k ar and 39ar 40ar whole rock dating of zeolite facies metamorphosed flood basalts the upper paleocene basalts of the faroe islands ne atlantic

trans atlantic correlation of the palaeogene volcanic successions in the faeroe islands and east greenland before continental break up in the ne atlantic the faeroe islands and central east greenland were within a distance of 100 120 km chemical and lithological data for complete sections through the 5 km thick piles of contemporaneous palaeogene flood basalts in the faeroe islands and in the nansen fjord area in east greenland show very similar basalt compositions and evolution patterns with time the faeroes lower basalt formation and the equivalent nansen fjord formation in east greenland form a pre break up succession overlain by a sediment horizon a syn break up succession consists of the faeroes middle and upper basalt formations and the equivalent milne land formation in east greenland in which five intervals can be correlated with a compositional evolution from ti rich magnesian basalts and picrites at the base to a dominance of morb like low ti basalts at the top the successions were generated in the same mantle melting column beneath a thinning continent with a rift zone that eventually ruptured the continent the evolution pattern is very similar to that seen on the se greenland margin but spreading according to the palmason model of 1973 was not yet established the pre and syn break up successions formed volcanic megasystems stretching across the rift zone with areal extents of 70 000 and 220 000 km 2 and volumes of 120 000 and 250 000 km 3 rocks from the pre and syn break up successions can be discriminated based on a simple major element plot the overlying succession was 3 3 5 km thick in e greenland but was thin or absent in the faeroes the energy source for the melting appears to have been concentrated on the greenland side

isotopic evidence for the age and origin of pitchstones and felsites isle of eigg nw scotland an 8 point whole rock and mineral rb sr isochron for the sgurr of eigg yields an age of 52 1 1 0 ma 2 this is the youngest accurate date yet reported for igneous activity in the british tertiary province and indicates that sub acid rocks of the small isles represent one of the latest episodes of hebridean magmatism pb isotope compositions of these rocks yield an apparent age of 2660 75 ma 2 interpreted as a mixing line between lewisian torridonian and mantle derived pb and indicating the presence of lewisian basement under eigg pb sr and nd isotopic evidence points to contamination of basaltic differentiates and granulite facies lewisian melts by torridonian sandstone but precludes significant involvement of amphibolite facies lewisian which may have been largely removed by erosion before deposition of the torridonian sediments in the small isles

40ar 39ar dating of mineral separates and whole rocks from the western ghats lava pile further constraints on duration and age of the deccan traps abstract there has been an ongoing debate for almost two decades on the age and duration of deccan trap volcanism some of the best available ages have been determined using the 40 ar 39 ar method but differences in sample location laboratory procedure type of analyzed material and ages of monitors may have been responsible for much of the apparent spread in the data we have collected and analyzed samples from close to the bottom and top of the main lava pile in the western ghats near bombay we confirm previous findings that alteration and ar recoil strongly affect these rocks particularly measurements made on whole rocks when mineral separates mostly plagioclase are analyzed the spread is much reduced we find a mean age of 65 4 0 7 ma for five flows near the base of the section jahwar and igatpuri formations and 65 2 0 4 ma for a dyke cross cutting the poladpur formation below the topmost mahabaleshwar formation this implies that at least 1800 m out of the 2500 m composite trap section were erupted in a short interval less than 1 ma long and close to 65 5 ma i e close to the cretaceous tertiary boundary we find that older ages near 67 ma found by several authors are essentially based on whole rock samples which display disturbed spectra when this can be checked i e when the spectra are published

a reappraisal of episodic burial metamorphism in the andes of central chile the jurassic to miocene sequences of the central andes east of santiago reputedly show repeated cycles of episodic sub greenschist facies burial metamorphism that are identified by sharp breaks in metamorphic grade at major stratigraphic boundaries this paper presents the first detailed petrochemical analysis of these low grade metamorphic sequences by examining the progressive development of secondary minerals reaction progress in mafic phyllosilicates and topological variations in the low grade assemblages as a means of testing this model the results indicate a progressive increase from zeolite facies through to close to the onset of greenschist facies from miocene to jurassic rocks combined analysis of reaction progress in mafic phyllosilicates and petrochemical relationships of chlorite pumpellyite actinolite in metabasites provides no evidence for sharp metamorphic breaks at major stratigraphic boundaries integrating the results presented here with the most recent models of stratigraphic tectonic development of the central andes shows that the metamorphism took place in two episodes and was not episodic on a 40 million year cycle an absence of sharp breaks in metamorphic grade in any part of the succession as demonstrated here shows that the original petrographic establishment of low grade facies provided insufficient resolution of changes in metamorphic conditions to establish definitive evidence of such breaks accordingly this study suggests that re assessment of metamorphic breaks reputedly developed in other areas of the andean cordillera is imperative in order to resolve the questions raised here about the origin of the low grade metamorphism

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

the tertiary lavas and sediments of northwest rhum inner hebrides several small outliers of tertiary lavas and sediments rest with strong unconformity on a buried landscape eroded from torridonian sediments and tertiary granophyre erosion continued during the period of sediment and lava accumulation four formations are recognized these are in order of increasing age the orval formation hawaiite and basaltic hawaiite lavas the guirdil formation icelandite lavas interbedded conglomerates the upper fionchra formation tholeiitic basaltic andesite lavas hyaloclastite deposits basal conglomerate and the lower fionchra formation alkali and transitional basalt basaltic hawaiite and hawaiite lava flows basal conglomerate each is separated by an erosional interval clasts in the conglomerates reveal a history of erosion of a terrain exposing gneisses torridonian sediments igneous rocks derived from the rhum tertiary central complex including allivalites and tertiary lavas of local origin but also including in the oldest conglomerates tholeiitic basalts not now preserved on or near rhum prior to and during lava and sediment accumulation erosion on rhum had cut down to a level similar to that of the present day although not to the extent that high grade thermally altered rocks which are a marked feature of the central complex were being eroded in any quantity a sequence of east west trending valleys possibly initiated on the line of the earlier main ring fault drained the area of the central complex which then as now must have been high ground small lakes occasionally formed in the valleys allowing the accumulation of fine grained sediment with plant remains and promoting the formation of hyaloclastite deposits when buried by later flows no source for any of the lava formations is preserved on rhum they are thought to have come from feeders north of rhum possibly near canna and to have ponded against the hills and valleys near and in the central complex the oldest tholeiitic lavas not now found in situ were followed by alkali and transitional flows compositionally similar to the skye main lava series but characteristically feldsparphyric the most mafic also contain phenocrysts of magnesian olivine with included cr al rich spinels and aluminous spinel both the early alkalic transitional basalts and the youngest hawaiites and basaltic hawaiites equilibrated at pressures apart from a few n s to nw se trending basalt dykes the lava formations represent the youngest tertiary igneous event on rhum

reaction pathways and reaction progress for the smectite to chlorite transformation evidence from hydrothermally altered metabasites the transformation from smectite to chlorite has been interpreted as involving either a disequilibrium chlorite smectite mixed layering sequence or an equilibrated discontinuous sequence involving smectite corrensite chlorite here analysis of the smectite to chlorite transition in different geothermal systems leads us to propose that the transformation proceeds via three contrasting reaction pathways involving i a continuous mixed layer chlorite smectite series ii a discontinuous smectite corrensite chlorite series and iii a direct smectite to chlorite transition such contrasting pathways are not in accord with an equilibrium mineral reaction series suggesting that these pathways record kinetically controlled reaction progress in the geothermal systems reviewed the style of reaction pathway and degree of reaction progress is closely correlated with intensity of recrystallization and not to differences in thermal gradients or clay grain size this suggests a kinetic effect linked to variation in fluid rock ratios and or a contrast between advective or diffusive fluid transport the mode of fluid transport provides a means by which the rates of dissolution nucleation growth can control the reaction style and the reaction progress of the smectite to chlorite transition slow rates of growth are linked to the first reaction pathway involving mixed layering while increasing rates of growth relative to nucleation promote the generation of more ordered structures and ultimately lead to the direct smectite to chlorite transition representative of the third pathway

mechanisms and patterns of o and h isotopic exchange during hydrothermal alteration of the north shore volcanic group and related hypabyssal sills midcontinent rift system minnesota abstract hydrothermal flow systems developed in volcanic and interflow sedimentary rocks in response to the emplacement of 1 1ga mafic plutonic rocks during intracontinental rifting in the midcontinent region oxygen and hydrogen isotopic studies of the north shore volcanic group nsvg northeastern minnesota and related hypabyssal sills demonstrate the importance of both permeability and temperature in controlling isotopic exchange in the rift related hydrothermal system oxygen isotopic values of mineral separates and volcanic whole rocks range from 5 5 to 17 7 and show a strong correlation with whole rock water content and intensity of hydrothermal alteration in the shallow portion of the stratigraphic sequence elevated 18 o values and mineralogical alteration are confined principally to highly permeable amygdaloidal flow tops and bottoms massive flow interiors show little or no perturbation from 18 o values considered normal for fresh basaltic rocks with increased depth and temperature isotopic exchange becomes more pervasive 18 o values remain elevated and oxygen isotopic profiles show no difference between flow margins and interiors at depths where greenschist facies mineral assemblages are found flow margins show a reversal to 18 o values as low as 6 6 and d values decrease to 18 o and d values enhanced rates for hydrogen isotopic exchange relative to oxygen are suggested by whole rock d values that tend to be uniform throughout flows that are less than 7 m thick d profiles that parallel those of 18 o values are found in thicker flows the correlation between degree of hydrothermal alteration and 18 o values suggests that solution recrystallization was the primary mechanism of isotopic exchange in the hydrothermal system individual feldspar phenocrysts from a single lava flow may show a range of 18 o values from 10 5 to 17 7 and ranges in individual altered phenocrysts may be as large as 4 8 feldspar isotopic heterogeneity suggests that permeability varied on a grain to grain scale and that flow through microfractures was important in controlling isotopic variations coexisting plagioclase and pyroxene from hypabyssal sills show a very steep slope on a 18 o plagioclase 18 o pyroxene plot also suggestive of kinetically dominated exchange assuming a fluid buffered system in the permeable flow margins a duration of isotopic exchange on the order of 40 000 years is calculated this time period is short compared to the lifetime of the intrusive activity in the rift and suggests that fluid flow was either episodic in nature or confined to the late stages of magmatic evolution

