nd sr isotopic composition of granulites and constraints on the evolution of the lower continental crust in a statistical study of granulite facies rocks we have analysed 32 samples with ages ranging from 2 900 to 200 myr for sm nd rb and sr and 143nd 144nd and 87sr 86sr ratios the sm nd ratio is found to be constant compared with the highly variable rb sr ratio using the model age formalism we show that in many cases internal differentiation of continental crust follows external differentiation with a large time interval

pegmatite emplacement in the serido belt northeastern brazil late stage kinematics of the brasiliano orogen abstract geometric and kinematic analysis was performed in an area located in the central part of the serido belt ne brazil where supracrustal rocks affected by polyphase deformation are well exposed the first event recognized in this area and regionally known as the d 2 deformation is characterized by top to the south thrust tectonics while a second one d 3 deformation is marked by upright folds strike slip or transpressive shear zones and the development of flower structures major pegmatite swarms were emplaced during and late as regards the second event dated ca 580 ma being part of the brasiliano orogeny similar dyke swarms are known from the nigerian shield these pegmatite swarms provide reliable kinematic markers of the late evolutionary stage of the neoproterozoic trans sahara borborema collisional belt mineralogical geometric and kinematic features support two stages of pegmatite emplacement during the strike slip event i older syn d 3 homogeneous pegmatites intruded mostly along lithological and structural discontinuities such as foliation surfaces ii late d 3 heterogeneous pegmatites were emplaced along tension gashes and other dilation structures the heterogeneous pegmatites are economically more important being exploited for precious metals and stones as well as industrial minerals

brazilian structural provinces an introduction abstract the territory of brazil coincides almost entirely with the south american platform the crystalline core of the continent its basement is composed of ancient metamorphic and igneous rocks and it has not suffered any tectonic regeneration since the beginning of the phanerozoic sedimentary rocks with almost horizontal bedding cover this crystalline basement this latter shows ages as old as early precambrian although ages between 500 and 1000 m y are conspicuously frequent the cratonic areas became consolidated more than 1700 m y ago whereas the fold belts formed essentially between 500 and 1700 m y ago the sedimentary cover accumulated from the early silurian in three large intracratonic basins until the platform became completely stabilized a late jurassic to early cretaceous reactivation caused by the break up of the ancient gondwana continent created another basin sequence chiefly along the atlantic continental margin based on the nature of the crystalline basement rocks and the sedimentary cover ten structural provinces can be distinguished 1 rio branco province in the north of the country occupied by the guyana shield and still only poorly known with an important fold belt developed during the so called transamazonian cycle 2000 200m y with high grade metamorphic rocks and only slight influence of later events 2 tapajos province in central brazil corresponding to the amazonas or guaporecraton showing chiefly precambrian crystalline rocks and behaving as an cratonic area during the phanerozoic 3 sa o francisco province located on the atlantic shield with its basement covered by rocks of different ages chiefly affected by the brasiliano cycle between 1000 and 500 m y and constituting another cratonic area 4 tocantins province between the amazonas and sa o francisco cratons with the oldest rocks in its centre ages over 2600 m y and at the eastern and western borders metamorphic sequences of various fold belts and almost no phanerozoic deposits 5 mantiqueira province located along the southern part of the atlantic coast affected chiefly by the brasiliano folding cycle 6 borborema province in the northeast brazilian fold belt affected by the brasiliano cycle in a very complex way and with important faulted zones it was reactivated in phanerozoic times when also sedimentary covers accumulated 7 amazonas province represented by the amazonas sedimentary basin syneclise and subdivided into four parts by three important arcs 8 parnaiba province coinciding with the piauf maranha o syneclise and filled with a rather thick sedimentary sequence 9 paranaprovince the sedimentary basin of southern brazil in which the well known late paleozoic glaciation features are found and which possesses an extensive cover of basaltic rocks of late jurassic to early cretaceous age 10 coastal province and continental margin the youngest structural unit developed during the separation of the continent and represented by rift valleys and coastal basins filled with mesozoic cenozoic deposits of various kinds

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

deformation and geochronology of syntectonic granitoids emplaced in the major gercino shear zone southeastern south america abstract the major gercino shear zone is one of the ne sw lineaments that separate the neoproterozoic dom feliciano belt of brazil and uruguay into two different domains a northwestern supracrustal domain from a southeastern granitoid domain the shear zone striking ne is composed of protomylonites to ultramylonites with mainly dextral kinematic indicators in santa catarina state southern brazil the shear zone is composed of two mylonite belts the mylonites have mineral orientations produced under greenschist facies conditions at a high strain rate strong flattening and coaxial deformation indicate the transpressive character while the role of pure shear is emphasized by the orientation of the mylonite belts in relation to the inferred stress field component the quartz microstructures point out that different dynamic recrystallization regimes and crystal plasticity were the dominant mechanisms of deformation during the mylonitization process additionally the fabrics suggest that the glide systems are activated for deformation conditions compatible with the metamorphism in the middle greenschist facies elongated granitoid intrusions belonging to two petrographically geochemically and isotopically distinct rock associations occur between the two mylonite belts the structures observed in the granites result from a deformation range from magmatic to solid state conditions points to a continuum of magma straining during and just after its crystallization conventional u pb analysis of multi crystal zircon fractions yielded essentially identical ages of 609 16 ma and 614 2 ma for the two granitic associations and constrain the transpressive phase of the shear zone k ar ages of biotites between 585 and 560 ma record the slow cooling and uplift of the intrusions some k ar ages of micas in regional mylonites are similar suggesting that thermo tectonic activity was intense up to this time probably related to the agglutination of the granite belt to the supracrustal belt nw of the mgsz

tightening up ne brazil and nw africa connections new u pb lu hf zircon data of a complete plate tectonic cycle in the dahomey belt of the west gondwana orogen in togo and benin the dahomey belt in togo and benin is an important segment of the larger west gondwana orogen here we review the geodynamic evolution of the dahomey belt and discuss new u pb and lu hf zircon data in light of similar data previously acquired on the geologically related northern borborema province in ne brazil eighteen samples from different tectonic settings and regions within the belt were collected for zircon isotopic investigation passive margin deposits of the atacora structural unit and lower units of the volta basin have detrital zircon signatures compatible with the flanking west africa craton the arc related magmatism resulted from the east dipping subduction of the goias pharusian oceanic lithosphere and is represented by a variety of granitoids emplaced in the benino nigerian shield between 670 and 610 ma these granitoids were mainly sourced from crustal reservoirs with subordinate juvenile input detrital zircon ages from syn orogenic deposits in benino nigerian shield suggest that arc development could have started as early as 780 ma the main period of melting in the internal part of the belt the benino nigerian shield is related to crustal thickening and occurs only ca 30 m y after initiation of the continental collision marked by the ca 610 ma ultra high pressure uhp metamorphism recorded at lato hills foreland development represented by the upper units of the volta basin developed soon after continental collision and persisted with the development of the west verging thrust front synchronously with the main period of crustal melting due to collision at ca 580 ma the subvertical transbrasiliano lineament in south america that corresponds to the kandi lineament in africa provides a present day fit between nw africa and ne brazil restoration of the movement of the transbrasiliano kandi lineament strike slip plate boundary places the dahomey belt and borborema province ne brazil along the same section of the west gondwana orogen this configuration would explain some of the misfits previously discussed in the literature and aligns the uhp eclogites in togo and ne brazil

boundary lines within petrologic diagrams which use oxides of major and minor elements use of some petrologic diagrams applied to analyses of volcanic rocks is unnecessarily difficult due to lack of data for construction of discriminant lines between rock series coordinates are provided for sufficient points to enable accurate plotting of the boundary lines within seven diagrams viz 1 tas total alkalies na2o k2o vs sio2 2 k2o vs sio2 3 afm 4 jensen 5 ktp k2o tio2 p2o5 6 fms feo mgo vs sio2 and 7 taktip k2o na2o k2o vs tio2p2o5 different versions of these boundaries are collated to indicate their variable position and it is demonstrated that inter laboratory analytical precision suffices to account for almost all of their spread on the tas and k2o vs sio2 diagrams

