granite ascent and emplacement during contractional deformation in convergent orogens based on a case study in the central maine belt of west central maine u s a it is proposed that crustal scale shear zone systems provide an eaective focussing mechanism for transfer of granite melt through the crust in convergent orogens during contractional deformation flow of melt in crustal materials at depths below the brittle plastic transition is coupled with plastic deformation of these materials the flow is driven by pressure gradients generated by buoyancy forces and tectonic stresses within the oblique reverse central maine belt shear zone system stromatic migmatite and concordant to weakly discordant irregular granite sheets occur in zones of higher strain which suggests percolative flow of melt to form the migmatite leuco somes and viscous flow of melt channelized in sheet like bodies possibly along fractures cyclic fluctuations of melt pressure may cause instantaneous changes in the eaective permeability of the flow network if self propa gating melt filled tensile and or dilatant shear fractures are produced due to melt enhanced embrittlement in homogeneous migmatite and schlieric granite occur in zones of lower strain which suggests migration of partially molten material through these zones en masse by granular flow and channelized flow of melt carrying entrained residue founded on the central maine belt case study we develop a model of melt extraction and ascent using the driving forces stress conditions and crustal rheologies in convergent especially transpressive orogens ascent of melt becomes inhibited with decreasing depth as the solidus is approached for intermedi ate a h2o muscovite dehydration melting the water saturated solidus occurs between 400 and 200 mpa near the brittle plastic transition during high t low p metamorphism where the balance of forces favors sub horizontal fracture propagation emplacement of melt may be accommodated by ductile flow and or stoping of wall rock and inflation may be accommodated by lifting of the roof at shallower crustal levels and or sink ing of the pluton floor the resultant plutons have sub horizontal tabular geometries with floors that slope down to the ascent conduits although these plutons may have locally discordant relations with country rock structures when viewed at the crustal scale granite ascent and emplacement in convergent orogens are syn tec tonic processes 1998 elsevier science ltd all rights reserved

short paper low grade metamorphism of the welsh basin lower palaeozoic succession an example of diastathermal metamorphism the lower palaeozoic succession of the welsh basin has suffered the effects of low grade metamorphism which shows features that do not accord with classical models of regional or burial metamorphism textural evidence demonstrates the development of greenschist facies metamorphism prior to deformation and cleavage development while mineralogical characters indicate a low pressure facies series in order to accommodate these features it is suggested that the area has suffered diastathermal metamorphism in which an enhanced thermal flux develops early in the tectonic cycle in response to an extensional setting temperatures remained relatively high during final closure associated with deformation of the basin sediments and consequent cleavage development such a model adequately resolves previously conflicting interpretations of burial and deformation related metamorphism

incipient metamorphism in the lower palaeozoic marginal basin of wales a diagenctic through anchizone to epizone transition is demonstrated in pelitic rocks of the lower palaeozoic marginal basin of wales by examination of variations in phyllo silicate mineralogy illite crystallinity and bo parameter of white micas this transition represents a temperature range from 150 c to 400 c and the metamorphism is of a low pressure facies series type with a geothermal gradient of 40 ckm 1 variations in grade can be correlated largely with the original basin and shelf form suggesting a depth related metamorphism however in areas closer to the site of caledonian plate collision an increasingly syn tectonic metamorphic event is apparent correlation of pelite data with metabasite assemblages is variable the most consistent relationship being between epizone crystallinity values andepidote actinolite greenschist facies assemblages diagenetic clay mineral assemblages are found associated with prehnite pumpellyite assemblages in metabasites and it is suggested that the latter represent non buffered and therefore non diagnostic assemblages

strike slip tectonics and granite petrogenesis transpression is a process that thickens the crust and therefore in obliquely convergent orogens as in normally convergent orogens there is the potential to generate granitic melts individual transcurrent shear zones may not only control the ascent paths siting and emplacement mechanisms of plutons but they may also cause the genesis of the granitoids themselves two contrasting situations are examined in the hercynian shear zones of iberia thickening together with hydrous fluxing along shear zones created intracrustal wet melting of fertile gondwanan sediments to produce syntectonic granites in the northern part of the british caledonides the association of compositionally expanded granitoids with a major mantle component and transcurrent shear zones may be explained by melting of continental crust at the lower limits of crustal transpressional faults detaching into the moho

geochemical variation in microstructural shell layers of the southern quahog mercenaria campechiensis implications for reconstructing seasonality abstract shells of the northern and southern quahogs mercenaria mercenaria and mercenaria campechiensis respectively from paleontological and archaeological deposits have been used to reconstruct climate environmental conditions and ecological relationships seasonal fluctuations in temperature and salinity are recorded in shell carbonate as variations in 18 o and 13 c values and sr ca ratios however these environmental proxies have not yet been calibrated for m campechiensis previous geochemical studies of quahog shells focused on the outer prismatic layer as an ecological archive the outer layer is often not preserved in fossil and archaeological shells moreover recent innovations in microsampling techniques allow high resolution sampling of the middle cross lamellar layer this study tests whether outer prismatic and middle cross lamellar layers record similar variations in 18 o and 13 c values and sr ca ratios in modern shells of m campechiensis and whether profiles are similar among individuals the last 2 years of shell growth from three individuals 6 7 and 9 years old collected alive near bokeelia southwest florida were microsampled for geochemical analyses 18 o varies seasonally relative to translucent summer and opaque winter growth increments variation of 18 o from outer and middle layers are identical within and among shells however the oldest individual did not preserve the most positive values represented in the two younger shells 13 c from both layers are similar within shells although the outer layer is more variable and the range of values varies somewhat among individuals sr ca ratios from the outer and middle layers within shells do not track each other well and profiles among individuals are different to evaluate whether a seasonal signal was present sr ca ratios were compared to 18 o values there was no co variation within the outer layer in any of the shells therefore this layer should be avoided when using sr ca ratios as a temperature proxy the middle layer from the 6 year old shell showed no co variation the 7 year old shell exhibited a moderate negative co variation r 2 0 55 p r 2 0 72 p 18 o and environmental interpretations made using 13 c values should be treated with caution

