nature of mantle source contributions and the role of contamination and in situ crystallisation in the petrogenesis of proterozoic mafic dykes and flood basalts labrador proterozoic tholeiitic dyke swarms share many compositional features with and pose similar petrogenetic problems to phanerozoic continental flood basalts but there are few extrusive equivalents of such swarms the mesoproterozoic 1 27 ga harp dyke swarm in labrador is one where possible extrusive equivalents exist in the seal lake group but are slightly displaced in space and time and can probably be related by models of progressive crustal extension here we try to evaluate the roles of crystal differentiation in situ crystallisation crustal assimilation and the relative contributions of asthenosphere and lithosphere derived melts in the petrogenesis of the mafic magmas modelling of the major and trace element variations both within individual dykes and between dykes and within the lava sequence does not suggest an important role for continental crust involvement while in situ crystallisation processes could account for some of the compositional variations the most successful models invoke mixing or contamination of asthenospheric magmas with by veined material in the lower lithosphere upper asthenosphere which carries the continental characteristics the results imply an important role for hydrous phases such as phlogopite and hornblende in the sub lithosphere mantle much of the low mgo character of mafic dykes may result from significant removal of mafic phases during in situ crystallisation within the lithosphere

deposition diagenetic and hydrothermal processes in the aptian pre salt lacustrine carbonate reservoirs of the northern campos basin offshore brazil abstract the discovery of large oil accumulations in the rift and sag pre salt sections of the campos and santos basins has revived interest in the exploration of the lacustrine carbonate reservoirs in the brazilian and african marginal basins more than half of brazilian oil production originates from the pre salt reservoirs of these offshore basins a study integrating systematic petrography cathodoluminescence scanning electron microscopy microprobe and x ray diffraction was performed on seven wells in the northern campos basin this study highlights the major primary diagenetic and hydrothermal features of the pre salt succession with the aim to improve our understanding of the factors that influence the porosity and permeability distribution in these important lacustrine carbonate reservoirs the pre salt deposits correspond to bioclastic grainstones and rudstones syngenetic crusts of fascicular calcite and intraclastic grainstones and rudstones of reworked crust fragments and calcite spherulites magnesian silicates are frequently associated with carbonate deposits in the sag phase stevensitic laminations constitute the substrate for the precipitation of calcite crusts and spherulites which displace and replace the syngenetic magnesian clay deposits in the rift section stevensitic ooids are mixed with bioclasts or form ooidal arenites pre salt carbonate reservoirs have undergone a complex and heterogeneous diagenetic evolution eodiagenetic processes include the dissolution neomorphism and cementation of bivalve bioclasts in the rift as well as the dissolution of magnesian silicates and their replacement by calcite spherulites silica and dolomite in the sag section burial alterations are commonly associated with hydrothermal fluids carried through faults and fracture systems these fluids promote dolomitization silicification and dissolution at varying degrees and intensities eodiagenetic precipitation and dissolution owing to variations in the lake water chemistry and the episodic flow of hydrothermal fluids under burial conditions control the creation redistribution and obliteration of porosity in the pre salt reservoirs

stratigraphy composition and form of the deccan basalts western ghats india in the western ghats between latitudes 18 20 n and 19 15 n 7000 km2 of deccan basalt have been mapped with the primary objective of establishing a flow stratigraphy as a guide to the volcanic history of the flood basalts using over 70 measured vertical sections major and trace element analyses of nearly 1200 samples and rare earth and87sr 86sr determinations for over 60 samples we divide the basalt into three subgroups and ten formations in this paper we describe the seven principal formations in the area and the most prominent individual flows

the tectonic evolution of the south atlantic from late jurassic to present an improved tectonic database for the south atlantic has been compiled by combining magnetic anomaly geosat altimetry and onshore geologic data we used this database to obtain a revised plate kinematic model starting with a new fit reconstruction for the continents around the south atlantic we present a high resolution isochron map from chron m4 to present fit reconstructions of south america and africa that require rigid continental plates result in substantial misfits either in the southern south atlantic or in the equatorial atlantic to achieve a fit without gaps we assume a combination of complex rift and strike slip movements 1 along the south american parana chacos basin deformation zone 2 within marginal basins in south america salado colorado basin and 3 along the benue trough niger rift system in africa these faults are presumed to have been active before or during the breakup of the continents our model describes a successive unzipping of rift zones starting in the southern south atlantic between 150 ma tithonian and approximately 130 ma hauterivian rifting propagated to 38 s causing tectonic movements within the colorado and salado basins subsequently between 130 ma and chron m4 126 5 ma the tip of the south atlantic rift moved to 28 s resulting in intracontinental deformation along the parana chacos basin deformation zone between chron m4 and chron mo 118 7 ma rifting propagated into the benue trough and niger rift inducing rift and strike slip motion after chron mo the equatorial atlantic began to open while rifting and strike slip motion still occurred in the benue trough and niger rift since chron 34 84 ma the opening of the south atlantic is characterized by simple divergence of two rigid continental plates

