geochemistry and u pb detrital zircon dating of paleozoic graywackes in east junggar nw china insights into subduction accretion processes in the southern central asian orogenic belt abstract the southern central asian orogenic belt caob is characterized by multiple and linear accretionary orogenic collages including paleozoic arcs ophiolites and accretionay wedges a complex history of subduction accretion processes makes it difficult to distinguish the origin of these various terranes and reconstruct the tectonic evolution of the southern caob in order to provide constraints on the accretionary history we analyzed major and trace element compositions of paleozoic graywackes from the huangcaopo group hg and kubusu group kg in east junggar the hg graywackes have relatively low chemical index of alteration cia values 50 to 66 suggesting a source that underwent relatively weak chemical weathering the identical average index of compositional variability icv values 1 1 for both the kg and hg samples point to an immature source for the paleozoic graywackes in east junggar which is consistent with an andesitic felsic igneous source characterized by low la th ratios and relatively high hf contents these graywackes are geochemically similar to continental island arc sediments and therefore were probably deposited at an active continental margin u pb dating of detrital zircons from the lower subgroup of the hg yielded a young age peak at 440 ma indicating a post early silurian depositional age however the youngest populations of detrital zircons from the kg graywackes and the upper subgroup of the hg yielded 206 pb 238 u ages of 346 ma and 355 ma respectively which suggest a post early carboniferous depositional age because of similarities of rock assemblages these two units should be incorporated into the early carboniferous nanmingshui formation the detrital zircon age spectrum of the early paleozoic hg graywackes resembles that of the habahe sediments in the chinese altai which suggests that the ocean between east junggar and the chinese altai was closed before the deposition of the sediments and that the armantai ophiolite was emplaced prior to the early devonian the differences in age spectra for detrital zircons from the post early carboniferous graywackes in east junggar and the harlik arc indicate that the emplacement of the kalamaili ophiolite postdates the early carboniferous therefore a long lasting northward subduction accretion process is suggested for the formation of east junggar and the reconstruction of the early paleozoic evolution of the southern caob

geochemistry of loess deposits in northeastern china constraint on provenance and implication for disappearance of the large songliao palaeolake the harbin loess typical of the loess in ne china is located in the easternmost margin of the eurasian loess belt and was recently investigated from the perspective of an asian aeolian dust system before this study the geochemical composition and provenance of the harbin loess remained poorly understood more importantly the study of the harbin loess also provides a unique opportunity to understand the geomorphological evolution process of the large songliao palaeolake in the northeast plain this study presents the results on the geochemical characteristics major elements trace elements rare earth elements and sr nd isotope of the harbin loess there are markedly consistent geochemical compositions in and between the loess and palaeosol sediments revealing stable dust sources and identical sources for the loess and palaeosol various indicators illustrating weathering and recycling indicate that the harbin loess palaeosol sediments were derived from sources with a low degree of weathering and from material with first cycle alluvial lacustrine deposits combinations of various provenance tracing indicators indicate that the harbin loess has a strong geochemical affinity with the songnen sandy land and to a certain extent the horqin sandy land but not with the hulun buir sandy land which indicates that these loess deposits are derived from a mixed provenance with the dominant source being the neighbouring songnen sandy land and a second distal source being the horqin sandy land finally we advocate that the harbin loess accumulation could serve as the direct record of the songliao palaeolake disappearance supplementary material data for the ne sandy lands of china and full provenance discrimination diagrams are available at https doi org 10 6084 m9 figshare c 3835009

geochemistry of lower vindhyan clastic sedimentary rocks of northwestern indian shield implications for composition and weathering history of proterozoic continental crust abstract the western margin of the 1600 ma vindhyan basin of the indian shield is marked by the presence of lower vindhyan group lvg comprising shales and sandstone with minor limestone conglomerate and mafic volcanic flows at the base major and trace element compositions of sandstones and shales of the lvg have been investigated to infer the nature of provenance weathering processes and tectonic setting provenance analysis using trace elements and petrography suggests that the archaean banded gneissic complex bgc of rajasthan is most likely the source of these clastic rocks the shale and sandstone comprising lower part of lvg namely khardeola formation are distinctive in having high lile such as th rb and k 2 o high ferromagnesian trace elements and also higher values of k 2 o na 2 o rb sr and la yb n ratios in comparison to succeeding sedimentary units comprising upper formations of the lvg the significantly different geochemical characteristics of khardeola formation suggest change in provenance with sediments in the lower formation being derived from proximal source and those of upper formations from distal source based on ree patterns and eu eu values the clastic sediments of khardeola formation can be modeled by a mixture of 60 berarch granite and 40 mafic volcanic rocks of hindoli belt on the other hand the upper formations are best modeled with a mixture of 60 bgc granitic gneisses 20 bgc mafic enclaves and 20 berach granite cia chemical index of alteration values and a cn k plot suggest that the source area experienced moderate to high degree of chemical weathering under warm and humid conditions the shift of sediment source from local in the beginning to distal during later period suggests sedimentation in a tectonically active basin

comments on the eu and th geochemistry of the archaean continental crust of the kaapvaal craton south africa chemical data from sedimentary and igneous rocks of the kaapvaal craton have been used to trace the occurrence of highly fractionated k2o rich granites with pronounced negative eu anomalies back to the early archaean from the sediments of the fig tree group about 3 4 ga old and granite pebbles in the overlying moodies group it was estimated that the igneous and metamorphic part of the continental crust at that time already contained up to about 35 of such granites their formation requires the presence of a thick continental crust to allow intracrustal partial melting with plagioclase as a residual phase and the intrusion of the resulting granites at high levels within the crust save for some small remnants these eu depleted archaean granites have since been removed by erosion shales of the 3 0 ga old pongola supergroup reflect only small source areas and contain considerable amounts of material derived from residual soils the eu anomalies of the pongola shales suggest up to 30 eu depleted granites in the source area the shales of the witwatersrand supergroup about 2 6 ga old with eu anomalies as low as 0 45 reflect a source area already having typical post archaean crustal compositions the th concentrations in south african shales support this conclusion and typical post archaean concentrations of above 10 ppm are found already in the early archaean sheba shales uranium and thorium mineralization has also occurred on the kaapvaal craton since the early archaean the south african late proterozoic and phanerozoic sedimentary record of eu anomalies and th concentrations confirms the trend observed on other cratonic areas and supports the concept of worldwide uniformity in crustal composition since the end of the archaean on the kaapvaal craton crustal growth and consolidation began 500 600 ma earlier than in other parts of the world the kaapvaal craton thus appears to represent a cold spot on the face of the earth

the composition and evolution of the continental crust rare earth element evidence from sedimentary rocks the composition of the present day upper crust inferred from the uniformity of sedimentary rock r e e rare earth element patterns is close to that of granodiorite a revised andesite model is used to obtain total crustal composition the lower crust is the composition remaining assuming that the upper crust one third of the total is derived from intracrustal partial melting the upper crustal r e e pattern has pronounced eu depletion eu eu 0 64 the lower crustal pattern has eu enrichment eu eu 1 17 and the total crust has no eu anomaly relative to chondritic abundances the eu depletion in the upper crust is attributed to retention of eu in plagioclase in the lower crust because plagioclase is not stable below 40 km 10 kbar the anomaly is intracrustal in origin the archaean upper crust has a different r e e pattern to that of the present day upper crust being lower in total r e e and la yb ratios and lacking an eu anomaly these data are used to infer the archaean upper crustal composition which resembles that of the present day total crust except that ni and cr contents are higher the archaean crustal composition can be modelled by a mixture of tholeiites and tonalite trondhjemites the latter have steep light r e e enriched heavy r e e depleted patterns consistent with equilibration with garnet and hence probable mantle derivation there is little reason to suppose that the archaean lower crust was different in composition from the upper crust except locally where partial melting episodes occurred the r e e evidence is consistent with isotopic and geological evidence for a low continental growth rate in the early archaean a massive increase to about 70 of the total crust between about 3000 and 2500 ma b p and a slow increase until the present day the change from archaean to post archaean r e e patterns in the upper crust is not isochronous but is reflected in the sedimentary rock r e e patterns at differing times in different continents on the basis of a model composition for the mantle 36 of the potassium 30 of uranium 15 of lanthanum and 3 of ytterbium are concentrated in the present continental crust this enrichment is related to ionic size and valency differences from common mantle cations e g mg fe pre 3 9 ga b p crusts were obliterated by meteorite bombardment no geochemical evidence exists for primordial anorthositic sialic or mafic crusts

potassium distribution and metasomatism in pelites and schists how and when relation to postdepositional events a large global database for over 9000 archean to holocene pelites in numerous basins has been developed to evaluate diagenetically unaltered and altered compositions the mineralogy and geochemistry of quaternary muds and ancient mudstones provide first order controls on diagenetic potassium metasomatism in pelites clay mineralogy of quaternary muds is dominated by smectite and kaolinite with minor detrital mica and illite thus k2o is 4 k al is 0 25 and k rb is 220 in most quaternary muds with diagenetic alteration of clays to illite smectite and illite k2o content increases to 4 8 and k al rises to 0 2 0 5 or more in extreme potassium metasomatism authigenic k feldspar develops sources of potassium are internal to shale sandstone units wherein detrital k feldspar is dissolved during diagenesis providing k for clay illitization in closed system isochemical alteration as in some gulf coast and north sea basin cases potassium in amounts greater than that available from closed system alteration is provided from extraformational circulating brines in open system diagenesis k rb with diagenesis varies according to whether the metasomatizing fluid was relatively poor in rb seawater derived brines or richer in rb as in many hydrothermal waters diagenetic fluids may originate from seawater or meteoric water enriched in cations by water rock interactions along their subsurface migration paths and or by dissolution of evaporites potassium metasomatizing fluids penetrate low permeability pelites through faults fractures and microfractures and by diffusion in the mineral matrix typically k metasomatism possibly with pb zn cu ba and f mineralization occurs long after sediment deposition and burial at temperatures of 50 to 150 c related to tectonic activity and fluid movement different modes of metasomatism are recognized and correlated with geological setting in the passive margin gulf coast tertiary basin open system k metasomatism where present is limited to areas adjacent to steeply dipping growth faults which act as conduits for episodic pulses of altering fluids migrated upward from deeper basin sources in contrast widespread pervasive open system k metasomatism produced by continuous long term gravity advective and diffusive flow of brine from the late paleozoic appalachian ouachita wichita mountains highland to altered earlier paleozoic shales plus sandstones bentonites and carbonates in the illinois and anadarko ardmore foreland basins this mode of k alteration following major mountain building events is characteristic of many precambrian and paleozoic collision orogens and associated foreland basins another mode is in rift basins with no external drainage in which indigenous waters are evaporated to brines these brines cause albitization of k feldspar to yield potassium for illitization of detrital clays

