white mica crystallinity finite strain and cleavage development across a large variscan structure nw spain x ray diffraction analyses were performed on white micas formed under strain and low grade metamorphic conditions from sandstones and siltstones along a transect through the narcea antiform a large structure that separates the external from the internal zones of the variscan belt of nw spain the results indicate a close spatial relationship between the onset of metamorphism and the cleavage front at the cleavage front strain is characterized by oblate ellipsoids that have x y z ratios from 1 to 1 5 the x y z ratio increases westward up to 3 0 as does the grade of metamorphism and is accompanied by an increase in the 9crystallinity9 index of the white mica from very low to low metamorphic grade this increase corresponds qualitatively to an increase in penetrativeness of the axial planar cleavage in the western part of the narcea antiform the major deformation event was the development of kilometre scale reverse shear zones because of polyphase deformation these rocks are not suitable for quantitative strain analysis but qualitatively finite strain can be observed to be prolate and invariably larger in this region than in the eastern sector the metamorphic grade is higher in the western part of the narcea antiform with local maxima occurring next to the base of the shear zones the b 0 cell parameter a semi quantitative geobarometer measured in the white mica is chaotic in the eastern sector showing a clear detrital inheritance in the western sector it is rational due to complete re equilibration during thermal resetting the inverse correlation between finite strain and white mica 9crystallinity9 is close showing that the anchizone epizone limit can be related to the increase of finite strain above values of r s 1 5 the close relation between white mica 9crystallinity9 and strain is suitable for tracking strain variations in extensive fine grained siliciclastic rocks where no appropiate strain markers are found

mobility and fractionation of rare earth elements in argillaceous sediments implications for dating diagenesis and low grade metamorphism abstract we report sm nd and rb sr data for the fine fractions of lower paleozoic argillaceous rocks from wales uk and new york usa spanning the range of low grade metamorphic conditions from the diagenetic zone zeolite facies to the epizone greenschist facies in all cases leaching of the fine fractions results in a high 147 sm 144 nd 0 09 0 29 acid soluble component and a complementary low 147 sm 144 nd 0 05 0 14 residual component the observed fractionation is an ancient feature related to diagenesis burial and metamorphism the magnitude of sm nd fractionation between leachates and residues as well as the resulting sm nd ages vary as a function of grain size and metamorphic grade uncleaved welsh mudrocks of the diagenetic zone yield sm nd leachate residue ages of 453 484 ma in agreement with their llanviian to caradocian biostratigraphic ages whereas higher grade rocks of the anchizone and epizone yield sm nd ages as young as 413 ma these ages are transitional between the time of deposition and the time of regional deformation related to the acadian orogeny at 390 ma distinct convex upward rare earth element ree patterns of the leachates suggest that the precipitation of early diagenetic apatite controls the trace element budget of the rock forcing a depletion of middle rees on the subsequently formed diagenetic phyllosilicates the amount of organic matter present and the extent of later prograde reactions are probable modifiers of this fractionation process ordovician and devonian clastic rocks associated with the trenton and onondaga limestones of new york yield single sample and multi sample sm nd isochron ages that agree well with their biostratigraphic ages of 454 ma and 390 ma respectively the ree fractionation observed in shale leachates of the ordovician utica formation is related to ca mg of the bulk rock and hence to the composition of the diagenetic carbonate cement in all cases the sm nd system remained closed subsequent to the peak of diagenesis or metamorphism including the north american rocks that show no evidence of being isotopically reset during widespread remagnetization of the subjacent limestone units in the late paleozoic

formation of chlorite during thrust fault reactivation record of fluid origin and p t conditions in the monte perdido thrust fault southern pyrenees the chemical and isotopic compositions of clay minerals such as illite and chlorite are commonly used to quantify diagenetic and low grade metamorphic conditions an approach that is also used in the present study of the monte perdido thrust fault from the south pyrenean fold and thrust belt the monte perdido thrust fault is a shallow thrust juxtaposing upper cretaceous paleocene platform carbonates and lower eocene marls and turbidites from the jaca basin the core zone of the fault about 6 m thick consists of intensely deformed clay bearing rocks bounded by major shear surfaces illite and chlorite are the main hydrous minerals in the fault zone illite is oriented along cleavage planes while chlorite formed along shear veins 50 m in thickness authigenic chlorite provides essential information about the origin of fluids and their temperature 18o and d values of newly formed chlorite support equilibration with sedimentary interstitial water directly derived from the local hanging wall and footwall during deformation given the absence of large scale fluid flow the mineralization observed in the thrust faults records the p t conditions of thrust activity temperatures of chlorite formation of about 240 c are obtained via two independent methods chlorite compositional thermometers and oxygen isotope fractionation between cogenetic chlorite and quartz burial depth conditions of 7 km are determined for the monte perdido thrust reactivation coupling calculated temperature and fluid inclusion isochores the present study demonstrates that both isotopic and thermodynamic methods applied to clay minerals formed in thrust fault are useful to help constrain diagenetic and low grade metamorphic conditions

textures in layered silicates progressive changes through diagenesis and low temperature metamorphism abstract phyllosilicates in samples from gulf coast cores and the martinsburg formation near lehigh gap pennsylvania which together represent a continuous sequence from mudstone to slate are shown by tem studies to possess imperfections on a massive scale and to exhibit a progressive increase in perfection through diagenesis and low grade metamorphism in shallow portions 1 750 and 2 450 m of the gulf coast core smectites have irregular and curved layers with high dislocation densities and many small angle grainlike boundaries in both the deeper portion 5 500 m of the gulf coast sediments and in the martinsburg mudstone samples gram perfection increases in illites but a complex mosaic structure with small angle grain boundaries and edge dislocations still prevails in the martinsbu g slate phyllosilicates exhibit layers which are continuous and straight and have few imperfections original clays which form at low temperatures have highly imperfect and metastable structures changes with increasing diagenesis are akin to annealing processes with increasing perfection of grains being an approach to a more stable strain free structure temperature pore water time and other variables affecting these changes act only to promote reaction rates to the defect free stable structures

