crustal contamination in palaeogene east greenland flood basalts plumbing system evolution during continental rifting abstract palaeogene central east greenland flood basalts have sr nd and pb isotope characteristics consistent with crustal contamination of icelandic type basalts in the lower basalts of central east greenland amphibolite facies gneiss is the dominant contaminant at low stratigraphic levels whereas contamination with granulite facies gneiss prevails in the upper part of the lava succession the contamination probably occurred by partial melting of the crust and can be modelled by assimilation fractionation crystallisation processes similar contaminants have been detected in lavas from odp leg 152 off the se greenland coast although in a reverse sequence contamination in lower basalts and odp leg 152 lavas which erupted through an archaean proterozoic substrate differs from lavas in ne greenland which overly the caledonian fold belt and large mesozoic sediment deposits in the ne greenland lavas the dominant contaminant appears to be a dacitic component formed by partial melting of mesozoic sediments changes in magma supply rates from the mantle may be the determining factor for the sequence of contaminants and may also control the depth of ponding in crustal chambers the compositional variation of the least contaminated east greenland basalts suggests that melts were produced from mantle sources similar to present day iceland and north atlantic morb at the time of continental breakup but distinction between mantle sources is very complicated ree systematics indicate a maximum in initial depth of mantle melting at kangerlussuaq during formation of the east greenland flood basalts consistent with a plume being centered in that region

a ce nd isotope study of crustal contamination processes affecting palaeocene magmas in skye northwest scotland twelve138ce 136ce isotope determinations 31 nd isotope analyses and 31 ree profiles are presented for tertiary basic to intermediate igneous rocks from the isle of skye nw scotland the aim of this work is to precisely identify the contamination mechanisms of basic magmas emplaced through old crust and to test the effectiveness of ce isotope analysis as a petrogenetic tool

deep crustal structure and magmatic processes the inception of seafloor spreading in the norwegian greenland sea summary an extensive investigation into the deep crustal structure of the conjugate norwegian and e greenland margins utilizing two ship expanded spread and wide aperture cdp profiling has led to the identification of a unique suite of crustal structures developed during the onset of seafloor spreading on both margins a region of low velocity crust was found proximal to marginal escarpments immediately seaward of the low velocity crustal block seaward dipping reflectors occur within thick crust that is interpreted to be wholly igneous a product of voluminous melt production enhanced by convective partial melting processes this earliest formed oceanic crust is actually several kilometres thicker than the low velocity crust or the adjacent continental crust normal oceanic crustal thicknesses are not produced until about 5 my after the initiation of spreading when convective partial melting has abated and the magma budget has reduced the extrusive unit reduces to normal thicknesses long before the intrusive and plutonic section indicating that the thickening or thinning of oceanic crust is accomplished primarily by variations in the latter

microstructural and fabric studies from the rocks of the moine nappe eriboll nw scotland abstract microstructures and quartz c axis fabric diagrams from mylonites and psammitic moine schists collected in traverses across the lower levels of the moine nappe in the eriboll area are presented on approaching the moine thrust from the kyle of tongue the following microstructural sequence is encountered interlayered coarse grained biotite psammitic and schistose tectonites being in part mylonitic with two platy slide zones one containing biotite and the other only muscovite and chlorite and both showing quartz microstructures indicative of post tectonic relaxation these pass into more mylonitic rocks nearer the thrust zone which in turn passes into the main chlorite grade mylonite belt and finally adjacent to the moine thrust into reworked lower chlorite grade mylonites although there is some local variation the overall quartz c axis fabric is an incomplete asymmetric type i girdle the main variation is the development of type ii girdles in the reworked ultrafine grained mylonites the extent of the mylonitization is more extensive than previously reported studies of folds within the mylonite belt have revealed eye structures and small scale folds many are sheath folds they cannot be unequivocally correlated with large scale recumbent folds within the moine nappe the results presented indicate that mylonitization is not limited to a single phase and raises the possibility that there may be earlier caledonian or possibly precambrian structural elements present in the eriboll region moines prior to much of the mylonitization

850 790 ma bimodal volcanic and intrusive rocks in northern zhejiang south china a major episode of continental rift magmatism during the breakup of rodinia abstract shrimp u pb zircon ages and geochemical and nd isotopic data are reported for mid neoproterozoic igneous rocks in northern zhejiang province south china the shenwu dolerite dykes the daolinshan granite diabase complex and the shangshu bimodal basalt rhyolite volcanic rocks were dated at 849 7 ma 794 9 ma and 792 5 ma respectively all the studied mafic rocks are tholeiitic in compositions and exhibit overall moderate enrichments in most incompatible trace elements resembling intraplate basaltic rocks in continental rifts the 850 ma shenwu dolerites are the earliest anorogenic basaltic rocks identified so far after the ca 1 1 0 9 ga sibao orogeny but seem to predate the neoproterozoic rifting our results are clearly at variance with the alternative tectonic model that an active continental margin existed along the southern margin of the yangtze block until ca 800 ma the intensive 790 ma bimodal volcanic and plutonic rocks part of a major phase of mid neoproterozoic rifting are concurrent with the widespread mid neoproterozoic intraplate magmatism throughout south china they are most likely related to the proposed mantle superplume that led to the breakup of the supercontinent rodinia

the kraemer island macrodyke east greenland solidification of a flood basalt conduit the kraemer island macrodyke that is exclusively exposed on kraemer island about 7 km west of the skaergaard intrusion belongs to a regional dyke swarm termed the skaergaard like dykes or fg 1 dykes weakly modally layered olivine gabbros dominate the exposed parts of the intrusion that has a width of 650 m to 1000 m plagioclase core an 68 2 and ca rich pyroxene core mg no 79 1 grains are normally zoned whereas olivine grains fo50 65 are homogeneous calculated mineral magma equilibria based on experimentally determined mg fe magma olivine and magma clinopyroxene partition coeffi cients suggest that the observed olivine and clinopyroxene compositions in the gabbros cannot have formed from a common parental magma the unzoned nature of olivine grains and their iron rich com positions relative to clinopyroxene suggest post cumulus mg fe exchange between olivine and interstitial melt a gabbroic pegmatite is developed in the centre of the intrusion along its entire exposed 5 km strike length here mineral zonation is limited and compositions are similar to rims of cumulus minerals in the enveloping olivine gabbros the pegmatite could therefore represent interstitial melt mobilized from gab broic cumulates that later accumulated and crystallized at its present stratigraphic location cumulus olivines in the gabbros are close to equilibrium with ca rich pyroxene in the pegmatite this is interpreted as reflecting interstitial melt mobility during the late stages of solidification of the macrodyke chilled margins are well preserved at the intrusion margins and are rich in fe 14 15 3 feo tot ti 3 3 3 tio2 and light rare earth elements la sm n 1 2 1 3 similar to magmas well represented in the overly ing sequence of contemporaneous high ti series flood basalts it is therefore likely that extensive wall rock melting adjacent to the macrodyke reflects continuous feeding of the overlying flood basalts through the kraemer island macrodyke

trans atlantic correlation of the palaeogene volcanic successions in the faeroe islands and east greenland before continental break up in the ne atlantic the faeroe islands and central east greenland were within a distance of 100 120 km chemical and lithological data for complete sections through the 5 km thick piles of contemporaneous palaeogene flood basalts in the faeroe islands and in the nansen fjord area in east greenland show very similar basalt compositions and evolution patterns with time the faeroes lower basalt formation and the equivalent nansen fjord formation in east greenland form a pre break up succession overlain by a sediment horizon a syn break up succession consists of the faeroes middle and upper basalt formations and the equivalent milne land formation in east greenland in which five intervals can be correlated with a compositional evolution from ti rich magnesian basalts and picrites at the base to a dominance of morb like low ti basalts at the top the successions were generated in the same mantle melting column beneath a thinning continent with a rift zone that eventually ruptured the continent the evolution pattern is very similar to that seen on the se greenland margin but spreading according to the palmason model of 1973 was not yet established the pre and syn break up successions formed volcanic megasystems stretching across the rift zone with areal extents of 70 000 and 220 000 km 2 and volumes of 120 000 and 250 000 km 3 rocks from the pre and syn break up successions can be discriminated based on a simple major element plot the overlying succession was 3 3 5 km thick in e greenland but was thin or absent in the faeroes the energy source for the melting appears to have been concentrated on the greenland side

continental flood basalts episodic magmatism above long lived hotspots abstract the eruption of continental flood basalt cfb may reflect episodic magmatism above long lived mantle plumes the iceland and yellowstone hotspots have generated successive cfb provinces large intrusive complexes anomalous uplift basin formation and rifting events and linear volcanic chains dating back 120 and 70 ma respectively amagmatic intervals occurred 1 when ascent of plumes to shallow levels was impeded by impenetrable lithosphere resulting in sub lithospheric ponding of plume mantle and 2 in response to dispersion by subducting oceanic lithosphere in convergent margin settings by comparison with the plume eruptive potential of typical oceanic hotspots it is apparent that preservation of only a small portion of plume mantle ponded during an amagmatic interval is necessary to account for large volume of cfbs thermal erosion lithospheric attenuation translation of ponded hotspot mantle to the base of thinner penetrable lithosphere and passage of plume mantle through slab windows in subducting oceanic lithosphere led to subsequent breakthrough and eruption of cfb since both mantle plume and plate tectonic processes have been operating since the archean it seems likely that the migration of continents over hotspots with attendant magmatic and tectonic consequences is a common occurrence in the geological record

