pre alpine and alpine evolution of the south alpine basement of the orobic alps new k ar age determinations on the gneisses and schists of the south alpine basement of the orobic alps and their separated minerals have been carried out to supplement and support the previous study ofbocchio et al 1981

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

from rifting to drifting tectonic evolution of the south alpine upper crust from the triassic to the early cretaceous abstract the tectonic evolution of the south alpine rifted margin is discussed on the base of four palinspastic upper crustal profiles extension related to the movements between adria and europe began in the norian after variscan orogeny late carboniferous to middle permian orogenic collapse and continental scale wrenching from the late triassic to the early liassic stretching was mostly limited to the lombardian basin during this time extension was mainly controlled by four major listric faults symmetrically centred around the late carboniferous early permian collio grabens smaller faults which also started in the norian were progressively de activated during the late triassic after the middle liassic faulting in the lombardian basin gradually ceased and the site of extension shifted westwards i e towards the future site of crustal separation extension was then controlled by a set of west dipping normal faults oceanic crust was formed not later than 157 ma the overall extension along the profile which had a final length of 290 km was 52 km which corresponds to a stretching factor of 1 22 for the whole length of the preserved margin the strain rate is ca 1 10 16 s the sedimentation history suggests that the extension of the south alpine margin was a continuous process from the norian to the middle jurassic the changes in tectonic pattern are related to progressive hardening of the lithospheric segment undergoing slow extension

from permo triassic lithospheric thinning to jurassic rifting at the adriatic margin petrological and geochronological record in valtournenche western italian alps abstract slices of polycyclic metasediments marbles and meta cherts are tectonically amalgamated with the polydeformed basement of the dent blanche tectonic system along a major alpine shear zone in the western alps becca di sale area valtournenche valley a combination of techniques structural analysis at various scales metamorphic petrology geochronology and trace element geochemistry was applied to determine the age and composition of accessory phases titanite allanite and zircon and their relation to major minerals the results are used to reconstruct the polyphase structural and metamorphic histories comprising both pre alpine and alpine cycles the pre alpine evolution is associated with low pressure high temperature metamorphism related to permo triassic lithospheric thinning in meta cherts microtextural relations indicate coeval growth of allanite and garnet during this stage at 300 ma textures of zircon also indicate crystallisation at ht conditions ages scatter from 263 to 294 ma with a major cluster of data at 276 ma in impure marble u pb analyses of titanite domains with variable al and f contents yield apparent 206pb 238u dates range from permian to jurassic chemical and isotopic data suggest that titanite formed at permian times and was then affected by extension related fluid circulation during the triassic and jurassic which redistributed major elements al and f and partially opened the u pb system the alpine cycle lead to early blueschist facies assemblages which were partly overprinted under greenschist facies conditions the strong alpine compressional overprint disrupted the pre alpine structural imprint and or reactivated earlier structures the pre alpine metamorphic record preserved in these slices of metasediments reflects the onset of the permo triassic lithospheric extension to jurassic rifting

the variscan structural and metamorphic evolution of the eastern southalpine basement the basement of the southern alps northern italy belongs to the southernmost part of the european variscan mountain belt in contrast to other areas of the alps the post variscan metamorphic and tectonic overprint is weak and therefore permits the unravelling of the variscan tectonometamorphic evolution of this region overprinting criteria and the mapping of the penetrative structural elements s2 and l2 allow three deformational events to be distinguished major variscan deformation d2 in the southern alps commenced in the carboniferous and lasted until the end of the late carboniferous tectonic movement during d2 was north directed and was accompanied and followed by greenschist facies metamorphism peak metamorphic conditions reached 450 550 c and 5 6 5 kbar in the brixen area and decreased in a southeasterly direction tectonic movement during d2 followed subduction of the plankogel terrane and collision of the noric terrane northernmost part of gondwana with laurasia and is interpreted to result from late orogenic extensional collapse of the variscan mountain chain extensional deformation caused exhumation of the basement rocks as evidenced by greenschist facies basement pebbles which show no substantial retrograde overprint in late carboniferous early permian conglomerates

formation of garnet clusters during polyphase metamorphism pre alpine garnets of variscan age from metapelitic basement units in northern italy were strongly retrogressed at near surface conditions prior to alpine contact metamorphism the replacement by sheet silicates caused a significant volume increase during retrogression producing pervasive fracturing up to several hundreds of angular fragments formed from each crystal electron backscatter diffraction analysis documents a maximum misorientation of 22 of some fragments as a result of local rotation during fracturing new garnet growth is observed on the garnet fragments during contact metamorphic overprinting resulting in garnet clusters fragments can be identified due to calcium rich domains fragment orientations were inherited and only minor new nucleation occurred these garnets develop features typically associated with multiple nucleation models but here they reflect multiple metamorphic events we propose that clusters can be indicative of multiple metamorphic events which were separated by a period of intense retrograde alteration

