magma evolution in the stillwater complex montana ii rare earth element evidence for the formation of the j m reef rare earth element data for 20 cumulus plagioclase feldspars from troctolite anorthosite zone i in the lower banded series and five cumulus and intercumulus plagioclase feldspars from samples that bracket the ultramafic series lower banded series contact of the stillwater complex are consistent with magma mixing as a fundamental process which produced cyclic units in troctolite anorthosite zone i the association of the j m reef with plagioclase rich cumulates in troctolite anorthosite zone i which have a crystallization order different from that observed in the ultramafic series suggests that a geochemically distinct type of magma was added to the chamber this new magma was characterized by a higher al 2 o 3 content ce yb n of 3 7 nd sm n of 1 5 and dy yb n of approximately 1 0 similar to group i gabbronorite dikes in the footwall of the stillwater complex helz 1985 this high al 2 o 3 magma was most likely derived by partial melting of a source of tholeiitic composition in the upper mantle or lower crust the higher al 2 o 3 content of this new magma suggests that it may have been near the sulfur saturation limit for melts of this composition even though the total sulfur concentration may have been low consistent with average sulfur concentrations in the group i gabbronorite dikes rare earth element data for cumulus orthopyroxenes from the ultramafic series presented in the first paper of this two part series lambert and simmons 1987 suggest that magmas parental to cumulates below troctolite anorthosite zone i were derived by multistage dynamic partial melting of a deeper upper mantle source rare earth element and sulfur data for group 2 high mgo gabbronorite dikes also in the footwall of the stillwater complex suggest that these liquid compositions may be similar to magmas parental to the ultramafic series dynamic partial melting of a platinum group element rich source may have permitted enrichment of the platinum group elements in the stillwater magma chamber during the formation of the ultramafic series perturbation of the tholeiitic trend of fractional crystallization observed in the ultramafic series olivine followed by orthopyroxene due to the introduction of an al 2 o 3 rich sulfur saturated magma into platinum group element enriched residual magma remaining in the chamber may have produced the cyclic modal and rare earth element cryptic layering in troctolite anorthosite zone i the identification of feldspars in troctolite anorthosite zone i below the j m reef which have rare earth element affinities with the ultramafic series suggests that progressive mixing of these two liquids was accomplished in a system which preserved the integrity of geochemically different liquid layers double diffusive convective mixing irvine et al 1983 of the two stillwater parental magmas is therefore preferred over mixing via a turbulent plume campbell et al 1983 rare earth element data suggest that complete mixing first occurred with the formation of the sixth olivine bearing member directly below the j m reef in troctolite anorthosite zone i o 5b of todd et al 1982 complete mixing of these two geochemically distinct magmas produced sulfur saturated hybrid magmas parental to the first laterally continuous troctolite in troctolite anorthosite zone i and the platinum group element enriched j m reef

eclogite facies rocks of the monotonous unit clue to variscan suture in the moldanubian zone bohemian massif abstract the formation and exhumation of variscan high to ultrahigh pressure metamorphic rocks from the moldanubian zone in the bohemian massif are the subject of controversy regarding their unclear relationship to possible subduction zones in this paper we present the results of a petrological study of eclogites that occur together with serpentinites within amphibolite facies gneisses in the moldanubian zone east of the tepla barrandian block more than 100 bodies of retrogressed eclogite and serpentinite follow an approximately 250 km long sw ne trending zone in the central part of the bohemian massif together with surrounding gneisses the eclogites share a medium to low pressure amphibolite facies metamorphism p t conditions estimated for the eclogite facies stage indicate a relatively low temperature geothermal gradient similar to those observed in the saxothuringian zone which occurs north and northwest of the tepla barrandian block the presence and distribution of eclogites and comparison of their p t and age data with hp uhpm rocks in other units in the bohemian massif allow us to constrain the variscan suture which straddles the se of the tepla barrandian block the existence of this suture in relation to available geotectonic models and its possible continuation through other allochthonous units along the european variscan belt are discussed

