petrology and geochemistry of potassic and ultrapotassic volcanism in central italy petrogenesis and inferences on the evolution of the mantle sources major trace element sr isotopic and mineral chemical data are reported for mafic volcanic rocks mg value 65 from the northern central sector of the potassic volcanic belt of central italy the rocks investigated range from potassic series ks and high k series hks to lamproitic lmp and kamafugitic kam through a transitional series trans thus covering the entire compositional spectrum of potassic and ultrapotassic magmas kam rocks are strongly silica undersaturated and compared with the other rock series have low sio2 al2o3 na2o sc and v and high cao k na na k al ks and hks have high al2o3 cao and variable enrichment in k2o and incompatible elements lmp rocks are saturated in silica and have high sio2 k2o k2o na2 mgo ni and cr and low al2o3 cao na2o sc and v trans rocks display intermediate compositional characteristics between lmp and ks all the rocks under study have fractionated hygromagmaphile element patterns with high lil hfs element values and negative anomalies of ti ta nb and ba negative sr anomalies are observed in the lmp and trans rocks lil elements show overall positive correlations with k2o whereas different trends of sr and hfse vs k2o are defined by lmp trans and ks hks kam 87sr 86sr range from about 0 710 to 0 716 ks hks and kam rocks have similar 87sr 86sr values clustering around 0 710 lmp and trans rocks have the highest 87sr 86sr values geochemical and isotopic data reported for the most primitive italian potassic and ultrapotassic rocks support the hypothesis that the interaction between crustal and mantle reservoirs was a main process in the genesis of italian potassic magmatism simple mass balance calculations exclude however an important role of crustal assimilation during ascent of subcrustal magmas to the surface and indicate that the sources of central italy volcanics underwent contamination with fluids and or melts released by upper crustal material previously brought into the mantle by subduction processes different trends of incompatible elements vs k2o observed in the studied rocks suggest distinct metasomatic processes for the sources of the investigated magmas liquids derived by bulk melting of pelitic sediments are believed to be the most likely contaminants of the source of lmp rocks fluids or melts rich in ca sr and with high lile hfse value and sr isotopic composition around 0 710 are the most likely contaminant of the source region of ks hks and kam volcanics variations in cao na2o and ferromagnesian element abundances and ratios suggest that in some zones the mantle source of potassic magmas experienced partial melting with extraction of basaltic liquids prior to metasomatism

the ultrapotassic rocks from southeastern spain the ultrapotassic rocks of orenditic affinity from southeastern spain have variable sio2 and mgo contents high k2o p2o5 ni cr ba ce th zr values and high k2ona2o nimgo ratios on the contrary tio2 nb y are moderate and feot cao v sc are low the parent magmas possibly generated from a phlogopite bearing mantle source enriched in many incompatible elements by metasomatic processes and probably they were characterized by very co2 co2 h2o ratio the low tip ratio and the moderate tio2 and nb contents in spite of the high alkaline element contents of the magmas suggest that titanium and niobium were retained in the source of the metasomatic fluids the low na contents may depend in part on the loss of this element via naf separation during the latest stage of crystallization crustal contamination cannot be excluded

magmatic evolution of a volcano studied by 230th 238u disequilibrium and trace elements systematics the etna case age determinations of several lava flows from etna through 230th 238u disequilibrium internal isochrons yield a precise chronology of the volcano s history for the last 200 000 years and emphasize the main episodes in the formation of this huge complex strato volcano study of 230th232th 0 initial ratios of lavas together with their trace element compositions yields a consistent model of magmatic evolution implying the existence for 200 000 years of a deep reservoir of alkalic magma periodically mixed with magmas of tholeiitic affinity these short periods of mixing appear to be related to the formation of the large calderas of etna in addition to these processes affecting the deep reservoir fractional crystallization also occurred in more superficial levels of the volcanic edifice thereby yielding several series of differentiation of relatively short duration as for its geochemistry etna s volcanism is of oceanic type but with its own characteristics and in a peculiar geodynamic context at the edge of the african plate

upper miocene submarine volcanism in the strait of sicily banco senza nome two basaltic fragments dredged from a volcanic seamount in the sicilian continental slope of the strait of sicily show petrochemical features typical of within plate alkalibasaltic lavas and k ar age of about 10 m y the data represent the first volcanic evidence of upper miocene tensional tectonics in this sector of the african plate

structural setting and magmatic evolution of montefiascone volcanic complex vulsini district central italy abstract a comparative study concerning volcanological petrographic and geochemical aspects of montefiascone volcanic complex has been carried out this complex together with those of paleo bolsena bolsena and latera forms the vulsini volcanic district located in the northernmost part of roman comagmatic province four groups of lavas belonging to montefiascone volcanic complex have been recognized on petrographic and geochemical grounds tephritic leucitites tel leucite bearing basanites lbsn evolved tephritic leucitites etel and leucititic tephrites lt lavas clearly show ti and nb anomalies further supporting the genesis of these magmas from a subduction related metasomatized mantle source the majority of the lava erupted comparable in volume with the pyroclastic products appears to be of leucititic affinity having suffered only a slight amount of differentiation major and trace element compositions very similar to those of etel and lt have been obtained from tel and lbsn by means of simple mass balance and rayleigh fractional crystallization model according to petrographic observations main liquidus phases are clinopyroxene leucite olivine and magnetite and magmatic differentiation acts essentially by fractional crystallization eruptive activity is spread over a large area and the volcanic centres are aligned following nnw sse wnw ese and e w directions explosive activity on the other hand was mainly located at montefiascone caldera the great abundance of nearly primary magmas in the area leads us to think that the structural setting was not suitable for developing a great unique magma reservoir but rather a series of small elongated bodies

petrology and isotope geochemistry of san vincenzo rhyolites tuscany italy two groups of rhyolites have been recognized at san vincenzo tuscany italy group a rhyolites are characterized by plagioclase quartz biotite sanidine and cordierite mineral assemblages they show constant mgo and variable cao and na2o contents initial87sr 86sr ratios in group a samples range between 0 71950 and 0 72535 whereas the nd isotopic compositions are relatively constant 0 51215 0 51222 group b rhyolites are characterized by orthopyroxene and clinopyroxene as additional minerals and show textural mineralogical and chemical evidence of interaction with more mafic magmas the sr and nd isotopic ratios range between 0 71283 0 71542 and 0 51224 0 51227 respectively magmatic inclusions of variable size 1 mm to 10 cm were found in groups b rhyolites these inclusions consist mainly of diopsidic clinopyroxene and minor olivine and biotite they are latitic in composition and represent blobs of hybrid intermediate magmas entrained in the rhyolitic melts these magmatic inclusions have relatively high sr contents 996 1529 ppm and sr and nd isotope ratios of 0 70807 0 70830 and 0 51245 0 51252 respectively 87sr 87sr data on minerals separated from both group a and b rhyolites and magmatic inclusions reveal strong isotopic disequilibria due to the presence of both restitic and newly crystallized phases in group a rhyolites and due to interaction of rhyolites with a mantle de rived magma in group b rhyolites isotopic data on whole rocks and minerals allow us to interpret the group a rhyolites as representative of different degrees of melting of an isotopically fairly homogeneous pelitic source conversely group b rhyolites underwent interactions with a mantle derived magma the crustal source as inferred from isotopic systematics would be characterized by87sr 86sr and143nd 144nd ratios close to 0 7194 and 0 51216 respectively the sub crustal magma would have sr isotopic composition close to 0 7077 and a143nd 144nd ratio greater than or equal to 0 51252 these isotopic features are different from those reported for the parental magmas postulated for vulsini and alban hills in the nearby roman magmatic province and are similar to those of the vesuvius and ischia magmas

