the peraluminous aurumina granite suite in central brazil an example of mantle continental crust interaction in a paleoproterozoic cordilleran hinterland setting abstract peraluminous granites and tonalites granodiorites of the aurumina suite crop out in the external zone of the brasilia fold belt on the western margin of the sao francisco craton rocks of this suite intrude retrograde medium grade schists and paragneisses of the ticunzal formation and display a magmatic muscovite biotite garnet assemblage and discrete graphite lamellae the suite is divided into six members au1 through au6 which grade from synkinematic to post kinematic in character with ages spanning from 2 11 to 2 16 ga according to zircon u pb data compositional features of the aurumina suite and associated coeval metaluminous mafic to intermediate plutons suggest an origin by source hybridization processes characterized by reaction between metasedimentary rocks and basaltic melts that would have taken place at shallow depths pressure

three dimensional mantle dynamics with an endothermic phase transition three dimensional convection for the spinel to perovskite phase change has been simulated numerically results for rayleigh ra numbers of 0 106 show intermittent layering with a strong robust plume rising through the phase boundary many descending instabilities are deflected but merging cold sheets come together at a junction a pool of cold material accumulates underneath in the phase transition zone a strong gravitational instability results which precipitates a rapid and massive discharge of upper mantle material

the ktb apatite fission track profiles building on a firm foundation abstract deep boreholes serve as natural laboratories for testing thermochronometers under geological conditions the kontinentale tiefbohrung ktb is an interesting candidate because the geological evidence suggests that approximate isothermal holding since the last documented exhumation in the late cretaceous to palaeocene is a reasonable assumption for the thermal histories of the ktb samples we report 30 new apatite fission track ages and 50 new mean confined track lengths determined on cores from the 4 km deep pilot hole the and external detector ages are consistent with the population ages from earlier studies and together define a clear age profile the mean track lengths from this and earlier studies reveal the effects of experimental factors the measured age and length profiles are compared with the predictions of 24 annealing models for isothermal holding there are clear discrepancies between the measured and calculated profiles down to 1 5 km depth the measured mean track lengths are shorter than the predicted the balance of methodological evidence indicates that this is due to seasoning i e a shortening of the fossil confined tracks without attendant age reduction from 2 5 to 4 0 km depth the mean track lengths are longer than the predictions this suggests that the bias model that weights the probabilities of observing tracks of different length and which is based on experiments relating surface track densities to mean track lengths is not appropriate for confined tracks experimental and methodological factors are sometimes difficult to disentangle but present a sufficient margin for there to be no need to go against the independent geological evidence unknown geological events cannot be ruled out but their existence cannot be inferred from the fission track data alone much less can the nature or magnitude of such events be specified

tectonic evolution of south america during the late proterozoic abstract this paper discusses the evolution and the late proterozoic assembly of the south american continent i e the west gondwana under the light of global tectonics the northern portion of the continent the amazonian craton is a pre brasiliano domain it may be a fragment derived from laurentia this domain behaved as a large continental plate which was accreted to the central and southeastern portions of the continent brasiliano domain the brasiliano domain had a more complex history and composition during the brasiliano cycle middle proterozoic to early paleozoic in the brasiliano domain several late proterozoic continental plates have been identified sao luiz west africa congo kasai sao francisco rio de la plata arequipa antofalla which acted as forelands to the brasiliano orogenic belts that surround and amalgamate these plates smaller fragments of pre brasiliano continental lithosphere are common in the interior of the brasiliano orogenic belts and played diversified roles microplates microcontinents structural highs some of them have acted as true backlands hinterlands flanked by arc magmatism two principal types of brasiliano orogenic belts are recognized between the continental plates a marginal orogenies to the late proterozoic plates with pelitic carbonatic rocks and discrete volcanism which were thrust by collision onto the margin of the plates and which seem to represent final products of previous passive margin sequences b distal orogenics internal belts within the continental masses sometimes a branching system of orogenics that contain varied clastic and minor carbonate sedimentary rocks accompanied by important bimodal volcanism and calc alkaline plutonism for these orogenies a greater variety of basinal scenarios and tectonic settings may be visualized the amalgamation of the continent during the late proterozoic i e the formation of the brasiliano structures indicates the existence of practically synchronous multilateral stresses within lithospheric plates this can be produced by complementary adjustment of the continental plates as a response to major global events elsewhere analogous to modern continental collisions two large and important tectonic zones may be candidates to such a scenario the hijaz mozambique zone site of connection of east and west gondwana and the transbrasiliano lineament which cuts across south america and extends to north africa following the zone of connection between pre brasiliano and brasiliano domains other small ocean openings and closings inside the various brasiliano areas dom feliciano gariep for instance were minor events and complementary to the other two major tectonic zones mentioned above

thermally softened continental extensional zones arcs and rifts as precursors to thickened orogenic belts abstract intra continental deformation of soft zones during continental collision requires weak continental lithosphere which is able to be shortened across considerable width during later convergence this enables significant thickening with formation of an orogenic root we have examined models with a history of lithospheric thinning by pure shear during an earlier phase of intra continental extension with associated heating geologically this situation is appropriate to intra continental rifts and back arc basins if thinning of elevated thermal structure is decoupled from the thinning of lithology then a weak soft lower crust and sub arc rift mantle result this weak structure has a favoured rheology for subsequent convergent thickening while the lithosphere is still hot these regions are associated with formation of granulites and metamorphic assemblages typical of high temperature low pressure ht lp if convergence starts while the heat input is still active then the failed rifts and arcs are shut off by lateral wedging of the hard lower crust and upper mantle of shoulder regions into the softened arc rift domain such sites are ideal for the formation and for the exhumation of metamorphic core complexes subsequent thickening during convergence leads to ht eclogites when the previous arc rift was hot and to medium t eclogites for a thickened standard geotherm these p t paths are counterclockwise and their shapes are strongly dependent on the amount of previous thinning and type of initial geotherm if the compression starts long after cessation of the extensional event and associated thermal anomaly then the geotherm of the extended area relaxes and the whole region hardens in this case no homogeneous thickening occurs and deep continental roots cannot form

apatite fission track age correction and thermal history analysis from projected track length distributions abstract the frequency distribution of fission track lengths in an apatite reflects the mineral s thermal history since the time of its last crossing into the partial annealing zone paz apatite thermal history analysis based on confined track length measurements is an already well established tool in hydrocarbon exploration in a less direct though more easily accessible way the thermal history is also reflected in the frequency distribution of the projected lengths of tracks intersecting the apatite surface a mathematical description of the probability distribution of the projected length of surface tracks ptld projected track length distribution corresponding to different types of thermal history is presented and a new method for correcting thermally lowered fission track ages is proposed this correction method gives the time of entry into the paz at 120 c but also the time when the sample left the paz to enter the total track stability zone at 60 c an age function is subsequently defined which allows us to distinguish between different types of thermal history experimental verification shows that although the proposed approach is less precise compared to existing methods based on projected track lengths it is possible to distinguish between different thermal histories provided that a sufficient number of tracks are measured on the other hand the age function provides information relative to the samples complete trajectory through the paz whereas existing methods are limited to temperatures below the closing temperature of the fission track system

denudation and cooling of the lake teletskoye region in the altai mountains south siberia as revealed by apatite fission track thermochronology abstract lake teletskoye occupies a narrow graben located in the northwestern sector of the altai fold belt in south siberia the lake basin is thought to have formed during the pleistocene as a distant result of the cenozoic collision of india and eurasia that caused a tectonic reactivation of the palaeozoic gorny altai ga and west sayan ws blocks the present work reports of a pilot fission track study performed on 13 apatite separates collected from rocks that were sampled along two profiles in close proximity of the lake the age length data and aft thermochronological modelling reveal two important phases of cooling in the altai mountains a first one during the late jurassic early cretaceous and a second one that started in the miocene pliocene and that persists until today the first event is interpreted to result from uplift induced denudation probably related to the closure of the mongol okhotsk ocean the second event can be linked to the young cenozoic movements that lie at the origin of the formation of the lake teletskoye basin

seismic studies of the brasilia fold belt at the western border of the sao francisco craton central brazil using receiver function surface wave dispersion and teleseismic tomography abstract the tocantins province in central brazil is composed of a series of ssw nne trending terranes of mainly proterozoic ages which stabilized in the neoproterozoic in the final collision between the amazon and sao francisco cratons no previous information on crustal seismic properties was available for this region several broadband stations were used to study the regional patterns of crustal and upper mantle structure extending the results of a recent e w seismic refraction profile receiver functions and surface wave dispersion showed a thin crust 33 37 km in the neoproterozoic magmatic arc terrane high average crustal vp vs ratios 1 74 1 76 were consistently observed in this unit the foreland domain of the brasilia foldbelt on the other hand is characterized by thicker crust 42 43 km low vp vs ratios 1 70 1 72 were observed in the low grade foreland fold and thrust zone of the brasilia belt adjacent to the sao francisco craton teleseismic p wave tomography shows that the lithospheric upper mantle has lower velocities beneath the magmatic arc and goias massif compared with the foreland zone of the belt and sao francisco craton the variations in crustal thickness and upper mantle velocities observed with the broadband stations correlate well with the measurements along the seismic refraction profile the integration of all seismic observations and gravity data indicates a strong lithospheric contrast between the goias massif and the foreland domain of the brasilia belt whereas little variation was found across the foldbelt craton surface boundary these results support the hypothesis that the brasilia foreland domain and the sao francisco craton were part of a larger sao francisco congo continental plate in the final collision with the amazon plate

