rare earth elements in the sediments of the north atlantic ocean amazon delta and east china sea reinterpretation of terrigenous input patterns to the oceans

transgressive dune formation along a cliffed coast at 75 ka in sardinia western mediterranean a record of sea level fall and increased windiness transgressive dunes develop frequently along strandplain coastlines however they may also form along rocky coasts dominated by cliffs and embayments two lithified transgressive dune systems developed along the cliffed alghero coast nw sardinia italy have been investigated one aeolian system forms a cliff front anchored aeolian dune accumulation the other is a valley head anchored sand ramp system optically stimulated luminescence ages indicate that both systems formed around 75 ka this period which corresponds to the beginning of mis 4 was characterized by a relatively low sea level stand 15 m below the present sea level and was preceded by a sea level highstand 1 m asl around 81 ka mis 5a our results show that this rapid sea level fall exposed an enormous amount of marine sand which was transported inland by strong northerly winds and deposited in front of cliffs or in valley heads therefore sediment supply and sea level fall seem to be critical factors controlling dune formation along rocky coasts which generate time transgressive aeolianites terra nova 00 000 000 2010

geochemistry and origin of archaean quartz cordierite gneisses from the godthabsfjord region west greenland we report new data on the major minor and trace element compositions of metasedimentary quartzcordierite gneisses qcg an important member of the archaean malene supracrustal suite found throughout the godthabsfjord region of west greenland the analyzed qcg contain assemblages of quartz cordierite biotite garnet anthophyllite gedrite staurolite sillimanite plagioclase with abundant accessory zircon and minor rutile monazite and allanite and broadly resemble cordierite orthoamphibole rocks found in a great number of other metamorphic terrains chemically the qcg are characterized by 1 high but variable sio2 59 87 wt relative enrichments in mgo feo and al2o3 mg 0 35 0 85 and depletions in na2o k2o and especially cao 2 low concentrations of sc cr co ni and sr 3 high concentrations of y nb zr hf ta th u and ree rare earth elements with prominent negative eu anomalies in each case 5 high concentrations of ga 18 55 ppm with variable ga al ratios that are significantly higher than average crustal material low cr and ni together with enriched and fractionated ree displaying negative eu anomalies distinguish the malene qcg from published accounts of most other archaean sedimentary rocks furthermore all of the above mentioned trace element characteristics distinguish the qcg from ordinary malene clastic metasediments quartzites psammites and pelites suggesting a separate origin for the qcg these data point towards chemically evolved felsic igneous rocks being the source of the qcg consequently we propose that the malene qcg represent metamorphosed felsic volcaniclastic sediments that underwent hydrothermal alteration by heated seawater prior to metamorphism which resulted in gain of mg and fe loss of alkalis and lime and possibly eu and sr the overall trace element characteristics of the qcg elevated ga zr nb ree etc are features shared by a type granites and their volcanic equivalents such igneous rocks may represent the ultimate source material for the qcg protolith

geochemically unravelling the sedimentary components of archaean metasediments from western australia major and trace elements of metasediments of the granulite facies narryer and the amphibolite facies jack hills metasediments have been used to devise plots which enable the original sedimentary components of these rocks to be unravelled despite their metamorphic state these plots overcome the predominant effect of quartz which ranges up to 100 and can be used to study other metasediment suites six components controlled the composition of the clastic metasediments 1 kaolinite 2 illite muscovite 3 chlorite montmorillonite 4 dolomite 5 quartz 6 resistant heavy minerals such as rutile zircon chromite and monazite identifying the first four of these components enables the trace element composition associated with the components to be estimated feldspar was largely destroyed by chemical weathering but there is a suggestion that traces of it formed a minor seventh component in the narryer rocks there was little or no mixing of the original kaolinite bearing and illite muscovite bearing samples indicating two distinct sources although both sets of samples contain 4 ga zircons suggesting a third zircon source the old zircons may have been derived from re worked sediment or metasediment as they occur in the matrix and in the quartzite clasts of conglomerates using bhatia9s criteria a passive margin is the likely site of deposition calc alkaline tholeiites in the narryer succession are intrusive and did not contribute to the sediments which were derived from extremely heavily weathered granites komatiites schists and reworked sandstones or quartzites

geochemistry of daihai lake sediments inner mongolia north china implications for provenance sedimentary sorting and catchment weathering abstract to advance the understanding of sediment distribution catchment weathering hydraulic sorting and sediment provenance in a tectonically stable basin the geochemistry of surface sediment samples from daihai lake in north china is presented mud bulk sediments were analyzed for 10 major and 30 trace elements organic carbon and nitrogen and for 87sr 86sr ratios in silicate fraction acid insoluble ai and carbonates acid soluble as enrichment in fe2o3 cao mno mgo k2o and p2o5 and their positive correlation with al2o3 rb sr ratios and bivariate discrimination diagrams of k vs rb and th u vs th suggest low chemical weathering intensity under semiarid conditions in the daihai catchment accumulation of clays in deeper water sites by sedimentary sorting results in an evident distribution of sediment composition with the lake water depth although similar chondrite normalized ree rare earth element patterns of lake sediment samples with lree light rare earth element enrichment negative eu anomalies and an almost flat hree heavy rare earth element pattern may suggest similar sedimentary source rocks original sediment provenance can be recognized on the basis of their distinctive 87sr 86sr ai ratios and immobile th zr ti and nb trace element signature separating from weathering and sedimentary processes as well as from grain size effect