crustal growth during island arc accretion and transcurrent deformation natal metamorphic province south africa new isotopic constraints the natal metamorphic province consists of from north to south the tugela mzumbe and margate terranes these were accreted to the southeastern margin of the kaapvaal craton in the late mesoproterozoic and followed by intrusion of a large suite of a type granitoid bodies new u pb data from zircon titanite and monazite further constrains the temporal framework of these geological events the tugela and mzumbe terranes record protracted magmatism in an island arc complex from 1200 ma to 1160 ma followed by the accretion of these terranes onto the southern margin of the kaapvaal craton at 1150 ma arc magmatism in the margate terrane continued until 1120 ma and was followed by extension and bimodal volcanism immediately prior to accretion to the kaapvaal mzumbe continental margin at 1090 ma this accretion was accompanied by high pressure and high temperature metamorphism juxtaposition of the mzumbe and margate terranes along the melville thrust and the formation of a number of syntectonic intrusive units derived from melting of the pre existing arc crust after accretion extensional collapse is evidenced by the intrusion of mafic ultramafic and alkaline intermediate magmatic suites at 1085 ma resulting from mafic underplating and or lower crustal delamination nd and hf isotopic data imply the magmatic rocks of the natal metamorphic province were derived from relatively juvenile continental crust initially generated by island arc magmatism and subsequently reworked during the accretion event s the combined kaapvaal nmp region the southern margin of the enlarged kalahari craton then experienced extensive sinistral transcurrent deformation centred along a series of discrete steep shear zones that are found from the kaapvaal cratonic margin to the southernmost portion of the natal metamorphic province this deformation is accompanied by low pressure ultra high temperature metamorphism isobaric cooling and intrusion of the voluminous a type oribi gorge suite porphyritic granites and charnockites throughout the mzumbe and margate terranes

petrogenetic significance of orthopyroxene free garnet clinopyroxene plagioclase quartz bearing metabasites with respect to the amphibolite and granulite facies orthopyroxene free garnet clinopyroxene plagioclase quartz bearing mineral assemblages represent the paragenetic link between plagioclase free eclogite facies metabasites and orthopyroxene bearing granulite facies metabasites although these assemblages are most commonly developed under p t conditions consistent with high pressure granulite facies they sometimes occur at lower grade in the amphibolite facies thus these assemblages are characteristic but not definitive of high pressure granulite facies compositional factors favouring their development at amphibolite grade include fe rich mineral compositions ca rich garnet and plagioclase and ti poor hornblende the generalized reaction that accounts for the prograde development of garnet clinopyroxene plagio clase quartz from a hornblende plagioclase quartz bearing amphibolite precursor is hbl pl qtz grt cpx liquid or vapour depending on whether the reaction occurs above or below the solidus there are significant discrepancies between experimental and natural constraints on the p t conditions of orthopyroxene free garnet clinopyroxene plagioclase quartz bearing mineral assemblages and therefore on the p t position of this reaction semi quantitative thermo dynamic modelling of this reaction is hampered by the lack of a melt model and gives results that are only moderately successful in rationalizing the natural and experimental data

seismotectonics of sierra pie de palo a basement block uplift in the andean foreland of argentina we operated a digitally recording seismic network from september 1987 to may 1988 in the san juan province of northwestern argentina the network was located in the thin skinned precordillera and thick skinned sierras pampeanas tectonic provinces of the andean foreland our data provide a detailed view of the crustal seismicity of sierra pie de palo one of the most seismically active mountain blocks of the sierra pampeanas and the site of the destructive m 7 3 caucete earthquake in 1977 most of the well located shallow earthquakes we recorded occurred directly beneath the range between 9 and 30 km depth with a maximum activity around 25 km depth the morphology of the range seismicity pattern and focal mechanisms strongly suggest sierra pie de palo is cut into two blocks by a northeast southwest striking fracture in the northern block seismicity occurs along a well defined west dipping seismic zone while the southern block is characterized by east dipping seismic layers both dipping zones connect to a subhorizontal mid crustal detachment that we interpret as the lower boundary of the brittle failure domain the complex fault geometry and large variety of focal mechanism orientations we observe suggest that the ongoing seismic activity is a continuation of the aftershock sequence of the 1977 caucete earthquake we therefore interpret the two principal dipping zones to be buried thrusts activated during the complex rupture of the 1977 earthquake inversion of the focal mechanism data for the orientation of the regional deviatoric stress tensor shows the maximum compressive stress u oriented at an azimuth of n107 e this direction is approximately orthogonal to the trend of the sierra but rotated 27 clockwise with respect to the current nazca south america convergence direction this result suggests that local boundary effects within the sierras pampeanas province partially control the shortening direction in the crust it may also indicate that a component of sinistral strike slip motion exits within the precordillera to the west of the sierras pampeanas