the goias massif implications for a pre columbia 2 2 2 0 ga continent wide amalgamation cycle in central brazil abstract the goias massif is a collage of archean paleoproterozoic terranes that crop out in the central brazil northern brasilia belt and acted as the continental margin onto which neoproterozoic brasiliano orogenic events took place most belts in the massif formed from 2 2 to 2 0 ga a widespread age bracket in the sao francisco congo craton pericratonic areas and other terranes associated with the formation of the atlantica supercontinent however the genesis of the goias massif has always been a matter of debate due to a sharp crustal thickness contrast marked on surface by the trace of the rio maranhao thrust and widely hypothesized as a suture zone the hypothesis that this suture would have been formed when a microcontinent was accreted to the rest of the massif during the neoproterozoic brasiliano orogeny is tested in this paper through mapping and zircon geochronology of rocks along the rio maranhao thrust our study indicates that units contain paleoproterozoic ages have similar geological features to other goias massif 2 2 2 0 ga terranes and show no evidence of syn collisional neoproterozoic magmatism we therefore reinterpret the available data to propose instead that the amalgamation of the massif occurred fully in the paleoproterozoic first the gravimetric contrast marked by the rio maranhao thrust represents in fact a neoproterozoic lower crust delamination event that affected both the brasiliano orogen and the terrane it was accreted against the campinorte arc second serra da mesa suite mesoproterozoic granites intruded both sides of the rio maranhao thrust and thus indicate the two domains were amalgamated prior to the mesoproterozoic finally mesoproterozoic volcanic and plutonic rocks follow the trace of the rio maranhao thrust suggesting a reactivated intracontinental structure rather than an original product of brasiliano collision the goias massif also show important similarities to the sao francisco craton a orogen building and magmatism from 2 2 to 2 0 ga b metamorphic peaks from 2 11 to 2 08 ga c zircon age peaks at 1 76 ga 1 58 ga and 1 2 ga largely attributed to crustal extension events and d contiguous seismic signature from massif to craton these common features allied to our reassessment of central brazil geochronology and the nature of central brazil sharp crustal thickness contrast indicate that the goias massif was fully assembled during a 2 2 2 0 ga orogeny as part of the western border of the sao francisco congo paleocontinent a comprehensive understanding of this orogeny is vital to test the validity of atlantica and to infer the landmasses later involved in the assembly of columbia

the effect of lithospheric phase transitions on subsidence of extending continental lithosphere abstract a new simple model for sedimentary basin formation which combines both stretching and phase transitions occurring together in the lithosphere is proposed and two important aspects in sedimentary basin formation are addressed 1 the explanation of the domal uplift preceding rifting and 2 the ability of the model to explain the contradiction between a small amount of crustal stretching and the large amount of thermal subsidence without the need to invoke differential stretching between the crust and mantle relationships for synrift initial and thermal post rift subsidence are derived the model predicts that extension of continental lithosphere could produce synrifting uplift up to a threshold amount of extension and then subsidence for greater extension three dimensionless parameters quantify the contribution of the phase transition to the subsidence history 1 the clapeyron slope normalized to the initial geothermal gradient 2 the ratio of depth of phase transition to the thickness of the lithosphere and 3 the ratio of density effect due to phase transition to the change in density due to thermal expansion an evaluation of these parameters has been carried out by using thermodynamical data for the garnet spinel transition for upper mantle rocks our simple model shows that by including the effects of lithospheric phase transition in existing stretching models of sedimentary basin formation we can obtain an extra significant parameter controlling the subsidence and the entire extensional tectonics of such basins

field and geochronological evidence for origin of the contendas mirante supracrustal belt sao francisco craton brazil as a paleoproterozoic foreland basin abstract the low grade contendas mirante meta volcano sedimentary belt cmb was previously interpreted as an archean greenstone belt with the uppermost formation deposited in the paleoproterozoic new zircon u pb la icp ms ages on sedimentary groups and granite sheets intrusive in the belt constrain the timing of basin development phyllites of the lower group and meta arenites of the upper group yielded similar young zircon populations at ca 2080 ma which are interpreted as the maximum deposition age for the entire sequence the unique exception is the 3273 3338 ma zircon bearing quartzite that occurs in spatial association with the 3300 ma rhyolite a granite sheet intrusive into metagraywacke of the lower group constrains the minimum deposition age of 2045 26 ma for the cmb altogether four main age clusters of detrital zircon grains were recognized namely 2075 2200 ma 2200 2440 ma 2500 2770 ma 3270 3380 ma these age intervals match the ages of igneous and metamorphic rocks in the different terranes that comprise the sao francisco craton maximum deposition age for both groups indicates that sedimentation occurred immediately prior the high grade metamorphism in the adjacent archean paleoproterozoic jequie block and itabuna salvador curaca orogen these observations along with coarsening upwards in the sediments indicate that the cmb represents a foreland basin developed along the eastern margin of the gaviao block during plate convergence paleoarchean basement rocks were reactivated and emplaced into the cmb during basin inversion and is timely correlated with the granitic intrusion recognition of older supracrustal fragments is ambiguous one main example is the 3273 3338 ma zircon bearing quartzite which correlates with the northern jacobina basin along the contendas mirante lineament e g bahia gold belt and may represent part of an archean intracontinental rift sequence

thermal expansion measurements at very high pressure systematics and a case for a chemically homogeneous mantle new data on thermal expansivity of iron to over 400 kbar in laser and electrically heated diamond anvil cells and on mgo and forsterite from spectroscopic data to over 200 kbar and previously measured adiabats boehler 1982 imply that at high pressure may be described by a constant value of 1n 1nv t 5 5 0 5 this behavior is supported by previous measurements at lower pressures on a large variety of materials densities of magnesio wustite and silicate perovskite in a pyrolitic ratio calculated at lower mantle conditions using a finite strain equation and at high pressures using the above systematics match those in prem through the entire lower mantle making a strong case for a uniform mantle

petrogenetic and mineralization processes in paleo to mesoproterozoic rapakivi granites examples from pitinga and goias brazil abstract the 1 8 ga pitinga granites agua boa and madeira massifs in amazonas northern brazil are within plate shallow level rapakivi granites associated with an extensional fracture system they comprise an early facies of pyterlitic to wiborgitic rapakivi granite a fine to medium grained biotite granite as well as topaz granite agua boa massif and albite granite madeira massif the granites are usually metaluminous to peraluminous the albite granite however is peralkaline they are enriched in sio2 k2o na2o f rb th nb y zr and the rare earth element ree and impoverished in mgo tio2 p2o5 and sr as the majority of sn mineralized granites sn contents range from 1 ppm in the rapakivi facies to 3400 ppm in the albite granite end at 1 8 ga values vary from 2 2 to 0 4 and nd model ages lie between 2 4 and 2 1 ga mineralization in the madeira massif includes disseminated magmatic cryolite zircon cassiterite pyrochlore columbite tantalite and xenotime and massive cryolite bodies in the f rich peralkaline albite granite in the agua boa massif sn mineralization is associated with greisens and episyenite the 1 77 ga g1 and 1 58 1 57 ga g2 rapakivi granites of goias central brazil are coeval with the arai rift basin granites of the g1 suite are metaluminous and alkaline while the g2 suite is metaluminous to peraluminous both are enriched in f sn rb y th nb ga and the ree primary micas range from fe rich biotite to zinnwaldite the micas of the more evolved granites and the metasomatic micas are strongly enriched in f with f li between 2 and 10 the initial end values of both suites show a considerable range 14 to 0 and indicate substantial compositional variation in source sn deposits in goias are hosted mainly by greisens indium is concentrated in quartz topaz rock and albitized g2d granite of the mangabeira massif and is always related to a cassiterite sulfide association this quartz topaz rock is of metasomatic origin probably generated by a hydrothermal fluid derived from the topaz albite granite mineralization in the studied deposits was essentially associated with f enrichment in the peralkaline madeira albite granite extremely high f contents favored disseminated mineralization while in the goias tin province gtp and agua boa massif greisenization was related to early fluid saturation the gtp and pitinga granites display tectonic petrogenetic geochemical isotopic and metallogenic similarities that can be applied in search of sn and rare metal deposits

