depositional age and sediment source of the auriferous moeda formation quadrilatero ferrifero of minas gerais brazil new constraints from u pb hf isotopes in zircon and xenotime abstract the moeda formation is a metasedimentary clastic sequence of witwatersrand like metaconglomerate and quartzite at the base of the minas supergroup quadrilatero ferrifero of minas gerais brazil which is also known for its world class iron ore deposits the moeda formation can be subdivided into three units units i to iii from bottom to top with each unit consisting of metaconglomerates at their base mostly auriferous grading into quartzite and locally into phyllite this study presents the first combined u pb and lu hf isotope data from detrital zircons and xenotimes of auriferous metaconglomerates and quartzites from units ii and iii these data indicate that deposition of sediments occurred 2 68 ga for unit ii and 2 62 ga for unit iii as supported by the youngest magmatic zircon grains the data further provide evidence that the source area of sediments was affected by crust re working during two major magmatic metamorphic events at 2 89 2 87 and 2 72 2 69 ga and minor events between 3 3 and 3 0 and at 2 80 and 2 65 ga this interpretation is supported by mainly subchondritic hf t between 10 and 1 4 and large hf t variations including all magmatic events a few superchondritic hf t obtained for the magmatic events at 3 3 2 9 and 2 7 ga point to limited juvenile crust addition the new dataset indicates that the clastic sediments of the auriferous moeda formation were mainly derived from an evolved continental crust in contrast to primitive magmatic arc sources which are suggested for the mesoarchaean auriferous sediments of the witwatersrand basin of south africa and those of the eastern goldfield province of australia

late paleoproterozoic geon 18 and 17 reactivation of the neoarchean great lakes tectonic zone northern michigan usa evidence from kinematic analysis thermobarometry and 40ar 39ar geochronology abstract the neoarchean great lakes tectonic zone in the midcontinent of north america exerted a strong control during the proterozoic stabilization of the superior craton in northern michigan this zone underlies the paleoproterozoic marquette synclinorium a penokean compressional feature that was reactivated during post penokean orogenic collapse and gneiss dome formation early north side up motion along steep reverse faults under lower greenschist facies conditions was related to basin inversion of the marquette trough sediments during penokean orogenesis in the western portion of the marquette synclinorium these early penokean fabrics are overprinted by a high grade event recorded by synkinematic mineral growth in high grade shear zones these late shear zones accommodate s side up motion with archean basement uplifted relative to proterozoic supracrustal sequences activity related to the emplacement of the ca 1800 ma humboldt granite metamorphic amphiboles from the center of a circular pattern of metamorphic isograds the republic node record plateau ages of 1730 ma with progressively older ages across the marquette synclinorium away from the central metamorphic high model archean ages are preserved in the most distal amphibole samples north of the marquette syncline reflecting the preservation of amphibole crystallization in the biotite zone mica ages record complete resetting and a similar age progression with the youngest ages 1715 ma in the central metamorphic high and older ages 1806 ma towards the margin thermobarometry indicates peak temperatures of 620 c for the central sillimanite zone decreasing to 500 c for the garnet zone and maximum pressures of 300 400 mpa gasp and grips assemblages collectively these characteristics are interpreted to reflect two separate reactivation episodes of the gltz during penokean accretion marquette syncline formation and post penokean formation of granite cored gneiss domes collapse

mesoarchean assembly and stabilization of the eastern kaapvaal craton a structural thermochronological perspective 1 understanding the construction and stabilization of archean continental lithosphere has important implications for models of tectonic and thermal regimes in the early earth as well as for the subsequent evolution of continents we provide new constraints for the amalgamation and stabilization of the eastern kaapvaal craton in the vicinity of the barberton greenstone belt circa 3 3 3 1 ga isotope dilution thermal ionization mass spectrometry u pb geochronology and thermochronology are combined with mapping and structural analysis around the margins of this belt in order to constrain movement on major shear zones that separate high grade orthogneiss terranes from low grade supracrustal rocks and to relate this movement to the timing of differential exhumation new and existing data are consistent with accretion at 3 23 ga within an oblique subduction plate boundary manifested in the bgb as an asymmetric flower like structural geometry for 100 ma following terrane assembly transform boundary tectonics manifested partially as transtension on reactivated faults led to large scale juxtaposition of middle to lower crustal basement orthogneiss complexes against upper crustal greenstone sequences and episodic emplacement of granitic batholiths stabilization of this portion of archean lithosphere involved at least a three stage process 1 creation of a thick and rigid mantle lithosphere during crustal growth subduction and terrane assembly from circa 3 30 to 3 23 ga 2 generation of a rigid crust during strike slip and transtensional tectonics coupled with migration of granites and heat producing elements into the upper crust between circa 3 2 and 3 1 ga and 3 erosion and removal of the uppermost crust during regional peneplanation that lasted until circa 2 9 ga

celebrating the centenary of the geology of central minas gerais brazil an insight from the sitio largo amphibolite abstractone century ago in 1915 harder and chamberlin established the stratigraphy on which the geology of the quadrilatero ferrifero of minas gerais is based one of their key observations was the occurrence of metavolcanic rocks in spatial association with the itabira iron formation an observation that has been ignored by most authors crucial for the understanding of the geology of the quadrilatero ferrifero but equally ignored is a 400 m thick unit of amphibolite that rests immediately on the itabira iron formation also known as the caue itabirite here we revitalize the sitio largo amphibolite by indicating its basaltic affiliation we present new results of u pb dating of zircon and rutile that show that the amphibolite protolith which covered an erosional surface on the caue itabirite is ca 2 18 ga old and underwent metamorphic overprint at ca 500 ma

experimental zircon melt and zircon garnet trace element partitioning and implications for the geochronology of crustal rocks garnet is the most commonly used mineral in thermobarometry whereas zircon is the most robust chronometer to date high grade metamorphic rocks to provide a basis for correlation of zircon and garnet growth we determined experimentally the trace element partitioningbetweenzircon ahydrousgraniticmeltandgarnetat20kbarand800 1000 cforp y rareearthelements ree zr hf th andu inrespecttomelt zirconpreferentiallyincorporatesallinvestigatedelementsapartfromreewithatomicnumberlowerthansm at this pressure and in the chosen composition the distribution coefficient between zircon and melt d zrn melt for ree increases with increasingatomicnumberofthereeandwithdecreasingtemperature dyb is 20at1000 c butmorethananorderofmagnitude higher at800 c the solubilityofzrinhydrousgraniticmelts buffered by zirconisabouta factoroftwo lower at20kbar than what has been previously established for mid crustal pressures large garnet produced in the experiments allowed determination of garnet melt trace element partitioning d grt melt at temperatures of 800 1000 c conditions relevant for partial melting of crustal rocks there is a systematic increase indree with decreasing temperature zircon contains significantly more heavy ree than garnet at temperatures of 800 850 c zircon garnetpartitioncoefficientsofheavy reedecreasewithincreasingtemperaturefromdlu of12at800 cto 1 4at1000 c middle reepartitioningisclosetounityforthewholeinvestigatedtemperaturerange differentempiricallydetermined zircon garnet partition coefficients from granulites and ultra high temperature granulites can potentially be explained by the experimentallydeterminedchangeofpartitioningasafunctionoftemperature thesedatacanassistinestablishingequilibriumbetween garnetandzirconzonesinnaturalrocks andintheconstructionofdetailedpressure temperature timepathsinhigh grademetamorphic rocks

recycling of foliations during folding bedding on the limbs of early formed regional folds generally lies oblique to the major directions of bulk shortening in the crust that is the horizontal and vertical during subsequent deformations matrix foliations on at least one limb of these folds may start to form but then be destroyed by reactivation of the bedding causing decrenulation and rotation into parallelism with the compositional layering consequently the schistosity parallel to bedding s0 s1 in multiply deformed rocks contains the relics of many deformation events and the 2 or 3 oblique foliations record only the very youngest history this lengthy early history is preserved as inclusion trails within porphyroblasts recycling of foliations depends on the shear sense acting on any newly developing foliation and the orientation of this foliation relative to s0 s1 for some orientations and combinations of shear senses both limbs of a pre existing fold can be reactivated from the commencement of a new deformation event this can result in the decrenulation and obliteration of a new foliation that is beginning to form before it shows any significant degree of development for other combinations one limb of a pre existing fold will behave in this manner whereas a new foliation does develop fully on the other limb however subsequent phases of deformation switch which limb shears versus develops an oblique new cleavage rotating the earlier formed oblique foliation into parallelism with s0 s1

archean magmatic hydrothermal fluid evolution in the quadrilatero ferrifero se brazil documented by b isotopes la mc icpms in tourmaline abstract in the archean quadrilatero ferrifero district se brazil tourmaline occurs as a major constituent in a leucogranitic intrusion and numerous pegmatitic aplitic veins within magmatic basement complexes as well as in quartzo feldspathic veins in quartz tourmaline rocks tourmalinites and as disseminated grains in the surrounding greenstone belt metasediments the chemical and boron isotope composition of these tourmalines was analysed by electron microprobe and la mc icp ms to determine the origin of the fluids involved and to shed light on the hydrothermal evolution of the region the tourmalines exhibit an overall decrease in fe fe mg ratio and a net increase in cr up to 0 75 wt from tourmaline hosted in the leucogranite the quartzo feldspathic veins the tourmalinites to the disseminated grains in the schists these variations mirror the bulk composition of the host schists and illustrate a strong protolith control on tourmaline major element composition the full range of tourmaline 11b is from 27 1 to 9 2 with a major cluster between 12 and 19 which includes the magmatic tourmaline in the leucogranite 15 2 to 12 5 most of these isotope compositions can be reconciled with a model involving tourmaline growth from late stage exsolved magmatic fluids percolating through the magmatic basement and into the nearby metasediments this model agrees well with the trends of major element compositions as well as with the critical observation that tourmaline occurrence is restricted to the vicinity of the basement complexes 11b values lower than 19 in our dataset are beyond the reach of isotope fractionation during magmatic fluid exsolution and temperature decrease and suggest the presence of a distinct fluid component we propose that an isotopically light fluid was released from mica breakdown in the country rocks during local upper amphibolite facies metamorphism in the dome border shear zone

proterozoic metamorphism and cooling in the southern lake superior region north america and its bearing on crustal evolution abstract metamorphism along the southern margin of the archean superior province has been historically attributed to the penokean orogeny a narrow corridor of amphibolite facies rocks north of the main suture does record 1 83 1 80 ga metamorphic monazite u th pb ages that mark the culmination of arc accretion however subsequent widespread amphibolite facies metamorphism and associated magmatism is recorded along the regions of greatest penokean crustal thickening the tectonically buried archean proterozoic continental margin in minnesota new monazite geochronology reveals a profound midcrustal metamorphic imprint caused by emplacement of the 1 775 ga east central minnesota batholith at moderate depths in northern wisconsin and upper peninsula michigan metamorphic monazite growth at 1 78 1 745 ga and far from geon 17 intrusions reflect a previously little recognized regional amphibolite facies metamorphic event associated with ca 1 76 ga yavapai interval accretion not solely penokean induced crustal collapse south of the penokean suture penokean terrane rocks were twice metamorphosed to upper greenschist facies first during yavapai accretion and again during geon 16 mazatzal accretion geon 16 overprinting also affected a small part of the continental margin in the northeast orogen the peavy metamorphic node south directed basement thrusts there likely accommodated substantial mazatzal foreland shortening suggesting thick skinned deformation mazatzal amphibolite facies metamorphism occurred throughout iowa and southernmost wisconsin south of the baraboo quartzite 40 ar 39 ar mineral cooling ages from eastern wisconsin reveal a limited metamorphic aureole associated with the intrusion of the 1 47 ga wolf river batholith in part reflecting its rapid emplacement at shallow crustal levels a local area of anomalously young 40 ar 39 ar mica ages away from known midcontinent rift exposures probably reflect shallow crustal level reheating during rifting not exhumation the pattern and degree of proterozoic metamorphism preserved across the northern interior of the north american midcontinent is highly variable and is a reflection of multiple accretionary and intrusive events between 1 87 and 1 47 ga collectively the geo and thermochronologic data reveal a progressive tectonic younging toward the south and overall waning of proterozoic metamorphism in this region these results demonstrate a changing tectonic environment from active margin to continental interior as laurentia grew southward and stabilized over a few hundred million years

