igneous microstructures in migmatites clear differences between the microstructures of leucosomes and mesosomes occur in some nebulitic migmatites in the proterozoic arunta block central australia the leucosomes show crystal faces of k feldspar microcline microperthite cordierite andalusite and plagio clase against quartz indicating crystallization of a melt in contrast the mesosomes show predominantly polygonal grain shapes modified by 001 crystal faces of biotite and rare crystal faces in some porphyroblasts of garnet cordierite and orthopyroxene indicating re crystallization in the solid state the occurrence of abundant crystal faces in minerals such as feldspar cordierite and andalusite which typically grow crystal faces in magmas but generally not in metamorphic rocks is evidence of former melt therefore igneous microstructures can be preserved in leucosomes that have not undergone penetrative subsolidus deformation and recrystallization the cores of many of the crystals of cordierite and k feldspar have small locally oriented inclusions similar to those in the same minerals in the mesosomes these indicate that the cores remained solid during crystallization of the leucosome which produced inclusion free euhedral rims these crystals provide criteria for recognizing possible restite in granitoid magmas

deformation of incompletely crystallized magma systems granitic gneisses and their tectonic implications certain textures in two feldspar calc alkaline granitic gneisses have been identified as indicators of the presence of intercrystalline melt during deformation of crystal melt mushes fluid relocation or fracting is to be expected in an incompletely crystallized magmatic system as deviatoric stresses are applied to it relocation of late stage fluids capable of crystallizing k feldspar quartz microaplite and myrmekite to low pressure zones such as the pressure shadow region around earlier crystals occurs within some developing mylonitic gneiss fabrics and results in a distinctive assemblage of textures strain in these rocks is expressed by various brittle and ductile features and by fluid relocation textures the petrogenesis of these particular gneisses is therefore both magmatic and metamorphic contrasting with gneiss forming processes that had no participating magmatic component migmatitic complexes evolving by partial anatexis of non magmatic rocks in a tectonically active environment are not

shrimp u pb zircon geochronology of the fuping complex implications for formation and assembly of the north china craton abstract the fuping complex located within the central zone of the north china craton is composed of amphibolite to granulite facies orthogneisses interleaved with minor supracrustal rocks at similar metamorphic grade the oldest components recognised are hornblende gneiss enclaves within the predominant biotite orthogneiss which have a shrimp u pb zircon age of 2708 8 ma we consider these enclaves to represent fragments of 2 7 ga continental materials incorporated in the biotite gneiss the biotite gneiss has a shrimp u pb zircon age of 2513 12 ma interpreted to be time of magmatic crystallisation of the igneous precursor based on the igneous characteristics of the zircons this indicates a major magmatic episode at 2 52 ga in the fuping complex identical to the age of felsic volcanism within the low grade wutai complex which crops out immediately to the west a gneissic granite that intrudes the biotite gneisses has a poorly defined 207pb 206pb age of 2045 64 ma this is within error of the age of 2097 46 ma obtained from a fine grained gneiss interlayered with amphibolite of the wanzi supracrustal suite wss interpreted to be volcanic in origin zircons from both these samples have strong oscillatory zoning and provide the first indication of a palaeoproterozoic magmatic event in the area again similar in age to magmatic events recently recognised in the adjacent wutai complex these data indicate a comparable history for the fuping and wutai complexes and support geochemical evidence that they had a common origin and formed part of a late archaean arc affected by later palaeoproterozoic re activation low uranium zircons without oscillatory zoning separated from a sample of biotite gneiss yield data clustered at 1817 26 ma which is interpreted to reflect a period of new zircon growth during a major metamorphic event these data support the recently proposed tectonic model that amalgamation of the north china craton occurred due to collision of the eastern and western blocks along the central zone at 1 8 ga ago

palaeoproterozoic accretionary processes in fennoscandia abstract accretionary processes contributed to major continental growth in fennoscandia during the palaeoproterozoic mainly from 2 1 to 1 8 ga the composite svecofennian orogen covers c 1 10 6 km 2 and comprises the lapland savo fennia svecobaltic and nordic orogens it is a collage of 2 1 2 0 ga microcontinents and 2 02 1 82 ga island arcs attached to the archaean karelian craton between 1 92 and 1 79 ga andean type vertical magmatic additions especially at c 1 89 and c 1 8 ga were also important in the continental growth the palaeoproterozoic crust is the end product of accretionary growth continental collision and orogenic collapse preserved accretional sections are found in areas where docking of rigid blocks has prevented further shortening the pirkanmaa belt represents a composite accretionary prism and other preserved palaeosubduction zones are identified in the gulf of bothnia and the baltic sea areas in the southern segment of the lapland savo orogen collision between the archaean continent lower plate and the palaeoproterozoic arc microcontinent assembly upper plate produced a special type of lateral crustal growth the archaean continental edge decoupled from its mantle during initial collision and overrode the arc and its mantle during continued collision

paleoproterozoic accretionary orogenesis in the north china craton a shrimp zircon study abstract the inner mongolia suture zone imsz and the trans north china orogen tnco incorporate the major paleoproterozoic accretionary orogens in the north china craton ncc with the jiao liao ji belt jljb representing the third one here we investigate the paleoproterozoic tectonics of the imsz and tnco through zircon shrimp geochronology on a representative suite of rocks comprising metasediments and arc magmatic rocks shrimp analysis of zircons with textures indicating extreme recrystallization under ultrahigh temperature uht conditions from the metapelites at heling er in the southern domain of the imsz reveals a single population with a weighted mean 207 pb 206 pb age of 1913 17 ma the zircons in another uht granulite from this locality yield a weighted mean 207 pb 206 pb age of 1910 18 ma these data correlate with the ca 1 92 ga age reported from zircons in sapphirine bearing uht granulites further north and confirm the regional extent of the paleoproterozoic uht metamorphism within the imsz zircons in a charnockite from the southern margin of the khondalite belt fringing the uht granulites in the imsz show two distinct age groups an older population with a magmatic paragenesis and a weighted mean 207 pb 206 pb age of 1932 24 ma and a younger group of metamorphic zircons with an age of 1858 26 ma we also report zircon ages from charnockites in two localities around xing he in the huangtuyao belt belonging to the huai an complex within the westernmost domain of the tnco at the junction with the imsz the charnockite from first locality carries two distinct zircon populations with the older group yielding a weighted mean 207 pb 206 pb age of 2477 2 ma and the younger population showing an age of 1807 38 ma the internal structure as revealed from cl images and the overall high th u values up to 2 42 of the older zircons suggest their magmatic affinity whereas the younger group with extremely low th u 0 02 0 09 is of metamorphic origin zircons from the charnockite in the second locality also define two distinct age clusters with a dominant older magmatic group having a weighted mean 207 pb 206 pb age of 2147 11 ma and a minor younger group with an age of 1958 25 ma the range of ages from 2477 to 2147 ma from magmatic zircons in the charnockites from the eastern periphery of the imsz within the western margin of the tnco in combination with similar ages reported in recent studies from zircons in magmatic complexes within the imsz suggest a prolonged history of subduction related arc magmatism and accretionary tectonics analogous to those in some of the phanerozoic belts such as the central asian orogenic belt and the western pacific subsequent progressive collision and suturing of the continental blocks were accompanied by the exhumation of high pressure hp and uht metamorphic rocks the available data from the imsz and tnco suggest long lived convergent margins associated with the southward subduction of the yinshan block and westward subduction of the eastern block in a double sided subduction realm prior to the final amalgamation of the ncc and its incorporation within the columbia supercontinent in the late paleoproterozoic