a complete wilson cycle recorded within the riacho do pontal orogen ne brazil implications for the neoproterozoic evolution of the borborema province at the heart of west gondwana abstract the riacho do pontal orogen rpo is part of the extensive brasiliano pan african orogenic system in northeastern south america borborema province this region is crucial for understanding the neoproterozoic evolution of west gondwana because it occupies a strategic position between three of its most important constituents namely the sao francisco congo amazon and west africa cratons based on lithostratigraphic lithochemical geochronological and isotopic data a five stage model for the geodynamic evolution of the rpo and adjacent areas in the borborema province is proposed 1 rift phase development of a triple junction rift system at ca 900 820 ma ago probably related to the emplacement of a mantle plume and leading to intense mafic ultramafic magmatism of the brejo seco layered intrusion ca 900 ma this was followed by the development of a continental rift system testified by the paulistana complex metavolcanosedimentary sequence ca 882 ma 2 drift phase with continued lithospheric thinning the rift system further evolved to a broad passive margin in the northern sao francisco craton edge represented by the barra bonita formation platformal sediments the drift phase culminated in the development of true oceanic crust represented by the monte orebe ophiolite around 820 630 ma ago 3 convergence phase at circa 630 620 ma onset of subduction in the borborema province caused the inversion of the basins obduction of oceanic crust slices and sedimentation of the mandacaru formation syn orogenic greywackes 4 collisional phase continental collision between the sao francisco craton lower plate and the pernambuco alagoas block upper plate occurred around 620 590 ma with the stacking of the casa nova nappes upon the lower plate crustal thickening deformation metamorphism melt generation and intrusion of the syn collisional rajada suite two mica granites 5 lateral escape phase the collisional stage was followed by a lateral escape tectonics stage around 590 530 ma that generated the western branch of the e w trending pernambuco shear zone which truncates the northern part of the orogen and displaces its counterparts in the transversal zone in a dextral sense shear this phase was accompanied by extensive alkaline magmatism in the form of the serra da aldeia caboclo suites thus the rpo represents a complete plate tectonic cycle that spans the whole neoproterozoic and preserve excellent time markers for both the break up and dispersion of the rodinia supercontinent fragments and for the re amalgamation of those fragments in west gondwana the interpretation of a complete wilson cycle in this area challenges current views of the borborema province as a coherent block since paleoproterozoic times part of the atlantica supercontinent suggesting instead a dynamic setting where multiple plates interacted to form the heart of west gondwana

the paranapanema lithospheric block its importance for proterozoic rodinia gondwana supercontinent theories abstract this work considers the tectonics of the southeastern portion of the south american platform based on new geological and geophysical grounds for the last decade only three amazonic sao francisco and la plata of the many other cratonic blocks have been attributed remarked to the south america portion for most of the usual rodinia reconstitutions the possibility of the existence of other blocks has rarely been mentioned the postulation of the presence of a considerable paleoproterozoic pre brasiliano fragment as part of parana basin basement is highly probable in order to infer the basement structure of parana basin previous to the sedimentation process an isostatic modeling was applied to a large scale gravity survey looking to correlate topographic and gravity anomalies caused by sub surface loads the bouguer anomaly obtained from the gravity survey represents the crustal contribution of crystalline basement in addition to the sedimentary and volcanic layers of the basin following the isostatic modeling and the basin load stripping the residual anomaly allows observing similarities between the basement gravity signature and outcropping units besides the stress pattern of the two earlier events obtained through the back stripping analysis presents a geographically coincident maximum and a new e se high emerging for the second event suggesting continuous change of the stress field as a precursor for south american plate rotation the evident correlation between gravity highs and main attenuation suggests the presence of some pre existing suture zones the weakened lithosphere during ordovician and carboniferous provided the magma conduits to form in early cretaceous tectonic stress field pattern the resultant mosaic of gravity blocks and the main faults site give support to the presence of this cratonic proterozoic unit here on referred to as the paranapanema block which had been neglected in most of the models reported for the reconstruction of gondwana and rodinia

control of granite emplacement by regional deformation abstract a combination of structural measurements and gravity data is used to investigate the mode of granite emplacement in various tectonic settings the deepest zones revealed by gravity data provided those zones also show vertical lineations in outcrop are interpreted as magma feeder channels these structures or root zones reflect the flow pattern of the upwelling magma while being emplaced they can be interpreted within the regional stress field active during that time granites intruded during transcurrent shearing have only a few feeder channels these roots are not connected to the main shear zone all of them lie within locally extensional areas of the regional stress field this situation is also observed in granites emplaced during a compressional phase of deformation when emplaced during a regionally extensional phase the granitic plutons are very thin with many small deeper zones in which variously evolved granite types may be intrusive these regions vertically differentiated are also interpreted as feeder channels the different morphology at depth as well as the difference in the number of root zones suggest that deformation controls the emplacement of granitic plutons the root zones are oriented at a high angle to the direction of maximum compression when magma is emplaced in a plastic crust if the pluton is emplaced higher in a brittle crust then the roots are aligned with the maximum compression according to the andersonian theory of fracture the orientation of the root zone along the direction of extension in a plastic crust may be explained by shearing which acts as a valve mechanism for segregating magma in all the examples the geometry of the root zone relative to the major fault plane implies that the shear zone does not induce magma generation and that it does not serve as a guide for magma emplacement

a suture related accretionary wedge formed in the neoproterozoic aracuai orogen se brazil during western gondwanaland assembly abstract the aracuai orogen represents a branch of the brasiliano orogenic system developed between the sao francisco and congo cratons in neoproterozoic time we conducted detailed studies on a complex schist belt located to the west of the rio doce magmatic arc along the assumed suture zone of the aracuai orogen this 30 km wide and 100 km long ns trending belt includes pelitic schists with intercalations of quartzites metaultramafic schists and diopsidites intruded by collisional granites u pb ages from detrital zircon grains point to distinct provenances for different portions of the schist belt the lower succession shows an age spectra and maximum depositional age 819 ma similar to passive margin deposits of the precursor basin the upper succession yielded a maximum deposition age around 600 ma pointing toward the rio doce arc as the main sediment source thrust to the west onto the guanhaes basement and separated on the east from the ediacaran rio doce magmatic arc by reverse dextral faults the schist belt exhibits the architecture of an asymmetric flower structure developed in transpressional regime the distribution of metamorphic assemblages across the schist belt characterizes a collisional barrovian type metamorphic zoning in which the sillimanite kyanite staurolite and garnet zones are locally duplicated by thrusts p t conditions range from 700 c at 7 5 kbar at the western base of the pile to 550 c at 5 5 kbar at the eastern top of the package zircon u pb ages record metamorphic overprinting on the sheared top of the basement at 560 20 ma and crystallization of collisional granites at 544 10 ma our results allow us to interpret the schist belt as a suture related accretionary wedge and suggest that basin closure during the assembly of west gondwanaland lasted to the ediacaran cambrian boundary in the aracuai orogen

timing of crust formation deposition of supracrustal sequences and transamazonian and brasiliano metamorphism in the east pernambuco belt borborema province ne brazil implications for western gondwana assembly abstract the main structural feature of the central domain of borborema province ne brazil is a network of dextral and sinistral shear zones these shear zones rework an older regionally developed flat lying foliation in orthogneisses and supracrustal belts which in the east pernambuco belt was formed under amphibolite facies conditions this study reports la icp ms u pb zircon ages of metaigneous and metasedimentary rocks aiming to constraint the pre transcurrent tectonothermal evolution in the eastern pernambuco domain ages of 2125 7 and 2044 5 ma in a mafic layer of banded orthogneiss are interpreted as the age of the protolith of the orthogneiss and of high grade transamazonian metamorphism respectively the latter age is consistent with the occurrence of low th u metamorphic zircon xenocrysts dated at 2041 15 ma in the leucosome of a migmatitic paragneiss a granitic orthogneiss dated at 1991 5 ma reflects late to post transamazonian magmatic event a similar age 1972 8 ma was found in rounded zircon grains from a leucocratic layer of banded orthogneiss ages of detrital zircons in a paragneiss sample indicate derivation from sources with ages varying from the archean to neoproterozoic with peak ages at ca 2220 2060 1940 1200 1150 and 870 760 ma detrital zircons constrain the deposition of the supracrustal sequence to be younger than 665 ma magmatic zircons with the age of 626 15 ma are found in the leucosome of a migmatitic paragneiss and constrain the age of the brasiliano high temperature metamorphism a lower intercept age of 619 36 ma from a deformed granodiorite dated at 2097 5 ma and the crystallization age of 625 24 ma of the felsic layer of banded orthogneiss also confirm the late neoproterozoic metamorphism these results show that the present fabric in basement and supracrustal rocks was produced during the brasiliano orogeny paleoproterozoic ages reported in this study are similar to those found in other sectors of the borborema province the cameroon and nigeria provinces and the sao francisco congo craton they show the importance of the transamazonian eburnean event and suggest that these tectonic units may have been part of a larger single continental landmass likewise similarities in post transamazonian metamorphic and magmatic events in the borborema nigeria and cameroon provinces suggest that they shared a common evolution and remained in close proximity until the opening of the atlantic ocean

