u pb dating of monazite and its application to geological problems monazite is an underutilized mineral in u pb geochronological studies of crustal rocks it occurs as an accessory mineral in a wide variety of rocks including granite pegmatite felsic volcanic a

geochemical evidence for the geotectonic setting of early proterozoic metavolcanic sequences in lapland systematic geochemical study of early proterozoic metavolcanic rocks from the northern part of the baltic shield supports the concept that a complex mosaic of continental masses volcanic arcs and marginal basins were welded into one cratonic massif during the svecokarelian orogeny 1900 1750 ma in the north in finnmark and northern norrbotten tholeiitic metabasalts together with thick shallow water clastic metasediments form the main part of the karelian supracrustal sequences which in turn overlie archaean basement the trace element signatures of these metabasalts are comparable to those of post palaeozoic continental tholeiites which erupted in zones of lithosphere attenuation and which suffered contamination by sialic crust it is not however possible to determine by geochemistry alone whether the continental rift basin in question lay in an intraplate or back arc setting metabasalts with a volcanic arc geochemical signature become an increasingly important component of supracrustal sequences as the diffuse southern margin of the karelian continent is approached in the skellefte field an ensialic volcanic arc developed and fed considerable quantities of immature clastic sediment into the deep water norrland geosyncline which may have been fully oceanic in character subduction magmatism continued during the svecokarelian orogeny giving rise to synorogenic metavolcanic suites such as the kiruna porphyries the evidence for marked spatial variation of magmatic activity in the early proterozoic is in contrast to the rather monotonous pattern of greenstone belt development reported from the archaean we believe that this spatial variability is a consequence of geotectonic processes similar to those of modern day plate tectonics and present a plate tectonic model for the early proterozoic development of the baltic shield

mesoproterozoic arc magmatism in se india petrology zircon u pb geochronology and hf isotopes of the bopudi felsic suite from eastern ghats belt abstract the southern segment of the eastern ghats mobile belt egmb in india was an active convergent margin during mesoproterozoic prior to the final collision in neoproterozoic during the assembly of the rodinia supercontinent here we present mineralogical whole rock geochemical zircon u pb and hf isotopic data from a granitoid suite in the bopudi region in the eggb the granitoid complex comprises quartz monzodiorite with small stocks of rapakivi granites the monzodiorite locally porphyritic contains k feldspar megacrysts plagioclase quartz biotite and ortho amphibole the presence of mantled ovoid megacrysts of alkali feldspar embaying early formed quartz and the presence of two generations of the phenocrystic phases in the rapakivi granites indicate features typical of rapakivi granites the k feldspar phenocrysts in the rapakivi granite are mantled by medium grained aggregates of microcline ab7 or93 which is compositionally equivalent to the rim of kfs phenocryst and pl an23 24 ab75 the geochemistry of both the granitoids shows arc like features for ree and trace elements la icp ms zircon analyses reveal 207pb 206pb ages of 1582 mswd 1 4 for the rapakivi granite 1605 3 ma mswd 3 9 for the monzodiorite the zircons from all the granitoid samples show high ree contents prominent hree enrichment and a conspicuous negative eu anomaly suggesting a common melt source the zircons from the monzodiorite have a limited variation in initial 176hf 177hf ratios of 0 28171 0 28188 with ehf t values of 2 2 to 2 8 correspondingly their two stage hf isotope model ages tdm2 ranging from 2 15 to 2 47 ga probably suggest a mixed source for the magma involving melting of the paleoproterozoic basement and injection of subduction related juvenile magmas the prominent mesoproterozoic ages of these granitoids suggest subduction related arc magmatism in a convergent margin setting associated with the amalgamation of the columbia derived fragments within the neoproterozoic rodinia assembly

metamorphic field gradients in the central alps metamorphic field gradients were determined across the entire amphibolite grade central alps c 50 100 km p t were calculated from 116 samples acquired from our own field work from samples provided to us by others and from rocks with mineral compositions described in the literature only fluid conserved equilibria were used to determine p t the use of an internally consistent thermodynamic database and mineral solid solution models makes the results robust and reduces relative errors the results are presented in contour maps temperature increases from 500 to 550 c along the limit of amphibolite grade metamorphism in the north and west to c 675 c toward the south at the insubric line near the town of bellinzona maximum recorded pressures of c 7 kbar are in a central region c 20 km north of the insubric line and decrease both to the north 5 5 kbar and south 4 5 kbar the p t results indicate that there is a relatively large area that reached conditions in the sillimanite stability field but developed neither sillimanite nor fibrolite this is interpreted as a result of kinetic constraints on nucleation and growth because of the small amounts of thermal overstep 40 c of the kyanite sillimanite phase boundary comparison of p t conditions with carbonate isograds in the region indicate that fluids present during metamorphism were not dominated by a homogeneous external source examination of the two dimensional distribution of pressure and temperature in the context of thermal and tectonic models indicates that two thermal pulses aected the central alps during the tertiary in the second heat aected only the southern parts of the area and overprinted the previously established p t gradients

precambrian ophiolites and related rocks developments in precambrian geology 13 t m kusky editor pp 800 elsevier 2004 isbn 0 444 50923 2 price 198 the purpose of this collection of papers is to address the still controversial question as to the continuity of modern plate tectonic processes back through earth s early geologic history by cataloguing worldwide precambrian ophiolite sequences for the economic geologist and explorationist this volume provides an opportunity to acquaint themselves with geological and geochemical characteristics of precambrian ophiolite assemblages and of phanerozoic analogues this in turn will allow them to recognize this distinctive type of sea floor assemblage in the field and from geochemical study as well as the implications of its presence within certain precambrian volcano sedimentary terranes the editors organized the series of 22 papers into four sections proterozoic ophiolites and related rocks archean ophiolites and related rocks models for the evolution of oceanic crust with time and analogies to precambrian ophiolites the editor s introduction to these sections is somewhat lengthy and repetitious but at the same time it is a good summary of the different ophiolite settings and their characteristics and also serves as a volume synopsis

seismically imaged shallow and deep crustal structure and potential field anomalies across the eastern dharwar craton south indian shield possible geodynamical implications abstract the time bound crustal evolution and subsequent deformation of the cuddapah basin nellore schist belt and eastern ghats terrain of eastern dharwar craton which have undergone sustained geodynamic upheavals since almost 2 0 billion years remain enigmatic an attempt is made here to integrate newly available potential field data and other geophysical anomalies with deep seismic structure to examine the generative mechanism of major crustal features associated with this sector our study indicates that the initial extent of the cuddapah basin sedimentation may have been much larger extending by almost 50 60 km west of tadipatri during paleoproterozoic period which subsequently shrank due to massive erosion following thermal uplift caused by sw cuddapah mantle plume below this region crust is still quite warm with moho temperatures exceeding 500 c similarly nallamalai fold belt rocks bounded by two major faults and extremely low gravity may have occupied a large terrain in western cuddapah basin also before their abrasion no geophysical signatures of thrusting are presently seen below this region and thus it could not be an alien terrain either in contrast nellore schist belt is associated with strikingly high positive gravity possibly caused by a conspicuous horst structure and up dipping mafic crustal layers underneath that resulted due to india east antarctica collision after the cessation of prolonged subduction 1 6 0 95 ga further the crustal seismic and gravity signatures would confirm presence of a totally distinct geological terrain east of the cuddapah basin but the trace of eastern ghats belt is all together missing instead all the geophysical signatures point out to presence of a proterozoic sedimentary terrain east of nellore schist belt it is likely that the extent of prorerozoic sedimentation was much larger than thought today in addition presence of a seismically detected gondwana basin over nellore schist belt apart from some recently discovered similar subsurface gondwana occurrences in intracratonic parts would indicate that dharwar craton was rifting even during gondwana period thereby challenging the long held view of cratonic stability

archean greenstone belts are not oceanic crust archean greenstone belts with a preponderance of mafic volcanic rocks often preserved in tectonically complex sequences are obvious candidates in the search for remnants of archean ocean crust we review the tectonic setting and stratigraphy of a number of greenstone sequences previously interpreted as archean ophiolites and conclude on the basis of basal unconformities presence of xenocryst zircons geochemical and isotopic evidence for crustal contamination intrusive relationships with older basement and their internal stratigraphy that none of these examples is derived from archean oceanic crust thermal tectonic and isostatic constraints imply that archean oceanic basins did exist and were covered with several kilometers of water we consider it possible indeed probable that relict archean oceanic crust is preserved in granite greenstone terrains or other archean tectonic settings but that an unequivocal archean ophiolite has not yet been recognized

electron microprobe and la icp ms study of monazite hydrothermal alteration implications for u th pb geochronology and nuclear ceramics this paper presents new data that quantify the response of magmatic monazite to three main types of hydrothermal alteration namely sericitization chloritization and greisenization the samples were taken from three paleozoic granites the st nectaire granite massif central france the carnmenellis granite cornwall england and the skiddaw granite lake district england fluid inclusion thermometry and hydrothermal parageneses indicate that alteration took place at temperatures ranging from 260 to 340 c and salinities from 3 to 18 wt nacl equivalent possible monazite alteration mechanisms found in the course of this study using backscattered scanning electron microscopy bse sem electron microprobe laser raman spectroscopy and laser ablation inductively coupled plasma mass spectrometry la icp ms include cationic substitutions monoclinic to hexagonal structure transition accompanied by chemical exchanges selective th removal dissolution reprecipitation and dissolution with replacement by a different mineral these results show that despite the compositional and crystallographic simplicity of monazite it exhibits varied chemical responses to different mineral fluid interactions elemental and isotopic measurements by la icp ms for the unaltered monazites yield statistically significant 232th 208pb and 238u 206pb magmatic crystallization ages in some cases it was possible to give a reasonable estimate of the age of mineral fluid interaction using the altered parts of the monazites or newly precipitated crystals for other minerals 232th 208pb and 238u 206pb systematics were strongly disturbed by variable inputs and or depletions of u th and pb the data suggest that monazite like nuclear waste forms would be good hosts for tetravalent actinides but may release the lanthanides and actinides with lower and higher valencies especially if the fluids are oxidizing or tend to dissolve monazite

experimental resetting of the u th pb systems in monazite abraded fragments 200 400 m of a large chemically homogeneous and non metamict brazilian monazite crystal characterised by a concordant u pb ages of 474 1 ma 208pb 206pb 19 5 were hydrothermally treated at varying temperatures with solutions of different compositions product monazites were analysed with scanning electron microscope sem electron microprobe emp secondary ion mass spectrometer sims and isotope dilution thermal ionisation mass spectrometer id tims experiments with pure water over a temperature range of 800 1200 c at 700 mpa and durations ranging from 5 to 60 days showed that even at 1200 c any dissolution and recrystallization of new monazite is confined to the outermost surface of the grain neither pb diffusion at the emp scale nor significant discordancy were observed we performed experiments at 800 and 1000 c for different durations using different fluid compositions at quartz saturation a 10 wt cacl2 fluid a 10 wt srcl2 fluid a 10 wt nacl fluid and a fluid containing nbs 982 pb standard 208pb 206pb 1 for all runs emp traverses revealed no pb diffusion profiles significant overgrowths of newly formed monazite are documented by sem analyses they occurred only in the 1000 c experiments when either cacl2 or pb bearing fluids were present in the cacl2 experiment two zones could be distinguished within the crystal a core possessing the initial monazite composition and a rim consisting of newly formed monazite produced by dissolution precipitation which was enriched in ca and pb free id tims dating of single grains treated with srcl2 and cacl2 solutions at 1000 c are significantly discordant experiments employing the nbs pb standard produced sub concordant monazite for which the 207pb 206pb apparent age has become older than prior to the experiment 544 ma at 800 c and 495 ma at 1000 c the newly grown monazite rim had obviously incorporated pb from the fluid none of our reaction products contained a detectable diffusion profile the only resetting mechanism we detected involved dissolution precipitation the extent of the dissolution precipitation process depends on fluid composition and is a more efficient mechanism than diffusion for controlling the resetting of monazite in natural rocks

kinematic analysis of melange fabrics examples and applications from the mchugh complex kenai peninsula alaska permian to cretaceous melange of the mchugh complex on the kenai peninsula south central alaska includes blocks and belts of graywacke argillite limestone chert basalt gabbro and ultramafic rocks intruded by a variety of igneous rocks an oceanic plate stratigraphy is repeated hundreds of times across the map area but most structures at the outcrop scale extend lithological layering strong rheological units occur as blocks within a matrix that flowed around the competent blocks during deformation forming broken formation and melange deformation was noncoaxial and disruption of primary layering was a consequence of general strain driven by plate convergence in a relatively narrow zone between the overriding accretionary wedge and the downgoing generally thinly sedimented oceanic plate soft sediment deformation processes do not appear to have played a major role in the formation of the melange a model for deformation at the toe of the wedge is proposed in which layers oriented at low angles to s1 are contracted in both the brittle and ductile regimes layers at 30 458 to s1 are extended in the brittle regime and contracted in the ductile regime and layers at angles greater than 458 to s1 are extended in both the brittle and ductile regimes imbrication in thrust duplexes occurs at deeper levels within the wedge many structures within melange of the mchugh complex are asymmetric and record kinematic information consistent with the inferred structural setting in an accretionary wedge a displacement field for the mchugh complex on the lower kenai peninsula includes three belts an inboard belt of late triassic rocks records west to east directed slip of hanging walls a central belt of predominantly early jurassic rocks records north south directed displacements and early cretaceous rocks in an outboard belt preserve southwest northeast directed slip vectors although precise ages of accretion are unknown slip directions are compatible with inferred plate motions during the general time frame of accretion of the mchugh complex the slip vectors are interpreted to preserve the convergence directions between the overriding and underriding plates which became more oblique with time they are not considered indicative of strain partitioning into belts of orogen parallel and orogen perpendicular displacements because the kinematic data are derived from the earliest preserved structures whereas fabrics related to strain partitioning would be expected to be superimposed on earlier accretion related fabrics 1999 elsevier science ltd all rights reserved