a white mica crystallinity study of the berwyn hills north wales white mica crystallinity studies have been carried out on 90 samples of mudrocks six of spotted slate and five of accretionary lapilli tuff from the area around the berwyn hills north wales strain was measured for some of the spotted slate and tuff samples the metamorphic grade increases from southeast to northwest with values of the kubler index varying from 0 64 to 0 20 2 metamorphic zonal boundaries follow the strike of bedding and cleavage but crystallinity values increase into stratigraphically younger rocks on the northwest side of the berwyn dome this effect is attributed mainly to a rapid increase in the thickness of synmetamorphic overburden to the northwest comprising exposed silurian turbidites and inferred lower devonian non marine sediments strain variations have a more local influence on crystallinity and lateral variations in the contemporary geothermal gradient cannot be ruled out however only with unrealistically high gradients would the need for a thick lower devonian component to the overburden be removed this reasoning implies that the metamorphic peak was coeval with the acadian late caledonian event rather than with an early diastathermal event

the gem lake shear zone cretaceous dextral transpression in the northern ritter range pendant eastern sierra nevada california the gem lake shear zone is a northwest striking steeply dipping dextral transpressional shear zone that provides the first direct evidence for dextral deformation in wall rock pendants in the central part of the sierra nevada batholith the gem lake shear zone is a minimum of 30 km in length and extends at least from the north end of the northern ritter range pendant to the southeast edge of the ritter range pendant the amount of displacement on the zone is uncertain but matching fault slivers of pennsylvanian marble in the northern ritter range pendant to similar exposures north of the mount morrison pendant suggests a minimum dextral offset of 20 km the gem lake shear zone was active in early late cretaceous time from before 91 ma the age of the syntectonic granodiorite of kuna crest to at least 80 ma 40ar 39ar age of syndeformational mica from the shear zone in the ritter range pendant deformation in the gem lake shear zone is characterized by combined dextral simple shear and subvertical stretching which are variably partitioned in anastomosing high strain zones in the shear zone at gem lake predominantly dextral deformation is indicated by porphyroclast asymmetries s c fabric and asymmetric crenulations a component of subvertical stretching is indicated by a moderately to steeply plunging stretching lineation in a segment of the shear zone at kuna crest a strongly developed stretching lineation indicates predominantly subvertical stretching with a lesser component of dextral strike slip the gem lake shear zone is considered to be part of a proposed regional system of shear zones in the eastern sierra nevada the sierra crest shear zone system this dextral transpressional system was active prior to and synchronous with intrusion of the late cretaceous sierra nevada batholiths deformation ceased shortly after pluton emplacement the sierra crest shear zone system includes possibly related shear zones as far north as saddlebag lake pendant and at least as far south as oak creek pendant and the owens valley indicating a possible strike length greater than 150 km

transpressional shearing and strike slip partitioning in the late cretaceous sierra nevada magmatic arc california the rosy finch shear zone rfsz represents an example of synmagmatic strike slip tectonics in the east central sierra nevada and provides information on transpressional tectonics within magmatic arcs the rfsz 1 4 km wide and 80 km long is contained in late cretaceous granitoids 92 83 ma of the mono pass intrusive suite dextral strike slip deformation is indicated by a continuous band of orthogneiss mylonitic and cataclastic deformation characterized by subvertical foliations with subhorizontal lineations the style of dextral shearing in the rosy finch shear varies along strike from wide zones of ductile deformation in the youngest plutons to narrow zones of cataclastic deformation in the older plutons deformation of the youngest granitoids is synmagmatic as constrained by both field observations and isotopic studies dextral shearing occurs concurrently in contemporaneous intrusive suites to the north and south tuolumne and mount whitney suggesting strike slip movement along the axis of active plutonism field studies also suggest that contraction across the arc acted concurrently with strike slip movement consistent with a transpressional setting for the late cretaceous sierra nevada magmatic arc comparison of the field data with strain models of oblique plate convergence suggests that the rfsz is a preserved ductile signature of transcurrent motion of strike slip partitioning within a transpressional orogenic system recent plate reconstructions indicate a switch from sinistral to dextral oblique convergence at 95 ma which is consistent with the timing constraints on the dextral movement on the rfsz

subsidence in late paleozoic basins in the northern appalachians during the interval between continental collision in the devonian and continental breakup in the triassic the northern appalachians became the site of a wide plate boundary zone of dominantly right lateral strike slip as is typical of intracontinental transforms tectonism was both diachronous and rapidly variable along strike through regimes of pure strike slip transpressional deformation and rapid subsidence of extensional basins up to 9 km of mainly nonmarine clastic sediments accumulated in these local depocenters which subsided episodically in two stages 1 an initial phase of stretching and thinning of the lithosphere when subsidence was rapid fault controlled and often accompanied by volcanism and 2 a subsequent phase of gradual thermal subsidence during which the depositional basins expanded to bury the earlier border faults and progressively younger sedimentary units onlapped basement the largest depocenter the magdalen basin opened as a pull apart between strike slip faults in newfoundland and new brunswick from late devonian to early carboniferous subsequent thermal subsidence affected a large area during medial and late carboniferous a phenomenon that is well recorded to the north and west where no later tectonism occurred in areas to the south and east of the basin strike slip on other faults continued into the time of thermal subsidence introducing complications such as localized transpressional deformation and rapid subsidence in smaller pull aparts

new u pb zircon ages and the duration and division of devonian time newly determined u pb zircon ages of volcanic ashes closely tied to biostratigraphic zones are used to revise the devonian time scale they are 1 417 6 1 0 ma for an ash within the conodont zone of icriodus woschmidti i w hesperius early lochkovian 2 408 3 1 9 ma for an ash of early emsian age correlated with the conodont zones of po dehiscens lower po inversus 3 391 4 1 8 ma for an ash within the po c costatus zone and probably within the upper half of the zone eifelian and 4 381 1 1 3 ma for an ash within the range of the frasnian conodont palmatolepis punctata pa punctata zone to upper pa hassi zone u pb zircon ages for two rhyolites bracketing a palyniferous bed of the pusillites lepidophyta spore zone are dated at 363 8 2 2 ma and 363 4 1 8 ma respectively suggesting an age of363 ma for a level within the late famennian pa g expansa zone these data together with other published zircon ages suggest that the base and top of the devonian lie close to 418 ma and 362 ma respectively thus lengthening the period by20 over current estimates we suggest that the duration of the middle devonian eifelian and givetian is rather brief perhaps no longer than 11 5 myr 394 ma 382 5 ma and that the emsian and famennian are the longest stages in the period with estimated durations of15 5 myr and 14 5 myr respectively 1998 elsevier science b v all rights reserved

