origin of the alxa block western china new evidence from zircon u pb geochronology and hf isotopes of the longshoushan complex abstract the nw se trending longshoushan is in the southwestern margin of the alxa block which was traditionally considered the westernmost part of the north china craton ncc precambrian crystalline basement exposed in the longshoushan area was termed the longshoushan complex this complex s formation and metamorphism are significant to understand the geotectonics and early precambrian crustal evolution of the western ncc in this study field geology petrology and zircon u pb and lu hf isotopes of representative orthogneisses and paragneisses in the longshoushan complex were investigated u pb datings reveal three paleoproterozoic magmatic episodes ca 2 33 ca 2 17 and ca 2 04 ga and two subsequent regional metamorphic events ca 1 95 1 90 ga and ca 1 85 ga for metamorphic granitic rocks in the longshoushan complex u pb dating of the detrital magmatic zircons from two paragneisses yields concordant 207pb 206pb ages between 2 2 ga and 2 0 ga and a small number of metamorphic zircon rims provide a ca 1 95 ga metamorphic age suggesting that the depositional time of the protolith was between 2 0 and 1 95 ga and that the sedimentary detritus was most likely derived from the granitic rocks in the longshoushan complex itself zircon lu hf isotopic analyses indicate that nearly all magmatic zircons from ca 2 0 ga to ca 2 17 ga orthogneisses have positive ehf t values with two stage hf model ages tdmc ranging from 2 45 to 2 65 ga peak at ca 2 5 ga indicating that these paleoproterozoic granitic rocks were derived from the reworking of the latest neoarchean early paleoproterozoic juvenile crust detrital magmatic zircons from two paragneisses yield scattered 176hf 177hf ratios ehf t and tdmc values further indicating that the sedimentary detritus was not only derived from these plutonic rocks but also from other unreported or denuded paleoproterozoic igneous rocks the ca 2 15 ga detrital magmatic zircons from one paragneiss have negative ehf t values with tdmc ranging from 2 76 to 3 04 ga indicating another important crustal growth period in the longshoushan region these data indicate that the longshoushan complex experienced neoarchean early paleoproterozoic crustal growth approximately ca 2 3 2 0 ga experienced multiphase magmatic events and approximately ca 1 95 1 90 ga and ca 1 85 ga experienced high grade metamorphic events the sequence of tectonothermal events is notably similar to that of the main ncc together with the datasets from an adjacent area we suggest that the western alxa block was most likely an integrated component of the ncc from the neoarchean to the paleoproterozoic

decoupling of u pb dates from chemical and crystallographic domains in granulite facies zircon abstract high spatial resolution 12 m spot size and high density up to 22 spots per crystal u pb ion probe data were acquired for zircon crystals from a mafic granulite in the canadian shield the zircon grains are metamorphic in origin and were subjected to a subsequent high pressure granulite facies event 207pb 206pb dates span 650 ma and can be used to define discrete intracrystalline domains of dates by contouring the age results the boundaries of these domains cut at high angles across compositional zonation revealed by cathodoluminescence backscatter hf and u chemical mapping and electron backscatter diffraction misorientation data the results indicate decoupling of radiogenic pb behavior from other trace elements in the zircon crystals previously published u pb id tims data define a highly linear discordia array with an upper intercept at 2554 5 4 3 ma associated with initial zircon growth during granulite facies metamorphism and a lower intercept at 1896 0 18 0 ma associated with a second granulite facies metamorphic event that caused u pb discordance the intracrystalline patterns of ion probe dates are incompatible with multiple zircon growth events and are inconsistent with reported examples of zircon recrystallization in which resetting of the u pb systematics was accompanied by modification of zircon chemistry and cl patterns grains lack discernible core rim variations expected for diffusion profiles and diagnostic indicators of low temperature radiation damage enhanced pb loss are absent rather the data appear to require heterogeneous pb loss from non metamict metamorphic zircon during high temperature metamorphism implying that under certain circumstances other intracrystalline fast pathways facilitate pb loss in metamorphic zircon crystals this result is incompatible with conventional thinking regarding pb mobility in zircon and indicates that important aspects of pb behavior in zircon are not yet fully understood

zircon u pb chronology of the jianping complex implications for the precambrian crustal evolution history of the northern margin of north china craton abstract the neoarchean to paleoproterozoic jianping complex in the western liaoning province is an important component of the precambrian basement of the north china craton this region connects the neoarchean fengning chengde complex in the west the neoarchean to paleoproterozoic north chaoyang complex in the northeast and the yixian fuxin archean greenstone belt in the east the precambrian jianping complex is dominantly composed of metamorphosed supracrustal sequence and dioritic to granitoid gneisses tonalite trondhjemite granodiorite ttg here we present results from la icpms zircon u pb isotope dating from the various lithological units in this complex which reveal that the magmatic precursors of the metavolcanic rocks associated with the supracrustal sequence were generated during 2555 2550 ma and up to 2615 ma a major magmatic pulse of dioritic to granitic suite occurred during 2538 to 2495 ma this was followed by ca 2485 ma granulite facies metamorphism and a retrograde event at ca 2450 2401 ma our data also reveal a major charnockite emplacement event in this region at ca 1694 ma integrating our new results with the available geologic and previous geochronological data we identify three major growth stages in the crustal evolution history of the northern margin of the eastern continental block of the north china craton at ca 2550 2495 ma ca 2403 2394 ma and ca 1700 ma the la icp ms zircon data provide new insights on the neoarchean to paleoproterozoic tectonothermal evolution history in the northern margin of the eastern continental block of the north china craton

particle transport of 234u 238u in the kalix river and in the baltic sea the role of particles for u isotope transport was investigated in the kalix river watershed a particle poor fe mn rich river in northern sweden and in the baltic sea estuary particles 0 45 m are strongly enriched in u and contain 20 50 of the total riverine uranium budget and 1 of the total u in brackish waters 3 7 psu the particles have high 234 u which is close to that of dissolved u in the associated water indicating that u on particles is dominantly nondetrital and isotopically exchanges rapidly with the ambient dissolved u particles at the river mouth are dominated by nondetrital fe mn oxyhydroxides uranium and fe are strongly correlated clearly demonstrating that secondary fe oxyhydroxide is the major carrier of u in river water there is no evidence for significant association of u with mn oxyhydroxide apparent u distribution coefficients k d fe were calculated for u between the authigenic fe on particles and the solution these values appear to be relatively constant throughout the year this suggests an equilibrium between fe in solution and authigenic fe oxyhydroxides on detrital particles high values of k d fe calculated for one summer as well as high u concentrations in brackish waters can be explained by u scavenging by biogenic phases with low authigenic fe content

geochronological evidence for archaean and palaeoproterozoic polymetamorphism in the central zone of the limpopo belt south africa abstract the central zone of limpopo belt is a polymetamorphic and complexly deformed precambrian terrane whose tectonic evolution is still uncertain because details on the timing of its evolution are not well established we report zircon ages for newly discovered rocks in the central zone which document three distinct high grade tectono metamorphic events at 3 22 2 62 and 2 02 ga the youngest of these has been well established but the older two events were so far largely based only on rare zircon overgrowth rims most of our samples were collected from a large enclave in the 2 61 ga bulai pluton and constitute a strongly deformed brecciated high grade assemblage of supracrustal rocks meta anorthosite and granitoid gneisses some of which are most likely equivalents of the pre 3 33 ga beit bridge complex magmatic zircon ages of 3 34 3 33 ga for anorthositic and tonalitic gneisses in the enclave breccia support this correlation ca 3 22 ga metamorphic zircons possibly reflecting the d1 m1 event were only found in one felsic granulite sample and their regional significance remains uncertain abundant metamorphic zircons grew at ca 2 62 ga under granulite facies conditions d2 m2 and some of these experienced variable degrees of recrystallization and or pb loss during the third granulite facies event at ca 2 0 ga d3 m3 rocks outside the bulai enclave contain abundant newly formed ca 2 02 ga metamorphic zircons but only rarely preserve evidence for the ca 2 62 ga event since these late archaean zircons show strong recrystallization and or pb loss at ca 2 0 ga we suggest that the bulai pluton protected the rocks inside the enclave from severe overprinting at ca 2 0 ga because these rocks remained dry whereas the assemblages outside the bulai pluton experienced fluid assisted metamorphic overprinting at ca 2 0 ga our new data sugggest that the tectonic evolution of the central zone was more complex than previously thought we favour a scenario where collision involving the limpopo belt occurred in the neoarchaean whereas the high grade palaeoproterozoic event was most likely intracontinental and records far field effects of plate movements elsewhere the tectonic setting controlling the early archaean d1 m1 event remains unknown largely because of insufficient data and extensive overprinting during later times

earliest paleoproterozoic supracrustal rocks in the north china craton recognized from the daqingshan area of the khondalite belt constraints on craton evolution abstract the upper wulashan subgroup at daqingshan in the khondalite belt of the western block of the north china craton contains an early paleoproterozoic 2 5 2 45 ga supracrustal sequence incorporating banded iron formations which we refer to as the daqingshan supracrustal rocks they contain rounded to elliptical zircon grains that commonly show core rim or core mantle rim structures and precise shrimp u pb dating of these domains allows the discrimination of a series of tectono thermal events that straddle the archean proterozoic boundary detrital zircon cores with oscillatory zoning have formation ages ranging from 2 55 to 2 50 ga indicating derivation from late neoarchean magmatic rocks as a result of exhumation and erosion these became incorporated in earliest paleoproterozoic sediments that were deposited between 2 50 and 2 45 ga and then underwent high grade metamorphism at 2 45 2 40 ga this event variously recrystallized the cores into two main domains a dark inner domain and a gray outer domain as imaged in cathodoluminescence the gray domains commonly show sector zoning and have th u ratios of 0 1 0 5 a feature commonly noted in zircons recrystallized under high grade metamorphic conditions metamorphic mantles are generally more homogeneous in structure and have lower th u ratios commonly

provenance of meso to neoproterozoic cover sediments at the ming tombs beijing north china craton an integrated study of u pb dating and hf isotopic measurement of detrital zircons and whole rock geochemistry abstract the north china craton ncc was subjected to an extensional regime after the luliang movement at 1 8 ga and then was covered by an extensive meso to neoproterozoic sedimentary succession namely the changcheng jixian and qingbaikou groups in ascending order we report age spectra for detrital zircons and monazites hf isotopic systematics of detrital zircons and whole rock chemical and nd isotopic compositions for sediments from the succession in the ming tombs area beijing one of the typical meso to neoproterozoic areas in the ncc detrital zircons of six sedimentary samples have two distinct age peaks at 2 52 ga and 1 85 ga there are some detrital zircons at 2 4 2 0 ga but none at 2 3 ga and only a few 2 7 ga the detrital zircon age spectra change with time sediments in the lower succession changcheng group and in the upper successions jixian and qinbaikou groups are dominated by significant detrital zircon populations of late neoarchean and late paleoproterozoic ages respectively the 2 5 ga detrital zircons of the changcheng group have ehf 2 5 ga values and tdm hf model ages mainly ranging from 2 to 7 and 2 8 to 2 7 ga respectively detrital monazites of a sample from the jixian group exhibit a major age peak between 1 95 and 1 80 ga with some data between 2 0 and 1 95 ga the sedimentary rocks of the changcheng group are characterized by high k2o contents mostly 7 09 15 20 and insignificant eu anomalies eu eu 0 71 1 16 they have tdm nd model ages ranging from 2 70 to 2 43 ga being older than the tdm nd ages 2 11 and 1 99 ga of sedimentary samples from the qingbaikou group based on a comparison with ages for the early precambrian 1 8 ga basement of the ncc it can be concluded that 1 the sediments of the meso to neoproterozoic cover were undoubtedly derived from the ncc itself or once neighboring terranes 2 variations in the detrital zircon age spectra from the lower to the upper successions reflect provenance evolution in that the lower crustal late paleoproterozoic rocks were exposed at the surface after the upper crustal late neoarchean rocks had already been eroded

