the geochemical fingerprints of different tectonomagmatic environments using hygromagmatophile element abundances of tholeiitic basalts and basaltic andesites abstract using hygromagmatophile element abundances normalized to primordial mantle composition it is possible to discriminate between the tectonic settings of tholeiitic basalts and basaltic andesites 47 0 53 5 sio2 based on field and chemical evidence data from provinces or grouped localities have been compiled in order to establish chemical reference data sets for ocean island continental ocean ridge and floor subduction zone and back arc environments the disturbing effects of crustal contamination and hydrothermal alteration on the hygromagmatophile h element patterns are of minor significance in the recognition of plate tectonic settings on the basis of the data sets magmas from a number of ambiguous tectonic settings have been recognized using their h element patterns attention is directed to problems concerning rifting and drifting related basalts in an apparent continental setting and to the origin of ophiolite complexes using available data the pillow lavas of the troodos ophiolite complex cyprus are recognized as basalts erupted in a subduction zone environment

upb la mc icp ms dating of rutile new reference materials and applications to sedimentary provenance in response to the general lack of sufficiently abundant and high quality rutile usingle bondpb reference materials for in situ geochronology we have characterised two new potential rutile 1 8 ga reference materials sugluk 4 and pca s207 from granulite facies belts of the canadian shield namely the northern cape smith belt of quebec and the snowbird tectonic zone sasatchewan characterisation includes id tims and la icp ms usingle bondpb dating imaging and trace element analysis we compare these materials with existing rutiles used already r19 and r10 luvizotto et al 2009 and zack et al 2011 and show that the measured usingle bondpb compositions i e including any common pb of our rutiles are considerably more homogeneous this makes possible a usingle bondpb normalisation procedure not reliant upon a common pb correction that results in a significant decrease in the uncertainty contribution from the common pb correction and better reproducibility of reference materials and unknowns for provenance analysis and other applications the reproducibility is 2 4 2rsd for 206pb 238u and 207pb 206pb only slightly greater than long term data for zircon reference materials we show in a rutile provenance study from young orogens bhutan himalaya and canadian cordillera that the sensitivity of our analytical set up allows dating of 90 of rutiles in a sediment using a 50 m laser ablation spotsize within samples containing rutile as young as 10 20 ma and obviates the requirement for u concentration pre screening thus reducing or eliminating rutile selection bias unsuccessful analyses are due to poor quality rutiles with predominant common pb 207pb signal below detection or u content below 1 2 ppm we have used the 207pb method using the tera wasserburg diagram to correct for substantial common pb in very young and or very low u rutiles rather than developing an on line correction since rutile ages reflect mainly the time of cooling rutile is a sensitive recorder of metamorphic thermochronological information and therefore is an excellent complement to detrital zircon u207pb data the contrast between zircon and rutile signatures in himalayan samples with rutile as young as 10 ma is shown to be very dramatic most zircons from the same sample are 480 ma with only a few grains or metamorphic rims reflecting miocene metamorphism as such rutile provides complementary information about the thermal events within the source regions of the grains rutile usingle bondpb dating is an underexploited provenance method with wide applicability to sedimentary provenance studies

striding athabasca mylonite zone complex archean deep crustal deformation in the east athabasca mylonite triangle northern saskatchewan the geophysically defined snowbird tectonic zone is manifested in northernmost saskatchewan as a deep crustal multistage mylonitic structure the east athabasca mylonite triangle the triangle located at the northeastern apex of a stiff crustal scale lozenge is composed of mid archean annealed mylonites and late archean ribbon mylonites formed during two granulite facies events 850 1000 c 1 0 gpa the flow pattern in the mylonites is geometrically and kinematically complex and corresponds to that expected adjacent to the apex of a stiff elliptical volume subjected to subhorizontal regional extension parallel to its principal axis the late archean mylonites are divided into an upper structural deck entirely occupied by a dip slip shear zone and an underlying lower deck the latter is divided into two upright conjugate strike slip shear zones separated by a low strain septum which deformed by progressive coaxial flow the flow pattern in the mid archean mylonites is compatible with that of

in situ la icp ms u pb dating of metavolcanics of norrbotten sweden records of extended geological histories in complex titanite grains in situ u pb dating of a variety of mineral phases is an important goal in petrology this study reports data chiefly from titanite but also from rutile and apatite obtained using the laser ablation la icp ms methodology on polished thick sections in order to retain as much petrologic information as possible and allowing trace element analyses from adjacent areas to the u pb analyses the samples analysed come from svecofennian intermediate to acid volcanic rocks of the porphyry group within the major iron ore province of norrbotten northern sweden where titanite is a common phase associated with the mineralisation using a laser beam of 30 45 m in diameter accurate and relatively precise 2 3 2 on 206pb 238u ratios data can be obtained on titanite similar precision on pb u ratios can be obtained in rutile but accuracy cannot be assessed because of the lack of well characterised standards however the data are a priori accurate as they are geologically reasonable apatite shows reverse discordance which could be explained by a number of scenarios however the 207pb 206pb ages are fairly precise 1 3 2 and the calculated age is geologically reasonable suggesting that the data may be accurate the titanite grains studied show complex internal structures rare earth element ree analysis by la icp ms demonstrates that the core and rim zones are distinct and supports a model for two stage evolution in two out of three samples with the rims in these samples being enriched in total ree and particularly in lree in one sample and displaying a positive y anomaly in contrast to the cores in situ u pb analysis reveals that core zones from all samples retain distinct older ages of c 2050 ma whereas the reworked rims have either distinct younger or strongly reset ages the older cores are interpreted as representing the first alteration metamorphism of the volcanic pile titanite rims record a u pb age of 1870 24 ma 2 at luossavaara and 1826 15 ma 2 at the smaller fe oxide apatite body at gruvberget in both cases the ages are consistent with previous age determinations from the fe oxide apatite deposits a sample from malmberget records a slightly different scenario where u pb ages spread along concordia from c 1920 to 1708 ma we attribute this pattern to metamorphic reworking of primary metasomatic titanite rutile ages from inclusions within the core of gruvberget titanite essentially mimic the ages obtained from the surrounding titanite and suggest that they have undergone the same history apatite inclusions within titanite at malmberget record a younger age of 1584 12 ma 2 which we attribute to later events known to have affected the area these data indicate that the porphyry group volcanics must be at least c 2050 ma in age in contrast to previous age determinations from zircon which suggested a deposition age of c 1960 to 1880 ma the large grain size of the titanites studied coupled with the ability for the first time to obtain in situ u pb ages from small areas has allowed probing of this hitherto unrecognised event smaller zircon grain size coupled with extremely saline hot pressurised high p t metasomatic fluids relating to regional magnetite apatite mineralisation in the period 1900 to 1800 ma may have resulted in resetting of zircon u pb systematics 2007 elsevier b v all rights reserved

the brahmaputra tale of tectonics and erosion early miocene river capture in the eastern himalaya the himalayan orogen provides a type example on which a number of models of the causes and consequences of crustal deformation are based and it has been suggested that it is the site of a variety of feedbacks between tectonics and erosion within the broader orogen fluvial drainages partly reflect surface uplift different climatic zones and a response to crustal deformation in the eastern himalaya the unusual drainage configuration of the yarlung tsangpo brahmaputra river has been interpreted either as antecedent drainage distorted by the india asia collision and as such applied as a passive strain marker of lateral extrusion latest neogene tectonically induced river capture or glacial damming induced river diversion events here we apply a multi technique approach to the neogene paleo brahmaputra deposits of the surma basin bengal basin bangladesh to test the long debated occurrence and timing of river capture of the yarlung tsangpo by the brahmaputra river we provide u pb detrital zircon and rutile isotopic sr nd and hf and petrographic evidence consistent with river capture of the yarlung tsangpo by the brahmaputra river in the early miocene we document influx of cretaceous paleogene zircons in early miocene sediments of the paleo brahmaputra river that we interpret as first influx of material from the asian plate transhimalayan arc indicative of yarlung tsangpo contribution prior to capture the predominantly precambrian paleozoic zircons indicate that only the indian plate was drained contemporaneous with transhimalayan influx reflecting the river capture we record arrival of detrital material affected by cenozoic metamorphism as indicated by rutiles and zircons with cenozoic u pb ages and an increase in metamorphic grade of detritus as recorded by petrography we interpret this as due to a progressively increasing contribution from the erosion of the metamorphosed core of the orogen whole rock sr nd isotopic data from the same samples provide further support to this interpretation river capture may have been caused by a change in relative base level due to uplift of the tibetan plateau assuming such river capture occurred via the siang river in the early miocene we refute the tectonic aneurysm model of tectonic erosion coupling between river capture and rapid exhumation of the eastern syntaxis since a time interval of at least 10 ma between these two events is now demonstrated this work is also the first to highlight u pb dating on detrital rutile as a powerful approach in provenance studies in the himalaya in combination with zircon u pb chronology

reply to comment by wang et al on paleoproterozoic arc continent collision in the north china craton evidence from the zanhuang complex by li et al 2016 precambrian research 286 281 305

derivation of isotope ratios errors and error correlations for u pb geochronology using 205pb 235u 233u spiked isotope dilution thermal ionization mass spectrometric data a comprehensive treatment of the derivation of u pb isotope ratios and their corresponding uncertainties from isotope dilution thermal ionization mass spectrometric measurements is presented standard parametric statistical methods of error propagation are utilized to convolve uncertainties associated with instrumental mass fractionation tracer subtraction blank pb and u subtraction and initial common pb correction derivations include errors and error correlations for total sample u pb and pb isotope ratios including radiogenic and initial common pb for two and three dimensional isochron calculations radiogenic u pb and pb isotope ratios for concordia and radiogenic model age calculations and the propagation of model age errors from radiogenic isotope ratios

