orogenesis and basin development u pb detrital zircon age constraints on evolution of the late paleozoic st marys basin central mainland nova scotia abstract the st marys basin along the southern flank of the composite late paleozoic magdalen basin in the canadian appalachians and along the avalon meguma terrane boundary contains late devonian early carboniferous continental clastic rocks of the horton group that were deposited in fluvial and lacustrine environments after the peak of the acadian orogeny shrimp ii geological survey of canada data on approximately 100 detrital zircons from three samples of horton group rocks from the st marys basin show that most of the zircons have been involved in a multistage history recycled from clastic rocks in the adjacent meguma and avalonian terranes although there is a minor contribution from early silurian 411 ma and late devonian suites ca 380 370 ma neoproterozoic ca 700 550 ma and paleoproterozoic ca 2 0 2 2 ga zircon populations predominate with a minor contribution from ca 1 0 1 2 and 1 8 ga zircons published u pb single zircon analyses on clastic sedimentary rocks indicate th

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

single zircon dating constraining the maximum age of the barberton greenstone belt southern africa one of the central issues in early precambrian crustal evolution is the age and genetic relationship between greenstone belt supracrustal assemblages and nearby high grade gneiss terranes the crucial question in most cases is which of the two is older we report zircon ages using the new single grain evaporation technique from a metaquartzite and a felsic volcanic flow of the onverwacht group in the barberton greenstone belt bgb south africa and swaziland that constrain the age of this basal part of the greenstone sequence between 3451 15 and 3438 6 ma this is almost 100 ma younger than previous age estimates assigned to the onverwacht and agrees well with recent sm nd pb pb and zircon ion microprobe dating tonalitic orthogneisses of the ancient gneiss complex agc adjacent to the bgb have ages up to 3644 4 ma and are thus demonstrably older than the greenstones also they cannot be derived from them by partial melting as has been proposed previously this age relationship as well as the presence of metaquartzites and thrust slices of pregreenstone tonalite in the onverwacht tectonic sequence rule out an entirely oceanic evolution for the early history of the bgb and suggest that continental crust was either below or nearby

late proterozoic and devonian plutonic terrane within the avalon zone of rhode island the u th pb radiometric age of zircons demonstrates that much of rhode island consists of a late proterozoic plutonic complex that subsequently was intruded by a large devonian alkalic to subalkalic igneous complex two groups of late proterozoic rocks can be recognized 1 the esmond granite and related plutonic rocks in northern rhode island ranging in composition from gabbro to granite and yielding an upper concordia intercept of 621 8 m y and 2 quartz rich recrystallized gneissic rocks ten rod granite gneiss and hope valley alaskite gneiss in southern rhode island giving an upper concordia intercept age of 601 5 m y the lower intercept of the zircon discordia for the gneissic rocks indicates the isotopic systems were disturbed during the late paleozoic alleghanian orogeny whereas zircons from the esmond granite and related plutonic rocks have been subjected mainly to loss of lead attributable to recent dilatancy rocks determined to be of devonian age include the scituate granite and plutonic rocks of the east greenwich plutonic suite the quincy granite of quinn 1971 in northeastern rhode island may be of a similar devonian age or it may be slightly younger zircon data from these rocks fall upon a chord that gives an upper concordia intercept of 370 7 m y and a lower intercept close to the origin collectively these rocks form a composite pluton with an area 700 km 2 the devonian rocks exhibit petrologic characteristics distinct from slightly older acadian orogenic rocks that occur in lithostratigraphic zones to the west and are more comparable to anorogenic ordovician to devonian alkalic granitoids that trend north northeast across massachusetts and the gulf of maine the new age determinations confirm that much of rhode island contains plutonic rocks intruded during the avalonian orogeny but that a hertofore unrecognized major devonian plutonic episode also occurred the distinctly different ages probably had gone unrecognized because of generally similar lithologic and textural features exhibited by the late proterozoic and devonian rocks we present new petrologic and geochemical criteria that should assist in distinguishing the rocks of different ages as additional mapping and geologic study proceeds

