mid archaean evolution of the eastern indian craton geochemical and isotopic evidence from the bonai pluton abstract the archaean eastern indian craton consists of several granitoid batholiths intruding greenstone like sequences the bonai pluton one such body in the central sector of the craton consists of a dominant porphyritic and a subordinate equigranular variety ranging in composition from trondhjemite to granite xenoliths of massive trondhjemite migmatite and quartzite occur in both phases the granitoid xenoliths represent remnants of a sialic crust and are the oldest recognizable element in the bonai pluton these are high al 2 o 3 trondhjemites with a highly fractionated rare earth element ree pattern a pb pb isochron age of 3369 57 ma has been obtained from a few spatially related xenoliths the porphyritic and equigranular granitoids show similar petrographic characteristics and most of the samples show a calc alkaline trondhjemitic association a few granite samples however belong to a normal calc alkaline association the trondhjemitic samples are of the low al 2 o 3 type with fractionated light ree lree flat heavy ree hree and a marked negative eu anomaly granites of the calc alkaline association have an identical ree pattern pb isotopic data for all samples together yield a pb pb isochron age of 3163 126 ma the ree characteristics of the two trondhjemite suites can be generated by partial melting of a mafic source with variable residual phases at different depths on the other hand the normal calc alkaline granite may be generated by partial melting of trondhjemite similar in composition to that represented by the 3369 ma xenoliths isotopic ages and the chemical nature of the bonai pluton are similar to those identified in the adjacent singhbhum granite complex it is suggested that the entire eastern indian craton has a common mid archaean history the earliest sialic crust formed at 3400 ma and was followed by trondhjemitic addition at 3300 ma during this phase restricted reworking of the older sialic crust produced calc alkaline granites

residence of ree y th and u in granites and crustal protoliths implications for the chemistry of crustal melts a systematic study with laser ablation icp ms scanning electron microscopy and electron microprobe revealed that 70 95 wt of ree except eu y th and u in granite rocks and crustal protoliths reside within reeythu rich accessories whose nature composition and associations change with the rock aluminosity the accessory assemblage of peraluminous granites migmatites and high grade rocks is composed of monazite xenotime in low ca varieties apatite zircon thorthosilicate uraninite and betafite pyrochlore metaluminous granites have allanite sphene apatite zircon monazite and thorthosilicaie peralkaline granites have aeschinite fergusonite samarskite bastnaesite fluocerite allanite sphene zircon monazite xenotime and th orthosilicate granulite grade garnets are enriched in nd and sm by no less than one order of magnitude with respect to amphibolite grade garnets granulitegrade feldspars are also enriched in lree with respect to amphibolite grade feldspars accessories cause non henrian behaviour of ree y th and u during melt solid partitioning because elevated fractions of monazite xenotime and zircon in common migmatites are included within major minerals their behaviour during anatexis is controlled by that of their host settling curves calculated for a convecting magma show that accessories are too small to settle appreciably being separated from the melt as inclusions within larger minerals biotite has the greatest tendency to include accessories thereby indirectly controlling the geochemistry of ree y th and u we conclude that ree y th and u are unsuitable for petrogenetical modelling of granitoids through equilibrium based trace element fractionation equations

coastal land loss in texas an overview abstract nearly 1 500 acres of prime coastal real estate and productive wetlands are destroyed each year in texas along the gulf shoreline near bay margins and within alluvial valleys primarily as a result of erosion and submergence wetland losses constitute about 75 of the total land losses historical analyses of maps and aerial photographs indicate that land losses are accelerating and that human activities are either directly or indirectly responsible for the increased losses natural decreases in sediment supply have been exacerbated by 1 river basin projects that reduce the volume of sediment transported to the coast and 2 coastal structures and navigation projects that prevent redistribution of littoral sediments along the coast erosion caused by high wave and current energy and an inadequate supply of sediment is responsible for higher local rates of land loss than submergence erosion losses are also more perceptible especially after major storms when the greatest losses occur boat traffic causes substantial erosion along some navigation channels the principal components of submergence are subsidence and the eustatic rise in sea level submergence converts uplands to wetlands and wetlands to open water these surficial changes occur mostly on the coastal plain but are also observed on barrier islands on bayhead deltas and within entrenched valleys recent accelerated submergence is induced by extraction of shallow ground water or production of hydrocarbons at moderate depths faults activated by the withdrawal of these fluids concentrate the subsidence near fault planes coastal land losses directly caused by dredging marinas subdivisions canals rig locations are less than those caused by erosion and submergence but they constitute a growing percentage of total land losses in texas

provenance of cenozoic sedimentary rocks from the sulaiman fold and thrust belt pakistan implications for the palaeogeography of the indus drainage system abstract the provenance of middle eocene to early miocene sedimentary rocks cropping out in the sulaiman fold and thrust belt has been determined examining the mineralogy bulk rock major and trace elements and nd sr isotopes the older 50 30 ma deposits are characterized by a mixed orogenic provenance with a major contribution from the karakorum and the tethyan belt c 80 as the 50 30 ma deposits have a provenance distinct from that of coeval subathu khojak and ghazij shallow marine formations of india and pakistan we propose that they were deposited as a distinct delta system that once fed the palaeo indus fan we document a major change in provenance that occurred before the early late oligocene transition at c 30 ma this change in provenance is marked by the appearance of chlorite and monazite and a shift toward more radiogenic nd sr isotopic compositions we interpret this change as the result of the exhumation and erosion of the proto higher himalaya the 30 15 ma sampled rocks are characterized by a major contribution from the tethyan belt and the higher himalayan crystallines 70 90 and a subordinate contribution 10 30 from the karakorum ophiolitic suture and trans himalaya as the end 0 values of our 30 15 ma samples are similar to those of the palaeo indus fan deposits we suggest that the 30 15 ma sedimentary rocks of the sulaiman fold and thrust belt were the fluvial onshore record of the indus fan other coeval deposits of india and pakistan recorded similar increasing exhumation of the higher himalaya range so that we postulate that these sedimentary rocks all derived from the palaeo indus drainage basin this would suggest that the modern indus drainage basin is no younger than 30 ma

the localized role of base level lowering on channel adjustment of tributary streams in the trinity river basin downstream of livingston dam texas usa the purpose of this study is to describe and explain river channel cross sectional change in tributary streams within the 60 km scour zone of the trinity river texas downstream of livingston dam this study investigates changes within the tributary systems to base level changes triggered by incision associated with scour and channel migration within the dammed trunk stream the dam represents a precise moment and place of a system perturbation and therefore represents an opportunistic geomorphic experiment geomorphological effects of the lower trinity river tributaries were investigated through three different types of data resurveys of bridge cross sections planform change as measured from aerial photographs and field mapping of changes in channel geometry using fallen trees undercut banks and other geomorphic indicators of change while field evidence of channel change was found at sites well upstream of tributary confluences with the trinity these were not consistent with responses to lower base level downcutting and lateral changes in response to lowered base level were confined to lower tributary reaches this suggests distance decay and lag times in transmission of responses upstream and or masking of dam induced effects by other processes and controls the most dynamic locations of geomorphic change are near the confluences of the tributaries with the trinity river where the response to dam induced incision is strongly influenced by local trunk stream morphology indicating a high degree of historical and geographical contingency

