the junction between pan african mobile belts in namibia its structural history abstract in namibia there is a junction between a pan african age 650 450 m y orogenic system which follows the western rim of africa and an ene trending arm which branches off into eastern namibia and zambia the se verging structures of this eastern arm were superimposed on earlier westward verging folds of the northern coastal belt the eastern arm was a zone of high temperature metamorphism and granite intrusion early in its history and this zone of high heat flow affected the style intensity and orientation of structures of the coastal belt these change from open flexural folds in the north away from the zone of high heat flow to intense recumbent folds associated with high grade metamorphism in the south where they deform rocks of the eastern arm the change in deformation intensity was largely responsible for a change in trend of the folds to produce a pronounced arcuate fold belt at the junction between the coastal and eastern arms

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

key ring structure gradients and sheath folds in the goantagab domain of nw namibia abstract the concept of deformation phases is one of the corner stones of structural geology but despite its simplicity there are situations where the concept breaks down in the goantagab domain of nw namibia structures in an area of complex deformation can be subdivided into at least four sets attributed to four deformation phases on the basis of overprinting relations three of these sets of structures however formed during the same tectonic event under similar metamorphic circumstances but slightly different flow conditions these sets of structures show gradational transitions in space that can be understood by a concept of key ring structure gradients where older d a structures are reoriented and overprinted by new structures d a 1 that have similar orientation and seem to grade into d a structures outside the overprinted area this kind of behaviour may be common in inhomogeneous non coaxial flow in the core of the goantagab domain d 2 structures are thus reoriented and overprinted by local d 2b folds and foliations that have the same orientation and style as d 2 structures outside the domain core

geochronology and petrogenesis of pan african syn tectonic s type and post tectonic a type granite namibia products of melting of crustal sources fractional crystallization and wall rock entrainment abstract the oetmoed granite migmatite complex ogmc central damara orogen namibia consists mainly of 526 to 516 ma garnet and cordierite bearing granite and subordinate 488 to 494 ma hornblende and titanite bearing granite in the form of planar sheets and dykes additionally a slightly elongated granite body occurs in the center of the complex the garnet and cordierite bearing granite has major and trace element characteristics of s type granite but the hornblende and titanite bearing granite has higher hfse and ree contents similar to a type granite whereas the garnet and cordierite bearing granite contains numerous restitic xenoliths the hornblende and titanite bearing granite is xenolith free the country rocks are cordierite sillimanite k feldspar garnet bearing metasedimentary rocks and migmatite cordierite and garnet rich xenoliths in the s type granite do not represent primary restite their depleted chemical composition is best explained by varying and large degrees of partial melting of incorporated country rocks most chemical variations among the garnet and cordierite bearing granite can be explained by processes linked with fractional crystallization of plagioclase biotite and accessory phases mostly monazite and zircon major and trace element data and high 18 o values suggest that the least evolved members of the garnet and cordierite bearing granite were derived from metapelitic rocks at ca 800 c as inferred from monazite and apatite dissolution thermometry higher cao and na2o but lower sio2 contents and lower rb sr ratios as well as lower 18 o values of the hornblende and titanite bearing granite suggest that they are more likely generated by partial melting of non pelitic sources metagranitoids at temperatures in excess of 900 c decreasing tio2 na2o feotot mgo cao total ree content but increasing al2o3 and k2o indicate fractionation of mainly hornblende and titanite in the case of the hornblende and titanite bearing granite the differing compositions of the garnet and cordierite bearing granite and the hornblende and titanite bearing granite are attributed to different source rocks metapelite instead of metagranitoid and different temperatures during melting as inferred from accessory phase dissolution thermometry furthermore significant entrainment of country rock in some samples played a major role during petrogenesis of the garnet and cordierite bearing granite but was not important during the evolution of the hornblende and titanite bearing granite intrusion of such hot felsic magmas close to the inferred peak of metamorphism has probably caused in part the high temperature metamorphism and anatexis of the country rocks at relatively low pressures

petrogenesis of cogenetic nepheline and quartz syenites and granites northern damara orogen namibia enriched mantle versus crustal contamination abstract geochemical and nd sr o whole rock isotope data and pb isotope data from leached feldspars for a suite of synorogenic ca 540 ma syenites and granites from the northern part of the proterozoic damara orogen namibia constrain their sources and petrogenesis major rock types are mildly potassic nepheline syenites and quartz syenites that were primarily generated by fractional crystallization from a mantle derived alkaline magma even the most primitive samples show pronounced depletion in nb ti sr and p on a primitive mantle normalized diagram indicating the involvement of a recycled crustal component in the source extrapolation of the sr nd pb o isotope composition of the syenites back to a hypothetical parental melt with 8 wt 10 wt mgo suggests derivation from a moderately enriched lithospheric upper mantle documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage t

