composite titanomagnetite ferrian ilmenite grains and correlative magnetic components in a dacite with self reversed trm abstract electron microprobe and reflected light microscopic examinations confirm the presence of composite grains of ferrian ilmenite with x ilm 0 53 and titanomagnetite with x usp 0 13 in a dacite with self reversed trm a parallel trm component associated with titanomagnetite and a reversed component associated with self reversing ferrian ilmenite are the principal nrm components a subordinate parallel component is associated with ferrian ilmenite which is not magnetically coupled to an phase the natural self reversing properties are mainly a consequence of the dacite s high quenching temperature calculated at 862 864 c using the fe ti oxide geothermometer and are most consistent with the self reversal mechanism proposed by lawson et al 9 the conduction of thermal demagnetization and trm induction tests in air had a much greater effect on the fe ti oxides than did natural cooling and resulted in significant oxidation with the consequent modification of some magnetic properties and the formation of another reversed trm component the subdivision of titanomagnetite grains by oxidation along fractures decreased its effective grain size and caused an apparent increase in its magnetic intensity in addition to a slight increase in its resistance to alternating field demagnetization the phase associated with the reversed nrm component with 0 42 x ilm 0 31 became fe enriched during the earlier stages of heat treatment it is suggested that after heating at 600 c for two hours or more this phase exsolves as titanohematite with x ilm the ferrian ilmenite host is consequently enriched in ti and another phase much closer in composition to the host generates a reversed trm component with t b

partial self reversal of trm in baked soils and ceramics from ecuador abstract partial self reversed thermoremanent magnetizations srtrms were observed in samples of baked soils hearths and ceramics from the rumipamba archeological site near quito ecuador and ceramics from sites near the town of esmeraldas ecuador the srtrms were recognized at room temperature on few samples but cooling the samples in liquid nitrogen enhanced the intensity of the srtrm and measurement at 77 k enables its rapid detection in many samples from these sites alternating field demagnetization of the srtrm indicate median destructive field of the order of 50 mt and thermal demagnetization give unblocking temperatures in the temperature range 280 380 c the magnetic carriers of the srtrm are stable to heating in air or in vacuum up to 600 c suggesting that titanomaghemite should not be the magnetic carrier of the srtrm the studied baked clays and ceramics contain detrital material of mainly volcanic origin ti poor titanomagnetite is the main magnetic carrier identified by strong field data or susceptibility measurements versus temperature ilmeno hematite grains were recognized with microscope observations under reflected light scanning electron microscope observations with energy dispersive x ray spectrometry indicate a ti fe ratio corresponding to an ilmenite content of 0 55 we also compared the magnetic properties of the partially self reversed baked clays with those of the self reversed pinatubo pumices the srtrms were measured upon cooling from room temperature to 20 k with the mpms upon cooling to 20 k the srtrm show a nearly tenfold increase in intensity with respect to the room temperature measurement the baked clay and ceramics from ecuador carrying the srtrm share similar magnetic properties with the pinatubo pumices unblocking temperatures low temperature behavior supporting the interpretation that detrital hemoilmenite originating from the holocene activity of the numerous ecuadorian volcanoes is the main carrier of the srtrm in the archeological samples in some samples anisotropy tensors of srtrm show large anisotropy degree 2 the high anisotropy observed when the total magnetization is the sum of a normal thermoremanent magnetization trm and a srtrm is likely amplified because the anisotropy carried by the srtrm is larger than the one carried by the normal component a component of srtrm may be hidden in archeological materials containing detrital hemoilmenites related to dacitic andesitic volcanism and this may affect the determination of paleointensity of the earth magnetic field especially when non thermal method are used

domain observations of the magnetic phases in volcanics with self reversed magnetization the temperature dependence of magnetic domain pattern in titanomagnetite and titanohematite of self reversing andesites from the 1985 eruption of the nevado del ruiz volcano colombia has been observed using the bitter technique during heating cooling cycles in the laboratory the natural remanent magnetization loses the self reversal properties between 110 c and 180 c the titanohematite domains disappear in the same temperature interval large grain regions without visible colloid contrast are observed between room temperature and 180 c utilizing the high resolution of a magnetic force microscope it is found that these regions contain either domains with wall spacing of about 3 m or a buckling pattern along the phase boundary between a titanomagnetite and a titanohematite grain but within the titanohematite a 2 m wide chemically different domain has been observed it has a higher iron content than the main titanohematite the two titanohematite phases represent two phases with higher curie temperature tc but lower spontaneous magnetization ms and lower tc but higher ms respectively negative interaction between these phases is called upon to explain the self reversing mechanism

u pb zircon and sm nd model ages of high grade moldanubian metasediments bohemian massif czechoslovakia we report single grain u pb ion microprobe as well as conventional bulk size fraction ages for zircons from 3 metasediment samples of the moldanubian complex bohemian massif one of the largest crystalline complexes of the hercynian foldbelt in europe these are complemented by whole rock sm nd model ages the metasediments are of upper amphibolite to granulite grade and come from the bory massif in moravia nw of brno sample aa 1 and from the varied group aa 2 and monotonous group aa 3 in the ceske budejovice region of sw bohemia

identification of magnetic minerals carrying nrm in pyroclastic flow deposits the ikeshiro pyroclastic flow deposit at yufu volcano kyushu in japan is a typical block and ash flow generated by collapse of the ikeshiro lava dome erupted ca 2000 years ago clasts in the ikeshiro pyroclastic flow deposit were found to have a low temperature magnetic component that is parallel to the present earth s magnetic field we carried out paleomagnetic and rock magnetic experiments in order to examine the origin of the component by identifying magnetic minerals as a result the clasts were classified into two types based upon magnetic mineralogy type a is carried by titanomagnetite tc 480 485 c whose grain size is mainly md type b is carried by ti poor titanomagnetite tc 500 580 c hematite and titanohematite fe2 ytiyo3 with y 0 5 tc 215 220 c the low temperature magnetic component of type a was carried by titanomagnetite with very low coercivity whereas that of type b was principally carried by titanohematite the viscous remanent magnetization vrm acquisition and demagnetization results indicated that titanomagnetite with low coercivity in type a acquires vrm easily whereas titanohematite in type b hardly acquires vrm therefore the low temperature magnetic component of type b originates not from vrm but from thermoremanent magnetization whereas a part of the low temperature component of type a originates from vrm emplacement temperature of the ikeshiro pyroclastic flow deposit which is estimated from the unblocking temperature of type b samples is between 220 and 400 c the different magnetic mineral assemblage between the two types reflects the difference in oxidation state samples of type b are strongly oxidized whereas those of type a are not oxidation had probably occurred at the ikeshiro lava dome before generation of the ikeshiro pyroclastic flow