geochronology and trace element geochemistry of zircon monazite and garnet from the garnetite and or associated other high grade rocks implications for palaeoproterozoic tectonothermal evolution of the khondalite belt north china craton abstract it is well documented that the khondalite belt of the north china craton formed by the collision of the yinshan and ordos blocks during ca 1 95 ga but the onset of the post collisional exhumation stage has not been well dated the garnetite and garnet bearing quartz rich lens that are regarded as residue melt phases occur occasionally in the foliated metasedimentary rocks in the jining terrane of the khondalite belt they are free of regional gneissosity and may be generated during the post collisional exhumation stage zircon u pb dating monazite chemical dating and trace element analyses of zircon monazite and garnet have been carried out on the garnetites and associated other high grade metamorphic rocks the geochronological results reveal a major age group of ca 1890 ma from the zircon grains overgrowths and inclusion type monazites within the garnet poikiloblasts in the garnetites the trace elements analyses show that zircon hosted in the mantle of the garnet poikiloblasts in the garnetite contains the highest hree and y contents whereas zircon hosted in the garnet rims shows higher hree and y contents than matrix zircon grains inclusion type monazite in the garnetite shows higher lan smn ratio than matrix monazite grains large garnet poikiloblasts in the garnetite exhibit bell shaped hree and y zoning profiles attributed to rayleigh fractionation during their growth compared to garnet poikiloblasts coronal garnet shows higher gro cr and li contents but lower xmg sm hree zn and zr contents and weaker eu negative anomalies reflecting their different formation environments based on evidences from microstructural analyses ree concentrations and patterns hree partition coefficients and the ti in zircon thermometer it is proposed that most of the zircon grains overgrowths in the garnetite formed coevally with garnet poikiloblasts consequently ca 1890 ma represents the timing of garnet poikiloblast growth during the partial melting of the protolith coronal garnet or zn rich spinel plus cordierite coupled with the matrix type monazite formed after garnet poikiloblasts growth during near isothermal decompression given the high metamorphic temperature and pressure revealed by the garnetite 820 850 c and up to 950 c 8 5 9 5 kbar it is suggested that the ca 1890 ma age group represents the beginning of extension exhumation event in the khondalite belt which is also recorded in the associated other high grade rocks such as the garnet bearing quartz rich lenses sillimanite garnet bearing quartzo feldspathic gneisses garnet bearing quartzo feldspathic gneisses and a pure quartz vein

multiple mafic magmatic and high grade metamorphic events revealed by zircons from meta mafic rocks in the daqingshan wulashan complex of the khondalite belt north china craton abstract meta mafic rocks in the daqingshan wulashan complex in the central segment of the paleoproterozoic khondalite belt of the north china craton are composed predominantly of garnet bearing mafic granulites mafic granulites and amphibolites they occur mainly as irregular lenses and as a set of deformed dyke sill swarms within dioritic granitic orthogneisses and al rich paragneisses an integrated study involving laser raman and scanning electron microscope analyses of mineral inclusions from zircons cathodoluminescence imaging and la icp ms u pb dating of zircons shows that magmatic zircons in these meta mafic rocks record four age groups of 2500 2450 2300 2100 1970 1930 and ca 1850 ma indicating multiple mafic magmatic events occurred within the daqingshan wulashan complex in addition four groups of metamorphic zircons with ages of ca 2450 ca 1950 ca 1900 and ca 1850 ma are present within these meta mafic rocks zircons of the first oldest group contain medium to low pressure mlp granulite facies mineral inclusions of orthopyroxene clinopyoxene amphibole plagioclase quartz and yield ages of ca 2450 ma representing the timing of the early mlp granulite facies metamorphic event within the daqingshan wulashan reworked basement complex in contrast zircons of the second group are free of mineral inclusions and yield ages of ca 1950 ma which is considered to be the timing of the regional high grade metamorphic and structural event probably caused by the collision of the yinshan and ordos blocks zircons of the third group contain mlp granulite facies mineral inclusions of orthopyroxene clinopyoxene amphibole plagioclase quartz similar to the zircons of the first group but yield younger ages of ca 1900 ma interpreted as the timing of the regional mlp granulite facies metamorphism that most likely occurred during the early stage of post orogenic exhumation of the daqingshan wulashan complex zircons of the fourth group contain mlp amphibolite facies mineral inclusions of clinopyoxene amphibole plagioclase quartz and yield the youngest ages of ca 1850 ma which probably represent the timing of the regional amphibolite facies metamorphism during the late stage of post orogenic exhumation of the daqingshan wulashan complex combined with previous geological data of the khondalite belt it is speculated that the multiple mafic magmatism and contemporaneous high grade metamorphism of the daqingshan wulashan complex from the khondalite belt were most likely generated by an orogenic process involving early subduction collision and late exhumation in an active continental margin arc

time scale and conditions of weathering under tropical climate study of the amazon basin with u series the rio solimoes amazonas amazon river and its major tributaries have been analyzed for u series nuclides 238 u 234 u 230 th 226 ra disequilibria have been measured in the dissolved 0 2 lm as well as bed sands u series disequilibria are closely related to major and trace element compositions and therefore reflect elemental fractionation during chemical weathering moreover while the dissolved load records present day weathering suspended particles integrate the erosion his tory over much longer time scales 100 ka lowland rivers are characterized by long time scales of chemical erosion p100 ka result ing in a high weathering intensity moreover exchange between suspended particles and the dissolved load may explain the u series signature for these rivers by combining u series and pb isotopes in suspended particles we show that erosion in the rio madeira basin occurred as a multi step process whereby the pristine continental crust was eroded several hundreds of ma ago to produce sediments that have then been integrated in the cordillera by crustal shortening and are currently eroded in contrast recent erosion of a pristine crust is more likely for the rio solimoes amazonas 10 ka the suspended particles of the rivers draining the andes solimoes ama zonas madeira suggest time scales of weathering ranging between 4 and 20 ka this indicates that suspended particles transported by those rivers are not stored for long periods in the andean foreland basin and the tropical plain the sediments delivered to the ocean have resided only a few ka in the amazon basin 6 3 1 ka for the rio amazonas at obidos nevertheless a large fraction of the sediments coming out from the andes are trapped in the foreland basin and may never reach the ocean erosion in the andes is not operating in steady state u series systematics shows unambiguously that rivers are exporting a lot more sediments than predicted by steady state erosion and that is a consequence of soil destruction greater than production by relating this observation to the short time scales of weathering inferred for the andes a few ka it appears that the erosion regime has been recently perturbed resulting in high denudation rates a possible explanation would be the increase in precipitation less than 5 ka proposed by recent paleoclimatic studies our results indicate that erosion responds rapidly to high frequency climatic fluctuations

tectonic attribution of the langshan area in western inner mongolia and implications for the neoarchean paleoproterozoic evolution of the western north china craton evidence from la icp ms zircon u pb dating of the langshan basement abstract the langshan area in western inner mongolia china is situated on the conjunction of the alxa block in the west the northern north china craton ncc in the east and the central asian orogenic belt in the north but its tectonic attribution has long been in debate because the alxa block may be an independent precambrian block from the ncc and the boundary between the two blocks may lie along the bayanwulashan to the west of the langshan instead of around the helanshan area to the east of the langshan in this study new la icp ms zircon u pb ages of four gneisses one amphibolite and three granites combined with previously published age data from langshan basement rocks indicate that the langshan area may be subdivided into the main langshan to the north of a series of nnw trending faults and the southern langshan in the south the main langshan basement rocks were mainly formed during neoarchean paleoproterozoic with five major stages of 2 7 ga 2686 ma 2 6 ga 2619 2563 ma 2 5 ga 2534 2491 ma 1 95 ga 1969 1938 ma and 1 7 ga 1672 ma these stages are largely consistent with the major tectonothermal events of the northern ncc during neoarchean paleoproterozoic but significantly different from the principal formation period ca 2 5 1 9 ga of alxa basement rocks the southern langshan is characterized by the presence of late paleoproterozoic 1 9 ga and neoproterozoic 0 9 ga rocks and the latter is consistent with the neoproterozoic magmatism in central alxa block which might be formed in a orogenesis related compressional environment in contrast to contemporaneous rift related extensional tectonic setting within the ncc therefore the main langshan and the southern langshan are most likely closely related to the northern ncc and the alxa block respectively

late paleoproterozoic tectono thermal event in the northwestern north china craton evidence from u pb dating and o hf isotopic compositions of zircons from metasedimentary rocks north of hohhot city inner mongolia northern china abstract the khondalite belt is located in the northwestern north china craton and contains late paleoproterozoic metasedimentary rocks that generally underwent high grade metamorphism only a few supracrustal sequences experienced low grade metamorphism such as the meidaizhao sequence in the daqingshan area the erdaowa sequence north of hohhot city inner mongolia is another late paleoproterozoic supracrustal unit showing relatively low grade metamorphism compared with the adjacent metapelitic khondalites it is composed of metavolcanic as well as clastic metasedimentary rocks and marble it was traditionally subdivided into the halaqin and hongshangou formations we undertook an integrated study of u pb zircon dating and o hf isotopic analysis combined with whole rock chemistry on metasedimentary rocks of the erdaowa sequence in order to understand the relationship between the low and high grade rocks and their tectono thermal history in the khondalite belt cores and metamorphic domains of detrital zircons from a two mica schist of the halaqin formation have 207 pb 206 pb ages of 2 0 and 1 86 ga 18 o values of 6 69 9 45 and 6 68 12 15 and hf t values of 7 7 to 16 2 and 3 3 to 20 9 respectively a recrystallized core fir tree domain and altered domain of zircon from a grt st ky two mica gneiss have 207 pb 206 pb ages of 2 0 ga 1 88 ga and 1 86 ga 18 o values of 12 10 12 75 11 57 12 65 and 11 57 12 27 and hf t values of 0 7 to 5 4 2 0 to 4 7 and 2 0 to 0 5 respectively these data limit the time of deposition of the sedimentary rocks between 2 0 and 1 88 ga and show a high maturity similar to khondalites of the same sequence compared with previous studies we infer that the erdaowa sequence consists of rocks of different origins and metamorphic grades including khondalites which were brought up to the same crustal level as tectonic slices due to late paleoproterozoic tectonic processes

