unusual bulk rock compositions in eclogite facies rocks from syros and tinos cyclades greece implications for u pb zircon geochronology low temperature eclogite facies rocks from syros and tinos cyclades greece include meta ophiolitic blocks of unclear origin meta olistostrome or tectonic melange these blocks occur in a matrix of altered serpentinite and or semipelitic to tuffitic schists some samples are characterized by very high trace element concentrations for example eclogites from tinos contain up to 4950 ppm zr and up to 480 ppm y understanding of the geochemical controls on the petrogenesis of these rocks has important consequences for interpretation of geochronological results the emphasis of this study is on the behaviour of zr under eclogite facies p t conditions due to remarkably high modal amounts of zircon some samples from the meta ophiolite suite are suitable for u pb zircon chronology the geological significance of zircon ages can only be correctly interpreted by assessing whether zircon crystallized from a melt or formed during subsequent hydrothermal or metamorphic processes the results of this study lead us to conclude that infiltration of trace element enriched fluids during eclogite facies metamorphism caused local scale compositional changes textural observations document the presence of a fracture network favouring fluid infiltration under high pressure conditions such a system of microfractures might have provided infiltration paths for trace element enriched fluids during earlier stages of high pressure metamorphism in most cases subsequent recrystallization erased any evidence of the early infiltration channels but inclusion of high pressure phases in zircon provide evidence for this model suggesting metasomatic alteration in a subduction zone environment the original fluid source and the conditions e g high f environment highly saline brines under which commonly immobile elements were mobilized remain unclear it is speculated that eclogitization of closely associated metagabbros may have released zr and other trace elements the results of this contribution suggest that new zircon formed during high pressure metamorphism cretaceous u pb zircon ages are interpreted to date subduction metamorphism and not magmatic crystallization

thermochronology of the south cyclades shear zone ios greece effects of ductile shear in the argon partial retention zone micas and potassium feldspars from the south cyclades shear zone ios cyclades greece yield varied and complex 40ar 39ar apparent age spectra a correlation exists between 40ar 39ar apparent ages and the relative timing of different episodes of recrystallization and grain growth as indicated by fabric and microstructural analysis the 40ar 39ar apparent age spectra record the effects of variation in the degree of recrystallization and grain growth and partial to complete resetting of argon systematics in potassium bearing minerals during hercynian m0 alpine m1 and late oligocene early miocene m2 metamorphism deformation was strongly partitioned within the shear zone and this led to localized recrystallization and heterogeneous resetting of argon systematics within preexisting minerals modeling suggests the oligo miocene thermal events were of insufficient magnitude and or duration to completely reset the isotopic systematics in these samples our data lead to the concept of the argon partial retention and resetting zone prz defined as that portion of the crust where temperatures are insufficient to completely reset argon systematics within preexisting potassium bearing minerals within the prz some radiogenic argon in preexisting potassium bearing minerals will be outgassed and only partially retained tectonic exhumation of the prz involves movement on crustal scale ductile shear zones accompanied by strongly partitioned deformation and localized recrystallization recrystallization leads to resetting of argon systematics and thus will result in heterogeneous 40ar 39ar age distributions within these ductile shear zones e g in the south cyclades shear zone

40ar 39ar and oxygen isotope studies of polymetamorphism from tinos island cycladic blueschist belt greece petrological oxygen isotope and 40ar 39ar studies were used to constrain the tertiary metamorphic evolution of the lower tectonic unit of the cyclades on tinos polyphase high pressure metamorphism reached pressures in excess of 15 kbar based on measurements of the si content in potassic white mica temperatures of 450 500 c at the thermal peak of high pressure metamorphism were estimated from critical metamorphic assemblages the validity of which is confirmed by a quartz magnetite oxygen isotope temperature of 470 c some 40ar 39ar spectra of white mica give plateau ages of 44 40 ma that are considered to represent dynamic recrystallization under peak or slightly post peak high pressure metamorphic conditions early stages in the prograde high pressure evolution may be documented by older apparent ages in the high temperature steps of some spectra eclogite to epidote blueschist facies mineralogies were partially or totally replaced by retrograde greenschist facies assemblages during exhumation oxygen isotope thermometry of four quartz magnetite pairs from greenschist samples gives temperatures of 440 470 c which cannot be distinguished from those deduced for the high pressure event the exhumation and overprint is documented by decreasing ages of 32 28 ma in some greenschists and late stage blueschist rocks and ages of 30 20 ma in the lower temperature steps of the ar release patterns of blueschist micas almost flat parts of ar ar release spectra of some greenschist micas gave ages of 23 21 ma which are assumed to represent incomplete resetting caused by a renewed prograde phase of greenschist metamorphism oxygen isotope compositions of blueschist and greenschist facies minerals show no evidence for the infiltration of a 18o enriched fluid rather the compositions indicate that fluid to rock ratios were very low the isotopic compositions being primarily controlled by those of the protolith rocks we assume that the fundamental control catalysing the transformation of blueschists into greenschists and the associated resetting of their isotopic systems was the selective infiltration of metamorphic fluid a quartz magnetite sample from a contact metamorphic skarn taken near the miocene monzogranite of tinos gave an oxygen isotope temperature of 555 c and calculated water composition of 9 1 the value of 18o obtained from this water is consistent with a primary magmatic fluid but is lower than that of fluids associated with the greenschist overprint which indicates that the latter event cannot be directly related to the monozogranite intrusion

kinematic reconstructions and magmatic evolution illuminating crustal and mantle dynamics of the eastern mediterranean region since the late cretaceous the relationship between subduction dynamics and crustal deformation in the mediterranean region has been recently studied using three dimensional 3d numerical models such models require however a detailed information concerning the past geological evolution we use stratigraphic petrologic metamorphic structural paleomagnetic and magmatic data to build new kinematic reconstructions of the eastern mediterranean region since the late cretaceous using the principle of non rigid domains the motions of the 56 deforming domains defined in this work are calculated based on published paleomagnetic rotations the directions and amounts of displacement on crustal scale shear zones and the burial and exhumation histories of the main metamorphic units extracted from these reconstructions paleotectonic maps and lithospheric scale cross sections illustrate that the present day subduction zone has been continuously retreating southward since the late cretaceous and has accreted several small continental domains in the process we find evidence for two back arc related extensional events 1 slow extension along the balkans and the pontides in the late cretaceous while the trench was long and linear and 2 faster extension in the rhodope aegean west anatolian region since the eocene oligocene rapid rotation of the hellenides between 15 and 8 ma probably indicates a slab tearing event below western anatolia that could have further accelerated this extensional kinematics spatial distribution and the geochemical signature of magmatic centers integrated in these reconstructions allow us to trace mantle related processes revealing the deep dynamics that controls both the magma genesis and the crustal deformation

crustal zircons and mantle sulfides archean to triassic events in the lithosphere beneath south eastern sicily three types of zircon coexist in an unusual lower crustal xenolith from the valle guffari diatreme hyblean plateau sicily igneous type 1 near euhedral weakly zoned ce ce 1 partially recrystallised type 2 ovoid structureless weak ce anomaly hydrothermal type 3 sugary spongy textured probably related to f rich aqueous fluids u pb dating by lam icpms supported by in situ hf isotope analysis suggests that both type 1 and type 2 zircons were originally archean ca 2 7 ga though many of these grains have experienced severe pb loss the u pb ages of the hydrothermal zircons cluster around 246 ma interpreted as the timing of the hydrothermal event their ehf 8 5 to 1 2 indicates the mixing of old crustal components and material from a juvenile source in situ os isotope analyses of sulfides hosted in peridotite xenoliths from valle guffari show paleoproterozoic archean trdminimum ages corresponding to the age of the oldest zircon grains in the crustal xenolith other peaks of trd ages suggest that multiple metasomatic events have affected the lithospheric mantle these observations suggest that the lower crust and the upper part of the lithospheric mantle beneath the hyblean plateau represent the northernmost portion of the african plate these two units have coexisted since at least late archean time and have remained linked through several episodes of crustal modification including the permo triassic hydrothermal event which was probably related to the onset of rifting in the ionian basin