quantifying rates of dome and keel formation in the barberton granitoid greenstone belt south africa abstract the barberton granitoid greenstone belt is a classic dome and keel province characterized by kilometer scale gneiss domes and elongate keels of largely folded supracrustal rocks combined u pb shrimp data and structural mapping demonstrate that the geometry of the barberton belt reflects events that occurred over 30 million year interval from ca 3230 and 3203 ma early deformation with nw se shortening in the upper crust was accompanied by emplacement of tonalite trondhjemite granodiorite ttg magmas at 3234 12 and 3226 9 ma much of the structural grain of the greenstone belt relates to a long episode of post orogenic extension with ne directed extension in the lower crust leading to exhumation of high grade gneisses in the southern barberton terrane advective heat transfer during emplacement of kilometer scale ttg plutons around the margins of the greenstone belt facilitated the infolding of the relatively denser and colder greenstone sequence the end of this tectonic cycle is punctuated by the emplacement of the undeformed 3203 7 ma dalmein pluton which sharply truncates not only anticlines and synclines in the greenstone belt but also the dominant fabric in the high grade gneisses

permo triassic changes in bulk crustal shortening direction during deformation and metamorphism of the taebaeksan basin south korea using foliation intersection inflection axes implications for tectonic movement at the eastern margin of eurasia during the songrim indosinian orogeny abstract the permo triassic songrim indosinian orogeny in south korea was a major tectonic event involving complicated continental collisions at the eastern margin of eurasia previous studies have examined the structural and metamorphic features of the songrim orogeny in each of the paleozoic terranes of the orogenic belt i e the taebaeksan basin the okcheon basin and the imjingang belt but correlations of these features among the terranes remain uncertain the aim of this paper is to reveal deformation history including bulk crustal shortening directions in the taebaeksan basin and to correlate the tectono metamorphic evolution of the taebaeksan basin with other phanerozoic mobile belts in eastern asia based on a combined analysis of foliation intersection inflection axes fia trends and metamorphic p t and t t time paths the orientations and relative timing of fia preserved as inclusion trails within porphyroblasts of andalusite chloritoid garnet and staurolite reveal two age groups of inclusion trails in the pyeongan supergroup at the northeastern margin of the taebaeksan basin these microstructures indicate the development of early nnw nne trending structures and fabrics followed by later e w trending ones these observations suggest a change in the orientation of bulk crustal shortening from e w to n s during the songrim orogeny based on the similar microstructures and temperature time paths of the three paleozoic terranes we interpret that the e w bulk crustal shortening influenced the eastern part of the korean peninsula during the early stages of the songrim orogeny presumably related to amalgamation between the proto japan terrane and the eastern margin of eurasia whereas the n s bulk crustal shortening was stronger in the western part of the peninsula during the later stages of the orogeny related to collision between the south and north china blocks

paleoproterozoic basement cover infolding and thick skinned thrusting in hearne domain nunavut canada intracratonic response to trans hudson orogen enveloped by orogenic belts during the assembly of laurentia 2 0 1 7 ga the rae and hearne domains of the western churchill province experienced a complex sequence of tectonic events historically referred to as the hudsonian orogeny in hearne domain diverse thick skinned intraplate structures developed in response to accretionary processes in trans hudson orogen these structures include infolds of archean basement and paleoproterozoic cover and broken foreland thrusts in central hearne domain an intracratonic basin succession of continental to shallow marine strata hurwitz group 2 45 1 91 ga is exposed in outliers that constitute the keels of doubly plunging basement cover synclinoria defining a fold train with wavelengths of 35 45 km with the exception of local out of syncline shear zones basement and hurwitz group are welded formed under greenschist facies conditions the basement cover infolds are open concentric and commonly display complex structures in their interiors in the poorfish windy outlier southwestern hearne domain continental rift deposits of the kiyuk group unconformably overlie the hurwitz group in contrast to basement cover infolding to the north and east archean basement the hurwitz group and the kiyuk group are imbricated by thick skinned northwest vergent thrusts and folds thrusting in the poorfish windy belt was completed by 1815 10 ma as indicated by new u pb data from hydrothermal metamorphic titanite recovered from recrystallized cements at the top of the kiyuk group we interpret that shortening from shallow level basement cover infolding is balanced at depth by intrabasement faulting and distributed ductile strain and that the geometric link between central hearne basement cover infolding and poorfish windy thrusting is via intrabasement faults that cut to the surface the main controls on thick skinned deformation were the high competency of the lower hurwitz group previous thermal weakening basement heterogeneities and possibly shallow subduction preceding terminal collision in trans hudson orogen 2002 elsevier science b v all rights reserved

anatomy of zircons from an ultrahot orogen the amalgamation of the north china craton within the supercontinent columbia abstract we report shrimp u pb rare earth element hf isotope and laser raman spectroscopic data on zircons from ultrahigh temperature uht granulites from the jining complex of the khondalite belt in the western block of the north china craton ncc these uht rocks form part of an ultrahot orogen that formed along the collisional margin of the ncc associated with the tectonics of assembly of the paleoproterozoic supercontinent columbia despite the core rim textures displayed by some of the uht zircons their age values sharply converge within error yielding weighted mean 207pb 206pb ages of ca 1 92 ga indicating growth by recrystallization under extreme thermal conditions in general the zircons show moderate heavy rare earth element enrichment with sharp positive ce and negative eu anomalies the hf isotope data from the uht zircons also display a fairly uniform character with the majority of them characterized by positive ehf values without any indication of mixing between reworked crust and

hydrography above the mid atlantic ridge 33 40 n and within the lucky strike segment as part of the french american ridge atlantic program the french american zero angle photon spectrometer and rocks fazar cruise conducted water column studies between 33 and 40 n along the mid atlantic ridge mar to detect hydrothermal activity and map its influence this paper describes the large scale hydrography within the axial valley with particular emphasis on the hydrothermally active lucky strike segment 37 17 n the fazar study area is affected by the presence of the azores current and mediterranean water mw although the mw core has been mapped as far north as 50 n off the ridge the northern boundary of the mw within the mar in the fazar study area exists as a strong front south of the azores platform this front is most likely caused by the shallower ridge crest becoming a physical barrier to the mw the lucky strike segment lies within this front and as a result has complicated hydrography which can obscure hydrothermal temperature and salinity anomalies

smaller gold deposits in the archean rio das velhas greenstone belt quadrilatero ferrifero brazil abstract the quadrilatero ferrifero is a classic gold province on a global scale with hundreds of individual gold occurrences in the archean greenstone belt comprising the rio das velhas supergroup there are numerous small gold deposits including faria esperanca iii bicalho bela fama juca vieira brumal boa vista fernandes moita roca grande bico de pedra and pari as well as the world class deposits of morro velho and cuiaba all these deposits whether large or small are structurally controlled and related to either shear zones or folds extensive down plunge continuity is a consistent feature of the deposits in the rio das velhas greenstone belt six main styles of gold mineralization are recognized these are deposits hosted within 1 lapa seca e g morro velho bicalho bela fama 2 banded iron formations e g cuiaba sao bento raposos faria brumal roca grande 3 quartz veins e g juca vieira fernandes 4 disseminated sulfides with quartz veinlets e g moita 5 amphibolites e g pari and 6 disseminated to massive base metal sulfides e g bico de pedra the first four types of deposits are epigentic orogenic gold deposits similar to those found in greenstone belts worldwide the last two are unusual types of gold deposit peculiar to the quadrilatero ferrifero bico de pedra is a polymetallic au ag zn pb cu deposit related to an aplite intrusion whereas pari is a stratiform au bearing banded iron formation metamorphosed to epidote amphibolite metamorphic facies

porphyroblast rotation versus nonrotation conflict resolution data on foliation intersection and or inflection axes preserved in porphyroblasts fias indicate that no porphyroblast rotation occurs during ductile deformation relative to spatial coordinates this contrasts with 99 of investigations of rigid objects in non coaxially deforming media where the objects rotate when anastomosing shear zone formation around relatively strong objects in a weaker matrix is modeled no porphyroblast rotation occurs formation of these anastomosing zones controls the development of this phenomenon called gyrostasis if such zones are absent porphyroblasts rotate in weak materials the gyrostatic situation arises because the superposition of simple shearing deformation normal to initial coaxial shortening results in only small rotations of principal axes of stress since shear zones are controlled by the orientations of principal axes of stress initial anastomosing zones retain their orientations and positions during subsequent non coaxial deformation the porphyroblast is isolated from the embedding non coaxially deforming matrix but this material close to the porphyroblast continues to deform coaxially no local rotation occurs this has major tectonic significance because allowing for the effects of rotation due to brittle deformation porphyroblasts can now be routinely used to access lengthy structural and or metamorphic histories destroyed in the matrix by reactivation such as movement directions shear senses and extended pressure temperature time paths

gold deposit formation during collapse tectonics structural mineralogical geochronological and fluid inclusion constraints in the ouro preto gold mines quadril atero ferrifero brazil the gold deposits of the ouro preto area minas gerais brazil occur along the main tectonic contact responsible for the thrusting of the paleoproterozoic units minas supergroup onto the gneissic and metasedimentary archean units rio das velhas supergroup during the brasiliano ca 600 ma tectonic event these deposits are claimed to be the most complex of the quadrilatero ferrifero with orebodies lying parallel to the thrust related stretching lineation based on structural mineralogical geochronological and fluid inclusion constraints our study demonstrates that the gold bearing quartz veins were formed in response to late orogenic collapse tectonics that led in this area to gravity backsliding of the nappes during these movements sigmoidal voids were created and filled by elongate quartz grains associated with sulfides tourmaline and carbonates inferred hydrothermal effects were responsible for the formation of illite sulfides carbonate garnet and green biotite within the matrix large arsenopyrites within the veins were affected by three stages of fracturing during which the metallic phases and associated gold were deposited gold wherever it is in contact with bi related minerals occurs in the form of electrum maldonite or aurostibite fluid inclusion studies show that this event was associated with the entrapment of both co2 rich and h2o salt rich fluids the process was initiated after the brasiliano thrusting event ca 500 c 8 kbars and in p t conditions close to 300 to 400 c and 2 kbars an age of ca 485 to 490 ma has been obtained by 40ar 39ar dating of hydrothermal biotite and muscovite this study demonstrates that the ouro preto gold deposits were formed in a context of late orogenic collapse tectonics drastically different from the conventional auriferous shear zone model this could account for their relative paucity compared to other mesothermal gold deposits

insights into orogenic processes from drab schists and minor intrusions southern sao francisco craton brazil abstract minor altered intrusions and drab retrogressed schists can easily be overlooked in geological studies but this contribution explores these rocks within the archaean and palaeoproterozoic southern sao francisco craton ssfc brazil using geological relationships and accessory mineral in situ analyses in the context of cratonic assembly three magmatic pulses are documented i archaean and ii palaeoproterozoic felsic intrusions both hosted by archaean protoliths and iii palaeoproterozoic felsic intrusions in palaeoproterozoic supracrustal sequences archaean felsic intrusions confirm the palaeoarchaean age of the mafic ultramafic sequence of the rio das velhas greenstone belt and rhyacian intrusions mark the collisional stage of the mineiro belt with the sfc at c 2130 ma greenstone belt schists show a wide distribution of rounded soccer ball archaean detrital and metamorphic zircon grains ranging in age from 3200 to 2750 ma with an interpreted overprinting high grade metamorphic event at c 2700 to 2680 ma most high grade metamorphic rims have th u 0 1 negative ehf t values and ree pattern consistent with eclogite granulite metamorphic facies reinforcing the hypothesis of a dehydrated refractory crust formed during the stabilization of the ssfc even though no such protoliths are preserved this event links crustal thickening and partial melting of archaean lower crust archaean rutile crystals from the greenstone belt schist were reset during the palaeoproterozoic event but still preserve the early archaean high grade metamorphic signature the presence of unstable ilmenite replaced by rutile in the schist associated to felsic intrusions with the same age at c 2130 ma suggest high pressure low temperature prograde metamorphism during the collisional stage of the palaeoproterozoic orogen elongate and prismatic zircon grains from the rhyacian intrusions have low ehf t signature and crystallised from partial melting of sedimentary protoliths accretionary events produced thicker and more differentiated crust by the end of rhyacian time easily overlooked rocks in this study when studied have revealed a rich multi event history of cratonic evolution

