formation and maintenance of shear zones the processes that initiate and maintain prograde ductile shear zones are not well understood we have performed shear experiments 1 to 3 on a fine grained 100 150 m gneiss 59 quartz 28 plagioclase 13 aligned but not interconnected biotite to determine the evolution of deformation mechanisms that produce dramatic strain weakening and localization at conditions 1 5 gpa 800 c dot gamma 2 10 5 s where pure quartz aggregates deform homogeneously by dislocation creep initial yield occurs where stress concentrations at the tips of weak biotite grains produce semibrittle deformation in intervening quartz or plagioclase allowing local biotite interconnection by slip on 001 and initiating strain weakening after yield the interconnected biotites kink and thus strengthen but the highly strained parts of grains react to a fine grained mixed phase assemblage which deforms by grain size sensitive creep allowing further strain weakening and localization at 3 3 the strain and strain rate in the 100 m thick shear zone are 100 times that of the enclosing host rock similar to the localization observed in natural shear zones thus the processes that initiate strain localization are not necessarily the same as those that preserve the weak shear zone and once formed a shear zone may be permanently weakened

archaean granulites of the limpopo belt zimbabwe one slow exhumation or two rapid events the peak of high temperature low pressure anticlockwise granulite facies metamorphism in the northernmost part of the northern marginal zone of the limpopo belt has been dated at 2591 4 ma using conventional u pb zircon geochronology on a syntectonic charnockite intrusion the thermal peak of 800 c at 0 5 gpa reflects a strong perturbation of the geotherm and was reached during compressional deformation and coincides with vast granite intrusion into the overlying craton the coincidence of the temperature t maximum and compression is thought to be a response to lithospheric mantle thinning that resulted in granite emplacement and a consequent change in upper crustal rheology after initial exhumation and cooling these high temperature low pressure rocks resided at a comparatively shallow depth 10 km for 620 ma field microstructural and geochronological investigation of a low temperature overprint establishes that reworking and final exhumation occurred at 1 97 ga along discrete greenschist facies thrusts the 40ar 39ar laser ablation of hornblende and biotite and pb pb step leaching of sphene are shown to be useful geochronometers in dating the low grade assemblages our results indicate that early proterozoic deformation lasted 10 to 20 ma however a number of ages intermediate between the events at 2 6 and 2 0 ga have been reported for the northern marginal zone these include pb pb garnet ar ar step heating amphibole and u pb zircon ages such ages suggest that these minerals were partially rejuvenated below their commonly believed closure temperature reinterpreting these intermediate ages as partial overprint or inheritance was possible in the northern marginal zone due both to the large time gap between the two tectonometamorphic events and the use of a multimethod multichronometer approach these observations call for great caution when extrapolating mineral ages into cooling histories especially so when the resulting cooling is slower than thermally and or mechanically possible and suggests that the geochronometry in other regions that underwent multiple tectono metamorphic events may need to be reassessed

mechanisms of weak phase interconnection and the effects of phase strength contrast on fabric development abstract to determine how deformation mechanisms and fabrics evolve during the strain weakening and localization that accompany the transition from a load bearing framework to an interconnected weak phase shear experiments were conducted on a fine grained gneiss 58 quartz interconnected 28 plagioclase dispersed 13 biotite aligned but dispersed at 745 and 800 c 1500 mpa and two strain rates 2 10 5 and 2 10 6 s 1 in experiments with a high phase strength contrast hpsc 25 and 45 between dispersed biotite and framework quartz grains biotite grains become interconnected due to stress concentration at their tips that allow local semi brittle flow of intervening quartz and cataclasis of intervening plagioclase in the higher temperature hpsc samples the biotite partially dehydrated in high strain zones a single narrow shear zone formed because the psc between the quartz and biotite reaction product layers remained high in the lower temperature hpsc samples there was no reaction and a penetrative s c fabric formed the fabric is defined by the many multiply interconnected biotite strands that formed because the psc between the quartz and biotite decreased as the biotites kinked in slower strain rate experiments with a much lower psc 10 biotite interconnection occurs by shearing into quartz quartz boundaries where new weak strain free recrystallized grains form at low strain a weak s c fabric forms but it evolves to an s c fabric as the psc decreases with strain thus the magnitude of the strength contrast between a weak phase and its matrix influences the mechanism of weak phase interconnection the degree of strain localization and the fabric

