characterization of an early metamorphic stage through inclusions in zircon of a diamondiferous quartzofeldspathic rock from the erzgebirge germany about 1000 zircon grains from a diamondiferous quartzofeldspathic rock of the saxonian erzgebirge were investigated for inclusions using optical microscopy and confocal laser raman spectroscopy cathodoluminescence imagery was applied to characterize the growth zone of zircon where the inclusions occurred the most abundant inclusion minerals are microdiamonds coesite was not detected however garnet and jadeite occur as rare inclusions in zircon cores where diamonds are lacking jadeite was detected for the first time in quartzofeldspathic rocks from the crystalline complex of the erzgebirge the compositions of the pristine garnets in the zircons are similar to those of core areas of millimeter sized garnets but the original garnet composition of the early metamorphic stage is only preserved in zircon intracrystalline diffusion at temperatures as high as 1000 c resulted for instance in higher ti concentrations in garnet cores compared with garnet enclosed in zircon rutile quartz and the compositions of jadeite and garnet inclusions in zircon and of phengite inclusions in cores of large garnets were applied for geothermobarometry the results related to an early metamorphic stage are 650 c and 18 kbar which represent conditions at the base of a thickened continental crust before deeper subduction

dehydration melting of amphibolite at 10 kbar the effects of temperature and time we have simulated the dehydration melting of a natural low k calcic amphibolite 67 4 hornblende 32 5 anorthite in piston cylinder experiments at 10 kbar and 750 1000 c for 1 9 days the solidus temperature is lower than 750 c garnet appears at 850 c the overall reaction is hb p l cpx al hb ca hb ga opx three stages of reaction are 1 melting dominated by the growth of clinopyroxene and garnet with little change in composition of liquid or garnet 2 a reversal of this reaction between 875 c and 900 c with decreases in the amounts of liquid and garnet and 3 a large increase in liquid along with the loss of hornblende and decrease of plagioclase while clinopyroxene and garnet increase garnet is enriched in pyrope and zoned from fe cores to mg edges range 3 mol pyrope liquid composition is enriched first in an to 950 c and then in ab the liquids are more calcic and aluminous than natural tonalites which is attributed to the plagioclase composition an90 the formation of peraluminous liquid from the metaluminous amphibolite is caused by anorthite not h2o saturated conditions the results are consistent with an amphibolite phase diagram with relatively high solidus temperatures in the garnet absent field 900 1000 c but with a solidus backbend at 7 9 kbar coincident with the garnet in boundary hornblende breakdown due to garnet formation in a closed system must make h2o available for h2o undersaturated melting right down to the h2o saturated solidus below 700 c which defines a large low temperature pt area where hydrous granitoid melts can be generated with residual garnet and hornblende

adakites from continental collision zones melting of thickened lower crust beneath southern tibet adakites are geochemically distinct intermediate to felsic lavas found exclusively in subduction zones here we report the first example of such magmas from southern tibet in an active continental collision environment the tibetan adakites were emplaced from ca 26 to 10 ma and their overall geochemical characteristics suggest an origin by melting of eclogites and or garnet amphibolites in the lower part 50 km of thickened tibetan crust this lower crustal melting required a significantly elevated geotherm which we attribute to removal of the tectonically thickened lithospheric mantle in late oligocene time the identification of collision type adakites from southern tibet lends new constraints to not only the himalayan tibetan orogenesis how and when the indian lithosphere started underthrusting asia can be depicted but also the growth of the early continental crust on earth that consists dominantly of the tonalite trondhjemite granodiorite suites marked by adakitic geochemical affinities

ultrahigh pressure minerals and metamorphic terranes the view from china abstract ultrahigh pressure uhp metamorphism refers to mineralogical modifications of continental and oceanic crustal protoliths associated mafic ultramafic rocks initially formed or emplaced in shallow levels of the lithosphere but which subsequently have experienced p t conditions within or above the coesite stability field 2 7 gpa 700 c typical products include eclogite garnet peridotite and uhp varieties of metapelite quartzite marble paragneiss and orthogneiss uhp metamorphic assemblages require relatively cold lithospheric subduction to mantle depths some recrystallization even occurs under forbidden p t conditions characterized by a geotherm of the current explosion of research on continental uhp terranes reflects their significance for mantle dynamics and the tectonics of continental subduction collision exhumation mantle slab interactions and geochemical recycling a further characterization of uhp phases and positive identification of uhp minerals requires new experimental studies coupled with state of the art analyses for example the very rare occurrence of microdiamond inclusions in zircons from dabie sulu uhp rocks may reflect high f o 2 attending recrystallization inasmuch as epidote is rather common rutile needles within garnets from sulu uhp eclogitic rocks may not be the result of exsolution so in such cases the apparent uhp pressure may have been over estimated hadean igneous microdiamond inclusions in jack hills detrital zircons could have originated from mantle xenoliths whereas abundant detrital phanerozoic diamonds containing inclusions of coesite and other eclogitic minerals from new south wales might have been derived from unexposed uhp metamorphic terranes micro mineral intergrowth and nano size minerals may hold important key to deciphering the actual p t paths of subduction and mantle return flow although most exhumed terranes have returned surfaceward relatively rapidly after short time of uhp condition the long duration of storage at great depth and slow exhumation for the largest uhp terranes remain as major problems

supersilicic clinopyroxene and silica exsolution in uhpm eclogite and pelitic gneiss from the kokchetav massif kazakhstan abundant exsolved quartz rods occur in matrix clinopyroxene of eclogite from the kokchetav massif kazakhstan these rocks are diamond grade ultrahigh pressure uhp metamorphic rocks that recrystallized at p 6 gpa and t 1000 c zircon is an excellent container which effectively protects peak uhp metamorphic phases from retrogression therefore to ascertain the pre exsolution composition of the clinopyroxene we analyzed clinopyroxene inclusions in zircon of the eclogite and a diamond bearing biotite gneiss clinopyroxene in zircon has an excess of fe 3 vi al over iv al na k and calculated cation totals significantly less than 4 0 per six o atoms the stoichiometry of these pyroxenes can be reconciled if the ca eskola end member ca 0 5 0 5 alsi 2 o 6 is considered the zircon hosted clinopyroxene in the eclogite contains up to 9 6 mol of the ca eskola component and in the biotite gneiss up to 18 2 mol whereas the matrix clinopyroxene contains much less 1 3 mol on average recalculation of the composition of the matrix clinopyroxene prior to exsolution of quartz rods yields 6 8 mol ca eskola component which is consistent with the composition of the clinopyroxene inclusions in zircon we conclude that the ca eskola component in the peak metamorphic clinopyroxenes broke down by a retrograde reaction 2 ca 0 5 square 0 5 alsi 2 o 6 rightarrow caal 2 sio 6 3 sio 2 resulting in exsolution of the quartz rods in the matrix clinopyroxene subducted crustal and supracrustal rocks form the ca eskola clinopyroxene at high pressures and temperatures the vacancy containing clinopyroxene may have an important bearing on the physico chemical properties of the subducted slab at upper mantle depth

microdiamond daughter crystals precipitated from supercritical coh silicate fluids included in garnet erzgebirge germany microdiamonds associated with phlogopite quartz paragonite phengite apatite and rutile are found as regular constituents of minute polyphase inclusions in garnet of ultrahigh pressure p metamorphic gneiss lenses within migmatites of the gneiss eclogite unit erzgebirge germany these aggregates are interpreted to represent original inclusions of a supercritical dense coh fluid rich in k na and sio 2 from this fluid diamond was precipitated as a daughter crystal due to cooling at ultrahigh p conditions brittle failure of the garnet host due to overpressure during release of confining pressure is demonstrated by healed radial cracks during further cooling the silicate phase assemblage of the inclusions crystallized at reduced internal pressures outside the stability field of diamond as indicated by the presence of quartz paragonite and plagioclase it is proposed that the mica dominated mineral assemblage of the inclusions formed by reaction between the fluid and the garnet host the previously formed diamond daughter crystals being preserved metastably these diamond bearing inclusions provide an unequivocal record of dense supercritical coh fluids rich in alkalies and silica within subducted continental crust during ultrahigh p metamorphism

