the troodos cyprus and kizildag s turkey ophiolites as structural models for slow spreading ridge segments the troodos cyprus and kizildag southern turkey ophiolites in the eastern mediterranean region represent remnants of the neotethyan ocean which evolved as a result of seafloor spreading north of afro arabia in mesozoic time both ophiolites display numerous planar to listric normal faults and structural grabens in their sheeted dike complexes suggestive of tectonic extension and complex intrusive relations in their plutonic units indicate recurring and intermittent magmatic activities detachment surfaces within the lower crustal sequence troodos or at the boundary between the crustal and mantle sequences kizildag define a brittle plastic transition zone along which tectonic thinning and extension within the crustal units above was accommodated by mylonitization and denudation in lower crustal units or mantle rocks below the variation in depth to these accommodation zones is inferred to be a function of the vertical temperature gradient in the oceanic lithosphere which may in turn point to the

relicts of the early cretaceous seamounts in the central western yarlung zangbo suture zone southern tibet abstract despite more than 30 years studies in the yarlung zangbo suture zone yzsz southern tibet is still a region of discovery and the geological evolution of the neo tethys here remains controversial in this paper we present the new field observation petrography zircon u pb dating and geochemistry of the zhongba mafic rocks in the western segment of the yzsz zircon u pb analyses from a diabase using sims secondary ion mass spectrometry yielded a concordant age of 125 7 0 9 ma all the zhongba mafic rocks exhibit lree enrichment lan ybn 4 7 15 8 without eu anomalies eu 0 97 1 01 their primitive mantle normalized trace element patterns are similar to those of the average oib and hawaii alkaline basalts as well as other oib type rocks already reported in the central western yzsz these observations lead us to suggest the existence of a series of seamounts within the neo tethys during the early cretaceous these oib type rocks could be originated from 1 hotspot mantle 2 near ridge seamounts by melting induced mixing of two distinct mantle sources 3 late stage magmatism fed by melts that originated from an asthenospheric window due to slab delamination combining literature data from the entire yzsz which include studies of the morb and iab type rocks and mantle peridotites we prefer the third explanation that the oib type rocks represent a late stage magmatic activity above an intra oceanic supra subduction zone within the neo tethys

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

comparisons between tethyan anorthosite bearing ophiolites and archean anorthosite bearing layered intrusions implications for archean geodynamic processes

crustal structure and evolution of the mariana intra oceanic island arc a new high resolution velocity model of the mariana arc backarc system obtained from active source seismic profiling demonstrates velocity variations within the arc middle and lower crusts of intermediate to felsic and mafic compositions the characteristics of the oceanic island arc crust are a middle crust with velocity of 6 km s laterally heterogeneous lower crust with velocities of 7 km s and unusually low mantle velocities petrologic modeling suggests that the volume of the lower crust composed of restites and olivine cumulates after the extraction of the middle crust should be significantly larger than is observed suggesting that a part of the lower crust especially the cumulates is seismically a part of the mantle

the almacik mafic ultramafic complex exhumed sakarya subcrustal mantle adjacent to the istanbul zone nw turkey the almacik mountains in northwestern turkey expose an upper amphibolite facies complex consisting of alternating ultramafic harzburgitic and websteritic and mafic metagabbroic rock types in the eastern part of this complex are island arc meta tholeiites and transitional to calc alkaline metabasites that are chemically quite similar to those of the permo triassic cele mafic complex north of bolu and this suggests an equivalence however much of the section exposes structurally deeper and chemically different mafic and ultramafic rocks which have no equivalent in the cele mafic complex and isotopic dating has suggested that these rocks also formed during the permian period and underwent triassic and jurassic metamorphism furthermore sparse inherited ages unlike those from istanbul zone granitoids suggest a link with north african derived armorican type basement and hence the sakarya zone rather than amazonia derived avalonian basement alternating mafic and ultramafic rocks suggest structural repetition supported by the exposure of discrete high strain zones or poorly exposed shattered rock west of each outcrop of ultramafic rocks the high grade of metamorphism and the absence of either extrusive lavas or sheeted dyke rocks suggests that the almacik complex was not an ophiolite but formed instead as subcontinental lower crust and subjacent mantle dominantly calc alkaline geochemistry suggests that it formed the basement to an active continental margin bounding the north side of the sakarya continent with s dipping subduction of palaeotethys the almacik complex was uplifted as a late result of compression against the southern margin of the istanbul zone in the jurassic period lack of coeval high grade metamorphism in the istanbul zone indicates that the latter was overthrust southwards over the sakarya margin and that there was therefore a change of subduction polarity in the triassic period the evidence further casts doubt on the existence of a mesozoic intra pontide ocean in northwestern turkey and suggests that the latest permian magmatism with subsequent triassic and jurassic metamorphism was instead related to the closure of the palaeotethyan ocean

petrology and origin of primitive lavas from the troodos ophiolite cyprus parental magmas to the troodos ophiolite are characterised by low tio2 and al2o3 and high sio2 extremely fresh and chemically primitive high mgo rocks are found within the upper pillow lavas and along the arakapas fault belt of cyprus and contain forsteritic olivine enstatite and groundmass clinopyroxene set in glass or plagioclase with accessory magnesiochromite and sometimes hornblende they are quartz normative and may have originally contained up to 3 wt h2o geochemically there are three distinct groups of primitive lavas based on tio2 and zr contents but also reflected by cao na2o and ree abundances these groups cannot be related by crystal fractionation and are considered to have been generated by incremental melting of a variably depleted source region the parental magma to the least depleted group group i was that of the major portion of the troodos plutonic complex and is similar to those postulated for other low ti ophiolites chemically it has close affinities with komatiitic basalts the most depleted lavas group iii all have u shaped ree profiles and variable 143nd 144nd ratios interpreted in terms of metasomatism of the source region by an incompatible element enriched component which was probably derived from a subducted slab these lavas represent an intermediate step in the development of boninite series rocks

tectono magmatic evolution of the west philippine mariana region and the origin of boninites re examination of published data on the tectomagmatic evolution of the west philippine mariana region indicate that arc magmatism and back arc extensional pulses are not synchronous but are largely asynchronous arc volcanism ceases within a few million years of the development of a back arc basin and recommences oceanward on a new arc during the final stages in the development of the back arc basin following karig s model we believe that ascent beneath the arc axial chain of a line of diapirs of morb source mantle interferes with processes of arc magma generation and arc magmatism wanes partial melting of the diapirs produces voluminous morb type tholeiites which split the arc and form the crust of a widening back arc basin we show for both the west philippine mariana region and occurrences of boninite like lavas in ophiolites that boninites appear to be erupted after arc magmatism and immediately before eruption of morb type lavas passage of a diapir of hot morb source mantle through refractory sub arc peridotite which has been enriched in lile and h2o will result in limited partial melting of the hydrous peridotite and the restricted production of boninitic magmas these are more likely to erupt modified or only slightly modified by crystal fractionation through the thin forearc crust continued ascent of the morb source mantle diapir will result in it partially melting to yield morb tholeiites which erupt shortly after and in considerably more voluminous amounts than the boninites finally we suggest that forearcs regions are likely to be incorporated into foldbelts and that boninite or low ti lava bearing ophiolites may characterize such regions

the genesis of refractory melts in the formation of oceanic crust refractory primary liquids arising in various oceanic plate tectonic settings are characterized by high mgo sio2 ca na low tio2 and generally low incompatible element abundances relative to primary liquids parental to morb we propose that the former melts segregate from upper mantle peridotite which has earlier been depleted by extraction of picritic melts which were parental to morb a compositional range in the second stage melts is expected depending on the extent of previous depletion of the peridotite the temperature and pressure of melt segregation and the possible influence of volatile phases c h o present during melting

