trondhjemitic 1 35 1 31 ga gneisses of the mount holly complex of vermont evidence for an elzevirian event in the grenville basement of the united states appalachians a newly recognized suite of trondhjemite tonalite and dacitic gneiss forms a 10 km wide belt of rocks within the mount holly complex in the central part of the green mountain massif of vermont field relationships and chemistry indicate that these gneisses are calc alkaline volcanic and hypabyssal plutonic rocks older than the middle proterozoic regional deformation that affected the mount holly complex u pb zircon dates indicate ages as great as 1 35 ga for crystallization of the volcanic protoliths and for intrusion of crosscutting trondhjemite tonalitic plutonism continued until 1 31 ga map scale contacts between the trondhjemitic tonalitic dacitic gneisses and the paragneiss sequence of the mount holly complex are sharp suggesting that the volcanic rocks of the trondhjemite tonalite suite underlie the paragneiss units and do not intrude them these relationships suggest that the trondhjemite tonalite suite is either considerably older than and unconformable beneath the paragneiss cover rocks or

could bacteria have formed the precambrian banded iron formations banded iron formations bifs are prominent sedimentary deposits of the precambrian but despite a century of endeavor the mechanisms of their deposition are still unresolved interactions between microorganisms and dissolved ferrous iron in the ancient oceans offer one plausible means of mineral precipitation in which bacteria directly generate ferric iron either by chemolithoautotrophic iron oxidation or by photoferrotrophy on the basis of chemical analyses from bif units of the 2 5 ga hamersley group western australia we show here that even during periods of maximum iron precipitation most if not all of the iron in bifs could be precipitated by iron oxidizing bacteria in cell densities considerably less than those found in modern fe rich aqueous environments those ancient microorganisms would also have been easily supported by the concentrations of nutrients p and trace metals v mn co zn and mo found within the same iron rich bands these calculations highlight the potential importance of early microbial activity on ancient metal cycling

bridging the gap between the foreland and hinterland i geochronology and plate tectonic geometry of ordovician magmatism and terrane accretion on the laurentian margin of new england

pressure temperature and time indicators in mafic schist their application to reconstructing the polymetamorphic history of vermont mineral chemistry and overgrowth relationships in mafic schist from vermont are shown to be sensitive indicators of pressure temperature and relative time and to be good chronicles of the paleozoic history of this polymetamorphic terrane within the common assemblage amphibole chlorite epidote plagioclase quartz ti phase carbonate k mica fe 3 oxide electron microprobe analyses show that increasing metamorphic grade as defined by intercalated pelitic schist is recorded by an increase in the edenite glaucophane and tschermakite contents of amphibole in the anorthite content of plagioclase and in the substitution of al vi f 2 ti al iv for fe 2 mg mn si in biotite chlorite and muscovite with increasing pressure the glaucophane component of amphibole increases for medium pressure metamorphism the albite oligoclase gap is in the garnet zone where amphibole has between 1 2 and 1 8 formula proportion al iv and al vi fe 3 ti cr this gap is below the garnet isograd in low pressure mafic schist where al iv and al vi fe 3 ti cr in the amphibole are both less than 0 6 mineral growth periods observed are characterized by metamorphic grade and facies series and are assigned to two ordovician taconic and two devonian acadian events silurian devonian mafic schist in northeastern vermont records two periods of low pressure devonian metamorphism these events are expressed by medium pressure and low pressure metamorphism in the cambrian and ordovician rocks to the west and south in the pre silurian rocks high pressure and medium pressure metamorphism is assigned to the ordovician the observed high pressure metamorphism is confined to a 110 by 40 km area along the green mountain anticlinorium axis in north central vermont

early obduction related deformation features of the taconic allochthon analogy with structures observed in modern trench environments an early generation of probably recumbent isoclinal folds and veined and slickensided faults has now been documented at several localities in the western taconic allochthon these folds and thrust faults predate development of the regional slaty cleavage and associated folds the early generation of structures either predates or developed synchronously with obduction of the taconic allochthon on the coeval early paleozoic shelf of eastern north america during early stages of the taconic orogeny evidence for this early phase of deformation is threefold 1 recognition of small scale downward facing folds at single outcrops 2 local occurrences of downward younging directions structural facing directions projected on slaty cleavage surfaces and 3 large scale inversion of stratigraphic succession subsequently refolded by the slaty cleavage generation of structures clear examples of case 3 are seen at willard mountain and near granville new york the amplitude and wavelength of the early folds are measured in kilometres but structural and stratigraphic observations are inconsistent with a regionally recumbent structure of the entire giddings brook slice of the taconic allochthon these examples provide convincing evidence for complex polyphase deformation of the taconic allochthon prior to thrusting into its present structural position these observations accord well with recent large scale models that suggest emplacement of the taconics in a collision related subduction accretion environment deep sea drilling project dsdp and geophysical data from modern trenches suggest that similar scale and style of deformation may occur during incorporation of thick already lithified sedimentary sequences into the trenchward side of accretionary prism complexes

