revised empirical garnet biotite muscovite plagioclase geobarometer in metapelites the garnet biotite muscovite plagioclase gbmp barometer was empirically revised for p t conditions of 1 14 kbar and 450 840 c using 263 metapelitic rock samples from all over the world this barometer is based on activity models for garnet biotite and plagioclase identical to those of the well calibrated garnet biotite thermometer and the garnet aluminosilicate plagioclase quartz gasp barometer the gbmp barometer is less temperature dependent than the gasp barometer and can be applied to either al2sio5 absent or al2sio5 bearing metapelites the total error of the gbmp barometer is estimated to be about 1 2 kbar on considering input temperature error and analytical errors of chemical compositions of the phases involved the random error of the gbmp barometer is evenly distributed with respect to pressure temperature and mineral composition simultaneous application of the gbmp barometer and the garnet biotite thermometer identifies the correct stability field for al2sio5 bearing metapelites application of the gbmp barometer to metapelitic rocks within the same geological terranes or thermal contact aureoles yielded similar pressures within error a spreadsheet for implementing the proposed gbmp geobarometer is supplied on the journal s website

paleo and neoproterozoic magmatic and tectonometamorphic evolution of the isla cristalina de rivera nico perez terrane uruguay the isla cristalina de rivera crystalline complex in northeastern uruguay underwent a multistage magmatic and metamorphic evolution based on shrimp u pb zircon th u pb monazite chime epma method and k ar age data from key units several events can be recognized 1 multistage magmatism at 2 171 2 114 ma recorded on zircon of the granulitic orthogneisses and their 2 093 2 077 ma overgrowths 2 a distinct amphibolite facies metamorphism at 1 980 ma recorded by monazite 3 greenschist facies reworking and shearing at ca 606 ma monazite and k ar on muscovite along the rivera shear zone and finally 4 intrusion of the post tectonic sobresaliente and las flores granites at around 585 ma lithological similarities geographic proximity and coeval magmatic and metamorphic events indicate a similar tectonometamorphic evolution for the isla cristalina de rivera the valentines block in uruguay and the santa maria chico granulitic complex in southern brazil since at least 2 1 ga

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

pacific subduction coeval with the karoo mantle plume the early jurasssic subcordilleran belt of northwestern patagonia the early mesozoic magmatism of southwestern gondwana is reviewed in the light of new u pb shrimp zircon ages 181 2 ma 181 3 ma 185 2 ma and 182 2 ma that establish an early jurassic age for the granites of the subcordilleran plutonic belt in northwestern argentine patagonia new geochemical and isotopic data confirm that this belt represents an early subduction related magmatic arc along the proto pacific margin of gondwana thus subduction was synchronous with the initial phase of chon aike rhyolite volcanism ascribed to the thermal effects of the karoo mantle plume and heralding rifting of this part of the supercontinent overall there is clear evidence that successive episodes of calc alkaline arc magmatism from late triassic times until establishment of the andean patagonian batholith in the late jurassic involved westerly migration and clockwise rotation of the arc this indicates a changing geodynamic regime during gondwana break up and suggests differential rollback of the subducted slab with accretion of new crustal material and or asymmetrical scissor like opening of back arc basins this almost certainly entailed dextral displacement of continental domains in patagonia

recrystallization or cooling ages in situ uv laser 40ar 39ar geochronology of muscovite in mylonitic rocks the intra grain 40 ar 39 ar age distributions of muscovite from greenschist facies mylonite zones are shown to be sensors of metamorphic history textures compositions and in situ uv laser ablation 40 ar 39 ar data from two extensional shear zones demonstrate that cooling ages can be distinguished from neo or recrystallization ages deformed muscovite porphyroclasts and recrystallized shear band muscovite from the pogallo shear zone ivrea zone southern alps reveal variable intra grain 40 ar 39 ar ages with internal age variations of more than 65 ma the wide range of 40 ar 39 ar ages within compositionally homogeneous grains is consistent with diffusion dominated argon loss controlled by observable intra grain microstructures in contrast intra grain 40 ar 39 ar ages of muscovite fish from proterozoic mylonite of the porsgrunn kristiansand shear zone southern norway display only minor age dispersion and age variations are correlated with changes in muscovite composition the sympathetic age composition correlations in different muscovite microstructures directly relate to sequential neo and recrystallization of muscovite and ultimately provide insight into the protracted recrystallization history during extensional deformation along the porsgrunn kristiansand shear zone

th diffusion in monazite abstract diffusion of th has been characterized in synthetic monazite under dry conditions the synthetic monazites either pure cepo4 ndpo4 or a mixed lree phosphate containing ce nd and sm were grown via a na2co3 moo3 flux method the sources of diffusant for the experiments were either synthesized thsio4 or cath po4 2 powders mixed with cepo4 experiments were performed by placing source and monazite in pt capsules and annealing capsules in 1 atm furnaces for times ranging from 3 weeks to a few hours at temperatures from 1350 to 1550 c the th distributions in the monazite were profiled by rutherford backscattering spectrometry rbs the following arrhenius relation was obtained for th diffusion in monazite d 6 7 10 1 exp 740 63 kj mol 1 r t m 2 s 1 the diffusivity of th was found to be similar for monazites containing a single ree ce or nd and the mixed lree phosphates th diffusion was also similar for experiments run using the th silicate and ca th phosphate sources suggesting that the substitutional mechanism for th in monazite i e th 4 si 4 ree 3 p 5 for the thsio4 source and th 4 ca 2 2ree 3 for the cath po4 2 source does not significantly affect th diffusivities and that th is likely the rate limiting species in diffusion th diffusion in monazite is about 3 orders of magnitude slower than pb diffusion cherniak d j watson e b grove m harrison t m 2004a pb diffusion in monazite a combined rbs sims study geochimica et cosmochimica acta 68 829 840 this contrasts with findings of gardes et al gardes e jaoul o montel j m seydoux guillaume a m wirth r 2006 pb diffusion in monazite an experimental study of pb2 th4 2nd3 interdiffusion geochimica et cosmochimica acta 70 2325 2336 who determined that diffusivities measured in pb th ree exchange which would under most circumstances be rate limited by the slowest diffusing species are comparable to those measured by cherniak et al cherniak d j watson e b grove m harrison t m 2004a pb diffusion in monazite a combined rbs sims study geochimica et cosmochimica acta 68 829 840 for pb th diffusion in zircon cherniak d j hanchar j m watson e b 1997 diffusion of tetravalent cations in zircon contributions to mineralogy and petrology 127 383 390 is about an order of magnitude slower than in monazite but with similar activation energy for diffusion the lower diffusivities in zircon may be a consequence of the larger disparity in size between th and the zr site in zircon as compared with th and the ree site in monazite nonetheless th is essentially immobile in monazite with respect to exchange by volume diffusion under most geologic conditions these findings may also have implications for containment of high level actinide based nuclear waste in monazite ceramic waste forms but it should be noted that mechanisms other than diffusive exchange e g recrystallization dissolution reprecipitation may be important to consider along with diffusion when evaluating the retentivity of monazite for th and other actinide elements