evolution of reworked paleoproterozoic basement rocks within the ribeira belt neoproterozoic se brazil based on u pb geochronology implications for paleogeographic reconstructions of the sao francisco congo paleocontinent abstract geological geochemical and geochronological data suggest that the basement exposed in the occidental terrane of the neoproterozoic ordovician ribeira belt represents the continuation of the sao francisco paleocontinent which was amalgamated at the end of the rhyacian transamazonian orogeny the mantiqueira complex represents a cordilleran type active margin of the archean paleocontinent while the juiz de fora complex represents juvenile paleoproterozoic material probably developed within an oceanic setting primitive arc or plateau the latter unit was probably accreted to the active margin of the paleocontinent during the latest stages of the transamazonian orogeny around ca 2 05 ma causing the deformation and metamorphism of the mantiqueira complex at this time to the west and on the cratonic foreland concurrent deformation of the minas passive margin and sedimentation of the sabara basin took place subsequent to the transamazonian orogeny the basement associations were intruded by intraplate alkaline and basic rocks related to rifting events between ca 1 7 and 1 3 ga that resulted in the development of intracratonic basins the paleoproterozoic suture located between the mantiqueira and juiz de fora paleoproterozoic terranes probably controlled the neoproterozoic passive margin tectonics and also the development of a major thrust surface during the neoproterozoic cambrian brasiliano convergence

the neoproterozoic cambrian paraguay belt central brazil part i new structural data and a new approach on the regional implications abstract together with the araguaia and brasilia belts the paraguay belt forms in central brazil the tocantins province that is one of the largest orogens of western gondwana the corumba area occupies the site where the northern and southern parts of the paraguay belt form together with the chiquitos tucavaca aulacogen stretching e w in the adjacent bolivian territory an r r r basin system opened filled in the 700 650 540 ma interval within the amazon rio apa paleo continent the sedimentary volcanic rocks of the jacadigo and corumba groups found around the corumba city record part of the neoproterozoic cambrian passive margin precursor of the paraguay belt our pioneer structural analysis reveals that these rocks experienced progressive deformation phases d 1 d 2 d 3 and low grade metamorphism during the brasiliano cycle 540 513 ma the crystalline basement was also involved according to structural data and k ar ages in the literature the paleo passive margin was thickened during the d 1 d 2 deformation and was lately shortened d 3 in two orthogonal directions se nw d 3p and sw ne d 3t developed co axially and verging to nw d 1 d 2 d 3p structures record the closure of the basin precursor of the paraguay belt whereas d 3t structures seem related to the inversion of the aulacogen although the tectonic transport to nw as observed in the corumba area matches the reported transport of paraguay belt s supracrustal rocks towards the eastern margin of the rio apa block and araguaia belt s rocks towards the amazon craton the transport direction is opposite in other parts of the paraguay belt our comprehensive discussion of these facts brings to light profound regional implications

u pb and hf isotope study on detrital zircons from the paranoa group brasilia belt brazil constraints on depositional age at mesoproterozoic neoproterozoic transition and tectono magmatic events in the sao francisco craton abstract the eastern portion of the brasilia belt part of the neoproterozoic tocantins province in central brazil is mainly constituted by metasedimentary rocks that partially represent the evolution of a passive margin developed during the meso neoproterozoic in the western margin of the sao francisco paleocontinent new in situ u pb and lu hf isotope analyses on detrital zircons by la mc icp ms were obtained for clastic sedimentary rocks of the paranoa group considered to be the oldest unit of the this sedimentary sequence the u pb data define a maximum depositional age of 1 54 ga and a minimum depositional age of 1 04 ga the latter obtained on diagenetic xenotime overgrowth on zircon grains combined u pb and lu hf data permitted to reconstruct the tectonomagmatic evolution of the potential source areas identified in the sao francisco craton two zircon populations at 1 54 and 1 78 ga with hf t values from 5 1 to 5 5 are possibly related with the mesoproterozoic extensional magmatism registered in the arai and espinhaco rifts located in the brasilia belt and in the sao francisco craton respectively the most important paleoproterozoic zircon population at 2 0 2 25 ga shows positive hf t 0 to 8 and paleoproterozoic tdm values indicating a continuous long period of magmatism with generation of juvenile crust during the paleoproterozoic two minor zircon populations at 2 1 and 2 15 ga showing negative hf t values 0 to 10 1 and tdm values between 2 65 and 3 22 ga may represent arc related magmatism with reworking of archean crust during the evolution of an active margin of the sao francisco craton a third minor zircons population at 2 05 ga with strongly negative hf t values between 8 6 and 17 7 and tdm values between 2 93 and 3 46 ga may represent a collisional magmatism that characterized the final stage of an active margin evolution with reworking of early archean crust several minor zircon populations of archean ages 2 7 2 85 3 1 and 3 42 ga derived from the archean nuclei in the interior of the sao francisco craton suggest the presence of different magmatic pulses that repeatedly reworked a juvenile paleoarchean crust the oldest event of generation of juvenile crust is registered by zircons with u pb age of 3 42 ga positive hf t values of 3 0 and tdm of 3 54 ga

petrogenetic interpretation of granitoid rock series using multicationic parameters abstract granitoid rock compositions from a range of tectonic environments are plotted on a multicationic diagram devised by de la roche and his coworkers this shows that there is a systematic change through an orogenic cycle which leads progressively to the ultimate development of alkaline magmas possible source materials and mechanisms of magma generation are considered from analysis of mineral compositional vectors these suggest that most granitoid series result from a two stage process first fractional crystallisation of clinopyroxene olivine and calcic plagioclase from a basic source with tholeiitic affinities produces a magma of intermediate composition this magma then undergoes periodic mixing with a felsic magma followed by in situ fractionation to generate individual intrusions within granitoid series

high resolution records of location and stratigraphic provenance from the rare earth element composition of fossil bones abstract bone apatite acts as a natural timed sampling device scavenging trace elements from local pore waters over timescales of ca 1 50 ka the rare earth element ree and u th composition of fossil bones reflects associated pore water compositions during the period of recrystallisation the ree composition of fossil bones is controlled by partitioning of ree between pore waters and particle surfaces and the ree composition of fossil bones reflects the ree composition of pore waters which vary spatially and temporally light ree are preferentially sorped onto particle surfaces thus the high la yb values seen in many bones from coastal marine and aeolian environments are best explained by release of ree from light ree enriched particles to local pore waters and subsequent immobilisation in recrystallising bones the ree compositions of bones recovered from pedogenically altered diatomite sediments of the olorgesailie formation of southern kenya vary over spatial scales of less than 10 m location accounts for 48 of the observed variation in bone chemistry and bones recovered from eight discrete excavations within the same time equivalent stratigraphic layer can be assigned to their excavation location with 70 accuracy based on a discriminant analysis of ree u and th composition despite this within layer variation bones recovered from different stratigraphic horizons within the olorgesailie formation can also be distinguished on the basis of their trace element composition bones recovered from four stratigraphic horizons spanning ca 0 5 million years were assigned to their correct stratigraphic layer with 90 accuracy where sedimentological conditions are favourable the trace element composition of fossil bone may be used to test stratigraphic provenance and burial location in excavated bone with a temporal resolution of

granulite formation during continental extension in fiordland new zealand the formation of the high pressure high temperature rocks known as granulites has traditionally been attributed to metamorphism in the deeper parts of continental collision zones and magmatic arcs1 3 more recently greater consideration has been given to the possibility of granulite formation during continental rifting4 6 and in particular4 to whether granulites are forming today beneath the north american basin and range province here we report thermobarometric data and single zircon u pb ages for an analogous but much more deeply exhumed extensional terrain in fiord land new zealand7 8 where early cretaceous granulites underwent renewed granulite facies metamorphism at significantly shallower crustal levels following the extensional collapse of magmatically thickened continental crust our data show that granulite facies metamorphism can take place during continental extension and demonstrate that in western new zealand it was accompanied at higher structural levels by the emplacement of one or more metamorphic core complexes7 9 in keeping with the model postulated4 for the basin and range province

the relationship between track shortening and fission track age reduction in apatite combined influences of inherent instability annealing anisotropy length bias and system calibration abstract the relationship between reduced track density 0 and reduced mean confined track length l l 0 during annealing has been studied for both spontaneous and induced tracks in four apatite samples with different chemical composition compositional range and origin and with spontaneous tracks giving mean confined track lengths varying from 14 5 m down to 12 3 m in all four apatites induced tracks show a relationship between l l 0 and 0 composed of two regions at higher values the two appear to be equal while at lower values l l 0 is greater than 0 this can be understood qualitatively in terms of the anisotropy of annealing and the bias against revelation of short tracks data from three apatites of uniform composition show near identical relationships in a sample showing a range of apatite compositions the point of divergence from the 1 1 relationship is somewhat higher than in the uniform composition apatites because individual grains anneal at different rates and the presence of highly annealed grains affects the track density more strongly than the mean track length spontaneous tracks show similar relationships to those observed for induced tracks in each sample when the data are renormalised to allow for the natural annealing present in the spontaneous tracks previous studies which led to the idea of length reduction without density reduction for spontaneous tracks did not renormalise density data as well as length data thereby distorting the apparent relationship data for the multi compositional apatite sample where annealing has taken place in geological conditions also agrees with the laboratory annealing data for both spontaneous and induced tracks this consistency suggests that a single process is responsible in all three cases implying that a laboratory based description of annealing can be applied to geological conditions with confidence recognition that spontaneous tracks in apatite are commonly annealed suggests that in general the fission track age should be interpreted as a reflection of both the time over which tracks have been retained and the distribution of etchable track lengths since the latter is a manifestation of the thermal history the fission track age should be interpreted in similar terms