on the porphyroblastic eclogites of the śnieznik mountains in the polish sudetes abstract a rare textural variety of eclogite characterized by a two fold occurrence of omphacite as older microlites and as later porphyroblasts enclosing numerous microlites has been found at only two localities in the polish sudetes wojtowka and nowa wie the chemical compositions of 27 mineral grains in four samples of such eclogites have been determined by electron microprobe spot analysis all omphacite analyses of both porphyroblasts and microlites show almost identical compositions except for one microlite enclosed in an armoured relic of tremolite this microlite displays a lower na content and is considered to belong to an earlier stage of metamorphic evolution preceding the stage of eclogitization it is deduced that these porphyroblastic eclogites experienced three successive stages of metamorphic evolution 1 1 pre eclogite stage when the primary rocks developed the mineral composition of a low temperature metabasite greenschist facies 2 2 eclogite stage when the rock gained typical eclogite mineral compositions as a consequence of a strong increase in total pressure unaccompanied by an increase in ph2o eclogite facies 3 3 post eclogite stage when the garnet omphacite association ceased to be stable and was transformed into a hornblende plagioclase association as a result of the decrease of total pressure which became equal to ph2o amphibolite facies

exact timing of granulite metamorphism in the namche barwa eastern himalayan syntaxis new constrains from sims u pb zircon age zircon grains separated from 2 granulites from the eastern himalaya were investigated by raman spectroscopy cathodoluminescence imaging and secondary ion mass spectrometry these grains have a thin homogeneous rim and an oscillatory inner zone domain with or without a relict inherited core garnet kyanite and rutile inclusions were identified within only the rim domain of zircon grains indicating that the rim had formed during peak granulite facies metamorphism u pb zircon data record three distinct age populations 1 805 ma for the inherited core ca 500 ma oscillatory inner zone as well as 24 25 ma and ca 18 ma for the metamorphic rim these new precision ages suggest that the peak metamorphic age for the hp granulite is at ca 24 25 ma and subsequent amphibolite facies retrograde metamorphism occurred at ca 18 ma

vertical extrusion and horizontal channel flow of orogenic lower crust key exhumation mechanisms in large hot orogens a large database of structural geochronological and petrological data combined with a bouguer anomaly map is used to develop a two stage exhumation model of deep seated rocks in the eastern sector of the variscan belt an early sub vertical fabric developed in the orogenic lower and middle crust during intracrustal folding followed by the vertical extrusion of the lower crustal rocks these events were responsible for exhumation of the orogenic lower crust from depths equivalent to 18 20 kbar to depths equivalent to 8 10 kbar and for coeval burial of upper crustal rocks to depths equivalent to 8 9 kbar following the folding and vertical extrusion event sub horizontal fabrics developed at medium to low pressure in the orogenic lower and middle crust during vertical shortening fabrics that record the early vertical extrusion originated between 350 and 340 ma during building of an orogenic root in response to se directed saxothuringian continental subduction fabrics that record the later sub horizontal exhumation event relate to an eastern promontory of the brunia continent indenting into the rheologically weaker rocks of the orogenic root indentation initiated thrusting or flow of the orogenic crust over the brunia continent in a north directed sub horizontal channel this sub horizontal flow operated between 330 and 325 ma and was responsible for a heterogeneous mixing of blocks and boudins of lower and middle crustal rocks and for their progressive thermal re equilibration the erosion depth as well as the degree of reworking decreases from south to north pointing to an outflow of lower crustal material to the surface which was subsequently eroded and deposited in a foreland basin indentation by the brunia continental promontory was highly noncoaxial with respect to the se oriented saxothuringian continental subduction in the early visean suggesting a major switch of plate configuration during the middle to late visean