late devonian carboniferous detachment faulting and extensional tectonics in western cape breton island nova scotia canada abstract shallow dipping detachment faults have played a key role in the evolution of the late devonian to carboniferous maritimes basin of western cape breton island the faults are responsible for large scale post orogenic crustal thinning and denudation of mid crustal levels tournaisian and westphalian clastic sedimentation can be related to detachment faulting along the margaree shear zone msz and the ainslie detachment ad respectively the msz consists of thick shallow dipping retrogressive mylonite overprinted by cataclastic horizons brittle detachment faults and chloritic breccia the shear zone crosscuts and transports low grade late devonian bimodal volcanic rocks and redbeds of the fisset brook formation in its hanging wall towards the west and juxtaposes them against exhumed medium to high grade metamorphic rocks and basement in its footwall locally the mylonite is 200 700 m thick and outcrops along strike from north to south for approximately 60 km tournaisian coarse clastic units and debris flow deposits of the horton group unconformably overlie ductile mylonite of the msz due to incision of brittle faults into the msz regional transgression and marine flooding of the extensional complex and exhumed units are marked by the thick visean carbonate and evaporite deposits of the windsor group the carbonates reflect a relatively quiescent tectonic regime and widespread subsidence was likely due to the cessation of extension related thermally induced buoyancy in a tectonically thinned crust the ainslie detachment nucleated as a regional flat lying fault along the carbonate evaporite contact at the top of the macumber formation forming calc mylonite near the base of the windsor group significant stratigraphic omissions in the windsor group above the detachment provide clear evidence of an extensional regime during faulting clastic deposits of the westphalian cumberland group are bounded by the detachment and sedimentation is related to listric normal faults which cut down section towards the west southwest the documentation of detachment faults has widespread implications for the interpretation of the post orogenic evolution of the northern appalachians an extensional regime can now be recognized deep reflection seismic data are interpreted in light of the new findings and can be related to late devonian carboniferous basin formation

the transition from magmatic to high temperature solid state deformation implications from the mount stuart batholith washington criteria for syntectonic emplacement of plutons are commonly ambiguous the strongest single criterion is probably the preservation of a continuous transition from submagmatic to high temperature solidstate deformation this transition is not commonly documented an exception is the mount stuart batholith which displays a variety of syntectonic structures within its sheared northeastern margin including the following 1 s c fabrics that range from submagmatic to high temperature solid state 2 solid state foliation and lineation defined by amphibolite facies assemblages that overprint and are parallel to magmatic foliation and lineation 3 folds of magmatic and solid state foliation that have hinge lines parallel to magmatic and solid state lineation and to equivalent structures in wall rocks 4 dikes that were folded and boudinaged over a wide range of rheological states with boudin necks locally filled by tonalite 5 pegmatite dikes and mineralized joints that are sub perpendicular to magmatic and solid state lineation and 6 submagmatic and high temperature solidstate ductile shear zones we suggest that many plutons emplaced during regional deformation do not preserve evidence for syntectonic deformation because of the transitory nature of the submagmatic state and the obscuring effects of postemplacement deformation syntectonic features are most likely preserved in plutons cooled at slow to moderate rates or in plutons deformed at high strain rates during emplacement the optimum conditions for preservation may occur in plutons emplaced along fault zones in the mid crust such as the mount stuart batholith and in intrusions at deeper levels that were rapidly exhumed and or intruded during the waning stages of regional deformation

a microstructural and microchemical study of cleavage lamellae in a slate abstract the results of a detailed microstructural and microchemical study of a slate from the chewton area victoria australia are reported in which it was found that cleavage developed from microcrenulations phyllosilicates in p domains differed in their chemistry from those in q domains and were concentrated in the p domains by localized metamorphic reactions which were aided by mechanical rotation and solution processes wide dark cleavage lamellae often taken as evidence for the passive concentration of insoluble minerals appear to have had a similar origin the p domains exhibited two types of microstructure one which was consistent with annealing and the other with deformation within the domain cataclasites had formed along some p domains suggesting that they had become preferential sites for late phase brittle deformation

magma assisted strain localization in an orogen parallel transcurrent shear zone of southern brazil in a lithospheric scale orogen parallel transcurrent shear zone of the pan african dom feliciano belt of southern brazil two successive generations of magmas an early calc alkaline and a late peraluminous have been emplaced during deformation microstructures show that these granitoids experienced a progressive deformation from magmatic to solid state under decreasing temperature conditions magmatic deformation is indicated by the coexistence of aligned k feldspar plagioclase micas and or tourmaline with undeformed quartz submagmatic deformation is characterized by strain features such as fractures lattice bending or replacement reactions affecting only the early crystallized phases high temperature solid state deformation is characterized by extensive grain boundary migration in quartz myrmekitic k feldspar replacement and dynamic recrystallization of both k feldspar and plagioclase decreasing temperature during solid state deformation is inferred from changes in quartz crystallographic fabrics decrease in grain size of recrystallized feldspars and lower ti amount in recrystallized biotites final low temperature deformation is characterized by feldspar replacement by micas the geochemical evolution of the synkinematic magmatism from calc alkaline metaluminous granodiorites with intermediate 87sr 86sr initial ratio to peraluminous granites with very high 87sr 86sr initial ratio suggests an early lower crustal source or a mixed mantle crustal source followed by a middle to upper crustal source for the melts shearing in lithospheric faults may induce partial melting in the lower crust by shear heating in the upper mantle but whatever the process initiating partial melting lithospheric transcurrent shear zones may collect melt at different depths because they enhance the vertical permeability of the crust these zones may then act as heat conductors by advection promoting an upward propagation of partial melting in the crust synkinematic granitoids localize most if not all deformation in the studied shear zone the regional continuity and the pervasive character of the magmatic fabric in the various synkinematic granitic bodies consistently displaying similar plane and direction of flow argue for accommodation of large amounts of orogen parallel movement by viscous deformation of these magmas moreover activation of high temperature deformation mechanisms probably allowed a much easier deformation of the hot synkinematic granites than of the colder country rock and consequently contributed significantly to the localization of deformation finally the small extent of the low temperature deformation suggests that the strike slip deformation ended approximately synchronously with the final cooling of the peraluminous granites the evolution of the deformation reflects the strong influence of synkinematic magma emplacement and subsequent cooling on the thermomechanical evolution of the shear zone magma intrusion in an orogen scale transcurrent shear zone deeply modifies the rheological behavior of the continental crust it triggers an efficient thermomechanical softening localized within the fault that may subsist long enough for large displacements to be accommodated therefore the close association of deformation and synkinematic magmatism probably represents an important factor controlling the mechanical response of continental plates in collisional environments