zr and hf diffusion in rutile abstract chemical diffusion of zr and hf under anhydrous conditions has been measured in synthetic and natural rutile the sources of diffusant used were hafnon or zircon powders or a hafnon rutile mixture experiments were run in crimped pt capsules in air or in sealed silica glass capsules with solid buffers to buffer at nno or qfm rutherford backscattering spectrometry rbs was used to measure zr and hf diffusion profiles from these measurements the following arrhenius relations were obtained for zr diffusion parallel to c over the temperature range 750 1100 c d zr c 9 8 10 15 exp 170 30 kj mol 1 rt m 2 s 1 for hf diffusion parallel to c over the temperature range 800 1000 c d hf c 9 1 10 15 exp 169 36 kj mol 1 rt m 2 s 1 for hf diffusion normal to c over the temperature range 750 1050 c d hf c 2 5 10 12 exp 227 62 kj mol 1 rt m 2 s 1 diffusivities for experiments buffered at qfm and nno are similar to those run in air diffusivities in synthetic and natural rutile are likewise similar indicating that these findings can be applied directly in determining zr diffusivities in rutile in natural systems these data indicate that rutile should be moderately retentive of zr chemical signatures with zr diffusivities within an order of magnitude of those for pb in rutile over most geologic conditions when applied in evaluation of the relative robustness of the recently developed zr in rutile geothermometer t zack r moraes a kronz temperature dependence of zr in rutile empirical calibration of a rutile thermometer contributions to mineralogy and petrology 148 2004 471 488 e b watson d a wark j b thomas crystallization thermometers for zircon and rutile contributions to mineralogy and petrology 151 2006 413 433 these findings suggest that zr concentrations in rutile will be somewhat more likely to be affected by later thermal disturbance than the geothermometer based on zr concentrations in titanite l hayden e b watson d a wark a thermobarometer for sphene abstract 16th v m goldschmidt conference 2006 and much less resistant to diffusional alteration subsequent to crystallization than the ti in zircon geothermometer e b watson d a wark j b thomas crystallization thermometers for zircon and rutile contributions to mineralogy and petrology 151 2006 413 433 e b watson t m harrison zircon thermometer reveals minimum melting conditions on earliest earth science 308 2005 841 844

circulation and nutrients in middepth atlantic waters isopycnal analyses of distributions of salinity oxygen apparent oxygen utilization nitrate and silica in the depth range 800 3000 m in the north and tropical atlantic ocean are carried out using data from the transient tracers in the oceans meteor cruise 56 leg 5 atlantis ii cruise 109 legs 1 and 3 and the geosecs atlantic study the results are summarized in isopycnal distribution maps and property property plots the layer between 800 and 1500 m is oxygen poor and nutrient rich and is poorly ventilated below this is a better aerated layer with the deep western boundary current part of which separates along the equator regeneration of nutrients and consumption of oxygen persist into the deep ocean in regions off the west coast of africa the mediterranean outflow water apparently is causing significant cross isopycnal mixing through salt fingering the resultant cross isopycnal velocity may be large enough to cause significant vorticity stretching

zircon u pb and lu hf record from high grade complexes within the mantiqueira complex first evidence of juvenile crustal input at 2 4 2 2 ga and implications for the palaeoproterozoic evolution of the sao francisco craton abstract the mantiqueira complex together with the mineiro belt and juiz de fora complex characterize a long lived system of oceanic and continental magmatic arcs the proposed accretion of these terranes in the rhyacian resulted in the closure of the minas basin a fundamental turning point for the geology of this area understanding this event is critical for determining the evolution of the south american continent despite its importance there is a paucity of zircon u pb and lu hf data available for the mantiqueira complex and no data from two complexes within the later u pb zircon data were obtained from interpreted metasediments of those two complexes the granulite facies acaiaca complex and pedra dourada granulite four targeted samples yield cores with ages between 3 5 and 2 2 ga and a large proportion of metamorphic grains and rims at ca 2 1 2 0 ga the similarity between the detrital zircon signatures of the acaiaca complex and pedra dourada granulite and the archaean inheritance of the mantiqueira complex indicates that these three units are related potentially the acaiaca complex and pedra dourada granulite either sourced the mantiqueira complex or the mantiqueira intrusive suites reworked the protoliths of the acaiaca complex and pedra dourada granulite hf in zircon data indicate two discrete sources for both the acaiaca complex and pedra dourada granulite an archaean source model ages of 3 2 2 6 ga which is consistent with the suggested origin of the mantiqueira complex as recycled archaean crust sourced from the sao francisco craton and reworked during the ca 2 1 ga transamazonian orogeny a second more juvenile source model ages of 2 4 2 2 ga recorded exclusively within the more mafic samples is identical in age and hf to that found in the juiz de fora complex this could indicate a close link between the mantiqueira and juiz de fora complexes in the period 2 2 to 2 0 ga alternatively these juvenile zircons may represent an as yet unidentified juvenile portion of the mantiqueira arc the results of this study are consistent with the prevailing theories for formation of the mantiqueira complex as a continental arc with cumulative proportion curves indicating an active margin type depositional setting

the drawbacks of sectioning rocks relative to fabric orientations in the matrix a case study from the robertson river metamorphics northern queensland australia understanding the relationships of inclusion trail geometries in porphyroblasts relative to matrix foliations is vital for unravelling complex deformation and metamorphic histories in highly tectonized terranes and the approach used to thin sectioning rocks is critically important for this two approaches have been used by structural and metamorphic geologists one is based on fabric orientations with sections cut perpendicular to the foliation both parallel p and normal n to the lineation whereas the other uses geographic orientations and a series of vertical thin sections studies using p and n sections reveal a simple history in comparison with studies using multiple vertical thin sections the reason for this is that inclusion trails exiting the porphyroblasts into the strain shadows in p and n sections commonly appear continuous with the matrix foliation whereas multiple vertical thin sections with different strikes reveal that they are actually truncated such truncations or textural unconformities are apparent from microstructures textural relationships compositional variations and fia foliation intersection axis trends a succession of four fia trends from ene wsw e w n s to ne sw in the robertson river metamorphics northern queensland australia suggests that these truncations were formed because of the overprint of successive generations of orthogonal foliations preserved within porphyroblasts by growth during multiple deformation events at least four periods of orogenesis involving multiple phases of porphyroblast growth can be delineated instead of just the one previously suggested from an n and p section approach

age of felsic volcanism and the role of ancient continental crust in the evolution of the neoarchean rio das velhas greenstone belt quadrilatero ferrifero brazil u pb zircon dating of volcaniclastic graywackes abstract a felsic eruptive event within the rio das velhas greenstone belt is mainly represented by volcaniclastic and resedimented volcaniclastic rocks besides rare lava flows of dacitic composition the volcaniclastic rocks comprise four lithofacies 1 monomictic and polymictic breccias 2 conglomerate graywacke 3 graywacke sandstone and 4 graywacke argillite shrimp and id tims u pb zircon dating of three graywacke samples dated the eruptive event at 2792 11 2773 7 and 2751 9 ma indicating a range of about 40 ma for the felsic magmatism this time range may also encompass the whole magmatic activity within the belt because of the association of the felsic rocks with the more extensive basaltic flows volcaniclastic graywackes are characterized by an abundance of detritic xenocrystic zircon grains displaying a polymodal age spectrum ranging from 2 8 to 3 4 ga older zircon grains must be derived from magma contamination and or post eruptive sedimentary reworking and mixing with felsic detritus from other sources it may be concluded that felsic magmas were in contact with older granitoid crust and that this basement was also exposed as source for detritic material in the quadrilatero ferrifero the ancient complexes of ttg gneisses and migmatites make up most of the exposed archean terrains and its present juxtaposition with the supracrustal assemblage suggests an authochthonous to parauthocthonous development for the rio das velhas greenstone belt in a continental or near continental tectonic setting

the application of zircon cathodoluminescence imaging th u pb chemistry and u pb ages in interpreting discrete magmatic and high grade metamorphic events in the north china craton at the archean proterozoic boundary abstract two distinct late neoarchean to early paleoproterozoic magmatic events are recognized at the northern margin of the eastern block of the north china craton based on interpretation of zircon structure cathodoluminescence signature th u pb chemistry and age the first event is represented by emplacement of the chaihe trondhjemite and xianjinchang tonalite with magmatic zircon core ages ranging from 2559 2553 7 ma respectively these rocks experienced a subsequent high grade metamorphic event constrained by partial recrystallization of primary magmatic zircon and growth of new zircon rims between 2517 and 2490 ma a second magmatic episode generated by partial melting of crustal rocks during this metamorphic event is represented by the hongshilazi syenogranite with an age of 2502 11 ma the short time interval between initial magmatism and high grade regional metamorphism signifies they were related to the same major tectonothermal event when considered along with evidence of a similar sequence of events at this time 800 km to the south in eastern hebei and the previously established 2500 ma age of granulite facies metamorphism in mafic rocks across the craton it is proposed that a large mantle plume developed beneath the north china craton at the close of the archean

extensional v contractional origin for the southern menderes shear zone sw turkey tectonic and metamorphic implications the southern menderes massif in southwest turkey consists mainly of orthogneisses and overlying palaeozoic middle paleocene schists and marbles the contact between the two distinct rock types is almost everywhere structural herein named the southern menderes shear zone a s facing high angle ductile shear zone that separates metamorphic rocks of differing grade although there is a consensus that the shear zone was associated with top to the s ssw shearing and is of tertiary age its origin and nature have been highly debated over the last decade some claim the contact is a thrust fault while others have argued for an extensional shear zone integration of field and microstructural data the identification of different fabrics associated kinematics and overprinting relationships with fission track thermochronology and the p t paths of the rocks above and below the shear zone supports the conclusion that the southern menderes shear zone is an extensional shear zone and not a thrust the data are consistent with a model that the exhumation and cooling of the southern menderes massif occurred after a period of extensional deformation pervasive top to the n nne high temperature medium pressure ductile shear structures d 2 deformation overprint an early hp event d 1 deformation the subsequent top to the s ssw greenschist shear band foliation d 3 deformation developed mostly around the orthogniess schist contact and forms the most characteristic features of the massif the top to the n nne structures are attributed to the main alpine constructional deformation that developed during back thrusting of the lycian nappes during the latest palaeogene collision between the sakarya continent and the anatolide tauride platform across the neotethyan ocean the top to the s ssw structures are interpreted to be the result of the exhumation of the massif during the activity of the southern menderes shear zone the presence of these two distinct fabrics with differing kinematics suggests that the southern menderes shear zone operated as a top to the n nne thrust fault during early alpine contractional deformation but was later reactivated with an opposite sense of movement top to the s ssw during subsequent oligocene miocene extensional collapse

conductive incubation and the origin of dome and keel structure in archean granite greenstone terrains a model based on the eastern pilbara craton western australia 1 the archean east pilbara granite greenstone terrane epggt of the pilbara craton in western australia preserves a prolonged record of voluminous mafic dominated volcanism and felsic plutonism commencing at circa 3515 ma and ultimately resulting in the development of spectacular dome and keel structures early crustal growth was dominated by basaltic magmatism and the regional development of a 12 18 km thick greenstone sequence by circa 3335 ma overlying widespread 3470 3430 ma tonalite trondhjemite granodiorite ttg suites emplaced into the middle crust stratigraphic and structural relationships imply that voluminous granitoid plutonism and crustal scale dome and keel formation in the southeastern epggt was not initiated until circa 3325 ma but proceeded rapidly over the interval 3325 3308 ma this scenario is consistent with a conductive incubation period of 10 100 myr duration following the burial of radiogenic granitic crust beneath the accumulated greenstone pile for heat production parameters appropriate to the 3515 3308 ma epggt we show that the burial of 3470 3430 ma granitoid rich crust beneath the 3335 ma euro basalt and its precursors led to long term crustal temperature increases of 170 234 c at 35 km depth with a corresponding reduction in effective viscosity of 2 5 orders of magnitude in combination with the greenstone over granitoid density inversion the changes in effective viscosity of the mid to deep crust due to the burial of heat sources may have provided the crucial trigger for the initiation of dome and keel formation similarly cooling and strengthening of the crust during syndoming exhumation of the heat production potentially terminated large scale dome and keel amplification