alkali basalts and leucitites in an extensional intracontinental plate setting the late cenozoic calatrava volcanic province central spain the calatrava volcanic province cvp of central spain is characterised by a intracontinental plate magmatic association of leucitites melilitites nephelinites and olivine basalts extruded during the late miocene to quaternary most of the rocks represent relatively primitive magmas and less than 45 have experienced small degrees 25 of crystal fractionation melilitites nephelinites and olivine basalts form a suite generated by variable degrees of partial melting f 5 17 of a nearly homogeneous enriched up to x18 chondritic values for the highly incompatible elements and 3 for the moderately incompatible mantle source composed of ol opx cpx gt phl the leucitites appear to be derived by low degrees of partial melting 4 from a different mantle source characterized by higher rb ba k and 87sr enrichment and the presence of residual apatite phlogopite contributed to the leucititic liquids in a higher proportion than in the basaltic suite whereas clinopyroxene participated in lower amounts the geochemical characteristics of the primary liquids suggest a contribution from two source components 1 a subcontinental lithospheric component which appears to be a strongly enriched garnet lherzolite with phlogopite apatite and 2 an asthenospheric diapir component with characteristics similar to the himu reservoir these conclusions combined with geophysical observations indicate that the cvp magmatism could develop in two stages in a first stage a himu like mantle diapir would trigger magma generation in the overlying subcontinental lithosphere by melting of pervasive enriched streaks or veins with phlogopite apatite giving rise to small amounts of leucititic liquids and to initial extension of the upper crust in subsequent steps the extension developed the lithosphere becomes stripped of its enriched components and the asthenospheric diapir starts to melt giving rise to the basaltic liquids which are virtually oib like magmas this model is also supported by the available k ar radiometric ages as the leucitites were extruded first 7 6 ma followed by the basaltic suite 4 6 1 75 ma this volcanic region is linked with the rift system which developed from late miocene to quaternary times in western central europe and shows strong geochemical similarities with this volcanism in which both lithospheric and asthenospheric components have been detected

the western mediterranean extensional basins and the alpine orogen the western mediterranean late oligocene miocene basins alboran valencia and provencal basins are a coherent system of interrelated troughs in all basins normal faults and thermal subsidence migrated toward the east progressively moving to the miocene to pleistocene algerian and tyrrhenian basins all those troughs appear elements of the back arc opening related to the eastward roll back of the w directed apennines maghrebides subduction zone similarly to western pacific back arc settings these late oligocene early miocene basins nucleated both within the betic cordillera e g alboran sea and in its foreland valencia and provencal troughs the n40 70 direction of grabens is oblique to the coexisting n60 80 trending orogen and shows its structural independence from the orogenic roots thus as the extension cross cuts the orogen and developed also well outside the thrust belt front the westernmost basins of the mediterranean had to develop independently from the alps betics orogen therefore the alboran extension considered a classic example of a basin generated by the collapse of an orogen cannot be ascribed to the detachment or annihilation of the lithospheric root in contrast with the eastward migrating extensional basins the betic balearic thrust front was migrating westward producing interference or inversion structures

comparison of young volcanic associations of western and eastern anatolia formed under a compressional regime a review abstract turkey is characterised by widespread post oligocene volcanism in western anatolia volcanic activity began during the late oligocene early miocene in a compressional regime a widespread suite of andesitic and dacitic calc alkaline rocks was erupted in this period they were accompanied by locally developed high level granitic intrusions the abrupt change from n s compression to n s stretching in the middle miocene was accompanied by a gradual transition to alkali basaltic volcanism in eastern anatolia the collision related compressional tectonics and associated volcanic activity began in the late miocene to pliocene and continued almost without interruption into historical times one minor and two major volcanic phases may be distinguished an initial minor eruptive phase of basic and intermediate alkaline rocks was followed by widespread eruptions of andesitic and dacitic calc alkaline rocks during the pliocene partly penecontemporaneous with this a second more important major phase of volcanism involving alkaline and transitional lavas began and continued during the quaternary the calc alkaline lavas of both regions were erupted at a time when the compressional regime led to crustal thickening these lavas display many geochemical and isotopic signatures that may be interpreted in terms of contamination of magma involving continental crustal materials the alkaline lavas in both regions began during the final stage of the compressional regime and form two groups one group is geochemically and isotopically similar to the calc alkaline rocks and the other is more typically alkaline the volcanic evolution of these two regions of turkey followed a somewhat similar pattern producing highly contaminated calc alkaline lavas at the beginning and evolving towards uncontaminated alkaline lavas at the end

petrogenesis of calc alkaline shoshonitic and associated ultrapotassic oligocene volcanic rocks from the northwestern alps italy along the western alps there is geological evidence of late alpine oligocene magmatic activity which clearly postdates the lepontine eocene early oligocene metamorphism and related deformation of the alpine nappe pile this magmatic activity was notably delayed in relation to the most important convergent processes and may be related to buoyancy of lithosphere tensional tectonics and thermal updoming subsequent to the collision between the eurasian and african plates the geochemical features of the rocks and the geophysical characteristics of the alpine chain suggest that a shoshonitic and calcalkaline melts may have been generated by partial melting of metasomatized peridotitic material and subsequent fractional crystallization and crustal contamination silicic andesites and latites however could have been also derived from metasomatized eclogite or deep continental crust material b the ultrapotassic lamprophyres with high k p lree th zr u and high 87sr 86sr ratios were generated by partial melting of strongly metasomatized mantle the varied sr isotopic ratios may partially also reflect additional radiogenic component from the continental crust following magma segregation from the source