mineral deposits of the mesoproterozoic midcontinent rift system in the lake superior region a space and time classification abstract the mesoproterozoic midcontinent rift system mrs of north america hosts a diverse suite of magmatic and hydrothermal mineral deposits in the lake superior region where rift rocks are exposed at or near the surface historically hydrothermal deposits such as michigan s native copper deposits and the white pine sediment hosted stratiform copper deposit were major mrs metal producers on going exploration for and potential development of copper nickel sulfide deposits hosted by the duluth complex of minnesota and the opening of the eagle nickel mine in michigan indicate an expanding interest in mrs magmatic deposits mrs hydrothermal and magmatic mineral deposits many of which are significant past present and likely future providers of critical minerals here are placed into a space and time metallogenic framework to construct this framework regional mrs mineral deposits extracted from the u s geological survey mineral resources data system mrds and the ontario ministry of energy northern development and mines mineral deposit inventory mdi were supplemented by other known and recently recognized mineral deposits described in the literature all mineral deposits were classified by deposit type host rock age and type and estimated timing of mineralization deposits were then put into a tectonic evolutionary framework stages for the mrs which shows that deposits formed within discrete spatial and temporal stages of rift evolution each stage of rift evolution is characterized by specific mineral deposit types that are largely confined both by their physical location in rift rocks and type and timing of mineralization examples include mrs nickel rich conduit type magmatic sulfide deposits which are restricted to an early magmatic stage mrs history when magma compositions were characterized by ni rich picrites and high mg basalts in contrast contact type magmatic sulfide deposits with cu ni were derived from more evolved al rich tholeiitic magmas that dominated a later time of voluminous magmatic activity hydrothermal sediment hosted stratiform chalcocite mineralization along the margins of western lake superior in michigan is economic only where fluid flow through red beds was concentrated by structures along the margins of a post volcanic mrs sedimentary basin widespread native cu and native ag mineralization occurred about 40 million years after formation of host basalt lava flows and interflow sedimentary rocks these descriptions of the diverse mineral deposits in the lake superior region cataloged within the space of the local and regional geology and over the more than 60 million year mineralizing history of the mrs are indicative of the complex and at times overlapping magmatic and hydrothermal mineral systems that operated within this major large igneous province

stable isotopic studies of mafic sills and proterozoic metasedimentary rocks located beneath the duluth complex minnesota abstract thin 3 20 m picritic to noritic sills are found in the proterozoic metasedimentary footwall rocks of the duluth complex minnesota the sills were emplaced prior to the major intrusions that comprise the duluth complex and underwent contact metamorphism along with the country rocks oxygen hydrogen and sulfur isotopic compositions of the sills indicate a varied history of isotopic exchange between minerals melts and hydrothermal fluids in the high temperature environment below the major plutonic bodies of the midcontinent rift system the pre duluth complex sills exhibit a range in 18o values from 4 9 to 14 8 with values between 6 and 7 generally found in sill interiors high 18o values near sill contacts with high 18o metasedimentary rocks of the pelitic virginia formation or biwabik iron formation coupled with a smooth sigmoidal isotopic profile centered at the contact suggest that oxygen diffusion was an important exchange mechanism the elevated 18o values near the center of the thickest sills are thought to reflect the emplacement of isotopically contaminated basaltic magma dehydration reactions in the pelitic rocks of the contact aureole liberated high 18o fluids that enhanced subsolidus diffusive exchange advective displacement of the diffusion profiles toward the sill interior is less than 40 cm and suggests that layer parallel flow dominated in the dehydration of the contact aureole elevated but uniform 18o values 9 7 to 10 5 in thinner sills suggests that oxygen diffusivity was increased relative to country rocks due to enhanced porosity perhaps related to extensive development of microcracks although d values of the pelitic country rocks record a history of dehydration systematic variations of d 64 to 143 and h2o 0 15 to 5 40 wt content are not found in the sills 18o values of coexisting plagioclase and pyroxene from the sills indicate a close approach to isotopic equilibrium and are consistent with a diffusion dominant exchange process at temperatures near 500 c results of diffusion modelling suggest a duration of isotopic exchange that may have extended from tens of thousands of years to 1 4 ma depending on local controls of porosity and permeability as well as rates of fluid production in the contact aureole localized areas of 18o and d depletion in the sills values as low as 4 9 and 143 respectively denote exchange with meteoric water after interaction with the high 18o metamorphic fluids in the contact aureole although all of the elevated 18o samples in the contact environment may have suffered 18o depletion most exchange with meteoric water appears to be spatially localized and is thought to reflect highly channelized fracture controlled fluid flow sulfur isotopic values of the sills are variable 2 7 to 11 2 and indicative of an evolution involving pre emplacement contamination of basaltic magma and sub solidus exchange with an h2s bearing metamorphic fluid sulfur contents exceed 3 0 wt only within troctolitic to melatroctolitic sills and 34s values of 7 8 to 8 3 are strongly suggestive of pre emplacement contamination by sulfur derived from a proterozoic sedimentary unit high 18o and 34s rocks particularly at sill margins are consistent with either hydrothermal precipitation of fine grained sulfide minerals or isotopic exchange between magmatic sulfides and an h2s bearing metamorphic fluid

geochemistry of precambrian sedimentary rocks used to solve stratigraphical problems an example from the neoproterozoic volta basin ghana abstract the neoproterozoic volta basin of ghana 115 000 km 2 depth up to 5 7 km consists of flat lying sedimentary rocks mainly sandstones that unconformably overlie the crystalline basement of the west african craton the stratigraphical column has been subdivided into three main units in upward succession the bombouaka oti and obosum groups but poor exposure has resulted in major disagreements on stratigraphical correlations and on the areal extents of these units geochemical data major and trace element concentrations as well as rb sr pb and sm nd isotope data on siltstones and mudstones intercalated with the sandstones from the different units were used in an attempt to solve some of these problems siltstones and mudstones from the bombouaka group can be unequivocally distinguished from similar rocks from the oti and obosum groups by higher k 2 o and rb larger eu anamalies higher 87 sr 86 sr and more negative nd values geochemical distinction between samples from the oti and obosum groups is ambiguous because published geological maps differ with respect to the relative extents of the oti and obosum groups rb sr isotope data combined with high degrees of correlation between the concentrations of k and rb and ca and sr indicate that mobility of these elements did not significantly change their concentrations during surface weathering the clear geochemical distinction between mudstones and siltstones from the bombouaka group and similar rocks from the oti and obosum groups is used to solve one of the outstanding controversies regarding the stratigraphy of the volta basin

application of palynological data to the chronology of the palaeogene lava fields of the british province implications for magmatic stratigraphy new high precision ages determined from palynomorph assemblages within intercalated sedimentary deposits are presented for the palaeogene lava fields skye mull and antrim of the british province these data reveal very rapid averaged eruption rates 1 m 200 years and the non synchronous formation of the lava fields eruption of the volumetrically dominant transitional to mildly alkaline lavas of the skye 58 25 58 0 ma and mull post 55 ma lava fields the skye main lava series and the plateau group of mull respectively is separated by the eruption of the morb like preshal more flows of olivine tholeiite found at the top of the preserved sequence on skye and at the base of the mull lava field the lower formation of the antrim lava field correlates with the skye lava field and the upper formation correlates with the preshal more flows the new ages indicate that the eruption of the preshal more flows was synchronous with the main ocean floor spreading event which occurred 500 km to the nw at c 55 ma during chron 24r a combined thinspot and channelized plume model may best explain the temporal and spatial distributions of the lava fields and associated subvolcanic complexes of the british province

hydrothermal alteration of the ediacaran volyn brest volcanics on the western margin of the east european craton abstract the ediacaran continental flood basalts and associated tuffs were studied to identify and quantify alteration processes by means of xrd and chemistry supplemented by mossbauer and ftir spectroscopies petrography oxygen and iron isotopes k ar dating and organic geochemistry two superimposed alteration processes were identified the ediacaran hydrothermal alteration induced by meteoric waters heated and put in motion by the cooling basalt and the caledonian and or variscan potassic alteration the degree of basalt alteration was quantified using as an index the sum of primary minerals in the bulk rock the sequence of minerals dissolved and crystallized during the hydrothermal alteration was established the alteration resulted in the loss of ca dissolution of plagioclases compensated by the gain of water and mg crystallization of clays and proceeded from the edges of the basalt flows in an oxidizing environment evidenced by the increasing amount of hematite and fe3 fe2 ratio of the bulk rock cyanobacteria were active in the hydrothermal system most probably responsible for the measured negative 56fe values and more reducing conditions at the stage of intense alteration chlorophaeite palagonite following quartz as the earliest petrographically identifiable basalt alteration product was found to vary systematically from fully isotropic to birefringent the chlorophaeite was identified as a mixture of fe montmorillonite and fe saponite identical with griffithite and oxysmectites probably preceded by a finer grained ferrosaponite at the isotropic stage ree content of chlorophaeite indicates basaltic volcanic glass sideromelane as the major source of material ree in clays are contained mostly in the dioctahedral smectite while in the bulk rock mostly in phosphates the smectite characteristics and mg enrichment are indicative of the hydrothermal basalt alteration process which perhaps was dominant also on mars both mineral and chemical composition of tuffs vary continuously from basaltic to felsic the latter close to the measured rhyodacite composition dominated by quartz and feldspars the basaltic tuffs resemble the most altered basalts but contain also abundant albite and chlorite indicative of higher alteration temperatures up to 220 c tuff composition indicates stronger component of felsic volcanism in the trap formation than evidenced by the preserved bodies of effusive rocks

