the fluid dynamics of a basaltic magma chamber replenished by influx of hot dense ultrabasic magma this paper describes a fluid dynamical investigation of the influx of hot dense ultrabasic magma into a reservoir containing lighter fractionated basaltic magma this situation is compared with that which develops when hot salty water is introduced under cold fresh water theoretical and empirical models for salt water systems are adapted to develop a model for magmatic systems a feature of the model is that the ultrabasic melt does not immediately mix with the basalt but spreads out over the floor of the chamber forming an independent layer a non turbulent interface forms between this layer and the overlying magma layer across which heat and mass are transferred by the process of molecular diffusion both layers convect vigorously as heat is transferred to the upper layer at a rate which greatly exceeds the heat lost to the surrounding country rock the convection continues until the two layers have almost the same temperature the compositions of the layers remain distinct due to the low diffusivity of mass compared to heat the temperatures of the layers as functions of time and their cooling rate depend on their viscosities their thermal properties the density difference between the layers and their thicknesses for a layer of ultrabasic melt 18 mgo a few tens of metres thick at the base of a basaltic 10 mgo magma chamber a few kilometres thick the temperature of the layers will become nearly identical over a period of between a few months and a few years during this time the turbulent convective velocities in the ultrabasic layer are far larger than the settling velocity of olivines which crystallise within the layer during cooling olivines only settle after the two layers have nearly reached thermal equilibrium at this stage residual basaltic melt segregates as the olivines sediment in the lower layer depending on its density the released basalt can either mix convectively with the overlying basalt layer or can continue as a separate layer the model provides an explanation for large scale cyclic layering in basic and ultrabasic intrusions the model also suggests reasons for the restriction of erupted basaltic liquids to compositions with mgo 10 and the formation of some quench textures in layered igneous rocks

the magmatic plumbing system of the askja central volcano iceland as imaged by seismic tomography the magmatic plumbing system beneath askja a volcano in the central icelandic highlands is imaged using local earthquake tomography we use a catalog of more than 1300 earthquakes widely distributed in location and depth to invert for the p wave velocity vp and the vp vs ratio extensive synthetic tests show that the minimum size of any velocity anomaly recovered by the model is 4 km in the upper crust depth 8 km below sea level bsl increasing to 10 km in the lower crust at a depth of 20 km bsl the plumbing system of askja is revealed as a series of high vp vs ratio bodies situated at discrete depths throughout the crust to depths of over 20 km we interpret these to be regions of the crust which currently store melt with melt fractions of 10 the lower crustal bodies are all seismically active suggesting that melt is being actively transported in these regions the main melt storage regions lie beneath askja volcano concentrated at depths of 5 km bsl with a smaller region at 9 km bsl their total volume is 100 km3 using the recorded waveforms we show that there is also likely to be a small highly attenuating magmatic body at a shallower depth of about 2 km bsl

paleomagnetism of the 1 1 ga lackner lake complex and tectonics of the kapuskasing structural zone the middle proterozoic lackner lake complex is a circular alkalic syenite carbonatite stock with a diameter of about 5 5 km it intrudes granulite rank archean gneisses in the kapuskasing structural zone of the wawa subprovince in the superior province of the canadian shield it adjoins the ivanhoe lake fault zone which forms the boundary with the abitibi subprovince and is the probable locus of maximum motion between the subprovinces specimens from 18 sites in the complex were analyzed paleomagnetically by alternating field and thermal demagnetization and by saturation isothermal remanence tests large recent viscous remanence components required removal before a stable remanence with a mean direction of 305 4 64 1 95 5 2 was isolated its pole of 53 7 n 156 5 w dp 6 7 dm 8 3 indicates emplacement at 1108 10 ma during a brief normal interval in a predominantly reversed polarity time this study indicates that there has been no postintrusion tilting of the kapuskasing structural zo

deformation of grimsvotn volcano iceland 1998 eruption and subsequent inflation 1 grimsvotn is a subglacial volcano under the vatnajokull ice cap at the center of the icelandic hotspot this highly active volcano erupted in december 1998 gps measurements at a single station on a nunatak at the caldera rim were made 7 times during 1992 2001 the measurements prior to the 1998 eruption reveal pre eruption inflation but subsidence of more than 15 cm was measured during the eruption following the eruption re inflation occurred initially at a rate of 20 mm month then declined to 5 mm month measurements were fitted to a mogi model assuming that the source was located under the center of the grimsvotn caldera complex results indicate a source depth of at least 1 6 km the calculated amount of magma outflow during the eruption is comparable to field estimates of the erupted volume grimsvotn continues to inflate but has not reached its 1998 pre eruption level

origin of the compositional diversity in the basalt to dacite series erupted along the heiðarsporður ridge ne iceland the hei arspor ur ridge located in the northern volcanic zone of iceland was formed approximately 9000 years ago by a volcanic episode known as ludent fires the episode produced a broad spectrum of different magma types forming approximately 50 small scoria cones and two larger craters ludent and hraunbunga the bulk compositions cluster in five distinct groups 1 olivine basalts 2 fe ti basalts 3 basaltic icelandites 4 icelandites and 5 dacites major and trace element trends together with mineral chemistry and isotopic ratios suggest that the dominant process involved in generating the evolved magmas was crystal fractionation occurring at variable depth an origin by polybaric differentiation is confirmed by melts modeling magma mixing played a dominant role in the formation of the basaltic icelandites additionally the fe ti basalts which erupted shortly after the dacites and used approximately the same vent area display unusually high concentrations of fe ti p and sr their composition is best explained by some pyroxene dominated fractionation prior to fe ti oxide stability and by entrainment of some crystal cumulate material at shallow depth mostly left over from the silicic differentiation stage textural and chemical features of the minerals e g presence of glomerocrysts two populations of plagioclase in these basalts support this interpretation of evolved cumulate remobilization fe ti basalts with the same field compositional and textural characteristics have also been erupted in the nearby but magmatically independent krafla volcanic system suggesting that a similar differentiation trend occurs also in this larger central volcano

volcano geodesy and magma dynamics in iceland abstract here we review the achievements of volcano geodesy in iceland during the last 15 years extensive measurements of crustal deformation have been conducted using a variety of geodetic techniques including leveling electronic distance measurements campaign and continuous global positioning system gps geodesy and interferometric analysis of synthetic aperture radar images insar results from these measurements provide a comprehensive view of the behavior of icelandic volcanoes between inflation intrusion and eruption episodes volcanoes are likely to deflate or show no sign of seismic activity subsidence rates are often in the range of a few millimeters to a few centimeters a year reducing progressively with time since the last eruption or intrusion at the volcano subsidence can be caused by cooling and contraction of magma outflow of magma it can be related to plate spreading volcano subsidence or lack of deformation is often interrupted by episodic magma flow towards near surface locations such magma recharge has been observed geodetically at hengill hekla eyjafjallajokull katla grimsvotn and krafla volcanoes with inflow inferred to last from a few months up to two decades in the last 15 years five volcanic eruptions three intrusive events and two m6 earthquakes have occurred in recent years the grimsvotn and katla volcanoes have exhibited continuous inflation of a few centimeters per year which at grimsvotn culminated in an eruption on 1 november 2004 hekla and torfajokull volcanoes have inflated at rates an order of magnitude less subsidence is occurring presently at the askja and krafla volcanoes within the period of geodetic measurement signals consistent with no deformation are typical for most of the 35 active volcanoes in iceland

experimental constraints on the low pressure evolution of transitional and mildly alkalic basalts the effect of fe ti oxide minerals and the origin of basaltic andesites one atmosphere melting experiments controlled at the fayalite magnetite quartz oxygen buffer on mildly alkalic and transitional basalts from iceland show that these begin to crystallize fe ti oxide minerals magnetite and or ilmenite at 1105 5 c apparently independently of bulk composition and the order of silicate and oxide mineral crystallization most samples crystalline plagioclase and olivine as the first two crystalline phases augite as the third phase and an fe ti oxide mineral as the fourth phase the main effects of fe ti oxide crystallization are a marked decrease in feo and tio2 in the liquid and a notable increase in sio2 and al2o3 and the minor oxides k2o and p2o5 with decreasing temperature the most silicic glasses are compositionally mugearitic and shoshonitic basaltic andesites because the smallest amount of glass that could be analyzed with the microprobe represents 20 55 percent liquid remaining it can be expected that more silicic liquids will occur at lower temperatures on normative pseudoternary projections the general effect of fe ti oxide crystallization for mildly alkalic and transitional basalts is a marked increase in normative quartz this is caused by a strong systematic convergence with the appearance of fe ti oxides of the bulk solid precipitates toward the liquid compositions as projected on the triangle plagioclase diopside olivine for alkalic basalts the bulk solid precipitate shows an increase in normative diopside with falling temperature and fe ti oxide crystallization this causes the liquids to move toward decreasing normative diopside and relatively little variation in nepheline the experimental observations imply that mildly alkalic and transitional magmas without stabilizing a fe ti oxide mineral will not evolve toward early silica saturation