crustal profile of mountain belt cocorp deep seismic reflection profiling in new england appalachians and implications for architecture of convergent mountain chains relatively shallow reflections from beneath the taconic allochthon and green mountain anticlinorium are suggested to mark a zone of detachment that carries precambrian grenville basement of the green mountains in its upper plate the data suggest that tectonic thickening of the shelf sedimentary and synorogenic flysch sequence beneath the taconic allochthon has occurred and that major thrust fault systems champlain thrust of western vermont which imbricate shelf lithologies project downplunge into the seismic profile eastward dipping reflections beneath the east flank of the green mountains appear to project across the connecticut river valley into new hampshire because some of these reflections can be projected into an exposed zone of probable thrust imbricated precambrian grenville basement and younger metasediments there is a strong implication that similar rock types exist in the deeper crust beneath new hampshire the gross geometry of the eastward dipping zone is a large thrust ramp that facilitated faulting of continental basement in the green mountains to higher structural levels thus suggesting the anticlinorium is a large hanging wall structure or ramp anticlinorium highly deformed satellitic massifs of grenville basement such as the chester dome and sub silurian and silurian devonian rocks of the bronson hill anticlinorium and merrimack synclinorium are probably transported along intracrustal detachments as well data suggest the possiblity of a tectonically buried continental margin transition zone much like that interpreted for the southern appalachians this zone would have been preserved from prior rifting of the proto atlantic ocean and its inherited geometry could have controlled subsequent convergent deformation during appalachian orogenesis although regional deep seismic reflection profiling has not been carried out across the alps existing data imply lateral changes in crustal reflection character similar to those observed in the appalachians deep seismic reflection data from convergent mountain belts raises the possibility of such preserved transition zones beneath transported rocks and provides an alternative interpretation to lithosphere penetrating sutures in explaining lateral geologic and geophysical contrasts in the crust modified journal abstract

the comparative behaviors of yttrium and lanthanides in the seawater of the north pacific yttrium has long been recognized as an ekalanthanide because of its chemical contiguity relative to the chemistries of rare earth elements which are in recent years intensively utilized for elucidation of metal scavenging processes in the ocean here we present the first detailed depth profile of y iii in seawater together with the lanthanides in the north pacific ocean the concentrations of y iii range 70 290 pmol kg and show the nutrient like profile best resembling that of ho iii amongst the other rare earth elements the results agree well with an expectation based on the similarity in their ionic radii and hence stability constants of complexation with carbonate ions yet the ho iii y iii ratios in seawater systematically increase with depth suggesting that y and ho are fractionated during scavenging by natural marine particulates this is likely to result from the different complexation behavior in that y iii is more weakly complexed than ho iii with soft organic ligands on the surface of particulate matter during scavenging in the surface water but once released into seawater in the deep sea y iii is complexed with carbonate ions equally or stronger than ho the pattern of deep water enrichment in the lanthanide series appears to be consistent with the recent observation of partitioning between suspended particles and seawater our precise measurements also indicated that pr and tb best resemble nd and dy respectively in their oceanic behavior whereas ho and tm are intermediate between their neighboring rare earth elements

geochemistry of pyrite from diamictites of the boolgeeda iron formation western australia with implications for the goe and paleoproterozoic ice ages abstract sediments of the ca 2400 ma turee creek group of western australia are coincident with the oxygenation of earth s surface associated with the great oxidation event goe diamictite within the boolgeeda iron formation from the boundary ridge section at the duck creek syncline was previously correlated to the glaciogenic sediments of the meteorite bore member of the turee creek group at the hardey syncline martin 1999 the meteorite bore member is considered correlative and time equivalent with the paleoproterozoic glacial diamictites of the 2 45 2 22 ga huronian supergroup in north america if the diamictite units at boundary ridge correspond to worldwide paleoproterozoic glaciations they should also record the disappearance of mass independently fractionated mif sulfur triple s isotope compositions for pyrites from the boundary ridge sections were measured in situ by a multi collector ion microprobe and yielded both mass dependently fractionated mdf and mif sulfur isotope values 33 s values from 0 44 to 6 27 trace element heterogeneities are indicated by measurements at multiple spatial scales within rounded pyrites in the boundary ridge section consistent with multiple generations of pyrite that incorporated sulfur processed in an anoxic atmosphere a compilation of s isotope data from pyrite in the boundary ridge diamictites analyzed in this study and previous work williford et al 2011 defines multiple 34 s vs 33 s arrays consistent with an origin of detrital pyrite from the underlying hamersley and fortescue groups sulfides in a shale unit from the boundary ridge section plot along the terrestrial fractionation line but still retain positive mif s this and detrital pyrite suggest low atmospheric oxygen immediately before the initiation of paleoproterozoic ice ages this data is inconsistent with a correlation to glacially influenced successions within the north american huronian supergroup in which the mif s signal permanently disappears the diamictites at the duck creek syncline are older than the meteorite bore member because their stratigraphic position is within the boolgeeda iron formation that underlies the turee creek group which is separated from the meteorite bore member by nearly 1000 m of the kungarra shale at the hardey syncline