la icp ms u pb geochronology of detrital zircons from the jining complex north china craton and its tectonic significance abstract la icp ms u pb zircon dating and cathodoluminescene cl image analysis were carried out to determine the protolith and metamorphic ages of high grade al rich gneisses named as khondalites from the jining complex of the north china craton ncc the analytical results of more than 200 detrital zircon grains from the khondalites show three main age populations 2060 ma 1940 ma and 1890 ma these data indicate that the provenance of the jining khondalites is paleoproterozoic in age but not archean as previously suggested and the sediments were derived from a provenance different from the eastern block and the yinshan terrane of the ncc the nearly concordant youngest age of 1842 16 ma 207pb 206pb age for the detrital zircons is interpreted as the maximum depositional age of the khondalites overgrowth rims of detrital zircons yield an age of 1811 23 ma which we interpret as the metamorphic age the new age data are consistent with the recent three fold tectonic subdivision of the ncc and support that the eastern and western blocks collided at 1 8 ga to form the coherent ncc

erosion deposition waves in shallow granular free surface flows debris flows can spontaneously develop regular large amplitude surge waves that are interspersed by periods in which the channel fill is completely stationary these are important because each individual surge is much more destructive than a steady uniform flow with the same mass flux in this paper small scale experiments that exhibit similar behaviour are described the flow consists of carborundum particles that flow down a rough inclined chute covered with a static erodible layer of the same grains for inflow conditions close to the minimum depth required for steady uniform flows to exist small disturbances are unstable creating waves that rapidly coarsen and grow in size as the waves become sufficiently large the troughs between the wave crests drop below a critical thickness and come to rest a series of steadily travelling waves develop which erode the static layer of particles in front of them and deposit grains behind them to form a layer that is again stationary this is in turn re eroded and deposited by the next wave we term these waves granular erosion deposition waves although erosion and deposition problems are notoriously difficult a simple model is developed which uses a depth averaged version of the i rheology and pouliquen and forterre s extended friction law the viscous dissipation combines with dynamic intermediate and static friction regimes to generate finite length waves with static and mobile regions the existence of stationary layers fundamentally distinguishes erosion deposition waves from granular roll waves which form in slightly deeper flows and are always completely mobilized numerical simulations show that the system of equations is able to model both erosion deposition waves and granular roll waves moreover the computed wave amplitude wavespeed and coarsening dynamics are in good quantitative agreement with experiments

upb zircon geochronology of late archaean metamorphic rocks in the taihangshan wutaishan area north china abstract a geochronological study has been made in the taihangshan wutaishan area shanxi province on a sequence of precambrian supracrustal rocks and stratigraphically related intrusives these constitute part of the north china basement block the oldest rocks of the fuping group consist predominantly of mafic granulites paragneisses quartzites and amphibolites this group was always considered archaean and early k ar measurements yielded some minimum ages as old as 2300 ma and others as young as 1500 1900 ma the unconformably overlying wutai group is considered as the base of the proterozoic succession and consists of amphibolite grade paragneisses carbonates and amphibolites in its lower part and greenschist grade clastic sediments and volcanics in its upper part it has given apparent k ar mineral ages between 1500 and 2000 ma present investigations have employed the u pb zircon approach on selected parts of the sequences to provide direct depositional ages or age constraints for the fuping and wutai groups and to find the age s of subsequent metamorphic processes a minimum age for the fuping group and maximum age for the wutai group has been derived from zircon age measurements on the lanzhishan granite 2560 9 ma which intrudes the fuping group and is unconformably overlain by the wutai group this late archaean result can be interpreted as the age of an igneous crystallization event and therefore a maximum for the depositional age of the wutai group the above result is well substantiated by zircon dating of the wutai group itself zircons from a low grade metamorphosed keratophyric lava in the upper part of the group give a volcanic crystallization age of 2520 17 ma this provides close constraints for the age of deposition of the wutai group at between 2560 and 2520 ma ago if the wutai group is indeed basal proterozoic the archaean proterozoic boundary in this part of china is also dated at between 2560 and 2520 ma further evidence as to the antiquity of the wutai group comes from a u pb zircon age of 2520 30 ma for the ekou granite which intrudes the wutai group sequence the age of the underlying late archaean rocks in the tainhangshan wutaishan area the fuping group could not be directly determined because of the paucity of volcanic markers and the higher grade of metamorphism its minimum age is 2560 ma see above but only an approximate maximum age can be gleaned from the zircon systematics in granulite and amphibolite grade paragneiss samples these data indicate a degree of pb inheritance which combined with metamorphic disturbance and more recent pb loss inhibit any further interpretation other than that they indicate an approximate maximum age of around 2800 ma several of the paragneisses and one amphibolitic schist contain zircon systems which are interpreted as having completely responded to a well defined amphibolite facies metamorphic event at 2500 ma ago a younger but less well defined metamorphism caused complete pb loss from zircon and recrystallization in the amphibolite and granulite facies terrains at between 2000 and 1800 ma ago rocks which show evidence of this younger metamorphic event were not recognized from the field or structural data base but from the distribution of the few such samples analyzed it seems possible that this younger terrain may now be interpretable as a cross cutting proterozoic mobile belt this proterozoic metamorphic province is also reflected by k ar mica ages between 1600 and 2000 ma and some preliminary rb sr total rock ages at 1700 ma on nearby paragneisses whose u pb zircon age is 2500 ma