early cretaceous rifts of western and central africa an overview abstract the structure and evolution of early cretaceous rift basins in western and central africa are described two stages of rift development and fracturing have been identified 1 from neocomian to early aptian roughly e w and nw trending troughs upper benue n cameroon s chad sudan etc opened in response to a submeridian extensional regime in central africa while in western africa the n s trending transsaharian fault zone acted as a sinistral wrench 2 from middle aptian to late albian large northwest trending troughs e niger sudan sirte etc opened in response to a northeast extensional regime while the central african fault zone from benue to sudan exhibited strike slip movements generating pull apart basins these rift and fracture systems delimit three large blocks within the african plate a western block an arabian nubian block and an austral block the arabian nubian block tends to separate from the two other blocks migrating towards the north during the first stage of basin development and then towards the ne during the second stage the opening of the atlantic ocean was the dominant driving force for the western and austral blocks while the arabian nubian block probably moved in response to the opening of the indian ocean and to the evolution of the tethyan margin

basaltic lava flows covering active aeolian dunes in the parana basin in southern brazil features and emplacement aspects abstract burial of active aeolian dunes by lava flows can preserve the morphology of the dunes and generate diverse features related to interaction between unconsolidated sediments and lavas in the study area located in southern brazil burial of aeolian deposits by cretaceous basaltic lava flows completely preserved dunes and generate sand deformation features sand diapirs and peperite like breccia the preserved dunes are crescentic and linear at the main contact with basalts and smaller crescentic where interlayered with lavas the various feature types formed on sediment surfaces by the advance of the flows reflect the emplacement style of the lavas which are compound pahoehoe type four feature types can be recognized a type 1 features are related to the advance of sheet flows in dune interdune areas with slopes 5 b type 2 is formed where the lava flows advance in lobes and climb the stoss slope of crescentic dunes slopes 8 12 c type 3 is generated by toes that descend the face of linear dunes slopes 17 23 and d type 4 occurs when lava lobes descend the stoss slope of crescentic dunes slopes 10 15 the direction of the flows the disposition and morphology of the dunes and the ground slope are the main factors controlling formation of the features the injection of unconsolidated sand in lava lobes forms diapirs and peperite like breccias sand diapirs occur at the basal portion of lobes where the lava was more solidified peperite like breccias occur in the inner portion where lava was more plastic favoring the mingling of the components the generation of both features is related to a mechanical process the weight of the lava causes the injection of sand into the lava and the warming of the air in the pores of the sand facilitates this process the lava sediment interaction features presented here are consistent with previous reports of basalt lavas with unconsolidated arid sediments and additional new sand deformation features formed by lava breakouts and sand diapir injections are presented

evolution of the walvis ridge rio grande rise hot spot system implications for african and south american plate motions over plumes crystallization ages of volcanic rocks dredged or drilled from the walvis ridge ten sites and the rio grande rise one site have been determined by the 40ar 39ar incremental heating technique the fundamentally age progressive distribution of these basement ages suggests a common hot spot source for volcanism on the island of tristan da cunha along the walvis ridge and rio grande rise and for the formation of the continental flood basalts located in namibia africa and brazil south america the walvis ridge rio grande rise volcanic system evolved along a section of the south atlantic spreading axis as the african and south american plates migrated apart astride or in close proximity to an upwelling plume reconstructions of the spatial relationship between the spreading axis the tristan hot spot and the evolving walvis ridge rio grande rise volcanic feature show that at about 70 ma the spreading axis began to migrate westward away from the hot spot the resulting transition to intraplate hot spot volcanism along the walvis ridge and associated termination of rio grande rise formation also involved a northward migration of previously formed african seafloor over the hot spot rotation parameters for african motion over fixed hot spots i e absolute motion have been recalculated such that the predicted trail of the tristan hot spot agrees with the distribution of radiometric and fossil basement ages along the walvis ridge african absolute motion has been extended to the south and north american plates by the addition of relative motion reconstruction poles

