heat flow data from the western margin of the west african craton mauritania eighteen new heat flow determinations in shallow boreholes 100 230 m at three sites in west africa mauritania are presented the first site is located in the archaean province reguibat shield of the west african craton temperature measurements were performed in iron formation and needed correction for refraction the average value is 50 mw m 2 and radioactive element distribution in surface rocks provides an upper crust heat contribution of 1 8 w m 3 the second site located near the craton margin and the third site located in the senegal coastal basin display a slightly higher value of 59 mw m 2

rare earth thhfta and large ion lithophile element variations in metabasites from the proterozoic amphibolite granulite transition zone at arendal south norway abstract the proterozoic arendal metabasites are minor intrusions which possess wholly metamorphic textures and mineral assemblages data are presented for eight selected samples chondrite normalised ree patterns are relatively flat between 10 and 20 chondrite abundances they are characterised by 1 ce n yb n n nd n 1 normalised to primordial mantle values the incompatible elements reveal variable enrichment in the large ion lithophile elements except th and low ta they are similar to many destructive plate margin basalts on a th hf ta diagram the rocks plot in a sub linear trend reflecting variation in th hf samples with high th hf ratios come from the zone of highest metamorphic grade where the host gneisses are deficient in th consequently the patterns in the metabasites could not have been produced by any secondary metamorphic fractionation or by bulk contamination by the country rocks there is no evidence for any post magmatic mobility of any of the elements except rb which is depleted in the highest grade rocks relative to their lower grade counterparts the trace element distributions are largely reflecting those of the parental magmas but consideration of bulk distribution coefficients shows how they may have been modified in some aspects e g ce n nd n by the imposition of the metamorphic mineralogies the data are consistent with a model involving a derivation from a mantle source region which had been affected by subduction related hydrous metasomatism or partial melting and b crystallisation of the magmas directly under the high grade conditions

morphology internal architecture and emplacement mechanisms of lava flows from the central atlantic magmatic province camp of argana basin morocco abstract the morphology internal architecture and emplacement mechanisms of the central atlantic magmatic province camp lava flows of argana basin in morocco are presented the volcanic pile was produced by two volcanic pulses the first represented by the tasguint formation corresponds to a succession of 3 13 individual flows created by 1 8 eruptions the second alemzi formation is composed of 2 7 individual flows formed by 1 4 eruptions these formations geochemically distinct are separated by thin silty or sandy horizons or by palaeosols they include compound pahoehoe flows and simple flows the first type is almost exclusive of the lower formation while the second type dominates the upper formation the lava flows show clear evidence of endogenous growth or inflation the characteristics of the volcanic pile suggest slow emplacement during sustained eruptive episodes and are compatible with a continental basaltic succession facies model

opening of a trans pangaean marine corridor during the early jurassic insights from osmium isotopes across the sinemurian pliensbachian gssp robin hood s bay uk the hispanic corridor represents a significant phase of continental reorganisation of the early jurassic that eventually provided connectivity between the western tethyan and eastern pacific oceans along the central atlantic rift zone although the initiation of this marine corridor profoundly impacted oceanic circulation and marine faunal exchange patterns the timing of its formation hitherto remains poorly constrained with estimates spanning both the hettangian and sinemurian the sinemurian pliensbachian global stratotype section and point gssp at robin hood s bay uk comprises a succession of well exposed immature organic rich sediments only previously characterised by strontium oxygen and carbon isotope geochemistry new re and os isotope profiling indicates substantial variation in seawater chemistry at this time initial osmium isotope data become increasing unradiogenic 0 40 to 0 20 across the boundary providing evidence for a continual flux of unradiogenic os into the oceans during the latest sinemurian the initial unradiogenic 187os 188os values indicate the occurrence of low temperature hydrothermal activity associated with the formation of the hispanic corridor during the breakup of pangaea therefore combined with biogeography and faunal exchange patterns the os isotope data demonstrates that connectivity between the eastern pacific and tethyan oceans initiated during the latest sinemurian as a result this study better constrains the timing of establishment of the hispanic corridor which was previously limited to poorly defined biogeography

oxide and sulphide minerals in highly oxidized rb depleted archaean granulites of the shevaroy hills massif south india oxidation states and the role of metamorphic fluids the shevaroy hills of northern tamil nadu southern india expose the highest grade granulites of a prograde amphibolite facies to granulite facies deep crustal section of late archaean age these highly oxidized quartzofeldspathic garnet charnockites generally show minor high tio2 biotite and amphibole as the only hydrous minerals and are greatly depleted in the incompatible elements rb and th peak metamorphic temperatures garnet orthopyroxene and pressures garnet orthopyroxene plagioclase quartz are near 750 degrees c and 8 kbar respectively pervasive veinlets of k feldspar exist throughout dominant plagioclase in each sample and show clean contact with orthopyroxene they are suggested to have been produced by a low h2o activity migrating fluid phase under granulite facies conditions most likely a concentrated chloride carbonate brine with high alkali mobility accompanied by an immiscible co2 rich fluid silicate oxide and sulphide mineral assemblages record high oxygen fugacity pyroxenes in the felsic rocks have high mg mg fe 0 5 0 7 the major oxide mineral is ilmenite with up to 60 mole per cent exsolved hematite utilizing three independent oxygen barometers ferrosilite magnetite quartz ferrosilite hematite quartz and magnetite hematite in conjunction with garnet orthopyroxene exchange temperatures samples with x hm ilm 0 1 yield a consistent oxygen fugacity about two log units above fayalite stability less oxidized samples x hm ilm 0 4 pyrite is the dominant sulphide and pyrrhotite is absent pyrite grains in these samples have marginal alteration to magnetite along the rims signifying a high temperature oxidation event moderately oxidized samples 0 1 x hm ilm 0 4 have abundant co existing pyrrhotite pyrite and magnetite the most reduced granulite samples have pyrrhotite as the dominant sulphide with little or no pyrite and no coexisting magnetite chalcopyrite is a common accessory mineral of pyrite and pyrrhotite in all the samples textures in some samples suggest that it formed as an exsolution product from pyrrhotite extensive vein networks of magnetite and pyrite associated principally with the pyroxene and amphibole give evidence for a pervasive highly oxidizing fluid phase thermodynamic analysis of the assemblage pyrrhotite pyrite and magnetite yields consistent high oxidation states at 700 800 degrees c and 8 kbar the oxygen fugacity in our most oxidized pyrrhotite bearing sample is 10 12 65 bar at 770 degrees c there are strong indications that the shevaroy hills granulites recrystallized in the presence of an alkali rich low h2o activity fluid probably a concentrated brine it cannot be demonstrated at present whether the high oxidation states were set by initially oxidized protoliths or effected by the postulated fluids the high correspondence of maximally rb depleted samples with the highest recorded oxidation states suggests that the rb depletion event coincided with the oxidation event probably during breakdown of biotite to orthopyroxene k feldspar we speculate that these alterations were effected by exhalations from deep seated alkali basalts which provided both heat and high oxygen fugacity low a h2o fluids it will be of interest to determine whether greatly rb depleted granulites in other precambrian terranes show similar highly oxidizing signatures

mapping and characterization from aeromagnetic data of the foum zguid dolerite dyke anti atlas morocco a member of the central atlantic magmatic province camp abstract a high resolution aeromagnetic survey was carried out in the anti atlas morocco covering the main areas traversed by the great camp foum zguid dyke fzd this doleritic dyke belongs to the central atlantic magmatic province camp a large igneous province which is associated with the fragmentation of the supercontinent pangaea and the initial stages of rifting of the central atlantic ocean it also coincides in time with the mass extinction of the triassic jurassic boundary based on the study of geological maps and google earth satellite images it appears that the fzd is poorly exposed and often covered by quaternary deposits this work proposes aeromagnetic modelling and interpretation of the fzd in order to better constrain its structural extent the data have allowed i mapping of the dyke over great distances under the quaternary deposits and through areas where it was poorly characterized on the geological map ii identifying major tectonic lineaments interpreted as faults iii recognizing magnetic anomalies related to mafic intrusive bodies and iv informing about regional structural context

permian magmatism and basin dynamics in the southern pyrenees a record of the transition from late variscan transtension to early alpine extension

discovery of early cretaceous rocks in new caledonia new geochemical and u pb zircon age constraints on the transition from subduction to marginal breakup in the southwest pacific abstract new u pb dating of detrital zircon and geochemical features of permian mesozoic arc derived volcanic rocks and volcaniclastic turbidites graywackes when compared with those of the volcanic rocks associated with unconformable late cretaceous shallow water sediments reveal that subduction in new caledonia once thought to be extinct in the late jurassic ca 150 ma was still active at least from ca 130 to 95 ma the accumulation of volcanic arc derived sediments during the late early cretaceous suggests that as in new zealand active margin activity went on for a short time in spite of the assumed subduction jamming by the hikurangi plateau at ca 100 ma meanwhile the rift related magmatic activity that preceded the marginal breakup migrated eastward from ca 130 ma 130 95 ma in eastern australia to 110 ma 110 82 ma in new zealand and finally to ca 89 ma 89 83 ma in new caledonia and generated large volumes of silicic magma in contrast marginal basins opened synchronously at

asthenospheric and lithospheric sources for mesozoic dolerites from liberia africa trace element and isotopic evidence abstract combined elemental and sr and nd isotopic data are presented for mesozoic dolerite dikes of liberia africa which are related to the initial stage of opening of the atlantic ocean the large scatter of both trace element and isotopic data allows the identification of five groups of dolerites which cannot be related to each other by simple processes of mineral fractionation from a common source on the contrary the observed chemical and isotopic variation within some dolerites groups i and ii may result either from variable degrees of melting of an isotopically heterogeneous source or mixing between enriched and depleted oceanic type mantle for the other dolerites groups iii v mixing with a third mantle source with more radiogenic sr and with element ratios characteristic of subduction environments is suggested this third source is probably the subcontinental lithospheric mantle finally no significant modification by interaction with continental crust is apparent in most of the analyzed samples