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

large scale melt depletion in granulite terranes an example from the archean ashuanipi subprovince of quebec this study uses field petrographic and geochemical methods to estimate how much granitic melt was formed and extracted from a granulite facies terrane and to determine what the grain and outcrop scale melt flow paths were during the melt segregation process the ashuanipi subprovince located in the north eastern superior province of quebec is a large 90 000 km2 metasedimentary terrane in which 85 of the metasediments are of metagreywacke composition that was metamorphosed at mid crustal conditions 820 900 c and 6 7 kbar in a late archean dextral transpressive orogen decrease in modal biotite and quartz as orthopyroxene and plagioclase contents increase together with preserved former melt textures indicate that anatexis was by the biotite dehydration reaction biotite quartz plagioclase melt orthopyroxene oxides using melt orthopyroxene ratios for this reaction derived from experimental studies the modal orthopyroxene contents indicate that the metagreywacke rocks underwent an average of 31 vol partial melting the metagreywackes are enriched in mgo cao and feot and depleted in sio2 k2o rb cs and u have lower rb sr higher rb cs and th u ratios and positive eu anomalies compared to their likely protolith these compositions are modelled by the extraction of between 20 and 40 wt granitic melt from typical archean low grade metagreywackes a simple mass balance indicates that about 640 000 km3 of granitic melt was extracted from the depleted granulites the distribution of relict melt at thin section and outcrop scales indicates that in layers without leucosomes melt extraction occurred by a pervasive grain boundary porous flow from the site of melting across the layers and into bedding planes between adjacent layers in other rocks pervasive grain boundary flow of melt occurred along the layers for a few to tens of centimetres followed by channelled flow of melt in a network of short interconnected and structurally controlled conduits visible as the net like array of leucosomes in some outcrops the leucosomes contain very little residual material 5 biotite orthopyroxene indicating that the melt fraction was well separated from the residuum left in situ as melt depleted granulite only 1 3 vol percentage melt remained in the melt depleted granulites hence the extraction of melt generated by biotite dehydration melting in these granulites was virtually complete under conditions of natural melting and strain rates in a contractional orogen

melt segregation in the continental crust distribution and movement of melt in anatectic rocks the grain and outcrop scale distribution of melt has been mapped in anatectic rocks from regional and contact metamorphic environments and used to infer melt movement paths at the grain scale anatectic melt is pervasively distributed in the grain boundaries and in small pools consequently most melt is located parallel to the principal fabric in the rock typically a foliation short branched arrays of linked melt bearing grain boundaries connect melt depleted parts of the matrix to diffuse zones of melt accumulation protoleucosomes where magmatic flow and alignment of euhedral crystals grown from the melt developed the distribution of melt leucosome and residual rocks normally melanocratic in outcrop provides different but complementary information the residual rocks show where the melt came from and the leucosomes preserve some of the channels through which the melt moved or sites where it pooled different stages of the melt segregation process are recorded in the leucosome melanosome arrays regions where melting and segregation had just begun when crystallization occurred are characterized by short arrays of thin branching leucosomes with little melanosome a more advanced stage of melting and segregation is marked by the development of residual rocks around extensive branched leucosome arrays generally oriented along the foliation or melting layer places where melting had stopped or slowed down before crystallization began are marked by a high ratio of melanosome to leucosome because most of the melt has drained away very few leucosomes remain to mark the melt escape path this is common in melt depleted granulite terranes many migmatites contain abundant leucosomes oriented parallel to the foliation mostly these represent places where foliation planes dilated and melt drained from the matrix via the branched grain boundary and larger branched melt channel leucosome arrays collected melt collected in the foliation planes was partially or fully expelled later when discordant leucosomes formed leucosomes or veins oriented at high angles to the foliation layering formed last and commonly lack melanocratic borders hence they were not involved in draining the matrix of the melting layer discordant leucosomes represent the channels through which melt flowed out of the melting layer

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

archaean ttgs as sources of younger granitic magmas melting of sodic metatonalites at 0 6 1 2 gpa two natural low k2o na2o ttg tonalitic gneisses one hornblende bearing and the other biotite bearing were partially melted at 0 8 1 2 gpa fluid absent the chief melting reactions involve the breakdown of the biotite and hornblende the hornblende tonalite is slightly less fertile than the biotite tonalite but melt volumes reach around 30 at 1 000 c this contrasts with results of most previous work on more potassic ttgs which generally showed much lower fertility though commonly producing more potassic melts garnet is formed in biotite bearing tonalitic protoliths at p 0 8 gpa and at 1 0 gpa in hornblende bearing tonalitic protoliths all fluid absent experiments produced peraluminous granitic to granodioritic melts typically with sio2 70 wt for the biotite tonalite increasing t formed progressively more melt with progressively lower k2o na2o however the compositions of melts from the hornblende tonalite do not vary significantly with t with increasing p melts from the biotite tonalite become less potassic due to the increasing thermal stability of biotite for the hornblende tonalite again there is no consistent trend fluid absent melting of sodic ttgs produces melts with insufficient k2o to model the magmas that formed the voluminous late potassic granites that are common in archaean terranes reconnaissance fluid present experiments at 0 6 gpa imply that h2o saturated partial melting of ttgs is also not a viable process for producing magmas that formed these granites the protoliths for these must have been more potassic and less silicic nevertheless at granulite facies conditions sodic ttgs will produce significant quantities of broadly leucogranodioritic melt that will be more potassic than the protoliths upward abstraction of this melt would result in some lile depletion of the terrane younger k rich magmatism is unlikely to represent recycling of ttg crust on its own and it seems most likely that evolved crustal rocks and or highly enriched mantle must be involved

shrimp u pb zircon ages of the fuping complex implications for late archean to paleoproterozoic accretion and assembly of the north china craton the fuping complex is situated in the central part of the north china craton and consists of four major lithological assemblages fuping tonalitic trondhjemitic granodioritic gneisses longquanguan augen gneisses wanzi supracrustal assemblage and nanying granitic gneisses shrimp u pb geochronology combined with u th and cathodoluminescence cl imaging of zircon enables resolution of magmatic and metamorphic events that can be directed towards understanding the late archean to paleoproterozoic history of the fuping complex cl images reveal the coexistence of magmatic and metamorphic zircons in nearly all rock types of the fuping complex the metamorphic zircons occur as either single grains or overgrowth or recrystallization rims surrounding and truncating oscillatory zoned magmatic zircon cores and are all characterized by nebulous zoning or being structureless with extremely high luminescence and very low th contents these features make them distinct from magmatic zircons that are characterized by concentric oscillatory zoning comparatively low luminescence and high th and u contents shrimp u pb analyses on magmatic zircons reveal that the tonalitic trondhjemitic and granodioritic plutons of the fuping gneisses were emplaced at 2523 14 ma 2499 10 ma and 2486 8 ma respectively whereas the monzogranitic and granitic plutons of the longquanguan augen gneisses were intruded respectively at 2510 22 ma and 2507 11 ma prismatic and oscillatory zoned zircons dominate in the pelitic rocks of the wanzi supracrustal assemblage and are interpreted as detritus from igneous source rocks the concordant and discordant u pb ages of 2502 5 ma and 2507 14 ma obtained from two pelitic rock samples indicate these rocks must have been deposited no earlier than 2507 ma ago in addition a zoned zircon grain in one pelitic rock sample has a near concordant age of 2109 5 1 ma which may provide a maximum depositional age for the wanzi supracrustal rocks shrimp results also reveal that granitic magmatism assigned to the nanying granitic gneisses occurred over a protracted interval from 2077 13 ma to 2024 21 ma the nebulously zoned zircon grains and overgrowth recrystallization zircon rims from different rocks yielded similar concordant 207pb 206pb ages in the range 1875 to 1802 ma interpreted as approximating the age of regional metamorphism of the fuping complex timing of primary zircon crystallization and regional metamorphism of the fuping complex is in general agreement with recent u pb zircon ion probe results for the wutai and hengshan complexes that bound the fuping complex to the northwest these areas are characterized by the emplacement of major granitoid bodies at around 2 50 ga to 2 48 ga ago deposition of supracrustal rocks in the paleoproterozoic intrusion of paleoproterozoic granitic bodies at 2 1 to 2 0 ga and regional metamorphism at 1 875 to 1 802 ga these data indicate that the fuping and hengshan complexes do not represent an older crystalline basement to the wutai complex as suggested in previous tectonic models but together with the wutai complex represent elements of a single late archean to paleoproterozoic magmatic arc system that has been subsequently tectonically disrupted and juxtaposed during the collision of the eastern and western north china blocks at 1 85 ga which resulted in the final assembly of the north china craton

