shrimp zircon age constraints from the larsemann hills region prydz bay for a late mesoproterozoic to early neoproterozoic tectono thermal event in east antarctica this paper reports a geochronological study of granulite facies rocks from the larsemann hills of prydz bay east antarctica shrimp zircon ages were obtained for a total of 14 samples that cover various rock types including felsic gneiss mafic gneiss paragneiss enderbite granitic gneiss and leucogneiss these age results combined with zircon geochemical data and cathodoluminescence images enable us to explore the morphological complexity in zircons that record polytectonic events features of multiple age zoning for example were observed in zircon separates from three felsic gneiss samples studied which contain magmatic cores of ca 1 13 ga and metamorphic mantles and rims of ca 1 0 and 0 53 ga respectively this suggests a period of magmatism and crust formation in the late mesoproterozoic and two subsequent phases of high grade metamorphism during the early neoproterozoic and early paleozoic respectively similar phenomena of zircon overgrowth were also observed in other rock types although early paleozoic tectonic activity has been considered as most significant in antarctic crustal evolution our study provides the first convincing age evidence for the existence of a prydz bay late mesoproterozoic mobile belt the ca 1 0 ga metamorphic record preserved in diverse rocks from the study area furthermore supports the proposal of a continuous circum east antarctica late mesoproterozoic grenville age orogenic belt within this framework the prydz bay orogen is proposed to have been located in the central part of the rodinia assembly that brought the eastern ghats of india together with antarctica at ca 1 0 ga

syntectonic anatexis and magmatism in a mid crustal transpressional shear zone an example from the hercynian rocks of the eastern pyrenees abstract hercynian metamorphism and magmatism in the cap de creus peninsula eastern pyrenees occurred contemporaneously with non coaxial deformation in a transpressive regime an example of this has been taken from a migmatite complex in the northern coast the studied area is formed by partially melted sillimanite schists together with two different suites of igneous rocks a calc alkaline magmatic sequence consisting of small granitoid bodies and later peraluminous leucogranites and pegmatites all these rocks occur within a ne sw to e w trending sub vertical high strain zone where a first schistosity is tightly folded and transposed the relations between folds and crenulation cleavage in the metasediments and deformational fabrics in the granitoids and migmatites provide good evidence for the penecontemporaneity of deformational processes magmatism and migmatization furthermore the analysis of cross cutting relationships between different granitoids as well as the observation of their different degrees of deformation verifies that magmatism in this area took place as synkinematic consecutive intrusions from the more basic magmas to the peraluminous acid dykes

preservation of chemical residue melt equilibria in natural anatexite the effects of deformation and rapid cooling a combined petrologic textural and chemical approach is used to interpret the relationship between chemical exchange and deformation during partial melting of a granodioritic gneiss in the contact aureole of the bergell pluton central alps in contrast to most regional metamorphic anatexites chemical equilibrium between residue and segregated melt was attained and preserved by several elements as shown by their present distribution equilibrium involved both the major and the trace elements hosted in the rock forming minerals and the light rare earth elements lree hosted primarily in allanite equilibrium with respect to lrees was achieved because allanite had a reactive behaviour during water present melting in contrast to the refractive behaviour of xenotime which mostly controls the distribution of the hree s the attainment and preservation of equilibrium between leucosome and residue requires peculiar boundary conditions we propose that the achievement of equilibrium between melt and residue was promoted by deformation operating via a mechanism of melt present granular flow i e dissolution accommodated grain boundary sliding active during partial melting microstructural observations indicate that melt present granular flow was the dominating deformation mechanism in these anatexites the preservation of residue melt equilibrium is inferred to result from rapid cooling under contact metamorphic conditions and from the lack of deformation below the solidus

a sequence of partial melting reactions at mt stafford central australia metasedimentary gneisses show a rapid change in grade in a 10 km wide low p high t regional aureole at mt stafford in the arunta block central australia migmatite occurs in all but the lowermost of five metamorphic zones which grade from greenschist zone 1 through amphibolite zones 2 3 to granulite facies zones 4 5 the sequence of partial melting reactions inferred for metapelitic rocks is dependant upon protolith temperature and fluid conditions the metapelite solidus in zone 2 reflects vapour present melting at p 3 kbar and t 640 c melting having initially been controlled by the congruent breakdown of the assemblage crd kfs bt qtz at slightly higher temperature andalusite in leucosome formed via the reaction kfs qtz bt h2oand melt and melt having been stabilized by the presence of boron sillimanite coaxially replaces andalusite in the high grade portion of zone 2 in zone 3 large aluminosilicate aggregates in leucosome are armoured by spl crd grt symplectites garnet partially pseudomorphs biotite cordierite or spinel in high grade portions of zone 3 zone 4 grt crd opx bearing metapsammite assemblages and garnet bearing leucosome reflect t 800 c and p 2 2 0 9 kbar in the model kfmash system the principal vapour absent melting step reflected significant modal changes related to the breakdown of the as bt tie line and the establishment of the spl crd tie line the bulk rock geochemistry of migmatite samples straddle the spl crd tie line the aluminous bulk rock composition of the common bedded migmatite restricted its potential to witness garnet forming and orthopyroxene forming reactions minor textural and modal changes in and above zone 3 reflecting biotite destablization in biotite poor assemblages

electrical conductivity in a partially molten crust from measurements on metasedimentary enclaves abstract the complex electrical impedance of a garnet biotite sillimanite residual enclave in the neogene dacite of el hoyazo se spain has been determined up to 978 c at 200 300 mpa this well studied material represents a direct sampling of the alboran domain thinned lower crust undergoing partial melting the paragenesis is garnet biotite sillimanite plagioclase graphite cordierite coexisting with widespread 10 wt rhyolitic melt occurring as inclusions and interstitial glass and developed during regional anatexis at 850 50 c and 500 700 mpa the samples were used in cyclic measurements consisting in heating cooling ramps at progressively higher maximum temperature to observe the effect of reactions on conductivity in the first cycle up to 850 c at 2 3 kbar re melting of the interstitial glass is achieved with no additional reactions and logarithmic conductivity s m is up to 1 5 at t 950 c new melt with orthopyroxene ilmenite is produced from biotite partial breakdown and the logarithmic conductivity s m is up to 0 7 application of mixing models as hashin shtrikman bounds or archie s law shows that once interconnected melt controls the electrical conductivity of the enclave the electrical conductivity of the rock can be simulated with the electrical conductivity data obtained on the starting matrix glass of the enclave and with the new melt our experimental results obtained on a graphitic metasedimentary material evidence show that graphite does not contribute to the conductivity enhancement and that the electrical anomaly observed below the betics can be explained as effect of partial melting of a residual crust comparison with previous works on mafic granulites shows that similar conductivity values can be achieved both by melting of a residual felsic crust or by melting of mafic materials and suggests that pelitic rocks can be more common at deep levels than expected

contrasting behaviour of boron during crustal anatexis the study of volatile elements in crustal anatectic terranes may provide information about the role of fluid flow and fluid pathways during high temperature metamorphism we have studied the distribution of li and b in two migmatitic areas the pena negra anatectic complex central spain and the migmatites and leucogranites associated with the ronda peridotites southern spain they represent two contrasting cases in the behaviour of volatiles particularly b during anatexis the pena negra migmatites are dominated by diatexites where it is possible to distinguish granitic leucosomes melanosomes metric to decimetric leucogranitic segregates and decimetric to metric sillimanite rich restitic enclaves all lithologies are characterized by low b concentrations mean values 30 ppm and b is enriched in melanosomesrrestites with respect to leucosomesrleucogranites this may be due to the fact that a notable proportion of b can reside in restitic sillimanite b concentrations in leucosomes and leucogranites can be explained by closed system anatexis at ronda metatexitic and diatexitic migmatites underlie the ronda peridotites and diatexite and leucogranite dikes intrude the peridotites diatexites underlying the peridotites are characterized by cordierite q biotite garnet as ferromagne sian phases with rare or absent tourmaline low b contents 10 ppm and cordierite garnet leucosomes leucogranite dikes have high b concentrations mean valuef 400 ppm with tourmaline being a common and sometimes the most abundant ferromagnesian phase a genetic relationship between diatexites and leucogranites is strongly suggested by the presence of dikes of diatexites intruding the bottom of the peridotites we conclude that b in the leucogranites may stem from the dissolution of tourmaline originally present in the metasediments or infiltration of b rich aqueous fluids into the anatectic zone according to recent experimental work textures in some tourmaline rich ronda leucogranites suggest initial b o 23 contents in the melt higher than the present measured concentrations preliminary experimental results presented in this paper propose that this difference is not due to water activities in the melt rapid crystallization of the melts following undercooling may explain the existence of abundant euhedral tourmaline in leucogranites with low aluminium saturation index asis molar al o r caoq na oq k o and relatively low b contents q 2001 elsevier science b v all rights 23 2 2 reserved