fluid influence on mineral reactions in ultrahigh pressure granulites a case study in the śnieznik mts west sudetes poland small tectonic slices of undeformed eclogites and ultrahigh pressure granulites occur in three tectonic units of the nieznik mts sw poland ultrahigh pressure granulite eclogite transitions with peak metamorphic conditions between 21 and 28 kbar at 800 to 1000 c occur only in the zlote unit conventional u pb multigrain analyses of zircons from a mafic granulite provided 207pb 206pb ages between 360 to 369 ma which are interpreted to approximate timing of original crystallisation from a melt diffusion kinetics and the restricted availability of a fluid phase mainly controlled the conversion from granulite to eclogite although some bulk chemical differences were also recognised the ultrahigh pressure granulites from the zlote unit exclusively contain h2o rich inclusions with variable salinities which distinguishes them from high temperature ht granulites world wide this is also in contrast to the fluid regime h2o n2 co2 recognised in the lower temperature eclogites 600 800 c from the closely associated miedzygorze and nieznik units the variation in fluid composition between the lower temperature eclogites and ultrahigh pressure granulites on the one hand and ultrahigh pressure granulites and ht granulites on the other hand probably indicates contrasting p t t paths as a result of different tectonic environments

exhumation of eclogitized continental basement during variscan lithospheric delamination and gravitational collapse sudety mountains poland a variscan deep crustal level eclogite facies continental basement massif in western poland the snieznik complex was tectonically exhumed crustal penetrating mylonite zones record three main kinematic events early top to the north directed thrusting right slip transpression tension and late top to the south and east normal faulting thrusting resulted in extreme crustal thickening and associated eclogite facies metamorphism right slip movements produced retrogressive crystal plastic simple shear zones normal faults flank carboniferous to early permian terrigenous sedimentary basins documenting tectonic and erosional denudation of the snieznik complex during lithospheric extension sm nd isotopic dates previously reported for the in situ eclogite facies metamorphic mineral assemblages are 341 337 and 329 ma brueckner et al 1991 40 ar 39 ar isotopic dates for metamorphic hornblende 338 333 and 332 ma muscovite 329 and 329 ma and biotite 328 ma reflect times of cooling through the 500 350 and 300 c isotherms respectively these nearly concordant mineral dates document rapid cooling from 850 c eclogue facies temperatures to 300 c rapid denudation of these deep crustal rocks 19 22 kbar pressures 70 km depth is attributed to processes similar to those of metamorphic core complexes that operated during lithospheric delamination and gravitational collapse the sequence of late paleozoic alleghanian crustal thickening followed by right slip transpression tension followed by normal faulting recognized in the u s appalachians implies that this tectonic pattern may exist throughout the afleghanian variscan belt

high pressure granulites from the sudetes south west poland evidence of crustal subduction and collisional thickening in the variscan belt two high grade gneissic complexes of the western sudetes the gory sowie block and the nieznik area complex contain small predominantly felsic granulitic inliers with minor cpx bearing intercalations the p t conditions of the granulite facies events and of the subsequent re equilibration are estimated using the ternary feldspar thermometer and the geo calc computer program version twq jan 92 in the gory sowie granulites the peak granulitic event occurred at c 18 20 kbar and 900 c and the late decompressive re equilibration within a range of 4 10 kbar and temperatures decreasing to 600 700 c the latter event is thought to have coincided with the main metamorphic phase in the surrounding gneisses the p t estimates are more scattered in the nieznik granulites but the peak conditions for the granulitic event are estimated at pressure over 22 kbar possibly around 30 kbar and temperature exceeding 900 c the analysed samples from the nieznik area bear no significant evidence of lower pressure re equilibration integrating the thermobarometric data and some age constraints indicates that the gory sowie granulites belong to the early stage type i granulites of the variscan belt c 400 ma old which are interpreted as fragments of continental crustal materials subducted to mantle depths in the earliest stages of the variscan orogeny the nieznik granulites are more problematic they may belong to a younger high p suite c 350 ma old widespread in the southern and eastern parts of the bohemian massif and possibly related to the climax of the variscan continent continent collision