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

an experimental study of grain scale melt segregation mechanisms in two common crustal rock types creation of pathways for melt to migrate from its source is the necessary first step for transport of magma to the upper crust to test the role of different dehydration melting reactions in the development of permeability during partial melting and deformation in the crust we experimentally deformed two common crustal rock types a muscovite biotite metapelite and a biotite gneiss were deformed at conditions below at and above their fluid absent solidus for the metapelite temperatures ranged between 650 and 800 c at pc 700 mpa to investigate the muscovite dehydration melting reaction for the biotite gneiss temperatures ranged between 850 and 950 c at pc 1000 mpa to explore biotite dehydration melting under lower crustal conditions deformation for both sets of experiments was performed at the same strain rate e 1 37 10 5 s 1 in the presence of deformation the positive v and associated high dilational strain of the muscovite dehydration melting reaction produces an increase in melt pore pressure with partial melting of the metapelite in contrast the biotite dehydration melting reaction is not associated with a large dilational strain and during deformation and partial melting of the biotite gneiss melt pore pressure builds more gradually due to the different rates in pore pressure increase melt enhanced deformation microstructures reflect the different dehydration melting reactions themselves permeability development in the two rocks differs because grain boundaries control melt distribution to a greater extent in the gneiss muscovite dehydration melting may develop melt pathways at low melt fractions due to a larger volume of melt in comparison with biotite dehydration melting generated at the solidus this may be a viable physical mechanism in which rapid melt segregation from a metapelitic source rock can occur alternatively the results from the gneiss experiments suggest continual draining of biotite derived magma from the lower crust with melt migration paths controlled by structural anisotropies in the protolith

migmatites formed by water fluxed partial melting of a leucogranodiorite protolith microstructures in the residual rocks and source of the fluid abstract the opatica subprovince in the canadian shield is a late archaean 2761 2702 ma plutonic arc formed above a north dipping subduction zone anatexis 2690 2677 ma of leucogranodiorite and leucotonalite orthogneisses in the opatica generated migmatites in an area of north vergent back thrusts visible at the surface and in l ithoprobe seismic profile 48 schollen diatexite migmatites occur in the thrusts and metatexite migmatites between them the modal mineralogy microstructure and whole rock major trace and oxygen isotope compositions of the protolith and migmatites were investigated to 1 determine the melting reaction 2 find microstructural criteria for identifying residual rocks in leucocratic systems where there is no melanosome and 3 to determine the source of the fluid involved in anatexis partial melting of the protolith did not change the mineral assemblage but the abundance of quartz and microcline both declined and plagioclase and biotite increased in the residual rocks quartz plagioclase and microcline show evidence for dissolution and biotite does not thus water fluxed melting of quartz plagioclase microcline occurred a mass balance indicates 25 30 partial melting the melting reaction consumed the microcline and created essentially monomineralic domains of plagioclase extraction of 80 90 of the melt left a thin film of melt on the grain boundaries and crystallization of these in the plagioclase domains created diagnostic microstructures microcline fills the last remaining pore space and forms high aspect ratio crystals between plagioclases or triangular crystals at grain junctions quartz shows a range of morphologies from high aspect ratio films through the string of beads to isolated rounded grains as the microstructure progressively equilibrated after crystallisation most accessory phases including zircon remained in the residuum however almost all the schollen migmatites have high contents of th u nb ta and ree relative to the protolith due to contamination by accessory phases derived from mafic rocks disaggregation of the mafic rocks may have been facilitated by the high strain in the back thrusts where the schollen diatexites formed average whole rock 18 o for the protolith and migmatites are similar ca 8 2 and the small difference between melt rich 8 6 and residuum rich rocks 8 0 is consistent with fractionation thus the fluid that caused melting was probably of metamorphic origin with 18 o similar to the protolith the seismic profile shows several reflectors extending to a present depth of 20 km ca 40 km in the late archaean under the migmatites these are the paths along which the metamorphic fluid migrated and generated the migmatites now at the surface a new type of neosome reported in this study may have formed along fractures that the fluids migrated along however these are peripheral pathways in the metatexites adjacent to the back thrusts and schollen diatexites

development of migmatites and the role of viscous segregation in high t metamorphic complexes example from the ryoke metamorphic complex mikawa plateau central japan abstract the ryoke metamorphic complex of the mikawa plateau central japan contains well preserved geological and thermal structures formed during cretaceous high temperature high t metamorphism these structures can be explained by a pervasive melt migration model however the estimated flux of melt is much higher than that expected for compaction flow of felsic melt an additional tectonic force and or melt segregation to network of veins or layers is therefore required this paper presents the results of numerical modeling of the structural evolution of immiscible viscous fluids under simple shear deformation and discusses the role of viscous segregation in enabling pervasive melt flow in hot crust the results of numerical modeling of simple shear deformation of immiscible viscous fluids reveal that low viscosity fluids segregate into lenses and layers this segregation is a type of viscous segregation that takes place to minimize the dissipation of energy in the system the rate of energy dissipation is proportional to viscosity therefore this process is expected to occur universally in immiscible viscous fluids with large viscosity contrasts and is an important process in focusing melt in partially melted rocks the structures produced by the numerical simulation are similar to those of migmatites the simulation results suggest that melts distributed randomly in partially melted rock will segregate into lenses or layers with progressive simple shear deformation during deformation enhanced segregation a melt network will form dynamically if this process occurs at small to large scales in the hot crust melts will be effectively transported upward due to buoyancy forces it is possible that pervasive migmatites at low structural levels within high t metamorphic complexes may represent fossil melt pathways formed by crustal scale viscous segregation

shrimp u pb zircon geochronology of palaeoproterozoic metasedimentary rocks in the north china craton evidence for a major late palaeoproterozoic tectonothermal event abstract although metasedimentary successions are widely distributed across the north china craton those of palaeoproterozoic age are restricted to three orogenic belts namely the trans north china orogen the khondalite belt and the jiao liao ji belt prior to this work some of those successions were considered to be archaean in age the palaeoproterozoic metasedimentary rocks contain detrital igneous zircon grains with ages ranging from 3 0 to 2 1 ga with the majority recording an age between 2 3 and 2 0 ga this latter group define a series of magmatic events that occurred between formation of the archaean basement and deposition of the metasedimentary rock protoliths the metasedimentary rocks occur as two associations older volcano sedimentary units that formed between 2 37 and 2 0 ma and younger sedimentary units that formed mainly between 2 0 and 1 88 ga all these rock associations were then subjected to metamorphism during the late palaeoproterozoic 1 88 and 1 85 ga tectonothermal event named the luliang movement in early chinese literature which was a continent continent collision that led to the amalgamation of the north china craton