crustal structure and magmatism of north atlantic continental margins enormous variations exist along rifted margins of the north atlantic in the magmatic activity that accompanied continental breakup the north atlantic margins exhibit a full range of magmatic behaviour one end of the spectrum commonly termed volcanic margins is found in the northern north atlantic off greenland norway and northwest britain where huge volumes of igneous rock were added to the continental crust as it rifted and the adjacent oceanic crust is considerably thicker than normal the other end of the spectrum so called non volcanic margins occurs further south off france and spain where only minor volcanism accompanied rifting and the oceanic crust immediately adjacent to the continental margin is thinner than normal oceanic crust results from seismic profiles are used to determine the volume and distribution of igneous crust accumulated as the continents broke up and seafloor spreading commenced the volume of melt produced during rifting is controlled mainly by the temperature of the underlying asthenospheric mantle and by the amount and rate of decompression as it rises beneath stretching and thinning lithosphere large quantities of igneous rock were produced as the continents broke up to form the north atlantic where the newly initiated iceland plume brought abnormally hot mantle beneath the rift much smaller quantities of melt were produced further south where mantle temperatures were normal during rifting the distribution of igneous rocks across the continental margin and the adjacent oceanic crust is controlled by the rate at which melt is produced and by the ease with which it can intrude laterally in the crust where the rate of stretching is very slow and the rifting is spread over many millions or even tens of millions of years as is common during the initial stages of rifting then the rising asthenospheric mantle has time to cool by conduction heat is lost from the asthenospheric mantle both vertically into the overlying water layer and laterally into the colder wedges of continental lithosphere on either side of the rifted region as a consequence of the reduced mantle temperatures considerably less partial melting occurs in the slowly rising asthenospheric mantle than in mantle which rises rapidly beneath rifts this explains the abnormally thin oceanic crust found adjacent to the non volcanic rifted continental margins in the north atlantic as the new ocean widens the spreading centre becomes isolated from the continental margins on either side by newly formed warm oceanic lithosphere and the rate of rifting generally increases the oceanic crust then attains its equilibrium thickness as conductive heat loss from the rising asthenospheric mantle reduces and all the melt formed by decompression of the mantle solidifies close to the spreading axis basaltic melts can intrude continental crust far more easily than oceanic crust basaltic dykes may extend many hundreds of kilometres away from the magma source beneath continental rifts this redistributes melt laterally towards the continent in both volcanic and to a lesser extent in non volcanic margins and may be a contributory factor in leaving unusually thin crust in the first formed ocean immediately adjacent to some rifted margins it also allows melt formed under volcanic rifts to migrate large distances laterally as is seen in the rockall area northwest of britain

temporal evolution of a long lived syenitic centre the kangerlussuaq alkaline complex east greenland abstract we present new mineral and whole rock compositions and sr nd hf pb o h isotope data on samples from the kangerlussuaq alkaline complex 1000 km2 in central east greenland part of the north atlantic igneous province this complex mainly consists of the kangerlussuaq intrusion but includes at least 13 separate satellite intrusions emplaced in the uppermost crust close to the unconformity between archaean gneisses and overlying palaeogene flood basalts the complex is divided into i older satellite intrusions 55 53 ma composed of multiple syenites and granites and minor gabbros and peridotites ii the voluminous kangerlussuaq intrusion 50 ma which displays a gradual transition from quartz syenites nordmarkites at the margin to nepheline syenites foyaites in the centre and iii younger satellite intrusions 47 45 ma of minor syenites granites and diorites concentrated southeast of the kangerlussuaq intrusion the complex displays a temporal evolution in which sio2 decreases 74 56 wt and total alkalis 6 16 wt amphibole na k content 206pb 204pbmeas endi and ehfi 3 to 11 increase from the older intrusions through the nepheline syenites this is followed by a reversal to higher silica 62 73 wt and lower total alkalis 9 12 wt amphibole na k content 206pb 204pbmeas endi and ehfi 13 to 2 in the younger satellite intrusions temporal changes in the location of magma plumbing systems and in magma production rates played a profound role in controlling silica content alkalinity and degree of crustal contamination during development of the complex phonolitic magma was only generated after prolonged magmatism had shielded the conduits from interaction with country rock the parental magmas were probably basanitic to alkali olivine basaltic in composition the older satellite intrusions and the kangerlussuaq intrusion have low 18o magma values 1 to 6 compared to the younger more crustally contaminated satellite intrusions 4 5 to 7 it appears that the magmas only had sufficient over pressure to intrude the basalt cover when larger volumes of less contaminated trachyte magma were produced resulting in the generation of low 18o magmas due to dehydration of hydrothermally altered basalt xenoliths

evidence from the rare earth element record of mantle melting for cooling of the tertiary iceland plume widespread flood basalt volcanism and continental rifting in the northeast atlantic in the early tertiary period 55 myr ago have been linked to the mantle plume now residing beneath iceland1 2 3 4 5 although much is known about the present day iceland plume6 7 8 9 its thermal structure composition and position in the early tertiary period remain unresolved estimates of its temperature for example range from 1 600 c in some plume models3 to 1 500 c based on the volume and composition of basaltic crust10 11 12 several recent studies4 have emphasized similarities in the thermal and chemical structure of the tertiary and present day plumes to argue for stability of the mantle anomaly whereas others12 13 relate variations in basalt volumes and compositions to changes in plume flux moreover some authors1 2 13 have assumed that the plume was rift centred for its entire history whereas others argue that it became ridge centred only after plate separation14 15 here we report compositional data for 6 000 metres of flood basalts erupted in east greenland close to the inferred plume axis that we use to constrain the tertiary plume structure rare earth element systematics place limits on the pressures and extents of mantle melting and show that the mantle was initially moderately hot 1 500 c but that its temperature declined during flood volcanism these observations are difficult to reconcile with current plume head models and call for important lithospheric control5 10 16 17 18 on actively upwelling mantle along the rifted margin

new faults v fault reactivation implications for fault cohesion fluid flow and copper mineralization mount gordon fault zone mount isa district australia fluid flow leading to mineralization can occur both on newly formed faults and on faults that are reactivated subsequent to their initial formation conventional models of fault reactivation propose that under high pore fluid pressures misorientated faults may reactivate due to low fault cohesion timing and orientation data for a mineralized palaeo to mesoproterozoic terrain mount gordon fault zone mgfz indicate that multiple successive new orientations of predominantly strike slip faults developed between 1590 and c 1500 ma requiring that during the younger deformations some earlier formed faults were too cohesive and or had insufficient pore fluid pressures or other potential fault weakening effects to induce reshear low pore fluid pressures were probably not to blame for failed reactivation on all older faults because some young faults did form or reactivate due to high pore fluid pressures as evidenced by jigsaw fit dilatant breccias hypogene copper mineralization in veins and breccia infill and subvertical tensile quartz veins aligned subparallel to 1 the assumption that old faults consistently have little or no cohesion appears to be incorrect in this terrain many older faults display prominent quartz veins along their length which may explain this conclusion furthermore faults with high cohesion may have acted as barriers and compartments so that intersections between them and newly formed faults host mineralization not because of reactivation but because of interaction between new faults and cohesive materials defined either by fault precipitates or rock juxtaposition together these results and observations provide new simple tools to stimulate copper exploration within the region and in fault hosted terrains

hotspots mantle plumes flood basalts and true polar wander persistent long lived stationary sites of excessive mantle melting are called hotspots hotspots leave volcanic trails on lithospheric plates passing across them the global constellation of fixed hotspots thus forms a convenient frame of reference for plate motions through the orientations and age distributions of volcanic trails left by these melting anomalies hotspots appear to be maintained by whole mantle convection in the form of upward flow through narrow plumes evidence suggests that plumes are deflected little by horizontal flow of the upper mantle mantle plumes are largely thermal features and arise from a thermal boundary layer most likely the mantle layer just above the core mantle boundary experiments and theory show that gravitational instability drives flow beginning with the formation of diapirs such a diapir will grow as it rises fed by flow through the trailing conduit and entrainment of surrounding mantle the structure thus develops a large spherical plume head and a long narrow tail on arrival at the base of the lithosphere the plume head flattens and melts by decompression producing enormous quantities of magma which erupt in a short period these are flood basalt events that have occurred on continents and in ocean basins and that signal the beginning of major hotspot tracks the plume supported hotspot reference frame is fixed in the steady state convective flow of the mantle and is independent of the core generated axial dipole paleomagnetic reference frame comparison of plate motions measured in the two frames reveals small but systematic differences that indicate whole mantle motion relative to the earth s spin axis this is termed true polar wander and has amounted to some 12 since early tertiary time the direction and magnitude of true polar wander have varied sporadically through the mesozoic probably in response to major changes in plate motions particularly subduction zone location that change the planet s moments of inertia

deformation mode switches in the penninic units of the urtier valley western alps evidence for a dynamic orogen abstract multiple deformation mode switches from shortening to extension have been detected in the penninic units of the urtier valley to the north of the gran paradiso massif an early shortening phase whose traces are poorly preserved led to the tectonic burial of rock units to high pressure conditions subsequent extensional shearing sz gr 1 led to the formation of mylonitic fabrics during this event the eclogitic piemonte unit was exhumed to greenschist facies conditions p 0 2 0 3 gpa in the footwall of the blueschist piemonte unit exotic slices of continental basement and mesozoic continental shelf carbonates are found along their contact implying the existence of older thrust stacks dissected by the younger extensional shear zones the extensional structures were later affected by upright folding fu indicating yet another deformation mode switch which led to re burial to p 0 6 0 8 gpa later multi stage deformation culminated in an episode of extensional shearing with top to the west kinematics sz gr 2 therefore at least two shortening extension cycles affected the piemonte units of the urtier valley existing geochronological data allow less than 20 myr for the complex deformation history discussed here shortening extension cycles can be inferred also from published studies from other sections of the western alps however while the first shortening extension cycle was widespread in the penninic units the lateral extent and relative timing of the second cycle in different areas are still unclear

the development of structural geology and the historical context of the journal of structural geology a reflection by bruce hobbs abstract this paper sets the historical context for the journal of structural geology when it was first published in 1979 the major advances in structural geology over the past 40 years are then highlighted finally ten grand challenges for the next decade are nominated

deep seismic and geochemical constraints on the nature of rift induced magmatism during breakup of the north atlantic abstract the deep crustal structure of the norwegian and east greenland conjugate passive margins has been investigated with two ship multichannel reflection and refraction techniques in the region from the iceland faeroe ridge to the greenland fracture zone the data demonstrate the presence of thick 15 20 km igneous crust that was emplaced along the conjugate margins during the initiation of seafloor spreading seaward dipping units comprise the uppermost 3 6 km of the thick crust the thickness of igneous crust diminishes seaward with oceanic layer 2 approaching normal thickness nearer to the margin than does oceanic layer 3 wide angle reflection techniques used in conjunction with expanded spread profiling reveal deep moho reflections beneath 20 km thick oceanic crust that continue seaward into the ocean basin margins that have experienced unusually prolific magmatic activity during breakup have been termed volcanic to distinguish them from non volcanic margins that have evolved into accreting plate boundaries without excessive volcanism the origin of volcanic margins cannot be determined from seismically derived crustal thicknesses alone however thermal and chemical properties of the mantle source may be revealed in the compositions of magmatic products emplaced within volcanic passive margins a combined investigation of crustal structure and associated basalt composition could better resolve processes that result in volcanic passive margins on the voring plateau of the norwegian margin seismic lines coincide with dsdp leg 38 drill sites whole rock major element abundances in the freshest basalts are compatible with moderate extents of partial melting of mantle much like that believed to underlie modern mid ocean ridge systems and would be expected to result in the emplacement of oceanic crust about 9 2 km thick this is about the thickness of oceanic crust located seaward of the dipping basaltic units off the hatton more and voring plateau margins and 10 km south of iceland on the reykjanes ridge these results together with regional heat flow depth gravity and geoid anomalies suggest that the mantle beneath this region has experienced a moderate long lived temperature increase compared to mantle beneath normal ocean basins the much greater thicknesses of igneous crust adjacent to the margins do not appear to reflect an unusually hot flux associated with their construction since this would lead to much greater extents of partial melting than are inferred from the basalt geochemistry evidently the difference between volcanic and non volcanic margins is not simply proximity to a hot spot instead we suggest that the magmatism is closely related to the continental rifting phase tectonism resulting in an abrupt transition from continental to oceanic lithosphere may induce small scale convection that results in the circulation of a greater amount of mantle material into the pressure temperature field where partial melting begins than would occur through passive upwelling alone initial crustal thicknesses are enhanced as a result of the convective partial melting crustal thickness is expected to diminish as this secondary convection slows but the degree of partial melting of the source should not change dramatically

