tectonothermal evolution of the mayuan assemblage in the cathaysia block implications for neoproterozoic collision related assembly of the south china craton the mayuan assemblage in the cathaysia block southeast china consists of felsic paragneiss pelitic schist greenschist amphibolite marble calcsilicate and quartzite that underwent three episodes of deformation d1 d3 and four episodes of metamorphism m1 m4 in the early neoproterozoic the m1 assemblage consists of mineral inclusions defining an early foliation s1 within porphyroblasts represented by chlorite muscovite biotite plagioclase quartz in pelitic schist and actinolite chlorite epidote albite quartz enclosed in amphibolite m2 coincides with the development of the regional schistosity s2 and represents the formation of the porphyroblasts and growth of matrix minerals resulting in development of prograde metamorphic zones chlorite biotite garnet staurolite and kyanite zones m3 is simultaneous with the third phase of deformation d3 and produced sillimanite bearing mineral assemblages in pelitic schist and hornblende bearing assemblages in amphibo lite the last metamorphic episode m4 gave rise to the retrogressive assemblage chlorite muscovite in pelitic rock and actinolite chlorite epidote in amphibolite the sequence of mineral assemblages and history of metamorphic reactions built from the petrogenetic grid of pelites suggest a near isothermal decompression clockwise p t path for the mayuan pelitic schists using the tweequ software program the garnet biotite thermometer and garnet muscovite biotite plagioclase barometer yield p t conditions for m1 of 5 5 to 6 0 kb and 450 to 500 c and conditions for the garnet staurolite and kyanite zones of m2 of 6 0 to 7 0 kb and 550 to 600 c 6 0 to 7 5 kb and 600 c and 11 0 to 11 5 kb and 600 c the p t conditions of m3 were estimated at 570 to 625 c and 4 0 to 4 5 kb using the muscovite biotite thermometer and hornblende plagioclase geothermo barometer the garnet chlorite thermometer yields temperatures of 300 to 400 c for m4 but the pressures of m4 cannot be quantitatively estimated because of the lack of a suitable geobarometer these p t estimates also define an near isothermal decompression clockwise p t path which is involved in initial crustal thickening followed by rapid exhumation and final cooling and is related to amalgamation of the cathaysia and yangtze blocks to form the south china craton

late ordovician to early devonian adakites and nb enriched basalts in the liuyuan area beishan nw china implications for early paleozoic slab melting and crustal growth in the southern altaids abstract we report newly defined nb enriched basalts adakites and dacites from the beishan nw china of the southern altaids based on field geochemical isotopic and geochronology studies two phases of adakites adakite i and adakite ii have been defined which are calc alkaline and characterized by high na 2 o k 2 o ratios 1 49 1 71 and 2 32 3 64 and sr contents 494 1213 ppm and 325 494 ppm negligible to positive eu anomalies strong depletion of hree e g yb 0 48 0 93 ppm and 0 50 0 99 ppm and y 6 87 9 80 ppm and 6 02 10 30 ppm and enriched in rb sr ba k and depleted nb and ti they are characterized by relatively low e nd t values 0 8 to 0 9 and 0 6 to 3 8 and relatively constant high 87 sr 86 sr i ratios 0 70635 0 70636 and 0 70583 0 70651 the zircons of adakite i have relatively low e hf t 0 8 to 2 7 the nb enriched basalts are sodium rich n 2 o k 2 o 1 31 4 44 with higher tio 2 p 2 o 5 zr and nb contents and nb th pm nb la pm and nb u ratios than typical arc basalts they are relatively enriched in rb ba u pb and k depleted in nb and minor negative to positive ba zr sr and ti they have low positive e nd t 0 9 to 2 3 and relatively high 87 sr 86 sr i 0 70556 0 70691 ratios the dacites are typical arc magmas with moderately enriched lile distinctly negative eu nb sr and ti anomalies they have positive e nd t 2 2 and relatively high 87 sr 86 sr i 0 70786 we argue that the liuyuan adakites were most probably related to the melting of young hot subducted crust of the paleo asian ocean which included tectonically subducted radiogenic crustal material and or inheritance from highly radiogenic oceanic crust e g oib the nb enriched basalts likely resulted from mantle wedge peridotites metasomatized by adakites and or further changed by components other than adakites e g minor slab derived fluids based on own zircon sims u pb dating of these key rock types we further propose that from the late ordovician to early devonian large volumes of magma consisting of late ordovician nb enriched basalts 451 ma and dacites 442 ma late silurian adakite i 424 ma early devonian adakite ii 374 ma and i s a type granites 436 ma 380 ma developed in the southern altaids together with other geochronological data from the literature we conclude that subducted oceanic slab melting was frequent from 470 ma to 370 ma our results suggest that frequent hot and or young oceanic crustal subduction and slab melting were important mechanisms in the accretionary growth of the southern altaids

paleozoic laurentia gondwana interaction and the origin of the appalachian andean mountain system laurentia the rift bounded precambrian nucleus of north america may have broken out from a neoproterozoic supercontinent between east and west gondwana several lines of evidence suggest that the appalachian margin of laurentia subsequently collided with the proto andean margin of the amalgamated gondwana supercontinent in different relative positions during early and mid paleozoic time in route to final docking against northwest africa to complete the assembly of pangea hence the appalachian and andean orogens may have originated as a single mountain system the overall hypothesis retains the same paleomagnetic and paleobiogeographic controls as previous global reconstructions for the paleozoic era laurentia gondwana collisions may help to explain contemporaneous unconformities in the paleozoic sedimentary cover of the laurentian gondwanan and baltic cratons

mo deposits in northwest china geology geochemistry geochronology and tectonic setting abstract northwest china covering northern xinjiang northern gansu and westernmost inner mongolia mainly includes junggar basin and its surrounding mountains such as chinese altay junggar chinese tianshan and beishan it lies at the junction of siberia tarim and kazakhstan plates and is a key sector of the central asian orogenic belt caob characterized by multistage phanerozoic continental growth herein at least nine mo only or mo dominated fourteen cu mo two w mo and one be mo deposits have been discovered these 27 deposits occur in altay west jungar west tianshan east tianshan and beishan areas and have been formed during accretionary or collisional orogenies the majority of the deposits are porphyry type followed by the skarn and quartz vein types the orebodies occur mainly as veins lens pods in the positions from inner intrusions through contact zones to the hostrocks distal to causative intrusions the host rocks are variable in lithologies including granites porphyries volcanic breccias and tuffs and sedimentary rocks outward from orebodies to hostrocks the wallrock alteration is zoning from potassic k feldspar quartz mica through phyllic quartz sericite chlorite epidote to propylitic or argillic alterations with skarn specifically occurring in skarn type systems hydrothermal mineralization generally includes four stages from early to late represented by 1 potassic feldspar quartz veins or veinlets 2 quartz molybdenite stockworks 3 quartz polymetallic sulfide stockworks and 4 quartz carbonate fluorite veins or veinlets the ore forming fluids were initially magmatic in origin and shew high temperature and high salinity containing daughter mineral and or co2 bearing fluid inclusions and eventually evolved to low temperature low pressure low salinity and co2 poor meteoric water the porphyry mo deposits can be further subdivided into two subtypes i e dabie and endako types the endako type mo deposits e g suyunhe and hongyuan together with all the cu mo systems were formed in the palaeozoic subduction related magmatic arcs the dabie type porphyry mo deposits represented by giant donggebi and baishan together with the mo only mo dominated and w mo or be mo deposits were formed in syn to post collisional tectonic setting with isotope ages ranging 260 213 2 ma with the kumutage skarn type mo system being an exception aged 319 ma the dabie type porphyry mo deposits are characterized by the co2 bearing fluid inclusions that cannot be observed in the endako type porphyry mo systems the re contents in molybdenites from porphyry and porphyry skarn cu mo systems are mainly 100 ppm suggesting a source significantly contributed by the mantle whereas the re contents in molybdenites from the mo only or w mo deposits are mainly