geochemical composition of the last deglacial lacustrine sediments in east tibet and implications for provenance weathering and earthquake events abstract identifying and understanding the respective influence of climate and tectonic processes becomes a frontier topic in tectonically active mountain ranges in the eastern tibetan plateau earthquake occurred frequently and usually led to formation of many dammed lakes in this study a last deglacial section of lacustrine sediments at xinmocun in eastern tibet is selected for conventional x ray fluorescence xrf and scanning xrf sxrf elemental analysis the high correlation of major and trace element abundances between the xinmocun lacustrine samples and the loess soil samples from the chinese loess plateau clp supports the previous view of the eolian origin of the xinmocun lacustrine sediments analysis of major element abundances ratios and trace element ratios indicates that the dust provenance of the xinmocun lacustrine sediments is different from that of the clp and is similar to that of the loess at ganzi and hongyuan nearby the study area the significant variations of most geochemical elements and their close coupling with grain size variations cannot be reasonably explained by the changes in transport dynamics of eolian dust and is possibly caused by the intermittent changes in available dust provenance frequent earthquakes triggered abundant landslides and provided large amounts of dust for the study area accordingly in the 16 m fraction of the xinmocun grain size record many element abundances and their ratios such as sio2 al2o3 tio2 al2o3 cao al2o3 sr al2o3 rb sr and na2o al2o3 can be regarded as sensitive indicators of earthquake events in the tectonically active regions sxrf measurements at the u channel surface can provide some cost effective indicators of seismic events such as si al ti al ca al sr al zr rb and rb sr significant variations in most element abundances and their ratios of the xinmocun lacustrine sediments correlate closely with those of its grain size record they all show no increasing or decreasing trends with time these suggest that tectonic activities characterized by seismic events possibly had a major role on the landscape erosion in the eastern tibetan plateau while the climatic influence seems minor

early eocene carbonaceous shales of tadkeshwar formation cambay basin gujarat india geochemical implications petrogenesis and tectonics abstract we report the early eocene cambay sedimentation petrogenesis and tectonics of a rift graben basin of western india using geochemistry the carbonaceous shales of tadkeshwar formation from tadkeshawar and rajpardi lignite mines of cambay basin were studied using major trace and rare earth elements the study reveals that the protolith of these shales is likely to be mafic to ultramafic in nature based on the high abundance of fe2o3 avg of 8 68 mgo avg of 5 78 and tio2 avg of 8 80 with a lower sio2 content avg of 44 74 when compared to paas values paas normalized ree distribution patterns of carbonaceous shales indicate relatively depleted lree with enriched hree coupled with small magnitude of eu eu strongly supports the protolith of these shales to be mafic to ultramafic weathering indices confirm these shales were derived from a source subjected to intense weathering low na2o and high perseverance of al2o3 are proxies for smectite to illite transformations of shales in a gaining temperature regime across the basin we speculate that the high geothermal gradients prevailed during 60 65 ma along the western margin of indian plate might be responsible for the intense weathering of the marine mafic protolith redox sensitive trace element ratios and negative ce anomalies suggest that the shales were deposited under suboxic to oxic environment the link between early eocene marine transgression followed by basin subsidence is inferred for the deposition of these carbonaceous shales in suboxic to oxic environments the immobile trace element ratios and geochemical discrimination diagrams k2o na2o vs si2o la th sc reveals the protoliths of the tadkeshwar formation were derived from a transitional domain of intra ocean island arc to active continental margin followed by convergence leading to the deposition of immature shales

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

geochemistry and isotope sr nd o study of al mg granulites from the in ouzzal archaean block hoggar algeria the in ouzzal al mg granulites are found within sedimentary units deposited after 2 7 ga the whole association being metamorphosed under extreme temperature conditions c 1000 c at 2 ga the al mg granulites are interlayered with other metasediments including metapelites quartzites and magnetite bearing quartzites forsterite spinel marbles and a few meta igneous rocks mainly pyroxenites they do not occur at a specific position in the sedimentary suite and they do not reflect any particular structural control the major and trace element compositions of al mg granulites especially the high cr ni co contents show that their peculiar refractory chemistry is more compatible with premetamorphic sedimentary characteristics rather than with metasomatic metamorphic or partial melting processes sedimentary admixtures of a common mature detrital component coming from the weathering of the local acidic igneous crustal protoliths normal pelitic component with an extremely immature component derived from reworking of basic ultrabasic lithologies al mg cr co ni rich chloritic component is consistent with the geochemistry of such rocks as in other instances the quartz garnet oxygen isotopic thermometer here records an apparent temperature close to the peak metamorphism c 1000 c although the persistence of pre existing 18o variations on a small scale during the metamorphism does not support a major pervasive fluid flow during metamorphism it does not rule out the presence of syn to post metamorphic co2 the low 18o c 5 to 6 of the most typical al mg granulites indicate that the chloritic component in these rocks was derived from hydrothermally altered mafic ultramafic protoliths rather than dominantly from palaeosols it is suggested that the presence of such al mg cr co ni rich sediments is indirect evidence for the presence of greenstone belts in the local crust of the in ouzzal at 2 6 2 7 ga

archean upper crust transition from mafic to felsic marks the onset of plate tectonics the archean eon witnessed the production of early continental crust the emergence of life and fundamental changes to the atmosphere the nature of the first continental crust which was the interface between the surface and deep earth has been obscured by the weathering erosion and tectonism that followed its formation we used ni co and cr zn ratios in archean terrigenous sedimentary rocks and archean igneous metaigneous rocks to track the bulk mgo composition of the archean upper continental crust this crust evolved from a highly mafic bulk composition before 3 0 billion years ago to a felsic bulk composition by 2 5 billion years ago this compositional change was attended by a fivefold increase in the mass of the upper continental crust due to addition of granitic rocks suggesting the onset of global plate tectonics at 3 0 billion years ago

geochemical evidence of windblown origin of the late cenozoic lacustrine sediments in beijing and implications for weathering and climate change abstract the origin of lacustrine sediments in the middle latitudes of asia remains controversial in this study we conducted major and trace element analyses on 86 lacustrine samples from the x5 core in beijing to determine their origin and provenance the results indicate that their abundances all varied in relatively narrow ranges similar to those of the samples in the chinese loess plateau clp a high correlation of major and trace element abundances exists between our lacustrine samples and the clp samples ucc normalized major and trace element abundances also have a similar pattern between the lacustrine and the clp samples with the exception of a few elements with high solubility such as mg in the ternary diagrams of a cn k la th sc and zr 10 th sc and the correlation maps of major trace elements and their ratios four groups of the x5 lacustrine samples with different sedimentation stages 3 16 2 7 ma 2 7 1 8 ma 1 8 0 9 ma and 0 9 0 ma overlap well with each other and with the clp samples indicating that the x5 lacustrine samples are likely of windblown origin and possibly shared common unchanged dust source regions with the clp since the late pliocene this interpretation is supported by the overlap of the lacustrine samples from beijing with the clp samples in the correlation maps of ms vs sorting ms vs skewness ms vs kurtosis and kurtosis vs skewness analysis of the frequency and cumulative frequency curves indicates that the x5 lacustrine sediments were dominantly transported by the winter monsoon and the westerly circulation given more scattered distributions of different elemental ratios and grain size parameters of the x5 core lacustrine sediments compared to those of the clp samples local dust particles must have contributed occasionally which deserves further investigation in the future several chemical ratios such as rb sr na 2 o al 2 o 3 cao al 2 o 3 and chemical index of alteration cia were calculated for the lacustrine samples in beijing comparison of these ratios variations with those of the sea level of the south china sea and the global benthic 18 o record indicates that na 2 o al 2 o 3 and cia can be regarded as sensitive indicators of chemical weathering of lacustrine dust sediments in the mid latitudes of northern hemisphere

geochemical evolution of peraluminous paleoproterozoic bandal orthogneiss nw himalaya himachal pradesh india implications for the ancient crustal growth in the himalaya abstract paleoproterozoic 1900 100 ma granites along a 2000 km linear belt in the lesser himalaya are significant in terms of their tectonic setting and emplacement as they form the basement for younger sedimentary cover rocks the geochemistry and petrogenesis of one granitic body bandal orthogneiss 1904 70 ma from the lesser himalayan belt has been studied and correlated with the other plutons in the belt the paleoproterozoic bandal orthogneiss covering 500 sq km surface area occupies the core of the kulu rampur window himachal pradesh nw lesser himalaya the orthogneiss is s type peraluminous and can be subdivided into two suites a muscovite biotite granite suite i coarse grained porphyritic gneiss cpg and fine grained gneiss fg and a tourmaline muscovite granite suite ii leucogranite lg the two mica granites suite i form the bulk of the bandal orthogneiss whereas the tourmaline bearing granite which cuts across suite i occurs in the central part of the main pluton comparison of their bulk composition with melt experimental data suggest formation of lg from a water undersaturated magma of minimum melt composition at pressures around 3 4 kbar whereas the tourmaline free granites may have been derived by melting due to biotite dehydration at pressures 5 kbar high concentrations of mgo feo ba sr rb sr and ti of the two mica granites reflect higher extents of melting derived from biotite dehydration melting of a metasedimentary source tourmaline leucogranites with higher rb sr and low ba sr ratios represent low fraction melts extracted from their protoliths under conditions of low water activity the striking similarities in terms of the geological setting geochemical characteristics and the age data between bandal orthogneisses and the adjoining wangtu gneisses suggest that they were formed under similar tectono magmatic environment during paleoproterozic period the wide spread occurrence of paleoproterozoic gneisses in the lesser himalayan belt strongly suggest that they represent the granitic basement on which the younger sedimentary rocks were deposited the geochemical studies on the overlying sedimentary rocks chail and jutogh formations in the lesser himalaya suggest that the huge volumes of granitic magmatism played a significant role in changing the crustal composition of the himalayan rocks from mafic to felsic during the post archean

weathering and sediment generation in the archean an integrated study of the evolution of siliciclastic sedimentary rocks of the 3 2ga moodies group barberton greenstone belt south africa abstract alluvial and braided fluvial deposits of the 3 2 ga moodies group in the barberton greenstone belt south africa are analyzed petrographically and geochemically to identify their source rocks provenance and assess the degree and type s of weathering required to produce the observed moodies sediment compositions conglomerate sandstone and shale data give somewhat differing pictures of provenance and weathering due to derivation from different components within the source terrane and size and compositional fractionation during transport the results suggest that shale geochemistry provides the most accurate estimate provenance although both rare earth element and trace element th sc zr cr and ti data must be used in combination to give the best results the source area for moodies group sediments was dominated by tonalite felsic volcanic rock komatiite basalt and granite based on mineralogical and major element divergence from estimated source area composition the moodies group sediments are remnants of an aggressive weathering environment labile materials such as komatiite basalt and coarse plagioclase grains decomposed almost entirely to clays and solutes and the chemical index of alteration for moodies shale is well above the global average an aggressive weathering environment in the archean may have been achieved by increased rainfall higher temperatures and or higher atmospheric p c o 2 more likely a combination of these conditions worked to offset the inhibitory weathering effects of a plant free environment