aluminous granulites from the pulur complex ne turkey a case of partial melting efficient melt extraction and crystallisation abstract in the pulur complex ne turkey a heterogeneous rock sequence ranging from quartz rich mesocratic gneisses to silica and alkali deficient fe mg and al rich melanocratic rocks is characterized by granulite facies assemblages involving garnet cordierite sillimanite ilmenite spinel plagioclase quartz biotite corundum rutile and monazite textural evidence for partial melting in the aluminous granulites particularly leucosomes is largely absent or strongly obliterated by a late stage hydrothermal overprint however inclusion relations high peak p t conditions the refractory modes bulk and biotite compositions of the melanocratic rocks strongly support a model of partial melting the melt was almost completely removed from the melanocratic rocks and crystallised within the adjacent mesocratic gneisses which are silica rich bear evidence of former feldspar and show a large range in major element concentrations as well as a negative correlation of most elements with sio2 peak conditions are estimated to be 800 c and 0 7 0 8 gpa subsequent near isothermal decompression to 0 4 0 5 gpa at 800 730 c is suggested by the formation of cordierite coronas and cordierite spinel symplectites around garnet and in the matrix sm nd rb sr and 40ar 39ar isotope data indicate peak conditions at 330 ma and cooling below 300 c at 310 ma

cordierite as a sensor of fluid conditions in high grade metamorphism and crustal anatexis cordierite h2o and co2 volatile saturation surfaces derived from recent experimental studies are presented for p t conditions relevant to high grade metamorphism and used to evaluate fluid conditions attending partial melting and granulite formation the volatile saturation surfaces and saturation isopleths for both h2o and co2 in cordierite are strongly pressure dependent in contrast the uptake of h2o by cordierite in equilibrium with melts formed through biotite dehydration melting controlled by the distribution of h2o between granitic melt and cordierite dw dw wt h2o melt wt h2o crd is mainly temperature dependent dw 2 5 6 0 for the h2o contents 0 4 1 6 wt percentage typical of cordierite formed through biotite dehydration melting at 3 7 kbar and 725 900 c this range in dw causes a 15 30 relative decrease in the total wt of melt produced from pelites cordierite in s type granites are h2o rich 1 3 1 9 wt and close to or saturated in total volatiles signifying equilibration with crystallizing melts that achieved saturation in h2o in contrast the lower h2o contents 0 6 1 2 wt percentage preserved in cordierite from many granulite and contact migmatite terranes are consistent with fluid absent conditions during anatexis in several cases including the cooma migmatites and broken hill granulites the cordierite volatile compositions yield ah2o values 0 15 0 4 and melt h2o contents 2 2 4 4 wt compatible with model dehydration melting reactions in contrast h2o leakage is indicated for cordierite from prydz bay antarctica that preserve h2o contents 0 5 0 3 wt which are significantly less than those required 1 0 0 8 wt for equilibrium with melt at conditions of 6 kbar and 860 c the co2 contents of cordierite in migmatites range from negligible 0 1 wt to high 0 5 1 0 wt and bear no simple relationship to preserved cordierite h2o contents and ah2o in most cases the cordierite volatile contents yield total calculated fluid activities ah2o aco2 that are significantly less than those required for fluid saturation at the p t conditions of their formation whether this reflects fluid absence dilution of h2o and co2 by other components or leakage of h2o from cordierite is an issue that must be evaluated on a case by case basis

the surface tectonics of mantle lithosphere delamination following ocean lithosphere subduction insights from physical scaled analogue experiments many postulated lithospheric removal events occur in regions with an earlier history of subduction but the relationship between the two processes has not been explored in this work we use physical scaled analogue experiments to investigate the evolution from ocean lithosphere subduction to collision and possible delamination of the mantle lithosphere from the crust we test how varying the magnitude of plate convergence alters the behavior of the subduction delamination model our experiments show that a retreating ocean proplate can evolve to continental mantle lithosphere delamination negative surface topography is supported at the delamination hinge and this migrates back with the peeling lithosphere with high plate convergence delamination is suppressed rather the crust and mantle lithosphere split at the collision zone in a form of flake tectonics as oncoming procrust is accreted on top of the retroplate and the promantle lithosphere subducts below localized high topography develops at this zone of crustal accretion and thickening the results suggest that delamination may be a continental continuation of plate retreat and that lithospheric removal is triggered by the transition from one process to another

timing of thrust activity in the high zagros fold thrust belt iran from u th he thermochronometry 1 apatite and zircon u th he cooling ages are used to quantify the timing of exhumation associated with thrust faulting in the high zagros fold thrust belt single grain cooling age data were collected from 1 cambrian sandstone in various thrust sheets 2 sandstone and basement clasts derived from structurally controlled salt plugs or fault bounded slices and 3 syntectonic neogene siliciclastics strata in the northwestern kuhrang and central kuh e lajin high zagros apatite u th he ahe ages range from 26 7 to 0 38 ma most cooling and exhumation occurred in the early to middle miocene constrained by ahe ages 19 15 ma from the high zagros thrust sheet localized faults and reset cooling ages from bakhtiyari deposits in the southeastern high zagros kuh e dinar cooling occurred later with ahe ages ranging from 16 5 to 0 79 ma here most cooling and exhumation occurred in the late miocene constrained by ahe ages 12 8 ma from the high zagros fault and exhumed paleozoic clasts in synorogenic strata zircon u th he zhe ages from bedrock samples across the high zagros are reflective of the precollisional thermal history the preservation of precollisional zhe ages limits the pre miocene maximum burial temperature of the exhumed strata to 180 c and indicate 7 9 km of maximum exhumation in the central zagros this study shows that thrust activity in the high zagros and continental suturing along the zagros suture was underway by at least 19 ma and initiated no later than latest oligocene to early miocene time 23 ma

buildup of a dynamically supported orogenic plateau numerical modeling of the zagros central iran case study the iranian plateau is a vast inland region with a smooth average elevation of c 1 5 km formed at the rear of the zagros orogen as a result of the arabia eurasia collision i e over the last 30 35 myr this collision zone is of particular interest due to its disputed resemblance to the faster himalayan collision which gave birth to the tibetan plateau around 50 myr ago recent studies have suggested that a recent 10 5 ma slab break off event below central iran caused the formation of the iranian plateau here we test several hypotheses through large scale 3082 3 590 km numerical models of continental subduction mod els that incorporate a free upper surface erosion rheological stratification brittle elastic ductile rheologies and metamorphic phase changes density and physical properties and account for the specific crustal and thermal structure of the arabian and iranian continental lithospheres we test the impact of the transition from oceanic to continental subduction and the topographic consequences of the progressive slowdown of the convergence rate during continental subduction our results demonstrate the role of mantle flow beneath the overriding plate initiated as an indirect consequence of slab break off this flow creates a dynamic topography support during continental subduction and results in delamination of the overriding plate lithospheric mantle followed by isostatic readjustment hence of further uplift and maintenance of a plateau like topography without significant crustal thickening the slowdown of the convergence rate dur ing the development of the continental subduction collision phase largely contributes to this process by controlling the timing and depth of slab break off