the development of contrasting structures during the cooling and crystallisation of a syn kinematic pluton structural elements of a syn kinematic pluton in the pilbara craton of western australia reflect evolving rheology during progressive crystallisation representing a transition from magmatic through high temperature sub solidus to low temperature deformation magmatic elements include 1 nne trending sub vertical kilometre scale folds outlined by xenolith trains that have a magmatic axial planar foliation defined by aligned k feldspar 2 cross cutting ene trending sub vertical shear zones intruded by numerous metre scale granite sheets and 3 a narrow concordant magmatic k feldspar foliation in the granite host rock adjacent to the shear zones microstructures such as fractured feldspars healed by quartz that is continuous with matrix grains further suggest that deformation occurred while the pluton was magmatic high temperature structural elements include checkerboard sub grain boundaries in quartz whereas deformation at lower temperatures is indicated by kinked feldspar recrystallised microcline and undulose quartz a nne trending sub vertical sub solidus foliation defined by elongate quartz aggregates overprints all the above mentioned features generation of these sequential structural elements correspond to the transition from newtonian fluid through bingham type to newtonian solid behaviour as the crystal fraction of the felsic magma increased the orientation of the structures is consistent with a regional ese wnw shortening direction throughout magma crystallisation this study indicates how evolving magma rheology and regional strain regimes control the micro to macro structural elements of a syn tectonic pluton

water saturated and undersaturated melting of metaluminous and peraluminous crustal compositions at 10 kb evidence for the origin of silicic magmas in the taupo volcanic zone new zealand and other occurrences

age and evolution of a late archean to paleoproterozoic upper to lower crustal section in the wutaishan hengshan fuping terrain of northern china abstract the taihangshan wutaishan area forms part of the central north china craton and consists of three main components the hengshan and fuping complexes containing predominantly ductilely deformed late archean to paleoproterozoic high grade granitoid orthogneisses intruded by mafic dykes of gabbroic composition and the low grade late archean wutai greenstone belt developed between the high grade terrains and consisting of bimodal volcanic rocks and metasediments associated with coeval granitoids zircon dating shows that both the hengshan and fuping complexes were intruded by major granitoid bodies between 2 52 and 2 48 ga with rare occurrences of 2 7 ga gneisses wutai granitoids adjacent to the fuping complex also contain 2 7 ga xenocrysts all possibly reflecting the same older basement rare granitic gneisses in the hengshan have emplacement ages of 2 35 ga and show the same structural features as the older rocks indicating that the main deformation occurred after 2 3 ga deposition of supracrustal rocks and emplacement of mafic dykes occurred in the paleoproterozoic together with intrusion of granitic bodies at 2 1 2 0 ga and all underwent regional metamorphism at 1 88 1 85 ga we interpret the hengshan and fuping granitoid gneisses as the lower plutonic part of a late archean to early paleoproterozoic japan type magmatic arc with the upper volcanic part represented by the wutai complex components of this arc may have evolved at a continental margin as indicated by the 2 7 ga zircons virtually all magmatic events are recorded in all three complexes however the 1 85 ga metamorphic event is only recorded in the higher grade portions of the wutai since low grade and presumably high crustal level precluded metamorphic zircon growth shear zones in the hengshan are interpreted as major lower crustal discontinuities post dating the peak of high pressure granulite metamorphism we suggest that they formed during orogenic collapse and uplift of the hengshan complex in the late paleoproterozoic 1800 ma

deformation history of the hengshan wutai fuping complexes implications for the evolution of the trans north china orogen abstract the trans north china orogen separates the north china craton into two small continental blocks the eastern and western blocks as one of the largest exposure in the central part of the orogen the hengshan wutai fuping complexes consist of four lithotectonic units the wutai hengshan and fuping complexes and the hutuo group the hengshan complex contains high pressure mafic granulites and retrograded eclogites structural analysis indicates that most of the rocks in these complexes underwent three distinct episodes of folding d 1 to d 3 and two stages of ductile thrust shearing stz 1 between d 1 and d 2 and stz 2 after d 3 the d 1 deformation formed penetrative axial planar foliations s 1 mineral stretching lineations l 1 and rarely preserved small isoclinal folds f 1 in the hengshan and fuping complexes in the wutai complex however large scale f 1 recumbent folds with sw vergence are displayed by sedimentary compositional layers penetrative transposition resulted in stacking of thrust sheets which are separated by ductile shear zones stz 1 the kinematic indicators of stz 1 in the hengshan and wutai complexes show top to the s230 w thrusting likely related to northeastward oblique pre collisional subduction d 1 resulted in crustal thickening with resultant prograde peak metamorphism the hutuo group did not undergo the d 1 deformation either because sedimentation was coeval with the d 1 deformation or because it was at a high structural level and was not influenced directly by the early deformation the d 2 deformation produced nw verging asymmetric and recumbent folds the d 2 deformation is interpreted to have resulted from collision between the eastern and western blocks of the north china craton in the hutuo group and the fuping complex the development of ese verging asymmetric tight folds is associated with d 2 the structural pattern resulting from superimposition of d 1 and d 2 is a composite synform in the hengshan wutai fuping complexes all four lithotectonic units were superposed during the later d 3 deformation the d 3 deformation developed nw trending open upright folds ongoing collision led to development of transpressional ductile shearing stz 2 forming the transpressional zhujiafang dextral ductile shear zone between the northern hengshan complex and the southern hengshan complex and generating the sinistral longquanguan ductile shear zone between the fuping complex and the wutai complex respectively the stz 1 and d 2 deformation were possibly responsible for fast syn collisional exhumation of the high pressure mafic granulites and retrograded eclogites the structural patterns and elucidation of the deformation history of the hengshan wutai fuping complexes places important constraints on the tectonic model suggesting that an oceanic lithosphere between the eastern and western blocks underwent northeastward directed oblique subduction beneath the western margin of the eastern block and that the final closure of this ocean led to collision between the two blocks to form the coherent basement of the north china craton

seismicity patterns prior to the 2011 el hierro eruption abstract the first fully monitored volcanic eruption in the canary islands started on 10 october 2011 2 km south of the island of el hierro this submarine eruption was preceded by various precursory signals the most evident of which were the seismic series that started in july 2011 consisting of almost 10 000 low magnitude earthquakes located by manual picking this seismicity revealed a 20 km horizontal migration from the north of the island to the south at depths of between 10 and 17 km the deeper events occurring further south we applied a relative location algorithm hypodd to improve hypocentral locations the resulting distribution is more compact slightly shallower and reveals a clearer difference in depths from north to south tests performed to check the reliability of the results gave maximum uncertainties of 400 m in the relocations furthermore new features were found including the origin of the seismicity in the center of the island and the presence of two alternating seismogenic zones in the north of the island during the first month of activity online material figures of waveforms from cross correlation data statistics stereogram of ray parameters ray tracing tests and table of hypodd relocations