prograde metamorphism of metapelites garnet biotite thermometry and prograde changes of biotite chemistry in high grade rocks of west uusimaa southwest finland abstract the metamorphic grade in west uusimaa sw finland increases from low to intermediate amphibolite facies in the west island of kemio up to granulite facies in the middle and eastern parts mineral assemblages in metapelitic rocks indicate metamorphic conditions of 700 825 c 3 5 kbar for the high amphibolite and granulite facies region which is in agreement with independent estimates these metamorphic conditions are used to evaluate garnet biotite thermometry zoning in garnet fe enrichment and mg depletion towards rims bordered by biotite or cordierite and a distinct fe depletion mg enrichment of biotites in contact with garnet are caused by fe mg exchange of these minerals during retrogression 550 c the general prograde metamorphic trend from medium to high grade in west uusimaa is reflected by a systematic change in the composition of biotites thermometry and biotite chemistry are used to indicate that the granulite facies metamorphism in west uusimaa is probably caused by a thermal event

evolution of primary producers and productivity across the ediacaran cambrian transition abstract nearly all extant animal phyla first appeared during the ediacaran cambrian transition a revolution in primary producer ecology and marine productivity has been proposed as a bottom up ecological driver for this rapid diversification of metazoans however the control s driving the evolution of primary producers and primary productivity around the ediacaran cambrian transition and their potential relationship to the cambrian explosion are not fully understood in this study we measured the nitrogen content and the nitrogen isotopic composition of kerogen in the piyuancun and hetang formations using samples collected from the chunye 1 well on the lower yangtze block in western zhejiang our isotope results suggest that nitrate was the dominant n source fueling primary productivity during deposition of the entire piyuancun formation and lower hetang formation with only a minor contribution from n 2 fixation quantitative reconstructions of paleoproductivity based on organic carbon and pyrite content and sedimentation rate indicate that productivity either stayed the same or increased during the transition from the piyuancun formation to the overlying hetang formation thus fixed n availability was not the ultimate factor limiting primary productivity during the ediacaran cambrian transition combining our data with published nitrogen isotope records from the early middle ediacaran to cambrian stage 4 suggests that no 3 was the dominant n species fueling phytoplanktonic productivity during the broad cambrian explosion interval from its earliest roots in the ediacaran through the main episode in cambrian stage 3 eukaryotes lack the ability to fix nitrogen and preferentially assimilate nitrate the no 3 dominated euphotic zone from the early middle ediacaran to cambrian stage 3 may have promoted the radiation of relatively large eukaryote phytoplankton which may have been an important driver of early animal diversification

from granulites to eclogites in the sesia zone italian western alps a record of the opening and closure of the piedmont ocean the sesia zone italian western alps offers one of the best preserved examples of pre alpine basement reactivated under eclogite facies conditions during the alpine orogenesis a detailed mineralogical study of eclogitized acid and basic granulites and related amphibolites is presented in these rare weak to undeformed rocks microstructural investigations allow three main metamorphic stages to be distinguished a a medium to low p granulite stage giving rise to the development of orthopyroxene garnet plagioclase brown amphibole ilmenite biotite in basic granulites and garnet k feldspar plagioclase cordierite sillimanite biotite ilmenite in acid granulites b a post granulite re equilibration associated with the development of shear zones producing discrete amphibolitization of the basic granulites and widespread development of biotite sillimanite cordierite spinel in the acid rocks c an eo alpine eclogite stage giving rise to the crystallization of high p and low t assemblages in an effort to quantify this evolution independent well calibrated thermobarometers were applied to basic and acid rocks for the granulite event p t estimates are 7 9 kbar and 700 800 c and for subsequent retrograde evolution p t was 4 5 kbar and 600 c for the eo alpine eclogite metamorphism pressure and temperature conditions were 14 16 kbar and 550 c the inferred p t path is consistent with an uplift of continental crust produced by crustal thinning prior to the subduction of the continental rocks in the light of the available geochronological constraints we propose to relate the pre alpine granulite and post granulite retrograde evolution to the permo jurassic extensional regime the complex granulite eclogite transition is thus regarded as a record of the opening and of the closure of the piedmont ocean