early paleozoic high mg diorite granodiorite in the eastern kunlun orogen western china response to continental collision and slab break off abstract phanerozoic high mg diorites and granodiorites are known to be related to diverse subduction styles at convergent margin settings but have not been linked to collision processes the santonggou complex in the eastern kunlun orogen is composed of a dominant granodiorite intruded by small stocks and dykes of porphyritic granodiorites and diorites which are all characterized by high mgo and ni but low yb and y contents mafic microgranular enclaves mmes occur in the granodiorite and the porphyritic granodiorites la icpms u pb analyses of zircon from the granodiorite porphyritic granodiorite and porphyritic diorite yield similar ages of 432 2 ma 432 2 ma and 427 3 ma respectively plagioclase in the granodiorite exhibits complex zonations with resorbed and patchy cores plagioclase and hornblende phenocrysts in the porphyritic granodiorites and diorites also show textural and compositional zonations the highest e nd t value of 0 6 in mmes suggests asthenosphere derived mafic magma these lines of mineralogical and geochemical evidence together with results from geochemical modeling suggest that the porphyritic diorites were derived from mantle metasomatized slab melts the porphyritic granodiorites and the granodiorite were generated by simultaneous assimilation and fractional crystallization of the magma approximated by the porphyritic diorites plus mixing of asthenosphere derived mafic and evolved granodioritic magmas pressure estimates from the al in hornblende geobarometer suggest that the granodiorite had been uplifted to near the surface before the emplacement of the porphyritic granodiorites partial melting of the slab upwelling of the asthenosphere and rapid orogenic uplift at 430 ma suggest that the cessation of subduction and the initiation of collision as well as slab break off occurred almost simultaneously in the eastern kunlun orogen

early cretaceous adakitic granites in the northern dabie complex central china implications for partial melting and delamination of thickened lower crust abstract to date few adakitic rocks have been reported in direct association with contemporary intra continental extensional structures which has cast doubt on genetic models involving partial melting of the lower crust this study presents early cretaceous 143 129 ma new sensitive high resolution ion microprobe shrimp zircon u pb ages adakitic granites which are directly associated with a contemporary metamorphic core complex i e the northern dabie complex in the dabie area these granites exhibit relatively high sr contents negligible to positive eu and sr anomalies high la yb and sr y ratios but very low yb and y contents similar to subducted oceanic crust derived adakites they are also characterized however by very low mgo or mg and ni values and nd sr isotope compositions end t 14 6 to 19 4 and 87sr 86sr i 0 7067 0 7087 similar to triassic continent derived eclogites subducted in the dabie sulu orogen additionally late granitic dikes in the adakitic intrusions exhibit low sr contents clearly negative eu and sr anomalies low la yb and sr y ratios but relatively high yb and y contents similar to 118 105 ma granites in the northern dabie complex based on composition and geochronology data of neoproterozoic amphibolites and orthogneisses triassic high to ultra high pressure metamorphic rocks and early cretaceous mafic ultramafic intrusive rocks and the constraints provided by experimental melt data for tonalites metabasaltic rocks and eclogites we suggest that the adakitic granites were most probably generated by partial melting of thickened amphibole or rutile bearing eclogitic lower crust as a consequence of triassic middle jurassic subduction and thrusting the late dikes probably originated from plagioclase bearing intermediate granulites moreover we suggest that late mesozoic delamination or foundering of thickened eclogitic lower crust is also a more plausible mechanism for the petrogenesis of early cretaceous mafic ultramafic intrusive rocks in the dabie area and probably involved partial melting of a mixed source comprising eclogitic lower crust that had delaminated or foundered into upper lithospheric or asthenospheric mantle peridotite asthenospheric upwelling in response to post collisional delamination of lithospheric mantle was likely to have provided the heat source for the cretaceous magmatism

δ13c signature of early graphite and subsequently formed microdiamond from the saxonian erzgebirge germany microdiamond and graphite occur in quartzofeldspathic rocks saidenbachites of the central erzgebirge in saxony zircon from such rocks contains not only relatively high but also variable amounts of early formed graphite and later crystallized diamond however no carbonate six purified zircon concentrates were analysed by mass spectrometry for 13c pdb values which were between 24 and 33 the carbon inclusions resulted in c contents of the concentrates as high as 0 5 wt it is hypothesized that the observed 13c signature results from organic material in the sedimentary protoliths of the saidenbachites probably deposited not earlier than mid devonian this material was transformed to graphite during early metamorphism and subsequently to diamond after an anatectic event at great depths 150 km during these processes the 13c signature of the carbon did not significantly change

multiphase solid inclusions in zoisite bearing eclogite evidence for partial melting of ultrahigh pressure metamorphic rocks during continental collision abstract multiphase solid ms inclusions in both garnet and omphacite were investigated for zoisite bearing eclogite from the ultrahigh pressure uhp metamorphic zone in the sulu orogen the results provide petrological evidence for local anatexis during exhumation of deeply subducted continental crust there are three types of ms inclusions 1 plagioclase quartz 2 plagioclase quartz k feldspar and 3 barite plagioclase k feldspar zoisite epidote the host minerals mostly exhibit radial fractures surrounding the ms inclusions the first and second types of ms inclusions were analyzed for their bulk compositions yielding high sio 2 and na 2 o but very low feo mgo tio 2 with variable k 2 o for the second type trace element analyses of representative ms inclusions yield generally very low concentrations except such large ion lithophile elements as sr ba and rb these features suggest different origins for the three types of ms inclusions the first type of ms inclusions would be primarily derived from dehydration melting of paragonite whereas the second type of ms inclusions would be derived from dehydration melting of both paragonite and phengite the third type of ms inclusion may result from the interaction between metamorphic fluid and host mineral in view of the occurrence of barite as a filling phase in the fractures of host minerals zoisite breakdown is also indicated by its highly cuspate shape in coexistence with quartz providing components for growth of garnet during the exhumation therefore the anatexis of zoisite bearing uhp eclogite is primarily driven by the breakdown of hydrous minerals such as phengite and paragonite the major and trace element compositions of ms inclusions in the eclogites mainly depend on the species of hydrous minerals involved in the mineralogical reactions of dehydration melting in subduction channel

multiple zircon growth during fast exhumation of diamondiferous deeply subducted continental crust kokchetav massif kazakhstan diamondiferous rocks from the kokchetav massif kazakhstan represent deeply subducted continental crust in order to constrain the age of ultra high pressure uhp metamorphism and subsequent retrogression during exhumation zircons from diamondiferous gneisses and metacarbonates have been investigated by a combined petrological and isotopic study four different zircon domains were distinguished on the basis of transmitted light microscopy cathodoluminescence trace element contents and mineral inclusions mineral inclusions and trace element characteristics of the zircon domains permit us to relate zircon growth to metamorphic conditions domain 1 consists of rounded cores and lacks evidence of uhp metamorphism domain 2 contains diamond coesite omphacite and titanian phengite inclusions providing evidence that it formed at uhp metamorphic conditions p 43 kbar t 950 c domain 3 is characterised by low pressure mineral inclusions such as garnet biotite and plagioclase which are common minerals in the granulite facies overprint of the gneisses p 10 kbar t 800 c this multi stage zircon growth during cooling and exhumation of the diamondiferous rocks can be best explained by zircon growth from zr saturated partial melts present in the gneisses domain 4 forms idiomorphic overgrowths and the rare earth element pattern indicates that it formed without coexisting garnet most probably at amphibolite facies conditions p 5 kbar t 600 c the metamorphic zircon domains were dated by shrimp ion microprobe and yielded ages of 527 5 528 8 and 526 5 ma for domains 2 3 and 4 respectively these indistinguishable ages provide evidence for a fast exhumation beyond the resolution of shrimp dating the mean age of all zircons formed between uhp metamorphic conditions and granulite facies metamorphism is 528 3 ma indicating that decompression took place in less than 6 ma hence the deeply subducted continental crust was exhumed from mantle depth to the base of the crust at rates higher than 1 8 cm year we propose a two stage exhumation model to explain the obtained p t t path fast exhumation on top of the subducted slab from depth 140 to 35 km was driven by buoyancy and facilitated by the presence of partial melts a period of near isobaric cooling was followed by a second decompression event probably related to extension in a late stage of continental collision