formation crystallization and migration of melt in the mid orogenic crust muskoka domain migmatites grenville province ontario migmatitic orthogneisses in the muskoka domain southwestern grenville province ontario formed during the ottawan stage c 1080 1050ma of the grenvillian orogeny stromatic migmatites are volumetrically dominant comprising granodioritic gneisses with 2 5cm thick granitic leucosomes locally rimmed by thin melanosomes that constitute 20 30 vol and locally 40 50 vol of the outcrops patch migmatites in dioritic gneisses form large 10m pinch and swell structures within the stromatic migmatites and consist of decimetre scale irregular patches of granitic leucosome surrounded by medium grained hornblende plagioclase melanosomes interpreted as restite the patches connect to larger networks of zoned pegmatite dykes petrographic and geochemical evidence suggests that the patch leucosomes formed by 20 40 fluid present equilibrium melting of the dioritic gneiss followed by feldspar dominated crystallization the dyke networks may have resulted from hydraulic fracturing probably when the melts reached water saturation during crystallization field and geochemical data from the stromatic migmatites suggest a similar petrogenesis to the patch migmatites but with significant additions of externally derived melts indicating that they acted as conduits for melts derived from deeper structural levels within the orogen we hypothesize that the muskoka domain represents a transfer zone for melts migrating to higher structural levels during grenvillian deformation

single zircon grains record two paleoproterozoic collisional events in the north china craton abstract the western block of the north china craton consists of the yinshan block in the north and the ordos block in the south which were amalgamated along the east west trending khondalite belt at 1 95 ga the western block then collided with the eastern block to form the coherent basement of the north china craton along the north south trending trans north china orogen at 1 85 ga the huaian complex a high grade terrrane located at the conjunction of the khondalite belt and trans north china orogen records metamorphic events associated with both collisions the complex consists of lithologies from both the khondalite belt and trans north china orogen of which the former consist of graphite garnet sillimanite gneiss garnet quartzite felsic paragneiss calc silicate rock and marble together called the khondalite series zircons in the graphite garnet sillimanite gneiss can be divided into three types 1 spherical grains without internal structures 2 grains with a core and rim structure and 3 grains with a dark core surrounded by double rims except for the dark core in type 3 all other types of zircon domains are structureless and highly luminescent with very low th u ratios typical of a metamorphic origin analyses on the cores of type 2 and the inner rims of type 3 from two samples yield upper intercept ages of 1946 26 and 1947 22 ma similar to previously determined metamorphic ages from the khondalite belt and thus interpreted as the time of collision between the yinshan and ordos blocks analyses on type 1 zircons rims of type 2 and the outer rims of type 3 from the same two samples give ages of 1850 15 and 1857 16 ma interpreted as the time of collision between the eastern and western blocks thus zircons in the graphite garnet sillimanite gneiss of the huaian complex record both of the paleoproterozoic collisional events in the north china craton

origin of migmatites by deformation enhanced melt infiltration of orthogneiss a new model based on quantitative microstructural analysis a detailed field study reveals a gradual transition from high grade solid state banded orthogneiss via stromatic migmatite and schlieren migmatite to irregular foliation parallel bodies of nebulitic migmatite within the eastern part of the gfohl unit moldanubian domain bohemian massif the orthogneiss to nebulitic migmatite sequence is characterized by progressive destruction of well equilibrated banded microstructure by crystallization of new interstitial phases kfs pl and qtz along feldspar boundaries and by resorption of relict feldspar and biotite the grain size of all felsic phases decreases continuously whereas the population density of new phases increases the new phases preferentially nucleate along high energy like like boundaries causing the development of a regular distribution of individual phases this evolutionary trend is accompanied by a decrease in grain shape preferred orientation of all felsic phases to explain these data a new petrogenetic model is proposed for the origin of felsic migmatites by melt infiltration from an external source into banded orthogneiss during deformation in this model infiltrating melt passes pervasively along grain boundaries through the whole rock volume and changes completely its macro and microscopic appearance it is suggested that the individual migmatite types represent different degrees of equilibration between the host rock and migrating melt during exhumation the melt topology mimicked by feldspar in banded orthogneiss forms elongate pockets oriented at a high angle to the compositional banding indicating that the melt distribution was controlled by the deformation of the solid framework the microstructure exhibits features compatible with a combination of dislocation creep and grain boundary sliding deformation mechanisms the migmatite microstructures developed by granular flow accompanied by melt enhanced diffusion and or melt flow however an ams study and quartz microfabrics suggest that the amount of melt present did not exceed a critical threshold during the deformation to allow free movements of grains

contrasted tectonic styles for the paleoproterozoic evolution of the north china craton evidence for a 2 1 ga thermal and tectonic event in the fuping massif structural analysis along with 40ar 39ar and u pb datings in the fuping massif provide new insight into the evolution of the eastern part of the trans north china belt north china craton from 2 7 ga to 1 8 ga d1 is responsible for the development of a dome and basin structure coeval with crustal melting giving rise to migmatite and nanying gneissic granites at 2 1 ga this dome and basin architecture resulted from the interference between a n s compression of a weak ductile crust and gravity driven vertical flow in a high thermal regime the next events involved flat lying ductile thrusting d2 and normal faulting d3 dated at around 1880 ma and 1830 ma respectively the d2 and d3 events belong to the trans north china orogeny that results in the final amalgamation of the north china craton the d1 deformation is considered as evidence for an earlier orogen developed around 2 1 ga prior to the trans north china orogeny the change in the deformation style between the 2 1 ga and 1 8 ga could be viewed as a consequence of the cooling of the continental crust in the north china craton

archean blocks and their boundaries in the north china craton lithological geochemical structural and p t path constraints and tectonic evolution an examination of lithological geochemical geochronological structural and metamorphic p t path data suggests that the basement of the north china craton can be divided into eastern and western blocks separated by major crustal boundaries that roughly correspond with the limits of a 300 km wide zone called the trans north china orogen the eastern block consists predominantly of late archean domiform tonalitic trondhjemitic granodioritic ttg batholiths surrounded by anastomosing networks and linear belts of open to tight synforms of minor volcanic and sedimentary rocks metamorphosed from greenschist to granulite facies at 2 5 ga with anticlockwise p t paths some early to middle archean rocks are locally present in the eastern block but their tectonic history is unclear due to reworking by the 2 5 ga tectonothermal event the western block has a late archean assemblage structural style and metamorphic history similar to that of the eastern block but it differs in the absence of early to middle archean assemblages and in being overlain by and interleaved with paleoproterozoic khondalites which were affected by a 1 8 ga metamorphic event involving clockwise p t paths a mantle plume model is proposed for the formation and evolution of late archean basement rocks in the eastern and western blocks based on a combination of extensive exposure of ttg gneisses affinities of mafic rocks to continental tholeiitic basalts presence of voluminous komatiitic rocks dominant diaprism related domiform structures anticlockwise p t paths and a short time span from the primary emplacement of ttg and ultramafic to mafic rocks until the onset of regional metamorphism between the two blocks is the trans north china orogen which is bounded by two major fault systems and is composed of late archean to paleoproterozoic ttg gneisses and granitoids interleaved with abundant sedimentary and volcanic rocks that are geochemically interpreted as having developed in magmatic arc and intra arc basin environments these rocks underwent multiple phases of compressional deformation and peak high pressure metamorphism followed by rapid exhumation during the late paleoproterozoic at 1 8 ga as a result of collision between the eastern and western blocks resulting in the amalgamation of the north china craton

