identifying granite sources by shrimp u pb zircon geochronology an application to the lachlan foldbelt the potential genetic link between granites and their host sediments can be assessed using zircon age inheritance patterns in the lachlan fold belt southeastern australia granites and associated high grade metasedimentary rocks intrude low grade ordovician country rock this relationship is well exposed in the tallangatta region northeast victoria part of the wagga omeo metamorphic complex in this region granites two i types and two s types have intruded during the mid late silurian between approximately 410 430 ma based on the ages of magmatic zircons the age spectra for inherited zircons from the granites have been compared with those of detrital zircons from the enclosing low and high grade metasediments in broad terms both for detrital zircons in all four sediments and for inherited zircons in three of the four granites the dominant ages are early paleozoic and late precambrian with sporadic older precambrian ages extending up to 3 5 ga the ages of the youngest detrital zircons from the low grade lockhart and talgarno terranes limit the time of sedimentation to ca 466 ma or younger the youngest detrital zircons from two samples of the high grade gundowring terrane are 473 ma making these sediments ordovician or younger not cambrian as originally suggested however the individual age spectra for the four selected metasediments are not well matched when closely examined the age spectra of the inherited zircons in the granites also do not adequately match those in any of the metasediments thus the metasediments might not be representative of the actual source rocks of the granites while the exact source of the granites cannot be identified from the analysed samples the existence of a large population of ca 495 ma inherited zircon grains in the s type granites requires that the granite source contains a significant proportion of cambrian or younger material this does not preclude the existence of a precambrian basement to the lachlan fold belt but indicates that at the level of s type magma generation a cambrian and or younger protolith is required

separate episodes of eclogite and blueschist facies metamorphism in the aegean metamorphic core complex of ios cyclades greece

metamorphic evolution of the attic cycladic metamorphic belt on naxos cyclades greece utilizing 40ar 39ar age spectrum measurements 40ar 39ar age spectrum analysis of phengite separates from naxos part of the attic cycladic metamorphic belt in greece indicates that cooling following high pressure low to medium temperature metamorphism m1 occurred about 50 ma ago phengite has 40ar gradients that suggest that part of the scatter observed in conventional k ar ages was caused by diffusion of radiogenic argon from the minerals during a younger metamorphism m2 in central naxos this metamorphism m2 has overprinted the original mineral assemblages completely and is associated with development of a thermal dome excellent 40ar 39ar plateaus at 15 0 0 1 ma 11 8 0 1 ma and 11 4 0 1 ma obtained on hornblende muscovite and biotite respectively from the migmatite zone indicate that relatively rapid cooling followed the m2 event and that no significant thermal overprinting occurred subsequent to m2 toward lower m2 metamorphic grade 40ar 39ar plateau ages of hornblendes increase to 19 8 0 1 ma concomitantly the proportion of excess 40ar in the spectra increases as well we propose that the peak of m2 metamorphism occurred beween 15 0 and 19 8 ma ago k ar ages of biotites from a granodiorite on the west coast are indistinguishable from those found in the metamorphic complex and hornblende k ar ages from the same samples are in the range 12 1 13 6 ma as the latter ages are somewhat younger than most ages obtained from the metamorphic complex intrusion of the granodiorite most likely followed the peak of the m2 metamorphism the metamorphic evolution of naxos is consistent with rapid crustal thickening during the cretaceous or early tertiary causing conditions at which supracrustal rocks experienced pressures in the range 900 1500 mpa transition to normal crustal thicknesses ended the m1 metamorphism about 50 ma ago the m2 metamorphism and granodiorite intrusion occurred during a period of heat input into the crust possibly related to the migration of the hellenic volcanic ar c in a southerly direction through the area

prolonged residence times for the youngest rhyolites associated with long valley caldera 230th 238u ion microprobe dating of young zircons abstract we describe a new ion microprobe method for dating magmatic zircon growth that is based on in situ measurement of the magnitude of238u 230th disequilibrium our results support independent inferences that zircon can remain suspended for long periods 100 ka in the convecting potions of the magma from which it crystallizes because the crystallization ages date when the magma cooled to its zircon saturation temperature even when the zircons have long magmatic residence times 238u 230th zircon dating can be used to constrain the thermochemical evolution of silicic magmas 238u 230th ages have been determined for individual zircons from rhyolites associated with the long valley magmatic system of eastern california the samples are from deer mountain an 115 3 ka low silica moat rhyolite and from the coarsely porphyritic low silica rhyolite of south deadman dome one of the 0 6 ka inyo domes previous investigations have suggested that the two lavas were derived from the same magma reservoir a few of the zircon model ages calculated with respect to the isotopic characteristics of the whole rocks are within error of that for eruption of the deer mountain rhyolite however the majority of zircons from both lavas cluster around an age of 230 ka this common interval of zircon nucleation and growth for petrologically similar lavas suggests that the younger inyo dome lava may have tapped the same magma body from which the deer mountain rhyolite erupted more than 100 ka before on the other hand most of the zircon model ages are younger than previous episodes of silicic volcanism in the long valley caldera suggesting that the rhyolites may have been generated during development of a silicic upper crustal magma chamber in the western portion of long valley caldera zircon saturation temperatures for the rhyolites studied 795 810 c are the same as those obtained from coexisting fe ti oxides 809 4 c showing that the magma cooled to

