chemical and boron isotopic compositions of tourmaline from the paleoproterozoic houxianyu borate deposit ne china implications for the origin of borate deposit abstract the houxianyu borate deposit in northeastern china is one of the largest boron sources of china hosted mainly in the paleoproterozoic meta volcanic and sedimentary rocks known as the liaohe group that are characterized by high boron concentrations the borate ore body has intimate spatial relationship with the mg rich carbonates silicates of the group with fine grained gneisses meta felsic volcanic rocks as main country rocks the presence of abundant tourmalinites and tourmaline rich quartz veins in the borate orebody provides an opportunity to study the origin of boron the nature of ore forming fluids and possible mineralization mechanism we report the chemical and boron isotopic compositions of tourmalines from the tourmaline rich rocks in the borate deposit and from the tourmaline bearing fine grained gneisses tourmalines from the fine grained gneisses are chemically homogeneous showing relatively high fe and na and low mg with 11b values in a narrow range from 1 22 to 2 63 tourmalines from the tourmaline rich rocks however commonly show compositional zoning with an irregular detrital core and a euhedral overgrowth and have significantly higher mg ree and more pronounced positive eu anomalies v 229 1852 ppm and sr 208 1191 ppm than those from the fine grained gneisses they show varied b isotope values ranging from 4 51 to 12 43 which plot intermediate between those of the terrigenous sediments and arc rocks with low boron isotope values as represented by the 11b 1 22 to 2 63 of the fine grained gneisses of this study and those of marine carbonates and evaporates with high boron isotope values in addition the rim of the zoned tourmaline shows notably higher mg ti v sn and pb and ree particularly lrees but lower fe co cr ni zn mn and lower 11b values than the core these data suggest that 1 the sources of boron of the borate ore body are mainly the paleoproterozoic meta volcanic and sedimentary rocks and 2 the ore forming fluids should be the high temperature metamorphic fluids related to the amphibolite facies metamorphism of the paleoproterozoic foldbelt which leach boron from the boron rich meta volcanic and sedimentary rocks of the liaohe group and the boron rich metamorphic fluids subsequently interacted with the marine mg rich carbonates and evaporates forming borate deposit the tourmaline overgrowth in the rim and the tourmaline rich rocks

provenance and tectonic setting of the paleoproterozoic metasedimentary rocks from the liaohe group jiao liao ji belt north china craton insights from detrital zircon u pb geochronology whole rock sm nd isotopes and geochemistry abstract geochronological geochemical and isotopic studies were carried out on the paleoproterozoic metasedimentary rocks from the jiao liao ji belt north china craton to constrain their provenance and tectonic setting as well as to evaluate the effects of weathering and sedimentary processes on the source rock signature u pb isotopic dating using the la icp ms method on zircons from the paleoproterozoic metasedimentary rocks reveals that the depositional age can be constrained in the period between 1 98 and 1 9 ga geochemical data reveal low intermediate degrees of weathering of the source compositionally immature and poorly sorted detailed analysis of detrital zircon u pb geochronology whole rock sm nd isotope and geochemistry show that these metasedimentary rocks are derived from a mixed provenance the predominant derivation is from metavolcanic rocks of the liaohe group and paleoproterozoic gneissic granitoids there is also an input of archean continental crust the metasedimentary rocks were probably deposited in a back arc basin within an arc system thus supporting the paleoproterozoic arc continent collision model as already proposed by some authors

multiple 3 8 3 1 ga tectono magmatic events in a newly discovered area of ancient rocks the shengousi complex anshan north china craton abstract near anshan city in the north china craton the oldest rocks in asia 3300 ma have been thoroughly documented at two localities baijiafen and dongshan in this paper we report the full geological context for more ancient rocks from a third anshan locality the polyphase migmatite shengousi complex shrimp u pb zircon dating indicates a protracted tectono magmatic history for the shengousi complex the oldest recognised component is banded trondhjemitic gneiss 3773 6 ma which is veined by strongly deformed granitic pegmatite these occur with a second generation of trondhjemitic rocks 3454 8 and 3448 9 ma the next generation of plutonic rocks is a composite suite of iron enriched mafic dykes 3332 6 and 3331 8 ma with broadly coeval felsic veins 3311 4 ma finally there was intrusion of monzogranite 3129 6 ma strong deformation has generally brought these 3773 3129 ma plutonic phases into close concordance to form banded rocks however locally cross cutting relationships are preserved in small lower strain domains that give a relative chronology agreeing with the absolute zircon u pb chronology all of this complex history is recorded in a single some of the the 3450 ma igneous phases discovered in the shengousi complex are new ages for igneous rocks in the anshan area the precambrian geology of the anshan area is thus marked by polyphase eoarchaean orthogneisses the newly recognised 3450 ma phases widespread 3360 3300 magmatic activity found in association with mesoarchaean plutonic rocks and the neoarchaean anshan group and palaeoproterozoic liaohe group metasedimentary rocks this shows that the geology of the anshan area has a strong similarity with the narryer gneiss complex of western australia which contains the polyphase eoarchaean meeberrie gneisses the 3490 3440 ma eurada gneiss association 3300 ma granites and migmatisation and is intercalated with younger precambrian metasedimentary rocks of different ages containing hadean detrital zircons no other ancient gneiss complexes in the world show such a close match in their history with the anshan area rocks this means that the anshan area is prospective for locating new occurrences of hadean crustal components intercalated with younger sedimentary rocks

boiling sand springs dismal river nebraska agents for formation of vertical cylindrical structures and geomorphic change boiling motion not temperature sand springs fed by ground water moving upward along distinct conduits occur along the dismal river in the nebraska sand hills the cylindrical conduits are developed in alluvium and are as large as 10 m in diameter and 44 m deep sides of the spring conduits are firm boiling sand is overlain in several places by a clear layer of water in the spring time varying wave patterns and boils can be observed along the interface between these layers sediment within the conduits is generally well sorted although some debris is present because of their large size firm sides and persistence in time the conduits have the potential for preservation as vertical cylindrical structures comparable to those reported in sedimentary rocks of several ages the discovery of preserved structures is evidence of paleo ground water discharge and therefore the structures are a useful mapping tool for determining position in landscape evolution

spatial distribution and budget for submarine groundwater discharge in eckernforde bay western baltic sea submarine groundwater discharge sgd from subseafloor aquifers through muddy sediments was studied in eckernforde bay western baltic sea the fluid discharge was clearly traced by 222 rn enrichment in the water column and by the chloride profiles in pore water at several sites a considerable decrease in chloride to levels less than 10 of bottom water concentrations was observed within the upper few centimeters of sediment studies at 196 sites revealed that 22 of the seafloor of the bay area was affected by freshwater admixture and active fluid venting a maximal discharge rate of 9lm 22 d 21 was computed by modeling pore water profiles based on pore water data the freshwater flow from subseafloor aquifers to eckernforde bay was estimated to range from 4 3 10 6 to 57 3 10 6 m 3 yr 21 therefore 0 3 4 1 of the water volume of the bay is replaced each year owing to negligible surface runoff by rivers sgd is a significant pathway within the hydrological cycle of this coastal zone high resolution bathymetric data and side scan sonar surveys of pockmarks depressions up to 300 m long were obtained by using an autonomous underwater vehicle steep edges with depths increasing by more than 2 m within 8 10 m in lateral directions equivalent to slopes with an angle of as much as 118 were observed the formation of pockmarks within muddy sediments is suggested to be caused by the interaction between sediment fluidization and bottom currents fluid discharge from glacial coastal sediments covered by mud deposits is probably a widespread but easily overlooked pathway affecting the cycle of methane and dissolved constituents to coastal waters of the baltic sea

petrochemistry and sr pb and nd isotopic geochemistry of the paleoproterozoic kuandian complex the eastern liaoning province china abstract a geochemical and isotopic study has been carried out on the paleoproterozoic kuandian complex and some associated granitoids in the eastern liaoning province china the kuandian amphibolite and granite give an imprecise 1 9 0 4 ga all errors are 2 rb sr errorchron with 87 sr 86 sr o 0 711 0 006 a 2 11 0 06 ga pb pb isochron with a first stage growth 8 43 and a 2 31 0 10 ga smnd isochron with 143 nd 144 nd o 0 50970 0 00009 or nd t 1 1 nd depleted mantle model dates tdm of amphibolite are 2 75 to 2 46 ga those of granite are 2 53 to 2 36 ga zircons from the kuandian granite give a minimum 2 14 0 05 ga upper intercept date the kuandian amphibolite is chemically similar to the high mg member of columbia river basalts and the kuandian granite is an anorogenic granite they are genetically related and formed 2 4 to 2 3 ga ago fractional crystallization of olivine pyroxene and plagioclase from precursor magma of the kuandian amphibolite can produce magma with composition of the kuandian granite the paleoproterozoic mantle source in the eastern liaoning province is less depleted than that inferred from the average mantle evolution curve in contrast with the archean mantle in the sino korean craton which is more depleted than the average this study also reveals that the lishan granite of the eastern liaoning province is older than 3 ga and that some granites and dykes previously mapped as proterozoic are of mesozoic age

is the paleoproterozoic jiao liao ji belt north china craton a rift as a typical example of the paleoproterozoic crust in the eastern block of the north china craton the paleoproterozoic jiao liao ji belt consists principally of the liaohe group and its equivalents liaoji granites and mafic intrusions previous studies indicate that the evolution of the jiao liao ji belt has been mainly attributed to the opening and closing of an intracontinental rift along the eastern continental margin of the north china craton here we synthesize the paleoproterozoic magmatism sedimentation metamorphism and metallogeny against the rift model and propose a process of arc continent collision between the northern longgang and the southern nangrim blocks this conclusion is consistent with the observations including that a the 2 0 to 2 2 ga magmatism shows a typical subalkaline series rather than a bimodal distribution since the mafic rocks mostly have arc affinities and the acidic intermediate rocks belong to the calc alkaline series b the main source of the 1 9 to 2 0 ga sedimentary rocks is the paleoproterozoic arc materials indicating a fore arc or back arc basin setting c a couple of big borate deposits occur in the boron rich volcanic rocks that were formed in convergent continental margins d the north and south liaohe groups show different rock associations and metamorphic histories p t paths and e the nangrim and longgang blocks vary in lithological units geochronology and metamorphic features thus an arc continent collision tectonic scenario for the paleoproterozoic jiao liao ji belt is involved a a southward subduction in the period 2 0 2 2 ga b sedimentation during the period 1 9 2 0 ga c arc continent collision at ca 1 9 ga and d post collisional extension at 1 82 1 87 ga marking the end of the paleoproterozoic tectonothermal event