an elastic plate model for interseismic deformation in subduction zones geodetic observations of interseismic surface deformation in the vicinity of subduction zones are frequently interpreted using simple kinematic elastic dislocation models edm in this theoretical study we develop a kinematic edm that simulates plate subduction over the interseismic period the elastic subducting plate model espm having only 2 more degrees of freedom than the well established back slip model bsm an elastic plate thickness and the fraction of flexural stresses due to bending at the trench that are released continuously unlike the bsm in which steady state deformation in both plates is assumed to be negligible the espm includes deformation in the subducting and overriding plates owing to plate thickness while still preserving the correct sense of convergence velocity between the subducting and overriding plates as well as zero net steady state vertical offset between the two plates when integrated over many seismic cycles the espm links elastic plate flexure processes to interseismic deformation and helps clarify under what conditions the bsm is appropriate for fitting interseismic geodetic data at convergent margins we show that the espm is identical to the bsm in the limiting case of zero plate thickness thereby providing an alternative motivation for the bsm the espm also provides a consistent convention for applying the bsm to any megathrust interface geometry even in the case of nonnegligible plate thickness the deformation field predicted by the espm reduces to that of the bsm if stresses related to plate flexure at the trench are released either continuously and completely at shallow depths during the interseismic period or deep in the subduction zone below 100 km however if at least a portion of these stresses are not continuously released in the shallow portion of the subduction zone via seismic or aseismic events then the predicted surface velocities of these two models can differ significantly at horizontal distances from the trench equivalent to a few times the effective interseismic locking depth

detrital and metamorphic 40 ar 39 ar ages from muscovite and whole rock samples meguma supergroup southern nova scotia 40ar 39ar dates from eight muscovite separates and two whole rock samples of slate siltstone and sandstone provide detrital metamorphic and post metamorphic overprinting ages for the low grade metamorphic rocks of the meguma supergroup in the mahone bay region of southern nova scotia variably reset detrital muscovite ages of 600 550 ma agree with previously obtained u pb dates from detrital zircon separates of foliation parallel muscovite yield precise 40ar 39ar ages of 395 388 ma petrographic and metamorphic data indicate that these are metamorphic mica growth ages the range overlaps with the younger limit of previously reported whole rock ages 410 380 ma but is more tightly constrained an apparent inconsistency between previous 40ar 39ar whole rock ages as old as 410 ma and regional deformation that affected sedimentary rocks as young as 385 ma has been resolved by the younger ages reported here and by recent adjustments to the early paleozoic time scale which suggest that the lochkovian to

syntectonic monazite in low grade mudrocks a potential geochronometer for cleavage formation monazite paragenesis during low grade metamorphism is poorly understood the first unequivocal instance of the mineral having crystallized synchronously with compressional cleavage formation is reported compositionally these monazites resemble diagenetic forms but differ texturally by displaying syntectonic microfabrics their predominant association with organic walled fossils implies that organic material may play an important role in controlling the behaviour of light rare earth elements lree during low grade metamorphism probably by releasing diagenetically sequestered lree and organically bound phosphorus during thermal maturation devolatilization and or by promoting a favourable environment for monazite precipitation potentially such monazites could provide a powerful geochronometer for cleavage formation

40ar 39ar illite dating of late caledonian acadian metamorphism and cooling of k bentonites and slates from the welsh basin u k the vacuum encapsulation laser 40ar39ar technique allows extremely small 10 6 g samples of fine grained materials such as diagenetic clays to be dated here we show that the method can be extended to higher grade clay minerals the integration of transmission electron microscopic tem characterization with 40ar39ar dating of vacuum encapsulated samples permits the resolution of the timing of metamorphic growth cooling from the time of diagenesis we have applied this technique to well characterized lower paleozoic slates and k bentonites from the welsh basin which span the transition from anchizonal to epizonal grade which had been previously studied using rbsr and smnd dating tem observations of epizonal k bentonites and slate showed that illite in these samples is of 2m1 polytype of muscovite like composition and oriented parallel to cleavage suggesting that they are of metamorphic origin total gas ages equivalent to conventional kar ages for encapsulated epizonal k bentonites and slate 340 408 ma are considerably variable the ar retention ages calculated from 39ar and 40ar atoms retained in the sample after irradiation are more consistent 383 411 ma the 39ar recoil losses are minor for illites from whole rock samples of epizonal k bentonites but very significant for clay separates of epizonal slate plateaus in age spectra were observed in epizonal k bentonites and slate the plateau ages 414 421 ma and retention ages 383 411 ma can be correlated with the onset of acadian metamorphism and culmination of uplift and inversion of the welsh basin respectively these ages are significantly younger than the 450 ma ages previously reported for diagenetic clays using the same method suggesting that diagenetic history has been lost in these epizonal k bentonites and slate tem observations of anchizonal slates showed that there are two modes of illite the first mode is similar to that observed in epizonal samples suggesting a metamorphic origin the second mode consists of the 1md polytype has typical diagenetic illite composition and is oriented parallel to bedding suggesting a diagenetic origin total gas ages for encapsulated anchizonal slates vary considerably 361 422 ma the retention ages are more consistent 413 432 ma than the total gas ages the 39ar recoil losses are more significant than those for epizonal k bentonites and slate plateaus in age spectra are generally not observed however the consistent retention ages for the anchizonal slates correspond to the plateau ages for the epizonal samples and are inferred to represent the onset of acadian metamorphism these data when combined with our previously published results for diagenetic shales suggest that thermal conditions near the boundary of anchizonal and epizonal grades are necessary to completely reset ar systems in shales and slates