magnetic properties of granitic rocks from the southern half of the idaho batholith influences of hydrothermal alteration and implications for aeromagnetic interpretation the measured magnetic susceptibilities of rocks from more than 600 outcrops in the southern idaho batholith have a good spatial correspondence with the observed intensities of magnetic anomalies koenigsberger ratios are low 0 3 in 75 of cored samples indicating that remanent magnetizations are unimportant relative to induced magnetizations which are related to susceptibilities in aeromagnetic anomaly interpretations in this region extremely low susceptibilities 25 10 5 within the eastern and western parts of the batholith are the sources of positive aeromagnetic anomalies the associated lithologies include mesozoic tonalite and granodiorite as well as a diverse tertiary group of epizonal stocks and batholiths intense meteoric hydrothermal activity associated with the emplacement of these tertiary plutons caused a reduction in in some rocks although coarse igneous magnetite is surprisingly resistant to hydrothermal corrosion and oxidation however under conditions of lower water rock ratios a significant increase in commonly occurred in the leucocratic plutons generally reflecting growth of hydrothermal magnetite because the measured susceptibilities do not necessarily represent more than the uppermost part of the 14 to 20 km thick crust that gives rise to magnetic anomalies an apparent discordance between susceptibility measurements and susceptibilities inferred from aeromagnetic data can occur such discordance patterns occur near the margins of the batholith where granitic rocks are thin and also in hydrothermally altered regions where subsurface tertiary plutons are present magnetic models suggest that the idaho batholith thickens to 20 km in its east central part which is underlain by enormous tertiary intrusive bodies

density changes during the fractional crystallization of basaltic magmas fluid dynamic implications the dynamical behaviour of basaltic magma chambers is fundamentally controlled by the changes that occur in the density of magma as it crystallizes in this paper the term fractionation density is introduced and defined as the ratio of the gram formula weight to molar volume of the chemical components in the liquid phase that are being removed by fractional crystallization removal of olivine and pyroxene whose values of fractionation density are larger than the density of the magma causes the density of residual liquid to decrease removal of plagioclase with fractionation density less than the magma density can cause the density of residual liquid to increase during the progressive differentiation of basaltic magma density decreases during fractionation of olivine olivine pyroxene and pyroxene assemblages when plagioclase joins these mafic phases magma density can sometimes increase leading to a density minimum calculations of melt density changes during fractionation show that compositional effects on density are usually greater than associated thermal effects in the closed system evolution of basaltic magma several stages of distinctive fluid dynamical behaviour can be recognised that depend on the density changes which accompany crystallization as well as on the geometry of the chamber in an early stage of the evolution where olivine and or pyroxenes are the fractionating phases compositional stratification can occur due to side wall crystallization and replenishment by new magma with the most differentiated magma tending to accumulate at the roof of the chamber when plagioclase becomes a fractionating phase a zone of well mixed magma with a composition close to the density minimum of the system can form in the chamber the growth of a zone of constant composition destroys the stratification in the chamber a chamber of well mixed magma is maintained while further differentiation occurs unless the walls of the chamber slope inwards in which case dense boundary layer flows can lead to stable stratification of cool differentiated magma at the floor of the chamber in a basaltic magma chamber replenished by primitive magma the new magma ponds at the base and evolves until it reaches the same density and composition as overlying magma successive cycles of replenishment of primitive magma can also form compositional zonation if successive cycles occur before internal thermal equilibrium is reached in a chamber in a chamber containing well mixed plagioclase saturated magma the primitive magma can be either denser or lighter than the resident magma in the first case the new magma ponds at the base and fractionates until it reaches the same density as the evolved magma mixing then occurs between magmas of different temperatures and compositions in the second case a turbulent plume is generated that causes the new magma to mix immediately with the resident magma

paleomagnetism of dykes from the groundhog river block northern ontario implications for the uplift history of the kapuskasing structural zone the groundhog river block gb forms the central segment of the kapuskasing structural zone a fault bounded belt of proterozoic crustal uplift in the southern superior province dykes adjacent to t

formation of oskjuvatn caldera at askja north iceland mechanism of caldera collapse and implications for the lateral flow hypothesis the oskjuvatn caldera at askja volcano north iceland was formed as a result of an explosive eruption at askja on 28th 29th march 1875 oskjuvatn is one of the youngest collapse calderas on earth and its initiation and growth were observed and documented by explorers and geologists working in the askja region between 1875 and 1932 it is an example of caldera formation where the volume of the caldera collapse far exceeds the volume of associated erupted products the discrepancy between the collapse volume and associated erupted products has been used by previous workers as a justification for the hypothesis that the injection of lateral dykes from askja fed the february to october 1875 n jahraun fissure eruptions at the sveinagja graben some 60 km north of askja however historical accounts documenting the caldera formation in sufficient detail show clearly that oskjuvatn formed by piecemeal collapse taking over 40 years to reach its current form we use these accounts to undertake a detailed examination of the stages of caldera collapse and to compare them with the known magmatic output of askja in the years following the 1875 eruption arcgis software was used to calculate the volume of oskjuvatn during the various stages of collapse and the volume of material erupted after 1875 while a dyke extending between askja and sveinagja may be implied to account for the missing volume our results demonstrate that the volume of oskjuvatn in july 1875 closely corresponds to the volume of rhyolitic material erupted on 28th 29th march 1875 in addition geochemical evidence indicates that the n jahraun magmas were sourced from a deeper reservoir rather than by lateral injection from askja s shallow crustal magma chamber we therefore suggest that the injection of sills and dykes in a coherent intrusion complex beneath askja central volcano combined with background deflation of the magma storage zone beneath oskjuvatn caldera over 30 to 40 years following 1875 satisfactorily accommodates the volume required to form oskjuvatn caldera

mechanical interaction between active volcanoes in iceland 1 we test the possibility of mechanical interaction between eight central volcanoes in the central part of the active iceland rift zone the average distance between the volcanoes is 30 km all are thought to have shallow magma chambers and many contain collapse calderas we analyzed many finite element models with the volcanoes subject to a tensile stress of 5 mpa equal to the maximum in situ tensile strength of the crust in a direction parallel to the spreading vector n105 e the results show zones between many nearby volcanoes where the tensile stresses exceed the in situ tensile strength of the crust the results indicate that mechanical interaction between volcanoes in a pair such as simultaneous dike emplacement seismogenic faulting and deformation may be common in this part of iceland in agreement with observations

slow dense replenishments of a basic magma chamber the layered series of the newark island layered intrusion nain labrador the newark island layered intrusion is a composite layered intrusion within the nain anorthosite complex labrador the intrusion comprises a lower layered series ls dominated by troctolites olivine gabbros and oxide rich cumulates and an upper hybrid series hs characterized by a wide range of mafic granitic and hybrid cumulates and discontinuous layers of chilled mafic rocks wiebe 1988 the hs crystallized from a series of replenishments of both silicic and basic magmas the ls crystallized from periodically replenished basic magmas the ls has a lower zone that consists mainly of olivine plagioclase cumulates and contains minor cryptic reversals in mineral compositions that resulted from replenishments of relatively primitive magma an upper zone is dominated by olivine plagioclaseaugite ilmenite cumulates cumulus titanomagnetite and pyrrhotite occur within some oxide rich cumulates and the stratigraphically highest layers contain cumulus apatite at intermediate levels in the sequence cumulus inverted pigeonite occurs in place of olivine several prominent regressions in the stratigraphy of the upper zone are marked by fine grained troctolitic layers with much higher mg no 100 mgo mgo feo and anorthite than underlying cumulates these layers coarsen upward and grade back to oxide bearing olivine gabbros within thicknesses ranging from 10 cm to 15 m dikes that cut the ls have major and trace element compositions that strongly suggest that they are feeders for the replenishments in the lower zone when olivine and plagioclase were the only cumulus phases replenishments were less dense than the resident magma and rose as plumes and mixed with it precipitation of cumulus oxides in the upper zone lowered the density of resident magma so that subsequent replenishments were more dense than resident magma replenishments that occurred after oxides began to precipitate had small injection velocities these post oxide injections flowed along the interface between resident magma and the cumulate pile and precipitated flow banded fine grained troctolites