unroofing of active metamorphic core complexes in the d entrecasteaux islands papua new guinea metamorphic core complexes in the d9entrecasteaux islands are forming as a result of active extension at the western end of the woodlark basin spreading center high grade metamorphic rocks exist in the cores of gneissic domes including eclogites formed at minimum depths of 45 75 km i e 13 21 kbar high temperatures were maintained during uplift of basement rocks from 13 21 kbar t 730 900 c through 7 11 kbar t 570 730 c to 5 6 kbar t 730 c subsequent rapid cooling 100 c m y of the basement occurred between 1 and 2 ma on the basis of 40 ar 39 ar geochronology the core zone rocks were transected by kilometre scale amphibolite facies ductile shear zones active at depths of 25 35 km extensive intrusion of granodiorite plutons occurred and the age of granodiorite intrusions constrains late stage shear zone activity to 2 ma and younger unroofing was accomplished by faults and shear zones bounding the gneiss domes juxtaposing them against ultramafic rocks and sedimentary deposits surface uplift continues today and has led to topographic elevations of up to 2 5 km

on the hellenic subduction zone and the geodynamic evolution of crete since the late middle miocene abstract in recent syntheses of the geodynamic evolution of the aegean area the hellenic subduction zone and in particular its initiation plays a central role two basic aspects of the recent comprehensive model put forward by le pichon angelier and co workers are a initiation of subduction along the hellenic arc is inferred to have taken place approximately 13 ma ago whereas several others have suggested approximately 5 ma and b migration of the trench system to the south southwest giving rise to an extensional regime in the back arc region which led to general subsidence and formation of the present day aegean sea new data on the structure of the upper mantle underneath the aegean area indicate that the former aspect requires modification the seismic velocity structure indicating a northward dipping slab down to a depth of at least 600 km and the depth distribution of earthquake hypocenters can not be reconciled with straightforward initiation of subduction approximately 13 ma ago estimates of about 5 ma for the time of initiation of the present subduction zone are even more difficult to reconcile assuming 400 km as an upper bound for the effect of stretching of the aegean sea and of other processes contributing to the separation of crete from the eurasian mainland we arrive at an age for the hellenic subduction zone of at least 26 ma as to aspect b geological data of crete lend strong support to the idea that on crete compressional tectonics is of much greater importance than current models imply combining geophysical and geological data we propose a modification of the tectonic evolution for the cretan segment of the hellenic arc whereas the basic process of extension caused by a migrating trench system roll back in a land locked basin remains a very useful and sound concept the tectonics of the margins of a region subject to stretching in this case crete may be more complicated than hitherto thought in our model the documented fragmentation of crete into several basins which started about 12 ma ago is not attributed to initiation of subduction but to inception of the roll back process compressional tectonics on crete is taken to be associated with deformation of the crust resulting from the overall tensional stress regime

isotopic dating of pre alpidic rocks from the island of ios cyclades greece the lower tectonic unit of ios provides evidence of an at least four stage metamorphic and intrusive history which well might be generalized for large parts of the internal pelagonian

dolerites of the woodlark basin papuan peninsula new guinea a geochemical record of the influence of a neighbouring subduction zone abstract the moresby seamount located in the woodlark basin is a fragment of oceanic crust dolerites and gabbros generated 66 4 1 5 my ago before being obducted during the eocene on the australian margin since 8 my the motion of a normal fault related to the opening of the woodlark basin is responsible for the unroofing of the moresby seamount the latter was sampled during the odp campaign leg 180 in 1998 geochemical compositions of dolerites can be explained by a process of fractional crystallization in which the magnesian spinel was involved therefore fractionation processes took place at relatively high pressure 1 gpa most likely during the upward migration of deep magmas throughout the mantle the fe rich gabbros more differentiated than the dolerites could have derived from an early differentiated magma evolving in the crust at relatively low pressures the trace element content of dolerites indicates that their source was a depleted oceanic mantle slightly influenced by arc related magmas this study suggests that the ophiolites fragment of the moresby seamount was created in the vicinity of a subduction zone this model being in agreement with the regional tectonic setting of se asian since the cainozoic

