petrogenesis and tectonic setting of late precambrian ensimatic volcanic rocks central eastern desert of egypt abstract early stages in the geologic evolution of the central eastern desert of egypt ced reflect an intense episode of ensimatic volcanic activity similar to modern magmatism of the ocean floors and island arcs this paper reports results from studies of the petrology and petrogenesis and interprets the significance of these late precambrian volcanic rocks a three fold stratigraphy is preserved in the basement of the ced a basal section of oceanic crust includes ultramafics gabbros and pillowed basalts these older metavolcanics omv are conformably succeeded by dominantly volcanogenic metasediments which are in turn succeeded by a dominantly andesitic calc alkaline sequence of younger metavolcanics ymv the omv and ymv are largely restricted to the ced in egypt but analogous terranes are found in northern arabia 40 400 ppm and ni 30 260 ppm they are poor in k 2 o 0 05 0 92 rb 0 3 5 0 ppm and ba 11 89 ppm on ti zr cr v ni p discriminant diagrams the omv plot in the field of modern abyssal tholeiites high k rb 450 1800 and light ree depletions support this inference although k ba 25 45 is lower than modern mid ocean ridge basalts morb the sum of omv geochemical characteristics requires that these magmas were derived by the fractional fusion of the mantle it is suggested that the omv were generated by 20 25 fractional melting of previously depleted mantle at depths of less than 60 km relatively little fractionation accompanied ascent to the surface where the omv were erupted in a primitive crustal environment either a small oceanic rift or a back arc basin metamorphism of the ymv resulted in little elemental redistribution these andesites have sub alkaline clinopyroxenes and major element geochemical characteristics indistinguishable from modern calc alkaline andesites ymv andesites in the central and western ced have k rb 400 600 k ba 20 40 and are light ree enriched and heavy ree depleted high concentrations of cr 50 150 ppm and ni 20 100 ppm and low initial 87 sr 86 sr ratios 0 7028 0 7030 indicate that these magmas were generated by melting in the mantle modelling studies and consideration of experimental data indicate that these andesites were formed by 2 10 fractional fusion of hydrous undepleted garnet therzolite at depths of 65 km or more in the mantle the data show that an intense episode of instability convection and widespread melting occurred in the mantle beneath afro arabia at the end of the precambrian

late proterozoic lavas of the central arabian shield evolution of an ancient volcanic arc system a petrological assessment of 11 volcano sedimentary belts in the central arabian shield cas using 118 new analyses is presented the same samples have been used in a rb sr dating programme also reported in this volume the oldest volcano sedimentary belts sequence c 900 ma are chemically immature bimodal suites of low k tholeiites and sodic dacite rhyolite depleted in lithophile elements these lavas have chemical characteristics similar to immature island arcs such as the tonga kermadee and lesser antilles arcs younger lavas of sequence b 900 700 ma are slightly more mature in composition being predominantly calc alkaline and low k arc tholeiite series with higher lithophile element contents they are comparable with pacific island are averages the youngest voluminous lavas sequence a 700 570 ma are generally calc alkaline or high k calc alkaline series lavas with moderately high lithophile element abundances they are comparable to volcanic arcs such as central america and indonesia which are transitional between island arcs and continental margin volcanic arcs the cratonization process in the cas thus involved the superimposition of three volcano sedimentary sequences formed in progressively maturing volcanic arcs early formed immature island arcs were succeeded by chemically more mature island arc deposits and then by deposits formed in volcanic arc s transitional between island arcs and continental margins the evolution ceased at this stage and probably did not advance to the equivalent of an andean continental margin

age determinations in the precambrian basement of the wadi araba area southwest jordan abstract the precambrian basement of jordan belongs to the northern margin of the arabian nubian shield age determinations by u pb isotopic analyses on sized and magnetic zircon fractions a monazite and an apatite sample and by rb sr isotopic studies on whole rocks and minerals of metasedimentary rocks granodiorites granites and dykes have elucidated the following events 1 a major regional high grade metamorphism accompanied by migmatization and synkinematic plutonism occurred at about 800 ma according to u pb zircon ages of metasediments and granites 2 during a postkinematic plutonic event between 615 and 600 ma extensive masses of granodioritic to granitic composition and dykes were emplaced the u pb data of zircons of the rocks yielded upper intercept ages with the concordia consistent with rb sr biotite ages the rb sr mineral ages of the older metasedimentary rocks document the resetting of the rb sr system due to the thermal pulse at this time 3 a younger plutonic event produced diorites and dykes at about 570 ma the plutonic events are related to the pan african orogenic phase the low initial87sr 86sr ratios of the plutonic rocks 0 7032 0 7046 correspond to values reported from equivalent rocks throughout the arabian nubian shield and suggest that no significant portions of ancient sialic crustal material contributed to the generation of the granitic to granodioritic magmas

