petrology and geochronology of muscovite age standard b4m muscovite b4m distributed in 1961 as an age standard was ground under ethanol five grain size fractions were obtained and characterized by x ray diffraction they display a mixing trend between a phengitic enriched in the fraction 20 m high pressure phengite is preserved as a relict in retrograde muscovite electron microprobe analyses of the distributed mineral separate reveal at least four white mica populations based on si al mg na fe and f rb k ratios vary by one order of magnitude rb sr analyses link the mineralogical heterogeneity to variable rb sr and 87sr 86sr ratios the grain size fractions define no internal isochron relict fine grained phengite gives older ages than coarse grained retrograde greenschist facies muscovite the inverse grain size age relationship also characterizes 39ar 40ar analyses cl k anticorrelates with step ages cl rich coarse muscovite is younger than cl poor fine relict phengite sr and ar preserve a similar isotopic inheritance despite peak metamorphism reaching 635 20 c a suitable mineral standard requires that its petrological equilibrium first be demonstrated relicts and retrograde reaction textures are a guarantee of isotopic disequilibrium and heterogeneous ages within single crystal at the micrometre scale

the kinematics of movements along the insubric line and the emplacement of the ivrea zone schmid sm zingg a and handy m 1987 the kinematics of movements along the insubric line and the emplacement of the ivrea zone in h j zwart m martens i van der molen c w passchier c spiers and r l m vissers editors tectonic and structural processes on a macro meso and micro scale tecta a hys 135 47 66 the insubric line west of locamo is characterized by a 1 km thick greenschist facies mylonite belt east of locamo these mylonites are overprinted by a discrete brittle fault the mylonites are derived from basement units of the central alps sesia zone and the southern alps ivrea zone as well as from the permo mesozoic cover of the southern alps canavese sense of shear criteria indicate that the mylonites accomm ated backt s g followed by dextral strike slip motion mylonitization during backtbrusting was synchronous with backfolding of the central alpine nappes under higher metamorphic conditions a horizontal temperature gradient resulted from the rapid juxtaposition of the warm central alpine block against the cold southern alpine block mylonites formed during the later dextral strike slip event are related to large transcurrent displacements in the central alps deduced from regional kinematic considerations thus both mylo t ation events are contempor eous with deformation to the north and south of the insubric line insubric phase extensively modifying the pre insubric crustal configuration of the alps the insubric phase post dates the bergell intrusion 30 m y the emplacement of the geophysical ivrea body is a combined effect of vertical uplift due to e w directed crustal thinning during the early jurassic and underplating by continental crust associated with late cretaceous compression a deep crustal normal fault pogallo line subsequently rotated during tertiary alpine orogenesis separates deeper parts of the southern alpine crust ivrea zone from intermediate crustal levels strona ceneri zone the rigid ivrea body localir ed large strains within the insubric mylonite belt and is responsible for the present curvature of the insubric line in uetion

metamorphic field gradients in the central alps metamorphic field gradients were determined across the entire amphibolite grade central alps c 50 100 km p t were calculated from 116 samples acquired from our own field work from samples provided to us by others and from rocks with mineral compositions described in the literature only fluid conserved equilibria were used to determine p t the use of an internally consistent thermodynamic database and mineral solid solution models makes the results robust and reduces relative errors the results are presented in contour maps temperature increases from 500 to 550 c along the limit of amphibolite grade metamorphism in the north and west to c 675 c toward the south at the insubric line near the town of bellinzona maximum recorded pressures of c 7 kbar are in a central region c 20 km north of the insubric line and decrease both to the north 5 5 kbar and south 4 5 kbar the p t results indicate that there is a relatively large area that reached conditions in the sillimanite stability field but developed neither sillimanite nor fibrolite this is interpreted as a result of kinetic constraints on nucleation and growth because of the small amounts of thermal overstep 40 c of the kyanite sillimanite phase boundary comparison of p t conditions with carbonate isograds in the region indicate that fluids present during metamorphism were not dominated by a homogeneous external source examination of the two dimensional distribution of pressure and temperature in the context of thermal and tectonic models indicates that two thermal pulses aected the central alps during the tertiary in the second heat aected only the southern parts of the area and overprinted the previously established p t gradients

alpine and pre alpine magmatism in the root zone of the western central alps the highest grade of metamorphism and associated structural elements in orogenic belts may be inherited from earlier orogenic events we illustrate this point using magmatic and metamorphic rocks from the southern steep belt of the lepontine gneiss dome central alps the u pb zircon ages from an anatectic granite at verampio and migmatites at corcapolo and lavertezzo yield 280 290 ma i e hercynian ages these ages indicate that the highest grade of metamorphism in several crystalline nappes of the lepontine gneiss dome is pre alpine alpine metamorphism reached sufficiently high grade to reset the rb sr and k ar systematics of mica and amphibole but generally did not result in crustal melting except in the steep belt to the north of the insubric line where numerous 29 to 26 ma old pegmatites and aplites had intruded syn and post kinematically into gneisses of the ductile simplon shear zone the emplacement age of these pegmatites gives a minimum estimate for the age of the alpine metamorphic peak in the monte rosa nappe the u pb titanite ages of 33 to 31 ma from felsic porphyritic veins represent a minimum age estimate for alpine metamorphism in the sesia zone a porphyric vein emplaced at 448 5 ma u pb monazite demonstrates that there existed a consolidated caledonian basement in the sesia zone

protracted fluid induced melting during barrovian metamorphism in the central alps the timing and dynamics of fluid induced melting in the typical barrovian sequence of the central alps has been investigated using zircon chronology and trace element composition multiple zircon domains in leucosomes and country rocks yield u pb ages spanning from 32 to 22 ma the zircon formed during alpine melting can be distinguished from the inherited and detrital cores on the basis of their age th u 0 1 and trace element composition ti in zircon thermometry indicates crystallization temperatures around 620 700 c their composition allows discriminating between 1 zircon formation in the presence of early garnet 2 zircon in equilibrium with abundant l mree rich accessory phases allanite titanite and apatite typical of metatonalites and 3 zircon formed during melting of metasediments in feldspar dominated assemblages the distribution of zircon overgrowths and ages indicate that repeated melting events occurred within a single barrovian metamorphic cycle at roughly constant temperature that in the country rocks zircon formation was limited to the initial stages of melting whereas further melting concentrated in the segregated leucosomes that melting occurred at different times in samples a few meters apart because of the local rock composition and localized influx of the fluids and that leucosomes were repeatedly melted when fluids became available the geochronological data force a revision of the temperature time path of the migmatite belt in the central alps protracted melting over 10 my followed the fast exhumation of alpine eclogites contained within the same region and preceded fast cooling in the order of 100 c ma to upper crustal levels

shear zone patterns and strain distribution at the scale of a penninic nappe the suretta nappe eastern swiss alps abstract shear zone patterns within the suretta nappe of the swiss alps are studied in order to understand finite strain and bulk tectonic displacements the suretta nappe is chosen for two main reasons the availability of late variscan granites deformed during alpine tectonics only and because of continuous outcrop from bottom to top of the nappe this alpine nappe recorded two major ductile deformation phases which developed under low grade metamorphic conditions the analysis of shear zone patterns in the roffna granite reveals that the first deformation is well developed in the entire nappe the second deformation however is more localized in the lower and upper parts the bulk asymmetry of the shear zone patterns confirms the kinematics recorded by other penninic nappes in this area first a stacking of the nappe towards the nw followed by the top to the e shearing during post thickening extensional tectonics the shear zone patterns and strain distribution allow us to deduce kinematics at the nappe scale

nondestructive chemical dating of young monazite using xrf 1 design of a mini probe age data for samples from the central alps and comparison to u pb tims data the analytical capabilities of an improved mini xrf instrument for trace element analysis are explored to obtain u th pb ages for single monazite grains as small as 50 m the detection limits for u th and pb with counting times of 10 min are 10 ppm counts collected for 40 min are sufficient to date single monazite grains as young as 15 ma the precision of the age data depends essentially on the counting statistics of the main x ray peak npb l 1 2 used to analyze for lead the age uncertainty is proportional to the age of the grain the total contents of th and u and the squared integration time the reliability of the chemical dating method is tested using monazite separates from amphibolite facies metamorphic monazite samples from the central alps for which the chemical ages are compared to mass spectrometric u pb tims ages we show that populations of analyses obtained using the new instrument on single grains as young as 25 30 ma agree within error with the tims data for monazite separates with complex age patterns such as polymetamorphic samples yielding discordant isotopic ages the statistical analysis of chemical age data obtained from populations of 26 50 grains is advantageous in that it may provide additional insight into the reasons for the discordancy chemical th u pb dating of monazite by the xrf method offers considerably improved accuracy and precision compared with chemical dating by electron microprobe which typically becomes impractical for samples less than 100 200 ma in age conversely the minimum grain size required for the new instrument 50 m is well above the spatial resolution of the electron microprobe 2 m a companion paper scherrer et al this volume describes further developments towards microanalysis by xrf with improved spatial resolution