diagenetic origin for quartz pebble conglomerates the occurrence of quartz pebble conglomerates qpc in the rock record increases backward through time from the tertiary through the precambrian the positive correlation between qpc abundance and age is valid both for numbers of reported qpc and for qpc as a percentage of all conglomerate and at both the era and the period level qpc are usually interpreted as being due to intense chemical weathering protracted transport or sediment recycling but none of these can account for the age distribution of qpc which is the opposite of the global mass age distribution for sedimentary rocks precambrian and tertiary conglomerates with similar sources and sedimentology have vastly different clast populations nonquartzose clasts being much more abundant in the younger rocks comparison of the petrology of qpc and polymict conglomerates shows that qpc have consistently higher proportions of diagenetic secondary matrix and pressure solved grain contacts we conclude that diagenetic factors play an important role in qpc formation by preferentially destroying less durable clasts

propagating rifts and continental breakup a simple quantitative model is proposed whereby continental breakup starts with a phase of continental rifting and continues with the propagation of rifts toward locked zones these zones are deformed in the rifting process while oceanic crust is generated in the wake of the rift tip the deformed zones will cause overlaps when a continental reassembly is attempted the model accounts for the shape and distribution of magnetic quiet zones at ocean continent boundaries and for the detailed geometry of some v shaped zones of oceanic magnetic anomalies the former provide an estimate of the scale at which the hypothesis of rigid plates fails and the latter are used to measure the velocity of rift propagation illustrations are provided for present rifting in the red sea and gulf of aden and for early rifting of the south atlantic and breakup of pangea

dam to delta sediment inputs and storage in the lower trinity river texas livingston dam on the trinity river in se texas usa disrupts the transport of sediment to the lower trinity river and the trinity bay galveston bay estuary however a sediment budget of the lower basin shows that the effects of this disruption are undetectable in the lower river sediment trapped in lake livingston is partly offset by channel erosion downstream of the dam and by inputs from the lower basin most importantly however the lower coastal plain reaches of the trinity are characterized by extensive alluvial storage and are a bottleneck that buffers the bay from effects of upstream changes in sediment flux storage is so extensive that the upper trinity basin and the lowermost river reaches were essentially decoupled in the sense that very little upper basin sediment reached the lower river long before the dam was constructed whereas sediment storage in lake livingston is extensive alluvial storage on the trinity flood plain is even more extensive dam related sediment starvation effects are noted for about 52 km downstream and the sediment budget suggests that a majority of the sediment in this reach is likely derived from channel scour and bank erosion the capacious alluvial storage in the lower trinity not only limits flux to the bay but the large amount of remobilizable alluvium also allows the system to adjust to localized sediment shortages as illustrated in the dam to romayor reach internal adjustments within the lower trinity river valley thus buffer the bay from changes in sediment supply upstream

dating the oldest terrestrial rocks fact and fiction abstract this paper is a review of well documented geochronological and related isotopic evidence for the age and total crustal residence time of the oldest known terrestrial rocks mainly using rb 1bsr sm 1bnd and u 1bpb decay schemes the oldest known stable sialic crust of true continental character predominantly composed of calc alkaline orthogneisses was produced during major mantle differentiation some 3 7 3 5 ga ago and is represented on several continents enclaves of older supracrustal rocks as well as a major remnant of a volcano sedimentary sequence of greenstone belt affinity dated at 3 8 ga are preserved within the surrounding gneisses in southern west greenland recent ionmicroprobe evidence for 4 1 ga old detrital zircon grains in a younger early archaean metasedimentary sequence in western australia is also discussed several cases are reviewed where claims for early archaean 3 4 ga dates are believed to be erroneous and to result from oversimplistic and incorrect interpretation of isotopic data some of the geological and geochemical mechanisms which have given rise to claims for spuriously old sm 1bnd u 1bpb and pb pb dates are critically discussed to supplement published data we present several new sets of whole rock sm 1bnd and pb pb isotopic data from west greenland labrador zimbabwe and eastern india relevant to the overall topic of this review

sr and nd isotopes in river sediments from the ganga basin sediment provenance and spatial variability in physical erosion 1 sr and nd concentrations and isotope compositions in sediment of the ganga river from gangotri to rajmahal and its tributaries have been measured to determine provenance and the spatial variability in physical erosion among the ganga subbasins sr and nd in silicates range from 37 to 138 and from 10 to 36 g g with 87sr 86sr and nd of 0 7474 0 8428 and 25 5 to 15 5 respectively the results suggest that 65 of ganga mainstream sediments are derived from the higher himalayan crystallines highlighting intense physical erosion in this region the 87sr 86sr values of sediments in the gangetic plain show nearly identical trends during two seasons with a sharp and significant decrease at barauni downstream of gandak confluence this brings out the major impact of the sediment contribution from the gandak to the ganga mainstream model calculation suggests that about half of the ganga sediment at rajmahal is sourced from the gandak the erosion rates in the himalayan subbasins of the ganga range between 0 5 and 6 mm a where a is years with the gandak having the highest erosion rate high relief and intense precipitation over the headwater basins of the gandak appear to drive the rapid and focused erosion of this basin the results of this study and those in literature suggest that the eastern syntaxis brahmaputra the western syntaxis indus and the gandak have much higher physical erosion rates than the other himalayan basins focused erosion in the hot spots of these river basins contributes significantly to the global riverine sediment budget and influence regional tectonics

sm nd isotopic and geochemical study of the archean tonalite amphibolite association from the eastern indian craton we report the results of a sm nd isotopic major element and rare earth element ree study of the older metamorphic group omg tonalite amphibolite association of the eastern indian craton the older metamorphic tonalite gneisses omtg have been previously dated to be 3 8 ga using sm nd isotope systematies and 3 2 3 4 ga by rb sr and pb pb dating the results of this study indicate that the protoliths of the omg amphibolites are 3 3 ga isochron age 3 30 0 06 ga nd 0 9 0 7 and therefore the omtg which intrude into the associated amphibolites cannot be any older than 3 3 ga the amphibolites display light ree enrichment ce yb n 2 2 6 7 la 30 100 x chondrite and nearly flat heavy ree patterns tb lu n 1 2 1 9 the basaltic parents of the amphibolites were probably generated by the partial melting of a spinel lherzolite mantle strong linear relationships between the amphibolites and tonalites in 147sm 144nd 143nd 144nd space isochron age 3 29 0 04 ga nd 0 8 0 8 imply that they are genetically related the tonalites display fractionated ree patterns la 100 300 x chondrite with moderate heavy ree depletions tb lu n 1 9 3 4 the isotopic major element and ree data are consistent with the derivation of the omtg from partial melting of omg amphibolites or equivalent rocks at amphibolegarnet stabilization depths an initial nd t value of 0 9 0 7 for the amphibolites indicates the presence of a slightly depleted mantle source at 3 3 ga with 147sm 144nd between 0 20 and 0 22 it is suggested that the growth of continental crust in the eastern indian craton occurred in response to magmatic underplating in a plume setting

