the petrology of a large intra oceanic silicic eruption the sandy bay tephra kermadec arc southwest pacific abstract the pumiceous pyroclastic deposits known as the sandy bay tuff on macauley island in the kermadec group represent a medium scale silicic eruption in an oceanic subduction setting the sandy bay eruption occurred about 6310 yr bp forming a large submarine caldera in the summit of the mainly submarine and essentially basaltic macauley volcano the sandy bay magma contained

the influence of post perovskite strength on the earth s mantle thermal and chemical evolution abstract we have investigated the influence of post perovskite ppv viscosity on mantle convective dynamics and stirring efficiency using numerical modeling and simple analytical theory our results show that the strength of ppv has a dramatic influence on convective dynamics the presence of a weak ppv enhances heat transfer across the bottom thermal boundary layer resulting in higher temperatures lower mantle viscosities and considerably larger convective velocities this leads to a significant increase in stirring efficiencies with decreasing the ppv strength by at least one order of magnitude in addition using a simple parameterized convection evolution that includes the influence of ppv coupled to a mixing model we show that during the long term history of the earth s mantle the presence of ppv yields systematically hotter thermal evolution and more efficient convective stirring such a strong effect of ppv strength on mantle stirring efficiency suggests that the influence of ppv phase must be considered when interpreting both geochemical and geophysical observations

relative roles of source composition fractional crystallization and crustal contamination in the petrogenesis of andean volcanic rocks there are well established differences in the chemical and isotopic characteristics of the calc alkaline basalt andesite dacite rhyolite association of the northern n v z central c v z and southern volcanic zones s v z of the south american andes volcanic rocks of the alkaline basalt trachyte association occur within and to the east of these active volcanic zones the chemical and isotopic characteristics of the n v z basaltic andesites and andesites and the s v z basalts basaltic andesites and andesites are consistent with derivation by fractional crystallization of basaltic parent magmas formed by partial melting of the asthenospheric mantle wedge containing components from subducted oceanic lithosphere conversely the alkaline lavas are derived from basaltic parent magmas formed from mantle of within plate character recent basaltic andesites from the cerro galan volcanic centre to the se of the c v z are derived from mantle containing both subduction zone and within plate components and have experienced assimilation and fractional crystallization a f c during uprise through the continental crust the c v z basaltic andesites are derived from mantle containing subduction zone components probably accompanied by a f c within the continental crust some c v z lavas and pyroclastic rocks show petrological and geochemical evidence for magma mixing the petrogenesis of the c v z lavas is therefore a complex process in which magmas derived from heterogeneous mantle experience assimilation fractional crystallization and magma mixing during uprise through the continental crust

synthesis and equation of state of post perovskites in the mg fe 3al2si3o12 system abstract the formation and properties of the post perovskite cairo 3 type phase were studied in fe rich compositions along the pyrope almandine mg fe 3 al 2 si 3 o 12 join natural and synthetic garnet starting materials with almandine fractions from 38 to 90 mol were studied using synchrotron x ray diffraction in the laser heated diamond anvil cell single phase post perovskite could be successfully synthesized from garnet compositions at pressures above 148 gpa and temperatures higher than 1600 k in some cases evidence for a minor amount of al 2 o 3 post perovskite was observed for alm38 and alm54 compositions in the perovskite post perovskite two phase region pressure volume data for the post perovskite phases collected during decompression show that incorporation of fe leads to a systematic increase of unit cell volume broadly similar to the variation observed in the mg fe sio 3 system the presence of al 2 o 3 increases the stability of perovskite relative to post perovskite requiring higher pressures 148 gpa for synthesis of pure post perovskites our data together with those of tateno et al 2005 also suggest that in the al rich system the presence of fe has no strong effect on the pressure required to synthesize the pure post perovskite phase but the two phase perovskite and post perovskite region may be broad and its width dependent on fe content our results suggest that any regions highly enriched in al 2 o 3 may consist of either the perovskite phase or a mixture of perovskite and post perovskite phases throughout the entire thickness of the d region the observed synthesis pressures 148 gpa for a pure post perovskite phase are beyond that at the earth s core mantle boundary 135 gpa

experimental development of patch perthite from synthetic cryptoperthite microstructural evolution and chemical re equilibration in this study deuteric coarsening of a lamellar cryptoperthite to a patch perthite has been experimentally induced for the first time an homogeneous alkali feldspar bulk composition ab60or40 was produced by molten salt ion exchange with a gem quality orthoclase from madagascar this was then annealed isothermally for 32 days at 550 c this resulted in a coherent lamellar cryptoperthite periodicity 28 nm probably by spinodal decomposition and subsequent coarsening the cryptoperthite was then reacted with 20 wt h2o or 0 1 m hcl at 400 and 500 c at 200 400 and 1000 mpa for 96 h and for 192 h at 1500 mpa at 1000 and 1500 mpa and 500 c the cryptoperthitic crystal fragments were partially replaced by a mosaic of albite and sanidine subgrains in the form of a patch perthite other than the gibb s free energy of reaction as derived from the chemical potentials of the reactant and product phases the second principle mechanism responsible for driving this reaction is interface coupled dissolution reprecipitation driven by a reduction in the coherency strain energy stored within the initial cryptoperthite the main indicator of this is a change in the microtexture due to the destruction of the coherent lamellar cryptoperthite and its replacement by an assemblage of incoherent subgrains detailed mass balancing based on xrd and epma data indicate that conversion of cryptoperthite to patch perthite is not isochemical experimental formation of patch perthite appears to take place in two steps between the original cryptoperthite and the patch perthite tem investigation identified a transition zone characterized by a significant coarsening of the preexisting lamellar microstructure without any sign of a loss in lattice coherency this zone is probably the result of hydrous species diffusing into the cryptoperthite causing the h2o enhanced diffusion of na and k and hence lamellar coarsening in the second step this coarsened microstructure is replaced by the patch perthite the polycrystalline patch perthite is characterized by a very complex distribution of albite and sanidine sub grains even the larger albite or sanidine patches are polycrystalline this inevitably results in a three dimensional network of grain boundaries characterized by high defect densities resulting from the structural misfit between the albite and sanidine sub grains an increase in average grain size with distance from the interface indicates that secondary coarsening of the initially very fine grained intergrowth occurred this was driven by the reduction of interface and surface energy similar to the coarsening observed in multiphase ceramics in addition the newly formed patch perthite is highly porous despite the fact that the results obtained in this study show that the replacement of cryptoperthite by patch perthite is nearly isovolumetric v 0 6 the observed reaction induced porosity results from differences in relative solubilities between the cryptoperthite and the patch perthite porosity abundance has been estimated at approximately 10 to 11 vol based on the amount of quartz formed during the experiments three dimensional investigations via fib serial sectioning indicate almost no interconnectivity between pores a consequence of albite and sanidine sub grain coarsening is the constant redistribution of the porosity and the sub grain boundaries within the patch perthite mosaic over time this suggests that a dynamically evolving porosity could provide an important transport mechanism for fluid transport through rocks even when the abundance and interconnectivity of the reaction induced porosity is low

the effect of temperature on the seismic anisotropy of the perovskite and post perovskite polymorphs of mgsio3 using elastic constants determined by ab initio molecular dynamics over a range of temperatures we show that the seismic anisotropy of mgsio3 perovskite is significantly temperature dependent at 90 gpa the direction of greatest shear wave splitting changes from 001 at 0 k to 010 at 3500 k in addition there is no shear wave splitting in the 100 direction at 0 k but a significant amount of splitting is exhibited at 3500 k we have also calculated the first set of high temperature elastic constants of the new post perovskite phase by ab initio molecular dynamics methods we find that in contrast to mgsio3 perovskite temperature has little effect on the acoustic anisotropy of this phase we determine ln vs ln vp for the post perovskite phase and find it to be very low about 0 9 this may be an important parameter for mapping out the post perovskite phase in the lowermost mantle

routine characterization and interpretation of complex alkali feldspar intergrowths almost all alkali feldspar crystals contain a rich inventory of exsolution twin and domain microtextures that form subsequent to crystal growth and provide a record of the thermal history of the crystal and often of its involvement in replacement reactions sometimes multiple microtextures strongly influence the subsequent behavior of feldspars at low temperatures during diagenesis and weathering they are central to the retention or exchange of trace elements and of radiogenic and stable isotopes this review is aimed at petrologists and geochemists who wish to use alkali feldspar microtextures to solve geological problems or who need to understand how microtextures influence a particular process we suggest a systematic approach that employs methods available in most well founded laboratories the crystallographic relationships of complex feldspar intergrowths were established by the 1970s mainly using single crystal x ray diffraction but such methods give limited information on the spatial relationships of the different elements of the microtexture or of the mode and chronology of their formation which require the use of microscopy we suggest a combination of techniques with a range of spatial resolution and strongly recommend the use of orientated sections sections cut parallel to the perfect 001 and 010 cleavages are the easiest to locate and most informative techniques described are light microscopy scanning electron microscopy using both backscattered and secondary electrons including the use of surfaces etched in the laboratory electron probe microanalysis and analysis by energy dispersive spectrometry in a scanning electron microscope transmission electron microscopy we discuss the use of cathodoluminescence as an auxiliary technique but do not recommend electron backscattered diffraction for feldspar work we review recent publications that provide examples of the need for great care and attention to pre existing work in microtextural studies and suggest several topics for future work

simultaneous volume measurements of post perovskite and perovskite in mgsio3 and their thermal equations of state abstract simultaneous volume measurements of mgsio 3 post perovskite ppv and perovskite pv were performed in a diamond anvil cell dac combined with synchrotron x rays an externally heated dac was used in addition to a laser heated dac for the volume measurement experiment at high temperatures the volume data were collected in the stability field of post perovskite from 115 to 130 gpa the temperature generated in the externally heated and the laser heated dacs for the volume measurement were up to 832 and 2330 k respectively using two different but complementary heating techniques we collected the data at a wide temperature range from 300 to 2330 k the obtained p v t data for ppv and pv were fitted to a third ordered birch murnaghan equation of state eos for a precise comparison of the volume between the two phases the eoss were constructed based on the same pressure scale of mgo the simultaneous volume measurements and the volumes calculated from the determined eoss demonstrate that the volume difference between ppv and pv of about 1 5 is almost constant with increasing temperature to 4000 k at the transition at the base of the mantle this density difference corresponds to a temperature anomaly of 1300 k without the phase transition due to the very small thermal expansivity of minerals which has a significant effect on mantle dynamics the thermal expansivity contrast between the top and the bottom of the mantle is a factor of 3 6 from a mantle convection study this value suggests that huge and hot plumes are formed at the core mantle boundary

raman spectroscopy of perovskite and post perovskite phases of mggeo3 to 123 gpa abstract raman spectra of the perovskite pv and post perovskite ppv phases of mggeo 3 were measured in the laser heated diamond cell up to 123 gpa between 14 and 51 gpa we observed a total of seven pv modes the frequencies and relative intensities of which are similar to mgsio 3 pv with systematically lower frequencies we also observe a total of ten ppv modes which are much more intense than pv modes indicating strong polarizability of the bonds in ppv the observed raman mode frequency range of ppv extends to much higher frequencies than pv which can be related to the edge sharing of the geo 6 octahedra in ppv which would make ge o bonds stronger the spectroscopic gruneisen parameters of pv and ppv are constrained to be 1 56 0 10 and 1 15 0 06 respectively at the ppv transition we also estimated that the thermal expansion parameter decreases by 25 10 across the ppv transition if a similar change occurs in mgsio 3 the low thermal expansivity would dynamically stabilize the ppv layer at the lowermost mantle using our raman measurements and a kieffer type model the clapeyron slope of the ppv transition is constrained to be 20 10 mpa k the strong positive slope is consistent with the prediction for mgsio 3 and is related to a positive shift of phonon density of states at high frequency region resulting from the formation of the edge sharing among the geo 6 or sio 6 octahedra in ppv

