melting of newly formed mafic crust for the formation of neoproterozoic i type granite in the hannan region south china abstract neoproterozoic magmatism in the hannan region at the northwestern margin of the yangtze block is characterized by numerous felsic plutons associated with minor mafic ultramafic intrusions the felsic plutons are either adakitic or normal arc granitic in composition the adakitic plutons are 735 ma in age and are interpreted as having formed by partial melting of a thickened lower mafic crust among the normal arc related felsic plutons the tianpinghe pluton is the largest and has a shrimp zircon u pb age of 762 4 ma older than the adakitic plutons in the region rocks from the tianpinghe pluton have relatively high sio2 67 1 70 1 wt and k2o na2o 7 8 8 6 wt and relatively low mgo 0 7 1 3 wt and al2o3 contents 14 5 15 6 wt with al2o3 cao k2o na2o a cnk values ranging from 0 95 to 1 08 they have arc affinity trace element compositions that are characterized by enrichment of large ion lithophile elements and depletion of high field strength elements nb ta with strong

viscosity of liquid fayalite up to 9 gpa abstract the viscosity of liquid fayalite fe 2 sio 4 was determined up to 9 2 gpa and 1850 c using in situ falling sphere viscometry and x ray radiography imaging the viscosity of liquid fayalite was found to decrease both along the melting curve and an isotherm therefore temperature is thought to have little effect on liquid fayalite viscosity at high pressure the results are in contrast with previous studies on depolymerised silicate melts which found viscosity to increase with pressure in accordance with recent in situ structural measurements on liquid fayalite the viscosity decrease is likely a result of the increase in fe o coordination with pressure the results show that liquid silicate viscosities need to be considered on an individual basis and can be strongly dependent on the melt structure and composition this has important implications for models of planetary differentiation in particular terrestrial bodies with high fe contents and reducing mantle conditions are likely to have had very mobile melts at depth

neoproterozoic adakitic plutons in the northern margin of the yangtze block china partial melting of a thickened lower crust and implications for secular crustal evolution abstract numerous neoproterozoic felsic and mafic ultramafic intrusions occur in the hannan region at the northern margin of the yangtze block among these the wudumen and erliba plutons consist of granodiorites and have shrimp zircon u pb ages of 735 ma the rocks have high k 2 o 0 8 3 6 wt and na 2 o 4 4 6 4 wt and low mgo 0 4 1 7 wt they also have high sr y 32 209 and la yb n ratios 4 4 38 6 their e nd values range from 0 41 to 0 92 and zircon initial 176 hf 177 hf ratios from 0 282353 to 0 282581 these geochemical features are similar to those of adakitic rocks produced by partial melting of a thickened lower crust our new analytical results combined with the occurrence of voluminous arc related mafic ultramafic intrusions emplaced before 740 ma lead us to propose that the crustal evolution in the northern margin of the yangtze block during neoproterozoic involved 1 rapid crustal growth and thickening by underplating of mafic magmas from the mantle which was modified by materials coming from the subducting oceanic slab from 1 0 to 0 74 ga and 2 partial melting of the thickened lower crust due to a thermal anomaly induced by upwelling of asthenosphere through an oceanic slab window producing the 735 ma adakitic wudumen and erliba plutons our model suggests that the crustal thickness was more than 50 km at the northern margin of the yangtze block at 735 ma and rule out the possibility of a mantle plume impact causing the 735 ma magmatism in the region

elasticity and pressure induced structural changes in vitreous mgsio3 enstatite to lower mantle pressures abstract knowledge of the pressure density relations and elastic properties of silicate melts at high pressure and temperature conditions is fundamental for a better understanding of the dynamics and properties of the earth s interior here we report the sound velocities and elastic properties of vitreous mgsio 3 enstatite a simple analog for melts in the earth s mantle determined up to 33 4 1 2 gpa in a diamond anvil cell by brillouin scattering spectroscopy the adiabatic bulk and shear moduli of mgsio 3 glass at ambient conditions are k s0 78 4 6 gpa and 0 41 1 3 gpa corresponding to compressional and shear wave velocities of v p 6 969 30 km s and v s 3 868 20 km s on compression both longitudinal v p and transverse v s modes exhibit anomalous change in slope at 8 gpa and between 15 0 1 and 17 9 2 gpa that are associated with polyamorphic phase transitions in the glass the acoustic data were used to determine the density and compressibility of mgsio 3 glass at high pressure in both compression and decompression experiments in contrast to velocities and elastic moduli the density increases smoothly without singularities upon compression over the investigated pressure range densities of recovered glasses at room pressure are 1 5 to 2 4 higher than uncompressed glasses indicating pressure induced irreversible densification the anomalous changes in velocities and compressibility with pressure are consistent with structural modifications that have been identified as the main densification mechanisms of the glass by in situ spectroscopic studies and molecular dynamic calculations the results suggest that the residual densification of recovered glasses is associated with irreversible structural changes in the intermediate range order i e ring statistics rather than with changes in the coordination number of si upon pressurization if the changes in compressibility of mgsio 3 glasses identified in the present brillouin study also occurs in the analog melts they may have important implications for the present day dynamics of the mantle and early evolution of the earth during a magma ocean phase

petrogenesis of permian triassic felsic igneous rocks along the truong son zone in northern laos and their paleotethyan assembly abstract felsic igneous rocks in northern laos are key elements of the truong son and song ma tectonic zones and are important for constraining the paleotethyan evolution in southeastern asia this paper presents new geochronological geochemical and sr nd hf o isotopic data from granitoids and rhyolites in northern laos geochronological data show that these samples have zircon u pb ages ranging from ca 281 ma to 221 ma these samples can be subdivided into four groups based on their lithology and geochemical composition group 1 samples are granodiorite and granite with i type geochemical characteristics they have lower 87sr 86sr i ratios 0 7036 0 7059 and higher end t values 0 7 0 8 than group 2 biotite granite samples 87sr 86sr i 0 7087 0 7129 and end t 5 3 to 9 2 and group 3 rhyolite samples 87sr 86sr i 0 7092 0 7103 and end t 5 9 to 8 9 zircon grains from group 1 samples yield positive ehf t values of 5 1 13 8 tdm ages of 0 37 0 71 ga and 18o values of 5 6 6 3 in contrast zircons from group 2 and group 3 samples have mainly negative ehf t values 9 6 to 1 6 and 4 7 to 14 7 older tdm ages 1 08 1 70 ga and 1 42 1 97 ga and higher 18o values 5 7 10 3 and 6 7 8 0 respectively our geochemical data suggest that the group 1 granite samples are the products of the partial melting of newly underplated mafic rocks whereas the group 2 and 3 samples originated from a mixed source of ancient metaigneous and metagreywacke components group 4 samples include late triassic i type granodiorite and granite and have end t 1 9 to 3 9 87sr 86sr i 0 7079 0 7084 zircon ehf t 0 0 7 5 and 18o 5 1 6 9 indicating a derivation from a metagreywacke source with a component of juvenile mafic crust our data suggest that the permian felsic igneous rocks in northern laos formed in an arc setting related to the song ma paleotethyan branch bab subduction whereas the late triassic i type granitoids formed in a post collisional setting in response to the gravitational collapse of thickened crust during the collision of the south china and indochina blocks the temporal spatial pattern of igneous rocks along the truong son zone is related to the paleotethyan evolution and resembles that along the ailaoshan song ma zone and its eastward extension in northern hainan

magmatic effects of the lunar late heavy bombardment abstract large volumes of mare basalts are present on the surface of the moon located preferentially in large impact basins mechanisms relating impact basins and mare basalt eruptions have previously been suggested lunar impacts removed low density material that may have inhibited eruption and created cracks for fluid flow icarus 139 1999 246 and lunar basins have long been described as catchments for magma e g rev geophys space phys 18 1980 107 and references therein we present a new model for melt creation under near side lunar basins that is triggered by the impacts themselves magma can be produced in two stages first crater excavation depressurizes underlying material such that it may melt in situ second the cratered lithosphere rises isostatically warping isotherms at the lithosphere asthenosphere boundary which may initiate convection in which adiabatic melting can occur the first stage produces by far the largest volume of melt but convective melting can continue for up to 350 ma we propose that giant impacts account for a large portion of the volume and longevity of mare basalt volcanism as well as for several compositional groups including high alumina high titanium kreep rich and picritic magmas

fractional crystallization of mantle derived melts as a mechanism for some i type granite petrogenesis an example from lachlan fold belt australia the mt buller igneous suite in the southeastern lachlan fold belt encompasses a large variety of rock types from gabbros through diorites to granites mafic rocks and enclaves have primitive mantle like initial 87 sr 86 sr values 0 7037 0 7045 and nd values 5 6 to 4 1 the granites do not show distinctively more radiogenic initial 87 sr 86 sr values suggesting they are derived from the same parent as the gabbros most of the mt buller rocks do not show a positive correlation between silica and initial 87 sr 86 sr or between silica and 143 nd 144 nd values indicating that neither crustal contamination nor mixing between mantle and crustal derived melts has been important and that these rock types are likely to have formed by fractional crystallization from a common mantle derived parent some scatter in compatible trace elements particularly in granites and diorites may be explained by mixing geochemical modelling confirms a comagmatic origin of the suite magma generation probably took place in the subduction modified upper mantle this mechanism may play an important role in i type granite petrogenesis in particular tectonic settings such as during an efficient slab roll back or sinking of oceanic crust in divergent subduction conditions

proto to paleo tethyan evolution of the eastern margin of simao block abstract the simao block northern extension of indochina block was located along the proto tethyan subduction zone in early paleozoic and subducted by the paleo tethyan ailaoshan ocean along its eastern margin in permo triassic the location of simao block in the proto tethyan paleogeography and the permo triassic consumption process of the ailaoshan ocean were unclear detrital zircons from silurian to triassic strata and the representative xin anzhai intrusion in the eastern margin of simao block are studied to constrain the proto to paleo tethyan evolution the major age peaks of detrital zircons in early silurian early devonian strata are 0 95 ga and 0 45 ga which are consistent with those of detrital zircons in the time equivalent strata in the simao indochina block rather than yangtze block this implied that the paleo tethyan suture of the ailaoshan ocean was located along the ailaoshan tengtiaohe fault the dominant and diagnostic 450 ma detrital zircons with large variation of ehf t in the above strata were derived from arc igneous rocks associated with the evolution of proto tethyan oceanalong the western margin of simao block this feature contrasts with the equivalent strata from qiangtang and tethyan himalaya blocks located along gondwana indian margin which was dominated by 0 95 ga detrital zircons that derived from the orogenic belt within gondwana this difference indicates that the simao block located outboard of the indian margin of gondwana before the opening of paleo tethyan ocean during the consumption of paleo tethys the i type xin anzhai granitic intrusion emplaced at ca 257 ma geochemical compositions of the intrusion can be modeled by the partial melting of proterozoic gneiss with an addition of 30 45 arc basaltic magma age histogram of detrital zircons from upper triassic sediments is featured by a peak of 254 ma similar to the emplacement age of the xin anzhai intrusion this suggests an extensive magmatism occurred at ca 255 ma which can be explained by the rollback of the paleo tethyan slab the subsequent granitoids emplaced in 247 237 ma show different features of high silica peraluminous and a derivation from meta sedimentary rocks and were suggested to be produced in syn collision setting the early paleozoic strata in the eastern margin of simao block generally show a nw se foliation and ne sw striking stretching lineation reflecting a ne sw compression resulted from block amalgamation after the paleo tethyan closure in the angularly unconformable overlying upper triassic strata these structural traces are absent it means that the area has evolved into post collision extensional setting in late triassic