ophiolite obduction pulses as a proxy indicator of superplume events abstract a major new synthesis of ophiolite geochronology and map of global phanerozoic distribution indicates that previously noted pulses of ophiolite obduction can be linked to superplume related tectonism a marked cyclicity is evident in obduction ages obtained from minerals in ophiolite metamorphic soles and obduction related minor intrusions this episodicity is in phase with periods of predominantly uniform polarity of the geomagnetic field formation of massive carbon rich deposits sea level highstands and formation of large flood basalt provinces all generally considered to be superplume proxy indicators a key to interpreting ophiolite obduction as a further proxy is the mid cretaceous superplume associated ocean margin compressional deformation during this event thermal rejuvenation and increased buoyancy of ocean lithosphere caused arc terrane collision marginal basin shoaling back arc basin closure and ophiolite obduction convergent margins were placed in compression with increased coupling between subducting and overriding plates resulting in major ocean margin deformation being directly datable and tectonically and petrologically distinctive makes ophiolite assemblages of considerable use for identifying superplume events

geochemistry and sr nd pb isotopic composition of the central atlantic magmatic province camp in guyana and guinea abstract the central atlantic magmatic province camp is one of the largest igneous provinces on earth extending more than 5000 km north to south on both sides of the atlantic ocean its emplacement occurred about 200 ma ago at the triassic jurassic boundary and is linked to the initial breakup of pangaea two areas of the province are studied here french guyana surinam south america and guinea west africa in order to document the petrogenesis and geodynamical significance of high ti and low ti basaltic magmas from the camp in guyana doleritic and gabbroic dykes are located on the edge of the guiana shield and represent limited volumes of magma they display low sio2 47 50 high tio2 2 5 3 5 and high feo tholeiitic trends and show variably enriched trace element patterns la yb n 1 5 5 1 their isotopic signature and ratios of very incompatible elements endi 5 8 to 4 2 87sr 86sr i 0 703 0 705 207pb 204pb i 15 46 15 64 match a depleted prema prevalent mantle like source their genesis can be modeled by ca 15 partial melting of a lherzolite source and a subsequent limited fractional crystallization 5 10 or a slight upper crustal assimilation fractional crystallization afc r 0 1 proterozoic contaminant in guinea in contrast huge volumes of camp magmas were intruded along the rockelides suture and the west african craton forming the fouta djalon sills and the kakoulima laccolith the laccolith is more than 1000 m thick these features consist of gabbros dolerites diorites and mafic gabbro and ultramafic dunite wherlite cumulates guinean tholeiites show high sio2 51 58 low tio2 0 7 1 2 and feo trends with high lile hfse ratios and slight negative nb ta anomalies isotopic signatures endi 0 4 to 5 3 87sr 86sr i 0 705 0 710 207pb 204pb i 15 57 15 66 indicate a more enriched source than for guyana as well as a higher rate of magma upper crust interaction through an afc process r 0 3 birimian crust contaminant and probably an additional upper crustal contamination for the most differentiated sample this geochemical study supports the prevalence in guinea as for other low ti camp tholeiites of a lithospheric mantle source previously enriched during ancient subduction events and preferentially reactivated in late triassic times by edge driven convection between cratonic and mobile belt domains a larger contribution from a depleted asthenospheric source is required to generate high ti tholeiites in guyana which may reflect the development of camp rifting towards the initiation of the central atlantic oceanic crust

petrologic evidence for k feldspar metasomatism in granulite facies rocks abstract we present evidence for k feldspar metasomatism in charnockitic granulites from two well known terranes the shevaroy hills massif s india 750 c 8 kbars and the bamble sector s e norway 790 c 7 5 kbars in the form of k feldspar veins principally along plagioclase and quartz grain boundaries and in the form of highly variable antiperthitic patches of k feldspar in an uneven scattering of plagioclase grains with one exception orthopyroxene or amphibole grains in contact with these k feldspar veins show no alteration to secondary biotite indicating that the h 2 o activity of the fluids responsible for these veins must have been relatively low a high ba concentration in these veins also suggests a metasomatic origin point counted back scattered electron photomicrographs along with microprobe analyses provide reintegrated k feldspar and plagioclase compositions for 8 to 12 predetermined random areas per thin section for three samples from bamble and three samples from the shevaroy hills these reintegrated feldspar compositions plot over a range of temperatures on the feldspar ternary for each sample and indicate saturation temperatures above the mean temperature for either region with a few reintegrated compositions above the 1000 c isotherm and with the lower cut off temperatures at 700 c and 600 c for the shevaroy and bamble samples respectively these patterns suggest that exsolution alone could not have been responsible for the formation of the k feldspar veins and patches in these rocks we suggest that these veins are due to the influx of complex supercritical low h 2 o activity brines shortly after peak metamorphic conditions that this influx continued during the initial phases of post peak metamorphic uplift and that these fluids represent the first stage in a series of fluid influxes in which the h 2 o activity increased as uplift continued reaction of these brines with potassium undersaturated plagioclase grains formed k feldspar veins along grain boundaries and fractures as well as diffusing into the plagioclase which became supersaturated and exsolved k feldspar as antiperthite patches during uplift and cooling formation of secondary biotite halos around orthopyroxene in one sample associated with these k feldspar veins can be explained by heightened h 2 o activity in the brines due to enrichment in h 2 o emplacement of the veins at lower pressure during uplift or both compared to the other shevaroy and bamble samples

mesozoic and cenozoic exhumation history of the sw iberian variscides inferred from low temperature thermochronology abstract the post paleozoic tectonothermal evolution of the sw iberian variscides is poorly known mainly due to the scarce low temperature geochronological data available we have obtained new apatite fission tracks and apatite u th he ages to constrain the mesozoic and cenozoic tectonic evolution of this portion of the iberian massif located just north of the betic rif alpine orogen we have obtained nine apatite fission track ages on samples from variscan and pre variscan granitoids these ages range from 174 4 10 8 to 54 1 4 9 ma with mean track lengths between 10 3 and 13 9 m we have also performed 5 u th he datings on some of the same samples obtaining ages between 74 6 1 6 and 18 5 1 4 ma time temperature path modeling of these low temperature geochronological data leads us to envisage four post paleozoic tectonically controlled exhumation episodes in the sw iberian variscides three of these episodes occurred in mesozoic times middle triassic to early jurassic early cretaceous and late cretaceous at rates of 1 1 to 2 5 c ma 1 separated by periods with almost no cooling we relate these mesozoic cooling events to the formation of important marginal reliefs during the rifting and opening of the central and northern atlantic realm the fourth exhumation episode occurred in cenozoic times at rates of 3 2 to 3 6 c ma 1 being only recorded in samples next to faults with topographic escarpments these samples cooled below 80 c at 20 ma at rates of 3 13 c ma 1 due to roughly n s oriented compressional stresses affecting the whole iberian plate which in the particular case of sw iberia reactivated some of the previous late paleozoic thrusts

chronology of the central atlantic magmatic province implications for the central atlantic rifting processes and the triassic jurassic biotic crisis abstract the central atlantic magmatic province camp is among the largest igneous provinces on earth emplaced synchronously with or just prior to the triassic jurassic t j boundary ca 200 ma in great part due to the controversial connection between the occurrence of camp and the events of the t j boundary the demand for better constraints on the duration and eruptive chronology of this province has increased more than 100 new 40 ar 39 ar ages have been published in the last 15 years with more than half of these in the last 3 years a careful review and selection of available ages as well as the publication of 16 new ages from the carolinas newark basin usa french guyana and morocco are presented judicious selection yields a total of 58 accepted age determinations for camp magmatism ranging from 202 to 190 ma covering every part of the camp a more complete picture develops with intrusive camp magmatism commencing as early as 202 ma extrusive activity initiated abruptly 200 ma reaching peak volume and intensity around 199 ma on the african margin the main period of camp magmatism is confirmed as brief but is suggested to consist of at least two phases over 1 5 ma with magmatism commencing along the africa north american margins and slightly later along the south american margin two volumetrically minor but distinctive magmatic peaks centered at 195 and 192 ma are mirrored in data from all three continents and highlighted by our statistical approach models describing rifting and thermal input and magma production on these timescales are explored despite significant advances in our understanding of the chronology of camp more data of better quality and broader geographical coverage are needed to completely characterize the evolution of the camp and infer its geodynamic origin in addition lack of a well defined t j boundary age as well as the absence of a relevant basis for comparison between u pb and 40 ar 39 ar data for this time period remain limiting factors to unambiguously linking camp in time with the events of the t j boundary

assessing the record and causes of late triassic extinctions accelerated biotic turnover during the late triassic has led to the perception of an end triassic mass extinction event now regarded as one of the big five extinctions close examination of the fossil record reveals that many groups thought to be affected severely by this event such as ammonoids bivalves and conodonts instead were in decline throughout the late triassic and that other groups were relatively unaffected or subject to only regional effects explanations for the biotic turnover have included both gradualistic and catastrophic mechanisms regression during the rhaetian with consequent habitat loss is compatible with the disappearance of some marine faunal groups but may be regional not global in scale and cannot explain apparent synchronous decline in the terrestrial realm gradual widespread aridification of the pangaean supercontinent could explain a decline in terrestrial diversity during the late triassic although evidence for an impact precisely at the boundary is lacking the presence of impact structures with late triassic ages suggests the possibility of bolide impact induced environmental degradation prior to the end triassic widespread eruptions of flood basalts of the central atlantic magmatic province camp were synchronous with or slightly postdate the system boundary emissions of co2 and so2 during these eruptions were substantial but the contradictory evidence for the environmental effects of outgassing of these lavas remains to be resolved a substantial excursion in the marine carbon isotope record of both carbonate and organic matter suggests a significant disturbance of the global carbon cycle at the system boundary release of methane hydrates from seafloor sediments is a possible cause for this isotope excursion although the triggering mechanism and climatic effects of such a release remain uncertain d 2003 elsevier b v all rights reserved