end permian to mid triassic termination of the accretionary processes of the southern altaids implications for the geodynamic evolution phanerozoic continental growth and metallogeny of central asia the altaids is one of the largest accretionary orogenic collages in the world with the highest rate of phanerozoic continental growth and significant metallogenic importance it is widely accepted that subduction related orogenesis of the altaids started in the late precambrian and gradually migrated southward present coordinates however it is uncertain when and how the building of the altaids was finally completed based on structural geology geochemical geochronological and paleomagnetic data this paper presents late paleozoic to early mesozoic accretionary tectonics of two key areas north xinjiang in the west and inner mongolia in the east together with neighboring mongolia the late paleozoic tectonics of north xinjiang and adjacent areas were characterized by continuous southward accretion along the wide southern active margin of siberia and its final amalgamation with the passive margin of tarim which may have lasted to the end permian to early mid triassic in contrast in inner mongolia and adjacent areas two wide accretionary wedges developed along the southern active margin of siberia and the northern active margin of the north china craton which may have lasted to the mid triassic the final products of the long lived accretionary processes in the southern altaids include late paleozoic to permian arcs late paleozoic to mid triassic accretionary wedges composed of radiolarian cherts pillow lavas and ophiolitic fragments and high pressure ultrahigh pressure metamorphic rocks permian alaskan type zoned mafic ultramafic complexes intruded along some major faults of the tien shan we define a new tarim suture zone immediately north of the tarim craton that is probably now buried below the tien shan as a result of northward subduction of the tarim block in the cenozoic the docking of the tarim and north china cratons against the southern active margin of siberia in the end permian to mid triassic resulted in the final closure of the paleoasian ocean and terminated the accretionary orogenesis of the southern altaids in this part of central asia this complex geodynamic evolution led to formation of giant metal deposits in central asia and to substantial continental growth

using hydrogeophysical methods to assess the feasibility of lake bank filtration abstract lake tisso is the fourth largest lake in denmark with sufficient continuous surface water input making lake bank filtration an optimal surface water extraction method provided that there are suitable shallow aquifers with a good hydraulic connection to the lake the shallow subsurface under the lake was mapped by five waterborne electrical resistivity tomography ert and eight ground penetrating radar gpr surveys to locate potentially suitable aquifers distributed temperature sensing dts was used for the general characterisation of the hydraulic connection between the lake and aquifer the waterborne ert surveys showed several zones of coarser material along the shore extending up to five meters depth below the lake surface while on land ert surveys confirmed that these zones extend to the lakeshore with similar thickness both waterborne ert and gpr profiles agreed that at a selected field site the highest possibility of coarse sediments is 60 140 m from the lakeshore lakebed temperatures measured in december 2016 may and june 2017 all indicated potential groundwater discharge to the lake at approximately 135 m from the shoreline supporting the results of geophysical surveys showing coarser sediments in that area a 2d numerical flow model of the area with a geological setup based on geophysical information and slugtest data also showed upward groundwater discharge peaks at 60 and 140 m from the shoreline confirming the findings of the field surveys thus this study shows that the combination of waterborne ert and gpr surveys with dts measurements is a fast and efficient way to assess the feasibility of lake bank filtration in these settings this approach of combined hydrogeophysical methods is robust and has a potential to site lake bank filtration areas not only next to large rivers where coarse sediments are found in abundance but even in different sedimentary environments where suitable layers for lake bank filtration are more difficult to identify

geochemistry and evolution of the qingyuan archaean granite greenstone terrain ne china the qingyuan archaean granite greenstone terrain is a complex synclinal belt surrounded by granites the first grade structure is an s type regional fold the metavolcano sedimentary series of amphibolite facies is divided into 3 formations from the lower to the upper basic volcanic formation shipenzi fm calc alkaline volcanic formation hongtoushan fm and sedimentary formation nantianmen fm according to the ree distributions the qingyuan basaltic komatiites belong to groups i and ii of jahn et al most tholeiitic rocks belong to the th1 dat type of condie that exhibits lree depletion only a few samples of the hongtoushan fm belong to the th2 eat type the distinguishing feature of the f1 dsv type felsic rocks of the shipenzi fm are their strong depletion in hree but the felsic rocks of the hongtoushan fm belong to the f2 type usv with which the famous hongtoushan massive sulphide copper ore deposit is closely associated the supracrustal rocks overlie a basement of high grade gneissic rocks a lot of tonalitic and monzonitic granites of 2 7 2 9 ga age intruded the greenstone belt the rock association of the qingyuan greenstone succession is similar to the volcanic association in a continental rift setting it is possible that the archaean greenstone belt erupted in a palaeo continental rift which was generated above hot spots or hot lines in the mantle 2 7 2 9 ga ago

re os dating of chalcopyrite from selected mineral deposits in the kalatag district in the eastern tianshan orogen china the kalatag cu zn au district contains a number of economically important cu deposits in eastern tianshan in xinjiang nw china due to the lack of precise mineralization ages the metallogenesis of this area has long been a matter of debate in this study chalcopyrite re os isotope methods are used to date the south meiling cu zn and hongshi cu deposits in the eastern part of kalatag area the south meiling cu zn deposit is hosted in volcanic sedimentary rocks of the late ordovician to early silurian daliugou formation the deposit consists of two parts a concordant massive sulfide ores and discordant vein type ores located in the footwall strata the principal ore minerals are pyrite chalcopyrite sphalerite minor tetrahedrite galena and pyrrhotite gangue minerals include quartz sericite and barite and minor chlorite plagioclase and carbonate minerals the hongshi cu deposit represents a hydrothermal vein system hosted in the mafic volcanic rocks of daliugou formation the orebodies are associated with quartz veins and controlled by subsidiary faults of the kalatag fault the ore forming process can be divided into the early middle and late stages and is characterized by quartz pyrite quartz chalcopyrite pyrite and quartz carbonate gypsum veins respectively re os analyses of chalcopyrite from the south meiling cu zn deposit yield an isochron age of 434 2 3 9 ma and initial 187os 188os ratio of 0 647 0 098 mswd 0 59 re os analyses of chalcopyrite from the hongshi cu deposit yield an isochron age of 431 8 2 7 ma and initial 187os 188os ratio of 0 165 0 075 mswd 0 77 since chalcopyrite is the primary copper mineral we interpret these isochron ages as the timing of cu mineralization based on field geology and petrographic evidence these results suggest that the re os ages presented here provide for the first time a direct constraint on an early paleozoic cu mineralization event of the eastern tianshan orogen the high initial 187os 188os ratios 0 647 0 098 ratio of 434 ma chalcopyrite from the south meiling deposit suggest that the metal was sourced from a two end member mixing of crust and mantle materials moreover we propose that the vms mineral system and hydrothermal vein system of the kalatag district were related to the south dipping subduction of the kalamaili oceanic plate during the late ordovician silurian

