the coastal terrane of the kaoko belt namibia outboard arc terrane and tectonic significance abstract the coastal terrane or westernmost part of the kaoko belt outboard of the three palms mylonite zone has distinct feldspathic arenite sedimentary sequences no basement distinct nd sediment signatures excluding the archaean and suggesting mesoproterozoic neoproterozoic provenance sources and primitive arc like geochemical signatures for i type granitoids it contains evidence for an older metamorphic event at 650 645 ma not present in any other part of the kaoko belt this metamorphism m1 was of high t low p granulite to upper amphibolite facies and includes migmatisation associated with i type granitic magmatism of arc affinity arc growth at 650 640 ma took place outboard within the adamastor ocean while swakop group facies turbidite sedimentation continued inboard along the passive margin overprinting m1 and m2 fabrics and metamorphic assemblages constrain the docking to have occurred between 650 and 580 ma lack of ophiolite fragments and evidence of intermediate to high p metamorphism along the three palms mylonite zone suggest that it is not a suture and more likely to be part of an arc backarc wrench shear system that developed inboard of an e dipping subduction system where the adamastor ocean was subducted beneath the leading edge of the attenuated congo craton oblique collision between 650 and 580 ma due to docking of this outboard coastal terrane with arc affinities caused i crustal scale oblique overriding of the arc terrane over the passive margin of the congo craton ii crustal scale sinistral shear partitioned into two major ductile shear zones iii high t granulite facies metamorphism in an extruded shear zone bounded core and iv outwards or cratonwards verging basement cored fold nappes

upb geochronology of the central ribeira belt brazil and implications for the evolution of the brazilian orogeny abstract the ribeira belt in southeastern brazil was generated during the brazilian orogeny the south american equivalent of the pan african orogeny 700 450 ma the central sector of the belt is characterised by a series of northwest vergent crustal scale thrust stacks emplaced under amphibolite grade conditions major shear zones separate four lithotectonic domains andrel ndia to the northwest juiz de fora paraiba and coastal domains analyses of zircon monazite and titanite from a variety of rock types from the four domains lead to the following conclusions all domains record an important tectono thermal event at 590 565 ma represented by partial melting granitoid intrusion and remobilisation of older basement gneisses and corresponding to thrusting and development of dextral shear zones also recorded are older metamorphic ages 611 604 ma of unknown significance only the paraiba and coastal domains in the south record a younger metamorphic event m2 at 535 520 ma most likely associated with another episode of thrusting and shear zone development the absence of m2 metamorphism in the andrel ndia and juiz de fora domains can be explained by post 535 520 ma emplacement of the paraiba domain late to post tectonic activity is represented by 503 492 ma metamorphism precursors of the basement gneisses are 2185 2134 ma old rocks generated during the transamazonian orogeny some of which contain a significant archean component with minimum ages of 2981 2846 ma the ages presented above define specific metamorphic events for the brazilian orogeny for the first time in southeastern brazil in addition together with previously published data they indicate that the basement rocks found in the allochthonous domains of the central ribeira belt represent an extension of the units present in the southern sao francisco craton and indicate that the foreland was more extensive than the presently defined craton