quantifying extension of passive margins implications for sea level change 1 passive margins form by rifting and extension of continental crust extension of continental crust associated with subduction elsewhere decreases ocean basin volume forcing sea level to rise the amount that the continental crust has been extended can be quantified using total tectonic subsidence which is determined using sediment thickness and water depth data coupled with a model of the rifting process we calculate total tectonic subsidence the amount of extension or stretching factor and thickness of extended continental crust at a 5 min grid resolution for passive margins between 77 n and 70 s our modeled crustal thicknesses correlate well with seismic refraction data from north america and australia and can be used to estimate the change in ocean volume and the impact of extension on sea level the duration of extension is constrained by estimating the onset of rifting and the breakup age which is the time when rifting ceased and seafloor spreading began our results indicate that extension of continental crust has increased sea level by 20 8 m since the start of pangea rifting

the role of premagmatic rifting in shaping a volcanic continental margin an example from the eastern north american margin both magmatic and tectonic processes contribute to the formation of volcanic continental margins such margins are thought to undergo extension across a narrow zone of lithospheric thinning 100 k

the moho in extensional tectonic settings insights from thermo mechanical models the lithospheric memory is key for the interplay of lithospheric stresses and rheological structure of the extending lithosphere and for its later tectonic reactivation other important factors are the temporal and spatial migration of extension and the interplay of rifting and surface processes the mode of extension and the duration of the rifting phase required to lead to continental break up are to a large extent controlled by the interaction of the extending plate with slab dynamics the finite strength of the lithosphere has an important effect on the formation of extensional basins this applies both to the geometry of the basin shape as well as to the record of vertical motions during and after rifting we demonstrate a strong connection between the bulk rheological properties of europe s lithosphere and the evolution of some of europe s main rifts and back arc systems the thermo mechanical structure of the lithosphere has a major impact on continental break up and associated basin migration processes with direct relationships between rift duration and extension velocities thermal evolution and the role of mantle plumes compressional reactivation has important consequences for post rift inversion borderland uplift and denudation as illustrated by poly phase deformation of extensional back arc basins in the black sea and the pannonian basin region

the impact of the central atlantic magmatic province on climate and on the sr and os isotope evolution of seawater abstract there is a wide range of geochemical sedimentological and palaeontological evidence suggesting that the interval at the end of the triassic and the beginning of the jurassic some 200 ma ago was a time of major environmental change the supercontinent pangaea began to rift eustatic sea level rose and there was a mass extinction in marine and terrestrial biota the eruption of at least 3 10 6 km 3 of basaltic lavas that comprise the central atlantic magmatic province camp also took place at that time and was intimately associated with the onset of rifting of pangaea emplacement of this large igneous province occurred within a relatively very short period of perhaps no more than 1 ma although relatively little of camp now remains the original province was extremely large extending from present day nova scotia to bolivia and from central southern usa to morocco of major relevance to the environmental impact of the camp was its relatively rapid emplacement and its original location across the equator recent advances in our understanding of the weathering of silicate rocks and in particular of fresh basaltic rocks have highlighted the importance of the weathering process in influencing environmental change in this contribution we examine the details of the seawater sr and os isotope records that span the triassic jurassic boundary in order to understand the nature and course of environmental change at that time correlation of sr isotope data from mainland europe and the uk shows that the rapid rate of decrease in the 87 sr 86 sr ratio of seawater which had started in the early rhaetian late triassic came to an end in the late rhaetian close to the triassic jurassic boundary at that time the 187 os 188 os ratio of seawater had also decreased to particularly low values in the late rhaetian the seawater 87 sr 86 sr ratio rapidly increased from 0 70765 to 0 70775 and then it and the seawater 187 os 188 os ratio both remained approximately constant for the duration of the hettangian the first 3 ma of the jurassic apart from a transient excursion to radiogenic os isotope values i e high 187 os 188 os ratios near the triassic jurassic boundary the unradiogenic 187 os 188 os ratio of seawater during the hettangian is interpreted as reflecting a major perturbation to global weathering patterns that was caused by the rapid weathering and erosion of the camp immediately after its emplacement we suggest that the rapid increase of seawater 87 sr 86 sr ratios in the late rhaetian and the approximately constant 87 sr 86 sr ratios throughout the hettangian were caused by the input of a large amount of additional radiogenic sr to the oceans which was likely to have been derived from a readily weathered lithology such as carbonate or evaporite at the start of the sinemurian the 87 sr 86 sr ratio of seawater began to decrease while at the same time the 187 os 188 os ratio of seawater increased to more radiogenic values we interpret the trends in the seawater sr and os isotope compositions at the start of the sinemurian as evidence that much of the camp had been removed by chemical weathering by which time the camp had ceased to have a major influence on earth s environment the patterns of change in the seawater sr and os isotope records that occurred in the latest triassic and earliest jurassic show many similarities with those that took place in the pliensbachian toarcian 183 ma ago overlapping the eruption of the karoo ferrar igneous province in both cases the seawater sr and os isotope records indicate that there was a relatively very brief interval during which rates of continental weathering increased greatly the similarities in the responses of the seawater sr and os isotope records at those times strongly suggest that the eruption and subsequent erosion of basaltic large igneous provinces have played a major role in defining the course of environmental change in the past

co2transfer between the upper mantle and the atmosphere temporary storage in the lower continental crust the co2 atmospheric content has shown large variations over geological times high contents up to one order of magnitude more than present day values ultimately correspond to discrete episodes of mantle degassing either juvenile or subduction related carbon recycling a number of arguments e g the continuous volume increase of carbonate bearing sediments with time suggest that throughout the earth s history juvenile co2 has formed a major contribution to the global carbon budget of the earth the absence of a direct relationship between major volcanic episodes and the average co2 atmospheric content suggests that volcanoes might not be the only way by which mantle co2 is transported to the surface it is proposed that large quantities of juvenile co2 could temporarily be stored in the lower continental crust during major episodes of granulite formation these are primarily caused by magmatic underplating and they result in a vertical accretion of the crust by accumulation of co2 bearing mantle derived magmas most of the co2 migrates through the crust during post metamorphic evolution and isostatic restoration of the normal continental thickness however large quantities of co2 can still be present in some areas notably as high density fluids enclosed in minerals

anomalously low temperature orthopyroxene spinel and sapphirine occurrences in metasediments from the bamble amphibolite to granulite facies transition zone south norway possible evidence for localized action of saline fluids a reply the question has been raised whether isolated granulite facies islands which occur throughout the amphibolite facies of the regional amphibolite to granulite facies transition zone in the central part of the bamble sector southern norway are the result of prograde or retrograde metamorphism we have studied one of these islands the sapphirine granulites at hauglandsvatn despite the preponderance of amphibolite facies assemblages in the surrounding rocks and the small scale of distribution of the granulite facies assemblages the current assemblage is considered to be prograde and has developed at the expense of biotite in the regional main foliation leaving relict biotite separated from quartz by a continuous corona of orthopyroxene and k feldspar other high temperature low ah2o reactions resulted in the formation of spinel corundum albite oligoclase symplectites at the expense of biotite and sillimanite the symplectites have partially been replaced by symplectites of tiny blue sapphirine

high rate flexure of the east greenland volcanic margin constraints from 40ar 39ar dating of basaltic dykes abstract in the north atlantic domain the se greenland volcanic margin developed in response to continental break up at 57 54 ma progressive tilting and intrusion of dykes reflect a major tectonic seaward dipping flexing of the continental crust we report eight new ages of tilted pre flexure and vertical post flexure dykes determined by 40ar 39ar incremental heating experiments despite strong excess argon on plagioclase measured 40ar 39ar plateau and mini plateau ages of dykes from detailed step heating experiments on mineral separates appear reliable and a systematic difference of ages is found between tilted and vertical dykes in both studied areas kap wandel 66 20 n and kap gustav holm 66 40 n tilted dykes show ages of 54 55 ma whereas vertical dykes are younger with ages of 51 ma therefore it appears that the crustal flexing of the volcanic passive margin is a short duration event

from rifting to passive margin the examples of the red sea central atlantic and alpine tethys abstract evolution of the red sea gulf of suez and the central atlantic rift systems shows that an initial transtensive rifting phase affecting a broad area around the future zone of crustal separation was followed by a pre oceanic rifting phase during which extensional strain was concentrated on the axial rift zone this caused lateral graben systems to become inactive and they evolved into rift rim basins the transtensive phase of diffuse crustal extension is recognized in many intra continental rifts if controlling stress systems relax these rifts abort and develop into palaeorifts if controlling stress systems persist transtensive rift systems can enter the pre oceanic rifting stage during which the rift zone narrows and becomes asymmetric as a consequence of simple shear deformation at shallow crustal levels and pure shear deformation at lower crustal and mantle lithospheric levels preceding crustal separation extensional denudation of the lithospheric mantle is possible progressive lithospheric attenuation entails updoming of the asthenosphere and thermal doming of the rift shoulders their uplift provides a major clastic source for the rift basins and the lateral rift rim basins their stratigraphic record provides a sensitive tool for dating the rift shoulder uplift asymmetric rifting leads to the formation of asymmetric continental margins corresponding in a simple shear model to an upper plate and a conjugate lower plate margin as seen in the central atlantic passive margins of the united states and morocco this rifting model can be successfully applied to the analysis of the alpine tethys palaeo margins such as rif and the western alps

paleomagnetic study of french guyana early jurassic dolerites hypothesis of a multistage magmatic event a detailed paleomagnetic and anisotropy of magnetic susceptibility ams study was carried out on 34 sites of early jurassic dolerite dykes from french guyana which formed during the initial opening of the central atlantic ocean four types of ams fabrics are recognized i normal fabric 21 dykes defined by clustering of k1 k2 axes on the dyke plane whereas the k3 axis is nearly perpendicular to it this fabric is interpreted as due to magma flow the subhorizontal inclination of the k1 axis permitted to suggest that the french guyana dykes could be fed by horizontal magma fluxes from a distant magma source ii reversal fabric 8 dykes is characterized by the k2 k3 plane close to the dyke plane and the k1 perpendicular to dyke orientation such fabric was attributed to the local shearing stress iii intermediate fabric 1 dyke is defined by k1 k3 axes close to the dyke plane and k2 axis is perpendicular to this plane it was interpreted as due to vertical compaction of a static magma column iiii other fabric 4 dykes does not show any preferential orientation scanning electronic microscope and susceptibility versus temperature experiments show that minerals of the titanomagnetite family are main magnetic remanence carriers two magnetic components were isolated ages of magnetic remanences are estimated at 198 3 2 0 ma to 192 3 1 5 ma their virtual geomagnetic poles are calculated pole a v 73 2 n p 15 3 e k 288 8 a95 3 4 n 8 and pole b v 81 6 n p 89 1 e k 69 8 a95 4 2 n 18 these two groups probably correspond to two distinct magmatic events which occurred in a short period this hypothesis is consistent with published 40 ar 39 ar radiometric ages though with mini plateau spectra these paleomagnetic results suggest the presence of magmatic pulses led to the construction of the central atlantic magmatic province in french guyana during the early jurassic fl 2000 elsevier science b v all rights reserved

