balanced section in thrust belts part 1 construction balanced geological cross sections are an important aid to understanding thrust belt structures and to estimating their hydrocarbon bearing potential two approaches to section balancing have been taken in the past construction of retrodeformable sections from raw data and modification of previous interpretations after an evaluation procedure these approaches may be unified by use of a langrangian grid projection of datum locations dips and stratigraphic thicknesses onto the section plane is performed directly by calculation or graphically using plunge lines spline interpolation and parallel fold modeling are the simplest ways of filling gaps in the sectional data but isogon interpolation yields rheologically more realistic results faults are interpolated using cutoff geometries as constraints fault tips are located from distance displacement plots whereas the depth to detachment is obtained by a modified chamberlin construction interpolations made on the section are directed back to the geologic map via plunge lines although these techniques of section construction do not guarantee a balance especially when complexities such as growth structures diapirs or strike slip faults are present they eliminate many potential errors

late cenozoic tectonic evolution of the northwestern tien shan new age estimates for the initiation of mountain building the tien shan are the quintessential intracontinental range situated more than 1000 km north of the suture between india and asia their initiation and growth in the cenozoic however remain poorly understood in this study we present stratigraphic detrital fission track and magnetostratigraphic results that provide a basis for reconstructing the cenozoic tectonic evolution of the kyrgyz range and adjacent chu basin in the northwestern tien shan detrital fission track thermochronology indicates that the northwestern tien shan was tectonically quiescent for much of the cenozoic prior to uplift and exhumation in the late miocene the kyrgyz range was buried by sediments shed from highlands to the south and or east paired bedrock fission track and u th he ages from a sampling transect of 2 4 km relief demonstrate that rapid exhumation commenced at ca 11 ma initial thrusting in the hinterland was followed by evaporite accumulation 0 4 km m y which coincided with erosion of the pre 11 ma strata that mantled the kyrgyz range between 10 and 3 ma bedrockexhumation rates decreased to 0 3 km m y while sedimentation rates decelerated initially to 0 25 km m y before accelerating to 0 4 km m y at 4 5 ma detrital fission track results indicate that by 4 5 ma

immiscibility synthesis as an indication of cooling rates of basalts abstract silicate liquid immiscibility of iron rich and silica rich compositions have been experimentally reproduced at various cooling rates to evaluate the possible effect of cooling rate on the size of the globules the experiments were performed at constant cooling rates from 1 to 32 c hr under atmospheric pressure and controlled oxygen fugacity a tholeiitic basalt which occurs as a 36 cm thick dike in the izu peninsula japan was used as starting material the sample was melted at the liquidus temperature 1190 5 c and cooled to 1000 c two immiscible liquids occur in the charges as do in the dike in the experiments the size of the globules is strongly related to the cooling rate the diameter of the globules decreases exponentially with increasing cooling rate and the number of globules decreases with decreasing cooling rate these facts together with the typical coalescence shape suggests a growth of globules by coalescence rather than a decrease of the nucleation rate compositional variations of the liquid just prior to splitting within each charge do not affect those of the two immiscible liquids however the compositional differences between the two liquids slightly increase with decreasing cooling rate using the cooling rate vs size relationships the size of globules in basalts allows a cooling rate estimate in the izu dike the 10 largest silica rich globules range from 2 3 m to 0 8 m in the center and the margin suggesting cooling rates of about 15 c hr and greater than 32 c hr respectively at temperatures near 1000 c other estimates from natural samples show a good agreement between deduced cooling rates and observed textures

textural difference between pahoehoe and aa lavas of izu oshima volcano japan an experimental study on population density of plagioclase the groundmass textures of 1778 a d pahoehoe and 1986 a d aa lavas of izu oshima volcano japan differ in population density of plagioclase by about two orders of magnitude the pahoehoe lavas are coarser grained and have a population density of lo cme3 while the aa lavas are finer grained and have a population density of 109 3 cma3 the groundmasses of these texturally different lavas however have nearly the identical chemical compositions one atmosphere melting crystallization experiments on the lavas showed that a 20 c difference in initial melting temperature near the liquidus temperature can cause about five orders of magnitude difference in the population density of plagioclase after annealing at about 100 c below the liquidus temperature yet more than two orders of magnitude difference in cooling rate of the experiments only bring about less than one order of magnitude difference in population density the large effect of the initial melting temperature on the population density of plagioclase is interpreted to reflect nucleation induced by the transformation of polymerized clusters in the melt into crystal nuclei by a reduction in the critical size of the nuclei the initial size distribution of clusters in the melt largely affects the population density of plagioclase during natural eruptive processes degassing of magmas produces strongly undercooled conditions and it is proposed that a slight difference in the degree of undercooling of magmas before final degassing and eruption may have caused the large difference in the population density of plagioclase of the pahoehoe and aa lavas

the lower zone critical zone transition of the bushveld complex a quantitative textural study the lower zone critical zone boundary of the bushveld complex is an intrusion wide major stratigraphic transition from ultramafic harzburgite and pyroxenite in the lower zone to increasingly plagioclase richpyroxenitesandnoritesinthecriticalzone quantitative textural and compositional data for 29 samples through this transition show the following lower zone orthopyroxene grains are larger have higher aspect ratios are better foliated and have a lower trapped liquid component than those of the critical zone the larger grain size of the lower zone results in crystal size distribution plots that are rotated to lower slopes and intercepts relative to those in the critical zone although all rocks show differing amountsoffoliation mineral lineations are weak to absent these data are consistent with significant compaction driven recrystallization in the study section numerical modeling of concurrent compaction and crystallization provides a quantitative model of how the lower zone critical zone transition may have formed plagioclase is rare in the lower zone because compaction removes interstitial liquid before it reaches plagioclasesaturation however asthecrystalpilegrows plagioclase saturation is reached in the interstitial liquid before compaction is complete in more evolved pyroxenites producing more abundant but still modest amounts of plagioclase characteristic of the lower critical zone it is concluded that both the textures and the modal mineralogy are largely controlled by compaction and compactiondriven recrystallization primary magmatic textures are not preserved

evidence for silicate liquid immiscibility in monzonites and petrogenesis of associated fe ti p rich rocks example from the raftsund intrusion lofoten northern norway

a crystal size distribution study of the kiglapait layered mafic intrusion labrador canada evidence for textural coarsening crystal size distributions csd of plagioclase olivine and clinopyroxene from the kiglapait intrusion labrador were measured for a series of 13 samples spanning most of the exposed intrusion all csds lack small crystals and are slightly curved to the right concave up all csds of a mineral sweep out a fan as expected where there is relatively little variation in phase proportion csd dispersion can be replicated using the communicating neighbours model of textural coarsening ostwald ripening most of the textural variation in the intrusion is the result of variable degrees of coarsening it is proposed that coarsening occurred in the outer part of the crystallisation boundary layer where release of latent heat and changes in the intercumulus liquid composition resulted in a reduction in undercooling suppression of nucleation and coarsening particularly coarse grained areas reflect focusing of circulating magmatic fluids during coarsening the effects of crystal settling were not observed in the csds even within single well developed layers they were probably concealed by the pervasive textural coarsening csds do not suggest that mechanical compaction has played an important role in the formation of texture although it is very likely to have occurred however samples from the lower part of the intrusion show textural evidence of pressure solution compaction

crystal size distribution csd in rocks and the kinetics and dynamics of crystallization crystal size distributions csds measured in metamorphic rocks yield quantitative information about crystal nucleation and growth rates growth times and the degree of overstepping t of reactions during metamorphism csds are described through use of a population density function n dn dl where n is the cumulative number of crystals per unit volume and l is a linear crystal size plots of ln n vs l for olivine pyroxene and magnetite in high temperature 1000 c basalt hornfelses from the isle of skye define linear arrays indicating continuous nucleation and growth of crystals during metamorphism using the slope and intercept of these linear plots in conjunction with growth rate estimates we infer minimum mineral growth times of less than 100 years at t 10 c and nucleation rates between 10 4 and 10 1 cm3 s garnet and magnetite in regionally metamorphosed pelitic schists from south central maine have csds which are bell shaped we interpret this form to be the result of two processes 1 initial continuous nucleation and growth of crystals and 2 later loss of small crystals due to annealing the large crystals in regional metamorphic rocks retain the original size frequency distribution and may be used to obtain quantitative information on the original conditions of crystal nucleation and growth the extent of annealing increases with increasing metamorphic grade and could be used to estimate the duration of annealing conditions if the value of a rate constant were known finally the different forms of crystal size distributions directly reflect differences in the thermal histories of regional vs contact metamorphosed rocks because contact metamorphism involves high temperatures for short durations resulting csds are linear and unaffected by annealing similar to those produced by crystallization from a melt because regional metamorphism involves prolonged cooling from high temperatures primary linear csds are later modified by annealing to bell shapes

