archaean lode gold deposits in africa crustal setting metallogenesis and cratonization abstract archaean rocks in africa are preserved in seven important tectonic and physiographic terranes the zimbabwe craton the kaapvaal craton of south africa the tanzanian shield the north zaire massif which extends westwards into the central african republic the kasai angola massif of southern zaire the atlantic rise of cameroun gabon and congo extending southwards to the malanje strip of northern angola the guinea rise of guinea sierra leone and liberia and the reguibat rise of mauritania morocco and northwest algeria dilational vein and brittle to ductile shear zone mineralization is common in the zimbabwe craton distribution of the gold being controlled by the permeability characteristics of the hydrothermal conduits and by the competence and chemistry of the wallrocks within the kaapvaal craton the barberton greenstone belt has the highest productivity with a marked concentration of lode type mineralization at or near the tectonized contact between the basalt dominated onverwacht group and the metasediments of the fig tree group smaller lode gold deposits are also present in the greenstone belts of the central and northern kaapvaal craton the more important being the stibnite gold mineralization that accompanied late brittle deformation of carbonatized high mg basalts in the murchison greenstone belt the archaean basement and supracrustal sequences of the guinea rise the reguibat rise the atlantic rise and the kasai angola massif are dominated by medium and high grade metamorphic rocks and with the exception of small greenschist facies sequences in central sierra leone the guinea rise are mostly devoid of gold mineralization however vein type gold lodes are more numerous in the low metamorphic grade greenstone belts of the north zaire massif and the tanzanian shield the important gold producing terranes are characterized by a complex tectonic history of strike slip and reverse dip slip shearing the productive lodes commonly being hosted by second or higher order shear zones these shear zones demonstrate an important component of late reverse dip slip activation or reactivation which provided optimum conditions for crustal de watering during prograde metamorphism and emplacement of tonalitic magmas the stratigraphic sequences and tectonic evolution are generally compatible with but do not unequivocally demonstrate cordilleran type accretionary regimes a link between gold metallogeny and the tectonothermal events that preceded and ultimately culminated in cratonization is commonly evident however cratonization in africa in a broad sense was diachronous ranging from ca 3 0 ga in the barberton greenstone belt through ca 2 6 2 5 ga in the zimbabwe and tanzanian cratons to the ca 2 0 ga eburnean event of west africa the latter contributing to the important gold mineralization of southwest ghana this multiplicity of tectonothermal events and the evidence for higher and more persistent crustal heat fluxes in the early precambrian terranes of west and equatorial africa have important ramifications for crustal evolution metallogeny and gold exploration

possible crustal contamination of midcontinent rift igneous rocks examples from the mineral lake intrusions wisconsin interlayered mafic telsic intrusions from the mineral lake intrusive complex in northwest wisconsin reflect the typical bimodal basalt rhyolite compositional pattern of the midcontinent rift flood

the ocean continent boundary off the western continental margin of iberia i crustal structure at 40 30 n summary the western continental margin of the iberian peninsula has the characteristics of a rifted non volcanic margin with half graben and tilted fault blocks seen in several places the ocean continent boundary ocb is therefore expected to be where thinned continental crust and oceanic crust are juxtaposed as at many similar margins worldwide it is particularly useful to locate the ocb off western iberia in order to constrain the pre rift fit of north america to iberia and by implication the shape of the proto bay of biscay the fit is only marginally constrained by sea floor spreading magnetic anomalies because anomaly 34 is believed to be far to the west of the ocb and it is even possible that all older oceanic crust was created during the cretaceous constant polarity interval the best way to distinguish oceanic crust from thinned continental crust appears to be the crustal seismic velocity structure therefore in 1986 a series of seismic refraction lines was shot parallel to and normal to the continental margin these lines enabled us to bracket the location of the ocb a further constraint on the location was obtained by modelling an east west magnetic profile which included the enigmatic j anomaly this anomaly can be explained as either just pre anomaly mo or as part of the cretaceous constant polarity interval depending on whether spreading began about 127 or after 118 myr ago respectively the evidence favours the former explanation lastly the depth to acoustic basement was contoured from a compilation of seismic reflection profiles this indicated a new fracture zone at 41 15 n which offsets the ocb a few key reflection profiles also suggest that the ocb can be identified by an abrupt landward step down in acoustic basement we conclude that the ocb in the eastern iberia abyssal plain lies between 12 10 and 12 30 w and has a trend just east of north this westerly location is consistent with recent estimates of the location of the ocb off the grand banks but brings into question the proposed location at about 11 w of the ocb in the tagus abyssal plain