geochemistry and provenance of the altendorf feldspathic sandstone in the middle bunter of the thuringian basin germany an integrated mineralogical and geochemical study of the altendorf feldspathic sandstone in the middle bunter of the thuringian basin germany was carried out to obtain more information on its provenance sedimentological history and geochemistry the fine to coarse grained sandstone has an average composition of 66 63 71 quartz 30 27 35 feldspar and a fine grained illitic matrix the heavy mineral assemblage consists mainly of tourmaline zircon rutile apatite and opaque fe ti minerals it is inferred that this terrigenous material was laid down at the margin of the bunter basin high abundances of igneous quartz grains determined by cathodoluminescence cl the lack of metamorphic heavy minerals as well as alkali feldspar with an average k2o na2o ratio of 14 9 point to material of mostly granitic origin regional geologic and paleogeographic observations suggest that granites of the older and younger intrusive complex of the western erzgebirge are possible source rocks of the feldspathic sandstone the geochemical signature of the alkali feldspar k2o na2o ratio bao content points to the bergen and kirchberg granites as possible parent rocks comparing the mineralogical and geochemical data of the presumed source granites with those of the altendorf sandstone provides information on the sedimentation processes weathering fluvial transport and sedimentation under arid climatic conditions have caused complete decomposition and removal of plagioclase and biotite lowered the bulk ree content and led to the development of a positive eu anomaly the immobile trace elements e g ta th zr sc co ti are best suited for provenance determination because of their relatively low mobility during sedimentary processes elemental ratios such as la sc th sc co th and th ta have inherited the signature of the parent rocks and thus may be particularly useful for provenance studies to evaluate the control of specific minerals and grain size fractions on the distribution of trace elements in the sandstone separated fractions of quartz feldspar the 20 m fraction and heavy minerals were additionally analyzed the concentration densities elemental concentration in the fraction fraction weight reveal that with exception of hf the influence of the heavy mineral fraction on the whole rock geochemistry is insignificant although this fraction generally plays the most important role as carrier of trace elements quartz feldspar and the 20 m fraction have similar concentration densities between 20 and 40 for the ree with exception of eu which is mainly hosted in feldspar strong positive eu anomaly the influence of the 20 m fraction on the distribution of the ree and the elements u th sc ta strongly depends on the abundance of this fraction in the sediment alkali feldspar contains most of the ba sr rb and cs in the samples concentration densities of up to 97 thus differences in the abundances of these elements in whole rock samples can be related to changes in the mineralogical composition quartz feldspar ratio of the sediment

geochemical investigation of dry and wet deposited dust during the same dust storm event in harbin china constraint on provenance and implications for formation of aeolian loess abstract a strong dust storm event occurred in harbin china on may 11 2011 the dry and wet deposited dust depositions in this dust storm event together with the surface sediments from the potential sources were collected to study grain size distributions carbonate content and carbon isotopic composition of carbonate major element trace element and rare earth elements ree and sr nd isotopic compositions the results indicate as follows the dry deposited dusts are characterized by bimodal grain size distributions with a fine mode at 3 6 m and a coarse mode at 28 m whereas the wet deposited dusts are indicative of unimodal grain size modes with a fine mode at 6 m the dust storm depositions are influenced to a certain extent by sedimentary sorting and are of a derivation from the recycled sediments based on identifying the immobility of element pairs before constraining sources of dust storm deposits using geochemical elements in conjunction with ree and especially sr nd isotopic compositions the primary and strengthening sources for the dust storm event were detected respectively the hunsandake sandy land as the primary source and the horqin sandy land as the strengthening source were together responsible for the derivation of dust depositions during dust storm event the hunsandake sandy land however contributes less dust to the dust storm event in harbin compared to the horqin sandy land and the hulun buir sandy land is undoubtedly excluded from being one of the sources for dust storm depositions in harbin there are not notable differences in geochemical especially sr nd isotopic compositions between dry and wet deposited dusts indicating that the wet deposited dust is of identical derivation to the dry deposited dust based on our observations it is of interest to suggest that fine and coarse particles in the clp chinese loess plateau loess possibly have the same sources

ree geochemistry of recent clastic sediments from the kaveri floodplains southern india implication to source area weathering and sedimentary processes abstract the kaveri river drains through archean gneissic and charnockitic terranes in south india the rare earth element ree chemistry of the floodplain sediments along with their major and trace element composition in different size fractions are discussed we observe a strong coherence between ree and tio2 contents in the sediments which is suggestive of strong control by titanite or its weathered products on the ree chemistry ree concentrations also show a general increase with decreasing size of bulk sediments different size fractions and of light and heavy mineral fractions this suggests the presence of ree in the fe oxy hydroxide surface coatings on clastic grains formed by the weathering of hornblende and titanite in the granodioritic to dioritic protoliths we also interpret the data as showing that ree patterns of clastic sediments notably eu anomaly are affected by the degree of source rock weathering as well as by the fluvial processes of sorting and mineral differentiation therefore for provenance study sediments having all granulometric grades such as the floodplain sediments are preferable because they most closely represent the source particularly when the source region suffered little chemical weathering in the present case the sediments were derived from the high standing hills of archean charnockites because of their recent uplift and physical denudation

geochemistry of the floodplain sediments of the kaveri river southern india abstract the floodplain sediments of the kaveri river southern india derived from archean gneissic and charnockitic source regions show interbedding of silty 4 4 7 and sandy units 1 4 3 7 the geochemistry of silty beds is remarkably uniform at a given location and over a lateral distance of nearly 250 km the sandy beds have more variable chemical compositions yet are comparable to those of silty beds except for the diluting effect of quartz silty sediments retain the geochemical signature of prominently exposed source rocks for almost all elements and provide evidence of tectonic instability in the source region the floodplain sediments contain all size grades sand silt and clay which may have resulted in minimizing the biases imposed on suspended and bedload sediments due to sorting the low chemical index of alteration cia the dominance of unweathered primary minerals and the minor proportion of smectitic clay all suggest that the region has been subjected to little chemical weathering this is possible if the region has undergone recent uplift exposing fresh archean rock to surface denudation the formation of fertile farmland along the kaveri river course and its delta is related to these recent geological processes

sedimentology of archean greenstone belts signatures of tectonic evolution abstract stratigraphic styles in archean greenstone belts are compared to those of modern and phanerozoic depositional basins in order to test the conformity of tectonic style through time six lithological associations in greenstone belts are recognized 1 mafic ultramafic volcanic 2 calc alkaline volcanic 3 bimodal volcanic 4 quartz arenite iron formation or carbonate 5 conglomerate wacke and 6 conglomerate arenite examples of the associations are described from the kaapvaal superior and zimbabwe provinces and the pilbara block each association differs only slightly between the four regions thereby emphasizing a common tectonostratigraphic theme sedimentary rocks are a minor component of the mafic ultramafic volcanic association they were deposited in two types of volcanic basin environments one similar to barred lagoons and bays around oceanic volcanic islands and the other similar to sediment starved platforms adjacent to coalesced volcanoes in inter arc intra arc and back are basins in contrast sedimentary rocks in the calc alkaline volcanic association comprise thick wedges of epiclastic and volcaniclastic detritus deposited in elongate sedimentary basins and thick wedges of pyroclastic and volcaniclastic detritus deposited adjacent to volcanic centers sedimentary facies were identical to those of forearc trench and marine volcanoplutonic arc settings chemogenic lithofacies in both volcanic associations were deposited in shallow to deep marine waters although shallow water profiles were selectively preserved in some terrains sedimentary rocks of the bimodal volcanic association are thick wedges of siliciclastic and volcaniclastic deposits interbedded with subaerial to submarine erupted flows of komatiite basalt and rhyolite depositional environments ranged from braid plain and braid delta to coastal marine and submarine fan starved basin deposits include banded iron formation and sulfidic black shale the association is identical to that of cratonic extensional basins in arc continent and intracontinental rifts sedimentary rocks of the quartz arenite iron formation or carbonate association onlap weathered basement or bimodal volcanic association rocks depositional environments were either transitional from fluvial to siliciclastic and carbonate marginal marine to offshore pelagic hemipelagic or from inshore intrabasinal clastic and volcaniclastic to offshore pelagic hemipelagic siliciclastic deposits are supermature two tectonic sites are represented those with siliciclastic facies represent continent adjacent syn to post rift stable shelves and those dominated by banded iron formation with volcaniclastic deposits represent arc adjacent post rift stable shelves sedimentary rocks of the conglomerate wacke association are represented by thick wedges of siliciclastic or volcaniclastic sandstone with minor volcanic components these were deposited in braid plain braid delta and submarine fan environments within elongate sedimentary basins coastal plain and pelagic basin facies are also present siliciclastic detritus was derived from syndepositional magmatic and metasedimentary thrust belts two stratigraphic styles are preserved one that records a flysch like stage of deep to marginal marine environments and the other that records a molasse like stage of marginal marine to fluvial environments the association is identical to that of compressional foreland basins of arc continent collisional and compressional arc tectonic affinities the conglomerate arenite association is preserved in elongate structural basins that transgress crustal components and all other supracrustal units it is dominated by thick wedges of siliciclastic sandstone although bimodal volcanic and volcaniclastic rocks can also be present depositional environments ranged from talus and alluvial fan and braid plain to either lacustrine or coastal marine local basin margins often show lateral offsets from source terrains and vertically skewed facies patterns longitudinal facies geometries are typical the association is similar to that of strike slip collisional graben in hinterland tectonic escape and terrane accretion orogens the examples show that despite all the ramifications of secular geophysical geothermal and geochemical global change the stratigraphic style of sedimentary rocks in archean greenstone belts can be matched with modern tectonic analogues thereby emphasizing the conformity of stratigraphic style through time the inseparable link between stratigraphic and tectonic styles implies that the tectonic style of greenstone belts was not temporally unique major differences exist with regards to the chemical composition of certain magmatic and sedimentary products but these are compensated for by their constant stratigraphic function in terms of depositional processes depositional environments and tectonic sites