zircon u pb ages and hf isotope compositions of migmatite from the north dabie terrane in china constraints on partial melting migmatite gneisses are widespread in the dabie orogen but their formation ages are poorly constrained eight samples of migmatite including leucosome melanosome and banded gneiss were selected for u pb dating and hf isotope analysis most metamorphic zircon occurs as overgrowths around inherited igneous cores or as newly grown grains morphological and internal structure features suggest that their growth is associated with partial melting according to the hf isotope ratio relationships between metamorphic zircon and inherited cores three formation mechanisms for metamorphic zircon can be determined which are dissolution reprecipitation of pre existing zircon breakdown of zr bearing phase other than zircon in a closed system and crystallization from externally derived zr bearing melt four samples contain magmatic zircon cores yielding upper intercept u pb ages of 807 35 768 12 ma suggesting that the protoliths of the migmatites are neoproterozoic in age the migmatite zircon yields weighted mean two stage hf model ages of 2513 97 894 54 ma indicating reworking of both juvenile and ancient crustal materials at the time of their protolith formation the metamorphic zircons give u pb ages of 145 2 120 2 ma the oldest age indicates that partial melting commenced prior to 145 ma which also constrains the onset of extensional tectonism in this region to pre 145 ma the youngest age of 120 ma was obtained from an undeformed granitic vein indicating that deformation in this area was complete at this time two major episodes of partial melting were dated at 139 1 and 123 1ma the first episode of partial melting is obviously older than the timing of post collision magmatism corresponding to regional extension the second episode of partial melting is coeval with the widespread post collision magmatism indicating the gravitational collapse and delamination of the orogenic lithospheric keel of the dabie orogen which were possibly triggered by the uprising of the cretaceous mid pacific superplume

zircon u pb hf isotopic and whole rock geochemical studies of paleoproterozoic metasedimentary rocks in the northern segment of the jiao liao ji belt china implications for provenance and regional tectonic evolution abstract this study examines the original nature tectonic setting and evolution of paleoproterozoic metasedimentary rocks of the north liaohe and laoling groups of the northern segment of the jiao liao ji belt in the northeastern margin of the eastern block of the north china craton using new petrological whole rock geochemical geochronological and lu hf isotope data zircons in the metasediments are magmatic and metamorphic in origin yielding peak ages of 2 03 ga for the deposition of the protoliths of the metasedimentary rocks and 1 90 and 1 85 ga for their metamorphism geochemical analyses indicate the protoliths contained sandy wacke and claystone material with minor shale derived from paleoproterozoic granites coeval acidic meta volcanic rocks meso and neo archean basement granites and paleoproterozoic meta mafic rocks using regional geological and new lu hf isotopic data we propose that the protoliths of the metasedimentary rocks of the north liaohe and laoling groups in the northern segment of the jiao liao ji belt were deposited in a back arc basin setting this region experienced two crustal growth events at 2 57 2 46 and 2 28 2 08 ga as well as two metamorphic events at ca 1 90 and 1 85 ga

micromorphology of atlantic rankers on the coast of northern portugal summary the micromorphological examination of atlantic rankers on the coast of northern portugal shows the importance of alternating phases of landscape stability and instability during the genesis of the profiles archaeological artifacts and marine deposits allow the dating of these events recent pedogenesis is characterized by the formation of a microgranular structure and the illuviation of amorphous organic matter remnants of an older leached phase in the gelfa section reflect an initial weakly base desaturation stage during historical times the soil was buried under eolian sands

late eocene lawsonite eclogite facies metasomatism of a granulite sliver associated to ophiolites in alpine corsica abstract the metamorphic history of two samples of continental slivers from alpine corsica is reconstructed on the basis of detailed petrology trace and major element geochemistry of the bulk rock and of metamorphic minerals and u pb dating of zircon a lawsonite fels with a ca rich and si na k poor bulk composition and a garnet bearing schist were investigated they both exhibit strongly zoned garnet crystals characterised by cores with high mg 0 25 0 33 low grossular component 0 03 0 09 and ree patterns with a negative eu anomaly u pb dating of zircons from the lawsonite fels yielded scattered inherited core ages from 912 to 575 ma and a first metamorphic rim that is dated at 292 5 3 3 ma ti in zircon thermometry the trace element pattern of the zircon and the composition of the garnet cores indicate that both samples experienced permian upper amphibolite to granulite facies metamorphism these high grade metamorphic rocks are associated to serpentinites metabasalts and metasediments that are typical of the alpine tethys indicating that the continental slivers were incorporated into an ocean continent transition during rifting alpine eclogite facies metamorphism is characterised by low temperature conditions as demonstrated by lawsonite crystallisation in both samples the lawsonite fels preserved the most complex history with two stages of garnet growth corresponding to a first prograde path possibly in the lawsonite blueschist facies and a second stage at lawsonite eclogite facies conditions this second stage was marked by strong ca metasomatism that is responsible for the peculiar composition of the sample and the massive crystallisation of lawsonite the strong metasomatism is also responsible for the partial recrystallisation of zircon and the formation of alpine rims the age of these zircon rims 34 4 0 8 ma represents the first u pb age for alpine high pressure metamorphism in corsica

extraneous argon in high pressure metamorphic rocks distribution origin and transport in the cycladic blueschist unit greece the 40ar 39ar geochronological technique has significantly contributed to the study of crustal deformation however the frequent occurrence of extraneous argon in high pressure terrains can make the interpretation of 40ar 39ar ages problematic this contribution attempts to i characterize the distribution and origin of extraneous argon in the cycladic blueschist unit cbu greece by applying the 40ar 39ar dating technique on three cycladic islands syros tinos and ios which span a complete transect of the cbu and ii discuss regional implications for the tectonometamorphic evolution of the cbu our results combined with ages available in the literature show that periods of peak burial conditions 55 49 ma and syn 45 38 ma to post orogenic exhumation 37 21 ma are well constrained in the cbu by the 40ar 39ar method moreover variscan apparent ages are preserved in the cycladic basement of ios indicating that the 40ar 39ar system was not completely reset during the eocene metamorphic episode additionally our 40ar 39ar data show that the occurrence of extraneous argon is heterogeneously distributed in the cbu while high pressure rocks of syros and tinos are only locally affected extraneous argon is clearly evidenced in the cbu of ios yielding geologically meaningless ages up to 900 ma on each island the most affected ages were obtained in microstructures where fluid circulation is enhanced such as shear bands or strain shadows around garnets we suggest that the cycladic basement a polymetamorphic continental unit behaved as an argon reservoir that has been mobilized by fluid circulation at the eocene these argon enriched fluids have then heterogeneously contaminated metamorphic rocks when percolating through the cbu finally we claim that a minimum closure temperature of 550 c must be accepted for the phengites of the cbu and conclude that volume diffusion is not the main process resetting the argon isotopic system in high pressure rocks compare to recrystallization enhanced by fluid circulation and deformation

crystallization and age of zircon from corsican ophiolitic albitites consequences for oceanic expansion in jurassic times abstract in corsica albitites and albite granites are considered to be the latest products of an ophiolitic magma two albitites have been selected for a study of the morphology typology and age of their zircons applying for the first time the u pb method to western tethyan ophiolites both samples come from the rospigliani series possibly formed in a transform fault zone the first one shows a cumulate texture whereas the second has features indicative of more rapid cooling zircons have precipitated at the end of the fractional crystallization process after a strong enrichment in alkali and incompatible elements in a typological diagram they plot at high temperature and high agpaicity indexes these abundant large euhedral to subhedral zircons sometimes with fine zoning may also contain inclusions of granitic composition which testifies to k fractionation in ophiolitic magmas the u pb age of 161 3myr is in the range of the ages obtained for alpine and apennines ophiolites all these ophiolitic remnants belonging to the liguro piedmont basin may represent a portion of the central atlantic ocean created during middle and upper jurassic times

leaching and differential recrystallization of metamict zircon under experimental hydrothermal conditions we report results of hydrothermal experiments on four alluvial zircons from sri lanka which cover a wide range of radiation damage at 450 c and 1 3 kbar for 744 h with 2 m cacl2 solution as reactive fluid after the hydrothermal treatment the most metamict samples show micrometer thick reaction rims which surround apparently unreacted zircon as revealed by cathodoluminescence cl and nomarski differential interference contrast ndic images these rims have sharp curved and transgressive boundaries with unreacted zircon and are in some cases spread out along cracks the thickness of reaction rims increases with increasing cumulated dosage of the starting materials the reaction rims are strongly enriched in ca up to 7000 ppm and a water species and depleted in radiogenic pb zr and si as revealed by electron microprobe analyses a significant th loss from the reaction rims was detected in the case of the most metamict sample whereas u remained in the structure ft infrared spectrometry and x ray diffraction measurements revealed that the bulk run products were recrystallized using micro raman spectrometry we were able to demonstrate that differential recrystallization took place the reaction rims are strongly recrystallized whereas the unreacted grain interiors underwent only minor recrystallization recrystallization of the rims is accompanied by an enhancement of the integral cl intensity it is suggested that recrystallization in the reaction rims was catalyzed by water infiltration and ion exchange and prevented significant congruent zircon dissolution under the given experimental conditions previous zircon studies have shown that 1 a transgressive morphology 2 a reduced th u ratio and 3 an enhanced cl emission are also characteristics of rims in zircons from high grade metamorphic rocks based on these similarities between natural and experimentally produced rims it is suggested that leaching catalyzed recrystallization is an important alteration process in zircon under wet geological conditions and can account for many complex core rim structures found in natural zircons furthermore the strong enrichment of ca in the reaction rims supports previous assumptions that high ca concentrations in natural zircons are of secondary origin it is suggested that lower u pb concordia intercept ages obtained from single phase zircons with high ca contents date a leaching event