extent of oceanic crust in the labrador sea1 this paper examines the mode of formation of the northern margins of the labrador sea observations of faulted and rotated crustal blocks across the northwest margin of greenland had led to the suggestion that this region is underlain by extended continental crust a detailed analysis of all geophysical data from this region presented here suggests on the contrary the possibility that it could have been formed by an excessively slow seafloor spreading process and ridge axis propagation this gave rise to rotated fault blocks at a time when spreading was mainly confined to the greenland side prior to its shift to the west supporting evidence for such a reconfiguration of the spreading axis comes from gravity and magnetic observations of this and neighbouring regions modelling of magnetic anomalies in the area based on the average rate of spreading for the entire labrador sea shows that a gradual shift of the ridge axis to the west started after the formation of anomaly 32 because the earliest seafloor spreading anomalies are highly attenuated in amplitudes other models like extended and intruded continental crust are possible if one only considers the magnetic anomalies however an extended continental crustal model poses problems in accounting for other geophysical observations based on a slow spreading model it is shown that oceanic crust extends fairly close to the foot of the slope with a narrow belt of extended continental crust along the west greenland margin while a much wider zone of perhaps attenuated continental crust extends off labrador breakup of the continental landmass occurred much closer to the west greenland coast than to labrador the applicability of a slow spreading model to other passive margins where similar geophysical observations have been reported is discussed

getting over continent ocean boundaries the idea of a simple linear boundary between continental and oceanic crust at extended continental margins is widely recognized to be an oversimplification despite this such boundaries continue to be mapped because of their perceived utility in palinspastic and plate kinematic reconstructions to examine whether this perception is justified we review the data and models on which basis continent ocean boundaries are interpreted and map a set of such interpretations worldwide from more than 150 publications the maps show that the location of the continent ocean boundary is rarely consistently estimated within the 10 100 km observational uncertainty that might be expected of the geophysical data used for doing so that this is the case regardless of whether the transition zone behind the boundary is classified as magma rich or magma poor and that the geographical separation of estimates exceeds the width of single study continent ocean transition zones the average of global maximum separations across sets of three or more estimates is large 167 km and mostly a consequence of interpretations published over the last decade we interpret this to indicate an extra component of uncertainty that is related to authors understanding of the range of features that are interpretable at extended continental margins we go on to discuss the implications of this uncertainty for palinspastic and plate kinematic modelling using examples from the literature and from the south atlantic ocean we conclude that a precise continent ocean boundary concept with locational uncertainty defined from the ensembles is of limited value for palinspastic reconstructions because the reconstruction process tends to bunch the ensemble within a region that is i of similar width to the observational uncertainties associated with continent ocean boundary estimates ii narrower than the regions of uncertainty about rotated features implied by the propagation of uncertainties from plate rotation parameters and iii coincident within all the above uncertainties with the more easily mapped continental shelf gravity anomaly secondly we conclude that estimated continent ocean boundaries are of limited use in developing or testing plate kinematic reconstructions because i reconstructions built using them as markers do not within uncertainty limits defined from the ensembles differ greatly from those using more easily determined bathymetric or gravity anomaly contours and ii because it is impossible to segment and date them with useful precision to use as markers of the edges of rigid oceanic lithosphere outside of the constraints of a pre existing plate kinematic model

pb isotopes in ascension island rocks oceanic origin for the gabbroic to granitic plutonic xenoliths abstract the pb isotopic compositions and u and pb concentrations of the lava series alkali basalt to comendite and of their plutonic xenoliths gabbro to alkaline granite of ascension island are reported the data are used to evaluate the source of the xenoliths which formed two differentiation suites the acidic and intermediate xenoliths together with most of the lavas on the one hand and the gabbroic xenoliths and a basaltic tuff on the other hand the pb isotopic compositions imply a mantle origin for the source magmas of the xenoliths and confirm the possibility of generating granitic rocks in an oceanic environment by fractional crystallization of a mantle derived magma whose geochemical and isotopic characteristics are comparable to the source magmas of oceanic island basalts

biogenic caliches in texas the role of organisms and effect of climate abstract biogenic constituents are ubiquitous and abundant in the caliches of texas investigation of 51 caliche profiles on various host strata alluvium limestone igneous rocks etc across approximately 900 km of texas from subhumid east to arid west has shown that 43 of these profiles exhibit prominent biogenic constituents these profiles exhibit significant differences in thickness varying from centimeters to meters and maturity varying from i to vi all of the different caliche facies are composed of low mg calcite biogenic features generally occur in the upper part of the profiles including the uppermost portion of massive caliche horizons platy horizons laminar crusts and pisoids the main biogenic caliche facies include rhizoliths calcified root structures stromatolite like laminar crusts and coated grains compared to the abiogenic massive micritic to microsparitic calcite groundmass biogenic constituents are morphologically distinct these biogenic constituents are composed of several microscopic mineral components including calcified filaments needle fiber calcite e g single crystalline needles and needle pairs triangular crystals and polycrystalline chains of rhombohedrons spherulites micro rods and nano spheres a large number of calcified root cellular structures and micro organisms e g fungal filaments actinomycetes and rod like bacteria are also present plant roots as well as soil biota produce distinctive structures and also enhance lithification by inducing calcite precipitation in the caliches i e biologically controlled or influenced processes host strata did not significantly influence the abundance nor type of biogenic features in the caliches in contrast climate had an evident effect on the development of biogenic constituents in these caliches in terms of the amount as well as type the thickness of laminar crusts and grain coatings and the abundance of biotic constituents within those facies decrease as the climate shifts from subhumid and subarid in southeast and central areas to subarid and arid in west and northwest texas in addition root structures and micro rods diminish significantly from the subhumid east to the arid west