the transpressional connection between dom feliciano and kaoko belts at 580 550 ma a new u pb shrimp age of 551 4 ma on a mylonitic porphyry that intruded into the sierra ballena shear zone southernmost dom feliciano belt uruguay and a review of relevant published data make possible a more refined correlation and reconstruction of brasiliano pan african transpressional events paleogeographic reconstruction kinematics and timing of events indicate a connection between the shear systems of the dom feliciano and kaoko belts at 580 550 ma sinistral transpression recorded in shear zones accommodates deformation subsequent to collision between the congo and rio de la plata cratons the correlation is strengthened by the similarity of magmatic and metamorphic ages in the coastal terrane of the kaoko belt and the punta del este terrane of the dom feliciano belt this post collisional sinistral transpression brought these units near to their final position in gondwana and explains the different evolution at 550 530 ma while in the kaoko belt an extensional episode resulted in exhumation as a consequence of collision in the damara belt in the dom feliciano belt sinistral transpression occurred associated with the closure of the southern adamastor ocean due to kalahari rio de la plata collision

530 ma syntectonic syenites and granites in nw namibia their relation with collision along the junction of the damara and kaoko belts abstract the lower ugab and goantagab structural domains are located at the junction between the n s trending kaoko and the e w trending damara belts nw namibia where neoproterozoic metavolcano sedimentary sequences were intruded by several syenitic granitic plutons we present here new u pb ages on zircon grains from the voetspoor and doros plutons together with petrological geochemical and structural data we evaluate the timing of the deformation and relation to the geodynamics during the final stage of gondwana amalgamation the plutons are composed of three main rock types hornblende quartz syenite syenodiorite and biotite granite the two former are predominant and show genetic correlation such as magma mingling structures and similar geochemical signatures the biotite granite occurs in the sw parts of the intrusions and clearly cuts the syenitic rocks although the plutons are mainly isotropic the structures around them demonstrate that their intrusion occurred during a second deformation phase d2 with a component of sinistral solid state rotation with respect to the wall rocks in response to d2 transpression four samples were dated using u pb shrimp methodology in single zircon grains a hornblende monzodiorite from the voetspoor pluton yielded an age of 534 4 5 ma a hornblende monzonite from the doros pluton produced an age of 528 5 ma the biotite granite facies was sampled in the doros intrusion and yielded an age of 530 4 5 ma in addition a granitic vein folded by d2 close to the northeastern contact of the doros pluton with the encasing phyllites amis river formation was also dated yielding an age of 533 6 ma the data show that all granite syenite from doros and voetspoor intrusions are contemporaneous and crystallized in the period between 539 and 522 ma within the errors d1 d2 deformational phases took place under greenschist facies biotite zone conditions and during d3 the metamorphic grade was slightly lower we interpret that the plutons are coeval to peak metamorphism of the region 530 520 ma and that d2 and d3 sinistral transpressional phases are due to collision in the damara belt the e w compressional event and second metamorphic episode in the kaoko belt occurred between 580 and 560 ma and are apparently unrelated to the thermo tectonic evolution described here although d1 might be partially related to this event the sinistral transpressional d2 phase resulted probably from the position of the area considered at the junction between the belts and not in the frontal damara collision further to the east this new interpretation is consistent with the ar ar ages for the region about 500 ma interpreted to reflect cooling of the orogen the enrichment in lree k rb ba and sr and depletion in nb of these basic to intermediate alkalic rocks could indicate that they partially derived from melting of a subcontinental lithospheric mantle that was affected by subduction and the granitic rock types represent lower crust contamination we interpret that they could be related to heating in the mantle caused by asthenosphere influx in a zone of slab breakoff during collision between kalahari and congo cratons