timing of subduction zone metamorphism during the formation and emplacement of troodos and baer bassit ophiolites insights from 40ar 39ar geochronology the troodos ophiolite in cyprus and baer bassit ophiolite in syria together form part of the tethyan ophiolite belt they were generated in a supra subduction zone setting in late cretaceous times as with many of the ophiolite occurrences in this belt the sequences are closely associated with tectonic coloured melange zones which contain among a variety of lithologies metre to kilometre size blocks of metamorphic rocks precise 40 ar 39 ar laser step heating experiments performed on four amphibolites from sw cyprus and six from nw syria yield plateau ages ranging from 75 7 0 3 ma to 88 9 0 8 ma in cyprus and 71 7 0 5 to 88 4 0 4 ma in syria the older limits of these time spans are coeval with the age of the formation of the associated ophiolites unlike other metamorphic sole rocks which seem to form in relatively short time spans these metamorphic rocks found in cyprus and syria are interpreted to have formed in late cretaceous times by accretion below the overriding troodos and baer bassit crust for a period of 15 18 ma the metamorphic complexes were exhumed by extension and crustal thinning associated with subduction roll back and the rotation of the overriding plate until the cessation of subduction in maastrichtian times in cyprus the exhumed metamorphic complex was incorporated into an accretionary prism constructed primarily of the collapsed mamonia passive margin sequence intercalated with rocks of the troodos ophiolite during plate collision in the maastrichtian concomitantly in syria the baer bassit ophiolite and subcreted metamorphic complex were emplaced onto the arabian passive margin and fragmented into blocks and knockers forming the baer bassit melange

terminology for layered intrusions the terminology of igneous cumulates was originally formulated on the concept that the rocks were concentrations of minerals accumulated by crystal settling it is now well established for several major intrusions that plagioclase that had been typed as cumulus should in fact have floated in its parental magmatic liquid nevertheless the terminology has been valuable in the classification of the rocks of these and other intrusions and it has been useful in certain aspects of their genetic interpretation the name cumulate is therefore redefined so that crystal settling is a possible but not essential process in the origins of the rocks to which it is applied the overall nomenclature of layered intrusions is then reviewed and definitions of names are refined and systematized to clarify their applications careful distinction is made between descriptive and genetic names a cumulate is defined as an igneous rock characterized by a framework of touching mineral crystals and grains that evidently were concentrated through fractional crystallization of their parental magmatic liquids a layer is a sheetlike unit of cumulate that is a distinctive entity in its compositional and or textural features a lamina is a thin 2 5 cm sharply defined layer igneous lamination is a general name for planar structure that is pervasive through cumulates on the scale of their grain size layering is the overall structure and fabric manifest through combinations of layers laminae and lamination the difference between layers and layering is essentially the difference between individuals and a population and the terms that describe one are not necessarily appropriate to the description of the other but the fundamental variants of all of layering lamination grading in layers and layer contacts can essentially be described by one or more of the four modifiers grain size modal textural and cryptic where the last refers to cumulus miner al compositions a system for subdividing layered intrusions is proposed in which distinction is made between general divisions ranked as series zones and subzones and prominent units that are called groups members rhythmic units and cyclic units applications to the skaergaard bushveld muskox stillwater and other intrusions are briefly described

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

seafloor spreading and the ophiolitic sequences of the troodos complex a principal component analysis of lava and dike compositions the troodos complex of cyprus has been interpreted as a slice of oceanic crust formed from ultramafic magma in a back arc basin recent investigations however have suggested that only the upper gabbros the sheeted dike complex and the lower lava sequence formed as a direct consequence of seafloor spreading and that the primary magma was of basaltic composition according to this model the upper lava suite and related ultramafic cumulates are not represented in the sheeted dike complex a multivariate statistical analysis of major element compositions of 480 dikes and 130 volcanic glasses supports this suggestion statistically 98 the dikes are similar to the glasses of the lower lava suite this supports the model that the seafloor spreading related sequence of the troodos ophiolite is relatively thin and of basaltic composition the magma chambers represented by the ultramafic cumulates and associated gabbros and the upper lavas appear to have been emplaced after or during the termination of the major spreading events

petrogenesis and implications for tectonic setting of cambrian suprasubduction zone ophiolitic rocks in the central beishan orogenic collage northwest china abstract the hongliuhe ophiolite is a fragment of the early paleozoic oceanic crust situated in a suture zone central to the beishan orogenic collage which is a subset to the central asian orogenic belt in northwest china it has lithologic geochemical and chronological similarities to the yueyashan xichangjing ophiolite 400 km east along strike allowing for correlative interpretations of marginal seafloor spreading processes the hongliuhe ophiolite is explored in this study with complete petrologic geochemical and structural descriptions the ultramafic body comprises cumulate layered lherzolite and gabbro plutonic lithologies tectonized with syn magmatic extension a u pb age of 520 3 5 8 ma was obtained by shrimp analysis of zircons from a pegmatitic gabbro the ultramafic rocks are variably depleted and some show refertilization of lile s consistent with slab dehydration in a supra subduction mantle wedge mineral chemistry shows high cr 36 82 populations of spinel and wide ranging lower mg 8 50 consistent with known forearc peridotite chemistry partially re equilibrated by tectonism or through staged magmatism olivine and pyroxene chemistries are consistent with this interpretation a thin volcanic cover is interlayered with red cherts and has a bimodal set of geochemical signatures both island arc tholeiite and calc alkaline they bear distinct nb ta anomalies consistent with slab derived fluid initiated melting in a supra subduction environment coarse volcaniclastic turbidites infilled paleo topography along the ophiolite and they are interbedded with quartzofeldspathic sandstones indicating an initial proximity to an arc the sandstones contain detrital spinel with analyses showing relation to both the ophiolite and possibly a secondary alkalic volcanic source the supracrustal sequence continues with a thick contiguous covering of margin related rocks including interlayered limestone pyroclastic and siliciclastic rocks and basalts as a constraint on emplacement we provide an u pb zircon shrimp date 413 6 3 5 ma from an undeformed granitoid body intruded within the overlying stratigraphy the entire analysis paints a picture of subduction rollback related extension affecting the margin of an emerging or mature arc just prior to a known magmatic cycle that ended with a collision in the devonian at the latest this suture is one of many in the beishan and our interpretation provides evidence in a long running debate supporting accretionary orogenesis in an island arc archipelago as the main system for growing sialic crust in the paleozoic central asia