pulses of plagioclase laden magmas and stratigraphic evolution in the upper zone of the bushveld complex south africa cumulate rocks of the upper main zone and upper zone uumz of the bushveld complex south africa contain the world s major resources of fe ti v hosted in ti magnetite and apatite and are commonly considered as having crystallized from the last major injection of magma into the magma chamber in this study we present the petrography modal proportions whole rock major element chemistry 260 samples electron microprobe data 10 000 analyses for plagioclase olivine and pyroxene and compiled analyses of cr in magnetite 239 samples for the uumz sampled over 2 1 km of the bierkraal drill cores in the western limb of the complex the uumz section exhibits a broad normal fractionation trend upwards but a series of reversals to more primitive anorthite contents in plagioclase mg in pyroxenes and olivine cr in whole rocks and cr in magnetite separates are observed accompanied by the appearance or disappearance of various minerals anorthosite or leucogabbro layers are closely linked to these reversals the reversals in an of plagioclase are used as boundaries to divide the uumz into 18 cycles these cycles are interpreted as indications of magma chamber replenishment by plagioclase laden magmas up to 20 vol plagioclase and are also marked by spikes in cr content in addition abundant fe ti oxide bearing plagioclase rich rocks are identified in the lower half of the uumz these have crystallized from a hybrid melt produced by the mixing of a new plagioclase bearing magma batch and the resident magma further crystallization of this hybrid liquid may lead to the formation of magnetite layers in the lower part of the uumz the bushveld uumz therefore grew by multiple emplacements of crystal laden magmas coming from deep seated chambers slow cooling in a shallow chamber explains the systematic bottom up compositional evolution in the cumulate pile within individual cycles the residual melt reached silicate liquid immiscibility soon after the saturation of apatite thereafter segregation of conjugate fe rich and si rich melts and crystallization of the paired melts produces cumulates with a smooth upward decrease in fe ti oxides whereas plagioclase mode increases in each apatite bearing cycle a comparison of systematic geochemical analyses and a detailed lithological stratigraphy between the bushveld limbs demonstrates the possible connectivity between the western and eastern upper zone but indicates notable differences from the bellevue section of the northern limb

palaeozoic collisional tectonics and magmatism of the chinese tien shan central asia abstract the chinese tien shan range is a palaeozoic orogenic belt which contains two collision zones the older southern collision accreted a north facing passive continental margin on the north side of the tarim block to an active continental margin on the south side of an elongate continental tract the central tien shan collision occurred along the qinbulak qawabulak fault southern tien shan suture the time of the collision is poorly constrained but was probably in in the late devonian early carboniferous we propose this age because of a major disconformity at this time along the north side of the tarim block and because the youshugou ophiolite is imbricated with middle devonian sediments a younger probably late carboniferous early permian collision along the north tien shan fault northern tien shan suture accreted the northern side of the central tien shan to an island arc which lay to its north the north tien shan arc this collision is bracketed by the middle carboniferous termination of arc magmatism and the appearance of late carboniferous or early permian elastics in a foreland basin developed over the extinct arc thrust sheets generated by the collision are proposed as the tectonic load responsible for the subsidence of this basin post collisional but palaeozoic dextral shear occurred along the northern suture zone this was accompanied by the intrusion of basic and acidic magmas in the central tien shan late palaeozoic basic igneous rocks from all three lithospheric blocks represented in the tien shan possess chemical characteristics associated with generation in supra subduction zone environments even though many post date one or both collisions rocks from each block also possess distinctive trace element chemistries which supports the three fold structural division of the orogenic belt it is unclear whether the chemical differences represent different source characteristics or are due to different episodes of magmatism being juxtaposed by later dextral strike slip fault motions because the southern collision zone in the tien shan is the older of the two the tarim block sensu stricto collided not with the eurasian landmass but with a continental block which was itself separated from eurasia by at least one ocean the destruction of this ocean in late carboniferous early permian times represented the final elimination of all oceanic basins from this part of central asia

pipe vesicles in basalt trails left by dense immiscible melt droplets sinking through a viscous basal thermal boundary layer abstract pipe vesicles tubular cavities found in the basal parts of p hoehoe lava flows worldwide constitute one of their fundamental primary structures like other vesicles pipe vesicles have been ascribed to gas bubbles whether rising buoyantly through viscous lava or trapped in an inward moving solidification front these models involve special and unrealistic conditions and still poorly explain characteristic features of pipe vesicles in a new model proposed here pipe vesicles are postulated to be hollow trails left by droplets of dense immiscible iron rich silicate or sulphide melt sinking through a viscous thermal boundary layer just above the lower chilled margin of a p hoehoe flow unit driven by gravity the droplets sink vertically in static lava but obliquely in mobile lava such as lava undergoing gravitational spreading viscous and fluid dynamic forces tend to deform the sinking droplets and resist their motion the characteristic inverted y inverted v and lambda shapes of individual pipe vesicles reflect downward branching with a large droplet breaking up in partially crystallized highly polymerized basalt in the basal thermal boundary layer pipe vesicles do not yield lava flow directions even on a local centimeters to meters scale but are indirect indicators of widespread liquid immiscibility processes in basalts and potential commercial fe cu ni mineralization as pipe vesicles are not vesicles cavities left by gas escape a new term is necessary and the term vermiform p hoehoe having good descriptive power but no genetic connonations is suggested for p hoehoe with pipe vesicles

dynamic observations of vesiculation reveal the role of silicate crystals in bubble nucleation and growth in andesitic magmas abstract bubble nucleation and growth control the explosivity of volcanic eruptions and the kinetics of these processes are generally determined from examinations of natural samples and quenched experimental run products these samples however only provide a view of the final state from which the initial conditions of a time evolving magmatic system are then inferred the interpretations that follow are inexact due to the inability of determining the exact conditions of nucleation and the potential detachment of bubbles from their nucleation sites an uncertainty that can obscure their nucleation location either homogeneously within the melt or heterogeneously at the interface between crystals and melts we present results of a series of dynamic real time 4d x ray tomographic microscopy experiments where we observed the development of bubbles in crystal bearing silicate magmas experimentally synthesized andesitic glasses with 0 25 0 5 wt h 2 o and seed silicate crystals were heated at 1 atm to induce bubble nucleation and track bubble growth and movement in contrast to previous studies on natural and experimentally produced samples we found that bubbles readily nucleated on plagioclase and clinopyroxene crystals that their contact angle changes during growth and that they can grow to sizes many times that of the silicate on whose surface they originated the rapid heterogeneous nucleation of bubbles at low degrees of supersaturation in the presence of silicate crystals demonstrates that silicates can affect when vesiculation ensues influencing subsequent permeability development and effusive vs explosive transition in volcanic eruptions

crystal mush compaction in the cohassett flood basalt flow hanford washington abstract the 40 m thick central part of the cohassett flow which was probably emplaced by inflation of an earlier more ti rich flow exhibits textural evidence of crystal mush compaction during solidification although most of the crystallization started in the roof zone as water circulated through fractures in the crust plumes of dense crystal mush must have dripped from the roof to the floor of the sheet thus displacing upward the colonnade entablature boundary the mush initially consisted of loosely stacked plagioclase crystals and liquid surrounding rapidly grown pyroxene oikocrysts containing radial chadacrysts of plagioclase upon compaction in the floor zone the liquid between the loosely stacked plagioclase crystals was expelled as the crystals packed tightly together to form sub parallel grains wrapped around the pyroxene oikocrysts which recrystallized into granular aggregates of augite and pigeonite stacks of sinking plagioclase crystals and rising liquid accumulated on the upper and lower sides respectively of horizontal plagioclase phenocrysts this asymmetric texture clearly indicates the relative motion of solids and liquid the degree of compaction as determined from variations in the amount of interstitial liquid and quantitative measures of textural anisotropy accounts for the chemical variation through the central part of the flow with compatible elements being concentrated in the lower part of the flow and incompatible elements concentrated in the upper part modeling of the compaction process using the iridium melt infiltration and reaction program of boudreau a 2003 iridium a program to model reaction of silicate liquid infiltrating a porous solid assemblage computers and geosciences 29 423 429 shows that the observed chemical and compaction dilation profiles are to be expected in a 40 m thick sheet of basaltic magma solidifying on the earth s surface compaction of crystal mush should be even more likely in slowly cooled intrusive bodies

power scaling and seasonal changes of floe areas in the arctic east siberian sea the cumulative number versus floe area distribution of seasonal sea floes from six satellite images of the arctic ocean during the summer breakup and melting is fit by two scale invariant power law scaling regimes for floe areas ranging from 30 to 28 400 000 m2 scaling exponents for larger floe areas range from 0 6 to 1 0 with an average of 0 8 scaling exponents for smaller floe areas range from 0 3 to 0 6 with an average of 0 5 the inflection point between the two scaling regimes ranges from 283 102 to 4850 102 m2 and generally moves from larger to smaller floe areas through the summer melting season the stability of the power scaling results is demonstrated for two of the images by dividing each in half and analyzing each half separately with the result that the scaling exponents and the size of the inflection points are nearly the same for each half as for the whole image we propose that the two scaling regimes and the inflection between them are established during the initial breakup of sea ice solely by the process of fracture the distributions of floe size regimes retain their scaling exponents as the floe pack evolves from larger to smaller floe areas from the initial breakup through the summer season due to grinding crushing fracture and melting the scaling exponents for floe area distribution are in the same range as those reported in previous studies of arctic floes and for the single scaling exponents found for crushed and ground geologic materials including streambed gravel lunar debris and artificially crushed quartz the single scaling exponent found for fault gouge falls below the range for floes possibly because the fracturing and grinding process in fault gouge takes place under high confining pressure a probabilistic model of fragmentation is proposed that generates a single power law scaling distribution of fragment size

