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

the properties and geological environments of helicoids axially symmetric surfaces in torsional and non torsional deformations abstract a helicoid is a non developable surface swept out by a line the generator which rotates with constant sense about another non parallel line the heliaxis whilst moving parallel to that line in a constant direction torsion may produce helicoids though this is not a necessity previous references to helicoids and helices in structural geology are reviewed and the basic properties of helicoids are compared with those of cylindrical and conical surfaces a map and stereogram technique is developed for the recognition of nested initially parallel equally rotated coaxial macroscopic helicoids the typically sigmoidal outcropping traces of nested helicoids show dip direction reversals along trend on stereograms linear loci each representing the poles from a single helicoid trace diverge from a common pole a nested set of macroscopic helocoidal cleavage surfaces from the hill end trough southeastern australia are modelled and interpreted to have formed in two stages of torsion associated with accommodation of an adjacent granitoid intrusion the volume affected by torsion is bound by curviplanar scissor faults helicoids may be found associated with rotational porphyroblasts deformed cylindrical bodies or strip like surfaces e g intrafolial fold axial planes cylindrical diapirs with helical flow patterns cylindrical scissor faults ductile shear zones zones of constrictional strain and en echelon segmented dykes and fracture planes

alteration and geochemical patterns in the 3 7 3 8 ga isua greenstone belt west greenland abstract volcanic rocks of the 3 7 3 8 ga isua greenstone belt have undergone heterogeneous poly phase post magmatic alteration resulting in mobilisation of many elements including lree and th in contrast nb ta ti zr y and hree were relatively immobile during alteration processes the degree of element mobility appears to have been controlled by chemical composition primary volcanic structures carbonate and silica enrichment and proximity to ultramafic units and shear zones on the basis of the geochemical characteristics of the least altered samples two distinct associations have been recognised in structurally separated lithotectonic sequences these magmatic associations are a low hfse association tio2 0 20 0 40 wt zr 12 30 ppm nb 0 13 0 80 ppm y 6 14 ppm in the central arc sequence resembling tertiary boninites and a high hfse association tio2 0 50 1 14 wt zr 34 77 ppm nb 1 2 2 7 ppm y 11 28 ppm in the outer and inner arc sequences comparable to phanerozoic island arc basaltic to picritic volcanic rocks these two geochemically distinct sequences were likely to have been juxtaposed as a consequence of phanerozoic style plate tectonic processes operating in the early earth the isua volcanic rocks are characterised by sub chondritic nb ta 10 16 ratios post magmatic alteration can be ruled out as the cause of these low ratios rather these ratios reflect a compositional characteristic of the paleoarchean mantle source s the sub chondritic nb ta ratios may have resulted from either preferential sink of nb into the core as a weakly siderophile element or preferential retention of nb in the subducted oceanic crust during subduction zone petrogenetic processes alternatively the building blocks of the earth were not composed of ci like chondritic material

geological and isotopic geochemical constraints on the evolution of the fuping complex north china craton abstract the neoarchean to paleoproterozoic fuping complex together with the adjacent hengshan and wutaishan complexes form the middle part of the central zone that separates the north china craton into the eastern and western archean continental blocks the fuping complex comprises three major lithological assemblages the 2 50 ga fuping trondhjemite tonalite granodiorite ttg gneisses the wanzi paragneisses and the 2 08 and 2 02 ga nanying granitic gneisses the ttg gneisses contain abundant enclaves of highly metamorphosed supracrustal rocks including 2 70 ga mafic granulites and amphibolites with endt of 3 29 to 3 96 and tdm of 2 72 2 82 ga and are characterized by tdm of 2 76 3 04 ga and a linear correlation between endt 1 64 to 0 96 and 1 nd indicative of mixing of melts derived from the 2 70 ga metabasic rocks and 3 0 ga crustal materials mafic dikes along the chengnanzhuang shear zone were most likely emplaced at 2 31 ga and have endt of 2 24 to 4 78 and tdm of 2 34 2 59 ga the nanying granitic gneisses with endt of 4 04 to 5 24 were most likely related to partial melting of the 2 70 ga supracrustal rocks and or the 2 50 ga ttg gneisses these geological relationships and isotopic data suggest that the fuping complex probably formed from the following five major tectonomagmatic events 1 mafic magmatism and early crustal growth at 2 7 ga with the possible existence of older 3 0 ga crust s 2 collision related crustal thickening underplating of basaltic magma high grade metamorphism crustal anatexis and ttg magmatism at 2 5 ga 3 extensional deformation and emplacement of the mafic dikes and the nanying granitic plutons along the chengnanzhuang shear zone 2 5 to 2 0 ga 4 s type granitic magmatism and associated amphibolite facies metamorphism 2 0 to 1 80 ga and 5 emplacement of granitic pegmatite dikes at 1 80 ga

dynamics of the polish and eastern slovakian parts of the carpathian accretionary wedge insights from palaeostress analyses the arcuate outer carpathian accretionary wedge formed in front of the east alpine carpathian pannonian alcapa megablock during the eocene sarmatian the wedge accreted sediments of the subducting remnants of the carpathian flysch basin a large oceanic tract left in front of the alpine orogen the palaeostress data for the orogenic hinterland particularly the data related to the early miocene extension that was expanding towards the ne combined with coeval subduction related volcanism that was expanding towards the ne indicate that the uneven roll back of the subduction zone was the main mechanism controlling the development of the northern west carpathian arc the palaeostress data for the tertiary accretionary wedge from the same time period are characterized by outward fanning 1 trajectories that changed gradually during the wedge development in contrast the palaeostress data for the hinterland are characterized by preferred directional stress events that changed abruptly during the wedge development these palaeostress results are in accordance with the behaviour of the wedge and the hinterland as the wedge behaved as a weak continuum and the hinterland behaved as a block mosaic with weak boundaries the fault traces of the northern west carpathian arc converge to both ends of the arc and suggest that the pre existing basin was the factor that controlled the arc location these fault trace patterns are asymmetric indicating a slightly oblique overall convergence in a ne sw direction in accordance with this convergence the palaeostress data for the accretionary wedge indicate that the western part of the wedge which is characterized by nw se oriented maximum principal compressional stress 1 was undergoing sinistral transpression meanwhile the eastern part which is characterized by ne sw oriented 1 was undergoing compression apparently the dynamics of the accretionary wedge was further influenced by the shape of an elongated ne sw trending alcapa megablock which was located behind the wedge and advanced in the direction of the general early miocene convergence during the most pronounced stages of the wedge development this megablock served as the local indenter as its strength surpassed that of the accretionary wedge located to its front further dynamic complexities were added because of the complex shape of the magura unit which was located in the most proximal portion of the wedge and was stronger than the units in front of it wedge outcrops indicate that the large scale shortening which is characterized by the development of detachments and ramps was preceded by an initial layer parallel shortening this is indicated by scaly fabrics and minor reverse faults that rotated into locked positions during the later accretion several outcrops with a wedge detachment fault indicate that there was a relatively low amount of friction during its development the decollement zone is several hundred metres thick and shows evidence of transient fluid flow that was driven by pressure gradients this is documented by frequent hydrofracturing sandstone dykes and fibrous veins that opened against the weight of the whole wedge all of which indicate cycles of higher pore fluid pressures that lowered the basal friction

regional isostatic response to messinian salinity crisis events abstract the salinity crisis of the mediterranean during messinian time was one of the most dramatic episodes of oceanic change of the past 20 or so million years resulting in the deposition of kilometer thick evaporitic sequences a large and rapid drawdown of the mediterranean water level caused erosion and deposition of non marine sediments in a large lago mare basin both the surface loading by the lower messinian evaporites and the removal of the water load resulted in isostatic flexural rebound that significantly affected river canyons and topographic slopes we use flexure models to quantitatively predict possible signatures of these events and verify these expectations at well studied margins the highly irregular shape of the reconstructed basin calls for a three dimensional model near basin margins plate bending effects are most pronounced which is why flexure is particularly important for a relatively narrow basin like the mediterranean we focus on one specific sea level scenario for the messinian salinity crisis where most of the evaporite load was deposited during a sea level highstand followed by a rapid desiccation evaporite loading at current sea level is expected to cause subsidence of the deep basins by hundreds of meters and simultaneous uplift of continental parts of the margins differential uplift may lead to significant slope angle changes and thus gravity flows the relative scarcity of lower evaporite sequences along the margins may be a result of these phenomena normal faulting of lower evaporite and older sediments and rocks is expected on the margins desiccation enhances erosion of the freshly exposed continental shelf and slope subsidence and riverbed sedimentation occurs on the continental margins and significant uplift towards the basin center reverse faulting is predicted at the margins finally regional isostatic uplift following zanclean flooding is predicted to destabilize margin slope deposits and to cause marginal uplift river down cutting and normal faulting