growth of early archaean crust in the ancient gneiss complex of swaziland as revealed by single zircon dating abstract we present new single grain zircon ages derived from ion microprobe and evaporation analysis for rocks of the ancient gneiss complex agc of swaziland southern africa that enable us to document a more extended history of early archaean crustal growth in the eastern kaapvaal craton than previously documented the oldest rocks appear to be strongly flattened tonalitic trondhjemitic orthogneisses dated as 3644 4 ma compston and kroner 1988 which are in faulted contact with rocks of the barberton greenstone belt bgb in an agc enclave in northwest swaziland and which also record subsequent thermal metamorphic plutonic events at 3580 3500 3430 3200 and 3000 ma these rocks are tectonically interbanded with 3200 ma old granodiorites and k rich leucogranites thereby providing evidence for a severe and important post 3 2 ga deformation event that is also inferred for 3500 to 3200 ma old tonalitic to trondhjemitic gneisses in the agc of northeast swaziland in central swaziland zircons from the oldest tonalitic gneiss of the agc have an age of 3560 ma whereas younger additions of granitoid which also intrude the dwalile greenstone enclave in the agc record ages of 3450 3460 ma intense ductile deformation has repeatedly obliterated original intrusive contacts so that preserved field relationships often cannot resolve the above chronology chemical and whole rock isotopic data are consistent with derivation of some agc gneisses from mantle sources and thus imply very short crustal residence times for their granitoid precursors but this is not generally true for the entire agc we suggest that at least some gneisses resulted from intracrustal melting perhaps during crustal thickening events as recorded in granulite parageneses within the agc documentation of 3 5 ga or older granitoid crust in all major regions of the agc now exposed suggests the presence of continental terrains of up to at least 100 km in diameter before the barberton greenstone succession was deposited the combined geochronologic and structural metamorphic data for the agc and bgb reveal a complex history of growth and deformation for the eastern kaapvaal craton that lasted for at least 600 ma from 3 64 to about 3 ga ago the magmatic and tectonic evolution of the agc was broadly similar to that of the bgb and suggests that early archaean greenstone and gneiss terrains were coeval but formed and evolved in different settings and at different crustal levels our age data and the structural metamorphic evolution of the eastern kaapvaal craton are compatible with models suggesting crustal growth through vertical magmatic underplating and early crustal thickening uplift and erosion as a result of tectonic interleaving due to repeated horizontal shortening

a new non archie model for pore structure numerical experiments using digital rock models summary tortuosity is a key parameter for investigating the pore structure of sedimentary rock the conventional model for calculating the tortuosity is derived from archie s law which is only valid for one conductive phase contained in a porous rock however there have been increasingly cases to show the non archie phenomenon in instances where the rock matrix has extra conductive phases such as surface conductivity caused by the electrical double layer therefore such model may be inapplicable in these cases and in calculations involving partial melting we investigated the relation between formation factor and porosity f o ver aw ide range of porosities by the lattice gas automata method lga with the electrical double layer the digital rock samples were constructed by packing up solid grains with different shapes and size distribution as previously measured in the laboratory on real rock specimens our purpose was to identify the origin of non archie behaviour of the f relation sometimes observed the simulations show that archie s law may only be an approximation of the f relation in a high porosity range based on our lga simulations and additional laboratory experimental data we developed new equations for non archie f relation and tortuosity potentially useful for improving the evaluation of pore structure

u pb isotopic and geologic evidence for late paleozoic anatexis deformation and accretion of the late proterozoic avalon terrane southcentral connecticut