evolution of clay mineral assemblages in the tinguiririca geothermal field andean cordillera of central chile an xrd and hrtem aem study abstract hrtem textural evidence shows that clay minerals in the tinguiririca geothermal field andean cordillera central chile are the result of direct alteration of former volcanic glass and minerals by hydrothermal fluids at similar temperatures to the present day they show the classical pattern of diagenetic transformation from smectite at the top to illite at the bottom with the progressive formation of corrensite and chlorite the high fluid rock ratio disposability of necessary cations and absence of previous detrital phyllosilicates allow the consideration of this area as a natural laboratory to establish the extreme ideal conditions for very low t reactions transformations from smectite to r1 illite smectite i s and from these to r3 mixed layers occur respectively at 80 120 c and 125 180 c in spite of ideal genetic conditions the new formed minerals show all the defective character and lack of textural and chemical equilibrium previously described in the literature for diagenetic and hydrothermal low temperature assemblages chemistry of smectite illite phases evolves basically through a diminution of the pyrophyllitic component toward a theoretical muscovite si 4 al 3 k however a second chemical vector si 4 mg 2 al 3 al 3 that is decreasing of the tschermack component also contributes to the evolution toward the less si more al rich muscovite in relation to the original smectite residual mg and fe from the latter reaction is consumed in the genesis of chloritic phases nevertheless as a consequence of the lack of chemical equilibrium probably because of the short time scale of the geothermal alteration processes the composition of clay minerals is highly heterogeneous at the level of a single sample consequently the respective fields of smectite r1 i s and r3 i s overlap each other making the distinction among these three phases impossible based exclusively on chemical data

the architecture of the buem structural unit implications for the tectonic evolution of the pan african dahomeyide orogen west africa abstract the buem structural unit bsu occurs about 50 km west of the high pressure hp rocks of the pharusian suture zone psz of the dahomeyide orogen west africa shale and sandstone intercalations form the base of the bsu followed by serpentinised peridotite gabbro massive and pillow basalts fragment of oceanic crust jasper chert and limestone dolostone with sandstone and diamictite forming the top of the bsu the package was deformed at lower greenschist facies conditions during the pan african orogeny the n s trending bsu has been affected by three deformational events the main deformation event d2 which transposed bedding parallel d1 fabrics produced a steeply ene dipping s2 tectonic foliation s2 is axial planar to upright or west verging isoclinal and chevron f2 folds top to the west d2 shear zones in the sandstone shale and serpentinised peridotite are associated with a down dip ene plunging l2 stretching lineation the mafic ultramafic rocks occur as d2 tectonic slices marking the boundaries between thrust sheets within the bsu the d2 related n s trending structural and short wavelength 1 km magnetic anomalies that are continuous along trend with the regional structures of the west african craton wac and are interpreted as signals from the basement rocks of the bsu d2 deformation is interpreted to result from overthrusting of the bsu onto the southeastern margin of the wac during collision of the wac and benino nigerian shield bns meter scale steep and open f3 kink and crenulation folds with steeply se dipping axial planes are sporadically observed throughout the entire bsu d3 deformation is inferred to reflect a period of nw se shortening deformation history of the bsu shares structural similarities with the one of the psz we therefore interpret the bsu as an allochthonous fold and thrust belt with a geodynamic evolution involving subduction of the wac beneath the bns and the ensuing collision between the two terranes the tectonic framework of the bsu suggests a geodynamic evolution of the dahomeyide orogen evolving from initial accretion of oceanic crust and deposition of passive margin sedimentary sequences and their inversion due to continent continent collision during the pan african orogeny

hydrothermal flow systems in the midcontinent rift oxygen and hydrogen isotopic studies of the north shore volcanic group and related hypabyssal sills minnesota abstract rift related lavas of the north shore volcanic group nsvg are intruded by plutonic rocks of the duluth complex along the unconformity between the nsvg and the underlying proterozoic metasedimentary rocks animikie group and archean volcano sedimentary and plutonic rocks heat associated with the emplacement of the mafic intrusions generated fluid flow in the overlying plateau lavas 18o values for whole rocks from the nsvg and hypabyssal sills range from 5 5 to 17 7 and 5 3 to 11 5 respectively and most values are higher than those considered normal for basaltic rocks 5 4 to 6 0 in general there is a positive correlation between whole rock 18o and water content which suggests that elevated 18o values are related primarily to secondary mineral growth and isotopic exchange during hydrothermal alteration and metamorphism 18oh2o values computed from amygdule filling minerals such as smectite chlorite and epidote found in low to high temperature metamorphic zones range from 1 to 6 with an average value of 3 smectite in the lower grade zones gives computed dh2o values between 26 and 83 whereas epidote in the higher grade zones gives dh2o values of 15 to 6 fluid isotopic compositions computed from epidote and smectite values are suggestive of the involvement of at least two fluids during the early stages of amygdule filling fluid d and 18o values determined from epidote at the higher metamorphic grades indicate that seawater dominated the deeper portions of the system where greenschist facies assemblages and elevated 18o values were produced in flow interiors as well as margins smectite isotopic compositions suggest that meteoric water was predominant in the shallower portions of the system the increase in 18o values of massive flow interiors with depth is interpreted as a result of rock interaction with a fluid of constant oxygen isotopic composition with increasing temperature the stable isotopic data are supportive of previous suggestions that seawater was involved in the hydrothermal system associated with the midcontinent rift although the origin of the seawater remains problematic it appears that marine incursions may have occurred during the late stages of portage lake volcanism and periodically thereafter

atacamite inclusions in rock forming feldspars and copper bearing smectites from the radomiro tomic mine chile copper insoluble mineral occurrences recovery of copper from the radomiro tomic deposit in chile employs innovative large scale hydrometallurgical extraction methods with sulfuric acid used for heap leaching here we report on improvements in understanding mineral components of copper oxide ores which are insoluble in acid leach solutions in order to advance leaching technology using optical electron microprobe x ray diffraction energy dispersive spectroscopy and least squares inversion methods two different occluded mineral occurrences were discovered in oxide ores which present two distinct metallurgical challenges for heap leaching but offer important new opportunities in process mineralogy first in a high chloride to sulfate ratio feldspar stable portion of the orebody at radomiro tomic disseminated atacamite cu4cl2 oh 6 inclusions 1 to 8 m in diameter occur in microfractured feldspars and biotites these account for the acid insoluble fraction often as high as 30 percent of the total contained copper about 70 percent of the total copper occurs generally as atacamite in cracks which upon crushing is exposed along surfaces of the rock fragments and is hence soluble in contrast in more intensely hydrothermally altered feldspar destructive weak argillic alteration zones insoluble copper exists more probably within the crystal structure of well crystallized saponitic smectite clays in nonexchangeable octahedral crystallographic sites the two different forms of applied acid insoluble components are products of contrasting microchemical environments one in a reactive potassic alteration gangue and the second in a nonreactive gangue in both cases however the nature of hydrothermal alteration though pervasive was relatively weak and left intact rock mineral buffers capable of neutralizing acidic and reducing fluids hence the insoluble components with respect to the applied sulfuric acid reflect strong wall rock mineral assemblage control on the behavior of copper on all scales the macrofield zonal scale the microscale of individual crystals in cracks and the atomic scale of octahedral sites in the clay structure the metallurgical implications of these atacamite microinclusions in rock forming minerals and structurally bound copper in smectites are different in the former case exposing ore inclusions on fragment surfaces would require extremely costly grinding to a grain size well below 400 mesh secondly chemical extraction of microinclusion copper from feldspar host phases would entail high acid consumption as hydrolysis of feldspars by chemical reaction with applied acid would occur causing neutralization in the case of cu smectites since the copper occurs in structurally bound nonexchangeable sites processes to remove the copper via ion exchange or acidic leach cannot work alternatives to ion exchange could involve destroying the smectite host mineral perhaps by its conversion to kaolinite that may only incorporate copper or other divalent cations in negligible amounts

a new geomagnetic polarity time scale for the late cretaceous and cenozoic we have constructed a magnetic polarity time scale for the late cretaceous and cenozoic based on an analysis of marine magnetic profiles from the world s ocean basins this is the first time since heirtzler et al 1968 published their time scale that the relative widths of the magnetic polarity intervals for the entire late cretaceous and cenozoic have been systematically determined from magnetic profiles a composite geomagnetic polarity sequence was derived based primarily on data from the south atlantic anomaly spacings in the south atlantic were constrained by a combination of finite rotation poles and averages of stacked profiles fine scale information was derived from magnetic profiles on faster spreading ridges in the pacific and indian oceans and inserted into the south ariantic sequence based on the assumption that spreading rates in the south atlantic were smoothly varying but not necessarily constant a time scale was generated by using a spline function to fit a set of nine age calibration points

disturbed 40ar39ar spectra from hornblendes thermal loss or contamination abstract accurately weighed aliquots of the 500 ma old interlaboratory standard hornblende mmhb 1 were mixed with known quantities of a 100 ma old himalayan biotite and subjected to normal 40 ar 39 ar incremental heating analysis the addition of the biotite created monotonically increasing apparent ages similar to profiles interpreted as showing ar diffusive loss in the literature the release of ar from biotite dominates the low temperature portion of the spectra and lowers the observed age lower ca k ratios in the first increments are consistent with contamination but do not distinguish the composition of the contaminant phase a large number of published amphibole diffusive loss profiles may in fact be due to contamination by biotite or other phases which release ar at low extraction temperatures

miocene fossil hydrothermal system associated with a volcanic complex in the andes of central chile abstract cenozoic deposits in the andes of central chile have been affected by very low grade burial metamorphism at about 33 s in the cuesta de chacabuco area approximately 53 km north of santiago two oligocene and miocene volcanic units form a ca 1300 m thick rock pile the miocene unit corresponds to a volcanic complex composed of two eroded stratovolcanoes secondary mineral assemblages in both units were studied petrographically and using x ray diffraction and electron microprobe analyses most of the igneous minerals are wholly or partially preserved and the ubiquitous secondary minerals are zeolites and mafic phyllosilicates the alteration pattern observed is characterized by a lateral zonation in secondary mineralogy related to a lateral increase in temperature but not to stratigraphic depth the following three zones were established mainly based on the distribution of zeolites zone i comprises heulandite thomsonite mesolite stilbite and tri smectite zone ii contains laumontite yugawaralite prehnite epidote and chlorite and zone iii comprises wairakite epidote chlorite diopside biotite and titanite for each zone the following temperature ranges were estimated zone i 100 180 c zone ii 180 270 c and zone iii 245 310 c the alteration episode was characterized by a high p fluid p total ratio ca 1 0 although slightly variable a high geothermal gradient of ca 160 c km 1 and fluid pressures below 500 bars although temperature was the main control on the mineral zonation several interrelated parameters mainly fluid composition porosity and permeability were also important hot near neutral to slightly alkaline ph alkali chloride hydrothermal fluids with very low dissolved co 2 contents deposited the secondary minerals the alteration pattern is the result of depositing fluids in outflow regions from a hydrothermal system developed inside a volcanic complex during the miocene the hydrothermal system has been eroded to a maximum depth of 1 7 km