hard turbulent thermal convection and thermal evolution of the mantle this article summarizes the results of hard turbulent convection obtained in laboratory experiments and numerical simulations its applications to mantle convection are illustrated by two dimensional numerical solutions to 1 newtonian 2 non newtonian convection and 3 newtonian convection with multiple phase transitions in newtonian mantle convection the transition from soft to hard turbulence is marked by the appearance of disconnected plumes spectral analysis of the time series of the nusselt number reveals the presence of a spectral scaling subrange for hard turbulence but not for soft turbulence in hard turbulence there is correspondence between the spectra in frequency and wavenumber domains the slope of the seismic wave spectra measured from seismology suggests that the mantle convection today is strongly time dependent the transition to hard turbulence takes place at much lower nusselt numbers for non newtonian than for newtonian rheology for the mantle this would have important ramifications non newtonian plumes behave quite differently from newtonian ones in that large curvatures are developed in their trajectories in the hard turbulent regime mantle convection with phase transitions tends to become more layered with increasing rayleigh numbers the style of mantle convection might have changed from a layered to a more whole mantle type of flow with time catastrophic overturns associated with strong gravitational instabilities in the transition zone could be responsible for superplume events

timing and role of the maranhao river thrust in the evolution of the neoproterozoic brasilia belt and tocantins province central brazil abstract a low angle thrust fault places high pt granulites hangingwall of the internal zone of the neoproterozoic brasilia belt tocantins province central brazil in contact with a lower grade footwall external zone comprised of nappes of distal passive margin and back arc basin related supracrustals the footwall units were emplaced at 750 ma onto proximal sedimentary rocks paranoa group of the sao francisco paleo continent passive margin the high pt belt is comprised of 645 630 ma granulite facies paragneiss and orthogneiss and mafic ultramafic complexes that include three major layered intrusions and metavolcanic rocks granulitized at 750 ma these complexes occur within lower grade metasedimentary rocks in the hangingwall of the maranhao river thrust which forms the internal zone external zone boundary fault to the north of the pirineus zone of high strain detailed lithostructural studies carried out in maranhao river thrust hangingwall and footwall metasedimentary rocks between the niquel ndia and barro alto complexes and also to the east of these indicate the same lithotypes and sm nd isotopic signatures and the same d1 d2 progressive deformation and greenschist facies metamorphism additionally footwall metasedimentary rocks exclusively display a post d2 deformation indicating that the maranhao river thrust propagated through upper crustal rocks of the paranoa group relatively late during the tectonic evolution of the belt fault propagation was a consequence of intraplate underthrusting during granulite exhumation the results allow for a better tectonic understanding of the brasilia belt and the tocantins province as well as explaining the presence of the pirineus zone of high strain

standardless fission track dating of the durango apatite age standard abstract five dating strategies were used for determining the standardless fission track age of the durango apatite these use the same fossil track densities but differ in the manner in which the induced track densities are determined a conventional age calculation without correction for experimental factors gives inconsistent ages with method related differences 15 correcting for these factors brings the ages in line with each other and with the reference age but leaves no room for a partial annealing correction based on the confined track lengths three further reasons suggest that a length correction is not appropriate 1 the evidence for length based corrections is inconclusive 2 the plateau age of the durango apatite is consistent with its apparent fission track age to within 1 3 the calculated effective etchable length of the fossil tracks agrees within error with that of the induced tracks both are further consistent with the measured mean length of confined induced tracks the circumstance that the u th he ages of the accepted and proposed apatite age standards are consistent with their reference ages leaves no margin for a lowered fission track age resulting from partial annealing although the case of the durango apatite itself is inconclusive because of its exceptional crystal size it is conjectured that the shortening of the fossil tacks in the durango apatite is due to a lowering of the track etch rate over time in this case annealing equations fitted to induced track data underestimate the extent of confined track length reduction in geological samples

fission track dating of the south eastern bohemian massif waldviertel austria thermochronology and long term erosion apatite fission track thermochronology was applied to the outcropping variscan basement of the south eastern bohemian massif in order to resolve its post variscan cooling and denudation history nineteen samples could be dated by the grain population method the ages range from 235 to 90 ma middle triassic to middle cretaceous neglecting some local variations they show a decrease from the western edge of the moravian thaya window towards the west and the south fission track length histograms mainly exhibit broad distributions with arithmetic means between 12 3 and 13 4 m and standard deviations between 1 5 and 2 0 m thermochronological model calculations of the measured data demonstrate that the apatite fission track memory of the samples reaches back until the begin of the cretaceous and for some samples even until the permian the regional fission track cooling pattern is not related to the limitations and the internal structure of the moravian thaya window which appears to be an elder probably late carboniferous to permian tectonic feature also the vitis fault does not delimit areas of different thermochronological behaviour therefore significant vertical offset on that fault can be excluded for the cretaceous and cenozoic thermochronological model calculations of fission track ages and length data are consistent with the assumption of regional sedimentary reburial during the late cretaceous the amount of reburial could have attained 1 km the cenozoic bulk denudation was in the order of 1 to 3 km

geochemistry and sm nd isotopic characteristics of bimodal volcanic rocks of juscelândia goias brazil mesoproterozoic transition from continental rift to ocean basin abstract the ca 1 28 ga juscel ndia sequence in goias brazil comprises a unit of bimodal metavolcanic rocks amphibolite and felsic gneiss intercalated with metapelite and metachert the bimodal nature of the magmatism is evident as the metavolcanic rocks have basalt and rhyolite compositions bimodality is also shown by the trace elements and nd isotope data amphibolites yield positive e nd t values that can be separated into two groups in the first group amphibolite samples from lower and middle portion of the sequence are characterized by e nd between 2 8 and 5 4 these fairly positive values and enrichment in lree and lile suggest they were derived from a depleted mantle source with subsequent assimilation of continental crustal material the second group is formed by amphibolite of the upper portion of the sequence that have e nd around 6 0 and their low lile and lree contents resemble normal mid ocean ridge basalts n morb initial 143 nd 144 nd ratios represent the composition of the depleted mantle at 1 28 ga felsic rocks yield negative e nd t values between ca 2 8 and 4 5 and paleoproterozoic model ages t dm values in the interval between 1 8 and 2 1 ga the original felsic magmas represent the product of melting of older paleoproterozoic continental crust with trace element chemistry indicating that the felsic magmas were the product of continental crustal melts with small mantle contribution previous models proposing an oceanic setting for the origin of magmatism of the juscel ndia rocks within a mid ocean ridge or back arc basin are not supported by the data presented here transition from a continental rift to an ocean basin represents the most likely tectonic setting for the origin of the juscel ndia rocks

the paleoproterozoic campinorte arc tectonic evolution of a central brazil pre columbia orogeny abstract the campinorte arc is a poorly exposed 2 19 2 07 ga paleoproterozoic terrane in contact with the neoproterozoic goias magmatic arc by the rio dos bois fault in the northern brasilia belt central brazil the campinorte arc is divided into pau de mel suite metatonalites to metamonzogranites and the campinorte volcano sedimentary sequence pau de mel suite whole rock geochemistry indicates at least three separate coeval parental magmas compatible with arc signatures u pb geochronology data of paragranulite and mafic granulite exposed in the region as part of the campinorte arc provide additional information on this paleoproterozoic orogenic cycle the formation and preservation of these granulites was due to tectonic switching of the back arc basin and consequent lithospheric thinning from 2 14 to 2 09 ga with metamorphic peak from 2 11 to 2 08 ga a pau de mel suite granodiorite sample dated at ca 2 08 ga indicates post peak magmatism the arc was thereafter rapidly contracted preserving paleoproterozoic high metamorphic grade mineral assemblages additionally the campinorte arc and the neighboring crixas guarinos faina greenstone belts may have shared the same source of sediments as attested by geographic proximity coeval maximum sedimentation felsic volcanism and the occurrence of similar rock types gravimetric and seismic data also support a common basin hypothesis the formation of the campinorte arc is contemporaneous to other northern brasilia belt basement terranes and along with similar arcs within and at the sao francisco craton edges indicate a continental crust formation event that eventually led to the assemblage of columbia