the evolution of the serie dei laghi strona ceneri and scisti dei laghi the upper component of the ivrea verbano crustal section southern alps north italy and ticino switzerland abstract the serie dei laghi is the upper intermediate component of the exposed crustal section of massiccio dei laghi in the southern alps the latter including also the ivrea verbano zone its oldest event is a sedimentary cycle at least of early paleozoic age it starts with a pelitic sequence scisti dei laghi that later suffered a tectono metamorphic event of rather high pressure this event is recorded by rare mineral and textural relics detected in the scisti dei laghi of the m gambarogno area later an arenaceous sequence strona ceneri was deposited it is now represented at the base by basaltic metatuffites resulting from the reworking of an ophiolite belt then followed by paragneisses with detrital phenoclasts ceneri gneiss and by fine grained gneisses gneiss minuti during the ordovician conspicuous tonalitic to granitic masses were emplaced the induced thermal disturbance and the circulation of pegmatitic stage magmatic residuals caused contact metamorphism and granitization in the sedimentary country rocks during the hercynian a main prograde event generated the regional foliation with isoclinal geometry and transposed character a syn kinematic stage produced medium grade parageneses of relatively high pressure static recrystallization and sillimanite growth then followed during the uplift due to the combined action of hydration and decompression margarite bearing coronitic textures grew around kyanite a retrograde metamorphism was then recorded by the appearance of greenschist facies parageneses at particular structural sites the formation of the cossato mergozzo brissago cmb and pogallo fault system against the ivrea verbano zone as well as the schlingen folds in the northeast is of late hercynian age along the cmb fault calc alkaline dykes and stocks appinites were intruded probably towards the end of the upheaval but deep enough to induce partial melting in their country rocks between val d ossola and valsesia the hercynian evolution is concluded by the early permian magmatic cycle that gave rise to both extrusive rhyolitic sheets and to granitic epiplutonic bodies accompanied by mafic dykes in mesozoic times the serie dei laghi was dissected by many extensional faults these partly reactivated the hercynian lineaments such as the cmb and pogallo lines during the alpine orogenic cycle the serie dei laghi was affected by s verging cataclastic overthrusts

syn tectonic magmatism and the development of compositional layering ungava orogen northern quebec canada abstract layer and foliation parallel emplacement of granitic veins was an important process in the regional development of compositional layering in now exhumed middle crustal sections of both archean and paleoproterozoic age in the northern ungava peninsula quebec canada in the archean superior province diorite and tonalite plutons were penetratively deformed and metamorphosed at granulite facies conditions coeval with voluminous granitic magmatism the paleoproterozoic narsajuaq arc contains evidence for contemporaneous magmatism transpressional deformation and granulite facies metamorphism prior to its collision with the superior province basement in both plutonic domains regional compositional layering is defined by 1 metre to kilometre scale alternation of generally well foliated tonalite and quartz diorite bodies and 2 centimetre to kilometre scale variably deformed granitic veins and sheets that lie parallel to layering tectonic foliation in the host rocks syn tectonic intrusion of a substantial portion of the veins along extension fractures sub parallel to layering foliation i e at high angle to the regional shortening direction is interpreted to have occurred due to the combination of a strong anisotropy and high magma pressures compositional layering generated and or enhanced by this process may contribute to the overall seismic reflectivity of the middle and lower crust

modelling flanking structures using deformable high axial ratio ellipses insights into finite geometries flanking structures develop locally around material inhomogeneities the evolution of these structures is investigated here by modelling peturbations around a deformable ellipse using a 2d analytical solution non unique instantaneous and finite geometries are predicted and it is shown that in terms of slip and curvature complex histories are possible under simple shear whereas under pure shear instantaneous geometries map directly into finite state geometries single flanking structures are of limited use in kinematic analysis but can help constrain the kinematics when interpreted in conjunction with other structural features flanking structures exhibiting a range of ce cross cutting element orientations have more potential as kinematic indicators flanking structures serve as an excellent example of the role that material homogeneity can play in locally producing complex structures in a relatively simple bulk flow field

