u pb evidence for polymetamorphic history of huronian rocks within the grenville front tectonic zone east of sudbury ontario canada abstract this paper presents u pb geochronological data for zircon titanite monazite apatite allanite and rutile from street township east of sudbury that help unravel two main facets of the geology of the grenville front tectonic zone gftz they document the presence of extensive huronian supergroup magmatic ca 2480 2460 ma and sedimentary rocks inside the gftz and highlight the significance of bimodal magmatism in the genesis of the huronian supergroup the data also provide further insights into the complex orogenic evolution of the gftz showing in particular that amphibolite facies metamorphism migmatization and deformation during grenvillian compression at 995 980 ma overprinted but did not totally erase the memory of earlier metamorphic events at ca 1720 and 1470 ma from the point of view of u pb systematics the results emphasize the nearly ubiquitous state of isotopic disequilibrium in these rocks most of the minerals investigated comprise two or more generations and are generally in a state of partial resetting because of the superimposed effects of younger events the preservation of pre grenvillian ages in titanite and allanite demonstrates that processes related to final grenvillian collision and exhumation occurred at great speed only apatite and to some degree rutile yield ages that are clearly post tectonic thereby reflecting the final stages of exhumation and cooling of the region

metallogeny of the grenville province revisited four new petrogenetic and metallogenic models are proposed herein to explain the formation of important mineral deposits in the grenville province providing a framework from which to reappraise grenvillian mineral potential recognition of a high pressure metamorphic belt within the grenville province suggests a potential for eclogite hosted rutile deposits an important and much sought commodity a recently developed norwegian model proposes that anorthosite genesis occurred through lower crust underplating and coeval partial melting rather than by plume magmatism applied to the grenville province the new petrogenetic model may provide insight into the widespread occurrence of platinum group element pge poor nickel showings and the distribution of chromite ti rich and low ti iron oxide deposits within the grenville and adjacent terranes a new type of sedimentary exhalative sedex mineralization formed by oxidized brines has been defined following the discovery of new deposits in australia appl

effects of overbank deposition on the quality and maceral composition of the herrin no 6 coal pennsylvanian of southern illinois abstract a detailed study of the effects of overbank deposition on the quality and maceral composition of the herrin no 6 coal member carbondale fm middle pennsylvanian in southern illinois was done using proximate total sulfur and maceral analyses channel samples were taken of the herrin no 6 coal and descriptions of the rocks overlying the coal were made at nine locations the herrin no 6 coal in the study area is known to 3ave a high ash content toward the walshville channel a channel sandstone possibly representing a distributary channel that existed during herrin peat deposition sulfur content is knwon to in crease markedly where marine or marginal marine rocks directly overlie the coal and decrease where overlain by thick non marine shale three types of overbank deposits are found in the study area accounting for the changes in coal quality and composition overbank flooding as sheet flow off the levee of the walshville channel introduced suspended material into the peat swamp the suspended material either produced numerous shale laminae or intermixed with the peat producing an increased ash content of the resultant coal close to the walshville channel contemporaneous with the introduction of clastic material by overbank flooding partial oxidation of the peat occurred oxygenated flood water was able to oxidize the peat increasing the pseudovitrinite content of the resultant coal fire splay deposits forming the partings occurred when the swamp was flooded after a dry period or a swamp fire the build up of crevasse splay deposits forming the energy shale helped decrease sulfur content of the herrin no 6 coal in portions of the study area the non marine shaly distal crevasse splay facies formed an impermeable barrier to downward movement of sulfate ion bearing marine water during deposition of the marine units above the energy shale high sulfur coal is associated with the sandy crevasse distributary channel fill facies and bay fill facies of the energy shale or where the energy shale is not present the crevasse distributary channel fill facies allowed marine water to permeate through it reaching the peat and resulting in high sulfur coal high sulfur coal found where the bay fill facies or other marine units directly overlie the coal is a result of marine water being able to permeate the peat directly

geology of the mineralized zone of the wanapitei complex grenville front ontario the wanapitei complex 6 km 2 5 km lying 0 4 km southeast of the grenville front consists of a northwestern zone of gabbro and folded injection breccia and a southeastern layer of intensely folded hornblendeplagioclase gneiss disseminated ni cu sulphides are unevenly distributed in a zone between the injection breccia and the folded gneiss

geology of the east bull lake gabbro anorthosite pluton algoma district ontario the east bull lake pluton a layered gabbro anorthosite intrusion 90 km west of sudbury ontario is in an inward dipping lopolith and is 780 m thick in the centre and elliptical in plan view 13 5 km 3 5 km it intruded archean metavolcanic and metaplutonic rocks of the superior province during the early proterozoic 2480 ma the intrusion consistes of a basal anorthositic unit succeeded by rhythmic layered gabbro troctolite layered gabbro and massive and dendritic gabbro units it is offset by the west northwest striking subvertical folson lake fault south of this fault only anorthosite and massive and dendritic gabbro are exposed north of the fault subsurface lithologies intersected by me boreholes correlate with surface lithologies troctolite the most distinctive marker that can be correlated between boreholes and surface exposures confirms the general attitude and shape of the layers and lopolith chemical composition of the intrusion ranges from high mg tholeiite to calc alkaline high

chalcophile and platinum group element pge concentrations in the sulfide minerals from the mccreedy east deposit sudbury canada and the origin of pge in pyrite magmatic sulfide deposits consist of pyrrhotite pentlandite chalcopyrite pyrite and platinum group minerals pgm understanding the distribution of the chalcophile and platinum group element pge concentrations among the base metal sulfide phases and pgm is important both for the petrogenetic models of the ores and for the efficient extraction of the pge typically pyrrhotite and pentlandite host much of the pge except pt which forms pt minerals chalcopyrite does not host pge and the role of pyrite has not been closely investigated the ni cu pge ores from the south range of sudbury are unusual in that sulfarsenide pgm rather than pyrrhotite and pentlandite are the main carrier of pge probably as the result of arsenic contribution to the sulfide liquid by the as bearing metasedimentary footwall rocks in comparison the north range deposits of sudbury such as the mccreedy east deposit have as poor granites in the footwall and the ores commonly contain pyrite our results show that in the pyrrhotite rich ores of the mccreedy east deposit os ir ru rh ipge and re are concentrated in pyrrhotite pentlandite and surprisingly in pyrite this indicates that sulfarsenides which are not present in the ores were not important in concentrating pge in the north range of sudbury palladium is present in pentlandite and together with pt form pgm such as ptpd tebi 2 platinum is also found in pyrite two generations of pyrite are present one pyrite is primary and locally exsolved from monosulfide solid solution mss in small amounts 2 wt together with pyrrhotite and pentlandite this pyrite is unexpectedly enriched in ipge as pt and the concentrations of these elements are oscillatory zoned the other pyrite is secondary and formed by alteration of the mss cumulates by late magmatic hydrothermal fluids this pyrite is unzoned and has inherited the low concentrations of ipge and re from the pyrrhotite and pentlandite that it has replaced

the nature age and petrogenesis of the cartier batholith northern flank of the sudbury structure ontario canada abstract the cartier batholith exposed about 50 km northwest of sudbury ontario canada is a late archean k rich granitic batholith along with the levack gneiss complex it forms the footwall of the north range of the sudbury igneous complex the cartier batholith dominantly monzogranitic to granodioritic is relatively homogeneous unfoliated and contains k feldspar megacrysts it shows several distinctive chemical features including high concentrations of zr th u and lree and low concentrations of nb ti and hree several discriminants suggest a post orogenic tectonic environment for the batholith this inference is consistent with a u pb age of 2642 1 ma obtained on zircon only slightly younger than the inferred timing of high grade metamorphism of the levack gneiss complex the cartier batholith could have formed as a result of approximately 33 partial melting of the levack gneiss complex

a special issue devoted to the mineral deposits of the sudbury basin the magmatic ni cu platinum group element pge deposits in the sudbury area were discovered in 1883 giblin 1984 as ni has grown from a chemical curiosity through early use in armor plating to its present day pervasive role in modern industry resources have continued to be discovered around the periphery of the sudbury igneous complex and even richer ore zones have been identified in the footwall rocks making sudbury one of the world s largest and most productive mining camps the premining resource production current resource of the sudbury mining camp has been estimated to be 1 648 million metric tons at 1 2 percent ni and 1 0 percent cu naldrett and lightfoot 1993 as of 2000 the camp had produced 9 69 million metric tons of ni 9 59 million metric tons of cu 69 6 thousand metric tons of co 116 metric tons of au 319 metric tons of pt 335 metric tons of pd 37 6 metric tons of rh 23 3 metric tons of ru 11 5 metric tons of ir 3 710 metric tons of ag 3 002 metric tons of se and 256 metric tons of te from 781 million metric tons of ore ontario ministry of northern development and mines information and marketing services section 2002 sudbury remains one of the world s largest producers of ni with an annual production of 114 thousand metric tons plus 137 thousand metric tons of cu 1 8 metric tons of au and 11 4 metric tons of total platinum group minerals ontario ministry of northern development and mines information and marketing services section 2000 because of its great economic significance the sudbury basin has been studied intensively by local mining companies federal and provincial survey geologists and numerous university researchers landmark papers include the post war synthesis by stevenson 1963 the first recognition of the impact origin of the structure by dietz

petrographical palynological and geochemical analyses of the hub and harbour seams sydney coalfield nova scotia canada implications for facies development abstract benched channel samples and continuous block samples of the hub and harbour seams from the sydney basin nova scotia were studied to define the variations in petrography geochemistry and palynology of the seams and to interpret the depositional environments of the precursor mires petrographic compositions of the seams are very similar with high vitrinite content low inertinite moderate liptinite and negligible clastic mineral content pyrite content is moderate there is little variation in petrographic composition between benches in either seam there are marked variations in petrographic compositions at the lithotype scale petrographic facies parameters indicate deposition of the bulk of both seams under fairly uniform conditions in relatively wet mires unusually wet or dry periods were rare the palynological assemblages of both seams indicate a middle to late westphalian d age the common to abundant palynomorphs are derived from arborescent and herbaceous lycopsids tree ferns and other ferns and sphenopsids the palynological assemblages support the petrographic interpretation of relatively wet mires cordaites pollen florinites is rare suggesting that there was little contribution from plants growing in dry or upland areas the absence of the densospore miospore phase and the petrographic data indicate there were no periods in which domed mires occurred the distinctive dull bands occurring in the hub and harbour seams are characterised by low vitrinite contents high inertinite content high liptinite and common detrital minerals sporinite showing varying degrees of degradation is commonly highly concentrated in thin bands the geochemistry is marked by much higher hi values than normally reported for coals but other indices are typical for coals of this rank the palynological assemblages of the dull bands are dominated by tree fern spores and other ferns with only a minor contribution by sphenopsid spores the dull bands were deposited during periods of inundation of mires relatively low mineral content indicates that the distributary channels which flooded the mires were quite distant from the depositional sites of the samples

observations on the nature and origin of fusain abstract evidence for a wildfire origin of fusain is examined charcoal produced by natural fires and artificial charring resembles fossil fusain both in physical size and shape and in optical reflectance artificially charred leaves and naturally burnt wood are illustrated using the sem reflectance studies on modern charcoals suggest that much semifusinite may be produced by pyrolysis the ecological implications of a wildfire origin of fusain are discussed and it is concluded that fires have been a feature of terrestrial ecosystems from at least the late devonian