extreme element mobility during transformation of neoarchean ca 2 7ga pillow basalts to a paleoproterozoic ca 1 9ga paleosol schreiber beach ontario canada abstract this study reports petrographic major and trace element and oxygen isotopic data for a subaerial weathering profile at schreiber beach ontario canada the weathering profile developed on neoarchean ca 2 7 ga pillow basalts and is unconformably overlain by the paleoproterozoic ca 1 88 ga gunflint chert and basal conglomerates this stratigraphy suggests that the basalts were uplifted and subaerially weathered prior to deposition of the gunflint formation rocks at schreiber beach have been classified as pillow cores pillow rims hyaloclastites weathered red basalts and weathered brown to green basalts there are gradual textural mineralogical and geochemical transitions from unweathered basalts to intensely weathered hematite bearing basalts with stratigraphic height rocks at schreiber beach underwent seafloor hydrothermal alteration and greenschist facies regional metamorphism in the neoarchean following the subaerial weathering they were buried and re metamorphosed to greenschist facies in the paleoproterozoic given the preservation of primary igneous textures and trace element patterns in pillow cores they are used as a protolith to estimate element mobility during seafloor alteration metamorphism and subaerial weathering chloritized pillow rims and hyaloclastites are characterized by moderate losses or gains of major si ca na fe ti k and trace lile ree hfse and transition metals elements during neoarchean seafloor alteration alteration of these rocks at relatively low temperatures is indicated by enrichment in 18 o 18 o 10 4 to 12 7 relative to fresh basalt 5 5 subaerial weathering resulted in significant textural mineralogical e g hematite precipitation and geochemical variations in metamorphosed basalts causing additional redistribution of most major and trace elements and further enrichment in 18 o the highest 18 o values 16 1 to 17 1 occur at the silica rich base of the weathering profile many elements including lile ree and hfse are strongly enriched 100 or depleted up to 96 relative to their original abundances alkaline metasomatism which is a characteristic of precambrian paleosols is also evident at schreiber beach where extreme enrichment in k 280 to 1000 rb 390 to 660 and cs 1160 to 2560 occurs in the weathering profile

geometry and kinematics of vertical axis rotations in fold and thrust belts abstract vertical axis rotations indicated by palaeomagnetic study are a common and important component of deformation in fold and thrust belts the kinematics of rotating thrust sheets may be best described as a combination of a far field rotation or translation in a planar geometry and a near field or local rotation so that the instantaneous rotation axis lies outside the body of the sheet this simplifies the thrust sheet kinematics significantly as it means that all the points on the sheet have a component of forward motion and the bounding fault is a thrust over all its area the consequences of a simple thrust sheet rotation for the interpretation of fault lineation data are investigated the lineations recorded in the footwall and the hangingwall are likely to be different the hangingwall lineations being skewed towards the direction of rotation the principal mechanism for local thrust sheet rotation is differential shortening either on folds or thrusts or by a combination of these structures the forward propagation of thrusting towards the foreland can accumulate large 120 rotations on the uppermost sheets these rotational thrust sheets cannot extend indefinitely along strike instead they terminate against lateral ramps or transfer faults sidewall duplexes may form as a consequence of rotation over a lateral ramp examples of rotational geometries from the subbetic zone of southern spain are described on a larger scale obliquely convergent orogenic wedges can evolve by rotation of faults with an initial thrust geometry into progressively greater components of strike slip motion reliable palaeomagnetic data are not always available for fold and thrust belts so independent evidence for rotational deformation is of great importance fold and thrust geometries particularly along strike variability in fold geometries highly variable axial traces of folds evidence for near synchronous refolding and the deviation of regional trends may all be a consequence of thrust sheet rotation detailed mapping and particularly the identification of the position of footwall and hangingwall cut offs can also provide strong evidence for rotation

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

progressive rollover fault propagation folding a possible kinematic mechanism to generate regional scale recumbent folds in shallow foreland belts the occurrence of strongly overturned to recumbent anticlines in shallow foreland thrust fold belts is not well explained by the available geometric models of thrust related folding when a very large amount of forward shear cannot be assumed and for growth anticlines when the tectonic uplift rate is greater than the sedimentation rate we have developed new geometries for fault propagation folding progressive rollover fault propagation folding that are suitable for overcoming these problems in our configuration an outer right way up sector and an overturned sector can grow simultaneously in the forelimb of the tip anticline they are linked to a recumbent leading syncline a semiquantitative computation of fracture strain during folding allowed us to define six homo eneously strained rock panels the location of the most strained panels suggests that an anticlinal breakthrough or a synclinal breakthrough is likely to affect the structure later in its evolution the outer morphology of the tip anticline in progressive rollover fault propagation folding resembles a fault bend fold this model produces geometries suitable for hydrocarbon trapping

palaeomagnetic study of the ronda peridotites betic cordillera southern spain abstract a palaeomagnetic study of the ronda peridotites southern spain has been carried out on 301 samples from 20 sites spread along the three main outcrops of the ultrabasic complex ronda ojen and carratraca massifs different lithologies and outcrops with different degrees of serpentinization have been sampled and analysed rock magnetic experiments have been carried out on a representative set of samples these measurements include curie curves hysteresis cycles isothermal remanent magnetization irm acquisition curves thermal demagnetization of irm imparted along three orthogonal axes and magnetic bulk susceptibility results indicate that magnetite is the main magnetic mineral present in the samples stepwise thermal and alternating field af demagnetization of the natural remanent magnetization nrm reveals the presence of a characteristic remanent magnetization chrm carried by magnetite and in some sepentinized samples a northward component with variable unblocking temperatures up to 250 575 c the appearance and the relative intensity of this northward component are strongly related to serpentinization degree taking into account the geological history of the peridotites the chrm has been considered as a thermo chemical remanent magnetization acquired during the first serpentinization phase associated to the post metamorphic cooling of this unit on the basis of radiometric and fission track analysis the chrm is proposed to have been acquired between 20 and 17 18 ma the inclination of the mean direction of the chrm statistically coincides with the expected inclination for stable iberia during the oligocene miocene the declination of the chrm differs from the expected declination indicating clockwise block rotations of 41 12 about vertical axes since the cooling of the peridotites when applying a compositional layering correction the chrm directions fail to pass this kind of fold test thus the compositional layering was not a palaeohorizontal during chrm acquisition time normal and reversed polarities of the chrm are reported showing that at least one reversal of the earth s magnetic field took place during chrm acquisition process a tentative polarity zonation within the peridotitic outcrops is also suggested no evidence is found from these data for the previously proposed simultaneity between post metamorphic cooling and rotation of the peridotites

formation age and tectonic setting of the shirengou neoarchean banded iron deposit in eastern hebei province constraints from geochemistry and sims zircon u pb dating abstract the shirengou iron deposit a typical bif deposit located in eastern hebei province in the north china craton is hosted in neoarchean metamorphic rocks the metamorphic rocks include hornblende plagiogneiss plagioclase amphibolite and magnetite quartzite we use contents and ratios of major elements for protolith reconstruction showing that protoliths of the hornblende plagiogneiss and plagioclase amphibolite should be dacite and basaltic andesite respectively based on geology and geochemistry of the hosting rocks we infer that the shirengou bif is an algoma type deposit formed in an arc related basin in the neoarchean constrains from sims u pb dating and oxygen isotopic analysis of zircon from wall rocks and interlayer of the shirengou iron bodies can be summarized as follow 1 igneous zircons from two samples of amphibolite and gneiss yield u pb ages of 2541 21 ma and 2553 31 ma respectively providing the age of the neoarchean protolith because the shirengou bif is an algoma type deposit determined u pb age 2541 2553 ma for zircons from hosting volcanic rocks can constrain the bif forming time 2 metamorphic overgrowth zircons from amphibolite and gneiss give two u pb ages of 2512 13 ma and 2510 21 ma respectively corresponding to the neoarchean matemorphism 3 the zircons with 18o values of 5 9 7 6 also indicate magmatic origin the zircons have 18o values of 6 8 9 9 suggesting that exotic high 18o fluids were involved during metamorphism based on the geology and geochemistry of bif and combing with zircon age and oxygen isotopic systematics we suggest that the shirengou iron deposit was formed in a submarine volcanic setting related to subduction of an oceanic slab