perpotassic granulites from southern bohemia garnetiferous perpotassic granulites associated with voluminous felsic calc alkaline garnet kyanite granulites in the blansk les massif are described as a new rock type a k2o content near 13 wt zr content ranging from 17 to 5877 ppm primary ca mg fe garnet pyrope up to 33 mol and the absence of primary plagioclase and quartz characterize the new type of granulite calcium is incorporated only in garnet and apatite owing to the content of garnet the bulk syenitic compositions are metaaluminous to peraluminous the perpotassic granulites are accompanied by subordinated melanocratic perpotassic granulite and alkali feldspar melanocratic granulite containing up to 20 vol quartz these rock types also contain highly variable amounts of zircon high pressure noneutectic partial melting of crustal rocks and crystallization under upper mantle conditions is suggested as the main process in generation of perpotassic granulites a polyphase ductile to brittle deformation and recrystallization in pyroxene granulite and later in amphibolite facies accompanied obduction of the granulite massif which carries numerous enclaves of pyrope peridotite euhedral equant zircon crystals up to 4 mm long enclosed in large garnet crystals and in the alkali feldspar matrix of perpotassic granulite represent a uniform high temperature population with a nearly concordant u pb system aftalion et al 1989 the hercynian age of this zircon indicates the role of the hercynian anatexis of crustal rocks under upper mantle conditions in generation of granulites in the moldanubian zone of the bohemian massif

a detailed magnetic and mineralogical study of self reversed dacitic pumices from the 1991 pinatubo eruption philippines 39 dacitic pumice and lithic samples from the 1991 eruption of mount pinatubo were investigated through both magnetic and mineralogical means as in a previous study natural remanent magnetization nrm is found to be reversed for most of the samples with respect to the direction of the actual geomagnetic field direction a few samples amongst them ancient lithics transported by pyroclastic flows show scattered nrm directions from thermal demagnetization of these particular samples it is concluded that their orientation changed after emplacement the emplacement temperature is estimated to be more than 460 c from thermal demagnetization of lithic samples two magnetic minerals with large grain sizes are observed under the optical microscope titanomagnetite tm and haemo ilmenite hem ilm microprobe analyses yield x 0 10 for tm and y 0 52 and y 0 54 for two hem ilm phases in agreement with the observed curie temperatures 480 c for tm and 250 c for hem ilm the hem ilm particles display chemical zonation which seems to be correlated with a change of the domain structure typically a ferrimagnetic fm phase with slightly higher titanium content is observed in the central part whilst the crystal margin which is weakly ferromagnetic wf due to spin canted antiferromagnetism is slightly poorer in titanium two different mechanisms for the origin and formation of the two observed phases are discussed 1 chemical zonation of hem ilm crystals due to a change in conditions in the magma chamber shortly before eruption 2 similar to the microstructures observed from synthetic samples this zonation in the large natural hem ilm could be the result of migration of the wf phase towards the grain boundary during residence below the order disorder transition temperature in the magmatic chamber the room temperature hysteresis loop which seems to be dominated by tm provides multidomain md like parameters jrs js 0 01 and hcr hc 20 the large coercivity of remanence 15 40 mt which is attributed to hem ilm may be intrinsic or due to interactions between wf and fm phases the field dependence of the magnitude of the thermoremanent magnetization trm is not linear it increases first reaches a maximum negative value for an applied field h close to 0 5 mt then decreases steadily by extrapolation it is estimated that the trm should be zero for a field of about 12 mt and become positive beyond this total trm is in fact the sum of several components af demagnetization of trms acquired in different fields shows the presence of at least three components a self reversed sr component that contains both hard and moderately hard components and a soft normal component independently of the value of h the median destructive field of the sr component is of the order of 70 mt which suggests that the fm phase is not entirely locked by exchange interaction with the wf phase and can experience noticeable wall movements however some 20 per cent of the sr trm component remains after af treatment of 220 mt although we believe that exchange interaction plays a key role in the process of sr trm acquisition torque curves in the high field of samples both with and without sr trm do not show clear evidence for unidirectional anisotropy similarly we do not observe any significant shift of major hysteresis loops or non vanishing hysteresis losses two characteristics generally considered as indicative of exchange anisotropy and usually observed for synthetic single domain pseudo single domain sd psd hem ilm indeed in addition to the exchange interaction the magnetostatic coupling must play some role here because of the large size of the hem ilm grains

self reversal of natural remanent magnetization in andesitic pumice abstract the natural remanent magnetization of andesitic pumice erupted from the nevado del ruiz volcano colombia in 1985 is reversed with respect to the present geomagnetic field because of self reversal laboratory thermoremanent magnetization acquired in fields of

variszische duktile tektonik im bohmischen massiv die verformungsanalyse und richtungsverteilung der streckungslineationen in den duktilen bereichen des bohmischen massiv zusammen genommen mit der lage der foliationsflachen deuten darauf hin das die blattverschiebungen wahrend der variszischen deformation die entscheidende rolle gespielt haben die kompatibilitat der deformation im gesamtbereich wurde durch uberschiebungen und transpression in den durch die blattverschiebungen begrenzten blocke erreicht das variszische tektogen wurde als blattverschiebungsorogen auf einer kontinentalen oberproterozoischen kruste angelegt die zonen mit intensiver krustenverkurzung und damit groster krustenmachtigkeit sind durch weitraumige anatexis und granitische intrusionen gekennzeichnet

separating multidomain and single domain like remanences in pseudo single domain magnetites 215 540 nm by low temperature demagnetization it has long been hypothesized that the pseudo single domain properties of submicron magnetites are a result of coexisting multidomain and single domain like remanences we have separated these two types of remanence by low temperature demagnetization ltd in which samples are cooled through the low temperature magnetite isotropic point and rewarmed to room temperature all in zero field at the isotropic point the first crystalline anisotropy constant vanishes and multidomain remanence due to displaced domain walls will be unpinned we measured stepwise alternating field af and thermal demagnetization curves of weak field thermoremanent magnetization trm and anhysteretic remanent magnetization and of saturation isothermal remanent magnetization irms both with and without prior ltd for magnetites with mean particle sizes of 215 390 and 540 nm the memory fraction of remanence recovered after ltd which represents the single domain like component decreased from 44 for 215 nm grains to 23 for 540 nm grains in the case of irms memory of trm was approximately twice as large af decay curves were soft and multidomain like before ltd but became harder and single domain like in shape after ltd ltd removed most low unblocking temperature trm and irms distributed low unblocking temperatures are therefore a multidomain phenomenon probably resulting from successive adjustments of domain walls to changing internal fields and pinning strengths during heating