ocean continent boundary in the iberia abyssal plain from multichannel seismic data abstract the ocean continent boundary of the west iberia margin is marked by a basement ridge trending n s four segments of this ridge are recognized each of them being progressively offset westward from 40 n to 43 n because the setting and seismic character of the ridge in the iberia abyssal plain are similar to those of the galicia margin ridge which is made of serpentinized peridotite we think that the southern segments of the ridge are also made of the same mantle material the segmentation of the ridge suggests that the northward propagation of the continental break up during the north atlantic opening in early cretaceous times was discontinuous each segment possibly corresponding to a propagation step east of the ridge the basement of the whole iberia abyssal plain consists of highly thinned continental crust locally resting on a seismic reflector that resembles the s reflector previously recognized off galicia by analogy with the galicia margin we propose that the tilted crustal blocks lay on serpentinized peridotite derived from the upper mantle the s reflector corresponding to the contact between crustal rocks and serpentinite

deep structure of the ocean continent transition in the southern iberia abyssal plain from seismic refraction profiles ocean drilling program legs 149 and 173 transect we present a wide angle seismic refraction study of an 80 40 km region of the southern iberia abyssal plain south of galicia bank an intersecting grid of two e w and four n s wide angle reflection refraction profiles is used to define variations of the basement velocity structure within this unusually wide ocean continent transition oct these structures can be systematically linked to variations in acoustic basement morphology and to results from ocean drilling program odp boreholes lateral changes in the velocity structure of the basement occur abruptly over distances of 20 km where complex variations may be found thinned upper continental crust 2 5 km thick with velocities of 5 0 6 6 km s is limited to a series of n s fault blocks immediately south of galicia bank this crust is underlain by a high velocity layer 7 3 7 9 km s of weakly serpentinized i e 0 25 peridotite which exists throughout the eastern part of the survey area basement within the oct appears to consist dominantly of a broad region of exposed upper mantle that has been serpentinized heterogeneously both vertically and horizontally in the southeast sector of our survey where basement topography deepens and becomes subdued continental fault blocks are absent instead basement contains an upper layer of more pervasively serpentinized i e 25 45 peridotite that is 2 km thick this layer is characterized by low velocity at the top of basement 4 2 km s that increases rapidly with depth and it probably corresponds to a seismically unreflective layer previously identified in reflection profiles to the south of our survey in the western section of our survey beneath a series of elevated basement ridges velocities are reduced within both the upper basement layer 3 5 6 0 km s and lower layer 6 4 7 5 km s these changes suggest that both upper and lower layers have become more highly serpentinized with values of 60 100 in the upper layer and 25 45 in the lower layer probably during the last stages of rifting and immediately before formation of oceanic crust a normal or slow spreading oceanic crustal structure is not found within the survey region thus it appears that the onset of seafloor spreading occurs in the region west of the peridotite ridge sampled at odp site 897 and east of the j magnetic anomaly

sem imaging of bacteria and nannobacteria in carbonate sediments and rocks abstract bacteria and nannobacteria 0 1 m dwarfed forms are abundant in both non etched and gently etched travertines deposited by hot springs in viterbo italy they are also plainly visible in bahama ooids and hardgrounds and in great salt lake aragonite ooids and cements in addition nannobacteria are found in paleozoic and mesozoic rocks because of their extraordinary abundance in some specimens they probably play a prominent role in catalyzing the precipitation of carbonate materials

a rendzina from the lower carboniferous of south wales a thin calcrete crust horizon from the lower carboniferous llanelly formation of south wales consists of two parts an upper laminated unit and a lower peloidal unit the former is interpreted as a subaerial stromatolite and the latter as an a horizon of a palaeosol comparisons are made with the a horizons of rendzinas and it is concluded that the calcrete crust represents a complete rendzina profile this fossil rendzina contains abundant evidence of a soil fauna in the form of fecal pellets and small burrows

thin crust at the western iberia ocean continent transition and ophiolites western iberia is bounded by a nonvolcanic rifted continental margin made up of three apparently independent segments the age of breakup decreases from south to north seismic refraction and reflection profiles and magnetic and gravity data from each segment show a consistent pattern of geophysical observations across the ocean continent transition oct zone which is a few tens of kilometers wide we emphasize here the discovery of thin 2 4 km oceanic crust underlain by 7 6 km s 1 material within the oct the available evidence favors the suggestion that the 7 6 km s 1 layer is serpentinized peridotite and that the thin oceanic crust is primarily the result of a poor magma supply for a few million years immediately after continental breakup this thin crust may be the source of some ophiolites which exhibit thin crustal sections and continental margin affinities

