argon and halogen geochemistry of hydrothermal fluids in the loch ainort granite isle of skye scotland highly turbid alkali feldspars from the loch ainort granite 59 ma isle of skye have been analysed using the 40ar 39ar method to obtain chronological and chemical k cl br i information concerning their origin and hydrothermal alteration three methods of gas extraction have been applied to neutron irradiated samples laser probe spot fusion of feldspars in vacuo crushing of a feldspar quartz separate and laser stepped heating of the crushed residue apparent ages obtained by laser probe spot fusion are mostly similar to the 59 ma intrusion age analyses of relatively pristine regions give some high apparent ages 59 ma indicating the presence of small amounts of 40are excess 40ar crushing releases significant amounts of 40are but is dominated by an 40ara atmospheric 40ar component 84kr 36ar values obtained by crushing are higher than air and are consistent with air equilibration with fresh water at low temperature 20 c therefore 40ara was most probably introduced as palaeoatmospheric argon dissolved in the circulating hydrothermal fluids that interacted with the granite thus supporting a meteoric origin for the fluids stepped heating gives a flat age spectrum and an age of 56 4 ma crushing and stepped heating both released significant amounts of halogens with high br cl and i cl ratios excess xe is also present as indicated by the high 132xe 36ar values it seems likely that the halogen and possibly xe enrichments resulted from interaction of the meteoric fluids with jurassic sedimentary country rocks

laccolithic as opposed to cauldron subsidence emplacement of the eastern mourne pluton n ireland evidence from anisotropy of magnetic susceptibility the structural evolution and emplacement of the eastern mourne pluton was investigated using anisotropy of magnetic susceptibility ams measurements carried out on 112 oriented block samples and structural data from the host rocks from these new data cauldron subsidence as the emplacement mechanism is disputed and evidence for an alternative laccolithic style model involving inflation is presented this includes deflection and uplift of host rock bedding close to contacts and the magnetic fabric pattern which has a gentle dome geometry even close to contacts the magnetic lineations usually plunge down dip near the external margins but otherwise have a general ssw nne trend that diverges northward this suggests a northward directed inflow direction the model for the emplacement of the eastern mourne pluton is a laterally fed laccolith emplaced south to north the eastern margin is interpreted as a faulted contact facilitating the inflation of an asymmetrical breached laccolith

the neogene to recent rallier du baty nested ring complex kerguelen archipelago taaf indian ocean stratigraphy revisited implications for cauldron subsidence mechanisms 1 ups cnrs fre 2566 orsayterre department des sciences de la terre universite de paris sud f 91405 orsay cedex france bbonin geol u psud fr 2 cnrs umr 6524 magmas et volcans department de geologie petrologie geochimie universite jean monnet f 42023 saint etienne cedex 2 france 3 cnrs ep 125 department des sciences de la terre universite de nice sophia antipolis f 06108 nice cedex 2 france 4 department of geology university college dublin belfield dublin 4 eire 5 gemoc macquarie university sydney australia

hydrothermal alteration of tertiary igneous rocks from the isle of skye northwest scotland hydrothermal alteration of tertiary granites from skye involed the reaction of igneous amphibole biotite ferroaugite fayalite alkali feldspar plagioclase feldspar magnetite and ilmenite with aqueous fluid primarily to combinations of secondary alkali feldspar chlorite and montmorillonite lesser amounts of muscovite calcite epidote prehnite zeolite and pyrite also developed during mineralogical alteration of the granites there was a net addition to rock of h and a net loss of ca fe and probably na there is no positive evidence for a significant change in their al si mg or k contents hydrothermal alteration of skye granite involved not only large scale migration of 18o 16o d and h but also of ca fe and probably na mineral thermometry combined with textural observations suggests that chemical isotopic and most mineralogical alteration were synchronous and occurred at 350 450 c textural observations further suggest that the mechanism of isotopic exchange between granite and hydrothermal fluid primarily was solution reprecipitation