differentiation of tholeiitic basalt to a type granite in the sept iles layered intrusion canada the undeformed 564 ma sept iles layered intrusion quebec canada is a large igneous body of c 20 000km3 from the base to the top it consists of a layered series dominated by troctolite and gabbro an anorthositic upper border series and a dominantly granitic upper series the parent magma of the layered series is inferred to be an iron rich tholeiitic basalt 48 wt sio2 15wt feot whole rock compositions from the chilled margin dykes cross cutting the layered series and silicic rocks from the upper series display continuous major and trace element geochemical trends ranging from basalts to ferroan metaluminous a type granites 77 wt sio2 initial 143nd 144nd 0 51201 0 51207 and 87sr 86sr 0 70353 0 70548 indicate a juvenile mantle source and minimal contamination by old crust 1 2 during crystallization geochemical modeling using the melts thermodynamic calculator combined with equations predicting mineral melt equilibria from experiments on tholeiitic basalts indicate that basaltic to monzonitic melt compositions are in equilibrium with the troctolites and gabbros of the layered series fe ti oxides saturate early in the layered series after 14 fractionation of plagioclase olivine cumulates further fractionation of fe ti oxide bearing gabbros drives the residual liquids toward silica enrichment and iron depletion major and trace element modeling indicates that the a type granites from the upper series were produced by protracted fractional crystallization of an iron rich basaltic parent magma at a fraction of residual liquid of only 8 the observed relative volumes of mafic cumulates and silicic rocks in the intrusion are in agreement with the calculations most of the intermediate compositions correspond to magmatic mafic enclave bearing granitoids and display geochemical evidence of hybridization intermediate compositions produced by fractional crystallization are scarce and a daly gap occurs from 57 to 67 wt sio2 this gap could result either from the fractional crystallization process or from silicate liquid immiscibility during that compositional interval

orientation of tabular mafic intrusions controls convective vigour and crystallization style the microstructure in basaltic dykes is significantly different from that in sills and lava lakes of the same bulk composition for a given width of intrusion or depth of lava lake vertical tabular bodies are coarser grained than horizontal bodies with an invariant plagioclase shape across the intrusion when comparing samples from sills and dykes for which the average grain size is the same the dyke samples contain fewer small grains and fewer large grains than the sill samples in contrast the variation of median clinopyroxene plagioclase plagioclase dihedral angles in dykes correlates precisely with that observed in sills and is a function of the rate of diffusive heat loss these patterns can be accounted for if the early stages of crystallization in dykes primarily involve the growth of isolated grains suspended in a well mixed convecting magma with the final stage during which dihedral angles form occurring in a crystal rich static magma in which heat loss is primarily diffusive in contrast crystallization in sills occurs predominantly in marginal solidification fronts suggesting that any convective motions are insufficient to entrain crystals from the marginal mushy layers and to keep them suspended while they grow an exception to this general pattern is provided by members of the mull solitdykes which propagated 100 1000 km se from the mull palaeogene igneous centre scotland through the shallow crust these dykes where sampled 100 km from mull have a microstructure indistinguishable from that of a sill of comparable thickness we suggest that sufficient nucleation and crystallization occurred in these dykes to increase the viscosity sufficiently to damp convection once unidirectional flow had ceased

india eurasia collision chronology has implications for crustal shortening and driving mechanism of plates the motion of the indian plate is determined in an absolute frame of reference and compared with the position of the southern margin of eurasia deduced from palaeomagnetic data in tibet the 2 600 900 km of continental crust shortening observed is shown to have occurred in three different episodes subduction of continental crust intracontinental thrusting and internal deformation and lateral extrusion the detailed chronology of the collision and plate reorganizations in the indian and pacific oceans supports the hypothesis that slab pull is a dominant driving mechanism of plate tectonics

a paleomagnetic study of tertiary formations from the kyrgyz tien shan and its tectonic implications as part of a general investigation of cenozoic deformation in central asia we studied the paleomagnetism of tertiary red beds and lava flows from intermontane basins in the tien shan region of kyrgyzstan we collected 532 cores and hand samples from 78 sites at 12 localities and progressively demagnetized them thermally or with alternating fields for most sites there are magnetic components with unblocking temperatures higher than 600 c we infer that the magnetic carriers are mainly hematite and magnetite for most localities the high temperature component appears to predate tectonic folding for all localities mean inclinations are shallower than expected from apparent polar wander paths inclination anomalies range from 16 5 for the issyk kul basin to 26 7 for the fergana basin if due to changes in latitude these anomalies imply at least 2000 km of northward displacement of the tien shan during the tertiary for which there is no tectonic evidence we thus consider that the paleomagnetic reference directions cannot be directly compared with our tertiary data we explored other possible reasons for this anomaly which has also been reported from other parts of the alpine belt but we could not find a satisfactory explanation absolute rotations cannot be accurately determined because of problems with the reference direction nevertheless the mean declination for the fergana basin lies counterclockwise by 20 11 from the mean declination of the issyk kul basin this result is consistent with the counterclockwise rotation inferred for the fergana basin from the pattern of cenozoic faults and folds it suggests a cenozoic right lateral displacement of 110 60 km on the talas fergana fault

new space geodetic constraints on the distribution of deformation in central asia abstract results from repeated measurements across a 90 sites global positioning system network in central asia provide direct evidence of current high rates of tectonic deformation far north of the india eurasia suture zone the data help to quantify both the partitioning of deformation within the seismically active tien shan and northern pamirs as well as the ongoing rotation of virtually undeformed blocks such as the tarim with respect to stable eurasia the nnw to sse shortening rates derived from the computations reach values of 23 3 mm yr 1 over the ranges studied suggesting that regional deformation rates have increased considerably within the study area since the onset of cenozoic shortening 20 25 myr ago

chemical evolution of intercumulus liquid as recorded in plagioclase overgrowth rims from the skaergaard intrusion the intercumulus liquid of a crystal mush fills pore spaces and typically solidifies to form overgrowths on cumulus grains and poikilitic post cumulus minerals if the liquid is immobile solidification produces zoned intercumulus minerals as a result of progressive fractionation of the residual liquid convection within the mush results in buffering of the liquid composition and thus limits mineral zonation for fully solidified cumulates fossil changes in liquid composition or porosity are difficult to identify however detailed study of immobile minor components of plagioclase overgrowth rims can provide information about the progressive solidification of intercumulus material ti contents of plagioclase overgrowths in samples from the lowermost parts of the skaergaard intrusion show strong variations with anorthite content with decreasing xan ti concentrations first rise and then fall consistent with changing tio2 contents of the intercumulus liquid during solidification tio2 in plagioclase decreases sharply at an55 reflecting local saturation of fe ti oxides ti in clinopyroxene oikocrysts also falls rimward but zoning in faster diffusing species fe mg is limited other than slight reverse zones that may occur on the plagioclase margins xan falls continuously during crystallization the reverse zoning is interpreted as the result of compaction driven dissolution and reprecipitation of plagioclase the continual decrease in xan is exploited together with back scattered electron images of the cumulates to produce calibrated images showing regions of progressive crystallization this allows the regions crystallizing at each stage of solidification to be visualized these images show that the final remnants of interstitial melt were present in triangular pockets and as thin grain boundary melt films this approach can provide information about the progressive reduction of porosity during cumulate solidification

silicate liquid immiscibility within the crystal mush late stage magmatic microstructures in the skaergaard intrusion east greenland late stage microstructures developed during the last stages of solidification of the skaergaard intrusion comprise a wide array of reactive and non reactive intergrowths reactive microstructures ascribed to open system behaviour include serrated grain boundaries between pyroxene and plagioclase primocrysts fish hook pyroxenes and mafic symplectites they form by the addition of fe and ca and removal of alkalis and silica other microstructures include those formed by internally generated redox reactions between olivine fe ti oxides and pyroxene non reactive microstructures include closely spatially associated granophyric intergrowths and previously undescribed ilmenite rich intergrowths that are interpreted as a consequence of crystallization of separated conjugate immiscible liquids the open system reactive microstructures occur predominantly in the cumulates on the chamber floor appearing in lzb and disappearing to be replaced by a granophyre ilmenite rich intergrowth association in the upper zone they are not common in the marginal border series their distribution mirrors that of efficient expulsion of trapped liquid and can be attributed to the gravitationally driven loss of a si rich immiscible component from the interstitial liquid the loss of the si rich component causes the remaining fe rich liquid to react with the primocrysts bulk rock major element data are consistent with little or no preferential loss of the si rich liquid from the mush but the resolution of the available data is not sufficient to assess the effect of this relative movement on the liquid line of descent of the bulk magma the first appearance of the paired conjugate non reactive intergrowths in mz marginal border series points to the early onset of immiscibility in the bulk liquid