geochemical features of collision related volcanic rocks in central and eastern anatolia turkey in central and eastern anatolia volcanism related to continental collision occurred in neogene to quaternary times major and trace element compositions 87sr86 sr and k ar ages were determined for volcanic rocks from selected volcanoes in this region 87sr86 sr for basalts from erciyes volcano in the eastern part of central anatolia as well as ararat nemrut and karacadag volcanoes in eastern anatolia are in the range 0 7035 0 7040 these ratios are higher than those of n type morb but overlap arc volcanic rock values this suggests that the mantle source which was possibly contaminated by fluids and melts with higher87sr86 sr during the pre collisional events has similar sr isotopic compositions in wide areas of the eastern part of central anatolia and eastern anatolia even across the collision plate boundary in addition chemical compositions of the mantle source are not homogeneous with respect to hfs element contents indicating that the slab derived components resulting from past subductions are not homogeneously distributed the mantle source beneath the western part of central anatolia which corresponds to the transitional region from collision tectonics in the east to subduction tectonics in the west has different chemical and sr isotopic signatures from other parts of anatolia in the western part of central anatolia the87sr86 sr of basaltic rocks from three regions characterized by monogenetic volcanoes are in the range 0 70427 to 0 70581 and differ from region to region in spite of indistinguishable chemical compositions this indicates either source heterogeneity on a small scale or different contributions of local crustal contaminants calc alkaline lavas from ararat and kars in eastern anatolia have chemical compositions with subduction signatures inherited from pre collision subduction events in contrast the chemical and sr isotopic features of volcanic rocks from erciyes volcano in central anatolia indicate that the combined process of fractional crystallization of basaltic magma and crustal assimilation without injection of slab derived components is the dominant process generating calc alkaline andesite under the collision tectonics regime operating in this region

the geochemistry of potassic lavas from vulsini central italy and implications for mantle enrichment processes beneath the roman region major and trace element and 143nd 144nd 0 51209 0 51216 and 87sr 86sr 0 70879 0 71105 isotope analyses are presented on a representative group of lavas from the vulsini district of the roman magmatic province three distinct series are identified the high k and low k series are similar to those described from other italian volcanoes while the third is represented by a group of relative ly undifferentiated leucite basanites which are thought to be near primary mantle melts major and trace element variations within the high k series are consistent with fractional crystallisation from a parental magma similar to the most magnesian leucitites crustal contamination resulted in an increase in 87sr 86sr with increasing fractionation but it was superimposed on magmas which had already inherited a range of incompatible element and isotope ratios from enrichment processes in the sub continental mantle these are reviewed using the available results from vulsini roccamonfina and ernici transition element abundances and ta yb ratios indicate that the pre enrichment mantle was similar to that of e type morb and that these elements were not mobilised by the enrichment process mixing calculations suggest that three components were involved in the enrichment process mantle comparable with the source of morb and two other components rich in trace elements one the low k component had high sr nd th ta and ba nb and no europium anomaly while the second had lower sr nd a negative europium anomaly and very high th ta it was also characterised by low nb ba and high rb ba ratios similar to those reported from phlogopite rich peridotite xenoliths the trace element enrichment processes are therefore thought to have occurred in the mantle wedge above a subduction zone with the trace element characteristics of the high k end member reflecting the subduction of sediments and the stabilisation of mantle phlogopite

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

slab breakoff a model for syncollisional magmatism and tectonics in the alps slab breakoff is the buoyancy driven detachment of subducted oceanic lithosphere from the light continental lithosphere that follows it during continental collision in a recent paper davies and von blanckenburg 1994 have assessed the physical conditions leading to breakoff by quantitative thermomechanical modeling and have predicted various consequences in the evolution of mountain belts breakoff will lead to heating of the overriding lithospheric mantle by upwelling asthenosphere melting of its enriched layers and thus to bimodal magmatism breakoff will also lead to thermal weakening of the subducted crustal lithosphere thereby allowing buoyant rise of released crustal slices from mantle depths in this paper we present a test of this model in the tertiary evolution of the european alps in the alps both basaltic and granitoid magmatism occur between 42 and 25 ma following the closure of oceanic basins by subduction and continental collision the granitoids are now well established to result from mixing of basalt with assimilated continental crust to identify the tectonically crucial origin of the partial mantle melts we have compiled all published geochemical and isotopic data of numerous mafic dykes occurring throughout the whole alpine arc their trace element and isotopic composition suggests that they have been formed by low degree melting of the mechanically stable lithospheric mantle we see no evidence for melting of asthenospheric mantle it was thus not decompressed to depths shallower than 50 km once initiated rapid lateral migration of slab breakoff will result in a linear trace of magmatism in locally thermal weakened crust this explains why all alpine magmatic rocks intruded almost synchronously along a strike slip fault the periadriatic lineament a compilation of ages from penninic high pressure rocks subducted to depths of up to 100 km shows that subduction took place at circa 55 40 ma followed by uplift at 40 35 ma from the short time interval between their uplift and the onset of magmatism we infer that both processes have been induced by the breakoff the slab breakoff model fulfills its predictions in the case of the alps and therefore supports the assumptions made in the theoretical model on a geological basis we believe that the characteristic association of magmatic activity with the return of high pressure rocks to the surface allows the identification of this process in the earth s mountain belts

the primitive lavas of roccamonfina volcano roman region italy new constraints on melting processes and source mineralogy within a large collection of lavas from the roccamonfina volcano are rocks which represent the most mafic samples yet recorded from roccamonfina and which are amongst the least differentiated lavas found in the roman co magmatic region as a whole these rocks extend both high k and low k series to more primitive values however petrographic and geochemical considerations rule out a primary origin and even these mafic samples appear to record the effects of repeated episodes of fractional crystallization and hybridization relatively potassic samples from the low k series are apparently transitional between low k and high k series as previously delineated however these intermediate k samples are not transitional in their sr isotopic composition suggesting that there is no continuum between low k and high k magma source regions rather the compositional range within the low k series appears predominantly to reflect variation in the degree of melting of a common mantle source analysis of the low k series data using an inverse method suggests a source containing amphibole and garnet and indicates that these phases were consumed during the melting processes responsible for the low k series magmas the role of amphibole is further indicated by the association of low k2o with elevated rb concentration and for example higher ce yb such variations are taken to reflect the consumption of high k rb amphibole during the initial phase of partial melting

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

extensional collapse related to compressional uplift in the alpine chain off northern tunisia central mediterranean abstract geophysical and geological marine data recently collected allow to outline the structure of the basement involved fold thrust belt developed during the late cenozoic between sardinia and tunisia along the africa europe plate boundary by integrating these with inland data it is possible to document step by step the progression of crustal thickening from north to south and the collapse of the first uplifted northern units while collision was still going on the geodynamic setting suggests that coupled extensional collapse and fold thrust propagation were driven by island arc drifting above a southward retreating subduction zone

neogene extensional collapse in the western mediterranean betic rif alpine orogenic belt implications for the genesis of the gilbraltar arc and magmatic activity the betic rif orogenic belt crops out in the western mediterranean and constitutes the westernmost part of the alpine edifice the building up of this belt by compressional nappe tectonics occured during a series of alpine deformational episodes subsequent neogene extension resulted from the collapse of the previous compressional edifice through major extensional detachment systems this episode was accompanied by cafe alkaline to shoshonitic and lamproitic volcanism tectonic denudation of metamorphic core complexes and emplacement of peridotite bodies as a final consequence of the evolution of detachment systems furthermore we relate the generation of the gibraltar arc and the opening of the alboran sea to this major extensional event finally two main stages of tectonic denudation of these detachment systems are defined 1 nevado filabride stage with ex humation of metamorphic core complexes and 2 ronda beni boussera stage with a more advanced degree of denudation leading to the exhumation of upper mantle peridotites which may be tentatively termed mantle core complexes