geochemistry of the mesoproterozoic lakhanda shales in southeastern yakutia russia implications for mineralogical and provenance control and recycling abstract shales of the lakhanda group of late mezoproterozoic age 1050 1000 ma from the southeastern siberian craton in russia have been analyzed for major elements and a number of trace elements including the ree s shales along the maya river formed as platform sediments in a deeper shelf facies whereas shales along the belaya river formed in more active and open environments of an upper shelf carbonate ramp the log of most elemental compositions to al2o3 ratios are the same in the maya and belaya river samples suggesting a similar source rock composition for rocks in the two areas the log of sio2 mgo na2o k2o rb ba and ni to al2o3 ratios are significantly higher and the log of tio2 to al2o3 ratios are significantly lower in shales from the belaya river than the maya river sections the cia chemical index of alteration is thus significantly lower in the belaya shales than the maya shales suggesting less weathering of the in the belaya shales than the maya shales the icvs index of compositional variability fe2o3 k2o na2o cao mgo tio2 al2o3 of the lakhanda shales are less than 1 suggesting that they are compositionally mature and were likely dominated by recycling several samples have icv 1 suggesting some first cycle input the low k2o al2o3 ratios of these shales suggest that minimal first cycle alkali feldspar was present in the initial source most shales of the lakhanda plot parallel and along the a k line in a cn k plots suggestive of intense chemical weathering high cia and do not indicate any clear cut evidence of k metasomatism or direct weathering back to the original source if k metasomatism produced these rocks then they could have formed from tonalites to basalts if weathering produced these rocks then they could have been produced from varied amounts of mostly granodiorite to granite elemental ratios critical of provenance la sc la cr la co th sc th cr th co and eu eu are not significantly different between the maya river and belaya river shales and the ratios are similar to fine fractions derived from the weathering of mostly granitoids and not basic rocks the eu eu th sc and low k2o al2o3 ratios of most shales suggest weathering from mostly a granodiorite source rather than a granite source consistent with a source from old upper continental crust some samples at the bottom of the belaya river section contain very low eu eu 0 35 suggesting significant input of first cycle detritus from highly differentiated granitoids similar to those from the aldan shield

two types of neoarchean basalts from qingyuan greenstone belt north china craton petrogenesis and tectonic implications abstract geochronological geochemical and isotopic studies were carried out on the basalts from the qingyuan greenstone belt to understand the petrogenesis of the basalts and the mantle source characteristics providing insights into neoarchean geodynamic evolution of the north china craton the qingyuan greenstone belt is analogous to the typical barberton type greenstone belt in lithological units u pb isotopic dating using the la icp ms method on zircons from the basalts and the associated andesite reveals that they erupted at 2510 2545 ma and were subjected to subsequent 2486 2491 ma metamorphism the basalts are high fe tholeiitic showing coherently positive end t and ehf t values the geochemical and isotopic studies strongly imply that they experienced significant fractional crystallization and minor no crustal contamination during magma evolution on the basis of chondrite normalized ree patterns the basalts can be further divided into two types viz the enriched and the depleted the enriched basalts are abundant in lree and rees contents depleted in hfses nb zr ti endowed with arc like geochemical peculiarities modeling reveals that the parent magma of the enriched basalts has been derived from 70 to the depleted basalts the basalts from the neoarchean qingyuan greenstone belt witness the episode of the vertical growth of continental crust at ca 2 5 ga combined with other geological considerations a large archean mantle plume model is favored to account for the generation of the late neoarchean basalts from qingyuan greenstone belt in the eastern block north china craton

zircon u pb hf isotopic and whole rock geochemical studies of paleoproterozoic metasedimentary rocks in the northern segment of the jiao liao ji belt china implications for provenance and regional tectonic evolution abstract this study examines the original nature tectonic setting and evolution of paleoproterozoic metasedimentary rocks of the north liaohe and laoling groups of the northern segment of the jiao liao ji belt in the northeastern margin of the eastern block of the north china craton using new petrological whole rock geochemical geochronological and lu hf isotope data zircons in the metasediments are magmatic and metamorphic in origin yielding peak ages of 2 03 ga for the deposition of the protoliths of the metasedimentary rocks and 1 90 and 1 85 ga for their metamorphism geochemical analyses indicate the protoliths contained sandy wacke and claystone material with minor shale derived from paleoproterozoic granites coeval acidic meta volcanic rocks meso and neo archean basement granites and paleoproterozoic meta mafic rocks using regional geological and new lu hf isotopic data we propose that the protoliths of the metasedimentary rocks of the north liaohe and laoling groups in the northern segment of the jiao liao ji belt were deposited in a back arc basin setting this region experienced two crustal growth events at 2 57 2 46 and 2 28 2 08 ga as well as two metamorphic events at ca 1 90 and 1 85 ga

the palaeoproterozoic boron deposits in eastern liaoning china a metamorphosed evaporite abstract we have carried out a field study of the palaeoproterozoic borate deposits of eastern liaoning province china the borates are hosted within mg silicate and mg carbonate rocks and have previously been interpreted as either skarns or metamorphosed volcanogenic exhalative deposits the deposits contain textures that are compatible with a sedimentary origin of the borates and there is a lack of the associated mineralization that is characteristic of magnesian skarns in addition preliminary boron isotope data indicate that the boron within the borates was not derived from the footwall rocks that have been proposed as the source of the skarn fluids a purely exhalative model can be dismissed on the basis of geochemical arguments and comparison with modern environments which show that borates are too soluble to precipitate directly from hydrothermal fluids the geology and field relationships show a number of similarities with recent non marine evaporite settings and young continental rifts in arid environments we conclude therefore that the liaoning deposits are metamorphosed evaporites in which the boron was originally present as primary evaporite borate minerals this model places the liaoning borates in the same category as the majority of other economic borate deposits in the world

u pb dating of serpentinization hydrothermal zircon from a metasomatic rodingite shell sudetic ophiolite sw poland abundant zircon grains were found within a metasomatic chlorite corrensite trioctahedral regularly interstratified chlorite smectite shell that surrounds rodingite from the jordanow gogolow jg serpentinite massif sudetic ophiolite bohemian massif the zircon grains bear two types of primary fluid inclusions containing either liquid co2 or aqueous solutions the inclusions were used to assign the p t conditions of the zircon formation 270 300 c ca 1 kbar using the crossed isochore method abraded single zircon grains give a u pb age of 400 4 3 ma which is interpreted as the age of zircon crystallization this zircon formed during rodingitization i e a metasomatic process directly connected with the serpentinization and thus provides the opportunity of direct dating of the early serpentinization process the zircon age from the jordanow gogolow massif is consistent with the crystallization age of metagabbro protoliths from the sudetic ophiolite as determined by oliver et al j geol soc lond 150 1993 355 a ce anomaly combined with the mineralogical record of greenschist facies metamorphism of the mafic rocks from sudetic ophiolite suggests that the sm nd whole rock isotopic data of pin et al lithos 21 1988 195 353 21 ma possibly characterize the age of later metamorphic event

lithostratigraphic subdivision of the karoo rocks of the luwegu basin tanzania and their biostratigraphic classification based on microfloras macrofloras fossil woods and vertebrates for the first time a comprehensive stratigraphic classification of the karoo deposits of the luwegu basin in southern tanzania is presented the karoo strata attain a total thickness of almost 6000 metres and are composed of deltaic lacustrine and fluvial deposits the sedimentary sequence has been subdivided into nine major lithostratigraphic units the results of biostratigraphic investigations microfloras macrofloras fossil woods vertebrates indicate that the karoo sequence ranges in age from upper permian to lower jurassic the palynological sequence comprises microfloras of the upper permian the upper triassic onslow and ipswich type and the lower jurassic their age is assessed from correlation with palynofloras of malagasy australia and zimbabwe resulting from this regional as well as intercontinental correlations with the karoo strata of east and south africa the karoo type strata of north africa and the gondwana sequences of australia and india can be proposed furthermore on the base of radiometric datings of hitherto unknown occurrences of volcanic rocks a karoo volcanism and an early tertiary volcanism can be distinguished

deformation history of the paleoproterozoic liaohe assemblage in the eastern block of the north china craton the paleoproterozoic liaohe assemblage and associated liaoji granitoids represent the youngest basement in the eastern block of the north china craton various structural elements and metamorphic reaction relations indicate that the liaohe assemblage has experienced three distinct deformational events d1 to d3 and four episodes of metamorphism m1 to m4 the earliest greenschist facies event m1 is recognized in undeformed or weakly deformed domains wrapped by the s1 schistosity suggesting that m1 occurred before d1 the d1 deformation produced small mostly meter scale isoclinal and recumbent folds f1 an associated penetrative axial planar schistosity s1 a mineral stretching lineation l1 and regional scale ductile shear zones concurrent with d1 was m2 metamorphism which occurred before d2 and produced low to medium pressure amphibolite facies assemblages regionally divergent motion senses reflected by the asymmetric f1 folds and other sense of shear indicators together with the radial distribution of the l1 lineation surrounding the liaoji granitoids imply that d1 represents an extensional event the d2 deformation produced open to tight f2 folds of varying scales s2 axial crenulation cleavages and ene ne striking thrust faults involving broadly nw se compression following d2 was m3 metamorphism that led to the formation of sillimanite and cordierite in low pressure type rocks and kyanite in medium pressure rocks the last deformational event d3 formed nw wnw trending folds f3 axial planar kink bands spaced cleavages s3 and strike slip and thrust faults which deflect the earlier d1 and d2 structures d3 occurred at a shallow crustal level and was associated with or followed by a greenschist facies retrograde metamorphic event m4 the liaohe assemblage and associated liaoji granitoids are considered to have formed in a paleoproterozoic rift the late spreading of which led to the occurrence of the early extensional deformation d1 and the m1 and m2 metamorphism and the final closing of which was associated with the d2 and d3 phases of deformation and m3 and m4 metamorphism

asthenospheric and lower lithospheric mantle contributions to continental extensional magmatism an example from the british tertiary province abstract the compositions of palaeocene basic magmas in skye and mull british tertiary igneous province btip shed light on the general problem of the relative importance of lithospheric and asthenospheric mantle as sources of continental magmatism during extension after the effects of crustal contamination in some magmas have been detected and the samples showing it eliminated from consideration the remainder show a significant diversity of trace element compositions basic lavas from skye and mull which lack crustal contamination have contents of compatible and moderately compatible elements e g ni cr y yb and also of moderately incompatible elements e g ti p zr hf sr mree which are within the range shown by the tholeiitic and mildly alkalic ocean island basalts oib as from iceland in contrast their contents of strongly and very strongly incompatible elements e g k rb ba u th nb ta lree are within the range shown by normal mid ocean ridge basalts these btip magmas are interpreted as second stage melts from mantle which had already lost a very small melt fraction the latter is thought to be represented by the strongly alkalic permo carboniferous dyke swarms associated with rifting at that time in nw scotland residual uppermost asthenosphere from the late palaeozoic magma genesis event is postulated to have become attached to the thickening lithosphere beneath nw scotland and thus to have remained coupled to it throughout the mesozoic until reactivated by palaeocene stretching occasional hawaiites in the top third of the skye lava pile remnant and also basalt dykes of the swarm cutting the granites of the central complex the last significant magmatic event at this centre have the elemental characteristics of typical north atlantic oib notably la ta 10 12 these compositions are thought to demonstrate input from asthenospheric source basic magmas throughout the lithospheric source dominated skye and mull magmatism it is suggested that all the basic magmas in this province are ultimately derived from asthenospheric sources but that the larger magma batches became contaminated during uprise by melting and assimilating the most fusible parts of overlying lithospheric mantle followed by lower crust in many cases smaller magma batches traverse the overlying lithosphere with little or no contamination perhaps the difference between the two lies in their flow behaviour laminar or turbulent during uprise