evolution of palaeoproterozoic mafic intrusions located within the thermal aureole of the sudbury igneous complex canada isotopic geochronological and geochemical evidence abstract impact cratering and their resultant geological phenomena are recognised as significant factors in the lithological and biologic evolution of the earth age dating of impact events is critical in correlating cause and effects for these catastrophic processes the falconbridge and drury township twp intrusions were emplaced at the contact between neoarchaean basement and palaeoproterozoic volcanosedimentary rocks and also lie at the southeast and southwest edges of the sudbury igneous complex sic within its thermal contact aureole the falconbridge twp intrusion is dated at 2441 3 ma by u pb zircon with evidence of archaean inheritance from its host granitoids granitoids from the southernmost abitibi subprovince are dated here between 2670 11 ma for an undeformed algoman granite and 2696 18 ma for a foliated granitoid consistent with existing data from the abitibi greenstone belt and from the wawa subprovince major and trace element geochemical evidence common pb isotopic compositions and end2440 values between 0 and 1 are all consistent with a palaeoproterozoic origin for the falconbridge twp intrusion and support inclusion in the east bull lake type suite of leucogabbroic plutons and sills in contrast the drury twp intrusion gives a u pb zircon age of 1859 13 ma coincident with the date of sic emplacement while the major and trace element compositions are comparable to the falconbridge data the drury displays significant heterogeneity in end2440 with values ranging from 3 7 to 0 1 and contains more radiogenic pb isotopic compositions field geochemical and isotopic evidence clearly distinguishes this intrusion from constituents of the sic itself and indicates that the drury too is a palaeoproterozoic intrusion this requires that apparently unshocked undeformed magmatic looking zircon has been grown or reset in a postmagmatic setting this has significant implications for the identification of mantle derived magmas and crustal remelts associated with large impact craters a resetting mechanism involving aggressive hydrothermal alteration of zircon facilitated by halogen complexing is proposed inducing rapid postshock lead loss and subsequent annealing

agglutinated foraminifera and thecamoebians from the late carboniferous sydney coalfield nova scotia paleoecology paleoenvironments and paleogeographical implications abstract agglutinated foraminifera and arcellaceans thecamoebians were examined from carboniferous late westphalian stephanian cyclothems in the sydney basin of nova scotia four distinctive assemblages are documented from the coal bearing strata and indicate deposition took place on an extensive coastal platform the eastern part of the basin is dominated by ammobaculites ammotium assemblages that indicate estuarine low marsh paleoenvironments relationships between test size grain size within the test and grain size of the substrate are used to differentiate between upper mudflat and lower mudflat sub facies within estuarine facies mixed assemblages dominated by trochammina ammotium and ammobaculites indicate vegetated substrates analogous to modern lower marsh environments and flanked the estuarine system in the eastern and western areas of the basin rich trochammina and thecamoebian assemblages are restricted to the western area of the basin indicating deposition within the upper part of the tidal range trochammina assemblages typically found underlying coal seams may indicate vegetated substrates equivalent to modern high marsh facies thecamoebian assemblages dominated by an agglutinated difflugid type indicate freshwater paleoenvironments encystment of the thecamoebian populations suggests paleoenvironments were short lived

petrogenesis and tectonic implications of paleoproterozoic mafic rocks in the black hills south dakota abstract four suites of mafic to intermediate intrusions and flows that occur in the precambrian core of the black hills were analyzed for major and trace elements to establish tectonic settings of magmatism during the late archean and early proterozoic history of the region most of the mafic rocks were later metamorphosed to amphibolites during the black hills orogeny the collision of the wyoming and superior cratons which began at 1760 ma compositions of the mafic suites define at least two distinct tectonic settings of magmatism between 2480 and 1880 ma the 2 48 ga blue draw intrusion located on the eastern edge of the precambrian core is differentiated with compositions ranging from gabbro to granodiorite it is a calcalkaline intrusion with relatively high sio2 from 51 8 to 60 5 wt and k2o mostly 1 wt ree patterns are slightly concave upward and show lree enrichment the intrusion is also enriched in ba rb th and zr these and other geochemical discriminants are typical of modern calcalkaline basalts the intrusion s calcalkaline character is similar to the neighboring slightly older 2 56 ga little elk granite gneiss these two intrusions support the presence of late archean early proterozoic arc magmatism on the eastern margin of the wyoming craton the three other mafic suites comprising metamorphosed flows and sills occur within a sequence of deep marine sediments including black shales limestones iron formations and cherts u pb zircon ages suggest their emplacement between 2 01 and 1 88 ga mafic rocks in the eastern half of the precambrian core in the mt rushmore and pactola dam quadrangles and near bear mountain on the western margin of the precambrian core have slightly depleted to enriched tholeiitic compositions with sio2 50 wt and k2o generally 2 5 wt and the lowest sio2 concentrations typically

ecological changes in pennsylvanian asturian and early cantabrian coal floras inferred from lycophyte microspore abundances abstract the distribution and relative amounts of six genera of lycophyte microspores lycospora densosporites crassispora cirratriradites endosporites and cadiospora are compared in the pennsylvanian coals from the dobrudzha and forest of dean coalfields clastic sediments from the glace bay borehole in the cape breton coalfield and the southern crop of the south wales coalfield inverse relationships are shown between lycospora and densosporites and between lycospora and crassispora in samples from all of the coalfields the changes in microspore assemblages suggest there were at least five different communities of lycophyte plants growing in the pennsylvanian swamps changes in microspore assemblages at the asturian cantabrian boundary mirror the previously described macrofloral changes at this time

neoarchean trondhjemitic and tonalitic orthogneisses identified within the northern grenville province in ontario by precise upb dating and petrologic studies abstract integrated field petrologic and geochronological studies reveal the presence of heretofore unrecognized neoarchean trondhjemitic and tonalitic orthogneisses north and northwest of lake nipissing in the northern grenville province of ontario the high al type trondhjemitic orthogneiss termed warren trondhjemite forms several intrusions with a total exposed area of 300 km 2 the tonalitic orthogneiss including the tilden tonalite forms a discontinuous zone of intrusions about 20 km long and up to 1 6 km wide both trondhjemitic and tonalitic orthogneisses are geochemically distinct from paragneiss that forms the country rocks precise u pb zircon and titanite dating of the warren trondhjemite at six locations and of the tilden tonalite at one location indicates that the former was emplaced at 2679 2 ma whereas the latter was intruded at 2669 4 ma a 996 to 975 ma metamorphism shown by the lower intercept ages of concordia plots of all samples caused partial pb loss from primary neoarchean zircons and titanites and also induced new growth of zircons in a local pegmatitic patch and of titanite in orthogneisses intrusive relationships between the orthogneiss and the country rock paragneiss indicate that the paragneiss is also archean in age the orthogneiss and the paragneiss together constitute an archean crustal sequence southeast of the grenville front boundary fault within the ontario segment of the grenville province this archean sequence resembles the pontiac subprovince of the superior province in western quebec in several important ways including the dominance of compositionally similar metasediments comparable depositional ages of the metasediments and contemporaneous granitoid plutonism at about 2680 to 2670 ma differences also exist between the two for example the major granitoids in the pontiac subprovince are generally more k rich

the role of komatiitic and picritic magmatism and s saturation in the formation of ore deposits abstract the introduction of high temperature high magnesium komatiitic and picritic magmas into the earth s upper crust has given rise either directly or indirectly to many of the world s major ore deposits there is a direct association between komatiitic magmatism and kambalda style ni cu pge deposits the link between magma type and mineralization is not as clear cut in the case of the noril sk cu ni pge deposits but nevertheless there is an association between picritic magmatism and ore deposit development komatiitic and or picritic magmatism may also have contributed to the formation of many types of hydrothermal ore deposits recent nd isotope data generated by kent et al 1994 provide strong evidence for the hypothesis proposed by keays in 1983 that the au in archaean au deposits was derived either directly or indirectly from komatiites through the action of metamorphic fluids the importance of komatiitic and picritic magmas in ore forming processes arises from the fact that they are among the few mafic ultramafic magma types that are s undersaturated at the time of magma formation s saturation of magmas leads to marked depletion in the chalcophile metals for example czamanske and moore 1977 estimated that a third of the original cu and ni contents of morb were lost from the magmas due to extraction into magmatic sulphides the depletion in au and the pge is much greater due to their much larger partition coefficients keays and scott 1976 komatiitic and picritic magmas are s undersaturated because they are high temperature magmas produced by large degrees of partial melting of upper mantle sourc regions that were already depleted in s through earlier partial melting events most mafic magmas produced by less than 25 partial melting are s saturated and hence depleted in the ore forming chalcophile elements in contrast komatiitic and picritic magmas have their full complement of chalcophile metals when they do become s saturated they form sulphides that are strongly enriched in ni cu au pge and other strongly chalcophile metals these may accumulate to form ore deposits directly e g magmatic ni cu pge sulphides or be dispersed in komatiitic picritic rocks or their fractionation products making these source rocks for hydrothermal au or vms deposits

u pb baddeleyite ages of the kerns and triangle mountain intrusions nipissing diabase ontario u pb ages have been determined for two diabase intrusions from kerns and hudson townships that are part of the nipissing diabase suite from the proterozoic southern province canada the ages of the kerns and triangle mountain intrusions are 2217 2 4 and 2209 6 3 5 ma respectively based on analysis of primary baddeleyite from both intrusions together with a published u pb age for the castle mine intrusion these data indicate that the nipissing sills were emplaced separately over a time span of 2 8 1 6 ma given by the maximum and minimum overlap of errors on the u pb ages

northeast extension of proterozoic terranes of mid continental north america granite and rhyolite ranging in age from 1 8 1 34 m y are known to underlie much of mid continental north america west of the phanerozoic michigan basin u pb zircon ages of similar rocks exposed adjacent to the grenville province in ontario suggest that the mid continental terranes extend northeast beneath and beyond the michigan basin prevalence of similar u pb zircon ages obtained from metaplutonic rocks southeast of the grenville front and affected by the 1 2 1 0 ga grenvillian orogeny prompts the interpretation that deformed equivalents of the mid continental proterozoic rocks form the bulk of the crust in the central gneiss belt of the grenville province east of lake huron farther east pre 1 34 ga terranes appear to be tectonically occluded by 1 28 1 22 ga volcanic sedimentary and plutonic rocks of the central metasedimentary belt themselves deformed metamorphosed and intruded by younger grenvillian plutons

terminology for layered intrusions the terminology of igneous cumulates was originally formulated on the concept that the rocks were concentrations of minerals accumulated by crystal settling it is now well established for several major intrusions that plagioclase that had been typed as cumulus should in fact have floated in its parental magmatic liquid nevertheless the terminology has been valuable in the classification of the rocks of these and other intrusions and it has been useful in certain aspects of their genetic interpretation the name cumulate is therefore redefined so that crystal settling is a possible but not essential process in the origins of the rocks to which it is applied the overall nomenclature of layered intrusions is then reviewed and definitions of names are refined and systematized to clarify their applications careful distinction is made between descriptive and genetic names a cumulate is defined as an igneous rock characterized by a framework of touching mineral crystals and grains that evidently were concentrated through fractional crystallization of their parental magmatic liquids a layer is a sheetlike unit of cumulate that is a distinctive entity in its compositional and or textural features a lamina is a thin 2 5 cm sharply defined layer igneous lamination is a general name for planar structure that is pervasive through cumulates on the scale of their grain size layering is the overall structure and fabric manifest through combinations of layers laminae and lamination the difference between layers and layering is essentially the difference between individuals and a population and the terms that describe one are not necessarily appropriate to the description of the other but the fundamental variants of all of layering lamination grading in layers and layer contacts can essentially be described by one or more of the four modifiers grain size modal textural and cryptic where the last refers to cumulus miner al compositions a system for subdividing layered intrusions is proposed in which distinction is made between general divisions ranked as series zones and subzones and prominent units that are called groups members rhythmic units and cyclic units applications to the skaergaard bushveld muskox stillwater and other intrusions are briefly described