metal silicate partitioning of tungsten at high pressure and temperature implications for equilibrium core formation in earth abstract tungsten partitioning between liquid metal and liquid silicate d w from 1723 2673 k 0 5 18 gpa and over a wide range of metal and silicate compositions provides constraints on planetary core formation models we find that w partitioning is extremely sensitive to the carbon content of the metal alloy becoming about an order of magnitude more siderophile at carbon saturation activity composition corrections based on interactions between w c and fe c in metal solution shift calculated d w to more lithophile values and calculated oxygen fugacities fo2 to more oxidizing values than uncorrected data w exists as a highly charged cation in silicate solution displaying a mixture of oxidation states from 4 to 6 in experiments at fo2 of 0 5 2 log units below the iron wustite buffer at constant fo2 the average calculated valence decreases with pressure from 5 5 at 0 5 gpa to 4 at 11 18 gpa as a result of its high oxidation state w partitioning is strongly dependent on silicate melt polymerization and fo2 in contrast to some previous results we find that d w may decrease slightly in response to increasing temperature in the pressure range of our experiments pressure exerts a non monotonic effect d w increases with pressure up to 3 to 4 gpa but decreases at higher pressures previous models for the effects of pressure and temperature on w partitioning that conflict with our results appear to result from a conflation of the intensive parameters of pressure temperature and carbon content the mantle abundance of w could have been set by single stage metal silicate equilibration along the liquidus in a deep peridotite magma ocean at pressures from 20 50 gpa and at oxygen fugacities consistent with the mantle s present iron budget iw 2 to 2 5 equilibration at higher pressure is viable if the core forming metal contained a significant but not unreasonable abundance of carbon 2 wt recent continuous accretion models involving multi stage metal silicate equilibration in a deepening magma ocean with progressive oxidation of the silicate remain permissible given our new treatment of w partitioning data

geophysical consequences of planetary scale impacts into a mars like planet all planetary bodies with old surfaces exhibit planetary scale impact craters vast scars caused by the large impacts at the end of solar system accretion or the late heavy bombardment here we investigate the geophysical consequences of planetary scale impacts into a mars like planet by simulating the events using a smoothed particle hydrodynamics sph model our simulations probe impact energies over two orders of magnitude 2 10 27 6 10 29 j impact velocities from the planet s escape velocity to twice mars orbital velocity 6 50 km s and impact angles from head on to highly oblique 0 75 the simulation results confirm that for planetary scale impacts surface curvature radial gravity the large relative size of the impactor to the planet and the greater penetration of the impactor contribute to significant differences in the geophysical expression compared to small craters which can effectively be treated as acting in a half space the results show that the excavated crustal cavity size and the total melt production scale similarly for both small and planetary scale impacts as a function of impact energy however in planetary scale impacts a significant fraction of the melt is sequestered at depth and thus does not contribute to resetting the planetary surface complete surface resetting is likely only in the most energetic 6 10 29 j slow and head on impacts simulated a crater rim is not present for planetary scale impacts with energies 10 29 j and angles 45 but rather the ejecta is more uniformly distributed over the planetary surface antipodal crustal removal and melting is present for energetic 10 29 j fast 6 km s and low angle 45 impacts the most massive impactors with both high impact energy and low velocity contribute sufficient angular momentum to increase the rotation period of the mars sized target to about a day impact velocities of 20 km s result in net mass erosion from the target for all simulated energies and angles the hypothesized impact origin of planetary structures may be tested by the presence and distribution of the geochemically distinct impactor material

solidus and liquidus profiles of chondritic mantle implication for melting of the earth across its history abstract we investigated the melting properties of a synthetic chondritic primitive mantle up to core mantle boundary cmb pressures using laser heated diamond anvil cell melting criteria are essentially based on the use of x rays provided by synchrotron radiation we report a solidus melting curve lower than previously determined using optical methods the liquidus curve is found between 300 and 600 k higher than the solidus over the entire lower mantle at cmb pressures 135 gpa the chondritic mantle solidus and liquidus reach 4150 150 k and 4725 150 k respectively we discuss that the lower mantle is unlikely to melt in the d layer except if the highest estimate of the temperature profile at the base of the mantle which is associated with a very hot core is confirmed therefore recent suggestions of partial melting in the lowermost mantle based on seismic observations of ultra low velocity zones indicate either 1 a outer core exceeding 4150 k at the cmb or 2 the presence of chemical heterogeneities with high concentration of fusible elements our observations of a high liquidus temperature as well as a large gap between solidus and liquidus temperatures have important implications for the properties of the magma ocean during accretion not only complete melting of the lower mantle would require excessively high temperatures but also below liquidus temperatures partial melting should take place over a much larger depth interval than previously thought in addition magma adiabats suggest very high surface temperatures in case of a magma ocean that would extend to more than 40 gpa as suggested by siderophile metal silicate partitioning data such high surface temperature regime where thermal blanketing is inefficient points out to a transient character of the magma ocean with a very fast cooling rate

magma ocean formation due to giant impacts current understanding of the last stages of planetary accretion suggests that mass and energy accumulation are dominated by a few large impacts an important thermal consequence of such impacts is melting and formation of a large melt pond if the planet s isostatic adjustment time scale is short compared to the magma pond s cooling time scale the melt may be extruded onto the surface and form a magma ocean of approximately uniform depth although a giant impact striking at 10 15 km s 1 deposits enough energy to melt the entire planet the distribution of that energy is important in determining the thermal outcome of the collision we examine the thermal effects of giant impacts by estimating the melt volume generated by the initial shock wave and corresponding magma ocean depths additionally we examine the effects of the planet s initial temperature on the generated melt volume the hugoniot curve plotted in pressure entropy space is used to determine the shock pressure required to completely melt the material for room temperature dunite this pressure is about 150 gpa and the partial melt region is narrow for dunite initially at the solidus temperature this pressure is about 115 gpa the partial melt region extends throughout the planet once the melting pressure is known an impact melting model based on the second hugoniot equation the linear shock particle velocity relationship and the empirical particle velocity distance relationship is used to estimate the radial distance melting occurred from the impact site once the distance is found the melt region s geometry determines the associated melt volume partial melt volume is also estimated the melt fraction that is not excavated during crater formation is estimated and magma ocean depths resulting from both excavated and retained melt are calculated the model is also used to estimate the fraction of a planet melted by the initial shock wave nominal conditions of the moon forming giant impact projectile planet mass 0 14 impact speed 15 km s 1 generate melting of 30 65 of the planet depending on its initial temperature whole planet melting requires projectile planet mass ratios of 0 4 for a 15 km s 1 impact if the planet was near its solidus before the impact isostatic relaxation may generate a significant volume of additional melting

geochemical and textural investigations of the eoarchean ukaliq supracrustals northern quebec canada abstract structural geochronological and geochemical studies of pre 3 75 ga rocks of volcano sedimentary protoliths in the inukjuak domain in the superior province in quebec have been mostly focused on the nuvvuagittuq supracrustal belt nsb the porpoise cove outcrops at the southwestern limit of the nsb are the de facto type locality for the supracrustals of the inukjuak complex yet it remains unclear whether the nsb rocks are geochemically distinct from or are in fact common to a host of other supracrustal enclaves locked in the dominantly gneissic inukjuak domain here we report detailed textural and geochemical studies for a suite of rocks from the ukaliq supracrustal belt usb located approximately 3 km northeast of the nsb we find that the usb and nsb have a similar protracted metamorphic history both experienced amphibolite grade metamorphism and contain granitoid gneiss sheets quartz magnetite rocks banded iron formation s l and quartz biotite schists within amphibolitized rocks of basaltic affinity with local retrogressions if the various inukjuak supracrustal belts were once a part of a larger coherent now dismembered terrane they should show similar emplacement ages and source chemistry new zircon u pb geochronology from five gneissic units and two quartz biotite metasedimentary schists reveal the oldest emplacement ages across all units of each individual rock type to be 3 68 0 07 ga granitoid gneisses and 3 65 0 06 ga quartz biotite schists these new ages are similar to those documented as likely minimum emplacement ages of the nsb determined by u pb geochronology zircons from the quartz biotite schist were also analyzed by ion microprobe for their u pb geochronology and were found to yield statistically identical albeit more precise ages than those obtained by la icp ms possible detrital zircons from the usb quartz biotite schists were analyzed by ion microprobe for their coupled 30sinbs28 and 18ovsmow values with respective values between 0 75 and 0 07 30sinbs28 and 5 61 and 6 59 18ovsmow the 18ovsmow values which are on average above mantle derived zircon indicate contribution of altered non mantle derived material into the source of the rocks that weathered to form the quartz biotite schists zircon mineral inclusions quartz feldspar apatite biotite muscovite and other unrecognized fe al si rich phases along with the major and trace element contents for the rocks were analyzed to substantiate this interpretation together with 30sinbs28 18ovsmow our results suggest that lithologies like authigenic silica and serpentinized rocks re melted to form the parent melts that gave rise to zircons found in the usb quartz biotite schists additional lu hf studies reported here on the same zircons also show similarities with nsb zircons the ehf values showed a positive correlation with the measured u pb age from 22 7 0 8 to 1 9 1 1 the lu hf system also reveals that the usb extracted at ca 3 8 ga carries isotopic signatures of an older hadean reservoir having been formed from an eoarchean mafic melt that incorporated them taken together this supports a co genetic origin for the nsb and the usb

the sudbury structure controversial or misunderstood the origins of the sudbury structure and associated igneous complex have been controversial most models call for a major impact event followed by impact induced igneous activity although totally igneous models are still being proposed much of the controversy is due in our opinion to a misunderstanding of the size of the original sudbury structure by analogy with other terrestrial impact structures the spatial distribution of shock features and huronian cover rocks at the sudbury structure suggest that the transient cavity was 100 km in diameter which places the original final structural rim diameter in the range of 150 200 km theoretical calculations and empirical relationships indicate that the formation of an impact structure of this size will result in 104 km3 of impact melt more than sufficient to produce a melt body the size of the igneous complex present volume 4 8 103 km3 for the igneous complex to be an impact melt sheet it must have a composition similar to that of the target rocks evidence for this has been presented previously for sr and nd isotopic data which suggest a crustal origin here we also present new evidence from least squares mixing models that the average composition of the igneous complex corresponds to a mix of archean granite greenstone terrain with possibly a small component of huronian cover rocks this is a geologically reasonable mix based on the interpreted target rock geology and the geometry of melt formation in an impact event of this size the igneous complex is differentiated which is not a characteristic of previously studied terrestrial impact melt sheets this can be ascribed however to its great thickness and slower cooling that large impact melt sheets can differentiate has important implications for how the lunar samples and the early geologic history of the lunar highlands are interpreted if this working hypothesis is accepted namely that both the sudbury structure and the igneous complex are impact in origin then previous hybrid impact igneous hypotheses can be discarded and the sudbury structure can be studied specifically for the constraints it provides to large scale cratering and the formation of basin sized multiring impact structures

partial melting of an ancient sub continental lithospheric mantle in the early paleozoic intracontinental regime and its contribution to petrogenesis of the coeval peraluminous granites in south china abstract the appinite granite association has been found in various tectonic regimes related to recent subduction arc continent or continent continent collision and post collision orogen and appinites generally originate from recently subduction modified lithospheric mantle we conducted a study on a rarely reported appinite granite association formed in an intracontinental regime the zhangjiafang qinglongshan complex zqc in which the appinites were derived from an ancient sub continental lithospheric mantle sclm the zqc is located in the western wugongshan domain and consists of basaltic to intermediate appinites and granitoids ten dated samples including massive and gneissoid granitoids hornblende gabbro and diorite give zircon 206pb 238u ages ranging from 444 3 ma to 452 4 ma indicating that these various lithologies were emplaced synchronously the basaltic appinites show radiogenic 87sr 86sri 0 71016 0 71431 and negative end t 6 1 to 8 9 and zircon ehf t 4 2 to 7 5 values combined with regional geological background an origin from the neoproterozoic metasomatised sclm can be inferred for the appinites in the wugongshan domain the granitoids are peraluminous and almost high k calc alkaline to shoshonitic they exhibit a wide range of isotopic compositions 87sr 86sri 0 70828 0 71857 end t 6 2 to 10 5 zircon ehf t 9 5 to 26 6 some of the granitoids display the most evolved sr nd isotopic signatures among all the studied lithologies which are consistent with those of the middle to upper crust suggesting a pure crustal origin other granitoids show relatively mafic composition and less evolved isotopic signature the intermediate appinites have intermediate chemical compositions between those of the basaltic appinites and granitoids and similar sr nd isotopic compositions to those of the basaltic appinites that have relatively evolved composition and isotopic signature the petrographical and the geochemical studies as well as magma mixing and mingling between mafic magma and felsic magma observed in field work indicate that the zqc was formed by several complex magmatic processes including assimilation and fractional crystallization afc of a basaltic magma derived from an enriched mantle magma mixing and mingling fractional crystallization of a crust derived felsic magma the contribution of the neopreoterozoic subduction modified lithospheric mantle to the early paleozoic granitoids in south china has been poorly constrained due to its evolved isotopic signatures such as negative end t and ehf t values high initial ratio of 87sr 86sr and few studies on the early paleozoic basaltic felsic rock association our work here suggests that the wugongshan appinite granite association was formed in an intracontinental orogen and the ancient enriched mantle may have had a significant chemical contribution to the formation of some of the paleozoic peraluminous granites in south china