grustal shortening in the alpine orogen results from deep seismic reflection profiling in the eastern swiss alps line nfp 20 east the deep crustal seismic line nfp 20 east crosses almost the entire swiss alps despite the complex geometry of the well exposed nappe structure and the considerable axial plunge of some of the units the vibroseis survey yielded coherent reflections from several individually identifiable nappe contacts in the northern part of the survey the vibroseis data closely match the internal structure of the helvetic nappes and the underlying autochthonous parautochthonous mesozoic sediments on the northern flank of the aar massif an external basement uplift these mesozoic sediments seem to rise from a depth of approximately 7 8 km below sea level to the surface in a series of steps which is interpreted to represent crustal shortening achieved by a combination of folding and thrusting in the southern part of the survey it was possible to image a number of thin slivers of mesozoic carbonates pinched between slabs of penninic basement nappes as well as nappe contacts between lithologically contrasting units in addition it seems that the insubric fault zone which marks the contact between the penninic zone and the southern alps and which outcrops about 30 km to the south of the survey shows up as steeply north dipping reflections the lower european crust in the northern part of the survey is relatively transparent as opposed to the adriatic lower crust whose reflective nature may stem from shear zones associated with mesozoic crustal stretching the base of both the european and adriatic crust coincides with a 1 s thick band of laterally discontinuous reflections this reflection moho drops to greater depths going from the north toward the center of the alpine chain where it disappears with a steep southerly dip the moho reappears as a reflection band farther south this moho gap is situated above the lithospheric root and may be caused by perturbations related to subduction of lower crustal material the crustal scale structure obtained from the vibroseis data may be interpreted as a continent continent collision with wedge shaped indentation of a piece of adriatic crust into the european crust and vertical escape of the material in the core of the orogen along steeply dipping faults

cooling and uplift patterns in the lepontine alps south central switzerland and an age of vertical movement on the insubric fault line 96 new fission track ft apatite and zircon k ar and rb sr biotite and muscovite ages are presented for 19 samples mainly acid gneisses from a 40 km traverse through the lepontine alps in the maggia valley south central switzerland plotting measured mineral ages against assumed system closure temperatures yields cooling rates for each sample the entire profile shows a fairly uniform late neogene recent mean uplift rate of 0 5 mm a confirmed by a gradient of ft apatite age with elevation cooling from higher temperatures occurred earlier in the south where uplift rates of 2 2 mm a in the steep belt root zone indicate 9 km early miocene uplift of the northern pennine block this uplift started before 23 ma and is interpreted as resulting from a major phase of backthrusting along the insubric line and as dating the formation of the mylonite belt estimated cooling rates constrain the timing of lepontine mid tertiary metamorphism 3 schematic models are proposed which also consider published rb sr k ar mica and hornblende and u pb monazite ages slow cooling differential initial heating and subsequent cooling of different parts of the central alps and post 38 ma cooling with syntectonic metamorphism at 27 ma are postulated as alternative interpretations of isotopic data and geologic evidence from extrapolation between k ar and rb sr mica ages and apatite ft ages 240 50 c is proposed as the closure temperature for the retention of fission tracks in zircon

geochronological evidence for continuous exhumation through the ductile brittle transition along a crustal scale low angle normal fault simplon fault zone central alps 1 major low angle normal faults juxtapose different structural levels of the crust that record both brittle and ductile deformation field relationships alone cannot establish whether these different responses to deformation represent 1 parts of a single process of exhumation along the detachment or 2 two separate events with the later more discrete brittle detachment exhuming a fossil ductile shear zone from depth these two general models are critically assessed for the low angle normal simplon fault zone sfz in the central alps the sfz shows a spatial transition from a broad ductile mylonitic shear zone to a discrete brittle detachment with identical kinematics the age of the ductile shear zone and ductile brittle transition is controversial we present a detailed geochronological study based on fission track 40ar 39ar and rb sr microsampling dating coupled with structural petrological and chemical analyses that provides tight constraints on sfz timing discontinuous mineral cooling ages over a broad range of temperatures across the fault zone argue for fault activity between 20 and 3 ma on the basis of synkinematic white mica in low temperature shear zones and necks of foliation boudinage the brittle ductile transition in the footwall could be dated at 14 5 10 ma overall the data presented here are consistent with a continuous transition from ductile shearing to a more localized zone of brittle deformation within the same geological framework over a period of 15 ma the sfz is therefore an example of a telescoped crustal section within a single major low angle fault involving a continuous period of exhumation rather than a two stage structure

magnetic study of a late alpine dike crosscutting the regional foliation abstract dikes crosscutting existing folds or foliation are one of the classic time markers used in establishing relative geological chronology we examine the paleomagnetic directions and magnetic fabric in a deformed gneiss from the southern steep belt which lies immediately north of the insubric fault and in an aplite dike that transects this foliation no clear foliation or other evidence of internal deformation is discernible in the aplite dike in the field and it was originally interpreted as post tectonic with its intrusion age placing a young limit on the timing of deformation in the adjacent gneiss the aplite dike does not carry a stable remanent magnetization and cannot be used to resolve the age of intrusion the characteristic paleomagnetic remanence in the gneiss is compatible with eurasian paleomagnetic directions during the last 20 ma these results suggest that the timing of magnetization postdates dextral motion along the insubric and rhine rhone faults the low field anisotropy of magnetic susceptibility ams in the gneiss is controlled by the regional schistosity high field ams suggests that both paramagnetic and ferromagnetic minerals contribute to the magnetic fabric whereby the ferromagnetic minerals show a stronger lineation this is further confirmed by the anisotropy of anhysteretic remanent magnetization aarm rock magnetic experiments show that multi domain magnetite is the main ferromagnetic mineral in the gneiss unexpectedly the ams results from the aplite dike show that it also has a magnetic fabric which is shown by high field ams aarm and rock magnetic experiments to be carried by superparamagnetic magnetite the presence of magnetic fabric in the dike demonstrates that regional deformation was still on going in the southern steep belt of the central alps at the time of its intrusion

metamorphic rates in collisional orogeny from in situ allanite and monazite dating the prograde sequence of rare earth minerals recorded in metapelites during regional metamorphism reveals a series of irreversible reactions among silicates and phosphates in individual samples from the northern lepontine central alps allanite is partly replaced by monazite at 560 580 c relic allanite retains its characteristic growth zoning acquired at greenschist facies conditions 430 450 c coexisting monazite and allanite were dated in situ to delimit in time successive stages of the barrovian metamorphism in situ sensitive high resolution ion microprobe shrimp u th pb dating of allanite 31 5 1 3 and 29 2 1 0 ma and monazite 18 0 0 3 and 19 1 0 3 ma constrains the time elapsed between 430 450 c and 560 580 c which implies an average heating rate of 8 15 c m y combined with new fission track ages zircon 10 9 ma apatite 7 5 6 5 ma metamorphic rates of the entire orogenic cycle from prograde to final cooling can be reconstructed

metalliferous mining geophysics state of the art after a decade in the new millennium mining exploration was very active during the first decade of the twenty first century because there were numerous advances in the science and technology that geophysicists were using for mineral exploration development came from different sources instrumentation improvements new numerical algorithms and cross fertilization with the seismic industry in gravity gradiometry kept its promise and is on the cusp of becoming a key technology for mining exploration in potential field methods in general numerous techniques have been developed for automatic interpretation and 3d inversion schemes came into frequent use these inversions will have even greater use when geologic constraints can be applied easily in airborne electromagnetic em methods the development of time domain helicopter em systems changed the industry in parallel improvements in em modeling and interpretation occurred in particular the strengths and weaknesses of the various algorithms became better understood simpler imaging schemes came into standard use whereas layered inversion seldom is used in the mining industry today improvements in ground em methods were associated with the development of squid technology and distributed acquisition systems the latter also impacted ground induced polarization ip methods developments in borehole geophysics for mining and exploration were numerous borehole logging to measure physical properties received significant interest perhaps one reason for that interest was the desire to develop links between geophysical and geologic results which also is a topic of great importance to mining geologists and geophysicists

alpine and pre alpine subduction events in polycyclic basements of the swiss alps coupling macrostructural and microstructural observation with metamorphic petrology of polymetamorphic mafic rocks occurring in basement rocks of the central alps clearly provides evidence that some part of this basement underwent at least two different and independent events of high pressure metamorphism the older eclogitic event occurred at middle to high temperature and must have been pre alpine because late variscan plutons crosscut the high pressure structures the younger event occurred at high pressure and low temperature conditions and is tertiary in age the separation of alpine from pre alpine eclogite facies events is discussed for several alpine nappes

titanium in quartz thermometry on synkinematic quartz veins in a retrograde crustal scale normal fault zone abstract previous studies have suggested that estimation of deformation temperatures in quartz mylonites by titanium in quartz geothermometry is only possible at temperatures 500 c above which efficient ti exchange is achieved via grain boundary migration recrystallization based on quartz mylonite samples collected across the simplon fault zone sfz we demonstrate that deformation temperatures of dynamic recrystallization can be obtained down to 350 c a prerequisite for such temperature estimates at the low temperature end of ductile deformation of quartz is the formation of synkinematic quartz veins and their immediate overprint either by subgrain rotation sgr or bulging recrystallization blg it is the slow growth of the synkinematically precipitating vein quartz that allows for equilibration of ti in the vein quartz this ti concentration may only slightly be modified during sgr hence ti in qtz thermometry provides a close approach to the vein formation temperature ti concentrations are partially reset during blg and resulting temperatures are thus maximum temperatures of quartz recrystallization importantly undeformed vein quartz always yield vein formation temperatures investigation of the dynamic recrystallization processes overprinting synkinematic quartz veins thus allows for a critical independent evaluation of the ti in quartz temperatures obtained for the sfz there is a decrease in recrystallized grain sizes towards the fault plane and a change in the dominant recrystallization process associated with a narrowing of the shear zone as indicated by the ti in quartz temperature estimates this strain localization correlates with cooling from 560 c in the oldest microstructures at the periphery of the shear zone down to 350 c in the youngest microstructures of the footwall near the hanging wall contact a great benefit of the approach presented here is that intermediate to low temperature plastic deformation in quartz can now also be assessed such novel temperature constraints on quartz crystallization are essential for better constraining deformation and rheology in the upper earth s crust