spatial distribution of 1950 1800 ma metamorphic events in the north china craton implications for tectonic subdivision of the craton abstract great progress has been made in the last twenty years in understanding the unification of the north china craton according to a prevailing model it developed by amalgamation of four sub blocks by three spatially and temporally separate orogenic belts the first at 1950 ma formed the western block the second at 1900 ma created the eastern block and the third at 1850 ma unified the eastern and western blocks however new data show that the 1950 ma orogen was affected by granulite facies reequilibration at 1800 ma and parts of the 1850 ma orogen have evidence of metamorphism at 1950 1900 ma new zircon u pb ages enable the paleoproterozoic metamorphic events to be statistically sub divided into four at 1955 ma m 1 1920 ma m 2 1885 ma m 3 and 1850 ma m 4 the m 1 2 events always appear together mainly in two belt areas including the helanshan qianlishan northern ordos yinshan liangcheng huai an lushan jiaobei and liaodong regions whereas the m 3 4 events are distributed over larger areas and were superimposed on the m 1 2 activities this age distribution is inconsistent with previous models of three separate orogens it should also be noted that the igneous rocks contemporary with the regional ultra high temperature high pressure granulite facies metamorphisms m 1 2 were previously described as arc related series i e the gabbro norites i s type granites and trimodal volcanics based on the spatial distribution of the paleoproterozoic metamorphic events m 1 4 and of the coeval igneous rocks we speculate that the assembly of the ncc was created by one orogen which formed between the eastern and western cratons in two tectonic stages two marginal arcs the korean and xuwujia developed against both eastern sides of the two sub cratons and were both accreted and deformed at 1965 1900 ma m 1 2 subsequently the amalgamation of the two sub cratons resulted in metamorphism on the western side of the eastern craton and in metamorphism that was superimposed on both accreted arcs at 1900 1790 ma m 3 4

diagenetic relationships of methanogenesis nutrients acoustic turbidity pockmarks and freshwater seepages in eckernforde bay abstract high organic loading 4 5 wt and sedimentation rates 1 4 mm yr 1 in eckernforde bay sediments lead to anaerobic conditions within the uppermost 15 cm intense bacterial sulphate reduction 0 011 0 15 mm so 4 red 2 yr 1 exhausts dissolved sulphate around 150 cm sediment depth resulting in methanogenesis at greater sediment depth by carbonate reduction 1 8 8 5 m ch 4 yr 1 extensive regions of eckernforde bay are characterized by acoustically turbid sediments 300 cm sediment depth resulting from deeper sediments supersaturated in methane forming free gas accumulation zones methane migrating upward into the sulphate reduction zone is effectively consumed by sulphate reducing bacteria at rates 3 10 times greater than methanogenesis i e 46 95 m ch 4 ox yr 1 the pathways of methane formation and anaerobic oxidation are confirmed by stable carbon and hydrogen isotope evidence pockmarks i e shallow surface depressions in some eckernforde bay sediments are not associated with gas ebullition rather these physical features result from the expulsion of freshwater these episodic springs or seepages of groundwater from the underlying holocene glacial lags and sands into the basin at pockmark sites have characteristic low chloride and methane concentrations the geochemical evidence indicates that the commercial accumulation of petroleum in the lower cretaceous dogger beta hauptsandstein schwedeneck field in eckernforde bay 1500 mbsf has no surface manifestation and does not influence either the occurrence of acoustic turbidity or pockmarks

sm nd isotopic systematics as tectonic tracers an example from west avalonia in the canadian appalachians abstract the tectonothermal evolution of west avalonia in mainland nova scotia appalachian orogen provides an excellent example of the application of sm nd isotopic analyses to tectonic studies regional syntheses indicate that west avalonia originated in the neoproterozoic as one of a number of terranes collectively known as peri gondwanan terranes along the gondwanan margin these terranes are characterized by late neoproterozoic voluminous arc related magmatism west avalonia became detached from gondwana in the early ordovician was accreted to laurentia by the silurian and was deformed by post accretionary strike slip tectonics in the late devonian and carboniferous associated with the amalgamation of pangea all stages of this evolution can be recognized in the sm nd isotopic signature in various neoproterozoic and paleozoic volcanic and sedimentary successions in mainland nova scotia crustally derived felsic volcanic rocks of late neoproterozoic ca 615 ma early cambrian and early silurian ages define an envelope that provides gross constraints on the sm nd isotopic composition of the underlying basement source e nd values range from 0 78 to 2 92 in the neoproterozoic rocks from 2 0 to 5 0 in the cambrian rocks and from 0 1 to 2 5 in the silurian rocks t dm model ages for each of these suites are remarkably similar they typically range from 0 9 to 1 2 ga suggesting that the sm nd isotopic composition of these suites was profoundly influenced by repeated melting of the same basement source this sm nd envelope provides a reference by which other suites may be compared and interpreted for example turbidites in a late neoproterozoic volcanic arc basin have sm nd compositions that lie within this envelope suggesting that they are derived from the coeval volcanic rocks that flank the basin the genetic significance of the model ages can be investigated by the analysis of felsic and metasedimentary rocks that pre date the main arc phase two felsic complexes have compositions that lie within the envelope and have similar model ages the presence of this signature in early avalonian rocks suggests that the model ages represent a genuine tectonothermal event at ca 1 0 to 1 2 ga that formed juvenile proto avalonian crust by implication the main arc phase in avalonia recycled this crust sm nd isotopic analyses of the metasedimentary sequences are incompatible with previous interpretations of deposition in a passive margin setting that pre dated avalonian arc activity instead the data indicate derivation from two distinct sources ancient gondwanan basement and juvenile avalonian arc suggesting deposition in a western pacific type back arc environment this re interpretation has important implications for neoproterozoic reconstructions the sm nd isotopic composition of west avalonian basement contrasts with that of other peri gondwanan terranes for example similarly aged felsic rocks in the cadomian terrane of nw france are characterized by more negative e nd values and older t dm model ages 1 0 1 9 ga these data together with detrital u pb data suggest that west avalonia was developed along the amazonian margin whereas cadomia was developed along the west african margin of gondwana sm nd isotopic analyses of early silurian clastic rocks in mainland nova scotia show fundamental differences with unconformably underlying neoproterozoic and early paleozoic avalonian successions these data have e nd values ranging from 5 7 to 6 1 with t dm model ages of about 1 4 to 1 7 ga indicating that the clastic rocks cannot have been derived from underlying successions a variety of geologic data and regional constraints suggests that they may have been derived from the caledonide orogen of western europe thereby providing a minimum age for the accretion of west avalonia to laurentia baltica sm nd isotopic analyses from late devonian early carboniferous continental clastic rocks deposited along the southern flank of avalonia in mainland nova scotia also show little influence from avalonian rocks and are compatible with derivation from the meguma terrane immediately to the south these data are consistent with regional synthesis which indicate uplift of the meguma terrane relative to west avalonia during dextral transpression

nature of three episodes of paleoproterozoic magmatism 2180 ma 2115 ma and 1890 ma in the liaoji belt north china with implications for tectonic evolution abstract three episodes of paleoproterozoic magmatism with distinctively different nature in the liaoji belt of the eastern north china craton are discussed here the 2200 2140 ma liaoji a type granites the 2160 2110 ma haicheng mafic sills and the 1890 ma qingchengzi i type granites the mafeng monzogranitic pluton representative of the liaoji a type granites gives a sims u pb zircon age of 2181 6 ma n 20 mswd 4 3 the qingchengzi plagiogranitic pluton representative of the qingchengzi i type granites gives a sims u pb zircon age of 1891 10 ma n 8 mswd 1 8 the mafeng monzogranites are high in fe ti k but low in mg al and ca they have high abundance of total rees ree 213 346 ppm they are relatively depleted in feldspar compatible elements e g eu and sr and hfses e g nb ta p and ti they have high 10000 ga al ratios of 3 19 3 61 and zr nb ce y concentrations of 472 656 ppm which are typical for a type granite the qingchengzi plagiogranites have relatively high al ca na but low fe ti and k contents they have low abundance of total rees ree 17 6 21 6 ppm they are enriched in liles e g ba k and sr but depleted in hfses e g nb ta p ti and y these features combined with extremely high sr y ratios 327 413 are comparable with those of typical modern adakites the liaoji a type granites show a source same to that of the haicheng mafic sills and they originated from the archean subcontinental lithospheric mantle plus with contamination from the archean granites however the qingchengzi i type granites are interpreted to originate from subducted oceanic crust with significant contributions from sediments the liaoji a type granites and the coeval bimodal volcanism in the liaohe group as well as the haicheng mafic sills may represent episodic magmatism which was related to a protracted intra continental rifting caused by lithospheric extension the ca 1890 ma qingchengzi i type granites and coexisting s type granites as well as the coeval regional metamorphism are favored to represent an active continental magmatism linked to paleoproterozoic subduction

the influence of connected heterogeneity on groundwater flow and salinity distributions in coastal volcanic aquifers abstract geological heterogeneity is a key factor affecting rates and patterns of groundwater flow and the evolution of salinity distributions in coastal aquifers the hydrogeologic systems of volcanic aquifers are characterized by lava flows that can form connected geologic structures in the subsurface surface based geostatistical techniques were adopted to generate geologically realistic statistically equivalent model realizations of a hydrogeologic system based on that of the big island of hawaii conduit models the density dependent groundwater flow and solute transport code seawat was used to perform 3d simulations to investigate subsurface flow and salt transport through these random realizations statistically equivalent geological systems generated with sequential indicator simulation sis and equivalent homogeneous systems were also simulated for comparison simulation results show that conduit realizations tend to have more complex salinity distributions with larger mixing zones for which the center of mass is farther offshore compared to homogeneous and sis realizations the zone of groundwater discharge is also farther offshore than for homogeneous and sis models with higher point fluxes of both fresh and saline water and greater spatial variability this highlights the importance of the geometry of geologic features in coastal groundwater flow and solute transport processes in highly heterogeneous aquifers it also points to the importance of preferential flow with implications for both coastal groundwater resource management and solute fluxes to the ocean

adakitic rocks from slab melt modified mantle sources in the continental collision zone of southern tibet abstract major trace element and sr nd pb isotopic data are presented for newly discovered adakitic rocks in the western gangdese belt southern tibet the miocene 26 10 ma adakitic rocks from the southern tibetan continental collision zones exhibit distinct differentiation trends typical of arc magmas these rocks display geochemical affinities similar to those of cretaceous 136 80 ma adakitic rocks derived from the partial melting of subducted neotethyan slab in southern tibet the whole rock geochemical and isotope characteristics of the post collision adakitic rocks reveal that their magmas likely originated from an upper mantle region previously metasomatized by slab melts during the cretaceous subduction event an interesting observation is that the e w trending belt of adakitic rocks along the yarlung tsangpo suture zone occupies a fore arc position reflecting a geotectonic setting compatible with the genesis of adakitic magmas the widespread occurrences of cretaceous adakitic rocks in this region are interpreted to testify to the former location of the adakite metasomatized mantle our favoured interpretation is that the spatial isotopic variation in the post collision adakitic rocks is mostly linked to a westward increase in sediment input in the tibetan mantle region under the framework of our paleo subduction model a slab break off event that initiated at around 25 ma would have allowed an asthenospheric upwelling beneath southern tibet which was instrumental in generating the post collision adakitic magmatism in southern tibet