mantle source magma differentiation and sulfide saturation of the 637ma zhouan mafic ultramafic intrusion in the northern margin of the yangtze block central china abstract the sill like zhouan mafic ultramafic intrusion in the northern margin of the yangtze block formed at 637 4 ma and consists of a lherzolite unit and an olivine gabbronorite unit with a marginal zone between the lower margin of the olivine gabbronorite unit and the underlying country rocks composed of marble and schist of the meso to neoproterozoic zhujiashan formation rocks from the lherzolite unit have 41 4 44 0 wt sio2 and 29 0 38 4 wt mgo and contain olivine with fo values ranging from 80 to 85 mole indicating crystallization from a moderately evolved magma they commonly have we propose that moderately evolved magmas may have reached sulfide saturation due to fractionation of olivine in a deep seated staging magma chamber forming an olivine laden crystal mush with minor sulfide droplets in the lower part and more evolved pge depleted magmas in the upper part of the chamber the evolved pge depleted magmas were forced out of the staging chamber and entered the shallow magma chamber to form the olivine gabbronorite unit the lherzolite unit formed from the emplacement of the olivine laden crystal mush into the same chamber because of paucity of extensive crustal contamination only minor amounts of sulfide segregation occurred in the olivine gabbronorite unit and the marginal zone the zhouan intrusion thus may not be a good target for economic ni cu sulfide deposit

evidence of chemical heterogeneity within lunar anorthosite parental magmas abstract lunar anorthosites are known for displaying a limited range of plagioclase an content an 94 to 98 here we demonstrate that plagioclase trace element variations from apollo ferroan anorthosites fan samples collected by the apollo 15 and 16 missions display more significant chemical heterogeneity e g chondrite normalized la sm 0 33 5 42 than previously reported we report mineral plagioclase pyroxene and olivine major and trace element abundances for a suite of apollo fan samples in addition to anorthositic clasts within apollo 16 regolith breccias this suite of data extends the compositional range currently reported for apollo anorthosites and for anorthositic clasts previously found within lunar meteorites petrological classifications of the regolith breccia clasts e g anorthosite versus noritic anorthosites cannot always be accurately assessed due to the limited size

impact megadomes and the origin of the martian crustal dichotomy abstract we show that a sufficiently energetic impact can generate a melt volume which after isostatic adjustment and differentiation forms a spherical cap of crust with underlying depleted mantle depending on impact energy and initial crustal thickness a basin may be retained or impact induced crust may be topographically elevated retention of a martian lowland scale impact basin at impact energies 3 1028 3 1029 j requires an initial crustal thickness greater than 10 km formation of impact induced crust with size comparable to the martian highlands requires a larger impact energy 1 3 1030 j and initial crustal thickness

magmatism along the southeast margin of the yangtze block precambrian collision of the yangtze and cathysia blocks of china the time and the environment of formation of igneous rocks along the southern margin of the yangtze continental block in southern anhui province and northeastern jiangxi province china are discussed sm nd dating for samples from two ophiolite suites give ages of about 1 ga late proterozoic granodiorites in the study area have ages of about 0 9 ga and have normal intrusive relations against banxi group metamorphic country rocks trace element abundances and initial nd isotope ratios for ophiolite and proterozoic granodiorites are consistent with formation in an island arc environment in contrast sr and nd isotopes for mesozoic granitoids of the region suggest that they are products of remelting of crustal sources similar to the basement of the yangtze block these relations do not support a recently proposed mesozoic overthrust tectonic model for southeast china but rather favor a late precambrian collision history for the yangtze and cathysia blocks

turbulent mixing of metal and silicate during planet accretion and interpretation of the hf w chronometer in the current view of planet formation the final assembly of the earth involved giant collisions between proto planets 1000 km radius with the moon formed as a result of one such impact at this stage the colliding bodies had likely differentiated into a metallic core surrounded by a silicate mantle during the moon forming impact nearly all metal sank into the earth s core we investigate to what extent large self gravitating iron cores can mix with surrounding silicate and how this influences the short lived chronometer hf w used to infer the age of the moon we present fluid dynamical models of turbulent mixing in fully liquid systems attempting to place constraints on the degree of mixing erosion of sinking cores driven by rayleigh taylor instability does lead to intimate mixing and equilibration but large blobs 10 km diameter do not emulsify entirely emulsification is enhanced if most of the accreting metal cores deform into thin structures during descent through the earth s mantle yet only 1 20 of earth s core would equilibrate with silicate during earth s accretion the initial speed of the impactor is of little importance we proceed to evaluate the mixing potential for shear instabilities where silicate entrainment across vertical walls causes mixing the turbulent structure indicates that vortices remain at the largest scale and do not mix to centimeter length scale where diffusion operates and isotopes can equilibrate thus incomplete emulsification and equilibration of accreting iron cores is likely to occur the extent of metal silicate equilibration provides key information for interpretation of siderophile budgets and the timing of core formation using the hf w chronometer the time scale of core formation derived from the hf w chronometer is usually tied to the last major metal silicate re equilibration believed to coincide with time of the moon forming impact however we show that large cores have limited ability to reset the hf w system in the silicate earth excess 182 w in bulk silicate earth is more sensitive to early core formation processes than to radiogenic ingrowth after the last giant impact

thermal perturbations caused by large impacts and consequences for mantle convection united states national aeronautics and space administration planetary geology and geophysics program

zircon u pb geochronology and elemental and sr nd hf o isotopic geochemistry of post collisional rhyolite in the chiang khong area nw thailand and implications for the melting of juvenile crust volcanic rocks are widely exposed within the chiang khong lampang tak igneous zone in nw thailand a representative rhyolite sample from the chiang khong area yielded a zircon u pb age of 230 7 1 1 ma n 20 mswd 0 98 the chiang khong rhyolites are characterized by low tio2 0 29 0 62 wt and mgo 0 04 0 82 wt with a cnk values of 0 95 1 06 one outlier at 1 47 and can be classified as highly fractionated i type rhyolites they are enriched in liles and lrees and depleted in hfses two representative samples have 87sr 86sr i ratios of 0 70497 and 0 70527 and the end t values fall at 1 1 and 1 3 respectively ehf t and 18o in zircon are between 4 7 to 12 0 and 5 3 to 5 9 respectively our geochemical data suggest that the chiang khong rhyolites formed by partial melting of juvenile mafic lower crust in a post collisional setting deep crustal anatexis was probably induced by upwelling asthenospheric mantle shortly after slab detachment subsequent to closure of the paleo tethys

fluid dynamics of local martian magma oceans the evolution of a melt region produced by a large impact during mars formation is addressed while some impact induced melt is redistributed during crater excavation sufficiently large impacts much larger than basin forming impacts generate an intact melt region which is retained beneath the excavation zone i e a local magma ocean local magma ocean evolution depends on the effective rheology controlling large scale deformation of the solid part of the planet mechanism of crystallization and melt region size within the uncertainties of various parameters two scenarios are possible for sufficiently weak rheology or large melt region size evolution is characterized by rapid extrusion and formation of a global magma ocean for sufficiently strong rheology or small melt region size in situ crystallization to a partially molten solid state occurs prior to isostatic adjustment subsequent to in situ crystallization local magma ocean evolution depends on melt region size and efficiency of lateral redistribution compared to bulk conductive cooling for large melt regions lateral spreading occurs via plastic deformation and results in an asymmetric global partial melt layer for small melt region size viscous spreading viscous can result in bulk cooling below the solidus prior to formation of a global layer a hypothesis for the origin of the hemispherical crustal dichotomy and tharsis rise is suggested the dichotomy is associated with a global partial melt layer produced by evolution of a large local magma ocean after dichotomy formation evolution of a second smaller local magma ocean is related to tharsis development

isotopic outcomes of n body accretion simulations constraints on equilibration processes during large impacts from hf w observations abstract most estimates of planetary core formation timescales using hafnium tungsten hf w isotopes employ analytical expressions assuming either continuous planetary growth or instantaneous core formation in contrast dynamical modelling of planetary accretion suggests that the final stage of terrestrial planet formation is punctuated by multiple large and stochastic impacts such giant impacts have significant thermal and isotopic consequences we present a framework for calculating the hf w isotope evolution of individual bodies based on the results of an n body accretion simulation and assuming constant partition coefficients the results show that smaller bodies exhibit a larger range in isotopic values than larger bodies because the latter have suffered more impacts the analytical core formation timescales calculated using these isotopic values can differ very significantly from the timing of the final giant impact each planet actually experiences simulations in which 1 even the largest impactors undergo re equilibration with the target s mantle rather than the cores merging directly and 2 the original planetary embryos possessed radially variable iron silicate ratios produce results which are consistent with the observed physical and isotopic characteristics of inner solar system bodies varying w partition coefficients due to changing mantle oxidation state or initial planetesimal hf w ratios might produce similar isotopic outcomes and potentially permit core mergers without violating the isotopic constraints the style of re equilibration required suggests that magma oceans were present on mars sized and larger bodies an alternative for bodies of mars size and smaller is that the bulk of the mass was delivered as impactors much smaller than the target for mars we conclude that a prolonged 10 myr accretion process is both dynamically and isotopically plausible we also predict likely pd ag isotopic anomalies for vesta mars and earth size bodies

the origin of the moon the origin of the moon is considered within the theory of formation of the terrestrial planets by accumulation of planetesimals the theory predicts the occurrence of giant impacts suggesting that the moon formed after a roughly mars sized body impacted on the protoearth the impact blasted portions of the protoearth and the impacting body into geocentric orbit forming a prelunar disk from which the moon later accreted although other mechanisms for formation of the moon appear to be dynamically impossible or implausible fundamental questions must be answered before a giant impact origin can be considered both possible and probable

formation of the giant chalukou porphyry mo deposit in northern great xing an range ne china partial melting of the juvenile lower crust in intra plate extensional environment abstract the chalukou porphyry mo deposit 2 46 mt 0 087 mo located in the northern great xing an range ne china is the largest mo deposit discovered in china so far the host rocks consist of aplite porphyry granite porphyry and quartz porphyry and are intruded into lower ordovician intermediate felsic volcanic sedimentary rocks and pre ore monzogranite and are cut by post ore feldspar porphyry diorite porphyry and quartz monzonite porphyry here we present the zircon u pb ages whole rock geochemistry sr nd isotopic and zircon hf isotopic data for the pre ore syn ore and post ore intrusive rocks the chalukou ore forming porphyries intruded during 147 148 ma and have high silica alkali rich metaluminous to slightly peraluminous compositions and are oxidized they are enriched in large ion lithophile elements e g k rb u and th light ree and depleted in high field strength elements e g nb p and ti depletions in eu ba sr nb ta p and ti suggest that they have experienced strong fractional crystallization of plagioclase biotite hornblende and accessory minerals the pre ore monzogranite 172 ma also belongs to the high k calc alkaline series highly fractionated ree patterns la yb n 19 6 21 7 high values of sr y 54 69 and la yb 29 32 are adakite like geochemical features the post ore rocks 141 128 ma have similar geochemical characteristics with ore forming porphyries except that quartz monzonite porphyry shows no ba sr negative anomaly all intrusive rocks have relative low initial 87sr 86sr 0 705413 0 707889 and end t values 1 28 to 0 92 positive ehf t values 2 4 to 10 1 and young two stage nd and hf model ages tdm2 nd 863 977 ma tdm2 hf 552 976 ma these geochemical and isotopic data are interpreted to demonstrate that the ore forming porphyries formed by partial melting of the juvenile lower crust caused by underplating of mafic magmas in an intra plate extensional setting the pre ore monzogranite formed by partial melting of thickened lower crust in a collisional setting caused by closure of mongol okhotsk ocean the post ore feldspar porphyry shares a similar magma source with ore forming porphyry but the quartz monzonite porphyry has a relatively deeper magma source region and has not experienced as much fractional crystallization the transformation from middle jurassic compression to late jurassic extension created favorable conditions for the generation and emplacement of the ore forming magma the juvenile lower crust provided the main source of molybdenum for chalukou deposit enrichment of mo by fractional crystallization played an important role in concentrating mo during formation of the chalukou mo deposit the age 147 ma high fluorine and associated pb zn deposits are all different from other major porphyry mo deposits in ne china chalukou is a new mineral deposit type in the great xing an range

