geochemistry zircon u pb and hf isotope for granitoids nw sanandaj sirjan zone iran implications for mesozoic cenozoic episodic magmatism during neo tethyan lithospheric subduction abstract the sanandaj sirjan zone ssz of iran comprises sedimentary and metamorphic basement rocks which are generally regarded as having been derived from the southern active margin of the eurasian plate within the ssz a number of mesozoic to cenozoic granitoid intrusions of various size elongated in a nw se direction are exposed with the benefit of precise age dating geochemical and isotopic data over the past decade the magmatic history of these intrusions has become clearer this study presents further geochronological and geochemical data for previously dated and undated granitoids together with considerable zircon lu hf isotopic data which were limited in the past combined with previous work these new data including the finding of ca 170 ma adakite lead to improved constraints on the meso cenozoic tectonic evolution of neo tethyan lithospheric subduction the dominant early middle jurassic magmatism is proposed to have occurred with a major contribution of crustal components during the initiation of neo tethyan subduction subsequent late jurassic magmatism suggests the involvement of lower crust partial melting in an extensional tectonic setting cretaceous magmatism almost ceased after formation of a flat slab caused by a trench retreat rate exceeding that of slab roll back late paleocene eocene magma sources in the ssz are dominated by mantle derived input through an asthenospheric window with subordinate crustal contamination during the subduction of the neo tethyan ridge spreading center this interpretation differs from paleocene eocene magmatic flare up proposed for the northern iranian interior which may have been driven by an episode of slab retreat or slab roll back following cretaceous flat slab subduction

the evolution of the southern menderes massif in sw turkey as revealed by zircon dating this geochronological study concentrates on the evolution of the southern part of the menderes massif a metamorphic core complex in w turkey by dating single zircons with the pb pb evaporation and the u pb methods we have analysed zircons from granitic augen gneisses of the core as well as from quartzites of the metasedimentary envelope zircons from the granitic core gneisses are euhedral with typical igneous morphologies the single zircon ages of these gneisses indicate a time span for magmatic activity from 520 to 570 ma with a major event at about 550 ma some gneisses contain older zircons of c 660 ma which are interpreted as inherited grains zircons from the quartzites are detrital as indicated by their rounded shapes and pitted surfaces their ages vary from 526 to 2094 ma these data partially coincide with the ages of the gneisses but also indicate an older source region for the sediments neither a tertiary origin nor metamorphic overgrowth is documented in the pb ages of the zircons the results suggest that the southern part of the menderes massif developed during a major magmatic episode in latest proterozoic times thus suggesting a relation with other pan african regions in n africa

panafrican basement and mesozoic gabbro in the zagros orogenic belt in the dorud azna region nw iran laser ablation icp ms zircon ages and geochemistry abstract the dorud azna region of the sanandaj sirjan metamorphic zone ssmz in nw iran is part of the zagros orogenic belt and exposes an amphibolite grade metamorphic succession of mixed continental e g the granitic galeh doz orthogneiss and oceanic or rift e g various amphibolites origin which is intruded by the darijune gabbro laser ablation icp ms u pb zircon ages of 608 18 ma and 588 41 ma of the granitic galeh doz orthogneiss indicate a panafrican basement similar to that of the yazd block of central iran we therefore interpret this part of the ssmz to represent a previously subducted part of the panafrican basement of the iranian microcontinent based on geochemistry amphibolites and metagabbros from the amphibolite unit representially potentially the cover on the panafrican galeh doz orthogneiss are interpreted to have their origin in a likely upper paleozoic to mesozoic rift or e morb ophiolite succession one of the metagabbros here termed dare hedavand metagabbro yield a 206pb 238u mean age of 314 6 3 7 ma talc bearing greenschists to epidote amphibolites of the triassic june complex formed in a similar but younger environment furthermore the metamorphic complex is intruded by the darijune gabbro which yields a mean icp ms u pb zircon age of 170 2 3 1 ma the final stage of likely jurassic greenschist facies grade metamorphism postdates the gabbro intrusion we interpret the darijune gabbro to have resulted from the initial subduction of neotethyan oceanic lithosphere in a continental arc setting the new data combined with data from literature indicate that the ssmz represents a tectonic element of a mixed continental and oceanic origin likely accreted to central iran during a jurassic tectonic process the new data also constrains thick skinned nappe stacking within the sanandaj sirjan zone

the sanandaj sirjan zone in the neo tethyan suture western iran zircon u pb evidence of late palaeozoic rifting of northern gondwana and mid jurassic orogenesis abstract the zagros orogen marking the closure of the neo tethyan ocean formed by continental collision beginning in the late eocene to early miocene collision was preceded by a complicated tectonic history involving pan african orogenesis late palaeozoic rifting forming neo tethys followed by mesozoic convergence on the ocean s northern margin and ophiolite obduction on its southern margin the sanandaj sirjan zone is a metamorphic belt in the zagros orogen of gondwanan provenance zircon ages have established pan african basement igneous and metamorphic complexes in addition to uncommon late palaeozoic plutons and abundant jurassic plutonic rocks we have determined zircon ages from units in the northwestern sanandaj sirjan zone golpaygan region a sample of quartzite from the june complex has detrital zircons with u pb ages mainly in 800 1050 ma with a maximum depositional age of 547 32 ma latest neoproterozoic earliest cambrian a shrimp u pb zircon age of 336 9 ma from gabbro in the june complex indicates a carboniferous plutonic event that is also recorded in the far northwestern sanandaj sirjan zone together with the permian hasanrobat granite near golpaygan they all are considered related to rifting marking formation of neo tethys scarce detrital zircons from an extensive package of metasedimentary rocks hamadan phyllite have ages consistent with the triassic to early jurassic age previously determined from fossils these ages confirm that an orogenic episode affected the sanandaj sirjan zone in the early to middle jurassic cimmerian orogeny although the cimmerian orogeny in northern iran reflects late triassic to jurassic collision of the turan platform southern eurasia and the cimmerian microcontinent we consider that in the sanandaj sirjan zone a tectonothermal event coeval with the cimmerian orogeny resulted from initiation of subduction and closure of rift basins along the northern margin of neo tethys

a thermo chemical regime in the upper mantle in the early earth inferred from a numerical model of magma migration in a convecting upper mantle abstract a numerical model of mantle magmatism in a convecting upper mantle has been developed to study the thermo chemical evolution of the upper mantle of the early earth the solid state convection in the upper mantle is modeled by a convection of a binary eutectic material with a newtonian temperature dependent rheology in a two dimensional rectangular box placed on a heat bath as a model of the lower mantle the density depends on the chemical composition and melt content as well as temperature of the material the material contains heat producing elements that are incompatible and exponentially decay with time mantle magmatism is modeled by a permeable flow of melt generated by a pressure release melting induced by the solid state convection through matrix the permeable flow is driven by a buoyancy due to the density difference between the melt and the matrix the thermo chemical evolution in the box occurs in two stages if the deeper part of the box is not so strongly depleted in heat producing elements in spite of the upward migration and concentration of heat producing elements into a crustal layer along the top surface boundary due to magmatism in the earlier stage active magmatism occurs because of a strong internal heating due to the heat producing elements a chemically stratified structure develops well in the box with dense magmatic products in the deeper part and less dense residual materials in the shallower part and the temperature distribution becomes strongly superadiabatic over the entire box the temperature at the base of the box becomes as high as the solidus temperature the chemically stratified structure is however suddenly destroyed by convective mixing and the temperature in the deeper part of the box suddenly drops by several hundred degrees when the internal heat source becomes too weak owing to the decay of heat producing elements which sustain the active magmatism and hence keep the effect of chemical differentiation due to the magmatism stronger than the effect of convective mixing in the later stage of the evolution the box becomes chemically homogeneous and magmatism occurs only mildly if heat producing elements are efficiently transported into the crustal layer and the deeper part of the box becomes strongly depleted in heat producing elements owing to the magmatism only a mild magmatism occurs even at the beginning a chemically stratified structure does not develop well and the temperature in the box rapidly decreases to a stationary value the regime of hot and chemically stratified upper mantle suggested from the earlier stage of the case with mild depletion of heat producing elements at depth fits in with many observations of the archean continental crust

role of the lithosphere in mantle convection plate geometry and kinematics generally reflect the mechanical properties of the solid lithosphere rather than those of the fluid mantle underneath and plate formation and subduction account for most of the heat transport from the earth s interior correspondingly mantle convection models must incorporate a stiff but mobile boundary layer like the lithosphere before they can reproduce the main features of mantle convection a relatively easy way to accomplish this in numerical models is to combine a temperature dependent viscosity with an imposed piecewise constant surface velocity boundary condition it is shown how surface heat flux topography gravity and geoid but not plate velocities or stresses can then be derived numerical models confirm that a lithosphere has a first order effect on the underlying flow structure for internally heated models approx imating the case of whole mantle convection the calculated surface signatures are in first order agreement with observations a level of empirical success which hitherto has not been approached by models of mantle convection companion papers exploit the observations more fully to constrain the main features of mantle convection