constraints on provenance stratigraphic correlation and structural context of the volta basin ghana from detrital zircon geochronology an amazonian connection abstract we have dated 1060 detrital zircon grains from the neoproterozoic to lower palaeozoic volta basin ghana and from sandstones in the adjoining thrust sheets belonging to the pan african dahomeyide belt all dated zircons in the lower voltaian bombouaka group are older than 1000 ma while samples from the middle voltaian oti group and the upper voltaian obosum group contain numerous zircons of 600 1000 ma the samples we have studied from the dahomeyide thrust sheets buem and togo structural units have zircon age spectra similar to those from the bombouaka group confirming a correlation of the investigated sandstones with lower voltaian strata the bombouaka group was deposited between 1000 and 600 ma perhaps shortly after 1000 ma as suggested by earlier rb sr data on clay minerals deposition of the oti and obosum groups took place shortly after 600 ma perhaps continuing into the lower palaeozoic most samples contain palaeoproterozoic zircons with ages of 2000 2200 ma that probably have been derived from the surrounding crystalline birimian basement archaean zircons present in smaller proportions may have come from archaean rocks in the west african craton most zircons of 1000 1900 ma were probably derived from sources outside the west african craton the amazonian craton being a plausible source region zircons with ages around 1200 ma are believed to have been derived from grenvillian orogenic belts perhaps those that fringe the amazonian craton if this is so no major seaways could have been present between these belts and the west african craton during the early neoproterozoic zircons younger than 1000 ma were probably derived from an eastern continental block that collided with the west african craton during the pan african orogeny 600 ma ago

a rift to drift record of vertical crustal motions in the faroe shetland basin nw european margin establishing constraints on ne atlantic evolution the upper paleocene eocene rock record in the faroe shetland basin is punctuated by a series of unconformities that reflect a persistent tectonic instability throughout the syn to early post breakup period a duration of about 20 myr a particular focus is on a late paleocene subaerial unconformity herein termed the flett unconformity which has been argued to have formed in response to a transient pulse of mantle convective uplift associated with a proto iceland plume however ambiguity over its presumed correlation with the faroe islands basalt group combined with stratigraphic and palaeogeographical analysis of the upper paleocene eocene succession indicates that it is just one of a series of subaerial unconformities that are spatially linked to the same part of the faroe shetland basin most of these late paleocene eocene unconformities post date volcanism and their formation coincides with vertical motions associated with phases of uplift inversion and compressional deformation linked to the growth of structures such as the wyville thomson and munkagrunnur ridges and the judd anticline these deformation phases are broadly coeval with intraplate and plate boundary events in the wider ne atlantic region the possibility that the flett unconformity had a similar tectonic origin should not be discounted

volcaniclastic rocks of the reydarfjordur drill hole eastern iceland 2 alteration volcaniclastic rocks of basaltic to rhyolitic composition interlayered with basaltic to icelanditic lava flows in a 1918 m drill hole in eastern iceland are altered to zeolite and pumpellyite prehnite facies assemblages major alteration minerals are chlorites illite chlorite mixed minerals illites k feldspar laumontite quartz and sphene others include julgoldite and secondary allanite smectites as well as mordenite and heulandite are restricted to the upper 900 m epidote and minor pumpellyite prehnite wairakite and anhydrite are restricted to the core below 900 m the boundary between zeolite and prehnite pumpellyite facies is marked by the appearance of chlorite epidote quartz assemblages at around 900 m the typical paragenesis prehnite pumpellyite epidote chlorite occurs near the base of the drill hole at 1855 m textures indicate generally prograde metamorphic conditions the broad compositional spectrum ranging from basalt to rhyolite abundance of vitric fragments and high initial porosity and permeability of the volcaniclastic rocks were major factors governing alteration the site of growth crystal shard vesicle pore and vein of a secondary phase strongly influences composition habit amount paragenesis and time of crystallization for example sphene and to a lesser degree epidote are less rich in fe and better crystallized in plagioclase than in the adjacent matrix the different mineral assemblages developed within plagioclase crystals through the section most clearly reflect the metamorphic changes the abundance of vitric rhyolitic clasts shards and pumice favored k al si enriched fluids from which phases like illite and adularia crystallized these phases were not found in interlayered basalt flows mehegan et al this issue low ca activity is also indicated by the occurrence of wairakite and pumpellyite rather than prehnite which is common in the basalts secular changes during metamorphism in pore fluid composition are shown by zonations in mineral compositions for example the depletion of epidote and juldoldite pumpellyite in fe with time appears to reflect decreasing o2 bulk rock chemical changes are more intense in the volcaniclastic rocks compared to the basalt lavas h2o fe3 fe2 and in part co2 values are higher in the upper part of the hole reflecting the higher porosity and glass content of the clastic rocks they decrease markedly downhole approaching values of the lava flows due to more pervasive metamorphic alteration and thus similar mineral assemblages and physical properties

thermal and geodynamic setting of the buem volcanic rocks near tiele northwest benin west africa abstract the mafic volcanic rocks of tiele occur as intercalations within very low grade metasediments mainly metapelites and meta arenites these proterozoic supracrustal rocks are considered as part of the buem structural unit bsu which is the most external tectonic unit of the pan african dahomeyide fold belt the geodynamic significance of the bsu is still debated it could represent either a portion of the west african passive margin during the rifting and the drifting of the pan african ocean affaton 1990 or a continental rift zone jones 1990 or the pan african oceanic crust itself burke and dewey 1972 and burke and dewey 1973 shackleton 1976 the mafic lavas derived from tholeiitic magmas which originated by partial melting of the lithospheric mantle with a possible asthenospheric contribution they are quite similar to morbs and free of any crustal contamination they are related to a strongly attenuated continental lithosphere and a passive margin would be the most likely tectonic environment for the coeval volcanic and sedimentary activities on the other hand these rocks demonstrate effects of early static recrystallizations related to a very low grade metamorphism of the prehnite pumpellyite facies developed under temperatures of 200 to 300 c such a thermal imprint clearly marks an extensional tectonic stage later the early parageneses were re equilibrated under slightly higher pressures during the pan african collision both magmatic features and early metamorphic characters of these metavolcanic rocks are in good agreement with a context of lithospheric thinning characterized by mantle derived magmatism and high thermal gradient these results fit with previous interpretations mainly based on sedimentary and structural features the bsu represents a portion of a proterozoic passive margin which suffered strong lithospheric stretching with deep seated normal faults and mantle derived magmatism and a subsequent severe subsidence evidenced by the prominent detrital sedimentation such a tectonic setting developed during the pan african oceanic drifting

low grade metamorphism and geotectonic setting of the macuchi formation western cordillera of ecuador the cretaceous eocene basic to intermediate marine volcanic rocks of the mucuchi formation constitute the western cordillera in northern ecuador their chemical features mostly correspond to those of tholeiitic basalts with some calc alkaline affinities and suggest an oceanic island arc setting the macuchi rocks are affected by low grade non deformative metamorphism characterized by zeolite prehnite pumpellyite and lower greenschist facies assemblages depth zonation is suggested by the downward mineral sequence i laumontite pistacitic epidote pumpellyite prehnite ii pumpellyite prehnite pistacitic epidote iii actinolite biotite pistacitic epidote chlorite this broad zonation and the chemistry of individual minerals point to an interaction between the volcanic rocks and sea water under a moderate to high thermal gradient 75 c km alteration appears to have been dependent primarily on fluid control volume pressure composition temperature and reaction kinetics which together partly overshadow the role of load pressure compositional variations of a mineral species at the scale of a contiguous flow or even at the scale of a thin section show that intensity of alteration was spatially uneven depending on rock permeability and consequently metastable equilibrium commonly exists however a progressive approximation to equilibrium as a result of p t control is shown by the mineralogy a high fo2 of the fluid phase is evident from the mineral chemistry the metamorphism of the macuchi volcanics is similar to the hydrothermal burial type produced during the development of a volcanic arc where lavas and volcanoclastics accumulated in a shallow marine environment however some of its characteristics point to a transition toward systems defined by a higher t p ratio such as those found in ocean floor metamorphism a model is proposed in which the macuchi volcanics are assigned to an oceanic island arc generated contemporaneously with a marginal basin which has opened as the outcome of progressive north south attenuation of the continental crust due to mantle diapirism

how natural remanent magnetization of basaltic units can dominate the reduced to pole magnetic value a case study from the faroe islands abstract early attempts to utilize magnetic data to understand the volcanic and subvolcanic succession on the faroese continental shelf have shown that conventional interpretation and modelling of magnetic data from this area leads to ambiguous results interpretation of the aeromagnetic data on the faroese continental shelf shows that some previously identified basement highs coincide with reduced to pole magnetic highs whereas others coincide with negative or mixed magnetic features similarly igneous centres are characterized by different polarity magnetic anomalies palaeomagnetic analysis of the onshore volcanic succession has demonstrated that the thermoremanent magnetization of the basaltic lavas is stronger than the induced magnetism and both reversely and normally magnetized units are present we have tested this with 2 d profile modelling using the palaeomagnetic information to correlate high amplitude magnetic anomalies with basalt successions containing changes in magnetic polarity this approach has enabled us to map the termination of the differently magnetized units offshore and thereby extend the mapping of the faroe island basalt group on the faroese platform and into adjacent areas

precise 40ar 39ar age for the initiation of palaeogene volcanism in the inner hebrides and its regional significance sanidine crystals from two tuff layers at the base of the palaeocene eigg lava formation on the isle of muck inner hebrides yield step wise ar release plateau ages of 62 8 0 6 2 and 62 4 0 6 ma using the laser 40 ar 39 ar dating technique these determinations which are in accord with but significantly more precise than earlier results provide a definitive age for the inception of igneous activity in the small isles from stratigraphical inferences the muck ages provide precise maximum constraints on the possible age of skye magmatism and possibly most of the british tertiary volcanic province the new ages significantly predate the oldest ages determined so far for tertiary magmatism in ne greenland but are similar to ages obtained from w greenland and offshore se greenland and support the concept that the initiation of regional volcanism was primarily controlled by lithospheric thin spots the muck sanidine age appears to precisely date the first manifestation of the impingement of the iceland mantle plume on the lithosphere beneath nw scotland