rayleigh fractionation of heavy rare earths and yttrium during metamorphic garnet growth we present laser ablation microprobe inductively coupled mass spectrometry analyses of y yb er dy and gd in garnet crystals from high grade metamorphic and migmatitic rocks from the sierras pampeanas of argentina these rocks contain large garnet porphyroblasts 4 mm that are compositionally zoned rare earth element concentrations in the rims are typically one order of magnitude lower than those in the cores a notable feature of this zonation is the inversion of yb er and yb dy ratios from 1 in the cores to 1 in the rims we show quantitatively that the spatial distribution of these trace elements in garnet can be most simply and effectively explained as arising from rayleigh fractionation during garnet growth we also analyzed a small garnet crystal 2 mm representative of a garnet population that is associated with migmatitic leucosomes this crystal displays uniformly low concentrations of all trace elements that are virtually identical in both absolute and relative magnitudes to those in the rims of the larger garnet porphyroblasts the small crystals and perhaps part of the porphyroblast rims are likely to be peritectic products of incongruent melting reactions we argue that melts formed from garnet bearing sources may generally be unable to equilibrate with metamorphic garnet cores and may thus be more strongly depleted in heavy rare earth elements and y than modeling based on bulk source rock abundances of these elements would indicate

the spatial distribution of grains and crystals in rocks characterisation and analysis of the spatial distribution pattern sdp of grains or crystals in rocks is potentially a powerful technique which can be used to constrain the processes which operate in the formation of rocks a method to quantify the sdp of grains in thin section is presented the distance betwen the centre of a grain and the centre of its nearest neighbour is calculated for all the grains in the sample area to produce a distribution of distances that characterises the spatial pattern of grains in the rock this distribution is then normalised to a random distribution of points with the same population density to give a descriptive value r values of r for rock samples are plotted against porosity modal abundance of other phases in igneous and metamorphic rocks to characterise the sdp the sdp of randomly packed distributions of equal size spheres varies systematically with porosity producing a line on a porosity versus r plot termed the random sphere distribution line rsdl rocks which plot above the rsdl have an ordered sdp and those that plot below a clustered sdp the effects of variation in grain packing order grain sorting compaction and random crystallisation overgrowth on determined r values were investigated using a combination of 3 d sphere models and 2 d texture models the maximum possible value of r is 2 148 corresponding to a perfect section through hexagonal cubic close packing of grains the minimum value of r is dependent on the proportion of grains in the sample volume and may be as low as 1 2 for a sample volume with 30 grains which are clustered variations in size sorting can cause r to vary by approximately 0 25 mechanical compaction of a loose framework of grains results in a higher packing order and an increase in r continued compaction creates a fabric in the texture and r decreases as cluster patterns are developed perpendicular to the principal stress the overgrowth of grains in a touching framework alters the location of the geometric grain centre as the crystal shape changes leading to an increase in r as porosity decreases results reveal that grain sorting compaction and grain overgrowth produce different r versus porosity relationships which can be identified in and therefore used to constrain the relative importance of these processes in rock samples

the chimalpahad anorthosite complex and associated basaltic amphibolites nellore schist belt india magma chamber and roof of a proterozoic island arc abstract new major and trace element data on the proterozoic chimalpahad layered anorthositic complex and associated basaltic amphibolites of the nellore schist belt of south india provide new constraints on their petrogenesis and geodynamic setting the complex consists of layered anorthosites leucogabbros gabbros ultramafic rocks and is spatially associated with basaltic amphibolites despite deformation and metamorphism primary cumulate textures and igneous layering are locally well preserved throughout the complex whereas the amphibolites display diverse ree systematics the chimalpahad anorthositic gabbroic rocks are characterized by moderately depleted to strongly enriched lree patterns and by flat to depleted hree patterns the field relations major and trace element compositions of the basaltic amphibolites suggest that they are petrogenetically related to the anorthositic gabbroic rocks by fractional crystallization the anorthositic rocks and the basaltic amphibolites share the depletion of nb relative to th and la on primitive mantle normalized diagrams they exhibit signatures of arc magmatic rocks such as high lile and lree relative to the hfse and hree as well as high ba nb ba zr sr y la yb ratios that mimic chondrite normalized ree and primitive mantle normalized trace element patterns of arc magmas similarly on log transformed tectonic discrimination diagrams the chimalpahad rocks plot within the field of phanerozoic magmatic arcs consistent with a subduction zone origin on the basis of field relations and geochemical characteristics the chimalpahad complex is interpreted as a fragment of a magma chamber of an island arc which is tectonically juxtaposed against its original volcanic cover a new preliminary sm nd date of anorthosite from the chimalpahad complex indicates a model age of 1170 ma

monazite xenotime garnet equilibrium in metapelites and a new monazite garnet thermometer prograde suites of pelitic rocks were examined with electron microprobe introduction and laser ablation inductively coupled plasma mass spectrometry to monazite ce la th po4 plays an important role in determine the systematics of element partitioning between coexisting studies of igneous and metamorphic petrogenesis foremonazite xenotime and garnet monazite grains that grew in most monazite is used to date specific events in a equilibrium with xenotime are enriched in y and dy compared with petrogenetic sequence e g parrish 1990 harrison et al monazite that grew in xenotime absent assemblages y and heavy 1997 hawkins bowring 1999 foster et al 2000 rare earth element contents of monazite coexisting with xenotime monazite may contain a large percentage of the sample increase with rising temperature monazite xenotime y gd and rare earth element ree budget and thus exert an y dy partitioning is systematic within a metamorphic grade and important influence on the evolution of melt composition increases slightly with increasing metamorphic grade suggesting wark miller 1993 bea 1996 the thermobarometric that monazite xenotime pairs approached partitioning equilibrium potential of monazite coexisting with xenotime has been garnet and monazite in both xenotime bearing and xenotime absent recognized leading to calibration of monazite geoassemblages show a strong r 0 94 systematic relationship thermometers gratz heinrich 1997 1998 heinrich between inverse temperature and ln keq for the net transfer equiet al 1997 andrehs heinrich 1998 in addition the librium yag oh ap 25 4 qtz 5 4 grs 5 4 an recognition that nearly all lead in monazite is radiogenic 3ypo4 mnz 1 2h2o suggesting that garnet and monazite parrish 1990 montel et al 1994 has led to development crystallized in compositional equilibrium the following temand application of electron microprobe monazite dating perature keq relationship for the equilibrium above has been derived techniques suzuki adachi 1991 montel et al 1996 williams et al 1999 t c 1 45p bars 447772 32052 567 40 rln keq 273 15 full realization of monazite as a thermochronometer requires detailed understanding of the specific reactions responsible for its formation previous studies of monazite with a precision of some 30 c for temperature estimates our petrogenesis have focused on monazite compositional observations suggest that major and accessory phases interact in a zonation zhu o nions 1999a 1999b monazite coupled fashion during metamorphism and also approach a state growth kinetics ayers et al 1999 behavior of monazite of compositional equilibrium as reactions proceed during hydrothermal alteration poitrasson et al 1996 crowley ghent 1999 and melting events watt 1995 or to a limited extent the influence of major phase mineral assemblage in monazite reactivity zhu

the genesis of the northern kettle moraine wisconsin interpreting past glacial dynamics from the glacial record requires that the depositional environments of glacial sediments and landforms be understood in the case of interlobate deposits models that incorporate various components of pro supra and subglacial deposition have been developed and tested in the northern kettle moraine nkm wisconsin a large interlobate deposit that formed between the green bay and lake michigan lobes of the laurentide ice sheet during the last deglaciation in this paper we interpret a new genesis for the nkm using sediment analysis and distribution along with landform distribution in sheboygan county the nkm consists of two steep sided high relief hummocky ridges separated by a low elevation and low relief central axis gravel in the bounding hummocky ridges is well sorted and well rounded some bedding is collapsed large isolated moulin kames are restricted to the axis area and composed of relatively poorly sorted more angular gravel and diamicton the distribution of these different sediments and landforms are explained by the accumulation of supraglacial debris that insulated the ice below the axis of the nkm while the melting of cleaner ice on either side formed channels on the ice surface as deglaciation proceeded a substantial thickness of well rounded stream deposited sand and gravel accumulated on ice in the bounding channels eventual collapse of this sediment formed the two hummocky ridges poorly sorted debris along the axis fell and slid into moulins and larger collapse areas in the ice thus differential debris insulation and ice ablation controlled the mainly supraglacial deposition of this part of the nkm

seismic imaging across the eastern ghats belt cuddapah basin collisional zone southern indian shield and possible geodynamic implications abstract the evolutionary nature of the area encompassing cuddapah basin nellore schist belt and the ongole domain of eastern ghats belt south india which contains paleo neoproterozoic imprints of thick sedimentation intense magmatism subduction accretion and possible collision between eastern dharwar craton and east antarctica is still being debated an attempt has been made here to decipher the deep crustal structure and tectonics of this region by reprocessing deep seismic sounding data acquired earlier through the modeling of first arrival refraction and wide angle reflection travel times the derived crustal seismic velocity structure reveals that the intracratonic proterozoic cuddapah basin containing only 4 km thick sediment is bounded by two major faults the moho reaching fault detected on its eastern boundary demarcates cuddapah basin from the nellore schist belt although a deep normal fault is mapped within the nallamalai fold belt but no thrusting signatures are apparent almost all the crustal layers beneath nellore schist belt and the ongole domain of eastern ghats belt show distinct eastward dipping trend conforming to an upthrusted feature suggesting possible presence of a collisional suture besides the areas lying east of cuddapah basin appear to be an accreted orogenic terrain beneath which lower crust has upwarped substantially the entire stretch of the studied region is underplated by unprecedently thick 20 km high velocity 7 0 7 4 km s magma layer above the moho indicating strong crust mantle thermal perturbation and massive subcrustal erosion further an expression of a deep seated mantle thermal anomaly has also been found below the parnapalle region of the sw cuddapah basin beneath which deeper crustal layers have exhumed

the significance of intergranular diffusion to the mechanisms and kinetics of porphyroblast crystallization the spatial disposition compositional zoning profiles and size distributions of garnet crystals in 11 specimens of pelitic schist from the picuris range of new mexico usa demonstrate that the kinetics of intergranular diffusion controlled the nucleation and growth mechanisms of porphyroblasts in these rocks an ordered disposition of garnet centers and a significant correlation between crystal radius and near neighbor distances manifest suppressed nucleation of new crystals in diffusionally depleted zones surrounding pre existing crystals compositional zoning profiles require diffusionally controlled growth the rate of which increases exponentially as temperature increases with time an acceleration factor for growth rate can be estimated from a comparison of compositional profiles for crystals of different sizes in each specimen crystal size distributions are interpreted as the result of nucleation rates that accelerate exponentially with increasing temperature early in the crystallization process but decline in the later stages because of suppression effects in the vicinity of earlier formed nuclei simulations of porphyroblast crystallization based upon thermally accelerated diffusionally influenced nucleation kinetics and diffusionally controlled growth kinetics quantitatively replicate textural relations in the rocks the simulations employ only two variable parameters which are evaluated by fitting of crystal size distributions both have physical significance the first is an acceleration factor for nucleation with a magnitude reflecting the prograde increase during the nucleation interval of the chemical affinity for the reaction in undepleted regions of the rock the second is a measure of the relative sizes of the porphyroblast and the diffusionally depleted zone surrounding it crystal size distributions for the picuris range garnets correspond very closely to those in the literature from a variety of other localities for garnet and other minerals the same kinetic model accounts quantitatively for crystal size distributions of porphyroblastic garnet phlogopite sphene and pyroxene in rocks from both regional and contact metamorphic occurrences these commonalities indicate that intergranular diffusion may be the dominant kinetic factor in the crystallization of porphyroblasts in a wide variety of metamorphic environments

precambrian blueschist terranes of the world blueschists are most common in post paleozoic orogenic belts higher geothermal gradients and a thinner lithosphere in the precambrian are widely thought to have disfavored blueschist formation however several precambrian blueschist belts have been reported from eurasia on the basis of established geologic and geochronologic constraints these belts are grouped as suspect possible and definite precambrian blueschists suspect ones include those in alaska in anglesey u k in the scandinavian caledonides and in yunnan china possible precambrian blueschist belts are those in delhi india in central china in heilongjiang china and in the urals and altai sayan tien shan u s s r definite precambrian blueschist terranes are those in western africa and in aksu china these blueschist belts are described with emphasis on the age constraints the recently investigated aksu blueschist terrane is presented as an example this high p t coherent blueschist greenschist terrane at aksu is unconformably overlain by a thick unmetamorphosed sedimentary sequence ranging in age from late sinian ca 600 800 ma through eocambrian to paleozoic the basal sinian conglomerates contain subrounded to rounded clasts of the underlying schist clearly demonstrating the pre sinian age of blueschist facies metamorphism characteristic features of the above mentioned blueschist belts include 1 1 the blueschist terranes are coherent and primary compositional lithological layers are well preserved melange belts are not common 2 2 protoliths include pelite sandstone bimodal volcanics limestone and small quantities of pillow basalt and pelagic sediment ultramafics are rare 3 3 the protoliths were subjected to multiple deformation and transitional blueschist greenschist facies metamorphism typical precambrian blueschist contains the assemblage crossitic amphibole epidote chlorite albite quartz actinolite rare lawsonite jadeitic pyroxene and aragonite have been described in few belts 4 4 the metamorphic facies series belongs to the high pressure intermediate type of miyashiro 1961 the precambrian blueschist and other pre mesozoic intracontinental blueschist of eurasia formed before rather than during final closure of intervening ocean basins between pairs of continents they do not usually mark the location of terminal sutures but are the result of earlier accretion and continental growth by subduction underplating and imbrication of accretionary wedges including both oceanic and continent derived elastics the rare preservation of precambrian blueschist is due to greenschist overprinting when the heat flow and geothermal gradient was high rather than reflecting lack of production their existence in china and africa demonstrates that requisite conditions for blueschist facies metamorphism existed at least 700 ma ago