large scale folding in the upper part of the ivrea verbano zone nw italy new geological mapping has led to a new interpretation of the large scale superimposed folding in the upper part of the ivrea verbano zone italian alps the region is widely held to represent an upended section through lower continental crust of northern italy the dominant fold structure extending some 40 km along strike is the massone tight to isoclinal antiform with a hinge line strongly curved through 115 this folds pre existing large scale folds that formed during regional migmatization probably during the hercynian orogeny to form a type 2 interference geometry the region then suffered post orogenic mafic magmatic underplating and other magmatism accompanied by crustal stretching with contact metamorphism and migmatization causing the imposition of the final pattern of metamorphic isograds the ivrea verbano zone was brought into contact with the overlying metamorphic rocks of the serie dei laghi on a major shear zone sub solidus stretching continued though displacements on low angle high temperature shear zones most of the ivrea verbano zone was finally tilted to the vertical and emplaced into its present position after the mesozoic era and probably during alpine orogenesis forming the vertical limb of a crustal scale double kink

on the valency state of radiogenic lead in zircon and its consequences in zircon u pb systematics extreme robustness up to the temperatures of granulite facies and anatexis contrasts with apparently easy loss of radiogenic pb at low temperatures often without any metamorphic event being in evidence here we propose that this paradoxical behaviour can be understood with the hypothesis that radiogenic pb in zircon is tetravalent we review data and arguments in favour of this hypothesis diffusion profiles calculated for pb sup 2 in a 25 micro m radius zircon xenocryst in a melt at 1000 c combined with the incompatibility of pb sup 2 or for a zircon core inside a younger zircon rim at this temperature show age effects that should have been observed in sims dating further in zircon evaporation as well as in leaching experiments common pb is generally released preferentially to radiogenic pb after removal of less radiogenic pb the evaporation record generally shows pure radiogenic pb during the final evaporation steps the distribution of residual pb in a leached titanite revealed by pixe is similar to that of ti lastly xanes spectra of a 1 ga old titanite predominantly radiogenic pb and an alpine one predominantly common pb are significantly different although the former does not resemble that ofmore pbo sub 2 the arguments why radiogenic pb should be tetravalent are based on analogies with studies relating to the tetravalent state of sup 234 th and the hexavalent state of sup 234 u which show that alpha recoil in silicates generates a strongly oxidizing environment at the site where the recoiling nucleus comes to rest further a zircon grain being small should remain highly oxidizing in its interior by the constant loss of beta particles maintaining the 4 state of radiogenic pb from its effective ion radius similar to that of zr sup 4 and its charge pb sup 4 has to be compatible in the zircon lattice also by analogy with u sup 4 th sup 4 and hf sup 4 its diffusivity should be several orders of magnitude lower than that measured for pb sup 2 in zircon these factors can account for the extreme retentivity of u pb clocks even at high temperatures on the other hand radiogenic pb situated in alpha recoil damaged sites could be leached out by any electrolyte solution that reduces it to the divalent state making it both incompatible and soluble thus discordia can be generated in weathering the curious observation that discordant archaean zircon suites generally define trends to lower intercepts at up to 800 ma instead of zero appears to reflect a c 14 to 28 better average accessibility to leaching of radiogenic sup 207 pb compared to sup 206 pb in accord with the somewhat higher recoil energy of the last branched alpha decays in the sup 235 u chain less

multistage accretion and exhumation of the continental crust ivrea crustal section italy and switzerland the ivrea crustal section exposes in map view all levels of the southern alpine continental crust from ultramafic mafic and felsic granulite facies rocks of the deep crust ivrea verbano zone through medium grade basement rocks strona ceneri zone and val colla zone to unmetamorphosed permo mesozoic sediments the oldest part of the crustal section is preserved in the medium grade basement units which are interpreted to be the overprinted remains of an ordovician 440 480 ma magmatic arc or forearc complex during variscan subduction this arc was technically underplated by a carboniferous accretion subduction complex 320 355 ma containing metasediments and slivers of rheic oceanic crust presently found in the ivrea verbano zone during the late stages of variscan convergence 290 320 ma lithospheric delamination triggered magmatic underplating and lead to polyphase deformation under amphibolite to granulite facies conditions this was broadly coeval with extensional exhumation and erosion of the variscan overprinted ordovician crust presently exposed in the strona ceneri and val colla zones post variscan transtensional tectonics 270 290 ma were associated with renewed magmatic underplating mylonitic shearing and incipient exhumation of the lower crust in the ivrea verbano zone this coincided with the formation of elongate basins filled with volcanoclastic sediments in the upper crust early mesozoic tethyan rifting of the southern alpine crust 180 230 ma reduced crustal thickness to 10 km or less in the lower crust most of this thinning was accommodated by granulite to retrograde greenschist facies mylonitic shearing the lower crust was exhumed along a large noncoaxial mylonitic shear zone that was linked to asymmetrical rift basins in the upper crust the composite structure resulting from this complex evolution is probably typical of thinned late variscan continental crust on the passive margins of western europe alpine faulting and folding 20 50 ma fragmented the crustal section the originally deepest levels of the crustal section in the ivrea verbano zone as well as some segments of the basement cover contact were steepened whereas other parts of the crustal section particularly the strona ceneri zone underwent only minor to moderate alpine rotation