discovery of a neoproterozoic basin in the prydz belt in east antarctica and its implications for gondwana assembly and ultrahigh temperature metamorphism abstract detrital zircon u pb age data from ultrahigh temperature metasedimentary rocks 950 c and 10 12 kbar in the rauer group and a granulite grade metasedimentary rock 850 c and 7 8 kbar in the bolingen islands in prydz bay east antarctica indicate the existence of a neoproterozoic ediacaran cover sequence that was deposited on basement ranging in age between approximately 2800 ma and 940 ma the neoproterozoic succession has a maximum depositional age of approximately 600 ma with derivation from igneous and metamorphic sources of archaean palaeoproterozoic and neoproterozoic age the minimum depositional age is constrained to be approximately 575 ma based on existing th u pb monazite metamorphic ages from the uht metasedimentary rocks the identification of late neoproterozoic protoliths to the uht granulites in the rauer group demonstrate that ultrahigh temperature metamorphism in prydz bay must be ediacaran to cambrian in age the timing of basin development ca 600 575 ma is coincident with the amalgamation of eastern gondwana the palaeogeography of the prydz bay region during the amalgamation of gondwana is uncertain but in our proposed reconstruction we suggest that inherited zircons were derived from indo antarctic crust ocean closure is inferred to have occurred along a suture zone to the southeast of prydz bay and the prince charles mountains our palaeogeographic model invites further work on the possibility of a shared history between india and a large portion of east antarctica prior to the final amalgamation of eastern gondwana during the ediacaran cambrian

evidence for reaction induced microfracturing in granulite facies migmatites water undersaturated partial melting at granulite facies conditions followed by accumulation and upward migration of the resulting granitic melt is one of the principle causes of crustal differentiation microfracturing caused by small positive volume changes associated with water undersaturated melting reactions may facilitate rapid extraction of melts from melting sites we have successfully imaged annealed microfractures in residual quartz from three granulite facies migmatites annealed microfractures have been discovered in 1 quartz inclusions in peritectic garnet 2 quartz inclusions in k feldspar and 3 quartz occurring as entrained grains in the latter oscillatory zoned quartz has overgrown fractures in the residual core indicating that fracturing occurred prior to melt crystallization and that postanatectic volume changes are not responsible for the observed fracturing fractures are generally 5 m wide are parallel sided and have low sinuosity crack densities are two to three times higher than experimental studies involving muscovite melting and approach theoretical interconnectivity thresholds for percolation implying that reaction induced microfracturing is a viable mechanism contributing to melt interconnectivity during anatexis under granulite facies conditions

fluorine as a tool in the petrogenesis of quartz bearing magmatic associations applications of an improved f oh biotite apatite thermometer grid abstract fluorine contents of whole rock and biotites allow to discriminate quartz bearing magmatic associations through the nature of protoliths and petrogenetic processes f and rb are used to discriminate crustal and mantle derived melts with the exception of calcalkaline association in which mantle contribution may be significant the other quartz bearing associations are essentially derived from crustal melting by biotite and or amphibole vapor absent reactions based on experimental data and on an improved biotite apatite thermometer grid closure temperatures from near liquidus to near solidus conditions as well as fh2o fhf evolution during cooling are determined for the different magmatic associations it appears that the liquidus temperature and fh2o fhf decreases in the order high silica alkaline high k calcalkaline and calcalkaline associations while the solidus temperature follows an inverse order the fh2o fhf values for the peraluminous associations vary over a wide range and overlap the fh2o fhf values of the high k calcalkaline and calcalkaline associations

a classification scheme for mantle derived garnets in kimberlite a tool for investigating the mantle and exploring for diamonds abstract a new empirical method has been devised for classification of mantle derived garnets in kimberlite simple chemical screens have been developed to distinguish between garnets from different parageneses based on mg fe ca cr ti and na values of published analyses of garnets from 2000 ultramafic xenoliths in kimberlite although crustal garnets are typically uncommon as xenocrysts in kimberlite the first step in the classification is to screen these from the mantle population using data from 600 garnet bearing crustal rocks such a screen may also prove useful in evaluating the source crust vs mantle of garnet in kimberlite exploration samples subsequent steps divide mantle garnets into eclogite peridotite and cr poor megacryst groupings and sub groups of the peridotite lherzolite harzburgite wehrlite and eclogite groups i and ii and a b c and grospydite populations important features of this classification include the fact that it is based on distinctions between groups of fundamental geological significance e g peridotite vs eclogite and it is based on a large well documented and well understood xenolith database as it utilizes oxide values and molar ratios of major and minor elements the rationale for the screens is readily understood and it is simple to use

tertiary sequence of deformation in a thin skinned thick skinned collision belt the zagros folded belt fars iran 1 we describe how thin skinned thick skinned deformation in the zagros folded belt interacted in time and space homogeneous fold wavelengths 15 8 5 3 km tectono sedimentary evidence for simultaneous fold growth in the past 5 5 2 5 ma drainage network organization and homogeneous peak differential stresses 40 15 mpa together point to buckling as the dominant process responsible for cover folding basin analysis reveals that basement inversion occurred 20 ma ago as the arabia eurasian plate convergence reduced and accumulation of neogene siliciclastics in foreland basin started by 10 ma ongoing contraction occurred by underplating of arabian crustal units beneath the iranian plate this process represents 75 of the total shortening it is not before 5 ma that the zagros foreland was incorporated into the southward propagating basement thrust wedge folds rejuvenated by 3 2 ma because of uplift driven by basement shortening and erosion since then folds grew at 0 3 0 6 mm yr and forced the rivers to flow axially a total shortening of 65 78 km 16 19 is estimated across the zagros this corresponds to shortening rates of 6 5 8 km ma consistent with current geodetic surveys we point out that although thin skinned deformation in the sedimentary cover may be important basement involved shortening should not be neglected as it requires far less shortening moreover for such foreland folded belts involving basement shortening underplating may be an efficient process accommodating a significant part of the plate convergence

poly phase metamorphism of garnet bearing mafic granulite from the larsemann hills east antarctica p t path u pb ages and tectonic implications abstract a rare garnet bearing mafic granulite boulder has recently been identified from the larsemann hills east antarctica with the development of two stage decompression textures detailed petrographic study and p t estimates suggest three distinct metamorphic stages 1 peak ultrahigh temperature uht stage m1 represented by coarse grained garnet orthopyroxene bearing porphyroblastic assemblage with p t conditions of 9 3 kbar and 900 950 c 2 post peak decompression stage i m2 represented by relatively coarse grained orthopyroxene plagioclase ilmenite bearing corona assemblage with p t conditions of 7 2 7 6 kbar and 810 830 c and 3 post peak decompression stage ii m3 represented by fine grained orthopyroxene plagioclase ilmenite bearing symplectite assemblage on garnets and along garnet cracks with p t conditions of 6 2 6 9 kbar and 730 770 c combined with p t pseudosection calculation in the nckfmashto system these p t data define a clockwise p t path with post peak two stage decompression accompanied by important cooling la icp ms zircon u pb age dating results show two weighted mean ages of 927 13 ma and 547 30 ma suggesting that m1 uht stage occurred in the early neoproterozoic rayner orogeny 990 900 ma whereas m2 m3 stages occurred during the early palaeozoic pan african tectonic event 550 500 ma the peak p t conditions post peak two stage decompression p t path and age data resemble those of the early neoproterozoic garnet bearing mafic granulite on sostrene island it is inferred that the mafic granulite boulder may have been derived from the bedrock under inland ice sheet of the larsemann hills thus the early neoproterozoic metamorphism m1 may reflect crustal thickening followed by post orogenic extension likely associated with the assembly of rodinia continent while the overprinting of early palaeozoic metamorphism m2 m3 may reflect an extensive intra continental reworking