contrasted monazite hydrothermal alteration mechanisms and their geochemical implications in spite of the major importance of monazite as a repository for the rare earths and th in the continental crust for u th pb geochronology and as a possible form for high level nuclear waste very little work has been carried out so far on the behaviour of this mineral during fluid rock events this contribution describes two contrasting examples of the hydrothermal alteration of monazite the first case comes from a sample of the carnmenellis granite cornwall southwest england chloritized at 284 16 c whereas the other occurs in the skiddaw granite lake district northwest england which underwent greisenization at 200 30 c an integrated study involving backscattered scanning electron microscopy electron microprobe analyses and laser ablation inductively coupled plasma mass spectrometry la icp ms reveals that the chloritization event was characterized by the coupled substitution 2ree3 th4 ca2 in the altered parts of the monazite thus leaving the p o framework of the crystal untouched in contrast greisenization led to the coupled substitution ree3 p5 th4 si4 and therefore involved a partial destruction of the phosphate framework the resulting rare earth element patterns are quite different for these two examples with a maximum depletion for dy and er in the altered parts of the carnmenellis monazite whereas the skiddaw monazite shows a light rare earth depletion but an yb and er enrichment during alteration this latter enrichment accompanied by an increase in u but roughly unchanged pb concentrations probably resulted from a decrease in the size of the 9 coordinated site in monazite thereby favouring the smaller rare earths these contrasted styles of monazite alteration show that the conditions of fluid rock interaction will not only affect the aqueous geochemistry of the lanthanides actinides and lead and the relative stability of the different minerals holding these elements variations in these conditions will also lead to various possible chemical exchanges between the crystalline phases and the hydrothermal fluids the occurrence of common lead along penetrative cracks in the carnmenellis monazite shows that only a leaching prior to the u pb analyses of the whole grain will permit an accurate determination of the magmatic crystallization age in contrast for the skiddaw case it may be possible to date the fluid rock event by in situ 207pb 206pb geochronology the observation that the altered parts of both monazite examples display nd leaching and no significant sm nd fractionation indicates that they should not affect the host whole rock nd isotopic signatures finally it appears that monazite like ceramics designed for the containment of high level nuclear wastes will retain th and the geochemically equivalent transuranic elements during fluid rock events similar to those documented in this study but may release nd u and the corresponding radionuclides to the environment

dismembered ophiolites in paleoproterozoic nappe complexes of kandra and gurramkonda south india abstract the constitution and structural architecture of ophiolites is important in the understanding of cratonic growth through subduction collision accretion history in the precambrian terrains the nellore schist belt nsb sandwiched between the eastern ghats belt egb and the eastern dharwar craton south india is known to have deformed remnants of ocean plate stratigraphy ops as at kanigiri and kandra suggesting multiple cycle of ocean opening and closure in the paleoproterozoic mesoproterozic interval the outcrop details and structural architecture of the paleoproterozoic kandra complex with geochemical character of subduction related ophiolites is described and analyzed to better constrain the accretion collision history along the southeastern margin of the indian craton imbricate thrust slices between kandra and arimanupadu host slivers of hornblende metagabbro thick massive to pillowed metabasalt sheets sheeted meta dolerite dykes and thin metapsammite and metapelite interstratified with metabasalt comparable to layer 2a of oceanic crust contractional deformation in interlayered metabasalt and quartzite around kandra is accommodated by south dipping imbricate thrust systems and n ne vergent folds the wnw trending thrust system with dominant tectonic transport toward northeast is oblique to overall n s trend of the nsb or the egb further east intercalated metabasalts deep water turbidites and thin metacherts around gurramkonda are deformed into sw vergent imbricate thrust system here occurrence of metabasalt sheets along individual thrusts suggest syntectonic emplacement of some basalts as the subducted and scraped upper oceanic crust was incorporated in the acrcetionary prism the rock sequences around kandra and gurramkonda comprising incomplete ops are tectonically stacked over granites of the eastern dharwar craton margin bivergent thrust systems around kandra possibly root into a detachment separating layers 1 2 from more rigid deeper layers cf layer 3 of oceanic crust as indicated by absence of significant volume of ultramafic rocks at the current outcrop level

field and geochemical characteristics of the mesoarchean 3075ma ivisaartoq greenstone belt southern west greenland evidence for seafloor hydrothermal alteration in supra subduction oceanic crust the mesoarchean ca 3075ma ivisaartoq greenstone belt in southern west greenland includes variably deformed and metamorphosed pillow basalts ultramafic flows picrites serpentinized ultramafic rocks gabbros sulphide rich siliceous layers and minor siliciclastic sedimentary rocks primary magmatic features such as concentric cooling cracks and drainage cavities in pillows volcanic breccia ocelli interpreted as liquid immiscibility textures in pillows and gabbros magmatic layering in gabbros and clinopyroxene cumulates in ultramafic flows are well preserved in low strain domains the belt underwent at least two stages of calc silicate metasomatic alteration and polyphase deformation between 2963 and 3075ma the stage i metasomatic assemblage is composed predominantly of epidote now mostly diopside quartz plagioclase hornblende scapolite and occurs mainly in pillow cores pillow interstitials and along pillow basalt gabbro contacts the origin of this metasomatic assemblage is attributed to seafloor hydrothermal alteration on the basis of the common presence of epidote inclusions in diopside and the local occurrence of epidote rich aggregates the stage i metasomatic assemblage is interpreted as relict epidosite the stage ii metasomatic assemblage occurs as concordant discontinuous layered calc silicate bodies to discordant calc silicate veins commonly associated with shear zones the stage ii metasomatic assemblage consists mainly of diopside garnet amphibole plagioclase quartz vesuvianite scapolite epidote titanite calcite scheelite given that the second stage of metasomatism is closely associated with shear zones and replaced rocks with an early metamorphic fabric its origin is attributed to regional dynamothermal metamorphism the least altered pillow basalts picrites gabbros and diorites are characterized by lree enriched near flat hree and hfse especially nb depleted trace element patterns indicating a subduction zone geochemical signature ultramafic pillows and cumulates display large positive initial end values of 1 3 to 5 0 consistent with a strongly depleted mantle source given the geological similarities between the ivisaartoq greenstone belt and phanerozoic forearc ophiolites we suggest that the ivisaartoq greenstone belt represents mesoarchean supra subduction zone oceanic crust 2006 international association for gondwana research published by elsevier b v all rights reserved

blueschist facies metamorphism in the ophiolitic melange of the late proterozoic delhi fold belt rajasthan india abstract the south delhi fold belt of late proterozoic age contains ophiolitic melange of contrasting ensialic rift and arc trench marginal basin tectonic domains the trench metagreywacke contains relict blueschist assemblages which have been overprinted by regional greenschist facies metamorphism

time transgressive deposits of repeated depositional sequences within interlobate glaciofluvial esker sediments in koylio sw finland abstract evidence of conspicuous repeated seasonal to annual deposition of glaciofluvial and glaciolacustrine sequences within a structurally complex interlobate esker segment in sw finland is presented the time transgressive overlapping depositional sequences consist of deposits from two successive melt seasons including three vertically stacked lithofacies associations 1 massive to stratified coarse gravels summer deposits 2 trough and ripple cross stratified fine grained deposits autumn to winter deposits and 3 sandy stratified beds spring deposits the depositional environment of each lithofacies association involves a transition from subglacial or submarginal tunnel to a subaqueous re entrant environment which then passes to a proglacial glaciolacustrine environment the study also presents evidence of headward extension of subglacial tunnel deposits related to the rapid shifting of a tunnel expansion point during the increasing spring discharge which occupied the old tunnel exit this mode of annual deposition has not been reported previously in esker studies the good preservation of the rhythmic lithofacies associations is suggested as resulting from interlobate depositional conditions associated with rapidly decaying icestreams therefore the depositional model may provide a key to recognizing time transgressive interlobate eskers that form an important geomorphological and sedimentological record of meltwater activity during the last deglaciation of the fennoscandian and laurentide ice sheets the identification of time transgressive interlobate eskers and associated palaeo icestream behaviour is an essential step forward for more accurate models of ice sheet behaviour and palaeoclimatic reconstructions

mantle and slab contributions in arc magmas destructive plate margins are major sites of terrestrial magmatism that have long had a key role in models for the generation of continental crust and the development of chemical heterogeneities in the upper mantle in these models it is important to evaluate the nature and size of contributions from preexisting crust in the generation of magmas above subduction zones and to constrain the size of the fluxes of different elements from the subducted crust and the overlying mantle wedge in arc magmas kay 1 980 hawkesworth ellam 1 989 hawkesworth et al 1 99 1 a mcculloch gamble 1 99 1 the more mobile components of the subducted oceanic crust are the products of element fractionation processes in the upper continental crust the ocean basins and hydrothermal systems on the ocean floor they therefore have distinctive isotope and even trace element ratios that in principle should be readily distinguished from those in peridotite in the mantle wedge prior to subduction in practice however element ratios are almost certainly fractionated during shallow level dehydration of sub ducted material and at greater depths where material from the slab appears to contribute in the generation of arc magmas moreover the isotope signal of slab derived material may be heavily diluted by exchange with

monazite and xenotime petrogenesis in the contact aureole of the makhavinekh lake pluton northern labrador high temperature 700 900 c metamorphism in the contact aureole of the makhavinekh lake pluton mlp northern labrador led to the growth of monazite and xenotime during progressive replacement of regional garnet bearing assemblages m1 by lower pressure symplectitic coronas of orthopyroxene cordierite m2 in the inner aureole 500 m from the contact where m1 garnet is strongly resorbed high y hree monazite xy hree 0 14 0 18 occurs as small isolated grains and as discontinuous rims on partially resorbed pre m2 monazites that were liberated from garnet xenotime also occurs as small isolated grains within m2 coronas ion microprobe dating of thin high y rims indicates that new monazite growth occurred during m2 monazite xenotime miscibility gap temperatures are consistent with al solubility in orthopyroxene thermometry estimates indicating that peak temperatures in the inner aureole are accurately recorded and preserved by monazite m2 monazite records therefore the temperature and timing of m2 metamorphism two net transfer reactions modelled using singular value decomposition in the system p y hree lree are proposed to account for the growth of m2 phosphates 1 38 grt1 1 mnz1 1 13 mnz2 and 2 737 grt1 1 ap 1 mnz2 3 4 xno2 reaction 1 conserves p and gave rise to locally coronitic high y overgrowths on partially resorbed pre m2 monazite whereas reaction 2 accounts for the growth of small new monazite and xenotime grains both reactions were highly localized within individual m2 coronas due to slow intergranular diffusion accompanying fluid undersaturated metamorphism in the mlp aureole similar monazite forming reactions are expected in other polymetamorphosed granulites

structural metamorphic evolution of late proterozoic ophiolites and precambrian basement in the central asian foldbelt of mongolia abstract the late proterozoic and palaeozoic central asian foldbelt is regarded as one of the best examples which demonstrates that processes of lateral crustal growth and obduction accretion tectonics took place since at least late precambrian times in mongolia the precambrian and early cambrian rocks were formed in two different environments continental and oceanic the archaean to early proterozoic complexes are autochthonous and constitute a metamorphic continental basement the late proterozoic to early cambrian ophiolite complexes represent remnants of obducted oceanic crust and occur as intensely deformed nappes and tectonic melanges overthrust on the older basement during the caledonian orogeny distinctly differentiated tholeiitic calc alkaline and boninite rocks series together with the lithological characteristics of associated volcanogenic sedimentary formations indicate that the mongolian ophiolite complexes were generated in a forearc marginal basin setting the culmination of ophiolite formation occurred in late precambrian to early cambrian time five phases of progressive regional metamorphism have been identified in the basement complex and overlying ophiolite association each of these was accompanied by folding and granitoid plutonism and is characterised by a distinct composition of fluid inclusions

u pb zircon chronology of the pangidi kondapalle layered intrusion eastern ghats belt india constraints on mesoproterozoic arc magmatism in a convergent margin setting abstract the pangidi kondapalle layered intrusion is emplaced within high grade supracrustal rocks in the southern sector of the eastern ghats granulite belt proximal to the south eastern periphery of the indian plate this layered complex consists mainly of anorthosites and gabbronorites with pyroxenite cumulates containing orthopyroxenites and chromitites in a regressive intrusive sequence here we report laser ablation icp ms data and ree geochemistry of zircons from three co spatial anorthositic bodies in this complex our results show mean 207 pb 206 pb ages of 1634 18 ma mswd 0 55 for the anorthosite from pangidi two anorthosite samples from kondapalle yield mean 207 pb 206 pb ages of 1628 36 ma mswd 0 54 and 1693 110 ma mswd 5 5 the zircons from all the anorthosite samples show high ree contents prominent hree enrichment and a conspicuous positive eu anomaly suggesting a common melt source the prominent mesoproterozoic ages revealed from our u pb dating of zircons from these layered complexes suggest subduction related magmatic underplating in a convergent margin setting associated with the amalgamation of the columbia derived fragments within the neoproterozoic supercontinent