structural analysis of basement tectonites from the aegean metamorphic core complex of ios cyclades greece abstract the island of ios in the cyclades aegean sea greece exposes high pressure metamorphic rocks that were subjected to miocene continental extension subsequent to the alpine collisional orogeny the pre alpine history of the lower plate of this aegean metamorphic core complex involves intrusion of granitoids into metasediments which subsequently underwent hercynian amphibolite facies metamorphism during the alpine orogeny d2 these rocks were overthrust by the cyclades blueschist nappe later the lower plate was tectonically exhumed during oligo miocene continental extension and associated plutonism final exhumation occurred as a result of the operation of a south directed crustal scale shear zone and low angle normal faults this study shows that the ios core complex reflects an intrinsic relationship between early thrusting and later extensional tectonism basement rocks have been exhumed from beneath the nappes that originally overrode them

the tertiary geodynamical evolution of the aegean arc a paleomagnetic reconstruction abstract the paleomagnetic results obtained in the last 5 6 years from tertiary formations in the aegean domain indicate that the lower miocene arc was almost rectilinear with an e w trend and that its curvature has been acquired tectonically in two major phases during the middle miocene a first phase of deformation is characterized by rotations occurring at the two terminations of the arc clockwise in the west epirus anticlockwise in the east southeastern anatolia a second phase of rotation occurring in the last 5 ma about a pole situated in the southern adriatic sea has affected only the northwestern part the data also indicate that the antalya basin has not been involved in the geodynamical evolution of the arc so that the bey daglari represent its easternmost unit a more complex rotational pattern is observed in the internal zones the large rotations measured in evia 48 and skyros 26 probably occurred in the last 5 ma and result from rotation of fault bounded blocks in a shear zone connecting the north aegean trough to the external arc in the izmir region in the karaburun peninsula of western anatolia and in the island of lesbos significant counterclockwise clockwise or null rotations of coherent blocks have taken place during the neotectonic extensional regime furthermore the values of the inclination obtained from both sedimentary and volcanic oligo miocene formations are systematically shallower than expected on the basis of a geocentered dipole field this suggests that the entire studied area has undergone a northward drift of over 1000 km since the middle miocene

north south extension within the convergent himalayan region recent work by burg et al 1984 indicates the presence of east west striking gently north dipping normal faults in the higher himalayas and southern tibet that formed during postcollisional convergence of india and tibet these faults extend for at least 800 km along strike we interpret these normal faults as probable late miocene extensional features with perhaps several tens of kilometres of down to the north displacement a simple elastic model suggests that these normal faults may have formed during gravitational collapse of the miocene topographic front between india and tibet in this interpretation gravitational collapse occurred by southward motion relative to india and tibet of a wedge of crustal rocks bounded above by gently north dipping normal faults and below by thrust faults that probably dip north north south extension produced in this way is probably confined to upper crustal levels only and does not reflect regional extension of the entire lithosphere

low angle faults above and below a blueschist belt tinos island cyclades greece new observations from the island of tinos greece allow a better definition of the structural position of the alpine eocene blueschist belt exposed in the islands of the aegean sea these blueschists over a significant part of the aegean sea are delimited from below by a low angle thrust fault while from above they are delimited by a low angle normal type fault which omits a substantial crustal interval both underlying and overlying rocks were not affected by the high metamorphism the rapid uplift and exhumation of the high rocks was therefore mainly the result of fault movements rather than erosion and whole crust uplifting the low angle normal fault apparently had a major role in the uplift of the blueschists

the mobility of zirconium and other immobile elements during hydrothermal alteration abstract development of zircon and other zr phases in hydrothermal deposits indicates that zr can be highly mobile in these systems mobility is most common in but not restricted to f rich hydrothermal systems related to alkalic f rich igneous suites these suites can range from peralkaline through metaluminous to peraluminous a few examples are neither alkalic nor f rich three locations in the trans pecos magmatic province texas u s a demonstrate this hydrothermal zr mobility all three igneous systems are alkalic and f rich but vary in alkali al ratios peralkaline rhyolites and trachytes in the christmas mountains contain as much as 2100 ppm zr mostly in aegirine or arfvedsonite zircon is rare or absent fluorspar replacement deposits in limestone at contacts with the rhyolites contain as much as 38 000 ppm zr occurring as small disseminated zircons the deposits also are enriched in a variety of incompatible elements including be rare earth elements ree y nb mo hf pb th and u the sierra blanca intrusions a series of mildly peraluminous f rich rhyolite laccoliths contain as much as 1000 ppm zr mostly as zircon hydrothermal zircon occurs as overgrowths on magmatic grains as veinlets connected to overgrowths and in fluorspar replacement bodies in adjacent limestone the highest zr concentrations in fluorspar are 200 ppm metaluminous quartz monzonite from the infiernito caldera contains 400 600 ppm zr mostly as zircon euhedral zircon in quartz fluorite veins in the quartz monzonite indicates mobility of zr zirconium concentrations in the veins are unknown but the paucity of zircon suggests little zr enrichment relative to the host zircon and more rarely zirconolite occur in skarn in the ertsberg district of irian jaya indonesia unlike in texas related igneous rocks are metaluminous and the hydrothermal system was f poor worldwide hydrothermal zircon other zr phases and ti and al bearing phases occur in skarn epithermal precious metal veins volcanogenic massive sulfide deposits and mylonites we propose that differences in zr mineralogy of igneous source rocks is an important factor in determining the availability of zr to hydrothermal fluids although zr concentrations in the sierra blanca and christmas mountains rhyolites are similar zr enrichment in fluorspar was much greater in the christmas mountains we suggest that hydrothermal solutions could easily break down aegirine and arfvedsonite to release zr but that zircon was only moderately attacked therefore far more zr was available for transport and subsequent deposition in the christmas mountains than at sierra blanca availability of other trace elements probably is also governed by their mineral host although zr mobility is most common in f rich hydrothermal systems related to alkalic and f rich igneous systems mobility at ertsberg may have been promoted by sulfate complexing

