short paper terrane rotations in the irish caledonides palaeozoic palaeomagnetic data from scotland and ireland have been compared on a terrane model and early caledonide clockwise rotation of a terrane through 65o 30 connemara massif and later anticlockwise rotation of up to 50 in the vicinity of clew bay have been deduced these are the firfirst estimates of terrane rotations in the european part of the caledonian appalachian orogen terrane analysis of orogenic belts is currently a major field of study to a large extent this has been stimulated by studies in the north american cordillera where large rotations and translations have been determined palaeomagnetically see beck et al 1986 and references therein terrane analysis may be applied to older orogenic belts such as the caledonides which have been particularly intensively studied previously published palaeomagnetic data from connemara morris tanner 1977 and nw galway morris et al 1973 had indicated local block rotations but furnished few constraints on either their age and magnitude or on the identities of the blocks this paper relates the currently available palaeomagnetic dataseto a terrane model of w ireland hutton dewey 1986 see fig 1 new data new palaeomagnetic data from the irish caledonides comprise the following connemara gabbro robertson 1988 the dalradian rocks of the connemara massif are unique in the british caledonides in lying to the south of a caledonian ophiolite and also south of a thick sequence of ordovician shales and volcanics of low metamorphic grade in clew bay and the south mayo trough respectively the structural

al rich objects in ordinary chondrites related origin of carbonaceous and ordinary chondrites and their constituents ca al rich inclusions cais common constituents of carbonaceous chondrites have been studied extensively because they contain refractory high temperature minerals suggesting that they formed at high temperatures during the early history of the solar system few cais have previously been discovered in ordinary chondrites here we describe 169 al rich objects arbitrarily defined as having 10 wt al2o3 from 24 types 3 and 4 ordinary chondrites five regolith breccias containing unequilibrated material the unique meteorite kakangari and a few types 5 and 6 ordinary chondrites based on shape and texture we divide al rich objects into chondrules round with igneous textures irregularly shaped inclusions similar to type f and spinel rich complex cais and fragments probably fragments of al rich chondrules and inclusions for descriptive purposes we further subdivide al rich chondrules into compositional subgroups although they are entirely transitional conclusions 1 a al rich chondrules and irregularly shaped inclusions and fragments are wide spread albeit rare constituents of ordinary chondrites and are similar to cais except the coarse grained ones in carbonaceous chondrites thus providing strong support for a related origin of ordinary and carbonaceous chondrites and their constituents 2 b al rich chondrules formed as independent freely floating molten droplets and cooled rather rapidly 3 c al rich chondrules like the common mg fe rich chondrules formed by melting of millimeter to submillimeter sized dust balls in the case of al rich chondrules the dust balls consisted largely of mixtures of variable amounts of at least two solid precursor materials one refractory rich and the other volatile rich phases which we interpret as unmelted relicts in some of these objects support this model 4 d the wide ranges in bulk compositions of al rich chondrules from refractory rich to volatile rich ones yet complete transition between them and the existence of chondrules transitional in composition between the al rich and common mg fe rich chondrules of ordinary chondrites suggest that all chondrules formed by related processes differences in bulk composition result from mixing of different proportions of refractory volatile and mg fe rich and other solid precursor materials into dust balls which were then melted 5 e most irregularly shaped inclusions show no indication of a once molten origin secondary alteration has probably changed the original mineral assemblage mainly volatile rich materials entered the refractory objects as a gas reacted with the constituents and produced secondary phases rims probably formed both by decomposition and reaction with solar gas as well as by accretion 6 f coarse grained inclusions as found in allende were not observed in ordinary chondrites

palaeomagnetism and rb sr ages of the ratagan and comrie intrusions the newer igneous rocks in the scottish highland region which post date the main grampian metamorphism and tectonism range in composition from basic to granitic two members of this suite were chosen for palaeomagnetic study in an attempt to refine the apparent polar wander path for scotland in the palaeozoic this paper describes the results of the detailed palaeomagnetic analysis which has been carried out thermal and alternating field af demagnetization was supplemented by rock magnetic experiments on a limited selection of specimens from each sampling site to help characterize the remanence carriers these consisted of thin section observations and the measurement of thermomagnetic and hysteresis properties each complex provides an example of the widespread variability in remanence behaviour which may be encountered in a single intrusion for an accurate interpretation of palaeomagnetic data from any formation an estimate of the age of the rock is required brown miller grasty 1968 give a poorly defined k ar now at bp research centre str epd chertsey road sunbury on thames middlesex tw16 7ln

multicomponent magnetizations from the mississippian mauch chunk formation of the central appalachians and their tectonic implications previous paleomagnetic study of mississippian red beds of the mauch chunk formation from the central appalachians reported shallow inclination directions a positive fold test was obtained after thermal demagnetization typically to only 550 c the near equatorial position for north america indicated by these results has been used to support the idea of a circa 15 latitudinal tectonic offset of the acadia displaced terrain in the northern appalachians the present study of 153 samples from 29 sites occupied in the mauch chunk formation of eastern and southern pennsylvania reveals multicomponent magnetizations a characteristic component declination d 161 0 inclination i 27 8 alpha95 a95 7 9 with unblocking temperatures usually concentrated above 650 c and with dual polarity 10 sites normal 13 sites reversed is isolated in 23 sites and passes a fold test at the 99 confidence level it is interpreted as an early acquired hematitic detrital or chemical magnetization of pre alleghanian orogeny and pre kiaman reverse polarity interval age a southern hemisphere paleolatitude of 15 for the sampling area is indicated another component has generally lower unblocking temperatures which however can extend up to 650 c or more and therefore is also likely to be carried by hematite this secondary magnetization d 170 6 i 5 3 a95 5 0 isolated in 25 sites is of uniform reverse polarity and application of fold tests suggests that it is of synfolding origin a near equatorial paleolatitude 3 n at the time of acquisition is obtained consistent with permo carboniferous kiaman directions from north america incomplete removal of the secondary component can explain the previous results from the mauch chunk comparison of the present paleomagnetic results with those from similar age rocks in the canadian maritimes does not support a latitudinal offset of acadia with respect to interior north america in the early carboniferous in fact according to these and other paleomagnetic data thought to be representative of this age the atlantic bordering continents already were near to a pangea configuration the alleghanian and particularly the hercynian orogenies therefore may not be associated with the closure of a large latitudinal ocean later in the carboniferous and permian alleghanian deformation however may have resulted in the partial bending of the pennsylvanian salient on the basis of apparently systematic deviations in paleomagnetic declinations

origin of chondrule rims and interchondrule matrices in unequilibrated ordinary chondrites optical microscopy electron probe microanalysis scanning electron microscopy and analytical transmission electron microscopy were used to study the interchondrule matrix and opaque chondrule rims in bishunpur krymka semarkona chainpur all ll3 tieschitz and sharps both h3 the size distribution of grains in matrix in bishunpur obeys a power law rims around chondrules tend to be finer grained than matrix and may be layered all rims and matrices studied contain clasts of forsteritic olivine and low ca pyroxene in bishunpur the clasts may be cemented by amorphous glue rich in normative albite and partly altered to smectite rims in krymka sharps chainpur and tieschitz have a densely packed olivine groundmass with a grain size of about 0 1 m in krymka groundmass olivine is fo20 30 and forms interlocking dendrites various fe oxides are present in rims and matrices grain size distribution and chemical data indicate that clastic olivines and pyroxenes are derived from chondrules from analyses matrices rims in the ll group apparently are enriched in a feldspathic component tieschitz rims are slightly enriched but here the feldspathic component is present as white matrix sharps rims are not enriched in a feldspathic component opaque interchondrule matrices and chondrule rims probably formed from the products of fragmentation of chondrules partly induced by contraction of protopyroxene on inversion to clinopyroxene fragmented silica rich chondrule mesostases reacted with fe oxides and na to form the groundmass of fayalitic olivine and feldspathic glue a low temperature nebular or pre solar component is limited to 3 vol of each meteorite so equilibrium condensate was not the carrier of volatiles such as tl and bi

paleomagnetism of the dewey lake formation late permian northwest texas end of the kiaman superchron in north america the remanent magnetization of red beds of the late permian dewey lake formation from northwest texas was investigated using thermal chemical and alternating field demagnetization after removal of a small present day viscous overprint the dewey lake shows univectorial decay of the remanence revealing a dual polarity characteristic direction of d 152 4 i 11 1 k 39 5 95 5 9 n 16 sites three polarity transitions were observed defining at least four polarity zones the observed polarity zones are roughly layer parallel and may correlate with the illawarra magnetozone at the end of the kiaman if this is so k ar age determinations of an interbedded ash layer kolker and fracasso 1985 suggest that the kiaman superchron ended before 251 4 ma the paleopole located at 51 4 n 126 2 e confirms the reliability of previously reported late permian poles from southwest united states which fall significantly off the late permian segment of the paleomagnetic euler pole pep track of gordon et al 1984 this implies that a more segmented set of small circles is required to model the north american apparent polar wander path diminishing the usefulness of the pep method

caledonian strike slip terrane accretion in w ireland insights from very low grade metamorphism illite chlorite crystallinity and b0 parameter analysis of pelites with detrital white micas in the clew bay galway bay segment of the irish caledonides indicates that b0 data from whole rock and 2 m fractions generally show differences smaller than the errors of the method irrespective of 001 illite crystallinity values probably due to metamorphic recrystallization intermediate pressure metamorphism of the ordovician silurian clew bay group indicates slow subduction allowing partial thermal re equilibration before exhumation in contrast the croagh patrick group laurentian shelf sediments underwent high pressure alteration suggesting rapid subduction exhumation synchronous with strike slip faulting the murrisk group which underwent high intermediate pressure metamorphism in an ordovician back arc forms a separate terrane to the croagh patrick group to the north and also to the ordovician lough nafooey and tourmakeady groups and rosroe formation in the south in which low intermediate pressure alteration occurred these together with the silurian north galway group may have undergone heating due to movement over or deposition on the hot gowlaun detachment as the connemara dalradian was exhumed the south connemara group also underwent a high pressure alteration consistent with its inferred subduction environment evidence of contact alteration due to known or inferred buried late to post caledonian granitoid plutons has been found in the clew bay louisburg clare island croagh patrick murrisk and south connemara groups these show evidence of lower pressure alteration than the surrounding country rocks

