geochemical characteristics and hydrocarbon generation modeling of the jurassic source rocks in the shoushan basin north western desert egypt abstract the shoushan basin is an important hydrocarbon province in the western desert egypt but the origin of the hydrocarbons is not fully understood in this study organic matter content type and maturity of the jurassic source rocks exposed in the shoushan basin have been evaluated and integrated with the results of basin modeling to improve our understanding of burial history and timing of hydrocarbon generation the jurassic source rock succession comprises the ras qattara and khatatba formations which are composed mainly of shales and sandstones with coal seams the toc contents are high and reached a maximum up to 50 the toc values of the ras qattara formation range from 2 to 54 wt while khatatba formation has toc values in the range 1 47 wt the ras qattara and khatatba formations have hi values ranging from 90 to 261 mghc gtoc suggesting types ii iii and iii kerogen vitrinite reflectance values range between 0 79 and 1 12 vrr rock eval t max values in the range 438 458 c indicate a thermal maturity level sufficient for hydrocarbon generation thermal and burial history models indicate that the jurassic source rocks entered the mature to late mature stage for hydrocarbon generation in the late cretaceous to tertiary hydrocarbon generation began in the late cretaceous and maximum rates of oil with significant gas have been generated during the early tertiary paleogene the peak gas generation occurred during the late tertiary neogene

element geochemistry and neodymium isotope systematics of the neoarchean banded iron formations in the qingyuan greenstone belt north china craton abstract pure marine chemical sediments such as banded iron formations bif are excellent archives of ancient seawater and may also provide constraints on the coeval hydrosphere atmosphere system however on the bases of interpreted depositional settings it has long been argued that algoma type bif merely recorded the intrabasinal volcanic or hydrothermal conditions here we couple systematic major and trace elements data with samarium sm neodymium nd isotopic analyses of 2 56 ga old algoma type bifs and associated schists from different stratigraphic horizons of the qingyuan greenstone belt qgb in the north china craton ncc to characterize the source and depositional environment over a short period 30 ma meanwhile we also compare our results with data for other archean to paleoproterozoic superior and algoma type bifs especially those near contemporaneous counterparts in the ncc to further evaluate the reliability of qgb bifs as proxies for the neoarchean seawater low al2o3

zircon u pb age hf isotopes and geochemistry of shuichang algoma type banded iron formation north china craton constraints on the ore forming age and tectonic setting abstract the shuichang iron deposit is an algoma type bif in the north china craton the shuichang bif is interlayered with plagioclase hornblende gneiss indicating that these two lithologies are temporally closely associated we use the content of major elements and their ratios in order to define the protolith of the plagioclase hornblende gneiss sims u pb dating of zircons from the plagioclase hornblende gneiss suggests that the shuichang bif was formed at ca 2547 7 ma and was subjected to metamorphism at 2513 4 ma the major elements and ree data of the ore as well as individual magnetite and chert bands indicate that the shuichang bif precipitated from hydrothermal fluids discharging on the sea floor and that iron and silica may be associated with submarine volcanic activity in situ lu hf analysis of zircons from the plagioclase hornblende gneiss yielded initial 176 hf 177 hf ratios from 0 280903 to 0 281197 and e hf t values from 10 5 to 0 4 with a weighted mean value of 8 72 0 38 the single stage hf model age ranges from 2836 ma to 3249 ma we infer that the protolith of the plagioclase hornblende gneiss originated from the enriched mantle mixed with a small amount of subducted crust materials the shuichang bif formed at 2 5 ga and is coeval with other bifs in north china and in other cratons in the world we propose that the shuichang bif was formed in a subduction related back arc basin or intra arc basin based on the geochemical signatures of the wall rocks we further point out that the 2 5 ga global bif formation peak not only predates the amalgamation of the north china craton but probably also record an ancient supercontinent event

hydrocarbon generation and expulsion characteristics of eocene source rocks in the huilu area northern pearl river mouth basin south china sea implications for tight oil potential abstract in recent years new oil reservoirs have been discovered in the eocene tight sandstone of the huilu area northern part of the pearl river mouth basin south china sea indicating good prospects for tight oil exploration in the area exploration has shown that the huilu area contains two main sets of source rocks the eocene wenchang e2w and enping e2e formations to satisfy the requirements for further exploration in the huilu area particularly for tight oil in eocene sand reservoirs it is necessary to re examine and analyze the hydrocarbon generation and expulsion characteristics based on mass balance this study investigated the hydrocarbon generation and expulsion characteristics as well as the tight oil resource potential using geological and geochemical data and a modified conceptual model for generation and expulsion the results show that the threshold and peak expulsion of the e2w source rocks are at 0 6 vitrinite reflectance and 0 9 vitrinite reflectance respectively there were five hydrocarbon expulsion centers located in the western eastern and northern huizhou sag and the southern and northern lufeng sag the hydrocarbon yields attributed to e2w source rocks are 2 4 1011 tons and 1 6 1011 tons respectively with an expulsion efficiency of 65 the e2e source rock threshold and peak expulsion are at 0 65 vitrinite reflectance and 0 93 vitrinite reflectance respectively with hydrocarbon expulsion centers located in the centers of the huizhou and lufeng sags the yields attributed to e2e source rocks are 1 1 1011 tons and 0 2 1011 tons respectively with an expulsion efficiency of 20 using an accumulation coefficient of 7 13 the eocene tight reservoirs could contain approximately 1 3 1010 tons to 2 3 1010 tons with an average of 1 8 1010 tons of in place tight oil resources highest recoverable coefficient can reach 17 18 indicating that there is significant tight oil potential in the eocene strata of the huilu area

petroleum origin and accumulation in basin evolution a quantitative model basin data geologic geophysical geochemical hydrodynamic and thermodynamic can be combined for quantified hydrocarbon prediction a three dimensional deterministic dynamic basin model can be constructed to calculate all the measurable values with the help of mass and energy transport equations and equations describing the physical and or physicochemical changes in organic matter as a function of temperature input data consist of heat flux initial physical and thermal properties of sediments paleobathymetric estimates sedimentation rate and amount and type of organic matter subsequently the model computes pressure temperature physical and thermal properties of sediments maturity of organic matter and the hydrocarbon potential of any source rock as a func ion of space and time thus the complex dynamic processes of petroleum formation and occurrence in a given sedimentary basin can be quantified for example hydrocarbon potential maps for any given source rock and any geologic time slice of the basin evolution can be provided as computer printouts the computer model can be applied to any stage of an exploration campaign and updated as more information becomes available

madbi biyadh qishn petroleum system in the onshore masila basin of the eastern yemen abstract the masila basin is one several onshore mesozoic basins in yemen however a full petroleum system with its essential elements and processes has not been assigned yet in this study one petroleum system in the masila basin was identified and named madbi biyadh qishn petroleum system an extensive overview study showed that the main source and reservoir rocks in the masila basin are the upper jurassic madbi shales and lower cretaceous biyadh qishn sandstones respectively hence it appears that the most important effective petroleum system of the masila basin becomes the madbi biyadh qishn petroleum system organic rich shales of the upper part of the madbi formation deposited during the upper jurassic constitute the main source rock of the crude oils in the basin these shales contain high total organic carbon content toc 2 0 wt and predominantly algal type ii with minor type i kerogen the madbi shales are mature and at the present time are within the oil window with determined vitrinite reflectance values in the range of 0 53 0 91 ro the biyadh qishn sandstones are the main reservoir and were deposited in braided river channels shoreface and shallow marine environments and consist of fine to coarse grained moderate to well sorted grains seal rocks are cretaceous limestone and shales of the qishn and harshiyat formations and tertiary shales overburden rocks include the cretaceous rocks and the tertiary sediments deposited simultaneously with development of the traps during oligocene middle miocene time burial thermal histories models demonstrate that hydrocarbon generation from the madbi shales commenced in the late cretaceous and maximum rates of hydrocarbon expulsion occurred during the early tertiary geochemical oil source rock correlation studies showed that regional distribution of the oil fields almost overlap with the previously constructed pod of active madbi shales indicating that oil migrated from the active madbi source rock to trapping sites via vertical migration pathways through faults as a result of tertiary rifting during oligocene middle miocene time

tectonic systems and the deposition of iron formation abstract the iron formation group of chemically precipitated cherty iron rich sediments ranges in age from early precambrian to recent and is composed of a variety of lithological facies iron formations are classified as lake superior and algoma types to emphasize the contrasting conditions in the spectrum of sedimentary tectonic environments in which they formed thick stratigraphic units of lake superior type iron formation which form the most extensive iron ranges of the world are part of sedimentary tectonic systems that developed along the margins of cratons or continental platforms and were deposited over extended periods of time under relatively stable tectonic conditions the algoma type iron formations were deposited with volcanic rocks and greywacke in various tectonic volcanic systems formed under more dynamic tectonic conditions but are otherwise comparable to present day spreading ridges on the ocean floor the thick sequences of lake superior type iron formation deposited between 2 7 and 2 0 ga form part of major sedimentary tectonic systems that extended along the margins of continents direct biogenic factors and the composition of the atmosphere probably had only limited influence on the precipitation of these chemical sediments

two stage collision related extrusion of the western dabie hp uhp metamorphic terranes central china evidence from quartz c axis fabrics and structures abstract the western dabie orogen also known as the hong an block forms the western part of the dabie sulu hp uhp belt central china rocks of this orogen have been subjected to pervasive ductile deformation and include numerous quartz schists and felsic mylonites cropping out in ductile shear zones quartz textures in these mylonites contain important clues for understanding the movement sense of late collisional extrusion and exhumation of high pressure ultrahigh pressure hp uhp rocks from the lower crustal level to the upper crustal level during middle triassic and early jurassic the orientation and distribution of quartz crystallographic axes were used to confirm the regional shear sense across the orogen the asymmetry of c axis patterns consistently indicates top to the southeast thrusting across the orogen in early structural stages later stages of deformation show different senses of movement in northern and southern parts of the orogen with top to the northwest sinistral shearing recorded in rocks north of the xinxian hp uhp eclogite facies belt and top to the southeast dextral shearing south of the same unit based on our study on quartz c axis fabrics and marco to micro scale structures simultaneous southeastward shearing within a large part of the orogen and normal faulting north of the xinxian hp uhp unit is explained by upward extrusion in early stages of deformation the extrusion process has been attributed to syn and late collisional processes accounting for some earlier deformation in the western dabie orogen such as metamorphic sequences around the core of the xinxian hp uhp eclogite facies unit much higher pressure of deformation is also indicated in the aligned glaucophane and omphacite from blueschist and eclogite in the field an orogen parallel eastward extrusion of the xinxian hp uhp eclogite facies unit however occurred diachronously in later stages of deformation therefore a tectonic model combining an early upward extrusion with a later eastward extrusion is presented two different stages and types of extrusion for exhumation of hp uhp rocks are suitable to all of east central china geochronological data shows that the first upward extrusion occurred during middle triassic the second eastward extrusion occurred during late triassic to early jurassic these two extrusions are correlative with two stages of rapid exhumation of the dabie hp uhp rocks respectively these two stage late collisional middle triassic to early jurassic extrusion events bridge the gap between syn collisional early to middle triassic vertical extrusion and post collisional cretaceous eastward directed lateral escape and provide vital clues to understanding the more detailed processes of exhumation of hp uhp rocks

2 9 ga magmatism in eastern hebei north china craton abstract we report the results of a comprehensive isotopic investigation of zircons from metamorphosed orthogneisses from caochang village in the zunhua complex eastern hebei north china craton ncc a detailed investigation of the internal structures of these zircons combined with sims u pb and icp ms lu hf zircon analyses as well as a whole rock geochemical study revealed a previously unknown 2 9 ga magmatic event in eastern hebei the identified mesoarchean gneisses are mainly felsic gneisses and amphibolites amphibolites with positive end t values were derived from the depleted mantle with some crustal contamination during ascent zircon hf isotopes indicate that the high silica gneisses originate from the high temperature mixing of mafic magma derived from a depleted mantle source and the subsequent felsic magma derived from the melting of ancient crustal materials with the subsequent high degree fractional crystallization of the parental magma some of the 2 9 ga felsic gneisses may record an anatectic event at 2 5 ga the widespread ancient inherited and detrital zircons 2 6 ga and the existence of mesoarchean rocks imply a possible ancient continent in eastern hebei it may also imply that the mesoarchean era especially at 2 9 ga represents another important magmatic event in the eastern block of the ncc the identification of 2 9 ga rocks indicates that the magmatic history of eastern hebei is far more complex than previously thought

source rock characteristics and hydrocarbon generation modelling of upper cretaceous mukalla formation in the jiza qamar basin eastern yemen abstract the upper cretaceous mukalla coals and other organic rich sediments which are widely exposed in the jiza qamar basin and believed to be a major source rocks were analysed using organic geochemistry and petrology the total organic carbon toc contents of the mukalla source rocks range from 0 72 to 79 90 with an average toc value of 21 50 the coals and coaly shale sediments are relatively higher in organic richness consistent with source rocks generative potential the samples analysed have vitrinite reflectance in the range of 0 84 1 10 ro and pyrolysis t max in the range of 432 454 c indicate that the mukalla source rocks contain mature to late mature organic matter good oil generating potential is anticipated from the coals and coaly shale sediments with high hydrogen indices 250 449 mg hc g toc this is supported by their significant amounts of oil liptinite macerals are present in these coals and coaly shale sediments and py gc s 2 pyrograms with n alkane alkene doublets extending beyond nc 30 the shales are dominated by type iii kerogen hi one dimensional basin modelling was performed to analysis the hydrocarbon generation and expulsion history of the mukalla source rocks in the jiza qamar basin based on the reconstruction of the burial thermal maturity histories in order to improve our understanding of the of hydrocarbon generation potential of the mukalla source rocks calibration of the model with measured vitrinite reflectance ro and borehole temperature data indicates that the present day heat flow in the jiza qamar basin varies from 45 0 mw m 2 to 70 0 mw m 2 and the paleo heat flow increased from 80 ma to 25 ma reached a peak heat flow values of approximately 70 0 mw m 2 at 25 ma and then decreased exponentially from 25 ma to present day the peak paleo heat flow is explained by the gulf of aden and red sea tertiary rifting during oligocene middle miocene which has a considerable influence on the thermal maturity of the mukalla source rocks the source rocks of the mukalla formation are presently in a stage of oil and condensate generation with maturity from 0 50 to 1 10 ro oil generation 0 5 ro in the mukalla source rocks began from about 61 ma to 54 ma and the peak hydrocarbon generation 1 0 ro occurred approximately from 25 ma to 20 ma the modelled hydrocarbon expulsion evolution suggested that the timing of hydrocarbon expulsion from the mukalla source rocks began from 15 ma to present day