petrogenesis and tectonic implications of 2 45 ga potassic a type granite in the daqingshan area yinshan block north china craton abstract an archean paleoproterozoic a p a type granite 15 km wide and 25 km long has been identified in the daqingshan area south of the yinshan block yb north china craton ncc known as the jinpen pluton this body is the largest individual granite pluton in the daqingshan area its crystallization ages of 2460 2450 ma has been determined precisely using the secondary ion mass spectrometer sims zircon u pb method the jinpen pluton is lithologically homogeneous consisting mainly of gneissic monzogranites with metaluminous affinity and high k2o na2o 1 08 2 06 ga al and zr nb ce y values with high zircon saturation temperatures tzr of 889 837 c these features are typical of a type granite most samples contain enriched large ion lithophile elements liles and light rare earth elements lrees high y and yb abundances with pronounced eu negative anomalies low sr y and la yb n ratios and low mgo cr and ni contents implying a shallow crustal source for the pluton its whole rock nd isotope composition end t 2 5 0 6 and zircon hf isotope composition ehf t 5 2 1 7 tdmc 2 88 2 65 ga indicate that it mainly originated from pre existing crustal materials the zircon 18o values of 6 58 5 71 suggest a crustal source of orthogneiss with little metasedimentary material previous studies have demonstrated that the yb experienced significant crustal growth at 2 8 2 7 ga and further crustal reworking at about 2 5 ga this study reveals that the jinpen potassic a type granite was generated at 2 46 2 45 ga by partial melting of pre existing crustal materials at relatively low pressure p and high temperature t conditions symbolizing the first and essential cratonization of the lithospheric stabilization of the ncc

geochronology geochemistry and nd hf isotopes of the xiaokouzi granite from the helanshan complex constraints on the paleoproterozoic evolution of the khondalite belt north china craton abstract the khondalite belt is one of the three paleoproterozoic mobile belts in the north china craton and was formed by a continental collision between the yinshan and ordos blocks at ca 1 95 ga although the metamorphism geochemistry and geochronology of the khondalite belt have been extensively studied the provenance and depositional setting of the khondalite protoliths remain controversial in this study u pb zircon ages and geochemistry of the xiaokouzi granite in the southern helanshan complex part of the khondalite belt are investigated zircon u pb dating indicates that the xiaokouzi granite was formed between 2023 14 ma and 2034 16 ma the granite shows variable sio2 64 33 74 14 wt moderate al2o3 14 75 17 23 wt and a cnk values 1 09 1 66 low cao 0 33 2 35 wt tio2 0 30 0 66 wt tfe2o3 2 58 5 76 wt mgo 0 82 2 32 wt and k2o 2 57 4 96 wt contents these features indicate that the xiaokouzi granite is a peraluminous s type granite the xiaokouzi granite has cr and ni contents of 32 7 84 5 ppm and 10 7 19 3 ppm respectively the granite is enriched in cs k pb and zr and depleted in nb ta p and ti the granite displays variable nd t values 1 7 to 3 5 with a corresponding two stage nd model age of 2 2 2 5 ga in addition the granite has high initial 176hf 177hf ratios 0 281569 0 281789 and positive hf t values 1 1 to 8 1 with a two stage hf model age of 2 2 2 6 ga suggesting a juvenile source the crystallization age of the xiaokouzi granite is synchronous with the peak in the detrital zircon age population from metasedimentary rocks in the khondalite belt in addition hf isotope values from the xiaokouzi granite are consistent with the detrital zircons from metasedimentary rocks together with other paleoproterozoic igneous rocks in the khondalite belt it is inferred that paleoproterozoic magmatic rocks in the khondalite belt may be the potential provenance for khondalite protoliths combined with previous geological data from the khondalite belt we propose that the helanshan khondalite protoliths were most likely deposited in an active continental margin near the ordos block

paleoproterozoic accretionary orogenesis in the north china craton a shrimp zircon study abstract the inner mongolia suture zone imsz and the trans north china orogen tnco incorporate the major paleoproterozoic accretionary orogens in the north china craton ncc with the jiao liao ji belt jljb representing the third one here we investigate the paleoproterozoic tectonics of the imsz and tnco through zircon shrimp geochronology on a representative suite of rocks comprising metasediments and arc magmatic rocks shrimp analysis of zircons with textures indicating extreme recrystallization under ultrahigh temperature uht conditions from the metapelites at heling er in the southern domain of the imsz reveals a single population with a weighted mean 207 pb 206 pb age of 1913 17 ma the zircons in another uht granulite from this locality yield a weighted mean 207 pb 206 pb age of 1910 18 ma these data correlate with the ca 1 92 ga age reported from zircons in sapphirine bearing uht granulites further north and confirm the regional extent of the paleoproterozoic uht metamorphism within the imsz zircons in a charnockite from the southern margin of the khondalite belt fringing the uht granulites in the imsz show two distinct age groups an older population with a magmatic paragenesis and a weighted mean 207 pb 206 pb age of 1932 24 ma and a younger group of metamorphic zircons with an age of 1858 26 ma we also report zircon ages from charnockites in two localities around xing he in the huangtuyao belt belonging to the huai an complex within the westernmost domain of the tnco at the junction with the imsz the charnockite from first locality carries two distinct zircon populations with the older group yielding a weighted mean 207 pb 206 pb age of 2477 2 ma and the younger population showing an age of 1807 38 ma the internal structure as revealed from cl images and the overall high th u values up to 2 42 of the older zircons suggest their magmatic affinity whereas the younger group with extremely low th u 0 02 0 09 is of metamorphic origin zircons from the charnockite in the second locality also define two distinct age clusters with a dominant older magmatic group having a weighted mean 207 pb 206 pb age of 2147 11 ma and a minor younger group with an age of 1958 25 ma the range of ages from 2477 to 2147 ma from magmatic zircons in the charnockites from the eastern periphery of the imsz within the western margin of the tnco in combination with similar ages reported in recent studies from zircons in magmatic complexes within the imsz suggest a prolonged history of subduction related arc magmatism and accretionary tectonics analogous to those in some of the phanerozoic belts such as the central asian orogenic belt and the western pacific subsequent progressive collision and suturing of the continental blocks were accompanied by the exhumation of high pressure hp and uht metamorphic rocks the available data from the imsz and tnco suggest long lived convergent margins associated with the southward subduction of the yinshan block and westward subduction of the eastern block in a double sided subduction realm prior to the final amalgamation of the ncc and its incorporation within the columbia supercontinent in the late paleoproterozoic

a neoarchean arc back arc system in eastern hebei north china craton constraints from zircon u pb hf isotopes and geochemistry of dioritic tonalitic trondhjemitic granodioritic dttg gneisses and felsic paragneisses abstract dioritic tonalitic trondhjemitic granodioritic dttg gneisses are widely exposed in the zunhua qinglong microblock of eastern hebei north china craton ncc la icp ms zircon u pb isotopic dating for the dttg gneisses in the qinglong area reveals that their magmatic precursors were emplaced between 2517 10 and 2505 23 ma zircons of these dttg gneisses exhibit positive hf t values of 0 0 to 6 5 with adakite like geochemical features represented by relatively high mgo mg and la yb n ratios petrogenetic studies reveal that dioritic magmas were derived from depleted mantle peridotites that were strongly modified by slab derived melts fluids and that the tonalitic trondhjemitic granodioritic ttg magmas were formed by a high degree of fractional crystallization in the dioritic magmas with hornblende as the primary fractionated phase la icp ms zircon u pb isotopic analyses reveal that the protoliths of the felsic paragneisses in the northern zunhua area were deposited in the late neoarchean with a maximum depositional age of 2517 20 ma most of the detrital zircon grains display apparent 207pb 206pb ages from 3432 20 to 2502 21 ma with negative hf t values of 15 4 to 2 4 and calculated tdmc ages between 3995 and 3368 ma suggesting that they were formed from the recycling of eo to paleoarchean crustal materials closely related to the southeastern ancient continental crust of the interior eastern block these data together with previous investigations reveal that the basement rocks of zunhua qinglong microblock formed in a neoarchean arc back arc system with the subduction direction from present northwest to southeast

ca 2 9 ga granitoid magmatism in eastern shandong north china craton zircon dating hf in zircon isotopic analysis and whole rock geochemistry abstract eastern shandong is one of the important early precambrian areas in the north china craton where extensive 2 5 2 7 and 2 9 ga magmatic activities have been identified followed by very strong late neoarchean and paleoproterozoic tectono thermal events we carried out shrimp zircon dating hf in zircon isotope analysis and a whole rock geochemical study on ca 2 9 ga mesoarchean magmatic rocks these include gneissic diorite gneissic quartz diorite gneissic tonalite and gneissic high si trondhjemite and were identified in five new areas all these rocks contain ca 2 5 ga metamorphic zircons the dioritic and tonalitic rocks are enriched in large ion lithosphile lil elements k rb ba and depleted in nb and p with moderate to highly fractionated ree patterns similar in chemical compositions to archean ttg rocks worldwide the high si trondhjemites are characterized by very high sio2 and very low feot mgo the magmatic zircon grains from all these rocks are similar in their hf isotopic compositions and have hf t values ranging from 1 3 to 9 9 combined with previous work our main conclusions are 1 eastern shandong underwent a major magmatic event at ca 2 9 ga 2 diorites and tonalites formed by melting of thickened lower crust or the root of an oceanic plateau whereas the high si trondhjemites were derived from melting of intermediate to felsic rocks 3 the mesoarchean ca 2 9 ga was a major period of juvenile crustal addition from the depleted mantle earlier than the major crustal growth period at 2 7 2 8 ga in the north china craton 4 the 2 7 2 9 ga rocks underwent strong metamorphism at ca 2 5 ga probably as a result of magmatic underplating