i type plutonism in a continental back arc setting miocene granitoids and monzonites from the central aegean sea greece miocene extension in the aegean sea was accompanied by the intrusion of granitoid and monzonitic magmas at upper crustal levels with increasing distance from the trench the metaluminous to slightly peraluminous i type intrusives show a systematic regional variation in their compositions medium to high k granodiorites in the wsw high k low silica granites in the central region and high k high silica granites monzonites monzogranites and composite lamprophyric monzonitic dikes in the ene the granodioritic and low silica granitic plutons comprise similar ranges of sio2 63 5 71 6 wt and initial nd sr and o isotopic values end i 7 0 to 9 8 87sr 86sr i 0 7092 0 7115 18o 9 7 11 5 but display significant differences in their chemical compositions compared with the granodiorites the low silica granites have higher abundances of large ion lithophile light rare earth and some high field strength elements initial isotopic signatures of the high silica granites in the ene end i 9 0 to 9 6 87sr 86sr i 0 7128 0 7144 18o 10 8 12 0 are slightly more crust like than those of the low silica granites in chemical variation diagrams samples from the high silica granites continue the trends defined by those from the low silica granites it is suggested that all granitoid magmas originated by dehydration melting of metaluminous older crustal sources most probably metagreywackes u pb systematics of zircon populations in conjunction with zr sio2 systematics and zircon saturation temperatures suggest a high proportion of restitic or assimilated crustal material in these magmas monzonitic to monzogranitic rocks show chemical and isotopic characteristics end i 0 8 to 3 6 87sr 86sr i 0 7059 0 7071 18o 9 4 10 6 that are significantly different from those of the granitoids a small portion of an older crustal component in these magmas is indicated by u pb systematics of zircon populations lamprophyric dikes display chemical and isotopic signatures end i 0 5 87sr 86sr i 0 7044 18o 8 9 that are compatible with an origin from either a mantle source previously enriched by slab derived hydrous fluids melts as suggested by negative nb ta anomalies or a new k rich basaltic underplate abundant composite dikes indicate the coexistence of and mingling mixing between lamprophyric melts and monzonitic magmas

decrepitated uhp fluid inclusions about diverse phase assemblages and extreme decompression rates erzgebirge germany minute polyphase inclusions in garnet of quartzofeldspathic rocks saidenbachite from the saxonian erzgebirge germany contain microdiamond or graphite phlogopite quartz paragonite phengite and other minerals in minor amounts these inclusions are interpreted to represent an original dense coh silicate fluid trapped in crystallizing garnet at depths of 150 km inspection of the inclusion population in a single garnet by sem reveals two characteristic features i the shape of most inclusions indicates healed radial cracks in the host garnet and thus the buildup of a significant differential pressure p i e a contrast in pressure between the inclusion pi and the host mineral pe the mineral assemblages sealing the cracks and showing an equilibrated microstructure indicate temperatures of 750 50 c and pressures below 2 5 gpa ii the diverse types of inclusions appear to be randomly distributed in the garnet host thus the variable phase assemblages do not reflect a compositional evolution of the fluid trapped in the garnet combining observations i and ii we propose that the diversity of the phase assemblage in the inclusions is the result of decrepitation at different times and thus of distinct histories of pi as p at decrepitation is primarily controlled by inclusion size and shape applying a flow law for dislocation creep of garnet a low strength of garnet at 750 50 c for low geological strain rates is predicted thus differential pressure should have been kept low i e pi pe by continuous stretching of the inclusion for typical exhumation rates of metamorphic rocks to attain the differential pressure pi pe required for catastrophic brittle failure of the garnet host the decompression rate must have been extremely high as a robust lower bound a minimum exhumation rate on the order of 100 m year 1 is suggested which corresponds to ascent rates of magma

collision driven slab breakoff magmatism in northwestern anatolia turkey abstract postcollisional eocene magmatism in northwestern anatolia produced two e w trending linear belts of plutons along and north of the izmir ankara erzincan suture zone iaesz whose geochemical features and age relations support a slab breakoff model for their petrogenetic oinevolution the suture zone granitoids szgs in the southern belt have ages around 54 48 ma are intrusive into blueschist rocks of the iaesz and are composed of diorite quartz diorite granodiorite and syenite the marmara granitoids mgs in the northern belt are slightly younger 48 35 ma intrusive into the paleozoic mesozoic crystalline basement rocks of the sakarya continent and composed of monzogranite granite and granodiorite both szgs and mgs have moderately to highly evolved medium to high k calc alkaline compositions and are predominantly metaluminous i type granitoids nd sr isotope systematics documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepacka

generation of sodium rich magmas from newly underplated basaltic crust sodium rich rocks of trondhjemite tonalite dacite ttd or granodiorite ttg suites form much of precambrian continental crust1 they are thought to have formed by partial melting of subducted oceanic crust2 3 a process that would have been much more widespread early in earth history than at present owing to the higher thermal gradients prevailing at that time4 phanerozoic ttd suites do exist however and seem also to relate to subduction zones5 defant and drummond6 proposed that these suites form where young 25 myr hot oceanic lithosphere is subducted and melts thus locally simulating the conditions that led to widespread crustal growth in the archaean here we describe plutonic and volcanic rocks from the cordillera blanca complex in peru which have characteristics of the high al ttd suite but which were produced above a subduction zone containing a 60 myr old slab we present evidence that the complex formed by partial melting of newly underplated basaltic crust and argue that this mechanism should be considered more generally as an additional way of generating sodium rich arc magmas

exsolution of thortveitite yttrialite and xenotime during low temperature recrystallization of zircon from new caledonia and their significance for trace element incorporation in zircon recrystallized zircon grains in a phengite epidote chlorite schist from north eastern new caledonia contain as inclusions a mineral assemblage consisting of celadonite kaolinite quartz fe oxy hydroxide smectite chlorite xenotime y thortveitite yttrialite and allanite ce this assemblage formed during low temperature 100 c seafloor alteration of a plagioclase rich mafic rock and represents the first documented evidence of this alteration event in the high p belt of northeastern new caledonia the survival of this low temperature mineral paragenesis in zircon of a rock that has undergone subsequent eclogite facies metamorphism testifies to the strength and value of zircon as a container for mineral inclusions thortveitite sc2si2o7 and yttrialite y2si2o7 inclusions in the altered zircon cores represent a new occurrence for these minerals and suggest that they may be more common than is currently recognized the altered zircon cores generally have lower trace element contents than the pristine igneous zircon cores and we suggest that thortveitite yttrialite and xenotime y formed as a result of trace element expulsion from zircon during low temperature recrystallization trace element concentrations in the zircon cores indicate that trivalent cations ree y sc in zircon cannot be charge balanced by xenotime substitution alone pentavalent cation concentrations nb as are also insufficient for charge balance the presence of thortveitite sc2si2o7 and yttrialite y2si2o7 suggests that trivalent cations in zircon might be charge balanced either by monovalent anions substituting for oxygen or by small monovalent cations such as h occurring interstitially in the zircon lattice

zircon u pb dating geochemistry and sr nd pb hf o isotopes for the nan getan granodiorites and mafic microgranular enclaves in the east kunlun orogen record of closure of the paleo tethys abstract the east kunlun orogen in the northern qinghai tibet plateau is an ideal region to investigate the geodynamic processes of magmatism related to the closure of the paleo tethys ocean here we report petrology zircon u pb geochronology whole rock geochemistry and multiple isotope data from granodiorites and the associated dioritic enclaves in a dominant nan getan granitoid in the east kunlun orogen zircon u pb ages indicate that the host granodiorites and dioritic enclaves were synchronously emplaced at 243 ma the granodiorites are medium to high k calc alkaline metaluminous a cnk 0 93 0 98 with high al 2 o 3 content 15 28 16 10 mg 47 49 very low sr y ratios 127 217 high abundances of incompatible elements y 3 87 8 36 ppm nb 3 04 5 71 ppm th 3 04 5 71 ppm low 87 sr 86 sr i 0 7050 0 7079 negative whole rock e nd t 8 2 to 5 8 206 pb 204 pb 243ma of 18 520 to 18 772 207 pb 204 pb 243ma of 15 611 to 15 650 208 pb 204 pb 243ma of 38 227 to 38 528 18 o smow 6 8 9 1 e hf t of 1 2 to 2 4 the dioritic enclaves sio 2 51 08 56 29 have mg values of 48 51 with negative eu anomalies eu 0 59 0 79 low 87 sr 86 sr i 0 7058 0 7080 negative whole rock e nd t 8 2 to 5 8 206 pb 204 pb 243ma of 18 376 to 18 809 207 pb 204 pb 243ma of 15 606 to 15 661 208 pb 204 pb 243ma of 38 244 to 38 540 18 o smow 5 6 10 0 e hf t of 3 5 to 1 7 these isotopic features of arc type rocks from the east kunlun suggest that the parental magmas of the nan getan granodiorites and the dioritic enclaves originated from an enriched lithospheric mantle the nan getan granitoids might have recorded the northward subduction of the paleo tethys ocean lithosphere following the initial collision of the bayan har songpan ganzi east kunlun terrane and the closure of the paleo tethys ocean at 243 ma