mineralogy of fine grained material in the krymka ll3 1 chondrite two dark lithic fragments and matrix of the krymka ll3 1 chondrite were mineralogically and chemically studied in detail these objects are characterised by the following chemical and mineralogical characteristics which distinguish them from the host chondrite krymka 1 bulk chemical analyses revealed low totals systematically lower than 94 wt due to high porosity 2 enrichment in feo and depletion in s mgo and sio2 due to a high abundance of fe rich silicates and low sulfide abundance 3 fine grained almost chondrule free texture with predominance of a porous cryptocrystalline groundmass and fine grains 4 occurrence of a small amount of once molten material microchondrules enclosed in fine grained materials 5 occurrence of accretionary features especially unique accretionary spherules 6 high abundance of small calcium aluminium rich inclusions cais in one of the fine grained fragments it is suggested that the abundance of cais in this fragment is one of the highest ever found in an ordinary chondrite accretionary fine grained spherules within one of the fragments bear fundamental information about the initial stages of accretion as well as on the evolution of the clast its incorporation and history within the bulk rock of krymka the differences in porosity bulk composition and mineralogy of cores and rims of the fine grained spherulitic objects allow us to speculate on the following processes 1 low velocity accretion of tiny silicate grains onto the surface of coarse metal or silicate grains in a dusty region of the nebula is the beginning of the formation of accretionary porous fluffy silicate spherules 2 within a dusty environment with decreasing silicate metal sulfide ratio the porous spherules collected abundant metal and sulfide particles together with silicate dust which formed an accretionary rim variations of the silicate sulfide metal ratio in the dusty nebular environment result in the formation of multi layered rims on the surface of the silicate rich spherules 3 soft accretion and lithification of rimmed fluffy spherules fine grained silicate rich dust metal sulfide particles cais silicate rich microchondrules and coarse silicate grains and fragments followed 4 after low temperature processing of the primary accretionary rock collisional fragmentation occurred the fragments were subsequently coated by fine grained material which was highly oxidized and depleted in sulfides 5 in a final stage this accretionary dusty rock was incorporated as a fragment within the krymka host

paleomagnetic study of the ordovician table head group port au port peninsula newfoundland a paleomagnetic study of the ordovician table head group in the port au port peninsula of western newfoundland reveals a simple two component magnetization history comprising a reversely magnetized stable southeasterly remanence with a shallow to moderate inclination and an unstable present day overprint pole positions for the stable component both with and without tectonic tilt correction correspond with the early to middle ordovician pole positions for north america suggesting this remanence is early although the nature of the geomagnetic field in the ordovician is not well known the polarity observed is consistent with that reported from other mid ordovician studies and appears to reflect a predominance of reverse polarity for this time interval evidence of significant rotation of any of the sites studied is absent indicating that the continental margin in this region acted in an integral rather than a fragmented fashion during deformation previously published and new but preliminary resul

an exotic kind of cosmic material graphite containing xenoliths from the krymka ll3 1 chondrite abstract seven graphite containing xenoliths were found in the krymka ll3 1 chondrite the xenoliths have the following chemical and mineralogical characteristics which distinguish them from the krymka host 1 low totals in bulk chemical analyses obtained by electron microprobe 2 high bulk fe abundances 3 a uniform recrystallized chondrule free texture 4 the presence of euhedral graphite and carbon rich material 5 higher quantities of troilite and metal 6 a relatively homogeneous composition of silicates 7 a distinctive composition of metal chromite and phosphate 8 isotopically heavy c in graphite compared to both bulk krymka and graphite in other ordinary chondrites the xenoliths are mineralogically similar but not identical to the krymka carbonaceous clast k1 which bears graphite microcrystals organic compounds and mysterite they resemble carbonaceous chondrites both chemically and isotopically the mineralogical chemical and isotopic data for the graphite containing fragments suggest that this material represents metamorphosed varieties of a previously unknown type of unequilibrated carbonaceous matter most likely the graphite has a metamorphic origin and was crystallized from c containing precursor materials through the following transformation sequence organic compounds c rich material graphite

strike slip duplexes abstract strike slip fault systems often contain zones of steep imbricate faults geometrically similar to imbricate fans and duplexes in dip slip thrust and normal fault systems they are evident in map view rather than in vertical sections examples of duplexes are cited from both active and ancient systems and from theoretical and physical models duplexes may form at bends on strike slip faults by a process kinematically analogous to the sequential imbrication of ramps on dip slip faults however some may form and many may initiate as non sequential riedel fractures at fault offsets or on straight fault segments this process is more marked than in dip slip systems where primary anisotropy such as bedding exerts more control on fault geometry strike slip duplexes may be shunted along the fault system parallel to the regional slip vector however duplexes or individual horses will usually also move up or down perpendicular to the slip vector because of the unconstrained upper surface to the fault system these factors mean that no section through a strike slip system should be expected to area balance the faults of strike slip duplexes and imbricate fans may root in kinematically necessary low dip faults or may converge downwards and appear in vertical sections as flower structures

acfer 182 and paired samples an iron rich carbonaceous chondrite similarities with alh85085 and relationship to cr chondrites three samples of a new fe rich chondrite were found in the sahara in 1990 and 1991 acfer 182 acfer 207 acfer 214 the samples are paired and the meteorite will be designated as acfer 182 the chondrite is chemically texturally and mineralogically similar to the allan hills meteorite alh85085 one important difference between the two meteorites is the smaller average chondrule size in alh85085 the major components of acfer 182 in decreasing abundance are 1 1 highly altered by terrestrial weathering matrix 2 2 mineral and polymineralic silicate fragments and aggregates 3 3 chondrule fragments 4 4 chondrules 5 5 metal 6 6 fine grained dark inclusions the abundance of chondrules is lower and the average chondrule size 90 m smaller than in most other chondrites chondrule fragments are often so large that they cannot be derived from the present chondrule population apparently size sorting has prevented accumulation of the larger parent chondrules several spectacular ca al rich inclusions were found rich in ca dialuminate hibonite or zr y sc bearing phases the chemical composition of acfer 182 and of alh85085 are almost indistinguishable major chemical signatures are 1 1 uniform enrichment of fe and other nonvolatile metals relative to ci chondrites by about 70 2 2 absence of enrichment in refractory lithophiles characteristic of most type 2 and 3 carbonaceous chondrites 3 3 strong depletion of volatile and moderately volatile elements based on the oxygen isotopic composition the chemical composition and the abundances of chondrules and matrix acfer 182 should be classified as a carbonaceous chondrite considering their affinity to carbonaceous chondrites and their high bulk iron content the two meteorites acfer 182 and alh85085 are designated as ch chondrites there are mineralogical and chemical similarities among acfer 182 alh85085 and cr chondrites which distinguish these meteorites from other types of carbonaceous chondrites 1 1 low feo contents of olivine and pyroxene and correspondingly high metal contents 2 2 high cr content in olivine 3 3 abundant fine grained dark inclusions 4 4 abundant ca dialuminate caal4o7 in cais 5 5 similarities in oxygen isotopic composition 6 6 low contents of moderately volatile elements 7 7 low refractory element contents 8 8 presence of a unique component of subsolar rare gases these observations suggest similar conditions of formation for the components of these meteorites a single common parent body is unlikely in view of the differences in chemical composition and in the size distribution of individual components

clasts in the cm2 carbonaceous chondrite lonewolf nunataks 94101 evidence for aqueous alteration prior to complex mixing clasts in the cm2 carbonaceous chondrite lonewolf nunataks lon 94101 have been characterized using scanning and transmission electron microscopy and electron microprobe analysis to determine their degrees of aqueous alteration and the timing of alteration relative to incorporation of clasts into the host the provenance of the clasts and the mechanism by which they were incorporated and mixed with their host material are also considered results show that at least five distinct types of clasts occur in lon 94101 of which four have been aqueously altered to various degrees and one is largely anhydrous the fact that they have had different alteration histories implies that the main part of aqueous activity occurred prior to the mixing and assimilation of the clasts with their host further the presence of such a variety of clasts suggests complex mixing in a dynamic environment involving material from various sources two of the clasts one containing approximately 46 vol carbonate and the other featuring crystals of pyrrhotite up to approximately 1 mm in size are examples of unusual lithologies and indicate concentration of chemical elements in discrete areas of the parent body ies possibly by flow of aqueous solutions

reclassification of villalbeto de la pena occurrence of a winonaite related fragment in a hydrothermally metamorphosed polymict l chondritic breccia na meteorite clearly bears consequences for the previous classification of the rock the oxygen isotope data clearly show that an exotic eye catching black and plagioclase maskelynite rich clast is not of l chondrite heritage villalbeto de la pe is consequently reclassified as a polymict chondritic breccia the oxygen isotope data of the clast are more closely related to data for the winonaite tierra blanca and the anomalous silicate bearing iron meteorite lew 86211 than to the ordinary chondrite groups the ree pattern of the bulk inclusion indicates genetic similarities to those of differentiated rocks and their minerals e g lunar anorthosites eucritic and winonaitic plagioclases and points to an igneous origin the an content of the plagioclase within the inclusion is increasing from the fragment host meteorite boundary approximately an10 toward the interior of the clast approximately an52 this is accompanied by a successive compositionally controlled transformation of plagioclase into maskelynite by shock as found for plagioclase compositions of individual spinels enclosed in plagioclase maskelynite also vary from the border toward the interior of the inclusion in addition huge variations in oxygen isotope composition were found correlating with distance into the object the chemical and isotopical profiles observed in the fragment indicate postaccretionary metamorphism under the presence of a volatile phase

the ordovician evolution of the south mayo trough western ireland the south mayo trough a broad synclinorium containing a 10 km sequence of early ordovician turbidites passing up into shallow water sandstones is interpreted as a forearc basin with an ophiolitic basement that formed a backstop to an accretionary prism on the northern edge of a north facing arc the arc collided with the laurentian margin in early llanvirn times to deform the dalradian supergroup rifted margin clastic sequence prior to the late llandovery sinistral transcurrent faulting transposed the connemara dalradian terrane to its present position on the outboard ocean ward side of the south mayo trough the south mayo trough was fed principally by arc and ophiolite derived elastics during the arenig with an increasing metamorphic component during the llanvirn the late ordovician transpressional docking of connemara provided the southerly source for high grade metamorphic clasts in the deny veeny formation

accretionary rims on inclusions in the allende meteorite abstract many inclusions in allende particularly those with irregular shapes are surrounded by a sequence of thin layers which differ from one another in texture mineralogy and mineral chemistry the layer underlying all others contains either ia pyroxene needles olivine clumps of hedenbergite and andradite ib olivine doughnuts or ic rectangular olivine crystals the next layer outward ii contains tiny