origin and migration timing of hydrothermal fluids in sedimentary rocks of the parana basin south america abstract clay authigenesis induced by migrating hydrothermal fluids related to the early cretaceous serra geral magmatism in the parana basin was investigated by mineralogical geochemical and isotopic means stratigraphically equivalent sedimentary samples from the permian rio bonito in southeastern brazil and san miguel in eastern paraguay formations were collected close to and far from major intrusive bodies the rocks are generally cemented in addition to the clay material by a variety of minerals such as albite k feldspar calcite dolomite zeolite and quartz the authigenic clay minerals mainly consist of different types of illite rich mixed layers illite smectite and chlorite corrensite and smectite were also observed measured and estimated k ar ages indicate that the varied authigenic illite assemblages were most probably generated 140 to 130 ma ago during the serra geral magmatic event hydrothermal fluids depleted in lree and sometimes enriched in eu promoted the formation of the well crystallized illite minerals in rocks characterized by high water rock ratios additional authigenic assemblages with paas type ree patterns probably formed in less porous rocks with low water rock ratios also some of the authigenic clays yield hree depleted patterns which point to the occurrence of organic matter during crystallization

chemical stratigraphy of the parana lavas south america classification of magma types and their spatial distribution a new classification scheme has been developed to assign the lava flows of the parana continental flood basalt province south america into geochemically distinct magma types with six basaltic major and trace element abundances and or ratios by mapping out the spatial distribution of these magma types within the lava sequences it has been possible to determine the internal stratigraphy of the lava pile on a regional scale previous studies on road profiles traversing the well exposed coastal serra geral escarpment of southern brazil are summarised together with results from some new sampled sections more widespread stratigraphical investigations of the parana lavas have been hampered by the lack of sufficient topographic relief and the cover of sedimentary rocks however access to drill core chippings from nine boreholes in the central parana region has provided a unique opportunity to investigate the stratigraphy of the otherwise inaccessible deeper levels of the lava pile and to map out stratigraphic variations in three dimensions the borehole samples have indicated cated a stacking of units of different magma types all overlapping towards the north which suggests that the main locus of magmatism moved northwards with time within the parana basin this migration could be related to the northward propagation of rifting during the initiation of the south atlantic ocean maps of the surface distribution of samples of each magma type show a pattern consistent with the stratigraphy inferred from the boreholes although suggesting that the shift in magmatism may have been towards the northwest on the basis of geochemical similarities between magma types and their inferred stratigraphical relationships it is proposed that the parana can be divided into two principal magmatic centres 1 an older one in the south comprising the gramado esmeralda and urubici magma types and 2 a younger one developed about 750 km to the north formed by the pitanga paranapanema and ribeira magma types

mantle plumes and flood basalt stratigraphy in the parana south america geochemical studies of the parana continental flood basalts in brazil have led to the recognition of distinct magma types which have been used to infer the internal stratigraphy of the lava pile the overstepping of stratigraphic units toward the north and a similar compositional change in sills within the underlying parana basin sedimentary rocks imply that the site of magnatism migrated 750 km toward the north during this volcanic event this has important implications for recent plume related models that have argued whether fithospheric rifting must accompany the presence of a mantle plume in order to generate continental flood basalt magmatism it is difficult to ascribe the magnitude and direction of the shift in the locus of the parana magmatism to movement of the brazilian lithosphere relative to the underlying tristan mantle plume instead it is suggested that the observed lava distribution and internal structure of the parana etendeka flood basalt province were imposed by the intracacies of the rifting process during the initial opening of the south atlantic ocean

40ar 39ar dating of quaternary feldspar examples from the taupo volcanic zone new zealand using a continuous laser and resistance furnace we have measured ages on quaternary plagioclase with an absolute precision of about 30 ka and on quaternary sanidine with a relative precision of better than 1 such precision was achieved by using low temperature heating steps to remove much of the nonradiogenic argon contamination plagioclase is one of the most common mineral phases in volcanic rocks thus these procedures will be widely applicable to many problems for which precise radiometric age control has not been available we studied plagioclase and plagioclase sanidine concentrates from the oldest and the three largest silicic ash flow deposits of the taupo volcanic zone new zealand one of the world9s largest and most active volcanic systems the results are in close agreement with new magnetostratigraphic data suggest that existing fission track age determinations significantly underestimate the age of older units and shift the inception of taupo volcanic zone volcanism back to at least 1600 ka the improved precision has permitted the first correlations between proximal and distal units previous correlations with deep sea ash flows in the western pacific require major revision