precipitation at lake qinghai ne qinghai tibet plateau and its relation to asian summer monsoons on decadal interdecadal scales during the past 500 years knowledge of the variability of precipitation at lake qinghai and its relation to the asian summer monsoons is helpful in constraining global climatic dynamics based on the high resolution precipitation indicators of 13 c of the organic matter 13 corg c n atomic ratio and the detrended total organic carbon content tocdetrended we found that the trend of precipitation at lake qinghai is inversely correlated to that of the indian summer monsoon ism inferred from layer thickness of a stalagmite s3 in southern oman on decadal interdecadal scales the chinese drought flood d f indices which can indicate the dryness wetness over large geographic areas are also used to indicate the intensity of the monsoon rainfall the d f index of xining near lake qinghai is synchronous with those of the regions in northern china where the east asian summer monsoon easm dominates while it is anti phase with those of southwestern china where ism prevails these materials suggest that during the past 500 years the source of moisture to lake qinghai on decadal interdecadal scales is controlled mainly by the easm but not by the ism it is also suggested that the intensity of easm is inversely related to that of the ism on decadal interdecadal scales the decadal interdecadal variability of enso may be responsible for the inverse relationship between the intensity of easm and that of ism 2007 elsevier b v all rights reserved

insolation values for the climate of the last 10 million years new values for the astronomical parameters of the earth s orbit and rotation eccentricity obliquity and precession are proposed for paleoclimatic research related to the late miocene the pliocene and the quaternary they have been obtained from a numerical solution of the lagrangian system of the planetary point masses and from an analytical solution of the poisson equations of the earth moon system the analytical expansion allows the direct determination of the main frequencies with their phase and amplitude numerical and analytical comparisons with the former astronomical solution ber78 are performed so that the accuracy and the interval of time over which the new solution is valid can be estimated the corresponding insolation values have also been computed this analysis leads to the conclusion that the new values are expected to be reliable over the last 5 ma in the time domain and at least over the last 10 ma in the frequency domain

geochemistry of pan african volcanic arc sequences in southeastern sinai peninsula and plate tectonic implications abstract major and trace elements of lavas dykes and plutonic rocks of the late proterozoic orogenic sequence in se sinai the kid group have been analysed the dykes and lavas of the southermost sequences the tarr complex and heib formation are calc alkaline whereas the lavas of the malhak formation and the sharira gabbro to the north of the heib formation show both calc alkaline and tholeiitic trends the trace element characteristics of the tarr complex and the heib and malhak formations despite between sequence variations are all comparable with ensialic island arc magmatism whereas the sharira gabbro shows some morb characteristics in addition to the island arc imprints the sharira gabbro and the lavas of the malhak formation possibly formed in a developing back arc basin behind a continental marginal ensialic island arc the tarr complex and heib formation during the pan african orogeny the constituent units of the kid group were mutually juxtaposed along major ductile shear zones of thrust fault character this plate convergence regime involved initial magmatic arc development following northward subduction and subsequent collision between the arc complex and the proterozoic continental margin

last deglaciation in the okinawa trough subtropical northwest pacific link to northern hemisphere and tropical climate 1 detailed deglacial and holocene records of planktonic delta o 18 and mg ca based sea surface temperature sst from the okinawa trough suggest that at similar to 18 to 17 thousand years before present kyr b p late spring early summer ssts were approximately 3 degrees c cooler than today while surface waters were up to 1 practical salinity unit saltier these conditions are consistent with a weaker influence of the summer east asian monsoon eam than today the timing of suborbital sst oscillations suggests a close link with abrupt changes in the eam and north atlantic climate a tropical influence however may have resulted in subtle decoupling between the north atlantic and the okinawa trough eam during the deglaciation okinawa trough surface water trends in the holocene are consistent with model simulations of an inland shift of intense eam precipitation during the middle holocene millennial scale alternations between relatively warm salty conditions and relatively cold fresh conditions suggest varying influence of the kuroshio during the holocene