trace element rich brines in eclogitic veins implications for fluid composition and transport during subduction primary and pseudosecondary fluid inclusions occur in oscillatory and sector zoned omphacite in eclogitic veins from the monviso ophiolitic complex in the western alps the inclusions contain aqueous brines and daughter crystals of halite sylvite calcite dolomite albite anhydrite and or gypsum barite baddeleyite rutile sphene fe oxides pyrite and monazite this daughter mineral suite indicates high solubilites of na k ca mg fe zr ti p ba ce la th and s species and provides direct evidence for transport of high fieldstrenght large ion lithophile and light rare earth elements as dissolved species during subduction fluid inclusion heterogeneities preserved within and between adjacent grains in the veins however suggest that the scale of fluid equilibration was small a crack seal geometry in some of the veins implies that fluid release in pulses rather than steady flow controlled mineral deposition and growth in the veins from these observations we develop a model of fluid release and entrapment in which pulses of fluid are associated in time with increments of shear and tensile failure the rate of fluid release and the reduction in porosity both depend on the rate of plastic flow vein fluids may initially be derived from decreptitation of early fluid inclusions in the host eclogites small scale fluid heterogeneities implied by the fluid inclusions in the veins are best interpreted in terms of limited fluid flow and hence limited metasomatism we conclude that element recycling into the mantle wedge during subduction will depend at least as strongly on fluid transport mechanisms as on element solubilities in the fluid phase at monviso despite evidence for high trace element solubilities in saline brines the elements were not removed from the downgoing slab prior to teaching depths of 40 km

radiogenic isotopes in fluid inclusions radiogenic isotopes studied in fluid inclusions are still a limited field with great potential for expansion as analytical techniques improve the main limitation for sr ar and he isotope work is the very small number of radiogenic atoms produced in a typical fluid inclusion the requirements to analysts are correspondingly high examples show that isotopic tracing on fluid inclusion fluids can be a decisive tool in solving geological problems

the application of zircon cathodoluminescence imaging th u pb chemistry and u pb ages in interpreting discrete magmatic and high grade metamorphic events in the north china craton at the archean proterozoic boundary abstract two distinct late neoarchean to early paleoproterozoic magmatic events are recognized at the northern margin of the eastern block of the north china craton based on interpretation of zircon structure cathodoluminescence signature th u pb chemistry and age the first event is represented by emplacement of the chaihe trondhjemite and xianjinchang tonalite with magmatic zircon core ages ranging from 2559 2553 7 ma respectively these rocks experienced a subsequent high grade metamorphic event constrained by partial recrystallization of primary magmatic zircon and growth of new zircon rims between 2517 and 2490 ma a second magmatic episode generated by partial melting of crustal rocks during this metamorphic event is represented by the hongshilazi syenogranite with an age of 2502 11 ma the short time interval between initial magmatism and high grade regional metamorphism signifies they were related to the same major tectonothermal event when considered along with evidence of a similar sequence of events at this time 800 km to the south in eastern hebei and the previously established 2500 ma age of granulite facies metamorphism in mafic rocks across the craton it is proposed that a large mantle plume developed beneath the north china craton at the close of the archean

temperature time history of subducted continental crust mount olympos region greece in the mt olympos region of northeastern greece continental margin strata and basement rocks were subducted and metamorphosed under blueschist facies conditions and thrust over carbonate platform strata during alpine orogenesis subsequent exposure of the subducted basement rocks by normal faulting has allowed an integrated study of the timing of metamorphism its relationship to deformation and the thermal history of the subducted terrane alpine low grade metamorphic assemblages occur at four structural levels three thrust sheets composed of paleozoic granitic basement and mesozoic metasedimentary cover were thrust over mesozoic carbonate rocks and eocene flysch thrusting and metamorphism occurred first in the highest thrust sheets and progressed downward as units were imbricated from ne to sw 40ar 39ar spectra from hornblende white mica and biotite samples indicate that the upper two units preserve evidence of four distinct thermal events 1 293 302 ma crystallization of granites with cooling from 550 c to 20 km extension in the olympos region was synchronous with extension in the mesohellenic trough and the aegean back arc and concurrent with westward progressing shortening in the external hellenides

archaean hydrothermal zircon in the abitibi greenstone belt constraints on the timing of gold mineralisation abstract hydrothermal zircons have been found in archaean mesothermal au veins and altered wallrock selvages at val d or in the abitibi greenstone belt of canada the zircons are paragenetically associated with vein quartz tourmaline mica carbonate scheelite pyrite and gold zirconium mobility and the consequent occurrence of hydrothermal zircon may be associated with the intense tourmalinisation characteristic of archaean gold deposits in this district the shrimp ion microprobe has been used to analyse hydrothermal zircons from four separate mines spatially associated with the bourlamaque batholith and has yielded ages constraining formation of the au bearing vein systems to within 20 ma of emplacement of the pluton the ion microprobe data reveal multiple stages of hydrothermal zircon growth in the vein systems contemporaneous with the regional metamorphic peak and late kinematic activity along regional structures younger including proterozoic ages previously obtained for the veins using other minerals and isotopic schemes must reflect either alteration or renewed mineral growth during much later reactivation of fluids along the same structures up to 400 ma after initial formation of the veins

on the loss of 40ar from muscovite during polymetamorphism abstract recent laser studies have documented 40ar concentration gradients in micas that correspond to age variations of tens to hundreds of millions of years a fundamental problem to interpretation of such variations is to evaluate whether they represent loss via lattice diffusion or alternatively loss during chemical reactions and deformation new data of this study and comparisons with previous work suggest that muscovite is far less prone to lose accumulated 40ar via lattice diffusion during overprinting events than has been inferred previously kyanite muscovite assemblages in proterozoic schist along the eastern flank of the green mountain massif central vermont appalachians formed during proterozoic metamorphism at ca 1 1 ga and were remetamorphosed in the greenschist facies during both taconian ca 465 ma and acadian ca 390 ma events the overprinting events resulted in deformation of earlier kyanite and muscovite growth of chemically and isotopically distinct rims on earlier muscovite and growth of neoblastic muscovite and chloritoid with each event at inferred minimum temperatures of ca 425 c laser spot analyses of the grenville muscovite range from 988 11 ma in the centers of porphyroclasts to 397 7 ma along rims ages younger than 750 ma occur only on the deformed edges of porphyroclasts and in highly deformed areas within crystals parts of early formed muscovite that escaped late deformation and chemical reaction seem remarkably retentive of accumulated 40ar laser spot analyses along the outer 100 m of an undeformed growth face yield ages close to the timing of proterozoic metamorphism and cooling the present study suggests episodic heating events are ineffective for producing measurable diffusive argon loss along the edges of large muscovite crystals ca 1 mm in diameter particularly in comparison with deformation and reaction mechanisms and are unlikely to result in complete 40 ar 39 ar age resetting of such crystals the results of this study further emphasize that the argon diffusivity in muscovite is lower than commonly considered in previous work

single grain argon laser probe dating of phengites from the blueschist to greenschist transition on sifnos cyclades greece measurements of 40ar 39ar age spectra on single white mica crystals of tertiary age is a new means of obtaining detailed geochronological information of alpine blueschists internal consistency of the data set as well as excellent agreement with previous conventional k ar results demonstrates that this new technique can be used with confidence to obtain information on the thermal evolution of young metamorphic belts