sedimentary fractionations between al ti and zr and the genesis of strongly peraluminous granites sedimentary sorting produces alternating shales and sandstones with complementary chemistries the preferential partitioning of quartz and zircon in the coarse grained fraction of the sediments results in a strong fractionation between sio 2 zr and other components al 2 o 3 tio 2 as a consequence shales may be defined chemically as having higher al 2 o 3 sio 2 and tio 2 zr ratios than their source rocks sandstones have the opposite characteristics and vary in composition according to the maturity of their source and the intensity of sorting processes most sedimentary suites display a good correlation between al 2 o 3 sio 2 and tio 2 zr this result is used to evaluate an average bulk composition for recent clastic materials and to model their composition on a ternary al ti zr diagram because the al 2 o 3 tio 2 ratio of igneous rocks is highly sensitive to magmatic differentiation igneous and sedimentary trends crosscut in the al ti zr di

chemical signals from submarine fluid advection onto the continental shelf we report unambiguous signals of subsurface fluid input on the continental shelf 40 80 km from shore in water depths of 20 45 m the site of this input is near the continental shelf break off the south carolina coast between 32 33 n and 78 5 79 5 w repeated transects over this area reveal enrichments of 226ra 228ra and ba by factors of 3 4 above ambient ocean concentrations the short lived 223ra and 224ra are enriched by over an order of magnitude the ra and ba enrichments are not caused by up welling of deep water as no such 226ra or ba concentrations occur in the upper 2000 m of the atlantic ocean sediments in the region are largely relic sand and do not offer a viable source for 226ra 228ra and ba enrichments we conclude that the source is the discharge of submarine fluids enriched in 226ra 228ra and ba the salinity of water in the ra ba enriched zone is diluted by 0 2 0 6 relative to water at the same depth farther offshore pronounced salinity and density anomalies characterize one site of active discharge estimates of 226ra and ba concentrations in the fluids and the replacement time of water in the enriched zone suggest the fluid flux is similar to the summertime flow of major rivers in the area the region of ra ba enrichment is also characterized by a strong fluorescence signal indicating high concentrations of chlorophyll in the water it is probable that the leaking fluids supply nutrients that stimulate productivity in the bottom waters which are within the photic zone because the fluorescence signal is restricted to depths greater than 20 m it is unlikely that the signal could be recognized by satellite or aircraft sensors

impact of late cretaceous to neogene plate tectonics and quaternary ice loads on supra salt deposits at eastern gluckstadt graben north german basin

prediction of some weathering trends of plutonic and volcanic rocks based on thermodynamic and kinetic considerations abstract the exposed crust consists mainly of plagioclase 35 quartz 20 k feldspar 11 volcanic glass 12 biotite 8 and muscovite 5 quartz is a resistate thus feldspars and glass represent approximately 75 percent of the labile minerals the weathering characteristics of these constituents are summarized in the context of thermodynamic mass balance and kinetic considerations experimentally determined release rate constants were used to predict the proportions of ca na and k released by feldspars of plutonic rocks granites to gabbros to weathering solutions the chemical weathering trends of the weathered residues calculated from the kinetic data conform closely to the initial trends observed in some recent weathering profiles demonstrating the accuracy of the predictions since the weathering of feldspars is controlled by processes that should not change through geological time the relative release rates of ca na and k from the feldspars of granitic rocks can be calculated for future and past episodes of continental weathering experimentally determined release rate constants are not available for a wide range of volcanic glass compositions but the limited data indicate that compositional trends are predictable in weathering profiles developed on volcanic rocks the kinetic data available for rhyolitic glasses accurately predict the initial weathering trends observed in a recent rhyolite weathering profile

estimating the groundwater contribution into florida bay via natural tracers 222rn and ch4 groundwater may represent a significant pathway for nutrients and other dissolved solutes into florida bay especially near the keys where wastewater disposal practices add large amounts of nitrogen and phosphorus to the subsurface each year previously we suggested that high water column inventories of the tracers 222rn and ch4 may be indicative of groundwater discharge in this study we employed mass balance calculations to determine that the total benthic fluxes required to maintain the measured water column tracer inventories were significantly larger than diffusive fluxes and varied between 4 2 5 6 dpm m 2 min 1 and 5 8 15 4 nmoles m 2 min 1 for 222rn and ch4 respectively independent estimates of the diffusive flux and porewater activities concentrations allowed us to calculate an advective groundwater velocity assuming that the difference between the total benthic flux given above and the diffusive flux is driven by seepage driven porewater advection these calculated velocities ranged from 0 2 to 4 3 cm d 1 for all sites tracers and sampling periods with a best estimate of approximately 1 7 cm d 1 these estimates of groundwater velocities compare very well with previous measurements of groundwater flux 1 3 cm d 1 at the same sites via seepage meters

geochemical and isotopic studies of the sedimentary and granitic rocks of the altai orogen of northwest china and their tectonic implications the altai orogen northwest china represents the southwestern margin of the central asian orogenic belt geochemical and nd sr isotope analyses were carried out on the palaeozoic sedimentary and granitic rocks in order to trace their sources and to evaluate the pattern of continental growth of the orogen nd isotopic data for both the granites and sediments suggest a significant proportion of middle proterozoic crust beneath the altai orogen however addition of juvenile material arc back arc oceanic crust during palaeozoic times is also significant trace elements and isotopic data of sediments suggest their sources were immature they represent mixtures between a palaeozoic juvenile component and an evolved continental crust the early palaeozoic sediments show e nd t 3 4 to 5 0 t dm 1 5 1 8 ga and i sr 0 710 0 712 they represent a passive margin setting with a predominance of evolved crustal material in the source the devonian sequences however might have been deposited in a back arc basin setting produced by subduction of the junggar oceanic crust along the irtysh fault a significant addition of arc material into the sedimentary basin is responsible for the highly variable e nd values 6 to 0 and i sr 0 711 0 706 the carboniferous rocks were also deposited in a back arc basin setting but with predominantly arc material in the source as suggested by an abrupt increase in e nd t 6 to 3 and decrease in i sr 0 7045 0 7051 voluminous syn orogenic granitoids have e nd t 2 1 to 4 3 i sr 0 705 0 714 and t dm 0 7 1 6 ga they were not derived by melting of local metasedimentary rocks as suggested by previous workers but by melting of a more juvenile source at depth post orogenic granites have higher e nd t 4 4 than the syn orogenic granitoids indicating their derivation from a deeper crustal level where juvenile crust may predominate

the liaohe group an insight into the paleoproterozoic tectonic evolution of the jiao liao ji belt north china craton abstract the jiao liao ji belt represents one of the largest exposed regions of paleoproterozoic rocks along the eastern margin of the north china craton paleoproterozoic basement material in this area is dominated by deformed and metamorphosed volcano sedimentary successions with minor amounts of granitic and mafic intrusions this study presents the results of a systematic zircon u pb geochronological whole rock geochemical and sm nd isotopic analysis of the volcano sedimentary successions within the liaohe group of the jiao liao ji belt these data provide insights into the multiple paleoproterozoic tectonothermal events that have affected the belt as well as the petrogenesis of the paleoproterozoic meta volcanics and metasedimentary rocks on the liaodong peninsula accurate u pb age and trace element determinations of zircons from the meta volcanics in this region using laser ablation inductively coupled plasma mass spectrometry indicate that these units formed at ca 2 2 ga and were metamorphosed at ca 1 9 ga respectively these data indicate that the paleoproterozoic geochronological framework of this region is dominated by 2 2 2 0 ga magmatic 1 98 1 90 ga sedimentary ca 1 90 ga metamorphic and 1 87 1 82 ga post tectonic magmatic events the meta volcanics can be split into basalt basaltic andesite andesite dacite and rhyolite units that define a continuous calc alkaline dominated magmatic sequence that is similar to the sequences commonly found within phanerozoic magmatic arcs the meta basalts are enriched in the light rare earth elements cr co and ni and are depleted in the heavy rare earth elements and the high field strength elements e g nb ta and ti indicating that they formed from magmas generated by the partial melting of lithospheric mantle material that was previously metasomatized by subduction zone derived fluids melts the large geochemical and isotopic variations within the metamorphosed andesite dacite and rhyolite units all provide evidence of magma mixing between felsic melts generated by the partial melting of paleoarchean neoarchean basement material and the mafic melts that may also formed the meta basalts within the liaohe group or evolve via fractional crystallization prior to mixing the geochemistry of the metasedimentary rocks indicate that the pre metamorphism sedimentary units ranged from being compositionally mature to being immature and poorly sorted and were weakly to moderately weathered these sediments were derived primarily from metamorphosed felsic volcanics within the liaohe group as well as the paleoproterozoic liaoji granites they also contain archean continental crustal material that is derived mainly from ancient intermediate mafic basement units the combined geochronological geochemical and geological data indicate that a paleoproterozoic arc continent collision model can well explain the nature and formation of the jiao liao ji belt

are the south and north liaohe groups of north china craton different exotic terranes nd isotope constraints abstract the liaohe group is an important paleoproterozoic stratigraphic unit in the northeastern part of the north china craton and is traditionally subdivided into the north and south liaohe groups associated with both the north and south liaohe groups are voluminous paleoproterozoic granitoid rocks named the liaoji granitoids different tectonic models including terrane amalgamation continent arc collision and rift closure have been proposed to interpret the tectonic setting and evolution of the north and south liaohe groups and associated liaoji granitoids at the centre of the controversy between these models is whether or not the north and south liaohe groups developed on the same archean basement nd isotopic geochemistry of the liaoji granitoids provides important constraints on this controversial issue the liaoji granitoids associated with the north and south liaohe groups display similar e nd values restricted to a narrow range from 0 to 2 implying that these granitoid rocks were derived from the same or a similar magma source moreover the liaoji granitoids associated with the north and south liaohe groups have similar nd model ages t dm ranging from 2 4 to 2 6 ga suggesting that the protoliths of the liaoji granitoids associated with both groups may have formed simultaneously and that the basement rocks underneath the liaoji granitoids and associated north and south liaohe groups belong to the same continental block rather than two different blocks combining lithological structural and geochronological considerations we interpret the north and south liaohe groups as having developed on a single late archean basement that underwent paleoproterozoic rifting associated with the intrusion of the liaoji granitoids and the formation of the liaohe group and closed upon itself in the paleoproterozoic