time dependent convection in the earth s mantle abstract the range of rayleigh numbers accessible to 2 dimensional numerical models of mantle convection has been increased by almost two orders of magnitude through the use of high resolution numerical grids with up to 200 intervals in each of the horizontal and vertical directions the range of rayleigh numbers studied is believed to encompass conditions which would have existed in a hot archaean mantle although the models are highly idealized in that they neglect the compressibility and variations of viscosity in the earth s mantle they suggest that during the archaean and earlier boundary layer instabilities developed more frequently than at present and did so with a scale and separation comparable to the thickness of the thin thermal boundary layers rather than to the depth of the convecting layer itself the rising and sinking plumes which develop from these instabilities drive transient flows which nevertheless have a horizontal scale comparable to the depth of the convecting layer thus although surface motions during the archaean may have been more variable than at present in terms of speed and direction they might have occurred on a horizontal scale which is similar to that of present day plate motion the numerical models indicate that motions at the upper surface of time dependent convection cells are 1 predominantly controlled by transient sinking plumes and 2 converge directly towards the source region of the sinking plumes on the other hand rising plumes have a more diffuse influence on the upper surface and surface divergence does not always occur immediately above a rising plume if oceanic plates on earth constitute the upper surface of mantle wide convection cells the more pronounced geophysical signature of subduction zones compared to oceanic ridges may be simply due to the fact that buoyancy is most concentrated at the earth s surface in the form of sinking plumes the total surface heat flow is found to fluctuate significantly in the time dependent convection models since estimates of the present day heat loss from the earth s interior are at best instantaneous measurements it is possible that these estimates are not representative in a time averaged sense of the present thermal state of the planetary interior

gondwanan eoarchean neoproterozoic ancient crustal material in iran and turkey zircon u pb hf isotopic evidence the sanandaj sirjan zone is a basement culmination northeast of the neo tethys suture in iran in this zone near azna granite has a magmatic zircon u pb age of 568 11 ma with 900 800 ca 2400 and ca 3600 ma inherited cores the ca 3600 ma inherited zircon is the oldest crustal component yet detected in iran near chadegan orthogneiss has a magmatic zircon u pb age of 637 15 ma and carries ca 1000 and 2000 ma inherited zircons inherited 900 1000 ma zircons have juvenile initial ehf values of ca 8 to 9 whereas the younger 630 and 568 ma magmatic zircons show lower initial ehf values however the 3600 ma core has initial ehf 0 0 a neoproterozoic rim on the inherited 3600 ma core has the most extreme initial ehf value of 18 the hf isotopic data indicates generation of the magmatic protoliths from a mixture of juvenile neoproterozoic and archean sources previous studies showed that in turkey the central anatolian crystalline complex is underlain by neo eoarchean rocks the menderes mas

the anarak jandaq and posht e badam metamorphic complexes in central iran new geological data relationships and tectonic implications abstract the anarak jandaq and posht e badam metamorphic complexes occupy the nw part of the central east iranian microcontinent and are juxtaposed with the great kavir block and sanandaj sirjan zone our recent findings redefine the origin of these complexes so far attributed to the precambrian early paleozoic orogenic episodes and now directly related to the tectonic evolution of the paleo tethys ocean this tectonic evolution was initiated by late ordovician early devonian rifting events and terminated in the triassic by the eocimmerian collision event due to the docking of the cimmerian blocks with the asiatic turan block the variscan accretionary complex is a new name we proposed for the most widely distributed metamorphic rocks connected to the anarak and jandaq complexes this accretionary complex exposed from sw of jandaq to the anarak and kabudan areas is a thick and fine grain siliciclastic sequence accompanied by marginal sea ophiolitic remnants including gabbro basalts with a supra subduction geochemical signature new 40ar 39ar ages are obtained as 333 320 ma for the metamorphism of this sequence under greenschist to amphibolite facies moreover the limy intercalations in the volcano sedimentary part of this complex in godar e siah yielded upper devonian tournaisian conodonts the northeastern part of this complex in the jandaq area was intruded by 215 15 ma arc to collisional granite and pegmatites dated by id tims and its metamorphic rocks are characterized by some 40ar 39ar radiometric ages of 163 156 ma the variscan accretionary complex was northwardly accreted to the airekan granitic terrane dated at 549 15 ma later from the late carboniferous to triassic huge amounts of oceanic material were accreted to its southern side and penetrated by several seamounts such as the anarak and kabudan this new period of accretion is supported by the 280 230 ma 40ar 39ar ages for the anarak mild high pressure metamorphic rocks and a 262 ma u pb age for the trondhjemite rhyolite association of that area the triassic bayazeh flysch filled the foreland basin during the final closure of the paleo tethys ocean and was partly deposited and or thrusted onto the cimmerian yazd block the paleo tethys magmatic arc products have been well preserved in the late devonian carboniferous godar e siah intra arc deposits and the triassic nakhlak fore arc succession on the passive margin of the cimmerian block in the yazd region the nearly continuous upper paleozoic platform type deposition was totally interrupted during the middle to late triassic local erosion down to lower paleozoic levels may be related to flexural bulge erosion the platform was finally unconformably covered by liassic continental molassic deposits of the shemshak one of the extensional periods related to neo tethyan back arc rifting in late cretaceous time finally separated parts of the eocimmerian collisional domain from the eurasian turan domain the opening and closing of this new ocean characterized by the nain and sabzevar ophiolitic melanges finally transported the anarak jandaq composite terrane to central iran accompanied by large scale rotation of the central east iranian microcontinent ceim due to many similarities between the posht e badam metamorphic complex and the anarak jandaq composite terrane the former could be part of the latter if it was transported further south during tertiary time

a precambrian microcontinent in the indian ocean ridges of thick raised crust on the indian ocean floor were thought to be mostly volcanic seamounts formed above the reunion mantle plume dating of zircon minerals in mauritian lavas however indicates that fragments of an ancient microcontinent may be preserved beneath the seamounts contributing to the thickened crust

the dergaon h5 chondrite fall classification petrological and chemical characteristics cosmogenic effects and noble gas records a multiple fall of a stony meteorite occurred near the town of dergaon in assam india on march 2 2001 several fragments weighing 2 kg and a single large fragment weighing 10 kg were recovered from the strewn field which extended over several tens of square kilometers chemical petrographic and oxygen isotopic studies indicate it to be in most aspects a typical h5 chondrite except the unusually low k content of 340 ppm a cosmic ray exposure of 9 7 ma is inferred from the cosmogenic noble gas records activities of eleven cosmogenic radionuclides were measured 26al and 22na activities as well as the 22na 26al activity ratio are close to the values expected on the basis of solar modulation of galactic cosmic rays the low 60co activity 1 dpm kg is indicative of a small preatmospheric size of the meteorite cosmic ray heavy nuclei track densities in olivine grains range from 10 6 cm 2 in samples from the largest fragment to approximately 4 9 105 cm 2 in one of the smaller fragments the combined track radionuclide and noble gas data suggest a preatmospheric radius of 20 cm for the dergaon meteorite

u pb dating and emplacement history of granitoid plutons in the northern sanandaj sirjan zone iran abstract the sanandaj sirjan zone ssz which runs parallel to the zagros fold and thrust belt of iran underwent a multistage evolution starting with neotethys initiation its subsequent subduction below the iranian continental crust and eventual closure during convergence of arabia towards central iran plutonic complexes are well developed in the northern part of the ssz and we have dated a number of them by id tims u pb on zircon the new data record the following events a mid jurassic period that formed the boroujerd plutonic complex 169 ma the astaneh pluton 168 ma and the alvand pluton 165 ma late jurassic emplacement of the gorveh pluton 157 149 ma mid cretaceous 109 ma formation of a i type phase in the hasan salary pluton near saqqez followed by early paleocene 60 ma intrusion of a type granite in the same pluton and the youngest intrusive event recorded so far in the ssz with the intrusion of granite in the gosheh tavandasht complex near boroujerd at 34 9 ma these different events reflect specific stages of subduction related magmatism prior to the eventual miocene collision between the two continental blocks

numerical experiments on coupled magmatism mantle convection system implications for mantle evolution and archean continental crusts numerical experiments on the simple models of coupled mantle magmatism mantle convection system were made to study the thermal and chemical evolution of the earth s mantle mantle convection is modeled by the convection of a two component newtonian constant viscosity fluid in an internally heated two dimensional rectangular region the density of the convecting fluid is assumed to depend on its chemical composition as well as its temperature the two components of the convecting fluid are assumed to constitute an eutectic system mantle magmatism i e magma segregation in the mantle is modeled by differentiation of the eutectic mixture from the convecting fluid this chemical differentiation is assumed to occur when the temperature t increases to the solidus temperature ts assumed to increase linearly with depth owing to convective heat transport taking into account the change in density distribution due to this chemical differentiation we solve the basic equations for convection numerically as an initial value problem we find that how the mantle evolves depends strongly on two quantities h internal heating rate and r m e m is the density of undifferentiated mantle material at 0 c e is the density of the eutectic mixture at 0 c and 0 is a measure of density change in the mantle due to thermal expansion r represents the magnitude of the dependence of density on chemical composition 1 when 4 r 2 and h hc tav increases with time while q continues to be nearly constant magmatism occurs to produce a residue which occupies the uppermost part of mantle throughout the evolutionary history of the mantle 3 when 1 1 and h hc tav increases with time while q decreases with time mantle magmatism occurs to produce a residue which accumulates at the bottom of mantle throughout the evolutionary history of the mantle we also find that mantle magmatism occurs episodically when r 1 and r 2 while mantle magmatism occurs continuously when 1 r 1 from an estimate of r appropriate for the earth s mantle and known features of the archean continental crusts we conclude that the type 1 mantle evolution occurs beneath the continental crusts many features of the archean continental crusts fit in with the framework of the type 1 mantle evolution