geochemistry of archean shales from the pilbara supergroup western australia archean clastic sedimentary rocks are well exposed in the pilbara block of western australia shales from turbidites in the gorge creek group ca 3 4 ae and shales from the whim creek group ca 2 7 ae have been examined the gorge creek group samples characterized by muscovite quartzchlorite mineralogy are enriched in incompatible elements k th u lree by factors of about two when compared to younger archean shales from the yilgarn block alkali and alkaline earth elements are depleted in a systematic fashion according to size when compared with an estimate of archean upper crust abundances this depletion is less notable in the whim creek group such a pattern indicates the source of these rocks underwent a rather severe episode of weathering the gorge creek group also has fairly high b content 85 29 ppm which may indicate normal marine conditions during deposition rare earth element ree patterns for the pilbara samples are characterized by light ree enrichment lanybn 7 5 and no or very slight eu depletion eueu 0 82 0 99 a source comprised of about 80 felsic igneous rocks without large negative eu anomalies felsic volcanics tonalites trondhjemites and 20 mafic ultramafic volcanics is indicated by the trace element data very high abundances of cr and ni cannot be explained by any reasonable provenance model and a secondary enrichment process is called for

mesoarchaean palaeoproterozoic stratigraphic record of the singhbhum crustal province eastern india a synthesis the mesoarchaean palaeoproterozoic stratigraphic record of the singhbhum crustal province eastern india implies sedimentation and volcanism in a changing tectonic scenario and thus assumes immense geological significance although efforts have been made by many researchers in the past several decades to summarize various geological aspects of the singhbhum crustal province a critical synthesis of various stratigraphic issues was long overdue the present contribution is an updated critical synthesis of the mesoarchaean palaeoproterozoic stratigraphic record of the singhbhum crustal province we have pointed out the problematic stratigraphic issues of the singhbhum crustal province that deserve careful scrutiny in order to gain better insights into the mode of stratigraphic sequence building

emplacement of clay into sand by infiltration abstract laboratory experiments and examination of core samples from holocene sand bodies demonstrate that vadose infiltration of muddy water through sand is an effective mechanism by which clay can be emplaced into sand infiltrated clays form coatings on and bridges between sand sized framework grains clay particles are mostly oriented parallel to grain surfaces in coatings and perpendicular to grain surfaces in bridges the amount of clay emplaced as coatings and bridges is determined by complexly interacting variables that include sand grain size clay particle size clay particle shape concentration of suspended sediment and clay composition of these relatively coarse sand sizes and high concentrations of suspended sediment appear to promote the emplacement of the most infiltrated clay in holocene sands the formation of grain coatings and bridges is facies dependent infiltrated clays are most effectively emplaced in environments characterized by high suspended sediment concentrations fluctuating water levels and minimal sediment reworking e g point bar and delta plain in contrast virtually no infiltrated clay is emplaced in environments characterized by low suspended sediment concentration and continuous reworking e g beach and tidal delta

sedimentary rocks and crustal evolution tectonic setting and secular trends a significant change in the composition of turbidites deposited at active tectonic settings is seen at the archean post archean transition in comparison to post archean active margin turbidites archean greenstone turbidites exhibit more uniform th sc ratios but with a similar average greater hree depletion gd n yb n 2 less eu depletion eu eu mostly 0 85 and an absence of low th u ratios th u 3 these data indicate that differing mantle sources and or conditions of mantle melting existed during the archean and that intracrustally differentiated rocks older cratons or young differentiated arcs were relatively unimportant as provenance components for archean turbidites in contrast some sedimentary rocks preserved in archean platformal sequences of high grade terranes display significant negative eu anomalies similar to the majority of post archean shales indicating that the general process of cratonization including intracrustal differentiation has taken place since at least 3

continental rifting and the implications for plate tectonic reconstructions previous plate tectonic reconstructions have tried to recreate the pre rifting pangea configuration of the continents by matching contours or lineaments that are thought to represent the continental boundaries such reconstructions have the inherent assumptions that no extension occurs within the continent during rifting that the continental boundaries are isochrons and that the continents rift without distortion this paper proposes a model for continental breakup with distortion where ocean basins are formed by propagating rifts this model challenges the assumptions made by previous reconstructions and presents a new method for representing the prerift geometry of the continents as rifts propagate through a continent the region in front of the rift extends by continental faulting and crustal thinning while the region behind the rift expands by seafloor spreading the propagating rift model implies that large amounts of continental extension up to 150 km occur and that the continental boundaries are not isochrons this extension due to rifting results in apparent overlap when the continents are returned to their original configuration because of the gradational nature of this extension the best representation of the pre rift configuration is obtained by matching the initial rifting point and having overlap increase in the direction of rift propagation the overlap is equivalent to the amount of extension that has taken place on both continental edges reconstructions based on this model are presented for the gulf of california the gulf of aden the norwegian greenland sea and the south atlantic ocean they are able to resolve the discrepancies that occur when rigid plate tectonics are applied this model has implications that can be used to predict the age and structure of the continental shelves and the location and orientation of the oldest magnetic isochrons

continental break up of the south china sea stalled by far field compression the outcome of decades of two dimensional modelling of lithosphere deformation under extension is that mechanical coupling between the continental crust and the underlying mantle controls how a continent breaks apart to form a new ocean however geological observations unequivocally show that continental break up propagates in the third dimension at rates that do not scale with the rate of opening here we perform three dimensional numerical simulations and compare them with observations from the south china sea to show that tectonic loading in the direction of propagation exerts a first order control on these propagation rates the simulations show that in the absence of compression in that direction continental break up propagates fast forming narrow continental margins independently of the coupling when compression is applied propagation stagnates forming v shaped oceanic basins and wide margins changes in out of plane loading therefore explain the alternation of fast propagation and relative stagnation these new dynamic constraints suggest that the west to east topographic gradient across the indochinese peninsula prevented continental break up propagation through the 1 000 km wide continental rift of the central and west basin of the south china sea until the direction of stretching changed 23 million years ago resulting in bypassing and acceleration of continental break up propagation tectonic loading in the direction of propagation exerts an important control on the propagation of continental break up according to three dimensional simulations of the south china sea

self consistent rolling hinge model for the evolution of large offset low angle normal faults the nature of the physical processes responsible for the formation of continental and oceanic metamorphic core complexes is widely debated the controversy focuses primarily on whether the low angle normal faults observed in these environments formed and slipped at low angles or were rotated from an original high angle orientation after large offsets we describe a self consistent numerical model for the extension of a brittle layer that can spontaneously produce normal fault structures in our formulation a fault or faults form because strength is locally reduced with increasing strain if the reduction in fault strength is 10 of the total strength of the layer then faults lock after an offset smaller than the layer thickness and new faults form larger strength reduction leads to single faults that continue to slip no matter how large the fault offset if the strength reduction occurs by the loss of cohesion then we see the unlimited offset faults for layers 11 22 km thick for reasonable values of cohesion the key result of this study is that structures very similar to those observed in both oceanic and continental core complexes are produced by rotation of the inactive part of the model fault after very large offset