last glacial holocene change in intermediate water ventilation in the northwestern pacific abstract the modern north pacific is the terminus for deep water circulation while no deep water mass is formed today the region is the source of north pacific intermediate water 300 800 m formed primarily in the sea of okhotsk however the source and strength of intermediate water in the glacial north pacific was likely different from the present intermediate water here we present microfossil evidence of late quaternary ventilation changes associated with changes in intermediate water intermediate water ventilation decreased drastically at the glacial termination when it switched to the modern circulation system the ventilation change appears to have been widespread and synchronous in the north pacific glacial microfossil assemblages indicate that there was relatively strong ventilation of intermediate water which seems to have been cold and well oxygenated as if produced by brine rejection during the formation of sea ice on the basis of the glacial distribution of microfossils in the north pacific we suggest that intermediate water was derived from the bering sea during the glacials rather than the sea of okhotsk

geochronology of the arabian nubian shield in southern israel and eastern sinai the metamorphic and igneous orogenic basement rocks of southeastern israel and eastern sinai evolved as part of the arabian nubian shield during latest proterozoic time pelitic schist of the elat association from wadi nahal shlomo south of eilat israel is dated at about 800 m y but the geologic significance of this date is not clear in southeastern sinai metapelitic rocks of the kid group have a date of diagenesis or metamorphism of about 615 m y syn orogenic and post orogenic plutons which intrude the layered metamorphic complex s are dated at 600 m y near eilat and 590 m y at wadi kid other dated plutons include the timna granite at 592 7 m y in the area of nevi ot at 595 m y and in the vicinity of dahab at 588 2 m y and 595 0 3 m y the radiometric dates and low initial 87 sr 86 sr ratios 0 7034 imply that pre 1000 m y continental sialic crust was not involved in the formation of the shield in southeast israel and eastern sinai

lagged response of summer precipitation to insolation forcing on the northeastern tibetan plateau during the holocene the precipitation changes on the northeastern tibetan plateau during the holocene remain unclear due to discrepancies among different proxy records we compared proxy records with the results from a transient simulation performed using the kiel climate model forced by orbital variations to analyse summer precipitation changes in this area during the holocene 9 5 0 ka bp the model results suggested increasing amounts of summer precipitation from 9 5 to 6 2 ka bp and a persistent decline thereafter which matched well with pollen records but was inconsistent with ostracod 18o records the holocene climatic optimum lagged the northern hemisphere summer insolation maximum by 3 5 ka caused by the interplay between the east asian summer monsoon easm circulation and the mid latitude westerlies the ostracod 18o values reflected the strength of the easm circulation a strong easm circulation increased the transport of water vapour towards the northeastern tibetan plateau from the northwestern pacific weakened mid latitude westerlies increased the incursion of cold air masses into the northeastern tibetan plateau during the early holocene relatively strong mid latitude westerlies resulting from an enhanced arctic oscillation reduced summer precipitation on the northeastern tibetan plateau in spite of a strong insolation driven easm circulation the weakening easm circulation and the strengthening westerlies together induced the decreasing trend of summer precipitation from the middle to late holocene in addition summer precipitation variations were further modulated by sea surface temperatures in the northwestern pacific through weakening the strength of the easm

rare earth element evidence concerning the origin of voluminous mid tertiary rhyolitic ignimbrites and related volcanic rocks sierra madre occidental chihuahua mexico abstract the mid tertiary volcanic sequence of the central sierra madre occidental in chihuahua mexico is about one kilometer thick and is composed predominantly of rhyolitic ignimbrites basaltic andesite to dacitic lavas are interbedded with the rhyolites but they are of minor volumetric importance rare earth element ree data are used to constrain a crustal anatexis model for the origin of the voluminous ignimbrites and to test a fractional crystallization model the ree patterns indicate that if the rhyolites were formed by direct crustal anatexis the residue from partial melting could contain no more than a few percent garnet or about 20 hornblende this eliminates residues with the mineralogy of amphibolite eclogite or garnet granulite but melting of a garnet free granulite source is permitted the crustal anatexis model is difficult to evaluate critically because of a lack of knowledge concerning the mid tertiary geothermal gradient and the composition of the crust beneath the sierra madre occidental in contrast the fractional crystallization model can be tested rigorously rayleigh fractionation calculations are used to closely model ree patterns in the basaltic andesite to rhyolite series the minerals involved are those occurring as phenocryst phases and the mineral proportions were generated by leastsquares major element calculations the results of the calculations are consistent with the hypothesis that the voluminous rhyolites originated by plagioclase dominated crystal fractionation