ne atlantic continental rifting and volcanic margin formation the ne atlantic rift system which has been under extensional deformation since the caledonian orogeny experienced a series of rift episodes in late paleozoic to early cenozoic times fig 1 late paleozoic rifting is poorly constrained particularly with respect to timing however rifted basin geometries inferred to be of this age are observed at depth in seismic data from the flanks of the younger rift structures

thermal evolution of the central atlantic passive margins continental break up above a mesozoic super plume a near synchronous early jurassic 200 ma magmatic event along the north american and west african passive continental margins can be explained by the upwelling of a large scale mantle plume super plume beneath the west african craton lateral deflection of the outflow from the plume to the ne by the ambient sub lithospheric mantle flow could account for tholeiitic basaltic magmatism extending over a distance of nearly 5000 km from brazil to southern spain continental break up is considered to occur above the stagnation streamline along which cooler material eroded from the base of the continental lithosphere by the plume is returned to the convecting mantle the dynamic model is a scaled version of that recently proposed for the hawaiian plume and satisfactorily explains the apparent lack of involvement of thermally anomalous mantle in the generation of central atlantic oceanic crust

rift related magmatism of the central atlantic magmatic province in algarve southern portugal abstract lower jurassic volcanic sedimentary successions of the algarve basin 198 ma are associated with the central atlantic magmatic province camp that was one aspect of pangean intracontinental rifting volcanic products of the algarve camp include subaerial lava flows pyroclastic deposits and peperites and contemporaneous sedimentation is dominated by mudstones and conglomerates often containing volcanic fragments intercalated in the volcanic sequence these lithological characteristics and associations are compatible with a facies model typical of continental basaltic magmatism volcanic rocks are fractionated low ti tholeiites enriched in large ion lithophile elements relative to high field strength elements the compositional spectrum e g ba nb zr nb is consistent with derivation of primary magmas by partial melting of heterogeneous sources initially continental lithospheric mantle progressively replaced by an asthenospheric source fractional crystallization processes occurred in magma chambers located at depths close to the crust mantle boundary from density and geobarometric constraints reflecting magma underplating during the initial stages of continental rifting sporadic occurrence of crustal xenoliths widespread evidence for basement carbonate assimilation as well as sr 87sr 86sr 0 0 7050 and oxygen 18o 7 40 isotope ratios that are higher than those typical of continental lithospheric mantle indicate that crustal contamination also contributed to the observed geochemical variations the carnian norian 228 0 to 203 6 ma or older age estimated for the basal silves sandstones suggests that the inception of rift related sedimentation preceded volcanism by 20 to 30ma these results lead us to propose a passive rifting model for the algarve sector of camp magmatism the orientation of which was conditioned by strain localization along pre existing lithospheric mechanical anisotropies

titaniferous magnetite ilmenite thermometry and titaniferous magnetite ilmenite orthopyroxene quartz oxygen barometry in granulite facies gneisses bamble sector se norway implications for the role of high grade co 2 rich fluids during granulite genesis oxygen fugacities have been estimated for a wide distribution of samples from the granulite facies terrane region c of the bamble sector se norway using both the titaniferous magnetite ilmenite and orthopyroxene titaniferous magnetite quartz oxygen barometers these oxygen fugacities are estimated using temperatures calculated from the titaniferous magnetite ilmenite thermometer of ghiorso and sack 1991 and are both internally consistent with each other as well with the thermometer in samples for which the estimated temperature is high the two oxygen barometers show good agreement whereas agreement is poor for low temperature samples in these low temperature samples oxygen fugacities estimated from titaniferous magnetite ilmenite are considerably less than those estimated from orthopyroxene titaniferous magnetite quartz an increase in this discrepancy with decrease in temperature appears to reflect preferential resetting of the hematite component in the ilmenite grains without significant alteration of the more numerous titaniferous magnetite grains this is due in part to greater re equilibration of the ilmenite grains during retrograde interoxide resetting between the ilmenite grains and the titaniferous magnetite grains the mean temperature for the non reset samples 791 17 c 1 is in good agreement with temperatures obtained from garnet orthopyroxene kd exchange thermometry in the same region 785 795 c 1 harlov 1992 2000a most non reset oxygen fugacities range from log10 f o2 14 to 11 8 or approximately 0 5 1 5 log units above quartz fayalite magnetite at 7 5 kbar both these temperatures and the range of oxygen fugacities are in good agreement with those estimated using the titaniferous magnetite ilmenite thermometer oxygen barometer of andersen et al 1991 the quilp equilibrium quartz ulvospinel ilmenite pyroxene is used to project self consistent equilibrium temperatures and oxygen fugacities for samples reset due to hematite loss from the ilmenite grains these projected temperatures and oxygen fugacities agree reasonably well with the non reset samples the mean projected quilp temperature is 823 6 c 1 this result supports the conclusion that low titaniferous magnetite ilmenite temperatures down to 489 c and accompanying low oxygen fugacities are the result of hematite loss from the ilmenite grains non reset oxygen fugacities lie approximately 1 5 log10 units above the upper graphite stability curve indicating that the stable c o h fluid phase interacting with these gneisses whether regionally or locally was co2 this is borne out by the presence of numerous co2 rich fluid inclusions in these rocks

the anari and tapirapua jurassic formations western brazil paleomagnetism geochemistry and geochronology the anari and tapirapua formations are very similar from the point of view of paleomagnetic geochronological and geochemical results they date from around 197 ma and the flows are mainly tholeiitic basalts with a low tio2 and incompatible element content the magnetic carriers in rocks from these two formations were highly oxidized titanomagnetites maghemites and probably titanomaghemites paleomagnetic analysis has shown that the magnetizations are all normal in polarity and virtual geomagnetic poles obtained for both formations are indistinguishable at the 95 confidence level the calculated mean for these poles is 250 3 e 65 5 s n 15 a95 3 6 ksc 1578 this pole is compatible with jurassic poles which have been determined for south america or transposed from africa

the cretaceous peri atlantic alkaline pulse paap deep mantle plume origin or shallow lithospheric break up abstract although the early jurassic initial phases of atlantic rifting were marked by the tholeiitic basalt flows and dikes of the central atlantic magmatic province camp the cretaceous era was characterised by widespread alkaline igneous activity on widely separated peri atlantic continental margins nearly half the peri atlantic alkaline rocks dated between 250 to 50 ma fall within the 125 80 ma range this period of intense alkaline activity during the mid cretaceous formed the peri atlantic alkaline pulse paap the mechanisms for the opening of the atlantic ocean as well as for the camp and later paap magmatic events have been attributed either to deep mantle plumes or to a combination of tensional forces lithospheric rifting and structural controls we describe the paap event for the first time and test the plume and structural models the paap displays evidence of strong structural control and appears to be related to reactivation of pre existing lithospheric structures throughout the peri atlantic zone early stages of continental separation and abrupt changes in plate motion were the most likely mechanisms for the reactivation of favourably oriented pre existing structures mid cretaceous pulses in alkaline magmatic activity were closely associated with major atlantic tectonic events the opening of the south atlantic the change in intensity and direction of both seafloor spreading and plate motion and the consequent fluctuation in intra plate stresses were responsible for two spikes in peri atlantic alkaline magmatism at 125 ma and 85 80 ma the general lack of evidence for a deep hotspot associated with the peri atlantic zone seems incompatible with a mantle plume origin on the other hand the spatial distribution of the paap along passive continental margins suggests an ideal setting for the development of edge driven convection we propose that the periodic reactivation of deep seated pre existing zones of weakness during the major stages of atlantic tectonic evolution combined with coeval asthenospheric upwelling due to edge driven convection and continental insulation flow enhanced the ascent of alkaline magmas this mechanism of shallow small scale upwelling during periodic structural reactivation provides a more likely explanation for the paap than the commonly proposed deep mantle plume scenario

40ar 39ar dating and geochemistry of tholeiitic magmatism related to the early opening of the central atlantic rift tholeiitic effusive and intrusive magmatism from iberia morocco algeria and mali related to the early opening of the central atlantic rift was dated by the 40ar 39ar step heating method four plateau ages ranging from 203 7 2 7 to 197 1 1 8 ma were obtained on plagioclase from dykes from the taoudenni area mali and two lava flows from morocco the messejana dyke iberia which previously yielded discrepant conventional kar dates did not furnish any 40ar 39ar plateau dates however there is a clear inverse relationship between apparent age and k ca ratio for gas fractions from a plagioclase separate proportional to the alteration degree which combined with dates obtained on amphibole biotite and pyroxene allows us to determine an age of around 200 ma for this body these data and those obtained on the foum zguid morocco and the ksi ksou algeria dykes give evidence of a brief magmatic event between 206 and 195 ma ago which affected a large area ca 2500 km long trace element modelling shows that most of these formations originated from a homogeneous enriched source material such a brief magmatic episode related to the opening of a continental rift is in agreement with findings in other magmatic provinces e g the deccan traps and the red sea rift precisely dated by the 40ar 39ar method as well