mesoproterozoic ophiolitic melange from the se periphery of the indian plate u pb zircon ages and tectonic implications abstract the kanigiri melange within the proterozoic nellore khammam schist belt in southern peninsular india includes ophiolitic fragments that represent the remnants of an oceanic plate the ophiolitic units were accreted along a ne trending suture that juxtaposes the proterozoic eastern ghats granulite belt eggb against the archean nellore schist belt of the dharwar craton the ophiolite components in the kanigiri melange include plagiogranites and gabbros which show mutually intrusive relations indicating their coeval nature we report laser ablation icp ms age data and ree geochemistry of zircons from the gabbro and granite the zircons from both gabbro and granite show high ree contents prominent hree enrichment and a conspicuous negative eu anomaly suggesting a common melt source zircon ree abundances and normalized patterns show little intersample and intrasample variations u pb dating of the zircons reveals prominent mesoproterozoic ages for the plagiogranite with the ca 1 33 ga age of the kanigiri ophiolitic melange offering important clues for arc continent collision during the final stages of amalgamation of the columbia derived fragments within the neoproterozoic supercontinent assembly

blueschist metamorphism during accretion in the lachlan orogen south eastern australia low t intermediate to high p assemblages indicative of the prehnite pumpellyite greenschist and blueschist facies are preserved in melange zones and slivers of oceanic crust within two major fault zones of the turbidite dominated lachlan orogen in one of these fault zones governor fault zone blueschists occur as franciscan like blocks in a serpentinite talc matrix that is interleaved with phyllites and slates and structurally overlain by a fault slice or duplex of predominantly pillow basalt chert and turbidite the blueschist metavolcanics are interpreted to have formed at 450 c and at a depth of approximately 21 27 km the presence of blue amphibole in the blocks rinds and matrix indicate that the metavolcanics were emplaced in the matrix prior to blueschist metamorphism blocks and matrix were partially exhumed interleaved with tectonic slices of phyllite and slate and subsequently folded at about 10 12 km depth inferred from bo values of the dominant mica fabric in the phyllites and slates metamorphic p t is highest in the structurally lowest slice melange zone and lowest in the overlying ophiolitic fault slice suggestive of an accretionary burial metamorphic pattern formed by underplating of the melange in the other fault zone heathcote fault zone blueschists transitional to greenschist facies are interpreted to have formed at 450 c and at a depth of approximately 15 21 km they occur as blocks in serpentinite talc matrix melange and are also associated with fault slices of oceanic crust textural and mineralogical evidence suggests that the protoliths for the blueschists in both fault zones were boninitic pillow lavas the metamorphic facies and patterns and the structural and lithological associations can be interpreted in terms of disruption of oceanic crust and overlying sediments during subduction and formation of serpentinite matrix melange overprinted by blueschist metamorphism either prior to or during underplating of the melange and duplex formation the presence of blueschist metavolcanics indicate that these processes occurred at considerable depth these interpretations have implications for the evolution of large scale fault zones in noncollisional convergent oceanic settings

redistribution of rare earth elements thorium and uranium over accessory minerals in the course of amphibolite to granulite facies metamorphism the role of apatite and monazite in orthogneisses from southwestern norway the amphibolite to granulite facies transition has been studied in a high k calc alkaline hornblende biotite k feldspar megacrystic augen gneiss series from the rogaland vest agder rog va sector of the sveconorwegian province southwest norway hornblende begins to break down mainly to clinopyroxene at the cpx in isograd and biotite mainly to orthopyroxene at the opx in isograd abbreviations for minerals following kretz 1983 the magmatic accessory mineral association comprises titanite and allanite which begin to break down before the cpx in isograd titanite is preserved as relict inclusions in other minerals at higher grade monazite and thorite formed in the breakdown of allanite monazite abundance reaches a maximum between the cpx in and opx in isograds the middle and heavy rare earth element m hrees except eu content of apatite steeply increases with increasing metamorphic grade and is directly correlated to the decrease of the modal abundances of titanite hornblende and biotite the u contents of apatite are low and do not increase with metamorphic grade the light rare earth elements lrees and th content of apatite are not correlated to the breakdown of allanite around the cpx in isograd but increase around the opx in isograd a simplified equation is proposed for monazite crystallisation in the vicinity of the cpx in isograd that accounts for the allanite titanite and hornblende breakdowns and the m hree substitution in apatite 3 m hree 2o3in hornblende and litanite 3 lree 2in allaniteo3 2ca5 po4 3apatite f oh 6sio2quartz 6 lree pmonaziteo4 2ca2 m hree 3 sio4 3 f oh lessingite in apatite 6caoin plagioclase at the opx in isograd the increase of lrees and th content of apatite results either from the breakdown of some monazite 3 lree po4monazite 3sio2quartz 4caoin plagioclase f2 h2o fluid ca5 po4 3 f oh apatiteca2 lree 3 sio4 3 f oh less ingite in apatite or the breakdown of the remaining allanite 3 lree 2o3in allanite 6sio2quartz 4caoin plagioclase f2 h2o fluid 2ca2 lree 3 sio4 3 f oh lessingite in apatite the release of fluorine from the breakdown of biotite at the opx in isograd may increase apatite stability relatively to monazite in the granulite facies when compared to amphibolite facies the granulite facies augen gneisses do not show any u or th depletion this means that changes in the accessory mineral associations and the simultaneous breakdown of hydrous minerals at the amphibolite granulite facies transition do not inevitably result in depleted granulite facies rocks but rather to an isochemical element redistribution the coexistence of small amounts of metamorphic monazite and of relict inclusions of titanite in upper amphibolite facies augen gneisses suggests that high k calc alkaline orthogneisses are a suitable material to date with the u pb method prograde path of amphibolite facies regional metamorphism on monazite and the cooling path on titanite

nondestructive chemical dating of young monazite using xrf 2 context sensitive microanalysis and comparison with th pb laser ablation mass spectrometric data a newly developed xrf microprobe at the institute of mineralogy and petrology university of bern switzerland has been applied for precise chemical th u pb dating of individual monazite grains separated from pb free polished petrographic thin sections the nondestructive nature of the xrf measurement permitted a comparative study of dating methods by sequentially applying chemical dating by electron microprobe analysis empa chemical dating by xrf microprobe analysis and isotopic 208pb 232th dating by laser ablation plasma ionisation multi collector mass spectrometry la pimms analysis as an example the 2 precision achieved with the xrf microprobe for well characterised reference material monazite fc 1 tims age 54 3 1 ma xrf age 55 3 2 6 ma doubly polished to 30 m in thickness is below 5 after 90 min integration time 50 kv 30 ma at a spatial resolution of 90 m at 38 m spatial resolution the uncertainty is 35 for the same integration time the sample characteristics are 200 300 ppm of pb xrf 3 8 5 1 wt of th empa and 0 4 1 4 wt u empa combined with an electron microprobe and conventional optical microscopy the xrf microprobe is thus a competitive low cost and nondestructive alternative to more costly isotopic methods the xrf microprobe is easy to use and maintain

electron microprobe dating of monazite because monazite is extremely rich in u and th radiogenic pb pb accumulates very quickly and reaches in about 100 ma a level where it is possible to analyse it with the electron probe assuming that common pb is negligible and that partial loss of pb has not occurred the simultaneous measurement of u th and pb allows to obtain a geologically meaningful age from a single electron probe analysis here we present the results of two years of systematical investigations aiming to define both the limits and potential of this method a specific statistical method to deal with the large number of data which can be obtained on a single sample is described and several guidelines illustrated by examples are suggested to optimize the method electron probe measurements carried out on samples of known age from 200 ma to 3 1 ga yield ages that always fall inside the confidence interval of the isotopically determined age demonstrating that this method is reliable the younger age limit is approximately 100 ma although it can be younger in some favourable cases in old monazites extremely high pb contents have been found up to 5 wt indicating that monazite can tolerate high radiation doses without experiencing lead loss the final precision on the age for a normal monazite is 30 50 ma for a total counting time of 600 s a complete dating procedure can be completed in less than 1 h first results indicate that old ages can be preserved in monazite either in small relict cores in crystals or by the coexistence of several generations of monazites in a sample this method has all the advantages of the electron probe it is non destructive has an excellent spatial resolution monazites as small as 5 i m can be dated and because it is possible to work on normal polished thin sections the petrographical position of the dated crystal is known this method offers a large number of geologists access to an in situ dating technique at moderate cost

shallow marine to pelagic sediments from a dismembered ophiolite kandra southern india glimpses of ancient subduction zone related sedimentation abstract a suprasubduction zone oceanic back arc setting for the paleoproterozoic kandra ophiolite complex koc in southern india has been suggested from geochemical signatures the telescoped segments of thin deformed sedimentary successions of shallow marine to pelagic affinity overlying a basaltic substrate and preserved within thrust slices of the koc are tectonically juxtaposed against the eastern dharwar craton margin in the northern thrust slice kandra village succession about 150 m of sedimentary strata show intercalation of quartz arenite and basaltic flow in the lower part grading upwards to heterolithic sandstone mudstone deposited above the storm wave base in the southeastern part of the koc gurramkonda succession deep water greywacke turbidite pelagic chert mafic tuff and volcaniclastics and quartz arenite deposited below the storm wave base are preserved as thrust bound packets intermittent basaltic outpourings punctuated deeper water deposition as evidenced by alternate metachert and metabasalt layers and emplacement of basaltic rocks along small thrusts which transpose stratification craton margin sediments consists of immature coarse terrigenous clastics intercalated with thin mafic tuff suggesting influence of mass flow processes giving way to fluvial sedimentation in the lower part of the udaigiri group further up fine grained plane laminated siltstone shale with rippled sandstone lenses grade upward to compositionally mature quartz arenite deposited close to the craton margin with signatures of tidal and wave reworking the association of stratigraphic successions of two contrasting depositional environments in the koc adds to the spectrum of variation of sedimentary collage of the ocean plate stratigraphy the kandra village and gurramkonda successions of the koc possibly represent ancient arc trench milieu and shallower part of oceanic marginal basin respectively paleoproterozoic subduction accretion process led to collapse of these basins and tectonic emplacement of the koc against the eastern dharwar craton margin which hosted near shore sedimentary succession of the udaigiri group occurring west of the koc

proterozoic orogens in southern peninsular india contiguities and complexities abstract the precambrian terranes of southern peninsular india have been central to discussions on the history of formation and breakup of supercontinents of particular interest are the proterozoic high grade metamorphic orogens at the southern and eastern margins of the indian shield skirting the 3 4 ga dharwar craton which not only preserve important records of lower crustal processes and lithospheric geodynamics but also carry imprints of the tectonic framework related to the assembly of the major neoproterozoic supercontinents rodinia and gondwana these proterozoic orogens are described as southern granulite terrane sgt in the southern tip and the eastern ghats mobile belt egmb in the eastern domains of the peninsula the contiguity of these orogens is broken for a distance of 400 km and disappears in the bay of bengal these orogens expose windows of middle to lower crust with well preserved rock records displaying multiple tectonothermal events and multiphase exhumation paths recent studies in these orogens have led to the recognition of discrete crustal blocks or terranes separated by major shear zone systems some of which represent collisional sutures the sgt and egmb carry several important features such as fold thrust tectonics regional granulite facies metamorphism of up to ultrahigh temperature conditions in some cases multiple p t paths development of lithospheric shear zones emplacement of ophiolites presence of alkaline and anorthositic complexes development of crustal scale flower structures transpressional strains and reactivation tectonics a heterogeneous distribution of different metamorphic and magmatic assemblages with distinct spatial and temporal strain variations in shaping the fabric elements in different blocks is identified both egmb and sgt share a common transpressional deformation history during the latest neoproterozoic characterized by the steepening of the initial low angle crustal scale structures leading to a subvertical grain conducive to reactivation tectonics our synthesis of the spatial distribution geometry kinematics and the transpressional strain of the shear zone systems provides insights into the tectono metamorphic history of the proterozoic orogens of southern india and their contiguity and complexities recent understanding of subduction accretion and collisional history along these zones together with a long lived transpressional tectonic regime imply that these orogens witnessed identical tectonic regimes at different times in earth history although the major and common structural architecture was built during the final assembly of the gondwana supercontinent

petrological and geochemical characterization of proterozoic ophiolitic melange nellore khammam schist belt se india abstract the ophiolitic melange occurs close to a major fault within the metavolcanics of kanigiri area in the western margin of the khammam nellore schist belt andhra pradesh india the unique feature of kanigiri melange is the occurrence of ophiolitic bodies of various sizes most of which display petrological and geochemical characteristics typical of supra subduction zone oceanic crust the ophiolitic melange is composed mainly of blocks of variably deformed and metamorphosed pillow basalts serpentinized ultramafic rocks meta gabbros cherts and pelagic sediments in a pervasive scaly argillite matrix the melange has a tectonic contact with a granite intrusion and numerous thrust faults cut across the melange exposures primary magmatic features such as drainage cavities in pillow basalts magmatic layering in gabbros and clinopyroxene cumulates in ultramafic rocks are well preserved in low strain domains mafic components enclosed in the melange are characterized by low pressure secondary mineral assemblages retrograded from amphibolite down to greenschist facies as commonly observed in subducted and exhumed orogens all amphiboles in the studied fragments are calcic and cluster in the range from winchite actinolite to hornblende amphibole compositions are characterized by al 2 o 3 2 4 13 8 feo 5 13 k 2 o 0 04 0 7 and x mg 0 67 0 77 the metabasalt and wherlite samples exhibit light rare earth element lree enriched patterns and negative high field strength element hfse zr and ti consistent with a suprasubduction zone geodynamic setting a notable feature of both metabasalt and wherlite samples is their pronounced negative ce anomaly and a positive eu anomaly as observed in a number of modern island arc suites as the petrological features and the geochemical characteristics of the remnant fragments we analyzed in this study are reminiscent of their phanerozoic ophiolite counterparts we suggest that these rocks were derived from a proterozoic oceanic plate belonging to a suprasubduction ophiolite assemblage subduction zone geochemical signatures and ophiolite like crustal lithologies in the study region are consistent with the accretion of juvenile oceanic fragments to the pre existing continental blocks through a forearc continent collision