magnetic anisotropy of the redencao granite eastern amazonian craton brazil implications for the emplacement of a type plutons abstract a magnetic fabric study was performed on the redencao pluton in an attempt to understand its emplacement history the redencao pluton is part of the 1 88 ga anorogenic a type jamon suite that intruded 2 97 2 86 ga old archean granitoids of the rio maria granite greenstone terrane in the eastern amazonian craton northern brazil previous gravity survey indicates that the pluton is a 6 km thick tabular intrusion it is characterized by a concentric distribution of facies with rings of seriated and porphyritic granite that cut across the main facies of even grained monzogranites the whole set is intruded by leucogranites that occupy the center of the pluton petrographic examination magnetic susceptibilities coercivity spectra and thermomagnetic curves indicate that the magnetic fabric is primarily carried by coarse grained multidomain magnetite this is reinforced by the coincidence of magnetic susceptibility and remanence anisotropy principal axes the absence of solid state deformation features and the low anisotropy degrees indicate that the magnetic fabric is magmatic in origin the magnetic fabric displays a systematic pattern with all facies including the rings of porphyritic granite being characterized by concentric gently dipping foliations associated with gently plunging lineations only the central leucogranitic facies shows a slightly discordant pattern with steeply dipping fabrics at its northeastern sector an emplacement model by vertical stacking of successive magma batches is proposed for the construction of the redencao pluton which reconciles the tabular shape of the intrusion the petrographic and geochemical zoning and the magnetic fabric pattern initially two magma batches were emplaced as sills first the even grained monzogranite then the seriated and porphyritic granites which formed by mingling of a leucogranitic melt with the host biotite monzogranitic magma as attested by geochemical data and field evidence the final shape of the pluton was acquired after the intrusion and inflation of the central leucogranite giving raise to the concentric pattern of facies in map view

magnetic polarity stratigraphy of the tertiary igneous rocks of skye scotland summary palaeomagnetic results are given for 115 sites mainly from the central igneous intrusive complex and also radiometric dates by the 40ar 39ar step heating method these are combined with published dates and palaeomagnetic results for the lavas and regional dyke swarms with the aim of establishing the magnetic polarity stratigraphy igneous activity spanned at least the approximate interval 61 54 ma and a polarity sequence of at last r n r n r but could span seven polarity intervals r n r n r n r depending on the age of the loch ainort epigranite however these polarities cannot be identified with specific chrons of either the harland et al 1982 or berggren et al 1985 polarity time scales the skye igneous activity therefore spans more polarity intervals than in mull ardnamurchan or rhum and this is consistent with the radiometric ages interpretations of the results for the coire uaigneich and related granites and for the loch ainort epigranite present problems and partial solutions are offered

granite emplacement in an extensional setting an ams study of the magmatic structures of monte capanne elba italy abstract monte capanne elba is an excellent example of a granite pluton 42 km 2 emplaced 6 m y ago in a well known extensional environment that of the tyrrhenian sea its internal fabric has been studied using the magnitude of magnetic susceptibility and anisotropy measurements at 74 regularly distributed stations susceptibility magnitudes reveal a normal and asymmetrical petrographical zoning in the pluton the magnetic lineations are remarkably well organized mostly of nw se azimuth with slight plunges and always normal to the walls of the late aplitic dykes these lineations are also parallel to the stretching lineations of the immediate country rocks of the pluton in the western and probably deepest part of the pluton a ne sw trending magnetic magmatic lineation is also imprinted the following scenario is proposed for the emplacement of monte capanne during the opening of the tyrrhenian sea an approximately e w trending sinistral transform fault is taken as being responsible for the creation of a pull apart void that permitted upwelling of the magma orientations of the magmatic structures at depth are parallel to the elongation of the feeding zone of this void i e ne sw the very top part of the pluton was sheared parallel to the granite country rock interface i e nw se parallel to the local stretching of the tyrrhenian sea at the time of emplacement

the lebombo monocline and associated feeder dyke swarm diagnostic of a successful and highly volcanic rifted margin abstract as part of the karoo large igneous province lip the 700 km long and 182 ma old lebombo monocline outcrops inland of a younger mozambique lowland cover that separates it from the indian ocean detailed mapping of a particularly well exposed olifant river traverse across this monocline correlates basaltic and rhyodacitic lava units within the lebombo group to at least three hosted generations of intrusions within the monocline parallel northern lebombo dyke swarm implying that a much denser feeder dyke swarm is covered by younger flows measured attitudes and thicknesses of up to 197 lavas and 791 dykes statistically quantify a structural cross section from which a properly weighted lava log of nearly 5 km basalts inter bedded with an overlying 3 5 km of rhyodacites is constructed the geological section is compatible with a slightly deeper sub lava section through the innermost 30 km s of a relatively narrow east greenland margin to which the lebombo monocline can be rigorously compared in both cases field relationships are consistent with early tectonic extension being overwhelmed by dyke dilation within margin parallel swarm segments that may have been injected laterally from central magma chambers thus it is argued that the lebombo monocline is part of a successfully rifted margin that furthermore belongs to a distinct class of highly volcanic and relatively narrow margins which only form proximal to classical triple rift lip centers where an elevated magma flux sustains a magmatic mode of extension for the karoo lip this conforms with 1 a tight gondwana reconstruction of africa with antarctica 2 successful breakup during intense 182 ma emplacement of the lebombo group 3 triple rift formation together with a short lived 181 178 ma okavango dyke swarm and 4 anomalously thick igneous crust formation during continued 2 5 cm yr separation into a loose 155 ma plate configuration of africa and antarctica