thermoelastic properties of post perovskite phase mgsio3 determined experimentally at core mantle boundary p t conditions we have performed in situ x ray diffraction measurements on mgsio3 up to 144 5 gpa and 2535 k to obtain a precise experimental determination of the elastic properties of the post perovskite polymorph of mgsio3 at high pressure and temperature the complete dataset is comprised of 24 data points at ambient temperature and an equal number at high temperature fitted physical properties are the room temperature bulk modulus the gruneisen parameter with its volume dependency the thermal expansion with its temperature dependency and the linear incompressibilities as well as thermal expansions and their temperature dependency on each axis elastic anisotropy is observed the structure being much more compressible along the b axis and less compressible along the c axis in agreement with theoretical calculations we also demonstrate with the present study that experiments can provide precise information on the elasticity of complex materials at extreme pressure and temperature conditions

role of magma mixing in the pre eruptive dynamics of the aeolian islands volcanoes southern tyrrhenian sea italy abstract we combined literature and experimental data to determine the role of magma mixing in the pre eruptive dynamics of the aeolian islands volcanoes as a first step we systematically reviewed the evidence supporting the hypothesis of mixing triggered eruptions in the aeolian archipelago providing textural chemical and rheological constraints the existing data highlighted the significant role of magma mixing in many eruptions within the aeolian archipelago examples include the upper pollara and porri volcano eruptions at salina monte guardia and the ad 1230 monte pilato eruption at lipari as well as the present day activity at stromboli then we focused on vulcano island chosen as a case study because it represents one of the volcanoes posing the highest risk in the aeolian archipelago at vulcano island we highlighted the role of magma mixing in the ad 1739 and 1888 90 eruptions finally we investigated mixing to eruption timescales for the ad 1739 eruption performing mixing experiments and evaluated the progressive decay of the chemical concentration variance with time results pointed to mixing to eruption timescales of the order of 29 9 h and magma ascent rates ranging between 3 10 2 and 5 10 2 m s 1 we finally emphasized that the presented results may have significant implications in the context of volcanic hazard mitigation and planning of emergency activities

k ar age and geochemistry of the sw japan paleogene cauldron cluster implications for eocene oligocene thermo tectonic reactivation abstract systematic k ar dating and geochemical analyses of paleogene cauldrons in the sanin belt of sw japan have been made to explore the relationship between the timing of their formation and the paleogene subduction history of sw japan documented in the shimanto accretionary complex we also examine the magma sources and tectonics beneath the backarc region of sw japan at the eastern plate boundary of eurasia fifty eight new k ar ages and 19 previously reported radiometric age data show that the cauldrons formed during middle eocene to early oligocene time 43 30 ma following a period of magmatic hiatus from 52 to 43 ma the hiatus coincides with absence of an accretionary prism in the shimanto belt resumption of the magmatism that formed the cauldron cluster in the backarc was concurrent with voluminous influx of terrigenous detritus to the trench as a common tectono thermal event within a subduction system the cauldrons are composed of medium k calc alkaline basalts to rhyolites and their plutonic equivalents these rocks are characterized by lower concentrations of large ion lithophile elements lile including k2o ba rb th u and li lower la yb n ratios lower initial sr isotopic ratios 0 7037 0 7052 and higher end t values 0 5 to 3 5 relative to late cretaceous to early paleogene equivalents there are clear trends from enriched to depleted signatures with decreasing age from the late cretaceous to the paleogene the same isotopic shift is also confirmed in lower crust derived xenoliths and is interpreted as mobilization of pre existing enriched lithospheric mantle by upwelling depleted asthenosphere relatively elevated geothermal gradients are presumed to have prevailed over wide areas of the backarc and forearc of the sw japan arc trench system during the eocene to oligocene newly identified late eocene low silica adakites and high mg andesites in the sanin belt and early eocene a type granites in the sw korea peninsula probably formed due to upwelling of hot asthenosphere and subduction of a young plate the backarc region was an extensional tectonic setting and some paleogene rift basins and sanin belt cauldrons occur in linear arrays the eocene oligocene sanin se korea continental arc lies on the ne extension of the east china sea basin the initial stage of which probably formed by continental arc rifting this rifting may have been triggered by upwelling of hot asthenosphere into the wedge space created by rollback of the subducted slab in response to decreased convergence rate between the pacific and eurasian plates

eight phase alkali feldspars low temperature cryptoperthite peristerite and multiple replacement reactions in the klokken intrusion eight feldspar phases have been distinguished within individual alkali feldspar primocrysts in laminated syenite members of the layered syenite series of the klokken intrusion the processes leading to the formation of the first four phases have been described previously the feldspars crystallized as homogeneous sodian sanidine and exsolved by spinodal decomposition between 750 and 600 c depending on bulk composition to give fully coherent strain controlled braid cryptoperthites with sub m periodicities below 500 c in the microcline field these underwent a process of partial mutual replacement in a deuteric fluid producing coarse up to mm scale turbid incoherent patch perthites we here describe exsolution and replacement processes that occurred after patch perthite formation both or and ab rich patches underwent a new phase of coherent exsolution by volume diffusion or rich patches began to exsolve albite lamellae by coherent nucleation in the range 460 340 c depending on patch composition leading to film perthite with 1 m periodicities below 300 c misfit dislocation loops formed which were subsequently enlarged to nanotunnels ab rich patches bulk composition ab91or1an8 in one sample exsolved giving peristerite with one strong modulation with a periodicity of 17 nm and a pervasive tweed microtexture the ab rich patches formed with metastable disorder below the peristerite solvus and intersected the peristerite conditional spinodal at 450 c this is the first time peristerite has been imaged using tem within any perthite and the first time peristerite has been found in a relatively rapidly cooled geological environment the lamellar periodicities of film perthite and peristerite are consistent with experimentally determined diffusion coefficients and a calculated cooling history of the intrusion all the preceding textures were in places affected by a phase of replacement correlating with regions of extreme optical turbidity we term this material ultra porous late feldspar uplf it is composed predominantly of regions of microporous very or rich feldspar mean ab2 5or97 4an0 1 associated with very pure porous albite ab97 0or1 6an1 4 implying replacement below 170 90 c depending on degree of order in tem uplf has complex irregular diffraction contrast similar to that previously associated with low temperature albitization and diagenetic overgrowths replacement by uplf seems to have been piecemeal in character ghost like textural pseudomorphs of both braid and film parents occur formation of patch perthite film perthite and peristerite occurred 104 105 year after emplacement but there are no microtextural constraints on the age of uplf formation

deformation and texture development in cairo3 post perovskite phase up to 6 gpa and 1300 k at near ambient conditions cairo3 is isostructural with the high pressure polymorph mgsio3 post perovskite ppv mgsio3 ppv is thought to be a major phase in the earth s lowermost mantle cairo3 can thus serve as an analog for studying deformation of the ppv phase under conditions achievable with a multi anvil deformation apparatus here we study the rheologic behavior of cairo3 ppv at a variety of pressure and temperature conditions from 2 gpa to 6 gpa and 300 k to 1300 k and various strain rates sintered polycrystalline cairo3 ppv cylindrical in shape was deformed in the d dia multi anvil press in several shortening cycles up to 20 axial strain at each temperature and pressure shortening cycles were followed by lengthening back to 0 strain quantitative texture information was obtained using in situ synchrotron x ray diffraction and the rietveld method to analyze images in all cases we find that 010 lattice planes align perpendicular to the compression direction upon shortening and that there is little change in texture with temperature or pressure this texture pattern is consistent with slip on 010 100 the texture observed here is different from that produced in room temperature diamond anvil cell dac experiments on mggeo3 and mgsio3 ppv which both display textures of 100 and 110 lattice planes at high angles to the compression direction this implies that cairo3 ppv may not be a good analog for the plastic behavior of mgsio3 ppv 2008 elsevier b v all rights reserved

experimental evidence for perovskite and post perovskite coexistence throughout the whole d region article i nfo since the phase diagram for fe al bearing mgsio3 compositions at the p t conditions of the core mantle boundary remains ambiguous we investigated the fe distribution among the silicate perovskite pv and post perovskite ppv polymorphs using tandem synchrotron analyses of x ray diffraction and x ray absorption spectroscopy we performed measurements at the fe k edge of the partitioning of iron between pv and ppv up to more than 150 gpa after annealing at about 3300 k we obtain a unique solution for kfe pv ppv of 4 2 0 5 our results evidence that the two silicates should coexist over the whole d region with the main post perovskite phase being largely depleted in fe compared to the perovskite as fe and al have a dominant effect on the phase diagram these new results challenge recent determinations of the temperature profile in the lowermost mantle based on the clapeyron slope of the pv to ppv transition for pure mgsio3 composition also it appears clear that variations in the molar fractions of perovskite and post perovskite phases should be expected radially or laterally in the d region in relation with thermal or compositional heterogeneities this can help explaining the seismological anomalies observed for this mantle region finally we predict a significant increase of the feo activity in the d region which should greatly affect the chemical exchange between mantle and core

u pb and re os geochronological evidence for the jurassic porphyry metallogenic event of the xiongcun district in the gangdese porphyry copper belt southern tibet prc abstract two large copper gold porphyry deposits no i and no ii were discovered in the last decade within the xiongcun district which is located along the south margin of the gangdese porphyry copper belt these two deposits are hosted by two porphyries that were emplaced into the early jurassic volcano sedimentary rock sequences of the xiongcun formation the porphyries are both quartz diorite bodies with one of these distinguished by its large quartz eyes our new molybdenite re os and zircon u pb dates for no i and no ii deposits and host intrusions reveal the presence of two middle jurassic ore forming episodes in the xiongcun district the no ii deposit is related to the 181 175 ma quartz diorite porphyry and formed ca 172 6 2 1 ma the no i deposit is related to the 167 161 ma quartz diorite porphyry with large quartz eyes and formed ca 161 5 2 7 ma geochronological and geochemical features of the jurassic ore bearing porphyries in the xiongcun district indicate that the porphyry copper gold mineralization formed in an island arc setting which is related to the northward subduction of the neo tethys oceanic plate these observations in combination with geochronological data reported at gangdese porphyry copper belt indicated four major metallogenic events existed in the gangdese porphyry copper belt at 172 161 ma 65 48 ma 40 23 ma and 20 12 ma we have identified two mineralization episodes comprise middle jurassic metallogenic event 172 161 ma in the xiongcun district which provides new constrains for the middle jurassic metallogenic event in the gangdese porphyry copper belt

structure and evolution of the himalaya tibet orogenic belt the 1981 french chinese expedition to tibet focused on the lhasa block extending earlier coverage 400 km north of the tsangpo suture the lhasa block stood between 10 and 15 n latitude over most of the upper cretaceous and eocene and if gondwanian in origin had detached from gondwana by early permian seismic profiles reveal a complex moho topography resulting both from multiple continental thrusting and large scale strike slip faulting subduction related granitoids representing mixtures of mantle and crustal components and anatectic granitoids have been analysed and dated this study emphasizes the role of smaller blocks in the accretion of the continental mosaic

in situ observations of phase transition between perovskite and cairo 3 type phase in mgsio 3 and pyrolitic mantle composition abstract in situ observations of the perovskite cairo3 phase transition in mgsio3 and in pyrolitic compositions were carried out using a laser heated diamond anvil cell interfaced with a synchrotron radiation source for pure mgsio3 the phase boundary between the orthorhombic mg perovskite and cairo3 type phases in the temperature range of 1300 3100 k was determined to be p gpa 130 3 0 0070 0 0030 t 2500 k using platinum as a pressure calibrant we confirmed that the cairo3 type phase remained stable up to pressures of at least 156 gpa and temperatures of 2600 k the consistency of our results with previous theoretical calculations leads us to conclude that the 2700 km seismic discontinuity at the bottom of the lower mantle can be attributed to a phase transition to the cairo3 type phase the phase change from an orthorhombic mg perovskite to a cairo3 type bearing assemblage in a pyrolitic mantle composition was also observed at p 125 gpa which corresponds to the same mantle depth as the seismic discontinuity the phase boundary between the orthorhombic mg perovskite and cairo3 type bearing assemblage was determined to be p gpa 124 4 0 008 0 005 t 2500 k using gold as a pressure calibrant this transition boundary indicates that the temperature at a depth of 2700 km is about 2600 k and the adiabatic temperature gradient in the lower mantle is estimated to be 0 31 k km the partition coefficients and the effect of some elements on the phase equilibrium between the orthorhombic mgsio3 perovskite and cairo3 type mgsio3 were estimated from ab initio calculations our experimental and theoretical results indicate that the d layer consists of a cairo3 type bearing assemblage which is likely to have significant effect on the chemical and thermal evolution of the earth s mantle