la icp ms u pb zircon ages of the liaohe group in the eastern block of the north china craton constraints on the evolution of the jiao liao ji belt abstract as a major lithostratigraphic unit of the palaeoproterozoic jiao liao ji belt in the eastern block of the north china craton the liaohe group consists of an assemblage that is transitional from a lower arkose and volcanic rich sequence langzishan li eryu and gaojiayu formations through a middle carbonate rich sequence dashiqiao formation to an upper argillaceous sequence gaixian formation tectonically associated with the liaohe group are large volumes of granitoid rocks named the liaoji granite whose emplacement age has been dated at about 2 1 2 2 ga regionally the liaohe group and the associated liaoji granite extend along a nee trending belt that is flanked by 2 45 2 55 ga basement gneisses u pb zircon data and cathodoluminescence cl imaging are used to constrain the maximum depositional and metamorphic ages of the liaohe group which are critical in understanding the tectonic setting and history of the jiao liao ji belt cathodoluminescence images reveal that most detrital zircon grains from the liaohe group are characterized by concentric oscillatory zoning with comparatively low luminescence and high th u ratios suggesting magmatic origin but minor zircon grains have structureless overgrowth or recrystallization rims that are characterized by low th u ratio indicating metamorphic origin results of this study show that the detrital zircon grains of the langzishan formation the lowest sequence of the liaohe group have concordant u pb ages ranging from 2 05 to 2 24 ga suggesting that the liaohe group was deposited at some time after 2 05 ga although most overgrowth or recrystallization rims are too narrow to be analyzed some rims yield concordant or upper intercept ages around 1 93 ga interpreted as the peak metamorphic age of the liaohe group detrital zircon grains from the dashiqiao formation in the middle sequence of the liaohe group have concordant u pb ages ranging from 2 25 to 2 53 ga some of which are similar to those of the basement gneisses in the area this study indicates that the liaohe group was deposited after the emplacement of the liaoji granite and the latter may be a major component of the provenance for the langzishan formation in contrast the protoliths of the middle sequence of the liaohe group were most probably derived from the basement gneisses these new zircon ages for the liaohe group and recent data for the liaoji granite combined with lithological structural and metamorphic considerations support a rifting model for the evolution of the jiao liao ji belt

shrimp u pb zircon geochronology of palaeoproterozoic metasedimentary rocks in the north china craton evidence for a major late palaeoproterozoic tectonothermal event abstract although metasedimentary successions are widely distributed across the north china craton those of palaeoproterozoic age are restricted to three orogenic belts namely the trans north china orogen the khondalite belt and the jiao liao ji belt prior to this work some of those successions were considered to be archaean in age the palaeoproterozoic metasedimentary rocks contain detrital igneous zircon grains with ages ranging from 3 0 to 2 1 ga with the majority recording an age between 2 3 and 2 0 ga this latter group define a series of magmatic events that occurred between formation of the archaean basement and deposition of the metasedimentary rock protoliths the metasedimentary rocks occur as two associations older volcano sedimentary units that formed between 2 37 and 2 0 ma and younger sedimentary units that formed mainly between 2 0 and 1 88 ga all these rock associations were then subjected to metamorphism during the late palaeoproterozoic 1 88 and 1 85 ga tectonothermal event named the luliang movement in early chinese literature which was a continent continent collision that led to the amalgamation of the north china craton

the influence of sediment recycling and basement composition on evolution of mudrock chemistry in the southwestern united states this paper reports systematic changes in mudrock composition through time on a single con tinental cmstal block the changes reflect both sediment recycling processes and changes through time in the composition of crystalline material being added to the sedimentary system and are related to tectonic evolution as the block matures from a series of accreted arc terranes to a stable craton the major and trace element distributions reflect different aspects of the provenance of the mudrocks in this study major elements record sediment recycling processes as well as changing proportions of sedi mentary and first cycle source rocks with the exceptions of kzo which tends to increase and sioz and a o3 which show no trend most major oxides tend to decline in relative abundance in younger mud rocks patterns shown by the index of compositional variability fe03 kro naao cao mgo mno tiojialro and by k20 a1203 indicate that the major oxide trends are due to decreasing proportions of nonclay silicate minerals and a concomitant increase in the proportion of clay minerals probably due to decreasing input of first cycle detritus coupled with recycling of sedimentary material excursions from progressive trends marked by increases in mgo k20 and cao reflect episodes of large scale input of nonclay first cycle minerals from crystalline source rocks due to large scale basement uplift the chemistry of low solubility trace elements in contrast is not sensitive to recycling effects and reflects the composition of first cycle input incompatible elements are progressively enriched relative to compatible elements in younger mudrocks and values for chondrite normalised rare earth elements also increase in addition the eu anomaly becomes systematically more negative in younger samples these trends cmnot be explained by diagenetic or weathering processes and therefore indicate that the proportion of fnc tionated granitic first cycle detritus being added to the sedimentary system becomes greater with time these results confirm the importance of tectonic setting in controlling mudrock chemistry and also demonstrate that there is a dynamic relationship between the tectonic evolution of a continental block and the composition of its sedimentary mantle

shrimp u pb zircon geochronology of the liaoji granitoids constraints on the evolution of the paleoproterozoic jiao liao ji belt in the eastern block of the north china craton abstract the paleoproterozoic jiao liao ji belt lies at the eastern margin of the eastern block of the north china craton and is composed mainly of the liaohe group and the liaoji granitoids the liaoji granitoids are divisible into pre tectonic magnetite and hornblende biotite monzogranitic gneisses and post tectonic or anorogenic porphyritic monzogranites and alkaline syenites shrimp u pb zircon geochronology combined with cathodoluminescence cl imaging has enabled for resolution of magmatic and metamorphic events that can be directed towards understanding the history of the paleoproterozoic jiao liao ji belt shrimp u pb zircon analyses reveal that the pre tectonic magnetite monzogranitic gneisses were emplaced in the period 2176 2166 ma and metamorphosed at 1914 ma and the pre tectonic hornblende biotite monzogranitic gneisses were emplaced at 2150 2143 ma these data demonstrate that the pre tectonic monzogranitic gneisses of the liaoji granitoids were not younger than the deposition of the liaohe group as previously considered since the latter contains a large amount of 2100 2000 ma detrital zircons in contrast the pre tectonic monzogranitic gneisses of the liaoji granitoids or similar aged granitoids may have been an important component of the provenance for the liaohe group shrimp u pb zircon analyses also reveal that the post tectonic porphyritic monzogranites and granites in the jiao liao ji belt were emplaced at 1875 10 and 1856 31 ma respectively simultaneously with the emplacement of the alkaline syenites that was dated at 1857 20 and 1843 23 ma similar post tecotnic or anorogenic granites are not only limited to the jiao liao ji belt but are also found in the north and south korea suggesting that the 1 85 ga post tectonic or anorogenic granitic magmatic event may have occurred in both the korean peninsula and the eastern block of the north china craton the shrimp zircon ages of this study combined with lithological structural metamorphic and geochemical data is consistent with a rift closure model which suggests that the archean complexes on the northern and southern sides of the jiao liao ji belt were originally situated on a single continental block that underwent paleoproterozoic rifting associated with the formation of the liaohe group and liaoji granitoids and closed upon itself at 1 9 ga

boninite like volcanic rocks in the 3 7 3 8 ga isua greenstone belt west greenland geochemical evidence for intra oceanic subduction zone processes in the early earth abstract the 3 7 3 8 ga isua greenstone belt of southwest greenland is characterized by variably metamorphosed metasomatised and deformed lithotectonic successions of volcanic and sedimentary rocks the voluminous mafic volcanic rocks are composed primarily of pillow basalts intercalated with ultramafic units the sedimentary rocks consist mainly of banded iron formation cherts conglomerates and siliciclastic turbidites the least altered metavolcanic amphibolites the garbenschiefer unit from the central tectonic domain of the isua greenstone belt are characterized by high mg number 0 60 0 80 mgo 7 18 wt al2o3 14 20 wt ni 60 645 ppm and cr 60 1920 ppm contents but low tio2 0 20 0 40 wt zr 12 30 ppm y 6 14 ppm and rare earth element ree concentrations these compositional features collectively represent a coherent mafic to ultramafic suite chondrite normalized ree patterns are concave upward on the primitive mantle normalized extended trace element diagrams they are characterized by relative depletion of nb but with an enrichment of zr relative to neighboring rees alteration deformation and crustal contamination can be ruled out as the cause of the distinct and coherent composition the average initial end value of these metavolcanic rocks is 2 collectively these geochemical characteristics are comparable to those of phanerozoic boninites given the observation that in the tertiary boninites are exclusively associated with intra oceanic subduction environments e g izu bonin mariana subduction system this suggests that intra oceanic subduction zone like geodynamic processes were operating as early as 3 7 3 8 ga

evidence for major mass transfer and volume strain during regional metamorphism of pelites systematic examination of published sedimentary and metamorphosed pelite analyses has revealed evidence of significant mass transfer and volume strain during regional metamorphism statistical analysis of the data shows that barrovian zone metamorphism of pelitic schist generally causes increases in the whole rock concentrations of the low solubility elements ti and all the observed increases in ti and al contents as functions of metamorphic grade are almost certainly due to residual enrichment caused by the removal of other more soluble species application of mass balance principles to the petite compositional trends indicates that the average pelite may lose as much as 30 of its mass and volume during progressive metamorphism from subgreenschist to amphibolite facies conditions the bulk of the lost mass is silica not volatiles in addition other elements particularly ca na and k appear to be highly mobile in deep crustal metamorphism contrary to conventional interpretations it is concluded that the regional metamorphism of pelites is not an isochemical process

active pockmarks in a large lake lake constance germany effects on methane distribution and turnover in the sediment in the eastern part of lake constance the second largest prealpine lake in europe about 500 pockmarks morphological depressions on the lake floor were recently discovered the diameters of these pockmarks are as large as 16 m and about 24 of them continuously release methane in the form of visible bubbles the isotopic composition of the escaping gas indicated that the methane was of biogenic origin and was predominantly produced by the co2 reduction pathway in shallow water pockmarks 9 m and 12 m pore water analysis revealed increased methane concentrations of 1100 mol l 1 in the sediment diffusive methane fluxes and oxidation rates within these shallow pockmarks were much higher than in sediments outside pockmarks an estimated methane ebullition into the water column of about 40 l h 1 per pockmark showed that methane ebullition is by far the dominant pathway of methane release from these shallow pockmarks meiofauna abundance and organic carbon contents of the sediment were also higher in the shallow pockmarks but there were no differences in macrofauna in a deep pockmark 80 m even though bubble release was observed the sediment inside the pockmark had almost the same methane concentrations and isotopic compositions as a site outside of the pockmark at both sites the shallow and deep pockmark act mainly as sedimentation traps the emanating gas has only minor influence on the methane inventory of the sediments and therefore gas bubbles enter the water column essentially unaffected

chemistry and mineralogy of precambrian paleosols at the base of the dominion and pongola groups transvaal south africa abstract we have studied alteration zones found between sedimentary rocks of the lowest proterozoic dominion and archean pongola groups and underlying granitic rocks transvaal south africa the alteration zones have a transitional lower boundary primary igneous minerals are gradually destroyed upward toward the contact with overlying sedimentary rocks ca mg na p and mn are progressively lost upwards in the zone the chemical and mineralogical variations and available stratigraphic evidence are all consistent with the interpretation that these zones are paleosols formed by precambrian weathering 2 8 3 0 ga ago after formation the paleosols underwent metasomatism and metamorphism sericite now present in the paleosols was probably formed by low temperature potassium metasomatism of original clays andalusite in the pre pongola paleosol was probably formed by metamorphism of original clays in impermeable zones which prevented groundwater flow and potassium metasomatism the dominion paleosol was investigated at three localities the effects of weathering were similar at the three sites except for the behavior of iron one site lost about 20 of iron whereas the other sites lost 75 and 80 of initial iron content the differences in iron loss may be due to variation in soil ventilation and diffusion of atmospheric gases as a function of grain size and topography variations in iron loss may be used to place some constraints on the composition of the precambrian atmosphere given constraints imposed by dominion paleosols overlying uraniferous conglomerates requirements for chemical weathering and the greenhouse effect and possible presence of non diffusing photochemical oxidants the partial pressures of oxygen and carbon dioxide gases probably are in the ranges po1 0 02 to 0 5 pal present atmospheric level and pco2 5 to 30 pal