40ar 39ar dating of white micas from an alpine high pressure metamorphic belt on naxos greece the resetting of the argon isotopic system overprinting of white micas from high pressure low to medium temperature m1 metamorphic assemblages in pelitic schists on naxos during subsequent thermal dome m2 metamorphism ranges from minor in the southeast of the island to complete recrystallization in the amphibolite facies rocks near the migmatites in the centre of the dome the original m1 minerals are phengites si4 6 7 7 0 and the overprinting minerals are muscovites si4 6 0 6 45 40ar 39ar step heating analyses of white mica separates from rocks in the area where phengite and muscovite occur together yield complex age spectra characterized by low apparent ages in the first and the last stages of gas release and high apparent ages in between these upward convex age spectra are shown to be caused by mixing of two generations of micas each of which has a different age spectrum and argon release pattern seemingly good plateaus in some age spectra from white micas of the area must be interpreted as providing meaningless intermediate ages further the upward convex age spectra have been used to trace the isotopic signature of phengites toward increasing m2 metamorphic grade and suggest that as long as phengites can be observed in the rocks upward convex age spectra occur on naxos crystallization of muscovite at the expense of phengite appears to be the main mechanism of resetting argon isotopic ages in white micas however there is also good evidence for argon loss by volume diffusion from phengites simple diffusion calculations suggest that the m2 metamorphism was caused by a shortlived heat source

tectonic evolution of the north qinling orogen from subduction to collision and exhumation evidence from zircons in metamorphic rocks of the qinling group abstract the north qinling block nqb is an important segment of the qinling orogen in central china here we report the results from sims geochronology and oxygen isotopes as well as la mc icpms hf isotopic analyses on zircon grains from a suite of metamorphic rocks felsic gneisses garnet plagioclase amphibolites and retrograde eclogite dikes in the qinling group of the nqb the age data show that these rocks underwent at least two episodes of metamorphism with the peak at 483 501 ma followed by 454 470 ma retrograde metamorphism these results are generally coeval with the periods of 500 480 ma for peak metamorphism and 460 420 ma for retrograde metamorphism previously obtained from the hp uhp metamorphic rocks of the nqb during the prograde and retrograde metamorphism widespread fluid and melt circulation within the block has been identified from the geochemical features of the metamorphic zircons the fluids that circulated in the felsic gneisses and retrograde eclogite dikes originated from the dehydration of altered oceanic basalts as inferred from the exceedingly low th u ratios positive ehf t 5 and extremely 18o 10 01 13 91 values in metamorphic zircons in contrast the melt involved in the formation of garnet plagioclase amphibolites appears to have been derived from continental sediments interlayered with the oceanic basalts since zircons crystallized during the peak and retrograde metamorphism show typical magmatic features with high u and th contents and th u ratios and enriched hf ehf t 5 42 to 0 18 and oxygen isotope composition 18o around 8 geochronological and geochemical features of the magmatic cores of the clear core rim textured zircons demonstrate that the protoliths of the gneisses were intermediate acid volcanic rocks erupted before neoproterozoic 800 ma which is further supported by the intrusion of basaltic magma of asthenospheric origin as represented by protoliths of retrograde eclogite dikes with the oldest magmatic zircon formed at 789 ma the protoliths of garnet plagioclase amphibolites appear to be altered oceanic basalts but had been significantly affected by the melt during the metamorphism combined with the previous studies the qinling group experienced overall subduction in the early paleozoic the nqb as represented by the qinling group was most likely a discrete micro block in the neoproterozoic and underwent deep subduction in the cambrian 483 501 ma and exhumation in ordovician 454 470 ma we propose that the nqb preserves a complete cycle of tectonic evolution of an orogen from an oceanic basin spreading and micro continent formation to deep subduction and exhumation

hydrous species in zircon a survey of hydrous species in zircon from a number of localities with a range of occurrences and structural states was carried out using infrared spectroscopy and electron microprobe analysis stepwise heating experiments on a crystalline and a nearly metamict zircon were also performed the region of ir spectra of oh bearing crystalline zircon from 4000 to 2500 cm 1 exhibits anisotropic oh bands resulting from at least three oh sites one at vacant tetrahedra and two that may be at si occupied tetrahedra one site associated with si occupied tetrahedra is the most important producing ir absorption bands at 3420cm 1 e c and 3385cm 1 e c oh maybe introduced during crystal growth by a coupled substitution of the form m 3 h 4 zr 4 the sites at vacant tetrahedra account for considerably less oh producing relatively weak ir absorption bands at 3510 cm 1 e c e c oh may be introduced during crystal growth by the hydrogrossular substitution of the form 4 oh sio 4 4 an additional oh band at 3270 cm 1 appears to be associated with an occupied tetrahedron heating experiments produced two more oh environments and indicate that h can migrate among the sites at high temperatures some crystalline samples are entirely free of oh ir spectra of metamict zircon exhibit broad asymmetric isotropic oh bands similar to h 2o bands but the absence of the h 2o bending mode band indicates that oh is the dominant hydrous species present ir spectra of partially metamict zircon samples retain some anisotropy the breadth of their oh bands is a function of degree of metamictizatron the maximum oh observed in any metamict or partially metamict zircon sample was approximately 0 1wt expressed as h 2o sucho h is probably secondary the maximum primary oh observed is 0 05 wt partially metamict zircon from a hydrothermal vug crystalline crustal zircon contains more than an order of magnitude less oh the maximum oh content observed in crystalline zircon 0 01 wt i s in a mantle zircon sample from a kimberlite suggesting high activity of h 2o in the mantle additional oh is present in some metamict and partially metamict zircon samples in the form of inclusions of hydrous silicates such as kaolinite fluid inclusions containing h 2o occur in some samples this may be a partial explanation of the high h 2o contents reported for some radiation damaged zircon based on loss on heating measurements

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

tectonically driven dolomitization of cambrian to lower ordovician carbonates of the quruqtagh area north eastern flank of tarim basin north west china dolomites occur extensively in cambrian to lower ordovician carbonates in the tienshan orogen of the quruqtagh area north east tarim basin where thick up to 1 km dark grey lenticular limestones of semi pelagic to pelagic origin are prominent the dolomites generally occur as beige anastomosed geobodies that cross cut well stratified limestones based on detailed field investigations and petrographic examination three types of matrix dolomite are identified fine crystalline planar e md1 fine to coarse crystalline planar s e md2 fine to coarse crystalline non planar a md3 dolomites one type of cement dolomite the non planar saddle dolomite cd is also common the preferential occurrence of md1 along low amplitude stylolites points to a causal link to pressure dissolution by which minor mg ions were probably released for replacive dolomitization during shallow burial compaction type md2 md3 and cd dolomites commonly co occurring within the fractured zones have large overlaps in isotopic composition with that of host limestone implying that dolomitizing fluids inherited their composition from remnant pore fluids or were buffered by the formation water of host limestones through water rock interaction however the lower 18o and higher 87sr 86sr ratios of these dolomites also suggest more intense fluid rock interaction at elevated temperature and inputs of mg and radiogenic sr from the host limestones with more argillaceous matter and possibly underlying neoproterozoic siliciclastic strata secondary tensional faults and fractures within a compressional tectonic regime were probably important conduits through which higher temperature mg rich fluids that had been expelled from depth were driven by enhanced tectonic compression and heating during block overthrusting forming irregular networks of dolomitized bodies enclosed within the host limestones this scenario probably took place during the late hercynian orogeny as the tarim block collided with tienshan island arc system to the north and north east subsequent downward recharges of meteoric fluids into the dolomitizing aquifer probably terminated dolomitization as a result of final closure of the south tienshan ocean or palaeo asian ocean and significant tectonic uplift of the tienshan orogen this study demonstrates the constructive role of notably tensional or transtentional faulting fracturing in channelling fluids upward as a result of intense tectonic compression and heating along overthrust planes on the convergent plate margin however a relatively short lived low fluid flux may have limited the dolomitization exclusively within the fractured faulted limestones in the overthrust sheets this article is protected by copyright all rights reserved

the crystal chemical basis for ar retention in micas inferences from interlayer partitioning and implications for geochronology cation partitioning data for coexisting muscovite and biotite are shown to be useful indicators of relative interlayer bond length strength in these minerals these data therefore provide a useful crystal chemical perspective on relative mass transfer kinetics of radiogenic isotopes and account for the observation that biotite is generally less retentive of 40ar and 87sr than coexisting muscovite partitioning behavior of trace elements underscores three reasons why overall interlayer bonding in biotite is weaker than in muscovite first the preferences of large rb cs in biotite and of small la3 and na in muscovite indicate a relatively spacious interlayer volume in biotite suggesting a longer mean k o bond second the preference of interlayer vacancies in biotite with some all possibly h2o h3o filled suggests that its adjacent 2 1 sheets are connected by fewer interlayer bonds per unit cell than those of muscovite third the relative exclusion of large ba2 from biotite despite its large interlayer sites is attributed to o h bonds pointing into the interlayer cavity sub normal to 001 k ba2 h repulsion thereby induced by the bare proton both destabilizes ba2 and weakens k o bonds in contrast muscovite offers a more favorable electrostatic environment for ba2 substitution since its o h bonds are directed into the vacant m1 octahedral site sub parallel to 001 this hypothesis is supported by the observation that progressive f oh 1 exchange enhances ba2 partitioning into biotite phlogopite relative to coexisting muscovite these crystal chemical differences between biotite and muscovite are mirrored in calculated values of ionic porosity z i defined here as the percentage of their interlayer unit cell volume not occupied by ions a monitor of ionic packing density and geometry z i is inversely correlated with k o bond strength which appears to be the rate determining kinetic common denominator for a variety of processes affecting micas including those responsible for loss of radiogenic isotopes in biotite and muscovite accordingly the relatively longer weaker k o bonds of biotite are envisioned as being more easily stretched during volume diffusion or broken during recrystallization or retrograde alteration this in turn accounts for common observations of enhanced radiogenic ar sr loss and younger 40ar 39ar and rb sr ages in natural biotite high z i relative to coexisting muscovite lower z i significantly this pattern may arise irrespective of isotopic loss mechanism diffusion or recrystallization etc and it follows that any age discordance observed between muscovite and biotite cannot be ascribed uniquely to one mechanism or the other without appropriate field petrographic and petrologic constraints extension of this partitioning porosity based synthesis leads to prediction of corollary age retentivity composition effects among chemically diverse trioctahedral and dioctahedral micas which are best field tested in terranes that cooled slowly under dry static conditions pressure effects on argon retention are also inferred from the porosity model