crystallographic preferred orientation development by dissolution precipitation creep abstract crystallographic preferred orientations cpos in deformed rocks are commonly interpreted as resulting from crystal plastic deformation mechanisms where deformation is achieved by the movement of dislocations in this paper we investigate the possibility of cpo development by dissolution precipitation creep or pressure solution a numerical model is presented which simulates the development of a grain aggregate that deforms by reaction controlled dissolution precipitation creep grains are simulated as rectangular boxes that change their shape by growth or dissolution of their surfaces depending on the normal stresses acting on the individual surfaces grains can also rotate due to an applied vorticity for non coaxial deformation and if they have a non equidimensional shape for each strain increment stress that is applied to the grains is the same for all grains while individual grains deform and rotate by different amounts a variety of cpos develop at moderate strains depending on the reaction rates of the different crystal surfaces and type of deformation uni axial shortening plane strain pure shear and simple shear the modelling results confirm that dissolution precipitation creep may play a role in cpo development in rocks

petrology and geochemistry of the boolgeeda iron formation hamersley basin western australia abstract the boolgeeda iron formation and overlying turee creek group hamersley basin western australia represent a conformable succession of sediment deposited between 2 45 and 2 22 ga this interval of geologic history is of significant interest because it spans the great oxidation event goe when oxygen first accumulated in earth s atmosphere here we present geochemical and petrographic data from the uppermost 45 m of the boolgeeda iron formation and an additional 30 m of the overlying kungarra formation mudstones as sampled from the turee creek drilling project 1 drill core tcdp1 this core captures the termination of bif deposition in the hamersley basin and coincides with a global decline in bif deposition in the paleoproterozoic we provide a continuous high resolution chemostratigraphic dataset of major and trace element concentrations as well as fe speciation data to assess the relationship between the rise of atmospheric oxygen and the subsequent decline in bif deposition we also highlight the interplay between local and global controls on the preservation of redox signatures including the rise and fall of local base level input of weathered continental material influx of reduced sulfur species associated with the continental weathering and global increases in atmospheric oxygen we interpret boolgeeda deposition to have taken place under shallow water oxic conditions overlying anoxic ferruginous deep water intermittent periods of oxidative weathering led to influxes of redox sensitive trace elements from land as continental sulfide minerals were weathered this also led to the temporary disappearance of mif s but o2 remained below the threshold capable of completely eliminating s isotope signatures associated with the mif s recycling

geochemistry of the krivoy rog banded iron formation ukraine and the impact of peak episodes of increased global magmatic activity on the trace element composition of precambrian seawater abstract pure superior type banded iron formation bif samples from the krivoy rog supergroup ukraine are excellent archives of ambient early precambrian seawater they show low concentrations of incompatible elements such as zr hf and th and shale normalized rare earths and yttrium rey sn patterns similar to those of modern seawater i e heavy rey sn enriched patterns with positive la sn gd sn and y sn anomalies lack of ce sn and presence of positive eu sn anomalies indicate rey contributions to anoxic ferruginous seawater from high temperature hydrothermal fluids the depositional age of the krivoy rog bif is ill defined but a late archean to paleoproterozoic age has been suggested based on u pb zircon ages for units stratigraphically above and below the bif we determined sm nd isotopic compositions of pure and impure samples from the krivoy rog bif which yield an errorchron with an apparent age of 2406 350 ma mswd 15 that falls within this broad age range all pure bif samples show chondrite normalized subscript cn rey patterns with strong positive eu cn anomalies that are typical for archean but rather rare and much less pronounced in proterozoic bifs associated schists also show archean rather than post archean style rey distributions the rey geochemistry of both chemical and epiclastic sediments therefore is more consistent with a late archean rather than a post archean depositional age of the krivoy rog supergroup initial nd values of impure bifs and of associated schist reveal variable contributions from ttgs less radiogenic in nd and a more radiogenic component possibly comprised of basement amphibolites or mafic volcanics of the stratigraphically underlying new krivoy rog group the purest krivoy rog bif representing local krivoy rog seawater displays an nd 2 60 ga value of 2 3 this value is less radiogenic than impure krivoy rog bifs or other near contemporaneous neoarchean pure chemical sediments to preserve this specific local isotopic fingerprint in anoxic archean seawater the krivoy rog bif must have been deposited in an isolated sea basin with limited exchange with ferruginous deep waters of the open ocean a compilation of rey data for high purity precambrian bifs reveals that eu cn eu cn ratios of precambrian seawater follow a general global evolution curve that shows specific peaks which reflect times of increased high temperature hydrothermal rey input into seawater following declining eu cn eu cn ratios from the eoarchean to the mesoarchean the ratios suddenly rise at 2 7 ga and reach a maximum at 2 6 ga indicating an increased flux of high temperature hydrothermal rey to neoarchean seawater which supports the hypothesis that times of widespread bif deposition coincided with periods of intense submarine hydrothermal activity probably triggered by major mantle plume events this association is supported by a strong increase of the nd t values of pure seawater archives at 2 7 2 6 ga which reflects an increased flux of mantle nd into seawater these results suggest that eu rey systematics and potentially nd systematics are robust tools to indentify episodes of enhanced mantle plume activity