207pb 206pb zircon ages and the evolution of the singhbhum craton eastern india an ion microprobe study1this paper is dedicated to prof a k saha one of the senior co authors who passed away after submission of the paper 1 abstract we used an ion microprobe to measure 207 pb 206 pb ages of single zircons from four key lithologies from the singhbhum craton of eastern india detrital zircons from the oldest recognized sedimentary sequence of the craton the older metamorphic group yielded 3 5 3 6 ga ages indicating that crustal formation had already been initiated by 3 6 ga in this region this data also established an older limit of 3 5 ga for the age of sediment deposition the first appearance of extensive continental crust in the singhbhum region marked by the emplacement of the older metamorphic tonalite gneisses is dated at 3 44 ga our data from both of these units reveal that a 3 2 ga event induced overgrowths within each of these pre existing zircon populations we determine an age of 3328 7 ma for the singhbhum granite phase ii the most widespread of this granite s three phases the emplacement of the two phases of the mayurbhanj granite marking the youngest major episode of acid plutonism in this craton took place in rapid succession at 3 1 ga marking the broad scale stabilization of this terrain

rare earth elements in the indian banded iron formation abstract the normalised patterns of rare earth elements ree of the banded iron formation bif associated rocks and ores from eastern india show similarities to the patterns of the bif from other parts of the world but their absolute content is rather low for the chemically precipitated rocks of india the process of formation appears to have influenced the relative concentration as the clastic phases are richer in ree compared to the chemically precipitated rocks the observed ce and eu anomalies are similar to those reported for such rocks and have been ascribed to mineralogy and process of formation rather than to their temporal position or the source rock the bif of precambrian and younger age could very well be discriminated by the eu sm ratio the ree are selectively enriched in the fe oxides compared to silica during diagenetic differentiation but neither diagenesis nor metamorphism affects the distribution patterns of the ree as diagenetically unaffected supergene products and highly metamorphosed bif exhibit analogous normalised patterns

heat production heterogeneity of the indian crust beneath the himalaya insights from the northern indian shield abstract crustal thermal structure of the himalaya depends partly on the thermal properties of the indian crust that has subducted beneath the himalayan range and partly accreted to it in most studies the indian crust is assigned uniform thermal properties mostly derived from the data from southern india we show here that this might actually not be a good assumption based on the in situ heat production measurements at 279 sites in the bundelkhand craton chottanagpur gneissic complex and singhbhum craton the dominant rocks granites and gneisses in the bundelkhand craton and chottanagpur gneissic complex are characterized by high heat production 3 93 6 52 w m 3 whereas the rocks in the singhbhum craton are low in heat production 1 41 3 27 w m 3 thereby suggesting a large variation of heat production in the northern indian shield the available surface heat flow values also show consistently large variation from 56 to 96 mw m 2 the crustal heat production heterogeneity within the indian crust has to be duly considered in the interpretation of thermobarometric and thermochronological data of the himalaya

a comparison of factors controlling sedimentation rates and wetland loss in fluvial deltaic systems texas gulf coast abstract submergence of coastal marshes in areas where rates of relative sea level rise exceed rates of marsh sedimentation or vertical accretion is a global problem that requires detailed examination of the principal processes that establish maintain and degrade these biologically productive environments using a simple 210 pb dating model we measured sedimentation rates in cores from the trinity lavaca navidad and nueces bayhead fluvial deltaic systems in texas where more than 2000 ha of wetlands have been lost since the 1950s long term average rates of fluvial deltaic aggradation decrease southwestward from 0 514 0 008 cm year 1 in the trinity 0 328 0 022 cm year 1 in the lavaca navidad to 0 262 0 034 cm year 1 in the nueces the relative magnitudes of sedimentation and wetland loss correlate with several parameters that define the differing fluvial deltaic settings including size of coastal drainage basin average annual rainfall suspended sediment load thickness of holocene mud in the valley fill and rates of relative sea level rise there is some evidence that upstream reservoirs have reduced wetland sedimentation rates which are now about one half the local rates of relative sea level rise the extant conditions indicate that fluvial deltaic marshes in these valleys will continue to be lost as a result of submergence and erosion

growth of granite greenstone terranes at convergent margins and stabilization of archean cratons abstract archean granite greenstone terranes represent juvenile continental crust formed in a variety of plate tectonic settings and metamorphosed through a complex series of structural and magmatic events most archean granite greenstone terranes appear to have acquired their first order structural and metamorphic characteristics at convergent plate margins where large accretionary wedges similar in aspect to the chugach makran and altaids grew through offscraping and accretion of oceanic plateaux oceanic crustal fragments juvenile island arcs rifted continental margins and pelagic and terrigenous sediments buoyant slabs of parts of archean oceanic lithosphere may have been underplated beneath these orogens forming thick crustal roots characterized by interleaving between the depleted slabs and undepleted asthenosphere back stepping of the subduction zones after accretion of plateaux and arcs caused the arcs magmatic fronts to migrate trenchward through the accretionary wedges dehydration of the subducting slabs hydrated the mantle wedges below the new arcs and generated magmas sanukitoid suite in the mantle wedge whereas other magmas tonalite trondhjemite granodiorite or ttg suite appear to have been generated by melting of hot young subducted slabs eventual collision of these juvenile orogens with other continental blocks formed anatectic granites then thickened the crust beyond its ability to support its own mass which initiated gravitational collapse and decompressional release of syn to late tectonic granitoids from wedges of fertile mantle trapped between underplated oceanic lithospheric slabs and aided in the cratonization of the granite greenstone terranes deeply penetrating structural discontinuities such as shear zones and sutures provided pathways for fluids and granitoids to migrate into the mid and upper crust forming ore deposits and plutons most preserved granite greenstone terranes have been tectonically stable since the archean and form the cratonic interiors of many continents

a review of hadean to neoarchean crust generation in the singhbhum craton india and possible connection with pilbara craton australia the geochronological perspective abstract the precambrian singhbhum craton sc of the eastern india holds a prolonged record of sporadic crust generation and evolution starting from as ancient as the hadean till late archean like every craton episodic nature of crust generation in the sc has been indicated by several workers based on numerous stratigraphic and geochronological evidence long before however it is only in the past decade that a growing zircon u pb hf database has resolved its evolution in detail such as the timing of juvenile crust versus reworking events integration of over a thousand geochronological data n 1558 from hadean 4 ga till the end of archean 2 5 ga exhibits multiple prominent crustal growth peaks during c 3320 ma 3300 ma and 2780 ma combined with moderate peaks at c 3460 ma 3380 ma and 2820 ma integral crust modelling of hf isotopezircon indicates major juvenile crust addition during the periods c 3760 3640 ma 3620 3440 ma while prominent age frequency peak at c 2780 ma defined by u thbaddeyelite ages of regional mafic ultramafic dyke swarm provides a direct proxy of large scale juvenile crust generation the interplay between the growth of new crust and reworking of older crust combined with multiphase deformation and metamorphic events till the end of archean constructed the critical juxtaposition of diverse crustal components of this important craton constraining crust generation and deformation events from an updated geochronological database of this craton reveal remarkable similarity in crustal growth pattern with the archean pilbara craton australia moreover paleomagnetic studies of the neoarchean mafic dyke swarms of both the cratons indicate similar paleo positions therefore compiled field geochronological paleomagnetic parameters invoke the possibility that the archean singhbhum and pilbara cratons might have been connected and shared similar crustal history at least till the neoarchean era