early neoproterozoic multiple arc back arc system formation during subduction accretion processes between the yangtze and cathaysia blocks new constraints from the supra subduction zone ne jiangxi ophiolite south china the ne jiangxi ophiolite in the eastern jiangnan orogen is a tectonic melange that mainly consists of individual tectonic blocks comprising pyroxenite gabbro basalt diorite granite and chert in a matrix of serpentinite or tuffaceous greywacke a combined geochemical and geochronological study of the ne jiangxi ophiolite was undertaken to constrain the timing and tectonic setting of its formation the basalts were geochemically subdivided into three groups with different feot and tio2 contents group 1 basalts have the lowest feot 12 17 13 07 wt and tio2 1 48 1 62 wt contents and the lowest nb yb 0 80 0 88 and th nb 0 02 0 03 ratios furthermore they have normal mid ocean ridge basalt n morb like trace element patterns suggesting derivation from an n morb type mantle source without subduction input group 2 fe ti basalts have the highest feot 15 52 16 30 wt and tio2 3 06 3 23 wt contents nb yb and th nb ratios from 1 75 to 1 89 and from 0 11 to 0 15 respectively and trace element patterns similar to those of back arc basin basalts the geochemical characteristics suggest that group 2 basalts were derived from a slightly enriched morb type mantle source with a minor subduction contribution in contrast group 3 fe ti basalts have moderate feot 12 98 13 40 wt and tio2 2 37 2 71 wt contents and nb yb and th nb ratios from 1 28 to 1 45 and from 0 27 to 0 30 respectively these basalts further display markedly negative nb ta anomalies and show a geochemical affinity to island arc basalts iab indicating a slightly enriched morb type mantle source that was significantly influenced by subduction derived fluids and or melts sims zircon u pb dating on gabbros gave ages of 995 22 ma and 993 12 ma which are interpreted as the formation age of the ne jiangxi ophiolite positive zircon ehf t 8 8 to 13 8 values for the gabbros and whole rock end t 5 5 to 6 6 values for the basalts indicate that the ne jiangxi ophiolite originated from an isotopically homogeneous depleted mantle source the diversity of morb to iab like basalts and the presence of fe ti basalts favor a formation of the ne jiangxi ophiolite during the initial rifting phase of an intra oceanic back arc basin between an oceanic arc huaiyu terrane and the continental margin of the yangtze block jiuling terrane at ca 990 ma both the present and previous studies imply that multiple arc back arc systems formed during long lasting subduction accretion processes between the yangtze and cathaysia blocks during the early neoproterozoic

submarine arc volcanism in the southern mariana arc as an ophiolite analogue supra subduction zone ssz ophiolites are being recognized more and more frequently in the geologic record submarine volcanism in intra oceanic arcs is one tectonic setting in which such ophiolites might form the southern seamount province of the mariana arc represents the best documented example of an active intra oceanic submarine arc which can serve as a modern analogue for some ssz ophiolites the southern seamount province comprises nine submarine volcanoes at least seven of which are active or dormant these edifices have erupted plagioclase clinopyroxene olivine phyric basalts andesites dacites and dacitic pumices these lavas are similar to those of the subaerial edifices in the central marianas sio2 49 70 tio2 0 4 1 3 mgo 1 2 5 9 ba 140 450 ppm 87sr 86sr 0 70346 0 70354 but are somewhat more enriched in the light rare earth elements the geochemical characteristics of subaerial mariana arc lavas are preserved in the submarine lavas and the ssp lavas are slightly enriched in incompatible elements relative to those of the subaerial edifices we are ignorant of the composition of any lavas erupted between the principal edifices which may be expected to dominate the volcanic section of ssz ophiolites nevertheless the characteristic high ratio lil hfsc low ti ni and cr and diagnostic mineral compositions calcic plagioclase relatively fe rich olivine are likely to be preserved in unaltered submarine arc lavas the mariana active arc is built on back arc or thinned frontal arc crust and any ophiolite formed from such an arc is likely to exhibit complex intrusive and eruptive relationships of plutonic hypabyssal and volcanic rocks of back arc frontal arc and active arc origin such complicated intrusive relationships if preserved in structurally disrupted ophiolites will make it difficult to reconstruct the geologic history of an ssz ophiolite

structure of the kizildag ophiolite a slow spread cretaceous ridge segment north of the arabian promontory the cretaceous kizildag ophiolite in southern turkey is part of the neotethyan ophiolite belt around the arabian promontory and consists of two structurally distinct massifs that are separated by a northwest striking high angle oblique fault this fault is interpreted as an accommodation zone that permitted differential movements between the adjacent crustal blocks during the evolution of the ophiolite the main massif to the west includes an antiformal peridotite core adjoined on the southeast by the normal fault bounded plutonic sequence and sheeted dike complex which collectively form a structural graben reminiscent of slow spreading ridge axes the inferred spreading axis and axis parallel fault contacts are intersected and offset at right angles by tear faults that are geometrically and kinematically analogous to transfer faults in modern oceanic lithosphere at slow spreading ridge environments the secondary massif to the east consists mainly of serpentinized peridotites directly overlain along tectonic and or intrusive contacts by lava flows rotated dike blocks and plutonic rocks and can be interpreted as a segment of oceanic lithosphere evolved at and near the accommodation zone the general structure of the ophiolite suggests its evolution via magmatic accretion and subsequent tectonic extension along a low angle shear zone at a slow spreading ridge environment

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

geochemical variability of the oman ophiolite lavas relationship with spatial distribution and paleomagnetic directions 1 the multielemental composition of 31 lavas sampled in the extrusive section of the oman ophiolite was determined by inductively coupled plasma source mass spectrometry icp ms this study allowed us to define clear geochemical criteria to characterize the different lavas types in oman most of the oman ophiolite extrusive sequence is composed of lavas of composition similar to present day morb with the exception of slight nb ta negative anomalies v1 magmatism v1 lavas display ree patterns and zr hf ratios in the range of morb zr hf 33 15 38 7 in contrast the overlaying v2 lavas which outcrop only in the northern part of the ophiolite are ree depleted relative to v1 and display low zr hf ratios 23 6 30 5 v2 lavas may be further classified into two sub groups v2 type i lavas display a continuous decrease from hree to lree the upper v2 type ii lavas are more depleted in hree and mree but they are enriched in lree they are also distinguished by their low nb ta ratios 10 53 11 65 relative to other oman basalts v1 12 5 14 v2 type i 12 35 14 4 as for n morb melting of an upwelling mantle beneath an oceanic spreading centre formed v1 lavas v2 resulted from fluid enhanced melting of previously depleted mantle residual after v1 extraction this process probably occurred at different melt rock ratios thus resulting in the two v2 sub groups and was enhanced in the northern part of the ophiolite v3 lavas overlie the v1 v2 sequence in the salahi area they display incompatible element rich patterns lree mree hree and a high nb ta ratio typical of within plate basalts in the different studied areas the transition from one lava type to the other seems to correlate with the block rotations revealed by paleomagnetic data

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

petrogenesis of neoarchean metavolcanic rocks in changyukou northwestern hebei implications for the transition stage from a compressional to an extensional regime for the north china craton abstract the 2 5 ga metavolcanic rocks in changyukou northwestern hebei can be classified into three groups based on major and trace elements high mg basalts tholeiitic basalts and the calc alkaline series basaltic andesites andesites and dacites rhyolites both high mg basalts and tholeiitic basalts have negative anomalies of nb zr ti and heavy rare earth elements hree as well as enrichments of sr k pb ba and light rare earth elements lree and show typical subduction zone affinities the petrogenesis of high mg basalts can be ascribed to high degree partial melting of an enriched mantle source in the spinel stability field that was previously enriched in large ion lithophile elements lile and lree by slab derived hydrous fluids melts supercritical fluids as well as the subsequent magma mixing processes of different sources at different source depths with little or no influence of polybaric fractional crystallization the flat hree of tholeiitic basalts indicates they may also originate from the spinel stability field but from obviously shallower depths than the source of high mg basalts they may form at a later stage of the subduction process when rapid slab rollback leads to extension and seafloor spreading in the upper plate we obtain the compositions of the archean lower crust of the north china craton based on the archean wutai jining section by compiling the average tonalite trondhjemite granodiorite ttg components average mafic granulite components and average sedimentary rock components the modeling results show that the generation of high al basalts basaltic andesites and andesites can be attributed to assimilation by high mg basalts primary basalts of relatively high al 2 o 3 thickened lower crust and the subsequent crystallization of prevailing mafic mineral phases while al 2 o 3 rich plagioclase crystallization is suppressed under high pressure and nearly water saturated conditions dacites and rhyolites may be the result of further fractional crystallization of basaltic andesites high al basalts and andesites mixing of magmas at various stages along the fractionation course of basaltic andesites high al basalts toward rhyolites promotes the trend of the calc alkaline series to reconcile the 2 55 to 2 5 ga ttgs derived from overthickened crust the 2 51 to 2 50 ga calc alkaline volcanic rocks derived from thickened crust tholeiitic basalts representing low pressure and an extensional tectonic setting 2493 ma leucosyenogranites derived from overthickened crust 2437 ma biotite monzogranites derived from slightly thinner crust than leucosyenogranites but still thickened as well as the clockwise hybrid itd and ibc p t paths of the hp granulites and widespread extension and rifting setting within the ncc from 2300 ma we propose a model of an evolving subduction process among them the composition of the 2 5 ga changyukou volcanic rocks and potassic granites as well as the clockwise hybrid itd and ibc p t paths of the hp granulites may reveal that the tectonic setting in northwest hebei was in a transition stage from a subduction related compressional regime to an extensional regime related to plate rollback