stabilization and reactivation of cratonic lithosphere from the lower crustal record in the western canadian shield new u pb geochronology for an extensive exposure of high pressure granulites in the east lake athabasca region of the western canadian shield is consistent with a history characterized by 2 55 ga stabilization of cratonic lithosphere 650 million years of lower crustal residence and cratonic stability and 1 9 ga reactivation of the craton during lithospheric attenuation and asthenospheric upwelling high precision single grain and fragment zircon data define distinctive discordia arrays between 2 55 and 1 9 ga u pb ion microprobe spot analyses yield a similar range of u pb dates with no obvious correlation between date and cathodoluminescence zonation we attribute the complex u pb zircon systematics to growth of the primary populations during a 2 55 ga high pressure granulite facies event 1 3 gpa 850 c recorded by the dominant mineral assemblage of the mafic granulite gneisses with subsequent zircon recrystallization and minor secondary zircon growth during a second high pressure granulite facies event 1 0 gpa 800 c at 1 9 ga the occurrence of two discrete granulite facies metamorphic events in the lower crust separated by an interval of 650 million years that included isobaric cooling for at least some of this time suggests that the rocks resided at lower crustal depths until 1 9 ga we infer that this phase of lower crustal residence and little tectonic activity is coincident with an extended period of cratonic stability detailed structural and thermochronological datasets indicate that multistage unroofing of the lower crustal rocks occurred in the following 200 million years extended lower crustal residence would logically be the history inferred for lower crust in most cratonic regions but the unusual aspect of the history in the east lake athabasca region is the subsequent lithospheric reactivation that initiated transport of the lower crust to the surface we suggest that a weakened strength profile related to the 1 9 ga heating left the lithosphere susceptible to far field tectonic stresses from bounding orogens that drove the lower crustal exhumation an ultimate return to cratonic stability is responsible for the preservation of this extensive lower crustal exposure since 1 7 ga

interpretation and significance of combined trace element and u pb isotopic data of detrital rutile a case study from late ordovician sedimentary rocks of saxo thuringia germany the u pb age and trace element composition of detrital rutile provide information about the metamorphic history of the source region that cannot be constrained by traditional u pb dating of detrital zircon previous provenance investigations focussed on only one of these methods based on a large la icp ms trace element and u pb isotopic dataset of detrital rutile and u pb isotopic data of detrital zircon from late ordovician sedimentary rocks of saxo thuringia germany this paper discusses the application and significance of combining these methods in provenance investigations u pb age spectra from the detrital zircons analysed show multiple age components multimodal age spectra in all samples this is in contrast with the detrital rutile data as only one sample yielded a multimodal u pb age distribution multimodal age spectra of detrital rutile are most likely preserved in sediments from large catchment areas with complex geological histories they may also be related to specific sedimentation events such as glacial washout during the retreat of large ice shields e g the hirnantian glaciation of gondwana unimodal age spectra are however not restricted to small catchment areas if the provenance region is characterized by a pervasive thermal overprint such as the pan african orogeny throughout gondwana unimodal age distributions may further consist of overlapping age cluster detectable by different trace element composition of the detrital rutile grains the combined u pb age and trace element data from detrital rutile grains demonstrate that rutile sourced from metapelitic rocks yield reliable and precise u pb ages in contrast detrital rutile classified to be of metamafic origin generally has too low uranium concentrations to be dated reliably by la icp ms detrital rutile records low to medium grade metamorphic events in the source region and therefore has the potential to better constrain the maximum depositional age of sedimentary rocks in comparison to u pb dating of detrital zircon

petrology and geochronology of rutile rutile tio2 is an important accessory mineral that when present offers a rich source of information about the rock units in which it is incorporated it occurs in a variety of specific microstructural settings contains significant amounts of several trace elements and is one of the classical minerals used for u pb age determination here we focus on information obtainable from rutile in its original textural context we do not present an exhaustive review on detrital rutile in clastic sediments but note that an understanding of the petrochronology of rutile in its source rocks will aid interpretation of data obtained from detrital rutile for further information on the important role of rutile in provenance studies the reader is referred to previous reviews e g zack et al 2004b meinhold 2010 triebold et al 2012 coarse rutile is the only stable tio2 polymorph under all crustal and upper mantle conditions with the exception of certain hydrothermal environments smith et al 2009 as such we will focus on rutile rather than the polymorphs brookite anatase and ultrahigh pressure modifications in this chapter we first review rutile occurrences trace element geochemistry and u pb geochronology individually to illustrate the insights that can be gained from microstructures chemistry and ages then in the spirit of petrochronology we show the interpretational power of combining these approaches using the ivrea zone italy as a case study finally we suggest some areas of future research that would improve petrochronologic research using rutile rutile is a characteristic mineral in moderate to high pressure metapelitic rocks in high pressure metamorphosed mafic rocks and in sedimentary rocks e g force 1980 frost 1991 zack et al 2004b triebold et al 2012 rutile also occurs rarely in magmatic rocks e g anorthosites as well as in some hydrothermal systems coarse grained

modest movements spectacular fabrics in an intracontinental deep crustal strike slip fault striding athabasca mylonite zone nw canadian shield geometry and strain partitioning within lower crustal intraplate strike slip shear zones can be extremely complex compared with analogous structural levels of interplate strike slip shear zones sited at plate margins striding athabasca mylonite zone canadian shield is a spectacular ca 500 km long granulite facies continental intraplate shear zone the shear zone is composed of middle archean granulite facies annealed mylonites ca 3 13 ga and late archean ca 2 62 2 60 ga granulite facies ribbon mylonite belts which thread a sinuous course along a chain of crustal scale lozenges cored by relatively stiff rocks of mafic to intermediate composition to the northeast the mylonites form a n s trending 5 10 km thick dextral strike slip belt to the southwest this bifurcates into a pair of conjugate strike slip shear zones overlain by a contemporaneous dip slip shear zone striding athabasca mylonite zone was kinematically inefficient as a strike slip fault and cannot have accommodated large wallrock displacements nevertheless spectacular granulite facies ribbon mylonites were formed throughout the shear zone reflecting the very high temperatures ca 850 1000 c high recrystallization rate strain rate ratios and the transpressive nature of the deformation wk 1 possibly accommodated by significant volume loss by magma migration

geochronological constraints on the legs lake shear zone with implications for regional exhumation of lower continental crust western churchill province canadian shield the legs lake shear zone marks the southeastern boundary of an extensive region 20 000 km2 of high pressure 0 8 1 5 gpa granulite facies rocks in the western churchill province canada the shear zone is one of the largest exhumation related structures in the canadian shield and coincides with the central segment of the 2 800 km long snowbird tectonic zone the movement history of this shear zone is critical for the development of models for the exhumation history of the high pressure region we used electron microprobe u th pb dating of monazite with supplemental id tims u pb geochronology to place constraints on the timing of shear zone activity combining these and other data we suggest that regional exhumation occurred during at least three distinct phases over an 150 million year period the first phase involved high temperature decompression from 1 0 to 0 8 0 7 gpa shortly following 1 9 ga peak metamorphism possibly under an extensional regime the second phase involved rock uplift and decompression of the hanging wall to 0 5 0 4 gpa during east vergent thrusting across the legs lake shear zone at ca 1 85 ga this phase was likely driven by early collision related convergence in the trans hudson orogen the final phase of regional exhumation involving the removal of 15 20 km of overburden from both footwall and hanging wall likely occurred after 1 78 ga and may have been related to regional extensional faulting

fluid flow in sedimentary basins abstract the pore waters in sedimentary basins are ultimately derived from sea water meteoric water mineral bound water or from the underlying basement geochemically these waters initially have different signatures however the pore water compositions are easily altered by reactions with minerals or by mixing with other fluids meteoric water is driven downwards into sedimentary basins by the gravitational potential which approximately corresponds to the elevation of the ground water table above the sea or lake level it is difficult to model such flow however because the permeability on a large scale is almost impossible to represent realistically the continuity of confined sandstone and limestone aquifers plays an important role in determining the flux of meteoric water received by rocks in different parts of the basin but this is again difficult to predict pore water flow driven by compaction typically has velocities several orders of magnitude lower than what is commonly found in meteoric water flow regimes the average rate of upwards flow is lower than the rate of subsidence and the difference is the rate of incorporation of sea water in the topmost layer at the sea floor the salinity of formation water provides important constraints on fluid flow in sedimentary basins in the north sea basin and the gulf coast basin these types of data indicate that vertical mixing by compaction driven flow and convection is limited compaction driven flow obeys darcy s law but there are complex interdependencies between pressure permeability and compaction given the low compressibility of water the flow which can result from pressure release alone is very limited except on a local scale along permeable faults and fractures the main flow of pore water during burial is driven by compaction which is a slow process even when overpressure is released abruptly with a resulting increase in the net effective stress in the case of high permeability faults the flow into the fault from low permeability sedimentary rocks at depth may be rate limiting the displacement of pore water in adjacent sediments near the top of the faults may also slow down fluid flow on fractures that do not reach the sea floor modelling of fluid flow in sedimentary basins should be based on permeability distributions which are mostly determined by primary facies diagenesis and the tectonic history of the basin extensional tectonics produce a predominantly vertical fracture network which may serve as conduits for pore water flowing deep into the crust upward flow of hot fluids in basement fractures must have a high velocity in order to produce large thermal anomalies concentrations of dissolved components precipitate at the surface and form deposits of quartz and ore minerals if basement fractures are overlain by a thick sequence of soft sediments the rate of flow on the fractures will be reduced because of the low permeability in the sedimentary cover and conductive heat transport will dominate flow of water at high enough velocities to bring hot water to the surface and produce hot springs is most likely to occur in fractured basements rocks or well cemented sedimentary rocks concentrated precipitation of dissolved components will occur near the surface where the rate of cooling is highest also sediment hosted ores precipitated from flow of hot water on fractures are therefore likely to form before accumulation of thick overlying sedimentary sequences

in situ rutile u pb dating based on zircon calibration using la icp ms geological applications in the dabie orogen china abstract rutile u pb dating by laser ablation inductively coupled plasma mass spectrometry la icp ms has received increasing attention in geological applications previous natural rutile reference materials are insufficient to meet the growing demand of working standards for laser u pb dating in present study we have developed a new calibration strategy for la icp ms rutile dating using the most popular zircon as reference material the matrix dependent elemental fractionation of rutile and zircon could effectively decrease to a better level by optimized analytical parameters and with large ablation spot size 44 90 m the acceptable pb u correction coefficient between rutile and zircon could be obtained while the diameter depth ratio of the crater is above 1 after assessing elemental fractionation of different ablation size five rutile were analyzed by zircon calibration and their ages show strong agreement with those obtained by secondary ion mass spectrometry sims or thermal ionization mass spectrometry tims analysis with both precision and accuracy within 2 two rutiles of eclogitic rocks from the dabie orogen yield the ages of 207 0 2 1 ma and 205 6 2 4 ma respectively both ages are conceivably younger than that previously reported rutile ages 218 ma for eclogites the quite different u pb ages of rutile from different localities most likely indicate the different exhumation history of the subducted continental crust la icp ms rutile u pb dating using zircon as an external calibration reference material would assuage rutile reference material insufficient supply and improve its widespread application