inclusion localised crystal plasticity dynamic porosity and fast diffusion pathway generation in zircon abstract a population of oscillatory zoned igneous zircon grains in a javanese andesite contains fluid and mineral inclusions up to 10 m across trapped during zircon growth orientation contrast imaging and orientation mapping by electron backscatter diffraction reveal that crystal plastic deformation overprints growth zoning and has localized around 1 10 m pores and inclusions cumulative crystallographic misorientation of up to 25 around pores and inclusions in zircon is predominantly accommodated by low angle 010 slip a distinctly different population of sub micron pores is present along subgrain boundaries and their triple junctions these are interpreted to have formed as a geometric consequence of dislocation interaction during crystal plasticity dislocation creep microstructures are spatially related to differences in cathodoluminescence spectra that indicate variations in the abundance of cl active rare earth elements the extent of the modification suggests deformation related fast pathway diffusion distances that are over five orders of magnitude greater than expected for volume diffusion this enhanced diffusion is interpreted to represent a combination of fast diffusion pathways associated with creep cavitation dislocations and along low angle boundaries these new data indicate that ductile deformation localised around inclusions can provide fast pathways for geochemical exchange these pathways may provide links to the zircon grain boundary thus negating the widely held assumption that inclusions in fracture free zircon are geochemically armoured once they are physically enclosed

time of formation and peak of variscan hp ht metamorphism of quartz feldspar rocks in the central erzgebirge saxony germany the variscan erzgebirge represents an antiform with a core of gneisses and mica schists surrounded by a phyllitic mantle the gneiss eclogite unit geu in the central part is a composite tectonometamorphic assemblage characterized by a hp ht imprint and comprises migmatitic para and orthogneisses ht mylonites hp granulites eclogites and garnet peridotites it is tectonically sandwiched between two major units with distinctly lower pt histories the geu experienced a characteristic kinked retrograde pt path after hp ht equilibration with 1 strong near isothermal decompression at high temperatures 2 extensive re equilibration at medium pressures followed 3 by rapid cooling during continued uplift we dated zircons pb pb evaporation from granitoid orthogneisses and metapelites of the geu the orthogneisses contain euhedral long prismatic zircons of igneous origin that provided protolith ages between 470 and 524 ma metapelites retain well preserved granulite facies mineral assemblages and contain spherical multifaceted metamorphic zircons that grew near the peak of hp ht metamorphism inclusions of prograde hp phengite 15 kbar and rutile are included in one such zircon metamorphic zircons of three samples from different localities yielded identical 207pb 206pb ages of 340 5 0 7 ma 341 2 0 5 ma and 341 6 0 5 ma respectively consideration of these zircon ages with published 39ar 40ar white mica ages suggests fast cooling and uplift rates in excess of 50 c ma and 4 km ma this is typical for large scale extensional tectonic unroofing of the ultra deep part of a fossil thickened variscan continental crust 60 km during continuing continental collision and orogenic collapse

elevator tectonics and orogenic collapse of a tibetan style plateau in the european variscides the role of the bohemian shear zone variscan collision of peri gondwanan terranes led to a doubly vergent crustal wedge that was thicker than 55 km in the area of the bohemian massif this crustal thickness resulted in a highly elevated bohemian plateau with a topographic height 3 4 km the bohemian plateau was covered with unmetamorphic paleozoic strata all of which are today well preserved in the tepla barrandian unit because of crustal scale vertical slip along the bohemian shear zone bsz the bsz forms a subvertical ca 500 km long and up to 2 km wide belt of dip slip mylonites which show several 90 deflections in map view tepla barrandian down movements were active under retrograde metamorphic conditions starting with granulite and ceasing with greenschist facies conditions as slip along the bsz was largely vertical and led to a minimum throw of 10 km this type of crustal scale deformation is referred to as elevator tectonics the elevator style movements caused the juxtaposition of the supracrustal tepla barrandian lid the elevator against high grade rocks of the extruding orogenic root the bsz has further governed the foci of mantle derived plutonism new u pb zircon and monazite tims dating of six plutons suggest that emplacement of mantle derived melts along the bsz lasted for at least 20 m y starting with the emplacement of the klatovy granodiorite at 347 4 3 ma and ceasing with the emplacement of the drahotin pluton at 328 1 ma when taking into account the new ages of synkinematic plutons the simultaneous vertical slip along the individual segments of the bsz north west and central bohemian shear zone is bracketed to the period 343 337 ma elevator tectonics was probably controlled by delamination of thickened mantle lithosphere that caused a dramatic thermal turnover and heating up of the orogenic root the overheated lower crust was thermally softened by anatexis and diffusion creep resulting in channel flow vertical extrusion fast uplift and exhumation of the orogenic root

new evidence of blueschist facies rocks and their geotectonic implication for variscan suture s in the bohemian massif blueschist facies rocks exposed within consolidated continental blocks provide some of the best evidence for the existence of previous suture s they usually occur as lenses or layers embedded within greenschist or amphibolite facies rocks and indicate reequilibration at medium to low pressure conditions in the bohemian massif a few occurrences of blueschists have been reported and here new evidence of high pressure hp metamorphism in various lithologies is presented that suggests a larger extent of blueschist facies rocks along the northern border of this massif an earlier blueschist facies metamorphism is documented by inclusions of glaucophane in garnet epidote and titanite from metabasites along with zoned white mica having a phengitic core and a muscovite rim in metapelites and orthogneisses the estimated p t conditions obtained using pseudosections and mineral isopleths correspond to blueschist and low temperature eclogite facies conditions 1 1 2 0 gpa at 350 550 c together with medium temperature eclogites from different units in the bohemian massif they indicate a geothermal gradient of 8 10 c km 1 which is typical for cool subduction radiometric dating on phengite from metapelites confirms an early palaeozoic cooling age of c 360 ma for this hp metamorphic event the presence of blueschist facies rocks their p t relations and age constraint together with those from eclogite facies rocks allows us to locate the variscan suture which straddles the se margin of saxothuringian zone from erzgebirge to sudetes and its possible continuation to the moldanubian zone where eclogite facies and uhpm rocks are abundant

dehydration melting of ultrahigh pressure eclogite in the dabie orogen evidence from multiphase solid inclusions in garnet several types of multiphase solid ms inclusions are identified in garnet from ultrahigh pressure uhp eclogite in the dabie orogen the mineralogy of ms inclusions ranges from pure k feldspar to pure quartz with predominance of intermediate types consisting of k feldspar quartz silicate plagioclase or epidote barite the typical ms inclusions are usually surrounded with radial cracks in the host garnet similar to where garnet contains relict coesite barite aggregates display significant heterogeneity in major element composition with total contents of only 57 73 and highly variable sio2 contents of 0 32 25 85 that are positively correlated with bao and so3 contents the occurrence of ms inclusions provides petrographic evidence for partial melting in the uhp metamorphic rock the occurrence of barite aggregates with variably high sio2 contents suggests the coexistence of aqueous fluid with hydrous melt under hp eclogite facies conditions thus local dehydration melting is inferred to take place inside the uhp metamorphic slice during continental collision this is ascribed to phengite breakdown during hot exhumation of the deeply subducted continental crust as a consequence the aqueous fluid is internally buffered in chemical composition and its local sink is a basic trigger to the partial melting during the continental subduction zone metamorphism

petrographic and geochemical comparison between the copper age ljubljana type axes and similar lithotypes from eisenkappler diabaszug complex southern austria the petrographic and geochemical features of ljubljana type axes including lt1 3 groups one of the largest assemblage of copper age stone artefacts in the caput adriae region made from magmatic and metamorphosed igneous rock types have been compared to rocks specifically sampled from the eisenkappler diabaszug complex southern austria recently considered as the most probable provenance area both the axes and the geological samples show similar mineralogical and petrographic features and are characterized by a common oib like signature metamorphic and spilitization features the collected data confirm that the lt3 group originates from one outcrop located in the eastern part of the complex while the precise provenance of the lt1 2 axes is still to be identified in the same geological area