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

rb sr ages of micas from the kathmandu complex central nepalese himalaya implications for the evolution of the main central thrust the kathmandu complex comprising the bhimphedi and phulchauki groups which range in age from precambrian to lower palaeozoic is situated in the frontal part of the main central thrust sheet although it is likely that a tectonothermal event antedated the intrusion of early ordovician granites rb sr dating of muscovites in deformed pegmatites suggests that the high grade metamorphism in the bhimphedi group is of tertiary age these pegmatites cooled through c 500 c at 18 14 ma biotite rb sr data indicate cooling through c 300 c at 7 5 ma thermobarometric and radiometric data from central nepal suggest an evolutionary model for the emplacement history of the main central thrust sheet involving cooling from a peak temperature c 660 c to c 500 c in 7 ma and loss by erosion or tectonic denudation of c 4 11 km of section

chlorite mica stacks in low strain rocks from central wales weakly deformed low grade lower palaeozoic metasediments from central wales contain abundant stack like intergrowths of chlorite and white mica that closely resemble stacks described from the devonian hunsruckschiefer of west germany the ordovician martinsburg slate new jersey u s a and elsewhere several theories have been proposed to explain the origin of such stacks including a detrital origin strain controlled growth of chlorite on a detrital mica nucleus and strain controlled intergrowth during metamorphism none of these satisfactorily explains the central wales stacks a detrital origin is precluded by the presence of many stacks with shapes too delicate to have survived transportation and a lack of hydrodynamic equivalence between the stacks and the clastic host grains features inconsistent with strain controlled growth are constant alignment parallel to bedding but non systematic orientation with respect to tectonic cleavage their common occurrence in undeformed rocks and petrographic evidence that they precede the tectonic cleavage it is proposed that the stacks formed during diagenesis and low grade metamorphism and before the onset of deformation through mimetic growth on a primary bedding fabric composed of clay minerals

evidence for syntectonic crystallization for the mudstone to slate transition at lehigh gap pennsylvania u s a data primarily for phyllosilicates have been obtained for the continuous transitional sequence from mudstone to slate with well developed slaty cleavage at lehigh gap pennsylvania and for slates from quarries in the same area samples were studied by optical microscopy powder x ray diffraction and electron microprobe analysis with emphasis on transmission and analytical electron microscopy mineral grains are virtually free of deformation induced strain concomitant with the gradual development of cleavage normal to bedding the following changes are observed or confirmed 1 the orientation of phyllosilicate grains changes discontinuously from being preferentially parallel to bedding to being parallel to cleavage 2 crystal imperfections as expressed in layer terminations low angle grain boundary like features and other defects decrease in density 3 complex mixed layering is replaced by homogeneous packets of layers of single phases and 4 illite transforms to muscovite with increase in k a1 and change from a lmd to 2m polytype slaty cleavage apparently develops due in part to pressure solution of phyllosilicates oriented parallel to bedding mass transport of components and crystallization to form new grains parallel to cleavage it reflects transitions from imperfect metastable phases toward ordered stable phases in a low temperature 225 c metamorphic environment

the metamorphic history of the concealed caledonides of eastern england and their foreland white mica illite crystallinity data derived mostly from borehole samples have been used to generate a contoured metamorphic map of the concealed caledonide fold belt of eastern england and the foreland formed by the midlands microcraton the northern subcrop of the fold belt is characterized by epizonal phyllites and quartzites of possible cambrian age whereas anchizonal grades characterize silurian to lower devonian strata of the anglian basin in the southern subcrop of the fold belt regional metamorphism in the anglian basin resulted from deep burial and acadian deformation beneath a possible overburden of 7 km assuming a metamorphic field gradient of 36 c km 1 late proterozoic volcaniclastic rocks forming the basement of the microcraton show anchizonal to epizonal grades that probably developed during late avalonian metamorphism cambrian to tremadoc strata showing late diagenetic alteration rest on the basement with varying degrees of metamorphic discordance during early palaeozoic times much of the microcraton was a region of slow subsidence with overburden thicknesses of 3 3 5 5 km however concealed tremadoc strata in the northeast of the microcraton reach anchizonal grades and may have been buried to depths of 7 km beneath an overburden of uncertain age

behavior of illite and chlorite during pressure solution of shaly limestone of the kalkberg formation catskill new york textural and chemical changes occurring in illite and chlorite concomitant with pressure solution of limestone were studied in samples from the kalkberg formation of catskill new york using xrd and tem aem samples on one limb of an anticline are massive shaly limestones but those on the other have undergone extensive pressure solution and well developed cleavage is present illite and chlorite from the uncleaved shaly limestone are found in small individual packets 100 800 thick dispersed throughout the carbonate matrix with crystal morphologies characteristic of burial diagenesis phyllosilicates from the limb more affected by pressure solution occur in larger units 1 urn thick as stacks of subparallel packets 150 500 a thick such stacks are inferred to represent coalescence of smaller packets these data imply that the phyllosilicates are largely passive during pressure solution of limestone however localized solution recrystal lization is required by the coherent to semi coherent packet boundaries and the crystal morphologies present in the pressure solution sample the largely passive role is in contrast with the more active role of phyllosilicates in many shales and slates xrd data for illite show an increase in crystallinity in the pressure solution sample under isothermal conditions differences in illite crystallinity are adequately explained in large part by differences in crystal size with some contribution due to strain the data demonstrate that illite crystallinity cannot be unambiguously used in determining absolute or even relative temperatures