provenance analysis of upper cretaceous strata in the tethys himalaya southern tibet implications for timing of india asia collision abstract the provenance of upper cretaceous strata in the tethys himalaya provides critical constraints on the closure time of the neo tethys ocean and the initial india asia collision this paper reports detailed petrographic studies in situ detrital zircon u pb ages and lu hf isotopic analyses whole rock nd isotopes and cr spinel electronic microprobe data from upper cretaceous clastic sedimentary rocks of the tethys himalaya near gyangze southern tibet the berriasian coniacian jiabula formation consists of black mudstone chert and minor quartz arenite and is dominated by detrital zircons with archean cambrian u pb ages which were most likely derived from the indian continent overlying conformably is the santonian maastrichtian chuangde formation which consists of red shale limestone and chert the chuangde formation is in turn overlain by the late maastrichtian late paleocene zongzhuo formation which is composed of black mudstone and lithic sandstone enclosing various olistoliths of sandstone limestone and chert the rilang conglomerate is a lens which is located within the upper part of the zongzhuo formation and consists of an upward thinning and fining succession of volcanic conglomerate sandstone and black mudstone the zongzhuo formation and the rilang conglomerate record an abrupt influx of cretaceous zircon grains with juvenile hf isotopic compositions arc related cr spinels and positive end 0 sediments suggesting an arc and suture zone provenance the change in provenance of upper cretaceous strata from the southern indian continent to a northern arc and suture zone is attributed to either 1 initial collision between the indian plate and lhasa terrane or 2 initial collision between the indian plate and an intra oceanic arc we prefer option 1 that the initiation of india asia collision occurred during maastrichtian 70 65 ma

from continental rifting to seafloor spreading insight from 3d thermo mechanical modeling abstract continental rifting to seafloor spreading is a continuous process and rifting history influences the following spreading process however the complete process is scarcely simulated using 3d thermo mechanical coupled visco plastic numerical models we investigate the complete extension process and the inheritance of continental rifting in oceanic spreading our modeling results show that the initial continental lithosphere rheological coupling decoupling at the moho affects oceanic spreading in two manners 1 coupled model a strong lower crust mechanically couples upper crust and upper mantle lithosphere generates large lithospheric shear zones and fast rifting which promotes symmetric oceanic accretion i e oceanic crust growth and leads to a relatively straight oceanic ridge while 2 decoupled model a weak ductile lower crust mechanically decouples upper crust and upper mantle lithosphere generates separate crustal and mantle shear zones and favors asymmetric oceanic accretion involving development of active detachment faults with 3d features complex ridge geometries e g overlapping ridge segments and curved ridges are generated in the decoupled models two types of detachment faults termed continental and oceanic detachment faults are established in the coupled and decoupled models respectively continental detachment faults are generated through rotation of high angle normal faults during rifting and terminated by magmatism during continental breakup oceanic detachment faults form in oceanic crust in the late rifting early spreading stage and dominates asymmetric oceanic accretion the life cycle of oceanic detachment faults has been revealed in this study

causes and rate limiting mechanisms of ridge propagation a fracture mechanics model all known propagating rifts pr s are growing away from shallow ridge axis seafloor we examine a model for rift propagation in which the ridge axis conduit like a crack in the lithosphere propagates in response to a stress concentration at its tip the stress concentration near the tip which causes progressive failure of the lithosphere is primarily due to excess gravity spreading stresses associated with anomalously shallow ridge axis along the growing ridge this stress concentration near the tip is characterized by a stress intensity factor k which must exceed a threshold value to cause failure of the plate a remotely applied tensile stress would make a positive contribution to k but would act equally on both ridge segments and not favor the propagation of either the pressure in the crack may depending on its distribution make either a positive or negative contribution to k for a high enough propagation rate viscous flow induced suction due to the growth of the ridge axis as well as the reduced vertical flow near the pr tip would make k 0 this simple model thus includes a mechanism to control the speed of propagation and is consistent with the axial morphology of the spreading segment associated with the well studied 95 5 w pr of the galapagos spreading center analytical solutions for the contribution to the stress intensity factor of viscous forces resisting ridge axis growth and causing the pr axial topography are consistent with the observed propagation rate for asthenosphere viscosities of about 1019 pa s 1020 p we also examine a more general model of a ridge transform ridge system on the basis of energy conservation in terms of energetics this more general model reduces to that of a single propagating ridge axis crack when both the work done by forces driving propagation and the viscous dissipation due to forces resisting propagation are generated along the growing ridge axis however the more general model also includes mechanisms that may stop ridge propagation propagation will stop either when the energy needed for transform migration is larger than that available for ridge axis growth or when the strain energy absorbed at the dying ridge axis is comparable to that released at the growing ridge axis i e if the stress intensity factors for both ridge axes are similar both of these possibilities may occur when the propagating rift completely propagates through a ridge segment as at 85 w in the galapagos finally in this model the slight systematic differences in ridge axis orientation of the growing and dying ridges are a consequence rather than a cause of ridge propagation

continental rifting and initial sea floor spreading in the woodlark basin our understanding of the processes by which continents rift and sea floor spreading initiates is derived primarily from studies either of old passive margins and oceanic crust or of young regions of intra continental extension where spreading has not yet started it has been thought that continental rifting ceases when sea floor spreading begins1 2 that oceanic fracture zones develop from transfer or transform faults within continental rifts3 4 and that linear magnetic anomalies correlate with the onset of sea floor spreading during times of magnetic reversals5 6 here we present a marine geophysical survey of one of the few active examples of continental rifting and spreading initiation the western woodlark basin papuan peninsula region of new guinea which shows that in detail these assumptions do not hold the data confirm models of the rifting to spreading transition that invoke both ridge propagation and nucleation of discrete spreading cells7 10 and provide an unambiguous example of a spreading centre reorienting by synchronous jumping rather than propagation

the paleogene record of himalayan erosion bengal basin bangladesh a knowledge of himalayan erosion history is critical to understanding crustal deformation processes and the proposed link between the orogen s erosion and changes in both global climate and ocean geochemistry the most commonly quoted age of india asia collision is 50 ma yet the record of paleogene himalayan erosion is scant either absent or of low age resolution we apply biostratigraphic petrographic geochemical isotopic and seismic techniques to paleogene rocks of the bengal basin bangladesh of previously disputed age and provenance our data show that the first major input of sands into the basin in the 1 km thick deltaic barail formation occurred at 38 ma our biostratigraphic and isotopic mineral ages date the barail formation as spanning late eocene to early miocene and the provenance data are consistent with its derivation from the himalaya but inconsistent with indian cratonic or burman margin sources detrital mineral lag times show that exhumation of the orogen was rapid by 38 ma the identification of sediments shed from the rapidly exhuming southern flanks of the eastern central himalaya at 38 ma provides a well dated accessible sediment record 17 myr older than the previously described 21 ma sediments in the foreland basin in nepal discovery of himalayan detritus in the bengal basin from 38 ma 1 resolves the puzzling discrepancy between the lack of erosional evidence for paleogene crustal thickening that is recorded in the hinterland 2 invalidates those previously proposed evidences of diachronous collision which were based on the tenet that himalayan derived sediments were deposited earlier in the west than the east 3 enables models of himalayan exhumation e g by mid crustal channel flow to be revised to reflect vigorous erosion and rapid exhumation by 38 ma and 4 provides evidence that rapid erosion in the himalaya was coincident with the marked rise in marine 87sr 86sr values since 40 ma whether 38 ma represents the actual initial onset of vigorous erosion from the southern flanks of the east central himalaya or whether older material was deposited elsewhere remains an open question