regional stratigraphy of the zagros fold thrust belt of iran and its proforeland evolution the latest neoproterozoic through phanerozoic stratigraphy of the zagros fold thrust belt of iran has been revised in the light of recent investigations the revised stratigraphy consists of four groups of rocks each composed of a number of unconformity bounded megasequences representing various tectonosedimentary settings in the lowest group ranging in age from latest precambrian to devonian the uppermost neoproterozoic to middle cambrian rocks constitute a megasequence of evaporites siliciclastic deposits and interlayered carbonates which were deposited in pull apart basins that developed by the najd strike slip fault system this megasequence is overlain by a second one middle to late cambrian in age which consists of shallow marine siliciclastic and carbonate rocks representing deposition in an epicontinental platform the overlying shales siltstones and partly volcanogenic sandstones of ordovician silurian and devonian age are local remnants of stratigraphic units that were extensively eroded during development of several major unconformities the second group consists of two megasequences one permian and the other triassic composed of widespread transgressive basal siliciclastic rocks and overlying evaporitic carbonates of an equatorial epi pangean very shallow platformal sea the third group is composed of four megasequences formed of shallow and deep water carbonates with some siliciclastic and evaporite deposits which accumulated on a neo tethyan continental shelf during earliest jurassic through late turonian time the fourth group comprises siliciclastic and carbonate deposits of a largely underfilled nw to se trending forward and backward migrating late cretaceous to recent proforeland basin which has evolved as an integral part of the zagros orogen this last group consists of three megasequences ix x and xi with distinctive lateral and vertical facies variations which reflect specific tectonic events megasequence ix comprises uppermost turonian to middle maastrichtian prograding and retrograding siliciclastic and carbonate deposits whose accumulations reflect emplacement obduction of ophiolite slivers and subsequent collisional events in the zagros orogen megasequence x consists of uppermost maastrichtian to upper eocene siliciclastic and carbonate rocks which deposited first progradationally in front of the zagros orogenic wedge with reduced contractional tectonic activity and then retrogradationally due to intensified thrust stacking in the interior parts of the orogen megasequence xi consists of oligocene and lower miocene carbonate strata deposited retrogradationally shortly after a period of intensified late eocene thrust faulting in the deformational wedge and an overlying succession of upward coarsening northeasterly derived siliciclastic deposits of lower miocene to recent age which are composed of erosional byproducts of the southwest vergent zagros thrust sheets

systematic changes in orientation of linear mylonitic fabrics an example of strain partitioning during transpressional deformation in north golpaygan sanandaj sirjan zone iran abstract kilometer scale shallowly dipping nw striking top to the ne reverse and dextral strike slip shear zones occur in metamorphic rocks of north golpaygan these metamorphic rocks are exposed at the ne margin of the central part of the sanandaj sirjan zone in the hinterland of the zagros orogen nw striking top to the ne normal shear zones were also found in a small part of the study area structural evidence of three deformation stages were found pre mylonitization metamorphic mineral growth happened during d1 the main mylonitization event was during the d2 deformational event following coaxial refolding synchronous to retrograde metamorphism of amphibolite to greenschist facies in the late cretaceous paleocene and before d3 folding and related mylonitization we documented the systematic changes in the orientations of d2 linear fabrics especially stretching lineations and superimposition relations of structures it is concluded that the dextral strike slip and dip slip shear zones were coeval kinematic domains of partitioned dextral transpression the shallowly dipping reverse and strike slip shear zones are compatible with partitioning in a very inclined transpressional model fabric relations reflect that the top to the ne normal shear zones were not produced during deformation partitioning of inclined dextral transpression the late cretaceous paleocene strain partitioning was followed by later n s shortening and ne extension in the north golpaygan area

the dependence of convection planform on mode of heating a fundamental property of a convecting fluid is its planform the distribution in the horizontal plane of hot rising regions and cold sinking regions for the earth s mantle the planform might he visualized as a map of subduction zones hotspots and possibly ocean ridges here i report numerical experiments of convection at high rayleigh number which show a strong dependence of planform on heating mode when heat generation is distributed uniformly through the box the preferred planform consists of an ensemble of time dependent cold axial sinkers distributed in a hot diffuse upward flow when half of the heat is generated within the box and the other half is input through the base the preferred planform consists of an array of hot axial plumes and elongated cold sheets in the former case the mean horizontal wavelength is about equal to the layer depth for the latter it is about twice the layer depth

origin and global tectonic significance of early archean cherts from the marble bar greenstone belt pilbara craton western australia abstract five sections of bedded chert in mafic ultramafic rocks of the archean warrawoona group in the marble bar greenstone belt pilbara craton were analyzed in order to understand their depositional environment and to provide some constraints on early archean tectonics the sections are divisible into two types based on their field occurrence mineralogy and geochemistry thicker ones a and b that overlie fe rich low k tholeiites and thinner ones c1 c2 and c3 overlying komatiitic basalts the thickest ferruginous section a 45 m thick is the marble bar chert of the towers formation and is interpreted to have been an in situ precipitate derived from a high t hydrothermal solution emanating from a mid oceanic ridge mor the following geochemical features are similar to those of modern hydrothermal iron rich sediments at a mor i p v zn and y are positively correlated with fe ii a positive eu anomaly normalized to chondrite decreases from 6 6 to 1 3 and the magnitude of a negative ce anomaly decreases from 0 6 to 1 0 as ree and lree hree increase the 13 m thick b section in the apex basalt dominated by sio2 and containing significant amounts of ba up to 4330 ppm originated from a low t mor hydrothermal solution this section is characterized by an association with massive black gray silica veins that were hydrothermal feeders in normal fault zones in the spreading center geochemical evidence from the greenstones underlying the b section indicates that they are of morb origin in contrast to a and b c1 c2 and c3 sections are intercalated conformably with komatiitic basalts of the apex basalt and are 3 6 m thick the geochemical signatures of these three sections suggest that they were most likely formed by low t weak hydrothermal activity that may have been associated with hot spot volcanism they show strong enrichment of cr and ni reflecting a significant contribution of komatiitic basaltic detritus during sedimentation the presence of volcaniclastic chert in their uppermost beds indicates a decrease of hydrothermal silica precipitation due to waning hydrothermal activity among the three sections the uppermost c3 exhibits greater enrichment in zr nb hf and th and higher th sc and la yb n relative to the lower c1 and c2 sections this could mean that the depositional environment of c3 was relatively closer to a continent or to island arcs composed of granitoids and or their volcanic equivalents siliceous mudstones and sandstones of the uppermost clastic rocks t section panorama formation have geochemical signatures analogous to those of felsic plutonic volcanic rocks high th sc negative eu anomalies and high ree in some siliceous mudstones from this unit imply that differentiated granitoids or volcanic equivalents were already exposed erupted at least locally in the pilbara at this time this study shows these warrawoona group cherts were deposited in a variety of environments ranging from a mid oceanic spreading center to a convergent plate boundary via a hot spot this variation was most likely due to horizontal plate motions which accordingly support the operation of plate tectonics in the early archean

mantle convection and early crustal evolution abstract the existence of peridotitic komatiites in the archaean suggests that the archaean mantle was significantly hotter than the modern mantle this evidence is contradicted by estimates of archaean continental geothermal gradients based on the pressure and temperature recorded in metamorphic rocks which suggest that there is no marked difference between archaean and modern continental geothermal gradients numerical modelling shows that small changes in the mantle temperature can have an important influence on convection if the average temperature of the upper mantle is increased by 200 c convection within the mantle becomes chaotic and an upper mantle partial melt zone encircles the globe the crust formed during this period will be komatiitic in composition but will be unstable and will be mixed back into the mantle by subduction later when the mantle temperature falls to 100 c above its present level the upper mantle partial melt zone contracts away from subduction areas it is suggested that the first primitive felsic magmas were generated at subduction zones the appearance of these magmas at 3 8 ga permitted the formation of buoyant continents and eventually led to crustal thickening as a consequence of this thickening the proto continents consisting of a bimodal suite of basalts and sodic granodiorites contained two types of latent energy 1 radioactive energy held in elements such as th k and u and 2 potential energy resulting from the elevation of the continents above sea level the potential energy of the continents led to sedimentation the increase in the rate of sedimentation during the archaean resulted from increased crustal buoyancy at the same time heat released by radioactive elements in the deep crust built up under the insulating blanket of the upper crust this caused a major metamorphic metasomatic and crustal melting event which produced the potassic granites of the late archaean once the radioactive elements had been removed from the lower crust that region of the continent become tectonically stable the proterozoic shelf sediments were deposited at the margins of these stable cratons convection models of the archaean mantle show hot diapirs rising from the boundary layer above the core mantle interface we suggest that these diapirs began to melt at a depth of 450 km giving rise to komatiitic magmas this model requires the average temperature of the archaean upper mantle to be 100 c above that of the modern mantle the similarity between archaean and modern continental geothermal gradients can be explained if archaean continents formed above subduction zones raising the temperature of the archaean mantle by 100 c 1 halves the thickness of the oceanic lithosphere 2 increases the oceanic geothermal gradient at the mid point of a convection cell 3 decreases the viscosity of the mantle by at least an order of magnitude the combination of these effects produces a marked decrease in the strength of the archaean lithosphere and mantle thus the form of archaean tectonics can be expected to have been very different from modern tectonics

constraints on early archean crustal extraction and tholeiitic komatiitic volcanism in greenstone belts of the northern superior province numerous small remnants of archean greenstone belts in the northern superior province ca 2875 2710 ma have chemical characteristics similar to those of the larger greenstone belts of the southern superior province and preserve direct evidence of crustal conditions prior to the major volcanic events of the late archean wawa abitibi subprovinces ca 2760 2700 ma three of the best preserved belts are engulfed in tonalite intrusions of the faribault thury complex ftc and exhibit common chemical characteristics which may imply a similar origin the dominant tholeiitic basalts typically have mgo contents 7 wt tio2 15 low cao al2o3 1 2 and chondritic gd yb ratios similar to 2 7 ga al undepleted komatiites the trace element ratios of komatiitic rocks are indistinguishable from those of the associated tholeiites suggesting either a derivation from similar mantle sources or a comagmatic relationship nb thpm 0 8 1 1 la cepm 0 9 1 3 nb ce 0 7 0 9 y hopm 1 and th lapm 0 7 1 1 numerical modelling of trace and major elements during low pressure crystal fractionation reproduces the spectrum of both inferred liquid and cumulate compositions and is consistent with a comagmatic origin between the komatiites and tholeiites the relatively low nb th ratios of these mid archean volcanic rocks relative to both modern day basalts and late archean basalts may indicate that they were derived from a mantle source that had not lost its crustal components nor seen significant recycled oceanic crust high nb th the extraction of continental crust from this archean mantle source might then postdate the ftc volcanism and may be associated with the generation of the voluminous tonalites that engulf the belts 445