the subduction accretion history of the bangong nujiang ocean constraints from provenance and geochronology of the mesozoic strata near gaize central tibet abstract the mesozoic strata within the bangong nujiang suture zone in central tibet recorded critical information about the subduction accretion processes of the bangong nujiang ocean prior to the lhasa qiangtang collision this paper reports detailed field observations petrographic descriptions sandstone detrital zircon u pb ages and hf isotopic analyses from an accretionary complex preserved as mugagangri group and the unconformably overlying shamuluo formation near gaize the youngest detrital zircon ages together with other age constraints from literature suggest that the mugagangri group was deposited during late triassic early jurassic while the shamuluo formation was deposited during late jurassic early cretaceous based on the differences in lithology age and provenance the mugagangri group is subdivided into the upper middle and lower subunits these units are younging structurally downward southward consistent with models of progressive off scrapping and accretion in a southward facing subduction complex the upper subunit comprising mainly quartz sandstone and siliceous mud shale was deposited in abyssal plain environment close to the qiangtang passive margin during late triassic with sediments derived from the southern qiangtang block the middle and lower subunits comprise mainly lithic quartz sandstone and mud shale containing abundant ultramafic ophiolitic fragments the middle subunit of late triassic early jurassic age records a transition in tectono depositional setting from abyssal plain to trench wedge basin with sudden influx of sediments sourced from the central qiangtang metamorphic belt and northern qiangtang magmatic belt the appearance of ultramafic ophiolitic fragments in the middle subunit reflects the subduction initiation the lower subunit was deposited in a trench wedge basin during early jurassic with influx of jurassic aged zircons originating from the newly active southern qiangtang magmatic arc the lower subunit records the onset of arc magmatism related to the northward subduction of the bangong nujiang ocean the shamuluo formation comprising mainly lithic feldspar sandstone with limestone interlayers was deposited in a post collisional residual sea or pre collisional trench slope basin with sediments derived entirely from the qiangtang block

effect of fe enrichment on seismic properties of perovskite and post perovskite in the deep lower mantle recent experimental measurements of the equation of state of perovskites and post perovskites in the mg fe sio3 and mg fe al fe al si o 3 systems over a wide range of iron contents are used to constrain the effects of fe and al on density and bulk modulus of these phases at deep mantle pressures the density of fe bearing perovskite follows a linear relationship with fe content at a representative mid mantle depth of 1850 km 80 gpa 80 g cm 3 5 054 1 1 270 3 x fe the bulk modulus of silicate perovskite is not sensitive to fe content and follows the relationship k 80 gpa 546 2 12 25 x fe the velocity heterogeneity parameter partial derivative ln v b partial derivative x fe determined by experimental values for the bulk sound speed is 0 10 1 in agreement with theory and the behaviour of other fe bearing silicates near the core mantle boundary fe rich post perovskite is observed to be more compressible than the mg end member in contrast to theoretical predictions from experimental data the densities of perovskite and post perovskite at 125 gpa 2700 km depth are 125 pv g cm 3 5 426 11 1 38 4 x fe and 125 ppv g cm 3 5 548 1 1 41 3 x fe the density contrast across the post perovskite transition is similar to 2 per cent irrespective of fe content but the contrast in bulk sound speed increases with fe content al rich silicates exhibit no significant differences in density or compressibility relative to al free silicates but may be responsible for seismic heterogeneities due to differences in the depth and width of the post perovskite transition observations of increased densities in large low shear velocity provinces and ultra low velocity zones may be consistent with local iron enrichment from mg 90 to mg 78 88 and mg 50 respectively

the role of al defects on the equation of state of al mg fe sio3 perovskite abstract we performed compression curves of aluminous silicate perovskite al pv synthesized under various conditions of pressure temperature and mgo or sio 2 activities using laser heated diamond anvil cell at the esrf grenoble we refined bulk moduli k 0 from 235 to 270 gpa in agreement with the wide range of values reported in the literature we observe that al pv phase synthesized at high temperature in the sio 2 rich system is more compressible than al pv phase synthesized at high pressure in the mgo rich system as suggested by various authors the resolution of this controversy rests on a better understanding of the crystal chemistry of al in perovskite which involves at least two competitive mechanisms substitution of si in the octahedral site only or a coupled substitution on both mg and si sites the vacancy mechanism is expected to reduce the k 0 significantly due to the presence of oxygen vacancies all compression curves performed in this study can be explained by considering that the vacancy mechanism is favored at high temperatures and that the coupled mechanism is favored at high pressures these trends agree well with previous reports for mg fe si al o 3 perovskite compositions relevant to the lower mantle the two previous reports and our new data set for a morb type perovskite phase agree well with each other with higher k 0 values between 260 and 270 gpa compared with k 0 253 gpa for the pure mgsio 3 phase suggested from previous studies in these compounds coupled substitution of al 3 and fe 3 cations leads to a well constrained crystal chemistry therefore the low k 0 value observed in some of the previous studies for fe free al pv is likely to be irrelevant for mantle perovskite some questions may remain only for the mantle region just below the 670 km discontinuity where pressures remain moderate thus potentially allowing for at most 2 of oxygen vacancies however it is clear that using low k 0 values to extrapolate to greater depths is unjustified

effect of pressure and composition on lattice parameters and unit cell volume of fe mg sio3 post perovskite we obtained unit cell volumes v for fe sub 0 2 mg sub 0 8 sio sub 3 and fe sub 0 4 mg sub 0 6 sio sub 3 post perovskite ppv over a pressure p range of 80 145 gpa by in situ x ray diffraction xrd measurements in laser heated diamond anvil cells dacs our study suggests that the ppv structure is highly resistant to back transformation to pv and fe sub 0 4 mg sub 0 6 sio sub 3 ppv was preserved down to 80 gpa upon laser heating during decompression before transforming to pure perovskite pv the bulk modulus k sub 0 of ppv was determined to be 215 1 and 198 1 gpa for fe sub 0 2 mg sub 0 8 sio sub 3 and fe sub 0 4 mg sub 0 6 sio sub 3 ppv respectively with a fixed k sub 0 4 the substitution of fe for mg in ppv results in the largest expansion along the b axis among all three axes and the expansion of the b axis is nearly independent of pressure whereas the expansion along the a and c directions slightly decreases with increasing pressure combining our results with previous studies on fe sub x mg sub 1 x sio sub 3 ppv x 0 0 09 0 1 shows that the bulk modulus of ppv decreases with increasing fe content over a composition range x 0 to 0 4 varying from 231 gpa for x 0more to 198 gpa for x 0 4 with zero pressure volumes scaled to composition v sub 0 angstrom sup 3 162 2 19 5x and fixed k sub 0 4 0 the increase of fe content in ppv from x 0 to 0 4 results in a 6 3 increase in density and a 5 3 decrease in bulk sound velocity at 130 gpa less

3d imaging of subducting and fragmenting indian continental lithosphere beneath southern and central tibet using body wave finite frequency tomography abstract we perform a finite frequency tomographic inversion to image 3d velocity structures beneath southern and central tibet using teleseismic body wave data recorded by the tibet 31n passive seismic array as well as waveforms from previous temporary seismic arrays high velocity bodies dip 40 northward beneath the himalaya and the lhasa terrane we interpret these high velocity anomalies as subducting indian continental lithosphere icl the icl appears to extend further north in central tibet than in eastern tibet reaching 350 km depth at 31 n along 85 e but at 30 n along 91 e low p and s wave velocity anomalies extend from the lower crust to 180 km depth beneath the tangra yum co rift yadong gulu rift and the cona rift suggesting that rifting in southern tibet may involve the entire lithosphere the anomaly beneath tangra yum co rift extends down to about 180 km whereas the anomalies west of the yadong gulu rift and east of the cona rift extend to more than 300 km depth the low velocity upper mantle west of the yadong gulu rift extends furthest north and appears to connect with the extensive upper mantle low velocity region beneath central tibet thus the northward subducting indian plate is fragmented along north south breaks that permit or induce asthenospheric upwellings indistinguishable from the upper mantle of northern tibet

monzo anorthosite from the taguei ring complex air niger a hybrid rock with cumulus plagioclase and an infiltrated granitic intercumulus liquid an unusual hybrid rock composed of cumulus plagioclase with the interstices occupied by abundant quartzo feldspathic material occurs in the taguei ring complex niger the monzo anorthosite consists of about 75 plagioclase in large tabular to elongate crystals with interstices occupied in two stages firstly by clinopyroxene titanomagnetite and apatite and then by an intergrowth of quartz and alkali feldspar associated with brown green amphibole and zircon secondary green amphibole chlorite epidote and calcite may occur four stages in the crystallization history were identified 1 cumulus stage represented by the cores of the plagioclase laths an56 an66 with reversed oscillatory zoning and rare clinopyroxene t 1 150 c 2 early intercumulus stage with a wide overgrowth zone on plagioclase an62 an15 with oscillatory zoning and increase in or clinopyroxene apatite titanomagnetite t 1 150 1 050 c 3 late intercumulus stage with alkali feldspar quartz opaque oxide brown green amphibole apatite and zircon t 750 700 c alkali feldspar gave cryptoperthite on further cooling 4 deuteric stage with development of turbidity in the alkali feldspar and plagioclase rims and formation of patch perthite with microcline and secondary minerals t 400 c

the origin and pre cenozoic evolution of the tibetan plateau different hypotheses have been proposed for the origin and pre cenozoic evolution of the tibetan plateau as a result of several collision events between a series of gondwana derived terranes e g qiangtang lhasa and india and asian continent since the early paleozoic this paper reviews and reevaluates these hypotheses in light of new data from tibet including 1 the distribution of major tectonic boundaries and suture zones 2 basement rocks and their sedimentary covers 3 magmatic suites and 4 detrital zircon constraints from paleozoic metasedimentary rocks the western qiangtang amdo and tethyan himalaya terranes have the indian gondwana origin whereas the lhasa terrane shows an australian gondwana affinity the cambrian magmatic record in the lhasa terrane resulted from the subduction of the proto tethyan ocean lithosphere beneath the australian gondwana the newly identified late devonian granitoids in the southern margin of the lhasa terrane may represent an extensional magmatic event associated with its rifting which ultimately resulted in the opening of the songdo tethyan ocean the lhasa northern australia collision at similar to 263 ma was likely responsible for the initiation of a southward dipping subduction of the bangong nujiang tethyan oceanic lithosphere the yarlung zangbo tethyan ocean opened as a back arc basin in the late triassic leading to the separation of the lhasa terrane from northern australia the subsequent northward subduction of the yarlung zangbo tethyan ocean lithosphere beneath the lhasa terrane may have been triggered by the qiangtang lhasa collision in the earliest cretaceous the mafic dike swarms ca 284 ma in the western qiangtang originated from the panjal plume activity that resulted in continental rifting and its separation from the northern indian continent the subsequent collision of the western qiangtang with the eastern qiangtang in the middle triassic was followed by slab breakoff that led to the exhumation of the qiangtang metamorphic rocks this collision may have caused the northward subduction initiation of the bangong nujiang ocean lithosphere beneath the western qiangtang collision related coeval igneous rocks occurring on both sides of the suture zone and the within plate basalt affinity of associated mafic lithologies suggest slab breakoff induced magmatism in a continent continent collision zone this zone may be the site of net continental crust growth as exemplified by the tibetan plateau c 2012 international association for gondwana research published by elsevier b v all rights reserved