rare earth element and mineralogic changes in holocene soil and stream sediment a case study in the wet mountains colorado u s a size fractions of soil and stream sediment developed on plutonic metamorphic or mixed plutonic metamorphic sources in the wet mountains colorado have been analyzed for ree rare earth elements and mineralogy the clay sized fractions 0 5 m and 0 5 2 m or 2 m have ree contents that are similar to more enriched than the source eu sm ratios that are similar to the source and la lu ratios that are higher than their corresponding source the 0 5 m fractions have lower ree contents than the corresponding 0 5 2 m fraction in every stream sediment for example unweathered san isabel batholith near the sampled stream sediment contains ree 511 712 ppm and eusmratio 0 17 0 19 and lalu cn 9 8 10 3 the corresponding 0 5 and 0 5 2 m fractions of sediment formed from san isabel batholith are enriched in the ree ree 1135 and 2225 ppm respec are slightly lower in eu sm ratios 0 16 and 0 14 respec and are higher in la lu ratios 12 2 and 16 3 respec than the unweathered rocks the silts have lree light ree contents and eu sm ratios very similar to the source due to similar mineralogy but silts have hree heavy ree contents higher than the source due to high zircon for example silt sized fractions in stream sediment draining the san isabel batholith contain ree 808 ppm eusmratio 0 18 and lalu cn 7 4 in contrast the sands contain lower ree contents similar to higher eu sm ratios and lower la lu ratios than the corresponding source due to depletion in heavy minerals concentrated in the ree and containing lower eu sm ratios relative to quartz and feldspar depleted in the ree and containing higher eu sm ratios for example the sand fraction from stream sediment from the san isabel batholith has ree 206 ppm eusmratio 0 17 and lalu cn 5 6 small variations of accessory minerals concentrated in the ree like allanite or sphene may cause drastic variation in ree content in the sand fraction clays and silts derived only from the plutonic rocks have higher ree and similar to lower eu sm ratios than corresponding fractions derived only from migmatites e g ree of silt fractions from migmatites 224 ppm and from plutons 375 808 ppm eu sm ratios from migmatites 0 17 and from plutons 0 11 0 18 the ree content and the eu sm ratio of clays and silts derived from migmatites are much more similar to the average of mcps mid continent platform sediment than corresponding fractions derived from the plutons mcps has ree 192 ppm eusm 0 19 lalu 8 0 the eusm ratio of sands derived from plutons and migmatite are lower to mostly higher than the mcps the ree content of sands from plutons are not necessarily higher than the mcps due to variation in heavy minerals like allanite or sphene concentrated in the ree sediments derived from mixed sources have ree contents in fine fractions that may not necessarily reflect the ree content of the nearest source for example 0 5 2 m and silt fractions developed on the san isabel batholith with a migmatite upstream have the high ree content of the batholith ree in 0 5 2 m fraction 1100 ppm silt 573 ppm while the corresponding 0 5 m fraction has the very low ree content of the migmatites ree 110 ppm these results suggest fine grained material near the source may sample a much larger area than coarser material near the source thus analysis of fine grained sediment associated with coarse grained material near the source may give a much different impression of the source than would determination of the source using classic petrographic examination of the sand or gravels

a review of the superior province of the canadian shield a product of archean accretion abstract the superior province consists of northern and southern high grade gneiss subprovinces and a central region of alternating granite greenstone and metasedimentary belts subprovinces are commonly fault bounded and display contrasting lithological assemblages metamorphic and structural styles geophysical characteristics and ages they are comparable to phanerozoic suspect terranes the middle and late archean rocks of the superior province are of mantle and juvenile crustal origin with little evidence of inheritance of components older than ca 3 1 ga high grade gneiss subprovinces are mainly deeper level equivalents of the supracrustal rich belts but one minnesota river valley is part of an older ca 3 6 ga terrane that was juxtaposed late in the tectonic history granite greenstone terranes are characterized by low grade volcano sedimentary sequences tholeiitic komatiitic submarine lava plain tholeiitic calc alkalic submarine to subaerial central complexes early platform type quartz arenite stromatolitic carbonate late timiskaming type shoshonitic alkalic volcanic fluvial sediment that range in age from ca 3 0 ga to 2 7 ga the volcanics represent oceanic island arc and continental arc volcanism the quartz arenite carbonate sequences deposition on relatively stable early formed ca 3 0 ga crust and the timiskaming type sequences deposition in pull apart basins developed along strike slip faults late archean ca 2 75 2 70 ga greenstone sequences occur throughout the superior province middle archean ca 3 0 2 8 ga sequences are common in northern and central superior province rare or absent in the south volcanism was accompanied by mafic to trondhjemitic plutonism the metasedimentary belts consist of variably metamorphosed volcanogenic turbidites with detrital zircons ranging in age from ca 3 0 ga to 2 7 ga the metasediments were deposited after major volcanism and probably represent accretionary prisms late archean orogenesis accompanied and followed by calc alkalic plutonism youngs southward from about 2 73 2 72 ga to 2 70 2 68 ga it involved large scale north south compression and dextral transpression and displays thin to thick skinned transitions the contemporaneity of events along the lengths of the belts and their southward younging coupled with the evidence of major compression are consistent with subduction driven oblique accretion of oceanic and continental volcanic arcs accretionary sedimentary wedges older microcontinental fragments etc in a convergent margin setting analogous to parts of the pacific rim differences eg abundance of komatiites and tonalitic plutonic suites are attributable to a hotter archean mantle