new constraints on the chronology magnitude and distribution of deformation within the central andean orocline 1 constructing an accurate kinematic model for crustal thickening in the central andes is hampered by the inability of 2 d balanced cross sections to resolve the 3 d displacement field within the poorly studied orocline core this study presents new structural and thermochronometer data from the orocline axis in the central andes that constrains the magnitude and timing of deformation through 1 a balanced cross section quantifying shortening perpendicular to strike 2 field observations that constrain translational displacements parallel to strike and 3 thermochronology that brackets the age of exhumation and deformation mapping results show that north directed right lateral motion has been accommodated on at least one major strike slip fault in the eastern cordillera ec shortening perpendicular to fault strike is 179km 41 for the ec and inter andean zone ia and 86km 39 for the sub andes sa apatite fission track cooling ages indicate rapid ec ia exhumation from 46 to 18ma and 15 to 0ma in the sa zircon and apatite u th he ages show that rapid exhumation began 10 5ma in the sa these results are integrated with existing paleomagnetic and gps rotation data in map view reconstructions to quantify the magnitude of translational faulting related to oroclinal bending the reconstructions suggest that at least 85km of north directed translational displacement has been accommodated within the orocline core orogen parallel displacement and vertical axis rotations have focused crustal thickening at the orocline core and facilitated development of a high elevation plateau suggesting a genetic link between large scale oroclines and plateaus

cold spots during the cenozoic evolution of the eastern alps thermochronological interpretation of apatite fission track data abstract the tectonical and morphological evolution of the eastern alps during the cenozoic is discussed in the light of sedimentary record and thermochronological data of crystalline basement rocks the regional distribution of apatite fission track data in the central eastern alps reveals the differential cenozoic exhumation in contrast to the penninic of the tauern window which has been strongly uplifted and eroded since the early miocene the amount of neogene denudation was much smaller in most areas east of the hohe tauern a sudden change in the regional cooling behaviour is observed at the katschberg fault system which must have been active in post oligocene times within the austroalpine only the schladming tauern have experienced a rather strong denudation the seckau core complex east of the pols line and the area of the northern koralpe cooled to below 100 c at the end of the cretaceous and in the early eocene respectively and have never been heated again during the cenozoic these parts of the austroalpine nappe pile were always near to the surface and therefore may be summarized under the term cold spots a major post cretaceous thickening of the austroalpine can be excluded because it would have produced more uplift and erosion any remnants of miocene or older landforms are certainly not preserved in the area of the tauern window the uppermost planation surfaces in the crystalline basement at the western border of the styrian basin koralpe probably developed in the late miocene

fluid assisted granulite metamorphism a continental journey abstract lower crustal granulites which constitute the base of all continents belong to two series high pressure granulites generated by crustal thickening subduction and ultra high temperature granulites associated with crustal extension fluid inclusions and metasomatic features indicate that the latter were metamorphosed in the presence of low water activity fluids high density co2 and brines which have invaded the lower crust at peak metamorphic conditions fluid assisted granulite metamorphism high pressure and ultra high temperature granulites commonly occur along elongated paired belts they were formed from the early proterozoic onwards during a small number of active periods lasting a few hundreds of m y these periods were separated from each other by longer periods of stability each period ended with the formation of a supercontinent whose amalgamation coincided with low to medium pressure ultra high temperature granulite metamorphism immediately before continental break up it is proposed that large quantities of mantle derived co2 stored in the lower crust at the final stage of supercontinent amalgamation are released into the hydro and atmosphere during breakup of the supercontinent fluid assisted granulite metamorphism therefore appears to be an important mechanism for transferring deep mantle fluids towards the earth s surface possible consequences were for example the sudden end of proterozoic glaciations as well as the post cambrian explosion of life

u pb and sm nd constraints on the nature of the campinorte sequence and related palaeoproterozoic juvenile orthogneisses tocantins province central brazil

u pb zircon geochronology of precambrian tin bearing continental type acid magmatism in central brazil abstract u pb geochronological data for alkali rich granitic and associated rhyolitic rocks from two different tin bearing sub provinces of the goias tin province in central brazil reveal two distinct episodes of mid proterozoic continental acid magmatism at ca 1770 ma and 1600 ma ago u pb zircon data for the sucuri and soledade granites of the eastern rio parana sub province rps define upper intercept ages of 1767 10 ma and 1769 2 ma respectively these ages are indistinguishable from a u pb age of a rhyolite from the basal portion of the arai group 1771 2 ma these results attest to the contemporaneity of the intrusion of the granites and the crystallization of the rhyolitic lavas in the rio parana sub province u pb isotopic results from zircons from the serra da mesa granite in the western rio tocantins sub province rts indicate that the granitic igneous activity in that area is ca 150 ma younger than that in the rps the inheritance pattern observed in the zircons from the arai rhyolite suggests that the sialic basement in the rps region may include an early proterozoic ca 2 2 ga component inheritance of possibly archaean age was also found in zircons from the sucuri granite lower intercept age of 658 ma for the soledade granite zircons indicate pb loss during the late proterozoic brasiliano pan african thermo tectonic event 650 550 ma similar associations of tin bearing alkali rich granites with rhyolites and succeeding continental sediments are known in other parts of the precambrian shield of brazil this indicates that extensive areas of this shield experienced extensional tectonics in intracontinental settings between ca 1700 and 1900 ma ago

fission track dating of the western border of the bohemian massif thermochronology and tectonic implications since 1985 apatite fission track analysis was applied to more than 70 samples from surface outcrops and shallow boreholes at the western margin of the bohemian massif apatite ages were determined by the grain population method additional information from the frequency distributions of fully confined spontaneous tracks was used for modelling of t t paths in the low temperature range 120 c seven zircon samples were dated by the external detector method zircon ages between 283 and 215 ma indicate unroofing during the permian molasse stage and the triassic tectonic quiescence and slow subsidence prevailed from the jurassic until the middle cretaceous in the basement area south of weiden a mesozoic partial annealing zone for apatite fission tracks is now exposed at the surface farther north the basement was affected by stronger cretaceous and palaeogene erosion which yielded cooling ages between 110 and 49 ma this second period of post variscan denudation was correlated to reverse faulting along the franconian line

l apatite de durango mexique analyse d un mineral standard pour la datation par traces de fission abstract durango apatite is analyzed by four analysts using three different dating techniques both the annealing technique and the external detector technique are applied to basal and prismatic sections cut from single crystals the population technique is applied to mounts of randomly oriented grains provided an individual determination of the geometry factor is performed all ages are indistinguishable from the reference age of 31 4 0 5 ma there is no noticeable influence of either the dating technique or the crystallographic orientation of the dated section on the ft age based on the projected lengths of surface tracks spontaneous track length reduction varies from 4 to 10 mean 6 depending on the analyst two simple corrections are applied to eliminate the net effect of the missing short tracks on the mean projected length after correction the amount of shortening of the spontaneous tracks is found to be slightly larger mean 8 but is still dependent on the analyst 4 to 12 the mean is in fairly good agreement with the value obtained from confined track length measurements 9 finally an isochronal 1 h plateau annealing procedure was applied to a separate set of basal and prismatic sections cut from the same single crystals annealing temperatures up to 300 c do not affect the ft age erratic ages at 350 c are ascribed to the occurrence of unetchable gaps it is argued that the absence of a distinct plateau up to temperatures where it could be expected from theoretical considerations indicates that the ft age of durango apatite needs no correction

u pb ages of detrital zircon grains for the canastra group and passos nappe units and u pb and lu hf isotope analyses from orthogneisses provenance and tectonic implications southern brasilia belt brazil abstract the age and tectonic setting of mesoproterozoic to neoproterozoic covers that occur along the southwestern margins of the sao francisco craton sfc are still uncertain this study reports new 207 pb 206 pb la icpms ages of detrital zircon grains from metasedimentary rocks of the passos nappe units and canastra group and combined u pb and lu hf analyses on igneous zircon from orthogneisses within the metasedimentary units the u pb age spectra of detrital zircon grains of the canastra group comprises a wide range from 1 30 2 71 ga with a significant rhyacian component source 2 1 ga the second major source is represented by statherian grains 1 6 1 7 ga and are similar to the statherian interval at espinhaco basin the youngest ages fall around 1 30 ga this peak age is compatible with the initial extension and sedimentation of the carandai and middle espinhaco basins which mark the second intracratonic rifting of the sao francisco craton zircon grains from the metasedimentary rocks the passos nappe yielded a very prominent rhyacian peak at ca 2 10 ga with some statherian and neoarchean contributions all samples from the metasedimentary units show a prominent peaks at ca 2 10 ga suggesting that most sedimentary sources derived from a rhyacian substratum similar to the mineiro belt natividade cavalcanti block campinorte complex and juiz de fora complex rhyacian age is compatible with u pb crystallization ages of felsic orthogneisses 2122 67 and 2117 23 enclosed as tectonic lenses in the passos nappe and the canastra group these rocks show initial e hf t values between 2 1 to 13 0 and 10 0 and 1 8 indicating that these orthogneisses are derived from rhyacian granitic rocks which were formed either by fractionation or re melting of juvenile magmas and by the reworking of more ancient mainly archean crust the new data and regional geologic correlations suggest evidence of paleoproterozoic terrain in the southwestern margin of the sfc