circulation of tampa bay driven by buoyancy tides and winds as simulated using a finite volume coastal ocean model 1 the circulation of tampa bay is investigated using a high resolution three dimensional density dependent finite volume coastal ocean model fvcom that includes tampa bay the intracoastal waterway and the inner portion of the west florida continental shelf model performance over the three month interval september to november 2001 is assessed against available tide gauge and velocity profiler data before using the model to describe the circulation as driven by rivers tides and winds because of a mean wind velocity vector directed down the estuary axis we ran a parallel model experiment without winds to distinguish the estuarine circulation by gravitational convection from the mean wind effects with or without winds tampa bay exhibits a robust two layered estuarine circulation that concentrates on the deep channels the mean outflow at the surface tends to converge on the channels where the free surface elevation is locally minimum the mean inflow near the bottom also concentrates in the channels where the baroclinic pressure gradient force is largest geometry thus guides the mean circulation and salinity distributions at the tampa bay mouth mean outflows exist both in the deeper egmont channel and the shallower south pass whereas a mean inflow is limited to the egmont channel a residence time based on the egmont channel influx is estimated to be about 100 days consistent with previous studies we conclude that gravitational convection is a major contributor to the water property distributions of tampa bay and that the fvcom is suitable for estuary shelf interaction studies

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

magmatic underplating extension and crustal reequilibration insights from a cross section through the ivrea zone and strona ceneri zone northern italy the thermal impact of magmatic underplating at various crustal levels is studied along a traverse through the ivrea verbano zone and strona ceneri zone in northern italy geochronological and petrologic data are compared to a two dimensional thermal kinematic model field data and numerical simulation show the strong disturbance of the temperature field in the lower and intermediate crust in relation to magmatic underplating leading to granulite to amphibolite facies metamorphism as well as reequilibration of mineral chemical and isotopic systems magmatic underplating leaves a crust with an apparently heterogeneous tectonometamorphic evolution as information on the earlier history is preserved only at upper crustal levels

single mineral pbpb dating a new stepwise leaching procedure applied to ca bearing silicates enables the selective recovery of radiogenic and common pb from a mineral and thus makes single phase pbpb dating possible this permits independent dating of rock forming minerals within a paragenetic sequence and the direct combination of age data with petrological results furthermore the stepwise leaching technique has the advantage over multi phase pbpb dating in that it does not require initial pb isotope equilibrium among the phases but may be used to test it the presence of microscopic solid inclusions which complicate dating of single phases if they are not in equilibrium with the host can be traced via the behaviour of the 208pb 204pb ratio through the various leaching steps this constitutes a major advantage over conventional bulk phase pbpb dating in addition the precision of an isochron age can be improved relative to that of the bulk mineral approach since the 207pb 204pb and 206pb 204pb data spread is significantly enhanced therefore the technique may both be used to date younger phases and or minerals with less favourable u pb ratios and to extend the applicability of self consistent p t t modelling

westward continuation of the craton limpopo belt tectonic break in zimbabwe and new age constraints on the timing of the thrusting the umlali thrust zone has been mapped between the granite greenstone rocks of the zimbabwe craton and the granulite facies rocks of the limpopo belt a tectonic break had not been previously recognized in the southwest because of the intrusion of syn to post kinematic porphyritic granites these intrusions locally obliterate evidence for the tectonic break although in some localities the associated mylonites are preserved as xenoliths within the intrusive porphyritic granite bodies the porphyritic granite bodies were emplaced during and after the main regional fabric forming event a u pb zircon age of 2627 7 ma obtained from a late kinematic microgranite is interpreted to be the minimum time of thrusting a whole rock rb sr age of 2583 52 ma obtained from plutonic rocks and retrogressed zones within the north marginal zone falls within error of the zircon age suggesting widespread magmatic and hydrothermal activity coeval with the thrusting