geochronology and geochemistry of neoarchean granitoids from the western shandong province north china craton implications for crustal evolution and cratonization abstract granites are keys to the understanding of crustal stabilization and cratonization processes here we present an integrated study of the lithology geochronology and geochemistry of late neoarchean granites and associated ttgs tonalite trondhjemite granodiorite and migmatites from the western shandong province wsp of the north china craton ncc the ttg xenoliths in the lushan massive granite formed at 2529 7 ma the menglianggu and yishan migmatite containing 2 70 2 52 ga inherited zircons defines regional anatectic event at 2512 6 ma the menglianggu lushan and yishan granites yield 207 pb 206 pb ages ranging between 2517 to 2488 ma which are indistinguishable from the regional metamorphic anatectic events of the wsp based on the sio 2 contents and ree patterns the late neoarchean granites exposed in wande sishui qixingtai and menglianggu areas can be subdivided into three groups group 1 exhibits low sio 2 68 6 69 2 wt high ree 345 4 475 3 ppm contents moderate negative eu anomalies 0 55 0 73 and depletion in u nb ta and sr contents group 2 is composed of most of the studied samples and show moderate sio 2 70 1 75 7 wt and ree 135 8 357 1 ppm content with obvious negative eu anomalies 0 33 0 58 excluding an outlier sample ts1310 and pronounced depletions in ba sr nb and ta contents group 3 displays high sio 2 74 9 76 3 wt and low ree 18 6 71 6 ppm abundance they are characterized by lack of negative eu and sr anomalies geology and geochemical comparisons suggest that the major part of the granitic magma originate from re melting of ttg type rocks at low pressure the negative u anomalies of group 1 granite are inherited from the source which is different from the case of ttgs the correlation of sr vs ba and rb suggests fractional crystallization of k feldspar plagioclase and minor hornblende in the precursor magmas of group 2 these geochemical features are similar to those associated with crustal stabilization in other cratons our studies on the wsp also suggest that the coeval to slightly earlier regional anatexis is also an important process during cratonization

effect of sedimentary heterogeneities on hydrocarbon accumulations in the permian shanxi formation ordos basin china insight from an integrated stratigraphic forward and petroleum system modelling abstract sedimentary heterogeneities are ubiquitous in nature and occur over a range of scales from core reservoir to basin scales they may thus exert significant influences on hydrocarbon generation migration and accumulation the sedimentary heterogeneities of the permian shanxi formation in the ordos basin china were modelled using sedsim a stratigraphic forward modelling program the simulation results were then used to construct a 3d petroleum system model using petromod the effects of sedimentary heterogeneities on hydrocarbon accumulations were evaluated by comparing the integrated sedsim petromod model with the classic 3d basin model the sedsim simulation shows that considerable sedimentary heterogeneities are present within the shanxi formation as a result of the interplay of the initial topography tectonic subsidence base level change and sediment inputs a variety of lithologies were developed both laterally and vertically within the shanxi formation at kilometre and metre scales respectively with mudstones mainly developed in the depositional centre while sandstones developed in the southern and northern margin areas a typical source ward retrogradation is well developed within the lower shanxi formation a base case classic 3d basin model was constructed to quantify the permian petroleum system in the ordos basin the geological and thermal models were calibrated using vr and borehole temperature data the source rocks of the upper paleozoic became mature r o 0 5 and high mature r o 1 2 in the late triassic and late jurassic respectively in the central and southern areas during the early cretaceous a tectonically induced geothermal event occurred in the southern ordos basin this caused the source rocks to reach over maturity r o 2 0 quite rapidly in the early late cretaceous in the central and southern areas all the source rock transformation ratios tr at present are greater than 70 in the p 1 coal and p 1 mudstone layers with tr values approaching 100 in the central and southern areas the transformation ratios of the p 1 limestone are close to 100 over the entire interval in the base case model a large amount of hydrocarbons appear to have been expelled and migrated into the shanxi formation but only a minor amount was accumulated to form reservoirs in the model the shanxi formation sandstone layer was set to be homogeneous vertically and there was no regional seal rocks present at the top of the shanxi formation therefore hydrocarbons could not be trapped effectively with only minor accumulations in some local structural highs where hydrocarbons are trapped both at the top and in the up dip direction by the adjacent mudstone facies in contrast the integrated sedsim petromod model takes into account of the internal lithological and sedimentary facies heterogeneities within the shanxi formation forming complex contiguous sandstone mudstone stacking patterns hydrocarbons were found to have accumulated in multiple intervals of lithological traps within the shanxi formation the results indicate that lithological distinctions controlled by sedimentary heterogeneities in three dimensions can provide effective sealing in both the top and up dip directions for hydrocarbon accumulations with gas being mainly accumulated near the depocentre where lithological traps usually formed due to frequent oscillations of the lake level

a review of the western part of the altaids a key to understanding the architecture of accretionary orogens abstract the altaids is one of the largest accretionary collages in the world and the tectonic styles of the accretionary processes have been interpreted in several ways including as an amalgamation of multiple terranes as a result of oroclinal bending of a long single arc or as a caledonian continental collision based on recent tectonostratigraphic analyses together with paleomagnetic data the tectonic styles of the neoproterozoic to paleozoic accretionary processes of the altaids are discussed the western altaids is the main focus of the study which was mainly composed of several independent linear components such as arcs and microcontinents with proterozoic basement and cover rocks various kinds of arcs existed in the paleo asian ocean including a complicated type of arc alaskan type which is a combination of the japan and mariana type intra oceanic arcs and the cordillera type continental arcs these linear components rotated and collided with each other with multiple subduction polarities which could have been an important result of multiple linear element amalgamation and which has contributed greatly to the architecture of the eurasian continent the basic tectonic styles of the altaids can be summarized as arc arc collision oroclinal bending and large scale rotation and multiple subductions with a complicated archipelago paleogeography these basic features of accretionary orogens in general can be attributed to the amalgamation of complicated multiple linear elements some mesozoic to cenozoic accretionary orogens in the world are also characterized by processes of multiple linear element amalgamation more attention should be paid to the multiple linear element amalgamation of ancient accretionary orogens which will shed light on lateral and vertical continental growth

hydrocarbon potential and depositional paleoenvironment of a middle jurassic succession in the falak 21 well shushan basin egypt integrated palynological geochemical and organic petrographic approach abstract the topmost lower and middle jurassic formations of organic rich clastics with minor carbonate deposits were recognized as very significant source reservoir rocks in the north western desert of egypt although several studies are devoted to their source rock characteristics and petroleum potential the depositional paleoenvironment and paleoredox conditions are still poorly understood in the current study these sediments were assessed in terms of their organic matter richness hydrocarbon generative potential and thermal maturity total sulfur ts contents and organic petrographic characteristics depositional paleoenvironment was investigated using dinoflagellate cysts dinocysts and the composition of particulate organic matter along with organic petrographic characteristics redox conditions were assessed based on ts total organic carbon toc relationship and type of kerogen that was derived mainly from palynofacies analysis toc contents show fair to excellent generative potential of hydrocarbons mainly of kerogen type iii gas prone the thermal maturity based on tmax and rr indicates mature stage of oil window palynological analysis yielded poor to rich intervals of moderately preserved biostratigraphically useful dinocysts in the upper part of the khatatba formation an assemblage of 81 species belonging to 54 genera of moderately diverse dinocysts spores and pollen grains was identified freshwater algae acritarchs and microforaminiferal test linings are sporadically documented within the studied interval based on the first downhole appearance fda of the recovered marker dinocyts the upper part of the khatatba formation was palynologically dated as late bathonian callovian the depositional environment was interpreted and three environments were deduced the lower interval representing the topmost ras qattara and yakout red shale member was deposited in a fluvial lacustrine environment where oxic conditions prevailed as deduced from the color of reddish brown sandstone and red shale the middle interval spans unit iib kabrit and unit iii which accumulated in a deltaic to shallow marine environment under dysoxic suboxic settings the upper interval covers unit i and unit iia that have been deposited in a fluvio deltaic to shallow marine with high paleoproductivity under suboxic conditions one sample in the upper interval in unit iia reflected anoxic setting

mineral prospectivity analysis for bif iron deposits a case study in the anshan benxi area liaoning province north east china abstract banded iron formation bif deposits are the most important source of iron in china and they are dominantly distributed in the north china craton ncc the anshan benxi area is situated in the northeastern part of the ncc and is considered to be the most fertile and productive iron metallogenic province in china the recent successive discoveries of large bif deposits in this area especially the world class dataigou bif deposit indicates that there is great potential for iron resources in this area the bif deposits are well defined by magnetic and gravity anomalies and thereby extensive petrophysical and geophysical studies are critical for bif prospective mapping a data driven weights of evidence wofe analysis was used for bif prospectivity modeling the results indicated that prospective areas are generally confined to the precambrian greenstone belt basin and high prospectivity zones class a are mainly distributed in the middle and northeastern parts of the study area moderate prospectivity zones class b are mainly distributed in southwestern and northeastern parts of the study area finally we employed selective human intervention to redefine priority targets to be more reliable the results suggest the method is successful as 82 target areas were narrowed to class a 20 and 11 class b targets the total of class a and class b targets was just over one third of the total number of targets offering a much smaller but effective selection of targets for consideration by mineral explorers

a new evaluation of palaeo heat flows and eroded thicknesses for the carboniferous ruhr basin western germany palaeo heat flows and thicknesses of eroded carboniferous overburden in the coal bearing ruhr basin western germany were estimated using one dimensional basin modelling techniques thermal and burial histories for 11 localities were calibrated by comparing measured and calculated vitrinite reflectance data based on the assumption of pre tectonic maturation in contrast to former studies the kinetic easy r0 approach applied for the calculation of vitrinite reflectance lopatin s tti algorithm was used in addition for one well in order to demonstrate the impact of the selection of the algorithm for vitrinite reflectance calculation on the simulation results simulated palaeo heat flows for the time of maximum burial most probably in the stephanian range from 64 to 83 mw m2 and decrease towards the south these heat flows are lower than values computed during earlier studies but are still relatively high in comparison to recent foreland basins the regional distribution of palaeo heat flows indicates that the crust beneath the ruhr basin was relatively thin and thickened towards the south at the end of the carboniferous after the stephanian 2200 3500 m of carboniferous overburden were eroded an observed southward increase in eroded thicknesses is caused by an uplift of the area studied which decreased towards the north at the end of the variscan folding large thicknesses of deposited and later eroded carboniferous sediments have to be postulated to explain the maturity data even along the present southern margin of the ruhr basin this indicates that the late carboniferous sediments originally extended far to the south geological arguments suggest that the deposition of carboniferous sediments in the entire ruhr basin continued until the stephanian

meso and neoarchean banded iron formations and genesis of high grade magnetite ores in the anshan benxi area north china craton a reply

changing provenance of late neoarchean metasedimentary rocks in the anshan benxi area north china craton implications for the tectonic setting of the world class dataigou banded iron formation abstract the anshan benxi anben area is situated in the northeastern part of the north china craton and is considered to be the most important iron metallogenic province in china the giant dataigou banded iron formation bif has the potential to be the largest iron deposit in this area even in asia here we present in situ zircon u pb hf o isotopic and whole rock geochemical data for metasedimentary rocks metapelites and metaarenites and trondhjemitic gneiss associated with this bif in order to provide constraints on the setting of the deposit with implications for geological and exploration models sims zircon u pb dating results indicate that the trondhjemitic gneiss crystallized at 3450 19 ma and thus it is the oldest rock so far indentified in the benxi area the youngest group of detrital zircons from the metaarenite samples constrains their maximum depositional age at 2 54 ga in combination with the earlier metamorphic age of 2 51 ga for the metaarenites the deposition age of the dataigou bif can be constrained between 2 54 and 2 51 ga the metasediments have undergone varying degrees of source weathering source rocks of the metapelites have undergone moderate to severe chemical weathering whereas those of the metaarenites have subjected to relatively weak chemical weathering diagnostic geochemical features such as the al2o3 tio2 values trace element ratios and ree patterns suggest that the metaarenites were predominantly derived from felsic igneous sources whereas the metapelites were sourced mainly from mafic tholeiitic rocks the dominant late neoarchean detrital zircons further indicate that they were most likely sourced from almost synchronous volcanic rocks in the anben area taking into account the lithostratigraphic geochemical and geochronological data the dataigou bif is interpreted as having been deposited in a back arc basin most probably on the margin of an ancient continental crust continuous provenance variations of these metasediments during highstand times and transgression can be ascribed to changes in natures of nearby volcanisms integrated with previous zircon hf isotopic data from other supracrustal rocks in the anben area our hf o data reveal major juvenile crustal growth stages at 3 0 ga and during 2 8 2 7 ga and a crustal reworking event with minor juvenile addition at 2 5 ga in the studied area

geochemical characteristics of archean cherts and other sedimentary rocks in the pilbara block western australia evidence for archean seawater enriched in hydrothermally derived iron and silica abstract geochemical and mineralogical characteristics of archean sedimentary rocks such as cherts banded iron formations bifs shales and sandstones collected from the pilbara block western australia were studied lithologic features of various types of chert occurring within the pilbara supergroup are closely related to their chemical compositions red and brown bands of banded cherts generally show a higher fe 2 o 3 total iron as fe 2 o 3 concentration than do other cherts whereas grey greenish grey and pale yellowish grey cherts are characteristic of relatively higher concentrations of al and ti white cherts are generally composed almost exclusively of sio 2 geochemical characteristics of the cherts suggest that they were precipitated from hydrothermal solutions evidence for the hydrothermal origin includes 1 low mno fe 2 o 3 values 2 low concentrations of heavy metals 3 positive euanomalies 4 low co zn and ni zn values and 5 high concentrations of mn and fe in carbonates positive correlations between fe and heavy metals such as ni and zn imply the intermittent precipitation of fe oxides hydroxides along with these heavy metals in the ancient ocean cherts except for grey greenish grey and pale yellowish grey cherts are highly depleted in detrital materials this may have been caused by rapid precipitation of hydrothermally derived fe and si the archean pilbara ocean is believed to have been enriched in fe and si derived from hydrothermal activity

basin modeling of the late miocene zeit source rock in the sudanese portion of red sea basin implication for hydrocarbon generation and expulsion history abstract the late miocene zeit formation is exposed in the red sea basin of sudan and represents an important oil source rock in this study five 5 exploratory wells along red sea basin of sudan are used to model the petroleum generation and expulsion history of the zeit formation burial thermal models illustrate that the red sea is an extensional rift basin and initially developed during the late eocene to oligocene heat flow models show that the present day heat flow values in the area are between 60 and 109 mw m2 the variation in values of the heat flow can be linked to the raise in the geothermal gradient from margins of the basin towards offshore basin the offshore basin is an axial area with thick burial depth which is the principal heat flow source the paleo heat flow values of the basin are approximately from 95 to 260 mw m2 increased from oligocene to early pliocene and then decreased exponentially prior to late pliocene this high paleo heat flow had a considerable effect on the source rock maturation and cooking of the organic matter the maturity history models indicate that the zeit formation source rock passed the late oil window and converted the oil generated to gas during the late miocene the basin models also indicate that the petroleum was expelled from the zeit source rock during the late miocene 7 ma and it continues to present day with transformation ratio of more than 50 therefore the zeit formation acts as an effective source rock where significant amounts of petroleum are expected to be generated in the red sea basin