a 3500 ma plutonic and volcanic calc alkaline province in the archaean east pilbara block variably foliated predominantly granodioritic plutonic rocks from the northern part of the shaw batholith in the east pilbara archaean craton are dated at 3 499 22 ma 2 errors by a whole rock pb pb isochron these rocks intrude the surrounding greenstone sequence and their age is indistinguishable from that sequence high strain grey gneisses which occupy much of the western and southern shaw batholith are chemically and isotopically similar to the north shaw suite and are inferred to have been derived from this suite by tectonic processes felsic volcanics within the greenstones together with a major portion of the granitic batholiths apparently formed in a calc alkaline volcanic and plutonic province at 3 500 ma this volcanic and plutonic suite is similar to modern calc alkaline suites on the basis of major element rare earh element and most other trace element contents the archaean suite contrasts with modern equivalents only in having lower concentrations of hree and higher concentrations of ni and cr

origin of komatiite at high pressures abstract experimental melting relations to 20 gpa demonstrate that the commonly accepted model of komatiite formation by non invariant melting olivine liquid is too restrictive komatiites could also have formed by pseudoinvariant melting olivine pyroxene garnet liquid modified spinel majorite liquid along the anhydrous solidus at high pressures the mgo content of komatiites cannot be used to infer the degree of partial melting in the mantle there would have been little change in mgo at a high pressure pseudoinvariant point even for degrees of partial melting that ranged from very small to over 80 instead the mgo content of komatiites may be a better indication of the depth of partial melting than it is the degree of partial melting late archean komatiites containing 20 28 mgo could have been generated at depths ranging from about 130 to 260 km early archean komatiites from the barberton mountain land could have formed at depths corresponding to the present day transition zone or at shallower levels from an unusually primitive mantle source komatiites which originated by partial melting in a lower mantle formed by majorite fractionation should have geochemical characteristics that have not yet been reported this indicates that either melting did not extend into the lower mantle or if it did the upper and lower mantles were isolated by a chemical and thermal boundary layer in archean times

eocene gold ore formation at muteh sanandaj sirjan tectonic zone western iran a result of late stage extension and exhumation of metamorphic basement rocks within the zagros orogen a multidisciplinary study including field geology microstructure analysis 40ar 39ar dating and fluid inclusion microthermometry and raman spectrometry has been carried out on the muteh gold deposit located in a greenschist to amphibolite facies metamorphic rock complex of the sanandaj sirjan tectonic zone zagros orogen iran the muteh gold deposit has been previously interpreted as genetically related to precambrian granitic intrusions as an exhalative hot spring deposit related to paleozoic rhyolitic acidic tuffs and to local metamorphic processes host rocks of the gold deposit are predominantly schist and gneiss and subsidiary amphibolite and quartzite and are intruded by leucogranites in the vicinity of the gold deposit the metamorphic and granitic rocks display a subhorizontal mylonitic foliation containing a northeast oriented stretching lineation field investigations in one of the producing open pits show that the gold orebodies are controlled by northwest oriented normal faults and joints dipping to the northeast and the southwest and crosscutting the ductile fabric of the host rocks both the ductile fabric of the host rocks and the gold ore controlling brittle structures are interpreted to have formed within a single continuous extensional event which started with ductile deformation and gradually changed into brittle deformation hydrothermal alteration associated with ore formation consists of quartz muscovite pyrite dolomite ankerite and albite which crosscuts the ductile fabric and overprints the metamorphic minerals of the host rocks pyrite is the dominant opaque mineral and is the major phase associated with gold chalcopyrite marcasite bismuth galena sphalerite and pyrrhotite are subsidiary to rare phases 40ar 39ar incremental heating and in situ laser ablation age data reveal a coherent sequence of cooling and hydrothermal events in the metamorphic complex hosting the muteh gold deposit muscovite samples from the alteration zone and from one quartz vein from the orebodies yield 40ar 39ar ages between 55 7 and 38 5 ma and show that gold mineralization is the youngest among the different dated events this eocene age is consistent with the young structural setting of the gold orebodies revealing an emplacement along northwestoriented normal faults which can be correlated with tertiary extensional tectonic events reported by previous field investigations the data also indicate that gold ore formation is coeval with magmatism in the adjacent tertiary urumieh dokhtar magmatic belt and with intrusive activity within the sanandaj sirjan zone with a 40ar 39ar age of 54 ma near muteh according to our study the remaining 40ar 39ar data reveal a cretaceous to early tertiary metamorphic magmatic and deformation history of the complex hosting the muteh gold deposit the fluid inclusion study showed that distinct fluids were present during the regional geologic evolution predating ore formation and later eocene gold ore formation early regional fluids consist of an h2o co2 nacl fluid with subsidiary n2 and ch4 that underwent local unmixing the salinities of the liquid rich inclusions resulting from unmixing range between 2 2 and 17 5 wt percent nacl equiv and those of the co2 rich vaporrich inclusions range between 2 8 and 7 2 wt percent nacl equiv the high salinity of the regional liquid rich fluids is attributed to metamorphism of evaporite bearing sedimentary rocks magmatic intrusions retrograde hydration reactions metamorphism of impure marbles and migmatization the fluid inclusions spatially associated with the gold veins postdating regional metamorphism and ductile deformation of the host rocks contain a low salinity aqueous fluid with a dilute low density co2 component and maximum salinities between 2 6 and 5 8 wt percent nacl equiv gold deposition at muteh occurred during or after late stages of eocene brittle extension as a result of exhumation of the host metamorphic complex possibly during magmatic activity in the sanandaj sirjam zone it is most likely linked to the introduction of a distinct low salinity low co2 content aqueous fluid during crustal extension these fluids are younger than and therefore unrelated to the highly saline and co2 rich regional metamorphic magmatic fluids our results refute previous models advocating precambrian to paleozoic ages of the muteh gold deposit

tectonics of the zagros orogenic belt of iran new data and interpretations abstract the zagros orogenic belt of iran is the result of the opening and closure of the neo tethys oceanic realm and consists from northeast to southwest of three parallel tectonic subdivisions 1 1 the urumieh dokhtar magmatic assemblage 2 2 the sanandaj sirjan zone and 3 3 the zagros simply folded belt recent detailed and reconnaissance mapping and stratigraphie structural analyses in various parts of the belt indicate that the tectonics of the sanandaj sirjan zone is dominated by either large scale composite duplex structures or low and high angle ne dipping imbricate systems these thrust systems have transported numerous horses and sheets nappes of metamorphosed and non metamorphosed phanerozoic rocks for tens or possibly hundreds of kilometers stratigraphic evidence and synorogenic conglomerates indicate that thrusting was initiated in the late cretaceous and has continued episodically since that time shear sense indicators in various parts of the zone reveal a ne sw direction of transport thrust systems generated in each episode are superimposed upon the previous structures cutting across and displacing them and producing complicated stacks of thrust sheets southwestward transport and stacking of the thrust sheets has resulted in an increase of about 10 15 km in the thickness of the continental crust in the southwestern part of this zone the so called z main zagros thrust which has been traditionally considered as the boundary between the sanandaj sirjan zone and the zagros simply folded belt is by no means a single high angle reverse fault nor is it a narrow zone of crushed rocks but a series of low angle thrusts locally cut by normal faults these thrusts are indistinguishable from those mapped in the sanandaj sirjan zone and are geometrically part of the sanandaj sirjan zone structures there is no single thrust among them that can be singled out and referred to as the main one the boundary of the sanandaj sirjan zone with the urumieh dokhtar magmatic assemblage to the northeast is characterized by a series of structural depressions formed by compression rock sequences including ophiolite complexes exposed along this boundary are strongly dismembered sheared and mylonitized shearing planes and foliations are either subvertical or dipping at high angles to the northeast this zone is suggested to be the suture between the afro arabian and the iranian plates overall the intensity of deformation increases northeastward from the persian gulf mesopotamian basin where strata are undeformed and horizontal through the zagros simply folded belt which is characterized by en echelon doubly plunging flexural slip folds gentle to open in the southwest and close to locally sw overturned in the northeast the intensity of deformation continues to increase further northeastward through the sanandaj sirjan zone where thrust faulting is dominant this pattern suggests that the deformational front has migrated through time from the northeastern part of the sanandaj sirjan zone i e the suture zone to its present position in the central portion of the persian gulf mesopotamian foreland basin

zagros orogeny a subduction dominated process this paper presents a synthetic view of the geodynamic evolution of the zagros orogen within the frame of the arabia eurasia collision the zagros orogen and the iranian plateau preserve a record of the long standing convergence history between eurasia and arabia across the neo tethys from subduction obduction processes to present day collision from 150 to 0 ma we herein combine the results obtained on several geodynamic issues namely the location of the oceanic suture zone the age of oceanic closure and collision the magmatic and geochemical evolution of the eurasian upper plate during convergence as testified by the successive sanandaj sirjan kermanshah and urumieh dokhtar magmatic arcs the p t t history of the few zagros blueschists the convergence characteristics across the neo tethys kinematic velocities tomographic constraints subduction zones and obduction processes together with a survey of recent results gathered by others we provide lithospheric scale reconstructions of the zagros orogen from 150 to 0 ma across two sw ne transects the evolution of the zagros orogen is also compared to those of the nearby turkish and himalayan orogens in our geotectonic scenario for the zagros convergence we outline three main periods regimes 1 the mid to late cretaceous 115 85 ma corresponds to a distinctive period of perturbation of subduction processes and interplate mechanical coupling marked by blueschist exhumation and upper plate fragmentation 2 the paleocene eocene 60 40 ma witnesses slab break off major shifts in arc magmatism and distributed extension within the upper plate and 3 from the oligocene onwards 30 0 ma collision develops with a progressive sw migration of deformation and topographic build up sanandaj sirjan zone 20 15 ma high zagros 12 8 ma simply folded belt 5 0 ma and with partial slab tear at depths 10 ma to present our reconstructions underline the key role played by subduction throughout the whole convergence history we finally stress that such a long lasting subduction system with changing boundary conditions also makes the zagros orogen an ideal natural laboratory for subduction processes