the sudbury igneous complex viscous emulsion differentiation of a superheated impact melt sheet the sudbury igneous complex of ontario canada is the remnant of a voluminous melt sheet produced in a few minutes by impact of a massive meteorite into continental crust 1 85 ga ago the transient cavity and melting zone reached the moho and instantly 2 min relaxed to form a more familiar large shallow crater holding a thick superheated 1700 c melt sheet covered by 2 km of breccia there is little about the resulting bimodal igneous complex that resembles crystallization of well known sheet like bodies of similar composition yet the norite and granophyre exhibit a remarkable similarity in isotopic and trace element compositions suggesting an intimate common parentage from the surrounding crust this petrogenetic enigma is explained here as a natural unavoidable consequence of the impact process in the rapid formation of a superheated magmatic emulsion which we take as the high temperature equivalent of breccia a wide spectrum of viscously discrete interdispersed parcels of mafic and felsic liquids reflecting the compositional heterogeneity of the target crustal materials formed the emulsion within days to months the emulsion components separated according to their relative densities into a bimodal norite granophyre assemblage that formed the basic structure of the present sudbury igneous complex there is clear evidence of this emulsion in the earliest dikes i e offsets which likely give the earliest state of the nascent melt sheet immediately following emulsion separation the strongly superheated bimodal melt sheet underwent vigorous thermal convection in each layer these convective motions homogenized and rapidly cooled the magma to the liquidus temperatures whereupon convection ceased the pattern of convection was pinned in place by the embayment topography of the crater floor which in turn played a pivotal role in directing sulfide deposition into the embayments all further cooling was by conduction of heat through the upper and lower boundaries during which time solidification fronts were established and propagated inward from the upper and lower margins there is clear evidence of solidification from the floor upward and the roof downward minimal differentiation and compositional modification took place throughout cooling and solidification nevertheless during the solidification stage granitic rock fragments on the crater floor and rafts of fallback breccia from the thick overlying onaping formation became unstable and entered the melt sheet and the partially melted remnants collected at the interface between the norite and granophyre some interstitial melt from the norite also percolated upward and altogether the blocks and melt produced the distinctive chemical and physical characteristics of the unusual transition zone the sudbury melt sheet is in essence a full scale magmatic experiment the conditions of formation relative to any other large terrestrial magma are precisely known thus the clear lack of any significant modal layering the overall homogeneous nature of each unit and the lack of any significant chemical differentiation through crystal fractionation establish sudbury as a valuable example of what does not happen under the initial conditions long assumed to prevail at the formation of most large magma chambers

albitization at 1700 2 ma in the sudbury wanapitei lake area ontario implications for deep seated alkalic magmatism in the southern province u pb geochronology of hydrothermal monazite in albitized rocks from two gold deposits east of the sudbury complex indicates that albitization in the sudbury wanapitei lake area occurred at 1700 2 ma and was coeval with a period of granitic plutonism in the southern structural province between 1750 and 1700 ma a variety of rare earth element ree minerals such as two generations of hydrothermal monazite bastnasite synchysite and gadolinite were identified in the albitized huronian sediments in the espanola sudbury wanapitei lake areas the presence of these ree minerals the extraordinary light rare earth element enrichment in rocks from the sheppard gold property east of the sudbury igneous complex and the elevated ree concentrations in some albitized rocks suggests that sodium rich fluids may have been generated by carbonatitic or alkalic intrusions at depth gold mineralization occurs in rocks that have been altered by at least two different types of fluids 1 peralkaline na ree bearing a

archaean boninite like rocks in an intracratonic setting abstract boninites and high mg norites show considerable compositional overlap while the petrogenesis of both may involve large ion lithophile element lile replenishment of a mantle source boninites appear restricted to convergent margin settings while many high mg norite suites intrude intracratonic rifts rocks with boninitic compositions are extremely rare in archaean terranes where they are also interpreted to have formed at convergent margins high mg lile enriched basaltic rocks form a ca 2950 ma sub volcanic sill within the 3010 2935 ma mallina basin of the pilbara craton northwest australia they are spatially associated with low ti tholeiitic intrusions and a high mg diorite sanukitoid suite they have a strong boninitic affinity with high sio 2 52 54 wt mg 62 67 and al 2 o 3 tio 2 62 73 low tio 2 0 24 0 28 wt and strong lile enrichments primitive mantle normalised la gd and gd yb ratios of 4 5 and 0 6 respectively produce prominent u shaped normalised trace element patterns despite such close compositional affinity to boninite these rocks probably formed more than 40 myr after regional magmatism and 60 myr after the last regional event linked to subduction geological evidence suggests that the mallina basin formed in an intracontinental setting not at a convergent margin and so high mg norite provides a more realistic analogy for the mallina boninite like rocks in terms of tectonic setting in old terranes the tectonic setting of rocks with boninite like compositions must be interpreted with caution trace element modelling shows that contamination by felsic crust is a plausible explanation for lile enrichments in the mallina boninite like rocks but the scarcity of suitable contaminants makes this explanation unlikely petrogenesis of the boninite like rocks was closely associated with that of the high mg diorite sanukitoid suite lile enrichments in high mg diorite require that the mantle beneath the entire mallina basin was variably metasomatically enriched by an earlier subduction event at or before ca 3015 ma the refractory source for the boninite like rocks may have also been enriched during this event

anisotropy and shear velocity heterogeneities in the upper mantle long period surface waves are used to map lateral heterogeneities of velocity and anisotropy in the upper mantle the dispersion curves are expanded in spherical harmonics up to degree 6 and inverted to find the depth structure the data are corrected for the effect of surface layers and both love and rayleigh waves are used shear wave velocity and shear polarization anisotropy can be resolved down to a depth of about 450 km the shear wave velocity distribution to 200 km depth correlates with surface tectonics except in a few anomalous regions below that depth the correlation vanishes cold subducted material shows up weakly at 350 km as fast s wave anomalies in the transition region a large scale pattern appears with fast mantle in the south atlantic s anisotropy at 200 km can resolve uprising or downwelling currents under some ridges and subduction zones the pacific shows a nw se fabric

the geochemistry and petrogenesis of the agnew intrusion canada a product of s undersaturated high al and low ti tholeiitic magmas

st john s island red sea a new geophysical model and its implications for the emplacement of ultramafic rocks in fracture zones and at continental margins abstract new gravity and magnetic data from the northern red sea reveal the extent of the large gravity anomaly 164 mgal and the presence of significant magnetic anomalies over st john s island spectral transformation and three dimensional potential field modelling delineate the surface configuration and vertical extent of the causative body and the enormous density contrast required 1 2 g cm 3 suggests that it is composed of unserpentinised peridotite density 3 4 g cm 3 to a depth of at least 8 km st john s island is uniquely located not only at a passive continental margin but also within a fracture zone at the transition from plate separation by seafloor spreading to extension by lithospheric attenuation this precludes several suggested mechanisms for the emplacement of ultramafic bodies in fracture zones thermal contraction serpentinite diapirism and changes in the poles of rotation do not seem possible mechanisms in this tectonic environment and the emplacement is most probably related to the spreading readjustment necessary to create a continent to continent fracture zone a post mesozoic age of emplacement associated with the onset of continental rifting and the rejuvenation of a pre existing continental fracture seems most probable

a petrochemical study of calcic amphiboles from the east bull lake anorthosite gabbro layered complex district of algoma ontario calcic amphiboles are ubiquitous in the east bull lake anorthosite gabbro complex northeastern ontario the mode of occurrence suggests the amphiboles replaced clinopyroxene throughout the stratigraphic levels of the pluton but they are also prolific in the top younger units three types of amphiboles namely 1 tremolite actinolite 2 actinolitic hornblende and 3 hornblende were identified the composition of the amphiboles shows that they define a continuous series without any obvious compositional miscibility gap the analyses further indicated that the amphiboles evolved from actinolite to hornblende through coupled substitution involving edenite and tschermakite end members among the three coexisting amphiboles chlorine is concentrated in hornblende this is due to preferential location of hornblende along grain boundaries and or its favourable structure

controls on the formation of contact type platinum group element mineralization in the east bull lake intrusion the east bull lake intrusion is the type example of a series of 2 48 to 2 49 ga layered mafic intrusions that developed within the rifted southern margin of the superior craton central ontario the east bull lake intrusion contains contact type pge mineralization e g strata bound pge rich disseminated sulfide mineralization occurring within 200 to 300 m of the contact with footwall and sidewall units although the mineralization locally occurs in massive plagioclase cumulates anorthosite zone most of the sulfides are concentrated within an underlying heterolithic inclusion rich unit inclusion bearing zone that grades into an intrusive breccia disseminated sulfides also occur in fine grained gabbroic veins within brecciated footwall rocks border zone geochemical characteristics of the contact type sulfide mineralization and its host rocks include 1 high average pd abundances throughout the east bull lake intrusion including an average of ca 200 ppb in the lower series 2 low average s contents typically contact type pge mineralization in the east bull lake intrusion reflects flow enhanced gravitational settling of a cu rich sulfide liquid followed by local fractionation of ni rich monosulfide solid solution to produce minor amounts of cu and pt rich residual sulfide liquid most of the observed compositional variation in the sulfides can be modeled using variable silicate liquid sulfide liquid mass ratios field and geochemical observations suggest that the parental magmas to the mineralized parts of the east bull lake intrusion were s saturated and pge rich at the time of their emplacement these fertile parent magmas most likely originated as buoyant feldspathic residual magmas that escaped from a deeper more primitive subchamber although first described from the east bull lake intrusion contact type pge mineralization is now recognized in all of the major east bull lake suite intrusions similar styles of magmatic sulfide mineralization are developed in the contact environments of the bushveld igneous complex platreef deposits and the duluth complex minnesota e g dunka road deposit these contact type deposits are part of the family of basal accumulation magmatic sulfide deposits the economics of low sulfide tenor contact type pge cu ni mineralization such as that developed in the east bull lake intrusion will be most favorable in systems in which 1 the parent magmas possessed high initial pge tenors and reached s saturation prior to emplacement 2 the parent magmas experienced vigorous convection in the early stages of chamber development and 3 segregated sulfide was not diluted by coprecipitating silicate minerals a scenario that is most likely to occur in feldspathic magmas in which plagioclase is the principal high temperature liquidus mineral

tectonic influences on the morphometry of the sudbury impact structure implications for terrestrial cratering and modeling impact structures developed on active terrestrial planets earth and venus are susceptible to pre impact tectonic influences on their formation this means that we cannot expect them to conform to ideal cratering models which are commonly based on the response of a homogeneous target devoid of pre existing flaws in the case of the 1 85 ga sudbury impact structure of ontario canada considerable influence has been exerted on modification stage processes by late archean to early proterozoic basement faults two trends are dominant 1 the nnw striking onaping fault system which is parallel to the 2 47 ga matachewan dyke swarm and 2 the ene striking murray fault system which acted as a major paleoproterozoic suture zone that contributed to the development of the huronian sedimentary basin between 2 45 2 2 ga sudbury has also been affected by syn to post impact regional deformation and metamorphism the 1 9 1 8 ga penokean orogeny which involved nnw directed reverse faulting uplift and transpression at mainly greenschist facies grade and the 1 16 0 99 ga grenville orogeny which overprinted the se sector of the impact structure to yield a polydeformed upper amphibolite facies terrain the pre syn and post impact tectonics of the region have rendered the sudbury structure a complicated feature careful reconstruction is required before its original morphometry can be established this is likely to be true for many impact structures developed on active terrestrial planets based on extensive field work combined with remote sensing and geophysical data four ring systems have been identified at sudbury the inner three rings broadly correlate with pseudotachylyte friction melt rich fault systems the first ring has a diameter of 90 km and defines what is interpreted to be the remains of the central uplift the second ring delimits the collapsed transient cavity diameter at 130 km and broadly corresponds to the original melt sheet diameter the third ring has a diameter of 180 km the fourth ring defines the suggested apparent crater diameter at 260 km this approximates the final rim diameter given that erosion in the north range is 6 km and the ring faults are steeply dipping impact damage beyond ring 4 may occur but has not yet been identified in the field one or more rings within the central uplift ring 1 may also exist this form and concentric structure indicates that sudbury is a peak ring or more probably a multi ring basin these parameters provide the foundation for modeling the formation of thisrelatively large terrestrial impact structure