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

40ar 39ar thermochronology of the pan african damara orogen namibia with implications for tectonothermal and geodynamic evolution abstract forty three 40ar 39ar step heating experiments on white mica biotite hornblende and whole rocks from the damara orogen namibia revise and refine the regional perspective on cooling exhumation and tectonic reactivation across the orogen these data also document post orogenic motion on major shear zones and the thermal effects of late syn to post tectonic granite intrusions these data show regional cooling through temperatures of 300 350 c from 545 to 520 ma in the central kaoko zone 530 510 ma in the eastern kaoko zone and 495 480 ma in the southern zone of the damara belt discordant 40ar 39ar age spectra relate to 1 partial resetting of k ar isotopic system in micas by late stage activity in the major kaoko belt shear zones such as the purros and three palms mylonite zones giving apparent ages of 467 6 481 3 and 492 ma and 2 thermal effects from granitic plutions particularly in the ugab domain of the southern kaoko belt 500 475 ma and in the granite dominated central zone of the damara belt 490 460 ma phyllites of the lower grade flank regions of the orogen record apparent 40ar 39ar mica crystallization ages of 517 ma eastern kaoko zone 526 ma northern zone damara belt and 568 553 ma southern foreland zone damara belt the southern foreland samples however are maximum ages due to variable influence of detrital mica rates of cooling vary between zones within the kaoko belt in the central kaoko belt hornblende biotite and white mica apparent 40ar 39ar ages are similar suggestive of rapid cooling through the 300 550 c temperature range whereas in the orogen core or western kaoko belt hornblende and biotite apparent ages are older than the white mica ages because the white micas record the reactivation of the main structures these data support models of diachronous deformation and metamorphism of the component parts of the orogen and suggest that pan african orogenesis in the damara is broadly bracketed between 580 and 500 ma with final movements through 480 ma and spans through the terminal amalgamation of gondwana in the kuunga orogeny through the initiation of subduction under the gondwana continental margin

juvenile contribution of the neoproterozoic rio negro magmatic arc ribeira belt brazil implications for western gondwana amalgamation abstract the ca 790 600 ma rio negro complex rnc of the ribeira belt brazil consists of a plutonic portion of a magmatic arc built by the e vergent subduction of the ese border of the sao francisco paleoplate during the amalgamation of western gondwana the plutonic series comprises low to medium k granitoids ca 790 620 ma and high k granitoids and shoshonite rocks ca 610 605 the age span of 185 m y is suggestive of a long history of arc related magmatism continuously or not in time the nd isotopic signatures of the rnc consist of end t ratios from 3 to 5 for the medium k series shoshonite series and from 14 to 3 for the younger high k group this time dependent trend of nd isotopes is indicative of progressive maturity of the arc over time the same evolution is indicated by sr data as the medium k rocks have 87sr 86sr initial ratios the role of transform fault zones such as the luanda shear zone is emphasized in order to explain the consumption of a wide oceanic plate in the inner portion of western gondwana

timing of terrane accretion in the neoproterozoic eopaleozoic ribeira orogen se brazil abstract new id tims u pb geochronological data of the oriental terrane combined with detailed geological data of other terranes support the presented tectonic evolutionary model for the central segment of the neoproterozoic ribeira belt southeastern brazil that can be subdivided in four major tectonic units a the occidental terrane regarded as the reworked margin of the sao francisco craton to the northwest b the paraiba do sul klippe c the oriental terrane comprising arc complexes and d the cabo frio terrane to the southeast the oriental terrane and the cabo frio terrane were progressively accreted to the reworked continental margin of the sao francisco paleoplate the occidental terrane contains pre 1 8 ga basement slivers interlayered with neoproterozoic passive margin successions andrel ndia group the oriental terrane focus of the work was firstly accreted to the sao francisco margin at ca 580 ma it displays three structural domains thrusted to nw toward the reworked sao francisco margin from west to east a the cambuci domain that is interpreted as the fore arc association b the costeiro domain that contains a distal passive margin succession intruded by the ca 790 and 635 620 ma rio negro arc complexes and c the italva domain that comprises marbles psamitic gneisses and amphibolites of ca 840 and is regarded both as the proximal passive margin of the oriental terrane or a back arc basin syn to late collisional granitoids and three metamorphic episodes ca 605 580 and 550 ma indicate a complex history for the collision i they were also described for the kaoko damara and west congo only the youngest one pan african belts located at occidental africa and are supposed to record the main episode of convergence in western gondwana a second collisional stage at ca 530 ma accreted the cabo frio terrane to the oriental terrane and is possibly related with the closure of a back arc basin located between the oriental terrane and kalahari craton that remained open until the ordovician the effects of the collision ii were also detected at the oriental terrane finally late to post tectonic granites marked the tectonic collapse of the orogenic building between 513 480 ma