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

sample preparation for determination of comminution ages in lacustrine and marine sediments abstract the uranium isotope composition 234u and 238u of detrital matter has become an essential tool for evaluating the response of erosion and catchment dynamics to climate variability on geological time scales relative variability of the 234u 238u activity ratio can be used as an estimate of the time that has elapsed since physical and chemical weathering has formed grains combined u isotopes and gas adsorption surface area analyses on two sediment samples from the mediterranean sea yield comminution ages up to 5 times older after sample treatment which highlights the importance of a comprehensive removal of non detrital matter from the bulk sediment composition moreover gas sorption analysis allowed determining whether a fractal correction for calculation of the recoil fraction should be applied precise estimates of the recoil fraction are crucial for calculating the comminution ages as it governs the loss of 234u from detrital matter samples analyzed in this study display type ii isotherms suggesting a non porous or macroporous surface micro and mesopores which increase the surface area during gas adsorption analyses but do not contribute to the loss of 234u are absent thus a fractal correction to account for micro and mesopores is not required

timing of late neoarchean to late paleoproterozoic events in the north china craton shrimp u pb dating and la icp ms hf isotope analysis of zircons from magmatic and metamorphic rocks in the santunying area eastern hebei abstract santunying is an important area for revealing nature of the late neoarchean tectono magmato thermal events in the eastern hebei part of the north china craton it is mainly composed of meta intrusive rocks supracrustal rocks sporadically occur in the meta intrusive rocks the meta intrusive rocks are subdivided into the santunying tonalitic gneiss qiuhuayu tonalitic trondhjemitic gneiss xiaoguanzhuang dioritic gneiss and qingyangshu meta gabbro respectively shrimp u pb zircon dating on fourteen samples yielded weighted mean 207pb 206pb ages of 2525 2537 2532 2546 2530 2544 and 2531 ma for magmatic zircons from them dioritic gneiss of the xiaoguanzhuang gneiss contain abundant 2544 3487 ma xenocrystic zircons shrimp u pb dating on a garnet biotite gneiss sample yielded a weighted mean 207pb 206pb age of 2537 ma for detrital zircons all rocks underwent strong metamorphism deformation and anatexis resulting in formation of leucosomes and residues with some leucosomes concentrating to form large veins they record a strong late neoarchean event by metamorphic zircon ages of 2489 2519 ma some rocks also record metamorphic zircon ages of 1772 1843 ma magmatic zircons from the magmatic rocks show large variations in ehf t values ranging from 1 7 to 8 7 combined with early studies conclusions are 1 intrusive rocks with the involvement of mantle derived materials have a narrow range of magmatic zircon ages from 2525 to 2546 ma and supracrustal rocks were formed during the same period 2 ancient crustal remnants 2600 ma are present consistent with the late neoarchean arc magmatism involving older continental crust similar to phanerozoic andean margins 3 the archean basement underwent a strong tectonothermal event at the end of the neoarchean with the metamorphic zircon ages being 10 30 million years younger than the timing of magmatism a common feature of the north china craton 4 a late paleoproterozoic tectonothermal event widely occurred in the western part of eastern hebei which is linked with regional ductile deformation

late paleoproterozoic crustal evolution in the daqingshan area evidences from adakitic and a type granitoids in the guyang changshengqu goldfield khondalite belt north china craton abstract granitic magmatism related to the collisional orogenic belt provides important information for the evolution of continental crust we performed petrological geochemical and zircon u pb hf isotopic studies on four granitoid samples from the guyang changshengqu goldfield daqingshan area khondalite belt north china craton the zircon u pb dating results indicate that the granitoids emplaced at 1950 11 ma 1935 12 ma 1846 11 ma and 1812 8 6 ma represent two periodic granitic magmatism in the daqingshan area the 1 95 and 1 93 ga granites exhibit high sio2 k2o and na2o contents with high sr y and la yb n ratios but low mgo cr and ni contents indicating that they were produced by the partial melting of a thickened lower crust under relatively high pressure conditions the granites show negative dominated zircon ehf t values 5 7 1 9 and their zircon tdm2 hf values range between 2 9 and 2 5 ga indicating an ancient crust source with minor juvenile crustal material involved the 1 85 and 1 81 ga quartz monzonites exhibit high sio2 k2o na2o zr nb ce and y but low mgo cr and ni contents with high ga al and feot feot mgo ratios indicating an a type granite affinity the whole rock geochemistry suggests that they were formed by dehydration re melting of ttg and quartz diorite at high temperatures and shallow depths the quartz monzonites show homogeneous ehf t values 10 6 6 1 and the zircon tdm2 hf values range between 3 0 and 2 8 ga suggesting that they were derived from a neoarchean ancient crust the 1 95 1 93 ga adakitic magmatism is associated with the 1 93 1 92 ga mafic magmatism and uht metamorphic in the eastern part of the khondalite belt this suggests a tectonic transition from collisional compression to post collisional extension the 1 85 1 81ga a type granitic magmatism indicates evident continental crust thinning and asthenospheric upwelling the two unrelated magmatism are possibly induced by two collisional events from the khondalite belt and the trans north china orogen respectively the collision of the western and eastern blocks at ca 1 85 ga may have played an important role in the a type quartz monzonites origin and influenced the crustal evolution as far as the daqingshan area in the khondalite belt

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

late neoarchean paleoproterozoic arc continent accretion along the khondalite belt western block north china craton insights from granitoid rocks of the daqingshan wulashan area abstract late neoarchean early paleoproterozoic granitoid rocks are widely exposed in the daqingshan wulashan area in the central segment of the khondalite belt kb western block north china craton ncc and are characterized by widespread evidence of ductile deformation and anatexis to improve our understanding of the formation and evolution of the kb in this study we analyzed zircon u pb ages zircon lu hf isotopes and whole rock geochemistry of the granitoid rocks la icp ms u pb analysis of zircons from eight representative granitoid samples indicates the granitoid rocks were emplaced at ca 2 45 and ca 2 50 ga and underwent multi stage metamorphism at ca 1 90 1 95 2 45 and 2 50 ga zircon lu hf isotopic analysis shows that the granitoids have positive zircon ehf t values with an average of 4 36 and two stage depleted mantle zircon hf model ages tdm2 clustering around 2 7 3 0 ga indicative of a juvenile crustal source that formed at ca 2 7 3 0 ga furthermore whole rock geochemical analysis of 34 granitoid samples reveal them to be magnesian low to high k calcic to calc alkalic and metaluminous to weakly peraluminous alumina saturation index asi 0 74 1 17 they are characterized by high total rare earth element ree contents 57 45 346 96 ppm enrichment in light ree and depletion in heavy ree with la yb n ratios of 8 33 224 16 they have weak eu anomalies eu eu 0 37 2 53 average 1 08 high sr contents 300 ppm and relatively low y and yb concentrations average 10 55 and 0 96 ppm respectively primitive mantle normalized trace element diagrams show that the granitoid samples are depleted in high field strength elements nb ta u and ti and enriched in large ion lithophile elements ba k rb and sr in general the samples have geochemical characteristics consistent with cordilleran i type granitoids which form in subduction related magmatic arcs therefore our results together with the occurrence of synchronous mafic rocks and other available geological data indicate the kb contains multiple magmatic arcs and underwent long term deposition and multi phase metamorphism analogous to phanerozoic accretionary orogens such as the cordilleran orogen consequently we infer that the kb underwent long term arc continent accretion along the southern margin of the yinshan block during the late neoarchean paleoproterozoic and late stage continent continent collision between the yinshan and ordos blocks at ca 1 90 1 95 ga

vegetation and climate in southern hemisphere mid latitudes since 210 ka new insights from marine and terrestrial pollen records from new zealand abstract paleo vegetation records developed from marine sedimentary sequences offer considerable potential for examining changes in terrestrial climate beyond the range of 14 c dating because they can be independently dated by 18 o stratigraphy here we present the first pollen record of vegetation from a marine core site in the tasman sea tan0513 14 42 18 s 169 53 e 110 km west of new zealand s south island an independent chronology provided by correlating the globigerina bulloides 18 o record at tan0513 14 to a global isotope stack shows that the record extends back to 210 ka glacial to interglacial changes in palynomorph content are characterised by shrub and podocarp broadleaf forest taxa respectively and are correlated with similar changes in the ca 150 kyr long terrestrial pollen record from okarito pakihi bog 110 km to the south southeast both records are placed on the same timescale by matching variations in dacrydium cupressinum and fuscospora between sites with a unique tie point provided by the ca 25 4 ka kawakawa tephra our southern hemisphere mid latitude vegetation records show forest extent is greatest during periods of low ice volume high mean annual sea surface temperature masst and anti phased with local insolation intensity however there are several features not attributable to changes in mean annual temperature first a fundamental change in forest composition occurred at termination ii tii with a loss of southern beech nothofagus from the study area second the amplitude of masst change through mis 5 is not reflected in corresponding changes in forest extent suggesting other feature s of regional climate seasonality frostiness ice cover exert important controls over vegetation patterns at these latitudes

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

fractionation of u th ra and pb from boreal forest soils by sequential extractions abstract to determine the mobility of natural radionuclides in boreal forest soil a five step sequential extraction procedure was carried out on soil samples taken from various depths down to 3 m on olkiluoto island finland where there are plans to construct a spent nuclear fuel disposal repository in the bedrock the extracted fractions studied were exchangeable acid soluble reducible oxidizable and tightly bound it was found that the extractability of most of the radionuclides studied was dependent on the sample grain size and depth all the elements were concentrated in the smallest grain size samples

the sedimentology paleoenvironment and diagenesis of the asencio formation of western uruguay abstract the uppermost sandstones of the asencio formation of southwestern uruguay are characterized by a striking iron rich geochemical overprint which has been interpreted as paleopedogenic in origin among the notable features of the asencio is the extension of the overprint into tubes that extend several meters into the underlying mercedes formation field observations and geochemical analyses of the asencio formation and its parent lithologies from four localities around uruguay allow the identification of the asencio overprint as a ferricrete displaying a number of the features typically associated with ferricrete formation iron and aluminum enrichment conversion of labile minerals to kaolinite vermiform pseudoconglomeratic and mottled textures and loss of bulk density in comparison to the unmodified parent the nature and uniformity of the geochemical profiles through the asencio formation suggests that this paleopedogenic modification occurred during a single phase of intense tropical weathering on a stable land surface possibly over a geologically short time period paleoenvironmental proxies suggest this weathering could have occurred at temperatures around 18 c with about 1500 mm of precipitation although these proxies are not developed for paleosols lacking a strong bt horizon like the asencio no existing hypotheses can satisfactorily explain the nature of the tubes extending from the base of the asencio formation nor do any of the observed features aid in the dating of the asencio

zircon indicators of fluid sources and ore genesis in a multi stage hydrothermal system the dongping au deposit in north china abstract the syenite hosted dongping gold deposit represents the largest gold deposit on the northern margin of the north china craton here we present new u pb age trace element concentration and hf isotope data from magmatic and hydrothermal zircons to better constrain the timing and genesis of gold mineralization geochronological data combined with field and petrologic results document two mineralization events the earlier event in the mid devonian 380 ma was associated with the intrusion of the shuiquangou syenite whereas the later event in the early cretaceous 140 ma was related to the emplacement of the shangshuiquan granite devonian hydrothermal zircons extracted from gray auriferous quartz veins were formed via metasomatic alteration of magmatic zircons derived from the syenite by an oxidizing f rich fluid in contrast cretaceous hydrothermal zircons hosted in the altered shangshuiquan granite and milky gold rich quartz ores were precipitated directly from a high temperature zr saturated aqueous fluid that was contaminated by basement materials possessing a substantially different hf isotopic composition from the contemporaneous magmatic zircons we propose that the devonian au mineralization event at dongping involved a magmatic fluid and auto metasomatism during crystallization of the shuiquangou syenite complex whereas the cretaceous au mineralization event involved a fluid exsolved from the shangshuiquan alkaline pluton results from this study highlight the potential utility of zircons as indicators of fluid sources and ore genesis in a multi stage hydrothermal system