petrochemistry and tectonic significance of carboniferous volcanic rocks in new brunswick carboniferous volcanic rocks from the new brunswick platform in the maritimes basin are divided into three age groups late tournaisian to early visean volcanic rocks are tholeiitic basalts and andesites that in southern new brunswick are inter bedded with abundant calc alkalic rhyolite late visean to namurian volcanic rocks consist of an interbedded sequence of alkalic basalts and trachyandesites late westphalian volcanic rocks change in composition up section from trachyte to peralkalic rhyolite all three age groups display petrochemical features indicative of an intraplate tectonic setting the volcanic geochemistry is consistent with the development of the maritimes basin either as a failed rift formed along the margin of a late paleozoic ocean or as a rhomb graben formed within a transcurrent zone the former model is preferred the change in basaltic composition from tholeiitic to alkalic apparently coincided with a decrease in rate of extension between the tournaisian and namurian local peral

stratigraphic significance of upper devonian and lower carboniferous miospores from the type area of the horton group nova scotia late devonian late famennian miospores have been found in the lowermost 7 3 m of the horton group on harding brook in the type area windsor subbasin nova scotia below tournaisian miospores of the emphanisporites rotatus indotriradites explanatus zone extending the age of the horton group in the type area down into the latest devonian shows that the lowest beds of the 1000 m thick group are coeval with latest devonian rocks elsewhere in the maritimes basin that have been excluded from the horton group by some authors evidence presented here favours the argument that late devonian rocks lithologically similar to the horton group deposited on the acadian unconformity should be included in that group miospore evidence indicates a minimum age of about 355 ma for exhumation of the northeastern part of the south mountain batholith

pluton emplacement by wall rock thrusting hanging wall translation and extensional collapse latest devonian plutons of the cobequid fault zone nova scotia canada

the age and significance of carboniferous plutons of the western cobequid highlands nova scotia a series of granite plutons with minor gabbro diorite phases outcrops immediately north of the cobequid fault in the western cobequid highlands nova scotia the plutons were cut by numerous dykes and parts of some plutons and some dykes subsequently underwent ductile deformation four rb sr whole rock k feldspar isochron dates ranging from 338 to 356 ma have been obtained from these plutons and the dykes that cut them the ages of these plutons are indistinguishable from an age of 341 ma obtained for the nearby fountain lake group volcanic rocks within estimated uncertainties measured carboniferous granite ages are uncommon in the appalachian orogen of eastern canada the geochemical character of these plutons indicates that they result from crustal extension this extension and the compression observed in the granites with ductile deformation are related to transcurrent motion between the avalon and meguma terranes along the minas fault zone preliminary rb sr whole rock data from poorly exposed pluto

microgranitic enclave shapes and magmatic strain histories constraints from drop deformation theory microgranitic mafic enclaves are common components of plutonic bodies and may represent the result of mingling of mafic and felsic magmas these enclaves are often ellipsoidal or elongate in shape in order to constrain the conditions under which such enclaves were generated and deformed by their host pluton we apply drop deformation theory to model their distortion our model takes into account the change in viscosity of both the host and enclave with temperature composition and the amount of suspended solids in enclave host systems and it assumes that simple shear flow predominates in the plutonic system we focus on the deformational history of enclaves over the time period in which they remain liquid and are either more or less viscous than their granitic host we find that the most stable enclaves with respect to being broken up by their entraining flow are generally the most mafic enclaves those with silica contents near those of the host granite are highly unstable this is in reasonable accord with field observations of the abundance of enclaves of different chemistry most deformation of microgranitic enclaves is likely to occur at relatively high temperatures probably between 950 c and 1050 c and therefore the enclaves record magmatic strain of the host over only a limited temperature time range in the host s cooling history furthermore the observation of apparent deformation of these enclaves in a liquid regime coupled with our calculated dependences of enclave deformation on flow velocity implies that magmatic flow velocities are likely to be below 10 m yr in enclave bearing plutonic systems

crustal thickness under the gulf of st lawrence northern appalachians from gravity and deep seismic data we have determined crustal thickness in the gulf of st lawrence an area that corresponds to an offset of the main northern appalachians units a complete bouguer anomaly was calculated from recent depth to basement compilations and sediment densities from well data the moho surface was obtained by inverting the bouguer anomaly assuming a single density contrast at depth and using an average depth provided by deep reflection seismic data the resulting crustal model shows a moho depth of 42 44 km beneath the grenville craton north of the appalachian deformation front south of this front the depth to moho displays a pronounced thinning of the crust beneath the carboniferous magdalen basin this is in striking contrast to the deep seismic data which give a moho depth of about 43 km the modelling of the bouguer anomaly in the magdalen basin taking into account the seismic reflection and refraction data reconciles these different results and suggests that a 43 km deep moho beneath the basin is as

crustal scale en echelon p shear tensional bridges a possible solution to the batholithic room problem the apparent paradox of voluminous granitoid emplacement in an overall compressional magmatic are has been somewhat alleviated by the idea of pluton emplacement in tension cracks and dilational jogs within arc parallel strike slip fault systems as an alternative hypothesis we propose that an en echelon p shear array provides a continuous batholith scale zone of dilation along the trend of active magmatism and that it is favored by transpressional tectonics in the sierra nevada california the late cretaceous granitoids of the cathedral range intrusive epoch are consistent with emplacement into bridges between p shears because 1 they are elongated oblique to the magmatic trend consistent with p shear orientation within a dextral system 2 they were all emplaced within 5 10 m y along a 300 km trend resulting in a displacement rate of 1 cm yr across the arc and 3 syn to late magmatic dextral shear zones bound and crosscut the plutons as in a p shear tensional bridge