paleomagnetism of the powder mill group michigan and wisconsin a reassessment of the logan loop the apex and the western arm of the 1 1 ga logan loop a prominent feature of the north american precambrian apparent polar wander path are defined by a large body of data from keweenawan rocks of the lake superior basin until now the eastern arm of this paleomagnetic feature has been largely defined by two data points from the powder mill group of michigan a reexamination of this sequence reveals a primary reversed magnetization which plots close to the apex of the logan loop data from the mellen complex in wisconsin together with our data from the powder mill group suggest that for steeply dipping volcanic units the true direction of a prefolding reversed magnetization is not obtained by simple structural rotation about presently observed strike reversely magnetized sites from localities where the powder mill volcanics have shallow dips yield a pole 146 w 46 n a95 9 2 on the apex of the loop a well defined stratigraphically continuous normal to reversed polarity change near the base of the powder mill group is not confirmed normally magnetized flows have been found at two localities only one of which is at the base of the lava sequence the higher metamorphic grade of normally magnetized sites suggests that normal magnetizations within the powder mill group are secondary and should not be used in definition of the eastern arm of the logan loop

magma chamber processes in central volcanic systems of iceland constraints from layered gabbro of the austurhorn intrusive complex new field work and petrological investigations of the largest gabbro outcrop in iceland the hvalnesfjall gabbro of the 6 7 ma austurhorn intrusive complex have established a stratigraphic sequence exceeding 800 m composed of at least 8 macrorhythmic units the bases of the macrorhythmic units are composed of 2 10 m thick melanocratic layers rich in clinopyroxene and sometimes olivine relative to the thicker overlying leucocratic oxide gabbros while the overall compositional variation is limited mg clinopyroxene 72 84 an plagioclase 56 85 the melanocratic bases display spikes in mg and cr2o3 of clinopyroxene and magnetite indicative of magma replenishment some macrorhythmic units show mineral trends indicative of up section fractional crystallisation over up to 100 m whereas others show little variation two populations of plagioclase crystals large an rich and small less an rich indicate that the recharge magma carried plagioclase xenocrysts high an type the lack of evolved gabbros suggests formation in a dynamic magma chamber with frequent recharge tapping and fractionation modelling of these compositional trends shows that the parent magma was similar to known transitional olivine basalts from iceland that had undergone about 20 crystallisation of olivine plagioclase and clinopyroxene and that the macrorhythmic units formed from thin magma layers not exceeding 200 300 m such a mushy magma chamber is akin to volcanic plumbing systems in settings of high magma supply rate including the mid ocean ridges and present day magma chambers over the iceland mantle plume the austurhorn central volcano likely formed in an off rift flank zone proximal to the iceland mantle plume during a major rift relocation

terminology for layered intrusions the terminology of igneous cumulates was originally formulated on the concept that the rocks were concentrations of minerals accumulated by crystal settling it is now well established for several major intrusions that plagioclase that had been typed as cumulus should in fact have floated in its parental magmatic liquid nevertheless the terminology has been valuable in the classification of the rocks of these and other intrusions and it has been useful in certain aspects of their genetic interpretation the name cumulate is therefore redefined so that crystal settling is a possible but not essential process in the origins of the rocks to which it is applied the overall nomenclature of layered intrusions is then reviewed and definitions of names are refined and systematized to clarify their applications careful distinction is made between descriptive and genetic names a cumulate is defined as an igneous rock characterized by a framework of touching mineral crystals and grains that evidently were concentrated through fractional crystallization of their parental magmatic liquids a layer is a sheetlike unit of cumulate that is a distinctive entity in its compositional and or textural features a lamina is a thin 2 5 cm sharply defined layer igneous lamination is a general name for planar structure that is pervasive through cumulates on the scale of their grain size layering is the overall structure and fabric manifest through combinations of layers laminae and lamination the difference between layers and layering is essentially the difference between individuals and a population and the terms that describe one are not necessarily appropriate to the description of the other but the fundamental variants of all of layering lamination grading in layers and layer contacts can essentially be described by one or more of the four modifiers grain size modal textural and cryptic where the last refers to cumulus miner al compositions a system for subdividing layered intrusions is proposed in which distinction is made between general divisions ranked as series zones and subzones and prominent units that are called groups members rhythmic units and cyclic units applications to the skaergaard bushveld muskox stillwater and other intrusions are briefly described

paleomagnetism of the late precambrian coldwell complex ontario canada abstract the coldwell complex is a large multiphase alkaline intrusion on the north shore of lake superior it was emplaced into archean metavolcanics of the superior province in at least three distinct magmatic episodes during the late precambrian giving isotopic ages ranging from 1010 to 1188 ma detailed af and thermal demagnetization was done on 416 specimens from 33 sites representing all petrologic phases additional data are included from an unpublished 1972 study by w a robertson the oldest magnetic component pca was found in 19 episode i sites with a reversed mean direction of d 121 0 i 70 9 104 95 3 3 its pole position falls on the keweenawan apwp at an age of 1109 5 ma the second component pcb was found in 11 sites with a normal mean of d 301 8 i 60 1 49 95 5 8 that indicates an age of 1103 5 ma the youngest component pcc was found in nine episode iii sites and gives a reversed mean of d 119 1 i 54 3 101 95 5 1 that indicates an age of 1095 5 ma the assigned ages agree closely with recent u pb zircon ages but are distinctly older than the k ar and rb sr ages these paleomagnetic results require some changes in existing models for the petrologic evolution of the complex and indicate a much more rapid three stage cooling history than previously postulated they also provide strong evidence against the proposed concept of an asymmetrical geomagnetic field in middle keweenawan time

an experimental study of the influence of oxygen fugacity on fe ti oxide stability phase relations and mineral melt equilibria in ferro basaltic systems

the crustal magma chamber of the katla volcano in south iceland revealed by 2 d seismic undershooting summary results of a 2 d seismic undershooting experiment on the katla central volcano in south iceland are reported large localized traveltime anomalies 0 4 s are observed on an array within the katla caldera the traveltimes are forward modelled using a wavefront tracker developed in appendix a thus non linear effects encountered in traveltime tomography are avoided as well as common problems with ray tracing in the presence of strong lateral heterogeneity an extreme variation in compressional velocity is required to extend over a significant volume in order to model the data the resulting model is not unique but constraints on the allowable range of velocities 2 5 6 0 km s render the basic features well constrained a clear s wave shadow is closely associated with delays in traveltime due to a shallow slow anomaly low amplitude p waves go hand in hand with early arrivals due to thin structural features flanking the slow anomaly the model is interpreted in terms of a magma chamber containing extensively molten rock the magma chamber is shallow with a bottom at a depth of about 1 5 km below sea level 3 0 km below surface and measures about 5 km across the depth of the chamber is roughly at the level of buoyant equilibrium for basaltic melt in the crust owing to poor vertical resolution at shallow depths in the undershooting geometry the top of this shallow magma chamber is not well resolved on the other hand the bottom of the chamber is well resolved the chamber is underlain by rocks of average or high velocity for that depth the magma chamber is a persistent feature big enough 10 km3 to supply magma for large eruptions and to supply heat to permit remelting of hydrated basaltic crust to produce silicic magmas at shallow levels the chamber is fed by magma fracturing from below the model agrees with phenomenological models of magma chambers in iceland based on geological observations and provides a quantification of those models in terms of depth and size on the other hand it is fundamentally different from recent models of magma chambers at mid ocean ridges which may be more akin to the pervasive region of partial melt at depth beneath iceland this underlines the important effect of the icelandic hotspot on tectonics and volcanism in iceland and implies a substantially different crustal and thermal structure in iceland from that of normal mid ocean ridges

a comparative 40ar 39ar study of the kapuskasing structural zone and the wawa gneiss terrane thermal and tectonic implications 40ar 39ar studies were performed on rocks from two traverses within the southern superior province of the canadian shield the racine traverse across the wawa gneiss terrane and the shawmere traver

archean crust as revealed in the kapuskasing uplift superior province canada in the central superior province of the canadian shield a 120 km wide transition from the low grade michipicoten greenstone belt to the high grade kapuskasing structural zone represents an oblique section through some 20 km of archean crust uplifted along a northwest dipping thrust fault the restored vertical section through the upper and middle crust consists of three megalayers with undulating boundaries 1 0 to 25 km a high grade heterogeneous gneissic assemblage in part older than the upper supracrustal succession

magma ascent and crustal accretion at ultraslow spreading ridges constraints from plagioclase ultraphyric basalts from the arctic mid ocean ridge plagioclase ultraphyric basalts pubs with up to 54 plagioclase phenocrysts were dredged in the rift valley and adjacent flanks of the ultraslow spreading mohns and knipovich ridges the pubs show large variations in crystal morphologies and zoning the large variations suggest that single basalt samples contain a mixture of plagioclase crystals that aggregated at different levels in the magma conduits resorbed crystals and repeated reverse zones suggest that the magma reservoirs were replenished and heated several times thin concentric zones with melt inclusions and sharp reductions in the anorthite content of 3 7 are common between the reverse zones these zones and skeletal crystals with distinctly lower anorthite contents than massive crystals are interpreted to be the result of rapid crystalliztion during strong undercooling the changes between short periods of cooling and longer periods with reheating are explained by multiple advances of crystal rich magma into cool regions followed by longer periods of gradual magma inflow and temperature increase the porphyritic basalts are characterizd by more depleted and more fractionated compositions than the aphyric basalts with lower la sm n k 2 oa nd mg numbers this relationship and the observation that pubs are sampled only close to segment centres along these ridges suggests that the pubs formed by higher degrees of melting and evolved in more long lived magma reservoirs we propose that the zoning patterns of plagioclase crystals and crystal morphologies of these pubs reflect the development and flow of magma through a stacked sill complex like conduit system whereas the aphyric equivalents represent later flow of magma through the conduit the formation of voluminous higher degree melts may trigger the development of the magma conduits and explain the generally depleted compositions of pub magmas