hydrothermal anomalies in the lucky strike segment on the mid atlantic ridge 37 17 n abstract as part of the fara french american ridge atlantic project the northern mid atlantic ridge mar was surveyed for hydrothermal activity between 33 and 40 n during the fazar cruise in august october 1992 after detection of hydrothermal activity in the lucky strike segment 37 17 n a detailed water column survey was undertaken in this region during a follow up cruise with the dsrv alvin several active high temperature vents were found on the summit of an central seamount here we focus on the results of the water column survey undertaken during the fazar cruise the influence of mediterranean water and the site s shallow depth complicates the hydrography at lucky strike background temperature and salinity gradients are usually too complex to indicate hydrothermal anomalies clearly light transmission anomalies within the lucky strike region were small but were detected throughout the segment from mid depth 1500 m all the way to the valley seafloor 2300 m depth the large depth range covered by these anomalies probably results from vertical mixing as well as the presence of multiple vent sources at various depths methane anomalies and a temperature anomaly observed deeper than the vents documented by alvin further support the idea of additional deeper vents within this segment particle samples of the deep particle layer are enriched in fe and trace metals

intense mixing of lower thermocline water on the crest of the mid atlantic ridge the mixing of warm upper ocean water with the colder water beneath is an essential component of global ocean circulation because it increases the buoyancy of deep water but it is not clear where this mixing takes place louis st laurent and andreas thurnherr have measured the mixing that occurs as water flows through a narrow passage on the crest of the mid atlantic ridge the rates they observe are unusually high if similar mixing rates occur in the other narrow passages on the mid atlantic ridge in the north atlantic basin these sites could account for as much buoyancy flux at the depth of the passages as the rest of the basin combined so mixing in narrow passages on mid ocean ridges sites that have previously been overlooked may make a significant contribution to buoyancy flux at the global scale observations from the crest of the mid atlantic ridge suggest that passages in rift valleys and ridge flank canyons provide the most energetic sites for oceanic turbulence measurements show that large diffusivities characterize the mixing downstream of a sill in a well stratified boundary layer with mixing levels remaining of the order of 10 4m2 s 1 at the base of the main thermocline buoyancy exchange between the deep and the upper ocean which is essential for maintaining global ocean circulation mainly occurs through turbulent mixing1 2 this mixing is thought to result primarily from instability of the oceanic internal wave field3 but internal waves tend to radiate energy away from the regions in which they are generated rather than dissipate it locally as turbulence4 and the resulting distribution of turbulent mixing remains unknown another more direct mixing mechanism involves the generation of turbulence as strong flows pass through narrow passages in topography but the amount of turbulence generated at such locations remains poorly quantified owing to a lack of direct measurements here we present observations from the crest of the mid atlantic ridge in the subtropical north atlantic ocean that suggest that passages in rift valleys and ridge flank canyons provide the most energetic sites for oceanic turbulence our measurements show that diffusivities as large as 0 03 m2 s 1 characterize the mixing downstream of a sill in a well stratified boundary layer with mixing levels remaining of the order of 10 4 m2 s 1 at the base of the main thermocline these mixing rates are significantly higher than the diffusivities of the order of 10 5 m2 s 1 that characterize much of the global thermocline and the abyssal ocean5 our estimates suggest that overflows associated with narrow passages on the mid atlantic ridge in the north atlantic ocean produce as much buoyancy flux as has previously been estimated for the entire romanche fracture zone6 7 a large strait in the mid atlantic ridge that connects the north and south atlantic basins this flux is equivalent to the interior mixing that occurs in the entire north atlantic basin at the depth of the passages suggesting that turbulence generated in narrow passages on mid ocean ridges may be important for buoyancy flux at the global scale

flow and mixing in the rift valley of the mid atlantic ridge high levels of diapycnal mixing and geothermal heating near midocean ridges contribute to the buoyancy fluxes that are required to close the global circulation in topographically confined areas such as the deep median valleys of slow spreading ridges these fluxes strongly influence the local hydrography and dynamics data from a segment scale hydrographic survey of the rift valley of the mid atlantic ridge and from an array of current meters deployed there during an entire year are analyzed in order to characterize the dominant hydrographic patterns and dynamical processes comparison with historic hydrographic data indicates that the temporal variability during the last few decades has been small compared to the observed segment scale gradients the rift valley circulation is characterized by inflow from the eastern ridge flank and persistent unidirectional alongsegment flow into a cul de sac therefore most of the water flowing along the rift valley upwells within the segment with a mean vertical velocity 1025 ms 21 the observed streamwise hydrographic gradients indicate that diapycnal mixing dominates the rift valley buoyancy fluxes by more than an order of magnitude in spite of the presence of a large hydrothermal vent field supplying several gigawatts of heat to the water column hydrographic budgets in the rift valley yield diffusivity values of order 5 3 1023 m2 s 21 consistent with estimates derived from statically unstable overturns the largest of which were observed downstream of topographic obstacles in the path of the along segment flow this suggests vertical shear associated with cross sill flows as the dominant contributor to the mechanical mixing in the rift valley

a model of an atlantic and pacific hydrothermal plume a turbulent entrainment model of a hydrothermal plume was modified to include the effects of vertical structure in the background temperature and salinity fields owing to the stably stratified salinity field in the pacific relatively warm salty water spreads at an equilibrium level where the plume density equals the ambient density in contrast the unstable salinity profile in the atlantic gives the plume added buoyancy and it continues to rise to an equilibrium level where it is relatively cool and fresh these attributes result from the mixing between the plume and the surrounding seawater at each level below the spreading layer the equilibrium temperature anomaly is not directly proportional to the source strength but depends on the mixing and background salinity gradient the net entrainment of surrounding seawater is greater in the atlantic plume model than in the pacific plume model for the same buoyancy flux recent data from the tag hydrothermal field at 26 n on the mid atlantic ridge and data from a vent field on the endeavor segment of the juan de fuca ridge are compared to model results the model predicts the penetration height and s characteristics of the atlantic plume and it explains some of the differences between the observed atlantic and pacific hydrothermal plumes

paleo to mesoarchean polymetamorphism in the barberton granite greenstone belt south africa constraints from u pb monazite and lu hf garnet geochronology on the tectonic processes that shaped the belt discussion cutts et al 2014 1 combined thermodynamic modeling of compositional change in metamorphic garnet with metamorphic peak and retrograde pressure temperature p t data derived from quantitative p t pseudosections to determine clockwise p t paths for several samples from the tjakastad

the ancestry and magmatic evolution of archaean ttg rocks of the quadrilatero ferrifero province southeast brazil abstract combined la icp ms and shrimp u pb ages from quadrilatero ferrifero qf province reveals for the first time three main periods of ttg magmatism in the southern part of the sao francisco craton brazil these periods described here as the santa barbara sb rio das velhas i rvi and rio das velhas ii rvii events embody a significant part of a protracted tectonomagmatic history of the craton spanning from 3220 to 2770 ma the initial stages of ttg magmatism the sb event led to formation of a core of palaeoarchaean ttg crust via magmatic additions of juvenile felsic rocks into mafic ultramafic rocks at ca 3212 3210 ma the following event rvi event saw the growth of this palaeoarchaean core into a more composite polydefomed continental segment through magmatic additions of juvenile ttg rocks and tectonic accretion of mafic ultramafic greenstone belt terrains our u pb data and regional constraints suggest that much of the ttg crust exposed in the qf region today was produced during the rvi event between 2930 and 2900 ma the final stages of ttg crust production rv ii event were marked by convergence related magmatism affecting an extensive palaeo to mesoarchaean continental block the rvi block the timing and duration of the latest ttg event coincides with the felsic volcanism and deposition of turbiditic wackes of the main greenstone belt sequence

regional shortening followed by channel flow induced collapse a new mechanism for dome and keel geometries in neoarchaean granite greenstone terrains the lateral flow and extrusive exhumation of ductile migmatitic gneisses due to a horizontal gradient in lithostatic pressure a process termed channel flow or lateral protrusion has previously been proposed as an important tectonic process in large hot orogens centrifuge simulation of i layer parallel shortening followed by ii collapse of a cover sequence during ductile flow of underlying layers in an analogous manner to channel flow suggests a new mechanism for the development of structures within neoarchaean granite greenstone terrains the centrifuge model incorporates an upper package of silicone modelling clay microlaminates that simulate a greenstone sequence that overlies slightly less dense ductile silicone putties whose rheological properties simulate migmatitic felsic gneiss a low viscosity low density layer along half of the infrastructure superstructure interface simulates the presence of granitoid melt during initial layer parallel shortening upright folds form in the upper greenstone package and the interface with underlying ductile layers is folded in basal silicone layers recumbent to overturned non cylindrical and upright folds form with increasing distance from the moving end wall removal of material parallel to fold hinges at the foreland end of the model i e furthest from the ram used to shorten models in several stages simulates erosion and the difference in gravitational loading thus created induces ductile flow and lateral extrusion of basal silicone layers models aim to reproduce features comparable to those developed during extrusive channel flow and focused exhumation of basement migmatitic gneiss in nature early formed recumbent to inclined folds are then accentuated during simulated channel flow while new recumbent isoclinal folds in basal layers develop broad antiforms and tight synforms similar to the dome and keel geometry that typifies many archaean granite greenstone belts are produced as a late feature in the model channel flow and collapse of a fold thickened crust is therefore proposed as a potential alternative mechanism for the formation of structures in some neoarchaean granite greenstone terrains by analogy with other physical and numerical models channel flow in the neoarchaean may be enhanced by impingement of an upper mantle wedge into the base of the crust our results imply that both lithospheric shortening and non diapiric gravitational instabilities may be responsible for the formation of some archaean dome and keel structures and may account for the juxtaposition of some granite greenstone and high grade gneiss terrains in the archaean model results also show similarities to structures produced during lateral flow and withdrawal of salt in fold belts

currents fronts and eddy fluxes in the canary basin hydrographic data from two cruises in the canary basin meteor 57 july 1981 poseidon 86 april 1982 are analysed with respect to current distribution and lateral heat flux in the azores madeira region the first part of the data base consists of long transects of xbt and g e k measurements between cape finisterre north west spain and the northern canary basin where several year long current meter records exist further information is obtained by thermosalinograph surface data and by expendable current profilers xcp geostrophic currents are derived from xbt profiles using the tight temperature salinity relationship in the depth range of the warmwassersphare the results compare well with the g e k and xcp current observations the second part consists of ctd data from an eddy resolving box shaped ctd survey 500 500 km2 centered at the mooring location 33 n 22 w the observations are supplemented by satellite buoy trajectories horizontal parameter distribution is shown in terms of objectively contoured maps bands of spatially enhanced energetic structures seen in the long transects are further resolved by the box survey as a deep jet like current system cross the canary basin in a west east direction associated with this azores current is a frontal zone with near surface temperature and salinity steps of order 2 k and 0 3 practical salinity units the dynamic topography field can be decomposed into a linear background field a rossby wave and a mesoscale eddy field we find that major contributions to the meridional eddy heat flux are confined to the vicinity of the azores current frontal zone it is shown that the principal balance in the temperature equation is between heating by the mean horizontal advection terms and cooling by the eddy flux divergence

hydrography within the central and east basins of the bransfield strait antarctica abstract the hydrography in the central and east basins of the bransfield strait is examined using data collected along a track 340 km long through the strait in november 1995 in conjunction with historical data from the nodc database circumpolar deep water cdw was seen throughout most of the central basin and the western east basin in contrast to earlier studies that have shown cdw being absent or only weakly present in the strait in the western part of the central basin the cdw was present as a narrow band near deception island with strong thermal gradients at its southern boundary consistent with previous observations local deep water formation produces bottom waters in the central and east basins that are distinct from one another and distinct from the surrounding deep waters outside the strait the east basin has a more complex stratification than the central basin that must be the result of multiple sources a model presented here explains the deep water in the east basin as a mixture betwee

from thickening to extension in the variscan belt kinematic evidence from sardinia italy the variscan nappe stack of se sardinia originated as a result of several stages of nappe imbrication during the lower carboniferous phases of the variscan orogeny the crustal shortening caused regional ssw and w directed thrusting greenschist facies metamorphism and open to isoclinal polyphase folding the final stage of shortening produced large scale antiforms and synforms post collisional deformation resulted in inversion of earlier thrusts as normal faults development of low angle normal faults and refolding of earlier foliation and thrust planes by asymmetric folds with subhorizontal axial planes facing directions of these latest folds are directed horizontally outward from the hinge zones of main antiforms suggesting that they cannot be regarded as parasitic folds of the latest thickening phase but instead are the consequence of vertical shortening during gravitational collapse of dome like km scale antiforms leading to denudation of antiformal culminations