the application of zircon cathodoluminescence imaging th u pb chemistry and u pb ages in interpreting discrete magmatic and high grade metamorphic events in the north china craton at the archean proterozoic boundary abstract two distinct late neoarchean to early paleoproterozoic magmatic events are recognized at the northern margin of the eastern block of the north china craton based on interpretation of zircon structure cathodoluminescence signature th u pb chemistry and age the first event is represented by emplacement of the chaihe trondhjemite and xianjinchang tonalite with magmatic zircon core ages ranging from 2559 2553 7 ma respectively these rocks experienced a subsequent high grade metamorphic event constrained by partial recrystallization of primary magmatic zircon and growth of new zircon rims between 2517 and 2490 ma a second magmatic episode generated by partial melting of crustal rocks during this metamorphic event is represented by the hongshilazi syenogranite with an age of 2502 11 ma the short time interval between initial magmatism and high grade regional metamorphism signifies they were related to the same major tectonothermal event when considered along with evidence of a similar sequence of events at this time 800 km to the south in eastern hebei and the previously established 2500 ma age of granulite facies metamorphism in mafic rocks across the craton it is proposed that a large mantle plume developed beneath the north china craton at the close of the archean

unidirectional slip vectors in the western betic cordillera implications for the formation of the gibraltar arc we have investigated the kinematics of deformation in a sector of the gibraltar arc in sw spain by synthesizing lineation data from major faults the variation in structural trends and palaeomagnetically determined vertical axis rotations the kinematic data show that transport directions throughout formation of the arc were consistently wnw regardless of structural trend which varies between 050 and 350 in the study area the data suggest that this part of the gibraltar arc can be explained simply by oblique convergence with a curved indenter taken together with other published data it appears likely that the variation in thrust transport direction within the betic cordillera as a whole may be considerably less than that in the moroccan part of the arc unidirectional convergence combined with the presence of palaeomagnetic rotations implies either eycient overprinting of old rotated lineations and or a convergence rate that is much greater than the rotation rate also required is a low degree of partitioning in the thrust wedge the latter probably increases east of the study area where the structural trend becomes more oblique to the transport direction and major strike slip faults occur

remobilisation of the continental lithosphere by a mantle plume major trace element and sr nd and pb isotope evidence from picritic and tholeiitic lavas of the noril sk district siberian trap russia the late permian to early triassic siberian traps have been sampled by drill core core sg 9 and from surface exposure section 1f in the noril sk region of the siberian platform russia combined major trace element and nd sr and pb isotope data on selected samples through the siberia trap offer new chemostratigraphic criteria for the identification and characterisation of two fundamentally different magma types and 9 of the 11 formations of lava developed near noril sk a lower sequence of sub alkalic basalts tholeiites and picritic basalts upwards these are the ivakinsky syverminsky and gudchichinsky formations are overlain by an upper sequence of picritic basalts and tholeiites interbedded with tuffs upwards these are the khakanchansky tuklonsky nadezhdinsky morongovsky mokulaevsky and kharayelakhsky formations the gudchichinsky and tuklonsky formations contain both picritic and tholeiitic lavas the tuklonsky formation tholeiites and picrites have moderate gd yb 1 6 1 8 low tio2 0 45 0 95 wt a significant negative ta and nb anomaly nb la 0 42 0 57 and unradiogenic nd nd chur to 4 6 in contrast both the gudchichinsky formation tholeiites and picrites have high gd yb 2 3 3 1 and tio2 1 2 2 3 wt no significant nb or ta anomaly nb la 0 8 1 1 and radiogenic nd nd chur to 7 3 the low ti and nb la high la sm and unradiogenic nd isotope signatures of the picritic tuklonsky formation lavas and the tholeiitic lavas of the upper sequence are characteristic of magmas strongly influenced by material from the continental lithosphere whereas the high ti and nb la low la sm and radiogenic nd isotope signatures of the lower sequence are more comparable to deeper asthenospheric mantle plume generated lavas similar to oceanic island basalts the lavas overlying the tuklonsky formation have mg numbers of 0 63 to 0 68 and are more evolved than the tuklonsky mg number 0 62 and have more radiogenic nd chur tuklonsky 0 03 to 4 66 mokulaevsky 0 60 to 1 61 but have many of the incompatible trace element features of the tuklonsky sky type magma these lavas show a progressive upwards decline in sio2 55 49 wt la sm 4 6 2 0 and ur sr 67 to 13 which has previously been attributed to a decrease in the proportion of crustal material contributed to the magma this paper explores and alternative model where a component of the crustal contribution might be derived from within an ancient region of the mantle lithosphere as recycled sediment rather than from the overlying continental crust

a morb arc basalt adakite association in the 2 5 ga wutai greenstone belt late archean magmatism and crustal growth in the north china craton abstract the wutai fuping and hengshan complexes form part of a major 2 5 ga late archean arc system in the northern part of the north china craton the wutai complex is a well preserved greenstone belt dominated by mafic intermediate and felsic volcanic rocks with a mid ocean ridge basalt morb arc back arc basalt adakite association multi element plots for morb type mafic volcanic rocks are characterised by depletion of th nb ta and la nearly flat or lree depleted ree patterns and nb u and nb th ratios greater than primitive mantle values back arc basin basalt babb type rocks have nearly flat trace element patterns with nb ti and y troughs and lree depleted or near flat ree patterns island arc basalt iab type rocks are characterized by negative anomalies of nb ta zr hf ti and y and by lree enrichment the felsic volcanic rocks conform to most compositional criteria for cenozoic adakites including low y and yb high la smn la yb and zr sm ratios and similar trace element and ree patterns trends defined by the association are consistent with magma mixing between morb like melts and subduction slab related melts intra oceanic subduction coupled with contemporary morb type mantle upwelling related to back arc basin extension can account for the interaction between the two components subduction accretion or back arc basin collapse led to accretion of the wutai intra oceanic arc back arc complexes with the magmatic arcs of the fuping and hengshan terranes that are dominated by high grade tonalite trondhjemite granodiorite ttg gneisses this geodynamic interpretation is different from many tectonic models for archean greenstone belts in which magmatism and crustal growth were dominated by mantle plumes or interaction between subduction zones and mantle plumes the final disposition of the arc complexes was controlled by the collision of the eastern and western blocks of the north china craton during the paleoproterozoic resulting in extensive tectonic interleaving within a major orogenic belt that extended throughout the central portion of the north china craton

the significance of 3000 ma granulite facies mafic dikes in the central zone of the limpopo belt southern africa abstract structural petrographic and chemical relationships indicate that a suite of mafic dikes possibly 3000 ma old were emplaced at high crustal levels deformed and metamorphosed epicontinental supracrustal rocks of the central zone of the limpopo belt these dikes were subsequently metamorphosed at granulite grade conditions probably during the limpopo orogeny about 2700 ma ago the earlier metamorphism was under conditions of at least amphibolite grade and isotopic data suggest that it may have occurred between about 3200 ma and 3100 ma ago these relationships indicate that the rocks comprising the central zone did not follow a single p t time loop but instead were involved in two distinct orogenies the orogenies were separated by a period of crustal stability during which the supracrustal rocks presently and originally exposed at the surface were again moved near to the surface this tectonic history also implies that the supracrustal rocks of the central zone and the sand river gneisses are of great antiquity if the sand river gneisses with their unique suite of older mafic dikes form a previously deformed basement to the supracrustal rocks then the sand river gneisses have been involved in three orogenies pb isotopic data indicate that the mafic dikes were derived from a mantle source with a high average ratio at some stage in its evolution prior to 3200 ma all archean lithologic units within the central zone of the limpopo belt are characterized by having been derived from rocks that had a large mu ratio at some time in their history and this relationship suggests that a link existed between the crustal rocks of the central zone and this mantle source from at least 3200 ma ago until it was destroyed by or after the limpopo orogeny the tectonic history of the central zone is different from those of the lithologies forming the adjacent terranes and suggests that it evolved separately from these terranes until they were juxtaposed during the limpopo orogeny

neoarchean to paleoproterozoic continental evolution and tectonic history of the north china craton a review rocks of archean age are widely distributed in the north china craton ncc and can be divided into high grade regions and granite greenstone belts even though most underwent amphibolite to granulite facies metamorphism quartzo feldspathic gneisses occupy 70 to 85 of the total exposure of archean rocks in the craton supracrustal rocks commonly consist of intercalated volcano sedimentary rocks including metabasites meta intermediate to acid rocks banded iron formations metapelites and minor marbles the oldest rocks outcrop over a small area of 4 km2 and are a quartzite amphibolite sequence that yield 3 8 3 5 ga isotopic ages chemical and geochronological data constrain the earliest main crust forming episode at ca 2 9 2 7 ga when most rocks were metamorphosed to upper amphibolite or granulite facies at moderate pressures mafic granulites can be classified into two textural types referred to as near isobaric cooling ibc and near isothermal decompression itd types high pressure granulites and retrograded eclogites occur mainly near the junction of the provinces of hebei shanxi and inner mongolia in the central ncc and in eastern shandong province of the eastern ncc as well as locally in henan province in the southern ncc these high pressure rocks show clockwise p t paths indicating decompression from 14 kb to 5 6 kb an exposed section interpreted as representing precambrian continental lower crust has also been identified in the northern part of the craton although some mafic ultramafic metamorphic rocks have been interpreted as remnants of ancient oceanic crust and mantle by a number of geologists geochemical and petrological evidence demonstrates that many supracrustal orthogneiss associations have island arc affinities therefore several tectonic models have been proposed including continent continent collision and island arc terrain accretion achieved by arc arc or arc microcontinent collision the ncc underwent multiple high grade metamorphic events of which the two most important took place during the intervals 2600 2450 ma and 1950 1750 ma which are considered to be related to early precambrian supercontinental cycles

