collapse of archean orogens and the generation of late to postkinematic granitoids intrusion of late stage granitoids into archean granite greenstone terranes may be spatially and temporally related to late orogenic upper crustal extensional collapse and formation of metamorphic core complexes the late to postkinematic granitoid magmas are derived through partial melting of upper mantle and lower crustal sources and are locally associated with late stage extensional structures thick roots of depleted lithospheric mantle of archean age tectosphere beneath archean cratons makes melting of the upper mantle difficult it is proposed that the archean tectosphere is made of imbricated slabs of buoyant archean oceanic crust and lithosphere together with trapped wedges of fertile mantle this would explain several seemingly contradictory phenomena decompression from upper crustal extension generates significant amounts of basaltic melts from the trapped wedges of fertile mantle these basaltic magmas intrude and partially melt the lower crust becoming more silicic in composition they migrate upward to solidify in the middle and upper crust forming the late to postkinematic granitoid suite the tectosphere in this model is less dense and colder than surrounding asthenosphere and is consistent with eclogitic xenoliths from subducted oceanic lithosphere found entrained in kimberlites the direction and magnitude of shear wave anisotropy in the upper mantle is also explained in this model through the orientation of olivine crystals in the underplated slabs of oceanic lithosphere

strain patterns decollement and incipient sagducted greenstone terrains in the archaean dharwar craton south india the archaean dharwar craton is characterized by two greenstone successions the 3 ga sargur group and the 3 0 2 5 ga dharwar supergroup examples of both successions are described from the region of jayachamarajapura where they are also distinguished by different tectonic patterns the younger greenstones have undergone only minor deformation and are only slightly metamorphosed and so provide a good case study of the relative behavior of greenstones in relation to their granite gneiss country rocks a detailed structural analysis indicates two strain fields associated with two deformational episodes d1 and d2 the d1 episode produced dome and basin structures and affected merely the older greenstones and the gneisses the mapped strain field is compatible with the hypothesis that it is associated with the development of diapiric type gravitational instabilities the d2 episode affects only the younger greenstone belt which has the overall geometry of a complex syncline it is discordant over a complex of gneisses and older greenstones that was deformed during the d1 episode the base of the discordant cover sequence is tectonized and constitutes a decollement surface kinematic criteria at this surface have opposite sense and converge towards the belt axis these structural features are interpreted in terms of progressive deformation compatible with the incipient development of a sagducting trough these results are consistent with those obtained from other parts of the craton where the tectonic evolution appears to reflect mainly relative vertical displacements facilitated by the reheating of continental crust during two major archaean tectonometamorphic episodes copyright 1996 elsevier science ltd

magmatic record of neoarchean arc polarity reversal from the dengfeng segment of the central orogenic belt north china craton abstract the dengfeng segment of the archean central orogenic belt in the north china craton ncc includes circa 2 66 2 5 ga metavolcanics metasedimentary and plutonic rocks all units are intruded by 2 5 2 4 ga mafic dykes and a suite of coeval circa 2 5 2 45 ga potassic granites and leucogranitic veins mafic dykes are characterized by mixed alkaline and calc alkaline affinities enriched lree and negative nb and zr anomalies indicating a lithospheric mantle source region potassic granitic dykes exhibit highly fractionated lree patterns and negative hfse anomalies and positive zircon ehf t values which are similar to those of ttg gneisses in the area leucogranitic veins cutting across the diorites are characterized by high contents of sio2 and k2o lree enriched patterns positive eu and sr anomalies negative nb and ti anomalies we suggest that the granitic dykes and leucogranitic veins were derived from partial melting of thickened older crusts combined with other reported 2 5 ga mafic dykes and potassic granites from the ncc we propose that they demonstrate an ancient example of arc polarity reversal and provide a record of the formation of a new plate boundary in the archean the underplating of the mafic dyke source beneath the newly accreted eastern block may have generated the widespread 2 5 2 47 ht lp metamorphism with anticlockwise p t paths in the eastern block

a reconnaissance teleseismic study of the upper mantle and transition zone beneath the archean slave craton in nw canada abstract the objective of this study is to investigate upper mantle structure below the archean slave craton site of the oldest known rocks on earth and numerous diamondiferous kimberlites and thence to gain an understanding of early craton formation and kimberlite genesis to this aim a temporary array consisting of 13 sites equipped with broadband seismometers recorded teleseisms between november 1996 and may 1998 this data set has been augmented with recordings from the yellowknife seismic array our three most robust observations and their interpretation are 1 p wave travel time tomography reveals the oldest part of the craton the central slave basement complex to be underlain by the fastest seismic velocities indicating that this block remains distinct well into mantle depths 2 receiver function analysis requires only the moho as major s wave velocity discontinuity and points to a fairly constant crustal thickness throughout the slave province and 3 sks splitting analysis shows little variation in delay times and fast polarization directions across the array leading us to conclude that the present day plate motion of north america is the primary cause for mantle fabric beneath the entire array furthermore the data let us rule out the possibility that the mackenzie plume had any seismologically detectable effect on the slave lithosphere more speculative results of our investigation namely a possible genetic link between a low seismic velocity anomaly at depth with the overlying lac de gras kimberlite field and a possible archean origin of two shallow low velocity anomalies will require further investigation

structural relationships along a greenstone shallow water shelf contact belingwe greenstone belt zimbabwe the belingwe greenstone belt zimbabwe contains a thick tholeiitic basalt and komatiite sequence structurally above shallow water shelf rocks that in turn rest nonconformably over an older gneiss terrain the nature of the contact between the basalt komatiite sequence and underlying shallow water shelf rocks represents one of the most critical relationships upon which general models for the tectonic setting of greenstone belts and the petrotectonic environment of komatiites are based this contact is a 20 to 200 m thick high strain zone characterized by a wide variety of shear zone tectonites derived from both the shelf and overlying lava succession in the ultramafic part of the high strain zone tectonites include ductile ultramafic mylonites ultramafic ultramylonites ultramafic phyllonites and serpentinite schists enclosing less deformed ultramafic serpentinite blocks in the part of the shear zone derived from shallow water sedimentary rocks shear zone tectonites include chert c s mylonite phyllonite and iron rich argillaceous shear bands both parts of the shear zone are cut by brittle fault gouge fabrics we suggest that the tholeiitic basalt and komatiite sequence is allochthonous with respect to the shallow water shelf and refer to it as the mberengwa allochthon contrary to prior suggestions there is no evidence that the tholeiitic basalt and komatiite succession of the belingwe greenstone belt sits conformably over the shallow water shelf rocks documentation of the shear zone at the base of the mberengwa allochthon lends support to the idea that the allochthon represents a fragment of an off axis oceanic plateau structurally emplaced over a shallow water shelf during a convergent plate interaction prior to 2 6 ga