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

lower crustal melting and the role of open system processes in the genesis of syn orogenic quartz diorite granite leucogranite associations constraints from sr nd o isotopes from the bandombaai complex namibia abstract the bandombaai complex southern kaoko belt namibia consists of three main intrusive rock types including metaluminous hornblende and sphene bearing quartz diorites allanite bearing granodiorites and granites and peraluminous garnet and muscovite bearing leucogranites intrusion of the quartz diorites is constrained by a u pb zircon age of 540 3 ma quartz diorites granodiorites and granites display heterogeneous initial nd and o isotope compositions end 540 ma 6 3 to 19 8 18o 9 0 11 6 but rather low and uniform initial sr isotope compositions 87sr 86srinitial 0 70794 0 70982 two leucogranites and one aplite have higher initial 87sr 86sr ratios 0 70828 0 71559 but similar initial end 11 9 to 15 8 and oxygen isotope values 10 5 12 9 the geochemical and isotopic characteristics of the bandombaai complex are distinct from other granitoids of the kaoko belt and the central zone of the damara orogen our study suggests that the quartz diorites of the bandombaai complex are generated by melting of heterogeneous mafic lower crust based on a comparison with results from amphibolite dehydration melting experiments a lower crustal garnet and amphibole bearing metabasalt probably enriched in k2o is a likely source rock for the quartz diorites the granodiorites granites show low rb sr 1 and are most likely generated by biotite dehydration melting of heterogeneous felsic lower crust all segments of the lower crust underwent partial melting during the pan african orogeny at a time 540 ma when the middle crust of the central damara orogen also underwent high t medium p regional metamorphism and melting geochemical and isotope data from the bandombaai complex suggest that the pan african orogeny in this part of the orogen was not a major crust forming episode instead even the most primitive rock types of the region the quartz diorites represent recycled lower crustal material

partial melting of diverse crustal sources constraints from sr nd o isotope compositions of quartz diorite granodiorite leucogranite associations kaoko belt namibia the torrabaai koigabmond complex southern kaoko belt namibia consists of three main intrusive rock types including metaluminous hornblende and titanite bearing quartz diorites metaluminous hornblende and biotite bearing granodiorites and peraluminous garnet and muscovite bearing leucogranites uranium pb zircon data obtained on the granodiorites and leucogranites indicate concordia upper intercept ages of 553 40 ma although pb 207 pb 206 ages of ca 650 ma in zircon from the granodiorites suggest some inheritance of older material uranium pb monazite data obtained on the leucogranites give concordant ages of 550 ma 3 ma these ages are similar to the rb 87 sr 86 whole rock age of 584 35 ma obtained on the granodiorites and leucogranites although the rb sr age seems to be biased towards older ages due to limited assimilation of older material in contrast to other plutonic complexes from the kaoko belt the quartz diorites granodiorites and granites show a restricted range in their initial nd sr and o isotope compositions quartz diorites epsilon nd init 5 4 to 6 7 delta o 18 8 3 9 4 parts per thousand sr 87 sr 86 0 7081 0 7098 granodiorites epsilon nd init 6 1 to 7 7 delta o 18 9 9 10 9 parts per thousand sr 87 sr 86 0 7071 0 7105 leucogranites epsilon nd init 4 9 to 8 7 delta o 18 9 8 11 3 parts per thousand sr 87 sr 86 0 7060 0 7125 enclaves are found in the granodiorites and leucogranites but not in the quartz diorites they have a granodioritic composition with quartz plagioclase k feldspar and hornblende and some have additional garnet relative to the country rock gneisses the so called nk formation enclaves are depleted in sio2 na2o k2o sr ba and enriched in cao feo total mgo tio2 sc v cr ni rb and y rare garnet bearing enclaves are additionally depleted in lree and enriched in hree relative to the granodiorites these features are qualitatively consistent with the hypothesis that these enclaves may represent moderately depleted melting residues of nk formation gneisses in comparison with experimentally derived melts and based on low al2o3 feo mgo tio2 ratios and high al2o3 feo mgo tio2 values it is suggested that the quartz diorites are generated by dehydration melting of a mafic amphibole and plagioclase bearing lower crustal source of pan african age the granodiorites likely represent fractionation products of the quartz diorites however it is also possible that the granodiorites represent partial melting products of a mafic to intermediate lower crustal source but experienced likely slightly lower degrees of melting probably at water present conditions the leucogranites display higher al2o3 feo mgo tio2 ratios but lower al2o3 feo mgo tio2 values and are most likely generated by biotite dehydration melting of felsic crustal sources major and trace element and isotope variations indicate that fractional crystallization with only limited crustal contamination was the major rock forming mechanism it is suggested that most of the isotope variation reflects pre existing heterogeneities of the sources consequently interpretation of geochemicaland isotope data from the complex suggests that the pan african igneous activity in this part of the damara kaoko belt was not a major crust forming episode and all rock types represent reprocessed crustal material c 2008 elsevier b v all rights reserved less

