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

the composition and evolution of the continental crust rare earth element evidence from sedimentary rocks the composition of the present day upper crust inferred from the uniformity of sedimentary rock r e e rare earth element patterns is close to that of granodiorite a revised andesite model is used to obtain total crustal composition the lower crust is the composition remaining assuming that the upper crust one third of the total is derived from intracrustal partial melting the upper crustal r e e pattern has pronounced eu depletion eu eu 0 64 the lower crustal pattern has eu enrichment eu eu 1 17 and the total crust has no eu anomaly relative to chondritic abundances the eu depletion in the upper crust is attributed to retention of eu in plagioclase in the lower crust because plagioclase is not stable below 40 km 10 kbar the anomaly is intracrustal in origin the archaean upper crust has a different r e e pattern to that of the present day upper crust being lower in total r e e and la yb ratios and lacking an eu anomaly these data are used to infer the archaean upper crustal composition which resembles that of the present day total crust except that ni and cr contents are higher the archaean crustal composition can be modelled by a mixture of tholeiites and tonalite trondhjemites the latter have steep light r e e enriched heavy r e e depleted patterns consistent with equilibration with garnet and hence probable mantle derivation there is little reason to suppose that the archaean lower crust was different in composition from the upper crust except locally where partial melting episodes occurred the r e e evidence is consistent with isotopic and geological evidence for a low continental growth rate in the early archaean a massive increase to about 70 of the total crust between about 3000 and 2500 ma b p and a slow increase until the present day the change from archaean to post archaean r e e patterns in the upper crust is not isochronous but is reflected in the sedimentary rock r e e patterns at differing times in different continents on the basis of a model composition for the mantle 36 of the potassium 30 of uranium 15 of lanthanum and 3 of ytterbium are concentrated in the present continental crust this enrichment is related to ionic size and valency differences from common mantle cations e g mg fe pre 3 9 ga b p crusts were obliterated by meteorite bombardment no geochemical evidence exists for primordial anorthositic sialic or mafic crusts

timing of tibetan uplift constrained by analysis of volcanic rocks the tibetan plateau has had a central role in the development of recent models for the mechanics of mountain belts1 2 and cenozoic global climate change3 the present elevation and extensional deformation of the plateau probably result from uplift owing to convective thinning of the underlying lithospheric mantle1 2 an age for the uplift would provide a valuable constraint on these models but because recently proposed indicators of uplift are all climate dependent they are equivocal possibly reflecting global cooling rather than regional uplift4 here we present new geochemical data on post mid miocene lavas from the plateau which show that the lavas were derived from the lithospheric mantle simple thermal arguments indicate that the generation of such magmas also necessitates thinning of the lithospheric mantle thus volcanism is coincident with uplift providing a climate independent means of dating the onset of uplift dating by the laser 40ar 39ar technique places the beginning of this volcanism and therefore the time of uplift of the tibetan plateau at 13 myr ago

asthenospheric and lithospheric sources for mesozoic dolerites from liberia africa trace element and isotopic evidence abstract combined elemental and sr and nd isotopic data are presented for mesozoic dolerite dikes of liberia africa which are related to the initial stage of opening of the atlantic ocean the large scatter of both trace element and isotopic data allows the identification of five groups of dolerites which cannot be related to each other by simple processes of mineral fractionation from a common source on the contrary the observed chemical and isotopic variation within some dolerites groups i and ii may result either from variable degrees of melting of an isotopically heterogeneous source or mixing between enriched and depleted oceanic type mantle for the other dolerites groups iii v mixing with a third mantle source with more radiogenic sr and with element ratios characteristic of subduction environments is suggested this third source is probably the subcontinental lithospheric mantle finally no significant modification by interaction with continental crust is apparent in most of the analyzed samples

magnesian andesites from the island of skyros greece geochemistry and regional significance high magnesian andesites outcrop in a subvolcanic complex in the island of skyros in the central aegean sea mineralogical indicators of this high mg content include bronzite and clinopyroxene phenocrysts the former with high cr 2 o 3 and some phlogopite plagioclase oligoclase andesine and fe ti oxides were late phases to appear at the liquidus whole rock geochemistry shows trace element features similar to those of subduction related rocks radiometric dating indicates an early serravallian middle miocene age approximately synchronous with the onset of post orogenic rifting and extension of the aegean sea this tectonic setting is consistent with an origin by adiabatic decompression of mantle peridotite previously modified by subduction related fluids