rare earth elements in synthetic zircon part 1 synthesis and rare earth element and phosphorus doping zircon crystals were grown from a li mo flux 7 5 mol li2moo4 86 5 mol moo3 to which equal molar proportions zro2 and li2sio3 were added 3 mol each the crystals were initially grown undoped but later dy was added to the flux without any other charge compensating species with dy and p added in equal molar proportions the zircon crystals incorporated 1 37 mol 6 99 wt dy and 1 36 mol 1 33 wt p a factor of 5 3 increase in dy over the crystals grown without p intentionally added to the flux the other p ree doped zircon crystals displayed an approximately 1000 fold increase in ree and p from la through lu a result of decreasing ionic radii the incorporation of p5 allowed in a general sense the xenotime type coupled substitution of ree3 y3 p5 zr4 si4 from la to nd however p exceeds ree from sm to gd the ree are generally equal to p and from tb to lu the ree exceed p the y and p doped zircon behaved more like middle ree mree doped zircon than heavy ree hree doped zircon crystals in their ability to incorporate y and p and to maintain charge neutrality to investigate the incorporation of dy with no p added to the flux the p to lree excess and the hree to p excess in the doped zircon secondary ion mass spectrometry sims analyses were completed on selected crystals in the dy doped crystals the sims analyses revealed minor amounts of p li and mo in the crystals these elements contributed to charge balance required by the excess dy in ree and p doped zircon the sims analyses detected li and mo and the li and mo may also provide charge balance for excess ree in the hree p doped crystals

compositional modification and trace element decoupling in rutile insight from the capricorn orogen western australia abstract the trace element composition of rutile is widely used to constrain the temperature and timing of geological events the provenance of detrital grains and as a potential indicator of proximal ore mineralisation understanding the processes of trace element mobility in rutile is therefore paramount to the successful application of rutile geochemistry to geological research using a combination of electron backscattered diffraction and laser ablation split stream icp ms we studied the microtextures trace element contents and u pb isotopes of rutile from the siliciclastic rocks of the mesoproterozoic edmund group capricorn orogen western australia the capricorn orogen experienced multiple tectonothermal events during the proterozoic under conditions spanning from low to high grade metamorphism rutile textures u pb ages trace element variations and calculated zr in rutile temperatures reveal different generations of rutile including detrital or metamorphic hydrothermal grains u pb age data of detrital rutile indicate a wide span of ages ca 2 7 1 7 ga of the source rocks and cr nb compositions point to metamafic metapelitic and granitic origins for this rutile the youngest detrital ages of 1 68 ga constrain the maximum depositional age of the sedimentary package the comparison of age spectra across the stratigraphy indicates a change in provenance with sourcing from circa 1 7 ga rocks during early stages of deposition followed by unroofing of older basement during deposition of the youngest units of the edmund group partial resetting of detrital rutile is observed in the increase of 207pb 206pb ratios accompanied by a decrease of zr in rutile calculated temperatures lower temperature metamorphic hydrothermal rutile yielded imprecise mesoproterozoic ages 1 3 1 1 ga this rutile has significant variations of zr hf and nb ta extending to values both lower and higher than primitive mantle which may record crystallisation from or interaction with hydrothermal fluids partial remobilisation of trace elements in metamorphic hydrothermal rutile is demonstrated by sub grain domains with sharp geochemical variations and by evidence of element diffusion at grain margins orders of magnitude differences in trace element concentrations including nb cr and zr in metamorphic hydrothermal rutile grains suggest thorough investigation of rutile textures needs to be carried out prior to using the trace element concentrations in rutile for provenance and zr in rutile temperature studies

the extensional messaria shear zone and associated brittle detachment faults aegean sea greece structural thermochronological and metamorphic data are used to elucidate the tectonic nature and evolution of the ductile extensional messaria shear zone and the associated brittle messaria and fanari detachment faults which exhumed their footwall from mid crustal depths on the island of ikaria in the aegean thermobarometric data indicate that the messaria shear zone formed at 350 400 c and 3 4 kbar i e at a depth of c 15 km normal faulting was accompanied by the intrusion of two granites which together with the thermobarometric data indicate a relatively high thermal field gradient of 25 35 c km 1 zircon and apatite fission track and apatite u th he ages demonstrate rapid cooling in the footwall of the messaria detachment from c 400 c to c 40 c between 11 and 3 ma age distance relationships of the data suggest that the messaria shear zone and the messaria detachment slipped at apparent rates of c 6 9 km ma 1 kinematic indicators show a consistent top to the nne shear sense for the extensional faults however at the southern part of the messaria detachment some late stage shear sense indicators are top to the ssw and are assumed to be associated with updoming of the footwall numerous deformed pegmatite veins in the messaria shear zone allow the reconstruction of deformation and flow parameters the mean kinematic vorticity number ranges from 0 13 to 0 80 indicating that shearing deviated significantly from simple shear that is extensional shearing was associated with vertical ductile thinning which contributed to tectonic exhumation finite strain shows oblate geometries and axial ratios of the finite strain ellipse in sections parallel to tectonic transport and normal to the mylonitic foliation range from 1 8 to 19 9 we calculate using a 1d numerical model that vertical ductile thinning contributed c 20 to exhumation during extensional shearing normal faulting was the major agent exhuming the footwall from c 15 km depth

compressional metamorphic core complexes low angle normal faults and extensional fabrics in compressional tectonic settingscompressional core complexesm searle t lamont metamorphic core complexes mccs are interpreted as domal structures exposing ductile deformed high grade metamorphic rocks in the core underlying a ductile to brittle high strain detachment that experienced tens of kilometres of normal sense displacement in response to lithospheric extension extension is supposedly the driving force that has governed exhumation however numerous core complexes notably himalayan karakoram and pamir domes occur in wholly compressional environments and are not related to lithospheric extension we suggest that many mccs previously thought to form during extension are instead related to compressional tectonics pressures of kyanite and sillimanite grade rocks in the cores of many of these domes are c 10 14 kbar approximating to exhumation from depths of c 35 45 km too great to be accounted for solely by isostatic uplift the evolution of high grade metamorphic rocks is driven by crustal thickening shortening regional barrovian metamorphism isoclinal folding and ductile shear in a compressional tectonic setting prior to regional extension extensional fabrics commonly associated with all these core complexes result from reverse flow along an orogenic channel channel flow following peak metamorphism beneath a passive roof stretching fault in naxos low angle normal faults associated with regional aegean extension cut earlier formed compressional folds and metamorphic fabrics related to crustal shortening and thickening the fact that low angle normal faults exist in both extensional and compressional tectonic settings and can actively slip at low angles c 60 is needed