the spatial temporal patterns of asian summer monsoon precipitation in response to holocene insolation change a model data synthesis highlights slice and transient simulations of holocene climate change were performed spatial temporal patterns of holocene asian summer precipitation are investigated a tripole pattern of summer precipitation can be seen over monsoonal asia insolation change is a key factor for holocene asian summer monsoon change internal feedbacks are important to holocene asian summer precipitation changes abstract paleoclimate proxy records of precipitation effective moisture show spatial temporal inhomogeneous over asian monsoon and monsoon marginal regions during the holocene to investigate the spatial differences and diverging temporal evolution over monsoonal asia and monsoon marginal regions we conduct a series of numerical experiments with an atmosphere ocean sea ice coupled climate model the kiel climate model kcm for the period of holocene from 9 5 ka bp to present 0 ka bp the simulations include two time slice equilibrium experiments for early holocene 9 5 ka bp and present day 0 ka bp respectively and one transient simulation ht using a scheme for model acceleration regarding to the earth s orbitally driven insolation forcing for the whole period of holocene from 9 5 to 0 ka bp the simulated summer precipitation in the equilibrium experiments shows a tripole pattern over monsoonal asia as depicted by the first modes of empirical orthogonal function eof1 of h0k and h9k the transient simulation ht exhibits a wave train pattern in the summer precipitation across the asian monsoon region associated with a gradually decreased trend in the strength of asian summer monsoon as a result of the response of asian summer monsoon system to the holocene orbitally forced insolation change both the synthesis of multi proxy records and model experiments confirm the regional dissimilarity of the holocene optimum precipitation effective moisture over the east asian summer monsoon region monsoon marginal region and the westerly dominated areas suggesting the complex response of the regional climate systems to holocene insolation change in association with the internal feedbacks within climate system such as the air sea interactions associated with the el nino southern oscillation enso and shift of the intertropical convergence zone itcz in the evolution of asian summer monsoon during the holocene

two years at meridiani planum results from the opportunity rover the mars exploration rover opportunity has spent more than 2 years exploring meridiani planum traveling 8 kilometers and detecting features that reveal ancient environmental conditions these include well developed festoon trough cross lamination formed in flowing liquid water strata with smaller and more abundant hematite rich concretions than those seen previously possible relict hopper crystals that might reflect the formation of halite thick weathering rinds on rock surfaces resistant fracture fills and networks of polygonal fractures likely caused by dehydration of sulfate salts chemical variations with depth show that the siliciclastic fraction of outcrop rock has undergone substantial chemical alteration from a precursor basaltic composition observations from microscopic to orbital scales indicate that ancient meridiani once had abundant acidic groundwater arid and oxidizing surface conditions and occasional liquid flow on the surface

palaeoclimatic changes in the qinghai lake area during the last 18 000 years abstract multi proxy analysis of a sediment core from qinghai lake including pollen carbonate toc tn and 13 c of organic matter was used to document regional climatic changes during the last 18 000 years climate was very cold and dry before 16 9 cal kyr bp the warm and wet period which began at about 14 1 cal kyr bp culminated at 6 5 cal kyr bp and came to its end at about 4 5 cal kyr bp after that the climate gradually became colder and drier during the transitional period from the late glacial to the holocene the climate fluctuated frequently the short term climatic oscillations including the bolling allerod younger dryas and the cooling event at ca 8 2 cal kyr bp appear to be recorded in the qinghai lake sedimentary record the climatic evolution in the qinghai lake area during the past 18 000 years indicates that solar insolation changes on the ten thousand year scale are the likely driving force of the east asia monsoon

the origin of reversed geochemical zoning in the northern new hebrides volcanic arc integration of petrographic and geochemical data on each island in the banks group northern vanuatu new hebrides has revealed a decrease in k2o and related incompatible elements across the islands away from the new hebrides trench the correlation with depth to the benioff zone is therefore the reverse of that typically found in island arcs ree evidence and source modelling indicates that this variation represents a progressive depletion in lree hree in the upper mantle laterally away from the trench and a progressive increase in partial melting of the source in the same direction these variations in chemistry are attributed to an earlier west dipping miocene subduction system and the variation in degree of partial melting to the location of the islands with respect to the active back arc basin an enrichment of the upper mantle in the miocene is ascribed to the ascent of hydrous fluids enriched in incompatible elements from the west dipping benioff zone and subsequent reaction with the overlying upper mantle the model thus envisages approximately contemporaneous development of the central chain volcanoes and the backarc basin in the late pliocene with partial melting of hydrous laterally zoned upper mantle resulting from the convective thermal regime associated with the rifting apart of the back arc basin the role of the present subduction regime in magma generation is considered to be very limited