volcanic glass compositions of the troodos ophiolite cyprus fresh volcanic glass is preserved throughout the extrusive section of the troodos ophiolite which indicates that the lavas have not been pervasively metamorphosed glass compositions reveal the existence of two major magma suites apparently corresponding to distinct stratigraphic intervals the basal 400 500 m of the sequence consists of an andesite dacite rhyolite assemblage containing abundant hyaloclastites the remainder of the section comprises a basalt basaltic andesite assemblage with high mgo and low tio 2 and total iron the lower sequence is interpreted as an evolved arc tholeiite suite the upper has some similarities to boninitic lavas the close association in time and space of these two suites is similar to that observed in the mariana and bonin arcs and suggests that all of the troodos lavas were erupted in a subduction zone environment most probably in an incipient arc or fore arc

stability of the boundary separating antarctic bottom water from north atlantic deep water in the western south atlantic a comparison of data collected between 1911 and 1937 with those obtained during save 1987 89 and ajax 1984 expeditions reveals no significant change in the position of the boundary separating antarctic bottom water and north atlantic deep water in the western south atlantic ocean

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

equivalent radiolarian ages from ophiolitic terranes of cyprus and oman radiolarian biostratigraphy shows that umberiferous strata overlying the troodos ophiolite in cyprus are turonian in age and are thus essentially contemporaneous with similar strata that overlie the samail ophiolite in oman however this radiolarian age is markedly older than campanian isotopic ages measured on the underlying rocks of the troodos ophiolite the revised age for the umbers indicates that the troodos lavas were formed no later than turonian time the presence of overlying autochthonous maastrichtian chalks restricts the emplacement of the troodos ophiolite to the late cretaceous santonian to early maastrichtian

geochemistry and intrusive directions in sheeted dikes in the troodos ophiolite implications for mid ocean ridge spreading centers 1 sheeted dikes at mid ocean ridge volcanoes represent the link between deep magma production and storage processes and shallow processes such as volcanism and hydrothermal activity as such they are crucial for the interpretation of many observations at mid ocean ridges or other volcanoes with pronounced rift zones including topography hydrothermal systems petrology and geochemistry we carried out a structural magnetic and chemical investigation of a 4 10 km sheeted dike section in the troodos ophiolite cyprus on the basis of major and trace element geochemistry we distinguish dikes that may be correlated with the basal high ti series hts lavas from those of the overlying low ti series lts lavas all dikes studied are nearly parallel to each other with vertical or steeply dipping planes whose strike likely indicates the orientation of the spreading center anisotropy of magnetic susceptibility measurements suggests that the hts and lts dikes intrude in fundamentally different ways hts dikes reflect the intrusive behavior of dikes in the vicinity of a magma supply system and define ridge parallel intrusive sheets that radiate out from the magma chamber lts dikes show a bimodal orthogonal set of intrusive directions one shallow and one near vertical near lateral propagating dikes provide a means for delivery of magma into distant portions of a rift system and near vertical dike propagation directions are probably associated with feeder dikes to down rift surface flows our study suggests that the types of dike intrusive behavior in the troodos ophiolite may also be typical for normal mid ocean ridges or other major shield volcanoes with well developed rift zones

geochemical characteristics of soil radon and carbon dioxide within the dead sea fault and karasu fault in the amik basin hatay turkey abstract the study area is close to the boundary of three tectonic plates anatolian arabian and african plates and is characterized by important tectonic lineaments which consist mainly of the dead sea fault dsf the karasu fault and the east anatolian fault eaf systems to understand the origin of soil gas emanation and its relationships with the tectonics of the amik basin hatay a detailed soil gas sampling was systematically performed together with co2 flux measurements 220 soil gas samples were analyzed for rn and co2 concentrations the distribution of soil rn kbq m3 co2 concentration ppm and co2 flux g m2 day in the area appears as a point source spot and or diffuses halo anomalies along the buried faults fractures due to crustal leaks the results revealed that rn and co2 concentrations in the soil gas show anomalous values at the specific positions in the amik basin the trace of these anomalous values is coincident with the n s trending dsf co2 is believed to act as a carrier for rn gas based on the rn and co2 concentrations of soil gases at least three gas components are required to explain the observed variations in addition to the atmospheric component two other gas sources can be recognized one is the deep crust component which exhibits high rn and co2 concentrations and is considered the best indicator for the surface location of fault fracture zones in the region the other component is a shallower gas source with high rn concentration and low co2 concentration moreover he isotopic compositions of representative samples vary from 0 94 to 0 99 ra illustrating that most samples have a soil air component and may have mixed with some crustal component without significant input of the mantle component based on the repeated measurements at a few sites soil gas concentrations at the same site were observed to be higher in 2014 than in 2013 which may be associated with the activity of the dsf in 2013 2014 this suggests that soil gas variations at fault zone are closely related to the local crustal stress and hence are suitable for monitoring fault activities

structural development of sumisu rift izu bonin arc geophysical swath mapping multichannel seismic profiling and ocean drilling data are used to document the structural evolution of sumisu rift and to analyze the pattern of strain resulting from extension of an intraoceanic island arc the 120 km long 30 50 km wide sumisu rift is bounded to the north and south by structural and volcanic highs west of the sumisu and torishima calderas and longitudinally by curvilinear border fault zones with both convex and concave dip slopes the zig zag pattern of normal faults average strikes 337 and 355 indicates extension oriented 076 10 orthogonal to the volcanic arc three oblique transfer zones divide the rift along strike into four segments with different fault trends and uplift subsidence patterns differential strain across the transfer zones is accommodated by interdigitating rift parallel faults and sometimes by cross rift volcanism rather than by strike or oblique slip faults from estimates of extension 2 5 km the age of the rift 2 ma and the accelerating subsidence we infer that sumisu rift is in the early synrift stage of back arc basin formation following an early sag phase half graben formed with synthetically faulted structural rollovers facing large offset 2 2 5 km throw border fault zones in the three northern rift segments the largest faults are on the arc side and dip 60 75 w whereas in the southern segment they are on the west side and dip 25 50 e the present full graben stage is dominated by hanging wall antithetic faulting basin widening by footwall collapse and a concentration of subsidence in an inner rift the hanging wall collapses but not necessarily as a result of border fault propagation from adjacent rift segments whereas the border faults may penetrate the theologically weak lithosphere te 3 km many of the hanging wall and footwall collapse structures are detached only a few kilometers below the seafloor back arc volcanism usually erupted along faults occurs in the rift and along the protoremnant arc during both stages where drilled the arc margin has been uplifted 1 1 0 5 km concurrently with 1 1 km of rift basin subsidence extremely high sedimentation rates up to 6 m kyr in the inner rift have kept pace with synrift faulting created a smooth basin floor and resulted in sediment thicknesses that mimic the differential basin subsidence a linear zone of weakness caused by the greater temperatures and crustal thickness along the arc volcanic line controls the initial locus of rifting rifts are better developed between the arc edifices intrusions may be accommodating extensional strain adjacent to the arc volcanoes no obvious correlations are observed between the rift structures and preexisting cross arc trends

