolite facies rocks were subsequently metamorphosed at ca 6 5 7 kbar 650 700 c when coronal garnet appeared in pyroxene bearing lithologies and d the rocks were affected by non pervasive hydrous fluid flux either prior to or during the later metamorphism it is suggested that the deep crustal granulite facies rocks were juxtaposed against the amphibolite facies rocks along the ductile shear zone during this later phase of metamorphism interpretation of available geochronologic data suggests that the granulite facies metamorphism occurred prior to 1 7 ga and that the entire terrain was possibly reworked at ca 1 ga grenvillian orogeny

evolution and tectonic setting of the malani nagarparkar igneous suite a neoproterozoic silicic dominated large igneous province in nw india se pakistan abstract the neoproterozoic malani igneous suite mis in nw india along with analogous magmatic rocks from the adjoining nagarparkar region in se pakistan can be collectively classified as a silicic dominated large igneous province slip this magmatic event includes bimodal predominantly felsic volcanism granite emplacement and felsic and mafic dyke intrusions felsic rocks have typical a type affinity as indicated by high abundance of silica alkali high field strength and large ion lithophile element concentrations and low cao and mgo contents their nb negative anomalies and zr saturation parameters indicate significant crustal input and high temperature melting mafic volcanics and dykes show geochemical homogeneity and derivation from a depleted continental mantle source without any significant crustal contamination low u and th contents and no visible nb anomaly the region extending from the mount abu batholith in the east to jaswantpura in the west 2700 km2 representing a transition from the metamorphic sirohi terrane to the undeformed mis was evaluated through an integrated structural including satellite image analysis geochemical and geochronological study during the initial stage prior to 760 ma the granitic basement erinpura granites and overlying sirohi metasediments behaved in a brittle manner that led to development of linear fractures and nne trending rift structures and bimodal volcanic activity emplacement of voluminous granitic bodies in response to progressive extension of the crust is inferred during the more evolved second stage younger than 760 ma mirpur granite a representative of this younger granitic suite jalor type pink granite has yielded 753 9 ma zircon u pb crystallization age granitic plutons mark regions of crustal extension as seen in parallel alignment of plutonic bodies jaswantpura granitic belt and parallel mafic dyke swarms 340 transecting the granites structural analysis further identified an episode of crustal convergence which is documented in folding and faulting of the sindreth basin sequence and in tectonic overprint of early stage mafic rocks rifting and bimodal magmatic activity in mis is coeval with similar rock types in nagarparkar in se pakistan further traceable into the seychelles microplate and central madagascar considering the neoproterozoic paleogeography and our observations an extensional setting and an active continental margin position for mis is inferred

geochemical constraints on anorogenic felsic plutonism in north delhi fold belt western india abstract ajitgarh pluton along the northwestern fringe of indian peninsular shield is a composite trondhjemite alkali granite body both the granitoid types intrusive into metasediments of delhi supergroup represent different pulses of anorogenic magmatism the granitoids post dating the major deformational phase demonstrate a time relationship with each other subtle variations in mineralogy and geochemistry indicate distinctive petrogenetic lineage for both the granitoids their tectonic setting mineralogical compositions and geochemical signatures are consistent with a type granites and an igneous source ajitgarh granitoids are quite distinct from other syn orogenic s type granite massifs 1700 1500 ma of north delhi fold belt and appear to be related to the late proterozoic anorogenic magmatism associated with rejuvenation of the rift system that controlled the delhi sedimentation all the granitoid bodies within north delhi fold belt have so far been presumed to be syn orogenic and coeval at 1700 1500 ma

back arc extension in the okinawa trough the okinawa trough lying to the east of china is a back arc basin formed by extension within continental lithosphere behind the ryukyu trench arc system middle to late miocene uplift associated with normal faulting of the initially adjacent ryukyu nonvolcanic arc and the taiwan sinzi folded belt corresponds to the first rifting phase the timing of rifting is supported by the presence of marine sediments of corresponding age drilled in the northern okinawa trough the rifting occurred after a major early miocene change in the motion of the philippine plate with respect to eurasia and ceased during the pliocene a second rifting phase started about 2 m y ago at the plio pleistocene boundary and has continued until the present time it has proceeded to a more advanced stage in the middle and southern okinawa trough than it has farther north detailed bathymetric sea beam seismic reflection and magnetics data collected during the pop 1 cruise of the r v jean charcot reveal the principal features of the extensional processes the back arc spreading phase started very recently in the southern and middle okinawa trough as exemplified by several en echelon and in some cases overlapping active central graben oriented n70 e n80 e some of these depressions are intruded by volcanic ridges of fresh back arc basalt with associated large magnetic anomalies transform faults between these en echelon active rifts are not obvious we suggest that the major part of the southern okinawa trough is underlain by a thinned continental crust and that except for the system of en echelon rifts of the southern okinawa trough the back arc basin oceanic domain is limited to a width of a few tens of kilometers or less in the axial portion of the trough the system of axial back arc volcanic ridges that occur in the rifts ends at the latitude of okinawa island whereas active volcanoes in the ryukyu arc occur only north of okinawa island we refer to this transition between active arc and back arc volcanism as the volcanic arc rift migration phenomenon vamp globally back arc volcanism propagated from the southern okinawa trough to the okinawa vamp area rifting continues to occur in the northern okinawa trough but is not yet accompanied by associated volcanism the okinawa vamp area is characterized by a series of parallel basaltic ridges oriented n75 e with associated linear magnetic anomalies characteristic of dyke intrusions we suggest that the formation of the back arc oceanic domain took place along the axial back arc extensional zone trending n75 e and that this zone presently ends at the southern extremity of the active volcanic chain the initial phase of formation of back arc basin oceanic crust is non steady state and is characterized by the lack of a developed fracture zone pattern the termination of the vamp area in the direction of the volcanic zone of the arc is consistent with the suggestion of molnar and atwater that the volcanic arc is a fundamental line of weakness which determines where initial back arc spreading occurs apparently back arc extension initially occurred within the continental lithosphere located westward of the ryukyu arc along its whole length but the subsequent back arc volcanism was initiated in the southernmost portion of the region and then moved northward this migration was accompanied by the shutting down of volcanic activity along the abandoned portions of the arc it is this transfer of volcanism that we call the vamp process thus arc and back arc basin volcanism seem to be associated in such a manner that spreading tends to migrate simultaneously with a cessation in volcanic activity along the arc this interplay of arc and back arc activity is probably linked to changes in the parameters of plate convergence since the plate motion in the phillippine sea is oblique to the trench at least in the southern part of the okinawa trough we suggest that the oblique resisting force applied to the edge of the overriding plate engenders the development of en echelon extensional features within and behind the arc the motion of the ryukyu platelet with respect to eurasia is consequently an extensional strike slip motion trending roughly parallel to the okinawa trough

geochemical and geochronological characterization of granitoids of the north delhi fold belt implications for the tectonic evolution of east gondwana