age sediment source and tectonic setting of the ore hosting jinwozi formation at the jinwozi gold deposit in beishan orogen nw china evidence from detrital zircon u pb ages and lu hf isotopes abstract the jinwozi deposit is the most economically important gold au deposit in the beishan orogen eastern xinjiang nw china the au orebodies occur as quartz veins or altered lodes in the jinwozi formation clastic rocks and the jinwozi granite the age detrital source and tectonic setting of the jinwozi formation however have not been well understood in this paper we reported new la icp ms detrital zircon u pb ages and lu hf isotopes of ore hosted clastic rock of the jinwozi formation detrital zircons from three samples yield 334 u pb ages varied from 330 to 2598 ma showing complex detrital sources the youngest detrital zircon population suggests that the jinwozi formation deposited no earlier than 330 ma in the late carboniferous or permian the majority of the detrital zircons 291 out of 334 have ages between 330 and 533 ma suggesting a dominant paleozoic detrital source much of these age data 282 out of 334 range from 470 to 370 ma broadly coeval with the zircon u pb ages of the jinwozi granite zircon hf isotope data also implicate their similar origin to the jinwozi granite the source rocks as well as the jinwozi granite may have formed in the early paleozoic caledonian orogeny during which the supercontinents of gondwana and laurasia assembled this orogeny process may have provided plenty of detritus for the clastic rocks of the jinwozi formation precambrian detrital zircon ages from the jinwozi formation cluster around 2400 2600 ma 1800 2100 ma 800 1300 ma which corresponds to the supercontinent assembly events of kenor columbia and rodinia respectively the age populations of 2200 2300 ma and 600 660 ma are possibly related to the breakup of kenor and rodinia supercontinents this understangding is supported by zircon hf isotope data

paleozoic multiple accretionary and collisional processes of the beishan orogenic collage in the paper by xiao and others american journal of science v 310 p 1553 1594 1 please substitute figure 16 reproduced here for figure 16 that appears between pages 1573 and 1574 the original figure 16 2 was printed at an improper size figure 3 1 lookup volpage 310

the latest neoarchean paleoproterozoic evolution of the dunhuang block eastern tarim craton northwestern china evidence from zircon u pb dating and hf isotopic analyses abstract the dunhuang block in the easternmost segment of the tarim craton exposes precambrian mafic granulite and felsic gneiss to better understand the poorly known formation and evolution of the tarim craton we carried out detailed field based petrological and geochronological investigations on the major lithologies of the dunhuang block u pb dating and hf isotopic analyses on the mafic granulite and felsic gneiss ttg gneiss of the dunhuang block reveal two distinct age populations the latest neoarchaean 2 5 ga and the late palaeoproterozoic 1 82 1 85 ga the zircon magmatic cores and metamorphic rims of tonalitic gneisses yield similar ages of 2 5 ga supporting the existence of archean rocks in the dunhuang block the short time interval between latest neoarchean magmatism and high grade metamorphism suggests that they were related to the same latest neoarchean tectonothermal event the 2 5 ga zircons show hf depleted mantle model ages between 2 55 ga and 2 8 ga with a peak at 2 7 ga suggesting that 2 7 ga juvenile crust of the dunhuang block was reworked during a 200 300 m y period at the end of the neoarchean the age of 1 85 ga obtained from the mafic granulites and felsic gneisses is interpreted as the age of high pressure granulite facies metamorphism combined with the hf isotopic data these data suggest that the dunhuang block experienced 2 7 ga crustal growth a 2 5 ga magmatic metamorphic event and a 1 82 1 85 ga hp granulite facies metamorphic event this sequence of events is very similar to that of the north china craton implying that the evolutionary history of the dunhuang block was consistent with that of the north china craton from the neoarchean to paleoproterozoic

upb zircon geochronology of late archaean metamorphic rocks in the taihangshan wutaishan area north china abstract a geochronological study has been made in the taihangshan wutaishan area shanxi province on a sequence of precambrian supracrustal rocks and stratigraphically related intrusives these constitute part of the north china basement block the oldest rocks of the fuping group consist predominantly of mafic granulites paragneisses quartzites and amphibolites this group was always considered archaean and early k ar measurements yielded some minimum ages as old as 2300 ma and others as young as 1500 1900 ma the unconformably overlying wutai group is considered as the base of the proterozoic succession and consists of amphibolite grade paragneisses carbonates and amphibolites in its lower part and greenschist grade clastic sediments and volcanics in its upper part it has given apparent k ar mineral ages between 1500 and 2000 ma present investigations have employed the u pb zircon approach on selected parts of the sequences to provide direct depositional ages or age constraints for the fuping and wutai groups and to find the age s of subsequent metamorphic processes a minimum age for the fuping group and maximum age for the wutai group has been derived from zircon age measurements on the lanzhishan granite 2560 9 ma which intrudes the fuping group and is unconformably overlain by the wutai group this late archaean result can be interpreted as the age of an igneous crystallization event and therefore a maximum for the depositional age of the wutai group the above result is well substantiated by zircon dating of the wutai group itself zircons from a low grade metamorphosed keratophyric lava in the upper part of the group give a volcanic crystallization age of 2520 17 ma this provides close constraints for the age of deposition of the wutai group at between 2560 and 2520 ma ago if the wutai group is indeed basal proterozoic the archaean proterozoic boundary in this part of china is also dated at between 2560 and 2520 ma further evidence as to the antiquity of the wutai group comes from a u pb zircon age of 2520 30 ma for the ekou granite which intrudes the wutai group sequence the age of the underlying late archaean rocks in the tainhangshan wutaishan area the fuping group could not be directly determined because of the paucity of volcanic markers and the higher grade of metamorphism its minimum age is 2560 ma see above but only an approximate maximum age can be gleaned from the zircon systematics in granulite and amphibolite grade paragneiss samples these data indicate a degree of pb inheritance which combined with metamorphic disturbance and more recent pb loss inhibit any further interpretation other than that they indicate an approximate maximum age of around 2800 ma several of the paragneisses and one amphibolitic schist contain zircon systems which are interpreted as having completely responded to a well defined amphibolite facies metamorphic event at 2500 ma ago a younger but less well defined metamorphism caused complete pb loss from zircon and recrystallization in the amphibolite and granulite facies terrains at between 2000 and 1800 ma ago rocks which show evidence of this younger metamorphic event were not recognized from the field or structural data base but from the distribution of the few such samples analyzed it seems possible that this younger terrain may now be interpretable as a cross cutting proterozoic mobile belt this proterozoic metamorphic province is also reflected by k ar mica ages between 1600 and 2000 ma and some preliminary rb sr total rock ages at 1700 ma on nearby paragneisses whose u pb zircon age is 2500 ma