zircon u pb and hf isotope constraints on the mesozoic tectonics and crustal evolution of southern tibet the first in situ hf and u pb isotope analyses of zircon separates from mesozoic granites in southern tibet identify a significant previously unknown stage of magmatism igneous zircons n 34 from a granite within the gangdese batholith show a weighted mean 206 pb 238 u age of 188 1 1 4 ma and e hf t the parts in 10 4 deviation of initial hf isotope ratios between the zircon sample and the chondritic reservoir values between 10 4 and 16 8 suggesting predominantly early jurassic intrusive activity with a juvenile mantle contribution of 40 inherited zircons from two cretaceous s type granites in the northern magmatic belt 23 delineate a slightly older 206 pb 238 u age cluster between 188 and 210 ma these zircons have e hf t values from 3 9 to 13 7 yielding crustal hf model ages from ca 1 4 to 2 1 ga suggesting a major episode of crustal growth in proterozoic time and remelting of this crust in early jurassic time combining these with literature data we interpret the jurassic gangdese magmatism as an early product of the neo tethyan subduction that played a long lasting role in the tectonic evolution of southern tibet prior to the india asia collision

archean komatiites and geotherms solution to an apparent contradiction the high liquidus temperatures of archean komatiites and the low geothermal gradients in archean continents appear to give contradictory indications of temperatures in the archean mantle we resolve this contradiction in terms of an idealized model of convection in the archean mantle we suggest that temperatures in the archean mantle were no more than 100 c higher than at present that komatiites originate from the hot lower boundary layer of a non steady convection cell and that archean continents were formed near subduction zones where near surface temperature gradients were lower than elsewhere in the convecting mantle

the complex age of orthogneiss protoliths exemplified by the eoarchaean itsaq gneiss complex greenland shrimp and old rocks abstract field studies integrated with cathodoluminescence petrography and shrimp u pb dating of zircons from 150 orthogneisses and metatonalites from the eoarchaean itsaq gneiss complex southern west greenland shows that only a minority contain 3840 ma zircons whereas the majority carry only younger ones rocks containing 3840 ma zircons vary from very rare single phase metatonalites to more common complexly banded tonalitic migmatites the former metatonalites have simple oscillatory zoned 3840 ma zircon with limited recrystallisation and overgrowth whereas the more common migmatites have much more complicated zircon populations with both 3840 ma and 3650 3600 ma oscillatory zoned zircon more extensive recrystallisation and widespread complex core rim multiple growth relationships with only 100 160 ppm zr in the tonalites and likely melt generation temperatures of 1000 c the experimentally determined zircon solubility melt composition relationships established by other workers shows that the precursor melts to the itsaq complex tonalites were strongly undersaturated in zircon thus any entrained xenocrystic zircon would have been rapidly dissolved therefore the 3840 ma oscillatory zoned zircons crystallised out of tonalitic melt and gives magmatic age of the rock in which they occur with an established igneous age of 3840 ma established from such relationships we interpret the correlated variation between the field nature of these rocks and their zircon petrography age structure as due to superimposition onto 3840 ma tonalite protoliths of variable amounts of heterogeneous strain heterogeneous distribution of melt patches formed during in situ anatexis at up to 800 c plus granitic veining this explains why geologically simple metatonalites have simple zircon populations whereas complex orthogneisses have complex zircons the large amount of integrated field geochemical and zircon data rule out an alternative interpretation that the 3840 ma zircons represent an igneous xenocrystic component present in younger rocks to varying degrees if this were true then the structurally simple less reworked rocks should still display complex zircon populations gneisses with 3840 ma zircon are commonest on akilia and neighbouring islands in itilleq fjord 65 km east akilia and on the north of ivisaartoq 150 km northeast of akilia these include from itilleq a 3891 6 ma gneissic tonalite with minor neosome which is currently the oldest rock recognised in the itsaq gneiss complex overall the 3840 ma tonalites are a widespread and unevenly distributed in the itsaq gneiss complex and they are a volumetrically minor component compared with 3800 3750 and 3700 ma tonalite generations using the subset of our data covering itilleq and the neighbouring fjords migmatite samples with 3800 ma igneous zircon are mutually exclusive from migmatite samples with 3700 ma igneous zircon this suggests that prior to an amalgamation event followed by 3660 3600 ma high grade metamorphism 3840 ma tonalites might have resided in a terrane discrete from 3700 ma tonalites this is in accord with interpretation of the non migmatised part of the complex in the isua area where a terrane of 3800 ma tonalites with a minor associated 3840 ma component and a terrane with 3700 ma tonalites were tectonically juxtaposed at 3660 ma

numerical models of mantle convection with secular cooling summary a 2 d time dependent finite difference numerical model is used to investigate the thermal character and evolution of a convecting layer which is cooling as it convects two basic cooling modes are considered in the first both upper and lower boundaries are cooled at the same rate while maintaining the same temperature difference across the layer in the second the lower boundary temperature decreases with time while the upper boundary temperature is fixed at 0 c the first cooling mode simulates the effects of internal heating while the second simulates planetary cooling as mantle convection extracts heat from and thereby cools the earth s core the mathematical analogue between the effects of cooling and internal heating is verified for finiteamplitude convection it is found that after an initial transient period the central core of a steady but vigorous convection cell cools at a constant rate which is governed by the rate of cooling of the boundaries and the viscosity structure of the layer for uppermantle models the transient stage lasts for about 30 per cent of the age of the earth while for the whole mantle it lasts for longer than the age of the earth consequently in our models the bulk cooling of the mantle lags behind the cooling of the coremantle boundary models with temperature dependent viscosity are found to cool in the same manner as models with depth dependent viscosity the rate of cooling is controlled primarily by the horizontally averaged variation of viscosity with depth if the earth s mantle cools in a similar fashion secular cooling of the planet may be insensitive to lateral variations of viscosity

chronology of the mount magnet granite greenstone terrain yilgarn craton western australia implications for field based predictions of the relative timing of granitoid emplacement abstract in this paper we report ion probe u pb ages for zircons from thirteen granitoids and five supracrustal rocks from the mount magnet area of the archaean murchison province of western australia the oldest granitoids in the area are two porphyry dykes and three small apparrently underformed bodies that intrude the greenstones the porphyries have ages of 2715 and 2752 ma and the undeformed granitoids range in age between 2696 and 2716 ma the external monzogranites that surround the greenstone belt are larger than the internal granitoids and show variable evidence of deformation from a foliated texture to recrystallization at grain boundaries with the exception of a gneissic sample which has an age of 2710 ma they are younger than the undeformed granites with ages between 2640 and 2694 ma a late leucocratic vein in one sample is 20 ma younger than the mesocratic foliated rock it cuts the young age for the external monzogranite is unexpected and it implies that the supracrustal rocks away from the margins of the mount magnet greenstone belt and the small granitoids that intrude them were cold and rigid at the time of emplacement of the monzogranites so that they behaved as a coherent block we suggest that the stress associated with the deformation even that produced foliation gneissic textures and recrystallization in the external granitoids was dissipated at the margins of the greenstone belts so that the cores of these belts were little affected by this event the age difference of 20 ma between the melanocratic and leucocratic phases in the same rock suggest slow cooling and crystallization in the lower or middle crust followed by tectonic emplacement into the upper crust the mount magnet granitoids contain abundant xenocrystic zircons that show three pronounced peaks in their age distribution a broad peak between 2910 and 2950 ma and sharp peaks at 2800 ma and 2730 ma these ages correlate with eruption of volcanic rocks in the murchison we suggest that the thermal events that produced these periods of volcanism were also recorded in the lower crust which later melted to produce the mount magnet granitoids two felsic volcanics from near the top of the stratigraphy yield ages of 2704 and 2727 ma and a chert the warramboo galtee moore chert of the windaning formation an age of 2798 ma not 3 0 ga as had previously been thought the 2727 ma felsic volcanic unit unconformably overlies the warramboo galtee moore chert and is in turn overlain by the wattle creek basalt of the mount farmer group this suggests that the mount farmer group has an age of 2 7 ga and that 2 7 ga mafic volcanism is present in the murchison the younger external monzogranites which make up the bulk of the granitoids in the mount magnet region may have derived their heat from the mantle plume head that is believed to be responsible for basaltic and komatiitic volcanism of the norseman wiluna greenstone belt however the smaller internal granitoids are older than the plume related volcanic activity of the eastern yilgarn and require an additional heat source to have affected the northern part of the yilgarn craton