microdistribution of primordial ne and ar in fine grained rims matrices and dark inclusions of unequilibrated chondrites clues on nebular processes the low temperature fine grained material in unequilibrated chondrites which occurs as matrix rims and dark inclusions carries information about the solar nebula and the earliest stages of planetesimal accretion the microdistribution of primordial noble gases among these components helps to reveal their accretionary and alteration histories we measured the ne and ar isotopic ratios and concentrations of small samples of matrix rims and dark inclusions from the unequilibrated carbonaceous chondrites allende cv3 leoville cv3 and renazzo cr2 and from the ordinary chondrites semarkona ll3 0 bishunpur ll3 1 and krymka ll3 1 to decipher their genetic relationships the primordial noble gas concentrations of semarkona and with certain restrictions also of leoville bishunpur and allende decrease from rims to matrices this indicates a progressive accretion of nebular dust from regions with decreasing noble gas contents and cannot be explained by a formation of the rims on parent bodies the decrease is probably due to dilution of the noble gas carrying phases with noble gas poor material in the nebula krymka and renazzo both show an increase of primordial noble gas concentrations from rims to matrices in the case of krymka this indicates the admixture of noble gas rich dust to the nebular region from which first rims and then matrix accreted this also explains the increase of the primordial elemental ratio 36ar 20ne from rims to matrix larger clasts of the noble gas rich dust form macroscopic dark inclusions in this meteorite which seem to represent unusually pristine material the interpretation of the renazzo data is ambiguous rims could have formed by aqueous alteration of matrix or as in the case of krymka by progressive admixture of noble gas rich dust to the reservoir from which the renazzo constituents accreted the leoville and krymka dark inclusions as well as one dark inclusion of allende show noble gas signatures different from those of the respective host meteorites the allende dark inclusion probably accreted from the same region as allende rims and matrix but suffered a higher degree of alteration the leoville and krymka dark inclusions must have accreted from regions different from those of their respective rims and matrices and were later incorporated into their host meteorites the noble gas data imply a heterogeneous reservoir with respect to its primordial noble gas content in the accretion region of the studied meteorites further studies will have to decide whether these differences are primary or evolved from an originally uniform reservoir

palaeomagnetism of the early carboniferous deer lake group western newfoundland no evidence for mid carboniferous displacement of acadia early carboniferous visean to possibly earliest namurian sedimentary rocks of the deer lake group of western newfoundland rest unconformably on grenvillian basement rocks of the canadian shield which form the western border of the early palaeozoic appalachian orogen in addition to magnetically soft magnetizations directed along the present field three families of magnetization directions are found two of them referred to as n north and s south are antiparallel and prefolding and were probably acquired during the depositional process n and s are roughly of equal frequency they have a mean direction irrespective of sign of 0 7 35 2 k 40 95 8 9 and a palaeopole 21 5 n 121 8 e 10 3 6 0 corresponding to a palaeolatitude of 20 6 s this agrees with the palaeolatitude 17 5 s determined from the somewhat older early carboniferous tournaisian terenceville formation of the avalon platform on the eastern side of the appalachian orogen in newfoundland the third magnetization referred to as h roughly horizontal has a mean direction 156 8 13 3 k 37 95 10 1 and a palaeopole 45 4 n 140 3 e 10 3 5 3 corresponding to a palaeolatitude of 7 4 n we interpret this to be an early kiaman latest carboniferous to early permian overprint probably acquired chemically the palaeolatitude determined from the h overprint agrees with that determined from early carboniferous rocks of cratonic north america west of the appalachians therefore we argue early carboniferous palaeofield determinations for cratonic north america have been strongly biased by unremoved kiaman overprints because of this and because of the good agreement between early carboniferous palaeolatitudes obtained from opposite margins of the appalachian orogen we suggest that there is at present no palaeomagnetic evidence for the previously proposed 1500 km displacement from the south of an eastern portion of the appalachians acadia relative to cratonic north america during the carboniferous

matrix material in type 3 chondrites occurrence heterogeneity and relationship with chondrules abstract matrix material in type 3 chondrites forms rims on chondrules metal sulfide aggregates ca al rich inclusions and chondritic clasts it also forms lumps up to a millimeter in size which may contain coarser silicates chondrules of all types were found with internal matrix lumps that appear to have entered the chondrules before the latter had crystallized mean concentrations of mg na al and ca in matrix occurrences show up to fivefold variations in a single chondrite variations between mean matrix compositions of individual type 3 ordinary chondrites are almost as large and partly reflect systematic differences between h l and ll matrices such variations are probably a result of nebular separation of feldspathic material and ferromagnesian silicates compositions of chondrules and their matrix rims are normally unrelated although rim compositions are correlated with those of matrix lumps inside chondrules a single chondrule was found with a composition nearly identical to that of its internal matrix lump suggesting that some chondrules may have formed from matrix material matrix lumps are as heterogeneous as chondrules but mean chondrule and matrix compositions differ even allowing for possible loss of metallic fe ni during chondrule formation since bulk compositions of matrix lumps and rims have probably not changed significantly since their formation except for fe mg exchange our matrix samples cannot represent typical chondrule precursor materials

brecciation among 2280 ordinary chondrites constraints on the evolution of their parent bodies abstract after accretion of meteorite parent bodies larger and smaller collisions have led to significant modifications of these bodies involved processes include excavation of material thermal metamorphism melting mixing of different materials re accretion and re lithification all these processes can be repeated several times in this study polished thin sections pts of 2280 chondrites 1193 h 947 l and 140 ll chondrites were investigated in order to obtain the abundance of brecciated rocks among the ordinary chondrites in addition we have determined the abundance and characteristics of shock vein bearing h l and ll chondrites and of impact melt rock clasts we also recognized xenolithic components based on o isotope studies noble gas data were considered in order to detect regolith breccias and to discuss late impact histories the investigation of 2280 samples shows that 23 276 of 1193 of the h chondrites 23 220 of 947 of the l chondrites and 79 110 of 140 of the ll chondrites are brecciated considering the heavily brecciated ll chondrites in 63 of the 140 chondrites 45 shock veins were clearly detected 57 of these 63 chondrites are brecciated rocks the investigation of the h and l chondrites has shown that about 26 310 of 1193 of the h chondrites and 40 379 of 947 of the l chondrites contain shock veins in our data set 20 of the h chondrites and 8 3 of the ll chondrites but only 3 0 of the l chondrites contain solar noble gases remarkably about 62 of all brecciated h chondrites with noble gases analyzed contain solar noble gases compared to only around 11 and 10 of the brecciated l and ll chondrites respectively the identification of xenolithic clasts e g ci cm and ureilite like lithologies in primitive type 3 chondrites indicates simultaneous accretion of clasts and chondrules these clasts must have been formed early within the first 2 ma on subsequently destroyed precursor first generation parent bodies the formation of complex breccias witnesses the collisions between asteroids of very different lithologies and heritage although the onion shell configuration of primordial parent bodies is necessary in order to form the chondrites with different degrees of metamorphic overprint petrologic types 3 6 subsequent catastrophic fragmentation and reassembly to form asteroids with a rubble pile structure are required to explain certain features discussed in this work however distinct peaks in the cosmic ray exposure age distributions indicate that not too many impacts in the last 100 ma were responsible to deliver the majority of the ordinary chondritic meteoroids to earth yet this certainly does not tell anything about the number of last generation parent bodies that exist in the asteroid belt since s type asteroids are the most abundant type of asteroid in the inner main belt and thought to be the parent bodies of ordinary chondrites

a geological and tectonic cross section of the caledonides of western ireland geological and geophysical criteria allow division of the caledonides of western ireland into four principal zones zone 1 in the north contains grenvillian basement with late proterozoic supra crustals zone 2 contains the westport complex the deer park complex and the south mayo trough connemara and the delany dome formation occupy zone 3 zone 4 contains the south connemara group and mid to late ordovician and silurian sediments and volcanic rocks the various tectonic models proposed for these zones are discussed and a minimum tectonic model is presented which involves early ordovician collision of the northern part of zone 2 with the laurentian margin zone 1 followed by subduction flip northern part of zone 4 the site of early collision between zones 1 and 2 subsequently underwent orogenic collapse and strike slip dismemberment leading to the sinistral transpressive emplacement of zone 3 assemblages in zone 4 were accreted to the now south facing arc northwards stress transfer consequent on suturing deformed the thinned crust and overlying silurian sequences in zones 2 and 3

the cooling history of connemara western ireland from k ar and rb sr age studies k ar age determinations are reported on 26 hornblendes 17 coexisting muscovites and biotites nine muscovites and nine biotites from the dalradian connemara schists metagabbros and gneisses these are supplemented by 19 rb sr whole rock mica ages and four 40 ar 39 ar incremental heating ages on micas a long at least 75 ma and complex cooling history without a simple pattern of ages has been obtained the ages are not correlated with the grain sizes or the chemical compositions of the analysed minerals or the topographic elevation from which the samples came but there is an irregular increase of hornblende k ar ages to 480 10 ma with increase of metamorphic grade southwards towards the metagabbros and gneiss the preservation of the complex cooling pattern is ascribed to the action of syn and post metamorphic slides or faults which moved slices of the massif upwards by varying amounts closely spaced samples give results which rule out simple tilting of the massif rapid cooling c 30 ma 1 at 480 10 ma arenig llanvirn during the f3 fold phase was followed by slower cooling until 455 10 ma caradoc when renewed rapid uplift occurred possibly correlated with the southward thrusting of connemara and with synchronous renewed uplift in the scottish dalradian rocks widespread biotite k ar and rb sr ages of 440 5 ma end ordovician throughout connemara and the dalradian rocks of scotland are ascribed to the major taconic tectonic event which docked the dalradian rocks against the flanking folded ordovician successions of the midland valley and the south mayo trough biotites giving k ar ages of about 415 ma may have been reset during the late silurian folding of the cover rocks the importance of very close sampling integrated with structural studies is emphasized

paleomagnetic and petrographic investigation of the taum sauk limestone southeast missouri paleomagnetic results from 17 sites in one quarry of the cambrian dresbachian stage taum sauk limestone member bonneterre formation reveal two components of magnetization on the basis of thermal demagnetization coercivity spectrum analysis and petrographic evidence both components reside in hematite sites can be divided into three groups based on behavior during thermal demagnetization and petrographic criteria the first group consists of massive pink limestones and contains a component with declination 270 inclination 11 corresponding to a cambrian pole at 175 6 e 3 6 s the second group of sampling sites consists of specimens with numerous burrows filled with hematite intergrown with calcite the sites are dominated by the second component of magnetization which has declination 147 and inclination 10 and corresponds to a reversed pole position at 133 1 e 37 3 n a significant fraction of this component which is interpreted to be carboniferous in age resides in hematite intergrown with calcite in the burrows petrographic relationships indicate that the calcite and hematite were acquired before dolomite which places a maximum age on dolomitization the third group consists of massive pink limestones with variable amounts of burrowing and contains both directions of magnetization in roughly equal proportions with closely overlapping blocking temperature spectra the presence of two stable directions of magnetization from one location with neither being dominant or even present throughout the quarry indicates the significant effect that diagenetic and sedimentologic processes may have on the magnetization of sedimentary units