the age of the buyuk menderes graben west turkey and its tectonic implications the age of formation of the buyuk menderes graben middle burdigalian middle serravallian is earlier than previously considered tortonian this supports the contention of seyitog lu scott that north south extensional tectonics in west turkey commenced at the latest during early miocene time following a palaeogene compression

miocene magnesian andesites and dacites evia greece adakites associated with subducting slab detachment and extension abstract mid miocene volcanic rocks are rare in the aegean region although early miocene and late miocene quaternary volcanism is widespread at oxylithos island of evia 14 ma dacites form a dome or sub volcanic complex phreatomagmatic eruptions formed nearby rhyolitic pyroclastic surge deposits the calc alkaline dacites contain bronzite mg rich clinopyroxene phlogopite and plagioclase phenocrysts the magma results from mixing of a mg rich andesitic magma similar to that in the nearby island of skyros with more felsic magmas represented by the rhyolitic pyroclastics the dacites are geochemically similar to adakites which are derived by partial melting of eclogitic subducted oceanic crust and have low y and yb and high sr y ratio 87 sr 86 sr 0 7095 is found in both dacite and rhyolite lead isotopic composition from the high mg andesite from skyros with 207 pb 204 pb 15 70 and 208 pb 204 pb 38 90 forms a linear trend with evia dacite and rhyolite with 207 pb 204 pb 15 71 and 208 pb 204 pb 39 05 the high temperatures required to produce such magma resulted from decompression due to extension of the aegean basin at the same time as the initial intrusion of the detached subducted slab in the western aegean that has been imaged by seismic tomography the oxylithos rocks extend the known occurrences of adakite series rocks this series is not restricted to sites with subduction of young oceanic crust

trace element geochemistry of high alumina basalt andesite dacite rhyodacite lavas of the main volcanic series of santorini volcano greece trace element systematics throughout the cal calkaline high alumina basalt basaltic andesite andesite dacite rhyodacite lavas and dyke rocks of the main volcanic series of santorini volcano greece are consistent with the crystal fractionation of observed phenocryst phases from a parental basaltic magma as the dominant mechanism involved in generating the range of magmatic compositions marked inflection points in several variation trends correspond to changes in phenocryst mineralogy and divide the main series into two distinct crystallisation intervals an early basalt to andesite stage characterised by calcic plagioclase augite olivine separation and a later andesite to rhyodacite stage generated by plagioclase augite hypersthene magnetite apatite crystallisation percent solidification values derived from ratios of highly incompatible trace elements agree with previous values derived from major element data using addition subtraction diagrams and indicate that basaltic andesites represent 47 69 andesites 70 76 dacites ca 80 and rhyodacite ca 84 crystallisation of the initial basalt magma least squares major element mixing calculations also confirm that crystal fractionation of the least fractionated basalts could generate derivative main series lavas though the details of the least squares solutions differ significantly from those derived from highly incompatible element and addition subtraction techniques main series basalts may result from partial melting of the mantle asthenosphere wedge followed by limited olivine pyroxene cr spinel crystallisation on ascent through the sub aegean mantle and may fractionate to more evolved compositions at pressures close to the base of the aegean crust residual andesitic to rhyodacite magmas may stagnate within the upper regions of the sialic aegean crust and form relatively high level magma chambers beneath the southern volcanic centres of santorini the eruption of large volumes of basic lavas and silicic pyroclastics from santorini may have a volcanological rather than petrological explanation

isotope geochemistry of recent magmatism in the aegean arc sr nd hf and o isotopic ratios in the lavas of milos and santorini geodynamic implications in this comparative study of variations in the isotopic compositions sr nd o and hf of the calc alkaline magmas of the largest two volcanoes milos and santorini of the aegean arc eastern mediterranean we demonstrate the complexity of the processes governing the evolution of the magmas on the scale both of the arc and of each volcano on santorini the crustal contamination processes have been limited effecting the magma gradually during its differentiation the most differentiated lavas rhyodacite and pumice are also the most contaminated on milos by contrast these processes are very extensive they are expressed in the143nd 144nd vs 87sr 86sr diagram as a continuous mixing curve between a mantle and a crustal end member pole defined by schists and metavolcanic rocks outcropping on these volcanoes in contrast with santorini the least differentiated lavas on milos are the most contaminated these isotopic singularities can be correlated with the geodynamic evolution of the aegean subduction zone consisting of alternating tectonic phases of distension and compression the genesis of rhyolitic magmas can be linked to the two phases of distension and the contamination of the calc alkaline mantle derived magmas with the intermediate compressive phase the isotopic characteristics of uncontaminated calc alkaline primitive magmas of milos and santorini are directly comparable to those of magmas generated in subduction zones for which a contribution of subducted sediments to partial melts from the mantle is suggested such as in the aleutian sunda and lesser antilles island arcs however in spite of the importance of the sediment pile in the eastern mediterranen oceanic crust 6 10 km the contribution of the subducted terrigenous materials remains of limited amplitude