the paleoproterozoic north hebei orogen north china craton s collisional suture with the columbia supercontinent abstract understanding the geologic history and position of the north china craton in the paleoproterozoic columbia supercontinent has proven elusive paleoproterozoic orogenic episodes 2 00 1 85 ga are temporally associated with ultimate stabilization of the north china craton ncc followed by the development of extensive craton wide rift systems at 1 85 1 80 ga the age difference between the sedimentary cover and the metamorphic basement is up to 500 700 ma suggesting that uplift and doming of cratonic basement occurred in the latest paleoproterozoic mafic dike swarms 1 80 1 77 ga and anorogenic magmatism 1 80 1 70 ga record the extensional breakup and dispersal of the north china craton during this stage the late paleoproterozoic tectonic framework and geological events documented provide important constraints for reconstruction of the ncc within the late paleoproterozoic supercontinent of columbia an east west striking thousand kilometer long belt of khondalites granulite facies metapelites stretches along the northern margin of the north china craton on the cratonward side of the northern hebei orogenic belt this granulite belt includes mg al sapphirine bearing granulites that reached ultrahigh temperature peak metamorphic conditions of 1000 c at 10 kbars at 1927 11 ma following peak ultrahigh temperature conditions the rocks underwent initial isobaric cooling and subsequent isothermal decompression and these trajectories are interpreted to be part of an overall anti clockwise p t evolution indicating that the northern margin of the craton experienced continental collision at 1 93 1 92 ga the position of the khondalite belt south of the northern hebei orogenic belt makes it analogous to tibet a continental collision related plateau characterized by double crustal thicknesses and granulite facies metamorphism at depth we suggest that the tectonic evolution of the ncc during this period was closely related to the assembly and break up of the columbia supercontinent and that the ncc was adjacent to the baltic and amazonian cratons in the period 2 00 1 70 ga craton wide extension occurred within 100 150 ma of collision along the northern margin of the craton at 1 93 1 92 ga it is concluded that mantle upwellings are chiefly responsible for the breakup of the ncc from the paleoproterozoic supercontinent

partial melt topology in statically and dynamically recrystallized granite microstructures of the murray granite pluton central ontario canada show evidence of both static and dynamic crystallization subsequent to partial melting backscattered electron analyses reveal interstitial k feldspar and plagioclase at triple junctions of strain free isometric quartz grains the geometry of the quartz feldspar boundaries mimics the original topology of the quartz melt contacts during crystallization this conclusion is suggested by the occurrence of both rounded and planar faces of quartz grains and by low 27 dihedral angles of quartz quartz feldspar boundaries similar to dihedral angles in experimentally crystallized quartz quartz silicic melt systems in contrast feldspar seams in deformed granites have high axial ratios are usually elongated perpendicular to the foliation plane and are located preferentially along individual grain boundaries quartz grains are dynamically recrystallized and occasionally transected by feldspar seams indicating that fracturing occurred in the presence of melt during crystal plastic deformation of quartz the subparallel orientation in quartz grains of intragranular feldspar bearing fractures and interstitial feldspar seams suggests that these features originated as intragranular and intergranular fractures respectively partial melt topology was therefore controlled by intergranular and occasionally by intragranular fracturing

incongruent melting of biotite to spinel in a quartz free restite at el joyazo se spain textures and reaction characterization in the grt bt sil restitic xenoliths of el joyazo cerro de hoyazo hercynitic spinel is a minor phase commonly associated with biotite the possible reaction relationships among biotite and spinel are studied in reaction textures developed around biotites at their contact with patches of fibrolitic sillimanite and rhyolitic melt in these textures resorbed biotite crystals about 1 mm long are rimmed by a layer of glass 5 mm away from the reaction texture biotite is chemically zoned cores bt 1 have xmg 0 35 0 02 and ti 0 58 0 01 atoms whereas the outer rims bt 2 have xmg 0 45 0 01 and ti up to 0 68 atoms the hercynite rich spinel spl has low zno content 0 80 wt and xmg 0 26 0 04 the chemical compositions of the mix aggregate represent linear combinations between sillimanite and a silica rich melt this melt melt 1 is different from that of the layer around biotite melt 2 which is also richer in ca and alkalis garnet rims grt have low ca and mn and xmg 0 14 plagioclase is characterized by large homogeneous cores pl 1 an31 2 and more calcic rims pl 2 an49 6 matrix analysis in the 9 component al ca fe k mg mn na si ti 9 phase bt1 bt2 grt spl ilm melt2 mix pl1 pl2 system provides the mass balance in mole units

shrimp data on zircons from the wanzi series constraints on the rock formation time and implications of migmatization at 2 1 2 0 ga in the fuping complex north china craton in most of the rocks of the fuping complex in the trans north china orogen a large quantity of leucocratic veins dykes and granitic intrusions are present as migmatites the incongruent partial melting of biotite to hornblende suggests the water fluxed anatexis of the migmatization of which the melt is manifested by the euhedral crystallization of some quartzofeldspathic minerals and presence of albite rim on earlier plagioclase or k feldspar importantly some melt may be segregated in the sillimanite gneisses or extracted and injected as faint dispersed melt into the competent metasedimentary quartzite of the wanzi series rocks the quartzite has plenty of zircon ages cluster at c 2 5 ga and some at c 2 1 2 0 ga the former may be detrital zircon age while the latter represents the age of migmatization or crystallization of the injected melt not detrital zircon ages as conventionally believed the data have constrained that the sedimentation time of the protolith of the quartzite of the wanzi series was between c 2 5 ga and c 2 1 2 0 ga not younger than c 2 1 2 0 ga as once proposed therefore it must be cautious in applying detrital zircon ages in constraining the time of sedimentation if the rocks have undergone middle high grade metamorphism especially when migmatization was involved in the metamorphism process