remobilisation of the continental lithosphere by a mantle plume major trace element and sr nd and pb isotope evidence from picritic and tholeiitic lavas of the noril sk district siberian trap russia the late permian to early triassic siberian traps have been sampled by drill core core sg 9 and from surface exposure section 1f in the noril sk region of the siberian platform russia combined major trace element and nd sr and pb isotope data on selected samples through the siberia trap offer new chemostratigraphic criteria for the identification and characterisation of two fundamentally different magma types and 9 of the 11 formations of lava developed near noril sk a lower sequence of sub alkalic basalts tholeiites and picritic basalts upwards these are the ivakinsky syverminsky and gudchichinsky formations are overlain by an upper sequence of picritic basalts and tholeiites interbedded with tuffs upwards these are the khakanchansky tuklonsky nadezhdinsky morongovsky mokulaevsky and kharayelakhsky formations the gudchichinsky and tuklonsky formations contain both picritic and tholeiitic lavas the tuklonsky formation tholeiites and picrites have moderate gd yb 1 6 1 8 low tio2 0 45 0 95 wt a significant negative ta and nb anomaly nb la 0 42 0 57 and unradiogenic nd nd chur to 4 6 in contrast both the gudchichinsky formation tholeiites and picrites have high gd yb 2 3 3 1 and tio2 1 2 2 3 wt no significant nb or ta anomaly nb la 0 8 1 1 and radiogenic nd nd chur to 7 3 the low ti and nb la high la sm and unradiogenic nd isotope signatures of the picritic tuklonsky formation lavas and the tholeiitic lavas of the upper sequence are characteristic of magmas strongly influenced by material from the continental lithosphere whereas the high ti and nb la low la sm and radiogenic nd isotope signatures of the lower sequence are more comparable to deeper asthenospheric mantle plume generated lavas similar to oceanic island basalts the lavas overlying the tuklonsky formation have mg numbers of 0 63 to 0 68 and are more evolved than the tuklonsky mg number 0 62 and have more radiogenic nd chur tuklonsky 0 03 to 4 66 mokulaevsky 0 60 to 1 61 but have many of the incompatible trace element features of the tuklonsky sky type magma these lavas show a progressive upwards decline in sio2 55 49 wt la sm 4 6 2 0 and ur sr 67 to 13 which has previously been attributed to a decrease in the proportion of crustal material contributed to the magma this paper explores and alternative model where a component of the crustal contribution might be derived from within an ancient region of the mantle lithosphere as recycled sediment rather than from the overlying continental crust

stable carbon and oxygen isotopes in quaternary soil carbonates as indicators of ecogeomorphic changes in the northern chihuahuan desert usa stable carbon and oxygen isotopes in soil carbonates provide an additional technique for investigating quaternary ecogeomorphic changes in arid and semiarid regions this study presents 13c and 18o values in surface and buried soils in a basin and range region of southern new mexico and west texas three landform categories were investigated for their isotopic content 1 middle piedmont slopes 2 a lower piedmont slope and adjacent playa depressions and 3 an intermontane basin floor soils of the middle piedmont slopes had greatest 13c variability ranging from 0 6 in buried soils to 11 1 pdb in surface soils soils of the basin floor had greatest variability in 18o values ranging from 0 6 to 7 6 pdb fossil pollen patterns roughly paralleled 13c shifts with cheno am pollen exhibiting greatest changes both isotope and pollen data across stratigraphic discontinuities suggest that a conversion from c4 grasses to c3 desert shrubs accompanied holocene erosion

insight into the nature of the ocean continent transition off west iberia from a deep multichannel seismic reflection profile a high quality multichannel seismic reflection profile across the west iberia margin confirms the existence of a 130 km wide transition zone between tilted continental fault blocks and unequivocal oceanic crust the zone is characterized by a 1 0 2 5 km thick seismically unreflective upper basement layer lying over reflective basement that contains major landward dipping reflectors that mostly truncate against the base of the upper layer and penetrate to depths of 10 19 km below the basement surface and minor seaward dipping reflectors moho reflections are absent in this zone we suggest that the unreflective layer represents tectonically exposed upper mantle that has undergone extensive serpentinization enhanced by vigorous seawater circulation about 40 km farther north the transition zone narrows to 35 km and the unreflective layer is less continuous and widespread

the influence of source rock type chemical weathering and sorting on the geochemistry of clastic sediments from the quetico metasedimentary belt superior province canada abstract the geochemistry of clastic sediments is controlled by at least four factors source rock composition weathering of the source rocks sorting of the weathered detritus and post depositional alteration during diagenesis and metamorphism in order to evaluate the relative importance of these factors major trace and rare earth element contents were determined for distal and proximal clastic turbidites from the archaean quetico metasedimentary belt the degree of metamorphism of the samples ranges from middle greenschist to upper amphibolite facies simple mixing of archaean mafic and acidic rocks of similar composition to those found in the adjacent granite greenstone terrains will account for the tio2 feot mgo cr sc y and hree contents of the clastic turbidites the metasediments are enriched in lree relative to the mafic acidic rock mixing line indicating that the acidic source was enriched in the lree compared to archaean terrains from other parts of the world some quetico turbidites have an additional fe rich source component possibly a chemical sediment incorporated at the time of turbidite slumping cao na2o k2o rb and sr form trends on bivariant plots that are oblique to the mafic acidic rock mixing line and hence cannot be accounted for by simple mixing of the source rocks chemical weathering of feldspars prior to deposition can account for cao na2o k2o rb and sr trends since cao na2o and sr are lost to the weathering solutions and k2o and rb are retained on the product clays the higher na 2 o cao k 2 o na 2 o rb k 2 o and sr cao ratios but lower cao content of the distal samples suggest that they were derived from a bulk sediment that had experienced a greater degree of chemical weathering than the proximal sediments sorting is the factor that most strongly influences the composition of individual samples within the proximal sample group the coarse grained metagraywackes with their higher quartz feldspar and heavy mineral contents are enriched in sio2 cao na2o sr lree zr and hf relative to the fine grained clay rich metapelites metapelites are enriched in tio2 al2o3 feot mgo k2o cr zn sc y hree cs and rb as a result of their higher clay contents thus the use of clastic sediment compositions in determining average crustal compositions e g for the ree can only give satisfactory results when all grain size fractions are sampled in their correct relative proportions the chemical differences observed between the high and low grade samples can be accounted for by source rock type chemical weathering of the source rocks or sorting post depositional element mobility does not appear to be an important factor in determining the composition of the quetico metasediments