high resolution acoustic mapping to understand the ore deposit in the bayonnaise knoll caldera izu ogasawara arc we collected deep sea multibeam side scan and subbottom profiler data using an autonomous underwater vehicle at the bayonnaise knoll a submarine caldera located in the rift zone of the izu ogasawara arc we aimed to reveal topographic and geological features and the origin of a hydrothermal field called the hakurei site in the caldera we performed seafloor classification by textural analysis using calibrated side scan sonar data which provided an effective means to understand the geology and to highlight potential areas of hydrothermal constructions the high resolution bathymetric map illustrates that the hakurei hydrothermal field is distributed over a landslide landform in the caldera wall the distribution of hydrothermal vents indicates that the slip surface has served as a major route of hydrothermal fluids the radial alignment of chimneys and mounds indicates radial routes of hydrothermal fluid and or belching along fragile lines in the landslide landform various postcaldera activities are inferred including the formation of a lava dome a pyroclastic cone and subsequent phreatic explosions a general volcano tectonic structure extending across the caldera in a nw se direction is interpreted as an inferred boundary fault of the north myojin rift analogous to the hokuroku basin and land kuroko deposits it is suggested that the main contributing factor in the formation of kuroko deposits was volcano tectonic activity that dominated the margin of the back arc rift basin the intersections between the margin of a rift basin and the surrounding knolls have a high potential for ore forming areas

reos isotopes and major and trace element geochemistry of carbonaceous shales aravalli supergroup india impact of post depositional processes abstract this study presents re os isotopes and major and trace elemental geochemistry of carbonaceous black shales and a weathering profile developed on them from the paleoproterozoic deposits of the jhamarkotra formation lower aravalli group india these measurements provide information on the impact of post depositional processes particularly hydrothermal fluid interactions on the 187re 187os geochronology and element mobility in shales in contrast to previously reported compositions for black shales the re concentrations of the jhamarkotra shales are in general lower than their corresponding os concentrations the 187os 188os ratios of the black shales vary widely from 3 5 to 10 5 without any discernible systematic trend with 187re 188os range 0 4 to 17 7 excluding an outlier these 187re 188os ratios are lower by a few orders of magnitude than those generally reported for fresh black shales these observations indicate that the jhamarkotra shales though they seem visually well preserved have undergone post depositional modification resulting in open system behavior of re and os with re more mobile than os higher sio2 tio2 and k2o na2o ratios of the jhamarkotra shales compared to their sediment provenance suggest interaction of the shales with hydrothermal possibly si and k rich fluids the sio2 tio2 ratios an index for hydrothermal alteration show negative correlation with re corg and positive correlation with corg n ratios 42 154 indicating loss of re and nitrogen during hydrothermal fluid interaction earlier studies have shown the presence of re porphyrins n containing organic compounds in black shales thus the observed co variations of sio2 tio2 with re corg and corg n ratios suggest mobilization of re from the porphyrins in the shales further positive correlation between sio2 tio2 and 187os 188os ratios indicates that significant amounts of radiogenic os component were re distributed throughout the jhamarkotra shales during the post depositional processes in the weathering profile developed on black shales the al2o3 normalized abundance ratios of mobile major elements are in general lower in the upper layers relative to those in deeper sections attesting to their preferential mobility during chemical weathering on the contrary the al2o3 normalized ratios of trace elements mostly showed insignificant depth variations in the weathering profile suggesting limited mobility over the depth of the profile the depth profiles of re al2o3 and 188os al2o3 and 187os 188os ratios in the weathering profile by and large reflect compositions of their presumed parent shales which have gone through significant post depositional changes

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

tectonic significance of fissure veins associated with pseudotachylites of the kui chitraseni shear zone aravalli mountain nw india abstract kui chitraseni shear zone of the precambrian aravalli mountain nw india is marked by a nearly 400 m wide ne sw trending belt of cataclasites developed on the granites and metasediments of the delhi supergroup the cataclasites are criss crossed by pseudotachylite and fissure veins while the pseudotachylites are aphanitic to microlitic the fissure veins are characterized by syntaxial bitaxial as well as unitaxial growth of elongate quartz grains textural study of the fissure veins suggests crystallization of quartz grains in a dilational environment in hydrous condition contrarily the pseudotachylites bear the evidence of frictional melting of rocks under anhydrous conditions since in many instances it is observed that the fissure veins have replaced the pseudotachylites following the crack seal mechanism the formation of fissure veins is considered to be latekinematic to pseudotachylites hence it is interpreted that the pseudotachylites and fissure veins were formed in two distinct tectonic events while pseudotachylites were formed in a compressional setting resulted from thrusting along the shear zone the fissure veins have been formed in an extensional setting during a late stage normal faulting in light of this tectonic rejuvenation along the kui chitraseni shear zone various lineaments present within the aravalli mountains need further study for understanding the evolution of the region

la icp ms u pb zircon age constraints on the paleoproterozoic and neoarchean history of the sandmata complex in rajasthan within the nw indian plate abstract the sandmata complex in rajasthan within the nw indian shield represents a metamorphosed volcano sedimentary sequence incorporated within a late paleoproterozoic suture zone here we report laser ablation icp ms age data and ree geochemistry of zircons from four rock types in the sandmata complex a foliated biotite quartz gneiss a leucocratic gneiss a biotite gneiss and a porphyritic granite the zircons in the biotite gneiss and porphyritic granite show magmatic textures with a single paleoproterozoic age population whereas the leucocratic gneiss and biotite quartz gneiss samples carry zircons with an archean provenance history and metamorphic characteristics u pb dating of zircon domains in these samples shows two dominant age populations paleoproterozoic and neoarchean and indicate that a granulite facies metamorphism biotite quartz gneiss ca 1708 ma and the granitic magmatism porphyritic granite ca 1733 ma were synchronous and b the igneous protoliths of the leucocratic gneiss ca 2750 ma and biotite gneiss ca 2698 ma were formed in the archean the zircons from both gniesses and granite show high ree contents prominent hree enrichment and a conspicuous negative eu anomaly suggesting a common melt source we correlate the paleoproterozoic magmatic ages from sandmata complex with a subduction accretion history prior to the final collisional assembly of the continental fragments in the neoproterozoic comparable with a similar history all along the central indian tectonic zone as well as from the eastern indian suture probably indicating the continuity of the suture all along peninsular india our zircon data also reveal the vestiges of archean rocks within the paleoproterozoic suture

caldera forming processes and the origin of submarine volcanogenic massive sulfide deposits certain volcanogenic massive sulfide vms ore deposits form in submarine calderas this association is well known but the link between caldera formation and the origins of the deposits remains poorly understood here we show that the size and location of a vms deposit within a submarine caldera may be determined by how and when the caldera formed these spatial temporal conditions control development of the hydrothermal system associated with the vms deposit we propose that caldera opening along outward dipping faults promotes magma degassing seawater influx and high temperature leaching resulting in a metal rich hydrothermal fluid these outward dipping faults are considered to provide critical pathways for ore forming fluids responsible for some caldera hosted vms deposits and may also be fundamentally important for the formation of many other caldera hosted ore deposit types

east pacific rise at 21 n the volcanic tectonic and hydrothermal processes of the central axis abstract photographs obtained by the angus survey system at 21 n reveal many similarities to the geological processes delineated at other spreading centers and in particular those observed in the galapagos rift at 86 w the region of recent volcanism is restricted to a narrow zone zone 1 approximately 1 km wide this suggests that the width of the magma chamber feeding these flows is also narrow at the top variations in sediment cover are used to subdivide the flows within zone 1 into three sheet flow pillow flow pairs the youngest pair appears to have erupted form a linear fissure 8 km long running parallel to the valley axis this fissure is part of a larger en echelon pattern of eruptive fissures active hydrothermal vents associated with the youngest flows are situated directly above the apparent eruptive fissure the high 350 c temperature for some of the fluids exiting from the vents suggests a highly restricted circulation system involving at times little to no mixing with cold seawater the lava terrain in the remainder of zone 1 and bordering regions is characterized by extensive fracturing further suggesting a narrow upper dimension to the magma chamber and also that subsurface hydrothermal deposition of massive sulfides may lead to a reduction in the mixing processes