siluro devonian palaeomagnetism terrane emplacement and rotation in the caledonides of western ireland summary the palaeomagnetism of a silurian succession lough mask formation lavas and sediments and intrusive rocks kilbride dolerites and andesites and microgranodiorites killary harbour area in the connemara mayo segment of the irish caledonides is described magnetizations are related to the regional tectonic history with the aid of fold conglomerate and contact tests they describe sequences of shallow to intermediate inclinations in both the eastern and western sectors of this caledonian inlier the shallow components appear to pre date siluro devonian folding while the intermediate ones post date this episode the change in palaeofield inclination is identical to the transition described by contemporaneous deep level silurian to shallow and surface level lower devonian bodies in the british and scandinavian caledonides but declinations are rotated consistently to the west the lower middle silurian vector is rotated clockwise by c 100 this difference is progressively reduced to c 50 by the time of the siluro devonian d2 folding showing that this fold generation now oriented e w developed in parallelism with contemporaneous folds on the ne strike continuation of the caledonides post folding remanences continue to deviate in a clockwise sense from the remainder of the caledonides showing that rotation continued into middle upper devonian times and finally ceased during carboniferous times the integrated effect of this rotation is recognized in ordovician magnetizations from the dalradian metamorphic terrane of south connemara it can be explained by block rotations within a zone of distributed deformation according to the mckenzie jackson model and illustrated by many neovolcanic zones palaeomagnetism defines a tectonic regime commencing with rotations of c 3 myr and lasting from mid silurian to carboniferous times western ireland is sited within a closure gap along the iapetus suture extending from ireland to newfoundland and these terranes were apparently emplaced by progressive elimination of this gap during sinistral transpression across the caledonian orogen

regional distribution of white k mica polymorphs and their phengite content in the central alps some 150 white k micas from the central alps were analysed for their polymorph and phengite content

microchondrule bearing clast in the piancaldoli ll3 meteorite a new kind of type 3 chondrite and its relevance to the history of chondrules in the piancaldoli ll3 chondrite we found a mm sized clast containing 100 chondrules 0 2 64 m in apparent diameter much smaller than any previously reported that are all of the same textural type radial pyroxene fs1 17 this clast like other type 3 chondrites has a fine grained ferich opaque silicate matrix sharply defined chondrules abundant low ca clinopyroxene and minor troilite and si and cr bearing metallic fe ni however the very high modal matrix abundance 63 8 vol unique characteristics of the chondrules and absence of microscopically observable olivine indicate that the clast is a new kind of type 3 chondrite most chondrules have feo rich edges and chondrule size is inversely correlated with chondrule core feo concentration the first reported correlation of chondrule size and composition chondrules acquired fe by diffusion from fe rich matrix material during mild metamorphism possibly before final consolidation of the rock microchondrules those chondrules 100 m in diameter are also abundant in another new kind of type 3 chondrite clast in the rio negro l chondrite regolith breccia in other type 3 chondrite groups microchondrule abundance appears to be anticorrelated with mean chondrule size viz 0 02 0 04 vol in h and co chondrites and 0 006 vol in l ll and cv chondrites microchondrules probably formed by the same process that formed normal sized droplet chondrules melting of pre existing dustballs because most compound chondrules in the clast and other type 3 chondrites formed by collisions between chondrules of the same textural type we suggest that dust grains were mineralogically sorted in the nebula before aggregating into dustballs the sizes of compound chondrules and chondrule craters which resulted from collisions of similarly sized chondrules while they were plastic indicate that size sorting of dustballs occurred before chondrule formation probably by aerodynamic processes in the nebula we predict that other kinds of type 3 chondrites exist which contain chondrule abundances size ranges and proportions of textural types different from known chondrite groups

mineralogy and composition of matrix and chondrule rims in carbonaceous chondrites abstract the degree of compositional variation of fine grained minerals displayed by the members within any carbonaceous chondrite group i e ci cm cv cr is a direct reflection of the range of aqueous alteration assemblages present matrix and fine grained chondrule rims within any particular carbonaceous chondrite are mineralogically nearly identical to one another but not necessarily similar in bulk elemental composition even though they have subsequently experienced post accretional secondary processing aqueous alteration under identical conditions this may indicate preferential alteration of one component brecciation exchange of components between rims and matrix and or accretion of matrix and rims from mineralogically or compositionally distinct materials we propose that co chondrites experienced parent body conditions of low f o2 low water rock ratios and temperatures below 50 c cr chondrites experienced higher water rock ratios potentially higher temperatures 150 c and a wide range of f o2 the alteration mineralogy of cv chondrites indicates water rock ratios at the high end at least of the range for cr chondrites essebi and mac 87300 cm chondrites experienced temperatures below 50 c low f o2 and low water rock ratios except eet 83334 which probably experienced relatively higher f o2 and b 7904 and y 86720 which experienced postalteration temperatures in the range 500 700 c most ci chondrites experienced temperatures between 50 and 150 c relatively high water rock ratios and variable f o2 with a late stage increase in the latter due possibly to brecciation and consequent release of methane y 82162 witnessed post alteration heating possibly as high as 400 c

paleomagnetism of lower cambrian volcanics and a cross cutting cambro ordovician diabase dyke from buckingham quebec the buckingham area quebec is characterized by cambrian volcanic flows 573 32 ma cut by a cambro ordovician diabase dyke which is dated at 497 14 ma by k ar method the volcanics yield multicomponent magnetizations a hard characteristic magnetization and a relatively soft one these components could be separated easily and the intensity decay curves of the different components could also be obtained separately using zijderveld s method three different groups of site mean characteristic magnetizations can be distinguished each representing a different stratigraphic level the paleopoles derived from these magnetizations are from oldest to youngest 186 e 10 s 173 e 01 n and 154 e 06 n this is interpreted as a rapid motion of the paleopole for the north american craton during the early cambrian the paleopole derived from the hard characteristic dyke direction is 123 e 03 s this magnetization direction was also found at one of the sites in the volcanics this means that the contact test

the matrices of unequilibrated ordinary chondrites implications for the origin and history of chondrites abstract the matrices of sixteen unequilibrated ordinary chondrites all witnessed falls were studied microscopically in transmitted and reflected light and analyzed by electron microprobe selected specimens were also studied by scanning electron microscopy these studies indicate that the fine grained opaque silicate matrix of type 3 unequilibrated chondrites is compositionally mineralogically and texturally distinct from the chondrules and chondrule fragments and may be the low temperature condensate proposed by larimer and anders 1967 1970 examination of the matrices of unequilibrated chondrites also shows that each meteorite has been metamorphosed with the alteration ranging in intensity from quite mild where the matrix has been only slightly altered to a more severe metamorphism that has completely recrystallized the opaque matrix most of the metamorphic changes in the matrix occurred without significant effects on the compositions or textures of the chondrules the metamorphic alteration probably resulted from a combination of processes including thermal metamorphism and the passage of shock waves the present appearance of each unequilibrated chondrite is a result of the particular temperature and pressure conditions under which it and its components formed plus the subsequent metamorphic alteration it experienced

accretionary dust mantles in cm chondrites evidence for solar nebula processes abstract the texture and modal composition of fourteen carbonaceous chondrites of the cm group were studied in detail to unravel the origin of their various components and the evolution of their parent body ies the analysis and mapping of complete thin sections by scanning electron and optical microscopy revealed textural units in most samples in which all coarse grained primordial chondritic components chondrules chondrule fragments refractory inclusions particles rich in hydrous silicates pcp and various mineral fragments are coated by fine grained serpentine and tochilinite rich dust the dust mantled particles have spheroidal shapes and form a special lithology in which they are densely aggregated and compacted without any interstitial clastic material the dust mantles termed accretionary dust mantles and their host lithology called primary accretionary rock are interpreted as products of accretion processes in the solar nebula an origin by processes on the parent body is excluded for textural and mineralogical reasons most cm chondrites are affected by secondary impact induced brecciation of variable intensity except for yamato 791198 which is composed entirely of a primary accretionary rock obviously unaltered by secondary parent body processes such as aqueous alteration or brecciation the cm chondrites y74662 haripura cold bokkeveld kivesvaara mighei pollen nogoya y793321 alh 83100 murray and murchison consist of clasts of primary accretionary rock embedded in a fine grained clastic matrix derived from primary rock material the volume percentage of the clastic matrix ranges from 5 y74662 to 76 murchison bells and essebi are brecciated and affected by aqueous alteration to an extent that fragments of primary rock are not preserved the evaluation of literature data indicates that solar wind implanted noble gases are absent in the primary accretionary rock component of cm chondrites they are only present in brecciated samples it must be concluded from the relation between the content of trapped solar wind and degree of brecciation that 1 the solar noble gases are residing exclusively in the clastic matrix and 2 some cm chondrites are regolith breccias and others are fragmental or monomict breccias both types were formed on the parent body although the latter were never exposed to the solar wind based on the results of textural chemical and mineralogical investigations we conclude that a major portion of the water bearing phases in cm chondrites were produced by the hydration of refractory phases prior to the formation of dust mantles and prior to the formation of the recent cm parent body this process requires special physicochemical conditions which allow the crystallization of oh bearing silicates e g higher density of the nebula gas or special precursor planetesimals of the cm chondrite parent body

oroclinal bending in the caledonides of western ireland new palaeomagnetic data from the caledonides of western ireland indicate that the silurian rocks of south mayo underwent oroclinal bending following folding in siluro devonian time bending was accommodated on faults cutting the silurian sequence and was driven by strike slip motion across the antrim galway line a recently recognized major curvilinear lineament we propose that the silurian rocks in the east of the region suffered up to 30 clockwise rotation increasing towards the west to approximately 70 this newly inferred differential rotation of the silurian rocks about a sub vertical axis implies that the underlying dalradian rocks of the connemara massif to the south must also have undergone clockwise rotation in late silurian early devonian time this is consistent with published palaeomagnetic data from the connemara gabbro and accounts for the swing in strike of the caledonian orogen in this part of western ireland from this we deduce that the swing in strike of the caledonian rocks elsewhere on the west coast of ireland might have a similar explanation

a middle late silurian paleomagnetic pole from limestone reefs of the wabash formation indiana u s a a middle late silurian paleomagnetic pole has been obtained from reefal limestones in north central indiana eleven of 25 collected sites yielded well clustered characteristic directions after stepwise alternating field demagnetization one additional site gave the same characteristic direction after great circle analysis these 12 site level directions were used to calculate the mean paleomagnetic pole reef flank beds dip steeply about 38 away from reef centers field and petrographic studies demonstrate that part of this dip is a primary depositional dip and that part is due to a tilt caused by early postdepositional differential compaction of underlying mudstone beds present attitudes of geopetal surfaces indicate that the postdepositional component of the observed bedding dip is about 12 a 12 tilt correction to paleomagnetic directions from oppositely dipping flank beds brings the mean directions into significantly better agreement indicating that the magnetization was acquired early the presence of both normal and reversed polarities also suggests early acquisition of the characteristic remanence the mean of the site paleomagnetic poles with sites from steeply inclined flank beds corrected for 12 of tilt is 16 8 n 124 9 e k 74 a95 5 1 in good agreement with the middle silurian rose hill pole for north america