microstructural evolution of silicate immiscible liquids in ferrobasalts an experimental study of the microstructural evolution of an immiscible basaltic emulsion shows that the fe rich liquid forms homogeneously nucleated droplets dispersed in a continuous si rich liquid together with droplets heterogeneously nucleated on plagioclase magnetite and pyroxene heterogeneous nucleation is likely promoted by localised compositional heterogeneities around growing crystals the wetting angle of fe rich droplets on both plagioclase and magnetite increases with decreasing temperature droplet coarsening occurs by a combination of diffusion controlled growth and ostwald ripening with an insignificant contribution from coalescence characteristic microstructures resulting from the interaction of immiscible fe rich liquid with crystal grains during crystal growth can potentially be used as an indicator of liquid unmixing in fully crystallised natural samples in magma bodies 10 m in size gravitationally driven segregation of immiscible fe rich droplets is unlikely to be significant

on the identification of textural equilibrium in rocks using dihedral angle measurements three dimensional numerical simulations have been constructed to investigate the distributions of apparent dihedral angles expected in unequilibrated rock textures for comparison with those expected in equilibrated rock textures for unequilibrated fully crystallized monomineralic rocks the simulation shows that the distribution of apparent dihedral angles is nonrandom and similar to that expected from an equilibrated rock and is largely independent of crystal shape and crystal nuclei distribution this new distribution together with the one previously derived for an equilibrated texture defines a pair of apparent dihedral angle curves which encompass curves corresponding to any degree of textural equilibrium within a monomineralic rock the location of a measured curve within this range allows a quantitative assessment of the degree of textural equilibrium within that rock the simulations also show that unequilibrated partially crystallized rocks generate apparent dihedral angle distributions for closed triangular pores that depending on crystal shape may mimic equilibrated dihedral angle distributions they also indicate that unless a rock is in complete equilibrium different dihedral angle distributions are produced by measuring the apparent dihedral angles of closed triangular or open nontriangular pores an important conclusion of this work is that apparent dihedral angle curves for unequilibrated textures are similar to those for equilibrated textures therefore care must be taken when interpreting textural equilibrium from apparent dihedral angle curves

the skaergaard trough layering sedimentation in a convecting magma chamber the upper parts of the floor cumulates of the skaergaard intrusion east greenland contain abundant features known as troughs the troughs are gently plunging synformal structures comprising stacks of crescentic modally graded layers with a sharply defined mafic base that grades upward into plagioclase rich material the origin of the troughs and layering is contentious attributed variously to deposition of mineral grains by magmatic currents descending from the nearby walls or to in situ development by localised recrystallisation during gravitationally driven compaction they are characterised by outcrop scale features such as mineral lineations parallel to the trough axis evidence of erosion and layer truncation associated with migration of the trough axis and disruption of layering by syn magmatic slumping a detailed microstructural study of the modal trough layers using electron backscatter diffraction together with geochemical mapping demonstrates that these rocks do not record evidence for deformation by either dislocation creep or dissolution reprecipitation instead the troughs are characterised by the alignment of euhedral plagioclase crystals with unmodified primary igneous compositional zoning we argue that the lineations and foliations are therefore a consequence of grain alignment during magmatic flow post accumulation amplification of the modal layering occurred as a result of differential migration of an unmixed immiscible interstitial liquid with upwards migration of the si rich conjugate into the plagioclase rich upper part of the layers whereas the fe rich immiscible conjugate remained in the mafic base both field and microstructure evidence support the origin of the troughs as the sites of repeated deposition from crystal rich currents descending from the nearby chamber walls

immiscible silicate liquid partition coefficients implications for crystal melt element partitioning and basalt petrogenesis this study investigates partitioning of ele ments between immiscible aluminosilicate and boro silicate liquids using three synthetic mixtures doped with 32 trace elements in order to get a good spatial separation of immiscible liquids we employed a high temperature centrifuge experiments were performed at 1 050 1 150 c 1 atm in sealed fe and pt containers quenched products were analysed by electron micro probe and la icp ms nernst partition coefficients d s between the fe rich and si rich aluminosilicate immiscible liquids are the highest for zn 3 3 and fe 2 6 and the lowest for rb and k 0 4 0 5 the plots of d values against ionic potential z r in all the composi tions show a convex upward trend which is typical also for element partitioning between immiscible silicate and salt melts the results bear upon the speciation and structural position of elements in multicomponent silicate liquids the ferrobasalt rhyolite liquid immis cibility is observed in evolved basaltic magmas and may play an important role in large gabbroic intrusions such as skaergaard and during the generation of unusual lavas such as ferropicrites

the effect of crystallization time on plagioclase grain shape in dolerites the average aspect ratio of plagioclase measured in thin section varies systematically through dolerite sills with a symmetrical m shaped profile observed in sills thinner than 200 m thicker sills show the same marginal reversal at the base but average aspect ratios appear to continue increasing towards the top creating an s shaped profile a distorted m shaped profile is visible in the stratigraphic variation of plagioclase average aspect ratios in the prehistoric makaopuhi lava lake with the centre of the m in the olivine enriched horizon close to the base of the lake higher overall values of average aspect ratio are observed in thin sills compared to thicker sills demonstrating that the plagioclase in more slowly crystallized bodies is more equant than that in more rapidly crystallized bodies there is a strong correlation between the plagioclase average aspect ratio in the central parts of the sills and the crystallization time calculated using a simple one dimensional thermal model assuming conductive cooling the cause of the marginal reversals in average aspect ratio is not well understood but may result from early grain impingement in the developing crystal mushy layer

on the pseudomorphing of melt filled pores during the crystallization of migmatites pseudomorphs of melt filled pores recognized by their generally cuspate shape are used as diagnostic for the former presence of partial melt they are commonly observed in migmatites from the mid to deep crust although they occur in the smaller pores in migmatites from shallower levels 1 2 kbar the pseudomorphing of meltfilled pores is controlled by the kinetics of nucleation and is a consequence of the greater supersaturation required for nucleation in a small pore compared with a larger one we examine three migmatites in detail a contact metamorphosed cherty band from an iron formation an archaean regional granulite from an accretionary prism and an amphibolite facies sample from the roots of an archaean mountain chain the greater undercooling required for nucleation in progressively smaller pores is recorded by the composition of plagioclase pseudomorphs a study of dihedral angles at the corners of pseudomorphed pores demonstrates that melt solid textural equilibrium was probably attained only in the contact aureole the regional granulite preserves an almost unmodified reaction controlled melt distribution with little evidence for either melt solid textural equilibration or solid solid re equilibration whereas the reactioncontrolled melt distribution in the regional amphibolite facies example has been modified by a partial approach to solid solid textural equilibrium it is not clear whether the differences in dihedral angle population are due to differences in uplift and exhumation rates or due to the presence of h2o on grain boundaries

monitoring the crystallization of water saturated granitic melts in real time using the hydrothermal diamond anvil cell critical aspects of the crystallization dynamics of h2o saturated melts of a typical granitic composition as well as granitic melts enriched in lithium 8800 ppm were investigated in real time experiments using the hydrothermal diamond anvil cell at 480 700 c and 220 960 mpa complete crystallization of the charges was achieved within 5 118 h with average crystal growth rates ranging from 3 to 41 cm year for quartz and from 18 to 58 cm year for alkali feldspars demonstrating that crystals formed from a silicate melt in the presence of a coexisting aqueous phase crystallize rapidly the combination of substantial nucleation delays low nucleation densities and rapid growth rates for quartz and alkali feldspars led to the formation of euhedral megacrysts of these minerals in the vicinity of clusters comprised of much smaller muscovite or spodumene crystals subsolidus replacement processes initiated during crystal fluid interactions after the silicate melt was consumed were directly observed in the experiments the experimental results underscore the important role of water as a medium for the transport of essential elements such as si al na and k from the silicate melt to the newly formed crystals and provide important insights into the crystallization of miarolitic pegmatites

a no lid zone in the central chang thang platform of tibet evidence from pure path phase velocity measurements of long period rayleigh waves pure path phase velocities of fundamental mode rayleigh waves in the period range 30 80s are obtained for different paths across the tibet plateau using a two event method with events located in tibet and aligned with world wide standard seismograph network stations at teleseismic distance evidence is found for significant lateral variations of structure of the crust and uppermost mantle in tibet in particular phase velocities obtained on paths contained in the chang thang platform are higher than average in tibet at periods shorter than 60s while remaining very low at longer periods when interpreting these phase velocities in terms of simple layered s velocity models it is found that contrary to other areas in tibet they are incompatible with the existence of a high velocity lid s velocities of 4 4 4 5 km s are found immediately beneath the moho and the crust is somewhat thinner 50 60 km than average in tibet this indicates a possible upwelling of hot asthenospheric material compatible with landsat evidence for subrecent basaltic volcanism and independent seismic evidence such as inefficient transmission of sn waves on the other hand a 70 km thick crust and very high upper mantle s velocities are found on the western border of tibet karakorum hindu kush compatible with previous finer scale results concerning the associated zone of intermediate depth seismicity