self reversed magnetization carried by titanomaghemite in oceanic basalts abstract self reversal the remarkable ability of some rocks to acquire a magnetization antiparallel to the earth s magnetic field was of great interest in the 1950s and 1960s but it was subsequently found to be rare limited to a few hemoilmenite bearing igneous rocks here we show through thermal demagnetization experiments that pillow basalts from the northwestern pacific ocean lacking hemoilmenite contain two nearly antiparallel components of natural remanent magnetization partial thermoremanent magnetization experiments indicate that the component isolated at lower temperatures is a self reversal of the primary magnetization the self reversal is carried by titanomaghemite formed by low temperature oxidation because maghemitization of oceanic crust is ubiquitous self reversals may be an important factor contributing to the decrease with age in marine magnetic anomaly amplitudes observed for crust formed during the last 30 million years

book reviewthe techniques of modern structural geology vol 1 strain analysis j g ramsay and m i huber academic press london 1983 xiii 307 pp us 55 00 28 00 hardcover

high paleointensities of the geomagnetic field from thermomagnetic studies on rift valley pillow basalts from the mid atlantic ridge nineteen pillow basalts dredged within the rift valley of the mid atlantic ridge at 36 8on were studied by the thellier stepwise heating method in order to determine the paleointensity of the geomagnetic field when they erupted on to the sea floor previously reported fission track ages are 2 000 to 6 000 years for the youngest rocks mainly olivine basalts and 10 000 to 100 000 years for the others mainly plagioclase basalts and pyroxene basalts all but three pillow basalts meet the conditions commonly considered as indicative of quite reliable paleointensity estimates stability of the direction of nrm during its thermal demagnetization constant ratio of nrm trm natural remanent magnetization to thermoremanent magnetization over 50 or more of the original nrm intensity 80 to 94 for 11 specimens and reproducibility of low temperature partial trm ptrm however strong field thermomagnetic measurements indicate that 11 of these 16 samples display a significant increase in curie temperature 15 to 80oc during the paleointensity experiments below 250oc notwithstanding the linearity of the nrm trm plot in this temperature interval this alteration probably due to low temperature oxidation of the specimens seems typical of young pillow basalts and may result in paleointensity estimates which are too high this result shows that excellence of remanence tests nrmtrm linearity and ptrm stability does not ensure the absence of chemical changes during the thellier experiments and therefore the validity of the paleointensity obtained the assumption that reliable paleointensities are obtained when the curie point increase is 10oc or less led to the selection of five of our specimens all from the youngest group their mean paleointensity is 64 2 20 5 xt standard deviation and the corresponding virtual axial dipole moment vadm is 11 5 3 7 x 1022 a m 2 given the variations of the vadm of the earth s magnetic field over the last 6 000 years as established from archeomagnetic studies our paleointensity results suggest that the latest eruptions on the inner floor of the rift valley at 36 8on occurred 1 500 1 000 years ago

reversed magnetization in pyroclastics from the 1985 eruption of nevado del ruiz colombia one of the fundamental principles of palaeomagnetism and rock magnetism states that the direction of natural remanent magnetization nrm is acquired parallel to the applied field however ferromagnetic mineral phases with complex microstructures may violate this rule1 2 and produce a magnetization vector which is antiparallel to the direction of the external field andesitic pumice which was hurled several kilometres during the disastrous 1985 eruption of the nevado del ruiz volcano colombia carries a stable but reversed nrm with southerly declination and negative inclination heating experiments show that this magnetization is due to a self reversal mechanism which also induces a reversed thermoremanent magnetization trm in the laboratory field thus we have used the nevado del ruiz 1985 pumice which was magnetized in the present well known geomagnetic field to demonstrate for the first time that self reversal has actually controlled the process of nrm acquisition

paleomagnetic evidence for hot pyroclastic debris flow in the shallow submarine shirahama group upper miocene pliocene japan the cooling history of coarse andesite blocks erupted and deposited proximally in the shallow marine environment has been investigated by paleomagnetic studies and heat conductivity calculations six to 13 samples were cored and measured from six essential andesite blocks 1 5 m in diameter thermal demagnetization techniques were used to detect the directional change of remanence through demagnetization steps each representing different blocking temperatures tb each sample has three components of remanent magnetization c1 c2 and c3 c1 is characterized by a tb of 250 c and an orientation coincident with the present geomagnetic field c2 has a tb of 250 500 c and shows a consistent direction with reversed polarity c3 has a tb of 500 600 c and its orientation is nearly consistent from samples collected from nearby parts of the same clast and are random from samples from different clasts on the basis of the remanence direction of each component the differing tb and the petrography of the samples c1 is interpreted as secondary viscous remanence whereas c2 and c3 are the partial thermoremanent magnetizations obtained during and immediately after emplacement c3 was probably acquired while the blocks were rolling or sliding downslope and c2 formed during continued cooling of the blocks after they came to rest heat conductivity calculations suggest that the blocks were emplaced less than an hour after being erupted if the eruptive vent was only 2 3 km distant as suggested by field relations a downslope transport rate of less than 36 km h is implied