development of shear zone related lozenges in foliated rocks abstract tectonic lozenges are elongate bodies bounded by relatively more deformed rocks the focus of this study is on the 2 d structure of tectonic lozenges developed during ductile shear in rocks with a pre existing mechanical anisotropy on the basis of a detailed analysis of shear zones in foliated rocks from the cap de creus area variscan of the eastern pyrenees five mechanisms to explain the development of different types of lozenges in foliated rocks are suggested these mechanisms are explained on the basis of the orientation of the previous foliation relative to the bulk shearing direction it is shown that the prevailing mechanism does not majorly depend on the bulk kinematics but on the angular relationship between the pre existing foliation and the bulk kinematic axes and on shear zone interaction this has implications on the use of lozenge shapes in tectonic interpretations the fact that there is a wide range of initial orientations propagation modes and coalescence types implies that the final lozenge geometry is not univocally related neither to the type of strain nor to the kinematic regime

thrust tectonics in the north patagonian massif argentina implications for a patagonia plate 1 in the northeastern segment of the north patagonian massif southern argentina s to sw directed thrusting affected a late proterozoic to cambrian phyllite succession and the silurian to lower devonian sierra grande formation thrust tectonics was partly combined with the formation of mylonites and followed by the intrusion of the navarrete granodiorite mylonites of the tardugno granitoids are attributed to a ductile shear horizon in the subsurface as second stage of deformation after cessation of thrusting nw se compression led to the formation of widely distributed fold structures around ne sw trending axes as well as se directed reverse faults it was followed by the late kinematic intrusion of a porphyric granite as final pulse of the navarrete intrusive complex local folding around n s to nw se trending axes represents the final stage of the compressive deformational history thrust tectonics plus subsequent stages of deformation can be assigned to the late paleozoic time interval probably the permian they are seen in conjunction with the formation of granite mylonites within the north patagonian massif the cerro de los viejos granite mylonite of the la pampa province and the compressive structures in the sierras australes fold and thrust belt north of the boundary between patagonia and gondwana south america also the lack of early paleozoic famatinian deformation in this part of the north patagonian massif and the inferred boundary of the early paleozoic cuyania precordillera terrane north of patagonia support the interpretation that extra andean patagonia collided with gondwana south america during the gondwanide orogeny it is possible that patagonia was part of a plate and that the long gondwanide fold belt represented its collision zone

vrancea source influence on local seismic response in bucharest the mapping of the seismic ground motion in bucharest due to the strong vrancea earthquakes is carried out using a complex hybrid waveform modeling method that allows easy parametric tests starting from the actually available strong motion database we can make realistic predictions for the possible ground motion the basic information necessary for the modeling consists of a the representative mechanisms for the strong subcrustal events b the average regional structural model and c the local structure for bucharest two scenario earthquakes are considered and the source influence on the local response is analyzed in order to define generally valid ground motion parameters to be used in the seismic hazard estimations the source has its own detectable contribution on the ground motion and its effects on the local response in bucharest are quite stable on the transversal component t while the radial r and vertical v components are sensitive to the scenario earthquake although the strongest local effects affect the t component both observed and synthetic a complete determination of the seismic input for the built environment requires the knowledge of all three components of motion r v t the damage observed in bucharest for the march 4 1977 vrancea event the strongest earthquake to strike the city in modern times is in agreement with the synthetic signals and local response

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

tectonic development of the north patagonian andes and their related miocene foreland basin 41 30 43 s 1 the northern patagonian andes have been constructed through multiple mechanisms that range from tectonic inversion of extensional structures of early to middle jurassic age in the main andes to oligocene in the precordilleran region these have acted during two distinctive orogenic stages first in late early cretaceous and later in miocene times late oligocene extension separates these two contractional periods and is recorded by half grabens developed in the retroarc region the last contractional stage coexists with an eastward foreland expansion of the late miocene arc whose roots are presently exposed as minor granitic stocks and volcanic piles subordinately in the main andes east of the present arc as a consequence of this orogenic stage a foreland basin has developed having progressed from 18 ma in the main north patagonian andes where the mountain front was flooded by a marine transgression corresponding to the base of the nirihuau formation to 11 ma in the foreland area cannibalization of this foreland basin occurred initially in the hinterland and then progressed to the foreland zone blind structures formed a broken foreland at the frontal zone inferred from growth strata geometries during pliocene to quaternary times most of the contractional deformation was dissipated in the orogenic wedge at the time when the arc front retracted to its present position

thermobarometric modelling of zircon and monazite growth in melt bearing systems examples using model metapelitic and metapsammitic granulites u pb age data collected from zircon and monazite are used to draw fundamental inferences about tectonic processes in the earth despite the emphasis placed on zircon and monazite ages the understanding of how to relate the timing of growth of zircon and monazite to an evolving rock system remains in its infancy in addition few studies have presented large datasets of geochronological data from zircon and monazite occurring in the same metamorphic rock sample such information is crucial for understanding the growth of zircon relative to monazite in a systematic and predictive manner as per this study the data that exist support the generally held conception that zircon ages tend to be older than monazite ages within the same rock here experimental data for zircon and monazite saturation in melt bearing rocks are integrated with phase diagram calculations the calculations constrain the dissolution and growth behaviour of zircon and monazite with respect to evolving pressure temperature and silicate mineral assemblages in high grade melt bearing metasedimentary rocks several key results emerge from this modelling first that in aluminous metapelitic rocks i e garnet cordierite sillimanite assemblages zircon ages are older than monazite ages in the same rock second that the growth rate of accessory minerals is nonlinear and much higher at and near saturation than at lower temperatures and third that the difference in zircon and monazite ages from the same rock may be ascribed to differences in the temperature s at which zircon and monazite grow rather than differences in closure temperature systematics using our methodology the cooling rate of granulites from the reynolds range central australia have been constrained at 4 c myr 1 this study serves as a first pass template on which further research in applying the technique to a field study can be based