the occurrence of grossite caal4o7 in chondrites the mineral grossite caal4o7 ca dialuminate also termed ca2 is a rare phase in terrestrial and extraterrestrial rocks in meteorites grossite was found in ca al rich inclusions cais from allende cv3 vigarano cv3 leoville cv3 efremovka cv3 colony co3 murchison cm2 alh 85085 ch acfer 182 ch acfer 059 e1 djouf 001 cr2 and the anomalous c chondrite adelaide in ca al rich inclusions from most of these meteorites grossite is an accessory phase however in many cais from alh 85085 acfer 182 and acfer 059 e1 djouf 001 it is a major phase in this paper we summarize the results of literature reports on grossite bearing inclusions and focus in detail on the occurrence mineralogy and chemistry of caal4o7 containing ca al rich inclusions in acfer 182 and acfer 059 e1 djouf 001 in these carbonaceous chondrites abundant cais with high modal abundances of grossite 30 vol often 60 vol were discovered to classify ca al rich inclusions with such high modal abundances of caal4o7 we suggest to introduce a new type of ca al rich inclusion grossite rich inclusions in acfer 059 e1 djouf 001 and acfer 182 grossite containing inclusions occur with different petrographic characteristics the most common type consists of a core of grossite rimmed by melilite ca pyroxene spinel hibonite in an other type anhedral polycrystalline grossite complexes of irregular shape are enclosed in melilite in some spherical ca2 inclusions abundant euhedral to subhedral grossite and or hibonite crystals are embedded in melilite in two inclusions a core of hibonite is mantled by grossite additionally some grossite bearing inclusions with unique texture and or mineralogy were found one of these inclusions is rich in zr y rich perovskite and contains phases rich in the ultrarefractory elements sc and or zr almost all of the grossite containing inclusions are rich in al2o3 50 wt even if grossite is only a minor phase based on textural appearance and or on ree contents in some inclusions core minerals with group ii patterns a condensation history for most inclusions is suggested a single inclusion rich in perovskite and other phases containing high concentrations of ultrarefractory elements appears to have formed as an early condensate from the solar nebula

paleomagnetism of the cambrian royer dolomite and pennsylvanian collings ranch conglomerate southern oklahoma an early paleozoic magnetization and nonpervasive remagnetization by weathering the cambrian royer dolomite in the arbuckle mountains oklahoma contains an early paleozoic magnetization and a nonpervasive late paleozoic remagnetization caused by meteoric fluids the royer was uplifted and underwent karst conditions and eroded clasts were deposited in the collings ranch conglomerate during the pennsylvanian an early paleozoic depositional or chemical remanent magnetization dec 109 inc 10 tilt corrected residing in magnetite is found in gray brown ferroan dolomite a pennsylvanian permian chemical remanent magnetization crm residing in hematite dec 147 inc 4 in situ is associated with dedolomitized rocks in an alteration zone around and below the karst features clasts of the royer in the pennsylvanian collings ranch conglomerate correlated with the parent rock by petrographic isotopic and rock magnetic methods also contain two magnetizations the clast centers contain a predepositionai magnetization whereas the clast margins contain a postdepositional crm dec 152 inc 4 in situ conglomerate tests using these magnetization components constrain the timing of remanence acquisition in the royer dolomite field relations and the results of oxygen and carbon stable isotope analyses indicate that the dedolomitization events were caused by near surface meteoric fluids that altered the royer around the margin of the karst dissolution caves and the outer rims of royer dolomite clasts remagnetization was not pervasive and an early paleozoic magnetization is preserved in the dolomite below the zone affected by weathering

k ar age determinations on phengites from the internal part of the sesia zone western alps italy k ar age determinations have been carried out on various well defined phengite populations from a small area of the internal part of the sesia zone lower aosta valley there the rocks have suffered high pressure metamorphism attributed to early alpine subduction in the stability field of jadeite quartz p 15 1 kbar at t 550 50 c coarse grained phengites from well preserved high pressure parageneses and phengites re crystallized early during decompression at still elevated temperatures in the stability field of albite yield identical ages close to 80 ma in the most external sample high pressure phengites yield 72 ma the ages around 80 ma are interpeted as essentially undisturbed cooling ages they are not notably influenced by paragenesis chemical composition polymorphism grain size deformation and recrystallization at higher temperatures this part of the sesia zone has cooled down about 80 ma ago to below the blocking temperature for the k ar system of white mica some of the samples show pronounced retrogression of the high pressure assemblages especially formation of albite and fine grained phengite at the expense of jadeite and are affected by intense late deformation at temperatures of about 300 to 350 c estimated from the small grain size of dynamically recrystallized quartz the ages obtained from extremely kinked or sheared coarse high pressure phengites scatter considerably being partly higher partly lower than 80 ma the fine grained phengite fractions from these samples yield lowered ages down to 70 ma these values are interpreted as mixed ages resulting from variable mixtures of fragmented high pressure phengites and new phengite replacing jadeite the k ar data suggest that the new phengites have crystallized between 60 and 70 ma b p all phengites formed at low temperatures at the expense of jadeite reveal high si contents these range up to 6 9 for the coarser grained earlier generations and up to 6 7 for the very fine grained last generation relatively high pressures are required to stabilize phengites so high in si this implies that cooling had not been achieved by uplift and erosion obviously the thermal structure of the crust was still strongly perturbed by continuing subduction and thrusting during the late cretaceous

the behavior of shape preferred orientations in metamorphic rocks amphiboles and jadeites from the monte mucrone area sesia lanzo zone italian western alps abstract in the eclogitic micaschists of the monte mucrone area sesia lanzo zone italian western alps we have measured in detail the eclogitic lineation at several outcrops we present observations at the markers scale this scale allowing the whole range of progressive deformations to be described populations of glaucophanes and jadeites show different statistical features which must be at least for a part produced by the distinct mechanical behavior for these markers during the alpine history the glaucophanes show a positive correlation between the intensity of preferred orientation and their aspect ratio due to a mainly rigid behavior the observed differences in shape and orientation of jadeites also show different behavior related to different retromorphic overprints we briefly discuss the implications of these observations on the significance of metamorphic lineations

palaeomagnetism of the foyers and strontian granites scotland abstract palaeomagnetic data from the newer granites of foyers and strontian show similar directions of magnetization with southerly declinations and horizontal to intermediate mostly downward dipping inclinations these palaeomagnetic directions which accord with previous data from the helmsdale granite are thought to be of siluro devonian age the present results are not sufficiently precise to justify discussion of possible lateral displacements along the great glen fault of the order of a few hundred kilometres but it is clear that the recent idea of a c 15 20 latitudinal offset along the fault in the carboniferous has to be disregarded

end silurian modifications of ordovician terranes in western ireland the ordovician elements in the caledonides of western ireland may be divided into the clew bay south mayo north connemara and south connemara sub zones evidence suggests the presence of a number of proximal suspect terranes and the remnants of one or possibly two arc complexes palaeontological and sedimentary evidence suggests an allochthonous relationship between the silurian rocks of croagh patrick and north galway these successions may therefore not represent a single sedimentary overlap to the ordovician suspect terranes end silurian deformation was the result of transpression which was responsible in part for the present configuration of the area s geology deep seated shear zones with fuchsite and gold mineralization were foci for lateral translation concomitant with southward directed thrusting northward directed back thrusts and variable degrees of metamorphism being of higher grade in the north the end silurian event is intermediate between the high grade metamorphism in parts of the appalachians and the non metamorphic silurian sequences of scotland

a basin model for the silurian of the midland valley of scotland and ireland identification of terranes is frequently based on the apparent disparity of source area for sediment supply palaeontological evidence and the presence of faults whose sense and amount of displacement is sometimes conjectural as an example of such superficially unrelated stratigraphies the silurian successions of the midland valley of scotland and the northern part of ireland are compared a diachronous late llandovery early middle telychian transgressive episode which is earlier in the midland valley inliers is common to many of the successions the transgression resulted in the development of comparable sedimentary facies and fossil assemblages the clast compositions of llandovery and wenlock conglomerates suggest erosion of a common provenance of volcanic rocks founded on a metaquartzite basement areas of active volcanism appear to have migrated in a north westerly direction with time a similar sequence is evident in canada it is suggested that the common features of the scottish irish successions may be accommodated in a unified basin model for silurian sedimentation and volcanism such a basin may have formed in an intra arc environment and been controlled by oblique slip fault mechanisms although evidence for the presence of a silurian arc is equivocal

statistical significance of the fold test in palaeomagnetism summary the significance of the change in direction dispersion on applying the fold test in palaeomagnetism is used as the criterion for deciding whether or not the fold test has statistical significance a set of tables is compiled which can be used for quick application of the test

palaeomagnetic and geochemical evaluation of a terrane linking ignimbrite evidence for the relative position of the grampian and midland valley terranes in late silurian time the proposed correlation of lower old red sandstone dacitic ignimbrites between the northern midland valley and grampian terranes of central scotland has been investigated in a combined palaeomagnetic and geochemical study statistically indistinguishable late silurian palaeolatitudes grampian terrane 27 0 s 3 5 northern midland valley terrane 22 9 s 1 8 comparable remagnetization histories a shared normal polarity for the primary magnetization and equivalent gyroremanence acquisition favour correlation across the highland boundary fault similar chemistry for biotite phenocrysts further corroborates correlation which was initially made on the basis of a similar field appearance and petrography this correlation together with evidence from provenance studies suggests that the separation of the northern midland valley and grampian terranes inferred for the ordovician period was largely removed by late silurian times with scope for relative displacements of the order of only tens of kilometres after lower old red sandstone deposition

matrix and fine grained rims in the unequilibrated co3 chondrite alha77307 origins and evidence for diverse primitive nebular dust components abstract the mineralogical and chemical characteristics of dark matrix and fine grained rims in the unequilibrated co3 chondrite alha77307 have been investigated in detail by scanning electron microscopy electron microprobe analysis and transmission electron microscopy matrix and fine grained rims on chondrules and other objects are compositionally homogeneous on the scale of 10 m and there are no compositional differences between matrix and rims mineralogically both the matrix and rims are extremely diverse and consist of a highly unequilibrated assemblage of si and fe rich amorphous material olivine pyroxene fe ni metal magnetite pentlandite pyrrhotite anhydrite and mixed layer phyllosilicate phases several distinct components can be recognized within the matrix and rims based on their textural and compositional characteristics which appear to represent basic fine grained units of nebular dust the microstructures of these different components show that they have experienced significantly different thermal histories unlike the ordinary chondrites an origin for any significant component of the matrix or rims in alha77307 from chondrules is improbable based on compositional and microstructural evidence the matrix and rim components in alha77307 formed by disequilibrium condensation processes as fine grained amorphous dust that is represented by the abundant amorphous component in the matrix such condensation could have occurred under a variety of conditions at different times and locations within the solar nebula or possibly earlier in a circumstellar environment subsequent thermal processing of this primitive condensate material in a variety of environments in the nebula resulted in partial or complete recrystallization of the fine grained dust the intimate association of fine grained components with disparate compositions and thermal histories shows that mixing of fine grained dust within the nebula must have been extremely thorough