correlation of neoproterozoic terranes between the ribeira belt se brazil and its african counterpart comparative tectonic evolution and open questions

neoproterozoic glacial deposits from the aracuai orogen brazil age provenance and correlations with the sao francisco craton and west congo belt article i nfo glacigenic diamictite successions of the macaubas group are widespread in the western domain of the aracuai orogen east of the sao francisco craton brazil diamictites also occur on this craton and in the african counterpart of the aracuai orogen the west congo belt detrital zircon grains from the matrix of diamictites and sandstones from the macaubas group were dated by the u pb shrimp technique the geochronological study sets the maximum depositional age of the glacial diamictites at 900 ma and indicates multiple sources for the macaubas basin with ages ranging from 900 to 2800 ma sm nd tdm model ages determined on whole rock samples range from 1 8 ga to 2 5 ga and get older up section comparison of our data with those from the cratonic area suggest that these glacial deposits can be correlated to the jequitai and carrancas diamictites in the sao francisco craton and to the lower mixtite formation of the west congolian group exposed in africa the 900 1000 ma source is most probably represented by the zadinian mayumbian volcanic rocks and related granites from the west congo belt however one of the most voluminous sources with ages in the 1 1 1 3 ga interval has not been detected in the sao francisco congo craton possible sources for these grains could occur elsewhere in africa or possibly from within the brasilia belt in western central brazil

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

geochemistry and crustal evolution of volcano sedimentary successions and orthogneisses in the sao gabriel block southernmost brazil relics of neoproterozoic magmatic arcs abstract precambrian metaplutonic rocks of the sao gabriel block in southernmost brazil comprise juvenile neoproterozoic calc alkaline gneisses cambai complex the connection with associated ultra mafic metavolcanic and metasedimentary rocks palma group is not well established the whole complex was deformed during the brasiliano orogenic cycle both metasedimentary and metavolcanic rocks as well as metaplutonic rocks of the cambai complex have been sampled for geochemical analyses in order to get constraints on the tectonic setting of these rocks and to establish a tectonic model for the sao gabriel block and its role during the assembly of west gondwana the major element compositions of the igneous rocks palma group and cambai complex indicate a subalkaline character most orthogneisses have a calc alkaline chemistry many metavolcanic rocks of the palma group show signatures of low k tholeiitic volcanic arc basalts trace element data especially ti zr y nb of most igneous samples from both the lower palma group and the cambai complex indicate origin at plate margins i e in a subduction zone environment this is corroborated by relative enrichment in lree low contents of nb and other high field strength elements and enrichment in lile like rb ba and th the data indicate the possible existence of two suites an oceanic island arc and a continental arc or active continental margin however some ultramafic samples of the lower palma group in the western sao gabriel block indicate the existence of another volcanic suite with intra plate character which possibly represents relics of oceanic island basalts oib trace element data indicate contributions from andesitic to mixed felsic and basic arc sources for the metasedimentary rocks the patterns of chondrite and n morb normalized spider diagrams resemble the patterns of the igneous rocks i e lile and lree enrichment and hfs depletion the geochemical signatures of most igneous and metasedimentary samples and their low 87sr 86sr t ratios suggest only minor contribution of old continental crust a geotectonic model for the sao gabriel block comprises east ward subduction and following accretion of an intra oceanic island arc to the eastern border of the rio de la plata craton at ca 880 ma and westward subduction beneath the newly formed active continental margin between ca 750 and 700 ma the sao gabriel block represents relics of an early brasiliano oceanic basin between the rio de la plata and kalahari cratons this ocean to the east of the rio de la plata craton might be traced to the north and could possibly be linked with neoproterozoic juvenile oceanic crust in the western brasilia belt goias magmatic arc