geochemistry and isotopic characteristics of mafic phulad ophiolite and related rocks in the delhi supergroup rajasthan india implications for rifting in the proterozoic abstract regionally metamorphosed mafic rocks phulad ophiolite in the delhi supergroup n w india have tholeiitic compositions the data indicate that there are two independent rock suites phulad barr jetgarh pbj meta basalts with element abundances and ratios resembling midocean ridge basalts morb and ranakpur desuri r d meta basalts and meta gabbros with relatively high al2o3 and low tio2 for similar mgo and high lil element rb ba sr coupled with low hfs zr nb and ree sm nd abundances the compositions of these rocks are characteristic of ocean arc basalts dioritic rocks at ranakpur exhibit low k tholeiitic differentiation trends and have close chemical affinity with the spatially associated meta basalts ranakpur diorites define a sm nd whole rock isochron corresponding to an age of 1012 78 ma with an initial nd isotopic composition that is similar to bulk earth sm nd mineral isochrons for a meta gabbro diorite and quartz diorite at ranakpur correspond to ages of 838 36 835 43 and 791 43 ma respectively these younger ages reflect metamorphic re equilibration of isotope compositions during widespread granite and granodiorite emplacement in this region during the late proterozoic model sm nd ages for the meta basalts calculated relative to undepleted mantle vary between 1300 1700 ma the chemistry of both mafic rock suites indicates derivation from source regions which had been depleted during previous melting episodes so the model ages are a maximum age constraint ranakpur desuri mafic rocks have negative initial nd 1 0 to 4 5 and low sm nd ratios suggesting crustal contamination or reworking with a few exceptions the pbj meta basalts have positive initial nd 1 5 to 4 8 indicating that they were derived from heterogeneously enriched although longterm depleted mantle the chemical data support the hypothesis that the pbj meta basalts are fragments of proterozoic ocean crust

submarine volcanism in the sumisu rift izu bonin arc submersible and deep tow camera results abstract multi vent volcanic extrusions in the sumisu back arc rift have formed individual cones in the outer rift and short segment en echelon ridges in the inner rift alternating or coincident phases of constructional volcanism and mass wasting have produced a terrain consisting mainly of large basalt pillow lavas lesser amounts of fresh looking lobate ropy and sheet flows along the ridge crests and volcanic debris ubiquitous throughout the rift a rhyolite volcano and inactive hydrothermal chimneys occur at the step in the largest en echelon ridge at present constructional volcanism predominates over tectonism on the features studies the development of en echelon basaltic ridges is greater in other western pacific back arc rifts with more extension continued extension in sumisu rift may cause a shift in the concentration of volcanism from the cross rift trend to an along axis orientation eventually leading to the coalescing of the inner rift volcanic ridges into a back arc spreading system

a 2 5 ga porphyry cu mo au deposit at malanjkhand central india implications for late archean continental assembly abstract eight re os ages from six molybdenite samples representative of the cu mo au mineralization at the giant malanjkhand deposit in madhya pradesh were obtained using id ntims these data provide a clear late archean early paleoproterozoic age for this deposit and by implication for its calc alkaline granitoid host among other diverse models the origin of malanjkhand has been debated as orogenic lode style associated with development of a quartz reef in a shear zone or as porphyry style associated with a pink granitoid intrusion that is conspicuously present in the vicinity of the brittle ductile deformed quartz reef previous models included middle proterozoic ages for the cu mo au mineralization based on poorly constrained rb sr and k ar data direct dating of molybdenite shows that stringer mineralization in the quartz reef and disseminated and vein mineralization in the granite were contemporaneous at 2490 8 ma 2489 5 1 4 ma based on regression without uncertainty in the 187re decay constant mswd 0 5 n 5 additional molybdenite from the quartz reef dated at 2475 and 2450 ma represents at least two pulses of reworking that we suggest configured the elongate and arcuate quartz reef into its present position we suggest that the quartz reef is a high silica stockwork cap embedded in its primary potassic alteration halo and subsequently tilted so that it lies convex to the east with deeper levels exposed to the north petrographic evidence coupled with high re concentrations for molybdenites 400 650 ppm support this scenario and suggest that malanjkhand is a subduction related stockwork porphyry style cu mo au deposit involving mantle malanjkhand geology shares little affinity with three surrounding and tectonically juxtaposed terranes we suggest that the malanjkhand cu mo au deposit and the surrounding batholith containing multiple cu mo au prospects may be the vestiges of a microplate captured along the north margin of the nearly amalgamated southern indian craton at 2 5 ga as the southern craton began to converge with the northern indian craton or other protocontinents now embedded or consumed within the central indian tectonic zone citz this scenario could imply an early paleoproterozoic age for the earliest development of the citz at least in the region of the central indian shear suture cis whose marked ene trend imprints the northwestern part of the malanjkhand batholith and the cu mo au deposit based on a compilation of 2 5 ga events in both the southern and northern indian cratons we suggest that some units in the citz linked an extensive and continuous late archean early paleoproterozoic orogenic belt that may extend to the moyar bavali bhavani attur palghat and cauvery shear zones in southern india and include inboard units of the eastern ghats and the aravalli delhi orogenic belts robust geochronology that can be tied to the early history within these orogenic belts is scarce and extensive reworking during grenvillian and pan african time creates a pervasive overprint at some localities this late archean early paleoproterozoic belt preserves sutures within a coalesced late archean continent that included east antarctica napier and vestfold hills complexes at 2 5 ga

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

an archean oceanic felsic dyke swarm in a nascent arc the hunter mine group abitibi greenstone belt canada abstract the 2730 ma old hunter mine group hmg a dominantly felsic subaqueous volcanic sequence was formed during early arc construction in the abitibi greenstone belt quebec canada the western part of the hmg contains a felsic dyke swarm up to 1 5 km wide and traceable up section for 2 5 km five distinct generations were identified 1 aphanitic to feldspar phyric dykes 2 quartz feldspar phyric dykes with nd values suggest that an older sialic substrate was not involved in their genesis we infer that the felsic rocks were generated by melting of mafic oceanic crust the swarm was emplaced during nascent oceanic island arc development and was related to rifting of the arc the conformably overlying morb like basalts and basaltic komatiites of the stoughton roquemaure group used the same conduits and further indicate splitting of the arc hmg and associated parts of the abitibi greenstone belts bear a strong resemblance to modern rifted intraoceanic arcs of the western pacific

alvin seabeam studies of the sumisu rift izu bonin arc abstract bimodal volcanism normal faulting rapid sedimentation and hydrothermal circulation characterize the rifting of the izu bonin arc at 31 n analysis of the zigzag pattern in plan view of the normal faults that bound sumisu rift indicates that the extension direction 080 10 is orthogonal to the regional trend of the volcanic front normal faults divide the rift into an inner rift on the arc side which is the locus for maximum subsidence and sedimentation and an outer rift further west transfer zones that link opposing master faults and or rift flank uplifts further subdivide the rift into three segments along strike volcanism is concentrated along the ene trending transfer zone which separates the northern and central rift segments the differential motion across the zone is accommodated by interdigitating north trending normal faults rather than by ene trending oblique slip faults volcanism in the outer rift has built 50 700 m high edifices without summit craters whereas in the inner rift it has formed two multi vent en echelon ridges the largest is 600 m high and 16 km long the volcanism is dominantly basaltic with compositions reflecting mantle sources little influenced by arc components an elongate rhyolite dome and low temperature hydrothermal deposits occur at the en echelon step in the larger ridge which is located at the intersection of the transfer zone with the inner rift the chimneys veins and crusts are composed of silica barite and iron oxide and are of similar composition to the ferruginous chert that mantles the kuroko deposits a 1 2 km transect of seven alvin heat flow measurements at 30 48 5 n showed that the inner rift bounding faults may serve as water recharge zones but that they are not necessarily areas of focussed hydrothermal outflow which instead occurs through the thick basin sediments the rift basin and arc margin sediments are probably dominated by permeable rhyolitic pumice and ash erupted from submarine arc calderas such as sumisu and south sumisu volcanoes

