eruptive history of earth s largest quaternary caldera toba indonesia clarified single grain laser fusion sup 40 ar sup 39 ar analyses of individual sanidine phenocrysts from the two youngest toba indonesia tuffs yield mean ages of 73 plus minus 4 and 501 plus minus 5 ka in addition glass shards from toba ash deposited in malaysia were dated at 68 plus minus 7 ka by the isothermal plateau fission track technique these new determinations in conjunction with previous ages for the two oldest tuffs at toba establish the chronology of four eruptive events from the toba caldera complex over the past 1 2 m y ash flow tuffs were erupted from the complex every 0 34 to 0 43 m y culminating with the enormous 2500 3000 km sup 3 youngest toba tuff eruption caldera formation and subsequent resurgence of samosir island timing of this last eruption at toba is coincident with the early wisconsin glacial advance the high precision sup 40 ar sup 39 ar age eruption of such magnitude may provide an important marker horizon useful as a baseline for research and modeling of the worldwide climatic impact of exceptionally large explosive eruptions

experimental calibration of the aluminum in hornblende geobarometer with application to long valley caldera california volcanic rocks a new geobarometer based on the al content of igneous hornblendes in equilibrium with melt fluid biotite quartz sanidine plagioclase sphene and magnetite or ilmenite has been calibrated experimentally the calibration was performed by equilibrating the required phase assemblage over the pressure range 2 8 kbar at 740 780c and then analyzing euhedral hornblendes in equilibrium with glass melt experiments were performed on natural samples of both volcanic and plutonic rocks earlier empirical calibrations of this geobarometer relied on analyzing natural hornblendes from plutons with the required phase assemblage and inferring pressure from nearby metamorphic country rocks the experimental calibration differs from the empirical calibrations especially above 5 kbar and shows that the al content of hornblendes in equilibrium with the required phase assemblage is greater for a given total pressure than previously thought the geobarometers uncertainty is dramatically reduced the derived equation is p plus minus 0 5 kbar 3 minus 3 46 plus minus 0 24 4 23 plus minus 0 13 al sup t the geobarometer is applied to post bishop tuff volcanic rocks from long valley caldera california and reveals that most rhyodacites in this complex erupted from depths of about 6 km these eruptions occurred over 500 000 yr suggesting that the rhyodacitic magma reservoir beneath longmore valley had reached a steady p depth t state less

birth death and resurrection the life cycle of suprasubduction zone ophiolites 1 suprasubduction zone ssz ophiolites display a consistent sequence of events during their formation and evolution that suggests that they form in response to processes that are common to all such ophiolites this sequence includes the following 1 birth which entails the formation of the ophiolite above a nascent or reconfigured subduction zone this stage is typically characterized by the eruption of arc tholeiite lavas and the formation of layered gabbros and sheeted dike complex 2 youth during which is continued melting of refractory asthenosphere depleted during birth occurs in response to fluid flux from the subducting slab with extensional deformation of the older plutonic suite eruption of refractory lavas and the intrusion of wehrlite pyroxenite 3 maturity with the onset of semistable arc volcanism typically calc alkaline as the subduction zone matures and stabilizes and the intrusion of quartz diorite and eruption of silicic lavas and 4 death which is the sudden demise of active spreading and ophiolite related volcanism which in many cases is linked to collision with an active spreading center and the onset of shallow underthrusting of the buoyant spreading axis expressed as dikes and lavas with oceanic basalt compositions that crosscut or overlie rocks of the older suites 5 resurrection with emplacement by obduction onto a passive margin or accretionary uplift with continued subduction the early stages 1 3 may be diachronous and each stage may overlap in both time and space the existence of this consistent progression implies that ophiolite formation is not a stochastic event but is a natural consequence of the ssz tectonic setting

u pb shrimp and nd isotopic data from the western bohemian massif bayerischer wald germany implications for upper vendian and lower ordovician magmatism u pb shrimp dating of zircons of metamagmatites from the bayerischer wald germany reveals a complex evolution of this section of the moldanubian zone exposed in the western bohemian massif of the central european variscan belt in the south western part of the bayerischer wald upper vendian magmatism is constrained by pooled 206pb 238u mean ages of 555 12 549 7 and 549 6 ma from metarhyolites and a metabasite inherited zircon cores were not observed zircon overgrowths yielding pooled 206pb 238u ages of 316 10 and 319 5 ma provide evidence for variscan metamorphic zircon growth cathodoluminescence imaging reveals a two stage metamorphic overprint

fission track ages of late cenozoic distal tephra beds in the yukon territory and alaska six distal tephra beds from the yukon territory and alaska have been dated by the fission track method zircon and glass ages were determined for the fort selkirk and lost chicken tephra beds but only glass ages for the others assuming that no track fading has occurred in the glass old crow and dawson tephra beds are younger than 120 000 and 52 000 years bp respectively mosquito gulch tephra is 1 22 ma old fort selkirk tephra is about 1 ma old the ester ash bed is 0 45 ma old and the best estimate of the age of lost chicken tephra is the range 1 7 2 6 ma it is evident from these results and from the known abundance of tephra beds within late cenozoic deposits of the yukon territory and alaska that application of the fission track method to distal tephra in conjunction with detailed characterization studies offers great potential for elucidation of the late cenozoic geologic history of alaska and the yukon territory

the volcanic stratigraphy and petrogenesis of the oman ophiolite complex the volcanic stratigraphy and trace element geochemistry of the oman ophiolite complex indicate a multistage magmatic origin comprising 1 magmatism due to sea floor spreading in a marginal basin 2 magmatism associated with discrete submarine volcanic centres or seamounts 3 magmatism associated with crustal uplift and rifting and 4 magmatism associated with continent arc collision trace element petrogenetic modelling is used to investigate the nature of the mantle source region and the partial melting and fractional crystallization history for each magmatic event the petrogenetic pathway for the sea floor spreading lavas requires a high degree of melting of a mantle that was depleted in incompatible elements prior to subduction but subsequently selectively enriched in certain elements mostly lil elements and h2o from an underlying subduction zone it also requires magma mixing in an open system magma chamber prior to eruption the seamount lavas were probably derived by a similar degree of partial melting of a similar source but fractional crystallization was restricted to smaller high level magma chambers the rifting lavas were derived from a mantle source that was more depleted than the seamount lavas prior to subduction but which was later modified by a larger subduction zone component the syn collision lavas were however derived from an enriched mantle source which probably underlay the passive continental margin rather than the marginal basin complex results such as these may provide considerable insight into the petrogenetic changes accompanying the transitions from spreading to arc volcanism in a supra subduction zone setting

rapid crustal accretion and magma assimilation in the oman u a e ophiolite high precision u pb zircon geochronology of the gabbroic crust 1 new high precision u pb zircon geochronology from the oman united arab emirates u a e ophiolite provides insight into the timing and duration of magmatism and the tectonic setting during formation of the lower crust the new data come from a well preserved and exposed crustal section in the center of the wadi tayin massif single grain and grain fragment 206pb 238u dates from upper level gabbros tonalites trondhjemites and gabbroic pegmatites corrected for initial th exclusion range from 112 55 0 21 to 95 50 0 17 ma with most data clustered between 96 40 0 17 to 95 50 0 17 ma zircon dates from upper level gabbros are most consistent with the ophiolite forming at a fast spreading ridge with half rates of 50 100 km ma dates from tonalites trondhjemites and from a gabbroic pegmatite associated with a wehrlite intrusion overlap with dates from adjacent upper level gabbros suggesting that any age differences between these three magmatic series are smaller than the analytical uncertainties or intrasample variability in the dates three of the dated upper level gabbros and a single gabbroic pegmatite from the base of the crust have 1 ma intrasample variability in single grain dates suggesting assimilation of older crust during the formation or crystallization of the magmas whole rock end t of seven samples including the upper level gabbros with variable zircon dates have tightly clustered initial values ranging from end 96 ma 7 59 0 23 to 8 28 0 31 the end values are similar to those from other gabbros within the ophiolite suggesting that any assimilated material had a similar isotopic composition to primitive basaltic magmas the new dates suggest that the studied section formed at a fast spreading mid ocean ridge between 96 4 95 5 ma the large intrasample variability in zircon dates in some samples is unexpected in this setting and may be related to propagation of a younger ridge into older oceanic lithosphere

p t paths from high temperature shear zones beneath ophiolites mctamorphic rocks of the st anthony complex of north western newfoundland are best interpreted in terms of a high temperature shear zone formed between down going continental margin rocks and overriding oceanic lithosphere in a subduction zone high grade rocks immediately beneath the oceanic lithosphere peridotite display retrograde meta morphism in high strain zones whereas lower grade rocks near the base of the metamorphic complex display prograde metamorphism in high strain zones mylonite zones in meta basitcs at all levels in the complex contain the assemblage epidote hornblende albite sodic oligoclase these observations suggest that the inverted metamorphic gradient within the st anthony complex results from the fortuitous preservation of residual metamorphic assemblages from different crustal levels within an epidote amphibolite facies shear zone the degree of re equilibration is strongly dependent on the degree of strain and is best achieved in synmetamorphic mylonite zones this interpretation of the st anthony complex can be extended to other sub ophiolite metamorphic sheets which show very similar relationships it is proposed that most metamorphic sheets beneath ophiolites are high temperature shear zones the p t paths of which preserve records of burial and exhumation in subduction zones