constraints on the timing and genetic link of the large scale accumulation of proximal w sn mo bi and distal pb zn ag mineralization of the world class dongpo orefield nanling range south china abstract the world class dongpo orefield located in the nanling range south china is famous for its large scale accumulation of w sn mo bi pb zn ag deposits which includes the giant shizhuyuan w sn mo bi jinchuantang sn bi yejiwei sn cu and congshuban pb zn ag deposits the polymetallic mineralization around the qianlishan pluton exhibits systematic zoning and comprises proximal w sn mo bi skarn greisen and distal pb zn ag veins however the timing and genetic link of its proximal skarn greisen w sn mo bi and distal vein type pb zn ag mineralization are not well constrained here we present systematic geochronological data to constrain the geochronological framework and genetic model of the giant dongpo orefield the muscovite 40ar 39ar age of the greisen type ore in the shizhuyuan w sn mo bi deposit is 151 1 ma which is consistent with the formation of the skarn type ore and the qianlishan pluton thus indicating that they are genetically related and both formed contemporaneously at 151 ma the large scale accumulation of w sn mo bi in the shizhuyuan deposit may have occurred in a short duration of 1 ma the muscovite 40ar 39 ar ages 155 5 1 1 and 156 8 1 1 ma and molybdenite re os age 153 8 4 2 ma of the ma naoshan w sn fe mn pb zn deposit as well as the zircon u pb age 152 7 0 9 ma of the yejiwei sn cu deposit in the dongpo orefield are consistent with the age of the qianlishan pluton thus indicating that the proximal skarn greisen w sn mo bi mineralization in the dongpo orefield is genetically related to the qianlishan pluton in addition the rb sr dating of sphalerite indicates that the vein type pb zn ag mineralization in the congshuban area formed at 152 2 ma which is consistent with the la mc icp ms zircon u pb age 156 155 ma of the equigranular biotite granite indicating that the distal vein type pb zn ag mineralization is genetically associated with the qianlishan equigranular biotite granite by integrating the high precision geochronological data of this study with those of previous studies the new genetic model of the large scale accumulation of w sn mo bi pb zn ag in the dongpo orefield suggests that the proximal skarn greisen w sn mo bi and distal vein pb zn ag mineralization in the dongpo orefield are genetically related to the qianlishan pluton and that the mineralization type is mainly controlled by the characteristics of the wall rock

large scale thin skinned thrusting in the southern alps kinematic models the tectonics of the southern alps are dominated by thrusting on extensive ramp flat systems and by folding some of which is ramp folding basement slivers are involved that frontally are not thicker than 5 km the most unequivocal section is that of the grigna mountains east of lake como for which kinematic modeling based on material balance considerations results in a shortening of 40 km it is separated from the grona section in the west and the valtorta section in the east by probably pre existing transverse zones that cut through the important decollement horizons and define blocks of different depths to basement local tectonics across these transverse zones change abruptly while over all shortening remains constant in particular both east and west of the grigna section basement ramp folds were wedged into the very incompetent raibl beds provoking back thrusting of the thick hauptdolomit and higher sediments with the substitution of the original cover of the frontal limb of the basement fold this new kinematic interpretation is compatible with the information facilitates the construction of balanced sections and is required for lateral constancy of shortening

thermal modeling of extensional tectonics application to pressure temperature time histories of metamorphic rocks one and two dimensional finite difference models are used to generate theoretical pressure temperature time ptt paths for rocks uplifted from deep and intermediate crustal levels during extension via ductile stretching of the entire lithosphere pure shear and via movement of crustal blocks along rooted large scale low angle normal faults simple shear the temperature time paths of rocks uplifted by these pure shear and simple shear mechanisms have similar morphologies although rocks from pure shear settings reach their final depth at higher temperatures than do rocks uplifted by simple shear and experience a greater amount of posttectonic cooling the depth temperature paths of rocks uplifted from deep crustal levels 30 km depth via pure shear extension are often characterized by a period of nearly isothermal high dp dt uplift during the early stages of extension a modified pure shear extension model that includes enhanced heating in the lower lithosphere during extension generally produces ptt paths characterized by a more protracted period of nearly isothermal uplift in contrast simple shear uplift along low angle normal faults produces almost linear depth temperature paths with moderate dp dt and isothermal uplift is not observed for a given initial geotherm the single factor controlling the ptt paths of rocks unroofed below gently dipping normal faults that dip less than 30 is the rate at which unroofing takes place unroofing rate is defined as the rate of uplift relative to the surface and the syntectonic ptt paths are insensitive to fault dip or horizontal displacement rate current geothermometric and geobarometric techniques yield typical uncertainties in real pressure temperature and temperature time data of 1 kbar and 50 c thus it will be difficult to distinguish between these various extensional mechanisms on the basis of ptt data derived from a single sample or from a single structural level although use of data from samples collected from a broad range of structural levels may improve resolution ultimately the best prospect for distinguishing between simple shear and pure shear mechanisms through petrologic work lies in improving the precision of metamorphic pressure and temperature measurements to 0 5 kbar and 25 c