the effect of the instability of muscovite during in vacuo heating on 40ar 39ar step heating spectra abstract the goal of this study was to ascertain the nature of the ar release mechanism s for a single well characterized muscovite heated incrementally in vacuo 1 at temperatures and time intervals typical of 40ar 39ar step heating experiments and 2 isothermally at 500 c and 700 c for time intervals much longer than typical 40ar 39ar experiments the rate of delamination was monitored by sem and the rate of dehydroxylation was determined by xrd analyses the rate of in vacuo ar loss was compared to that for muscovite heated at 700 c and 2 kbar h2o pressure for all experiments the rate of ar loss was found to be consistent with the rates of dehydroxylation and delamination at 500 c ar loss behavior for a 3000 m diameter muscovite grain was approximated by volume diffusion but the ar release pattern became increasingly non fickian with decreasing grain size we propose that the cause of the non fickian behavior is delamination the portions of the isothermal runs exhibiting diffusion like profiles also yielded discordant 40ar 39ar ages and low atmospheric ar contamination we attribute this phenomenon to the preferential concentration of atmospheric ar in interlayer extended defects and of 40ar in interlayer vacancies whereas 39ar is uniformly distributed throughout the muscovite structure by recoil the 40ar 39ar age spectrum of the muscovite heated under hydrothermal conditions has a staircase shape comparable to that usually interpreted as due to 40ar diffusive loss whereas 40ar 39ar laser spot analyses did not allow this interpretation the in vacuo ar release rate for this sample however indicates that it had been altered during the experiment therefore we propose that this 40ar 39ar age spectrum reflects the differential release of ar from the alteration phases vs muscovite for our sample dehydroxylation and delamination control the in vacuo ar release at temperatures less than 900 c whereas ar diffusion from thin dehydroxylated slabs may dominate the ar release mechanism for temperatures greater than 900 c this model would predict that the recovery of fossil 40ar concentration gradients from muscovite by typical incremental heating schedules is not possible the rates of delamination in vacuo and the rate of ar diffusion for muscovite in nature however both depend upon its the chemical composition structural integrity and grain size consequently for samples with either a naturally occurring broad range of grain sizes such as detrital muscovite and authigenic clay from sedimentary rocks or a compositionally or structurally heterogeneous population of grains such as muscovite from shear zones 40ar 39ar incremental heating may be able to detect intergranular variations in the k ar age

diffusion in zircon despite its low abundance in most rocks zircon is extraordinarily useful in interpreting crustal histories the importance of u th pb isotopic dating of zircon is well and long established davis et al this volume parrish et al this volume zircon also tends to incorporate trace elements useful as geochemical tracers such as the ree y and hf a number of characteristics of zircon encourage the preservation of internal isotopic and chemical variations often on extremely fine scale which provide valuable insight into thermal histories and past geochemical environments the relative insolubility of zircon in crustal melts and fluids as well as its general resistance to chemical and physical breakdown often result in the existence of several generations of geochemical information in a single zircon grain the fact that this information is so frequently retained as evidenced through backscattered electron or cathodoluminescence imaging that often reveal fine scale zoning down to the sub micron scale has long suggested that diffusion of most elements is quite sluggish in zircon in this chapter we present an overview of the findings to date from laboratory measurements of diffusion of cations and oxygen in zircon because of its importance as a geochronometer attempts have been made to measure diffusion especially of pb for over 30 years but only in the last decade or so have profiling techniques with adequate depth resolution been employed in these studies resulting in a plethora of new diffusion data these findings have important implications for isotopic dating interpretation of stable isotope ratios closure temperatures and formation and preservation of primary chemical composition and zoning in zircon efforts have been made for some time to quantify and characterize diffusion in zircon most notably of pb in deference to its significance in interpreting pb isotopic signatures and refining understanding of thermal histories as is evident from

the timing of partial melting barrovian metamorphism and granite intrusion in the naxos metamorphic core complex cyclades aegean sea greece abstract the relationship between partial melting and granite intrusion in a classic barrovian metamorphic terrane has been assessed thirteen samples were dated by shrimp u pb zircon geochronology from the island of naxos greece one of the aegean metamorphic core complexes the effect of partial melting during peak barrovian metamorphism on naxos is recorded by fine

redistribution of hfse elements during rutile replacement by titanite titanite growth at the expense of rutile during retrograde hydration of eclogite into amphibolite is a common phenomenon we investigated an amphibolite sample from the tromso eclogite facies terrain in northern norway to gain insight into the trace element distribution between rutile and titanite during incomplete resorption of the rutile by titanite patchy compositional zoning of al ti and f in titanite relates to the presence of a fluid with variable ti al and or f during its growth laser ablation icp ms and electron microprobe data for high field strength elements hfse nb zr ta and hf of rutile resorbed by titanite indicate a pronounced enrichment of these elements in the rim of a large single rutile crystal 8 mm and a systematic decrease towards uniform hfse contents in the large core hfse contents of smaller rutile grains 0 5 mm and rutile inclusions 100 m in the titanite overgrowth are similar or higher than in the rims of large rutile crystals element profiles from the rim inward demonstrate that hfse enrichment in rutile is controlled by diffusion hfse ratios in diffusion altered rutile show systematic variations compared with the uniform core composition of the large rutile modelling of zr and nb diffusion in rutile indicates that diffusion coefficients in rutile in fluid dominated natural systems must be considerably higher than those determined experimentally at 1 bar in dry systems variations of hfse contents in the newly formed titanite show no systematic spatial distribution hfse ratios in titanite and the rims of rutile are different indicating different solid fluid distribution coefficients in these minerals element fractionation by diffusion into the relict rutile and during fluid mediated growth of new titanite could substantially change the hfse budget of these minerals and could affect their use for geochemical tracing and other applications such as zr based geothermobarometry

40ar 39ar laserprobe direct dating of discrete deformational events a continuous record of early alpine tectonics in the pelagonian zone nw aegean area greece abstract the pelagonian zone of the hellenides which occurs in the northwestern aegean region of the eastern mediterranean contains remnants of a high pressure metamorphic belt which subsequently underwent localized reworking during low temperature deformational events formation of alpine deformational structures in the ossa massif northern pelagonian zone occurred at metamorphic temperatures which failed to reset most pre alpine 40ar 39ar cooling ages in muscovite porphyroclasts the low metamorphic temperatures at alpine times have preserved the argon isotopic signature in white mica bearing deformational structures which allows direct dating of these structures and refinement of the alpine tectonic history in the region after separation of carefully selected and characterized mica generations application of sensitive 40ar 39ar laserprobe dating has resulted in a continuous record of the early alpine tectonic history of the ossa massif from the early cretaceous well into the eocene the early alpine history of the ossa massif records cooling of basement thrust slices which are mostly hercynian granites and metamorphics below ca 350 c by 100 ma while lower tectonostratigraphic levels of the basement had already cooled below these temperatures at around ca 285 ma blueschist facies mylonite fabrics related to a top to ene direction of tectonic transport yield ages as old as 84 5 3 3 ma exhumation of the blueschist facies sequences was initiated at ca 54 ma and involved tectonic activity along blueschist facies and greenschist facies mylonites zones final wsw ward transport of the metamorphic sequence across the structurally lowest and supposed autochthonous series occurred at around 45 ma and resulted in termination of the ductile deformation history in the studied area this study confines the onset of high pressure metamorphism in the pelagonian zone to an interval between 100 ma and 85 ma and has shown that high pressure metamorphism had terminated by ca 54 ma the recognition of an early to middle alpine cycle lasting over 30 million years and involving crustal shortening high pressure metamorphism exhumation and subsequent crustal shortening is a key contribution to a better understanding of the early alpine tectonic history of the aegean region and eastern mediterranean

constraining the prograde and retrograde p t t path of eocene hp rocks by shrimp dating of different zircon domains inferred rates of heating burial cooling and exhumation for central rhodope northern greece ion microprobe shrimp dating was carried out on different zircon domains from metamorphic rocks of the hp ht terrane of central rhodope northern greece to constrain the timing of prograde and retrograde stages within a single tectono metamorphic cycle a well determined p t t relative path for the metamorphic rocks of this terrane was used as a petrological basis for the geochronological investigations ion microprobe work was assisted by cathodoluminescence cl images of the zircon crystals the geochronological results revealed that hercynian continental crust was subducted during the eocene several stages of the eocene tectono metamorphic cycle including both the prograde and retrograde parts of the p t path above ca 300 c 0 3 gpa were dated using zircons from the following rock types 1 a deformed quartz vein probably formed at ca 300 c 0 3 gpa zircons in this vein precipitated from a hydrothermal fluid they yielded an age of 45 3 0 9 ma which corresponds to the time of a low t prograde stage of metamorphism 2 in kyanite eclogites zircons were entirely reset during eclogite facies metamorphism resetting was very probably enhanced by the presence of fluids derived by h2o liberating reactions close to the p peak they yielded an age of 42 2 0 9 ma 3 orthogneisses surrounding the kyanite eclogites contained zircons with magmatic oscillatory zoned cores which yielded hercynian ages of 294 8 ma age of granitic protolith formation whereas cl bright metamorphic rims yielded like the eclogite zircons ages of 42 0 1 1 ma therefore both the eclogites and orthogneisses are interpreted to have approached maximum depth at around 42 ma 4 in a leucosome of a migmatized orthogneiss oscillatory zoned zircons yielded an age of 40 0 1 0 ma at this time the rocks reached maximum temperatures during early decompression 5 a late pegmatite crosscutting the schistosity of amphibolites contained oscillatory zoned zircons that yielded a crystallization age of 36 1 1 2 ma thus the whole tectono metamorphic cycle above ca 300 c 0 3 gpa lasted from 45 3 0 9 ma to 36 1 1 2 ma that is 9 2 ma with an extreme error value of 2 1 ma based on combined shrimp and petrological data the average rates of heating and burial during subduction above ca 300 c 0 3 gpa are 94 c ma and 15 mm a respectively rates of cooling and exhumation also above 300 c 0 3 gpa are calculated to be 128 c ma and 7 7 mm a the eocene age of metamorphism in central rhodope implies that the terrane of at least central rhodope and the cyclades very probably was part of the same continental crust