fracture control of late archean pluton emplacement in the northern slave province canada the structural and geophysical characteristics of 2590 2580 ma leucogranites in the northern slave province have been studied to determine the role and conditions of late archean regional deformation during their emplacement the irregularly shaped straight sided c 10 km long ulu pluton is located in the southern part of the high lake greenstone belt northern slave province fabrics in the pluton and its wall rocks indicate that it was emplaced at mid to upper crustal levels during regional e w late archean compression a gravity survey and resulting three dimensional gravity model of the pluton show that it has several linear deep 6 km root zones which feed a relatively thin 2 km tabular body the orientations of the major root zone along the pluton s western side and three subsidiary roots correspond to those of the pluton contacts to within 5 landsat tm data reveal several sets of lineaments interpreted to represent fractures formed during the early assembly of the slave province which cut both the greenstone belt and the younger plutons analysis of these lineaments defines several maxima whose orientations correspond with those of both the feeder zones and the contacts of the ulu pluton the pluton therefore appears to have been emplaced via an intersecting set of vertical fractures magma was arrested by and spread along an unknown horizontal structure subsequent inflation of the pluton was also facilitated by vertical translation on pre existing fractures

lithospheric density structure beneath the tarim basin and surroundings northwestern china from the joint inversion of gravity and topography abstract intraplate strain generally focuses in discrete zones but despite the profound impact of this partitioning on global tectonics geodynamics and seismic hazard the processes by which deformation becomes localized are not well understood such heterogeneous intraplate strain is exemplified in central asia where the indo eurasian collision has caused widespread deformation while the tarim block has experienced minimal cenozoic shortening the apparent stability of tarim may arise either because strain is dominantly accommodated by pre existing faults in the continental suture zones that bound it essentially discretizing eurasia into microplates or because the lithospheric scale strength i e viscosity of the tarim block is greater than its surroundings here we jointly analyze seismic velocity gravity topography and temperature to develop a 3 d density model of the crust and upper mantle in this region the tarim crust is characterized by high density v s v p and v p v s consistent with a dominantly mafic composition and with the presence of an oceanic plateau beneath tarim low density but high velocity mantle lithosphere beneath southern southwestern tarim underlies a suite of permian plume related mafic intrusions and a type granites sourced in previously depleted mantle lithosphere we posit that this region was further depleted dehydrated and strengthened by permian plume magmatism the actively deforming western and southern margins of tarim the tien shan kunlun shan and altyn tagh fault are underlain by buoyant upper mantle with low velocity we hypothesize that this material has been hydrated by mantle derived fluids that have preferentially migrated along paleozoic continental sutures such hydrous material should be weak and herein strain focuses there because of lithospheric scale variations in rheology rather than the pre existence of faults in the brittle crust thus this world class example of strain partitioning arises not simply from the pre existence of brittle faults but from the thermo chemical and therefore rheological variations inherited from prior tectonism

are wilson cycles preserved in archean cratons a comparison of the north china and slave cratons a review and comparison of the tectonic history of the north china and slave cratons reveal that the two cratons have many similarities and some significant differences the similarities rest in the conclusion that both cratons have a history of a wilson cycle having experienced rifting of an old continent in the late archean development of a rift to passive margin sequence collision of this passive margin with arcs within 100 200 ma of the formation of the passive margin reversal of subduction polarity then eventual climactic collision with another arc terrane microcontinental fragment or continent this cycle demonstrates the operation of paleozoic style plate tectonics in the late archean the main differences lie in the later tectonic evolution the slave s post cratonization history is dominated by subduction dipping away from the interior of the craton and later incorporation into the interior of a larger continent whereas the north china craton has had a long history of subduction beneath the craton including presently being located above the flat lying pacific slab resting in the mantle transition zone placing it in a broad back arc setting with multiple mantle hydration events and collisions along its borders the hydration enhances melting in the overlying mantle and leads to melts migrating upwards to thermochemically erode the lithospheric root this major difference may explain why the relatively small slave craton preserves its thick archean lithospheric root whereas the eastern north china craton has lost it

controls on the distribution of deep sea sediments deep sea sediments represent the largest geological deposit on earth and provide a record of our planet s response to conditions at the sea surface from where the bulk of material originates we use a machine learning method to analyze how the distribution of 14 400 deep sea sediment sample lithologies is connected to bathymetry and surface oceanographic parameters our probabilistic gaussian process classifier shows that the geographic occurrence of five major lithologies in the world s ocean can be predicted using just three parameters sea surface salinity and temperature provide a major control for the growth and composition of plankton and specific ranges are also associated with the influx of non aerosol terrigenous material into the ocean while bathymetry is an important parameter for discriminating the occurrence of calcareous sediment clay and coarse lithogenous sediment from each other we find that calcareous and siliceous oozes are not linked to high surface productivity diatom and radiolarian oozes are associated with low salinities at the surface but with discrete ranges of temperatures reflecting the diversity of planktonic species in different climatic zones biosiliceous sediments cannot be used to infer paleodepth but are good indicators of paleotemperature and paleosalinity our analysis provides a new framework for constraining paleosurface ocean environments from the geological record of deep sea sediments it shows that small shifts in salinity and temperature significantly affect the lithology of seafloor sediment as deep sea sediments represent the largest carbon sink on earth these shifts need to be considered in the context of global ocean warming

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

sea level changes an integrated approach sea level changes an integrated approach in october 1985 sepm sponsored a four day conference entitled sea level changes an integrated approach the purpose of the conference was to provide a forum for an interdisciplinary exchange of ideas on sea level changes and to provide an opportunity for integrating various types of evidence in approaching unresolved issues the conference was successful in bringing together scientists from industry academia and government representing all of the major geosciences disciplines presentations of many new papers plus significant releases of data that were previously held proprietary provided fertile ground for discussion this much cited volume represents the best of the material presented at the conference includes the early vail chart

imaging the continental upper mantle using electromagnetic methods abstract the internal structure of the continental lithosphere holds the key to its creation and development and this internal structure can be determined using appropriate seismic and electromagnetic methods these two are complementary in that the seismic parameters usually represent bulk properties of the rock whereas electrical conductivity is primarily a function of the connectivity of a minor constituent of the rock matrix such as the presence of a conducting mineral phase e g carbon in graphite form or of a fluid phase e g partial melt or volatiles in particular conductivity is especially sensitive to the top of the asthenosphere generally considered to be a region of interconnected partial melt knowledge of the geometry of the lithosphere asthenosphere boundary is important as this boundary partially controls the geodynamic processes that create modify and destroy the lithosphere accordingly collocated seismic and electromagnetic experiments result in superior knowledge than would be obtained from using each on its own this paper describes the state of knowledge of the continental upper mantle obtained primarily from the natural source magnetotelluric technique and outlines how hypotheses and models regarding the development of cratonic lithosphere can be tested using deep probing electromagnetic surveying the resolution properties of the method show the difficulties that can be encountered if there is conducting material in the crust examples of data and interpretations from various regions around the globe are discussed to demonstrate the correlation of electromagnetic and seismic observations of the lithosphere asthenosphere boundary also the observations from laboratory measurements on candidate mineralogies representative of the mantle such as olivine are presented