rare earth major and trace element composition of monterey and dsdp chert and associated host sediment assessing the influence of chemical fractionation during diagenesis abstract chert and associated host sediments from monterey formation and deep sea drilling project dsdp sequences were analyzed in order to assess chemical behavior during diagenesis of biogenic sediments the primary compositional contrast between chert and host sediment is a greater absolute sio2 concentration in chert often with final sio2 98 wt this contrast in sio2 and si al potentially reflects precursor sediment heterogeneity diagenetic chemical fractionation or both sio2 concentrations and si al ratios in chert are far greater than in modern siliceous oozes however and often exceed values in acid cleaned diatom tests compositional contrasts between chert and host sediment are also orders of magnitude greater than between multiple samples of the host sediment calculations based on the initial composition of adjacent host observed porosity reductions from host to chert and a postulated influx of pure sio2 construct a chert composition which is essentially identical to observed sio2 values in chert thus precursor heterogeneity does not seem to be the dominant factor influencing the current chert composition for the key elements of interest in order to assess the extent of chemical fractionation during diagenesis we approximate the precursor composition by analyzing host sediments adjacent to the chert the sio2 concentration contrast seems caused by biogenic sio2 dissolution and transport from the local adjacent host sediment and subsequent sio2 reprecipitation in the chert along with sio2 other elements are often added with respect to al to monterey and dsdp chert during silicification although absolute concentrations decrease the two monterey quartz chert nodules investigated in contrast to the opal ct and quartz chert lenses formed primarily by extreme removal of carbonate and phosphate thereby increasing relative sio2 concentrations dsdp chert formed by both carbonate phosphate dissolution and sio2 addition from the host manganese is fractionated during chert formation resulting in mno al 2 o 3 ratios that no longer record the depositional signal of the precursor sediment ree data indicate only subtle diagenetic fractionation across the rare earth series ce ce values do not change significantly during diagenesis of either monterey or dsdp chert eu eu decreases slightly during formation of dsdp chert la n yb n is affected only minimally as well during formation of one monterey opal ct chert lens ree al ratios show subtle distribution changes at gd and to a lesser extent near nd and ho ree compositional contrasts between diagenetic states of siliceous sediment and chert are of a vastly smaller scale than has been noted between different depositional environments of marine sediment indicating that the paleoenvironmental ree signature is not obscured by diagenetic overprinting

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

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

tectonic evolution in the archaean and proterozoic kre high heat flow probably favoured global hotspot activity with crustal growth of iceland type through vertical magmatic accretion the tectonic environment for the generation of about 3 9 2 5 ga granite gneiss greenstone terrains with volurnlnous production of tonalite trondhjemite granodlorite magmas is still uncertain and both piate ma and intrapiate scenarios arc possible deformanon atylea in greenstones and adjacent high grade orthognersses with intercalated shallow water supracrustal assemblages suggest extensive horizontal shortening and crustal interstacking this probably resulted from collisional and or rotational motion and produced significant intracrustal melting as early as 3 9 ga by the end of the archaean and locally much earlier crustal thicknesses reached 35 50 km or more and the cxtl aordinary hhlding to erogenic deformation may have been due to delamination and crustal subduction towards the end of the proterozoic evidence for modern style plate tectonics becomes abundant through the recognition of laland a16 accretion ophiolite obducdon foreland thrust and fold belts and exotic terranes one of the best documented example for this tectonic environment is the arabian nubian shield 4lthough it is doubtful whether modern plate tectonic settings can serve as analogues for all precambrian tectonic styles it i clear that global crustal evolution since the early archaean was governed by the motion of plates which repeatedly came togethel tv fotm supercontinents and then dlspersetl again we shall not understand the detailed mechanism of precambrian crustal itlton cs before we learn horn lo irltejpret recent large scale crustal deformation and its causes