origin of cryptoperthites in the bishop tuff and their bearing in its thermal history the bishop tuff is a large rhyolitic ash flow which erupted from long valley caldera in eastern california and contains abundant sanidine cryptoperthite phenocrysts which can be used to constrain the time between the two major cooling units the cryptoperthites bulk composition or66ab33an1 have exsolved in two principal stages followed by a third minor stage the earliest exsolution which produced patches of relatively wide lamellae 100 800 nm without wedge shaped ends wse occurred by coherent nucleation and was probably related to a high flux of water associated with fumarolic activity in this area the spacing of these lamellae shows no correlation with depth or thermal history the second major stage of exsolution occurred by spinodal decomposition which produced lamellae with wse having spa ein g s of from 80 to 170 nm and the spacing of these lamellae varies with depth in a pattern consistent with the two known cooling units heat flow calculations indicate that this pattern of lamellar spacings is consistent with an interval of about 1 0 2 0 years between the eruption of these two cooling units

dynamics and time evolution of a shallow plumbing system the 1739 and 1888 90 eruptions vulcano island italy abstract in this work we analyze the morphology of latitic enclaves occurring in the rhyolitic lava flow of pietre cotte island of vulcano italy we show that enclave morphology is a feature inherited from the shallow plumbing system of the volcano during the invasion of the latitic magma into the rhyolitic magma the complexity of enclave morphology is quantified by fractal analyses using the empirical relationship given by perugini et al 2005 relating the fractal dimension and viscosity ratio the range of viscosity ratios that developed during the injection of the latitic magma into the rhyolitic one is estimated thermodynamical and rheological modeling indicates that the most plausible scenario to explain the variability of observed viscosity ratios is represented by a plumbing system where a large volume of latitic magma intruded a smaller volume of rhyolitic magma the comparison of volume proportions of magmas on the outcrop with those estimated by the modeling allows us to infer that most of the latitic magma remained in the plumbing system after the ending of the 1739 eruptive cycle the strong similarity of compositions from the pietre cotte lava flow and products erupted during the last volcanic activity of vulcano 1888 1890 is interpreted as the reactivation of the latitic magma that possibly evolved through fractional crystallization to produce trachy rhyolitic compositions during the 1888 1890 eruptive cycle we suggest that the methodological approach presented here can represent a further tool for hazard mitigation in volcanic areas in particular it allows obtaining information about the dynamics of plumbing systems of active volcanoes and their time evolution

the postperovskite transition the discovery of the postperovskite ppv transition has profound impact on our understanding of the core mantle boundary cmb region unlike perovskite pv the ppv phase has a layered structure of the sio6 octahedra which may lead to a large contrast in some properties with pv recent studies have proposed unusual properties of ppv such as a large positive clapeyron slope a large sensitivity of the transition depth to iron a decrease in bulk sound speed at the transition and a development of significant lattice preferred orientation many of the proposed properties can provide explanations for the intriguing seismic observations at the cmb region yet significant discrepancies still exist however rapid developments in mineral physics will continue to improve our knowledge on the changes across the ppv transition

towards a more practical two feldspar geothermometer the thermodynamic basis of several recent attempts to formulate a simple two feldspar geothermometer is discussed together with a review of earlier empirical geothermometers and ones based on experimental studies in the ternary feldspar system it is shown that double binary thermometers which involve the combination of regular solution mixing models for the binary alkali feldspar system with ideal mixing in plagioclases do not give a satisfactory representation of two feldspar relations especially for albite rich compositions where a critical point exists thermometers based on mixing parameters for ordered alkali feldspar frameworks are even more unjustified both because low plagioclases are certainly non ideal and because of uncertainty in knowing the degree of al si order in the alkali feldspar when exchange equilibrium was achieved a thermodynamic thermometer requires knowledge of ternary activities which are at present unknown experimental determinations of relationships in the ternary feldspar system are reviewed and the correct general form of the thermometer constructed using mainly the experimental data of seck 1971a and smith and parsons 1974 chemographic tests for equilibrium between feldspar pairs are suggested and petrographie features discussed in an appendix new values are given of margules parameters calculated for binary disordered alkali feldspars from recent solvus data up to 15 kbars and their physico chemical basis examined we suggest that accurate representations of the mixing properties of disordered alkali feldspars using margules parameters are at present premature

geochemical criteria for a refined tectonic discrimination of orogenic andesites abstract minor trace element and sr isotope data are compiled for tertiary recent orogenic andesites of three tectonic settings 1 oceanic island arcs divided into low k and other andesites 2 continental island arcs and thin continental margins and 3 thick continental margins andean from 1 to 3 there is a steady overlapping increase in la ce ree la yb la y th th u zr y hf yb and ni co and a decrease in k la p la sc cr and sc ni optimum discriminationis achieved with a plots of la yb vs th and la yb vs sc ni and b cluster analysis using the critical parameters listed above shoshonitic andesites and high mg andesites boninites are considered separately orogenic andesites are distinguished from non orogenic andesites by high contents of al low contents of ti zr ree y nb ta and ga and the absence of pronounced negative eu anomalies

argon loss by alkali feldspars abstract ar loss is generally thought to preclude the use of alkali feldspars for dating old rocks by the k ar method and has been thought to occur via structural features such as the boundaries of perthite lamellae or twins we have done a 40 ar 39 ar step heating study of a suite of alkali feldspars from the klokken syenite intrusion u pb age 1166 4 ma which have been fully characterized by transmission electron microscopy all are low albite low microcline intergrowths but some are non turbid strain controlled cryptoperthites while others are turbid coarse microperthites geological and textural evidence suggests that the turbidity developed by fluid rock interactions during the initial cooling of the pluton in the stability field of low microcline 5 a after crystallization the finest cryptoperthites have numerous fully coherent perthite lamellar boundaries lamellar periodicity 40 nm and twin composition planes the have retained radiogenic 40 ar and yield total degassing ages as high as 1125 16 ma and maximum ages on the age spectra of 1162 16 ma the coarse turbid microperthites with patchy intergrowths and twins on the mm scale gave total degassing ages as low as 650 ma maximum age 750 ma argon leakage occurs least in feldspars with the most lamellar interfaces but in proportion to the degree of turbidity which is thought to be caused by large numbers of micropores strain controlled intergrowths in perthites allow the recognition of volumes of alkali feldspar crystals which have not interacted with deuteric fluids these retain argon and can give reliable k ar ages in old rocks saddle shaped age spectra appear to result from the preferential degassing at 1000 c of part of the feldspar structure which has lost more of its argon in nature the laboratory degassing does not occur in a manner analogous to that which has occurred naturally

tectonic evolution of the qinghai tibet plateau abstract the qinghai tibet plateau composed of several continental slivers within the eastern tethyan domain is one of the pivotal sites to examine to better understand the theory of plate tectonics and the orogenic evolution on earth this plateau is generally inferred to be a collage of several continental blocks that rifted from gondwanaland and subsequently accreted to the asian continent however recent recognition of over twenty ophiolite melange zones and their associated island arcs indicates that the traditional model of tectonic evolution requires revision based on 177 recently finished 1 250 000 scale geological maps and related studies we summarize the main tectonic context of the qinghai tibet plateau and propose a new integrated model to account for the new findings the complex orogen of the immense qinghai tibet plateau consisting of multiple island arc basin systems that developed at different stages while surrounded by the north china yangtze tarim and indian plates is emphasized the entire orogen surrounded by suture zones that mark the locations of oceanic closure is investigated by examining i the first order tectonic units and ophiolitic melanges including arc arc continent collision zones and ii their internally enclosed blocks as the second order tectonic units therefore the qinghai tibet plateau is divided into three major orogenic systems namely from northeast to southwest the early paleozoic qinling qilianshan kunlunshan qin qi kun the late paleozoic triassic qiangtang sanjiang and the late paleozoic to cenozoic gangdese himalaya orogenic systems which are separated by the kangxiwa muzitagh maqin mianxian and the bangong shuanghu changning menglian sutures respectively we propose that the formation and evolution of the qinghai tibet plateau to have been intrinsically related to those of the eastern tethys recorded by the longmu co shuanghu ophiolite melange zone the southern qiangtang paleozoic accretionary arc basin system the bangong nujiang suture zone and their associated composite island arc basin systems the present day bangong shuanghu changning menglian suture system marks the final closure of the tethyan ocean the yarlung zangbo ocean opened as a back arc basin in response to the southward subduction of the tethyan ocean lithosphere in the middle triassic and closed as a result of the india asia collision at the end of cretaceous followed by the northward indention of the indian plate that resulted in significant intra continental deformation and plateau uplift in the cenozoic

the nature of potassium feldspar exsolution microtextures and development of dislocations as a function of composition in perthitic alkali feldspars transmission electron microscope data on the morphology of exsolution lamellae the nature of the potassium feldspar and the development of dislocations at lamellar interfaces in coherent cryptoperthites and fine microperthites are reviewed dislocations have been reported previously in only two crystals and periodic dislocations noted in only one an or rich microperthite periodic dislocations spacing 100 150 nm are here described from a ternary mesoperthite or26 ab52 an22 small crystallites 30 nm of other phases have sometimes nucleated on the dislocations the 020 lattice fringes of the feldspar phases have been imaged the difference in 020 spacings can be almost entirely accommodated by the regular dislocations so that the boundaries may be termed nearlyperfectly semicoherent

development of microporosity diffusion channels and deuteric coarsening in perthitic alkali feldspars turbidity is an almost universal feature of alkali feldspars in plutonic rocks and has been investigated by us in alkali feldspars from the klokken syenite using sem and tem it is caused by the presence of myriads of tubular micro inclusions either fluid filled micropores or sites of previous fluid inclusions and is associated with coarsening of microperthite and development of sub grains micropores are abundant in coarsened areas in which porosities may reach 4 5 but are almost absent from uncoarsened pristine braind microperthite areas the coarsening is patchy and involves a scale increase of up to 103 without change in the composition of the phases low albite and low microcline or in the bulk composition of the crystal it occurs abruptly along an irregular front within individual crystals which retain their original shapes the coherent braid microperthite gives way across the front to an irregular semi coherent film perthite over a few m and then to a highly coarsened irregular patch perthite containing numerous small sub grains on scales of a few hundred nm in both phases the coarsening and micropore formation occured at a t 400 450 c and it is inferred to have been driven by the release of coherent strain energy low angle grain boundary migration being favoured by a fluid the patchy nature of the coarsening and the absence of a relationship with initial grain boundaries suggest that the fluid was of local origin possibly arising in part through exsolution of water from the feldspar the sub grain texture and microporosity modify profoundly the permeability of the rock and greatly enhance the subsequent reactivity of the feldspars