emplacement and deformation of the vinukonda meta granite eastern ghats india implications for the geological evolution of peninsular india and for rodinia reconstructions abstract the u pb and rb sr systematics of zircon populations from two samples and biotite white mica pairs from four samples constrain the time of emplacement and deformation of the vinukonda meta granite eastern ghats india sm nd and lu hf systematics of whole rock samples provide information on the magma source and its evolution the granitic magma intruded into granitic gneisses of the vinjamuru domain krishna province andhra pradesh india which were already deformed and metamorphosed at epidote amphibolite facies peak conditions consistent sm nd and lu hf whole rock model ages suggest an isotopically homogeneous crustal source and a multi stage evolution of the granitic magma u pb analyses of prismatic zircon multi grain fractions define two discordias with identical upper intercepts at 1588 4 7 1 and 1589 7 5 7 ma interpreted to date magma emplacement rb sr ages of strongly annitic biotite xmg 0 07 0 20 and phengitic white mica 3 13 3 23 si pfu show a complex age pattern due to low temperatures during deformation the rb sr system only partly re equilibrated in biotite synkinematically grown phengitic white mica evidence a low grade metamorphic overprint at 501 474 ma the deformation was associated with w nw directed nappe stacking which caused the actual geological configuration of southern andhra pradesh with the arcuate nallamalai fold belt as most prominent feature regional considerations lead to the proposition that thrusting in the krishna province is the expression of intracratonic deformation in response to the amalgamation of the eastern ghats rayner province terrane with subcontinental india in early palaeozoic time

the queensborough mafic ultramafic complex a fragment of a meso proterozoic ophiolite grenville province canada abstract the queensborough mafic ultramafic complex occurs as a fault bounded block up to 10 km wide and having an area 220 km2 it lies in the grimsthorpe domain of the bancroft elzevir mazinaw sharbot lake terrane in the central metasedimentary belt of the grenville province it has been suggested without adequate supporting data that the complex may represent oceanic crust a fragment of an ophiolite or even a metavolcanic sequence made up of basaltic and komatiitic flows the geological and tectonic significance of the complex is assessed using field relationships petrography and geochemistry structurally the lowest part of the complex comprises a series of ultramafic rocks characterized by metre scale compositional layering represented by several different metamorphic assemblages of talc chlorite carbonate anthophyllite and actinolite tremolite these assemblages indicate that the original rocks were cumulate peridotites and pyroxenites the ultramafic rocks are overlain structurally by a series of mafic rocks predominantly massive to highly sheared gabbros the gabbros are penetrated by a series of mafic dykes and include a few small enclaves of pillowed mafic volcanics major and trace element chemistry shows that the mafic rocks represent a fractionally crystallized sequence of tholeiitic gabbros lavas and mafic dykes and that the ultramafic cumulates are co genetic the regional geological setting and the trace element signatures of the mafic rocks suggest that they were formed in a back arc basin comparison of the queensborough complex with proterozoic and phanerozoic igneous complexes suggests that it represents a partially preserved crustal section of a mesoproterozoic ophiolite in addition the rocks of the queensborough complex are petrographically and geochemically similar to those of the vavilov basin which occurs in the deepest part of the tyrrhenian sea by analogy with this neogene back arc basin we suggest that the central metasedimentary belt which contains the queensborough complex formed by attenuation of the continental crust the complex is preserved along a suture and is interpreted as having formed during the collision of a continental island arc with the continental margin of laurentia the characteristics and preservation of the queensborough mafic ultramafic complex are very similar to those of many phanerozoic ophiolites and suggest that mesoproterozoic thermotectonic processes are also similar to those presently operating

geology geochemistry and tectonic significance of mafic ultramafic rocks of mesoproterozoic phulad ophiolite suite of south delhi fold belt nw indian shield abstract a thick sequence of mafic ultramafic rocks occurs along a major shear zone phulad lineament running across the length of aravalli mountain range for about 300 kms it has been suggested that this sequence may represent a fragment of ophiolite or a rift related metavolcanic suite made up of basalts and fractionated ultramafics the geological and tectonic significance of the complex is assessed using field relationships petrography and geochemistry structurally the lowest part of the complex comprises a discontinuous band of plastically deformed harzburgite mantle component followed by layered cumulus gabbroic rocks crustal component a complex of non cumulus rocks comprising hornblende schists gabbros sheeted dykes and pillowed basalts structurally overlies layered gabbros huge bodies of diorite intrude volcanics geochemical classification suggests that all non cumulus mafic rocks are sub alkaline basalts except one variety of dykes which shows mildly alkaline character the sub alkaline rocks are tholeiite to calc alkaline with boninite affinity tectono magmatic variation diagrams and morb normalised patterns suggest a fore arc tectonic regime for the eruption of these rocks the mafic rocks of phulad ophiolite suite are zoned across the strike in terms of their distribution from west to east the hornblende schists and basalts are exposed at the westernmost margin followed by gabbros and dykes the alkaline dyke occurs at the easternmost part the rocks of phulad suite are juxtaposed with shallow water sediments in the east followed by platformal sediments and then continental slope sediments in the further east indicating gradual thickening of the crust from west to east and an eastward subduction the geochemical interpretation presented in this study together with discussion of lithological association is used to decipher the tectonic evolution of the mesoproterozoics of nw indian shield

an early proterozoic ophiolite the jormua mafic ultramafic complex northeastern finland abstract the 1 96 ga old jormua complex comprises a fault bounded unit 2 5 20 km in the centre of the kainuu schist belt at the eastern margin of the early proterozoic svecokarelides the complex was dismembered and metamorphosed to lower amphibolite facies during the svecokarelian orogeny in the intact complex the basal unit was formed by serpentinites the middle unit of mafic dykes and the uppermost extrusive unit of basaltic pillow lavas gabbroic rocks are present as intrusive bodies in the serpentinites and narrow interdyke screens within the dyke unit trondhjemitic rocks occur as minor segregations in the gabbroic rocks or as cross cutting narrow dykes in the mafic dyke unit the dyke unit is locally composed exclusively of subparallel metabasalt and metadolerite dykes in places interdyke metagabbro and serpentinite screens are abundant well preserved chilled margins are common and many dykes are split and intruded by younger dykes the chemical composition of the serpentinites indicates derivation from harzburgitic dunitic mantle peridotites the gabbroic rocks which represent obvious tholeiitic cumulates show extreme differentiation similar to gabbros of the high ti ophiolites and gabbros dredged from present midocean ridges in chemistry the trondhjemitic rocks are close to granitic rocks from mid ocean ridges and high ti ophiolites the similar chemical composition of the dykes and lavas of the complex indicates that they are cogenetic both being tholeiitic metabasalts with chondrite normalized ree patterns ranging from slightly lree depleted to slightly lree enriched ones compared with modern rocks the jormua metabasalts most closely resemble transitional midocean ridge basalts it is suggested that the jormua complex and related mafic ultramafic bodies in eastern finland originated as divergent margin ophiolites concomitantly with the break up of the archaean craton in the extensional stage of the svecokarelian orogeny which culminated in the opening of a major ocean after 1 97 ga the tectonic emplacement of these ophiolites into their present locations occurred in connection with thrusting during the compressional stage of the orogeny about 1 9 ga ago

tectonics and surface effects of the supercontinent columbia abstract assembly of the supercontinent columbia at about 1 85 1 90 ga coincided with several events that affected the entire earth the oldest worldwide network of orogenic belts formed at the same time although some granite granodiorite gg suites had formed earlier the gg suites became common in the 1 8 1 9 ga orogenic belts these suites succeeded the older tonalite trondhjemite granodiorite ttg suites which were not produced after 1 8 ga changes on the earth s surface at 1 8 1 9 ga include rapid increase in the concentration of oxygen in the atmosphere and oceans and probably the evolution of eukaryotes all of these surface changes occurred as columbia accreted and the assembly of columbia may have contributed to the drastic changes in the earth s surface environment as well as to the evolution of primitive life forms

a neoarchean dismembered ophiolite complex from southern india geochemical and geochronological constraints on its suprasubduction origin abstract ophiolites the remnants of ancient oceanic lithosphere have been described from collisional sutures of various ages with only few examples from archean terranes here we report the discovery of a neoarchean ophiolite suite from the southern margin of the dharwar craton in india tectonically intercalated within a neoproterozoic suture zone the metamorphosed and variably dismembered ophiolite suite exposed around devanur comprises altered ultramafic units websterite gabbros mafic dykes amphibolites trondhjemites and pegmatites associated with ferruginous metachert structural and petrographic studies indicate that the rocks represent a highly sheared and metamorphosed suite emplaced as a thrust sheet the major and trace element geochemistry of the mafic dykes indicate derivation from basaltic andesitic magmas with tholeiitic to calc alkaline characteristics the rocks display negative nb anomalies with enrichment of lile k rb ba th and depletion in hfse ti nb hf tb the tectonic discrimination of these rocks based on various geochemical plots suggests that they were generated in a suprasubduction zone setting we present new shrimp zircon u pb data for two trondhjemite samples from this complex which yield 238u 206pb ages of 2528 61 and 2545 56 ma the neoarchean age from the trondhjemites obtained in our study is closely comparable to similar ages obtained in recent studies from magmatic zircons in charnockites and orthogneisses in the area the suprasubduction zone assemblages and arc magmas suggest a neoarchean ocean closure along the southern margin of the dharwar craton

grain coarsening in contact metamorphic carbonates effects of second phase particles fluid flow and thermal perturbations under contact metamorphic conditions carbonate rocks in the direct vicinity of the adamello pluton reflect a temperature induced grain coarsening despite this large scale trend a considerable grain size scatter occurs on the outcrop scale indicating local influence of second order effects such as thermal perturbations fluid flow and second phase particles second phase particles whose sizes range from nano to the micron scale induce the most pronounced data scatter resulting in grain sizes too small by up to a factor of 10 compared with theoretical grain growth in a pure system such values are restricted to relatively impure samples consisting of up to 10 vol micron scale second phase particles or to samples containing a large number of nano scale particles the obtained data set suggests that the second phases induce a temperature controlled reduction on calcite grain growth the mean calcite grain size can therefore be expressed in the form d 1 4 c2 eq rt dp fp m where c2 is a constant q is an activation energy t the temperature and m the exponent of the ratio dp fp i e of the average size of the second phases divided by their volume fraction however more data are needed to obtain reliable values for c2 and q besides variations in the average grain size the presence of second phase particles generates crystal size distribution csd shapes characterized by lognormal distributions which differ from the gaussian type distributions of the pure samples in contrast fluid enhanced grain growth does not change the shape of the csds but due to enhanced transport properties the average grain sizes increase by a factor of 2 and the variance of the distribution increases stable d18o and d13c isotope ratios in fluid affected zones only deviate slightly from the host rock values suggesting low fluid rock ratios grain growth modelling indicates that the fluid induced grain size variations can develop within several ka as inferred from a combination of thermal and grain growth modelling dykes with widths of up to 1 m have only a restricted influence on grain size deviations smaller than a factor of 1 1 to summarize considerable grain size variations of up to one order of magnitude can locally result from second order effects such effects require special attention when comparing experimentally derived grain growth kinetics with field studies

geologically constraining india in columbia the age isotopic provenance and geochemistry of the protoliths of the ongole domain southern eastern ghats india abstract the ongole domain in the southern eastern ghats belt of india formed during the final stages of columbia amalgamation at ca 1600 ma yet very little is known about the protolith ages tectonic evolution or geographic affinity of the region we present new detrital and igneous u pb hf zircon data and in situ monazite data to further understand the tectonic evolution of this columbia forming orogen detrital zircon patterns from the metasedimentary rocks are dominated by major populations of palaeoproterozoic grains ca 2460 2320 2260 2200 2100 2080 2010 1980 1920 1850 and 1750 ma and minor archaean grains ca 2850 2740 2600 and 2550 ma combined u pb ages and lu hf zircon isotopic data suggest that the sedimentary protoliths were not sourced from the adjacent dharwar craton instead they were likely derived from east antarctica possibly the same source as parts of proterozoic australia magmatism occurred episodically between 1 64 and 1 57 ga in the ongole domain forming felsic orthopyroxene bearing granitoids isotopically the granitoids are evolved producing ehf values between 2 and 12 the magmatism is interpreted to have been derived from the reworking of archaean crust with only a minor juvenile input metamorphism between 1 68 and 1 60 ga resulted in the partial to complete resetting of detrital zircon grains as well as the growth of new metamorphic zircon at 1 67 and 1 63 ga in situ monazite geochronology indicates metamorphism occurred between 1 68 and 1 59 ga the ongole domain is interpreted to represent part of an exotic terrane which was transferred to proto india in the late palaeoproterozoic as part of a linear accretionary orogenic belt that may also have included south west baltica and south eastern laurentia given the isotopic geological and geochemical similarities the proposed exotic terrane is interpreted to be an extension of the napier complex antarctica and may also have been connected to proterozoic australia north australian craton and gawler craton

neoproterozoic massif type anorthosites and related magmatic suites from the eastern ghats belt india implications for slab window magmatism at the terminal stage of collisional orogeny abstract the jugsaipatna massif type anorthosite complex jac in the eastern ghats belt of india comprises anorthosite leuconorite norite in the central part and gabbros gabbronorites and porphyritic granites in the periphery in this study we report laser ablation icp ms zircon u pb data and ree geochemistry from the anorthosites gabbros and porphyritic granites from the jac the zircon data yield weighted mean 207 pb 206 pb ages of 918 33 ma mswd 2 2 for the anorthosite and 931 38 ma mswd 2 2 and 928 35 ma mswd 0 99 for two leuconorites the gabbros bordering the anorthosite body yield weighted mean 207 pb 206 pb ages of 984 10 ma mswd 0 41 and 969 12 ma mswd 1 5 zircons from the associated porphyritic granites define two concordant age groups an older group with 207 pb 206 pb weighted mean age of 996 11 ma mswd 2 1 and a younger group with an age of 964 29 ma mswd 2 1 zircons from another granite sample yield an age of 957 17 ma mswd 1 1 identical to the neoproterozoic age data obtained from the zircons in the anorthosites and gabbros of the jac the zircons from anorthosites show moderate ree contents prominent hree enrichment and a conspicuous positive eu anomaly the zircons from the granites show high ree contents prominent hree enrichment and a conspicuous positive eu anomaly suggesting a common melt source the ages reported in study correlate well with similar ages of 983 2 5 ma for the anorthosites from chilka lake complex and the ca 930 ma for bolangir anorthosite in the eastern ghats belt the early neoproterozoic ages reported from the magmatic suite in this study remarkably coincide with the timing of ultrahigh temperature metamorphism reported from various localities in the eastern ghats belt in recent studies the coeval nature of mantle derived magmatism and ultrahigh temperature metamorphism in a collisional orogen following a prolonged subduction accretion history along the eastern periphery of the indian lithosphere suggests asthenospheric upwelling probably through a slab window mechanism we correlate the geodynamic setting to post collisional slab break off at the terminal stages of the orogeny