structure and strain variation at mid crustal levels in a transpressional orogen a review of kaoko belt structure and the character of west gondwana amalgamation and dispersal abstract the neoproterozoic cambrian kaoko belt is an orogen scale 800 180 km transpressional system important in the amalgamation of west gondwana mid crustal transpression at amphibolite to granulite facies conditions is dominated by two major 400 km exposed strike slip shear zones bounding a 20 40 km wide high grade orogen core to the east a deeply buried nappe dominated escape zone has inverted metamorphic sequence and verges outwards onto a platformal foreland to the west an arc like neoproterozoic coastal terrane was amalgamated and variably reworked during transpression the major purros and three palms mylonite zones have calculated shear displacements on the order of 120 180 km these shear zones are moderately to steeply dipping mylonite zones of 1 5 km width are arcuate and curvilinear in map view and show along strike variation in slip kinematics also highly curved in vertical section the shear zones define a flower to half flower geometry for the orogen core an oblique network of mylonitic shear zones akin to riedel shears links the major shear zones and defines regional scale shear lozenges internally deformed by tight upright folding and shear fabrics these shear zones create domains in the orogen core with varying dominance of pure shear in shear lozenges and simple shear in shear zones however absence of dip slip domains and the smoothly continuous traces of sub horizontal to shallow and acute oblique stretching lineations across all parts of the belt preclude marked kinematic partitioning and the internal part of the belt resembles large scale triclinic shear clast aspect ratios boudin train extension sheath fold aspect geometry degree of rotation of planes producing flanking folds composite s c foliations pressure fringes on pyrite and garnet porphyroclasts provide a semi quantitative measure of strain intensity average strain ratios are x z 40 1 for the major shear zones x z 12 1 for the orogen core x z 8 1 for the escape zone and x z 3 1 for the coastal terrane a more continuous pattern of strain intensity across the whole belt is mapped using a qualitative foliation intensity index foliation traces have a sigmoidal pattern in the orogen core swinging from sub parallel to the boundary shear zones to higher acute angles in the internal parts deformation character also varies from upright open folding in amphibolite facies domains in the north upright tight chevron folding in a low grade central domain to a high grade domain of tight to isoclinal inter folded basement and cover with inclination decreasing towards the south the kaoko belt is a well exposed sector of an extensive 3000 km long broad 400 km arcuate orogenic system adamastor orogen that amalgamated west gondwana bringing the south american sao francisco and rio de la plata cratons and african kalahari and congo cratons components together though a complex system most sectors involved oblique collision and accretion of magmatic arcs of 660 610 ma age followed by peak metamorphism and main phase transpressional orogenesis between 585 and 560 ma with shear zones remaining active until 530 ma this e w amalgamation immediately pre dates the final n s amalgamation of gondwana along the kuunga orogen between 535 and 510 ma the large scale adamastor orogen consisting of kaoko dom feliciano ribeira aracuai and west congo mobile belts also shows broadly similar and symmetric structural architecture throughout the high grade thermally softened core partitioned intense wrench dominated strains and networks of transcurrent shear zones that dip inwards with listric form either side of the internal zone containing amalgamated arcs and high grade core are nappe fold and thrust belts that rework attenuated passive margin basement and adamastor ocean sediments and structures verge outward at moderate to high angles onto both foreland margins the kaoko belt well illustrates the highly partitioned nature of transpressional systems in general and patterns in common throughout the greater adamastor orogen such as metamorphic zonation and heterogeneous distribution of deformation style flow regime and highly variable degrees of reworking strain and recrystallization this highly partitioned and steep structural grain localized lithospheric extension and rifting 415 ma later during breakup and dispersal of gondwana