phyllosilicates in hydrothermally altered basalts from dsdp hole 504b leg 83 a tem and aem study phyllosilicates occurring as replacements of olivine clinopyroxene and interstitial materials and as veins or fracture fillings in hydrothermally altered basalts from dsdp hole 504b leg 83 have been studied using transmission and analytical electron microscopy the parageneses of phyllosilicates generally change systematically with depth and with the degree of alteration which in turn is related to permeability of basalts saponite and some mixed layer chlorite smectite are the dominant phyllosilicates at the top of the transition zone chlorite corrensite and mixed layer chlorite corrensite occur mainly in the lower transition zone and upper levels of the sheeted dike zone chlorite talc and mixed layer talc chlorite are the major phyllosilicates in the sheeted dike zone although replacement of talc or ohvine by saponite is observed the phyllosilicates consist of parallel or subparallel discrete packets of coherent layers with packet thicknesses generally ranging from 100 a to a few hundred a the packets of saponite layers are much smaller or less well defined than those of chlorite corrensite and talc indicating poorer crystal linity of saponite by contrast chlorite and talc from the lower transition zone and the sheeted dike zone occur in packets up to thousands of a thick the si si al ratio of these trioctahedral phyllosilicates increases and fe fe mg decreases in the order chlorite corrensite saponite and talc these relations reflect optimal solid solution consistent with minimum misfit of articulated octahedral and tetrahedral sheets variations in composition of hydrothermal fluids and precursor minerals especially in si si al and fe fe mg ratios are thus important factors in controlling the parageneses of phyllosilicates the phyllosilicates are generally well crystallized discrete phases rather than mixed layered phases where they have been affected by relatively high fluid rock ratios as in high permeability basalts in veins or areas adjacent to veins intense alteration in basalts with high permeability indicating high fluid rock ratios is characterized by pervasive albitization and zeolitization minimal alteration in the basalts without significant albitization and zeolitization is characterized by the occurrence of saponite mixed layer chlorite smectite in the low temperature alteration zone and mixed layer chlorite corrensite or mixed layer talc chlorite in the high temperature alteration zone textural non equilibrium for phyllosilicates is represented by mixed layering and poorly defined packets of partially incoherent layers the approach to textural equilibrium was controlled largely by the availability of fluid or permeability

cambrian and ordovician metabentonites and their relevance to the origins of associated mudrocks in the northern sector of the lower palaeozoic welsh marginal basin

kinematic and metamorphic aspects of cleavage formation in very low grade metamorphic slates abstract fabric studies on first cleavages in very low grade metamorphic slates from the rheinisches schiefergebirge and other very low grade metamorphic fold belts reveal two types of slaty cleavage 1 1 cleavage without syntectonic phyllosilicate recrystallization these are generally crenulation cleavages 2 2 cleavage with syntectonic phyllosilicate recrystallization with increasing syntectonic metamorphism the crenulation fabric is modified by solution of grains along cleavage planes and precipitation of material within the cleavage lamellae with further increase of metamorphism the crenulation fabric is gradually obliterated the finite fabric results from progressive solution crystallization and recrystallization superimposed with bending kinking and internal rotation achieved by slip parallel to the basal planes of phyllosilicates

the relationship of phyllosilicate orientation x ray diffraction intensity ratios and c b fissility ratios in metasedimentary rocks of the helvetic zone of the swiss alps and the caledonides of jamtland central western sweden phyllosilicate preferred orientations x ray diaraction intensity ratios and c b fissility ratios were determined from 23 metasedimentary rocks from the helvetic zone of the swiss alps and from the caledonides of jamtland sweden the relationships between these parameters of phyllosilicate orientation depend strongly on the lithology and the cleavage morphology diaerences in orientation direction and in degree of preferred orientation reflect both deformation intensities and lithologies whereas polyphase deformed fine grained samples generally have phyllosilicates that are oriented parallel to cleavage showing high orientation intensities phyllosilicates in less deformed and or coarse grained rocks are oriented parallel to bedding with low orientation intensities weak orientation intensities in samples with phyllosilicates that are oriented parallel to bedding reflect compaction strain the higher strain in samples with micas and chlorites that are parallel to cleavage is a combination of compaction and tectonic strain due to the development of crenulation cleavage 2000 elsevier science ltd all rights reserved

chlorite illite muscovite interlayered and interstratified crystals a tem stem study phyllosilicates in rocks which are transitional from mudstone to slate from lehigh gap pa have been studied by a variety of techniques including high resolution transmission electron microscopy and analytical electron microscopy the principal minerals are white mica which is transitional from illite in mudstone to ordered twolayer mica in slate and chlorite 7a berthierine occurs more rarely dioctahedral and trioctahedral layers are shown to be interleaved in individual crystals at all scales between the following two end members 1 both random and regular 1 1 interlayering at the scale of individual layers as shown in part by lattice fringe images 2 packets of trioctahedral and dioctahedral layers up to a few thousand angstroms or microns in thickness detectable with ordinary optical techniques the complete range of intermediate structures is represented in samples which are in transition to slate bulk analytical empa x ray diffraction or other measurements are shown to result in averages over both kinds of layers when tem techniques are not used

diagenetic alteration of detrital biotite in proterozoic sedimentary rocks from sweden abstract the alteration of detrital biotites has been studied in two clastic proterozoic sedimentary sequences from southern and central sweden namely the visingso group and the dala sandstone respectively the results showed that the biotites are altered through replacement by clay minerals illite chlorite and less commonly kaolinite biotite may either be replaced pseudomorphically one biotite crystal by one clay mineral crystal or by fine crystalline clay minerals which merge into the interstitial clay matrix in reddish rocks biotite may be associated with pigmentary hematite and in greenish rocks with pyrite the variation in the two textural habits of biotite alteration i e pseudomorphic or fine crystalline is related to physico chemical properties of pore solution and nature of biotite particles

low grade metamorphism and geotectonic setting of the macuchi formation western cordillera of ecuador the cretaceous eocene basic to intermediate marine volcanic rocks of the mucuchi formation constitute the western cordillera in northern ecuador their chemical features mostly correspond to those of tholeiitic basalts with some calc alkaline affinities and suggest an oceanic island arc setting the macuchi rocks are affected by low grade non deformative metamorphism characterized by zeolite prehnite pumpellyite and lower greenschist facies assemblages depth zonation is suggested by the downward mineral sequence i laumontite pistacitic epidote pumpellyite prehnite ii pumpellyite prehnite pistacitic epidote iii actinolite biotite pistacitic epidote chlorite this broad zonation and the chemistry of individual minerals point to an interaction between the volcanic rocks and sea water under a moderate to high thermal gradient 75 c km alteration appears to have been dependent primarily on fluid control volume pressure composition temperature and reaction kinetics which together partly overshadow the role of load pressure compositional variations of a mineral species at the scale of a contiguous flow or even at the scale of a thin section show that intensity of alteration was spatially uneven depending on rock permeability and consequently metastable equilibrium commonly exists however a progressive approximation to equilibrium as a result of p t control is shown by the mineralogy a high fo2 of the fluid phase is evident from the mineral chemistry the metamorphism of the macuchi volcanics is similar to the hydrothermal burial type produced during the development of a volcanic arc where lavas and volcanoclastics accumulated in a shallow marine environment however some of its characteristics point to a transition toward systems defined by a higher t p ratio such as those found in ocean floor metamorphism a model is proposed in which the macuchi volcanics are assigned to an oceanic island arc generated contemporaneously with a marginal basin which has opened as the outcome of progressive north south attenuation of the continental crust due to mantle diapirism