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

geochemical constraints on the origin of bimodal magmatism at the okinawa trough an incipient back arc basin abstract the magmatism at the axial zone of the middle okinawa trough a young continental back arc basin comprises a bimodal basaltic rhyolitic suite accompanied by minor intermediate rocks we report major and trace element and sr nd isotopie data for the intermediate to silicic suites to provide constraints on their petrogenesis the rhyolites recovered as lava and pumice fall into three geochemical groups type 1 2 and 3 rhyolites type 1 rhyolites have 87sr 86sr 0 7040 0 7042 and 143nd 144nd 0 5128 0 5129 identical to those of associated basalts and are characterized by highly fractionated ree patterns petrogenesis of type 1 rhyolites is explicable in terms of fractional crystallization of the associated basalt in contrast type 2 rhyolites and andesite have slightly higher 87sr 86sr 0 7044 0 7047 but similar 143nd 144nd 0 5128 compared to those of the basalts the compositions of type 2 rhyolite and andesite can be explained by assimilation and fractional crystallization afc processes of the basalt magma quantitative analysis suggests assimilation fractional crystallization ma mc ratios of 0 05 hybrid andesite generated by mixing of evolved basalt and type 1 rhyolite is also present we emphasize that mechanical extension in this part of the okinawa trough involves gabbro ic lower crust that resulted from fractionation of mantle derived basaltic magmas type 3 rhyolite occurs only as pumice which makes its derivation questionable this rhyolite has major and trace element compositions and sr nd isotopie ratios which suggests that it may be derived from volcanic activity on the southern ryukyu volcanic front and arrived in the okinawa trough by drifting on the kuroshio current

remnants of a cretaceous intra oceanic subduction system within the yarlung zangbo suture southern tibet extensive field investigations along the yarlung zangbo suture zone in southern tibet reveal the presence of now fragmented remnants of a south facing intra oceanic subduction system this system developed within tethys during the cretaceous the associated arc forearc ophiolite and subduction complex were emplaced onto the leading edge of india at the end of the cretaceous rapid sedimentation in oblique slip basins and disruption of water saturated sediments into melange was widespread and concomitant with ophiolite emplacement we describe the tectonic entities that developed during this previously unrecognized phase of tethys tibet evolution and present a new model for the evolution of this portion of tibet

petrogenesis of early to middle jurassic granitoid rocks from the gangdese belt southern tibet implications for early history of the neo tethys abstract the gangdese belt tibet records the opening and closure of the neo tethyan ocean and the resultant collision between the indian and eurasian plates mesozoic magmatic rocks generated through subduction of the tethyan oceanic slab constitute the main component of the gangdese belt and play a crucial role in understanding the formation and evolution of the neo tethyan tectonic realm u pb and lu hf isotopic data for tonalite and granodiorite from the xietongmen nymo segment of the gangdese belt indicate a significant pulse of jurassic magmatism from 184 ma to 168 ma the magmatic rocks belong to metaluminous medium k calc alkaline series characterized by regular variation in major element compositions with sio2 of 61 35 73 59 low to moderate mgo 0 31 2 59 with mg of 37 45 these magmatic rocks are also characterized by lree enrichment with concave upward trend in mree on the chondrite normalized ree patterns and also lile enrichment and depletion in nb ta and ti in the primitive mantle normalized spidergrams these rocks have high zircon ehf t values of 10 94 to 15 91 and young two stage depleted mantle model ages tdm2 of 192 ma to 670 ma the low mgo contents and relatively depleted hf isotope compositions suggest that the granitoid rocks were derived from the partial melting of the juvenile basaltic lower crust with minor mantle materials injected in combination with the published data it is suggested that northward subduction of the neo tethyan slab beneath the lhasa terrane began by the late triassic which formed a major belt of arc related magmatism

partial assimilative recycling of the mafic plutonic roots of arc volcanoes an example from the chilean andes at long lived arc volcanic centers open system interactions between new magma and the plutonic roots of the arc will increase with time relative to interactions with older crust assimilative recycling of mafic plutonic arc roots will have different isotopic consequences than incorporation of aged continental crust in the tatara san pedro complex 30 comagmatic basaltic lavas are characterized by xenocrysts and microxenoliths of olivine augite and plagioclase derived from solidified cumulates wide but poorly correlated ranges of incompatible and compatible major and trace elements and limited isotopic variability this mineralogical chemical signature reflects assimilation wherein grain boundary melting and disaggregation of xenoliths led to blending of incompatible element enriched melts derived mainly from hornblende phlogopite and plagioclase in combination with variable retention of xenocrysts hence variable but high compatible element concentrations

experimental constraints on the origin of the 1991 pinatubo dacite crystallisation and mantle interaction experiments have been performed on the 1991 pinatubo dacite to constrain its petrogenesis in the dacite h2o system at 1000 mpa magnetite and either clinopyroxene or amphibole are the liquidus phases no garnet is observed at this pressure dacite mantle interaction at 920 mpa produces massive orthopyroxene crystallisation in addition to amphibole phologopite amphibole crystallising in dacite at 1000 mpa faithfully reproduces aluminium rich hornblende preserved in the cores of amphibole phenocrysts of the 1991 dacite suggesting a high pressure stage of dacite crystallisation under high melt h2o contents 10 wt and low temperatures 950c altogether plagioclase amphibole and melt inclusion compositions suggest that the pinatubo dacite was water rich oxidized and not much hotter than 900c when emplaced in the shallow reservoir in which most phenocrysts precipitated until the onset of the 1991 eruptions the strongly fractionated ree pattern demands garnet somewhere during dacite genesis which in turn requires high pressure derivation partial melting of subducted oceanic crust gives melts unlike the pinatubo dacite interaction of these slab melts with sub arc peridotite is unable to produce such type of dacite nor is a direct mantle origin conceivable on the basis of mantle dacite interaction experimental results dehydration melting of underplated basalts requires temperatures too high to match with petrological constraints and does not allow production of dacite melts having the low feo mgo high h2o ni and cr contents typical of pinatubo like magmas on the face of chemical and phase equilibrium constraints the most plausible origin of the pinatubo dacite is via high pressure fractionation in the upper mantle of an hydrous oxidised primitive basalt that crystallises amphibole and garnet upon cooling as shown by recent phase equilibrium work dacite melts so produced are directly expelled from the uppermost mantle or lower crust to shallow level storage reservoirs from which they erupt occasionally following injection of h2o poor mantle derived basalts that can ascent without massive crystallisation unlike their hydrous counterparts parental to dacite magmas that rest at mantle depths dacites such as the pinatubo one may thus witness particular h2o rich conditions of the sub arc mantle rather than the melting of a young and hot subducting oceanic plate

the zedong terrane a late jurassic intra oceanic magmatic arc within the yarlung tsangpo suture zone southeastern tibet an overturned sequence of igneous and volcaniclastic rocks crops out along the yarlung tsangpo suture zone ytsz in southeastern tibet these rocks are remnants of an intra oceanic island arc the zedong terrane that once lay between india and asia 40ar 39ar dating and u pb ion microprobe analyses reported here reveal that this arc was active during the jurassic u pb dating of zircon from a dacite breccia from the middle portion of the zedong terrane yields an age of 161 2 3 ma 1 40ar 39ar dating of hornblende from a cross cutting andesite dyke yields the youngest age of 152 2 3 3 ma 1 40ar 39ar results from hornblende from andesite dykes and an andesite breccia from the upper portion yield a mean age of 156 0 3 ma 1 additional u pb and 40ar 39ar dating of zircon and hornblende from quartz diorite yields a mean age of 156 8 0 8 ma 1 geochronological data reported here and other published work indicate that the intra oceanic subduction system to which the zedong terrane belonged was active from at least late jurassic to early to mid cretaceous

geochemical changes in baslts and andesites across the archean proterozoic boundary identification and significance abstract to identify accurately changes in rock composition across the archean proterozoic boundary it is necessary to compare rocks from similar lithologic associations to constrain the effect of tectonic setting most basalts and andesites from the greenstone association volcanic dominated submarine supracrustal rocks possess a geochemical subduction zone component similar to their counterparts from modern arc systems basalts with island arc geochemical affinities dominate in archean greenstones while those with calc alkaline affinities are most abundant in proterozoic greenstones basalts with morb or oceanic within plate geochemical characteristics are rare in precambrian greenstones of all ages preserved early archean greenstone basalts 3500 ma reflect mantle sources less depleted than late archean greenstone basalts 2500 3500 ma proterozoic greenstone basalts are derived from relatively enriched mantle sources compared to all archean sources a feature which may be due to recycling of continental sediments into the mantle following rapid late archean continental growth precambrian andesites are geochemically similar to andesites from modern arcs but archean andesites are unique in that they are depleted in heavy ree and y results are consistent with archean andesite production by partial melting of descending mafic crust with amphibole garnet remaining in the residue while proterozoic and younger andesites are produced by fractional crystallization of basalts

a tem and microprobe study of a two perthite alkali gabbro implications for the ternary feldspar system an unusual magmatic three feldspar syenogabbro occurs 3 m inside the contact of the klokken gabbro syenite stock it contains plagioclase mesoperthite and cryptoperthite together with a low temperature symplectite intergrowth textural relationships have been investigated by cathodoluminescence bulk chemistry by microprobe and exsolution microtextures and intracrystalline boundaries by tem the mesoperthite has a bulk composition around or26ab52an22 well outside accepted limits of ternary feldspar solid solution it is a mainly coherent lamellar intergrowth of sodic andesine and orthoclase with incipient mtwinning the cryptoperthite bulk composition around or61ab33an6 is a coherent lamellar intergrowth of orthoclase and sodic low oligoclase the compositions of the exsolved phases have been estimated the meso and cryptoperthite crystals have sharp boundaries with each other

geological characteristics and genesis of the jurassic no i porphyry cu au deposit in the xiongcun district gangdese porphyry copper belt tibet abstract the xiongcun district located in the western segment of the gangdese porphyry copper belt gpcb hosts the only known jurassic mineralization in the gpcb tibet prc the no i deposit in the xiongcun district is related to the middle jurassic quartz diorite porphyry 167 161 ma and the mineralization was formed at ca 161 5 2 7 ma ore bearing middle jurassic quartz diorite porphyry emplaced into the early jurassic volcano sedimentary rock sequences of the xiongcun formation veinlets and disseminated mineralization developed within the middle jurassic quartz diorite porphyry and the surrounding metamorphosed tuff hosting a measured and indicated resource of 1 04 mt copper 143 31 t gold and 900 43 t silver with an average grade of 0 48 copper 0 66 g t gold and 4 19 g t silver the mineralization can be assigned to four stages including three main stages of hypogene mineralization and one epigenetic stage the main alteration associated with mineralization is potassic seven mineralization related hydrothermal veins have been recognized including quartz sulfide biotite sulfide magnetite sulfide quartz molybdenite sulfide chalcopyrite pyrite pyrrhotite pyrite and polymetallic veins the s and pb isotopic compositions of the ore sulfides and the re contents of the molybdenite suggest a mantle source for the ore forming materials with minor contamination from the subducted sediments hydrogen and oxygen isotope compositions of quartz in the ores suggest that both magmatic and meteoric waters were involved in the ore forming process the ore bearing porphyry 167 161 ma and ore forming 161 5 2 7 ma ages of the no i deposit correspond to the time of northward subduction of neo tethys oceanic slab the geochemical data of the ore bearing porphyry indicate that the no i deposit formed in an intra oceanic island arc setting and the ore bearing porphyry originated from the partial melting of mantle with limited contribution of subducted sediments the genesis of the ore bearing porphyry and no i deposit is interpreted as being related to northward intra oceanic subduction of neo tethys oceanic slab in the middle jurassic time 167 161 ma

boninite like volcanic rocks in the 3 7 3 8 ga isua greenstone belt west greenland geochemical evidence for intra oceanic subduction zone processes in the early earth abstract the 3 7 3 8 ga isua greenstone belt of southwest greenland is characterized by variably metamorphosed metasomatised and deformed lithotectonic successions of volcanic and sedimentary rocks the voluminous mafic volcanic rocks are composed primarily of pillow basalts intercalated with ultramafic units the sedimentary rocks consist mainly of banded iron formation cherts conglomerates and siliciclastic turbidites the least altered metavolcanic amphibolites the garbenschiefer unit from the central tectonic domain of the isua greenstone belt are characterized by high mg number 0 60 0 80 mgo 7 18 wt al2o3 14 20 wt ni 60 645 ppm and cr 60 1920 ppm contents but low tio2 0 20 0 40 wt zr 12 30 ppm y 6 14 ppm and rare earth element ree concentrations these compositional features collectively represent a coherent mafic to ultramafic suite chondrite normalized ree patterns are concave upward on the primitive mantle normalized extended trace element diagrams they are characterized by relative depletion of nb but with an enrichment of zr relative to neighboring rees alteration deformation and crustal contamination can be ruled out as the cause of the distinct and coherent composition the average initial end value of these metavolcanic rocks is 2 collectively these geochemical characteristics are comparable to those of phanerozoic boninites given the observation that in the tertiary boninites are exclusively associated with intra oceanic subduction environments e g izu bonin mariana subduction system this suggests that intra oceanic subduction zone like geodynamic processes were operating as early as 3 7 3 8 ga