pan african structures ophiolites and melange in the eastem desert of egypt a traverse at 26 n a detailed structural traverse across the basement rocks of the eastern desert of egypt shows that they consist apart from intrusions of four broadly recumbent tectonic units the lowest of arkosic metasediments of continental shelf facies is exposed in a dome this unit is overlain by an allochthonous ophiolitic melange containing complete and dismembered ophiolitic masses in a matrix of deep oceanic graphitic pelites and turbidites a near horizontal schistose to mylonitic fabric most intense near and below the base of the melange indicates extensive lateral tectonic transport of the ophiolitic melange over the shelf metasediments but the melange originated as an olistostrome before being tectonically transported the melange extends over an area of at least 10 000 km 2 it is locally overlain by calc alkaline volcanics in which deformation is less intense but increases and schistosity flattens downwards indicating some translation over the melange unconformable on the melange and calc alkaline volcanics is a molasse facies series itself also locally strongly deformed late tectonic granites preceded and locally post date the molasse facies sediments still later diapiric peralkaline riebeckite granites locally up domed the recumbent structures the tectonic evolution is related to a late proterozoic subduction zone to the se of the region

the common origin and alteration history of the hypabyssal and volcanic phases of the wadi tarr albitite complex southern sinai egypt new data and interpretations are presented for the igneous albitites of the wadi tarr area southern sinai egypt the albitite masses are isolated in outcrop from any granitic intrusions and have intrusive contacts against the country rocks without any structural control they have marginal zones of breccias with jigsaw fit angular clasts suggesting explosive in situ formation the albitites are of two types the western medium grained hypabyssal albitite and the eastern fine grained porphyritic albitite the field relations suggest emplacement at different levels in a magmatic cupola the hypabyssal texture and steeply dipping slope of the upper contact of the western albitite imply deeper emplacement whereas the gently dipping contacts and porphyritic texture of the eastern albitite masses indicate that they define the probable location of the cupola apex both types of albitites consist of albite 92 97 with minor amounts of quartz k feldspar and biotite the accessory minerals include fe oxides augite sulphides zircon rutile xenotime titanite allanite and monazite the whole rock compositions of the hypabyssal and porphyritic albitites are closely related but the porphyritic type has lower abundances of sr ba y nb th and zr we show that the hypabyssal and porphyritic albitites have a common petrogenetic origin most likely as late stage cumulates from a fractionating strongly alkaline a type magma consistent with the compositions of the mafic minerals the source magma was probably a tephritic liquid we use melts models to show that only a sufficiently alkaline magma follows a differentiation path that both avoids quartz saturation and encounters the alkali feldspar solvus reaching a residual liquid in equilibrium with highly sodic feldspar although the melts results show a chemically consistent means of forming igneous albitite they are incomplete in that physical segregation mechanisms are still required to isolate the albite from mafic minerals and or a low temperature aqueous alteration stage is needed to leach k from the feldspar alteration surrounding the wadi tarr albitites is extensive and dominated by alkali metasomatism similar to fenitization alteration in the marginal breccia zone of the albitite is dominated by precipitation of amphibole and carbonate in veins and in the breccia matrix whereas the volcanic country rocks show replacement of feldspars by sericite carbonate and epidote as well as vein carbonate the altered volcanic country rocks show lower concentrations of fe 2o 3 sr cu pb ba and ce accompanied by higher concentrations of na2o and mgo compared to unaltered equivalent samples

evaluation of oxygen isotopes in carbonate as an indicator of lake evolution in arid areas the modern qinghai lake qinghai tibet plateau abstract the oxygen isotopic composition of carbonate in lakes has been used as a useful indicator in palaeolimnological research and has made some important contributions to our understanding of lacustrine systems for modern lakes in arid or cold areas however there are few data available to test the effect of lake salinity and temperature on the oxygen isotopic composition of various carbonate sources such as ostracod bulk carbonate and fine grained carbonate for arid high altitude lake qinghai our results suggest that variations in the 18o values of carbonate in lake qinghai are mainly controlled by the oxygen isotope ratio of the lake water changing with water salinity as a secondary effect increasing water depth leads to cooler bottom and surface water which may result in more positive 18o values of ostracod and bulk carbonate