response of basement and cover rocks to multiple deformations a study from the precambrian of rajasthan western india abstract metasedimentary rocks of the aravalli group of early proterozoic age 2000 2500 ma are underlain by migmatitic gneisses forming the banded gneissic complex bgc with radiometric ages varying widely from 3500 ma to much younger both the basement and the cover rocks are however involved in superposed folding of identical style and sequence in the metasedimentary cover rocks isoclinal folds of the first set f 1 with an axial planar cleavage have been affected by coaxial folds f 14 in some places and have been overprinted by a set of open upright folds f 2 with n s striking axial planes these structures are affected by small scale conjugate folds and kink bands f 3 with subhorizontal axial planes caused by a vertical compression and by upright kinks and conjugate folds f 4 developed by longitudinal shortening the f 1 f 1a and f 2 structures match in their entirety with those in the migmatites forming the basement gneisses the migmatization being synkinematic with the first folding this suggests that the bgc in its present state represents a largely mobilized basement

mineralogical evolution of indium in high grade tin polymetallic hydrothermal veins a comparative study from tosham haryana state india and goka naegi district japan abstract two tin polymetallic vein type deposits widely separated in time and space but with strong similarities in terms of mineralization style ore mineralogy and chemistry have been studied comparatively with the aim of understanding the mineralogical evolution of in rich hydrothermal systems the tosham deposit bhiwani district haryana india is of neoproterozoic age and constitutes a sn cu prospect with unusually high in content the disseminated crude stockwork and vein mineralization is hosted by greisenised metasedimentary rocks intruded by a porphyritic granite stock and by later rhyolitic effusives the goka deposit naegi district japan is probably of uppermost cretaceous age and occurs close to a well fractionated ilmenite series granitoid body the tin polymetallic vein in the goka deposit is hosted by a welded tuff unit close to a subvolcanic granodiorite porphyry the main host minerals of indium in the tosham and goka ores are sphalerite stannite unidentified zn cu fe in sn s phases and chalcopyrite up to 0 48 wt in has been noted in the goka chalcopyrite whereas at tosham the mineral has a maximum in concentration of 1220 ppm at goka the sphalerite contains up to 1 89 wt in whereas in bearing stannite carries up to ca 9 wt of the metal roquesite is the other indium mineral present in the tosham ores but is absent in goka the mineral chemistry of the tosham and goka ores suggest that the in bearing minerals belong to a multi component zn cu fe ag sn in s system based on various triangular plots of the atomic proportions of the main metals it is inferred that there are end member phases roquesite and stannite in the tosham ores co existing with chalcopyrite the sphalerite is both pure end member and cu in bearing in both the tosham and goka ores some of the analysed stannite grains in tosham ores could possibly be petrukite the zn cu fe sn in s system in the two ores has a sn poor high in solid solution phase and also a sn rich low in solid solution phase it seems possible that these two solid solutions were the first to form during hydrothermal ore deposition at high temperatures from a disordered solid solution located at the cu ag zn fe in sn 3 5 2 intersection in the cu ag zn fe in sn field with decreasing temperatures the sn poor in rich solid solution exsolved the zn in mineral of ohta ohta e 1980 mineralization of izumo and sorachi veins of the toyoha mine hokkaido japan bulletin geological survey of japan 31 585 597 in japanese with english abstract and sphalerite while the sn rich in poor solid solution was broken down to stannite and relatively cu rich sphalerite

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

ion microprobe 207pb 206pb ages of zircons from the bundelkhand massif northern india implications for crustal evolution of the bundelkhand aravalli protocontinent abstract 207pb 206pb ages of zircon from gneissic rocks and granitoids representing all major lithological units of the bundelkhand massif northern india have been determined using an ion microprobe the presence of early archaean granitic crust in this region is established from these ages ages for the kuraicha and the mahoba gneisses and associated metabasic enclaves suggest the oldest partial and metamorphic melting event in this region occurred at 3 3 ga the presence of younger gneisses with ages of 2 7 and 2 5 ga suggests the existence of multiple protolith age components within the massif three major granitoid suites hornblende biotite leuco granitoids were emplaced at 2 5 ga within a time span of a few tens of millions of years felsic volcanism also occurred around this time an age of 2 49 ga for the youngest phase of leucogranitoid marks the time of overall stabilization of this massif the geochronological history of the bundelkhand massif appears to be remarkably similar to that of the basement complex of the aravalli delhi orogenic belt another prominent archaean crustal block to the west of this massif geochemical features of the different rock types in these two regions are also very similar we propose that these two apparently discrete blocks evolved as a single crustal unit with stabilization taking place at 2 5 ga

geology geochemistry and tectonic significance of mafic ultramafic rocks of mesoproterozoic phulad ophiolite suite of south delhi fold belt nw indian shield abstract a thick sequence of mafic ultramafic rocks occurs along a major shear zone phulad lineament running across the length of aravalli mountain range for about 300 kms it has been suggested that this sequence may represent a fragment of ophiolite or a rift related metavolcanic suite made up of basalts and fractionated ultramafics the geological and tectonic significance of the complex is assessed using field relationships petrography and geochemistry structurally the lowest part of the complex comprises a discontinuous band of plastically deformed harzburgite mantle component followed by layered cumulus gabbroic rocks crustal component a complex of non cumulus rocks comprising hornblende schists gabbros sheeted dykes and pillowed basalts structurally overlies layered gabbros huge bodies of diorite intrude volcanics geochemical classification suggests that all non cumulus mafic rocks are sub alkaline basalts except one variety of dykes which shows mildly alkaline character the sub alkaline rocks are tholeiite to calc alkaline with boninite affinity tectono magmatic variation diagrams and morb normalised patterns suggest a fore arc tectonic regime for the eruption of these rocks the mafic rocks of phulad ophiolite suite are zoned across the strike in terms of their distribution from west to east the hornblende schists and basalts are exposed at the westernmost margin followed by gabbros and dykes the alkaline dyke occurs at the easternmost part the rocks of phulad suite are juxtaposed with shallow water sediments in the east followed by platformal sediments and then continental slope sediments in the further east indicating gradual thickening of the crust from west to east and an eastward subduction the geochemical interpretation presented in this study together with discussion of lithological association is used to decipher the tectonic evolution of the mesoproterozoics of nw indian shield