a u pb zircon paleoproterozoic age for the metasedimentary host rocks and gold mineralization of the crixas greenstone belt goias central brazil abstract the crixas greenstone belt central brazil consists of a greenschist to low amphibolite facies metamorphic volcanosedimentary sequence that contains the sixth largest gold reserve 70 t au in the average 12 g t of brazil and the largest gold only deposit of central brazil the gold orebodies are hosted by low angle thrust faults within the stratigraphically upper metasedimentary rocks and are crosscut by a set of post mineralization mafic dykes for decades the sequence has been interpreted as a typical archean greenstone belt of the brazilian shield and the gold mineralization considered either archean or neoproterozoic this study shows the results of in situ u pb shrimp and la icp ms dating of detrital zircon grains from graywacke and mafic dyke samples as well as pb and nd isotopic signatures of the ores the oldest detrital zircon grains of the graywacke samples yielded u pb ages of 3354 40 ma and the youngest between 2209 28 ma and 2222 30 ma which indicates a mixture of archean and paleoproterozoic rhyacian source area and that the maximum depositional age is of ca 2 2 ga magmatic zircon of the post mineralization mafic dykes yielded the u pb age of 2170 17 ma which suggests that the basin stage its deformation metamorphism and mineralization were restricted to the rhyacian and took place during a time span of about 30 40 m y the paper thus provides a new insight into the age of the crixas greenstone belt rocks and of its gold only deposits and reveals that mid paleoproterozoic was a more widespread metallogenetic epoch related to greenstone belts of the brazilian precambrian platform then previously interpreted the observed pb and nd isotopic signatures of the ores suggest that metabasalts carbonaceous schist and metagraywackes could provide the adequate sources for the ore bodies and that the initiation of fluid circulation may have resulted by the emplacement of a high level granite

on the densities of etchable fission tracks in a mineral and co irradiated external detector with reference to fission track dating of minerals abstract the ratio r of the densities of etchable fission tracks in a mineral and co irradiated external detector is shown to depend on the length ratios of the latent l f l f and etched fission tracks l e l e in the two media calculations of l f for apatite titanite zircon and l f for muscovite external detectors allow a precise determination of r and confirm that neglecting r could constitute the most significant source of systematic error in fission track dating with the external detector method the ratios of the measured fission track densities in an external and internal surface of apatite and a muscovite external detector are calculated from r and the track counting efficiencies q the fact that the results are consistent with the experiment supports the accuracy of r and of the q factors the external detector ages of a basal and prismatic section of durango apatite are consistent with its reference age without the need for a length correction a critical examination of the uncertainties related to the thermal neutron fluence measurements fission decay constant calculated r value independently determined q factors and the reference age of the standard demonstrates that the assumption that a reduction of the confined track length of the spontaneous tracks in durango apatite necessitates a proportional correction of its apparent fission track age is the element most in question published u th he ages lend support to the conclusion that such a correction is not required the significance of r and of the ratio q of the q factors for the induced and spontaneous track counts leads us to propose a formalisation of the age equation in which these are included as well as a formal length correction l which is however not set equal to the ratio of the confined track lengths of the induced and spontaneous tracks the accuracy of the thermal neutron fluence and the decay constant leads us to propose a possible deconvolution of the calibration factor if for suitable age standards l 1 the proposed deconvoluted calibration factor 0 is a function of r and q which can be determined in ways that are independent of the reference ages of the standards this can either be interpreted as a multiple calibration or as establishing the fission track method as an independent dating method

fission track thermochronology vertical kinematics and tectonic development along the western extension of the north anatolian fault zone 1 we have investigated the low temperature history of pre neogene basement areas adjacent to the western extension of the north anatolian fault zone nafz by apatite fission track thermochronology of 33 samples taken from marmara island kapidag peninsula both in the sea of marmara samothrake island and chalkidike peninsula both in the north aegean region together with already published apatite fission track data from 30 other sampling localities of the region these data 63 in total have been evaluated mainly with regard to the transitional period from the mesohellenic orogeny eocene oligocene to the plate tectonical individualization of the aegean antolian microplate to gain a better understanding of the tectonical chronology that has led to modern east mediterranean plate tectonics apatite fission track investigations of the area under discussion reveal two major events of cooling and exhumation of pre neogene rocks a first frequency maximum of cooling ages between 35 and 25 ma late eocene to early oligocene is due to postorogenic regional erosion after the climax of the mesohellenic orogeny which was caused by continental collision of the pelagonian platform and the rila rhodope zone after the final closure of the axios vardar ocean a second frequency maximum of cooling ages between 17 and 11 5 ma early to middle miocene depicts more localized uplift and exhumation in consequence of transpressive and or transtensive movements along early structural discontinuities that gave rise to the later fault systems in the western extension of the nafz the earliest fault structures of this system have developed shortly after the orogenic collapse and crustal extension of the rila rhodope area but quasi simultaneously with the onset of subduction along the modern hellenic trench the regional distribution of fission track ages of the miocene age group 17 to 11 5 ma and corresponding thermochronological calculations based on frequency distributions of confined track lengths suggest that an early fault propagation has occurred from east to west during the miocene the onset of subduction along the hellenic trench and subsequent gravitational retreat of the subducting oceanic slab are considered as a major cause for transtension of the aegean crust especially with regard to the development of north aegean pull apart basins such early fault structures have facilitated the westward propagation of a continuous nafz that took place during the pliocene from the easternmost marmara region to the gallipoli peninsula and to the north aegean trough

mantle phase transitions and layered chaotic convection we report preliminary results from a sequence of very high resolution simulations of the mantle convection process based upon an axisymmetric anelastically compressible spherical model with both olivine spinel and spinel post spinel phase transitions our analyses strongly suggest that at earth like rayleigh numbers near 107 the circulation may be quite strongly layered although a very long timescale transient exists that is characterized by brief excursions into the whole mantle state we suggest the use of a new diagnostic tool that may be employed to determine whether or not the layered state is characteristic of the present day earth

the effects of temperature dependent viscosity on mantle convection with the two major phase transitions abstract we have studied the consequences of including temperature dependent viscosity in mantle convection with both of the major phase transitions included we have employed a two dimensional cartesian model in a box of aspect ratio four the time dependent equations have been cast within an extended boussinesq framework in which each of the phase changes has been simulated by an enhanced thermal expansivity both latent heat release and viscous dissipation have been considered two different forms of temperature dependent viscosity the arrhenius and the frank kamenetzky forms have been employed we have considered both models with a constant temperature equilibrium boundary condition at the core mantle boundary and models with a time varying non equilibrium temperature boundary condition from cooling of the core the effects of temperature dependent viscosity are to enhance the tendency of the convective system to be layered because of the influences of the latent heat release and the mechanical heating on creating a sharp viscosity drop across the transition zone we have found that there is a non linear coupling between latent heat release from the interaction of the hot plume with the endothermic phase transition and the temperature dependent viscosity this positive feedback process can give rise to the production of very high temperatures in the transition zone which may cause melting in the region between the two major phase transitions flushing events produced by the non equilibrium boundary condition are much more dramatic than those generated by the equilibrium boundary condition the non linear coupling between latent heat release and temperature dependent viscosity is also stronger in the non equilibrium models comparison with models without the presence of latent heat release and viscous dissipation shows that the models without the heating terms produce a greater degree of mass flux across the endothermic phase boundary because of the lack of the viscosity stratification in the interior of the convective flow results of this moded can be applied to other types of endothermic phase transition found in the lithosphere or to the spinel perovskite transition in the deep portion of the martian mantle

the role of lithospheric phase transitions on seafloor flattening at old ages abstract uplift of old ocean seafloor relative to the familiar t 1 2 curve is generally presumed to be a consequence of an additional heat source at the base of the oceanic lithosphere several geodynamical mechanisms have been invoked to explain this behavior we have taken a petrological rather than geodynamical approach and considered whether the causes of the flattening can be found within the conductive portion of the lithosphere where viscous creep is insignificant accordingly self consistent calculations of the phase assemblages of several candidate mantle compositions have been performed subject to the constraint of experimentally determined calorimetric measurements we have found that in general for temperatures below around 800 c and pressures greater than 10 kbar the phase change from spinel peridotite to garnet peridotite has a significantly negative clapeyron slope which could cause some amount of uplift of old seafloor the transition for representative thermal profiles should be depressed from about 35 km at 40 ma to 45 km at 160 ma since spinel peridotite is of lower density the net effect is to raise the seafloor topography the extent of the uplift which depends primarily on the al 2 o 3 content of the peridotite and on the effective thermal diffusivity of the lithosphere should be on the order of 140 220 m

an empirical test of helium diffusion in apatite borehole data from the otway basin australia we have analyzed helium ages of apatites from several boreholes in the otway basin australia to evaluate whether laboratory helium diffusivity can be accurately extrapolated to conditions relevant in nature downhole apatite helium ages define a broad swath of values from 78 71 ma at the surface 15 c to nearly zero at depths corresponding to ambient temperatures of 80 c the width of the swath results from uncertainties in corrected borehole temperatures differences in the thermal history experienced by the various boreholes and possibly from slightly different helium diffusivities among the detrital apatite grains studied in the eastern otway basin the shape and position of the helium age profile is in good agreement with predictions based on the extrapolation of laboratory diffusivity data for durango apatite coupled with published thermal histories for this part of the basin in contrast helium ages are much younger than predicted in the western otway basin based on measured ages from otway sediments which have been essentially isothermal over the last few million years an empirical diffusivity d 2 of 2 10 15 s 1 can be assigned to apatites residing at downhole temperatures of 67 97 c this empirical diffusivity is consistent with laboratory diffusion measurements demonstrating that such measurements are reasonably accurate and can be applied with confidence to natural geologic settings given this confirmation of the laboratory diffusivity data the discrepancy between the observed and modeled helium age profiles in the western otway basin suggests that these sediments recently experienced higher temperatures than presently supposed