diffusion reaction in a thermal gradient implications for the genesis of anorthitic plagioclase high alumina basalt and igneous mineral layering abstract piston cylinder experiments investigating the interaction between basaltic andesite melt and partially molten gabbro in a thermal gradient provide insight into melt rock reaction processes occurring during magma differentiation in the crust in two experiments juxtaposing basaltic andesite and gabbro at 0 5 gpa pressure for durations of either 13 or 26 days diffusive chemical exchange between the two materials results in mineral layering and notable mineral compositions such as anorthitic plagioclase specifically the basaltic andesite gains al2o3 mgo and cao from the gabbro and loses na2o k2o sio2 and feo to it with a plagioclase rich layer developing at the interface between the two materials in a process termed diffusion reaction the percent crystallinity of the basaltic andesite increases during the process and the plagioclase crystals within the interface region develop anorthitic cores up to an90 that abruptly shift in composition to thin rims that are in na ca exchange equilibrium with the co existing melt both the mineralogical layering and bulk compositional change occurring at the interface are reproduced in model simulations of diffusion reaction isotopic tracers 45ca 6li 84sr and 136ba initially deposited at the basaltic andesite gabbro interface in the 13 day experiment were detected in the cores of the anorthitic plagioclase after the experiment demonstrating that the melt chemically communicates with the plagioclase cores over the duration of the diffusion reaction experiment the formation of anorthitic plagioclase during diffusion reaction may explain its widespread occurrence in terrestrial volcanic rocks without requiring the presence of ultra calcic melts textures and mineralogical changes in the gabbro indicate that chemical transport occurs throughout the experiments despite temperatures at the cold end of the experimental capsule approaching 500 c for instance apatite fenis olivine and almost pure albite occur at distinct specific horizons in the gabbro within the 26 day experiment because the bulk element profiles indicating chemical transport reflect analyses of almost completely solid gabbro equilibration between minerals and fluids melts must be rapid the overall effect of the diffusion reaction process is to make an ascending magma more primitive in composition and in this case produce anorthitic plagioclase while making surrounding crustal wall rocks more evolved several observations within igneous rocks support the occurrence of this process suggesting that the genesis of porphyritic high alumina basalt ubiquitously observed at convergent margins could reflect a diffusion reaction process in the crust

experiments on liquid immiscibility along tholeiitic liquid lines of descent crystallization experiments have been conducted on compositions along tholeiitic liquid lines of descent to define the compositional space for the development of silicate liquid immiscibility starting materials have 46 56 wt sio2 11 7 17 7 wt feotot and mg number between 0 29 and 0 36 these melts fall on the basaltic trends relevant for mull iceland snake river plain lavas and for the sept iles layered intrusion where large scale liquid immiscibility has been recognized at one atmosphere under anhydrous conditions immiscibility develops below 1 000 1 020 c in all of these compositionally diverse lavas extreme iron enrichment is not necessary immiscibility also develops during iron depletion and silica enrichment variations in melt composition control the development of silicate liquid immiscibility along the tholeiitic trend elevation of na2o k2o p2o5 tio2 promotes the development of two immiscible liquids increasing melt cao and al2o3 stabilizes a single liquid field new data and published phase equilibria show that anhydrous low pressure fractional crystallization is the most favorable condition for unmixing during differentiation pressure inhibits immiscibility because it expands the stability field of high ca clinopyroxene which reduces the proportion of plagioclase in the crystallizing assemblage thus enhancing early iron depletion magma mixing between primitive basalt and fe ti p rich ferrobasalts can serve to elevate phosphorous and alkali contents and thereby promote unmixing water might decrease the temperature and size of the two liquid field potentially shifting the binodal solvus below the liquidus leading the system to evolve as a single melt phase

earthquake focal mechanisms deformation state and seismotectonics of the pamir tien shan region central asia this paper presents a review of the seismotectonics of the pamir tien shan collision zone in the garm region tajikistan based on geological structure seismicity and focal mechanism solutions the region is dominated by horizontal compression manifested by imbricate low angle thrust faults that separate the upper crust into a series of tectonic sheets these thrust systems verge northward from the tajik depression toward the southern tien shan and southward toward the northern pamir the pattern of seismicity across the region suggests that similar low angle thrust faults exist within the crystalline basement as well in order to reconstruct the present day stress strain state of the region we used data from over 15 000 earthquake focal mechanism solutions for small earthquakes m 1 0 gathered over the 27 year period 1963 1989 the method of reconstruction of the seismotectonic deformation std field involves dividing the data set into small spatial windows and summing the individual focal mechanism solutions to form an average earthquake mechanism tensor for each cell the std state of that cell can then be represented by the orientation of the principal axes of compression and tension the intensity or relative uniformity of the average mechanism and the lode nadai coefficient which defines the relation between the magnitudes of the principal strain components we observe throughout the study area a general predominance of subhorizontal compression manifested in a mixture of thrust and strike slip deformation normal fault deformation is observed in only two small portions of the study area the orientation of principal compression varies from nearly n s in the northern pamir and the southern tien shan to nw se in the peter the first range in general the std field has a clearly developed mosaic structure defined by spatial groupings of nearly uniform orientations of compression and tension axes the boundaries of these groups cannot in most cases be directly correlated with known geological contacts the std structure also shows some variation with depth with more or less uniform std orientation in the upper crust 0 14 km depth changing significantly at greater depths examination of various subsets of the focal mechanism catalog shows the spatial structure of the std field to be largely stable with respect to both time and magnitude the reliability of the reconstructed std field was verified using three approaches 1 comparison of individual focal mechanism determinations for a large subset of the data using both soviet and u s algorithms 2 comparison of std reconstruction results using the two independent focal mechanism catalogs and 3 examination of the effect of spatial sampling on the results the dominance of generally north oriented subhorizontal compression is interpreted to be primarily the result of convergence between the pamir and tien shan ranges in turn caused by the ongoing collision of india and eurasia

active tectonics of the pamirs and karakorum the source parameters of moderate to large sized earthquakes and the spatial distribution of earthquake hypocenters are used to investigate the active tectonics and three dimensional configuration of the subducted lithosphere in the pamir karakorum intracontinental convergence zone the continental collision has resulted in intense deformation in the upper crust the deformation is distributed over a broad area and is absorbed by thrust and strike slip faulting along the northern margin of the pamirs where the deformation due to continental collision is most intense the northern pamir thrust and several major fault zones mark the present boundary most of the earthquakes that occurred in these fault zones are shallow crustal events with focal mechanism solutions correlated with the tectonic features of the region along the pamir front thrust faulting dominates while on the western and eastern edges of the pamirs the style of deformation is characterized by oblique thrusting with a component of strike slip motion thrust type events beneath the tadjik depression indicate that both the sedimentary rocks and the basement are involved in shortening right lateral strike slip motion is observed on the eastern edge of the pamirs where they border the tarim basin and in the talas fergana kun lun fault zone a southward dipping seismic zone beneath the pamirs and karakorum indicates that the asian lithosphere has been subducted along the pamir front to a depth of at least 200 km this interpretation is consistent with published geological and geophysical data focal mechanism solutions of some intermediate depth events beneath the pamirs show strike slip faulting with approximately n s horizontally oriented p axes indicating that nearly horizontal compression at the intermediate depth is the predominant mode of deformation a 90 km deep event beneath the karakorum is interpreted as occurring in the subducted asian lithosphere the fact that the p axis of this event is oriented parallel to the descending plate suggests downdip compression within the asian lithosphere the overall tectonics of the region is interpreted as a consequence of the underthrusting asian lithosphere being impinged upon by the shallow northward underthrusting indian lithosphere

immiscible iron and silica rich liquids in the upper zone of the bushveld complex the bushveld complex south africa is the largest layered intrusion on earth and plays a considerable role in our understanding of magmatic differentiation and ore forming processes in this study we present new geochemical data for apatite hosted multiphase inclusions in gabbroic cumulates from the bushveld upper zone inclusions re homogenized at high temperature 1060 1100 c display a range of compositions in each rock sample from iron rich 35 wt feotot 28 wt sio2 to silica rich 5 wt feotot 65 wt sio2 this trend is best explained by an immiscible process and trapping of contrasted melts in apatite crystals during progressive cooling along the binodal of a two liquid field the coexistence of both si rich and fe rich immiscible melts in single apatite grains is used to discuss the ability of immiscible melts to segregate from each other and the implications for mineral and bulk cumulate compositions we argue that complete separation of immiscible liquids did not occur resulting in crystallization of similar phases from both melts but in different proportions however partial segregation in a crystal mush and the production of contrasting phase proportions from the fe rich melt and the si rich melt can be responsible for the cyclic evolution from melanocratic fe ti p rich to leucocratic plagioclase rich gabbros which is commonly observed in the upper zone of the bushveld complex where it occurs at a vertical scale of 50 to 200 m