two contrasting styles of interaction between basic magmas and continental crust in the british tertiary volcanic province some of the magmas that were extruded to form the 59 ma early plateau forming basalt dominated floods in the british tertiary volcanic province mixed with small amounts of sialic melt during their uprise through the continental crust while others underwent concomitant fractional crystallization and crustal assimilation magma batches of the first group originated in the uppermost asthenosphere and mostly ponded at the moho where they crystallized fractionally to form a basalt benmoreite suite the major element compositions of members of this series show that they underwent negligible further fractional crystallization between the moho and the surface nevertheless they paused long enough during their uprise to dissolve their high pressure phenocrysts and in many cases to mix with small amounts of acid melt from lower crustal granulite facies archaean lewisian leucogneisses basalts and tholeiitic andesites of the second group outcrop locally at the base of the lava piles in mull and skye their major element compositions show that they equilibrated within the upper third of the crust under conditions approximating to anhydrous 1 atm cotectic equilibria most of these lavas are in sw mull where proterozoic moine metasediments form the uppermost crust the major and trace element and isotopic compositions of the sw mull basal lavas show that they assimilated substantial amounts of moine metasediments progressively as they fractionated within the upper crust pb isotope data reveal that before their final upper crust evolution some of the sw mull basal lavas had already mixed with small amounts of lower crust acid melts

north atlantic oscillation dynamics recorded in greenland ice cores carefully selected ice core data from greenland can be used to reconstruct an annual proxy north atlantic oscillation nao index this index for the past 350 years indicates that the nao is an intermittent climate oscillation with temporally active coherent and passive incoherent phases no indication for a single persistent multiannual nao frequency is found in active phases most of the energy is located in the frequency band with periods less than about 15 years in addition variability on time scales of 80 to 90 years has been observed since the mid 19th century

classification seasonality and persistence of low frequency atmospheric circulation patterns abstract orthogonally rotated principle component analysis rpca of northern hemisphere 1 month mean 700 mb heights is used to identify and describe the seasonality and persistence of the major modes of interannual variability the analysis is detailed and comprehensive in that 1 a high resolution approximately equal area 358 point grid is used for the virtually maximum possible 35 year period of record 2 a positive bias in the nmc data base in the early 1950s in the subtropics is largely eliminated for the first time and 3 homogeneous separate analyses of each month of the year are carried out detailing the mouth to month changes in the dominant circulation patterns winter results are similar to those of other recent rpca and teleconnection studies except that some less obvious patterns are identified and further detail of the better known patterns is provided two north south dipole patterns are found over the pacific ocean west pacific oscillation and east pacific pattern and over the atla

the interpretation of stretching lineations in multiply deformed terranes an example from the hualapai mountains arizona usa stretching lineations are commonly assumed to be genetically related to the foliation plane in which they occur and are generally interpreted to represent the finite elongation direction or tectonic transport direction in highly strained rocks in multiply deformed terranes however lineations may be reoriented rendering their interpretation less straightforward this situation could lead to misinterpretation of the kinematics or strain signature of the second event an example from the hualapai mountains arizona shows that where two discrete deformational events occurred under similar physical conditions an early l1 lineation developed on s1 could be misinterpreted as an l2 lineation developed on s2 here a gently dipping s1 fabric was reoriented by folding into a steeply dipping composite s1 s2 fabric in areas of little d2 overprint the l1 lineation is essentially down dip f2 folding of l1 resulted in reorientation of the lineations to very shallow rakes d1 fabrics are exposed only in extremely limited areas due to nearly complete overprinting by d2 structures therefore unless thorough field mapping reveals the local presence of d1 structures it would be easy to infer mistakenly that the l1 lineation seen on the widespread s2 foliation formed during d2 thereby leading to an incorrect interpretation of the strain signature and kinematics of the second event

the transition from the passive to active continental margin a case study of brittle fractures in the miocene loshui sandstone on the hengchun peninsula southern taiwan 1 in the hengchun peninsula southern taiwan the miocene loshui sandstone of deep sea fan facies provides an opportunity and challenge to investigate the brittle deformation history of an emerging accretionary prism a survey of brittle fractures was carried out along the coastline the brittle fractures include joints of various sets a few faults and veins the intricacy of joints lies in the large number of joint sets that often vary not only from one site to another site but in bed thickness at a given site as well and in the complicated abutting relationships between the joint sets in order to investigate the joint sequence we define a full spectrum of possible systematic joint sets and develop an algorithm to determine the shortest joint sequence from the observed abutting relationships little difference between the shortest joint sequences for the southern and northern parts of the study transect attests to the evolution of overall uniform stress throughout the area that is responsible for these various systematic newly formed or reactivated joint sets the fracture sequence derived reveals a complicated stress evolution during the transition from the passive to active continental margin

repeated parallel deformation in part of the eastern sierra nevada california and its implications for dating structural events study of a thick section of late paleozoic to mid cretaceous sedimentary and volcanogenic rocks in eastcentral sierra nevada has revealed an involved structural succession not readily apparent when analysed under the traditional assumptions of structural analysis e g parallel structures are of the same age earliest structures in the area occur as sparse folds in late paleozoic rocks whereas in triassic to mid cretaceous rocks earliest structures occur as penecontemporaneous slumps upon these earliest structures are superimposed slaty cleavage with associated lineations and subsequent crenulations the slaty cleavage across the area is statistically parallel as are the axial planes of crenulations which fold the slaty cleavage such a succession would traditionally be interpreted as representing two periods of deformation the first forming the slaty cleavage and the second the crenulation of the slaty cleavage there is evidence however to indicate that the slaty cleavage itself was formed during more than one period of deformation and the same may be true for the crenulations dykes emplaced in jurassic rocks have been dated u pb as mid cretaceous and lie parallel to what is probably an early slaty cleavage direction the dykes however also bear a slaty cleavage albeit weaker than in the host rock in addition quantitative strain determinations of rocks in the area show that the older units are more strongly deformed than the younger units these and other data suggest that the statistically parallel slaty cleavage and related structures folds lineations etc found in the jurassic and older rocks have formed during at least two and possibly three increments of strain each increment separated by a lengthy period of geologic time possibly as much as 45 ma or more crenulations of the slaty cleavage at any point subsequently formed after each period of slaty cleavage formation may even predate slaty cleavage formed later at another nearby point while it is possible to set up a chronology between earlier tectonic and or penecontemporaneous slumps and later structures slaty cleavage folds lineations etc it is not valid to designate for the entire area a relative time sequence of formation of slaty cleavage and crenulations in the jurassic or older rocks by the usual methods e g s2 s3 f2 f3 etc these later structures can only be designated as only in the youngest stratigraphic unit in the area which has been subjected to one deformation mid cretaceous can a valid structural succession be applied areally we suggest that multiphase parallel structures comparable to those we have described may be a relatively common phenomenon in orogenic belts until one arrives at a thorough understanding of the detailed stratigraphy and the absolute ages of units in key relationships to the structures it may only be possible to delineate the broadest of time sequences for the structures concerned

tertiary mineralization and magmatism east greenland lead isotope evidence for remobilization of continental crust abstract well defined lead isotope signatures of sulphides from mineral occurrences associated with tertiary igneous rocks in east greenland fall in two distinct groups 1 sulphide samples from the region south of scoresby sund kangerlussuaq igneous complex have lead characterized by retarded uranogenic evolution and slightly elevated thorogenic isotopic ratios 2 sulphide samples from the region north of scoresby sund werner bjerge kap simpson and kap broer ruys igneous complexes have lead compositions typical of modern average terrestrial lead these distinct signatures in both conventional and 208 pb 204 pb 207 pb 204 pb diagrams lie within linear trends defined by previously published lead isotope data for unmineralized tertiary basalts and intrusive rocks the linear trends are mixing lines between the lead in the tertiary mantle derived magmas and lead remobilized from continental crust tertiary magmas in the region south of scoresby sund remobilized and incorporated lead from low high 208 pb 204 pb crustal sources similar in isotope composition to archaean high grade metamorphic gneisses exposed in the area non to sub parallel distinct lead mixing lines in the 208 pb 204 pb 206 pb 204 pb diagram for the tertiary igneous complexes in this region suggest strong heterogeneity of the remobilized crust more than one half of the lead from sulphide mineral occurrences within the kangerlussuaq alkaline intrusive complex may be derived from local continental crust in the region north of scoresby sund the tertiary magmas remobilized moderately radiogenic lead from local basement and sedimentary basins palaeoproterozoic mesoproterozoic gneisses neoproterozoic ordovician sediments caledonian granites and palaeozoic mesozoic sediments these crustal rocks lack the very low 206 pb 204 pb and 207 pb 204 pb ratios typical of the gneisses from the kangerlussuaq region and mixing trends between mantle derived and crustal lead are therefore much less extensive however because the mantle derived magmas had very low total lead contents even small additions of crustal lead are traceable sulphide mineralized alkaline intrusive complexes are shown to be highly contaminated with crustal lead and it is further shown that the pre tertiary rocks of north east greenland are likely local crustal sources of lead for contaminated tertiary plateau basalts many stratabound sediment hosted hydrothermal base metal occurrences in the palaeozoic mesozoic jameson land basin are thought to be tertiary in age but do not appear to contain tertiary igneous derived lead on the contrary the hydrothermal alteration zones associated with several of the mineralized tertiary igneous complexes attest to extensive fluid rock mass transfer lead and other base metals were leached from the country rocks homogenized and incorporated into sulphide minerals