structures of the zagros fold thrust belt in iran the zagros fold thrust belt in iran forms the external part of the zagros active orogenic wedge it includes a sequence of heterogeneous latest neoproterozoic phanerozoic sedimentary cover strata 7 to 12 km thick and composed of alternating incompetent and competent layers overlying precambrian crystalline basement with a complex pre zagros structural fabric balancing structures of the cover statigraphic units in the zagros fold thrust belt requires in sequence and out of sequence involvement of the precambrian basement in the deformation six detailed balanced and retrodeformable cross sections which are constructed based on geological and geophysical data across various sectors of the belt show fault bend and fault propagation folds interpreted to have formed by slip on their subjacent thrusts out of sequence basement rooted thrusts as the interpretations in the constructed cross sections suggest have breached the cover basement interface and using incompetent cover strata for propagation cut across the cover structures and have created associated new folds superimposed upon the pre existing structures this style of deformation which has resulted in structural complexity of the belt characterizes a 200 to 300 km wide zone of distributed partly synchronous deformation with along strike and across strike variations it also implies that the zagros fold thrust belt as the external part of the orogenic wedge is still in its subcritical condition with internal deformation to achieve a critical taper in creating structural complexity in addition to out of sequence thrusting mechanically weak layers evaporites and mudstones of the cover strata have played a significant role by providing several detachment horizons shortening estimates across the belt are variable based on the constructed cross sections and their restorations minimum shortening estimates range from 16 percent to 30 percent in different sectors of the belt

intrusion age of pan african augen gneisses in the southern menderes massif and the age of cooling after alpine ductile extensional deformation

mantle convection as a boundary layer phenomenon summary the boundary layer nature of vigorous thermal convection is explored using high resolution numerical solutions to the governing hydrodynamic equations these solutions are obtained for a series of idealized models of the earth s mantle in which the viscosity is assumed to be constant a detailed analysis of the local energy balance within the horizontal and vertical thermal boundary layers is presented in terms of which a test of the fundamental assumptions of boundary layer theory is provided the results of this test have important geophysical consequences since the asymptotic predictions of boundary layer theory have been employed extensively in the context of thermal history modelling although boundary layer theory closely predicts the correct power law behaviour of various quantities it does not determine their absolute values accurately vertical advection is shown to play an important role in the energy balance within horizontal boundary layers at all rayleigh numbers horizontal and vertical advection dominate the energy balance within vertical plumes while horizontal diffusion plays a very minor role when heating is partially from within the fluid vertical advection into the upper thermal boundary layer can produce significant departures in the thermal structure from that found when heating is entirely from below for a free upper boundary this results in a relative flattening of the variations of surface topography and heat flow across the convection cells for a constant velocity upper boundary similar to plate motion the bathymetry flattens but the heat flow does not this result agrees with marine observations rigidity of the thermal boundary layer below the upper surface is not included explicitly in the model and it is not known whether the inclusion of this feature in future models would significantly alter the topographic expression if not the observed departure of the oceanic bathymetry from a 6 dependence at old ocean floor ages could be attributed to a small amount of internal heating in a mantle wide convective circulation

long lived eocene miocene stationary magmatism in nw iran along a transform plate boundary abstract the eocene miocene mianeh hashtroud igneous district in nw iran is part of the turkish caucasus iranian collision zone a key region to decipher the assembly and differentiation of gondwana derived terranes along the alpine himalayan convergence zone major inherited tectonic structures control in space and time the mesozoic cenozoic transition from oceanic subduction to continental collision in the region the geology of the study area is dominated by a polyphase long lived magmatic activity spanning from 45 to 6 ma the igneous products are subalkaline to alkaline with intermediate to acid compositions and a high k calcalkaline to shoshonitic affinity evidence of crustal contamination is attested by inherited zircons in the oldest eocene oligocene samples with ages spanning from neo archean to paleocene the sr nd isotopic compositions of the eocene oligocene samples plot close to the bulk silicate earth estimate whereas the miocene samples document stronger crustal contamination the lack of correlation between nd sr isotopes and sio2 supports a scenario of magma differentiation of different magma batches rather than crustal contamination major oxide and sr nd isotopic variation lead us to suggest that magmatism is the consequence of re melting of earlier underplated mesozoic tertiary magmatic products controlled by amphibole dominated fractionation processes regional scale correlations show long lived cenozoic magmatism in nw iran and caucasus region where the main porphyry and epithermal deposits occur we propose that the cenozoic collisional magmatism and the associated mineralisation at the junction between nw iran and caucasus was controlled by the activity of a major lithosphere scale inherited boundary transverse to the convergence zone in such a geodynamic setting the along strike segmentation of the lithosphere slab generated asthenospheric melts their upwelling into the metasomatised supra subduction mantle wedge and the potential activation of different mantle and crustal sources with consequent mineral endowment in the region

effects of chemical fractionation of heat producing elements on mantle evolution inferred from a numerical model of coupled magmatism mantle convection system abstract a numerical model of a coupled magmatism mantle convection system has been developed to study how a strong chemical fractionation of heat producing elements as a result of mantle magmatism influences the evolution of the uppermost mantle mantle convection is modeled by a convection of a binary eutectic material a b 1 with a newtonian rheology in a two dimensional rectangular box a stands for olivine and b stands for a mixture of pyroxene and garnet the viscosity depends strongly on temperature and the density increases with decreasing and temperature the material contains decaying heat producing elements mantle magmatism is modeled by an extraction of melt generated at depth by a pressure release partial melting and a deposit of the extracted melt in a thin crustal layer at the top of the box the melt extraction and deposit give rise to heterogeneity in the distribution of heat producing elements as well as in distribution a dense low magmatic product is more enriched in heat producing elements than is a less dense high residual material the positive thermal buoyancy that a low material gains owing to its enrichment with heat producing elements causes a catastrophic change in the chemical structure of the uppermost mantle with time when the viscosity in the top thermal boundary layer lithosphere is sufficiently high a chemically stratified structure with a high residual material in the upper part and a low material in the lower part of the box develops at the beginning when active magmatism occurs owing to strong internal heating as the heat producing elements decay and the magmatism becomes weaker a steep gradient or a discontinuity in distribution develops between the high part and the low part of the box and the convection occurs temporarily as a layered convection the chemically layered structure is however catastrophically destroyed and the box suddenly becomes more chemically homogeneous at a subsequent time when the positive thermal buoyancy of the low material at depth becomes stronger than the chemically induced negative buoyancy of the low material the catastrophic change in chemical structure induces a drop in temperature at depth and a sudden enhancement of mantle magmatism when the viscosity in the lithosphere is sufficiently low in contrast the chemically layered structure is stable and the catastrophic change from a chemically layered structure to a more chemically homogeneous structure does not occur a catastrophic destruction of a chemically layered structure in the upper mantle as a result of strong fractionation of heat producing elements may have occurred in the early earth

the neotethyan sanandaj sirjan zone of iran as an archetype for passive margin arc transitions the sanandaj sirjan zone of iran is a northwest trending orogenic belt immediately north of the zagros suture which represents the former position of the neotethys ocean the zone contains the most extensive best preserved record of key events in the formation and evolution of the neotethys from its birth in late paleozoic time through its demise during the mid tertiary collision of arabia with eurasia the record includes rifting of continental fragments off of the northern margin of gondwanaland formation of facing passive continental margins initiation of subduction along the northern margin and progressive development of a continental magmatic arc the latter two of these events are critical phases of the wilson cycle that elsewhere in the world are poorly preserved in the geologic record because of superimposed events our new synthesis reaffirms the similarity between this zone and various terranes to the north in central iran late paleozoic rifting preserved as a type granites and accelerated subsidence was followed by a phase of pronounced subsidence and shallow marine sedimentation in permian through triassic time marking the formation and evolution of passive margins on both sides of the suture subduction and arc magmatism began in latest triassic early jurassic time culminating at 170 ma the extinction of arc magmatism in this zone and its shift northeastward to form the subparallel urumieh dokhtar arc occurred diachronously along strike in late cretaceous or paleogene time post cretaceous uplift transformed the zone from a primarily marine borderland into a marine archipelago that persisted until mid tertiary time

zircon u pb ages and hf o isotopic composition of migmatites from the zanjan takab complex nw iran constraints on partial melting of metasediments abstract we study migmatites and other metamorphic rocks in the zanjan takab region of nw iran and use these results to report the first evidence of oligocene core complex formation in iran four samples of migmatites associated with paragneisses including leucosomes and associated para amphibolite melanosomes were selected for u pb dating and hf o isotopic analysis zircon cores interpreted as originally detrital zircons have variable ages that peak at ca 100 110 ma but their sedimentation age indicated by the youngest 206 pb 238 u ages is ca 35 40 ma new zircons associated with incipient melting occur as overgrowths around zircon cores and or as newly grown grains morphologies and internal structures suggest that rim growth and formation of new zircons were associated with partial melting all four samples contain zircons with rims that yield 206 pb 238 u ages of 28 25 ma indicating that partial melting occurred in late oligocene time 18 o values for zircon rims vary between 8 2 and 12 3 significantly higher than expected for mantle inputs 18 o 6 and consistent with equilibrium with surface materials zircon rims yield ehf t between 2 2 and 12 4 and two stage hf model ages of 448 562 ma indicating that the region is underlain by cadomian caledonian crust according to the hf o isotopic values the main mechanism forming zircon rims was dissolution of pre existing detrital zircons with reprecipitation of new zircon shortly thereafter oligocene ages indicate that partial melting accompanied core complex formation in the zanjan takab region extension melting and core complex formation in south central iran are eocene in age but younger ages of oligocene miocene in nw iran and turkey indicate that extension was distributed throughout the region during cenozoic time