lower and upper neoproterozoic magmatic records in itaiacoca belt parana brazil zircon ages and lithostratigraphy studies abstract the itaiacoca belt is a sequence of metavolcanic and metasedimentary rocks that crop out east of parana and southeast of sao paulo states in southern brazil this geologic geochronologic study supports division of the itaiacoca belt into two major lithologic sequences the older is a carbonate platform sequence dolomitic meta limestones metamarls calc phyllites carbonate phyllites with minimum deposition ages related to the end of the mesoproterozoic beginning of the neoproterozoic 1030 908 ma u pb zircon of metabasic rocks the younger sequence contains mainly clastics deposits meta arkoses metavolcanics metaconglomerates metapelites with deposition ages related to the neoproterozoic 645 628 ma u pb zircon of metavolcanic rocks these ages are quite close to k ar ages fine fraction of the 628 610 ma interval associated with metamorphism and cooling of the itaiacoca belt the contact between the dolomitic meta limestones and meta arkoses is marked by intense stretching and high angle foliation suggesting that the discontinuity between these associations resulted from shearing it is proposed here that the term itaiacoca sequence should represent the dolomitic meta limestones and the term abapa sequence represents the meta arkoses metavolcanics phyllites in a major tectonic context these periods are related to the break up of rodinia supercontinent 1030 908 ma and the amalgamation of the gondwana supercontinent 645 628 ma

comparative k ar and rb sr age determinations of retrograde processes on rocks from the ktb deep drilling project conventional k ar and rb sr age determinations were carried out on fine mineral fractions of the ktb rocks and of rocks from the surrounding area in order to get an idea of the timing of retrograde processes which the rocks had suffered in the late variscan and afterwards coarse grained mica minerals were used as reference minerals it can be demonstrated that conventional k ar dating on fine mineral fractions is a useful tool to date retrograde processes in crystalline rocks for methodical reasons the rb sr method is less suitable a complex age pattern was found which could be divided into different age groups representing different periods of either penetrative retrograde overprint or formation and reactivation of distinct cataclastic fault zones whereas the rocks of the ktb have undergone a penetrative retrograde overprint during the permo triassic without any visible deformation the jurassic brought a first reactivation of cataclastic fault zones in cretaceous time an overprint led to a pronounced second reactivation of cataclastic fault zones this latter process turned out to be the important one for the stacking and a repetition of the drilled profile in the ktb the data and interpretations obtained on the fine mineral fractions coincide excellently with fission track data on sphene from the drill site with sedimentological records concerning a larger regional scale and with hydrothermal alterations and ore forming events in mid europe

an ediacaran back arc basin preserved in the transversal zone of the borborema province evidence from geochemistry geochronology and isotope systematics of the ipueirinha group ne brazil abstract the ipueirinha group is a ca 75 km long by 13 km wide succession of metavolcano sedimentary rocks situated in the westernmost transversal zone borborema province ne brazil it is mostly composed of marine terrigenous sedimentary rocks represented by immature metarhythmites and quartzites local metaultramafites rhyolitic metavolcaniclastic rocks and rare lenses of metamarl the metavolcaniclastic rocks show highly fractionated chondrite normalized ree patterns lan ybn 3 68 8 94 with slight to moderately negative eu anomalies eu eu 0 72 0 94 incompatible trace element signatures similar to those of volcanic arc related rocks including a nb ta trough and negative nd t ranging from 3 4 to 12 2 metarhythmites show lree enrichment lan ybn 4 83 15 32 moderate eu anomalies eu eu 0 68 0 81 and incompatible element signatures consistent with provenance from a continental arc nd isotope data indicate important detrital contribution from juvenile sources for these metarhythmites as reflected by the nd t values between 3 6 and 0 3 detrital zircon la icp ms u pb data reveal that the main sources of detritus for the metarhythmites are neoproterozoic rocks maximum age peak at ca 845 ma and younger zircons at ca 636 ma hf isotope data obtained from detrital zircons show evidence for an important period of juvenile magmatism in the borborema province from 895 to 834 ma hf t 2 0 5 7 and suggest that the neoproterozoic crustal evolution of this province involved mixing between older sources with negative hf t and younger juvenile sources metaultramafites associated with the ipueirinha group show nd t ranging from 1 3 to 8 5 and tdm 1 33 1 64 ga the whole unit was submitted to four deformation phases and to heterogeneous metamorphic conditions from lower to upper greenschist facies additional geochronological data obtained for a granitic sill 578 13 ma zircon u pb along with monazite u pb data from a chloritite sample 595 29 ma indicate that strike slip deformation metamorphism and syn orogenic melt generation coexisted in the ipueirinha group during the late ediacaran we suggest that the ipueirinha group comprises a cryogenian ediacaran back arc basin to the tamboril santa quiteria magmatic arc which is one of the few records of initial convergence tectonics in the west gondwana margin of the borborema province

the transition from magmatic to high temperature solid state deformation implications from the mount stuart batholith washington criteria for syntectonic emplacement of plutons are commonly ambiguous the strongest single criterion is probably the preservation of a continuous transition from submagmatic to high temperature solidstate deformation this transition is not commonly documented an exception is the mount stuart batholith which displays a variety of syntectonic structures within its sheared northeastern margin including the following 1 s c fabrics that range from submagmatic to high temperature solid state 2 solid state foliation and lineation defined by amphibolite facies assemblages that overprint and are parallel to magmatic foliation and lineation 3 folds of magmatic and solid state foliation that have hinge lines parallel to magmatic and solid state lineation and to equivalent structures in wall rocks 4 dikes that were folded and boudinaged over a wide range of rheological states with boudin necks locally filled by tonalite 5 pegmatite dikes and mineralized joints that are sub perpendicular to magmatic and solid state lineation and 6 submagmatic and high temperature solidstate ductile shear zones we suggest that many plutons emplaced during regional deformation do not preserve evidence for syntectonic deformation because of the transitory nature of the submagmatic state and the obscuring effects of postemplacement deformation syntectonic features are most likely preserved in plutons cooled at slow to moderate rates or in plutons deformed at high strain rates during emplacement the optimum conditions for preservation may occur in plutons emplaced along fault zones in the mid crust such as the mount stuart batholith and in intrusions at deeper levels that were rapidly exhumed and or intruded during the waning stages of regional deformation

magma assisted strain localization in an orogen parallel transcurrent shear zone of southern brazil in a lithospheric scale orogen parallel transcurrent shear zone of the pan african dom feliciano belt of southern brazil two successive generations of magmas an early calc alkaline and a late peraluminous have been emplaced during deformation microstructures show that these granitoids experienced a progressive deformation from magmatic to solid state under decreasing temperature conditions magmatic deformation is indicated by the coexistence of aligned k feldspar plagioclase micas and or tourmaline with undeformed quartz submagmatic deformation is characterized by strain features such as fractures lattice bending or replacement reactions affecting only the early crystallized phases high temperature solid state deformation is characterized by extensive grain boundary migration in quartz myrmekitic k feldspar replacement and dynamic recrystallization of both k feldspar and plagioclase decreasing temperature during solid state deformation is inferred from changes in quartz crystallographic fabrics decrease in grain size of recrystallized feldspars and lower ti amount in recrystallized biotites final low temperature deformation is characterized by feldspar replacement by micas the geochemical evolution of the synkinematic magmatism from calc alkaline metaluminous granodiorites with intermediate 87sr 86sr initial ratio to peraluminous granites with very high 87sr 86sr initial ratio suggests an early lower crustal source or a mixed mantle crustal source followed by a middle to upper crustal source for the melts shearing in lithospheric faults may induce partial melting in the lower crust by shear heating in the upper mantle but whatever the process initiating partial melting lithospheric transcurrent shear zones may collect melt at different depths because they enhance the vertical permeability of the crust these zones may then act as heat conductors by advection promoting an upward propagation of partial melting in the crust synkinematic granitoids localize most if not all deformation in the studied shear zone the regional continuity and the pervasive character of the magmatic fabric in the various synkinematic granitic bodies consistently displaying similar plane and direction of flow argue for accommodation of large amounts of orogen parallel movement by viscous deformation of these magmas moreover activation of high temperature deformation mechanisms probably allowed a much easier deformation of the hot synkinematic granites than of the colder country rock and consequently contributed significantly to the localization of deformation finally the small extent of the low temperature deformation suggests that the strike slip deformation ended approximately synchronously with the final cooling of the peraluminous granites the evolution of the deformation reflects the strong influence of synkinematic magma emplacement and subsequent cooling on the thermomechanical evolution of the shear zone magma intrusion in an orogen scale transcurrent shear zone deeply modifies the rheological behavior of the continental crust it triggers an efficient thermomechanical softening localized within the fault that may subsist long enough for large displacements to be accommodated therefore the close association of deformation and synkinematic magmatism probably represents an important factor controlling the mechanical response of continental plates in collisional environments