paleoproterozoic structural evolution of the southern segment of the jiao liao ji belt north china craton abstract the paleoproterozoic jiao liao ji belt separates the eastern block of the north china craton into two small sub blocks the northern longgang and the southern rangrim blocks however it still remains unknown or controversial about the subduction polarity collisional deformation and kinematics between two sub blocks the southern segment of the belt consists of the paleoproterozoic fenzishan and jingshan groups and paleoproterozoic high pressure mafic granulites and serpentinites blocks which are located in the jiaodong complex all of which are separated from the jiaodong complex of neoarchean ttg gneisses by stz1 ductile shear zones structural analysis in this study indicates that most of the rocks in all the units of the southern segment of the jiao liao ji belt underwent three distinct episodes of folding d1 to d3 and two stage of ductile thrust shearing stz1 coeval to d1 and d2 stz2 between d2 and d3 the d1 deformation formed penetrative axial planar foliations s1 bedding parallel ductile shear zone mineral stretching lineations l1 and rarely preserved small isoclinal d1 folds in the jingshan and fenzishan groups in the jingshan group however penetrative deformational transposition resulted in stacking of sedimentary compositional layers which are separated by bedding parallel ductile shear zones stz1 at a period of about 1956 ma to 1914 ma the kinematic indicators of stz1 in the jingshan group with resultant prograde peak metamorphism up to granulite facies grade and the fenzishan group with peak metamorphism up to amphibolite facies grade indicate nw directed compression d2 resulted in crustal thickening with retrograded medium pressure granulite facies grade at about 1914 1893 ma the d2 deformation produced nw verging asymmetric and recumbent folds interpreted to have resulted from basement involved thicken skin structures the jiaodong complex was also involved into the development of wnw verging asymmetric tight folds associated with d2 in the jingshan and the fenzishan groups ongoing collision led to the development of transpressional ductile shearing stz2 forming the transpressional taipingzhuang dextral ductile shear zone between the jingshan group and the southern archean complex and the transpressional tading xiadian sinistral ductile shear zone between the jingshan group and the northern archean complex all three lithotectonic units were superposed during the late d3 deformation with amphibolite facies metamorphism the d3 deformation developed wnw trending open to tight upright folds at about 1893 1875 ma the structural pattern resulting from superimposition of d2 and d3 is a composite synform in the fenzishan and jingshan groups the structural events of d1 and stz1 and d2 and stz2 deformation were possibly responsible for fast syn collisional exhumation of the high pressure mafic granulites the structural patterns and deformational history of the fenzishan and jingshan groups suggest a southeastward directed oblique subduction beneath the northwestern margin of the rangrim block and that the final scissor shaped closure of the rift led to collision between the two blocks to form the coherent basement of the eastern block of the north china craton

implications based on the first shrimp u pb zircon dating on precambrian granitoid rocks in north korea abstract precambrian basement rocks in north korea consist of supracrustals and granitoid rocks of which the former comprise the late archean nangrim group and the paleoproterozoic jungsan and machollyong groups whereas the latter include four major types grey ttg gneisses garnet sillimanite bearing s type granites massive porphyritic granites and hornblende bearing granites shrimp u pb zircon geochronology combined with cathodoluminescence cl imaging has enabled for resolution of magmatic and metamorphic events that can be directed towards understanding the precambrian history of north korea and its relations to north china shrimp u pb zircon analyses on two grey gneiss samples po501 and k14 reveal that they were emplaced at 2538 14 ma and 2636 49 ma respectively and metamorphosed at 2462 2433 ma these data for the first time confirm the existence of late archean rocks in the korean peninsula moreover one grey gneiss sample k14 contains several zircons with ages of 3 1 3 4 ga suggesting possible middle archean crust in north korea rocks with such ages are widely distributed in north china implying that north china and north korea have similar archean basement one grey gneiss sample r53 yielded a shrimp zircon age of 1933 12 ma indicating that some grey gneisses in north korea formed during the paleoproterozoic two garnet and or sillimanite bearing s type granite samples gave 207 pb 206 pb ages of 1908 31 ma and 1903 49 ma interpreted to reflect the time of metamorphism of the paleoproterozoic jungsan and machollyong groups since these s type granites are considered to be derived from the partial melting of pelitic rocks from these groups the 1 9 ga metamorphic event has also been revealed in paleoproterozoic rocks in south korea and north china shrimp zircon data also show that massive porphyritic monzogranites in north korea evolved over a protracted interval from 1865 ma to 1843 ma similar aged porphyritic monzogranites are also found in north china and south korea and their origin is considered to be related to a post tectonic extensional event of regional extent the last phase of precambrian magmatism recognized in north korea is grenvillian aged represented by the hornblende bearing granite that yields a shrimp zircon age of 1195 4 ma and is considered to be the source rock of the adjacent yushulazi group north china that contains numerous 1 1 1 2 ga old detrital zircons

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

provenance determined by ree th and sc analyses of metasedimentary rocks boyden cave roof pendant central sierra nevada california abstract ree th and sc analyses of metasedimentary rocks collected from the western sequence of the boyden cave roof pendant when compared with similar analyses of uppermost precambrian to cambrian miogeoclinal mudstones from the nopah range support the idea that rocks from the former area are a part of a large tectonically displaced fragment of the cordilleran miogeocline as proposed by r a schweickert and m m lahren for example ree distribution diagrams for samples from the western sequence of the boyden cave pendant show parallel light ree enrichment trends and negative eu anomalies that range from 0 61 to 0 86 and average 0 69 likewise ree distribution patterns for samples from the uppermost precambrian to cambrian cordilleran miogeoclinal section in the nopah range also show p rallel light ree enrichment trends and eu anomalies that range from 0 65 to 0 75 and average 0 68 on a la th se diagram samples from the boyden cave pendant cluster with specimens from the nopah range finally sediments from both areas have broadly similar th sc and th u ratios these results suggest that samples from the western sequence of the boyden cave pendant are composed of detritus like that shed from differentiated western north american upper continental crust during the early development of the cordilleran miogeocline such an interpretation is consistent with 1 the quartz rich to slightly feldspathic character of sandstones in the boyden cave pendant which implies a continental to transitional continental block provenance 2 a precambrian u pb age for detrital zirco extracted from a single sandstone bed and 3 the complete absence of volcanic material in the western sequence of the boyden cave pendant our results support the regional correlations and tectonic model proposed by r a schweickert and m m lahren and further suggest that although underutilized ree th and sc analyses are useful in evaluating stratigraphic sedimentologic and tectonic problems in complexly metamorphosed and deformed metaclastic terranes

paleoproterozoic metavolcanic rocks in the ji an group and constraints on the formation and evolution of the northern segment of the jiao liao ji belt china abstract in this paper we present new petrological and whole rock geochemical data for the paleoproterozoic metavolcanic rocks in the lower part of the ji an group within the northern segment of the jiao liao ji belt china as well as u pb dates and lu hf isotope data for the zircons in these rocks the new data help in understanding the original nature of these volcanic rocks as well as their tectonic setting and the formation and evolution of the belt the zircons can be divided into two groups of which one is of a typical magmatic origin whereas the other shows a metamorphic origin zircon u pb dating and lu hf isotopic results show that the magmatic zircons have peak ages at 2184 2365 2503 and 2710 ma with ehf t values ranging from 1 40 to 8 95 10 86 to 1 48 14 68 to 7 53 and 1 05 and with corresponding model ages of 2 18 2 69 2 80 3 57 2 54 3 83 and 3 22 ga respectively the metamorphic zircons show two peak ages at 1857 and 1914 ma geochemical data show that the protolith of the metavolcanic rocks consists mainly of basaltic andesite andesite and dacite along with minor basalt and rhyolite and that these rocks formed a continuous calc alkaline magmatic sequence in this sequence the intermediate basic rocks have relatively low sio2 contents are enriched in mgo feot the light rare earth elements lrees cr co and ni and depleted in heavy rare earth elements hrees and high field strength elements hfses such as nb ta and ti indicating derivation by partial melting of a depleted lithospheric mantle that had been metasomatized by fluids or melts derived from a subducted slab on the other hand the acidic rocks have high contents of sio2 and low contents of mgo and feot are enriched in alkalis ba th u and k and depleted in nb ta p and ti these geochemical characteristics together with their hf isotope data indicate the acidic rocks could have been derived from the partial melting of juvenile crust and a small amount of ancient crustal material possibly as old as late archean we consider therefore that the metavolcanic rocks in the lower part of the ji an group were formed at ca 2 18 ga as an assemblage of volcanic rocks typical of an active continental margin and that they were affected by two stage metamorphic events at 1 90 and 1 85 ga taking into account the regional geological data we consider that the evolution of the northern segment of the jiao liao ji belt was related to an arc continent collision before the middle paleoproterozoic

the controls on the major and trace element variation of shales siltstones and sandstones of pennsylvanian permian age from uplifted continental blocks in colorado to platform sediment in kansas usa shales siltstones and sandstones of pennsylvanian permian age from near the source in colorado to those in the platform in eastern colorado and kansas have been analyzed for major elements and a number of trace elements including the rees the near source sandstones are significantly more enriched student t test at better than the 99 confidence level in sio2 and na2o concentrations and more depleted in al2o3 fe2o3 total tio2 th hf sc cr cs rees y and ni concentrations and laco and lani ratios than the near source shales and siltstones most likely due to more plagioclase and quartz and less clay minerals in the sandstones than in the shales and siltstones there are no significant differences in k2o and sr concentrations and eueu lalu lase thsc thco and crth ratios between the near source sandstones and the near source shales and siltstones samples of the molas hermosa and cutler formations near the source that were formed in different environments in the same area contain no significant difference in eueu lalu lasc thsc thco and crth ratios so a generally silicic source and not the environment of deposition was most important in producing these elemental ratios for example crth ratios of near source shales siltstones and sandstones range from 2 5 to 17 5 and eueu range from 0 48 to 0 78 which are in the range of sources of sediments derived from mainly silicic and not basic sources near source shales and siltstones contain significantly higher student t test and more varied concentrations of most elements al2o3 fe2o3 mno tio2 ba th hf ta co sc rees nb y but significantly lower concentrations of na2o and eueu than platform shales and siltstones in kansas e g la 65 7 40 and eueu 0 55 0 07 in near source shales and siltstones and la 23 7 8 7 and eueu 0 64 0 08 in platform shales and siltstones the sio2 and cao concentrations are not significantly different in platform shales and siltstones compared to the near source shales and siltstones so dilution of other minerals by quartz and calcite is not the main reason for the lower concentration of most elements in the platform relative to the near source shales and siltstones rather the lesser concentrations of most elements in clay minerals of the platform shales and siltstones can account for the lower concentration of most elements compared to corresponding near source shales and siltstones the lower concentrations of many elements in clay minerals in the platform shales and siltstones may be a result of having been derived from recycling of clay minerals from older rocks the greater homogeneity of elemental concentrations of the platform shales and siltstones compared to those in the source is also consistent with homogeneous mixing of such recycled material also there is no significant difference in thsc laco thco lani and crth ratios of the near source sedimentary rocks in colorado to the platform shales and siltstones in kansas and the latter are also consistent with derivation from mostly silicic source rocks