revealing the temporal dynamics of subsurface temperature in a wetland using time lapse geophysics summary there is growing recognition of the need to understand the subsurface hydrological mechanisms within wetlands given their potentially significant role in the hydrological and biogeochemical function of catchments as well as their effects on the wetland habitat itself conventional subsurface hydrological sampling and monitoring techniques are often limited in such environments because of the invasive nature of such approaches and the sensitivity of the environment in this study we use ground penetrating radar gpr and electrical resistivity tomography ert to characterise the stratigraphy at a small riparian wetland site then through time lapse ert measurements over a 12 month period we demonstrate how changes in resistivity may provide additional value about localised recharge we compare direct measurements of water table depth and pore fluid electrical conductivity and temperature with subsurface resistivity derived from the ert measurements and reveal that temporal fluctuations in temperature of the subsurface dominates the change in resistivity over the monitored period from this we use the ert images of resistivity to estimate spatio temporal changes in subsurface temperature and thus infer localised zones of groundwater recharge due to suppressed seasonal variation in temperature although we focus here on one 2 d vertical profile through the wetland the results highlight the potential value of non invasive time lapse geoelectrical surveys for mapping 3 d thermal patterns within a wetland environment

petrology and geothermobarometry of granulite facies metapelites from the hisoy torungen area south norway new data on the sveconorvegian p t t path of the bamble sector the high grade rocks metapelite quartzite metagabbro of the hisoy torungen area represent the south westernmost exposures of granulites in the proterozoic bamble sector south norway the area is isoclinally folded and a metamorphic p t t path through four successive stages m1 m4 is recognized petrological evidence for a prograde metamorphic event m1 is obtained from relict staurolite chlorite albite staurolite hercynite ilmenite cordierite sillimanite fine grained felsic material quartz and hercynite biotite sillimanite within metapelitic garnet the phase relations are consistent with a pressure of 3 6 0 5 kbar and temperatures up to 750 850 c m1 is connected to the thermal effect of the gabbroic intrusions prior to the main m2 sveconorwegian granulite facies metamorphism the main m2 granulite facies mineral assemblages quartz plagioclase k feldspar garnet biotite sillimanite are best preserved in the several metre wide al rich metapelites which represent conditions of 5 9 9 1 kbar and 790 884 c these p t conditions are consistent with a temperature increase of 80 100 c relative to the adjacent amphibolite facies terranes no accompanying pressure variations are recorded up to 1 mm wide fine grained felsic veinlets appear in several units and represent remnants of a former melt formed by the reaction bt sil qtzgrt lq this dehydration reaction together with the absence of large scale migmatites in the area suggests a very reduced water activity in the rocks and xh2o 0 25 in the c o h fluid system was calculated for a metapelitic unit a low but variable water activity can best explain the presence or absence of fine grained felsic material representing a former melt in the different granulitic metapelites the strongly peraluminous composition of the felsic veinlets is due to the reaction grt former melt silcrd bt qtz h2o which has given poorly crystalline cordierite aggregates intergrown with well crystalline biotite the cordierite and biotite producing reaction constrains a steep first stage retrograde relative to m2 uplift path decimetre to metre wide strongly banded metapelites quartz plagioclase biotite garnet sillimanite inter layered with quartzites are retrograded to m3 amphibolite facies assemblages a p t estimate of 1 7 5 6 kbar 516 581 c is obtained from geothermobarometry based on rim rim analyses of garnet biotite plagioclase sillimanite quartz assemblages and can be related to the isoclinal folding of the rocks m4 greenschist facies conditions are most extensively developed in millimetre wide chlorite rich calcite bearing veins cutting the foliation

high grade k feldspar veining in granulites from the ivrea verbano zone northern italy fluid flow in the lower crust and implications for granulite facies genesis we present observations of extensive networks of k feldspar microveins associated with quartz and plagioclase in granulite facies rocks from the ivrea verbano zone northern italy back scattered electron images of the granulite facies samples show a continuous system of k feldspar veins along quartz plagioclase and plagioclase plagioclase grain boundaries that can be traced across the entire length of a thin section quartz grain rims in contact with these veins show evidence of extensive corrosion in contrast contact between the veins and the orthopyroxene and garnet grains is very clean with no signs of secondary alteration k feldspar quartz plagioclase of the same compositions as the veins are commonly found as near rim inclusions within garnet microprobe traverses from the interiors of the k feldspar veins to their rims show increasing ba indicating a metasomatic origin some veins display a boudinaged texture possibly related to the d2 deformation event in the ivrea verbano zone and the

geochemistry and geochronology of early mesozoic tholeiites from central morocco from central morocco central high atlas middle atlas haute moulouya continental tholeiites were investigated geochemically and geochronologically these tholeiites are intercalated within continental redbeds of the early mesozoic triassic liassic the major trace and rare earth element contents classify these volcanic rocks as basaltic to andesitic basaltic quartz normative tholeiites some trace element ratios e g zr nb zr y y nb ti v suffer a heterogeneous source with a composition similar to morb p to n type the enriched lile contents the negative nb anomaly and the inital87sr 86sr ratios 0 7064 0 7069 reveal the presence of a crustal component up to 13 17 wt 87sr 86sr ratios of carbonate mineral separates from different lava flows show different cycles of alteration however the major and trace element chemistry together with sr isotope evidence indicate that the alteration phases are not submarine in origin 40ar 39ar age determinations on translucent plagioclase phenocrysts yield extrusion ages which range between 210 4 2 1 ma and 196 3 1 2 ma these ages correspond to a stratigraphic period between the norian rhaetian and the upper sinemurian

p t conditions for the arendal granulites southern norway implications for the roles of p t and co2in deep crustal lile depletion the orthopyroxene clinopyroxene garnet orthopyroxene and garnet clinopyroxene geothermometers and the garnet orthopyroxene plagioclase garnet clinopyroxene plagioclase and anorthite ferrosilite grossular almandine quartz geobarometers are applied to metabasites and the garnetplagioclase sillimanite quartz geobarometer is applied to a metapelite from the proterozoic arendal granulite terrain bamble sector norway p t conditions of metamorphism were 7 3 0 5 kbar and 800 60 c this terrain shows a regional gradation from the amphibolite facies into normal lile content granulite facies rocks and finally strongly lile deficient granulite facies gneisses neither p nor t vary significantly across the entire transition zone the change in grade parallels the increasing dominance of co2 over h2o in the fluid phase lile depletion is not a pre condition of granulite facies metamorphism granulites may have either depleted or normal chemistries the results presented herein show that lile deficiency in granulite facies orthogneisses is not necessarily related to variations in either p or t the important mechanisms in the arendal terrain were a direct synmetamorphic crystallization from magma with primary lile poor mineralogies imposed by the prevailing fluid regime and b metamorphic depletion involving scavenging of liles during flushing by mantle derived co2 rich fluids the latter process is constrained by u pb and rb sr isotopic work to have occurred no later than 50 ma after intrusion of the acid intermediate gneisses and was probably associated with contemporary basic magmatism in a tectonic environment similar to a present day cordilleran continental margin

halogen geochemistry of fluid during amphibolite granulite metamorphism as indicated by apatite and hydrous silicates in basic rocks from the bamble sector south norway abstract the halogen chemistry of apatites biotites amphiboles and titanites from metabasites in the proterozoic bamble sector amphibolite granulite facies transition zone norway has been studied to gather data on the behaviour of halogens during original magmatism and subsequent metamorphism three groups of apatites could be recognized 1 f oh apatites with more than 50 mole f and virtually no cl 2 f oh apatites with up to 15 mole of cl 3 cl rich apatites with up to 6 73 wt cl the latter are the most cl rich metamorphic apatites ever reported the f contents of coexisting biotite do not follow this division but their cl contents do log z sfnc hf z sfnc h 2 o values for the coexisting fluid phase calculated from biotite are slightly lower than those calculated from apatite log z sfnc hcl z sfnc h 2 o values in the coexisting fluid phase calculated from biotite are substantially higher than those calculated from apatite the log z sfnc hf z sfnc h 2 o reflects the composition of the peak metamorphic fluid phase the apatites show three types of patterns of f contents 1 w shaped 2 rimward increasing and 3 rimward decreasing pattern 2 reflects progressive metamorphism pattern 3 is thought to reflect growth starting near peak metamorphism and going on during retrograde dilution of the peak metamorphic fluid by a newly introduced aqueous one some apatites show local resorption effects along cracks no regional trend in calculated log z sfnc hf z sfnc h 2 o values is present as different from other granulite terrains the bamble biotites and amphiboles from granulite facies samples do not have higher halogen contents as those from the amphibolite facies area the samples show that halogen bearing fluids were controlled at a very restricted scale during peak as well as retrograde metamorphism

oblique terrane assembly in the late paleoproterozoic during the labradorian gothian orogeny in southern scandinavia the earliest deformation of the curved kongsberg bamble belt in norway produced a northwestward directed tectonic wedge suggesting oblique nw se collision during the labradorian gothian orogeny in southern scandinavia oblique terrane assembly with the outboard western units colliding last explains the trapping of the ostfold marstrand island arc formed at c 1760 ma between younger 1700 1600 ma terranes the apparent lack of svecofennian crust west of the trans scandinavian igneous belt and the apparent lack of labradorian gothian age crust in the british isles the labradorian of north america may have also resulted from oblique terrane assembly

high density carbonic fluid inclusions in charnockites from eastern ghats india petrologic implications abstract charnockite rocks a significant litho unit in eastern ghats granulite belt eggb are characterized by the presence of dry mineral assemblage the anhydrous conditions required for the formation of ganulites are believed to be controlled by the lowering of water activity through the influx of co2 bearing fluids here we present the results of systematic petrographic and microthermometric studies characterizing the nature composition and density of co2 rich fluid inclusions in charnockitic rocks from gangarajumadugula in this study we have combined mineralogic thermobarometry with microthermometric data of the carbonic fluid inclusion population of high density in the charnockite samples from gangarajumadugula area to substantiate desiccation of lower crustal environment producing anhydrous conditions in the eastern ghats granulite belt close to peak pressure temperature calculations for core composition using convergence technique in present study indicate that these charnockites have experienced maximum p t around 860 c at 9 kbar which is broadly consistent with independent p t estimates inferred for gangarajumadugula area the reaction textures preserved by these charnockites and the thermobarometric data characterize decompression followed by cooling history high density carbonic fluid inclusions 1 08 g cm3 in quartz provide potential evidence for the involvement of co2 rich fluid during granulite facies metamorphism in eggb occurrence of co2 rich inclusions and elevated p t conditions open up the possibility favoring the mechanism of vapour deficient dehydration melting accompanied by co2 infiltration to account for charnockite formation from precursor crustal rocks our study documents that fluid inclusions in high grade metamorphic rocks have enormous potential in retaining worthwhile information of peak metamorphic conditions and understanding the metamorphic processes