temporal and spatial variability of seafloor spreading processes in the northern south atlantic the paper reports the results of an aeromagnetic study of the mid atlantic ridge in the northern south atlantic the study consists of 38 000 km of low altitude magnetics tracks centered on the ridge covering nearly 700 000 km2 south of ascension island most tracks extend out to crust of anomaly 5b age large areal coverage coupled with high resolution allows comparison of crustal generation processes over space along ridge strike and time across strike temporal heterogeneities revealed in the data include a pattern of change in spreading rate and direction of plate motion the total spreading rate increased sharply at 10 ma and then decreased between 6 ma and the present the time variation of spreading rate is similar to that seen in other areas such as the north atlantic and the southern and eastern pacific the plate boundary has evolved over time in response to the shift in plate motion direction through fragmentation and rotation of ridge segments between fracture zones in several cases the location of the spreading axis has shifted through jumps or propagation across fracture zones the periods of plate boundary modification correlate with the increase and decrease in spreading rate spatial heterogeneities are caused by the influence of the ascension hotspot on the mid atlantic ridge in the northern portion of the study area and include two apparent seamount chains formed by the hotspot ascension island and several of the seamounts appear to have been produced some distance away from the hotspot location presently about 225 km southeast of ascension island by channeled subaxial magma flow beneath the spreading center the seamount chains therefore do not mark hotspot tracks large local variations in spreading rate and degree of asymmetry along strike are also found

an aem tem study of weathering and diagenesis abert lake oregon i weathering reactions in the volcanics abstract abert lake in south central oregon provides a site suitable for the study of sequential weathering and diagenetic events in this first of two papers transmission electron microscopy was used to characterize the igneous mineralogy subsolidus alteration assemblage and the structural and chemical aspects of silicate weathering reactions that occur in the volcanic rocks basalts basaltic andesites and dacitic rhyolitic extrusive and pyroclastics that outcrop around the lake olivine and pyroxene replacement occurred topotactically whereas feldspar and glass alteration produced randomly oriented smectite in channels and cavities the tetrahedral octahedral and interlayer compositions of the weathering products largely dioctahedral smectites varied with primary mineral composition rock type and as the result of addition of elements released from adjacent reaction sites weathering of the highly evolved fe rich jug mountain complex at the north end of the lake produced a homogeneous smectite assemblage that contrasts with the heterogeneous smectite assemblage replacing the volcanics along the eastern margin of the lake the variability within and between the smectite assemblages highlights the microenvironmental diversity fluctuating redox conditions and variable solution chemistry associated with mineral weathering reactions in the surficial environment late stage exhalative and aqueous alteration of the volcanics redistributed many components and formed a variety of alkali and alkali earth carbonate chloride sulfate and fluoride minerals in vugs and cracks overall substantial mg si na ca and k are released by weathering reactions that include the almost complete destruction of the mg smectite that initially replaced olivine the leaching of these elements from the volcanics provides an important source of these constituents in the lake water the nature of subsequent diagenetic reactions resulting from the interaction between the materials transported to the lake and the solution will be described in part ii banfield et al 1991

potassium argon dating of fine grained basalts with massive ar loss application of the 40ar39ar technique to plagioclase and glass from the kirkpatrick basalt antarctica 40ar39ar incremental heating measurements are reported for separated plagioclase and matrix fractions and a glass sample of the kirkpatrick basalt from victoria land antarctica these results are used to address the limitations of whole rock analyses of glassy or very fine grained basalts the use of plagioclase for determining ages of such rocks the ar behavior of matrix glass and feldspar components the use of k cl ratios in interpretation of step heating results and the age of the kirkpatrick basalt the precise age of the kirkpatrick basalt has been a long standing problem the wide range of reported k ar dates has been attributed to loss of ar despite massive ar loss from the matrix plagioclase is shown to yield good 40ar39ar plateaus plateaus are observed for increments above 700 c and are defined not only by consistent apparent ages but also appropriately low k ca and high k cl ratios cl relations help identify ar released from small amounts of matrix which can seriously contaminate a plagioclase separate an age of 176 8 1 8 ma defines the time of formation of the youngest lava at mt frustum in north victoria land a glass sample from south victoria land gives the same age indicating that terrestrial basaltic glasses as old as jurassic do not inherently lose ar the step heating profile of glass with very fine crystals shows discordance which is an artifact of ar recoil during irradiation