modelling petroleum generation of late cretaceous dabut formation in the jiza qamar basin eastern yemen abstract the jiza qamar basin in eastern yemen is still undergoing hydrocarbon exploration and hydrocarbon potential has not been assigned yet in this study subsurface samples from two onshore and offshore wells were collected to characterize the source rocks of dabut formation and to incorporate into basin modelling in order to know and determine the timing of petroleum generation and expulsion of the dabut source rocks based on organic geochemical analysis the dabut sediments have variable toc content in the range of 0 5 2 5 wt indicating fair to good source rock generative potential the organic matter of dabut source rock is dominated by type iii kerogen with minor mixed iii ii kerogen contributions and is thus considered to be mainly gas prone and some oil prone this is supported by hi values in the range of 50 212 mg hc g toc vitrinite reflectance in the range of 0 63 0 88 ro and pyrolysis tmax in the range of 427 454 c generally indicate that the dabut source rocks contain mature organic matter one dimensional basin modelling was performed to analyse the hydrocarbon generation and expulsion history of the dabut source rocks in the jiza qamar basin based on the reconstruction of the burial and thermal maturity histories in order to improve our understanding of the hydrocarbon generation potential calibration of the model with measured vitrinite reflectance ro and borehole temperature bht data indicates that the paleo heat flow was constant during the late cretaceous paleogene but increased during the late paleogene neogene and then decreased exponentially from neogene to present day the modelled maturity history predicts that the oil generation 0 5 ro in the dabut source rock began from about 56 ma to 42 ma and the peak hydrocarbon generation 0 86 ro occurred approximately from 14 ma to 13 ma these models also indicate that the main phase of oil generation was at 56 ma to 25 ma whereas the main phase of gas generation began from about 25 ma to 14 ma and still generated in the present day

source rock evaluation and nature of hydrocarbons in the khalda concession shushan basin egypt s western desert abstract organic geochemical studies were undertaken on 241 jurassic cretaceous ditch cuttings samples and 9 gases and 10 crude oils from some productive mesozoic pay zone intervals in the khalda concession shushan basin northern western desert egypt we aim to investigate source rock characteristics origin of gases nature of oils and their inferred source rocks and to establish genetic relationships the middle jurassic khatatba formation and older rocks contain organic rich mature coaly and carbonaceous shales which are the main source for hydrocarbons in egypt s western desert including the shushan basin which generally contains kerogen type iii with some contribution of type iii ii kerogen molecular and isotopic composition indicates that the khalda gases are of thermogenic origin a mature 0 6 2 0 ro source rock with terrigenous organic matter kerogen type iii was identified as a precursor of wet thermogenic gases in the shushan basin geochemical results indicate that the oil samples were derived from a source rock deposited mainly under oxic conditions and containing terrigenous organic matter biomarker data indicate that paraffinic oils and condensates from the shushan basin were derived from mature source rocks within the oil window the light hydrocarbon data suggests that the analyzed oils can be classified into two groups as a result of evaporative fractionation and maturation this signal of a secondary reservoir transformation which is not revealed by biomarker analysis emphasizes the importance of integrating biomarker and light hydrocarbon data to understand variations in the chemical composition and origin of oils

geochemistry of metamorphosed volcanic rocks in the neoarchean qingyuan greenstone belt north china craton implications for geodynamic evolution and vms mineralization abstract the qingyuan greenstone belt qgb situated in the northeastern section of the north china craton ncc is considered to be the unique metallogenic province in china that hosts both banded iron formations bifs and volcanogenic massive sulfide vms deposits traditionally the qgb is divided into the hunbei and hunnan terranes separated by the hunhe fault and grouped into the lower shipengzi middle hongtoushan and upper nantianmen formations which have undergone amphibolite to granulite facies metamorphism exposed supracrustal rocks are composed of plagioclase amphibolite and biotite plagioclase gneiss with minor amounts of bif marble and schist in this study detailed field and petrographic observations lithogeochemical u pb geochronological as well as whole rock sm nd isotope analyses were conducted on the metamorphosed mafic and felsic volcanic rocks i e plagioclase amphibolite and biotite plagioclase gneiss respectively in the qgb in order to discuss implications for tectonic evolution and comitant vms mineralization the supracrustal rocks of both hunbei and hunnan regions were formed synchronously from 2 57 to 2 53 ga and then had undergone 2 52 2 48 ga regional metamorphism specifically the lower section of the shipengzi formation is composed dominantly of transitional tholeiitic basalts with negative nb anomalies suggesting that they were likely generated by partial melting of a mantle source that was previously altered by variable amounts of slab derived fluids or melts in contrast the lithologic assemblages grade upward into interlayered bimodal volcanic rocks i e n morb type tholeiitic basalts and fi to fii type dacites of transitional to calc alkalic affinities representing a change in tectonic regime related to subduction from compression to extension among them all the basalts are characterized by consistently positive initial nd compositions end t 2 7 to 3 4 further indicate a depleted mantle source whereas those dacites possess negative to positive initial nd values end t 4 6 to 3 0 suggesting they were formed by partial melting of depleted mafic source likely lower mafic crust and accompanied with variable addition of old crustal material significantly vms deposits are genetically linked to this extensional event and closely associated with the fii type dacites that were generated by higher melting degree at high temperature conditions and shallow crustal level likely 10 15 km in all geochemical features e g negative nb anomalies and end value together with contamination from continental crusts for tholeiitic mafic rocks and inherited zircons from these volcanic rocks imply a continental arc setting where the qgb was evolved

calculation of dike trajectories from volcanic centers 1 patterns of dike swarms around volcanic centers or above mantle plumes are interpreted by a mechanical analysis of regional and dike induced stresses in which dike emplacement is controlled and guided by the stress state comparisons of dike patterns with patterns of principal stress trajectories caused by a source and a regional stress system are commonly used to infer paleostresses however dike trajectories are determined by a complex interaction between dike induced stresses and the source regional stress system we present numerical calculations based on a novel boundary integral formulation which examines the simultaneous effects of regional stresses magma pressure and dike injection on the local stress state around a continuously curving dike dike paths are calculated from the condition that dikes propagate by mode i failure our results suggest that the magnitude of the regional stresses would be 2 5 times higher than previous estimates based on principal stress trajectory analysis

geochemical characterization of solid bitumen migrabitumen in the jurassic sandstone reservoir of the tut field shushan basin northern western desert of egypt abstract asphaltene rich solid bitumen was identified within the pore spaces of the middle jurassic khatatba sandstone reservoir in the tut field shoushan basin egypt despite good reservoir properties have been detected the solid bitumen forms barriers in the reservoir which prevents economic petroleum production petrographical and organic geochemical techniques were applied in order to characterize the source rock of the bitumen and its formation mechanisms organic geochemical results confirm the presence of actively migrating non biodegraded hydrocarbons within the khatatba sandstones biomarker data suggest that the solid bitumen originated from a marine shale source rock that was deposited in mildly anoxic to suboxic conditions biomarker oil source rock correlation with the organic rich sediments of the khatatba formation indicates that the shale and coaly shale within the khatatba formation are an effective source rock in the area the natural deasphalting is the predominant mechanism for the formation of the asphaltene rich solid bitumen in the khatatba sandstones adsorption of asphaltenes onto authigenic kaolinite played a major role in the fixation of bitumen furthermore gas deasphalting caused by gas injection may have contributed to a phase change in the reservoir fluid and deposition of solid bitumen the injected gas is generated from khatatba type iii kerogen or older source rocks in the shoushan basin during the mature stage of hydrocarbon generation

discovery of sapphirine bearing mg al granulites in the north china craton implications for paleoproterozoic ultrahigh temperature metamorphism abstract we report here for the first time the occurrence of mg al granulites within the khondalite belt of the north china craton and provide robust evidence for extreme crustal metamorphism at ultrahigh temperature uht conditions in this region the uht indicator assemblages in these rocks include equilibrium sapphirine quartz high alumina orthopyroxene sillimanite quartz and high temperature perthites the surrounding khondalites also carry spinel quartz equilibrium assemblage the sapphirines occurring in various microstructural associations are markedly magnesian x mg 0 69 0 77 with a composition varying from 7 9 3 to 2 2 1 end members spinel is a solid solution of mg spinel and hercynite x mg 0 41 0 54 with zno content varying from 0 48 to 1 79 wt garnet shows a compositional range between pyrope and almandine x mg 0 39 0 48 with minor grossular 2 6 3 8 mol and spessartine 0 5 1 8 mol the orthopyroxenes display marked compositional variations in terms of al 2 o 3 content with the core of the medium grained orthopyroxene adjacent to sillimanite in the matrix showing the highest al 2 o 3 content 8 4 wt the history of prograde to peak metamorphism of these rocks is indicated by inclusions of sapphirine spinel quartz and sillimanite within garnet sapphirine formed during the peak stage through the reaction spl qtz sil spr occurrence of sapphirine and quartz inclusions trapped in sillimanite suggests the reaction spr qtz grt opx sil a series of retrograde reactions are observed in these rocks as corona and symplectite textures around early coarse grained minerals such as opx sil qtz crd grt sil qtz crd grt qtz crd opx grt o 2 crd qtz fe 2 o 3 in mag or ilm and finally opx kfs h 2 o bt qtz whereas the stability of sapphirine quartz and orthopyroxene sillimanite quartz in the matrix assemblage suggests t 1000 c and p 10 kbar peak metamorphism conventional geothermobarometric estimates confirm the ultrahigh temperature nature and also trace the retrograde conditions 930 970 c at p 10 kbar al in opx 900 1000 c perthite 930 990 c grt opx and 910 940 c sapphirine spinel electron microprobe dating of monazites in textural association with the uht assemblages yield a sharply defined isochron age of 1917 48 ma which correlates with the 1927 11 ma obtained in a previous study from monazites in the surrounding khondalites carrying spinel quartz assemblage an evaluation of the assemblages and textures using appropriate petrogenetic grids indicate that following peak ultrahigh temperature conditions the rocks underwent initial isobaric cooling and subsequent isothermal decompression and we interpret these trajectories to be part of an overall anti clockwise p t evolution the study area located within the northern margin of the north china craton defines a collisional orogenic belt where continental collision and deep subduction was possibly followed by extensional collapse of the orogen evidence for the extensional phase is provided by the several mafic magmatic intrusives of paleoproterozoic age occurring in the region which suggest a possible link between the thermal anomaly associated with extreme crustal metamorphism in this region with mantle derived magmas the uht metamorphism in north china craton coincided with the assembly of the paleoproterozoic supercontinent columbia

the application of zircon cathodoluminescence imaging th u pb chemistry and u pb ages in interpreting discrete magmatic and high grade metamorphic events in the north china craton at the archean proterozoic boundary abstract two distinct late neoarchean to early paleoproterozoic magmatic events are recognized at the northern margin of the eastern block of the north china craton based on interpretation of zircon structure cathodoluminescence signature th u pb chemistry and age the first event is represented by emplacement of the chaihe trondhjemite and xianjinchang tonalite with magmatic zircon core ages ranging from 2559 2553 7 ma respectively these rocks experienced a subsequent high grade metamorphic event constrained by partial recrystallization of primary magmatic zircon and growth of new zircon rims between 2517 and 2490 ma a second magmatic episode generated by partial melting of crustal rocks during this metamorphic event is represented by the hongshilazi syenogranite with an age of 2502 11 ma the short time interval between initial magmatism and high grade regional metamorphism signifies they were related to the same major tectonothermal event when considered along with evidence of a similar sequence of events at this time 800 km to the south in eastern hebei and the previously established 2500 ma age of granulite facies metamorphism in mafic rocks across the craton it is proposed that a large mantle plume developed beneath the north china craton at the close of the archean

thermal maturity history and petroleum generation modelling for the upper jurassic madbi source rocks in the marib shabowah basin western yemen abstract widely exposed in the marib shabowah basin the early upper jurassic madbi formation is divided into two units lam and meem youngest to oldest believed to be major source rocks the madbi source rocks are characterized by relatively high total organic carbon contents toc ranging from 0 5 to 10 0 wt meeting the standard as a fair to excellent source rock generative potential they contain type ii kerogen grading to mixed type ii iii kerogens and type iii kerogen with hi values between 40 and 620 mg hc g toc vitrinite reflectance values range from 0 41 to 2 61 r o indicating sufficient thermal maturity for both oil and gas generation burial thermal history models indicate that the upper jurassic madbi source rocks passed the peak of oil generation stage and convert the oil to dry gas in the late jurassic to late cretaceous time oil generation began immediately after deposition in the late jurassic time 150 145 ma and maximum rates of oil with significant gas have been generated during late cretaceous time 110 65 ma the peak gas generation occurred during the early tertiary the modelled hydrocarbon expulsion evolution suggests that the timing of hydrocarbon expulsion from the madbi source rocks began in the early cretaceous 140 ma persisted to present day the modelling results also indicate that the peak hydrocarbon generation and expulsion from madbi source rocks in the studied area occurs well after deposition of the seal rock and formation of traps ensuring entrapment and preservation of migrated hydrocarbons

organic facies and maturation of jurassic cretaceous rocks and possible oil source rock correlation based on pyrolysis of asphaltenes scotian basin canada abstract this paper establishes source rock characterization and oil oil and oil source rock correlations of selected organic rich shale source rocks and selected light oils and condensates from jurassic and cretaceous formations of the scotian basin multiple source rocks of kerogen types iia oil prone iia iib oil and condensate prone iib condensate and gas prone iii gas prone and iv nonsource are identified in various stratigraphic horizons vitrinite reflectance and t inmax data indicate variable maturation histories in different parts of the basin which are related to differences in lithology related heat conductivity and the presence of older sediments closer to areas of rifting multiple linear regression and cluster analyses of aromatic hydrocarbon ratios reveal two oil families some kerogen type iib rock extracts match many of the light oil and condensates in terms of the aromatic hydrocarbon distributions a second family of oils could not be correlated with any of the analyzed source rock extracts pyrolysis gas chromatography of asphaltenes from the cohasset a 52 and sable island 3h 58 wells are significantly different in n alkane aromatics and phenol content the asphaltene pyrolyzates of samples from venture b 52 and s desbarres o 76 have a high aromatic content low content of n alkanes and high phenol values s desbarres which is typical of a terrestial source they correlate well with one another and also with sable island light oil the n triumph g 43 whole rock and extract asphaltene pyrolyzates are dominated by normal hydrocarbons with aromatic phenolic and thiophenic compounds in low concentrations suggesting an algal source algal type organic matter may have made a contribution to the cohasset petroleum as its asphaltene pyrolyzate is also dominated by normal hydrocarbons

time and temperature in petroleum formation application of lopatin s method to petroleum exploration n v lopatin in the soviet union has developed a method for taking both time and temperature into account as factors in thermal maturation of kerogen lopatin s time temperature index of maturity tti values correlate with the thermal regimes corresponding to generation and preservation of hydrocarbons because such information is potentially of great interest for oil exploration a calibration and evaluation have been made of lopatin s method within the limitations of the data presently available the following statements can be made 1 the rate of the chemical reactions involved in thermal maturation of organic material appears to double with every 10 c 18 f rise in temperature 2 threshold values of lopatin s time temperature index of maturity tti are 15 onset of oil generation 75 peak oil generation 160 end oil generation 500 40 oil preservation deadline 1 000 50 oil preservation deadline 1 500 wet gas preservation deadline 65 000 dry gas preservation deadline 3 tti values calculated from lopatin reconstructions consistently agree with other maturation parameters commonly used by petroleum geochemists potential applications of lopatin s method for oil exploration include timing of oil generation calculation of volume of hydrocarbons generated within a basin and determination of economic deadlines