large scale silicate liquid immiscibility during differentiation of tholeiitic basalt to granite and the origin of the daly gap the dearth of intermediate magmatic compositions at the earth s surface referred to as the daly gap remains a major issue in igneous petrology the initially favored explanation invoking silicate liquid immiscibility during evolution of basalt to rhyolite has lost support because of the absence of any firm geological evidence for separation of fe and si rich liquids in igneous rocks this work presents a record of large scale magmatic differentiation due to immiscibility in the tholeiitic sept iles intrusion canada one of the largest layered plutonic bodies on earth gabbroic cumulate rocks from the critical zone of this intrusion show a bimodal distribution in density and p2o5 content despite identical major element chemistry of the principal magmatic phases immiscibility is supported by the presence of contrasting fe rich and si rich melt inclusions trapped in cumulus apatite phase diagrams and well documented occurrences of small scale immiscibility confirm that liquid liquid unmixing and the separation of fe rich and si rich liquids may contribute significantly to the daly gap along the tholeiitic liquid line of descent

silicate liquid immiscibility within the crystal mush evidence from ti in plagioclase from the skaergaard intrusion a key target in the study of layered intrusions is to constrain the liquid line of descent of the magma however the evolution of the interstitial liquid is rarely considered and its liquid line of descent is often assumed to be equivalent to that of the bulk magma because of extensive sub solidus and diffusional changes that occur in slowly cooled rocks clues to the composition of the interstitial liquid can only be obtained using very slowly diffusing trace elements and components this study uses the ti concentrations and anorthite contents of interstitial plagioclase to consider the compositional evolution of the interstitial liquid in the skaergaard intrusion ti xan zoning of interstitial plagioclase does not follow the same cryptic variations that develop in plagioclase primocrysts as a function of stratigraphic height demonstrating that the bulk and interstitial liquid lines of descent are not equivalent after fe ti oxides start to crystallize ti concentrations decrease in both primocryst and interstitial plagioclase as a result of decreasing melt ti however in the interstitial plagioclase within a single thin section divergent trends develop adjacent to fine grained interstitial pockets containing diverse mineral assemblages which are interpreted to represent the crystallized products of late stage immiscible liquids these trends vary systematically as a function of stratigraphic height and spatial location within the intrusion the distribution and compositions of these plagioclase zoning trends are used to comment on the spatial distribution and differential movement of interstitial immiscible liquids within the intrusion the author 2010 published by oxford university press all rights reserved

late cenozoic tectonics of the kepingtage thrust zone interactions of the tien shan and tarim basin northwest china the kepingtage kalpin thrust zone northwest china is an actively deforming part of the india asia collision system it lies south of the tien shan has an area of 16 000 km2 and consists of arcuate emergent imbricates overall vergence is toward the interior of the tarim basin to the south thrust sheets typically expose upper cambrian to permian platformal strata thrusting is largely thin skinned thin upper cambrian evaporites are likely to be the main decollement horizon the kepingtage thrust zone is the only margin of the tarim basin to deform in this style in the cenozoic it is replaced to the east and west by thrust zones which have propagated shorter distances into the interior of the tarim basin thick up to 10 km mesozoic and cenozoic clastic successions are present and deformed in these regions such successions are not present in the kepingtage thrust zone or the adjacent bachu uplift within the tarim basin because of episodic activity on steep northwest southeast trending thrusts which define the margins of the bachu uplift these mesozoic cenozoic strata may have suppressed the ability of regions along strike from the kepingtage thrust zone to deform by thin skinned thrusting utilizing the upper cambrian decollement the along strike variation in active thrusting at the southern margin of the tien shan is an example of syntectonic sedimentation controlling thrust belt deformation style a balanced section across the thrust zone indicates 28 shortening equivalent to 35 km this is equivalent to an average slip rate of 1 8 mm yr 1 and a strain rate of 4 4 10 16 s 1 assuming deformation began at circa 20 ma active deformation is focused along the frontal thrust the kepingtage fault the northern boundary of the kepingtage thrust zone is formed by the south tien shan fault this major north dipping thrust is seismically active with published earthquake focal depths at midcrustal levels 14 and 18 km it juxtaposes upper carboniferous sedimentary rocks of different facies and different paleozoic deformation histories speculatively it represents a reactivation of a late paleozoic thrust which originated at the boundary between the platformal interior of the tarim block and a deeper water foreland basin to its north

least squares restoration of tertiary thrust sheets in map view tajik depression central asia the tajik depression located west of the pamirs and south of the tien shan is a compressional intermontane basin bounded by basement overthrusts and filled with mesozoic and cenozoic sediments the internal structure is typical of a thin skinned tbld and thrust belt kinematic data available in the literature suggest that indentation of the pamirs into asia durino the cenozoic collision of india and asia has been accommodated in various ways within the depression including westward extrusion thickening combined with wrenching along n s tblds and thrusts and counterclockwise rotations these various detbrmation processes can be analyzed and quantified by reconstruction of the predeformed state of the depression however the combina tion of thrusting wrenching and block rotations implies a nonplane deformation which cannot be restored properly using balanced cross sections alone we have therefore developed a numerical method for restoration of stratigraphic surfaces designed for regions of nonplane compressional tectonics the deformed region is represented in map view as a mosaic of fault bounded blocks overlapping each other along the faults blocks are separately unfolded and then numerically packed together by least squares minimization of overlaps yielding fields of finite horizontal translations and rotations about vertical axes to analyze the deformation postdating the collision of india and asia we have restored a stratigraphic surface at the base of the cenozoic first in order to test the numerical method we restored a map that had previously been restored by a purely manual method restoration of a second map drawn from newly available subsurface data leads to geometrical inconsistencies overlaps and gaps which cannot be reduced remain in the restored state after correction of these inconsistencies restoration yields a complex mode of deformation for the depression individual thrust slices have undergone counterclockwise rotations about verti cal axes the magnitudes increasing from west to east up to a maximum of 40 o near the pamirs horizontal shortening is 150 km 35 in the center of the depression and 240 km 85 in the northeastern part between the pamirs and the tien shan strike slip motions commonly not revealed by balancing cross sections are associated with thrusting on faults striking n s inde pendent paleomagnetic data and slip directions on small scale faults provide positive checks on the results of our restoration

crystal size distributions and scaling laws in the quantification of igneous textures this study demonstrates the value of the combined use of scaling of the chemical evolution of cooling magmas have investigated the detailed relationships between temanalysis and crystal size distribution csd measurements in the interpretation of igneous textures and outlines applications of these perature phase abundance and phase composition the reaction series bowen 1922 and the cipw norm cross methods to other fundamentally important problems theoretical calculations and measurements of natural samples are used to et al 1903 have been supplemented by modern thermodynamic models that predict the compositions and abundcharacterize the relationship between igneous texture and cooling history the total number nt and mean length l of crystals ances of minerals in equilibrium with melt of a given composition and temperature e g ariskin et al 1993 in a sample are correlated through a scaling relationship of the form n t l this proportionality depends on the mineralogy ghiorso sack 1995 although there are a few areas of uncertainty the chemistry of igneous petrogenesis is of the rock and so a modal normalization factor c is introduced the csd slope s and intercept ln n are uniquely related to fairly well understood but chemical models cannot address the central probeach other through the equation ln n 4ln s ln c where s l this overall relationship allows the texture of a rock lem of igneous crystallization which is the physical process of generating a set of crystals from magma two to be related to the local duration of cooling through an arbitrary crystal growth model quantification of the link between the texture magmas with the same composition will normally produce the same minerals in the same proportions however of a rock and its cooling history makes it possible to predict spatial variations in texture we show an example of this method using depending on their thermal histories these two magmas can solidify into rocks with drastically different physical the sudbury igneous complex canada additional applications include relating the textures of volcanic rocks to spatial and temporal appearances e g basalt and gabbro the set of minerals is the same but what distinguishes them is the abundance variations in the magma chamber and extracting kinetic parameters from suites of comagmatic rocks and size of the constituent crystals the fine grained rocks in the chilled margins of most basic intrusions cooled rapidly and the coarse grained rocks in the interiors of plutons cooled slowly this has

the pamir hindu kush seismic zone as a strain marker for flow in the upper mantle we introduce a new hypothesis to explain the unusual pattern of intermediate depth earthquakes in the pamir hindu kush region of central asia in our model the seismicity of the zone is defined by a remnant piece of oceanic crust that became neutrally buoyant and now hangs in the mantle underneath the center of the active mountain belt we interpret an abrupt cutoff in seismicity shallower than 70 km under the pamir as evidence that the slab has become decoupled from the surface deformation and we suggest that the remnant slab has acted as a strain marker for flow in the upper mantle the contortion of the slab can be explained by superimposed vertical and lateral simple shear related to the western edge of the indian plate this model requires relatively low strain rates of the order of 10 15 s 1 consistent with strain rates estimated from earthquake moment release in benioff zones in contrast the double facing subduction zone model applied by a number of previous authors requires strain rates of the order of 10 13 s 1 which are much higher than inferred strain rates in subducting slabs the results are important for understanding mantle dynamics providing a unique way to measure strain in the upper mantle the implied flow field also has significant implications for the kinematics of the entire collisional plate boundary in tibet and the tien shan