the petrographic and geomechanical properties of some sandstones from the newspaper member of the natal group near durban south africa abstract sandstones are by far the most important rock type in the natal group which is mainly a terrestrial succession formed during ordovician silurian times the most notable member of the group is the newspaper member from which sample material was obtained most of the material sampled was fine to medium grained arkosic sandstone they are well cemented with generally low porosity accordingly sandstones of the newspaper member tend to be very strong and of low deformability few of the petrological properties of these sandstones have a notable influence on the mechanical properties however increasing clay content tends to lower the unconfined compressive strength whilst increasing quartz content tends to enhance it on the other hand many of the mechanical properties are interrelated significant or highly significant relationships occur amongst the strength parameters and between hardness and strength young s modulus also has significant relationships with strength and hardness another property which has a significant influence on strength and deformability is porosity

sedimentary provenance in the central ribeira belt based on laser ablation icpms207pb 206pb zircon ages abstract 207 pb 206 pb ages from 420 zircons from 9 upper amphibolite and granulite facies quartzites of the post 1 8 ga metasedimentary cover from the occidental and oriental terranes from the neoproterozoic central ribeira belt in eastern brazil are discussed ages from the occidental terrane show that proterozoic ages predominate with a modal class at 2 1 2 2 ga and that archean ages are less common and lie within the 2 5 3 6 ga range a wider dispersion on proterozoic ages down to 1 1 ga and youngest neoproterozoic ages 550 700 ma were also observed and could be related to the brasiliano panafrican metamorphic events the data show that detrital sediments of the occidental terrane were derived predominantly from erosion of a 1 8 2 2 ga paleoproterozoic terrane with a small proportion of a 2 6 3 6 ga archean crust ages from the oriental terrane are greatly dispersed but show a significant proportion of ages between 1 7 and 2 1 ga although the great majority are between 0 5 and 1 6 ga only 3 out of 144 grains yield archean ages between 2 5 and 2 9 ga based on the greatest errors observed on the data from the oriental terrane 100 300 ma and a continuum of obtained ages between 2072 ma paleoproterozoic and 517 ma cambrian the more plausible interpretation is that the dispersion of ages results from partial resetting of the u pb system by brasiliano metamorphism in proterozoic archean zircons although a mesoproterozoic source could not be discarded

sensitivity of sandstone strength and deformability to changes in moisture content the loss in uniaxial compressive strength between ucsdry and ucssat for 35 british sandstone types is discussed the difference between ucsdry and ucssat for the clay rich cretaceous greensand was 78 while for the siliceous sandstone the strength decreased by only 8 over the whole range of sandstones however the view that the weaker varieties are more sensitive to moisture content is not supported the degree of sensitivity to moisture content is controlled primarily by the proportions of quartz and clay minerals present and to a lesser extent by the rock microfabric research indicates that the development of pore pressure during loading is negligible especially in pure sandstones and hence does not play a significant role in moisture related strength reduction the work has demonstrated a large variation in sensitivity to moisture content throughout the range of sandstones studied and has shown that an increase in moisture content of as little as 1 from the dry state can have a marked effect on both strength and deformability a classification scheme for moisture content sensitivity is proposed a

a new zealand record of sea level rise and environmental change during the paleocene eocene thermal maximum abstract the global warming associated with the paleocene eocene thermal maximum petm ca 55 5 myr ago is the most dramatic identified short term temperature increase of the cenozoic one direct consequence of a warming world is a rise in sea level due primarily to the thermal expansion of water as oceans warmed the kumara 2 core south island new zealand spans the paleocene eocene transition and provides a rare southern hemisphere continental margin record of the petm lithology palynology and compound specific stable isotope compositions of higher plant leaf wax n alkanes reveal a 4 85 m petm and a carbon isotope excursion cie of ca 4 5 larger than the 2 5 to 3 5 cies generally recorded by deep sea foraminifera there is a shift from a terrestrial to a marine potentially anoxic sedimentary depositional environment at the base of the petm interpreted as being the result of a local sea level rise coincident with the onset of the cie is the appearance of pollen associated with thermophilic conditions and the development of nypa mangrove swamps moreover there is a reorganisation of the angiosperm pollen assemblage during the petm and an initial increase in fern spores and decrease in gymnosperms crucially all of these changes occur below the horizon characterised by the most negative 13c values suggesting that 1 the recorded negative excursion of 4 5 may indeed reflect the shift in atmospheric co2 isotopic composition and 2 that the large input of 13c depleted carbon into the ocean atmosphere system was not geologically instantaneous with at least some of the added carbon lagging warming sea level rise and vegetation change furthermore compound specific hydrogen isotope analyses show a large degree of variability both directly before and during the cie suggesting that the petm in new zealand was characterised by complex and transient changes in the hydrological regime similar to those reported in north america the arctic and eastern africa thus the new petm record from kumara 2 reveals local climatic and biotic responses driven by a combination of global warming and the consequential change in local depositional environment induced by sea level rise