strong current events related to a subtropical front in the northeast atlantic long term temperature and current meter records from moorings in the northern canary basin display strong current events with time scales between one and three months and large vertical scales of several thousand meters the data are compared to hydrographic surveys in the area that show a meandering subtropical front the strong current events are found to be related to the passage of the front through the mooring positions an analysis of composite time series for selected depths indicates cases of westward and of eastward propagation of frontal meanders the frontal pattern is also found in geopotential anomalies inferred from historical xbt data sets suggesting that the front is a persistent feature of the density field in two cases strong current events appear to be related to a mediterranean water lens

transamazonian orogeny in the southern sao francisco craton region minas gerais brazil evidence for paleoproterozoic collision and collapse in the quadrilatero ferrifero abstract the paleoproterozoic transamazonian orogeny yielded a series of discrete orogens in brazil the present field study in the quadrilatero ferrifero qf indicates that the southern sao francisco craton region of the brazilian highlands preserves a portion of one of these orogens two sets of transamazonian structures occur in this region the first consists of northwest verging folds and thrusts affecting supracrustal sequences it is suggested that this set formed in a fold thrust belt setting shortly after 2 125 ga during the closure of a passive margin basin that had initiated along the margins of a preexisting continental mass at ca 2 5 ga the second set consists of structures defining the prominent dome and keel architecture of the qf this set a consequence of the emplacement of basement domes against supracrustal rocks at 2 095 ga may reflect the consequences of orogenic collapse narrow conglomerate filled intermontane basins may have formed coevally with dome emplacement formation of an ocean basin east of the present day sao francisco craton eventually occurred in late mesoproterozoic in effect the transamazonian orogen of the qf represents the collision and collapse stages of a paleoproterozoic wilson cycle the contractional phase of the transamazonian orogeny probably represents accretion of an offshore arc to the eastern and southeastern margin of the present day sao francisco craton region the arc and an associated suture may be traced in the brasiliano pan african orogen east of the sao francisco craton northwards into the northeastern lobe of the sao francisco craton clearly initial assembly of crustal blocks to form a larger continent involving south america occurred during the paleoproterozoic 2 1 ga post transamazonian rifting of this continent created the basins which were later inverted during the brasiliano assembly of gondwana

rhyacian evolution of subvolcanic and metasedimentary rocks of the southern segment of the mineiro belt sao francisco craton brazil abstract the tiradentes suite in conjunction with the coeval estacao tiradentes metasedimentary sequence makes up a distinct tectonic element within the paleoproterozoic framework of the mineiro accretionary belt in the southern sao francisco craton this suite is the second occurrence of volcanic and subvolcanic rocks in the mineiro belt and shows petrographic and geochemical similarities with the nearby serrinha suite 2 23 2 21 ga the first reported rhyacian volcanism in the mineiro accretionary belt the tiradentes suite is represented by mafic andesites dacites granophyres and tonalites and shows petrographic features of volcanic rocks such as spherulites amygdales granophyric intergrowths and porphyritic textures with plagioclase phenocrysts the rocks of this suite are metaluminous to peraluminous enriched in alkaline elements compatible with low al2o3 trondhjemites and show primary fractionation of plagioclase apatite hornblende biotite and ilmenite high hree content coupled with incipient negative eu anomaly and high y values are consistent with a garnet amphibolite protolith with plagioclase and hornblende as present in the residual phases the samples of the tiradentes suite show geochemical signatures of magmatic arcs which are also observed in rocks from the serrinha suite rocks of the tiradentes suite yield the following u pb zircon crystallization ages i mafic andesite 2217 23 ma ii dacitic dike 2204 11 ma iii subvolcanic tonalite 2213 9 ma the deposition of the estacao tiradentes metasedimentary sequence probably took place after ca 2088 12 ga although it may be as young as 1990 39 ma according to the u pb detrital zircon geochronology of selected samples diamictite ferruginous quartzite and carbonaceous phyllite the tiradentes rocks show three tdm model ages ranging between 2 4 and 2 3 ga indicating short crustal residence for the magmatic protoliths this inference is also supported by the nd 2 2 ga values 1 1 to 2 3 whereas the respective 87sr 86sr t ratios 0 7018 0 7032 are consistent with the involvement of low rb sr material in the magma genesis such as a paleoproterozoic oceanic lithosphere from a tectonic perspective the above data are consistent with the idea that a paleoproterozoic seafloor developed outboard of the archean siderian passive margin setting minas basin of the sao francisco proto craton the 2 23 2 21 ga volcanic subvolcanic rocks of the tiradentes and serrinha suites were associated with a rhyacian oceanic arc the estacao tiradentes metasedimentary sequence turbidites and distal phyllites probably formed in back arc setting concomitant with the tiradentes suite as indicated by detrital zircon geochronology

deformation style way back when thoughts on the contrasts between archean paleoproterozoic and contemporary orogens abstract in many important ways archean and paleoproterozoic older orogens differ structurally from contemporary examples this essay examines the premise that contrasts between older orogens and contemporary orogens reflect long term changes in the temperature of the continental crust in the density of supracrustal sections and in exhumation rates for example if continental crust were warmer and exhumation rates faster earlier in earth history then higher grade rocks would occur closer to the surface of older orogens and the orogens would be lower and wider this situation might contribute to the formation of wide belts of high grade gneiss found in ancient crust if the high strength layer of the crust were thinner and supracrustal sequences denser earlier in earth history then regional extensional tectonism might lead to crustal scale boudinage and diapirism this situation might explain formation of the extensive dome and keel provinces found in ancient crust testing such speculations through the application of structural analysis coupled with petrologic studies dating and rheological modelling will constrain models of earth s long term physical evolution

significance of dextral reactivation of an e w transfer fault in the formation of the pennsylvania orocline central appalachians 1 three hypotheses for orocline development including 1 primary plate boundary shape 2 arc rotation and 3 arc overprinting were evaluated for the formation of the pennsylvania orocline maryland usa using foliation intersection inflection axes preserved within porphyroblasts fia the distribution and the timing of sequences of fia data measured from garnet porphyroblasts within the loch raven formation are statistically the same across the pennsylvania orocline suggesting that all subregions of the arc have experienced the same deformation history this may suggest that the geometry of the paleocontinental margin controlled the basic shape of this orocline however if the orocline is treated as three separate regions defined by the sw we ne trending portions of its slightly staircase shaped outline a much higher proportion of the nne ssw trending fias set ii are preserved in the w e trending region than those to either side this geometric relationship could have resulted from wnw ese directed bulk stress causing a zone of dextral shear along what is now the w e trending portion of this orogen it appears that what could have been an early formed w e trending sinistral transform shear zone was preferentially dextrally reactivated during the taconian orogeny

reconstructed mediterranean salt lens trajectories the existence of energetic anticyclonic mid depth vortices of mediterranean water meddies questions the validity of a conventional advective diffusive balance in the eastern atlantic subtropical gyre a mesoscale experiment in the azores madeira region reveals a link of these meddies to large scale subsurface meanders for the first time it is shown that meddies may have strong surface vorticity indicative of a generation process involving the azores current a deep reaching near surface jet

hydrothermal vents near a mantle hot spot the lucky strike vent field at 37 n on the mid atlantic ridge the lucky strike hydrothermal field occurs in the summit basin of a large seamount that forms the shallow center of a 65 km long ridge segment near 37 n on the mid atlantic ridge the depth and chemistry of the ridge segment are influenced by the azores hot spot and this hydrothermal field is the first atlantic site found on crust that is dominated by a hot spot signature multiple hydrothermal vents occur over an area of at least 300 m by 700 m vent morphologies range from flanges and chimneys with temperatures of 200 212 c to black smoker chimneys with temperatures up to 333 c cooler fluids from northern vents have higher chlorinities and lower gas volumes while hotter southern fluids have chlorinities 20 below seawater with higher gas volumes suggesting phase separation has influenced their compositions all gas volumes in fluids are higher than those at tag and snake pit hydrothermal fields black smokers exhibit their typical mineralogy except that barite is a major mineral particularly at lower temperature sites which contrasts with previously investigated atlantic sites the fluid chemistry distribution of the relict sulfide deposits on the seamount summit in the areas investigated using dsv alvin and contact relationships between active vent sites and surrounding basaltic and sulfide substrate suggest that the hydrothermal system has a long history and may have recently been rejuvenated fauna at the lucky strike vent sites are dominated by a new species of mussel and include the first reported sea urchins the lucky strike biological community differs considerably from other vent fauna at the species level and appears to be a new biogeographic province the lucky strike field helps to constrain how variations in the basaltic substrate influence the composition of hydrothermal fluids and solids because basalt compositions at lucky strike are 10 30 times enriched in incompatible elements compared to other atlantic hydrothermal sites such as tag snake pit and broken spur the incompatible element

combining compositional zoning and foliation intersection axes fias in garnet to quantitatively determine early p t t paths in multiply deformed and metamorphosed schists north central massachusetts usa quantitative compositional and microstructural analysis of garnet porphyroblasts in kyanite staurolite garnet grade rocks from the northeastern flank of the pelham dome north central massachusetts distinguishes the effects of acadian deformation and metamorphism from extensive overprinting alleghanian shearing the p t conditions and the metamorphic path during the acadian were determined using samples preserving well defined stages in a lengthy tectonic history revealed by a succession of five foliation intersection axis trends preserved within porphyroblasts fias this acadian succession extends at least 120 km to the north into rocks where no evidence has been found of an alleghanian overprint for each sample where garnet first nucleated during one of these stages in the tectonic history the pt of core growth was determined by plotting the intersection of the mn fe and ca isopleths calculated for the core composition on a p t pseudosection for that sample using thermocalc combining the pt data from all these samples indicates that the temperature and pressure increased throughout acadian orogenesis causing episodic garnet growth during the alleghanian locally intense shearing especially against the margin of the pelham dome formed the dominant schistosity which truncated all foliations defined by inclusion trails in porphyroblasts and obliterated all remains of acadian deformation and metamorphism in the rock matrix shearing was accompanied by near complete homogenization of the compositional zoning in garnet porphyroblasts and an associated apparent increase in the temperature of the matrix to 700 c in those rocks lying directly adjacent to the pelham dome and resulted from the rocks of the northfield syncline being thrust a large distance southwards over the gneisses in the dome

growth of granite greenstone terranes at convergent margins and stabilization of archean cratons abstract archean granite greenstone terranes represent juvenile continental crust formed in a variety of plate tectonic settings and metamorphosed through a complex series of structural and magmatic events most archean granite greenstone terranes appear to have acquired their first order structural and metamorphic characteristics at convergent plate margins where large accretionary wedges similar in aspect to the chugach makran and altaids grew through offscraping and accretion of oceanic plateaux oceanic crustal fragments juvenile island arcs rifted continental margins and pelagic and terrigenous sediments buoyant slabs of parts of archean oceanic lithosphere may have been underplated beneath these orogens forming thick crustal roots characterized by interleaving between the depleted slabs and undepleted asthenosphere back stepping of the subduction zones after accretion of plateaux and arcs caused the arcs magmatic fronts to migrate trenchward through the accretionary wedges dehydration of the subducting slabs hydrated the mantle wedges below the new arcs and generated magmas sanukitoid suite in the mantle wedge whereas other magmas tonalite trondhjemite granodiorite or ttg suite appear to have been generated by melting of hot young subducted slabs eventual collision of these juvenile orogens with other continental blocks formed anatectic granites then thickened the crust beyond its ability to support its own mass which initiated gravitational collapse and decompressional release of syn to late tectonic granitoids from wedges of fertile mantle trapped between underplated oceanic lithospheric slabs and aided in the cratonization of the granite greenstone terranes deeply penetrating structural discontinuities such as shear zones and sutures provided pathways for fluids and granitoids to migrate into the mid and upper crust forming ore deposits and plutons most preserved granite greenstone terranes have been tectonically stable since the archean and form the cratonic interiors of many continents