experiments on silicate melt immiscibility in the system fe2sio4 kalsi3o8 sio2 cao mgo tio2 p2o5 and implications for natural magmas the effect of cao and mgo with or without tio2 and p2o5 on the two melt field in the simplified system fe2sio4 kalsi3o8 sio2 has been experimentally determined at 1 050 1 240 c 400 mpa despite the suppressing effect of mgo cao and pressure on silicate melt immiscibility our experiments show that this process is still viable at mid crustal pressures when small amounts 0 6 2 0 wt of p2o5 and tio2 are present our data stress that the major element partition coefficients between the two melts are highly correlated with the degree of polymerisation nbo t of the sio2 rich melt whatever temperature pressure or exact composition experimental immiscible melt compositions in natural systems at 0 1 mpa from the literature lunar and tholeiitic basalts plot on similar but distinct curves compared to the simplified system these relations between melt polymerisation and partition coefficients which hold for a large range of compositions and fo2 are extended to various volcanic and plutonic rocks this analysis strengthens the proposal that silicate melt immiscibility can be important in volcanic rocks of various compositions from tholeiitic basalts to lamprophyres however the majority of proposed immiscible compositions in plutonic rocks are at least not coexisting melts but may have suffered accumulation of early crystallized minerals

crystallization processes in an artificial magma variations in crystal shape growth rate and composition with melt cooling history a large 4 8 m3 1 3x107 g artificial mafic melt with a bulk composition similar to that of a basalt but with a high cao content of 17 wt was generated during a demonstration of in situ vitrification and was allowed to cool naturally during the melting process convection was vigorous resulting in a chemically and thermally homogeneous melt body once heating was complete the cooling rate was rapid with the temperature dropping from 1500 c to 500 c in 6 days within the interior of the 3 m diameter 1 5 m thick body a 20 h period of constant temperature 1140 c observed during colling was the result of latent heat released by widespread crystallization the final crystalline assemblage consists of diopsidic to hedenbergitic pyroxene and anorthitic feldspar with a subordinate amount of potassic feldspar plus a small amount of evolved glass the compositions and proportions of phases agree well with those predicted by the melts thermodynamic model thermal and textural evidence suggest that convection within the melt ceased coincident with formation of the first crystals textural investigation of core samples reveals large up to 1 cm in length acicular diopsidic pyroxenes in a matrix of smaller feldspar and zoned pyroxene crystals 500 m in length crystal shape and pyroxene composition vary as a function of position within the solidified body as a function of cooling rate both crystal size and degree of crystallinity are highest in the central most slowly cooled parts of the rock crystal shape is characterized by tabular equilibrium growth forms in the slowly cooled areas grading to highly skeletal dendritic forms at the rapidly cooled edges of the body the pyroxene crystals are dominantly homogeneous diopside but crystals are characterized by thin fe rich hedenbergitic rims these rims were deposited when mg in the melt was depleted by diopside growth and melt temperature had cooled sufficiently to allow fe rich pyroxene growth crystal growth rates can be calculated based on thermal behavior of the melt reinforced by thermodynamic modelling and are determined to be between 10 7 and 10 8 cm s in the central part of the melt these estimates agree well with growth rates in natural systems with similar cooling rates pyroxene crystals that formed at a higher cooling rates are characterized by higher al and lower mg contents relative to tabular equilibrium crystalline forms presumably as a result of disequilibrium melt compositions at the crystal melt interface

dual origin of fe ti p gabbros by immiscibility and fractional crystallization of evolved tholeiitic basalts in the sept iles layered intrusion we present a detailed study of two ca 200 m thick apatite bearing ferrogabbro horizons of the sept iles layered intrusion canada these rocks are the most evolved cumulates of the megacyclic units mcu i and ii and mark the transition between basaltic and silicic magmatism they are made up of plagioclase an55 34 olivine fo66 21 clinopyroxene mg 75 55 ilmenite magnetite apatite pigeonite and are a significant source of fe ti p ore ferrogabbros have relatively uniform bulk rock compositions in mcu i but are bimodal in mcu ii the liquid lines of descent for major elements in equilibrium with cumulates of mcu i and ii have been calculated using a forward model formalism both trends evolve towards sio2 enrichment and feot depletion after saturation in fe ti oxides however because of magma mixing in mcu ii they do not follow the same path evolved liquids from mcu ii are shown to enter the experimentally determined two liquid stability field while mcu i liquids do not immiscibility in mcu ii and its absence in mcu i are supported by the presence of contrasted reactive symplectites in cumulate rocks apatite bearing ferrogabbros in mcu ii have crystallized from distinct immiscible fe rich and si rich silicate melts which have physically segregated in the slow cooling magma chamber two different types of cumulate rocks are thus produced leucocratic and melanocratic gabbros this is consistent with the presence of si rich and fe rich melt inclusions in apatite in contrast homogeneous ferrogabbros from mcu i were produced by simple fractional crystallization of a homogeneous liquid our data suggest that immiscibility could also explain the large geochemical variability of ferrogabbros in the upper zone of the bushveld complex south africa

physical constraints on the migration of immiscible fluids through partially molten silicates with special reference to magmatic sulfide ores the migration of dense fe rich immiscible liquids through partially molten silicates is of critical importance to several issues including the formation of magmatic sulfide ore deposits and the formation of metallic cores in terrestrial planets the migration of immiscible dense liquids can be considered to occur in three distinct regimes dictated by the relative sizes of the dense liquid droplets and the crystals making up the silicate crystal mush or partially melted silicate rock if dense microdroplets are smaller than the diameter of the narrowest portions of pore throats between silicate crystals then downward motion is possible the maximum rate of downward settling in loosely packed crystal mushes occurs when the droplet size is approximately one tenth the crystal size as long as the silicate melt between crystals remains microdroplets are capable of migrating distances of hundreds to thousands of meters vertically through crystal mushes contained in cooling dikes with typical grain sizes stranding of larger droplets similar in size to the crystals results from capillary forces preventing droplet deformation as they attempt to pass into pore throats narrower than themselves only in very coarse grained mushes with grain sizes greater than about 2 cm can droplets the size of intergranular pores migrate downwards on the other hand coalescence of many microdroplets into pre existing stranded droplets may generate connected net textured domains of the dense liquid that are many times larger than the grain size of the mush when the vertical height of the connected domain is great enough the pressure gradient inside the dense phase exceeds the capillary force impeding downward motion through narrow pore throats and the immiscible phase is able to move down along vertically oriented networks these networks should not suffer stranding until they reach a solid layer or the base of the intrusion we suggest that in most cases sulfide droplets formed by fractional segregation in large magma bodies should come to rest at the top of the crystal pile whereas sulfides introduced to magmas by assimilation of fine grained sulfidic sediments should be capable of passing downward through crystal mushes to form massive or net textured accumulations at the bottom of the partially molten zone

heterogeneous nucleation and epitaxial crystal growth of magmatic minerals electron backscatter diffraction analysis of dendritic clinopyroxene cpx forming in rapidly cooled basalt reveals two features that are unexpected for phases growing from a liquid 1 helical growth about 010 cpx the crystallographic b axis with incremental rotation up to 0 4 m 1 within branch segments and large rotational jumps 10 46 between closely spaced branch segments and 2 strong crystallographic preferred orientation cpo between cpx and titanomagnetite timt decorating branch tips such that 010 cpx aligns with one of the six symmetrically equivalent 110 timt face poles more than 80 of timt crystals occur within 5 of the cpo with cpx substrate as do 22 of timt crystals in contact with euhedral cpx in natural etna basalt the probability distribution of distances between an arbitrary unit vector and the nearest 110 timt normal unit vector was found by numerical simulation and indicates that the observed alignment to within 5 occurs with frequency 0 024 thus the cpo occurs 9 33 times more commonly than expected from randomly distributed crystals the cpo matches previous observations of epitaxial relationship between spinel and host pyroxene during subsolidus exsolution but has not previously been reported among magmatic cpx and timt conspicuous contiguity among phenocryst phases is inferred to result from heterogeneous nucleation facilitated by epitaxy associated with cpo an inference supported by high resolution electron microscopy observations of 150 nm timt crystals adhering preferentially to silicate substrates epitaxial relationships among phases nucleating from a melt may contribute to fabrics in magnetic properties dictate the textures of igneous rocks by promoting development of crystal clusters and even influence magma transport and eruption styles through a control over magma rheology