formation mechanisms of illite chlorite and mixed layer illite chlorite in triassic volcanogenic sediments from the southland syncline new zealand clay minerals from the three principal kinds of zeolitic sediments from the type area for zeolite facies alteration the triassic murihiku supergroup southland new zealand have been studied by tem bentonitic tuff consists largely of smectite and heulandite with minor illite they occur as replacements of glass shards and are inferred to be direct alteration products of tuff alteration both analcime and laumontite rich tuffs contain chlorite illite and mixed layer illite chlorite including 1 1 mixed layer sequences subhedral to euhedral phyllosilicate crystal shapes and other textural features imply that phyllosilicates crystallized from solution derived in part by dissolution of precursor smectite intralayer transitions involving illite and chlorite are inferred to be products of crystallization rather than direct alteration and replacement petrographically similar bentonitic and analcimized tuffs overlap each other in the stratigraphic section supporting earlier observations that there is no systematic change in smectite relative to the illite plus chlorite derived from smectite in sections up to 8 5 km thick the data imply that smectite may be metastable relative to illite plus chlorite permeability and fluid chemistry are inferred to be as significant as temperature in promoting reactions in clay minerals as well as zeolites during burial metamorphism

low grade metamorphism around the down dip limit of seismogenic subduction zones example from an ancient accretionary complex in the shimanto belt japan abstract reactions involving clay minerals during low grade metamorphism at the depth of an ancient accretionary complex in the shimanto belt kyushu japan were studied by integrated transmission electron microscopy energy dispersive x ray spectroscopy and x ray diffraction analyses of the bulk rock and clay fraction the analyzed metasediment the kitagawa unit contain an incipient sub horizontal slaty cleavage illite crystallinity data and mica b dimensions indicate that the conditions of metamorphic deformation were anchizone epizone grade and intermediate pressure cleavage formation was linked to two reactions involving clay minerals 1 the recrystallization of 1m dominant matrix mica inherited from the original sedimentary fabric into thick defect free 2m1 packets along cleavage planes and 2 the formation of chlorite from 7 a berthierine balanced equations among the clay phases based on compositional data and their relative abundance suggest that the decomposition of matrix mica resulted in the formation of paragenetic mica and chlorite along the cleavage planes without significant elemental outflux although a modal increase in phyllosilicates is not indicated by the data the growth of chlorite and mica along cleavage planes may have a large influence on the rheological properties of a decollement and may be related to the occurrence of the seismic aseismic transition at 350 c

relationship of concretions and chlorite muscovite porphyroblasts to the development of domainal cleavage in low grade metamorphic deformed rocks from north central wales great britain abstract spaced or domainal cleavage is widespread in deformed rocks of low metamorphic grade this study presents evidence on the origin of spaced cleavage in deformed pelites from silurian turbidite sequences in north central wales the variable development of cleavage folia is related to the presence of concretions which served to concentrate strain effects of well developed cleavage folia in the matrix in some zones and to produce strain shadows few or no cleavage folia in others compositional differences between matrix narrow transition zones and concretions also influenced the development of cleavage folia chlorite muscovite porphyroblast growth was initiated before cleavage formation but its further growth and development took place during cleavage formation the relationship of cleavage to the concretions and to the chlorite muscovite porphyroblasts indicates that cleavage developed in a matrix that must have been in an advanced state of lithification anchimetamorphic to lowermost greenschist facies of metamorphism and not during soft sediment deformation

the influence of strain lithology and stratigraphical depth on white mica illite crystallinity in mudrocks from the vicinity of the corris slate belt wales implications for the timing of metamorphism in the welsh basin low and very low grade metamorphism in a mudrock dominated succession ranging from upper cambrian to wenlock is characterized by the white mica illite crystallinity technique kubler indices indicate that grade ranges from late diagenetic to low epizonal the prograde reaction most easily observed by xrd study is the progressive ordering of k na micas diagnostic 2m 1 polytypic reflections become recognizable at 0 4 2 and strengthen as grade advances grade generally correlates with thickness of overburden in the 9 km thick succession however strain exercises additionally a profound influence on grade and the distribution of strain was related to decollement resulting from inhomogeneities and ductility contrasts within the succession strain energy is seen as an additional and important variable in the development of white mica crystallinity in slates the distribution of metamorphic grade implies that the lower palaeozoic succession was augmented by several kilometres of lower devonian old red sandstone strata at the metamorphic culmination the culmination was syntectonic and represents an acadian rather than an early diastathermal event subsequent dextral movement on the bala fault displaced the regional isocryst pattern by c 4 km retrogressive smectite appears to be localized to the vicinity of the bala fault and is probably of variscan age

chlorite mica aggregates morphology orientation development and bearing on cleavage formation in very low grade rocks abstract chlorite mica aggregates in slates from northern spain have been investigated in very thin thin sections specimens from various parts of a fold show different morphologies that are dependent on the operating deformation mechanism from the limb to the hinge rigid body rotation becomes less important and folding and intragranular kinking are more common this is associated with an increasing aspect ratio width length for the aggregates chlorite mica aggregates were in part present before cleavage development and are statistically parallel to the bedding enveloping surface variation in degree of distortion of chlorite suggests continuous solution and growth during deformation muscovite on the other hand is generally more highly deformed suggesting that it was all early splitting of micas along 001 planes occurs creating extension sites where chlorite growth can take place thus an early diagenetic metamorphic formation with mimetic growth of chlorite on detrital micas is favoured for the origin of the aggregates the behaviour of the aggregates during deformation provides information on a mechanism for cleavage development as a result of intragranular kinking high angle boundaries are created with parts of the deformed grains at a low angle to the cleavage plane mechanical rotation appears to be the important mechanism for cleavage development in these rocks modification by recrystallization solution and growth processes can account for the observed microstructure