gneiss greenstone relationships in the ancient gneiss complex of southwestern swaziland southern africa and implications for early crustal evolution the ancient gneiss complex agc of swaziland is made up of a variety of archaean gneisses derived from granitoid intrusions of ttg composition emplaced between 3 6 and 3 2 ga ago and infolded amphibolite grade greenstone remnants locally referred to as dwalile supracrustal suite dss the greenstones were considered to postdate the oldest phases of the agc known as ngwane gneiss and this is now confirmed by conventional u pb zircon geochronology and shrimp single zircon ages on detrital grains from dwalile clastic metasediments suggesting that these rocks were derived from a granitoid source some 3 42 to 3 57 ga in age trondhjemitic ngwane gneiss structurally underlying the greenstones has a zircon age of 3521 23 ma and contains a 3683 10 ma old xenocryst this together with nd tdm model ages between 3 7 and 3 9 ga and a pronounced negative euanomaly for the metasediments strongly argues for the presence of very old crustal components in the agc nd isotopic systematics of dwalile metavolcanics suggest these rocks to be contaminated with older crustal material of ngwane type and resulting nd model ages 3 60 3 99 ga are likely to be too old the major and trace element chemistry of the dwalile metavolcanics is remarkably similar to that of onverwacht group rocks in the nearby barberton greenstone belt bgb and this together with a similar age for these two suites points to a genetic link and a similar mechanism of formation our data as well as field relationships do not favour an intra oceanic origin for the dss as has been inferred for the onverwacht group nor do they support a juvenile origin for the exposed ngane gneisses of the agc the ngwane precursors were apparently derived from older crustal protoliths whose subsequent modification particularly through remelting and deformation gave rise to the various compositional varieties now seen in the ngwane suite

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 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

redistribution of rare earth elements during diagenesis of turbidite hemipelagite mudrock sequences of llandovery age from central wales

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 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

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

seismic reflection response of folded structures and implications for the interpretation of deep seismic reflection profiles seismic reflection characteristics of several common folded structures have been investigated by forward synthetic modeling in order to interpret correctly the seismic data from ancient and modern orogenic belts the studied folds include single order open folds close folds and asymmetric inclined folds as well as multi order folds the results show that complex folded structures and particularly those in deep crust cannot be accurately imaged by conventional 2d reflection profiling it is possible that widely observed subhorizontal discontinuous reflections in the continental crust of compressional folded belts do not necessarily correspond to subhorizontal compositional lamination multi order folded structures may equally enable the formation of such reflections furthermore trains of asymmetric inclined folds produce straight discontinuous dipping reflections that are potentially misinterpreted as thrusting faults or shear zones thus structural geologists must be extremely cautious when trying to convert directly seismic reflection sections to geological profiles for the continental crust

geochemical oxygen and neodymium isotope compositions of metasediments from the abitibi greenstone belt and pontiac subprovince canada evidence for ancient crust and archean terrane juxtaposition abstract the abitibi greenstone belt agb and pontiac subprovince ps in the southwestern superior province are adjacent greenstone plutonic and metasedimentary dominated terranes respectively separated by a major fault zone metasediments from these two contrasting terranes are compared in terms of major and trace element and o and nd isotope compositions and detrital zircon ages the following two compositional populations of metasediments are present in the low grade abitibi southern volcanic zone 1 1 a mafic element enriched population mep characterized by flat depleted ree patterns enhanced mg cr co ni and sc low incompatible element contents and minor or absent normalized negative troughs at nb ta and ti and 2 2 a low mafic element population lmep featuring lree enriched patterns enhanced rb cs ba th and u contents and pronounced normalized negative troughs at nb ta and ti these geochemical features are interpreted to indicate that the mep sediments were derived from an ultramafic and mafic dominated oceanic provenance whereas the lmep sediments represent mixtures of mafic and felsic arc source rocks the ps metasediments are essentially indistinguishable from abitibi lmep on the basis of majorelement and transition metal abundances suggesting comparable types of source rocks and degrees of maturity but are distinct in terms of some trace elements and o isotope compositions the pontiac metasediments are depleted in 18o and enriched in cs ba pb th u nb ta hf zr and total ree and also have higher ratios of rb k cs rb ba rb ta nb th la and ba la relative to the abitibi lmep two subtypes of ree patterns have been identified in ps metasediments both have moderately fractionated distributions with la yb n 3 9 17 and 7 8 13 6 respectively but the first has minor eu anomalies eu eu 0 89 1 16 whereas the second features pronounced negative anomalies eu eu 0 52 0 81 coupled with high concentrations of incompatible elements such as k rb th u nb and ta the first subtype is interpreted to be derived from provenances of mixed mafic and felsic volcanic rocks whereas the eu depleted type has geochemical features that are typical of post archean sediments or archean k rich granites and volcanic equivalents abitibi metasediments have nd 2 7 ga values of 0 71 to 1 83 corresponding to model tchur 2 82 2 55 ga with the majority around 2 65 ga close to the depositional age of 2 7 ga in contrast ps metasediments are characterized by nd 2 7 ga 0 95 to 3 51 corresponding to t dhur 2 7 to 3 1 ga consistent with a detrital zircon age spectrum of 2 69 ga to 3 1 ga pontiac subprovince and abitibi metasediments have contrasting provenances suggesting their separate initial geodynamic settings and later tectonic juxtaposition major and trace elements o and nd isotopic compositions and detrital zircon ages collectively suggest that some of the ps metasediments were derived mainly from juvenile rocks whereas others require various degrees of contribution from older 3 0 3 5 ga evolved crust mixed with juvenile 2 7 ga rocks older components were derived either directly from erosion of an ancient continental arc or from contemporaneous volcanic rocks which may contain recycled older components if the ps represents in part an accretionary prism to the abitibi southern volcanic zone then oblique subduction may have been involved which allowed the pontiac subprovince to juxtapose or sample exotic source rocks