seismic evidence for preservation of the archean uchi granite greenstone belt by crustal scale extension abstract in the westernmost superior province of canada the east west alignment of granite greenstone belts and the adjacent highly deformed gneiss belts led to the first proposals that plate tectonics existed before 2 5 ga ago with the belts thrust against one another by east west oriented subduction zones here we present seismic reflection data which demonstrate that in this region the present juxtaposition of the uchi granite greenstone belt and the north caribou gneiss terrane occurred along a late southeast dipping extensional shear zone that extends from the surface into the lower crust the preservation of the uchi belt and probably the english river metasedimentary belt is directly related to their dropping along extensional shear zones which limited subsequent erosion the relative lateral transport of these greenstone rocks implies that they were neither derived from the immediately underlying crust nor preserved by vertical crustal movements as might occur in the absence of plate tectonics extension may have been associated with the emplacement of mantle derived magmas at 2700 ma which has been linked to slab break off or lithospheric delamination making the extension approximately coeval with local gold mineralisation since crustal scale faults can facilitate the circulation of gold bearing fluids we suggest that greenstone rocks preserved in the hanging walls of syn to post accretion extensional shear zones may preferentially host archean lode gold deposits in the westernmost superior province our seismic observations imply that some of the late structures in the well developed belts defined by surface mapping arose through the collapse of a collage of laterally accreted terranes

lithospheric domains and controls on kimberlite emplacement slave province canada evidence from elastic thickness and upper mantle composition 1 we have mapped the deep structure of the slave craton by combining analysis the effective elastic thickness te with data on mantle samples from numerous kimberlites three dimensional mapping of the subcontinental lithospheric mantle sclm using mantle derived xenoliths and xenocrysts in kimberlites has shown that much of the craton is underlain by a strongly layered sclm a highly depleted upper layer low in basaltic components ca al fe is separated from a relatively fertile lower layer by a sharp boundary this boundary lies at 140 150 km depth in the lac de gras area and shallows to 100 km in the northern and southern parts of the craton weak lithosphere te 56 km in the younger eastern part of the craton is separated from the older western part by a zone of steep te gradient parallel to the major locus of kimberlite intrusion which may map the deep extension of the boundary between the two domains another strong te gradient across the kilohigok basin accompanies a marked compositional change in the upper layer of the lithospheric mantle the basin probably marks a major translithospheric fault correlations between te and mantle composition suggest that te is strongly influenced by the rheology of the upper mantle

sinking of the dharwar basin south india implications for archaean tectonics abstract archaean tectonic regimes can be constrained by structural analysis of greenstone belts and adjacent granite gneiss masses the late archaean bababudan greenstone belt dharwar craton south india contains a low grade volcanosedimentary basin sequence the dharwar supergroup which unconformably overlies a previously eroded basement made up mainly of granite and gneiss fabric trajectories kinematic indicators and superimposed deformation patterns show that greenstones have undergone a radial converging downward flow with respect to underlying basement rocks as well as strike slip shearing this is attributed to the progressive sagduction gravity sinking of the greenstone belt into its basement along a decollement horizon and to late transcurrent shearing we propose a deformation model for the bababudan belt that may explain common features of many greenstone belts such as decollements and superimposed upright fold patterns which are attributed to the progressive sinking of the supracrustals into their basement the results of our study and other field investigations in the crust of the dharwar craton as well as structural evidence from other cratons zimbabwe and pilbara raise the possibility that a unique continental tectonic regime existed during the archaean controlled by the emplacement of dense volcanic traps mantle underplating and thermal blanketing of the continental crust this tectonic regime differs from that observed within the modern lithosphere as it is characterized by the pervasive development of gravity i e rayleigh taylor instabilities between the volcanic traps and the rest of the continental crust it allowed heat and material transfer through the crust and has to be seen as an efficient recycling mechanism for the young continental crust during periods of mantle underplating the diapiric phenomenon is independent of the boundary forces acting at the limits of the protocontinents and is associated in time and space with large scale accretion events of the continental crust perhaps in hot spot environments

extensional structures during deposition of the 3460 ma warrawoona group in the eastern pilbara craton western australia abstract structures in the granite greenstone terrain of the eastern pilbara were mapped in both the coongan belt greenstones and shaw batholith granitoids important structures that are coeval with an early magmatic event in both the granitoids and the greenstones were identified in the coongan greenstones a brittle extensional fault array is syn kinematic with felsic volcanics of the duffer formation dated at 3452 16 ma pidgeon 1978a in the underlying shaw batholith a major ductile shear zone was mapped at the contact with the coongan belt although the foliation in the shear zone forms part of a domal structure the stretching lineations constantly plunge to the ene the sense of shear as recorded in the shear zone is consistently east up with subsidiary sinistral strike slip movement on the e w trending part and dextral movement on the n s trending part a wedge shaped granitoid intruded into the shear zone during deformation this granitoid is part of the north shaw suite which has been dated at 3467 6 ma mcnaughton et al 1988 the structures in the greenstones and in the batholith have therefore formed during the same event at different crustal levels the structural kinematic magmatic metamorphic and geochronological data are consistent with a model in which these structures represent the downdip part of a dome with a core complex type of geometry in turn this implies that the domal geometry of the shaw batholith was initiated at mid crustal levels at a very early stage namely during deposition of the warrawoona group in the evolution of the pilbara craton as extension occurred during intrusion and extrusion of large volumes of magma the original distribution and thickness of granitoid bodies and greenstones were probably largely determined by the extensional geometry the initial domal shape of the shaw batholith has been enhanced during later deformation phases which caused steepening of n s trending structures

partial convective overturn of archaean crust in the east pilbara craton western australia driving mechanisms and tectonic implications strain varies systematically from weakly developed outward dipping s tectonites in the 3320 3310 10 ma mount edgar batholith to intensely deformed subvertical l tectonites in greenstones of the warrawoona syncline a consistent greenstone down batholith up sense of shear is recorded in batholithic domal margins and adjacent high grade supracrustal rims lineations converge to a central zone of subvertical extension zone of sinking along the synclinal axis at domal margins early kinematic granitoid sheets and intrusive diatexites are subconcordant to a well developed dome parallel schistosity but late to post kinematic intrusives are discordant high level plutons all granitoids are the same age within analytical error these syn doming features conform with structural tests for diapirs and differ from those expected during metamorphic core complex formation and cross folding diapirism is part of a larger process involving partial convective overturn of the crust based on strain patterns and kinematic criteria we argue that deformation was initiated by sinking of greenstones which rapidly subsided the domes then rose passively as accommodation structures ongoing doming d4 partitioned strain to the southern rim of the batholith and uplifted a wedge of the warrawoona syncline keel we consider that crustal overturn occurred in response to mantle plume activity eruption of a 5 10 km thick 3325 ma mafic ultramafic greenstone pile onto an older granite greenstone terrain created a negatively buoyant crust but convective overturn occurred some 5 20 ma later triggered by widespread anatexis thermal softening convective overturn may have been a common archaean process occurring in response to a hotter mantle and represents an end member deformation mechanism that includes thrust accretion of lithotectonic assemblages in other archaean terranes such as the superior province of canada