a quantitative appraisal of airborne and ground based transient electromagnetic tem measurements in denmark the last decade has seen growing use of ground based transient electromagnetic tem methods in denmark for hydrogeological purposes due to an intensified mapping campaign airborne tem methods were proposed as a possible tool for mapping large areas the first test flights were flown in june 2000 using the geotem system traditional approximate interpretation tools for airborne data are insufficient in hydrogeological investigations where a quantitative model specifying model parameter reliability is needed we have carried out full nonlinear one dimensional inversion on the field amplitude of airborne synthetic and field data and compared the airborne method with the traditional ground based protem 47 system that has found extensive use in denmark an improved measuring procedure for airborne systems is suggested to facilitate the estimation of noise that is necessary in a quantitative inversion the analyses of synthetic data demonstrate the differences in resolution capability between ground based and airborne data ground based data typically resolve three or four layer models and occasionally up to five layers airborne data resolve three layers as a maximum one or two layers being common the airborne geotem system detects layers to depths of more than 300 m bearing only little information about the top 50 70 m the ground based protem 47 system has a maximum penetration of approximately 170 m with higher resolution capabilities in the top 100 m coupling to man made conductors is a serious problem for all tem methods in densely populated areas and results in distorted data coupling influences the airborne data from denmark on two thirds of the area covered these data must be eliminated to avoid misinterpretation

robust estimation of the band limited inductive limit response from impulse response tem measurements taken during the transmitter switch off and the transmitter off time theory and an example from voisey s bay labrador canada modern transient electromagnetic systems are able to take measurements in the transmitter on time integrating measurements taken during the transmitter switch off and those collected in the transmitter off time yield an estimate of the primary field plus the secondary inductive limit response if the transmitter loop position is known and the position and orientation of the receiver dipoles are known it is possible to calculate the primary field when the theoretical primary field is subtracted from the measured inductive limit plus primary response the inductive limit response can be isolated an anomalous inductive limit response is a diagnostic feature of highly conductive ore bodies on and off time protem data collected in a drill hole proximal to the reid brook zone one of the voisey s bay deposits in labrador canada shows a strong inductive limit anomaly corresponding to an off hole conductor a drill hole targeted to test this conductor intersected 20 4 m of mineralization including 8 25 m

the maggia nappe an extruding sheath fold basement nappe in the lepontine gneiss dome of the central alps the lepontine gneiss dome represents a unique region of the central alps where oligocene miocene amphibolite facies grade rocks and fold nappes of the deepest tectonic level of the alpine orogenic belt are exposed in a tectonic window the cenozoic structures of the maggia nappe reveals a giant tens of kilometre scale tubular fold structure that cross cut through the surrounding lower penninic nappes from its root situated in the southern steep belt of the alps near bellinzona the mesozoic sedimentary cover of the maggia nappe is typical for the helvetic stratigraphic domain the age of formation of the lower penninic fold nappes by ductile detachment of the upper european crust during its underthrusting below the higher penninic and austroalpine orogenic lid and adriatic indenter was estimated between 40 and 30 ma maximal pressures of 8 9 kbars and temperatures of 600 700 c were attended during and after the nappe emplacement some 30 22 ma ago the maggia and surrounding nappes are crosscut by the isograds of the barrovian regional metamorphism

combined high precision chronometry and geochemical tracing using accessory minerals applied to the central alpine bergell intrusion central europe abstract the behaviour of nd u th and pb isotopes in granitic rocks is usually controlled by accessory minerals in this study an approach is adopted which uses the initial 143 nd 144 nd as a geochemical tracer together with high precision u th pb geochronology for accessory phases and their respective whole rock this combination of techniques is applied to zircon allanite sphene apatite and thorite from both a tonalite and a granodiorite from the tertiary alpine bergell intrusion inherited isotopic signatures have been identified in bulk zircon fractions by inverse discordance in the u pb system in the case of the granodiorite the initial nd in the zircon is 1 77 units lower than the bulk rock this result indicates that nd isotopes in zircon may serve as a useful tool for tracing crustal precursors in silicic rocks all other accessories yield an initial nd equal to their respective whole rock and u th pb ages equal to or lower than the respective intrusion ages this indicates either the absence of inherited cores or that such cores have exchanged their nd and pb in the case of apatite also sr by diffusion where available experimental mineral melt equilibria and diffusion models are consistent with this behaviour u pb geochronology of carefully selected core free zircons yields a precise intrusion age of 31 88 0 09 ma for the tonalite in contrast granodioritic zircons are partially reset due to radiation damage caused by high u concentrations however an alternative means of dating young intrusions is to use 232th 208pb ages of allanite in the case of allanite the presence of a large excess of 206pb derived from 230th indicates that no total pb loss has occurred since crystallization and the nd isotopes indicate the absence of inheritance using both the allanite age and a 238u 206pb age for sphene an intrusion age of 30 13 0 17 ma is indicated for the granodiorite taken together with an allanite th pb age for the tonalite these results strongly support the high closure temperatures for the allanites studied here 700 800 c at high post magmatic cooling rates all other accessory mineral ages are reset to values similar to hornblende 39ar 40ar ages and reflect the effect of post magmatic cooling superimposed on post metamorphic cooling

the composite magnetic anomaly map of the east antarctic abstract airborne and marine magnetic observations in east antarctica and adjacent seas of the indian ocean were compiled for a magnetic anomaly map of the antarctic for east antarctica over 260 000 line km of russian reconnaissance magnetic data were used that had been collected since 1955 mainly at line spacings of about 5 20 and 50 km for the offshore areas magnetic data from american australian german japanese and russian marine expeditions were incorporated digitally recorded data and data digitized from published and unpublished maps and profiles were included in the compilation local grids of these data were developed and merged into a regional grid at an interval of 5 km the prime product of this compilation is a shaded relief map that shows the most complete and coherent perspective to date of the region s magnetic character in combination with other types of data the compilation provides new insight on the tectonic features and history of this largely inaccessible region of the world it maps out approximately 4300 km of the antarctic continental margin magnetic anomaly acmma related to gondwana breakup new cratons and mobile belts and large submarine igneous provinces

eocene ultra high temperature uht metamorphism in the gruf complex central alps constraints by la icpms zircon and monazite dating in petrographic context the gruf complex in the lepontine alps is one of the rare occurrences of phanerozoic uht metamorphism in the world but its age is still a matter of debate here we present la icpms dating in petrographic context of zircon and monazite from an uht restitic granulite zircons and monazites are both included in large crystals and in retrograde symplectites in such restitic rocks partial melting or fluid interactions are unlikely precluding resetting of the monazite chronometers zircon cores yield permian ages interpreted as age of charnockitisation they are sometimes surrounded by a narrow rim at 32 ma monazites are strongly zoned but all yield a 31 8 0 3 ma 15 age interpreted as the time of complete re crystallisation during the uht paragenesis we propose that the zircons dated a post hercynian metamorphism which is responsible of the widespread formation of granulites in the southern alps and the crust differentiation this fluid absent melting event produced refractory lithologies such as restites in charnockites we suggest that gruf uht paragenesis is alpine in age and cristallised from these refractory lithologies we conclude that the lower restitic crust produced at the permian time had the ability to achieve uht conditions during the fast exhumation and heating related to lithospheric thinning in alpine time

a ternary solid solution model for omphacite and its application to geothermobarometry of eclogites from the middle adula nappe central alps switzerland a ternary solid solution model for omphacite with the end members jadeite naalsi2o6 diopside camgsi2o6 and hedenbergite cafesi2o6 was derived from experimental data from the literature the subregular solution model fitted by linear programming is best suited to omphacites with very little aegirine component in common eclogites applying this solution model to the calculation of equilibrium phase diagrams of eclogites from the adula nappe central alps switzerland results in large stability fields for common eclogite assemblages garnet omphacite quartz h2o kyanite within this field the compositions of garnet and omphacite show very little variation a precise determination of the peak pressure and temperature is not possible the occurrence of amphibole overgrowing the peak pressure assemblage in fresh eclogite suggests retrograde re equilibration still under eclogite facies conditions the computation of isopleths for garnet and pyroxene end members allows the estimation of the pressure and temperature conditions of this re equilibration event 19 21 kbar c 700 c

dehydration melting and water saturated melting of basaltic and andesitic greenstones and amphibolites at 1 3 and 6 9 kb