geochemical character and petrogenesis of pan african amspoort suite of the boundary igneous complex in the kaoko belt nw namibia abstract in the kaoko belt of northwestern namibia syn tectonic granitoids of the boundary igneous complex intruded a nnw sse trending suture between the neoproterozoic coastal terrane and archaean neoproterozoic congo craton derived foreland typical representatives of this syn collision igneous association are ca 550 ma old k feldspar phyric bt cam granites granodiorites of the amspoort suite with minor cpx gabbro and rare two pyroxene dolerite bodies the scarce opx cpx bt dolerites could come from a chur like mantle source 87sr 86sr550 0 7045 e550nd 0 the rest of the amspoort suite is interpreted as a product of ht anatexis of a heterogeneous lower crust built mainly by immature metapsammites rich in arc derived detritus with minor metabasite and intermediate metaigneous bodies the most likely source appears to be the anatectic coastal terrane gneisses yet partial melting of so far little constrained congo craton cover if formed by immature and youthful detritus unrelated to the basement cannot be discounted in any case the rather primitive sr nd isotopic compositions of the amspoort suite apart from dolerites e550nd 3 4 to 5 3 87sr 86sr550 0 7063 0 711 rule out any major role for the congo craton basement derived material which is in sharp contrast to other more southerly pan african granitoid complexes of the kaoko belt

cloud water in windward and leeward mountain forests the stable isotope signature of orographic cloud water 1 cloud water can be a significant hydrologic input to mountain forests because it is a precipitation source that is vulnerable to climate change it is important to quantify amounts of cloud water input at watershed and regional scales during this study cloud water and rain samples were collected monthly for 2 years at sites on windward and leeward east maui the difference in isotopic composition between volume weighted average cloud water and rain samples was 1 4 18o and 12 2h for the windward site and 2 8 18o and 25 2h for the leeward site with the cloud water samples enriched in 18o and 2h relative to the rain samples a summary of previous literature shows that fog and or cloud water is enriched in 18o and 2h compared to rain at many locations around the world this study documents cloud water and rain isotopic composition resulting from weather patterns common to montane environments in the trade wind latitudes an end member isotopic composition for cloud water was identified for each site and was used in an isotopic mixing model to estimate the proportion of precipitation input from orographic clouds orographic cloud water input was 37 of the total precipitation at the windward site and 46 at the leeward site this represents an estimate of water input to the forest that could be altered by changes in cloud base altitude resulting from global climate change or deforestation

pan african rifting and orogenesis in southern to equatorial africa and eastern brazil abstract the upper proterozoic pan african belts of africa and brasiliano belts of south america are assumed to have evolved from an elaborate system of continental rifts which formed on the west gondwana continent 1100 1000 ma ago reviews of the damara belt kaoko belt gariep belt saldania belt west congolian belt lufilian arc and zambezi belt of southern and equatorial africa and the ribeira mantiqueira belt of brazil show the applicability of the continental rifting model to all these belts strict application of the rifting model describing the formation and further evolution of rift structures in the continental crust leads to the conclusion that the west congolian belt as traditionally described is incomplete and has to be supplemented by the mayumbian belt and a rift structure situated farther to the west the lufilian arc is assumed to have evolved from two rift structures separated from each other by the later domes region the golfe du katanga which branches off northeastwards from the lufilian arc is interpreted as an aulacogen shaba aulacogen the pan african brasiliano rift system is comparable to the mesozoic arctic north atlantic rift system with regard to size and distribution of rifts ocean floor spreading and opening of a proto south atlantic ocean most probably occurred along a main line of rifting situated between the west coast of africa and eastern brazil the wedge like ocean may have terminated at the sao francisco congo cratonic bridge between northeastern brazil and gabon cameroon there is no evidence of ocean floor spreading in the zambezi belt the lufilian arc and the traditional west congolian belt which are arranged along another major line of rifting closure of the pan african brasiliano rift system took place during two successive orogenic episodes the zambezi belt lufilian arc and west congolian belt underwent their main orogenic deformation with prevailing ene and wsw oriented tectonic transportation directions during the katangan episode at 900 750 ma the transcontinental sinistral mwembeshi shear zone adjusted opposite movement directions in the zambezi belt and lufilian arc and at its western prolongation concurrently supported opening of the khomas trough in the southern damara belt the proto south atlantic ocean still opening during the katangan episode was gradually closed during the damaran episode at 750 500 ma closure proceeded from north to south and was accompanied by northwest directed subduction underneath the brazilian plate and southeast directed tectonic transportation in the kaoko damara and gariep belts in the previously formed west congolian belt lufilian arc and zambezi belt a second deformation phase characterized by strike slip faulting and local emplacement of nappes occurred during this episode