gravity anomalies over the ferghana valley central asia and intracontinental deformation gravity anomalies over the ferghana valley a deep sedimentary basin in the western tien shan differ from those expected for local airy isostatic equilibrium by 120 mgal and imply a deficit of mass beneath the basin equivalent to a moho deeper by 10 km than prevails where isostasy holds although the ferghana valley is underlain by thick mesozoic and cenozoic sediment measurements of density show that this material cannot account for the deficit of mass gravity anomalies over both the surrounding mountain belts and the kazakh platform farther north imply that local airy isostatic equilibrium is approached flexure of a relatively thin effectively elastic plate thickness 10 15 km loaded by the present topography can account for the range of bouguer anomalies of 50 to 450 mgal over these regions we infer that approximately north south shortening of the relatively thin lithosphere has created mountains north and south of the basin has warped the basement of the immediate surroundings of the basin up by folding the mantle lithosphere and has forced the basin floor down beneath the ferghana valley it appears that the preexisting thermal structure and variations in crustal thickness have dictated the styles and distribution of deformation in this region

differentiation and magma mixing on kilauea s east rift zone the lavas of the 1955 east rift eruption of kilauea volcano have been the object of considerable petrologic interest for two reasons first the early 1955 lavas are among the most differentiated ever erupted at kilauea and second the petrographic character and chemical composition of the lava being erupted changed significantly during the eruption this shift from more differentiated mgo 5 0 5 7 to more magnesian mgo 6 2 6 8 lava has been variously interpreted as either due to systematic excavation of a zoned differentiated magma body or to invasion of the differentiated magma by more primitive magma followed by rapid mixing and eruption of the resulting hybrid magmas petrologic examination of several nearvent spatter samples of the late 1955 lavas shows abundant evidence for magma mixing including resorbed and or reversely zoned crystals of olivine augite and plagioclase in addition the compositional ranges of olivine plagioclase and groundmass sulfide are very large implying that the assemblages are hybrid core compositions of olivine phenocrysts range from fo85 to fo77 the most magnesian olivines in these samples must have originally crystallized from a melt containing 8 0 8 5 mgo which is distinctly more magnesian than the bulk composition of the late 1955 lavas the majorelement and trace element data are either permissive or supportive of a hybrid origin for the late 1955 lavas in particular the compositional trends of the 1955 lavas on plots of cao vs mgo and the virtual invariance of al2o3 and sr in these plagioclase phyric lavas are more easily explained by magma mixing than by fractionation the pattern of internal disequilibrium re equilibration in the late 1955 spatter samples is consistent with reintrusion and mixing having occurred at least twice during the latter part of the 1955 eruption plagioclase zonation preserves possible evidence for additional earlier reintrusion events least squares modelling the mixing of early 1955 bulk compositions with various summit lavas olivine pick the 1952 summit lava as most like the primitive component the results also indicate the primitive component had mgo 7 5 8 0 corresponding to liquidus temperatures of 1165 1175 c the absence of fe ti oxide phenocrysts in the late 1955 lavas implies that the cooler component of the hybrid had t 1110 c thus the thermal contrast between the two components may have been as much as 55 65 c sufficient to produce the conspicuous disequilibrium effects visible in the spatter samples

late cenozoic tectonic development of the intramontane alai valley pamir tien shan region central asia an example of intracontinental deformation due to the indo eurasia collision 1 the pamir indentor of the northwestern himalayan syntaxis is a first order feature demonstrating partly the northward extent of deformation due to the cenozoic indo eurasia collision the alai valley of kyrgyzstan at the northern end of the indentor is a strategically positioned e w trending intramontane basin that constrains the timing and extent of crustal deformation in this area of the collision zone to quantify the convergence accommodated across the alai valley during the late cenozoic we collected structural and stratigraphic field data reviewed existing soviet literature and analyzed migrated seismic reflection profiles to construct and restore two regional sections crosscutting the basin our study suggests that the development progressive closure and conversion of this formerly marine basin into a terrestrial intramontane basin result from two main deformation events 1 distributed north south contraction took place during the late oligocene early miocene accommodated one third to half of the total shortening and was followed by the formation of a regional erosion surface and 2 n s shortening resumed in the mid miocene and continues today during this second episode the thrust front migrated southward localized along the trans alai ranges and failed to reactivate earlier neogene structures horizontal shortening of about 35 across the alai valley implies relatively low strain rates and displacement rates of about 4 18 4 69 10 16 s 1 and 0 66 0 78 mm yr 1 respectively for the last 25 myr our study confirms other regional observations indicating that contractional deformation occurred far in the interior of the asian continent as early as the late oligocene

paleozoic accretion and cenozoic redeformation of the chinese tien shan range central asia the tien shan range in central asia contains two late paleozoic sutures the older southern suture marks the collision of a passive margin at the north of the tarim block and an active continental margin subduction under the latter was to the north the younger northern suture separates a northern carboniferous island arc from an active continental margin developed over a south dipping subduction zone the subduction direction under the island arc is unknown mesozoic elastics were deposited over the doubly sutured orogen rate and energy of sedimentation waned until deposition of oligocene conglomerates above a regional unconformity interpreted as marking the onset of deformation induced by the india asia collision molasse deposition accelerated in pliocene and quaternary time and deposition continues today as active thrusts generate relief paleozoic structures control the gross divergence of cenozoic thrusts across the orogen

late paleozoic tectonic amalgamation of northwestern china sedimentary record of the northern tarim northwestern turpan and southern junggar basins sedimentary rocks contained in basins adjacent to the tian shan provide a long and complex record of the late paleozoic continental amalgamation of northwestern china complementing that provided by rocks preserved within the range this record which comprises dramatic changes in sedimentary facies sediment dispersal patterns sandstone provenance and basin subsidence rates broadly supports previous interpretations of a two part evolution of the tian shan late devonian to early carboniferous collision of the tarim continental block with the central tian shan followed by collision of this combined block with island arcs in the north tian shan and bogda shan in late carboniferous early permian times the first collision resulted in widespread angular unconformities within the tarim basin continued convergence following the collision created a long lived flexural foredeep along the northern margin of the tarim block which received at least 2000 m of lower carboniferous through lower permian fluvial and marine sediment derived from the interior of tarim subsequent early permian continental extension of the northern tarim basin resulted in the deposition of interbedded nonmarine siliciclastic sedimentary rocks and mafic to felsicvolcanic rocks sandstone within this interval was derived from the paleo tian shan and is composed predominantly of lithic volcanic grains similar to the rhyolite in contrast to the tarim basin calc alkaline volcanic rocks and volcanogenic sedimentary rocks dominated carboniferous and permian sedimentation in the northern turpan and northwestern junggar basins volcanic arcs remained active in the north tian shan and bogda shan through the early late carboniferous depositing a kilometers thick interval of deep marine sediment gravity flows in the northwestern junggar basin major arc magmatism ceased in the late carboniferous in response to closure of the oceanic basin between the combined tarim central tian shan block and the north tian shan bogda shan arcs upper carboniferous through lower permian rocks in the northwestern junggar basin compose the sedimentary fill of a bathymetric basin of oceanic depth on the northern side of the volcanic arcs culminating in a 1000 m thick marine regressive sequence middle to upper permian sandstones were derived from the uplifted paleo tian shan and bear the distinctive provenance imprint of granitic rocks presently exposed within the range late permian subsidence of the junggar basin accommodated 5 km of nonmarine sediments however the cause of this subsidence and its relationship to regional tectonic events remain controversial

interfacial tension between magmatic sulfide and silicate liquids constraints on kinetics of sulfide liquation and sulfide migration through silicate rocks we have measured the interfacial tension sf between sulfide and silicate melts over the temperature range from 1250 to 1325 c by in situ measurement of the shapes of sessile drops of sulfide immersed in silicate melt sitting upon level alumina substrates the silicate melt was an alkali free synthetic basaltic liquid the sulfide melts were synthetic mixtures of fe cu ni s or fe s o the interfacial tension ranges from about 500 mj m 2 for sulfide of fes composition to approximately 600 mj m 2 for sulfide containing 16 wt cu or 6 5 wt ni and up to 650 mj m 2 for fe s o liquid containing 4 wt o contact angles vary from 150 to 180 showing that sulfide liquid does not wet oxide minerals in silicate magmas the density of sulfide melt does not depend strongly on cu or ni content remaining between 4 0 and 4 5 g cm 3 over the range of cu and ni contents expected in sulfide magmas but increases substantially to about 5 5 g cm 3 in fe s o melts containing 4 wt o our results can be combined with conventional concepts of nucleation theory to indicate that when silicate magmas exist at small degrees of supersaturation sulfide drops may nucleate rarely and at widely spaced intervals leading to kinetic control of the compositions of the resulting droplets in light of our results we use simple scaling arguments to argue that sulfide magmas are not capable of migrating through partially molten silicate rocks by capillary forces alone but may be forced through narrow grain boundaries or grain edges only by the flow of the enclosing silicate melt during melt extraction by compaction of partially molten mantle peridotite it is highly unlikely that silicate flow rates could be sufficiently rapid to permit the entrainment of droplets of sulfide liquid at temperatures below the solidus of enclosing silicate rocks sulfide melt will be free to travel along fractures or grain boundaries the mobility of sulfide melts through completely solidified silicate rocks may account for the widespread observation that late stage highly fractionated sulfide liquids have escaped from cooling bodies of massive sulfide to form veins and disseminations of cu rich sulfide minerals in their silicate host rocks