lunar magma ocean crystallization revisited bulk composition early cumulate mineralogy and the source regions of the highlands mg suite abstract crystallization of the lunar magma ocean lmo has been numerically modeled and its products inferred from sample observations but it has never been fully tested experimentally this study is a reexamination of the lmo hypothesis by means of the first experimental simulation of lunar differentiation two end member bulk moon compositions are considered one enriched in refractory lithophile elements relative to earth and one with no such enrichment a two stage model of magma ocean crystallization based on geophysical constraints is simulated and features early crystal suspension and equilibrium crystallization followed by fractional crystallization of the residual magma ocean an initially entirely molten moon is assumed part 1 of this study presented here focuses on stage 1 of this model and considers the early cumulates formed by equilibrium crystallization differences in mantle mineralogy resulting from different bulk moon compositions and implications for the source regions of the highlands mg suite refractory element enriched bulk moon compositions produce a deep mantle that contains garnet and trace cr spinel in addition to low ca pyroxene and olivine in contrast compositions without refractory element enrichment produce a deep dunitic mantle with low ca pyroxene but without an aluminous phase the differences in bulk composition are magnified in the residual melt the residual lmo from the refractory element enriched composition will likely produce plagioclase and ilmenite earlier and in greater quantities both compositions produce mg rich early cumulate piles that extend from the core mantle boundary to 355 km depth if 50 equilibrium crystallization and whole moon melting are assumed these early lmo cumulates provide good fits for the source regions for a component of the high mg ni and co poor parental magmas of the mg suite cumulates if certain conditions are called upon the olivine in early lmo cumulates produced by either bulk moon composition is far too rich in cr to be reasonable for the source regions of the mg suite meaning either core formation in the presence of s and or c must be invoked to deplete the lmo and the crystallizing olivine in cr or that current estimates of the bulk lunar cr content are too high we infer that melts meeting the criteria of the mg suite parents could be produced from early lmo cumulates by solid state kreep and plagioclase hybridization near the base of the crust and subsequent partial melting additionally we propose a revised model for mg suite petrogenesis

petrogenesis of the jiaoziding granitoids and associated basaltic porphyries implications for extensive early neoproterozoic arc magmatism in western yangtze block abstract middle neoproterozoic ca 860 750 ma granitoids are widely distributed in the western margin of the yangtze block china yet their magma sources and tectonic settings are unclear the geochronology and geochemistry of the granitoids and associated basaltic porphyries which intruded the 970 ma tongmuliang arc volcanic rocks in the jiaoziding area east of pingwu county were investigated in this study la icp ms zircon u pb dating indicates that the jiaoziding granitoids and basaltic porphyries were formed at 795 6 ma and 790 20 ma respectively the granitoids have high sio 2 69 2 76 9 wt k 2 o 2 3 5 6 wt and na 2 o 3 2 5 1 wt contents and a low al 2 o 3 12 4 14 5 wt content the basaltic porphyries contain high concentrations of tio 2 3 wt and high field strength elements have steep rare earth element patterns and are depleted in nd and hf isotopes batch partial melting modelling indicates that the jiaoziding granitoids could have been derived by 5 and 50 70 partial melting of tongmuliang mafic rocks and quartz keratophyres respectively formation of the basaltic porphyries by melting of upwelling asthenospheric mantle would have been facilitated by extensive lithospheric delamination during the neoproterozoic this study established a link between mid neoproterozoic granitic magmatism and 970 ma juvenile arc crust indicating that extensive early neoproterozoic juvenile arc crust and partial melting of this crust in an extensional setting favoured the formation of middle neoproterozoic granitic rocks along the w nw margin of the yangtze block

differential melt scaling for oblique impacts on terrestrial planets abstract analytical estimates of melt volumes produced by a given projectile and contained in a given impact crater are derived as a function of impact velocity impact angle planetary gravity target and projectile densities and specific internal energy of melting applications to impact events and impact craters on the earth moon and mars are demonstrated and discussed the most probable oblique impact 45 produces 1 6 times less melt volume than a vertical impact and 1 6 and 3 7 times more melt volume than impacts with 30 and 15 trajectories respectively the melt volume for a particular crater diameter increases with planetary gravity so a crater on earth should have more melt than similar size craters on mars and the moon the melt volume for a particular projectile diameter does not depend on gravity but has a strong dependence on impact velocity so the melt generated by a given projectile on the moon is significantly larger than on mars higher surface temperatures and geothermal gradients increase melt production as do lower energies of melting collectively the results imply thinner central melt sheets and a smaller proportion of melt particles in impact breccias on the moon and mars than on earth these effects are illustrated in a comparison of the chicxulub crater on earth linked to the cretaceous tertiary mass extinction gusev crater on mars where the mars exploration rover spirit landed and tsiolkovsky crater on the moon the results are comparable to those obtained from field and spacecraft observations other analytical expressions and hydrocode simulations

compositional dependence of structural transition pressures in amorphous phases with mantle related compositions properties of silicate melts are key to understanding the evolution of the mantles of the earth and terrestrial planets although remarkable progress has been made in first principle calculations for melts in recent years structural measurements of silicate melts at in situ high p t remain one of the most challenging tasks the study of glasses kinetically frozen melts at high pressure can provide valuable insights into related melts in the mantle we report raman scattering of mgsio 3 glass revealing a structural transition at 19 38 gpa which is associated with increases in the si o coordination number and another transition at 65 70 gpa however in casio 3 and mg 2 sio 4 glasses the former transition occurs at higher pressures by 5 10 gpa and the latter transition is not observed to our maximum pressure 80 gpa indicating that a less polymerized si o network increases the transition pressures our results suggest that the pressure for the structural transitions in these glasses is influenced strongly by the concentration of network former cations and the ionic size of the network modifiers this observation may have important implications for compositional differentiation in the early magma ocean and the present day mantle

lifting the cover of the cauldron convection in hot planets convection models of planetary mantles do not usually include a specific treatment of near surface dynamics in all situations where surface dynamics is faster than internal dynamics the lateral transport of material at the surface forbids the construction of a topography that could balance the internal convective stresses this is the case if intense erosion erases the topography highs and fills in the depressions or if magma is transported through the lithosphere and spreads at the surface at large distances in these cases the usual boundary condition of numerical simulations that the vertical velocity cancels at the surface should be replaced by a condition where the vertical flux on top of the convective mantle equilibrates that allowed by the surface dynamics we show that this new boundary condition leads to the direct transport of heat to the surface and changes the internal convection that evolves toward a heat pipe pattern we discuss the transition between this extreme situation where heat is transported to the surface to the usual situation where heat diffuses through the lithosphere this mechanism is much more efficient to cool a planet and might be the major cooling mechanism of young planets even the modest effect of material transport by erosion on earth is not without effect on mantle convection and should affect the heat flow budget of our planet

thermal effects of late accretion to the crust and mantle of mercury abstract impact bombardment on mercury in the solar system s late accretion phase ca 4 4 3 8 ga caused considerable mechanical chemical and thermal reworking of its silicate reservoirs crust and mantle depending on the frequency size and velocity of such impactors effects included regional and global scale crustal melting and thermal perturbations of the mercurian mantle we use a 3d transient heating model to test the effects of two bombardment scenarios on early pre tolstojan mercury s mantle and crust results show that rare impacts by the largest 100 km diameter bodies deliver sufficient heat to the shallow mercurian mantle producing high temperature ultra magnesian komatiitic s s melts impact heating leading to effusive flood volcanism can account for the eponymous high magnesium region hmr observed during the mercury surface space environment geochemistry ranging messenger mission we find that late accretion to mercury induced volumetrically significant crustal melting 58 vol mantle heating and melt production which combined with extensive resurfacing 100 also explains why its oldest cratering record was effectively erased consistent with crater counting statistics

neoproterozoic arc related mafic intrusions along the northern margin of south china implications for the accretion of rodinia abstract south china has been considered as part of the rodinian supercontinent during neoproterozoic time although its paleogeographic position within this supercontinent is still a matter of debate the wangjiangshan and bijigou complexes along the northern margin of south china are among the largest mafic intrusions in china new shrimp zircon u pb results indicate that these two intrusions have crystallization ages of 820 ma and 780 ma respectively enrichment of large ion lithophile and light rare earth elements and depletion of high field strength elements in these intrusions suggest derivation from an active arc along a continental margin this interpretation of these intrusions as part of a continental arc assemblage is in contrast with the previous view that they were products of a neoproterozoic mantle plume that initiated the breakup of rodinia the presence of a neoproterozoic magmatic arc suggests that rodinian oceanic lithosphere was subducted beneath the present northern margin of south

amphibole out phase boundary in partially melted metabasalt its control over liquid fraction and composition and source permeability a number of experimental studies in recent years have demonstrated that partial melting of hydrated basalt at lower crustal to upper mantle pressures is capable of generating up to 40 high sio2 liquids closely comparable in many geochemical respects to tonalitic trondhjemitic granitoids ttg from these studies it is apparent that the initiation of melting coincides with the beginning of amphibole dehydration but that a broad reaction interval over which amphibole coexists with high sio2 liquids is defined by the wet basalt solidus on one side and the amphibole out phase boundary on the other the phase relations of an alkali rich tholeiitic metabasalt have been examined in the outer half of this region up to and beyond the amphibole out phase boundary results indicate that amphibole exerts a strong control over the amount and composition of coexisting liquid over this interval melt fraction increases slowly and gradually from 0 up to 20 30 between the wet basalt solidus and up to amphibole out but increases substantially from 30 to 50 across and immediately beyond it liquids are strongly to moderately peraluminous molar al2o3 cao na2o k2o a cnk 1 0 up to amphibole out neutral at the boundary a cnk 1 0 and increasingly metaluminous a cnk 1 0 beyond it up to amphibole out potassium is strongly partitioned into liquid relative to amphibole i e mineral melt kd 0 2 0 3 whereas titanium is strongly partitioned into liquid kd 2 0 6 0 physical and chemical criteria suggest that efficient melt segregation leading to ttg plutonism requires more than 20 30 batch melting of a garnet bearing basaltic protolith near or beyond the amphibole out phase boundary leaving dry residues of eclogite or garnet granulite and producing metaluminous liquids that are strongly depleted in heavy rare earth elements hrees and yttrium y lower degrees of melting may involve critical melting where melt is squeezed out of the residue by deformation of the crystalline matrix resulting in disequilibrium melt compositions

impact origin for the martian crustal dichotomy half emptied or half filled 1 one possible mechanism for martian hemispherical dichotomy formation is excavation of the northern lowland basin in the existing crust by a single giant impact here we explore a related scenario in which the impact melt volume is sufficient to fill the excavated cavity produce a thickened crust and generate a topographic basin centered on the opposite side of the planet we investigate this mechanism by formulating a model for viscous spreading of a partially molten layer over a spherical surface we derive numerical solutions and calculate the final area of the spreading layer as a function of impactor size and shape of the layer boundary due to an initially asymmetric melt region results are compared with observations including lowland basin size shape of the dichotomy boundary age difference between crustal provinces and crustal thickness

a model for the layered upper mantle abstract numerical modeling of mantle convection by liu 1994 science 264 1904 1907 favors a two layer convection if the results are reinterpreted for the correct phase relations in mg fe 2sio4 the resulting chemical isolation of the upper and lower mantle suggests a highly differentiated and layered upper mantle to account for the discrepancy between the observed compositions of mantle xenoliths and the cosmic abundances of elements it is shown that a layered upper mantle with a hidden reservoir can have a structure consistent with the observed seismic velocity profiles and an average bulk composition corresponding to the cosmic abundances the evolution of the upper mantle and the origin of komatiites are discussed in the context of the proposed model