hotspot tracks and the early rifting of the atlantic abstract many hotspot tracks appear to become the locus of later rifting as though the heat of the hotspot weakens the lithosphere and tens of millions of years later the continents are split along these weakened lines examples are the west coast of greenland east coast of labrador madeira hotspot the south coast of mexico north coast of honduras guyana hotspot and the south coast of west africa north coast of brazil st helena hotspot a modern day analog of a possible future rift is the snake river plain where the north american continent is being pre weakened by the yellowstone hotspot track this conclusion is based on reconstructions of the motions of the continents over hotspots for the past 200 million years the relative motions of the plates are determined from magnetic anomaly isochrons in the oceans and the motion of one plate is chosen ad hoc to best fit the motions of the plates over the hotspots however once the motion of this one plate is chosen the motions of all the other plates are prescribed by the relative motion constraints in addition to the correlation between the predicted tracks and sites of later continental breakup exposed continental shields correlate with the tracks their exposure may be the result of hotspot induced uplift which has led to erosion of their former platform sediment cover

petrogenesis of coexisting sio2 undersaturated to sio2 oversaturated felsic igneous rocks the alkaline complex of itatiaia southeastern brazil abstract the itatiaia alkaline complex is a late cretaceous intrusion 72 myr made up of felsic differentiates with syenitic rocks dominant throughout and with presence of both nepheline and quartz rich varieties dykes with phonolitic or trachytic composition cross cut the coarse grained facies the rocks are arranged concentrically with the core of the complex being formed by sio 2 oversaturated syenites with a small outcrop of granites and are radially displaced by faults related to regional tectonic lineaments the minerals show gradual but significant changes in composition salitic and augitic to aegirine rich pyroxenes hastingsite and actinolite to richterite and arfvedsonite amphiboles sodic plagioclase to orthoclase feldspars and so on and the whole rock trends are broadly consistent with fractional crystallization processes dominated by alkali feldspar removal sr isotopic data indicate more radiogenic ratios for the sio 2 oversaturated rocks 0 7062 0 7067 against 0 7048 0 7054 for the sio 2 undersaturated syenites consistent with small amounts of crustal input the favored hypothesis for the petrogenesis of the different syenitic groups is the prolonged differentiation starting from differently sio 2 undersaturated mafic parental magmas potassic alkali basalts to ankaratrites present in the late cretaceous dyke swarms of the area accompanied by variable crustal contamination prior to the final emplacement the lack of carbonatite as a significant lithotype the potassic affinity of the itatiaia complex and the relatively high sr isotopic ratios match the characteristics of the other complexes of the rio de janeiro sa o paulo states coastline and confirm the ultimate derivation of these differentiated rocks from an enriched lithospheric mantle source

quartz latite rheoignimbrite flows of the etendeka formation north western namibia the etendeka formation of north western namibia consists of a sequence of interbedded quartz latites and tholeiitic basalts and forms part of the karoo igneous province in southern africa the age of the etendeka formation is approximately 130 135 ma the quartz latites make up a significant proportion of the stratigraphic succession 25 of the total stratigraphic thickness and form as much as 60 of the outcrop area in the southern etendeka apart from some systematic differences between pitchstones and devitrified quartz latite largely explained by alteration processes individual quartz latite units exhibit remarkably uniform compositions with no significant vertical or lateral variation geochemistry can be used as a primary criterion for the correlation of major quartz latite units over much of the southern etendeka area enabling the reconstruction of the etendeka formation stratigraphy in this region individual quartz latite units occur as voluminous 400 2600 km3 widespread up to 8800 km2 sheet like deposits typically between 40 and 300 m thick each unit consists of basal main and upper zones the main zone generally constitutes over 70 of the thickness of the unit and typically consists of texturally featureless devitrified quartz latite in contrast the basal and upper zones of the flow are characterised by flow banding pitchstone lenses and breccia with rare occurrences of pyroclastic textures the quartz latites are sparsely porphyritic 10 phenocrysts with glassy or devitrified groundmass textures the phenocrysts consist of plagioclase pyroxene titanomagnetite and rare ilmenite pyroxene geothermometry indicates high 1000 1100 c temperatures of crystallisation which coupled with the absence or primary hydrous phases indicates that the quartz latites were relatively hot h2o undersaturated magmas the quartz latites display features common to both rhyolite lavas and ignimbrites and are clearly the products of an unusual eruption style the local preservation of pyroclastic textures and the broad areal extent of these units lead to the conclusion that the quartz latites are high temperature rheomorphic ignimbrites i e rheoignimbrites a combination of high eruption temperature and relatively low viscosity helps to explain the often completely welded and homogeneous textures observed in most quartz latite outcrops in the etendeka area