the illite crystallinity technique a critical appraisal of its precision analysis of the precision of the illite crystallinity technique shows that machine errors are 0 8 it is necessary that contours should be at intervals of 0 1 2 which is equivalent to subdivision of the anchizone into upper and lower units where previous interpretations have relied upon an isocryst method of contouring at less than 0 1 2 the conclusions must be regarded as unsubstantiated centrifuge separation of clay fractions based on a stokes law application gives separations in which a significant but variable percentage of grains have long axes greater than the size calculated for the typical 2 m fraction utilized some 20 of grains lie in the 2 4 m range although the proportion is not believed to have a significant effect upon crystallinity values the formula is applicable for grain sizes down to 0 5 m illite crystallinity values on samples prepared by an ultrasonic disaggregation method show a small increase on those prepared by ball mill crushing the differences are minimal at the epi anchizone level but increase to some 10 at the anchizone diagenetic level the effect on grade determinations is again thought to be minimal and indicates that concern over unsuitability of the ultrasonic disaggregation method is unfounded

stoped blocks in plutons paleo plumb bobs viscometers or chronometers abstract stoped blocks potentially provide information about paleohorizontal during emplacement of plutons and about the timing of and viscosities and kinematics during magmatic fabric formation in the mount stuart batholith washington we have examined magmatic fabrics patterns around stoped blocks in tonalite located near the pluton roof and completed 1 1 scale three dimensional mapping of fabric patterns around stoped blocks in diorite located 1000 m below the roof in both settings no structural record of magma ascent early emplacement or settling of the blocks is preserved instead fabric formation largely post dates the settling and trapping of stoped blocks two dimensional finite difference thermal modeling and calculations of sinking rates support short

compositional controls on coexisting prehnite actinolite and prehnite pumpellyite facies assemblages in the tal y fan metabasite intrusion north wales implications for caledonian metamorphic field gradients the tal y fan intrusion is a 110 m thick sub concordant metabasite sheet intruded into volcaniclastic and pyroclastic rocks of ordovician age in north wales despite low grade metamorphism primary textural zones resulting from initial cooling of the sheet are preserved and retain primary mineralogical and chemical variations which influenced the nature and extent of metamorphic recrystallization this has resulted in a vertical sequence of secondary mineral assemblages through the intrusion during early hydrothermal alteration k feldspar replaced plagioclase micropheno crysts in the marginal and contact zones and olivine in the central zone was replaced by saponite subsequent regional metamorphism resulted in the development of metastable prehnite pumpellyite epidote assemblages in two sub zones characterized by high fe2o3 elsewhere the assemblage prehnite actinolite epidote developed except in the contact and marginal zones where activity of co2 suppressed both prehnite and pumpellyite both assemblages contain excess albite quartz and chlorite and on the basis of uniform mineral compositions over the area of an individual thin section are considered to represent buffered equilibrium assemblages indicative of prehnite pumpellyite and prehnite actinolite facies conditions a metamorphic temperature of 310 c at 1 85 kbar is obtained using the p t x grid of liou maruyama cho 1985 which implies a field gradient of 44 c km 1 assuming that metamorphism relates to burial an overburden thickness of 7 km is indicated total maximum thicknesses however of ordovician silurian and lower devonian strata in the area do not exceed 6 km indicating a field gradient of 52 c km 1 these relatively high gradients may possibly be related to concealed late caledonian intrusions or alternatively may result from high heat flow as a consequence of crustal thinning rapid sedimentation and intense magmatic activity in a marginal basin setting

late devonian palinspastic reconstruction of the avalon meguma terrane boundary implications for terrane accretion and basin development in the appalachian orogen abstract a late devonian early carboniferous palinspastic reconstruction of atlantic canada restoring subsequent motion along the avalon meguma terrane boundary and other major faults facilitates re examination of the spatial distribution of important accretionary and post accretionary events in the northern appalachian orogen the reconstruction suggests a genetic relationship between 415 and 360 ma tectonothermal activity in both these terranes and the collision between laurentia and gondwana in a zone of anomalously intense shortening opposite the st lawrence promontory in the appalachian orogen erosional products of this collision are an order of magnitude greater than the thin ca 1 km veneer of latest devonian early carboniferous clastic rocks that were deposited in intermontane basins across the orogen this missing detritus may be found in the thick deposits of the catskill clastic wedge in the central appalachians

significance of a hiatus in down temperature fabric development within syn tectonic quartz diorite complexes channel islands uk detailed fabric development histories have been deduced for neoproterozoic foliated quartz diorite complexes of the channel islands uk the plutons were emplaced during regional d2 deformation magmatic fabrics formed at low crystal contents pre rheologically critical melt percent are only locally preserved due to overprinting by solid state deformation which occurred at moderate temperatures 400 550 c this is demonstrated by brittle microcracking of plagioclase and amphibole with only local ductile bending and dynamic recrystallization marginal recrystallization and myrmekite development on alkali feldspar ductile deformation of quartz to form types 2 3 ribbons quartz recrystallization by sub grain rotation associated with a slip but not c slip biotite recrystallization within micro shears and variable amphibole recrystallization the deformation histories display a distinct division between magmatic state and moderate temperature solid state deformation features with little evidence for intervening high temperature solid state fabrics this contrasts with criteria widely considered diagnostic of syn tectonic plutons which envisage development of a continuum of magmatic through high temperature solid state deformation fabrics however we consider the plutons syn tectonic and attribute the hiatus to contrasting strain and pluton cooling rates length and nature of deformation events and host rock ambient temperatures intermediate magmas emplaced at moderate to shallow levels in the crust may be more prone to develop such apparently punctuated deformation histories than sio 2 richer granitoids