u pb ages from sw poland evidence for a caledonian suture zone between baltica and gondwana faunal and palaeomagnetic evidence suggests the existence of a c 3000 km wide tornquist sea between gondwana and baltica in early ordovician times which narrowed to 50 2 ma an unmetamorphosed ophiolite gives a 420 20 2 ma age whilst in two other terranes amphibolite facies island arc lavas have fossiliferous lower ludlow 424 415 ma sequences all are unconformably overlain by unmetamorphosed upper devonian conglomerates a sinistral transpressive regime is observed in regionally extensive mylonite zones titanite and zircon ages 338 2 3 and 339 4 ma record variscan magmatism the data suggest considerable narrowing of the tornquist sea during the ordovician continuing ocean floor and island arc activity in the silurian and final sinistral transpressive closure by the mid devonian

the 40ar 39ar ages from the west sudetes ne bohemian massif constraints on the variscan polyphase tectonothermal development

pb u and th diffusion in natural zircon zircon zrsio4 is one of the most widely used minerals for determining the age origin and thermal history of rocks by u th pb geochronology but the parameters describing the solid state volume diffusion rates of these elements in natural zircon which are themselves important for establishing the limits on the applicability of zircon for geochronological studies have remained poorly quantified1 this is because of the measurement difficulties associated with the low p p m concentrations and low diffusion rates of these elements in natural zircon and the chemical and physical heterogeneity present in most crystals here we present direct measurements of the uranium thorium and lead loss from a thermally treated gem quality natural zircon and show that lead diffuses much faster than uranium or thorium we find that the u th pb isotopic system in natural zircon will typically have a closure temperature greater than 900 c which explains why zircon is apparently such a robust geochrometer and is capable of remaining isotopically closed through extended periods of high grade metamorphism and partial melting of the host rock

new evidence of blueschist facies rocks and their geotectonic implication for variscan suture s in the bohemian massif blueschist facies rocks exposed within consolidated continental blocks provide some of the best evidence for the existence of previous suture s they usually occur as lenses or layers embedded within greenschist or amphibolite facies rocks and indicate reequilibration at medium to low pressure conditions in the bohemian massif a few occurrences of blueschists have been reported and here new evidence of high pressure hp metamorphism in various lithologies is presented that suggests a larger extent of blueschist facies rocks along the northern border of this massif an earlier blueschist facies metamorphism is documented by inclusions of glaucophane in garnet epidote and titanite from metabasites along with zoned white mica having a phengitic core and a muscovite rim in metapelites and orthogneisses the estimated p t conditions obtained using pseudosections and mineral isopleths correspond to blueschist and low temperature eclogite facies conditions 1 1 2 0 gpa at 350 550 c together with medium temperature eclogites from different units in the bohemian massif they indicate a geothermal gradient of 8 10 c km 1 which is typical for cool subduction radiometric dating on phengite from metapelites confirms an early palaeozoic cooling age of c 360 ma for this hp metamorphic event the presence of blueschist facies rocks their p t relations and age constraint together with those from eclogite facies rocks allows us to locate the variscan suture which straddles the se margin of saxothuringian zone from erzgebirge to sudetes and its possible continuation to the moldanubian zone where eclogite facies and uhpm rocks are abundant