development of the north china craton during the late archaean and its final amalgamation at 1 8 ga some speculations on its position within a global palaeoproterozoic supercontinent abstract new data obtained over the past few years indicate that by the end of the archaean the north china craton consisted of two major crustal entities identified as the eastern and western blocks the formation of island and magmatic arcs along the western margin of the eastern block record the latest archaean events so far identified the location of the western block at this time is unknown however the occurrence of palaeoproterozoic passive margin sediments similar in age to those occurring in the eastern block indicates essentially similar conditions and possible separation by an ocean of unknown dimensions re initiation of east dipping subduction beneath the western margin of the eastern block resulted in closure of this palaeoproterozoic ocean bringing the western block into close proximity continued subduction resulted in a major continental continental collision and the western margin of the eastern block was again reactivated leading to extensive thrusting high pressure metamorphism and the generation of crustal melts this occurred between 1 88 and 1 79 ga ago resulting in the formation of the trans north china orogen and final amalgamation of the north china craton although information is sparse to non existent we have reviewed the possible location of the north china craton at the time of collision and note a similarity in terms of age lithology and distribution of major crustal components between the north china craton and baltica although speculative we present this model in the hope that it will stimulate debate and lead to further research in these areas

a mantle derived bimodal suite in the hercynian belt nd isotope and trace element evidence for a subduction related rift origin of the late devonian brevenne metavolcanics massif central france in the brevenne series ne massif central a low grade bimodal association of metabasalts and metarhyolites is exposed together with intrusive trondhjemite bodies zircon u pb dating constrains their magmatic emplacement at 366 5 ma and 358 1 ma respectively the metabasalts are characterized by a distinct enrichment in incompatible elements e g th and lree and positive ndi from 5 to 8 combined isotope and trace element systematics rule out crustal contamination of mafic melts as a suitable cause of the lile large ion lithophile element enrichment rather a mixing process between a component similar to mid ocean ridge basalts and an enriched end member with ndi 5 is suggested an enriched mantle source of ocean island basalt affinity is precluded by the relative depletion of high field strength elements especially nb which shows negative anomalies in chondrite normalized patterns on the contrary a subduction related origin for the lile enrichment would be more consistent it may be inferred that arc like melts enriched in th and lree light rare earth elements and depleted in nb with ndi 5 were produced through partial melting of a depleted mantle source to which a small amount of crustally derived component had been added the metarhyolites are enriched in lile and have a close genetic relationship with the metabasalts as evidenced by their high ndi from 4 7 to 6 8 although the chemical evidence remains ambiguous it is suggested that fractional crystallization accompanied by subordinate assimilation is the petrogenetic process most consistent with the data the trondhjemites are isotopically distinct from the metarhyolites their ndi values from 1 0 to 2 2 reflect an important contribution of continental crust to their genesis and disprove their inferred cogenetism with the felsic volcanics a review of modern environments in which such bimodal suites are exposed shows that settings involving incipient rifting of a volcanic arc fringing a continental margin or built upon young thin continental crust might provide suitable analogues geodynamic reconstructions are complicated by subsequent tectonic events which disrupted the initial patterns and by mesozoic cenozoic sedimentary cover however this subduction related magmatism enlarges the growing body of evidence for southward subduction processes until the late devonian during the evolution of the northern flank of the european variscides as a general implication it is suggested that the combined use of the sm nd system with incompatible elements relatively resistant during alteration and low grade metamorphism ree th zr nb may provide diagnostic criteria for recognizing the tectonic setting of bimodal metaigneous suites in ancient orogenic belts

precise determination of phanerozoic zircon pb pb age by multicollector sims without external standardization 1 zircon has long been recognized as the best geochronometer and the most important timekeeper in geosciences modern microbeam techniques such as sims and la icpms have been successfully applied to in situ u pb zircon age determinations at spatial resolutions of 20 30 m or less matrix matched calibration by external standardization of well characterized natural zircon references is a principal requirement for precise microbeam u pb zircon age determination due to fractionation effects between pb and u which usually result in an external age error exceeding 1 alternatively zircons with a closed u pb system can be directly dated by measurement of 207pb 206pb isotopic ratio without external standardization which has been a common practice for zircons older than 1 0 ga but not for relatively young 1 0 ga and particularly phanerozoic ones because of limitations of analytical precision we describe in this paper a method of 207pb 206pb measurement on phanerozoic zircons using a new generation of large radius magnetic sector multicollector cameca ims 1280 sims in combination with multicollector mode a nuclear magnetic resonance nmr magnet controller and oxygen flooding techniques we achieve precisions of 207pb 206pb ratio of 0 1 and 0 1 0 2 propagating to pb pb age errors 0 4 and 1 3 excluding u decay constant uncertainties for zircons of latest neoproterozoic and late paleozoic to mesozoic age respectively therefore the multicollector sims is capable of direct determination of zircon pb pb ages as young as mesozoic age with uncertainties of geological significance this technique is useful for direct dating of zircons in thin sections moreover it has significance for dating of some other u rich minerals i e baddeleyite and zirconolite that are not suitable for sims u pb dating by external standardization

tectonic evolution and metallogenesis of north china craton the north china craton ncc is one of the oldest cratons in the world with the age of the old crust up to 3 8 ga it has a complicated evolution history and has recorded almost all the important geological events of the earth the main early precambrian geological events and key tectonic issues are as follows old continental nuclei and main crustal growth in the ncc took place at 2 7 2 9 ga by 2 5 ga the micro blocks had amalgamated to form a coherent craton the 2300 1950 ma paleoproterozoic mobile belts came into being the 1800 ma lower crust uplifted as a whole and there occurred mafic dyke swarm continental rifting and intrusion of orogenic magmatic association in the period of neoproterozoic and paleozoic the ncc was tectonically inactive but was affected by the central asian orogenic belt the north china craton probably experienced disruption and reconstruction in mesozoic and belonged to the circling pacific orogenic belt in cenozoic the north china craton has abundant ore deposits corresponding in age to the above important geological events mesozoic metallogenesis was closely related to mesozoic tectonic inversion and lithospheric thinning the main ore forming types include porphyry mo polymetallic deposits and large scale metallogenic gold explosion related to partial melting of the basement and granitic intrusion

formation age and genesis of the banded iron formations from the guyang greenstone belt western north china craton abstract previous studies focused on banded iron formations bifs from the eastern north china craton ncc few were conducted on those of the western ncc this paper concentrates on two bifs sanheming and gongyiming from the guyang greenstone belt gb western ncc sims u pb analyses of zircons from the amphibolite interlayers of sanheming and gongyiming bifs gave upper intercept ages of 2562 14 ma and 2569 78 ma respectively coupled with 2555 56 ma for the tonalite with bif xenoliths the formation age of the bifs is inferred to be 2 56 2 57 ga protolith reconstruction based on geochemistry and field occurrence of the amphibolites shows that they are originally basaltic trace elements reveal that the basalts of sanheming and gongyiming are t morb and n morb respectively metabasites from other locations of the guyang gb in previous studies display island arc geochemical characteristics plus the lithologic assemblage of komatiite basaltic komatiite boninite tholeiite high mg andesite and nb enriched basalt identified by previous work the geodynamic setting of the bifs from guyang gb is deduced to be a mantle plume erupting into the subduction zone and they were deposited in arc related basins relatively low contents of al 2 o 3 tio 2 and hfse for the bifs indicate negligible detritus involvement paas normalized rey profiles are characterized by lree depletion and hree enrichment positive la gd y and eu anomalies weak negative ce anomalies and y ho weight ratios of 26 41 indicating a mixture of submarine high temperature hydrothermal fluids and seawater chondrite normalized ree patterns for the bifs are similar to those of the komatiites occurring at the bottom of the gb collectively it is speculated that submarine high temperature hydrothermal fluids leached out the fe and si of oceanic volcanics during circulation and they deposited due to the changed physiochemical conditions when the fluids mixed with the seawater the hydrothermally altered amphibolites proximal to the gongyiming upgraded iron ores are enriched in various elements added by the hydrothermal fluids plus the 18 o values of magnetites and quartzs the genesis of the gongyiming upgraded iron ores is proposed to be hydrothermal interaction with low grade bif

neoarchean algoma type banded iron formations from eastern hebei north china craton shrimp u pb age origin and tectonic setting abstract algoma type bif hosted iron deposits occur in the shirengou xingshan erma lingshan and sijiaying areas of eastern hebei the deposits are hosted in archean rocks and their formation was related to volcanic and sedimentary processes in small volcano sedimentary basins supracrustal rocks of the qianxi and dantazi groups were metamorphosed from granulite facies to amphibolites facies available data indicate that these bif deposits mainly formed during paleoarchean and neoarchean times the paleoarchean bif hosted iron deposits are represented by the xingshan deposit that yielded a shrimp u pb zircon age of 3389 5 7 6 ma the other bif hosted iron deposits formed in the late neoarchean represented by the shirengou shuichang lingshan erma and sijiaying deposits that formed between 2605 ma and 2503 ma the late neoarchean bif iron deposits are considered to have formed under a mantle plume environment the eu anomalies of the ore samples from eastern hebei show that the bif ores formed in a hydrothermal related volcanic submarine environment where submarine hydrothermal effluents associated with volcanic activities may have supplied both the iron and silica for the bifs from eastern hebei

guidelines for evaluating petroleum source rock using programmed pyrolysis rock eval pyrolysis is used to rapidly evaluate the petroleum generative potential and thermal maturity of rocks accurate conclusions require programs every 30 60 ft 9 18 m understanding of interpretive pitfalls and supporting data such as visual kerogen vitrinite reflectance and elemental analyses the generative potential of coals is commonly overestimated by pyrolysis and is best determined by elemental analysis and organic petrography most coals show high s2 s3 5 and low hi values 300 mg hc g toc migrated oil and mud additives which alter rock eval data can sometimes be removed by special processing for immature rocks bimodal s2 peaks and pi values over 0 2 indicate contamination pyrolysis downgrades organic poor clay rich rocks which show lower hi and higher tmax values than isolated kerogen because of adsorption of pyrolyzate on the clays tmax values for small s2 peaks 0 2 mg hc g toc are unreliable tmax is affected by maturation organic matter type contamination and the mineral matrix s3 is sensitive to inorganic and adsorbed carbon dioxide and to instrumentation problems acidification of carbonate rich samples and proper maintenance improves s3 measurement constant sample weights 100 mg are recommended below a threshold weight tmax increases by up to 10 c and other parameters decrease organic rich samples which overload the detector can be diluted with carbonate detector linearity is determined by pyrolyzing various weights of an organic rich rock

timing and tectonic setting of the sijiaying banded iron deposit in the eastern hebei province north china craton constraints from geochemistry and sims zircon u pb dating the north china craton ncc one of the oldest continental blocks in asia has a complicated evolutionary history with the age of the old crust up to 3 8 ga and records the important geological events of the earth the sijiaying bif the largest banded iron formation located in eastern hebei province in the east part of ncc records an important thermo tectonic event previous studies focused on the geological description but the timing and tectonic setting have been unclear so far the sijiaying bif is hosted in neoarchean metamorphic rocks which includes biotite leptynite hornblende plagioclase gneiss and biotite plagioclase gneiss using major element contents and ratios of the host rocks the protoliths of the hornblende plagioclase gneiss and biotite plagioclase gneiss are shown to be dacite whereas those of the biotite leptynites are sedimentary rocks based on geology and geochemistry of the host rocks we infer that the sijiaying bif is an algoma type deposit sims zircon u pb dating shows igneous zircons from the biotite plagioclase gneiss and hornblende plagioclase gneiss have u pb ages of 2535 8 ma and 2543 14 ma respectively zircons from the biotite leptynite have an age of 2537 13 ma we infer that the ages of 2543 2535 ma represent the time of the sijiaying bif paas normalized rey profiles of the ore samples are characterized by lree depletion and hree enrichment positive la and eu anomalies and an average y ho weight ratio of 32 indicating a mixture of submarine hydrothermal fluids and deep seawater ree data was combined with the 18o analyzed by previous researcher of the ore from individual magnetite bands to infer that the sijiaying bif precipitated from hydrothermal fluids discharging on the sea floor the hornblende plagioclase gneiss and biotite plagioclase gneiss that are enriched in lile and lree and depleted in high field strength elements hfse nb ta and ti have geochemical signatures that are similar to those of island arc volcanic rocks the biotite leptynite shows the characteristics of continental island arc based on the geology and geochemistry of associated wall rocks and ore samples we suggest that the sijiaying bif formed in a back arc basin in neoarchean

petroleum generation modeling of the organic rich shales of late jurassic early cretaceous succession from mintaq 01 well in the wadi hajar sub basin yemen late jurassic early cretaceous shales of the naifa safer and madbi formations were studied to evaluate source rock characterization the results of the source rock were then incorporated into basin modeling to understand the timing of hydrocarbon hc generation and expulsion the late jurassic early cretaceous shales have low to high organic matter with total organic carbon toc values in the range of 0 50 28 01 indicating fair to excellent source rock potential main oil and gas are anticipated to be generated from the naifa safer and lam shale samples with types i and or ii and types ii iii kerogens in contrast the meem samples are dominated by type iii kerogen hydrogen index hi 200 mg hc g toc and are thus considered to be gas prone the late jurassic early cretaceous shale samples have temperatures of maximum pyrolysis yield tmax in the range of 337 515 c consistent with immature to post mature stages the tmax data also indicate that the safer and madbi shale samples h

assembling north china craton within the columbia supercontinent the role of double sided subduction abstract an evaluation of recent s wave receiver functions s wave velocities and two versions of p wave tomographic images along various transects in the north china craton provides some clues on the subduction collision history of the different crustal blocks and their final amalgamation within the paleoproterozoic columbia supercontinent interpretation of a n s seismic section of the craton suggests thick slab debris sinking to various depths in the mantle the w e seismic corridors show the preservation of a thick 200 km lithospheric root tectosphere beneath the ordos block and its variable and extensive erosion towards the yanliao block eastern block this zone is characterized by layers with marked velocity contrast and suggests repeated stacking of the remnants of underplated and accreted paleoproterozoic oceanic lithosphere the present day lithosphere asthenosphere boundary beneath this region probably marks the erosional plane along which decratonization occurred through subduction erosion from the east and thermal and material erosion by upwelling asthenosphere from below resulting in the partial destruction of the tectosphere and its thinning towards the east within the asthenosphere below the yanliao block younger and thinner slabs predominate in the absence of any prominent thick high velocity layers these younger slabs define a westward polarity and constitute a mega scale duplex formed by underplating through phanerozoic subduction process particularly the pacific plate subduction from the east the lithologic associations within the inner mongolia suture zone dividing the yinshan block to the north and ordos block to the south correspond to an accreted ocean plate stratigraphic sequence with the tonalite trondhjemite granodiorite ttg gneisses charnockites and calc alkaline granites representing a continental arc built up through subduction from the north the seismic transects bring out a contrasting polarity in the subduction regime with an oblique east to southward subduction of the yinshan block and a westward subduction of the yanliao block here i propose a double sided subduction history for the ncc similar to the ongoing subduction process in the western pacific such double sided subduction is considered to promote rapid amalgamation of continental fragments within supercontinents and the subduction polarities and mantle dynamics of ncc are therefore considered to be critical in evaluating the final assembly of the paleoproterozoic supercontinent columbia