high pressure metamorphism in the aegean eastern mediterranean underplating and exhumation from the late cretaceous until the miocene to recent above the retreating hellenic subduction zone 1 we report 40ar 39ar ages from various tectonic units in the aegean and westernmost turkey on the basis of published geochronologic data and our 40ar 39ar ages we propose that the aegean is made up of several high pressure units which were successively underplated from the late cretaceous until the miocene ages for high pressure metamorphism range from 80 83 ma in parts of the vardar izmir ankara suture zone in the north to 21 24 ma for the basal unit in the cyclades and the external high pressure belt on crete in the south published seismic data suggest that high pressure metamorphism is currently occurring underneath crete younging of high pressure metamorphism in a southerly direction mimics the southward retreat of the hellenic subduction zone we propose that distinct stages of high pressure metamorphism were controlled by the underthrusting of fragments of mainly thinned continental crust and that these punctuated events were superposed on progressive slab retreat by far most of the exhumation of the high pressure units occurred early during the orogenic history in a forearc position

an extended episode of early mesoproterozoic metamorphic fluid flow in the reynolds range central australia the products of metamorphic fluid flow are preserved in zones within the marbles and metamorphosed semipelites of the upper calcsilicate unit in the granulite portion of the late palaeoproterozoic reynolds range group northern arunta block central australia the zones of retrogression characterized by minerals such as wollastonite grossular and clinohumite local resetting of oxygen isotopic compositions and local major element metasomatism were channelways for water rich fluids derived from granulite facies metapelites u th pb isotopic ages measured by the shrimp ion microprobe on zircon and monazite from a granulite facies semipelite an early semiconcordant aluminous quartz rich fluid flow segregation and a late discordant quartz rich segregation record some of the extended thermal history of the area zircon cores from the semipelite show its likely protolith to be an igneous rock 1812 11 ma old itself derived from a source containing zircon as old as 2 2 ga low th u overgrowths on the zircon grew during granulite facies metamorphism at 1594 6 ma monazite cooled to its blocking temperature at 1576 8 ma zircon cores from the semiconcordant segregation are dominantly 2 3 ga old indicating that the source of the fluids was not the particular metamorphosed semipelite studied two generations of low th u overgrowths on the zircon give indistinguishable ages for the older and younger of 1589 8 and 1582 8 ma respectively the monazite age is the same 1576 12 ma zircon from the late discordant segregation gave 1568 4 ma fluid flow occurred for at least 18 3 ma and ended 26 3 ma after the peak of metamorphism suggesting a very slow cooling rate of 3 c ma 1 the last regional high grade metamorphism in the reynolds range occurred at 1 6 ga not 1 78 ga as previously thought the high grade event at 1 78 ga is a separate event that affected only the basement to the reynolds range group

hydrothermal alteration of magmatic zircon related to nacl rich brines diffusion reaction and dissolution reprecipitation processes magmatic zircon from altered gabbros adjacent to the 1 07 ga lala fe cu deposit sw china was modified by nacl rich brines related to the fe cu mineralization the modified zircon grains are composed mostly of both inclusion free and porous domains and some grains also have overgrowth rims the inclusion free domains commonly overgrown by the porous domains are roughly homogeneous under bse imaging but display oscillatory or sector zoning under cl imaging in contrast the porous domains are distinctly mosaic like under cl imaging and contain abundant pores and mineral inclusions such as thorite xenotime ree rich phases actinolite albite biotite and calcite the inclusion free domains have concentrations of non formula elements for example al ca and fe much higher than the magmatic zircon and are thus interpreted to be products of interaction between magmatic zircon possibly metamictized and the fluids via a diffusion reaction process however these resultant domains have retained the th and u contents ree patterns u pb ages and hf isotopes of the precursor magmatic zircon on the other hand the porous domains have stoichiometric end member compositions that is lowered th u ree y and p and given that they are porous and inclusion rich we proposed that they have formed from the precursor element rich inclusion free domains or magmatic zircon via a fluid induced dissolution reprecipitation process it is notable that the porous domains retained the 177hf 176hf ratios of the precursors but possess modified and meaningless u pb ages instead a meaningful u pb age 1006 62 ma similar to the timing of the lala deposit is well recorded by the overgrowth rims with distinctly high lrees la 50 700 ppm and elevated hf isotopic ratios our new results reveal that in the presence of the nacl rich brines the nature of the precursor magmatic zircon that is high element budgets and or metamictization plays a key role on hydrothermal alteration of zircon

deciphering subduction from exhumation in the segmented cycladic blueschist unit central aegean greece the cycladic blueschist unit cbu represents the northern passive margin of the adria continental block and ophiolites that are the remnants of the pindos ocean which were affected by high pressure low temperature metamorphism in the blueschist and eclogite facies during the eocene prior to the lutetian the ophiolitic and margin units were thrust towards the sw inside a ne dipping subduction zone two subsequent exhumation stages are characterized by top to the ne senses of shear i from mantle depths to lower crustal levels before 37 ma which was accommodated by an extensional inversion of the vardar suture zone and ii from deep crustal levels to the near surface between 30 and 20 ma which was accommodated by the north cycladic detachment it is during this late stage of ductile exhumation that the central cyclades core complex was exhumed segmentation of the cbu by normal faults started at 12 ma as recorded by the low temperature thermochronometers and was coeval with block rotation and folding with ns trending axes which were predominantly controlled by the myrthes ikaria strike slip fault in the centre of the cyclades a map restoration of the cbu geometry prior to segmentation has been carried out using available structural and paleomagnetic data the restoration shows that following the cbu exhumation from mantle depths to crustal levels a single core complex was exhumed along a single ne dipping detachment

zircons from syros cyclades greece recrystallization and mobilization of zircon during high pressure metamorphism zircons were studied from high pressure low temperature metamorphosed meta igneous lithologies from syros these rocks carry several zircon generations related to each other by dissolution reprecipitation processes one generation is pristine zircon that shows growth zoning relatively elevated contents of trivalent cations and high th u ratios the other end member is a skeletal zircon generation with negligible trivalent cation contents and low th u ratios 0 1 texturally between these two there is a range of zircon crystals with complex inclusion populations of y hree th phases and fluid inclusions showing variable progress of replacement recrystallization both end members yield distinct sensitive high resolution ion microprobe shrimp u pb ages the pristine generation has an age of 80 2 1 6ma from a metagabbro and 76 4 2 1ma from a meta plagiogranite dyke the skeletal low th u zircon generation yields an age of 52 4 0 8ma the older late cretaceous zircons are interpreted to date emplacement of the magmatic protoliths in a small segment of oceanic crust the younger eocene age however dates a zircon recrystallization event which possibly coincides with high solubility and mobility of high field strength elements in a high pressure aqueous fluid phase intergrowth relations between zircon and peak metamorphic garnet and excellent agreement of the u pb ages with white mica ar ar ages for the same samples support the conclusion that eocene is the true age of high pressure metamorphism on syros