petrogenesis of a tholeiite boninite sequence from ayios mamas troodos ophiolite evidence for splitting of a volcanic arc lavas and intrusives from ayios mamas at the western end of the limassol forest complex south of the arakapas fault belt troodos comprise three groups a a basal sequence of pillow lavas intruded by dikes and sills with the phenocryst assemblage plag cpx mt ol b a middle sequence of pillows and massive flows cpx plag ol and c an upper sequence mostly of pillows and breccia ol opx cpx sp the lower group is tholeiitic and the upper two groups boninitic group c whole rock and glass mg numbers exceed 72 and range up to 78 with corresponding olivine compositions of fo92 they are strongly depleted in high field strength elements and on the basis of cr y variation and major element mass balances are interpreted to reflect partial melting of a basalt depleted cpx harzburgite source group a tholeiites in contrast are interpreted to derive from a more fertile lherzolite source magmaphile and compatible trace element variation is best modelled after an open magma system involving periodic replenishment tapping and fractionation although neither tholeiites nor boninites represent steady state compositions the boninites belong to a cao rich sio2 poor variant resembling those from guam but contrasting with those from bonin and cape vogel morb normalized enrichment of low field strength elements k sr ba and rb in all groups indicates subduction derived source contamination it is proposed that the ayios mamas section reflects splitting of a volcanic arc prior to back arc spreading in an environment analogous to that of the present day andaman sea

extreme tectonic rotations within an eastern mediterranean ophiolite baer bassit syria palaeomagnetic results from 27 sites at five localities within the dismembered bae r bassit ophiolite of northern syria are presented the ophiolite forms part of a series of thrust sheets emplaced over mesozoic carbonates of the arabian platform in the middle maastrichtian a positive inclination only area wide tilt test applied to four locality mean remanences and positive fold and reversal tests from palaeohorizontal units pillow lavas lava flows within one of these localities indicate that the ophiolite preserves pre deformation magnetisations variable directions of remanence between localities demonstrate that the ophiolite has experienced extreme relative anticlockwise rotations on a kilometric scale within the most extensively sampled ophiolite massif bassit sheet there is a progressive increase in rotation from north to south the southernmost units at the lowest structural level in the imbricate thrust stack record the highest rotation exceeding 200 although tectonic rotation during imbricate thrusting has been reported in a number of orogenic belts the pattern of rotations in the bassit sheet is difficult to explain by differential thrust sheet rotation instead regional comparisons with the hatay ophiolite of southern turkey and the troodos ophiolite of cyprus suggest that a significant component of rotation may be ascribed to intraoceanic deformation of a coherent region of oceanic crust within the southern neotethyan basin prior to ophiolite emplacement the partially rotated bae r bassit ophiolite was then emplaced and structurally dismembered by thrust faulting during the late tertiary the ophiolitic units were further rotated during the initiation and development of a major sinistral strike slip fault zone linking the cyprus subduction zone to the dead sea transform system the extreme rotations observed in the study are therefore of composite origin and reflect the complex development of structural fabrics within the ophiolite 2002 elsevier science b v all rights reserved

nd sr isotopic study of primitive lavas from the troodos ophiolite cyprus evidence for a subduction related setting primitive lavas from the troodos ophiolite have a large variation in initial 143 nd 144 nd ratios from nd 7 9 to 0 8 which precludes a mid ocean ridge origin initial 87 sr 86 sr ratios vary from 0 70415 to 0 70705 the range of nd values may be accounted for by a two component mixing model whereby variable amounts of a light rare earth element ree enriched component with nd between 5 and 7 has interacted with a depleted iherzolite source with nd 8 this is consistent with an arc related setting where fluids derived from a subducting slab induce further melting in an overlying iherzolite source

evidence for forearc seafloor spreading from the betts cove ophiolite newfoundland oceanic crust of boninitic affinity abstract the ordovician betts cove ophiolite of newfoundland has a well developed cumulate sequence in which is rooted a sheeted dyke complex that grades up into pillow lavas dominant chromite olivine orthopyroxene cumulate peridotites and orthopyroxenites have phase assemblages and mineral chemistries consistent with crystallization from boninitic magmas dykes and lavas have phenocrysts of olivine high cr al chromite orthopyroxene low tio2 clinopyroxene they have high sio2 and mgo contents and depleted u shaped trace element profiles indistinguishable from those of bonin islands boninites field data imply that cumulates dykes and lavas all are comagmatic while geochemical and mineralogical data indicate that all are of boninitic affinity since boninites are only found in forearcs this implies that the betts cove ophiolitic crust formed in a forearc since the entire oceanic crustal section at betts cove is of boninitic affinity then this implies that true seafloor spreading can initiate in forearc

light rare earth enrichment of supra subduction zone mantle evidence from the troodos ophiolite cyprus geochemical data including rare earth elements ree are presented for the primitive high mgo glassy lavas from the upper sections of the troodos ophiolite al2o3 tio2 ratios are used as an index of degrees of mantle melting and or successive depletion events the sources of the lavas have undergone both a ubiquitous introduction of low field strength elements and a selective addition of light ree suggesting a two stage mantle contamination model correlations between al2o3 tio2 ratios depletion index and the amount of light ree enrichment present in the lavas suggest a component induced melting model in this model progressive addition of a light ree component induced higher degree batch melts or enabled melting of highly refractory source material the light ree enrichment event is shown to be of the same character across the ophiolite but it is limited to the older ocean crust and a transform influenced regime

a neoarchean dismembered ophiolite complex from southern india geochemical and geochronological constraints on its suprasubduction origin abstract ophiolites the remnants of ancient oceanic lithosphere have been described from collisional sutures of various ages with only few examples from archean terranes here we report the discovery of a neoarchean ophiolite suite from the southern margin of the dharwar craton in india tectonically intercalated within a neoproterozoic suture zone the metamorphosed and variably dismembered ophiolite suite exposed around devanur comprises altered ultramafic units websterite gabbros mafic dykes amphibolites trondhjemites and pegmatites associated with ferruginous metachert structural and petrographic studies indicate that the rocks represent a highly sheared and metamorphosed suite emplaced as a thrust sheet the major and trace element geochemistry of the mafic dykes indicate derivation from basaltic andesitic magmas with tholeiitic to calc alkaline characteristics the rocks display negative nb anomalies with enrichment of lile k rb ba th and depletion in hfse ti nb hf tb the tectonic discrimination of these rocks based on various geochemical plots suggests that they were generated in a suprasubduction zone setting we present new shrimp zircon u pb data for two trondhjemite samples from this complex which yield 238u 206pb ages of 2528 61 and 2545 56 ma the neoarchean age from the trondhjemites obtained in our study is closely comparable to similar ages obtained in recent studies from magmatic zircons in charnockites and orthogneisses in the area the suprasubduction zone assemblages and arc magmas suggest a neoarchean ocean closure along the southern margin of the dharwar craton