crystallization processes in migmatites an important prerequisite for interpreting microstructures in plutonic and metamorphic rocks is understanding crystallization processes this study uses crystal size distribution csd and grain shape parameters to investigate mineral reaction controlled crystal growth and melt solidification in migmatites of the bayerische wald variscan orogeny germany partially molten rocks were studied because they allow the simultaneous investigation of crystallization of a melt and the growth of solid products of a metamorphic reaction csd and shape factors were obtained for three different minerals 1 cordierite in a diatexite where the melt remained in the system 2 k feldspar in a segregated k feldspar quartz leucosome and 3 plagioclase from a segregated plagioclase quartz leucosome cordierite crystals exhibit a nearly linear csd pattern and subeuhedral crystal shapes k feldspar grains display an approximately linear csd except for a marked increase at large crystal size classes k feldspar grains are characterized by strongly lobate grain boundaries the csd of plagioclase crystals is nearly linear with a slight decrease at small crystal size classes plagioclase shape factors also indicate lobate grain boundaries however with lower values than k feldspar cordierite csd and shape data are consistent with interface controlled crystallization the csd indicates that nucleation and growth is the rate limiting step during the cordierite producing reaction in contrast to porphyroblast growth in solid rocks where mass transfer is typically rate limiting csd combined with information on the duration of crystallization yields growth rates of 10 17 to 10 18 m s the plagioclase data are consistent with magmatic crystallization and are well described by the communicating neighbor theory the k feldspar data also indicate magmatic crystallization the overproduction of large grains of k feldspar reflects the interplay between nucleation and growth rates at the initial stage of crystallization because of different environments between solid and anatectic rocks the csd may help to decipher metamorphic from anatectic migmatites

monazite th pb age depth profiling the significant capabilities of the ion microprobe for thermochronometric investigations of geologic materials remain largely unexploited whereas sup 208 pb sup 232 th spot analysis allows approximately 10 mm scale imaging of pb loss profiles or overgrowths in sectioned monazite grains the spatial resolution offered by depth profiling into the surface region of natural crystals is more than two orders of magnitude higher the authors document here the ability of the high resolution ion microprobe to detect sup 208 pb sup 232 th age differences of 1 m y with better than 0 05 micro m depth resolution in the outer micron of tertiary monazites from the hanging wall of the himalayan main central thrust age gradients on this scale are inaccessible to ion microprobe spot analysis or conventional thermal ionization mass spectrometry interpretation of the near surface sup 208 pb distributions with available monazite pb diffusion data illustrates the potential of the approach for recovering continuous high temperature thermal history information not previously available

textural chemical and isotopic insights into the nature and behaviour of metamorphic monazite monazite is a mineral of choice for dating metamorphism in amphibolite and granulite grade metapelites however there exist a number of difficulties that complicate the interpretation of monazite geochronological data and prevent its application to many geological problems the two main obstacles addressed in this contribution are firstly the minor but significant e g 1 30 ma dispersal in duplicate isotope dilution thermal ionisation mass spectrometry id tims u pb age data commonly recorded from a single rock and secondly the difficulty of attaching monazite age data to pressure and temperature information through a multidisciplinary approach utilising tims and laser ablation multicollector inductively coupled plasma mass spectrometry la mc icpms isotope data quantitative and qualitative emp chemical analyses of monazite and textural studies we assess the significance of pb loss older components and continuous and episodic monazite growth in the generation of dispersed age data three samples from the canadian cordillera and one sample from the himalaya of pakistan are examined each sample exhibits an age dispersion of between 1 and 12 ma for single crystal and multi grain tims u pb monazite age determinations consideration of the closure temperature for pb diffusion in monazite and the metamorphic temperatures experienced by these samples suggests diffusive pb loss did not play a significant part in generating this age dispersal the la mc icpms study indicates that an older component 100 ma older than the tims ages contributed to the age dispersal in three of the four samples in all the samples however chemical analyses identified that the majority of monazites examined exhibited significant intra crystalline zoning in y content the la mc icpms analysis of one sample that was constrained to zones of distinct y content indicates that these zones are of distinct age we suggest that monazite grown before the appearance of garnet and during garnet breakdown is relatively rich in y whereas monazite grown after garnet is relatively poor in y a combination of these chemical data with textural observations suggests that once monazite had entered the mineral assemblage it grew or recrystallised episodically throughout the prograde and retrograde paths of the metamorphic event this behaviour contributes to and in one of the samples controls the observed age dispersal this recognition allows the generation of pressure temperature time points by combining textural and chemical information of monazite with in situ age determinations and pressure temperature information from garnet thus the episodic growth of compositionally distinct monazite throughout a metamorphic event provides the geochronologist with a very valuable chronological tool

sm nd and rb sr dating of amphibolite from the nellore khammam schist belt se india constraints on the collision of the eastern ghats terrane and dharwar bastar craton the nellore khammam schist belt se india is sandwiched in between the proterozoic eastern ghats terrane and the archaean dharwar bastar craton we report sm nd and rb sr mineral isochron ages of amphibolite from the schist belt the sm nd and rb sr ages are 824 43 ma and 481 16 ma respectively the sm nd age indicates the timing of peak metamorphism whereas the rb sr age indicates the pan african thermal overprint the peak metamorphism was related to collision of the eastern ghats terrane with the dharwar bastar craton which occurred during early neoproterozoic time

structural styles in mesozoic and cenozoic melanges in the western cordillera of north america the term melange is currently used to describe several different kinds of mudstone rich rocks that are broadly characterized by an obscure stratigraphy stratal disruption or a chaotic block in matrix fabric four types of melange which can be defined in outcrop on the basis of mesoscopic fabric and lithologic composition are particularly widespread and distinctive type i includes sequences of originally interbedded sandstone and mudstone that record incipient to thorough disruption and fragmentation of strata accomplished largely by layer parallel extension type ii consists of similarly deformed thin layers of green tuff radiolarian ribbon chert and minor sandstone originally interbedded with black mudstone disruption in both types i and ii which probably occurred while the sediments were incompletely consolidated has been ascribed to either imbricate faulting in accretionary wedges or gravitationally driven deformation type iii comprises inclusions of diverse shapes sizes and compositions enveloped in a locally scaly pelitic matrix the ultimate source of fragments is obscure because the majority were not derived by either the progressive disruption of interbedded sediments or in situ tectonic plucking and abrasion of adjacent rocks although some type iii melanges may have originated deep within accretionary prisms final emplacement as olistostromes muddy debris flow deposits or mud diapirs seems likely type iii melanges are mechanically analogous to scaly sheared serpentinites many probably have been tectonically remobilized or even intruded into shallow level fault zones type iv consists of lenticular inclusions bounded by an anastomosing network of subparallel faults their fabric records progressive slicing in brittle fault zones each of the four types of melange described here could in theory have formed in a variety of settings on or within an accretionary wedge at an active convergent margin none can yet be singled out as a uniquely diagnostic subduction melange

accretion of the archean slave province continental rift models have long been applied to the archean slave province of northwestern canada a reassessment of these models shows them to be incompatible with observed geological relations and suggests that contractional tectonic models may be more appropriate than extensional ones regions composed of different rock suites e g orthogneisses vs mafic volcanics are separated by high strain zones recording large displacements it is proposed that the high strain zones separate four distinct terranes that have been juxtaposed during collisional orogenesis from west to east these include the anton terrane interpreted as an archean microcontinent the sleepy dragon terrane possibly an exhumed more eastern part of the anton terrane the contwoyto terrane a westward verging fold and thrust belt containing tectonic slivers of greenstone volcanics and the hackett river volcanic terrane interpreted as an archean island arc the contwoyto and hackett river terranes represent a paired accretionary prism and island arc system that formed above an east dipping subduction zone these collided with the anton microcontinent producing a basement nappe expressed as the sleepy dragon terrane during the main accretion event within the slave province the whole tectonic assemblage was intruded by late kinematic to postkinematic granitoids

multi stage deformation in the nallamalai fold belt cuddapah basin south india implications for mesoproterozoic tectonism along southeastern margin of india abstract the common occurrence of small scale refolded folds deformed lineations and overprinting of foliation in the northern nallamalai fold belt nfb within the intracratonic cuddapah basin south india suggest three phases of deformation d 1 d 2 and d 3 during the late paleoproterozoic to neoproterozoic d 1 structures are represented by tight to isoclinal folds a slaty cleavage and local development of mylonites d 2 structures include ne trending tight to open folds f 2 with variable plunge indicating control of large domal structures and a steep crenulation cleavage developed in phyllites and other schistose rocks the above structures are overprinted by e w trending d 3 folds and cleavage which affect mesoproterozoic rocks of the srisailam formation uppermost nallamalai group in the e w trending vami konda range and metasediments of the palnad area thought to be equivalent of the neoproterozoic kurnool group taking into account the ages of intrusive vellaturu granite 1575 ma which is post d 2 deformation in the nfb and the chelima lamproite 1400 ma which intrudes the folded nallamalai rocks we suggest that the earlier episodes of multiple deformation in the nfb represent either late paleoproterozoic or early mesoproterozoic nw se compression in this part of the east gondwana the e w fold thrust structures affecting the srisailam quartzite and adjacent metasediments in the palnad area represent late mesoproterozoic and or neoproterozoic event the granite gneisses and schists of the nellore schist belt bordering the nfb are affected by both early and later events and thrust over rocks of the nallamalai group we propose that early deformation in the nfb lying southwest of the eastern ghats and roughly parallel to the present southeastern coast of india was unrelated to and occurred prior to the main deformation in the eastern ghats which is generally correlated with the 1000 ma old grenville event alternatively in view of the complex protracted evolution of the eastern ghats and published geochronological data supporting pre grenvillian tectonism particularly in the western charnockite zone of the eastern ghats d 1 and d 2 deformation in the nfb may be linked to this older 1600 1400 ma tectonic disturbance in the eastern ghats

age constraints on late paleozoic evolution of continental crust from electron microprobe dating of monazite in the peloritani mountains southern italy another example of resetting of monazite ages in high grade rocks in situ monazite microprobe dating has been performed for the first time on trondhjemite and amphibolite facies metasediments from the peloritani mountains in order to obtain information about the age of metamorphism and intrusive magmatism within this still poorly known sector of the hercynian belt all samples show single stage monazite growth of hercynian age one migmatite and one biotitic paragneiss yielded monazite ages of 311 4 and 298 6 ma respectively these ages fit with previous age determinations in similar rocks from southern calabria indicating a thermal metamorphic peak at about 300 ma at the same time as widespread granitoid magmatism the older of the two ages might represent a slightly earlier event possibly associated with the emplacement of an adjacent trondhjemite pluton previously dated by shrimp at 314 ma no evidence for pre hercynian events and only a little indication for some monazite crystallization starting from ca 360 ma were obtained from monazite dating of the metasediments suggesting either a single stage metamorphic evolution or a significant resetting of the monazite isotope system during the main hercynian event ca 300 ma rare monazite from a trondhjemite sample yields evidence for a late hercynian age of about 275 ma this age is interpreted as representing a post magmatic stage of metasomatic monazite crystallization which significantly postdates the emplacement of the original magmatic body

collapse of sediment filled crevasses associated with floods and mass flows in the proximal zone of the pernunnummi sandurdelta iii salpausselka sw finland abstract we aim to explain the complex geomorphology and sedimentology of the proximal zone of the pernunnummi sandurdelta which forms the most prominent portion of the iii salpausselka ice marginal complex of the former scandinavian ice sheet in sw finland we focus on the sedimentology of distinct diamicton covered glaciofluvial and glaciolacustrine landforms kames making accurate interpretations of both proximal diamictons and related sedimentary facies a crucial approach in the reconstruction of the depositional history of ice marginal zones we propose that the build up of the sandurdelta was followed by the formation of a glaciofluvially dominated supraglacial landsystem within a stagnant ice zone which fringed a glacial embayment between sub lobes of different deglacial dynamics the role of an integrated meltwater system including a major subglacial tunnel and water filled crevasses was crucial in constructing the controlled topography of this supraglacial landsystem the stagnant ice zone contributed to the incremental retreat of the ice margin within a shallow water environment where growing end moraines and proglacial deposits led to the development of supra aquatic conditions our results also evoke temporary dammed water bodies which were supported by ice marginal frozen bed conditions within the baltic sea icestream the collapse of crevasses with ice contact glaciofluvial and glaciolacustrine sediments during the continued deglaciation led to outbursts of dammed meltwater accompanied by extensive mass flows that formed the diamictons partially covering the ice proximal zone because ice marginal and esker related glaciofluvially dominated landform complexes with disintegration topography are relatively common along the main ice marginal deposits of the last great ice sheets the interpreted sedimentary record must be more common than previously reported thus extensive surficial and proximal diamictons of the ice marginal complexes do not always indicate either a re advance or a major oscillation of the ice front this study promotes the understanding of deglacial icestream dynamics with no known modern analogues and hence helps to improve both palaeogeographic reconstructions and the future compilation of even more accurate ice sheet models