deducing source rock lithology from detrital rutile geochemistry an example from the erzgebirge germany abstract this study evaluates the applicability of rutile trace element geochemistry to provenance studies the study area is the erzgebirge in eastern germany where metamorphic rocks ranging from lower greenschist facies conditions up to granulite facies conditions are exposed we collected sand and rock samples from small catchment areas for a comparative analysis of rutile geochemistry using wavelength dispersive electron microprobe our results show that rutile geochemistry is a powerful tool in provenance studies allowing for the identification of source lithologies and an evaluation of the host orogen s metamorphic history the log cr nb ratio has proven to be decisive in discriminating between mafic and metapelitic lithologies it is also useful for identifying different source rocks when plotted versus a third element or proxy furthermore our results suggest that rutile thermometry can be applied to a much wider range of lithologies than previously assumed a quantification of temperature populations within single sand samples shows that at high grade metamorphic conditions such as those found in the erzgebirge more than 65 of rutiles do not re equilibrate during retrograde metamorphism and thus retain their peak temperature chemistry such samples which have equilibrated at recent metamorphic conditions can be identified by their 2 standard deviations of less than 120 c below 550 600 c no complete equilibration is reached rutiles from greenschist facies and lower metamorphic conditions in the erzgebirge still inherit relict temperatures from a former metamorphic cycle they partly record very high temperatures 950 c and supposedly derive from erosion of the west african craton in ordovician time

petrography and geochemistry of basaltic rocks from the conrad fracture zone on the america antarctica ridge intrusive and extrusive basaltic rocks have been dredged from the conrad fracture zone transecting the slow spreading america antarctica ridge the majority of rocks recovered are holocrystalline with the dominant mineral assemblage being plagioclase plus clinopyroxene with or without minor fe ti oxides olivine occurs in only three samples and many of the samples show evidence of extensive alteration secondary minerals include chlorite actinolite k and na feldspar analcite and epidote in terms of bulk chemistry the rocks are characterized by their generally evolved and highly variable compositions e g mg 0 65 0 35 tio2 0 7 3 6 zr 31 374ppm nb 3 21ppm y 17 96ppm ni 100 9ppm but with respect to the immobile incompatible element ratios e g zr nb y nb la sm are similar to normal or depleted mid oceanic ridge basalts quantitative major and trace element modelling indicate that most of the variation observed can be attributed to low pressure fractional crystallization of plagioclase plus clinopyroxene in approximately equal proportions with or without minor fe ti oxides the range in composition can be accounted for by up to 76 fractional crystallization although ferrobasalts have not frequently been associated with slow spreading ridges the extreme differentiation observed in the conrad fracture zone basalts implies some additional constraint other than spreading rate on the formation of ferrobasalt and reaffirms the importance of extensive crustal differentiation during the production of this basalt type

geochemistry of basalt lavas from vestfjella and adjacent areas dronning maud land antarctica abstract k ar ages major and trace element compositions and sr isotope data are presented for basalt lavas from vestfjella dronning maud land antarctica the new conventional k ar age data have yielded ages from 171 2 to 695 11 ma but the youngest i e middle jurassic ages are preferred mineralogical and chemical data show that the majority of the basalts are tholeiites petrographic mixing calculations ree modelling and the sr isotope data suggest that they were derived by partial melting of garnet free lherzolites with variable ree patterns and subsequently modified by fractionation of olivine ca rich pyroxene and plagioclase incompatible trace element data from nearby middle jurassic basalt lavas from kirwanryggen and heimefrontfjella suggest a different source and ree modelling indicates generation from garnet lherzolites

an exhumation history of continents over billion year time scales the continental lithosphere contains the oldest and most stable structures on earth where fragments of ancient material have eluded destruction by tectonic and surface processes operating over billions of years although present day erosion of these remnants is slow a record of how they have uplifted eroded and cooled over earth s history can provide insight into the physical properties of the continents and the forces operating to exhume them over geologic time we constructed a continuous record of ancient lithosphere cooling with the use of uranium lead u pb thermochronology on volcanically exhumed lower crustal fragments combining these measurements with thermal and pb diffusion models constrains the range of possible erosion histories measured u pb data are consistent with extremely low erosion rates persisting over time scales approaching the age of the continents themselves

tempo of burial and exhumation within the deep roots of a magmatic arc fiordland new zealand the u pb thermochronology of titanite apatite and rutile from a crustal profile through a mesozoic magmatic arc in fiordland new zealand is used to constrain the timing and duration of significant vertical movements during arc construction and evolution titanite data from deep crustal 12 13 kbar basement and cover rocks of central fiordland imply that contractional arc thickening 25 km occurred by 111 1 113 4 ma within a few million years of a major phase of mid to deep crustal magmatism this finding suggests that this cycle of magmatism arc thickening and high grade metamorphism occurred in 6 2 m y in contrast to rapid burial significant unroofing of the central fiordland gran ulites was more protracted requiring an additional 40 45 m y these new data are con sistent with continued residence of the granulites in thickened arc crust for 15 20 m y with subsequent major unroofing recorded by rutile cooling to 450 8c by ca 70 ma such temporal constraints are essential for comprehensive models of the growth modifi cation and unroofing of magmatic arc systems

geological evidence for a proterozoic to mesozoic link between southeastern africa and dronning maud land antarctica comparison of crustal provinces in southeastern africa and dronning maud land antarctica reveals considerable similarity in their evolution from the archaean until the mesozoic separation archaean granites and mid proterozoic supracrustal successions in these regions are so comparable that correlation is suggested a major mid to late proterozoic orogenic terrain in dronning maud land comprising the h u sverdrupfjella and heimefrontfjella subprovinces and termed the maudheim province is very similar in age lithology structural style and metamorphic history to the mozambique and natal orogenic provinces of kibaran age 1000 ma in southeastern africa deformed supracrustal sequences in all three provinces host syn tectonic granites intruded during upper amphibolite to granulite facies metamorphism isoclinal folding was accompanied by thrusting towards adjacent cratonic areas development of orogenic provinces of kibaran age in southeastern africa and antarctica reflects accretion of marginal basin volcanic arc sequences onto older continents the 500 ma pan african event was a widespread predominantly thermal overprinting of parts of the older orogenic provinces faulting and rifting of the supercontinent preceded break up and influenced the stratigraphy of phanerozoic sedimentary successions in se africa

plio pleistocene exhumation of the eastern himalayan syntaxis and its domal pop up the eastern termination of the himalayan orogen forms a structural syntaxis that is characterised by young from 10 to 7 km in elevation in the core of the syntaxis very high erosion rates documented in sediment downstream of the gorge at the foot of the himalaya contribute 50 of total detritus to the sediment load of the brahmaputra the initiation of very high rates of exhumation has been attributed either to the extreme erosive power of a river flowing across a deforming indentor corner and the associated positive feedback or to the geometry of the indian plate indentor with the corner being thrust beneath the asian plate resulting in buckling which accommodates shortening both processes may be important the northern third of the syntaxis corresponds to a steep domal pop up structure bounded by the india asia suture on three sides and a thrust zone to the south within the dome greater himalaya rocks equilibrated at 800 c and 25 30 km depth during the miocene with these conditions potentially persisting into the latest miocene and possibly the pliocene with modest decompression prior to 4 ma this domal pop up corresponds to the area of youngest bedrock ages on a wide variety of thermochronometers and geochronometers in this paper we review the extensive scientific literature that has focused on the eastern syntaxis and provide new chronological data on its bedrock and erosion products to constrain the age of inception of the very rapid uplift and erosion we then discuss its cause with the ultimate aim to reconstruct the exhumation history of the syntaxis and discuss the tectonic context for its genesis we use zircon and rutile u pb white mica ar ar and zircon fission track dating methods to extract age data from bedrock brahmaputra modern sediments including an extensive compilation of modern detrital chronometry from the eastern himalaya and neogene palaeo brahmaputra deposits of the surma basin bangladesh numerical modelling of heat flow and erosion is also used to model the path of rocks from peak metamorphic conditions of 800 c to 250 c our new data include u pb bedrock rutile ages as young as 1 4 ma from the namche barwa massif and 0 4 ma from the river downstream of the syntaxis combined with existing data our new data and heat flow modelling show that i the detrital age signature of the modern syntaxis is unique within the eastern himalayan region ii the rocks within the domal pop up were 575 75 c only 1 2 myr ago iii the neogene surma basin does not record evidence of the rise and erosion of the domal pop up until latest pliocene pleistocene time iv pleistocene exhumation of the north easternmost part of the syntaxis took place at rates of at least 4 km myr with bedrock erosion of 12 21 km during the last 3 ma v the inception of rapid syntaxial exhumation may have started as early as 7 ma or as late as 3 ma and vi the yarlung tsangpo is antecedent and subsequently distorted by the developing antiform together our data and modelling demonstrate that the domal pop up with its exceptional erosion and topographic relief is likely a pleistocene feature that overprinted earlier structural and metamorphic events typical of himalayan evolution

high sensitivity u pb rutile dating by secondary ion mass spectrometry sims with an o2 primary beam abstract we present a secondary ionization mass spectrometry sims technique for u pb geochronology of rutile at high spatial resolution and sensitivity using an o 2 primary ion beam coupled with surficial o 2 gas deposition o 2 flooding the o 2 beam is 10 more intense than conventionally applied o or o 2 beams at the same lateral resolution natural and synthetic rutile was determined to be conductive under o 2 bombardment permitting higher excavation sputter rates than conventional sims using negatively charged o beams without detrimental effects of sample charging the main advantage of o 2 is rapid sputtering at shallow primary ion penetration depths this minimizes the contribution of surface derived common pb and generates a high secondary ion flux at high sensitivity with useful yields uy detected ions atoms removed from target for pb in rutile of 4 and 3 for o and o 2 respectively in addition o 2 flooding reduces spread in the pb u vs uo 2 u calibration by mitigating crystal orientation dependent variability of sputter yields calibrated against primary rutile standard r10b 1090 ma o 2 generated sims u pb and pb pb age averages are accurate within 2 bombardment can also be advantageous for sims analysis of other conductive minerals such as cassiterite columbite tantalite hematite ilmenite and magnetite