granulite facies metamorphism palaeo isotherms and disturbance of the u pb systematics of zircon in anorogenic plutonic rocks from the adirondack highlands in the adirondack highlands of new york state the effect of granulite facies metamorphism on the physical and isotopic characteristics of zircon from anorogenic plutonic rocks has a distinct geographical pattern the location of zircon populations which appear to have been altered describes a roughly circular area where metamorphic palaeotemperatures have been determined to be in excess of 750 c zircons from anorogenic plutonic rocks outside this area were undisturbed during metamorphism and yield well constrained ages granitic charnockitic and mangeritic anorogenic plutonic rocks peripheral to the marcy anorthosite massif have large euhedral prismatic zircons that display fine internal magmatic growth zonations and abundant randomly orientated mineral inclusions co genetic zircon fractions yield linear discordant arrays and well constrained upper intercepts of 1125 1157 ma metamorphic zircon is limited to sporadically developed and volumetrically insignificant clear low u overgrowths or protuberances in marked contrast zircons from petrographically and geochemically identical rocks adjacent to or within the marcy anorthosite massif are typically large limpid anhedral to subhedral crystals or crystal fragments lacking internal features except for tubular cavities and co 2 rich inclusions co genetic zircon fractions yield nearly concordant non linear clusters with 207pb 206pb minimum ages of 1073 1095 ma metamorphic overgrowths cannot be readily identified by optical or cathodoluminscence techniques however many grains show complex and unusual external boundaries suggestive of post crystallization modification these data indicate that temperatures as low as 750 c in combination with other factors may have been sufficient to facilitate recrystallization and diffusion of radiogenic pb from the zircon crystal structure during the complex protracted metamorphism of the adirondack highlands

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

multivariate mixing and mass balance m3 calculations a new tool for decoding hydrogeochemical information to support and help hydrochemical evaluation a multivariate mathematical tool named m3 multivariate mixing and mass balance calculations has been created within the aspo hard rock laboratory research programme the computer code can be used to trace the origin of the groundwater and calculate the mixing portions and mass balances from ambiguous groundwater data groundwater composition data used traditionally to describe the reactions taking place in the bedrock can now be used to trace the effect from present and past groundwater flow with increased accuracy the m3 model consists of the following 3 steps multivariate analysis called principal component analysis pca is used to summarise the information from the data set the summarised information shown in the pca plots is used for finding relationships patterns extreme waters and for further m3 modelling from the pca plot mixing calculations are used to calculate the effect of the groundwater mixing on the obtained groundwater composition this so called ideal mixing model is used to calculate the mixing proportions given in for all the groundwater samples the final step in m3 calculations is the mass balance calculations deviations from the ideal mixing model are used to trace the sources and sinks of elements given in mg l which can be due to mass balance reactions the tested margin of error of the model is 10 for the aspo site data but depends on the data to be modelled a mixing portion of less than 10 is regarded as under the detection limit of the model and such calculations are therefore uncertain this method can be used to trace the origin and calculate the mixing portions and effects from the reactions on the observed groundwater composition with a higher resolution and convenience compared to many standard methods

revealing hidden structures the application of cathodoluminescence and back scattered electron imaging to dating zircons from lower crustal xenoliths abstract the use of cathodoluminescence cl and or back scattered electron bse imaging techniques with in situ ion probe analyses of zircons can help unravel complex crustal histories of metamorphic rocks that otherwise might remain elusive using these techniques we have imaged zircons from three lower crustal xenolith suites that have previously been dated by shrimp sensitive high resolution ion microprobe in all three cases the zircons are featureless in transmitted light but cl and bse reveal internal structures that correlate with distinct growth events generally cl and bse images reveal similar structures with cl showing finer detail neither imaging technique is capable of delineating all growth features in every sample and the best results are obtained using a combination of the two techniques igneous cores in zircons commonly emit a different color cl emission in many cases zoning in the cores is truncated indicating that the zircons either spent time in the supracrustal environment after their initial crystallization and prior to the granulite facies event s or that part of the core zircon was resorbed during the subsequent metamorphic event metamorphic rims when present are commonly 10 to 30 m thick and are nearly always unzoned and featureless igneous cores and metamorphic overgrowths commonly have distinctive cl emission spectra and trace element concentrations however the cl spectra can only be used to qualify chemical differences as a linear relationship has not been shown to exist between cl intensity and trace element concentration in natural zircons in many cases the hf y p u and the heavy rare earth elements hrees concentrations can be correlated to igneous and metamorphic growth using a combination of cl and bse imaging techniques and in situ trace element analyses with either the electron microprobe or pixe particle induced x ray emission