perspectives on the origin of plagiogranite in ophiolites from oxygen isotopes in zircon abstract the formation of oceanic plagiogranite has been attributed primarily to either 1 extreme fractional crystallization of a mantle melt or 2 partial melting of hydrated mafic crust with support for the latter from field evidence and recent melting experiments remelting of hydrothermally altered ocean crust could yield rocks and minerals with diverse primary magmatic 18 o values forming proximal to the magmatic center where crustal growth is occurring to constrain the magmatic 18 o of a wide range of silicic rocks in oceanic crust and evaluate their petrogenesis we characterized the 18 o of zircons in 22 plagiogranite samples tonalite and trondhjemite from 8 different ophiolites and one dacite sampled along the east pacific rise using secondary ion mass spectrometry sims the 18 o values of 202 magmatic zircons from ophiolites range from 3 9 to 5 6 n 244 spots average 4 9 0 6 2sd extending 1 below typical zircon in equilibrium with mantle and from gabbroic massifs along slow spreading mid ocean ridges 4 7 5 9 east pacific rise dacite zircons range from 4 6 to 5 0 n 12 spots plagiogranite from the dike gabbro transition zone of the northern oman ophiolite yield the lowest 18 o zrn with rock average values of 4 3 5 0 the low 18 o values are best explained by remelting of crust altered by hydrothermal fluids with seawater like isotopic compositions at high temperatures possibly due to vertical migration of the boundary between an active magma chamber and a vigorous high temperature hydrothermal system in the overlying crust if the partial melt was assimilated into a fractionating melt lens with morb like 18 o as envisioned for km scale plagiogranite bodies in oman up to 20 contamination by a protolith with 18 o 2 would be required previous oxygen isotope constraints from quartz in oman plagiogranite suggested melting of both high and low 18 o crust had occurred comparison of quartz zircon pairs indicates that quartz has been modified in most samples and we find no evidence for the involvement of high 18 o rocks during plagiogranite formation

the difficulties of dating mafic dykes an antarctic example archaen gneisses of the vestfold hills of east antarctica are transected by several compositionally discrete suites of tholeiitic dykes a representative of one of those suites which has been dated in the present study shows that not only rb sr whole rock isochrons but also u pb zircon techniques if not properly applied can produce erroneous crystallisation ages two zircon populations were recovered from the mafic dyke itself one of which is 2 483 9 ma in age and clearly of xenocrystic origin the other yields an age of 1 025 56 ma which is not ascribed to the magmatic crystallisation of the dyke but rather to the time that it underwent metamorphic metasomatic alteration as a response to high grade tectonism in the adjacent mobile belt it is presumed that the zircon in question formed by the breakdown of another mineral or minerals possibly magmatic baddeleyite due either to ingress of a siliceous fluid or more probably by the release of silica from the breakdown of pyroxene to amphibole a cogenetic 1 2 cm wide felsic vein of late magmatic early hydrothermal origin also contains two zircon populations again most of the grains therein which are interpreted as of xenocrystic origin grew at 2 483 9 ma however a few euhedral zircons with very high u and th contents grew at 1 248 4 ma which is taken to be the formation age of both the felsic vein and the enclosing mafic dyke

mobility of trace elements and oxygen in zircon during metamorphism consequences for geochemical tracing this study addresses the mobility of the rare earth elements ree o u and pb during the formation of metamorphic zircon by overgrowth and coupled dissolution reprecipitation for this purpose the texture and ree 18o and u pb compositions of zircon crystals from a metabasite an anatectic granite and a metapelite sampled on naxos island greece have been characterised the behaviour of o and ree during the formation of the zircon metamorphic domains shows that two end members can be distinguished i closed or partially closed system behaviour for oxygen and trace elements in zircon formed by coupled dissolution reprecipitation metamorphic domains partially preserve the textural and geochemical characteristics of the precursor zircon chemical or isotopic equilibrium between the metamorphic zircon and the matrix is not completely reached for all the elements such zircons have to be used with caution for geochemical tracing when re equilibration is not complete the u pb system is also not completely reset and the metamorphic domains yield old scattered ages ii open system behaviour for oxygen and trace elements in zircon formed by overgrowth these metamorphic zircons do not show an inherited shape and are devoid of relic magmatic zoning their composition reflects growth in an open system at the scale of the zircon grain with the fluid phase and the surrounding minerals because equilibrium between the metamorphic zircon and the matrix was thus favoured zircon crystallisation can be more confidently related to the p t history and fluid circulation events by geochemical tracing

early cambrian oceanic plagiogranite in the silvretta nappe eastern alps geochemical zircon u pb and rb sr data from garnet hornblende plagioclase gneisses garnet hornblende plagioclase gneisses rich in incompatible elements occur in the crystalline basement of the austro alpine silvretta nappe and are associated with clinopyroxene norites and harzburgite cumulates it is proposed here that the gneisses were formerly oceanic plagiogranites an end 530 value of 5 6 for the gneisses as well as initial87sr 86sr values of 0 7036 0 7037 for the gabbroic rocks and 0 7026 0 7027 for the ultramafic rocks suggest a mantle source for this rock association the geochemical characteristics of the garnet hornblende plagioclase gneisses indicate that their precursors were derived by fractional crystallization from a basaltic parent magma by the same process which produced the adjacent clinopyroxene norites and ultramafic cumulates as well the combined u pb upper intercept ages of zircons from two gneiss samples yield an igneous crystallization age of 532 30 ma similar to previously dated mostly calcalkaline orthogneisses in the same area high quality transparent zircons showed the least degree of discordance but contain extremely low u and pb levels the rock suite including this plagiogranite was emplaced within oceanic crust which formed in the latest precambrian early palaeozoic off the northern continental margin of gondwana

what caused the denudation of the menderes massif review of crustal evolution lithosphere structure and dynamic topography in southwest turkey the deformation of earth s lithosphere in orogenic belts is largely forced externally by the sinking slab but can also be driven by internal delamination processes caused by mechanical instabiliti

dating the indian continental subduction and collisional thickening in the northwest himalaya multichronology of the tso morari eclogites multichronometric studies of the low temperature eclogitic tso morari unit ladakh india place timing constraints on the early evolution of the northwest himalayan belt several isotopic systems have been used to date the eclogitization and the exhumation of the tso morari unit lu hf sm nd rb sr and ar ar a ca 55 ma age for the eclogitization has been obtained by lu hf on garnet omphacite and whole rock from mafic eclogite and by sm nd on garnet glaucophane and whole rock from high pressure metapelites these results agree with a previously reported u pb age on allanite and together these ages constrain the subduction of the indian continental margin at the paleocene eocene boundary during exhumation the tso morari rocks underwent thermal relaxation at about 9 3 kbar characterized by partial recrystallization under amphibolite facies conditions ca 47 ma as dated by sm nd on garnet calcic amphibole and whole rock from metabasalt rb sr on phengite apatite and whole rock and ar ar on medium si phengite from metapelites ar ar analyses of biotite and low si muscovite from metapelites which recrystallized at 5 kbar toward the end of the exhumation show that the tso morari unit was at upper crustal levels ca 30 ma these results indicate variable exhumation rates for the tso morari unit beginning with rapid exhumation while the indian margin subduction was still active and later proceeding at a slower pace during the crustal thickening associated with the himalayan collision

u pb zircon geochronology of unusual eclogite facies rocks from syros and tinos cyclades greece low temperature eclogite facies rocks from syros and tinos cyclades greece include meta igneous blocks of unclear origin meta olistostrome or tectonic melange with very high trace element concentrations e g zr up to 4850 ppm y up to 475 ppm the unusual geochemistry is considered to be the result of metasomatic alteration in a subduction zone environment locally metasomatic exchange with an ultramafic matrix further enhanced compositional anomalies a concordant u pb zircon age of 78 1 ma recorded for an omphacitite from syros is interpreted as the time of pre eocene high pressure metamorphism in the cyclades zircon dates of 61 and 63 ma for a jadeitite from tinos possibly indicate an additional discrete event deformation these results are a first geochronological indication that high pressure metamorphism in the cyclades commenced signif icantly earlier than indicated by previous white mica chronology which provided ages between c 50 and 40 ma

thermochronologic evaluation of postcollision extension in the anatolide orogen western turkey 1 to better understand the driving mechanisms behind the transition from collision to extension in a convergent orogen data from multiple low temperature thermochronometers were obtained from the simav detachment fault sdf the earliest developed major extensional structure recognized in the western anatolide orogen of western turkey twenty two zircon fission track ft 26 apatite ft 12 apatite u th he ages and 26 apatite ft track length analyses are presented the data establish that the sdf was a major extensional fault active between 25 and 19 ma the coincidence in timing of magmatism with cessation of sdf activity implies the detachment became locked owing to doming induced by magma emplacement zircon ft ages away from the influence of miocene magmatism record rapid footwall cooling between 25 and 21 ma at a slip rate of up to 15 km myr and demonstrate that active ductile deformation along the sdf migrated northward with time apatite ft ages from the footwall of the sdf are spatially invariant over 100 km and consistently 2 3 myr younger than the zircon fission track ages from the same samples these data are consistent with a regional but relatively rapid erosion linked cooling phase that removed 2 3 km of overburden following cessation of sdf activity but before deposition of sedimentary rocks on the detachment surface at 16 4 ma postcollision extension of the anatolide orogen with development of the sdf can be best explained as the result of postcollision magmatism and thermal weakening of the crust inducing the extensional reactivation of an earlier major out of sequence thrust