multistage exhumation and juxtaposition of lower continental crust in the western canadian shield linking high resolution u pb and 40ar 39ar thermochronometry with pressure temperature deformation paths 1 new u pb titanite apatite rutile and 40ar 39ar hornblende muscovite biotite data are linked with existing pressure temperature deformation paths to impose detailed temporal constraints on the juxtaposition and unroofing of domains in an extensive 20 000 km2 region of exhumed lower crust in the east lake athabasca region of the western canadian shield in the 200 m y between circa 1 9 ga high pressure granulite facies metamorphism and circa 1 7 ga unconformable deposition of athabasca basin sediments on the exhumed rocks our analysis reveals at least three distinct phases of unroofing at different rates specifically we distinguish 1 an early phase of extensional unroofing in the chipman domain at rates of 1 5 2 0 km m y from 1 0 gpa to 0 8 gpa associated with mafic magmatism and metamorphism 2 an episode of regional contractional uplift and associated unroofing along the legs lake shear zone at circa 1850 ma from 0 7 0 8 gpa to 0 4 0 5 gpa and 3 a final period of extensional unroofing at rates of 0 2 0 3 km m y from 0 4 to 0 5 gpa that culminated in transport of current exposures to near surface conditions the cooling patterns and retrograde assemblages are consistent with pauses between exhumational phases the apparent convergence of disparate higher temperature histories in several deep crustal domains at circa 1 89 1 88 ga implies their juxtaposition at 0 7 0 8 gpa conditions regional east directed thrusting of the deep crustal domains as a coherent unit at 1 85 ga juxtaposed the granulites with middle crustal hearne domain rocks at 0 4 0 5 gpa conditions the detailed exhumational history allows correlation with changing regional tectonic regimes associated with the amalgamation of laurentia the temporal and spatial heterogeneity of exhumation patterns in the east lake athabasca region may be a common feature of the unroofing histories of lower crustal rocks

aspects of jurassic magmatism and faulting in western dronning maud land antarctica implications for gondwana break up

compositional modification and trace element decoupling in rutile insight from the capricorn orogen western australia abstract the trace element composition of rutile is widely used to constrain the temperature and timing of geological events the provenance of detrital grains and as a potential indicator of proximal ore mineralisation understanding the processes of trace element mobility in rutile is therefore paramount to the successful application of rutile geochemistry to geological research using a combination of electron backscattered diffraction and laser ablation split stream icp ms we studied the microtextures trace element contents and u pb isotopes of rutile from the siliciclastic rocks of the mesoproterozoic edmund group capricorn orogen western australia the capricorn orogen experienced multiple tectonothermal events during the proterozoic under conditions spanning from low to high grade metamorphism rutile textures u pb ages trace element variations and calculated zr in rutile temperatures reveal different generations of rutile including detrital or metamorphic hydrothermal grains u pb age data of detrital rutile indicate a wide span of ages ca 2 7 1 7 ga of the source rocks and cr nb compositions point to metamafic metapelitic and granitic origins for this rutile the youngest detrital ages of 1 68 ga constrain the maximum depositional age of the sedimentary package the comparison of age spectra across the stratigraphy indicates a change in provenance with sourcing from circa 1 7 ga rocks during early stages of deposition followed by unroofing of older basement during deposition of the youngest units of the edmund group partial resetting of detrital rutile is observed in the increase of 207pb 206pb ratios accompanied by a decrease of zr in rutile calculated temperatures lower temperature metamorphic hydrothermal rutile yielded imprecise mesoproterozoic ages 1 3 1 1 ga this rutile has significant variations of zr hf and nb ta extending to values both lower and higher than primitive mantle which may record crystallisation from or interaction with hydrothermal fluids partial remobilisation of trace elements in metamorphic hydrothermal rutile is demonstrated by sub grain domains with sharp geochemical variations and by evidence of element diffusion at grain margins orders of magnitude differences in trace element concentrations including nb cr and zr in metamorphic hydrothermal rutile grains suggest thorough investigation of rutile textures needs to be carried out prior to using the trace element concentrations in rutile for provenance and zr in rutile temperature studies

paleoproterozoic amphibolite facies retrogression and exhumation of archean metapelitic granulites in the southern marginal zone of the limpopo belt south africa abstract granulite facies terrains that have undergone extensive retrogression provide insights into the recycling of volatiles in the crust during orogenesis the timing of retrogression is key to understanding the geodynamic circumstances of such events this study has investigated amphibolite facies retrogression of melt depleted granulite facies metapelites from the south marginal zone smz of the limpopo belt the granulites formed via incongruent partial melting reactions that consumed biotite and produced garnet and rutile as prominent peritectic products at 2 7 ga under conditions of 11 kbar and 875 c peak metamorphic garnet is replaced by cordierite orthopyroxene symplectites with cordierite subsequently partially replaced by fine grained intergrowths of amphibolite facies assemblages including rutile zr in rutile thermometry yielded a temperature range of 731 846 c for the granulite facies rutiles and 629 690 c for rutile in the retrograde assemblage all rutile generations yielded a youngest u pb age of 2 1 ga some granulite facies rutiles record older dates 2 55 2 25 ga which are non systematic with regard to crystal size and represent non reset relicts of metamorphic recrystallization during a potentially complex history of amphibolite facies metamorphism between 2 7 and 2 2 ga the youngest 2 1 ga rutile u pb age represents the time of final exhumation related cooling of the rocks following a discrete paleoproterozoic amphibolite facies retrograde event coexisting orthoamphiboles in the peak metamorphic assemblage indicate a maximum peak temperature of 625 c the lack of age size correlation and intragrain age zoning in rutile suggest fast cooling following the amphibolite facies retrograde event which is inferred to be not much older u pb dating of zircon from the same rocks confirms that the metapelites underwent granulite facies metamorphism at 2695 35 ma thus the metapelites of the smz record both an archean granulite facies event and a polymetamorphic paleoproterozoic amphibolite facies history that culminated in a mid amphibolite facies peak inducing partial retrogression of the granulites the paleoproterozoic amphibolite facies event is tentatively correlated with mantle melting documented between 2 3 and 2 1 ga to produce mafic and intermediate volcanic rocks in the pretoria group on the kaapvaal craton

anatomy of impactites and shocked zircon grains from dhala reveals paleoproterozoic meteorite impact in the archean basement rocks of central india abstract the dhala structure in central india has been a topic of global interest ever since the report of an ancient meteorite impact event there here we present an integrated study of the petrology geochemistry and zircon u pb zircon geochronology and rare earth element geochemistry from the structure along with and an analysis of the grain morphology and textural features our results provide new insight into the nature and timing of the impact event the zircon grains from the impactites show textures typical of shock deformation which we correlate with the impact event we also identified the presence of reidite based on raman spectroscopy and characteristics such as a persistent planar fracture bright backscattered electron images and a lack of zoning which are all diagnostic features of this mineral formed during an impact event our zircon u pb data from the various rock types in the basement show magma emplacement at ca 2 5 2 47 ga and the pb loss features suggest that the impact might have occurred between ca 2 44 ga and ca 2 24 ga another minor group of late paleoproterozoic zircons with concordant ages of 1826 and 1767 ma in the brecciated quartz reefs along the margins of the impact crater from unfractured grains represent an younger thermal event after the impact the rare earth element patterns of the neoarchean to early paleoproterozoic zircon population reflect the effects of hydrothermal alteration on a peralkaline host rock the abnormally high concentration of k2o in the impactite up to 15 91 wt is also consistent with metasomatic alteration associated with the impact event

u pb zircon and rutile ages from granulite xenoliths in the slave province evidence for mafic magmatism in the lower crust coincident with proterozoic dike swarms zircon and rutile in two garnet bearing mafic granulite xenoliths from a kimberlite in the archean slave province record proterozoic ages similar to the 1 27 ga mackenzie dike swarm or similar to the ca 2 23 2 21 ga malley mckay diabase dike swarms the xenolith sample site currently overlies thick lithosphere more than 700 km to the southeast of the proposed mackenzie plume center because lower crust was effectively shielded from the thermal effects of an upwelling plume by thick lithospheric mantle heat was most likely advected into the lower crust by lateral intrusion of mafic sills over distances of hundreds of kilometres from magma generation zones nearer the plume center

geochemistry of basalts from central and southern new hebrides arc implication for their source rock composition abstract the basaltic rocks from the central and southern islands of the new hebrides aneityum tanna erromango efate emae tongoa and epi have geochemical features typical of island arc volcanics they are enriched in lile and depleted in zr hf nb and ta compared to n type morb the rocks were derived from a similar upper mantle source as n type morb but with a higher degree of partial melting in addition their source was enriched in lile k rb sr ba and lree probably by migrating hydrous fluids released during the dehydration of the subducted oceanic slab the basalts from futuna island which is located farther from the trench display characteristics typical of calc alkaline rocks the futuna basalts were generated from a different lile enriched upper mantle source it seems that this upper mantle source was modified by interaction with partial melts from the subducted oceanic lithosphere

constraints on the u pb systematics of metamorphic rutile from in situ la icp ms analysis abstract in situ laser ablation icp ms u pb dating of metamorphic rutile from granulite facies metapelitic rocks of the archaean pikwitonei granulite domain manitoba canada provides constraints on pb diffusion and characterizes the closure behavior of rutile the analysis of transects of 35 m spots across 15 rutile grains having a size of 120 to 280 m yielded concordant ages with core ages of ca 2450 ma and core to rim younging towards 2280 ma age profiles indicate that volume diffusion of pb occurs in rutile implying that the ages represent cooling ages to investigate the closure behavior of pb in rutile closure temperature profiles t c x were constructed based on different models combined with experimentally determined diffusion parameters the classical t c x model of dodson 1986 mat sci forum 7 145 154 indicates a rapid decrease of t c in the rims of grains providing unrealistic estimates for the cooling rate when combined with u pb ages a new t c x model was constructed based on the analyzed age profiles that are described by an error function this model shows a more steady decrease in t c throughout the grain from ca 640 c in the core depending on grain size to a rim intercept t c rim of 490 c 7 2 which is interpreted to be the extrapolated theoretical absolute temperature of insignificant pb diffusion in rutile the new model provides a better description of the relation between age and t c for the analyzed grains however both t c x models demonstrate that even in small grains the variations of t c can be significant making it impossible to derive one representative t c for pb in rutile the error function based t c x model allows the determination of cooling rates which show a decrease over time from ca 2 2 to 0 4 c ma agreeing well with previous estimates for the pikwitonei granulite domain this consistency supports the validity of our model and indicates that cooling rates can be estimated from single grains by la icp ms u pb dating of rutile providing constraints on the cooling history of a metamorphic terrane the slow cooling rates imply that exhumation was slow