crustal structure study at the izu bonin subduction zone around 31 n implications of serpentinized materials along the subduction plate boundary abstract the izu bonin trench ibt has characteristics different from other trenches in many aspects a chain of serpentine seamounts is exposed at the forearc slope 40 50 km west of the izu bonin trench axis in this area few large earthquakes m7 0 have occurred at shallow depths 0 100 km and many large earthquakes have occurred at greater depths 400 km to reveal the mechanism of serpentine diapiring and the relation between heterogeneity in earthquake occurrence and plate subduction seismic refraction reflection studies at the forearc slope with two lines perpendicular and parallel to the izu bonin trench axis were carried out in 1998 detailed seismic velocities were obtained for both lines by seismic tomography using refracted first arrivals and reflected arrivals on the e w line the subducting plate interface and the moho of the subducting oceanic plate were recognized the velocity of the mantle wedge is lower than that of the average velocity of oceanic mantle along the subducting plate a layer between the two plates plate boundary layer pbl with a low velocity appears just beneath the serpentine diapir and forms a sill shape which becomes thicker with increasing velocity from the diapir toward the west and connects to the mantle wedge the low velocity materials in the pbl are interpreted as chrysotile which is a low temperature phase of serpentine produced by the hydration of peridotite by water supplied by the subducting slab the chrysotile in the pbl might act as a lubricant and decrease seismic activity along the subduction zone and this can explain the characteristics of seismicity in the izu bonin subduction zone ibsz in contrast to the e w line the crustal structure along the n s line is rather homogeneous

upper eocene to middle miocene ostracode faunas and paleo oceanography of the north coastal belt jamaica west indies the ostracode fauna of the montpelier group upper eocene middle miocene exposed in the western part of the north coastal belt of jamaica contains representatives of three paleo environments allochthonous shelf and littoral species characterized by abraded carapaces and eye tubercles and a diverse group of archibenthal slope forms are present throughout species restricted to the world ocean psychrosphere 1000 m notably bradleya dictyon australoecia tipica agrenocythere hazelae and macrocypris spp first occur in sediments of earliest miocene age planktonic foraminiferal zone n4 their presence establishes a minimum age for the entry into the cayman trench of frigid 8 10 c water masses drawn from the atlantic thermohaline stratification upper eocene and oligocene assemblages are dominated by smooth blind genera such as krithe messinella and bythocypris and lived in lower thermospheric conditions 10 c at or near the base of the thermocline 700 900 m during the early miocene downfaulting of the north island slope of jamaica in response to southward tilting of the nicaraguan rise brought the montpelier depositional site into contact with progressively deeper and colder water masses this subsidence culminated in the late early miocene faunas of this age contain large numbers of bradleya dictyon agrenocythere hazelae macrocypris sp 1 bairdia oarion and bairdoppilata cassida they suggest paleodepths of 1500 2000 m and the local presence of a 3 4 water mass tentatively identified as the north atlantic deep water if this interpretation is correct then the maximum sill depth and water mass configuration of the miocene cayman trench have persisted to the present middle miocene assemblages indicate a return to paleodepths of 1000 1500 m heralding the pronounced upper miocene to mid pleistocene tectonism which elevated montpelier strata above sea level an alternative view attributes the miocene ostracode succession in the montpelier to episodes in the development of the atlantic circulation as recognized in the northeast atlantic montpelier deposition took place either on a broad step intervening between the northern edge of the clarendon platform and the abyssal basins of the cayman trench or on a tertiary analog of the existing north island slope these results have emphasized the value of detailed taxonomic and morphologic studies of deep sea ostracodes in geohistorical reconstruction

heishan mafic ultramafic rocks in the qimantag area of eastern kunlun nw china remnants of an early paleozoic incipient island arc abstract the heishan mafic ultramafic complex in the qimantag mountains of the eastern kunlun orogen consists of olivine pyroxenites pyroxenites gabbros and massive basalts zircons from the gabbros have magmatic sector zoning with th u ratios of 0 5 2 3 and yield a shrimp age of 486 6 ma mswd 0 31 which is taken as the time of magma crystallization spinel pyroxene and plagioclase mineral compositions indicate that the rocks formed in an island arc environment the cumulates have low la yb n ratios 0 6 0 8 and depleted lree patterns high abundances of cr 3031 1185 ppm co 119 50 ppm and ni 953 291 ppm and end t values of about 8 0 the massive basalts have somewhat higher contents of tio2 1 0 1 3 na2o 2 6 2 8 total ree and mg s 55 63 than those of the cumulates the la yb n ratios of the basalts range from 0 8 to 1 2 and the chondrite normalized ree patterns are flat end t values of the basalts range from 7 7 to 9 4 the compositions of the cumulates and volcanic rocks especially their similar nd isotopes suggest that the rocks were derived from similar depleted mantle sources the primary magmas were probably generated in an initial arc environment we conclude that the heishan mafic ultramafic complex in the qimantag area preserves a record of proto tethyan oceanic lithosphere formed in the early paleozoic it possibly marks a suture between the qaidam and north kunlun terranes in the western segment of the eastern kunlun orogen

tectonic evolution of the troodos ophiolite within the tethyan framework a new tectonic model reconciles conflicting structural and geochemical evidence for the origin of the troodos ophiolite a well preserved remnant of neotethyan oceanic crust grabens and normal faults within the sheeted dike complex and the extrusive sequence of the troodos ophiolite resemble those of oceanic spreading centers diverse intrusive and tectonic contact relationships between the sheeted dike complex and the underlying plutonic sequence indicate multiple and episodic intrusion of magma and along and across strike variation in volcanic and tectonic activity during development of oceanic crust coupled with the existence of the arakapas transform fault to the south these structural and intrusive relationships suggest origin at an intersection between a spreading center and a transform fault the arclike chemistry of sheeted dikes and related extrusive rocks and the inferred highly depleted and hydrous nature of the mantle source of the late stage intrusive and extrusive rocks argue however for generation of part of the ophiolite within a subduction zone environment regional reconstructions suggest that the mesozoic neotethys may have evolved as a marginal basin both to the afro arabian continent and the paleotethyan ocean over an active or recently active south dipping subduction zone the troodos ophiolite and other eastern mediterranean ophiolites whose magma compositions were affected by the subducted paleotethyan slab may have formed along east west trending spreading centers separated by north south trending transform faults within this marginal basin a rapid change in relative plate motion in late cretaceous time between eurasia and afro arabia created a regional compressive regime that may have resulted in plate boundary reorganizations within the neotethyan realm and in initiation of north dipping subduction zone s beneath the troodos and other ophiolites in the region the apparent forearc setting of the troodos ophiolite is a consequence of this intraoceanic displacement after its formation and thus is unrelated to its generation

volcanism in the earliest stage of back arc rifting in the izu bonin arc revealed by laser heating 40ar 39ar dating abstract the back arc region of the izu bonin arc has complex bathymetric and structural features which due to repeated back arc rifting and resumption of arc volcanism have prevented us from understanding the volcano tectonic history of the arc after 15 ma the laser heating 40ar 39ar dating technique combined with high density sampling of volcanic rocks from the back arc region of this arc successfully revealed the detailed temporal variation of volcanism related to the back arc rifting based on the new 40ar 39ar dating results 1 back arc rifting initiated at around 2 8 ma in the middle part of the izu bonin arc 30 30 n 32 30 n volcanism at the earliest stage of rifting is characterized by the basaltic volcanism from north south trending fissures and or lines of vents 2 following this earliest stage of volcanism at ca 2 5 ma compositionally bimodal volcanism occurred and formed small cones in the wide area this volcanism and rifting continued until about 1 ma in the region west of the currently active rift zone 3 after 1 ma active volcanism ceased in the area west of the currently active rift zone and volcanism and rifting were confined to the currently active rift zone the volcano tectonic history of the back arc region of the izu bonin arc is an example of the earliest stage of back arc rifting in the oceanic island arc age data on volcanics clearly indicate that volcanism changed its mode of activity composition and locus along with a progress of rifting