differential displacement and rotation in thrust fronts a magnetic calcite twinning and palinspastic study of the jones valley thrust alabama us appalachians to test whether a displacement gradient along a curved fault structure requires rotation we studied the northeast striking northwest verging large displacement jones valley thrust fault of the appalachian thrust belt in alabama paleomagnetism anisotropy of magnetic susceptibility ams and calcite twinning analysis complemented by balanced cross sections were used to evaluate the presence and magnitude of any rotation remanence directions from the silurian red mountain formation reveal a prefolding magnetization acquired in the pennsylvanian whereas magnetic analysis shows a strong bedding parallel compaction fabric with a tectonic lineation paleomagnetic directions and magnetic lineations reveal no relative rotation between the hanging wall and footwall of the thrust fault rather than rotation therefore we interpret the jones valley thrust sheet as a structure that developed in a self similar fashion with lateral growth of the fault surface occurring coincident with growth into the foreland

conical folds and apparent rotations in paleomagnetism a case study in the southern pyrenees abstract the so called apparent rotation was defined as the angular deviation between a local paleomagnetic direction after the standard bedding correction and their corresponding paleomagnetic reference j geophys res 85 1980 3659 in this paper we make a theoretical exploration on this concept and we conclude that depending on the number sequence orientation and magnitude of the deformation axes that have affected to the rock volume the apparent rotation may be the addition of a vertical axis rotation plus a spurious rotation the later is an error whose origin is the inappropriate application of the bedding correction during the restoration which does not fit the reverse sequence of deformations then apparent rot s l spurious rot vertical axis rot conical folds are complex geometries that cannot be restored by using the bedding correction however appearance of apparent and spurious rotations has not been studied even though the presence of this kind of folds is very common in fold and thrust belts in this paper we show a way to restore these structures and its associated paleomagnetic data by means of forward modelling on a stereographic projection the modelling has to be based on a good characterization of the geometry fold axis orientation and understanding of the kinematics of the fold general modelling has also allowed us to predict the apparent rotation in conical synclines its magnitude depends on the semiapical angle and on the degree of development of the fold the sense of the rotation clockwise or counter clockwise will depend on the sense of rotation the fold axis the western external sierras provide an excellent case study of apparent rotations due to the presence of a conical fold in the footwall ebro foreland basin of the south pyrenean sole thrust in addition a vertical axis clockwise rotation up to 47 32 in average has been detected in the hagingwall an apparent rotation up to 28 20 in average is observed in the footwall of the structure when a simple bedding correction is used this deviation does not fit with the expected ebro basin direction reference and is caused by the effect of the riglos conical syncline developed by the flexure of the foot wall ramp of the south pyrenean sole thrust the forward modelling carried out considering the geometry and kinematics non significant rotations in the autochthonous foot wall of this structure predicts very well the paleomagnetic observations in the field geographic coordinates with angular departures of only 5 in average the only application of the bedding correction would introduce errors spurious rotations up to 21 12 in average related to the conical geometry that would not allow the differentiation of these distinct structural units

slab breakoff as a mechanism for flipping of subduction polarity in taiwan we propose a model that involves breakoff of the east dipping eurasian slab beneath the taiwan orogen to account for the flipping of subduction polarity in northern taiwan the breakoff was initiated in southern ryukyu in the early pliocene and then propagated southwestward into taiwan like an opening zipper detachment of the eurasian slab has created a mantle window for the north dipping philippine sea plate to move in laterally causing a switch in the subduction polarity slab breakoff not only provides a testable model for the lithospheric structure of taiwan but also accounts for the high heat flow rapid uplift synorogenic extension deep focus earthquakes and north diminishing crustal contraction in the taiwan mountain belt

late archaean granites a typology based on the dharwar craton india abstract extensive field work in the eastern dharwar craton associated with petrographic and geochemical major and trace elements investigations allows four main types of late archaean granitoids to be distinguished 1 na rich granitoids of trondhjemitic tonalitic and granodioritic composition ttg that are characterised by strongly fractionated ree patterns and low hree contents and generally interpreted as slab melts generated by partial melting of metamorphosed hydrated basalt most likely in a subduction environment 2 sanukitoids which are k and mg rich monzonites and granodiorites with ttg like ree patterns associated with marked lile enrichment and considered to result from the reaction of slab melts generated in a subduction environment with and assimilation of mantle wedge peridotite 3 uncommon high hfse mg and k granites with strongly ree and lile enrichment that probably formed by partial melting of an enriched mantle source unlike in the genesis of sanukitoids in this case the slab melt is considered to be wholly consumed by reaction with mantle minerals resulting in mantle enrichment subsequent melting of this enriched mantle probably in a post subduction setting gives rise to high hfse mg and k magmas as demonstrated for the closepet granite dharwar craton the hot mantle derived magma can induce melting of continental crust and then mix with the anatectic products 4 k rich mg poor anatectic biotite granites with ree patterns that are less fractionated and show negative eu anomalies these granites result from the remelting of old basement or recently accreted plutons both with ttg compositions such anatexis can occur either in a subduction or in a post subduction environment this typology based on the well exposed dharwar craton can be extended to late archaean granitoids from all over the world about such 500 analysis were compiled and a number of discriminant diagrams based on both major and trace elements are proposed the most likely tectonic setting for the observed rock types is an accretionary orogen with accretion of continental blocks above a subduction system followed by thermal reworking of the newly accreted continental material however it has been found that archaean subduction related granitoids are significantly different from their modern counterparts implying progressive changes in the modes of magma generation at convergent margins from the archaean to the present

geochronology and petrology of migmatites from the proterozoic damara belt importance of episodic fluid present disequilibrium melting and consequences for granite petrology abstract the oetmoed granite migmatite complex ogmc central damara orogen namibia consists of grt and crd bearing s type granites and hbl and titanite bearing a type granites that intruded into crd sil kfs bearing metasedimentary rocks stromatic migmatites and nebulites stromatic migmatites formed by limited in situ partial melting of metapelites under h2o saturated conditions at 700 c and 4 5 kbar this partial melting event took place close to the peak of regional metamorphism at 510 ma as revealed by sm nd garnet whole rock ages as well as u pb monazite ages the newly formed melt remained more or less at the site of origin melanosomes of the stromatic migmatites do not resemble true residues instead they represent reaction zones between in situ melt and the metasedimentary host rock leucosomes from the stromatic migmatites are lree and hfse depleted which is typical for the low melt fractions generally observed in migmatite terranes mass balance calculations suggest that these leucosomes may be disequilibrium melts similar 18 o whole rock values between 13 and 14 observed in leucosomes and corresponding melanosomes suggest homogenization of oxygen isotopes between the different rock types via the melt and an internal fluid phase during melting other leucosomes with a major element chemistry similar to the leucosomes from the stromatic migmatites but higher ree contents suggest significant incorporation of lree enriched phases monazite from the country rocks for these leucosomes sm nd garnet whole rock ages are 473 ma indicating a second phase of melting nebulites mainly result from injection of granitic melts into the country rocks but their residual chemistry indicates that partial melting and limited melt removal must have occurred monazites from these nebulites record concordant u pb ages between 540 ma and 470 ma indicating episodic migmatization during high grade regional metamorphism intrusive peraluminous granites are likely generated by partial melting of pelitic sources in the lower crust and different source rocks are probably not the controlling factor for the different chemistry of leucosomes and granites therefore different melting conditions fluid absent vs fluid present and different modes of entrainment and solubilities of accessory phases control the elemental budgets of the leucosomes large scale migmatite granite complexes are a substantial part of some high grade terrains but the distinctive geochemical composition of some leucosomes make these rocks unlikely precursors of large scale granitic bodies