building the zagros collisional orogen timing strain distribution and the dynamics of arabia eurasia plate convergence he zagros mountains are the result of the arabia eurasia collision initiated at 35 ma as the rifted arabian lithosphere was underthrusted beneath the iranian plate due to its negative buoyancy the onset of crustal thickening started at 25 ma as recorded by the hinterland exhumation and foreland clastic deposition deformation throughout the arabia eurasia collision zone and the uplift of the iranian plateau occurred after 15 12 ma as a result of shortening thickening of the thin iranian crust we emphasize that only 42 of the post 35 ma convergence is partitioned by shortening within central iran tomographic constraints show ongoing slab steepening or breakoff in the nw zagros whereas underthrusting of the arabian plate is observed beneath central zagros the current subduction dynamics can be explained by the original lateral difference in the buoyancy of the distal margin that promoted slab sinking in nw zagros and underthrusting in central zagros critical wedge approach applied to the zagros favors the hypothesis of strong brittle crust detached above a viscous lower crust in contrast the weak sedimentary cover deforms by buckling of a thick multilayered cover thrust faulting associated with folding occurs in the competent layers and is responsible for most of the earthquakes there is evidence that the role of the slab pull force in driving the arabian plate motion was reduced after 12 ma large scale mantle flow induced by mantle upwelling at the afar plume appears to be the main driver of the arabia plate motion we stress that the main kinematic change in the zagros region occurred at 15 12 ma as the zagros uplifted before the arabian slab detached the zagros appears key to investigate coupling between continental rheology plate driving forces and mountain building in which the role of rift inheritance appears to be central

petrogenesis of the carboniferous ghaleh dezh metagranite sanandaj sirjan zone iran constraints from new zircon u pb and 40ar 39ar ages and sr nd isotopes

the mechanisms of petrogenesis of the archaean continental crust comparison with modern processes the petrographic and chemical composition of magmatic rocks generated during the archaean appears to be different from that of post archaean rocks komatiites are widespread before 2 5 ga and rarely occur afterwards in addition the archaean continental crust is primarily ttg tonalitic trondhjemitic and granodioritic in composition exhibiting typical trondhjemitic differentiation trends whereas modern equivalents are granodioritic to granitic following classical calc alkaline differentiation trends this distinction becomes more prominent when rare earth elements ree are taken into account archaean ttg are yb poor ybn 8 5 and have high layb ratios 5 layb n 150 in comparison the post 2 5 ga granitoids emplaced in subduction zone geodynamic environments have high yb content 4 5 ybn 20 with very low layb n ratios 20 theoretical calculations and experimental petrology have shown that the ttg can be produced by partial melting of an archaean tholeiite transformed into garnet bearing amphibolite consequently the low heavy ree content of the ttg is explained by the influence of both residual garnet and hornblende in their source after 2 5 ga the role of these minerals in calc alkaline magma genesis becomes progressively less important which is interpreted in terms of a cooling earth model in modern subduction zone environments the subducted oceanic slab is relatively old and cold and the geothermal gradient along the benioff plane in low ca 10 c km consequently the down going lithosphere undergoes dehydration before partial melting is able to occur the liberated fluids are light ree and lile enriched and ascend into the overlying mantle wedge where they induced partial fusion the produced magmas separate from their mantle source region leaving a residue mainly composed of olivine and pyroxenes mantle derived magmas typically exhibit high yb contents due to low kdyb values for olivine and pyroxenes during the archaean the subducted lithosphere was relatively young and hot providing high geothermal gradients along the benioff zone thus partial melting of the subducted slab was possible at lower temperatures before dehydration would take place garnet and hornblende are the main residual phases accounting for the low yb contents of the archaean ttg this model can be tested using a modern analogue of archaean like subduction processes in south chile an oceanic ridge has subducted and all thermodynamic calculations indicate that this creates locally high geothermal gradients along the benioff zone thus in very small areas archaean like environments may be simulated in modern subduction zones the modern andesites produced in this environment show archaean geochemical characteristics with low ybn 5 whereas the majority of andesites along the andean arc have modern patterns with ybn ranging from 8 to more than 17 this conclusion was generalised to all young subducted lithospheres all over the world in conclusion it appears that since the archaean there has been a change in the site of continental crust genesis the location of calc alkaline magma source in subduction zone environments has migrated through time from the subducted slab to the mantle wedge this is a direct consequence of the progressive cooling of the earth

komatiites i eruption and flow because of their high eruption temperatures and ultrabasic composition komatiite lavas had low viscosities which typically ranged from 0 1 to 10 pa s a major consequence of this low viscosity is that most lavas erupted as turbulent flows an analysis of their ascent through the lithosphere suggests ascent velocities in the range of 1 to over 10ms 1 and reynolds numbers much greater than the critical value of 2000 the lavas would have remained turbulent for most or all of their subsequent flow and emplacement typical horizontal flow rates are estimated to range from 0 5 to 100 m2 s per unit width of flow such turbulent lava flows would have lost their heat by convection to the surroundings at rates which are orders of magnitude greater than the rates for laminar flows which cool by conduction a quantitative analysis of the cooling of komatiites indicates cooling rates from over 1000 c hr to a few c hr 1 while the flows remained turbulent these rates are in an appropriate range to cause phenomena such as high nucleation rates strong supersaturation of the lava delayed nucleation of olivine and skeletal or dendritic crystal morphologies komatiites often flowed over ground composed of rocks with lower melting temperatures it is proposed that the turbulent lavas melted the ground to form deep thermal erosion channels melting rates at the lava source are calculated at several metres per day and deep troughs with depths of several metres to hundreds of metres and lengths of several kilometres probably formed laboratory experiments performed to simulate thermal erosion show qualitative agreement with the theory with channel depth decreasing downstream the experiments also revealed that the channel margins become undercut during thermal erosion to form overhanging sides of the channel some sinuous rilles observed in the mare regions of the moon are thought to have formed by thermal erosion hulme 1973 they provide analogues of the channels postulated to form in komatiite eruptions where conditions were in fact more favourable for thermal erosion an assessment of the role of olivine crystals precipitated in the cooling turbulent flows indicates that they will remain in suspension until the lava has come to rest contamination of komatiite lava by underlying rock can be as much as 10 per cent some illustrative calculations show how the major element and trace element compositions of residual melts can be significantly modified by combined assimilation and fractional crystallization in a moving flow assimilation of tholeiitic basalt into a komatiite can cause incompatible trace element ratios such as ti zr and y zr and the rare earth patterns of derivative lavas to vary substantially some of the variations in such geochemical parameters which are often ascribed to mantle heterogeneity also could have resulted from assimilation of the ground assimilation could have modified the isotope geochemistry of lava suites and led to apparent ages which differ from the true eruption age the thermal erosion model also provides an explanation of the formation of some nickel sulphide ores found at the bottom of thick komatiite flows it is proposed that ores can form by assimilation of sulphur rich sediment which combines with ni from the komatiite to form an immiscible liquid

helium isotopic evidence for a lower mantle component in depleted archean komatiite archean magnesium rich komatiites require hot and presumably deep mantle sources but their trace element composition and radiogenic isotope composition are similar to those of modern mid ocean ridge basalts which originate in the upper mantle the isotopic composition of helium extracted by sequential crushing of fresh olivines separated from two archean and one mid proterozoic komatiites varies over three orders of magnitude between a radiogenic end member rich in helium 4 and a component rich in helium 3 such helium 3 enrichment suggests the presence of a lower mantle component in archean komatiites

metamorphism denudation and sea level in the archean and cooling of the earth abstract over long enough time scales an antagonistic balance acting between tectonics uplift and rifting and the sedimentary cycle erosion and deposition probably keeps continental surfaces close to sea level we argue that this balance constitutes the primary mechanism for keeping long term continental freeboard constant by regulating the continental thickness an important implication is that the long term uplift or down warp history of ancient continental segments provides direct information on secular changes in the isostasy of the adjacent ocean basins through time the amount of uplift can be estimated from the regional metamorphic grade the burial pressures at present exposure of ten undisturbed archean granite greenstone segments are estimated to be 1 5 0 5 kb from this we infer a maximum long term uplift and erosion of 5 2 km for stable continent since 3 0 ga implying a mean continental thickness of 46 km at the time of crustal stabilization for a constant mean ocean depth isostasy requires a palaeo oceanic crust 14 2 km thick in the middle archean alternatively if little continent existed then the oceans would have been shallower and the palaeo oceanic crust 21 2 km thick assuming plate spreading did occur in the archean potential temperatures in the shallow mantle at 3 ga were 90 30 c or 170 20 c for no continent hotter than those prevailing today these temperatures translate into time averaged cooling rates for the shallow mantle of 57 c ga 1 and are comparable to estimates of the present day cooling rate for the whole earth the low burial pressures are a strong argument against the existence of catastrophic mantle flush events in the geological past which have been predicted by some mantle convection models taken together these constraints suggest a plume related geological setting i e oceanic plateau like flood basalt for the initial stage of greenstone belt evolution

convergence history across zagros iran constraints from collisional and earlier deformation the zagros orogen provides a unique opportunity within the alpine system to evaluate the interplay between a young tertiary collision and earlier subduction obduction processes within the crush zone and the sanandaj sirjan internal zone separating the zagros fold belt from central iran we document several major tectonic events taking place at the end of the cretaceous of the eocene and from the mio pliocene onwards ca 20 15 ma contrary to recent interpretations our data cross sections and description of the overall deformation style strongly suggest that the main zagros thrust mzt is deeply rooted possibly to moho depths and that the suture zone effectively runs along the mzt field observations show that the final resorption of the oceanic domain took place slightly after 35 ma and that collision must have started before ca 23 25 ma in northern zagros the shortening rate across the crush zone since the mid miocene 20 15 ma is estimated at a minimum 3 4 mm year shear movements in the crush zone during the eocene oligocene period and extensional strike slip movements in the internal zones during the late cretaceous point to an oblique setting early in the convergence history a geotectonic scenario for convergence from the time of obduction to the present is finally proposed

arabia eurasia collision and the forcing of mid cenozoic global cooling the end of the eocene greenhouse world was the most dramatic phase in the long term cooling trend of the cenozoic era here we show that the arabia eurasia collision and the closure of the tethys ocean gateway began in the late eocene at 35 ma up to 25 million years earlier than in many reconstructions we suggest that global cooling was forced by processes associated with the initial collision that reduced atmospheric co2 these are 1 waning volcanism across southwest asia 2 increased organic carbon storage in paratethyan basins e g black sea and south caspian 3 increased silicate weathering in the collision zone and 4 a shift towards modern patterns of ocean currents associated with increased vigour in circulation and organic productivity