geochemistry of pelitic and psammopelitic precambrian metasediments from southwestern sri lanka implications for two contrasting source terrains and tectonic settings most of the pelitic and psammopelitic metasediments of southwestern sri lanka are compositionally transitional between shales and greywackes and are of two isotopically distinct types metasediments of the northern part have nd model ages of 1 5 ga and belong to the relatively younger nd model age province of wanni complex wc model ages of two southerly located samples are 2 8 ga being compatible with those of the highland complex hc in accordance with mostly mesoproterozoic to late archaean crustal residence ages cr ni la th and ree data also suggest that the sources of these metasediments are plagioclase fractionated post archaean upper crustal material crustal differentiation of source terrains corresponding to the hc and the wc probably occurred at distinctly different time intervals significant differences exist between the major and trace element chemistry of the metapelites from the wc north and the hc south suggesting derivation from two different source terrains metasediments of the south are slightlymmore siliceous low in sc and v and highly enriched in lree 220 ppm their sources have been chemically evolved felsic igneous rocks and or recycled sediments relatively high v sc feo mgo tio2 and slightly low sio2 in the rocks of the wc indicate contribution of more basic sources cia chemical index of alteration estimates and comparative alkali alkaline earth element patterns suggest relatively fresh source material the o isotope data indicate that the pre metamorphic isotope signatures are preserved and the clay component of the original sediments mainly consisted of illite typifying continental weathering conditions sediments of the south hc were probably deposited in a passive continental margin continental rift environment while those of the northern part wc were probably formed at an andean type active continental margin the two units were tectonically juxtaposed prior to pan african granulite metamorphism

geochemistry of fine grained clastic sediments in the archean abitibi greenstone belt canada implications for provenance and tectonic setting abstract archean sandstone and shale samples from turbiditic tf and alluvial facies af in the southern volcanic zone of the abitibi greenstone belt canada have been analyzed for major and trace elements the sandstones from tf are mainly greywackes whereas those from af are lithic arenite sio 2 ranges from 51 to 68 geochemically the clastic sediments cluster into mafic element mep and low mafic element lmep populations the mep sediments which include turbiditic facies sediment samples from the larder lake group in the kirkland lake area are characterized by elevated abundances of mgo 5 ni 200 ppm cr 400 ppm sc 22 ppm and feo 7 5 abundances lower contents of incompatible elements such as th u ta hf zr and total rees n ratios are 4 5 with eu eu 1 the lmep sediments which include samples from the beatty dome and three nations lake formations in timmins the kewagama group noranda area and timiskaming group in the kirkland lake area however show reverse abundance distributions the geochemical gaps shown by the major and trace elements define two populations of sediments suggesting contrasting provenances for them the mep sediments require a dominantly mafic and ultramafic 90 source with 10 felsic contribution whereas the lmep sediments require a mixture of felsic 75 and mafic 25 rocks this compositional distribution of the sediments is closely related to their tectonic environments the mep was formed in basins proximal to or within an active oceanic rift consequently having ultramafic and mafic rocks as the major source for the lmep calc alkaline volcanic sequences formed in an island arc like setting were the major source these two contrasting volcanic plutonic terranes were tectonically juxtaposed during late archean collisional tectonics

geochemical evolution of archean shales from south africa i the swaziland and pongola supergroups abstract the fig tree group and moodies group part of the swaziland supergroup are exposed in the barberton mountain land and are 3 3 3 5 ga old shales from the fig tree group are characterized by variable rare earth element ree patterns with lan ybn 5 5 1 8 95 confidence and eu eu 0 97 0 18 shales from the moodies group have more fractionated ree patterns lan ybn 8 7 2 0 and less variable eu anomalies eu eu 0 92 0 02 both groups are characterized by very high abundances of cr and ni modelling of those trace elements which are relatively unfractionated during sedimentary processes ree th sc is consistent with the derivation of these sedimentary rocks from a mixture of the archean bimodal igneous suite mafic volcanics tonalites trondhjemites felsic volcanics with the moodies group requiring a larger percentage of the felsic end member the data for shales from the swaziland supergroup support models suggesting that the archean exposed crust was of more mafic composition than the post archean upper crust and that intracrustal melting was relatively unimportant in differentiating the archean crust the pongola supergroup is younger than the swaziland supergroup but older than 2 9 ga shales and mudstones from the nsuze and mozaan groups show ree characteristics intermediate between the older shales of the swaziland supergroup and typical post archean shales one quartzite sample from the witwatersrand supergroup ca 2 6 ga has a ree pattern virtually parallel to average post archean shales but with much lower abundances the change from typical archean to post archean sedimentary trace element patterns has been documented in early proterozoic successions from canada and australia this change has been related to a short interval of intense growth and chemical differentiation of the continental crust during the late archean 2 7 2 5 ga in southern africa this change appears to have taken place much earlier 3 2 2 9 ga but can be related to similar processes thus the period of crustal evolution related to the archean proterozoic boundary while virtually episodic regionally occurred over a protracted period world wide during the period ca 3 2 2 5 ga

geochemistry of asara shale member of karaj formation central alborz iran provenance source weathering and tectonic setting abstract the asara shale member of the karaj formation middle to upper eocene in age includes mixed volcaniclastic siliciclastic and carbonate strata in the alborz mountains northern iran these strata have been geochemically analyzed to constrain their source area provenance most of the major and trace element contents of asara shale samples are generally similar to the upper continental crust ucc values and their chemical index of alteration cia values 31 7 50 9 suggesting a semi arid climate in a weathering limited source area moreover correlation of major element oxides suggests the controlling effect of plagioclase and clay minerals on these data when xrd analysis and sem images indicate clastic sourced illite and chlorite in the samples probably due to the weathering of volcanic ash and glass in an aqueous transport and depositional environment however the chondrite normalized ree plot of mudrocks of the asara shale show a similarity with the ucc composition mineral composition and geochemistry of the asara shale mudrocks reveal the rapid reworking of penecontemporaneous volcaniclastic deposits which had covered the surrounding areas instead of their epiclastic genesis related to erosion and transport major trace and rare earth element ree data imply a felsic composition of the volcanic source related to an active continental margin continental island arc and most probably back arc tectonic setting for the depositional basin inferring a mid upper eocene extensional volcanism in a subduction complex

geochemistry of the archean yellowknife supergroup the archean yellowknife supergroup slave structural province canada is composed of a thick sequence of supracrustal rocks which differs from most archean greenstone belts in that it contains a large proportion 80 of sedimentary rocks felsic volcanics of the banting formation are characterized by hree depletion without eu anomalies indicating an origin by small degrees of partial melting of a mafic source with minor garnet in the residua granitic rocks include synkinematic granites hree depleted low 87sr86sr i post kinematic granites negative eu anomalies high 87sr86sr i and granitic gneisses with ree patterns similar to the post kinematic granites sedimentary rocks turbidites of the burwash and walsh formations have similar chemical compositions and were derived from 20 mafic intermediate volcanics 55 felsic volcanics and 25 granitic rocks jackson lake formation lithic wackes can be divided into two groups with group a derived from 50 mafic intermediate volcanics and 50 felsic volcanics and group b characterized by hree depletion derived almost exclusively from felsic volcanics ree patterns of yellowknife sedimentary rocks are similar to other archean sedimentary ree patterns although they have higher lanybn these patterns differ significantly from typical post archean sedimentary ree patterns supporting the idea that archean exposed crust had a different composition than the present day exposed crust

on the geochemical evolution of sedimentary rocks the average major element compositions of clastic sedimentary rock assemblages i e carbonates evaporites and iron formations not considered for various times in earth history archean early proterozoic late proterozoic phanerozoic are reported based largely on the compilations of f l schwab uncertainties in the averages are considerable there is little evidence of secular change in the average composition of sedimentary rocks during post archean times such data do not support the idea of a gradual change in composition of the upper continental crust from mafic to felsic composition trace element patterns in sedimentary rocks particularly ree and th distributions support the idea of constancy of upper crustal composition during the post archean the average composition of archean clastic sedimentary rocks differs significantly from the post archean average being enriched in na mg and ca and depleted in si and k such differences are consistent with the archean upper crust having a more mafic composition than post archean upper crust the trace element data further support this idea it is noted that the average composition of post archean clastic sedimentary rocks predicts to within a factor of 1 2 the composition of the exposed continental crust for the elements si al fe k mg and ti the abundances of ca and na are depleted in the sedimentary rocks due to the fact that the compositions of carbonates evaporites and seawater were not considered the average composition of archean clastic sedimentary rocks does not compare favourably with an independent estimate of archean upper crust composition being enriched in si k and depleted in ca fe mg and ti such a comparison indicates that the archean upper crust may have had a more felsic major element composition than previously thought

comparative sedimentology of the principal volcanic sequences of archean greenstone belts in south africa western australia and canada implications for crustal evolution abstract significantly different assemblages of sedimentary rocks are present in the volcanic sequences of archean greenstone belts in the barberton mountain land pilbara block and canadian shield thin sedimentary layers in the 3 55 3 3 ga old onverwacht and warrawoona groups in the barberton mountain land and eastern pilbara respectively are made up of silicified volcaniclastic sediments banded cherts carbonaceous cherts silicified evaporites and locally stromatolites deposited under shallow water anorogenic conditions the volcanic sequences in these belts essentially lack terrigenous debris derived by weathering and erosion algoma type iron formation volcanogenic massive sulfide deposits hyaloclastic breccias and evidence for the existence during their accumulation of older sialic basement sedimentary rocks in the volcanic sequences of the 2 8 2 6 ga old superior and slave provinces canada and perhaps in other late archean belts in the rhodesian craton and yilgarn block include generally unsilicified volcaniclastic units mafic hyaloclastic breccias terrigenous clastic layers abundant algoma type iron formation and massive sulfide deposits sedimentation took place under relatively deep water conditions local tectonic activity was widespread and older sialic basement was present these younger volcanic sequences contain few shallow water deposits such as carbonaceous cherts evaporites and stromatolites the volcanic sequences of both generations of greenstone belts seem to have formed platforms that stood topographically above the surrounding sea floor differences between older and younger greenstone belt volcanic sequences appear to reflect the increasing abundance and influence of sialic crust from early to late archean time and an accompanying increase in ocean water depth over the volcanic platforms evolution of the sialic crust can be traced through at least five archean and early proterozoic stages 1 1 an early period of ensimatic shallow water greenstone belt formation 3 55 3 3 ga ago 2 2 an early episode of intrusive and metamorphic activity from ca 3 3 2 9 ga ago during which some of the older greenstone belts were cratonized 3 3 a late period of greenstone belt formation 2 8 2 6 ga ago during which volcanism occurred under relatively deep water conditions upon and or adjacent to areas of older sialic basement 4 4 a late archean interval of continental crust formation during which the younger greenstone belts were cratonized 2 7 2 5 ga ago 5 5 a latest archean and early proterozoic period dominated by stable cratons and relatively minor greenstone belt activity

the cr th ratio in precambrian pelites from the kaapvaal craton as an index of craton evolution the cr th ratio in precambrian pelites from the kaapvaal craton in southern africa correlates with pelite th sc and co th ratios and appears to monitor changes in source composition it is a more sensitive indicator of provenance than other commonly used ratios including eu eu la yb and th u pelite cr th ratios are controlled most strongly by the effects of local provenance and local tectonics secular decreases in this ratio the most prominent of which occurred at 2 4 2 3 ga in pelites of the kaapvaal craton record rapid uplift exposing granite decreases in the ratio reflect increases in the abundance of komatiite basalt in sediment sources in response to arc collisions rift hotspot volcanism or intracratonic uplift of older greenstones relatively constant cr th ratios appear to require widespread uplift or and efficient sediment mixing during tectonic stability changes in cr th ratio of kaapvaal pelites reflect a complex tectonic history of the craton from about 3 3 ga to 1 8 ga and do not favor single stage cratonization in the late archean