magma evolution recorded in plagioclase zoning in 1991 pinatubo eruption products plagioclase the most abundant phenocryst at mount pinatubo displays varying textures and compositions within the 1991 eruption products in june 7 14 dome forming andesite plagioclase phenocrysts show prominent rims with higher mgo 0 04 0 06 wt jo fe20j 0 6 0 8 wt o and k20 at given an than the interiors the compositions of the rims are identical to those of microlites which are abundant in the groundmass glass white dacitic pumice the most voluminous product of the june 15 eruption contains partly corroded plagioclase phenocrysts but no prominent rims and no microlites the interiors of phenocrysts from the dacitic pumice and the dome forming andesite are remarkably similar in terms of textures and compositions they show oscillatory zoning mostly an35 6o low mgo 0 02 wtojo and fe20j 0 10 0 30 wt jo and similar k20 at given an this similarity indicates that the two types of plagioclase phenocrysts formed in the same rhyolitic melt the oscillatory zoning likely formed by temperature fluctuations in the convecting magma and incorporation and degassing of external fluids a portion of the felsic magma 800 c mixed with a mafic melt 1000 1100 c to become an andesitic magma that extruded to form the june 7 14 dome all plagioclase phenocrysts in the andesite were derived from the felsic magma the mixing caused destabilization of phenocrysts forming sieve textures dusty zones and partial resorption extrusion of the mixed magma resulted in overgrowths on once resorbed phenocrysts and the nucleation of plagioclase microlites in the groundmass glass in the unmixed remaining portion of the felsic magma some plagioclase underwent partial resorption but did not develop overgrowths this lack of overgrowths and the absence of micro lites in the groundmass glass of the june 15 dacitic pumice indicate rapid magma ascent during the eruption and a short time span between the injection of a mafic melt and the cataclysmic eruption supporting the linkage between the two

relative permeability and trapping of co2 and water in sandstone rocks at reservoir conditions 1 we report the results of an experimental investigation into the multiphase flow properties of co2 and water in four distinct sandstone rocks a berea sandstone and three reservoir rocks from formations into which co2 injection is either currently taking place or is planned drainage relative permeability and residual gas saturations were measured at 50 c and 9 mpa pore pressure using the steady state method in a horizontal core flooding apparatus with fluid distributions observed using x ray computed tomography absolute permeability capillary pressure curves and petrological studies were performed on each sample relative permeability in the four samples is consistent with general characteristics of drainage in strongly water wet rocks measurements in the berea sample are also consistent with past measurements in berea sandstones using both co2 brine and oil water fluid systems maximum observed saturations and permeabilities are limited by the capillary pressure that can be achieved in the experiment and do not represent endpoint values it is likely that maximum saturations observed in other studies are limited in the same way and there is no indication that low endpoint relative permeabilities are a characteristic of the co2 water system residual trapping in three of the rocks is consistent with trapping in strongly water wet systems and the results from the berea sample are again consistent with observations in past studies this confirms that residual trapping can play a major role in the immobilization of co2 injected into the subsurface in the mt simon sandstone a nonmonotonic relationship between initial and residual co2 saturations is indicative of a rock that is mixed or intermediate wet and further investigations should be performed to establish the wetting properties of illite rich rocks the combined results suggest that the petrophysical properties of the multiphase flow of co2 water through siliciclastic rocks is for the most part typical of a strongly water wet system and that analog fluids and conditions may be used to characterize these properties further investigation is required to identify the wetting properties of illite rich rocks during imbibition processes

paleomagnetic evidence for high temperature emplacement of the 1883 subaqueous pyroclastic flows from krakatau volcano indonesia the paroxysmal 1883 eruption of krakatau volcano in indonesia discharged at least 6 5 km3 dense rock equivalent of pyroclastic material into the shallow waters of the sunda straits within a 15 km radius of the volcano progressive thermal demagnetization studies of individually oriented pumice clasts from a core sample of the submarine pyroclastic deposits show that 41 out of 47 clasts exhibit single component remanence with mean inclination of 2 4 the partial thermoremanent magnetization components of both pumice and lithic clasts are well grouped in orientation indicating that substantial cooling of clasts must have occurred following deposition estimated subaqueous emplacement temperature for such clasts is greater than 500 c rare two component lithic fragments exhibit inflection points on vector endpoint diagrams that mark the temperature below which the fragments acquired magnetization of similar orientation these inflection points range from 350 to 550 c indicating a minimum subaqueous emplacement temperature of 350 c paleomagnetic evidence for high emplacement temperature supports the hypothesis that proximal 1883 submarine pyroclastic deposits resulted from entrance of hot subaerially generated pyroclastic flows into the sea similar deposits have been interpreted from the geologic record but this is the first documented example of submarine pyroclastic flows from a historic eruption the krakatau deposits thus serve as an important modern analog for the study of pyroclastic flow seawater interactions

eclogites with a short lived granulite facies overprint in the moldanubian zone czech republic petrology geochemistry and diffusion modelling of garnet zoning the petrology and geochemistry of a newly discovered suite of high pressure garnet clinopyroxene bearing rocks from the monotonous series of the moldanubian zone of the bohemian massif southwest czech republic have been investigated three types common eclogites quartz kyanite clino zoisite eclogites and garnet hornblende clinopyroxenites are distinguished by petrography and geochemistry all underwent a significant degree of partial breakdown under granulite and amphibolite facies conditions during exhumation important features include the growth of orthopyroxene in breakdown domains after garnet and omphacite and anorthite spinel corundum exceedingly peraluminous sapphirine replacing kyanite garnet zoning and inclusion patterns support a prograde evolution from low pressures for at least some of the samples the post eclogite stage granulite facies overprint indicates that high temperatures prevailed during exhumation but preservation of zoning in some garnets and the results of diffusion modelling suggest that this overprint took place over a very short time scale the geochemical and petrological results allow characteristic differences to be recognized between these eclogites and metabasites found in other tectonic units of south bohemia and consequently the assigning of all high pressure rocks to a single now disrupted tectonic unit is a gross simplification that seriously misrepresents the tectono metamorphic history of the region

feti oxide mineralogy and the origin of normal and reverse remanent magnetization in dacitic pumice blocks from mt shasta california detailed mineralogical analyses and rock magnetic experiments have made it possible to directly identify the fe ti oxide phases responsible for the normal and reverse magnetic components of two dacitic pumice blocks from mt shasta california both samples contain a normal component carried by 100 m size multi domain md titanomagnetite usp11 24 one sample also contains a second normal component carried by 10 m size pseudo single domain psd or single domain sd ti free magnetite usp1 found in the dacitic glass the md titanomagnetite and psd or sd magnetite dominate the strong field magnetic signal but only the psd or sd magnetite has any influence on the remanence signal unlike the strong field signal the remanence signal of both samples is dominated by a reverse nrm component this reverse component is carried by 100 m size ferrian ilmenite ilm53 65 the compositions of the ilmenites in both samples are within the range of compositions ilm50 75 known to have the ability to acquire self reversing thermoremanent magnetizations trm the results of the lowric fuller test indicate that the remanence signal is dominated by psd or sd carriers because one sample contains only large md titanomagnetite and no sd ti free magnetite in addition to ferrian ilmenite the ferrian ilmenite must be a psd or sd carrier oxide and pyroxene geothermometry indicate the fe ti oxides in the pumice crystallized at temperatures between 880 and 945 c this temperature range is within the disordered region of the ilmenite hematite phase diagram for ilm53 65 previous work on synthetic ilm70 and ilm80 has shown that cooling through the order disorder transition into the ordered region develops a transformation induced microstructure consisting of cation ordered domains with disordered domain boundaries an ilm58 59 grain from one of the mt shasta samples was examined in the transmission electron microscope and was found to contain 100 200 a diameter cation ordered domains these domains arose during cooling through the transition temperature which is estimated at 800 c for ilm58 59 the presence of the disordered domain boundaries provides an explanation for the magnetic behavior of the ferrian ilmenite 1 the disordered boundaries are the higher curie point phase necessary for the operation of the self reversal mechanism 2 the disordered domain boundaries either inhibit the formation of magnetic domain walls or restrict magnetic domain wall movement accounting for the psd or sd behavior of the ferrian ilmenite