sequential porphyroblast growth and crenulation cleavage development during progressive deformation abstract six stages of crenulation cleavage development during a single deformation event d2 can be recognised in both matrix and porphyroblast inclusion trails in the robertson river formation n e australia these stages progress from undeformed to crenulation and differentiated crenulation of s1 through development of differentiated crenulation cleavage s2 and differentiated schistosity s2 to homogeneous foliation s2 these rocks underwent prograde metamorphism during d2 and chloritoid garnet staurolite andalusite and sillimanite isograds have been mapped most rocks are rich in porphyroblasts which commonly contain well defined inclusion trails the geometry of these trails varies from one mineral type to the next depending on the timing of porphyroblast growth relative to the stage of crenulation cleavage development in the schistose matrix the deformation history involved progressive bulk inhomogeneous shortening and the strain is very heterogeneous on all scales partly as a consequence of this hence each of the six stages of crenulation cleavage development formed locally at various times during d2 therefore if a porphyroblast grew late in the deformation it could overgrow all stages of crenulation cleavage development including the very early ones the temperature increased during metamorphism in any one locality and consequently some porphyroblasts overgrew others that were unstable in the new conditions preserving them from destruction hence the precise timing of mineralogical reactions deduced from isograds and mineral chemistry relative to the stages in schistosity development can also be determined in the andalusite zone the sequence of porphyroblasts in decreasing order of age is biotite garnet biotite staurolite andalusite dissolution of garnet occurred during growth of staurolite and especially andalusite and dissolution of staurolite accompanied andalusite growth

contrasting thermomechanical evolutions in the southalpine metamorphic basement of the orobic alps central alps italy in the southern alps a progressive metamorphic zonation with an increase in the geothermal gradient from ne to sw has been widely proposed however recent investigations have shown that the greenschist metamorphic imprint of the low grade zone corresponds to a metamorphic retrogression following amphibolite facies conditions on the other hand in the medium grade zone a later low pressure high temperature lpht metamorphic event has also been proposed in an attempt to resolve these different interpretations new petrological and partly new structural data have been obtained for two sectors of the orobic alps traditionally attributed to different metamorphic zones thermo barometric determinations supported by microstructural analysis indicate the following different pressure retrograde paths in each sector 1 in the val vedello basement wb rocks a first metamorphic imprint characterized by p 7 9 kbar and t 570 610 c was followed by a greenschist retrogression p 4 kbar and t 500 c 2 in the lario basement lb rocks the first detectable metamorphic stage characterized by mineral assemblages indicating p 7 9 kbar and t 550 630 c was followed by a lpht event synkinematic with f2 extensional deformation a greenschist retrogression marks the final uplift of these rocks reinterpretation of the available geochronological data indicates a diachronism for the two thermo mechanical evolutions in the light of these data we interpret the retrograde p t t path of the vvb rocks as a pre permian post thickening uplift and the retrograde p t t evolution of the lb rocks as a permo mesozoic uplift related to the extensional tectonic regime of the tethyan rifting