seismically derived gondwana and proterozoic sediments east of cuddapah basin south indian shield and its possible geotectonic implications the study area situated east of the intracratonic proterozoic cuddapah basin that covers nellore schist belt ongole domain of the eastern ghats belt and adjacent east coast terrain forms a geodynamically important segment of the dharwar craton this region came into focus due to proposition that during paleoproterozoic to mesoproterozoic era it underwent persistent oceanic subduction continent continent collision and multi stage accretional growth in view of this we have reprocessed in detail the deep seismic sounding data over this region and integrated the obtained results with other geophysical studies to decipher the deep subsurface crustal structure underneath the derived shallow seismic section across this region showed the presence of a thin layer 250 m of gondwana sediments with a distinct velocity of 4 20 km s and a lateral dimension of about 40 km around vinjamuru region of the nellore schist belt another important finding has been the depiction of 5 3 and 5 5 km s velocity sediments comparable to upper and lower cuddapah sediments between nellore schist belt and the east coast indicating the possible presence of an additional proterozoic sedimentary terrain named here as east coast sedimentary basin overlying the concealed archean crystalline basement the presence of this sedimentary terrain which is separated from the cuddapah basin by an exhumed horst like feature situated below nellore schist belt would indicate proterozoic sedimentation in a much larger area during columbia and rodinia supercontinental assembly periods than hitherto known importantly no seismic trace of the eastern ghats belt has been found between nellore schist belt and the east coast it appears that during the gondwana period entire eastern segment of the east dharwar craton has been persistently rifting uplifting and eroding together with other gondwana grabens in india thereby leaving behind only a thin veneer of gondwana sediments at many places in view of the presence of number of gondwana occurrences all along the east coast it is felt that the rifting during the gondwana period may have been a prelude to india antarctica breakup with final separation between the two taking place much later during early cretaceous

geochemistry and significance of metavolcanic rocks from the bou azzer el graara ophiolite morocco abstract the bou azzer el graara ophiolite has been interpreted as oceanic or backarc crust overlain by a calc alkalic arc sequence or as a composite forearc and accretionary melange seventy nine new major and trace element whole rock analyses of the metavolcanic rocks from throughout the complex support the latter interpretation the results of this study indicate that the bou azzer metavolcanics comprise three petrographically and geochemically distinct groups 1 a a siliceous metavolcanic series ranging from basaltic andesites to rhyolites with calc alkalic characteristics is the principal component of the northern section of the complex it includes distinct quartz bearing rhyolite flows isolated massive pyroclastic layers are typically associated with this series these rocks are interpreted as forearc basin volcanics and pyroclastics produced during late proterozoic north dipping subduction 2 b low ti metabasalts which display island arc tholeiitic affinities occur as isolated flows or blocks in most of the ophiolite these rocks may represent fragments of the forearc basement below the siliceous series 3 c high ti metabasalts which range from enriched tholeiitic zr y 4 compositions are the principal volcanic block in the southern melange like terrane of the complex these have high ba and sr contents 100 200 ppm average ba 260 ppm average sr they are identical to basaltic compositions from tachdamt and bleida on the southern margins of the ophiolite which have previously been interpreted as a rifted continental margin sequence these rocks could represent seamount volcanic products which have been thrust into an accretionary prism along the north dipping subduction zone or rift margin basalts which have been assembled in a tectonic melange as a consequence of a forearc continent collision both the low ti and high ti metabasalt groups include samples previously identified as ocean ridge or back arc volcanics however there is no compelling evidence for the occurrence of these lava compositions in this region the distribution of rock types within the ophiolite is most consistent with a previously proposed model which views the complex as three allochthonous terranes which were juxtaposed during collision to form the bou azzer el graara complex

effects of matrix grain size on the kinetics of intergranular diffusion a linear relationship exists between the mean volume of garnet porphyroblasts and the squared inverse of mean matrix grain diameter for six samples of garnetiferous mica quartzite with identical thermal histories and similar mineralogy and modes this relationship accords with theoretical predictions of the dependence of intergranular diffusive fluxes on the volume fraction of grain edges that function as diffusional pathways during porphyroblast growth the impact of matrix grain size is large compared to a rock with a 1 mm matrix a rock with a 10 m matrix would experience rates of diffusion controlled porphyroblast growth that are 10 000 times faster and characteristic length scales for chemical equilibration that are 100 times larger precursor grain sizes may therefore exert a major influence on crystallization kinetics if matrix coarsening occurs during prograde reaction a decrease in the volume fraction of diffusional pathways will tend to counteract the exponential thermal increase in diffusive fluxes the impact of such matrix grain growth although difficult to assess without firm knowledge of coarsening rates in polymineralic aggregates might be significant for matrices finer than c 100 m at temperatures above c 500 600 c but is likely negligible for coarser grain sizes and lower temperatures

collision tectonics in the late precambrian eastern ghats mobile belt mesoscopic to satellite scale structural observations regional mapping of a section across the eastern ghats mobile belt egmb north of the godavari graben in eastern peninsular india by using landsat thematic mapper data enables recognition of a number of shear zones lineaments and structural domes and basins a conspicuous megashear occurs at the western boundary of the granulite facies rocks of the egmb adjacent to the archean granite greenstone craton the confinement of a suite of alkaline igneous rocks to this shear zone is a notable feature the strike extensions of this shear belt extend through to the elchuru alkaline complex prakasam district andhra pradesh and the syenite plutons of koraput district orissa the contrasting lithologies metamorphism and structural history on either side of the shear zone suggests that it might be a precambrian suture zone the mesoscopic structural features in the egmb include prominent foliation with moderate to steep dips folds faults shears s c fabrics pinch and swell structures and other linear fabric elements these observations favour the consideration of drastic crustal shortening and thickening and a complex deformational sequence the major rock units in this part of egmb comprise garnetiferous sillimanite gneisses quartzites and calc granulites forming the khondalitic suite of rocks and a wide variety of charnockitic rocks the contact of the two rock units is generally sheared and often migmatised the structural fabric suggests two major tectonic events an essentially horizontal tectonic regime resulting in thrust systems and associated structures subsequently followed by strike slip tectonics characterized by high shear strains features such as westward verging thrusts large scale recumbent folds major shear zones structural domes and basins indications of tectonic crustal shortening extensive calc alkali magmatism and widespread migmatization in the region are attributed to collisional processes during proterozoic times the spatial disposition of the egmb and its linkage with the distribution of similar rock units during the late precambrian time in a global tectonic scenario are discussed

stratigraphic and structural development of the precambrian cuddapah basin s e india abstract the present work recognizes a subdivision of the cuddapah basin into 3 separate main units of deposition some of the earlier correlations proved to be untenable for practical reasons king s widely used stratigraphy is retained where possible in a revised form we recognise the cuddapah group 2000 1490 1450 ma nallamalai group 1490 1450 1090 870 ma and the kurnool group 1090 870 500 ma on the basis of surface geology mapping and photo interpretation we discern 5 tectonic zones the boundaries of these zones coincide with deep crustal faults encountered in a deep seismic sounding survey three phases of tectonic activity are 1 1 crustal movements resulting in broad folds affecting the cuddaph rocks exposed in the western area followed by 2 2 folding of the nallamalai rocks east of the rudravaram line and 3 3 later folding and metamorphism in the eastern tectonic zones structural sections allow an interpretation of an eastward continuation of the cuddapah basin rocks into the highly metamorphosed series underlying the eastern ghats complex a colour geological map and sections are included

evidence for archean ocean opening and closing in the southern slave province archean mafic volcanic and plutonic rocks of the cameron and beaulieu river greenstone belts outcrop in the southern slave structural province of the canadian shield lower portions of the greenstone belts have ophiolitic characteristics but young felsic units may represent superimposed arc magmatism the greenstone belts are separated from older circa 2 7 3 2 ga quartzo feldspathic gneisses of the sleepy dragon complex by a regional decollement zone patterson lake structural complex consisting of deformed mafic volcanic rocks thin slivers of gneiss ultramafic rocks and banded iron formation mixed in a highly deformed phyllonitic and phacoidally cleaved metapelitic matrix high synmetamorphic strains are indicated by abundant intrafolial folds transposed layering rare sheath folds and by the parallelism of elongated primary features and metamorphic mineral lineations the patterson lake structural complex is similar to melanges of phanerozoic mountain belts adjacent quartzo feldspathic gneisses are strongly mylonitized the melange and deeper mylonites formed when the greenstone belts were emplaced as thrust sheets wildflysch shed from tips of emergent thrusts was in part incorporated into the basal melange as the allochthon overrode its own detritus continued compression involved the basement in the deformation uplifting the sleepy dragon complex in a structural culmination mantled by the imbricated greenstone stack late granodioritic to granitic intrusions formed regional thermal aureoles which overprint earlier metamorphic fabrics

evaluation of the tectonic setting of precambrian dalma volcanic belt eastern india using major and trace element characters geochemical characterisation of dalma basalt the upper member of a volcanic belt occupying the spine of a precambrian basin has been used to elucidate the eruptive environment dalma basalt in general is high magnesian in character as well as depleted in incompatible elements the major and trace element distributions are strongly indicative of ocean ridge magmatism with a feeble impression of arc volcanism such a transitional tectonic environment corroborated by the overall geologic setting of the volcanic belt is strongly indicative of a marginal basin situation during proterozoic time in this part of the indian shield

three dimensional quantitative textural analysis of metamorphic rocks using high resolution computed x ray tomography part ii application to natural samples three dimensional quantitative textural analysis coupled with numerical modelling has been used to assess the dominant mechanisms governing crystallization of garnet porphyroblasts in rocks from diverse regional metamorphic environments in every case spatial dispositions crystal size distributions and compositional zoning patterns of porphyroblasts indicate the dominance of diffusion controlled nucleation and growth mechanisms nine samples from three geological areas were studied a suite of semi pelitic rocks from the picuris mountains new mexico usa a suite of mafic samples from the llano uplift texas usa and a kyanite schist from mica dam british columbia canada the semi pelitic suite exhibits post deformational garnet growth whereas garnet in the mafic suite and in the kyanite schist grew synkinematically in rocks displaying weak and strong penetrative fabrics respectively for each sample the centres and radii of thousands of garnet crystals were located and measured in three dimensions using images produced by high resolution computed x ray tomography statistical measures of the degree of ordering and clustering of nucleation sites and estimates of crystal isolation for each porphyroblast were then computed from the measured spatial dispositions these measures can be reproduced in simple numerical models only by diffusion controlled nucleation and growth mechanisms normalized radius rate relations computed from compositional zoning patterns in the garnets require thermally accelerated diffusion controlled growth providing independent confirmation of the conclusions based on textural analysis the unexpected similarity of results from all samples indicates that diffusion controlled nucleation and growth mechanisms may govern porphyroblast crystallization in many metamorphic regimes

an empirical garnet yag xenotime thermometer a pronounced negative correlation between the yttrium concentration in garnet y grt and temperature has been observed in xenotime ypo4 bearing metapelites from central new england usa the y grt decreases roughly two orders of magnitude 5500 to less than 100 ppm y over a 150 c interval a regression of ln y grt against estimated reciprocal temperature yields the following relationship

structural trends in the cuddapah basin from deep seismic soundings dss and their tectonic implications abstract structural trends in the upper proterozoic cuddapah basin at the basement level and at the moho level have been discussed based on deep seismic sounding dss studies results of dss studies along the alampur koniki profile profile 2 of fig 1 crossing the northern part of the cuddapah basin have been discussed in detail these results combined with the results of the kavali paranpalle section of the kavali udipi dss profile profile 1 of fig 1 kaila et al 1979 crossing the basin on its southern flank along with geological data and earthquake epicentral locations are used to explain the structural trends of the area it has been shown that the cuddapah basin was first created in its western part by downfaulting of the crustal block between faults 7 and 14 towards the west and fault 6 in the east fig 1 subsequently the eastern part was downfaulted against fault 6 before the commencement of upper cuddapah sedimentation further downfaulting towards the north along fault 5 created the srisailam block minor scale downfaulting between faults 7 and 13 in the west and fault 6 in the east and fault 8 in the north gave rise to the kurnool sub basin at a later stage similar downfaulting east of fault 9 and north of fault 5 gave rise to the palnad sub basin both these sub basins received kurnool sediments after the close of kurnool sedimentation the blocks between faults 4 and 6 along profile ii and between 11 and 6 along profile i were uplifted at the basement level thus giving rise to the nallamalai hills and iswarakuppam dome fig 1 the low angle thrust fault 3 on the eastern margin of the cuddapah basin might be a post cuddapah phenomenon the low angle thrust fault 2 probably occurred in the post dharwar period faults 1 17 and 10 near the east coast of india seem to be comparatively younger probably of mesozoic time along which the coastal block is downfaulted giving rise to the sedimentary basins