the mineralogy and origin of some silurian bentonites welsh borderland u k thirty clay rich horizons from the wenlock and ludlow series of the welsh borderland were analysed to investigate their mineralogy and possible origins two mineralogical assemblages were determined an assemblage of illite smectite and kaolinite of volcanic origin which was associated in many samples with an assemblage of illite and chlorite of detrital origin immobile chemical discriminants zr tio2 versus nb y infer an originally rhyolitic ash composition with a possible temporal progression from the wenlock to ludlow to more differentiated compositions the thin nature of the bentonites and lack of juxtaposed subduction related calc alkaline magmatism suggests that the tuffs were derived from distant explosive eruptions from volcanic centres possibly in ne europe

reactions leading to the formation and breakdown of stilpnomelane in the otago schist new zealand semi pelitic rocks ranging in grade from the prehnite pumpellyite to the greenschist facies from south eastern otago new zealand have been investigated in order to evaluate the reactions leading to formation and breakdown of stilpnomelane detrital grains of mica and chlorite along with fine grained authigenic illite and chlorite occur in lower grade rocks with compactional fabric parallel to bedding at higher grades detrital grains have undergone dissolution and metamorphic phyllosilicates have crys tallized with preferred orientation sub parallel to bedding leading to slaty cleavage stilpnomelane is found in metapelites of the pumpellyite actinolite facies and the chlorite zone of the greenschist facies but only rarely in the biotite zone of the greenschist facies illite or phengite is ubiquitous whereas chlorite occurs only rarely with stilpnomelane upgrade of the pumpellyite out isograd chemical and textural relationships suggest that stilpnomelane formed from chlorite phengite quartz k feldspar and iron oxides stilpnomelane was produced by grain boundary replacement of chlorite and by precipitation from solution overprinting earlier textures some relict 14 achlorite layers are observed by tem to be in the process of transforming to 12 astilpnomelane layers the aem analyses show that fe is strongly partitioned over mg into stilpnomelane relative to chlorite k d 2 5 and into chlorite relative to phengite k d 1 9 modified aaefm diagrams projected from the measured phengite composition rather than from ideal kal 3 si 3 o 10 oh 2 are used to elucidate reactions among chlorite stilpnomelane phengite and biotite in addition to pressure temperature and bulk rock composition the stilpnomelane in isograd is controlled by variations in k fe3 fe2 o oh and h 2 o contents and the locus of the isograd is expected to vary in rocks of diverent oxidation states and permeabilities biotite quartz and less phengitic muscovite form from stilpnomelane chlorite and phengite in the biotite zone projection of bulk rock compositions from phengite naalo 2 sio 2 and h 2 o reveals that they lie close to the polyhedra defined by the aaefm minerals and albite other extended aaefm diagrams such as one projected from phengite naalo 2 caal 2 o 4 sio 2 and h 2 o may prove useful in the evaluation of other low grade assemblages

the smectite chlorite transition in drillhole nj 15 nesjavellir geothermal field iceland xrd bse and electron microprobe investigations abstract in well nj 15 of the nesjavellir geothermal field iceland the transition of discrete smectite into discrete chlorite has been studied from drill cuttings recovered at depths of less than 1714 m and over a continuous range of temperatures between 60 and 300 c at temperatures below 180 c the clay fractions contain mixtures of di and trioctahedral smectites whose layer charge increases with depth between 200 and 240 c discrete smectites have transformed into smectite rich randomly interstratified chlorite and smecite r0 c s because the abundance of chlorite interlayers in this c s is generally 20 its presence can be detected only by electron microprobe techniques and not by x ray diffraction between 245 and 265 c both regularly r1 and randomly interstratified c s are the predominant layer silicates discrete chlorite first appears at approximately 270 c and coexists with minor amounts of r0 c s at higher temperatures r0 and r1 c s form a nearly complete compositional series between trioctahedral saponite and discrete chlorite end members the interlayer cation and si content of smectites and c s decrease with increasing temperature the mg mg fe content of smectite c s and chlorite is unrelated to temperature the percentage of chlorite in c s as determined by electron microprobe analyses increases continuously with increasing temperature except for occurrences of smectite rich c s in fresh basaltic dykes which have not thermally equilibrated with the higher grade country rocks

constraining absolute deformation ages the relationship between deformation mechanisms and isotope systematics isotope diffusion in a mineral is strongly temperature dependent but is also a function of grain size deformation must therefore be an important consideration in the interpretation of isotopic data because it provides a means of modifying grain size and shape we illustrate the range of different deformation mechanisms common in micas and use simple models to investigate the relationship between these and isotope diffusion we consider three different thermal scenarios with deformation taking place during a the prograde heating path b at the closure temperature of the deforming mineral and c at temperatures significantly below the closure temperature we have modelled these simple systems using a finite difference algorithm that simulates argon diffusion profiles and bulk ages this modelling illustrates that obtaining deformation ages is critically dependent on an understanding and recognition of the different deformation mechanisms that have affected the sample in the cases where deformation causes a change in grain size it is important to characterise both the temperature at which deformation takes place and the closure temperature of grains formed during the deformation the development of grains with tc greater than the deformation temperature may record a deformation age examples of this condition include i neocrystallisation ii grain size reduction occurring at temperatures below tc of the reduced grain size where the deformation mechanism has reset the grains and iii deformation induced grain coarsening