the araa uaa west congo orogen in brazil an overview of a confined orogen formed during gondwanaland assembly abstract the neoproterozoic adamastor brazilide ocean was generated during the breakup of the rodinia supercontinent and remnants of its oceanic lithosphere have been found in the brasiliano pan african orogenic system that includes the aracuai west congo brasilia ribeira kaoko dom feliciano damara and gariep belts the aracuai and the west congo belts are counterparts of the same neoproterozoic orogen the first belt comprises two thirds of the aracuai west congo orogen this orogen is rather unique owing to its confined nature within the embayment outlined by the sao francisco and congo cratons in spite of this the presence of ophiolitic remnants and a calc alkaline magmatic arc indicate that the basin orogen evolution comprise both oceanic spreading and consumption it is assumed that coeval paramirim and sangha aulacogens played a key role by making room for the aracuai west congo basin sedimentary successions record all major stages of a basin that evolved from continental rift when glaciation related sedimentation was very significant to passive margin rifting started around 1 0 0 9 ga the oceanic stage is constrained by an ophiolitic remnant dated at 0 8 ga if the cratonic bridge that once linked the sao francisco and congo palaeocontinental regions did not hinder the opening of an ocean basin it certainly limited its width as a consequence only a narrow oceanic lithosphere was generated and it was subducted afterwards this is also suggested by orogenic calc alkaline granitoids occuping a small area of the orogen geochronological data for pre syn and late collisional granitoids indicate that the orogenic stage lasted from 625 ma to 570 ma a period of magmatic quiescence was followed by intrusion of postcollisional plutons at 535 500 ma the features of the aracuai west congo orogen suggest the development of a complete wilson cycle in a branch of the adamastor ocean which can be interpreted as a gulf with limited generation of oceanic lithosphere

a geomechanical survey of some different facies in relation to stability at a mine in the eastern transvaal coalfield south africa abstract the stability of roofs and floors in bord areas in coal mines is affected by a number of properties of the strata above and below the coal seam that is worked these include the lithological character of the rocks the sedimentary features contained within them the geomechanical properties of the rocks notably their strength and deformation moduli the durability of the rocks on exposure and the incidence and geometry of discontinuities accordingly core material was obtained from a mine in the eastern transvaal coalfield in order to investigate the influence of certain of these properties on the stability of the roof and floor of the mine the investigation showed the existence of a number of sedimentary facies that had different geological characteristics and geomechanical properties a large number of compressive and tensile strength tests were carried out on the core material and the geodurability of the rocks of the argillaceous facies was also determined the geodurability is assessed in terms of the free swell coefficient and the uniaxial compressive strength it indicated that some of the argillaceous material was of very poor quality the geodurability and tensile strength would appear to provide indications of the potential behaviour of the roof rocks as mining occurs and the uniaxial compressive strength helps in the understanding of failure of floors in coal mines in particular poor roof and floor conditions were found to be associated with rocks of the argillaceous facies the most stable strata were the fine grained arenaceous facies whilst those of the coarse grained facies were somewhat less stable the triaxial compressive strength and deformation moduli of some of the sandstones also were determined