importance of titanohematite in detrital remanent magnetizations of strata spanning the cretaceous paleogene boundary hell creek region montana intermediate composition titanohematite fe2 ytiyo3 with 0 5 y 0 7 is seldom the focus of paleomagnetic study and is commonly believed to be rare in nature while largely overlooked in magnetostratigraphic studies intermediate titanohematite has been identified as the dominant ferrimagnetic mineral in an array of late mesozoic and early cenozoic laramide clastic deposits throughout the central united states intermediate titanohematite is ferrimagnetic and has similar magnetic properties to titanomagnetite except its unique self reversing property due to these similarities and with detrital remanent magnetizations masking its self reversing nature intermediate titanohematite is often misidentified in sedimentary deposits past studies relied upon nonmagnetic techniques including x ray diffraction and electron microprobe analysis while these techniques can identify the presence of intermediate titanohematite they fail to test whether the mineral is the primary recorder to facilitate the identification of intermediate titanohematite in sedimentary deposits we characterize this mineral using low temperature magnetometry and high temperature susceptibility experiments and present a new identification technique based on titanohematite s self reversing property for sediments that span the cretaceous paleogene boundary hell creek region montana results from the self reversal test indicate that the majority of remanence is held by minerals that become magnetized parallel to an applied field but that intermediate self reversing titanohematite y 0 53 0 63 is an important ancillary carrier of remanence while earlier literature suggests that intermediate titanohematite is rare in nature reanalysis using specialized rock magnetic techniques may reveal that it is more abundant in the rock record particularly within depositional basins adjacent to calc alkaline volcanics than previously thought

some aspects of magnetic viscosity in subaerial and submarine volcanic rocks summary the magnetic viscosity of 334 upper tertiary and pre bruhnes quaternary volcanic rocks from the massif central france and the steens mountain oregon and of 40 basaltic cores from dsdp leg 37 has been investigated thellier s viscosity index follows a log normal distribution with mean values equal to 6 and 3 5 per cent for subaerial and submarine rocks respectively for subaerial rocks the average intensity of the viscous remanent magnetization vrm acquired in the earth s field since the beginning of the bruhnes polarity epoch t 0 7 myr is estimated to be equal to one quarter of the average intensity of the primary remanence alternating field demagnetization of vrms acquired in low fields for acquisition times t ranging from 2 day to 32 month indicates the resistance to alternating fields is quite different from sample to sample and increases linearly with log t neel s diagnostic parameter of domain structure of the grains involved in magnetic viscosity shows that hard vrm is carried by single domain grains and soft vrm carried by multidomain particles single domain particles carrying hard vrm in subaerial volcanic rocks are almost equant magnetite intergrowths with size near the superparamagnetism threshold resulting from high temperature oxidation of titanomagnetite soft vrm is carried by low curie point homogeneous titanomagnetite unlike his single domain theory neel s multidomain theory of magnetic viscosity does not account quantitatively for the resistance of vrm to alternating fields

hercynian thrust related shear zones and deformation of the varied group on the contact of granulites southern moldanubian bohemian massif the three deformation phases inferred from the detailed structural analysis of the cesky krumlov varied group record the hercynian development of the southern moldanubian of the bohemian massif the deformation is related to the nw se thrusting of the large crustal units including granulites the formation of the ns and nw se trending shear zones is connected with the thrust movement the structural development begins with f1 isoclinal fold formation that could originate in unmetamorphosed sediments in the final stage they were strongly flattened and b1 boudinage developed in the rocks aplite dykes and migmatitization of paragneisses occurred at the same time the rocks were metamorphosed the d2 deformational phase was produced by the simple shear deformation of the unit and folds of various styles around rigid inclusions and the strike slip shear zone near the boundary of the granulite and the varied group were formed the f1 and f2 folds are parallel with the stretching and mineral lineation indicating a ns to nw se direction of the thrusting the youngest deformation is characterized by spectacular boudinage and by folding of the vertically oriented planes

micromagnetic rockmagnetic and mineralogical studies on dacitic pumice from the pinatubo eruption 1991 phillipines showing self reversed trm detailed micromagnetic rockmagnetic and mineralogical investigations on dacitic pumice from the eruption of mt pinatubo 1991 phillipines showing reversed nrm trm are presented two intergrown hemoilmenite phases in chemically zoned particles were detected as being responsible a rim or covering phase consisting of a weak ferromagnetic disordered hemoilmenite phase low saturation magnetization but magnetically hard like hematite fetio 3 content 53 57 mol percent and a core phase consisting of a ferrimagnetic ordered hemoilmenite phase high saturation magnetization but magnetically softer constant fetio 3 content 58 mol percent preliminary studies were carried out in order to elucidate the magnetic interactions possibly exchange coupling responsible for the self reversal of the nrm trm a preliminary and schematic model is presented which summarizes the main points of acquisition of a self reversed trm according to out results

hemoilmenite as a carrier of srtrm in dacitic pumice from akagi ontake and sambe volcanoes japan abstract hemoilmenite crystals 0 7 x 0 4 x ilmenite molecular fraction in ilmenite hematite solid solution series in dacite pumice in japanese arc volcanoes are common however only a small percentage of those pumices show classic self reversed thermo remanent magnetization srtrm such as that first found in a haruna dacite pumice the dacite pumice samples from akagi volcano in central japan about 110 km nw of tokyo n36 33 e139 12 however acquired an intense srtrm like that of the haruna volcano samples bitter patterns of the oriented hemoilmenite crystals from both the haruna and akagi volcanoes indicate that ordered ferrimagnetic and disordered weak ferromagnetic phases are inter grown in relatively comparable amounts with an irregular texture this pattern is consistent with rapid quenching of these pumices we conclude that not only the chemical composition tio 2 content or x of the hemoilmenite but also the cooling speed or thermal condition on cooling control srtrm acquisition in hemoilmenite of appropriate composition