stratigraphic relationships and detrital composition of the medial ordovician flysch of western new england implications for the tectonic evolution of the taconic orogeny a reply to rodgers we present new stratigraphic and petrographic data on the easterly derived medial ordovician flysch sequence of western new england and eastern new york state a conformable relationship exists between the flysch and underlying early paleozoic continental rise sediments within the western taconic allochthon farther east in the allochthon we observe a possibly disconformable relationship contrary to previous interpretations these facts suggest that at any place within this paleogeographic realm the sediments were undeformed and untransported at the initiation of flysch deposition at least three sources contributed detritus to the flysch 1 taconic sequence sediments 2 deformed and metamorphosed pelitic psammitic and volcanic lithologies and 3 igneous rocks of dominantly mafic ultramafic affinities these three sources are respectively interpretable as continental rise accretionary prism and ophiolitic volcanic arc assemblages which are recognized in the regional geology of western new eng

archean inheritance in zircon from late paleozoic granites from the avalon zone of southeastern new england an african connection abstract in southeastern new england the narragansett pier granite locally intrudes carboniferous metasedimentary rocks of the narragansett basin and yields a monazite u pb permian emplacement age of 273 2ma zircon from the narragansett pier granite contains a minor but detectable amount of an older inherited component and shows modern loss of lead zircon from the late stage aplitic westerly granite exhibits a more pronounced lead inheritance permitting the inherited component to be identified as late archean such old relict zircon has not been previously recognized in proterozoic to paleozoic igneous rocks in new england and may be restricted to late paleozoic rocks of the avalon zone we suggest that the archean crustal component reflects an african connection in which old archean crust was underplated to the avalon zone microplate in the late paleozoic during collision of gondwanaland with avalonia

age and setting of the bronson hill magmatic arc a re evaluation based on u pb zircon ages in southern new england new u pb zircon ages of selected rocks from the bronson hill anticlinorium in southern new england are as follows microcline gneiss of the dry hill gneiss in the pelham dome believed to be metamorphosed alkali rhyolite is confirmed as late proterozoic yielding an age of 613 3 ma another sample shows strong evidence of zircon inheritance with a minimum mean age of 1402 ma plagioclase rich gneisses with associated amphibolites swanzey pauchaug monson and fourmile gneisses now believed to represent metamorphosed intrusive igneous rocks yield ages from 454 to 442 3 2 ma thus ranging from early late through latest ordovician consistent with other absolute ages of the oliverian plutonic series in the region quartz phyric rhyolite from the upper member of the ammonoosuc volcanics and rhyolite tuff from the partridge formation are dated at 453 2 ma and 449 3 2 ma respectively indicating that eruption of felsic lavas in the overlying volcanic section spanned an interval of approximately 4 m y additional zircon ages are reported from a crosscutting but deformed tonalite dike in the swanzey gneiss 381 3 ma and a coarse grained gabbro from the prescott intrusive complex 407 3 2 ma intrusive into the partridge formation metamorphic overprinting of the region during both the acadian and alleghanian orogenies is indicated by u pb titanite ages of 357 5 ma and 292 5 ma from a tonalite gneiss on prescott peninsula and the dry hill gneiss respectively the above ages and other zircon ages as well as petrography and geochemistry of the fourmile monson swanzey pauchaug and other plagioclase rich gneisses of the bronson hill anticlinorium suggest that they represent the plutonic root of a calc alkaline magmatic arc at least 250 km long that was produced partly or entirely on a continental margin in late ordovician time ca 454 442 ma the physically overlying cover sequence of chemically different tholeiitic arc volcanic and sedimentary rocks of the ammonoosuc volcanics and partridge formation previously thought to rest unconformably on the plagioclase rich plutonic gneisses is now shown to have had a magmatic history overlapping with them the zircon ages in the plutonic gneisses and the cover sequence indicate that both are the same age or slightly younger than the late ordovician caradocian emplacement of the giddings brook slice of the taconian allochthons and too young to have formed above an early and medial ordovician subduction zone

