diatom inferred salinity in palaeolakes an indirect tracer of climate change lakes in arid and semi arid regions respond to climatic change through shifts in lake water volume and ionic concentration because diatom distribution is highly correlated with lake hydrochemistry diatoms can be used to infer changes in salinity and brine composition and thus to infer past climates here we critically examine the use of diatom inferred salinity as a climate proxy with examples taken from both modern waterbodies and sedimentary profiles sediment records may contain assemblages mixed from periods or sites of differing hydrochemistries because of the high degree of spatial and temporal heterogeneity in saline systems dissolution and diagenesis in saline brines may further complicate interpretation of sedimentary assemblages furthermore threshold effects salinity regulation via groundwater seepage antecedent conditions and other aspects of local hydrology may modify the relationship between salinity change and climatic forcing these complexities necessitate critical examination of the ecology and taphonomy of sedimentary assemblages as well as evaluation of potential non linearities in the salinity climate relationship through comparison of diatom inferred salinity with other proxy records of hydrochemical change historical documents where they exist and with other regional lake systems

biological remains geochemistry and stable isotopes for the reconstruction of environmental and hydrological changes in the holocene lakes from north sahara abstract a detailed reconstruction of the holocene palaeoenvironments and palaeohydrology is presented for wwamp and lacustrine calcareous sediments which lie in closed depressions along the northern margin of the great western erg algeria it is based on a multidisciplinary approach using numerous techniques the chronological framework is established on the basis of 23 14 c dates on inorganic calcite and mollusc shells ranging from ca 9300 to 3000 yr b p and on an isolated 14 c age of ca 10 900 yr b p the significance of various environmental markers is discussed biological remains are abundant and diversified about 150 taxa including diatoms chrysophytes charophytes ostracods molluscs and foraminifera they provide information on the water chemistry total salinity ionic ratio ph and partial pressure of carbon dioxide and on the habitats water depth development of aquatic plants ephemerality etc emphasis is given to the total salinity individual organisms and palaeobiological assemblages are ordered as a function of their salt requirements and tolerances calibration based on modern diatom samples allows a tentative estimate of the palaeosalinities which range from fresh to marine like and even hypersaline waters attention is drawn to the possible colonization of marine organisms in continental basins without any connection with the sea stable isotope analyses have been carried out on both inorganic and biogenic mollusc and ostracod shells charophyte gyronites carbonates the 18 o contents demonstrate wide changes in the hydrological balance fluctuations in 13 c contents inform on water mixing residence time and on the importance of biological effects on total dissolved inorganic carbon geochemical analyses were performed on inorganic calcite and on ostracod shells high contents in fe and mn indicate reducing environments during stages of high salinity sr ca and mg ca are used as salinity indicators zn mg ratios suggest that the major water supply was the aquifer of the great western erg today down to about 50 m rather than surface waters from the atlas mountains salinities resulted from the balance between inputs from the water table evaporation rate leaching of pre existing salt crusts and seepage through the lake bottom a reconstruction of the evolution of the palaeosystems through time and space has been attempted with special attention to the instability of the water and ionic balances for different time scales short term fluctuations occurring during a single hydrological cycle annual or seasonal are deduced from the apparent discrepancies between the various indicators found in a given sample long term 10 2 10 3 years changes confirm the existence of a climatic phase being wetter than today from at least ca 9300 and may be from ca 10 900 yr b p to 3000 yr b p for the first time the salinity deduced from diatoms and the 18 o content of the water deduced from that of the carbonate at 25 c are used simultaneously to model the hydrological history of the basins and to estimate the relative humidity of the atmosphere during the holocene major environmental trends recorded from ca 9300 to 3000 yr b p do not show a clear synchronims between the different profiles studied therefore they cannot be interpreted in terms of palaeoclimate but instead demonstrate the importance of local hydrology in the evolution of closed basins associated with groundwater a common situation in arid and semi arid zones

abrupt return of the summer monsoon 15 000 years ago new supporting evidence from the lower white nile valley and lake albert abstract the last glacial period ended with an abrupt return to warmer and wetter conditions at a number of sites in intertropical africa asia australia and south america 15 000 years 15 ka ago similar abrupt warming is also evident at this time in high latitudes and in air trapped in greenland ice we here report new supporting evidence of this event from a climatically sensitive region in africa the white nile valley of the central sudan and lake albert in the uganda headwaters during the last glacial maximum the white nile valley was even more arid than it is today with desert dunes active as far south as latitude 12 s the sudden overflow of lake victoria in the ugandan headwaters of the white nile 14 5 ka confirmed here by sr isotope statigraphy caused extensive flooding in the lower white nile valley and severe flooding in egypt the volume of fresh water flowing into the eastern mediterranean at this time ultimately resulted in accumulation of highly organic sediments sapropel 1 on the floor of the mediterranean sea flooding in the central sudan attained an elevation of 382 m along a north south distance of 400 km the recessional shoreline of the 382 m white nile is indicated by concentrations of now buried freshwater gastropod shells and beach sands dated respectively by calibrated ams radiocarbon and osl to 15 16 ka this mega flood event marks the abrupt return of the summer monsoon and may reflect a globally synchronous event

mantle derived noble gases in ore forming fluids of the granite related yaogangxian tungsten deposit southeastern china more than 90 of the tungsten resources of china are in the nanling region of south china and the yaogangxian vein deposit is the largest tungsten deposit in this region the tungsten deposits have ages of 150 160 ma and are spatially temporally and genetically related to granites which were previously believed to be produced by crustal anatexis this paper provides he and ar isotope data of fluid inclusions in pyrite and arsenopyrite from the yaogangxian w veins 3he 4he ratios range from 0 41 to 3 03 ra where ra is the 3he 4he ratio of air 1 39 10 6 and 40ar 36ar ratios from 328 to 1 191 moreover there are excellent correlations between he and ar isotopic compositions the results suggest that the ore forming fluids are a mixture between a crustal fluid containing atmospheric ar and crustal 4he and a fluid containing mantle components it is likely that the former is a low temperature meteoric fluid and the later is a fluid exsolved from the w associated granitic magma which formed by crustal melting induced by intrusion of a mantle derived magma

tertiary and quaternary tectonic faulting in southernmost illinois abstract tertiary and or quaternary tectonic faulting is documented in three areas of southernmost illinois the fluorspar area fault complex fafc in pope and massac counties the ste genevieve fault zone sgfz in alexander and union counties and the commerce fault zone cfz in alexander county in the fafc faults that strike ne and nne displace mounds gravel late miocene to early pleistocene and locally the metropolis terrace gravel pleistocene pre woodfordian no woodfordian or younger deposits are deformed faults typically outline narrow linear grabens that formed under tension with a component of strike slip north south to nw trending vertical faults near the southeast end of the sgfz displace eocene sediments again faults outline narrow grabens and show indications of strike slip deformed quaternary sediments have not been observed the cfz which trends northeast displaces mounds gravel in illinois and units as young as peoria silt woodfordian in missouri quaternary movement has been interpreted as right lateral strike slip the cfz coincides with a subtle gravity and magnetic lineament and seems to reflect a major feature in the basement surface expression in illinois is subtle but mafic and ultramafic intrusions hydrothermal alteration and small faults align with the commerce geophysical lineament earthquake foci in missouri and illinois lie on or close to the cfz some focal mechanisms fit the fault trend among these structures only the cfz exhibits slip that conforms to the current stress field principal compressive stress axis e w to ene wsw possibly the stress field changed during neogene time alternatively high fluid pressures or local stress concentrations may have induced slip on less favorably oriented fractures tighter constraints are needed on timing magnitude and direction of neogene displacement

a pleistocene diatomite from the western piedmont of jebel marra volcano darfur western sudan and its hydrological and climatic significance abstract diatoms collected at 10 cm intervals from a 6 3 m pleistocene lacustrine diatomite section in the now arid piedmont west of jebel marra volcanic caldera darfur western sudan were analyzed the lake probably formed as a result of a rhyolite lava flow damming a former river the lake was initially shallow but in the upper 5 3 m diatom composition and a salinity reconstruction indicate persistent deep freshwater conditions the diatomite is overlain by an acheulian stone tool assemblage with a probable age range of 0 8 0 3 ma the lake may be coeval with the oil palm fossils discovered 90 km to the southwest which are underlain by developed oldowan early acheulian artifacts some in primary context and so of lower middle pleistocene age both the diatomite and the oil palm fossils indicate a much wetter climate than today

seismic imaging of the crust and upper mantle under beijing and surrounding regions abstract we determined a high resolution 3d p wave velocity model of the crust and upper mantle under beijing and surrounding regions in north china by jointly inverting 48 750 arrival times from 2973 local events and 12 249 relative residuals from 234 teleseismic events recorded by the newly built capital seismic network our results show considerable differences in the lithospheric structure down to about 200 km depth between the north china basin the taihangshan uplift and the yanshan uplift which are the three main tectonic units in the study area at depths of 50 150 km a pronounced low velocity anomaly exists under north china basin and bohai bay and right beneath it a high velocity anomaly extends down to about 200 300 km depth suggesting that this area has experienced large scale lithospheric thinning and lithospheric detachment has occurred the taihangshan front fault and the zhangjiakou penglai fault zone may have cut through the crust to lithosphere and they become flimsy tectonic boundaries and the asthenospheric materials may upwell from the two channels we consider that the deep and shallow structures beneath this region are closely related to dynamic processes that result from the deep subduction of the pacific slab and its stagnancy in the mantle transition zone under east asia

the arid humid transition in the sahara and the sahel during the last deglaciation at the time of the last glacial maximum the sahara and sahel regions of north africa were extremely dry new records of rainfall show that during the subsequent deglaciation the transition from arid to humid conditions in these regions occurred synchronously in two main steps comparison with other records of palaeoclimate in europe and the north atlantic ocean shows that certain common factors controlled changes in ocean and atmosphere dynamics during the deglaciation

elastic strength of the slave craton at 1 9 gyr and implications for the thermal evolution of the continents the secular cooling of the earth since its formation together with the law of radioactive decay imply that the earth s internal heat generation and heat loss must have been two to three times higher in archaean and proterozoic times than today but this inference is difficult to test directly here we present a flexural analysis of the proterozoic kilohigok basin in the archaean slave craton of canada which shows that 1 9 gyr ago the craton had an effective elastic thickness of only 12 4 km because the present effective elastic thickness of the region is 100 25 km these results imply a two to fourfold decrease in the thermal gradient beneath the slave craton since 1 9 gyr and indicate that at 1 9 gyr 700 myr after the accretion of the continental lithosphere forming the craton the thick continental root beneath this part of the canadian shield had not yet formed

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

active tectonics of the north and central aegean sea summary in this paper we examine the connection between the westward motion of turkey relative to europe and the extension in and around the aegean sea the principal new data available since the last attempt to synthesize the tectonics of this region by mckenzie 1978 are much improved focal mechanisms of earthquakes constrained by p and sh body wave modelling as well as by first motions these mechanisms show that the faulting in the western part of the aegean region is mostly extensional in nature on normal faults with a nw to wnw strike and with slip vectors directed nnw to nne there is evidence from palaeomagnetism that this western region rotates clockwise relative to stable europe in the central and eastern aegean and in nw turkey distributed right lateral strike slip is more prevalent on faults trending ne to ene and with slip vectors directed ne palaeomagnetic data in this eastern region is more ambiguous but consistent with very small or no rotations in the northern part and possibly anticlockwise rotations relative to europe in the south the strike slip faulting that enters the central aegean from the east appears to end abruptly in the sw against the nw trending normal faults of greece the kinematics of the deformation is controlled by three factors the westward motion of turkey relative to europe the continental collision between nw greece albania and the apulia adriatic platform in the west and the presence of the hellenic subduction zone to the south as the right lateral slip on the north anatolian fault enters the aegean region it splays out becoming distributed on several parallel faults the continental shortening in nw greece and albania does not allow the rotation of the western margin of the region to be rapid enough to accommodate this distributed e w right lateral shear and thus leads to e w shortening in the northern aegean which is compensated by n s extension as the southern aegean margin can move easily over the hellenic subduction zone the dynamics of the system once initiated is self sustaining being driven by the high topography in eastern turkey and by the roll back of the subducted slab beneath the southern aegean the geometry of the deformation resembles the behaviour of a system of broken slats attached to margins that rotate in spite of its extreme simplicity a simple model of such broken slats is able to reproduce quantitatively most of the features of the instantaneous velocity field in the aegean region including the slip vectors and nature of the faulting in the eastern and western parts the senses and approximate rates of rotation the overall extensional velocity across the aegean and the distribution of strain rates as seen in the seismicity and topography or bathymetry and using geodetic measurements as part of this study we re examined the relation between the surface faulting and the focal parameters determined seismologically for the three large 1981 gulf of corinth earthquakes and reassessed the evidence for associating particular earthquakes in the sequence with observed surface faulting an important result is that in one of the events there is a resolvable discrepancy between the slip vector measured at the surface and that determined from the seismic body waves

r a daly s early model of seafloor generation 40 years before the vine matthews hypothesis an outstanding theoretical achievement inspired by field work on st helena in 1921 1922 large scale lateral mobility of the earth s lithosphere mobilism was a hotly debated issue in earth sciences during some two decades following publication of wegener s 1912 theory of continental displacement the final acceptance of lithospheric mobility was brought about with the plate tectonics revolution during the late 1960s support for mobilism was rather popular in certain european countries during the 1920s whereas the reactions in north america were mostly hostile one of the very few influential mobilists in the new world was reginald aldworth daly of harvard university the present paper discusses his model of continental displacement which is very remarkable in many aspects we focus on the hitherto neglected fact that daly proposed in the mid 1920s a mechanism to create oceanic crust which would have been totally consistent with the vine matthews hypothesis of seafloor generation published in 1963 it is furthermore suggested that daly s geotectonic proposals were inspired by small scale