decoding multiple tectonothermal events in zircons from single rock samples shrimp zircon u pb data from the late neoarchean rocks of daqingshan north china craton abstract a complex sequence of tectonothermal events shaped the neoarchean paleoproterozoic crustal evolution in the north china craton ncc here we present a systematic shrimp zircon u pb investigation on a suite of metamorphic rocks from the hademengou area of daqingshan within the khondalite belt of the inner mongolia suture zone we analyzed zircons from two meta volcanic rocks of the sanggan group and four meta igneous rocks of late neoarchean to early paleoproterozoic zircons from these rocks display typical core rim or core mantle rim structures the relict magmatic zircons record crystallization ages of ca 2 5 ga followed by variable pb loss showing a range of ages in the concordia up to 2 1 ga the recrystallized domains of zircons show sector zoning or are composed of inner dark domains and outer lighter domains the overgrowth mantles are homogeneous the recrystallized zircons and overgrowth mantles show similar age data distribution along a concordia from 2 5 to 2 1 ga the overgrowth rims are poorly developed but record ages 1 96 1 84 ga similar to those displayed by metamorphic zircons in the late paleoproterozoic upper wulashan subgroup as well as the charnockites and uht granulites reported in previous studies from this region a meta syenogranite intruding the sanggan group and hypersthene dioritic gneiss contains 1 95 ga magmatic and 1 85 ga metamorphic zircons magmatic zircons of the late neoarchean early paleoproterozoic metavolcanic rocks have e hf t of 1 2 to 7 0 t dm1 hf model ages of 2687 to 2532 ma and t dm2 cc hf model ages of 2837 to 2559 ma the recrystallized zircons and mantle domains display hf isotope composition similar to that of the magmatic domains our study is the first report from this region that identifies multiple tectonothermal events ranging from late neoarchean through early to late paleoproterozoic from zircons within single rock samples in the ncc a strong overprint of late paleoproterozoic metamorphism traced commonly in all the rock units indicates that these rocks were involved in the tectonothermal event associated with the final amalgamation of the ncc

distribution and genesis of bauxite on the mambilla plateau se nigeria abstract only small low grade occurrences of bauxite are known in nigeria associated with ferricretes on the jos plateau widespread bauxite has been discovered on the mambilla plateau laterite is developed on gneisses and basalt while on trachytic parent rocks low silica bauxite occurs with a thickness of more than 5 m and covering an area of at least 1 km2 this bauxite consists of gibbsite and goethite in contrast to the ferricretes of the jos plateau weathering on the mambilla plateau led to the formation of stone line profiles ferricretes were developed on the upper jurassic gondwana surface under contrasting climate and are preserved as relicts in the southern part of the mambilla plateau where weathered precambrian basement occurs in a highly uplifted position dismantling of these ferricretes is attributed to a gradual change of paleoclimate towards more humid conditions after the extrusion of volcanic rocks in the northern part of the plateau presumably during lower to mid miocene these volcanics underwent intense chemical weathering bauxite and laterite formed depending on the type of parent rock weathering profiles are mostly truncated and overlain by reworked material allochthonous components occur both within and also below the stone line this is shown by mineralogical indicators such as ilmenite or quartz and by a shift in the zr ti ratio similar profiles are missing further north on the jos plateau where ferricretes formed instead this is attributed to regional differences in paleoclimatic conditions which were more humid at mambilla while jos was under the influence of dryer conditions the widespread formation of laterite on different parent rocks on the mambilla plateau could be related to a global mid miocene climatic optimum which led to a general intensification of chemical weathering processes

possible southwestward extrusion of the ordos block in the late paleoproterozoic constraints from kinematic and geochronologic analysis of peripheral ductile shear zones abstract the paleoproterozoic is a pivotal period of the tectonic evolution of the north china craton ncc various models have been postulated by previous researchers through the study of petrology and metamorphism but the lack of structural analyses has hampered further understanding of the tectonic evolution of the ncc a series of well exposed ductile shear zones developed on the northern northwestern and eastern edges of the ordos block in the western part of the ncc we carried out detailed field studies along with isotopic dating focused on the kinematics and geochronology of the ductile shear zones on the north margin of the ordos block the wulashan daqingshan ductile shear zone is characterized by an e w trending shearing foliation that steeply dips 75 80 to the north or south with subhorizontal stretching lineation plunging 10 15 toward the east or west and dextral strike slip shearing kinematics on the west margin of the ordos block is the zongbieli ductile shear zone with top to the nw thrusting kinematics on the east margin of the ordos block is the xueling ductile shear zone with top to the se thrusting accompanied by sinistral strike slip shearing kinematics the 40ar 39ar ages of deformed minerals from mylonitic high grade metamorphic rocks and la icp ms u pb dating results of zircons from syn tectonic anatectic granites reflect that the ductile shear zones surrounding the ordos block deformed between 1 85 and 1 81 ga the similar deformation ages indicate that the ductile shear zones might be genetically correlated the geometry kinematics and geochronology characteristics of the wulashan daqingshan ductile shear zone on the north margin the zongbieli ductile shear zone on the west margin and the xueling ductile shear zone combined with the zhujiafang sinistral ductile shear zone on the east margin defined the possible southwestward extrusion of the ordos block in the late paleoproterozoic which might have resulted from the westward subduction collision of the eastern block under the western block at approximately 1 85 ga and which is correlated with the amalgamation of the columbia supercontinent

late palaeoproterozoic depositional age for khondalite protoliths in southern india and tectonic implications abstract shrimp dating of detrital zircons from garnet sillimanite cordierite paragneisses khondalites of the trivandrum block southern india revealed nanometer scale pb isotopic inhomogeneity in most grains that we ascribe to annealing processes during ultra high temperature metamorphism at ca 570 ma our age data for zircons from six representative khondalite samples do not document any neo or mesoproterozoic detrital grains and we conclude from the concordant ages and discordant minimum 207pb 206pb ages that the khondalite precursor sediments were deposited more than 2 1 ga ago and were subsequently intruded by granitoid rocks at ca 1765 2100 ma some detrital zircons in the khondalites contain late palaeoproterozoic metamorphic domains suggesting that these grains are derived from an unknown crustal source that most likely experienced late palaeoproterozoic high grade metamorphism both the metasedimentary assemblage and granitoids were severely ductilely deformed metamorphosed and migmatized during the pervasive pan african event at ca 550 580 ma this caused many detrital zircons in the khondalites to become variably recrystallized and to develop metamorphic rims proper interpretation of cathodoluminescence images and zircon morphology is important in interpreting detrital populations the southern indian khondalites were part of an extensive palaeoproterozoic metasedimentary assemblage that may have extended from southern madagascar via the trivandrum block to the highland complex of sri lanka and seems to reflect a continental margin sequence that was deposited on an archaean to early palaeoproterozoic continental terrane possibly the southern margin of the indian dharwar craton the tectonic history of this terrane remains obscure due to the pervasive overprint during the pan african event

analyses of high iron sedimentary bedrock and diagenetic features observed with chemcam at vera rubin ridge gale crater mars calibration and characterization curiosity investigated a topographic rise named vera rubin ridge vrr in gale crater for which a distinct hematite like signature was observed from orbit however the chemcam and apxs instruments on board the rover did not record any significant iron enrichment in the bulk of the ridge compared to previous terrains for this study we have re verified chemcam iron calibration at moderate abundances and developed more accurate calibrations at high iron abundances using iron oxide mixtures in a basaltic matrix in order to complete the chemcam calibration database the high iron calibration was first applied to the analysis of dark toned diagenetic features encountered at several locations on vrr which showed that their chemical compositions are close to pure anhydrous iron oxides then we tracked iron abundances in the vrr bedrock and demonstrated that although there is no overall iron enrichment in the bulk of the ridge 21 2 1 8 wt feo t compared to underlying terrains the iron content is more variable in its upper section with areas of enhanced iron abundances in the bedrock up to 26 6 0 85 wt feo t since the observed variability in iron abundances does not conform to the stratigraphy the involvement of diagenetic fluid circulation was likely an in depth chemical study of these fe rich rocks reveals that spatial gradients in redox potential eh may have driven iron mobility and reactions that precipitated and accumulated iron oxides we hypothesize that slightly reducing fluids were probably involved in transporting ferrous iron mobile fe could have precipitated as iron oxides in more oxidizing conditions

influence of chemical weathering on the composition of the continental crust insights from li and nd isotopes in bauxite profiles developed on columbia river basalts abstract mineralogical chemical and li and nd isotopic compositions of two drill cores 8 9 m deep through bauxites developed on the miocene columbia river basalts document the changes associated with basalt weathering provide insights into the processes involved and allow us to examine the overall influence of chemical weathering on juvenile basaltic crust gibbsite hematite kaolinite halloysite goethite and maghemite are the weathering products in the bauxites quartz is observed near the tops of the cores and its abundance decreases progressively with depth no quartz is observed below five meters depth in either core most major and trace elements including mobile and some immobile elements are severely depleted in the bauxites niobium is less mobile relative to the rare earth elements thus chemical weathering attenuates the negative nb anomaly in the continental crust li and nd are strongly depleted relative to fresh basalt and both increase systematically towards the surface in the quartz bearing samples while 7li and end values decrease systematically towards the surface in these same samples both li and nd were likely lost from the bauxites through leaching the systematic enrichment of li nd and quartz as well as the less radiogenic nd isotopic composition at the tops of both profiles reflects 20 60 wt addition of an eolian component to the soils the eolian dust is unlikely to have experienced significant post depositional weathering due to the relatively high li contents near the tops of the profiles and therefore the low 7li and end values suggest that the dust came from an old weathered region of the continent our results demonstrate that lithium isotopes are sensitive tracers of chemical weathering particularly in extreme weathering settings and support the hypothesis that chemical weathering influences the mass and composition of the continental crust

episodic paleoproterozoic 2 45 1 95 and 1 85 ga mafic magmatism and associated high temperature metamorphism in the daqingshan area north china craton shrimp zircon u pb dating and whole rock geochemistry abstract in the daqingshan area within the khondalite belt of the north china craton paleoproterozoic gabbro and dolerite intrusions show amphibolite to granulite facies metamorphism u pb zircon dating and whole rock geochemistry were undertaken on these rocks in order to understand the timing of mantle magmatism and thermal processes ten samples yielded zircon u pb formation ages of 2 46 2 44 ga 1 97 1 92 ga and 1 84 ga and metamorphic ages of 1 95 1 83 ga they are variable in geochemical composition with some being enriched in light rare earth and large ion lithosphere elements but depleted in nb ta combined with previous results it is concluded that mafic magmatism at 2 45 2 37 ga and 1 97 1 92 ga indicates two paleoproterozoic extensional phases with the latter accompanied by ht uht metamorphism as a result of underplating of mantle magma

u pb and hf isotopic study of detrital zircons from the wulashan khondalites constraints on the evolution of the ordos terrane western block of the north china craton abstract detrital zircons from the wulashan khondalites in the ordos terrane western block of the north china craton give u pb ages between 1 84 and 2 32 ga showing that their provenance was dominated by paleoproterozoic rocks much younger than those exposed in the eastern block these zircons have oscillatory growth zoning and possess e hf values between 8 and 9 suggesting derivation of their precursor magmas from old crust and juvenile materials from the mantle the lowest e hf values of zircons with different ages give a line that intersects the depleted mantle line at about 2 6 ga in a e hf vs time diagram this may imply that the precursor magma source in the deep level of the crust was largely separated from the mantle 2 6 ga ago and therefore 2 6 ga may be an important crustal formation period for the terrane one khondalite sample has detrital zircons with a single age population at 2 0 ga and positive e hf values from 1 to 9 clearly recording the significant addition of juvenile materials at 2 0 ga in the source region this may inspire us to seek whether there was a connection between a mantle superevent and the formation of columbia supercontinent data of this study exclusively demonstrate that the ordos terrane was developed independently from the eastern block until assembly of the columbia supercontinent therefore tectonic models involving that the whole north china craton was cratonized 2 5 ga ago should be dismissed