the role of dextral transpressional faulting in the evolution of an early carboniferous mafic felsic plutonic and volcanic complex cobequid highlands nova scotia canada abstract the wentworth plutonic complex consisting of gabbro and granite was emplaced in the earliest carboniferous in the cobequid shear zone of the northern appalachians the plutonic complex is coeval with a 5 km thick pile of volcanic rocks early alkalic a type granite correlates with thick felsic pyroclastics and minor basalt which are overlain by 1 5 km thick basalts that correlate with a large gabbro pluton that is intruded in turn by late granites the basalt and gabbro are fe rich tholeiites the geochemistry of the late granites suggests that they formed by differentiation of a granodioritic magma resulting from assimilation of early granite by the gabbroic magma the wentworth plutonic complex lies on the north side of the dextral rockland brook fault near the western tip of wedge shaped basement block of the avalon terrane field observations of mesoscopic structures and map contacts show that the plutonic bodies at all structural levels are related to transpressive strike slip faults dykes parallel to the mylonitic foliation in the rockland brook fault zone and at the contacts between igneous phases suggest that the plutons developed largely through dyke to pluton construction the plutonism was initiated by dyking related to major faults under transpression that was partitioned into shear zone bounded blocks while the sinking of those blocks finally provided the space for mafic magma emplacement dyking was active over at least a 10 ma time period the overall location of plutonism in the cobequid shear zone appears related to its position at the intersection of the shear zone bounding the southwestern margin of the magdalen basin and the e w transpressional contact of the avalon and meguma terranes magmatism enabled thermomechanical softening of the crust and the vertical and lateral extrusion of the wedge shaped basement blocks whose movement controlled the localisation of the voluminous magmatic activity

the mortagne granite pluton france emplaced by pull apart along a shear zone structural and gravimetric arguments and regional implication the emplacement mechanism and kinematics and the 3 d geometry of the variscan granite pluton of mortagne are interpreted using systematic measurements of the granite structure allied with micro structural observations from a detailed gravimetric study it is proposed that magma infilled a pull apart structure which formed along a jog of a northwest southeast trending sinistral zone of shear a second episode of shearing under a dextral regime and independent of emplacement was responsible for perigranitic tectonites these successive transcurrent opposite movements already documented for variscan times in western france are discussed concerning the age of the mortagne pluton 313 15 m y

interlaboratory standardization and calibration of day mineral crystallinity and crystallite size data clay mineral crystallinity and crystallite domain size data determined by x ray diffraction xrd are methods extensively used in the characterization of very low grade metamorphic conditions however the lack of sufficient interlaboratory standardization has made comparisons between different research groups unreliable due to significant variations in numerical results obtained a consequence of the different machine conditions measurement methods and sample preparations used during analysis a calibration approach to the standardization of data using rock chip standards is presented which allows data sets produced by different research groups to be directly and quantitatively compared a standardized scale the crystallinity index standard cis is proposed with illite crystallinity anchizonal boundary limits of 0 25d 2 and 0 42d 2 and equivalent illite crystallite sizes of 52 and 23 nm respectively determined by the warren averbach method calibrating both old and new data will enable more reliable comparisons between similar and contrasting geological environments and should improve the accuracy and reliability of correlations made between xrd data and other indicators of very low grade metamorphism hence increasing the value of such clay mineral studies

the great tonalite sill emplacement into a contractional shear zone and implications for late cretaceous to early eocene tectonics in southeastern alaska and british columbia the late cretaceous to early tertiary great tonalite sill of southeast alaska and british columbia is a very long 1 000 km and thin 25 km orogen parallel composite batholith which lies between the insular superter rane including the alexander and wrangellia terranes and the intermontane superterrane including the stikine and cache creek terranes the batholith is delineated by many steep sheet like plutons which are dominated by northwest southeast striking concordant fabrics with steep lineations that formed during deformation in a country rock shear zone prior to the complete crystallization of the magmas deformation in this shear zone is dominated by northeast southwest directed contraction orthogonal to the orogenic strike associated with a component of northeast over southwest high angle shear the steep multiple dike like nature of the body and its emplacement during orogenic contraction imply that ascent and emplacement have been achieved by dike wedging mechanisms along the deep reaching shear zone the remarkable narrowness of the great tonalite sill is probably the result of melting at the base of a very localized zone of thickened crust produced by the associated narrow contractional shear zone extending along the orogen length such a shear zone of late cretaceous to early tertiary age lying along 800 km of the possible boundary between the insular and intermontane superterranes implies that it may represent the actual boundary between them if this hypothesis is correct it implies that the large scale tectonic regime during emplacement of the great tonalite sill was predominantly orthogonal and not obliquely dextral as has been indicated from paleomagnetic data

relationships between syn tectonic granite fabrics and regional pttd paths an example from the gander avalon boundary of ne newfoundland abstract syn tectonic granitic plutons and their host rocks not only provide a potential wealth of information about the timing and nature of regional kinematics but also record information about thermal conditions successive syn tectonic plutons emplaced into the gander zone of ne newfoundland preserve fabric overprinting formed during cooling to regional ambient thermal conditions ambient conditions have been approximated for each pluton by careful analysis of microstructures and consideration of potential cooling histories and have illustrated regional exhumation during a tectonic reversal from silurian devonian sinistral transpression to devonian dextral transpression because the time period of emplacement is relatively short by comparison to orogenic events typically by more than an order of magnitude individual plutons may record a snap shot of the regional history by combining information from a number of plutons emplaced sequentially during a period of regional orogenesis a picture may be built up defining time temperature deformation ttd paths such information may compliment regional metamorphic pt studies in helping to establish orogenic pttd paths

tectonic deformation and magmatism along the southern flank of the maritimes basin the northeastern cobequid highlands nova scotia distributed crustal scale faulting in the cobequid highlands in the middle devonian to carboniferous resulted from the oblique convergence of the meguma and avalon terranes in the northeastern cobequid highlands seismic reflection profiles show neoproterozoic and lower paleozoic rocks together with enigmatic foliated rocks overlying the early carboniferous fountain lake group the foliated rocks form the hanging wall of a north vergent thrust fault their protolith is inferred from petrography and geochemistry to be principally neoproterozoic rhyodacitic tuff and late paleozoic hypabyssal intrusions the age of thrusting is stratigraphically constrained to the late tournaisian mid visean and sericite from mylonite yielded a tournaisian k ar age of 352 8 ma the thrusting occurs at the base of a tectonic escape sheet and resulted from a restraining bend in the rockland brook master fault farther west where the rockland brook fault trends almost east west tournaisian extensional features include