archean magmatism and deformation were not products of plate tectonics abstract the granite and greenstone terrains that dominate upper crust formed from about 3 6 to about 2 6 ga and record magmatic and tectonic processes very different from those of a younger time they indicate heat loss by the archean earth primarily by voluminous magmatism from a mantle much hotter than that of the present plate tectonic processes were not then operating the distinctive array of petrologic structural and stratigraphic features that characterize phanerozoic convergent plate systems ophiolites magmatic arcs accretionary wedges fore arc basins etc have no viable analogues in archean terrains purported archean plate tectonic indicators consist merely of rock types that even superficially resemble actual phanerozoic indicators only when considered in isolation from their association and structure archean ultramafic and mafic volcanic rocks neither resemble ophiolitic rocks in petrology nor occur in ophiolite type successions they often depositionally overlie felsic basement rocks and often overlie and are intercalated with sedimentary and felsic volcanic rocks and they require a mantle about 200 c hotter than now archean graywackes are coherent strata derived from nearby volcanic rocks late in the histories of their regions and they lack the setting and disruption that characterize modern accretionary wedges the lithologic structural and stratigraphic assemblages that typify proterozoic and phanerozoic rifted and reassembled margins also lack archean analogues and no evidence has been found for archean rifting rotation and reassembly of continental plates conversely characteristic archean assemblages lack modern equivalents in any tectonic setting ultramafic lavas erupted at liquidus temperatures are voluminous granite and greenstone terrains have no modern analogues greenstone belts are typically anastomosing networks of upright synforms formed by crowding aside by and sinking between large rising diapiric elliptical composite batholiths the batholiths include both the products of new crustal melts and variably remobilized mid crustal gneisses the greenstone belts are defined by late deformation of regionally semiconcordant volcanic and sedimentary successions and are not relics of successively formed volcanic chains little deformation generally preceded the diapirism and metamorphism was primarily of contact type the regionally uniform areal density and accordant crustal level of the diapiric batholiths their contacts primarily against the oldest strata of the synforms their general age 10 20 million years younger than most of the flanking stratiform rocks and considerations of high archean radiogenic heat productivity all fit the explanation that the batholiths were mobilized by partial melting of hydrous lower crust by radiogenic heating diapirism was accompanied by modest regional orthogonal shortening and extension of the hot upper crust producing the orientations of the batholiths rise of the batholiths greatly increased the petrologic fractionation of the crust and the concentration of radionuclides high in it resulting in cooling of the deeper crust and subjacent mantle and thus cratonization the upper crust containing the granite and greenstone aggregates was decoupled from the gneissic middle crust which underwent flattening and extension subparallel to the elongation of the shallow batholiths this deep deformation may have been driven by flow of dense restites toward delamination loci from which they sank into the mantle the early earth was probably wholly molten the surface of the earth like that of the moon must have been wholly recycled by impacts before 3 9 ga and heavily modified by them until 3 8 ga zircons as old as 4 2 ga have been found as clastic grains in much younger archean quartzites and polycyclic migmatites last partly melted and reconstituted under hydrous conditions only after 3 6 ga contain relict zircons as old as 4 0 ga the lithologies of the early earth protoliths in which these zircons formed have not been established but impact melts and breccias must be represented and magma ocean fractionates may be the nature of the transition in tectonic style into the granite and greenstone mode is unknown plate tectonic rifting and convergence were operating by about 2 0 ga and were in an essentially modern mode by about 0 8 ga the nature of the transition from the granite and greenstone mode at about 2 6 ga to plate mode by about 2 0 ga has yet to be defined the change may have been facilitated by the increasing content of water and carbon dioxide in the mantle as dense but hydrated delaminated archean crust sank into it

the red lake greenstone belt superior province evidence of plume related magmatism at 3 ga and evidence of an older enriched source abstract the red lake greenstone belt of the superior province contains a number of volcano sedimentary assemblages that range in age from 2 99 ga to 2 74 ga the oldest of these is the 2 99 2 96 ga balmer assemblage which is part of the north caribou terrane one of the oldest portions of the superior province the balmer assemblage dominates the red lake greenstone belt and is significant in that it contains evidence for plume generated magmatism the assemblage is dominated by mafic volcanic flows with intercalated komatiitic lavas mafic and ultramafic sills and shallow water clastic and chemical sediments the basalts have 5 12 mgo and mantle normalized signatures which range from slightly light rare earth element ree depleted through essentially flat to light ree enriched some lavas display slight negative nb and ti anomalies indicating selective crustal contamination the komatiites have 12 14 mgo are the al undeplated type and are strongly depleted in light ree they have e nd values of ca 2 the geochemistry combined with sm nd isotope data indicates that the komatiites came from a long term enriched source that was probably generated at ca 3 4 ga that source underwent a melting event that depleted it in light ree prior to komatiite generation the presence of komatiites from a long term enriched source and the strong contrast in the ree profiles of the basalts and the komatiites are consistent with a plume model for the generation of the balmer assemblage the komatiites having originated in the plume core and the basalts from entrained upper mantle or mantle melted by the plume

potential of river bank filtration rbf in egypt drinking water supply in egypt is based on surface water abstraction 91 4 groundwater 8 3 and desalination 0 24 as egypt is currently facing problems with the pollution of surface water by industrial agricultural and municipal inflows riverbank filtration would offer a low cost alternative for pre treatment of raw water for potable use the objective of this paper is to evaluate the potential of riverbank filtration in egypt based on drilling at potential sites and monitoring of both water level and water quality parameters the evaluation of six sites receiving a bank filtration share of more than 50 and producing drinking water has proven the feasibility of riverbank filtration in egypt favorable hydrogeological conditions exist both along the river nile in upper egypt and on main canals in the desert fringes key issues for the feasibility of riverbank filtration in egypt were found to be the hydraulic connection between surface waters and the adjacent aquifer and the landside groundwater quality the river nile in upper egypt has fully to partially cut through surficial clayey sediments and is in hydraulic connection with a quaternary aquifer that is comprised of sandy riverine sediments the sandy riverbed and coarse aquifer materials have a sufficient hydraulic conductivity for riverbank filtration islands and stretches of the river channel that have convex sides are favorable for siting rbf schemes to receive a high portion of bank filtrate and limit mixing with land side groundwater which contains high concentrations of iron and manganese the main benefits of rbf would be the removal of pathogens algae and turbidity it is recommended that wells are constructed at short distances from the river bank to get a high share of bank filtrate

middle cambrian to permian subduction related accretionary orogenesis of northern xinjiang nw china implications for the tectonic evolution of central asia abstract middle cambrian to permian subduction related records are widely distributed in northern xinjiang which can be grouped into the chinese altay east junggar eastern tien shan west junggar yili and tarim domains by integrating paleogeographic and geological data we suppose that the chinese altay east junggar eastern tien shan domain was more closely located to siberia while the west junggar and yili domains occupied an intermediate position near the kazakhstan block in the early paleozoic paleoasian ocean distribution of andean type magmatic arcs island arcs accretionary wedges ophiolitic slices and or microcontinents shows an archipelago paleogeography forming a huge accretionary active margin sequences the tarim domain was on the opposite side of the early paleozoic paleoasian ocean remaining passive margin these tectonic units drifted northwards and approached the southern active margin of the siberian craton in the late paleozoic leading to termination of the paleoasian ocean and formation of a complicated orogenic collage between siberian craton and the tarim block between the end permian and triassic these multiple accretion processes significantly contributed to the lateral growth of central asia