rare earth element systematics of the chemically precipitated component in early precambrian iron formations and the evolution of the terrestrial atmosphere hydrosphere lithosphere system irrespective of provenance age and metamorphic grade the chemically precipitated component in early precambrian 2 3 ga iron formations ifs displays smyb cn 1 which reflects the corresponding ratios of contemporaneous seawater in conjunction with end if end shale jacobsen and pimentel klose 1988a b this rare earth element ree signature reveals that the ree distribution in early precambrian ifs must be explained by mixing between a marine bottom and a surface water component and that the rees and by analogy the fe cannot be derived from weathering of a continental source eusm n ratios of detritus free ifs are controlled by the marine bottom water component where eusm cn 1 results from ree input from high temperature high t hydrothermal fluids which received their ree signature during high t alteration of oceanic crust at spreading centres decreasing eusm cn of ifs with decreasing depositional age reflect the vaning importance of high t compared to low t 200 c hydrothermal fluids hence oceanic crust alteration for the ree budget of marine bottom waters and are related to decreasing upper mantle temperatures smyb n ratios of detritus free ifs are controlled by the marine surface water component whose ree distribution is affected by the same processes which operate today in the entire ocean mixing calculations reveal that smyb cn in early precambrian marine surface waters was significantly lower than it is today to explain this difference two mechanisms are discussed on the basis of higher pco2 and lower po2 levels of the precambrian atmosphere 1 1 higher co32 in precambrian river and coastal waters and 2 2 redox cycling of the rees at an oceanwide chemocline which separated anoxic and oxic water masses however these processes are synergetic and could have operated in unison the ree distribution in precambrian ifs is described as a result of mixing in a multicomponent system where high t and low t hydrothermal fluids contributed to the marine bottom water and ree input from the dissolved ree pool in river waters after some modification in estuaries dominated the ree distribution in marine surface waters after mixing of bottom and surface waters their combined ree load was quantitatively scavenged onto precipitating iron oxihydroxides or iron carbonates and deposited with the chemically precipitated component after addition of variable amounts of clastic detritus this eventually resulted in the ree distribution observed in precambrian ifs

reactivation of prethrusting synconvergence normal faults as ramps within the ordovician champlain taconic thrust system we present new geologic maps of the lake champlain region of west central vermont and east central new york state this region contains a shallow crustal section of an ordovician foreland basin and a far traveled thrust system that transported the upper cambrian lower ordovician platform sequence allochthonous rise facies pelites and arenites and middle ordovician basinal shales and flysch early foreland and shelf sections are cut by post depositional cross faults within the thrust system and the required stratigraphic throw across the cross faults is not matched by the amount they offset the thrust planes as thrust structures these cross faults control structural style and kinematics by laterally bounding regions that contrast sharply in amount of thrust imbrication duplexing and net transport the dominant set of normal faults in the autochthon which had time correlative slip with the thrust system cross faults is oriented parallel to the paleotrench though there is also a subordinate transverse linking set the normal faults developed in response to the migrating flexural forebulge during convergence between the laurentian margin and the accretionary prism above an outboard facing subduction zone within the thrust system any prethrust normal faults that were parallel to the paleotrench are largely masked by thrust structures however an out of sequence thrust which emplaced shelf rocks above the westernmost allochthon and surrounding parautochthonous shale may have localized on a paleotrenchparallel normal fault in the outer shelf we also identify a postthrust normal fault that significantly obscures the original thrust map pattern reactivation of synconvergence prethrust flexure induced normal faults adequately explains many otherwise puzzling stratigraphic and structural relationships in the taconic foreland

age origin and tectonic significance of mesoproterozoic and silurian felsic sills in the berkshire massif massachusetts discontinuous sills of felsic gneiss in the interior and western margin of the berkshire massif and granite sills on the eastern margin of the massif were correlated by ratcliffe 1984a 1984b 1985 and ratcliffe and hatch 1979 and interpreted by them as syntectonic anatectic melts that intruded taconic thrusts we dated three felsic gneiss sills and two granite sills in an attempt to constrain the age of taconic thrusting but discovered that the sills are not coeval rather they were intruded during two widely separated episodes one during the mesoproterozoic at approximately 1000 ma and the other during the silurian at approximately 430 to 435 ma the 1000 ma sills of felsic gneiss in the interior of the massif are located in mesoproterozoic units and many of the mapped taconic thrusts within the massif closely follow the distribution of this unit here informally called the felsic gneiss of harmon brook these 1000 ma sills formed during the ottawan or rigolet orogeny and they have no connection to the taconic orogeny the 430 to 435 ma granite sills along the eastern margin of the massif informally called the granite of becket quarry are found in both mesoproterozoic basement and the neoproterozoic hoosac formation the sills are too young to have intruded during the ordovician taconic orogeny but they may have formed during later faulting near the contact between mesoproterozoic basement and neoproterozoic cover rocks the tyringham gneiss is one of the most common mesoproterozoic units in the berkshire massif zircons from the tyringham gneiss contain cores with oscillatory zoning and thin unzoned rims the weighted average of eight 206 pb 238 u analyses from the cores is 1179 9 ma whereas nine spot analyses from the rims yield an age of 1004 9 ma we interpret these two ages to represent the crystallization of the tyringham gneiss protolith and a subsequent high grade metamorphism coeval with the intrusion of the felsic gneiss of harmon brook the western contact between mesoproterozoic rocks of the berkshire massif and underlying early paleozoic rocks is clearly a thrust but there is no independent evidence that movement occurred during the taconic orogeny displacement may also have occurred during the silurian salinic or the devonian acadian orogeny many contacts mapped as taconic thrusts within the berkshire massif follow the distribution of the 1000 ma felsic gneiss of harmon brook the age of the sills is clearly incompatible with this interpretation and evidence for faulting along these mapped thrusts is lacking instead of being deformed into an imbricate stack the massif behaved as a rigid block during paleozoic uplift finally the age of granite sills along the eastern margin of the massif does not constrain the basement cover contact to be a taconic thrust as previously interpreted the contact may be a silurian fault possibly related to extension and the opening of the connecticut valley trough as a back arc basin according to this model the magma for the granite sills was generated above a west dipping subduction zone under the laurentian margin which developed after the taconic orogeny