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

improvements of precision and accuracy in in situ hf isotope microanalysis of zircon using the laser ablation mc icpms technique abstract we have developed a new analytical technique for precise and accurate in situ hf isotope ratio measurements for zircons by means of laser ablation multiple collector inductively coupled plasma mass spectrometry la mc icpms newly designed sample cell and n 2 mixing technique provided higher elemental sensitivity with smoother signal intensity profiles in order to improve the accuracy of the hf isotope ratio analysis previously reported correction methods for isobaric interferences on 176 hf by 176 yb and 176 lu were rigorously evaluated and we found that the use of independent mass bias factors for hf and yb in the isobaric interference correction resulted in more reliable hf isotopic data in order to test the reliability of the hf isotopic data from 35 m diameter ablation pits that are smaller than the typical size of most zircon grains lu hf isotopes for three zircons harvard 91500 antarctic zircon pma7 and acasta zircon ay199 were measured the present 176 hf 177 hf isotope ratio of 0 282321 46 2 s d and initial 176 hf 177 hf isotope ratio of 0 282315 47 2 s d were obtained for zircon standard 91500 the resulting present 176 hf 177 hf isotope ratio shows an excellent agreement with the previously reported data our in situ isotopic data revealed that the lu hf ratio varied significantly within the 91500 grain 176 lu 177 hf 0 000130 0 000345 suggesting the existence of slight isotopic heterogeneity in present 176 hf 177 hf approximately 15 ppm for antarctic zircon pma7 despite the smaller size of the single grain 50 100 m wide 80 300 m long and the presence of various mineral inclusions resulting analytical precision for the initial 176 hf 177 hf ratio was 128 ppm 0 281208 36 2 s d for acasta zircon ay199 despite the high yb hf 0 05 and lu hf 0 01 isotope ratios with the early archean crystallization age 3 74 ga resulting analytical precision for the initial 176 hf 177 hf ratio was 187 ppm 0 280160 52 2 s d these results demonstrate clearly that in situ hf isotope ratio microanalysis developed here has the potential to become a significant tool for zircon lu hf isotopic study

ophiolites of the kunlun mountains china and their tectonic implications abstract three ophiolite belts ranging in age from cambrian to triassic provide valuable data on the tectonic evolution of the kunlun mountains which lie along the northern margin of the tibetan plateau all of the ophiolites contain dismembered but nearly complete assemblages of peridotite mafic and ultramafic cumulates pillow and massive lavas and radiolarian cherts 1 the oldest belt contains ophiolites of cambrian age but the initial spreading of the ocean basin most likely began in the late proterozoic these ophiolites outcrop in the central part of the eastern kunlun extend for at least a few hundred kilometers and are believed to reflect a small ocean basin closure of this ocean led the accretion of south kunlun block to the north kunlun block and tarim craton ophiolitic lavas in this belt have island are affinities 2 a second belt of early carboniferous ophiolites extends nearly 600 km along the northern margin of the western kunlun lavas in this belt are basalt basaltic andesite and andesite all of which have compositions characteristic of volcanic arcs this ocean basin possibly developed on the basis of the early paleozoic oceanic basin closure of this ocean basin by southward subduction in the early permian produced a suite of calc alkaline volcanic rocks in what is now the central part of the western kunlun 3 the third belt extends nearly 1200 km along the southern margin of the eastern kunlun and contains numerous ophiolites of early permian to middle triassic age these ophiolites are highly tectonized containing volcanic rocks with the geochemical characteristics of mid ocean ridges oceanic islands and volcanic arcs this belt is tentatively interpreted as the suture zone between gondwana and eurasia

luminescence of diamonds from metamorphic rocks this study explores the effects of metamorphism on luminescence of diamond four diamond suites extracted from metamorphic rocks were characterized using optical cathodoluminescence ftir spectroscopy photoluminescence at the liquid nitrogen temperature and cathodoluminescence spectroscopy the studied diamonds are from sedimentary conglomerate and lamprophyric breccia metamorphosed to the greenschist facies wawa canada and from the ultrahigh pressure uhp terranes of kokchetav kazakhstan and erzgebirge germany the majority of studied diamonds that experienced metamorphism have distinct cathodoluminescence and photoluminescence when compared to diamonds from unmetamorphosed rocks i e kimberlites or placers diamonds in metamorphic rocks do not show the prevalent blue cathodoluminescence emittance at 415 440 and 480 490 nm typical of common kimberlite and placer diamonds ultrahigh pressure diamonds are mostly non luminescent whereas diamonds from greenschists display green yellow orange and red cathodoluminescent colors emitting light at 520 and 576 nm and between 586 and 664 nm the shift of cathodoluminescence in octahedrally grown type ia diamonds metamorphosed in the greenschist facies results from the presence of optical centers with zero phonon lines at 575 and 637 nm detected via photoluminescence spectroscopy the centers may have formed due to irradiation of diamonds in the upper crust followed by annealing at t 500 c this process creates vacancies that diffuse through the diamond lattice and get trapped at n sites creating nv o and nv optical centers the distinct luminescence of metamorphosed diamonds can persist for billions of years in the crust but is annealed in the mantle

tectono thermal events in east kunlun northern tibetan plateau evidence from zircon u pb geochronology abstract the widely distributed high grade gneisses in the east kunlun orogenic belt ekob are keys to understand the precambrian tectonic evolution of the northern tibetan plateau in this study new la icp ms zircon u pb ages from paragneiss and schist of the proterozoic jinshuikou group and quartzite of the proterozoic binggou group are reported in an attempt to evaluate the neoproterozoic and paleozoic tectono thermal events of the ekob these geochronologic data can be classified into 4 groups group 1 ages ranging from 2243 ma to 3701 ma are represented by inherited zircons from protolith and confirm the existence of eoarchean to paleoproterozoic continental nucleus in the source region of the jinshuikou group group 2 ranging from 928 ma to 1849 ma yields lower intercept ages of 0 9 1 0 ga which represent the neoproterozoic tectono thermal event this event similar to that of the northern margin of qaidam might be a response to the assembly of rodinia group 3 ranges from neoproterozoic to early paleozoic with lower intercept ages which are identical to the weighted mean ages of group 4 these two age groups confirm the tectono thermal event related to paleozoic oceanic subduction moreover based on the youngest age of 2 2 ga in group 1 and the upper intercept age of 1 8 ga in group 2 the depositional timing of the jinshuikou and binggou groups can be defined as paleoproterozoic and mesoproterozoic respectively

a comparison of the evolution of diamondiferous quartz rich rocks from the saxonian erzgebirge and the kokchetav massif are so called diamondiferous gneisses magmatic rocks abstract the petrography and chemical composition of minerals of quartz rich diamondiferous rocks from the kokchetav massif especially the zonation of garnet were studied and compared with diamondiferous quartzofeldspathic rocks from the saxonian erzgebirge many compositional and textural features were found to be similar for instance microdiamonds are enclosed systematically in a specific intermediate growth zone of garnet in these rocks on the basis of experimental data a magmatic scenario was constructed to check if the quartz rich diamondiferous rocks are of magmatic origin by this the p t paths derived here for the kokchetav rocks and the textural observations it is concluded that the minerals of the diamondiferous rocks have crystallized from silicate melts these melts originated by anatexis of deeply submerged metasediments erzgebirge at t as high as 1200 c kokchetav massif at 50 100 c lower t and ascended from at least 200 km depth relics of the pre anatectic evolution are still present for instance as garnet cores after ascent and emplacement of the magma in deep portions of thickened continental crust kokchetav massif 45 50 km close to 800 c erzgebirge 55 60 km at 30 50 c lower t considerable quantities of white and or dark micas formed by peritectic reactions from melt for instance garnets could be resorbed at this stage and biotite grew instead after the magmatic stage retrogression took place much stronger in the kokchetav massif this was accompanied by deformation transforming broadly the magmatic texture of quartz rich diamondiferous rocks from the kokchetav massif to a gneissic texture

kokchetavite a new potassium feldspar polymorph from the kokchetav ultrahigh pressure terrane kokchetavite a new polymorph of k feldspar kalsi3o8 has been identified as micrometer size inclusions in clinopyroxene and garnet in a garnet pyroxene rock from the kokchetav ultrahigh pressure terrane kazakhstan kokchetavite has a hexagonal structure with a 5 27 1 a c 7 82 1 a v 188 09 a 3 z 1 and is found to be associated with phengite a b cristobalite or quartz siliceous glass phlogopite titanite calcite zircon occurring as multi phase inclusions in clinopyroxene and garnet it is concluded that kokchetavite could not be an exsolution phase in host minerals instead it might be metastably precipitated from an infiltrated k rich melt during rock exhumation alternatively although less likely kokche tavite might be derived from dehydration of k cymrite which in turn was formed at high pressures in either case kokchetavite is a metastable polymorph of k feldspar

heishan mafic ultramafic rocks in the qimantag area of eastern kunlun nw china remnants of an early paleozoic incipient island arc abstract the heishan mafic ultramafic complex in the qimantag mountains of the eastern kunlun orogen consists of olivine pyroxenites pyroxenites gabbros and massive basalts zircons from the gabbros have magmatic sector zoning with th u ratios of 0 5 2 3 and yield a shrimp age of 486 6 ma mswd 0 31 which is taken as the time of magma crystallization spinel pyroxene and plagioclase mineral compositions indicate that the rocks formed in an island arc environment the cumulates have low la yb n ratios 0 6 0 8 and depleted lree patterns high abundances of cr 3031 1185 ppm co 119 50 ppm and ni 953 291 ppm and end t values of about 8 0 the massive basalts have somewhat higher contents of tio2 1 0 1 3 na2o 2 6 2 8 total ree and mg s 55 63 than those of the cumulates the la yb n ratios of the basalts range from 0 8 to 1 2 and the chondrite normalized ree patterns are flat end t values of the basalts range from 7 7 to 9 4 the compositions of the cumulates and volcanic rocks especially their similar nd isotopes suggest that the rocks were derived from similar depleted mantle sources the primary magmas were probably generated in an initial arc environment we conclude that the heishan mafic ultramafic complex in the qimantag area preserves a record of proto tethyan oceanic lithosphere formed in the early paleozoic it possibly marks a suture between the qaidam and north kunlun terranes in the western segment of the eastern kunlun orogen