epithermal deposits in north xinjiang nw china the north xinjiang region nw china is an important part of the central asia orogenic belt situated at the junction of siberia tarim and kazakhstan plates it is an area characterized by multiple stages of phanerozoic continental growth during which several porphyry and epithermal systems were formed the relationship of these mineral systems to the geodynamic evolution of the region has not yet been well understood in this paper we list the main geological characteristics of 21 significant epithermal precious and base metal deposits in north xinjiang and classify them into high sulfidation and low sulfidation styles with the latter being predominant we have selected seven epithermal deposits representing different styles formed under different tectonic regimes and discuss their geology and geochemistry in some detail the deposit scale geology and geochemistry of epithermal systems in north xinjiang are essentially similar to those in other parts of the world all epithermal deposits in north xinjiang are hosted in volcanic rocks with ages ranging from devonian to triassic with the early carboniferous volcanic sequences being the most important followed by the permian and triassic the devonian early carboniferous host rocks belong to the calc alkaline series that developed in pre collisional arc back arc basin systems whereas the permian triassic host volcanic rocks of shoshonite series formed in post subduction regimes available isotopic ages of these epithermal systems cluster in two periods early carboniferous 320 ma and late carboniferous triassic 320 220 ma reflecting two metallogenic episodes that occurred during subduction related accretion and post subduction collision regimes respectively accordingly three groups of epithermal deposits in north xinjiang can be recognized as 1 pre collisional deposits without or with negligible collisional related modification 2 deposits formed in collision regime and 3 ore systems strongly overprinted by fluid flow in post subduction collision regime

early paleozoic ca 465 ma eclogites from beishan nw china and their bearing on the tectonic evolution of the southern central asian orogenic belt abstract the beishan orogen is situated in the southern margin of the central asian orogenic belt caob it is connected with the tianshan orogen to the west and the mongolia xing anling orogen to the east this orogen exhibits a well preserved neoproterozoic to late paleozoic sequence containing ophiolites accretionary complexes and eclogites in this paper we provide new geochronological data for eclogites and their country paragneiss to constrain the tectonic evolution of the beishan orogen u pb zircon dating of eclogites yielded two concordant ages 886 4 ma for the cores and 465 10 ma for the rims the zircon cores display bright luminescence with homogeneous or planar sector zoning intermediate th u ratios 0 17 0 47 and hree enriched patterns with negative eu anomalies eu eu 0 16 0 58 these features coupled with their formation temperatures of 680 710 c suggest that the core age represents the time of an earlier upper amphibolite facies metamorphic episode by contrast the zircon rims have dark luminescence with homogeneous or fir tree zoning low th u ratios 12 kbar for the eclogite facies metamorphism on the other hand zircon dating of a paragneiss near the eclogite produced two major groups of ages ca 1450 ma for a magmatic event and ca 900 ma for a metamorphic event it seems that eclogites and their country rocks have suffered the same earlier metamorphic episode based on the available age data obtained for eclogites ophiolites and granitoids from the beishan area a tectonic scenario can be constructed as follows the ca 1000 ma oceanic crust representing by protoliths of the eclogites was involved in early neoproterozoic orogenesis to have formed a microcontinental block during the early paleozoic island arcs and microcontinental blocks were progressively accreted to the northern margin of the tarim craton dunhuang block this was followed by the closure of paleoasian ocean and final amalgamation between the tarim craton and kazakhstan block in the late paleozoic

the neoproterozoic hongliujing a type granite in central tianshan nw china la icp ms zircon u pb geochronology geochemistry nd hf isotope and tectonic significance abstract located between the turpan hami junggar and tarim blocks the central tianshan zone is an important component of the central asian orogenic belt caob and crucial linkage between the siberian kazakhstan junggar turpan hami and tarim blocks the hongliujing granite associated with nb ta mineralization in the central tianshan zone dated at ca 740 ma using zircon la icp ms dating is the first reported neoproterozoic intrusion with a reliable and precise age in the chinese central tianshan the hongliujing granite shares all the characteristics of a type granites it contains predominant alkali feldspar and is characterized by high contents of sio 2 na 2 o k 2 o k 2 o and high field strength elements such as nb ta zr ga and y and low contents of cao mgo ba and sr with high feo t feo t mgo and ga al ratios typical of a type granites based on the geochemistry and zircon hf isotope data we propose that the hongliujing granite was most likely produced by partial melting of basic rocks in the lower crust which may have been derived from mantle magmas the hongliujing granite belongs to a1 type granites which indicate a rifting formation environment suggesting that like the tarim block the central tianshan zone recorded neoproterozoic rift related igneous events related to the breakup of the rodinia supercontinent our study verifies that not only the tarim block is related to the breakup of the rodinia supercontinent but also it is true for some key blocks in caob such as the central tianshan our new geochemical and geochronologic data also support and strengthen the notion that the central tianshan zone may be a part of the tarim block

contrasting sources for upper and lower continental crust the greenstone connection abstract greenstones may be useful in tracking the role of oceanic plateaus in the growth of continents lithologic proportions th ta ratios and ni concentrations in greenstone basalts of all ages show that arc related greenstones greatly exceed oceanic plateau and morb related greenstones in abundance this distribution may be accounted for by the preferential obduction of arcs formed on top of oceanic plateaus during collision of plateaus with continents because thick oceanic plateaus resist subduction a significant volume may be accreted to continental margins during collisions and over time these plateaus may evolve into lower continental crust this idea has important implications for continental development in that the lower continental crust may come chiefly from accreted oceanic plateaus while upper continental crust forms by subduction related processes consistent with this model are high seismic wave velocity layers in the lower crust of proterozoic cratons similar to high velocity layer

plume formation in the d double prime layer and the roughness of the core mantle boundary numerical simulations of the formation of thermal plumes in the d layer at the base of the earth s mantle show that plumes are initiated by coalescence of small scale convective instabilities within the low viscosity region immediately above the core mantle boundary cmb these instabilities support topographic roughness on the cmb having horizontal scales of 20 50 km and provide a source for scattered p waves seen as precursors to the phases pkikp and pkkp the calculated structure of fully developed plumes emerging from d consists of 5 50 cm yr 1 flow confined to 50 100 km thick vertical conduits with strongly temperature dependent viscosity plumes exhibit time dependent behaviour including upward propagating solitary conduit waves which may contribute to episodicity in hotspot volcanism

tectonothermal history of the basement rocks in the western zone of the north china craton and its tectonic implications the basement of the north china craton can be divided into the eastern central and western zones based on lithological structural metamorphic and geochronological data the western zone comprises two different petrotectonic units archaean tonalitic trondhjemitic granodioritic ttg grey gneisses and metamorphic mafic rocks and palaeoproterozoic khondalite series the former is characterized by isobaric cooling ibc type anticlockwise p t paths in the north northwestern part of the zone and near isothermal decompression itd type clockwise p t paths in the eastern part adjacent to the central zone on the other hand the tectonothermal evolution of palaeoproterozoic khondalite series rocks is characterized exclusively by nearly isothermal decompression following the peak of metamorphism and then cooling defining clockwise p t paths the archaean ttg gneisses and associated mafic rocks with anticlockwise metamorphic p t paths reflects an origin related to underplating and intrusion of mantle derived magmas which may be derived from mantle plumes they represent a late archaean continental block in the western part of the north china craton the palaeoproterozoic khondalite series rocks represent passive continental margin deposits they were metamorphosed and deformed in the late palaeoproterozoic during the amalgamation of the western continental block with another continental block in the east part of the north china craton the itd type clockwise p t t paths of the palaeoproterozoic khondalite series rocks record the tectonothermal histories of the collision of the western and eastern continental blocks which resulted in the final assembly of the north china craton at c 1800 ma