proterozoic first order sedimentary sequences of the sao francisco craton eastern brazil abstract the sao francisco craton in eastern brazil hosts sedimentary sequences deposited between the paleoarchean 3300 ma and late neoproterozoic 580 ma proterozoic successions occurring in this region comprise five 1st order sedimentary sequences which besides episodes of global significance record major basin forming events the ca 8000 m thick minas itacolomi 1st order sequence exposed in the brazilian mining district of the quadrilatero ferrifero and containing as marker bed the lake superior type caue banded iron formation tracks the operation of a wilson cycle in the paleoproterozoic era the quartz arenite dominated espinhaco i and ii sequences record at least two major rift sag basin forming events which affected the host continent of the sao francisco craton at around 1 75 ga and 1 57 ga the macaubas sequence and its correlatives in the extracratonic domains witness the individualization of a sao francisco congo plate in synchronicity with the break up of rodinia in the cryogenian period the sao francisco congo plate together with various fragments derived from rodinia reassembled to form gondwana in the ediacaran period in the course of the gondwana amalgamation orogenic belts developed along the margins of the craton its interior converted into foreland basins received the shallow water carbonates and pelites of the bambui 1st order sequence and its correlatives

meandering of the subtropical front south east of the azores a subtropical front was observed in the area south and southeast of the azores during cruises of fs meteor and fs poseidon in early 1982 the front has a basically west east extension with considerable meandering observed meso scale eddies are found on both sides the overall flow pattern corresponds to earlier results on geopotential differences in the upper northeast atlantic but the baroclinic transport of the order of 107 m3 s 1 is found to be concentrated in a 60 km wide jet we suggest here that the current band is part of the gyre circulation resulting from a branching of the north atlantic current

paleoarchean partial convective overturn in the singhbhum craton eastern india abstract analyses of fold superposition structures and kinematics of shear zones along a 120 km long e w corridor across the singhbhum craton eastern india are combined with u pb th chemical ages of texturally constrained monazites in supracrustals gneisses and granitoids to reconstruct the paleoarchean tectonic history of the western part of the craton metamorphic sub greenschist facies shallow marine to platformal sediments of the iron ore group iog and the accordantly deformed jagnnathpur meta lavas were accreted onto a basement composed of 3 4 ga amphibolite facies para amphibolites quartzites muscovite schists older metamorphic group omg and anatectic older metamorphic tonalite gneisses omtg crustal shortening was followed by emplacement ascent of the singhbhum granitoid pluton sgp 3 2 3 3 ga synchronous with sinking of supracrustal screens along n nne trending steep dipping sinistral normal ductile shear zones deformation microstructures indicate the bulk of the pluton responded to exhumation strain by low t 500 c ductile flow of quartz and k feldspar with plagioclase being largely unaffected but granitoids along the western margin of pluton emplaced concurrent with westward retreating exhumation of the thickened crust was deformed at t 650 c since all granitoids in the pluton share the same n nne trending tectonic fabric microstructural features imply phased emplacement of the pluton during exhumation strain the transition from crustal shortening to exhumation interrupted by phased granitoid emplacement between 3 2 and 3 4 ga is explained by a two stage partial convective overturn model involving structures typical of supracrustal down granitoid up sense of movement the tectonic restoration in the western part of the singhbhum craton is new and complements growing evidence that accretion and convective overturn may have operated during the paleoarchean

an epigenetic origin for the passagem de mariana gold deposit quadrilatero ferrifero minas gerais brazil abstract the gold tourmaline quartz vein deposit of passagem de mariana in the southeastern part of the quadrilatero ferrifero produced more than 60 tonne of gold from the end of the 17th century until 1954 the mine has not operated since 1985 orebodies are veins composed of white quartz 60 vol carbonate ankerite tourmaline sericite and sulfides tourmaline dravite up to 10 vol of the vein occurs as subhedral coarse commonly zoned crystals and is concentrated along vein boundaries and on the edges of host rock inclusions in the veins tourmaline is present in all rock types in the mine but the chemical composition of the host rocks determinates the intensity of tourmalinization with the alteration being greater in sericitic phyllites graphite sericite phyllites and calcareous rocks the most abundant sulfide is arsenopyrite which is normally associated with pyrite and pyrrhotite minor amounts of chalcopyrite galena lollingite berthierite and maldonite are present throughout the deposit sulfides are concentrated at veins boundary or are dispersed in the veins arsenopyrite is associated most commonly with calcareous rocks and graphite sericite phyllite pyrrhotite is usually found at the base of itabirites gold abundance is directly proportional to sulfide concentration hydrothermal alteration associated with the veins includes silicification tourmalinization and sulfidation the mineralized zone is a shear zone associated with a bedding parallel thrust fault that juxtaposes the itabirite lower proterozoic minas supergroup over other units this shear zone thrust fault extends for tens of km beyond the passagem mine and hosts numerous gold deposits the richest orebodies are along the itabirite footwall contact and within the graphite sericite phyllite main orebody although many lithologic units were mineralized the graphite sericite phyllite appears to have been most favorable for gold deposition the area underwent three phases of deformation d1 d2 and d3 mineral assemblages indicate upper greenschist to lower amphibolite conditions of regional metamorphism retrograde metamorphism characterized by chloritization of biotite and chloritization and biotitization of garnet developed locally the gold bearing veins crosscut the main foliation and lithologic contacts at low angle and occur within or are in contact with all lithotypes field and laboratory data indicate that gold mineralization at passagem de mariana is epigenetic gold deposition occurred after the peak of metamorphism within the late to post d2 period of deformation which is correlated with second set of structures of trasamazonian age of alkmim and marshak alkmim f f marshak s 1998 transamazonian orogeny in the southern sao francisco craton region minas gerais brazil evidence for paleoproterozoic collision and collapse in the quadrilatero ferrifero precambrian research 90 29 58 indicating that the gold mineralization occurred between 2 124 and 2 04 ga we choose to regard passagem de marina as an orogenic gold deposit as defined by groves et al groves d i goldfarb r j gebre mariam m hagemann s g robert f 1998 orogenic gold deposits a proposed classification in the context of their crustal distribution and relationship to other gold deposit types ore geology reviews 13 7 27 i e an epigenetic structurally hosted lode gold vein system in a deformed metamorphic terrane

the tectonic evolution of the quadrilatero ferrifero minas gerais brazil abstract the structural framework of the quadrilatero ferrifero is the result of two main deformational events the first is an extensional event of palaeoproterozoic age between 2 100 and 1700 ma the transamazonian event which resulted in the formation of archaean granite gneiss domes and the nucleation of regional synclines in the overlying rio das velhas and minas supergroups strata such regional synclines are continuous with each other and are bordered by structural granite gneiss highs at the contact between these units ductile brittle to ductile extensional shear zones are developed representing the dislocation surfaces of archaean blocks this tectonism is probably related to the evolution of a metamorphic core complex formed in the hinterland to the west of a transamazonian collision zone which structured the paramirim craton the second event is compressive and associated with the closure of the pan african brasiliano proto ocean 650 500 ma situated to the east of quadrilatero ferrffero qf during this event a west verging fold and thrust belt ftb developed causing inversion amplification translation and rotation of the basinal synclines the ftb affected mainly the eastern portion of qf and obliterated many of the tectonic features of the extensional event

flow kinematics in the deeper part of a subduction channel as inferred from inclusion trails in plagioclase porphyroblasts from the sambagawa metamorphic rocks south west japan the subduction and exhumation of accretionary prism metasedimentary rocks are accompanied by large strain ductile deformations which may be recorded in microstructures porphyroblast microstruc tures have been a key to unravel the kinematics in such deformed belts shape preferred orientation spo of epidote and amphibole inclusions that define s shaped trails in prograde cores of plagioclase porphyroblasts were analysed from the high p t sambagawa metamorphic rocks inclusions are found to be elongate parallel to the 010 and 001 directions respectively and their long axis orientations define an internal foliation si best fit great circle and lineation li maximum on the si s shaped inclusion trails in the orthogonal sections do not exhibit the same geometries but rather are grouped into two types where the foliation intersection axes fias are nearly perpendicular and parallel to li respectively these two types of s shaped inclusion trails are seen in the sections inclined at low and high angles to the li respectively however the latter type commonly consists of composite trails where the si is first rotated about an fia perpendicular to the li i e unique axis and then about an fia parallel to the li the s shaped inclusion trails are interpreted to have formed by the successive overgrowth of matrix minerals and rotation of the plagioclase porphyroblast cores about a unique axis in non coaxial deformation the rotation of si about an fia nearly parallel to the li is perhaps an apparent rotation caused by the deflection of foliation around the growing prismatic plagioclase grain prior to inclusion into the porphyroblast this study has for the first time documented the 3 d geometry of s shaped inclusion trails in porphyroblasts from accretionary prism metasedimentary rocks and identified their origin which helps to understand the flow kinematics in the deeper part of a subduction channel

detection of hydrothermal plumes on the northern mid atlantic ridge results from optical measurements abstract several water column surveys conducted on the northern mid atlantic ridge have been instrumental in the discovery of three new hydrothermal venting sites and seven other active segments hydrothermal sources have yet to be pinpointed in six of these segments in this study the observed frequency of high temperature venting from 23 n to 41 n on the mid atlantic ridge mar a distance of approximately 1900 km is one venting system per 150 km of ridge combining these data with those of other investigators for the region from 12 n to 41 n leads to an approximate venting frequency of one site per 90 km of ridge although venting frequencies on sections of the mar within this region are as high as one site per 25 km to support this frequency of venting and considering the larger dimensions of the mar rift valley the ratio of axial to off axis heat loss must be higher on the mar than on faster spreading ridges

dome emplacement and formation of kilometre scale synclines in a granite greenstone terrain quadrilatero ferrifero southeastern brazil the quadrilatero ferrifero is a portion of the brazilian precambrian shield containing a granite greenstone terrain characterized by granite gneiss domes encircled by kilometre scale synclines integration of porphyroblast matrix microtextures c axis fabrics analysis and strain analysis with outcrop scale structural relationships indicates that the main phase of domes and synclines evolution occurred during the westerly directed 0 8 0 6 ga pan african brasiliano regional shortening this tectonic event has produced the single pervasive foliation observed in the folded supracrustal sequences of the quadrilatero ferrifero dip slip ductile shear zones developed at the dome supracrustal interface are synchronous with dome evolution during the pan african brasiliano deformation as indicated by contemporaneous growth of aureole porphyroblasts in the shear planes these shear zones facilitated steeply directed downward flow of country rock material around the granite gneiss domes the marked density contrast between granite gneiss material and the iron rich supracrustals provided a gravitationally favourable environment for downward wallrock flow variations in shear zone kinematics away from the domes combined with kilometer scale viscous drag contributed to produce kilometre scale synclines in the supracrustal rocks constrained between upward moving domes

the development of spiral shaped inclusion trails during multiple metamorphism and folding three periods of mineral growth and three generations of spiral shaped inclusion trails have been distinguished within folded rocks of the qinling dabie orogen china using the development of three successive and differently trending sets of foliation intersection axes preserved in porphyroblasts fias this progression is revealed by the consistent relative sequence of changes in fia trends from the core to rim of garnet porphyroblasts in samples with multiple fias the first and second formed sets of fias trend oblique to the axial planes of macroscopic folds that dominate the outcrop pattern in this region the porphyroblasts containing these fias grew prior to the development of the macroscopic folds yet the fias do not change orientation across the fold hinges the youngest formed fias set 3 lie subparallel to the axial planes of these folds and the porphyroblasts containing these fias formed in part as the folds developed the deformation associated with all three generations of spiral shaped inclusion trails in garnet porphyroblasts involved the formation of subhorizontal and subvertical foliations against porphyroblast rims accompanied by periods of garnet growth pervasive structures have not necessarily formed in the matrix away from the porphyroblasts the macroscopic folds are heterogeneously strained from limb to limb doubly plunging and have moderately dipping axial planes the consistent orientation of set 1 fias indicates that the development of spiral shaped inclusion trails in porphyroblasts with fias belonging to set 2 did not involve rotation of the previously formed porphyroblasts the consistent orientation of sets 1 and 2 fias indicate that the development of spiral shaped inclusion trails in porphyroblasts with fias belonging to set 3 did not involve rotation of the previously formed porphyroblasts during folding this requires a fold mechanism of progressive bulk inhomogeneous shortening and demonstrates that spiral shaped inclusion trails can form outside of shear zones