gabbroic pegmatite intrusions iberia abyssal plain odp leg 173 site 1070 magmatism during a transition from non volcanic rifting to sea floor spreading the unroofed mantle grows younger outboard just as is the case for on the iberia abyssal plain ocean drilling program site 1070 normal volcanic sea floor gabbroic pegmatites and related rocks 127 4 ma u pb zircon intrude upper mantle that was subsequently exposed and serpentinized during early cretaceous non volcanic rifting the pegmatites include a 3 4 m dike or sill the main pegmatite numerous dikelets of

evidence from ocean drilling program leg 149 mafic igneous rocks for oceanic crust in the iberia abyssal plain ocean continent transition zone leg 149 of the ocean drilling program explored the ocean continent transition oct on the iberia abyssal plain and its role in the opening of the atlantic ocean approximately 130 ma mafic igneous rocks recovered from holes 899b and 900a have mid ocean ridge basalt morb trace element and isotopic characteristics indicating that a spreading center was active during the opening of the iberia abyssal plain oct the hole 899b weathered basalt and diabase clasts have transitional to enriched morb rare earth element characteristics and the hole 900a metamorphosed gabbros have morb initial epsilon nd values between 6 and 11 during the opening event the iberia abyssal plain oct is envisioned to have resembled the central and northern parts of the present red sea with localized spreading centers and magma chambers producing localized patches of morb mafic rocks the lack of a normal ocean floor magnetic anomaly pattern in the iberia abyssal plain means that a continuous spreading center similar to that observed in the present southern red sea was not formed before spreading ceased in the iberia abyssal plain oct and jumped to the present mid atlantic ridge

hydrothermal alteration in oceanic ridge volcanics a detailed study at the galapagos fossil hydrothermal field abstract the galapagos fossil hydrothermal field is composed of altered oceanic crust and extinct hydrothermal vents within the eastern galapagos rift between 85 49 w and 85 55 w the discharge zone of the hydrothermal system is revealed along scarps thus providing an opportunity to examine the uppermost mineralized and highly altered interior parts of the crust altered rocks collected in situ by the submersible alvin show complex concentric alteration zones microsamples of individual zones have been analysed for major minor trace elements and strontium isotopes in order to describe the complex compositional details of the hydrothermal alteration interlayered chlorite smectite and chlorite with disequilibrium compositions dominate the secondary mineralogy as replacement phases of primary glass and acicular pyroxene phenocrysts and matrix grains of plagioclase are unaffected during alteration using a modification of the gresens equation we demonstrate that the trivalent rare earth elements rees are relatively immobile and calculate degrees of enrichment and depletion in other elements strontium isotopic ratios increase as sr concentrations decrease from least altered cores to most altered rims and cross cutting veins in individual samples and can be modeled by open system behaviour under low fluid rock ratio

terrestrial stromatolites and laminar calcretes a review abstract laminated carbonates are prominent features of subaerial exposure profiles and many types of calcareous soils including calcretes previously much attention was given to differentiating these laminites from marine or lacustrine microbial stromatolites and oncoids however recent research has shown that many subaerial laminated carbonates are biogenically formed mainly but not wholly by microbial activity this paper reviews the criteria for distinguishing terrestrial carbonate laminites from non terrestrial stromatolites and oncoids and for differentiating biogenic from abiogenic terrestrial laminites