partial melting of mantle wedge and evolution of island arc crust evolution of the crust in the northeast honshu arc is discussed on the basis of the melting experiments of a peridotite under hydrous conditions and the crustal compositions estimated from deep crustal xenoliths and seismic velocity data the chemical compositions of melts formed by partial melting of a hydrous peridotite within the pressure temperature conditions for shallow parts of the mantle wedge 1 2 2 0 gpa have been experimentally determined with the sandwich experiment technique the melts range from olivine tholeiitic to magnesian andesitic compositions sio2 48 56 wt mgo 9 14 wt beneath the volcanic front in the northeast honshu arc and are slightly less silicic beneath the coast of the sea of japan the crustal compositions of the volcanic zone in the northeast honshu arc estimated from deep seated crustal inclusions in volcanic rocks and the velocity structures of the crust are andesitic in the central zone and high alumina basaltic near the coast of the sea of japan the compositions of possible primary melts formed in the mantle wedge are significantly more mafic than these crustal compositions consequently if the major part of the crustal materials in the volcanic zone was formed by magmas generated in the mantle wedge thick up to 30 km ultramafic cumulates including olivine pyroxenite must have been formed beneath moho during fractional crystallization and would constitute the upper part of the mantle wedge the volume of the mantle wedge consisting entirely of even fertile peridotite was not sufficient to produce crustal materials and cumulates in the volcanic zone by such processes in the northeast honshu arc a static mantle wedge model is therefore not satisfactory for the growth of the crust in northeast honshu arc a dynamic model is suggested in which the mantle materials have been supplied into the mantle wedge from the back arc side by a convective flow

assimilation and high pressure fractional crystallization afc recorded by paleo proterozoic mafic dykes southeast greenland abstract the northern margin of the nagssugtoqidian mobile belt in southeast greenland exposes a suite of moderately fractionated fe rich tholeiitic dykes of paleo proterozoic age the dykes were intruded during extension of the crust prior to the development of the nagssugtoqidian mobile belt although the dykes recrystallized under amphibolite facies conditions during proterozoic orogenesis they suffered little deformation excluding two very evolved samples the compositions range between 7 8 4 6 wt mgo 44 5 52 9 wt sio2 and 1 8 6 9 wt total alkalis one group of mafic dykes shows distinct mantle normalized trace element patterns with high abundance of low field strength elements and light rare earth elements and low abundances of high field strength elements these characteristics are consistent with a process of fractional crystallization coupled with assimilation of the regional granulitic crust relatively high rates of assimilation to fractional crystallization 0 7 are required to generate the level of incompatible trace elements this points to lower crustal conditions and the assimilation is believed to have taken place at the base of the continental crust trace element variations indicate fractionation at high pressure involving clinopyroxene as the main extracted phase we evaluate two fractionation models corresponding to a pressure of 0 9 and 1 5 gpa respectively and show that the trace element variations require polybaric fractionation at pressures from a maximum of 1 5 gpa to a minimum of 0 9 gpa

early tertiary magmatism in ne greenland abstract between 73 and 74 n in ne greenland a lower series of quartz tholeiite lavas is overlain by an upper series which varies from quartz tholeiitic to basanitic the lower series basalts are attributed to large melt fractions produced east of the present outcrop where lithospheric thinning permitted the iceland plume to ascend to shallow levels relocation of eruption sites up to 150 km further west preceeded upper series activity in this small degree melts generated principally from garnet lherzolite facies sources were followed by products of more extensive shallower melting which gave rise successively to the upper series lavas a post upper series basaltic dyke swarm and its associated caldera complex palaeomagnetic observations and 40 ar 39 ar dating suggest that the lower series and part of the upper series erupted in c24r at 58 57 ma rejuvenescent activity at c 32 ma marking the termination of volcanism may relate to decompressive mantle melting associated with late regional uplift

major element records of variable plume involvement in the north atlantic province tertiary flood basalts major element variations in north atlantic tertiary province primitive early erupted alkaline transitional tholeiite basalts recalculated to a restricted value of mgo give insights into the process of plume related magmatism basalts primitive enough to be crystallizing only olivine were recalculated to a proposed primary magma composition of 15 wt mgo the recalculated dataset shows clear inter element correlations including a strong significant negative correlation between fe and si indicating polybaric melt segregation overlap between basalt compositions and experimental melts from a fertile fe rich low mg number 85 5 peridotite suggests that relative to normal peridotite with mg number 89 the north atlantic basalt source was fe rich linear regression of the experimental data gives apparent pressures of magma segregation of 17 5 37 kbar with intra region variability in the depth derivation from the melt column for each sample thus suggesting that lithospheric thickness lid effect control on magma generation may have been overemphasized in recent studies comparable source composition magma segregation depth and calculated mantle potential temperature 1440 1460 c throughout the province supports the previously suggested plume impact model arriving below east greenland derived from a variably enriched and depleted lower mantle source given the good agreement between conclusions drawn from major element data and previously published results we suggest that restricted mgo recalculated datasets may be usefully applied to study other large igneous provinces

chemostratigraphy of the neoproterozoic alona bay lavas ontario a basal sequence of flood basalt lavas associated with the neoproterozoic midcontinent rift system crops out in alona bay along the southeastern shore of lake superior in ontario the alona bay lava succession is about 1200 m thick and lies just north of the well studied contemporaneous mamainse point formation detailed chemostratigraphy of the alona bay lavas suggests they are grossly correlative with the basal portion of the mamainse point formation for instance like the basal part of mamainse point formation the alona bay section contains numerous high mgo lavas and can be subdivided into 4 5 groups with distinct chemical characteristics chemical variations within the alona bay groups are largely the result of fractional crystallization likely at moderate pressures one small group of alona bay lavas also carries the compositional imprint of crustal contamination the remaining inter group chemical distinctions at alona bay are the consequence of temporal changes in partial melting and source ch

reconstruction of sea level pressure fields over the eastern north atlantic and europe back to 1500 spatially and temporally high resolution estimates of past natural climate variability are important to assess recent significant climate trends the mid latitude atmospheric circulation is the dominant factor for regional changes in temperature rainfall and other climatic variables here we present reconstructions of gridded monthly sea level pressure slp fields back to 1659 and seasonal reconstructions from 1500 1658 for the eastern north atlantic european region 30 w to 40 e 30 n to 70 n these were developed using principal component regression analysis based on the combination of early instrumental station series pressure temperature and precipitation and documentary proxy data from eurasian sites the relationships were derived over the 1901 1960 calibration period and verified over 1961 1990 under the assumption of stationarity in the statistical relationships a transfer function derived over the 1901 1990 period was used to reconstruct the 500 year large scale slp fields systematic quality testing indicated reliable winter reconstructions throughout the entire period lower skill was obtained for the other seasons although meaningful monthly reconstructions were available from around 1700 onwards when station pressure series became available the quality and the reconstructed slp fields for two exceptionally cold years 1573 1740 are discussed and climatologically interpreted an eof analysis of the 1500 1999 winter slp revealed firstly a zonal flow pattern with pronounced decadal to centenial time scale variations secondly a monopole pattern over northwest europe and thirdly a pattern modulating the meridional flow component over europe these 500 year slp reconstructions should be useful for modelling studies particulary for analyses of low frequency atmospheric variability and for circulation dynamics

the e greenland rifted continental margin an examination of the coastal flexure the structure of the e greenland coastal flexure a major structure developed during continental break up is interpreted on the basis of field observations an earlier model which involves normal inland dipping antithetic faulting and seaward rotation of fault blocks is confirmed and amplified this model requires that regional dips are very much less than dips measured on bedding planes thus leading to considerably reduced thicknesses for the tertiary volcanic succession in e greenland detailed mapping confirms the validity of these thickness estimates and leads to the conclusion that the volcanic extrusion rate was similar to that of present day iceland thus obviating the need for special mechanisms of magma generation comparable structures occur in other regions of continental break up and voluminous basalt discharge and it is suggested that the flexure developed by collapse of the attenuated crust subsequent to the massive extrusion of plateau basalts

inferring the timing of porphyroblast growth in the absence of continuity between inclusion trails and matrix foliations can it be reliably done abstract the timing of porphyroblast inclusion trails can be confidently interpreted relative to surrounding external foliations only where there is continuity between the two where this continuity is broken timing is ambiguous where single or multiple growths of two or more different porphyroblastic minerals have occurred during a relatively complex deformation history the risk of misinterpreting the relative timing of porphyroblast growth is high and can lead to wrong inferences about pressure temperature time deformation p t t d paths misinterpreting porphyroblast timing can also have considerable consequences for determining rates of fabric evolution relative to changes in metamorphic conditions the effects of porphyroblast rotation vs non rotation relative to an externally fixed reference frame on inferred p t t d paths are poorly understood however the d part of the path can differ considerably depending on whether or not porphyroblasts are inferred to have rotated the effect on the p t t part of the path depends on what effect inferences about porphyroblast rotation have on the inferred sequence of porphyroblast growth

evolution of the iceland hotspot mantle geochemical enrichment beneath the iceland faeroes hotspot has been episodic suggesting that blobs rather than a continuous plume may be rising in the region recurring rift propagation southward from central iceland causing progressive melting of the lower crust followed by the upper mantle best explains the geochemical variations found along the south west and south east neovolcanic zones and their degree of thermal maturity

seasonality of the north atlantic oscillation abstract monthly sea level pressure slp data from the national centers for environmental prediction reanalysis for 1948 99 are used to develop a seasonally and geographically varying mobile index of the north atlantic oscillation naom naom is defined as the difference between normalized slp anomalies at the locations of maximum negative correlation between the subtropical and subpolar north atlantic slp the subtropical nodal point migrates westward and slightly northward into the central north atlantic from winter to summer the naom index is robust across datasets and correlates more highly than eof coefficients with historical measures of westerly wind intensity across north atlantic midlatitudes as measured by this mobile index the nao s nodes maintain their correlation from winter to summer to a greater degree than traditional nao indices based on fixed stations in the eastern north atlantic azores lisbon iceland when the naom index is extended back to 1873 its annual values during

progressive folding and foliation development in a sheared coticule bearing phyllite abstract an evolutionary sequence of fold and foliation development interpreted to be the result of a single progressive deformation is seen in exposures of strongly sheared phyllite and coticule 10 km south of the e w trending chedabucto fault in the meguma terrane of nova scotia canada transposition sensu sander at mesoscopic and microscopic scales is fundamental to this development as are fold hinge rotation and migration the transposition process is cyclic with folds of layering and foliation developing rotating and being refolded fold hinges and axial surfaces repeatedly tend towards stable end orientations but do not reach them due to the cyclic transposition process the foliation is a steady state feature continually reconstituted by microscale transposition and dynamic recrystallization several important consequences of these contemporaneous processes are 1 geometrical relationships between folds and foliation are variable in the same outcrop even though both developed synchronously and many folds have the geometrical relationships of cleavage transected folds 2 the foliation is transient in orientation and therefore cannot track the xy principal plane of total finite strain and 3 fold hinge migration may be a common process especially in deformations involving a large component of simple shearing the observed relationships may be applicable to study of fold and foliation development in orogenic belts in general