water depth affecting thaumarchaeol production in lake qinghai northeastern qinghai tibetan plateau implications for paleo lake levels and paleoclimate abstract archaeal glycerol dialkyl glycerol tetraethers gdgts are increasingly popular and versatile tool for palaeolimnology studies but their applications in paleohydrology are scarce especially for thaumarchaeol which is specific for the newly proposed phylum thaumarchaeota after investigating our published gdgt data of lake qinghai we found that both the concentration of thaumarchaeol and the relative abundance of thaumarchaeol to total archaeal gdgts thaum in core top sediments increased significantly with increasing water depth r 0 88 and 0 95 respectively with lower concentrations of 5 5 ng g in shallow areas water depth 10 m this is likely because that the producers of thaumarchaeol thaumarchaeota prefer living in the relative deeper zone in lacustrine systems where probably both competition of ammonium the substrate from other microbes and light intensity are low therefore we proposed that thaumarchaeol was mainly produced in situ and changes in thaum might reflect water depth variations in this closed basin lake the application of thaum as a water depth indicator in a holocene sediment sequence of core qh 2011 provided a high resolution relative lake level history of lake qinghai which resembles that inferred from the 13corg value obtained in the same core this supports the use of thaum as an indicator of lake water depth in paleohydrology studies especially for medium lakes moreover the records of the two independent proxies in core qh 2011 confirmed a shallow lake qinghai in the early holocene and a late holocene highstand highlighting the importance of local temperature and evaporation loss in controlling effective moisture in the arid semi arid region

geochemical characterization of pan african dyke swarms in southern sinai from continental margin to intraplate magmatism abstract the chemical and mineralogical features of three pan african dyke swarms occurring in the wadi feiran area southern sinai have been examined two swarms striking approx 35 exhibit calcalkaline chemical properties the oldest s1 can be related to calcalkaline rocks from active continental margins by means of incompatible element plots displaying negative nb ta anomalies they dykes were emplaced synchronously with post orogenic alkaline granites to which they cannot be assigned chemically a second dyke generation l1 with the same strike shows apparent similarities with continental flood basalts e g columbia river basalts indicating a growing contribution of continental components to melt chemistry subduction related features are displayed by negative nb ta anomalies the youngest dyke swarm s2 striking approx 135 and crosscutting s1 displays transitional to mildly alkaline properties without negative nb ta anomalies its chemistry is fully consistent with intraplate basalts of continental areas the source regions of the three swarms cannot be related via assimilation of crustal material because decreasing lile percentages coupled with increasing amounts of nb and ta cannot be explained by this means the overlap of m values of rocks to be found with the three swarms indicates the unlikeliness of a development of the three magmas via fractional crystallization the emplacement of two totally different types of dykes in the wadi el sheikh area requires a source region which is either chemically inhomogeneous in a more or less vertical scale or changing its composition and melting conditions with time although the dykes were emplaced in an extensional tectonic setting the subcrustal lithosphere still produced calcalkaline melts documenting its subduction related magmatic history

evolution of the kid group southeast sinai peninsula thrusts melanges and implications for accretionary tectonics during the late proterozoic of the arabian nubian shield the late proterozoic age kid group in the southeast sinai peninsula is a volcano sedimentary sequence of island arc affinities the group deformed in four main phases with a thermal peak after d1 followed by a regional greenschist facies overprint during d2 recumbent folding followed by thrusting began during d2 with the folds being propagated southward toward the arc foreland until d4 the last folding event the thrusts are responsible for the juxtaposition of four megatectonic units above a substrate of unknown composition these units probably allochthonous comprise a basal complex of infracrustal gneiss and three arc related units including inferred vestiges of a forearc accretionary prism and a marginal basin narrow slices of ophiolitic rocks were emplaced tectonically along the boundaries of some of these units melanges attributed to gravitational collapse of the forearc edifice and thrust tectonics formed in particular along the arc forearc transition zone the evolution of the kid group coupled with data from the rest of the arabian nubian shield can be well accommodated within the concept of oceanic crust subduction magmatic arc development and ultimate collision between the arc complex and a continental margin