mylonite fabric development on naxos greece abstract the miocene structural evolution of the metamorphic complex on naxos occurred within a mid to lower crustal sub horizontal shear zone the deformation style and the regional stress field in the aegean during the miocene are consistent with exhumation of the metamorphic complex during ductile crustal thinning deformation in this shear zone initiated possibly before and continued after the peak of localized high temperature barrovian style metamorphism kinematic indicators in non coaxial fabrics indicate that upper structural levels were displaced to the nne throughout the miocene structural history of the metamorphic complex this displacement was accompanied during its latter stages by compression in an e w direction

the tectonic development of the hellenic arc and the sea of crete a synthesis abstract we use field analyses together with interpretation of aerial photographs orbital imagery and sea beam data to define more accurately the geometry of fault systems in the hellenic arc the tectonics of the area are dominated by extensional processes related to normal faulting we then use sea beam data previous oceanographic studies and extrapolation of field analyses on surrounding islands to describe the fault pattern of the cretan sea basin which is found consistent with the land fault pattern the mechanisms of the late neogene and quaternary deformation of the central and southern aegean region are analysed in terms of stress trajectories as revealed by field studies of fault populations we then examine the subsidence of the sea of crete in terms of a model of uniform stretching of the lithosphere the opening of the sea of crete was cylindrical and did not result in bending of crete this leads us to modify the southern portion of an earlier kinematic reconstruction of aegea in agreement with recent paleomagnetic results the implications of the opening of the sea of crete on the tectonic evolution of the straits to the west and east of crete are discussed finally the uplift of the outer arc is explained by underplating due to subduction

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

plutonism and the origin of metamorphic core complexes the formation of metamorphic core complexes may be triggered by plutonic activity during episodes of continental extension pulses of ductile deformation have taken place during short lived thermal events initiated by the heat input from intruded plutons sills or dikes such intrusions may be the underlying cause for differential uplift of the footwall during tectonic denudation of metamorphic core complexes fast cooling inferred from 40 ar 39 ar apparent ages may have taken place after periods of magma arrival and need not be the result of rapid erosional or tectonic denudation heterogeneity of 40 ar 39 ar apparent ages can be explained by rapid cooling of deforming mylonites formed at shallow crustal levels in the thermal aureoles of intruded sills or sill swarms under such circumstances apparently isothermal decompression paths may actually link pressure temperature points set during periods of transient mineral growth during and after periods of igneous activity

melt enhanced deformation a major tectonic process convergent tectonics between continental crustal blocks result in deep burial and anatexis of supracrustal rocks anatectic and or mantle derived melts combine to form a melt weakened zone in the thickened lower crust the accumulation of melt eventually leads to crustal failure along melt lubricated shear zones rapid 1 mm yr vertical component movements of large crustal blocks result we refer to these crustal displacements as tectonic surges the melt lubricated shears are characterized by the close association of sheared country rocks with foliated or massive igneous sills and plutons rocks that formed at different crustal levels are juxtaposed across these shear zones one result of surges with large lateral component of movement is metamorphic inversion with high pressure and high temperature metamorphic rocks structurally over lower pressure and temperature assemblages large and rapid vertical surges may displace crust containing abundant melt and may result in high t p including granulite facies metamorphism and preservation of metamorphic textures caused by rapid decompression

alpine deformation on naxos greece a detailed structural study of naxos attic cycladic massif greece reveals two major deformation events the first one is associated with large scale thrusting and high pressure low temperature metamorphism during an early alpine subduction episode the second event occurred during continental extension and the associated development of localized thermal domes where lower crustal rocks were brought into contact with upper crustal units along a major shallow dipping shear zone we agree with a model of naxos as a cordilleran type metamorphic core complex however our observations show that the sense of shear was upper plate moving north during the second event calling for a reinterpretation of existing tectonic models of the cyclades