strain partitioning in banded and or anisotropic rocks implications for inferring tectonic regimes abstract among the many concepts that have contributed to the development of structural geology in the last half century two are particularly important these are i the link between the symmetry of the structure the vorticity of the associated deformation process and the tectonic regime and ii the widespread partitioning of strain however a clear understanding of these concepts and the links between them are not always apparent in structural analyses carried out to determine the prevailing tectonic regime of an area through the use of structural symmetries and kinematic indicators this contribution based on field studies from cap de creus experimental data and theoretical concepts highlights some of the problems encountered when attempting to deduce the type of strain and associated regional tectonic regime from field structures the relation between the symmetry of a structure and its associated kinematics is not unique for example it is demonstrated that in mechanically anisotropic materials symmetry variations do not necessarily reflect marked vorticity variations but rather the angular variations between the kinematic framework and the anisotropy planes because of widespread strain partitioning local and general kinematics do not generally match this mismatch is especially accentuated in rheologically heterogeneous rocks nevertheless correct interpretations are possible by performing continuous multi scale structural analyses in which the impact of strain partitioning and of the presence of material anisotropies and heterogeneities are considered

chronological zonation of granitic plutonism in the northern patagonian andes of argentina the migration of intrusive cycles abstract many k ar age determinations on mineral concentrates from granitoids of the patagonian cordillera between 40 and 42 s have supported the distinction of a zonation and an apparent migration from east to west of the intrusive loci in geological times this zonation is based upon the evidence of a marginal eastern belt of granitic rocks considered as upper paleozoic triassic an intermediate area of upper cretaceous granitoids and an even internal belt of tertiary age this progressive lower age of magmatic episodes towards the west is also corroborated by stratigraphical data on the chilean slope a similar reverse zonation has been proposed although conflicting data have been found

structurally controlled fluid flow high grade silver ore shoots at martha epithermal mine deseado massif argentina abstract the role of structurally controlled fluid flow during epithermal ore shoot formation is addressed through a structural study of the high grade martha mine epithermal deposit deseado massif patagonia argentina martha is a silver rich deposit characterised by a complex mineralogy dominated by ag as sb sulphosalts and cu pb zn sulphides mineralised veins can be subdivided into two groups master structures that trend approximately nw se and second order structures that trend e w and coalesce into the master structures the martha vein system is interpreted as a hard linked step over zone between two nw se trending fault segments furthermore the geometry of ore shoots at martha mine suggests a first order structural control on their location and orientation at the deposit scale mineralisation is concentrated in the step over zone mineralisation and presumably hydrothermal fluid flow was concentrated at the intersection between the two main vein groups finally at the scale of single veins jogs and subordinate step overs were important channels for hydrothermal fluids and the formation of high grade ore shoots

stages of late palaeozoic deformation and intrusive activity in the western part of the north patagonian massif southern argentina and their geotectonic implications analyses of structures in the western part of the north patagonian massif southern argentina suggest a polyphase evolution accompanied by continuous intrusive activity the first two deformations d 1 d 2 and metamorphism affected the upper palaeozoic partly possibly older cushamen formation clastic succession and different intrusive rocks a second group of intrusions emplaced after the second deformational episode d 2 in many places contain angular xenoliths of the foliated country rocks indicating high intrusive levels with brittle fracturing of the crust deformation of these magmatic rocks presumably began during the final stage of cooling and continued under solid state conditions it probably coincided with the third deformational event d 3 in the country rocks based on published u pb zircon ages of deformed granitoids the d 2 deformation and younger event along with the regional metamorphism are likely to be permian in age an onset of the deformational and magmatic history during carboniferous times however cannot be excluded the estimated w e to ne sw compression during the d 2 deformation also affecting the first group of intrusive rocks can be related to subduction beneath the western patagonia margin or an advanced stage of collisional tectonics within extra andean patagonia the younger n s to ne sw compression might have been an effect of oblique subduction in the west and or continuing collision related deformation as a cause for its deviating orientation younger block rotations during strike slip faulting cannot be excluded the previous d 2 event presumably also had an effect on compression at the northern patagonia margin that was interpreted as result of patagonia s late palaeozoic collision with the southwestern gondwana margin with the recently proposed carboniferous subduction and collision south of the north patagonian massif the entire scenario might suggest that patagonia consists of two different pieces that were amalgamated with southwestern gondwana during late palaeozoic times

ocean basin evolution and global scale plate reorganization events since pangea breakup we present a revised global plate motion model with continuously closing plate boundaries ranging from the triassic at 230 ma to the present day assess differences among alternative absolute plate motion models and review global tectonic events relatively high mean absolute plate motion rates of approximately 9 10 cm yr 1 between 140 and 120 ma may be related to transient plate motion accelerations driven by the successive emplacement of a sequence of large igneous provinces during that time an event at 100 ma is most clearly expressed in the indian ocean and may reflect the initiation of andean style subduction along southern continental eurasia whereas an acceleration at 80 ma of mean rates from 6 to 8 cm yr 1 reflects the initial northward acceleration of india and simultaneous speedups of plates in the pacific an event at 50 ma expressed in relative and some absolute plate motion changes around the globe and in a reduction of global mean plate speeds from about 6 to 4 5 cm yr 1 indicates that an increase in collisional forces such as the india eurasia collision and ridge subduction events in the pacific such as the izanagi pacific ridge play a significant role in modulating plate velocities