evolution of intra oceanic arc trench systems abstract intra oceanic island arc trench systems represent a locus of crustal thickening through the addition of mantle derived magmas which form the volcanoes and subjacent stratiform plutons the crust of immature island arcs and adjacent back arc basins is oceanic in chemistry in that it is dominated by mafic e g tholeiitic magmas a magma sequence is recognized in many arc systems which progress from boninite to arc tholeiite to calc alkaline and in some arcs to shoshonites the adjacent back arc basins are formed of morb like tholeiite arc systems may be dominated by extension rather than compression if the subducted and overriding plates are decoupled extension of the arc system may be associated with seaward retreat or roll back of the trench axis alternate cycles of extension and compression of the arc system can lead to both tectonic erosion of the fore arc and accretion of fragments from the incoming sea floor the izu bonin mariana tonga yap and palau trench arc systems are examples of different stages in a dynamic process which starts when subduction begins along old fracture zone traces crustal extension causes both fore arc subsidence and opening of back arc basins island arc volcanic chains and back arc basins are new crust formed largely of fractional melts of the mantle the evolution of an arc system may stop when blocking of the subduction zone or changes in relative plate motion causes overthrusting of back arc lithosphere on to former volcanic arcs many ophiolites preserve remnants of arc system crust and upper mantle

oceanic crust generation in an island arc tectonic setting se anatolian orogenic belt turkey a number of late cretaceous ophiolitic bodies are located between the metamorphic massifs of the southeast anatolian orogenic system one of them the goksun ophiolite northern kahramanmaras which crops out in a tectonic window bounded by the malatya metamorphic units on both the north and south is located in the ew trending nappe zone of the southeast anatolian orogenic belt between goksunand afsin northern kahramanmaras it consists of ultramafic mafic cumulates isotropic gabbro a sheeted dyke complex plagiogranite volcanic rocks and associated volcanosedimentary units the ophiolitic rocks and the tectonically overlying malatya keban metamorphic units were intruded by syn collisional granitoids 85 ma the volcanic units are characterized by a wide spectrum of rocks ranging in composition from basalt to rhyolite the sheeted dykes consist of diabase and microdiorite whereas the isotropic gabbros consist of gabbro diorite and quartzdiorite the magmatic rocks in the goksunophiolite are part of a co magmatic differentiated series of subalkaline tholeiites selective enrichment of some lil elements rb ba k sr and th and depletion of the hfs elements nb ta ti zr relative to n morb are the main features of the upper crustal rocks the presence of negative anomalies for ta nb ti the ratios of selected trace elements nb th th yb ta yb and normalized ree patterns all are indicative of a subduction related environment all the geochemical evidence both from the volcanic rocks and the deeper levels sheeted dykes and isotropic gabbro show that the goksun ophiolite formed during the mature stage of a suprasubduction zone ssz tectonic setting in the southern branch of the neotethyan ocean between the malatya keban platform to the north and the arabian platform to the south during late cretaceous times geological geochronological and petrological data on the goksun ophiolite and the baskil magmatic arc suggest that there were two subduction zones the first one dipping beneath the malatya keban platform generating the baskil magmatic arc and the second one further south within the ocean basin generating the goksun ophiolite in a suprasubduction zone environment

petrology of the tekirova antalya ophiolite southern turkey evidence for diverse magma generations and their tectonic implications during neotethyan subduction the sw antalya complex is an assemblage of mesozoic carbonate platform margin and ophiolitic rocks which record the formation and tectonic emplacement of a small mesozoic ocean basin the late cretaceous ophiolitic rocks are located at two localities namely the relatively intact tekirova ophiolite to the east of kemer zone and the dismembered godene ophiolite to the west of kemer zone the tekirova antalya ophiolite comprises harzburgitic tectonites ultramafic to mafic cumulates isotropic gabbros and sheeted dikes numerous isolated dikes ranging in thickness from 5 cm to 10 m intruded the crustal rocks at different structural levels the isotropic gabbros are represented by gabbro diorite and quartz diorite rocks with granular to ophitic subophitic textures the isolated dikes are characterized by dolerite diabase and microdiorite with ophitic intersertal and microgranular textures these rocks exhibit tholeiitic to alkaline compositions new geochemical data presented in this paper from the isolated dikes and isotropic gabbros suggest that there are three main types of parental basic magmas that form the oceanic crustal rocks of the tekirova antalya ophiolite these are 1 iat series which can be referred to the group i isolated dikes and isotropic gabbros 2 low ti boninitic series characterized by the group ii isolated dike and isotropic gabbros and 3 oib type including the group iii isotropic gabbros the geochemical evidence suggests that the crustal rocks of the tekirova antalya ophiolite were generated from a progressive source depletion from island arc tholeiites iat to boninites therefore a fore arc tectonic setting seems likely for the generation of the crustal rocks from the tekirova antalya ophiolite in the southern branch of neotethys during the late cretaceous the oib type alkaline isotropic gabbros are thought to have resulted from either 1 a late stage magmatic activity fed by melts that originated within an asthenospheric window due to slab break off or 2 subduction of a ridge system which generated oib source across the asthenospheric window that has been no influence of fluids from the subducted slab into the overlying mantle wedge shortly before the emplacement of the tekirova antalya ophiolite onto the tauride platform

miocene remagnetisation of carbonate platform and antalya complex units within the isparta angle sw turkey abstract a palaeomagnetic study has been carried out within the mesozoic and tertiary units of the relatively autochthonous carbonate platforms and the allochthonous deep sea volcanics and sediments of the antalya complex exposed around the isparta angle sw turkey the antalya complex is interpreted as a mosaic of carbonate platforms basinal sediments volcanic and ophiolitic rocks which formed within a southerly strand of the neotethyan ocean adjacent to gondwana the results indicate a widespread remagnetisation event negative fold tests show that the remanence at most sites is of secondary origin e g within the cirali lavas the magnetisation is carried by magnetite of presumed authigenic origin the remagnetisation event is believed to have occurred in the early middle miocene burdigalian langhian it was possibly triggered by the migration of orogenic fluids ahead of the advancing lycian nappes during their emplacement onto the carbonate platforms subsequent to remagnetisation a large segment of the isparta angle underwent an anticlockwise rotation of 30 this rotation is attributed to the overall convergence and bending of the hellenic arc and the final stages of emplacement of the lycian nappes during the late miocene in agreement with previous studies previously southerly palaeolatitudes were inferred from late triassic extrusives of the godene zone calbali dag unit the post folding magnetisation identified here within the cirali lavas of the godene zone to the south implies that these low palaeolatitudes result from the inappropriate application of structural tilt corrections the available data cannot be used to substantiate an origin for the antalya units south of the equator in the early mesozoic instead a position close to the northern margin of gondwana is indicated

precise 40ar 39ar ages from the metamorphic sole of the mersin ophiolite southern turkey abstract the mersin ophiolite is located on the southern flank of the e w trending central tauride belt in turkey it is one of the late cretaceous neotethyan oceanic lithospheric remnants the mersin ophiolite formed in a suprasubduction zone tectonic setting in southern turkey at the beginning of the late cretaceous the mersin ophiolite is one of the best examples in turkey in order to study reconstruction of ophiolite emplacement along the alpine himalayan orogenic belt 40ar 39ar incremental heating measurements were performed on seven obduction related subophiolitic metamorphic rocks hornblende separates yielded isochron ages ranging from 96 0 0 7 ma to 91 6 0 3 ma all errors 1 five of the seven hornblende age determinations are indistinguishable at the 95 confidence level and have a weighted mean age of 92 6 0 2 2 ma we interpret these ages as the date of cooling below 500 c intraoceanic thrusting occurred 4 ma soon after formation of oceanic crust the sole was crosscut by microgabbro diabase dikes less than 3 m y later the final obduction onto the tauride platform occurred during the late cretaceous early paleocene our new high precision ages constrain intraoceanic thrusting for a single ophiolite mersin in the tauride belt

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

tethyan vs cordilleran ophiolites a reappraisal of distinctive tectono magmatic features of supra subduction complexes in relation to the subduction mode abstract supra subduction zone ssz ophiolites deserve special attention because they represent fundamental markers of intraoceanic convergence and generation of new lithosphere above subduction zones moreover owing to their structural characteristics and location in the overriding plate these complexes are far better represented and preserved than mid ocean ridge basalt morb ophiolites in orogenic belts in terms of their structure tectonics and magmatic features ssz ophiolites may be classified in two main types 1 tethyan complexes such as those of the albanide hellenide belt which mostly consist of complete and extensive volcanic dyke plutonic and mantle sections with prevalent island arc tholeiitic iat affinity and the significant presence of boninites obducted as relatively intact lithospheric slabs onto collisional continental margins 2 cordilleran complexes including the circum caribbean ophiolites of guatemala cuba and venezuela mostly represented by dismembered sections of arc volcanic plutonic and subordinate mantle sequences with tholeiitic to calc alkaline iac magmatic affinity and acidic differentiates commonly associated with metamorphic subduction complexes and tectonically emplaced onto or juxtaposed against the continental margin within polygenetic terranes these two types appear to be related to significantly different subduction modes and intraoceanic plate dynamics whereby ssz ophiolites were generated the tethyan complexes can best be compared with west pacific type subduction with accentuated steepening and retreat of the subducted slab accompanied by progressive decoupling of the converging plates intense mantle diapirism and tensional events in the upper plate with generation of large iat sheeted dike complexes in open oceanic spreading systems continuous slab sinking and roll back allow increasing asthenospheric diapirism and interarc spreading from the arc axis to the forearc region with generation of boninites and back arc basin basalts babb with intermediate morb iat composition by contrast the genesis of the cordilleran complexes requires a subduction mode characterized by a steady state regime with moderate and constant dip of the subducted slab and limited extension in the backarc region the magmatic evolution of these ophiolites from iat to iac and the significant presence of rhyodacite and tonalite differentiates coherently indicate a more mature stage of arc magmatism as well as the occurrence of efficient differentiation processes developing under nearly closed system conditions in independent magma chambers tethyan complexes are likely to be obducted as large and relatively intact slabs onto the continental margins through the interposition of metamorphic soles which represent relics of the morb lithosphere underplating the ssz ophiolites on the other hand the common emplacement of cordilleran complexes within polygenetic terranes appears to be controlled by prolonged accretionary mechanisms that trap against the continental margin parts of the arc structure subduction complexes melanges and volcanoclastic products