the magnetic susceptibility of granitic rocks as a proxy for geochemical composition example from the saruhan granitoids ne turkey the magnetic susceptibility of rocks is determined by their bulk chemistry and magnetic mineralogy in this case study of the saruhan pluton in turkey a detailed survey based on 925 field measurements and 37 whole rock analyses reveals a strong correlation between magnetic susceptibility and feot r 2 0 8 petrographic observations and calculations based on whole rock major element analyses indicate the presence of magnetite grains further calculations show that magnetite accounts for 88 of the magnetic susceptibility the spatial correlation between feototal and the magnetic susceptibility is particularly spectacular which suggests that km is a proxy for geochemical differentiation the zoning pattern of magnetic susceptibility across the pluton is concentric and reverse with the highest values of magnetic susceptibility in the center this type of zoning is consistent with a previously proposed petrogenetic model the comparison of the saruhan granites with a classic example of paramagnetic granites reveals two contrasted feot km trends this suggests that these variations can be a useful tool to distinguish between leucocratic paramagnetic and leucocratic ferrimagnetic granites 2007 elsevier b v all rights reserved

emplacement of the early miocene pinto peak intrusion southwest utah usa 1 in this contribution we report rock magnetic petrographic and anisotropy of magnetic susceptibility ams data from the pinto peak intrusion all of which bear on volcanic construction rock magnetic data indicate that the dominant magnetic mineral phase is low ti titanomagnetite of multidomain grain size the composition of which varies spatially across the intrusion the intrusion is a porphyritic andesite dominated by ca rich plagioclase 60 as well as biotite amphibole olivine and opaque minerals reflected light petrography reveals mostly euhedral subhedral titano magnetite crystals that often form clustered glomerocrysts and stringers of equant crystals without exhibiting a consistent mineral alignment fabric moderate to shallow plunging prolate magnetic susceptibility ellipsoids dominate the northern part of the intrusion while steeply dipping plunging magnetic susceptibility ellipsoids are generally restricted to the southern part of the intrusion the vent facies rocks yield moderate to steep oblate susceptibility ellipsoids we interpret the flow pattern in the north to reflect subhorizontal flow of magma filling a tabular sheet like body associated with propagation of the intrusion to the north we argue that the southern part of the intrusion represents the ascent site of the magma rising to shallow crustal levels along a steep feeder system the oblate magnetic fabrics in the vent area plausibly represent flattening against the conduit walls as evidenced by a weak planar flow foliation observed in the vent conduit rocks on reaching shallow crustal levels the magma deformed and uplifted the roof rocks leading to gravitational instability as the slide mass released from the roof an explosive eruption ensued resulting in the emplacement of the rocks of paradise tuff and associated effusive lava flows following eruption magma pressure decreased and the magma drained northward forming the northern intrusive phase

the emplacement of the granitic las tazas complex northern chile the relationship between local and regional strain abstract the coastal batholith of northern chile grew under transtensional extensional conditions that prevailed along the andean margin during the mesozoic the batholith hosts the atacama fault zone a major arc parallel fault system which was characterised by sinistral transtensional shearing during the early cretaceous the las tazas complex is a composite granitoid intrusion that was emplaced syntectonically along the atacama fault zone at 130 ma syntectonic emplacement is indicated by a consistent kinematic history between the complex and its wall rocks together with synchronous crystallisation and shearing ages in contrast to regional patterns the las tazas complex was emplaced during a local change from vertical east side down to dextral transcurrent displacement along the fault zone during intrusion strain was partitioned between non coaxial simple shearing within country rock mylonites and a flattening strain across the crystallising complex this combination indicates that the pluton was emplaced under temporary transpressive conditions that were localised around the pluton probably induced by magma emplacement such a difference between local and regional strain suggests that emplacement related structures should only be related to regional strain states with great care