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

tectonic significance and consequences of the gondwanide orogeny in northern patagonia argentina abstract the tectonic configuration of northern patagonia is recognized through geological and geophysical studies which define several areas characterized by positive and negative gravity anomalies with steep gravity gradients positive areas are interpreted as continental crust with grenvillian and pampean basement the pampean basement occurs to the north and south of the interpreted suture between patagonia and gondwana south america most negative anomalies are interpreted to represent gondwana igneous and sedimentary rocks areas characterized by steep gravity gradients are coincident with shear zones and mylonitic belts that were active during gondwana times a period of great magmatic and tectonic activity in northern patagonia dextral movement along the e w trending huincul fault zone resulted in block collage west northwest crustal translations and counterclockwise rotation of large crustal blocks belonging to the north patagonia massif the indentation of blocks within the north patagonia massif resulted in tectonic escape of the surrounding blocks each with a different trajectory escape tectonics may explain the diversity of stress directions during gondwana orogeny

the south american ancestry of the north patagonian massif geochronological evidence for an autochthonous origin new u pb and 40 ar 39 ar age data from deformed and undeformed granitoids of the north patagonian massif establish the presence of early cambrian and widespread ordovician magmatism in northern patagonia these data suggest that the pampean cambrian and famatinian ordovician magmatic belts of the sierras pampeanas are continuous into patagonia shrimp u pb age spectra from detrital zircons of cambro ordovician metasedimentary rocks show patterns very similar to those from equivalent units of the pampia block over 500 km farther north these results suggest that the north patagonian massif was likely part of the south american margin of gondwana in the early palaeozoic and strongly argue in favour of an authochtonous or para autochthonous origin for this block

geology of the earthquake source an introduction earthquakes arise from frictional stick slip instabilities as elastic strain is released by shear failure almost always on a pre existing fault how the faulted rock responds to applied shear stress depends on its composition environmental conditions such as temperature and pressure fluid presence and strain rate these geological and physical variables determine the shear strength and frictional stability of a fault and the dominant mineral deformation mechanism to differing degrees these effects ultimately control the partitioning between seismic and aseismic deformation and are recorded by fault rock textures the scale invariance of earthquake slip allows for extrapolation of geological and geophysical observations of earthquake related deformation here we emphasize that the seismological character of a fault is highly dependent on fault geology and that the high frequency of earthquakes observed by geophysical monitoring demands consideration of seismic slip as a major mechanism of finite fault displacement in the geological record

the thermodynamics of deformed metamorphic rocks a review abstract the deformation of rocks is a disequilibrium and strongly non linear phenomenon with a number of interacting chemical thermal and microstructural processes operating simultaneously we review progress in this area over the past 30 years deforming chemically reacting systems are dissipative systems and hence are characterised by highly ordered structures that develop through cooperative processes once parameters such as critical strains strain rates fluid infiltration rates damage densities or temperatures are attained such criticality is the hallmark of deformed rocks at all length scales and is the basis for a diverse range of structures such as foliations and lineations produced by metamorphic differentiation rotation recrystallisation folding boudinage and micro to regional scale fracture systems criticality is identified with classical criticality and not self organised criticality the first and second laws of thermodynamics are used to show that such structural diversity arises from reaction diffusion deformation equations criticality of the system is associated with the stored energy becoming non convex and structures arise in order to minimise this non convex energy these structures are scale invariant and hence are characterised by fractal and minimal surface geometries thermodynamics is a powerful discipline to integrate seemingly unrelated processes in structural geology and produce an integrated approach to the subject that crosses all length scales

retrowedge related carboniferous units and coeval magmatism in the northwestern neuquen province argentina the studied carboniferous units comprise metasedimentary guaraco norte formation pyroclastic arroyo del torreon formation and sedimentary huaraco formation rocks that crop out in the northwestern neuquen province argentina they form part of the basement of the neuquen basin and are mostly coeval with the late paleozoic accretionary prism complex of the coastal cordillera south central chile u pb shrimp dating of detrital zircon yielded a maximum depositional age of 374 ma upper devonian for the guaraco norte formation and 389 ma for the arroyo del torreon formation detrital magmatic zircon from the guaraco norte formation are grouped into two main populations of devonian and ordovician famatinian ages in the arroyo del torreon formation zircon populations are also of devonian and ordovician famatinian as well as of late neoproterozoic and mesoproterozoic ages in both units there is a conspicuous population of devonian magmatic zircon grains from 406 4 ma to 369 5 ma indicative of active magmatism at that time range the ehf values of this population range between 2 84 and 0 7 and the tdm hf are mostly mesoproterozoic suggesting that the primary sources of the devonian magmatism contained small amounts of mesoproterozoic recycled crustal components the chemical composition of the guaraco norte formation corresponds to recycled mature polycyclic sediment of mature continental provenance pointing to a passive margin with minor inputs from continental margin magmatic rocks the chemical signature of the huaraco formation indicates that a magmatic arc was the main provenance for sediments of this unit which is consistent with the occurrence of tuff mostly in the arroyo del torreon formation and very scarcely in the huaraco formation with a volcanic arc signature jointly indicating the occurrence of a carboniferous active arc magmatism during the deposition of the two units the guaraco norte formation is interpreted to represent passive margin deposits of mostly lower carboniferous age younger than 374 ma and older than 326 ma that precede the onset of the accretionary prism in chile and extend into the earliest stage of the accretion in a retrowedge position the arroyo del torreon and huaraco formations are considered to be retrowedge basin deposits to the early frontal accretionary prism eastern series of chile the presence of volcanism with arc signature in the units provides evidence of a mississippian magmatic arc that can be correlated with limited exposures of the same age in the frontal cordillera argentina the arc would have migrated to the west coastal batholith during pennsylvanian permian times coevally with the later basal accretionary prism western series the source of a conspicuous population of devonian detrital zircon interpreted to be of magmatic origin in the studied units is discussed in various possible geotectonic scenarios the preferred model being a magmatic arc developed in the chilenia block related to a west dipping subduction beneath chilenia before and shortly after its collision against cuyania gondwana at around 390 ma and not linked to the independent devonian mississippian arc developed to the south in patagonia