the distinction between caledonian burial and regional metamorphism in metapelites from north wales an analysis of isocryst patterns in snowdonia and ll n three prograde stages in metapelites gwna group to wenlock are recognized stage i with white mica crystallinity values 0 43 2 stage ii 0 43 0 26 2 stage iii isocrysts at 0 03 2 intervals are drawn separately for pelites above and below the sub arenig unconformity approximately concordant isocryst patterns characterize the unconformity in the northwestern and southeastern limbs of the snowdonia syncline whereas discordant patterns are developed northwest of the aber dinlle fault and at the closure of the ynyscynhaiarn anticline the contrasting patterns are the result of a pre arenig burial metamorphism in post tremadoc pelites belts of steep metamorphic gradient coincide with belts of high strain and a broad area of higher grade caradoc pelites coincides with the core of the snowdonia syncline the later metamorphism is related to caledonian deformation rather than to burial pelite alteration adjacent to small intermediate to acid plutons suggests emplacement before the caledonian regional metamorphism a close correlation exists between metabasite zones and metapelite stages

a transmission electron microscope study of white mica crystallite size distribution in a mudstone to slate transitional sequence north wales uk high resolution transmission electron microscopy hrtem measurements of the thickness of white mica crystallites were made on three pelite samples that represented a prograde transition from diagenetic mudstone though anchizonal slate to epizonal slate crystallite thickness measured normal to 001 increases as grade increases whereas the xrd measured 10 a peak profile the kubler index decreases the mode of the tem measured size population can be correlated with the effective crystallite size n 001 determined by xrd the results indicate that the kubler index of white mica crystallinity measures changes in the crystallite size population that result from prograde increases in the size of coherent x ray scattering domains these changes conform to the scherrer relationship between xrd peak broadening and small crystallite size lattice strain broadening is relatively unimportant and is confined to white mica populations in the diagenetic mudstone rapid increases in crystallite size occur in the anchizone coincident with cleavage development changes in the distribution of crystallite thickness with advancing grade and cleavage development are characteristic of grain growth by ostwald ripening the kubler index rapidly loses sensitivity as an indicator of metapelitic grade within the epizone

tem and aem constraints on the origin and significance of chlorite mica stacks in slates an example from central wales u k chlorite mica stacks grains of intergrown chlorite and white mica in a matrix of fine grained white mica and chlorite have been studied using xrd sem empa tem and aem methods the stacks occur in a weakly cleaved llandoverian mudstone central wales which has a white mica illite crystallinity index of 0 35 a20 corresponding to the lower anchizone white mica occurs as packets 100 to 8 m thick interleaved with dominant chlorite packets in stacks with both apparent coherent interfaces or cross cutting small angle boundaries with chlorite layers it is well crystallized 2m polytype with phengitic composition and low paragonite component na na k 0 0 07 chlorite in stacks is fe rich and relatively homogeneous tio2 crystals surround stacks and occur within chlorite and white mica is ti rich compared to matrix white mica fine grained white mica and chlorite in the matrix have two different orientations one parallel to bedding and one parallel to cleavage which is approximately 30 50 to bedding matrix white mica is predominantly a 2m1 polytype but some cleavage parallel white mica is 3 t and some bedding parallel white mica is 1md it is na rich na na k 0 14 0 42 and relatively heterogeneous some discrete paragonite and phengitic muscovite are observed to coexist in the cleavage orientation matrix white mica and chlorite each contain less fe than corresponding white mica and chlorite in stacks both matrix and stack chlorite are one layer polytypes the data imply that chlorite in the stacks is largely derived from the replacement of volcanogenic biotite and other ferromagnesian minerals probably via intermediate expandable trioctahedral phyllosilicates most intergrowths of chlorite and mica in stacks formed by mica replacement of chlorite and altered biotite along cleavage fissures subsequent deformation caused further modification of pre existing chlorite mica stacks whereas partial dissolution of stacks and bedding parallel matrix phyllosilicates resulted in the formation of cleavage parallel phyllosilicates

geochemical evolution of earliest carboniferous continental tholeiitic basalts along a crustal scale shear zone southwestern maritimes basin eastern canada abstract continental tholeiites were extruded locally within the extensional wrench maritimes basin from about 375 to 330 ma late devonian early carboniferous basalts of the early carboniferous diamond brook formation are up to 1 5 km thick and overlie a succession of predominantly felsic volcanic rocks reaching several kilometres in thickness the volcanic rocks and co genetic plutons occur along a crustal scale strike slip fault that facilitated the transport of relatively primitive magma resulting in basalts with high mg number ni and cr end 4 and relatively low tio2 v y and zr fractional crystallization of plagioclase and lesser clinopyroxene in the absence of iron oxide fractionation resulted in repetitive extrusion cycles hundreds of metres in thickness that show an upward increase in plagioclase phenocrysts or megacrysts and in tio2 and other high field strength elements hfses rare andesites are interpreted as products of partial melting of underplated gabbro the relatively primitive basalts resemble low ti basalts found in other continental flood basalt provinces and have trace element and isotopic characteristics that have been ascribed in the literature either to partial melting of or contamination of asthenospheric melts by subcontinental lithospheric mantle in the maritimes basin similar continental tholeiites were extruded intermittently over a 45 ma period high feot 12 normalised to 8 mgo suggests high potential temperature in the source mantle presumably within a mantle plume the youngest eruption products geochemically and isotopically resemble himu oib plume activity may have gradually migrated eastward from late devonian to permian from the maritimes basin through scotland to norway and north germany