geochemical and geochronologic constraints for paleozoic magmatism related to the orogenic collapse in the qimantagh south altyn region northwestern china abstract voluminous and discrete early paleozoic bimodal magmatic suites are thought to be the result of post collisional extension following the amalgamation of east kunlun and altyn tagh in this paper four representative magmatic units were studied for their geochemical fingerprint in conjunction with geochronological studies the 467 445 ma mangya mafic suite shows e morb type rare earth element ree patterns that are the result of asthenospheric interaction with a metasomatized subcontinental lithosphere high k calc alkaline granodiorites intruded at ca 450 ma are characterized by high mg the least fractionated ree pattern without an eu anomaly as well as high sr and low rb sr ratio we interpret these geochemical signals to result from lower crustal melting of garnet amphibolite at pressures between 16 and 22 kbar a 430 420 ma a type granite is interpreted to result from the melting of metaigneous rocks at middle to lower crustal depths lastly a late magmatic pulse occurs between 400 and 380 ma and is represented by the alk granite the alk granite is interpreted to be a product of metapelite melts and is associated with a smaller volume of mafic melts u pb zircon geochronology of paleozoic igneous rocks of the qimantagh south altyn reveals that most of the magmatic episodes are either coeval with or post extensional deformation this phase of extension is supported by the exhumation of hp uhp metamorphic rocks and crustal anatexis collectively the evolutionary stages documented in this study correspond to a succession of post collisional postorogenic and ultimately within plate magmatic episodes the overall features support orogenic collapse via removal of a thickened lithospheric root beneath the east kunlun altyn tagh collisional orogen during early paleozoic

the origin of amphibolites from metamorphic soles beneath the ultramafic ophiolites in evia and lesvos greece and their geotectonic implication abstract the dismembered jurassic ophiolitic complexes of the two greek islands evia and lesvos show significant similarities concerning their general structural petrographic and geochemical characteristics however their geotectonic position is quite different the evia ophiolites belonging to the western ophiolite belt of the internal hellenides whereas the lesvos ophiolites seem to be part of the eastern ophiolite belt representing a possible link to the pontides both ophiolites consist of large ultramafic masses which are emplaced onto continental crust the overthrust zones comprise flysch and ophiolitic melange as well as amphibolitic metamorphic soles beneath the ultramafics the identification of the protolithic character of the amphibolites as well as their metamorphic and deformation history allows a clarification of their complex emplacement processes from their original extensional oceanic ridge environment within the pindos and vardar marginal basins into a compressional supra subduction zone island arc and continental environment on the basis of relict magmatic minerals and textures as well as on major and trace element bulk chemical composition the medium to coarse grained evia amphibolites are interpreted as former gabbroic and cumulate rocks similar to equivalent amphibolites dredged from numerous fracture zones in the atlantic oceanic crust horizontal and vertical displacements which are able to produce such foliated and mylonitized gabbroic rocks suggest the existence of major deep reaching transform faults which may be considered as one of the most important locations for the breakup of oceanic crust and lithosphere and subsequent incipient subduction the evia amphibolites could have been formed by a two stage process first as gabbros that formed part of the gabbroic oceanic crust and secondly representing as small gabbroic and cumulate bodies within the ultramafic mantle either of these were transformed into low grade epidote amphibolites under oceanic metamorphic facies conditions and deformed by transform fault displacement mechanism subsequently within the pindos intra oceanic basin these transform faults also facilitated obduction of slices of oceanic mantle and crust onto an accretionary wedge in this respect the evia ophiolite complex shows a very similar emplacement process to that of the othris vourinos and pindos ophiolites which can be interpreted using a combination of an intra oceanic transform fault thrust and an obduction model in this respect the evia transform fault overthrust model sheds new light onto the discussion of the age of formation of oceanic crust as well as on the timing of subduction and obduction the so called sole amphibolites especially those of only low grade epidote amphibolite facies may have already been formed during mid ocean floor spreading and transform fault movements and not later during the ophiolite emplacement processes therefore all of the ages of oceanic crust and amphibolites both from sedimentary and paleontological evidence as well as from radiometric data indicate a specific time window between the generation of the older parts of oceanic crust around 170 ma bajocian and their emplacement processes at approximately 155 to 150 ma this leads to the conclusion that the pindos marginal oceanic basin could have at least an age spread of 15 to 20 ma in contrast the lesvos amphibolites are fine to medium grained with granoblastic and nematoblastc textures with relics of clinopyroxene and plagioclase cataclastic fabrics are also present as well as retrograde mineral assemblages from greenschist to prehnite facies metamorphic overprinting according to bulk chemical analyses these amphibolites are identified as morb type oceanic basalts as well as ssz supra subduction zone iat island arc tholeiite basalts and olivine gabbros the lesvos amphibolites are seen as basaltic and gabbroic slices which were incorporated into a supra subduction gabbro bearing island arc environment overridden by larger slices of depleted lherzolite and harzburgite mantle since the amphibolites show in many cases highly deformed fabrics and a rather high grade metamorphism as well as morb basaltic compositions an intra oceanic thrusting cannot be excluded their origin can rather be placed in the vardar oceanic basin than in the pindos basin

geochemistry and zircon u pb geochronology of granitic rocks in the buqingshan tectonic melange belt northern tibet plateau china and its implications for prototethyan evolution abstract the yikehalaer intrusives are distributed in the buqingshan tectonic melange belt at the southern margin of east kunlun china they tectonically intruded the conglomerate of permian gequ formation to the south and the marble of mesoproterozoic kuhai group to the north these intrusives mainly consist of off white coarse and fine grained granodiorite and granite u pb dating of zircons from the intrusive samples yielded weighted mean 206pb 238u ages of 436 7 ma and 437 6 ma indicating early silurian crystallization age the intrusives are high silica na rich i type calc alkalic rocks they are divisible into two subgroups on the basis of their trace element content adakitic rocks in the northern part and normal calc alkalic arc rocks in the southern part rocks in the northern part exhibit high sr 280 493 ppm low y 3 8 11 7 ppm high sr y ratios 23 9 125 0 high light rare earth elements lree heavy rare earth elements hree 15 1 35 7 and invisible eu eu they resemble adakites formed by slab melting in subduction zones however the rocks in the southern part possess the attributes of normal arc rocks in addition the ehf t values of the intrusives are consistently positive 0 6 7 6 implying that they were generated by melting of an isotopically depleted mantle source with insignificant contributions from crustal materials the source residues likely contained 10 20 garnet and hornblende and plagioclase was either absent or totally consumed during partial melting combining the regional tectonic data we conclude that the buqingshan paleo ocean may have started to subduct in middle cambrian until early silurian 436 ma generating an important adakitic magma event during the late stage of oceanic crust subduction moreover the proto tethyan ocean closed and the bayan har east kunlun and qaidam blocks collided during the middle late silurian

geochemical constraints on the petrogenesis of granitoids in the east kunlun orogenic belt northern tibetan plateau implications for continental crust growth through syn collisional felsic magmatism early triassic syn collisional granitoids with mafic magmatic enclaves mmes crop out along the entire east kunlun orogenic belt ekob at the northern margin of the tibetan plateau they are andesitic in composition and enriched in light rare earth elements lrees with a flat heavy ree hree pattern their average composition resembles that of the bulk continental crust the enclosed mmes have the same mineralogy as their host granitoids but contain a greater mode of mafic minerals amphibole and biotite and thus have higher hree abundances zircon u pb dating shows that both the granitoid hosts and mmes have the same crystallization age of similar to 250 ma and indistinguishable bulk rock sr nd pb hf isotope compositions i sr of 0 7080 0 7116 varying pb 206 pb 204 i of 18 53 19 32 essentially constant epsilon nd t of 5 3 to 2 1 and a small range of positive epsilon hf t mostly 1 7 5 2 the complete isotopic overlapping between the granitoid hosts and the mmes is understood to reflect that the mmes are disintegrated cumulates formed at an early stage of the granitoid magma evolution within the same magmatic system the isotopic data set reveals that the granitoids are variably evolved melts produced by partial melting of the subducted paleo tethyan oceanic crust with terrigenous sediments under amphibolite facies conditions in response to the continental collision c 2014 elsevier b v all rights reserved