assembling north china craton within the columbia supercontinent the role of double sided subduction abstract an evaluation of recent s wave receiver functions s wave velocities and two versions of p wave tomographic images along various transects in the north china craton provides some clues on the subduction collision history of the different crustal blocks and their final amalgamation within the paleoproterozoic columbia supercontinent interpretation of a n s seismic section of the craton suggests thick slab debris sinking to various depths in the mantle the w e seismic corridors show the preservation of a thick 200 km lithospheric root tectosphere beneath the ordos block and its variable and extensive erosion towards the yanliao block eastern block this zone is characterized by layers with marked velocity contrast and suggests repeated stacking of the remnants of underplated and accreted paleoproterozoic oceanic lithosphere the present day lithosphere asthenosphere boundary beneath this region probably marks the erosional plane along which decratonization occurred through subduction erosion from the east and thermal and material erosion by upwelling asthenosphere from below resulting in the partial destruction of the tectosphere and its thinning towards the east within the asthenosphere below the yanliao block younger and thinner slabs predominate in the absence of any prominent thick high velocity layers these younger slabs define a westward polarity and constitute a mega scale duplex formed by underplating through phanerozoic subduction process particularly the pacific plate subduction from the east the lithologic associations within the inner mongolia suture zone dividing the yinshan block to the north and ordos block to the south correspond to an accreted ocean plate stratigraphic sequence with the tonalite trondhjemite granodiorite ttg gneisses charnockites and calc alkaline granites representing a continental arc built up through subduction from the north the seismic transects bring out a contrasting polarity in the subduction regime with an oblique east to southward subduction of the yinshan block and a westward subduction of the yanliao block here i propose a double sided subduction history for the ncc similar to the ongoing subduction process in the western pacific such double sided subduction is considered to promote rapid amalgamation of continental fragments within supercontinents and the subduction polarities and mantle dynamics of ncc are therefore considered to be critical in evaluating the final assembly of the paleoproterozoic supercontinent columbia

geochemistry of neoarchean mafic volcanic rocks and late mafic dikes in the zanhuang complex central orogenic belt north china craton implications for geodynamic setting abstract an archean melange belt has been recognized in the zanhuang complex central orogenic belt cob north china craton ncc all units of the melange belt are intruded by a ca 2 5 ga granite pluton and cross cut by undeformed 2 5 ga pegmatites which constrains the archean formation age of the melange some exotic mafic ultramafic blocks are tectonically dispersed in the metasedimentary matrix to the melange and mainly consist of metabasaltic rocks deformed pillow lavas with epidosite cores gabbros and ultramafic rocks both metabasalts and epidosites are characterized by enrichment in light rare earth elements lree and pronounced negative nb and zr anomalies on chondrite and primitive mantle normalized diagrams these geochemical characteristics are consistent with supra subduction zone geochemical signatures based on the pillow structures and seafloor hydrothermal alteration combined with the geochemical characteristics an intra oceanic arc forearc setting is proposed for the formation of these exotic blocks furthermore a west dipping intra oceanic subduction zone is proposed to have formed in an ocean between the eastern block and an arc terrane prior to 2 5 ga the collision between the arc and the eastern block of the ncc happened at ca 2 5 ga resulting in the formation of the archean melange belt in the zanhuang complex all of the units of the melange belt were intruded by late mafic dikes which are present as boudinaged or lenticular shapes in the melange and surrounding gneisses and are intruded by younger ca 2 5 ga pegmatites geochemical characteristics of the late mafic dikes are consistent with an arc related mantle source region rather than an oib like source accordingly pending determination of their precise age an arc polarity reversal event is proposed to have occurred following the arc continent collision resulting in east dipping subduction beneath the newly accreted arc and eastern block forming the swarm of arc affinity mafic dikes the new subduction zone further resulted in ponded magma below the accreted arc block which induced the partial melting of thickened crust and the intrusion of ca 2 5 ga granitic and pegmatitic melts into the melange and the mafic dikes the mafic dikes were then deformed and metamorphosed when the remaining open part of the ocean between the arc collision modified margin of the eastern block collided with the western block of the ncc

the malaguide alpujarride boundary a major extensional contact in the internal zone of the eastern betic cordillera se spain abstract the internal zone of the betic cordillera in southern spain was formed by thrusting and horizontal ductile shortening in the late cretaceous palaeogene but was subjected to a major phase of vertical thinning during the neogene the contact between the malaguide the structurally highest thrust complex and the underlying alpujarride complex is an example of several major tectonic contacts across which the stratigraphy has been repeated by thrusting but the metamorphic grade increases abruptly downwards detailed work in the eastern betic cordillera documents the relationships across this contact in the sierra espuna petrographic and illite crystallinity studies show that the highest imbricate thrusts of malaguide rocks have only been diagenetically altered whereas the deeper parts of the thrust stack exhibit anchizone facies metamorphism in the northern sierra de las estancias apatite fission track studies confirm that there too malaguide rocks have never been above 200 c in both areas the underlying alpujarride rocks exhibit epizone or greenschist facies metamorphism and a major fault zone with both brittle and ductile structures separates the two complexes the observed rapid increase in metamorphic grade over very small structural thicknesses suggests that part of the original metamorphic zonation has been excised across the contact thus in its present form we interpret the malaguide alpujarride boundary as an extensional contact the extensional motion took place in early middle miocene times as constrained by fission track cooling ages the fault zone at the contact is sub horizontal in the sierra espuna but in the sierra de las estancias it is mainly sub vertical kinematic data from a 5 20 m thick calcmylonite layer within the fault zone in the sierra de las estancias corrected for later tilting indicates that the extensional movement was originally directed with the top to the east northeast the interpretation of the malaguide alpujarride boundary as an extensional contact is similar to recent interpretations of the nevado filabride alpujarride contact at deeper structural levels in the central and eastern betics the identification of another major extension fault system confirms that miocene extension was widespread at all structural levels during the early middle miocene the sierra espuna and the northern margin of the sierra de las estancias lay on the boundary between the region undergoing horizontal extension in the internal zones and the alboran sea to the south and the region undergoing horizontal shortening in the external zones to the north as a result the detailed history of the alpujarride malaguide contact is complicated extensional structures cross cut structures related to earlier alpine shortening and were then affected by further shortening as convergence with the external zones continued

vertical axis rotation associated with folding and thrusting an example from the eastern subbetic zone of southern spain the differential rotation about a vertical axis across a southeast vergent recumbent fold and thrust fault within a single nappe from the eastern subbetic zone of southern spain has been determined using paleomagnetism the upper limb has rotated clockwise by 68 22 relative to the lower limb mapping of the structure indicates that the syncline is an isoclinal fold in the northeast and an open fold in the southwest the differential vertical axis rotation across this structure appears to have been accommodated by a pro gressive increase in along strike horizontal shortening reaching 8 5 km at 3 7 km from the pivot point this differential shortening and the rotation are probably the result of increased drag on the underlying thrust close to the pivot point compared to the farthest extent of the structure the lateral boundary of the rotating sheet is a sinistral strike slip fault which is consistent with the rotational geometry pinning is probably enhanced by lateral heterogeneities within the stratigraphy this result ties rotational deformation to early middle miocene folding and thrusting rather than to late miocene holocene strike slip deformation rotation by pinning of thrust sheets may prove to be an important aspect of deformation in many fold and thrust belts

geochronology and geochemistry of 2 5 to 2 4 ga granitic plutons from the southern margin of the north china craton implications for a tectonic transition from arc to post collisional setting abstract the xutai and lujiagou plutons exposed in the songshan jishan area in the southern margin of the north china craton ncc are studied to constrain the tectonic regime of the ncc between 2 5 and 2 4 ga the xutai pluton dated at 2509 33 ma mainly consists of tonalite and monzogranite with a calc alkaline affinity rittmann index

shallow burial and exhumation of the peloritani mountains ne sicily italy insight from paleothermal and structural indicators we used vitrinite refl ectance and mixedlayered clay minerals to investigate levels of diagenesis of the oligocene miocene basin developed on the nappes of the alpine orogen exposed in the peloritani mountains ne sicily paleothermal indicators were integrated with stratigraphic and structural analyses and published apatite fi ssion track and u th sm he ages to defi ne the late evolutionary stages of the peloritani mountains this multimethod approach allowed us to reconstruct the paleogeothermal gradient of the basin in oligocene miocene times to constrain its burial evolution and discriminate between areas where it has been affected by sedimentary and or tectonic load in the southern area of the basin organic and inorganic thermal parameters increase as function of depth suggesting that their evolution was ruled by sedimentary burial they record a decrease in paleogeothermal gradient values marking the evolution of the basin from a forearc to a thrust top setting during the convergence collision process between the calabria peloritani arc and the african plate on the other hand in the northern edge of the basin vitrinite refl ectance values 0 46 0 58 indicate that the thermal evolution of this area was controlled by tectonic burial related to late langhian early serravallian out of sequence thrust tec tonics the tectonic overburden has been totally removed by extensional tectonics and or erosion since the late miocene the short time span at maximum temperature 2 m y elapsing between thrust stack emplacement and the beginning of tectonic overburden removal has allowed only vitrinite refl ectance and thermochronological indicators to record this compressive reactivation

integration of palaeomagnetic data basement cover relationships and theoretical calculations to characterize the obliquity of the altomira loranca structures central spain this work was funded by the project kinesal cgl2010 21968 c02 02 of the spanish ministry of economy and competitiveness and a phd igme grant for the first author it is a contribution of the geomodels research institute and the grup de recerca de geodinamica i analisi de conques 2014sgr467 supported by the agencia de gestio d ajuts universitaris i de recerca and the secretaria d universitats i recerca del departament d economia i coneixement de la generalitat de catalunya