archaean cambrian crustal development of east antarctica metamorphic characteristics and tectonic implications abstract the east antarctic shield consists of a variety of archaean and proterozoic cambrian high grade terranes that have distinct crustal histories and were amalgamated at various times in the precambrian cambrian high grade pan african tectonism at 600 500ma is recognized from four distinct belts the dronning maud land lutzow holm bay prydz bay and denman glacier belts these high grade belts juxtapose distinct mesoproterozoic and neoproterozoic crustal provinces maud rayner and wilkes the rauer terrane and have also marginally affected archaean cratonic remnants in the napier complex and southern prince charles mountains the wilkes province experienced its principal tectonothermal events prior to 1130ma and was not affected by the younger events that characterize the maud province 1150 and 1030 990ma the rayner province 990 920ma and the rauer terrane 1030 990ma these differences between the isotopic event records of the basement provinces now separated by the pan african belts require that the older provinces were not formerly parts of a continuous grenville belt as proposed in the sw us east antartic model east antarctica was not a single unified crustal block within either east gondwana or rodinia until the cambrian which is now demonstrated to be the key phase of high grade and ultrahigh temperature uht metamorphism associated with supercontinent assembly the high grade pan african tectonism is characterized by extensive infracrustal melting clock wise p t paths rapid post peak exhumation along isothermal decompression paths to shallow or mid crustal levels by 500 ma and the generation at least locally of uht conditions a significant flux of heat from the mantle into the deep and initially overthickened crust is required to produce these observed metamorphic effects whilst the thermal evolution can be explained by models that invoke the removal of most of the lithospheric mantle following crustal thickening and prior to rapid extension of the remaining crust these one dimensional models are inconsistent with present crustal thicknesses of 25 35km in the pan african domains of the east antarctic shield

ultrahigh temperature granulite metamorphism 1050 c 12 kbar and decompression in garnet mg70 orthopyroxene sillimanite gneisses from the rauer group east antarctica highly magnesian and aluminous migmatitic gneisses from mather peninsula in the rauer group eastern antarctica preserve ultrahigh temperature uht metamorphic assemblages that include orthopyroxene sillimanite quartz garnet sillimanite quartz and garnet orthopyroxene sillimanite garnet that ranges up to xmg of 71 5 coexists with aluminous orthopyroxene that shows zoning from cores with 7 5 8 5 wt al2o3 to rims with up to 10 6 wt al2o3 adjacent to garnet peak p t conditions of 1050 c and 12 kbar are retrieved from fe mg al thermobarometry involving garnet and orthopyroxene in very good agreement with independent constraints from petrogenetic grids in feo mgo al2o3 sio2 and related chemical systems sapphirine orthopyroxene and cordierite form extensive symplectites and coronas on the early phases the specific reaction textures and assemblages involving these secondary phases correlate with initial garnet xmg with apparent higher pressure reaction products occurring on the more magnesian garnet and are interpreted to result from an initial phase of ultrahigh temperature near isothermal decompression uht itd from 12 to 8 kbar at temperatures in excess of 950 c later textures that involved biotite formation and then partial breakdown along with garnet relics to symplectites of orthopyroxene cordierite or cordierite spinel may reflect hydration through back reaction with crystallizing melts on cooling below 900 850 c followed by itd from 7 to 8 kbar to c 5 kbar at temperatures of 750 850 c the tectonic significance of this p t history is ambiguous as the rauer group records the effects of archean tectonothermal events as well as high grade events at 1000 and 530 ma late stage biotite formation and subsequent itd can be correlated with the p t history preserved in the proterozoic components of the rauer group and hence with either 1000 or 530 ma collisional orogenesis however whether the preceding uht itd history reflects a temporally unrelated event e g archean or is simply an early stage of either the late proterozoic or pan african tectonism as recently deduced for similar uht rocks from other areas of the east antarctica remains uncertain

petrology of an intrusion related high grade migmatite implications for partial melting of metasedimentary rocks and leucosome forming processes intrusion related migmatites comprise a substantial part of the high grade part of the southern damara orogen namibia which is dominated by al rich metasedimentary rocks and various granites migmatites consist of melanosomes with biotite sillimanite garnet cordierite hercynite and leucosomes are garnet and cordierite bearing metamorphic grade throughout the area is in the upper amphibolite to lower granulite facies 5 6 kbar at 730 750 c field evidence petrographic observations chemical data and mass balance calculations suggest that intrusion of granitic magmas and concomitant partial melting of metasedimentary units were the main processes for the generation of the migmatites the intruding melts were significantly modified by magma mixing with in situ partial melts accumulation of mainly feldspar and contamination with garnet from the wall rocks however it is suggested that these melts originally represented disequilibrium melts from a metasedimentary protolith the occurrence of lile hfse and lree enriched and depleted residues within the leucosomes implies that both quartzo feldspathic and pelitic rocks were subjected to partial melting isotope ratios of the leucosomes are rather constant 143nd 144nd 500 ma 0 511718 0 511754 e nd 500 ma 3 54 to 5 11 and sr 87sr 86sr 500 ma 0 714119 0 714686 the metasedimentary units have rather constant nd isotope ratios 143nd 144nd 500 ma 0 511622 0 511789 e nd 500 ma 3 70 to 6 93 but variable sr isotope ratios sr 87sr 86sr 500 ma 0 713527 0 722268 the most restitic melanosome mel 4 has a sr isotopic composition of 87sr 86sr 500 ma 0 729380 oxygen isotopes do not mirror the proposed contamination process due to the equally high 18o contents of metasediments and crustal melts however the most lile depleted residue mel 4 shows the lowest 18o value 10 mass balance calculations suggest high degrees of partial melting 20 40 it is concluded that partial melting was promoted by heat transfer and release of a fluid phase from the intruding granites high degrees of partial melting can be reached as long as the available h2o derived from the crystallization of the intruding granites is efficiently recycled within the rock volume due to the limited amounts of in situ melting it seems likely that such regional migmatite terranes are not the sources for large intrusive granite bodies the high geothermal gradient inferred from the metamorphic conditions was probably caused by exhumation of deep crustal rocks and contemporaneous intrusion of huge masses of granitoid magmas the davetsaub area represents an example of migmatites formed at moderate pressures and high temperatures and illustrates some of the reactions that may modify leucosome compositions the area provides constraints on melting processes operating in high grade metasedimentary rocks

ultramafic to mafic granulites from the larsemann hills east antarctica geochemistry and tectonic implications the larsemann hills area is part of a reworked early neoproterozoic metamorphic terrain in southwestern prydz bay east antarctica ultramafic and mafic granulites whose origins remain controversial occur as lenses boudins or layered bodies within the para and ortho gneiss in the region the ultramafic and mafic granulites show spinel olivine bearing and two pyroxene bearing mineral assemblages recrystallized at 860 900 c their bulk rock analyses indicate an origin as igneous cumulates with high mg molar mgo mgo feo from 0 73 to 0 84 for ultramafic granulite and from 0 46 to 0 78 for mostly mafic granulite as well as high cr and ni contents cr and ni up to 1826 ppm and 1400 ppm respectively for ultramafic granulite and cr of 1460 ppm for mafic granulite trace element patterns show pronounced negative nb anomalies suggesting a subduction related tectonic setting for their precursors consistent with derivation from arc basalts also suggested by low tio2 and k enrichment in mafic granulites the ultramafic to mafic granulites may have been formed in a subduction related continental back arc basin environment simultaneously with peak metamorphism associated with arc continent collision during the early neoproterozoic 990 900 ma rayner orogeny