proterozoic evolution of eastern dharwar and bastar cratons india an overview of the intracratonic basins craton margins and mobile belts abstract based on critical analysis of stratigraphic succesions in the proterozoic intracratonic basins geochronological data and possible connections or disconnections with adjacent fold belts and craton margin mobile belts an overview of the proterozoic evolution of the eastern dharwar and bastar cratons in india is presented the chattisgarh and cuddapah basins in these cratons with archean nuclei host sedimentary successions ranging in age from the paleoproterozoic to the ediacaran period and representing several silicilastic and carbonate cycles the proterozoic successions of the pranhita godavari valley pgv basin along the dharwar bastar join record multiple rifting since late neoarchean suturing of the two cratons we postulate a first order stratigraphic correlation across the cratons and their join and discuss how major sea level fluctuations coupled with regional tectonic events shaped the development of unconformity bound successions adjoining fold belts and the mobile belts with supposed global tectonic connections the oldest paleoproterozoic intracratonic sedimentation is preserved in the cuddapah and pgv basins while sedimentation in the bastar craton is largely mesoproterozoic the fluviodeltaic to shallow marine neoproterozoic sedimentation followed emplacement of end mesoproterozoic kimberlites and lamproites indicating thermal rejuvenation of the indian continental crust the allochthonus nallamalai fold belt in eastern cuddapah basin and the early mesoproterozoic fold belt in pgv basin point to craton margin crustal convergences which punctuated the paleoproterozoic sedimentation widespread development of carbonate bearing sequences in the mesoproterozoic to neoproterozoic successions point to peneplanation of the cratons and stable platformal sedimentation the neoproterozoic sedimentation was terminated following the docking of the eastern ghats belt to the eastern margin of the dharwar bastar ensemble and possible final tectonic hiccups in the central indian tectonic zone bordering the bastar craton the available data and their analysis rule out any foreland basin type connection between the eastern ghats belt and cratonic sedimentation in bastar and eastern dharwar

sedimentary and structural evolution of a relict subglacial to subaerial drainage system and its hydrogeological implications an example from anglesey north wales uk subglacial drainage systems exert a major control on basal sliding rates and glacier dynamics however comparatively few studies have examined the sedimentary record of subglacial drainage this is due to the paucity of modern analogues the limited recognition and preservation of upper flow regime deposits within the geological record and the difficulty of distinguishing subglacial meltwater deposits from other meltwater sediments e g glacier outburst flood deposits within this study the sedimentological and structural evolution of a subglacial to subaerial ice marginal proglacial drainage system is examined particular emphasis is placed upon the genetic development and preservation of upper flow regime bedforms and specifically recognising them within a subglacial meltwater context facies are attributed to subglacial meltwater activity and record sedimentation within a confined but progressively enlargening subglacial channel system produced under dune to upper flow regime conditions bedforms include rare large scale sinusoidal bedding with syn depositional deformation produced by current induced traction and shearing within the channel margins subglacial sedimentation culminated with the abrupt change to a more ephemeral drainage regime indicating channel abandonment or a seasonal drainage regime retreat of the ice margin led to the establishment of subaerial drainage with phases of sheet flow punctuated by channel incision and anastomosing channel development under diurnal ablation related seasonal discharge the presence of extensive hydrofracture networks demonstrate that proglacial groundwater levels fluctuated markedly and this may have influenced later overriding of the site by an ice stream

rbsr geochronological studies on the granites of vinukonda and kanigiri prakasam district andhra pradesh india

magnitude and frequency in proglacial rivers a geomorphological and sedimentological perspective abstract proglacial fluvial sedimentary systems receive water from a variety of sources and have variable discharges with a range of magnitudes and frequencies little attention has been paid to how these various magnitude and frequency regimes interact to produce a distinctive sedimentary record in modern and ancient proglacial environments this paper reviews the concept of magnitude and frequency in relation to proglacial fluvial systems from a geomorphic and sedimentary perspective rather than a hydrological or statistical perspective the nature of the meltwater inputs can be characterised as low magnitude high frequency primarily controlled by ablation inputs from the source glacier or high magnitude low frequency primarily controlled by exceptional inputs the most important high magnitude low frequency inputs are catastrophic outburst floods often referred to by the term jokulhlaup icelandic for glacier burst glacier surges are an additional form of cyclical variation impacting the proglacial environment which briefly alter the volumes and patterns of meltwater input the sedimentary consequences of low magnitude high frequency discharges are related to frequent variations in stage the greater directional variability that sediment will record and the increased significance of channel confluence sedimentation in contrast the most significant characteristics of high magnitude low frequency flooding include the presence of large flood bars and mid channel jokulhlaup bars hyperconcentrated flows large gravel dunes and the formation of ice block kettle hole structures and rip up clasts glacier surges result in a redistribution of low magnitude high frequency processes and products across the glacier margin and small floods may occur at the surge termination criteria for distinguishing magnitude and frequency regimes in the proglacial environment are identified based on these major characteristics studies of quaternary proglacial fluvial sediments are used to determine how the interaction of the various magnitude and frequency regimes might produce a distinctive sedimentary record consideration of sandur architecture and stratigraphy shows that the main controls on the sedimentary record of proglacial regions are the discharge magnitude and frequency regime sediment supply the pattern of glacier advance or retreat and proglacial topography a model of sandur development is suggested which shows how discharge magnitude and frequency in combination with sandur incision and aggradation controlled by glacier advance and retreat can control sandur stratigraphy

equilibration and disequilibration between monazite and garnet indication from phase composition and quantitative texture analysis the integration of information which can be gained from accessory i e age t and rock forming minerals i e temperature t and pressure p requires a more profound understanding of the equilibration kinetics during metamorphic processes this paper presents an approach comparing conventional p t estimate from equilibrated assemblages of rock forming minerals with temperature data derived from yttrium garnet monazite ygm and yttrium garnet xenotime ygx geothermometry such a comparison provides an initial indication on differences between equilibration of major and trace elements regarding this purpose two migmatites two polycyclic and one monocyclic gneiss from the central alps switzerland northern italy were investigated while the polycyclic samples exhibit trace element equilibration between monazite and garnet grains assigned to the same metamorphic event there are relics of monazite and garnet obviously surviving independent of their textural position these observations suggest that surface processes dominate transport processes during equilibration of those samples the monocyclic gneiss on the contrary displays rare isolated monazite with equilibration of all elements despite comparably large transport distances with a nearly linear crystal size distribution of the garnet grain population growth kinetics related to the major elements were likely surface controlled in this sample in contrast to these completely equilibrated examples the migmatites indicate disequilibrium between garnet and monazite with a change in ree patterns on garnet transects the cause for this disequilibrium may be related to a potential disequilibrium initiated by a changing bulk chemistry during melt segregation while migmatite environments are expected to support high transport rates i e high temperatures and melt presence the evolution of equilibration in migmatites is additionaly related to change in chemistry as a key finding surface controlled equilibration kinetics seem to dominate transport controlled processes in the investigated samples this may be decisive information towards the understanding of age data derived from monazite

characterization of the sukinda and nausahi ultramafic complexes orissa india by platinum group element geochemistry abstract samples of 20 chromitite 14 ultramafic and mafic rock and 9 laterite and soil samples from the precambrian sukinda and nausahi ultramafic complexes orissa india were analyzed for platinum group elements pge the maximum concentrations are palladium 13 parts per billion ppb platinum 120 ppb rhodium 21 ppb iridium 210 ppb and ruthenium 630 ppb comparison of chondrite normalized ratios of pge for the chromitite samples of lower proterozoic to archean age with similar data from paleozoic and mesozoic ophiolite complexes strongly implies that these complexes represent precambrian analogs of ophiolite complexes this finding is consistent with the geology and petrology of the indian complexes and suggests that plate tectonic and ocean basin developement models probably apply to some parts of precambrian shield areas

accreted oceanic materials in japan abstract the phanerozoic circum pacific orogenic belts contain numerous ocean derived materials accreted through plate converging processes japanese islands in particular display various kinds of oceanic materials of different origins including fragments of seamounts oceanic reef limestone morb like rocks and oceanic mantle and pelagic sediments the compilation of these rocks in many subduction complexes of late permian to the present led to following conclusions accretion processes work effectively only for materials primarily composing the upper portion of subducting oceanic crust i e layer 1 and layer 2 many fragments of seamount with alkali basalt 600 hot spot seamount 26 oceanic reef limestone 291 morb like basalt 200 and numerous cherts more than 1000 are recognized as ancient oceanic materials accreted to the japanese islands however gabbros and mantle materials of layer 3 and lower parts of the oceanic lithosphere scarcely occur in subduction accretion complexes except for a few examples of back arc basin or fore arc origin accretion occurs episodically in southwest japan oceanic materials were accreted intermittently in 1 a end permian 2 b middle late jurassic 3 c late cretaceous times 4 d at ca 50 ma and 5 e in miocene times while in northeast japan and hokkaido this occurred in b middle late jurassic c late cretaceous and f early cretaceous times in contrast to the general belief on accretion of younger oceanic plates the majority of japanese subduction accretion complexes were formed during the subduction of plates up to 160 ma old the accretionary events in end permian and middle late jurassic times coincide with northward collision of ancient island arcs oceanic rises or seamount chains of hot spot origin with the asian continent accretion relevant to subduction of older plates may be controlled by the collision subduction process of these topographic reliefs on an oceanic plate in addition the chronological coincidence with the continent collision amalgamation between the sino korean and siberian platforms and between the sino korean and yangtze blocks also implies collision induced voluminous supply of elastics from back arc regions and its contribution to the formation of huge accretionary complexes accreted fragments of ancient seamounts are much smaller than the average size of modern seamounts this implies that most parts of a colliding seamount are not accreted but subducted together with the underlying oceanic crust to much deeper levels with respect to the metamorphic grades for japanese subduction complexes oceanic materials have less than 1 vol in the zeolite facies 15 20 in the prehnite pumpellyite metagraywacke facies and ca 30 in the greenschist glaucophane schist facies and albite epidote amphibolite fades this relationship indicates that the major process for landward accretion of oceanic materials is not off scraping or sedimentary mixing at the trench but underplating subcretion at much deeper levels of a subduction zone

petrology and geochemistry of paleoproterozoic a type granite at kanigiri in the nellore khammam schist belt andhra pradesh india abstract the kanigiri pluton 6 2 2 1 km 2 995 ma occurs close to a major fault and is intrusive into the meta volcanics in the western margin of the nellore khammam schist belt in andhra pradesh india the dominantly metaluminous pluton displays geochemical characteristics of a type granites including high abundances of nb zr y ta and ree except eu and low concentrations of sc ba sr and eu displaying distinct variation trends with increasing silica content it has less fractionated ree patterns and large negative eu anomalies the cluster analyses of major and trace elements indicate three main associations a a primary magmatic association characterized by the presence of la ce fe ti mn k ba and sr b ore related association consisting of zr hf nb y and c a volatile association with rb ta li and cs systematic variation in sr rb sr rb ba ratios along with and low k rb ratios reflects fractionation of feldspars variations in sc and ta result from fractionation of ferromagnesian silicates removal of zircon and allanite affected the hf ta and ce yb ratios limited variation in source sensitive y nb and yb ta ratios is consistent with the results of melting experiments and indicates that metaluminous granitoids of this pluton were probably derived through melting of lower crustal sources geochemical field and petrological data for the kanigiri granite suggest that the granite is a partial melting product of mafic rocks like gabbros that are also present in the study region

sedimentation in an interlobate outwash stream abstract sediments deposited in and adjacent to the wabash valley during the late wisconsinan were studied in outcrop and by drilling in order to determine their stratigraphic relationships and their mode of deposition the wabash valley at that time was the line of contact between ice lobes that each advanced several times into indiana from the lake michigan and lake huron erie basins both lobes supplied meltwater and sediment directly to the wabash when their ice was adjacent to the valley indirectly through preexisting tributaries that entered the valley downstream from ice margins and also by tributaries that originated as ice walled channels in downwasting ice adjacent to the river sedimentation occurred in a variety of depositional settings sediments that accumulated in braided channels in valleys and on outwash fans consist of deposits of longitudinal bars and 2 dimensional dunes interspersed laterally and vertically with braid channel fills sediments accumulated rapidly and show evidence of large variations in discharge sediments that accumulated in lakes consist of muddy bottomset beds sandy foresets and sand and gravel rich topset beds bottomsets and foresets show evidence of underflows that while acting continously varied greatly in intensity sediments filling intraice channels include lacustrine sands and muds debris flow deposits outwash sand and gravel and ablation debris in complex lateral and vertical juxtaposition repeated ice advances into the basin of the wabash valley along with the occurrence of the multiple lateral sediment sources that they produced resulted in a complex lateral and vertical arrangement of lithofacies in and adjacent to the valley

structural development of an archean orogen western point lake northwest territories the point lake orogen in the central archean slave province of northwestern canada preserves more than 10 km of structural relief through an eroded antiformal thrust stack and deeper anastomosing midcrustal mylonites fault restoration along a 25 km long transect requires a minimum of 69 km slip and 53 km horizontal shortening in the western part of the orogen the basal decollement places mafic plutonic volcanic rocks over an ancient tonalitic gneiss complex ten kilometers to the east in the keskarrah bay area slices of gneiss unroofed on brittle thrusts shed molasse into several submerged basins conglomerates and associated thinly bedded sedimentary rocks are interpreted as channel levee and overbank facies of this thrust related sedimentary fan system the synorogenic erosion surface at the base of the conglomerate truncates premetamorphic or early metamorphic thrust faults formed during foreland propagation while other thrusts related to hinterland progressing imbrication displace this unconformity tightening of synorogenic depositional troughs resulted in the conglomerates present localization in synclines to the west of associated thrust faults and steepening of structural dips eastern parts of the orogen consist of isoclinally folded graywackes composed largely of mutti and ricci lucchi turbidite facies b c and d interpreted as submarine fan deposits eroded from a distant volcanic arc thrust faults in the metasedimentary terrane include highly disrupted slate horizons with meter scale duplex structures and recrystallized calcmylonites exhibiting sheath folds and boudin trains with very large interboudin distances the sequence of fabric development and the overall geometry of this metasedimentary terrane strongly resembles younger forearc accretionary prisms conditions of deformation along the thrusts parallel the regional metamorphic zonation amphibolite facies in the basal decollement through greenschist facies shear zones to cataclastic crush zones in the region of emergent thrusts in keskarrah bay depth differences can account for only half of the metamorphic gradient thermal profiles which increased downwards in obducted greenstone belts and synthrusting plutonism explains other high metamorphic gradients a tectonic model involving the collision of an accretionary prism with a continental margin best explains the structural and sedimentological evolution of the orogen