application of younging tables to the construction of relative deformation histories 1 fracture systems abstract for a collection of age relationships there are several possible histories younging tables provide a simple and direct method with which all valid deformation histories can be identified the application of this technique to fracture systems is illustrated using two case studies these examples demonstrate how unobserved relationships synchronous structures and fractures that appear in more than one stage of a deformation history can be dealt with using younging tables suggestions are made for the presentation of relative deformation histories that highlight ambiguities and convey a visual impression of the confidence that may be placed on a particular history application of the approach to the analysis of grain fragmentation has identified a range of multi stage deformation histories all of these histories involve to a greater or lesser extent the operation of extension fractures shear fractures linked shear fractures and extension fractures and several sets of synchronous extension fractures analysis of an exposed fracture system using younging tables has raised the possibility of the simultaneous growth of two sets of orthogonal extension fractures with the implication that the scale of stress homogeneity may be only a few metres

contrasts in composition and evolution of tertiary cfbs between west and east greenland and their relations to the establishment of the icelandic mantle plume

generation and polybaric differentiation of east greenland early tertiary flood basalts major element trace element isotope and petrographic studies of magma diverentiation mantle melting the basal volcanics in the kangerdlugssuaq fjord region of the east greenland margin provide constraints on the generation and subsequent diverentiation and contamination histories of magmas during the early stage of continental rifting creating the north atlantic ocean basin most primitive lavas of the succession are introduction shown to have accumulated olivine corrections for crystal ac the distinctive geochemical features of continental flood cumulation yield estimates for erupted primitive liquids with mgo basalts cfbs are strongly controlled by the composition contents between 9 and 13 wt the occurrence of suspended and structure of the continental lithosphere and underfo88 olivine phenocrysts further requires the existence of parental lying mantle significant controversy remains concerning magmas with mgo contents up to 17 wt and feo contents up the balance of contributions from the lithosphere and to 14 wt quantitative modeling of fractionation indicates the mantle and the mechanisms by which the various that kangerdlugssuaq magmas primarily diverentiated at moderate source components contribute to basalt petrogenesis pressures 8 kbar to form dunitic and wehrlitic cumulates at how are basalt compositions influenced by generation depths of 25 km evolved east greenland flood basalts from the in anomalous regions in the deep mantle plumes by scoresby sund region record a more complex fractionation history interaction between rising magmas and the continental involving olivine clinopyroxene fractionation in the lower crust lithospheric mantle and by contamination with conas found for the lower basalts followed by olivine plagioclase tinental crust how are the diverentiation con clinopyroxene fractionation at shallow crustal levels cor tamination magma mixing and storage histories of cfbs relations between sio2 incompatible elements and sr nd isotopic controlled by the structure of the continental lithosphere ratios for the lower basalts are consistent with bulk assimilation and how do the thermal and chemical features of the of up to 15 20 archean leucogneiss with unradiogenic sr and mantle and the extensional history of the continental nd isotopic ratios characteristic of lewisian type lower crust lithosphere avect mantle melting and compositions of comparisons between inferred magma types for lower basalts and primary liquids estimated primary liquids from elsewhere in the north atlantic these questions are best addressed in the context of show complex relationships among feo mgo tio2 contents a geologically and geochemically well constrained cfb and cao al2o3 ratios that relate to regional and temporal changes province such as the north atlantic tertiary province in mantle temperatures lithospheric thickness and involvement of separation of east greenland from northern europe in at least two end member mantle sources we postulate that the the early tertiary at 60 myr ago was accompanied by lower basalts of east greenland were derived from iron poor voluminous volcanic activity the blosseville coast of mantle depleted by prior melt extraction and distinct from morb east greenland extending from kangerdlugssuaq fjord source mantle that was enriched in tio2 and lrees by a to scoresby sund fig 1 contains spectacularly exposed metasomatic event shortly before early tertiary magmatism we sections of lavas these sections begin at the basal contact associate this depleted mantle source with the ancestral iceland of the lavas with underlying cretaceous tertiary sediments and archean basement and continue to the plume anomaly

fracture geometry of pseudotachylyte generation zones a study of shear fractures formed during seismic events abstract the orientation and movement sense of shear fractures in pseudotachylyte generation zones developed in the ikertoq shear belt west greenland are described and the fracturing sequence is inferred the generation zones occur in strongly foliated gneisses and consist of concordant usually paired principal displacement shear fractures paired shears on which most melt was generated and a system of minor shear fractures minor shear fracturing was most intense between principal displacement shear fractures and most melt intruded from these paired shears inwards with respect to the zone intrusion of injection veins of pseudotachylyte off the principal displacement shear fractures is believed to be contemporaneous with minor shear fracturing in the generation zones generation zones affect intact rocks and individual principal displacement shear fractures have a maximum displacement of 4 6 m paired principal displacement shear fractures are up to 3 m apart and have a maximum observed length of 1 km in all the zones described the fracturing appears to be the result of a single episode of slip both dextral and sinistral generation zones occur within the study area and have different geometries in each case the displacement is strike slip

two lips and two earth system crises the impact of the north atlantic igneous province and the siberian traps on the earth surface carbon cycle the links between the siberian traps and the end permian mass extinction and between the north atlantic igneous province naip and the paleocene eocene thermal maximum petm demonstrate a critical role for large igneous provinces lips in the disruption of the earth surface carbon cycle escc high precision ages for both volcanic provinces and the associated environmental crises show that in both cases the crisis was contemporaneous with the volcanism the naip comprises two phases the earlier phase 1 c 61 ma and the much more voluminous phase 2 c 56 ma linked to the opening of the ne atlantic the latter triggered the petm the largest cenozoic hyperthermal the siberian traps are significantly more voluminous than the naip and triggered the end permian mass extinction the masses of volcanic co 2 emitted from these provinces may have been much greater than previously suggested as substantial gas may come from intrusive bodies deep within the crust precursory warming due to the accumulation of volcanic co 2 in the atmosphere likely triggered the release of low 13 c methane hydrate although the masses of methane hydrate alone may have been insufficient to account for the observed temperature rises the organic c was likely strongly supplemented by magmatically derived carbon and thermogenic carbon released during emplacement of sills and dykes into c rich sedimentary units more data are required on the volcanic flux rates in order to refine the cause effect relationships between lips and the escc

high pressure fractionation in rift related basaltic magmatism faeroe plateau basalts the lower 3000 m basalt stratigraphy on the faeroe islands shows strong positive correlation between degree of fractionation and zr y ratio this is ascribed to the presence of garnet in the cumulus assemblage during fractionation at the base of the continental crust or in the upper lithospheric mantle at 40 50 km depth later basaltic magmas fractionated at low pressures which is a typical shift in fractionation regime among continental flood basalts seismic velocity v p for the garnet bearing high pressure fractionates is in excess of 7 km s this is comparable to velocities observed in high v p bodies in the basal crust of precambrian shields high pressure fractional crystallization of basaltic magmas may thus be an important process in generating the chemical complexity of continental flood basalts and has implications for crustal underplating

construction and systematic assessment of relative deformation histories abstract the systematics of deformation histories have been investigated using basic combinatorial mathematics using this approach the number of deformation histories and the number of relationships associated with a given number of structures can be calculated the range of possible deformation histories can also be determined using this information the ambiguous nature of all deformation histories is examined together with the impact of unobserved relationships the number of possible deformation histories that are compatible with a collection of age relationships is used as a crude measure of confidence in those histories a procedure is described for the construction of deformation histories that is systematic repeatable and rapid and within which ambiguities can be easily recognised

tectonic sequence diagrams and the structural evolution of schists and gneisses in multiply deformed terranes the current practice of numbering deformation phases d 1 d n in a multiple deformed terrane obscures the link between data and inference and can lead to erroneous interpretations routine production of tectonic sequence diagrams tsds allows a method whereby all deformation and metamorphic mineral growth events their overprinting and duration based on relative timing are systematically recorded different hypotheses as to correlation of tsds across a region can then be assessed objectively including correlation of characteristic sequences that can often be recognized in tsds from single outcrops tsds ensure that absolute dating is applied to specific events within a sequence eliminating the practice where a date is imprecisely correlated tsds also place a focus on the independent character of events here confirmed by absolute dating here we demonstrate how tsds show that many fold nappes are recumbent folds transected by later unrelated ductile shear zones tsds also show that p t t paths and exhumation rates must include starting and stopping of events periods of quiescence and changes in tectonic mode from shortening to extension tsds allow objective analysis of the complexity present in many metamorphic tectonites but which is obscured by conventional methodology

xenoliths of gneisses and the conformable clot like granophyres in the marginal border group skaergaard intrusion east greenland results of this research and the earlier work of wager and his colleagues indicate that contamination influenced the overall character of the marginal border group of the skaergaard intrusion precambrian gneisses were a major source of contamination and are identifiable as xenoliths in the marginal border a variety of other xenoliths occur with a wide compositional range

effects of h2o on phase relations during crystallization of gabbros in the kap edvard holm complex east greenland in the kap edvard holm complex of east greenland layered monly results in the formation of discordant igneous bodies within crystallizing cumulus sequences in some olivine gabbros host numerous small discordant bodies of ultramafic cases partially molten cumulates appear to have reacted rock and magnetite gabbro that formed by late magmatic metasomatic with liquids or fluids that originated within the cumulus replacement of partially solidified gabbroic cumulates similar repile resulting in resorption of cumulus minerals crysplacive rocks are also present as large semi conformable sheet like tallization of intrusive or replacive bodies and rebodies within the layered gabbro sequence replacive bodies of all distribution of mass within the intrusion e g robins sizes are spatially associated with xenoliths of metabasaltic lava 1982 butcher 1985 butcher et al 1985 helz 1987 the small bodies cluster around xenoliths and the large bodies host irvine 1987 boudreau 1988 mcbirney sonnenthal swarms of xenoliths the replacive bodies are inferred to have formed 1990 larsen brooks 1994 scoon mitchell 1994 by reaction of gabbroic cumulates with pore liquids that were mathez 1995 meurer et al 1997 boudreau 1999 in enriched in h2o by degassing or dehydration melting of the other cases partially molten cumulates are inferred to metabasalts as these liquids migrated through the cumulus pile have reacted with externally derived magmas that inthey resorbed plagioclase and produced ultramafic bodies where the truded the cumulus pile resulting in the development liquids came to rest they reacted with gabbroic cumulates to produce of discordant replacive bodies via the reaction of the bodies of magnetite rich gabbro the most evolved pore liquids cumulates with intruding magmas e g morse et al 1987 segregated to form hornblende rich pegmatitic dikes of roughly dacitic bedard et al 1988 tegner wilson 1993 tegner composition suggesting that hydration and possibly oxidation of the robins 1996 because cumulus rocks preserve records magma shifted the liquid line of descent from a tholeiitic trend toward of magmatic fractionation and replenishment in large a calc alkaline trend these inferences imply that contamination of mafic intrusions recognizing and understanding postmagmas by dehydration of hydrothermally altered crustal rocks may cumulus processes such as these can provide insights significantly influence the evolution of mafic magma systems into the evolution of mafic magma chambers and the differentiation of basaltic magmas within the crust near the western margin of the kap edvard holm