paleomagnetic evidence for post 2 55 ga tectonic tilting and 1 1 ga reactivation in the southern kapuskasing zone ontario canada we report paleomagnetic results from two traverses in the southern part of the kapuskasing structural zone ksz of north central ontario which is hypothesized to be an oblique cross section through upper and middle archean crust exposed by uplift and tilting anorthosites and granulite grade gneisses were sampled to the east in the shawmere traverse while amphibolite grade tonalitic gneisses were sampled to the west in the racine traverse three distinct remanent magnetization rm components were isolated by alternating field and thermal demagnetization the a rm has mean directions d 50 i 62 k 138 95 5 n 5 sites 23 samples for the shawmere traverse and d 23 i 53 k 49 95 7 n 8 sites 46 samples for the racine traverse because of the high 500 c laboratory unblocking temperatures of the a rm in the shawmere granulite grade rocks we believe this magnetization was acquired in early slow uplift without tilting from about 24 km to about 16 km depths and has survived the later main ksz uplift and tilting event the racine a rm has laboratory unblocking temperatures down to 400 c and was blocked prior to uplift from a shallower crustal level about 8 km with tilt corrections of 30 and 15 respectively about the 35 striking axis of the ksz the shawmere and racine a paleomagnetic poles fall on the laurentian apparent polar wander path around 2 55 ga dating the time of initial uplift crustal tilting in the ksz occurred after 2 55 ga and was greater in the east than in the west the b rm found mainly in shawmere anorthosites has d 115 i 68 k 17 95 17 n 4 sites 14 samples with a paleopole at 144 w 49 n near 1 1 ga keweenawan reverse poles and the normal pole of the nearby 1 0 ga nemegosenda carbonatite the b rm is probably a thermochemical overprint related to reactivation of the ksz at the time of keweenawan rifting and volcanism in the lake superior region a novel c rm with d 64 i 2 k 18 95 13 n 6 sites 10 samples is either an unresolved hybrid of the a and b rms or an earliest keweenawan 1 3 ga overprint

experimental study of liquid evolution in an fe rich layered mafic intrusion constraints of fe ti oxide precipitation on the t f o 2 and t ϱ paths of tholeiitic magmas the newark island layered intrusion a composite intrusion displaying a similar fractionation sequence to the skaergaard has both dikes which preserved liquids fed into the intrusion and chilled pillows of liquids resident in the chamber this study reports experimentally determined one atmosphere liquid lines of descent of these compositions as a function of oxygen fugacity which varies from qfm quartz fayalite magnetite to 0 5 log10 units above iw iron wustite these experiments reveal a strong oxygen fugacity dependence on the order of appearance and relative abundances of the fe ti oxide minerals titanomagnetite saturates prior to ilmenite at qfm but the order is reversed at lower oxygen fugacities in the layered series of the newark island intrusion ilmenite arrives shortly before titanomagnetite and the titanomagnetite ilmenite ratio decreases monotonically after the cumulus appearance of titanomagnetite comparison of the crystallization sequence in the intrusion with that of the experiments requires that the oxygen fugacity in the intrusion increased relative to qfm before titanomagnetite saturation and decreased afterward but always remained between the qfm and iw buffers similar trends in the modes of the fe ti oxides ilmenite and titanomagnetite in the skaergaard kiglapait and somerset dam intrusions along with fe2o3 feo ratios in morbs suggest that such a temperature oxygen fugacity path may be typical of tholeiitic magma differentiation calculations of the temperature density paths of the experimental liquids indicate that at all possible oxygen fugacities the density must have decreased abruptly after fe ti oxide saturation accordingly liquids replenishing the intrusion after fe ti oxide saturation should pond at the bottom of the chamber quenching against older cumulates field observation at the newark island intrusion confirm this prediction the similarities in the fractionation paths of several other layered intrusions to that of the newark island intrusion suggest that the density of the liquids in these intrusions also decreased after fe ti oxide saturation experiments on a suggested initial skaergaard liquid are consistent with this model

surface deformation at the krafla volcano north iceland 1982 1992 extensive measurements of ground deformation at the krafla volcano iceland have been made since the beginning in 1975 of a series of eruptions and intrusions into the fissure system that extends north and south of the volcano i concentrate on measurements before and after the eruption of september 1984 the last event of this series when the largest volume of lava was erupted the patterns of ground deformation associated with the 1984 eruption determined by precision levelling electronic distance measurements and lake level observations were similar to earlier intrusions and eruptions in that the surface of the volcano subsided and the fissure system widened as magma moved laterally from a shallow central reservoir into the fissure system the shallow magma reservoir of krafla continued to expand for about five years after the eruption but a slow subsidence of the central area began in 1989 besides the presence of an inflating and deflating shallow magma reservoir at a depth of 2 5 km beneath the krafla caldera another inflating magma reservoir may exist at much greater depth below krafla the accumulation of compressive strain by numerous rift intrusions and eruptions since 1975 along the flanks of the north south krafla fissure swarm is being released slowly and will probably be reflected in the results of deformation measurements near krafla for the next several decades the total horizontal extension of the krafla rift system in 1975 1984 was about 9 m equal to about 500 years of constant plate divergence the extension is twice the accumulated divergence since previous rifting events and eruptions in 1724 1729

paleomagnetism of the clay howells carbonatite complex constraints on proterozoic motion in the kapuskasing structural zone superior province canada abstract the clay howells complex is located approximately 130 km east of hearst ontario at 49 50 n 82 05 w near the north end of the kapuskasing structural zone ksz of the superior province in the canadian shield the pluton is a large oviform pluton of about 16 km2 that is composed dominantly of syenite with minor carbonatite it was emplaced into an archean gneissic terrain of amphibolite to granulite facies with middle precambrian diabase dikes the complex is unmetamorphosed and late precambrian in age rb sr 1075 15 ma multistep alternating field af and thermal demagnetization was carried out on 194 specimens from 18 sites in the intrusion and three sites in the host rock dikes and gneisses the 18 syenite sites give a unit mean direction of 294 2 27 1 n 18 k 26 95 7 0 which yields a pole position of 178 8 e 26 5 n p 4 1 m 7 6 for the clay howells complex a crude contact test using the three host rock sites proved inconclusive the pole for the intrusion is concordant falling at about 1080 10 ma on the keweenawan apparent polar wander path which agrees with the rb sr age thus the complex has an untilted primary remanence that indicates that there has been no significant uplift

faroe iceland ridge experiment 2 crustal structure of the krafla central volcano the seismic velocity structure of the krafla central volcano is characterized by large variations in compressional velocity a 40 km wide high velocity dome extends from the lower crust 11 14 km depth beneath the volcano narrowing upward a magma chamber sits at its top near 3 km depth it is defined by both 0 2 0 3 s compressional wave delays and shear wave shadowing to be 2 3 km n s 8 10 km e w and 0 7 1 8 km thick the near surface structure uppermost 2 5 km of the krafla caldera is approximately flat lying with only minor lateral heterogeneities the crust beneath the magma chamber has low shear wave attenuation and anomalously high compressional and shear wave velocities shear waves reflected from a 19 km deep moho are clearly visible for some paths through the crustal volume below the magma chamber even though the more shallow diving s waves are severely attenuated the midcrust beneath the shallow magma chamber cannot contain partial melt or even be at near solidus temperatures the krafla central volcano plays a major role in crustal genesis along the plate boundary the high velocity dome in our view represents crust generated in and around the magma chamber which has subsequently been advected to greater depths