1 1 ga k rich alkaline plutonism in the sw grenville province u pb zircon and baddeleyite dating of six syenitic stocks establishes that the ultrapotassic potassic alkaline and shoshonitic magmatism with island arc affinities in the central metasedimentary belt cmb of the southwestern grenville province canada took place between 1089 and 1076 ma along a 400 km long northeast trending plutonic belt these ages indicate that ultrapotassic rocks with arc affinities are not unique to the phanerozoic west to east emplacement ages along a northern and southern cross section of this belt range from 1083 2 ma kensington through 1081 2 ma lac rouge to 1076 1 3 ma loranger in the north and from 1089 3 4 ma loon lake and 1088 2 ma calabogie to 1076 2 ma westport in the south although closely spaced in time in detail these ages suggest a slight younging of this magmatic activity to the southeast integration of the geochronological data with the spatial extent and potassic character of the plutons shows that the k rich alkaline suite is distinct from the nepheline syenite belt of the bancroft terrane and from the syenite monzonite suite of the frontenac terrane of the cmb and it is considered to be a magmatic episode unique to the elzevir terrane and its gatineau segment the timing and the postmetamorphic emplacement of these plutons indicate that the regional greenschist to granulite facies metamorphism of the country rock precise age unknown is older than 1089 ma throughout the entire elzevir terrane the potassic magmatism is interpreted as the initiation of the 1090 1050 ma ottawan orogeny in the elzevir terrane thus the regional metamorphism in this terrane previously assigned to the ottawan orogeny is an earlier event the contemporaneous emplacement of this postmetamorphic plutonic belt with keweenawan volcanism is at variance with current tectonic models which consider the keweenawan rift to be formed at the same time as regional metamorphism in the cmb

structure and u pb geochronology of central hoggar algeria a reappraisal of its pan african evolution discovery of large scale deep seated thrusts in central hoggar with a plurifacial evolution ranging from lower amphibolite facies to upper greenschist facies conditions and linked to a regional refoliation has led us to reconsider the pan african tectonic and metamorphic history in that region two areas are described and a review of other thrusts leads to an interpretative cross section of a large portion of reactivated continental crust the age and kinematics of this structural reworking have been approached using u pb zircon dating in the tamanrasset region despite the difficulty of estimating the age of the initiation of the assumed intracontinental a type subduction the results provide a time span of 30 40 m y between the climax of granitoid emplacement and a late retrogressive offset along the thrust planes some key ages were determined 1 2075 30 ma is the age given by granulite facies remnants which escaped from the refoliation the corresponding lower intercept at 530 70 ma confirms the pan african imprint 2 615 5 ma reflecting the age of syntectonic to late tectonic granitoids emplaced in reworked gneisses and in preserved granulites 3 580 ma the concordant age of sphenes and monazites from the same granitoids which is interpreted as corresponding to the end of medium grade conditions no evidence has been found of a 1000 ma age a kibaran event does not appear to exist in central hoggar the age similarity between the observed deep intracontinental evolution of central hoggar and the collision related tectonics of western hoggar and iforas suggests a common origin for both phenomena

distinguishing grenvillian basement from pre taconian cover rocks in the northern appalachians distinguishing grenvillian basement rocks from pre taconian cover sequences in the appalachians is a first order problem essential for accu rate structural interpretations the cavendish formation in southeastern ver mont presents a classic example of this problem doll and others 1961 showed the cavendish formation as younger than the middle proterozoic mount holly complex but older than the lithologically similar cambrian tyson and hoosac formations more recently the name cavendish formation has been informally abandoned and its metasedimentary units have been mapped as the tyson and hoosac formations of late proterozoic to cambrian age in a radical departure from these interpretations ratcliffe and others 1997 reassigned metasedimen tary rocks of the cavendish formation to the mount holly complex based on an inferred intrusive relationship between them and a 1 42 ga tonalite this new age assignment if correct requires a completely new structural interpretation of the region shrimp and pb evaporation ages of detrital zircons extracted from a quartzite layer from cavendish gorge near the proposed intrusive contact with the tonalite constrain the time of deposition of the cavendish formation grain shapes of the zircons vary from euhedral to nearly spherical virtually all the grains have pitted surfaces and show at least some rounding of edges and terminations grains exhibit oscillatory zoning typical of zircons that crystallized from a magma single grain pb evaporation analyses of ten zircons and shrimp analyses of 15 zircons all yield ages less than 1 42 ga seven of the grains are consistent with derivation from the bull hill gneiss that postdates the grenville orogenic cycle and predates deposition of the cavendish formation thus the metasedimentary units of the cavendish formation should not be assigned to the mount holly complex