pathline calibrated groundwater flow models of nile valley aquifers esna upper egypt abstract strongly concentrated agriculture along the river nile in egypt combined with hydrologic changes related to the construction of the aswan high dam in the 1970 s has led to increasing salinization and waterlogging of agricultural areas successful control and remediation of these problems requires accurate understanding of the shallow quaternary aquifers within the nile valley while extensive conceptual models have been developed by the egyptian rigw published numerical models have yet to incorporate all features of the conceptual model in particular marine affinity of some shallow groundwaters within the valley cl as the predominant anion indicates significant leakage from deeper cretaceous aquifers into the shallow quaternary aquifers a feature that is not present in current models in this study groundwater profile modeling incorporating the bedrock leakage demonstrates that its shallow appearance requires hydraulic separation of surficial from deep recharged zones of the quaternary aquifer this separation occurs near the boundary between reclaimed and traditional agricultural lands which is also the primary site of waterlogging apparently excessive recharge presumed to occur beneath the reclaimed lands does not penetrate deeply and therefore might be easily remediated with shallow drains profound similarities exist between the nile valley salinization cases and the occurrence of shallow nuisance water in desert southwestern u s cities e g las vegas the u s experience with this problem may provide useful guidance in addressing nile valley salinization and waterlogging issues in the future in general irrigation related recharge from the reclaimed lands in the nile valley may have a much more localized impact on traditional lands than previously thought

late quaternary calcareous nannofossil biozonation chronology and palaeo oceanography in areas north of the faeroe iceland ridge abstract the stratigraphic distribution of late quaternary calcareous nannofossils from the european arctic ocean the fram strait and the norwegian and greenland seas are summarised in a biozonation scheme the nannofossil events are directly correlated to oxygen isotope stratigraphy leading to an absolute chronology comprising approximately the last 500 ka the zones are defined by the qualitative and quantitative content of calcareous nannofossils zones n g 2 4 6 8 and 10 comprise intervals mainly barren of nannofossils zone n g 1 is characterised by high abundances of coccolithus pelagicus and emiliania huxleyi zone n g 3 is recognised by dominant gephyrocapsa muellerae and common e huxleyi zone n g 5 is characterised by dominant g aperta n g 7 by dominant g muellerae n g 9 by common g muellerae g aperta and g caribbeanica and zone n g 11 is recognised by dominant g caribbeanica and rare pseudoemiliania lacunosa intervals in which nannofossils occur in high abundances reflect comparatively warmer periods and influences of north atlantic surface water strong inflows of north atlantic water have occurred several times during the last 500 ka in oxygen isotope stage 1 the last 8 ka and in oxygen isotope stages 5 11 12 and 13 north atlantic surface water penetrated into the central parts of the norwegian and greenland seas also during oxygen isotope stage 9 nannofossil barren intervals are mainly formed during time periods with a year round ice cover

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

progressive accretionary tectonics of the beishan orogenic collage southern altaids insights from zircon u pb and hf isotopic data of high grade complexes abstract high grade gneissic rocks of the southern altaids in central asia bear one of the key elements needed to solve a basic architectural controversy in accretionary orogenesis high grade gneissic rocks widely crop out across the beishan orogenic collage southern altaids mapped as the beishan complex or dunhuang group which was previously regarded as neoarchean to paleoproterozoic crystalline basement but without systematic field geochronological and geochemical data this study provides field geochemical and zircon u pb and hf isotopic data for gneissic rocks from the beishan complex geochemical data show that the gneissic rocks were probably formed in a subduction environment most zircons from the beishan complex are magmatic origins because their th u ratios are mostly in the range of 1 0 0 1 they show age peaks at 494 ma 464 ma and 375 ma with a few precambrian ages zircon hf isotopic compositions indicate that both old and juvenile materials were involved in the formation of the beishan complex which is different from the basement rocks of the tarim craton both in radiometric age and hf isotopic composition all these data thus do not support a uniform precambrian basement in the beishan area as previously proposed but instead the beishan complex probably formed in a subduction accretion environment combined with regional geology and geochemical data we conclude that the beishan orogenic collage was characterized by progressive long lived accretionary tectonics during late precambrian to paleozoic time therefore shedding light on the controversy regarding the architecture of accretionary orogens

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

age and tectonic setting of late archean greenstone gneiss terrain in henan province china as revealed by single grain zircon dating the authors report precise u pb zircon ages for single grains of a metarhyodacite from the late archean dengfeng greenstone belt in henan province china near the southern margin of the north china craton most grains belong to an igneous population whose u pb isotopic systematics define a straight line intersecting concordia at 2512 12 ma and this is interpreted as the time of crystallization of the original greenstone volcanics several grains are distinctly older between 2576 9 and 2945 44 ma and the authors interpret the older grains as xenocrysts of pre greenstone continental crust that provide evidence for crustal derivation or crustal contamination of the original rhyodacitic lava the xenocrysts suggest evolution of the dengfeng greenstone belt in a continental environment that may be represented by the taihua high grade gneisses bordering the dengfeng greenstones and for which the authors obtained ages of 2806 7 and 2841 6 ma the data add evidence to the now widely held concept that most archean greenstones developed on or near older continental crust and were therefore prone to crustal contamination in such cases conventional zircon dating may not always record the precise age of rock formation

the wutaishan orogenic belt within the shanxi province northern china a record of late archaean collision tectonics abstract the archaean crust of northeastern shanxi province northern china is dominated by three distinct terranes the high grade gneiss terranes hengshan in the north and fuping in the southeast and the intervening low grade terrane of the wutaishan belt the 2 5 ga beitai arc complex which constitutes the major part of the wutaishan belt is interpreted as a pre collision magmatic arc there was a marginal sea basin between the beitai arc and the fuping terrane at the fore arc collision stage the passive margin deposits which occurred along the western margin of the fuping terrane became a foreland fold thrust belt after the closure of the marginal sea basin the remnants of sea floor occur as a melange zone along the suture zone between the beitai arc and fuping terrane nearly at the same time at 2 5 ga the back arc oceanic lithosphere started to subduct beneath the hengshan terrane then the hengshan terrane came into collision with the arc and the fuping terrane some dismembered sea floor rocks are preserved as a back arc melange zone between the hengshan terrane and the beitai arc the term wutaishan orogeny should be reserved for the processes which lasted from 2600 to 2450 ma