middle to late paleozoic acadian orogeny in the northern appalachians a laramide style plume modified orogeny the laramide orogeny of the western united states is proposed as a modern analogue for the silurian devonian acadian orogeny and subsequent diachronous voluminous short lived magmatism and basin formation in the northern appalachians shallowing of the benioff zone accounts for several enigmatic features associated with plate convergence in the northern appalachians including 1 wenlockian ludlovian termination of arc related magmatism in the avalon terrane followed by a period of relative magmatic quiescence from 395 to 380 ma and 2 diachronous migration of the acadian deformation front from ca 415 ma in the southeast to ca 370 ma in the northwest extending more than 600 km into the continental interior the flattening of the subduction zone is attributed to overriding of a plume by the convergent margin which may explain 1 the abrupt termination of magmatic quiescence by 380 370 ma and voluminous felsic magmatism and production of plume related lamprophyres in the southeast meguma terrane as the plume thermally eroded the oceanic lithosphere causing melting of the lower crust 2 late devonian regional high temperature low pressure metamorphism in the meguma terrane related to the thermal anomalies above a plume 3 synchronous devonian emplacement of meguma gold deposits and associated siderophile elements possibly derived from fluid circulation above an ascending plume 4 rapid late devonian uplift and erosion of as much as 10 km due to dynamic uplift over a plume 5 migration of magmatism to the north avalon terrane cobequid highlands so that plume related carboniferous magmatism occurred in and around the carboniferous permian maritimes basin 6 the high density lens at the base of the crust beneath the maritimes basin the product of plume derived underplated mafic rocks and 7 a subsidence mechanism for formation of the maritimes basin by cooling of a decapitated plume head

synkinematic granite emplacement in a shear zone the pleasant hills pluton canadian appalachians the 361 356 ma late devonian pleasant hills pluton comprises a series of sheets of mafic felsic and hybrid intermediate plutonic rocks that were emplaced along a positive flower structure associated with the crustal scale cobequid fault zone this dextral strike slip fault zone forms the boundary of the meguma and avalon terranes the sequence of emplacement is inferred from crosscutting structures radiometric dating shows that the pluton was crystallized within a few million years gabbroic magma was probably derived from partial melting of the mantle beneath the extensional magdalen basin and successive batches of mafic magma were progressively richer in h 2 o ti zr and nb felsic magmas were probably derived by partial melting of the lower crust small batches of felsic magma show differing trace element evolution that reflects changes in magma temperature and type of volatiles younger magmas being generally richer in high field strength elements and fluorine rapakivi textured granite is common heat transfer and mass transfer along the fault zone resulted in thermal softening and lubrication of all branches of the flower structure so that there was repeated magma flow through each branch the presence of magma controlled the localization of strain which is expressed as synmagmatic deformational structures and deformation continued under solid state conditions to produce mylonitic fabrics each branch of the flower structure acted as a pathway for many magma batches and only small volumes of magma reached the highest structural levels the pluton was emplaced by progressive widening of and magma emplacement along branches of the flower structure as a result of country rock translation this translation involved both lateral and upward movement of blocks within the cobequid fault zone dextral slip is recognized on the southern margin of the escaping block and reverse oblique sinistral slip on its northern margin the pleasant hills pluton shows progressive creation of space for magma along branches of the flower structure and gradual expansion of the pluton from a series of dike like bodies to a larger composite pluton through the emplacement of numerous small geochemically distinct magma batches

the emplacement of the rieserferner pluton eastern alps tyrol constraints from field observations magnetic fabrics and microstructures abstract within the alpine chain several tertiary intrusions e g bergell adamello intruded along the periadriatic lineament this study attempts to show how the rieserferner pluton vedrette di ries is related to the defereggen antholz vals dav line for a survey of the fabric pattern over the pluton the anisotropy of the magnetic susceptibility ams was determined on nearly 170 samples low bulk susceptibilities of k vol 6 si and a total degree of anisotropy p

deep seismic reflection data from the bay of fundy and gulf of maine tectonic implications for the northern appalachians three deep penetration seismic reflection profiles were collected off southwest nova scotia to determine the crustal structure and geometry beneath the avalon and meguma zones of the appalachian orogen in canada onshore geological features have been traced seawards using new gravity and magnetic anomaly maps the seismic data can also be correlated with the previous united states geological survey profile in the central gulf of maine two seismically distinct lower crustal blocks are identified the avalon and sable lower crustal blocks separated by a major north dipping reflection zone that cuts the entire crust the recognition of the sable block adds a fourth block to the three already identified in the canadian appalachians the sable block is overlain by the meguma zone the avalon zone overlies at least the northern part of the avalon lower crustal block although offshore extension of geological features is not unequivocal it appears that a north dipping reflection zone southwest of nova scotia m

observations from the floor of a granitoid pluton inferences on the driving force of final emplacement an east west profile across the tilted bergell pluton exposes a 10 km thick interval in terms of crustal depth consequently the floor as well as the root and side of the main intrusive body of the pluton crop out at the surface and a tentative three dimensional geometry is constructed at the highest crustal level the geometry and deformation features at the margin of the pluton indicate ballooning whereas the folded floor of the main intrusive body indicates synmagmatic shortening related to regional deformation these contrasting features are best explained by shortening of the base of the pluton which caused an expansion at a higher crustal level final emplacement of the pluton into higher crustal levels was therefore not driven primarily by buoyancy but rather by regional deformation within deeper levels of the crust

calibration of the anchizone a critical comparison of illite crystallinity scales used for definition instrumental settings for determination of illite muscovite crystallinity half height width of the 10 a x ray diffraction peak and the limits of the anchimetamorphic zone adopted by various authors fall into several groups the variation in the limiting peak widths between the authors that have adopted kubler s neuch tel boundary values of 0 42 and 0 25 2 can be interpreted in terms of variation in the instrumental settings the choice of time constants higher than those given by the formula causes peak broadening this effect is particularly marked at high scan rates the peak broadening is by constant increments that are virtually independent of the absolute peak width the differences between the kubler neuch tel derived limiting values and some other scales are appreciably greater than can be accounted for by these differences in instrumental settings many of these scales are not equivalent in particular the limits adopted by dunoyer de segonzac 1969 and subsequent workers at strasbourg are too broad their anchizone represents a range of grades of metamorphism lower than that of kubler widely overlapping the latter s diagenetic zone those adopted by some other mainly french authors are too narrow the limits of the anchizone should be calibrated by inter laboratory standards and the instrumental settings should be specified in full