regional shortening followed by channel flow induced collapse a new mechanism for dome and keel geometries in neoarchaean granite greenstone terrains the lateral flow and extrusive exhumation of ductile migmatitic gneisses due to a horizontal gradient in lithostatic pressure a process termed channel flow or lateral protrusion has previously been proposed as an important tectonic process in large hot orogens centrifuge simulation of i layer parallel shortening followed by ii collapse of a cover sequence during ductile flow of underlying layers in an analogous manner to channel flow suggests a new mechanism for the development of structures within neoarchaean granite greenstone terrains the centrifuge model incorporates an upper package of silicone modelling clay microlaminates that simulate a greenstone sequence that overlies slightly less dense ductile silicone putties whose rheological properties simulate migmatitic felsic gneiss a low viscosity low density layer along half of the infrastructure superstructure interface simulates the presence of granitoid melt during initial layer parallel shortening upright folds form in the upper greenstone package and the interface with underlying ductile layers is folded in basal silicone layers recumbent to overturned non cylindrical and upright folds form with increasing distance from the moving end wall removal of material parallel to fold hinges at the foreland end of the model i e furthest from the ram used to shorten models in several stages simulates erosion and the difference in gravitational loading thus created induces ductile flow and lateral extrusion of basal silicone layers models aim to reproduce features comparable to those developed during extrusive channel flow and focused exhumation of basement migmatitic gneiss in nature early formed recumbent to inclined folds are then accentuated during simulated channel flow while new recumbent isoclinal folds in basal layers develop broad antiforms and tight synforms similar to the dome and keel geometry that typifies many archaean granite greenstone belts are produced as a late feature in the model channel flow and collapse of a fold thickened crust is therefore proposed as a potential alternative mechanism for the formation of structures in some neoarchaean granite greenstone terrains by analogy with other physical and numerical models channel flow in the neoarchaean may be enhanced by impingement of an upper mantle wedge into the base of the crust our results imply that both lithospheric shortening and non diapiric gravitational instabilities may be responsible for the formation of some archaean dome and keel structures and may account for the juxtaposition of some granite greenstone and high grade gneiss terrains in the archaean model results also show similarities to structures produced during lateral flow and withdrawal of salt in fold belts

transamazonian orogeny in the southern sao francisco craton region minas gerais brazil evidence for paleoproterozoic collision and collapse in the quadrilatero ferrifero abstract the paleoproterozoic transamazonian orogeny yielded a series of discrete orogens in brazil the present field study in the quadrilatero ferrifero qf indicates that the southern sao francisco craton region of the brazilian highlands preserves a portion of one of these orogens two sets of transamazonian structures occur in this region the first consists of northwest verging folds and thrusts affecting supracrustal sequences it is suggested that this set formed in a fold thrust belt setting shortly after 2 125 ga during the closure of a passive margin basin that had initiated along the margins of a preexisting continental mass at ca 2 5 ga the second set consists of structures defining the prominent dome and keel architecture of the qf this set a consequence of the emplacement of basement domes against supracrustal rocks at 2 095 ga may reflect the consequences of orogenic collapse narrow conglomerate filled intermontane basins may have formed coevally with dome emplacement formation of an ocean basin east of the present day sao francisco craton eventually occurred in late mesoproterozoic in effect the transamazonian orogen of the qf represents the collision and collapse stages of a paleoproterozoic wilson cycle the contractional phase of the transamazonian orogeny probably represents accretion of an offshore arc to the eastern and southeastern margin of the present day sao francisco craton region the arc and an associated suture may be traced in the brasiliano pan african orogen east of the sao francisco craton northwards into the northeastern lobe of the sao francisco craton clearly initial assembly of crustal blocks to form a larger continent involving south america occurred during the paleoproterozoic 2 1 ga post transamazonian rifting of this continent created the basins which were later inverted during the brasiliano assembly of gondwana

neoarchean decoupling of upper and mid crustal tectonothermal domains in the southeast slave province evidence from the walmsley lake area the walmsley lake area of the southeastern slave province northwest territories exposes a crustal transect from greenschist through to upper amphibolite facies two tectonothermal crustal domains are proposed based on disparate tectonic histories and rock types within the upper tectonothermal domain d1 deformation reached lower amphibolite facies prior to 2614 ma and associated m1 metamorphic conditions outlasted d1 peak m2 conditions reached middle amphibolite facies prior to 2603 ma and outlasted d2 deformation deformation style and metamorphic sequencing during these two pre 2600 ma events are consistent with regional crustal shortening and thickening in the upper tectonothermal domain at mid crustal levels in the lower tectonothermal domain a third tectonothermal event d3 m3 produced uppermost amphibolite facies peak metamorphic conditions transposed preexisting fabrics to shallow dips and produced a subhorizontal foliation and recumbent folds at ca 2583 ma d3 structures are not present

the vertical distribution of radiolaria in the waters surrounding japan abstract geographic and vertical distribution patterns of living radiolaria are closely related to the characters of the water column we studied living radiolaria in samples collected at closely spaced depth intervals in the waters surrounding japan in order to understand their vertical distribution and its controlling factors such information is needed to reconstruct past vertical water mass structure the japanese islands are surrounded by the marginal japan sea and the north pacific ocean and from approximately 46 n to 26 n they thus extend across a large latitudinal range and are exposed to a similarly wide range of environmental conditions inducing the warm kuroshio tsushima and tsugaru currents and the cold oyashio current we performed plankton tows mesh size 63 mm from late may to early june 2002 at eight sites sampling the upper 200 m of the water column at seven depth intervals we recognized five radiolarian assemblages the upper surface assemblage 0 40 m the lower surface assemblage 40 80 m the surface assemblage 0 80 m the subsurface assemblage 80 200 m and the lower surface subsurface assemblage 40 200 m groups pseudodictyophimus gracilipes and tetrapyle octacantha juvenile show tropical submergence i e they live at the surface in high latitudes and at greater depth in low latitudes four taxa live at specific temperature and salinity acrosphaera spinosa larcopyle butschlii challengeron diodon and stichocorys seriata four taxa are associated with characteristic temperatures neosemantis distephanus arachnocorys umbellifura antarctissa sp 1 and saccospyris conithorax