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

paleo uplift and cooling rates from various orogenic belts of india as revealed by radiometric ages discussion 1 discussion sur l article de sharma k k bal k d parshad r nard lal nagpaul k k paru dans tectonophysics nld 1980 70 135 158

age and implications of the phosphatic birmania formation rajasthan india abstract the birmania inlier in western rajasthan india contains important phosphate deposits the depositional age of which is poorly constrained here we provide the first direct age constraints for the phosphate bearing birmania formation using a combination of paleontological and detrital zircon u pb data the occurrence of the multicellular algal fossil wengania exquisita in phosphatic chert of the birmania formation suggest that it was deposited during the ediacaran detrital zircon age distributions contain prominent populations of 1 7 1 9 ga grains with subordinate younger grains that range from 650 to 980 ma the distribution broadly resembles those from neoproterozoic strata from both cratonic and himalayan india but like zircon age distributions from the marwar group lacks 1 0 1 2 ga grains which may suggest that both areas shared similar local sources the lack of zircon grains younger than 650 ma is consistent with an ediacaran depositional age because almost all cambrian or younger strata from india have yielded cambrian or latest neoproterozoic age grains these findings raise the possibility of a previously unrecognized late neoproterozoic episode of phosphogenesis on the india craton the presence of wengania exquisita further supports strong palaeogeographic affinity between the india and south china during the neoproterozoic and cambrian

geochemistry and chemical dating of uraninite in the samarkiya area central rajasthan northwestern india implication for geochemical and temporal evolution of uranium mineralization abstract in the samarkiya area located at the central part of the aravalli delhi fold belt adfb uranium mineralization is hosted both by the basement mangalwar complex and the overlying supracrustal rocks of the pur banera belt the present study aims to appraise the geochemical and temporal evolution of uranium mineralization from the basement and the adjoining supracrustals in the samarkiya area integrating textural features geochemistry and in situ u th pb total dating of uraninite uraninite occurs as inclusions in the major rock forming minerals viz plagioclase quartz biotite and chlorite based on the shape location in the host mineral well inside at the grain boundary along or connected to micro cracks etc and association with other secondary uranium minerals uraninites are classified into different groups which are compositionally distinct barring some exceptions integrating texture geochemistry and in situ electron probe dating we propose that in addition to an old event at 1 88 ga in the basement rocks there are two major events of uraninite formation at 1 24 1 20 ga and 1 01 0 96 ga in both the basement and supracrustal rocks although none of the pristine unaltered uraninites that formed during the above mentioned events contain significant intrinsic minor or rare earth elements the basement uraninites are consistently much enriched in thorium compared to those from the supracrustal based on the compositions we propose that the basement uraninites formed from a high temperature magmatic metamorphic fluid whereas those in the supracrustal rocks crystallized from a low temperature presumably oxidized fluid back scattered electron images x ray elemental mapping of selected elements and epma spot analysis of large uraninite grains both from the basement and the supracrustals collectively demonstrate that subsequent to the major mineralizing event at 1 24 1 20 ga the mineralized rocks were subjected to fluid mediated alteration which resulted in ree y and si ca enrichment of existing 1 24 1 20 ga uraninites in the basement and supracrustal rocks respectively we cannot constrain the exact timing of this alteration event however as this event altered the 1 24 1 20 ga uraninites and as spot ages of the altered grains yield ages largely between 1 24 and 0 96 ga it is reasonable to place this event between the second and third stages of uranium mineralization mobilization at 1 20 ga and 1 01 ga respectively the last event that took place at 1 01 0 96 ga most likely represent an episode of recrystallization alteration of existing uraninite leading to complete pb loss and resetting of the isotopic clock however we do not entirely reject the possibility of neo mineralization the discrete events deciphered from uraninite in the samarkiya area can also be broadly linked to some major magmatic metamorphic events identified from other independent studies in the adfb for example the earliest 1 88 ga event displayed by basement uraninite is most likely related to a pervasive magmatic metamorphic event 1 86 1 82 ga that affected the basement whereas the last latest event 1 01 0 96 ga can be linked to a pervasive metamorphic event that affected perhaps the entire adfb this last episode can also be linked to the tectono thermal event related to the rodinian amalgamation the 1 24 1 20 ga event appears to be somewhat enigmatic in the context of well known geological events in the area however based on some very recently published data we interpret this to be a post peak metamorphic 1 37 1 35 ga hydrothermal event or even a new metamorphic event hitherto unknown

geochemistry and petrogenesis of late archaean aravalli volcanics basement enclaves and granitoids rajasthan abstract major trace and rare earth element data are presented for the basal volcanics of the aravalli supracrustal sequence for the mafic enclaves within the banded gneiss complex bgc adjacent to the supracrustals and for granitoids within the bgc and their petrogenesis discussed with regard to the problematic relationship between basement and cover in terms of major elements the aravalli lavas range from magnesian 22 mgo komatiites or picrites to fe rich tholeftes but differ from a typical archaean greenstone volcanic sequence in that their trace element patterns are relatively enriched in incompatible and light rare earth elements moreover the range of patterns observed requires a number of distinct mantle sources as well as different degrees of melting there are two distinct types of mafic enclave within the bgc the larger lenses correspond quite well with the range of variation seen in the aravalli sequence and could be feeders the other type close to the bgc aravalli junction has more alkalic characteristics high nb zr and could represent rift volcanics or tectonically accreted fragments of ocean islands the data suggest that the aravalli sequence is not an accreted oceanic terrane but are more consistent with interaction of a deep mantle plume with rather mature sub continental lithosphere the early archaean age of parts of the bgc the chemistry of the granitoids and the alkaline character of the enclaves are also broadly compatible with this model the character of the aravalli sequence is intermediate between that of an archaean greenstone belt and a phanerozoic flood basalt province

geochemical evolution and timing of uranium mineralization in the khetri copper belt western india abstract the khetri copper belt kcb located in the northwestern part of the aravalli delhi fold belt western india is famous for cu mineralization and known for au ag co fe ree u p occurrences the present study conducted in and around the madan kudan kolihan chandmari cu deposits of the kcb integrates the mode of occurrence mineralogical association textural relation and chemistry of hydrothermal minerals and in situ chemical dating of uraninite we propose that the u mineralization represented by type 1 to type 6 uraninites has evolved through six successive stages u mineralization type 1 uraninite of uncertain origin u mineralization type 2 uraninite during fe mg alteration u mineralization type 3 uraninite during ca na alteration cu fe co ree u mineralization type 4 uraninite during na ca k alteration u mineralization type 5 uraninite during chloritization fe mg alteration cu co ree u mineralization type 6 uraninite during k fe mg alteration the chemical ages of uraninite suggest that the hydrothermal mineralization associated with type 2 to type 5 uraninites formed during a hitherto unknown older event compared to the previously reported age of the mineralization in the kcb i e 850 ma which initiated during the second phase of metamorphism of 1 40 1 30 ga m2 and terminated well before the third phase of metamorphism of 985 920 ma m3 the k fe mg alteration and the associated mineralization most likely is time equivalent of the known alteration mineralization event of 850 ga the chlorite and biotite thermometry in tandem with u th ratios of uraninite suggest that uraninites crystallized at high temperature 390 c in all the hydrothermal stages the common presence of magnetite and ilmenite the occasional presence of graphite and the fe3 fe3 fe2 ratios of the co genetic gangue minerals such as amphibole biotite and chlorite suggest reduced environment below the haematite magnetite buffer during the crystallization of uraninite in all the hydrothermal stages based on the composition of gangue minerals apatite amphibole biotite chlorite and scapolite in alteration assemblages veins and physicochemical characters of the fluids we discuss the possibility of transportation of u as u4 chloride and fluoride complexes the high temperature and reduced nature of the fluids high th contents and low u th ratios of most of the uraninites similar age of mineralization type 2 to type 5 uraninite and m2 metamorphism and the absence of concomitant magmatic activity collectively suggest that hydrothermal mineralization related to type 2 to type 5 uraninites took place from metamorphic fluids we report for the first time a unique uraninite graphite association in the kcb which can be best explained by their co precipitation from a hydrothermal fluid during the earliest fe mg alteration