observations of laterally inhomogeneous anisotropy in the continental lithosphere elastic anisotropy may be an important factor when seismic data are used to determine the structure composition and dynamics of the earth s interior1 2 azimuthal anisotropy has been found in the oceanic and although less definitely in the continental lithosphere from field experiments where the pn wave velocity was observed to vary with direction3 5 these studies suggest a 180 periodicity in travel time variations as the only criterion of the presence of azimuthal anisotropy this criterion however may fail to distinguish between anisotropy and lateral inhomogeneity of the isotropic medium a new method for measuring azimuthal anisotropy has been described recently6 based on the phenomenon of shear wave splitting in anisotropic media which suggests three criteria instead of one to identify azimuthal anisotropy we have now applied this method to the records of permanent seismograph stations in southern germany our results provide strong evidence for the presence of azimuthal anisotropy in the lithosphere of the region and reveal pronounced lateral variations in the parameters of anisotropy on a scale of about 200 km

the tectonostatigraphic evolution of the huronian basement and the subsequent basin fill geological constraints on impact models of the sudbury event abstract whereas both meteorite impact and volcanic mechanisms have been proposed to explain the rocks and associated ore deposits within the sudbury structure 1 85 ga few models take into consideration the paleogeographic constraints implied by the history of the region before and following the sudbury event huronian depositional style south of the sudbury structure appears to have been strongly influenced by the early development of transtensional extensional basins these basins were initiated by sinistral strike slip movement along part of a paleoproterozoic continental margin which later evolved into a passive margin transition from a rifted strike slip margin to a passive margin was accompanied by major slope failure and soft sediment deformation south of the murray fault diabase dikes were intruded into unconsolidated sediments at considerable depth producing round stone breccias which are texturally similar to sudbury breccia minor compressional folding occurred prior to emplacement of the nipissing diabase suite at around 2 2 ga the abundance of diabase intrusions near the sudbury structure suggests that the area may have been a focal point for magma emplacement the post event fill of the paleoproterozoic sudbury basin reflects initial deposition in a deep water anoxic setting there is no evidence to suggest that sedimentation was influenced by the steep walls of an impact crater carbonate in the basal vermilion member is of sedimentary exhalative origin and was not derived from a shallow marine shelf turbidites in the chelmsford formation show no evidence of centripetal fill as might be expected from a restricted circular basin they appear to have been emplaced along the depositional axis of an elongate foreland basin developed in front of the rising penokean mountain chain penokean deformation continued after the sudbury event and influenced strata well into the archean craton deformation of huronian strata in paleovalleys north and west of the sudbury structure does not appear to be genetically related to the sudbury event if superfaults were generated around the sudbury structure in response to compression and crustal rebound following a meteorite impact these do not appear to have caused offset of pre existing steeply dipping contacts between the huronian and underlying archean basement a new model consistent with these observations needs to be developed

sulfur saturation and second stage melts application to the bushveld platinum metal deposits a model is presented for the formation of platinum group element ores by precipitation from sulfur deficient platinum group element rich second stage magmas numerous investigations indicate that basaltic magmas originating from undepleted or mildly depleted source regions are sulfur saturated at the time of eruption mantle derived peridotites residual from midocean ridge basalt magma genesis have pd abundances that are uniformly too high for it to have originated solely from silicate melt retained in the source region at the time of segregation they imply the presence of an accessory immiscible sulfide melt to host the platinum group elements the very high sulfide melt silicate melt partition coefficients of the platinum group elements would result in strong fractionation of these metals into the accessory sulfide component during the melting event the extreme platinum group element enrichment of this accessory sulfide component is evidenced by the discovery ofplatinoid minerals in residual mantle nodules containing pd and pt abundances at the low ppb level keays et al 1981 magmas generated by low to moderate degrees of partial melting of undepleted mantle are therefore sulfur saturated at the time of segregation these first stage magmas become impoverished in platinum group elements during the early stages of fractional crystallization because of coprecipitation of an immiscible sulfide phase e g midocean ridge basalt glasses average

the afar depression transition between continental rifting and sea floor spreading abstract detailed geophysical and geological studies were undertaken in the 1970 s in the afar depression ethiopia in order to study the processes of continental break up occurring in this unique setting the afar is located between the ethiopian and somali plateaus and the red sea we have re evaluated the available seismic refraction and gravity data using modern interpretation techniques and incorporated new information derived from magnetic gravity seismicity and seismic refraction data the results of the re evaluation indicate that the afar depression is a transition zone between the continental rifts of kenya and the present ongoing sea floor spreading of the red sea and the gulf of aden in the depression itself the crust is greatly attenuated and is underlain by a low velocity 7 4 7 5 km s high temperature upper mantle material however it is still a continental crust which thins to the northeast and east towards the red sea and the gulf of aden it is not a newly formed oceanic crust as has been suggested in the past the trends of the areas of crustal thinning are indicated by the gravity anomalies the trend of the ethiopian rift reaches south afar where the trend of crustal thinning changes and becomes parallel to the red sea trend it is offset en echelon from the spreading area of the red sea which is marked by an alignment of gravity maxima the gulf of aden trend continues west towards the ethiopian rift the gravity trends coincide with areas of maximum crustal thinning and may thus suggest a possible triple junction in development

upper mantle heterogeneities in africa deduced from rayleigh wave dispersion abstract rayleigh wave group velocities have been measured along 11 paths on the african continent north of the equator the results lead to a division of this part of the continent into three regions cratonic areas non cratonic areas mobile zones and a region located to the east of the meridian 31 e red sea neighbourhood ethiopia djibouti the highest velocities are found beneath the cratonic areas at 200 km depth 5 at greater depths the mobile zones exhibit higher velocities than the cratonic regions but the difference is not as marked 2 and the resolution is poor velocities in the eastern region are similar to those found under young oceanic regions

carboniferous marsh foraminifera from coal bearing strata at the sydney basin nova scotia a new tool for identifying paralic coal forming environments assemblages of agglutinated foraminifera from the carboniferous late westphalian to stephanian coal bearing strata of the sydney basin cape breton island documented for the first time in 150 yr of investigation of the nova scotian coal basins indicate that deposition took place on an extensive coastal platform not in a fluviolacustrine setting as previously suggested the assemblages contain representatives of genera that are widely distributed in modern estuarine and marsh environments allowing us to determine carboniferous paleoenvironments with unusual confidence seat earths beneath several coal seams contain assemblages dominated by trochammina which suggests deposition within vegetated zones analogous to modern high marsh environments found within the upper half of the tidal range a coal seam split and a coarse siltstone contain assemblages dominated by ammobaculites and ammotium which indicate paleoenvironments analogous to modern low marsh and estuarine environments mixed assemblages of trochammina ammobaculites and ammotium also indicate estuarine low marsh depositional environments detailed analysis of foraminiferal biofacies assists in the recognition of marine influence during deposition of these carboniferous rocks and may help distinguish areas of sulfur rich coal the findings indicate that environmental and morphological conservatism has characterized agglutinated marsh foraminifera for the past 300 m y

phytogeography of asturian westphalian d lycophytes throughout the euramerican belt of coalfields a summary is given of the lycophyte species as distributed in fifteen euramerican coalfields during asturian westphalian d times the datasets from the coalfields are analysed using unweighted pair group cluster analyses of jaccard s coefficients separate analyses were done on the full dataset the full list of morphospecies and on a more restricted dataset only the morphospecies of vegetative organs three groupings of coalfields were resolved in the analyses the two most clearly defined groupings represented the intramontane basin coalfields and the coalfields of the lowland basins on the variscan foreland where the lycophyte floras are well documented the third grouping included those basins with less well studied lycophyte floras where the apparently low species diversity has undoubtedly affected the results more work is needed on these poorly documented floras to produce a more rigorous phytogeographical analysis

palaeofloristics of middle pennsylvanian medullosaleans in variscan euramerica abstract medullosaleans were an abundant and widely distributed group of seed plants in the moscovian palaeotropical wetland forests coal forests using mainly a combination of cluster analysis and detrended correspondence analysis the distribution of medullosalean species has revealed five distinct palaeofloristic units within variscan euramerica two palaeofloristic units characterise lowland paralic areas the rhine palaeoprovince found in parts of the variscan foreland and the donets basin and the iberia palaeoprovince found in northern spain and portugal the silesia palaeoprovince represents somewhat lower diversity medullosalean macrofloras mainly associated with more marginal areas of the variscan foreland intramontane basins are characterised by two palaeofloristic units that are quite distinct from those of the more lowland areas and to a lesser extent different from each other the saar lorraine and central bohemia palaeoprovinces the latter found in the intrasudetic basin and zwickau as well as central bohemia except for the appearance of the iberia palaeoprovince in late moscovian times the basic palaeofloristic structure of variscan euramerica remained essentially unchanged through the moscovian age those palaeofloristic changes that did take place mainly reflect localised increased influxes of sediment or changes in water table the palaeofloristic data provides no evidence of regional scale environmental changes taking place such as marked climate change regional uplift or major palaeolatitudinal change

river valley pluton ontario a late archean early proterozoic anorthositic intrusion in the grenville province abstract the river valley pluton is a ca 100 km2 body of anorthositic and gabbroic rocks located about 50 km northeast of sudbury ontario the pluton is situated entirely within the grenville province but its western margin is a series of imbricate thrust faults associated with the grenville front tectonic zone it is dominated by coarse leuconorite and leucogabbro with lesser anorthosite gabbro and rare ultramafics igneous textured rocks are abundant and consist of plagioclase an60 70 charged with fe ti oxide inclusions low ca pyroxene orthopyroxene and or inverted pigeonite and augite the most unfractionated rocks are minor olivine gabbros with fo70 80 a variety of deformed and recrystallized equivalents of the igneous textured rocks is also present and these are composed largely of calcic plagioclase and hornblende ten samples including both igneous and deformed lithologies give a pb pb whole rock isochron of 2560 155ma which is our best estimate of the time of primary crystallization the river valley pluton is thus the oldest anorthositic intrusive yet reported from the grenville province but is more calcic and augitic than typical massifs and lacks their characteristic fe ti oxide ore deposits the river valley body may be more akin to similar gabbro anorthosite bodies situated at the boundary between the archean superior province and huronian supracrustal belt of the southern province west of the grenville front an sm nd isochron from 3 igneous textured leucogabbros and an augite mineral separate gives 2377 68 ma implying slight disturbance of the sm nd whole rock mineral system during later metamorphism the rb sr system has been substantially disturbed giving an age of 2185 105 ma which is similar to internal pb pb isochron ages of 2165 130 ma and 2100 35 ma for two igneous textured rocks it is uncertain whether these ages correspond to a discrete event at this time or represent a partial resetting of the rb sr and pb pb systems from a younger event such as the grenvillian orogeny of ca 1 0 ga none of the isotopic systems we investigated however gives an age near 1 0 ga suggesting that neither the river valley pluton nor the immediately surrounding gneisses were strongly affected by metamorphism associated with the grenvillian orogeny initial isotopic ratios for the river valley pluton correspond to single stage model parameters of 8 06 nd 0 to 3 and isr 0 7015 to 0 7021 collectively these suggest either an enriched mantle source or crustal contamination of a mantle derived magma the crustal component involved must have been older and more radiogenic than the majority of rocks exposed at the surface in the nearby superior province