using element geochemistry and detrital zircon geochronology to constrain provenance of the eocene sediments in the deep water area of the pearl river mouth basin south china sea abstract as the largest sag in the zhu ii depression within the pearl river mouth basin the baiyun sag bys lies in an area of interaction among the eurasian plate pacific plate and indian plate the bys mainly experienced the paleogene rift stage and neogene quaternary post rift stage thick syn rift sediments in the bys have recorded the nature of provenance and provide an excellent record of regional tectonic evolution prior to the opening of the south china sea in this study elemental geochemical analysis combined with published zircon detrital u pb dating results are applied to better understand the provenance change and tectonic setting of the bys during the eocene in this respect the chondrite normalized ree patterns of eocene samples from six wells are highly similar to upper continental crust ucc and post archean australian shale paas particularly with respect to lree enrichment significantly negative eu anomalies and slight ce anomalies geochemical results confirm that the bys eocene sediments were derived from a continental island arc to an active continental margin in addition the rock types of sources and the relatively narrow detrital zircon ages show that mesozoic intermediate felsic igneous and metamorphic rocks derived from local topographic highs contributed to infilling most of the bys furthermore the results show that the panyu low uplift began to accept large sediments transported by the paleo pearl river from the south china coast during the active syn rift stage prior to the opening of the south china sea

desilication in archean weathering processes traced by silicon isotopes and ge si ratios abstract the influence of continental weathering on oceanic seawater compositions in the precambrian is poorly understood here we combine si isotopes and ge si ratios of a mesoarchean paleosol 2 95 ga and overlying shales as proxies for weathering processes and si mass transfer at the early earth s surface results show that as with modern soils neoformation of secondary clay minerals in the paleosol was associated with the fractionation of si isotopes and ge si ratios in response to chemical weathering and soil desilication furthermore the loss of fe bearing minerals most likely fe bearing smectites produced additional controls on si and ge mobility through the coupled dissolution and re precipitation of clay minerals under reductive conditions opposite fractionation behaviors are observed neoformed secondary clay minerals acted as a 28 si and ge sink whereas the leaching of fe bearing minerals released 28 si and ge into soil solutions an attempt to estimate 30 si and ge si signatures of released dissolved elements into soil solution provided 30 si and ge si signatures of about 0 1 and 1 4 mol mol respectively furthermore shales deposited shortly after paleosol formation display 30 si and ge si compositions that may be explained as mixtures of the recognized paleosol components the shale record suggests that weathering induced desilication and leaching of fe bearing minerals to a lesser extent might have been widely effective during the mesoarchean for the transfer of si from the continents to the hydrosphere comparison of si isotopes recorded in detrital sediments and chemical precipitates over time might thus provide a useful supplementary tool to decipher the impact of putative increases in continental weathering and desilication relative to changes in hydrothermal and continent derived solute inputs to ocean and ocean cooling

geochemistry of archean shales from the witwatersrand supergroup south africa source area weathering and provenance with a few exceptions shales from the archean witwatersrand supergroup 2800 ma in south africa are depleted in na ca lile ree and hfse compared to phanerozoic shales cr co and ni are enriched in all witwatersrand shales and fe and mg are high in shales from the west rand groups wrg and lower central rand group crg shales from the crg and uppermost wrg are enriched in na al lile ree hfse and transition metals relative to shales from the lower wrg chondrite normalized ree patterns for all witwatersrand shales are enriched in light ree and exhibit small to moderate negative eu anomalies a positive correlation of ree and al2o3 contents in the shales suggests that ree are contained principally in clay minerals relative to shales from the crg shales from the wrg exhibit depletions of na ca and sr a feature probably reflecting intense chemical weathering of their source rocks cia indices in witwatersrand shales are variable chiefly 70 98 even within the same shale unit such variations reflect chiefly variable climatic zones or rates of tectonic uplift in source areas with perhaps some contribution from provenance and element remobilization during metamorphism compared to present day upper continental crust all but the orange grove roodepoort and k8 shales appear to have been derived from continental sources depleted in lile ree and hfse and enriched in transition metals computer mixing models based on six relatively immobile elements th hf yb la sc co and four source rocks indicate that the relative proportions of granite basalt and komatiite increased with time in sediment source areas at the expense of tonalite the contributions of basalt and komatiite appear to reach a maximum during deposition of the booysens shale and granite during deposition of the k8 shales and possibly during deposition of the orange grove shales

the archean proterozoic transition evidence from the geochemistry of metasedimentary rocks of guyana and montana abstract chemical and isotopic compositions of ancient sedimentary rocks have in some cases provided evidence of first order changes yin the composition and extent of continental crust from the archean to the proterozoic we reversed the typical sampling pattern comparing metapelites and carbonates from proterozoic greenstone belts with those from an archean continental margin sequence and found the reverse of the typical age composition relations metapelites and volcaniclastic metagreywackes from the early proterozoic greenstone belts of northern guyana resemble those of many archean greenstone belts in ree trace and major element compositions mean ratios la sm 5 0 la th 5 1 eu eu 0 80 th u 3 4 median k 2 o na 2 o 0 8 schists from an archean continental marginal sequence in montana have compositions like many post archean metapelites derived from continental sources mean ratios la sm 6 8 la th 2 7 eu eu 0 67 th u 5 2 median k 2 o na 2 o 1 7 other archean continental basin sediments also resemble their post archean counterparts implying that upper continental crusts of these eras may not have differed significantly in these respects however temporal change may have occurred in upper crustal contents of ni and cr many archean clastic metasedimentary rocks of both continental and greenstone belt provenance have contents of these compatible elements higher than in their post archean equivalents dolomitic limestone from a greenstone belt of northern guyana has a very low 87 sr 86 sr ratio of 0 7007 such low values may reflect the local influence of mantle derived volcanic rocks during carbonate deposition diagenesis or metamorphism and they may not be reliable indicators of the evolution of sr in seawater archean dolomitic marbles from the continental marginal sequence of montana have 87 sr 86 sr ratios of 0 7065 and higher significantly more radiogenic than many accepted values for carbonates of archean greenstone belts

the negative eu anomaly in archean sedimentary rocks implications for decomposition age and importance of their granitic sources abstract a negative correlation between eu anomaly eu eu and chemical index of weathering ciw is found for archean pelites in both greenstone belts and high grade terrains from southern africa western greenland north america western australia and china a similar negative correlation betweeneu eu and chemical index of alteration cia is also found for worldwide young deep sea sediments from various tectonic settings the increase in the indices of weathering and alteration coincide with both shifts in mineral composition and mineral decomposition from plagioclase k feldspar to mica kaolinite and from amphibole to chlorite smectite juvenile crustal materials from local sources are only slightly decomposed due to little or no transport they are characterized by low ciw cia and negligible eu anomalies in contrast crustal materials from large provenance areas have usually suffered long transport and recycling which results in decomposition of high temperature minerals and large ciw cia values that are negatively correlated with significant negative eu anomalies thus the observed trends are interpreted as mainly reflecting increasing contributions of recycled and decomposed materials from large provenances relative to juvenile crustal additions through arc magmatism at local sources in this context there is no obvious distinction between archean greenstone and high grade metasediments and young deep sea sediments the large provenance areas have contained a significant proportion of granitic and rhyolitic materials which in part have not resisted decomposition and erosion due to their occurrence in shallow crustal layers granites received their distinctive negative eu anomaly from equilibration with a feldspar rich residuum the negative eu anomaly of archean metaclastic sediments apparently increased with decreasing age trends of minimum and averageeu eu values of metaclastic sediments evolving with time are calculated for the archean and lead us to infer that the negative eu anormaly eu eu

ree geochemistry of modern eolian dust deposits in harbin city heilongjiang province china implications for provenance abstract there has long been disagreement about the provenance of sandy dust deposits in harbin city heilongjiang province china the present paper contributes to a better understanding of the source of sandy dust deposits in harbin by the application of ree geochemistry the ree composition of samples from sand dust deposit events in harbin was compared with that of three different grain size fractions

old model nd ages in namibian pan african rocks the nd isotope analytical technique is a powerful tool for studying many geological processes particularly the evolution of the upper mantle as seen through the isotope and trace element geochemistry of mantle derived volcanic rocks1 3 and magmatic processes along destructive plate margins4 5 however previous studies of continental crust6 8 have been largely restricted to archaean examples and concerned primarily with dating their time of derivation from the upper mantle in the present study we investigated the nd and sr isotope characteristics of rocks involved in a relatively young 800 450 myr orogenic event such events represent critical stages in the evolution of most of the earth s continental crust new material is added from the mantle and pre existing crust is remobilized by erosion and sedimentation deformation and magmatic activity nd and sr isotopes were used to outline the range in age and geochemical characteristics of rock sequences and provinces within both the upper mantle and the pre existing crust which were sampled during the orogeny and hence provided the major components of what is now a stable segment of continental crust

geochemistry of archean metasedimentary rocks from west greenland abstract archean metasedimentary rocks occur as components of the isua supracrustals akilia association and malene supracrustals of southern west greenland primary structures in these rocks have been destroyed by metamorphism and deformation their chemistry and mineralogy is consistent with a sedimentary origin but other possible parents e g acid volcanics altered pyroclastic rocks cannot be excluded for some of them there is little difference in the composition of metasedimentary rocks from the early archean isua supracrustals and probable correlative akilia association both have a wide range in rare earth element ree patterns with la n yb n ranging from 0 61 5 8 the ree pattern of one akilia sample with low la n yb n compares favourably with that of associated tholeiites and it is likely that such samples were derived almost exclusively from basaltic sources other samples with very steep ree patterns are similar to felsic volcanic boulders found in a conglomeratic unit in the isua supracrustals samples with intermediate ree patterns are best explained by mixing of basaltic and felsic end members metasedimentary rocks from the malene supracrustals can be divided into low silica 55 sio2 and high silica 77 sio2 varieties these rocks also show much variation in la n yb n 0 46 14 0 and their origin is explained by derivation from a mixture of mafic volcanics and felsic igneous rocks the wide range in trace element characteristics of these metasedimentary rocks argues for inefficient mixing of the various source lithologies during sedimentation accordingly these data do not rigorously test models of early archean crustal composition and evolution the systematic variability in trace element geochemistry provides evidence for the bimodal nature of the early archean crust