geochemical variation with time in the cenozoic high k volcanic rocks of the island of lesbos greece significance for shoshonite petrogenesis shoshonitic and calc alkaline volcanic rocks alternate in the miocene volcanic sequence of the island of lesbos in the eastern aegean sea the sequence and chronology of the lavas is well known which has allowed a study of both spatial and temporal geochemical variation early calc alkaline volcanism 21 5 ma was followed by a hiatus the main shoshonitic phase occurred from 17 to 17 7 ma and was followed by minor late calc alkaline volcanism the earliest shoshonitic lavas skoutaros formation were relatively anhydrous with f as an important volatile phase but later lavas sykaminea formation have abundant phenocrysts of hydrous minerals and show ree patterns indicating amphibole fractionation there is petrographic and mineralogic evidence for magma mixing throughout the volcanic sequence shoshonitic lavas enriched in k nb th and 206pb 204pb interbed with more calc alkaline lavas in both more hydrous and less hydrous stratigraphic units these variations in k nb and th cannot be accounted for by fractionation or crustal assimilation rather they reflect different mantle sources with different pb isotope signatures the k rich source is similar to source mantle for many alkaline ultrapotassic rocks the lower k source resembles normal calc alkaline source mantle both sources are unusually high in 207pb and 208pb suggesting strong th and u enrichment similar to that in roman province and sunda arc rocks the observed volcanic sequence is consistent with partial melting caused by a rising volatile front that initially triggered partial melting at higher levels in the mantle but as volatile content increased allowed partial melting at greater depths in less enriched mantle

the late alpine evolution of an extensional shear zone naxos greece the mid miocene structural evolution of the metamorphic complex on naxos involved deformation in a kilometre scale low angle ductile shear zone the structures described are consistent with exhumation of the metamorphic complex during crustal extension after and possibly during localized high temperature barrovian style metamorphism non coaxial fabrics developed throughout the structural history indicate that upper levels were transported to the north during ductile extension this shear sense is opposite that inferred previously for the development cordilleran style metamophic core complexes in the cvclades ductile deformation was accompanied during its later stages by e w directed compression

geochemical and petrological evidence of the subduction of delaminated adriatic continental lithosphere in the genesis of the neogene quaternary magmatism of central italy abstract serri g innocenti f and manetti p 1993 geochemical and petrological evidence of the subduction of delaminated adriatic continental lithosphere in the genesis of the neogene quaternary magmatism of central italy in m j r wortel u hansen and r sabadini editors relationships between mantle processes and geological processes at or near the earth s surface tectonophysics 223 117 147 the neogene quaternary magmatism of the northern apenninic arc took place in four phases separated in space and time which become progressively younger from west to east phase i 14 ma phase ii 7 3 6 0 ma phase iii 5 1 2 2 ma phase iv 1 3 0 1 ma this magmatism is the result of the activation of three physically separate sources 1 the adriatic continental crust extracted from the mantle in the late proterozoic 2 a strongly refractory recently k enriched harzburgitic mantle located in the mechanical boundary layer mbl of the lithosphere and 3 a recently metasomatized cpx rich mantle compositionally variable from iherzolite to wehrlite clinopyroxenite interpreted as an ephemerally k enriched asthenosphere the adriatic continental crust is the dominant source of the acid plutonic and volcanic rocks of the tuscan region the acid magmatism is mostly found inside an ellipsoidal area about 150 300 km centred on giglio island here defined as the tuscan crustal dome within this area mantle derived magmas unaffected by important crustal contamination processes and mixing with crustal anatectic melts have so far not been found pure crustal magmas are rare but are represented for example by some of the san vincenzo and roccastrada rhyolites virtually all the tuscan acid centres show evidence of mixing with potassic mantle derived magmas major and trace elements as well as 87 sr 86 sr and 143 nd 144 nd data on primitive rocks mg 65 reveal two groups of mantle derived magmas these define two distinct mantle enrichment trends both essentially due to the additions of k rich components which metasomatized separate compositionally diverse upper mantle sectors in both cases the most remarkable mineralogical effect of these enrichment processes is the production of variable amount of phlogopite through reaction between fluids and or melts with the mantle the rocks of group i ol hy and q normative lamproites ultrapotassic high mg latites ultrapotassic shoshonites and shoshonites saturated trend are considered to be derived by partial melting at low pressure 87 sr 86 sr 0 717 ce sr 0 3 and k 2 o na 2 o 6 7 and low ratios of 143 nd 144 nd 0 5121 0 5120 and ba la 87 sr 86 sr 0 712 and k 2 o na 2 o 8 9 values and low 143 nd 144 nd ba la 0 10 ratios these constraints do not allow to exclude a complete identity between the k rich components which metasomatized the mantle sources of the saturated and undersaturated trend magmas the geochemical and isotopic features of the components that metasomatized the mantle sources of the northern apenninic arc magmatism can be explained by a geodynamic process which causes a large amount of crustal materials to be incorporated within the upper mantle we propose that the delamination and subduction of the adriatic continental lithosphere related to the still ongoing northern apennine continental collision provide a viable mechanism to explain the genesis and eastward discontinuous migration of the magmatism in central italy the subduction of delaminated lithospheric mantle with lower crustal slivers would have exposed uppermost mantle adriatic mbl and crustal units previously imbricated in the apennine chain to the heating advected by the upwelling of a recently and ephemerally k enriched asthenospheric mantle wedge and by the underplating of magmas derived from it we consider that the diapiric uprising of a hot crustally contaminated asthenosphere occurs in the wake left above the sinking of the adriatic delaminated subducting continental lithosphere the delamination subduction process of the adriatic lithosphere has probably started in the early middle miocene but earlier than 15 14 ma ago as indicated by the age and petrologic characteristics of the first magmatic episode sisco lamproite of the northern apennine orogenesis