new insights on the stepwise collapse of the carboniferous coal forests evidence from cyclothems and coniferopsid tree stumps near the desmoinesian missourian boundary in peoria county illinois usa abstract the first phase in the stepwise collapse of the carboniferous coal forests occurred near the desmoinesian missourian boundary early kasimovian 307 ma and involved extirpation of lycospora producing lepidodendrids and some other lycopsids across most of tropical euramerica in this paper we follow up on historical reports of silicified tree stumps in peoria county northwest central illinois usa which have significant implications for understanding carboniferous coal forest collapse rooted near the paleoweathered top of the lonsdale limestone and widespread across an area of 250 km2 the silicified tree stumps belong to amyelon type coniferopsids a key feature of the fossil wood is the occurrence of abundant axial parenchyma arranged along irregular growth interruptions suggestive of climatic seasonality an inference consistent with silicic preservation the silicified fossil forest directly underlies the exline limestone and athensville coal the horizons that mark the us wide loss of lycospora and demonstrate that lowland areas were colonized by dryland coniferopsid forests following coal forest collapse placed in a cyclothem context the silicified fossil forest horizon lies above the maria creek mudstone paleosol top of piasa cyclothem in which earlier d18o analyses have identified a major pulse of global warming and coincides with the hanna city paleosol top of lonsdale cyclothem which is correlative with the seminole sandstone a midcontinent incised valley fill representative of one of the most profound glacioeustatic falls seen in the pennsylvanian record our new findings therefore demonstrate that coal forest collapse was closely linked to intensification of glacial cycle amplitude near the desmoinesian missourian boundary involving both extreme episodes of global warming and cooling

the impact of climate change tectonism and hydrology on the formation of carboniferous tropical intermontane mires the springhill coalfield cumberland basin nova scotia abstract both climate and tectonism are influential in determining basin and mire hydrology in the rapidly subsiding intermontane basins of euramerica which formed in response to the variscan hercynian alleghenian orogeny however the record of climate change has been muted and may therefore go unrecognized the complex interplay of climate and tectonism in a tropical continental setting is recorded in the 4 km thick westphalian a b fill of the cumberland basin thick peat accumulation was favoured at the southern basin margin where rheotrophic forested mires 4 9 km wide flourished between basin axis rivers and coalesced alluvial fans derived from the cobequid highlands as a result of their geomorphic setting the thicker coals 4 3 m exhibit distinct areal zones piedmont inner mire and riverine evidence that the forest swamps were nourished both by rainfall and by groundwater discharge from alluvial fans includes the stratigraphic affinity of thick coals and basin margin conglomerates interfingering of distal fan sheetflood and mire deposits petrographic evidence of elevated ph partial gelification of tissues and predominance of the arboreous lycopsids lepidodendron and paralycopodites whereas such groundwater supply is crucial to maintenance of modern wetlands in drier periods a subhumid climate is implied coals at joggins further removed from groundwater discharge at the basin margin are less well developed a 600 m thick coal window within the fining and reddening upward basin fill sequence at springhill is inferred to represent a coincidence of optimal groundwater supply and net subsidence the ultimate decline of peat accumulation in the basin may reflect decrease in catchment area and fan size and loss of rainshadow as the cobequid massif was denuded and the basin filled the coal window ca 6 9 10 5 yr is punctuated by non marine cyclothems of coal grey sideritic mudrock and multistorey sandstone that share similar time frames with astronomic cycles of axial precession ca 1 4 10 4 yr the stress of related climate change is inferred to have caused peat accumulation to fall into disequilibrium with sediment flux and basin subsidence the late quaternary history of certain african mires illustrates the dramatic impact of climate change especially temperature and seasonality and orographic effects on tropical continental peat accumulation aspects of the groundwater recharge model may be applicable to the westphalian stephanian intermontane mires of spain and mesozoic coal basins of northern china

paleoproterozoic boninites in an ophiolite like setting trans hudson orogen canada paleoproterozoic high mg andesites of the birch lake assemblage in the trans hudson orogen display trace element enrichments characteristic of cenozoic boninites occur in an analogous stratigraphic setting and share a metallogenic association with the younger rocks their geochemical compositions indicate substantial prior depletion and a selective reenrichment of the mantle sources consistent with metasomatism by siliceous melts these requirements are best accounted for in an island arc setting and indicate that early extensional stages of subduction zone development have not changed substantially over the past 2000 m y

regional gold enrichment of conglomerates in paleoproterozoic supergroups formed during the 2 45 ga rifting of kenorland abstract we describe the presence of witwatersrand type gold mineralization in paleoproterozoic strata indicating this style of deposit is not confined to the archean the discovery of basal and intraformational conglomerate hosted gold mineralization in the huronian supergroup s mississagi formation near sudbury ontario has indicated potential for witwatersrand type gold mineralization to occur also in early paleoproterozoic basin fills on the hearne wyoming and karelian cratons the paleoproterozoic hurwitz snowy pass and karelian supergroups located on these cratons are similar to the huronian supergroup on the superior craton in terms of sequence and litho stratigraphy and maximum age of deposition a review of the stratigraphy and past gold exploration from each supergroup has revealed each basin to contain similar fluvial poly to oligomictic pyritic conglomeratic strata with locally anomalous gold and or uranium concentrations at stratigraphic levels that correlate to those of the mississagi and matinenda formations of the huronian supergroup examination of gold and gold enriched pyrite from the mississagi formation conglomerates suggests that specific point sources existed for most of the gold therein each of the above basins forms part of a larger contiguous continental rift system prior to the breakup of the supercontinent kenorland at around 2480 ma as each basin fill was displaced to different geographic positions following this continental breakup and then subjected to separate orogenic and metamorphic overprints the gold and uranium mineralization in each of the basins is unlikely to be of a common post depositional hydrothermal origin but in all likelihood of a paleoplacer origin it is suggested that this period of gold enrichment is the result of truncation and reworking of underlying stratigraphically older units and or from former gold deposits occurrences and gold enriched greenstone dominated hinterlands the omnipresence of detrital and or syn depositional pyrite in conglomerates of each basin reinforces the idea that a lack of atmospheric oxygen at the time of kenorland rifting existed during their deposition and prior to the great oxidation event our comparative analysis of kenorland rift graben fills reveals that gold potential which can be determined by the presence of detrital pyrite as observed at the pardo project should exist in all stratigraphic equivalents of the mississagi and matinenda formations where preferentially reworked fluvial conglomerates rest on an erosional unconformity and where a corresponding au enriched archean hinterland existed

tectonic stratigraphic and geochemical comparisons between ca 2500 2440ma mafic igneous events in the canadian and fennoscandian shields abstract palaeoproterozoic mafic igneous rocks 2500 2440 ma are exposed in the form of layered intrusions dykes and volcanic rocks in the southern canadian shield and the fennoscandian shield both igneous events were associated with episodes of continental rifting the tectonic and regional geological evolution of the two regions during the palaeoproterozoic are strikingly similar further comparisons of the mineralogical and stratigraphic characteristics of the layered intrusions the east bull lake suite in the canadian shield and the fennoscandian suite in the fennoscandian shield show additional similarities earlier investigations have identified possible parental magmas from mafic dykes for each of the layered intrusion suites these include the so called streich dykes associated with the east bull lake suite and the viianki dykes with the fennoscandian suite the viianki dykes are more primitive in composition mg 0 66 0 78 relative to the streich dykes mg 0 62 0 65 and probably represent primary mantle derived magmas geochemical modelling suggest that these two parental magmas cannot be related through fractional crystallization the differences in ree characteristics la yb cn 3 6 9 1 for viianki dykes 3 6 3 7 for streich dykes and selected incompatible element ratios can however be accounted for by either differential amounts of upper crustal contamination or different degrees of partial melting from a similar garnet bearing mantle source the data suggest that the viianki dyke parental magmas may have been produced by a degree of partial melting that was more than a factor of two lower than that which gave rise to the more primitive parent of the streich dyke magmas we conclude that these early palaeoproterozoic igneous events in the southern canadian shield and the fennoscandian shield could have been derived from a similar mantle source that was characterized by partial melt extraction of amorb and enrichment by subduction like lree enriched fluids during the archaean

geologic transect across the grenville orogen of ontario and new york revised cross sections of the western grenville province incorporate new geologic results and reprocessed seismic reflection data the geology is presented in terms of three tectonic elements 1 pre grenvillian laurentia and its margin with ca 1740 and 1450 ma continental arc plutons and associated supracrustal rocks 2 composite arc belt of allochthonous 1300 1250 ma volcanic arcs and sedimentary rocks and 3 frontenac adirondack belt characterized by supracrustal and granitoid rocks and anorthosites of uncertain affinity that may represent a distinctive part of the composite arc belt or an offshore micro continent rocks of the composite arc and frontenac adirondack belts were amalgamated with each other by ca 1160 ma were then thrust over laurentia during ca 1080 1035 ma and ca 1010 980 ma phases of convergence and were dissected and exhumed by 1040 ma normal faults penetrative deformation was restricted to that part of the pre grenvillian laurentian margin that lies to the southeast of the grenville front and parts of the accreted composite arc and frontenac adirondack belts the laurentian rocks in the grenville province are bounded to the northwest and southeast by southeast dipping ductile thrust and or normal shear zones the composite arc and frontenac adirondack belts to the southeast are bounded by ductile and brittle ductile thrust and or normal faults that separate domains with contrasting cooling histories despite a long pre grenvillian tectonic and plutonic history the present crustal architecture and much of the seismic reflectivity were acquired during 1080 980 ma phases of compression and extension

geology and geochemistry of the east bull lake intrusion district of algoma ontario the east bull lake intrusion is a layered basic lopolith of early aphebian age that intrudes archean basement rocks at the southern margin of the superior province it contains at least two magmatic cycles cumulate olivine plagioclase and clinopyroxene form anorthosite troctolite and olivine gabbro norite in the lower portion of each cycle these rocks are overlain by two pyroxene gabbros norites in which orthopyroxene or pigeonite joins augite and plagioclase as the major cumulate minerals molecular proportion ratio mpr variation diagrams support this interpretation of the petrology of the intrusion and show that the major cumulate rock types represent magmatic liquids formed sequentially by fractionation of the above mentioned cumulate phases small scale phase layering is common in the intrusion size graded layers are absent and cross bedded and igneous lamination features are not common we conclude that this scale of layering formed in a boundary layer along the floor of the intrusion as a

late carboniferous and early permian drainage patterns in atlantic canada paleoflow data have been compiled for late carboniferous late westphalian a to early permian alluvial deposits over a large area of atlantic canada the data which include more than 36 000 measurements of large scale trough cross strata indicate a predominantly northeasterly paleoflow and suggest that a major source area lay to the southwest of the region throughout the 30 ma period represented uplands within the basin deflected paleoflow and probably formed important local drainage and sediment sources tectonostratigraphic analysis suggests that the drainage originated in the fold and thrust belt of the central appalachians and parts of the northern appalachians rivers probably followed northeast oriented structural lineaments through the older acadian mountains of the northern appalachians a considerable proportion of the rising orogen s drainage and probably detritus may have traversed basins along the strike of the mountain belt a situation analogous to that of the modern himalayas

a u pb age for mineralized nipissing diabase gowganda ontario concordant baddeleyite and rutile analyses from mineralized nipissing diabase at gowganda s castle mine yield a u pb age of baddeleyite is a primary phase of the diabase therefore the age dates the magmatic emplacement the genesis of rutile is less certain rutile could have formed during an episode of alteration veining and ag mineralization in which case the u pb age would also date the secondary event and demonstrate that ag mineralization was related to the nipissing magmatism however a primary magmatic origin of the rutile cannot be completely excluded for now thus the u pb age may not constrain the time of ag mineralization further research is needed to clarify this point