petrology of the east pacific rise crust and upper mantle exposed in hess deep eastern equatorial pacific the hess deep a rifted oval shaped depression located east of the galapagos triple junction at the tip of the cocos nazca ridge about 101 w 2 n was explored in 1988 during 21 submersible dives a total of 11 dives were devoted to the exploration of the e w trending intrarift ridge 15 km in length 3000 5400 m in depth north of the hess deep depression the intrarift ridge represents an outcrop of recent 1 m y crustal and subcrustal material created at the axis of the east pacific rise epr and emplaced during the lithospheric extention responsible for the westward propagation of the cocos nazca rift francheteau et al 1990 the lithospheric block has undergone cataclastic deformation and was dislocated by tectonic activity as attested to by the mixed and erratic distribution of rock types and by the occurrence of polygenic breccias and gabbroic mylonites the samples are metamorphosed to varying degrees but their protolith textures are generally well preserved their relic mineralogy indicates that they consist of harzburgites dunites gabbroic cumulates gabbronorites and olivine gabbros isotropic gabbros dolerites and basalts some samples of refractory harzburgites and most dunitic cumulates with local accumulation of chromite have been impregnated by wehrlitic and gabbroic primitive melts similar to those described from the mantle crust transition zone of the samail ophiolite complex oman the mineral chemistry indicate that the ultramafics partly reequilibrated with the magmatic impregnations in the liquidus solidus temperature range of 980 1100 c the dolerites and basalts have been derived from mid ocean ridge basalt primary melts presenting a broad range of incompatible element composition which suggests intermittent cycles of magmatic processes involving a progressive melting of a composite source with discontinous extraction of liquids as proposed for the epr volcanics near 13 n hekinian et al 1989 most of the rocks underwent partial retrograde metamorphism and recorded several episodes of recrystallization from the upper greenschist facies ultramafics and gabbros to diagenetic alteration volcanics the cumulate gabbronorites the isotropic gabbros and some dolerites were partially albitized and amphibolitized during the penetration of seawater in the ocean crust prior to serpentinization several samples of unfoliated amphibolites are believed to be completely metamorphosed gabbroic rocks the gabbroic cumulates and the plagioclase rich melt impregnations were variably rodingitized presence of various ca silicates such as epidote prehnite hydrogarnet and zeolite in relation to the serpentinization of the peridotites one dive located on the scarps forming the northern wall of the hess deep to the east of the explored area revealed the presence of in situ outcrops of isotropic gabbros doleritic dikes and extrusives and permitted to observe the contact between the sheeted dike complex and the high level isotropic gabbros

cumulate gabbros from the southwest indian ridge 54 s 7 16 e implications for magmatic processes at a slow spreading ridge a diverse volcanic and plutonic rock suite was recovered from the center of the 80 km long ridge segment of the southwest indian ridge 54 s 7 16 e between the islas orcadas and shaka fracture zones the cumulus nature of the gabbroic rocks in the suite is indicated by phase modal and cryptic layering igneous lamination and low incompatible element abundances we present a mass balance model for calculating the proportions and compositions of cumulus phases and crystallized intercumulus liquid from bulk rock major element compositions the model is based on the ability to define a compositional array of basaltic liquids and on the assumption that cumulus minerals are initially in equilibrium with trapped liquid calculated proportions of trapped liquid range from 3 15 values that are characteristic of adcumulates to mesocumulates models of postcumulus crystallization indicate significant enrichments of incompatible elements and buffering of compatible elements in residual trapped liquids thus explaining the high tio2 contents observed in magnesian clinopyroxenes cumulus phase assemblages and compositions suggest solidification in shallow level magma chambers but disequilibrium plagioclase compositions suggest some crystallization at greater depth furthermore basalt compositions projected onto the olivine clinopyroxenequartz pseudoternary suggest magma generation over a range of pressures from less than 10 to greater than 20 kb as well as polybaric fractional crystallization we suggest that the southwest indian ridge is characterized by low magma supply with small batches of melt that either ascend directly to the surface having undergone limited polybaric crystallization or are trapped in shallow crustal magma chambers where they evolve and solidify to form cumulate gabbros the adcumulus nature of the gabbros investigated here suggests slow cooling rates typical of large intrusions implying relatively large but ephemeral magma chambers below segments of the southwest indian ridge

archaean greenstone belt from the central african republic equatorial africa abstract the bandas belt one of two prominent archaean greenstone belts in the central african republic equatorial africa is ca 250 km long at the southernmost part of the belt a metasedimentary metavolcanic rock suite is preserved only in brachysynclines the suite can be divided into two lithostratigraphic units the lower unit is composed predominantly of volcanic rocks while the upper one contains mainly metasedimentary rocks the volcanic rocks which are part of a sequence ca 3600 m thick can be sub divided according to stratigraphic position lithology and geochemistry into three groups the lowermost group includes low k tholeiitic basalts depleted in light ree the second group consists of tholeiitic basalts with light ree enriched patterns and the third uppermost group includes andesites which are similar in several respects to recent calc alkaline andesites the tholeiitic basalts of the first two groups are probably related to different upper mantle sources the andesites of the third group were produced either by fractional crystallization from a basaltic magma enriched in light ree or equilibrium melting of eclogite or garnet amphibolite

the role of fungal biomineralization in the formation of early carboniferous soil fabrics paleosols in the lower carboniferous limestones of south wales commonly contain needle fibre calcite which is an unusual form of calcite recently shown to form by the calcification of fungal hyphae in present day soils the needle fibre calcite occurs in two associations in the paleosols as coatings on sediment grains and as rhizocretions the former can be compared with the microbial grain coatings of quaternary calcretes the latter represent the sites of fungal coats on roots and are interpreted as probable ectomycorrhizae a symbiotic fungal sheath root association these findings suggest that biomineralization was important in the formation of soil fabrics during the carboniferous as it is in present day soils