tectonic controls on magmatism associated with continental break up an example from the parana etendeka province the high and low ti basalts of the parana etendeka province were primarily derived from old trace element enriched source regions in the lithospheric mantle and they are associated with dyke swarms of different orientations these swarms appear to reflect different amounts of extension and it is inferred that the high and low ti magma types were characterised by different melt generation rates of 0 15 km3 yr 1 and 0 4 km3 yr 1 respectively stewart et al earth planet sci lett 143 1996 95 109 there is probably a gap of 2 myr between the end of the main phase of cfb magmatism and the oldest rocks on the adjacent ocean floor a simple numerical model has been used to constrain the amounts and rates of melt generated from the continental lithosphere and asthenosphere under finite duration extension melting in the mantle is assumed to be controlled by the dry peridotite solidus in the asthenosphere and the hydrous 0 2 h2o peridotite solidus in the lithosphere for a maximum of 4 and a duration of extension of 10 myr the derivation of melt from the asthenosphere by dry peridotite melting depends primarily on potential temperature tp and is relatively insensitive to the thickness of the mbl while the converse is the case for melt derived from the lithosphere by hydrous peridotite melting for a tp of 1450 50 c inferred from the crustal thickness estimates along the rio grande rise and walvis ridge the model successfully generates 2 4 km of lithosphere derived melt before producing significant volumes of asthenosphere derived melt it is concluded that increases of melt volume with time can be generated by decompression melting of the mantle lithosphere critically in areas of significant melt generation within the mantle lithosphere during extension and break up there is likely to be a gap in the volcanic record between the end of melt generation in the lithosphere and the onset of melting in the underlying asthenosphere no such gap is present if all melts are generated within the mantle plume and thus these models may in principle be tested in the geologic record

interpretation of the propagation of surface altimetric observations in terms of planetary waves and geostrophic turbulence 1 the interpretation of surface altimetric signals in terms of rossby waves is revisited rather than make the long wave approximation the horizontal scale of the waves is adjusted to optimally fit the phase speed predicted by linear theory to that observed by altimetry assuming a first baroclinic mode vertical structure it is found that in the tropical band the observations can be fit if the wavelength of the waves is assumed to be large of order 600 km or so however poleward of 30 it is more difficult to fit linear theory to the observations and the fit is less good than at lower latitudes the required scale of the waves must be reduced to about 100 km somewhat larger than the local deformation wavelength it is argued that these results can be interpreted in terms of rossby wave baroclinic instability and turbulence theory at low latitudes there is an overlap between geostrophic turbulence and rossby wave timescales and so an upscale energy transfer from baroclinic instability at the deformation scale produces waves at high latitudes there is no such overlap and waves are not produced by upscale energy transfer these ideas are tested by using surface drifter data to infer turbulent velocities and timescales that are compared to those of linear rossby waves a transition from a field dominated by waves to one dominated by turbulence occurs at about 30 broadly consistent with the transition that is required to fit linear theory to altimetric observations

magmatism and continental break up in the south atlantic high precision40ar 39ar geochronology abstract a detailed ar ar study of the parana etendeka continental flood basalts cfb has been undertaken using laser spot analysis data provide information not only on the age of the samples but also on the variability of the non radiogenic ar component and state of alteration the results indicate the parana etendeka cfb were erupted over 10 million years between 137 and 127 ma providing a minimum overall eruption rate of 0 1 km3 yr 1 this is an order of magnitude less than that previously proposed for this the deccan and siberian cfb provinces but it is similar to estimates for hawaii iceland and the columbia river cfb in detail the new ar ar analyses indicate that magmatism within the parana etendeka province migrated from nw to se prior to and during the opening of the south atlantic providing an explanation for the asymmetry of the cfb lavas about the south atlantic moreover chemically defined magma types were erupted at different times in different places and so within the paranathey may not be used as reliable chronostratigraphic units rather such magmatic units may reflect the extent of compositionally distinct source regions in the uppermost mantle and indicate that partial melting took place over a wide area under the paranabasin the nw se migration of the onset of magmatism might be interpreted as a plume trace but the inferred rate of movement is 3 times faster than that inferred from subsequent magmatism on the rio grande rise it is argued that the onset of magmatism reflected extensional tectonics normal to the ponta grossa dyke swarm given that magmatism preceded the main phase of rifting and that extension across the ponta grossa was moderate current plume models have difficulty in predicting sufficient melting within the asthenosphere