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

chemical characteristics and genesis of the quartz feldspathic rocks in the archean crust of greenland a total of 108 samples of meta tonalites metagranodiorites granites and meta tholeiites representing groups of early to late archean age and different metamorphic history from sw and se greenland have been analyzed for ca k and 28 trace elements there is no systematic change of the chemical composition with age observable the results support petrologic experiments which suggest that tonalites and granodiorites the most abundant rocks of the archean crust are partial melting products of a mafic lower crust modelling suggest that this crust consisted of garnet amphibolite derived from a source with a bulk composition resembling a slightly enriched rather than depleted mantle the cen ybn ratio is above 10 in the majority of tonalites most samples have no eu anomaly because of a balanced contribution from the minerals of a mafic rock or a plagioclase free source the positive eu anomaly of some granodiorites and of a minor proportion of tonalites can be explained as being caused by plagioclase accumulation during differentiation or by partial melting of plagioclase rich fractions modelling with zn excludes an origin of tonalitic melts by differentiation of basaltic to dioritic magmas the archean meta diorites meta tonalites and meta granodiorites from greenland have generally lost some k and s relative to their suggested magmatic protoliths loss of rb tl pb and k and relative gain of ca sr ba and sc connected with granulitization of meta tonalites can be explained in the majority of cases by separation of about 25 percent granitic partial melt high k rb k pb zn cd and nb th ratios of granulites plus low ratios of granites are almost in balance with intermediate ratios of amphibolite facies tonalites retrogression of granulites into amphibolites was accompanied by introduction of pb tl rb ba sr and k from na cl rich brines circulating on fractures a comparison of the abundance of 24 elements characterized by different compatibility in the archean crust of greenland with the present bulk crust reflects only minor changes th nb if at all in the chemical composition of the continental crust since the archean

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 ciw index a new chemical index of weathering abstract a new chemical index of weathering that can be applied to modern soils and precambrian paleosis is proposed ciw al2o3 the value of this index increases as the degree of weathering increases and the difference between ciw index values of the silicate parent rock and soil or sediment reflects the amount of weathering experienced by the weathered material

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

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

geochemistry across an exposed section of archaean crust at vredefort south africa with implications for mid crustal discontinuities abstract the central region of the vredefort structure consists of a semi circular multi layered sequence of crystalline rocks which are nearly vertical in attitude and which increase in metamorphic grade towards the core of the structure together with the overlying precambrian strata this sequence provides a cross section through almost the entire crustal section of the kaapvaal craton 36 km the upper part of the vredefort crystalline crust consists of a 3 0 ga sequence of differentiated felsic rocks in amphibolite facies the lower crust consists of a complex and heterogeneous both chemically and isotopically high grade metamorphic terrain of charnockites granulites mafic and felsic and supracrustal rocks the upper crust is separated from the lower crust by the vredefort discontinuity a brittle ductile shear zone characterised by a high concentration of pseudotachylite and brecciated rock petrographic chemical and isotopic evidence suggest that the upper and lower crust have undergone very different styles of evolution this is indicative of different geological environments prior to their present juxtaposition we speculate that the upper and the lower parts of the vredefort crystalline crust were juxtaposed during intracratonic thrusting 2 8 ga ago and that the entire 36 km section of crust including the vredefort discontinuity were rotated and uplifted into their present vertical position during the 2 0 ga vredefort catastrophe

geology and geochemistry of arenite quartzwacke from the late archaean sandur schist belt implications for provenance and accretion processes abstract detailed geological petrological and geochemical studies have been carried out on an arenite quartzwacke suite of rocks constituting a part of the late archaean sandur schist belt in dharwar craton southern india for understanding the nature of provenance for these sedimentary rocks the arenite quartzwacke consists of rounded to sub rounded and angular fragments of monocrystalline polycrystalline quartz quartzite and chert embedded in a fine grained matrix of quartz and sericite while arenites are more siliceous sio 2 80 92 wt the quartzwacke have relatively lower silica content ca 69 78 wt the arenites and quartzwackes have cia values ranging from 76 to 96 which suggest intense chemical weathering this is further corroborated by the positive correlation between al 2 o 3 and tio 2 in both these rock types the a cn k modeling of arenites and quartzwackes show evidence for addition of k 2 o during later metasomatic alteration in the a cnk fm ternary diagram all the samples plot along a mixing line between chlorite and sericite indicating alteration during k metasomatism and the presence of mafic rocks in the source the high concentration of hfse such as zr hf nb and ta and the trace element ratios th sc la sc th u and ce th in the arenite quartzwacke indicate a mixed provenance the rare earth element modeling of quartzwackes considering tonalite granite and amphibolite end members in the provenance suggests equal proportions of mafic and felsic end members a composition comprising of 25 tonalite 25 granite 50 amphibolite in the provenance appears to match with the observed range of ree patterns of quartzwackes the presence of higher proportions of granite in the provenance is evidenced by the large negative eu anomalies in these sediments field evidence and structural discordance suggest that the arenite quartzwacke suite is an allochthonous part of the sandur schist belt