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 role of selective alkali contamination in the evolution of recent historic lavas of mt etna abstract the recent and abrupt encounter of magma bodies with the phreatic system beneath the sedimentary basement of mt etna is considered to be responsible for the alkali anomaly detected in lavas emitted since 1970 geochemical work on the inclusions and the rocks of the sedimentary basement indicates that the piedimonte and the capo d orlando flysch units are the possible sources of contamination

space time patterns of magmatism along the tethysides a preliminary study five maps showing spatio temporal evolution of magmatism along the tethysides for the following time intervals late carboniferous and permian 320 248 ma triassic and early jurassic 247 188 ma

tectonics of the zagros orogenic belt of iran new data and interpretations abstract the zagros orogenic belt of iran is the result of the opening and closure of the neo tethys oceanic realm and consists from northeast to southwest of three parallel tectonic subdivisions 1 1 the urumieh dokhtar magmatic assemblage 2 2 the sanandaj sirjan zone and 3 3 the zagros simply folded belt recent detailed and reconnaissance mapping and stratigraphie structural analyses in various parts of the belt indicate that the tectonics of the sanandaj sirjan zone is dominated by either large scale composite duplex structures or low and high angle ne dipping imbricate systems these thrust systems have transported numerous horses and sheets nappes of metamorphosed and non metamorphosed phanerozoic rocks for tens or possibly hundreds of kilometers stratigraphic evidence and synorogenic conglomerates indicate that thrusting was initiated in the late cretaceous and has continued episodically since that time shear sense indicators in various parts of the zone reveal a ne sw direction of transport thrust systems generated in each episode are superimposed upon the previous structures cutting across and displacing them and producing complicated stacks of thrust sheets southwestward transport and stacking of the thrust sheets has resulted in an increase of about 10 15 km in the thickness of the continental crust in the southwestern part of this zone the so called z main zagros thrust which has been traditionally considered as the boundary between the sanandaj sirjan zone and the zagros simply folded belt is by no means a single high angle reverse fault nor is it a narrow zone of crushed rocks but a series of low angle thrusts locally cut by normal faults these thrusts are indistinguishable from those mapped in the sanandaj sirjan zone and are geometrically part of the sanandaj sirjan zone structures there is no single thrust among them that can be singled out and referred to as the main one the boundary of the sanandaj sirjan zone with the urumieh dokhtar magmatic assemblage to the northeast is characterized by a series of structural depressions formed by compression rock sequences including ophiolite complexes exposed along this boundary are strongly dismembered sheared and mylonitized shearing planes and foliations are either subvertical or dipping at high angles to the northeast this zone is suggested to be the suture between the afro arabian and the iranian plates overall the intensity of deformation increases northeastward from the persian gulf mesopotamian basin where strata are undeformed and horizontal through the zagros simply folded belt which is characterized by en echelon doubly plunging flexural slip folds gentle to open in the southwest and close to locally sw overturned in the northeast the intensity of deformation continues to increase further northeastward through the sanandaj sirjan zone where thrust faulting is dominant this pattern suggests that the deformational front has migrated through time from the northeastern part of the sanandaj sirjan zone i e the suture zone to its present position in the central portion of the persian gulf mesopotamian foreland basin

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

lithospheric boudinage in the western mediterranean back arc basin the western mediterranean is composed of irregular troughs formed as back arc basins in the hanging wall of the w directed apenninic subduction which retreated eastward during neogene and quaternary times the basins are progressively younger toward the east ageing from late oligocene early miocene valencia provencal alboran and algerian basins to middle miocene pleistocene tyrrhenian sea the basins isolated boudins of continental lithosphere the sardinia corsica block representing the largest the boudinage has a wavelength of 100 400 km and facilitated stretching of the continental lithosphere with formation of new oceanic crust in the provencal algerian vavilov and marsili basins the boudins developed both in the earlier alpine betic orogen alboran basin and in its foreland provencal and valencia troughs the extension appears clearly asymmetric due to its eastward polarity accommodated by e dipping master low angle normal faults moreover the thinning shows variations in boudinage wavelength and is characterized by several along strike transfer zones and heterogeneities the western mediterranean back arc setting is comparable with atlantic and western pacific back arc basins associated with w directed subduction zones that show similar large scale lithospheric boudinage

decompression and high temperature low pressure metamorphism in the exhumed floor of an extensional basin alboran sea western mediterranean leg 161 of the ocean drilling program odp has made a major contribution to our understanding of the origin of the alboran basin by demonstrating that it is underlain by rocks of continental origin that have undergone high temperature metamorphism and melting at exceptionally low pressure after exhumation and decompression basement rocks recovered from site 976 consist of high grade schist and gneiss derived from aluminous sediments and minor amounts of marble granitic dikes and migmatitic segregations of granitic material mineral assemblages and textural relations show that an early assemblage including biotite garnet staurolite plagioclase and rutile is overprinted by a second assemblage of biotite sillimanite plagioclase potassium feldspar and ilmenite both assemblages are overprinted by andalusite potassium feldspar and minor garnet migmatitic gneiss contains relict andalusite overprinted by sillimanite and cordierite coexisting with granitic leucosome preliminary pressure temperature estimates suggest that the metamorphic evolution followed an approximately isothermal decompression path from 7 to 3 kbar at temperatures in the range 580 to 630 c after decompression granitic melts formed at 670 c after andalusite breakdown and within the sillimanite stability field the cored rocks closely resemble high grade metamorphic rocks in the adjacent betic cordillera of southern spain which yield early miocene radiometric dates at odp site 976 they are overlain by middle miocene marine sediments the combination of exhumation in an extensional tectonic environment and the evidence for high and increasing temperature during exhumation provide support for and new constraints on current models for the basin that involve the removal of lithospheric mantle below a zone of continental collision accompanied or followed by extension

intraplate stresses and the subsidence history of the sirte basin libya abstract results of quantitative analysis of subsidence data from 20 wells covering six major basement blocks from the sirte basin libya are presented following the late cretaceous onset of extension and associated collapse of the sirte arch the tectonic history post dating this event can be separated into four distinct phases 1 1 minor subsidence during the late cretaceous 2 2 differential fault tectonics and reactivation of basement faults during latest cretaceous to paleocene 3 3 regional uplift and subsidence during the eocene 4 4 minor subsidence during the early oligocene and early miocene a late cretaceous paleocene stretching phase of finite duration can explain the overall patterns of observed tectonic subsidence during the syn and post rift evolution of the sirte basin strong short term deviations from thermally induced long term subsidence observed during the eocene can be explained by intraplate compression and subsequent stress release related to changes in atlantic flowline geometry modelling of the observed tectonic subsidence for the sirte basin yields estimates of extension in excess of estimates inferred from displacements along high angle normal faults the effect of late stage intraplate compression on tectonic subsidence may cause part of the observed discrepancy between estimates of crustal extension