architecture of late orogenic plutons in the aracuai ribeira fold belt southeast brazil abstract post collisional to late orogenic magmatism 580 to 480 ma in the aracuai ribeira fold belt se brazil is characterized by the predominance of high k metaluminous allanite titanite bearing granitoids small lenses of coronitic gabbro anorthosite pyroxenite and phlogopite peridotite are also common in deeper exposed areas of this fold belt in the region of southern espirito santo state the deep erosional level associated with a steep topography reveals the internal architecture of the intrusions a tendency to funnel shaped bodies with sub vertical hemi ellipsoidal conical roots changing upwards to shallow dipping tops associated stocks sills and dykes of basic and acid rocks generally intrude the enclosing gneisses along the foliation planes local ductile shear zones and parallel to fold hinges the contact between intrusions and the enclosing rocks is sharp in deeply eroded plutons e g santa angelica venda nova mimoso do sul and varzea alegre but at shallow levels e g castelo pedra azul and conceicao de muqui agmatic stoping zones occur along the borders a magmatic foliation within the granitoids is usually well marked but the schistosity in the surrounding gneisses wraps around the plutons the intrusions have a bimodal chemical distribution and generally are reversely zoned with mafic cores gabbro diorite to tonalite surrounded by a mingled marble cake zone where basic and acid rocks are interfingered and an external zone of syenomonzonite and granite widespread evidence of mingling and mixing between contrasting magmas of gabbroic and granitic and or syenomonzonitic compositions is characteristic for all intrusive complexes we suggest that replacement of lithospheric mantle by hot asthenospheric mantle induced partial melting of the crust the mantle exchange was due to lithospheric mantle delamination and slab breakoff following collisional orogenesis the bimodal plutons result from interaction among the contrasting magmas ascent and emplacement followed older regional structures such as regional fold hinges and ductile shear zones

tearing up rodinia the neoproterozoic palaeogeography of south american cratonic fragments rodinia was initially defined as a long lived supercontinent that assembled all the continental fragments around laurentia and remained stable from 1000 up to 750 ma nonetheless recent work has cast doubt on the rodinia palaeogeography and even on the timing of its assembly and break up the geochronological and palaeomagnetic databases accumulated for south america and africa in the last decade show that most of these continental fragments were not part of rodinia a wide brasiliano ocean separated most of the south american and african cratons from the laurentia amazonia rio apa west africa margin this ocean was closed between 940 and 630 ma along the pampean paraguay araguaia pharusian mobile belts moreover accretion along the south american and african platforms was a diachronous and long lived process that involved several intra oceanic and continental magmatic arcs and microcontinents this evolution started at around 1000 ma and ended at around 520 ma with the final assembly of gondwana

neoproterozoic crustal accretion in central brazil recent geochronological studies have demonstrated the existence of several neoproterozoic orthogneiss and metavolcanic are suites ca 900 to 600 ma in the southern part of the tocantins province a major neoproterozoic orogenic area in central brazil exposed between the amazon and sao francisco cratons nd and sr isotopic characteristics of these metamorphic suites are primitive initial 87 sr 87 sr ratios vary from 0 7024 to 0 7042 and initial nd values are positive varying between 0 2 and 6 9 the most primitive initial sr and nd isotopic compositions are very similar to model depleted mantle compositions at the time of formation of the original magmas the mantle like characteristics of these are suites in western goias contradict previous ensialic evolution models for the tocantins province and reveal that crustal accretion processes were important in large areas of central brazil during neoproterozoic time