ree fractionation and nd isotope disequilibrium during crustal anatexis constraints from himalayan leucogranites abstract the processes controlling rare earth element ree behaviour during crustal anatexis have profound implications not only for ree abundances of crustally derived granites but also for their model nd ages for typical protoliths of anatectic granites such as are exposed in metapelites from the high himalayan crystalline series hhcs a high proportion 90 of bulk rock lree and mree are sited in the accessory phases apatite and monazite and of hree in garnet and apatite mass balance constraints indicate that observed ree abundances of himalayan leucogranites cannot be generated from ree deficient reactant phases in their metasedimentary protoliths muscovite biotite plagioclase and quartz but must result from the dissolution of accessory phases ree and th abundances in anatectic melts are modelled by combining empirical expressions for monazite apatite and zircon solubilities as a function of temperature melt composition and melt water content with analysed accessory phase ree abundances this approach which allows the contribution to the melt from each accessory phase to be identified successfully predicts himalayan leucogranite trace element abundances the lree and th concentrations of a metapelite derived melt are buffered by monazite stability although apatite dissolution contributes up to 10 of lree to the melt together with 50 of the mree and 90 of the hree zircon contributes a negligible proportion of ree to the melt during anatexis dissolution of monazite and apatite in the melt results in 1 increasing ree abundances and lree hree ratios with increasing temperature of melting 2 a negative eu anomaly in the melt 3 high sm nd ratios in the melt relative to its metasedimentary protolith and 4 nd isotope disequilibrium between the melt and its protolith all of these features are observed in the ree geochemistry of himalayan leucogranites the combined effects of sm nd fractionation and nd isotope disequilibrium during anatexis imply that model nd ages calculated for anatectic granites derived from apatite and monazite bearing protoliths should be treated with considerable caution

coupling of anatectic reactions and dissolution of accessory phases and the sr and nd isotope systematics of anatectic melts from a metasedimentary source advances in field observations and experimental petrology on anatectic products have motivated us to investigate the geochemical consequences of accessory mineral dissolution and nonmodal partial melting processes incorporation of apatite and monazite dissolution into a muscovite dehydration melting model allows us to examine the coupling of the rb sr and sm nd isotope systems in anatectic melts from a muscovite bearing metasedimentary source modeling results show that 1 the sm nd ratios and nd isotopic compositions of the melts depend on the amount of apatite and monazite dissolved into the melt and 2 the relative proportion of micas muscovite and biotite and feldspars plagioclase and k feldspar that enter the melt is a key parameter determining the rb sr and 87 sr 86 sr ratios of the melt furthermore these two factors are not in practice independent in general nonmodal partial melting of a pelitic source results in melts following one of two paths in e nd 87 sr 86 sr ratio space a higher temperature fluid absent path path 1 represents those partial melting reactions in which muscovite biotite dehydration and apatite but not monazite dissolution play a significant role the melt will have elevated rb sr 87 sr 86 sr sm nd and e nd values in contrast a lower temperature fluid fluxed path path 2 represents those partial melting reactions in which muscovite biotite dehydration plays an insignificant role and apatite but not monazite stays in the residue the melt will have lower rb sr 87 sr 86 sr sm nd and e nd values than its source the master variables controlling both accessory phase dissolution and hence the sm nd system and melting reaction and hence the rb sr systematics are temperature and water content the complexity in sr nd isotope systematics in metasediment derived melts as suggested in this study will help us to better understand the petrogenesis for those granitic plutons that have a significant crustal source component

polyphase deformation of the fuping complex trans north china orogen structures shrimp u pb zircon ages and tectonic implications abstract the fuping complex is located in the central part of the trans north china orogen tnco a paleoproterozoic collisional belt along which the eastern and western blocks were assembled to form the north china craton three stages of deformation d 1 d 3 have been identified in the fuping complex d 1 formed tight to isoclinal folds f 1 penetrative foliation s 1 and mineral lineation l 1 which resulted from the early nw se orientated thrusting and shearing d 2 progressively overprinted d 1 and was dominated by mostly nw se shortening and top to the se e thrusting under a compressive environment d 1 and d 2 occurred in association with the peak metamorphism m 1 d 3 occurred during the exhumation after the crustal thickening producing regional scale nww see trending open folds f 3 and associated low angle detachment faults and was responsible for the isothermal decompression m 2 metamorphism of the complex following the d 3 deformation the complex was exhumed to shallow crustal levels where the rocks underwent the retrogressive cooling m 3 metamorphism syn tectonic leucocratic dykes are widespread in the fuping complex two leucocratic dykes that did not experience d 1 but underwent d 2 yield shrimp zircon ages of 1843 12 ma and 1844 18 ma which indicate that d 1 must have occurred at some time earlier than 1844 ma whereas d 2 must have occurred at some time later than this age two weakly deformed post d 2 leucocratic dykes yield shrimp zircon ages of 1817 14 ma and 1815 45 ma suggesting that the d 2 deformation occurred in the period 1843 1815 ma the new shrimp data indicate that the collision related deformation of the fuping complex must have occurred in the paleoproterozoic not in the late archean structural and geochronological data presented in this study in association with previous lithological and geochemical data suggest that the fuping complex may have been located in the hinterland of an eastward subduction zone and underwent intensive deformation and metamorphism involved in the final collision between the western and eastern blocks at 1 85 ga

geochemistry of the paleoproterozonic nanying granitic gneisses in the fuping complex implications for the tectonic evolution of the central zone north china craton abstract the nanying granitic gneisses represent a major 2 0 ga magmatic event in the fuping complex of the central zone of the north china craton they occur in the chengnanzhuang shear zone which is chiefly developed along the contact between the neoarchean fuping tonalite trondhjemite granodiorite ttg gneisses and the wanzi paragneisses the nanying granitic gneisses were derived from monzogranite and syenogranite with minor amounts of quartz diorite and granodiorite and belong to the metaluminous to peraluminous high potassium calc alkaline series correlations between major and trace elements and similarities in chondrite normalized rare earth element ree patterns support field relationships e g gradational contacts indicating that the nanying granitic gneisses originated from similar magmas the granodioritic monzogranitic and syenogranitic gneisses have similar ranges of initial 87sr 86sr values 0 7030 0 7093 0 7021 0 7193 and 0 7049 0 7111 respectively although these variations are partly attributable to local resetting of the isotopic system during late deformation hydrothermal alteration the whole rock 208pb 204pb 206pb 204pb and 207pb 204pb values of the nanying granitic gneisses are 37 32 76 8 15 24 23 82 and 14 92 17 92 respectively these sr pb isotope systematics together with previously published nd isotope compositions suggest that the nanying granitic gneisses were derived from partal melting of the 2 50 ga fuping ttg gneisses with local assimilation involving the wanzi paragneisses the nanying granitic gneisses are geochemically similar to the cca and ccl groups of granitoids except for a few samples which plot in the hlo group the nanying granitic gneisses exhibit depletion in rb sr ba ti and u and enrichment in th nb zr hf and ree similar to those of syn shear high potassium calc alkaline granitoids and late shear alkaline granitoids they therefore represent the product of a syn collisional to post collisional magmatic event this collisional event which is significantly older than the 1 84 1 80 ga event that has been proposed for the juxtaposition of the eastern and western archean continental blocks in the final stabilization of the north china craton supports a tectonic model involving multiple collisional events for the amalgamation of the internal lithotectonic domains within the central zone and the assembly of the north china craton as a whole