episodic magmatism and diverse plutonic products within the thverartindur central volcanic complex at the former icelandic plate margin oceanic spreading concurrent with icelandic plume activity during the last 16 million years has created the icelandic crust the tertiary thverartindur central volcanic complex was located at the icelandic spreading margin c 5 7 ma ago and reveals evidence for episodic magmatic activity intrusive rocks range from ultramafic through olivine and quartz tholeiitic gabbros and hybrid rocks to granitic rocks the mafic intrusions form sill like bodies with compositions close to those expected for magmatic liquids field evidence indicates that episodic emplacement of basaltic magma took place over a considerable time span and confirms the contemporaneous existence of primitive and more evolved mafic magmas granitic and ultramafic rocks represent the latest phase of magmatism at a spreading margin the thverartindur rocks are depleted in mgo ni and cr which is compatible with early fractionation of olivine and pyroxene calculated parental liquid for this complex is picritic 18 wt mgo and resembles picritic lavas from the reykjanes peninsula and western skye and dykes from rum and mull the compositions of the thverartindur high magnesian dykes olivine and quartz tholeiitic gabbros and granites can be modelled through decompressional fractional crystallization of this magma the gabbroic sills are crystallized at low pressure following equilibrium crystallization paths

the effect of trapped liquid crystallization on cumulus mineral compositions in layered intrusions a series of calculations have been carried out to evaluate the effect on cumulus mineral compositions of solidification of trapped intercumulus liquid in orthocumulates the calculation assumes local equilibrium between phases and that the system remains chemically closed during crystallization of the trapped liquid the latter assumption is held to be valid on a scale of tens to hundreds of centimeters it is not necessary to know the composition of the trapped liquid as the calculation only requires an estimate of feo content and trapping temperature the change in composition of a mineral from that of the initially precipitated cumulus crystals to the final composition after complete solidification is termed the trapped liquid shift its magnitude depends on the modal proportions of cumulus phases and the initial porosity and is only weakly dependent on initial phase compositions trapped liquid shifts are significant when compared with mineral composition changes occurring during fractional crystallization crystallization of 30 trapped liquid gives rise to shifts of up to 10 mol percent in mg number of olivine or pyroxene the size of the shift becomes greater when the initial cumulus assemblage has a lower total feo mgo content and vice versa as a result of the relationship between trapped liquid shift and cumulus mode mineral composition variations and trends may be generated in sequences of cumulates which originally had constant compositions of cumulus minerals for example in a cyclic unit grading from a pyroxenitic base to an anorthositic top crystallization of a uniform proportion of trapped liquid will result in an apparent iron enrichment trend from bottom to top of the cycle as has been observed in the upper critical zone of the bushveld complex

cyclicity in the main and upper zones of the bushveld complex south africa crystallization from a zoned magma sheet the major element composition of plagioclase pyroxene olivine and magnetite and whole rock sr sr data are presented for the uppermost 2 1 km of the layered mafic rocks upper main zone and upper zone at bierkraal in the western bushveld complex initial sr sr ratios are near constant 0 7073 0 0001 for 24 samples and imply crystallization from a homogeneous magma sheet without major magma recharge or assimilation the 2125 m thick section investigated in drill core comprises 26 magnetitite and six nelsonite magnetite ilmenite apatite layers and changes up section from gabbronorite an72 plagioclase mg 74 clinopyroxene to magnetite ilmenite apatite fayalite ferrodiorite an43 mg 5 clinopyroxene fo1 olivine the overall fractionation trend is however interrupted by reversals characterized by higher an of plagioclase higher mg of pyroxene and olivine and higher v2o5 of magnetite in the upper half of the succession there is also the intermittent presence of cumulus olivine and apatite these reversals in normal fractionation trends define the bases of at least nine major cycles we have calculated a plausible composition for the magma from which this entire succession formed forward fractional crystallization modeling of this composition predicts an initial increase in total iron near constant sio2 and an increasing density of the residual magma before magnetite crystallizes after magnetite begins to crystallize the residual magma shows a nearconstant total iron an increase in sio2 and decrease in density we explain the observed cyclicity by bottom crystallization initially magma stratification developed during crystallization of the basal gabbronorites once magnetite began to crystallize periodic density inversion led to mixing with the overlying magma layer producing mineralogical breaks between fractionation cycles the magnetitite and nelsonite layers mainly occur within fractionation cycles not at their bases in at least two cases crystallization of thick magnetitite layers may have lowered the density of the basal layer of melt dramatically and triggered the proposed density inversion resulting in close but not perfect coincidence of mineralogical breaks and packages of magnetitite layers

evolution of icelandic central volcanoes evidence from the austurhorn intrusion southeastern iceland the austurhorn intrusive complex in southeastern iceland represents an exhumed tertiary central volcano the geometry of the intrusion and geochemistry of the mafic and felsic rocks indicate austurhorn was a volcanic center analogous to eyjafjallajokull and torfajokull in iceland s eastern neovolcanic zone evz early transitional tholeiitic basalt magmatism at austurhorn formed a shallow crustal chamber 5 km in diameter apparent rhythmic modal layering of and intrusive contacts within the gabbro indicate the mafic chamber was replenished frequently as it cooled and crystallized felsic activity postdated near solidification of the gabbro numerous granitic magmas intruded along gabbro margins and within the adjacent crust field relations indicate that infrequent felsic replenishment prevented convective mixing of the austurhorn chamber during this time although commingled mafic and felsic magmas are observed in an extensive net veined complex late stage mafic dikes intrude the entire complex suggesting that magmatic heat was abundantly available throughout the evolution of the austurhorn system plagioclase and clinopyroxene compositions in mafic through felsic rocks including gabbros support a model of progressive differentiation field relations constrain the felsic magmas to originate at p 1 kbar presumably by fractional crystallization the structure and geochemistry of the austurhorn intrusive complex suggest formation in an immature rift environment similar to the modern evz the proposed rift segment was parallel to the western and eastern neovolcanic zones and probably resulted from a reorganization of plate boundaries 7 ma saemundsson 1979 helgason 1985 jancin et al 1985 triggered by activity of the iceland mantle plume

paleomagnetic evidence for proterozoic tectonism in the kapuskasing structural zone ontario the kapuskasing structural zone ksz in the superior province of north central ontario is thought to be an oblique cross section through middle archean crust samples from 30 sites in granulite gneisses and anorthosites of the ksz have been analyzed using alternating field and thermal demagnetization methods three remanent magnetization components were found with mean directions of d 23 i 50 k 52 95 11 n 5 sites for the a component d 73 i 68 k 50 95 5 n 18 for the b component and d 332 i 59 k 53 95 17 n 3 for the c component v constanzo alvarez and d j dunlop found similar a and b components in less metamorphosed rocks from the same units to the immediate west they attributed the a component to uplift at 2 55 ga with subsequent 15 to 30 wnw tilt and the b component to thermochemical overprinting at 1 1 ga as the ksz was reactivated during keweenawan rifting and volcanism we also attribute the a component to uplift during the 2 55 ga kenoran orogeny with concomitant 10 5 wnw downward tilting before intrusion of the 2 45 ga matachewan dikes to give its present paleopole of 33 e 67 dp 6 dm 9 n 13 using data from both studies we attribute acquisition of the b component to uplift at about 1 88 ga as suggested by recent thermal modeling and tectonic syntheses to give a paleopole of 124 w 30 n dp 7 dm 8 n 20 the c component is found in only three sites and may be either an hybrid of a 1 1 ga keweenawan crm with the b component or a fault block tilted b component near the ivanhoe lake cataclastic zone

seismic delineation of upthrust archaean crust in kapuskasing northern ontario the kapuskasing structure fig 1 is a 500 km long high grade metamorphic zone which cuts obliquely across the predominantly east west structural grain of the superior province of the canadian shield results from surface geological mapping and gravity profile analysis suggest that the structure is a partial cross section of archaean crust exposed by thrust faulting along the ivanhoe lake cataclastic zone during the early proterozoic1 2 similar exposures of the crust have been observed elsewhere3 4 but no seismic investigation has been conducted over a contiguous exposed cross section of archaean crust here we report results from a seismic refraction profile across the kapuskasing structural zone carried out to investigate the deep structure of this greenstone gneiss belt modelling of travel times and amplitudes yields a high velocity anomaly of 6 6 6 7 km s 1 extending from 20 km depth to close to the surface under the kapuskasing zone with a suggested dip of 15 2 to the west this anomaly is in agreement with a structure comprising upthrusting middle to lower crust

paleomagnetism of the keweenawan chipman lake and seabrook lake carbonatite complexes ontario the chipman lake complex crops out as a series of carbonatite and related alkalic mafic dikes in the wabigoon subprovince of the superior province whereas the seabrook lake complex crops out as an