confirmation of carboniferous clastic swamp communities tropical lowlands dominated by arborescent lycophytes are the principal foci for our understanding of carboniferous peat and non peat accumulating coastal swamps1 2 suggested palaeoeco logical interpretations of various lycophytes and accessory vegetation have been based on transported compression assemblages3 4 and palynomorphs5 6 or in situ permineralized peats7 where transported macrodetritus resulting in compression assemblages is used to reconstruct past vegetational patterns taphonomic factors tend to bias the preserved flora8 11 taken alone or out of depositional context the heterogeneity of an assemblage most probably bears little or no relationship to that of the original contributing communities gastaldo9 suggests that reconstructions should be based on the autecology of erect in situ plants because with peats permineralized in situ precise resolution in time of laterally equivalent vegetation is difficult here i document an erect in situ early pennsylvanian lycophyte swamp which provides the first confirmatory evidence for previously hypothetical ecological amplitudes

mafic intrusions in the footwall of the sudbury igneous complex origin of the sudbury impact melt sheet and its associated ore deposits abstract the majority of the mafic intrusions and volcanic rocks in the footwall of the sudbury igneous complex sic which is believed to have formed from a melt sheet produced by the impact of a very large bolide belong to one of two large mafic igneous suites viz the 2450 ma east bull lake ebl suite of intrusions or the 2200 ma nipissing diabase suite of intrusions this has been established by u pb la icp ms dating of zircons from the newly recognized frood intrusion which hosts the frood and stobie ore bodies and two of the sudbury gabbro intrusions and comparison of the geochemistry of these intrusions with that of the ebl intrusive suite and the nipissing diabase suite the age of the frood intrusion 2421 32 ma falls within error of other ebl type intrusions whereas the ages of the nairn 2203 11 ma and totten 2168 11 ma intrusions fall within error of published ages for the nipissing diabase moreover the primitive mantle normalized patterns of the petrogenetic trace elements on extended spidergrams of the nairn and totten sudbury gabbros are similar to those of the nipissing diabase as are those of the frood intrusion to other members of the ebl intrusive suite the ni cu pge sulfide deposits at sudbury are associated with the lower contact of the sic where a discontinuous inclusion rich norite contains much of the mineralization at the contact and an inclusion bearing quartz diorite is directly associated with the mineralization of the radial and concentric offset dykes the sublayer rocks and the overlying unit of melanocratic norites provide strong evidence for a relationship between the ore deposits and both inclusions and melts of mafic ultramafic target rocks the most common pre impact mafic ultramafic target rocks are intrusions of the ebl and nipissing diabase suites although there are also archean aged mafic ultramafic enclaves in the gneisses beneath the north range of the sic the totten orebody in the worthington offset and the kelly lake ore body in the copper cliff offset are directly associated with footwall rocks comprising sudbury gabbro and nipissing gabbro respectively whereas the frood stobie offset and the associated frood and stobie ore deposits are sandwiched in a breccia belt containing large rafts of amphibole megacrystic gabbro belonging to the extensively broken up frood intrusion olivine bearing mafic and ultramafic rocks some of which exhibit shock metamorphic features but others do not are common inclusion types and occur in diverse sublayer environments the unshocked melanorite and olivine melanorite inclusions share common geochemical characteristicsde indicating that they were derived from a similar source their geochemistry is also similar to that of the frood intrusion as well as the east bull lake intrusion this indicates that they may have been sourced from east bull lake type intrusions that were incorporated into the melt sheet at the time of melt generation mass balance calculations indicate that the felsic country rocks which account for 77 of the total volume of the sic and contain 18 ppm ni and 21 ppm cu together with the sulfide poor mafic country rocks which account for 23 of the sic and contain 133 ppm ni and 92 ppm cu have too little ni and cu to account for the ni and cu contents of the quartz diorite the composition of which provides an indication of that of the initial melt sheet calculations using the r factor equation of campbell and naldrett 1979 indicate that the magmas that formed the norites and the ore deposits had 137 ppm ni and 121 ppm cu mass balance calculations have been used to determine if the country rocks have sufficient ni and cu in them to generate the ore deposits the weighted median ni content of the sulfide poor

shear velocity structure of the laterally heterogeneous crust and uppermost mantle beneath the indian region abstract the shear velocity structure of the indian lithosphere is mapped by inverting regionalized rayleigh wave group velocities in time periods of 15 60 s the regionalized maps are used to subdivide the indian plate into several geologic units and determine the variation of velocity with depth in each unit the hedgehog monte carlo technique is used to obtain the shear wave velocity structure for each geologic unit revealing distinct velocity variations in the lower crust and uppermost mantle the indian shield has a high velocity 4 4 4 6 km s upper mantle which however is slower than other shields in the world the central indian platform comprised of proterozoic basins and cratons is marked by a distinct low velocity 4 0 4 2 km s upper mantle lower crustal velocities in the indian lithosphere generally range between 3 8 and 4 0 km s with the oceanic segments and the sedimentary basins marked by marginally higher and lower velocities respectively a remarkable contrast is observed in upper mantle velocities between the northern and eastern convergence fronts of the indian plate the south bruma region along the eastern subduction front of the indian oceanic lithosphere shows significant velocity enhancement in the lower crust and upper mantle high velocities 4 8 km s are also observed in the upper mantle beneath the ninetyeast ridge in the northeastern indian ocean

on the origin of the seismic anisotropy of the lithosphere summary it is known that flow in the mantle can produce preferred orientation in olivine crystals with seismic anisotropy as a consequence flow in the subcrustal lithosphere is unlikely because of the high viscosity lenses of high temperature and low viscosity anomalous mantle are located under the crust in many tectonically active regions and viscous flow can easily arise in such material resulting in seismic anisotropy after cooling such anomalous mantle acquires high viscosity and becomes incorporated into the lithospheric layer preserving the anisotropy produced by the flows which existed previously the interaction of the stresses with cracks in the upper crust can be one of the causes of anisotropy in this layer

up down or sideways emplacement of magmatic fe ni cu pge sulfide melts in large igneous provinces the preferential localization of fe ni cu pge sulfides within the horizontal components of dike sill lava flow complexes in large igneous provinces lips indicates that they were fluid dynamic tra

the westphalian d fossil lepidodendrid forest at table head sydney basin nova scotia sedimentology paleoecology and floral response to changing edaphic conditions copyright c 1996 elsevier science b v all rights reserved strata of westphalian d age on the western coast of the sydney basin expose a fossil forest of approximately 30 lepidodendrid trees within one of several clastic splits of the harbour seam a mutidisciplinary approach was employed to interpret the origins of the coal bed the depositional history of the site and the response of the fossil forest to changing edaphic conditions the megaspore and miospore records indicate that the mire vegetation was dominated by arboreous lycopsids especially paralycopodites with subdominant tree ferns petrographic palynological and geochemical evidence suggest that the harbour coal bed at table head originated as a rheotrophic cf planarr mire eutric histosolr the mire forest is interpreted to have been engulfed by prograding distributary channel sediments sparse protist assemblages are suggestive of a freshwater delta plain lake environment occasionally in contact with brackish waters lepidodendrids persisted as site colonizers of clastic substrates even after burial of the rheotrophic peatland and influenced the morphology of deposited sediment but apparently were unable to colonize distributary channels equivocal taxonomic data compression fossilsr show the fossil forest to have been composed of both monocarpic lepidodendronr and polycarpic diaphorodendron paralycopodites sigillariar lycopsids genera recorded in the palynology of the uppermost ply of the underlying coal bed comparatively rare within the clastic beds of the fossil forest however is the stem compression of paralycopodites whose dispersed megapores and miospores dominate the underlying coal bed tree diameter data recorded equivalent to breast height indicate a forest of mixed age these data would appear to suggest that some lepidodendrids employing a polycarpic reproductive strategy were better able to cross the ecological barrier imposed between peat and clastic substrates foliar compressions indicate that an understory or stand of psaronius type tree ferns co existed with the lepidodendrids on clastic substrates which developed as incipient gleysol soils the entombment of the forest can be ascribed to its distributary coastal setting local subsidence and a seasonal climate that fostered wildfire and increased sedimentation

common ground between two british pennsylvanian wetland floras using large first hand datasets to assess utility of historical museum collections abstract studies on the macrofloral record of the pennsylvanian palaeotropical peat forming wetlands coal swamps depend heavily on historical collections in museums this paper examines the robustness of such collections for macrofloral studies 110 000 individually recorded specimens from south wales field collected by david davies during the early 20th century were compared with the 6500 museum voucher specimens derived from them eventually intended for a representative teaching collection this showed that the voucher collections accurately reflect the order of abundances of the plant groups as documented in the field records but not their actual numerical abundances these welsh data were then compared with the available museum collections for similar aged macrofloras in the nearby bristol somerset coalfield cluster analyses and detrended correspondence analyses were performed 1 on the raw data recorded on the museum labels 2 on standardised data that were updated to reflect the currently accepted taxonomy for these fossils and 3 on standardised data in which single occurrences of taxa have been removed the results suggest that unstandardised museum collection data can reveal meaningful biostratigraphical and biogeographical patterns but that these become clearer with the standardised data the effect of removing the singleton taxa was less clear although it did provide a slightly enhanced resolution with cluster analysis whilst previous studies have considered these to be separate basins with the bristol somerset coalfield being proximal to south wales on the mid pennsylvanian variscan front results show an overall consistency between the two basins and suggest a common gene pool and vegetational physiognomy as well as a general stasis through time based on relatively stable environmental and growth conditions the study confirms that even unrevised historical collections can provide some data to help understand the dynamics of these systems however it also underlines the importance of taxonomy in such studies as it helps improve both the biostratigraphical and biogeographical resolution

the east bull lake intrusive suite remnants of a 2 48 ga large igneous and metallogenic province in the sudbury area of the canadian shield the 2 49 to 2 475 ga intrusions of the east bull lake intrusive suite occur in an east northeast trending discontinuous belt along the presently exposed boundary between the archean superior and the proterozoic southern provinces of the canadian shield near sudbury ontario the east bull lake intrusive suite is part of a regional paleoproterozoic magmatic event extending from 2 49 to 2 44 ga which also includes bimodal volcanic rocks felsic plutons and the regionally extensive hearst and matachewan dike swarms this regional magmatic event is thought to be the result of a mantle plume driven intracontinental rifting event that led to the development of a major basin to the south that was subsequently filled by sedimentary rocks of the huronian supergroup the east bull lake intrusive suite appears to have been emplaced along a major axial rift fault related to this rifting event compelling field and geochemical evidence summarized here indicates that the three largest east bull lake suite intrusions the east bull lake agnew lake and river valley intrusions crystallized from similar low ti high al tholeiitic parent magmas that originated in deeper more basic chambers it is proposed that the primary magmas for the east bull lake suite intrusions were second stage melts derived from a sublithospheric depleted mantle source that had been modified by neoarchean subduction and accretion events most of the petrological characteristics of the east bull lake suite intrusions reflect plagioclase dominated fractional crystallization that generated a pronounced fe enrichment trend in the residual magmas orthopyroxene is the high temperature pyroxene and occurs as a cumulus phase through most of the stratigraphy of each intrusion of the suite local olivine rich cumulates present in all of the major east bull lake suite plutons in some instances appear to be comagmatic with associated plagioclase rich cumulates a distinctive marginal facies comprising brecciated and locally thermally recrystallized and or partially melted footwall rocks is a common feature of the east bull lake suite plutons and indicates that a high energy flow regime existed during the initial stages of emplacement this marginal unit typically grades into a heterolithic inclusion rich gabbronorite that contains erratically distributed mafic autoliths and footwall derived xenoliths chemical contamination of the mineralized matrix of these inclusion bearing rocks can be extreme at a local scale the inclusion rich zone is overlain by a thick interval of undifferentiated plagioclase rich cumulates that locally display spectacular glomerocrystic textures rhythmically and irregularly modally layered leucogabbronorite and gabbronorite make up much of the middle parts of the stratigraphy massive to crudely layered ferrogabbro and ferrosyenite only occur in the uppermost part of the agnew lake intrusion varitextured gabbro and pegmatitic gabbro occur as irregular layers and local veins and pods throughout the stratigraphy the east bull lake suite is currently being explored for contact type platinum group element pge cu ni mineralization that has a clear spatial association with orthopyroxene rich cumulates which are otherwise poorly represented in these intrusions disseminated pge and cu rich sulfide mineralization is concentrated within the inclusion bearing zones but extends upward into overlying plagioclase cumulates and appears to have collected along the lower margins of these intrusions through the combined action of vigorous convection of the resident magma and downward percolation of dense mafic residual liquid the resultant narrow zones of sulfide mineralization were thus derived from a much larger volume of parent magma feeder dikes to the mineralized parts of the intrusions appear to have been saturated in sulfide upon emplacement and had relatively high background pge contents compared to those for other known magmatic pge deposits however the economic potential of the contact type mineralization hinges on the efficacy of the physical concentration process which upgraded the metal content and size of the original sulfide particles inside the magma chambers based on the proposed basal accumulation model the best prospective mineralization is most likely to occur in embayments along the sidewall and floor of the intrusions