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

middle miocene early pliocene wedge top basins of nw sicily italy constraints for the tectonic evolution of a non conventional thrust belt affected by transpression the study of geological evolution of a multiphase orogenic belt is complex expecially when the tectonic events are superimposed in a coaxial fashion the sicilian fold and thrust belt represents an interesting case study as a non coaxial superimposition of structures is recognizable owing to large synkinematic clockwise rotations during each of two subsequent compressional events these rotations involved also the syntectonic basins that developed in the wedge top depozone this study aims to constrain the tectono depositional evolution of the nw sicily fold and thrust belt and the associated wedge top depozone between the middle miocene and the early pliocene integrated analyses of stratigraphic sedimentological and structural field data allow us to better constrain the transition between the two tectonic events the syntectonic basins developed during the first late serravallian early tortonian tectonic event were relatively wide and characterized by marine sedimentation the onset of the second latest tortonian transpressional event induced localized deformation into the wedge top depozone and the syntectonic successions were accommodated as basin fill in progressively narrower and laterally discontinuous basins bounded by transpressional structures the lateral correlation of the wedge top successions suggests a latest miocene regional palaeoslope with a present day wsw dip which fits well with the tectonic transport calculated for the early compressional event

lower miocene deformation in the hanging wall of the internal external zone boundary of the betic cordillera deformation at the edges of vertical axis rotation domains in oblique convergent margins

a late archean foreland fold and thrust belt in the north china craton implications for early collisional tectonics abstract the archean north china craton is divided into the western and eastern blocks along the central orogenic belt a 1600 km long archean foreland basin and thrust belt fringes the eastern side of the central orogenic belt rocks in the orogen form tectonically stacked east vergent fold and thrust sheets including foreland basin sediments 2 50 ga ophiolitic melange and an island arc complex foreland basin sediments overlie a passive margin sequence and include a 2 50 ga deep water turbidite sequence that grades upward and westward into shallow water molasse now disposed in structurally imbricated east verging thrusts and asymmetric folds that gradually migrated craton ward with deformation uplift and erosion of the orogen there is a strong linked relationship of the formation of the foreland basin to collision of the east and west blocks of the north china craton along the central orogenic belt at 2 50 ga the qinglong foreland basin and central orogenic belt of the north china craton represents one of the best preserved archean orogen to craton transitions in the world its classic internal to external zonation and flexural response to loading demonstrate that convergent tectonics in the archean were broadly similar to phanerozoic convergent margin processes

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

a neoarchean subduction polarity reversal event in the north china craton abstract subduction polarity reversal events following arc continent arc arc or continent continent collisions have been well documented from cenozoic mesozoic and paleozoic orogens but not from the archean we here document a neoarchean subduction reversal event after an arc continent collision between the eastern block of the north china craton ncc and the fuping arc using field geochemical and geochronological data we focus our work on the wangjiazhuang granite in the zanhuang massif located along the eastern margin of the central orogenic belt cob of the ncc and a regional tectonic comparison with other granitic rocks with similar ages geochemical and petrogenetic characteristics the ca 2 5 ga a type wangjiazhuang granite intrudes the neoarchean zanhuang melange belt and contains mafic and felsic inclusions it has positive end t values 0 12 to 1 13 and tdm2 ages between 2784 ma and 2869 ma this work shows clearly from field structural relationships geochemistry and geochronology that the wangjiazhuang granite formed after an arc continent collision between the eastern block which is defined as a continental block and the fuping arc after a subduction polarity reversal event placed a new slab beneath the collisionally modified margin of the eastern block and converted it to an andean type margin the subduction polarity reversal event at ca 2 5 ga resulted in melting of the enriched mantle meanwhile the rising magma induced partial melting of the old and thickened ttg crust leading to the intrusion of ca 2 5 ga wangjiazhuang granite into the neoarchean zanhuang melange there are other granitic rocks with similar ages and geochemical and petrogenetic features in the central orogenic belt and eastern block of the north china craton suggesting that they formed in a similar tectonic setting as the circa 2 5 ga granites across the eastern block the neoarchean subduction polarity reversal event and prior arc continent collision provide strong evidence that plate tectonics was operating by the end of the neoarchean

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

a new proposal for the late cenozoic geodynamic evolution of the western mediterranean a simple mechanism of arcuate fold belt and back arc basin formation is presented based on the opening of mega continental tension gashes along pre existing deep parallel and steep faults that separate lithospheric units with different properties if plate convergence is parallel to these faults the fault bounded units open at right angles to the convergence vector adopting an arcuate shape with thrusting in front of the bowed out units and extensional basin opening between the separated units this model is applied to the cenozoic geodynamic evolution of the western mediterranean after the iberian collision 35 30 ma several ellipsoidal basins valencia alboran north algerian and liguro provencal developed by 10 ma along the eastern margin of the iberian plate the formation of these basins is attributed to an increase in ne sw horizontal tectonic palaeostress during early middle miocene times resulting from the post subduction collision of the tethyan oceanic lithosphere

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

tertiary extension development and extension compression interplay in the west carpathian mountain belt abstract this paper describes palaeostresses calculated from fault striae data the inferred palaeostrain patterns and determines the inter relation of the compression and extension during the tertiary development of the west carpathians the calculated stress and inferred strain patterns for the palaeogene burdigalian indicate that the ancestral west carpathians formed a more or less straight orogenic belt this belt underwent contraction and uplift in its narrow frontal zone stretching along its strike during the paleocene chattian and regional contraction and uplift during the chattian burdigalian the strain stress pattern is similar to the collision related pattern known from the eastern alps for the paleocene burdigalian during the burdigalian tortonian the calculated and inferred west carpathian stress and strain patterns indicate narrow frontal contractional and lateral sinistral transpressional zones in the orogenic front and broad extensional and dextral transtensional zones in the orogenic interior the stress strain pattern is similar to the subduction related pattern known from areas such as the hellenic or sunda banda arcs

origin of granites in an archean high grade terrane southern india archean deep level granites in southern india are similar geochemically to young granites from continentalmargin arc systems they exhibit light ree enriched patterns with variable but chiefly positive eu anomalies this is in striking contrast to the negative eu anomalies typical in high level archean granites in addition the deep level granites are relatively enriched in ba and sr and depleted in total ree and high field strength elements hfse one pluton the sankari granite has unusually low contents of ree and hfse most of the deep level granites appear to represent cumulates with variable amounts of trapped liquid and of minor phases resulting from fractional crystallization of a granitic parent such parental granitic magmas can be produced by batch melting of archean tonalite at middle to lower crustal depths the sankari granite requires a tonalitic source depleted in ree and hfse archean tonalites and tonalitic charnockites exhibit original igneous geochemical signatures and their average composition does not show a significant eu anomaly hence they cannot represent the positive eu anomaly complement to the negative eu anomaly high level granites our results suggest that archean deep level granites may represent this complement such granite may form in waterrich zones in the middle or lower crust and be produced in response to dehydration of the lower crust by a rising co2 rich fluid phase

zircon geochronology and metamorphic evolution of mafic dykes in the hengshan complex of northern china evidence for late palaeoproterozoic extension and subsequent high pressure metamorphism in the north china craton abstract magmatic and metamorphic zircons have been dated from ductilely deformed gabbroic dykes defining a dyke swarm and signifying crustal extension in the northern part of the hengshan complex of the north china craton these dykes now occur as boudins and deformed sheets within migmatitic tonalitic trondhjemitic granodioritic and granitic gneisses and are conspicuous due to relics of high pressure granulite or even former eclogite facies garnet pyroxene bearing assemblages shrimp ages for magmatic zircons from two dykes reflect the time of dyke emplacement at 1915 ma whereas metamorphic zircons dated by both shrimp and evaporation techniques are consistently in the range 1848 1888 ma the youngest granitoid gneiss yet dated in the hengshan has an emplacement age of 1872 17 ma these results complement recent geochronological studies from the neighbouring wutai and fuping complexes to the se of the hengshan showing that a crustal extension event occurred in the late palaeoproterozoic this preceded a major high pressure collision type metamorphic event in the central part of the north china craton that occurred in the palaeoproterozoic and not in the late archaean as previously thought our data support recent suggestions that the north china craton experienced a major craton wide orogenic event in the late palaeoproterozoic after which it became cratonized and acted as a stable block