new evidence and model on the evolution of the southeast anatolian orogen this paper describes the orogenic evolution of the southeast anatolian orogenic belt based mostly on new geologic data collected from its constituent tectonic units in the course of new mapping programs of the past decade the southeast anatolian orogenic segment of the alpides may be divided into three approximately east west trending zones from south to north they are the arabian platform followed by a zone of imbrication and then a zone of nappes the arabian platform includes a mostly marine sedimentary succession deposited from early cambrian to middle miocene time the zone of imbrication is a narrow belt sandwiched between the arabian platform and the zone of nappes it consists of imbricated thrust slices emplaced onto a late cretaceous to early miocene sequence the zone of nappes is the highest tectonic unit consisting of two stacks of nappes simply designated the lower and upper nappes the lower nappe is represented by the slices of a polyphase metamorpbic ophiolitic assemblage and the maden group the upper nappe rests on the lower nappe and is represented by the bitlis and poturge metamorphic massifs southeastern anatolia underwent two major episodes of alpide deformation the first occurred during the late cretaceous period when ophiolite was emplaced on the arabian platform this event was not the consequence of a continental collision the ophiolite obduction onto the arabian platform was followed by a regionwide extension and a new marine transgression over the platform immediately after the ophiolite obduction the second episode of deformation occurred during middle eocene miocene time in two distinct stages as a result of the progressive elimination and complete closure of the ocean s which led to the collision between the zone of nappes located to the north being a part of eurasia and the arabian plate the maden marginal basin formed north of the subduction zone that eliminated the ocean between eurasia and arabia along the southern margin of the taurus belt and abutted the arabian plate much later during the final collision the second episode of deformation formed the present orogenic segment leading to the amalgamation of different tectonic units these amalgamated nappes collided with the arabian plate and welded onto it at the latest stage of the orogenic evolution during the miocene epoch

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

miocene calc alkaline heritage in the pliocene postcollisional volcanism of monte arci sardinia italy abstract at monte arci alkaline hawaiites to trachytes subalkaline with a marked calc alkaline character basalts to dacites and rhyolitic lavas were erupted almost simultaneously in late pliocene time major and trace element chemistry microprobe mineralogy and isotopic data suggest a partial melting origin for both rhyolites and subalkaline rocks different sources are however inferred for two rock series homogeneous calc alkaline in nature for subalkaline rocks unhomogeneous richer in 87 sr for rhyolitic ones crystal fractionation differentiation from subcrustal alkali basalts should have been the main process in the genesis of alkaline rocks large scale contaminations with rhyolitic and or alkaline rocks are evident in many of these lavas such a complicated magmatic association characterizes an area where volcanism related to post collisional tensional movements in pliocene time superimposes to middle miocene calc alkaline basic volcanism related to previous subduction processes the pliocene volcanic history of monte arci emphasizes the influence of the paleogeodynamic environment on the nature of magmas erupted in post continental collision areas that are frequently difficult to arrange in the usual schemas connecting magma composition with tectonic setting

late cenozoic volcanic evolution of the northeastern aegean region abstract late cenozoic volcanism occurred in an extensional tectonic regime in the northeastern aegean activity commenced in northeastern greece during the early oligocene and in northwestern turkey during the late oligocene early miocene the beginning of activity was characterized by calc alkaline intermediate acid volcanism however alkaline intermediate basic volcanism became dominant from the late miocene until the pleistocene this change in nature of volcanism is not related to a change of regional tectonic regime as previously thought instead we propose that the volcanic evolution reflects an increasing asthenospheric contribution facilitated by a thinned extended lithosphere arising from the regional extensional tectonics

petrogenesis of monte vulture volcano italy inferences from mineral chemistry major and trace element data the paper presents major and trace element data and mineral compositions for a series of foiditic tephritic to phonolitic rocks coming from monte vulture southern italy and investigates their origin evolution and relationship with the other centres of the roman province

the early miocene onset of a ventilated circulation regime in the arctic ocean deep water formation in the northern north atlantic ocean and the arctic ocean is a key driver of the global thermohaline circulation and hence also of global climate1 deciphering the history of the circulation regime in the arctic ocean has long been prevented by the lack of data from cores of cenozoic sediments from the arctic s deep sea floor similarly the timing of the opening of a connection between the northern north atlantic and the arctic ocean permitting deep water exchange has been poorly constrained this situation changed when the first drill cores were recovered from the central arctic ocean2 here we use these cores to show that the transition from poorly oxygenated to fully oxygenated ventilated conditions in the arctic ocean occurred during the later part of early miocene times we attribute this pronounced change in ventilation regime to the opening of the fram strait a palaeo geographic and palaeo bathymetric reconstruction of the arctic ocean together with a physical oceanographic analysis of the evolving strait and sill conditions in the fram strait suggests that the arctic ocean went from an oxygen poor lake stage to a transitional estuarine sea phase with variable ventilation and finally to the fully ventilated ocean phase 17 5 myr ago the timing of this palaeo oceanographic change coincides with the onset of the middle miocene climatic optimum3 although it remains unclear if there is a causal relationship between these two events

when did sardinia stop rotating new palaeomagnetic results previous palaeomagnetic results from tertiary volcanic rocks of sardinia suggest that the island underwent a quick counterclockwise rotation between 20 5 and 19 ma in order to test this hypothesis a new palaeomagnetic study was carried out on sediments and volcanic rocks with well controlled biostratigraphic or radiometric ages younger than the suggested age for the end of the rotation unrotated palaeomagnetic directions obtained from the upper ignimbrites radiometric age 18 5 19 7 myr and the aquitanian tuffs collected in the anglona region n sardinia would suggest that the rotation ended earlier than 20 5 myr the directions found in the volcanics are perhaps the result of secular variation rather than differential rotation of the anglona region with respect to surrounding regions two sites belonging to the upper ignimbritic unit si2 in the monte traessu monte rispisu area logudoro region exhibited a large amount of rotation d 320 4 i 60 which may be age related radiometric age 19 9 myr four dikes radiometric age 18 3 16 7 myr in the arcuentu volcanic complex arburese region yield results d 348 4 i 33 8 that indicate a further amount of rotation after their emplacement stable end point directions were successfully isolated from three upper burdigalian langhian sedimentary sites biostratigraphic age 18 15ma from the marmilla and logudoro regions consistent paleomagnetic directions obtained from these sites d 352 i 49 indicate that a small amount of rotation was still to be completed in the langhian time this result is consistent with the history of sardinian volcanism which did not end until 13 ma