transition from the zeolite to prehnite pumpellyite facies in the karmutsen metabasites vancouver island british columbia

volcanic ash in a cored borehole w of the shetland islands evidence for selandian late palaeocene volcanism in the faeroes region

new paleomagnetic pole and magnetostratigraphy of faroe islands flood volcanics north atlantic igneous province abstract a paleomagnetic sampling was carried out along four sections altogether 86 lava flows 548 samples in the north atlantic igneous province outcropping in faroe islands denmark the four polarity zones in the 700 m thick exposed part of the faroes lower formation can be correlated with the geomagnetic polarity time scale as c26n c25r c25n c24r the seven lava flows erupted during c25n indicate a very low eruption rate in the upper part of the faroes lower formation of 1 70 kyr the faroes middle and upper formations composite thickness 2300 m are all reversely magnetized corresponding to c24r the eruption rate at the onset of middle formation volcanism was very high as evidenced by several thick lava sequences recording essentially spot readings of the paleomagnetic field the shift in eruption rate between the faroes lower and middle formations and evidence that onset of the faroes middle formation volcanism took place in c24r are of particular importance placing onset of middle formation volcanism in close temporal relation to north atlantic continental break up and the late paleocene thermal maximum after grouping flows recording the same field directions we obtained 43 independent readings of the paleomagnetic field yielding a paleomagnetic pole with coordinates 71 4 n 154 7 e a95 6 0 k 14 n 43 age 55 58 ma the pole is supported by a positive reversal test paleosecular variation estimated as the angular standard deviation of the virtual geomagnetic pole distribution 21 7 3 9 2 8 is close to expected for the given age and paleolatitude our new faroes paleomagnetic pole is statistically different from the majority of previously published poles from the british and faroes igneous provinces and we suggest that these older data should be used with care

low temperature oxidation in morb of titanomagnetite to titanomaghemite a gradual process with implications for marine magnetic anomaly amplitudes mid ocean ridge basalt morb samples varying in age from recent to jurassic were selected for electron microscopic and rock magnetic studies our observations indicate that the degree of oxidation of titanomagnetite in morb increases only gradually with sample age the titanomagnetite in recent morb 100 ka both electron microscopic observations and rock magnetic data support the notion that low temperature oxidation of titanomagnetite to titanomaghemite in morb is a gradual process moreover the rate of maghemitization is controlled by many factors on both macroscopic and microscopic scales including regional oceanic crustal structures lithological features grain size and surrounding matrices pillow lavas and the tops of massive flows tend to have higher degrees of oxidation than the interiors of massive flows owing to higher porosity and permeability of the former in contrast interstitial glass can protect fine titanomagnetite grains from alteration the natural remanent magnetization nrm intensity of morb varies as a function of age magnetic granulometry concentration of magnetic materials and the degree of alteration fine grained morb samples typically have higher nrm intensity the nrm intensity appears to decrease substantially with increasing degree of maghemitization and hence with age the envelope of the smoothed magnetic anomaly amplitudes resembles the change in nrm intensity for morb samples of the last 30 million years but the underlying assumption that the highest degree of maghemitization observed in the samples of a given age is entirely responsible for these intensity changes is not supported maghemitization of the micrometer sized and larger grains is probably only partly responsible whereas a significant contribution to nrm intensities is inferred from those submicrometer sized titanomagnetite grains that remained protected from oxidation by the surrounding matrix of interstitial glass

the stratigraphy and structure of the faroese continental margin this paper presents a summary of the stratigraphy and structure of the faroese region as the faroese area is mostly covered by volcanic material the nature of the pre volcanic geology remains largely unproven seismic refraction data provide some indications of the distribution of crystalline basement which probably comprises archaean rocks with the overlying cover composed predominantly of upper mesozoic cretaceous and cenozoic strata the cenozoic succession is dominated by the syn break up faroe islands basalt group which crops out on the faroe islands where it is up to 6 6 km thick and shelf areas post break up sediments are preserved in the adjacent deep water basins including the faroe shetland basin seismic interpretation of the post volcanic strata shows that almost every sub basin in the faroe shetland basin has been affected by structural inversion particularly during the miocene these effects are also observed on the faroe platform the munkagrunnur ridge and the fugloy ridge where interpretation of low gravity anomalies suggests a large scale fold pattern the structure of the iceland faroe ridge which borders the nw part of the faroe area remains ambiguous the generally thick crust together with the absence of well defined seawards dipping reflectors may indicate that much of it is underlain by continental material

occurrence of fe mg rich smectites and corrensite in the morron de mateo bentonite deposit cabo de gata region spain a natural analogue of the bentonite barrier in a radwaste repository the morron de mateo bentonite deposit is being studied as a natural analogue of the thermal and geochemical effects on a bentonite barrier in a deep geological repository of high level radioactive wastes this bentonite deposit and its host rocks were intruded by a rhyodacitic volcanic dome that induced a hydrothermal metasomatic process affecting the biocalcarenite beds close to the dome in this work the mineralogical and chemical features of the clay minerals of the hydrothermally altered pyroclastic white tuffs and epiclastic rocks mass flow located in the ne sector of the morron de mateo deposit are described white tuffs have a high content of phyllosilicates mainly composed of dioctahedral smectites while mass flow have a higher proportion of inherited minerals the neoformed phyllosilicates are dioctahedral smectites and an interlayer chlorite smectite mineral of corrensite type the chemical composition of smectites reflects the different nature of the parent rocks in such a way that smectites from white tuffs have a quite homogeneous chemical composition and their structural formulae correspond to montmorillonite type while smectites from mass flow show more chemical variability higher fe and mg contents and a mean structural formulae corresponding to fe mg rich beidellite and or to an intermediate smectite member between beidellite and saponite in addition chemical composition and textural features of corrensite like clay minerals in relation to fe mg rich smectites in the samples have also been studied suggesting that the former seems to be formed from fe mg rich smectites the presence of corrensite in the epiclastic rocks suggests that in the morron de mateo area a hydrothermal alteration process occurred after bentonite formation which transformed fe mg rich smectites into corrensite this transformation was probably favoured by the intrusion of the morron de mateo volcanic dome which produced a temperature increase in the geological media and a supply of fe mg rich solutions these physicochemical conditions were also responsible for the metasomatic transformations observed in the biocalcarenite beds located on the top of the bentonite deposit all these data suggest that the morron de mateo natural system could be a good natural analogue of both thermal and chemical effects on a bentonite barrier related to the radioactive decay of fission products and the interaction between the corrosion products of steel over packs and the bentonite these circumstances would favour the transformation of the candidate al rich smectites into fe mg rich smectites and corrensite as steps prior to formation of chlorite in this case all the physicochemical and mechanical properties of al rich smectites would disappear and the clayey barrier would fail

columnar jointed bentonite below a doleritic sill tideswell dale derbyshire uk formation during prograde contact metamorphism columnar jointed tuffs bentonites are located below the lower carboniferous tideswell dale sill derbyshire in an abandoned quarry there are three zones of prismatic joints columnar joints 60 cm thick and mean column width of 4 1 cm and massive conchoidally fractured rock sequentially from the dolerite contact downwards the rocks are very fine grained 1 10 m and consist mainly of sanidine interlayered illite smectite and hematite with minor quartz apatite montmorillonite anatase and detrital minerals high k2o and fe2o3 low na2o and low mgo are interpreted as due to alteration of felsic volcanic ash the bentonite was contact metamorphosed by the sill to temperatures of c 300 c the progression of fracture geometries thermal considerations and application of theories of column formation indicate that columns formed downwards away from the sill due to prograde contact metamorphism of the originally clay rich bentonite the formation of columnar fracture networks by shrinkage due to igneous heating may have implications for the effectiveness of bentonite as a barrier for disposal of high level radioactive waste regional dimensions include documenting widespread felsic explosive volcanism and inferring a triassic oxidation event from palaeomagnetic data

morphologies and emplacement mechanisms of the lava flows of the faroe islands basalt group faroe islands ne atlantic ocean the palaeogene faroe islands basalt group fibg comprises three eruptive sequences or formations all emplaced into a subaerial environment during the development of the extensive continental flood basalt province that stretches from east greenland through the faroe islands and into the faroe shetland basin the beinisvor formation having a tabular classic facies architecture is composed of a sequence of simple flows each comprising a single sheet lobe the beinisvor formation is overlain by the distinctly contrasting malinstindur formation that has a compound braided facies architecture the enni formation occurs at the top of the sequence and consists of a mixture of simple and compound flows with tabular classic and compound braided facies architectures respectively surface and internal characteristics of the sheet lobes of the beinisvor and enni formations indicate emplacement through inflation which is more obvious for the tube fed compound flows of the malinstindur and enni formations the difference between the simple and compound flow sequences of the fibg is most likely linked to the manner in which the lava was supplied during the eruption and the eruptive style of the volcanic system the sheet lobes were erupted over laterally extensive areas from fissure systems which had a continuous supply of lava which contrasts with the tube fed compound flows which were erupted in a gradual piecemeal manner from point sourced low shield volcanoes with limited areal extents

early cenozoic saucer shaped sills of the faroe islands an example of intrusive styles in basaltic lava piles abstract the early cenozoic igneous activity of the north atlantic igneous province that generated widespread sill complexes in sedimentary basins at the nw european margins also generated various intrusive systems in the contemporaneous basaltic lava pile of the faroe islands the faroe island basalt group comprises seven formations with a total thickness of about 6 5 km of which four major formations are built up of tholeiitic lava flows each being several hundred metres thick and three thinner formations are mostly built up of volcaniclastic lithologies each being a few metres to a few tens of metres thick the largest sills are exposed as partly saucer shaped bodies in the three uppermost formations where inner gently dipping basal sill sections gradually give way to more steeply inclined discordant outer rims that commonly cut several hundred metres into overlying lava flows numerous subvertical and moderately inclined dykes ranging in thickness from c 0 5 to c 4 0 m intersect the areas affected by sill intrusion but only inclined dykes or sheets have been positively identified as sill feeders locally controlled rotations of least principal stress axes 3 during initial sill intrusion or propagation may have been an important contributing factor in determining the overall geometry of the investigated intrusions