mantle 3he in hydrothermal vents and plume of the lucky strike site mar 37 17 n and associated geothermal heat flux abstract the helium isotopes composition of hydrothermal fluids at the lucky strike vent field has been determined by mass spectrometry 4he end member concentration is in the 1 2 10 5 2 6 10 5 cm3 stp g range and the 3he 4he ratio r is 8 13 0 05 ra the helium 3 budget over the active vent field was estimated from the 3he mapping of the plume advected along the rift valley using a 1 d advection box model constrained with current velocity measurements this leads to a 3he flux of 2 3 10 8 mol s the scaling of the heat flux from the lucky strike site to that of 3he using the 3he heat ratio measured in the hydrothermal fluids 5 2 10 18 7 7 10 18 mol j allows an initial evaluation of the total geothermal heat output from the site in the 3800 1200 mw range this flux of the same order as that of the endeavour segment juan de fuca ridge indicates the present high level of the hydrothermal activity in this section of the mid atlantic ridge

an integrated microtextural and chemical approach to zircon geochronology refining the archaean history of the napier complex east antarctica integrated textural and chemical characterisation of zircon is used to refine the u pb geochronology of the archaean ultra high temperature napier complex east antarctica scanning electron microscope characterisation of zircon and the rare earth element compositions of zircon garnet and orthopyroxene are integrated to place zircon growth in an assemblage context thereby providing tighter constraints on the timing of magmatic and metamorphic events data indicate that magmatism occurred in the central and northern napier complex at ca 2 990 ma a regional relatively low pressure metamorphic event occurred at ca 2 850 2 840 ma mineral ree data from garnet bearing orthogneiss indicate that ca 2 490 2 485 ma u pb zircon ages provide an absolute minimum age for the ultrahigh temperature uht foliation preserved in this rock internal zircon zoning relationships and estimated zircon garnet dree values from paragneiss suggest that an absolute minimum age of ultra high temperature metamorphism is ca 2 510 ma but that it is more likely to be older than ca 2 545 ma we suggest that the high proportion of published zircon u pb data with ages between ca 2 490 2 450 ma reflects late post peak zircon growth and does not date the timing of peak uht metamorphism

hydrography and high temperature heat flux of the rainbow hydrothermal site 36 14 n mid atlantic ridge on the basis of an extensive set of conductivity temperature depth lowered acoustic doppler current profiler ladcp and nephelometry profiles and tow yos the hydrography flow field and particle plume of the rainbow hydrothermal site on the mid atlantic ridge are analyzed in the rift valley the water column is less dense and stratified than both eastern and western off ridge water with t s characteristics consistent with inflow across a sill from the east the buoyant hydrothermal plumes rise into a strong boundary current flowing along the slope of a topographic high which partially blocks the rift valley below 1950 m the bulk of the neutrally buoyant plume is advected across a sill which forms both the narrowest and the shallowest part of the valley and acts as a hydraulic control point for the flow below 2000 m large amplitude internal waves consistent with tidal forcing are observed near the sill but ladcp measurements suggest that the tidal signal is not strong enough to lead to flow reversal at plume depth above 2000 m the mean current across the topography generates lee waves which radiate energy upward and downstream density averaged light scattering profiles show the hydrothermal particle plume to be gaussian in depth even in the near field where many of the individual profiles are characterized by multiple peaks and the horizontal variability is highest the temperature anomalies associated with the mean near source particle plume are of order 5 10 3 c that is the plume is cold fresh as expected from the background hydrography of the deep atlantic using the flow field light scattering and hydrographic anomaly observations the heat flux associated with the hydrothermal particle plume at rainbow is estimated to lie between 1 and 5 gw

the water masses of the central north atlantic in 1983 84 abstract hydrographic surveys carried out in 1983 84 along both sides of the mid atlantic ridge between 24 and 53 n provide a detailed description of the well known north atlantic water masses with particular emphasis on their meridional distribution and zonal dissymetry in the upper layers the dense horizontal sampling resolves the several narrow gulf stream extensions into the ocean interior giving the image in the central water density range of a mosaic of mode waters separated by fronts at intermediate depths a vertical shear in the distributions of the mediterranean water and labrador sea water stands out both water masses having their lower part displaced southwards relative to their upper parts bottom waters containing nearly 20 percent of pure antarctic bottom water are observed at 50 n in the eastern basin in contrast with the western basin where proportions greater than 10 percent were found only south of 36 n along our section this water mass analysis also gives indications that stron

gravity anomalies and crustal thickness variations along the mid atlantic ridge between 33 n and 40 n multibeam bathymetry and gravity data have been obtained along an 800 km long section of the mid atlantic ridge from just south of the hayes fracture zone at 33 n to the northern edge of the azores platform near 40 n a three dimensional analysis of these gravity and topography data combined with results from earlier seismic refraction studies in this area reveal two different scales of crustal heterogeneity a systematic along axis segment scale 8 9 km near the middle of spreading segments and the thinnest crust 3 4 km near segment offsets the magnitude of this along axis variation in crustal thickness is proportional to the length of the spreading segment and the size of the adjacent ridge offset there also is a distinct asymmetry in crustal structure across the rift valley near large segment offsets with gravity highs inferred to be thinner crust beneath the inside corner highs adjacent to these offsets this segment scale crustal heterogeneity is similar to that reported from the kane to atlantis section of the mid atlantic ridge and from parts of the intermediate spreading southern mid atlantic ridge it is superimposed on a second longer wavelength variation in gravity and crustal thickness associated with the azores hot spot the most pronounced effect of the azores hot spot on the mid atlantic ridge occurs between 38 n and 40 n where the ridge axis rapidly shoals by more than 1000 m the crust thickens by over 2 km and the rift valley largely disappears the absence of a deep axial rift valley on the azores platform and near the midpoints of some ridge segments along the mid atlantic ridge to the south agrees well with the predictions of recent ridge crest thermal and rheological models that suggest a dependence of axial morphology on both crustal thickness and spreading rate the transition from a rift valley to an axial high morphology at these spreading rates 11 mm yr half rate occurs at a crustal thickness of about 9 1km

dome and keel provinces formed during paleoproterozoic orogenic collapse core complexes diapirs or neither examples from the quadrilatero ferrifero and the penokean orogen paleoproterozoic dome and keel provinces in which troughs of deformed and metamorphosed paleoproterozoic supracrustal rocks surround domes of archean basement continue to puzzle geologists in current literature some authors refer to the domes as diapirs implying basement flowed vertically upward and others consider them to be cordilleran type metamorphic core complexes implying that the contact between basement and cover is an upwarped detachment geochronological studies suggest that dome emplacement occurred during extensional collapse of contractional orogens seemingly supporting the core complex analogy however structural analyses in the quadrilatero ferrifero brazil and the penokean orogen michigan demonstrate that the domes resemble diapirs in shape and in terms of surface kinematics the domes differ from diapirs however in that they did not flow penetratively within but were emplaced by movement along steeply dipping shear zones we suggest that contrasts between dome and keel and core complex provinces reflect contrasts between paleoproterozoic and phanerozoic crustal structure specifically during the paleoproterozoic when crust was warmer and supracrustal assemblages denser core complex like detachment faulting and associated plutonism resulted in juxtaposition of hot basement beneath a denser tectonically thickened supracrustal layer the resulting viscosity contrast and density inversion triggered vertical rise of diapir shaped basement domes by slip on steep shear zones

physical oceanography of the azores front abstract there is a considerable body of evidence for the existence of a permanent front sw of the azores surveys in the summer of 1981 show the structure to penetrate to a depth of 2 000 m and suggest that its meandering is related to the alignment of the atlantis gt meteor seamount chain the total volume transport above 2 000 m is between 10 and 12 106 m3 s 1 towards the east the main water mass contrasts across the front are confined to the upper 400 m of the water column only a small change in mediterranean water strength was seen across the front satellite tracked drogued buoys seeded into the front showed surface speeds of up to 40 cm sec 1 and revealed the formation of a closed cold core eddy some 100 115 km in diameter which propagated westward at 2 2 km day 1 over a 200 day period sea surface temperature contrasts across the frontal boundary are small and together with frequent cloud cover prevent the detection of the front and associated eddies by satellite infra red imagery the front apparently marks the eastward extension of the southern branch of the gulf stream recirculation and the northern limit of the 18 c water mass eddies formed at the front could propagate westward into the interior of the sargasso sea but have not yet been observed to cross the mid atlantic ridge

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

tectonic implications of precambrian sm nd dates from the southern sao francisco craton and adjacent aracuai and ribeira belts brazil abstract the archean and paleoproterozoic sao francisco craton of eastern brazil is surrounded on all sides by brasiliano pan african orogens the n ne trending orogen that separates the eastern edge of the southern sao francisco craton from the atlantic coast can be divided into the largely greenschist and amphibolite facies aracuai belt on the west and the largely granulite facies ribeira belt on the east a pronounced linear gravity and magnetic anomaly the abre campo discontinuity defines the boundary between these two belts we obtained sm nd mineral ages and whole rock sm nd model ages for garnet bearing metamorphic rocks along an e w transect across the southern sao francisco craton the southern aracuai belt and the ribeira belt at about latitude 20 s a recrystallization age of 2 1 ga from metasediments recrystallized during the development of the classic dome and keel province of the southern sao francisco craton the quadrilatero ferrifero indicates that dome emplacement occurred during the waning stages extensional collapse of the transamazonian collisional orogeny seven mineral ages from the southern aracuai and ribeira belts date the thermal peak of metamorphism at between 538 and 589 ma confirming that these belts were pervasively remobilized during the brasiliano event samples from the aracuai belt yield either archean 2 6 ga or transamazonian 2 1 2 3 ga tdm model ages indicating that the protoliths were either fragments of the sao francisco craton crust or were sediments derived from that craton which presumably had been stretched to form a thinned continental margin during meso and neo proterozoic rifting events notably a mixed meta pelite and metabasite sequence in the southern aracuai belt the dom silverio group has transamazonian ancestry and thus may represent oceanic sediments deposited on or east of this stretched margin and then thrust back onto the continent to mark a collisional suture between the sao francisco block and an archean crustal sliver to the east the rocks of the ribeira belt i e the region east of the abre campo discontinuity have younger model ages tdm 1 6 2 0 ga indicating that this belt was not originally part of the sao francisco craton it may represent an accreted transamazonian terrane therefore the abre campo discontinuity marks an important crustal boundary possibly a suture brasiliano shear zones appear to steepen and root into the abre campo discontinuity suggesting that it formed in brasiliano time

thrust related diapiric and extensional doming in a frontal orogenic wedge example of the montagne noire southern french hercynian belt abstract the montagne noire which is situated at the toe of the orogenic wedge of the french massif central south european variscides appears to be a well suited area for studying the origin and evolution of middle to upper crustal domes adjacent to foreland basins the data reported in the present paper show that the montagne noire dome is a particular type of basement involved frontal culmination in an orogenic wedge and foreland basin system this frontal culmination is characterized by a syn contractional ht decompression recorded by clockwise ptt paths and widespread strata overturning in thrust and fold structures which controlled the sedimentation in the adjacent foreland basin these unusual characteristics are interpreted to be a result of the succession of thrusting diapirism and extensional collapse antiformal stacking of syn metamorphic thrust sheets controlled the first stages of the foreland basin development diapirism was essentially responsible for the ht decompression and widespread strata overturning extensional doming was a result of late to post metamorphic collapse acting on the pre existing high amplitude dome diapirism and associated isothermal decompression metamorphism which constitute the essential difference between the montagne noire and ordinary frontal ridges in orogenic wedges were probably enhanced by a local partial melting of the upper to middle crust it is suggested that the occurrence of these phenomena in front of an orogenic wedge was related to local over thickening due to the superposition of an upper crustal antiformal stack on top of a lower crustal ramp anticline