crustal scale geometry and kinematics of late orogenic extension in the central aegean cyclades and ewia island abstract the aegean continental domain is known to be the site of widespread back arc extension since at least the middle miocene overprinting structures related to the mesozoic cenozoic hellenic orogeny features attributed to early thrusting include the overall ductile deformation within two broad belts that have suffered hp lt metamorphism across the aegean this study summarizes the results of a structural study in the central aegean area cyclades and evvia island examining in particular the relationship between ductile and brittle deformation on a regional scale extension appears to be responsible for most of the ductile deformation within hp rock units that have experienced penetrative greenschist facies and higher grade metamorphic overprinting on each island studied progressive extensional deformation has occurred through the development of a major normal sense detachment zone down to depths of about 18 25 km the geometry of the extensional system on the scale of the central aegean is described taking into account the characteristic features of the regional scale ductile strain field at least two probably three subparallel major north dipping detachment zones are identified trending nw se in the northwestern cyclades and e w in the southeastern cyclades both with initial dips estimated at around 30 45 the present geometry of the extensional system implies that these detachment zones and related metamorphic core complexes interfere with each other two main evolutionary models are considered which can account for such a kinematic link between parallel detachment zones structural data favour the sequential development of synthetic major detachments zones in a direction opposite to the sense of dip of the detachments early oligocene lower miocene detachment type extension in the area studied is related to a late orogenic post thickening back arc setting

mechanical consequences of granite emplacement during high t low p metamorphism and the origin of anticlockwise pt paths high temperature t low pressure p metamorphic belts preserve evidence of metamorphism under extremely perturbed and hence transient thermal regimes pt paths are commonly anticlockwise that is the maximumt was attained prior to or at the same time as the maximump with cooling at near constant or increasingp and in many terrains peak metamorphic temperatures prevailed during convergent deformation using a simple coupled thermal mechanical model that assumes a strongly temperature dependent rheology for the continental lithosphere we show that the coincidence of metamorphism with convergent deformation in high t low p terrains may reflect the thermal weakening attendant with rapid advective movement of heat within the lithosphere via granite magma ascent the model shows that the magnitude of the thermal weakening effect is sensitive to the critical temperature for granite segregation tm crit the depth of emplacement of the magmas and most importantly the granite forming mechanism during the prograde cycle material points attain maximum temperatures during a transient 3 ma high strain rate pulse withe up to 10 14 s 1 at levels near the site of granite emplacement the attainment of maximum pressures follows peak temperatures with rapid cooling paths that are close to isobaric or involve slight compression

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

rapid extension recorded by cooling age patterns and brittle deformation naxos greece the metamorphic core complex exposed as the island of naxos in the aegean sea records an unusually complete sequence of structures developed as a result of continental extension the structures formed during miocene rise and cooling from ductile upper amphibolite facies and anatectic conditions to brittle near surface conditions beneath the naxos detachment fault top to the north ductile fabrics in the footwall which initially developed during amphibolite facies prograde metamorphism were overprinted by a succession of north directed normal sense lower temperature brittle structures as the footwall was tectonically unloaded and unroofed pseudotachylite and cataclasite formation brittle faulting alteration and erosion of the footwall occurred during continued slip and tectonic denudation neogene conglomerate and megabreccia in part derived from exhumation of this footwall lie structurally above the peripheral naxos fault published k ar and 40ar 39ar ages for hornblende white mica and biotite in the footwall decrease northwestward apparent ages 17 50 ma in the southeast correspond to areas of low metamorphic grade where preextension argon was partially retained published ages 16 10 ma in higher grade rocks of the domal core in the north are cooling ages that for each of the three minerals show a component of younging in the nne direction of extension assuming this is the direction of unroofing we interpret the rate of this younging as the fault slip rate as the footwall rocks moved 20 km ssw relative to their hanging wall along the base of the naxos detachment fault the calculated rates of slip average 5 8 mm yr comparable to maximum rates reported in the basin and range province

shrimp u pb geochronological constraints on archean volcanic hosted massive sulfide and lode gold mineralization at mount gibson yilgarn craton western australia the base metal rich mount gibson gold deposits are hosted within the ductile mount gibson shear zone in the retaliation greenstone belt of the murchison province of the archean yilgarn craton the deposits are unique in the context of the yilgarn as they exhibit two main styles of hydrothermal alteration and sulfide mineralization consistent with overprinting of synvolcanic volcanic hosted massive sulfide style mineralization by synorogenic shear hosted lode gold mineralization on the basis of model ages based on sulfide pb isotope ratios these two events are separated by approximately 300 m y subvolcanic intrusive rocks from within the host sequence to mineralization at the orion two and hornet deposits have shrimp ii zircon u pb ages of crystallization of 2929 or 3 and 2934 or 6 ma respectively these ages are within error and provide a minimum age for the host sequence and consequently the volcanic hosted massive sulfide mineralizing event at mount gibson on the basis of geochemical evidence the intrusions are interpreted to have been derived from the same parent magma as the rocks they intrude and consequently ca 2930 ma is probably the approximate age of the host rocks to mineralization the regional granitoid which intrudes the base of the greenstone belt has a crystallization age of 2935 or 3 ma a post tectonic granite porphyry which crops out approximately 25 km north northeast of the mount gibson mine site has a shrimp ii zircon u pb crystallization age of 2623 or 7 ma this granitoid has identical field relationships similar whole rock geochemistry and similar pb isotope ratios to those of an internal granitoid which cuts gold mineralization in the deep south pit and is consequently interpreted to be cogenetic and contemporaneous the 2623 or 7 ma date is therefore interpreted to be a minimum age for synorogenic gold mineralization at mount gibson a morphologically and chemically distinctive population of zircons extracted from the sample of 2934 ma altered felsic schist from the hornet deposit gives a shrimp ii zircon u pb age of 2627 or 13 ma these zircons have characteristics which are consistent with a hydrothermal origin and the age is interpreted to represent the timing of synorogenic gold mineralization at mount gibson the interpreted mineralization age is within error of the inferred age of the postmineralization post tectonic granitoid the age is also within error of the majority of absolute ages for gold mineralization from across the yilgarn craton which generally fall in the range 2630 or 10 ma shrimp zircon u pb geochronology thus provides strong support for the two stage model proposed for mineralization at mount gibson synvolcanic volcanic hosted massive sulfide style mineralization occurred at ca 2930 ma and was overprinted by lode gold mineralization some 300 m y later at ca 2630 ma