magnetic susceptibility patterns in a cordilleran granitoid the las tazas complex northern chile the las tazas complex is a ferromagnetic i type granitoid that crops out in the coastal cordillera of northern chile magnetic susceptibility patterns across the complex are controlled by modal magnetite contents rather than overall mineralogy resulting in an ambiguous correlation between susceptibility magnitude variation and lithological zoning there is a good correlation between plagioclase magmatic fabric orientations and magnetic fabric orientation demonstrating that anisotropy of magnetic susceptibility ams measurements can accurately characterize magmatic fabric patterns in cordilleran i type granitoids however there is no simple quantitative relationship between magmatic and magnetic fabric intensities the magmatic and magnetic fabrics share a similar character and it is clear that ams fabrics are strongly influenced by the dominant plagioclase fabric across the las tazas complex but cannot be directly equated in quantitative strain analysis

mesoscopic and magnetic fabrics in arcuate igneous bodies an example from the mandi karsog pluton himachal lesser himalaya field microstructural and anisotropy of magnetic susceptibility ams data from the palaeozoic mandi karsog pluton in the lesser himalayan region reveal a concordant relationship between fabric of the proterozoic host rock and the granite the pluton displays a prominent arcuate shape on the geological map the margin parallel mesoscopic and magnetic fabrics of the granite and warping of the host rock fabric around the pluton indicate that this regional curvature is either synchronous or pre dates the emplacement of the granite body mesoscopic fabric magnetic fabric and microstructures indicate that the northern part of the pluton preserves its pre himalayan magmatic fabric while the central and southern part shows tectonic fabric related to the tertiary himalayan orogeny the presence of nw se trending aplitic veins within the granite indicates a post emplacement stretching in the ne sw direction shear sense indicators in the mylonites along the margin of the pluton suggest top to the sw shearing related to the himalayan orogeny based on these observations it is envisaged that the extension that gave rise to this rift related magmatism had a ne sw trend that is normal to the trend of the aplite veins subsequently during the himalayan orogeny compression occurred along this same ne sw orientation these findings imply that the regional curvature present in the himachal lesser himalaya is in fact a pre himalayan feature and the pluton has formed by filling a major pre himalayan arcuate extension fracture

experiments on cauldron formation a polygonal cauldron and ring fractures abstract the sequence of formation of the motojuku type cauldron fujita et al 1970 is summarized as follows 1 doming due to ascent of a magma body 2 development of normal faults which produce a polygonal cauldron and 3 eruption of acidic to intermediate pyroclastic material model experiments on a scale of 1 200 000 reveal that ascent of the magma body which was imitated by a ball of hardened putty produced a polygonal cauldron composed of radial and concentrically arranged short fractures on the roof of a dome no ring fractures were formed this type of cauldron develops near the surface on the other hand emission of magma which was imitated by evaporation of a ball of dry ice in brittle powdered material caused ring faults dipping outward and a circular cauldron without up or downwarping this type of cauldron develops upward from the magma body the former model is equated to the motojuku type cauldron and the latter model to the other types of cauldrons with ring fractures

magnetic fabric vs magnetite and biotite shape fabrics of the magnetite bearing granite pluton of gameleiras northeast brazil abstract a well defined orientation pattern has been evidenced by the magnetic fabric lineation and foliation of the granite pluton of gameleiras northeast brazil this magnetic fabric is roughly parallel to the magmatic fabric defined by the elongate mafic enclaves and by the planar arrangement of tabular k feldspar phenocrysts low field anisotropy of magnetic susceptibility properties as well as magnetic mineralogy studies and optical microscopy observations indicate that multidomian iron rich magnetite is the main carrier of the magnetic susceptibility and its anisotropy to examine in more detail the relationships between the magnetic and magmatic fabrics measurements of two dimensional shape fabrics of grain populations of biotite and magnetite have been carried out on a total of 500 grains using an automatic shape fabric analysis procedure on carefully selected and oriented thin sections angular departure has been calculated between the maximum elongation of the mineral shape fabric of the given grain population and the maximum susceptibility axis of the corresponding macroscopic specimen we found that the fabric of biotite correlates strongly with the magnetic fabric 7 8 the fabric of magnetite although not as well defined mainly because of its weak shape anisotropy still correlates with the magnetic fabric 12 18 origin of low field magnetic fabric in the pluton of gameleiras is concluded to be controlled principally by the statistical alignment of the long axes of inequant magnetite grains magnetic interactions between the grains of magnetite where they form clusters may account for the observed scattering or abnormal magnitudes of the magnetic anisotropy concerning the orientation of the magnetic fabric as the long dimensions of the magnetites preferentially align with the boundaries of the other minerals particularly biotite and titanite the magnetic fabric as defined by the grains of magnetite is concluded to approximate closely the magmatic fabric defined by the shape fabric of biotite however as no correlation exists between shapes or clustering of magnetites and strain undergone by the deforming magma the strength of the magnetic fabric in ferromagnetic granites cannot be used in a simple way as a measure of strain intensity