implications of early devonian poles from the canadian arctic archipelago for the north american apparent polar wander path lower devonian red beds from the upper member of the peel sound formation at prince of wales island canadian arctic yield two different paleopoles at 25n 099e and at 01n 091e the first one being

palaeozoic terrane accretion in the western irish caledonides the southern boundary of the early caledonides in the british isles the dalradian supergroup is represented by the major highland boundary fair head clew bay line in the west of ireland accretion along this laurasian margin occurred by sinistral strike slip during two phases the earliest was responsible for the docking of the connemara terrane a displaced part of the dalradian outboard of the south mayo trough in the llanvirnian llandeilian this event is recorded independently by radiometrically dated sinistral strike slip deformation in the ox mountains of western ireland following deposition of a silurian overlap sequence sinistral movements were renewed between the upper llandoverian and the siegennian this event is part of a major phase of sinistral strike slip throughout the british isles involving accretion of the cadomian terrane and the devonian accretion of the avalon terrane in the northern appalachians

zoning and recrystallization of phengitic micas implications for metamorphic equilibration white micas phengites in the metasediments of the scottish dalradian display a large range of compositions within single samples the variations in the composition of these phengites are strongly controlled by their structural age with early fabrics containing a paragonite poor celadonite rich phengite whereas in later fabrics the micas are generally paragonite rich and celadonite poor retrograde phengite growth identified using back scattered electron imaging occurs as celadonite rich rims on micas within all existing fabrics and appears to be preferentially developed along existing white mica plagioclase grain boundaries the presence of these chemically distinct phengite populations within single samples implies that chemical exchange between the individual micas was inefficient it is proposed that diffusion controlled exchange reactions in phengites have relatively high closure temperatures below which major element exchange is effectively impossible this closed system behaviour of micas questions the ease with which phengites may equilibrate with other phases during prograde greenschist and lower amphibolite facies metamorphism many of the chemical variations preserved in phengites from such metamorphic rocks may reflect deformation recrystallization controlled equilibria

palaeomagnetic re examination of the basal caithness old red sandstone aspects of local and regional tectonics abstract a palaeomagnetic re examination of the basal strata of the caithness old red sandstone has given results that are fully compatible with previous palaeomagnetic findings in this region after structural correction the dominant remanence component has d 205 i 3 95 6 4 n 27 the existence of this shallow inclined magnetization in the middle devonian strata of caithness invalidates the model proposed by van der voo and scotese 1981 involving a ca 2000 km sinistral offset along the great glen fault in the carboniferous however the available data are in favour of a few hundred kilometres sinistral movement along this fracture zone however the possibility of there having been a much larger transcurrent shift between europe and north america in late post devonian times accumulated along various fracture zones within the caledonian fold belt is discussed on the basis of an inferred overprinted magnetization it is tentatively concluded that the tectonic deformation of the old red sandstone of caithness has a mid jurassic or younger age

paleomagnetism of the maringouin and shepody formations new brunswick a namurian magnetic stratigraphy two hundred thirty five oriented cores were taken in a stratigraphic sequence of 575 m of red sediments from the namurian maringouin and shepody formations these samples were taken from the south

compositional control on the polymorphism 2m 1 3t of phengitic white mica from high pressure parageneses of the sesia zone lower aosta valley western alps italy the polymorphism 2m1 or 3t modification of phengites coexisting with jadeite quartz k feldspar in gneisses of granitic composition from the internal part of the sesia zone correlates with their mg al fe mg ratio for values 0 015 the 2m1 modification for values 0 025 the 3t modification is formed for mg al fe mg values between these limits both polymorphs coexist as distinct grains less than 1 of a large number of separated grains show an intergrowth of both modifications in these cases the predominant phase boundaries are hko planes rational at least with respect to the 2m1 lattice boundaries parallel 001 occur as subordinate steps in hko boundaries extensive intergrowth parallel 001 is not observed as far as revealed by optical microscopy the stacking sequence once established in a nucleus is retained during growth of the crystal

chemical isotopic and mineralogical evidence for the origin of matrix in ordinary chondrites abstract we report the first combined chemical isotopic and mineralogical study of fine grained opaque matrix material from a type 3 ordinary chondrite allan hills a77299 h3 7 electron microprobe and instrumental neutron activation analysis of a large matrix lump show that it has a major element composition typical of matrix material in type 3 chondrites unlike chondrules it shows no siderophile element depletions and is remarkably unfractionated relative to ci chondrites suggesting that it is primitive solar system material the matrix lump has an unique oxygen isotopic composition which lies below the terrestrial fractionation line and differs markedly from the composition of fine grained matrix lumps and chondrule rims from semarkona grossman et al 1987 transmission electron microscope tem studies of microtomed samples of the fine grained fraction m of the matrix lump show that it consists of rare angular clastic grains 1 m of pyroxene and olivine embedded within a groundmass of rounded fine grained m olivines the mineralogy and textures of the matrix lump are typical of other occurrences of fine grained matrix studied by tem in other type 3 ordinary chondrites we conclude that the matrix lump in alha 77299 consists largely of material that formed by annealing of amorphous presolar dust or nebular condensates and could not have been derived from chondrules by any reasonable mechanism however the observed compositional differences between the matrix lump and chondrules in ordinary chondrites are consistent with the idea that some chondrules formed by melting of matrix like material accompanied by loss of siderophile and volatile elements

a lower ordovician paleomagnetic pole from the oneota dolomite upper mississippi river valley of 149 oriented specimens of the lower ordovician oneota dolomite subjected to stepwise thermal or alternating field demagnetization 50 contained a characteristic shallow east southeasterly or west northwesterly component of magnetization identifiable on orthogonal vector projections for 48 other specimens the demagnetization trajectories defined greats circle paths which intersect close to the characteristic direction the site mean directions for the characteristic component were determined by using both the characteristic directions and great circle data and these yield a formation mean direction obtained from 10 sites with a declination of 105 an inclination of 2 and a precision parameter k of 17 6 95 11 9 the mean of the site virtual poles falls at 10 4 n 166 4 e k 48 0 a95 7 0 which is close to previously published late cambrian paleopoles both polarity groups have predominantly positive inclinations suggesting the presence of an uncleaned downward component whose unblocking temperature and coercivity spectra overlap very strongly with those of the characteristic magnetization we interpret this downward component as a drilling induced remanent magnetization and we hope to have removed its contribution by averaging the dual polarity results multicomponent magnetizations were also present in samples collected from blocks of oneota dolomite incorporated into the overlying new richmond sandstone lower ordovician but only six out of eight samples yielded well defined greats circle paths the best fit intersection of these defines a northerly and steep recent component while the directions of the higher stability components are different for each block due to the small number of samples and imperfect resolution of magnetic directions the conglomerate test is not conclusive nevertheless it suggests that the stable remanence in the oneota dolomite was acquired prior to the deposition of the new richmond sandstone

palaeozoic palaeogeography of laurentia and its margins a reassessment of palaeomagnetic data summary the recognition of wide spread remagnetization events during the palaeozoic has greatly added to the problems of reconstructing the palaeozoic drift history of laurentia and in recent years increasing emphasis has been placed on evaluating the relative reliability of palaeomagnetic data in an attempt to overcome some of these difficulties in the light of these developments a revised apparent polar wander apw path has been constructed following a rigorous evaluation of all published palaeozoic palaeomagnetic data from laurentia and its palaeozoic margins i e north america northern britain northern ireland and greenland the use of data from the laurentian margins when corrected for the opening of the north atlantic has resulted in a revision of previously published palaeozoic palaeogeographic reconstructions and these new palaeogeographic reconstructions for laurentia are in good agreement with faunal and lithological data a comparison of this new apw path with other apw paths from avalonia and baltica reveals a very close agreement between all three paths from mid silurian times onwards indicating that the closure of the iapetus ocean and the tornquist sea was essentially complete by this time laurentia occupied equatorial latitudes from cambrian to early silurian times and its drift history was dominated by anti clockwise rotation up to 0 7 ma 1 and low latitudinal drift rates up to 3 cm yr 1 late silurian and early devonian times were characterized by increased rates of anti clockwise rotation up to 1 3 ma 1 and southerly directed latitudinal drift rates up to 13 cm yr 1 of the continent which place laurentia in moderate southerly latitudes 30 40 s by the early devonian these palaeolatitudes are comparable with those from avalonia and baltica for this time indicating that the amalgamation of eurmerica had already taken place by late devonian time laurentia had begun to drift north rates up to 5 cm yr 1 towards equatorial latitudes by mid carboniferous time

matrices of type 3 ordinary chondrites primitive nebular records petrologic studies were made on the fine grained matrices of type 3 ordinary chondrites of the lowest petrologic subtype the matrix minerals in order of abundance are olivine fo99 to fo9 enstatite or bronzite augite or subcalcic augite albite fe ni metal troilite magnetite spinel mgal2o4 chromite and calcite fe and mg rich fluffy particles and albite like particles are also major constituents the chemical compositions of olivine and pyroxenes vary within and among the chondrites and are in gross disequilibrium showing that the matrix materials were hardly heated after their formation textural relationships indicate that magnesian olivine was formed after ca pyroxene followed by intermediate to iron rich olivine intermediate olivine was formed from enstatite and metallic iron under relatively oxidizing conditions the observations indicate that matrices of chondrites are neither the fragments of chondrules nor the precursors of chondrules they were mostly the products of condensation and reaction among solids and or between solids and the ambient gas mostly at low temperatures and thus they contain records of primitive processes in the nebula in order to explain the presence of olivines more iron rich than fo50 the presence of free sio2 or a high activity of sio2 in the gas is necessary which was not shown in previous thermochemical calculations mineral assemblages of matrix minerals of chondrites of different chemical groups differ systematically according to oxidation state of the parental meteorites indicating that they were formed at different oxygen fugacities the rims of chondrules and surrounding matrix materials must have accreted onto chondrules during turbulent movements of the nebula