geology and he ar s pb isotope constraints on the genesis of the sidaogou gold deposit in liaodong peninsula northeastern north china craton abstract the large sidaogou gold deposit in the liaodong peninsula is a typical alteration type gold system hosted in paleoproterozoic metamorphic rocks of gaixian formation ore paragenesis indicates sidaogou mineralization occurred in four stages 1 early ore stage s1 quartz albite 2 main ore stage s2 quartz sericite pyrite gold 3 secondary ore stage s3 quartz polymetallic sulfide scheelite biotite gold and 4 late ore stage s4 calcite ankerite quartz sulfide associated hydrothermal alterations in wall rocks are dominated by albitization s1 silicification s1 to s4 sericitization s2 sulfidation s2 to s3 and carbonatization s4 with minor biotitization s3 developed locally gold mineralization was primarily occurred in s2 stage with intensive silicification sericitization and sulfidation in metamorphic wall rocks and occurred to a lesser extent in s3 stage with weak alteration possibly due to fairly weak activity and small fluxes of the ore fluids at gold precipitation stages s2 to s3 three generations of ore related pyrite py1 py2 and py3 are identified nanosims in situ s isotope analyses show that both py1 and py2 from the s2 stage have relatively broad ranges of 34s values with significant intra grain variations py1 7 7 16 5 py2 3 7 14 8 varying between those of wall rocks 11 6 20 3 and typical magmatic sources as well as the regional contemporary magmatic hydrothermal gold system cmhgs this can be interpreted by a mixed process formed by magmatic hydrothermal fluids leaching different proportion of sulfur from wall rocks during extensive fluid rock interaction py3 in the s3 stage has relatively narrow range of 34s values py3 0 6 3 7 similar to those of adjacent cmhgs indicating a magmatic origin without notable wall rock contamination due to weak fluid rock interaction the s2 stage gold bearing pyrite py1 dominated shows 206pb 204pb ratios from 17 882 to 18 938 207pb 204pb ratios from 15 592 to 15 825 and 208pb 204pb ratios from 37 811 to 39 003 which are between those of nearby early cretaceous sanguliu granitic pluton and wall rocks 206pb 204pb i 17 958 20 085 207pb 204pb i 15 664 15 976 and 208pb 204pb i 37 474 40 021 implying that the lead was originated from mixture sources ore fluids from the s2 stage pyrite py1 dominated have 3he 4he ratios range from 0 21 0 01 to 0 57 0 03 ra average 0 34 ra indicating a crustal source with minor mantle contributions moreover the 3he 4he ratios are similar to those of the adjacent cmhgs demonstrating a similar magmatic origin for the ore fluids the 40ar 36ar ratios range from 547 to 11994 indicating that the ore fluids contained 48 to 98 crust derived radiogenic 40ar which most likely inherited from wall rocks combining the geologic evidences and isotopic results we propose that the sidaogou deposit was formed by magmatic hydrothermal fluids exsolved from sanguliu pluton or its deep magma during fluid rock interaction sulfur lead and 40ar were partly scavenged from the wall rocks and then sequestered by ore related pyrite nevertheless gold was mainly of magmatic origin our study reinforces the previous conclusion that s pb isotope compositions of ore minerals could be strongly modified by wall rocks during fluid rock interaction

geology re os ages sulfur and lead isotopes of the diyanqinamu porphyry mo deposit inner mongolia ne china the diyanqinamu porphyry mo deposit in the southern greater khingan range of the central asian orogenic belt contains 800 million metric tons mt of ore with an average grade of 0 097 molybdenum the deposit is hosted in late jurassic volcanic rocks of tuff andesite and volcanic breccia multiple stage hydrothermal activities have resulted in propylitic phyllic and argillic alteration in this deposit five stages i v of hydrothermal activity are identified stage i is represented by a mineral assemblage of epidote chlorite and magnetite with some discontinuous barren veinlets of quartz k feldspar fluorite magnetite epidote chlorite stage ii is marked by occurrence of quartz fluorite molybdenite magnetite pyrite sericite siderite veinlets veins with phyllic halos stage iii consists of fluorite siderite quartz molybdenite pyrite ankerite calcite chalcopyrite veins that are commonly related to phyllic alteration and dissemination of fluorite in the altered rocks stage iv has an assemblage of fluorite quartz pyrite ankerite calcite molybdenite chalcopyrite sphalerite galena in coarse veins 10 20 mm wide stage v consists of narrow 5 mm wide veinlets of calcite fluorite pyrite quartz molybdenite mainly occurs in stages ii and iii re os dating results for molybdenite samples from these two stages yielded an isochron age of 156 2 4 2 ma 2 mswd 0 96 n 10 most molybdenite samples have high 34s values 8 4 relative to other sulfide minerals i e galena sphalerite pyrite and chalcopyrite of stages ii to v 34s 2 5 8 3 n 22 molybdenite also has low 207pb 204pb and 208pb 204pb ratios relative to other sulfide minerals although there are minor overlaps in a diagram of 206pb 204pb versus 207pb 204pb these pb isotope data display a positive trend transecting the growth curves of crustal lead which could be invoked by mixing of crustal and mantle sources with distinct pb isotopes in combination with the s isotope data and mineral paragenesis we suggest that magmas were the main source of molybdenum whereas other metals i e pb zn and cu were possibly sourced from the country rocks

pbsl dating of garnet and staurolite constraints on the paleoproterozoic crustal evolution of the eastern block north china craton abstract it is well accepted that the north china craton formed by assembly of the eastern and western blocks along the trans north china orogen with cratonization in the paleoproterozoic 1 85 ma however there is controversy about the formation of the eastern and western blocks themselves especially the timing of cratonization of the eastern block the jiao liao ji orogenic belt is the most important paleoproterozoic orogenic belt in the eastern block and consists mainly of the north and south liaohe groups and the liaoji granitoids the timing of metamorphism of the liaohe group is the key to understanding the evolution of the eastern block of the north china craton because the isotope systematics of metamorphic minerals can be used to date parts of the prograde thermal histories of metamorphic belts dating of minerals such as garnet and staurolite has the distinct advantage that time information can be related directly to the pressure temperature deformation history recorded by the same mineral in this paper we present results of precise 207pb 206pb stepwise leaching pbsl dating of garnet and staurolite from the metasedimentary rocks of the south liaohe group the primitive pb and radiogenic pb are selectively extracted from garnet and straurolite by leach steps and determined by multiple collector mc icp ms the garnet and staurolite record two groups of 207pb 206pb ages at 1 93 1 91 ga and 1 86 ga respectively these constrain the time of peak metamorphism of the liaohe group during collision between the longgang block in the north and the liaonan nangrim block in the south and a subsequent post collisional event respectively our data for the south liaohe group and previously published data for the liaoji granite combined with key lithological structural and metamorphic information indicate that the cratonization of the eastern block of the north china craton occurred at 1 93 1 91 ga

ages of quaternary pluvial episodes determined by uranium series and radiocarbon dating of lacustrine deposits of eastern sahara abstract as documented by radiocarbon dating and geoarchaeological investigations the now hyperarid northwestern sudan and southwestern egypt experienced a period of greater effective moisture during early and middle holocene time about 10 5 ka we have used the uranium series technique to date lacustrine carbonates from bir tarfawi bir sahara east wadi hussein oyo depression and the great selima sand sheet localities results indicate five paleolakeforming episodes occurred at about 320 250 240 190 155 120 90 65 and 10 5 ka four of these five pluvial episodes may be correlated with major interglacial stages 9 7 5e and 1 the 90 65 ka episode may be correlated with substage 5c or 5a our results support the contention that past pluvial episodes in north africa corresponded to the interglacial periods farther north ages of lacustrine carbonates from existing oases and from the sand sheet fail to indicate pluvial conditions between about 60 and 30 ka age results and field relationships suggest that the oldest lake and ground water deposited carbonates were much more extensive than those of the younger period and that carbonate of the latest wet periods were geographically localized within depressions and buried channels

dynamic cause of marginal lithospheric thinning and implications for craton destruction a comparison of the north china superior and yilgarn cratons we present a comparative tectonic analysis of the north china craton ncc which has lost parts of its root with the yilgarn and superior cratons which preserve their roots we compare the geophysical structure and tectonic histories of these cratons to search for reasons why some cratons lose their roots while others retain them based on the comparison and analysis of geological geophysical and geochemical data it is clear that the lithospheric thinning beneath craton margins is a common phenomenon which may be caused by convergence between plates however craton destruction is not always accompanied by lithospheric thinning except for cratons that suffered subduction and collision from multiple sides the western block also known as the ordos block of the ncc yilgarn and superior cratons have not experienced craton destruction the common ground among them is that they are surrounded by weak zones e g mobile belts or orogens that sheltered the cratons from deformation which contribute

nd isotopic constraints on crustal formation in the north china craton abstract recent tectonic analysis suggests that the north china craton consists of two archean continental blocks called the eastern and western blocks separated by the paleoproterozoic trans north china orogen although the published geochronological data are not sufficient to constrain the detailed tectonothermal evolution of the craton the available nd isotopic data show some important differences in nd model ages between the tectonic units the eastern block shows two main nd model age peaks one between 3 6 and 3 2 ga and the other between 3 0 and 2 6 ga limited nd isotopic data from the western block show a large range of model ages between 3 2 and 2 4 ga these differences are consistent with the recently proposed model the nd isotopic data from mantle derived mafic rocks indicate that the mantle beneath the north china craton was depleted in the archean consistent with major crustal growth during this period in the paleoproterozoic however the mantle derived mafic rocks show negative end t values implying crustal contamination this may have resulted from subduction and collision between the eastern and western block implying that the mechanisms of crustal formation and evolution may have been different between the archean and paleoproterozoic the north china craton was re activated by addition of mantle derived magma into the lower crust in the late mesozoic resulting in rejuvenation of the lower crust this indicates that underplating is also an important mechanism for continental addition although in this case it may not equate to crustal growth since it was preceded by removal of lithospheric mantle and possible some lower crust

rapid intraplate strain accumulation in the new madrid seismic zone remeasurement of a triangulation network in the southern part of the new madrid seismic zone with the global positioning system has revealed rapid crustal strain accumulation since the 1950s this area experienced three large moment magnitudes 8 earthquakes in 1811 to 1812 the orientation and sense of shear is consistent with right lateral strike slip motion along a northeast trending fault zone as indicated by current seismicity detection of crustal strain accumulation may be a useful discriminant for identifying areas where potentially damaging intraplate earthquakes may occur despite the absence of large earthquakes during historic time

fluid inclusions and hear isotopes in pyrite from the yinjiagou deposit in the southern margin of the north china craton a mantle connection for poly metallic mineralization abstract the yinjiagou poly metallic deposit is located at the southern margin of the north china craton smncc pyrite the most economically important mineral is associated with cu mo au pb zn and ag mineralization two main mineralization stages have been identified and studied a porphyry stage including the quartz molybdenite and quartz sericite pyrite qsp veins and a vein stage including the early and the late economically most important pyrite stages three types of fluid inclusions are distinguished in quartz and pyrite including liquid rich vapor rich biphase lv and halite bearing inclusions the transmitted and infrared microthermometry produced several results 1 each of the three types of fluid inclusions is trapped in the quartz of the porphyry stage with homogenization temperatures of 335 419 c and salinities of 4 7 49 6 wt nacl equiv the coexistence of halite bearing inclusions and low density vapor rich biphase lv inclusions indicates that the fluid was boiling the boiling elevated the fe and s levels in the hyper saline fluid as reflected by the development of pyrite chalcopyrite and s daughter minerals in the halite bearing inclusions 2 the fluid in the early pyrite also displays features characteristic of boiling with homogenization temperatures of 352 400 c and salinities of 3 7 42 4 wt nacl equiv whereas the late pyrite only contains liquid rich biphase lv inclusions with homogenization temperatures of 263 354 c and salinities of 6 0 21 3 wt nacl equiv the relationship between the decreasing salinity and the corresponding decreasing homogenization temperature clearly indicates extensive mixing and dilution 3 finally the infrared calibration indicates that the impact of infrared light intensity on the microthermometric results for the yinjiagou pyrite hosted fluid inclusions is limited the he ar isotopic analysis of the fluid inclusions in the pyrite indicates two different fluid sources that involve abundant mantle input the 3he 4he ratios in the porphyry stage are relatively uniform ranging from 1 39 to 1 78 ra ra 1 39 10 6 for air corresponding to 16 22 mantle 4he contribution this relatively constant range represents the actual helium isotopic composition of the fluids emanating from the cooling intrusions at depth the range of 3he 4he ratios in the vein stage is wide varying from 0 80 to 5 26 ra corresponding to 9 65 mantle 4he the significant variation in the 3he 4he ratios in the vein stage reflects mixing between two fluids these data indicate that there are two different fluid sources and separate evolutionary processes for the porphyry and vein stages and support a multi stage mineralization model the veinlet molybdenite mineralization was induced by the boiling and associated cooling of a magma sourced fluid that emanated from intrusions at depth the qsp veins are likely to be pressure release zones and late fluid flow channels the early pyrite mineralization may be attributed to the influx of a dominantly mantle derived fluid with high 3he 4he ratio 5 26 ra which is most likely related to a later more mafic magmatic event the precipitation of the most economically important pyrite resulted from the mixing and diluting of this mantle derived fluid with a surface derived fluid additionally the high 3he 4he ratios indicate a strongly extensional setting which is most likely related to the late jurassic to early cretaceous lithospheric modification and thinning of the smncc

single crystal 40ar 39ar dating of pyrite no fool s clock single crystals of dispersed trace pyrite from two igneous rocks were dated by using the 40 ar 39 ar laser probe method and the resulting ages were compared to step heating ages of associated amphibole and phlogopite the isochron ages for 0 1 0 3 mm cubes of pyrite agree well with the respective amphibole ages of these igneous samples preliminary step heating analyses of single pyrite crystals yield internal isochrons with indistinguishable although less precise ages both pyrite and amphibole are significantly older than associated phlogopite in one sample which has undergone postcrystallization alteration these results suggest that pyrite can give reliable and precise 40 ar 39 ar ages even in the presence of subsequent alteration given the ubiquity of pyrite in many geologic environments this technique has great promise for application to the dating of ores sediments and hydrocarbon migration all of which have been very difficult to date directly