valid garnet biotite gb geothermometry and garnet aluminum silicate plagioclase quartz gasp geobarometry in metapelitic rocks abstract at present there are many calibrations of both the garnet biotite gb thermometer and the garnet aluminum silicate plagioclase quartz gasp barometer that may confuse geologists in choosing a reliable thermometer and or barometer to test the accuracy of the gb thermometers we have applied the various gb thermometers to reproduce the experimental data and data from natural metapelitic rocks of various prograde sequences inverted metamorphic zones and thermal contact aureoles we have concluded that the four gb thermometers perchuk l l lavrent eva i v 1983 experimental investigation of exchange equilibria in the system cordierite garnet biotite in saxena s k ed kinetics and equilibrium in mineral reactions springer verlag new york berlin heidelberg pp 199 239 kleemann u reinhardt j 1994 garnet biotite thermometry revised the effect of alvi and ti in biotite european journal of mineralogy 6 925 941 holdaway m j 2000 application of new experimental and garnet margules data to the garnet biotite geothermometer american mineralogist 85 881 892 model 6av kaneko y miyano t 2004 recalibration of mutually consistent garnet biotite and garnet cordierite geothermometers lithos 73 255 269 model b are the most valid and reliable of this kind of thermometer more specifically we prefer the holdaway holdaway m j 2000 application of new experimental and garnet margules data to the garnet biotite geothermometer american mineralogist 85 881 892 and the kleemann and reinhardt kleemann u reinhardt j 1994 garnet biotite thermometry revised the effect of alvi and ti in biotite european journal of mineralogy 6 925 941 calibrations due to their small errors in reproducing the experimental temperatures and good accuracy in successfully discerning the systematic temperature changes of the different sequences in addition after applying the gasp barometer to 335 natural metapelitic samples containing one kind of aluminosilicate and 33 natural metapelitic samples containing two kinds of aluminosilicate as well as to rocks in limited geographic areas and thermal contact aureoles we propose that the calibrations of holdaway holdaway m j 2001 recalibration of the gasp geobarometer in light of recent garnet and plagioclase activity models and versions of the garnet biotite geothermometer american mineralogist 86 1117 1129 and newton and haselton newton r c haselton h t 1981 thermodynamics of the garnet plagioclase al2sio5 quartz geobarometer in newton r c navrotsky a wood b j eds thermodynamics of minerals and melts new york springer verlag 131 147 based on kleemann and reinhardt s kleemann u reinhardt j 1994 garnet biotite thermometry revised the effect of alvi and ti in biotite european journal of mineralogy 6 925 941 thermometer are the most valid gasp barometers other gb thermometers and gasp barometers are not recommended

the effects of retrograde reactions and of diffusion on 40ar 39ar ages of micas the effects of metamorphic reactions occurring during decompression were explored to understand their influence on the ar ar ages of micas monometamorphic metasediments from the lepontine alps switzerland reached lower amphibolite facies during the barrovian metamorphism related to the collision between european and african adria continental plates mineral assemblages typically composed of garnet plagioclase biotite muscovite and paragonite or margarite were screened for petrological equilibrium to focus on samples that record a minimum degree of retrogression x ray diffraction data indicate that some mineral separates prepared for ar ar stepwise heating analysis are monomineralic whereas others are composed of two white micas muscovite with paragonite or margarite or biotite and chlorite in monomineralic samples ar ar and ar ar proportional to ca k and cl k ratios did not change and the resulting ages can be interpreted unambiguously in mineral separates containing two white micas ca k and cl k ratios were variable reflecting non simultaneous laboratory degassing of the two heterochemical ar reservoirs these ratios were used to identify each ar reservoir and to unravel the age in a chlorite margarite biotite calcschist equilibrated near 5608c and 0 65 gpa biotite margarite and muscovite all yield ages around 18ma at slightly higher grade 560 5808c 0 8 0 9 gpa the assemblage muscovite paragonite plagioclase is in equilibrium and remains stable during retrogression in this case muscovite and paragonite yield indistinguishable ages around 16 5ma above 5908c paragonite was mostly consumed to form plagioclase 45908c whereby the relict mica yields an age up to 5 6 ma younger than muscovite this partial or total resetting of the ar clock in paragonite is interpreted to reflect plagioclase growth during decompression where biotite is present within this same assemblage it systematically yields a younger age than muscovite by 0 5 2 ma however these biotites all show small amounts of retrograde chlorite formation we conclude that even very minor chloritization of biotite is apparently a more effective process than temperature in resetting the ar clock as is the formation of plagioclase from paragonite decomposition multi equilibrium thermobarometry is an excellent means to ensure that equilibrium in investigated samples is preserved and this helps to obtain geologically meaningful metamorphic ages however even samples passing such equilibrium tests may still show retrograde effects that affect the ar retention of micas a more robust interpretation of such ar ar results may require use of a second geochronometer such as u pb on monazite

empirical garnet muscovite geothermometry in metapelites abstract two empirical garnet muscovite geothermometers assuming no ferric iron model a and 50 ferric iron model b in muscovite respectively were calibrated under the physical conditions of p 3 0 14 0 kbar and t 530 700 c the input temperatures and pressures were determined by simultaneously applying the garnet biotite thermometer am mineral 85 2000 881 and the gasp geobarometer am mineral 86 2001 1117 to natural metapelites to confirm internal thermodynamic consistency holdaway s am mineral 85 2000 881 garnet mixing properties were adopted muscovite was treated as a symmetric fe mg al vi ternary solid solution and its margules parameters were derived in this work the resulting two formulae reproduced the input garnet biotite temperatures well within 50 c and gave identical results for a great body of natural samples moreover they successfully distinguished the systematic changes of temperatures of different grade rocks from a prograde sequence inverted metamorphic zone and thermal contact aureole pressure estimation has almost no effect on the two formalisms of the garnet muscovite geothermometer assuming analytical error of 5 for the relevant components of both garnet and muscovite the total random uncertainty of the two formulations will generally be within 5 c the two thermometers derived in this work may be used as practical tools to metamorphic pelites under the conditions of 480 to 700 c low to high pressure in the composition ranges xalm 0 51 0 82 xpyr 0 04 0 22 and xgros 0 03 0 24 in garnet and fe tot 0 03 0 17 and mg 0 04 0 14 atoms p f u in muscovite

styles of footwall uplift along the simplon and brenner normal fault systems central and eastern alps the simplon and brenner extensional shear zones of the central and eastern alps respectively are low angle detachments that accommodated orogen parallel extension and unroofed midcrustal rocks in their footwalls an integrated field and fluid inclusion study of the simplon low angle shear zone reveals several postmylonitic structure types that are consistent with passage of a complex rolling hinge through the simplon footwall including synthetic west down and antithetic east down semibrittle and brittle structures and strike slip extensional and contractional structures all of which are consistent with southwest directed extension along the main detachment zone fluid inclusion analysis of kinematically referenced inclusion arrays indicates that west down semibrittle and brittle shears were active at conditions above 300 450 c and 14 26 km antithetic deformation along brittle faults occurred at much shallower conditions of 300 c and 3 7 km deformation along these structures accommodated footwall uplift by a subvertical simple shear mechanism in the southern part of the simplon region these results are strikingly similar to those from the brenner shear zone in contrast with the brenner region however the northern simplon footwall shows strike slip dominated postmylonitic deformation related to slip along the rhone line the footwalls of these detachment systems define a unique class of core complex characterized by the development of a midcrustal synthetic lower hinge that acts to steepen the detachment system and accommodate footwall uplift by a subvertical simple shear mechanism both systems have poorly developed upper hinges that flatten the middle to upper crustal ramp by only a few degrees of dip which is in contrast to detachment fault systems of the western united states where evidence of a deep hinge is rarely observed and the upper hinge acts to deactivate the flattened updip part of the detachment our results emphasize the role of tectonic setting buoyancy forces and synkinematic fluids in controlling the observed style and mechanisms of large magnitude extension during collision between continental masses

tectonic overpressure in weak crustal scale shear zones and implications for the exhumation of high pressure rocks 1 a two dimensional numerical simulation of lithospheric shortening shows the formation of a stable crustal scale shear zone due to viscous heating the shear zone thickness is controlled by thermomechanical coupling that is resolved numerically inside the shear zone away from the shear zone lithospheric deformation is dominated by pure shear and tectonic overpressure i e pressure larger than the lithostatic pressure is proportional to the deviatoric stress inside the shear zone deformation is dominated by simple shear and the deviatoric stress decreases due to thermal weakening of the viscosity to maintain a constant horizontal total stress across the weak shear zone i e horizontal force balance the pressure in the shear zone increases to compensate the decrease of the deviatoric stress tectonic overpressure in the weak shear zone can be significantly larger than the deviatoric stress at the same location implications for the geodynamic history of tectonic nappes including high pressure ultrahigh pressure rocks are discussed

basement cover relationships in the tambo nappe central alps switzerland geometry structure and kinematics in the pennine zone of the central alps the tambo nappe forms a thin crystalline sliver embedded in the mesozoic cover four tertiary alpine deformations are observed the d1 ductile deformation is linked to the progressive eocene stacking of the nappes towards the northwest during d1 basement deformation is governed by imbricate tectonics whereas cover is thin skinned and intensively folded these different structures reflect the original strong rheological contrast between basement and cover during the heterogeneous and ductile d2 deformation the behaviour of the basement and cover became similar the strong vertical d2 shortening associated with a top to the east shear led to the folding of the prior se dipping structures developing se vergent folds with axes close to the e w l2 stretching lineation d2 corresponds to post collisional crustal thinning following d2 crustal thickening the d3 and d4 deformations occurred under retrograde conditions and can be correlated with uplift and late dextral movement on the insubric line respectively

mechanisms of mass and heat transport during barrovian metamorphism a discussion based on field evidence from the central alps switzerland northern italy 1 tectonic and metamorphic data for the central alps switzerland italy are used to discuss this classic example of a barrovian metamorphic terrain notably the evolution of its thermal structure in space and time available p t t data indicate variable contributions of advective and conductive heat transport during collision and subsequent cooling and exhumation some areas experienced a prolonged period of partial melting while other areas at the same time show but moderate heating the barrow type metamorphic field gradient observed in the final orogen is the result of two distinct tectonic processes with their related advective and conductive heat transport processes the two tectonic processes are 1 accretion of material within a subduction channel related to decompression and emplacement of high pressure units in the middle crust and 2 wedging and related nappe formation in the continental lower plate the second process postdates the first one wedging and underthrusting of continental lower plate material produces heat input into lower crustal levels and this process is responsible for predominantly conductive heat transport in the overlying units the interacting processes lead to different maximum temperatures at different times producing the final barrovian metamorphic field gradient the south experienced rapid cooling whereas the north shows moderate cooling rates this discrepancy principally reflects differences in the temperature distribution in the deeper crust prior to cooling differences in the local thermal gradient that prevailed before the cooling also determined the relationships between cooling rate and exhumation rate in the different areas citation berger a s m schmid m engi r bousquet and m wiederkehr 2011 mechanisms of mass and heat transport during barrovian metamorphism a discussion based on field evidence from the central alps switzerland northern italy tectonics 30 tc1007 doi 10 1029 2009tc002622