electron backscatter diffraction documents randomly orientated c axes in moonmilk calcite fibres evidence for biologically induced precipitation moonmilk represents a conspicuous but controversially discussed precipitate of cave settings here new electron backscatter diffraction microscopic and petrographic evidence on the origin of moonmilk calcite is presented calcite fibres in a moonmilk mat from the walls of an active cave tunnel cave in devonian massive limestones in the northern part of the rhenish massif germany show orientations of the crystallographic c axis independent of the morphological fibre orientation this observation and the morphology of the fibres are in agreement with microbially induced as opposed to abiogenic calcite precipitation carbon isotope data are higher 1 9 to 3 3 than those commonly measured regionally in speleothem calcite stalagmites stalactites and flowstones an observation attributed to kinetic effects in combination these findings add independent evidence to the complex interplay of inorganic and bio induced carbonate precipitation in cave environments

deep structure of the ocean continent transition in the southern iberia abyssal plain from seismic refraction profiles the iam 9 transect at 40 20 n we present a crust and mantle velocity structure for the west iberia passive continental margin derived from a 320 km long wide angle seismic profile acquired in the southern iberia abyssal plain we observe a 170 km wide ocean continent transition zone which includes a pair of overlapping peridotite ridges and is bounded by oceanic crust and landward by fault bounded blocks of continental crust the profile lies 40 km south of the transect sampled by ocean drilling program odp legs 149 and 173 the transition zone structure can be divided into an upper layer 2 4 km thick with velocities of between 4 5 and 7 0 km s 1 and generally a high velocity gradient 1 s 1 and a lower layer up to 4 km thick with a velocity of 7 6 km s 1 and a low velocity gradient a weak moho reflection in this zone was seen only on wide angle profiles at an offset of 30 km the upper layer has a distinctly lower velocity than thinned continental crust adjacent to the continental slope conversely the lower layer has too high a velocity to be magmatically intruded or underplated lower continental crust on the coincident seismic reflection profile fault bounded crustal blocks identified in unequivocal extended continental crust are not observed in the transition zone the upper layer has velocity bounds and gradient similar to oceanic layer 2 observed west of the peridotite ridges but no oceanic layer 3 velocity structure is present while magnetic anomalies have been identified within the transition zone they have not been modeled successfully as seafloor spreading magnetic anomalies nor do they generally form long linear margin parallel features finally odp boreholes 40 km north of our profile and within the interpreted transition zone have recovered up to 140 m thick sections of serpentinite and serpentinized peridotites with little evidence of mafic igneous material we conclude that the transition zone cannot be dominantly composed of either extended continental crust or oceanic crust although current melting models predict a considerably thicker crust of decompression melt products we interpret this region as exposed upper mantle peridotite with little or no synrift extrusive material and limited amounts of synrift material intruded within the serpentinized peridotite

travertines depositional morphology and the bacterially constructed constituents abstract investigation of travertine accumulations throughout central italy and the west central u s has shown that the carbonate is precipitated in response to both inorganic and organic processes individual deposits range up to 85 m thick and hundreds of square kilometers in areal extent all of the carbonate is low magnesian calcite water chemistry temperature and morphology of the accumulation greatly influence the constituents comprising these deposits harsh environmental situations favor inorganic deposits while increasingly more moderate conditions result in the formation of a greater abundance of organically precipitated material morphological variations of travertine deposits recognized include 1 waterfall or cascade 2 lake fill 3 sloping mound fan or cone 4 terraced ound and 5 fissure ridge gross morphology internal stratification and constituents comprising these deposits vary systematically depending upon the type of accumulation and chemistry of the waters bacterially precipitated calcite forms a large percentage of the carbonate in many travertine accumulations exceeding 90 of the framework grains comprising some of the lake fill deposits bacteria are among the first taxa to inhabit and reproduce in harsh spring environments and produce a variety of fascinating constituents the bacteria are primarily rods generally 0 2 m in diameter and less than 1 0 m in length the rods readily decay resulting in calcite crystals loaded with micropores the basic building block of bacterially constructed travertine is a clump of bacteria averaging 20 m in diameter enclosed in a single crystal of calcite aggregates of these crystals produce a variety of deposits including 1 crudely laminated carbonate mud 2 finely laminat d layers of mud 3 intraclasts 4 foam rock and 5 shrubs the shrubs are most striking commonly forming layers 1 3 cm thick but also producing bacterial pisoids at some locales bacterial stromatolites composed of layers of shrubs alternating with finely laminated layers of bacterial mud comprise essentially the entire deposit the shrub layers are the result of flourishing summer growth of bacteria and furthermore show remarkable daily laminae 0 1 0 5 mm thick the importance of bacteria in the formation of travertine and their universally recognized abundance in modern sediments provides an impetus for a reexamination of the role of bacteria in the origin of other types of ancient deposits