petrography and zircon u pb isotopic study of the bayanwulashan complex constrains on the paleoproterozoic evolution of the alxa block westernmost north china craton abstract the bayanwulashan metamorphic complex bmc exposes along the eastern margin of the alxa block the westernmost part of the north china craton ncc bmc is principally composed of metamorphic rocks with amphibole plagiogneiss biotite plagioclase gneiss and granitic gneiss our research has been focused on the petrography and zircon u pb geochronology of the bmc to better understand the evolution of the alxa block and its relationship with the ncc evidences from field geology petrography and mineral chemistry indicate that two distinct metamorphic assemblages the amphibolite and greenschist facies had overprinted the preexisting granitic gneiss and suggest that the bmc experienced retrograde metamorphic episodes the la icp ms zircon u pb ages reveal that the primary magmatic activities of bmc were at ca 2 30 2 24 ga and the two metamorphic events were at ca 1 95 1 91 ga and ca 1 88 1 85 ga respectively these ages indicate that bmc initially intruded during paleoproterozoic not as previously suggested at archean period the early paleoproterozoic metamorphic records and the magmatic thermochronological data in bmc exhibit different evolution paths between the alxa block and the ncc the alxa block was most likely an independent early paleoproterozoic terrain following different amalgamation processes the alxa block combined with western block at ca 1 95 ga and then united with ncc at ca 1 85 ga

geology and geochemical constrains on the origin and depositional setting of the kpwa atog boga banded iron formations bifs northwestern congo craton southern cameroon abstract the kpwa atog boga kab area lies within the paleoproterozoic nyong unit which defines the northwestern edge of the archean congo craton in southern cameroon rock types in the area are mainly comprises of garnet gneiss associated with banded iron formations bifs the bifs and its host rocks were subjected to amphibolite facies metamorphism the rare earth elements ree patterns and the discrimination ratios e g al2o3 tio2 cr th th sc la sc of the garnet gneiss suggest that they are metasedimentary rocks derived from a felsic parent sources that have undergone intensive weathering the kab bifs exhibit significant concentration of tfe2o3 sio2 93 wt and lower al2o3 and tio2 contents and high field strength elements e g hf nb ta and zr this involves little contamination with continental detritus at their depositional environment the post archean australian shale paas normalized ree patterns of the kab bifs show enrichment in heavy ree and the depletion in light ree with positive la anomalies which imply that these bifs precipitated in ambient seawater moreover the contribution of low temperature hydrothermal fluids during the deposition of the studied bifs were highlighted through their high superchondritic y ho ratio mean 31 48 weak positive eu eu sn ratio mean 1 83 and the low contents of transition metals this suggests that the geochemical characteristics of the kab bifs and those of the paleoproterozoic superior type bifs are almost similar in summary low temperature hydrothermal fluids mixed with seawater contributed to the deposition of the kab bifs on the continental margin this depositional setting similar to its host rocks is consistent with the paleoproterozoic geological history recorded within the nyong unit this preliminary study reveals that the kab bifs are associated with low grade gold

age and genesis of the neoarchean algoma type banded iron formations from the dengfeng greenstone belt southern north china craton geochronological geochemical and sm nd isotopic constraints abstract banded iron formations bifs are important for understanding the composition and redox condition of paleo oceans as well as the evolution of the precambrian crust the neoarchean dengfeng greenstone belt ca 2 54 2 50 ga in the southern north china craton ncc consists of metamorphosed igneous and sedimentary assemblages including some bifs here we present mineralogical geochronological geochemical and sm nd isotopic data of the dengfeng bifs for the first time to constrain their age origins and source characteristics and then discuss their paleo environmental implications the bifs are mainly associated with metabasaltic rocks and quartz rich schists and can be classified as algoma type bifs zircon u pb dating of quartz rich schists interbedded with the bifs coupled with ages of wall rock metabasites demonstrate that the dengfeng bifs formed at 2 53 2 51 ga most bif samples have high contents of sio2 and fe2o3t and extremely low contents of al2o3 tio2 and high field strength elements e g zr hf th u indicating that they are detrital free deposits in the post archean australia shale normalized multi element diagram the detritus free bif samples are characterized by depleted light rare earth elements positive la and y anomalies and suprachondritic y ho ratios resembling geochemical features of modern seawater the absence of negative ce anomaly indicates an anoxic condition positive eu anomalies and relatively high concentrations of transition metal elements suggest that high temperature hydrothermal fluids contributed to the source of bifs isotopically all bif samples have positive end t values 0 12 to 4 53 consistent with the range of those 0 10 to 4 31 of ca 2 66 2 50 ga dengfeng metabasites and ttg gneisses suggesting input of possible juvenile crustal fluxes the elemental and isotopic characteristics of the dengfeng bifs collectively suggest that they precipitated from anoxic seawater with various input of hydrothermal fluids and oceanic crust derived fluxes

geochronology and geochemistry of the neoarchean lulong complex in the eastern hebei province north china craton implications on regional crustal evolution abstract the lulong complex exposed in the southeastern part of the eastern hebei province in the north china craton ncc is mainly composed of meta basalt biotite plagioclase gneiss migmatite amphibolite and granitoids mineral assemblages indicate the rocks in this complex underwent upper amphibolite facies metamorphism zircon u pb and lu hf data constrain that both protolith of the biotite plagioclase gneiss and the homogeneous domain of the migmatite formed at 2 53 ga and were mainly derived from the reworking of 2 8 2 7 ga crustal source the similar mineral association zircon u th pb systematics and geochemical features suggest that their protolith should be dominated by felsic volcanic sequences with limited contributions from older crust the protolith of the amphibolite gabbro or diabase was metamorphosed at 2477 20 ma geochemistry and zircon lu hf systems indicate that these meta mafic rocks originated from an enriched subcontinental lithospheric mantle source the granitoids exposed in the lulong area yield zircon u pb ages of 2507 21 ma to 2473 38 ma and record 2 5 ga regional metamorphic event the initial zircon 176hf 177hf ratios of the granite are in the range of 0 280640 0 281678 and suggest a complicated source where as those from the granodiorite indicate melting of juvenile crustal materials geochemically the felsic meta volcanics of the lulong complex are comparable to rhyodacite dacite with calc alkaline affinities the granitoids suite comprises 2 47 ga peraluminous granite felsic dikes and 2 51 ga metaluminous granodiorite all these felsic intrusions are characterized by moderate differentiation of ree patterns and low sr y ratios

ree geochemistry of carbonates from the guanmenshan formation liaohe group ne sino korean craton implications for seawater compositional change during the great oxidation event abstract the worldwide 2 33 2 06 ga positive 13 c carb excursion has been correlated with the great oxidation event goe and termed as the lomagundi event the 2 3 1 85 ga guanmenshan formation in the liaohe group of the northeastern sino korean craton is characterized by positive 13 c carb excursion and is a potential candidate to evaluate the lomagundi event using rey rare earth element and yttrium ree y chemical fingerprints here we present major and trace element analysis of 42 samples from the guanmenshan formation which are pure marine chemical sediments and use the data to trace the seawater composition during 2 3 1 85 ga 15 least altered dolomicrite samples 600 m strata have ree values of 0 739 4 175 ppm 2 414 1 184 ppm and the y ho ratios of 34 5 56 6 44 1 5 7 they show uniform positive la sn la sn 1 04 0 27 and gd sn gd sn 1 64 0 40 anomalies and notable lree depletions indicated by nd sn yb sn values of 0 24 0 92 average 0 56 0 19 these features are consistent with the geochemistry of well oxygenated shallow ambient seawater and suggest that these samples provide a robust record of the primary rey signature of seawater during the lomagundi event the rey patterns of 15 silicified dolomites marbles locally with veinlets from the pb zn mining camps in the region with average eu cn eu cn 1 56 0 95 are identical to those of high temperature hydrothermal fluids 250 c characterized by a flat pattern and marked positive eu anomalies indicating that these rocks were subjected to metasomatism by hydrothermal fluids the guanmenshan formation shows average ce sn ce sn of 0 93 0 09 and sm cn yb cn of 1 which are higher than those of the archean 2 33 ga chemical sediments generally f o 2 and p co 2 the rey in the dolomicrite were mainly sourced from fluxes of solutes from terrestrial weathering and also from seafloor hydrothermal processes on a subordinate scale the guanmenshan dolomicrites have eu sn eu sn values of 1 34 2 55 i e around 1 53 indicating that they were deposited during 2 33 2 06 ga as the eu sn eu sn 1 53 can be used as a proxy for the 2 33 2 06 ga marine chemical sediments our study shows that the guanmenshan formation was formed at a critical turning point in earth history when the global atmosphere hydrosphere system witnessed a dramatic change from reducing to oxidizing conditions

multi proxy approach to screen the hydrocarbon potential of the jurassic succession in the matruh basin north western desert egypt abstract this paper discusses the hydrocarbon potential of the jurassic succession in a mature basin in egypt as an important element of a larger petroleum system study prior to any organic petrographic and organic geochemical analyses a palynological age dating of the originally undifferentiated jurassic sequence was carried out to identify different formations under investigation this was based on key bioevents of some recovered dinoflagellate cysts and the identified lithology palynological toc rock eval pyrolysis including modified rock eval methods and vitrinite reflectance vro data from a total of 14 samples taken from the uppermost lower upper jurassic sequence represented by the wadi natrun toarcian aalenian khatatba late bathonian callovian and masajid oxfordian formations in the abu tunis 1 well are presented in addition two samples from the abu tunis 1 well and the proximal 32 km to the east siqueifa 1 well having the highest remaining hydrocarbon potential s2 yields were analyzed and their results were compared using modified pyrolysis programs although the data showed a good correlation between the tai of the palynomorph assemblage vitrinite reflectance and tmax from rock eval pyrolysis in the shallower intervals the correlation between vro and tmax was poor in the lower half of the studied succession middle khatatba and wadi natrun formations the very low tmax values indicate immature om while vro and tai indicated middle stage of oil window to past peak oil generation the reason for this discrepancy is that the deeper samples are reservoir rocks not source rocks and the majority of the organic matter is not composed of reactive kerogen but consists of migrated hydrocarbons and nso compounds contamination due to oil based mud obm was eliminated because the abu tunis 1 well was drilled in 1969 prior to the extensive use of obm in drilling this study showed that a multi proxy approach is the best way to screen the hydrocarbon potential in a thick succession that contains interbedded source and reservoir rocks

genesis of the hanwang fe deposit in neoarchean granite greenstone succession of the eastern north china craton abstract research on history of the earth s continents and the genesis of mineral deposits has been one of the hotspots in geology but only limited studies have been completed on iron deposits hosted by banded iron formations bifs such as the large hanwang iron deposit located in the eastern north china craton ncc in this contribution we present the petrology zircon u pb geochronology lu hf isotopes and whole rock geochemistry on hanwang iron deposit hosted by the metamorphosed neoarchean to early paleoproterozoic taishan group and orthogneiss derived from tonalite trondhjemite granodiorite ttgs magnetite bearing amphibolite magnetite bearing quartz amphibolite and gneiss host the hanwang iron deposit which is characterised by subhedral to euhedral magnetite pyrite with minor galena and bif laminations gangue minerals include hornblende plagioclase and quartz with minor amounts of biotite chlorite epidote and microcline this mineral assemblage is indicative of amphibolite facies metamorphism with regression to greenschist facies the deposit is located in a transition zone between metamorphosed mafic to felsic volcanic rocks the age of the hanwang bif hosted iron deposit is late neoarchean lu hf isotope systematics indicate that the protolith for the biotite amphibolite is derived from a neoarchean mantle contaminated by the crust and orthogneiss is derived from the neoarchean crust the biotite amphibolite is characterised by high na fe and low k assays enrichment in light rare earth elements lree and relatively depletion in heavy rees hree without an obvious ce anomaly the geochemistry of the orthogneiss is characteristic of a volcanic arc granite although this must be accepted with caution given that the orthogneiss is metamorphosed and deformed at amphibolite facies it is proposed that hanwang iron deposit was deposited in a marine volcanic arc environment and derived from hydrothermally leached volcanic units and similar to the algoma type iron deposit most bifs occurring in neoarchean granite greenstone succession of the eastern ncc belong to the algoma type iron deposits globally this sequence appears to be coeval with the yilgarn craton in western australia

age and evolution of a late archean to paleoproterozoic upper to lower crustal section in the wutaishan hengshan fuping terrain of northern china abstract the taihangshan wutaishan area forms part of the central north china craton and consists of three main components the hengshan and fuping complexes containing predominantly ductilely deformed late archean to paleoproterozoic high grade granitoid orthogneisses intruded by mafic dykes of gabbroic composition and the low grade late archean wutai greenstone belt developed between the high grade terrains and consisting of bimodal volcanic rocks and metasediments associated with coeval granitoids zircon dating shows that both the hengshan and fuping complexes were intruded by major granitoid bodies between 2 52 and 2 48 ga with rare occurrences of 2 7 ga gneisses wutai granitoids adjacent to the fuping complex also contain 2 7 ga xenocrysts all possibly reflecting the same older basement rare granitic gneisses in the hengshan have emplacement ages of 2 35 ga and show the same structural features as the older rocks indicating that the main deformation occurred after 2 3 ga deposition of supracrustal rocks and emplacement of mafic dykes occurred in the paleoproterozoic together with intrusion of granitic bodies at 2 1 2 0 ga and all underwent regional metamorphism at 1 88 1 85 ga we interpret the hengshan and fuping granitoid gneisses as the lower plutonic part of a late archean to early paleoproterozoic japan type magmatic arc with the upper volcanic part represented by the wutai complex components of this arc may have evolved at a continental margin as indicated by the 2 7 ga zircons virtually all magmatic events are recorded in all three complexes however the 1 85 ga metamorphic event is only recorded in the higher grade portions of the wutai since low grade and presumably high crustal level precluded metamorphic zircon growth shear zones in the hengshan are interpreted as major lower crustal discontinuities post dating the peak of high pressure granulite metamorphism we suggest that they formed during orogenic collapse and uplift of the hengshan complex in the late paleoproterozoic 1800 ma

geochemical constraints on the genesis of the ekou banded iron formation shanxi province north china the ekou banded iron formation bif in the wutai area is hosted within the late archean baizhiyan formation the mineral assemblage is used to identify oxide and silicate facies the oxide facies is composed of magnetite and quartz and the silicate facies is characterized by the presence of silicate minerals a geochemical analysis shows that the major elemental compositions are dominantly sio2 and fe2o3t with very little al2o3 and tio2 and minor abundances of incompatible elements and transition elements these results indicate that negligible terrigenous materials were involved in the bif deposition the rare earth elements rees normalized by post archean australian shale exhibit the characteristics of light ree lree depletion heavy ree hree enrichment and positive la y and eu anomalies the y ho ratios are superchondritic these results indicate that the material that formed the ekou bif originated from the mixing of seawater and submarine hydrothermal fluids a ce anomaly deficiency and heavy fe isotope enrichment indicate that the ekou bif formed in an anoxic marine environment the 30sinbs 28 values for quartz in the ekou bif are similar to those of other bifs with distribution ranges of modern sinters and black smokers the 18ov smow distribution is similar to that of hydrothermal sedimentary siliceous rocks these results suggest that the formation of the ekou bif was closely connected to submarine volcanic exhalation activity