partial melting and p t evolution of the kodaikanal metapelite belt southern india abstract the kodaikanal region of the madurai block in southern india exposes a segment of high grade metamorphic rocks dominated by an aluminous garnet cordierite spinel sillimanite quartz migmatite suite designated herein as the kodaikanal metapelite belt kmb these rocks were subjected to extreme crustal metamorphism during the late neoproterozoic despite the lack of diagnostic ultrahigh temperature assemblages the rocks preserve microstructural evidence demonstrating initial heating dehydration melting to generate the peak metamorphic assemblage and later retrogression of the residual assemblages with remaining melt the peak metamorphic assemblage is interpreted to be garnet sillimanite k feldspar spinel fe ti oxide quartz melt which indicates pressure temperature p t conditions around 950 1000 c and 7 8 kbar based on calculated phase diagrams a clockwise p t path is proposed by integrating microstructural information with pseudosections we show that evidence for extreme crustal metamorphism at ultrahigh temperature conditions can be extracted even in the cases where the rocks lack diagnostic ultrahigh temperature mineral assemblages our approach confirms the widespread regional occurrence of uht metamorphism in the madurai block during gondwana assembly and point out the need for similar studies on adjacent continental fragments

coeval blueschist exhumation along thousands of kilometers implications for subduction channel processes we herein focus on understanding what controls the detachment and migration of oceanic crustal slices along subduction zones we provide evidence for the coeval middle to late cretaceous exhumation of oceanic blueschists in the same neotethyan subduction zone across more than 3000 km from depths around 30 40 km over a short time interval similar to 25 ma compared to the duration of subduction 100 ma we stress the importance of such a geodynamic process and suggest that this exhumation was promoted by a regional scale change in the long term interplate mechanical coupling on the basis of geological and geophysical evidence we propose that the upward migration of oceanic fragments in subduction zones is normally inhibited and only occurs discontinuously as the subduction channel opens up in response to a major change in plate geodynamics slab geometry and dynamics mantle wedge and slab hydration or a combination of these

geochemistry and geochronology of high grade rocks from the grove mountains east antarctica evidence for an early neoproterozoic basement metamorphosed during a single late neoproterozoic cambrian tectonic cycle abstract the grove mountains of east antarctica are an inland continuation of the prydz belt the high grade metamorphic complex in this area is composed of felsic orthogneisses and mafic granulites with minor paragneisses and calc silicate rocks u pb zircon analyses using shrimp and la icp ms techniques reveal that the protoliths of mafic granulites and felsic orthogneisses were emplaced during a short time interval of ca 920 910 ma mafic granulites can be divided into low and high ti groups they have initial nd values nd t ranging from 0 8 to 1 9 tio 2 is positively correlated with feo t mgo and la nb ratios whereas it shows a negative correlation with nd t values indicating that the petrogenesis of their protoliths involved partial melting of a weakly enriched subcontinental lithospheric mantle and fractional crystallization of the magma accompanied by minor crustal contamination felsic orthogneisses have an affinity of a 2 type granites characterized by enrichment in ree y zr th and ga and high ga al and y nb ratios most of them have nd t values of 0 7 to 3 5 and nd depleted mantle model ages t dm of 1 76 1 65 ga and a few have low nd t values of 10 4 to 10 6 and old t dm of 2 46 2 27 ga reflecting a heterogeneity in their source region their protoliths were probably produced by high temperature partial melting of tonalitic granodioritic rocks triggered by the underplating of mantle derived mafic magma during post orogenic extension u pb analyses also reveal a metamorphic age of ca 2050 ma from detrital zircons in a paragneiss suggesting that the sedimentary materials of the paragneiss may have come from an as yet undiscovered early paleoproterozoic orogen of unknown provenance voluminous mafic felsic intrusives and a small amount of sedimentary rocks constitute an early neoproterozoic basement of the grove mountains subsequently this basement experienced only a single late neoproterozoic cambrian metamorphic cycle at ca 550 535 ma the available data indicate that the prydz belt is a collage of multiple basement terranes and each of them has a distinct tectonic evolution this supports the suggestion that the prydz belt may represent a late neoproterozoic cambrian collisional orogen that resulted in the final phase of the gondwana assembly

crustal residence history and garnet sm nd ages of high grade metamorphic rocks from the windmill islands area east antarctica nd whole rock data from the windmill islands area yield early proterozoic to middle archaean nd model ages these crustal residence times are consistent with regional correlations with other parts of antarctica bunger hills denman glacier area and the albany fraser orogen of south western australia during the mid proterozoic and thus support reconstructions with a continuous mid proterozoic orogen in these areas the new nd isotope data provide strong evidence that no age boundary exists between the higher and lower grade parts of the windmill islands area and that the metamorphic complex represents a single terrane with a common crustal history the data support the notion of a time link between the occurrence of intrusive charnockites c type magmas and high grade metamorphism the magmatic rocks and orthogneisses in the area are interpreted to have a mixed source consisting of older crustal components i e older sediments ca 3 2 2 6 ga and a younger mafic component ca 1 9 ga two garnet sm nd isochrons yield ages of 1156 17 ma and 1137 2 5 ma and are identical to shrimp u pb results on monazite from these samples a garnet sm nd age of 1123 13 ma for the ford granite is significantly younger than the shrimp u pb zircon age for this sample the difference relates to the different closure temperature of each isotopic system and is thus interpreted as initial cooling after granulite facies metamorphism

th u pb monazite ages from al mg rich metapelites rauer group east antarctica the age of high temperature granulite facies metamorphism 800 850 c in the rauer group prydz bay east antarctica is relevant for establishing the metamorphic and temporal architecture of the prydz bay mobile belt monazites within al mg rich granulite facies metapelites give an overall tanh estimated pan african age of 511 4 ma 2r using in situ electron microprobe based th u pb chronology consistent with existing u pb zircon geochronology from the rauer group and prydz bay monazite occurs primarily within cordierite bearing coronae and symplectic min eral reaction textures and also within biotite pan african granulite facies metamorphism is preferred as responsible for the development of the cordierite bearing microstructures and probably peak coarse grained assemblages constrained using an integrated geologic geochronologic and metamorphic framework thus pan african granulite facies metamorphism affected the rauer group within the prydz bay mobile belt moreover integrated monazite geochronology may be used to decipher the temporal metamorphic histories of potentially complex high temperature terrains

geology and thermotectonic evolution of the western margin of the trans hudson orogen evidence from the eastern sub athabasca basement saskatchewan in the cree lake zone of northern saskatchewan reworked archean orthogneisses are overlain by a highly deformed supracrustal sequence the paleoproterozoic wollaston group this package of rocks was deformed and metamorphosed during the ca 1 8 ga continent continent collision of the trans hudson orogen tho forming the wollaston fold thrust belt that underlies the eastern athabasca basin the hudsonian structural metamorphic and magmatic evolution of the wollaston fold thrust belt in the eastern athabasca area involved six major stages 1 early collisional stage dp1 at 1860 1835 ma involved burial of wollaston group metasediments from surface to depths equivalent to 3 5vkbar 1 kbar 100 mpa by thrust pile stacking or imbrication tectonics prograde metamorphism with garnet growth and development of early leucosomes and emplacement of ca 1840 ma grey granite suite 2 collisional stage dp2a at 1835 1820 ma involved continued deeper burial of wollaston group metasediments along a prograde

superimposed tectonic events at 2450 ma 2100 ma 900 ma and 500 ma in the north mawson escarpment antarctic prince charles mountains abstract the north mawson escarpment in antarctica s prince charles mountains forms part of a palaeoproterozoic crustal complex which separates archaean cratonic material to the south from early neoproterozoic and early palaeozoic metamorphic belts to the north it consists of nappe like masses of grey gneiss and metasupracrustal rocks that were subjected to repeated ductile deformation under upper amphibolite to lower granulite facies metamorphic conditions in this paper we report zircon u pb dating results that constrain the principal tectonothermal events to the periods 2490 2420 ma 2180 2080 ma 940 880 ma and 530 490 ma the magmatic precursors of volumetrically important grey gneiss were produced and reworked at least partly during the 2490 2420 ma and 2180 2080 ma events the superimposition of these events is clearly documented by u pb ages from several composite zircons the formation of associated metasupracrustals possibly in a passive margin setting occurred between the 2490 2420 ma and 940 880 ma events both grey gneiss and metasupracrustals were subsequently reworked during the 940 880 ma and 530 490 ma events corresponding to the superposition of both the rayner and prydz belts onto palaeoproterozoic lithosphere of the lambert complex transposition and peak metamorphism preceded the intrusion of leucogranite dykes at 905 ma and based on metamorphic zircon growth at around 930 ma in metapelitic schist are related to the rayner orogeny it is suggested that the north mawson escarpment was assembled to most coastal regions of the antarctic sector between 45 and 80 e by at least 900 m y ago