layered intrusions and volcanic sequences in central brazil geological and geochronological constraints for mesoproterozoic 1 25 ga and neoproterozoic 0 79 ga igneous associations abstract a 350 km long belt of layered complexes and associated volcano sedimentary sequences forms a continental scale feature exposed along the internal portion of the neoproterozoic brasilia belt in central brazil this study provides new geochronological results and a critical review of the available data of these igneous associations of central brazil precise age dating combined with geological and petrological studies indicate that this belt consists of two distinct igneous rock associations the 1 25 and 0 79 ga igneous episodes are constrained by reliable u pb zircon ages shrimp and id tims obtained in different regions and lithotypes of this belt both igneous associations were affected by high grade metamorphism and tectonism at ca 0 76 ga which partially disrupted the original igneous stratigraphy of the layered complexes and volcano sedimentary sequences the present configuration of this belt results from later events probably caused by final ocean closure and continental collision between the sao francisco and amazonian continents at ca 0 63 ga the 1 25 ga magmatic event represents an igneous rock association comprising extensive bimodal volcanic rocks palmeiropolis juscel ndia and indaianopolis sequences and associated large mafic layered intrusions serra dos borges and serra da malacacheta complexes geochemical data of mafic volcanic rocks indicate compositions similar to morb suggesting a tectonic setting where continental rifting led to the opening of an ocean basin large layered intrusions coeval to the volcano sedimentary sequence consist mainly of olivine plus plagioclase cumulates with abundant troctolite and leucotroctolite associated with olivine gabbro gabbro anorthosite and pyroxenite these layered intrusions are characterized by moderately primitive olivine compositions fo 63 76 positive nd 1250ma values low contents of incompatible trace elements and lree depleted mantle normalized ree patterns these features suggest an origin from the same depleted mantle source that produced the extensive morb type volcanism intrusion of layered complexes possibly occurred since the early stages of rifting being emplaced both at the base of and within the volcanic pile different magmatic ages and fractionation trends of layered bodies suggest that they represent a cluster of intrusions that originated during the time span of the continental to oceanic ridge rifting event the 0 79 ga igneous association consists of three large layered intrusions niquel ndia barro alto and canabrava complexes despite intense tectonism several striking geological similarities between the three separate bodies have been used to suggest that they originally constituted a single continuous magmatic structure these layered intrusion are characterized by thick ultramafic zones highly primitive compositions of olivine up to fo 93 and extensive fe enrichment trends of pyroxenes layered mafic ultramafic rocks have abundant evidence of crustal contamination including highly negative nd 800ma values as well as older crustal xenoliths and zircon xenocrysts cyclic units consisting of successive layers of dunite harzburgite orthopyroxenite websterite and gabbronorite occur throughout the stratigraphy of these layered intrusions the niquelandia complex ranks among the thickest layered intrusions in the world and together with the barro alto and canabrava layered intrusions represent an enormous volume of mantle derived magma emplaced into the crust during a continental rifting event the 1 25 and 0 79 ga rifting events are not exposed in the brasilia belt or in the sao francisco craton this supports previous interpretations suggesting that these igneous rock associations are part of an exotic block in the brasilia belt equivalent mesoproterozoic ca 1 25 ga and neoproterozoic ca 0 79 ga extensional events have been reported in other continents e g north america these igneous associations described within an allochtonous block in the brasilia belt provide additional constraints therefore for continental reconstruction models before the break up of rodinia

ultra high temperature metamorphism in central brazil the barro alto complex the barro alto complex central brazil is a layered mafic ultramafic intrusion which was subjected to granulite facies metamorphism during the neoproterozoic ultra high temperature conditions are recorded by parageneses that occur in some lenses of quartz bearing rock metagranite metapelite and impure quartzite the peak paragenesis consists of spinel quartz cordierite leucosome recording the former presence of melt with quartz in excess which is replaced by either orthopyroxene sillimanite or garnet sillimanite quartz biotite sillimanite garnet symplectites are ubiquitous and indicate reactions between fe mg phases and melt late kyanite porphyroblasts have overgrown these symplectites the direct replacement of spinel quartz cordierite by orthopyroxene sillimanite or garnet sillimanite occurred around the sa invariant point which appears only in a petrogenetic grid with inverted topology the topology inversion occurs at conditions of high oxygen fugacity or due to the presence of zno bearing spinel minimum peak conditions of ultra high temperature metamorphism were calculated as c 980 c and c 7 9 kbar the succession of observed mineral textures can be explained by a near isobaric cooling p t path with a cooling stage occurring between c 980 and 750 c

dynamical consequences of depth dependent thermal expansivity and viscosity on mantle circulations and thermal structure the effects of both depth dependent thermal expansivity and depth dependent viscosity on mantle convection have been examined with two dimensional finite element simulations in aspect ratio ten boxes surface rayleigh numbers between 107 and 6 107 have been considered the effects of depth dependent properties acting singly or in concert are to produce large scale circulations with a few major upwellings the interior of the mantle is cooled by the many cold instabilities which are slowed down and eventually swept about by the large scale circulation the interior temperature of the mantle can be influenced by the trade off between depth dependent properties and internal heating for chondritic abundance of internal heating and depth dependent thermal expansivity the viscosity increase across the mantle can be no greater than a factor of around ten in order to keep the lower mantle adiabatic the thermal contrasts between the cold blobs and the surrounding mantle are strongly reduced by depth dependent properties whereas the lateral differences between the hot upwelling and the ambient lower mantle can be significant over several hundred degrees depth dependent properties also encourage the formation of a stronger mean flow in the upper mantle which may be important for promoting long term polar motions

deep seismic refraction and gravity crustal model and tectonic deformation in tocantins province central brazil abstract interpretation of seismic refraction data in the central sector of tocantins province central brazil has produced a seismic crustal model with well defined upper intermediate and lower crust layers having smooth velocity gradient in each layer the depths to moho vary from 32 to 43 km and mean crustal p velocity varies from 6 3 km s beneath goias magmatic arc on the western side to 6 4 km s below goias massif in the central portion and the foreland fold and thrust belt on the eastern side the behaviour of the lower crust layer allows an improved understanding of regional gravimetric features of the central and northern sectors of tocantins province and suggests subduction of the amazon plate in central brazil in the southeastern sector the refraction experiment resulted in the detection of a thinner crust 38 km below brasilia fold belt and a thicker crust 41 km below parana basin and sao francisco craton 42 km the upper crust beneath parana basin is around 20 km thick whereas it is less than 10 km thick below the craton these results bring new insights into the geological history of the central and southeastern sectors of tocantins province gravimetric measurements in the central sector of tocantins province delineate a high and a low anomaly separated by a steep gradient with a ne direction the axis of the gradient seems to bend still further to ne in the northern sector of that province whereas the gravimetric high continues northwards defining a separation between them this suggests that those features belong to different tectonic processes that occurred during tocantins province orogenesis the gravimetric model which incorporates seismically resolved structure beneath tocantins province better matches the observed gravimetric data although tectonic movements have only been monitored with high precision gps for short time interval 1999 2001 the results suggest observable deformations the main seismicity of central brazil the goias tocantins seismic belt seems to be spatially associated with the large gravimetric high anomaly and with the observed tectonic deformation

hot orogens tectonic switching and creation of continental crust many granulite terrains were too hot to have formed during continental collision rather along with many high grade metamorphic terrains that typify continental crust most formed in accretionary orogens during tectonic switching when prolonged lithospheric extension was interrupted by intermittent transient contraction based on modern and ancient examples tectonic switching occurs when slab retreat induces upper plate extension causing arc splitting formation of microcontinent slivers and backarc basins then intermittent arrival of buoyant oceanic plateaus induces transient flat subduction or slab flip and crustal thickening during slab retreat basaltic magmas produced from decompressed asthenosphere advect into the extending orogen causing granulite facies metamorphism and granite generation but subsequent thickening during flat subduction cools the region thickening is focused in the thermally softened backarc region and if sediment filled a hot short lived 10 m y narrow 50 100 km orogenic belt forms such thickening is often misleadingly ascribed to arc or microcontinent collision once slab retreat mode is reestablished lithospheric extension recommences and a new arc backarc system forms generally outboard arrival of another plateau will reverse the procedure and another short lived hot orogen will form within the orogenic system cycles of tectonic switching efficiently produce continental crust