pumpellyite bearing rocks in central sweden and extent of host rock alteration as a control of pumpellyite composition metamorphic assemblages and mineral compositions in basic to intermediate volcanic rocks of two precambrian units the sub jotnian and jotnian in the dala region central sweden reveal a history comprising two metamorphic episodes the sub jotnian rocks were metamorphosed during a first episode the metamorphic grade ranges from prehnite pumpellyite facies in a displaced block with sub jotnian lava occurring in the border zone of a major impact structure to pumpellyite actinolite facies and to greenschist facies in the lower part of the unit the unconformably overlying jotnian rocks were metamorphosed to prehnite pumpellyite facies during a subsequent episode which also left its imprint in the sub jotnian basement two other jotnian sequences in central sweden are in the same facies

an unusual new archosauromorph reptile from the upper triassic wolfville formation of nova scotia an unusual new reptile teraterpeton hrynewichorum is described based on a partial skeleton the holotype comes from the upper triassic wolfville formation newark supergroup of nova scotia the temporal region of the skull is of the euryapsid type the premaxillary rostrum and dentary symphysis are elongate and edentulous the dentition comprises distinctive occluding upper and lower teeth a phylogenetic analysis shows that teraterpeton hrynewichorum is an archosauromorph reptile trilophosaurus buettneri from the upper triassic dockum group of texas may represent a closely related species

calcite and quartz microstructural geothermometry of low grade metasedimentary rocks northern range trinidad abstract the northern range of trinidad is a mountainous exposure of deformed metasedimentary rocks with mesozoic depositional ages and tertiary metamorphic ages located in the caribbean south american plate boundary zone these rocks lack precise mineralogical indicators of metamorphic grade using temperature sensitive quartz and calcite microstructures and fission track data we identify describe quantify and map a nearly continuous spectrum of relatively low l50 c to relatively high 250 400 c deformation temperatures from east to west across the range average estimated rock exhumation rates also increase systematically along this trend the analysis illuminates a sharp east west trending major thermal discontinuity i e a fault along the southern boundary of the range kinematic analysis of faults and shear bands adjacent to this boundary and modeling indicate that the fault probably dips southward at 80 85 and accommodated normal dip slip at rates that decreased from west to east earlier experimental studies at high strain rates illustrated a strong strain rate dependency for quartz microstructural transitions our work indicates a possible strong temperature control for quartz microstructural transitions at natural strain rates

a chlorite solid solution geothermometer the los azufres mexico geothermal system chlorite constitutes a major hydrothermal alteration product of metamorphism of andesites in the active geothermal system of los azufres mexico electron microprobe analyses performed on a set of crystals from each sample show wide variations in composition correlation coefficients among chemical constituents were calculated it is shown that the tetrahedral charge is positively correlated with the octahedral vacancy and negatively with the iron content and there is almost no correlation with the octahedral aluminium and magnesium content a procedure is proposed to select end members and substitution vectors and to give a general formula for these chlorites

onshore evidence for progressive changes in rifting directions during continental break up in the ne atlantic abstract current models for the palaeogene aged continental break up in the north atlantic invoke a nw se extension vector throughout the rift to drift process accommodated by margin parallel normal faults and margin oblique strike slip transfer zones however palaeogene and younger faults in the faroe islands provide evidence for a progressive anticlockwise rotation in extension vectors immediately prior to and following north atlantic break up six deformation stages are recognized 1 east west to ne sw extension accommodated by dip slip north south and nw se trending faults 2 continued ne sw extension accommodated by nw se and nne ssw oriented dykes 3 north south extension accommodated by ene wsw and ese wnw conjugate dykes 4 crustal extrusion involving both east west shortening and further north south extension along ene wsw dextral and ese wnw sinistral conjugate strike slip faults 5 during the final stages of rifting the regional extension vector became nw se and was accommodated predominantly by slip along ne sw dextral oblique slip faults 6 pre existing structures were locally reactivated as tensional and extensional shear features characterized by the entrainment of clastic material along fault planes the present study reveals a distinct period of margin parallel extension immediately prior to break up that is not accounted for by existing models illustrating the importance of conducting field based studies to validate otherwise widely accepted margin scale models worldwide

constraining the post emplacement evolution of the hebridean igneous province hip using low temperature thermochronology how long has the hip been cool abstract the thermal history of the hebridean igneous province has been determined through the application of low temperature thermochronology to the four central complexes in the province the zircon u th he age 59 4 3 3 ma 1 n 18 and the ages from each complex 60 7 2 3 ma for skye 58 0 0 4 ma for mull 55 9 3 2 ma for ardnamurchan 55 6 2 7 ma for rum are indistinguishable from the crystallization ages apatite fission track ages 61 2 57 2 ma mean of 59 3 3 4 ma from the major plutonic units also overlap crystallization ages implying that on a regional scale the hebridean igneous province cooled rapidly to near surface temperatures immediately after emplacement however apatite fission track ages and track lengths and apatite u th he ages from some small volume intrusions in the skye and rum central complexes identify localized mid eocene 45 47 ma cooling forward and inverse modelling suggests a discrete heating cooling event at c 47 ma which may have been caused by structurally controlled localized advection of heat above shallow emplacement this is the first suggestion of eocene magmatism in the hebridean igneous province

geochemical evolution of a palaeolaterite the interbasaltic formation northern ireland abstract the interbasaltic laterites of northern ireland were formed during a period of relative volcanic inactivity by extensive chemical weathering of tertiary basalts they reach a maximum thickness of 30 m and once provided a major source of iron and aluminium ore an extensive database comprising major minor and trace elements has been compiled for 240 samples in order to study the effects of weathering in terms of the changes in whole rock chemistry and mineralogy from basalt through to iron rich crust percolating waters caused degradation of the parent basalt mineralogy and precipitation of neoformed phases principally through incongruent dissolution processes primary olivine plagioclase feldspar and augite were successively broken down and replaced by a mineral assemblage consisting of hematite gibbsite goethite anatase meta halloysite and kaolinite changes in mineralogy facilitated concomitant changes in element concentrations mass balance calculations indicate that all elements were depleted in the iron crust enrichment of al loi cr cu and v occurred in the laterite horizon while enrichment of al loi ba ce cr cu ni and rb occurred in the lithomarge notably yttrium was found to be mobile indicating that weathered basalts should not be used in discrimination of original tectonic environments the severe leaching conditions evidenced by yttrium depletion local aluminium redistribution and iron crust formation are indicative of weathering under a humid sub tropical monsoon climate

the development of cavities and clastic infills along fault related fractures in tertiary basalts on the ne atlantic margin field based geological observations have revealed the hitherto unrecognised development of post magmatic brittle deformation structures cutting tertiary volcanic rocks in the faroe islands these faults and fractures are characteristically associated with different styles of clastic sedimentary infill including 1 0 3 1 0 m thick clastic units infilling open fractures formed along pre existing steeply dipping to sub vertical faults 2 0 1 0 6 m thick sub horizontal clastic units displaying internal features consistent with deposition from flowing water passing through complex open subterranean cavity systems within fractured basalts 3 anastomosing mm scale and planar dm scale clastic intrusion features mobilised and emplaced during transient fault related overpressuring events along pre existing fractures cutting the surrounding volcanic units the infill features provide evidence for the existence of sustained open cavities in the sub surface the clastic materials are commonly internally affected by later fault related deformation and lack mineralisation unlike all preceding faulting episodes in the faroes region perhaps reflecting their near surface development we believe structures equivalent to these features may occur widely in other parts of the ne atlantic margin particularly along the outer arcs of gentle regional scale fold hinges the uncemented fracture hosted clastic infills potentially represent important fluid migration pathways within the otherwise low permeability cenozoic volcanic sequences of the ne atlantic region

diastathermal extensional metamorphism at very low grades and possible high grade analogues abstract a genetic link between some areas of very low grade of metamorphism and an extensional setting is argued a model is presented in which the metamorphism develops in response to enhanced thermal gradients generated by extensional processes and the pressure temperature time paths are anticipated as being anticlockwise in form examples of granulite facies metamorphism attributed by sandiford and powell 1 to extensional settings may represent the lower crustal analogue of the very low grade examples examined here extensional processes may thus provide the mechanisms by which metamorphism in a variety of crustal levels can develop the traditional association of regional metamorphism with convergent processes is no longer tenable and the termdiastathermal metamorphism proposed by robinson 2 is appropriate to demonstrate the fundamental difference between these two types of regional metamorphism

contribution of long term hydrothermal experiments for understanding the smectite to chlorite conversion in geological environments the smectite to chlorite conversion is investigated through long duration experiments up to 9 years conducted at 300 c the starting products were the wyoming bentonite mx80 79 smectite metallic iron and magnetite in contact with a na ca chloride solution the predominant minerals in the run products were an iron rich chlorite chamosite like and interstratified clays interpreted to be chlorite smectite and or corrensite smectite accompanied by euhedral crystals of quartz albite and zeolite the formation of pure corrensite was not observed in the long duration experiments the conversion of smectite into chlorite over time appears to take place in several steps and through several successive mechanisms a solid state transformation significant dissolution of the smectite and direct precipitation from the solution which is over saturated with respect to chlorite allowing the formation of a chamosite like mineral the reaction mechanisms are confirmed by x ray patterns and data obtained on the experimental solutions ph contents of si mg na and ca because of the availability of some nutrients in the solution total dissolution of the starting smectite does not lead to 100 crystallization of chlorite but to a mixture of two dominant clays chamosite and interstratified chlorite smectite and or corrensite smectite poor in smectite the role of fe fe mg in the experimental medium is highlighted by chemical data obtained on newly formed clay particles alongside previously published data the newly formed iron rich chlorite has the same composition as that predicted by the geothermometer for diagenetic to low grade metamorphic conditions and the quartz fe chlorite albite experimental assemblage in the 9 year experiment is close to that fixed by water rock equilibrium