corrensite and mixed layer chlorite corrensite in metabasalt from northern taiwan tem aem empa xrd and optical studies many chloritic minerals in low grade metamorphic or hydrothermally altered mafic rocks exhibit abnormal optical properties expand slightly upon glycolation expandable chlorite and or have excess alvi relative to aliv as well as significant ca k and na contents chloritic minerals with these properties fill vesicles and interstitial void space in low grade metabasalt from northern taiwan and have been studied with a combination of tem aem empa xrd and optical microscopy the chloritic minerals include corrensite which is an ordered 1 1 mixed layer chlorite smectite and expandable chlorite which is shown to be a mixed layer chlorite corrensite corrensite and some mixed layer chlorite corrensite occur as rims of vesicles and other cavities while later formed mixed layer chlorite corrensite occupies the vesicle cores the tem observations show that the mixed layer chlorite corrensite has ca 20 and the corrensite has ca 50 expandable smectite like layers consistent with xrd observations and with their abnormal optical properties the aem analyses show that high si and ca contents high alvi aliv and low fevi fe mg vi ratios of chlorites are correlated with interstratification of corrensite or smectite like layers in chlorite the aem analyses obtained from 200 500 a thick packets of nearly pure corrensite or chlorite layers always show that corrensite has low aliv siiv and low fevi fe mg vi while chlorite has high aliv siiv and high fevi fe mg vi this implies that the trioctahedral smectite like component of corrensite has significantly lower aliv siiv and fevi fe mg vi the ratios of fevi fe mg vi and aliv siiv thus decrease in the order chlorite corrensite smectite the proportions of corrensite or smectite like layers relative to chlorite layers in low grade rocks are inferred to be controlled principally by fe mg ratio in the fluid or the bulk rock and by temperature compositional variations of chlorites in low grade rocks which appear to correlate with temperature or metamorphic grade more likely reflect variable proportions of mixed layered components the assemblages of trioctahedral phyllosilicates tend to occur as intergrown discrete phases such as chlorite corrensite corrensite smectite or chlorite corrensite smectite a model for the corrensite crystal structure suggests that corrensite should be treated as a unique phase rather than as a 1 1 ordered mixed layer chlorite smectite

synchronous very low grade metamorphism contraction and inversion in the central part of the welsh lower palaeozoic basin a metamorphic map white mica crystallinity based on 663 data points has been generated for the recently published british geological survey 1 50 000 geological sheets 178 llanilar and 179 rhayader epizone upper anchizone lower anchizone and sub anchizone are delineated metamorphic cross sections are drawn and superimposed on structural cross sections assumptions made in order to draw metamorphic cross sections are 1 the anchizone spans 100 c 2 the metamorphic field gradient was 36 ckm 1 3 metamorphic surfaces were initially generated parallel to the contemporary ground surface restoration of the sections indicates that at the time of metamorphism a contractional phase had begun because reverse movement on earlier extensional faults development of back thrusts footwall short cuts and limited growth of precursor folds had taken place metamorphism continued locally during later stages of the contractional phase as a strain related phenomenon finally tensional faulting followed the cessation of folding and metamorphism

gradational development of domainal slaty cleavage its origin and relation to chlorite porphyroblasts in the martinsburg formation eastern pennsylvania abstract slaty cleavage frequently consists of narrow folia or domains of phyllosilicates alternating with microlithons richer in quartz within cleavage domains the 001 planes of phyllosilicates are usually parallel or subparallel to the domain boundaries but they may be at a small but persistent angle in microlithons the phyllosilicates are commonly not parallel to the cleavage domains domainal cleavage may grade into pervasive slaty cleavage with few or no microlithons and a parallel orientation of all the phyllosilicates or may grade within distances as short as a few millimeters into pelite with no cleavage and with the phyllosilicates either with no preferred orientation or a weak to strong orientation parallel to bedding study of cleavage specimens mainly from the martinsburg formation of pennsylvania and new jersey indicates that cleavage development is related to strain variation and to volume loss at least as measured by deformed quartz calcite veins in the xz plane of a fold mass transfer of the more soluble constituents under conditions of low grade metamorphism chlorite zone and under the influence of deviatoric stress leads to the development of solution ways initiated at heterogeneities in the fabric which result in solution cleavage that is extended by terminal and lateral propagation stress gradients result in gradational development of solution cleavage with minimal development in strain shadows the solution ways develop into cleavage domains by accumulation of inert materials particularly carbonaceous grains iron ores such as framboidal pyrite and phyllosilicates however the preferred orientation of phyllosilicates is mainly the result of crystallization of new grains in the domains because of the breakdown of detrital or diagenetic phyllosilicates and of other minerals such as feldspar or epidote under low grade metamorphic conditions this interpretation is supported by lack of bent or deformed phyllosilicates in and along the boundaries of cleavage domains larger detrital phyllosilicates with 001 at a high angle to cleavage commonly show deformation further support is provided by the occurrence of chlorite grains with their 001 planes approximately parallel to the cleavage in strain shadows on competent grains such as framboidal pyrite and by the shape change of diagenetic chlorite porphyroblasts as traced from zones with no domainal cleavage into zones with well developed domainal cleavage mechanical rotation appears to be of minimal importance in the development of cleavage domains that are unrelated to a crenulation structure there will be grain boundary sliding and other adjustments as a consequence of removal of material along the solution ways deformation of a rock possessing a strong preferred orientation of phyllosilicates parallel to bedding or to a previous cleavage may result in the development of a crenulation structure and thenec a differentiated crenulation cleavage mechanical re orientation will in this case be a more important element in formation of cleavage although mass transfer and crystallization are still necessary to produce a domainal cleavage

the origin and sedimentary diagenetic and metamorphic evolution of chlorite mica stacks in llandovery sediments of central wales u k chlorite mica stacks in the llandovery sedimentary rocks of central wales have been studied both within cleaved turbidite mudrocks and within early diagenetic apatite carbonate and monazite concretions stacks show a graded distribution within turbidite units indicating an origin as detrital particles concretions provided an arrested diagenetic environment which restricted subsequent deformation and so revealed the nature of the stack precursor minerals these included detrital mica and also probable pyroxenes amphiboles and volcanic rock particles relic textures of which can be seen preserved within the concretions diagenetic alteration to chlorite probably with some white mica intergrowth occurred most probably via smectite and then mixed layer or swelling chlorite intermediates subsequent modification during the caledonian cleavage forming episode determined the present shape and mineralogy of the stacks outside concretions chlorite pseudomorphs were rotated by tectonic compression crushed at their margins and pulled apart along 001 cleavage and slip planes growth of at least two generations of secondary mica infillings has been recognized and this was probably accompanied by the growth of secondary chlorite this resulted in barrel shaped stacks elongated parallel with cleavage and with the 001 plane rotated from the bedding towards the perpendicular to the cleavage within early diagenetic concretions it is evident that some stacks had formed prior to lithification however these have a relatively simple morphology and the original bedding parallel grain shapes are largely preserved