sea ice conditions in the adelie depression antarctica during besetment of the icebreaker rv xuelong during the 30th chinese antarctic expedition in 2013 14 the chinese icebreaker rv xuelong answered a rescue call from the russian rv akademik shokalskiy while assisting the repatriation of personnel from the russian vessel to the australian rv aurora australis rv xuelong itself became entrapped within the compacted ice in the adelie depression region analysis of modis and sar imagery provides a detailed description of the regional sea ice conditions which led to the 6day long besetment of rv xuelong the remotely sensed imagery revealed four stages of sea ice characteristics during the entrapment the gathering compaction dispersion and calving stages four factors characterizing the local sea ice conditions during late december 2013 and early january 2014 were identified surface component of the coastal current near surface wind ocean tides and surface air temperature this study demonstrates that shipping activity in ice invested waters should be underpinned by general knowledge of the ice situation in addition during such activity high spatio temporal resolution remotely sensed data should be acquired regularly to monitor local and regional sea ice changes with a view to avoiding the besetment of vessels

factors controlling the trace element distribution in fine grained rocks the albian kaolinite rich deposits of the oliete basin ne spain abstract this paper combines mineralogical data x ray diffraction petrographic and scanning electron microscopy techniques and geochemical analyses in order to investigate the composition of a set of samples claystones siltstones and sandstones which belong to the albian deposits of the oliete basin in spain the aims of the study are to characterize the distribution of major and trace elements in kaolinite rich rocks to infer the role played by accessory phases and clay minerals on the trace element distribution and to assess the source area composition of these materials mineralogical analyses show that these rocks are mainly composed of different proportions of quartz and clay minerals kaolinite and illite with minor feldspar micron accessory minerals such as monazite zircon pyrite marcasite fe oxides and ti oxides have also been detected the sandstone framework implies that the source area of these sediments was probably a granitoid source which underwent low rank metamorphism the major element distribution reflects the mineralogy of the rocks plots in the a cn k diagram as well as their cia values indicate that these sediments were generated from an intensely weathered source area this produced the development of relatively cation poor clays such as kaolinite and claystones with large rare earth elements ree contents the fact that transition element contents in claystones are greater than in siltstones and sandstones suggests that they are preferentially linked to clay minerals probably by surficial sorption in comparison with other lithologies the high abundances of high field strength elements in siltstones imply that accessory minerals such as zircon and nb bearing phases are mainly concentrated in siltstones it should be noted that siltstones are also the lithology which displays the highest ree abundances the statistical analysis of the geochemical data suggests that ree are mainly controlled by ti and nb bearing phases such as titanite and nb and ti oxides although clay minerals are not the main phases in the ree control they probably also have some influence as the analyses show in addition other accessory phases such as monazite secondary phosphates and zircon also contribute to the ree contents although much less than amorphous fe oxides in some samples according to the ree patterns and elemental ratios such as la sc th sc th co etc the source area of the studied outcrops was probably felsic and with intermediate composition between granite and granodiorite

stratigraphic record across a retroarc basin inversion rocas verdes magallanes basin patagonian andes chile the mesozoic evolution of the andean cordillera of southern patagonia is recorded in two formations that are now part of the fold thrust belt the zapata formation and the punta barrosa formation extension in the ultima esperanza region began in the late jurassic with deposition of the marine volcaniclastic tobifera formation and eventually resulted in the full fledged oceanic rocas verdes basin the zapata formation was deposited over a time span of 50 m y in an irregular basin ultimately bordered by an early cretaceous arc to the west it is characterized by interbedded shale and siltstone mostly deposited in shallow water and in areas of oceanic crust emplacement also as deep water hemipelagic deposits sediments of the zapata formation were derived initially from local oceanic upwarps and tobifera highs and later from the ande sitic cover of the juvenile arc and or exhumed oceanic crust the paucity of sandstone in the zapata formation in the ultima esperanza region indicates a highly irregular basin partitioned by prominent horsts with sand deposition confined to sub basins closer to the arc changes in depositional regimes and sediment dispersal patterns related to the onset of andean contraction and formation of the magallanes foreland basin are recorded by sediments of the overlying punta barrosa formation this formation records the evolution of a fold thrust belt on the basis of the multimodal mineralogical and geochemical character of its sandstone and shale the presence of an arc is indicated but nearby andean metamorphic terranes are more significantly represented in punta barrosa sediments stratigraphic details across the zapata and punta barrosa formations indicate that deformation and development of a proto andean cordillera in southern south america was initiated in the late mesozoic and involved conspicuous crustal shortening

petrography and geochemistry of granitoid and metamorphite pebbles from the early archaean moodies group barberton mountainland south africa pebbles of potassic granitoids and metamorphites constitute up to 5 of the basal conglomerate of the moodies group in a ratio of 2 1 the granitoid pebbles frequently show micrographic quartz feldspar intergrowth whereas the metamorphites of a modal composition similar to that of the granitoids are characterized by large quartz grains which could represent original quartz phenocrysts in felsic volcanic precursors the granitoids show high k2o sr k2o na2o and k rb small enrichment of light ree large negative eu anomalies and slightly depleted and fractionated heavy ree compared to the granitoids the metamorphites show higher fe2o3 tio2 and cr concentrations greater enrichment of light ree and also large negative eu anomalies there is little similarity between the moodies pebbles and the majority of the rocks of the ancient gneiss complex of swaziland agc there is only some similarity of the ree distribution patterns between the pebbles and the mkhondo metamorphic suite possibly an areally restricted phase of the agc the geochemical data and especially the large negative eu anomalies suggest that the moodies pebbles were derived from granites which represent residual magmas from which much plagioclase had been removed the granites crystallized at depths of 7 km from magmas with low h2o pressures in a rather thick sialic crust it appears possible that the pre moodies granitoids originated through partial melting of low al2o3 siliceous gneisses of the agc a chronologic connection of the formation of the granitoids with the late onverwacht group volcanicity is possible

mineralogical and chemical changes of soil and stream sediment formed by intense weathering of the danburg granite georgia u s a abstract a series of soil and stream sediments developed during intense weathering on the metaluminous danburg granite northeastern georgia u s a have been analyzed mineralogically and chemically the concentrations of ba na rb and cs in the silt and coarser fractions are controlled mainly by feldspars and biotite hf is controlled by zircon and the ree rare earth elements and th are largely controlled by sphene variations in feldspar sphene and zircon may produce small variations in eu sm and la lu ratios ferromagnesian minerals control ta fe co sc and cr concentrations the mineralogical and chemical composition of the danburg granite is more closely reflected in the silt than in the sand or gravel fractions of stream sediments in the silt the contents of rb ree th ta fe co and sc and the ratios of la sc th sc la co th co eu sm and la lu are similar to those in the unweathered granite in contrast these element contents or ratios in the sands and gravels are 0 05 3 the concentration in the unweathered granite ta and ba contents are an exception to the above the ta and ba contents of the sands and gravels are similar to those of the granite in the kaolinite halloysite clays the content of na is depleted relative to the source rb cs ba hf and ta are depleted or enriched in the clays relative to the source while the ree th fe co sc and cr are enriched the eu sm eu anomaly size and la lu ratios and the ree patterns of the clays are similar to those of the source thus the mineralogy and element contents of a siltstone developed from metaluminous granitic sources during intense weathering would be expected to be more similar to the source rock than the sandstones and conglomerates claystones should contain similar ree patterns and eu sm ratios as the source rock but such fine grained sediments might represent much larger areas of source rocks than the more locally derived sandstones or conglomerates

precambrian superplumes and supercontinents a record in black shales carbon isotopes and paleoclimates prominent maxima in black shale abundance and in black shale total shale ratio occur at 2 0 1 7 ga with less prominent peaks in the late neoproterozoic 800 600 ma and in the late archean 2 7 2 5 ga peaks in chemical index of alteration cia of shales at the same times suggest corresponding warm paleoclimates the peaks in cia and black shale abundance are correlated in time at a 94 confidence level the black shale and cia peaks may reflect the combined effects of mantle superplume events and supercontinent formation at 2 7 and 1 9 ga mantle superplume events may have introduced large amounts of co2 into the atmosphere ocean system increasing depositional rates of carbon and increasing global warming increased black shale deposition may reflect some combination of 1 increased oceanic hydrothermal fluxes introducing nutrients 2 anoxia on continental shelves and 3 disrupted ocean currents the apparent absence of carbon isotope anomalies at these times reflects an increase in the deposition and burial rate of both reduced and oxidized carbon peaks in black shale abundance at 2 1 ga and 800 600 ma correlate with peaks in 13 c in marine carbonates increases in atmospheric oxygen and with high cia values in shales these are all consistent with higher rates of organic carbon burial in black shales at these times these peaks may record the breakup of supercontinents at 2 2 2 0 ga and again at 800 600 ma which resulted in increased numbers of partially closed marine basins disruption of ocean currents and increased hydrothermal vents at ocean ridges all of which led to widespread anoxia 2001 elsevier science b v all rights reserved

sedimentation patterns in the barberton mountain land south africa and the pilbara block australia evidence for archean rifted continental margins abstract archean supracrustal sequences in the barberton mountain land south africa and the pilbara block australia consist of lower volcanic and upper dominantly terrigenous clastic intervals as evidenced by the paleoenvironments of intercalated sedimentary horizons volcanism occurred mainly in shallow waters the overlying ca 3 3 ga sedimentary intervals contain various common as well as unique paleoenvironments the understanding of which places significant constraints on archean crustal models lateral and vertical associations of inferred paleoenvironments are used to interpret the geotectonic history of the archean depositories the early sedimentary history of the greenstone belts is characterized by terrestrial and subaqueous graben fill associations of facies related to the initial rift stage of basin development continued rifting and initial spreading produced submarine grabens within which ironformations accumulated in response to waning volcanism source area uplift resulted in progradation of submarine fans across the basinal chemical sediments the turbidites are gradational directly into braided alluvial sediments in part of fan delta origin suggesting that the continental to marine transition occurred along a narrow continental shelf in the barberton mountain land the steep rift margin was succeeded by the development of a stable continental shelf or shelf rise margin through progradation of the turbidite wedge possibly in association with a eustatic rise in sea level related to continued spreading on this shelf extensive tidal deltaic and barrier beach sediments accumulated sedimentation was terminated by closure of the passive margin oceans the late archean pongola supergroup in south africa is considered to be the late orogenic molasse response to this closure and represents the completion of the wilson cycle

chemical variations in clay minerals of the archaean barberton greenstone belt south africa abstract four rock profiles of shales carbonates and greywacke type sediments were sampled from the mafic compositional influenced sedimentary fig tree group and from overlying more felsic related sedimentary moodies group of the archaean barberton greenstone belt in south africa clay separates of forty sedimentary rocks were examined by xrd and sem to study the mineralogical chemical and morphological variations of the constitutive illite and chlorite the clay minerals showed a significant k enrichment in the illites and mg and fe depletions in the chlorites upwards the stratigraphic sequence three independent parameters influenced these variations 1 a change in the origin of the source rocks 2 the host rock lithologies and 3 a tectono thermal recrystallization process