thermally induced inversion of al substituted titanomagnetite in basalts evidence for partial self reversal 1 we report properties of laboratory induced thermoremanence combined with detailed rock magnetic experiments on basaltic samples which contain aluminum substituted titanomagnetite fe2 24ti0 64al0 12o4 prior to thellier coe paleointensity experiments specimens were first demagnetized using an alternating field with a peak field of 90 mt and were then imparted an anhysteretic remanent magnetization arm to represent a natural remanent magnetization nrm results show that the partial thermal remanent magnetization ptrm gain and arm remaining are linearly correlated up to 300 c between 300 and 460 c specimens acquire thermoremanence with a direction antiparallel to the external field direction leading to intensity decreases this coincides with the onset of the mineral transformation from al titanomagnetite to a magnetic phase with higher tc our results support that phase interaction with distinct tc values in partially oxidized titanomagnetite can produce partial self reversal thermoremanence in rocks

low temperature behavior and memory of iron rich titanomagnetites mt haruna japan and mt pinatubo philippines we report low temperature remanence and memory of octahedral crystals of titanomagnetite from mt haruna japan and mt pinatubo philippines the crystals have curie temperatures of 460 490 c indicating a low ti content 0 11 x 0 16 saturation isothermal remanence sirm produced at 20 k decreased rapidly in warming through the isotropic temperature 42 k ti 55 k where the first magnetocrystalline anisotropy constant k1 changes sign and momentarily vanishes 96 99 of the original sirm was demagnetized at ti and none was recovered in recooling from 300 k to 20 k sirm produced at 300 k decreased continuously in the course of zero field cooling leveling out around ti on rewarming to 300 k 45 60 of the initial sirm was recovered this memory or recovered remanence is very large compared to that of magnetite and must be due to domain walls that are more strongly pinned probably magnetostrictively by crystal defects which are common in natural crystals the present low temperature experiments provide a quick non destructive method of identifying iron rich titanomagnetites in soils sediments and rocks

capillary heterogeneity trapping of co2 in a sandstone rock at reservoir conditions 1 the storage of co2 in deep subsurface porous rocks is being developed worldwide for the mitigation of emission from large industrial sources such as power plants and steel manufacturing a main concern of this technology is in ensuring that the upwardly buoyant co2 does not migrate to the surface simulation studies suggest that substantial amounts of co2 can be trapped within permeable sections of a reservoir by capillary forces and intra reservoir heterogenities but there is little experimental observation of these phenomena we report the results of co2 core flooding experiments at high pressure and temperature performed to investigate the impact of natural capillary heterogeneity in a sandstone rock on co2 saturation buildup and trapping co2 and water were injected through a mt simon sandstone core at 9 mpa pore pressure and 50 c the core had two regions of distinct capillarity an upstream 10 cm long region of the core consisted of a relatively high permeability and homogenous sand a downstream 3 cm long region of the core consisted of a low permeability region characterized by significant cross bedding and a high capillary entry pressure for co2 during a drainage process of co2 displacing water co2 builds up upstream of the capillary barrier once in place co2 on the upstream side of the barrier cannot be displaced during 100 water flooding leading to trapped saturations that are a factor 2 5 times higher than what would be expected from residual trapping alone

paleomagnetic dating of the smectite to illite conversion testing the hypothesis in jurassic sedimentary rocks skye scotland 1 a hypothesized connection between clay diagenesis and magnetite authigenesis is supported by the results of paleomagnetic rock magnetic geochemical and petrographic studies on jurassic sedimentary rock of skye scotland the results have implications for understanding remagnetization mechanisms and for the development of a paleomagnetic method to date clay diagenesis previous diagenetic studies indicate that rock in north skye contains abundant detrital smectite whereas the clays in the same age rock in south skye have altered to illite because of tertiary igneous activity geochemical 87sr 86sr 13c 18o studies confirm that sedimentary rocks in south skye are altered the magnetization in the rocks in north skye is weak and unstable in contrast the rocks in south skye contain a multicomponent magnetization at intermediate temperatures 225 450 c a magnetization with southerly declinations and negative inclinations or its antipodal equivalent is present at higher temperatures 450 580 c a magnetization with northerly declinations and positive inclinations or its antipodal equivalent is removed both magnetizations have directions similar to tertiary igneous rocks on skye rock magnetic studies indicate the presence of pyrrhotite and magnetite the intermediate temperature component resides in pyrrhotite and or magnetite and is interpreted as a thermoviscous remanent magnetization or a thermochemical remanent magnetization related to the heat and or hydrothermal activity associated with the igneous activity the high temperature component resides in magnetite and is a chemical remanent magnetization crm the presence absence test and the timing of acquisition for this crm suggest that magnetite authigenesis is related to the smectite to illite conversion and that clay diagenesis is a viable remagnetization mechanism

evidence from zircon dating for existence of approximately 2 1 ga old crystalline basement in southern bohemia czech republic zircon ages are reported for three moldanubian amphibolite grade orthogneisses from the southern bohemian massif obtained by conventional u pb analyses for two of these orthogneisses conventional u pb data are supported by ion microprobe single zircon ages or single grain evaporation data the amphibolite grade orthogneisses occurring in three small tectonic lenses within the varied group close to the south bohemian main thrust are of tonalitic granodioritic or quartz dioritic composition

kinematics of the variscan thrusting in the eastern moldanubicum bohemian massif czechoslovakia evidence from the naměst granulite massif abstract the tectonic history of the nam st granulite massif eastern part of the bohemian massif moldanubian zone czechoslovakia is interpreted in terms of polyphased generally north directed variscan nappe stacking original acid granulites formed during a presumed phase of lithospheric subduction 750 c 11 14 kbar 370 350 ma were converted into ht mylonites during the dominant deformation phase d 1 as a consequence of complex thrust wrench processes related to intra moldanubian deep crustal stacking 700 c 9 6 kbar 350 330 ma bringing granulites and the surrounding gfohl gneiss on top of the moldanubian nappe sequence this phase was followed by a northeastward superposition of the already stacked moldanubian nappe over the moravian zone phases d 2 d 3 650 400 c 5 3 kbar 335 325 ma which also marginally affected the cadomian basement of the brunovistulicum the variscan tectonic history was terminated by a namurian to lower permian phase of sinistral strike slip movements along nne ssw trending faults