terranes and terrane boundaries in the sudetes northeast bohemian massif in the sudetes seven distinct lithostratigraphic terranes exhibit a symmetric distribution a central region of basinal oceanic and ophiolitic rocks the central sudetic terrane is bordered respectively to the northwest and southeast by the sialic saxothuringian and moldanubian terranes these exhibit contrasting metasedimentary metavolcanic successions and tectonic metamorphic sequences but both are characterized by palaeozoic plutonism these are in turn bordered again respectively to the northwest and southeast by the lusatian and moravian terranes which are also sialic but contain cadomian granitoids and represent rifted and now widely separated fragments of gondwana along the southwestern flank of the sudetes the barrandian terrane largely covered by younger sediments extends to the southwestern margin of the bohemian massif the sowie gory terrane forms a klippe of high grade gneisses tectonically emplaced on top of low grade sheared ophiolites of the central sudetic terrane the sowie gory terrane exhibits a history of three distinct probably multi orogenic regional metamorphic events an early high pressure granulite eclogite metamorphism followed by medium to low pressure granulite and in turn by amphibolite facies metamorphism all the terrane boundaries are complex zones of ductile to brittle shearing modified by later brittle movements some such as the leszczyniec shear zone mark lines of old pre variscan rift and suture zones reactivated and overprinted during a series of variscan ductile to brittle events of extensional shearing with related metamorphism and plutonism

pb diffusion in zircon abstract chemical diffusion of ti under anhydrous conditions at 1 atm and under fluid present elevated pressure 1 1 1 2 gpa conditions has been measured in natural zircon the source of diffusant for 1 atm experiments was a zro2 tio2 zircon mixture with experiments run in crimped pt capsules diffusion experiments done in the presence of h2o co2 fluid were run in a piston cylinder apparatus using a source of ground tio2 zrsio4 and sio2 with oxalic acid added to produce h2o co2 vapor and partially melt the solid source material yielding an assemblage of rutile zircon melt vapor nuclear reaction analysis nra with the resonant nuclear reaction 48ti p 49v was used to measure diffusion profiles for both sets of experiments the following arrhenius parameters were obtained for ti diffusion normal to cover the temperature range 1350 1550 c at 1 atm d ti 3 33 10 2 exp 754 56 kj mol 1 r t m 2 s 1 ti diffusivities were found to be similar for experiments run under fluid present conditions a fit to all of the data yields the arrhenius relation d 1 34 102 exp 741 46 kj mol 1 rt m2 s 1 these data suggest that zircon should be extremely retentive of ti chemical signatures indicating that the recently developed ti in zircon crystallization geothermometer watson e b harrison t m 2005 zircon thermometer reveals minimum melting conditions on earliest earth science 308 841 844 will be quite robust in preserving temperatures of zircon crystallization titanium diffuses somewhat faster in zircon than larger tetravalent cations u th and hf but considerably more slowly than pb the ree and oxygen hence ti crystallization temperatures may be retained under circumstances when radiometric ages or other types of geochemical information are lost

slow cooling of deep crustal granulites and pb loss in zircon abstract we attempted to obtain the magmatic crystallization age of a metamorphosed granulite facies t 800 90 c p 8 1 kbar massif type anorthosite from the ankafotia body of southwest madagascar the sample studied is a coarse grained leuconorite with good preservation of igneous texture and mineralogy although plagioclase which contains abundant rutile and zircon inclusions has been slightly recrystallized thirty three isotope dilution u pb analyses of zircons representing single grain fragments 29 analyses and multi grain fractions 4 analyses yield a spectrum of concordant ages from 631 to 549 ma a time span of more than 80 myr back scattered electron and cathodoluminescence images show that most grains are either homogeneous structureless fragments 35 or are permeated to a variable degree by anastamosing cracks occupied by relatively u and or th enriched zircon 45 a smaller percentage of grains show relict magmatic zoning 20 thin u and or th rich overgrowths occur on about 25 of grains raman spot analyses demonstrate that all fragments are highly crystalline and non metamict there are marked correlations between zircon grain size and internal features such that the oldest grains are larger and show relict magmatic zoning the youngest grains are small fragments containing high u crack networks ion microprobe spot analyses show that each zircon grain preserves a distinct trace element signature rare earth element patterns show heavy ree enrichment with negative eu anomalies and positive ce anomalies we suggest that the ca 80 myr spread in concordant u pb ages in this sample is indicative of high temperature pb loss during one or more protracted periods of granulite facies metamorphism with only minor episodic or continuous metamorphic zircon growth volume diffusion and or fracture assisted diffusion seems to be the dominant mechanism of pb loss cooling curves calculated using recently measured pb diffusion parameters conform to the age size relationship and imply very slow cooling rates 1 2 c myr or less as might be expected for a terrane in which granulite conditions were maintained for an extended period of time our results therefore suggest a note of caution for interpretation of concordant zircon ages in meta igneous rocks affected by high grade metamorphism of long duration