seismotectonics of the norwegian continental margin the norwegian continental margin and surrounding areas are seismically less active than some other passive margins worldwide indicating a potential earthquake deficit the adjacent oceanic crust is mostly aseismic except for parts of the lofoten and norway basins which have experienced rapid deposition of glacial sediments the excess load has enhanced the local stress field and in turn the seismic activity the marginal highs along the continent ocean transition and the part of the more and voring basins which experienced crustal extension prior to the early tertiary breakup are practically aseismic this region is also underlain by high velocity lower crust emplaced during breakup suggesting crustal strengthening which also may increase the return periods for the largest events farther landward a spatial correlation of seismic activity and the glacial sediment wedge suggests a causal relationship expressed through preferential rejuvenation of late jurassic early cretaceous faults the locally high seismic activity in the coastal region occurs in areas where the continental crust is relatively thick 25 30 km but still thinner than in the shield area farther east 45 50 km although stress relations are complex we suggest that increased stress due to appreciable postglacial rebound gradients in the coastal region may be a contributing factor while the stress field along the margin complies in general with the ridge push force we infer that regional and local stress enhancement factors are not only present in this region but also necessary for explaining the earthquake activity the relative importance of local stress sources is supported by several cases of 90 stress rotations relative to the ridge push direction the superposition of regional and local stress together with the existence of weakness zones and faults yields potentials for earthquakes primarily through structural reactivation

the early cretaceous yangzhaiyu lode gold deposit north china craton a link between craton reactivation and gold veining the yangzhaiyu gold deposit is one of numerous lode gold deposits in the xiaoqinling district southern margin of the north china craton gold mineralization is hosted in neoarchean to early paleoproterozoic amphibolite facies metamorphic rocks and consists of auriferous quartz veins and subordinate disseminated ores in the vein proximal alteration zone ore related hydrothermal alteration is dominated by sericite quartz sulfide assemblages close to gold veins and biotite quartz pyrite chlorite epidote alteration generally distal from mineralization pyrite is the predominant sulfide mineral locally coexisting with minor amounts of chalcopyrite sphalerite and galena gold occurs mostly as free gold enclosed in or filling microfractures of pyrite and quartz and is also present in equilibrium with au bearing tellurides mainly petzite and calaverite coexisting with hessite tellurobismuthite and altaite fluid inclusion studies suggest that gold veins were deposited at intermediate temperatures 175 313 c from aqueous or aqueous carbonic fluids with moderate salinity 5 14 wt nacl equiv 34s values of sulfide minerals range mainly from 2 0 to 4 4 whereas auriferous quartz vein samples have 18o values of 12 4 to 9 6 with calculated 18oh2o values of 6 0 to 3 2 gold related pyrite grains yield elevated 3he 4he ratios 1 51 0 32 ra relative to crustal reservoirs and mantle like 20ne 22ne and 21ne 22ne ratios 9 90 9 68 and 0 029 respectively the stable and noble gas isotopes thus suggest deep seated most likely magmatic and mantle derived sources for the ore fluids sulfur and by inference other components in the ore system 40ar 39ar dating of ore related sericite and biotite separates indicates two episodes of gold genesis at 134 5 to 132 3 and 124 3 to 123 7 ma the mineralization ages overlap zircon u pb ages of 141 0 1 6 to 125 8 1 4 ma 2 for the wenyu and niangniangshan monzogranite plutons and a number of mafic to intermediate dikes intruding these plutons all being proximal to the yangzhaiyu gold deposit the synchronism of gold genesis and magmatism provides additional weights of evidence for a magmatic derivation of ore fluids and sulfur the geochronologic data also suggest that gold veining took place billions of years after the stabilization of the north china craton and associated metamorphism in the late archean to early paleoproterozoic this contrasts sharply to lode gold deposits in other precambrian cratons that formed predominantly in late archean to paleoproterozoic temporarily and genetically related to regional high grade metamorphism and compressional or transpressional tectonism available data have demonstrated that the north china craton was reactivated in the late mesozoic as marked by voluminous igneous rocks faulted basin formation high crustal heat flow and widespread metamorphic core complexes in the eastern part of the craton it is thus suggested that the yangzhaiyu gold deposit together with other deposits of similar ages in the xiaoqinling district were products of this craton reactivation event lithospheric extension and extensive magmatism related to the craton reactivation may have provided sufficient heat energy fluid and sulfur required for the formation of the gold deposits

helium and argon isotope geochemistry of alkaline intrusion associated gold and copper deposits along the red river jinshajiang fault belt sw china abstract the red river jinshajiang strike slip fault zone on the eastern margin of the tibetan plateau was originally produced by the india eurasia collision 60 70 myr ago numerous post collisional mantle derived alkaline igneous rocks with ages of 40 30 ma have been intruded along this fault zone in recent years several copper and gold deposits associated with the alkaline intrusions of this region were discovered such as the yao an and beiya gold deposits and the yulong and machangqing copper deposits studied in this paper the mineralised intrusions are felsic with sio 2 ranging from 61 4 to 67 7 wt k 2 o na 2 o from 8 1 to 11 5 wt and k 2 o na 2 o 1 the deposits are located at both the exo and endo contact zones of the intrusions the mineral deposits are of hydrothermal origin with the ore forming temperatures mainly in the range 150 450 c this paper presents he and ar isotope analyses of these four deposits the concentrations of 4 he trapped in fluid inclusions of pyrites from the ores are 0 7 54 1 10 6 cm 3 stp g 1 and those of 40 ar are 0 6 7 3 10 6 cm 3 stp g 1 3 he 4 he ratios are 0 3 2 5 ra ra represents the 3 he 4 he ratio of air 1 39 10 6 40 ar 36 ar ratios are 316 1736 and 3 he 36 ar ratios are 0 2 11 2 10 3 generally the 3 he 4 he 40 ar 36 ar and 3 he 36 ar ratios for the gold deposits are higher than those for the copper deposits we suggest that the ore forming fluids of both gold and copper deposits were differentiated from the mantle derived alkaline magmas but were diluted by modified air saturated water masw that experienced intensive interaction with crustal rocks however the magmatic fluids responsible for the gold deposits were less extensively diluted by masw resulting in higher 3 he 4 he 40 ar 36 ar and 3 he 36 ar ratios than the copper deposits

the 127 ma gold mineralization in the wulong deposit liaodong peninsula china constraints from molybdenite re os monazite u th pb and zircon u pb geochronology abstract the wulong deposit is a large tonnage deposit 80 t of au characterized by quartz vein style of gold mineralization in the liaodong peninsula china the auriferous quartz veins are mainly hosted by late jurassic gneissic two mica granite 165 157 ma and early cretaceous granodiorite 129 ma and are structurally controlled by nne and nw trending faults previous attempts to date the gold mineralization yielded ambiguous results 135 112 ma here we report the first in situ and direct age constraint for gold mineralization at wulong five molybdenite samples from mo bearing quartz pyrite veins yielded an isochron age of 127 6 2 3 ma in situ sims u th pb dating of hydrothermal monazite that coexists with gold bearing pyrite and quartz yielded a 208pb 232th age of 126 7 3 2 ma these robust ages together with new age data of the wulong host intrusion and post ore dike allow for a better interpretation of the ore genesis of the wulong deposit relative to a well constrained sequence of regional geological events zircon u pb dating of the wulong host intrusion gneissic two mica granite representing the oldest magmatic event at wulong gave a weighted mean age of 159 3 1 4 ma that is interpreted to be the magma emplacement age subsequent magmatic events are recorded by the early cretaceous dikes with zircon u pb ages between 127 and 113 ma a post ore diorite porphyry dike that crosscuts the early cretaceous dikes has a u pb zircon age of 121 5 1 5 ma all the lines of geochronology evidence suggest a distinct genetic link between the gold mineralization and the early cretaceous magmatism in the wulong orefield this episode of magmatism and mineralization corresponds to the lithospheric thinning within the northeastern north china craton which is triggered by the subduction of the paleo pacific plate

formation of gold deposits a metamorphic devolatilization model a metamorphic devolatilization model can explain the enrichment segregation timing distribution and character of many goldfields such as those found in archean greenstone belts slate belts and other gold only provinces in this genetic model hydrated and carbonated greenschist facies rocks particularly metabasic rocks are devolatilized primarily across the greenschist amphibolite facies boundary in an orogenic setting devolatilization operates on the scale of individual mineral grains extracting not just h2o and co2 but also s and in turn au elevated gold in solution is achieved by complexing with reduced s and by h2co3 weak acid buffering near the optimal fluid ph for gold solubility the buffering is more important than being at the point of maximum gold solubility low salinity ensures low base metal concentrations in the auriferous metamorphic fluid migration of this fluid upwards is via shear zones and or into hydraulic fracture zones in rocks of low tensile strength the geometry of the shear zones dictates the kilometre scale fluid migration paths and the degree of fluid focusing into small enough volumes to form economic accumulations of gold deposition of gold from solution necessitates breakdown of the gold thiosulphide complex and is especially facilitated by fluid reduction in contact with reduced carbon bearing host rocks and or by sulphidation of wallrocks to generate iron bearing sulphide and precipitated gold as such black slate carbon seams banded iron formation tholeiitic basalt magnetite bearing diorite and differentiated tholeiitic dolerite sills are some of the important hosts to major goldfields gold deposition is accompanied by carbonation sulphidation and muscovite biotite alteration where the host rock is of suitable bulk composition the correlation of major gold deposits with rock type even when the gold is primarily in veins argues for rock dominated depositional systems not fluid dominated ones as a consequence a general role in gold deposition for fluid mixing temperature decrease and or fluid pressure decrease and boiling is unlikely although such effects may be involved locally several geological features that are recorded at gold only deposits today reflect subsequent modifications superimposed upon the products of this generic metamorphic devolatilization process overprinting by higher grade metamorphism and deformation and or palaeo weathering may provide many of the most obvious features of goldfields including their mineralogy geochemistry geometry small scale timing features geophysical response and even mesoscopic gold distribution

middle and late pleistocene humid periods recorded in palaeolake deposits of the nafud desert saudi arabia present climate in the nafud desert of northern saudi arabia is hyper arid and moisture brought by north westerly winds scarcely reaches the region the existence of abundant palaeolake sediments provides evidence for a considerably wetter climate in the past however the existing chronological framework of these deposits is solely based on radiocarbon dating of questionable reliability due to potential post depositional contamination with younger 14c by using luminescence dating we show that the lake deposits were not formed between 40 and 20 ka as suggested previously but approximately ca 410 ka 320 ka 200 ka 125 ka and 100 ka ago all of these humid phases are in good agreement with those recorded in lake sediments and speleothems from southern arabia surprisingly no holocene lake deposits were identified geological characteristics of the deposits and diatom analysis suggest that a single perennial lake covered the entire south western nafud ca 320 ka ago in contrast lakes of the 200 ka 125 ka and 100 ka humid intervals were smaller and restricted to interdune depressions of a pre existing dune relief the concurrent occurrence of humid phases in the nafud southern arabia and the eastern mediterranean suggests that moisture in northern arabia originated either from the mediterranean due to more frequent frontal depression systems or from stronger indian monsoon circulation respectively however based on previously published climate model simulations and palaecolimate evidence from central arabia and the negev desert we argue that humid climate conditions in the nafud were probably caused by a stronger african monsoon and a distinct change in zonal atmospheric circulation

archeological and pedological evidence for large prehistoric earthquakes in the new madrid seismic zone central united states prehistoric liquefaction features have been identified by careful observation of their structural and stratigraphic relations to native american occupation horizons and their subtle soil characteristics the ages of these liquefaction features have been estimated from radiocarbon dating of wood associated with the features and native american artifacts found within bounding occupation horizons at three sites near blytheville arkansas in the central part of the new madrid seismic zone one sand blow crater formed between a d 800 and 1400 two sand blow deposits formed between a d 800 and 1670 and three possibly four sand dikes formed since 4035 b c where not found in association with native american occupation horizons and artifacts prehistoric liquefaction features can be difficult to distinguish from features that formed during the great new madrid earthquakes of a d 1811 and 1812 this raises the possibility that prehistoric liquefaction features may have been misinterpreted during previous studies in the area nevertheless a paleoearthquake chronology is beginning to emerge for the new madrid seismic zone our findings are consistent with paleoseismological studies in the northern part of the seismic zone and suggest a recurrence interval of hundreds of years for earthquakes large enough to induce liquefaction in this region m 6 4 by mapping the age distribution of liquefaction features a more accurate assessment of the long term earthquake potential of the region will be possible

palaeocene oligocene tectonics of nw europe s u m m a r y during the palaeogene in nw europe a gallic subplate was very nearly created this plate was almost entirely bordered by plate boundaries although not of equivalent ages to the n the n atlantic began to open about 56 ma ago the alpine convergent boundary extended southwestwards to the pyrenean subduction zone on the ne side a n british pataeocene early eocene fracture dyke boundary appears to pass southeastwards across the north sea into a lutetian to burdigalian rift system there is an age disparity between the fracturing and nw dyke intrusion in britain and the fracturing and formation of the rhine graben the last dykes in britain ypresian were intruded not long before the important subsidence of the rhine graben earliest lutetian but their causes were different the british dyke fractures propagated southeastwards via the cleveland dyke towards the north sea whereas the rhine graben propagated northwards towards the zuider zee depression however the structures from both sides converged in the sole pit basin in the north sea with the result that a continuous though not coevally originated fracture system developed from nw scotland to the alps during the early tertiary there was a wide range of tectonic activity throughout nw europe fig 1 in nw britain lavas were extruded and plutonic complexes and extensive dyke swarms were intruded in the north sea tufts were deposited as a result of volcanism along the rockall faeroe rift reactivation of faults con trolled magmatism localization of gas fields devolatilization of coal and inversion of basins caused uplift deformation and erosion along the rhine graben doming subsidence and rift propagation took place in association with alka line volcanism and plutonism coevally with the meso alpine compressive deformations that oc curred in the alps faulting controlled sedimen tation patterns during inversion of basins in southern england the english channel northern france and holland old rift faults were reacti vated in the western approaches basin thrust faults formed in the southern bay of biscay contemporaneously with thrusting in the pyre nees and variscan massifs were tilted and uplifted along steep reverse faults the main tectonic activity lasted from about 60 ma to about 30 ma but it was diachronous generally earlier in the nw and later in the se the igneous activity in northern britain was related to the opening of the n atlantic in the late palaeocene but the formation of the rhine graben and the inversion of sedimentary basins in the north sea and near the english channel were caused by major orogenic events in the alpine region in the late eocene to mid oligocene frisch 1981 tectonic phenomena in the north sea provide the link between those in northern britain and those in the rhine graben the aim of this paper is to demonstrate that these movements from the atlantic and alpine sides almost fragmented the lithospheric plates of nw europe during the creation of a gallic sub block an earlier northern boundary from nw scotland to northern germany being transected by a later eastern boundary along the rhine graben