the strait of sicily continental rift systems physiography and petrochemistry of the submarine volcanic centres abstract new morphological geophysical mineralogical and petrochemical data on the submarine volcanic centres of the strait of sicily continental rift systems lead to the following conclusions magma types both in the rift valleys pantelleria and linosa and plateau areas adventure bank suggest mantle sources and fractionation processes comparable to those of the ethiopian afar section of the east african rift despite their different tectonic regimes however the volume of the erupted magmas less than 2000 3000 km 3 indicates the similarity with low volcanicity dry rifts this is probably related to the peculiar tectonic setting of the pelagian block african foreland dominated by extensional transcurrent motions connected with a strong neogene compressive dextral e w megashear along the sicilian collisional front in the plateau regions the most evolved rocks are hawaiites graham bank which formed by fractionation in the uppermost mantle probably at the base of the crust magma types vary from quartz tholeiites to basanites in the rift valleys the range of magma types is more restricted transitional to alkali basalts and a considerable volume of highly evolved magmas particularly in the axial volcanic complex of the pantelleria rift was produced in crustal magma chambers isotopic trace element evidence suggests the involvement of transitional to enriched morb and oib type mantle sources in the genesis of the quartz tholeiites alkali basalts and basanites throughout the strait of sicily continental rift system including the iblean plateau

tertiary volcanism of the galatia province north west central anatolia turkey abstract large volumes of trachyandesitic dacitic lava flows and pyroclastics of miocene age are associated with small volumes of alkali basalt lava flows in the galatia volcanic province northwest central anatolia turkey the volcanism postdates continental collision occurring in a transtensional tectonic setting associated with movement along the north anatolian fault zone major and trace element including ree and sr nd isotope data and k ar ages for representative samples of mafic intermediate volcanic rocks have been obtained from a series of localities within the province the k ar age data indicate that alkali basalts were erupted during two distinct time periods in the early miocene 17 19 ma and late miocene

tomographic images of the upper mantle below central europe and the mediterranean results from delay time tomography of the european mediterranean upper mantle are discussed and where possible interpreted in terms of geodynamic processes slab like positive velocity anomalies of which the locations correlate well with deeper seismicity are found beneath spain the tyrrhenian basin and the aegean these structures are interpreted as images of subducted slabs large aseismic regions with positive velocity anomalies are found beneath the western mediterranean italy the alps dinarides the pannonian basin northern greece and the aegean these anomalies can also be linked to subducted lithosphere from the anomaly patterns it is deduced that subduction occurred below the western mediterranean and along both sides of the adriatic micro plate beneath the dinarides and northern greece the velocity structures suggest detachment of the slab from the surface

imaging a mantle plume beneath the french massif central abstract teleseismic tomography is now widely recognised as a very powerful tool for studying the velocity structure of the lithosphere asthenosphere system on both local and regional scales a prime objective is to identify velocity perturbations and to constrain their location and intensity here we use the arrival times of p waves from teleseismic events recorded by a dense network of mobile and permanent stations within the french massif central to derive a 3d p velocity model for an area of 425 km 300 km combined with knowledge of the basement geology of the area and the timing and geochemical characteristics of tertiary quaternary volcanic activity we consider that the velocity model provides unequivocal evidence for the ascent of a thermally and chemically anomalous mantle plume beneath the massif central during tertiary times our model is consistent with regional bouger gravity anomalies documented patterns of neogene recent uplift and deformation fabrics within lithosphere derived mantle xenoliths exhumed by the volcanics

petrogenesis and tectonic setting of the roman volcanic province italy abstract the volcanism in the roman province of italy can be modelled by the partial melting of heterogeneously enriched mantle sources the heterogeneity was created by materials derived from a subducted slab which can still be traced geophysically beneath the central apennines new petrographical and chemical data are presented for the high k calc alkaline and the shoshonitic volcanics of the campania region primary magmas are present only locally the existence of spatial zonation in the volcanism of campania is documented for the first time the shoshonitic leucite basanitic and leucititic volcanics of the phlegraean fields procida ischia and the somma vesuvius areas are at similar degrees of evolution about two times richer in nb and ba than those of northwestern campania and the latium part of the roman province accordingly distinct north western and south eastern subprovinces can be defined the evaluation of enrichment factors that is the abundance ratio between the average contents of each element in the relatively primitive lavas of the low k and the high k suites shows that the mantle sources prior to k metasomatism were different in the two sub provinces of the roman province in the north western one they resembled the sources of ocean island tholeiites and moderately enriched mor basalts in the south east they were similar to those of ocean island alkaline lavas and enriched morb s modelling based on k p ce sr rb ba th sm eu gd y nb and 87 sr 86 sr was carried out it indicates that the range of mantle sources of the volcanics in northwestern campania and latium can be modelled by the addition of 3 to 20 of materials derived by partial melting of carbonaceous pelites to a sr enriched mantle wedge comparable to the honolulu mantle source least enriched in nb the production of sr enriched mantle wedge requires either the action of fluids produced by dehydration of subducted oceanic crust or a small amount of metasomatism caused by the presence of carbonatite melts the near absence of ti ta nb yb and the highly fractionated ree in the metasomatizing component requires the presence of residual garnet and accessory ti rich minerals during the partial melting of the subducted sediments the writers propose that the mantle wedge overlying the subducted slab was hybridized by melts produced by partial fusion of subducted material derived from the continental crust probably sediments this process played a dominant role in the generation of the mantle sources from which the high k calc alkaline shoshonitic leucite basanitic and leucititic magmas of the roman province were derived

stratigraphy and structure of eastern syria across the euphrates depression abstract a n s crustal scale geotransect across the northern arabian platform in eastern syria reveals an alternating series of basement uplifts and basins separated by predominantly transpressional fault zones above an effectively uniform crust four major tectonic provinces are crossed along a 325 100 km corridor that extends from the iraqi border in the south to the turkish border in the north the rutbah uplift the euphrates depression the abd el aziz structural zone and the qamichli uplift these features are the manifestations of reactivated pre cenozoic structures that responded to forces acting along nearby arabian plate boundaries particularly cenozoic convergence and collision along the margins of the northern arabian platform i e the bitlis suture and the east anatolian fault in southern turkey and the zagros suture in iran and iraq the database for this study consists of 3000 km of industry seismic reflection data 28 exploratory wells and geologic and bouguer gravity maps the deep crustal structure and in part the basement geometry along this transect are inferred from two dimensional modeling of bouguer gravity whereas the shallow about 8 km structure is constrained primarily by well and seismic data features of the geotransect reveal 1 1 a relatively uniform crustal column approximately 37 km thick with only minor crustal thinning beneath the euphrates crustal thinning may be slightly more pronounced beneath the euphrates about 35 km to the southeast of the transect where the bouguer gravity anomaly is slightly higher 2 2 along the euphrates depression ongoing subsidence which began during the late cretaceous resulted in the deposition of at least 3 km of late cretaceous and cenozoic rocks the structural complexity of the paleozoic and most of the mesozoic sedimentary sections along the transect contrasts markedly with a relatively simple flat lying cenozoic section along most of the transect a notable exception is the abd el aziz uplift where cenozoic rocks are strongly deformed 3 3 while euphrates subsidence continued throughout the cenozoic the inversion of the e w trending abd el aziz structure into a fault bounded tilted block began in the miocene perhaps as a response to the last episode of intense miocene collision along the nearby bitlis and zagros suture zones