the pb sr nd isotope geochemistry of some recent circum mediterranean granites pb sr and nd isotopic compositions have been analyzed in recent granites from northern africa northern italy and greece lead isotope compositions of k feldspars are rather homogeneous and cluster close to the modern lead of stacey and kramers 1975 but with slightly higher207pb 204pb and208pb 204pb ratios the cyclades samples however have higher206pb 204pb ratios addition of mantle derived lead was probably very limited which supports a quasi closed system evolution of this element in the continental crust the sr nd data fall in the enriched part of the143nd 144nd vs 87sr 86sr diagram and define a smooth hyperbolic mixing curve over a wide area straddling different orogens most granites may be accounted for by a binary mixture between a recycled crustal component and a depleted mantle like component no correlation is observed between either pb and sr or nd isotopic ratios or any isotopic ratio and major element contents quantitative modelling suggests that two cases fit the sr and nd characteristics of these granites they both require anatexis of the crust on a scale large enough to average the isotopic properties of heterogeneous terranes in the first case the mantle derived component may be represented by differentiated island arc type magmas and the granites result from mixing these magmas with anatectic melts in the second case mantle derived igneous rocks such as obducted ophiolites are part of the crustal source and their variable involvement in the anatectic process causes isotopic variations

polybaric geochemical evolution of two shield volcanoes from santorini aegean sea greece evidence for zoned magma chambers from cyclic compositional variations

spatial and temporal variation in late cenozoic back arc volcanic rocks aegean sea region abstract the aegean sea and adjacent areas comprise thinned continental crust forming an extensional back arc basin behind the south aegean arc in this back arc region are many small volcanic centres with a wide range of volcanic products this late miocene to quaternary volcanism is volumetrically minor in contrast to the voluminous igneous activity of the south aegean arc the age and major and trace element geochemistry of these back arc volcanic rocks are reviewed from the literature where necessary new geochemical analysis have been made five magma types are recognised continental alkaline basalts sodic basalts with incompatible element enrichment and nb depletion trachytes shoshonites and calc alkaline andesites all but the continental alkaline basalts appear geochemically related to subduction processes there is a regular distribution of rock types in time and space the subduction related rocks occur in a broad arc between the modern south aegean arc and the early to middle miocene volcanicity of the central aegean region sodic basalts occur in the most inboard regions and are later than the trachytes shoshonites and calc alkaline magmas occur in the most outboard regions this back arc volcanism occurs only in areas of abundant shallow seismicity however areas of extension and seismicity in the northeast aegean distant from the subducting slab lack volcanoes with the exception of the continental alkaline basalts the magmas are derived only from mantle influenced by the aseismic part of the hellenic subduction slab between 200 and 400 km depth back arc extension associated with rotation of the aegean microplate and strike slip motion on the north anatolian fault system has created a fault system allowing these magmas to reach the surface analogous processes may be common in the final stages of closure of ancient orogens