origin and tectonic significance of a mesozoic multi layer over thrust system within the yangtze block south china abstract in the yangtze block south china a well developed mesozoic thrust system extends through the xuefeng and wuling mountains in the southeast to the sichuan basin in the northwest the system comprises both thin and thick skinned thrust units separated by a boundary detachment fault the dayin fault to the northwest the thin skinned belt is characterized by either chevron anticlines and box synclines to the northwest or chevron synclines to the southeast the former structural style displays narrow exposures for the cores of anticlines and wider exposures for the cores of synclines thrust detachments occur along silurian fs and lower cambrian fc strata and are dominantly associated with the anticlines to the southeast this style of deformation passes gradually into one characterized by chevron synclines with associated principal detachment faults along silurian fs cambrian fc and lower sinian fz strata there are however numerous secondary back thrusts therefore the thin skinned belt is like the valley and ridge province of the north american applachian mountains the thick skinned belt structurally overlies the thin skinned belt and is characterized by a number of klippen including the xuefeng and wuling nappes it is thus comparable to the blue ridge province of appalachia the structural pattern of this thrust system in south china can be explained by a model involving detachment faulting along various stratigraphic layers at different stages of its evolution the system was developed through a northwest stepwise progression of deformation with the earliest delamination along lower sinian strata fz analyses of balanced geological cross sections yield about 18 1 21 total 88 km shortening for the thin skinned unit and at least this amount of shortening for the thick skinned unit the compressional deformation from southeast to northwest during late jurassic to cretaceous time occurred after the westward progressive collision of the yangtze block with the north china block and suggests that the orogenic event was intracontinental in nature

origin of the ganymede callisto dichotomy by impacts during the late heavy bombardment jupiter s large moons ganymede and callisto are similar in size and composition but different in surface and interior characteristics simulations with geophysical models of core formation indicate that the difference in impact energy received by the two satellites during the period of late heavy bombardment can explain the dichotomy jupiter s large moons ganymede1 2 and callisto2 3 are similar in size and composition however ganymede has a tectonically evolved surface1 and a large rock metal core2 whereas callisto s surface shows no sign of resurfacing3 and the separation of ice and rock in its interior seems incomplete2 these differences have been difficult to explain4 5 6 7 8 9 10 11 here we present geophysical models of impact induced core formation to show that the ganymede callisto dichotomy can be explained through differences in the energy received during a brief period of frequent planetary impacts about 700 million years after planet formation termed the late heavy bombardment12 13 14 15 we propose that during the late heavy bombardment impacts would have been sufficiently energetic on ganymede to lead to a complete separation of rock and ice but not on callisto in our model a dichotomy between ganymede and callisto that is consistent with observations is created if the planetesimal disk that supplied the cometary impactors during the late heavy bombardment is about 5 30 times the mass of the earth our findings are consistent with estimates of a disk about 20 times the mass of the earth as used in dynamical models that recreate the present day architecture of the outer solar system and the lunar late heavy bombardment15 16

early crustal evolution in the western yangtze block evidence from u pb and lu hf isotopes on detrital zircons from sedimentary rocks abstract in situ u pb and lu hf data on detrital zircons from paleo to neoproterozoic sediments have been used to gain a clearer picture of the regional tectonic background and crustal evolution in the western part of the yangtze block the youngest concordant zircon ages for sedimentary units suggest maximum depositional ages of 1014 ma for the kunyang group 750 649 ma for the sinian sequences and 525 ma for the cambrian sediments the yinmin formation previously assigned to the middle part of the kunyang group was actually deposited after 1667 ma and contains zircons whose ages are dominantly from late archean to paleoproterozoic 2 8 2 7 ga 2 5 2 3 ga and 1 85 ga the heishantou formation in the lower part of the kunyang group has two major age populations of 1 0 ga and 1 8 1 6 ga the sinian and cambrian sedimentary rocks are dominated by neoproterozoic zircons with age peaks at 760 ma and 825 ma consistent with the ages of widespread igneous rocks around the yangtze block the lu hf isotope data suggest that a significant juvenile input took place during archean and neoproterozoic times respectively while crustal reworking was dominant during the paleoproterozoic time the archean paleoproterozoic detritus is isotopically distinct from the archean paleoproterozoic basement exposed in the northern part of the yangtze block suggesting that a subarea of old crust lies beneath the young sediments covering the craton the presence of abundant 1 85 ga zircons suggests that the yangtze block was probably part of the columbia supercontinent during paleoproterozoic time comparisons to the other parts of columbia suggest that the yangtze block could have been adjacent to the north china craton and or australia

accretion timescale and impact history of mars deduced from the isotopic systematics of martian meteorites abstract high precision sm nd isotopic analyses have been completed on a suite of 11 martian basaltic meteorites in order to better constrain the age of silicate differentiation on mars associated with the formation of their mantle sources these data are used to evaluate the merits and disadvantages of various mathematical approaches that have been employed in previous work on this topic ages determined from the sm nd isotopic systematics of individual samples are strongly dependent on the assumed nd isotopic composition of the bulk planet this assumption is problematic given differences observed between the nd isotopic composition of earth and chondritic meteorites and the fact that these materials are both commonly used to represent bulk planetary nd isotopic compositions ages determined from the slope of 146sm 142nd whole rock isochrons are not dependent on the assumed 142nd 144nd ratio of the planet but require the sample suite to be derived from complementary contemporaneously formed reservoirs in this work we present a mathematical expression that defines the age of formation of the source regions of such a suite of samples that is based solely on the slope of a 143nd 142nd whole rock isochron and is also independent of any a priori assumptions regarding the bulk isotopic composition of the planet this expression is also applicable to mineral isochrons and has been used to successfully calculate 143nd 142nd model crystallization ages of early refractory solids as well as lunar samples this permits ages to be obtained using only nd isotopic measurements without the need for 147sm 144nd isotope dilution determinations when used in conjunction with high precision nd isotopic measurements completed on martian meteorites this expression yields an age of formation of the martian basaltic meteorite source regions of 4504 6 ma because the sm nd model ages for the formation of martian source regions are commonly interpreted to record the age at which large scale mantle reservoirs formed during planetary differentiation associated with magma ocean solidification the age determined here implies that magma ocean solidification occurred several tens of millions of years after the beginning of the solar system recent thermal models however suggest that mars sized bodies cool rapidly in less than 5 ma after accretion ceases even in the presence of a thick atmosphere assuming these models are correct an extended period of accretion is necessary to provide a mechanism to keep portions of the martian mantle partially molten until 4504 ma late accretional heating of mars could either be associated with protracted accretion occurring at a quasi steady state or alternatively be associated with a late giant impact if this scenario is correct then accretion of mars sized bodies takes up to 60 ma and is likely to be contemporaneous with the core formation and possibly the onset of silicate differentiation this further challenges the concept that isotopic equilibrium is attained during primordial evolution of planets and may help to account for geochemical evidence implying addition of material into planetary interiors after core formation was completed

subduction related origin of the 750 ma xuelongbao adakitic complex sichuan province china implications for the tectonic setting of the giant neoproterozoic magmatic event in south china abstract the xuelongbao plutonic complex in the eastern margin of the tibetan plateau is dated at 748 7 ma using the shrimp zircon u pb method and represents part of the neoproterozoic igneous assemblage of south china rocks in the complex include tonalite and granodiorite and have sio2 ranging from 62 0 to 74 8 wt and al2o3 from 14 3 to 20 9 wt their na2o contents range from 4 2 to 6 7 wt and k2o from 0 47 to 1 96 wt indicating that they belong to the na series these rocks show chondrite normalized ree patterns depleted in hree and variably enriched in lree they have positive sr and negative nb and ti anomalies in the primitive mantle normalized trace elemental spider diagram their sr contents range from 320 to 780 ppm and y contents are lower than 10 ppm resulting in high sr y ratios 52 320 characteristic of typical adakites their end t and initial sr isotopic compositions range from 0 36 to 2 88 and from 0 7033 to 0 7054 respectively the geochemical features of the xuelongbao plutonic complex are consistent with an origin from adakitic magmas that were likely derived from partial melting of a subducted oceanic slab together with arc signatures of other granites and mafic intrusions in the region the 750 ma xuelongbao adakitic complex provides evidence for a major subduction related neoproterozoic magmatic event of south china

the flaw in the crustal zircon archive mixed hf isotope signatures record progressive contamination of late stage liquid in mafic ultramafic layered intrusions

scaling of melt production in hypervelocity impacts from high resolution numerical simulations the volume of melt produced in hypervelocity planetary impacts and the size and shape of the melted region are key to understanding the impact histories of solid planetary bodies and the geological effects of impacts on their surfaces and interiors prior work of pierazzo et al pierazzo e vickery a m melosh h j 1997 icarus 127 408 423 gave the first estimates of impact melt production in geological materials using a modern hydrocode and equation of state however computational limits at the time forced use of low resolution which may have resulted in low melt volumes our simulations with 50 times higher resolution provide independent confirmation of the pierazzo et al pierazzo e vickery a m melosh h j 1997 icarus 127 408 423 melt volumes in aluminum iron dunite and granite impacts at velocities between 20 and 80 km s in ice ice impacts we find that melt volumes depend on target temperature and are lower than predicted by pierazzo et al pierazzo e vickery a m melosh h j 1997 icarus 127 408 423 our melt volumes are directly proportional to impact energy for all materials over a wide range of impact velocity we also report new data for melt volume scalings for ice dunite and iron dunite impacts and the size and shape of melted region valuable for interpretation of cratering records and studies of impact induced differentiation

how to make a planet an introduction the special collection building planets the dynamics and geochemistry of core formation aims to combine cutting edge experimental analytical and modeling results with review articles defining the state of the science and current challenges to our understanding of the origin geophysics and geochemistry of planetary cores our goal is to highlight novel and interdisciplinary approaches that address aspects of core formation and evolution at the atomic grain and planetary scales

magma mixing in the genesis of hercynian calc alkaline granitoids an integrated petrographic and geochemical study of the sazava intrusion central bohemian pluton czech republic the devonian early carboniferous 354 1 3 5 ma conventional zircon u pb age sazava intrusion biotite amphibole quartz diorite tonalite and granodiorite of the central bohemian pluton cbp associated with bodies of olivine pyroxene amphibole gabbro gabbrodiorite quartz diorite and rare hornblendite gives an opportunity for a comparative study of a rather shallow calc alkaline magma mixing zone at two levels separated by a vertical difference of approximately 1 km the deeper section p ibram displays the direct evidence for the existence of a long lived periodically tapped and replenished floored magma chamber masli the contacts between the subhorizontal sheet like basic bodies and the surrounding commonly cumulus rich sazava granitoid are arcuate and cut by a series of veins and ascending pipes shallow dipping swarms of strongly elongated and flow aligned mafic microgranular enclaves mme concordant with the contacts of the basic bodies are commonplace the higher level teletin section shows relatively independent basic intrusions some of them distinctly hybrid in character and mainly of quartz dioritic composition surrounded by relatively homogeneous nearly cumulus free sazava tonalite rich in texturally variable mme larger quartz microdiorite bodies and the mme both interpreted as hybrids contain varying proportions of highly heterogeneous plagioclase megacrysts with complex zoning particularly well shown by cathodoluminescence cl most often the megacrysts have cores of labradorite anorthite partly resorbed and overgrown by andesine rims but some are strongly brecciated and fragments have been annealed by rim growth also characteristic are long prisms of apatite oikocrysts of quartz and k feldspar and zoned amphibole the latter has brown pargasite and magnesiohastingsite cores resorbed and overgrown by magnesiohornblende compositionally similar to the amphibole in the sazava tonalite the brown cores are probably relics of a higher p t stage with temperature exceeding 900 c plots of major element analyses of the teletin quartz microdiorite and the mme fall close to the line joining the compositions of the evolved sazava granodiorite tonalite and the associated gabbros the estimated minimal proportions of the acidic end member range from ca 15 to 20 for the quartz microdiorite and ca 40 to 60 for the enclaves field relationships macroscopic and mineralogical features and whole rock geochemistry are consistent with a hybrid origin of the quartz microdiorite masses and enclaves unzoned calcic plagioclase cores are interpreted as having crystallized from a basic magma prior to its injection into a high level acidic magma chamber the sudden decrease in ca activity and introduction of water due to magma mixing with tonalitic melt led to their resorption and brecciation quartz ocelli that are abundant locally in some basic bodies are most likely xenocrysts introduced from the felsic magma in the basic melt they were resorbed and became a substrate for heterogeneous nucleation of the amphibole or pyroxene coronas strong undercooling of the basic magma resulted in the growth of long prismatic apatite together with dendritic boxy cellular plagioclase both types of plagioclase cores were subsequently overgrown by more sodic mantles at the same time small unzoned plagioclase crystals nucleated in the groundmass following thermal re adjustment crystallization of the hybrid magma ended with considerably slower growth of quartz and k feldspar oikocrysts