phyllosilicates in very low grade metamorphism transformation to micas phyllosilicates produced mostly by weathering of magmatic metamorphic and sedimentary rocks are widespread in the surficial and near surface zones of the earth s crust these phyllosilicates display very fine grain sizes disordered crystal structures and greatly varying chemical compositions and they represent structurally and chemically metasable phases they are subjected to heat during diagenesis and concomitant metamorphism which provides a long life source of energy for transforming the metastable phases into thermodynamically more stable phases through a long series of partly continuous partly discontinuous reactions the present review summarizes some major mineralogical aspects of these processes with special reference to petrogenetic applications forty years have passed since the first petrogenetic application of the illite structural changes for characterizing diagenetic processes in sedimentary basins weaver 1960 weaver s sharpness ratio as well as kubler s 1964 1968 empirical illite crystallinity index have been easy to use x ray powder diffraction xrd measures of the manifold inter related changes that the hydrous mica like phyllosilicates experience during increasing burial in the first two decades after the introduction of the illite crystallinity concept it was applied to metamorphic petrogenetic studies in certain areas of europe especially to the external fold and thrust belts of the european alps variscides etc these studies were the starting point of important further developments which include awareness of the phyllosilicate transformations during deep burial in sedimentary basins hower et al 1976 inoue et al 1990 amouric and olives 1991 lanson and champion 1991 lindgreen and hansen 1991 eberl 1993 huang et al 1993 whitney and velde 1993 huggett 1995 elliott and matisoff 1996 nieto et al 1996 dong et al 1997 and recognition of the nature of interstratified clay minerals see reynolds and hower 1970 moreover using the theory of powder x ray powder diffraction e g

comparison of diagenetic and low grade metamorphic evolution of chlorite in associated metapelites and metabasites an integrated tem and xrd study chlorite is a common sheet silicate that occurs in various lithologies over a wide grade range involving diagenesis and low grade metamorphism thus the reaction progress of chlorite overs a unique oppor tunity for direct correlation of zonal classification of metasedimentary rocks based on illite crystallinity with metabasite mineral facies to provide such correlation chlorite crystallinity indices apparent mean crystallite sizes and lattice strains crystallite size distributions and compositions of chlorite from coexisting metapelites and metabasites were determined by x ray divraction xrd scanning electron microscopy sem transmission electron microscopy tem analytical electron microscopy aem and electron microprobe emp methods samples were from palaeozoic and mesozoic formations of the bukkium innermost western carpathians hungary that underwent alpine cretaceous orogenic metamorphism metapelites range in grade from late diagenesis to epizone whereas metabasites vary from prehnite pumpellyite through pumpellyite actinolite to greenschist facies despite significant diverences in composition mineral assemblages and textures reaction progress as measured in part by chlorite crystallinity in metapelites paralleled that in metabasites chlorite crystal linity and mean crystallite size increase and the proportion of mixed layers in chlorite decreases whereas the calculated lattice strain does not change significantly with increasing metamorphic grade similar trends but especially at higher grades significant diverences were found in mean crystallite size values using various methods for xrd line profile analyses the increase in crystallite size with increasing grade was demonstrated also by direct tem measurements on ion milled whole rock samples but with a larger scatter of data at higher grades in spite of the diverent kinds of mixed layering in chlorite mg rich smectitic mostly random local corrensite like units in metabasites and fe rich berthierine and diocta hedral smectite in metapelites xrd calculated and tem measured parameters were found to be reliable tools for measuring reaction progress and metamorphic grade of the same degree in both lithotypes

a refraction seismic transect from the faroe islands to the hatton rockall basin 1 the crustal structure of the faroe rockall plateau was studied by a 790 km long refraction seismic transect consisting of two intersecting lines the air gun shot spacing was 200 m and the signals were recorded by 77 ocean bottom seismometers a p wave velocity model was developed from forward and inverse modeling of the wide angle seismic data and incorporation of coincident multichannel reflection seismic data continental crust with velocities ranging from 5 6 to 6 8 km s can be traced from the faroe islands across the banks to the sw of the faroes and into the hatton rockall basin the thickness of the subvolcanic crust is up to 25 km on the banks but is as little as 8 km in the channels between the banks the thinning in the channels may be related to nw trending shear zones extending from major lineaments in ne rockall trough basalt layers are found along the entire transect with a total thickness of up to 4 km two layers with velocities of 4 9 5 2 and 5 3 5 6 km s are thought to represent paleogene flood basalts that can be correlated from the faroe islands to george bligh bank close to george bligh bank an 80 km wide and up to 9 km thick body with velocities of 6 5 km s is interpreted as intrusion a 5 km thick high velocity lower crustal layer 7 3 km s extends from the area of the intrusion into the northern hatton rockall basin at the northern flank of lousy bank the transition zone to oceanic crust was encountered

the smectite to chlorite transition in the chipilapa geothermal system el salvador clay mineralogical x ray diffraction and electron microprobe studies have been carried out on separated 2 m fractions from cutting and core material from three wells in the chipilapa geothermal system in el salvador the data indicate that the smectite to chlorite transition is prevalent but a secondary smectite to illite transition is also present at depths approximately 750 m smectite with very minor chlorite mixed layers approximately 15 is dominant and has a composition midway between a di and tri smectite at 750 m there is a very clear distinction and sharp transition into discrete chlorite with very minor smectite mixed layers approximately 10 corrensite is recorded only as a rare and minor phase smectite occurs in abundance at temperatures up to 200 c and the transition from a smectitedominant to chlorite dominant assemblage takes place over a narrow temperature range 150 to 200 c the stability range of smectite is very similar to that recorded in other geothermal systems whereas the smectite to chlorite transition differs greatly from that recorded in other systems the transition does not involve continuous chlorite smectite mixed layering but a marked step it is the sharpest and most discontinuous stepped sequence of this mineralogical transition recorded

understanding the offshore flood basalt sequence using onshore volcanic facies analogues an example from the faroe shetland basin flood basalts in associated volcanic rifted margins such as the north atlantic igneous province have a significant component of lavas which are preserved in the present day in an offshore setting a close inspection of the internal facies architecture of flood basalts onshore provides a framework to interpret the offshore sequences imaged by remote techniques such as reflection seismology a geological interpretation of the offshore lava sequences in the faroe shetland basin using constraints from onshore analogues such as the faroe islands allows for the identification of a series of lava sequences which have characteristic properties so that they can be grouped these are tabular simple flows compound braided flows and sub aqueously deposited hyaloclastite facies the succession of volcanic rocks calculated in this study has a maximum thickness in excess of 6800 m down to the top of the sub volcanic sediments the offshore volcanic succession has a thickness of about 2700 m where it can be clearly identified across much of the area with a further 2700 m or more of volcanic rock estimated from the combined gravity and seismic modelling to the north and west of the region a large palaeo waterbody is identified on the basis of a hyaloclastite front apron consisting of a series of clinoforms prograding towards the eastern part of the basin this body was 500 m deep must have been present at the onset of volcanism into this region and parts of the water body would have been present during the continued stages of volcanism as indicated by the distribution of the hyaloclastite apron

geochemical transitions in the ancestral iceland plume evidence from the isle of mull tertiary volcano scotland the mull tertiary volcano lies on the eastern edge of the north atlantic igneous province and was active soon after the initiation of the iceland plume detailed sampling of the mull lava pile has allowed temporal changes in the mantle source to be determined a significant change at 700 m above sea level from n morb like to icelandic marks the first arrival beneath the region of mantle from the core of the ancestral plume the earlier basalts represent an outer plume sheath of heated upper mantle from the initiation of magmatism in mull it took 1 9 0 4 ma for this transition to occur

mantle hotspots plumes and regional tectonics as causes of intraplate magmatism in continental areas it is often difficult to determine the cause of intraplate magmatism large volumes of magma high eruption rates and the presence of a hotspot trace on the adjacent ocean floor are all evidence for the presence of an anomalously hot mantle however in many continental magmas there are chemical variations with time which are inferred to reflect changes from asthenospheric to predominantly lithospheric source regions or vice versa it is argued that these chemical characteristics constrain whether magmatism was triggered by the emplacement of a mantle plume or by lithospheric extension

transition from diagenesis to metamorphism in extensional and collision settings areas in which there is a transition from diagenesis to metamorphism occur in a variety of tectonic settings some of which are of an extensional character rather than the collision setting normally associated with regional metamorphism in collision settings the diagenetic to metamorphic transition is typically followed by a continuum into the highest grades of metamorphism the baric type is usually of an intermediate pressure variety and metamorphism occurs at a late stage in the tectonic cycle in extensional settings such as marginal basins a restricted facies series is developed that ranges from diagenetic levels up to a low greenschist facies maximum the metamorphism is normally of a low pressure type and develops early in the tectonic cycle these characteristics suggest that it is possible to subdivide broadly the field of low grade regional metamorphism into compressional orogenic and extensional settings

geochronology of sedimentary and metasedimentary precambrian rocks of the west african craton abstract this contribution summarizes current knowledge of the geochronology of the upper proterozoic sedimentary rocks covering the west african craton this was done by using direct dating methods correlations between the northern edge of the tindouf basin and the northern and southern part of the taoudeni basin as well as the volta basin are proposed an important depositional lack over almost the entire craton is confirmed between 1 7 ga which represents the time of the latest eburnean activities or cooling effects and about 1 0 ga which is the start of the general upper proterozoic sedimentation this lack is remarkable as the other cratons from gondwana continent are covered by middle proterozoic sediments the upper proterozoic sedimentation on the west african craton may be divided into a basal mainly sandstone period ca 1000 900 ma a middle mainly limestone period ca 900 750 ma and an upper mainly siltstone period ca 750 650 ma tectonic volcanic and thermal activities in connection with the pan african orogeny are recorded only around the craton they induced either sedimentation lacks in morocco or sedimentation excesses in hoggar unsolved problems such as the precise stratigraphic position of the uppermost proterozoic tillitic episode and the correlation within the moroccan anti atlas are also raised