holocene east asian summer monsoon records in northern china and their inconsistency with chinese stalagmite δ18o records abstract monsoon precipitation over china exhibits large spatial differences it has been found that a significantly enhanced east asian summer monsoon easm is characterized by increased rainfall in northern china and by reduced rainfall in southern china and this relationship occurs on different time scales during the holocene this study presents results from a diverse range of proxy paleoclimatic records from northern china where precipitation variability is traditionally considered as an easm proxy our aim is to evaluate the evolution of the easm during the holocene and to compare it with all of the published stalagmite 18 o records from the asian monsoon region in order to explore the potential mechanism s controlling the chinese stalagmite 18 o we found that the intensity of the easm during the holocene recorded by the traditional easm proxy of moisture or precipitation records from northern china is significantly different from the chinese stalagmite 18 o records the easm maximum occurred during the mid holocene challenging the prevailing view of an early holocene easm maximum mainly inferred from stalagmite 18 o records in eastern china in addition all of the well dated holocene stalagmite 18 o records covering a broad geographical region exhibit a remarkably similar trend of variation and are statistically well correlated on different time scales thus indicating a common signal however in contrast with the clear consistency in the 18 o values in all of the cave records both instrumental and paleoclimatic records exhibit significant spatial variations in rainfall on decadal to centennial time scales over eastern china in addition both paleoclimatic records and modeling results suggest that holocene east asian summer monsoon precipitation reached a maximum at different periods in different regions of china thus the stalagmite 18 o records from the easm region should not be regarded as a reliable indicator of the strength of the east asian summer monsoon furthermore modern observations indicate that the moisture for precipitation in the east asian monsoon region is mainly derived from the indian ocean the moisture transport route from the indian ocean to the east asian monsoon region during the holocene is almost identical to that of modern precipitation therefore the strong correlation of 18 o records in the easm and indian summer monsoon ism regions and the similarity of the pattern of latitudinal changes in 18 o values in stalagmites and in modern meteoric precipitation along the water vapor transport route further suggests that the stalagmite 18 o records from the easm region are essentially a signal of the isotopic composition of precipitation which is determined mainly by changes in the 18 o of atmospheric vapor in the upstream source region over the indian ocean and indian monsoon region via the upstream depletion mechanism finally the main trends of the stalagmite 18 o records are strongly correlated with the known evolution of the ism and therefore these records reflect the history of the ism rather than that of the easm our findings support the conclusion that easm variability is mainly controlled by northern hemisphere summer insolation and was strongly modulated by ice volume during the last deglaciation and early holocene which delayed the response of the easm maximum to peak insolation forcing

total organic carbon isotopes a novel proxy of lake level from lake qinghai in the qinghai tibet plateau china abstract the isotopic compositions of total organic carbon toc in lakes have been widely used to interpret paleoclimatic changes and the depositional environments of lake sediments however the main factors that affect the carbon isotopes of toc source of organic material water condition and others may vary in different lake sediment records which have limited the applicability of organic carbon isotopes in explaining biogeochemical and environmental changes in lakes in this study the organic carbon isotopic compositions of aquatic plants and surface sediments from lake qinghai and the living terrestrial plants and surface soils around the lake were systematically investigated to identify the sources of toc in the sediments and the significance of the organic carbon isotopes of sedimentary toc we found that the aquatic plants in the deep water areas 10 m were primarily dominated by cladophora but submerged plants potamogeton and ruppia l are the dominant species in shallow water 13 c org values 33 6 to 28 6 that are caused by c 3 like photosynthesis but the submerged plants have enriched 13 c org values 17 8 to 15 4 that are caused by c 4 like photosynthesis in addition the 13 c org values of cladophora become more negative with increasing water depth because of the slow photosynthetic rate caused by the weak light intensity at depth the isotopic data indicate that the carbon isotopes of organic material in the surface sediments are primarily controlled by the types of aquatic plant and that the 13 c org values of toc can be used to indicate the variation of the water depth lake level the organic carbon isotopic data from the 1f core showed that the water was shallow