age and tectonic setting of late archean greenstone gneiss terrain in henan province china as revealed by single grain zircon dating the authors report precise u pb zircon ages for single grains of a metarhyodacite from the late archean dengfeng greenstone belt in henan province china near the southern margin of the north china craton most grains belong to an igneous population whose u pb isotopic systematics define a straight line intersecting concordia at 2512 12 ma and this is interpreted as the time of crystallization of the original greenstone volcanics several grains are distinctly older between 2576 9 and 2945 44 ma and the authors interpret the older grains as xenocrysts of pre greenstone continental crust that provide evidence for crustal derivation or crustal contamination of the original rhyodacitic lava the xenocrysts suggest evolution of the dengfeng greenstone belt in a continental environment that may be represented by the taihua high grade gneisses bordering the dengfeng greenstones and for which the authors obtained ages of 2806 7 and 2841 6 ma the data add evidence to the now widely held concept that most archean greenstones developed on or near older continental crust and were therefore prone to crustal contamination in such cases conventional zircon dating may not always record the precise age of rock formation

geochemical signature of provenance in sand size material in soils and stream sediments near the tobacco root batholith montana u s a abstract trace element geochemistry of sandstones are being used to determine provenance we have conducted preliminary and limited experiments to determine to what extent daughter sands retain the geochemical signature of parent rocks six sets of first order stream sediments soils from adjacent slopes and a variety of parent rocks were collected from southwestern montana u s a sampling in a low relief area ensured that climate and residence time of soils on slopes could be eliminated as variables sand size fractions of stream sediments and soils and the corresponding parent rocks granodiorite quartz monzonite granite gneiss biotite tonalite gneiss and amphibolite were analyzed for most major elements and selected trace elements petrologic modal analysis of the parent rocks and the 0 25 0 50 mm fraction of each sand was done to monitor major mineralogic control if any on chemical compositions of the samples our data show that the abundances of the si and al in sediments do not discriminate provenance abundances of ca mg fe and ti may broadly distinguish between sands derived from metamorphic and igneous source rocks at least in the area studied differences in abundances of the ba and th and the ratio of la lu between granitic tonalitic and amphibolitic parent rocks are preserved in the daughter sediments that we studied however the size of the eu anomaly in the ree patterns of different daughter sediments is not diagnostic of parent rocks abundances of co and sc distinguish between sediments derived from felsic and mafic rocks a better provenance discrimination is obtained if the ratios la sc th sc la co ba sc and ba co are used petrologic modal data show that mineral contents and chemical compositions of parent rocks are compatible with each other the chemical composition of the sands may be roughly correlated to the petrological modal data but the abundances of some minor and trace elements of sediments cannot be inferred from modal mineralogy this is expected because these elements may concentrate in accessory minerals and or may weather out into aqueous or clay mineral fractions it is also compatible with conclusions of previous studies that some of these elements do not reside in sand size fractions of siliciclastic sediments

evidence for archean ocean opening and closing in the southern slave province archean mafic volcanic and plutonic rocks of the cameron and beaulieu river greenstone belts outcrop in the southern slave structural province of the canadian shield lower portions of the greenstone belts have ophiolitic characteristics but young felsic units may represent superimposed arc magmatism the greenstone belts are separated from older circa 2 7 3 2 ga quartzo feldspathic gneisses of the sleepy dragon complex by a regional decollement zone patterson lake structural complex consisting of deformed mafic volcanic rocks thin slivers of gneiss ultramafic rocks and banded iron formation mixed in a highly deformed phyllonitic and phacoidally cleaved metapelitic matrix high synmetamorphic strains are indicated by abundant intrafolial folds transposed layering rare sheath folds and by the parallelism of elongated primary features and metamorphic mineral lineations the patterson lake structural complex is similar to melanges of phanerozoic mountain belts adjacent quartzo feldspathic gneisses are strongly mylonitized the melange and deeper mylonites formed when the greenstone belts were emplaced as thrust sheets wildflysch shed from tips of emergent thrusts was in part incorporated into the basal melange as the allochthon overrode its own detritus continued compression involved the basement in the deformation uplifting the sleepy dragon complex in a structural culmination mantled by the imbricated greenstone stack late granodioritic to granitic intrusions formed regional thermal aureoles which overprint earlier metamorphic fabrics