time partitioning in cratonic carbonate rocks dissimilarity in relations between thickness duration and accumulation rate in holocene sections metre scale phanerozoic cycles and epoch interval phanerozoic sequences reflects variation in the distribution of depositional and hiatal time at different scales of observation modern accumulation rates are strongly dependent on duration of deposition lower rates correspond to longer durations phanerozoic cycle thicknesses are similar to those of holocene sections suggesting that both formed while filling similar amounts of accommodation space however accumulation rates of phanerozoic cycles and sequences are about two orders of magnitude less than in modern settings and they show neither strong dependence on duration of accumulation nor significant variation over phanerozoic time hence net accumulation is primarily controlled by regional subsidence rate these relations also allow for determination of that fraction of cycle period recorded as rock and that corresponding to hiatal time at intercycle boundaries depending on subsidence rate and amount of compaction most cratonic carbonate units accumulated over 3 to 30 of the time represented by sequences in which they occur

u pb geochronological constraints on precambrian stratified units in the avalon composite terrane of nova scotia canada tectonic implications concordant u pb ages of detrital zircons in several proterozoic units in the avalon composite terrane of nova scotia along with previously published data indicate that deposition of low and high grade stratified units in the cobequid highlands gamble brook formation and central cape breton island variously assigned to the george river group or george river metamorphic suite and the bras d or gneiss started after 977 ma extended through a period of volcanic activity between 694 and 637 ma and ended before the 629 612 600 ma volcanic activity associated with deposition of the georgeville and jeffers groups in the antigonish and cobequid highlands and the east bay hills and coxheath groups in southern cape breton island these age data along with u pb analyses of metamorphic and igneous zircon and monazite also constrain three late proterozoic tectonothermal events i oblique sinistral ductile shear deformation and greenschist amphibolite facies metamorphism probably occurred between 694 an

structural implications of some radiometric ages of igneous rocks in southeastern new england u th pb zircon rb sr whole rock and k ar hornblende isotopic ages reveal two major episodes of igneous activity in southeastern new england one in late proterozoic z time 600 650 m y ago and the other in devonian to ordovician time 375 450 m y ago the dated rocks are discussed in the context of several individual structural provinces that evolved more or less independently of each other prior to middle to late paleozoic assembly these provinces generally are bounded by major faults and or abrupt changes in metamorphic grade and from east to west include i the milford dedham zone ii the putnam nashoba zone iii the merrimack synclinorium and iv the bronson hill zone from the present and other recently published studies reasonably accurate ages representative of the two major episodes can now be assigned to many of the igneous rocks other units remain less well constrained but probably may be placed into one or the other of the general age groupings case by case examination of the geochronologic data makes us cautious in accepting some apparent fine structure age resolution within each grouping until factors such as inherited zircon and metamorphic overprinting can be better evaluated with the above mentioned qualifications in mind igneous rock ages summarized according to structural province are as follows i milford dedham zone a late proterozoic z dedham granite and related rocks 630 15 m y 595 15 m y 612 646 m y milford granite 630 15 m y 591 50 m y hope valley alaskite gneiss and scituate granite gneiss ages uncertain unnamed granite near assonet 600 650 m y b devonian to ordovician alkalic rocks including quincy cape ann and peabody granites wenham monzonite and nahant gabbro ii putnam nashoba zone a late proterozoic z no plutonic rocks identified but may include some metavolcanic rocks b devonian to ordovician sharpners pond diorite 430 5 m y andover granite 408 22 m y 450 22 m y muscovite granite near west berlin age uncertain preston gabbro 424 5 m y ill merrimack synclinorium a late proterozoic z granitic orthogneiss unit of the massabesic gneiss complex 600 620 m y metavolcanic unit of the massabesic gneiss complex 646 m y or older b devonian to ordovician fitchburg complex at maiden hill 402 11 m y fitchburg complex at rollstone quarry 390 15 m y newburyport complex 450 15 m y ayer granite 433 5 m y chelmsford granite age uncertain muscovite biotite granite at millstone hill 372 7 m y canterbury gneisses 395 10 m y 392 9 m y iv bronson hill zone a late proterozoic z dry hill gneiss of the pelham dome 600 m y b devonian to ordovician monson gneiss 440 10 m y these distinctive age patterns together with contrasts in petrography and texture of the igneous rocks in stratigraphy of the country lock and in metamorphic grade accentuate the separate geologic development of each structural province no correlation involving preassembly geology should be attempted across province boundaries without due consideration of the exotic character of each province