3 d assessment of peak metamorphic conditions by raman spectroscopy of carbonaceous material an example from the margin of the lepontine dome swiss central alps this study monitors regional changes in the crystallinity of carbonaceous matter cm by applying micro raman spectroscopy to a total of 214 metasediment samples largely so called bundnerschiefer dominantly metamorphosed under blueschist to amphibolite facies conditions they were collected within the northeastern margin of the lepontine dome and easterly adjacent areas of the swiss central alps three dimensional mapping of isotemperature contours in map and profile views shows that the isotemperature contours associated with the miocene barrow type lepontine metamorphic event cut across refolded nappe contacts both along and across strike within the northeastern margin of the lepontine dome and adjacent areas further to the northeast the isotemperature contours reflect temperatures reached during the late eocene subduction related blueschist facies event and or during subsequent near isothermal decompression these contours appear folded by younger large scale post nappe stacking folds a substantial jump in the recorded maximum temperatures across the tectonic contact between the frontal adula nappe complex and surrounding metasediments indicates that this contact accommodated differential tectonic movement of the adula nappe with respect to the enveloping bundnerschiefer after maximum temperatures were reached within the northern adula nappe i e after late eocene time

fast exhumation of the ultrahigh pressure alpe arami garnet peridotite central alps switzerland constraints from geospeedometry and thermal modelling previous studies suggest that the metamorphic evolution of the ultrahigh pressure garnet peridotite from alpe arami was characterized by rapid subduction to a depth of c 180 km with partial chemical equilibration at c 5 9 gpa 1180 c and an initial stage of near isothermal decompression followed by enhanced cooling in this study average cooling rates were constrained by diffusion modelling on retrograde fe mg zonation profiles across garnet porphyroclasts considering the effects of temperature pressure and garnet bulk composition on the fe mg interdiffusion coefficient cooling rates of 380 1600 c myr 1 for the interval from 1180 to 800 c were obtained similar or even higher average cooling rates resulted from thermal modelling whereby the characteristics of the calculated temperature time path depend on the shape and size of the hot peridotite body and the boundary conditions of the cooling process the very high cooling rates obtained from both geospeedometry and thermal modelling imply extremely fast exhumation rates of c 15 mm yr 1 or more these results agree with the range of exhumation rates 16 50 mm yr 1 deduced from geochronological results it is suggested that the alpe arami peridotite passively returned towards the surface as part of a buoyant sliver caused as a consequence of slab breakoff

the tectonometamorphic history of the peridotitic chiavenna unit from mesozoic to tertiary tectonics a restoration controlled by melt polarity indicators eastern swiss alps abstract occurrences of mantle and lower crustal rocks are often preserved in mountain ranges when they occur in the internal part of collision belts their initial geometry and palaeogeography are sources of debate the mafic ultramafic chiavenna unit is one example in the pennine units located in the eastern swiss alps because of strong deformation and metamorphism during tertiary collision tectonics the pre collision geometry of this unit can only be restored using a qualitative geometrical analysis of the successive ductile deformation phases d1 d4 in situ melting and melt migration during the main alpine deformation phase d2 is a way up indicator and is of special importance to this restoration the restored prealpine geometry of the chiavenna unit indicates that a sub continental mantle close to a thinned continental margin becomes denuded during the mesozoic extension and subsequently covered by carbonate sediments taking into account the position of the chiavenna unit in the nappe pile and its pt path a northern brianconnais situation during mesozoic time becomes most probable

geochronological evolution of hp metamorphic rocks of the adula nappe central alps in pre alpine and alpine subduction cycles abstract shrimp dating of zircon from eclogites their country rock gneisses and quartz veins from the northern and middle adula nappe central alps accompanied by cathodoluminescence imaging and ree geochemistry of these dated zircon crystals revealed that the adula nappe has been affected by two pre alpine high pressure hp metamorphic events northern c 330 340 ma middle c 370 ma in the northern adula nappe loss of radiogenic pb of the c 330 340 ma zircon domains in combination with previous geochronological data argue for an overprinting alpine hp event in the middle adula nappe metamorphic zircon domains record in addition a 33 32 ma event interpreted to reflect ht metamorphism already identified in the southern part of the nappe where such shrimp ages have been related to ht lepontine metamorphism the alpine hp event indicated by previous shrimp data in the southern adula nappe at c 35 ma is not recorded in the zircons of the rocks dated herein the absence in this dataset is probably because either no hp c 35 ma zircon domains were formed or such domains were completely reset during the 33 32 ma ht event protolith ages of orthogneisses and eclogites are 470 457 ma and 595 587 ma respectively and maximum sedimentation ages of paragneisses 500 460 ma the lack of hercynian protolith zircon ages implies pre hercynian formation for the precursors of the dated hp rocks our data suggest that the adula nappe contains various hercynian basement slivers that underwent subduction to hp conditions at c 330 340 ma and c 370 ma prior to the alpine orogeny similar to many other parts of the hercynian basement in central and southern europe supplementary material u pb and th shrimp data and ree and trace element analyses are availaible at http www geolsoc org uk sup18367

the metamorphic evolution of garnet cordierite sillimanite gneisses of the gruf complex eastern pennine alps metapelitic gneisses occuring as lenses and bands within the migmatites of the gruf complex in the eastern pennine alps contain various combinations of the minerals quartz biotite cordierite garnet sillimanite plagioclase k feldspar spinel orthopyroxene anthophyllite and muscovite the most common rock type is represented by a darkschistosebiotiterichcordierite garnet sillimanite gneiss a consistent pressure temperature range of 3 4 kb and 600 650 c has been calculated for the last metamorphic equilibration from six geological thermobarometers

empirical garnet muscovite plagioclase quartz geobarometry in medium to high grade metapelites abstract based on the net transfer reactions among garnet muscovite plagioclase and quartz for both mg and fe end member models the garnet muscovite plagioclase quartz gmpq geobarometry was empirically calibrated under the physical conditions of p 1 0 11 4 kbar and t 505 745 c for 128 natural metapelitic rock samples collected from the literature the input temperatures and pressures were simultaneously determined by the garnet biotite thermometer and the garnet aluminosilicate plagioclase quartz gasp barometer the gmpq calibrations adopted the same asymmetric quaternary solid solution model of garnet and the same al avoidance asymmetric ternary model of plagioclase as the calibrations of the garnet biotite geothermometer and the gasp geobarometer a symmetric fe mg al vi ternary solid solution model of muscovite was adopted and the margules parameters of muscovite were obtained through regression the mg and fe model reactions along with the assumption of whether the ferric iron content in muscovite is 0 or 50 resulted in four gmpq barometry formulae the gmpq barometry formulae reproduce the input gasp pressures well within 1 0 kbar mostly within 0 5 kbar for both aluminosilicate bearing and aluminosilicate absent samples the gmpq barometry formulae yield identical pressures for every sample whether the sample was included or not in calibrating the barometers for each of the mg or fe model reaction the formulae gave identical pressures within 40 bars the random error of the gmpq barometry may be expected as 1 4 kbar the d p d t slopes of these gmpq formulae are close to that of the gasp barometer in the p t space applications of the gmpq barometry to aluminosilicate absent metapelites within a limited geographic area without postmetamorphic structural discontinuity generally show no pressure difference it may be concluded that the gmpq barometry formulae derived in this work may be used as practical tools for metamorphic pelites under the conditions of 505 745 c and 1 11 4 kbar in the composition range of x gros 3 in garnet and x an 17 in plagioclase

petrochronology based on ree minerals monazite allanite xenotime apatite ree minerals monazite xenotime and allanite are ree1 minerals sensu stricto because lanthanides la lu and yttrium are critical constituents in them apatite does not require ree but because it contains substantial ree in many rocks it is included in this review all four minerals also host unusually high radionuclide concentrations notably th and u forming the basis of their utility as geochronometers this quartet of accessory minerals is playing an increasingly important role in petrochronology because they provide ways to link robust spot ages to petrogenetic p t conditions so can lend petrogenetic context to chronology based on other minerals part i of this review assembles the basic requisites prior to integrative petrochronologic analysis individual characteristics of the four ree minerals are addressed first i e their crystal chemistry and stability relations thermobarometers and trace element geochemistry used for tracing petrogenesis are discussed next and finally their chronology is summarized part ii presents case studies to highlight the specific strengths of ree minerals used to resolve the dynamics of a broad range of processes from diagenetic to magmatic conditions finally a brief section at the end outlines a few of the current challenges and promising perspectives for future work to introduce the four ree minerals in style let us recall the origins of their names the three phosphates have well respected greek grandparents and allanite has solid scottish roots yet of all four of them show idiosyncracies in etymology or type material apatite had long puzzled naturalists as it shows great chemical and physical variability and can resemble other minerals once properly identified abraham gottlieb werner named it apatite his reasoning referred to the greek root and giving the precise latin translation decipio taken literally both mean i deceive or i mislead which sounds like an apt confession from this mineral for having fooled