from rodinia to gondwana a review of the available evidence from south america abstract in this work we will examine the available evidence for the position of the cratonic masses that formed west gondwana in the neoproterozoic such as amazonia west africa sao francisco congo rio de la plata and kalahari some emphasis will be given to south america and the smaller continental masses or microcontinents that are usually forgotten in the exercise of global reconstruction will also be considered among others such tectonic units are the central goias massif the borborema trans saharan block as well as the luiz alves parana and pampia cratonic fragments taking into account the relevant paleomagnetic and geochronological data it is necessary to consider a large time interval for the process of break up of rodinia signals of extension and fracturing are as old as 1000 1100 ma when mafic dyke swarms are observed on the sao francisco congo craton and final break out is reported as young as 750 ma on the other hand the formation of intra oceanic magmatic arcs within the large goias ocean occurred prior to 930 ma ago moreover for the make up of gondwana the first continental collisions are reported at ca 800 ma and the most important accretion episodes are the ones between 650 and 500 ma corresponding to the brasiliano pan african orogenic cycle therefore it is apparent that fragmentation of rodinia is partially synchronous with agglutination of gondwana

time frame of 753 680 ma juvenile accretion during the sao gabriel orogeny southern brazilian shield abstract the time frame of the three main geological events in the neoproterozoic cambai complex juvenile sao gabriel belt in the southern brazilian shield is established by integrating field mapping back scattered electron imaging and sensitive high resolution ion microprobe shrimp ii u pb dating of 96 zircon crystals from nine granitic and metasedimentary rock samples the three events are 1 voluminous flat lying paragneisses cambaizinho complex and orthogneisses vila nova gneisses between 735 and 718 ma 2 tonalite trondhjemite association lagoa da meia lua suite between 710 and 690 ma and 3 late granodiorite intrusions sanga do jobim suite at 680 ma an additional older volcanic event campestre formation was dated at 753 ma these results are most significant for the reconstruction of west gondwana

andean andesites and crustal growth over the last 200 ma the ensialic andean plate margin has been characterized by calc alkaline magmatism the early mesozoic activity was dominantly of basaltic volcanism while the cainozoic volcanism was of intermediate calc alkaline character the restriction of recent volcanism to parts of the andes underlain by thick wedges of asthenospheric mantle and the sr and nd isotopic relations indicate that the calc alkaline parental magmas are derived from the asthenospheric mantle there is no unequivocal geochemical and geophysical evidence that continental crust or sediment has contributed to the mantle source for andean magmatism the chemical compositions of the calc alkaline volcanic rocks of the active volcanic zones are controlled by fractional crystallization whereas o sr isotopic relations reflect crustal interaction of mantle derived parental magma with the sialic basement of the andes the variable extent of fractional crystallization partial melting and mixing of crustal contaminant are related to the variable thickness and age of crust in the different volcanic provinces calc alkaline magmatism was largely responsible for post mesozoic crustal growth in the andes and would have depleted the underlying mantle unless balanced by circulation within the asthenospheric mantle wedge in terms of net growth of the south american continent it is not certain where the balance lies between growth by magmatic addition and shrinking by erosion

closure of the clymene ocean and formation of west gondwana in the cambrian evidence from the sierras australes of the southernmost rio de la plata craton argentina abstract the formation of gondwana took place across a series of brasiliano pan african suture zones that record late neoproterozoic to earliest paleozoic collisions between precambrian cratons in south america an internal suture zone marks the disappearance of the clymene ocean that separated the amazon craton from the sao francisco and rio de la plata cratons new geochronological data from the southern end of this massive collision zone in the sierras australes of central eastern argentina document paleoproterozoic crust and suggest an ediacaran age for the oldest sedimentary rocks these two observations extend the known limit of the rio de la plata craton at least 200 km sw of previous estimates new data also confirm the occurrence of late ediacaran to late cambrian magmatism in the sierras australes the age of these hypabyssal to volcanic rocks corresponds to igneous events in the pampean belt along the western margin of the rio de la plata craton although the shallow level magma emplacement in the sierra da ventana study area contrasts with the deeply exhumed rocks of the pampean orogeny type locality these new age data are compared with a broad compilation of geochronological age clymene collision belts to the north the paraguai and araguaia belts the close overlap of the timing of orogenesis indicates the age of clymene ocean closure in its northern reaches in the south the pampean belt was unconfined allowing continued tectonic activity and crustal accretion throughout the paleozoic