collision leading to multiple stage large scale extrusion in the qinling orogen insights from the mianlue suture abstract the geologic framework of the phanerozoic qinling dabie orogen was built up through two major suturing events of three blocks from north to south these include the north china craton including the north qinling block the qinling dabie microblock and the south china craton including the bikou block separated by the shangdan and mianlue sutures the mianlue suture zone contains evidence for mesozoic extrusion tectonics in the form of major strike slip border faults surrounding basement blocks a late paleozoic ophiolite and a ca 240 200 ma thrust belt that reformed by 200 150 ma thrusts during a type intracontinental subduction the regional map pattern shows that the blocks are surrounded by complexly deformed devonian to early triassic metasandstones and metapelites forming a regional scale block in matrix melange fabric five distinct tectonic units have been recognized in the belt 1 basement blocks including two types of precambrian basement crystalline and transitional 2 continental margin slices including early paleozoic strata and late paleozoic fluviodeltaic sedimentary rocks proximal and distal fan clastics reflecting the development of a north facing rift margin on the edge of the south china plate 3 out of sequence oceanic crustal slices including strongly deformed postrift deep water sedimentary rocks sheeted dikes basalts and mafic ultramafic cumulates of a late paleozoic ophiolite suite developing independent of the rift margin in a separate basin 4 out of sequence island arc slices 5 accretionary wedge slices all the tectonic units were deformed during three geometrically distinct deformation episodes d 1 d 2 and d 3 during 240 200 ma units 2 4 involved southward thrusting and vertical then southward extrusion of about 20 km of horizontal displacement above the autochthonous basement during the d 1 episode thrust slices 20 km south of the mianlue suture are related to this vertical extrusion due to the same rock assemblages ages and kinematics the d 2 and d 3 episodes folded all the units in a thick skinned style about east west d 2 and west northwest d 3 axes in the mianlue suture zone an early foreland propagating sequence of accretion of late paleozoic rocks deposited above the yangtze craton is not involved in d 1 deformation but is temporally equivalent to the d 2 and d 3 deformation in the mianlue suture two stages of strike slip faulting mainly occurred at the end of d 2 and d 3 respectively during d 2 deformation the bikou block was obliquely indented to the ese into the mianlue suture rather than being thrust over the mianlue suture from the north as a part of the qinling dabie microblock during d 3 deformation however the bikou block was bounded by the south boundary fault of the mianlue suture and the yangpingguan fault on the south these faults are coeval strike slip faults but of opposite senses and accommodated minor southwestward extrusion of the bikou block into songpan ganze orogen the other basement blocks north of the mianlue suture were extruded eastward by about 20 km of lateral displacement based on the offset of the wudang dome during the d 3 episode due to the northeastward indentation of the hannan complex of the south china craton post d 3 emplacement of granite cutting across the strike slip faults such as the mianlue suture provides a minimum age of 200 ma for d 3 deformation therefore based on insights from the evolution of the mianlue suture the d 2 and d 3 episodes in the mianlue suture and its neighbors are not responsible for and associated with the two stage extrusion of the dabie uhp hp terranes from the foping dome to the present erosional surface more than 350 km

geochemistry of the krivoy rog banded iron formation ukraine and the impact of peak episodes of increased global magmatic activity on the trace element composition of precambrian seawater abstract pure superior type banded iron formation bif samples from the krivoy rog supergroup ukraine are excellent archives of ambient early precambrian seawater they show low concentrations of incompatible elements such as zr hf and th and shale normalized rare earths and yttrium rey sn patterns similar to those of modern seawater i e heavy rey sn enriched patterns with positive la sn gd sn and y sn anomalies lack of ce sn and presence of positive eu sn anomalies indicate rey contributions to anoxic ferruginous seawater from high temperature hydrothermal fluids the depositional age of the krivoy rog bif is ill defined but a late archean to paleoproterozoic age has been suggested based on u pb zircon ages for units stratigraphically above and below the bif we determined sm nd isotopic compositions of pure and impure samples from the krivoy rog bif which yield an errorchron with an apparent age of 2406 350 ma mswd 15 that falls within this broad age range all pure bif samples show chondrite normalized subscript cn rey patterns with strong positive eu cn anomalies that are typical for archean but rather rare and much less pronounced in proterozoic bifs associated schists also show archean rather than post archean style rey distributions the rey geochemistry of both chemical and epiclastic sediments therefore is more consistent with a late archean rather than a post archean depositional age of the krivoy rog supergroup initial nd values of impure bifs and of associated schist reveal variable contributions from ttgs less radiogenic in nd and a more radiogenic component possibly comprised of basement amphibolites or mafic volcanics of the stratigraphically underlying new krivoy rog group the purest krivoy rog bif representing local krivoy rog seawater displays an nd 2 60 ga value of 2 3 this value is less radiogenic than impure krivoy rog bifs or other near contemporaneous neoarchean pure chemical sediments to preserve this specific local isotopic fingerprint in anoxic archean seawater the krivoy rog bif must have been deposited in an isolated sea basin with limited exchange with ferruginous deep waters of the open ocean a compilation of rey data for high purity precambrian bifs reveals that eu cn eu cn ratios of precambrian seawater follow a general global evolution curve that shows specific peaks which reflect times of increased high temperature hydrothermal rey input into seawater following declining eu cn eu cn ratios from the eoarchean to the mesoarchean the ratios suddenly rise at 2 7 ga and reach a maximum at 2 6 ga indicating an increased flux of high temperature hydrothermal rey to neoarchean seawater which supports the hypothesis that times of widespread bif deposition coincided with periods of intense submarine hydrothermal activity probably triggered by major mantle plume events this association is supported by a strong increase of the nd t values of pure seawater archives at 2 7 2 6 ga which reflects an increased flux of mantle nd into seawater these results suggest that eu rey systematics and potentially nd systematics are robust tools to indentify episodes of enhanced mantle plume activity

geochemical constraints on the genesis of the algoma type banded iron formation bif in yishui county western shandong province north china craton abstract the yishui iron deposit is a typical algoma type banded iron formation bif located in yishui county taishan group shandong province in the eastern block eb of the north china craton ncc the bif is interlayered with amphibolite migmatitic granite gneiss and schist integrated petrological mineralogical and geochemical interpretations of the iron ores were used to reconstruct the genesis of the yishui bifs and their tectonic evolution the iron ore samples were collected from surface outcrops banding textures are not always present in the iron ores because the bif experienced complex high grade metamorphism after precipitation interpretations of the major elements and rare earth elements plus yttrium rey data of the iron ores suggests that the ore forming materials of the yishui bif precipitated from a mixture of seawater and high temperature hydrothermal fluids 2 and fe 2 o 3 t sio 2 fe 2 o 3 t 85 8 95 8 wt are consistent with the major minerals such as quartz and iron oxide which represent chemical precipitates chlorite amphibole plagioclase biotite and calcite are present as minor minerals the low contents of tio 2 0 01 0 09 wt al 2 o 3 0 42 1 18 wt and high field strength elements hfses indicate little to no detrital assimilation which implies that the iron ores preserved their original geochemical signatures based on geochemical interpretations we conclude that the yishui bif can be explained by a mantle plume model that describes not only the tectonic evolution but also the sources of iron and silica of the yishui bif

modeling of gas generation from the alam el bueib formation in the shoushan basin northern western desert of egypt the shoushan basin is an important hydrocarbon province in the northern western desert egypt but the burial thermal histories for most of the source rocks in the basin have not been assigned yet in this study subsurface samples from selected wells were collected to characterize the source rocks of alam el bueib formation and to study thermal history in the shoushan basin the lower cretaceous alam el bueib formation is widespread in the shoushan basin which is composed mainly of shales and sandstones with minor carbonate rocks deposited in a marine environment the gas generative potential of the lower cretaceous alam el bueib formation in the shoushan basin was evaluated by rock eval pyrolysis most samples contain sufficient type iii organic matter to be considered gas prone vitrinite reflectance was measured at eight stratigraphic levels jurassic cretaceous vitrinite reflectance profiles show a general increase of vitrinite reflectance with depth vitrinite reflectance values of alam el bueib formation range between 0 70 and 0 87 vrr indicating a thermal maturity level sufficient for hydrocarbon generation thermal maturity and burial histories models predict that the alam el bueib source rock entered the mid mature stage for hydrocarbon generation in the tertiary these models indicate that the onset of gas generation from the alam el bueib source rock began in the paleocene 60 ma and the maximum volume of gas generation occurred during the pliocene 3 2 ma

archean granulite gneisses from eastern hebei province china rare earth geochemistry and tectonic implications the granulite gneisses and their retrograded products of the qianxi group from eastern hebei province china have been investigated for their isotope and trace element geochemistry a consistent age of about 2 5 ae has been obtained by the rb sr and sm nd whole rock isochron methods in agreement with the zircon u pb data pidgeon 1980 d y liu unpubl geochemical arguments from initial isotopic ratios isr and ind and elemental distribution patterns have led us to conclude that this age of about 2 5 ae represents the time of granulite facies metamorphism which must have followed closely the primary emplacement of their protoliths previous claims for early archean ages 3 5 ae of these granulites are not substantiated the mineral isotope systematics register an important thermal event at about 1 7 ae roughly corresponding to the time of the widespread luliang orogeny ma and wu 1981 or chungtiao movement huang 1978

shrimp u pb and cameca 1280 oxygen isotope results from ancient detrital zircons in the caozhuang quartzite eastern hebei north china craton evidence for crustal reworking 3 8 ga ago the caozhuang quartzite located in eastern hebei province in the eastern part of the north china craton has long been known to contain the oldest detrital zircon population in china a suite of 30 zircons with preliminary ages 3 7 ga were selected for detailed shrimp u pb and cameca 1280 oxygen isotope analyses shrimp u pb dating reveals that ages range from 3860 to 3135 ma the former being the oldest 207 pb 206 pb zircon age so far obtained from the north china craton values of 18 o range from 5 4 to 7 5 permil similar to archean igneous zircons worldwide but the average value for the 30 zircons is 6 6 permil which is 0 3 permil higher than suite averages for detrital zircons from the jack hills and beartooth mountains and nearly 1 permil higher than magmatic zircons from igneous rocks of the superior province and barberton these elevated values of 18 o suggest reworking of a significant amount of juvenile crust older than 3 8 ga that underwent a low temperature supracrustal history and was distributed throughout the source region in northeast china

geology and geochemistry of the macheng algoma type banded iron formation north china craton constraints on mineralization events and genesis of high grade iron ores abstract the macheng iron deposit is located in the eastern hebei province of the north china craton ncc it is hosted in neoarchean metamorphic rocks of baimiaozi formation in the dantazi group consisting of biotite leptynite plagioclase gneiss plagioclase amphibolite migmatite migmatitic granite and quartz schist geochemical analyses of the host biotite leptynite and plagioclase amphibolites show that their protoliths are both volcanics inferred to be trachytic basalt and basaltic andesite respectively based on the geochemical signature of the host rocks together with geology of the iron deposit it is inferred that the macheng bif is an algoma type iron exhalative formation formed in an arc related basin in the neoarchean post archean australian shale paas normalized rare earth elements rees plus yttrium y concentrations of different bif ores with gneissic striated and banded structure in the macheng deposit show similar patterns with depletions in light rare earth elements lrees and middle rare earth elements mrees relative to heavy rare earth elements hrees and with apparently positive la y and eu anomalies y ho ratios of the gneissic striated and banded bif ores vary from 37 to 56 these geochemical features of the bif ores reveal their affinity with the sea water and the presence of a high temperature hydrothermal component indicating that both the seawater and high temperature hydrothermal fluids derived from alteration of oceanic basalts and komatiites may contribute to formation of the macheng bif geological mineralogical and geochemical studies of the macheng deposit recognized two kinds of high grade iron ores one is massive oxidized high grade ore fe2o3t 74 37 86 20 wt mainly consisting of hematite with some magnetite which shows geochemical characteristics of the gneissic striated and banded bif ores the other type is magnetite high grade ore also massive and consisting of magnetite with distinct characteristics in trace elements of the gneissic striated and banded bif ores but show similarity to those of the migmatitic iron ores with significantly negative eu anomalies the geochemical discrepancy or duality between the two types of high grade ores in macheng suggests that they formed by two different mechanisms one is related to supergene enrichment caused by oxidation of magnetite and the leaching of gangue minerals from bif to form high grade ore the other is probably related to intensive migmatization which produced high grade ores by altering the primary iron ores

organic geochemical palynofacies and petrographic analyses examining the hydrocarbon potential of the kharita formation albian in the matruh basin northwestern egypt abstract a recent study of selected samples from the cretaceous albian kharita formation of egypt revealed very good to excellent source rock sr potential for six intraformational organic rich intervals this work investigates the sr potential of the entire kharita formation across the matruh basin using samples from two wells the abu tunis 1x well from the central part of the basin and the siqeifa 1x well drilled on the eastern margin of the basin more strongly reducing conditions were developed in the centre of the basin and resulted in the deposition of more organic rich shales by comparison to the less reducing conditions that prevailed on the eastern basin margin where the shales contain less organic matter deltaic intraformational shales and carbonates in the kharita formation of abu tunis 1x constitute a significant 120 m net of the potential sr the lower kharita formation contains 34 m net shale sr of good to very good excellent organic richness yielding values of 1 14 11 59 wt total organic carbon toc the organic matter has low hydrogen index hi values 184 389 mg hc g toc and amorphous organic matter aom and relatively high non opaque phytoclast frequencies indicating mainly gas oil prone organofacies kerogen types iii ii the upper kharita is rather important containing 86 m net shale carbonate sr that has fair to good organic richness 0 8 1 8 wt toc and lower hi 126 250 mg hc g toc a dominance of non opaque phytoclasts and subordinate aom frequencies which together indicate gas oil prone organofacies kerogen iii ii in the siqeifa 1x well kharita deltaic intraformational shales and shaley dolostones comprise 80 m net sr which has mainly fair to good to less very good organic richness 0 8 2 1 wt toc whilst low hi 93 220 mg hc g toc dominance of non opaque phytoclasts and subordinate aom indicate gas prone organofacies kerogen type iii a relative upward increase in deposition of lignite and coaly carbonaceous material supports a gas prone organofacies whilst thermal maturity indices only point to immature to early mature pre to early oil window srs in both the abu tunis 1x and siqeifa 1x wells hydrocarbon exploration focussing on this potential source rock may be justified in areas to the southeast of the matruh basin where modelling indicates this unit may have reached the late mature oil to main gas generation windows

geologic framework and tectonic evolution of the qinling orogen central china abstract the geologic framework of the qinling orogen was built up through interplay of three blocks the north china block including the north qinling the south qinling and the south china block separated by the shangdan and mianlue sutures the shangdan suture resulted from middle paleozoic collision of the north china block and the south qinling the mianlue suture resulted from late triassic collision of the south qinling and the south china block present upper crust of the qinling is structured dominantly by thrust fold systems the north qinling displays thick skinned deformation with crystalline basement involved whilst the south qinling is characterized by thin skinned thrusts and folds detached above the lower sinian two types of precambrian basement crystalline and transitional are defined according to lithology and metamorphic grade and different in age stratigraphic and sedimentary architecture is characterized by distinct zonation the qinling orogen experienced a prolonged continental divergence and convergence between blocks during the period from late neoproterozoic to early paleozoic times the south qinling was the northern margin of the south china block and the north qinling was the southern margin of the north china block separated by a proto tethyan qinling ocean the north qinling evolved into an active margin when the proto tethyan qinling ocean subducted northward during ordovician time collision of the south and north qinling took place in middle paleozoic along the shangdan suture synchronous with the collision rifting occurred at the southern rim of the south qinling and was followed by the opening of the paleo tethyan qinling ocean during the late paleozoic resulting in the splitting of the south china block from the south qinling collision of the south qinling and the south china block came about in the late triassic along the mianlue suture the late triassic collisional orogeny caused extensive fold and thrust deformation and granitoid intrusions throughout the qinling and led to final amalgamation of the north and south china blocks