the redox state of pinatubo dacite and the ilmenite hematite solvus the oxidation state of the 1991 pinatubo dacite and many other oxidized silicic arc volcanic rocks has been overestimated we reproduce the compositions of magnetite and ferrian ilmenite found in pinatubo dacite by doing crystallization experiments at 780 degrees c 2 2 kbar and nno 1 7 or 0 2 log units the extra components in igneous ilmenite notably geikielite expand the solvus in a way that is not correctly accommodated in current formulations of the iron titanium oxide thermobarometer

magmatic rifting of pangaea linked to onset of south american plate motion abstract the causes of the transition from amagmatic to magmatic rifting during continental break up are not always clear and have been often linked to the break through of a plume however stress fields recorded in the crust may offer new insights into the relationship between changes in the stress field and the onset of magmatism in this paper stress fields recorded in the crust of north america are used to test possible causes of the break up of pangaea and the transition from amagmatic to magmatic rifting finite element models reveal that the most likely scenario for the break up involves the initial northwest motion of north america at 230 ma followed by a south southeast motion of south america at 200 ma with the initiation of magmatism and finally a weakened area between north and south america sometime soon after 200 ma which resulted in north america being dominated by northwest motion once again it was also determined that plate boundary structure and orientation play a large part in the recorded stress fields and must be taken into consideration when modeling continental break ups and rifting

crustal structure of the outer voring plateau offshore norway from ocean bottom seismic and gravity data four ocean bottom seismograph obs profiles acquired across the continent ocean transition zone of the outer voring plateau have been modeled by inversion and forward modeling of the obs vertical component traces p waves the thickness of the sedimentary layers deposited since the early eocene continental breakup varies from up to 2 2 km above the basaltic inner flows if to a minimum of 0 2 km about 20 km seaward of the voring escarpment ve with a velocity of 1 6 2 55 km s the thickness of the uppermost layer of basalt varies from 0 5 to 4 5 km and its velocity ranges from 3 4 to 5 km s the modeling of the two dip profiles suggests that strongly intruded preopening sedimentary rocks extend 40 km seaward of the ve farther seaward another layer of extrusives intrusives is found beneath the uppermost layer of basalt the lateral variations in velocity in the main crustal layer along the dip profiles indicate that the continent ocean transition extends over a 30 50 km wide zone in between strike profile 3 oceanic 6 6 7 35 km s and strike profile 4 continental 6 1 7 0 km s an up to 7 km thick lower crustal high velocity body 7 1 7 4 km s is interpreted in terms of intrusions in the lower crust beneath and landward of the continent ocean transition the total thickness of the normal oceanic crust is estimated to be 7 9 km the anomalously thick oceanic crust reaches a maximum thickness of about 25 km and the crustal thickness is estimated to be 17 22 km beneath the if gravity modeling of the profiles confirmed all main aspects of the velocity models and provided constraints on the crustal structures toward the ends of the profiles where the ray coverage was low

comparative oxygen barometry in granulites bamble sector se norway oxygen fugacities have been estimated for the high grade portion of the bamble granulite facies terrane se norway using both titaniferous magnetite ilmenite and orthopyroxene titaniferous magnetite quartz oxygen barometers the two oxygen barometers show good agreement for samples indicating high titaniferous magnetite ilmenite temperatures whereas agreement is poor for low temperature samples oxygen fugacities estimated from titaniferous magnetite ilmenite are considerably lower than those estimated from orthopyroxene titaniferous magnetite quartz this discrepancy increases with a decrease in temperature which appears to reflect preferential resetting of the hematite content in the ilmenite grains without much alteration of the more numerous titaniferous magnetite or orthopyroxene grains the mean temperature for the non reset samples 795 60 c 1 agrees well with temperatures obtained from garnet orthopyroxene k d exchange thermometry in the same region 785 60 c 1 most non reset o

mobilization of gold in the deep crust evidence from mafic intrusions in the bamble belt norway abstract the location of mesothermal gold deposits in the upper part of major transcurrent shear zones suggests that gold was derived from the deep wider portions of the shears the 30 km wide bamble belt south norway is a deep shear belt metamorphosed to upper amphibolite and granulite grade at about 1 1 ga preliminary studies showed that its rocks are strongly depleted in gold and other chalcophile elements mafic rocks were chosen for more detailed study of the mobilization of gold during metamorphism gold in mafic rocks largely partitions into the sulphide phase which is mainly comprised of pyrrhotite modification of this phase was required to liberate gold bamble has two main groups of mafic rock the first metabasites intruded before peak metamorphism are uniformly low in gold their o2 at peak metamorphism obtained from ilmenite orthopyroxene equilibria indicates a strongly oxidized mineral assemblage well above fmq pyrrhotite was stable at peak temperature of 800 c but on cooling below 600 c internal mineralogical buffering by the oxidized assemblage caused the o2 to cross into the stability field of pyrite following conversion of pyrrhotite to pyrite the latter was partly replaced by magnetite these serial changes in the mineralogical form of iron sulphide was conducive to the extraction of gold as was the oxidized nature of the mineral assemblage that would have buffered metamorphic fluids to high o2 the second group of mafic rocks hyperites intruded after peak metamorphism record intermediate steps in the extraction of gold these intrusions of coronitic gabbro were metamorphosed in whole or in part to amphibolite there is a decrease in gold and other chalcophile elements across a hyperite dyke from the pyrrhotite bearing coronitic gabbro interior to the pyrite bearing amphibolite margin

structural stratigraphic and sedimentological characterisation of a wide rift system the triassic rift system of the central atlantic domain abstract the extension driven break up of the pangean supercontinent in the present day central and north atlantic domains commenced during permo triassic times permian rift processes in this region are generally attributed to crustal collapse during the latter part of the hercynian variscan orogeny but the triassic rifting style is more complex and the presence of a single over riding first order control is equivocal basement terrane structures and boundaries provide pre existing zones of weakness that had significant influences on the location timing orientation and extent of triassic rifting the 201 ma central atlantic magmatic province camp a large igneous province lip increased lithospheric heat flow at the end of rifting produced uplift and doming and also contributed to deformation and subsequent continental breakup analysis of the triassic basin structures in the central atlantic and north atlantic domains respectively georgia to nova scotia and conjugate margin and newfoundland iberia shows that four main basin types are geographically distributed 1 southern basins georgia to virginia small to medium size with limited growth structures 2 basins in the central east domain new jersey nova scotia wide basins with growth structures along long lived inherited bounding faults 3 basins in the central west domain offshore ns and morocco that comprise numerous half graben sub basins in a lower unit and an upper very wide tabular unit and 4 northern basins newfoundland portugal that are tabular and extremely wide basin architectures suggest that subsidence was regional in extent and not necessarily controlled by main faults variations in crustal characteristics are considered to have led to the development of the four main basin types stratigraphic data show that the onset of deposition in the central and southern north atlantic domain was restricted to a central segment between two main structural lineaments overall crustal rheology and inheritance exhibited first order controls on subsidence and basin development triassic basins in the central and north atlantic domain were all filled by continental deposits in various palaeo environments such as alluvial fan mega fluvial systems terminal fluvial systems fan delta fluvial systems permanent oxic and anoxic lakes playa lakes and saline lakes some of them interbedded with aeolian and volcanic deposits most basins were far from any marine domain but marine ingressions from the tethys domain are inferred from dolomite and marine fauna in portugal and from thick salt deposits in morocco and offshore canada during the late rhaetian a series of palaeogeographic maps are presented utilising plate reconstructions based on sahabi et al 2004 in order to summarize continental scale palaeo environment distribution our tectono stratigraphic model suggests that initial strain was accommodated in a central segment by inherited faults during the early and middle triassic during the carnian the rift domain was increasingly extended with the development of half graben basins in the central segment and some in the northern segment in both segments the initial strain re activated inherited faults and later the rift zones evolved towards significantly wider basins marked by the presence of a major erosional truncation surface across the basins and followed by lacustrine sedimentation that eventually overlapped paleo topographic highs subsidence of such regional extent is likely driven by thermal processes following a phase of extreme lithospheric thinning and lower crustal removal associated with mantle uplift the architecture of the triassic rifting has the characteristics of the wide rift mode very large basins with some active faults for which subsidence is driven by lower crustal flow within a high heat flow region nevertheless some faults were active during late triassic sedimentation but accounted for a relatively minor part of the overall subsidence pulses of active subsidence and topographic rejuvenation led to fluvial dominated deposition within basins non rejuvenation of hinterland led to more lacustrine dominated sedimentation and eventual connection of basins across topographic highs due to regional thermal subsidence during the latest rhaetian basins with the lowest elevations were possibly flooded by marine waters resulting in precipitation of thick salt deposits

radiating 1 0 ga mafic dyke swarms of eastern brazil and western africa evidence of post assembly extension in the rodinia supercontinent abstract several mafic dyke swarms of similar composition and age tholeiite ca 1 0 ga occur on both sides of the atlantic ocean in eastern south america and western africa when assembled to their pre drift position in the mesozoic the brazilian coastal dyke province of bahia and the african dykes in cameroun ebolowa suite and congo comba and sembe ouesso provinces define a giant radiating pattern 1200 km 800 km similar to other dyke swarms elsewhere associated with large scale continental rifting magma flow indicators of the brazilian dykes and branching propagation styles of their african counterparts indicate that the dyke conduits were fed with magmas diverging from a source beneath the long axis of the meso neoproterozoic west congolian basin in africa there morb like metabasalts have been described in the la bikossi group of the mayombian supergroup whether the rifting event and intrusion of dyke swarms were triggered or not by a mantle plume beneath part of the rodinia subcontinental lithosphere remain to be confirmed