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

tectonic implications of exposure of lower continental crust beneath the iberia abyssal plain northeast atlantic ocean geophysical evidence we present new seismic data from a basement high beneath the southern iberia abyssal plain that appears from the latest ocean drilling program odp results and indirectly from various seismic observations to consist of lower continental crustal material this is a unique opportunity to use seismic profiles and other geophysical evidence to investigate the tectonic process which led to the exhumation of these rocks we infer that the lower crust was exhumed at the seafloor and then uplifted toward the end of rifting our results lead to the following new observations low angle detachment faults previously reported along the east west lusigal 12 profile across the southern iberia abyssal plain penetrate both the 3 10 km thick continental crust and unusually the uppermost mantle processing of new multichannel seismic reflection profiles across the basement high on which odp sites 900 1067 and 1068 are located reveals that the high is bounded by a previously unsuspected steep landward dipping normal fault on its east flank basement cores from the above odp sites also reveal that the high is not capped by the previously predicted early syn rift sediment but by rocks from the lower continental crust these unexpected observations are incorporated into a new tectonic model of the development of this part of the west iberia margin which is also consistent with other geophysical observations a novel feature of this model is the proposal that the seaward edge of the continental crust is thinned to 3 6 km by a currently poorly understood process the model implies three stages of deformation 1 lithospheric extension principally by symmetric pure shear which leads to a 10 km thick crust in which the lower crust is largely absent over the axial zone and the crust mantle boundary forms a shear zone 2 further thinning and then dissection of the most distal continental crust by seaward dipping low angle normal faults 3 inception of a high angle landward dipping normal fault that offsets the tectonized crust mantle boundary and uplifts the lower crust to the crest of the basement high on which odp sites 900 1067 and 1068 were drilled

first alkaline magmatism during iberia newfoundland rifting the age and origin of alkaline rocks emplaced into the sediments of the rift related continental lusitanian basin were investigated to constrain earliest magmatic activity occurring prior to oceanic plate formation between iberia and newfoundland the u pb titanite ages are 146 5 1 6 2 sterr 145 3 1 4 and 142 3 1 0 ma and initial pb isotopic ratios of feldspars lie at 18 418 18 978 for 206pb 204pb at 15 594 15 925 for 207pb 204pb and at 37 105 39 216 for 208pb 204pb initial 87sr 86sr ratios measured in the same feldspar fractions lie at 0 705409 0 706462 this episode of magmatic activity lasting for at least 4 2 2 6 myr most likely marks a phase of maximum lithospheric thinning during which zones of weakness were created to allow deep magmas to reach the surface such zones are preferentially re activated palaeozoic faults of the iberian plate the isotope data suggest that the dominant volume of alkaline magmas was generated by partial melting of the metasomatized subcontinental iberian mantle

the biogenic origin of needle fibre calcite needle fibre calcite is one of the most ubiquitous habits of calcite in vadose environments caves deposits soil pores etc its origin either through inorganic indirect or direct biological processes has long been debated in this study investigations at 11 sites in europe africa and central america support arguments for its biogenic origin the wide range of needle morphologies is the result of a gradual evolution of the simplest type a rod this rod is the elementary brick which by aggregation and welding builds more complex needles the absence of cross welded needles implies that they are welded in a mould or under a longitudinal and unidirectional constraint before being released inside the soil pores the difference between the lengthening of the needles and the c axis can be explained by the existence of needles observed under a scanning electron microscope in organic sleeves which can act as a mould during rod growth complex morphologies with epitaxial outgrowths on straight rods cannot have grown entirely inside organic microtubes they must result from soil diagenesis after the release of straight rods in a soil free medium whisker crystals are interpreted as the result of growth and coalescence of euhedral crystals on a rod rhomb chains are considered to be the consequence of successive epitaxial growth steps on a needle during variations in growth conditions isotopic signatures for needle fibre calcite vary from 16 63 to 1 10 and from 8 63 to 2 25 for 13c and 18o respectively the absence of high 18o values for needle fibre calcite precludes a purely physicochemical origin evaporative for this particular habit of calcite as epitaxial growth cannot precipitate in the same conditions as initial needles needle fibre calcite stable isotopic signatures should be used with caution as a proxy for palaeoenvironmental reconstructions in addition it is suggested that the term needle fibre calcite should be kept for the original biogenic form the other habit should be referred to as epitaxial forms of needle fibre calcite

geochemistry of north shore hypabyssal dikes and sills in the midcontinent rift of minnesota an example the 47th avenue sill this study presents geochemical data for several of the numerous small to large dikes and sills including the 47th avenue sill exposed along the shore of lake superior in and north of duluth minnesota these intrusions are late magmatic features of the proterozoic midcontinent rift system and together form the north shore hypabyssal group the dikes are geochemically distinct from the sills and when the two are exposed together the younger dike intrudes the older sill dikes are primitive with mg up to 68 have positive end values and are oriented approximately north south with steep westerly or near vertical dips the older sills are more evolved usually have end values near or below 0 and have the same gentle easterly dip as the thick sequence of north shore volcanic group flows they intrude dike compositions correlate best with a mixture of widespread basalt compositions types 4 and 5 with primitive geochemistry and positive end values whereas sill compositions are similar to widespread ba