deformation style way back when thoughts on the contrasts between archean paleoproterozoic and contemporary orogens abstract in many important ways archean and paleoproterozoic older orogens differ structurally from contemporary examples this essay examines the premise that contrasts between older orogens and contemporary orogens reflect long term changes in the temperature of the continental crust in the density of supracrustal sections and in exhumation rates for example if continental crust were warmer and exhumation rates faster earlier in earth history then higher grade rocks would occur closer to the surface of older orogens and the orogens would be lower and wider this situation might contribute to the formation of wide belts of high grade gneiss found in ancient crust if the high strength layer of the crust were thinner and supracrustal sequences denser earlier in earth history then regional extensional tectonism might lead to crustal scale boudinage and diapirism this situation might explain formation of the extensive dome and keel provinces found in ancient crust testing such speculations through the application of structural analysis coupled with petrologic studies dating and rheological modelling will constrain models of earth s long term physical evolution

growth of granite greenstone terranes at convergent margins and stabilization of archean cratons abstract archean granite greenstone terranes represent juvenile continental crust formed in a variety of plate tectonic settings and metamorphosed through a complex series of structural and magmatic events most archean granite greenstone terranes appear to have acquired their first order structural and metamorphic characteristics at convergent plate margins where large accretionary wedges similar in aspect to the chugach makran and altaids grew through offscraping and accretion of oceanic plateaux oceanic crustal fragments juvenile island arcs rifted continental margins and pelagic and terrigenous sediments buoyant slabs of parts of archean oceanic lithosphere may have been underplated beneath these orogens forming thick crustal roots characterized by interleaving between the depleted slabs and undepleted asthenosphere back stepping of the subduction zones after accretion of plateaux and arcs caused the arcs magmatic fronts to migrate trenchward through the accretionary wedges dehydration of the subducting slabs hydrated the mantle wedges below the new arcs and generated magmas sanukitoid suite in the mantle wedge whereas other magmas tonalite trondhjemite granodiorite or ttg suite appear to have been generated by melting of hot young subducted slabs eventual collision of these juvenile orogens with other continental blocks formed anatectic granites then thickened the crust beyond its ability to support its own mass which initiated gravitational collapse and decompressional release of syn to late tectonic granitoids from wedges of fertile mantle trapped between underplated oceanic lithospheric slabs and aided in the cratonization of the granite greenstone terranes deeply penetrating structural discontinuities such as shear zones and sutures provided pathways for fluids and granitoids to migrate into the mid and upper crust forming ore deposits and plutons most preserved granite greenstone terranes have been tectonically stable since the archean and form the cratonic interiors of many continents

origin of 2 5 ga potassic granite from the nellore schist belt se india textural cathodoluminescence and shrimp u pb data in a geochemical and geochronological investigation of archean and proterozoic magmatism in the nellore schist belt we conducted shrimp u pb analyses of zircons from two cospatial granitic bodies at guramkonda and vendodu the former is a ba and sr rich hornblende bearing tonalite whereas the latter is a rb zr pb th u and ree rich biotite bearing leucogranite the guramkonda tonalite displays a restitic texture with remnants of trapped granitic melt whereas the vendodu leucogranite contains residual partially melted plagioclase grains both rock types contain two generations of zircon tonalite contains a group of euhedral zoned zircons enclosed within plagioclase and a group of subhedral patchy zircons associated with trapped melt quartz feldspar matrix and leucogranite also contains a group of doubly terminated euhedral zircons included within orthoclase as well as a group of zircons with visible cores mantled by later rim growth cathodoluminescence images also clearly document two distinctly textured varieties of zircon the tonalite contains a population characterized by narrowly spaced uninterrupted oscillatory zoning and a second population lacking zoning but exhibiting a random distribution of dark u rich and light u poor regions the leucogranite contains u rich zoned zircons and u poor zircon cores mantled by u rich rims the ree chemistry of zircon cores from the vendodu leucogranite is very similar to the ree of zoned zircons from the guramkonda tonalite zircon ages from both plutons exhibit bimodal distributions in u pb concordia diagrams the tonalite defines an age of 2 521 ma 5 ma for zoned magmatic zircons and 2 485 ma 5 ma for unzoned newlyprecipitated zircons whereas the leucogranite has an age of 2 518 ma 5 ma for u poor zircon cores relics of the tonalite pluton and 2 483 ma 3 ma for u rich zoned magmatic zircons the trace element geochemistry of the 2 520 ma zircons is distinctly different from the 2 485 ma zircons irrespective of the host rock our textural cl image and shrimp u pb analyses document the origin of the leucogranite by partial melting of the tonalite high alkalis na2o k2o rb nb hree feot mgo and low ca al ba sr and large negative eu anomalies characterize the leucogranite as a thermal minimum melt whereas the very low k and rb of the tonalite attests to its residual nature we suggest that the leucogranite formed by high t 900 950 c moderate pressure 10 kbar dehydration partial melting of the tonalite under reducing conditions the calculated source compositions of the leucogranite melt and the tonalite residue show strong similarities to melts that are considered to have been produced in a subduction zone environment the leucogranite probably formed in a post collisional realm immediately after accretion of the tonalitic crust

phaeodarian skeletons their role in silica transport to the deep sea the skeletons of phaeodarian radiolaria transport and redistribute silica to the tropical deep oceans by dissolving in the water column and on the sea floor the skeletons are initially solid but within a few days to months become progressively more porous while settling through the water column phaeodarian radiolaria are rarely preserved in the bottom sediments in contrast polycystine radiolaria are the dominant radiolaria preserved in the fossil record this preservational difference may be due to differences in skeletal constituents

the 2 65 ga a type granite in the northeastern yangtze craton petrogenesis and geological implications abstract the presence of old residual continental crustal rocks in cratons is crucial for investigation of their early evolution due to the poor exposure of archean rocks the early evolution of the yangtze craton has not been well constrained yet in this study a combined study of zircon u pb and hf o isotopes as well as whole rock major and trace elements and nd isotopes have been conducted for a granite pluton at huji town in the northern part of the yangtze craton u pb dating for magmatic zircons yielded a weighted mean age of 2656 6 ma revealing another archean outcrop in the yangtze craton besides the kongling terrane the granite samples show relatively evaluated k 2 o na 2 o cao tfeo mgo ratios and zr nb ce ga and y contents sharing the same features as the a type granite they have pronounced eu sr and ba negative anomalies on the primitive mantle normalized trace element spidergram implying the residual of plagioclase in the magma sources their high zr contents of 440 529 ppm correspond to initial magma temperatures of 930 962 c the zircons have negative hf t values of 2 7 to 0 1 and hf model ages of ca 3 2 ga respectively the 18 o values of the zircons range between 4 94 and 5 72 and give an average of 5 26 0 14 suggesting no contribution of supracrustal rock into the host magma therefore the hf model ages indicate the generation of the huji granite by reworking of ca 3 2 ga juvenile crustal rocks combined with previous studies the finding of the neoarchean huji granites with ca 3 2 ga hf model ages suggests the widespread of archean basement in the yangtze craton the occurrence of the ca 2 65 ga huji granites likely records the thermally stable lithosphere and thus heralds the craton stabilization of the eastern part of the yangtze block