preliminary stratigraphic and structural architecture of bhutan implications for the along strike architecture of the himalayan system article i nfo preliminary mapping and stratigraphic correlation of lesser himalayan rock in eastern bhutan using field characteristics u pb detrital zircon dating and nd geochemistry define the first order stratigraphic architecture of the indian passive margin sequence in the eastern himalaya we use this new image of the lateral and vertical relationships of the original stratigraphy to determine the structural framework of the eastern himalayan fold thrust belt in bhutan we propose that lesser himalayan rock in bhutan can be divided into lower lesser himalayan rocks with a paleoproterozoic detrital zircon signal and upper lesser himalayan rocks with detrital zircon signals of 1000 500 ma the 500 ma detrital grains are from rocks in the frontal portions of the fold thrust belt north of the main boundary thrust as well as directly in the footwall of the main central thrust in the hinterland our preliminary stratigraphic study coupled with mapping allows us to construct a composite balanced cross section through the kuru chu valley in eastern bhutan which provides the first image of the geometry and amount of shortening through bhutan the main frontal thrust tilts a 6 km of thick section of neogene foreland basin deposits these deposits are separated from folded and faulted upper lesser himalayan rocks by the main boundary thrust north of the main boundary thrust we propose two duplex systems the southernmost duplex contains 9 repeated sections of upper lesser himalayan units including the permian gondwana sequence and the cambrian or younger baxa group the northernmost duplex is located in the footwall of the main central thrust and is comprised of two repeated sections of the proterozoic shumar and daling formations the southern boundary of this northern duplex is the shumar thrust which acts as a roof thrust for the southern duplex system and may correlate to the ramgarh thrust in nepal and india we propose that the development of the duplex systems passively folded overlying tethyan and greater himalayan rocks minimum shortening for this part of the himalayan fold thrust belt is 359 km all of which has occurred from 22 ma to present defining a long term shortening rate of 16 mm yr

low long term erosion rates and extreme continental stability documented by ancient u th he dates zircon and apatite crystals from the western canadian shield yield u th he dates that are the oldest yet reported for terrestrial rocks zircon dates from 1 73 to 1 58 ga are consistent with independent geological and thermochronological constraints and indicate that the rocks were at temperatures 180 c and crustal depths 7 10 km since ca 1 7 ga apatite dates from 0 95 to 0 55 ga suggest residence of rocks at temperatures 40 50 c and crustal depths 1 5 2 km for 1 0 0 6 b y when interpreted using conventionally accepted apatite he diffusion kinetics and considering the proposed effect of radiation damage on apatite he retentivity our analysis implies long term integrated unroofing rates of 2 5 m yr since ca 1 7 ga these rates are significantly lower than the long term rates suggested by previous thermochronological data sets in continental interiors but are within the range of short term erosional estimates the results are consistent with the extreme stability of this region since the proterozoic

age discordance and mineralogy observations of discordant ages meaning that an age given by one mineral geochronometer is different from the age given by another geochronometer from the same rock began in the early days of geochronology in the late 1950s and 1960s discordant u pb zircon ages were unquestioningly attributed to pb diffusion at high temperature later the mineralogical properties and the petrogenesis of the zircon crystals being dated was recognized as a key factor in obtaining concordant u pb ages advances in analytical methods allowed the analysis of smaller and smaller zircon multigrain fractions then the analysis of individual grains and even pieces of grains with higher degrees of concordancy further advances allowed a higher analytical precision a clearer perception of accuracy and a better statistical resolution of age discordance as for understanding the cause s of discordance belief revision followed the coupling of imaging cathodoluminescence cl and back scattered electrons bse to in situ dating by secondary ion mass spectrometry sims or by laser ablation inductively coupled plasma mass spectrometry la icpms discordant zircon and other accessory minerals e g monazite apatite etc often consist of young rims accreted onto into older cores age gradients are sharp and no pb diffusion gradients are observed as u pb discordance in crystalline non radiation damaged grains is caused by diachronous heterochemical mineral generations interpretations of mineral ages based on the exclusive role of diffusion are superseded and closure temperatures of zircon and monazite are irrelevant in geological reality other isotopic systems rb sr k ar were believed since the 1960s to be similarly controlled by the diffusivity of radiogenic daughters when zircon and monazite discordance were recognized as zone accretion reaction with sharp boundaries that showed little or no high temperature diffusive re equilibration the other chronometric systems were left behind and interpretations of mineral ages based on the exclusive role of diffusion survived the evidence from textural petrologic imaging cl bse and element mapping by electron probe microanalyzer epma or high spatial resolution sims or la icpms provides the decisive constraints all microcline and mica geochronometers that have been characterized in detail document patchy textures and evidence for mineral replacement reactions it is important not to confuse causes and effects heterochemical microstructures are not the cause of ar and sr loss rather they follow it argon and sr loss by dissolution of the older mineral generation occurs first heterochemical textures form later when the replacive assemblage recrystallizes heterochemical mineral generations are identified and dated by their ca cl k systematics in 39ar 40ar replacive reactions adding or removing cl such as e g sericite overgrowths on k feldspar retrograde muscovite intergrowths with phengite etc are detected by cl k vs ar k isotope correlation diagrams calcium poor reaction products such as e g young biotite intergrown with older amphibole adularia replacing microcline etc can be easily identified by ca k vs ar k diagrams supported by epma analyses mixed mineral generations are observed to be the cause of discordant staircase shaped age spectra while step heating of crystals with age gradients produces concordant plateaus age gradients are therefore unrelated to staircase age spectra there is a profound analogy between the u pb rb sr and k ar systems lead and ar diffusion rates are both much slower than mineral replacement rates for all t 750 c patchy retrogression textures are always associated with heterochemical signatures u th ratios ree patterns ca cl k ratios as a rule single generation minerals with low amounts of radiation damage give concordant ages whereas discordance is caused by mixtures of heterochemical resolvably diachronous mineral generations in petrologic disequilibrium this can also include sub grains that have accumulated significant amounts of radiation damage for accurate geochronology the petrologic characterization with the appropriate technique s of the minerals to be dated and the petrologic context at large are as essential as the mass spectrometric analyses

microstructural trace element and geochronological characterization of tio 2 polymorphs and implications for mineral exploration abstract the geochemistry of rutile tio2 has recently found its use in mineral exploration with some studies reporting anomalous concentrations of fe w v sn and sb in rutile associated with mineralized ore systems however the use of rutile as a prospecting tool is likely to be complicated by the systematic changes in trace element composition with tio2 polymorph type anatase brookite and rutile here we present tio2 trace element and u pb geochronological data from the mineralized and barren portions of the palaeoproterozoic moorarie supersuite and capricorn orogen western australia with a focus on the minnie creek molybdenum prospect in the northern part of the gascoyne province the barren samples contain all three tio2 polymorphs anatase brookite and rutile textures suggest anatase and brookite may have formed during low t metamorphism either through replacement of previous rutile grains or titaniferous minerals rutile grains from barren samples yield variable u pb ages ca 3 0 2 2 ga as well as variable textures and chemical compositions suggesting detrital origins thus most likely representing metasedimentary units intruded by the moorarie supersuite rutile grains from the minnie creek prospect yield palaeoproterozoic ca 1 77 1 75 ga u pb cooling ages and nb ta concentrations of up to 17 wt that along with inclusions of manganocolumbite oscillatory and patchy zonation of nb and fe suggest a magmatic origin the commonly used pathfinder elements for gold and base metal mineralisation fe cr v w sn and sb are shown to be systematically lower in anatase and brookite thus yielding false negatives if polymorph type is not identified during reconnaissance studies for this reason a ternary diagram was constructed based on the systematic changes in chemistry of tio2 polymorphs to provide a relatively fast and easy chemical discrimination of polymorphs in large volumes of reconnaissance data furthermore it is shown that high al concentrations are characteristic of brookite and to a lesser degree anatase but not rutile in addition sn nb ta and w concentrations in rutile may be more sensitive to igneous processes and may be used to track processes occurring in strongly fractionated granitic magmas such as pegmatites and associated deposits

rutile chemistry and thermometry as provenance indicator an example from chios island greece abstract in the present study we focused on detrital rutile separated from 12 psammitic samples that belong to three different sedimentary successions carboniferous permo carboniferous permo triassic occurring on chios island greece the ti cr al fe nb zr si and v contents of the rutiles were obtained by electron microprobe analyses to trace their provenance the cr and nb concentrations of the analysed rutile grains show a wide range and indicate that this mineral in the carboniferous succession is mainly derived from metamafic rocks whereas in the permo carboniferous and permo triassic successions stem from a metapelitic source the calculated formation temperatures using the zr in rutile thermometer range from ca 520 to 850 c with hotter rutile being encountered in the permo carboniferous and permo triassic successions this feature together with the rutile chemistry indicate a change in source rock lithology through time which could either reflect an increasing depth of erosion of an exhumed variscan nappe pile of heterogeneous composition in the hinterland or a change in the style of accretion and erosion of different terranes at the southern margin of laurussia during the subduction of a branch of the palaeotethys ocean in the late palaeozoic in general this study underscores the importance of rutile chemistry and thermometry in quantitative single mineral provenance analysis and in chemostratigraphic analysis of clastic sedimentary rocks

temperature time paths in the high p manicouagan imbricate zone eastern grenville province abstract the manicouagan imbricate zone miz eastern grenville province is a series of lithotectonic units assembled from different crustal levels during the grenvillian orogeny structural evidence along with u pb dating suggests that two separate stages of metamorphism occurred at ca 1050 and 1020 ma respectively the first stage is characterised by high pressure and temperature high pt metamorphism up to eclogite facies with a range of maximum recorded pressures between 14 and 20 kbar consistent with variations in burial depth maximum t conditions recorded from apparently different structural levels are rather similar 800 900 c suggesting high heat flow during eclogite facies metamorphism evidence of high temperatures 700 800 c recorded during the early stage of tectonic exhumation indicate near isothermal decompression the second stage is a high t event coeval with the emplacement of syn metamorphic granite and is recorded by rocks in the upper most tectonic slice boundary zone in the south east miz this event may also be responsible for amphibolite facies overprinting across the rest of the miz using the u pb and pt data sets along with published diffusion rates for pb in monazite titanite and rutile tt paths have been modelled for several of the lithotectonic units in most of the units suggested cooling rates appear to have been relatively fast during initial exhuamtion but show a prolonged period of slower cooling 5 10 c ma for 30 40 ma apparently at odds with the steep near isothermal pt paths preservation of eclogitic textures and preservation of garnet zoning in the highest grade rocks on the other hand in the boundary zone the recorded cooling rates are much faster 20 c ma indicative of tectonic exhumation this phase of rapid cooling in the boundary zone corresponds in time with the slow cooling rates at 1030 1000 ma in the rest of the miz thus thrusting of the boundary zone over the rest of the miz appears to have caused a thermal overprint which was superimposed on the tt paths taken as a whole the tt paths for the miz are interpreted as representing two separate metamorphic events which were both relatively short in duration