updated gondwana permian cretaceous earth history of australia abstract permo carboniferous glaciation confined to icecaps and mountain glaciers was followed by permian coal measures and early triassic barren measures and redbeds in the east terminally deformed in the mid triassic coal deposition resumed during the late triassic and tholeiite was erupted in the southeast after rifting the western margin was formed by the opening of the indian ocean at 156 and 132 ma at 140 ma a brief glaciation affected central australia by the 99 ma mid cretaceous the southern margin was finally shaped by the opening of the southeastern indian ocean the shoreline retreated to the present coast from the maximum aptian shoreline of an epeiric sea and the eastern highlands were uplifted to produce the present morphology of australia new data relate to the permo carboniferous and early cretaceous glaciations the kiaman reversed paleomagnetic interval events about the permian triassic boundary including possible impact craters advances in palynology invertebrate paleontology macrofloral paleontology and paleobiogeography the provenance of sediments by u pb ages and host rock affinity of zircons stable isotopes and biomarkers in petroleum systems coal environments calibrating the time scale with u pb ages of zircons fission track thermotectonic imaging geothermal energy and terranes split off the western margin

the 3 4 3 5 ga sao jose do campestre massif ne brazil remnants of the oldest crust in south america abstract the oldest fragment of continental crust recognized in south america occurs as an isolated archean enclave in northeastern brazil s borborema province ca 600 ma brasiliano pan african orogenic belt this archean fragment the sao jose do campestre massif is surrounded by large tracts of 2 2 2 0 ga paleoproterozoic gneisses and is located more than 600 1500 km from the much larger assemblages of archean rocks found in the sao fransciso and amazonian cratons located to the south and west respectively geochronological studies of the sao jose do campestre massif show that its oldest rocks contain zircons with u pb ages up to 3 5 ga and sm nd tdm model ages of more than 3 7 ga indicating that they represent reworked crust this older nucleus is flanked by both reworked and juvenile 3 25 and 3 18 ga rocks which are intruded by both 3 00 and 2 69 ga plutonic bodies the protracted evolution the sao jose do campestre massif is consistent with that of a larger continental mass as opposed to a small crustal fragment that grew in isolation as such the sao jose do campestre massif is interpreted as representing a detached piece of an evolved craton that became entrained with younger rocks during a subsequent paleoproterozoic accretionary orogenic event this hypothesis is bolstered by the presence of paleoproterozoic gneisses that envelop the sao jose do campestre massif as well as the existence of ca 2 0 ga metamorphic zircon and monazite within its rocks the occurrence of several different archean cratonic basement inliers within the greater paleoproterozoic crustal framework of the borborema province suggests that cratonic slices spalled off one or more larger archean masses prior to the ca 2 2 2 0 ga paleoproterozoic orogenic collage a important challenge is to link these older fragments to their parent cratons although results are not unique the pattern of ages and isotopic signatures observed in the sao jose do campestre massif is similar to that seen in parts of the sao francisco craton and it is possible that the sao jose do campestre massif is a fragment of an archean continental fragment formed during an episode of continental breakup prior to 2200 ma

neoproterozoic oceanic crust remnants in northeast brazil the borborema province of northeast brazil occupies a strategic position in the central portion of west gondwana linking three of its major cratonic constituents the sao francisco congo amazon and west africa cratons the southern portion of the province the riacho do pontal fold belt comprises an association of exhalative rocks with metabasalts having transitional mid oceanic ridge basalt geochemistry monte orebe complex that suggests the preservation of remnants of neoproterozoic oceanic crust this view is supported by geophysical data the riacho do pontal fold belt corresponds to the inflexion of a paired positive negative bouguer anomaly similar to other precambrian suture zones sm nd isotope data for the metabasalts yield a whole rock isochron age of 819 120 ma with an initial e nd t 4 4 indicating derivation from a depleted mantle source the onset of subduction within the borborema province ca 630 ma caused the inversion of basins and obduction of slices of oceanic crust in this scenario the riacho do pontal fold belt represents a complete late neoproterozoic plate tectonics cycle involving the collision of the sao francisco craton lower plate with the pernambuco alagoas block upper plate this interpretation challenges current views that the borborema province has acted as a coherent block since paleoproterozoic time part of the atlantica supercontinent suggesting instead a dynamic setting where multiple plates interacted during the proterozoic the monte orebe ophiolite provides a link with other cryogenian oceanic crust occurrences in central brazil and west africa indicating the preservation of a transcontinental neoproterozoic suture zone in the heart of west gondwana

microstructures in feldspars from a major crustal thrust zone the grenville front ontario canada abstract samples collected across the nw grenville front in ontario canada have been used to document the change in feldspar microstructures produced during shearing under metamorphic conditions that range from upper amphibolite to lower greenschist grade the differential displacement across this 5 km thick section of the grenville front tectonic zone is at least 10 km at upper to mid amphibolite conditions both plagioclase and potassium feldspars behave plastically grainsize reduction occurs through grain boundary migration and or subgrain rotation and produces a core and mantle structure at middle to lower amphibolite grade grain size reduction is accomplished through grain boundary migration at lower amphibolite to upper greenschist grade grain size reduction occurs by the nucleation and growth of new grains with a different composition the abundance of intracrystalline deformation at this grade is inversely proportional to the amount of fine grained matrix in ultramylonite with a high matrix to porphyroclast ratio few intracrystalline strain features occur other than ubiquitous undulatory extinction myrmekite is well developed on the high stress boundaries in k feldspar at upper to mid greenshist conditions feldspars deform mainly by slip on synthetic shear fractures deformation twins undulatory extinction deformation bands and kink bands occur in plagioclase and perthite flames and myrmekite occur principally on high stress boundaries in k feldspar grain size reduction in feldspar is accomplished through cataclasis at middle to lower greenschist facies deformation in feldspars is dominated by antithetic fractures the transition from the dominance of synthetic to antithetic fractures is gradational below lower greenschist grade rocks deform by pervasive cataclasis of all minerals

deformation of granitoids at low metamorphic grade i reactions and grain size reduction abstract deformation and chemical reaction in granitoids at greenschist facies conditions were investigated in samples from three shear zones two from the alps corvatsch granodiorite and aiguilles rouges granite switzerland and one from eastern australia wyangala granite when examined by light and electron microscopy all show similar features of deformation that progressed from coarse parent rock through to fine grained mylonite quartz deformed plastically while both k feldspar and plagioclase fractured and recrystallized in conjunction with chemical change which had as its end point an assemblage albite quartz white mica caal silicates k feldspar and calcic plagioclase depending on fluid chemistry from the parent rock were unstable at low metamorphic grades in the presence of aqueous fluid since ca bearing plagioclase was not stable in these environments myrmekite did not replace k feldspar in any of the rocks examined recrystallization of feldspar which should perhaps be termed neocrystallization was initiated predominantly at clast margins or along microfractures that are marked by fluid inclusions and twin offsets in old feldspar grains particularly in plagioclases dislocations exist in wall like structures that are commonly associated with voids suggesting origins in microfractures these dislocations had severely limited mobility and subgrain rotation does not appear to have contributed to recrystallization we conclude that recrystallization mainly occurred via a classical nucleation mechanism with minor contributions from twin boundary migration we argue that the 450 550 c lower limit for recrystallization of feldspar referenced throughout the literature is not applicable to the rock systems under investigation and should be discarded as a universal limit microfracturing microboudinage and reaction are seen as the prerequisites for mylonite formation from coarse grained granitic rocks at low metamorphic grades see part ii of this paper neocrystallization fluid access and nucleation appear to have accompanied or immediately followed the first deformation increments and were primarily responsible for continued grain size reduction transgranular fracture was comparatively important only at the onset of deformation since the formation of shear zones in granitoids commonly takes place under conditions of low metamorphic grade in the presence of aqueous fluids in the earth s crust breakdown reactions and nucleation recrystallization in feldspars may be important influences upon localization of deformation in granitoids