dating polygenetic metamorphic assemblages along a transect across the western alps multichronometric analyses were performed on samples from a transect in the french italian western alps crossing nappes derived from the brianconnais terrane and the piemonte liguria ocean in an endeavour to date both high pressure hp metamorphism and retrogression history twelve samples of white mica were analysed by 39ar 40ar stepwise heating complemented by two samples from the monte rosa nappe 100 km to the ne and also attributed to the brianconnais terrane one sm nd and three lu hf garnet ages from eclogites were also obtained white mica ages decrease from c 300ma in the westernmost samples zone houille re reaching c 3008c during alpine metamorphism to 548ma in the internal units to the east which reached c 5008c during the alpine orogeny the spatial pattern of eocene k ar ages demonstrates that si rich hp white mica records the age of crystallization at 47 48ma and retains ar at temperatures of around 5008c paleocene early eocene lu hf and sm nd ages recording prograde garnet growth before the hp peak confirm eclogitization in eocene times petrological and microstructural features reveal important mineralogical differences along the transect all samples contain mixtures of detrital syn d1 and syn d2 mica and retrogression phases d3 in greatly varying proportions according to local variations in the evolution of pressure temperature fluid activity deformation p t a d conditions samples from the zone houille re mostly contain detrital mica the abundance of white mica with si46 45 atoms per formula unit increases eastward across the whole traverse phengitic mica grown during hp metamorphism defines the d1 foliation syn d2 mica is more si poor and associated with nappe stacking exhumation and hydrous retrogression under greenschist facies conditions syn d1 phengite is very often corroded overgrown by or intergrown with syn d2 muscovite most importantly syn d2 recrystallization is not limited to s2 schistosity domains micrometrescale chemical fingerprinting reveals muscovite pseudomorphs after phengite crystals which could be mistaken for syn d1 mica based on microstructural arguments alone the cl k ratio in white mica is a useful discriminator as d2 retrogression was associated with a less saline fluid than eclogitization as petrology exerts the main control on the isotope record constraining the petrological and microstructural framework is necessary to correctly interpret the geochronological data described in both the present study and the literature our approach which ties geochronology to detailed geochemical petrological and microstructural investigations identifies 47 48ma as the age of hp formation of syn d1 mica along the studied transect and in the monte rosa area cretaceous apparent mica ages which were proposed to date eclogitization by earlier studies based on conventional thermochronology are due to ar inheritance in incompletely recrystallized detrital mica grains the inferred age of the probably locally diachronous greenschist facies low si syn d2 mica ranges from 39 to 43ma coexistence of d1 and d2 ages and the constancy of non reset d1 ages along the entire transect provides strong evidence that the d1 white mica ages closely approximate formation ages volume diffusion of ar in white mica activation energy e 250 kj mol 1 pressure adjusted diffusion coefficient d 050 03 cm2 s 1 has a subordinate effect on mineral ages compared with both prograde and retrograde recrystallization in most samples

tracking changes in crustal thickness during orogenic evolution with sr y an example from the north american cordillera global compilations indicate that the geochemistry of arc magmatism is sensitive to moho depth magmatic products are prevalent throughout the history of cordilleran orogenesis and can be employed to constrain the timing of changes in crustal thickness as well as the magnitude of those changes we investigate temporal variations in crustal thickness in the united states cordillera using sr y from intermediate continental arc magmas our results suggest that crustal thickening began during the late jurassic to early cretaceous and culminated with 55 65 km thick crust at 85 95 ma crustal thicknesses remained elevated until the mid eocene to oligocene after which time crustal thicknesses decreased to 30 40 km in the miocene the results are consistent with independent geologic constraints and suggest that sr y is a viable method for reconstructing crustal thickness through time in convergent orogenic systems

the gondwana connections of northern patagonia a multidisciplinary study u pb sensitive high resolution ion microprobe geochronology hf and o isotopes in zircon sr and nd isotopes in whole rocks as well as major and trace element geochemistry has been carried out on granitoid samples from the area west of valcheta north patagonian massif argentina these confirm the cambrian age of the tardugno granodiorite 528 4 ma and the late permian age of granites in the central part of the yaminue complex 250 ma together with petrological and structural information for the area we consider a previously suggested idea that the cambrian and ordovician granites of northeastern patagonia represent continuation of the pampean and famatinian orogenic belts of the sierras pampeanas respectively our interpretation does not support the hypothesis that patagonia was accreted in late palaeozoic times as a far travelled terrane originating in the central transantarctic mountains and the arguments for and against this idea are reviewed a parautochthonous origin is preferred with no major ocean closure between the north patagonian massif and the sierra de la ventana fold belt supplementary material u pb shrimp analytical data geochemical analyses and sample global positioning system locations are available at www geolsoc org uk sup18722

paleoenvironmental changes from stable isotopes in planktonic foraminifera from eastern mediterranean sapropels abstract parallel stable oxygen isotope analyses of six planktonic foraminiferal species from an eastern mediterranean deep sea core present clear evidence for abnormal hydrographic regimes during deposition of sapropels s6 and s7 samples from s7 are characterized by a strong depletion of 18 o in epipelagic species while meso and bathypelagic forms are much less depleted this indicates a shallow pycnocline during deposition of s6 all species show a tendency towards lower values pointing to a far more extended pycnocline density stratification of the watercolumn is suggested by the isotopic differences between the species neogloboquadrina dutertrei globigerina bulloides and globorotalia scitula the latter represents the deepest element and shows the highest 18 o values carbon isotopes show two species groups each containing an epi meso and bathypelagic species under normal marine conditions group i globigerinoides ruber neogloboquadrina dutertrei and globorotalia truncatulinoides has relatively high 13 c values while species of group ii globigerina bulloides globorotalia scitula and globigerina quinqueloba show relatively low 13 c values indicating a stronger vital effect during sapropel deposition 12 c isotopes are enriched which we interpreted as the result of increased biological activity due to low oxygen stress conditions g scitula shows different eco phenotypes during times of sapropel deposition and the same may be true for the other members of group ii failure to adapt to adverse environmental conditions may be the cause for the absence of the two group i members globigerinoides ruber and globorotalia truncatulinoides in sapropel s6