ultra high temperature metamorphism of metapelitic granulites from kondapalle eastern ghats belt implications for the indo antarctic correlation a suite of quartz and corundum bearing metapelitic granulites intruded by layered gabbronorite pyroxenite anorthosite at kondapalle eastern ghats belt preserves a multitude of reaction textures involving oxide and silicate minerals that attest to several prograde and retrograde reactions in the quartz bearing associations the reactions are a biotite sillimanite quartz garnet liquid b garnet sillimanite spinel magnetite quartz c fe2tio4 o2 ferrian ilmenite magnetite d reversal of reaction b e fe2o3 rich ilmenite plagioclase quartz fe2o3 poor ilmenite garnet o2 reactions in the corundum bearing associations are f spinel biotite sillimanite garnet liquid g biotite sillimanite garnet ti rich spinel corundum liquid h biotite sillimanite garnet corundum liquid i fe2tio4 feal2o4 o2 ferrian ilmenite fe3o4 al2o3 in ilmenite j garnet corundum spinel sillimanite to examine the paragenetic evolution of the metapelitic granulites a petrogenetic grid for the kfmash system at high temperatures and pressures involving quartz and corundum was constructed the sequence of inferred reactions documents an anticlockwise heating cooling path reintegrated compositions of spinel with 10 mol fe2tio4 and feldspars indicate ultra high temperature uht of metamorphism 1000 c comparable with the liquidus temperature of the enclosing magmatic rocks crystallization pressures inferred for the magmatic rocks and the pressure constraints imposed by the petrogenetic grid on the metapelite assemblages indicate that the emplacement of the igneous suite and the accompanying uht metamorphism occurred in the lower crust 8 kbar reported u pb cooling ages of monazite and allanite from a late pegmatite suggest the uht metamorphism to be older than 1600 ma the deduced p t history and the absence of grenvillian high grade metamorphism in the study area provide important constraints on the configuration of east gondwana in particular on the continuation of the napier rayner terrane boundary into the eastern ghats belt

observations and numerical modeling of the pearl river plume in summer season a cruise survey of the pearl river plume during southeasterly and southwesterly winds two typical wind patterns in summer in guangdong coastal waters is reported and the cruise data are analyzed to unveil the plume dynamics the kelvin number is derived from the in situ data revealing that the pearl river plume exhibits two different scales in response to the two kinds of the wind forcing numerical model simulations based on the regional oceanic modeling system roms with nesting domains are implemented to explore details of the plume dynamics validated by cruise observations and tidal gauge sea level data modeling results suggest that there is a subtidal anticyclonic recirculation bulge on the west side out of the estuary mouth under southeasterly winds showing a recirculating plume in the near field when the wind changes to the southwesterly however the recirculating plume vanishes the distinct supercritical tidal plume front occurs with southeasterly winds prevailing in spring tide due to the strong advection in the tidal plume the tidal salt deficit flux can reach as high as 12 5 of the mean current flux and illustrates the combined forcing of tide and wind key points the paper clarifies the pearl river plume anatomy under two typical winds spring tide facilitates the supercritical pearl river tidal plume front tide determines tidal salt deficit flux strength wind determines the direction

recognition of pan african aged metamorphism in the fisher terrane central prince charles mountains east antarctica the precambrian tectonic architecture of the east antarctic shield is subject to considerable uncertainty because of limited outcrop the fisher terrane located in the central region of the prince charles mountains evolved during the mesoproterozoic as a volcanic arc system and provides key data about the evolution of this region our study provides evidence that the fisher terrane formed in proximity to a proterozoic plate boundary and was subsequently metamorphosed during the late neoproterozoic to early cambrian during the final stages of gondwana amalgamation u pb detrital zircon geochronology reveals that metasedimentary rocks from within a metasedimentary metavolcanic package cropping out at fisher massif were deposited after c 1300 ma and contain detritus derived from the rayner eastern ghats terrane this suggests that the fisher terrane was not an isolated oceanic arc but rather formed on the same tectonic plate as the rayner complex metapelitic schists from within the same metasedimentary package yield metamorphic u pb monazite ages of c 512 509 ma corresponding to a regionally recognized pan african aged event this event has not been previously identified in the fisher terrane and demonstrates that pan african aged metamorphism affected all parts of the prince charles mountains calculated phase equilibria modelling constrains the metamorphic conditions during this event to 2 5 4 0 kbar and 550 615 c corresponding to apparent thermal gradients of 146 220 c kbar 1 such conditions plausibly relate to metamorphism taking place in an extensional setting supplementary material methods and summarized mineral chemistry data sample locations u pb zircon and monazite data and representative electron microprobe mineral analyses are available at https doi org 10 6084 m9 figshare c 4392710

the geological evolution of the tibetan plateau the geological evolution of the tibetan plateau is best viewed in a context broader than the india eurasia collision zone after collision about 50 million years ago crust was shortened in western and central tibet while large fragments of lithosphere moved from the collision zone toward areas of trench rollback in the western pacific and indonesia cessation of rapid pacific trench migration 15 to 20 million years ago coincided with a slowing of fragment extrusion beyond the plateau and probably contributed to the onset of rapid surface uplift and crustal thickening in eastern tibet the latter appear to result from rapid eastward flow of the deep crust probably within crustal channels imaged seismically beneath eastern tibet these events mark a transition to the modern structural system that currently accommodates deformation within tibet

is himalayan leucogranite a product by in situ partial melting of the greater himalayan crystalline a comparative study of leucosome and leucogranite from nyalam southern tibet abstract widespread leucogranites in the himalayan orogenic belt are thought to have originated by in situ partial melting of the greater himalayan crystalline ghc when it underwent high grade metamorphism during cenozoic orogenesis therefore the leucogranites and associated migmatites can be used to constrain the exhumation history of the ghc however the petrogenetic relationship between the ghc leucogranites and migmatites is not well constrained as such we carried out a detailed petrographic mineralogical geochronological and geochemical study of leucosomes and leucogranites from the nyalam region in southern tibet monazite u th pb dating indicates that anatexis of the ghc occurred during the late eocene and miocene 40 14 ma whereas leucogranite emplacement occurred from 27 to 14 ma there are marked differences between the leucosomes in migmatites and leucogranites in terms of field geology mineralogy and geochemistry suggesting different origins the leucosomes occur mainly as pockets or are interlayered with melanosomes in stromatic metatexites the leucosomes contain oligoclase and fe rich biotite and have whole rock compositions with high k2o contents 4 8 7 4 wt and k2o na2o ratios 1 35 2 97 positive eu anomalies eu eu 0 93 2 61 and low rare metal li be cs sn and ta contents these features are consistent with an origin by muscovite dehydration melting of the ghc however the leucogranites intrude the ghc and occur as small plutons along the south tibetan detachment system in contrast to the leucosomes the plagioclase and biotite in the leucogranites are albite and siderophyllite respectively the leucogranites have relatively low k2o 4 3 4 7 wt contents and k2o na2o ratios 1 04 1 24 high rare metal contents and marked negative eu anomalies eu eu 0 47 0 70 indicating an origin by extensive fractional crystallization we propose that the leucogranites were magmas produced in the deeper ghc during peak metamorphism and subsequent extensive fractional crystallization occurred during long distance upward migration of magma along the south tibetan detachment system stds during exhumation of the ghc during these processes the high grade metamorphic rocks of the ghc were partially melted resulting in the formation of leucosomes within migmatites therefore the leucosomes and leucogranites have different origins and should be considered separately in studies of himalayan orogenesis