geochemical and geological modeling of the late jurassic meem shale member in the al jawf sub basin yemen implications for regional oil and gas exploration abstract the late jurassic meem shale member is widely exposed within the sabatayn basin including in the al jawf sub basin in this study a total of 49 shale samples of the meem shale member from two exploratory wells dahmar ali 01 28 samples in the depth range 1441 2351 m and kamaran 01 21 samples in the depth range 2275 3011 m in the south eastern portion of the al jawf sub basin were studied as possible source rocks evaluation of source rock geochemistry including total organic matter toc and rock eval pyrolysis analyses was performed the results of this study provided information regarding significant exploration of oil and gas resources from the meem shale member in the al jawf sub basin both the incident light microscopic results and the rock eval geochemical study have indicated that the analyzed shale samples contain predominantly type iii kerogen with minor contributions of type i ii and iv so as to they can generate both oil and gas the values of hydrogen index hi are decreased with increasing thermal maturity and varied between 34 and 803 mg hc g toc based on the measured vro range of 0 63 and 2 00 the meem shales are currently within the oil and gas generation window thus they are considered as shale oil and gas resource possible plays the geochemical results were integrated with geological data of the al jawf sub basin and incorporated into a basin model to simulate petroleum generation and or retention in the meem shale member the constructed basin models illustrated that the meem shale member displayed a peak oil generation window during the late jurassic to early miocene where large amounts of oil were generated with a conversion ratio of 10 75 tr further the high heating rates from local volcanic rocks from the early miocene till date have caused gas generation from both of the original type iii kerogen and from secondary cracking of retained oil in the meem shale member however the main contribution of gas generation within the meem shale member came from secondary cracking processes through geologic time the good thickness 736 910 m of the organically rich high genetic potential and matured meem shale member is expected to provide large amounts of oil and gas within the south eastern portion of al jawf sub basin therefore the al jawf sub basin can be considered as a rich oil and gas exploration province

archean blocks and their boundaries in the north china craton lithological geochemical structural and p t path constraints and tectonic evolution an examination of lithological geochemical geochronological structural and metamorphic p t path data suggests that the basement of the north china craton can be divided into eastern and western blocks separated by major crustal boundaries that roughly correspond with the limits of a 300 km wide zone called the trans north china orogen the eastern block consists predominantly of late archean domiform tonalitic trondhjemitic granodioritic ttg batholiths surrounded by anastomosing networks and linear belts of open to tight synforms of minor volcanic and sedimentary rocks metamorphosed from greenschist to granulite facies at 2 5 ga with anticlockwise p t paths some early to middle archean rocks are locally present in the eastern block but their tectonic history is unclear due to reworking by the 2 5 ga tectonothermal event the western block has a late archean assemblage structural style and metamorphic history similar to that of the eastern block but it differs in the absence of early to middle archean assemblages and in being overlain by and interleaved with paleoproterozoic khondalites which were affected by a 1 8 ga metamorphic event involving clockwise p t paths a mantle plume model is proposed for the formation and evolution of late archean basement rocks in the eastern and western blocks based on a combination of extensive exposure of ttg gneisses affinities of mafic rocks to continental tholeiitic basalts presence of voluminous komatiitic rocks dominant diaprism related domiform structures anticlockwise p t paths and a short time span from the primary emplacement of ttg and ultramafic to mafic rocks until the onset of regional metamorphism between the two blocks is the trans north china orogen which is bounded by two major fault systems and is composed of late archean to paleoproterozoic ttg gneisses and granitoids interleaved with abundant sedimentary and volcanic rocks that are geochemically interpreted as having developed in magmatic arc and intra arc basin environments these rocks underwent multiple phases of compressional deformation and peak high pressure metamorphism followed by rapid exhumation during the late paleoproterozoic at 1 8 ga as a result of collision between the eastern and western blocks resulting in the amalgamation of the north china craton

the paleoproterozoic north hebei orogen north china craton s collisional suture with the columbia supercontinent abstract understanding the geologic history and position of the north china craton in the paleoproterozoic columbia supercontinent has proven elusive paleoproterozoic orogenic episodes 2 00 1 85 ga are temporally associated with ultimate stabilization of the north china craton ncc followed by the development of extensive craton wide rift systems at 1 85 1 80 ga the age difference between the sedimentary cover and the metamorphic basement is up to 500 700 ma suggesting that uplift and doming of cratonic basement occurred in the latest paleoproterozoic mafic dike swarms 1 80 1 77 ga and anorogenic magmatism 1 80 1 70 ga record the extensional breakup and dispersal of the north china craton during this stage the late paleoproterozoic tectonic framework and geological events documented provide important constraints for reconstruction of the ncc within the late paleoproterozoic supercontinent of columbia an east west striking thousand kilometer long belt of khondalites granulite facies metapelites stretches along the northern margin of the north china craton on the cratonward side of the northern hebei orogenic belt this granulite belt includes mg al sapphirine bearing granulites that reached ultrahigh temperature peak metamorphic conditions of 1000 c at 10 kbars at 1927 11 ma following peak ultrahigh temperature conditions the rocks underwent initial isobaric cooling and subsequent isothermal decompression and these trajectories are interpreted to be part of an overall anti clockwise p t evolution indicating that the northern margin of the craton experienced continental collision at 1 93 1 92 ga the position of the khondalite belt south of the northern hebei orogenic belt makes it analogous to tibet a continental collision related plateau characterized by double crustal thicknesses and granulite facies metamorphism at depth we suggest that the tectonic evolution of the ncc during this period was closely related to the assembly and break up of the columbia supercontinent and that the ncc was adjacent to the baltic and amazonian cratons in the period 2 00 1 70 ga craton wide extension occurred within 100 150 ma of collision along the northern margin of the craton at 1 93 1 92 ga it is concluded that mantle upwellings are chiefly responsible for the breakup of the ncc from the paleoproterozoic supercontinent

early paleozoic to devonian multiple accretionary model for the qilian shan nw china abstract the qilian shan located in the northern tibetan plateau of nw china has an excellent record of early to middle paleozoic subduction accretion that resulted from convergence between the alax and qaidam blocks but there is no consensus about its detailed tectonic history this paper summarizes its tectonic divisions and discusses its tectonic evolution from the cambrian to devonian the belt has the following tectonic divisions in the far north the southern passive margin of the alax block is juxtaposed against an early cambrian to ordovician marianan type intra oceanic arc north qilian the north qilian arc is separated from a japanese type arc central qilian to the south by a high pressure metamorphic belt composed mostly of ophiolitic slices and oceanic crusts the central qilian is bounded to the south by a wide melange zone south qilian consisting of ophiolitic slices and continental margin sequences farther to the south lies the oulongbuluk microcontinent that is separated from the qaidam block and farther south by an ultrahigh pressure metamorphic belt tectonostratigraphic analysis together with geochemical geochronological and geophysical data indicates a complex evolution by subduction accretion processes from the cambrian to the devonian before final amalgamation and docking to the northern alax block this model solves the long lasting discussion on the polarity of subduction in paleozoic time this multiple subduction accretion history sheds light on the continuity of paleozoic sutures along the qilian shan and the nature of the altyn tagh fault and thus contributes to an improved understanding of the tectonic architecture of central asia

egypt far western desert basins petroleum charge system as defined by oil chemistry and unmixing analysis abstract source rock analysis and maturity modeling in egypt s far western desert basins indicates that several jurassic and cretaceous petroleum source rocks could be mature and expelling hydrocarbons over much of the area highly variable oil characteristics suggest that more than one charge system may be present reservoir crude oil chemistry indicates that the fluids originated from a single source facies generated over a wide range of organic maturity secondary alteration mechanisms include water washing biodegradation and evaporative fractionation crude oils in the upper reservoirs tend to be most affected by biodegradation and water washing condensates and light crude oils in the deepest reservoirs appear to be the residual phase from evaporative fractionation the broad variation in fluid properties and chromatogram character can be attributed to a multi end member mixing system early mature waxy oil later high maturity volatile oil low level biodegradation migrated phase evaporative fractionation and residual phase evaporative fractionation a statistical unmixing algorithm was used to determine end member compositions and relative mixing proportions for each crude oil sample remigration and mixing of the products from multi phase generation and alteration results in complex reservoir fluid compositions such that no single reservoired oil represents a compositional end member rather the oil samples are mixtures of various amounts of multiple end members mixing proportions show regional trends and variations in different reservoirs within a single field the crude oil geochemical data and unmixing analysis support a complex charge history from a single source rock rather than mixing of oils from two source rocks

late neoarchean magmatic and subsequent metamorphic events in the northern north china craton shrimp zircon dating and hf isotopes of archean rocks from yunmengshan geopark miyun beijing abstract the miyun geopark is located in the northern part of the north china craton and includes a variety of archean granitoids and metamorphic rocks shrimp zircon dating documents their emplacement ages at 2567 2508 ma metamorphic events at ca 2 50 ga 2 44 ga and 1 82 ga late neoarchean basement rocks elsewhere in the north china craton document a similar evolutionary history with intrusion of granitoid rocks commencing at ca 2590 ma and reaching three peaks at about 2550 2520 and 2500 ma these rocks then underwent regional metamorphisms at ca 2 50 ga 2 44 ga 1 92 ga and 1 82 ga positive e hf t values of 1 7 to 7 6 with model ages younger than 3 0 ga for magmatic zircons from these late neoarchean intrusive rocks indicate that they are predominantly derived from juvenile crustal sources and suggest that significant crustal growth occurred in the northern north china craton during the neoarchean the intrusive rocks in the miyun area are part of an extensive province of late neoarchean magmatism and appear to be related to mantle plume activity at this time

geochronology and geochemistry of the nanfen iron deposit in the anshan benxi area north china craton implications for 2 55 ga crustal growth and the genesis of high grade iron ores abstract the nanfen iron deposit situated in the anshan benxi area of the north china craton ncc hosts scattered banded iron formations bifs and bif hosted high grade iron orebodies defined by feot 80 wt the associated roof and bottom rocks which occur in parallel to orebodies are mica quartz schist and chlorite hornblende schist locally interlayers respectively the paas normalized rare earth element ree patterns indicate that quartz magnetite banded iron formations bifs are enriched in heavy rees with positive la eu and y anomalies indicating that the nanfen bif precipitated from a mixture of ambient seawater and submarine high t hydrothermal fluids 0 1 in addition the bif exhibits low concentrations of al2o3 and tio2 with depletion in hfses high field strength elements e g nb ta zr hf and ti suggesting little contamination with continental detritus excepting lower eu eu values the high grade iron ores display similar geochemical features to the bif likely implying that they have a same iron source but did not form synchronously electron microprobe analyses empa and in situ la icp ms analyses reveal that in relative to the magnetite grains in high grade ores the magnetite grains in the bif contain higher mno mgo and zn concentrations and lower al2o3 and ree abundances implying different formation conditions thus we argue that high grade ores are likely to be alteration products of primary bifs chlorite hornblende schist meta basalt samples exhibit low sio2 contents 43 78 50 30 wt with flat chondrite normalized ree and n morb normalized trace element patterns which are similar to those of babb back arc basin basalt the zircon u pb dating results indicate that magmatic and metamorphic zircons from chlorite hornblende schist formed at approximately 2552 ma and 2482 ma respectively the former volcanic age could constrain the deposition age of the nanfen bif and the latter reflects a subsequent metamorphic event which is also reflected by the 2480 ma hydrothermal zircons selected from the bif moreover most zircons from chlorite hornblende schist have positive hf t values 0 22 7 96 close to the depleted mantle evolutionary curve implying that the source magma were derived from a depleted mantle with some contamination from ancient crust hence we propose that the nanfen bif is a neoarchean algoma type bif and its deposition was most likely benefited due to crustal growth

geochemistry of a hydrothermal sediment core from the obs vent field 21 n east pacific rise abstract we report a geochemical investigation of near vent hydrothermal sediment from the east pacific rise collected in a short 11 cm core by submersible from the obs vent site at 21 n the sediment has high concentrations of fe 23 25 cu 10 22 zn 4 13 and s 22 30 but low mn concentrations 108 279 ppm indicating a much fresher sulfidic input than in previously reported near vent sediments from the mid atlantic ridge further studies are required to fully characterise near field hydrothermal sediments but the data in this study allow the first comparisons with similar near vent material collected previously from the mid atlantic ridge as well as ridge flank data from the epr and juan de fuca ridge compared to ridge flank sediments the near field sediments exhibit high ratios of fe mn cu fe and zn fe unlike cu and zn pb is not so significantly enriched pb isotope compositions are indistinguishable from local morb indicating a dominant hydrothermal source for this element in these sediments this may not necessarily be the case for ridge flank sediments however where scavenging from seawater may also be important ree distributions throughout the core exhibit low concentrations and characteristics typical of vent fluids with lree enrichments and pronounced positive eu anomalies these patterns are similar to those of relatively fresh sulfides but there is also evidence for progressive oxidation downward into the core by ingressing seawater

extreme crustal metamorphism during columbia supercontinent assembly evidence from north china craton abstract a vast supracrustal belt of khondalites granulite facies metapelites occur along the northern margin of the north china craton we report here for the first time spinel quartz equilibrium assemblage from these rocks in two textural settings 1 high zno up to 14 47 wt spinel with quartz as inclusions within garnet porphyroblasts defining pressure above 12 kbar and temperature of 900 c and 2 low zno down to 1 2 wt spinel in association with quartz in the matrix assemblage formed during peak ultrahigh temperature conditions ca 975 c and 9 kbar we present a unique case of decompression where the metamorphic conditions of the rocks traversed mostly through the spinel quartz extended stability field monazite grains in textural association with both types of spinel quartz textures were analysed for age determination and the data define two age peaks at 1927 11 ma and 1819 11 ma since the peak thermal regime of the khondalites was close to or exceeded the theoretical closure temperature of pb in monazite we infer the 1819 11 ma age as the timing of ultrahigh temperature event in this craton our data lend support to the idea of ca 1 9 1 8 ga e w collisional orogen at the northern margin of the north china craton we correlate the extreme crustal metamorphism with tectonics associated with the assembly of the north china craton within the columbia supercontinent