paleomagnetism of alkaline complexes and remagnetization in the kapuskasing structural zone ontario canada the shenango and nemegosenda alkaline complexes of the southern kapuskasing structural zone ksz an uplifted crustal block in northern ontario preserve primary thermoremanent magnetizations with mean directions d 112 i 46 k 13 95 13 n 9 sites and d 304 i 51 k 31 95 10 n 7 sites for shenango and d 124 i 57 k 13 9520 n 4 sites and d 312 i 54 k 17 95 13 n 7 sites for nemegosenda their average paleomagnetic pole is 45 n 189 e dp 6 dm 8 falling on the keweenawan track of the north american apparent polar wander path apwp between 1098 1107 ma reverse polarity r and 1087 1098 ma normal polarity n poles of lake superior lavas and dikes unlike the keweenawan paleopoles which have a reversal asymmetry of 30 the shenango and nemegosenda r and n poles are almost 180 reversed which was found also by lewchuk and symons 1990b this symmetry of r and n poles casts doubt on the previously accepted predominance of asymmetric reversals of the earth s field in keweenawan time the archean shawmere anorthosite is overprinted by a b magnetization with mean direction d 97 i 68 k 12 95 10 n 18 sites the shawmere b paleopole at 38 n 226 e dp 13 dm l6 falls between the carbonatite poles and 1850 ma cratonic poles and the b magnetization seems to be a composite of middle and late proterozoic overprints sites in the western part of the shawmere body close to the carbonatite line of faults linking the various alkaline complexes are severely altered with an abundance of secondary magnetite and hematite virtual geomagnetic poles for these sites fall close to the keweenawan track and have an average pole of 189 53 n dp 15 dm 20 suggesting that the b nrm at these sites is an 1100 ma thermochemical overprint caused by hydrothermal fluids at the time of carbonatite intrusion more easterly sites are less altered their vgps resemble 1850 ma cratonic poles and average to 235 e 30 n dp 15 dm 17 these b natural remanent magnetizations are probably thermochemical remanent magnetizations impressed by middle proterozoic hydrothermal activity associated with intrusion of early alkaline bodies fault movement on the ivanhoe lake fault zone or late stage uplift of the ksz

volcanism and the dynamics of open magma chambers the repeated eruption of many volcanoes is now accounted for by events taking place in a subterranean magma chamber which is open in the sense that the inflow of material to the chamber from below leads to a magma pressure p which exceeds the lithostatic pressure pl by an amount greater than the tensile strength of the chamber walls before the contents of the magma chamber have reached thermodynamic and geochemical equilibrium this is the condition which must be satisfied when an eruption occurs we have now derived a relationship giving the critical volume of added material v required to cause an eruption as a function of magma compressibility and the volume by which the chamber expands

replenishment of magma chambers comparison of fluid mechanic experiments with field relations magma chambers are commonly replenished at low reynold s number by liquids denser than that already resident in the chamber and nearly always the injected liquid is less viscous than the resident liquid in the magma chamber we report fluid mechanic experiments that are designed to investigate this case specifically treating the injection process itself before a multilayered convecting system develops the injected fluid is emplaced as a viscous gravity current with a nose slightly elevated above the floor trapping a thin layer of ambient fluid beneath it further these injections develop a flow front instability that takes the form of fingers that extend in the direction of the flow using scaling arguments we model the height and the length of the current as functions of time and we use dimensional analysis and our experimental data to model the wavelength of the fingers the fingers and other structures observed in the experiments correlate well with similar features found in silicic intrusions that have been replenished with basic liquid we argue that these and associated mafic pillows are formed by this finger instability our scaling can be combined with finger widths measured in the field to estimate magma reinjection rates we suggest that structures observed in similar environments composite lava flows sills and dikes also formed by a flow front instability when applied to basic replenishments of basic magma chambers the scaling arguments constrain the time required for emplacement the emplacement time scale is short relative to the cooling time scale indicating that even when traveling tens of kilometers such injections cool little during emplacement

thin thrust sheet formation of the kapuskasing structural zone revealed by lithoprobe seismic reflection data regional and high resolution seismic reflection data across the kapuskasing structural zone in ontario canada image at least three significant thrust faults that are low angle merge into a flat detachment on the west and together were responsible for the uplift of amphibolite and granulite facies rocks their geometry resembles a ramp and flat style of deformation that results in a thin upper plate above the 10 12 km about 4 0 s detachment northwest southeast horizontal shortening is estimated to be at least 55 km this large amount of shortening implies that much of the superior province was detached during the formation of the kapuskasing structural zone

astronomical time scale of the paleogene lacustrine paleoclimate record from the nanxiang basin central china abstract the chronostratigraphy of the paleogene succession of the nanxiang basin a petroliferous basin in central china with a predominantly lacustrine fill remains controversial due to the lack of an accurate geological time scale in this work high resolution gamma ray gr logs from the b270 and bs1 boreholes from depocenters in the biyang depression of the nanxiang basin were used to analyze the cyclostratigraphy paleocene floodplain facies of red clastics and purple mudstone is conformably overlain by eocene through oligocene lacustrine facies of gray mudstone oil shale and clayey dolomite interbedded with siltstone sandstone the multitaper method of spectral analysis and gaussian bandpass filtering were used to identify milankovitch cycles based on the recognition of 405 kyr long eccentricity cycles the series was tuned to establish a 46 myr floating astronomical time scale ats which was then anchored to the paleogene neogene boundary age of 23 03 ma the ats enables the assignment of the ages to non marine biozones depositional facies and rifting episodes within the nanxiang basin the subsidence and rates of sediment accumulation within the rift basin of the nanxiang basin has an inverse relationship to the convergence rates between pacific plates and the eastern margin of eurasia the middle eocene slowing of the convergence rate at 50 ma coupled with the onset of the collision of the indian and eurasian plate corresponds to the beginning of the climax stage of rifting the development of deep lakes and the deposition of oil shales in the nanxiang basin the organic rich 3rd member of the hetaoyuan formation eh3 during this middle eocene interval preserved a range of climatic cycles from annual varves to a 1 2 myr modulation of obliquity cycles indicating the importance of climatic oscillations on the development of those hydrocarbon source rocks

paleomagnetism and upb geochronology of volcanic rocks from michipicoten island lake superior canada precise calibration of the keweenawan polar wander track abstract paleomagnetic poles have been determined for normally magnetized samples of late proterozoic volcanic rocks from 23 sites on michipicoten island lake superior canada the results are in general accord with an earlier less precise study by one of the authors and indicate a small polar shift between rocks of the mamainse point formation 186 e 38 n and the michipicoten island formation 175 e 26 n a quartz feldspar porphyry intruding the lower mamainse point formation but unconformably overlain by the michipicoten island formation was sampled for u pb zircon dating the zircons define an age of emplacement of 1086 5 1 3 3 0 ma this age and the pole positions are consistent with and supplement ages previously determined for the keweenawan paleomagnetic polar wander curve these data define an apparent polar wander rate of 14 cm a 1 this age combined with previously published u pb ages indicates a time span of 22 ma for most keweenawan igneous activity in addition the time of the keweenawan paleomagnetic reversal is constrained to be between 1086 5 1 3 3 0 and 1097 6 3 6 ma the age of the upper part of the reversely magnetized osler group the zircon population contained an inherited component from a source 1403 50 ma old this suggests that the porphyry magma was at least partly derived by melting of an anorogenic pluton emplaced during the mid proterozoic

included blocks and blocks within blocks in the skaergaard intrusion geologic relations and the origins of rhythmic modally graded layers the early eocene skaergaard intrusion of greenland includes enormous numbers of rocks of both exotic and cognate origins the lower parts of the marginal border series contain abundant fragments of feldspathic peridotite that are possibly autoliths intermixed with occasional xenoliths of precambrian gneiss and metasomatized cretaceous paleocene sediments derived from adjoining country rocks the upper border series includes one exceptionally large block of gneiss several hundred meters across and numerous smaller fragments these originating from the intrusion9s footwalls plus a few pieces of peridotite the layered series contains countless autoliths of troctolite gabbroic anorthosite and oxide magnetite ilmenite gabbro broken from parts of the upper border series that have otherwise been lost to erosion at the upper midlevel of its western half it contains a few xenoliths of basalt derived probably from the now eroded eocene roof of the intrusion a distinctive postintrusion composite basaltic dike at one place contains 40 or more xenoliths of troctolite olivine gabbro and gabbroic anorthosite that may represent parts of the layered series still hidden at depth the layered series autoliths range from fragments a few centimeters on a side to blocks more than 400 m across and they typically are coarser grained than their host cumulates being in this respect more like upper border series rocks the autoliths are spread stratigraphically through the lower 70 of the exposed 2500 m thickness of the layered series and are generally concentrated in three broad stratigraphic zones their physical relationships to their host rocks particularly the way they indent older layers beneath and are covered by younger layers above provide abundant evidence that there was generally a sharp well defined interface between the top of the cumulate pile and the main body of magma in the intrusion while the layered series was forming the distribution of the autoliths between and through the well known rhythmic thin modally graded layers shows that these layers were spread by magmatic currents and their relations to the more extensive macrorhythmic layering suggest that it too was significantly shaped by currents many of the larger autoliths are crudely layered internally and in places it is evident that their stratification existed before they broke loose therefore it must have formed in the upper border series one particularly large block of oxide gabbro exhibits extraordinarily well developed modal and textural layering and includes small troctolitic autoliths of an earlier generation and it provides evidence that currents also spread crystalline materials across the top of the magma body many of the very small autoliths in the layered series are highly anorthositic in composition apparently because they were leached of mafic minerals and some of the larger blocks show local patchy internal replacement by anorthosite most large blocks show little sign of postaccumulation modification and some have thin fine grained augite rich rims or rinds demonstrating that even though they were out of thermal and chemical equilibrium with their host cumulates they still were effectively armored against extensive chemical change also documented is a large block that was cut by several early basaltic dikes before it broke free from the top of the intrusion these early dikes transgress small anorthositic replacement pipes in the block showing that the replacement process also occurred in the upper border environment two mechanisms are described whereby graded cumulate layers can be sorted and deposited by magmatic crystal liquid suspension currents one involving density surge currents has been advocated previously the other is a new concept based on boundary flow separation and reattachment vortex cells the two mechanisms are used in complementary ways to illustrate the formation of 1 some of the principal skaergaard structures involving blocks and layers 2 modally graded layers in the layered series that rhythmically alternate with uniform layers and 3 modally sorted layers in the upper border series featuring underside draping beneath small included blocks explanations are provided for 1 why plagioclase did not float away from the tops of graded layers even though it was less dense than the liquid and 2 how the liquid part of a current was fractionated away from the crystalline materials modal and grain size data from skaergaard intrusion graded layers are shown to be in excellent accord with characteristics predicted for layers sorted by currents a synthesis diagram is presented illustrating how all the above processes may have functioned in concert in the intrusion