u pb zircon and sm nd dating of moldanubian hp ht granulites from south ohemia czech republic conventional multigrain u pb systematics of zircon and single grain evaporation data of zircons in combination with nd model ages are presented for pyroxene free felsic garnetiferous granulites and orthopyroxene bearing granulites of felsic mafic and intermediate composition from the blansky les mountains granulite complex s bohemia zircons from mafic and intermediate granulites yield concordia intercept ages between 346 5 and 351 6 ma as well as a single grain evaporation age of 346 12 ma these ages define the time of variscan high grade metamorphism in s bohemia and are supported by an internal sm nd garnet zircon plagioclase whole rock isochron age of 343 21 ma euhedral zircons from felsic granulites of probable granitic derivation yield u pb ages of 366 5 ma and 373 12 ma interpreted as intrusion ages of the granitic precursor an inherited proterozoic crustal component is documented by poorly defined concordia upper intercept ages around 1700 ma and nd model ages between 1 3 and 2 1 ga

physics of gravity fingering of immiscible fluids within porous media an overview of current understanding and selected complicating factors abstract while gravity fingering of water in unsaturated sands has been demonstrated and studied under constrained laboratory conditions its significance under field conditions in the vadose zone is still under debate we review current understanding of the gravity fingering process as developed through linear stability analysis and laboratory experiments with respect to many complicating factors inherent in the field this understanding is deficient in determining the field conditions where gravity fingering can be expected and the behavior of the field scale fingering process when it occurs this deficiency is exemplified in the results we present of a field experiment conducted in a sandy alluvial deposit while simple extrapolation of current understanding of the process predicted instability fingers did not occur to begin to understand the influence of complicating factors inherent in the field we conducted several laboratory experiments that address three important complicating factors uniform and non uniform initial moisture content media heterogeneity and the presence of macropores and fractures these factors can fundamentally alter the gravity fingering process its scale of expression and under many field conditions suppress its occurrence entirely while the significance of gravity fingering of water in a wettable vadose zone is still inconclusive gravity fingering will likely play a significant role in water napl non aqueous phase liquid or water napl air systems as occur within the saturated and vadose zones respectively at many contaminated industrial sites to demonstrate this role the results of a simple experiment are presented

thermobarometry diffusion modelling and cooling rates of crustal garnet peridotites two examples from the moldanubian zone of the bohemian massif abstract different origins for crustal garnet peridotites are illustrated by two variscan bodies in adjacent tectonically juxtaposed terranes in the moldanubicum of the bohemian massif eastern moravia the nove dvory garnet peridotite contains lenses of eclogite and is allochthonous and allofacial with respect to surrounding gfohl gneiss the mohelno spinel garnet peridotite is devoid of eclogite and has concordant contacts with surrounding felsic granulite the bulk of the mohelno body is spinel peridotite and garnet occurs only along its margins where it has formed at the expense of spinel thermobarometry of mineral cores from garnet bearing assemblages yields values for nove dvory from 1195 c 54 6 kbar to 970 c 41 8 kbar and for mohelno from 1265 c 27 9 kbar to 1070 c 23 4 kbar the compositional zoning profiles in garnets at both localities are matched closely by diffusion modelling in which bilinear cooling rates are utilized at nove dvory initial cooling rates of 50 200 c ma are followed by slower rates of 10 50 c ma in contrast at mohelno an extremely rapid initial cooling rate of 1 c year was followed by one of 500 c ma an idealized tectonic model is proposed for the emplacement of the mohelno peridotite to account for its cooling history the predicted garnet zoning profiles that result from conductive cooling agree closely with the observed zoning profiles both peridotites were derived from the upper mantle the nove dvory body was emplaced from a deep fault zone into the crust as garnet peridotite and the mohelno body was emplaced as a hot slab of spinel peridotite in which garnet formed only along contacts as the body cooled into the garnet stability field

remanent magnetization of andesitic and dacitic pumice from the 1985 eruption of nevado del ruiz colombia reversed due to self reversal the natural remanent magnetization of andesitic pumice emitted during the 1985 eruption of the nevado del ruiz volcano colombia has a direction opposite to the present geomagnetic field the self reversing mechanism can be re activated in the laboratory during cycles of heating and subsequent cooling in air and zero magnetic field laboratory produced thermoremanent magnetization is dominated by the same self reversal process in fields up to several mt microchemical optical and curie temperature analyses indicate that the ferromagnetic minerals are members of the magnetite ulvospinel and hematite ilmenite series with average compositions of fe2 73ti0 27o4 and fe1 38ti0 62o3 respectively in analogy with the magnetic behaviour of synthetically grown antiferromagnetic ferromagnetic femn feni films the self reversal can probably be interpreted in terms of an exchange field acting between a ti poor canted antiferromagnetic and a ti rich ferrimagnetic phase in the hemoilmenite grains

mineralogical change and self reversed magnetizations in pyrrhotite resulting from partial oxidation geophysical implications abstract low field thermomagnetic studies performed on natural and synthetic pyrrhotite samples showed that a very low oxygen fugacity is already sufficient to start the transformation into magnetite at 500 c the quantity of magnetite produced is a function of the oxidation time during which the temperature is maintained the remanent magnetization acquired by the magnetite during its formation in the applied field is a chemical remanent magnetization crm in the case of partial oxidation the close coexistence of pyrrhotite and magnetite in a single grain may produce a self reversed thermoremanent magnetization in pyrrhotite upon cooling in zero field if the initial pyrrhotite is hexagonal oxidation is necessary to obtain the transformation into the monoclinic variety which may also show the self reversed trm in contrast to the natural samples in synthetic pyrrhotite the magnetite produced by oxidation carries a crm but shows no self reversal upon cooling to room temperature a model for a self reversal and structural change processes following oxidation is presented in which the size and the shape of pyrrhotite grains play an important role the consequences for palaeomagnetism and magnetic anomaly interpretation are discussed