lithofacies associations and structural evolution of the archean rio das velhas greenstone belt quadrilatero ferrifero brazil a review of the setting of gold deposits abstract the rio das velhas greenstone belt is located in the quadrilatero ferrifero region in the southern extremity of the sao francisco craton central southern part of the state of minas gerais se brazil the metavolcano sedimentary rocks of the rio das velhas supergroup in this region are subdivided into the nova lima and maquine groups the former occurs at the base of the sequence and contains the major au deposits of the region new geochronological data along with a review of geochemical data for volcanic and sedimentary rocks suggest at least two generations of greenstone belts dated at 2900 and 2780 ma seven lithofacies associations are identified from bottom to top encompassing 1 mafic ultramafic volcanic 2 volcano chemical sedimentary 3 clastic chemical sedimentary 4 volcaniclastic association with four lithofacies monomictic and polymictic breccias conglomerate graywacke graywacke sandstone graywacke argillite 5 resedimented association including three sequences of graywacke argillite in the north and eastern at greenschist facies and in the south at amphibolite metamorphic facies 6 coastal association with four lithofacies sandstone with medium to large scale cross bedding sandstone with ripple marks sandstone with herringbone cross bedding sandstone siltstone 7 non marine association with the lithofacies conglomerate sandstone coarse grained sandstone fine to medium grained sandstone four generations of structures are recognized the first and second are archean and compressional driven from nne to ssw the third is extensional and attributed to the paleoproterozoic transamazonian orogenic cycle and the fourth is compressional driven from e to w is related to the neoproterozoic brasiliano orogenic cycle gold deposits in the rio das velhas greenstone belt are structurally controlled and occur associated with hydrothermal alterations along archean thrust shear zones of the second generation of structures sedimentation occurred during four episodes cycle 1 is interpreted to have occurred between 2800 and 2780 ma based on the ages of the mafic and felsic volcanism and comprises predominantly chemical sedimentary rocks intercalated with mafic ultramafic volcanic flows it includes the volcano chemical sedimentary lithofacies association and part of the mafic ultramafic volcanic association the cycle is related to the initial extensional stage of the greenstone belt formation with the deposition of sediments contemporaneous with volcanic flows that formed the submarine mafic plains cycle 2 encompasses the clastic chemical sedimentary association and distal turbidites of the resedimented association in the eastern sector of the quadrilatero ferrifero it was deposited in the initial stages of the felsic volcanism cycle 2 includes the coastal and resedimented associations in the southern sector in advanced stages of subduction in this southern sedimentary cycle it is also possible to recognize a stable shelf environment following the felsic volcanism cycle 3 comprises sedimentary rocks of the volcaniclastic and resedimented lithofacies associations largely in the northern sector of the area the characteristics of both associations indicate a submarine fan environment transitional to non marine successions related to felsic volcanic edifices and related to the formation of island arcs cycle 4 is made up of clastic sedimentary rocks belonging to the non marine lithofacies association they are interpreted as braided plain and alluvial fan deposits in a retroarc foreland basin with the supply of debris from the previous cycles

structural evolution of the warrawoona greenstone belt and adjoining granitoid complexes pilbara craton australia implications for archaean tectonic processes abstract the pilbara granite greenstone terrain 3 6 2 8 ga australia is quoted as the key area formed by passive gravity driven solid state diapirism but contrasting views have been reported recently which led us to re examine the key area the warrawoona greenstone belt and the adjoining mt edgar and corunna downs granitoid complexes we found i an asymmetric non synclinal distribution of metamorphic grade and deformation intensity across the warrawoona greenstone belt ii a major shear zone on the margin of the mt edgar granitoid complex with a unidirectional stretching lineation pattern iii a ne trending transcurrent shear zone within the central gneiss complex of the mt edgar granitoid complex iv coeval magmatic activity and doming of the granitoid complexes and v the loss of about 9 km of stratigraphic units in the warrawoona greenstone belt the area has had a prolonged history with at least four deformation phases between ca 3 47 and 2 8 ga the main domal geometry of the mt edgar granitoid complex was obtained during active ne sw extension at 3 31 ga d x 2 which appears to been in response to e w compression possibly the e w compression resulted in crustal thickening and partial melting probably of a 3 46 3 43 ga granitic basement causing crustal weakening collapse and extrusion of the 3 31 ga granitoids extension at 3 31 ga was accommodated by the mt edgar shear zone a mid crustal detachment and by the associated beaton well zone which acted as a transfer zone within the gneissic basement doming of the mt edgar granitoid complex is interpreted to have occurred in an active extensional core complex type mode an indication for deformation prior to 3 32 ga comes from syntectonically intruded gabbro diorite and dolerite sills within the mt edgar shear zone which intruded possibly as early as 3 46 3 45 ga d x 1 ultramafic schistose xenoliths within the gabbro diorite intrusions are the oldest observed deformed rocks formed during d x post doming deformation involved ne sw compression d x 3 followed by dextral transcurrent deformation d x 4 during nw se compression the latter may be related to the regional phase of deformation at 2 93 ga during which the eastern and western pilbara were amalgamated comparison of this deformation sequence to others within the marble bar domain suggests a common tectonic history from 3 46 ga onwards our structural and kinematic data especially the 3 31 ga unidirectional stretching lineation pattern lead us to reject solid state diapirism due to convectional overturn as the main cause for doming of the mt edgar granitoid complex we suggest that the cause lies in a plate tectonic process

topographic control of a dispersing hydrothermal plume deep sea hydrothermal vents represent a major source of heat and chemicals to the oceans and support endemic chemosynthetic biological communities to fully understand the impact of hydrothermal activity upon the oceans however requires investigation of both the physical and the biogeochemical processes which are active in hydrothermal plumes and which serve to determine the net hydrothermal flux to the oceans we have recently conducted a detailed multidisciplinary study of the lateral dispersion of the hydrothermal plume emitted from the rainbow vent site near 36 15 n mid atlantic ridge combining velocity measurements from a lowered adcp optical back scatter measurements from a deep tow ctd and methane measurements from bottle samples we are able for the first time in the atlantic to trace a neutrally buoyant plume for a distance of over 50 km the path of the plume is seen to be heavily controlled by the local topography with a general northeast movement of water both particle and methane concentrations decrease downstream over the length of the observed plume the dataset provides an excellent opportunity to study the mixing and biogeochemical processes active in a hydrothermal plume and estimate fluxes of biogeochemical constituents

garnet porphyroblast timing and behaviour during fold evolution implications from a 3 d geometric analysis of a hand sample scale fold in a schist detailed 3 d analysis of inclusion trails in garnet porphyroblasts and matrix foliations preserved around a hand sample scale tight upright fold has revealed a complex deformation history the fold dominated by interlayered quartz mica schist and quartz rich veins preserves a crenulation cleavage that has a synthetic bulk shear sense to that of the macroscopic fold and transects the axis in mica rich layers garnet porphyroblasts with asymmetric inclusion trails occur on both limbs of the fold and display two stages of growth shown by textural discontinuities garnet porphyroblast cores and rims pre date the macroscopic fold and preserve successive foliation inflection intersection axes fias which have the same trend but opposing plunges on each limb of the fold and trend nne ssw and ne sw respectively the fias are oblique to the main fold which plunges gently to the wsw inclusion trail surfaces in the cores of idioblastic porphyroblasts within mica rich layers define an apparent fold with an axis oblique to the macroscopic fold axis by 32 whereas equivalent surfaces in tabular garnet adjacent to quartz rich layers define a tighter apparent fold with an axis oblique to the main fold axis by 17 this potentially could be explained by garnet porphyroblasts that grew over a pre existing gentle fold and did not rotate during fold formation but is more easily explained by rotation of the porphyroblasts during folding tabular porphyroblasts adjacent to quartz rich layers rotated more relative to the fold axis than those within mica rich layers due to less effective deformation partitioning around the porphyroblasts and through quartz rich layers this work highlights the importance of 3 d geometry and relative timing relationships in studies of inclusion trails in porphyroblasts and microstructures in the matrix

implications of melting for stabilisation of the lithosphere and heat loss in the archaean abstract the increased depth and volume of melting induced in a higher temperature archaean mantle controls the stability of the lithosphere heat loss rates and the thickness of the oceanic crust the relationship between density distributions in oceanic lithosphere and the depth of melting at spreading centres is investigated by calculating the mineral proportions and densities of residual mantle depleted by extraction of melt fractions the density changes related to compositional gradients are comparable to those produced by thermal effects for lithosphere formed from a mantle which is 200 c or more hotter than modern upper mantle if archaean continental crust formed initially above oceanic lithosphere the compositional density gradients may be sufficient to preserve a thick archaean continental lithosphere within which the archaean age diamonds are preserved the amount of heat advected by melts at mid ocean ridges today is small but heat advected by melting becomes proportionally more important as higher mantle temperatures lead to a greater volume of melt and as the rate of production of oceanic plates increases archaean tectonics could have been dominated by spreading rates 2 3 times greater than now and with mantle temperatures between ca 1600 c and 1800 c at the depth of the solidus mid ocean ridge melting would produce a relatively thick but light refractory lithosphere on which continents could form protected from copious volcanism and high mantle temperatures

transpressive tectonism in the eastern penokean orogen canada consequences for proterozoic crustal kinematics and continental fragmentation abstract results of a structural analysis of archean basement huronian cover and various igneous rocks in the sudbury area ontario and regional tectonometamorphic correlations suggest that structures in the eastern penokean orogen formed dominantly by dextral transpression bulk horizontal shortening started with the ca 2 4 to 2 2 ga blezardian orogeny and was accompanied by peak metamorphism and local granitoid magmatism during the 1 89 to 1 83 ga penokean orogeny horizontal dextral shear parallel to the curved axis of the eastern penokean orogen gained in importance and is most likely related to docking of proterozoic magmatic arc terranes to the superior province margin this tectonic activity led to distributed heterogeneous deformation of archean basement rocks and exhumation of deep crustal rocks in the sudbury area and the kapuskasing structural zone but may have failed to accrete paleoproterozoic terranes in the lake huron area the lack of evidence for horizontal extension following blezardian and penokean contraction suggests that crustal fragmentation of the archean supercontinent started at 2 45 ga finally the great lakes tectonic zone and the murray fault master dislocations in the penokean orogen may have acted as crustal weakness zones which influenced the orientation and development of 1 1 ga midcontinent rift segments

partitioning of deformation along an orogen and its effects on porphyroblast growth during orogenesis an identical succession of foliation inflection intersection axis trends in porphyroblasts fias is present in three separate areas affected by acadian deformation and metamorphism along 130 kms of the appalachians from east central vermont to north central massachusetts the fias trend successively nw se sw ne w e nnw sse and ssw nne and appear to have formed during the same succession of deformations which have been dated in se vermont as follows not yet dated pre 425 ma 425 to 404 ma 404 to 385 ma and 385 to 360 ma respectively the two areas that are furthest apart in east central vermont and north central massachusetts have strikingly similar total distributions of fias in garnet porphyroblasts that show a somewhat inverse relationship to those observed in the chester athens dome area of se vermont that separates them that is the number of samples containing the first formed fia set is greatest for the two regions lying furthest apart and decreases in younger sets whereas the number of samples containing the first fia set is the least in the central region and increases for more recently developed sets partitioning of deformation into zones of progressive shearing and shortening has been demonstrated to control the sites of development of differentiated crenulation cleavage versus porphyroblast growth and sites of porphyroblast growth on one limb of a regional fold versus the other we argue that partitioning of the deformation around competent feldspathic gneiss bodies at an orogen scale produced the variation in the number of samples containing the various fia sets between the central and outer areas described above porphyroblasts most commonly grew during sw ne directed shortening to the north and south of the chester athens dome region of the appalachians but only rarely grew in the latter region this suggests that deformation at this time partitioned around an outlier of competent basement feldspathic gneiss that underlies the chester athens dome region and this protected the rocks above and in the strain shadow to the nw and se from the effects of the great bulk of the deformation that occurred over the period of time during which nw se trending fias formed we attribute the increase in the number of samples containing progressively younger fias in the chester athens dome area during subsequent periods of nw se n s wsw ene and wnw ese directed shortening to the rise in temperature and pressure recorded in the rocks above the chester athens dome with peak conditions occurring during the development of the nnw sse oriented fia set between 404 and 385 ma as the temperature and pressure increased the gneiss below would have become less competent allowing the deformation associated with the succession of fia sets to partition more pervasively through the dome core this provided more sites for porphyroblast nucleation and growth and hence the number of samples recording these fia sets increased the concomitant decrease in the number of samples in which porphyroblasts grew in the rock within the areas to the north and south with successive fias possibly resulted from a progressive increase in competency of these rocks as the number of porphyroblasts increased

the stratigraphy of the 3 5 3 2 ga barberton greenstone belt revisited a single zircon ion microprobe study abstract recent field and geochemical studies indicate a need to test the stratigraphy of the ca 3 5 ga barberton greenstone belt as it is presently adopted 1 2 this work uses the ion microprobe shrimp to attempt such a test results show that 1 volcaniclastic sediments of the theespruit formation