the precambrian khondalite belt in the daqingshan area north china craton evidence for multiple metamorphic events in the palaeoproterozoic era high grade pelitic metasedimentary rocks khondalites are widely distributed in the northwestern part of the north china craton and were named the khondalite belt prior to the application of zircon geochronology a stratigraphic division of the supracrustal rocks into several groups was established using interpretative field geology we report here shrimp u pb zircon ages and hf isotope data on metamorphosed sedimentary and magmatic rocks at daqingshan a typical area of the khondalite belt the main conclusions are as follows 1 the early precambrian supracrustal rocks belong to three sequences a 2 56 2 51 ga supracrustal unit the previous sanggan group a 2 51 2 45 ga supracrustal unit a portion of the previous upper wulashan group and a 2 0 1 95 ga supracrustal unit including the previous lower wulashan group a portion of original upper wulashan group and the original meidaizhao group the units thus do not represent a true stratigraphy 2 strong tectono thermal events occurred during the late neoarchaean to late palaeoproterozoic with four episodes recognized 2 6 2 5 2 45 2 37 2 3 2 0 and 1 95 1 85 ga with the latest event being consistent with the assembly of the palaeoproterozoic supercontinent columbia 3 during the late neoarchaean to late palaeoproterozoic 2 55 2 5 2 37 and 2 06 ga juvenile mantle derived material was added to the crust

uranium series comminution ages of continental sediments case study of a pleistocene alluvial fan obtaining quantitative information about the timescales associated with sediment transport storage and deposition in continental settings is important but challenging the uranium series comminution age method potentially provides a universal approach for direct dating of quaternary detrital sediments and can also provide estimates of the sediment transport and storage timescales the word comminution means to reduce to powder reflecting the start of the comminution age clock as reduction of lithic parent material below a critical grain size threshold of 50 mu m to test the comminution age method as a means to date continental sediments we applied the method to drill core samples of the glacially derived kings river fan alluvial deposits in central california sediments from the 45 m core have independently estimated depositional ages of up to 800 ka based on paleomagnetism and correlations to nearby dated sediments we characterized sequentially leached core samples both bulk sediment and grain size separates for u nd and sr isotopes grain size surface texture and mineralogy in accordance with the comminution age model where 234u is partially lost from small sediment grains due to alpha recoil we found that 234u 238u activity ratios generally decrease with age depth and specific surface area with depletions of up to 9percent relativemore to radioactive equilibrium the resulting calculated comminution ages are reasonable although they do not exactly match age estimates from previous studies and also depend on assumptions about 234u loss rates the results indicate that the method may be a significant addition to the sparse set of available tools for dating detrital continental sediments following further refinement improving the accuracy of the method requires more advanced models or measurements for both the recoil loss factor fa and weathering effects we discuss several independent methods for obtaining fa on individual samples that may be useful for future studies less

magnetic viscosity of tropical soils classification and prediction as an aid for landmine detection summary electromagnetic induction emi based metal detectors are the most widely used sensing techniques in landmine clearance operations however they are negatively influenced by frequency dependence of magnetic susceptibility a total of 466 rock and soil samples collected from across the tropics are investigated in this study the data show that frequency dependent susceptibility depends on the parent material as well as on the degree of weathering ultramafic and mafic rocks and their derivatives have higher susceptibility and absolute frequency dependence than material originating from intermediate felsic and sedimentary rocks within each parent material group absolute frequency dependence increases steadily with increasing alteration from unweathered rock to topsoil this effect is likely due to either the residual enrichment of weathering resistant ferrimagnetic minerals including superparamagnetic sp grains the comminution of larger ferrimagnetic minerals or the neoformation of sp minerals during soil formation relative frequency dependence is generally lower than 15 per cent for the investigated samples with a few exceptions it increases with alteration for igneous rocks but remains at the initially high level for sediments this finding indicates that the relative concentration of sp minerals changes with respect to the total magnetic fraction for igneous rocks but remains constant for sediments soils derived from ultramafic mafic and intermediate rocks show low relative frequency dependence and their magnetic susceptibility is mainly the result of multidomain lithogenic minerals in contrast soils derived from felsic rocks and sediments show the highest values and their susceptibility is due to sp minerals that are either formed during pedogenesis or residually enriched theaverageandextremevaluesoftheabsolutefrequencydependencewithineachsubgroup based on parent material and alteration grade are used to design a prognosis system for assessing the impact of the subsurface on emi sensors for landmine detection in general intermediate felsic and sedimentary rocks have no influence on the detectors and only a weak influence inthemostextreme cases soilsderived fromtheserockstypically have noinfluence however they can have a very severe influence in a few cases in contrast ultramafic and mafic rocks typically have a moderate influence and a very severe influence in extreme cases with the associated soils resulting in a severe influence in general the deduced prognosis system can be used by demining organizations to help them predict metal detector performance in tropical regions on the basis of geologic and or soil maps which do not supply information on electromagnetic properties in this way such a system may eventually help with the planning of demining campaigns and selection of appropriate sensors

is the ordos block archean or paleoproterozoic in age implications for the precambrian evolution of the north china craton the ordos block is a large but poorly exposed crustal segment in the western part of the north china craton little is known about its precambrian basement rocks and their evolution because of an extensive cover by younger sedimentary rocks we report shrimp zircon ages and hf in zircon isotopic compositions for five samples recovered from drill holes that penetrated into the ordos basement based on these data two age groups of rocks can be distinguished an early paleoproterozoic group that is represented by one sample from the northern margin of the ordos block and a late or middle late paleoproterozoic group consisting of the remaining four samples the early paleoproterozoic sample contains zircon cores and metamorphic rims with ages of 2 4 ga and 2 28 ga respectively the cores have ehf t values of 8 8 to 6 2 and hf crustal model ages of 2606 to 3221 ma the remaining rocks of late or middle late paleoproterozoic age contain zircons that may also show a core rim structure but with the cores having ages of 2 08 ga and the metamorphic rims recording ages of 1 9 ga the zircon cores have ehf t values of 9 1 to 10 1 and hf crustal model ages of 2039 to 3068 ma combined with data from earlier studies we draw the conclusion that late paleoproterozoic metasedimentary rocks are widespread in the ordos basement there is no evidence from our samples of an archean component and this calls into question the prevailing view that the ordos basement is an archean cratonic block the fact that the ordos basement was involved in a widespread late paleoproterozoic tectono thermal event indicates that most current models for the tectonic evolution of the western portion of the north china craton need to be revised

neoarchean metagabbro and charnockite in the yinshan block western north china craton petrogenesis and tectonic implications abstract properly calibrating the magmatic record of mantle and crustal origin in archean granite greenstone terranes is crucial for understanding the petrogenetic and geodynamic processes that generated early continental landmasses this geochronological and geochemical study documents a neoarchean bimodal metaplutonic suite of metagabbro and charnockite from the yinshan block north china craton the meta gabbroic rocks show an sio 2 range from 45 9 to 51 7 and high mgo content from 6 7 to 16 4 with enrichment in ba and light rare earth elements la n yb n 3 11 5 99 and depletion in high field strength elements together with their enriched whole rock nd nd t 0 23 to 1 49 and zircon hf hf t 0 1 6 9 isotopic signatures relative to the archean depleted mantle in the craton these rocks are supposed to originate from the second stage high temperature partial melting of refractory depleted mantle that experienced prior basaltic magma extraction and subsequent metasomatism by subduction related fluids the associated charnockites range in sio 2 from 61 3 to 69 6 and exhibit a magnesian calc alkalic and metaluminous character with variable sr y and la n yb n ratios these elemental features plus their evolved isotopic compositions nd t 0 93 1 85 zircon hf t 0 3 to 2 5 are consistent with partial melting of newly underplated mafic lower crustal protolith in combination with widespread occurrence of metasomatized lithospheric mantle derived magmas e g high mg basalts sanukitoid suites and juvenile crust extracted potassic granites in the ncc during late neoarchean time such a mafic and felsic magma association not only attests to the establishment of a craton scale subduction related metasomatized sub continental lithospheric mantle but also encapsulates a scenario of coupled lithospheric mantle crust formation at 2 7 ga and juvenile crustal reworking at 2 5 ga within a modern style convergent continental margin possibly featuring episodic slab break off events

single zircon grains record two paleoproterozoic collisional events in the north china craton abstract the western block of the north china craton consists of the yinshan block in the north and the ordos block in the south which were amalgamated along the east west trending khondalite belt at 1 95 ga the western block then collided with the eastern block to form the coherent basement of the north china craton along the north south trending trans north china orogen at 1 85 ga the huaian complex a high grade terrrane located at the conjunction of the khondalite belt and trans north china orogen records metamorphic events associated with both collisions the complex consists of lithologies from both the khondalite belt and trans north china orogen of which the former consist of graphite garnet sillimanite gneiss garnet quartzite felsic paragneiss calc silicate rock and marble together called the khondalite series zircons in the graphite garnet sillimanite gneiss can be divided into three types 1 spherical grains without internal structures 2 grains with a core and rim structure and 3 grains with a dark core surrounded by double rims except for the dark core in type 3 all other types of zircon domains are structureless and highly luminescent with very low th u ratios typical of a metamorphic origin analyses on the cores of type 2 and the inner rims of type 3 from two samples yield upper intercept ages of 1946 26 and 1947 22 ma similar to previously determined metamorphic ages from the khondalite belt and thus interpreted as the time of collision between the yinshan and ordos blocks analyses on type 1 zircons rims of type 2 and the outer rims of type 3 from the same two samples give ages of 1850 15 and 1857 16 ma interpreted as the time of collision between the eastern and western blocks thus zircons in the graphite garnet sillimanite gneiss of the huaian complex record both of the paleoproterozoic collisional events in the north china craton

in situ radionuclide transport and preferential groundwater flows at ineel idaho decay series disequilibrium studies uranium and thorium decay series disequilibria in groundwater occur as a result of water rock interactions and they provide site specific natural analog information for assessment of in situ long term migration of radionuclides in the far field of a nuclear waste disposal site in this study a mass balance model was used to relate the decay series radionuclide distributions among solution sorbed and solid phases in an aquifer system to processes of water transport sorption desorption dissolution precipitation radioactive ingrowth decay and recoil isotopes of u 238u 234u th 232th 230th 228th 234th ra 226ra 228ra 224ra and rn 222rn were measured in 23 groundwater samples collected from a basaltic aquifer at the idaho national engineering and environmental laboratory ineel idaho the results show that groundwater activities of th and ra isotopes are 2 4 orders lower than those of their u progenitors which average 1 35 0 40 dpm 238u l with 234u 238u ratios of 1 6 3 0 222rn activities range from 20 to 500 dpm l modeling of the observed disequilibria places the following constraints on the time scale of radionuclide migration and water rock interaction at ineel 1 time for sorption is minutes for ra and th time for desorption is days for ra and years for th and time for precipitation is days for th years for ra and centuries for u 2 retardation factors due to sorption average 106 for 232th 104 for 226ra and 103 for 238u 3 dissolution rates of rocks are 70 to 800 mg l y 4 ages of groundwater range from 10 to 100 years contours of groundwater age as well as spatial patterns of radionuclide disequilibria delineate two north south preferential flow pathways and two stagnated locales relatively high rates of dissolution and precipitation and recoil of 222rn occur near the groundwater recharging sites as well as in the major flow pathways decay of the sorbed parent radionuclides e g 226ra and 228ra on micro fracture surfaces constitutes an important source of their daughter 222rn and 228th activities in groundwater