pb sr nd isotope constraints on the fluid source of the dahu au mo deposit in qinling orogen central china and implication for triassic tectonic setting abstract the dahu au mo deposit is a structure controlled lode system occurring in the northern xiaoqinling terrane huaxiong block qinling orogen this paper reports a new sr nd pb isotope dataset obtained for ore sulfides and the hostrocks within the taihua supergroup in an attempt to constrain the source of the ore forming fluids from a new dimension 16 sulfide samples yield i sr ratios of 0 70470 0 71312 with an average of 0 70854 e nd t values between 13 5 and 18 1 with average of 15 1 and 206 pb 204 pb i 207 pb 204 pb i and 208 pb 204 pb i ratios of 17 033 17 285 15 358 15 438 and 37 307 37 582 with averages of 17 162 15 405 and 37 440 respectively 5 gneiss samples from the taihua supergroup yield i sr ratios of 0 70947 0 73201 averaging 0 72294 e nd t values of 20 0 to 31 1 averaging 25 1 and 206 pb 204 pb i 207 pb 204 pb i 208 pb 204 pb i ratios of 17 127 18 392 15 416 15 604 and 37 498 37 814 with averages of 17 547 15 470 and 37 616 respectively these data show that the ore sulfides have less radiogenic sr nd pb isotope systematics than the hostrocks and suggest that the ore forming fluids which interacted with the wallrocks to form ores must be sourced from a depleted mantle or a depleted subducted oceanic slab in combination with the spatial scenario and geochemical signatures of the triassic magmatites and mineral systems we argue that in the triassic the mianlue ocean was not completely closed and that the northward oceanic plate subduction still survived along the mian lue suture which caused the late triassic magmatism and associated mineralization in qinling orogen including the au mo dahu deposit

re os and sr nd pb isotope constraints on source of fluids in the zhifang mo deposit qinling orogen china abstract the zhifang mo deposit is located in the northeastern qinling orogen along the southern margin of the north china craton the deposit represents a quartz vein system hosted in the mesoproterozoic xiong er group volcanic rocks we identify three hydrothermal stages early middle and late characterized by veinlets of quartz pyrite quartz molybdenite pyrite chalcopyrite galena sphalerite and quartz carbonate assemblages respectively five molybdenite samples from the zhifang deposit yield re os ages ranging from 241 2 1 6 ma to 247 4 2 5 ma with an isochron age of 246 0 5 2 ma 2 mswd 7 4 and a weighted mean age of 243 8 2 8 ma 2 mswd 5 5 the re os age shows that the mo mineralization occurred during the indosinian orogeny and suggests that the mineralization is unrelated to the yanshanian magmatism or the paleo mesoproterozoic volcanic hydrothermal event this study also reports a new sr nd pb isotope dataset from ore sulfides in an attempt to constrain the source of the ore forming fluids ten sulfide samples from middle stage of the zhifang mo deposit yield i sr t ratios of 0 710286 0 711943 with an average of 0 711004 e nd t values between 19 5 and 14 8 with an average of 16 7 and 206 pb 204 pb i 207 pb 204 pb i and 208 pb 204 pb i ratios of 17 126 17 535 15 374 15 466 and 37 485 37 848 with averages of 17 380 15 410 and 37 631 respectively one pyrite from the early stage yield i sr t of 0 722711 0 722855 with an average of 0 722783 which is higher than those of the middle stage sulfides and suggests equilibration with wallrocks the e nd t values are in the range of 17 3 to 16 6 with a mean at 17 0 and 206 pb 204 pb i 207 pb 204 pb i and 208 pb 204 pb i ratios are 17 386 15 405 and 37 622 respectively the ore sulfides show higher pb isotope ratios higher e nd t and lower i sr t values than the host rocks the results suggest that the ore forming fluids had lower i sr t and higher e nd t values than the ore sulfides and were possibly sourced from the dengfeng complex the southward subduction of the north china craton beneath the huaxiong block during the triassic was possibly responsible for the formation of the waifangshan orogenic mo system

archean crust as revealed in the kapuskasing uplift superior province canada in the central superior province of the canadian shield a 120 km wide transition from the low grade michipicoten greenstone belt to the high grade kapuskasing structural zone represents an oblique section through some 20 km of archean crust uplifted along a northwest dipping thrust fault the restored vertical section through the upper and middle crust consists of three megalayers with undulating boundaries 1 0 to 25 km a high grade heterogeneous gneissic assemblage in part older than the upper supracrustal succession

eduard suess relations to the pre 1950 schools of thought in global tectonics two main schools of thought in global tectonics are recognized during the first half of the xxth century on the basis of their proponents attitude towards the principle of uniformitarianism and their belief in an inherent order and regularity vs disorder and irregularity in tectonics one group here called the wegener argand school had a unifomitarian approach to global tectonics and believed in an inherently irregular nature in which probability rather than determinism was believed to be a realistic approach the other here called the kober stille school was largely non uniformitarian neocatastrophist and believed in an orderly regular nature these two schools correspond to argand s mobilists and fixists respectively in this paper i show that eduard suess was a convinced uniformitarian as far as his views on global tectonics are concerned and denied any inherent regularity in tectonic phenomena temporal or spatial in his interpretations of the causes and nature of orogeny nature of geosynclines and causes and nature of stabilisation suess appears to be the predecessor of the wegener argandians i e of the mobilists and not of the kober stilleans as hitherto assumed although he remained a fixist and contractionist to the end suess seems to have paved the way for the mobilists much as he did for the nappists during the last quarter of the xixth century

n2 ar he compositions in fluid inclusions indicators of fluid source abstract a quadrupole mass spectrometer was used to measure bulk samples of conservative gas species n 2 ar and he in fluid inclusions from a variety of hydrothermal systems analyses of these tracer elements help determine 1 1 if gases extracted by bulk inclusion analyses can provide accurate measurement of n 2 ar he in active and fossil geothermal systems 2 2 if hydrothermal fluids associated with paleogeothermal systems in a continental setting follow n 2 ar he systematics similar to th the western pacific rim active geothermal systems specifically new zealand and 3 3 whether different deposit types systematically vary with regard to n 2 ar he the n 2 ar he ratios of fluid inclusion volatiles released from recently deposited minerals from the valles system are similar to those of present day valles thermal waters those inclusion samples from deep within the valles system below a regional aquitard increase in n 2 compositions for inclusions from the questa and copper flat porphyry deposits are n 2 rich similar to those of arc related volcanic gases whereas those from taylor creek sn deposit appear to be mixtures of magmatic and crustal components n 2 ar he ratios of the precambrian tribag deposit suggest a basalt source but significant levels of self generated he from u and th in the inclusion fluids are also possible inclusions from two epithermal deposits with low salinity inclusions have n 2 ar he ratios trending towards air saturated meteoric waters asw and those inclusions with higher salinities indicate minor to no asw component the n 2 ar he ratios in fresnillo and cochiti inclusions which have magmatic helium isotopic ratios indicate additions of magmatic gases to meteoric fluids inclusions from sediment hosted deposits that contain hydrocarbon bearing brines are he rich as are meteoric waters with a long residence time in the crust at relevant pressure temperature composition conditions henry s law constants of n 2 ar and he are similar and thus the relative amounts of these species trapped under boiling conditions will not vary appreciably from solubility controlled amounts in coexisting liquid furthermore there is no evidence of he loss or gain in inclusions by diffusion process data from the various systems that were examined indicate that fluid inclusion n 2 ar he compositions may be related to magmatic fluid sedimentary brine deep circulating meteoric fluid and shallow circulating meteoric fluid sources which we modified from those proposed by giggenbach 1986 relationships between the n 2 ar he tracer gases and other measurable quantities in fluid inclusions are indicated to be helpful in understanding fluid mixing processes in paleogeothermal systems

origin and assembly of the tethyside orogenic collage at the expense of gondwana land abstract the tethysides are a superorogenic complex flanking the eurasian continent to the south and consisting of the cimmerides and alpides products of palaeo and neo tethys respectively we here review their evolution mainly on the basis of new maps showing the distribution of sutures magmatic rocks certain palaeobiogeographically and palaeoclimatologically significant taxa and facies and fragments of pan african 900 450 ma orogenic system forming the basement of many tethyside blocks these are supplemented by palaeomagnetic data reported in the literature a fundamental tenet of this paper is that major sutures which contain ophiolite fragments represent tectonic sections between continental blocks where oceanic crust has been subducted palaeo tethys came into existence largely in late carboniferous time coevally it began to be consumed by both internal and peripheral subduction zones which continued into the permian some of these had been inherited from pre tethyan times in the later permian rifting subparallel with the northern margin of gondwana land began between the zagros and malaysia separating a cimmerian continent from n gondwana land and thus heralding the opening of neo tethys and other smaller oceans that were back arc basins of palaeo tethys this rifting possibly also extended farther west into crete and mainland greece however the north china block yangtze block huanan block the eastern moity of the qangtang block north tibet and annamia all originally pieces of the end proterozoic early palaeozoic gondwana land had already separated from it in pre late carboniferous times possibly during the devonian all of these blocks and the cimmerian continent were characterized by cathaysian floral elements in late palaeozoic time palaeomagnetic and palaeontological data showing the original gondwana land affinity of these continental blocks are supplemented by correlating late proterozoic early palaeozoic pan african sutures orogenic belts and sedimentary basin fragments across tethyside sutures late permian foraminiferal provinces are related to this palaeogeographical interpretation by triassic times most cimmeride subduction zones were already in existence the cimmerian continent accelerated its separation from gondwana land and locally in the late permian began disintegrating internally along the waser rushan pshart banggong co nu jiang mandalay ocean by late triassic time all of the chinese blocks except lhasa and annamia had collided with each other and with laurasia the resulting enormous orogenic collage had a soft cushion between itself and laurasia in the form of the enormous accretionary complex of the songpan ganzi this connection enabled laurasian land vertebrates to reach south east asia by late triassic time in late triassic to middle jurassic times most major cimmeride collisions were completed widespread aridity in central asia occurred in late jurassic time probably in the rain shadow of the newly formed cimmeride mountain wall neo tethyan subduction systems formed along the s margin of the cimmerides or within neo tethyan oceanic lithosphere during the jurassic most if not all were north or east dipping they continued the northerly migration of the tethyside blocks evolution of the tethysides influenced the distribution of marine and terrestrial organisms and affected sea level changes and patterns of atmospheric circulation during much of the mesozoic and cainozoic it is likely to have reflected the surface expression of a persistent trend in the large scale convective circulation in the mantle that continuously transported material northward into the tethyan domain

estimation of intraplate strain accumulation in the new madrid seismic zone from repeat gps surveys the new madrid seismic zone nmsz a region within the eastern portion of the north american plate where great earthquakes occurred in 1811 1812 provides a type example of large earthquakes within a relatively stable continental interior assessing the scale and causes of deformation in such regions as well estimation of the recurrence interval for the large earthquakes within them has long been recognized as an important but challenging issue for which the advent of space based geodesy provides an important new tool here we report initial results from two global positioning system gps surveys of a geodetic network spanning the new madrid seismic zone because the expected strain signal is small the data were analyzed in three different ways first site velocities were estimated and analyzed for spatial gradients parallel and perpendicular to the trend of the major strike slip faults inferred from seismicity this analysis suggests but does not require a few millimeters per year of right lateral shear across the southern portion of the seismic zone consistent with the earthquake focal mechanisms second we divided the network into near field subnetworks and found possibly significant shear associated with approximately e w shortening within the southern nmsz and possible n s shortening across the left step connecting the presumed strike slip faults third we compared the strain observed along individual baselines to that predicted from the results of earlier near field surveys which combined gps and triangulation data in the southern part of the seismic zone we found general agreement with the earlier study although our data suggest lower strain rates hence after only 2 years of measurements the gps data suggest but do not yet require detectable strain accumulation however given that great earthquakes occur in the area and that our results are reasonably consistent with the results of other geodetic studies using different data and with earthquake mechanisms it seems likely that we are seeing strain accumulation in the southern nmsz a simple interpretation of the data as showing about 3 to 5 mm yr 1 of right lateral fault parallel motion implies a 1000 to 3300 year recurrence for great earthquakes with 5 to 10 m slip assuming all the accumulating strain is released seismically this recurrence interval may be an underestimate if the observed strains are largely postseismic effects of the 1811 1812 earthquakes a possibility suggested by the observation that plate wide data show less motion across the nmsz future surveys should resolve this issue as the longer measurement spans yield more precise velocity estimates