glacial geologic confirmation of an intraplate boundary in the parana basin of brazil the predrift fit of south america and africa remains problematic given the inability to match all parts of the coastlines simultaneously early opening of the south atlantic ocean may have been facilitated by intraplate deformation in either south america or africa but the location of such deformation is controversial and poorly constrained broad scale remote sensing has suggested an intraplate boundary in south america stretching from the cochabamba bend in the andes to the rio grande walvis rise but the inferred boundary crosses the paranaa basin of brazil where any evidence of deformation is obscured by late jurassic flood basalts a subsurface study of the upper paleozoic glaciogenic infill of the paranaa basin the 1300 m thick itarare group ca 300 260 ma identifies repeated episodes of intracratonic rifting on either side of an accommodation zone crossing the basin along the trend of the inferred intraplate boundary renewed influence of this zone during the opening of the south atlantic is suggested by the asymmetric distribution of successive magma units within the serra geral lava pile and by the trend of dike swarms the intraplate boundary identified within the paranaa basin may extend westward along the northern margin of the chaco basin in bolivia

the cretaceous iron belt of northern chile role of oceanic plates a superplume event and a major shear zone the cretaceous constitutes a turning point in the tectonic magmatic and metallogenic history of chile the geological evidence indicates that a major change occurred in late neocomian time when superplume emplacement mid pacific superplume and plate reorganization processes took place in the pacific the superplume event resulted in a major ridge push force resulting in increased coupling between the subducting and overriding plates this completely changed the tectonic setting of chile ending the early cretaceous extensional period aborted rifting in the back arc basin and increasing stress at a crustal scale as a consequence overpressurized dioritic magmas were pushed up mainly along the best possible structural path in northern chile i e the atacama fault zone eventually forming a 500 km long belt of kiruna type iron deposits with reserves of 2 000 mt 60 fe a unique case in chile s geological history

kinematic keys of the santos namibe basins understanding the genesis of the very peculiar 600 km wide santos basin sa o paulo plateau system and its narrow conjugate namibe margin is a kinematic and structural problem several hypotheses have been proposed in order to explain the genesis of this system that imply the same amount of horizontal movement we investigate the consequences of the horizontal movement in the santos basin based in plate kinematic reconstructions the kinematic history of this system that we present here based on the interpretation of seismic profiles and kinematic constraints has the following consequences 1 there is no evidence of a ridge jump sensu stricto but rather a southwards propagation in the central segment of the south atlantic that starts in the northern part between the ne brazilian and gabonese margins 2 the namibe margin evolved as a transform passive margin 3 the opening direction of the santos basin sa o paulo plateau system is oblique to the general opening motions of the south american and african plates and 4 this opening is younger 6 ma than those of the other basins of the central segment of the south atlantic

rb sr age determinations of rocks from the okenyenya igneous complex northwestern namibia the okenyenya igneous complex is one of a suite of intrusions which define a prominent northeast trending linear feature in damaraland northwestern namibia precise rb sr internal isochron ages range from 128 6 1 to 123 4 1 4 ma for the major phases of intrusion identified within the complex the tholeiitic gabbros forming the outer rings of the complex and the later alkali gabbros which form the central hills cannot be distinguished in terms of rb sr ages although field relations clearly indicate the younger age of the latter the intrusionsof nepheline syenite and essexite comprising the mountain of okenyenya bergon the northern edge of the complex give ages of 123 4 1 4 and 126 3 1 ma respectively and form the final major phase of intrusion the ages obtained for early and late intrusive phases define a minimum magmatic life span of approximately 5 ma for the complex the determined age of the okenyenya igneous complex 129 123 ma when taken together with the few reliable published ages for other damaraland complexes 130 134 ma suggests that these sub volcanic complexes were emplaced contemporaneously with the widespread etendeka volcanics 130 ma and relate to magmatism associated with the breakup of southern africa and south america with the opening of the south atlantic ocean the linear distributionof intrusions in damaraland is interpreted to be due to magmatism resultingfrom the upwelling tristan plume being focused along a structural discontinuity between the pan african damaran terrain to the south and proterozoiccratonic basement to the north