monazite allanite phase relations in metapelites abstract calculations of the transition from allanite to monazite bearing assemblages in typical pelitic bulk compositions have been made using thermodynamic data estimated from oxide sums and inferred from natural parageneses calculations in the cfashpce and mnnckfmashpce systems place the allanite to monazite transition in the middle amphibolite facies 525 600 c for a bulk composition similar to shaw s average pelite the temperature of the transition is pressure dependent and strongly dependent on the bulk rock cao content consistent with inferences from natural parageneses the transition is also a function of the bulk al 2 o 3 content although the calculated result is opposite to that inferred from natural samples comparison with published results in the la mg system suggest that the nature of the ree phosphate does not greatly influence the conditions of the transition

provenance variations in the late paleozoic accretionary complex of central chile as indicated by detrital zircons abstract we present detrital zircon u pb shrimp age patterns for the central segment 34 42 s of an extensive accretionary complex along coastal chile together with ages for some relevant igneous rocks the complex consists of a basally accreted high pressure low temperature western series outboard of a frontally accreted eastern series that was overprinted by high temperature low pressure metamorphism eleven new shrimp detrital zircon age patterns have been obtained for meta turbidites from the central 34 42 s segment of the accretionary complex four from previously undated metamorphic complexes and associated intrusive rocks from the main andean cordillera and three from igneous rocks in argentina that were considered as possible sediment source areas there are no mesozoic detrital zircons in the accretionary rocks early paleozoic zircons are an essential component of the provenance and grenville age zircons and isolated grains as old as 3 ga occur in most rocks although much less commonly in the western series of the southern sector in the northernmost sector 34 38 30 s proterozoic zircon grains constitute more than 50 of the detrital spectra in contrast with less than 10 in the southern sector 39 42 s the youngest igneous detrital zircons in both the northern western 307 ma and eastern series 345 ma are considered to closely date sedimentation of the protoliths both oxygen and lu hf isotopic analyses of a selection of permian to neoproterozoic detrital zircon grains indicate that the respective igneous source rocks had significant crustal contributions the results suggest that early paleozoic orogenic belts pampean and famatinian containing material recycled from cratonic areas of south america supplied detritus to this part of the paleo pacific coast in contrast in the southern exposures of the western series studied here permian detrital zircons 253 295 ma dominate indicating much younger deposition the northern sector has scarce early to middle devonian detrital zircons prominent south of 39 s the sedimentary protolith of the northern sector was probably deposited in a passive margin setting starved of devonian achalian detritus by a topographic barrier formed by the precordillera and possibly chilenia terranes devonian subduction related metamorphic and plutonic rocks developed south of 39 s beyond the possible southern limit of chilenia where sedimentation of accretionary rocks continued until permian times

electron microprobe observations of pb diffusion in metamorphosed detrital monazites electron microprobe analyses have been made on monazite grains from paragneiss samples in the andalusite sillimanite transition 620 15 c and sillimanite orthoclase 680 15 c zones of the cretaceous ryoke metamorphic belt southwest japan monazites from pelitic gneisses are of metamorphic origin euhedral to subhedral and chronologically homogeneous giving chemical th u total pb isochron chime ages of 98 8 3 3 98 0 3 2 ma two psammitic gneisses of individual metamorphic grade contain both metamorphic monazite grains and detrital ones as old as ca 1700 ma most detrital monazite grains are heterogeneous in the tho2 and uo2 concentrations and have multiple or single rims as young as ca 100 ma several detrital monazite grains are well rounded in form exhibit homogeneous th and u distributions and show a pb diffusion profile in the margin the width of the diffusion zones is approximately constant throughout grains from each psammitic gneiss 18 22 m for 620 c and 48 58 m for 680 c assuming the isothermal diffusion of pb from homogeneous monazite spheres during a 5 ma duration of peak metamorphism we obtain diffusion coefficients of 1 9 0 3 10 21 and 1 5 0 3 10 20 cm2 s at 620 c and 680 c respectively these data derive an activation energy of 2 44 2 85 1 26 105 j mol and a frequency factor of 3 4 10 7 8 5 10 12 2 2 107 cm2 s taking account of uncertainties of 15 c in the temperatures and of 20 in the diffusion coefficients the diffusion parameters obtained from natural samples in this study provide a reliable insight into the closure temperature for pb in monazite that has been poorly understood so far

pb diffusion in monazite a combined rbs sims study abstract we report measurements of pb diffusion in both synthetic cepo4 and natural monazites run under dry 1 atm conditions powdered mixtures of prereacted cepo4 and pbzro3 were used as the source of pb diffusant for in diffusion experiments conducted in sealed pt capsules for durations ranging from a few hours to several weeks following the diffusion anneals pb concentration profiles were measured with rutherford backscattering spectroscopy rbs and supplemented by measurements with secondary ion mass spectrometry sims in order to evaluate potential compositional effects upon pb diffusivity and simulate diffusional pb loss that might occur in natural systems we also conducted out diffusion experiments on pb bearing natural monazites in these experiments monazite grains were surrounded by a synthetic zircon powder to act as a sink monazites from these experiments were analyzed with sims over the temperature range 1100 to 1350 c the arrhenius relation determined for in diffusion experiments on synthetic monazite is given by d 0 94 exp 592 39 kj mol 1 rt m 2 s 1 diffusivities for synthetic and natural monazites are similar as are results of measurements made on the same samples using both rbs and sims the activation energy for pb diffusion we determined is more than three times that for natural monazite reported previously with pb diffusivities at