evidence for anisotropy in north east africa from geographical and azimuthal distribution of rayleigh wave velocities and average upper mantle structure long period surface waves from wwssn gdsn sro and geoscope stations are analysed to obtain the dispersion of surface waves in the north east of africa lateral variations of rayleigh wave velocities are retrieved simultaneously with the average azimuthal anisotropy as a first result the low velocity anomaly under the red sea is apparent at all periods a second result is a nearly north south direction of maximum group and phase velocities this azimuthal anisotropy direction is in agreement with the direction of absolute plate motion in this region anisotropy is also evidenced by simultaneous inversion of love and rayleigh wave dispersion along the path tam agd in the resulting models isotropic and anisotropic very low mantle sv wave velocities extend up to the moho for sh waves we find a strong discrepancy with sv waves sh wave velocities being higher by 5 down to 200 km under the crust this is characteristic of lithospheric polarization anisotropy the s wave anisotropy polarization and azimuthal found in this region including the red sea is compatible with the absolute motion of the african plate it can be interpreted considering the low lithospheric s velocities as a consequence of plastic flow in the weak lithosphere of an anomalous mantle this anisotropy evidenced here on s waves may be due as pn and lithospheric s wave anisotropy found in other continental areas to dynamical processes related to plate motion

variation in trace element content of magnetite crystallized from a fractionating sulfide liquid sudbury canada implications for provenance discrimination abstract laser ablation icp ms analysis has been applied to many accessory minerals in order to understand better the process by which the rock formed and for provenance discrimination we have determined trace element concentrations of fe oxides in massive sulfides that form ni cu pge deposits at the base of the sudbury igneous complex in canada the samples represent the crystallization products of fractionating sulfide liquids and consist of early forming fe rich monosulfide solution mss cumulates and residual cu rich intermediate solid solution iss this study shows that fe oxide geochemistry is a sensitive petrogenetic indicator for the degree of fractionation of the sulfide liquid and provides an insight into the partitioning of elements between sulfide and fe oxide phases in addition it is useful in determining the provenance of detrital fe oxide in a sulfide melt all lithophile elements cr ti v al mn sc nb ga ge ta hf w and zr are compatible into fe oxide the concentrations of these elements are highest in the early forming fe oxide titanomagnetite which crystallized with fe rich mss upon the continual crystallization of fe oxide from the sulfide liquid the lithophile elements gradually decrease so that late forming fe oxide magnetite which crystallized from the residual cu rich liquid is depleted in these elements this behavior is in contrast with fe oxides that crystallized from a fractionating silicate melt whereby the concentration of incompatible elements such as ti increases rather than decreases the behavior of the chalcophile elements in magnetite is largely controlled by the crystallization of the sulfide minerals with only ni co zn mo sn and pb present above detection limit in magnetite nickel mo and co are compatible in fe rich mss and thus the co crystallizing fe oxide is depleted in these elements in contrast magnetite that crystallized later from the fractionated liquid with cu rich iss is enriched in ni mo and co because fe rich mss is absent the concentrations of sn and pb which are incompatible with fe rich mss are highest in magnetite that formed from the fractionated cu rich liquid at subsolidus temperatures ilmenite exsolved from titanomagnetite whereas al spinel exsolved from the cores of some magnetite locally redistributing the trace elements however during laser ablation icp ms analysis of these fe oxides both the magnetite and its exsolution products are ablated so that the analysis represents the original magmatic composition of the fe oxide that crystallized from the sulfide melt

paleomagnetism and u pb geochronology of easterly trending dykes in the dharwar craton india feldspar clouding radiating dyke swarms and the position of india at 2 37 ga abstract a u pb baddeleyite age of 2367 1 ma from a diabase dyke together with previously published age data suggests that a major early proterozoic dyke swarm cuts across the structural grain of the archean dharwar craton in india paleomagnetic data suggest the swarm is at least 300 km wide and 300 km long and has a fan angle of at least 30 with convergence to the west it was originally emplaced at high latitudes and together with the widgiemooltha dykes of the yilgarn block of australia may have been a segment of a larger radiating swarm related to a long lived plume event that was active for about 50 my from 2418 to 2367 ma a regional change in the intensity of brown feldspar clouding in the dykes suggests that the dharwar craton was tilted northwards in harmony with previous observations on the structure and metamorphism of the archean rocks towards the south the brown feldspar clouding becomes more intense and locally assumes a blacker more sooty appearance the black clouding whose precise origins remain unknown is accompanied by a remagnetization and appears to coincide closely with a region of carbonatite magmatism at 800 ma and with a shear zone and change in structural trend related to pan african deformation at 550 ma paleomagnetic studies suggest that i the high coercivity part of the remanent magnetization is carried by magnetite exsolved within either brown or black clouded feldspars and ii as a more general observation diabase with brown clouded feldspar can carry a primary magnetization but not if the clouding is black

the paleoproterozoic marathon large igneous province new evidence for a 2 1 ga long lived mantle plume event along the southern margin of the north american superior province abstract three paleoproterozoic diabase dyke swarms the marathon kapuskasing and fort frances which outcrop around the northern margin of lake superior radiate from a focal region south of the lake u pb dating shows that the marathon and kapuskasing swarms range in age between 2126 and 2101 ma and the fort frances between 2076 and 2067 ma the convergence of the dyke swarms to a common focal area and the lack of apparent polar wander during the time interval 2125 2067 ma suggest that a single periodically active plume was responsible this 60 m y period of magmatism manifest over an area of at least 400 000 km2 is designated as the marathon large igneous province only a single magnetic field reversal has been recognized throughout the 60 m y of plume activity this observation together with u pb geochronology has helped to place dykes within each swarm in a chronological order based on their magnetic polarity for the dyke population as a whole older dykes tend to show more enriched and more fractionated incompatible trace element patterns compared to younger ones a result which suggests that the marathon kapuskasing and fort frances swarms are genetically related and underwent a secular reduction both in the degree of crustal contamination and or depth of mantle melting over the course of plume activity a similar geochemical relationship has also been found for the 2171 2166 ma biscotasing swarm but determining whether it is a common feature of plume development must await further study

2480 ma mafic magmatism in the northern black hills south dakota a new link connecting the wyoming and superior cratons the laramide black hills uplift of southwest south dakota exposes a precambrian crystalline core of 2560 2600 ma basement granitoids nonconformably overlain by two paleoproterozoic intracratonic rift successions in the northern black hills a 1 km thick layered sill the blue draw metagabbro that intrudes the older rift succession provides a key constraint on the timing of mafic magmatism and of older rift basin sedimentation ion microprobe spot analyses of megacrysts of magmatic titanite from a horizon of dioritic pegmatite in the uppermost sill portion yield a 207pb 206pb upper intercept age of 2480 6 ma all age errors 2 comparable to two point 207pb 206pb errorchron ages obtained by pb stepwise leaching of the same titanites nearly concordant domains in coexisting magmatic zircon yield apparent spot ages ranging from 2458 16 to 2284 20 ma i e differentially reset along u pb concordia and hornblende from an associated metadiorite yields a partially reset date with oldest apparent a

first two station results for long period surface waves velocity from the geoscope stations in africa rayleigh and love phase and group velocity dispersion measurements are performed along a purely african path between the geoscope stations tam and agr for the leewards isl earthquake of march 16 1985 this regional study is important as it brings deep seated structure information in a seismologically blank region of africa it enables to improve the spatial resolution of larger scale models for periods longer than 160 s and up to 250 s the fundamental rayleigh wave dispersion is in agreement with prediction by global models for shorter periods the results fall below this prediction which is an indication of lithospheric s wave velocities lower than those of the models the results are also compared with previous regionalizations the rayleigh second higher mode is extracted at agr and the phase velocity dispersion obtained in the period range 50 140 s is in agreement with the theoretical dispersion for prem

investigation of upper mantle structure under young regions of the southeast pacific using long period rayleigh waves abstract long period rayleigh wave group velocity dispersion curves are presented for paths across the east pacific ocean the records are from the ipg site at pamatai tahiti and the ida station at nana for the first time direct path observations of group velocities up to 300 s have been obtained this study shows that for young oceanic regions group velocities are low even at long periods the observations are interpreted in terms of an s wave velocity model by a generalized inversion scheme in the models for young ages the low velocity zone under the asthenosphere lid is well developed with a strong velocity gradient at the bottom of this zone followed at larger depths by a plateau representing a lower velocity zone and a marked gradient at 400 km

organic clastic facies relationships and chronostratigraphy of the barataria interlobe basin mississippi delta plain abstract the lithofacies of barataria basin an interlobe basin of the mississippi delta plain were established from analyses of 115 boreholes five organic lithofacies are distinguished by organic matter content botanical origin and degree of decomposition organic rich facies accumulated in three stages a b and c during two depositional events stage a developed locally but stages b and c developed as blankets over abandoned delta lobes protected from marine influx by a shoreline stage b lasted from 2 300 1 000 yr bp and ended when clastic sediments from renewed deltaic sedimentation blanketed the peats expanding the basin about 50 km seaward peat accumulation of stage c lasted from 900 yr bp to the present and is currently being terminated by subsidence induced transgression high quality stage b peats accumulated beneath herbaceous freshwater floating and emergent marshes mainly above the footwall of a strike oriented reactivated growth fault above the footwall the basin was shallower than on the hanging wall where the deeper basin invited more overbank sedimentation which increased ash content and diluted peats thus both tectonic and sedimentary factors influenced organic facies the quality and quantity of barataria basin peats differ from those of many deltaic coals however these peats accumulated in only 2 300 yr immediately after a rapid sea level rise frequent delta lobe switching increased organic matter preservation potential but did not provide sufficient time for the accumulation of thick low ash peats the geometries of organic and clastic facies are similar to those of some coal bearing strata