40ar 39ar and k ar data bearing on the metamorphic and tectonic history of western new england 40ar 39ar ages of coexisting biotite and hornblende from proterozoic y gneisses of the berkshire and green mountain massifs as well as 40ar 39ar and k ar mineral and whole rock ages from paleozoic metamorphic rocks suggest that the thermal peak for the dominant metamorphic recrystallization in western new england occurred 465 5 m y ago taconian although textural data indicate a complex metamorphic tectonic history for paleozoic rocks no evidence in rocks at least as high as kyanite grade dictates an acadian age for the barrovian metamorphism available 40ar 39ar and k ar data suggest that the low grade metamorphism and cleavage formation in taconic allochthons and the higher grade metamorphism and emplacement of the berkshire massif allochthon are taconian 40ar 39ar age data from a poorly defined terrane beginning near the east margin of the green mountain massif and extending along the eastern one third of the berkshire massif as far south as otis massachusetts suggest that the area has been retrograded during a metamorphism that peaked at least 376 5 m y ago acadian available age and petrologic data from western new england indicate the presence of at least three separate metamorphic structural domains of taconian age 1 a small area of relict high pressure and low temperature metamorphism in northern vermont t 1 domain 2 a broad area in vermont and eastern new york of normal barrovian metamorphism from chlorite to garnet grade and characterized by a gentle metamorphic gradient t 2 domain and 3 a rather narrow belt of steep gradient barrovian series metamorphic rocks extending from near the cortlandt complex northeastward through dutchess county new york to the berkshire massif in western massachusetts t 3 domain areas of maximum metamorphic intensity within the t 3 domain coincide with areas of maximum crustal thickening resulting from imbricate thrusting berkshire massif or from recumbent folding manhattan prong of remobilized north american continental crust in the later stages of the taconic orogeny

petrology and isotopic composition of a grenvillian basement fragment in the northern appalachian orogen blair river inlier nova scotia canada mesoproterozoic metaplutonic rocks in northern cape breton island introduction nova scotia occur in a tectonic inlier within the appalachian in eastern north america mesoproterozoic laurentian orogen although they have been multiply metamorphosed and basement rocks and their neoproterozoic to mid paleovariably deformed the petrology and geochemistry of these rocks zoic volcanic and sedimentary cover preserve a record provide insight into the tectonomagmatic evolution of easternmost of the proterozoic to mid paleozoic geologic history of laurentia anorthosite syenite and granitoid plutons eastern laurentia the record includes mesoproterozoic 1100 980 ma intruded the sailor brook and polletts cove river orogeny on the ancient north american craton breakgneisses new nd isotopic data are presented from a biotite rich up of a neoproterozoic supercontinent development of part of the sailor brook gneiss ndi 0 7 two anorthosite a passive continental margin and mid paleozoic orogeny samples ndi 2 1 and 2 8 and a charnockite unit exposures of mesoproterozoic laurentian basement in ndi 0 4 new u pb zircon data from the anorthosite yield easternmost canada and the usa occur in a dispb pb ages between 975 and 1095 ma disrupted u pb continuous band of inliers in the appalachian orogen systematics preclude a unique age interpretation but the 1095 ma from newfoundland to alabama fig 1 they consist single grain date is a minimum age for the anorthosite field relations mostly of rocks formed or deformed during major and trace element geochemistry and isotopic characteristics 1190 980 ma grenvillian tectonic and magmatic indicate that the anorthosite and charnockite were probably coexisting events and are distinct from neoproterozoic and melts but not differentiates of a single parent magma the lithological paleozoic units of accreted appalachian terranes in that and chemical characteristics of the lowland brook syenite are they include granulite facies metamorphic rocks and similar to those of a contemporaneous belt of ultrapotassic plutons in many cases anorthosite charnockite mangerite suites in the southern grenville province and both have the chemical bartholomew 1984 and papers therein pettingill et al characteristics of certain modern continental margin arc magmas 1984 rankin et al 1989 1993 owen erdmer 1990 these data indicate that the mesoproterozoic units in the blair hughes et al 1997 river inlier were juvenile crustal additions to eastern laurentia the sinuous trend of the band of basement inliers during grenville related orogenic events broadly reflects promontories and re entrants that formed