p t t paths and tectonic history of an early precambrian granulite facies terrane jining district south east inner mongolia china abstract the widespread khondalite series of south east inner mongolia consists largely of biotite sillimanite garnet gneiss and quartzo feldspathic gneiss with some marble and mafic granulite layers it has experienced two metamorphic events at c 2500 and 1900 2000 ma a pre peak stage of the first metamorphism at t 600 700 c and p 6 7 kbar is recognized by the relict amphibolite facies assemblage ky grt bt pl qtz and protected inclusions of biotite hornblende sodic plagioclase and quartz in garnet or orthopyroxene the peak stage with t c 800 50 c and p 8 10 kbar is characterized by the widespread granulite facies assemblages sil grt bt kfs pl qtz in gneiss and opx cpx pl hbl grt in granulite the p t t path suggests that the supracrustal sequence was buried in the lower crust by tectonic thickening during d1 d2 the beginning of the second metamorphism is characterized by further temperature rise to 700 c or more at lower pressure this stage is manifested by the appearance of cordierite after garnet fibrolite sil2 after biotite in gneiss and transformation of hbl1 into opx2 and cpx2 in granulite coronas of symplectitic opx2 pl2 surrounding grt1 and cpx1 in mafic granulite are interpreted as products of near isothermal decompression the p t t path may be related tectonically to waning extension of the crust by the end of the early proterozoic

evidence for multiphase water escape during rafting of shelly marine sediments at clava inverness shire ne scotland abstract the pleistocene shelly glaciomarine sediments exposed at clava near inverness northeast scotland occur in a series of thrust bound rafts accreted at on the up ice side of a bedrock high 150 m above od on the se side of the valley of the river nairn these sediments originally formed part of a coarsening upwards deltaic or subaqueous fan sequence deposited in the marine fjord of the loch ness basin located some 50 km to the sw the geometry of these allochthonous rafts coupled with the associated thrusting and large scale folding of these bodies are typical of many glacially transported rafts described in literature however at clava macro and microscopic evidence indicates that these ice rafted sediments were not frozen with liquefaction hydrofracturing and water escape occurring repeatedly during their transport and accretion the presence of large scale detachments within the sequence has led to the development of a purely glacitectonic model for rafting at clava the detachments acted as a focus for fluid flow which lubricated these decollement surfaces aiding in the subglacial transport of the rafts

sm nd isotope study of early archean rocks qianan hebei province china a sm nd study of mafic amphibolite which occurs as lensoid masses in migmatitic quartz dioriticto granitic gneiss was undertaken in order to determine if it has an early archean age six amphibolite samples define an isochron yielding an age of 3 50 0 08 b y with an initial nd value of 3 3 0 3 three samples of quartz dioritic gneiss lie close to this isochron and have sm nd model ages of ca 3 5 b y referenced to a epleted mantle source a sample of k feldspar rich granitic gneiss gives a younger model age of about 2 9 b y the non zero initial nd value of the mafic amphibolites can be compared with chondritic meteorite samples measured in this laboratory and is therefore precisely determined if the initial nd value can be regarded as representative of the original igneous rocks then it indicates that the mantle magma source was severely depleted with respect to magmaphile elements this suggests that the geochemistry of the early archean mantle may have been similar to that of the modern upper mantle and may require that a large pre archean continental crust have been present on the earth

paleoproterozoic crustal evolution of the tarim craton constrained by zircon u pb and hf isotopes of meta igneous rocks from korla and dunhuang abstract the tarim craton is one of three large cratons in china presently there is only scant information concerning its crustal evolutionary history because most of the existing geochronological studies have lacked a combined isotopic analysis especially an in situ lu hf isotope analysis of zircon in this study precambrian basement rocks from the kuluketage and dunhuang blocks in the northeastern portion of the tarim craton have been analyzed for combined in situ laser ablation icp mc ms zircon u pb and lu hf isotopic analyses as well as whole rock elements to constrain their protoliths forming ages and magma sources two magmatic events from the kuluketage block at 2 4 ga and 1 85 ga are revealed and three stages of magmatic events are detected in the dunhuang block i e 2 0 ga 1 85 ga and 1 75 ga the 1 85 ga magmatic rocks from both areas were derived from an isotopically similar crustal source under the same tectonic settings suggesting that the kuluketage and dunhuang blocks are part of the uniform precambrian basement of the tarim craton zircon hf model ages of the 2 4 ga magmatism indicate that the crust of the tarim craton may have been formed as early as the paleoarchean period the 2 0 ga mafic rock from the dunhuang block was formed in an active continental margin setting representing an important crustal growth event of the tarim craton in the mid paleoproterozoic that coincides with the global episode of crust formation during the assembly of the columbia supercontinent the 1 85 ga event in the kuluketage and dunhuang blocks primarily involved the reworking of the old crust and most likely related to the collisional event associated with the assembly of the columbia supercontinent while the 1 75 ga magmatism in the dunhuang block resulted from a mixture of the reworked archean crust with juvenile magmas and was most likely related to a post collisional episode

composition and evolution of the archaean crust in central henan china the upper archaean in central henan province is composed chiefly of rocks of the dengfeng complex and the taihua complex rocks of the dengfeng complex show essential features of a granite greenstone terrane they consist of greenstone belts and surrounding granitic and migmatite gneiss terranes when the late superposed structures are removed the archaean conspicuously exhibits alternating gneiss domes and synformal basins which are occupied by granitic rocks and greenstone belts respectively the succession of the greenstone belts is divided into three groups 1 1 the lower group consists of ultramafic complexes which are only locally exposed 2 2 the middle group which comprises most of the greenstone belts contains tholeiitic metabasalts and felsic volcanics which are bimodal in character 3 3 the upper sedimentary group consists of molasse and turbidite formations the metamorphic grade in the dengfeng complex is up to the medium pressure almandine amphibolite facies the mafic volcanics are characterized by flat to slightly enriched ree patterns that resemble the th2 type of condie in most other elements the rocks are also similar to the tholeiitic metabasalts in typical greenstone belts throughout the world however they possess certain unique characteristics as for the granitic rocks the early plutons tend to be na rich and are geochemically similar to tonalitic rocks while the later intrusives tend to be k rich the taihua complex is composed primarily of felsic gneisses and metamorphosed supracrustal rocks interbedded with mafic ultramafic rocks the metamorphic grade in the complex ranges from granulite to upper amphibolite facies geochemical characteristics of the rocks are also similar to those of other high grade terranes throughout the world in general an overall structural grain characterizes the taihua complex which constitutes an elongate fold belt with characteristic features of linear mobile belts that are frequently found in high grade terranes the extensive dengfeng granite greenstone terrane and the linear taihua high grade gneissic terrane are bounded by thrusts or contacts of unknown nature and together make up the late archaean crust in central henan they may have developed more or less contemporaneously on a pre existing sialic crust in this region controlled by shallow convection cells in the upper mantle their development may have occurred in two different tectonic settings the rift type and the subsiding type setting respectively