structural and geophysical expression of the solea graben troodos ophiolite cyprus field studies show that extensional structures in the north central part of the troodos ophiolite including steep normal faults grabens dikes and low angle detachment faults are related to e w present coordinates spreading the solea graben directly north of the mafic ultramafic rock outcrops around mount olympus consists of a 10 to 15 km wide western portion that is underlain by a low angle detachment fault between the sheeted dike complex and the plutonic section structural and paleomagnetic evidence indicates that the sheeted dikes above this detachment have been rotated 40 or more about subhorizontal axes the eastern portion of the graben contains sheeted dike blocks with varying axes of rotation and significantly different style of intrusive activity most structural features in the graben formed simultaneously with or shortly after cessation of spreading at the graben axis based on cross cutting ophiolitic dikes and hydrothermal mineralization timing of the uplift of the central part of the troodos ophiolite is uncertain but is probably related to the tectonic thinning of the upper crust by low angle normal faulting the solea graben spreading center provides a model for the structure of modern mid ocean ridges that are spreading amagmatically

petrological and chemical evolution of the northeastern troodos extrusive series cyprus four magma series are distinguished in the northeastern troodos extrusive series a a lower low ti series lo lts of basaltic andesites b a high ti series hts of basaltic andesites to rhyodacites c a low ti series lts and d a depleted low ti series dlts the last two being of basaltic andesite trace element characteristics vary systematically from series a to d and are interpreted in terms of a variable contribution of three major source components scs lile enriched water rich fluids sci derived from dehydration of a subducted lithosphere slab were continuously added to the overlying mantle wedge increasing lile hfse and lile ree ratios and decreasing absolute hfse and ree concentrations from series a to d indicate progressive depletion of the actual mantle source scii a negative ta anomaly in the lavas decreases from series a to d and is interpreted to have resulted from partial melting of the lower crust scii where ta nb ti may be fractionated by ti rich accessory phases the contribution of sciii decreases when the eruptive sites successively move away from the central axial zone and the temperature of the lower crust decreases preventing partial melting of the lower crust chemical compositions of fresh glass separates and phenocrysts indicate a change of major petrogenetic processes from series a to d lo lts and hts lavas are intrepreted to be directly related by open system fractional crystallization in crustal magma chambers removal of observed phenocryst phases clinopyroxene orthopyroxene plagioclase and magnetite and repeated subsequent mixing of dacitic to rhyodacitic magmas with batches of replenishing basaltic andesites are the major processes possibly induced by vesicle formation in the mafic layer after a period of some crystallization lts and dlts magmas were directly fed to the surface without stagnating at crustal levels with feeder dykes positioned marginal to the central rift zone and thus by passing the central magma chambers these magmas apparently experienced only limited fractional crystallization of 10 15 wt olivine clinopyroxene chromite probably at the mantle crust boundary

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

simultaneous boninitic and arc tholeiitic volcanisms in the izu forearc region during early arc volcanism based on odp leg 125 site 786 early arc volcanism during eocene to oligocene in the izu forearc region was investigated during odp legs 125 and 126 in 1989 and effusive and intrusive volcanics were recovered from leg 125 site 786 these rocks were all classified into boninites and associated rocks by leg 125 shipboard scientific party and they concluded that boninitic volcanism had occurred before 40 ma and arc tholeiitic volcanism began after 40 ma in this study lava flows and breccias that classified into boninite series are divided into two groups tholeiite and boninite based on petrographical and petrological properties both series are also distinguished by bulk rock composition it is considered that the sources of both rock types have similar depleted compositions because of their similar very low bulk hfse concentrations we suggest that boninitic and tholeiitic volcanism occurred closely in time and space and reflected different temperature and water condition

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

supra subduction zone ophiolites from inner mongolia north china implications for the tectonic history of the southeastern central asian orogenic belt abstract the recently discovered diyanmiao ophiolite in the eastern part of the erenhot hegenshan ophiolite belt forms part of the southeastern domain of the central asian orogenic belt caob the ophiolite suite is composed of the northern baiyinbulage ophiolite and the southern naolaiketu ophiolite each extending for nearly 100 km and a width of around 6 km the suite preserves a well exposed and a nearly complete ophiolite stratigraphy and includes from bottom to top harzburgite bedded gabbro isotropic gabbros anorthosite spilite pillow basalt keratophyre baschtauite with overlying chert and plagiogranite veins locally intruding the pillow basalt and gabbro the pillow basalts possess the geochemical characteristics of hahajima island arc tholeiite iat indicating that the diyanmiao ophiolite formed in the incipient arc setting of the supra subduction zone la icp ms u pb dating of zircon grains from three gabbro samples yield ages of 361 13 ma 356 7 3 8 ma and 340 14 ma indicating an early carboniferous age for the diyanmiao ophiolite we suggest that the intra oceanic subduction initiation of the erenhot hegenshan paleo ocean during the early carboniferous resulted in the formation of an incipient arc oceanic crust and that the diyanmiao ophiolite represents an important north dipping subduction event of the paleo asian oceanic crust beneath the southern margin of the siberian plate after early carboniferous and early permian our study provides further insights into the evolution of the paleo asian ocean and eventual construction of caob the globe s largest phanerozoic orogen

isotopic and trace element composition of volcanic glasses from the akaki canyon cyprus implications for the origin of the troodos ophiolite abstract we report major elements k rb cs sr ba sc cr ni hf ta th and ree concentrations and isotopic compositions of sr nd pb and o of carefully handpicked volcanic glasses from the akaki river section of the troodos ophiolite complex on the basis of sr and o isotopic composition and fe 2 o 3 feo ratios nine of the ten glasses analyzed are considered to be fresh even with respect to those elements that are easily affected by alteration the glasses range in composition from basaltic andesite to dacite incompatible element concentrations in the mafic compositions are low 10 chondritic and the light ree are strongly depleted however the ratios of low field strength elements k rb cs sr th to high field strength elements ta hf ti and rare earth elements are higher than those in n morb nd isotopic compositions show a very small range with e nd t 80ma varying from 6 5 to 7 5 initial 87 sr 86 sr ratios range from 0 7033 to 0 7040 pb isotope ratios have a restricted range with 206 pb 204 pb 18 60 18 70 207 pb 204 pb 15 55 15 59 and 208 pb 204 pb 38 08 38 51 the oxygen isotopic composition for fresh glasses falls within the range reported for fresh mafic mantle derived volcanics with 18 o varying from 5 4 to 6 5 all geochemical characteristics of the troodos volcanic glasses are similar to those observed in supra subduction zone volcanics but we cannot distinguish between an early stage island arc or back arc origin for the troodos ophiolite on a geochemical basis only however considering our data and previously published geologic and geochemical data we suggest that the troodos ophiolite formed in the earliest stages of island arc development

tectonic history of the hatay ophiolites south turkey and their relation with the dead sea rift abstract the kizildag ophiolite massif lies in the hatay region of s turkey some structural relationships in the area are examined and two geological sections through the ophiolite are presented a sequence of structural events dealing with the emplacement and post emplacement history is established the significance of the events is discussed and it is suggested that the post emplacement structural history was dominated by movements along the dead sea rift