geochemistry and sedimentology of a facies transition from microbanded to granular iron formation in the early proterozoic transvaal supergroup south africa abstract a transition from microbanded kuruman to granular griquatown iron formation is described in terms of sedimentological petrographic and geochemical characteristics major minor trace and rare earth elements as well as whole rock carbon and oxygen isotopic compositions transitions between microbanded and granular iron formation are uncommon in the global stratigraphic record with microbanding absent from most known granular iron formations the kuruman griquatown transition zone differs from other granular iron formation sequences in being non oolitic i e peloidal and magnetite rich instead of hematite rich it is concluded that the coarsening upward microbanded granular iron formation units in the kuruman griquatown sequence represent upward shallowing storm dominated deposits that is primary microbanded relatively deep water fair weather deposits having been intermittently reworked by storm wave action form wavy sharp based graded and poorly sorted irregular interbeds of granular iron formation this is in contrast to typical hematitic oolitic iron formation units which represent true shoal deposits geochemical results strongly support the storm reworking hypothesis microbanded and granular iron formation beds have similar ree patterns with negative ce anomalies and depletion in light ree reflecting deposits in a marine setting below the zone of shallow surface water positive eu anomalies in the ree patterns suggest admixture of a hydrothermal component of the ocean water carbonates in the iron formation are depleted in 13c indicating involvement of organic matter during diagenetic alteration of the iron formations there are also indications that total dissolved carbon dioxide in deeper water was more depleted in 13c than in shallower water deposition of the iron formations was sporadically interrupted by influx of tuffaceous material now represented by centimeter thick shard and lapilli bearing stilpnomelane lutite beds these beds differ chemically from the iron formations in being enriched in elements such as al sc y zr nb hf ta th u and the lanthanides and displaying ree patterns with strong negative eu and slight positive ce anomalies the composition of the parent magma of the volcanics remains uncertain because of probable post depositional alteration of the volcanic glass however it was most probably of intermediate to acidic composition as concluded from the widespread distribution of the ash beds typical of plinian type deposits chemical variations in the minor trace and ree compositions of the iron formations are closely correlated with the amounts of admixed volcanic ash material most of the iron formations appear chemically very uncontaminated being highly depleted in all minor trace and ree and organic carbon contents the iron formations were therefore most probably deposited as a chemical precipitate on a submerged shallow cratonic platform with volcanic ash as the only exogenic material sporadically reaching the depository

tectonic synthesis of the taconian orogeny in western new england a tectonic synthesis based on stratigraphic and structural analysis of western new england is proposed for the ordovician taconian orogeny it emphasizes arc continental collision in which ocean floor continental margin and ensialic rift rocks were imbricated westward in a repeatedly deformed accretionary wedge continued compression displaced segments of the north american sialic crust to the west and deformed the earlier emplaced slices of the taconic allochthons which were derived from the continental margin critical arguments for this synthesis are 1 the west to east stratigraphic relations among the basal rift clastic rocks of the dalton pinnacle and hoosac formations of late precambrian to early cambrian age 2 the stratigraphic and sedimentological similarities between the rocks of the lower taconic sequence and rocks in the pinney hollow and underhill slices to the east and north of the green mountain massif 3 the environmental similarities between the cambrian and lower ordovician section of the giddings brook slice and the age equivalent section in the st albans synclinorium 4 the presence of carbonate platform rocks as slivers between each of the successively higher and younger premetamorphic slices groups 1 and 2 of the taconic allochthons 5 the presence of synmetamorphic fault related structures in the youngest and highest slices group 3 of the taconic allochthons 6 the recognition of extensive thrust zones in the pre silurian eugeoclinal sequence east of the middle proterozoic basement of the housatonic berkshire green mountain and lincoln massifs 7 the location of the taconic root zone within the pre silurian eugeoclinal sequence 8 the recognition of numerous faults in the serpentinite belt 9 the similarity between the rocks of the moretown formation and modern fore arc basin sequences 10 the recognition that the volcanic arc continental complexes of the ascot weedon and bronson hill have been displaced westward over the moretown and or hawley formations along such faults as the bristol and coburn hill thrusts 11 the allochthonous and internally imbricated nature of the north american basement in the berkshire massif 12 the proposition that the housatonic green mountain and lincoln massifs as well as the middle proterozoic cored domes of southeastern vermont are also thick sialic slices of north american basement 13 the recognition of medium high to high pressure metamorphic mineral assemblages in the pre silurian eugeoclinal rocks of vermont and 14 the recent synthesis of isotopic age data by sutter and others 1985 on the basis of an analysis of the foregoing arguments and relationships a chronological sequence of seven structural sections between albany new york and the bronson hill anticlinorium in central massachusetts is used to depict the evolution of the taconian orogeny retrodeformed distances are based on structural overlap and restoration of the taconic slices to their depositional setting along the ancient north american continental margin these easterly younging diverticulated slices formed as a result of horizontal compression rather than gravity sliding this palinspastic analysis implies the following 1 approximately 1 000 km of shortening has occurred during the emplacement of the taconic allochthons and the subsequent imbrication of north american basement as thick sialic slices approximately 330 km of this shortening is attributed to multiple cleavage generations 2 repeated movement along such major surfaces as the cameron9s line whitcomb summit belvidere mountain thrust zone has buried the taconic root zone we suggest that the northern extension of this root zone is exposed to the east of the lincoln massif in vermont where the underhill pinney hollow and hazens notch formations are exposed these formations here considered thrust slices disappear along the belvidere mountain whitcomb summit thrust zone as it is traced southward into western massachusetts and western connecticut 3 taconian metamorphic rocks particularly the older medium high pressure rocks in northern vermont have been transported westward on such reactivated surfaces as the belvidere mountain thrust 4 the anticlinorial form of the middle proterozoic basement in the green mountain and lincoln massifs may have resulted from fault bend folding on deep mantle involved thrusts that developed late in the taconian orogeny