shear zone hosted migmatites eastern india the role of dynamic melting in the generation of ree depleted felsic melts and implications for disequilibrium melting in the ranmal migmatite complex non anatectic foliated granite protoliths can be traced to polyphase migmatites structural microtextural relations and thermobarometry indicate that syn deformational segregation crystallization of in situ stromatic and diatexite leucosomes occurred at 8008c and 8 kbar the protolith the neosome and the mesosome comprise quartz k feldspar plagioclase hornblende biotite sphene apatite zircon and ilmenite but the modal mineralogy differs widely the protolith composition is straddled by element abundances in the leucosome and the mesosome the leucosomes are characterized by lower cao feo mgo mg number tio2 p2o5 rb zr and total rare earth elements ree and higher sio2 k2o ba and sr than the protolith and the mesosome whereas na2o and al2o3 abundances are similar the protolith and the mesosome have negative eu anomalies but protolith normalized abundances of ree depleted leucosomes show positive eu anomalies the congruent melting reaction for leucosome production is inferred to be 0 325 quartz 0 288 k feldspar 0 32 plagioclase 0 05 biotite 0 014 hornblende 0 001 apatite 0 001 zircon 0 002 sphene1 4melt based on the reaction large ion lithophile element ree and zr abundances in model melts computed using dynamic melting approached the measured element abundances in leucosomes for40 5 mass fraction of unsegregated melts within the mesosome disequilibrium accommodated dynamic melting and equilibrium crystallization of melts led to uniform plagioclase composition in migmatites and ree depletion in leucosome

petrology and u pb geochronology of high grade metavolcano sedimentary rocks from central xolapa complex southern mexico abstract the xolapa complex xc is a high grade metamorphic belt exposed along the southern margin of the north american plate in mexico its evolution includes an episode of widespread anatexis related to crustal thickening and subsequent uplift to upper crustal levels which culminated in the paleocene on the basis of field and petrographic work this study integrates a petrological modelling approach with la icp ms u pb zircon and monazite geochronology to elucidate the tectonothermal evolution of this metamorphic belt the study area in central xc includes a suite of alternating migmatitic paragneiss and garnet bearing mafic schist petrological evidence and phase equilibria calculations demonstrate that a staurolite kyanite grade metamorphism occurred at 640 670 c and 8 9 kbar before extensive migmatization took place differential anatexis of fertile and refractory layers occurred progressively via biotite and amphibole dehydration melting reactions and continued during peak metamorphism at granulite facies conditions of 800 820 c and 5 7 kbar according to ti in zircon thermometry melt generation mobilization progressed during cooling until crystallize at 700 c the age of prograde metamorphism was likely recorded by monazite cores that give mid to late cretaceous ages cooling and melt crystallization could be constrained by leucosome zircon dated at 61 8 0 6 ma both zircon and low y monazite from whole rock and melanosome portions respectively are consistent with an episode of growth around 60 9 0 5 ma monazite however would have continued its crystallization process for a period of ca 10 ma thin zircon and high y monazite rims show ages between 34 25 ma and are interpreted to reflect a stage of reheating by magmatic advection related to large oligocene plutonism in the region age and nature of central xc basement show good correlation with other early cretaceous volcano sedimentary basins of central and southern mexico a period of crustal thickening would have buried the sequence at depths of 30 km causing barrovian type metamorphism during mid to late cretaceous time and eventually anatexis that evolved at granulite facies conditions during orogenic collapse in the paleocene 62 ma the time span from leucosome crystallization to late stage reheating in central xc would imply a protracted high temperature evolution indicating a middle to upper crustal residence time of at least ca 30 ma

anatexis during high pressure crustal metamorphism evidence from garnet whole rock ree relationships and zircon rutile ti zr thermometry in leucogranulites from the bohemian massif in situ mineral garnet zircon trace element and garnet rock ree distribution data obtained on leucogranulites from the bohemian massif are consistent with an origin of these now strongly deformed rocks as migmatitic leucogranites formed through dehydration melting of muscovite bearing protoliths at high pressures during the variscan orogeny partial melting at p t conditions of 900 9408c and 1 6 1 8 gpa consumed mica and feldspar to produce peritectic kyanite and high ca 20 24 mol grossular high eu garnet along with leucogranitic melt dree grt rock values calculated from this garnet and its host rock are consistent with garnet melt equilibrium at these p t conditions ternary feldspar and rutile 2000 3200 ppm zr crystallized from the leucogranite melts in the migmatites at minimum temperatures of 4880 9208c these formed along with apatite prior to and accompanying the crystallization of garnet rims and the majority of the garnet that occurs in the leucogranulites this garnet is typified by strong depletion in eu and dree grt rock values consistent with garnet melt equilibrium only after the fractionation of apatite 50 3 wt and ternary feldspar 5 15 wt zircons in one leucogranulite are dominated by oscillatory zoned grains that yield pre variscan u pb ages 495 and 433 ma and crystallized from ti in zircon thermometry at 700 8108c in their original host magmas these xenocrystic grains are potentially preserved because of the short timespan of the high p t variscan event at 340 ma and the moderate amounts of leucogranitic partial melt present in the rocks estimated as at least 30 wt melt from zr mass balance the production of new zircon associated with the variscan event in this leucogranulite is limited to weakly zoned zircon rims characterized by flat heavy ree patterns that approach ree equilibrium with garnet rims this post peak zircon was formed at minimum p t conditions of 8408c and 0 8 1 0 gpa along the decompression cooling path defined by previous work for the bohemian massif granulites the garnet whole rock ree relationships observation of eu depletion between early and later garnet growth phases and zr ti thermometric estimates indicating temperatures of48408c for this evolution cannot be explained by the alternative interpretation of the leucogranulites as reworked and recrystallized pre variscan low p granitoids but support a model in which the bohemian leucogranulites were high p variscan age migmatites containing significant amounts of melt

dynamics of slab detachment 1 we investigate the dynamics of slab detachment around the detachment zone and evaluate the amount of time necessary for slabs to detach the study combines results of two dimensional 2d state of the art thermomechanical numerical simulations and a 1d analytical solution for viscous necking under gravity we show that the dominant deformation mechanism during slab detachment is viscous necking independent of the depth of slab detachment when the slab dip is moderate 35 70 slab detachment is partly affected by localized simple shearing in the colder parts of the slab brittle fracturing breaking plays a minor role during slab detachment our 2d thermomechanical models indicate that the duration of slab detachment quantified from the onset of slab thinning until the actual detachment i e vanishing of slab pull force is relatively short 250 km can occur within a short time interval 1 ma which has implications for geodynamic interpretations using slab detachment as explanation for processes such as melting exhumation or surface uplift the thinning of the slab during detachment observed in 2d simulations agrees well with predictions from a 1d analytical solution indicating that the 1d solution captures the first order features of the detachment process we also evaluate the impact of shear heating on the duration of slab detachment the predictions of a simple semi analytical solution based on dimensionless parameters agree well with our and previously published results

melt producing and melt consuming reactions in the achankovil cordierite gneisses south india migmatitic cordierite gneisses within the achankovil zone az of southern pan african india record melt producing and subsequent melt consuming mineral reactions early mineral assemblages bt sil qtz and bt sil spl deduced from inclusion textures in garnet prophyroblasts break down via successive dehydration melting reactions to high t phase assemblages e g grt crd liq opx liq spl crd liq later back reactions between the restite and the in situ crystallizing melt resulted in thin cordierite coronas separating garnet from the leucosome and partial resorption of garnet to opx crd or crd bt qtz symplectites leucosomes generally display a moderate low strain gneisses to strong high strain gneisses depletion of alkali feldspar attributed to mineral melt back reactions partly controlled by the degree of melt segregation using a kfmash partial petrogenetic grid that includes a melt phase and qualitative pseudosections for microdomains of high and low al si ratios the successive phase assemblages and reaction textures are interpreted in terms of a clockwise p t path culminating at about 6 7 kbar and 900 950 c this p t path is consistent with but more detailed than published results which suggests that taking a melt phase into account is not only a valid but also a useful approach comparing p t data and lithological and isotopic data for the az with adjacent east gondwana fragments suggests the presence of a coherent metasedimentary unit exposed from southern madagascar via south india az and sri lanka wanni complex to the lutzow holm bay in eastern antarctica