geochemical and nd pb isotopic characteristics of the tethyan asthenosphere implications for the origin of the indian ocean mantle domain abstract it is unclear why the pb nd and sr isotopic composition of the modern mid ocean ridge basalts morb from the indian ocean is different from that of the north atlantic and pacific oceans a possible explanation for this is that the indian morb type isotopic signature is a long lived regional feature of the mantle as evidently shown by the isotopic composition of the 350 ma morb like mian lue northern ophiolite which was formed in the same region presently occupied by the indian ocean however this hypothesis is in conflict with the lack of indian morb type isotopic signature in a number of 150 ma tethyan and indian ocean crusts to further constrain the origin of the indian morb type isotopic signature we analyze the geochemical and pb nd and sr isotopic composition of representative mafic rocks from four tethyan ophiolites ranging in age from 90 to 360 ma the sr isotopic composition of the samples is unreliable due to alteration but the age corrected nd and pb isotopic ratios and geochemical data indicate that these tethyan rocks were derived from a geochemically depleted asthenospheric source that had a clear indian morb type isotopic signature we therefore conclude that the bulk of the indian suboceanic mantle was most probably inherited from the tethyan asthenosphere a few regions in both the tethyan and indian oceans however are most probably underlain by pacific and north atlantic morb type mantle and vice versa because of the flow of the asthenosphere in response to tectonic plate reorganizations that lead to openings and closures of ocean basins the indian morb type isotopic signature of the western pacific marginal basin crusts could be due to either flow of the indian ocean mantle into the western pacific or to endogenous production of such an isotopic signature from delaminated east asian sublithospheric materials during closure of the tethys ocean

cryptoperthites and cooling rate in a layered syenite pluton a chemical and tem study alkali feldspars in the klokken layered syenite south greenland are optically either coarse turbid deuteric patch perthites or clear unaltered fine braid microor cryptoperthites irregular clear volumes can be found in most turbid grains electron microscopy shows that all clear crystals or volumes are cryptoperthites in which the periodicity of the exsolution lamellae increases systematically with distance from the roof of the intrusion from 40 nm to 300nm they are composed of low albite and diagonally associated maximum microcline a secondary coarsening is responsible for optically visible braid microperthites the layered series consists of interleaved granular syenites and coarser grained laminated syenites with mineral layering microprobe analyses show a downward decrease in ca and ba in the granular syenite feldspars consistent with chilling from the roof inwards most laminated syenite feldspars contain 1 an increasing to 5 in mafic layers the periodicity of the exsolution lamellae in the laminated syenite feldspars is consistently up to twice that in adjacent granular syenites periodicity is not influenced by ab or or an content but high an feldspars have continuous zig zag intergrowths while others are more lozengeshaped lamellar periodicities largely reflect cooling rate in the pluton cooling times in the exsolution interval were probably about 103 104 years at the top of the series whereas existing experimental data suggest that the periodicities would be reached in a few days or years the periodicities at the top of the granular series are only slightly greater than experimentally determined initial wavelengths for spinodal decomposition suggesting that little or no coarsening occurred despite the long cooling times coarsening did occur lower in the pile framework ordering and twinning must slow or stop coarsening the difference in periodicity between the granular and laminated syenite series may be caused by differences in amount of water incorporated in the feldspar structure from the magma the exsolution of this water at low temperatures could be the cause of the deuteric unmixed patch perthites small volumes of cryptoperthite which have escaped deuteric unmixing may be common in igneous rocks in general and may be useful relative indicators of cooling rate

eocene neotethyan slab breakoff in southern tibet inferred from the linzizong volcanic record abstract formation of the linzizong volcanic successions in the lhasa terrane southern tibet has long been related to northward subduction of the neotethyan oceanic slab under asia here we report new 40ar 39ar age results for the volcanic rocks recovered from a large area 29 n to 32 n and 85 e to 93 e that together with literature data delineate two discrete stages of volcanism these are a widespread cretaceous stage and an intense but spatially confined paleogene stage the latter occurring only in the southern part of the lhasa terrane resulted in the linzizong volcanic successions our data furthermore suggest southward migration and intensification of the volcanism in the lhasa terrane with magmatic flare ups at ca 50 ma while the volcanic successions consist dominantly of calc alkaline rocks typical of arc lava geochemistry those formed during the flare up period show significant compositional variations from low k tholeiitic through calc alkaline to shoshonitic magma suites these observations enable us to interpret the volcanic southward migration and following flare ups as the consequences of rollback and breakoff of the subducted neotethyan slab that occurred ahead and in the early stage respectively of the india asia collision our interpretation that involves a major eocene tectonomagmatic activity and concomitant topographic uplift in southern tibet is consistent with himalayan metamorphic constraints regional sedimentary records and seismic tomography

stability of fe bearing hydrous phases and element partitioning in the system mgo al2o3 fe2o3 sio2 h2o in earth s lowermost mantle abstract we performed high pressure temperature p t experiments on a model composition of hydrous subducted slabs in the mgo al2o3 fe2o3 sio2 h2o system using laser heated diamond anvil cells the phase assemblages were characterized combining in situ synchrotron x ray diffraction and ex situ transmission electron microscope techniques the hydrous phase alooh feooh mgsio2 oh 2 sio2 coexists with bridgmanite bdg post perovskite ppv or both in a broad p t range of 104 126 gpa and 1750 2500 k the hydrous pyrite type feoohx phase was observed in situ over a p t range of 112 123 gpa and 1750 2300 k coexisting with the ppv phase chemical analysis on recovered samples showed that considerable amount of fe2o2 oh 2 8 13 mol and sio2 9 13 mol in the phase does not reduce its thermal stability compared to the al endmember indicating that the fe bearing phase can transport water to the lowermost mantle along the mantle geotherm in this hydrous system we observed that al depletion in both the bdg and ppv phases can significantly reduce the width of the bdg to ppv transition in contrast to a wide two phase coexistence region in a dry al rich system meanwhile the fe enrichment in the ppv phase relative to the coexisting bdg phase lowers the transition pressure to the depth of the d discontinuity accordingly the depth and thickness of the bdg to ppv transition in subducted basaltic crustal materials can explain the seismically detected d discontinuity partial melting could be triggered by dehydration of the fe bearing hydrous phases due to a steep temperature gradient at the core mantle boundary cmb and therefore the ultralow velocity zones ulvzs might be the regions where partial melting occurs at the lowermost mantle

sequence and petrogenesis of the jurassic volcanic rocks yeba formation in the gangdese arc southern tibet implications for the neo tethyan subduction abstract the yeba formation volcanic rocks in the gangdese arc recorded important information regarding the early history of the neo tethyan subduction to explore their magmatic evolution and tectonic significance we performed a systematic petrological geochronological and geochemical study on these volcanic rocks our data indicated that the yeba formation documents a transition from andesite dominated volcanism which started before 182 ma and continued until 176 ma to bimodal volcanism 174 168 ma in the earliest middle jurassic the early stage andesite dominated volcanics are characterized by various features of major and trace elements and are interpreted as the products of interactions between mantle derived arc magmas and lower crustal melts their positive end t and ehf t values suggest a significant contribution of asthenosphere like mantle the late stage bimodal volcanism is dominated by felsic rocks with subordinate basalts geochemical signatures of the basalts indicate a composite magma source that included a subduction component an asthenosphere like upper mantle domain and an ancient subcontinental lithospheric mantle component the felsic rocks of the late stage were produced mainly by the melting of juvenile crust with some ancient crustal materials also involved we suggest that the occurrence and preservation of the yeba formation volcanic rocks were tied to a tectonic switch from contraction to extension in the gangdese arc which probably resulted from slab rollback of the subducting neo tethyan oceanic slab during the jurassic

zircon u pb geochronology and hf isotopic constraints on petrogenesis of the gangdese batholith southern tibet abstract during the mesozoic cenozoic northward neotethyan subduction and subsequent india asia collision gave rise to the extensive transhimalayan magmatism that stretches from burma and western yunnan through southern tibet to the ladakh and kohistan complexes to understand the age distribution and petrogenesis of the gangdese batholith the largest intrusive exposure along the transhimalayan magmatic belt fifty granitic samples were selected for in situ zircon u pb and hf isotopic analyses the u pb data suggest four discrete stages of magmatic activity i e 205 152 109 80 65 41 and 33 13 ma respectively with the 65 41 ma stage being the most prominent the hf isotopic data indicate that the gangdese batholith is overwhelmed by positive e hf t values which are comparable to those of the kohistan ladakh batholiths in the west but differ markedly from those of the chayu burma batholiths in the east most of the gangdese granites show similar and young hf model ages 1000 300 ma indicating their derivation from juvenile crust however those formed in the 65 41 ma stage exhibit more heterogeneous hf isotopic ratios with negative e hf t values being observed in some granites younger than 50 ma suggesting the involvement of old indian continental crust in their petrogenesis this age may thus mark the onset of the india asia collision the newly established zircon u pb age and hf isotope database of the gangdese batholith can be used as a powerful tracer or fingerprint when studying the source to sink relation of the sediments eroded from the southern tibetan plateau

primary magmas at the volcanic front of the ne japan arc coeval eruption of crustal low k tholeiitic and mantle derived medium k calc alkaline basalts at azuma volcano it has long been thought that the low k tholeiitic th and medium k calc alkaline ca lavas in the nejapan arc were produced by fractional crystallization from mantle derived basalt magmas and that the latter formed from mixing of mafic and felsic magmas both derived from a common primaryth basalt through fractionation an alternative view was recently proposed on the basis of sr isotope microanalysis of plagioclase phenocrysts from the zao volcano suggesting that 1 the low k th basaltic andesites formed by melting of lower crustal amphibolite and that 2 the medium k ca basalts to andesites formed by mixing of mantle derived basalt and crustal th melts to investigate further the origin of the primary low k th and medium k ca basalts we investigated basalts and andesites from azuma volcano azuma is a quaternary eruption center at the volcanic front in the ne japan arc that has erupted two types of basalt 1 radiogenic sr sr sr1 4 0 7058 0 7062 low k th basalt lavas without evidence of magma mixing and assimilation 2 unradiogenic sr sr sr1 4 0 7039 0 7041 medium k ca basalt lavas with subtle evidence for magma mixing associated intermediate lavas are voluminous and are all 3 mildly radiogenic sr sr sr1 4 0 7044 0 7055 medium k andesites all of which have ca affinities with evidence for rigorous magma mixing but no crustal assimilation the low k th basalt has an isotopic composition similar to that of crustal granitoids beneath azuma and has a composition indicating that it potentially formed from a high degree lower crustal amphibolite melt the medium kca basalt has a basaltic groundmass with mg rich olivine fo89 and calcic plagioclase phenocrysts an90 and the most unradiogenic sr sr sr1 4 0 7037 0 7038 suggesting that it originated from a primary mantle melt major and trace element microanalysis of the basaltic groundmass indicates that the primary magma composition is close to high k we conclude that the mantle derived basalt at azuma is the result of a highto medium k magma that was later mixed with a low k th basalt melt from the amphibolitic lower crust to form medium k ca basalts and andesites this supports the view of a lower crustal origin of the low kth basalts and simultaneously requires a reappraisal of the origin of the across arc variation in k contents of the mantle derived primary arc basalts as the highto medium k ca basalt is geochemically fairly similar to the high k rear arc basalt in the ne japan arc