tectonic significance of basement cover relationships in the green mountain massif vermont middle proterozoic basement rocks of grenvillian age form the core of the green mountain massif in southern vermont and two distinctive cover rock sequences of late proterozoic to middle ordovician age flank the massif on the west and east the eastern cover sequence is dominated by metamorphosed graywacke silt stone shale and mafic volcanics and unconformably overlies the basement rocks along the eastern margin and in the northern interior of the massif the western boundary of the massif south of clarendon vermont is an unconformity between middle proterozoic basement and western cover sequence rocks composed of metamorphosed conglomerate quartz sandstone dolomite and limestone the western boundary north of clarendon however is a thrust which carried grenville basement and eastern cover rocks over the western cover sequence the structural interpretation of these basement cover contacts and the contrast in lithology and inferred depositional environment of the western and eastern cover seq

deformation and metamorphism on the east side of the green mountain massif in southern vermont an early deformation in the jamaica vermont area on the east side of the green mountain massif produced kilometre scale north plunging f1 folds overturned to the west with an axial planar foliation s1 motion on two major thrust faults juxtaposed three similar but distinctive cover rock sequences after or late in f1 folding smaller scale f2 folds deformed f1 fold limbs and the thrust faults and have an axial planar crenulation cleavage s2 in the structurally higher central and eastern cover rock sequences an early stage of garnet growth began late in s1 development or after it a retrogression partially resorbed first stage garnet and was followed by a second prograde stage of garnet growth late in or after s2 development this metamorphic history resulted in distinctive textural unconformities and zoning anomalies in garnet the western cover rock sequence lacks evidence for a major retrogression f1 folding and f2 folding approximately bracket both the motion on the thrust faults and the retrogression the second prograde metamorphism and the younger deformation are acadian tentative correlation of the first garnet growth stage with ordovician metamorphism recognized in western new england and of motion on the thrust faults with taconian synmetamorphic thrusts in the region suggests that these early thermal and tectonic events occurred in the ordovician the retrogression recorded in the central and eastern cover sequences furthermore may reflect thrusting of these rocks to a structurally higher colder environment followed by reburial and later prograde metamorphism

a newly identified gondwanan terrane in the northern appalachian mountains implications for the taconic orogeny and closure of the iapetus ocean the taconic and salinic orogenies in the northern appalachian mountains record the closure of the iapetus ocean which separated peri laurentian and peri gondwanan terranes in the early paleozoic the taconic orogeny in new england is commonly depicted as an ordovician collision between the peri laurentian shelburne falls arc and the laurentian margin followed by silurian accretion of peri gondwanan terranes during the salinic orogeny new u pb zircon geochronology demonstrates that the shelburne falls arc was instead constructed on a gondwanan derived terrane preserved in the moretown formation which we refer to here as the moretown terrane metasedimentary rocks of the moretown formation were deposited after 514 ma and contain abundant ca 535 650 ma detrital zircon that suggest a gondwanan source the moretown formation is bound to the west by the peri laurentian rowe belt which contains detrital zircon in early paleozoic metasedimentary rocks that is indistinguishable in age from zircon in laurentian margin rift drift successions these data reveal that the principal iapetan suture in new england is between the rowe belt and moretown terrane more than 50 km farther west than previously suspected the moretown terrane is structurally below and west of volcanic and metasedimentary rocks of the hawley formation which contains laurentian derived detrital zircon providing a link between peri laurentian and peri gondwanan terranes the moretown terrane and hawley formation were intruded by 475 ma plutons during peak activity in the shelburne falls arc we propose that the peri laurentian rowe belt was subducted under the moretown terrane just prior to 475 ma when the trench gap was narrow enough to deliver laurentian detritus to the hawley formation interaction between peri laurentian and peri gondwanan terranes by 475 ma is 20 m y earlier than documented elsewhere and accounts for structural relationships early ordovician metamorphism and deformation and the subsequent closure of the peri laurentian taconic seaway in this scenario a rifted arc system on the gondwanan margin resulted in the formation of multiple terranes including the moretown that independently crossed and closed the iapetus ocean in piecemeal fashion