the hellenic subduction zone a tomographic image and its geodynamic implications new tomographic images of the hellenic subduction zone demonstrate slab penetration in the aegean upper mantle to depths of at least 600 km beneath greece the lower part of the slab appears to be detached at a depth of about 200 km whereas it still seems to be unruptured beneath the southern aegean schematically we derive minimum time estimates for the duration of the hellenic subduction zone that range from 26 to 40 ma this is considerably longer than earlier estimates which vary between 5 and about 13 ma

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

normal faults associated with volcanic activity arc abstract volcanic centers volcanoes fumaroles or solfatara fields epicenters of strong shallow earthquakes with focal depths up to 20 km and epicenters of intermediate depth strong earthquakes with focal depths between 120 and 160 km in the southern aegean volcanic arc can be grouped into five well defined linear clusters trending about n60 e this lineation of shallow earthquakes and volcanic activity is attributed to five corresponding normal faults which are named after the five corresponding volcanic centers sousaki methana milos santorini and nisyros this is supported by a similar trend of the geomorphological features grabens and islands and of geophysical features bouguer anomalies as well as by other seismological data fault plane solutions and the origins of tsunamis and geological information on the santorini caldera the greater volcanic activity in the eastern volcanic centers santorini and nysiros compared to the western volcanic centers sousaki methana and milos is attributed to the higher rate of extensional crustal deformation in the eastern part of the volcanic arc it is 26 mm yr in the west it is 2 mm yr the delineation of the epicenters of the intermediate depth earthquakes along the same five lines indicates the existence of five corresponding rupture zones in the lower leading part of the descending lithospheric slab at depths of 120 180 km these deep zones are probably the sources of hot material which is ascending vertically upwards and intrudes into the crust along its fracture zones the orientation of these zones explains the focusing of the macro seismic results of these deep shocks at narrow regions of the sedimentary arc peloponnesus crete etc

lead isotopic compositions of neogene volcanic rocks from the aegean extensional area abstract lead isotope composition has been determined for a diverse suite of cenozoic igneous rocks in the central and north aegean sea these include voluminous miocene shoshonitic and calc alkali volcanic rocks and correlative plutonic phases middle miocene adakitic high mg andesite and associated rhyolite and middle miocene to quaternary ne normative basalt trachyte and k rich andesite the trace element contents of most of the studied rocks suggests that pb isotopic composition is not a result of upper crustal contamination but rather is a feature of the mantle source for k rich rocks this mantle source is represented by primitive mafic to intermediate volcanic rocks with 87 sr 86 sr 0 7095 high 207 pb 204 pb and 208 pb 204 pb 1000 ppm sr and ba and 35 ppm pb the high 207 pb 204 pb requires a metasomatic origin that lowered u pb and increased th pb followed by a second stage residence time of 1 ga this k rich magma was generated as a result of late tertiary extension of the aegean in the south aegean arc there is more evidence for crustal contamination either from present crust or from subduction of nile derived sediment

regional similarities in lead isotopes and trace elements in galena of the cyclades mineral district greece with implications for the underlying basement abstract polymetallic ore deposits of the aegean sea greece are temporally and spatially associated with the emplacement of miocene granitoids and pliocene to quaternary volcanic units along the active volcanic arc and occur adjacent to the crustal scale detachment systems a variety of mineral deposits e g carbonate replacement skarn porphyry vein type and low to high sulfidation epithermal deposits formed during different stages of back arc evolution and can be found in the footwall and hanging wall of the major detachment systems and all tectono stratigraphic units new galena pb isotope data collected by laser ablation inductively coupled plasma mass spectrometry la icp ms analysis exhibit a range of isotopic ratios 206pb 204pb 18 68 18 91 207pb 204pb 15 67 15 75 and 208pb 204pb 38 83 39 18 with a resolvable geographic pattern together with compiled pb isotope data of cycladic galena large scale regional similarities in the pb isotope signatures are recognized suggesting distinct sources of lead in the underlying basement base metal rich deposits in the west cyclades have a common lead source with a more radiogenic signature whereas lead in the deposits in the north central cyclades was derived from a less radiogenic source similarities between the pb isotopic signature of galena and regional host rocks suggest that the pre alpine cycladic basement was the main source of lead for the deposits in the north and central and the metamorphosed volcano sedimentary lower cycladic blueschist nappe was the source of lead for the deposits in the west moreover there is a close spatial relationship of the 206pb 204pb 18 84 line and trace of the proposed trans cycladic thrust regional trends were also observed in the new la icp ms trace element analysis in galena galena in carbonate replacement and skarn deposits in the west is enriched in base metals and silver whereas galena in vein type deposits in the north and low to high sulfidation epithermal deposits along the active volcanic arc is enriched in gold tellurides and selenium combined trace element and isotopic data of galena indicate distinct metal sources in the cyclades heterogenous rock assemblages of mixed provenance and age account for the differences in lead isotopes whereas the base metals and silver are ultimately sourced from the basement the precious metals might be derived from the metasomatized lithospheric mantle