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

diatomaceous sediments and environmental change in the pleistocene olorgesailie formation southern kenya rift valley abstract the olorgesailie formation is comprised of lacustrine volcaniclastic and alluvial sediments that formed in the southern kenya rift between about 1 2 million and 490 000 years ago diatoms are common in much of the sequence and preserve a record of environmental change within the basin a high resolution diatom stratigraphy has been developed for these deposits the data document the presence of freshwater and saline lakes as well as wetlands transfer functions indicate that these water bodies ranged in conductivity between about 200 20 000 s cm 1 with ph varying between about 7 5 and 10 3 pedogenesis affected multiple horizons within the succession reflecting periods of emergence that encompass much of the time represented by the olorgesailie formation a variety of other sedimentological indicators carbonates rhizoliths mudcracks erosional channels also record periods when the sampled portions of the basin were dominated by terrestrial conditions stone tools are common at several levels indicating the use of the area by hominins lakes and wetlands were potentially usable as sources of potable water by hominins for part of the basin history but at other times were undrinkable other water sources springs rivers would have been necessary during these periods paleoenvironments in the basin were complex and changed frequently with time such shifts in resources and habitat distribution during olorgesailie formation time seem likely to have influenced the behavior and evolution of local plant and animal populations including homo

upper mantle velocity structure beneath the siberian platform summary we present a new velocity model for the continental upper mantle beneath central siberia based on observations of the 1982 rift deep seismic sounding dss profile three peaceful nuclear explosions pne were detonated to provide energy for the 2600 km long profile that extends from the yamal peninsula to the mongolian border se of lake baikal in this paper we model seismic recordings from the northernmost explosion since data from that shot shows unambiguous arrivals from the mantle transition zone discontinuities the analysis combines forward traveltime modelling and waveform matching using reflectivity synthetic seismograms our model for the lithosphere has velocities of 8 25 8 20 km s from the moho to 117km depth between 117 and 123km depth a strong velocity gradient 8 30 8 53 km s is required while a moderate gradient 8 53 8 55 km s l exists between 123 and 136km depth a low velocity zone from 136 to 210km depth terminates this phase arrival branch the gradient again rises between 210 and 233 km and depth culminating in a high gradient zone 8 63 8 80 km s between 233 and 235 km depth below the high gradient zone more moderate gradient 8 80 8 85 km s is required from 235 to 253 km depth terminating in a zone of lower velocity 8 62 8 64 km s from 253 to 400 km depth the upper mantle transition zone consists of two high gradient zones separated by a more moderate gradient the upper zone is best modelled as a 35 km thick velocity gradient 8 64 9 45 km s from 400 to 435 km depth the existence of the velocity gradient is based on the observation that arrivals from this feature can be identified starting at 1580 km range and rapidly become prominent with increasing distance a model with a first order discontinuity predicts significant arrivals at ranges closer than 1580 km our observations contain no compelling evidence for a 520 km discontinuity although a small discontinuity cannot be ruled out the lower transitionzone discontinuity is modelled as a 4 km thick gradient zone 10 25 10 62 km s i from 655 to 659 km depth

comparison of the hydrological and hydrochemical evolution of lake naivasha kenya during three highstands between 175 and 60 kyr bp abstract three diatomite beds exposed in the ol njorowa gorge south of lake naivasha central kenya rift document three major lake level highstands between 175 and 60 kyr bp diatom transfer function estimates of hydrological and hydrochemical parameters suggest that a deep and large freshwater lake existed during the highstands at 135 and 80 kyr bp in contrast a shallower but more expanded freshwater lake existed at 110 kyr bp the best analog for the most extreme highstand at 135 kyr bp is the highstand during the early holocene humid period from 10 to 6 kyr bp the environmental conditions as reconstructed from diatom assemblages suggest long lasting episodes of increased humidity during the high lake periods this contrasts to the modern situation with a relatively shallow lake naivasha characterized by rapid water level fluctuations within a few decades the most likely cause for the variable hydrological conditions since 175 kyr bp is orbitally driven insolation changes on the equator and increased lateral moisture transport from the ocean

lead isotope signatures of the igneous rocks and porphyry copper deposits from the kerman cenozoic magmatic arc se iran and their magmatic metallogenetic implications abstract copper deposits in the kerman cenozoic magmatic arc kcma of iran are genetically related to miocene porphyries the porphyries and their associated cu deposits were intruded and emplaced into eocene volcanic sedimentary successions lead isotope compositions of the eocene volcanic rocks exhibit homogeneous non radiogenic values 206 pb 204 pb 18 40 to 18 49 207 pb 204 pb 15 41 to 15 48 and 208 pb 204 pb 38 15 to 38 36 in contrast the miocene ore hosting porphyries and associated sulfide minerals also show a relatively radiogenic and homogenous pb isotopic values with orogen character 206 pb 204 pb 18 52 to18 60 207 pb 204 pb 15 58 to 15 64 and 208 pb 204 pb 38 57 to 38 80 the shift in the pb isotopic composition of igneous rocks with time towards higher pb isotopic values precludes derivation of the ore hosting porphyries from a reservoir of similar composition to those in the eocene volcanic rocks the non radiogenic pb isotope signature of the eocene volcanic rocks indicates an origin from subduction related mantle derived magmas whereas the higher pb isotopic values of the miocene ore hosting porphyries suggest an increasing involvement of crustal materials with lower u pb ratios than the upper crust in the source region of these rocks combining the pb isotope data with petrogenetic considerations suggests derivation of the middle miocene ore hosting porphyries from a mixed reservoir with slightly radiogenic pb values such as a hybridized lower continental crustal source this source probably formed because of the involvement of cretaceous oligocene subduction related mantle derived mafic magmas at the base of arc crust the contribution of the miocene collision related mantle derived mafic magma in melting of the hybridized lower continental crust caused generation of the middle miocene porphyries hosting large copper deposits with the least 206 pb 204 pb ratios among other porphyries the higher 206 pb 204 pb ratios of the late miocene pliocene porphyries hosting small copper deposits as well as barren porphyries may reflect a decreasing involvement of collision related mantle derived mafic magma in the source of the late miocene pliocene porphyries the overall similarity in pb isotope values between sulfide minerals and ore hosting porphyries suggests a magmatic source for most of the pb and possibly other metals the more radiogenic isotopic ratios of some late sulfide minerals suggest an insignificant metal contribution from an external source such as local country rocks during hydrothermal circulation

diatom inferred salinity and carbonate oxygen isotopes in holocene waterbodies of the western sahara and sahel africa abstract thirteen holocene palaeolakes in the western sahara and sahel have provided diatom records with carbonate oxygen isotope profiles available from eight of them most of these palaeolakes were groundwater fed lake water chemistry is reconstructed using diatom transfer functions lake water salinity and 18 o records are assembled with some isotopic and chemical groundwater data to better understand the response of the hydrological systems to climate changes over the past 15 000 yr data are in general agreement with climate simulations using coupled atmosphere ocean vegetation models which show a mid holocene wetting over the whole of northwest africa and a rapid drying by 6 4 ka the lake record also shows that at many sites the major lake infilling lags the end of the younger dryas by 1 2 ka regional differences also appear in the timing of the lake hydrological optimum ca 10 5 8 5 and 7 5 4 5 ka in the northern sahara 10 8 5 ka in the air tenere 10 5 7 or 4 5 in the sahel and 7 5 ka in lake chad the whole of the holocene is punctuated by short term drying events changes in water isotopic composition through time are partly explained by changes in rainfall amount and air humidity during the wet holocene period however the very low values in the southern sahara also imply changes in the moisture transport pattern or rainfall mechanisms data suggest an apparent decrease in 18 o content of precipitation along the monsoon flow in contrast with modern patterns changes in water availability and quality have driven population migrations in and out of the sahara sahel but relationships between climate and cultures are complex short term dry events might have driven inventive adaptations in the sahara drying at 5 4 5 ka coincides with both the collapse of the classical neolithic civilization and the settlement of new cultures

fluid inclusion noble gas and halogen evidence on the origin of cu porphyry mineralising fluids the naturally occurring noble gas isotopes 40ar 36ar 84kr and 129xe and halogens cl br i have been determined simultaneously in irradiated quartz vein samples by noble gas mass spectrometry quartz vein samples were collected from the potassic and propylitic alteration zones of six porphyry copper deposits pcd bingham canyon utah and silverbell ray mission pinto valley and globe miami in arizona in addition analyses of 3he 4he have been obtained from sulphide minerals hosted by the quartz veins at silverbell ray pinto valley and globe miami the majority of pcd fluids studied have br cl and i cl ratios that overlap those of fluids included in mantle diamond suggesting that the salinity had a juvenile origin the high i cl 121 10 6 mole m in samples from the propylitic zone of silverbell is attributed to the presence of sedimentary formation water 3he 4he ratios have r ra values in the range 0 3 to 1 72 and provide evidence for the involvement of a crustal component in addition to mantle volatiles 40ar 36ar ratios vary from meteoric values of 317 in the propylitic zone of bingham canyon and 323 in the skarn alteration of mission up to 3225 in the potassic zone of pinto valley fluids in both the potassic and propylitic alteration zones of every deposit are a mixture of a low salinity end member comprising meteoric water and air and a high salinity end member consisting of a mixed mantle and crustal fluid the 40ar cl ratio of fluid inclusions at pinto valley 10 4 m is similar to values obtained previously for mantle fluids the 40ar cl value is two orders of magnitude lower at bingham canyon where a depleted 36ar concentration 0 2 10 6 cm3 g below that of air saturated water asw and a range of highly fractionated noble gas compositions f84kr 13 and f129xe 160 indicate that boiling and pulsed fluid flow have occurred

strength and rheology of the western u s cordillera effective elastic thickness te depends primarily on temperature composition and state of stress of the lithosphere in this paper we examine high resolution spectral estimates of te and their relationships to regional heat flow age of the lithosphere seismic properties stress orientations and earthquake focal depths of the western u s cordillera the relationship of te to heat flow indicates that ductile flow accommodates long term 106 to 108 years isostatic response at different levels of the crust and upper mantle depending principally on age and by implication bulk composition of the lithosphere isostatic response is primarily controlled by the upper mantle in archean lithosphere of the middle rocky mountains whereas te depends on lower crustal flow in early proterozoic lithosphere of the colorado plateau the yellowstone snake river plain volcanic field and significantly extended regions in the basin range and northern rocky mountains are associated with latest proterozoic aged lithosphere and indicate middle to upper crustal control of long term te we also show that azimuthal variations of te reflect deviatoric stress in the lithosphere te is found empirically to approximate the 95th percentile focal depth of background seismicity the latter relationship is inconsistent with brittle ductile control of focal depth indicating that another rheological transition e g from stick slip to stable sliding frictional behavior is responsible tectonic and structural relationships expand upon the hypothesis that the geographic distribution of tectonic features depends fundamentally on spatial variations in strength of the lithosphere moreover we find a spatial correlation of the intermountain seismic belt to a marked transition in te implying that forces responsible for this active seismic zone are derived from local buoyancy anomalies rather than from current day plate boundary interactions

new light on the age of the white nile landsat 5 thematic mapper and 7 enhanced thematic mapper imagery reveal lake shorelines in the white nile valley as far south as lat 10 n the highest shoreline is at 386 m elevation and was eroded when the white nile formed a lake as wide as 70 km and 500 km long finely laminated green clays laid down on the floor of this lake are overlain by alluvial sands and clays dated by optically stimulated luminescence as 15 ka to older than 250 ka the alluvium was deposited during interglacial episodes of stronger summer monsoons and very high white nile floods the white nile paleolake is much older than marine oxygen isotope stage 7 and may have formed ca 400 ka during the exceptionally long stage 11 interglacial

isotope geochemistry of the northeast zone mount polley alkalic cu au ag porphyry deposit british columbia a case for carbonate assimilation mount polley is a late triassic 205 ma alkalic porphyry cu au ag deposit 226 3 thousand tonnes t cu 21 5 t au and 65 1 t ag hosted by silica undersaturated to silica saturated monzonitic intrusions of the mount polley complex located in british columbia canada the northeast ore zone at mount polley is hosted by magmatic hydrothermal breccia copper and precious metals occur in sulfide minerals primarily as coarse to fine grained breccia cement local wall rocks include equigranular to porphyritic diorite monzodiorite and monzonite alteration breccia cement and veins of the northeast ore zone formed in five paragenetic stages prebreccia stage 1 brecciation and main stage mineralization stage 2 late stage mineralization stage 3 unmineralized postbreccia dikes and veins stage 4 and epithermal style veins stage 5 intense pervasive k and fe metasomatism calcite and calc silicate alteration occurred prior to brecciation caused by the intrusion of megacrystic k feldspar phyric monzonite stage 2 fluids were silica undersaturated high temperature 350 c co2 enriched and of near neutral to alkaline ph potassic sodic and calc potassic assemblages precipitated with mineralization during stage 2 with moderate temperatures at the deposit periphery and in stages 3 and 4 evidence for more acidic and lower temperature conditions is preserved in stage 5 veins the 34ssulfide isotope compositions of stages 2 and 3 chalcopyrite pyrite and bornite range from 7 1 to 1 4 sulfur isotope compositions of anhydrite and gypsum are mostly between 6 2 and 9 8 these values together with the presence of hematite are consistent with deposition from an oxidized sulfate dominant high temperature magmatic hydrothermal fluid limited sulfur isotope geothermometry indicates that cu sulfides precipitated at temperatures from 480 to 250 c hydrothermal calcite occurs in all paragenetic stages at mount polley calcite 13c values range from 0 2 to 10 5 and 18o values from 4 0 to 20 9 the enriched c o isotope values are not consistent with simple precipitation from an entirely magmatic source of hydrothermal fluid interaction of the fluid and or magma with limestone is considered a likely process to explain the c and o isotope signature lead isotope data suggest mixing of mantle and crustal sources during mineralization main stage chalcopyrite and pyrite as well as late stage galena have 206 204pb values of 18 77 to 18 92 207 204pb of 15 56 to 15 59 and 208 204pb of 38 22 to 38 32 strontium isotope data 0 70331 0 70371 provide evidence of a strongly depleted mantle source of sr with minor crustal input epsilon nd values for main stage apatite range between 5 9 and 6 5 also indicating a depleted mantle source stage 5 carbonate 206 204pb values of 18 96 to 19 04 207 204pb of 15 57 to 15 59 and 208 204pb of 38 26 to 38 36 suggest superposition of an epithermal system onto the northeast ore zone potentially as late as 100 m y after breccia formation the data presented are consistent with the hypothesis that the silica undersaturated alkalic mount polley complex formed due to carbonate assimilation prior to mineralization this process can explain the 13c 18o isotope data calcite precipitation concurrent with cu au mineralization and silica undersaturation of the magma the co2 released during assimilation of carbonate also could have promoted magmatic hydrothermal brecciation silica undersaturated alkalic porphyry systems may preferentially form in arc terranes built on a carbonate bearing substrate

introduction writing about twentieth century geology in a classic paper by the late yale historian of science derek de solla price 1965 based mainly on the study of citations in a single scientific research field it was shown how citations in a developing research area have a strong immediacy effect citation was found to be at a maximum for papers about two and a half years old and the major work of a paper is finished after 10 years as judged by citations there were however some classic papers that continue to be cited over long periods of time and review papers specifically discussing the earlier literature there appears to be a need for such review papers after the publication of about thirty to forty research papers in a field and the knowledge is synthesized in book form from time to time de solla price saw citations as the means whereby activities at the research front are linked to what has gone before he wrote e ach group of new papers is knitted to a small select part of the existing scientific literature but connected rather weakly and randomly to a much greater part since only a small part of the earlier literature is knitted together by the new year s crop of papers we may look upon this small part as a sort of growing tip or epidermal layer an active research front he continued the total research front has never been a single row of knitting it is instead divided by dropped stitches into quite small segments and