eclogitic metamorphism in carbonate rocks the example of impure marbles from the sesia lanzo zone italian western alps the impure marbles of the internal sesia lanzo zone underwent a multi stage metamorphic evolution of alpine age and retain early alpine eclogitic assemblages partially recrystallized under blueschist to greenschist facies conditions these high p assemblages consist of carbonates phengite quartz omphacite grossular rich locally spessartinic garnet zoisite and al rich titanite retrogressive stages are characterized by the growth of glaucophane paragonite phlogopite tremolite and albite halogen rich biotite and amphibole are also present p t estimates of the early alpine metamophism have been calculated from these unique high p assemblages in order to test the applicability of some calibrations to impure carbonate systems in particular some gt cpx calibrations and the phengite geobarometer give results t 575 45 c at 15 kbar for the eclogitic climax and t 500 c at ph2o 9 kbar for early alpine retrogressive stages which are within the range obtained from the surrounding lithologies phase relationships in p t xco2 space indicate that mineral assemblages in the impure marbles coexisted with h2o rich fluids xco2 0 03 during their entire alpine evolution

yamato 792947 793408 and 82038 the most primitive h chondrites with abundant refractory inclusions in this paper we report petrological and chemical data of the unusual chondritic meteorites yamato y 792947 y 93408 and y 82038 the three meteorites are very similar in texture and chemical composition suggesting that they are pieces of a single fall the whole rock oxygen isotopes and the chemical compositions are indicative ofh chondrites in addition the mineralogy and the abundances of chondrule types opaque minerals and matrices suggest that these meteorites are h3 chondrites they were hardly affected by thermal and shock metamorphism the degree of weathering is very low we conclude that these are the most primitive h chondrites h3 2 3 4 si known to date on the other hand these chondrites contain extraordinarily high amounts of refractory inclusions intermediate between those of ordinary and carbonaceous chondrites the distribution of the inclusions may have been highly heterogeneous in the primitive solar nebula the mineralogy chemistry and oxygen isotopic compositions of inclusions studied here are similar to those in co and e chondrites

fayalitic olivine in matrix of the krymka ll3 1 chondrite vapor solid growth in the solar nebula fayalitic olivine fa54 94 is a ubiquitous component in the matrix of krymka ll3 1 as well as in other highly unequilibrated chondrites ordinary and carbonaceous in krymka the fayalitic olivine has an unusual anisotropic platy morphology that occurs in at least five types of textural settings that can be characterized as 1 isolated platelets 2 clusters of platelets 3 euhedral to subhedral crystals 4 overgrowths of platelets on forsteritic olivine and 5 fluffy porous aggregates from transmission electron microscope tem investigation the direction of elongation of the platy olivine overgrowths on forsteritic olivine substrates is along the c axis and in most cases it corresponds with the c axis of the substrate olivine which suggests that the fayalitic olivine grew in this unusual morphology and is not a replacement product of preexisting material the fayalitic olivine in the matrix of krymka is compositionally similar to olivine with platy morphology in the matrix of some cv3 chondrites and both have similar fe mn ratios but important morphological differences indicate that their relationship needs to be explored further textural and compositional data indicate that the fayalitic olivine in the matrix of krymka as well as in some other unequilibrated ordinary chondrites formed prior to final lithification of the meteorite and probably prior to parent body accretion we find that formation of the fayalitic olivine by vapor solid growth provides the best explanation for our observations and data and is the only feasible mechanism for the formation of fayalitic olivine in the matrix of krymka we propose that the fayalitic olivine formed by vaporization and recondensation of olivine rich dust during a period of enhanced dust gas ratio in the nebula

palaeomagnetism of silurian sediments in w ireland evidence for block rotation in the caledonides summary a palaeomagnetic study of mid silurian wenlock sediments of the salrock formation in nw galway yields a reliable silurian palaeomagnetic pole position for w ireland at latitude 1 6 s longitude 287 7 e dp 2 5 dm 4 6 n 27 sites a further 12 sampling sites were located in silurian redbeds on clare island s mayo where a palaeomagnetic remanence of silurian age was also identified both remanences pre date local late caledonian folding however the clare island result has an anomalous azimuth when compared with the salrock result and with published silurian results for britain the two irish results can be reconciled by anticlockwise block rotation of the clare island region through 40 50 the rotation brings both palaeomagnetic and structural trends within the two sampling regions into alignment strike slip displacement across the clew bay fault zone producing a sinistral shear couple might have enabled such a rotation the timing and geographic extent of the proposed block rotation is as yet poorly defined however a younger age limit for rotation is provided by a late devonian or carboniferous partial magnetic overprint observed throughout both study areas and from geological evidence it seems likely that rotation is confined to the clew bay fault zone the dalradian rocks of the connemara massif located to the south of the salrock district have an e w structural grain some workers have proposed clockwise rotation of the connemara massif of ca 30 at between 500 and 390 ma which would bring the structural trends in w ireland into parallelism with caledonian trends in scotland because the silurian salrock succession which rests unconformably on the dalradian basement of connemara shows no such anomaly it provides a younger age limit for the possible rotation of connemara

terrane amalgamation in the clew bay region west of ireland the caledonides of the west of ireland provide a well exposed and well mapped example of an oblique collision zone the east northeast trending deer park and achill beg fault system is a crustal scale ductile sinistral strike slip duplex of late ordovician age imbricating late precambrian granulite facies lower crustal rocks near eclogite facies supracrustal rocks up to amphibolite facies dalradian metasedimentary rocks and greenschist facies cambro ordovician rocks this fault system is correlated with a pre devonian component of the highland boundary fault system in southern scotland in the clew bay area the high pressure low temperature facies metamorphic rocks in tectonic contact with greenschist facies cambro ordovician rocks are together interpreted as an accretionary prism complex related to northwestward directed subduction both of these are allocthonous terrains with respect to the dalradian terrane to the north north west mayo to the south the cambro ordovician rocks docked with a probable dalradian block containing ultramafic intrusives deer park complex during the late ordovician the deer park complex and south mayo trough linked earlier during the arenig silurian and lower middle devonian redbed successions sit unconformably on the metamorphic rocks deposition and deformation of these cover rocks was controlled by oblique strike slip movements on the leek fault whose strike swings from west northwest to north northeast following earlier basement trends as it is traced eastwards from clew bay the leek fault system may be correlated with the leannan fault of northwest donegal a splay of the great glen fault system of central scotland east of clew bay this sinistral shear generated local dilation on the more northerly trending bend of the leek fault lower and middle old red sandstone redbeds were developed here the west northwest trend of the leek fault in clew bay acted as a compressional bend during these sinistral movements and transpressional southwest directed thrusting developed in silurian rocks post middle old red sandstone pre late tournaisian dextral displacement on the leek fault reversed this pattern with transtension in clew bay allowing intrusion of small carbonated peridotite bodies into silurian rocks and easterly directed thrusting of middle old red sandstone rocks east of the bay on the transpressional north south bend a tectonic model for the region is presented here this model involves a northwestward directed subduction system 150 to 750 km of arenig sinistral strike slip movement and eastwards insertion of the connemara block with formation of the ordovician south mayo trough as a pull apart basin subsequently a further 130 to 650 km eastward displacement of rocks took place south of the deer park fault in later ordovician times the magnitudes of these estimates are directly proportional to an assumed maximum wavelength of 1500 km for promontories on the original laurentian margin and using the current juxtaposition of terranes a minimum wavelength of 300 km is inferred

carbonaceous xenoliths in the krymka ll3 1 chondrite mysteries and established facts abstract the results of a detailed study on mineralogy chemistry and the carbon and oxygen isotopes of two exotic krymka carbonaceous xenoliths are presented in this article the investigated xenoliths are metamorphosed and shocked and have the following characteristics which distinguish them from the krymka host 1 resemblance of their sio 2 mgo ratio to that of carbonaceous chondrites 2 higher fe content and feo feo mgo ratio 3 lower concentration of si ca al and an enrichment of s and probably of ag 4 smaller sizes and lower content 10 vol of chondrules and their clasts and correspondingly higher content of matrix 5 dominance of porphyritic chondrules and lack of nonporphyritic chondrules 6 occurrence of an amoeboid olivine grain with 16 o rich composition 7 existence of carbon in three different forms graphite carbon rich material and organic compounds the bulk chemistry of the xenoliths is similar but not identical to that of carbonaceous chondrites suggesting that they represent a chondrite parent body that has not been previously sampled among any known type of meteoritic material the mineralogy of the xenoliths corresponds only to that of other krymka graphite containing xenoliths it differs however from the latter by having a lower grade of metamorphism we infer that metamorphism of the primary carbonaceous body of the xenoliths and or shock of the krymka parent body are responsible for the major metamorphic alteration of the xenoliths including the crystallization of graphite from primary organic compounds a comparison of the features of the krymka xenoliths with the inferred characteristics of cometary meteorites attests that their genetic relationship to cometary material remains highly inconclusive

magnetism of igneous rocks from the tourmakeady and glensaul inliers w ireland mode of emplacement and aspects of the ordovician field pattern summary results of palaeomagnetic and rock magnetic studies are reported mainly for quartz porphyry collected at 18 sites from an ordovician igneous sedimentary succession at tourmakeady and glensaul in co mayo eire both magnetite and haematite are shown to be prominent magnetic constituents the magnetite is usually in the form of large multidomain grains these are held responsible for a soft remanence component called the b magnetization with d i 000 75 95 3 7 i e practically identical to the direction of the present axial geocentric dipole field in the area hence it is inferred that the b magnetization which is dominant at most sites is of recent origin haematite which has a more patchy occurrence apparently formed through high temperature oxidation of magnetite haematite is the main carrier of the palaeomagnetically significant component the a magnetization which gave well grouped in situ directions 29 samples from 10 sites with a mean value of d i 182 6 corresponding to a north pole at 31 n 169 e dp dm 2 1 4 2 correction for folding along the glensaul tourmakeady are shows that the a magnetization is of post folding age an earlier suggestion that the rocks represent a lower ordovician extrusive suite is discussed and discounted it is inferred that the quartz porphyries constitute syntectonic intrusions having formed during the taconic orogeny these rocks as well as their characteristic magnetization are therefore regarded as late ordovician in age it is concluded that a c 50 relative clockwise shift between the mid ordovician and late ordovician silurian geomagnetic field axes previously demonstrated for scotland has now been recognized also in ireland