pb pb and u pb zircon dating of subvolcanic rhyolite as a time marker for hercynian granite magmatism and sn mineralisation in the eibenstock granite erzgebirge germany considering effects of zircon alteration tin mineralisation is closely related to rhyolite stocks and dykes which occur in the endo and exocontact of the eibenstock granite erzgebirge germany the same structures which cut the granite control the rhyolite emplacement and the location of ore bearing greisen bodies albitisation and greisenisation related to tin mineralisation as well as sericitisation and argillic alteration may be traced by changes in chemical and mineralogical composition of both rhyolite and granite wall rock comprehensive zircon studies by scanning electron microscopy secondary and backscattered electron as well as cathodoluminescence imaging edx measurements reveal that zircon from rhyolite and from the enclosing granite shows significant changes in chemical composition and crystallinity including distortion of the u pb isotope system when affected by greisenisation single evaporation analysis of zircon from rhyolite with little or no greisenisation gave a 207pb 206pb age of 290 5 ma whereas zircon from little altered granite gave a 207pb 206pb age of 320 8 ma the single evaporation age of zircon from rhyolite is confirmed by 238u 206pb shrimp ages of 297 8 ma the significant time gap of at least 20 ma between granite intrusion and rhyolite formation suggests that the late magmatic evolution of the eibenstock granite cannot be regarded as a source for tin ore forming fluids as previously assumed

applications of single grain zircon evaporation analyses to detrital grain studies and age discrimination in igneous suites abstract 207 pb 206 pb ages determined by evaporation of single zircon grains from rocks of the new england appalachians in the usa permit an assessment of the applicability of this method to rocks that have been strongly disturbed by orogenesis tests conducted on highly metamorphosed and deformed orthogneisses of known protolith age whose u pb upper intercept zircon ages obtained through isotope dilution methods show profound post crystallization discordance demonstrate that the evaporation method is successful in recovering protolith ages uncertainties are 10 15 my for rocks of paleozoic age or older preliminary results suggest that the method is particularly useful for provenance studies of sedimentary rocks and in the discrimination among groups of rocks that are known to differ in age but are otherwise difficult to separate on the basis of field or petrographic characteristics 207 pb 206 pb ages of detrital zircon grains from two metasedimentary units from opposite sides of the new england appalachians reveal the distinct provenances of these rocks the missisquoi formation in southeastern vermont usa is comprised in part of detritus derived from the 1 0 1 2 ga grenvillian province of laurentia whereas this component is conspicuously absent from the plainfield formation in easternmost connecticut the detrital zircon age data suggest that the missisquoi formation accumulated peripheral to the laurentian craton during the early paleozoic whereas the plainfield formation detritus was supplied by non laurentian ossibly west african sources during the late proterozoic or early paleozoic in southern vermont the evaporation method successfully distinguishes between two groups of augen gneisses with ages of 960 ma and 1120 ma which were first recognized through isotope dilution methods collectively these results indicate that the dating of single zircon grains by the evaporation method can be widely exploited in the study of complexly deformed and strongly metamorphosed rocks