the sole of an ophiolite the ordovician bay of islands complex newfoundland the early ordovician c 485 ma bay of islands ophiolite complex was obducted onto the laurentian rifted margin as the fore arc of an oceanic arc that collided with the margin during the mid ordovician c 470 ma the subduction zone was nucleated on an oceanic transform fracture zone part of whose remnants occur as the polyphase deformed and intruded mafic and ultramafic rocks of the coastal complex the ophiolite formed as a suprasubduction zone fore arc ophiolite at the spatial and temporal continuation of the ridge normal to the transform fracture zone subduction zone system and ne of a trench trench ridge triple junction a two pyroxene garnet granulite garnet amphibolite epidote amphibolite mafic metamorphic sole at the base of the ophiolite was generated roughly synchronously with the ophiolite by the metamorphism of mid ocean ridge basalt mafic rocks in the descending slab at about 10 kbar and quickly attached to the base of the overlying ophiolite during slab flattening and not by subduction zone extrusion the metamorphic sole is not a metamorphic aureole at the base of a hot obducting ophiolite plate slip vector triangles around the triple junction before collision and during obduction are constructed from the orientation of dykes in the sheeted complex and the trends of structures in the high to lower temperature parts of the sole and the obducted nappes of oceanic and continental margin rocks beneath the ophiolite linear structures in the sole amphibolites trend nnw the subduction direction those in the greenschist facies obducted oceanic and continental margin rocks trend wsw the obduction direction

k ar dating of some infra ophiolitic metamorphic soles from the eastern mediterranean new evidence for oceanic thrustings before obduction abstract k ar dating on separated minerals from the infra peridotitic metamorphic soles outcropping beneath various eastern mediterranean ophiolite complexes pindos nappe in greece lycian antalya beysehir hoyran mersin and pozanti karsanti nappes in the taurides belt turkey baer bassit nappe in syria shows that the main metamorphic recrystallization of those rocks occurred 25 30 m y prior to the tectonic emplacement of the ophiolite complexes onto a continental platform these data support the hypothesis of an early slicing of the oceanic crust before its obduction advocated recently by several authors

counterclockwise p t t trajectory from the metamorphic sole of a neo tethyan ophiolite turkey abstract the kiziltepe ophiolitic thrust sheet in the bolkar mountains of turkey occurs between two subparallel ophiolite belts bounding the tauride carbonate platform and represents a remnant of the cretaceous neo tethyan oceanic lithosphere it is underlain by foliated amphibolite that represents a metamorphic sole developed at the inception of an intra oceanic subduction zone in the neo tethys 92 90 ma blueschist facies overprinting of the amphibolite indicates that the metamorphic sole was dragged deeper into the subduction zone where it experienced increasing p t with cooling regional tectonic constraints suggest a maastrichtian age for the timing of this blueschist facies metamorphism sodic amphibole rich veins and crossite mg riebeckite rims on hornblende suggest that growth of blueschist facies minerals was facilitated by infiltration of fluid along fractures and grain boundaries we infer a counterclockwise p t t trajectory during which metamorphism was accompanied succeeded by rapid uplift along the northern edge of the tauride belt in late cretaceous early tertiary time

dynamics of intraoceanic subduction initiation 1 oceanic detachment fault inversion and the formation of supra subduction zone ophiolites subduction initiation is a critical link in the plate tectonic cycle intraoceanic subduction zones can form along transform faults and fracture zones but how subduction nucleates parallel to mid ocean ridges as in e g the neotethys ocean during the jurassic remains a matter of debate in recent years extensional detachment faults have been widely documented adjacent to slow spreading and ultraslow spreading ridges where they cut across the oceanic lithosphere these structures are extremely weak due to widespread occurrence of serpentine and talc resulting from hydrothermal alteration and can therefore effectively localize deformation here we show geochemical tectonic and paleomagnetic evidence from the jurassic ophiolites of albania and greece for a subduction zone formed in the western neotethys parallel to a spreading ridge along an oceanic detachment fault with 2 d numerical modeling exploring the evolution of a detachment ridge system experiencing compression we show that serpentinized detachments are always weaker than spreading ridges we conclude that owing to their extreme weakness oceanic detachments can effectively localize deformation under perpendicular far field forcing providing ideal conditions to nucleate new subduction zones parallel and close to or at spreading ridges direct implication of this is that resumed magmatic activity in the forearc during subduction initiation can yield widespread accretion of suprasubduction zone ophiolites at or close to the paleoridge our new model casts the enigmatic origin of regionally extensive ophiolite belts in a novel geodynamic context and calls for future research on three dimensional modeling of subduction initiation and how upper plate extension is associated with that

origin and emplacement of the late cretaceous baer bassit ophiolite and its metamorphic sole in nw syria abstract the baer bassit ophiolite of inferred late cretaceous age was emplaced from the south tethys ocean onto the leading edge of the arabian continental margin in latest cretaceous maastrichtian time dismembered sequences in different thrust sheets can be correlated to produce a complete ophiolite sequence with a metamorphic sole at the base overlain in turn by upper mantle tectonite rare cumulates massive and layered gabbros localized high level plagiogranites sheeted dykes basic extrusives and minor fe mn sediments umbers the restored ophiolite sequence is similar to that of the more intact troodos and hatay ophiolites but dissimilar to oman the baer bassit extrusives are magnesian and strongly depleted comparable to primitive island arc tholeiites and some boninitic lavas which favours a subduction related origin a well developed metamorphic sole is divisible into high grade amphibolite facies assemblages in the north and lower grade greenschist facies assemblages in the centre of the region but no complete inverted metamorphic gradient is preserved in any one local area the protoliths of both the high grade amphibolites and lower grade rocks greenschists are interpreted as alkali basalts and pelagic sediments including chert similar to the volcanic rocks in an underlying unmetamorphosed melange the metamorphic fabrics progressively evolved from ductile to brittle during tectonic transport towards the southeast most of the metamorphic fabrics evolved during intense shearing but some of the alkaline metabasites apparently were metamorphosed under relatively static strain conditions the apparent absence of preserved inverted metamorphic gradients in baer bassit may reflect a complex deformation history during emplacement alternatively differing p t conditions may be recorded at different depths at the base of the over riding mantle wedge during emplacement the front of the ophiolite was tectonically imbricated and overthrust by the main ultramafic slab bassit massif the thickest massif baer is underlain by a high grade metamorphic sole and was internally shortened but without thrust duplication of the entire ophiolite sequence following covering by late maastrichtian palaeogene marine calcareous sediments the area was subjected to mid tertiary regional folding this was followed by neogene dominantly left lateral strike slip deformation along the african eurasian plate boundary extending from south of cyprus to the dead sea transform fault as a result the originally emplaced thrust sheets were dissected into three main composite units baer bassit and the southeastern units separated by strongly faulted and sheared ophiolitic blocks and unmetamorphosed volcanic sedimentary melange

evolution of boninites and island arc tholeiites in the pindos ophiolite greece whole rock geochemical and sm nd isotope data are presented for a representative suite of crustal rocks from the pindos ophiolite in order to resolve the origin of the geochemical signature of the boninites comparison is made with triassic morb from the avdella melange and with other ophiolitic rocks of northwestern greece hydrothermal alteration results in large scatter in sr and k and some variability in ba th and u the pindos boninites contain high zr and hf with respect to ree characteristic of many boninites pb la sm and nb decrease with decreasing tio 2 from morb to iat but then increase in the boninites nd isotopic values expressed as e nd decrease systematically with decreasing tio 2 from 7 8 in iat to 0 6 3 0 in boninites as mantle wedge harzburgite became increasingly depleted another magma source contributed significant amounts of pb ree and probably nb the pb and other large ion lithophile elements may have been transported in aqueous solutions from the subducting slab but the ree and nb imply an ocean island basalt oib type source within the mantle wedge this oib source is a consequence of mantle plume activity during late triassic rifting