the upper hadrynian jeffers group cobequid highlands avalon zone of nova scotia a back arc volcanic complex the upper hadrynian jeffers group of the cobequid highlands avalon zone of northern nova scotia consists of volcanic rocks of the gilbert hills formation interfingering deeper water argillite of the humming brook formation and overlying turbidite lithic arkose and siltstone of the cranberry lake formation the jeffers group has a spatially inhomogeneous flat lying cleavage associated with thrust faults of late hadrynian age that were in part reactivated during carboniferous strike slip motion on the cobequid fault the basal rocks of the gilbert hills formation consist of felsic pyroclastics and minor mafic and felsic flows of the lakelands facies locally passing up into sandstone and marble overlain by the humming brook formation thrust over this sequence is the harrington river facies of the gilbert hills formation comprising flows ranging in composition from subalkalic basalt to rhyodacite and pyroclastic rocks two groups of igneous rocks are distinguished geochemically high ti basalts of the lakelands facies and associated hypabyssal intrusions have tholeiitic character enrichment in ba rb and k flat light ree spectra little or no depletion in nb and ta and high feo t and ti they are associated with voluminous felsic rocks the harrington river facies has a calc alkaline differentiation series from basalt to rhyodacite but with more enrichment in feo t p nb and zr than is found in typical calc alkaline rocks these two groups are also represented in hypabyssal rocks the jeffers group rocks are compared geochemically with other predominantly continental back arc basins in the aegean sea and bransfield strait all three basins show the presence of a rock phase derived from partial melting of mantle only slightly modified by subduction processes the lakelands facies tholeiites of the jeffers group they all also contain a more differentiated series showing more subduction related chemical characteristics the harrington river facies of the jeffers group geochemical differences between these rocks and typical calc alkali series are related to the lesser role of hydrous fluids in the back arc basins as a result titanomagnetite and amphibole are not important fractionating phases and nb and ta are not significantly depleted

distribution of oceanic and continental leads in the arabian nubian shield new common lead data for feldspar whole rock and galena samples from the arabian nubian shield together with data from previous work can be divided into two main groups group i leads have oceanic mantle characteristics whereas group ii leads have incorporated a continental crustal component of at least early proterozoic age the group i leads are found in rocks from the red sea hills of egypt and the western and southern parts of the arabian shield group ii leads are found in rocks from the northeastern and eastern parts of the arabian shield as well as from the southeastern shield near najran they are also found in rocks to the south in yemen to the east in oman and to the west at aswan egypt this distribution of data suggests that the arabian nubian shield has an oceanic core flanked by rocks that have developed at least in part from older continental material two mechanisms are suggested by which this older lead component could have been incorporated into the late proterozoic rocks and each may have operated in different parts of the shield the older lead component either was derived directly from an underlying early proterozoic basement or was incorporated from subducted pelagic sediments or sediments derived from an adjacent continent

tertiary granitoids of rhodope northern greece magmatism related to extensional collapse of the hellenic orogen abstract many granites that occur in orogenic belts such as the himalaya the hercynides or caledonides are thought to result from anatexis of the lower parts of over thickened thrust stacked crustal units just preceding termination of the orogenic cycle numerous tertiary granitoids occur in the rhodope zone of northern greece and adjacent regions of bulgaria a belt regarded as an eastern extension of the alpine system these granites could also be regarded as syn or late tectonic crustal melts in that some have been emplaced as sheet like bodies along major alpine thrust planes and show a strong marginal foliation parallel to the pervasive alpine compressional fabrics in detail however the geochemistry of the granitoids their timing and relationships with country rocks suggest they have been emplaced in an extensional regime that is related to the collapse of the hellenic orogen and more particularly to the crustal thinning that the aegean region has suffered since the mid tertiary rather than being products of crustal thickening it is suggested that their petrogenesis is related to the elevation of thermal gradients during the extensional phase which initiated thermal breakdown of hydrous minerals in the lithosphere and lower crust that had existed metastably during compression and the subduction of cool ocean lithosphere beneath the region in the late mesozoic early tertiary