aeromagnetic data from the southern weddell sea embayment and adjacent areas synthesis and interpretation a new regional aeromagnetic anomaly map covering the southern weddell sea and surrounding areas was produced by combining russian and british magnetic anomaly data acquired between 1973 and 1989 the map is divided into four zones on the basis of characteristic anomaly wavelength antarctic peninsula weddell sea embayment haag nunataks and the western coast of east antarctica over the southern weddell sea and the ronne and filchner ice shelves the magnetic field is relatively smooth indicating a thick sedimentary cover although there are some distinctive anomalies a broad positive anomaly is clearly visible running w e across the north of the area which is interpreted as representing the continent ocean boundary the most prominent magnetic feature is the elongate explora anomaly which runs parallel to the coast of east antarctica and continues over berkner island terminating at the dufek massif we interpret the source of this anomaly as rift related volcanic rocks probably emplaced around the same time as the dufek intrusion the rift itself is interpreted as running south between the andenes escarpment and east antarctica and terminating south of berkner island russian gravity profiles show that berkner island does not have an associated bouguer anomaly although a large positive anomaly approx 80 mgal is seen to the east of the island over the filchner ice shelf suggesting the presence of a second rift structure gravity modelling across these features shows thinned crust below the ronne and filchner ice shelves indicating that the whole area has undergone significant extension a gravity model across the flanks of berkner island is used to estimate the ratio of unstretched to stretched crustal thickness the resulting value of approximately 2 7 indicates that enough extension has taken place to induce melting supporting the idea that the source of the magnetic anomaly over berkner island is rift related volcanic rocks

partial melting of two amphibolites contrasting experimental results under fluid absent conditions a large portion of the lower continental crust may be amphibolitic in composition and without a free fluid phase as a consequence h2o undersaturated or fluid absent melting of amphibolites may be responsible for the formation of some granites and migmatites produced during major orogenic events in an attempt to determine the systematics of melting under fluid absent conditions a series of piston cylinder experiments was conducted on two natural amphibolites one a meta alkali basalt aba with a total water content of wt contained in hornblende and the other a meta island arc tholeiite iat which has 1 1 3 wt water contained in hornblende cummingtonite and biotite the experimentally determined melting ranges of the two amphibolites showed that the solidus temperatures and sta temperature interval over which amphibole was stable were controlled by the amphibolites different bulk compositions and their resulting metamorphic assemblages the volume of melt produced by melting of the two amphibolites were compared with estimated amounts based on burnham s 1979 water melt solubility model and the fluid absent melting model presented by clemens and vielzeuf 1987 the observed melt volumes were greater than estimated as the water content of melt largely detemines the volume of melt produced independent measurements of the water content of the glass formed during partial melting in the aba were made by thermogravimetric analyses the water content of the aba glass is 2 wt which is less than the assumed melt water content water content of the melt used in previous modeling of fluidabsent anatexis in mafic lithologies as a consequence more melt can be expected during fluid absent partial melting of mafic lower crust as is observed in the experiments a modification of the clemens and vielzeuf 1987 fluid absent melting model for mafic compositions has been made using the experimental data available on melting in basaltic systems and is presented here for pressures of 5 8 and 10 kbar tectonic scenarios in which the crust is thickened i e by collision then undergoes extension or where a previously thinned crust is later rethickened provide enough heat so that amphibolite melting under fluid absent conditions can become importan and hence responsible for some melts produced during post collisional magmatism the results may also have applications to melting in hydrated oceanic crust in subduction zones and in island arc terains

symmagmatic folding of the base of the bergell pluton central alps abstract evidence for magmatic submagmatic and solid state deformation in tonalite granodiorite and country rocks found at the deep seated floor 22 26 km of the bergell pluton demonstrates that final emplacement and crystallization occurred during regional deformation of the pluton and the underlying country rocks after northward emplacement over the country rocks but before complete crystallization the floor of the pluton was folded during simultaneous n s shortening and e w stretching this is evidenced by synmagmatic folds with e w striking nearly vertical axial planes and by regional east plunging stretching lineations in the country rocks which are parallel to the regional scale fold axes and the magmatic mineral lineations in the pluton opposite senses of shear from the well foliated occasionally mylonitic contact suggest that deformation was mostly accomplished by pure shear synmagmatic deformation is related to late stage n s shortening of the alpine orogen and shows that the still partially molten pluton responded to low differential stress very much like the country rocks deformed in the solid state at high temperatures post emplacement tilting associated with backthrusting along the insubric mylonites led to the exposure of the pluton s floor at its present day western margin

metamorphism of metasediments at the scale of an orogen a key to the tertiary geodynamic evolution of the alps major discoveries in metamorphic petrology as well as other geological disciplines have been made in the alps the regional distribution of late cretaceous tertiary metamorphic conditions documented in post hercynian metasediments across the entire alpine belt from corsica tuscany in the west to vienna in the east is presented in this paper in view of the uneven distribution of information we concentrate on type and grade of metamorphism and we elected to distinguish between metamorphic paths where either pressure and temperature peaked simultaneously or where the maximum temperature was reached at lower pressures after a significant temperature increase on the decompression path the results show which types of process caused the main metamorphic imprint a subduction process in the western alps a collision process in the central alps and complex metamorphic structures in the eastern alps owing to a complex geodynamic and metamorphic history involving the succession of the two types of process the western alps clearly show a relatively simple picture with an internal high pressure dominated part thrust over an external greenschist to low grade domain although both metamorphic domains are structurally very complex such a metamorphic pattern is generally produced by subduction followed by exhumation along a cool decompression path in contrast the central alps document conditions typical of subduction and partial accretion followed by an intensely evolved collision process often resulting in a heating event during the decompression path of the early subducted units subduction related relics and collisional decompressional heating phenomena in different tectonic edifices charac terize the tertiary evolution of the eastern alps the tuscan and corsica terrains show two differ ent kinds of evolution with corsica resembling the western alps whereas the metamorphic history in the tuscan domain is complex owing to the late evolution of the apennines this study confirms that careful analysis of the metamorphic evolution of metasediments at the scale of an entire orogen may change the geodynamic interpretation of mountain belts

mass transfer and porosity evolution during low temperature water rock interaction in gneisses of the simano nappe arvigo val calanca swiss alps late alpine fissures and fractures in amphibolite facies basement gneisses at arvigo val calanca swiss alps show distinct cm sized reaction selvages parallel to the fracture walls that composed of subgreenschist facies assemblages produced by the interaction of water present in the fracture porosity with the old high grade gneiss assemblages the process of selvage or reaction vein formation occurred in the brittle deformation regime and at temperatures characteristic of first the prehnite pumpellyite facies and then later of the zeolite facies the vein formation occurred during uplift and cooling at very late stages of the alpine orogeny the reaction veins are composed of a selvage of altered gneiss on both sides of the central fracture and a central zone with fissure minerals that have been growing in the open fracture pore space the central zone of the arvigo veins contains an early assemblage with epidote prehnite and chlorite and a late succession sequence of various species of zeolite the veins of the arvigo quarry are convincing evidence that fracture fluids in gneiss and granite have the potential to precipitate ca zeolite this is an important find because many fluids recovered from deep continental drill holes and from geothermal energy exploration are found to be oversaturated in respect to a number of ca zeolite species vein formation during late uplift and cooling of the alps occurred at continuously decreasing t and at hydrostatic pressure 1 coexisting prehnite epidote records temperatures of 330 380 c 2 chlorite formation at temperature of 333 32 c and 3 formation of zeolites 250 c in the selvages the prime reaction is the replacement of plagioclase by albite along a sharp reaction front that separates the selvage from unaltered gneiss in addition to albitisation chloritisation of biotite is the second important reaction in the alteration process the reactions release components for the formation of ca al silicates the water rock interaction is associated with a depletion of al si ca fe and k in the altered wall rock the overall reaction is associated with an increase in porosity of up to 14 2 2 2 in the selvage zone altered wall rock caused by the volume decrease during albitisation and the removal of biotite the propagation of the sharp reaction front through the gneiss matrix occurred via a dissolution reprecipitation mechanism zeolite formation is tied to the plagioclase alteration reaction in the rock matrix which releases components for zeolite formation to a co2 poor aqueous liquid