late neoarchean magmatic and subsequent metamorphic events in the northern north china craton shrimp zircon dating and hf isotopes of archean rocks from yunmengshan geopark miyun beijing abstract the miyun geopark is located in the northern part of the north china craton and includes a variety of archean granitoids and metamorphic rocks shrimp zircon dating documents their emplacement ages at 2567 2508 ma metamorphic events at ca 2 50 ga 2 44 ga and 1 82 ga late neoarchean basement rocks elsewhere in the north china craton document a similar evolutionary history with intrusion of granitoid rocks commencing at ca 2590 ma and reaching three peaks at about 2550 2520 and 2500 ma these rocks then underwent regional metamorphisms at ca 2 50 ga 2 44 ga 1 92 ga and 1 82 ga positive e hf t values of 1 7 to 7 6 with model ages younger than 3 0 ga for magmatic zircons from these late neoarchean intrusive rocks indicate that they are predominantly derived from juvenile crustal sources and suggest that significant crustal growth occurred in the northern north china craton during the neoarchean the intrusive rocks in the miyun area are part of an extensive province of late neoarchean magmatism and appear to be related to mantle plume activity at this time

configuration of the late paleoproterozoic supercontinent columbia insights from radiating mafic dyke swarms abstract remnants of 2 1 1 8 ga orogens can be recognized in nearly every craton assembled within reconstructions of the rodinia supercontinent although no particular pattern of laterally extensive orogenic belts emerges these cratons may be the fragments of an older supercontinent formed in response to the collision and accretion of even older cratons during the early earth history paleomagnetic constraints have played a limited role in many previous reconstructions of the early precambrian supercontinent mainly because of the poor age control and a large scatter of the paleomagnetic poles the geometry of giant radiating dyke swarms and orogenic belts provide important constraints for the reconstruction of the pre rodinian supercontinent some early unmetamorphosed and undeformed mafic dyke swarms in north china southern peninsular india and north america share coeval age of 1 85 ga if these continents are assembled the overall pattern exhibited by the mafic dyke swarms appears to constitute a giant radiating swarm with a piercing point between the cuddapah rift in south india and the xiong er aulacogen in north china this suggests that the north china craton indian craton and canadian shield may be fragmented from the same landmass the reconstruction of 1 85 ga giant radiating dyke swarm attempted in this paper suggests that the north china craton india craton and canadian shield were united together to form a landmass within the columbia supercontinent before its extension and break up the 1 90 1 85 ga andean style north orogenic belt suggests that a subduction zone existed on the northern margin of the north china craton wopmay orogenic belt developed in a 1 88 1 84 ga continental volcano plutonic arc which is interpreted as the product of eastward subduction of oceanic lithosphere the transantarctica orogenic belt in east antarctica may be another candidate to be linked with the subduction zone the north orogenic wopmay orogenic and transantarctica orogenic belts could be connected to form a continuous subduction zone the orogenic comparison strengthens the configuration of the columbia supercontinent proposed by the reconstruction of the giant radiating dyke swarm the laurentia west australia and east antarctica were relatively stable from 1 85 ga to 1 20 ga as inferred from the data on dyke swarms magmatism and the paleomagnetism these continents constituted the core of the columbia supercontinent in the late paleoproterozoic time the north china craton and indian craton were however separated from the columbia supercontinent during the mesoproterozoic time

geophysical and geological tests of tectonic models of the north china craton abstract the geometry and timing of amalgamation of the north china craton have been controversial with three main models offering significantly different interpretations of regional structure geochronology and geological relationships one model suggests that the eastern and western blocks of the ncc formed separately in the archean and an active margin was developed on the eastern block between 2 5 and 1 85 ga when the two blocks collided above an east dipping subduction zone a second presumes the eastern block rifted from an unknown larger continent at circa 2 7 ga and experienced a collision with an arc perhaps attached to the western block above a west dipping subduction zone at 2 5 ga and the 1 85 ga metamorphism is related to a collision along the northern margin of the craton when the ncc joined the columbia supercontinent a third model suggests two collisions in the central orogenic belt at 2 1 and 1 88 ga but recognizes an early undated deformation event recent seismic results reveal details of the deep crustal and lithospheric structure that support both the second and third models showing that subduction beneath the central orogenic belt was west directed and that there is a second west dipping paleosubduction zone located to the east of the cob dipping beneath the western block ordos craton the boundaries identified through geophysics do not correlate with the boundaries of the trans north china orogen suggested in the first model and the subduction polarity is opposite that predicted by that model high pressure granulite facies metamorphism at 1 85 ga is not restricted to the tnco as suggested by the first model but is documented across the ncc as predicted by the second model suggesting a major continent continent collision along the north margin of the craton at 1 85 ga further it has recently been shown that in the southern tnco there is no record of metamorphism at circa 1 85 ga but only at 2 7 2 5 ga showing that the tnco as defined as a circa 1 85 ga orogen does not exist this is further confirmed by recent re os isotopic studies which show that the subcontinental lithospheric mantle beneath the southern cob is late archean in age and that a province in the northern ncc is circa 1 8 ga correlating with the proposed collision belt of the ncc with the columbia supercontinent across the entire ncc the cob is an archean convergent belt re worked in the paleoproterozoic and the paleoproterozoic tectonism is widespread across the ncc as predicted by the model whereby the previously amalgamated eastern and western blocks experienced a continental collision with columbia at circa 1 85 ga but uplift exhumation rates are slow necessitating a re evaluation of the tectonic models of the ncc

syn collisional channel flow and exhumation of paleoproterozoic high pressure rocks in the trans north china orogen the critical role of partial melting and orogenic bending within the paleoproterozoic trans north china orogen the high pressure belt hpb is made of high pressure 15 kbar mafic granulites hosted in migmatitic gneisses in this contribution we document a set of structural analyses acquired over the whole hpb we also proposed a morphological subdivision of the partially molten rocks that compose the hpb according to changes in melt fraction a compilation of the p t and radiochronological data carried out over the last 15 years is presented the results highlight the concurrent effect of oroclinal bending and partial melting in controlling the exhumation of the deeply buried continental crust during ongoing compression of the thickening orogenic root onset of partial melting at peak metamorphism is responsible for a first strength drop that enhanced an eastward lateral flow radiometric ages show that the deep crust was partially molten over a 50 ma lasting period during which it evolved in a diatexite core mantled by metatexites this was responsible for a second strength drop with strain concentrated along the diatexite metatexite boundaries as exemplified by the newly documented datong chengde shear zone a 400 km long normal shear zone with a sinistral strike slip component that accommodated the final uprise of the high pressure rocks

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

paleomagnetism u pb geochronology and geochemistry of marathon dykes superior province and comparison with the fort frances swarm we report the first detailed study of the paleomagnetism u pb geochronology and major element geochemistry of paleoproterozoic north trending marathon dykes north of lake superior the paleomagnetic and geochemical results demonstrate that marathon dykes can be divided into two subsets one of normal magnetic polarity the other of reverse polarity normal and reverse marathon paleomagnetic poles at 43 n 196 e dm 9 dp 7 number of dykes n 16 and 51 n 175 e dm 9 dp 6 n 12 respectively are statistically distinct and may indicate different ages of normal and reverse dyke emplacement a u pb baddeleyite age of ma has been obtained at a normally magnetized marathon paleomagnetic site the reversely magnetized marathon dykes are undated but have a paleopole rather close to that of the reversely magnetized ma fort frances dykes and major element geochemical signatures as portrayed on jensen plots that are identical to those of the fort frances swarm therefore reverse marathon an