strike slip faulting at thebes gap missouri and illinois implications for new madrid tectonism numerous nne and ne striking strike slip faults and associated normal faults folds and transtensional grabens occur in the thebes gap area of missouri and illinois these structures developed along the northwestern margin of the buried reelfoot rift of precambrian cambrian age at the northern edge of the mississippi embayment they have had a long lived and complex structural history this is an area of recent moderate seismicity approximately 45 km north of the new madrid seismic zone stratigraphic evidence suggests that these faults were active during the middle ordovician they were subsequently reactivated between the early devonian and late cretaceous probably in response to both the acadian and ouachita orogenies deformation during this period was characterized by strongly faulted and folded ordovician through devonian rocks in places these deformed rocks are overlain with angular unconformity by undeformed cretaceous strata fault motion is interpreted as dominantly strike slip a still younger period of reactivation involved late cretaceous and cenozoic formations as young as the miocene or pliocene mounds gravel these formations have experienced both minor high angle normal faulting and subsequent major right lateral strike slip faulting en echelon north south folds ene striking normal faults regional fracture patterns and drag folds indicate the right lateral motion for this major episode of faulting which predates deposition of quaternary loess several nondefinitive lines of evidence suggest quaternary faulting similar fault orientations and kinematics as well as recent seismicity and proximity clearly suggest a structural relationship between deformation at thebes gap and tectonism associated with the new madrid area

closing the gap between regional and global travel time tomography recent global travel time tomography studies by zhou 1996 and van der hilst et al 1997 have been performed with cell parameterizations of the order of those frequently used in regional tomography studies i e with cell sizes of 1 2 these new global models constitute a considerable improvement over previous results that were obtained with rather coarse parameterizations 5 cells the inferred structures are however of larger scale than is usually obtained in regional models and it is not clear where and if individual cells are actually resolved this study aims at resolving lateral heterogeneity on scales as small as 0 6 in the upper mantle and 1 2 3 in the lower mantle this allows for the adequate mapping of expected small scale structures induced by for example lithosphere subduction deep mantle upwellings and mid ocean ridges there are three major contributions that allow for this advancement first we employ an irregular grid of nonoverlapping cells adapted to the heterogeneous sampling of the earth s mantle by seismic waves spakman and bijwaard 1998 second we exploit the global data set of engdahl et al 1998 which is a reprocessed version of the global data set of the international seismological centre their reprocessing included hypocenter redetermination and phase reidentification finally we combine all data used p pp and pwp phases into nearly 5 million ray bundles with a limited spatial extent such that averaging over large mantle volumes is prevented while the signal to noise ratio is improved in the approximate solution of the huge inverse problem we obtain a variance reduction of 57 1 synthetic sensitivity tests indicate horizontal resolution on the scale of the smallest cells 0 6 or 1 2 in the shallow parts of subduction zones decreasing to approximately 2 3 resolution in well sampled regions in the lower mantle vertical resolution can be worse up to several hundreds of kilometers in subduction zones with rays predominantly pointing along dip important features of the solution are as follows 100 200 km thick high velocity slabs beneath all major subduction zones sometimes flattening in the transition zone and sometimes directly penetrating into the lower mantle large high velocity anomalies in the lower mantle that have been attributed to subduction of the tethys ocean and the farallon plate and low velocity anomalies continuing across the 660 km discontinuity to hotspots at the surface under iceland east africa the canary islands yellowstone and the society islands our findings corroborate that the 660 km boundary may resist but not prevent present day large scale mass transfer from upper to lower mantle or vice versa this observation confirms the results of previous global mantle studies that employed coarser parameterizations

diatom based transfer functions for inferring past hydrochemical characteristics of african lakes abstract a new dataset of 282 modern diatom samples and associated environmental information has been created by merging existing regional datasets from north and east africa and niger the relationships between diatom species distributions and hydrochemistry are examined using canonical correspondence analysis cca and partial cca variables reflecting water conductivity ph and cation and anion composition account for significant and independent components of the total variation in the diatom data predictive models transfer functions are developed using the method of weighted averaging for conductivity r 2 0 87 ph r 2 0 77 and ratios between alkali and alkaline earth metals r 2 0 81 and carbonate bicarbonate and sulphate chloride ions r 2 0 82 prediction errors are estimated using the computer intensive method of jackknifing these transfer functions enlarge the potential domain for reconstruction of past hydrochemistry from fossil diatoms preserved in lake sediments

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

fluid inclusion and noble gas studies of the dongping gold deposit hebei province china a mantle connection for mineralization the dongping gold deposit 100 t au occurs about 200 km inboard of the northern margin of the north china craton the deposit is mainly hosted by syenite of a middle paleozoic alkalic intrusive complex that was emplaced into late archean basement rocks both groups of rocks are intruded by late jurassic to early cretaceous crustal melt granite dikes and stocks some within a few kilometers of the deposit the gold ores were deposited during this latter magmatic period at about 150 ma a time that was characterized by widespread regional north south compression that formed the east west trending yanshan deformational belt the ores include both the telluride mineral bearing low sulfide quartz veins and the highly k feldspar altered syenite with most of the resource concentrated in two orebodies 1 and 70 fluid inclusion microthermometry indicates heterogeneous trapping of low salinity e g 5 7 wt nacl equiv fluids that varied from a few to 60 mole percent nonaqueous volatile species laser raman spectroscopy confirms that the vapor phase in these inclusions is dominated by co2 but may be comprised of as much as 9 mole percent h2s and 20 mole percent n2 methane concentrations in the vapor phase are consistently 1 mole percent the variable phase ratios are consistent with fluid immiscibility during ore formation fluid inclusion trapping conditions are estimated to be 250 to 375 c and 0 6 to 1 0 kbar helium isotope studies of fluid inclusions in ore stage pyrites indicate 3he 4he ratios of 2 1 to 5 2 ra ra 1 4 10 6 for air for orebody 1 and 0 3 to 0 8 ra for orebody 70 the former data suggest that at least 26 to 65 percent mantle helium occurs in the fluids that deposited the veins in orebody 1 the lower values for orebody 70 which is characterized by a more disseminated style of gold mineralization are interpreted to reflect an increased interaction of ore fluids with surrounding crustal rocks which may have contributed additional 4he to the fluids a mantle source for at least some of the components of the gold forming fluid is consistent with upwelling of hot asthenosphere and erosion of as much as 100 to 150 km of cool archean lithosphere beneath the craton during this time the dongping deposit is located along the 100 km wide north south gravity lineament which marks the western border of the thinned crust as both regional metamorphism of mesoproterozoic and younger cover rocks and widespread granite magmatism also occurred at ca 150 ma it is unclear as to whether one or both of these also contributed fluid and or metals to the hydrothermal system importantly these new data suggest that economically significant gold deposits of similar mineral style and fluid composition which are scattered along the margins of the craton may all be products of a fluid originally partly sourced within the mantle

midcontinent u s fault and fold zones a legacy of proterozoic intracratonic extensional tectonism the u s continental interior midcontinent contains numerous fault and fold zones seismic and drilling data indicate that some of these zones first formed as proterozoic eocambrian rift faults but the origin of most remains enigmatic we propose that the enigmatic fault and fold zones also began as proterozoic eocambrian normal faults we base our hypothesis on the following 1 enigmatic zones parallel known rifts 2 the structural style of enigmatic zones mirrors the structural style of known rifts 3 the map pattern of some enigmatic zones e g the la salle deformation belt of illinois resembles the map pattern of contemporary rifts and 4 it is easier to rupture an intact craton by normal faulting than by reverse or strike slip faulting these zones along with known rifts represent the legacy of widespread extensional tectonism that brittlely broke up the craton into fault bounded blocks prior to deposition of phanerozoic platform cover once formed midcontinent fault and fold zones remained weak allowing cratonic blocks to jostle relative to one another during the phanerozoic thereby inverting faults and creating transpressional or transtensional structural assemblages localizing seismicity and channeling or releasing ore generating fluids

u th he thermochronology of metallic ore deposits in the liaodong peninsula implications for orefield evolution in northeast china abstract the liaodong peninsula is an important region where polymetallic ores are mined in northeast china but precise thermochronological data needed to effectively constrain the evolution of this orefield are presently lacking for this study fifteen samples of granitoid rocks were collected from the qingchengzi and wulong orefields located in the liaodong peninsula to undertake u th he study of zircon and apatite single grain zhe and ahe ages from the qingchengzi orefield range from 73 to 166 ma and from 22 to 48 ma respectively whereas single grain zhe and ahe ages from the wulong orefield range from 42 to 88 ma and from 21 to 56 ma respectively some samples yield dispersed ages comprehensive analyses indicate that the main causes of the dispersion in the zhe ages are heterogeneous distribution of u and th within the crystals radiation damage and cooling rates whereas the dispersion in the ahe ages is primarily due to the first two factors based on the helium ages we calculate the average exhumation and cooling rates of the two orefields during different periods and the associated denudation which are 4 3 km for the qingchengzi orefield and 4 7 km for the wulong orefield the mineralization depths are estimated using homogenization temperature and the salinity of fluid inclusions and are calculated to be 1 5 3 2 km and 1 0 2 2 km for the qingchengzi pb zn and au ag deposits respectively and 2 1 3 1 km for the wulong au deposit these results are much less than the total denudation suggesting that the orebodies have been heavily eroded in conjunction with the distribution of the helium ages the modeled low t thermal histories of the two orefields reveal that the liaodong peninsula experienced three stages of relatively rapid cooling during the early cretaceous ca 135 115 ma from the end of the late cretaceous to the early eocene ca 70 50 ma and from the late eocene to the oligocene ca 38 26 ma the last stage of cooling affected both orefields and represents a period of relatively intense tectonic activity finally these results are compared with those of previous studies from the adjacent jiaodong peninsula this discussion shows that the cooling experienced by the orefields in the liaodong peninsula is largely consistent with that experienced by the jiaodong peninsula these data provide valuable information for further studies of the evolution of orefields in northeast china

exposed cross sections through the continental crust implications for crustal structure petrology and evolution abstract the continental crust is exposed in cross section at numerous sites on the earth s surface these exposures which appear to have formed by obduction along great faults during continental collision may be recognized by exposures of deep crustal rocks exhibiting asymmetric patterns of metamorphic grade and age across the faults and by distinctive bouguer anomaly patterns reflecting dipping basement structure and an anomalously deep mantle from an examination of five complexes which meet these criteria it is concluded that the most prominent layering in the crust is not compositional but metamorphic the lower crust consists of granulite facies rocks of mafic to intermediate composition while the intermediate and shallow levels consist predominantly of amphibolite facies gneisses and greenschist facies supracrustal rocks respectively post metamorphic granitic intrusions are common at intermediate to shallow levels position of discontinuities in refraction velocity where present commonly correspond to changes in composition or metamorphic grade with depth the continental crust is characterized by lateral and vertical heterogeneities of varying scale which are the apparent cause of the complex seismic reflections recorded by cocorp field observations coupled with geochemical data indicate a complex evolution of the lower crust which can include anatexis multiple deformation polymetamorphism and reworking of older crustal material the complexity of the crust is thus the result of continuous evolution by recycling and metamorphism through time in a variety of tectonic environments

helium isotopes as tracers of trapped hydrothermal fluids in ocean floor sulfides helium isotopes have been measured in fluids trapped in minerals from three mid ocean ridge hydrothermal systems at bare rock sites and at the sediment covered site at middle valley northern juan de fuca ridge fluid inclusion 3 he 4 he ratios from the bare rock sites are indistinguishable from those of the vent fluids and are close to the local basalt values values for 34 s of the host sulfide are consistent with variable contributions of seawater sulfate s to the basalt derived s in the hydrothermal fluids this does not affect inclusion hosted 3 he 4 he because of the high he concentration of the hydrothermal fluids relative to seawater fluid inclusion 3 he 4 he ratios from middle valley are a mixture of basalt derived he and radiogenic he acquired from pore fluids in the overlying pleistocene turbidite sediments after seawater basalt interaction the radiogenic he accompanies isotopicalty heavy s demonstrating that he isotopes may be used to distinguish pore fluids from seawater in hydrothermal systems

age and isotopic constraints on pleistocene pluvial episodes in the western desert egypt abstract north africa has undergone drastic climatic changes over the past several hundred thousand years the timing of humid intervals called pluvials was investigated by uranium series disequilibrium dating of travertines from the kurkur oasis western desert egypt the youngest and best dated travertines 70 160 ka are found in wadi kurkur and include spring and lacustrine units exposed as 2 to 3 m high terraces travertines having an age of approximately 191 220 ka are exposed by differential erosion as linear mounds produced by spring systems over fracture zones in ancient wadis the oldest travertines having ages 260 ka are extensive cap limestone units above the oasis and were deposited in paludal and lacustrine environments oxygen isotope ratios were measured for the wadi travertines 18o values ranging from 16 7 to 19 1 smow and for spring mound travertines 18 5 20 5 equilibrium oxygen isotope fractionation calculations indicate that the kurkur travertines were deposited from waters having 18o values similar to ancient western desert groundwaters 11 sonntag et al 1978a b thorweithe 1990 sultan et al 1996 the ages of the travertines correspond to times of monsoonal maxima eustatic sea level high stands and interglacial maxima rainfall producing these groundwaters and travertines was significantly fractionated during atmospheric transport in contrast to modern meteoric waters 2 09 joseph et al 1992 implying a distant source for the pluvial waters increased precipitation recharge of western desert groundwaters and resultant travertine deposition are interpreted to be consequences of milankovitch insolation cycle forcing through enhanced atlantic and indian ocean monsoons during interglacial time periods

the daduhe gold field at the eastern margin of the tibetan plateau he ar s o and h isotopic data and their metallogenic implications the daduhe gold field comprises several shear zone controlled tertiary lode gold deposits distributed at the eastern margin of the tibetan plateau the deposits are hosted in a precambrian granite greenstone terrane within the yangtze craton the gold mineralization occurs mainly as auriferous quartz veins with minor sulphide minerals fluid inclusions in pyrite have 3 he 4 he ratios of 0 16 to 0 86 ra whereas their 40 ar 36 ar ratios range from 298 to 3288 indicating a mixing of fluids of mantle and crust origins the 34 s values of pyrite are of 0 7 4 2 n 12 suggesting a mantle source or leaching from the mafic country rocks 18 o values calculated from hydrothermal quartz are between 1 5 and 6 0 and d values of the fluids in the fluid inclusions in quartz are 39 and 108 these ranges demonstrate a mixing of magmatic metamorphic and meteoric fluids the noble gas isotopic data along with the stable isotopic data suggest that the ore forming fluids have a dominantly crustal source with a significant mantle component