exhumation of deep crustal metamorphic rocks and crustal extension in arc and back arc regions we adress the problem of exhumation of metamorphic rocks in arc and back arc positions based on observations of the aegean and tyrrhenian back arc regions in both regions back arc extension has been active for 25 ma at least while convergence and crustal thickening was going on at the thrust front compressional structures are reworked in the back arc domain as extensional flat lying detachments which allow unroofing of the metamorphic pile metamorphic rocks suffer first a dynamic burial during nappe stacking and then fast exhumation when they reach the extensional region we describe a continuous process of burial and exhumation that is contemporaneous with the migration of the thrust front toward external zones the front of extension follows behind at approximately one hundred kilometers at the convergence front continuous subduction of a cold oceanic slab or continuous accretion of continental units keeps a cool p t gradient and hp lt metamorphic rocks are formed at depth the overall cool environment leads to a deep brittle ductile transition and thus to a resistant crust which can sustain its large thickness while the thrust front propagates forward parts of the accretionary wedge which were formed earlier enter the back arc domain which can be already under extension and which is warmer because of a greater distance to the cold slab and the presence of the volcanic arc the brittle ductile transition rises in the thick crust which then becomes unstable and subsequently collapses the exhumation history of metamorphic rocks below detachments is two fold 1 a constantely thickening crust is unroofed by flat lying detachments which rework early thrusts this first period is characterized by a good preservation of hp lt parageneses 2 the second period is when rocks are in the zone of crustal scale extension of the back arc region deep crustal metamorphics are exhumed below flat lying detachements which affect the whole crust

physical and chemical response of zircons to deformation an investigation of u pb isotopic systematics in zircons from mylonitized henderson gneiss sinha and glover 1978 revealed that selected zircon fractions from the mylonite zone suffered total loss of radiogenic pb at 460 m y to further investigate the relationship between pb loss u gain and grain size reduction associated with increasing strain in the shear zone we have characterized the chemistry and morphology of zircons in the mylonitic rocks using both electron microprobe analysis and scanning electron microscopy

o sr isotopic variations in miocene granitoids from the aegean evidence for an origin by combined assimilation and fractional crystallization sr o and d h isotopic compositions have been analyzed in miocene metaluminous to slightly peraluminous i type granitoids of the central aegean individual plutonic complexes display significant variations in their 18o and initial87sr 86sr compositions d and 18o compositions of minerals and whole rocks are mostly in the magmatic range some samples from naxos and mykonos delos show low d and 18o values characteristic of meteoric water hydrothermal interaction but as a whole the changes in 18o and sr isotopic compositions as a result of hydrothermal alteration were slight even in instances where marked alteration is petrographically observable consequently the bulk rock variations of 18o from 8 1 to 12 0 and of87sr 86sr from 0 70438 to 0 71450 may be regarded as primary and indicative of the conditions of their evolution heterogeneous isotopic compositions observed in the individual plutons of serifos ikaria samos and kos may be caused by the multiple intrusion of chemically and isotopically distinct magma pulses with high viscosities and relatively rapid consolidation in most cases preventing complete homogenization the granitoids of serifos ikaria and kos display weak correlations between the initial87sr 86sr and 18o and 1 sr the granitoid province shows a positive correlation between87sr 86sr and 18o and a non linear relationship between87sr 86sr and 1 sr whereby 1 sr increases more rapidly than the isotopic ratio as the degree of fractionation of the rocks increases it is argued that assimilation of older continental material by mantle derived arc magmas with combined fractionation afc is the most plausible model to explain the chemical and isotopic characteristics of the granitoids and the geological situation in which rock types trend from granodiorites in the south west near the inferred oligocene miocene suture to granites in the center and monzonites in the north east of the province