tem studies and the shock history of a mysterite inclusion from the krymka ll chondrite the microstructure and composition of the matrix of one carbonaceous inclusion k1 in the krymka ll3 1 chondrite were studied using transmission electron microscopy tem k1 has previously shown an enigmatic nature and similarities with volatile rich fine grained dark inclusions of krymka called mysterite in the present study four minerals were identified by tem olivine pyroxene and pyrrhotite typically have grain sizes of one micrometer graphite occurs as flakes of a similar size olivine shows a moderately high dislocation density most probably caused by shock pyroxene shows coexisting ortho and clinoenstatite lamellae that probably originated from shear stress after a shock event or from the rapid cooling of the protoenstatite stability field however we demonstrate that in this case a shock trigger is more likely pyrrhotite in the studied sample occurs as a 4c monoclinic superstructure the graphite flakes in the fragment are well crystallized as can be seen by discrete spots in the diffraction pattern in graphite the degree of crystallization increases with the metamorphic grade based on the microstructure of this mineral we conclude that after a first moderate shock event the residual temperature between 300 c and 500 c led to thermal metamorphism a second shock event possibly at excavation from the parent body is responsible for the shock features observed in olivine pyroxene and graphite

strike slip terranes and a model for the evolution of the british and irish caledonides evidence is presented that many of the major strike faults in the british and irish caledonides were active as sinistral strike slip zones in the end silurian to pre mid devonian period some such as the highland boundary fault moved in this way at an earlier stage in the ordovician these data allow the caledonian rocks lying between the laurentian miogeocline whose basement is represented by the lewisian moine and possibly the dalradian and the gondwanaland miogeocline midland platform and welsh basin to be re analysed as a group of disorganized terranes which originated to the southwest in north america and southwest europe africa prior to the silurian the highland border terrane and northern belt terrane are interpreted as duplicated pieces of a mid ordovician sequence which was a back are to northwest subduction the midland valley terrane is interpreted as a slice of laurentian foreland onto which ophiolites were obducted in the lower ordovician but which became the basement of a continental margin arc to northwest subduction in the mid ordovician the cockburnland terrane is inferred to be part of the same arc repeated and then broken up and dispersed by continuing strike slip the connemara terrane is regarded as an allochthonous piece of the dalradian miogeocline and the south mayo terrane as a remnant of an early ordovician arc and fore arc which in mid ordovician times became a back arc marginal basin to northwest subduction the lake district wexford terrane is part of an arc to southeast subduction under gondwanaland whose activity climaxed in the mid ordovician the central terrane is interpreted as a silurian overstep assemblage which blankets the junction between laurentian and gondwanaland derived oceanic terranes and therefore iapetus is regarded as an ordovician ocean which closed prior to the silurian the model suggests that at the end of the silurian a clockwise rotating gondwanaland having carried laurentia into collision with baltica broke free and created a major sinistral strike slip zone which disrupted the ordovician palaeogeography in the british isles north american sector of iapetus

paleomagnetic results from the peterhead granite scotland implication for regional late caledonian magnetic overprinting abstract palaeomagnetic results from the peterhead granite silurian ne grampian highlands define a characteristic magnetization with overall parameters of dec 180 4 inc 22 3 and a 95 5 2 these results agree with other newer granite data from north scotland an east west trending dyke cutting the peterhead granite shows an extremely well defined paleomagnetic direction with dec 192 5 inc 19 1 and a 95 1 3 suggesting a permo carboniferous age the peterhead granite results are seen in conjunction with other palaeozoic palaeomagnetic data from scotland it is concluded that the observed directional spread of characteristic remanence directions previously reported from the aberdeenshire gabbros are not related to slow post orogenic cooling watts and briden but is rather seen as the result of late caledonian magnetic overprinting

a deep seismic reflection transect across the irish caledonides new reflection profiles recorded to 18s c 60km along a 600 km north south transect west of ireland wire lines provide a cross section of the irish caledonides from lewisian basement in the north to the variscan fold and thrust belt in the south and constrain the offshore extrapolations of major caledonide structures we interpret the profiles as showing the great glen fault and associated ne trending strike slip faults as near vertical structures which cut the whole crust and the uppermost mantle beneath the surface trace of the fair head clew bay line we image a north dipping zone which soles into horizontal layers just above the reflection moho the north dipping zone separates south dipping reflections above and to the north from north dipping reflections below and to the south from the fair head clew bay line as far south as the iapetus suture the lower crust contains north dipping packages of reflectors possibly related to acadian accretionary underplating the iapetus suture zone can be identified as a zone more than 50 km wide dipping north at 30 from 5 km depth offshore dingle peninsula to sole into the moho at c 30 km depth west of the shannon estuary in the upper crust the shallow part of the iapetus suture zone appears to be transported to the north by late caledonian retrocharriage or by variscan thrusting the iapetus suture does not penetrate through the present day reflection moho but there are scattered reflectors in the mantle not continuous with crustal reflectors perhaps relics of a subducted slab we recognize lower crustal reflectivity and the reflection moho along almost all the wire profiles regionally the travel time to the reflection moho varies from 9 s c 28 km offshore donegal northwest of ireland to 12 s c 35 km offshore kerry and cork southwest of ireland apparent moho offsets rapid lateral changes in the travel time to the reflection moho north of donegal and north of mayo are probably due to the great glen fault and associated faults the lower crustal reflectivity varies considerably in thickness and prevalent dip direction along the wire profiles in donegal bay this reflectivity is observed at depths less than 5 km c 1 5s whereas west of kerry prominent reflectivity is only observed beneath 6s c 17 km though the age of crustal reflectors cannot in general be determined beneath central ireland from donegal to dingle the lower crustal reflectors mimic in dip direction the structural vergence of the exposed dalradian and early palaeozoic rocks so that the lower crustal reflectivity patterns and probably much of the crust must be caledonian in age

shock melted material in the krymka ll3 1 chondrite behavior of the opaque minerals six large millimeter to centimeter size regions of one specimen of the krymka ll3 1 ordinary chondrite show evidence of having been completely or nearly completely shock melted in situ a phenomenon rarely observed in primitive chondrites the shock pressure nominally in the range of 75 90 gpa could only have been 30 35 gpa in a porous material like fine grained matrix the melted regions have an igneous texture and their silicates are zoned and unequilibrated large metal troilite intergrowths formed in these regions the metal has a nickel content corresponding to martensite and the troilite contains up to 4 2 wt nickel melting must have been very short and cooling very fast 100 c h at high temperature the metal contains up to 0 7 wt phosphorus abundant chromite crystals and sodium iron phosphate glass globules are found in troilite the differences in composition between the opaque phases found in the melted regions and those generally observed in unmetamorphosed chondrules are assigned to melting under closed system conditions surprisingly high co concentrations up to 13 wt were found in some metal grains in or at the periphery of melted regions they likely resulted from sulfurization of metal by sulfur vapor produced during the shock after solidification at least one other shock led to mechanical effects in the melted regions

paleozoic paleogeography of north america gondwana and intervening displaced terranes comparisons of paleomagnetism with paleoclimatology and biogeographical patterns new paleomagnetic data have become available in the past 5 yr that require modifications in previously published paleogeographic reconstructions for the silurian and devonian in this paper the new paleopoles are compared to published paleogeographic models based on paleoclimatologic and biogeographic data the data from the three fields of paleomagnetism paleoclimatology and biogeography are generally in excellent agreement and an internally consistent paleogeographic evolutionary picture of the interactions between north america gondwana and intervening displaced terranes is emerging during the interval of the ordovician silurian and devonian north america stayed in equatorial paleoposition while rotating counter clockwise the northwest african part of gondwana was in high southerly latitudes during the late ordovician and was fringed by peri gondwanide terranes such as southern europe armorica and avalonian basement blocks now found in eastern newfoundland nova scotia the boston basin the appalachian piedmont and northern florida subsequently gondwana and the peri gondwanide terranes displayed rapid drift with respect to the pole this drift translates into the following pattern of movement for northwest africa during the latest ordovician early silurian this area moved rapidly northward from polar to subtropical latitudes followed by equally rapid southward motion from subtropical to intermediate about 50 s paleolatitudes during the late silurian middle devonian it is likely that significant east to west motion accompanied the latter shift in paleolatitudes with the caledonian acadian orogeny the result of silurian to early devonian convergence and collision between gondwana and north america this collision sandwiched several of the intervening displaced terranes between gondwana and north america subsequent to this collision gondwana was separated in the late devonian by a medium width ocean from north america and the avalonian and southern european blocks which were left behind adjacent to north america this new ocean closed during the carboniferous and the resulting convergence and collision were the cause of the hercynian alleghanian orogenic belt problems remaining for future research besides the further gathering of reliable paleopoles involve the uncertain pre devonian position of the southern british isles in this scenario and the very rapid velocity with respect to the pole that results from the rapid late ordovician silurian apparent polar wander for gondwana

oroclinal bending in the caledonides of western ireland a mid palaeozoic feature controlled by a pre existing structural grain abstract the caledonides of the west of ireland mark a segment of the caledonian appalachian orogen where the structural grain swings from the ne sw trend that typifies the caledonides of northern britain to an e w orientation the origin of this arcuate structure has previously been proposed to be either primary reflecting the original geometry of promontories and embayments in the laurentian margin in this sector of the orogen or secondary as evidenced by palaeomagnetically determined clockwise rotations in silurian rocks in the region we report the results of a palaeomagnetic investigation of the early to middle ordovician mweelrea ignimbrites llanvirn from an accreted taconic arc the south mayo trough in that part of the western irish caledonides that has an e w trend after removal of a low temperature component of magnetisation l interpreted to represent a viscous component of the earth s present field a high temperature component of magnetisation h was isolated at fifteen sites in four ignimbrite flows h passes a palaeomagnetic fold test yielding a tilt corrected mean remanence direction d 132 6 i 29 1 95 17 9 which corresponds to a palaeopole at 36 4 e 9 9 s the pole is in reasonable agreement with established early to middle ordovician reference poles for laurentia and indicates that no significant rotation occurred between the e w south mayo trough and the laurentian margin after accretion and therefore that the curvature of this segment of the orogen is likely to be primary this contrasts with other results from silurian sequences to the south which record significant post silurian clockwise rotations examination of all reliable palaeomagnetic data from the region indicates that the clockwise rotations are restricted to the rocks south of the south mayo trough we propose that the rotations in the south are linked with the superposition of the pre existing structural grain on the younger rock units during late silurian and early devonian sinistral transpression

magnetic properties of the lower old red sandstone lavas in the midland valley scotland palaeomagnetic and tectonic considerations abstract thermal demagnetization studies of lavas in the strathmore area of the midland valley scotland support overall palaeomagnetic data found in previous studies of these rocks reduced directional scatter as compared to some earlier studies is attributed to more effective demagnetization resolving some of the directional complexity of previous studies combined magnetic fabric and directional analysis suggest that at least some deviating directions may be explained by local tectonism the existence of almost antiparallel directional groups and field tests give supporting evidence for a primary deuteric origin of the main magnetization of these rocks additionally a second remanence component having shallow reverse directions of magnetization is attributed to later remagnetization in old red sandstone time the midland valley results are seen in conjunction with other palaeozoic palaeomagnetic results and possible geodynamic implications are discussed