complex fault patterns transtension and structural segmentation of the lofoten ridge norwegian margin using digital mapping to link onshore and offshore geology an integrated onshore offshore study involving regional to outcrop scale fault analysis is used to develop a self consistent structural model for transtension along the lofoten ridge the lofoten vesteralen archipelago lva is a segmented basement high showing distinct lateral variations in trend deformational style and structural complexity this study investigates whether segmentation can be linked to differences in the obliquity of preexisting structures relative to plate movement vectors regional analysis of fault lineament patterns using geographic information systems gis reveals that the lva can be subdivided into a series of distinct lineament domains these domains are closely coincident with changes in ridge trend and variations in structure within offshore models derived from seismic reflection studies digital field mapping and spatial analysis of faulting in the north lofoten reveal that multimodal faulting is dominated by transtensional dip slip and oblique slip movements which are comparable to analogue models where the ridge axis is 30 oblique to regional extension the overall change in fault orientation fault geometry and deformation style are consistent with models for transtension where the ridge bounding structure becomes increasingly oblique to regional extension previously identified transfer zones simply reflect segment domain boundaries and are not reactivating basement structures this model is a possible analogue for other orthogonal and oblique rift structures on the norwegian and other margins

atmospheric circulation variability in the north atlantic european area since the mid seventeenth century based on monthly mean sea level pressure grids objectively reconstructed by luterbacher et al variations of dynamical modes of the atmospheric circulation for january and july are described by novel indices for running 31 year periods between 1659 and 1999 these indices reflect the continuous evolution of the atmospheric circulation not only with regard to frequency changes of major dynamical modes but also in terms of internal changes within each mode concerning both dynamic vorticity intensity and climatic properties central european temperature and precipitation during occurrence of each mode respectively results indicate the great importance of within mode variations the zonal circulation mode in january varying in frequency with long term cycles primarily changed its dynamic and climatic properties towards higher indices during the transition from the little ice age to modern conditions between 1800 and 1930 within the russian high mode of january a change in preference from easterly to westerly patterns above central europe occurred around 1850 for july a striking frequency maximum of the westerly mode at the end of the eighteenth century coincided with a period of marked summer warmth in central europe due to negative positive deviations in vorticity temperature during occurrence of this mode the long term evolution in july indicates a general increase of anticyclonic conditions strengthening during the last 50 years towards a unique phenomenon within the last centuries the strong increase in the winter time westerly circulation during the last decades however does not appear extraordinary in view of the low frequency variations of this mode

high strain zone deformation in the southern capricorn orogen western australia kinematics and age constraints abstract detailed structural mapping of a the kerba fault a high strain zone along the contact between the archaean to palaeoproterozoic yarlarweelor gneiss complex partly derived from the yilgarn craton and the palaeoproterozoic volcano sedimentary rocks of the bryah group indicate a temporal progression from foliation development to folding and finally brittle deformation structural data from the penetrative s1 foliation indicate a superficially simple geometry and kinematic history with a single pervasive greenschist facies foliation forming during dextral strike slip shearing however systematic spatial variations in the magnitude and orientations of mineral elongation lineations indicate a more complex deformation history involving a combination of general transpressive dextral shear and subsequent reactivation this deformation has previously been considered to develop between 1830 and 1780 ma ago during the capricorn orogeny relative age relationships around the kerba fault indicate that the high strain deformation postdates the intrusion of the kerba granite at 1808 6 ma analysis of available age data indicates that the minimum age of deformation is poorly constrained however evidence from regionally similar structures and geochronological data indicate that deformation associated with high strain zones in the southern capricorn orogen may be mesoproterozoic in age significantly younger than the capricorn orogeny

scale dependence strain compatibility and heterogeneity of three dimensional deformation during mountain building a discussion plate motion at convergent margins causes crustal shortening and orogenic thickening relative motion that is oblique to the plate margins is an inevitable consequence of plate kinematics on a sphere and results in non coaxial three dimensional deformation that cannot be approximated to simple shear models of mountain building that include a smoothly varying component of pure shear shortening allow strain compatibility to be maintained by deforming the upper free surface of the earth without disrupting the material continuum however such models do not reflect accurately the nature of deformation in many areas of high strain in the upper crust which are characterized by interconnected arrays of kinematically linked faults that can be active on several scales of magnitude simultaneously as brittle deformation increases the coherence of the material continuum is highly reduced in such situations strain compatibility is maintained by partitioning the deformation amongst structures of varying kinematic significance over a wide range of scales and not by smooth variations in strain magnitude acting on a single scale across a material continuum there is a marked tendency for such partitioned domains to be oriented parallel or sub parallel to the orogenic grain alignment of domains in this way represents a strong structural anisotropy which acts as a highly significant boundary condition that controls deformation at subordinate scales finite strain observed within an individual domain at a given scale need not therefore display the same magnitude or orientation as bulk finite strain at the plate scale and consequently data must be collected from as large an area as possible to relate outcrop scale structures to global scale tectonics

a well verified multiproxy reconstruction of the winter north atlantic oscillation index since a d 1400 abstract a new well verified multiproxy reconstruction of the winter north atlantic oscillation nao index is described that can be used to examine the variability of the nao prior to twentieth century greenhouse forcing it covers the period a d 1400 1979 and successfully verifies against independent estimates of the winter nao index from european instrumental and noninstrumental data as far back as 1500 the best validation occurs at interannual timescales and the weakest at multidecadal periods this result is a significant improvement over previous proxy based estimates which often failed to verify prior to 1850 and is related to the use of an extended reconstruction model calibration period that reduced an apparent bias in selected proxies associated with the impact of anomalous twentieth century winter nao variability on climate teleconnections over north atlantic sector land areas although twentieth century nao variability is somewhat unusual comparable periods of persistent positive phase

tertiary volcanism in the kangerdlugssuaq region e greenland trace element geochemistry of the lower basalts and tholeiitic dyke swarms

thermal and compositional evolution of a cooling magma chamber by boundary layer fractionation model and its application for primary magma estimation a boundary layer fractionation model for the thermal and compositional evolution of a basaltic magma chamber is presented the model utilizes a multicomponent thermodynamic approach for relating the compositional structure with the thermal structure of a cooling magma body the magma composition in the main magma body evolves by transportation of a fractionated interstitial melt from the mushy boundary layer the consideration of both the thermal and compositional evolution of the magma body enables a realistic simulation of magmatic differentiation as a function of time and space the model is used to evaluate a primary magma estimation from volcanic rock series it is shown that the common procedure of olivine addition gives an estimation which is significantly different from the true primary composition if the magma actually differentiated by boundary layer fractionation this can cause significant errors in estimating the conditions at which the primary magma was generated

assimilation and fractionation in adjacent parts of the same magme chamber vandfaldsdalen macrodike east greenland the vandfaldsdalen macrodike which lies in the skaergaard region of east greenland is a remarkably zoned fossil magma chamber with a granophyric cap overlying cumulate gabboros the intrusion is distinctly bimodal with a large compositional discontinuity at the contact between the gabbro and granophyre although the exposed part of the macrodike is in contact with tertiary basalts and sediments the granophyre originated by assimilation of xenoliths derived from the underlying archean basement sr and nd isotopic ratios throughout the cumulate sequence are remarkably similar indicating insignificant contamination of the gabbro by the granophyre modelling of the compositional effects of cooling and crystallization indicate that the cumulate pile resulted from fractional crystallization with the complicating effects of trapped liquid and post cumulus fractionation the uppermost rocks in the mafic part of the chamber sio2 62 feo 12 4 resulted from about 85 fractional crystallization a transgressive sill of strongly fractionated magma sio2 67 feo 8 8 formed from extracted intercumulus liquid that was the result of 90 fractional crystallization of the original magma mass balance indicates that typical granophyre is made up of about 75 dissolved xenoliths by weight and 25 mantle derived basaltic magma the magmas were not measurably affected by material exchange across the interface between the gabbro and granophyre this magma chamber evolved by both assimilation and fractional crystallization but the residual liquids formed by fractional crystallization were unaffected by assimilation heat exchange between were unaffected by assimilation heat exchange between the two parts of the chamber was obviously important but there was insignificant material exchange the inability of fractional crystallization and assimilation to affect the same liquid is related to the dynamic behavior of this particular magma chamber particularly the buoyancy of granophyre relative to evolving tholeiitic magma

early to middle miocene intra continental basaltic volcanism in the northern part of the arabian plate se anatolia turkey geochemistry and petrogenesis continental basalts ranging in age from 16 5 to 19 08 ma crop out throughout the northern part of the arabian plate the basalts have distinctive petrographic characteristics such as rounded and skeletal olivine phenocrysts with abundant melt inclusions implying the mixing of two distinct magmas all of the analysed basalts are tholeiitic in composition the presence of quartz xenocrysts with clinopyroxene rims in some samples indicates that crustal assimilation was probably an important process during magma ascent to the surface and low mg number and high sio 2 contents of the basalts clearly show that they have experienced fractional crystallization as well as crustal contamination variations of the major and trace elements versus mgo show that olivine clinopyroxene plagioclase were the main fractionating minerals in terms of incompatible trace elements the basalts have oib like signatures with a slight depletion at nb ta on primitive mantle normalized diagrams the basalts have slightly lree enriched patterns with la yb n 5 5 to 6 7 la nb ratios are close to unity suggesting the melts may have originated in the asthenospheric mantle partial melting modelling based on ree data imply that the melts were not produced from a single mantle source depth which is either purely a spinel or garnet peridotite end member the samples lie on a binary mixing line between low degree melts 10 from spinel peridotite sources on a plot of la yb v dy yb requiring interaction of melts derived from both garnet and spinel peridotite fields melts originating from both sources were initially tapped by distinct magma chambers which subsequently hybridized into a single flow hybridized magma ascended to the surface along neogene strike slip faults which are linked to the dead sea fault zone