deformation related recrystallization processes abstract recrystallization is a common microstructural transformation that occurs during deformation metamorphism and diagenesis of rocks studies on minerals and rock analogues have demonstrated that a wide range of recrystallization mechanisms can occur the range of mechanisms is related to the various ways in which two basic processes grain boundary migration and new grain boundary formation combine to transform the microstructure two recent papers drury et al 1985 urai et al 1986 have proposed different schemes for the description of recrystallization mechanisms the purpose of this paper is to provide a unified framework for the description of mechanisms recrystallization mechanisms are divided into three main types rotation mechanisms which principally involve the formation of new grain boundaries migration mechanisms which principally involve grain boundary migration and general mechanisms which involve both basic processes a further distinction is made on the basis of the continuity of the microstructural transformation with respect to time each of the three main types of mechanism can be divided into a number of sub types depending on whether the processes of grain boundary migration new grain boundary formation and new grain formation occur in a discontinuous or continuous manner with respect to time as the terms continuous and discontinuous have been used in the metallurgical literature to signify the spatial continuity of the microstructural transformation the terms discontinuai and continual are used to refer to the temporal continuity of the transformation it is recommended that the following aspects should be specified if possible in a general description of recrystallization mechanisms 1 1 how do the basic processes combine to transform the microstructure 2 2 if new grain development occurs what is the development mechanism and does new grain formation occur in a continual or discontinuai manner 3 3 if grain boundary migration is involved in the transformation what is the migration mechanism i e fast solute escape migration slow solute loaded migration fluid assisted migration etc and is migration a continual or discontinuai process the application of the unified scheme is illustrated by reviewing studies that have provided detailed information on the recrystallization mechanisms involved the complicating effects of solid solution impurities dispersed second phase particles and grain boundary fluid films are also considered and it is demonstrated that variations in content of these types of impurity can significantly effect the types of recrystallization that occur in a given material

inherited deformation structures in metasedimentary enclaves in granites as windows into deeper levels of the crust abstract deformation structures preserved in metasedimentary enclaves in granites can provide information on the deformation history of the granite source region up to the time of melt generation particularly where the inclusions record more deformations than were present in the country rock at the time of emplacement the data provide a window to the source that is additional to that derived from petrology and geochemistry the improved view can give extra information about the nature of the source rocks and their structural tectonic history if applied across a wide region such as an orogenic belt the approach could provide fundamental information on large expanses of unexposed crust and could help delineate suspected or unsuspected boundaries between terranes or basement terranes examples from two granitic plutons in the palaeozoic lachlan foldbelt of southeastern australia support independent assertions that at least in places the foldbelt structurally overlies pre ordovician late proterozoic cambrian continental crust rather than oceanic crust alternatively it overlies ordovician continental crust the structural and chemical composition of which are different from ordovician rocks at surface

metamorphic core complexes of cordilleran type in the cyclades aegean sea greece the islands of ios and naxos in the cycladic archipelago greece contain elongate domes over which a pervasive north to north northeast stretching lineation is warped these islands may be metamorphic core complexes of cordilleran type involving middle crustal rocks drawn upward and outward from underneath the sedimentary basins now 100 km to the south from ca 25 ma onward during uplift the crystalline rocks now exposed on the islands were deformed noncoaxially in the deeper levels of a major shallow dipping ductile shear zone this shear zone may have connected with low angle normal faults in its upper levels because it is probable that its later history involved the metamorphic complexes being dragged out from under a stretching and fracturing upper plate composed mainly of unmetamorphosed sedimentary and ophiolite suite rocks this model can be extended to explain the exhumation of the high pressure low temperature metamorphic rocks on crete which with the cyclades forms a distended paired metamorphic belt separated by an active sedimentary basin

the geology of the polymetamorphic complex of ios cyclades greece and its significance for the cycladic massif the geological structure of the island of ios cyclades greece is a dome consisting of an augengneiss core and a mantle of garnet mica schists that together form the basement which is overthrust by a marble schist series this series is mainly made up of meta volcanics and metamorphosed sediments presumably mesozoic in age it is a tectonically composed pile of marbles alternating with glaucophane schists actinolite schists and chlorite schists petrological relations and isotope dating indicate the polymetamorphic character of ios two alpine metamorphic phases m1 and m2 and relicts of a high grade metamorphic or magmatic phase mo that only affected the basement are demonstrated radiometric ages obtained for the basement confirm the interpretation that the mo phase is pre alpine the m1 and m2 were dated at 43 ma and 25 ma respectively the p t conditions of metamorphism are estimated as 9 11 kb and 350 to 400 c for the m1 phase and 5 7 kb and 380 420 c for the m2 phase the metamorphic history of the cyclades is discussed and it is suggested that pre alpine basement occurs also on sikinos and naxos