distribution anisotropy the cause of ams in igneous rocks understanding of ams in pristine basic igneous rocks in terms of flow orientation of ellipsoidal magnetite grains is thwarted by the fact that these grains 1 typically are irregularly equant or skeletal rather than distinctly ellipsoidal and 2 usually crystallize interstitially and late after magma flow has ceased however the distribution of such grains will be relatively anisotropic if they grew in residual liquid volumes within a preferredly oriented by flow silicate template this constraint may impose a slight preferred shape orientation to the irregular magnetite grains during their growth but in addition we suggest it is the anisotropic magnetic interaction between the grains which is the basic cause of ams experiments involving the casting of magnetite epoxy mixtures in various foliated or lineated glass templates have yielded samples with the expected ams symmetry i e k3 perpendicular to glass plates or k1 parallel to glass rods slicing of near isotropic synthetic blank and natural rock cores and interleaving with glass slides showed a progressive increase in anisotropy of the blank from 0 4 to 2 and the natural rock from 0 9 to 2 9 in both the k3 axis migrates to be perpendicular to the glass slides

palaeomagnetic and anisotropy of magnetic susceptibility data bearing on the emplacement of the western granite isle of rum nw scotland the western granite is the largest of several granitic bodies around the margin of the rum central igneous complex we report palaeomagnetic and anisotropy of magnetic susceptibility ams data that bear on the emplacement and deformation of the western granite the collection includes samples from 27 sites throughout the western granite five sites in adjacent feldspathic peridotite and two sites in intermediate to mafic hybrid contact aureole rocks palaeomagnetic data from 22 of the 27 sites in the granite provide an in situ group mean d 213 2 i 69 5 95 5 5 that is discordant to an early paleocene reverse polarity expected field about 184 66 95 4 3 the discrepancy is eliminated by removing an inferred 15 of northwest side down tilting about a best fit horizontal tilt axis trending 040 data from the younger peridotite and hybrid rocks of the rum layered suite provide an in situ group mean of d 182 6 i 64 8 95 4 0 which is statistically indistinguishable from an early paleocene expected field and imply no post emplacement tilting of these rocks since remanence acquisition the inferred tilt recorded in the western granite which did not affect the younger layered suite suggests that emplacement of the ultrabasic rocks resulted in roof uplift and associated tilt of the western granite to make space for mafic magma emplacement magnetic fabric magnitude and susceptibility parameters yield two subtle groupings in the western granite ams data set group 1 data defined by rocks from exposures to the east and south have comparatively high bulk susceptibilities kmean 29 51 10 3 in si system stronger anisotropies pj 1 031 and oblate susceptibility ellipsoids group 2 data from rocks in the west part of the pluton have lower values of kmean 15 89 10 3 si and pj 1 014 and triaxial susceptibility ellipsoids magnetic lineations argue for emplacement of the granite as a tabular sheet from the south southeast toward the north and west moderate to steeply outward dipping magnetic foliations together with deflection of the country rock bedding in the north are consistent with doming accompanying magma emplacement

repeated tabular injection of high level alkaline granites in the eastern bushveld south africa field observations petrography and magnetic susceptibility fabrics are used to characterize the structure and evolution of the nebo granite forming part of the bushveld complex in south africa the nebo granite is the dominant component of the lebowa granite suite the largest known anorogenic granitoid complex it is a composite tabular intrusion comprising alkali feldspar granites and syenogranites there are seven main petrographic units defined by differences in modal content crystallization sequence magmatic water content magnetic susceptibility and magnetic fabric each unit was emplaced independently and fed by vertical dykes anisotropy of magnetic susceptibility used to map magmatic fabrics within the granite shows that they are dominantly controlled by alignment of early crystallizing magnetite and are similar in outcrop and pluton scale magnetic foliations and lineations are horizontal reflecting vertical host rock compression and horizontal magma flow during emplacement space was created by magma pressure which resulted in roof uplift and floor depression displacements were accommodated by normal and reverse faulting and flexural slip along bedding planes in host rock metasediments