metamorphic fluids in the deep crust most of the inferences about fluid action in the deep crust are drawn from field petrographic and geochemical studies in the granulite facies terrains deductions on the nature of fluids are made from major and minor element distributions and isotopic patterns in metamorphic rocks calculations of fluid mineral equilibria based on observed assemblages fluid inclusions and field evidence of fluid pathways discussion of these features has focused attention on several major problems concerning the timing flow mechanisms and origin of the fluids a key issue concerns episodicity versus secular activity of fluids there are strong arguments that most fluid action in the deep crust is linked to thermal deformational events including magmatism and regional meta morphism discrete radiometric ages of high grade terrains often define relatively short periods of crystallization or recrystallization the late archean granulite facies metamorphism of southern india is an example where u pb systems of zircons buhl et al 1983 whole rock u pb data peucat et al 1 987 and sm nd systems of whole rocks and minerals bernard griffiths et al 1 987 all show a major high temperature recrys tallization event inferred by several workers to have involved pervasive fluid action centered closely around 2 5 ga another geochemical indi cation of episodic fluids comes from stable isotope studies of progressive metamorphic sequences as in the damara orogen namibia where whole rock b 180 increases slightly at isograds representing arrested dehydration events but is monotonous between isograds hoernes hoffer 1 985

imaging a mantle plume beneath the french massif central abstract teleseismic tomography is now widely recognised as a very powerful tool for studying the velocity structure of the lithosphere asthenosphere system on both local and regional scales a prime objective is to identify velocity perturbations and to constrain their location and intensity here we use the arrival times of p waves from teleseismic events recorded by a dense network of mobile and permanent stations within the french massif central to derive a 3d p velocity model for an area of 425 km 300 km combined with knowledge of the basement geology of the area and the timing and geochemical characteristics of tertiary quaternary volcanic activity we consider that the velocity model provides unequivocal evidence for the ascent of a thermally and chemically anomalous mantle plume beneath the massif central during tertiary times our model is consistent with regional bouger gravity anomalies documented patterns of neogene recent uplift and deformation fabrics within lithosphere derived mantle xenoliths exhumed by the volcanics

a comparison of eastern north america and coastal new england magma suites implications for subcontinental mantle evolution and the broad terrane hypothesis new england and maritime canada host two major suites of mesozoic diabase dykes the oldest is the coastal new england dykes that were emplaced between 225 and 230 ma these rocks are dominantly alkaline with trace element and isotopic compositions indicative of a high 238u 204pb mantle himu source the oldest of the 200 ma mesozoic rift magmas is represented by the talcott basalt of the hartford basin and its feeder dykes external to the basin is the compositionally equivalent higganum dyke the extension of the higganum the onway dyke in new hampshire is identical in major and trace element and isotopic compositions indicating that the dyke system represented a feeder to flows of flood basalt proportions the talcott system rocks have some trace element similarities with arc basalts and have been interpreted as representing melts of a subduction zone modified mantle beneath the laurentian gondwanan suture incompatible trace element ratios and ba th and u values are however unlike arc basalts

development of volcanic passive margins two dimensional laboratory models 1 continental breakup above an anomalously hot mantle may lead to the development of volcanic margins volcanic margins are characterized by 1 thick seaward dipping lava flow sequences 2 central intrusive complexes associated with dyke swarms parallel to the coast and 3 high seismic velocity bodies in the lower crust attributable to magma underplating a conceptual model for volcanic margins development has recently been proposed based on onshore studies of the greenland margins and the british tertiary igneous province it is proposed that the long lived central intrusions are genetically linked to underlying persistent zones of mantle fusion these localized melting domains or soft spots equivalent to small mantle diapirs may locally soften the extending continental lithosphere the low viscosity diapirs would 1 localize tectonic strain and 2 feed the volcanic margin with magma thus such soft spots can control the along strike magmatic and tectonic segmentation of volcanic margins recent geophysical investigations appear to show that the along strike structure of volcanic passive margins is compatible with such a segmentation process here we present a set of scaled experiments designed to study how such localized rheological heterogeneities in the sub moho mantle may have a mechanical effect on continental breakup four layer models were constructed using sand and silicone putties to represent the brittle and ductile layers of both crust and mantle the soft spots are simulated by low viscosity silicone putty emplaced within the brittle material at the scale of the entire breakup zone the soft spots display an oceanic type strength profile defining low strength zones where continental breakup is initiated the rift orientation and segmentation are strongly controlled by the distribution of the low viscosity heterogeneities rather than by the direction of regional extension the experiments are compared with the geometry and segmentation of the onshore part of the greenland volcanic margins

broad terrane jurassic flood basalts across northeastern north america early jurassic tholeiitic lavas that flowed across mesozoic basins of northeastern north america were comagmatic as shown by their geochemistry petrography paleomagnetism radiometric ages and stratigraphy the basin basalts are correlated with diabase dikes that intersect the basins but also extend far outside them if the dikes represent feeders to fissure vents a vast flood basalt province must have extended across the rifted terranes of eastern north america in early jurassic time the initial member of the basalt flood was the most uniform and widespread among present basin remnants of the basalts but subsequent flows may have covered some areas farther from the basins other younger jurassic flood basalts have been described along submarine sections of the continental shelf and shelf break and beneath cretaceous coastal plain sediments in the southeastern united states across the inland mesozoic rift terranes the initial fissure eruptions produced a temporary basaltic plain perhaps 1600 km long and 200 to 400 km wide or roughly 500000 km 2 rivaling other great continental flood basalt provinces

the ilmenite titano magnetite assemblage kinetics of re equilibration abstract compositions of coexisting ilmenite and titano magnetite commonly have been used to establish the conditions of formation for igneous and metamorphic rocks buddington lindsley geothermometer oxybarometer temperatures so determined for some rocks are lower than might be expected for magmatic crystallization these low temperatures are the result of re equilibration of the fe ti oxides during slow cooling this process is one in which titano magnetites are oxidized and ilmenites are reduced upon cooling along a buffer curve or in the presence of a fluid of constant composition textural studies indicate that the reduction of the ilmenite hematite phase may be the slower reaction consequently the rate controlling step of the re equilibration the kinetics of the reduction reactions in the two phase region between the joins for the ilmenite hematite and magnetite ulvospinel solid solution series were studied by thermogravimetric analysis at controlledfo2 with experiments conducted near the ni nio and qfm buffers annealed ilmenite ss synthetic charges were reduced isothermally the temperature range for the experiments was 900 1250 c with total gas flow rates of 0 35 and 1 0 cm s kinetic data were collected by measuring the weight change during each experiment as a function of time additionally a cooling rate 0 5 c hr experiment was conducted near the qfm buffer from 1150 to 985 c starting with an ilmenite saturated spinel phase at 1200 c and 0 35 cm s flow rate the reaction mechanism is an interfacial process where mass transport of the reacting gases is the rate controlling step in isothermal experiments at 1100 c and lower diffusion of reactants through a solid product layer was the controlling mechanism the activation energy for this diffusion process is 59 6 kcal mole significant enhancement of diffusion occurs along grain boundaries although the individual phases may re equilibrate internally upon cooling the fe ti contents must remain unchanged for proper use of this mineral indicator i e no external re equilibration it is imperative that the kinetics of the situation be appreciated these preliminary kinetic experiments indicate that extreme caution must be exercised when applying this ilmenite titano magnetite geothermometer oxybarometer these phases may only truly quench in the parameters of formation in the case of certain hypabyssal and extrusive occurrences

decrease in oceanic crustal thickness since the breakup of pangaea earth s mantle has cooled by 6 11 c every 100 million years since the archaean 2 5 billion years ago in more recent times the surface heat loss that led to this temperature drop may have been enhanced by plate tectonic processes such as continental breakup the continuous creation of oceanic lithosphere at mid ocean ridges and subduction at deep sea trenches here we use a compilation of marine seismic refraction data from ocean basins globally to analyse changes in the thickness of oceanic crust over time we find that oceanic crust formed in the mid jurassic about 170 million years ago is 1 7 km thicker on average than crust produced along the present day mid ocean ridge system if a higher mantle temperature is the cause of thicker jurassic ocean crust the upper mantle may have cooled by 15 20 c per 100 million years over this time period the difference between this and the long term mantle cooling rate indeed suggests that modern plate tectonics coincide with greater mantle heat loss we also find that the increase of ocean crustal thickness with plate age is stronger in the indian and atlantic oceans compared with the pacific ocean this observation supports the idea that upper mantle temperature in the jurassic was higher in the wake of the fragmented supercontinent pangaea due to the effect of continental insulation thicker oceanic crust forms from a hot mantle observations of unusually thick oceanic crust that formed 170 million years ago in the atlantic and indian oceans suggest that the ancient supercontinent pangaea helped insulate and warm the mantle

dynamic processes controlling evolution of rifted basins abstract the extension of the lithosphere controlling the development of rifted basins is driven by a combination of plate boundary forces frictional forces exerted on the base of the lithosphere by the convecting asthenosphere and deviatoric tensional stresses developing over upwelling branches of the asthenospheric convection system although mantle plumes are not a primary driving force of rifting they play an important secondary role by weakening the lithosphere and by controlling the level of rift related volcanic activity a distinction between active and passive rifting is only conditionally justified the extension of the lithosphere depending on its rate and magnitude and the potential temperature of the asthenosphere can cause by adiabatic decompression partial melting of the lower lithosphere and upper asthenosphere in rift systems the level and timing of volcanic activity is highly variable the lack of volcanic activity implies passive rifting an initial passive rifting stage can be followed by a more active one during which magmatism plays an increasingly important role magmatic destabilization of the moho may account for the frequently observed discrepancy between upper and lower crustal extension factors combined with evidence for thermal thinning of the mantle lithosphere this suggest that the volume of the lithosphere is not necessarily preserved during rifting as advocated by conventional stretching models the structural style of rifts is controlled by the rheological structure of the lithosphere the availability of crustal discontinuities that can be tensionally reactivated the mode orthogonal or oblique and amount of extension and the lithological composition of pre and syn rift sediments simple shear extension prevails in rifts that subparallel the structural grain of the basement pure shear extension is typical for rifts cross cutting the basement grain pre existing crustal and mantle lithospheric discontinuities contribute to the localization of rift systems the duration of the rifting stage of extensional basins is highly variable stress field changes can cause abrupt termination of rifting in major rift systems progressive strain concentration on the zone of future crustal separation entails abandonment of lateral rifts depending on constraints on lateral block movements crustal separation can be achieved after as little as 9 my and as much as 280 my of rifting activity syn rift basin subsidence is controlled by isostatic adjustment of the crust to mechanical stretching of the lithosphere its magmatic inflation and thermal attenuation of the mantle lithosphere post rift basin subsidence is governed by thermal reequilibration of the lithosphere asthenosphere system deep seated thermal anomalies related to syn rift pull up of the asthenosphere lithosphere boundary have decayed after 60 my by about 65 and after 180 my by about 95 the magnitude of post rift subsidence is a function of the rift induced thermal anomaly and crustal density changes the potential temperature of the asthenosphere and initial water depths intraplate stresses can have an overprinting effect on post rift subsidence stretching factors derived from post rift subsidence analyses must be corrected for such effects