formation of wehrlites through dehydration of metabasalt xenoliths in layered gabbros of the noe nygaard intrusion southeast greenland the noe nygaard intrusion is a 4 2 5 km stock composed of layered gabbros and wehrlites within the precambrian basement of the coastal mountains west of the kialineq plutonic complex transgressive relationships to tertiary mafic dykes and the occurrence of abundant metabasaltic xenoliths signify a tertiary age for the intrusion the intrusion is characterized by alternating zones of gabbro and wehrlite gabbro is both intruded and replaced by wehrlite and the wehrlite zones are characterized by abundant metabasaltic xenoliths based on 87 sr 86 sr ratios mica olivine and oxide gabbros are all cumulates crystallized at different differentiation stages from a common parental magma field relations together with similarities in strontium isotope ratios and in the major and rare earth element ree mineral chemistry between gabbros and wehrlites indicate that the wehrlite bodies were formed by the dissolution of plagioclase from a gabbro cumulate mush by h 2 o derived from dehydration and the partial assimilation of metabasaltic xenoliths in terms of their ree characteristics melts from which the noe nygaard intrusion crystallized are within the compositional range of melts for other early tertiary mafic ultramafic complexes of east greenland however they were generated at a greater mean melting pressure and have less radiogenic strontium isotope ratios than the nearby imilik mafic ultramafic complex supporting existing models for mantle heterogeneity at the time of continental break up the abundance of metabasaltic xenoliths in the noe nygaard intrusion provides further evidence for the lateral extent of the north atlantic flood basalt province which onshore has been mostly removed by glacial erosion south of 68 n in greenland

geochemical evidence for complex magmatic plumbing during development of a continental volcanic center the combined use of major element compositions and experimental petrology to determine the pre eruptive depth of equilibration and of trace elements and isotopes to elucidate the components and mechanisms of magma crust interaction has enabled reconstruction of the magmatic plumbing during the initial stages of evolution of the skye and mull centers of the british tertiary volcanic province early basic magmas were delayed at more than one level in the crust nevertheless taken as a whole the data require that each individual batch of magma had a distinctive pre eruption history and followed a different route through the crust the plumbing beneath other areas of extension related continental basic magmatism may be equally complex

basalts generated by decompressive adiabatic melting of a mantle plume a case study from the isle of skye nw scotland

convective partial melting 1 a model for the formation of thick basaltic sequences during the initiation of spreading stretching and thinning of continental lithosphere during rifting and initiation of seafloor spreading cause upwelling of mantle rocks into a pressure temperature environment where partial melting occurs the amount of melt liberated is however insufficient to account for the thickness of igneous crust emplaced near the continent ocean boundary at some passive margins these igneous structures are known from seismic reflection and deep refraction studies on numerous passive margins worldwide seismic studies also suggest that the transition from continental to oceanic crust on these volcanic margins is relatively abrupt their evolution apparently does not include a prolonged extensional phase preceding the inception of spreading to account for these observations we propose a model for the generation of partial melt in which the lateral temperature contrasts evoked by rifting and asthenospheric upwelling drive convection in the upper mantle deep hot mantle material is transported upward by the convection and hence much larger volumes flow through the region where partial melting occurs than during passive upwelling numerical calculations show that convective partial melting can provide substantial additions to melting due to passive upwelling alone using this model we propose an evolutionary scheme for volcanic passive margins emphasize the contrast with nonvolcanic margins and describe conditions which might lead to the development of both types

onshore evidence for progressive changes in rifting directions during continental break up in the ne atlantic abstract current models for the palaeogene aged continental break up in the north atlantic invoke a nw se extension vector throughout the rift to drift process accommodated by margin parallel normal faults and margin oblique strike slip transfer zones however palaeogene and younger faults in the faroe islands provide evidence for a progressive anticlockwise rotation in extension vectors immediately prior to and following north atlantic break up six deformation stages are recognized 1 east west to ne sw extension accommodated by dip slip north south and nw se trending faults 2 continued ne sw extension accommodated by nw se and nne ssw oriented dykes 3 north south extension accommodated by ene wsw and ese wnw conjugate dykes 4 crustal extrusion involving both east west shortening and further north south extension along ene wsw dextral and ese wnw sinistral conjugate strike slip faults 5 during the final stages of rifting the regional extension vector became nw se and was accommodated predominantly by slip along ne sw dextral oblique slip faults 6 pre existing structures were locally reactivated as tensional and extensional shear features characterized by the entrainment of clastic material along fault planes the present study reveals a distinct period of margin parallel extension immediately prior to break up that is not accounted for by existing models illustrating the importance of conducting field based studies to validate otherwise widely accepted margin scale models worldwide

the behaviour of platinum group elements in basalts from the east greenland rifted margin the continental flood basalts of the east greenland volcanic rifted margin were extruded during continental breakup above the ancestral iceland mantle plume at 55 ma three distinct magma types the low ti high ti and very high ti series lts hts and vhts respectively are found intercalated in the 6 km thick plateau lava sequence incompatible trace elements indicate that the lts are derived from a more depleted mantle source compared to hts and vhts the lts is characterised by increasing cu 105 to 248 ppm and pd 7 to 24 ppb constant cu pd ratio 10 000 and decreasing ir 1 1 to 0 05 ppb and ru 1 8 to 0 3 ppb concentrations during magmatic differentiation 16 to 7 wt mgo the constant cu pd ratio reflects silicate and chromite dominated fractionation without concurrent segregation of sulphide s undersaturated differentiation is also indicated in the hts which also displays increasing pd 6 16 ppb and decreasing ir concentrations 1 to 0 05 ppb during differentiation and the cu pd ratios for the entire series average 21 000 however some hts samples have elevated cu pd ratios up to 33 000 cu pd ratios in the hts do not correlate with mgo and this is interpreted to reflect varying cu pd ratios of hts parental magmas rather than s saturated differentiation during s undersaturated differentiation of the lts and hts pt pd ratios decrease from 1 3 to 0 11 and 1 1 to 0 2 respectively which indicates that pd is much more incompatible than pt during s undersaturated differentiation the vhts consists exclusively of highly evolved samples with low mgo 6 6 6 1 wt and pd ir ratios 98 228 here cu pd ratios increase from 17 500 to 35 000 with decreasing cr concentrations which indicate that these magmas experienced silicate fractionation with concurrent segregation of sulphide the lts represent melting of a depleted source and show high pge concentrations and constant cu pd ratios during s undersaturated differentiation melting of a normal depleted upper mantle source generates s saturated melts morb and the depleted lts source is therefore considered an extraordinary s poor component within the ancestral iceland plume of the three series the vhts contain the largest contribution from enriched mantle portions the vhts have similar pge but much higher nb concentrations for instance compared to the most evolved lts and hts samples indicating that the enriched source contributes nb but not pge

links between flood events in central europe since ad 1500 and large scale atmospheric circulation modes 1 based on documentary sources incidence variations of flood events can be reconstructed back to ad 1500 for several catchment areas in central europe links to atmospheric circulation modes derived from reconstructed large scale sea level pressure slp grids for the last 500 years have been identified on climatic time scales monthly to seasonal these relations are expressed in terms of several indices describing the particular importance of atmospheric circulation modes as a dynamical background for the varying incidence of flood events during winter the zonal circulation mode covers the largest part of these events in relation to mode frequency however other circulation modes become important during historical periods of increased flood frequency e g modes characterised by atlantic low and russian high pressure centres different subtypes of this mode reached maxima in flood association during different climatic periods of the past

palaeogene continental to oceanic magmatism on the se greenland continental margin at 63 n a review of the results of ocean drilling program legs 152 and 163 mantle melting zone accompanied by an increase in the average drilling along a 63 n transect off se greenland during ocean degree of melting with time from 4 to 12 these modest drilling program odp legs 152 and 163 recovered a succession degrees of melting imply mantle temperatures only 100 c hotter of volcanic rocks representing all stages in the break up of the than normal upper mantle upwelling mantle must therefore have volcanic rifted margin the rocks range from pre break up continental been fed dynamically to the melt zone to generate the igneous crust tholeiitic flood basalt through syn break up picrite to truly oceanic of 18 km thickness deduced from seismic and gravity studies nbasalt forming the main part of the seaward dipping reflector morb like magmas dominated the earliest part of the succession sequence sdrs all the lava flows recovered from the transect although a few flows of icelandic basalt were erupted in the prewere erupted in a subaerial environment ar ar dating shows break up phase in contrast the post break up magmas had an that the earliest magmas were erupted at 61 ma and has icelandic mantle source this suggests that the developing head of confirmed that the main part of the sdrs was erupted during c24r the ancestral iceland plume was compositionally zoned with a core 56 53 ma following continental break up magma represented by of icelandic mantle surrounded by a thick outer zone of hot depleted the pre break up lava flows was stored in crustal reservoirs where upper mantle it evolved by fractional crystallization and assimilation of continental crust trace element and radiogenic isotope data show that the contaminant changed through time from lower crustal granulite to a mixture of granulite and amphibolite suggesting storage of magma at progressively shallower levels in the crust the degree of contamination declined rapidly as break up proceeded and the youngest rocks sampled in the transect are uncontaminated by continental basement variation of for example sc zr and sm

the e greenland continental margin a transition between oceanic and continental magmatism the e greenland continental margin differs from typical margins in having a superimposed regional uplift which has exposed the early rift products for direct examination most of the igneous rocks in this region find counterparts in the oceanic setting with the proviso that the area is part of an aseismic ridge thus picrites basalts sheeted dyke swarms and layered gabbros are present and these can be used to shed light on oceanic magmatic events nephelinitic rocks are products of off axis volcanism and find no counterparts in the present oceanic rift system although they occur in marginal regions of the ocean basins and were generated in distal regions at the initiation of rifting in icelandic anomaly voluminous salic rocks in the area owe their generation largely to continental crust which has undergone extensive melting they find no direct counterparts in oceanic areas the outstanding degree of exposure in this area combined with its transitional nature and wide spectrum of rock types make it a unique laboratory for the elucidation of a variety of problems regarding both oceanic and continental volcanism together with the nature of rifted continental margins in general

magmatism at rifted continental margins the intense volcanism and uplift observed on many rifted continental margins forming basaltic seaward dipping reflector sequences is accompanied by the emplacement of a thick igneous section at depth partial melting by decompression of passively upwelling asthenosphere that is hotter than normal because it is near a hotspot explains both the thickness of the new igneous crust and the initial elevation of the rifted margins

multiply deformed terrains problems of correlation abstract the best tools available for carrying out a structural analysis in a complex area are overprinting style and patterns of orientation unfortunately all have limitations which are reviewed briefly and discussed in areas of continuous unfaulted outcrop the deformational history can be established purely on the basis of overprinting but even in such areas there can be problems since not all overprinting relationships are unambiguous most fold interference patterns are reliable overprinting relationships based on foliations however are commonly unreliable and various examples are cited and discussed even in areas where the sequence of deformational events can be established their temporal significance is unclear since the methods applied only determine local relative timing and absolute relationships must vary in space and time across an orogen