magma emplacement into the lemptegy scoria cone chaine des puys france explored with structural anisotropy of magnetic susceptibility and paleomagnetic data the lemptegy volcano is a small monogenetic scoria cone located in the chaine des puys auvergne france which erupted about 32 000 years ago a first edifice lemptegy 1 formed during a trachybasalt eruption as a group of satellite vents of the puy de gouttes scoria cone a second trachyandesitic edifice lemptegy 2 formed soon after and completely covered lemptegy 1 with an 80 m high breached cone since 1946 the lemptegy volcano has been quarried for scoria and today offers unprecedented three dimensional exposure of the subvolcanic plumbing system to map the internal flow architecture of the plumbing system and to study the subvolcanic deformation of lemptegy 2 structural mapping petrographic observations anisotropy of magnetic susceptibility ams rock magnetic and paleomagnetic data were collected field structural mapping and thin section study of tension gashes riedel shears striations as well as ductile shear zones and bubbles allow the direction and sense of the magma flow to be determined twenty ams sites were established in ten dikes one to four sites in each dike with 504 specimens analyzed and 479 specimens used to infer magma flow patterns structural data the maximum susceptibility axis k 1 and the imbrication of the magnetic foliation k 1 k 2 planes indicate both upward and downward sense of flow as well as flow toward and away from the central vent rock magnetic experiments reveal that a cubic fe ti oxide phase likely low ti titanomagnetite is the principal magnetic phase carrying both the remanence and anisotropy paleomagnetic data from some sites yield statistically distinct at the 95 confidence level remanence directions while at other sites the data are indistinguishable at the 95 confidence level the paleomagnetic results observed steeply tilted scoria layers internal unconformities and faults show that as each dike was emplaced it displaced earlier dikes evidencing subvolcanic deformation the lemptegy 2 volcano shares similarities in terms of inferred eruption style and structures with other scoria cones such as cerro negro nicaragua and thus provides an excellent field laboratory to investigate active scoria cones world wide

spatial variation of bakken or lodgepole oils in the canadian williston basin there are two opposing schools of thought that infer either the bakken formation or the lodgepole formation as the primary source rock for the madison reservoired oils in the canadian williston basin a recent geochemical study revealed evidence indicating the existence of significant mixing of bakken and lodgepole oils in the madison reservoirs to investigate the geographic distribution of the oil compositions we employed a multivariate statistical method to extract source and maturity specific geochemical signatures from a geochemical data set for spatial analysis oil mixing appears to be geographically dependent and restricted by a northeast southwest striking zone torquay rocanville trend in southeast saskatchewan thus fracture or fault systems are inferred to have provided high permeability zones allowing bakken derived oil to migrate upward across the lodgepole formation the areas without significant fault or fracture systems favor lateral oil migration along porous beds restricting bakken derived oil accumulation to pools in bakken reservoirs and lodgepole derived oils to occur primarily in overlying reservoir beds of the madison group

a theoretical and experimental comparison of the anisotropies of magnetic susceptibility and remanence in rocks and minerals summary susceptibility thermo remanent magnetization trm and isothermal remanent magnetization irm anisotropy ellipsoids have been determined for several rock samples the results indicate that the ellipsoid of initial susceptibility is less anisotropic than the trm and low field irm ellipsoids which are found experimentally to be of identical shape this suggests that palaeomagnetic data for anisotropic rocks may be corrected by using the anisotropy ellipsoid determined from magnetically non destructive low field irm measurements such irm measurements can also be used to obtain anisotropy axes of samples which are inherently anisotropic but which have a susceptibility which is too weak to be accurately measured the results for a series of artificial anisotropic samples containing magnetite particles of different sizes in the range 0 2 90 m were very similar to those for the rocks in contrast a comparison of the susceptibility and irm ellipsoids for anisotropic samples containing particles from a magnetic tape gave very different results in accordance with theory such results imply that susceptibility and irm ellipsoids could be used to determine whether anisotropic rocks contain uniaxial single domain particles magnetization confined to the easy axis or whether the particles are essentially multidomain