geochronology of accessory allanite and monazite in the barrovian metamorphic sequence of the central alps switzerland abstract the formation of accessory allanite monazite and rutile in amphibolite facies rocks across the barrovian sequence of the central alps switzerland was investigated with a combination of petrography and geochemistry and related to the known structural and metamorphic evolution of the lepontine dome for each of these minerals a specific approach was adopted for geochronology taking into account internal zoning and u th pb systematics in situ u th pb dating of allanite and monazite by ion microprobe revealed systematic trends for the ages of main deformation and temperature in the lepontine dome isotope dilution tims dating of rutile returns dates in line with this picture but is complicated by inheritance of pre alpine rutile and possible pb loss during alpine metamorphism allanite is generally a prograde mineral that is aligned along the main foliation of the samples and found also as inclusions in garnet prograde allanite formation is further documented by rutile inclusions with formation temperatures significantly lower than the maximum t recorded by the rock mineral assemblage allanite ages vary from 31 3 1 1 ma in orthogneisses in the east to 31 7 1 1 ma for a bundnerschiefer and 28 5 1 3 ma for a metaquartzite in the central area to 26 8 1 1 ma in the western part of the lepontine dome these ages are interpreted to date the main deformation events nappe stacking and isoclinal deformation of the nappe stack close to peak pressure conditions the timing of the thermal peak in the lepontine dome is recorded in monazite that grew at the expense of allanite and after a main episode of garnet growth at temperatures of 620 c monazite in the central area yields an age of 22 0 0 3 ma which is indistinguishable from the age of 21 7 0 4 ma from a metapelite in the western part of the lepontine dome in the central area some of the classical kyanite staurolite garnet schists directly underlying the metamorphosed mesozoic sediments contain monazite that records only a pre alpine variscan metamorphic event of upper greenschist to lower amphibolite facies conditions dated at 330 ma the new age data provide evidence that nappe stacking at prograde amphibolite facies conditions and refolding of the nappe stack occurred between 32 and 27 ma only a few million years after eclogite facies metamorphism in the adula cima lunga unit amphibolite facies metamorphism lasted for about 10 my to 22 ma allowing for multiple ductile deformation and recrystallization events the long lasting amphibolite facies metamorphism requires fast cooling between 20 and 15 ma in the central alps this fast cooling was not related to an increase in sedimentation rates in the foreland basins suggesting that tectonic exhumation was responsible for termination of amphibolite facies metamorphism in the lepontine dome

the potential of detrital garnet as a provenance proxy in the central swiss alps abstract detrital garnet is a promising candidate to reliably fingerprint sediment sources in the alps which has so far been complicated by the wide range and similarity of some of the lithologies garnet is present in most alpine sediments is easy to identify is fairly stable and most importantly reflects the type and the metamorphic grade of its source rock in its chemical composition this study aims to establish fingerprints based on detrital garnet composition for the most important tectonic units of the central alps including european penninic and adriatic basement rocks and their respective metasedimentary covers sediments collected from modern rivers which drain representative portions of the individual tectonic units contain a natural mixture of the various garnet populations present in each unit we selected six catchments in southwestern switzerland draining the external massifs helvetic sediments and the penninic nappe stack at the transition of alpine greenschist to amphibolite facies metamorphism in order to test the variability of alpine garnets and the role of inherited pre alpine garnets extraordinary grossular and spessartine rich garnets of the external massifs which experienced greenschist facies metamorphism are clearly distinguishable from generally almandine rich garnets supplied by the higher grade metamorphic penninic nappe stack the variable pyrope grossular and spessartine components of these almandine rich garnets can be used to further distinguish pre alpine alpine eclogite facies and low grade metasedimentary garnets this provenance proxy has the potential to be used for reconstructing sediment sources transport and dispersal patterns in a variety of settings throughout the alpine sedimentary record

microstructural chemical and textural records during growth of snowball garnet the growth history of two populations of snowball garnet from the lukmanier pass area central swiss alps was examined through a detailed analysis of three dimensional geometry chemical zoning and crystallographic orientation the first population collected in the hinge of a chevron type fold shows an apparent rotation of 360 the first 270 are characterized by spiral shaped inclusion trails gradual and concentric mn zoning and a single crystallographic orientation whereas in the last 90 crenulated inclusion trails and secondary mn maxima centred on distinct crystallographic garnet domains are observed microstructural geochemical and textural data indicate a radical change in growth regime between the two growth sequences in the first 270 growth occurred under rotational non coaxial flow whereas in the last 90 garnet grew under a non rotational shortening regime the second population collected in the limb of the same chevron type fold structure is characterized by a spiral geometry that does not exceed 270 of apparent rotation these garnet microstructures do not record any evidence for a modification of the stress field during garnet growth concentric mn zoning as well as a single crystallographic orientation are observed for the entire spiral electron backscatter diffraction data indicate that nearly all central domains in the snowball garnet are characterized by one 001 axis oriented sub parallel to the symmetry axis and by another 001 axis oriented sub parallel to the orientation of the internal foliation these features suggest that the crystallographic orientation across the garnet spiral is not random and that a relation exists among the symmetry axis the internal foliation and the crystallographic orientation

microfabric memory of vein quartz for strain localization in detachment faults a case study on the simplon fault zone abstract this manuscript deals with the adaptation of quartz microfabrics to changing physical deformation conditions and discusses their preservation potential during subsequent retrograde deformation using microstructural analysis a sequence of recrystallization processes in quartz ranging from grain boundary migration recrystallization gbm over subgrain rotation recrystallization sgr to bulging nucleation blg is detected for the simplon fault zone sfz from the low strain rim towards the internal high strain part of the large scale shear zone based on i the retrograde cooling path ii estimates of deformation temperatures and iii spatial variation of dynamic recrystallization processes and different microstructural characteristics continuous strain localization with decreasing temperature is inferred in contrast to the recrystallization microstructures crystallographic preferred orientations cpo have a longer memory cpo patterns indicative of prism and rhomb glide systems in mylonitic quartz veins overprinted at low temperatures 400 c suggest inheritance of a high temperature deformation in this way microstructural textural and geochemical analyses provide information for several million years of the deformation history the reasons for such incomplete resetting of the rock texture is that strain localization is caused by change in effective viscosity contrasts related to temporal large and small scale temperature changes during the evolution of such a long lived shear zone the spatially resolved quantitative investigation of quartz microfabrics and associated recrystallization processes therefore provide great potential for an improved understanding of the geodynamics of large scale shear zones

two and three dimensional thermal modeling of a low angle detachment exhumation history of the simplon fault zone central alps 1 two alternative models have been proposed to explain footwall exhumation along major low angle detachments 1 crustal scale exhumation along a detachment fault that maintained a low dip angle or 2 exhumation along a high angle fault passively rotated by isostatic rebound rolling hinge model these proposed models were tested against a well documented example of a low angle detachment fault in the european central alps the simplon fault zone sfz an extensive thermochronological data set provides the basis for 2 and 3 d thermokinematic models pecube coupled with a stochastic inversion algorithm the neighbourhood algorithm model results establish that the thermochronological pattern is better reproduced by a low angle detachment that maintained a 30 dip rather than by a rolling hinge model although a range of histories involving either steady state or variable exhumation rates is possible the preferred model of highest probability is for a variable rate with the fault zone initiated at 18 5 2 5 ma and active until the present day footwall exhumation was relatively fast until 14 5 1 5 ma 1 4 mm yr 1 this enhanced sfz footwall exhumation is similar in timing and kinematics to orogen parallel extension reported throughout the alpine orogen after 14 5 ma sfz footwall exhumation continued at a reduced rate 0 7 mm yr 1 until 4 ma the subsequent increase to 1 mm yr 1 reflects enhanced regional erosion rates across both footwall and hanging wall after circa 4 ma from 0 35 0 15 mm yr 1 to 0 70 0 15 mm yr 1 probably in response to climate changes during the pliocene

dislocation creep of dry quartz small scale shear zones within the permian truzzo meta granite developed during the alpine orogeny at amphibolite facies conditions in these shear zones magmatic quartz deformed by dislocation creep and recrystallized dynamically by grain boundary migration with minor subgrain rotation recrystallization to a grain size of around 250 750 m consistent with flow at low differential stresses fourier transform infrared ftir spectroscopy reveals very low water contents in the interior of recrystallized grains in the form of discrete oh peaks 20 h 10 6 si and very little broad band absorption 100 h 10 6 si the spectral characteristics are comparable to those of dry brazil quartz in ftir spectra magmatic quartz grains show a broad absorption band related with high water concentrations only in those areas where fluid inclusions are present while other areas are dry drainage of fluid inclusions and synkinematic growth of hydrous minerals indicates that a hydrous fluid has been available during deformation loss of intragranular water during grain boundary migration recrystallization did not result in a microstructure indicative of hardening these ftir measurements provide the first evidence that quartz with extremely low intragranular water contents can deform in nature by dislocation creep at low differential stresses low intragranular water contents in naturally deformed quartz may not be necessarily indicative of a high strength and the results are contrary to implications taken from deformation experiments where very high water contents are required to allow dislocation creep in quartz it is suggested that dislocation creep of quartz in the truzzo meta granite is possible to occur at low differential stresses because sufficient amounts of intergranular water ensure a high recovery rate by grain boundary migration while the absence of significant amounts of intragranular water is not crucial at natural conditions