oxygen isotope geochemistry of oceanic arc lavas variations of oxygen isotope ratios in arc related lavas can constrain the contributions of subducted crustal igneous rocks sediments and fluids to the sub arc mantle we have measured oxygen isotope ratios in 72 arc and back arc lavas from five ocean ocean subduction zone systems using laser fluorination analyses of olivine and other phenocrysts and glass eighty percent of our samples have delta 18o values for any given phase olivine plagioclase glass or biotite within 0 2 per thousand of the average value for that phase in upper mantle peridotites and mid ocean ridge basalt morb the range for each phase is 1 0 per thousand this result contrasts with previous studies of whole rock samples which are significantly more variable even after exclusion of samples believed to be altered or fractionated by magmatic differentiation and demonstrates that most arc related lavas contain 1 2 of 18o enriched crustal oxygen from any source i e assimilation or subducted contributions elevations in delta 18o that do occur in these basic arc derived magmas relative to values most common for mantle derived lavas are associated both with enriched radiogenic isotope signatures and even more strongly with chemical indices consistent with high integrated extents of melting of their peridotite sources we interpret these relationships as evidence that melting in the sources of the high delta 18o lavas we have studied was fluxed by addition of high delta 18o aqueous fluid or perhaps a hydrous melt from the subducted slab such that sources that contain relatively large components of slab derived fluid or melt are both relatively 18o enriched and also experienced relatively large amounts of melting we have developed a quantitative model linking the amount of melting to the extents of 18o radiogenic isotope and trace element enrichment in a mantle source being fluxed by addition of aqueous fluid comparison of this model with observed variations in the geochemistry of lavas from the vanuatu fiji new caledonia region the suite of related samples showing the greatest range in delta 18o observed in this study constrains the amounts and chemical and isotopic compositions of slab derived phases in the sources of these arc related lavas assuming a delta 18o value of 20 per thousand for the slab derived fluid 0 5 1 0 wt is added to the sources of most mantle derived arc magmas the maximum amount of slab derived flux in the sources of arc magmas according to our results is 2 5 wt

precambrian geology of china preface abstract the north china south china and tarim cratons compose the tectonic framework of china in the last decade extensive structural metamorphic geochemical geochronological and geophysical investigations have been carried out on these cratonic blocks producing an abundant amount of new data and competing interpretations which have resulted in refinement of and in some cases significant reinterpretations of the precambrian crustal formation and evolution of china this forms a justification for organizing this special issue in which we invited representative experts working in the north china south china and tarim cratons to present overviews or detailed studies on the formation and tectonic evolution of major precambrian blocks in china twenty four contributions collected in this special issue provide current knowledge of the components and tectonic evolution of major precambrian blocks in china and discuss some key issues based on new data and interpretations which will provide important insights into understanding the precambrian geology of china

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

behavior of isotopic systematics during deformation and metamorphism a hf nd and sr isotopic study of mylonitized granite a combined compositional and hf nd and sr isotopic study was performed on a suite of samples of progressively deformed granite from a mylonite zone in the harquahala mountains western arizona to evaluate the effects of deformation and metamorphism on the isotopic systematics of typical continental crustal rocks the 1 4 ga harquahala granite was deformed during mesozoic thrusting along the harquahala thrust granite in the resulting 60 m wide shear zone ranges from protomylonite to ultramylonite in most of these mylonites the protolith is not megascopically recognizable and can be discerned only by the progressive transition to undeformed granite isotopic analyses of hf nd and sr from the shear zone document the immobility of the hf and nd isotopic systems relative to that of the sr isotopic system during deformation the rb sr isotopic data show considerable scatter on an isochron plot exhibiting both gains and losses of rb and sr from the whole rock systems in contrast the sm nd and lu hf isotopic systematics are mostly well behaved on isochron diagrams plotting mostly in tight clusters or along 1 4 ga isochrons these results show that while the sr isotopic system in crustal rocks is quite susceptible to later tectonic disturbance both hf and nd isotopic systems can provide reliable model age information in continental crustal terranes even when the rocks have been subjected to low to medium grades of deformation and metamorphism

zircons from syros cyclades greece recrystallization and mobilization of zircon during high pressure metamorphism zircons were studied from high pressure low temperature metamorphosed meta igneous lithologies from syros these rocks carry several zircon generations related to each other by dissolution reprecipitation processes one generation is pristine zircon that shows growth zoning relatively elevated contents of trivalent cations and high th u ratios the other end member is a skeletal zircon generation with negligible trivalent cation contents and low th u ratios 0 1 texturally between these two there is a range of zircon crystals with complex inclusion populations of y hree th phases and fluid inclusions showing variable progress of replacement recrystallization both end members yield distinct sensitive high resolution ion microprobe shrimp u pb ages the pristine generation has an age of 80 2 1 6ma from a metagabbro and 76 4 2 1ma from a meta plagiogranite dyke the skeletal low th u zircon generation yields an age of 52 4 0 8ma the older late cretaceous zircons are interpreted to date emplacement of the magmatic protoliths in a small segment of oceanic crust the younger eocene age however dates a zircon recrystallization event which possibly coincides with high solubility and mobility of high field strength elements in a high pressure aqueous fluid phase intergrowth relations between zircon and peak metamorphic garnet and excellent agreement of the u pb ages with white mica ar ar ages for the same samples support the conclusion that eocene is the true age of high pressure metamorphism on syros

zircon u pb and lu hf isotopic constraints on archean crustal evolution in the liaonan complex of northeast china abstract the changhai area is located in the southeastern part of the liaonan complex within the eastern block of the north china craton ncc in china early precambrian rocks in this area consist mainly of various types of gneisses and schists we present cathodoluminescence images u pb dates and trace element and hf isotope data for zircons from these rocks in order to constrain the formation and evolution of continental crust in the liaonan complex our results show that the magmatic precursors of the biotite monzonitic gneiss granitic gneiss garnet bearing monzogranitic gneiss and biotite granodioritic gneiss that is relatively sodium rich gneisses in the area were emplaced at ca 2543 ma muscovite bearing granitic gneiss monzogranitic gneiss and biotite granitic gneiss that is relatively potassium rich gneisses in the area were emplaced at ca 2516 ma whereas the sedimentary protolith s of the associated schists along the gneissose fabric of the gneisses were deposited after 2518 ma and metamorphosed at ca 1885 ma the latter age may represent the time of metamorphism of the south liaohe group in the jiao liao ji belt large numbers of inherited zircons with ages of 2596 2842 ma coupled with the results of previous studies suggest that multiple meso to neoarchean magmatic events took place in the liaonan complex at ca 2 84 2 75 2 67 2 61 2 54 and 2 51 ga lu hf isotopic results for magmatic zircon grains with ages of 2 61 2 84 ga have positive e hf values 2 4 to 11 3 and average continental crust model ages tc dm of 2 63 3 05 ga with a peak at 2 76 2 98 ga of the zircons with ages of ca 2 54 ga and 2 51 ga 94 have positive e hf values 0 13 to 8 57 average 3 1 and tc dm model ages of 2 53 3 06 ga similar to those of zircons with ages of ca 2 61 2 84 ga suggesting that both sets of zircons were derived from partial melting of juvenile crust however the remaining zircons 6 with ages of 2 54 2 51 ga and all metamorphic zircon grains with ages of ca 1885 ma have negative e hf values of 0 12 to 3 00 and 8 8 to 15 9 respectively and similar older tc dm model ages of 3 11 3 56 ga and 3 08 3 23 ga respectively these data combined with those from previous studies of the ncc indicate that the basement rocks of the liaonan complex underwent two significant juvenile crustal growth events at ca 2 71 2 99 ga and 2 54 2 51 ga a local reworking event at ca 2 51 ga and a major reworking event at 1 88 ga

metallogeny of the north china craton link with secular changes in the evolving earth abstract the north china craton ncc has experienced a complex geological evolution since the early precambrian and carries important records of secular changes in tectonics and metallogeny here we synthesize the salient geological and tectonic features of the evolution and destruction of the ncc vis a vis major metallogenic events and the formation of potential ore deposits we identify a close relationship between the major geological events in the ncc and those reported elsewhere on the globe we trace the records of a regular change in the pattern of metallogeny mineral deposit character spatial distribution and genetic mechanisms which closely match the timing and styles of the major geological and tectonic events in this craton the ncc went through five major tectonic cycles 1 neoarchean crustal growth and stabilization 2 paleoproterozoic rifting subduction accretion collision with imprints of the great oxidation event goe 3 late paleoproterozoic neoproterozoic multi stage rifting 4 paleozoic orogenesis at the margins of the craton and 5 mesozoic extensional tectonics associated with lithospheric thinning and decratonization coinciding with these major geological events are five major metallogenic systems identified as follows 1 an archean bif system 2 paleoproterozoic cu pb zn and mg b systems 3 a mesoproterozoic ree fe pb zn system 4 a paleozoic orogenic cu mo system and 5 mesozoic intracontinental au and ag pb zn and mo systems the ore deposit types in each of these metallogenic systems show distinct characteristics and tectonic affinities from early precambrian through late precambrian to paleozoic and mesozoic the ncc records a transition from primitive to modern style plate tectonics evidence for imbricated oceanic plate stratigraphy in a subduction accretion setting and collisional orogenesis along at least three major zones of ocean closure are documented major transitions in tectonic style and surface environmental changes recorded in other parts of the world are also reflected in the geological history and metallogenic events in the ncc large scale gold deposits formed through intraplate tectonics during the mesozoic provide important insights into mantle dynamics and crust mantle interaction associated with lithospheric thinning and craton destruction the ncc provides one of the best examples for documenting secular changes in the geological history and metallogenic epochs of an evolving earth

discharge of nitrate containing groundwater into a coastal marine environment the discharge of nitrate containing fresh groundwater from a sandy coastal aquifer and into the adjacent shallow marine environment was investigated near esbjerg at the northern end of the wadden sea in denmark the geological structure of the coastal site was determined from drilling data and geoelectrical methods such as multi electrode profiling mep the distribution of shoreface sediment containing freshwater was mapped using mep underwater multi electrode profiling umep and the electrical conductivity measured on pore waters freshwater discharge fluxes were measured using seepage meters and estimated from observed head gradients and measured hydraulic conductivities in the aquifer sediments the nitrate distribution in the coastal sediments was obtained from water samples a groundwater flow model was established to quantify the groundwater flow and travel times and the distribution of freshwater discharge along the coastline results show that on the watershed scale the fresh groundwater discharge is controlled by sand filled buried channels that connect the aquifer with the shoreface sediments the freshwater discharge along the coastline is at this scale probably best estimated by the groundwater flow model however at a more detailed scale the distribution of freshwater discharge in the intertidal zone is controlled by small scale geological heterogeneity and models are unable to predict what happens at a small scale on the beach and offshore for that purpose umep seepage meters pore water sampling and local hydraulic gradients are more useful these measurements indicate that the freshwater discharge occurs in distinct zones and that the highest discharge is near the high tide line decreasing rapidly in a seaward direction nitrate is abundant in the shallow groundwater of the coastal aquifer and is also present in the discharging fresh groundwater at certain patches along the coast however on average very little nitrate is observed in the freshwater discharging at the coast the maximum travel time of groundwater through the aquifer until it discharges at the coast is 100 years and since nitrate leaching from soils has only taken place during the last 40 50 years part of the fresh groundwater discharging at the coast must be free of nitrate