dynamics of magma mixing in partially crystallized magma chambers textural and petrological constraints from the basal complex of the austurhorn intrusion se iceland the tertiary austurhorn intrusive complex in se iceland represents an exhumed magma chamber that has recorded an extensive history of magma mixing and mingling the basal part of the intrusion consists predominantly of granophyres that have been intensively and repeatedly intruded by more mafic magma this association of granophyres basic and hybrid rocks at austurhorn is referred to in the literature as a net veined complex but field relations suggest a much more complex emplacement history here we present petrological and physical constraints on the various processes that resulted in magma mixing and mingling and the formation of different generations of hybrid rocks at austurhorn the complexity of the mixing and mingling processes increases towards the inferred centre of the intrusion where chaotic hybrid rocks dominate the exposed lithology complex cross cutting relations between different hybrid generations strongly suggest multiple magma injection and reheating events in the basal part of the shallow magma chamber model calculations employing distribution coefficients based on rare earth element concentrations reveal that early stage hybrid magma generations formed by pure endmember mixing between felsic and mafic magma with about 10 mafic fraction in the hybrids with repeated injections of mafic magma into the base of the magma chamber the intruding magma interacted to a greater extent with pre existing hybrids this led to the formation of hybrid magma compositions that are shifted towards the mafic endmember over time with up to 30 of the mafic fraction in the hybrids these mixing processes are recorded in the zonation patterns of clinopyroxene and plagioclase phenocrysts the latter have been divided into four main groups by cross correlation analysis melt viscosity calculations were performed to constrain the possible conditions of magma mixing and the results indicate that the interaction of the contrasting magmas most probably occurred at temperatures of approximately 1000 c up to 1120 c this suggests that the initiation of effective magma mixing requires local superheating of the felsic magmas thereby confining the process to areas of localized substantial mafic magma injection

infrastructure and mechanics of volcanic systems in iceland the divergent plate boundary in iceland consists of more than two dozen systems where most of the volcano tectonic activity takes place at the surface the volcanic systems are characterised by 5 20 km wide and 40 100 km long swarms of tension fractures 102 m long normal faults 103 m long and volcanic fissures the holocene fissure swarms are confined to less than 10 000 year old basaltic lava flows mostly pahoehoe occurring near the centre of the active rift zone in any particular swarm the number of tension fractures exceeds that of normal faults all tension fractures and normal faults are vertical at the surface indicating that the surface parts were generated by an absolute tension in addition to a fissure swarm many volcanic systems have a central volcano some of which have developed collapse calderas in the late tertiary and pleistocene lava pile of iceland extinct volcanic systems are represented by local sheet swarms and regional dyke swarms the sheet swarms are normally circular or slightly elliptical several kilometres in radius and are confined to the extinct central volcanoes many swarms are associated with large plutons exposed at 1 2 km depth beneath the initial top of the rift zone and presumably the uppermost parts of extinct crustal magma chambers and in short traverses up to 90 of the rock may consist of sheets the sheets have a very variable strike dip on average 45 65 mostly towards the centre of the associated volcano and have an average thickness of about 1 m the regional dykes occur outside central volcanoes in swarms that are commonly 50 km long and 5 10 km wide in several kilometre long traverses commonly 1 5 of the rock consist of dykes but occasionally as much as 15 28 most regional dykes are subparallel and subvertical the average dyke thickness in the pleistocene swarms is less than 2 m but 4 6 m in the tertiary swarms while active the volcanic systems in the rift zone are supplied with magma from reservoirs located at the depth of 8 12 km at the boundary between the crust and upper mantle the reservoirs are partially molten with totally molten top regions and of cross sectional areas similar to those of the volcanic systems that they feed some active volcanic systems especially those that develop central volcanoes high temperature areas and calderas have in addition to the deep seated magma reservoirs shallow crustal magma chambers located at 1 3 km depth which in turn are fed by the deeper magma reservoirs it is proposed that the regional dyke swarms are supplied with magma largely from the deep seated reservoirs whereas the local sheet swarms are mainly fed from the associated crustal magma chambers because the volume of a crustal chamber is less than that of its deeper source reservoir a single magma flow dyke intrusion from the reservoir may trigger tens of magma flows sheet intrusions from the chamber which is one explanation for the enormous number of sheets associated with many central volcanoes the sheets follow the stress trajectories of the local stress fields around the source chambers whereas the regional stress field associated with the divergent plate movements controls the emplacement of the regional dykes it is suggested that many dykes develop as self affine some as self similar structures when applied to the krafla volcanic system in northern iceland model calculations suggest that a magma flow from the krafla reservoir with regional dyke formation should occur on average once every several hundred years and that tens of sheets might be injected from the shallow krafla chamber and triggered by a single magma flow from the krafla reservoir these results are in broad agreement with the available data

u pb zircon geochronology of the kapuskasing structural zone and vicinity in the chapleau foleyet area ontario geological development of the archean abitibi and wawa greenstone granite subprovinces of the superior province occurred in three main stages 1 a volcanic sedimentary depositional stage 2750 2696 ma ago 2 a metamorphic tectonic event 2700 2685 ma ago and 3 a late plutonic event at 2685 2668 ma the area between wawa in the west and ivanhoe lake in the east represents an oblique section through some 20 km of the crust where geochronological relationships can be examined at different structural levels much of the eastern wawa subprovince is characterized by tonalite granodiorite gneiss containing abundant mafic xenoliths and represents a crustal level intermediate between high level metavolcanic rocks near lake superior and the deep seated gneisses of the kapuskasing structural zone a tonalitic phase of the complex gneiss of the wawa subprovince has a minimum u pb zircon date of 2707 ma but the fine zircons are partly reset possibly in response to lit par lit injections of gneissic to foliat

the cumulate paradigm reconsidered the cumulate terminology outlined by wager and coworkers provided a logical system for describing the textures of the rocks of layered intrusions but recent work shows that this model fails to accommodate all the processes responsible for the petrographic features of coarse grained rocks textures of adcumulates and orthocumulates in the skaergaard intrusion cannot be correlated with concentrations of incompatible elements representing trapped liquid compaction and open system percolative processes have altered their textures modes and even bulk compositions although a rock may be a cumulate in the absence of other evidence it can be misleading to refer to it in terms of the cumulate nomenclature

two types of chemical remanent magnetization during the oxidation of magnetite abstract the chemical remanent magnetization crm m crm resulting from oxidation of equidimensional magnetite to a mixture of 90 hematite and 10 cation deficient magnetite is jointly controlled by the initial anhysteretic remanent magnetization m arm of the parent magnetite and the field h crm applied during oxidation m crm lies in the plane defined by h crm and m arm which were perpendicular in our experiments in an intermediate orientation determined by the relative strengths of h crm 0 05 0 1 and 0 2 mt and m arm 0 002 0 12 saturation remanence the crms are univectorial remanences that remain fixed in these stable intermediate directions throughout the course of stepwise alternating field and thermal demagnetization similar experiments in which acicular single domain cation deficient magnetite was oxidized to maghemite gave quite different results m crm was always parallel to m arm its direction was unaffected by field strengths as high as h crm 1 5 mt the essential factors controlling crm direction seem to be the number and crystallographic structure of oxidation products for single phase oxidation without change in crystal lattice crm remains parallel to the initial remanence of the parent mineral for multiphase oxidation to a mixture of rhombohedral and spinel phases both initial remanence and the field acting during oxidation influence the crm direction which therefore has an intermediate orientation that parallels neither m arm nor h crm