dehydration melting of a basaltic composition amphibolite at 1 5 and 2 0 gpa implications for the origin of adakites this study presents the results of dehydration melting experiments on a basaltic composition amphibolite under conditions appropriate to a hot slab geotherm 1 5 and 2 0 gpa and temperatures of 850 to 1150 c dehydration melting produces an omphacitic augite and garnet bearing residue coexisting with rhyolitic to andesitic composition melts at 1 5 gpa the amphibolite melts in two stages between 800 and 1025 c the 2 0 gpa data also define two melting stages at 2 0 gpa the first stage involves nearly modal melting of the original amphibolite minerals qtz pl amp to produce melt cpx grt during the second stage the eclogite restite melts non modally 0 86 cpx 0 14 grt 1 melt the experimental results were combined with data from the literature to generate a composite p t phase diagram for basaltic composition amphibolites over the 800 to 1100 c temperature range for pressures up to 2 0 gpa comparison of the major element compositions of the experimentally produced melts with compositions of presumed slab melts adakites shows that partial melting of amphibolite at conditions appropriate to a hot slab geotherm produces melts similar to andesitic and dacitic adakites except for significant mgo and cao depletions trace element modelling of amphibolite dehydration melting using the 2 0 gpa melting reactions produces ree abundances similar to those of adakites at 10 15 wt batch melting but the models do not reproduce the high sr y ratios characteristic of adakites taken together the major and trace element results are not consistent with the derivation of adakites by dehydration melting of the subducted slab with little or no interaction with the mantle wedge or crust if adakites are partial melts of the subducted slab they must undergo significant interaction with the mantle and or crust during which they acquire a number of their distinctive characteristics

petrogenesis of late triassic granitoids and their enclaves with implications for post collisional lithospheric thinning of the liaodong peninsula north china craton abstract major and trace element sr nd and hf isotope zircon u pb and hf isotope data are reported for mafic microgranular enclaves and host granitoids from the xiuyan pluton in the liaodong peninsula of the eastern north china craton in order to investigate their sources petrogenesis and tectonic implications the zircon u pb age of the enclaves 211 1 ma is identical to that of the host granite 210 1 ma establishing that the mafic and felsic magmas were coeval the enclaves have high mgo concentrations at low silica contents and positive ehf t values they are enriched in large ion lithophile and light rare earth elements and depleted in high field strength elements which can be explained as melts derived from a depleted asthenospheric mantle with some crustal contamination the host granitoids have high sio2 contents variable initial 87sr 86sr ratios and highly negative end t and ehf t values suggesting they were mainly derived from an ancient crustal source heated by the input of asthenospheric mantle derived magmas field observations geochronology geochemistry sr and nd isotopic and zircon hf isotopic compositions point to a complex petrogenesis where asthenospheric mantle and crust derived magma mixing was coupled with crystal fractionation thus explaining the genetic link between enclaves and host rocks identification of an asthenospheric mantle component in the xiuyan granitoids suggests asthenospheric mantle upwelling and decompressional melting of the mantle indicating that post collisional lithospheric thinning in the liaodong peninsula was initiated in the late triassic

mantle crustal and atmospheric noble gases in ailaoshan gold deposits yunnan province china abstract helium and argon isotopic and elemental compositions of fluids released by crushing pyrite grains are reported from the ailaoshan gold province china the province is a series of gold deposits hosted on a 200 km segment of a major normal fault formed during eocene extension the deposits have been well characterised using conventional geochemical and microthermometric techniques the results of which are consistent with a predominantly high temperature magmatic fluid present in the ore minerals in contrast analyses of noble gases from three gold deposits demonstrate the palaeofluids were a mixture between a mantle derived magmatic fluid and two different low temperature fluids similar to modern groundwaters the specific isotopic and abundance characteristics of helium and argon allow unequivocal identification of both a mantle derived component and a surface derived component to these fluids the magmatic fluid dominates deposits from the northern end of the fault the second groundwater is present only in deposits in the south of the province by assuming fluid inclusion homogenisation temperatures in coexisting quartz are representative of fluid temperature 3he heat ratios can be estimated the 3he heat ratios vary systematically along the fault from 30 10 12 cm3 stp j 1 in the northern deposits to

a possible dependence of tectonic strength on the age of the crust in asia in asia the intensity and style of active tectonics appears to depend on the age of the last orogenic activity the shields have remained essentially undeformed during the india eurasian collision but paleozoic and mesozoic orogenic belts apparently have been reactivated by this collision with a suggestion of greater reactivation and more diffuse deformation of the younger belts if the greater observed heat flow in regions of more recent orogenic activity reflects at least in part a greater heat flow from the mantle beneath the younger belts then the temperatures in the mantle beneath the younger belts should be higher than beneath older belts and shields because of the strong dependence on temperature of the creep strength of minerals particularly of olivine the crust and mantle beneath the hotter younger belts should be much weaker than those beneath older belts this difference in temperature and consequent difference in strength may be the cause of the greater reactivation of younger belts

a pleistocene lacustrine episode in southeastern libya fossiliferous lacustrine deposits of middle pleistocene age have been found in central fezzan libya 1 on each side of the east west depressions of the wadi shati south of the precambrian qarqaf anticline1 8 they represent the shoreline of a former brackish water lake fig 1 which some 130 000 yr bp covered an area about 100km long and 10 20 km wide at present the area may be classified as hyperarid since 1978 a collaborative study of the fossil assemblages has been undertaken it included macro and micropalaeontological determination th u dating stable isotope and chemical analyses in parallel with sedimentological and archaeological studies of the sites here we consider the th u dating and the isotopic palaeohydrology of the lacustrine episode

noble gases in pyrites from the guocheng liaoshang gold belt in the jiaodong province evidence for a mantle source of gold abstract the recent mineral exploration programme 2009 2014 in the jiaodong gold province identified the new guocheng liaoshang gold metallogenic belt which contains reserves of 92 tonnes t au in the main deposits the ore is fault hosted in massive sulfides that make up to 25 to 95 vol the he ar isotope compositions of ore forming fluids from 20 pyrite samples from the five main deposits are reported here 3 he 4 he range between 0 41 and 2 39 r a 40 ar 36 ar are 367 to 2112 and 40 ar 4 he are 0 40 3 78 the data require four gas sources a dominant mantle derived component plus sub ordinate crustal radiogenic meteoric and basin brine components the mantle end member has 3 he 4 he 3 32 4 00 r a this is lower than most estimates for sub continental lithospheric mantle sclm 6 7 r a implying that it was probably refertilized by subduction related fluid metasomatism this is consistent with he ar isotopes reported for sclm xenoliths from basalts in the shandong province within the mineralisation province the mine au reserve is positively correlated with the proportion of mantle derived he in the ore forming fluids this implies that the fluids and by inference the gold was largely derived from mantle melts during lithospheric extension given the conspicuous association of gold ore deposits with mantle derived magma in the jiaodong peninsula we envisage that our genetic model can be applied to some extent to evaluate the potential of some mineral exploration targets

petrogenesis of late triassic intrusive rocks in the northern liaodong peninsula related to decratonization of the north china craton zircon u pb age and hf o isotope evidence abstract major and trace element whole rock sr nd and hf isotopes and zircon u pb age and hf o isotope data have been determined for mafic to felsic intrusive rocks from the late triassic mayihe myh longtou chaxinzi xiaoweishahe lcx and nankouqian xidadingzi nx intrusions in the northern liaodong peninsula ne china in order to investigate their sources and petrogenesis related to decrationization of the north china craton the rocks include clinopyroxene diorite monzodiorite quartz diorite granodiorite monzogranite and mafic microgranular enclaves laser ablation inductively coupled plasma mass spectrometry la icpms and secondary ion mass spectrometer sims u pb analyses of zircon from myh lxc to nx intrusions yield late triassic ages of 224 218 ma establishing that the mafic and felsic magmas were coeval the clinopyroxene diorites from the myh pluton have high mgo concentrations at low silica contents with positive whole rock e nd t up to 3 8 and e hf t up to 6 2 and zircon e hf t values and some mantle like 18 o values they are enriched in large ion lithophile liles and light rare earth elements lrees and depleted in high field strength elements hfses which can be explained as melts derived from a depleted mantle with some subsequent crustal contamination however the diorites from the lxc and nx intrusions are high k rocks and have high mgo concentrations and mantle like 18 o values but with negative whole rock e nd t 12 9 to 7 6 and e hf t 14 0 to 5 6 and zircon e hf t values distinct from those of myh pyroxene diorites indicating that their parental magmas were derived from partial melting of an ancient lithospheric mantle the granitoids from three plutons have high sio 2 contents and low mgo concentrations suggesting they were mainly derived from crustal sources however the myh monzogranite and granodiorite have positive whole rock e nd t and e hf t up to 3 0 and zircon e hf t values indicating a juvenile crustal source whereas the lxc and nx granitoids have strong negative and variable whole rock e nd t and e hf t and zircon e hf t values indicating that they were derived from partial melting of ancient lower crustal materials with involvement of mantle components field observations geochronology geochemistry sr nd hf isotopic and zircon hf o isotopic compositions point to a complex petrogenesis where mantle and crust derived magma mixing was coupled with crystal fractionation thus explaining the genetic link between mafic and felsic rocks identification of four components i e depleted and enriched mantle and juvenile and ancient crust in the late triassic magmatism in the northern liaodong peninsula suggests a strong mantle crust interaction process related to decratonization of the north china craton

resolving mantle and crustal contributions to ancient hydrothermal fluids hear isotopes in fluid inclusions from dae hwa wmo mineralisation south korea abstract helium and argon isotopes from fluid inclusions in individual colour zones b c d and e of a large scheelite crystal from the 88 ma dae hwa w mo deposit south korea trace the source and history of the ore fluids a gradual decrease of the fluid 3 he 4 he 3 he 36 ar and 40 ar 36 ar from the core to the edge of the scheelite reflects the progressive dilution of a magmatic fluid by meteoric water and is consistent with the previously observed decrease of 18oh2o and fluid inclusion homogenisation temperatures th shelton et al 1987 the covariation of fluid inclusion he ar isotope systematics with 18o and th defines a magmatic component with 3 he 4 he 1 2 10 6 3 he 36 ar 0 01 and 40 ar 36 ar 1000 anomalously high helium and argon isotope ratios in zone d fluids represents undiluted magmatic noble gases this may reflect local variation in the magmatic gas flux or gas loss due to boiling of the hydrothermal fluids prior to mixing with magmatic gas helium and argon isotope systematics constrain mantle and crustal components in the hydrothermal fluids 40 ar 3 he 4 5 104 are close to the mid ocean ridge basalt value implying that 40ar is mantle in origin radiogenic isotope ratios of the mantle endmember 40 ar 4 he 0 69 0 06 are similar to contemporary geothermal fluids the coincidence of mantle derived he and ar in the fluids is strong evidence that mantle melting during late cretaceous subduction triggered the crystal melting responsible for granite formation 40 ar 4 he of the meteoric fluid 0 007 0 001 is far lower than the crystal production ratio 0 2 implying an origin in crust below 200 c

genesis of the wulong gold deposit northeastern north china craton constraints from fluid inclusions h o s pb isotopes and pyrite trace element concentrations abstract the wulong vein type gold deposit located along the northeastern margin of the north china craton ncc contains 80 tonnes of gold at an average grade of 5 35 g t a total of 380 auriferous quartz veins are hosted in late jurassic gneissic two mica granite and early cretaceous granodiorite a paragenetic sequence representing three hydrothermal stages has been identified 1 quartz pyrite 2 quartz polymetallic sulfide and 3 quartz calcite pyrite petrography fluid inclusion microthermometry and laser raman spectroscopy revealed three types of fluid inclusions co2 ch4 type i h2o co2 ch4 type ii and h2o nacl type iii fluid inclusion data indicate that during mineralization fluid temperatures evolved from 283 to 395 c to 219 328 c to 144 255 c and salinities varied from 0 70 to 8 95 wt to 0 18 7 86 wt to 0 18 4 96 wt nacl equivalent quartz yielded 18oh2o smow and dsmow values of 0 9 7 3 and 65 to 48 respectively indicating that the ore forming fluids were derived from magmatic water and mixed with meteoric water during the later stages of mineralization sulfides have 34scdt sulfur isotopic compositions ranging from 0 9 to 3 9 suggesting that the s was derived from a magmatic source pb isotopic compositions of sulfides 206pb 204pb 17 488 17 718 207pb 204pb 15 565 15 631 and 208pb 204pb 38 190 38 766 indicate that the pb was derived primarily from the upper crust with minor input from the mantle in situ laser ablation inductively coupled plasma mass spectrometry la icp ms trace element analyses of three generations of pyrite py1 py2 and py3 reveal that gold concentrations in py2 are higher than those in py1 and py3 which indicates that gold mineralization occurred primarily during the middle stage high co ni ratios of pyrite are consistent with a hydrothermal origin we suggest that the wulong gold deposit formed during lithospheric thinning associated with destruction of the ncc this process was triggered by subduction of the paleo pacific plate during the early cretaceous

new evidence for a reduced water balance in east africa during the last glacial maximum implication for model data comparison some syntheses of lake level data for the last glacial maximum lgm in east africa 10 n and 30 s east of 25 e show apparently wetter conditions than present for some basins whereas palaeovegetation reconstruction indicates a generally dry climate pmip gcm simulations for the lgm support both scenarios for this region when run under different boundary conditions here we compare three new records from lakes in the data poor southern part of east africa lake malawi lake massoko and lake rukwa we also re assess previously published lake level data and apply a salinity transfer function to the diatom record from lake manyara our results show that in contrast to previous interpretations these lakes were at least as low as today at the lgm and are thus in agreement with the palaeovegetation data relative drought across east africa is best simulated by gcms that use computed ssts rather than the higher climap values lower ssts and the presence of the northern hemisphere ice sheets must have been dominant over any monsoon precipitation rise caused by astronomically induced summer insolation enhancement in the southern african tropics