tectonometamorphic evolution of the bohemian massif evidence from high pressure metamorphic rocks the variscan orogenic belt of which the bohemian massif is a part is typically recognized for its characteristic low pressure high temperature metamorphism and a large volume of granites however there are also bodies of high pressure rocks eclogites garnet peridotites and high pressure granulites which are small in size but widely distributed throughtout the massif initially the high pressure rocks were considered to be relicts of a much older orogenic event but the increasing data derived from isotopic and geochronological investigations show that many of these rocks have palaeozoic protoliths metamorphic ages from the high pressure rocks define no single event instead a number of discrete clusters of ages are found between about 430 ma and the time of the dominant low pressure event at around 320 330 ma

extension analysis of stretched belemnites a comparison of methods abstract stretched belemnites and other fragmented rigid brittle inclusions offer great potential for estimating true extensions in deformed rocks in the standard ramsay method of analysis the elongation is calculated as 1 e l l o where l is the final end to end length of the fragmented belemnite and l o is the original length sum of the individual fragment lengths this method always underestimates the true extension usually by a substantial amount an apparently trivial modification of the standard method as used by hossain gives much improved extension estimates without any additional cost in time or effort the strain reversal method proposed by ferguson is the most accurate method currently available but involves more laborious data collection and subsequent computer processing the three methods were applied to 120 stretched belemnites in the external french alps the different extension values calculated by the three methods arise because ferguson s method is sensitive to the entire fracture and separation history hossain s method is sensitive to only part of that history and ramsay s method is history insensitive if strain history is of prime interest ferguson s method should be used but the ease speed and relatively good performance of hossain s method will make it attractive to many field geologists

low temperature partial magnetic self reversal in marine sediments by magnetostatic interaction of titanomagnetite and titanohematite intergrowths with various low temperature experiments performed on magnetic mineral extracts of marine sedimentary deposits from the argentine continental slope near the rio de la plata estuary a so far unreported style of partial magnetic self reversal has been detected in these sediments the sulphate methane transition smt zone is situated at depths between 4 and 8 m where reductive diagenesis severely alters the magnetic mineral assemblage throughout the sediment column magnetite and ilmenite are present together with titanomagnetite and titanohematite of varying compositions in the smt zone titano magnetite only occurs as inclusions in a siliceous matrix and as intergrowths with lamellar ilmenite and titanium rich titanohematite originating from high temperature deuteric oxidation within the volcanic host rocks these abundant structures were visualized by scanning electron microscopy and analysed by energy dispersive spectroscopy warming of field cooled and zero field cooled low temperature saturation remanence displays magnetic phase transitions of titanium rich titanohematite below 50 k and the verwey transition of magnetite a prominent irreversible decline characterizes zero field cooling of room temperature saturation remanence it typically sets out at 210 k and is most clearly developed in the lower part of the smt zone where low temperature hysteresis measurements identified 210 k as the blocking temperature range of a titanohematite phase with a curie temperature of around 240 k the mechanism responsible for the marked loss of remanence is therefore sought in partial magnetic self reversal by magnetostatic interaction of titano magnetite and titanohematite when titanohematite becomes ferrimagnetic upon cooling its spontaneous magnetic moments order antiparallel to the titano magnetite remanence causing an drastic initial decrease of global magnetization the loss of remanence during subsequent further cooling appears to result from two combined effects 1 magnetic interaction between the two phases by which the titano magnetite domain structure is substantially modified and 2 low temperature demagnetization of titano magnetite due to decreasing magnetocrystalline anisotropy the depletion of titanomagnetite and superior preservation of titanohematite is characteristic for strongly reducing sedimentary environments typical residuals of magnetic mineral assemblages derived from basaltic volcanics will be intergrowths of titanohematite lamellae with titanomagnetite relics low temperature remanence cycling is therefore proposed as a diagnostic method to magnetically characterize such alteration palaeo environments 2007 the authors journal compilation 2007 ras

self reversal and apparent magnetic excursions in arctic sediments the arctic oceans have been fertile ground for the recording of apparent excursions of the geomagnetic field implying that the high latitude field had unusual characteristics at least over the last 1 2 myrs alternating field demagnetization of the natural remanent magnetization nrm of core hly0503 6jpc from the mendeleev ridge arctic ocean implies the presence of primary magnetizations with negative inclination apparently recording excursions in sediments deposited during the brunhes chron thermal demagnetization on the other hand indicates the presence of multiple often anti parallel magnetization components with negative inclination components having blocking temperatures predominantly but not entirely below 350 c thermo magnetic tests x ray diffraction xrd and scanning electron microscopy sem indicate that the negative inclination components are carried by titanomaghemite presumably formed by seafloor oxidation of titanomagnetite the titanomaghemite apparently carries a chemical remanent magnetization crm that is partially self reversed relative to the detrital remanent magnetization drm carried by the host titanomagnetite the partial self reversal could have been accomplished by ionic ordering during oxidation thereby changing the balance of the magnetic moments in the ferrimagnetic sublattices

model of syn convergent extrusion of orogenic lower crust in the core of the variscan belt implications for exhumation of high pressure rocks in large hot orogens reflection seismic section field structural analysis and gravimetric modelling of orogenic lower crust in the core of a carboniferous orogenic root reveal details of the polyphase process of exhumation subvertical amphibolite facies fabrics strike parallel to former plate margins that collided in the nw the fabrics are developed in both mid crustal and lower crustal high pressure granulite units as a result of intensive nw se intraroot horizontal shortening driven probably by the west directed collision in granulites the steep fabrics originated as a result of extrusion of orogenic lower crust in a 20 km wide vertical ascent channel from lower crustal depths at 350 340 ma the large granulite bodies preserve older granulite facies fabrics documenting a two stage evolution during the exhumation process surface exposures of granulites coincide with the absence of subhorizontal seismic reflectors at depth suggesting preservation of the 20 km wide subvertical tabular structure reaching moho depths horizontal seismic reflectors surrounding the vertical channel structure corroborate a dominant flat migmatitic fabric developed in all tectonic units this structural pattern is interpreted in terms of subhorizontal spreading of partially molten orogenic lower crust in mid crustal levels 765 c and 0 76 gpa at 342 337 ma large massifs of extruded and progressively dismembered felsic granulites disturbed mid crustal fabrics in the surrounding horizontally flowing partially molten crust the horizontal mid crustal flow resulted in collapse of the supra crustal tepla barrandian unit interpreted as the orogenic lid along a large scale crustal detachment above the extruded lower crustal dome the presence of felsic granulites at the bottom of the orogenic root is considered to be a key factor controlling the exhumation of orogenic lower crust in large hot orogens