porphyroblast nucleation growth and dissolution in regional metamorphic rocks as a function of deformation partitioning during foliation development in regional metamorphic rocks the partitioning of deformation into progressive shearing and progressive shortening components results in strain and strain rate gradients across the boundaries between the partitioned zones these generate dislocation density gradients and hence chemical potential gradients that drive dissolution and solution transfer phyllosilicates and graphite are well adapted to accommodating progressive shearing without necessarily building up large dislocation density gradients within a grain because of their uniquely layered crystal structure however most silicates and oxides cannot accommodate strain transitions within grains without associated dislocation density gradients and hence are susceptible to dissolution and solution transfer as a consequence zones of progressive shearing become zones of dissolution of most minerals and of concentration of phyllosilicates and graphite exceptions are mylonites where strain rates are commonly high enough for plastic deformation to dominate over diffusion rates and therefore over dissolution and solution transfer porphyroblastic minerals cannot nucleate and grow in zones of active progressive shearing as they would be dissolved by the effects of shearing strain on their boundaries however they can nucleate and grow in zones of progressive shortening and this is aided by the propensity for microfracturing in these zones which allows rapid access of fluids carrying the material presumed to be necessary for nucleation and growth zones of progessive shortening also have a number of characteristics that help to lower the activation energy barrier for nucleation this includes a build up of stored strain energy relative to zones of progressive shearing in which dissolution is occuring porphyroblast growth is generally syndeformational and previously accepted criteria for static growth are not valid when the role of deformation partitioning is taken into account porphyroblasts in a contact aureole do not grow statically either as microfracturing associated with emplacement allows access of fluids in a fashion that is similar to microfracturing in zones of progressive shortening the criteria used for porphyroblast timing can be readily accommodated in terms of deformation partitioning reactivation of deforming foliations and a general lack of rotation of porphyroblasts with the spectacular exception of genuinely spiralling garnet porphyroblasts

growth and concentration of fibrous sillimanite related to heterogeneous deformation in k feldspar sillimanite metapelites in foliated k feldspar sillimanite metapelites fibrous sillimanite is commonly concentrated in folia that anastomose between lenticular pods of coarser grained aggregates rich in quartz feldspar and biotite with or without garnet cordierite and residual andalusite many of the folia appear to be limbs of crenulation microfolds the sillimanite concentrations may be due largely to the ability of fibrous sillimanite aggregates to undergo strong non coaxial deformation by grain boundary sliding fibre sliding without appreciable build up of dislocations whereas other minerals are unstable in these zones and so concentrate in lower strain interfolial zones initiation and especially concentration of the sillimanite in folia may be assisted by fluid flow and local base cation leaching whereby minerals unstable in zones of strong non coaxial strain are dissolved and removed from these zones

pressure temperature time paths of regional metamorphism i heat transfer during the evolution of regions of thickened continental crust the development of regional metamorphism in areas of thickened continental crust is investigated in terms of the major controls on regional scale thermal regimes these are the total radiogenic heat supply within the thickened crust the supply of heat from the mantle the thermal conductivity of the medium and the length and time scales of erosion of the continental crust the orogenic episode is regarded as consisting of a relatively rapid phase of crustal thickening during which little temperature change occurs in individual rocks followed by a lengthier phase of erosion at the end of which the crust is at its original thickness the principal features of pressure temperature time ptt paths followed by rocks in this environment are a period of thermal relaxation during which the temperature rises towards the higher geotherm that would be supported by the thickened crust followed by a period of cooling as the rock approaches the cold land surface the temperature increase that occurs is governed by the degree of thickening of the crust its conductivity and the time that elapses before the rock is exhumed sufficiently to be affected by the proximity of the cold upper boundary for much of the parameter range considered the heating phase encompasses a considerable portion of the exhumation decompression part of the ptt path in addition to the detailed calculation of ptt paths we present an idealized model of the thickening and exhumation process which may be used to make simple calculations of the amount of heating to be expected during a given thickening and exhumation episode and of the depth at which a rock will start to cool on its ascent path an important feature of these ptt paths is that most of them lie within 50 c of the maximum temperature attained for one third or more of the total duration of their burial and uplift and for a geologically plausible range of erosion rates the rocks do not begin to cool until they have completed 20 to 40 per cent of the total uplift they experience considerable melting of the continental crust is a likely consequence of thickening of crust with an average continental geotherm a companion paper discusses these results in the context of attempts to use metamorphic petrology data to give information on tectonic processes 1984 oxford university press

deep crustal extensional faulting in the ivrea zone of northern italy abstract the ivrea zone in northern italy consists of amphibolite and granulite facies metasedimentary metabasic and ultrabasic rocks and is thought to represent a section through the deep continental crust detailed mapping of the high grade part of this zone has revealed a series of high temperature shear zones which after the effects of later low temperature faulting and folding have been removed appears to accommodate extension of the sequence the shear zones formed at temperatures similar to those at which thermal equilibration occurred in the host rocks and locally show evidence for development under prograde conditions these observations fit well with thermal effects calculated from a simple heat flow model for extensional faulting in this model the effects of shear heating and displacement of cool hanging wall rocks against hotter footwall rocks are calculated these observations indicate that faulting as a mode of failure may be important during extension of the lower continental crust especially when basic rocks are the dominant components of the rock pile a restored section through the extended lower crust as observed in the ivrea zone is presented and the extent to which the features observed are able to explain seismic reflectivity of the lower crust is discussed