geochemical and petrological evidence for subduction accretion processes in the archean eastern indian craton the composition of sandstones often provides key evidence about the tectonic weathering and transport processes operating on the surface at the time of deposition petrologic and geochemical analyses of little metamorphosed middle late archean sandstones from the eastern indian craton show that the sedimentary rocks were derived from dominantly 3 3 ga old amphibolites of the older metamorphic group omg and tonalites of the older metamorphic tonalite gneisses omtg the two oldest lithologic units of this craton chondrite normalized rare earth element ree patterns of the sandstones show a light ree enriched signature with la sm n varying from 5 2 to 6 7 with no eu anomaly while the heavy rees display flat patterns with gd lu n values of 0 9 1 6 primitive mantle normalized incompatible and compatible trace element plots of these sandstones demonstrate an overall similarity with global proterozoic archean sandstones including strong nb ta negative anomalies in an fsm nd vs ond 0 diagram the sandstones plot precisely between the regional amphibolites and tonalites we infer from the ree abundances and the fsm nd vs ond 0 plot that the sandstones represent a bimodal mechanical mixture of omg and omtg the low ce pb ratios of these rocks of 1 4 indicate a variably pb enriched archean crust and that the ce pb ratio acquired the continental crustal signature distinctly different from those of the bulk silicate earth and mantle values at least as early as mid archean strong nb ta depletion relative to the primitive mantle suggests the sandstones were derived from subduction related magmatic arc sources this latter suggestion is strongly supported by the low nb ta and high zr sm ratios of these sandstones identical to archean tonalite trondhjemites that require based on recent trace element partitioning results their protoliths to have formed by subduction melting of lowmagnesium amphibolites or metamorphosed hydrous basalts 1 2 the average nd model ages of the sandstones are greater than the sm nd crystallization ages of the omg and omtg at v3 3 ga the geochemical data presented here can be collectively interpreted to suggest the presence of subduction accretion processes operational in the midarchean eastern indian craton because the omg and omtg the source rocks of the sandstones formed in a subduction related arc setting the basement rocks upon which this arc was constructed must have been older this observation and the depleted mantle nd model ages tdm of the sandstones ranging from 3 6 to 4 0 ga strongly indicate the presence of continental crust in this eastern indian craton older than 3 3 ga and possibly as old as

comparison of isotopic and petrographic provenance indicators in sediments from tertiary continental basins of new mexico abstract isotopic variations of nd in sedimentary rocks provide a new tool for provenance determination we have compared the isotopic technique with classic petrographic indicators of provenance in sediments from two tertiary basins of the rio grande rift north central new mexico the siliciclastic sediments in these basins were derived from varying proportions of precambrian basement and oligocene volcanic rocks which are isotopically and petrographically distinct in the analyzed sediments a good correspondence exists between petrographic and isotopic provenance indicators the data also suggest that the isotopic tracer is more sensitive to minor detrital input from a second source than is the framework petrology the similar sm nd ratios for all analyzed sediments and the identical isot pic values of interlayered mudstone and sandstone indicate that no significant fractionation of sm and nd occurs during the formation of the sediments thus the nd model ages of the sediments preserve the crust formation age of the source the sr isotopic and concentration data show trends generally similar to the nd data but they do not indicate the sedimentary provenance as clearly this ambiguity is a consequence of strong rb sr source heterogeneities and rb sr fractionation during the sedimentary cycle

2 8 ga old anorogenic granite acid volcanics association from western margin of the singhbhum orissa craton eastern india abstract the tamperkola granite acid volcanics association occurring at the western margin of the archaean singhbhum orissa iron ore craton soc eastern india is intrusive into the darjing group which represents a sequence of mobile belt metasediments in this part of the soc the darjing group rests unconformably on the bonai granite 3 2 ga old absence of any deformational imprints of the country rock metasediments on the tamperkola granite acid volcanics together with its undeformed and unmetamorphosed nature its alkali feldspar dominant mineralogy and its high sio 2 and na 2 o k 2 o and low mgo and cao contents suggest that this granite acid volcanics association is anorogenic in nature two representative samples one each from the granite and the acid volcanics have been dated by in situ 207 pb 206 pb zircon dating method using a small ion microprobe minimum age of crystallisation of the acid volcanics is found to be 2 8 ga strong peak in the 207 pb 206 pb frequency diagram and equality of the observed and expected errors in radiogenic 207 pb 206 pb ratios suggest that this age probably represents the true age of formation of the volcanics the age data place the deposition and metamorphism of the mobile belt metasediments of the darjing group in between 3 2 and 2 8 ga occurrence of 2 9 2 8 ga old small granitoid plutons alkali feldspar granite to syenogranite in composition is also known from the southern margin of the soc therefore it appears that around 2 9 2 8 ga small alkali granite bodies formed at the marginal part of this cratonic block after its stabilisation at 3 1 ga

generation and evolution of palaeoarchaean continental crust in the central part of the singhbhum craton eastern india abstract palaeoarchaean granitoids are exposed over wide area in the singhbhum craton whose origin and role in crustal evolution are not well constrained this study presents whole rock and mineral chemical data coupled with zircon u pb dating and hf isotope results on such granitoids from the central part of the craton to understand their petrogenesis tectonic setting and role in continental crustal evolution the first phase of granitoid magmatism in this area is represented by a 3 47 ga na rich gneissic tonalite belonging to the archaean ttg tonalite trondhjemite granodiorite suite this rock is characterized by high hree heavy rare earth elements negative eu anomaly low sr y ratio and positive zircon hf isotope signature ehft 2 1 to 4 8 it is interpreted to be formed by shallow melting of a juvenile mafic source at 3 35 ga a silicic ferroan porphyritic biotite granite formed it shows variable k na low y and high sr y moderately fractionated hree and positive zircon ehft 1 8 to 4 0 and is explained as a product of high temperature melting of a heterogeneous juvenile source consisting of tonalites and mafic rocks at lower crustal depth the final phase of granitoid magmatism is marked by a 3 30 ga non porphyritic ferroan silica rich biotite granite geochemical characteristics like moderately high k moderately fractionated hree low ca and sr y and zircon ehft 0 8 to 3 7 suggest that the granite was formed by high pressure melting of a tonalite dominated source with short crustal residence all the three granitoid phases display low mg ni and cr contents and magnesium number mg precluding direct involvement of mantle in their origin rather crustal reworking caused by episodic plume related mafic ultramafic magma underplating and intraplating in an oceanic plateau setting is suggested as the possible mechanism for formation of the granitoids successive crustal reworking events involved progressively greater amount of previously formed felsic crust generating more evolved k rich granitoids it appears to be a globally important process which led to effective crustal differentiation and maturing of the cratons during palaeoarchaean