formation of collapse calderas collapse calderas are common in many volcanic regions but the mechanics of their formation is not well understood i propose that collapse calderas may form as a result of the growth of associated shallow magma chambers excess magmatic pressure in a shallow chamber gives rise to tensile stresses at the surface but the location and magnitude of the maximum tensile stress depend on the shape of and depth d to the top of the chamber this model indicates that penny shaped chambers i e sill like chambers of circular horizontal cross sections are most likely to initiate collapse calderas for such a chamber with horizontal radius a the higher the a d radius depth ratio the greater the probability of forming a collapse caldera

a review of criteria for the identification of magmatic and tectonic foliations in granitoids abstract foliations in granitoids can form by magmatic flow submagmatic flow high temperature solid state deformation and moderate to low temperature solid state deformation a review of previous work suggests that no single criterion can consistently distinguish foliations in granitoids formed by flow during ascent diapiric emplacement and expansion emplacement during regional deformation or regional deformation post dating emplacement however a magmatic origin is favoured for foliations defined by the alignment of igneous commonly euhedral minerals particularly where the foliation is parallel to internal or external pluton contacts foliations formed during expansion or ballooning of diapirs may be strictly magmatic in origin although some studies suggest that solid state deformation also may occur if so we would hope to find evidence of deformation of crystal melt systems and that the solid state deformation occurred at high temperatures the inference of syntectonic foliations is most convincing where magmatic and high temperature solid state foliations are subparallel these foliations are continuous with regionally developed foliations in the wall rocks synkinematic porphyroblasts are present in the wallrocks and igneous minerals have the same age as metamorphic minerals associated with the regional cleavage a strictly tectonic origin for foliations in granitoids is favoured when the foliation is defined by metamorphic minerals no alignment of igneous minerals occurs the foliation is locally at high angles to pluton wallrock contacts and the foliation is continuous with a regionally developed cleavage

structure of a hypovolcanic acid complex inferred from magnetic susceptibility anisotropy measurements the western red hills granites skye scotland thulean igneous province the geochemistry and the injection mechanism of hypovolcanic ring dykes have been extensively studied but such is not the case for their internal fabric the tertiary western red hills epigranites of the isle of skye are a classic example of such intrusions using anisotropy of magnetic susceptibility measurements we present the first structural data of their internal magmatic fabric the magnetic foliations equated with the magmatic flow planes have strikes which roughly follow the walls of the different intrusions they dip steeply toward the convex wall of each intrusion the lineations or maximal magnetic susceptibility axes generally have shallow plunges except in the latest granite intrusion these structures appear to be related to the compressional deformation of each intrusion toward the end of its crystallization this shortening would be a consequence of a radial and compressive stress field acting after each injection of magma this radial stress field is interpreted as the effect of high magma pressures originating from the acid magma chamber underlying the ring dyke complex at a shallow depth

constraining models of the tectonic setting of the giant olympic dam iron oxide copper gold deposit south australia using deep seismic reflection data abstract in the gawler craton the completeness of cover concealing the crystalline basement in the region of the giant olympic dam cu au deposit has impeded any sufficient understanding of the crustal architecture and tectonic setting of its iocg mineral system to circumvent this problem deep seismic reflection data were recently acquired from 250 line km of two intersecting traverses centered on the olympic dam deposit the data were recorded to 18 s twt 55 km the crust consists of neoproterozoic cover in places more than 5 km thick over crystalline basement with the moho at depths of 13 14 s twt 40 42 km the olympic dam deposit lies on the boundary between two distinct pieces of crust one interpreted as the archean paleoproterozoic core to the craton the other as a meso neoproterozoic mobile belt the host to the deposit a member of the 1590 ma hiltaba suite of granites is situated above a zone of reduced impedance contrast in the lower crust which we interpret to be source region for its 1000 c magma the crystalline basement is dominated by thrusts this contrasts with widely held models for the tectonic setting of olympic dam which predict extension associated with heat from the mantle producing the high temperatures required to generate the hiltaba suite granites implicated in mineralization we use the seismic data to test four hypotheses for this heat source mantle underplating a mantle plume lithospheric extension and radioactive heating in the lower crust we reject the first three hypotheses the data cannot be used to reject or confirm the fourth hypothesis