geochemical and multi isotopic 87sr 86sr 143nd 144nd 238u 235u perspectives of sediment sources depositional conditions and diagenesis of the marcellus shale appalachian basin usa abstract we investigate sediment sources depositional conditions and diagenetic processes affecting the middle devonian marcellus shale in the appalachian basin eastern usa a major target of natural gas exploration multiple proxies including trace metal contents rare earth elements ree the sm nd and rb sr isotope systems and u isotopes were applied to whole rock digestions and sequentially extracted fractions of the marcellus shale and adjacent units from two locations in the appalachian basin the narrow range of end values from 7 8 to 6 4 at 390 ma is consistent with derivation of the clastic sedimentary component of the marcellus shale from a well mixed source of fluvial and eolian material of the grenville orogenic belt and indicate minimal post depositional alteration of the sm nd system while silicate minerals host 80 of the ree in the shale data from sequentially extracted fractions reflect post depositional modifications at the mineralogical scale which is not observed in whole rock ree patterns limestone units thought to have formed under open ocean oxic conditions have 238u values and ree patterns consistent with modern seawater the 238u values in whole rock shale and authigenic phases are greater than those of modern seawater and the upper crust the 238u values of reduced phases the oxidizable fraction consisting of organics and sulfide minerals are 0 6 greater than that of modern seawater bulk shale and carbonate cement extracted from the shale have similar 238u values and are greater than 238u values of adjacent limestone units we suggest these trends are due to the accumulation of chemically and more likely biologically reduced u from anoxic to euxinic bottom water as well as the influence of diagenetic reactions between pore fluids and surrounding sediment and organic matter during diagenesis and catagenesis

petrogenesis of archaean malene supracrustal rocks nw buksefjorden region west greenland geochemical evidence for highly evolved archaean crust abstract we report new data for major minor and trace elements in forty nine samples of malene supracrustal rocks nw buksefjorden region west greenland the samples consist principally of quartz cordierite biotite garnet anthophyllite gedrite sillimanite with abundant accessory zircon and minor rutile allanite and monazite plagioclase is rare to absent while muscovite occurs in only a few cases thus the malene paragneisses closely resemble cordierite orthoamphibole rocks found in other metamorphic complexes the nw buksefjorden malene paragneisses have on average high sio2 moderate al2o3 fe2o3 t and mgo and low k2o na2o and especially cao distinctive trace element characteristics include 1 low sc co cr ni and sr 2 high y zr nb hf ta th u and ree with prominent negative eu anomalies in nearly every case and 3 high ga up to 65 ppm with ga al ratios significantly higher than average crustal material on the basis of afm assemblage type the paragneisses can be divided into distinct groups which preserve the field distinctions noted by previous workers however these groupings primarily reflect variations in mg total range from 0 26 0 89 with no meaningful distinctions appearing among trace elements except possibly cu the trace element characteristics of the nw buksefjorden malene paragneisses are identical to those of the malene quartz cordierite gneisses found throughout the greater godthabsfjord region these gneisses are different from the malene quartzite psammite pelite group of clastic metasediments the trace element data of the quartz cordierite gneisses are compatible with a protolith dominated by material derived from felsic igneous precursors an interpretation supported by immobile element discriminant plots and ternary la th sc and th co hf plots however negative eu anomalies and low sr and ca suggest that plagioclase was lost from the protolith or from the precursor lithology or both the high concentrations of ga indicate either that the source rocks for the paragneisses were unusually ga rich or that assembly of the protolith was accompanied by a major fractionation of al from ga we propose that the protolith for most of the nw buksefjorden malene paragneisses was a quartz chlorite sericite schist derived from felsic volcaniclastic sediment hydrothermally altered by seawater prior to metamorphism this alteration resulted in gain of mg and possibly fe and loss of lime and alkalis and possibly sr and eu this hypothesis contrasts with earlier proposals that the protolith included mg rich clay palygorskite that precipitated directly from seawater but it is more consistent with the new data there is a remarkably close correspondence between the element enrichments and depletions shown by the malene paragneisses and those found in a type granitic rocks and their volcanic equivalents consequently we suggest a comparable source for the paragneisses the petrogenesis of a type granites is associated typically with thickened stable continental crust this implies that the protolith for the malene paragneisses formed after or in conjunction with a major event of archaean crustal thickening

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

rare earth element patterns in archean high grade metasediments and their tectonic significance abstract metasediments from two contrasting types of archean high grade terrains are interpreted as being derived from distinct tectonic settings the kapuskasing structural zone canada represents the deep roots of a typical greenstone belt whereas the limpopo province southern africa and western gneiss terrain australia mainly consist of shelf sediments deposited on a granitic basement and then buried to the depths required for granulite fades metamorphism upper amphibolite to granulite fades paragneisses from the kapuskasing structural zone have ree patterns similar to those of greenstone belt sediments except where partial melting has occurred forming restites with eu enrichment and melts with eu depletion except in this latter instance metamorphism has not affected the primary ree patterns ree patterns in archean upper amphibolite to granulite facies metasediments from the central limpopo province and western gneiss terrain show wide differences ranging from patterns which resemble those in post archean terrigenous sediments to typical archean sedimentary rock patterns the diversity in ree patterns for these shallow shelf metasediments is interpreted as resulting from derivation from local provenances samples with post archean patterns displaying eu depletion are interpreted as being derived from k rich granitic plutons which were portions of small stable early archean terrains precursors of the widespread late archean proterozoic episode of major cratonic development