on removing the primary field from fixed wing time domain airborne electromagnetic data some consequences for quantitative modelling estimating bird position and detecting perfect conductors in the process of removing the primary field from fixed wing time domain airborne em data the response is decomposed into two parts which are referred to here as the time domain in phase and quadrature components the time domain in phase component is dominated by the primary field which varies significantly as the transmitter receiver separation changes the time domain quadrature component comes solely from the secondary response associated with currents induced in the ground and this is the component that has traditionally been used in the interpretation of data from fixed wing towed bird time domain em systems in the off time the quadrature response is very similar to the total secondary response however there are large differences in the on time and even some small differences in the off time one consequence of these differences is that when airborne em data are to be interpreted using a synthetic mathematical model the synthetic data calculated should also be the quadrature component a second consequence relates to the time domain in phase component which is sometimes used to estimate the receiver sensor bird position the bird position estimation process assumes there is no secondary field in the in phase component if the ground is resistive the secondary contained in the in phase component is small so the bird position estimate is accurate to about 30 cm but in highly conductive areas the secondary contribution can be large and the position estimate can be out by as much as 5 m a third consequence arises for highly conductive bodies the response of which is predominantly in phase this means that any response from these types of body is lost in the component that has been removed in the primary field extraction procedure however if the bird position is measured very accurately the actual free space primary field can be estimated if this is then subtracted from the estimated primary actually free space primary plus secondary in phase response then the residual is the secondary in phase response of the ground using this methodology very conductive ore bodies could be detected however a sensitivity analysis shows that detection of a large vertically dipping very conductive body at 150 m depth would require that the bird position be measured to an accuracy of about 1 4 cm and the aircraft attitude to within about 0 01 such tolerances are very stringent and not easily attainable with current technology

a purely structural restoration of the nfp20 east cross section and potential tectonic overpressure in the adula nappe central alps common extrusion type models for the high to ultrahigh pressure adula nappe require a major normal fault along the top of this unit which is not conveyed in the structural record this implies that such a normal fault existed but was completely erased during later deformational stages however there is evidence that decompression occurred during top to the foreland thrusting we performed a new purely structural kinematic restoration of the central part of the nfp20 east cross section in order to estimate the burial depths of individual units without converting petrological pressure data into depth under the assumption that pressures were lithostatic the results show that pressures within most of the units were close to but somewhat higher than lithostatic for several stages of the tectono metamorphic history only for the maximum burial stage of the adula nappe we estimate local tectonic overpressures of 40 to 80 of the lithostatic pressures accepting such an amount of overpressure which is moderate compared to values theoretically possible the adula nappe was probably not subducted to subcrustal depth we propose that the structural record of the penninic nappe stack is quite complete and suggest that the decay of tectonic overpressure is a feasible explanation for decompression from eclogite to amphibolite facies conditions during thrusting consequently exhumation and convergence rates of the eocene to oligocene alps may be smaller than previously assumed

the tectono metamorphic history of the valaisan domain from the western to the central alps new constraints on the evolution of the alps petrographic and structural studies on metasedimentary rocks of the valaisan domain from the engadine window switzerland to the petit st bernard france show that this entire domain was subjected to high pressure low temperature conditions during the alpine orogeny the pressure and temperature conditions increased from east to west the metasedimentary rocks of the engadine window and the grisons area are characterized by blueschist facies conditions 12 13 kbar 350 400 c defined by mg carpholite and chloritoid occurrences toward the west in the petit st bernard and versoyen units the metamorphic conditions reached the eclogite facies 15 16 kbar 500 c the shortening direction associated with the high pressure metamorphic event was oriented northwest southeast all over the valaisan domain this direction is compatible with the direction of convergence between the european and apulian plates in early tertiary time 65 35 ma in the eastern and central alps blueschist facies rocks occur below the austroalpine nappes over an area of 300 20 km 2 from the tauern window to the grisons area and have a thickness around 10 km this volume of blueschist facies rocks is in contrast with eclogite facies rocks of western alps that form only a small slice with a thickness of 2 to 5 km we interpret the difference in volume and metamorphic conditions from east to west as being due to a change of the subduction style in the east we assume the formation of a large wedge with a thickness of 40 50 km such that rocks underwent blueschist metamorphism and were exhumed before collision between the european and apulian plates in the west the petit st bernard and the versoyen units were metamorphosed and extruded in a subduction channel above the subducting slab the data further allow us to conclude that the valaisan domain was an ocean that was independent of the piemontese ocean and that had an important place in the youngest phase of the evolution of the alps before the collision

equilibration and disequilibration between monazite and garnet indication from phase composition and quantitative texture analysis the integration of information which can be gained from accessory i e age t and rock forming minerals i e temperature t and pressure p requires a more profound understanding of the equilibration kinetics during metamorphic processes this paper presents an approach comparing conventional p t estimate from equilibrated assemblages of rock forming minerals with temperature data derived from yttrium garnet monazite ygm and yttrium garnet xenotime ygx geothermometry such a comparison provides an initial indication on differences between equilibration of major and trace elements regarding this purpose two migmatites two polycyclic and one monocyclic gneiss from the central alps switzerland northern italy were investigated while the polycyclic samples exhibit trace element equilibration between monazite and garnet grains assigned to the same metamorphic event there are relics of monazite and garnet obviously surviving independent of their textural position these observations suggest that surface processes dominate transport processes during equilibration of those samples the monocyclic gneiss on the contrary displays rare isolated monazite with equilibration of all elements despite comparably large transport distances with a nearly linear crystal size distribution of the garnet grain population growth kinetics related to the major elements were likely surface controlled in this sample in contrast to these completely equilibrated examples the migmatites indicate disequilibrium between garnet and monazite with a change in ree patterns on garnet transects the cause for this disequilibrium may be related to a potential disequilibrium initiated by a changing bulk chemistry during melt segregation while migmatite environments are expected to support high transport rates i e high temperatures and melt presence the evolution of equilibration in migmatites is additionaly related to change in chemistry as a key finding surface controlled equilibration kinetics seem to dominate transport controlled processes in the investigated samples this may be decisive information towards the understanding of age data derived from monazite

alpine orogenic evolution from subduction to collisional thermal overprint the 40ar 39ar age constraints from the valaisan ocean central alps 1 the investigated hp lt metasedimentary units of the valaisan and adjacent european domains occupy a key position in the alpine belt for understanding the transition from early subduction related hp lt metamorphism to collision related barrovian overprint and the evolution of mountain belts in general the timing of high pressure metamorphism subsequent retrogression and following barrow type overprint was studied by 40ar 39ar dating of biotite and several white mica generations that are well characterized in terms of mineral chemistry texture and associated mineral assemblages four distinct age populations of white mica record peak pressure conditions 42 40 ma and several stages of subsequent retrograde metamorphic evolution 36 25 ma biotite isotopic analyses yield consistent apparent ages that cluster around 18 16 ma for the barrow type thermal overprint the recorded isotopic data reveal a significant time gap in the order of some 20 ma between subduction related hp lt metamorphism and collision related barrovian overprint supporting the notion of a polymetamorphic evolution associated with a bimodal p t path

recalibration of the gasp geobarometer in light of recent garnet and plagioclase activity models and versions of the garnet biotite geothermometer the garnet al silicate plagioclase gasp geobarometer has been recalibrated using four recent garnet activity models four analogous garnet biotite temperature models and two recent plagioclase activity models a typical sillimanite bearing sample that formed at about 5 25 kbar 575 c shows a possible p range of 0 7 kbar due to t error 1 kbar due to range of garnet activity model 0 9 kbar due to range of plagioclase activity model and 5 4 kbar due to range of experimental end member reversals extended by one sigma calibrations were further constrained with the kyanite sillimanite k s phase boundary such that the best fit of 76 pelitic schist samples from 11 localities provides an individual end member calibration for each of the eight possible combinations of garnet and plagioclase activity models with the appropriate geothermometer samples with low grossular or anorthite component were rejected the end member calibrations are constrained to pass through the the best determined portion of the gasp experimental reversals at 1230 c 26 6 kbar these individual end member calibrations provide self consistent models that tend to compensate for error in the garnet and plagioclase activity expressions the models were also tested on a set of 59 samples from the alps the recommended calibration is the average garnet activity model and average garnet biotite t model of holdaway 2000 the fuhrman and lindsley 1988 plagioclase activity model and h grs 6628521 s grs 258 76 to combine with the remaining phases in the berman database to produce the optimum end member gasp curve these thermodynamic data are for the gasp geobarometer only error is about 0 8 kbar absolute and about 0 6 kbar relative geological error is the largest component of error in many of these samples care should be taken to be sure that analyzed plagioclase and biotite are near analyzed garnet that the peak t portions of garnet and plagioclase are selected that the peak t al silicate is determined and that the t calculated is the most accurate possible these calibrations represent an improvement over previous published gasp calibrations these eight models are available for distribution as three programs t p p t intersection for the dos based personal computer

observations from the floor of a granitoid pluton inferences on the driving force of final emplacement an east west profile across the tilted bergell pluton exposes a 10 km thick interval in terms of crustal depth consequently the floor as well as the root and side of the main intrusive body of the pluton crop out at the surface and a tentative three dimensional geometry is constructed at the highest crustal level the geometry and deformation features at the margin of the pluton indicate ballooning whereas the folded floor of the main intrusive body indicates synmagmatic shortening related to regional deformation these contrasting features are best explained by shortening of the base of the pluton which caused an expansion at a higher crustal level final emplacement of the pluton into higher crustal levels was therefore not driven primarily by buoyancy but rather by regional deformation within deeper levels of the crust

syn migmatization way up criteria abstract most geopetal way up structures are developed during primary formation or soon after in sedimentary or igneous rocks in migmatite terrains the possibly anomalous behaviour of rocks in which melt is present has often limited structural synthesis geopetal relationships may however be recognizable in at least some migmatites asymmetries in two structural relationships cauliflower structure and vein cluster involving neosome veins conformable with the main foliation of layered migmatites are interpreted as indicating the way up of these rocks when they were partially molten additional examples recognized in the literature suggest that similar structures can be seen in a wide range of geological environments these prove to be as reliable as sedimentary top and bottom features and suggest that the migmatites containing them were essentially sub horizontal while being strained and melted