pb isotopes during crustal melting and magma mingling a cautionary tale from the miki fjord macrodike central east greenland abstract pb isotopic data are presented for hybrid rocks formed by mingling between mantle derived tholeiitic magma of the eocene miki fjord macrodike east greenland and melt derived from the adjacent precambrian basement bulk mixing and afc processes between end members readily identified in the field fail to model the pb isotope systematics selective contamination during diffusional exchange which can explain the complex sr and nd isotope compositions of the hybrid rocks blichert toft et al 1992 cannot fully account for the variability of the pb isotopic data using the identified crustal end members the crustal anatectic end member although similar in sr and nd isotope composition has a markedly different pb isotopic composition than its source gneiss the differences are consistent with preferential incorporation of radiogenic pb from accessory phases such as metamict zircon or loosely bound pb from grain boundaries during disequilibrium melting of the host rock by the mafic magma the crustal melt involved in magma interactions was therefore heterogeneous with respect to pb isotopes on a metre scale these results illustrate the difficulties inherent in interpreting isotopic variations in contaminated mafic magmas even when the end members are well constrained by field relations we show that the pb isotopic composition of the crustal contaminants and contamination trajectories for the miki fjord hybrid magmatic lithologies are markedly different from regional basement gneisses and contaminated lavas of the associated flood basalt province that were contaminated by characteristically unradiogenic pb and sr lithologies resembling lewisian basement not exposed locally

the transition from sedimentation to flood volcanism in the kangerlussuaq basin east greenland basaltic pyroclastic volcanism during initial palaeogene continental break up primary basaltic pyroclastic deposits compose volumetrically c 35 50 of the lower palaeogene vandfaldsdalen mikis and haengefjeldet formations within the lower volcanic series of the east greenland flood basalt province the eruption and emplacement mechanisms of these deposits can be used to evaluate the volcanotectonic evolution of continental break up and initiation of flood basalt volcanism we present a revised stratigraphy for the pre volcanic sedimentary rocks and synvolcanic pyroclastic and epiclastic deposits and lavas from the kangerlussuaq basin east greenland primary basaltic pyroclastic deposits range from airfall tuffs to bomb beds surge deposits and vent sites with cinder cones early volcanism was apparently restricted to localized basins and formed shield volcano structures 30 40 km in diameter that are characterized by a heterogeneous sequence of intercalated hyaloclastites sediments pyroclastic deposits and lavas the lateral variation of deposit morphology reflects distance from source and indicates a migrating locus of volcanism along the axis of rifting towards the ne paired with continued down dropping of inactive volcanic centres the complex stratigraphic architecture of the initial volcanic deposits reflects local tectonic and sedimentary processes interacting with regional flood basalt volcanism and continental rifting

pressure conditions for the solidification of the skaergaard intrusion eruption of east greenland flood basalts in less than 300 000 years abstract primary granophyres are differentiated from olivine tholeiitic magma and occur interstitially throughout the cumulus stratigraphy of the skaergaard intrusion east greenland samples from the lower zones a c lza c the middle zone mz and the sandwich horizon sh are included in the present study together with granophyric accumulations in gabbroic pegmatite from lza c fluid inclusions in quartz and the mineral assemblage in the granophyres record the pressure under which the skaergaard intrusion crystallised pegmatitic granophyre from lza c consists mainly of quartz plagioclase an 4 7 and alkali feldspar or 40 80 enclosing an earlier formed assemblage of ferrohastingsitic and ferroedenitic hornblende fayalite fo 4 5 titanite biotite and fluor apatite granophyric quartz albite and alkali feldspar crystallised from water saturated granitic melts near eutectic minimum conditions between 680 and 660 c the pressure of granophyre crystallisation was modelled by the intercept between fluid inclusion isochores and the minimum melt solidus for granitic compositions pressures recalculated to the roof pendant of the intrusion are 0 7 0 5 for lza 2 0 0 2 for lzb c 2 3 0 8 for mz the triple group level and 3 3 1 3 kb for sh amphibole geobarometry independently confirm the pressure estimates for pegmatitic granophyres in lza c the granophyres formed as the intrusion cooled through the minimum melt solidus in lza lzb lzc mz and sh respectively the pressure increase from lza to sh granophyres is explained by progressive burial during cooling of the intrusion and contemporaneous outpouring of 5 3 6 3 2 7 km of flood basalts during the initial opening of the northeast atlantic ocean accordingly the skaergaard intrusion evolved from a subvolcanic magma chamber at emplacement to a more deep seated igneous system during terminal crystallization when the majority of the intercumulus phases formed the present cooling history suggests the intrusion cooled through the temperature of granophyre formation in less than 300 000 years the increase in the pressure of granophyre formation therefore implies that 5 3 6 3 2 7 km of flood basalts were emplaced within this time window

magma flow in the east greenland dyke swarm inferred from study of anisotropy of magnetic susceptibility magmatic growth of a volcanic margin summary volcanic passive margins vpms are characterized by large volumes of melt emplaced within the lithosphere during break up processes several data and a recently proposed conceptual model of volcanic margin development suggest that vpms are fed from localized crustal zones of magma storage underlying large polygenetic volcanoes localized above diapir like instabilities of the asthenosphere we investigated the magma flow pattern within the coastparallel dyke swarm of the east greenland vpm which is the only outcropping vpm over a distance of 125 km the 44 sampled dykes are representative of the successive families of intrusions igneous petrofabrics are constrained by the measurements of the anisotropy of magnetic susceptibility the magnetic fabrics are of medium to low anisotropy p 0 flow related fabrics are recorded in 75 per cent of the sampled dykes we infer the flow directions from the imbrication geometry of the magnetic foliation planes at the dyke margins and check the results by measuring the preferred orientation of plagioclase in thin sections cut in the magnetic principal planes due to probable fabric superposition the magnetic lineation represents the zone axis for the distribution of magnetic foliation plane we obtained 23 reliable flow directions that are predominantly horizontal and directed away from identified crustal reservoirs this flow pattern supports the proposed model of vpm growth and emphasizes the localized nature of the magma sources in the mantle the entire flood basalt sequence appears to have been fed by a restricted number of crustal reservoirs and associated dyke swarms

deciphering the variscan tectonothermal overprint and deformation partitioning in the cadomian basement of the tepla barrandian unit bohemian massif the tepla barrandian unit tbu has long been considered as a simply bivergent supracrustal median massif above the saxothuringian subduction zone in the variscan orogenic belt this contribution reveals a much more complex style of the variscan tectonometamorphic overprint and resulting architecture of the neoproterozoic basement of the tbu for the first time we describe the crustal scale ne sw trending dextral transpressional krakovec shear zone ksz that intersects the tbu and thrusts its higher grade northwestern portion severely reworked by variscan deformation over a southeastern very low grade portion with well preserved cadomian structures and only brittle variscan deformation the age of movements along the ksz is inferred as late devonian 380 370 ma on the basis of structural microstructural and anisotropy of magnetic susceptibility data from the ksz we propose a new synthetic model for the deformation partitioning in the tepla barrandian upper crust in response to the late devonian to early carboniferous subduction and underthrusting of the saxothuringan lithosphere we conclude that the saxothuringian tepla barrandian convergence was nearly frontal during 380 346 ma and was partitioned into pure shear dominated domains that accommodated orogen perpendicular shortening alternating with orogen parallel high strain domains that accommodated dextral transpression or bilateral extrusion the synconvergent shortening of the tbu was terminated by a rapid gravity driven collapse of the thickened lithosphere at 346 337 ma followed by or partly simultaneous with dextral strike slip along the baltica margin parallel zones driven by the westward movement of gondwana from approximately 345 ma onwards

petrogenesis of syenites at a rifted continental margin origin contamination and interaction of alkaline mafic and felsic magmas in the astrophyllite bay complex east greenland the astrophyllite bay complex in east greenland part of the palaeogene north atlantic igneous province consists of an alkaline diorite plug with detached trachyandesitic pillows surrounded by co magmatic syenite that was emplaced into archaean basement the diorite intrusion has yielded a 47 11 0 68 ma rb sr isochron age saw cut profiles through pillow syenite gneiss sections have been taken to resolve close spatial elemental and isotopic sr nd hf pb o variations the diorite and syenite formed from alkaline basaltic mantle derived melts with complex histories of prolonged assimilation and fractional crystallisation each evolved to different extents in separate magma chambers during the establishment of new plumbing systems in the kangerlussuaq area the diorite is dominated by lower crustal granulite facies contamination whereas the syenite shows evidence for greater degrees of upper crustal amphibolite facies contamination indicating stalling and fractionation of magmas at different levels within the crust the syenite and diorite magmas were subsequently emplaced as separate pulses into the basement gneisses at astrophyllite bay giving rise to superimposed local contamination trends between pillow syenite and syenite gneiss respectively

regional and local patterns of low grade metamorphism in the north shore volcanic group minnesota usa the north shore volcanic group in northern minnesota is part of the middle proterozoic keweenawan sequence one of the largest plateau lava provinces in the world the primary geochemistry of the basalts suggests that volcanism occurred in an intracontinental rift environment the subaerial lava flows mainly amygdaloidal olivine tholeiites and tholeiites have undergone low grade metamorphism from zeolite to lower greenschist facies on the basis of alteration phases replacing the primary magmatic minerals infilling amygdales and veins and replacing secondary minerals the following zones have been distinguished 1 thomsonite scolecite smectite 2 heulandite stilbite smectite 3 laumontitechlorite albite 4 laumontite chlorite albite prehnite pumpellyite and 5 epidote chlorite albite actinolite zone in addition to the overall zonation based on mineral parageneses zonations in the composition of the ab content of the newly formed albite replacing primary ca rich plagioclase and of the newly formed mafic phyllosilicates are observed within the sequence and within single flows mafic phyllosilicates in the upper part of the sequence mainly smectites and mixed layer smectite chlorites display high si and ca na k contents whereas in the lower part of the sequence the amounts of si and ca na k are markedly lower mainly chlorites and mixed layer chlorite smectites similar zonations are observed within the individual flows the albite content of the newly formed plagioclase is highest and the si and ca na k content of the phyllosilicates lowest in the amygdaloidal flow top while the opposite is true for the massive flow interior the above features suggest that the overall pattern is one of burial type metamorphism associated with extension in the rift setting in detail the mineral assemblages are controlled not only by the stratigraphic position but also by the flow morphology controlling permeability whose effect on the assemblages is most pronounced in the stratigraphically upper parts this suggests that at the first stages of alteration lowest grade the patterns of fluid flow were important effects in controlling the assemblages at greater burial depth assemblages are more homogeneous perhaps representative of a more even and pervasive flow pattern using the observed assemblages at face value to define grade and or facies different conditions would be assigned within the different morphological flow portions thus at low grade metamorphic conditions it is essential to integrate assemblages from different morphological flow portions in order to define satisfactorily the overall metamorphic conditions