petrology of ocellar lamprophyres in south delhi fold belt danva sirohi district rajasthan india abstract ocellar lamprophyres are reported from danva in sirohi district rajasthan which lies in the pindwara watera sector of the south delhi fold belt sdfb they intrude mafic metavolcanics and garnetiferous biotite schist of the ajabgarh group of the delhi supergroup these lamprophyres are unaltered show porphyritic and panidiomorphic textures with olivine clinopyroxene amphibole biotite and spinels constituting the phenocryst phase in a ground mass of clinopyroxene amphibole mica and analcime petrochemically these lamprophyres can be classified as analcime monchiquite alkaline lamprophyre though they have some affinity towards ultramafic lamprophyres and lamproites chemistry of the minerals suggests that the clinopyroxenes have a compositional range of diopside and salite amphiboles are typically kaersutites biotites are titanium rich and spinels are al rich ulvospinels these minerals plot in discriminant fields of minerals from alkaline and ultramafic lamprophyres the danva lamprophyres are characterized by a variety of ocellar features that include i porphyritic ocellus with microphenocrysts of kaersutites and biotite in ground mass of analcime ii zoned ocellus with concentric zones of carbonates and analcime and iii composite ocellus in which type i ocellus enclose the type ii ocellus these ocellar features are interpreted to represent late stage magmatic segregation and magmatic crystallization involving two immiscible magmatic liquids the danva lamprophyres are in strike continuity of the pipela lamprophyres and therefore confirm alkaline magmatism for over 20 kms in sdfb during end neoproterozoic tectono thermal event the close spatial association of the lamprophyres with cu zn au deposits of danva and pipela area and the first report of monchiquite with affinity towards ultramafic lamprophyres and lamproites may be significant in gold and possibly diamond exploration in the sdfb

automated display and statistical analysis of interpreted deep sea bottom photographs abstract an automated system for interpreting and displaying deep sea bottom photographs has been developed which allows rapid compilation and analysis of geological information initially collected photographs are classified using a scheme similar to those applied to deep sea volcanic and hydrothermal terrains by other workers towed camera positions from a transponder navigation system are smoothed using a cubic spline technique to obtain a more accurate model of the tow vehicle track line scan recorded data of the camera s height above the seafloor and camera lens characteristics are used to calculate the field of view for each photograph these data sources are integrated to produce a graphic output which combines the location orientation field of view geological and biological information for all photographs in a single color display an illustration of the technique is presented for a photographic data set collected within the caldera of axial seamount 45 57 n 130 03 w a site of active high and low temperature hydrothermal venting on the central juan de fuca ridge results confirm the existence and pinpoint the locations of several areas of known hydrothermal activity within the caldera quantitative analyses of the data base indicate statistical associations between various geological and hydrothermal parameters in particular rigorous statistical testing indicates that hydrothermal activity is associated with regions dominated by sheet flow lavas and fracturing

malani rhyolites a review abstract the late proterozoic malani bimodal volcanics constitute the largest suite of anorogenic acid volcanics in india the volcanism took place during 745 10 ma ago succeeding the granitic activity of abu pluton and ceased before the onset of marwar sedimentation on the basis of field evidences three stages of igneous activity have been recognised volcanics of the first stage are mostly basalt with occasional andesite or trachybasalts these are subsequently covered by the voluminous outpouring of peralkaline and peraluminous rhyolite basalt dacite and trachyte flows the third stage ceased with the outburst of ash flow deposits the dominant felsic volcanics are rhyolites and rhyodacites spread over an area of about 31 000 km 2 the other rock types associated with rhyolite are trachytes dacites pitchstone welded tuff vitric lithic and crystal ash ignimbrite obsidian pyroclastic slates agglomerate volcanic breccia and volcanic conglomerates majority of the acid volcanics are high potassic and a few are calcalkaline or low potassic in composition feldspar geothermometry suggests the temperature of equilibrium to be above 650 c similar results were obtained by magnetite ulvospinel geothermometry oxygen fugacity is estimated to be about 10 18 under fmq ni nio buffer conditions malani volcanism was essentially under terrestrial conditions although deposition by aqueous conditions are also indicated the volcanic eruptions have been through fissures shield volcanoes and central cones the volcanism was triggered in an extensional tectonic regime of continental crust where geotherm was raised by the repeated influx of basic magma the initial basaltic magma was possibly generated at deeper depth by hot spot activity this magma while migrating upwards supplied additional heat for the partial melting of lower sialic crust resulting in the generation of felsic magma the crustal extension has helped in the upward advancement of the felsic magma

variations of midlatitude transient dynamics associated with enso abstract the winter response of the atmosphere to el nino events in the pacific is studied both from a 14 year integration of the center for ocean land atmosphere gcm using observed ssts from january 1979 to february 1993 and from the corresponding analyses of ecmwf emphasis is put on the shift in the high frequency transients that define the pacific storm track during warm events warm and normal ensembles are defined on the basis of the gcm s diabatic heating field in the tropical pacific which falls in one of two states during the winters of 1982 83 1986 87 and 1991 92 the heating averaged between 6 s and 6 n lies in the range of 100 200 w m 2 all the way across the basin the remaining 10 normal years all show no large tropical diabatic heating anomalies in the mid or eastern pacific the difference between warm and normal ensembles for the mean fields of zonal wind u and height z indicates an eastward and equatorward extension of the midlatitude pacific jet associated with a similar extensio

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

volcanism in the earliest stage of back arc rifting in the izu bonin arc revealed by laser heating 40ar 39ar dating abstract the back arc region of the izu bonin arc has complex bathymetric and structural features which due to repeated back arc rifting and resumption of arc volcanism have prevented us from understanding the volcano tectonic history of the arc after 15 ma the laser heating 40ar 39ar dating technique combined with high density sampling of volcanic rocks from the back arc region of this arc successfully revealed the detailed temporal variation of volcanism related to the back arc rifting based on the new 40ar 39ar dating results 1 back arc rifting initiated at around 2 8 ma in the middle part of the izu bonin arc 30 30 n 32 30 n volcanism at the earliest stage of rifting is characterized by the basaltic volcanism from north south trending fissures and or lines of vents 2 following this earliest stage of volcanism at ca 2 5 ma compositionally bimodal volcanism occurred and formed small cones in the wide area this volcanism and rifting continued until about 1 ma in the region west of the currently active rift zone 3 after 1 ma active volcanism ceased in the area west of the currently active rift zone and volcanism and rifting were confined to the currently active rift zone the volcano tectonic history of the back arc region of the izu bonin arc is an example of the earliest stage of back arc rifting in the oceanic island arc age data on volcanics clearly indicate that volcanism changed its mode of activity composition and locus along with a progress of rifting