a new petrogenetic grid for low grade metabasites abstract we have used internally consistent thermodynamic data to present calculated phase equilibria for the system na2o cao mgo al2o3 sio2 h2o ncmash in the range 0 500 c and 0 1 10 kbar involving the phases anorthite glaucophane grossular heulandite jadeite laumontite lawsonite paragonite prehnite pumpellyite stilbite tremolite wairakite zoisite with excess albite clinochlore quartz and pure water average activity terms derived from published mineral chemical data were included for clinochlore glaucophane prehnite pumpellyite tremolite and zoisite the new petrogenetic grid delineates stability fields and parageneses of common index minerals in zeolite prehniteactinolite prehnite pumpellyite pumpellyite actinolite blueschist and greenschist facies metabasites the stability fields of mineral assemblages containing prehnite pumpellyite epidote actinolite albite chlorite quartz were analysed in some detail using activity data calculated from five specific samples for example the prehnite actinolite facies covers a p t field ranging from about 220 to 320 c at pressures below 4 5 kbar the transition from the prehnite actinolite and pumpellyite actinolite to greenschist facies occurs at about 250 300 c at 1 3 kbar and at about 250 350 c at 3 8 kbar p t fields of individual facies overlap considerably due to variations in chemical composition

the age and structural setting of limestones and basalts on the main ring fault in southeast rhum in southeast rhum a mesozoic tertiary sequence is preserved as a fault bounded and rotated wedge this is juxtaposed between precambrian rocks torridonian sediments and lewisian gneiss and caught up in the complex structure of the tertiary main ring fault mrf which shows three distinct phases of movement the mesozoic rocks comprise fossiliferous limestone sandstone and shale which show differing degrees of thermal metamorphism depending on their relationships to the layered complex on the basis of faunal and lithological evidence the mesozoic sediments have been correlated with the lower liassic broadford beds the rhum sediments are overlain by sheared tertiary basalts the contact between them probably representing the original landscape unconformity the presence of these younger strata caught up along the mrf provides crucial evidence for a major phase of central subsidence in the early history of the rhum tertiary volcanic centre

unusual tetrapod teeth from the upper triassic chinle formation arizona usa two teeth collected from the upper triasssic chinle formation of northeastern arizona are described here and named kraterokheirodon colberti gen et sp nov these teeth are novel in having an occlusal ridge with six cusps and a posterior shelf lacking dentine evidence for thecodont implantation of the root suggests amniote affinities for these teeth they do not match any teeth known for basal vertebrates or basal tetrapods although the teeth display some affinities with traversodont cynodonts there are significant differences that preclude a referral to this group these teeth most probably represent an unknown tetrapod clade and document the presence of a large amniote previ ously unknown from late triassic terrestrial faunas

the moroccan anti atlas the west african craton passive margin with limited pan african activity implications for the northern limit of the craton abstract the moroccan anti atlas region located south of the south atlas fault has been viewed traditionally as containing two segments separated by the anti atlas major fault these two segments are said to consist of a 600 700 ma pan african segment located in the northeast and b 2 ga eburnian segment situated to the southwest on the basis of observations in the zenaga and saghro inliers and of a recent literature review we suggest that this subdivision is inappropriate in that eburnian and pan african materials occur throughout the anti atlas region the entire anti atlas is underlain by eburnian crust unconformably overlain by a lower neoproterozoic passive margin allochthonous pan african ocean crustal slices were thrust onto the west african craton wac passive margin sequence 685 ma ago as a result of pan african accretion tectonics high level high k calc alkaline and alkaline granitoids locally intruded the anti atlas sequence as a whole at the end of the pan african orogeny at 585 560 ma the intervening 100 m y interval was marked by quiescence this succession of events can be related to the behaviour of one single rigid cratonic passive margin during an orogeny and correlated to the pan african events that occurred to the east in the tuareg shield and to the north in avalonian terranes this model implies that the actual northern limit of the wac is located at the south atlas fault saf and not at the anti atlas major fault aamf we propose that the aamf corresponds to the southwestern boundary of an aulacogen that formed along the northern margin of the wac during early neoproterozoic times this is consistent with the development of the gourma aulacogen on the eastern side of the wac this model further suggests that the northeastern boundary of the wac occurs north and not south of ougarta algeria support for this model is provided by geological and geophysical evidence

the geochemistry of indian bole horizons palaeoenvironmental implications of deccan intravolcanic palaeosurfaces deccan intravolcanic bole horizons represent weathering products formed during major hiatuses of a major volcanic episode in these quiescent periods weathering processes pervasively altered the newly formed volcanic landscape and subsequent flows covered and effectively fossilized the resulting weathered palaeosurfaces the current work is a detailed geochemical study which examines patterns of element mobilization during these intravolcanic weathering events the bole horizons normally rest on top of altered basaltic lavas both boles and altered lavas represent a comparatively early stage in weathering because the content of chemically residual elements such as aluminium and iron are closer to fresh basalt than laterite nevertheless these is clear evidence for significant chemical alteration because the more mobile elements such as calcium and sodium have been substantially removed a chemically distinctive nature for some boles can be demonstrated from chondrite normalized ree plots and in these instances strontium and neodymium isotopic compositions demonstrate that the fine grained bole material is derived from a chemically distinct source in addition thin sections reveal that these fine grained portions commonly contains glass shards and occasionally fresh phenocrysts it is therefore suggested that many deccan boles are in fact weathered pyroclastic material and that the pyroclastic content of the basaltic succession may be greater than previously supposed a significant pyroclastic input during the deccan eruptions has important palaeoenvironmental implications for the fate of late cretaceous flora and fauna in peninsular india

textural and mineralogical changes associated with the incipient hydrothermal alteration of glassy dacite at the submarine pacmanus hydrothermal system eastern manus basin variably altered dacite from the pacmanus vent field in the eastern manus back arc basin papua new guinea was studied to determine the textural and mineralogical characteristics of hydrothermal alteration taking place in the immediate subsurface of this modern seafloor hydrothermal system detailed textural investigations show that fluid flow through the glassy dacite has been strongly controlled by the primary volcanic textures quench fractures and networks of interconnected perlitic cracks linking vesicles provided pathways for hydrothermal fluids flowing up to the seafloor hydrothermal alteration along these pathways resulted in the formation of pseudoclastic textures textural evidence suggests that alteration of the glassy dacite has not been sustained the samples have been affected by incipient hydrothermal alteration that is typically not preserved in ancient volcanic rock hosted massive sulfide deposits interaction of the glassy dacite with hydrothermal fluids primarily resulted in the conversion of volcanic glass to dioctahedral smectite only minor amounts of trioctahedral smectite were formed destruction of the volcanic glass and the formation of smectite caused pronounced changes in the chemistry of the dacite samples in particular a decrease in the sio2 whole rock content and the na2o k2o ratio the two alkali elements behaved differently during hydrothermal alteration due to preferential incorporation of k into the interlayer position of the newly formed dioctahedral smectite smectite formation occurred under rock dominated conditions although the addition of mg was required to form trioctahedral smectite from the silicic volcanic glass primary plagioclase was resistant to hydrothermal alteration highlighting the fact that the destruction of volcanic glass and feldspar are not necessarily contemporaneous in massive sulfide forming hydrothermal systems incipient alteration of the glassy dacite close to the seafloor occurred at temperatures below 150 c in an environment that allowed the development of steep temperature gradients comparison of the new data to the findings of deep drilling during odp leg 193 suggests that the smectite rich alteration in the immediate subsurface of the pacmanus hydrothermal vent field represents the low temperature equivalent of illite and chlorite rich alteration associations forming in the upflow zones of the hydrothermal fluids in the deeper portion of the volcanic sequence

geochemistry of early mesozoic tholeiites from morocco abstract mesozoic dolerites from two areas of morocco the high atlas fold belt between marrakech and demnat and the anti atlas belt in the area of foum zquid are most high ti quartz normative tholeiites whichi in many respects resemble mesozoic dolerite dikes from eastern north america the dolerites display a wide range of major and trace element compositions some of which are due to fractional crystallization the doleritic sequences from high atlas also show vertical stratigraphic zonation which is characterized by a progressive depletion of lithophile elements toward the top this trend together with regularities of trace element ratio variations are indicative of a dynamic melting of an initially homogeneous source it is suggested that the continental upper mantle source for dolerites of morocco was enriched in several incompatible elements in comparison with the upper mantle source for ocean floor tholeiites

arctosaurus osborni a late triassic archosauromorph reptile from the canadian arctic archipelago arctosaurus osborni is known only from an incomplete cervical vertebra from the upper triassic heiberg formation of cameron island nunavut canada re examination of the unique specimen indicates that it represents an archosauromorph reptile possibly from the clade allokotosauria to date arctosaurus osborni represents the sole record of late triassic continental tetrapods from nunavut and the northernmost record of such animals anywhere in the world

metamorphic grade and porosity and permeability controls on mafic phyllosilicate distributions in a regional zeolite to greenschist facies transition of the north shore volcanic group minnesota the 8 km thick keweenawan metabasaltic north shore volcanic group in minnesota shows a continuous zeolite to greenschist facies progression through five zonal divisions mafic phyllosilicates are ubiquitous and a well developed smectite corrensite chlorite transition is observed this sequence provides an excellent framework in which the influence of regional thermal as opposed to local scale lithology porosity and permeability factors on this transition factors that are actively debated can be comprehensively documented on a regional scale at lowest grades zones 1 and 2 thomsonite scolecite and heulandite stilbite assemblages respectively tri smectitic phyllosilicates dominate and with increase in grade to zone 3 laumontite albite there is a marked step and compositional gap to corrensite with further increase in grade to zones 4 laumontite albite prehnite pumpellyite and 5 epidote albite actinolite chlorite becomes dominant although a spread in the x ray diffraction and microprobe data suggests that the corrensite to chlorite transition is continuous rather than stepwise analytically distinct corrensite and chlorite in one flow suggest a clear compositional gap between these two phases overall the data offer some support for a discontinuous smectite corrensite chlorite transition above the lowest grade zones 1 and 2 approximately 150 c the phyllosilicates vary on a local within flow scale related to morphological contrasts in flow lithology smectite is dominant in the massive flow units low porosity and permeability whereas corrensite or chlorite is dominant in the amygdaloidal flow tops high porosity and permeability these changes provide clear indication that once above a threshold of approximately 150 c fluid rock ratios have a strong control on the metamorphic process at low grade similarities in xmgo between smectite corrensite chlorite and xmgo of the whole rock show that even though smectite is a relict phase remaining in massive zones it continues to undergo crystal chemical change with increasing grade however contrasts between aliv in the same minerals are provisional indicators that other factors perhaps related to kinetic features linked to advective and diffusive modes of fluid transport may be involved