stratigraphic architecture and evolution of the early paleoproterozoic mcgrath trough western australia abstract palaeocurrents provenance and stratigraphic analysis of the upper part of the mount bruce supergroup and the wyloo group in the southwestern part of the hamersley province of the pilbara region of western australia have been used to define an early paleoproterozoic foreland basin the mcgrath trough in front of the northwards advancing ophthalmian fold belt the foreland basin succession commences with the boolgeeda iron formation and includes the overlying turee creek and lower wyloo groups the basin geometry is asymmetric thinning northwards away from the ophthalmian fold belt and the trough axis trends east southeast parallel to the ophthalmian folds multiple intraformational unconformities in the upper turee creek group record the advance of the fold belt into the axis of the mcgrath trough in the hardey syncline area east southeast trending folds and steep southward dipping cleavage post date deposition of the beasley river quartzite a shallow marine to fluvial sandstone that records an interval of tectonic quiescence during basin evolution deposition of the beasley river quartzite was followed by renewed uplift of the southern hinterland resulting in continued deposition of fluvial siliciclastics into the foreland basin which was eventually filled with the eruption of the cheela springs basalt the age of the mcgrath trough is constrained broadly between 2 45 ga the age of the underlying woongarra rhyolite and 2 2 ga the age of the cheela springs basalt at the top of the succession the sedimentary fill of the mcgrath trough was derived mainly from the south and becomes texturally and compositionally more mature upward reflecting its recycled orogen provenance two main sources are identified a dominant cratonic source interpreted to be similar to the granitoid greenstone basement of the pilbara craton and a volumetrically lesser source from banded iron formation bif and volcanic and sedimentary rocks of the mount bruce supergroup felsic volcanic detritus likely to have been derived from the underlying woongarra rhyolite is most common in the lower turee creek group and becomes increasingly less common upsection although it is reworked from underlying formations chert and bif clasts appear first in the upper turee creek group and are important components of the most proximal alluvial fan to braided stream deposits in the upper turee creek and lower wyloo groups grains of microplaty haematite identical in appearance to microplaty haematite in the giant hamersley iron ore deposits appear in conglomeratic units from the base of the beasley river quartzite upwards these fluvial sediments were derived from the south and record erosion of ore grade deposits from the uplifted ophthalmia fold belt during lower wyloo group times

a level prior to the upper capsian at medjez ii algeria archaeozoological and taphonomical evidence combined with archaeological data the paper summarizes the archaeozoological analysis of faunal remains from medjez ii excavated by h camps fabrer identification and quantification of the animal remains are combined with an investigation of the aspect of the finds and the traces observed on them the archaeozoological and taphonomical data are then compared with those concerning the lithic and bone artefacts the lowest part of the site consisting of layers 13 and 14 pertains to an epipalaeolithic occupation with iberomaurusian similarities this chrono stratigraphical phase i layers 11 14 has been attributed to such an occupation prior to the upper capsian

evidence for tectonic emplacement of ultramafic and associated rocks in the pre silurian eugeoclinal belt of western new england vestiges of an ancient accretionary wedge in northern vermont detailed 1 10 000 mapping of the hazens notch ottauquechee stowe and moretown formations in the 60 km 2 jay area has shown that metasedimentary rocks and serpentinites are highly faulted to produce a tectonic stratigraphy in which serpentinites and talc carbonate rocks occur as slivers along faults that separate contrasting lithic assemblages remnants of the ophiolite sequences of quebec in western massachusetts the ultramafic bearing section of the rowe schist and the western part of the moretown formation are largely equivalent to the rocks in northern vermont mapping at 1 24 000 scale and compilation work for the new bedrock map of massachusetts have shown that the rowe schist consists of numerous lenses of greenschist carbonaceous schist gray schist amphibolite and serpentinite although the geometric arrangement of the lenses and serpentinites is strikingly similar to that seen in the jay area higher grade taconian and acadian metamorphism has partially obscured the fault fabrics seen in vermont modified journal abstract

geo and cosmochemistry of the twin elements yttrium and holmium we have analyzed the y ho ratios in bulk chondrites chondrules and four ca and al rich inclusions cais from carbonaceous and unequilibrated ordinary and enstatite chondrites ec by laser ablation inductively coupled mass spectrometry la icpms we demonstrate that bulk rock sample preparation by containerless melting is a suitable method for preparation of bulk rock samples for high precision la icpms bulk chondrites have variable y ho ratios carbonaceous chondrites ci1 cm2 cv3 and ck4 have a common y ho ratio 25 94 0 08 2 that is regarded as the solar system y ho ratio the y ho ratio increases from carbonaceous through ordinary ll l h to enstatite chondrites el6 which show the highest y ho ratio of 27 25 we discuss the result with respect to the origin of fractionation of re and os between chondrite groups within analytical error y and ho show a good correlation in oc and cv3 chondrules and define an y ho ratio of 26 22 0 40 2 y ho fractionation in ca and al rich inclusions is related to differences in volatility the bulk silicate earth is suggested to have a solar y ho ratio and links the earth with carbonaceous chondrites y ho variations in primitive and differentiated terrestrial igneous rocks are discussed in framework of incompatibility of y and ho during partial melting applicability of y ho as tracer for or against a sedimentary origin of the putative host rock of the earth s oldest traces of life from the island of akilia is briefly discussed