epma and pixe dating of monazite in granulites from stary gieraltow ne bohemian massif poland abstract chemical th u total pb chime dating of monazite by electron probe microanalyzer epma and proton microprobe pixe was carried out on felsic granulites from stary gieraltow poland which represent part of the orlica nieznik dome in the ne bohemian massif analyzed monazite is characterized by mosaic zoning rather than simple core to rim growth and strontium contents of up to 750ppm an isochron age of 347 13ma represents timing of amphibolite facies metamorphism in agreement with previously published estimates

single zircon ages and whole rock nd isotopic systematics of early palaeozoic granitoid gneisses from the czech and polish sudetes jizerske hory krkonose mountains and orlice sněžnik complex granitoid orthogneisses make up the predominant rock type in the west sudetes from jizerske hory in the nw to the orlicke hory and sn nik mountains in the se these generally strongly foliated gneisses are calc alkaline in composition and display trace element characteristics suggesting generation in a volcanic arc setting single zircon ages reflecting the time of emplacement of the gneiss protoliths define a relatively narrow cambro ordovician range between 502 and 515 ma this is similar to previously reported zircon ages from the czech and polish west sudetes and documents an important and regionally extensive post cadomian magmatic event that we relate to continental arc magmatism on the margin of avalonia that developed during closure of the tornquist ocean an age of 492 ma for a microgranite dyke cutting deformed and metamorphosed orthogneisses in the orlicke hory shows the main deformation to be early paleozoic zircon xenocryst minimum ages range between 546 and 2070 ma and show maxima in the cadomian pan african 550 850 ma and grenvillian 1000 1300 ma time brackets the grenvillian event is also evident from nd mean crustal residence ages that vary between 1 34 and 1 87 ga from these data we suggest that the pre variscan granitoid gneisses of the czech west sudetes were largely generated by melting of a predominantly grenville age basement that was part of the northern margin of gondwana and may have been related to grenville age basement now identified in northern south america

challenges in high pressure granulite metamorphism in the era of pseudosections reaction textures compositional zoning and tectonic interpretation with examples from the bohemian massif high pressure granulites are an important record of geodynamic processes in overthickened or subducted continental crust orthopyroxene free assemblages in granitic ternary feldspar s quartz garnet kyanite rutile intermediate ternary feldspar s quartz garnet clinopyroxene kyanite rutile titanite and basic garnet clinopyroxene plagioclase quartz rutile compositions indicate formation conditions at mantle depths clinopyroxene compositions in variscan high pressure granulites are unusual in that they include omphacite in plagioclase bearing rocks thus not eclogite and al rich diopside i e indicating high ca tschermak content with both yielding temperatures above 900 c problems such as compositional zoning multiple generations of key phases in reaction domains and unmixing of high temperature solid solution phases during cooling ternary feldspars omphacite clearly indicate disequilibrium and require very careful interpretation as to which phases and compositions possibly represent a former equilibrium association pressure temperature p t determination by the pseudosection method although allowing prediction of mineral assemblages compositions and molar proportions for a fixed bulk composition for modelled p t conditions still requires reliable activity composition information for the key phases feldspar and clinopyroxene as well as an interpretation of former equilibrium compositions in the investigated samples i e the same restrictions applying to conventional thermobarometry the interpretations of some recently determined pseudosections for the composition of variscan clinopyroxene bearing high pressure granulites contradict numerous published p t paths however quantitative information from thermobarometry or pseudosections must be integrated with key petrographic observations in the case of the variscan example it is argued that petrographic observations and published p t paths are consistent with mineral assemblages predicted in pseudosections and support existing tectonometamorphic models