episodic formation of neotethyan ophiolites tibetan plateau snapshots of abrupt global plate reorganizations during major episodes of supercontinent breakup abstract neotethyan ophiolites along the bangong nujiang and yarlung tsangpo sutures tibetan plateau consist of mantle peridotite cumulates isotropic gabbro diabase plagiogranite and basaltic lava they comprise morb iab oib and boninitic rocks and mostly belong to supra subduction zone ssz type compositionally different rocks crystallized synchronously over a distance of 2000 km along the bangong nujiang and yarlung tsangpo sutures during the short periods of 190 182 and 132 120 ma respectively resembling the izu bonin mariana ibm proto arc the early jurassic bangong nujiang ophiolites formed in forearc settings during subduction initiation while the middle jurassic early cretaceous bangong nujiang ophiolites and the early cretaceous yarlung tsangpo ophiolites formed in forearc settings at pre existing subduction zones crystallization of the neotethyan ssz ophiolites from the eastern mediterranean middle east tibetan plateau to southeast asia and formation of the associated metamorphic rocks temporally coincided with major supercontinent breakup events 1 initial breakup of pangea in central atlantic and westernmost tethys 190 ma 2 separation of west and east gondwana 170 160 ma and initial opening of piemont ligurian tethys 175 164 ma 3 initial breakups of east and west gondwana 135 125 ma and 4 final breakup of gondwana 104 96 ma far field forces associated with abrupt global plate reorganizations may have triggered subduction initiation or transient interplate coupling decoupling at pre existing subduction zones leading to episodic formation of the neotethyan ssz ophiolites and rapid exhumation of metamorphic rocks in subduction zones the formation events of neotethyan ssz ophiolites and associated metamorphic rocks represent snapshots of global plate reorganizations during the major episodes of supercontinent breakup

early paleozoic tectonic evolution of the north qinling orogenic belt in central china insights on continental deep subduction and multiphase exhumation abstract the north qinling orogenic belt is an important component of qinling composite orogen in central china high pressure and ultra high pressure hp uhp rocks occur as lenses or layers surrounded by gneissic rocks in the northern central and southern qinling complex of the north qinling belt considered to have formed through deep continental subduction the magmatic protoliths of the hp uhp metamorphic rocks show formation ages ca 800 ma geochemical characteristics and pb nd isotopic compositions similar to those of the neoproterozoic igneous rocks of the south qinling belt continental materials of the south qinling belt were dragged down to mantle depths by the shangdan oceanic crust subducted and subjected to hp uhp metamorphism at ca 500 ma the erlangping and kuanping backarc basin formed in response to subduction of the shangdan ocean and might have developed into a limited small ocean basin at ca 500 ma the hp uhp rocks yielded retrograde metamorphic ages of ca 470 450 ma and ca 420 400 ma these ages are identical to the age of magmatic events in the north qinling hp uhp belt at 500 ma 450 ma and 420 ma related to deep subduction collision slab breakoff and crustal thinning during post collisional extension the dominant ca 500 400 ma ages of detrital zircons from the liuling group of the south qinling belt match well with those from the three stages of magmatic rocks and hp uhp rocks in the qinling complex this correlation suggests that the magmatic rocks and hp uhp metamorphic rocks in the north qinling belt initially exhumed to the surface eroded and were then deposited in the liuling basin in a post orogenic extensional setting during middle to late devonian new evidence suggests that the qinling complex is a tectonic complex rather than a uniform stratigraphic unit or a microcontinent as previously believed and is mainly composed of the exhumed hp uhp metamorphic rocks deep subduction exhumation related magmatic rocks and the early neoproterozoic granites together with the host rocks from the over riding plate at an active continental margin the early paleozoic tectonic history of the nqb includes oceanic slab subduction and formation of arc backarc spreading continental deep subduction arc continent collision break off and multi stage exhumation of the deep subducted slab as well as extension and thinning and associated erosion and deposition

multiple ophiolite generation preserved in the northern philippines and the growth of an island arc complex abstract although all oceanic arcs grow through the addition of subduction generated magmas the geology of the northern philippines demonstrates that a major contribution to arc crustal growth can come from repeated episodic intra arc back arc and or fore arc oceanic crust generation with subsequent preservation of the basic ultrabasic units in the arc complex at least five episodes of oceanic crust generation are represented in the northern philippines by preserved ophiolitic sequences and recent intra arc seafloor spreading each episode is distinct in age as confirmed by modern dating techniques with the ages ranging from pre jurassic to quaternary although the philippines is widely regarded as an amalgamation of allochthonous terranes a review of the available data shows that there is currently no compelling evidence that these ophiolites are of exotic origin and that they have been tectonically accreted to the philippine arc complex rather the evidence suggests that most and possibly all of the ophiolites were generated as back arc fore arc or intra arc crust within the philippine arc complex hence there is a close spatial association of several ophiolitic terranes of diverse ages spanning 150 myr that formed as part of the arc complex such an association may have arisen from episodic generation of oceanic crust during periods of local extension in a suprasubduction zone setting which has experienced changing and possibly overlapping subduction from the east and west sides in the current reference frame disruption of the ophiolitic basement terranes has been and continues to be effected primarily by wrench faulting this style of arc growth has implications for the paleotectonic interpretation of ancient ophiolite arc terranes in continents and the petrologic evolution of island arcs

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

island arc tholeiite to boninitic melt evolution of the cretaceous kizildag turkey ophiolite model for multi stage early arc forearc magmatism in tethyan subduction factories article i nfo the kizildag ophiolite in turkey is a remnant of the late cretaceous suprasubduction zone ssz oceanic lithosphere that was developed in southern tethys and is part of a peri arabian ophiolite belt including the troodos cyprus baer bassit syria and semail oman ophiolites in the eastern mediterranean region the internal structure petrology and geochemistry of the kizildag intrusive and extrusive rocks show a magmatic progression from island arc tholeiite iat to boninitic compositions similar structural architecture and geochemical trends in the coeval troodos and baer bassit ophiolites suggest that this multi stage ssz magmatism was common in most tethyan ophiolites the sheeted dikes and the majority of extrusive rocks in kizildag are composed of basalt basaltic andesite and andesite with relatively low la smn 0 5 0 7 low mgo 7 9 wt and high tio2 0 7 1 0 wt and exhibit high dy ybn ratios these iat suites also include rare younger dacitic dikes showing strong enrichment in the lree the late stage basaltic dikes and lavas sakalavites have high la smn 0 8 1 0 high mgo 9 11 wt and low tio2 0 3 0 4 wt contents and display strong depletion in both lree and hree this suite of rocks represents the boninitic component of the kizildag ophiolite that developed from shallow and greater degrees of melting of hydrous and highly refractory peridotites beneath the extended forearc environment the progressive evolution of kizildag magmas from iat to boninitic compositions in time and space was a result of variable mixing aggregation and differentiation of geochemically different melt batches which formed in different levels polybaric within the melting column rapid slab rollback induced corner flow mantle return flow and asthenospheric diapirism providing more fertile melt directly affected the evolution of this melting column and may have triggered advanced degrees of shallow melting of the hydrous and highly depleted forearc mantle producing refractory liquids of the boninitic stage these geochemical patterns of the kizildag ophiolite reflect a multi stage evolution of ssz magmatism in tethyan subduction factories that is analogous to the earlier arc volcanism in the eocene oligocene izu bonin mariana system