a review of the geodynamic setting of large scale late mesozoic gold mineralization in the north china craton an association with lithospheric thinning abstract abundant gold deposits are distributed along the margins of the north china craton ncc occurring throughout the precambrian basement and located in or proximal to mesozoic granitoids these deposits show a consistent spatial temporal association with late jurassic early cretaceous magmatism and are characterized by quartz lode or disseminated styles of mineralization with extensive alteration of wall rock their ages are mainly early cretaceous 130 110 ma and constrain a very short period of metallogenesis sr nd pb isotopic tracers of ores minerals and associated rocks indicate that gold and associated metals mainly were derived from multi sources i e the wall rocks precambrian basement and mesozoic granites and associated mafic rocks previous studies including high surface heat flow uplift and later basin development slow seismic wave speeds in the upper mantle and a change in the character of mantle xenoliths sampled by paleozoic to cenozoic magmas have been used to suggest that ancient cratonic mantle lithosphere was removed from the base of the ncc some time after the ordovician and replaced by younger less refractory lithospheric mantle the geochemistry and isotopic compositions of the mafic rocks associated with gold mineralization 130 110 ma indicate that they were derived from an ancient enriched lithospheric mantle source whereas the mafic dikes and volcanic rocks younger than 110 ma were derived from a relatively depleted mantle source i e asthenospheric mantle according to their age and sources relation to magmatism and geodynamic framework the gold deposits were formed during lithospheric thinning the removal of lithospheric mantle and the upwelling of new asthenospheric mantle induced partial melting and dehydration of the lithospheric mantle and lower crust due to an increase of temperature the fluids derived from the lower crust were mixed with magmatic and meteoric waters and resulted in the deposition of gold and associated metals

contribution of metapelitic sediments to the composition heat production and seismic velocity of the lower crust of southern new mexico u s a abstract granulite xenoliths erupted at kilbourne hole maar were recently extracted from the lower crust of southern new mexico garnet and sillimanite bearing quartzofeldspathic xenoliths had pelitic protoliths and were probably emplaced in the lower crust by tectonic underplating at a lower proterozoic subduction zone thus the kilbourne hole metapelitic xenoliths illustrate the potential role of tectonosedimentary processes at convergent margins in determining the ultimate composition of the crust average p wave velocities for metapelitic xenoliths from kilbourne hole are 7 km s at 6 kbar like those of mafic metagabbros and anorthosites however in contrast to mafic lithologies the major element composition of the representative pelitic paragneiss rpp described in this paper is relatively siliceous and like that of average upper crust except for depletions of u and cs the trace element characteristics of the rpp are like those of pelitic sediments and are 3 10 times higher than those typically estimated for the lower crust the heat production of the rpp is high 1 0 w m3 as are those of many granulite and amphibolite grade metapelites in general portions of the lower crust in which sediments are present may be high in light ion lithophile and rare earth element abundances heat production 18o and87sr 86sr moreover the high pb contents and unradiogenic pb isotope signatures of metapelites provide an important reservoir for unradiogenic pb in the earth as a whole

noble gases fingerprint a metasedimentary fluid source in the macraes orogenic gold deposit new zealand the world class macraes orogenic gold deposit 10 moz resource formed during the late metamorphic uplift of a metasedimentary schist belt in southern new zealand mineralising fluids metals and metalloids were derived from within the metasedimentary host helium and argon extracted from fluid inclusions in sulphide mineral grains three crush extractions from one sample have crustal signatures with no evidence for mantle input r ra 0 03 xenon extracted from mineralised quartz samples provides evidence for extensive interaction between fluid and maturing organic material within the metasedimentary host rocks with 132xe 36ar ratios up to 200 times greater than air similarly i cl ratios for fluids extracted from mineralised quartz are similar to those of brines from marine sediments that have interacted with organic matter and are ten times higher than typical magmatic mantle fluids the macraes mineralising fluids were compositionally variable reflecting either mixing of two different crustal fluids in the metasedimentary pile or a single fluid type that has had varying degrees of interaction with the host metasediments evidence for additional input of meteoric water is equivocal but minor meteoric incursion cannot be discounted the macraes deposit formed in a metasedimentary belt without associated coeval magmatism and therefore represents a purely crustal metamorphogenic end member in a spectrum of orogenic hydrothermal processes that can include magmatic and or mantle fluid input elsewhere in the world there is no evidence for involvement of minor intercalated metabasic rocks in the macraes mineralising system hydrothermal fluids that formed other smaller orogenic deposits in the same metamorphic belt have less pronounced noble gas and halogen evidence for crustal fluid rock interaction than at macraes but these deposits also formed from broadly similar metamorphogenic processes

paleohydrological indicators in playas and salt lakes with examples from canada australia and africa abstract the sedimentary records of playas and salt lakes provide some of the best evidence for hydrological change in a region these records are comprised of siliciclastic sediments and chemically precipitated minerals that typically vary temporally and spatially in some cases the interpretation is straightforward with mineralogical sequences related to changing brine concentrations and hydrology as well as to climate in other cases it is difficult to identify the controlling factors which include climate groundwater geological setting and basin morphology in addition diagenesis commonly affects the sedimentary record models of lake level fluctuations are given for basins in the canadian prairies and southeastern australia the lake manitoba model is based mainly on secondary changes associated with pedogenesis that alters the deeper water sediment during low water stages smaller canadian lakes in more arid regions display a larger variety of diagnostic paleohydrological indicators such as specific carbonate and salt mineralogy nature of bedding grain size and secondary alterations decreasing water levels in shallow lakes tend to show a progression from clastics to carbonates to salts and then back to clastics as groundwater levels fall below the lake floor similar parameters are used in australian lakes the hydrological model for the lake tyrrell playa in victoria whose present ionic composition is similar to seawater shows clastics being deposited when average lake levels are high followed by deposition of primary carbonates gypsum and halite once seasonal drying on the lake floor begins secondary gypsum grows within already deposited sediment and clay pelletization in the capillary fringe of surface sediment leads to basin deflation and the accumulation of adjacent eolian lunettes continued water level decline leads to pedogenesis subsequent lake level rises in all of these models may initiate sedimentation in any part of the hydrological cycle and a complete cycle in the sedimentary record appears to be uncommon

orogenic gold common or evolving fluid and metal sources through time abstract orogenic gold deposits of all ages from paleoarchean to tertiary show consistency in chemical composition they are the products of aqueous carbonic fluids with typically 5 20 mol co 2 although unmixing during extreme pressure fluctuation can lead to entrapment of much more co 2 rich fluid inclusions in some cases ore fluids are typically characterized by significant concentrations of ch 4 and or n 2 common estimates of 0 01 0 36 mol h 2 s a near neutral ph of 5 5 and salinities of 3 7 wt nacl equiv with na k ca mg this fluid composition consistency favors an ore fluid produced from a single source area and rules out mixing of fluids from multiple sources as significant in orogenic gold formation nevertheless there are broad ranges in more robust fluid inclusion trapping temperatures and pressures between deposits that support a model where this specific fluid may deposit ore over a broad window of upper to middle crustal depths much of the reported isotopic and noble gas data is inconsistent between deposits leading to the common equivocal interpretations from studies that have attempted to define fluid and metal source areas for various orogenic gold provinces fluid stable isotope values are commonly characterized by the following ranges 1 18 o for precambrian ores of 6 to 11 and for phanerozoic ores of 7 to 13 2 d and 34 s values that are extremely variable 3 13 c values that range from 11 to 2 and 4 15 n of 10 to 24 for the neoarchean 6 5 to 12 for the paleoproterozoic and 1 5 to 10 for the phanerozoic secular variations in large scale earth processes appear to best explain some of the broad ranges in the o s and n data fluid rock interaction particularly in ore trap areas may cause important local shifts in the o s and c ratios the extreme variations in d mainly reflect measurements of hydrogen isotopes by bulk extraction of waters from numerous fluid inclusion generations many which are not related to ore formation radiogenic isotopes such as those of pb sr nd sm and os measured on hydrothermal minerals are even more difficult to interpret for defining metal source particularly as the low salinity ore fluids transport limited amounts of these elements and significant amounts of these may be locally added to the minerals during alteration reactions at the sites of gold deposition noble gas and halogen data are equally equivocal fluid exsolution from granitoids emplaced into the upper and middle crust metamorphism of the crust or fluids entering trans crustal fault zones from below the crust all remain as permissive scenarios associated with orogenic gold formation as the abundant geochemical data are equivocal however geological and geochronological data weigh heavily against a magmatic hydrothermal model in the upper to middle crust there is no universal temporal association between orogenic gold and magmatism and where there is an overlap in age there is no specific type of magmatism consistently associated with gold formation nor element zonation around any specific pluton a crustal metamorphic model for fluid and metal sources is very consistent with geological geochronological and geochemical data although metamorphism on a regional scale that releases these components into major fault zones can be associated with many processes along active continental margins these can include crustal thickening and radiogenic heating slab rollback and heating during crustal extension or subduction of a spreading ridge heating the base of an accretionary prism in rare examples where phanerozoic orogenic gold deposits are hosted in precambrian high grade metamorphic terranes fluids and metals must however enter a transcrustal fault system from a sub crustal source this could either be a devolatilized subducted relatively flat perhaps stalled slab and its overlying sediment or the corner of the fertilized mantle wedge that releases a fluid during a thermal event

noble gas isotopes in corundum and peridotite xenoliths from the eastern north china craton implication for comprehensive refertilization of lithospheric mantle abstract noble gas isotopic ratios provide unique and important constraints for the evolution of the subcontinental mantle we present one step crushing noble gas isotope data from corundum and co existing mantle xenoliths and megacrystals from the changle volcanic fields in the middle part of the tan lu fault zone eastern north china craton the 3he 4he ratio is 7 6 8 3 times the atmospheric ratio ra in corundum indicating contributions of fluids from convective asthenosphere the 3he 4he ratio in titanomagnetite megacrystals ranges from 5 8 to 6 5 ra suggesting possible u th contamination in the crystal lattice with the 3he 4he ratio of host magma being higher than 6 5 ra the olivine and opx in lherzolite show high helium contents and homogeneous isotopic helium ratios 6 9 7 0 ra the cpx in wehrlite show significantly higher helium contents and slightly higher helium ratio 7 6 ra than co existing olivine 7 0 7 5 ra combined with isotope correlation diagrams the systematic difference in 4he abundance the 3he 4he and 4he 40ar ratios of cpx opx and olivine in pyroxenite lherzolite and wehrlite suggest comprehensive refertilization of lithospheric mantle in the eastern north china craton

a local weakening of the brittle ductile transition can explain some intraplate seismic zones abstract a decrease in the strength of a localized area of the lower crust would decrease the depth to the brittle ductile transition and concentrate stress in the stronger elastic crust around and above the zone of decreased strength deformation within the zone of weakened lower crust would occur through viscous or dislocation creep in response to regional plate stress as well as by elastic failure in small earthquakes two dimensional finite element models of zones of weakness subjected to a regional plate stress predict stress amplification of 10 100 surrounding the local decrease in strength a finite element model for an irregular zone of weakness in the lower crust can generate stress directions and relative magnitudes that satisfy the focal mechanisms and seismicity of southeastern tennessee an analysis of the displacements and stresses in the central area of decreased crustal strength suggests that strike slip faulting should dominate above this zone the strike slip faulting should exhibit a strong thrust component the compression and extension of the crust surrounding the weakened zone at the level of the brittle ductile transition predicts that the dominant strike slip faulting should exhibit components of normal faulting on the edges of the weak zone which are near a line through the weak zone parallel to the regional stress and reverse faulting on the edges which are near a line transverse to the regional stress the seismicity of southeastern tennessee is diffused over a narrow elliptical zone trending ne the greatest concentration of activity is near the center the central zone is characterized by deep focus strike slip events with predominantly n or e striking nodal planes these events are responding directly to compression in the direction of the regional compressive stress and extension perpendicular to the regional compressive stress within the weak zone the area surrounding the central zone is characterized by focal mechanisms with larger components of reverse or normal fault movements events with normal components are dominant on the edges in line with the regional stress as predicted by the stress model for a weak central zone under stress the agreement between observed earthquake focal mechanisms in southeastern tennessee and models of crustal stress surrounding a zone of weakness suggests that these events may be caused by a zone of weakness in the lower crust

continental interiors and cratons any relation abstract continental interior was originally defined as a geographic concept on the example of central asia later james dwight dana used it in a geological context when he argued that north america was an ideal continent with a low old stable interior and higher younger more active periphery this picture was thought satisfactory from the viewpoint of fixist tectonics for more than a century although it was clear that it did not account for the structure of eurasia neither in asia nor in europe cratons and or areas of gentle deformation outside orogenic belts coincide with the continental interior mobilist tectonics and especially its plate tectonics version made clear that in a world where continents are constantly united and re dispersed such non coincidence with continental interior however it may be defined is exactly what one would expect if however after isolation by rifting and or transform fault displacement no plate boundary cuts across a continent the lithosphere beneath it would cool and thicken if this lithosphere is also made up of high mg mg fe residual material left after making basalt it would be lighter than pristine mantle lithosphere such lithosphere would be resistant to subduction and to deformation it would thus protect the portion of the continent overlying it giving rise to a craton cratonic keels of 300 km or deeper can be generated by shortening a depleted mantle cushion that normally reaches down to some 150 km by 50 in the archaean high geothermal gradients would eclogitise the mafic bottom of a continent more mafic than those that are younger and prevent its upper surface from rising during such a shortening this would keep the tops of archaean cratons unmetamorphic or at low grades if a continent with such a deep keel is left alone for a time period on the order of 1 ga continuous cooling will render it very strong only very large strike slip systems can remain active as plate boundaries for a long time within a continental interior therefore if a continental interior can be kept away from such a boundary for about 1 ga it inevitably will turn into a craton with low relief probably a high percentage areawise of internal drainage and high climatic continentality late palaeozoic gondwana land was one such region pre miocene africa was perhaps another with many features inherited from gondwana land if india stops pushing it is likely that central asia will turn into yet another one plume controlled active rifting is the only way to destroy such a consolidated continental interior it is thus the composition and the thermal state plus the state of stress of the lithosphere underlying a continent that determines its tectonic behaviour and not its crustal structure nor its geography craton formation is largely a lithospheric process not a crustal one by the very nature of the process of craton formation cratons commonly but not always originate within continental interiors but continental interior alone can be assigned no tectonic connotation whatever