paleoarchean partial convective overturn in the singhbhum craton eastern india abstract analyses of fold superposition structures and kinematics of shear zones along a 120 km long e w corridor across the singhbhum craton eastern india are combined with u pb th chemical ages of texturally constrained monazites in supracrustals gneisses and granitoids to reconstruct the paleoarchean tectonic history of the western part of the craton metamorphic sub greenschist facies shallow marine to platformal sediments of the iron ore group iog and the accordantly deformed jagnnathpur meta lavas were accreted onto a basement composed of 3 4 ga amphibolite facies para amphibolites quartzites muscovite schists older metamorphic group omg and anatectic older metamorphic tonalite gneisses omtg crustal shortening was followed by emplacement ascent of the singhbhum granitoid pluton sgp 3 2 3 3 ga synchronous with sinking of supracrustal screens along n nne trending steep dipping sinistral normal ductile shear zones deformation microstructures indicate the bulk of the pluton responded to exhumation strain by low t 500 c ductile flow of quartz and k feldspar with plagioclase being largely unaffected but granitoids along the western margin of pluton emplaced concurrent with westward retreating exhumation of the thickened crust was deformed at t 650 c since all granitoids in the pluton share the same n nne trending tectonic fabric microstructural features imply phased emplacement of the pluton during exhumation strain the transition from crustal shortening to exhumation interrupted by phased granitoid emplacement between 3 2 and 3 4 ga is explained by a two stage partial convective overturn model involving structures typical of supracrustal down granitoid up sense of movement the tectonic restoration in the western part of the singhbhum craton is new and complements growing evidence that accretion and convective overturn may have operated during the paleoarchean

a practical model for organic richness from porosity and resistivity logs a practical method the delta log r technique for identifying and calculating total organic carbon in organic rich rocks has been developed using well logs the method employs the overlaying of a properly scaled porosity log generally the sonic transit time curve on a resistivity curve preferably from a deep reading tool in water saturated organic lean rocks the two curves parallel each other and can be overlain since both curves respond to variations in formation porosity however in either hydrocarbon reservoir rocks or organic rich non reservoir rocks a separation between the curves occurs using the gamma ray curve reservoir intervals can be identified and eliminated from the analysis the separation in organic rich intervals results from two effects the orosity curve responds to the presence of low density low velocity kerogen and the resistivity curve responds to the formation fluid in an immature organic rich rock where no hydrocarbons have been generated the observed curve separation is due solely to the porosity curve response in mature source rocks in addition to the porosity curve response the resistivity increases because of the presence of generated hydrocarbons the magnitude of the curve separation in non reservoirs is calibrated to total organic carbon and maturity and allows for depth profiling of organic richness in the absence of sample data this method allows organic richness to be accurately assessed in a wide variety of lithologies and maturities using common well logs

stabilization of the southern portion of the sao francisco craton se brazil through a long lived period of potassic magmatism abstract although the southern portion of the sao francisco craton in southeast brazil encompasses one of the largest segments of meso to neoarchaean crust in the south american plate there is little understanding of the processes leading to cratonization of this region our combined field and u pb age data show that the crust experienced massive intrusions of potassic k rich granitoids between 2750 and 2600 ma and that this magmatic event marked the beginning of a fundamental change in the tectonics of the terrain with the ensuing billion years being dominated by the deposition of a series of thick clastic and chemical sedimentary successions thus deep crustal partial melting and subsequent transport and intrusion of these granites in the upper crust is a likely mechanism by which this segment of the sao francisco crust attained buoyancy to survive recycling regional mapping indicates that the potassic granitoids are distributed over an area of 25 000 km 2 and are related to one of the most prolific periods of potassic magmatism in the craton la icp ms and shrimp u pb data from 16 samples of potassic granitoids together with published tims ages suggest a long lived period of magmatism from 2750 to 2700 ma and a small pulse of granite crystallisation at ca 2612 ma the u pb shrimp and la icp ms data also indicate that the archaean granitoids do not record evidence of u pb zircon resetting our interpretation is that partial melting and transport of granitoid melts rich in heat producing elements to the upper crust gave rise to a thermally stable lower crust and that this lower crustal segment in the craton area became sufficiently refractory and resistant to further partial melting

crust mantle coupling revisited the archean slave craton nwt canada abstract lithospheric domains in the slave province inferred from regional variations in chemistry of mantle derived garnets are parallel with northeasterly structural trends in neoarchean surface rocks and were used previously to infer late neoarchean crust mantle coupling and argue against the presence of diamondiferous mesoarchean lithosphere under the western slave craton prior to its late neoarchean orogenic climax a review of more recent evidence does not support this view but concludes that a mesoarchean lithospheric root was coupled to mesoarchean crust of the central slave superterrane csst prior to accretion of adjacent juvenile neoarchean terranes a significant part of this root survived the neoarchean break up of the csst neoarchean terrane accretion and a complex sequence of post accretion deformation granitoid plutonism and metamorphism it was least modified in the ultra depleted harzburgitic layer udl under the central part of the contwoyto terrane where it is trapped above the deepest part of an underplated paleoproterozoic slab which stabilized the remnants of the archean root and caused downward younging of the lithosphere upper mantle domains recognized on the basis of the distribution of peridotitic garnets are not primary neoarchean features but were either inherited from the original mesoarchean root or resulted from neoarchean and paleoproterozoic tectonic erosion and or metasomatism of this root the slave province resembles other diamondiferous cratons in that its diamond endowment consists of both mesoarchean and paleoproterozoic stones

geochemistry and sequence stratigraphy of regional upper cretaceous limestone units offshore eastern canada abstract hydrocarbons occur in two regional upper cretaceous limestone units the turonian coniacian petrel member and the santonian maastrichtian wyandot formation the units form important seismic markers beneath the scotian shelf and the grand banks of eastern canada they mainly consist of bioturbated chalk and minor amounts of calcareous mudstone a search for source rock using the log r technique showed intervals with source potential but testing of core and cuttings by rock eval analysis showed no source potential three issues are the main cause for the inconsistency 1 unconsolidated shales that likely included organic material were lost during sample washing 2 severe contamination by mud additives and 3 presence of gas the organic matter found on the shelf has been strongly oxidised but the distal facies of these limestone units and condensed shale units above and below may yet have potential to form source rock beyond the studied areas