age progressive volcanism in the new england seamounts and the opening of the central atlantic ocean radiometric ages k ar and oar 39ar methods have been determined on dredged volcanic rocks from seven of the new england seamounts a prominent northwest southeast trend ing volcanic lineament in the northwestern atlantic ocean the oar 39ar total fusion and increnental heating ages show an increase in seamount construction age from southeast to northwest that is consistent with northwestward motion of the north american plate over a new england hot spot between 103 and 82 ma a linear volcano migration rate of 4 7 cm yr fits the seamount age distribution these ages fall within a longer age progression from the corner seamounts 70 to 75 ma at the eastern end of the new england seamounts to the youngest phase of volcanism in the white mountain igneous province new england 100 to 124 ma the new england hot spot estimated from the new radio metric ages and motion of north america in the hot spot reference frame to be near 28on 33ow probably generated a short line of mid to late tertiary age seamounts on the african plate but appears to be presently inactive the hot spot reference frame is used to calculate the motion of the north american plate away from the african plate from early cretaceous time to the present rominent magnetic anomalies recorded in the central atlantic seafloor give the positions of the spreading ridge for a bange of known ages hot spots that now underlie the african plate in the eastern central atlantic new england canary madeira cape verde azores could have produced cretaceous seamount and island chains on the north american plate during the early opening of the central atlantic each of these hot spots has been overridden by spreading ridges at pre dictable times some of these hot spot crossings are expressed as geochemical anomalies in the oceanic crust now far removed from spreading ridges

the chon aike province of patagonia and related rocks in west antarctica a silicic large igneous province the field occurrence age classification and geochemistry of the mesozoic volcanic rocks of patagonia and west antarctica are reviewed using published and new information dominated by rhyolitic ignimbrites which form a bimodal association with minor mafic and intermediate lavas these constitute one of the largest silicic igneous provinces known equivalent in size to many mafic lips diachronism is recognized between the early middle jurassic volcanism of eastern patagonia marifil and chon aike formations and the middle jurassic earliest cretaceous volcanism of the andean cordillera el quemado ibanez and tobifera formations this is accompanied by a change in geochemical characteristics from relatively high zr and nb types in the east to subalkaline arc related rocks in the west although the predominance of rhyolites remains a constant factor all of the associated mafic rocks are well fractionated compared to direct mantle derivatives petrogenetic models favour partial melting of immature lower crust as a result of the intrusion of basaltic magmas possibly with some hybridisation of the liquids and subsequent fractionation by crystal settling or solidification and remelting the formation of large amounts of intracrustal silicic melt acted as a density barrier against the further rise of mafic magmas which are thus rare in the province

mantle plumes continental magmatism and asymmetry in the south atlantic melt production rates are estimated along the ocean tracks rio grande rise walvis ridge and the onshore parana etendeka flood basalt province for the mantle plume currently associated with tristan da cunha in the south atlantic assuming that the volcanic edifice is locally compensated with a thickened crust the maximum oceanic rate 0 3 km3 a 1 is similar to the value for the continental flood basalt calculated using the surface volume of lava and a duration of 5 m y however this latter estimate is conservative and the appropriate rate is probably at least a factor of 5 higher the maximum mantle potential temperature for the ocean plume tracks is estimated to have been 1450 1540 c assuming a dry peridotite source using a similar model temperatures in excess of 1600 c are required to explain the observed magma volumes onshore this is because there is no significant extension in the region where the oldest lavas occur it is argued that the observed volumes of melt can be generated with small amounts of extension and with plume temperatures of 1500 c if melting in the lithosphere is strongly influenced by the relative enrichment of volatiles moreover the model predicts the inferred change to an asthenosphere dominated source when significant extension occurs recent geochonological data have shown that the continental volcanics are 138 ma in the northwest of the parana province and become progressively younger to the southeast oceanic melt production rates decrease by a factor of 3 5 towards the mid ocean ridge tristan da cunha intriguingly melt production rates on the rio grande rise vary to more than twice that of the walvis ridge for oceanic crust of similar age this asymmetry is also seen in the continental regions most obviously in the preserved volume of lava in the parana and etendeka but also in the topography of the underplated continental margin and the orientation of dykes and oblique extension structures these observations reflect the combined roles of the plume temperature lithospheric thickness crustal tectonics and magma source region composition during the evolution of the south atlantic over the last 150 ma