geophysical and geological tests of tectonic models of the north china craton abstract the geometry and timing of amalgamation of the north china craton have been controversial with three main models offering significantly different interpretations of regional structure geochronology and geological relationships one model suggests that the eastern and western blocks of the ncc formed separately in the archean and an active margin was developed on the eastern block between 2 5 and 1 85 ga when the two blocks collided above an east dipping subduction zone a second presumes the eastern block rifted from an unknown larger continent at circa 2 7 ga and experienced a collision with an arc perhaps attached to the western block above a west dipping subduction zone at 2 5 ga and the 1 85 ga metamorphism is related to a collision along the northern margin of the craton when the ncc joined the columbia supercontinent a third model suggests two collisions in the central orogenic belt at 2 1 and 1 88 ga but recognizes an early undated deformation event recent seismic results reveal details of the deep crustal and lithospheric structure that support both the second and third models showing that subduction beneath the central orogenic belt was west directed and that there is a second west dipping paleosubduction zone located to the east of the cob dipping beneath the western block ordos craton the boundaries identified through geophysics do not correlate with the boundaries of the trans north china orogen suggested in the first model and the subduction polarity is opposite that predicted by that model high pressure granulite facies metamorphism at 1 85 ga is not restricted to the tnco as suggested by the first model but is documented across the ncc as predicted by the second model suggesting a major continent continent collision along the north margin of the craton at 1 85 ga further it has recently been shown that in the southern tnco there is no record of metamorphism at circa 1 85 ga but only at 2 7 2 5 ga showing that the tnco as defined as a circa 1 85 ga orogen does not exist this is further confirmed by recent re os isotopic studies which show that the subcontinental lithospheric mantle beneath the southern cob is late archean in age and that a province in the northern ncc is circa 1 8 ga correlating with the proposed collision belt of the ncc with the columbia supercontinent across the entire ncc the cob is an archean convergent belt re worked in the paleoproterozoic and the paleoproterozoic tectonism is widespread across the ncc as predicted by the model whereby the previously amalgamated eastern and western blocks experienced a continental collision with columbia at circa 1 85 ga but uplift exhumation rates are slow necessitating a re evaluation of the tectonic models of the ncc

sm nd geochronology and petrography of garnet pyroxene granulites in the northern sulu region of china and their geotectonic implication abstract abundant garnet bearing granulite lenses are widely distributed in the northern part of the sulu region and adjacent areas they are possibly re metamorphosed high pressure metamorphic rocks on the basis of detailed petrographic study samples wd01 wd04 and ml06 from laixi and wendeng were identified as high pressure granulites and wh1 from weihai as an original coesite bearing eclogite three high pressure granulite samples give mineral wr isochron ages of 1846 76 1743 79 and 1752 30 ma tdm ages are 3 3 3 0 and 2 8 ga the sm nd mineral wr isochron ages are interpreted to date as the metamorphic resetting within the medium pressure granulite facies representing an isotopic re homogeneity during uplifting of the high pressure granulites from deep continent crust it is important that sm nd chronological characteristics are the same as archaean high pressure granulites in the north china craton however sample wh1 from weihai demonstrates abnormal sm nd characteristics its whole rock end 0 value is 129 tdm age is 1 3 ga and constrains the minimum age of re metamorphosed eclogite protolith formation to the mid proterozoic this result is identical to those reported by jahn 1994 showing complicated processes of metamorphism and metasomatism the data in this paper provide further evidence to define the boundary between the north china craton and uhpm belt in eastern shandong and to understand the geotectonic nature of the boundary

the sequence stratigraphic concept and the precambrian rock record an example from the 2 7 2 1 ga transvaal supergroup kaapvaal craton abstract sequence stratigraphic concepts are applied to the 2 7 2 1 ga transvaal supergroup which constitutes the sedimentary floor to the bushveld igneous complex the transvaal succeeds the foreland fill of the witwatersrand supergroup and in its lowermost portions is partly synchronous with the ventersdorp supergroup lavas that followed upon the witwatersrand sediments in the stratigraphic record the unconformable contacts at the base and the top of the transvaal supergroup mark significant changes in the overall tectonic setting which qualifies them as first order sequence boundaries sedimentation within the transvaal basin was controlled by cycles of extensional and or thermal subsidence resulting in the accumulation of sequences separated by stages of base level fall generating sequence bounding subaerial unconformities the transvaal first order sequence is subdivided into five second order unconformity bounded depositional sequences i e the protobasinal black reef chuniespoort rooihoogte timeball hill and boshoek houtenbek sequences the span of time of these second order sequences averaging 130 ma is much longer than the duration of the second order phanerozoic cycles which is explained by the less evolved and thus slower plate tectonic processes that dominated the late archaean early proterozoic times each transvaal second order sequence preserves a variable number of systems tracts mainly as a function of the strength of the erosional processes associated with the ravinement surfaces and subaerial unconformities in a complete succession a sequence would include a basal lowstand systems tract lst followed by transgressive tst highstand hst and falling stage fsst systems tracts the protobasinal and rooihoogte timeball hill sequences only preserve the lst and tst due to the strong post depositional erosion the black reef sequence is composed of lst tst and hst the chuniespoort sequence preserves the tst hst and fsst but lacks the lst due to ravinement erosion the boshoek houtenbek sequence develops the complete succession of systems tracts common features may be emphasized between the same types of systems tract developed within different sequences the lst includes high energy coarse prograding facies such as alluvial fans and fan delta systems that fill the irregularities of the pre existing topography and lead to the peneplanation of the depositional areas the tst may be recognized from retrogradational stacking patterns that are associated with the transgression of marine environments the hst marks the normal regression of the previous marine environments which assumes coeval aggradation of fluvial and basinal facies within an overall progradational framework the fsst would normally include only basinal facies age equivalent with the correlative subaerial unconformities in our case study the stages of base level fall resulted invariably in fully nonmarine environments within the transvaal area generally leading to the development of subaerial unconformities in particular circumstances the nonmarine environment may still accumulate and preserve fluvial alluvial fan or lacustrine sediments as a function of the relative position between topography and local equilibrium profiles and base levels the interpretations of base level rise and fall resulting from the application of the sequence stratigraphic concept to the 2 7 2 1 ga transvaal supergroup enable estimation of sea level changes during deposition of this succession with an understanding of local tectonic conditions on the kaapvaal craton and of coeval sedimentation elsewhere in africa eustatic and relative sea level changes and variation in continental freeboard conditions may be suggested for the transvaal supergroup

organic geochemical characteristics and interpreted depositional environment of the khatatba formation northern western desert egypt the middle jurassic khatatba formation in the northern western desert of egypt was evaluated in terms of organic matter abundance type and thermal maturity as well as for some organic petrographic characteristics depositional environments were interpreted based on organic geochemical rock eval pyrolysis extract analysis and biomarker distributions and organic petrological methods organic carbon contents range between 1 0 and 32 5 wt the khatatba shale and coaly shale samples have hydrogen index values in the range of 63 to 261 mg hydrocarbon hc g total organic carbon with mixed types 2 3 and 3 kerogens mean vitrinite reflectance ro between 0 77 and 1 07 is in reasonably good agreement with pyrolysis tmax temperature at maximum of s2 peak data 438 459c organic rich sediments of the middle jurassic khatatba formation have very good source rock generative potential and have obtained thermal maturity levels equivalent to the oil window the main generation products are gas with very limited liquid hcs oil or condensate seven shale and coaly shale samples from khatatba formation were characterized using gas chromatography gc and gc mass spectrometry techniques the khatatba samples are characterized by the predominance of c14 c24 alkanes a pristane phytane ratio of less than 2 abundant c27 regular steranes and the presence of tricyclic terpanes these are consistent with the suboxic marine environment conditions for the khatatba source rock biomarker parameters for these samples generally indicate a mixture of land as well as marine derived organic matter input the maturity indicators based on c32 22s 22s 22r homohopane and c29 20s 20s 20r and sterane ratios reveal that the khatatba samples are thermally mature and have reached the peak oil window maturity supporting the ro data

organic geochemistry and basin modeling of late cretaceous harshiyat formation in the onshore and offshore basins in yemen implications for effective source rock potential and hydrocarbon generation abstract the purpose of this study is to present the source rock characteristics of late cretaceous harshiyat formation source rock in the onshore and offshore basins of yemen i e mukalla sayhut sayun masilah and jiza qamar source rock geochemical characterizations were completed and integrated to perform 1 d basin modeling to back predict source rock thermal maturation oil generation and expulsion the geochemical findings revealed that mudstone intervals within the harshiyat formation comprised favorable source rocks with total organic carbon toc content between 0 50 and 35 10 wt the organic matter intervals in the harshiyat mudstones are mainly types ii iii and iii kerogens with certain amount of type ii and i kerogens based on rock eval hi values ranging from 40 to 923 mg hc g toc mixed kerogen types were consistent with vitrinite and sapropelic organic matters as observed by visual kerogen examination and the distribution of biomarker fingerprints the existence of mixed types i ii and iii organic matter suggests that the harshiyat mudstones could contribute to oil and gas at sufficient thermal maturity levels the biomarker data of the organic matter in the harshiyat mudstones show that the source rock contains a mix marine phytoplanktonic bacteria terrestrial land plant enthronement and were deposited in suboxic anoxic conditions results of chemical and optical maturity indicators led to the conclusion of varying maturity stages from immature to late oil window levels for the mudstones of the harshiyat formation thermal maturation distributions inferred from the analyzed samples indicate that mudstone intervals in the offshore mukalla sayhut basin are extremely mature compatible with the late mature oil window and may have generated significant amounts of oil this interpretation was supported by strong geochemical similarity between the organic matter in the mudstones and those of the oils found in the offshore mukalla sayhut basin and demonstrated by the concentrations of n alkanes and isoprenoids as well as carbon isotopes in the offshore ras ghashwah 1x well burial and thermal models were constructed and used to address the effect of tectonic events in triggering oil generation and expulsion from mature harshiyat source rock in the offshore area kinetic database models for type ii and iii kerogen mixtures indicate that the initial conversion of kerogen to oil occurred since the late oligocene with transformation ratios tr that span between 10 and 50 furthermore the oil was expelled from organic matter intervals in the harshiyat source rock starting from late oligocene and continuing until late miocene with trs of 50 72 from that period up to the present the retained oil was cracked into wet gas in the gas window with an easy ro greater than 1 30 and with peak tr ratios greater than 72

hf isotopic compositions of the standard zircons and baddeleyites used in u pb geochronology abstract zircon and baddeleyite u pb geochronological dating is widely used in solid earth sciences and the advent of rapid in situ methods of analysis such sims and icp ms has re emphasized the importance of having uniform standards recently it has been shown that hf isotopic data can provide important information on these minerals since they contain high concentrations of hf but have low lu hf ratios which results in negligible age correction however the complex internal structures that result from multiple thermal events such as inherited cores and metamorphic overgrowths require that the hf isotopic data be measured with high spatial resolution however the isobaric interferences of 176 yb and 176 lu on 176 hf hamper the precise determination of the 176 hf 177 hf ratio during in situ laser ablation mc icpms analysis it is shown here that mass biases of yb yb and hf hf change with time during analyses and behave differently for solutions and solid material therefore it is suggested that the mean yb value of the individual spot be used to obtain the precise isotopic composition for in situ zircon and baddeleyite hf isotopic analyses for low yb hf 176 yb 177 hf yb value have little effect on the accuracy of the hf isotopic composition however the appropriate yb isotopic abundance is necessary for high yb hf 176 yb 177 hf 0 001 zircons and baddeleyites since the interference of 176 yb on 176 hf is significant using the mean yb value of the individual spot and newly published yb isotopic abundance data six standard zircons and two standard baddeleyites have been investigated using a neptune mc icpms with 193 nm laser for zircons the obtained 176 hf 177 hf ratios are 0 282307 31 2sd for 91500 0 282680 31 2sd for temora 0 281729 21 2sd for cz3 282177 17 2sd for cn92 1 0 282983 17 2sd for fm0411 and 281234 11 2sd for phalaborwa the baddeleyites from phalaborwa and sk10 2 have 176 hf 177 hf ratios of 0 281238 12 and 0 282738 13 2sd these results agree well with the values obtained by the solution method and indicate that these standards have different hf isotopic compositions in which the extremely low 176 lu 177 hf and 176 yb 177 hf values of cz3 zircon and phalaborwa baddeleyite make them excellent standards for machine calibration during in situ zircon hf isotopic measurement with the other standards being more suitable for the development of the correction method

geological process of late paleozoic shale gas generation in the eastern ordos basin china revelations from geochemistry and basin modeling abstract during the meso cenozoic the eastern ordos basin eob underwent a complex tectonothermal process influenced by the destruction of north china craton significantly affecting the thermal maturation of the late paleozoic source rocks tectonothermal modeling and maturation of coal bearing shales were performed in the eob to reveal the geological process of shale gas generation organic geochemical data for shales show that toc values vary from 0 96 wt to 21 36 wt and ro values range from 1 56 to 2 47 as well as that organic matter mainly consists of type iii kerogen indicating excellent potential and sufficient maturity for gas generation thermal modeling suggests that heat flow in the eob significantly increased as a result of intense magmatic activities during the yanshanian orogeny with peak values of 108 and 128 mw m2 in the northern and southern regions respectively consequently great variations in maturation history are seen in different regions of the eob with the taiyuan and shanxi shales reaching the wet gas and dry gas stages respectively in the northern region and all shales reaching the dry gas stage in the southern region hydrocarbons were generated at 237 ma and peaked around 162 ma whereas hydrocarbon expulsion began at 230 ma and peaked at 131 ma rapid subsidence in the triassic brought the shales to the oil generation window and early cretaceous magmatic activities led to rapid maturation of shales again reaching the gas generation stage accordingly extremely high levels of thermal maturity are well documented in hydrocarbons alkane composition and stable carbon isotope ratio gas geochemical data 13c1 vs c1 c2 3 13c1 vs c2 imply that that hydrocarbons are from highly mature thermogenic gas generated mainly from humus and this offers great support for thermal maturation modeling regionally coal bearing shales have been steadily uplifted and eroded since the late cretaceous causing partial destruction of gas reservoirs the tectonothermal evolution and hydrocarbon generation of eob were controlled primarily by magmatic thermal effects resulting from the westward subduction of the pacific plate

two stage triassic exhumation of hp uhp terranes in the western dabie orogen of china constraints from structural geology abundant exposures of widely distributed hp uhp metamorphic rocks in the western part of the dabie orogen enable us to study the tectonic evolution of hp uhp terranes associated with the world s largest preserved continental subduction zone previous tectonic models for the dabie orogen were based largely on metamorphic studies most of them lacking significant structural constraints we present a comprehensive structural analysis based on detailed structural geology the results suggest that syn uhp d0 at 241 231 ma and syn hp d1 at 225 215 ma southeast vergent thrusting formed a series of stacked structural slices this was followed by southeast vergent folding under amphibolite facies conditions d2 at 215 205 ma then a third generation of flexural folding occurred at shallow levels d3 at 200 184 ma this leads us to proposes a two stage triassic exhumation model in which initially rapid vertical extrusion d0 d1 from uhp to hp conditions to lower crustal levels is followed by slow southeastward extrusion d3 from lower crustal levels to the earth s surface the tectonic model combines the early southeastward vertical extrusion with the later southeastward lateral extrusion revealing two different stages and thus different types of triassic extrusion for the exhumation of hp uhp rocks in the dabie orogen the first stage extrusion occurred in the middle triassic whereas the second stage extrusion lasted from the late triassic to early jurassic these two extrusion episodes correlate with the two stages of triassic exhumation of the dabie hp uhp rocks respectively during continental collision