proterozoic transpressive deformation in the northern marginal zone limpopo belt zimbabwe a major tectono metamorphic event affected the southern half of the northern marginal zone of the limpopo belt at 2 0 ga structural analysis along a 43 km long profile perpendicular to strike shows extension inclined toward the northeast extension parallel to axes of folds that uplift the southeast of the section i e the central zone of the limpopo belt and dextral shear these deformation patterns developed coevally the lineations form distinct tightly clustered groups that occur together in the profile there is no evidence for superimposed deformation or deformed lineations nor are lineations observed cross cutting each other this single episode of deformation can therefore be called transpression petrographic and thermobarometric evidence support this kinematic picture in that decreases of both pressure 2 5 kb and temperature 150 c are documented from southeast to northwest across strike 39 ar 40 ar step heating of metamorphic hornblende from amphibolites has yielded ages of

zircon and monazite petrochronologic record of prolonged amphibolite to granulite facies metamorphism in the ivrea verbano and strona ceneri zones nw italy abstract in order to improve the understanding of thermal tectonic evolution of high grade terranes we conducted a systematic study of textures ree content and u pb ages of zircon and monazite grains extracted from migmatitic metapelites across the amphibolite to granulite facies metamorphic gradient exposed in the ivrea verbano and strona ceneri zones italy this study documents the behaviour of these accessory minerals in the presence of melt the absence of relict monazite grains in the metasediments and the gradual decrease in the size of inherited zircon grains from amphibolite to granulite facies cores indicate partial to total dissolution of accessory minerals during the prograde path and partial melting the retrograde path is marked by i growth of new zircon rims r1 and r2 around inherited cores in the mesosome ii crystallisation of stubby zircon grains in the leucosome especially at granulite facies and iii crystallisation of new monazite in the mesosome stubby zircon grains have a distinctive fir tree zoning and a constant th u ratio of 0 20 together these features reflect growth in the melt conversely the new zircon grains with r1 rims have dark prismatic habits and th u ratios in the ivrea verbano zone the interquartile range iqr of u pb ages on zircon and monazite are interpreted to represent the length of zircon and monazite crystallisation in the presence of melt accordingly they provide an indication on the minimum duration for high temperature metamorphism and partial melting of the lower crust 20 ma and 30 ma in amphibolite and granulite facies respectively in amphibolite facies zircon crystallisation between 310 and 294 ma iqr is interpreted to reflect metamorphic peak condition and earlier retrograde history conversely monazite crystallisation between 297 and 271 ma iqr reflects cooling under 750 c to a temperature close to the solidus in granulite facies zircon crystallisation between 295 and 265 ma iqr is interpreted to reflect high temperature conditions which were attained after peak of metamorphism during isothermal decompression and subsequent cooling under 850 950 c the observed decrease of u pb ages in metamorphic zircon and monazite from amphibolite to granulite facies i e from the middle to the lower crust is interpreted to record slow cooling and crystallisation of the variscan orogenic root at the transition from orogenic collapse to opening of the tethys ocean

ree fractionation and nd isotope disequilibrium during crustal anatexis constraints from himalayan leucogranites abstract the processes controlling rare earth element ree behaviour during crustal anatexis have profound implications not only for ree abundances of crustally derived granites but also for their model nd ages for typical protoliths of anatectic granites such as are exposed in metapelites from the high himalayan crystalline series hhcs a high proportion 90 of bulk rock lree and mree are sited in the accessory phases apatite and monazite and of hree in garnet and apatite mass balance constraints indicate that observed ree abundances of himalayan leucogranites cannot be generated from ree deficient reactant phases in their metasedimentary protoliths muscovite biotite plagioclase and quartz but must result from the dissolution of accessory phases ree and th abundances in anatectic melts are modelled by combining empirical expressions for monazite apatite and zircon solubilities as a function of temperature melt composition and melt water content with analysed accessory phase ree abundances this approach which allows the contribution to the melt from each accessory phase to be identified successfully predicts himalayan leucogranite trace element abundances the lree and th concentrations of a metapelite derived melt are buffered by monazite stability although apatite dissolution contributes up to 10 of lree to the melt together with 50 of the mree and 90 of the hree zircon contributes a negligible proportion of ree to the melt during anatexis dissolution of monazite and apatite in the melt results in 1 increasing ree abundances and lree hree ratios with increasing temperature of melting 2 a negative eu anomaly in the melt 3 high sm nd ratios in the melt relative to its metasedimentary protolith and 4 nd isotope disequilibrium between the melt and its protolith all of these features are observed in the ree geochemistry of himalayan leucogranites the combined effects of sm nd fractionation and nd isotope disequilibrium during anatexis imply that model nd ages calculated for anatectic granites derived from apatite and monazite bearing protoliths should be treated with considerable caution