the longmenshan tectonic complex and adjacent tectonic units in the eastern margin of the tibetan plateau a review abstract the longmenshan tectonic complex lstc along the eastern margin of the tibetan plateau the site of devastating earthquakes such as the magnitude 8 0 wenchuan earthquake on 12 may 2008 preserves an exceptionally complete history of the tectonic evolution of the yangtze block and its relations to adjacent tectonic units due to sequential tectonic superposition and tectonic reactivation the tectonic nature of the lstc and in particularly the older history has been profoundly debated and many different tectonic models have been proposed herein we summarize the current understandings of the major tectonic events that have shaped this important tectonic complex highlighting problems left to be solved by future work including 1 the nature and constraints for at least 6 regional tectonic events i e building of the metamorphic basement art the columbia nuna supercontinent pt1t the rodinia supercontinent pt3t the paleozoic passive continental margin pzt the paleotethys orogeny mzt and the neotethys orogeny kzt 2 metamorphic basement exposures and their tectonic implications including rock types and geochronological constraints for the archean paleoproterozoic and neoproterozoic basements 3 nature of the present lstc and its affinity with adjacent tectonic units 4 consideration of the ne striking longmenshan thrust belt and arcuate shape yanyuan muli thrust belt as parts of a single tectonic feature 5 mountain basin coupled systems recording past tectonic eposides we draw the following conclusions and tectonic models based on published research combined with our own recent studies 1 the well preserved archean yudongzi gneiss group in the lstc has a genetic affinity with the kongling group and thus belongs to the yangtze block 2 the paleoproterozoic hejiayan group juxtaposed adjacent to the archean yudongzi group may represent a 2000 1800 ma orogenic belt which corresponds to the supercontinent nuna columbia amalgamation event 3 a neoproterozoic trench arc basin system which is reconstructed based on identification of a neoproterozoic ophiolite complex arc type magmatic rock assemblages and volcaniclastic basinal deposits along the western margin of the yangtze block and the lstc may represent the record of eastward subduction of the neoproterozoic mozambique oceanic lithosphere beneath the yangtze block during the assembly of the rodinia supercontinent 4 a complete bidirectional wilson cycle was reconstructed by the formation of the late permian to the middle late triassic back arc ganze litang rift and ocean following the early paleozoic mianlue continental rift and ocean and subsequent closure of the ocean basin by simultaneous bidirectional northward and southwestward subduction and later collision this relatively uncommon bidirectional wilson cycle might be attributed to the formation of the three armed rift system in the eastern paleotethys associated with the late permian eemeishan large igneous province in the lstc 5 a three stage tectonic sequence of in sequence imbricate thrust in the lstc during india eurasian collisional orogeny at 55 15 ma extrusion from 15 to 5 ma and the plateau uplift since 5 ma resulting from lower crustal channel flow is proposed for the formation of the present lstc

the yanbian terrane southern sichuan province sw china a neoproterozoic arc assemblage in the western margin of the yangtze block abstract the magmatic and tectonic history of the yangtze block and its possible affinity with other neoproterozoic arc terranes are important elements in the reconstruction of neoproterozoic plate tectonics the yanbian terrane in the western margin of the yangtze block is a typical arc assemblage composed of a flysch type sedimentary sequence intruded by gabbroic and granodioritic plutons the sedimentary sequence consists chiefly of tuffaceous material with interlayered chert sandstone and pillow basalts laser ablation icp ms u pb dating of detrital zircons from the sandstones yield ages as young as 840 ma the gaojiacun and lengshuiqing mafic intrusions are dated at 812 3 ma and 806 4 ma respectively using the shrimp zircon u pb technique geochemical data show that both the gaojiacun and lengshuiqing intrusions have arc signatures with nd t values of 1 5 to 6 0 initial 87 sr 86 sr ratios of 0 705 0 706 and pronounced negative nb ta and zr hf anomalies their geochemical variations are best explained by fractional crystallization without major crustal contamination the yanbian terrane represents a typical arc assemblage formed on the western edge of the yangtze block during neoproterozoic time the sedimentary sequence was deposited in an oceanic setting probably in a back arc basin environment the depleted subduction modified lithospheric mantle wedge above the subduction zone was the source of melts from which the mafic plutons were crystallized this scenario suggests subduction of oceanic lithosphere eastward present day orientation underneath the yangtze block

geochemical evidence for proterozoic continental arc and continental margin rift magmatism along the northern margin of the yangtze craton south china abstract middle to late proterozoic volcanic intrusive magmatism of continental island arc and continental margin rift nature is well developed in the xixiang beiba area shaanxi province along the northern margin of the yangtze craton in the xixiang area volcanism evolved temporally from submarine low k arc tholeiite through calc alkaline to subaerial alkali or shoshonitic this was accompanied by gradational enrichment of lree and zr an increasing negative eu anomaly as well as strong depletion of compatible elements cr co v and sc the associated hannan intrusive complex evolved from gabbro through tonalite trondhjemite granodiorite to granite with the distribution of basic to felsic plutons suggesting arc polarity there is also a systematic lateral increase in k 2 o content k 2 o na 2 o ratio and la n yb n ratio towards the interior of the yangtze craton in these rocks this spatial framework and compositional variation resembles that found in circum pacific island arcs or active continental margins in contrast both volcanic and intrusive rocks in the beiba area are characterized by alkaline compositions and a bimodal habit they are composed of tholeiite gabbro and alkali rhyodacite rhyolite alkali granite and lack rocks of intermediate composition they may have formed in a continental margin rift setting geological and geochemical evidence further suggests that the xixiang paleo arc was a mature continental arc built on older sialic basement the emplacement of voluminous tonalites and trondhjemites thickened the crust and may have marked an important continent building event at the northern margin of the yangtze craton in the middle to late proterozoic

assessing uncertainty in geochemical models for core formation in earth abstract unraveling the conditions at which earth s metallic iron core formed yields important information about earth s early accretion and differentiation history multi variable statistical modeling of siderophile element partitioning between core forming metallic liquids and silicate melts form the basis for physical models of core formation while it seems clear that core segregation in a deep peridotitic magma ocean is generally consistent with many mantle siderophile element abundances there is considerable disparity among extant physical models in terms of the key parameters of pressure temperature and oxygen fugacity at which the core formed moreover there is ongoing debate over whether simple single stage equilibrium or more complex multi stage accretion models are required by the partitioning data here we consider how variations in the statistical regression of partitioning data affect the outcomes of physical models for core formation taking extant experimental data sets for four well studied siderophile elements ni co w and v as examples we find that the regression model exerts a fundamental control on physical model outcomes further the experimental data are currently too imprecise to discriminate among various single stage and continuous core formation scenarios progress in the development of physical models requires better isolation of the independent variables that affect partition coefficients and verification of activity models at high pressure and temperature in order to reduce the global uncertainty in multi variable statistical models

geochemical characteristics and organic matter enrichment mechanism of black shale in the upper triassic xujiahe formation in the sichuan basin implications for paleoweathering provenance and tectonic setting abstract the first third and fifth members henceforth referred to as t 3 x 1 t 3 x 3 and t 3 x 5 respectively of the upper triassic coal bearing xujiahe formation black shale are among the most significant hydrocarbon source rocks in the sichuan basin here we present geochemical data for the upper triassic black shales from core from well ld 1 to determine their paleoenvironmental conditions paleoweathering provenance transitions and tectonic setting the v v ni vs u th v cr vs u th and ni co vs u th bivariate plots and the tfe toc ts total fe total organic carbon total sulphur ternary diagram indicate that the synsedimentary redox regime of almost all the shale samples was oxidizing ba al sr al and p ti data combined with quantitative biogenic ba data indicate that moderate high primary paleoproductivity levels prevailed during deposition the sr cu ratio and c value combined with the sedimentary features are indicative of warm humid climate conditions the ti al deposition rate proxy and decompacted sedimentation rate are positively correlated with the total organic carbon toc content according to our multiproxy approach black shale development in the xujiahe formation was mainly controlled by the primary productivity level foreland basin setting high tectonic subsidence and sedimentation rate and exhibited a limited correlation with water column redox conditions through the compilation and calculation of various weathering indices for the xujiahe black shale we suggest that caution must be taken when inferring paleoclimate characteristics based on the chemical index of alteration cia because of the influence of multiple nonweathering factors provenance sensitive elemental ratios th sc vs zr sc and co th vs la sc indicate that the clastic contribution to xujiahe black shales was of a predominantly felsic character the black shales exhibit a transition from t 3 x 1 to t 3 x 3 to t 3 x 5 whereby t 3 x 1 was primarily sourced from the proterozoic to early paleozoic strata of the qinling orogeny t 3 x 3 was primarily sourced from the neoproterozoic complex in the longmen shan and t 3 x 5 was primarily sourced from the songpan ganzi flysch strata which contributed recycled material because of folding and strong southeastward thrusting

origin of mafic intrusions in the micangshan massif central china implications for the neoproterozoic tectonic evolution of the northwestern yangtze block abstract the mafic intrusions exposed in the micangshan massif preserve important information pertaining to the neoproterozoic tectonic evolution along the northwestern margin of the yangtze block in this paper we report new la icp ms zircon u pb ages lu hf isotopes and whole rock compositions for mafic sills in the micangshan massif to understand their petrogenesis and supplement the regional tectonic evolution xenocrystic zircon grains from two mafic sill samples yield weighted mean ages of 860 4 ma and 858 5 ma respectively suggesting the maximum emplaced age of the mafic sills was at ca 860 ma a granitic dyke sample which intruded into the mafic sills yields a crystallization age of 799 4 ma implying the minimum emplacement of the mafic sills was at ca 800 ma these results indicate that the mafic sills were formed within the span of ca 860 800 ma geochemical results show that most of these mafic sills are tholeiitic in composition and are characterized by low to moderate feototal and tio2 contents as well as moderate to high mgo contents and mg values they display distinct enriched light rare earth element lree patterns variable remarkable enrichment in large ion lithophile elements liles e g cs and ba and evident depletion of high field strength elements hfses e g nb ta and p resembling signatures of typical subduction related basaltic rocks in addition these sills exhibit comparatively high nb ta moderate nb yb as well as low nb la th yb and nb y ratios indicating that they probably originated from partial melting of a previously metasomatized mantle wedge source these observations together with regional tectonics suggest that these mafic sills in the micangshan massif might be formed in a subduction related continental arc setting therefore combining our new data with available results from previous studies in the adjacent regions our study supports the dominant subduction related tectonics along the northwestern margin of the yangtze block during the early neoproterozoic time

can a sinking metallic diapir generate a dynamo metallic diapirs may have strongly contributed to core formations during the first million years of planetary evolutions the aim of this study is to determine whether the dynamics induced by the diapir sinking can drive a dynamo and to characterize the required conditions on the size of the diapir the mantle viscosity and the planetary latitude at which the diapir sinks we impose a classical hadamard flow solution for the motion at the interface between a spherical sinking diapir and a viscous mantle on dynamical simulations that account for rotational and inertial effects in order to model the flow within the diapir the flows are confined to a velocity layer with a thickness that decreases with increasing rotation rate these 3d flows are is then used as input for kinematic dynamo simulations to determine the critical magnetic reynolds number for dynamo onset our results demonstrate that the flow pattern inside a diapir sinking into a rotating planet can generate a magnetic field large diapirs r 10 km sinking in a mantle with a viscosity ranging from 10 9 to 10 14 pa s provide plausible conditions for a dynamo equatorial sinking diapirs are confined to a thicker velocity layer and are thus possibly more favorable for dynamo generation than polar sinking diapirs in addition equatorial sinking diapirs produce stronger saturated magnetic fields however for the range of parameters studied here estimation of the intensity of diapir driven magnetic fields suggests that they could not have contributed to the lunar or martian crustal paleomagnetic fields