paleozoic northward drift of the north tien shan central asia as revealed by ordovician and carboniferous paleomagnetism abstract three new middle late ordovician and two new early carboniferous paleomagnetic poles have been obtained from the north tien shan zone ntz of the ural mongol belt in kyrgyzstan and kazakhstan paleolatitudes for the carboniferous are unambiguously northerly and average 15 5 n whereas the ordovician paleolatitudes 6 9 and 9 are inferred to be southerly given that a very large 180 rotation of the ntz would be necessary during the middle paleozoic if the other polarity option was chosen thus the ntz drifted northward during much of the paleozoic east west drift cannot be determined as is well known from paleomagnetic data in addition detailed thermal demagnetization analysis reveals two overprints one of recent age and the other of permian age which is a time of strong deformation in the ntz the paleolatitude of the combined permian overprint is 30 5 2 n the paleolatitudes collectively track those predicted for the area by extrapolation from baltica very well but are different from those of siberia for ordovician times this finding is compatible with sengor and natal in s sengor a m c natal in b a 1996 paleotectonics of asia fragments of a synthesis in yin a harrison m eds the tectonic evolution of asia cambridge univ press cambridge pp 486 640 model of tectonic evolution of the ural mongol belt and disagrees with the models of other researchers declinations of the ordovician and early carboniferous results range from northwesterly to northeasterly and are clearly affected by local relative rotations which seem characteristic for the entire ntz because the permian overprint declinations also show such a spread apparently the important latest paleozoic triassic deformation involved shear zone related rotations as well as folding and significant granitic intrusions

petrochemistry and sr pb and nd isotopic geochemistry of early precambrian rocks wutaishan and taihangshan areas china abstract in the wutaishan and taihangshan region china the lowermost high grade metamorphic complex of grey gneiss and amphibolite the fuping complex gives a 2 2 0 2 ga pb pb isochron age with a first stage growth 7 73 and a 2 37 0 07 ga sm nd isochron age with 143nd 144nd 0 0 50963 0 00005 or nd t 1 50 nd depleted mantle model dates tdm of amhibolites 2 48 to 2 60 ga those of gneisses are 2 43 to 2 46 ga the unconformably overlying medium to low grade metavolcanic complex the wutai complex gives a 2 0 0 1 ga rb sr isochron age with 87 86sr 0 0 7025 2 a 2 27 0 02 ga pb pb isochron date with afirst stage growth 7 73 and a 2 26 0 06 ga sm nd isochron first stage 143nd 144nd 0 0 5097 2 or nd t 1 08 individual sample nd depleted mantle model date are 2 5 ga metavolcanic samples from low grade metasedimentary rocks of the hutuo group all lie close to a 2 3 ga reference line on a rb sr isochron plot have nd tdm 2 32 2 34 and 2 62 ga from previous geological work published u pb zircon dating and ourisotopic and geochemical results we infer that the fuping rocks formed 2 6 ga ago in an environment like a modern island arc the wutai complex formed by 2 5 ga ago mostly formed in a tectonic setting similar to that of the fuping complex with the exception that one volcanic cycle formed in an environment like a modern mor and one unit formed in an environment transitional between modern within plate and plate margin settings the hutuo group formed 2 4 ga ago in a within plate tectonic environment there is no evidence for continental crust before 2 6 ga in the wutaishan and taihangshan region this contradicts previous assignment of the fuping complex to 2 8 ga ol older the fuping complex is not an early nucleus of the sino korean craton the new data also confirm that the wutai complex is older than its original assignment as early proterozoic the sm nd systems for metabasaltic rocks in the region either in amphibolite facies or in greenschist facies are all significantly disturbed in contrast with the undisturbed sm nd reported in previous studies of the sino korea craton this study provides further evidence that the archean igneous rocks in china formed at different times from heterogeneous and depleted mantle sources

deformational behaviour of migmatites and problems of structural analysis in migmatite terrains an understanding of the response of migmatites to deformation is crucial to an interpretation of their structures and in anatectic and intrusive migmatite terrains due consideration must be given to the modification of deformation processes imposed by melts in partially molten systems containing more than 30 melt the classical theories of fold formation are inapplicable the folds produced are of the viscous fold type their geometry is unpredictable and may mimic structures due to refolding this is particularly true at very high degrees of melting where flow of melt carrying rafts of solids begins failure to distinguish viscous folds from the products of refolding will lead to errors in interpreting the structural and metamorphic history of an area and to consequent misinterpretation of the true structural relations between geometrically complex high grade and more regularly deformed low grade areas

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

composition and history of ground water in the western nile delta abstract a hydrochemical and environmental isotope investigation was conducted in the nile delta and adjacent desert fringes within the framework of a routine hydrogeological mapping project the aim of the study was to obtain additional insight into the historical groundwater flow pattern to differentiate between fossil ground water and contemporaneous recharge by nile water an inventory of existing chemical analyses was made 252 data points and an additional 27 new samples were taken which were also analysed for stable isotope 18 o and 2 h content and for nine samples 14 c and tritium in the majority of analyses irrespective of total salinity an enrichment in sodium was observed relative to the concentration resulting from direct mixing of nile water with sewater the na rich water is thought to have originated in the extreme arid period from 14000 to 10000 years b p last phase of the weichsel glaciation in which by direct evaporation of scarce rain trona minerals were formed at the surface and the ground water in phreatic aquifers gradually became saline the succeeding humid phase from 8000 to 5000 years b p when sea level was relatively high caused freshening on the desert fringes only because the delta floodplain was covered by clay the present arid phase which started at around 3500 years b p also marked the start of widespread floodplain irrigation groundwater levels on the desert fringes dropped as a result of both aquifer depletion by discharge to the qattara depression in the west and climatic effects this in turn initiated a freshening process in the quaternary and moghra aquifer of the delta nile water has since then reached wadi natrun 55 km to the west of the western floodplain border

3 5 ga old amphibolites from eastern hebei province china field occurrence petrography sm nd isochron age and ree geochemistry amphibolite blocks frequently occur in association with grey gneisses within a small area near the villages of caozhuang and huangbaiyu in eastern hebei they are interpreted as remnants of an earlier bimodal suite now engulfed in late archaean k rich granitic magma and are further cut by pegmatite dykes they are considered together as giant enclaves within late archaean granite plutons u pb zircon ages 2 5 2 6 ga the amphibolites have been dated by the sm nd isochron method the results yielded t 3470 107 ma ind 0 50827 11 and nd t 2 7 0 6 this age represents the time of protolithic basalt eruption and possibly suggests a period of formation of primitive mafic crust this is the oldest reliable age so far obtained for the archaean rocks of china the sm nd results of grey gneiss samples yielded model ages tdm of about 3 24 ga it is not clear whether this younger age in comparison with that of the amphibolites indicates the intrusive nature of these rocks or is a result of contamination by later granite about 2 5 ga ago the rb sr and common pb isotopic systems have been highly disturbed and no isochron ages could be established the common pb data show retarded growth of radiogenic pb and hence imply reduction of u pb ratios in the past this is consistent with the petrographic study which suggests that the amphibolites have undergone an earlier period of granulite facies metamorphism and were subsequently retrogressed to the present amphibolite facies mineral assemblages the positive nd t value of 2 7 for the amphibolites implies that the protolithic basalt magmas were generated from mantle sources which had undergone long term depletion in light rare earth elements lree the depletion event must have taken place prior to 4 ga ago if their source had a chemical nature similar to the modern oceanic mantle it could have occurred as early as 4 5 ga ago if their source was less impoverished in lree if the mantle depletion was related to extraction of continental or sialic material the sm nd data of the caozhuang amphibolites lend support to the existence of sialic crust if not of a continental size before 4 ga ago