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

arc magmatism in the delhi fold belt shrimp u pb zircon ages of granitoids and implications for neoproterozoic convergent margin tectonics in nw india abstract the northwestern region of peninsular india preserves important records of precambrian plate tectonics and the role of indian continent within proterozoic supercontinents in this study we report precise shrimp zircon u pb ages from granitoids from the sirohi terrane located along the western fringe of the delhi fold belt in rajasthan nw india the data reveal a range of neoproterozoic ages from plagiogranite of peshua foliated granite of devala and porphyritic granite of sai with zircon crystallization from magmas at 1015 4 4 ma 966 5 3 5 and 808 3 1 respectively the plagiogranite shows high sio2 na2o and extremely low k2o rb ba comparable with typical oceanic plagiogranites these rocks possess low lree and hree concentrations and a relatively flat lree hree slope a well developed negative eu anomaly and conspicuous nb and ti anomalies compared to the plagiogranite the foliated devala granite shows higher sio2 and moderate na2o together with high k2o and comparatively higher rb ba sr and ree with steep ree profiles and a weak positive eu anomaly in contrast to the plagiogranite and foliated granite the porphrytic sai granite has comparatively lower sio2 moderately higher na2o extremely high y zr nb and elevated ree the geochemical features of the granitoids hfse depletion and lile enrichment nb and ta negative anomalies and their plots in the fields of volcanic arc granites and those from active continental margins in tectonic discrimination diagrams suggest widespread neoproterozoic arc magmatism with changing magma chemistry in a protracted subduction realm our results offer important insights into a long lived active continental margin in nw india during early and mid neoproterozoic consistent with recent similar observations on cryogenian magmatic arcs widely distributed along the margins of the east african orogen and challenge some of the alternate models which link the magmatism to extensional tectonics associated with rodinia supercontinent breakup

late proterozoic acid magmatism and associated tungsten mineralisation in northwest india abstract the crustal evolution of northwestern india is marked by several episodes of acid magmatism during late proterozoic period 900 700 ma these events occurred during the culmination of delhi orogenic cycle and continued till the emergence of a stable aravalli craton some of these granites emplaced within the delhi supergroup of rocks in central and southern parts of rajasthan are associated with tungsten mineralisation this includes three important tungsten deposits namely those of degana sirohi and sewariya areas the geological setting petrography and geochemical characteristics of the mineralised granites in these deposits are comparable while these features are quite different in non mineralised granites of this region comparison of mineralised granites in the three areas indicates that 1 these are relatively small plutons 2 they are emplaced at high crustal level 3 the roof of the pluton is either just exposed or lies closely beneath ground surface and 4 the granites are geochemically specialised with enrichment of alumina w b f rb and li and depletion of sr

cover sequences at the northern margin of the antongil craton ne madagascar abstract the island of madagascar is a collage of precambrian generally high grade metamorphic basement domains that are locally overlain by unmetamorphosed sedimentary rocks and poorly understood low grade metasediments in the antalaha area of ne madagascar two distinct cover sequences rest on high grade metamorphic and igneous basement rocks of the archaean antongil craton and the neoproterozoic bemarivo belt the older of these two cover sequences the andrarona group consists of low grade metasedimentary rocks the younger sequence the newly defined ampohafana formation consists of unmetamorphosed sedimentary rocks the andrarona group rests on neoarchaean granites and monzogranites of the antongil craton and consists of a basal metagreywacke thick quartzites and an upper sequence of sericite chlorite meta mudstones meta sandstones and a volcaniclastic meta sandstone the depositional age of the volcaniclastic meta sandstone is constrained in age by u pb laser ablation icp ms analyses of euhedral zircons to 1875 8 ma 2 detrital zircons of archaean and palaeoproterozoic age represent an input from the antongil craton and a newly defined palaeoproterozoic igneous unit the masindray tonalite which underlies the andrarona group and yielded a u pb zircon age of 2355 11 ma 2 thus constraining the maximum age of deposition of the basal part of the andrarona group the andrarona group shows a low grade metamorphic overprint in the area near antalaha illite crystallinity values scatter around 0 17 2 cuk which is within the epizone the ampohafana formation consists of undeformed polymict conglomerate cross bedded sandstone and red mudstone an illite crystallinity value of 0 25 2 cuk obtained from the rocks is typical of the diagenetic zone occurrences of rhyodacite pebbles in the ampohafana formation and the intrusion of a basaltic dyke suggest a deposition in a wsw ene trending graben system during the opening of the indian ocean in the upper cretaceous that was characterized by extensive rhyolitic to basaltic magmatism along madagascar s eastern coast

implications of the geochemistry of the pelitic granulites of the delhi supergroup aravalli mountain belt northwestern india abstract an isolated granulite terrain occurs within the linear zone of schists and amphibolites of the proterozoic delhi supergroup in the aravalli mountain belt of nw india the geochemistry of the pelitic granulites of such terrains suggests the arkosic to shaley nature of the protolith these rocks are characterized by a high maturity index low to intermediate fe2o3 mgo values 5 78 high la sc 8 3 th sc 4 0 and ni co 1 4 ratios and low ti zr 16 sc cr 0 16 and sc ni 0 16 ratios which imply a passive continental margin nature of the basin the high k2o value 3 0 high k 2 o na 2 o ratio 1 96 lree enrichment flat hree pattern consistent negative eu anomaly high la cn yb cn 10 6 and th sc 4 0 ratios and low la th 2 4 ratio low ni 30 ppm and cr 47 ppm contents and uniform zr hf ratio largely reflect continental provenance and probable proterozoic age these rocks also differ from archaean granulites of the fold belt with regards to k2o and th u values and deformational history

structure and tectonics of the proterozoic aravalli delhi fold belt in northwestern india from deep seismic reflection studies abstract seismic imaging of the crust along a 400 km long deep seismic reflection profile across the palaeo mesoproterozoic aravalli delhi fold belt in the northwestern indian shield brings into focus its complex structure and provides clues to understand the geological processes involved in the evolution of this belt the reflectivity pattern varies considerably for different crustal units along the profile the deep crustal reflection data image two sets of oppositely dipping strong reflection bands from upper to lower crustal levels these are identified as the signatures of the collision corresponding to aravalli and delhi orogeny the data also exhibit a clear moho and strong lower crustal reflections near the collision boundaries a stack of dipping reflections from the top of the moho to the surface is identified as a major thrust fault indicating that the proterozoic collision and deformation were primarily thick skinned in nature

the dharwar craton and the assembly of peninsular india peninsular india can be separated into five discrete crustal areas the bhandara singhbhum and aravalli bundelkhand cratons the eastern ghats and a block in southern india containing the dharwar craton and adjoining granulite terrain these areas are separated by four major joins eastern ghats front godavari rift mahanadi rift sukinda thrust and narmada son lineament and associated thrusts all joins except the godavari rift are former orogenic belts where thrusts juxtapose rocks of different metamorphic grade these thrusts and the absence of clear correlation of suites among different cratons suggest that the indian shield formed by accretion of separate continental fragments epicontinental late archean early pro terozoic sediments throughout india however may indicate that the joins are intracontinental formed within a coherent block the term dravidian shield is used for the southernmost of the five crustal blocks including all of india south of the deccan basalts and godavari rift

submersible observations of hydrothermal manganese deposits on the kaikata seamount izu ogasawara bonin arc abstract during dive 408 of shinkai 2000 on the flank of the kaikata seamount located on the volcanic front of the izu ogasawara bonin arc possibly recent hydrothermal manganese deposits were observed on sandy volcanic sediments at water depths of between 800 and 1200 m the deposits cover almost the entire surface sediment across the manganese belt on the northwestern flank on the seamount on a scale of kilometers the occurrence of the deposits seafloor morphology and temperature of the sediments together with previous shipboard results lead to a possible model for the formation of the manganese deposits a low temperature hydrothermal solution ascends through fractures or faults in the volcanic substrate into permeable volcanic sands and precipitates manganate minerals which cement several centimeters of surface sand as hardpan more intense and probably intermittent discharge of hydrothermal solution follows and forms many typical ridge like vents and mounds on the seafloor subsequently pure dense manganate layers developed beneath the semi consolidated volcanic sandstone and act as a cap to the ascending fluids the proposed mechanism may be the dominant type of low temperature hydrothermal mineralization in submarine island arc volcanoes and rifts similar hydrothermal manganese deopsits most probably occur in other active areas of the modern seafloors and in inactive island arcs