exsolution and coarsening mechanisms and kinetics in an ordered cryptoperthite series cryptoperthites from the klokken layered syenite intrusion were examined by tem to determine the role of exsolution ordering and twinning in the development of the coherent microtextures during slow cooling the stratigraphic position of the samples in the layered series giving an independent variable in determining their evolution both periodicity primary and secondary and morphology change with distance from the top of the series most samples contain low microcline in the diagonal association

the lhasa terrane record of a microcontinent and its histories of drift and growth article i nfo the lhasa terrane in southern tibet has long been accepted as the last geological block accreted to eurasia before its collision with the northward drifting indian continent in the cenozoic but its lithospheric architecture drift and growth histories and the nature of its northern suture with eurasia via the qiangtang terrane remain enigmatic using zircon in situ u pb and lu hf isotopic and bulk rock geochemical data of mesozoic early tertiary magmatic rocks sampled along four north south traverses across the lhasa terrane we show that the lhasa terrane has ancient basement rocks of proterozoic and archean ages up to 2870 ma in its centre with younger and juvenile crust phanerozoic accreted towards its both northern and southern edges this finding proves that the central lhasa subterrane was once a microcontinent this continent has survived from its long journey across the paleo tethyan ocean basins and has grown at the edges through magmatism resulting from oceanic lithosphere subduction towards beneath it during its journey and subsequent collisions with the qiangtang terrane to the north and with the indian continent to the south zircon hf isotope data indicate significant mantle source contributions to the generation of these granitoid rocks e g 50 90 0 70 and 30 100 to the mesozoic magmatism in the southern central and northern lhasa subterranes respectively we suggest that much of the mesozoic magmatism in the lhasa terrane may be associated with the southward bangong nujiang tethyan seafloor subduction beneath the lhasa terrane which likely began in the middle permian or earlier and ceased in the late early cretaceous and that the significant changes of zircon ehf t at 113 and 52 ma record tectonomagmatic activities as a result of slab break off and related mantle melting events following the qiangtang lhasa amalgamation and india lhasa amalgamation respectively these results manifest the efficacy of zircons as a chronometer u pb dating and a geochemical tracer hf isotopes in understanding the origin and histories of lithospheric plates and in revealing the tectonic evolution of old orogenies in the context of plate tectonics

did the indo asian collision alone create the tibetan plateau it is widely believed that the tibetan plateau is a late cenozoic feature produced by the indo asian collision however because tibet was the locus of continental accretion and subduction throughout the mesozoic crustal thickening during that time may also have contributed to growth of the plateau this portion of the geologic history was investigated in a traverse through the central lhasa block southern tibet together with earlier studies our mapping and geochronological results show that the lhasa block underwent little north south shortening during the cenozoic rather our mapping shows that 60 crustal shortening perhaps due to the collision between the lhasa and qiangtang blocks occurred during the early cretaceous this observation implies that a significant portion of southern tibet was raised to perhaps 3 4 km elevation prior to the indo asian collision

kinematic significance of morphological structures generated by mixing of magmas a case study from salina island southern italy morphological features of mixing mingling structures of heterogeneous juveniles from the 13 ka old upper pollara eruption salina island southern italy are studied these heterogeneous rocks result from the mixing between an andesitic and a rhyolitic magma concentration patterns generated by magma mixing are analyzed on digital images and results have been compared with those obtained from numerical simulations of mixing processes coupling chaotic advection and chemical diffusion results show that the upper pollara rocks were produced by mixing 24 49 of andesite and 76 51 of rhyolite multifractal analysis of the mixing structures is performed and the mixing intensity i e the degree of interaction between the two end member magmas has been deduced from the analysis of the singularity spectrum results show that narrower is the multifractal spectrum the higher is the degree of homogenization of magmas reynolds number re during mixing has been estimated from the geometrical analysis of mixing structures by using experimentally defined relationships between shape parameters and re results show that re is between ca 500 and 7000 there is a positive correlation between the estimated initial percentage of mafic magma in the different analyzed samples and re in agreement with the observation that the higher is the percentage of the mafic lower viscosity magma the higher is the turbulence of the mixing system in addition our analysis reveals an unexpected inverse relationship between the calculated re and the degree of magma homogeneity suggesting that energy dissipation may have had a major role in the controlling mixing process because dissipation is inversely proportional to the mixing efficiency results suggest that mixing processes between the andesitic and rhyolitic magmas mainly developed in the conduit it is also suggested that mixing occurred in a shear layer type or pipe type flow

structure constraints and instability leading to the post perovskite phase transition of mgsio3 abstract recent experience with rietveld refinement of structural analogues and literature surveys suggests anion anion repulsion limits the stability of the perovskite phase including in the mgsio3 perovskite to post perovskite transition assuming rigid octahedral coordination still to be tested experimentally the critical point where intra and inter octahedral anion anion distances are equal provides a useful metric for predicting the pressure of the perovskite post perovskite transition and the clapeyron slope of the phase boundary once pressure and temperature derivatives of relevant structure parameters are known the inter octahedral anion anion distances and the polyhedral volume ratio are rigorously formulated as a function of octahedral rotation in this work assuming the orthorhombic pbnm perovskite structure where regular octahedra share each corner and conform to the in and anti phase rotation schemes designated by space group symmetry these mathematical expressions are consistent with structure data from 70 perovskite structured materials surveyed in the literature at ambient as well as extreme conditions and define structure constraints such as the minimum polyhedral volume ratio which must be reached before the phase transition to the post perovskite structure type can proceed the formalism we present is general for perovskite pbnm and dependent on the accuracy with which structures can be determined from sometimes compromised high pressure diffraction data

in situ determination of fe fe3s phase diagram and liquid structural properties up to 65 gpa abstract lighter elements than iron such as sulphur are required in the earth s core to account of the core density deficit accurate determination of the evolution of the fe fes phase diagram at high pressure is essential to determine sulphur amount in the earth s core ab initio calculations predict extensive solubility of s in solid fe at core pressures of 330 gpa whereas multi anvil quench analysis exhibits deep eutectic system at moderate pressure of 21 gpa in this study we investigated the fe rich part of fe fes phase diagram up to 65 gpa and 2200 k using in situ angle dispersive x ray diffraction we report a uniform increase with pressure of the eutectic temperatures t eut of about 15 k gpa above 50 gpa we evidence a decrease of s content in eutectic liquid with increasing pressure extrapolating this trend to inner core boundary pressures suggests that s cannot account for the 10 wt outer core density deficit and that other light elements such as si and o are needed diffraction pattern recorded at 42 gpa and 2150 k was selected for structural investigations of the fe s liquid by applying liquid structure simulation based on gaussian distribution of atoms around crystalline positions a good agreement has been found with hcp fe model structure rather than with fe 3 s structure it suggests that s acts as an interstitial impurity in the liquid state therefore s could have a relatively minor effect on sound velocities in liquid outer core

early middle jurassic 182 170 ma ruocuo adakitic porphyries southern margin of the lhasa terrane tibet implications for geodynamic setting and porphyry cu au mineralization abstract jurassic adakitic rocks in the southern lhasa subterrane are relatively rare poorly documented and their petrogenesis and geodynamic setting are unclear to explore these issues we present zircon u pb ages along with hf isotopic whole rock geochemical and sr nd pb isotopic data for hornblende quartz diorite porphyry hqdp and quartz diorite porphyry qdp in the ruocuo area southern margin of the lhasa terrane tibet zircon u pb dating for these rocks indicates that they were emplaced in the early middle jurassic 182 170 ma geochemically hqdp and qdp both show characteristics of adakites with intermediate sio2 60 62 65 63 wt and 57 46 60 04 wt respectively relatively high al2o3 14 23 18 23 wt and 18 32 20 14 wt respectively and sr 344 571 ppm and 514 614 ppm respectively and low y 9 12 13 0 ppm and 11 0 14 9 ppm respectively and yb 0 93 1 31 ppm and 1 08 1 52 ppm respectively in addition they show relatively low ratios of 87sr 86sr i 0 7038 0 7045 206pb 204pb i 18 31 18 42 207pb 204pb i 15 56 15 66 and 208pb 204pb i 38 36 38 77 and relatively high values of end t 5 54 to 6 33 and ehf t 11 8 to 17 3 the sr nd pb hf isotopes of hqdp and qdp are similar to those of the yarlung tsangpo ophiolites indicating that they were predominantly generated by the partial melting of the subducted neo tethys oceanic slab the subducted neo tethys oceanic slab derived adakitic rocks that are present in the southern lhasa subterrane provide strong evidence for proving the presence of oceanic subduction during the early middle jurassic by combining our results with the previously reported results we suggest that the ruocuo adakitic rocks and early mesozoic late triassic to middle jurassic magmatic rocks in the southern lhasa subterrane formed in a magmatic arc setting related to the northward subduction of the neo tethys oceanic slab that the initial subduction of the neo tethys oceanic slab occurred prior to the early jurassic and that the upwelling asthenosphere triggered by the roll back of the subducted neo tethys oceanic slab provided the heat that was required for slab melting however contrasting these results with the geochemical characteristics of the no 2 deposit in the xiongcun cu au district leads us to consider that the early middle jurassic subducted oceanic slab derived adakitic porphyries contain a greater potential for porphyry cu au mineralization than the early mesozoic normal magmatic rocks in the southern lhasa subterrane furthermore mineralized jurassic porphyry has been reported in the ruocuo area adjacent to the northern xigaze forearc basin indicating that the northern xigaze forearc basin also has the potential for subduction related porphyry cu au mineralization

early cretaceous subduction related adakite like rocks of the gangdese belt southern tibet products of slab melting and subsequent melt peridotite interaction abstract the limited geochronology and geochemistry data available for the early cretaceous igneous rocks of the southern gangdese belt southern tibet has resulted in the proposal of conflicting geodynamic models for the generation of the widespread cretaceous igneous rocks in the middle and northern parts of the belt to explore this issue we present shrimp u pb zircon data and geochemical and sr nd pb hf isotopic data for the mamen andesites from the southern margin of the gangdese belt the mamen andesites emplaced at 136 5 ma are sodic na2o k2o 1 2 2 3 and have geochemical characteristics typical of adakites i e high al2o3 high la yb ratios and sr contents low y and hree contents and positive eu anomalies except for high cr ni and mgo contents the andesites have initial 87sr 86sr t ratios of 0 70413 0 70513 positive end t values of 3 7 5 8 and 206pb 204pb t ratios of 18 37 18 51 207pb 204pb t ratios of 15 59 15 65 and 208pb 204pb t ratios of 38 43 38 72 in situ hf isotopic analyses of zircons that had previously been dated by shrimp yielded positive initial ehf t values ranging from 11 0 to 15 5 a model calculation using trace element and sr nd pb isotopic data indicates that several percent of subducted sediment is required to generate the mamen andesites which were derived via the partial melting of subducted neo tethyan slab morb sediment fluid and subsequently hybridized by peridotite in the mantle wedge our data indicate that the neo tethyan oceanic crust was subducted northward beneath the gangdese belt during the early cretaceous at a high angle our results are inconsistent with a tectonic model that advocates the low angle or flat slab subduction of neo tethyan oceanic crust in generating the widespread cretaceous magmatism recorded in the gangdese belt

high temperature fluid rock interactions in a layered syenite pluton variation in mineral composition with stratigraphie height in layered igneous intrusions is generally taken to indicate progressive fractional crystallization and following pioneering work on the skaergaard complex1 petrologists have used this variation in modelling the evolution of magma bodies in cooling plutons however major chemical changes may occur in circulating deuteric fluids2 3 in the klokken south greenland layered syenite we have discovered that mineral compositions vary both along and normal to the strike of individual layers and we argue that major chemical changes have occurred in a circulating aqueous fluid constrained to move along relatively permeable high temperature aquifers in the layered series