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

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

whole grain evaporation for 207 pb 206 pb age investigations on single zircons using a double filament thermal ion source a technique has been developed and tested to analyse 207pb 206pb apparent ages by thermal evaporation of radiogenic lead directly from untreated whole zircon grains 0 3 mm the evaporation analyses are performed in the double filament arrangement of a thermal ion mass spectrometer thims the method is a powerful tool to distinguish between different lead components occurring in the same grain because differing activation energies of the competing lead components cause their sequential evaporation from the zircons the evaporation of test samples results in 207pb 206pb apparent ages in good agreement with u pb ages known from literature single zircons from a granite of the marble mountains california yield an age of crystallization of 1 410 30 ma ceylon zircons from heavy mineral bearing gravels yield 560 40 ma as age of crystallization of the pegmatitic gravel sources individuals from a heterogeneous zircon population of a diatexite from the southern schwarzwald sw germany indicate metamorphic zircon formation around 500 ma and the existence of middle proterozoic relics 1 95 0 05 ga

ion microprobe dating of zircon from quartz graphite veins at the bristol new hampshire metamorphic hot spot detrital zircons entrained in hydrothermal quartz graphite rutile veins found near the bristol new hampshire metamorphic hot spot are overgrown by thin rims ion microprobe analyses of these rims date their growth at 408 plus minus 6 ma these measurements quantitatively confirm textural evidence that the graphite veins were emplaced during peak metamorphism associated with the acadian orogeny and they provide a direct positive test of the hypothesis based on petrological and stable isotope evidence that the hydrothermal systems responsible for the quartz graphite veins were also responsible for the hot spot metamorphism

unsupported radiogenic pb in zircon a cause of anomalously high pb pb u pb and th pb ages ion microprobe u th pb isotopic analyses of zircons from a granodioritic orthogneiss from the napier complex mount sones enderby land antarctica have identified an unambiguous example of unsupported radiogenic pb in a 3 950 ma old crystal at one 40 m spot on the crystal an unusually large heterogeneity in pb content was found the concentration of radiogenic pb ranging from 5 to 50 percent higher than could have been generated in 3 950 ma by radioactive decay of the co existing u and th this relative excess of radiogenic pb is attributed to pb gain rather than to u and th loss because first the pb content varied by more than the u or th contents and secondly changes in the pb u pb th and pb isotopic composition correlated directly with changes in the pb concentration the individual 207pb 206pb apparent ages ranged from 4 000 ma to 4 145 ma all greater than the inferred age of the crystal a correlation between 207pb 206pb and pb u shows that the pb excess has not resulted from recent pb movement the spot apparently gained radiogenic pb about 2 500 ma ago at the same time as the majority of the other zircons in the rock suffered substantial pb loss the pb movement occurred in response to a discrete geologic event reverse discordance is a phenomenon that must be considered when interpreting zircon u pb ages especially 207pb 206pb ages of single crystals or portions of crystals

u pb zircon geochronology of the adirondack mountains and implications for their geologic evolution u pb zircon studies of metamorphosed igneous rocks in the adirondack mountains have yielded preliminary ages within the range 1420 990 ma several geochronologically and geochemically distinct episodes of igneous intrusion and at least one pre granulite facies dynamothermal metamorphic event are documented this information is consistent with recent field and geochronological studies throughout the grenville province and suggests that a complex sequence of events occurred in the adirondack mountains prior to the widespread deformation and metamorphism commonly attributed to the approximately 1100 1000 ma ottawan phase of the grenvillian orogenic cycle