a u pb age for mineralized nipissing diabase gowganda ontario concordant baddeleyite and rutile analyses from mineralized nipissing diabase at gowganda s castle mine yield a u pb age of baddeleyite is a primary phase of the diabase therefore the age dates the magmatic emplacement the genesis of rutile is less certain rutile could have formed during an episode of alteration veining and ag mineralization in which case the u pb age would also date the secondary event and demonstrate that ag mineralization was related to the nipissing magmatism however a primary magmatic origin of the rutile cannot be completely excluded for now thus the u pb age may not constrain the time of ag mineralization further research is needed to clarify this point

timing of india asia collision geological biostratigraphic and palaeomagnetic constraints 1 a range of ages have been proposed for the timing of india asia collision the range to some extent reflects different definitions of collision and methods used to date it in this paper we discuss three approaches that have been used to constrain the time of collision the time of cessation of marine facies the time of the first arrival of asian detritus on the indian plate and the determination of the relative positions of india and asia through time in the qumiba sedimentary section located south of the yarlung tsangpo suture in tibet a previous work has dated marine facies at middle to late eocene by far the youngest marine sediments recorded in the region by contrast our biostratigraphic data indicate the youngest marine facies preserved at this locality are 50 6 52 8 ma in broad agreement with the timing of cessation of marine facies elsewhere throughout the region double dating of detrital zircons from this formation by u pb and fission track methods indicates an asian contribution to the rocks thus documenting the time of arrival of asian material onto the indian plate at this time and hence constraining the time of india asia collision our reconstruction of the positions of india and asia by using a compilation of published palaeomagnetic data indicates initial contact between the continents in the early eocene we conclude the paper with a discussion on the viability of a recent assertion that collision between india and asia could not have occurred prior to 35 ma

the effect of grain orientation on secondary ion mass spectrometry sims analysis of rutile abstract in situ high precision u pb analysis of rutile by secondary ion mass spectrometry sims reveals that instrumental bias for isotope ratios and count rates vary due to crystal orientation electron backscatter diffraction ebsd techniques have been combined with sims data to show consistent and systematic crystal orientation effects whilst confirming that all analyses are on single crystals and that there is random variation from grain to grain the result of the orientation effect is to produce an extremely large calibration slope more than an order of magnitude larger than for other minerals which can result in highly inaccurate and spurious u pb ages from rutile if not taken into account we present a large standard dataset to highlight this effect and show that by collecting good standard data from grains in multiple orientations these effects can be negated and accurate u pb sims data for rutile can be obtained using a standard calibration slope of ln pb u vs ln uo2 uo 1 12 examples from the anantangiri region eastern ghats india are used to show the magnitude of these effects on the calibration of unknowns evidence is presented to show that the cause of these orientation effects is most likely a combination of channelling of primary ions into the crystal and preferential emission of secondary ions along preferred lattice directions

nature and origin of authigenic rhodochrosite and siderite from the paleozoic aquifer northeast mississippi u s a abstract botryoidal rhodochrosite siderite and pyrite are the principal authigenic phases in fractured chert comprising the paleozoic aquifer of northeast mississippi locally alternating bands of near end member composition rhodochrosite and siderite occur in veinlets up to 2 mm wide in the fractured chert carbon isotope data for the carbonates 13c values ranging from 9 6 to 11 3 pdb indicate that both inorganic and organic c were incorporated into the authigenic phases the paleozoic aquifer is hydraulically connected with the overlying upper cretaceous coastal plain strata groundwater in the coastal plain units is undersaturated with respect to calcite and has relatively high fe and mn contents the latter apparently caused by the dissolution of fe mn oxyhydroxides the natural downward recharge of these waters into the paleozoic aquifer causes calcite dissolution in limestone at the base of the chert which in turn leads to the precipitation of rhodochrosite and siderite additionally pyrite is precipitated as so4 reduction occurs in the same zone rhodochrosite has not been previously documented as an authigenic phase in a fresh water aquifer system suggesting that the geochemical environment necessary for its precipitation is not common the alternating bands of rhodochrosite and siderite observed in this study coupled with available thermodynamic data indicate that the groundwater a mn2 afe2 ratio must have fluctuated over time variations in the ratio are best explained by variable eh conditions at the site where fe mn oxyhydroxide dissolution occurs

eclogites of the snowbird tectonic zone petrological and u pb geochronological evidence for paleoproterozoic high pressure metamorphism in the western canadian shield eclogite occurs within the southern domain of the east athabasca mylonite triangle in northern saskatchewan situated at the boundary between the archean rae and hearne provinces of the western canadian shield the east athabasca mylonite triangle is a fundamental exposure of the 3 000 km long snowbird tectonic zone the eclogite occurs in association with a variety of lower crustal high pressure granulites that record a complex metamorphic history from 2 6 to 1 9 ga temperatures of the eclogite facies metamorphism are constrained by garnet clinopyroxene exchange thermometry at 920 1 000 c minimum pressure conditions are recorded by the jadeite quartz albite geobarometer at 1 8 2 0 gpa a near isothermal decompression path to granulite facies conditions is inferred from retrograde reaction textures involving the formation of granulite facies assemblages such as orthopyroxene plagioclase and pargasite plagioclase u pb idtims zircon geochronology of the eclogite yields a weighted mean 207pb 206pb date of 1 904 0 0 3 ma which we interpret as the time of peak eclogite facies metamorphism shrimp in situ analyses of metamorphic zircons included within omphacitic clinopyroxene support this interpretation with a weighted mean 207pb 206pb date of 1 905 19 ma inclusion suites of high pressure phases and the petrographic setting of zircon are a direct link between zircon growth and eclogite facies metamorphism zircon from one eclogite sample has older cores that are 2 54 ga which is a minimum age for the emplacement or earliest metamorphism of the gabbroic protolith u pb rutile data indicate slow cooling at 1 c ma below 500 c from 1 88 to 1 85 ga the formation and exhumation of the eclogites at ca 1 9 ga has important implications for the tectonic significance of the snowbird tectonic zone during the paleoproterozoic the eclogites described here are consistent with transport of continental crust to mantle depths during the paleoproterozoic followed by rapid buoyancy driven exhumation to normal lower crustal depths

dating of anatase forming diagenetic reactions in rotliegend sandstones of the north german basin diagenetic conditions controlling authigenic formation of anatase and in situ la icp ms u pb isotope dating of this phase are studied in upper rotliegend ii sandstones from two wells in nw germany anatase grew after breakdown of detrital ti phases e g ilmenite although local scale ti transport and mobilization from detrital clay hematite coats or condensed hydrocarbons cannot be ruled out the anatase forming reaction marks the change from a regime of reducing conditions imposed by first hydrocarbon generation to oxidizing conditions probably caused by influx of a fluid from evaporitic zechstein rocks this change in fluid influx is interpreted as response to enhanced normal faulting and halokinesis during accelerated burial in triassic times isotope data of u and pb indicate incorporation of crustal common pb from mesozoic pore fluids but precisely mark diagenetic growth of anatase at 224 3 5 1 5 6 ma in a tera wasserburg plot locally u pb isotope signatures are consistent with either additional incorporation of u and pb from detrital precursor phases or with uptake of uranogenic pb from hydrocarbons generated prior to 224 ma anatase is shown to be a valuable authigenic phase suitable for u pb chronometry of diagenetic events which appears to be unaffected by protracted burial and temperatures exceeding temperatures of crystallization

u pb geochronology of the 2 0 ga itapecerica graphite rich supracrustal succession in the sao francisco craton tectonic matches with the north china craton and paleogeographic inferences abstract la icpms u pb isotopic studies are conducted for key samples from the itapecerica graphite rich supracrustal succession and the crystalline basement in the southern sao francisco craton sfc the detrital zircon analyses in paragneiss indicated a maximum deposition age of 2080 ma the age spectra in the relative probability diagrams suggested the precursor basin was essentially fed with detritus from the accretionary complexes of the mineiro belt 2470 2000 ma which is the most preserved segment of the minas orogen the basement rocks yielded zircon u pb crystallization ages between 2753 81 and 2679 61 ma whereas the metamorphic overprint gave 2069 84 ma one nearby charnockitic gneiss yielded a sm nd garnet whole rock linear array with an age of 2086 19 ma the youngest ages estimate the timing of the regional prograde granulite facies metamorphism that also overprinted the itapecerica rocks given our upper intercept zircon u pb ages of 1997 20 ma and 1971 40 ma in paragneiss and quartzite respectively these ages also constrained the timing of the conversion of carbonaceous material into graphite in the itapecerica mine the second largest graphite mine in brazil the high grade metamorphism was associated with the final docking of the mineiro belt against the neoarchean foreland the minas orogen has geologic matches with the jiao liao ji orogen located in the present eastern margin of the north china craton where khondalites uht granulites anatectic charnockites as well as large graphite deposits are documented such as in the 1 97 1 88 ga korean arc the tectonic similarities between the two cratons support a relatively close position at the time frame the dynamic scenario considers broadly coeval accretionary collisional belts along the active margins of archean blocks the progressive closure of the paleoproterozoic ocean and final docking of the sao francisco congo paleocontinent against the active continental margin of the north china craton the work model allows new insights into the paleogeography of the core of supercontinent columbia at 1 90 ga and the subsequent 1 78 1 50 ga intraplate activity

upb geochronology of the western cape smith belt canada new insights on the age of initial rifting and arc magmatism abstract the cape smith belt of northern quebec is the foreland to hinterland transition of the ungava orogen a component of the paleoproterozoic trans hudson orogen of north america u pb ages for rift and volcanic arc related units from the cape smith belt are presented and used to evaluate models of its tectonic evolution a gabbroic sill intrusive into the rift related sediments near the base of the povungnituk group is 2038 2 4 ma old indicating that rifting of the superior province was initiated prior to that time 80 m y earlier than previously reported a granodiorite intrusive into the volcanic rocks of the povungnituk group is dated at 1991 2 ma indicating that magmatism associated with the rifting event was active at that time 30 m y earlier than formerly thought the age of parent group arc related magmatism ranges from 1874 3 4 ma the age of a diorite intrusion to 1860 2 ma the age of another diorite intrusion and of a rhyolite a granodiorite intrusive in the parent group yields an age of 1845 6 ma magmatism of the parent group is thought to be related to northward dipping subduction of the superior province margin below the watts group a gabbroic intrusion in the 2000 ma old watts group is dated at 1858 2 ma these ages are consistent with a tectonic model in which the rocks of the cape smith belt formed in a single continental rift to oceanic progression followed by subduction which led to an arc continent collision a comparison of u pb ages for similar events in different sectors of the trans hudson orogen in canada reveals that early events are diachronous and range from 2169 ma to 2038 ma the main collisional events are coeval in the ungava and new quebec orogens and in some of the western sectors at 1830 1860 ma younger ages of 1800 1820 ma are obtained for the late collisional stages throughout the trans hudson orogen