discovery of early paleozoic eclogite from the east kunlun western china and its tectonic significance abstract eclogites discovered in the eastern part of the east kunlun mountains western china are primarily composed of garnet omphacite quartz rutile the garnets show end member components of 48 55 almandine 1 2 spessartine 19 29 grossularite and 16 29 pyrope and the omphacite has a jadeite content of 21 63 the peak metamorphic assemblage of eclogites records a p t condition of 1 6 gpa and 590 c 650 c zircon u pb dating of the inherited magmatic zircons from fine grained eclogite reveals a neoproterozoic age of 934 ma representing the protolith age of the eclogite zircons from the coarse grained eclogite contain inclusions of garnet omphacite and rutile and yield a weighted mean age of 428 ma indicating the metamorphic age of eclogite the eclogites together with the late cambrian 508 ma high pressure granulite to the west constitute an early paleozoic high pressure metamorphic belt in the east kunlun

origin of mesozoic adakitic intrusive rocks in the ningzhen area of east china partial melting of delaminated lower continental crust to the best of our knowledge modern adakites have not been documented in a nonarc environment we report geochemical and isotopic data for early cretaceous anjishan adakitic intrusive rocks that are in a continental setting unrelated to subduction the anjishan adakitic intrusive rocks which are exposed in the ningzhen area of east china have high sr y and la yb ratios coupled with low yb and y as well as relatively high mgo contents and mg numbers mg 0 4 0 6 similar to products from slab melting however low n d t values 6 8 to 9 7 and high 8 7 sr 8 6 sr i 0 7053 0 7066 are inconsistent with an origin by slab melting the tectonics and geochemistry lead us to conclude that adakitic magmas were most likely derived from partial melting of mafic material at the base of the continental crust high sr y and la yb ratios of the adakitic intrusive rocks suggest that garnet was stable as a residual phase during partial melting implying that the crustal thickness exceeded 40 km in the early cretaceous the present thickness of the crust in the ningzhen area is only 30 km and therefore the crust appears to have been thinned by at least 10 km since the early cretaceous the relatively high mgo contents and mg of the anjishan intrusive rocks suggest that adakitic magmas interacted with mantle rocks possibly coinciding with lower crustal delamination which would also account for the observed thinning

anatomy of a diffuse cryptic suture zone an example from the bohemian massif european variscides the fate of the lower plate during continental collision can be examined in deeply eroded orogens such as the late paleozoic variscan belt in continental europe in particular the bohemian massif at its eastern extremity preserves well the evolution of an andean type orogen involved in continental collision this process included relamination of subducted light felsic material rich in radioactive elements underneath a dense mafic lower crust of the upper plate this led to gravity driven overturns and overprinting of the original suture by a broad zone of mixed upper and lower plate materials in the studied example this zone of interaction repeatedly reappears within the orogen forming a so called diffuse cryptic suture zone

post collisional variscan lamprophyres black forest germany 40ar 39ar phlogopite dating nd pb sr isotope and trace element characteristics abstract subsequent to high pressure and temperature metamorphism of the axial zone of the variscan foldbelt in central europe at ca 340 to 330 ma formation of lamprophyre dikes during transtensional tectonics may be viewed as the beginning of the post collisional stage of the orogeny we report the results of 40 ar 39 ar mica dating major and trace element data and isotope compositions for lamprophyre and rhyodacite samples from the southern black forest the chemical compositions of these rocks shed light on the upper mantle and crust at the end of the variscan orogeny 40 ar 39 ar plateau ages for four phlogopite biotite separates from lamprophyres indicate emplacement at 332 to 314 ma this event coincides with melting of the crust as indicated by 40 ar 39 ar biotite plateau ages of ca 332 ma for rhyodacite dikes which are probably related to coeval undeformed granites incompatible trace element patterns of the lamprophyre samples reveal the characteristics of evolved continental crust and are interpreted as evidence for melting of sediment in a subduction modified mantle nd sr and pb isotope compositions indicate an enriched mantle source with end values of 1 5 to 6 8 which is similar to variscan crust significant contamination of the lamprophyric melts by variscan crust can be ruled out as mantle derived phlogopite phenocrysts have similar end values as in the whole rock samples we propose that the isotope compositions and incompatible trace element characteristics of the lamprophyres were predominantly inherited from melted sediment the isotope compositions of variscan lamprophyres from western europe suggest that enriched upper mantle was only partly delaminated when ascending hot mantle triggered melting of the lower crust as has been invoked for the origin of post collisional granites the isotope compositions of tertiary basalts and mantle xenoliths indicate a depleted upper mantle under western europe implying that the enriched variscan material was efficiently removed and mixed into the convecting mantle

experimentally determined compositions of diopside jadeite pyroxene in equilibrium with albite and quartz at 1200 1350 c and 15 34 kbar abstract equilibrium compositions of diopside jadeite pyroxene coexisting with albite and quartz were experimentally determined at 25 different p t conditions using an electron microprobe for analysis the new data and the 600 c data of holland 1983 provided the following mixing properties of the diopside di jadeite jd solid solution j k g xs x jd x di 12600 9 45 t 12600 7 6 t x jd x di 21400 16 2 t x jd x di 2 the di jd solution is close to ideal above 1000 c but immiscible below 565 c the di jd solvus is slightly asymmetric with the crest at composition di 42 4 jd 57 6 excess enthalpy is positive but smaller than indicated by the enthalpy of solution measurements of wood et al 1980 disorder in the di jd solution is significantly smaller than complete disorder implied by the ionic two site model

pressure temperature and fluid evolution of quartzo feldspathic metamorphic rocks with a relic high pressure granulite facies history from the central erzgebirge saxony germany by 1 high pressure hp equilibration followed by near isothermal the gneiss eclogite unit is a composite tectonometamorphic unit decompression at high temperatures during which rocks from different within the variscan erzgebirge mega antiform it comprises migdepths were amalgamated and 2 extensive hydration and rematitic paraand orthogneisses high temperature ht mylonites equilibration at medium pressures followed by rapid cooling during kyanite bearing granulites eclogites and garnet peridotites four continued uplift when the entire unit came into contact with cooler different quartzo feldspathic assemblages are recognized in which now overand underlying units this scenario is attributed to maximum conditions of up to 830 c and 21 kbar were determined continent collision orogenic collapse and disintegration of the hp the assemblages are characterized by the nearly complete prograde unit during continuing collision crustal stacking and uplift controlled breakdown of biotite by high grossular content 23 47 mol by extension of garnet in the presence of albite and high si contents of phengite 3 3 3 4 per formula unit p f u water activities at this stage are variable and range from 0 4 the maximum pressures indicated for individual rock volumes may vary considerably between 12 and 24 kbar at 700 800 c so that non coherency

exsolution of rutile or apatite precipitates surrounding ruptured inclusions in garnet from uht and uhp rocks this study describes halos of rutile apatite needles and or plates centred on quartz inferred former coesite inclusions or multiphase inclusions in garnet both types of central inclusions are surrounded by cracks the multiphase inclusions contain mica or carbonate minerals and are interpreted to represent decrepitated fluid inclusions examples from two localities are examined i ultrahigh temperature uht metapelitic gneisses from the southern end of the central maine terrane in northeastern connecticut usa rutile only and ii ultrahigh pressure uhp diamondiferous saidenbachite from the saxonian erzgebirge rutile and apatite the needles of apatite or rutile are typically oriented in three directions within garnet chemical zonation in garnet shows clear ti or p depletion halos corresponding spatially with the rutile or apatite inclusion halos the radii of the inclusion halos when measured from the centre of the central inclusions are about two to several times the radii of the central inclusions we propose that the inclusion halos of rutile apatite formed by exsolution out of ti bearing and or p bearing garnet during retrogression because of its strength garnet can internally maintain higher pressures than the surrounding matrix nonetheless if the inclusion pressure is significantly greater than the confining lithostatic pressure then deformation of the host garnet can occur during exhumation high internal pressures relative to the matrix can result from retention of fluid entrapment pressure or a phase change with a large positive volume increase e g coesite quartz the differential stress imposed on the garnet adjacent to the central inclusion preceding and during mechanical failure would create dislocations and other crystalline defects which are ideal sites for exsolved precipitates to nucleate and grow the strain induced exsolution hypothesis is consistent with observations i the radii of the halos are roughly consistent with the pressure vessel model or the multi anvil model for an inclusion which predict that differential stress will drop off sharply away from the boundary of the central inclusion into the host garnet ii the rutile or apatite inclusions formed in ti or p bearing garnet adjacent to ruptured central inclusions rutile or apatite are rare or absent elsewhere in garnet even in areas of high ti or high p concentration in addition rutile is absent in areas surrounding the central inclusion that were depleted in ti prior to rupturing e g garnet rims that lost ti during retrogression thus both elevated ti p concentrations and proximity to inclusions which deformed the host garnet were necessary for halo formation iii reintegrated garnet ti or p contents in and adjacent to the halos obtained using wide beam electron probe microanalysis are consistent with local derivation of the ti or p necessary to form rutile and apatite from garnet elevated ti p concentrations in aluminous garnet are mostly found in uht hp uhp granulite and mantle environments consequently the phenomena described will most likely occur in such settings