tectonic history of the barberton greenstone belt south africa 490 million years of archean crustal evolution rocks of the 3570 3080 ma barberton greenstone belt record a geological history of at least 490 million years m y and were affected by regional thermal perturbations for over 1500 m y small tectonic wedges of 3570 3511 ma schistose tonalite are the oldest rocks yet recognized but little is known about their tectonothermal history the earliest well recognized tectonothermal events 3490 3450 ma represent mid ocean ridge like processes these were followed by two periods of arc related and trench related processes separated by 160 m y the first 3445 3416 ma recording an intraoceanic suprasubduction like environment and the second 3260 3225 ma intra arc and interarc like processes culminating in arc amalgamation accretion like convergent processes dominated between 3230 ma and 3080 ma overlapping with the assembly of the kaapvaal craton at 3080 ma the barberton greenstone belt experienced a sudden shift from dominantly convergent to dominantly transtensional tectonism and was mineralized with mesothermal gold along a regional shear system soon afterward transtension became distributed across the entire kaapvaal craton heralding the onset of formation of the witwatersrand basin by 3070 ma

a 2 5 ga fore arc subduction accretion complex in the dengfeng granite greenstone belt southern north china craton abstract the dengfeng granite greenstone belt dggb located in the southern segment of the central orogenic belt cob of the north china craton ncc consists of a volcano sedimentary assemblage intruded by tonalite trondhjemite granodiorite ttg suite diorite granites and late mafic dikes the volcano sedimentary assemblage in the dggb mainly consists of tectonically imbricated basaltic amphibolites meta gabbroic rocks with minor ultramafic rocks and metagreywacke marble and quartzite consistent with characterisitics of typical phanerozoic subduction accretion complexes the basaltic amphibolites yield a metamorphic zircon 207pb 206pb age of 2507 26 ma interpreted to represent the peak age of amphibolite facies metamorphism that took place during subduction accretion of the basaltic protolith the basaltic amphibolites are characterized by a tholeiitic affinity and flat lree patterns with minor negative nb and zr anomalies based on mixed morb and arc affinities the basaltic amphibolites in the dggb are interpreted to have formed in a fore arc tectonic setting one late potassic granite dike cutting across the fabrics of the volcano sedimentary assemblage yields an intrusion age of 2492 35 ma constraining the minimum deformation age for tectonic assembly of the package the ttg gneisses and diorites intruded the western margin and the center of the subduction accretion complex respectively one ttg sample and one diorite sample yield igneous zircon 207pb 206pb ages of 2514 26 ma and 2518 36 ma respectively constraining their intrusion ages the ttg gneisses display high ratios of la yb cn and sr y and depletion in hfse with negative nb ta and ti anomalies consistent with those of typical archean ttgs the ttg gneisses are therefore considered to be generated from partial melting of a shallowly subducting oceanic slab and or accreted arc amphibolites the diorites have high concentrations of mgo 2 89 6 05 wt ni 148 178 ppm and cr 85 7 120 6 ppm and highly fractionated ree patterns and are depleted in hfses with negative nb ta zr hf and ti anomalies we suggest that the high mg diorites in the dggb may have been derived from a hydrated mantle wedge which was previously metasomatized by subduction derived melts and or fluids collectively a neoarchean subduction accretion collision event is therefore proposed to have generated the dggb the volcano sedimentary assemblage in the dggb represents a fore arc subduction accretion complex which is interpreted to be related to the suture zone of ca 2 5 ga arc continent collision between a ttg dominated arc terrane in the cob and the eastern block of the ncc we further propose a long n s striking neoarchean suture zone occurring in the eastern margin of the cob mainly consists of the ca 2 5 ga subduction accretion complex in the dggb to the south the ca 2 5 ga melange belt in the zanhuang complex in the central and the ca 2 5 ga ophiolitic melange belt in the zunhua dongwanzi structural belt to the north which separate an arc terrane in the cob from the eastern block of the ncc

anton terrane revisited late archean exhumation of a moderate pressure granulite terrane in the western slave province the anton terrane of the western slave province has been interpreted as a remnant of a meso archean 3500 2800 ma microcontinent the largest coherent entity of the terrane is a previously unrecognized 50 by 250 km granulite complex herein termed the snare domain the granulite facies mineral assemblages give the domain a characteristic magnetic signature that is distinct from areas of exposed meso archean basement in the slave province the domain consists of interlayered 3100 2624 ma orthogneisses and paragneisses and 2600 2585 ma granitoid rocks its structure was largely established during 2610 2590 ma orogeny throughout the province that culminated in ca 2590 ma granulite facies metamorphism pressure temperature p t determinations yield peak conditions of 775 875 c at 6 7 kbar shortly thereafter 2 4 kbar of decompression occurred before the domain cooled below 450 c decompression and retrogression textures are widespread but are most pervasive adjacent to late extensional detachment faults u pb zircon age data suggest that the snare domain was partially exhumed from mid crustal levels ca 2585 2580 ma we attribute exhumation to tectonic denudation of an overthickened crustal welt in the western slave province localization of the crustal welt in this part of the slave province may be related to the presence there of meso archean crust

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

the 1991 1996 natmap slave province project introduction the 11 papers contributed to this issue on the geology of the slave province are summarized and where appropriate links between data from the various contributions are emphasized in the discussion we provide a synthesis of the present knowledge base on pre yellowknife supergroup basement rocks throughout the slave province all available age data on mesoarchean and paleoarchean basement rocks are compiled and in a first attempt to seek regional correlations 10 events of crustal growth and or reworking are recognized spanning well over 1 billion years of earth s history the spatial distributions of these events across the craton are discussed finally we pose a series of important questions that remain and that may help guide future research

lithosphere development in the slave craton a linked crustal and mantle perspective the late tectonic evolution of the slave craton involves extensive magmatism deformation and high temperature low pressure ht lp metamorphism we argue that the nature of these tectonic events is difficult to reconcile with early pre 2 7 ga development and preservation of a thick tectosphere and suggest that crust mantle coupling and stabilization occurred only late in the orogenic development of the craton the extent and repetitiveness of the tectonic reworking documented within the mesoarchean basement complex of the western slave together with the development of large volume extensional mafic magmatism at 2 7 ga within the basement complex argue against preservation of a widespread thick cool mesoarchean tectosphere beneath the western slave craton prior to neoarchean tectonism broad scale geological and geophysical features of the slave craton including orientation of an early f1 fold belt distribution of ca 2 63 2 62 ga plutonic rocks and the distribution of geochemical petrological and geophysical domains within the mantle lithosphere collectively highlight the importance of an ne sw structural grain to the craton these trends are oblique to the earlier ca 2 7 ga north south trending boundary between mesoarchean and neoarchean crustal domains and are interpreted to represent a younger structural feature imposed during northwest or southeast vergent tectonism at ca 2 64 2 61 ga extensive plutonism in part mantle derived crustal melting and associated ht lp metamorphism argue for widespread mantle heat input to the crust a feature most consistent with thin 100 km lithosphere at that time we propose that the mantle lithosphere developed by tectonic imbrication of one or more slabs subducted beneath the craton at the time of development of the d1 structural grain producing the early 2 63 2 62 ga arc like plutonic rocks subsequent collision external to the present craton boundaries possibly accompanied by partial delamination of some of the underthrust lithosphere produced widespread deformation d2 and granite plutonism throughout the province at 2 6 2 58 ga an implication of this model is that diamond formation in the slave should be neoarchean in age d 2003 elsevier b v all rights reserved