the ophiolite related mersin melange southern turkey its role in the tectonic sedimentary setting of tethys in the eastern mediterranean region the mersin melange underlies the intact mersin ophiolite and its metamorphic sole to the south of the mesozoic tauride carbonate platform in southern turkey the melange varies from chaotic melange to broken formation in which some stratigraphic continuity can be recognized based on study of the broken formation four lithological associations are recognized 1 shallow water platform association dominated by upper palaeozoic lower cretaceous neritic carbonates 2 rift related volcanogenic terrigenous pelagic association mainly upper triassic andesitic acidic volcanogenic rocks siliciclastic gravity flows basinal carbonates and radiolarites 3 within plate type basalt radiolarite pelagic limestone association interpreted as upper jurassic lower cretaceous seamounts with associated radiolarian sediments and upper cretaceous pelagic carbonates 4 ophiolite derived association including fragments of the upper cretaceous mersin ophiolite and its metamorphic sole locally the ophiolitic melange includes granite that yielded a k ar radiometric age of 375 7 10 5 ma late devonian this granite appears to be subduction influenced based on immobile element composition the mersin melange documents the following history 1 triassic rifting of the tauride continent 2 jurassic cretaceous passive margin subsidence 3 oceanic seamount genesis 4 cretaceous supra subduction zone ophiolite genesis 5 late cretaceous intra oceanic convergence metamorphic sole formation and 6 latest cretaceous emplacement onto the tauride microcontinent and related backthrusting regional comparisons show that the restored mersin melange is similar to the beysehir hoyran nappes further northwest and a northerly origin best fits the regional geological picture these remnants of a north neotethys inner tauride ocean were formed and emplaced to the north of the tauride carbonate platform they are dissimilar to melanges and related units in northern syria western cyprus and southwestern turkey which are interpreted as remnants of a south neotethys early high temperature ductile transport lineations within amphibolites of the metamorphic sole of the mersin ophiolite are generally orientated e w possibly resulting from vertical axis rotation of the ophiolite while still in an oceanic setting by contrast the commonly northward facing later stage brittle structures are explained by backthrusting of the ophiolite and melange related to exhumation of the partially subducted northern leading edge of the tauride continent

zircon reaction and stability of the u pb isotope system during interaction with carbonate fluid experimental hydrothermal study the interpretation of metamorphically induced u pb isotopic discordance requires a thorough understanding of zircon fluid interactions with this aim we have studied the behaviour of metamict and crystalline zircon phases and their u pb systems by cathodoluminescence after treatment by 2m na2co3 solution at t 200 800 c and p 1 5 kbar for 3 14 days x ray diffraction microprobe and isotope dilution analysis the data indicate that zircon transformation under hydrothermal conditions depends on the experimental conditions and the degree of structural damage reconstitution of defective and impurity enriched zones of metamict zircon homogenization of impure element concentrations and increase of crystallinity was observed at 400 c and p 1 kbar considerable lead and uranium loss occurred under these conditions as a result of zircon dissolution newly formed baddeleyite accommodating u from 2m na2co3 solution and zr na silicate were recognized this process intensified with increasing pressure study of crystalline zircon indicates that migration of u and pb took place only during dissolution of zircon at t above 650 c in the presence of carbonate ions essential u and pb amounts are lost from metamict zircon at a lower p t than is typical for greenschist facies metamorphism

microatoll record for large century scale sea level fluctuations in the mid holocene coral microatolls have been long used as precise indicators of past sea level but their use for precise definition of detailed sea level fluctuations is still rare here we report twelve high precision thermal ionization mass spectrometric th 230 ages for twelve rims of five mid holocene microatolls from all emerged reef terrace at leizhou peninsula northern south china sea this is a tectonically stable area enabling lis to reconstruct both the timing and trajectory of local sea level fluctuations accurately the elevations of these microatoll rims and cores were accurately determined relative to the surface of modern livin microatolls at the same site the results indicate that the sea level during the period of 7050 6600 yr bp years before ad 1950 was about 171 to 219 cm above the present with at least four cycles of fluctuations over this 450 yr interval sea level fluctuated by 20 40 cm on century scales crown copyright c 2009 published by university of washington all rights reserved

isothermal plateau fission track ages of hydrated glass shards from silicic tephra beds abstract the age of silicic tephra beds that have experienced a simple thermal history can be accurately and precisely determined by application of the isothermal plateau fission track technique to hydrated glass shards age estimates obtained by this means compare very well with those based on co existing mineral phases using the k ar 40ar 39ar external detector fission track and thermoluminescence techniques this concordance contradicts the popular belief that hydrated glass shards give unreliable ages distal tephra beds many of which are difficult to date because of their fine grainsize low abundance of crystals and common presence of detrital grains are readily dated by the isothermal plateau glass fission track method this new advance should greatly facilitate the development of detailed chronologies of tephra bearing sedimentary sequences located far from volcanic centres

subduction zone infancy examples from the eocene izu bonin mariana and jurassic california arcs a new model for the earliest stages in the evolution of subduction zones is developed from recent geologic studies of the izu bonin mariana ibm arc system and then applied to late jurassic ophiolites of cailfornia the model accounts for several key observations about the earliest stages in the evolution of the ibm system 1 subduction nucleated along an active transform boundary which separated younger less dense lithosphere in the west from older more dense lithosphere to the east 2 initial arc magmatic activity occupied a much broader zone than existed later 3 initial magmatism extended up to the modern trench over a region now characterized by subnormal heat flow 4 early are magmatism was characterized by depleted tholeiitic and ultra depleted boninitic magmas indicating that melting was more extensive and involved more depleted mantle than is found anywhere else on earth 5 early arc magmatism was strongly extensional with crust forming in a manner similar to slow spreading ridges and 6 crust production rates were 120 to 180 km 3 km ma several times greater than for mature arc systems these observations require that the earliest stages of subduction involve rapid retreat of the trench we infer that this resulted from continuous subsidence of denser lithosphere along the transform fault this resulted in strong extension and thinning of younger more buoyant lithosphere to the west this extension was accompanied by the flow of water from the sinking oceanic lithosphere to the base of the extending lithosphere and the underlying asthenosphere addition of water and asthenospheric upwelling led to catastrophic melting which continued until lithosphere subsidence was replaced by lithospheric subduction application of the subduction zone infancy model to the late jurassic ophiolites of california provides a framework in which to understand the rapid formation of oceanic crust with strong arc affinities between the younger sierran magmatic arc and the franciscan subduction complex provides a mechanism for the formation and subsidence of the great valley forearc basin and explains the limited duration of high t high p metamorphism experienced by franciscan melanges

tectonic significance of the geochemistry and petrology of ophiolites in southeast anatolia turkey abstract the southeast anatolian ophiolites form discontinuous linear belts of oceanic fragments immediately north of the bitlis zagros suture which marks a continental collision zone between the arabian platform to the south and the taurides to the north the late cretaceous ophiolites in the southeast anatolia are represented by the kizildag in the hatay area the goksun and the berit to the north of kahramanmaras ispendere in the malatya area and komurhan guleman in the elazig area the goksun berit ispendere komurhan and guleman ophiolites were attached to the base of the tauride platform i e the malatya keban along its northern active margin and intruded by the i type calc alkaline granitoids of late cretaceous age in contrast the kizildag hatay ophiolite was thrust over the arabian passive margin in the south the cumulate rocks in the ophiolites are represented by dunite wehrlite lherzolite olivine clinopyroxenite olivine gabbronorite olivine gabbro gabbronorite and gabbro highly magnesian olivines fo88 to 74 and pyroxenes mg 95 to 60 as well as highly calcic plagioclases an95 to 68 from the cumulate rocks differ from oceanic equivalents which mainly formed from a mor basaltic melt the order of crystallization in mineral phases whole rock and mineral chemistry data from the cumulates suggest that the primary magma is compositionaly similar to that observed in modern island arc tholeiitic sequences the volcanic and subvolcanic rocks exhibit tholeiitic compositions chondrite normalized ree n morb normalized multi element patterns and tectonic discrimination diagrams suggest the existence of two main types of parental basic magmas in the crustal rocks of the se anatolian ophiolites these are i iat series that characterize the kizildag goksun ispendere komurhan and guleman ophiolites and ii low ti boninitic series known only in the kizildag ophiolite the geochemistry of the crustal rocks suggests that they formed in a suprasubduction zone tectonic setting including arc forearc environments in the southern neotethys these well preserved plutonic oceanic crustal remnants mainly exhibit an intact ophiolite pseudostratigraphy and are overlain by a volcanic sedimentary unit made up of alternations of basic to acidic extrusive rocks debris flows volcaniclastic sandstones and pelagic limestones interpreted as a tholeiitic ensimatic island arc assemblage built on the suprasubduction zone type crust the metamorphism of some of the ophiolites i e berit in the se anatolia might be related to later stages of intraoceanic subduction during the evolution of the southern neotethys

tectonic development of the samail ophiolite high precision u pb zircon geochronology and sm nd isotopic constraints on crustal growth and emplacement 1 new high precision single grain u pb zircon geochronology and whole rock nd isotopic data provide insight into the magmatic and tectonic development of the samail ophiolite the analyzed rocks can be broadly divided into two groups based on their structural position dates and isotopic composition an older group related to on axis magmatism and a younger group of post ridge dikes sills and stocks on axis gabbros tonalites and trondhjemites yielded th corrected 206pb 238u dates from 96 441 0 062 to 95 478 0 056 ma these dates combined with dates from rioux et al 2012 suggest that most of the ophiolite crust formed at an oceanic spreading center in 1 ma the post ridge intrusions come from all depths in the crust the upper mantle and the metamorphic sole post ridge gabbros tonalites and trondhjemites from the crust and mantle yielded th corrected 206pb 238u dates of 95 405 0 062 to 95 077 0 062 ma a small trondhjemitic pod from the metamorphic sole yielded younger th corrected 206pb 238u dates of 94 90 0 38 to 94 69 0 12 ma isotopic data suggest two distinct sources for the post ridge magmas five of the gabbros and tonalites from the crust have end 96 ma 6 90 0 12 to 7 88 0 16 and two trondhjemites from the upper mantle and metamorphic sole have end 96 ma 7 77 0 08 and 7 01 0 16 the negative end t and u pb dates from the mantle dike require that subduction or thrusting was established below the ophiolite 0 25 0 5 ma after formation of the crust the bimodal isotopic composition of post ridge magmas may reflect coeval decompression and or fluid fluxed melting of the mantle and melting dehydration or assimilation of sediment in the down going plate at this time the new data place temporal constraints on mid ocean ridge and supra subduction zone models for ophiolite formation