geochemical evolution of a palaeolaterite the interbasaltic formation northern ireland abstract the interbasaltic laterites of northern ireland were formed during a period of relative volcanic inactivity by extensive chemical weathering of tertiary basalts they reach a maximum thickness of 30 m and once provided a major source of iron and aluminium ore an extensive database comprising major minor and trace elements has been compiled for 240 samples in order to study the effects of weathering in terms of the changes in whole rock chemistry and mineralogy from basalt through to iron rich crust percolating waters caused degradation of the parent basalt mineralogy and precipitation of neoformed phases principally through incongruent dissolution processes primary olivine plagioclase feldspar and augite were successively broken down and replaced by a mineral assemblage consisting of hematite gibbsite goethite anatase meta halloysite and kaolinite changes in mineralogy facilitated concomitant changes in element concentrations mass balance calculations indicate that all elements were depleted in the iron crust enrichment of al loi cr cu and v occurred in the laterite horizon while enrichment of al loi ba ce cr cu ni and rb occurred in the lithomarge notably yttrium was found to be mobile indicating that weathered basalts should not be used in discrimination of original tectonic environments the severe leaching conditions evidenced by yttrium depletion local aluminium redistribution and iron crust formation are indicative of weathering under a humid sub tropical monsoon climate

early neoarchean 2 7 ga tectono thermal events in the north china craton a synthesis abstract the north china craton ncc is characterized by major 2 5 ga tectono thermal events and is thus different from many other cratons worldwide where 2 7 ga events are well developed however whole rock nd and hf in zircon isotopic compositions of 2 5 ga crustally derived granitoids reveal that they mainly formed through reworking of late mesoarchean to early neoarchean continental material we review the spatial distribution rock types geochemical and nd hf isotopic compositions of 2 7 ga granitiods that are widely identified in the ncc including eastern shandong western shandong huoqiu hengshan fuping zanhuang zhongtiao and wuchuan these granitoids are mainly tonalitic in composition and frequently underwent strong metamorphism deformation and anatexis they show large variations in sio 2 total feo mgo and cao and can be subdivided into two types in terms of their ree patterns more importantly whole rock nd and hf in zircon isotopic compositions indicate that the strong 2 7 ga tectono thermal event mainly involved juvenile additions to the continental crust the early neoarchean was the most important period for rapid production of new continental crust in the ncc the main difference of the ncc from many other cratons worldwide is a strong and widespread superimposed tectono thermal event at 2 5 ga that stabilized the cratonic assemblage

geochemical differentiation in laterite and bauxite formation abstract the geochemical differences between laterite and bauxite formation are discussed under the aspect that both weathering products are predominantly formed by residual enrichment of iron and aluminium residual enrichment is corroborated by various arguments and clearly proved for laterite formation on ultramafic rocks tropical weathering of the most widespread acidic rocks leads in spite of their high al fe ratio generally to formation of laterites showing a high enrichment of iron whereas aluminium is not or only slightly enriched this chemical variation indicates that high amounts of al are removed additionally to si in soil solutions when saprolites on acidic rocks are transformed into laterites containing goethite hematite and kaolinite as main constituents dissolved free aluminium exist in equilibrium with kaolinite only in minimal contents hence the removal of si and al probably takes place in form of freshly formed colloidal or subcolloidal x ray amorphous particles with kaolinitic or similiar composition the frequently observed occurrence of two generations of secondary kaolinite in laterites corroborates this conclusion there is general agreement that bauxites are formed by stronger leaching compared with laterites increased leaching lowers the concentration of dissolved silica thus enabling gibbsite instead of kaolinite stability the concentration of dissolved free aluminium however is increased in equilibrium with gibbsite free aluminium shows in contrast to iron an effective solubility atph 5 giving rise to downward leaching and re precipitation at higher ph below a laterite crust which covers the bauxite zone in most of the bauxite deposits in spite of the stronger leaching bauxite formation implies a lower mass loss mainly si during weathering than the formation of laterites loss ofsi al related to their highly increased iron content

self organization in chemical lateritic weathering abstract ferruginous laterites consist of different superposed horizons characterized by their mineralogical and structural composition climatic changes are foremost among the parameters used to explain variations of mineralogy and structures however several types of structures can be explained as the spontaneous and autonomous change of the geochemical soil system without the intervention of external factors such as climate or tectonics two examples illustrate this self organization of lateritic horizons formation of a ferruginous nodular horizon and formation of a conglomeratic iron crust self organization results from reaction transfer feedbacks between permanently precipitated and dissolved minerals and continuously percolating solutions allowing the geochemical soil system to be in constant search of equilibrium

paleoproterozoic ultrahigh temperature granulites in the north china craton implications for tectonic models on extreme crustal metamorphism abstract ultrahigh temperature uht metamorphic rocks associated with the khondalite belt within the inner mongolia suture zone imsz provide robust evidence for extreme thermal metamorphism in the north china craton ncc the imsz marks the collisional suture between the yinshan block to the north and the ordos block to the south as the ncc was incorporated within the columbia supercontinent amalgam during paleoproterozoic here we present a synthesis of the salient features of the uht rocks from the ncc including petrologic indicators fluid characteristics and monazite and zircon chronometry on the extreme crustal metamorphism the granulites carry diagnostic uht mineral assemblages including sapphirine quartz low zn fe3 spinel quartz high alumina orthopyroxene sillimanite quartz and high temperature mesoperthite the stability fields of the typical mineral assemblages conventional geothermobarometry and phase equilibria modeling using pseudosections as reported in a number of recent studies converge to indicate that these uht rocks experienced metamorphic temperatures up to or in excess of 1000 c at ca 10 kbar followed by an isobaric cooling segment the rocks were exhumed along a near isothermal decompression path microstructures mineral reactions and phase equilibria modeling suggest an anti clockwise p t path similar to those displayed by metamorphic orogens developed in subduction collision settings the dominant category of fossil fluids preserved within the major uht minerals is co2 consistent with the stability of the broadly anhydrous mineral assemblage in these rocks both chemical and radiogenic isotopic ages from monazite and zircon chronometry suggest the timing of the uht event as around 1 92 ga the paleoproterozoic high grade metamorphism younging from 1 95 ga in the western domain to 1 92 ga in the eastern domain of the khondalite belt might suggest a scissor like closure of oblique collision between the yinshan and the ordos blocks the salient features of the uht metamorphism in the ncc include 1 extreme metamorphic temperatures at moderate pressures 2 dominantly anhydrous nature of the mineral assemblages typically the stability of orthopyroxene 3 common presence of co2 rich fluid inclusions as the trace of the ambient fluid 4 regional extent of the uht granulites and 5 the association of the uht orogen with an accretionary belt in a continental collisional suture we evaluate the diverse models on the generation of uht orogens including their formation in thickened and inverted back arcs orogen self heating through heat producing elements heat and co2 input by plume impingement below a carbonated tectosphere and asthenospheric upwelling through ridge subduction and slab window process or during post collisional slab break off the ultra hot and dry uht rocks in the ncc provide one of the well preserved examples from the paleoproterozoic globe for investigating extreme metamorphism and related tectonic processes within the plate tectonic paradigm

sediment transport time measured with u series isotopes results from odp north atlantic drift site 984 abstract high precision uranium isotope measurements of marine clastic sediments are used to measure the transport and storage time of sediment from source to site of deposition the approach is demonstrated on fine grained late pleistocene deep sea sediments from ocean drilling program site 984a on the bjorn drift in the north atlantic the sediments are siliciclastic with up to 30 carbonate and dated by 18o of benthic foraminifera nd and sr isotopes indicate that provenance has oscillated between a proximal source during the last three interglacial periods volcanic rocks from iceland and a distal continental source during glacial periods an unexpected finding is that the 234u 238u ratios of the silicate portion of the sediment isolated by leaching with hydrochloric acid are significantly less than the secular equilibrium value and show large and systematic variations that are correlated with glacial cycles and sediment provenance the 234u depletions are inferred to be due to recoil loss of 234th and are used to calculate comminution ages of the sediment the time elapsed between the generation of the small 50 m sediment grains in the source areas by comminution of bedrock and the time of deposition on the seafloor transport times the difference between comminution ages and depositional ages vary from less than 10 ky to about 300 to 400 ky for the site 984a sediments long transport times may reflect prior storage in soils on continental shelves or elsewhere on the seafloor transport time may also be a measure of bottom current strength during the most recent interglacial periods the detritus from distal continental sources is diluted with sediment from iceland that is rapidly transported to the site of deposition the comminution age approach could be used to date quaternary non marine sediments soils and atmospheric dust and may be enhanced by concomitant measurement of 226ra 230th 230th 234u and cosmogenic nuclides

neoarchean paleoproterozoic multiple tectonothermal events in the western alxa block north china craton and their geological implication evidence from zircon u pb ages and hf isotopic composition abstract the alxa block was traditionally considered to be part of the north china craton but its metamorphic basement has been poorly studied here we present a systematic zircon u pb and hf isotopic investigation on four orthogneiss samples in the beidashan area of the western alxa block the petrographic and geochemical data show that these rocks are granodioritic and trondhjemitic gneisses with ttg tonalite trondhjemite granodiorite characteristics zircons from the ttg gneisses display typical core rim or core mantle rim structures u pb datings and hf isotopic analyses reveal two distinct age populations the latest neoarchean 2 5 ga and the late palaeoproterozoic 1 85 ga the magmatic zircon cores and metamorphic mantles rims of the ttg gneisses were dated at similar ages around 2 5 ga supporting the existence of archean rocks in the western alxa block the short time interval between the latest neoarchean magmatism and the subsequent metamorphism suggests that they were related to the same latest neoarchean tectonothermal event the 2 5 ga zircons have hf t mainly between 0 8 and 5 0 t dm hf model ages mainly between 2 6 and 2 8 ga with a peak at 2 7 ga and t dmc hf model ages mainly between 2 7 and 3 0 ga with a peak at 2 8 ga the age of 1 85 ga obtained from two trondhjemitic gneisses is interpreted as the age of the late paleoproterzoic high grade metamorphism our combined datasets show that the ttg gneisses in the baidashan area of the western alxa block experienced a main 2 7 2 8 ga crust growth a 2 5 ga magmatic metamorphic event and a 1 85 ga high grade metamorphic event the sequence of events is very similar to that of the other north china craton a combination of the data of paleoproterozoic metamorphic rocks in the alxa block suggests that the alxa block is the western extension of the khondalite belt rather than the yinshan block