himalayan metamorphic sequence as an orogenic channel insight from bhutan the bhutan himalayas differ from the rest of the himalayas in two major ways i low grade sedimentary rocks lie above the greater himalayan sequence as klippen i e erosional remains of the south tibetan detachment and ii an out of sequence thrust the kakhtang thrust lies structurally above the klippen and it doubles the exposed thickness of the greater himalayan sequence our field observations and geochronological data constrain the main kinematic events in the bhutan himalayas crystallisation ages of leucogranite dykes deformed by the main central thrust and the south tibetan detachment indicate that these two structures operated together between 16 and 22 ma the out of sequence kakhtang thrust was active at 10 14 ma and was concurrent with reactivation of the south tibetan detachment restoration of the bhutan himalayas prior to the out of sequence thrusting shows that the greater himalayan sequence was the core of a long low viscosity crustal channel extending under the tibetan plateau we propose that the gravity driven southward extrusion of the channel material from underneath the tibetan plateau caused the inverted metamorphic sequence in the lesser himalayan sequence and in the greater himalayan sequence this process also led to occurrences of present day surface rocks that were derived from variable distances down dip but from similar crustal depths such an exhumation pattern can explain the similar peak pressures for the greater himalayan sequence along the length of the himalayas

in situ measurement of hafnium isotopes in rutile by la mc icpms protocol and applications article i nfo this work presents a rigorous assessment of the accuracy and precision with which 176 hf 177 hf can be measured in rutile by laser ablation la mc icpms for rutile with 40 ppm hf we demonstrate that 176 hf 177 hf can be measured accurately and reproducibly with adequate precision for application to petrological problems we present an analytical and data reduction protocol tailored to the specific challenges of measuring hf isotope ratios in this low hf mineral precision and accuracy are optimised by interpolating between baselines measured every 10 analyses for many rutiles the advantages of determining the hf mass bias coefficient hf during analysis are negated by the low precision with which it can be measured at low hf contents hf mass bias is therefore monitored by regular analysis of a synthetic rutile doped with 5000 ppm hf sr 2 to facilitate an external mass bias correction because rutile contains negligible yb the yb mass bias coefficient yb must be inferred from yb hf measured on zircon in the same session to account for a systematic instrument bias of 0 5 1 ehf units rutile analyses are normalised to sr 2 for which 176 hf 177 hf has

slow exhumation of uhp terranes titanite and rutile ages of the western gneiss region norway abstract u pb ages of titanite and rutile were obtained from the central western gneiss region norway to assess the style and timing of exhumation and cooling of the western gneiss uhp terrane approximately half of the titanite ages are concordant the majority of which yield a limited age range from 393 to 390 ma the other titanite data are discordant and define discordia arrays with upper intercept ages of either 938 ma or 1 6 ga and a lower intercept of 389 ma concordant rutile analyses range from 385 to 392 ma both titanite and rutile ages young wnw toward the core of the orogen and are 4 ma older than 40 ar 39 ar muscovite ages corresponding to a cooling rate of 90 c ma a well defined boundary between concordant and discordant titanite ages in combination with the wnw increasing p t gradient and the similarity between muscovite cooling ages in the east and eclogite ages in the west suggests that the wgr remained coherent throughout its exhumation history and was progressively unroofed from east to west a 390 2 0 8 ma titanite in the soroyane uhp domain indicates that exhumation occurred at a vertical rate of 7 mm yr for 12 ma these rates are slower than estimates from smaller uhp terranes but similar to other large uhp terranes suggesting that there may be fundamental differences in the mechanisms controlling the evolution of large uhp terranes that undergo protracted subduction and exhumation and smaller uhp terranes that undergo rapid subduction and exhumation

hydro geochemical processes in an area with saline groundwater in lower shire river valley malawi an integrated application of hierarchical cluster and principal component analyses abstract our ability to adapt to changes in groundwater quality arising from a changing climate and or local pressures is dependent on comprehension of the governing controls of spatial variation in groundwater chemistry this paper presents results of an assessment of dominant hydro geochemical processes controlling groundwater chemical composition using an integrated application of hierarchical cluster analysis hca and principal component analysis pca of a major ion dataset of groundwater from lower shire river valley malawi the area is in the southernmost part of the western section of the east african rift system ears and has localised occurrence of saline groundwater hca classified samples into three main clusters c1 c3 according to their dominant chemical composition c1 dominant composition na cl median tds 3436 mg l 1 c2 dominant composition na hco 3 median tds 966 mg l 1 and c3 dominant composition alkali earths hco 3 median tds 528 mg l 1 these clusters were in turn described by the principal components pc1 pc3 and pc2 respectively resulting from the pca the results of the pca and geochemical interpretation suggest that the spatial variation of groundwater quality in the area is influenced by the following processes c3 samples result mainly from h 2 co 3 weathering of aluminosilicate minerals by percolating water supersaturated with co 2 in addition to aluminosilicate weathering c2 samples are influenced by the processes of cation exchange of ca 2 and mg 2 in the water for na on clay minerals and carbonate precipitation the increase in ionic strength of c2 samples is attributed to mixing with high tds groundwater in proximity with c2 samples the saline brackish c1 groundwater results from the processes of evaporation for samples with high water table close to the shire marshes and dissolution of cl and so 4 evaporative salts followed by mineralised seep from sedimentary karoo and cretaceous lupata sandstones

crustal evolution of peri gondwana crust into present day europe the serbo macedonian and rhodope massifs as a case study abstract the continental crust of europe west of the trans european suture zone tesz is mostly a collage of peri gondwana cadomian and avalonian terranes that were formed on the late neoproterozoic active margins of the supercontinent rifted from it during consecutive opening of oceanic basins and variably involved in ordovician carboniferous variscan and cretaceous eocene alpine orogenies while in many localities it is possible to study isolated portions of the european geologic history few sites can hold evidence for the entire record in a relatively small area the balkan terranes of the serbo macedonian smm and the rhodope massifs which contain neoproterozoic cadomian remnant basement and were involved in multiple phanerozoic phases of magmatism and metamorphism provide an ideal locality in which we can study the entire crustal evolutionary history of europe west of the tesz here we present new zircon u pb hf and rutile u pb data from basement rocks of the smm and rhodope as well as of beach placers from mouths of the large rivers draining the balkan towards the aegean sea strymon nestos and evros rivers basement igneous rocks of the smm and circum rhodope were dated to the late neoproterozoic 588 ma middle ordovician ca 460 ma carboniferous 300 ma permo triassic 254 ma paleocene 65 ma and miocene ca 23 ma demonstrating the episodic nature of crustal evolution in this terrane assemblage hf in zircon shows these intrusions evolve from negative ehf t values in the ordovician towards positive values in the paleocene zircons in beach placer samples also show distinct age peaks that follow the pulses of igneous activity in the source terranes with the addition of jurassic ca 150 ma zircons the u pb system in rutiles from the beach placer samples was mostly reset and records the final metamorphic phase during alpine orogeny 35 ma with some rutiles preserving eo alpine 80 ma ages in contrast to the igneous samples detrital zircons from the beach placer samples follow a trend bound by evolution lines leading to hf tdm model ages between 0 6 and 1 6 ga as a whole since the late neoproterozoic the balkan crust was the subject of repeated reworking in a series of magmatic orogenic events some extent of juvenile igneous addition on this evolving cadomian substrate brings about the shallow rise in ehf t values we observe in the igneous basement rocks the end result observed in the balkan is of a continental crust comprising a neoproterozoic peri gondwana cadomian core reworked in multiple episodes of phanerozoic orogenies and augmented to some extent by variable mantle derived input we argue that this mode of crustal origin and evolution is also applicable to many peri gondwana terranes of western europe

petrology and geochemistry of jurassic basalt dykes from vestfjella dronning maud land antarctica abstract mineral and whole rocks analyses of 12 jurassic basalt dykes from vestfjella dronning maud land antarctica are presented and their genesis discussed on the basis of major oxides and norms the basalts may be classified as olivine and quartz tholeiites plotted in the plag cpx opx 4q and ol plag q projections the compositions are most compatible with fractional crystallization of olivine clinopyroxene and plagioclase from a basalt liquid at very low pressure the ratios between strongly incompatible elements such as rb cs zr hf ta and th vary considerably and petrographic mixing calculations give poor fits with respect to rb cs ta th and light ree initial 87 sr 86 sr ratios range between 0 70347 and 0 70687 and show no correlation with rb sr or any other sie ratios the trace element and sr isotope data thus do not suggest any simple cogenetic petrogenetic model it is concluded that the basalt melts most plausibly have been contaminated by or mixed with anatectic melts of crustal material rather than reflecting mantle heterogeneity

pb diffusion in rutile diffusion of pb was measured in natural and synthetic rutile under dry 1 atmosphere conditions using mixtures of pb titanate or pb sulfide and tio2 as the sources of diffusant pb depth profiles were then measured with rutherford backscattering spectrometry rbs over the temperature range 700 1100 c the following arrhenius relation was obtained for the synthetic rutile d 3 9 10 10exp 250 12 kj mol 1 rt m2s 1 results for diffusion in natural and synthetic rutile were quite similar despite significant differences in trace element compositions mean closure temperatures calculated from the diffusion parameters are around 600 c for rutile grains of 100 m size this is about 100 c higher than rutile closure temperature determinations from past field based studies suggesting that rutile is more resistant to pb loss through volume diffusion than previously thought

melting of the shallow upper mantle a new perspective detailed examination of liquidus phase relationships in binary and ternary joins of the cfmas cr system has permitted a rigorous determination of the dry melting path of an initially fertile spinel peridotite composition resembling bulk silicate earth or morb pyrolite it is demonstrated that it is impossible to model mantle melting accurately using only one set of ratios of phases entering the melt this implies that the melting process is primarily controlled by solid solution rather than eutectic behaviour the proportions of phases entering a melt depend on whether a phase reacts and or disappears from a system and on the choice of the initial and final peridotite compositions four discrete domains in the melting regime of upper mantle peridotites are distinguished each characterized by different phase melting coefficients relating to the melting of 1 lherzolites 2 clinopyroxene bearing harzburgites i e free clinopyroxene 3 clinopyroxene saturated harzburgites i e clinopyroxene in solid solution in orthopyroxene and 4 clinopyroxene free harzburgites i e no clinopyroxene the proposed non linear fashion in which mantle lithologies melt explains the inadequacy of all previous models to reproduce the observed compositions of upper mantle peridotite melting residues it is suggested that 1 olivine and orthopyroxene will melt cotectically 2 clinopyroxene and spinel will lose most of their aluminous component after 8 melting within the first 4 kb 12 km of ascent from the dry solidus and that 3 clinopyroxene will disappear completely from a morb pyrolite mantle after 42 melting although such a number is significantly higher than that dictated by the position of the clinopyroxene out curves from peridotite isobaric equilibrium melting experiments 22 it is emphasized that the latter are a gross oversimplification of the natural melting process and are not equivalent to melting during adiabatic upwelling it is concluded that the commonly postulated disappearance of clinopyroxene from fertile peridotite compositions at 22 melting is greatly in error if melting in an adiabatically rising mantle is considered thus providing an explanation for many unsuccessful attempts by various authors to model the behaviour of transition elements in sub oceanic and supra subduction zone mantle and derivative magmas