archean mantle fractionation nd isotopic data require that most archean igneous rocks are derived from depleted or undepleted mantle sources while geochemical model stuides necessitate enriched sources for most archean tonalites this apparent contradiction can be reconciled if the enriched sources are produced by short lived 150 m y enrichment of depleted sources extensive depleted mantle reservoirs were probably produced during early degassing of the upper mantle 4 4 b y ago and complementary enriched reservoirs collected at the base of the lithosphere sources for tonalites may be produced from short lived enrichment of basalt above convective downcurrents by metasomatic and magmatic components derived chiefly from depleted mantle and by degassing of sinking short lived komatiitic lithosphere

the deep crust beneath island arcs inherited zircons reveal a gondwana continental fragment beneath east java indonesia abstract inherited zircons in cenozoic sedimentary and igneous rocks of east java range in age from archean to cenozoic the distribution of zircons reveals two different basement types at depth the igneous rocks of the early cenozoic arc found along the southeast coast contain only archean to cambrian zircons in contrast clastic rocks of north and west of east java contain cretaceous zircons which are not found in the arc rocks to the south the presence of cretaceous zircons supports previous interpretations that much of east java is underlain by arc and ophiolitic rocks accreted to the southeast asian margin during cretaceous subduction however such accreted material cannot account for the older zircons the age populations of archean to cambrian zircons in the arc rocks are similar to gondwana crust we interpret the east java early cenozoic arc to be underlain by a continental fragment of gondwana origin and not cretaceous material as previously suggested melts rising through the crust feeding the early cenozoic arc picked up the ancient zircons through assimilation or partial melting we suggest a western australian origin for the fragment which rifted from australia during the mesozoic and collided with southeast asia resulting in the termination of cretaceous subduction continental crust was therefore present at depth beneath the arc in south java when cenozoic subduction began in the eocene

dating the oldest greenstone in india a 3 51 ga precise u pb shrimp zircon age for dacitic lava of the southern iron ore group singhbhum craton abstract this article reports a precise 3506 8 2 3 ma u pb shrimp zircon age for dacitic lava in a well preserved low grade metamorphic and low strained greenstone belt succession of the southern iron ore group singhbhum craton india this age makes the succession the oldest known greenstone belt succession in india and one of the oldest low strain greenstone successions in the world after the 3 51 ga coonterunah group of the pilbara craton western australia and the moderately deformed 3 54 ga theespruit formation of the barberton greenstone belt kaapvaal craton south africa the geochemical composition of the dacitic lava and related volcanic rocks suggests that they formed in a volcanic arc setting the succession also contains a major 120 m thick oxide facies banded iron formation that distinguishes it from the slightly older successions of the pilbara and kaapvaal cratons this banded iron formation may well be one of the oldest and most well preserved and together with associated volcanics

cretaceous tertiary convergence and continental collision sanandaj sirjan zone western iran abstract the sanandaj sirjan zone contains the metamorphic core of the zagros continental collision zone in western iran the zone has been subdivided into the following from southwest to northeast an outer belt of imbricate thrust slices radiolarite bisotun ophiolite and marginal sub zones which consist of mesozoic deep marine sediments shallow marine carbonates oceanic crust and volcanic arc respectively and an inner complexly deformed sub zone late palaeozoic mesozoic passive margin succession rifting and sea floor spreading of tethys occurred in the permian to triassic but in the sanandaj sirjan zone extension related successions are mainly of late triassic age subduction of tethyan sea floor in the late jurassic to cretaceous produced deformation metamorphism and unconformities in the marginal and complexly deformed sub zones deformation climaxed in the late cretaceous when a major southwest vergent fold belt formed associated with greenschist facies metamorphism and post dated by abundant palaeogene granitic plutons in the southwest of the zone a late cretaceous island arc passive margin collision occurred with ophiolite emplacement onto the northern arabian margin similar to that in oman final closure of tethys was not completed until the miocene when central iran collided with the northeast arabian margin

zircon hf isotopic constraints on magmatic and tectonic evolution in iran implications for crustal growth in the tethyan orogenic belt abstract this study presents coupled zircon u pb and hf isotopic analyses of 82 igneous rocks that are mainly from the urumieh dokhtar magmatic arc udma and sanandaj sirjan zone ssz iran together with additional 11 samples from armenia along the central part of the neotethyan orogenic belt the results provide an overview of the isotopic variations for several important magmatic stages which can improve our understanding of magmatic and crustal evolution regarding the life cycles of tethys oceans during the past 600 myr in iran and adjacent regions late neoproterozoic cambrian 595 511 ma basement rocks in the ssz and central iran which were part of the cadomian magmatic arc yield zircon ehf t values from 11 to 3 and are attributed to have partially evolved with juvenile magmas in the arabian nubian shield along the northern margin of gondwana carboniferous 349 311 ma a type granitoids in the northwestern ssz show mostly positive zircon ehf t values from 5 to 1 and they are interpreted as the product involving a moderate mantle contribution in an extensional environment affiliated with the initial rifting of the neotethys late triassic 219 203 ma granitoids in the eastern alborz give slightly lower zircon ehf t values between 3 and 3 as a result of the progress of closing the paleotethys middle jurassic 165 ma granitoids in the ssz and central udma representing an early phase of neotethyan subduction related magmatism exhibit similar zircon ehf t values from 5 to 3 implying an origin of mixed juvenile and reworked sources to form the magmas late cretaceous 81 72 ma intrusive rocks in the southeast udma yield bimodal zircon ehf t values from 15 to 11 in the jiroft and from 5 to 9 in the bazman areas suggesting that the latter involves a reworked crustal component at the latest cenozoic magmatic rocks with ages

the isotopic composition of yb and the precise and accurate determination of lu concentrations and lu hf ratios by isotope dilution using mc icpms 1 we report yb isotopic data for the purpose of accurately correcting measurements of lu and lu hf ratios during multicollector inductively coupled plasma mass spectrometer mc icpms analysis the yb isotopic ratios agree with recent results of chu et al 2002 and amelin and davis 2004 when corrected for mass fractionation to the same reference values protocols for lu measurement involving simultaneous subtraction of yb interference and application of a fractionation correction based on yb are described it is shown that lu concentrations and lu hf ratios determined by mc icpms using this methodology are more precise and potentially more accurate than results based on analysis of lu by thermal ionization mass spectrometry tims a number of mc icpms lu hf isotopic analyses of standard rocks and other geological samples previously analyzed by tims are presented these document reproducibility of lu hf within 0 2 where most discrepancies can be assigned to the unknown lu mass fractionation during tims analysis

the equation of state of a molten komatiite 2 application to komatiite petrogenesis and the hadean mantle new experimental data for the equation of state of a komatiitic liquid were used to model adiabatic melting in a peridotitic mantle if komatiites are formed by 30 partial melting of a peridotitic mantle then komatiites generated by adiabatic melting come from source regions that began their unmelted ascent in the lower transition zone 500 670 km or the lower mantle 670 km the great depth of incipient melting implied by this model suggests that komatiitic liquids may form in a pressure regime where they are denser than their coexisting crystals possibly the bulk mantle although komatiitic magmas are thought to separate from residual crystals in the mantle at a temperature 200 c greater than modem mid ocean ridge basalts morbs their ultimate sources are predicted to be diapirs that if adiabatically decompressed from initially solid mantle were more than 700 c hotter than the sources of morbs and were derived from great depth we also studied the evolution of an initially molten mantle i e a magma ocean our model considers the thermal structure of the magma ocean density constraints on crystal segregation and approximate phase relationships for a nominally chondritic mantle crystallization will begin at the core mantle boundary perovskite buoyancy at 70 gpa may lead to a compositionally stratified lower mantle with iron enriched magnesiowustite content increasing with depth large convective velocities in the magma ocean would prohibit crystal liquid fractionation by settling or flotation until quiescent boundary layers form such boundary layers could form when the crystal content of the magma reaches a critical value near 44 vol and in the late stages of crystallization around unmelted blocks of foundered protocrust matrix compaction and diapirism could also lead to fractionation effects the upper mantle could be depleted or enriched in perovskite components relative to the bulk mantle olivine neutral buoyancy may lead to the formation of a dunite septum in the upper mantle partitioning the ocean into upper and lower reservoirs but this septum must be permeable

continental arc volcanism and tectonic setting in central anatolia turkey abstract the neogene and quaternary volcanism of central anatolia represents the central sector of the anatolian volcanic arc related to continental collision between the afro arabian and eurasian plates it is closely associated with a complex system of tectonic depressions related to brittle deformations of transtentional type and which commenced in the late miocene the volcanism here considered can be divided into three main periods of activity separated by important deformative and erosive events the first period is represented by a mostly andesitic effusive activity the second period is characterized by the emplacement of a thick ignimbritic sequence and shows an areal distribution up to 11 000 km 2 seven ignimbrite units have been recognized the three main units were found at a distance of more than 100 km from the presumed source area geological and sedimentological data lead us to recognize the melendiz dag volcanic complex and the ciftlik caldera as the probable ignimbrites source vent during the third period great andesitic basaltic stratovolcanoes and a number of prevalently acid monogenic centres developed the relationship between the volcanic activity is clearly conditioned by the main transcurrent fault systems present in central anatolia the neogene quaternary volcanic activity prevalently developed along the ene wsw karaman sivas lineament most of the great central volcanoes developed at the intersection between the ene wsw trends and the ecemis and tuz golu transcurrent faults the structural interpretation of the quaternary monogenic centres is more difficult probably they are related to the very recent n s fault swarms which cross the anatolides and the taurus range