low differential stress during high pressure metamorphism the microstructural record of a metapelite from the eclogite zone tauern window eastern alps constraints on the state of stress at deep levels of convergent plate boundaries can be obtained from the microstructural record of high pressure metamorphic rocks provided that this record has remained unchanged during exhumation the inclusion patterns in poikiloblastic garnet of a metapelite from the eclogite zone of the tauern window eastern alps reveal a foam microstructure of quartz controlled by grain boundary free energy the garnet blasts show concentric chemical zonation thermobarometry based on correlation between compositional domains in garnet chloritoid and phengite coexisting with kyanite rutile and quartz shows that garnet growth commenced before peak p t conditions close to 25 kbar and 600 c had been reached and continued during the early stage of decompression this implies that the microstructure of quartz controlled by grain boundary free energy persisted continuously during the latest stage of burial to ca 80 km depth and the early stage of exhumation involving detachment from the downgoing slab and release into the subduction channel the foam structure of the overgrown quartz indicates that the differential stress was too low to drive dislocation creep during that period when the rock crossed the interface between the plates with a high differential stress resulting in tectonic overpressure or significant shear heating being ruled out outside the protective garnet the interfacial free energy controlled quartz microstructure has been destroyed completely due to dynamic recrystallization during deformation by dislocation creep this happened while the crustal slice was exhumed to a depth of about 45 km the preservation of this synkinematic quartz microstructure indicates a tectonic scenario where the final cooling was rapid and no marked stress peak was associated with the brittle plastic transition

diamond inclusions in garnets from metamorphic rocks a new environment for diamond formation diamonds commonly occur in kimberlites lamproites and alluvial sediments derived from these rocks more recently diamonds or their graphite pseudomorphs have been discovered in ultramafic massifs1 and picrites2 here we report the occurrence of diamonds in situ in crustal rocks highly retrograded high pressure metamorphic garnet pyroxene and pyroxene carbonate garnet rocks biotite gneisses and schists from the kokchetav massif northern kazakhstan ussr the diamonds are cubo octahedral averaging 12 m in size and occur in zircons and with euhedral graphite as inclusions in unzoned garnets we believe that the zircon and garnet matrices protected these diamonds from retrogressive transformation to graphite mica rutile titanite clinopyroxene kyanite and zircon also occur as inclusions in garnet often intergrown with the diamonds equilibration relations of inclusions and host garnets indicate that both diamonds and graphite crystallized from a fluid phase under static conditions at pressures of 40 kbar and temperatures 900 1 000 c

status report on stability of k rich phases at mantle conditions abstract experimental research on k rich phases and observations from diamond inclusions uhp metamorphic rocks and xenoliths provide insights about the hosts for potassium at mantle conditions k rich clinopyroxene kcpx km3 si2o6 can be an important component in clinopyroxenes at p 4 gpa dependent upon coexisting k bearing phases solid or liquid but not apparently upon temperature maximum kcpx content can reach 25 mol with 17 mol the highest reported in nature partitioning k d cpx liquid above 7 gpa 0 1 0 2 require ultrapotassic liquids to form highly potassic cpx or critical solid reactions e g between kspar and di phlogopite can be stable to about 8 gpa at 1250 c where either amphibole or liquid forms when fluorine is present it generally increases in phl upon increasing p and probably t to about 6 gpa but reactions forming amphibole and or kmgf3 limit f content between 6 and 8 gpa the perovskite kmgf3 is stable up to 10 gpa and 1400 c as subsolidus breakdown products of phlogopite upon increasing p m4 k substituted potassic richterite ideally k kca mg5si8o22 oh f 2 is produced in k rich peridotites above 6 gpa and in di phl from 6 to 13 gpa k content of amphibole is positively correlated with p al and f content decrease with p in the system 1kspar 1h2o k cymrite hydrous hexasanidine kalsi3o8 nh2o kcym is stable from 2 5 gpa at 400 to 1200 c and 9 gpa kcym can be a supersolidus phase formation of kcym is sensitive to water content not forming within experiments with h2o kspar phase x a potassium di magnesium acid disilicate k1 x n 2 mg1 nmn3 2si2o7h2x forms in mafic compositions at t 1150 1400 c and p 9 17 gpa and is a potential host for k and h2o at mantle conditions with a low t geotherm or in subducting slabs the composition of phase x is not fixed but actually represents a solid solution in the stoichiometries 2mg2si2o7h2 k mg2si2o7h k2mg2si2o7 vacancy apparently stable only near the central composition k hollandite kalsi3o8 is possibly the most important k rich phase at very high pressure as it appears to be stable to conditions near the core mantle boundary 95 gpa and 2300 c other k rich phases are considered

kyanite eclogite xenolith from the orthogneiss terrane of the tisza megaunit janoshalma area crystalline basement of southern hungary abstract new evidence for high pressure eclogite facies metamorphism in the crystalline basement of the tisza megaunit southern hungary is reported the retrogressed mafic eclogite forms a small lens in the orthogneiss and it was found in the borehole near janoshalma the carbonated eclogite contains the peak metamorphic assemblage omphacite garnet phengite kyanite clinozoizite rutile k feldspar quartz omphacite xjd 0 40 0 41 xdio 0 52 0 53 xhd 0 05 xaug 1 55 2 85 occurs in the matrix and as inclusions in garnet xpy 0 37 0 38 xgrs 0 21 0 22 xalm 0 39 0 40 xsps 0 0 01 xadr 0 0 01 and kyanite thermobarometry based on net transfer reactions between garnet omphacite kyanite and phengite yields p t conditions of 710 10 c and 2 6 0 75 gpa retrogression during decompression is manifested by formation of symplectites the most typical are diopside plagioclase after omphacite corundum spinel plagioclase after kyanite and biotite plagioclase after phengite carbonatization along the veins of the retrogressed eclogite was probably coeval with formation of these symplectites at places where carbonate is absent the rock was completely hydrated and retrogressed down to the greenschist facies with the development of actinolite similar eclogites together with abundant orthogneisses occur mainly in the eastern parts of the tisza megaunit suggesting the existence of an ancient possibly variscan subduction accretionary complex

geochemistry and zircon u pb geochronology of granitoids in the east kunlun orogenic belt northern tibetan plateau origin and tectonic implications abstract the east kunlun orogenic belt ekob in the northern margin of the tibet plateau is characterized by widespread granitic plutons which are keys to understanding the tectonic evolution of the ekob the zhiyu pluton newly recognized in the central part of the ekob mainly consists of monzogranites biotite granites and quartz diorites their la icpms zircon u pb results show formation ages of 447 1 6 ma 448 2 5 ma and 408 1 8 ma the monzogranites and biotite granites are characterized by relatively high sr 208 631 ppm low y 4 28 15 82 ppm and yb 0 44 1 59 ppm contents thus resulting in elevated sr y 30 105 and la yb n 4 79 ratios indicating geochemical features of adakitic rocks these adakitic granites are medium to high k calcic or calc alkaline in composition and display a weak peraluminous character they have low mgo 0 57 1 84 wt average 1 01 wt mg 40 53 average 45 as well as low cr 3 67 17 98 ppm average 7 19 ppm and ni 2 59 9 30 ppm average 4 71 ppm contents these rocks are enriched in lree and show negligible or variable positive eu anomalies eu eu 0 61 3 80 average 1 45 and significant negative nb and ta anomalies majority of the zircon grains from these adakitic granitic rocks have positive ehf t values of 0 09 5 21 with two stage model ages ranging from 1 1 ga to 1 6 ga these features are compatible with those of adakitic rocks derived from a thickened lower crust in the garnet stability field their formation is mainly controlled by the process of crust thickening following the closure of the qimantag ocean the younger quartz diorites belong to medium to high k calc alkalic or alkali calcic and metaluminous series and exhibit a relatively high mgo 2 23 5 18 wt and mg 40 56 average 50 11 with significant lree enrichment and negative eu anomalies as well as depletion of nb ta in addition the quartz diorites have an enriched ehf t values ranging from 5 25 to 3 19 combining the regional tectonic data we infer that the younger quartz diorites were derived from enriched lithospheric mantle sources and contaminated by the crustal materials in a post collision extensional setting