isotope geochemistry and geochronology of the nico perez terrane rio de la plata craton uruguay u pb shrimp zircon geochronology bulk rock geochemistry and sr nd isotopic data are herein presented in order to better constrain the tectonic evolution of the nico perez terrane npt uruguay the studies are focused on two key geological units located in the southern part of the npt namely carape complex cc and lavalleja metamorphic complex lmc the carape complex previously interpreted as part of a remnant magmatic arc formed at 1 75 ga mainly by partial melting of archean crust t dm model ages between 2 8 and 3 0 ga with strong negative e nd t values the lavalleja metamorphic complex previously interpreted as a back arc basin has volcanic units formed at 0 59 ga during the brasiliano cycle but may also contain older paleoproterozoic units the geochemistry of the lmc meta igneous rocks indicates a complex scenario two distinct signatures have been recognized one similar to morbs and the other similar oibs sm nd t dm model ages of basic rocks present a bimodal paleoproterozoic and archean distribution which is also recognized in several other units of the npt in the light of new results we conclude that the npt represents a cratonic margin setting characterized by the stacking of lithotectonic assemblages with archean paleoproterozoic and neoproterozoic age at the margin of the rio de la plata craton

proterozoic history of the borborema province ne brazil correlations with neighboring cratons and pan african belts and implications for the evolution of western gondwana 1 geological and geochronological correlations between borborema province ne brazil and neighboring cratons and brasiliano pan african belts indicate that the amazonian west african and sao francisco congo cratons and the basement of the araguaia borborema nigerian and cameroon provinces were part of the atlantica supercontinent this continent was established at the end of the transamazonian eburnean cycle 2 0 ga and apart from ubiquitous taphrogenesis in the 1 8 1 7 ga interval remained largely unaffected for the following 1 ga around 1 ga an important magmatic event in borborema province correlates with rifting episodes and anorogenic magmatism in the sao francisco congo and amazonian cratons these events are interpreted as failed attempts to break up atlantica which at this time may have been part of the larger rodinia supercontinent renewed extensional conditions in borborema province during the middle and late neoproterozoic are attributed to far field stresses transmitted to the interior of atlantica by outwardly dipping subduction zones that encircled its northern present day coordinates portion the rarity of petrotectonic assemblages typical of subduction zone environments indicates that extension did not evolve enough to form large oceans basins and thus that the borborema province essentially includes reworked intracontinental domains regional deformation and metamorphism starting at 650 640 ma and shear zone development beginning at 590 595 ma were continuously developed through time and were synchronous throughout most of the borborema araguaia cameroon and nigerian provinces postorogenic conditions were reached 540 530 myr ago while active deformation was still occurring in other belts that accreted around atlantica to form western gondwana

the influence of grain boundary fluids on the microstructure of quartz feldspar mylonites quartz rich domains in pre alpine water deficient amphibolite facies 510 580 c 250 450 mpa pegmatite mylonites from mont mary mm western alps preserve a fine dynamically recrystallized grain size without significant annealing despite the high synkinematic temperatures and subsequent static greenschist facies alpine overprint the microstructure is dramatically different from more typical water rich amphibolite facies mylonites such as from the simplon fault zone in the central alps where the recrystallized grain size is on the millimetre scale the difference reflects the dominant strain induced recrystallization mechanism 1 progressive subgrain rotation and grain boundary bulging for the dry mm examples and 2 fast grain boundary migration for the wet simplon examples the grain boundary microstructure imaged with sem is also very different with most grain boundaries in the dry mm samples lacking porosity whereas grain boundaries in the wet samples are decorated by a multitude of pores quartz grain boundaries from both wet and dry samples are locally coated by thin 100 s of nanometres possibly amorphous silica films despite the differences in microstructure the crystallographic preferred orientations cpos of quartz rich domains from both areas are very similar water deficient conditions hinder grain boundary mobility and thereby modify the dominant recrystallization mechanism s but apparently have little influence on the intracrystalline slip systems as reflected in the cpos strong c axis y maxima in mm mylonites both k feldspar and plagioclase an33 38 dynamically recrystallize consistent with the inferred metamorphic conditions under water deficient conditions mid to lower crustal rocks can deform heterogeneously under transitional ductile brittle conditions at high differential stress for mm ca 300 500 mpa as estimated for dry mohr coulomb failure and preserve this high stress microstructure because of the low mobility of dry grain boundaries

major precambrian nappes of the brazilian belt ceara northeast brazil the precambrian of ceara northeast brazil includes the reactivated archean basement and proterozoic metasediments that have gone through one single stage of metamorphism and deformation during the brazilian orogeny that culminated at about 600 ma preliminary petrostructural studies show that crystalline nappes with associated himalayan type reverse metamorphism have been thrust westward to southwestward over more than 150 km geometrical relationships indicate that some steep shear zones could be lateral ramps to the nappes thus this precambrian segment of the brazilian belt has the petrostructural signature of a former overthickened continental crust similar to that generated by himalayan type continent continent collisions

the age distributions of detrital zircons in metasedimentary sequences in eastern borborema province ne brazil evidence for intracontinental sedimentation and orogenesis detrital zircons in metasedimentary rocks attributed to three different complexes in the central and pernambuco alagoas domains of the borborema province northeastern brazil were dated by la icpms two samples from each of the following complexes were analyzed sert nia and surubim in the central domain and rio una in the pernambuco alagoas domain one sample of the sert nia complex yielded only paleoproterozoic zircons whereas neoproterozoic zircons predominate in the other with the youngest zircon indicating deposition after 642 ma one of the samples of the surubim complex yielded again only paleoproterozoic concordant zircon ages in the other sample although paleoproterozoic zircons predominate ages vary from the mesoarchean to the neoproterozoic the age of the youngest zircon in this latter sample indicates deposition after 850 ma and an overgrowth on an archean grain yielded an age of 623 6m a 2 interpreted as the age of peak metamorphism the two samples of the rio una complex yielded predominantly detrital zircons with ages in the ranges 2 2 2 1 ga and 1 7 1 6 ga in this complex the youngest detrital zircon suggests deposition after 917 ma and metamorphic zircons indicate peak metamorphic conditions between 600 ma and 617 ma the results of this study together with available data from the literature allow the following conclusions a the sert nia complex is a neoproterozoic sequence and not paleoproterozoic as previously thought b the similarity in lithological associations detrital zircon populations and c isotope signature between the sert nia and surubim complexes suggest that they belong to the same lithostratigraphic unit c although the rio una complex can be older than the sert nia surubim complex the lack of zircons younger than 900 ma can also result from insufficient sampling or absence of young rocks alongside the drainage system that collected detritus to this complex d the regional metamorphism related to the brasiliano orogeny attained its climax at 630 600 ma e the age spectra of detrital zircons require their derivation from erosion of rocks formed not only during known geological events in the borborema province and in the nearby sao francisco congo craton but also from the amazonian craton suggesting accumulation in an intracratonic setting within a preexisting large continent f the short timespan between deposition and orogenic deformation may explain the overall medium to high temperature metamorphism due to maintenance in the subsequent compressional event of the elevated geothermal gradients produced during broad scale lithosphere extension

magnetic fabric and pluton emplacement in a transpressive shear zone system the itaporanga porphyritic granitic pluton northeast brazil the itaporanga granitic pluton intruded into the lithological contact separating neoproterozoic pelitic metasediments from meso to paleoproterozoic rocks that constitute the cachoeirinha belt of northeastern brazil an aureole of pelitic gneisses containing sillimanite and andalusite indicates that emplacement occurred at 12 15 km depth the objective of this work is a detailed study of its magmatic structures mainly by means of anisotropy of magnetic susceptibility ams regularly spaced sampling 61 stations yielded a magnetic fabric map which shows two domains 1 a western lobe which has lineations close to vertical and e trending foliations that merge southwestward as the transcurrent igarassi shear zone is approached isz and 2 an eastern lobe showing se trending lineations associated with inward dipping magmatic foliation situated in the center of the lobe the magnetic fabric is mostly paramagnetic in origin i e related to crystallographic preferred orientation of biotite and hornblende a low susceptibility magnitude 0 4 10 3 si typically of the ilmenite series granites is found in porphyritic granites diorites or their hybrid products hysteresis data indicate that the ferrimagnetic contribution to the bulk susceptibility does not reach 30 the magma was emplaced into a chamber developed by local wnw ese extension induced by an overall n s shortening melt ascent may have been facilitated by the pre existence of a steep fault zone bounding crustal discontinuities the magma must have ponded along a mechanical barrier formed by the contact between phyllitic metapelites and its gneissic basement which controlled the final emplacement structures of the pluton