nd sr isotopic provenance and trace element geochemistry of amazonian foreland basin fluvial sands bolivia and peru implications for ensialic andean orogeny nd and sr isotopes and the trace element contents including the rare earths were determined for fluvial sands of lithic arenite composition from the madre de dios foreland basin of bolivia and peru on standard petrologic ternary diagrams the sands fall in the recycled orogen provenance field and thus are similar to typical ancient foreland basin composition the average rare earth elemental pattern of the sands is identical to the upper continental crustal average as estimated from post archean composite shales of different continents ratios ofth u co th la sc and th sc of the fluvial sands are intermediate between an average magmatic arc and an upper crustal average compositions the dispersion of some trace elemental patterns in the sands can be attributed to fractionation of dense minerals including zircon during the sedimentation process the variations of nd isotopes in conjunction with the petrographic parameters of lithic metamorphic lm and volcanic lv fragments allow a two fold classification of the sands these two sand types can be interpreted in terms of mixing among three different provenances one volcanic rock suite with less negativeend o parameter than the other volcanic suite and a third metasedimentary source withend o value of around 12 which is considered to be similar to the average western brazilian shield composition thus the overall compositions of the sands has been modeled as mechanical mixtures of two components an andean magmatic arc and the brazilian shield derived metasediments the model is strongly supported by a plot ofend o versusesr o of the sands in this plot the type 1 and 2 sands define two coherent hyperbolic trends contiguous with two different portions of the andean magmatic trend this relationship has been interpreted to indicate that the observed andean magmatic trend in anesr o end o diagram is the result of varying degrees of contamination of a primitive arc type magma by the precambrian continental crust of the western brazilian shield the depleted mantle average nd model age of 1 46 ga for the fluvial sands reflects the average age of the brazilian continental crustal source the development of the andean orogenic belt has been discussed schematically with the isotopic data of the sands the model describes a trailing edge prism of sediments derived from the brazilian shield during the late paleozoic early mesozoic the prism becomes part of the fold thrust belt during the andean orogeny in the neogene when the foreland basin develops with the basin fill partly derived from the fold thrust belt the sedimentary rocks in the fold thrust belt are also a major source of contaminants for the andean magmas the contiguous nature of the andean magmatic trend and the fluvial sand data in theesr o end o diagram suggests that the ensialic andean magmatic arc has remained connected to its parent continent the western brazilian shield throughout the development of the andean orogeny

the structural and geochemical evolution of the singhbhum granite batholithic complex india abstract the singhbhum granite batholithic complex of eastern india is composed mainly of a the older metamorphic group tonalite trondhjemite gneiss omtg 3800 ma old whch intruded synkinematically into the enveloping older metamorphic group omg 3800 ma old and b the singhbhum granite sbg consisting of three distinct but closely related phases of at least twelve separate magmatic units of granodiorite granite 2950 ma old and a number of patches of granitised omg orthoamphibolites geochemical studies including ree and lil elements suggest that a the omg orthoamphibolites were derived either by re melting of a k poor basalt lkt or of mantle peridotite with high concentration of lil elements b the omg tonalite was derived by partial melting of amphibolite and c the singhbhum granite magmas appear to have formed in two distinct modes i partial melting of amphibolite for the magmas of phases i and ii and ii partial melting of a siliceous garnet bearing granulite for magmas of the phase iii units

composition and weathering conditions of paleoproterozoic upper crust of bundelkhand craton central india records from geochemistry of clastic sediments of 1 9ga gwalior group abstract the 1 9 ga gwalior group represents one of the major paleoproterozoic depositional basin of bundelkhand craton and is comprised of 1 km thick unmetamorphosed and undeformed sedimentary succession with basal clastic sequence of sandstone shale association par formation and an upper chemogenic bif rich sequence morar formation the geochemical examinations of the basal clastic rocks are the subject of present study the compositional maturity and extreme quartz enrichment in sandstones of gwalior group suggest slow upliftment and high in situ weathering of basement terrains in a stable platformal setting these clastic sediments are characterized by strong depletion of mobile constituents like ca sr and na high plagioclase index of alteration pia values and strong positive correlation between al 2 o 3 and tio 2 indicating severe weathering of upper crust positive correlation between feo t and tio 2 suggests fe retention in paleoweathering profiles and is attributed to oxygenated atmosphere post depositional potash metasomatism is indicated in a cn k systematics of these sediments and k enrichment from near zero to as high as 38 is inferred with most of the shale samples show 30 extraneous k 2 o additions depleted transition element ni cr co sc contents elevated large ion lithophile element lile k rb ba th light rare earth elements lree contents fractionated rare earth element ree patterns strong negative eu anomalies in these sediments point toward k rich intracrustal granitic source component steeper ree patterns avg la yb n 12 57 compared to basement granites and conspicuous depletion of y over other liles in auc normalized spidergram also suggest a heavy rare earth element hree depleted ttg source component minor contribution from tholeiitic mafic rock is evidenced from th sc sc la sc sc th systematics and positive correlation of transition elements with feo t and tio 2 mixing calculations on the ree data suggest that upper continental crust of bundelkhand craton was constituted of 40 granite 40 ttg and 20 mafic rocks this is in contrast with the present day composition of bundelkhand granite gneiss complex bggc that contains 90 high k granite the data suggest unroofing of granite batholiths by means of erosion and upliftment of supracrustal ttg cover

archean greenstone belt magmatism and the continental growth mantle evolution connection constraints from th u nb lree systematics of the 2 7 ga wawa subprovince superior province canada abstract an extensive database including th u nb ree systematics for diverse magmatic and sedimentary lithologies of 2 7 ga wawa greenstone belts provide new constraints on the mechanism of crustal growth in the southern superior province and controls on its composition the greenstone belts are characterized by collages of oceanic plateaus oceanic island arcs and trench turbidites these lithotectonic fragments were tectonically assembled in a large subduction accretion complex following juxtaposition these diverse lithologies were collectively intruded by syn kinematic ttg tonalite trondhjemite granodiorite plutons and ultramafic to felsic dykes and sills with subduction zone geochemical signatures intra oceanic basalts are characterized by near flat ree patterns and nb u and nb th ratios generally greater than primitive mantle values consistent with positive nd values they are associated with komatiites the association being interpreted as an ocean plateau sequence erupted from a mantle plume bimodal arc volcanic sequences trench turbidites and contemporaneous ttg suites are characterized by fractionated ree with nb u and nb th ratios less than primitive mantle values mixing hyperbolae between oceanic plateau and magmatic arc sequences pass through the estimated composition of bulk continental crust suggesting that crustal growth in the late archean was by tectonic sedimentary and chemical mixing of oceanic plateau and arc sequences at convergent plate boundaries mixing calculations suggest that oceanic plateau and subduction zone components in the wawa continental crust are represented by 6 12 and 88 94 respectively high nb u and nb th ratios of plateau tholeiitic basalts are interpreted as a complementary reservoir to arc magmatism low nb u and nb th hundreds of millions of years prior to recycling of oceanic lithosphere through a subduction zone high nb u nb th and its incorporation into a mantle plume from which 2 7 ga plateau tholeiites erupted the variably high nb u ratios of the plateau basalts are consistent with early extraction of large quantities of the protoliths magmatic precursor of continental crust from the southern superior province asthenospheric mantle