samarium neodymium elemental and isotopic systematics in sedimentary rocks abstract trace element sm nd th sc and nd isotopic data for sedimentary rocks are examined in order to evaluate 1 1 possible secular trends in the composition of the sedimentary mass and upper continental crust 2 2 the role of tectonic setting in controlling the chemical and nd isotopic composition of the sedimentary mass when plotted against either sedimentation age or average provenance age determined mainly from nd model ages sm nd in sedimentary rocks decreases abruptly from about 0 21 to 0 19 at the archean proterozoic transition consistent with a change towards a more felsic upper crustal composition the change is only somewhat less pronounced for turbidite sediments than for those of continental affinity a trend toward increasing sm nd approaching or exceeding archean values is observed for phanerozoic turbidites this latter change is interpreted as indicating undifferentiated young crust as a component in the sediment which has been lost from the older post archean record due to sedimentary and igneous intracrustal recycling processes when plotted against sedimentation age th sc ratios exhibit a trend directly comparable to that of sm nd in this case th sc increases abruptly at the archean proterozoic boundary from about 0 5 to 1 0 and then decreases to about 0 7 during the phanerozoic nd isotopic composition of modern sediments is affected by tectonic association with those deposited at active margin settings tending to have higher nd reflecting younger provenance and higher and more scattered sm nd reflecting less homogenized and differentiated provenance it is likely that these distinctions can be traced back for about 1 2 ga but with the current data base are not readily resolved in older sedimentary rocks the degree to which the preserved sedimentary mass is open to new additions from the mantle either through replacement during sedimentary recycling or sediment mass growth is highly variable but averages about 15 25 during the phanerozoic

sedimentary dynamics and climatic implications of cretaceous loess like red beds in the lanzhou basin northwest china abstract the chinese loess has been widely studied in order to reconstruct cenozoic environmental change cretaceous loess like red beds with alternating red and gray layers in the lanzhou basin northwest china are well preserved and can be used for understanding the mesozoic environmental and climatic changes however the red beds and gray clays are less studied and their significance is not clear hence cretaceous red beds of the basin are studied the gray layers show horizontal lamination and are deposited in a lacustrine environment whereas the red layers show gradational change in texture and color and are analogous to paleosols the red and gray clays show similar immobile trace elemental compositions and identical grain size distributions suggesting that the sediments of the two deposits are derived from the same source and carried by identical transporting forces the dominant 2 10 m fine grain size component indicates that the deposits are sorted through long distance transportation in this context the magnetic properties of the red beds are mainly determined by post depositional environmental development hematite is the dominant magnetic mineral and some of the gray layers contains small quantities of magnetite therefore the red and gray layers develop on land and under water respectively the magnetic parameter irm 100mt eliminates the effects of ferromagnetic minerals and is sensitive to hard magnetic minerals thus it can be used as a proxy for paleoclimatic change its high values in the red layers correspond to a drier paleoclimatic environment

matched field processing in shallow water for range depth and bearing determination results of experiment and simulation recent theoretical studies have investigated high resolution methods for estimating source position in a shallow water waveguide in this paper an eigenvector method and a geophysical model based on several types of seismic data are used to estimate the bearing depth and range of a moving submerged source in the presence of an interfering surface source at the same range and bearing the receiver was a sparse horizontal array of four sensors on the bottom depth 82 m a variant of the eigenvector method known as the music method was employed for measured and test field matching and a normal mode model was employed to calculate the test field the data analyzed were obtained by towing a submerged source directly below the stern of the cfav endeavour so that the surface and submerged sources were essentially at the same range and bearing source bearing estimates for the data showed that source bearings were usually well defined although somewhat ambiguous the presence of the deep source was indicated

geochemistry and provenance of sediments from the pongola supergroup south africa evidence for a 3 0 ga old continental craton abstract the 3 0 ga pongola supergroup of south africa is one of the oldest well preserved supracrustal successions the sequence is divided into the nsuze chiefly volcanics quartzites and pelites and the overlying mozaan groups chiefly quartzites and pelites relative to north american shale composite nasc most pongola pelites have similar la yb and eu eu ratios and large ion lithophile elements lile high field strength elements hfse ree v and sc contents ni 30 300 ppm and cr 100 800 ppm contents are greater than those of nasc while cr ni ratios are generally higher than shales and pelites of all ages these trends may reflect residual cr enrichment during intense chemical weathering of komatiitic source rocks local populations of nsuze pelites based on major and trace element data may result from source heterogeneities and or contemporaneous volcanic input into the sedimentary basin pongola quartzite compositions and detrital modes are similar to those of sandstones from phanerozoic passive margin and basement uplift tectonic settings ree lile and hfse contents in pelites suggest that pongola source rocks were remarkably similar in composition to average phanerozoic upper continental crust trace element geographic and isotopic constraints favor 3 2 to 3 0 ga hood granites as a major source for pongola detritus moderate contributions from mafic sources are also required whereas relatively minor contributions from tonalite and komatiite are allowed these results indicate the existence of a chemically evolved felsic craton in southern africa by 3 0 ga this craton appears to have been more evolved than sources for either older moodies 3 3 ga or most of the younger witwatersrand sediments 2 8 2 7 ga pelites from the kaapvaal craton record at least two mafic to felsic source area compositional cycles in pelites deposited from the mid to late archean additional cycles may be masked by a depositional or erosional hiatus between the moodies and pongola sequences these sedimentary patterns suggest that crustal development in southern africa was episodic in nature during the early precambrian

geochemistry of mansar lake sediments jammu india implication for source area weathering provenance and tectonic setting abstract the sediment geochemistry including ree of surface and core samples from mansar lake along with mineralogical investigations have been carried out in order to understand the provenance source area weathering hydrolic sorting and tectonic setting of the basin the geochemical signatures preserved in these sediments have been exploited as proxies in order to delineate these different parameters the major element log values fe 2 o 3 k 2 o vs sio 2 al 2 o 3 and na 2 o k 2 o vs sio 2 al 2 o 3 demarcate a lithology remarkably similar to that exposed in the catchment area the chondrite normalized ree patterns of lake samples are similar to post archaean australian shale paas with lree enrichment a negative eu anomaly and almost flat hree pattern similar to a felsic and or cratonic sedimentary source however the la th sc plot of samples fall in a mixed sedimentary domain close to upper continental crust ucc and paas suggesting sedimentary source rocks for the mansar detritus it also indicates that these elements remained immobile during weathering and transportation the mineralogical characteristic rees and high field strength elements hfse together with the high percentage of metamorphic rock fragments in the siwalik sandstone support a metamorphic source for lower siwalik sediments a very weak positive correlation between zr and sio 2 poor negative correlation with al 2 o 3 negative correlation of la yb n and gd yb n ratios with sio 2 and positive correlation with al 2 o 3 suggest that zr does not dominantly control the ree distribution in mansar sediments the petrographic character and textural immaturity indicate a short distance transport for the detritus the distribution of elements in core samples reflect fractionation the higher zr th and zr yb ratios in coarse sediments and paas compared to finer grained detritus indicate sedimentary sorting plots of the geochemical data on tectonic discrimination diagrams suggest that the sediments derived from the lower siwalik were originated within a cratonic interior and later deposited along a passive margin basinal setting it therefore reveals lower siwalik depositional history

petrographic and geochemical interpretation of the late cretaceous volcaniclastic deposits from the hateg basin abstract the present paper provides new petrographic and geochemical insights into the tectonic and depositional environment of the late cretaceous volcaniclastic deposits from the hateg basin in the south carpathians these deposits are widespread in the rachitova stei unit which is tectonically delimitated from the south carpathians geological background by strike slip faults the conducted petrographic studies have revealed that the volcaniclastic rocks from the rachitova stei unit are mainly pyroclastic i e coarse tuffs tuff breccia and tuffites with a wide variety of volcanic pyroclastic fragments most commonly including andesites amphibole andesites biotite bearing amphibole andesites pyroxene bearing amphibole andesites pyroxene andesites and less frequently latite andesites latites rhyolites and dacites the rhyolites are found only in the lower sequence of the rachitova stei unit the volcaniclastic deposits were altered by diagenetic processes the celadonite is the most common diagenetic mineral formed in these rocks whose presence indicates that the pyroclastic fragments have interacted with sea water the high percentage of pyroclastic fragments with mainly angular forms and the rich hornblende content evidence a highly explosive volcanism the poorly sorted pyroclastic assemblages with larger blocks and with little reworked pyroclastic material indicate a very short distance of transportation and a deposition relatively close to the explosion centre the huge quantity of pyroclastic material from the rachitova stei unit suggests that the volcanic explosion had a centre of considerable dimensions but there is no evidence of such a volcano neither in the hateg basin nor anywhere else in the south carpathians the whole rock geochemical composition and the immobile trace element signature from the pyroclastic fragments indicate an island arc depositional environment that was placed close to an active continental margin in addition the andesitic nature of the pyroclasts indicates that the volcaniclastic rocks were most probably derived from a magma generated by the subduction of an oceanic plate under a thin continental plate the presence of the non volcanic pyroclasts i e granites quartz muscovite schists muscovite quartzites graphitic phyllites etc detached from the continental crust by explosion confirms that the overriding plate is of continental material the presence of the rhyolitic magma in the first stage of eruption also supports the continental nature of the overriding plate the magma having resulted by the partial melting of the rocks from the continental crust the petrographic and geochemical data have evidenced that the volcanism took place on a thin continental crust however this crust is not characteristic to the getic unit from the south carpathians which is commonly accepted to represent the basement of the hateg basin most probably the volcanism was generated in another geotectonic context respectively in a continental island arc with a thin overriding crust most likely situated at a lower latitude the volcaniclastic units of the hateg basin were displaced and moved from this tectonic context to the present location by strike slip movements

geochemistry and origin of the archaean beit bridge complex limpopo belt south africa the beit bridge complex bbc in the central zone of the limpopo belt in southern africa includes metasedimentary rocks of archaean age chiefly metapelites quartzites and carbonates amphibolites chiefly mafic dykes and sills and granitoids which comprise 50 of the complex the amphibolites are tholeiites with relatively unfractionated ree patterns and pronounced subduction zone geochemical signatures in terms of relatively immobile incompatible elements tonalites and granites which also exhibit such a signature can be formed by batch melting of a mafic source with residual garnet or amphibole most granites have negative eu anomalies and appear to represent residual liquids bbc metapelites have negative eu anomalies and other immobile element distributions similar to phanerozoic shales one group of metapelites that shows significant depletion in ca na and sr appears to have been derived from intensely weathered sources the negative eu anomalies in bbc metapelites suggest intracrustal melting of their sources and thus the presence of evolved continental crust in this part of southern africa by about 3000 ma bbc quartzites are quartz rich sandstones that are depleted in hfs elements ba and sr relative to phanerozoic counterparts a feature that probably reflects less recycling during the early archaean bbc banded iron formation bif is similar to other archaean bif and shows large positive eu anomalies bbc carbonates are dolostones depleted in ree hfs elements u th ba sr ni sc v and y and enriched in fe mn na rb and cs relative to phanerozoic marine carbonates