the geochemistry of postkinematic mafic dike swarms and subophiolitic metabasites pozanti karsanti ophiolite turkey evidence for ridge subduction doleritic and gabbroic dike swarms cut the pozanti karsanti ophiolite and basal metamorphic sole but do not extend into the underlying melange or platform carbonates dike emplacement therefore postdated formation of the ophiolite and metamorphic sole but predated final obduction onto the tauride anatolide platform the dikes are compositionally similar to island arc basalts and basaltic andesites and were affected by retrograde static hydrothermal metamorphism over temperatures ranging from 900 c to below 450 c geochemical modeling of trace elements including rees show that the dikes are possibly fractionated products of multiple compositionally variable parental magmas the range of parental magmas is interpreted to represent variably mixed melt increments generated through polybaric fractional or continuous melting within a column of rising mantle material melt increments extracted from shallower parts of the melting column at lower pressures are more refractory and apparently pooled less efficiently than melt fractions generated from deeper sources and higher pressures amphibolite and greenschist facies metabasalt from the subophiolitic sole represent metamorphosed alkalic basalt and morb like basalt respectively the pozanti karsanti ophiolite probably constituted part of the fore arc mantle wedge directly above an intra oceanic subduction zone whereas the metamorphic sole represents material progressively transferred and accreted to the base of the ophiolite from the downgoing oceanic plate during subduction since late stage pre obduction dike swarms transect the metamorphic sole a probable subduction plane melt generation must have occurred below the wadati benioff zone and not in the overlying mantle wedge a probable source of parental liquids for the dikes may therefore be a subducting neo tethyan spreading center

dynamics of intraoceanic subduction initiation 2 suprasubduction zone ophiolite formation and metamorphic sole exhumation in context of absolute plate motions analyzing subduction initiation is key for understanding the coupling between plate tectonics and the underlying mantle here we focus on suprasubduction zone ssz ophiolites and how their formation links to intraoceanic subduction initiation in an absolute plate motion frame ssz ophiolites form the majority of exposed oceanic lithosphere fragments and are widely recognized to have formed during intraoceanic subduction initiation structural petrological geochemical and plate kinematic constraints on their kinematic evolution show that ssz crust forms at fore arc spreading centers at the expense of a mantle wedge thereby flattening the nascent slab this leads to the typical inverted pressure gradients found in metamorphic soles that form at the subduction plate contact below and during ssz crust crystallization former spreading centers are preserved in forearcs when subduction initiates along transform faults or off ridge oceanic detachments we show how these are reactivated when subduction initiates in the absolute plate motion direction of the inverting weakness zone upon inception of slab pull due to e g eclogitization the sole is separated from the slab remains welded to the thinned overriding plate lithosphere and can become intruded by mafic dikes upon asthenospheric influx into the mantle wedge we propound that most ophiolites thus formed under special geodynamic circumstances and may not be representative of normal oceanic crust our study highlights how far field geodynamic processes and absolute plate motions may force intraoceanic subduction initiation as key toward advancing our understanding of the entire plate tectonic cycle

k ar ages of the metamorphic sole of the semail ophiolite implications for ophiolite cooling history k ar ages have been determined on micas and hornblendes in the basal metamorphic sequence and in metamorphic rocks squeezed into the mantle sequence of the semail ophiolite the hornblende ages of 99 0 5 and 102 0 8 ma and the 90 ma ages of coexisting micas from the high grade metamorphic portion of the sequence are interpreted as cooling stages following the peak of metamorphism t 800 850 c p 6 5 9 kbar the new pressure estimates are based on findings of kyanite in garnet amphibolite and cordierite in quartzitic rocks these data indicate a cooling rate of 10 30 c ma the oldest mica ages of 95 1 ma are observed in the lowest grade greenschists these also largely represent cooling ages but might in part also include formation ages the pattern of the muscovite ages across the metamorphic sole indicates that the cooling front moved from the low grade metamorphic zone through the high grade rocks and into the base of the overlying ophiolite radiometric ages of hornblendes 92 3 0 5 and 94 8 0 6 ma indicate that the crustal gabbro sequence cooled below 500 c later than the base of the ophiolite sequence metamorphism of the sole rocks occurred during subduction of oceanic sediments and volcanic or gabbroic rocks as they progressively came into contact with hotter zones at the base of the overriding plate the peak of metamorphism must have been contemporaneous with the main magmatism in the semail ophiolite one of the dated muscovites yields an age of 81 3 0 8 ma but this is related to discrete deformation zones that were active during late stage emplacement of the ophiolite

the role of deformation in the formation of metamorphic gradients ridge subduction beneath the oman ophiolite two tectonic scenarios have been proposed for genesis and emplacement of the oman ophiolite one suggests that the ophiolite was generated at a spreading center the other suggests generation within an intraoceanic arc an integrated thermal and kinematic model of the temperature stress rock type and displacement fields during early stages of the emplacement of the oman ophiolite was developed to test these two possibilities the thermal evolution was calculated by a finite difference algorithm for heat conduction considering heats of metamorphic reactions deformational heating heat advection by flowing rock mantle heat flow and radioactive heating the stress and displacement fields were calculated by an analytical model using a velocity boundary condition power law constitutive relations and a brittle frictional sliding relationship field observations in oman can be satisfied with the spreading center model but not with the arc model moreover simulations indicate that the ophiolite was probably 2 m y old at the time of intraoceanic thrusting and that shear stresses of 100 mpa were attained during thrusting in addition to satisfying currently available field constraints these simulations also indicate specific further work that will help resolve the controversy over thevolution of this unique orogen future work should be directed toward 1 using thermobarometry and thermochronology to constrain the spatial variation in ptt paths of the metamorphic sole 2 dating the igneous genesis of the ophiolite and 3 using quartzite and dunite recrystallized grain size paleopiezometers to infer stresses during thrusting these simulations point out the important effect synmetamorphic deformation can have on the formation of metamorphic field gradients in contractional fault zones metamorphic field gradients may be normal or inverted and contracted or extended relative to the initial thermobarometric gradient and adjacent fault zone rocks may display crossing pt paths unusually contracted or extended metamorphic field gradients may also develop in the footwalls of normal fault zones where portions of the heating hangingwall accrete to the uplifting cooling footwall

internal structures of zircons from archaean granites from the darling range batholith implications for zircon stability and the interpretation of zircon u pb ages internal structures in zircons from granitoids from the late archaean darling range batholith show secondary features revealed by hf etching which record reconstitution of the zircons and modification of the distribution of trace elements during post crystallisation cooling of the granitoid zircons from the granites commonly contain unzoned to weakly zoned cores surrounded by rims showing oscillatory zoning which has been modified by recrystallisation the most striking feature is the development of high trace element concentration areas found in zircons from a number of granites these structures range from enhanced trace element concentrations in primary zones to a single accumulation of most trace elements in one band about half way between the outer edge and the centre of the zircon in any zircon the extent of the concentration of trace elements towards the formation of a single trace element band appears to be inversely related to the fading and broadening of primary oscillatory zones in the outer rim this suggests that the trace element bands formed by migration of trace elements from the outer primary zones to new concentration sites on an inner set of primary zones this explanation is supported by the formation of multiple curved trace element bands that transgress primary zoning and the determination of younger shrimp ages on depleted zircon outer rims compared to remnant primary oscillatory zoned areas of the zircon and unzoned centres also observed in some granite zircons is a finely convoluted zoning which overprints oscillatory zoning in parts of a zoned zircon and in rare cases occurs throughout the zircon this structure is explained in terms of secondary migration and reconcentration of trace elements in curved bands all structures can be transgressed by generally rounded lobes and patches of low u weakly nebulously zoned zircon this is interpreted as a late stage interaction between the zircon and fluids formed during cooling and crystallisation of the granitoid resulting in recrystallisation of affected parts of the zircon with accompanying loss of trace elements from the zircon