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

average chemical compositions of post archean sedimentary and volcanic rocks from the qinling orogenic belt and its adjacent north china and yangtze cratons the north china and yangtze south china cratons are the most important tectonic units of china the two cratons finally coalesced through collision during the late paleozoic to middle triassic with the formation of the e w trending qinling dabie mountains we report the average compositions of 13 major 16 trace and 14 rare earth elements of post archean sedimentary arenaceous pelitic and carbonate and volcanic mafic with sio2 56 and felsic with sio2 56 rocks from the qinling orogenic belt including the north and south qinling and the adjacent southern margin of the north china craton and northern margin of the yangtze craton as well as 9 tectonostratigraphic subunits the results are based on systematic sampling and analyses of 3 100 individual rock samples taken over an area of 153 200 km2 the obtained average compositions of sedimentary and volcanic rocks of the study area generally compare well with previously published averages for corresponding rock types pb is a notable exception and is 1 5 3 times as high as the earlier estimates for sedimentary and mafic volcanic rocks also the felsic volcanic rock has a bulk composition of quartz keratophyre with na2o k2o which is in marked contrast with the rhyolitic composition for both average world wide tertiary recent orogenic and anorogenic felsic volcanic rocks although there are significant intra unit chemical variations persistent inter unit distinctions are also evident especially in major elements

heat production in an archean crustal profile and implications for heat flow and mobilization of heat producing elements we have measured concentrations of heat producing elements th u and k in 58 samples representative of the main lithologies in a 100 km transect of the superior province of the canadian shield from the michipicoten wawa greenstone belt near wawa ontario through a domal gneiss terrane of amphibolite grade to the granulite belt of the kapuskasing structural zone near foleyet this transect has been interpreted as an oblique cross section through some 25 km of crust uplifted along a major thrust fault and thus provides an opportunity to examine in detail a continuous profile into deep continental crust of archean age mean heat production values for these terranes based on aereal distribution of major rock types and calculated from their th u and k concentrations are michipicoten greenstone belt 0 72 w m 3 wawa domal gneiss terrane amphibolite grade 1 37 w m 3 kapuskasing granulites 0 44 w m 3 among the silicic plutonic rocks tonalites granites and their derivative gneisses the relatively large variation in heat production correlates with modal abundances of accessory minerals including allanite sphene zircon and apatite we interpret these variations as primary pre metamorphic the relatively high weighted mean heat production of the domal gneiss terrane can be accounted for by the larger proportion there of late stage th u and k rich granitoid plutons these may have been derived from the underlying kapuskasing granulite terrane leaving it slightly depleted in heat producing elements transport of th u and k therefore could have taken place in silicate melts rather than in aqueous or carbonic metamorphic fluids this conclusion is supported by the lack of a statistically significant difference in heat production between tonalites tonalite gneisses and mafic rocks of amphibolite versus granulite grade the pre metamorphic radioactivity profile for this crustal section is likely to have been uniformly low with a mean heat production value less than 1 w m 3 this result is distinctly different from measured profiles in more silicic terranes which show decreasing heat production with depth this implies fundamental differences in crustal radioactivity distributions between granitic and more mafic terranes and may be an important factor in selective reactivation of lithologically different terranes possibly resulting in preferential stabilization of basic terranes in the geological record our results indicate that a previously determined apparently linear heat flow heat production relationship for the kapuskasing area does not relate to the distribution of heat production with depth low but significant heat production 0 4 0 5 w m 3 continues to lower crustal depths with no correlation to the depth parameter from the linear relationship this low heat production may be a minimum average granulite heat production and suggests that in general heat flow through the moho is 8 10 mw m 2 lower than the reduced heat flow calculated from the heat flow heat production regression

archaean metasedimentary rocks from the northwestern pontiac subprovince of the canadian shield chemical characterization weathering and modelling of the source areas abstract the northwestern pontiac subprovince is dominated by a monotonous greywacke sequence metamorphosed and deformed during the late archaean kenoran orogeny limited chemical weathering of the source rocks is suggested by 1 low values of the chemical index of alteration 2 no preferential leaching of ca sr and ba 3 well defined correlations among mgo k2o and al2o3 and 4 small variations of cr ni the source was dominantly felsic as indicated by high th sc average 0 39 la co 1 06 zr cr 0 7 zr y 11 2 and sio2 mgo 19 4 the composition of these metasedimentary rocks can be modelled by mixing of three end member components 1 tonalites trondhjemites 65 in order to explain the sio2 al2o3 th u zr sr and ba abundances na k ratios strong depletion in hree and hfse relative to lree and lile low th y and k2o and fractionated la yb 19 4 2 komatiites 5 in order to explain the high cr 220 ppm and ni 66 ppm contents and 3 tholeiitic basalts 30 accounting for the observed tio2 0 62 wt v 144 ppm and sc 17 ppm concentrations the source region may be considered as a deeply eroded greenstone belt intruded by abundant tonalite trondhjemite plutons possibly now located to the south and east in the grenville province or possibly to the north underlying the dominantly volcanic abitibi subprovince