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

submarine groundwater discharge within a landslide scar at the french mediterranean coast abstract submarine groundwater discharge sgd the flow of fresh and saline groundwater from the seabed into the coastal ocean has been intensively investigated in the recent years this research has usually been restricted to shallow water and intertidal areas whereas knowledge about groundwater seepage in deeper water is mainly limited to point sources from karstic aquifers in this study we observed submarine groundwater seepage and a subterranean estuary in sediments at water depths of 20 44 m located within the ligurian margin western mediterranean sea here a catastrophic submarine landslide occurred near the nice airport french ligurian coast in the fall of 1979 after a period of heavy rainfall during two research cruises gravity cores were recovered in and around the area of the landslide scar pore water samples collected from sediment cores indicated sediments containing freshwater within the landslide scar pore water profiles of selected ions such as chloride ammonium manganese sulfate and barium were used to assess transport and reaction processes within the sediment a 1 dimensional transport model indicates in most cores upward pore water velocities of 2 3 8 8 cm yr 1 this study shows that submarine groundwater seepage along the french mediterranean coastline can occur at water depths reaching 44 m

geochronology and geochemistry of the paleoproterozoic meta basalts from the jiao liao ji belt north china craton implications for petrogenesis and tectonic setting abstract geochronology geochemical and isotopic studies were carried out on the paleoproterozoic meta basalts and related rocks from the jiao liao ji belt north china craton to understand the mantle source characteristics and the geodynamic setting u pb isotopic dating using the la icpms method on zircons from the metamorphosed volcanic rocks reveals that they formed at ca 2204 2158 ma and were metamorphosed at ca 1895 1919 ma respectively the meta basalts are of calc alkali affinity and have arc like geochemical compositions they are enriched in lrees la yb n 3 5 5 1 and liles sr ba rb depleted in hfses nb ta zr hf ti and show variable high nd t values 2 4 to 4 5 the basaltic rocks experienced significant fractional crystallization and minor crustal contamination during magma evolution modeling suggests that the parent magma of the meta basalts was derived from 20 partial melting of spinel garnet lherzolites that were previously metasomatized by subduction zone fluids melts in the late archean petrological geochronological and geochemical data suggest that the paleoproterozoic volcanic rocks formed in an active continental margin setting and were subsequently deformed and metamorphosed to amphibolite facies due to the arc continental collision at ca 1 9 ga rather than in a continental rifting setting as previously thought by most workers

a comparison of u pb and hf isotopic compositions of detrital zircons from the north and south liaohe groups constraints on the evolution of the jiao liao ji belt north china craton the jiao liao ji orogenic belt is one of the important paleoproterozoic mobile belts in the north china craton ncc and its central segment consists of the north and south liaohe groups and the liaoji granitoids the north and south liaohe groups consist of sedimentary and volcanic successions metamorphosed from greenschist to lower amphibolite facies and the liaoji granitoids are divisible into 2 2 2 1 ga pre tectonic monzogranitic gneisses and 1 88 1 85 ga post tectonic porphyritic monzogranites and alkaline syenites combined with cathodoluminescence cl imaging we report la icp ms laser ablation inductively coupled plasma mass spectrometry u pb and hf isotopic data for detrital zircons from the north and south liaohe groups zircons grains from both groups have igneous zircon cores that yielded two u pb age populations at 2 0 2 2 ga major and 2 5 ga minor indicating that their provenances were similarly dominated by paleoproterozoic liaoji granitoids and late archean basement rocks some structureless zircon rims in both groups yield concordant or upper intercept ages 1 9 ga interpreted as the peak metamorphic age of the two groups these results suggest that both groups were deposited in the period 2 0 1 9 ga shortly after the emplacement of the liaoji granitoids moreover zircons from both groups have similar hf model age peaks t c dm peaks at 2 66 2 75 ga and 2 80 3 00 ga

petrogenesis and tectonic significance of paleoproterozoic meta mafic rocks from central liaodong peninsula northeast china evidence from zircon u pb dating and in situ lu hf isotopes and whole rock geochemistry abstract the central liaodong peninsula is located in the northwestern part of the jiao liao ji belt in the eastern block of the north china craton surface rocks in this region are voluminous meta mafic rocks which mainly comprise amphibolite and metamorphic gabbro or diabase we present zircon cathodoluminescence images u pb dates and lu hf isotope data as well as whole rock geochemical data for these rocks in order to constrain the tectonic nature and evolution of the jiao liao ji belt zircons from these meta mafic rocks can be divided into two groups with one having a typical magmatic origin and the other a metamorphic origin zircon u pb dating and lu hf isotopic results show that the magmatic zircons have two peak ages at 2503 and 2154 ma with the former have hf 7 1 to 8 1 and single stage model ages t hf 2 50 2 55 ga indicating that these captured or inherited zircons formed at ca 2 50 ga by partial melting of coeval juvenile crust the younger magmatic zircons have hf 3 2 to 9 6 mainly varying from 7 4 to 9 6 average 8 7 and t hf 2 15 2 34 ga peak at 2 19 ga suggesting that the meta mafic rocks were emplaced at ca 2 15 ga and derived by partial melting of depleted mantle the metamorphic zircons yield a peak age of 1897 ma and have hf 4 5 to 6 8 these zircons can be further divided into three groups on the basis of hf isotope data i e hf 4 5 to 2 3 0 6 to 1 0 and 3 1 to 6 8 t hf 2 39 2 54 2 26 2 39 and 2 03 2 11 ga respectively indicating they were derived by reworking of neoarchean paleoproterozoic ancient crust and some paleoproterozoic juvenile crust thus there existed two crustal growth events at 2 50 and 2 15 ga and one reworking event at ca 1 90 ga in the eastern block geochemical data show that these meta mafic rocks are characterized by sio 2 47 3 49 8 wt tio 2 0 97 1 59 wt k 2 o 0 39 1 95 wt na 2 o k 2 o 1 03 4 85 mg 39 52 cr 29 247 ppm and ni 24 4 73 4 ppm these features combined with low total rare earth element concentrations ree relative enrichment in light ree significant positive rb ba k and p anomalies and depletions in high field strength elements e g nb ta and ti indicate derivation by partial melting of depleted lithospheric mantle that had been metasomatized by subduction derived fluids with little or no crustal contamination during magma ascent given our dating results and the regional geological framework we propose that the subduction in the eastern longgang block took place at least in age of 2 15 ga and that the meta mafic rocks in the central liaodong peninsula may have formed in a back arc basin however the tectonic location of this magmatic arc and its relationship with the nangrim block need to be further studied to confirm this hypothesis

two episodes of paleoproterozoic mafic intrusions from liaoning province north china craton petrogenesis and tectonic implications abstract mafic dyke swarms constitute a common occurrence within the cratonic provinces in the world since the archean proterozoic boundary and their geneses harbor critical information for facilitating regional tectonic and supercontinental reconstruction this zircon u pb dating and geochemical study documents two episodes of paleoproterozoic mafic intrusions from liaoning province north china craton ncc the haicheng suite hcs with an emplacement age of ca 2125 ma from the liaodong peninsula and the guohua suite ghs with an age of ca 1816 ma from the western liaoning terrane the hcs meta gabbros are tholeiitic in composition with an sio 2 range from 46 to 52 modest enrichment in light rare earth elements lree and depletion in high field strength elements hfse and possess radiogenic whole rock nd nd t 1 91 to 4 21 and zircon hf hf t 4 02 to 8 48 isotopic compositions these features endow the hcs with a juvenile subduction fluid metasomatized lithospheric mantle source by contrast the ghs exhibits a meta gabbroic to dioritic composition and a transitional character from tholeiitic to calc alkali with variably enriched lrees and more evolved isotopic signatures nd t 0 20 to 5 82 zircon hf t 0 09 to 4 91 than the hcs while the relatively intact high fe gabbroic samples are supposed to represent partial melts from a fertile cratonic sub continental lithospheric mantle source in the garnet spinel stability field the low fe dioritic ones tend to bear the imprints of subsequent assimilation and fractional crystallization process apart from making an important contribution to the present archive of the paleoproterozoic mafic dyke swarms in the ncc the present two mafic magmatic suites witnessed a shift in tectonic setting from a subduction related continental arc environment to a post orogenic extensional regime synthesizing two suites with coeval felsic igneous episodes as well as concomitant episodic metamorphic events in the jiao liao ji belt leads to the characterization of a possible divergent double subduction system in the eastern ncc during the time span of ca 2 2 1 80 ga synchronized beating of episodic magmatic and metamorphic pulses in the constituent mobile belts of the ncc carries further implication of attesting to the ubiquity and importance of divergent double subduction system in the cratonic amalgamation and its incorporation within the supercontinent columbia

3800 to 2500 ma crustal evolution in the anshan area of liaoning province northeastern china shrimp u pb zircon geochronology has revealed several generations of granitoids and gneisses from the anshan area of northeastern china two samples of the baijiafen granite sensu lato 3804 5 and 3812 4 ma chentaigou granite 3306 13 ma lishan granite 3142 7 ma east and west anshan granites 3001 4 and 2994 8 ma tiejiashan granite 2962 4 and 2964 6 ma and qidashan granite 2475 ma the 3800 ma baijiafen granite consists of banded migmatitic trondhjemitic gneisses and forms only a small part of the area nonetheless they are the oldest known rocks in asia and are one of very few locations in the world where rocks of this antiquity have been found the younger suites are predominantly granite sensu stricto in composition and although foliated they have not been transformed into banded gneisses nd isotopic data suggest that the 3800 ma granites were produced at least in part by remelting of very ancient lree enriched sources perhaps like the exposed remnants of the 3800 ma rocks the chentaigou supracrustals contain layered mafic and ultramafic rocks banded iron formation bif and siliceous layered rocks of probable volcanogenic origin a sample of the latter has yielded a concordant u pb zircon age of 3362 5 ma interpreted as the age of deposition a trondhjemitic sheet cutting these rocks has yielded a u pb zircon age of 3342 6 ma the chentaigou supracrustals are the oldest recognised in the area younger supracrustal sequences collectively known as the anshan group contains the main economic bif units in the area bif is intruded by the 2475 ma qidashan granite indicating that the anshan group is archaean or earliest proterozoic in age

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