the mid carboniferous eustatic event stratigraphic and paleontologic evidence from mid carboniferous namurian basin and shelf successions in widely scattered parts of the world indicates that a major eustatic event occurred about 330 ma the event began with a regression that is recorded in most shelf sequences the regression was followed by a brief transgression about 328 ma and the event ended with a transgression that flooded large shelf areas about 325 ma the mississippian pennsylvanian unconformity in north america is a well known product of this event but equally prominent and contemporaneous unconformity surfaces are also present in europe north africa and elsewhere the event is believed to have caused numerous extinctions and it resulted in marked fluctuations in faunal diversity 94 references 2 figures

eoalpine to neoalpine magmatic and metamorphic processes in the northwestern vardar zone the easternmost periadriatic zone and the southwestern pannonian basin abstract the occurrence of eoalpine neoalpine genetically different granitoids and penecontemporaneous volcanics are characteristic for the vardar zone these rocks are mostly associated with late cretaceous paleogene basinal sedimentary rocks underlain by ophiolites the late cretaceous paleogene rocks were progressively metamorphosed under mediumpressure conditions and intruded by synkinematic mesoalpine granitoids the high pressure blueschist metamorphism was related to a pre maastrichtian eoalpine metamorphic event eoalpine and mesoalpine magmatism and the related metamorphism of the vardar zone may have taken place along a magmatic arc i e the subduction zone the easternmost parts of the periadriatic zone are characterized only by oligocene granitoids accompanied by penecontemporaneous volcanic rocks here younger alpine metamorphic phases only overprint and mylonitize older pre alpine formations most of the mesoalpine granite plutonism both of the vardar zone and the easternmost area of the periadriatic zone was related to subsequent extension the southwestern and southern parts of the pannonian basin contain the products of four pulses of mesoalpine and neoalpine synsedimentary volcanic activity egerian eggenburgian ottnangian carpathian badenian and post badenian represented by predominant basalt andesites trachyandesites and trachydacites this post subduction volcanism was closely connected with the cyclic marine ingressions in the pannonian basin

late oligocene to recent episodes of magmatism in and around the tyrrhenian sea implications for the processes of opening in a young inter arc basin of intra orogenic mediterranean type abstract five episodes of magmatic activity as deduced from geochronological data affected the tyrrhenian and peri tyr rhenian areas from the late oligocene 32 ma to recent times the episodes of w e migration of magmatism from the islands of sardinia and corsica to the eolian islands and the roman comagmatic region of central italy are related to the anti clockwise drift movements of the apenninic chain and clockwise drifting of the sicilian part of the maghrebian chain magmatic activity in the tyrrhenian sea initiated in late miocene times 8 5 6 5 ma in concomitance with a significant change of the geodynamic regime from compressional to tensional the tyrrhenian bathyal plain was affected by two episodes of spreading the first episode occurred around 7 ma and the second from about 1 9 to 1 3 ma these spreading events apparently developed during times of magmatic quiescence in the peri tyrrhenian areas two subsequent episodes of volcanism of the seamount type occurred in the bathyal plain one from 5 to 1 8 and the other from 1 3 to recent they are contemporaneously with extensive manifestations in the peri tyrrhenian areas the different types of magmatism of the late miocene back arc spreading in the central bathyal plain and acidic manifestations of intra crustal anatectic origin in the northern continental platform and slope i e islands of capraia elba and montecristo and seamount vercelli are possibly related to movements of upwelling of the astenosphere which in the southern tyrrhenian sea developed with a much stronger intensity timing of the volcanism in relationship with the overall structural evolution through time in the basin and neighbouring lands is considered

active tectonism in the intracontinental middle atlas mountains of morocco synchronous crustal shortening and extension this material has been published in the journal of the geological society of london volume 153 the only definitive repository of the content that has been certified and accepted after peer review copyright and all rights therein are retained by the geological society of london copyright 2006 the geological society of london see also http www geolsoc org uk template cfm name journals jgs home page http atlas geo cornell edu morocco publications gomez1996 htm

genesis of collision volcanism in eastern anatolia turkey late cenozoic volcanism in eastern anatolia extends in a broad sw ne trending belt across the arabia eurasia collision zone from the arabian foreland basin in the southwest and to the kars plateau and lesser caucasus in the northeast foreland volcanism is dominated by basaltic shield and fissure eruptions of transitional tholeiitic alkaline composition volcanism on the thickened crust north of the bitlis thrust zone varies from the mildly alkaline volcano nemrut and older mus volcanics in the south through the transitional calc alkaline alkaline volcanoes bingol and suphan and the alkaline volcano tendurek to the calc alkaline volcano ararat and older kars plateau volcanics in the north isotope sr nd and trace element systematics indicate that the lavas from the foreland were derived from the mantle lithosphere of the arabian continent which had been enriched by small volumes of asthenospheric melts over a period of time and that lavas from the alkaline volcanic area around mus and the volcanoes nemrut and tendurek north of the bitlis thrust zone were derived from a lithospheric source of similar composition either from the same underthrust arabian continent or from the bitlis massif microcontinent by contrast the transitional lavas from bingol and suphan and the calc alkaline lavas from ararat and kars were derived from lithosphere carrying a distinct subduction signature inherited from pre collision subduction events positive correlations between 87sr 86sr and sio2 and rb nb and sio2 in the alkaline and transitional lavas suggest that combined assimilation and fractional crystallization was an important process within at least part of the thickened crust of the collision zone trace element covariation diagrams such as y rb indicate the importance of hornblende crystallization at depth and orthopyroxene at shallow levels within the calc alkaline provinces in contrast to the consistently anhydrous crystallization sequences of the alkaline lavas trace element diagrams based on the covariation of compatible and incompatible elements point to moderate low degrees of partial melting with residual clinopyroxene throughout and residual garnet in the foreland province consideration of mineral stabilities mantle solidi and geothermal gradients before and after collision suggest that lithospheric thickening should both increase the thickness of metasomatized lithosphere and depress the metasomatized zone to greater depths probably beneath the amphibole and dolomite breakdown curves perturbation of the thickened lithosphere by delamination of the thermal boundary layer perhaps coupled with local stretching associated with pull apart basins on strike slip fault systems is then sufficient to generate melt the composition of that melt being largely dependent on the enrichment history of the lithosphere in question in eastern anatolia volcanism appears to have started between about 8 and 6 ma ago some 5 ma after the start of rapid uplift of the east anatolian plateau