the serido group of ne brazil a late neoproterozoic pre to syn collisional basin in west gondwana insights from shrimp u pb detrital zircon ages and sm nd crustal residence tdm ages abstract the serido group is a deformed and metamorphosed metasedimentary sequence that overlies early paleoproterozoic to archean basement of the rio grande do norte domain in the borborema province of ne brazil the age of the serido group has been disputed over the past two decades with preferred sedimentation ages being either paleoproterozoic or neoproterozoic most samples of the serido formation the upper part of the serido group have sm nd tdm ages between 1200 and 1600 ma most samples of the jucurutu formation the lower part of the serido group have tdm ages ranging from 1500 to 1600 ma some basal units have tdm ages as old as 2600 ma reflecting proximal basement thus based on sm nd data most if not all of the serido group was deposited after 1600 ma and upper parts must be younger than 1200 ma cathodoluminescence photos of detrital zircons show very small to no overgrowths produced during ca 600 ma brasiliano deformation and metamorphism so that shrimp and isotope dilution u pb ages must represent crystallization ages of the detrital zircons zircons from meta arkose near the base of the jucurutu formation yield two groups of ages ca 2200 ma and ca 1800 ma in contrast zircons from a metasedimentary gneiss higher in the jucurutu formation yield much younger ages with clusters at ca 1000 ma and ca 650 ma zircons from metasedimentary and metatuffaceous units in the serido formation also yield ages primarily between 1000 and 650 ma with clusters at 950 1000 800 750 and 650 ma thus most if not all of the serido group must be younger than 650 ma because these units were deformed and metamorphosed in the ca 600 ma brasiliano fold belt during assembly of west gondwana deposition probably occurred ca 610 650 ma soon after crystallization of the youngest population of zircons and before or during the onset of brasiliano deformation the serido group was deposited upon paleoproterozoic basement in a basin receiving detritus from a variety of sources the jucurutu formation includes some basal volcanic rocks and initially received detritus from proximal 2 2 2 0 ga transamazonian to late paleoproterozoic 1 8 1 7 ga basement provenance for the upper jucurutu formation and all of the serido formation was dominated by more distal and younger sources ranging in age from 1000 to 650 ma we suggest that the serido basin may have developed as the result of late neoproterozoic extension of a pre existing continental basement with formation of small marine basins that were largely floored by cratonic basement subjacent oceanic crust has not yet been found immature sediment was initially derived from surrounding land as the basin evolved much of the detritus probably came from highlands to the south present coordinates alternatively if the patos shear zone is a major terrane boundary the basin may have formed as an early collisional foredeep associated with south dipping subduction in any case within 30 million years the region was compressed deformed and metamorphosed during final assembly of west gondwana and formation of the brasiliano pan african fold belts

detrital zircon u pb geochronology and whole rock nd isotope constraints on sediment provenance in the neoproterozoic sergipano orogen brazil from early passive margins to late foreland basins abstract shrimp u pb detrital zircon geochronology and depleted mantle nd model ages of clastic rocks were combined to understand the sediment provenance in the neoproterozoic sergipano belt the sergipano is the main orogenic belt between the borborema province and the sao francisco craton eastern south america it is divisible into several lithostratigraphic domains from north to south caninde poco redondo maranco macurure vaza barris and est ncia nd model ages tdm and detrital zircon u pb shrimp geochronology indicate that the protoliths of clastic metasedimentary rocks from the maranco and macurure domains were mostly derived from eroded late mesoproterozoic to early neoproterozoic rocks 1000 900 ma whereas detritus of similar rocks from the caninde domain came from a younger source ca 700 ma and 1000 ma samples from the vaza barris domain show the greatest scatter of both tdm and zircon ages amongst all domains but with important contributions from proterozoic sources 690 1050 ma and ca 2100 ma and less from archaean sources the est ncia domain samples have zircon population peaks at 570 ma 600 ma and 920 980 ma with a few older grains one diamictite contains only ca 2150 ma zircon grains our preliminary results support a model in which sediments of the maranco and macurure domains were deposited on a continental margin of the ancient borborema plate before its collision with the sao francisco craton the caninde domain is likely to be an aborted neoproterozoic rift assemblage within the southern part of the borborema plate pernambuco alagoas massif the basal units of the vaza barris and est ncia domains have clast sources from the sao francisco craton and are best interpreted as passive margin sediments however the uppermost units of the est ncia and vaza barris domains come from foreland basins formed during collision of borborema plate with the sao francisco craton

upper triassic shallow water carbonates from the naizawa accretionary complex hokkaido japan new insights from panthalassa abstract due to their accretion on the circum pacific area during the jurassic and the cretaceous upper triassic carbonates from the panthalassa occur very scarcely and with relatively poor preservation in accretionary complexes however they represent a unique opportunity to improve our knowledge of the depositional conditions in tropical regions of the panthalassa recognized as a cradle of life shallow water carbonates are also of great importance to understand of how life evolved out of the tethyan domain during the triassic since 2007 several sedimentological and biostratigraphic studies focused on the upper triassic shallow water limestone from the circum pacific area have been carried out as part of the reefcade project at the university of geneva carbonates were thus reported in the southern part of japan and in the russian far east hokkaido island in northern japan represents the missing link between those two areas to fill this gap five limestone outcrops so far poorly described in the literature were identified and sampled in the pippu and esashi areas central and northern part of hokkaido island respectively their related microfacies are presented in detail as well as their foraminiferal associations the obtained age based on foraminiferal biostratigraphy from both areas is defined as carnian and the facies similarities associated with specific modes of occurrence identify the outcrops from the two areas as part of the same depositional system based on microfacies interpretations a hypothetical depositional model is presented it corresponds to a intra oceanic depositional system developed on the flanks of an emergent volcanic seamount the strong similarity with synchronous systems from the sambosan accretionary complex southwestern japan is discussed

west gondwana amalgamation based on detrital zircon ages from neoproterozoic ribeira and dom feliciano belts of south america and comparison with coeval sequences from sw africa

granitoid emplacement during a thrusting event structural analysis microstructure and quartz c axis patterns an example from hercynian plutons in the french massif central in the french massif central the rouergue albigeois area consists of three tectonic units stacked during the hercynian orogeny the structural analysis of the units and particularly the quartz c axis textural and structural observations in the syntectonic pinet type plutons allow us to determine the deformation history a first thrusting event d1 is responsible for a southwestward emplacement of the high grade levezou nappe above the lower gneiss unit itself overlying the para autochthonous micaschists in late devonian early carboniferous this stack of nappes experienced a second event d2 characterized by a top to the nw emplacement under medium pressure medium temperature conditions of the para autochtonous unit upon the lower gneiss unit during d2 the pinet type plutons were deformed by a shearing consistent with the kinematics recorded in the country rocks in contrast to previous works which considered the pinet type plutons emplacement to be coeval with the d2 event we argue that the emplacement of the plutons occurred during the top to the sw d1 nappe stacking this work emphasizes the importance of the quartz c axis and microstructure analysis for the identification of the early structural history in polyphase deformed terranes

the timing of mineral growth across a regional metamorphic sequence if regional metamorphism occurs in response to crustal thickening then low grade rocks are likely to record earlier thermal conditions than will high grade rocks in the same exhumed terrain1 4 at sulitjelma north norway regional metamorphism during the caledonian orogeny created a type sequence of progressive metamorphic zones5 7 garnet and coexisting minerals from the high grade part of this terrain yield precise and concordant sm nd u pb and rb sr ages which relate directly to the pressure temperature conditions of mineral growth8 here we present pressure temperature time p t t information for lower grade rocks occurring at the garnet isograd taken together these data provide a precise estimate of the timing of mineral equilibration across a prograde metamorphic terrain and indicate that peak metamorphic conditions at the garnet isograd at 458 20 c were attained 8 3 3 0myr t sm nd or 8 7 3 2myr t u pb before the peak in the high grade sample at 544 16 c these results clearly illustrate that the exhumed p t array recorded in such terrains cannot be used to deduce any single geotherm that existed during metamorphism