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

sapphirine bearing granulites from abu balaram area gujarat state implications for india madagascar connection p t conditions of sapphirine bearing granulites from a new locality from the north eastern part of gujarat range from 8 2 8 8 kbars at t of 950 c the discovery of this uht assemblage in the north western part of the indian shield bordering the aravalli delhi fold belt provides additional evidences for india madagascar connection linking the aravalli malani sequences to similar lithologies in northern madagascar

high phosphate availability as a possible cause for massive cyanobacterial production of oxygen in the paleoproterozoic atmosphere abstract the deposition of major precambrian phosphorites was restricted to times of global change and atmospheric oxygenation at both ends of the proterozoic phosphorites formed after highly positive carbon isotope excursions in carbonates deposited during the paleoproterozoic lomagundi jatuli event and the neoproterozoic cryogenian and ediacaran periods the correlative step wise rise in atmospheric oxygen over the proterozoic has been linked to changes in the carbon cycle however the postulated relations between carbon isotope events phosphorites and atmospheric oxygenation remain unexplained paleoproterozoic carbonates of the aravalli supergroup india preserve evidence for cyanobacterial blooms in the form of tightly packed stromatolitic columns in the world s oldest significant sedimentary phosphate deposit restricted basins of the lower aravalli group with stromatolitic phosphorites in jhamarkotra udaipur jhabua and sallopat exhibit near zero 13 c carb values and large ranges of 13 c org values between 33 3 and 10 1 indicative of a complex carbon cycle because phosphate accumulates primarily in oxic sediments these eutrophic microbial ecosystems likely developed within the photic zone of the shallow oxygenated marine realm this is consistent with deposition during the time of increasingly more oxidizing conditions after the great oxidation event goe approximately contemporaneous basins without phosphate deposits from ghasiar karouli negadia umra and babarmal exhibit a range of positive 13 c carb excursions some with values up to 11 2 that suggest high rates of organic carbon burial and others with moderately high 13 c carb values around 6 or 3 that suggest smaller carbon cycle perturbations the 15 n values of all these rocks vary between 0 7 and 3 4 and are consistent with the predominance of nitrogen fixation during cyanobacterial blooms in all basin types such low nitrogen isotope values are interpreted to have arisen from the biological response to high phosphate availability we conclude that increased phosphate availability during and after the paleoproterozoic lomagundi jatuli event likely caused cyanobacterial blooms and was a key factor in the oxygenation of earth s atmosphere increasing oxygenation of the shallow ocean seafloor favored the removal of excess phosphate as authigenic apatite thus dampening effects of weathering increases on organic burial and marine 13 c carb after about 2 0 ga

on the evolution of marginal basins this paper presents a critique of the models proposed to explain the formation of back arc basins those marginal basins located behind active or inactive trench systems and whose origin is inferred to be subduction related several marginal basins are not generically back arc but rather are related to the initiation of subduction continental rifting or plate boundary readjustments models of back arc basin formation must be consistent with the observed characteristics of back arc basins which include the following 1 the crustal structure and magnetic lineation patterns of back arc and major ocean basins are similar implying that the processes of crustal accretion in both tectonic environments are also similar 2 arc trench systems without currently spreading back arc basins are in the majority 3 although petrographically similar to and within the compositional range of midocean ridge basalts morb back arc basin basalts babb show consistent geochemical differences from n type morb and are in many respects transitional toward island arc tholeiites iat 4 distinct babb and iat mantle sources may be in extremely close proximity during the initial rifting of an island arc 5 rifting of the volcanic arc may occur on either side of the line of active volcanoes 50 km and may vary from one side to the other along strike 6 a remnant arc is not always developed 7 back arc spreading may be initiated above subducted lithosphere but with time the center of spreading migrates away from the arc and no longer overlies a benioff zone and 8 there is no simple correlation between the timing of global plate reorganizations and the formation of back arc basins the three main classes of models proposed to explain back arc basin formation are mantle diapirism induced aesthenospheric convection and global plate kinematics because the driving mechanism for the first two classes of models is provided by the local subducting slab these models fail to explain the temporal and spatial distribution of back arc basins the third class of models proposes that back arc basins should form whenever global plate interactions require divergence between the overriding plate and the trench line the latter being observed to move only in a seaward direction with respect to a hot spot reference frame however the tectonic setting of several back arc basins suggests that they represent more than just a passive response to kinematic boundary conditions for example some dynamic back arc force seems required to move the new hebrides tonga and south sandwich forearc plates seaward neither the local nor the global models adequately specify the necessary and or sufficient conditions for back arc basin formation a better understanding of the processes associated with the initiation of rifting and of subduction is required arc rifting and back arc spreading are both secondary phenomena to subduction but they are also primary extensional phenomena fundamentally analogous to continental rifting and the initial spreading of major ocean basins

transformation of cataclastically deformed rocks to pseudotachylyte by pervasion of frictional melt inferences from clast size analysis abstract optical contrast between clasts and surrounding dark coloured matrix in thin sections has been used in a computer aided method which has been applied for clast size analysis of pseudotachylyte exposed in a 25 km long brittle deformation zone in the precambrian terrain of western india while the number of clasts n2 equal to or coarser than size u shows a power law distribution n2 u d the area a1 occupied by the clasts of different size fractions shows a log normal distribution the clast size distribution pattern indicates transformation of cataclastically deformed rocks to pseudotachylytes by pervasion of a rock melt generated at the frictional slip planes it is inferred that the melt pervaded the permeable cataclastically damaged zones and so occupied the intergranular space this melt mixed with the comminuted material flowed further as a slurry to form the pseudotachylyte veins the clast laden final product retained the grain size distribution pattern of the parent comminuted material with minor modifications fusion of clast rinds due to heat transfer from the melt slightly modified the initial grain size distribution pattern because of the short duration of the heat transfer process combined with the low conductivity and high specific heat of rocks the temperature at the centre of the coarse grains remained at the initial temperature ti and reduction w of radius r by fusion was a negligible fraction of r as a consequence the population of coarse clasts retained the pre fusion size distribution pattern in contrast w r of the smaller grains was considerable the initial grain size distribution pattern did not survive in the finer fraction of the clasts this difference in thermal response of the smaller versus larger grains to the heat transfer process is revealed in clast size distribution graphs for the pseudotachylytes moreover the observed proportion of matrix can not be explained by cataclasis alone the extent of comminution in the parent material and size of the openings through which the clast laden melt flowed to form veins controlled the clast matrix ratio in the pseudotachylytes

high primary productivity and nitrogen cycling after the paleoproterozoic phosphogenic event in the aravalli supergroup india the causes of atmospheric oxygenation in the paleoproterozoic are multifaceted yet poorly understood an increase in the level of primary productivity however may have been an important factor in producing large amounts of oxygen in the atmosphere a likely environmental consequence of atmospheric oxygenation is an increased abundance of seawater nitrate which would have resulted in a fundamental change from the archean nitrogen cycle in this study we investigate these questions using the carbon and nitrogen isotope compositions of carbonaceous shales from the paleoproterozoic aravalli supergroup india the shales occur stratigraphically above one of the oldest phosphogenic events that took place around 2 0 ga shales in the phosphate domain of the jhamarkotra formation from the lower aravalli group have distinct characteristics from coeval shales in the non phosphate domain carbonaceous shales of the phosphate domain contain relatively high v and u th typical of oxygen deficient environments these shales also have a high organic content of up to 14 wt generally homogeneous 13 corg values around 29 c nwr up to 360 and a large range of 15