early jurassic volcanic rocks in the xiongcun district southern lhasa subterrane tibet implications for the tectono magmatic events associated with the early evolution of the neo tethys ocean abstract early mesozoic late triassic to middle jurassic igneous rocks in the southern lhasa subterrane tibet record important information on the early tectono magmatic evolution of the neo tethys ocean this paper presents petrological geochronological and geochemical data for the xiongcun formation ignimbrites to constrain the petrogenesis and tectonic setting of these rocks in the southern lhasa subterrane the xiongcun formation is dominated by andesitic and dacitic rocks these rocks yield early jurassic 195 176 ma zircon u pb ages and display typical arc like geochemical signatures being enriched in large ion lithophile elements e g rb ba and u and depleted in high field strength elements e g nb ta and ti sr nd pb hf isotopic compositions are highly depleted these chemical and isotopic data suggest that the xiongcun formation ignimbrites probably formed in an intra oceanic island arc setting related to the northward subduction of the neo tethys oceanic slab the parental magmas were derived from a depleted mantle source modified by fluids released from the neo tethys oceanic slab combined with previously reported data we suggest that the volcanic rocks in the early jurassic xiongcun and yeba formations of the southern lhasa subterrane were generated by a single subduction system but developed on oceanic and continental crust respectively

on the increase in thermal diffusivity caused by the perovskite to post perovskite phase transition and its implications for mantle dynamics the thermal diffusivity of perovskite and post perovskite cairo3 has been measured at elevated pressure and temperatures up to 600 c using the x radiographic angstrom method at high temperatures we find that the thermal diffusivity of post perovskite is slightly below twice that of isochemical perovskite over the temperature range investigated assuming a similar effect occurs in mgsio3 post perovskite the effect of the contrasting thermal transport properties between perovskite and post perovskite on mantle dynamics has been investigated using simple two dimensional convection models these show a reduction in extent and increase in depth of post perovskite lenses as well as increased core mantle boundary heat flux broader upwellings and more vigorous downwellings when compared to the reference constant a case

sedimentary features of the nunarssuit and klokken syenites s greenland summary the sediment like features of two layered syenites are described and reviewed in the light of recent suggestions that layering in gabbros maybe produced by in situ crystal growth in a near rigid medium in the nunarssuit syenite igneous layering is normally graded sharp mafic bases becoming more feldspathic upwards and channel structures some with breccias and extreme concentrations of mafic phases are evidence of magmatic currents contorted bedding provides evidence of c 15 m of crystal mush in the chemically similar klokken laminated syenite the layers are inversely graded with well sorted horizons of hedenbergite or fayalite at the top of graded units current bedding and rare channels are superimposed o n this inverse grading and striking load structures occur at junctions with interbedded horizons of granular syenite demonstrating the presence of 3 m of crystal mush the klokken laminated syenites are slightly more fractionated than nunarssuit and approach the water rich termination of fractionation of olivine clinopyroxene and ternary feldspar cumulus phases in nunarssuit layers are chemically less fractionated than those in normal rock and occur with cumulus chevkinite and zircon consistent with separation of suspended crystals from melt but cumulus minerals at klokken do not show this type of chemical contrast calculated magma densities and viscosities show that crystal settling of all phases was feasible in both complexes if the magmas were hydrous druses at klokken suggest that the intercumulus liquid was water saturated during load structure formation sorting of locally derived crystals by currents accounts for the nunarssuit layering but at klokken the crystal supply was controlled by order of nucleation and growth rate effects although crystal settling occurred

sidewall crystallization in the klokken intrusion zoned ternary feldspars and coexisting minerals the syenitic layered series in the klokken intrusion is surrounded by a zone 500 m thick of nearly structureless unlaminated syenite followed outwards by a zone of vertically banded gabbro 200 m thick at the outer rim the unlaminated syenite is intrusive into the gabbro and develops a thin 2 m transition zone of syenodiorite at the contact a traverse across the vertical transition zone and inwards towards the layered series was sampled with a portable drill mafic silicates olivine clinopyroxene biotite show inward evolution in fe fe mg across the syenodiorite unlaminated syenite zones feldspars change rapidly across the syenodiorite zone from rocks dominated by plagioclase in some cases together with two alkali feldspars one a mesoperthite or cryptomesoperthite the other a cryptoperthite to rocks in which plagioclase is seen only rarely as cores to cryptomesoperthitic alkali feldspar crystals plagioclase is absent from the layered series

origin and evolution of multi stage felsic melts in eastern gangdese belt constraints from u pb zircon dating and hf isotopic composition this integrated study of whole rock geochemistry zircon u pb dating and hf isotope composition for seven felsic rocks from the nyingchi complex in eastern himalayan syntaxis has revealed a complex magmatic history for the eastern gangdese belt this involves multiple melt sources and mechanisms that uniquely identify the tectonic evolution of this part of the himalayan orogen our u pb zircon dating reveals five stages of magmatic or anatectic events 165 81 61 50 and 25 ma the jurassic granitic gneiss 165 ma exhibits ehf t values of 1 4 to 3 5 the late cretaceous granite 81 ma shows variable ehf t values from 0 9 to 6 2 indicating a binary mixing between juvenile and old crustal materials the paleocene granodioritic gneiss 61 ma has ehf t values of 5 4 to 8 0 suggesting that it originated from partial melting of a juvenile crustal material the eocene anatexis is recorded in the leucosome which has hf isotopic composition similar to that of the jurassic granite indicating that the leucosome could be derived from partial melting of the jurassic granite the late oligocene biotite granite 25 ma shows adakitic geochemical characteristics with sr y 49 3 56 6 the presence of a large number of inherited zircons and negative ehf t values suggest that it sourced from anatexis of crustal materials in contrast to the gangdese batholiths that are mainly derived from juvenile crustal source in central tibet the old crustal materials play an important role for the magma generation of the felsic rocks suggesting the existence of a crustal basement in the eastern gangdese belt these correspond to specific magmatic evolution stages during the convergence between india and asia the middle jurassic granitic gneiss resulted from the northward subduction of the neo tethyan oceanic slab the late cretaceous magmatism is probably related to the ocean ridge subduction the paleocene eocene magmatism metamorphism and anatexis are interpreted to result from roll back and break off of the subducted neo tethyan slab that occurred in the early stage of the india asian collision respectively the late oligocene adakitic rocks resulted from the break off of the subducted indian continental crust starting at 25 ma

fe al bearing post perovskite in the earth s lower mantle the combined effects of fe and al on the electronic spin and valence states as well as the equation of state eos of post perovskite have been investigated using synchrotron x ray diffraction and mossbauer spectroscopy in high pressure diamond anvil cells two post perovskite samples mg0 6fe0 15al0 5si0 75o3 and mg0 66fe0 13al0 28si0 86o3 were synthesized at approximately 165 gpa and 2200 2500 k and were subsequently investigated for these properties at 114 170 gpa and 300 k analyses of the high pressure mossbauer spectra show that fe2 and fe3 occupy the large bipolar prismatic sites in both of our samples and remain in the high spin state at 165 168 gpa 165 168 gpa and 300 k combining the mossbauer results with the obtained pressure volume relationship from x ray diffraction we have found that the unit cell volume of post perovskite can be significantly affected by the spin and valence states of fe and the al substitution mg0 6fe0 15al0 5si0 75o3 ppv with the predominantly high spin fe2 95 95 and a greater amount of al has a unit cell volume similar to that of mg0 66fe0 13al0 28si0 86o3 ppv in which 65 65 of fe is in the high spin fe3 state our results are used together with previous results regarding the eos parameters in fe bearing perovskite and post perovskite to model the density and bulk sound velocity variation between perovskite and post perovskite in the d d layer in which the enrichment of fe and al can produce an increase in density but substantially reduce the bulk sound velocity across the phase transition that is the combined effect of fe and al leads to an anti correlation between the enhanced density and the reduced bulk sound velocity at the pressure condition of the lowermost mantle our results indicate that fe al fe al rich silicate post perovskite existing in the d d region would be shown as a relatively high density and low velocity region in deep mantle seismic observations

when and where did india and asia collide timing of the collision between india and asia is the key boundary condition in all models for the evolution of the himalaya tibetan orogenic system thus it profoundly affects the interpretation of the rates of a multitude of associated geological processes ranging from tibetan plateau uplift through continental extrusion across eastern asia as well as our understanding of global climate change during the cenozoic although an abrupt slowdown in the rate of convergence between india and asia around 55 ma is widely regarded as indicating the beginning of the collision most of the effects attributed to this major tectonic episode do not occur until more than 20 ma later refined estimates of the relative positions of india and asia indicate that they were not close enough to one another to have collided at 55 ma on the basis of new field evidence from tibet and a reassessment of published data we suggest that continent continent collision began around the eocene oligocene boundary 34 ma and propose an alternative explanation for events at 55 ma

early carboniferous back arc rifting related magmatism in southern tibet implications for the history of the lhasa terrane separation from gondwana

origin of silicic magmas and the compositional gap at sumisu submarine caldera izu bonin arc japan abstract sumisu caldera volcano izu bonin arc japan is dominated by the bimodal assemblage of basalt basaltic andesite 66 wt sio2 andesites 56 61 wt sio2 are minor in volume and a compositional gap of 6 wt sio2 exists between the andesites and the dacites most andesites contain sieve textured plagioclase and some dacites have resorbed plagioclase containing many glass inclusions some orthopyroxene phenocrysts in andesites and dacites have overgrowth rims with a high ca content a feature consistent with crystallization from a higher temperature magma than their cores dacites rhyolites and glasses in the matrix and plagioclases possess compositions similar to those of partial melts produced by dehydration melting of calc alkaline andesite tanzawa tonalite at low pressure 3 kbar phenocryst assemblages of these andesites dacites and rhyolites resemble restite phase assemblages of the dehydration melting prolonged fractionation or partial melting of basalts in contrast is untenable based on petrography and fractionation calculations we suggest that the silicic magmas at sumisu caldera volcano have been produced by dehydration melting of solidified calc alkaline andesite protolith in the upper to middle crust which segregated efficiently from restite crystals in rare situations reheating and en mass remobilization permitted the same andesites to erupt as andesites 56 61 wt sio2 thus sumisu has a relatively narrow temperature difference 900 1100 c associated with a large compositional range 56 74 wt sio2

geochronology and geochemistry of volcanic rocks of the bima formation southern lhasa subterrane tibet implications for early neo tethyan subduction abstract bima formation volcanic rocks which record the history of neo tethyan subduction are found within the central and eastern segments of the southern lhasa subterrane tibetan plateau zircon u pb dating whole rock major and trace element analysis and sr nd pb hf isotopic compositions of bima formation volcanic rocks from the central segment of the southern lhasa subterrane were used to constrain the magmatic and tectonic evolution of the lhasa terrane during the early mesozoic zircon u pb dating of five samples yielded consistent ages of 184 3 2 4 to 176 8 3 5 ma the dominant volcanic rock types within the bima formation are basalts basaltic andesites andesites and dacites which are enriched in the large ion lithophile elements e g rb sr and ba and depleted in high field strength elements e g nb ta and ti 87sr 86sr t ratios are low 0 702900 0 704146 end t and ehf t values are high and positive 4 4 to 6 9 and 9 6 to 15 7 respectively and pb isotope ratios are homogeneous initial 206pb 204pb 18 28 18 40 207pb 204pb 15 53 15 56 208pb 204pb 38 21 38 38 combining the new data with those from a previous study of bima formation volcanic rocks from the eastern segment of the southern lhasa subterrane indicates that the bima formation formed between the middle triassic and early jurassic it suggests that more widespread early mesozoic volcanic rocks in the southern margin of the lhasa terrane the basaltic rocks of the bima formation were generated by partial melting of a depleted mantle wedge metasomatized by slab derived fluids and subsequently experienced fractional crystallization without significant crustal contamination the andesitic and dacitic rocks were formed by fractional crystallization of the basaltic magma our study indicates that the bima formation volcanic rocks were generated within a continental island arc setting related to northward subduction of the neo tethyan oceanic slab during the early mesozoic