c axis fabrics and microstructures in a recrystallized quartz vein deformed under fluid rich greenschist conditions the quartz v axis orientations and microstructures have been analyzed in an originally single crystal buckled quartz vein of greenschist metamorphic grade from the sambagawa metamorphic belt southwest japan the basal 0001 plane was inclined by ca 10 20 to the quartz vein surface before deformation and the flexure essentially kink which produced the fold was accomplished by basal 0001 slip alone in the hinges i e flexural slip folding in addition to crystal plasticity dissolution microstructures are ubiquitous in the quartz vein dynamic recrystallization resulted from both subgrain rotation and grain boundary migration rotation recrystallization associated with basal 0001 slip resulted in host controlled v axis orientation distribution along a great circle whereas migration recrystallization perhaps assisted by intergranular fluid resulted in a penetrative development of a weak x elongation maximum v axis fabric nowhere rotated by the folding the migration recrystallization is evidenced by large v axis misorientations across the grain boundaries as well as irregular grain shape based on the weak v axis fabric development and lack of grain shape fabric solution precipitation creep may have dominated in the fluid assisted grain boundary migration regime and hence it is inferred that the transition from rotation to migration recrystallization was caused by a local decrease of differential stress in the buckled quartz vein

magnetic fabrics in the cabanas granite ne brazil interplay between emplacement and regional fabrics in a dextral transpressive regime the late neoproterozoic cabanas granite northeastern brazil is a tabular body that was emplaced south of the dextral east pernambuco shear zone system anisotropy of magnetic susceptibility ams measurements reveal that the cabanas granite has fabric patterns more complex than usually reported in plutons intruded in similar settings most structures have ew to ene trending magnetic lineations consistent with the overall regional structural direction in the north these structures parallel the mylonitic fabric encountered in shear zones that bound or cross cut the pluton in the central and southern portions of the pluton magnetic foliation trajectories the overall banded disposition of the iso susceptibility lines and shape parameters of the ams ellipsoids suggest that the cabanas granite was subjected to ene trending folding along with its underlying and enclosing country rocks it is concluded that emplacement took place during regional dextral transpression a secondary but well developed family of structures consists of n to nw plunging lineations which is interpreted to represent the trace of the vertical conduits that fed the pluton this study emphasizes the utility of the ams to unravel both the internal structure of low anisotropy plutons and the interplay between emplacement and regional straining during magma crystallization

relationships between syn tectonic granite fabrics and regional pttd paths an example from the gander avalon boundary of ne newfoundland abstract syn tectonic granitic plutons and their host rocks not only provide a potential wealth of information about the timing and nature of regional kinematics but also record information about thermal conditions successive syn tectonic plutons emplaced into the gander zone of ne newfoundland preserve fabric overprinting formed during cooling to regional ambient thermal conditions ambient conditions have been approximated for each pluton by careful analysis of microstructures and consideration of potential cooling histories and have illustrated regional exhumation during a tectonic reversal from silurian devonian sinistral transpression to devonian dextral transpression because the time period of emplacement is relatively short by comparison to orogenic events typically by more than an order of magnitude individual plutons may record a snap shot of the regional history by combining information from a number of plutons emplaced sequentially during a period of regional orogenesis a picture may be built up defining time temperature deformation ttd paths such information may compliment regional metamorphic pt studies in helping to establish orogenic pttd paths

prism and basal plane parallel subgrain boundaries in quartz a microstructural geothermobarometer empirical data on quartz subgrain patterns from various metamorphic rocks show that at least up to 10 kbar in the stability field of low quartz prismatic subgrain boundaries are dominant whereas basal subgrain boundaries are not developed in the high quartz stability field both prismatic and basal subgrain boundaries occur and form typical rectangular chessboard patterns the likely reason behind the different occurrence of these subgrain patterns is that in high quartz prismatic glide becomes as easy as or probably even easier than basal glide the two types of subgrain patterns can be clearly distinguished by optical microscopy consequently the occurrence of chessboard subgrain patterns in quartz represents a practicable geothermobarometer the possibilities of its application are far reaching and include the specification of deformation conditions at high grade metamorphism the recognition of syntectonic intrusions and the distinction between pluton emplacement at lower and at higher crustal levels

electromagnetic constraints for subduction zones beneath the northwest borborema province evidence for neoproterozoic island arc continent collision in northeast brazil the borborema province in northeast brazil occupies a crucial position in the complex neoproterozoic west gondwana reconstruction puzzle however correlation attempts between northeast brazil and west africa have been hampered because key links in the internal structure of the borborema province have yet to be identifi ed to aid such an correlation a magnetotelluric study was undertaken along two subparallel profi les to image the deep electrical structure in the northwestern part of the province despite the occurrence of recurrent tectonothermal episodes that affected the region in the past two dimensional models show that a large scale signature of the assembled terrane during the neoproterozoic accretion and collision is plausibly preserved in the area two resistive features dipping from the upper crust into the upper mantle in downward convergence opposite directions are defi ned beneath one of the profi les that are interpreted to be related to remnants of former subduction slabs since the observed high resistivity zone is consistent with a dehydrated oceanic lithosphere depleted of sediments on the basis of geological and geochemical information a model of collision of an intraoceanic magmatic arc coalesced into an earlier passive margin is proposed for the neoproterozoic tectonic evolution of the province involving processes of reversal of subduction polarity and oceanic slab breakoff

trace element geochemistry and sedimentary provinces a study from the tasman geosyncline australia abstract the abundances of la th u and hf have been determined in flysch sequences of the tasman geosyncline australia the arc derived graywackes differ from those of rifted continental margins and marginal basins in containing low abundances of la th u and hf la 9 2 1 7 ppm th 1 4 0 6 ppm u 0 52 0 2 ppm hf 2 1 0 6 ppm reflecting similar low abundances in the andesitic source rocks graywackes of rifted continental margins and marginal basins in contrast are mature and characterised by high abundances of la th u and hf la 39 9 9 ppm th 16 0 6 ppm u 3 40 0 5 ppm hf 7 9 ppm mirroring increased abundances in the granitic and sedimentary source rocks sedimentary rocks deposited in inter arc basins show wide dispersion but have characteristics intermediate between these two types the th u ratio for magmatic arc sediments is 2 7 for most other sedimentary rocks it remains uniform at 4 5 0 3 for sediments having andesitic and dacitic rocks as provenance the la th ratios are 6 7 and 4 5 respectively this ratio remains constant at 2 6 for most quartzose sedimentary rocks

preserved magnetic fabrics vs annealed microstructures in the syntectonic recrystallised george granite south africa the saldanian basement of the cape fold belt of south africa outcrops in the kaaimans inlier with granite plutons intruded in low grade pelitic and quartzitic metasediments around 535 ma new field data support a ubiquitous saldanian top to the north thrust kinematics coeval with granite emplacement with no substantial cape tectonic overprint the granites and their contact aureoles display both synkinematic and post kinematic fabrics this and the high strain zone commonly observed all along the contact between the kaaimans inlier and the cape fold belt suggest a structural decoupling between the basement and its cover microstructures in the kaaimans inlier and in the george pluton establish a post kinematic pervasive and thermal overprint of saldanian age granites and country rocks record a medium temperature high strain deformation phase followed by a strong low temperature static recrystallisation two sets of andalusite porphyroblasts occur systematically in the contact aureoles of the studied plutons and cannot be explained by successive magmatic pulses the granites studied by the anisotropy of magnetic susceptibility ams technique are paramagnetic 20 km 300 si biotite is mostly at the origin of the bulk rock susceptibility although minor contributions of tourmaline or ferromagnetic phases may occur the contribution of biotite alone to the bulk magnetic susceptibility is supported by two quantitative models based respectively on whole rock compositions curie weiss law and on intrinsic mineral susceptibilities the magnetic foliations and lineations are homogeneous throughout the george pluton and are consistent with field structures the ams results mainly from the magneto crystalline anisotropy of biotite and from its lattice preferred orientation lpo in the rock the magnetic fabric reveals the biotite subfabrics that had been acquired before static recrystallisation and which was not modified by the subsequent thermal metamorphic event the magnetic fabric therefore preserves the emplacement related deformation fabric