displacement of migrating bowhead whales by sounds from seismic surveys in shallow waters of the beaufort sea seismic surveys for subsea oil deposits were conducted each summer 1996 98 mainly in water 20 m deep airgun arrays were used with 6 16 airguns and total volumes 560 1500 cu in low frequency sound pulses were created at intervals of 8 20 s effective source levels for horizontal propagation were lower than nominal source levels but pulses were often detectable to 50 km offshore westward autumn migration of bowhead whales near and offshore of the exploration area was monitored by aerial surveys flown daily weather permitting during the three seasons aerial survey data from days with and without airgun operations were compared most bowheads avoided the area within 20 km of the operating airguns bowheads were common there on days without airgun operations in 1998 numbers sighted 20 30 km away were also significantly reduced during airgun operations conversely sighting rates just beyond the avoidance zone were higher on days with airgun operations broadband received levels of airgun pulses at 20 km were typically 120 130 db re 1 pa rms over pulse duration lower than those previously demonstrated to cause avoidance by bowheads many migrating bowheads 20 to 50 km offshore were exposed to weaker but presumably detectable pulses work supported by western geophysical and bp exploration alaska inc

initial isotopic heterogeneity and secondary disturbance of the smnd system in fluorites and fluid inclusions a study on mesothermal veins from the central and western swiss alps abstract this paper presents sm nd data on hydrothermal fluorites fluid inclusions and leach experiments on samples from the mont blanc and gotthard units swiss alps both areas have experienced alpine greenschist facies overprint 350 and 450 c respectively mont blanc samples represent late variscan fluorites from three individual veins whereas gotthard samples were taken from one hand specimen of probable late variscan age nd concentrations of fluorites range from 3 6 to 23 ppm 147 sm 144 nd ratios from 0 12 to 0 35 none of the samples sets yielded an isochron relationship imperfect correlations indicate apparent ages between late variscan and alpine reflecting neither the formation nor the age of metamorphic overprint reasons are wall rock influence during formation and post formation disturbance secondary fluorite gotthard shows the highest nd concentration and a low sm nd ratio close to that of fluid inclusion fractions a leach experiment showed the same trend even more pronounced 7 of nd and 2 of sm were removed from fluorite by 0 18 n hcl at 100 c hence secondary fluids are likely to leach and fractionate sm and nd and may easily cause dissolution and reprecipitation of fluorite consequently the system is highly suspicious if secondary fluid flow cannot totally be ruled out this is in sharp contrast to previously published conclusions however evidence for rejuvenation of the sm nd system in fluorites is also found in published data sets

dating synmagmatic folds a case study of schlingen structures in the strona ceneri zone southern alps northern italy the strona ceneri zone southern alps contains folds with moderately to steeply inclined axial planes and fold axes and amplitudes of up to several kilometres so called schlingen these amphibolite facies folds deform the main schistosity of late ordovician metagranitoids and are discordantly overlain by unmetamorphic permian sedimentary rocks mutually cross cutting relationships between these folds and garnet bearing leucotonalitic dykes indicate that these dykes were emplaced during folding sm nd systematics and the strongly peraluminous composition of these dykes point to an anatectic origin pb step leaching of magmatic garnet from a leucotonalitic dyke yielded a 321 3 2 3 ma intrusive age rb sr ages on muscovites from leucotonalitic dykes range from 307 to 298 ma interpreted as cooling ages during retrograde amphibolite facies metamorphism conventional u pb data of zircons from an older granodioritic dyke that pre dates the schlingen folds yielded discordant u pb ages ranging from 371 to 294 ma these ages reflect a more complicated multi episodic growth history which is consistent with the observed polyphase structural overprint of this dyke schlingen folding was accompanied by prograde amphibolite facies metamorphism during the thermal peak of which the leucotonalitic dyke material was generated by partial melting in a deeper source region from where these s type magmas intruded the presently exposed level because partial melting may occur in a relatively late stage of a clockwise p t t path or even during decompression on the retrograde path we do not exclude the possibility that schlingen folding had already started in early carboniferous time schlingen folds also occur in penninic and austroalpine basement units with a very similar pre alpine history indicating that variscan folding affected large segments of the future alpine realm