intrusion mechanisms in a turbidite sequence the voetspoor and doros plutons in nw namibia abstract two syntectonic plutons of cambrian age intruded neoproterozoic metaturbidites in namibia at the junction of the ns trending kaoko and ew trending damara belts sinistral transpression in the kaoko belt produced km scale upright d1 folds overprinted by minor d2 folds d3 is associated with n s shortening in the damara belt the plutons show two main pulses of intrusion hornblende syenite intruded late during d1 or during d2 and biotite granite during d3 each tectonic event produced a strain shadow defined by the shape of folds and the foliation trend around the plutons the internal igneous fabric and the arrangement of wall rock xenoliths that locally make up 50 of the intrusion mass suggest that the plutons have a disk or wedge shape a marginal shear zone indicates that one of the syenite intrusions descended during emplacement with respect to the wall rock emplacement is therefore inferred to be at least partly accommodated by descent of the intrusion floor the biotite granite intruded into mega strain shadows and tension gashes alongside and in the syenite during d3 the plutons show evidence of sinistral solid state rotation with respect to the wall rocks in response to d1 d2 transpression

petrology of basement dominated terranes i regional metamorphic t t path from u pb monazite and sm nd garnet geochronology central damara orogen namibia abstract mineral chemistry combined with sm nd garnet whole rock and u pb monazite ages provides important constraints on the geological history of metapelites migmatites and granites from the high grade basement dominated part of the damara orogen namibia based on thermobarometric data the peak of regional metamorphism reached c 700 750 c and c 5 kbar u pb monazite and sm nd garnet mineral ages obtained from unmigmatized garnet and cordierite bearing metapelites indicate that these peak metamorphic conditions were reached between 525 2 and 504 3 ma some migmatites with restitic melanosomes record sm nd garnet ages of 518 3 ma indicating that high grade regional metamorphism also culminated in partial melting some other migmatites and granites give older sm nd garnet whole rock ages between 530 3 and 540 4 ma different initial nd isotope compositions of granites leucosomes and melanosomes indicate that in this case migmatization is intrusion related crust derived granites intruded at 515 2 512 3 and 509 5 ma and are probably related to the second peak of regional metamorphism the youngest granites intruded at 469 3 ma indicating that intrusion of melts is superimposed upon the regional metamorphic history age constraints for the period between c 540 and 510 ma provide substantial evidence for crustal thickening together with igneous activity whereas the high temperature event which occurred at c 470 ma must be related to extensional tectonics the concordance between the monazite and garnet ages suggest that monazite grew at high grade metamorphic conditions it is further demonstrated that combined monazite and garnet chronometry are useful in identifying the timing and nature of particular tectonometamorphic and intrusive events in polymetamorphic high grade terranes

the position of the amazonian craton in supercontinents abstract this paper examines the extensive regions of proterozoic accretionary belts that either formed most of the amazonian craton or are marginal to its southeastern border their overall geodynamic significance is considered taking into account the paleogeographic reconstruction of columbia rodinia and gondwana amazonia would be part of columbia together with laurentia north china and baltica forming a continuous continental landmass linked by the paleo to mesoproterozoic mobile belts that constitute large portions of it the rodinia supercontinent was formed in the mesoproterozoic by the agglutination of the existing cratonic fragments such as laurentia and amazonia during contemporary continental collisions worldwide the available paleomagnetic data suggest that laurentia and amazonia remained attached until at least 600 ma since all other cratonic units surrounding laurentia have already rifted away by that time the separation between amazonia and laurentia marks the final break up of rodinia with the opening of the lapetus ocean