petrogenesis and geochronology of paleoproterozoic magmatic rocks in the kongling complex evidence for a collisional orogenic event in the yangtze craton abstract the early precambrian crustal evolution of the yangtze craton especially the paleoproterozoic orogenic event has drawn great attention the early precambrian magmatism in the archean palaeoproterozoic basement kongling complex is critical for understanding the early precambrian tectonic evolution of the yangtze craton we report the presence of two distinct groups of paleoproterozoic felsic rocks in the kongling complex providing new insights into the paleoproterozoic orogenic event in the northern yangtze craton group 1 is composed mainly of deformed or weakly deformed monzogranite syenite and quartz monzonite and formed at 2 0 1 95 ga they have high sr y 60 71 and la ybcn 32 464 ratios and are inferred to have equilibrated with garnet rich residues suggesting that they have been generated by partial melting of thickened continental crust group 2 is represented mainly by undeformed to weakly deformed k feldspar granitic rocks with minor rhyolites and was emplaced at 1 87 1 84 ga in contrast to group 1 group 2 rocks display flat hree patterns and large negative eu sr and ti anomalies with low sr y zircons from group 1 rocks have ehf t values ranging from 9 4 to 14 9 whereas zircons from group 2 rocks have lower ehf t values ranging from 14 2 to 20 6 both groups have similar two stage hf model ages ranging between 3 2 and 3 6 ga the production of the two groups of granitoids in the huangling dome is ascribed to the reworking dehydration melting of archean rocks in the paleoproterozoic widespread paleoproterozoic magmatism and metamorphism indicate that the yangtze craton experienced an orogenic event at ca 2 0 ga and subsequent orogenic collapse caused by lithospheric extension at ca 1 85 ga

flat slab subduction trench suction and craton destruction comparison of the north china wyoming and brazilian cratons abstract the mechanisms of growth and destruction of continental lithosphere have been long debated we define and test a unifying plate tectonic driving mechanism that explains the numerous petrological geophysical and geological features that characterize the destruction of cratonic lithospheric roots data from three archean cratons demonstrate that loss of their roots is related to rollback of subducted flat slabs some along the mantle transition zone beneath the cratons during flat slab subduction dehydration reactions add water to the overlying mantle wedge as the subducting slabs roll back they suck in mantle material to infill the void space created by the slab roll back and this fertile mantle becomes hydrated the roll back causes concomitant lithospheric thinning of the overlying craton so the flux of newly hydrated mantle material inevitably rises causing adiabatic melting generating new magmas that gradually destroy the roots of the overlying craton through melt peridotite reactions calculated fluxes of new mantle material beneath cratons that have lost their roots range from 2 7 trillion to 70 million cubic kilometers which is sufficient to generate enough melt to completely replace the affected parts of the destroyed cratons cratonic lithosphere may be destroyed in massive quantities through this mechanism warranting a re evaluation of continental growth rates with time

timing of plutonism deformation and metamorphism in the yellowknife domain slave province canada u pb zircon titanite and monazite ages for plutonic rocks are used to constrain the timing of post 2660 ma deformational and metamorphic events in the yellowknife domain southern slave province diorite and tonalite plutons of the defeat suite yield ages of 2628 3 and 2624 3 ma older than previous less precise estimates for this suite of ca 2618 2620 ma the defeat suite plutons crosscut and postdate upright f1 folds in turbidites of the ca 2680 2660 ma burwash formation d1 deformation is therefore bracketed between 2660 and 2630 ma the d1 event significantly predates deposition of polymict conglomerates of the jackson lake formation a second folding event f2 represented by northwest to north trending steeply plunging folds of bedding and d1 structures affected the area after intrusion of the hidden lake pluton at 2606 2 ma and synchronously with intrusion of the 2596 2 ma sparrow lake pluton a member of the two mica granite prosperous suite the time of initiation and the duratio

a high resolution opal and radiolarian record from the subpolar north atlantic during the mid pleistocene transition 1069 779ka palaeoceanographic implications abstract a high resolution record of radiolarian assemblages from site u1314 was studied to reconstruct hydrographic and climatic changes in the north atlantic ocean during the mid pleistocene transition period 1069 779 ka besides the faunal record absolute accumulation of radiolarians and total biogenic opal are used to determine changes in surface water productivity results show that the north atlantic arctic front shifted back and forth repeatedly at a glacial interglacial timescale bringing the site under the influence of both cold arctic glacial and much warmer atlantic interglacial waters during glacial intervals and ice rafted debris ird events the deep dwelling taxon cycladophora davisiana was the greatest contributor of the radiolarian assemblage suggesting cold surface conditions melt water discharge and changes in deep and intermediate circulation interglacial intervals were characterized by abundant shallow dwelling taxa like pseudodyctiophimus gracilipes and lithomelissa setosa increased opal accumulation and higher radiolarian diversities indicating a northward flow of warm atlantic surface waters to the site u1314 area a marked change in the structure of the radiolarian assemblage occurred after mis 22 860 ka with large taxa differences between warm and cold periods probably in response to changing ocean conditions due to the higher amplitude of glacial interglacial changes as the dominant periodicity of high latitude climate oscillations shifted from 41 kyr to 100 kyr thus we conclude that the radiolarian assemblage from the north atlantic have changed drastically along with variations in ocean circulation in response to orbital and millennial scale climatic variations that occurred in the early and mid pleistocene

in situ dissolution experiment of radiolaria in the central north pacific ocean abstract monospecific phaeodarian radiolarian assemblages of castanidium longispinum were suspended in plastic cages built with 225 m nylon mesh at different water depths from 378 to 5582 m in the central north pacific weight losses of these samples after a suspension period of 61 days were used to determine dissolution rates the highest weight losses were observed at 378 m where samples lost 90 of their initial weight through the main thermocline weight losses decreased from 90 to 60 and reached a constant value of 40 below it these weight losses are roughly an order of magnitude higher than those reported by earlier workers the higher weight losses can be attributed in part to the more soluble nature of the phaeodarian radiolarian skeletons and in part to the improved experimental technique kinetic considerations show that temperature is the major factor that controls silica dissolution rates in the ocean using an arrehnius plot for the apparent rate constants it can be shown that in surface water dissolution rats should be two orders of magnitude higher than in deep water below the main thermocline