the cordierite bearing anatectic rocks of the higher himalayan crystallines eastern nepal low pressure anatexis melt productivity melt loss and the preservation of cordierite cordierite bearing anatectic rocks inform our understanding of low pressure anatectic processes in the continental crust this article focuses on cordierite bearing lithologies occurring at the upper structural levels of the higher himalayan crystallines eastern nepal himalaya three cordierite bearing gneisses from different geological transects from mt everest to kangchenjunga have been studied in which cordierite is spectacularly well preserved the three samples differ in terms of bulk composition likely reflecting different sedimentary protoliths although they all consist of quartz alkali feldspar plagioclase biotite cordierite and sillimanite in different modal percentages analysis of the microstructures related to melt production and or melt consumption allows the distinction to be made between peritectic and cotectic cordierite the melt productivity of different prograde assemblages from two mica metapelite metagreywacke to biotite metapelite has been investigated at low pressure conditions evaluating the effects of muscovite v biotite dehydration melting on both mineral assemblages and microstructures the results of the thermodynamic modelling suggest that the mode and type of the micaceous minerals in the prograde assemblage is a very important parameter controlling the melt productivity at low pressure conditions the two mica protoliths being significantly more fertile at any given temperature than biotite gneisses over the same temperature interval furthermore the cordierite preservation is promoted by melt crystallization at a dry solidus and by exhumation along p t paths with a peculiar dp dt slope of about 15 18 bar c 1 overall our results provide a key for the interpretation of cordierite petrogenesis in migmatites from any low p regional anatectic terrane the cordierite bearing migmatites may well represent the source rocks for the miocene andalusite bearing leucogranites occurring at the upper structural levels of the himalayan belt and low p isobaric heating rather than decompression melting may be the triggering process of this peculiar peraluminous magmatism

late cenozoic reorganization of the arabia eurasia collision and the comparison of short term and long term deformation rates the arabia eurasia collision deforms an area of 3 000 000 km2 of continental crust making it one of the largest regions of convergent deformation on earth there are now estimates for the active slip rates total convergence and timing of collision related deformation of regions from western turkey to eastern iran this paper shows that extrapolating the present day slip rates of many active fault systems for 3 7 million years accounts for their total displacement this result means that the present kinematics of the arabia eurasia collision are unlikely be the same as at its start which was probably in the early miocene 16 23 ma or earlier in some but not all active fault systems short term 10 year and long term 5 million year average deformation rates are consistent there is little active thickening across the turkish iranian plateau and possibly the interior of the greater caucasus these are two areas where present shortening rates would need more than 7 million years to account for the total crustal thickening and where there are structural and or stratigraphic data for pre late miocene deformation we suggest that once thick crust up to 60 km built up in the turkish iranian plateau and the greater caucasus convergence took place more easily by crustal shortening in less elevated regions such as the zagros simple folded zone the south caspian region and foothills of the greater caucasus or in other ways such as westward transport of turkey between the north and east anatolian faults the time and duration of this changeover are not known for certain and are likely be diachronous although deformation started or intensified in many of the currently active fault systems at 5 2 ma

a type medina batholith and post collisional anatexis in the aracuai orogen se brazil abstract the medina batholith and its host granitic migmatites record intriguing plutonic processes in the northern aracuai orogen se brazil this orogen shows a long lasting 630 480 ma succession of granite production events from the earliest pre collisional plutons to the latest post collisional intrusions the medina batholith includes granite intrusions ascribed to the post collisional stage they show high alkali and halogen contents low cao at sio2 71 na2o k2o 7 to 9 cao 1 6 and high feot feot mgo ratios 0 78 to 0 92 the medina granites are metaluminous to weakly peraluminous with asi molecular ratio al ca 1 67p na k values of 1 76 to 2 07 and have high concentrations of high field strength elements zr nb ce y 700 ppm as well as high ga al ratios accordingly the medina intrusions are typical ferroan a type granites u pb ages from zircon 501 2 ma and monazite 497 2 ma constrain the emplacement timing of the medina batholith surprisingly all monazite ages from host rocks also cluster around 500 ma despite their nature and distance from the batholith suggesting that they would have shared a same thermal process the studied host rocks are granitic migmatites varying from patch metatexite to nebulitic diatexite comprising paleosome of foliated sillimanite garnet biotite metagranite to gneiss and non foliated garnet cordierite neosome poor to free of biotite a metatexite r14 located relatively far from the medina batholith and a diatexite m26 found at the batholith contact were sampled for detailed studies the paleosome of foliated metagranite r14a only shows zircon grains with igneous features and th u ratio from 1 64 to 0 26 although the spreading of zircon spots the main cluster yields a concordia age at 556 6 ma constraining the protolith magmatic crystallization a minor cluster furnishes a concordia age at 499 7 ma in agreement with the u pb monazite age at 501 2 ma extracted from the same metatexite sample the non foliated garnet cordierite neosome r14b shows both igneous and metamorphic zircon domains with th u ratios ranging from 1 47 to 0 00 again the u pb spots cluster at two distinct concordia ages 562 3 ma and 499 3 ma the youngest of them fitting with the monazite age 495 3 ma constrains melt crystallization while the oldest age suggests paleosome inheritance the nebulitic diatexite m26 shows monazite 497 2 ma and zircon th u 1 7 to 0 0 concordia ages at 564 2 ma and 507 3 ma populations similar to the metatexite neosome also with the youngest ages bracketing the melt crystallization process around 500 ma accordingly all those ages at around 500 ma disclose a partial melting episode coeval with the medina batholith emplacement phase equilibrium modeling on a garnet cordierite neosome furnished p t conditions of 750 840 c at 2 4 3 5 kbar for that post collisional anatexis evidence for such a late thermal event are common in the aracuai orogen even far from the post collisional batholiths thus a possible major heat source can be envisaged like a mantle plume triggering crustal anatexis and regional fluid circulation during the gravitational collapse of the aracuai orogen

monazite as a monitor for melt rock interaction during cooling and exhumation alexander m prent andreas beinlich laura j morrissey tom raimondo chris clark andrew putnis

cordierite as an indicator of thermodynamic conditions of petrogenesis a refined thermodynamic model of h2o and co2 bearing cordierite based on recent data on volatile incorporation into cordierite thompson et al in contrib mineral petrol 142 107 118 2001 harley and carrington in j petrol 42 1595 1620 2001 reflects non ideality of channel h2o and co2 mixing the dependence of cordierite h2o and co2 contents on p t and equilibrium fluid composition has been calculated for the range 600 800 c and 200 800 mpa it has been used for establishing thermodynamic conditions of cordierite formation and the following retrograde p t paths of cordierite rocks from many localities estimates of the h2o and co2 activities have shown that cordierites in granites pegmatites and high pressure granulites were formed in fluid saturated conditions and wide range of h2o co2 relations very low cordierite h2o contents in many migmatites may be caused not only by fluid undersaturated conditions at rock formation and h2o leakage on retrograde p t paths but also by the presence of additional volatile components like ch4 and n2 the pressure dependence of cordierite bearing mineral equilibria on fluid h2o co2 relations has been evaluated

melt producing versus melt consuming reactions in pelitic xenoliths and migmatites abstract melt producing reactions have operated both in pelitic xenoliths and in pelitic migmatites but melt consuming reactions related to in situ melt crystallization are largely restricted to migmatites hence xenolith reaction textures may provide clues on the relative roles of both reaction types in migmatites a comparison of xenoliths and migmatites results in a catalogue of textural and chemical features some of which are argued to be diagnostic the results have implications for the identification of restite in igneous rocks and for the relative importance of prograde and retrograde processes in different domains of layered migmatites