high pressure granulites formation recovery of peak conditions and implications for tectonics high pressure granulites are characterised by the key associations garnet clinopyroxene plagioclase quartz in basic rocks and kyanite k feldspar metapelites and felsic rocks and are typically orthopyroxene free in both basic and felsic bulk compositions in regional metamorphic areas two essential varieties exist a high to ultrahigh temperature group and a group representing overprinted eclogites the high to ultrahigh temperature type formerly contained high temperature ternary feldspar now mesoperthite coexisting with kyanite is associated with garnet peridotites and formed at conditions above 900 c and 1 5 gpa clinopyroxene in subordinate basic rocks is al rich and textural evidence points to a high pressure high temperature melting history the second variety contains symplectite like or poikilitic clinopyroxene plagioclase intergrowths indicating former plagioclase free i e eclogite facies assemblages this type of rock formed at conditions straddling the high pressure amphibolite high pressure granulite field at around 700 850 c 1 0 1 4 gpa importantly in the majority of high pressure granulites orthopyroxene is secondary and is a product of reactions at pressures lower than the peak recorded pressure in contrast to low and medium pressure granulites which form at conditions attainable in the mid to lower levels of normal continental crust high pressure granulites of nonxenolith origin mostly represent rocks formed as a result of short lived tectonic events that led to crustal thickening or subduction of the crust into the mantle short times at high temperature conditions are reflected in the preservation of prograde zoning in garnet and pyroxene high pressure granulites of both regional types although rare are known from both old and young metamorphic terranes e g c 45 ma namche barwa e himalaya 400 340 ma european variscides 1 8 ga hengshan china 1 9 ga snowbird saskatchewan and 2 5 ga jianping china this spread of ages supports proposals suggesting that thermal and tectonic processes in the lithosphere have not changed significantly since at least the end of the archean

sm nd and u pb dating of high pressure granulites from the zlote and rychleby mts bohemian massif poland and czech republic in the orlica snie znik complex at the ne margin of the bohemian massif high pressure granulites occur as isolated lenses within partially migmatized orthogneisses sm nd different grain size fractions of garnet clinopyroxene and or whole rock and u pb isotope dilution thermal ionization mass spectrometry id tims single grain and sensitive high resolution ion microprobe shrimp ages for granulites collected in the surroundings of cervenydul czech republic and at stary gieraltow poland constrain the temporal evolution of these rocks during the variscan orogeny most of the new ages cluster at c 350 340 ma and are consistent with results previously reported for similar occurrences throughout the bohemian massif this interval is generally interpreted to constrain the time of high pressure metamorphism a more complex evolution is recorded for a mafic granulite from stary gieraltow and concerns the unknown duration of metamorphism single short lived metamorphic cycle or different episodes that are significantly separated in time the central grain parts of zircon from this sample yielded a large spread in apparent 206 pb 238 u shrimp ages c 462 322 ma with a distinct cluster at c 365 ma this spread is interpreted to be indicative for variable pb loss that affected magmatic protolith zircon during high grade metamorphism the initiating mechanism and the time of pb loss has yet to be resolved a connection to high pressure metamorphism at c 350 340 ma is a reasonable explanation but this relationship is far from straightforward an alternative interpretation suggests that resetting is related to a high temperature event not necessarily in the granulite facies and or at high pressures around 370 360 ma that has previously gone unnoticed this study indicates that caution is warranted in interpreting u pb zircon data of ht rocks because isotopic rejuvenation may lead to erroneous conclusions

ultrahigh pressure metamorphism in the śnieznik mountains sudetes poland p t constraints and geological implications the petrology and p t conditions of eclogites and associated gneisses from the nieznik mountains sudetes sw poland were investigated to establish an exhumation path for these ultrahigh pressure rocks peak pressure conditions are represented by the assemblage garnet omphacite phengite kyanite coesite rutile post dating this assemblage but still in the eclogite facies garnet with lower grossular content and a first generation of amphibole are formed under lower pressure conditions giving rise to the assemblage garnet omphacite phengite kyanite quartz rutile calcic amphibole i continuing depressurization caused the successive destabilization of omphacite garnet kyanite and phengite in some places the primary eclogite assemblage was completely transformed into amphibolite facies mineral assemblages estimated peak pressures in the nieznik and miedzygorze units lie in the coesite stability field p 27 kbar at temperatures of about 700 800 c and are not significantly different to the zlote un