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

development of concepts concerning the genesis and emplacement of tethyan ophiolites in the eastern mediterranean and oman regions this article uses evidence mainly from the eastern mediterranean and oman to help understand the origin and emplacement of ophiolites field studies in these regions emphasize the diversity of ophiolite related sedimentary and tectonic processes topics to be discussed here include the sedimentary covers of ophiolites their metamorphic soles related melanges and emplaced continental margin slope rise units modes of ophiolite emplacement will also be considered and tectonic reconstructions that have a direct bearing on the origin and emplacement of ophiolites the writer argues that there is compelling evidence that many ophiolites formed by spreading above intraoceanic subduction zones evidence from their intact sedimentary covers however indicates that such subduction was only incipient in most cases as directly associated volcanic arcs or back arc basins are rare metamorphic soles record fragments of oceanic lithosphere derived from settings ranging from open ocean to continental slope rise settings underlying melanges record the subduction of most of the ocean lithosphere formed at spreading ridges continental margin units beneath ophiolites demonstrate that these ophiolites were formed in ocean basins that were mainly initiated by continental rifting in the eastern mediterranean and oman regions ophiolite emplacement was commonly achieved by trench margin collision although strike slip transpression and large scale accretion along subducting margins played a role in some cases taking account of the mesozoic palaeotectonic setting of the eastern mediterranean region it is widely believed that ophiolites of two main ages mid jurassic and late cretaceous were formed in several small oceanic basins separated by rifted microcontinents the history of tethys in pre mesozoic time remains more open to question lastly giant subduction related ophiolites as in oman cyprus or the balkans may not be forming anywhere in the world ocean today but may instead relate to rare events in earth history when regional to global plate boundaries were drastically reorganised

the correlation between reduction in fission track diameter and areal track density in volcanic glass shards and its application in dating tephra beds abstract the relationship between the reduction in fission track diameter and the reduction in areal track density in hydrated glass shards has been investigated under varying annealing and etching conditions a near 1 1 relation is observed for adequately etched tracks following laboratory heating of induced tracks the partially reduced areal track density was renormalized with error

island arc elements and arc related ophiolites abstract all major structural elements in island arc systems fore arc magmatic arc back arc basins and remnant arcs are potential ophiolite sources and those features that allegedly characterise ophiolites of ocean ridge origin sheeted dyke complexes mantling pelagic rocks hydrothermal metamorphism and associated mineralization can also arise within arc settings age relations are critical in the interpretation of arc related ophiolites remnants of oceanic lithosphere identified by a pre arc initiation age are restricted to fore arc magmatic arc and remnant arc elements as are ophiolite masses generated at the inception of underthrusting the latter apparently common in ancient fore arc terrains form in nascent arc systems in which the rate of role back of the subduction hinge exceeds the rate of convergence spreading occurs above a foundering slab resulting in some arc like compositional features in simple arc systems later ophiolitic rocks have formed either in the active back arc basin or the magmatic arc only those ophiolites that have resided within or very close behind magmatic arcs should show calcalkaline or arc tholeiite magmatic affinities or be intruded or overlain by these rocks volcanic derived sediment or pelagic material may mantle ophiolites from all arc settings but pelagic rocks will generally dominate in stratigraphic sequences above remnant arcs and on back arc basin floors except adjacent to the magmatic arc ophiolites generated at major ocean ridges are unlikely to be immediately overlain by sediment with a significant volcanic component whereas such detritus may lie directly on arc inception arc and back arc ophiolites some arc derived ophiolites are preserved in their original tectonic position others can be identified from their internal features their relationship to other tectonic elements and the nature of associated rocks

jurassic accretionary complex and ophiolite from northeast turkey no evidence for the cimmerian continental ribbon permian triassic and late cretaceous accretionary complexes ascribed to the consumption of two distinct oceans the paleo and neo tethys are exposed over extensive areas in the eastern mediterranean region however a separating continental ribbon the so called cimmeride continent between the paleo and neo tethys during early mesozoic time cannot be defined here we report a previously unknown early jurassic metamorphic oceanic accretionary complex and ophiolite from northeast turkey bounded by oceanic accretionary complexes of permian triassic and late cretaceous age to the north and the south respectively without a continental domain in between this special tectonic position and widespread coexistence of permian triassic and late cretaceous accretionary complexes alongside the izmir ankara erzincan suture imply that 1 the southern margin of laurasia in the eastern mediterranean region grew by episodic accretionary processes from late paleozoic to end mesozoic time without involvement of a cimmerian continental ribbon and 2 the paleo tethys and northern branch of the neo tethys were not distinct oceans in the eastern mediterranean region

late triassic rifting and jurassic cretaceous passive margin development of the southern neotethys evidence from the adiyaman area se turkey evidence of rifting and continental break up to form the s neotethys is found within the volcanic sedimentary kocali complex this is a folded thrust imbricated succession that includes lavas volcaniclastic sediments pelagic carbonates radiolarites and manganiferous deposits interbedded ribbon cherts contain radiolarians of late triassic to late jurassic age the lower part of the succession of mid late triassic age tarasa formation is dominated by enriched mid ocean ridge basalt e morb the overlying late triassic to mid jurassic interval konak formation is characterised by intercalations of ocean island basalt and e morb taking account of structural position the basalts erupted within the outer part of a continent ocean transition zone continental break up probably occurred during the late triassic carnian norian early to mid jurassic lavas and volcaniclastic sediments record volcanism probably after continental break up in addition the karadut complex is a broken formation that is located at a relatively low structural position just above the arabian foreland pelagic carbonates redeposited carbonates and radiolarites predominate radiolarians are dated as early to mid jurassic and late cretaceous in age the pelagic carbonates include planktic foraminifera of late cretaceous age the karadut complex resulted from the accumulation of calcareous gravity flows pelagic carbonate and radiolarites in a relatively proximal base of slope setting after continental break up morb and ophiolitic rocks formed within the s neotethys further north tectonic emplacement onto the arabian platform took place by earliest maastrichtian time regional interpretation is facilitated by comparisons with examples of triassic rifting and continental break up in the eastern mediterranean region and elsewhere

metamorphic sole rocks and their mafic dykes in the eastern tauride belt ophiolites southern turkey implications for oib type magma generation following slab break off the metamorphic sole rocks observed between the pozanti karsanti ophiolite and the melange unit are located on the eastern part of the tauride carbonate platform they consist of ortho amphibolites at the top and metasedimentary lithologies at the base amphibolites from the metamorphic sole rocks are represented by oib morb and iat based on their major trace and ree compositions the isolated dolerite dykes intrude both the metamorphic sole rocks and the ophiolitic unit at different structural levels the dolerite dykes cutting the metamorphic sole rocks are enriched in lile and depleted in hfse enrichment in lile such as th relative to hfse is indicative of the presence of a subduction component flat lying ree patterns of the dolerite dykes also confirm an iat source pyroxenite and albitite dykes also cut the metamorphic sole rocks ree patterns of pyroxenite dykes show prominent lree enrichment similar to that observed in within plate alkaline basalts the alkaline isolated pyroxenite dykes were probably the result of late stage magmatism fed by melts that originated from an oib source shortly before the emplacement of the pozanti karsanti ophiolite onto the tauride carbonate platform a hydrothermal alteration stage is characterized by albitite formation in the joints of the metamorphic sole rocks and by secondary mineralization along the contact zones of dolerite dykes mineral parageneses in the metamorphic sole rocks exhibit amphibolite and greenschist facies assemblages geothermobarometric studies based on a newly recognized mineral assemblage e g kyanite and chemical compositions of minerals in the metamorphic sole rocks indicate that the metamorphic temperature during the metamorphism was 570 580 c and the pressure was around 5 6 kbar

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

hydrothermal anorthitization of plagioclase within the magmatic hydrothermal transition at mid ocean ridges examples from deep sheeted dikes hole 504b costa rica rift and a sheeted dike root zone oman ophiolite abstract the unique petrological record from the deep sheeted dikes of odp hole 504b gives insight into the magmatic hydrothermal transition and the hydrothermal reaction zone at mid ocean ridges a high temperature peak hydrothermal event at hole 504b defined as stage 1 corresponds to an early stage in the evolution of an axial reaction zone and perhaps to an initial high temperature phase of black smoker activity the temperatures and or the a ca a na 2 ratios of hydrothermal fluids overlapped and even exceeded those inferred for the reaction zones of many modern black smoker systems the volume of hydrothermally altered rock at this stage of evolution of the reaction zone was much less than 10 stage 1 is manifest in hand specimen by reaction halos adjacent to hornblende veins and by irregular alteration patches there is no epidote associated with stage 1 wallrock reactions involved the replacement of magmatic clinopyroxene by magnesiohornblende and of magmatic plagioclase by hydrothermal plagioclase these secondary minerals were not in equilibrium hydrothermal plagioclase varies in composition from an 54 to an 95 and has low mgo and feo tot it has distinctive bright cathodoluminescence and a porous appearance stage 1 at hole 504b occurred at pressures between 38 and 85 mpa 0 38 and 0 85 kbar and temperatures in excess of about 425 c these p t conditions closely approach the two phase field for seawater stage 1 is overprinted by retrograde alteration that caused further replacement within halos and patches as well as 10 40 alteration mostly to chlorite and albite of all the fresh sheeted dikes that were unaffected during stage 1 stage 1 alteration at hole 504b is unique among oceanic and ophiolitic samples however we report one area from the oman ophiolite that exhibits features similar to stage 1 at hole 504b early halos around hornblende veins contain secondary hornblende and porous hydrothermal calcic plagioclase an 55 an 78 with low mgo and feo tot fluid inclusions in quartz show that some early hydrothermal fluids had 5 6 2 3 wt nacl equivalent at temperatures constrained to be above 320 and below 510 c as well as a saline brine with 31 to 40 wt equivalent nacl and similar temperature constraints although evidence for stage 1 alteration of sheeted dikes is far from ubiquitous it is now identified in an oceanic setting hole 504b and in an ophiolitic example from oman