quantification of the effects of secondary matrix on the analysis of sandstone composition and a petrographic chemical technique for retrieving original framework grain modes of altered sandstones abstract most studies of sandstone provenance involve modal analysis of framework grains using techniques that exclude the fine grained breakdown products of labile mineral grains and rock fragments usually termed secondary matrix or pseudomatrix however the data presented here demonstrate that when the proportion of pseudomatrix in a sandstone exceeds 10 standard petrographic analysis can lead to incorrect provenance interpretation petrographic schemes for provenance analysis such as qfl and qfr should not therefore be applied to sandstones containing more than 10 secondary matrix pseudomatrix is commonly abundant in sandstones and this is therefore a problem for provenance analysis the difficulty can be alleviated by the use of whole rock chemistry in addition to petrographic analysis combination of chemical and point count data permits the construction of normative compositions that approximate original framework grain compositions provenance analysis is also complicated in many cases by fundamental compositional alteration during weathering and transport many sandstones particularly shallow marine deposits have undergone vigorous reworking which may destroy unstable mineral grains and rock fragments in such cases it may not be possible to retrieve provenance information by either petrographic or chemical means because of this pseudomatrix rich sandstones should be routinely included in chemical petrological provenance analysis because of the many factors both pre and post depositional that operate to increase the compositional maturity of sandstones petrologic studies must include a complete inventory of matrix proportions grain size and sorting parameters and an assessment of depositional setting

crustal structure and inferred rifting processes in the northeast south china sea abstract taiger project deep penetration seismic reflection profiles acquired in the northeastern south china sea scs provide a detailed view of the crustal structure of a very wide rifted continental margin these profiles document a failed rift zone proximal to the shelf a zone of thicker crust 150 km from the shelf and gradually thinning crust toward the cob spanning a total distance of 250 300 km such an expanse of extended continental crust is not unique but it is uncommon for continental margins we use the high quality images from this data set to identify the styles of upper and lower crustal structure and how they have thinned in response to extension and in turn what rheological variations are predicted that allow for protracted crustal extension upper crustal thinning is greatest at the failed rift uc 7 5 but is limited farther seaward uc 1 2 we interpret that the lower crust has discordantly thinned from an original 15 17 km to possibly less than 2 3 km thick beneath the central thick crust zone and more distal areas this extreme lower crustal thinning indicates that it acted as a weak layer allowing decoupling between the upper crust and the mantle lithosphere the observed upper crustal thickness variations and implied rheology lower crustal flow are consistent with large scale boudinage of continental crust during protracted extension

resetting of neodymium isotopes and redistribution of rees during sedimentary processes the early proterozoic chelmsford formation sudbury basin ontario canada abstract the rees abundances and neodymium isotope compositions of sandstones and siltstones from the early proterozoic chelmsford formation reveal that the rees in some samples were redistributed during the early proterozoic by surficial processes and that prior to redistribution the nd isotope system was partially reset the chelmsford formation is a coarse grained turbidite sequence the youngest of three clastic units that fill the sudbury basin in ontario it is constrained to have been derived from nearby late archean superior province rocks by petrography major and trace element chemistry and pb isotopic compositions of whole rocks and feldspar although ree patterns and nd model ages for samples from around the base of the formation reflect their derivation from undisturbed superior province rocks tdm 2800 ma five samples from higher in the stratigraphic section show evidence that up to 60 of their la has been removed and sm nd has increased by up to 60 these samples also have anomalously high nd model ages tdm 3400 to 4400 ma the presence of pronounced positive ce anomalies in the samples with anomalous tdm indicates the influence of surficial processes on the ree composition the data scatter about a vertical array on a plot of f sm nd vs ndt 1800 nd calculated at 1800 ma approximately the depositional age with values of f sm nd between 0 45 and 0 14 mixing of nd from other sources is difficult to reconcile with the petrography major and trace element geochemistry and nd and pb isotopes removal of an lree enriched phase from the sedimentary system provides a viable mechanism to produce the ree and nd isotopic characteristics of the chelmsford formation samples neodymium isotopes require that this phase was partially isotopically equilibrated with the matrix sediment near the time of sedimentation this study shows the dangers of indiscriminate use of nd model ages in provenance studies and demonstrates that the simultaneous examination of sm nd and nd isotopes on f sm nd vs nd or isochron plots is required for the recognition of sedimentary redistribution of the ree

geochemical and sm nd isotopic characteristics of the late archaean palaeoproterozoic dhanjori and chaibasa metasedimentary rocks singhbhum craton e india implications for provenance and contemporary basin tectonics abstract in significant contrast to other cratonic blocks of india the singhbhum cratonic successions record continuous depositional record from the palaeoarchaean to mesoproterozoic although the sedimentary facies characteristics and mode of stratigraphic sequence building of the dhanjori and chaibasa formations are well known sedimentary geochemistry provenance and tectonic milieu of deposition of these two formations are hitherto unknown the current manuscript presents geochemical and sm nd isotopic data from the dhanjori and chaibasa formations for the first time and combine previous sedimentological data with the goal to expand the framework for understanding the depositional and tectonic setting of these two formations the sm nd isotopic data for the chaibasa clastics is unambiguous with respect to provenance average nd t 2 2 ga 0 8 1 0 and average nd model age tdm 2 51 0 08 ga with average 147 sm 144 nd ratios 0 1114 0 0041 for phyllites and quartzites indicate an extremely homogeneous source signature consistent with a late archaean juvenile crustal provenance possibly a dominantly upper crustal provenance the sm nd isotopic data from the older dhanjori formation also indicate broadly similar provenance as comparable lithologies in the younger chaibasa formation our sm nd isotopic data is entirely consistent with the previous sedimentological data and confirms a terrestrial rift dominated tectonic setting for the dhanjori formation proximal sources poorly mixed provenance and a marginal marine to offshore setting for the more homogeneous nd isotopic signature of the chaibasa formation distal sources well mixed provenance

eastern indian 3800 million year old crust and early mantle differentiation samarium neodymium data for nine granitic and tonalite gneisses occurring as remnants within the singhbhum granite batholith in eastern india define an isochron of age 3775 89 x 10 6 years with an initial 143 nd 144 nd ratio of 0 50798 0 00007 this age contrasts with the rubidium strontium age of 3200 x 10 6 years for the same suite of rocks on the basis of the new samarium neodynium data field data and petrologic data a scheme of evolution is proposed for the archean crust in eastern india the isotopic data provide evidence that parts of the earth s mantle were already differentiated with respect to the chondritic samarium neodymium ratio 3800 x l0 6 years ago