lithosphere asthenosphere interaction and the origin of cretaceous tholeiitic magmatism in northeastern brazil sr nd pb isotopic evidence abstract rio ceara mirim swarm forms a 400 km long mainly e w trending dykes which have been emplaced in northeastern brazil during the opening of south and equatorial atlantic oceans at early cretaceous this swarm is dominated by high and low ti tholeiitic dykes showing em type chemical characteristics such as i strong to moderate enrichment in large ion lithophile and high field strength elements and ii radiogenic sr 0 70713 to 0 70473 and non radiogenic nd 0 512494 to 0 512 and pb 206pb 204pb 18 77 to 18 34 isotopic compositions they are compatible with the melting of an old tdm ages ca 1 0 ga enriched lithospheric mantle source indicating that both proterozoic and mesozoic related mantle processes accounted for the genesis of continental tholeiitic magmatism conversely a distinct chemical component comparable to himu type has been identified in local alkaline sub swarm such dykes have depleted sr 0 70367 0 7031 and nd 0 5129 0 512772 and strongly radiogenic 206pb 204pb 19 95 ratios the source of the rio ceara mirim basalts appears to be related to melting of an enriched lithospheric mantle triggered by a long lived thermal anomaly it could be related either to deep st helena mantle plumes or convective cells induced by lithospheric discontinuities

juvenile co2in enderbites of tromoy near arendal southern norway a fluid inclusion and stable isotope study the enderbites from tromoy in the central granulite facies part of the proterozoic bamble sector of southern norway contain dominantly co2 and n2 fluid inclusions co2 from fluid inclusions in quartz segregations in enderbites was extracted by mechanical crushing and thermal decrepitation and the 13c measured measurement was also made on samples washed in 10 hcl oxidized with cuo at high temperatures and step wise extracted with progressive heating results between the different techniques are systematic the main results show 13c of 4 5 1 5 for crushing and 7 2 for thermal decrepitation 13c is about constant for co2 extracted at different temperatures and points to a homogeneous isotopic composition due to the presence of carbonate particles and or induced contaminations for the extraction by thermal decrepitation the results for the crushing experiments are assumed the most reliable for fluid inclusion co2 very low values of 13c have not been found in enderbite samples and 13c combined with 18o of the host quartzes 8 11 indicates juvenile values in addition the fluid inclusions were examined by microthermometry and raman analysis and host quartz by acoustic emission and cathodoluminescence co2 fluid inclusions have varying densities with a frequency maximum of 0 92 g cm 3 and generally do not concur with trapping densities at granulite conditions textures show that co2 must have been trapped in fluid inclusions in one early event but transformed to different extents during late isothermal uplift without important fractionation of isotope compositions the present data support a model of intrusion and crystallization of a co2 rich enderbitic magma at granuiite conditions

anomalous isotopic compositions of sr ar and o in the mesozoic diabase dikes of liberia west africa the mesozoic diabase dikes of liberia are tholeiites whose 87sr 86sr and 87rb 86sr ratios scatter widely on the rb sr isochron diagram the problem is attributed to differences in the initial 87sr 86sr ratios of these rocks which range from 0 70311 to 0 70792 assuming a uniform age of 186 ma for the dikes and using 87rb 1 42 10 11y 1 the range of values is similar to that observed in the mesozoic basalt flows and dikes of other gondwana continents

three dimensional modeling of plume lithosphere interaction in order to understand better the dynamics of hot spots such as hawaii we present a three dimensional numerical model for the interaction of a thermal plume with a moving lithosphere the model domain is a rectangular box filled with fluid whose viscosity depends upon temperature and pressure the lithosphere is represented by a layer of cold highly viscous fluid moving with an imposed horizontal velocity u in the x direction and a thermal plume is generated by a circular temperature anomaly on the bottom of the box the steady flow is determined numerically using a hybrid spectral finite difference technique the flow is characterized by a stagnation streamline of width y x1 5 that represents the edge of the spreading plume material we illustrate the detailed behavior of the model using the example of the hawaiian plume our best fitting hawaiian model is obtained by adjusting the plume buoyancy flux b until the predicted topography anomaly matches the observed hawaiian swell topography we find b 4100 kg s 1 which implies a plume radius of 90 km for an assumed plume mantle temperature contrast of 300 c the predicted topography is supported primarily by density anomalies beneath the lithosphere and cannot be explained by lithospheric erosion we therefore conclude that the classical lithospheric reheating model is unable to account for hotspot swells the horizontal flux of buoyancy associated with the swell exceeds b by up to 80 suggesting that current estimates of b for mantle plumes are too high empirically derived scaling laws for the width of the stagnation streamline and for the topography anomaly exhibit power law dependencies on b and u that agree well with those predicted by the refracted plume model of olson 1990 the principal weakness of the model is that the predicted geoid topography ratio of 0 010 for the hawaiian swell is about twice the observed value of 0 004 0 006

geology petrography and mineral chemistry of iron oxide apatite occurrences ioa type western sector of the neoproterozoic santa quiteria magmatic arc ceara northeast brazil abstract iron oxide apatite ioa occurrences have recently been identified in the western sector of the neoproterozoic santa quiteria magmatic arc ceara state northeast brazil close to the boundary with the paleozoic parnaiba basin the ioa mineralization is hosted by albitized metadiorites and metavolcano sedimentary rocks which are crosscut by a late to post brasiliano pan african biotite granite intrusion with a u pb age approximately 548 4 ma within the metavolcano sedimentary sequence metavolcanic rocks are bimodal and consist of albitized metabasalt andesite and metarhyolites with a u pb age of 554 6 ma whereas the metasedimentary units are represented by calc silicate marble and pelitic gneisses the iron oxide apatite mineralization occurs as i banded stratabound lenses layers composed of magnetite 50 70 with or without ilmenite exsolution apatite 8 monazite and locally subordinate copper sulfides 2 ii massive magnetite specular hematite apatite bodies iii disseminated and vein magnetite apatite within albitized metadiorite iv massive rhombohedral hematite bodies and v garnet magnetite type the host rocks also display marialite diopside albite and epidote sodic calcic alteration biotite and k feldspar potassic alteration chlorite and epidote in lower t hydrothermal alteration assemblages mineral chemistry data reveal that i except for vein types apatite in all the other occurrences is f rich 2 4 f 4 4 and low in cl

kimberlites in western liberia an overview of the geological setting in a plate tectonic framework the liberian kimberlite field lies in the southern part of the west african craton closely associated kimberlites are present in sierra leone ivory coast guinea and mali and as a group are structurally controlled within the archean basement complex of the leo uplift three major kimberlite dike lineaments are recognized in the northwest sector of liberia all with prominently developed n ne trends these lineaments are termed the kumgbo wuese trend the border trend and the lofa trend the areas of dike intrusion are covered by the bopolu geological quadrangle map of liberia the border trend is close to sierra leone the kumgbo wuese trend is to the east and the lofa trend is coincident with the lofa river only one kimberlite pipe has been recognized in liberia mano godua but a second may be present and both lie along the kumgbo wuese trend the kimberlite bodies intrude the 2700 m y old liberian age province which is comprised of granitic and granodioritic gneiss complexes two prominently developed features dominate the province a general ne sw basement fabric and an en echelon dolerite dike system that parallels the coast dated at 180 m y and considered to represent intrusion at the onset of continental fragmentation transecting dikes and related faults suggest a relative age for the kimberlites of 180 m y an estimate that corresponds to the 92 to 140 m y age for the sierra leone field there is indirect evidence that kimberlites in guinea and mali are of a similar age based on the fact these are on strike with kimberlites in liberia and sierra leone respectively trans atlantic correlations between west africa and northeast south america reveal a cyclic eruption of magma types that were generated in response to plate and continental movements it is postulated that liberian kimberlite emplacement was related to postplate motion crustal flexuring that reactivated precambrian basement sutures these sutures are parallel to and correlate with recognizable paleorift and paleocompressional zones within and adjacent to the west african craton long sustained periods of tectonic activity can be demonstrated and it is proposed that if brittle fractures are continuously reactivated these will serve as conduits to the upper mantle tapping kimberlite source regions at depths in excess of 100 km

high velocity breakup related sills in the voring basin off norway multichannel seismic reflection profiles in the hel graben v0ring basin reveal a sill complex at approximately 5 km depth it is associated with exceptionally high 7 4 km s 1 seismic wide angle velocities the existence of observable wide angle arrivals shows that the sills act as efficient waveguides seismic reflection data and amplitude modeling constrain the thickness of individual sills to approximately 100 m sonic logs from sills of similar thickness on the nearby utgard high show an average velocity of 7 0 km s 1 such high velocities require an olivine gabbroic sill composition and emplacement under conditions which allowed growth of relatively large crystal sizes a possible reason for such an emplacement environment is the hei graben s role as an intrusion center during breakup volcanism this would provide the necessary duration of the magmatic activity as well as locally increased melt volumes and cooling times sill complexes of this kind decrease the accuracy of determined velocity fields and crustal geometries below the top of the sill complex affecting depth conversion and gravity modeling furthermore the results question the concept of lower crustal bodies as large scale homogeneous accumulations of mafic melt