uranium lead isotopic ages of plagiogranites from the troodos ophiolite cyprus and their tectonic significance zircon populations from two plagiogranites in the plutonic section of the troodos ophiolite yield u pb dates between 90 3 0 7 and 92 4 0 7 ma zircons from one of the two samples yield u pb dates of 90 3 0 7 and 92 1 0 7 ma this difference is greater than our analytical error and the zircons are therefore considered to be slightly discordant two fractions from another sample are internally and externally concordant and have an average date of 91 6 1 4 ma this date is adopted as the age of plagiogranite crystallization the plutonic section of the troodos ophiolite is complex in that several generations of intrusion can be identified the plagiogranites occur at the top of the intrusive section and represent fractionated residua of magma chambers related to the lower pillow lavas they are possibly slightly older than small ultramafic layered complexes that represent the final intrusive episode within the troodos ophiolite there is good agreement between our results and the ages of radiolaria which occur in umbers intercalated with the upper pillow lavas of troodos blome and irwin 1985 this shows that the 75 to 85 ma k ar dates on the lavas and sheeted dikes desmet et al 1978 delaloye and desmet 1979 delaloye et al 1980 staudigel et al 1986 represent age resetting unrelated to ophiolite formation the troodos u pb zircon ages are slightly lower than u pb zircon ages from the samail ophiolite in oman which vary between 93 5 and 97 9 ma and average 95 ma tilton et al 1981 the small difference in age between the troodos and the samail ophiolites and the geochemical similarities of basalts from both complexes to island arc volcanic rocks imply that a 3000 km axis of rift related island arcs formed between 91 and 98 ma emplacement of the ophiolites followed during the early maastrichtian

simulation of the metamorphic and deformational history of the metamorphic sole of the oman ophiolite emplacement of ophiolites onto less dense continental crust is an enigma the rates of emplacement the mechanisms of emplacement and the stress and temperature histories of the rocks are all incompletely understood the oman ophiolite is well exposed and has been studied extensively by field geologists an integrated thermal and kinematic model of the temperature stress rock type and displacement fields during the early stages of the emplacement of the oman ophiolite is developed here to explain field relations and provide insight into the development of the metamorphic rocks at the base of the ophiolite the thermal evolution was calculated by a finite difference algorithm for heat conduction considering heats of metamorphic reactions deformational heating and heat advection by rock the stress and displacement fields were calculated by an analytical model using a velocity boundary condition power law constitutive relations and a brittle frictional sliding relationship in simulations involving young hot subducted lithosphere a wider portion of the subduction zone deforms by power law creep and deformation occurs at lower stresses and slower strain rates minimizing the deformational heating compared to colder older subducted lithosphere dehydration occurs at an accelerated rate in younger hotter lithosphere removing heat from the subducted plate at an earlier stage and causing the zone of brittle deformation to propagate downward more rapidly power law creep of subducted basaltic rocks is limited to temperatures greater than 500 c and occurs at differential stresses of the order of 100 mpa the simulations predict that metamorphic field gradients and the spatial distribution of rock types in the metamorphic sole of tethyan ophiolites might be used to infer the emplacement direction of the ophiolite

a 3 m y record of pliocene pleistocene loess in interior alaska many distal tephra beds exist in the thick fossiliferous loess deposits near fairbanks interior alaska isothermal plateau fission track ages determined on glass shards from tephra beds in conjunction with tephrostratigraphic and magnetostratigraphic techniques indicate that loess deposition began in the late pliocene an antiquity previously unsuspected hence there is the opportunity now to reconstruct a detailed well dated record of environmental changes in interior alaska during the past 3 m y

geochemical and petrological aspects of dike intrusions in the lycian ophiolites sw turkey a case study for the dike emplacement along the tauride belt ophiolites lycian ophiolites located in the western taurides are cut at all structural levels by dolerite and gabbro dikes the dolerite dikes from this area are both pristine and metamorphosed the non metamorphosed dikes are observed both in the peridotites and in the metamorphic sole rocks accordingly the non metamorphosed dikes cutting the metamorphic sole were generated after cooling of the metamorphic sole rocks the metamorphosed dolerite dikes are only observed in the peridotites the physical conditions and timing of the metamorphism for the metamorphosed dolerite dikes are similar to those of the metamorphic sole rocks of the lycian ophiolites suggesting that the metamorphosed dolerite dikes were metamorphosed together with the metamorphic sole rocks therefore the dike injections in the western part of the tauride belt ophiolites occurred before and after the generation of the metamorphic sole rocks all metamorphosed and non metamorphosed dikes are considered to have the same origin and all of them are subduction related as inferred from whole rock geochemistry and lead isotopes lead isotope compositions of whole rocks of both dike groups cluster in a narrow field in conventional pb isotope diagrams 206pb 204pb 18 40 18 64 207pb 204pb 15 56 15 58 208pb 204pb 38 23 38 56 indicating a derivation from an isotopically homogeneous source on the 207pb 204pb versus 206pb 204pb diagram isotope compositions of the dikes plot slightly below the orogen curve suggesting contributions from mantle reservoir enriched by subducted oceanic lithosphere such a signature is typical of island arc magmatic rocks and supports the formation of the investigated rocks in a subduction related environment

precise 40ar 39ar ages from the metamorphic sole rocks of the tauride belt ophiolites southern turkey implications for the rapid cooling history the tauride belt ophiolites in southern turkey are located on both sides of the e w trending mesozoic tauride carbonate platform they comprise the lycian antalya beysehir mersin alihoca and pozanti karsanti ophiolites from west to east each ophiolite has a metamorphic rock unit either at the base of the peridotites or in the melange units the metamorphic sole rocks generally consist of amphibolite at the top and near the contact with the overlying tectonized harzburgite of the ophiolites and mica schists mostly at the base near the tectonic contact with the underlying ophiolitic melange 40 ar 39 ar measurements from the metamorphic sole rocks of the lycian antalya and beysehir ophiolites are the first precise ages dating intra oceanic thrusting and the cooling age history during the closure of the neotethyan ocean amphiboles and white micas from the metamorphic sole rocks of the ophiolites yielded 40 ar 39 ar ages between 90 7 0 5 ma and 93 8 1 7 ma and between 91 2 2 3 ma and 93 6 0 8 ma respectively hornblende plateau ages from the amphibolites of the lycian ophiolites near koyce iz agree with those of antalya indicating that they were metamorphosed simultaneously in the neotethyan ocean the white micas display plateau ages concordant with the amphiboles from the same units in koyce iz and yesilova lycian ophiolites and from the pozanti karsanti ophiolite suggesting that the metamorphic sole rocks were rapidly cooled after their generation

exhumation of blueschists along a tethyan suture in northwest turkey a blueschist belt formed during the mid cretaceous 88 ma high pressure metamorphism of a passive continental margin sequence of shale siltstone quartzite and limestone occurs immediately south of the tethyan suture in northwest turkey a transect across the tethyan suture was mapped to constrain the exhumation of blueschist facies assemblages the presence of jadeite glaucophane and lawsonite in the blueschists indicates p t conditions of 20 kbar and 430 c the blueschists are tectonically overlain by a cretaceous oceanic accretionary complex showing an incipient blueschist metamorphism a peridotite slab with subvertical layering lies along a low angle fault contact over the blueschists and the accretionary complex isolated slices of early cretaceous 101 4 ma garnet amphibolites occur at the base of the peridotite the garnet amphibolites which show an incipient blueschist facies overprint have probably formed during the oceanic subduction in the foot wall of the subduction zone and later underplated the hanging wall blueschists and peridotites are abruptly truncated in the north by the steeply dipping suture fault that juxtaposes unmetamorphosed and slightly deformed triassic to jurassic sediments of the northern continent against the peridotite and the accretionary complex tectonic juxtaposition of the lower pressure accretionary complex and peridotites over the higher pressure blueschists implies the removal of a 45 km thick oceanic mantle sequence above the blueschists regional geological and geometric constraints suggest that the exhumation of the blueschists was achieved by a combination of two simultaneously acting mechanisms the first was the detachment of the upper crustal blueschist sequence from its basement in the subduction zone and its incorporation to the hanging wall followed by buoyant ascent the second mechanism is the progressive shallowing of the subducted continental lithosphere resulting in the thinning of the mantle wedge above the subducted slab the blueschists were partly exposed prior to the paleocene continent continent collision post collisional eocene 52 48 ma calc alkaline plutons that were emplaced in the blueschists and the overlying peridotite at 10 km depth are interpreted to have formed by crustal anatexis from advective heating by mantle melts these have probably been generated during the upwelling of the asthenosphere under the shallowing continental lithosphere

rapid emplacement of young oceanic lithosphere argon geochronology of the oman ophiolite 40 ar 39 ar dates of emplacement related metamorphic rocks beneath the samail ophiolite in oman show that cooling to