basin structure and tertiary magmatism on skye nw scotland the emplacement of igneous material into upper crustal rocks of sedimentary basins is likely to be strongly controlled by the geometry of the pre existing basin structures these controls are investigated using examples from the tertiary igneous complexes of skye part of the sea of hebrides basin of nw scotland the basin consists of an array of half graben related to se dipping normal faults these pre volcanic mesozoic structures are traced near the igneous complexes using geological relationships preserved unconformably beneath the widespread basaltic lava fields the unconformity represents a period of cretaceous uplift and denudation of the basin and its flanks entirely pre dating the tertiary volcanism of nw scotland this unconformity seals stratigraphically the major basin faults preserving field relationships that permit the tracing of these faults in the country rocks to the tertiary intrusions the major camasunary fault is separated from the raasay fault via a series of minor graben linked by a series of steep nw se trending faults that transfered mesozoic displacements between the principal fault strands a broad range of igneous material of various compositions was intruded into part of the mesozoic sea of hebrides basins and their flanks during palaeocene times different emplacement styles and different structural controls are found the major gabbroic centres do not appear to be controlled by upper crustal structures having been emplaced into the footwalls of major faults however minor synmagmatic displacements on the basin faults may have been sufficient to generate dilatational sites in these footwall positions thereby facilitating emplacement in contrast the granitic melts have been emplaced as sheets and domed into the sediments and overlying lava pile reactivating segments of the basin fault network doming occurred from an array of sills the stratigraphic levels of which can be reconstructed using structural relationships preserved in the roofs and walls of the intrusions the sill levels and their transgressive forms are strongly related to inferred mesozoic basin structures the major fold structures of tertiary age in southern skye are interpreted as accommodating granitic emplacement rather than crustal shortening the nw se mesozoic transfer fault trend appears to have strongly influenced the segmentation of the granite domes these interpretations are illustrated using field relationships mapped in the vicinity of the coire uaigneich granophyre it is concluded that although the higher parts of the basin faults were reactivated to facilitate the doming of granitic intrusions the deeper levels of the mesozoic faults show no evidence of substantial reactivation

controls on emplacement of the caledonian ross of mull granite nw scotland anisotropy of magnetic susceptibility and magmatic and regional structures abstract anisotropy of magnetic susceptibility ams was measured from 139 sites across the exposed portion of the caledonian ross of mull granite argyllshire nw scotland to investigate the internal architecture of the pluton field and petrographic observations support the results of the ams study and a complementary suite of rock magnetic experiments was used to quantify the magnetic mineralogy of the granite ams data reveal a fabric that records a partial tectonic overprint of an emplacement related magma inflow fabric the partially preserved inflow pattern indicates a south to north emplacement of subhorizontal sheets that coalesced to form a tabular pluton the ams data are remarkably consistent across the intrusion with clear north south strikes and trends dominating the magnetic foliation and lineation patterns respectively these ams fabrics are discordant to internal zonation in the granite in the southernmost part of the granite ams data are parallel to bedding cleavage orientations preserved in many large 100 m proterozoic moine blocks the scarcity of moine blocks in the northern part of the intrusion and the prevalence of randomly oriented stoped blocks suggest that this part of the intrusion is nearer to the ceiling of the pluton the partial tectonic overprint consistent with east west compression occurred during the latter stages of emplacement and mainly affected the younger granite facies rm2 while the oldest granite facies rm1 and certain magmatic structures diorite enclaves preserve the original emplacement fabric an alternative explanation interprets the magnetic lineations as an intersection fabric between a shallowly dipping emplacement fabric and a steep tectonic fabric we argue that emplacement was not associated with eastward orogenic collapse of the scandian moine nappes as proposed by previous workers but occurred either before orogenic collapse or during a period of compressional reactivation

magnetic susceptibility and ams of the bushveld alkaline granites south africa the bushveld complex in south africa includes one of the world s largest anorogenic alkaline granite intrusions 66 000 km2 the granite forms a composite laccolith of 350 250 km in area and about 2 km in thickness which was emplaced at about 5 km depth into sediments overlying the kaapvaal craton at 2054 ma the bushveld granite and its roof rocks have long been mined for sn w and f the bushveld granites have high magnetic susceptibilities km from 1000 to 4000 si and a quantitative model is presented suggesting that susceptibility fabrics are primarily carried by ferromagnetic minerals the measured ams foliations coincide with observed subhorizontal mineral lineations and compositional layering magnetic lineation trends vary considerably within the horizontal plane the existence of a weak planar fabric and an almost absent linear component may reflect a laccolithic emplacement by roof uplift causing flattening magmatic fabrics or b emplacement of largely crystal free magma crystallizing in situ and developing horizontal compositional layering from thermal chemical diffusion fronts and gravity driven mechanisms weak magnetic fabrics like those identified in the bushveld granites require specific sampling schemes and procedures in addition to rigorous constraint of magnetic mineralogy and crystallization sequence