exhumation of early tertiary coesite bearing eclogites from the pakistan himalaya early tertiary coesite bearing eclogites crop out in the internal zones of the indian plate in the kaghan valley of north pakistan field and geochronological data suggest that uhp metamorphism at c 46 ma accompanied subduction of the leading edge of continental india beneath kohistan a new u pb rutile age of 44 1 1 1 ma can be interpreted as either a growth or a cooling age given the uncertainty of the closure temperature of rutile for pb but clearly documents a very short lived metamorphic peak with cooling occurring between 44 and 40 ma eclogitic assemblages are preserved in permo triassic mafic rocks in both cover and basement sequences with the best preserved eclogites hosted in the anhydrous basement the eclogite facies rocks lie on the hanging wall of a major south directed ductile thrust zone which placed basement rocks that contain coesite eclogites southward onto lower pressure cover metasediments shear criteria show that those sequences that contain eclogite facies rocks are flanked above by a zone of pervasive north directed extensional shearing mineral assemblages contained within both the thrust and extensional shear zones are consistent with them both having operated synchronously during the amphibolite to greenschist facies transition amphibole ar ar cooling history data suggest that this was at c 40 42 ma as in the alps this implies exhumation from the metamorphic peak to greenschist facies conditions within a few million years this short time scale has important implications as it is likely that both the thrust and extensional shear zones represent the late stage of a deformation continuum that commenced at uhp and brought the uhp rocks back to mid crustal levels this late eocene extensional shearing and associated exhumation of hp or uhp rocks has not previously been recorded in the pakistan himalaya the thrusts and shears were later folded by major east trending folds themselves cut by miocene aged north vergent extensional faults that currently separate the indian plate from the structurally overlying kohistan island arc

la icp ms u pb dating of detrital rutile and zircon from the reynolds range a window into the palaeoproterozoic tectonosedimentary evolution of the north australian craton abstract the palaeoproterozoic reynolds range of the arunta region central australia comprises a series of shallow marine clastic sediments reynolds range group which overlies deeper water sequences of turbidites lander rock formation and minor sandstones unnamed sandstone u pb age data collected from detrital rutile and zircon in these rocks indicates these sequences contain very similar age spectra although with a notable and important shift to younger ages within the stratigraphically younger reynolds range group detrital zircons from the unnamed sandstone directly underlying the unconformity with the reynolds range group contain a major age component at ca 1860 ma together with a wide spread of ages between ca 3 2 and 2 0 ga and a minor age component of 1830 1805 ma detrital rutile from the same rock yield a unimodal age spectrum with a mean age of ca 1840 ma the detrital zircon age spectrum from the reynolds range group is similar although the youngest cluster of ages is younger and dates to between 1800 ma and 1780 ma detrital rutile from these rocks yield a unimodal age spectrum with a mean age of ca 1790 ma although the observed differences in ages are subtle we suggest these nevertheless mark a significant change in provenance we attribute the predominance of 1860 1820 ma detrital zircon and 1840 ma detrital rutile within the unnamed sandstone and the characteristic ca 1860 ma detrital zircon age cluster in previously published provenance studies from the lander rock formation to uplift and erosion of the halls creek pine creek orogen this orogenic belt marks collision between the north australian and kimberley cratons an event dated to between 1835 ma and 1810 ma and which incorporates marginally older rocks dating to 1860 ma from the foreland of the kimberley craton by contrast we attribute the influx of younger rutile and zircon together with a distinct facies change observed with the onset of the deposition of the reynolds range group to a shift in provenance region to the yapungku orogen which marks the collision between the west australian and north australian cratons and is dated to ca 1800 1765 ma a change in provenance is additionally consistent with significant differences in rutile trace element composition the lander rock formation and the reynolds range group are stratigraphically correlated with sedimentary rocks that overly large parts of the north australian craton we suggest this indicates spatially continuous basinal conditions within the north australian craton between ca 1840 ma and 1780 ma although the erosion and drainage systems feeding this basin were strongly influenced by the collision and orogenesis along the northwest and southwest margins of the north australian craton

early carboniferous back arc rifting related magmatism in southern tibet implications for the history of the lhasa terrane separation from gondwana

terrane boundaries within trans hudson orogen quebec baffin segment canada changing structural and metamorphic character from foreland to hinterland within the trans hudson orogen in northern quebec and southern baffin island parautochthonous archean basement and paleoproterozoic cover as well as allochthonous accreted paleoproterozoic units are exposed in a series of antiformal culminations and complementary synclinoria the parautochthonous rocks and two assemblages of accreted paleoproterozoic units crustal terranes are separated by major tectonic boundaries and associated deformation zones field petrological and isotopic data document an increase in the width of the deformation zones and a concomitant increase in p t conditions of metamorphism from orogenic foreland to hinterland tectonostratigraphic and geochronological criteria allow footwall and hangingwall units to be consistently distinguished in spite of the changing structural and metamorphic aspect of the terrane boundaries across the width of the orogen

sims u pb rutile age of low temperature eclogites from southwestern chinese tianshan nw china article i nfo the geological evolution in the southwestern chinese tianshan orogen in nw china is poorly understood because of controversial geochronological results of the low temperature eclogites it is likely due to sm nd and rb sr isotopic disequilibrium in different mineral fractions excess 40 ar in the high pressure k bearing minerals and multi stage growths of zircon in u pb system rutile as a typical eclogite facies mineral with high u pb ratio is a feasible candidate to constrain the eclogite facies metamorphic event in this study we measured u pb rutile age data using sims cameca ims 1280 for low temperature eclogites from southwestern chinese tianshan orogen nw china we demonstrate that sims is a powerful technique for in situ u pb dating of rutile with u content as low as 1 ppm sims u pb rutile dating yielded an age of 318 7 ma which may record the time when the rocks cooled down to the closure temperature of pb in rutile ca 500 c but should be very close to the time of eclogite facies metamorphism in terms of common fast exhumation of coesite bearing eclogite this new proof strongly suggests a late carboniferous collision between kazakhstan yili block and central tianshan microcontinent

channel flow ductile extrusion and exhumation in continental collision zones this collection of 27 review and research papers provides an overview of the geodynamic concepts of channel flow and ductile extrusion in continental collision zones the focal point for this volume is the proposal that the middle or lower crust acts as a ductile partially molten channel flowing out from beneath areas of over thickened crust such as the tibetan plateau towards the topographic surface at plateau margins this controversial proposal explains many features related to the geodynamic evolution of the plateau and for example extrusion and exhumation of the crystalline core of the himalayan mountain chain to the south in this volume thermal mechanical models for channel flow extrusion and exhumation are presented and geological and geophysical evidence both for and against the applicability of such models to the himalayan tibetan plateau system as well as older continental collision zones such as the hellenides the appalachians and the canadian cordillera are discussed

spatial and temporal trends in exhumation of the eastern himalaya and syntaxis as determined from a multitechnique detrital thermochronological study of the bengal fan the bengal fan provides a neogene record of eastern and central himalaya exhumation we provide the first detrital thermochronological study apatite and rutile u pb mica ar ar zircon fission track of sediment samples collected during international ocean discovery program iodp expedition 354 to the mid bengal fan our data from rutile and zircon fission track thermochronometry show a shift in lag times over the interval 5 59 3 47 ma the oldest sample with a lag time of 6 m y has a depositional age of 5 59 4 50 ma and the zircon and rutile populations then show a static peak until 12 ma this interval from 5 59 4 50 ma to 12 ma is most easily interpreted as recording passive erosion of the greater himalaya however single grains with lag times of 4 m y but with high analytical uncertainty are recorded over this interval for sediments older than 10 ma these grains were derived from the greater himalaya which was exhuming rapidly until ca 14 ma in sediments younger than 10 ma these grains could represent slower yet still rapid exhumation of the syntaxial antiform to the south of the massif lag times 1 m y are again recorded from 14 5 ma to the base of the studied section at 17 ma reflecting a period of greater himalayan rapid exhumation mica 40ar 39ar and apatite u pb data are not sensitive to syntaxial exhumation we ascribe this to the paucity of white mica in syntaxial lithologies and to high levels of common pb resulting in u pb ages associated with unacceptably high uncertainties respectively

sediment alteration and clay mineral diagenesis in a regional ground water flow system mississippi gulf coastal plain abstract high na hco3 pore fluids present in many areas of the shallow gulf coastal plain are the product of pervasive meteoric diagenesis of siliciclastic host sediments this diagenesis is driven in part by the subsurface generation of co2 and the reaction of carbonic acid with na bearing silicates while a significant body of water analyses exists there has been comparatively little study of the mineralogy and petrography of the altered sediments themselves an analysis of sediment samples from shallow boreholes and wells along a 100 km hydrologic flow path in upper cretaceous sands and clays in northeastern mississippi reveals the presence of spatially systematic variations in sediment alteration and clay mineral diagenesis kaolinite is the dominant clay mineral phase in the updip upgradient portion of the flow system and has apparently been produced in part by the in situ breakdown of feldspars the relative proportion of kaolinite progressively decreases downgradient and smectite becomes the dominant clay mineral in the distal portions of the flow system smectite is most abundant where meteoric waters are mixing with high nacl fluids migrating up from depth siderite is a common authigenic cement in the updip portions of the system and pyrite is present in some downgradient samples the results of the study are thus in general agreement with the earlier work of lee 1985 who inferred from systematic variations in the chemistry of waters along this same flow path that mineral diagenesis involving aluminosilicates and fe bearing phases exerts an important control on the geochemical evolution of these fluids of interest in deeper basinal studies is the role that pervasive shallow diagenetic precipitation of carbonate cements and authigenic clays such as we have documented may play in the subsequent porosity and permeability development of gulf coast sediments during deep burial