clarification of some misconceptions about antidune geometry and flow character estimates of palaeoflow conditions based on antidune geometries generally are obtained using equations derived from potential flow analysis unfortunately in several previous studies and review articles misunderstanding and improper substitution of density terms has resulted in two misconceptions a third is discussed in detail elsewhere both concern the commonly derived ratio of antidune wavelength to critical flow depth l d previously published results which suggest that for a given set of flow conditions in a two layer system l d increases with increasing density contrast are wrong correct substitutions indicate the opposite there is a slightly inverse relationship between the value of l d and the second misconception concerns the two values reported for critical flows l d 6 3 or 2 and l d 12 6 or 4 despite insinuations that the latter is wrong by a factor of two both are correct in this case what has been overlooked is that the former value is derived for antidunes on free surface flows a two layer system in which 2 1 and the latter for antidunes at the interface of a two layer system in which p 0 i e 2sime 1

placer formation in gravel bedded rivers a review the literature on placer formation processes within fluvial systems is widespread and ranges between detailed laboratory studies of the hydrodynamic segregation processes through to the intuitive interpretation of the distribution of minerals within geological sections however there are few if any comprehensive reviews of the literature surprisingly given the economic importance of placers the theoretical framework relating to the hydrodynamics of physical grain sorting is not well developed and there are relatively few detailed laboratory hydraulic investigations to inform theory in this wide ranging review the history of the development of principles of placer formation is explored as far as possible in a non technical fashion a consideration is given to the hydrodynamics of physical grain sorting above lower stage and upper stage bedforms and the typical internal sedimentary structures associated with placer concentrations are detailed finally examples of the depositional environment of diamond tin and gold placers are considered

a major oligo miocene detachment in southern rhodope controlling north aegean extension plutonic and metamorphic rocks of the southwest part of the rhodope massif in greece correspond to ductile lower crust exhumed and deformed along a major detachment during post thickening extensional tectonics extension started during the oligocene and is responsible for the development of miocene quaternary sedimentary basins both brittle and ductile deformations result from gravity collapse of previously thickened lithosphere as proposed for others large extended terranes this interpretation disagrees with the previous models which attributed tertiary ductile deformation to alpine thrusting and brittle extensional deformation to back arc tectonics above a subduction zone

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

the high k2o volcanism of northwestern tibet geochemistry and tectonic implications cenozoic volcanic rocks sampled during the 1989 sino french kunlun geotraverse have been analyzed to constrain the origin of widespread tertiary to neogene tibetan basic magmatism samples ranging from phonotephrites to trachyandesites are high k lavas k na always greater than 1 which exhibit typical calc alkaline characteristics although they are highly enriched in mobile trace elements and ree 600 times chondrites for la isotopic compositions exhibit radiogenic sr and pb 87sr 86sr 0 708 208pb 204pb 39 39 4 207pb 204pb 15 6 15 7 206pb 204pb 18 7 and unradiogenic nd 143nd 144nd 0 5123 0 5124 constancy of sr and nd isotopic values contrasts with variations in pb isotopes and highlights the problem of crustal contamination afc and simple mixing models are compatible with isotopic variations but were probably buffered by elemental concentration in sr and nd while pb isotopic values for the most primitive magmas are probably uncontaminated isotopic features are therefore proposed to be representative of a mantle source in which heterogeneity could be highlighted in the pb isotopic values such a source highly enriched in trace elements and radiogenic sr and pb is proposed to lay in the subcontinental mantle isolated from the overall convection movements the calc alkaline characteristics can date from the succession of enrichment phases linked to the numerous subduction events that have affected this part of asia while the isotopic characteristics are proposed to be the result of the aging of the concomitant lile enrichments the broad geographical geochemical and timing aspects of recent basic cenozoic magmatism are discussed in the light of our new data and we suggest that two mechanisms are acting together 1 a limited delamination of the base of the lithosphere or a thermal reequilibration of isotherms subsequent to thickening and 2 intracontinental subduction under the kunlun such mechanisms appear the most probable as they respect at the same time recent geophysical data and other geochemical studies