geochemical and sr nd pb isotopic data bearing on the origin of pan africangranitoids in the kid area southeast sinai egypt the kid plutonic suite in southeast sinai egypt is composed of 1 a gabbro diorite complex 2 quartz monzodiorite 3 granodiorite monzogranite association 4 syenogranite major and traceelement analyses and radiogenic isotope data rb sr sm nd and pb pb are presented for these rocks rb sr isochrons indicate ages of 581 11 ma 576 11 ma and 570 4 ma for the quartz monzodiorite granodiorite monzogranite association and syenogranite respectively the gabbro diorite complex is a layered intrusion varying in sio2 from 42 to 59 wt it shows a calc alkaline trend and the trace element characteristics of arc lavas subsequently this magma fractionated to give quartz monzodiorite andsyenogranite magmas the quartz monzodiorite has low initial 87 sr 86 sr ratios 0 703 and high nd values 4 4 0 3 with trace element characteristics of volcanic arc granites the granodiorite monzograniteassociation has higher initial sr ratio 0 7044 lower nd 1 5 2 7 and relatively high 207 pb 204 pb and 208 pb 204 pb significantly different from those of the older rocks isotopic data and ree modelling suggestthat it formed by partial melting of an early pan african amphibolitic lower crust the syenogranite isa typical highly fractionated post orogenic granite with a deep negative eu anomaly hfse and reeenrichment a 87 sr 86 sr initial ratio of 0 7033 nd of 4 5 0 5 and distinct variations in the 206 pb 204 pbratio the isotopic compositions of sr nd and pb indicate that the syenogranite and the quartzmonzodiorite are comagmatic increasing fluorine concentration from the quartz monzodiorite to thesyenogranite suggests that fluid fractionation involving fluorine complexing played an important role inthe genesis of the syenogranite this process led to the enrichment of hfse and hree to extensivefractional crystallization to changes in the u pb ratios and a distinct variation in 206 pb 204 pb

tectonic magmatic stages of shield evolution the pan african belt in northeastern egypt abstract the arabian nubian shield illustrates an example of plate tectonics during the pan african orogenic event ca 950 550 ma it is considered one of the most remarkable proterozoic shield areas yet known although there is now a general agreement that the shield has evolved by a series of magmatic arcs and terrain accretion the evolution of its northernmost part exposed in northeastern egypt remains controversial two contrasting tectonic models have been proposed for the evolution of this region regional crustal extension and magmatic arc regimes there is no convincing evidence of older archean sialic materials within the region although the region is flanked by archean crust the neoproterozoic crustal components exposed in northeastern egypt preserve a record for its evolution a large gabbro diorite tonalite complex gdt 881 ma emplaced during the early stage of the pan african orogeny is geochemically primitive exhibits a low initial 87 sr 86 sr ratio 0 704 and shows trace element characteristics of oceanic arc related lavas the gdt complex was produced by fractionation from a mantle derived tholeiitic magma formed within an island arc tectonic environment a synorogenic stage characterized by extensive volcanism that produced the dokhan volcanic suite dvs 620 ma forms a continuous basalt to rhyolitic ignimbrite calc alkaline magma series exhibiting features of arc related volcanism with ree profiles similar overall to andean andesites and ignimbrites the dokhan suite was produced in a continental arc setting a granodiorite adamellite leucogranite composite batholith 552 ma emplaced during the late stage of the pan african orogeny exhibits typical features of i type complexes trace element traits of volcanic arc granites and is also interpreted to have been formed in an andean type setting a trondhjemite pluton was also formed at this late orogenic stage by partial melting of gdt host rocks at depth cooling and relaxation of the newly formed pan african crust caused extensive fracturing which was followed by intrusion of ne sw and nnw sse trending dyke swarms at 493 ma this episode marks a fundamental inversion from orogenic compressional to anorogenic extensional processes around the neoproterozoic paleozoic boundary anorogenic magmatism was locally associated with such extension induced structures the mount gharib peralkaline granite 476 ma for example exhibits trace element traits of a type within plate granites and a high initial 87 sr 86 sr ratio 0 711 and was formed in a rift environment thus the region is characterized by lengthy episodes of discontinuous subduction 880 550 ma during which it evolved to form a primitive crust by early pan african island arcs and late pan african continental arcs this was followed by an even longer period of crustal extension 550 90 ma that produced localized anorogenic magmatism

petrogenesis of oceanic andesites the andesites and cogenetic rocks of the intraoceanic island arc systems vary in silica content from 49 50 sio2 to about 76 sio2 the most primitive compositions having contents of about 50 sio2 are the basaltic andesites which are primary compositions generated by partial melting of the uppermost section of the subducted oceanic crust the oceanic andesites cannot have assimilated material from the continental crusts but their petrogenesis bears on the origin of the continental crust the fractionation of andesitic magmas occurs at relatively low pressures within the stability range of plagioclase basaltic andesites fractionate mainly orthopyroxene plagioclase while andesites fractionate clinopyroxene orthopyroxene plagioclase the petrogenesis of the continental andesites is more complex as the primary andesites contain sedimentary material from the continental crusts