magma differentiation rates from 226ra 230th and the size and power output of magma chambers we present a mathematical model for the evolution of the 226ra 230th activity ratio during simultaneous fractional crystallization and ageing of magma the model is applied to published data for four volcanic suites that are independently known to have evolved by fractional crystallization these are tholeiitic basalt from ardoukoba djibouti morb from the east pacific rise alkali basalt to mugearite from vestmannaeyjar iceland and basaltic andesites from miyakejima izu bonin arc in all cases 226ra 230th correlates with indices of fractional crystallization such as th and the data fall close to model curves of constant fractional crystallization rate the best fit rates vary from 2 to 6 10 4 yr 1 consequently the time required to generate moderately evolved magmas f 0 7 is of the order of 500 to 1500 yrs and closed magma chambers will have lifetimes of 1700 to 5000 yrs these rates and timescales are argued to depend principally on the specific power output i e power output per unit volume of the magma chambers that are the sites of fractional crystallization equating the heat flux at the epr to the heat flux from the sub axial magma chamber that evolves at a rate of ca 3 10 4 yr 1 implies that the magma body is a sill of ca 100 m thickness a value which coincides with independent estimates from seismology the similarity of the four inferred differentiation rates suggests that the specific power output of shallow magma chambers in a range of tectonic settings covers a similarly narrow range of ca 10 to 50 mw km 3 their differentiation rates are some two orders of magnitude slower than that of the basaltic makaopuhi lava lake hawaii that cooled to the atmosphere this is consistent with the two orders of magnitude difference in heat flux between makaopuhi and the east pacific rise 226ra 230th data for magma suites related by fractional crystallization allow the magma differentiation rate to be estimated and from this the thermal budget of the magma chamber addressed and where an independent measurement of heat flux exists to place constraints on the size of the magma chamber such results have the potential to constrain the likely timescale and size of future eruptions of evolved magmas

structure of the grimsvotn central volcano under the vatnajokull icecap iceland the subglacial grimsvotn central volcano lying within a volcanic zone directly above the core of the iceland mantle plume is one of the most active in iceland local regional and teleseismic earthquake data recorded on a temporary seismometer array across western vatnajokull icecap during the summer of 1998 have provided a three dimensional image of the shallow crustal structure of the volcano microearthquake activity at depths of 1 4 km along the grimsvotn caldera rim coincided with inflation of a shallow magma chamber beneath the caldera which culminated in a 0 1 km3 eruption in december 1998 tomographic inversion of these earthquakes define the extent of a low velocity body beneath grimsvotn with a volume of 20 km3 extending to 3 km below the surface this low velocity body is flanked by high velocities under the caldera rim delays in the p wave arrival times through the grimsvotn caldera from regional and teleseismic earthquakes and from two detonations 150 km east of grimsvotn are 0 10 0 15 s greater than the delays through the uppermost 3 4 km of crust shown by local earthquake arrivals this suggests the presence of a further low velocity body at depths greater than 3 4 km beneath grimsvotn presumed to be due to the presence of melt using the distribution of local seismicity and shear wave attenuation we estimate the maximum lateral extent of the region containing partial melt to be 7 8 km e w and 4 5 km n s p wave delays require a thickness of less than 1 km of pure high percentage partial melt assuming a sill like magma chamber

seismic reflection profiles across deep continental crust exposed in the kapuskasing uplift structure seismic imaging of precambrian cratons holds the promise of deep structural mapping and interpretation of fundamental crustal construction processes deep structures may be identified by tracing reflections to the surface1 particularly in exposed crustal cross sections2 elevated along deeply penetrating faults but many such cross sections have structural orientations too steep for successful reflection imaging like the ivrea zone3 or occur in plate boundary settings not representative of continental interiors by contrast the archaean greenstone terrain exposed in the kapus kasing uplift of canada4 5 has gently dipping structures and an intracratonic setting within the superior province thus providing an opportunity to examine the third dimension of continental crust down to 25 30 km palaeodepth6 here we present new seismic reflection data which although generally supportive of structural models based on surface geology4 gravity modeling5 and seismic refraction studies7 8 also indicate that the kapuskasing structure is a relatively thin thrust sheet containing seismically reflective sequences of high grade rock which may be analogues of seismi cally reflective in situ lower crust

determination of domain structure in igneous rocks by alternating field and other methods abstract af alternating field demagnetization arm anhysteretic remanent magnetization and strong field hysteresis properties of a large collection of mostly continental igneous rocks are reported here the collection included rocks whose magnetic carriers were believed from previous work to be of one of three types md multidomain sd psd single domain pseudo single domain or a bimodal mixture of md grains e g discrete opaques and sd psd material e g silicate inclusions two series of subaerial basalts with a full range of deuteric oxidation classes included examples of all three classes of behaviour sd psd rocks have relatively hard inflected af decay curves decay rate initially increasing then decreasing md rocks have soft exponential like decay curves and bimodal rocks have a combination of these characteristics relative hardnesses of normalized decay curves of remanences acquired in weak intermediate and strong fields the lowrie fuller test are also distinctively different for the three classes and the results support the theory developed in an accompanying paper 1 that lowrie fuller characteristics are an expression of the shapes of decay curves the lowrie fuller test although its result can be expressed as a numerical parameter is not capable of fine scale classification of domain structure or grain size the shape of the arm induction curve does have a quasi continuous variation with grain size however the parameter ar j rs initial anhysteretic susceptibility normalized to saturation remanence which is easily measured with standard paleomagnetic instrumentation is potentially useful for magnetic granulometry although ar itself was not diagnostic of grain size

emplacement and crystallization time for the bushveld complex the bushveld complex formed by the crystallization of successive basaltic magmatism may be of the order of several million years for example the columbia river basalts hooper injections of magma which were sufficiently closely spaced in time that each previous magma had not cooled and differentiated 1988 were erupted in the period 17 12 ma with minor eruptions for a further 5 my although most outpouring significantly before the addition of the next one to constrain the emplacement and crystallization times a thermal model is presented occurred within the first 2 my it is now recognized that large intrusions were not emplaced in a single pulse but which permits the investigation of the rate of cooling of magma in an intrusion repeatedly subjected to magma addition and subresult from multiple magma injection the question is how rapidly were magma chambers such as the bc traction such modelling indicates that magmas injected into the bushveld complex were emplaced within 75 000 years at that filled and how much magma was involved answers are relevant to the dynamics of melt production storage and time injection into the complex ceased the volume of rock in the eastern and western limbs is 370 000 600 000 km however transport in the mantle and crust this paper describes a thermal modelling technique not previously applied a quantitative evaluation of the cr budget in the formation of chromitite layers indicates that large volumes of magma cannot be to magma chambers which can be used to analyse this process and to obtain an estimate of the emplacement accounted for in the preserved rock sequence similarly an evaluation of the incompatible trace element abundances such as those for zr time to present this model it is necessary to discuss the stratigraphy size and connectivity of the different limbs and k suggests that the chamber was open and that large volumes of differentiated magma escaped the volume of magma therefore of the bushveld complex and to consider the extent of tapping of the magma chamber as well as its filling greatly exceeded the preserved volume of cumulate rocks giving an estimated magma volume of over 1 10 km an average the term bushveld complex has been given several meanings in the literature and according to the south emplacement rate of 13 km year is indicated by these calculations african commission on stratigraphy 1980 includes not only the ultramafic mafic layered rocks but also the sills beneath the intrusion volcanic rocks which pre date the

age of the firesand river carbonatite complex from paleomagnetism the 2 3 km diameter firesand river complex intrudes archean volcanics and granites of the wawa subprovince in the superior province about 8 km east of wawa ontario it has given differing middle proterozoic k ar biotite ages of 1018 50 and 1097 ma alternating field and thermal step demagnetization of specimens from three calcific carbonatite sites five ferruginous dolomitic carbonatite sites and one lamprophyre dike site isolated a stable mean direction of 290 33 95 12 isothermal remanent magnetization tests indicate the remanence is held by single to pseudosingle domain magnetite and hematite in the carbonatite the dike remanence is keweenawan in age thereby confirming its genetic relationship to the complex and it gives a positive partial contact test with its host rock indicating no postintrusive remagnetization the blocking temperature spectra indicate that postintrusive uplift has not exceeded about 4 km the pole position for the complex is 183 e 27 n dp 8 dm 13 this

interpretation of magnetic susceptibility a new approach to geophysical evaluation of the degree of rock alteration magnetic susceptibility measurements on cores from boreholes in the eye dashwa lakes pluton an archean granitic pluton near atikokan ontario give a quantitative evaluation of the alteration leve