evolution of the late paleozoic accretionary complex and overlying forearc magmatic arc south central chile 38 41 s constraints for the tectonic setting along the southwestern margin of gondwana stratigraphic structural metamorphic and geochronologic studies of basement rocks in the andean foothills and coast ranges of south central chile 39 41 s suggest a protracted late paleozoic to middle mesozoic deformational and metamorphic history that imposes important constraints on the tectonic development of the southwestern gondwana margin in the study area the late paleozoic paired metamorphic belt coeval magmatic arc and overlying triassic sedimentary units preserve a record of late carboniferous to early permian subduction and arc magmatism subsequent deep exhumation of the western series subduction complex and diminished uplift and erosion of the eastern series arc forearc region by the late triassic late paleozoic structural elements and metamorphic assemblages formed during early subduction and arc magmatism collectively referred to as dl are largely erased in the western series by the dominant d2 schistosity and lower greenschist grade metamorphism d1 structural features as well as original sedimentary textures are relatively well preserved in the less penetratively deformed eastern series the regional distribution of late paleozoic arc magmatism suggests that the late paleozoic convergent margin deviated from a n s trend north of this area to a nw se trend near this latitude and faced an open marine environment to the southwest a transition from f2 isoclinal folding to more open larger scale f3 folds interpreted as change in ductility during differential uplift of the western series is not apparent in the eastern series despite a lesser degree of uplift during the main exhumational d2 event delineation of unconformities and u pb dating of detrital zircons and intrusions into the eastern series allow tighter constraints to be placed on timing of uplift and denudation of the eastern series than on that in the western series a regional unconformity exposed in the lake district that separates more highly deformed eastern series lithologies from late triassic shallow marine to continental deposits suggests that substantial uplift also affected the inner forearc and magmatic arc region during the d2 event we propose that dextral oblique convergence initiated during the middle permian along this segment of the gondwana margin resulted in the transpressional uplift and juxtaposition of high pressure temperature p t western series against low p t eastern series lithologies and culminated with deposition of late triassic continental to shallow marine coarse clastic sedimentary rocks in fault bounded strike slip basins adjacent to the exhumed western series large scale dextral transpression and northward displacement of the accretionary complex during late permian to late triassic time along the chilean margin of gondwana are synchronous and kinematically compatible with widespread regional transpression extension and silicic magmatism inboard of the southern gondwana margin at this time we thank c mpodozis m gardeweg and j munoz of the servicio de geologia y mineria de chile sernageomin for their support of this work fruitful discussions with n blanco f herve h moreno c mpodozis and f munizaga have aided in our understanding of the geology of the region the hard work by the staff of sernageomin s puerto varas office is graciously appreciated we thank j d walker and w r van schmus at the university of kansas for allowing mwm use of their u pb and mass spectrometer facilities and j vargas and the staff of sernageomin s geochemistry laboratory for their assistance in this project f munizaga allowed us to cite an unpublished 40ar 39ar date we thank g ya nez for access to aeromagnetic data t kato wishes to thank w g ernst comments by i dalziel s kay and v ramos helped clarify ideas presented in this paper and are greatly appreciated this work is dedicated to our friend and colleague alberto campos c who died in a climbing accident on calbuco volcano 1996

fluid mediated partial alteration in monazite the role of coupled dissolution reprecipitation in element redistribution and mass transfer monazite ce lree th u ca p si o4 with complex zoning in th and other elements is commonly observed in metamorphic and igneous rocks the hypothesis that this alteration is a product of fluid mediated element mass transfer has been tested in the piston cylinder press caf2 assembly cylindrical graphite oven at 1 000 mpa and 900 c and in cold seal autoclaves on a hydrothermal line at 500 mpa and 600 c experiments included a relatively homogeneous monazite ce 7 8 wt tho2 from a heavy mineral sand plus a series of alkali bearing fluids including 2n naoh 2n koh and na2si2o5 h2o experiments were conducted using bse imaging emp analysis and both tem and hrtem a subset of monazite grains from each experiment show evidence of partial alteration in the form of areas enriched in th si with sharp curvilinear compositional boundaries extending from the grain rim into the monazite interior these thsio4 enriched textures are similar to those commonly seen in natural examples of metasomatised monazite in both magmatic and metamorphic rocks in the na2si2o5 h2o experiments scarce inclusions of britholite formed in the altered monazite the altered monazite is also characterised by strong depletion in pb ca and y thorium and si mobility coupled with the formation of britholite inclusions during partial alteration in the monazite grain is considered to be the product of fluid aided coupled dissolution reprecipitation as opposed to solid state diffusion since other fluids including nacl and kcl brines do not result in the formation of these textures the experimental replication of thsio4 enriched areas in the monazite strongly suggests that similar textures in monazite observed in nature are fluid induced specifically by alkali bearing fluids if true complex metasomatically induced textures in monazite could yield information concerning the nature of the fluid responsible for their formation as well as allow for the dating of the metasomatic event presuming that all the original radiogenic pb has been removed

did the taconic appalachians continue into southern south america the appalachian mountains now terminating abruptly at the gulf of mexico coastal plain may have formerly continued into southern south america rocks forming the basement of the argentine andes can be interpreted as remnants of an early paleozoic orogen the famatinian belt not unlike the taconic appalachians both orogens are bordered to the west present coordinates by lower paleozoic carbonate platforms bearing the olenellid trilobite fauna that is characteristic of laurentia paleomagnetic and geologic data indicate that they could have formed as one continuous mountain chain possibly extending into antarctica during ordovician closure of an ocean basin southern iapetus between laurentia and gondwana the taconic and famatinian segments of the chain may have been truncated during late ordovician separation of laurentia and gondwana along the preexisting late neoproterozoic to cambrian rift system that initiated formation of the ouachita embayment and the southern margin of north america

structure and metamorphism of the gondwanan basement in the bariloche region north patagonian argentine andes in the bariloche region the gondwanan basement consists of metamorphic and deformed igneous rocks intruded by mesozoic and tertiary granitoids metasediments and amphibolites have been affected by three main deformation events d1 d2 and d3 d1 s1 foliation is well shown preserved in the microlithons of the s2 regional foliation and in albite and garnet porphyroblasts both foliations have been folded by upright open folds with associated crenulation s3 locally developed in the d3 folds hinge zones an early stage syn d1 subduction episode can be invoked based on evidence of high pressure hp metamorphism in schists and amphibolites as well as on the presence of pre collisional i type granitoids d2 d3 developed under regional intermediate pressure low pressure ip lp metamorphic conditions high temperature ht conditions were reached as deduced from regional migmatization and the intrusion of syn to late orogenic s type granitoids the transition from hp to ip metamorphic conditions could be associated with the emplacement of ne directed crustal scale thrusts during d2

inception of a devonian subduction zone along the southwestern gondwana margin 40ar 39ar dating of eclogite amphibolite assemblages in blueschist boulders from the coastal range of chile 41 s coarsely crystalline residual blueschist boulders from the coastal range of south central chile 41 s contain relict eclogite amphibolite assemblages that provide evidence of pre carboniferous hig