geochronology mantle source composition and geodynamic constraints on the origin of neoarchean mafic dikes in the zanhuang complex central orogenic belt north china craton abstract mafic granulitic and amphibolitic boudins dispersed in archean felsic gneisses are widely distributed in the central orogenic belt cob and the eastern block of the north china craton ncc and are considered to constitute deformed mafic dike swarms previous studies have demonstrated that the mafic dikes in the zanhuang complex of the ncc intruded the fabrics of an archean melange belt and were boudinaged during younger deformation igneous zircons from an undeformed mafic dike yield a 207 pb 206 pb age of 2535 30 ma which is interpreted as the crystallization age in addition pegmatites cutting across the mafic dikes in the field also yield an igneous zircon 207 pb 206 pb age of 2504 16 ma providing strong evidence that the mafic dikes in the ncc intruded during the neoarchean metamorphic zircons from one deformed mafic dike sample yield a metamorphic 207 pb 206 pb age of 2090 83 ma and another four samples from deformed mafic dikes have a consistent metamorphic zircon age of ca 1850 ma indicating that the mafic dikes underwent at least two generations of paleoproterozoic metamorphism of ca 2 1 ga and ca 1 85 ga previously reported trace element systematics of the mafic dikes are consistent with an arc related lithospheric mantle source region rather than an ocean island basalt oib like source region the new whole rock nd isotopic composition nd t 0 71 to 3 70 is relatively more evolved compared to that of the depleted mantle at 2 5 ga indicating an enriched lithospheric mantle source accordingly the mafic dikes are proposed to have been formed in a subduction related environment and their enriched mantle source was metasomatized by the melts and fluids derived from the subducted slab based on previous studies of the ncc and new geochronological and isotopic data in this contribution a new comprehensive tectonic model is proposed for the evolution of the ncc between 2 7 ga and 1 85 ga 1 from 2 7 to 2 5 ga an oceanic arc terrane belt developed in what is now the cob of the ncc while a passive margin sequence was deposited on the western edge of the eastern block on the margin of the intervening ocean 2 the oceanic arc terrane collided with the passive margin sequence at 2 5 ga above a west dipping subduction zone 3 the subduction polarity was reversed at ca 2 5 ga and a new east dipping subduction zone gave rise to the intrusion of the 2 5 ga mafic dikes of the ncc 4 the final collision between the western block and the eastern block occurred at ca 2 1 ga 5 the whole ncc then collided with the columbia supercontinent along the north margin of the craton at 1 9 1 8 ga

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

an overview of the geochemistry of eoarchean to mesoarchean ultramafic to mafic volcanic rocks sw greenland implications for mantle depletion and petrogenetic processes at subduction zones in the early earth abstract this study reviews the geochemical characteristics of eoarchean to mesoarchean ultramafic to mafic volcanic rocks now amphibolites in sw greenland and compares them with those of cenozoic oceanic island arc basalts to evaluate archean subduction zone petrogenetic processes emphasis is placed on the th ree hfse zr ti and nb systematics of the ca 3800 and ca 3700 ma arc suites in the isua greenstone belt the ca 3075 ma ivisaartoq ujarassuit greenstone belt and amphibolites associated with the ca 2970 ma fiskenaesset layered anorthosite complex on n morb normalized diagrams the isua ivisaartoq ujarassuit and fiskenaesset volcanic rocks are all characterized by depletion of nb relative to th and lree consistent with a supra subduction forearc arc backarc geodynamic setting similarly on the th yb nb yb projection these suites plot within the field of cenozoic oceanic island arc basalts on log transformed immobile trace element ratio nb th la th sm th and yb th diagrams they display a trend projecting from morb mid ocean ridge basalt to iab island arc basalt on the iab crb continental rift basalt oib ocean island basalt morb diagram as for cenozoic oceanic island arc basalts accordingly these trace element compositions are interpreted as reflecting the enrichment of archean depleted upper mantle morb source by subduction derived melts and fluids following the initiation of intra oceanic subduction and arc migration concentrations of mgo and ni in sw greenland archean basalts overlap with but extend to 2 to 4 times higher than those in cenozoic oceanic island arc counterparts in contrast the majority of archean basalts have ree and hfse concentrations 2 to 4 times lower than cenozoic oceanic island arc basalts consistent with more depleted sub arc mantle wedge peridotites in the archean than in cenozoic counterparts such depletion reflects the extraction of large volumes of mafic to ultramafic melts from hotter archean mantle we infer that less refractory compositions of the present day depleted upper mantle the source of morb and conservative hfse in arc basalts have resulted from re mixing with the less depleted to enriched deeper mantle material transferred to shallower depths by mantle convection and plumes in post archean times archean basalts in sw greenland share the negative nb and ti anomalies of cenozoic oceanic island arc basalts on average archean basalts however have lower th nb la nb sm tio2 and gd tio2 ratios than cenozoic oceanic island arc counterparts given that rutile controls the nb and ti budgets in arc magmas the lower th nb la nb sm tio2 and gd tio2 ratios in sw greenland basalts are attributed to the mobility of nb and ti in slab derived melts involving rutile fusion in archean subduction zones these elements are less mobile in fluids originating from cenozoic subducting oceanic crust where rutile appears to be generally stable accordingly it is suggested that higher geothermal gradients in the archean may have provided optimized conditions for slab melting and metasomatism of the sub arc mantle wedge by slab derived melts

arc continent collision and subduction reversal in the cenozoic evolution of the northwest pacific an example from kamchatka ne russia abstract the principal new feature in the cenozoic evolution of the northwest pacific is based on the fact that two arc terranes of upper cretaceous paleogene age were incorporated in the kamchatka orogen during the early tertiary and at the end of the miocene respectively these two arc terranes so called achaivayam valaginskaya campanian lower paleocene and kronotskaya coniacian eocene arcs are separated by a suture zone that contains deformed slices of young paleocene lower eocene oceanic crust paleomagnetic data on the original paleolatitudes of the arc positions as well as geochemical data characterized the arcs as intraoceanic originated in the northwest pacific the arcs were separated by the vetlovka oceanic plate suggested to be a trapped fragment of the kula plate in which a new spreading zone developed in the early tertiary the principal event in the reconstructed evolution is the eocene collision of the extended achaivayam valaginskaya arc terrane with the continental margin of asia the margin is interpreted as the southeastern facing the pacific cretaceous accretionary boundary of the okhotsk microplate docked against the asian continent at 55 65 ma the arc continent collision developed progressively northward from the end of the paleocene early eocene in southern kamchatka to the middle eocene in the olutorka region the different stages of the collision occurred simultaneously along strike of the achaivayam valaginskaya arc according to the geodynamic model based on geological data for southern kamchatka collision included 1 the entrance of the continental margin in the subduction zone deformation of the overriding plate and subduction of fore arc block 2 arc obduction on the continental margin blocking of continental margin subduction and 3 subduction reversal with onset of oceanic subduction at the eastern side of the deformed arc this subduction reversal produced deformation and reverse faulting along the eastern side of the arc thus allowing accretion of tectonic slices of young oceanic crust of the vetlovka plate which began to subduct during the subduction reversal the subducted plate dipping to the southeast was broken off and detached at depth under the arc area the subduction polarity reversal the most characteristic feature of the collision model is strongly supported by structural and geologic data of southern kamchatka and explains the main tectonic metamorphic and magmatic events related to and postdating the collision

hydrothermal processes at mid ocean ridges submarine hydrothermal systems are an integral component of crustal construction along the global system of mid ocean ridges thermally induced circulation of seawater through the permeable parts of the crust and upper mantle has been estimated to account for 34 of the predicted global oceanic heat flux which in turn comprises close to 25 of the total heat flux of the earth stein and stein 1994 and references therein discharge of hydrothermal fluids is manifest along mid ocean ridges as high temperature 200 400 c focused and lower temperature 200 c diffuse fluid flow off axis fluid flow may extend out to the crustal sealing age of 65 10 million years and may be responsible for more than 70 of the hydrothermal heat flux stein and stein 1994

new palaeomagnetic data from the betic cordillera constraints on the timing and the geographical distribution of tectonic rotations in southern spain a palaeomagnetic investigation has been carried out at 14 sites on jurassic red nodular limestones from the central and eastern part of the external zones of the betic cordillera subbetic and prebetic zones progressive thermal demagnetisation of samples from the subbetic zone reveals the presence of two stable magnetic components of the natural remanent magnetisation 1 a secondary neogene syn folding component and 2 the original jurassic magnetisation as similar characteristics have been reported in jurassic limestones from the western subbetic zone a widespread remagnetisation event took place within 106 years in the entire subbetic region during neogene times in contrast in the prebetic region no evidence for a secondary overprint has been detected palaeomagnetic jurassic declinations indicate variable and locally very large clockwise rotations 35 140 but the two sites in the north westernmost part of the investigated region are not rotated the use of both components of magnetisation and the incremental fold test results allowed the timing of block rotations in the subbetic zone to be constrained rotations in the western subbetic occurred after the acquisition of the secondary overprint whereas in the central part of the subbetic zone they were completed by the time of the remagnetisation event

thrust emplacement of the hispaniola peridotite belt orogenic expression of the mid cretaceous caribbean arc polarity reversal articulo sobre el emplazamiento fallero en la zona de peridotitas de la hispaniola republica dominicana estudiando si es la expresion orogenica de la polaridad reversa en la zona del caribe en el cretacico medio

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