structure and petrology of tauride ophiolites and mafic dike intrusions turkey implications for the neotethyan ocean cretaceous neotethyan ophiolites occur in four east west trending subparallel zones within the tauride tectonic belt in southern turkey the ophiolites of the inner intermediate and outer zones tectonically overlie the mesozoic platform carbonates of the tauride belt and are commonly underlain by a cenomanian ophiolitic melange these ophiolites consist mainly of tectonized mantle rocks mafic ultramafic cumulates and gabbros and commonly lack sheeted dike complexes and extrusive rocks of a complete ophiolite sequence metamorphic soles that are several hundred meters thick occur as thrust faulted slices beneath these ophiolites and show well developed metamorphic field gradients ophiolitic units and the metamorphic soles are intruded by mafic dike swarms that are truncated at the contact with the underlying melange unit dike rocks are made of subalkalic basalt to andesite typical of evolved island arc tholeiites they display large compositional variations with sio 2 content between 50 and 60 wt and mgo between 8 and 4 wt and contain higher ti augite phenocrysts and significantly less calcic plagioclase than their host cumulates the majority of the analyzed dike rocks show a slight depletion in light rare earth elements ree with low la smn ratios and are depleted in both high field strength hfs and heavy rees while enriched in large ion lithophile elements lile relative to normal mid ocean ridge basalt morb these characteristics suggest a mantle source that underwent previous melt extractions and subsequent metasomatism by lile and light ree enriched fluids geochemical modeling of trace elements shows that melting occurred at relatively low pressures under hydrous conditions and that it may have required the existence of an asthenospheric window in which the dike magmas developed through tapping and mixing of melts generated within a rising melting column starting slightly within the garnet stability field or in a transitional zone between the garnet and spinel stability fields at about 60 km depth this asthenospheric window was probably created during subduction of a neotethyan ridge system magmas ascending from the melt column within this window generated dikes that crosscut the metamorphic soles and were injected into the overlying mantle wedge and oceanic lithosphere the new 40 ar 39 ar hornblende dates of 92 90 ma and 90 91 ma from the metamorphic soles and dike swarms respectively show that evolution of these two geologic units was closely related in time and space and that they formed at the same intraoceanic subduction zone within the inner tauride seaway these data suggest that the tauride ophiolites within the three zones to the north originated from the same root zone situated north of the tauride carbonate platform and that they constitute remnants of a single ophiolitic nappe sheet derived from the inner tauride seaway within the neotethyan ocean

early middle jurassic intra oceanic subduction in the izmir ankara erzincan ocean northern turkey abstract the izmir ankara erzincan suture zone near cankiri comprises an ophiolite related rock suite of amphibolite pyroxene amphibolite and epidote amphibolite the geochemical data reveal that the basaltic protoliths of the cankiri amphibolite suite are derived from different mantle sources amphibolites showing a lree enrichment in chondrite normalized ree pattern relatively high la n yb n 8 5 13 1 sm yb 2 8 3 7 ce sm 8 8 9 8 nb yb 14 4 20 5 th nb 1 1 2 6 originate from an oib like type i amphibolites mantle source those with flat chondrite normalized ree pattern low la n yb n 0 6 sm yb 1 0 ce sm 2 7 nb yb 0 6 suggest derivation from a depleted morb like type ii amphibolites mantle source a third type of amphibolite is characterized by la n yb n 1 54 sm yb 1 1 ce sm 4 5 nb yb 1 1 and th nb 0 3 it is derived from an enriched morb source and shows characteristics similar to island arc basalts amphiboles from these rocks yielded 40 ar 39 ar plateau ages between 177 08 0 96 ma and 166 9 1 1 ma these dates are interpreted as metamorphic ages of the amphibolitic rocks they differ significantly from previous ages of amphibolitic rocks the metamorphic sole rocks related to turkish ophiolites however similar metamorphic ages have been reported for metamorphic sole rocks of the hellenic dinaric ophiolites further west the metamorphic sole rocks have been generated in intra oceanic environments via intra oceanic subduction thrusting events the new age data of the cankiri amphibolite suite suggest that the izmir ankara erzincan ocean was subducted around early middle jurassic times i e roughly coeval with the oceanic basin s generating the hellenic dinaric ophiolites in the early middle jurassic time the tectonic regime in the izmir ankara erzincan ocean was compressional and the ocean was closing at that time the new age data presented here could imply that jurassic granites which cross cut basement rocks of the pontides are related to the northward subduction of the izmir ankara erzincan ocean during jurassic time

isothermal plateau correction for partial fading of fission tracks in hydrated glass shards abstract the age of hydrated glass shards from the pleistocene ester ash bed at fairbanks alaska is determined using the population subtraction fission track method the age so determined is then corrected for partial annealing by use of the isothermal plateau fission track itp ft technique in this paper we describe the mathermatical basis of a procedure for obtaining the correct fission track age using the itp ft technique

isothermal plateau fission track age of the late pleistocene old crow tephra alaska hydrated glass shards of the widespread old crow tephra in alaska have been dated by the isothermal plateau fission track method which corrects for partial track fading if present the weighted mean age of 149 13 ka is older than previous estimates which range from 86 to 109 ka these new data support the view that old crow tephra was deposited during the cold interval that immediately preceded the last interglacial which is dated in the marine record at 125 ka b p it is likely therefore that the buried forest beds and numerous ice wedge pseudomorphs preserved in sediments just above old crow tephra in interior alaska relate to the last interglacial and not a younger warm event of early wisconsin age

rapid emplacement of the oman ophiolite thermal and geochronologic constraints in understanding of ophiolite emplacement requires knowledge of the elapsed time between igneous crystallization and intraoceanic thrusting and the rate and duration of that thrusting hornblende 4oar 39ar ages demonstrate that the igneous oceanic crust in oman crystallized and cooled to 825 k in 1 2 m y hornblende ages from metamorphic rocks and from cross cutting dikes require that the basal metamorphic thrust fault beneath the ophiolite also cooled below 825 k in 1 2 m y motion along the sole thrust accounted for 200 km of displacement at a rate of 100 200 mm yr on the basis of age relationships and thermal considerations we favor a two stage model for the initial stages of samall ophiolite emplacement intraoceanic thrusting over 2 m y old lithosphere at 150 km m y parallel to a spreading ridge for 1 2 m y followed by equally rapid and brief thrusting over cold and old lithosphere preservation of the samail ophiolite is a direct result of its young age and positive buoyancy at the time of ocean closure and we propose that all ophiolites that originated near spreading centers and were emplaced onto continents were young at the time of intraoceanic thrusting

geochemistry and tectonic significance of metamorphic sole rocks beneath the beysehir hoyran ophiolite sw turkey abstract the beysehir hoyran ophiolite is situated in the western part of the tauride belt sw turkey and crops out at two localities north of the lake beysehir it mainly comprises harzburgitic peridotites that were tectonically emplaced to their present position during the late eocene the ophiolites themselves are tectonically overlain by either slope basin deposits with lava blocks egirler formation or massive triassic limestone blocks deliktas formation high grade sub ophiolitic metamorphic rocks i e epidote amphibolite amphibolite and pyroxene amphibolite together with minor quartzite and calcschist are observed at the base of the ophiolite sequence where they occur as thin tectonic slices with an inverted metamorphic gradient average p t conditions of 630 770 c and c 6 1 5 kbar are calculated for the metamorphism of the amphibolites by conventional geothermobarometry corresponding to a burial depth of 18 20 km both the sub ophiolitic metamorphic rocks and the overlying mantle tectonites were intruded by isolated tholeiitic nb y 0 041 0 108 diabase dikes which do not transect the tectonic contact between the two units geochemical investigations of the amphibolites of the sub ophiolitic rock suite show two different geochemical affinities with the first group being alkaline in character nb y 1 3 86 and the second one being tholeiitic nb y 0 064 0 13 ree patterns trace element plots and tectonomagmatic discrimination diagrams indicate that the most probable protoliths for alkaline amphibolites are within plate type alkali basalts whereas those of the tholeiitic group resemble tholeiitic island arc basalts similarities between the geochemical characteristics of the amphibolites and those of the volcanic rocks of the egirler formation strongly suggest that the latter are the protoliths of the amphibolites

characteristics of ophiolite related metamorphic rocks in the beysehir ophiolitic melange central taurides turkey deduced from whole rock and mineral chemistry small outcrops of the metamorphic rocks of the beysehir ophiolite appear to the west of gencek and to the south of durak south of beysehir lake in the central tauride belt in turkey amphibolitic rocks in the ophiolitic melange have an igneous origin protoliths of these rocks were probably alkali basalts gabbros or some ultramafic cumulates such as pyroxenite the amphibolites of the beysehir ophiolite can be divided into four groups 1 amphibolecgarnetcplagioclasegepidote as secondary minerals gopaque such as ilmeniteg accessory minerals such as sphene and apatite 2 amphibolecpyroxenecplagioclasegepidotegaccessory minerals such as sphene apatitegchlorite calcite as secondary mineral 3 amphibolegplagioclasegopaquegaccessory minerals 4 amphibolec plagioclasegepidotegbiotite and muscovitegopaquegaccessory minerals these metamorphic rocks show mainly granoblastic grano nematoblastic porphyroblastic and or poikiloblastic textures all amphiboles in the amphibolites are calcic and cluster in the range from magnesio hastingsite pargasite to actinolite amphibole compositions are characterized by sio2z 38 02 54 3 al2o3z 1 5 12 8 feoz 10 03 14 67 k2oz 0 2 1 8 mgoz 5 5 15 7 mg z 0 3 0 8 the amphibolites show an alkaline to subalkaline character however the primitive mantle normalized incompatible trace element diagram shows close similarity with the typical ocean island basalt oib pattern the rock chondrite normalized ree diagram of the amphibolites also confirms their oib signature tectonomagmatic discrimination diagrams based on the immobile trace elements suggest a mostly within plate alkali basalt wpb environment beysehir ophiolitic melange contains amphibolites from ophiolite related metamorphic rocks but the matrix of the beysehir ophiolitic melange is not metamorphosed blocks of metamorphic rocks and the ophiolitic rocks may have been incorporated into the ophiolitic melange in an oceanic environment during the late cretaceous by tectonic forces q 2005 elsevier ltd all rights reserved