lack of bedrock grain size influence on the soil production rate abstract our study deals with the part played by bedrock grain size on soil formation rates u and th series disequilibria were measured in two soil profiles developed from two different facies of the same bedrock i e fine and coarse grain size granites in the geomorphically flat landscape of the experimental breuil chenue forest site morvan france the u and th series disequilibria of soil layers and the inferred soil formation rate 1 2 mm ky 1 are nearly identical along the two profiles despite differences in bedrock grain size variable weathering states and a significant redistribution of u and th from the uppermost soil layers this indicates that the soil production rate is more affected by regional geomorphology than by the underlying bedrock texture such a production rate inferred from residual soil minerals integrated over the age of the soil is consistent with the flat and slowly eroding geomorphic landscape of the study site it also compares well to the rate inferred from dissolved solutes integrated over the shorter time scale of solute transport from granitic and basaltic watersheds under similar climates however it is significantly lower than the denudation or soil formation rates previously reported from either cosmogenic isotope or u series measurements from similar climates and lithologies our results highlight the particularly low soil production rates of flat terrains in temperate climates moreover they provide evidence that the reactions of mineral weathering actually take place in horizons deeper than 1 m while a chemical steady state of both concentrations and u series disequilibria is established in the upper most soil layers i e above 70 cm depth in such cases the use of soil surface horizons for determining weathering rates is precluded and illustrates the need to focus instead on the deepest soil horizons

the mount saddleback bauxite deposit southwestern australia bauxite within the laterite horizon was first identified as being of economic significance in the mt saddleback area of western australia in the 1960 s the mt saddleback bauxite is derived from a mafic volcanic greenstone parent material and has high fe and low silica contents compared with other bauxite in the region it occurs as two distinct lithological units a hard massive fe rich layer hardcap overlying a more friable nodular earthy horizon b zone the bauxite averages 6 m in thickness and is underlain by a deep clay horizon the major minerals of the bauxite are gibbsite goethite and hematite with kaolin increasing with depth towards the base of the b zone minor components are quartz and anatase with traces of corundum boehmite and maghemite in hardcap

is the ordos basin floored by a trapped oceanic plateau abstract the ordos basin in china has about 10 km of neoarchean to quaternary sediments covering an enigmatic basement of uncertain origin the basement is tectonically stable has a thick mantle root low heat flow few earthquakes and has been slowly subsiding for billions of years the basement has geophysical signatures that indicate it is dominantly intermediate to mafic in composition and is similar to some other cratons world wide and also to several major oceanic plateaus it was accreted to the amalgamated eastern block and central orogenic belt of the north china craton ncc in the paleoproterozoic then involved in several proterozoic tectonic events including being over thrust by an accretionary orogen and intruded by andean arc related magmas and then involved in a continent continent collision during amalgamation with the columbia supercontinent thus the basement rocks are deformed metamorphosed to granulite facies and determining their initial origin is difficult we suggest that the data is consistent with an origin as an oceanic plateau that accreted to the ncc and later experienced different episodes of differentiation associated with later subduction and collisions formation of cratonic lithosphere by accretion of oceanic plateaus may be one mechanism to create stable cratons other cratons that apparently formed by partial melting of underplated and imbricated oceanic slabs are stable in some cases but also re activated and de cratonized in some cases in asia where they have been affected by younger subduction hydration slab roll back and melt peridotite reactions this suggests that the initial mode of craton formation may be a factor in the preservation of stable cratons and de cratonization is not only influenced by younger tectonic activity

the geochemistry of indian bole horizons palaeoenvironmental implications of deccan intravolcanic palaeosurfaces deccan intravolcanic bole horizons represent weathering products formed during major hiatuses of a major volcanic episode in these quiescent periods weathering processes pervasively altered the newly formed volcanic landscape and subsequent flows covered and effectively fossilized the resulting weathered palaeosurfaces the current work is a detailed geochemical study which examines patterns of element mobilization during these intravolcanic weathering events the bole horizons normally rest on top of altered basaltic lavas both boles and altered lavas represent a comparatively early stage in weathering because the content of chemically residual elements such as aluminium and iron are closer to fresh basalt than laterite nevertheless these is clear evidence for significant chemical alteration because the more mobile elements such as calcium and sodium have been substantially removed a chemically distinctive nature for some boles can be demonstrated from chondrite normalized ree plots and in these instances strontium and neodymium isotopic compositions demonstrate that the fine grained bole material is derived from a chemically distinct source in addition thin sections reveal that these fine grained portions commonly contains glass shards and occasionally fresh phenocrysts it is therefore suggested that many deccan boles are in fact weathered pyroclastic material and that the pyroclastic content of the basaltic succession may be greater than previously supposed a significant pyroclastic input during the deccan eruptions has important palaeoenvironmental implications for the fate of late cretaceous flora and fauna in peninsular india

dissolution of kaolinites and accumulation of iron oxides in lateritic ferruginous nodules mineralogical and microstructural transformations abstract in situ geochemical and structural microanalysis of non disturbed samples of laterite and mineralogical identifications of microsamples demonstrate an orderly succession of mineralogical and structural transformations these transformations result in the formation of fe nodules they start from a micaceous phase whose micropores are occupied by hematite because the source of iron is external to micas this hematite corresponds to an absolute accumulation in the meaning of brewer 1964 the following transformations constitute a succession of three mineral phases corresponding to relative accumulations with the removal of some components and in situ reutilization of residual materials 1 phase of fe kaolinite accumulation during which the structures inherited from micas are preserved 2 phase of al hematite accumulation resulting from a dissolution of fe kaolinite with development of pedoplasma and vesicular porosity 3 phase of accumulation of more or less aluminous goethite which ultimately results in the crystallization of geodes at the borders of voids

the epigenetic replacement of kaolinite by hematite in laterite petrographic evidence and the mechanisms involved abstract this paper describes the petrographical and mineralogical transformations accompanying the accumulation of iron oxides within the deep mottled layer of a laterite from burkina faso in this layer the proportions of purple red hematitic spots or nodules which form at the expense of volumes occupied by a soft microporous kaolinite plasma increase as the surface of the soil is approached the antagonism between hematite and kaolinite was observed at different levels in all cases from the border to the core of the hematitic domains kaolinite experiences corrosion soon followed by complete destruction at this later stage hemalite crystals are organized into thin lamellar structures showing clearly the shapes of former kaolinite platelets a mechanism explaining coincidence of the accumulation of hematite with the destruction of kaolinite is proposed we postulate that in the deep layers of laterites fed by fe 2 solutions the protons necessary to dissolve kaolinite are generated by a reaction similar to the oxidation hydrolysis step of ferrolysis in the mottled layer of this laterite therefore the progressive building up of a high concentration of iron appears to be the result of two closely related processes i e the precipitation of iron oxides from solution and the concommitant dissolution of kaolinite

archean to paleoproterozoic continental crust growth in the western block of north china constraints from zircon hf isotopic and whole rock nd isotopic data abstract this study presents new zircon hf isotopic data which combined with previous zircon u pb geochronological in situ hf and whole rock nd isotopic data from the western block of the north china craton ncc provide a new insight into global scale continental crustal growth and evolution of early earth in particular igneous zircons and detrital zircons from the yinshan block yield 207 pb 206 pb ages of 2446 2733 ma with an age peak at 2524 ma of those 86 have positive e hf t values and archean hf model ages t dm c 2 6 2 9 ga peaking at 2840 ma this indicates that those basement rocks of the western block are most likely derived from a depleted mantle source implying a crust mantle differentiation event at 2 8 2 7 ga followed by extensive reworking at 2 5 ga more than 95 detrital zircons from the khondalite belt have ages converging on 2 2 2 0 ga and their e hf t values vary from 5 43 to 9 54 of which 80 are positive and yield hf model ages t dm c peaking at 2250 ma these characteristics of e hf t values in the khondalite belt are also documented in the basement rocks of the ordos block under same age frame of 2 2 2 0 ga this suggests that the ordos block may be a juvenile crust with addition of older materials the initiation of subduction under the southern margin of the ordos block leads to the formation of a 2 2 2 0 ga magmatic arc which is likely related to the assembly of the supercontinent columbia

uranium series isotopes in river materials insights into the timescales of erosion and sediment transport abstract the uranium series isotope signatures of the suspended and dissolved load of rivers have emerged as an important tool for understanding the processes of erosion and chemical weathering at the scale of a watershed these signatures are a function of both time and weathering induced fractionation between the different nuclides provided appropriate models can be developed they can be used to constrain the residence time of river sediment this chronometer is triggered as the bedrock starts weathering and the inferred timescale encompasses the residence time in the weathering profile storage in temporary sediment deposits e g floodplain and transport in the river this approach has been applied to various catchments over the past five years showing that river sediments can reside in a watershed for timescales ranging from a few hundreds of years iceland to several hundreds of thousands of years lowlands of the amazon various factors control how long sediment resides in the watershed the longest residence times are observed on stable cratons unaffected by glacial cycles or more generally climate variability and human disturbance shorter residence times are observed in active orogens andes or fast eroding recently glaciated catchments iceland in several cases the residence time of suspended sediments also corresponds to the time since the last major climate change the u series isotope composition of rivers can also be used to predict the river sediment yield assuming steady state erosion is reached by comparing this estimate with the modern sediment yield obtained by multi year sediment gauging it is clear that steady state is seldom reached this can be explained by climate variability and or human disturbance steady state is reached in those catchments where sediment transport is rapid iceland or where the region has been unaffected by climate change and or human disturbance u series are thus becoming an important tool to study the dynamics of erosion

lateritisation of gabbro granophyre rock units of the ezhimala complex of north kerala india abstract laterites are ubiquitous in kerala and are developed over different rock types the laterite profiles developed over gabbro granophyre of the ezhimala complex are discussed here the principal mineral assemblages of the laterites are kaolinite gibbsite goethite hematite and quartz with minor amounts of zircon amorphous ferric hydroxide al hydroxide and al silicate are also present samples from various units of the weathering profile have been analysed for major and trace elements major element data show a steady depletion of si and enrichment of sesquioxides and tio2 during weathering it is also observed that the more the content of the mobile elements si mg ca k na in the parent rock the greater is its depletion during lateritisation the trace elements co cr v cu and ni show an enrichment while zn and ba show depletion over both the rock types pb shows almost a consistent behaviour

further evidence for 1 85 ga metamorphism in the central zone of the north china craton shrimp u pb dating of zircon from metamorphic rocks in the lushan area henan province abstract this paper reports shrimp zircon u pb dating of precambrian supracrustal and granitic rocks from the lushan area henan province in the southern portion of the central zone also referred to as the trans north china orogen of the north china craton a graphite garnet sillimanite gneiss sample tw0006 1 of the shangtaihua group gives a range of inherited zircon ages from 2 73 to 2 26 ga and a metamorphic zircon age of 1 84 0 07 ga a garnet bearing gneissic granitoid sample twj358 1 which is considered to intrude the shangtaihua group gives a magmatic zircon age of 2 14 0 02 ga and a metamorphic zircon age of 1 87 ga the metamorphic zircon ages of 1 87 1 84 ga obtained in this study indicate that an important tectonothermal event occurred at the end of the paleoproterozoic in the lushan area this supports the southern continuation of a central zone in the north china craton that workers have recently considered to result from continent continent collision it is also evident that the shangtaihua group was formed during the paleoproterozoic between 2 26 and 2 14 ga and not during the archean as previously considered