the himalayan main central thrust pile and its quartz rich tectonites in central nepal as a consequence of the india asia collision a deeply rooted crustal slice has been moved southwards over the northern boundary of the indian plate at the present level of observation the main central thrust pile a rock pile of several kilometers thickness is affected by shear deformation which imprinted a flat foliation and a constant n s trending lineation these tectonites were deformed under a wide range of metamorphic conditions grading from lower greenschists up to the sillimanite field the m c t shear zone is studied from the view point of the quartz rich rocks over 100 samples helped to define a microstructural zonation roughly parallel to the thermal one 35 samples particularly rich in quartz have been used for the preferred orientation study of their c axes and a few of them for their a axes strong lattice orientations when undisturbed by mainly secondary recrystallization attest to large plastic strains in quartz the c axes are mostly distributed along unequally populated crossed girdles which may evolve into an inclined single girdle the a axes concentrate perpendicular to the more populated girdle concerning quartz the following is suggested 1 1 the deformation has a dominant shear component responsible for the dominant stretching lineation it could be simple shear in case of inclined single girdles 2 2 one set of a axes line up parallel to the shear direction slip on various planes in the a direction may be an important deformation mechanism 3 3 asymmetry in the lattice preferred orientations could be used to predict the sense of shear for the himalayan thrusting in more than 80 of cases

rare earth element geochemistry of australian paleozoic graywackes and mudrocks provenance and tectonic control abstract in the paleozoic graywackes of eastern australia there is an increase in the total abundance of rare earth elements ree a light to heavy ree ratio and a decrease in the chondrite normalized eu anomaly with the increase in sio2 al2o3 and k2o na2o ratios this is due to the change in the dominant source rocks from andesite to dacite to granite gneiss and sedimentary rocks similarly an increase in total ree and a decrease in eu anomaly is also seen in the associated mudrocks with the increase in total clay content this correlation in paleozoic sedimentary rocks suggests that the ree characteristics of terrigenous sedimentary rocks reflect the depositional tectonic setting of the basin the oceanic island arc type graywackes are characterised by their lower total ree abundance only slight enrichment of lree over hree and the absence of a negative eu anomaly on chondrite normalised plots the continental island arc type graywackes are discriminated by their higher ree abundance and la yb ratio the presence of a small negative eu anomaly on the chondrite normalised plots and the significant enrichment of eu compared to paas post archean average australian shale the sedimentary rocks deposited on andean type active continental margins passive margins platform and cratonic basins are all characterised by the high enrichment of lree over hree and the presence of a pronounced negative eu anomaly on chondrite normalised plots and can not be discriminated from each other by ree patterns alone

age and geotectonic setting of late neoproterozoic juvenile mafic gneisses and associated paragneisses from the ribeira belt se brazil based on geochemistry and sm nd data implications on gondwana assembly abstract mafic gneisses and associated paragneisses from the cabo frio tectonic domain in the southeastern part of the ribeira belt along the coast of rio de janeiro state in southeast brazil were subjected to a geochemical and sm nd isotope study four lithotypes are distinguished aluminous paragneisses mainly sillimanite kyanite garnet biotite gneiss calcsilicate lenses quartzo feldspathic metasedimentary gneisses and mafic ultramafic lenses the whole rock major and trace including rare earth element distributions in the mafic ultramafic intercalations indicate that derivation from subalkaline basalt gabbro of tholeiitic affinity with e morb signature from a non subduction environment these mafic rocks have positive end t and tdm of 1 1 ga the metasedimentary rocks have negative end t and tdm of 1 7 ga a sm nd whole rock isochron of mafic rocks yielded an age of 604 38 ma for the crystallization this matches with the age of some detrital zircon grains from the paragneisses the depositional basin named buzios palmital was active at least until 620 ma age of the youngest detrital zircon and was subsequently deformed and metamorphosed at ca 525 ma age of metamorphic zircons during the buzios orogeny it is interpreted as a back arc basin with relation to the 630 ma magmatic arc of the oriental terrane in the ribeira belt to the nw however after 600 ma the buzios palmital basin changed to an active margin setting because the arc collided with the continental margin and the subduction shifted to the back arc environment by 610 ma most of the brasiliano belts registered collisional events related to multiple convergent blocks the stress fields and paleocontinent shapes would have allowed the occurrence of extensional areas with not only sedimentary deposition but also ocean floor spreading its remnants are preserved in this brazilian coastal region as an ancient suture reworked intensively during the mesozoic rifting events the reconnaissance of late neoproterozoic basins in the brasiliano pan african belts is of major importance to partially unravel the final amalgamation events of sw gondwana considering that the buzios palmital basin rock units are mostly covered by the marginal atlantic basins it is possible that other evidence could be preserved in the coastal regions of sw africa and se south america

dislocation creep regimes in quartz aggregates abstract using optical and tem microscopy we have determined that three regimes of dislocation creep occur in experimentally deformed quartz aggregates depending on the relative rates of grain boundary migration dislocation climb and dislocation production within each regime a distinctive microstructure is produced due primarily to the operation of different mechanisms of dynamic recrystallization at lower temperatures and faster strain rates the rate of dislocation production is too great for diffusion controlled dislocation climb to be an effective recovery mechanism in this regime recovery is accommodated by strain induced grain boundary migration recrystallization with an increase in temperature or decrease in strain rate the rate of dislocation climb becomes sufficiently rapid to accommodate recovery in this regime dynamic recrystallization occurs by progressive subgrain rotation with a further increase in temperature or decrease in strain rate dislocation climb remains sufficiently rapid to accommodate recovery however in this regime grain boundary migration is rapid thus recrystallization occurs by both grain boundary migration and progressive subgrain rotation the identification of the three regimes of dislocation creep may have important implications for the determination of flow law parameters and the calibration of recrystallized grain size piezometers in addition the identification of a particular dislocation creep regime could be useful in helping to constrain the conditions at which a given natural deformation has occurred

a connection between the neoproterozoic dom feliciano brazil uruguay and gariep namibia south africa orogenic belts evidence from a reconnaissance provenance study abstract a provenance study of neoproterozoic siliciclastic successions in the stratigraphically and tectonically lowermost and uppermost parts of the pan african gariep belt stinkfontein subgroup and oranjemund group respectively in southwestern africa as well as in the rocha group of the punta del este terrane dom feliciano belt in uruguay revealed that the oranjemund and rocha groups can be correlated and most likely formed in the same basin thus the rocha group is considered to represent the fill of the westernmost part of a re activated ediacaran gariep basin the lower parts of the oranjemund and rocha groups reflect erosion of mafic rocks whereas the upper parts are derived from a predominantly felsic source area first major and trace element geochemical data are consistent with oceanic islands of within plate geochemistry in the immediate vicinity as the most likely source of the mafic input into the lower part of the oranjemund group whereas a combination of continental margin and island arc is indicated as source for the bulk of the younger sediments in the upper oranjemund group age spectra obtained by shrimp u pb analyses of detrital zircon grains from the stinkfontein subgroup port nolloth group the oranjemund group and the rocha group are very similar except for a lack of the youngest age group around 600 ma in the stinkfontein subgroup in all three units zircon grains of 1000 1200 ma dominate with a further peak in the age distribution between 1700 and 2000 ma these ages compare well with the pre gariep basement geology in southwestern africa where the former age range corresponds to magmatic and high grade metamorphic activity in the mesoproterozoic namaqua natal belt and the latter to an extensive palaeoproterozoic andean type volcanic arc richtersveld terrane comparable ages are conspicuously absent in the basement of the rio de la plata craton in south america derivation of the rocha group sediments from a similar source as the contemporaneous oranjemund group sediments is therefore suggested the most likely source of the youngest detrital zircon grains in these two stratigraphic units is the 640 590 ma magmatic arc of the dom feliciano belt this interpretation explains the continental island arc component in the sediment provenance and takes into account the absence of comparable magmatism in the gariep belt two stages of basin evolution are distinguished for the gariep basin an early failed continental rift that subsequently became re activated as back arc basin related to the cryogenian dom feliciano arc consequently the gariep basin does not represent the main suture between the rio de la plata and the kalahari cratons this suture is now suspected to be west of the dom feliciano volcanic arc and might be connected with the sierra ballena major gercino lineament