metal silicate partitioning of ni and co in a deep magma ocean abstract the pattern of siderophile iron loving element abundance in the silicate portion of the earth is a consequence of metal separation during core formation the apparent excess of nickel and cobalt in mantle derived rocks has been attributed to metal silicate equilibration in a deep terrestrial magma ocean based on the extrapolation of phase equilibria and metal silicate partitioning results obtained at lower pressure p and temperature t previous estimates of the p t of equilibration are all greater than 25 gpa and 3000 k using the laser heated diamond anvil cell we have extended metal silicate partitioning measurements for ni and co to 75 gpa and 4400 k exceeding the liquidus temperatures for both metal and silicate basalt or peridotite and therefore achieving thermodynamic conditions directly comparable to those of the magma ocean the metal silicate partition coefficients of nickel and cobalt decrease with increasing pressure and reach the values required to yield present mantle concentrations at 50 gpa at these conditions silicon and oxygen concentrations measured in the metallic liquid allow to solve the seismically constrained core density deficit above 60 gpa the partition coefficients become too low resulting in an overabundance of ni and co in the silicate mantle our data therefore support the paradigm of core formation in a deep mama ocean providing an upper bound for the depth at which earth s core may have formed and explaining the main geophysical density and geochemical excess siderophile elements observables

magma mixing and the production of compositional variation within granite suites evidence from the granites of southeastern australia granite suites are groups of plutons possessing characteristic features that are a result of their derivation from source material of a specific composition variation within suites has been ascribed to a variety of processes magma mixing or mingling is a popular hypothesis generally proposed in terms of blending between a crustal melt and mafic material from the mantle that caused that melting when the compositions of pain of suites from the bega batholith of southeastern australia are compared any differences seen at either end of the range in composition are also seen at the other limit so that both the most mafic and mostfelsic rocks show similar relative abundances of particular elements similar relationships are seen for other granites in the region these observations are not consistent with largescale magma mixing or mingling and although those processes may operate on a small scale they cannot have been responsible for the major compositional variations likewise assimilation of country rocks had no significant role in producing variation in the granites of southeastern australia the production of variation by differential separation of melt from residual solid source material or restite must be favoured for many of the granite suites of this region

mid pleistocene lavas from the seguam volcanic center central aleutian arc closed system fractional crystallization of a basalt to rhyodacite eruptive suite in contrast to adjacent volcanic centers of the modern central aleutian arc seguam island developed on strongly extended arc crust k ar dates indicate that mid pleistocene late pleistocene and holocene eruptive phases constitute seguam this study focuses on the petrology of the mid pleistocene 1 07 07 ma turf point formation tpf which is dominated by an unusual suite of porphyritic basalt and basaltic andesite lavas with subordinate phenocryst poor andesite to rhyodacite lavas increasing whole rock feo mgo from basalt to dacite the anhydrous plag ol cpx opx mt phenocryst assemblage groundmass pigeonite and the reaction ol liq opx preserved in the mafic lavas indicate a tholeiitic affinity thermometry and comparison to published phase equilibria suggests that most tpf basalts crystallized plag ol cpx mt at 1160 c between about 3 5 kb 1 2 h2o andesites crystallized plag cpx opx mt at 1000 c between 3 4 kb with 3 5 h2o and dacites crystallized plag cpx opx mt at 1000 c between 1 2 kb with 2 3 h2o all lavas crystallized at f o 2 close to the nno buffer mineral compositions and textures indicate equilibrium crystallization of the evolved lavas petrographic evidence of open system mixing or assimilation is rare mgo cao al2o3 cr ni and sr abundances decrease and k2o na2o rb ba zr and pb increase with increasing sio2 50 71 lree enrichment ce yb n 1 7 0 2 characterizes most tpf lavas total ree contents increase and eu anomalies become more negative with increasing sio2 relative to other aleutian volcanic centers tpf basalts and basaltic andesites have lower k2o na2o tio2 rb ba sr zr y and lree abundances 87sr 86sr ratios 0 70361 0 70375 and ratios of 206pb 204pb 18 88 18 97 207pb 204pb 15 58 15 62 208pb 204pb 38 46 38 55 are the highest measured for any suite of lavas in the oceanic portion of the aleutian arc conversely end values 5 8 to 6 7 are among the lowest from the aleutians sr nd and pb ratios are virtually constant from basalt through rhyodacite whereas detectable isotopic heterogenity is observed at most other aleutian volcanic centers major and trace element ree and sr nd and pb isotopic compositions are consistent with the basaltic andesitic andesitic dacitic and rhyodacitic liquids evolving from tpf basaltic magma via closed system fractional crystallization alone fractionation models suggest that removal of 80 wt cumulate 61 plag 17 cpx 12 opx 7 ol and 3 mt can produce 20 wt rhyodacitic residual liquid per unit mass of parental basaltic liquid petrologic and physical constraints favor segregation of small batches of basalt from a larger mid crustal reservoir trapped below a low density upper crustal lid in these small magma batches the degree of cooling crystallization and fractionation are functions of the initial mass of basaltic magma segregated the thermal state of the upper crust and the magnitude of extension tholeiitic magmas erupted at seguam evolved by substantially different mechanisms than did calc alkaline lavas erupted at the adjacent volcanic centers of kanaga and adak on unextended arc crust these variable differentiation mechanisms and liquid lines of descent reflect contrasting thermal and mechanical conditions imposed by the different tectonic environments in which these centers grew at seguam intra arc extension promoted eruption of voluminous basalt and its differentiates unmodified by interaction with lower crustal or upper mantle wallrocks

trace element partitioning in pantellerites and trachytes abstract in order to investigate the effect of increasing melt peralkalinity on partitioning partition coefficients have been determined using neutron activation analyses of coexisting phenocrysts and glass of five samples from pantelleria spanning the range trachyte to pantellerite alkali feldspar partition coefficients for fe rb ba sr and eu vary with melt peralkalinity due to changes in melt polymerization and to the systematic increase in xor and decrease in xan of the feldspar in going from trachyte to pantellerite fe partition coefficients increase from 0 04 to 0 10 presumably because fe 3 increasingly substitutes in the feldspar tetrahedral site as melt activity of al declines and fe concentrations increase partition coefficients for trivalent light rees decrease and the partitioning pattern becomes flatter the most evolved samples having some of the lowest published values for feldspar the hundredfold decline in eu partition coefficients 2 5 to 0 024 and the decrease in the size of the positive partitioning anomaly are attributed to increasing eu3 eu2 in the melt as it becomes more peralkaline as well as to concomitant decrease in the ca content of feldspar as a result the behavior of eu during fractional crystallization of peralkaline suites is fundamentally different from that in metaluminous suites absolute abundances rise and the size of the negative eu anomaly changes little with fractionation beyond pantelleritic trachyte barium and strontium do not decline to extremely low concentrations in pantellerites as they do in highly evolved metaluminous rhyolites because the feldspar partition coefficients drop precipitously with peralkalinity similarly rb contents do not get large because the partition coefficient increases slightly 0 11 to 0 25 with peralkalinity and is sufficient to make rb only weakly incompatible in the feldspar dominated assemblage partition coefficients calculated for sr and ba from natural phenocryst matrix pairs can be gross over estimates in these feldspar rich felsic rocks if phenocrysts are zoned this effect is severe for assemblages with bulk distribution coefficients d greater than 4 even at very small crystal contents and for 4 d 2 it is significant if crystal contents exceed about 25 some of the very high values and much of the variability reported in the literature for ba and sr feldspar partition coefficients may be a result of this effect in contrast to feldspar partition coefficients for apatite and mafic phases change little with peralkalinity partition coefficients for rees and the compatible elements sc co and cr are all less than half those in fe rich clinopyroxene and olivine of similar composition in high silica rhyolite although absolute levels of partition coefficients are a function of melt polymerization the shapes of the ree partitioning patterns appear to be related to the composition of the clinopyroxene scandium is most strongly concentrated in clinopyroxene partition coefficient 27 60 having larger values in more fe rich compositions apatite shows a wide range in composition within the suite the concentrations of si fe rees and y increasing and mg ca and p decreasing in more evolved samples these trends however parallel those in coexisting glasses so that partition coefficients are essentially constant in peralkaline suites cs is the most incompatible of commonly analyzed elements but its great mobility during alteration and weathering limits its use as a differentiation index next in order of incompatibility are th u hf zr and ta which have small but finite values of d due to the partitioning of th and u into apatite 2 the accommodation of zr and hf by clinopyroxene 0 4 and titanomagnetite 0 2 and relatively high concentrations of ta in cossyrite and fe ti oxides the next most incompatible are the lightest and the heaviest rees and finally the middle trivalent rees and rb plots of pairs of relatively incompatible elements form straight line trends over large compositional ranges in peralkaline suites because the replacement of olivine titanomagnetite by cossyrite as the melt becomes more peralkaline causes little change in d the ree patterns of titanomagnetite and cossyrite are both approximately flat at values less than 0 1 and the relatively high titanomagnetite partition coefficients for ta 2 zr and hf when combined with the very low partition coefficients for these elements for olivine approximate the moderate partition coefficients for cossyrite

tectonics of the qinling belt build up and evolution of eastern asia the structure and history of the qinling range which separates the north and south china cratons are a key to the tectonic evolution of eastern asia the mountain range is composed of a northern palaeozoic belt and a southern mesozoic belt the palaeozoic orogeny probably involved the closure of an oceanic basin along a north dipping subduction zone followed by south vergent obduction of ophiolites and intracontinental thrusting during the devonian the mesozoic belt appears to be the product of purely intracontinental shallow crustal deformation over a large decollement with at least 100 km of horizontal transport since the devonian both belts have been sliced by huge east west sinistral strike slip faults

supracrustal input to magmas in the deep crust of sierra nevada batholith evidence from high δ 18 o zircon oxygen isotope ratios of zircon zc from intrusives exposed in the tehachapi mountains southern california reveal large inputs of high 18 o supracrustal contaminant into gabbroic and tonalitic magmas deep 30 km in the cretaceous sierra nevada batholith high 18 o zc values 7 8 0 7 predominate in the deep parts of the batholith but lower values 6 1 0 9 occur in shallower parts this indicates a larger gradient in 18 o with depth in the batholith than occurs from west to east across it oxygen sr and nd isotope data show that the supracrustal contaminant was likely young paleozoic or mesozoic hydrothermally altered upper oceanic crust or volcanic arc sediments such rocks were subducted or underthrust beneath the sierran arc during accretion of oceanic terranes onto north america this component yielded high 18 o magmas that were added to the base of the batholith on average gabbros in the southern sierra contain at least 18 of the subducted supracrustal component some tonalite and granodiorite magmas were additionally contaminated by kings sequence metasedimentary rocks as evidenced by 18 o zc and initial 87 sr 86 sr that trend toward values measured for the kings sequence besides high 18 o values in the southern sierra xenoliths in the central sierra also have elevated 18 o which confirms the widespread abundance of supracrustal material in the sub arc lithospheric mantle in contrast to 18 o zc whole rock 18 o values of many samples have undergone post magmatic alteration that obscures the magmatic contamination history of those rocks such alteration previously prevented confident determination of the mass of young hydrothermally altered mantle rocks that contributed to sierran granitoids

zircon u pb age hf isotopic compositions and geochemistry of the silurian fengdingshan i type granite pluton and taoyuan mafic felsic complex at the southeastern margin of the yangtze block abstract this work presents an integrated study of zircon u pb ages and hf isotope along with whole rock geochemistry on silurian fengdingshan i type granites and taoyuan mafic felsic intrusive complex located at the southeastern margin of the yangtze block filling in a gap in understanding of paleozoic i type granites and mafic intermediate igneous rocks in the eastern south china craton scc the fengdingshan granite and taoyuan hornblende gabbro are dated at 436 5 ma and 409 2 ma respectively the fengdingshan granites display characteristics of calc alkaline i type granite with high initial 87sr 86sr ratios of 0 7093 0 7127 low end t values ranging from 5 6 to 5 4 and corresponding nd model ages t2dm of 1 6 ga their zircon grains have ehf t values ranging from 2 7 to 2 6 and model ages of 951 1164 ma the taoyuan mafic rocks exhibit typical arc like geochemistry with enrichment in rb th u and pb and depletion in nb ta they have initial 87sr 86sr ratios of 0 7053 0 7058 end t values of 0 2 1 6 and corresponding t2dm of 1 0 1 1 ga their zircon grains have ehf t values ranging from 3 2 to 6 1 and model ages of 774 911 ma diorite and granodiorite from the taoyuan complex have initial 87sr 86sr ratios of 0 7065 0 7117 end t values from 5 7 to 1 9 and nd model ages of 1 3 1 6 ga the petrographic and geochemical characteristics indicate that the fengdingshan granites probably formed by reworking of neoproterozoic basalts with very little of juvenile mantle derived magma the taoyuan complex formed by magma mixing and mingling in which the mafic member originated from a metasomatized lithospheric mantle both the fengdingshan and taoyuan plutons formed in a post orogenic collapse stage in an intracontinental tectonic regime besides the paleozoic fengdingshan granites and taoyuan hornblende gabbro other neoproterozoic and indosinian igneous rocks located along the southeastern and western margin of the yangtze block also exhibit decoupled nd hf isotopic systemics which may be a fingerprint of a previous late mesoproterozoic to early neoproterozoic oceanic subduction