partial resetting of the upb isotope system in monazite through hydrothermal experiments an sem and upb isotope study abstract natural monazites of different grain sizes were treated experimentally under hydrothermal conditions and their u pb isotope systematics measured before and after the runs runs were carried out with monazite grain size fractions ranging from 40 to 125 m in diameter and on 207 pb 206 pb ages close to 380 ma u concentrations remained always unchanged all recrystallized monazites from the powdered starting material define a discordia with intercept ages of 379 3 2 3 and 7 18 ma the latter indicating the hydrothermally induced pb loss at the time of the experiment sem observations of monazite grains from hydrothermally treated grain size fractions exhibited only minor dissolution features confined to the grain surfaces run products from monazite powders showed recrystallization and grain growth induced by a dissolution precipitation mechanism increasing temperature resulted in a distinct enlargement of grain sizes at 650 and 750 c a dissolution precipitation process in presence of a fluid phase rather than pb volume diffusion acts as a very efficient mechanism for the resetting of the u pb isotope system of monazites even at low temperatures this process is most important in shear zones where channelized fluid flow may occur

metamorphic response in orogens of different obliquity scale and geometry abstract investigation of material flow within transpressional orogens must involve integration of structural and metamorphic datasets to illustrate the problems in documenting flow vectors we present integrated structural metamorphic datasets from two transpressional systems the kaoko belt in namibia and the kalinjala shear zone in south australia these orogens experienced widely differing metamorphic responses to transpressional deformation integration of kinematic and metamorphic datasets from the kaoko belt indicate shallow up plunging extrusion trajectories in the orogen core and show that the maximum stretching direction pattern matches the inferred flow vectors high grade domains 800 840 c and 7 0 8 0 kb in the orogen core developed low angle upward verging maximum stretching direction trajectories whereas a low grade domain 575 600 c and 5 0 5 5 kb in the orogen core has downward verging lineation trajectories the barometric differential between these high grade and low grade domains is entirely consistent with the angle of plunge of maximum stretching directions within the high grade domains that were extruded obliquely for the amount of lateral shear estimated for the orogen core the kalinjala shear zone in south australia contrasts strongly with the kaoko belt in this example the high grade and high strain shear zone core of the orogen experienced high t high p metamorphism with low thermal gradients of 21 26 c km and steep decompressive p t paths the lower grade external domains experienced lower t lower p metamorphism with high thermal gradients of 35 37 c km sub horizontal maximum stretching directions do not match the vertical extrusional flow in the high grade core that is indicated by the metamorphic data this comparison shows that in general and on a gross scale maximum stretching directions do not necessarily correlate with the real flow vectors experienced during orogenesis in some cases maximum stretching direction recorded by deformation structures is to some degree decoupled from the vertical component of material flow consequently information pertaining to flow is often partitioned into information derived from deformation structures and information derived from the metamorphic record these two datasets must be used in concert to obtain realistic constraints on first order material flow trajectories at orogenic scales the horizontal component of flow is typically best recorded by structural fabrics maximum stretching direction and sense of shear whereas the vertical component is typically best recorded by metamorphic information such as p t paths temperature over depth ratio g and metamorphic field gradients i e t p and g across the orogen

the late paleozoic tectonometamorphic evolution of patagonia revisited insights from the pressure temperature deformation time p t d t path of the gondwanide basement of the north patagonian cordillera argentina oriolo sebastian conicet universidad de buenos aires instituto de geociencias basicas aplicadas y ambientales de buenos aires igeba argentina

mid cretaceous polar standstill of south america motion of the atlantic hotspots and the birth of the andean cordillera article i nfo a paleomagnetic study on mid cretaceous rocks from the san bernardo foldbelt patagonia yields high unblocking temperature and high coercivity magnetizations the results indicate absence of relative vertical axis rotations during development of the foldbelt with the associated pole position being highly concordant with coeval poles from brazil and patagonia taken together mid cretaceous poles derived from studies in widely distributed localities provide supportive evidence that south america was essentially motionless with respect to the paleomagnetic axis from ca 125 to at least 100 ma the paleolatitudes of south america are not consistent with the occurrence of mid cretaceous true polar wander suggesting that the previously observed discrepancy between the paleomagnetic and the fixed indo atlantic hotspot reference frames be related to motion of the atlantic hotspots in agreement with this the discrepancy is diminished by half when the cretaceous poles of the americas are observed in a moving hotpots reference frame with the residual offset being comparable to that seen for younger time intervals the south american paleopoles and the moving hotspot framework provide a kinematic scenario that allows relating the extensional tectonics in the early stages of andean evolution with episodic divergence between the trench and the continental interior likewise the beginning of contractional events correlates with model predicted westward acceleration of south america in the late cretaceous suggesting that the continent episodically overrode the andean trench by those times we argue that this change in andean tectonic regime is associated to major plate reorganization at ca 95 ma

thermochemistry of monazite la and dissakisite la implications for monazite and allanite stability in metapelites thermochemical properties have been either measured or estimated for synthetic monazite lapo4 and dissakisite calamgal2 sio4 3oh the mg equivalent of allanite a dissakisite formation enthalpy of 6 976 5 10 0 kj mol 1 was derived from high temperature drop solution measurements in lead borate at 975 k a third law entropy value of 104 9 1 6 j mol 1 k 1 was retrieved from low temperature heat capacity c p measured on synthetic lapo4 with an adiabatic calorimeter in the 30 300 k range the c p values of lanthanum phases were measured in the 143 723 k range by differential scanning calorimetry in this study la oh 3 appeared as suitable for drop solution in lead borate and represents an attractive alternative to la2o3 pseudo sections were calculated with the theriak domino software using the thermochemical data retrieved here for a simplified metapelitic composition la ree y and considering monazite and fe free epidotes along the dissakisite clinozoisite join as the only ree bearing minerals calculation shows a stability window for dissakisite clinozoisite epidotes t between 250 and 550 c and p between 1 and 16 kbar included in a wide monazite field the p t extension of this stability window depends on the bulk rock ca content assuming that synthetic lapo4 and dissakisite la are good analogues of natural monazite and allanite these results are consistent with the ree mineralogy sequence observed in metapelites where 1 monazite is found to be stable below 250 c 2 around 250 450 c depending on the pressure allanite forms at the expense of monazite and 3 towards amphibolite conditions monazite reappears at the expense of allanite