manganese mineralization in archean greenstone belt joda noamundi sector noamundi basin east indian shield abstract a mesoarchean greenstone belt 3 5 3 0 ga in the western part of the east indian shield comprising the iron ore group of the noamundi basin contains economic resources of both iron and manganese ores in the nne plunging regional synclinorium manganese mineralization in the central and eastern parts of this synclinorium particularly in joda noamundi sector has taken place in multiple cycles starting from syngenetic sedimentary and exhalative type through mobilization and remobilization in different stages of tectonism deformation and hydrothermal activities to latest lateritic or supergene type a relatively high temperature metamorphic jacobsite hausmannite bixbyite braunite assemblage low temperature hydrothermal pyrolusite psilomelane hollandite assemblage and supergene pyrolusite manganomelane groutite polianite assemblage are present and were formed by recycling of manganese in different stages of mineralization a detailed structural study of the manganese ore bodies as well as their ore petrographic and mineralogical characteristics with mineral chemistry has revealed systematic mineralization and their relation to deformational phases such recycling of manganese and its structural control of mineralization in different phases is unique of its kind in comparison with other archean manganese deposits in the world

stratigraphy and petrology of the archaean nsuze group northern natal and southeastern transvaal south africa abstract the nsuze group is the lower dominantly volcanic division of the pongola supergroup that accumulated on a sialic basement between 3 1 and 2 9 ga the nsuze group is subdivided into a lower sedimentary unit 800 m thick a middle volcanic unit 7500 m thick and an upper volcaniclastic sedimentary unit 5 600 m thick the predominant sediments in the lower unit are immature medium to very coarse grained quartz wackes with thin intercalated lenses of quartz and arkosic arenites and minor conglomeratic wackes these sediments were deposited in a distal braided stream environment there followed a major period of volcanism during which lavas showing a continuous spectrum of compositions from basalt to rhyolite were extruded subaerially flows of both different and similar compositions are complexly interfingered on both regional and local scales as volcanism waned pyroclastic and sedimentary rocks became dominant in the upper unit the nsuze group is gently dipping and is metamorphosed to low greenschist grade the nsuze group is significant in that it provides evidence for the existence of high standing continental sialic crust in the southeastern part of the kaapvaal province at ca 3 0 ga volcanism and sedimentation in the pongola supergroup are more typical of proterozoic basins than of archaean environments despite their age komatiitic and high mg basalts were however being extruded in zimbabwe contemporaneously with the nsuze lavas

geochemistry of 1 8 1 67 ga mudstones and siltstones from the mount isa inlier queensland australia provenance and tectonic implications abstract sedimentary and volcanic rocks overlying basement in the mount isa inlier australia are subdivided into three cover sequences the upper two sequences range in age from 1800 to 1670 ma and each consist of rift and overlying sag phases of sedimentation fine grained siliciclastic sedimentary rocks in the upper two sequences define two distinct geochemical groups samples from groups i and ii are mainly from the lower and upper rift and upper thermal phases respectively one sample from the lower rift phase has affinities with group ii whereas samples from the lower thermal phase belong to both groups i and ii eu eu values are 0 70 to 0 85 for group iand 0 55 to 0 70 for group ii group i samples are characterized by lower la sc and th sc ratios and higher cr th ratios than group ii cr niand co concentrations are greater in group i than group ii groups i and ii also are distinguished on plots of eu eu vs lan smn and eu eu vs gdn ybn two samples from group i have elevated lree hree and lan ybn ratios that are attributed to intense weathering as revealed by their high chemical index of weathering values the geochemical data reflect a combination of local and more distant regional provenances and can be related to the tectonic evolution of the mount isa inlier group i samples are attributed to dilution of a mature post archean source by addition of recycled mafic volcanics group ii samples are typical post archean sediments derived from differentiated upper continental crust with no dilution by local sources the lower rift phase displays great geochemical variability samples from both groups that is typical of active tectonic settings such as rifts the great variability of samples from the lower sag phase both groups is surprising but probably reflects the diverse rock types beneath this basin stable tectonic conditions during this phase of sedimentation are indicated instead by major element geochemistry that records severe weathering similarity of the upper rift phase sample with some samples from the lower sag phase reflects recycling of the latter samples from the upper sag phase are remarkably uniform in composition group ii and reflect the stable tectonic conditions under which this basin developed

holocene evolution of the palar river southern india tracking history of migration provenance weathering and tectonics abstract this study examines the catchment and source morphodynamics of the palar river southern peninsular india here we applied a multidisciplinary approach remote sensing techniques lineament analysis geochemistry and ground penetration radar for a better understanding of its evolution during holocene times the major lineaments present in the palar river basin are showing the ne sw trend there are five major faults identified in the palar river basin with a transition zone where frequent smaller magnitude earthquakes have occurred the major fault f1 a strike slip fault occurs in the upper part of the palar river following the ne sw trend other major faults present are f2 and f3 with a transition zone where frequent minor and major tremors are documented fault f4 traverses in parallel to the cheyyar river a rapid change in the river course has resulted due to the movement along these strike slip faults f 5 nearer to the east coast indicates a passive tectonic activity regime the after effects of tectonic activity in the basin is further implicit from the gpr profile the sediments of active palar river are dominantly litharenite arkose wacke while the paleochannels are shale in composition weathering proxies such as cia pia elemental ratios along with a cn k plot indicate that a post depositional intense weathering due to climatic variability for all paleochannel sediments due to ongoing tectonic activity in the sources region along with subsequent aggradation and degradation in fluvial regimes the sediments of active palar river shows low to moderate weathering moreover the geochemical data also reveals that the sediments from the palar river and paleochannels are predominately from active continental and passive continental margins respectively moreover the major oxide trace elements and ree elements data indicate that palar river sediments are from a felsic source while paleochannels are from mafic source hence our study suggests that the catchment area of the palar river has been shifted to the south during the holocene period due to tectonic uplift subsequently the sediments of the paleochannels have undergone post depositional weathering due to ongoing tectonic activity and monsoonal changes in the region enhances rapid erosion in the catchment area results in the deposition of the huge thickness of sediments from the middle to lower reaches of active palar river

weathering of the ganga alluvial plain northern india implications from fluvial geochemistry of the gomati river abstract in the northern part of the indian sub continent the ganga alluvial plain gap feeds its weathering products to the ganga brahmaputra river system one of the world s largest fluvial systems the authors present a geochemical study of the gap weathering products transported by the gomati river the ganga river tributary to understand weathering processes of an alluvial plain in a humid sub tropical climate a total of 28 sediment samples were collected during the monsoon season and were analysed by x ray fluorescence spectrometry for 25 major and trace elements bulk chemistry of the channel flood and suspended sediments mostly consists 90 80 and 75 respectively of three elements al si and fe major element concentrations normalised with respect to upper continental crust ucc show strong depletion of highly mobile elements na ca and enrichment of immobile elements ti si silica enrichment in the sand fraction is probably caused by chemical weathering of feldspar mineral sorting during fluvial transportation acts as the single important factor that controls the geochemistry of these weathering products and also strongly influences major and trace element distribution in the individual sediment samples trace element ba cr cu nb ni pb v and zn concentrations were strongly correlated with major element si al fe mn and k concentrations indicating that the abundance of trace elements is controlled by the same processes that control the major element distribution in these sediments the gap weathering products were geochemically distinguished as arkose to litharenite in rock classification chemical mobility normalised with respect to tio 2 in ucc indicates that si na zr ba and sr mainly derived from feldspar muscovite and biotite are lost during weathering iron and zn remained immobile during weathering and were strongly adsorbed by phyllosilicates and concentrated in fine grained sediment fractions the chemical index of alteration indicates that the gap has experienced chemical weathering of incipient to moderate intensity the gap weathering products also demonstrated a progressive incomplete alteration in the alluvial sequence made up of the himalayan derived sediments a model has been proposed to better understand weathering processes and products of the gap in temporary storage of 50 ka in a humid sub tropical climate

continentally derived solutes in shallow archean seawater rare earth element and nd isotope evidence in iron formation from the 2 9 ga pongola supergroup south africa abstract the chemical composition of surface water in the photic zone of the precambrian ocean is almost exclusively known from studies of stromatolitic carbonates while banded iron formations ifs have provided information on the composition of deeper waters here we discuss the trace element and nd isotope geochemistry of very shallow water if from the pongola supergroup south africa to gain a better understanding of solute sources to mesoarchean shallow coastal seawater the pongola supergroup formed on the stable margin of the kaapvaal craton 2 9 ga ago and contains banded iron formations ifs that represent the oldest documented superior type iron formations the ifs are near shore pure chemical sediments and shale normalized rare earth and yttrium distributions reysn exhibit positive lasn gdsn and ysn anomalies which are typical features of marine waters throughout the archean and proterozoic the marine origin of these samples is further supported by super chondritic y ho ratios average y ho 42 relative to older isua ifs 3 7 ga from greenland and younger kuruman ifs 2 5 ga also from south africa the pongola ifs are depleted in heavy rare earth elements hree and appear to record variations in solute fluxes related to sea level rise and fall sm nd isotopes were used to identify potential sediment and solute sources within pongola shales and ifs the nd t for pongola shales ranges from 2 7 to 4 2 and nd t values for the coeval iron formation samples range 1 9 to 4 3 are generally indistinguishable from those of the shales although two if samples display nd t as low as 8 1 and 10 9 the similarity in nd isotope signatures between the shale and iron formation suggests that mantle derived rey were not a significant nd source within the pongola depositional environment though the presence of positive eu anomalies in the if samples indicates that high t hydrothermal input did contribute to their rey signature isotopic mass balance calculations indicate that most 72 of the nd in these seawater precipitates was derived from continental sources if previous models of fe nd distributions in archean ifs are applied then the pongola ifs suggest that continental fluxes of fe to archean seawater were significantly greater than are generally considered

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

geochemical constraints on the growth of the continental crust several lines of evidence indicate that the archean upper crust was considerably more mafic than the present day upper crust there has been no significant change in ree and th abundances in post archean clastic sedimentary rocks suggesting that there has been no change in the composition of the upper crust during the post archean this indicates that if there have been any additions to the post archean upper crust they must have had similar composition to the upper crust itself geochemical modelling of ree and th abundances in sedimentary rocks suggests that the minimum ratio of post archean to archean upper crustal composition required to eliminate the archean upper crustal trace element signature within analytical uncertainty is about 4 1 such a model also is supported by isotopic data using plausible assumptions regarding the volume of archean crust isostatic relations and extreme models of the earth s degassing history it is proposed that approximately 65 75 of the continental crust formed