quest for palladium application of the groves hypothesis to precambrian terranes of northeast north america a compilation of all types of magmatic deposits in precambrian terranes of northeast north america revealed a continental scale metallic zonation nickel is homogeneously distributed palladium and chromite are centered on the archean craton as predicted by the groves hypothesis whereas low ti cu u rare earth element iron oxide deposits occur just south and east of the archean and paleoproterozoic margin within the grenville province ti is concentrated along this margin in association with anorthosite nickel is still abundant there but palladium and chromite are remarkably absent this grenvillian ni pd cr paradox suggests that the fractionation process invoked in the classical plume model is inadequate to explain the geochemical characteristics of the anorthosite parental magmas the crustal tongue melting model seems to better explain the metallic zonation observed at the margin of the archean and proterozoic terranes

iridium and palladium as discriminants of volcanic exhalative hydrothermal and magmatic nickel sulfide mineralization neutron activation of pd au and ir in samples from 1 economically significant ni sulfide deposits of western australia 2 the host rocks of these deposits 3 hydrothermal and or metasomatic ni sulfide mainly millerite mineralization in peridotite at nullagine western australia 4 hydrothermal replacement volcanic exhalative ni cu sulfide mineralization at sherlock bay western australia and 5 volcanic exhalative sulfides in inter flow sediments at kambalda are used to show that ir and pd can be used to discriminate between the various origins magmatic volcanic exhalative hydrothermal metasomatic and metamorphic ascribed to ni sulfide deposits all the economically significant ni sulfide deposits in western australia are strongly enriched in the precious metals relative to other crustal rocks and have similar pd ni and ir ni ratios to each other these ratios persist regardless of whether the nickel sulfides occur at the base of volcanic ultramafic piles e g kambalda or as centrally disposed interstitial sulfides in intrusive dunite bodies the strong similarities of the pd ir ratios of the ni sulfide ores and of the spinilex textured komatiites which represent ultramafic liquids from which the ni sulfide liquids were derived together with other features of the precious metal geochemistry of the ores and host rocks of the ni sulfide deposits are used to show that the deposits had magmatic origins in contrast the pd ir and ir ni ratios of the hydrothermal and or metasomatic ni sulfide mineralization at nullagine are distinctly different from those of the major ni deposits the former contains high au moderate but variable pd and exceptionally low ir similarly the volcanic exhalative sulfides at kambalda and sherlock bay contain negligible quantities of pd and ir all the ir in both the interflow sediments and the hydrothermal metasomatic mineralization is contributed by the host rocks and not by the sulfides the western australian data together with precious metal data for other examples of hydrothermal ni cu sulfide mineralization strongly suggest that although ni au and pd may be transported in aqueous solution ir cannot hence the relative abundance of ir can be used as a discriminant of magmatic sulfides

paleoecology of the springfield coal member desmoinesian illinois basin near the leslie cemetery paleochannel southwestern indiana abstract the springfield coal member carbondale group petersburg formation of indiana is split locally in warrick and gibson counties indiana by clastic rocks of the folsomville member carbondale group petersburg formation that represent the leslie cemetery paleochannel part of a large interconnected paleochannel system in the springfield coal bed this study incorporated analysis of miospore and megaspore assemblages coal petrography plant compression fossils and conodonts from the coal and clastic split to document changes in the swamp and its vegetation in response to the activity of the leslie cemetery paleochannel palynological and petrographic data indicate that environmental conditions and vegetation in the lower bench of coal near the leslie cemetery paleochannel were similar to those found in profiles through the coal bed at sites near the larger more extensive galatia paleochannel miospore floras of the high vitrinite lower bench of coal were dominated by tree fern miospores with those of lycopsids ranking second in abundance megaspore assemblages were dominated by lycopsid megaspores near the contacy between the lower bench of coal and clastic split medullosan prepollen increases in abundance and fusain bands are more common which may indicate the occurrence of fire either within or outside the swamp vegetation in the folsomville member and upper bench of coal differ markedly from that of the lower bench of coal plant megafossils from the folsomville member indicate dominance by pteridosperms and or lycopsids and folsomville member miospore assemblages are dominated by lycopsid miospores most typically found in mudstones including lycospora torquifer and higher than normal abundances of granasporites medius in the transition from clastic rocks of the folsomville member to the upper bench of coal miospores of ecotonal lycopsids paralycopodites reach their peak abundance pteridosperms dominate megafossil assemblages and inertinite levels are relatively high at higher levels of the upper bench of coal vitrinite levels are higher and miospore assemblages are dominated by lycopsid miospores typical of coal swamps lycospora granulata l pusilla the presence of conodonts in coalballs in the upper bench of coal and spirorbid worms in the split indicate that the paleoswamp may have received some marine or brackish water influence higher than normal salinity levels may explain the vegetational changes observed in the upper bench of coal near the leslie cemetery paleochannel

on the structure of the crust and upper mantle beneath the afro arabian region from surface wave dispersion abstract the deep structure beneath the afro arabian region was investigated using rayleigh and love waves in the period range 20 250 s models of crustal and upper mantle structure were inferred by simultaneous inversion of both phase and group velocities our choice of model parametrization allowed for a transverse isotropic medium and a variable depth of the moho the crustal thicknesses were found to vary from 43 to 45 km on the african plate the moho is deeper in the arabian plate 45 47 km but in the vicinity of the red sea the crust becomes much thinner 30 km the upper mantle under arabia is more similar to that of an african mobile zone than to that of a shield a low velocity zone lvz with an s wave velocity of 4 14 4 2 km s was found in the depth range 120 140 km however this lvz is less pronounced than that found in the vicinity of the red sea s wave velocity 4 08 km s in agreement with previous studies the anisotropy v sh v sv v sv has been verified for the afro arabian region we interpret the anisotropies uisng the hypothesis of lattice preferred orientation of the olivine crystals in the upper mantle the inference is that a vertical flow exists up to near the surface in the vicinity of the red sea rift vsv is 8 9 faster than vsh under the african and arabian sides the flow is essentially vertical below depths of 200 40 km and becomes essentially horizontal above with a vsh from 6 to 14 faster than vsv

geochemistry and mineralogy of platinum group element mineralization in the river valley intrusion ontario canada a model for early stage sulfur saturation and multistage emplacement and the implications for contact type ni cu pge sulfide mineralization the river valley intrusion within the 2 48 ga east bull lake intrusive suite in ontario canada is an example of a mafic igneous intrusion with contact type ni cu pge sulfide mineralization along its base whereas many contact type deposits are thought to form from in situ contamination of the magma by the addition of crustal s during emplacement there are some intrusions including the river valley intrusion which appear to have a much more complex history where the timing of s saturation and thus the critical ore genesis processes may have occurred much earlier prior to emplacement the river valley intrusion is made up of a basal 100 m of unlayered inclusion bearing units overlain by layered cumulates the basal units contain autoliths of gabbroic rocks and inclusions of footwall gneiss and amphibolites all within a gabbroic matrix platinum group element rich magmatic sulfide mineralization occurs throughout both the inclusions and the matrix as blebby and disseminated sulfides the matrix and inclusions can be separated into two distinct textural types hydrothermally altered greenschist assemblages and unaltered metamorphic amphibolite assemblages the platinum group mineral pgm assemblages vary only between textural types and not between inclusions and matrix being dominated tellurides in all rock types the hydrothermally altered rocks however have fewer tellurides and an increased amount of sb and as bearing pgm indicative of minor fluid interaction although the pgm have not been mobilized significantly away from the base metal sulfides precious and base metal geochemistry shows all rock types to have an excellent correlation between all the platinum group elements pge indicating the presence of a single well homogenized pge rich sulfide liquid however au and cu appear to be decoupled from the pge at low concentrations although correlate well with each other which is interpreted to be due to minor fluid redistribution and alteration of sulfide bleb margins the overlying layered units above the mineralized units are not pge depleted trace element data including th yb pm and nb th pm ratios demonstrate that all river valley rocks were formed from crustally contaminated magmas following interaction with local country rocks in a deeper subchamber although some samples have s se ratios indicative of crustal s most have s se ratios lower than the mantle range indicative of s loss we propose a multistage model for the formation of the mineralization in the river valley intrusion with a major contamination event at depth with the addition of s from local crustal rocks inducing sulfide saturation sulfide droplets were then enriched in pge within a conduit system with possible further upgrading of sulfide metal tenors and reduction of s se ratios via partial dissolution of sulfide the pge enriched sulfide liquid then settled in a staging chamber and partially crystallized before a major pulse of magma entrained sulfide liquid eroded blocks of precrystallized and mineralized gabbro and footwall rocks and emplaced an inclusion bearing package as the lower 100 m or so of the river valley intrusion later emplacement of main river valley magma was from an s undersaturated pge fertile magma the river valley intrusion is thus an example whereby contact type mineralization is purely a function of the earliest magma intruded containing preformed sulfide mineralization rather than contamination triggering sulfide saturation in situ in such cases processes at depth determine the generation and subsequent tenor of the mineralization in particular dissolution of the sulfide can upgrade metal tenor but subsequently will reduce s se ratios masking the signature of crustal contamination in addition a multistage emplacement such as this will not necessarily preserve the characteristic increase in cu pd ratios in the overlying cumulates that is often used in exploration for pge deposits in mafic intrusions thus a full understanding of all the field geochemical and mineralogical factors is required to construct genetic models for such deposits and especially in the interpretation of s se and cu pd ratios as an indicator of crustal contamination and the presence of pge mineralization

anisotropy of the african continent inferred from surface waves abstract a no block regionalization is performed over the african continent to retrieve simultaneously local surface wave velocities and the 2 azimuthal variation of rayleigh waves the geographical distribution of phase and group velocities shows general agreement with global models but with enhanced lateral resolution a correlation with surface tectonics is found even at long periods low velocities occur in the tectonically active regions and high velocities in the shield areas the azimuthal anisotropy analysis results in a general north south fastest direction for rayleigh waves which could be correlated with the african plate s absolute motion the amplitude is of the order of 2 an isotropic inversion with data corrected for effects of surficial layers is performed very slight differences appear between the corrected and uncorrected results lateral heterogeneities of rayleigh and love waves are then inverted simultaneously with the 2 azimuthal terms of rayleigh waves by vectorial tomography to retrieve the 3d structures of s wave velocity and anisotropy throughout the african continent good correlation between low velocities and small anisotropies is found and their location at depth suggests an origin for african hotspots

age petrogenesis and tectonic setting of the thessalon volcanic rocks huronian supergroup canada abstract the 2 5 ga huronian supergroup volcanic rocks in northern ontario represent one of the better preserved paleoproterozoic continental flood basalt sequences on earth the youngest age of huronian supergroup volcanism is refined to 2452 5 6 2 ma based on new u pb zircon analyses of the copper cliff rhyolite recent mapping and geochemical analyses of thessalon formation volcanic rocks define a well developed chemostratigraphy a lower series consisting of four geochemically distinct mafic lava sequences and an upper basalt andesite volcanic series these two mafic volcanic series are separated by felsic volcanic rocks rhyolite and dacite the mafic lavas are low ti tholeiites dominated by basalt and andesite and are characterized by enriched light rare earth element profiles and high th contents up to 12 ppm most basalt andesite sequences show some internal geochemical variation reflecting a combination of fractional crystallization and in some cases limited crustal contamination but each sequence represents a discrete magma batch derived by different degrees of partial melting in the subcontinental lithospheric mantle all magma batches display prominent negative nb ta and ti anomalies on primitive mantle normalized multi element diagrams interpreted to reflect melting of a heterogeneous and previously metasomatized garnet lherzolite that acquired some chemical characteristics of a source typical of subduction zone magmatism at least three distinct mantle source compositions are required to explain the range in the observed lava compositions the most unusual of these reservoirs has a number of geochemical features akin to upper continental crust and is characterized by high th yb 10 and low zr nb 11 this reservoir is interpreted to be peridotite modified through metasomatism and entrapment of arc magmas during northward neoarchean subduction along the southern superior craton margin