contained reflected turbidity currents from the middle ordovician cloridorme formation quebec canada an alternative to the antidune hypothesis the parautochthonous cloridorme formation is a syn orogenic flysch succession that was deposited in an elongate foredeep basin as mainly lower middle fan outer fan and basin floor deposits the basin floor deposits about 1 5 km thick are confined to members 1 2 and 1 and are characterized by graded thick 1 10 m mud rich calcareous greywacke beds previously interpreted as deposits of concentrated muddy unidirectional turbidity currents that locally generated backset antidune lamination in internally stratified flows the dominant flow directions were from east to west but west to east transport also occurred as seen in the orientation of ripples climbing ripples flutes consistently overturned flames and grain imbrication we believe that the flows that deposited these thick calcareous greywacke beds reversed by roughly 180 one or more times during deposition of the lower sandy part of the beds flow reversals are consistent with the sharp grain size breaks and mud partings within sandy divisions measurement of grain fabric relative to stratification in the most celebrated antidune bedforms indicates flow from west to east thus the bedforms were produced by west to east migration of megaripples not by the upcurrent migration of antidunes the thick muddy beds were deposited by large volume muddy flows that were deflected and reflected from the side slopes and internal topographic highs of a confined basin floor much like the contessa and similar beds of the italian apennines large quantities of suspended mud were ponded above the irregular basin floor and settled to produce the thick silty mudstone caps seen on each bed because of their mode of emplacement we propose that these beds be called contained turbidites

geochemistry of a transitional ne trachybasalt q trachyte lava series from patmos dodecanesos greece further evidence for fractionation mixing and assimilation trace element and preliminary sr and o isotopic data are reported for a transitional alkaline sub alkaline lava series mvs from patmos greece the lava types belonging to this series are ne trachybasalt hy trachybasalt hy trachyandesite and q trachyte rb sr and ba contents as well as k rb ratios of the ne trachybasalts differ from those of alkali basalts of oceanic islands and those of k rich alkaline lavas of continental regions and are consistent with the occurrence of these volcanics in a destructive plate margin environment qualitatively the variations shown by many trace elements throughout the mvs are explicable in terms of magma evolution via fractional crystallization involving removal of the observed phenocryst phases cross cutting ree patterns can be explained by removal of small amounts of apatite however certain features of the data cannot be reconciled with the operation of fractional crystallization alone these are a the compatible behavior of ba throughout the mvs b the moderately as opposed to highly incompatible behavior of zr rb and nb relative to th and c the significant decrease of k th rb th zr th zr nb nb th yb th ta th u th and zr ta ratios especially but not exclusively in the mafic part of the series quantitative modeling indicates that the hy trachybasalts are anomalously enriched in both highly incompatible and highly compatible elements and these lavas are shown to be hybrids formed by mixing of ne trachybasalt and hy trachyandesite mixing proportions of the end members calculated from incompatible element abundances 19 ne trachybasalt differ from those calculated from compatible element abundances 62 ne trachybasalt and are inconsistent with proportions calculated from published mineral chemical data in addition mixing cannot account for the observed variations in incompatible element ratios and this is taken as evidence for the simultaneous operation of assimilation isotopic variations 87sr 86sr from 0 7049 to 0 7076 and 18o 16o from 4 7 to 8 6 and the positive correlation of isotope ratios with sio2 and th contents provide conclusive proof that assimilation occurred calculations show that the isotopic characteristics and the concentrations of many trace elements in the q trachytes can be explained by fractional crystallization of ne trachybasalt combined with assimilation of average continental crust 87sr 86sr 0 710 and that large amounts of assimilation are not necessary ma mc 0 55 ree data are not well explained by this model and suggest a crustal end member enriched in lree relative to the average crust zr and hf data are also not well explained and indicate that the assimilant was depleted in hfse relative to average crust or that hfse are held back in relatively insoluble phases such as zircon in the restite during assimilation nevertheless the results of the modeling demonstrate that ba concentrations may decrease during afc processes and that high sr contents 1500 ppm in the mvs ne trachybasalts do not render mafic parental magmas immune to the effects of assimilation in terms of their 87sr 86sr ratios the results of this study confirm conclusions based upon major oxide and mineral chemical data for the mvs lavas but more importantly show that careful analysis of trace element data allows the various processes involved in magma evolution to be identified and quantified even in the absence of major oxide and isotopic data finally it is reiterated that magma mixing and assimilation may be coupled processes in the magma chambers beneath many volcanic centers and recognition of this fact has profound implications for studies of magmas erupted at continental margins and through continental crust