new chronological constraints for cryogenian to cambrian rocks in the three gorges weng an and chengjiang areas south china abstract the neoproterozoic and cambrian were two of the most dramatic periods in the history of the earth because large multi cellular animals first appeared then in the so called cambrian explosion to better understand this event many paleontological and geochemical studies now focus on rocks in south china because of the fossiliferous succession and good exposure since the recognition of the yangtze gorges and chengjiang area as type localities of the sinian ediacaran and meishucunian early cambrian systems both sections have been intensively investigated in order to decipher the relationships between the evolution of life and surface environmental changes it is necessary to understand their paleontological geochemical and geo chronological constraints this study presents new chronological constraints for the cryogenian to cambrian rocks in the three gorges weng an and chengjiang areas south china we discovered two tuff layers one at the base of the shuijingtuo fm at three gorges and the other at the bottom of the dahai member in chengjiang in addition we collected sandstones from neoproterozoic strata in the three gorges chengjiang and weng an areas for provenance analysis zircons separated from the tuff layers provide new nano sims u pb ages of 526 4 5 4 ma at the base of the shuijingtuo fm and 523 9 6 7 ma at the bottom of the dahai member the tuffaceous beds occur at an unconformity and nodules are common in the three gorges meishucun and taoying sections indicating that major and relatively wide scale volcanic and sedimentological events occurred at ca 525 ma moreover carbonate carbon isotope chemostratigraphies at morocco siberia three gorges and meishucun display different characteristics during the tommotian one possibility is that the south china ocean was separated from an outer ocean at that time detrital zircons in sandstones have age populations at ca 2 7 1 8 1 6 1 0 and from 0 9 to 0 75 ga indicating that the paleo hinterland of the nanhua and kangdian rift basins was geologically complex despite the lack of ca 1 6 ga rocks in the yangtze and cathaysia blocks these data are nevertheless interpreted to indicate derivation of the zircons from basement rocks in the yangtze craton

neoproterozoic adakitic plutons and arc magmatism along the western margin of the yangtze block south china abstract neoproterozoic adakitic plutons that crop out along the western margin of the yangtze block south china from kangding on the north to panxi on the south provide constraints on the origin of the giant jinningian magmatic event of south china representative plutons include the xuelongbao 750 ma datian 760 ma and dajianshan intrusions the latter two bodies consist mainly of granodiorite with relatively high sio2 51 0 73 4 wt and mg s 0 36 0 55 they have fractionated rare earth element patterns with la yb n ratios ranging from 2 6 to 101 8 and are characterized by high sr 344 1018 ppm and low y 4 3 17 9 ppm yielding sr y ratios ranging from 27 to 111 on primitive mantle normalized trace element diagrams these rocks show enrichment of large ion lithophile elements and depletion of high field strength elements nb ta with positive zr hf and negative ti anomalies consistent with an arc related setting they have relatively constant initial whole rock 87sr 86sr ratios 0 70

grain size variations in the earth s mantle and the evolution of primordial chemical heterogeneities 1 chemical heterogeneities with relatively high viscosity are known to be poorly mixed by mantle convection one of the factors which can cause viscosity variations is the grain size analysis of various physical processes in the earth s mantle suggests that variation in the grain size can be extremely large even outside of subducting slabs the largest grains are formed in the early molten earth where ostwald ripening is controlled by liquid state diffusion the smallest grains are formed due to recrystallization by the spinel perovskite phase transformation in downwelling flow and subsequent ostwald ripening the viscosity contrast between primordial large grain size material and recrystallized material is likely to be many orders of magnitude preliminary numerical simulations show that strongly time dependent mantle convection can process primordial large grain size heterogeneities through the spinel perovskite phase boundary only after a few overturns this reduces the viscosity contrasts and leads to rapid mixing by mantle convection a more steady flow in the lower mantle may lead to the formation of stagnant poorly mixed regions which may never have had a chance to cross the 660 km discontinuity and be recrystallized by the spinel perovskite phase transformation heterogeneities can also avoid grain size reduction and mixing if their density is slightly larger than the surrounding mantle in this case they accumulate at the core mantle boundary

marine eemian in the danish eastern north sea abstract the distribution of marine eemian deposits along the west coast of jutland denmark is presented based on offshore shallow seismic mapping and a review of results from analysis of marine cores and samples from onshore borings the results are compiled from publications and unpublished reports in the archives of geological survey of denmark and greenland the eemian deposits in the area are characterised by the presence of lusitanian foraminifera such as elphidium lidoense and or elphidium translucens and characteristic mollusc faunas the eemian along the west coast of jutland seems to have been deposited in sheltered fjords straits or tidal flats in an archipelago it was probably protected by a row of islands towards the open north sea the eemian foraminiferal fauna has been divided into nine assemblages representing different palaeoenvironments the sea level during the eemian highstand was 0 10 m lower than the present sea level in the southeastern north sea area the eemian sea level curve constructed for northern germany and the netherlands seems to be a good approximation for western jutland the highest level of the marine eemian deposits is consistent along the northwest european north sea coast suggesting an almost uniform net subsidence history during the last 125 000 years

contrasting geochemical and sm nd isotopic compositions of archean metasediments from the kongling high grade terrain of the yangtze craton evidence for cratonic evolution and redistribution of ree during crustal anatexis abstract twenty three clastic metasediments from the kongling high grade terrain of the yangtze craton south china were analyzed for major trace and rare earth elements and sm nd isotopic ratios associated dioritic tonalitic trondhjemitic dtt and granitic gneisses as well as amphibolites were also analyzed in order to constrain provenance the results show that the clastic metasediments can be classified into 3 distinct groups in terms of mineralogical geochemical and sm nd isotopic compositions group a is characterized by having no to slight negative eu anomalies eu eu 0 82 1 07 being high in cr 191 396 ppm and ni 68 137 ppm and low in th 3 3 7 8 ppm and ree ree 99 156 ppm these characteristics are similar to those of metasediments from archean greenstone belts in addition the group a metasediments have the value of the chemical index of alteration ciw close to felsic gneisses their sm nd isotopic ree and trace element compositions can be interpreted by mixtures of the dtt gneisses and amphibolites dating of detrital zircons from 2 group a samples by shrimp reveals a major concordant age group of 2 87 3 0 ga which is identical to the age of the trondhjemitic gneiss these results strongly suggest that group a was principally the first cycle erosion product of the local kongling dtt gneiss and amphibolite moreover the higher than amphibolite cr content and slight eu depletion exhibited by some samples from this group infer that ultramafic rocks like komatiite and granite of probably 3 0 3 3 ga in age also played a role group b is characterized by the presence of graphite and shows a more evolved composition similar to post archean shales with a prominent negative eu anomaly eu eu 0 48 0 77 and high ciw on paired cr th vs la co and co th plots group b samples conform to a two end member mixing line of the kongling granitic gneiss and amphibolite however data on nd model age and ciw suggest that the granite component should be younger than the sampled granitic gneiss and derived from a distal source both groups a and b exhibit a clear positive correlation between ciw and tdm and a negative one between ciw and eu eu these correlations point to the crustal evolution of the yangtze craton towards coupled increasing ciw and eu depletion with decreasing age this in turn reflects the change of granitoid magmatism from local na rich dioritic tonalitic trondhjemitic rocks to widespread k feldspar granite the change led to the intracrustal differentiation stabilization and growth of the craton group c is restite and contains abundant sillimanite and garnet and unusually high ilmenite 7 11vol which can be seen to be dehydration melting products of biotite under the microscope this group shows extremely varied ree distributions from lree enriched to depleted and from negative to strong positive eu eu 1 63 eu anomalies compared to groups a and b group c is severely depleted in na2o k2o lree rb and ba whereas tio2 co v sc and hree and y are considerably enriched this is accompanied by anomalous high sm nd 0 21 0 28 147sm 144nd 0 1361 0 1738 and 143nd 144nd 0 511589 0 511958 ratios tdm correlates clearly with sm nd ratio and 2 out of 3 samples give significantly older to unrealistic tdm 3 9 4 9 ga the results document redistribution of ree and an open behavior of the sm nd isotope system during the biotite dehydration melting of metasediments

shock compression of liquid silicates to 125 gpa the anorthite diopside join we determined the equation of state eos of three silicate liquid compositions by shock compression of preheated samples up to 127 gpa diopside di ca 2mg 2sio 6 at 1773 k anorthite an caal 2si 2o 8 at 1923 k and the eutectic composition di 64 an 36 at 1673 k were previously studied by shock compression to 38 gpa the new data extend the eos of each composition nearly to the earth s core mantle boundary the previously reported anomaly at 25 gpa for di 64 an 36 eutectic was not reproduced rather all data for this composition fit within error a straight line hugoniot in particle velocity vs shock velocity di also displays a linear hugoniot consistent with ultrasonic data and a third order finite strain eos the full anorthite data set is complex we examine a model with a transition between two structural states and a fourth order finite strain model excluding two points that may not display relaxed behavior we also report an experiment on room temperature solid di 64 an 36 aggregate that clearly demonstrates increase upon compression of the gruneisen parameter of this liquid much as experiment and theory have shown for forsterite and enstatite liquids we construct isentropes and isotherms from our hugoniots using mie gruneisen thermal pressure and evaluate the model of ideal mixing of volumes volume may mix almost linearly at high temperature but deviates strongly when calculated along an isotherm it remains difficult to reach a firm conclusion we compare the densities of liquids to lower mantle solids our results suggest that basaltic liquids rich in cao and al 2o 3 are notably denser than liquids in the mgo sio 2 binary and subject to uncertainties in the behavior of feo and in corrections for thermal pressure such liquids may be the most likely candidates for achieving negative buoyancy in the lowermost mantle

neoproterozoic crustal growth and reworking of the northwestern yangtze block constraints from the xixiang dioritic intrusion south china abstract neoproterozoic magmatism along the northwestern margin of the yangtze block south china produced voluminous adakitic and normal arc felsic intrusions associated with many mafic ultramafic bodies dioritic plutons are rare but are important for understanding the crustal growth and secular evolution of the region the xixiang dioritic intrusion at hannan on the northwestern margin of the yangtze block provides a good opportunity to investigate the petrogenesis of intermediate plutons and to document the neoproterozoic continental crustal evolution of the region the pluton has a zircon u pb age of 764 9 ma rocks from the pluton have relatively low sio 2 59 3 63 0 wt and k 2 o na 2 o 5 8 6 2 wt and high mgo 2 3 3 1 wt and al 2 o 3 15 8 17 1 wt they have chondrite normalized ree patterns enriched in lree la ybn 7 2 14 7 with negative eu anomalies their primitive mantle normalized trace element patterns are characterized by enrichment of lile and depletion of hfse nb and ta with positive pb and negative ti anomalies they have a narrow range of initial 87 sr 86 sr ratios 0 7033 to 0 7035 with positive end values 1 94 to 3 16 their zircon ehf values range from 5 5 to 9 8 with an average of 7 32 these isotopic data are similar to those of the neoproterozoic mafic ultramafic intrusions in the hannan region suggesting that the diorites were generated by partial melting of newly emplaced basaltic rocks the diorites have mg values 50 3 54 4 higher than those of experimental melts of mafic igneous rocks in addition they contain fe rich clinopyroxene suggesting that partial melts of the newly formed mafic lower crust interacted with the mafic residue based on its arc related geochemical affinity and emplacement preceding voluminous adakitic magmatism the xixiang intrusion is proposed to have been generated in a neoproterozoic arc setting during crustal growth and reworking that lasted from 950 ma to