neoproterozoic igneous complex emplaced along major tectonic boundary in the kaoko belt nw namibia ion probe and la icp ms dating of magmatic and metamorphic zircons granitoid intrusions of the boundary igneous complex separate segments with different ages of high grade metamorphism in the kaoko belt nw namibia two granitoids of this complex were dated at 575 10 ma secondary ionization mass spectrometry sims or 571 9 ma laser ablation inductively coupled plasma mass spectrometry la icp ms and 562 11 ma sims or 572 4 ma la icp ms respectively the age of granulite facies metamorphism in the eastern part of the western kaoko zone was established at 549 5 ma sims by analysing metamorphic overgrowths of older c 1850 1000 ma zircons from melt segregations in amphibolites the coastal part of the western kaoko zone consists of horizons of migmatitic metasedimentary rocks that are intercalated with fine grained orthogneisses and amphibolites resembling metamorphosed sequences of bimodal volcanic rocks zircons from felsic members of two bimodal suites have sims ages of 805 4 ma and 810 840 ma respectively that are interpreted as dating their respective igneous protoliths melt segregations in the mafic member of the lower bimodal suite contain two populations of zircon dated at 650 5 ma sims or 645 5 ma la icp ms and 629 6 ma sims or 630 5 ma la icp ms respectively the later age is indistinguishable from the age of 630 4 ma sims or 625 10 ma la icp ms obtained from melt patches present in overlying metagreywackes the available age data suggest that the boundary igneous complex masks the suture between the coastal terrane and the rest of the kaoko belt ages of granitoid intrusions in this igneous complex are indicative of magmatic activity between 580 and 550 ma

closing the clymene ocean and bending a brasiliano belt evidence for the cambrian formation of gondwana southeast amazon craton we report new paleomagnetic and geochronological data from ediacaran rift drift carbonates in the paraguai belt at the southern end of the suture zone between the amazon craton and the sao francisco and rio de plata cratons south america early thrusting resulted in remagnetization ca 528 36 ma or later the mean age is established by 40 ar 39 ar encapsulation dating of mixed authigenic and detrital illite from remagnetized carbonates from the unmetamorphosed fold thrust belt this remagnetization overlaps with a 525 ma gondwana reference pole metamorphic illite from the slate belt yields 40 ar 39 ar ages of 496 484 ma the timing of peak regional metamorphism oroclinal bending of the paraguai belt was caused by a 90 clockwise rotation of the east west limb after ca 528 ma probably refl ecting the irregular margin of the southeast amazon craton the age of the paraguai belt overlaps with that of the pampean orogeny farther south along the western margin of the rio de plata craton suggesting a coeval closure for the clymene ocean separating the amazon craton from the sao francisco and rio de plata cratons

variable late neogene exhumation of the central european alps low temperature thermochronology from the aar massif switzerland and the lepontine dome italy 1 several recent studies proposed an important increase in exhumation rate in the western european alps since circa 5 4 ma in order to assess potential spatial differences in exhumation histories we present new apatite fission track aft and apatite u th he ahe ages from the central aar massif guttannen area switzerland and the western lepontine dome formazza area italy internal u th zoning in apatites explains alpha ejection corrected ahe ages that are older than the corresponding aft ages in this study a qualitative interpretation of aft and ahe age elevation relationships suggests a two phase 9 7 and 5 3 ma exhumation scenario affecting the central alps with a stronger expression of the pliocene signal in the formazza area however a quantitative evaluation of exhumation scenarios using the 3 d heat equation solver pecube highlights the existence of several other likely scenarios casting doubt on the validity of a qualitative interpretation of the age elevation relationships in formazza scenarios suggested by quantitative modeling include continuous denudation at a rate of 750 m ma and a one step exhumation rate change from 300 to 1000 m ma at 5 ma in guttannen they include continuous denudation at a rate of 400 m ma with valley deepening and two periods of higher exhumation rate increasing from 300 to 700 m ma repeatedly at 9 7 and at 5 3 ma contingent upon further flexural isostatic modeling the magnitude of exhumation recorded in the axial region of the alps since circa 5 ma does not appear sufficient to solely explain the denudation recorded in the north alpine foreland basin

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

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

a model for water transport and ice lensing in freezing soils a one dimensional model for water transport and ice lensing in incompressible saturated and solute free soil specimens is proposed for the simulation of small scale frost heave tests in the laboratory the model considers 1 open system freezing in which the variables t and pw are independent 2 an ice lens that continues to grow as long as enough energy is available in the frozen fringe to produce the work required for mass transfer to the ice lens and 3 a new ice lens that forms when the vertical strain in the frozen soil reaches the instantaneous tensile failure strain the proposed frost heave model is amenable to computer simulation procedures which provide predictions of rate of frost heave and rate of pore freezing front penetration as functions of applied load thermal and water flow regimes and soil properties position time of initiation and ultimate thickness of individual ice lenses emerge also as a part of the solution a quantitative comparison of observed and simulated responses for various boundary conditions is shown for a reference soil known as devon silt the model was found to exhibit many of the characteristics observed in frost heave tests on devon silt and predicted the effects of overburden pressure on frost heave rate