calc alkaline magmatism and rifting of the deep water volcano of marsili aeolian back arc tyrrhenian sea abstract geochemistry and mineral chemistry data indicate that new lava samples recovered from a large depth interval of marsili volcano southern tyrrhenian sea are calc alkaline the deep to intermediate portions of the volcano are made up by calc alkaline basalts whereas the summit consists of high potassium calc alkaline andesites the contents of mgo cao total iron cr ni and v decrease with increasing silica while k 2 o rb ba th nb and lree exhibit the opposite behavior the andesite lavas of the top at waterdepth shallower than about 800 m are younger than 0 2 ma composition of the basalts varies with the waterdepth overall the deepest products of marsili are geochemically more mafic than those erupted in the higher physiographic positions geochemical features suggest the existence of a mantle source that was modified by metasomatic acquirement of subduction derived incompatible elements the seamount lies on oceanic crust which formed above subducting lithosphere not earlier than about 2 ma ago it is currently undergoing extension along linear faults the major distensional fractures trend n s to n15 e paralleling the physiographic elongation of the volcano at the summit later faults have an oblique ne sw orientation which is associated with the eruption of high potassium andesites the transition from basalts to high potassium andesites is discussed as a possible response to the extension and rapid subsidence that dominate the volcano and surrounding region

chronology and geodynamic setting of cretaceous cenozoic rifting in west and central africa abstract the development of the early cretaceous to palaeogene west and central african rift system which extends from nigeria benue trough to kenya anza trough can be related to the build up of intraplate tensional stresses during the break up of gondwana which caused reactivation of pre existing zones of lithospheric weakness repeated changes in the intraplate stress regime of africa are reflected by phases of crustal extension alternating with episodes of compression many of these events can be correlated with changes in rates of seafloor spreading in the central and south atlantic oceans as reflected in flowline patterns the west and central african rifts can be considered as typical passive rifts which evolved in response to the build up of intraplate stresses however the st helena hot spot appears to have been located beneath the equatorial plate boundary at approximately 120 ma and may have played an important role in weakening the lithosphere during extension

styles of back arc extension in the central mediterranean in the central mediterranean two back arc basins the liguro provencal lpb and the tyrrhenian basin tb opened progressively and consecutively from the late eocene oligocene to the present evolution in space and time of rifting and drifting processes along three different transects across these basins shows differences in the style of extension lpb opened with the formation of a narrow single rift while in the tb deformation and magmatism is spread over a wide area moreover at the northern end of the tb the locus of extension progressively migrated towards the east whereas in the southern tb the locus of extension and magmatism migrated inside the basin inducing continental break up and drifting of the previously formed older conjugate basins we propose that these different styles of back arc extension depend upon internal conditions such as prerift rheology linked with its geological heritage and external conditions e g the style of subduction

an early miocene arc tholeiitic magmatic dike event from the alboran sea evidence for precollisional subduction and back arc crustal extension in the westernmost mediterranean a discussion is made on the general significance of an early miocene 22 23 ma k ar dating basaltic dike swarm that outcrops within large areas of the central and western betic zone of the betic cordilleras southern spain this dike swarm is thought to represent the remains of one of the earliest magmatic episodes within the the neogene volcanic province that is associated with the late orogenic evolution of the alboran region and undoubtedly offers the best preserved igneous material related to this early stage that is known at present the dike rocks hypabyssal equivalents of andesitic basalts and basaltic andesites have both major and trace element abundance patterns that are akin to those of arc tholeiitic suites the field relationships and areal distribution of the dike swarm suggest on the other hand that a major e w trending rifting belt could have been developing within the alboran block in oligo miocene times previous to its collision with adjacent continental borderlands in the early middle miocene

evidence for crustal assimilation by turbulently convecting mafic alkaline magmas geochemistry of mantle xenolith bearing lavas from northern sardinia abstract alkali basalt trachybasalt and basanite magmas containing abundant xenoliths of upper mantle origin were erupted during the plio pleistocene 2 4 0 14 ma in northern sardinia the magmas are enriched in k rb th and ba relative to mid ocean ridge basalts morb and most ocean island basalts oib resulting in high k nb th nb ba nb and rb nb ratios the large number of spinel peridotite inclusions in these lavas suggests that these chemical features cannot be explained by combined assimilation and fractional crystallization within the continental crust however volcanic rock chemistry can be explained by the assimilation of sialic rocks by turbulently convecting mafic magmas during their ascent to the surface fractionation of ba and k from the light rare earth elements lree is required to explain the positive correlation of k la and ba la with 87 sr 86 sr i consequently bulk assimilation of crystalline basement rocks by rising hot basaltic magmas cannot explain the observed chemical trends and preferential melting of a low melting quartzo feldspathic crustal component probably occurred leaving the ree in residual phases such as apatite zircon sphene and amphibole alternatively large ion lithophile element lile enrichment may have been related to interaction of rising mafic lavas with metasomatized lithospheric mantle or enriched asthenosphere

travel time tomography of the european mediterranean mantle down to 1400 km abstract the 3 d p wave velocity structure of the mantle below europe the mediterranean region and a part of asia minor is investigated this study is a considerable extension of an earlier tomographic experiment that was limited to imaging upper mantle structure only here the earth s volume under study encompasses the mantle to a depth of 1400 km and we increase the number of international seismological centre isc data for inversion by a factor of four by taking more years of observation and by including data from teleseismic events the most important departure from the earlier study is that we do not use the jeffreys bullen model as a reference model but an improved radially symmetric velocity model the pm2 model which is appropriate for the european mediterranean mantle our inversion procedure consists of two steps first the radial model pm2 is determined from the isc delay times by a nonlinear trial and error inversion of the data as opposed to the jeffreys bullen model the new reference model has a high velocity lithosphere a low velocity zone and seismic discontinuities at depths of 400 and 670 km next the isc data are corrected for effects related to the change in reference model and inverted for 3 d heterogeneity relative to the pm2 model we follow this two step approach to attain a better linearizable tomographic problem in which ray paths computed in the pm2 model provide a better approximation of the actual ray paths than those computed from the jeffreys bullen model hence the two step scheme leads to a more credible application of fermat s principle in linearizing the tomographic equations inversion results for the 3 d heterogeneity are computed for both the uncorrected isc data and for the pm2 data the data fit obtained in the two step approach is slightly better than in the inversion of isc data using the jeffreys bullen reference model a comparison of the tomographic results demonstrates that the pm2 data inversion is to be preferred to assess the spatial resolution an analysis is given of hit count patterns sampling of the mantle by ray paths and results of sensitivity tests with 3 d synthetic velocity models the spatial resolution obtained varies with position in the mantle and is studied both in map view and in cross section in the well sampled regions of the mantle the spatial resolution for larger scale structure can qualitatively be denoted as reasonable to good and at least sufficient to allow interpretation of larger scale anomalies a comparison is made of the results of this study with independent models of s velocity heterogeneity obtained in a number of investigations and with a prediction of the seismic velocity structure of the mantle computed from tectonic reconstructions of the mediterranean region in the context of this comparison interpretations of large scale positive anomalies found in the mediterranean upper mantle in terms of subducted lithosphere are given specifically addressed are subduction below southern spain below the western mediterranean and italy and below the aegean in the last region a slab anomaly is mapped down to depths of 800 km