inter layered clay stacks in jurassic shales as the average shale contains 60 clay minerals1 an understanding of the physical and chemical characteristics of these minerals is central to the question of burial diagenetic reactions and hydrocarbon generation much existing evidence concerning burial diagenesis relies on x ray diffraction xrd data particularly of the clay size 2 m fraction of shales2 here we use scanning electron microscopy sem in the backscattered electron bse mode together with energy dispersive x ray microanalysis edxa to show that lower jurassic shales from the north sea basin contain large numbers of clay mineral stacks up to 150 m in size by studying polished shale sections with bse and edxa the size shape orientation textural relationships and internal compositional variations of the clays can be observed in situ we present here preliminary evidence which suggests that the clay stacks studied are authigenic and may have formed at shallow burial depths during early diagenesis

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

40ar 39ar muscovite dating of thrust activity a case study from the axial zone of the pyrenees abstract the pyrenees belt features well exposed deformation structures that make it ideally suited to study thrust faults in this paper we present the first 40ar 39ar age of the pic de port vieux thrust ppvt located in the south western part of the pyrenean axial zone the ppvt is a secondary thrust related to the major gavarnie thrust gt with a footwall comprising upper cretaceous limestones and a hanging wall made of lower triassic pelites and sandstones a vertical transect in the fault hanging wall was investigated to characterize the deformation mechanism and to date the thrust activity by using 40ar 39ar step heating technique on muscovite petrographic observations confirm that pressure solution and recrystallization dissolution processes in presence of fluids are the main mechanisms that controlled the chemical textural changes in the fault core zone three groups of muscovite are identified in the fault zone i e detrital diagenetic and newly formed muscovites the detrital and diagenetic muscovites are mainly present in the damage zone while the newly formed muscovite is abundant in the core zone and is synkinematic to the fault activity the fractions 50 16 m 16 2 m 2 0 5 m

characterization of non equilibrium and equilibrium occurrences of paragonite muscovite intergrowths in an eclogite from the sesia lanzo zone western alps italy coexisting muscovite and paragonite have been observed in an eclogite from the sesia lanzo zone western alps italy the p t conditions of this eclogite reached 570 650 c and 19 21 kbar and the rocks show several stages of mineral growth during their retrograde path ranging from the subsequent lower p eclogite facies to the blueschist facies and then the greenschist facies muscovite and paragonite are very common in these rocks and show two texturally different occurrences indicating equilibrium and non equilibrium states between them in one mode of occurrence they coexist in equilibrium in the lower p eclogite facies in the same rock muscovite albite also replaced paragonite during a greenschist facies overprint as evidenced by unique across 001 layer boundaries the chemical compositions of the lower p eclogite facies micas plot astride the muscovite paragonite solvus whereas the compositions of the greenschist facies micas lie outside the solvus and indicate disequilibrium the tem observations of the textural relations of the greenschist facies micas imply structural coherency between paragonite and muscovite along the layers but there is a sharp discontinuity in the composition of the octahedral and tetrahedral sheets across the phase boundary we propose that muscovite formed through a dissolution and recrystallization process since no gradual variations toward the muscovite paragonite interfaces occur and no intermediate homogeneous na k phase has been observed because a solid state diffusion mechanism is highly unlikely at these low temperatures 300 500 c especially with respect to octahedral and tetrahedral sites it is assumed that h2o plays an important role in this process the across layer boundaries are inferred to be characteristic of such non equilibrium replacement processes the characterization of these intergrowths is crucial to avoiding erroneous assumptions regarding composition and therefore about the state of equilibrium between both micas which in turn may lead to misinterpretations of thermometric results

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

interlayering and interlayer slip in biotite as seen by hrtem partial interlayers of brucite like sheets which form with the adjacent talc like layers of the mica single chlorite layers interstratified in the mica structure brucite interlayering is interpreted as a brucitization of an interlayer level of the mica where partial slip or cleavage has previously occurred example of a chemical process at the atomic scale favored by deformation microstructures modified journal abstract

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

microstructures of intergrown phyllosilicate grains from verrucano metasediments northern apennines italy interleaved phyllosilicate grains ipg of various compositions are widespread in low grade verrucano metasediments of the northern apennines italy they are ellipsoidal or barrel shaped up to 300 400 m long and they are often kinked and folded phyllosilicate packets occur as continuous lamellae or as wedge shaped layers terminating inside the grain using electron microscopy techniques sem tem six types of ipg have been distinguished on the basis of their mineralogical composition 1 chl ms kln 2 chl ms pg kln 3 ms prl pg 4 ms prl su 5 ms prl chl su 6 su ms types 1 and 2 are mainly composed of chlorite with ms and pg as minor phases kln grows on ms in highly weathered samples types 3 4 5 and 6 are composed of muscovite with intergrown prl chl su and new formed muscovite the ipg show all kinds of contacts from coherent grain boundaries with parallel basal planes and along layer transitions to low and high angle grain boundaries the ipg formed on pristine minerals such as chlorite and muscovite the transformations took place during the prograde and retrograde metamorphic path of the rocks they were facilitated by deformation and they occurred in equilibrium with a fluid phase which allowed cation diffusion prograde reactions chl ms or pg ms prl ms chl involve dehydration and sometimes a decrease in volume whereas retrograde reactions ms kln ms su involve hydration and an increase in volume these transformations do not simply occur through an interchange of cations but often involve deep structural changes transitions from one phyllosilicate to another generally proceed through dissolution recrystallization reactions in conclusion verrucano ipg represent microstructural sites which have not completely equilibrated with the whole rock and whose mineral assemblage depends on the original composition of the microstructural sites