seismic characteristics of central brazil crust and upper mantle a deep seismic refraction study 1 a two dimensional model of the brazilian central crust and upper mantle was obtained from the traveltime interpretation of deep seismic refraction data from the porangatu and cavalcante lines each approximately 300 km long when the lines were deployed they overlapped by 50 km forming an e w transect approximately 530 km long across the tocantins province and western sao francisco craton the tocantins province formed during the neoproterozoic when the sao francisco the paranapanema and the amazon cratons collided following the subduction of the former goias ocean basin average crustal vp and vp vs ratios moho topography and lateral discontinuities within crustal layers suggest that the crust beneath central brazil can be associated with major geological domains recognized at the surface the moho is an irregular interface between 36 and 44 km deep that shows evidences of first order tectonic structures the 8 05 and 8 23 km s 1 p wave velocities identify the upper mantle beneath the porangatu and cavalcante lines respectively the observed seismic features allow for the identification of 1 the crust has largely felsic composition in the studied region 2 the absence of the mafic ultramafic root beneath the goias magmatic arc and 3 block tectonics in the foreland fold and thrust belt of the northern brasilia belt during the neoproterozoic seismic data also suggested that the bouguer gravimetric discontinuities are mainly compensated by differences in mass distribution within the lithospheric mantle finally the goias tocantins seismic belt can be interpreted as a natural seismic alignment related to the neoproterozoic mantle domain

independent fission track ages ϕ ages of proposed and accepted apatite age standards and a comparison of ϕ z ζ and ζ0 ages implications for method calibration abstract the results of 94 determinations of the independent fission track ages ages of proposed and accepted apatite age standards show that the ages of the accepted standards are consistent with their reference ages but that those of the proposed age standards are not the fct and dur data have been used to calculate z and 0 calibration factors the 0 values agree with independent measurements and with calculated values the z and 0 ages t t z t and t 0 of 71 samples dated in the course of geological investigations are on the whole consistent although systematic deviations from the mean t m exceed the statistical uncertainties these are however 2 and traceable to statistical variation including counting statistics and an extra poisson component resulting from somewhat inconsistent track identification criteria the overall agreement between t z t and t 0 implies that measurements of s i d and are reproducible but says nothing about their intrinsic accuracies the fact that t also agrees with t z t and t 0 t t m 1 000 0 001 is inconsistent with the 10 length reduction of confined spontaneous tracks in the standards the same applies to the t t ref ratio mean 1 009 0 010 it is concluded that the agreement between t and t ref and between t and t z t and t 0 is an indication of the fact that the calculated ages are indeed accurate this implies that the 10 reduction of the spontaneous track lengths in fct and dur has no measurable effect on their fission track ages to within the

intermittent layered convection in a model mantle with an endothermic phase change at 670 km it is not yet known whether convection in the earth s mantle occurs in cells that extend throughout the entire depth of the mantle or if the upper and lower mantle convect in independent layers both possibilities seem to be consistent with geochemical and seismologi cal observations1 4 it is generally accepted however that the seismologically observed boundary between the upper and lower mantle at 670 km depth arises from the endothermic phase change spinel perovskite mgo here we investigate the results of including this phase change in a model of mantle convection for realism and to eliminate scaling problems our calculations use commonly accepted values of geophysical parameters for a clapeyron slope y 2 106 a value close to experimental results and theoretical expectations we observe local intermittent mixing between the upper and lower mantle such behaviour may offer a way to reconcile the existence of geophysical evidence for both whole mantle and layered convection

petrological and geochronological constraints on the origin of the palime amlame granitoids south togo west africa a segment of the west african craton paleoproterozoic margin reactivated during the pan african collision abstract the palime amlame pluton pap in southern togo consists of silica rich to intermediate granitoids including enclaves of mafic igneous rocks and of gneisses they are commonly called the anatectic complex of palime amlame and without any convincing data they were interpreted either as synkinematic pan african granitoids or as reworked pre pan african plutons new field and petrological observations mineral and whole rock chemical analyses together with u pb zircon dating have been performed to evaluate the geodynamic significance of the pap within the pan african orogenic belt with regard to these new data the granitoids and related enclaves probably result from mixing and mingling processes between mafic and silicic magmas from respectively mantle and lower crust sources they display mg calc alkaline chemical features and present some similarities with late archaean granites such as transitional k rich ttgs and sanukitoids the 2127 2 ma age obtained from a precise u pb concordia on zircon points out a paleoproterozoic age for the magma crystallization and a lower intercept at 625 29 ma interpreted as rejuvenation during pan african tectonics and metamorphism based on these results a pan african syn to late orogenic setting for the pap i e the so called anatectic complex of palime amlame can be definitively ruled out moreover according to its location within the nappe pile and its relationships with the suture zone the pap probably represents a fragment of the west african craton reactivated during the pan african collision

zircon u pb age and trace element evidence for paleoproterozoic granulite facies metamorphism and archean crustal rocks in the dabie orogen abstract zircon u pb ages and trace elements were determined for granulites and gneiss at huangtuling which are hosted by ultrahigh pressure metamorphic rocks in the dabie orogen east central china cl images reveal core rim structure for most zircons in the granulites the cores show oscillatory zoning relatively high th u ratios and hree enriched patterns consistent with a magmatic origin they gave a weighted mean 207pb 206pb age of 2766 9 ma interpreted as dating magma emplacement of the protolith the rims are characterized by sector or planar zoning low th u ratios negative eu anomalies and flat hree patterns consistent with their formation under granulite facies metamorphic conditions zircon u pb dating yields a weighted mean 207pb 206pb age of 2029 13 ma which is interpreted to record a metamorphic event possibly during assembly of the supercontinent columbia the gneiss has a protolith age of 1982 14 ma which is younger than the zircon age of the granulite facies metamorphism suggesting a generally delay between ht metamorphism and the intrusion of post collisional granites a few inherited cores with igneous characteristics have 207pb 206pb ages of 2 90 3 28 and 3 53 ga suggesting the presence of mesoarchean to paleoarchean crustal remnants in the yangtze craton a few cretaceous metamorphic ages were also obtained suggesting the influence of post collisional collapse in response to cretaceous extension of the dabie orogen it is inferred that the recently discovered archean basement of the yangtze craton occurs as far north as the dabie orogen

apatite in igneous systems apatite is a minor but ubiquitous mineral in most igneous rocks although the modal proportion of apatite in common rocks is generally low it can reach high concentrations in enclaves cumulates and other rocks of low abundance i e rocks that constitute a small volume of the crust and mantle e g nelsonites the presence of apatite in most rocks is due not only to its low solubility in naturally occurring melts and aqueous solutions but also to the limited ability of common rock forming minerals to accept the amount of phosphorus that occurs in most rocks into their structure in this paper we will discuss some aspects of the occurrence texture composition physical chemistry and petrogenetic significance of apatite in felsic rocks i e andesite to rhyolite and their plutonic analogs in mafic i e basalts and related rocks and plutonic analogs and ultramafic rocks of the earth s crust and mantle fluorapatite is by far the most common member of apatite family found in igneous rocks however most natural fluorapatite contains some chlorine and hydroxyl as well and these constituents can attain high concentrations in some cases the other halogens bromine and iodine also occur in apatite but their concentrations are much lower than chlorine and fluorine many cations commonly substitute for calcium and phosphorus in apatite however they rarely reach concentrations that warrant the definition of a separate mineral species apatite can be described by the general formula a5 xo4 3z following sommerauer and katz lehnert 1985 the a site accommodates large cations e g ca2 sr2 pb2 ba2 mg2 mn2 fe2 ree3 eu2 cd2 na pan and fleet ch 2 in this volume and comprises two sites that exhibit vii fold

granites and the geodynamic history of the neoproterozoic brasilia belt central brazil a review abstract recent field and geochronological studies have demonstrated the importance of granitic magmatism in the evolution of the neoproterozoic brasilia belt in central brazil this is an orogenic belt developed in response to the convergence between the amazon sao francisco congo and parana continental blocks the presence of neoproterozoic juvenile arc rocks and syn collisional peraluminous granites challenged previous intracontinental evolution models for the belt the granitoid intrusions reviewed in this paper record the different stages of evolution of the orogen and their field and isotopic characteristics can be used to reconstruct the tectonic history of the belt the main field and isotopic characteristics of four granite suites associated with the brasilia belt are reviewed i 1 77 1 58 ga old rift related a type granite intrusions ii ca 0 8 0 7 syn collisional granitoids iii arc metatonalites and metagranodiorites ca 0 9 to 0 63 ga and iv bimodal post orogenic suite ranging in age from ca 0 59 to 0 48 ga these rocks suggest that during most of the neoproterozoic the western margin of the sao francisco continent faced a large oceanic basin where subduction and oceanic lithosphere consumption started at ca 0 9 ga roughly coeval with the initial stages of the break up of rodinia final ocean closure happened at ca 0 63 0 60 ga with crustal thickening uplift and erosion post orogenic extension related magmatism took place between ca 0 6 and 0 5 ga and was partially contemporaneous with the deposition of the paraguay and tucavaca sedimentary successions resulting from the rifting event related to the break up of laurentia from southwestern gondwana