mid ocean ridge and island arc affinities in ophiolites from iran palaeographic implications complementary reference abstract fifty six samples of ophiolitic rocks from various late cretaceous eocene suture zones in iran have been analyzed for major and trace elements in order to assess their petrogenetic affinity and to determine their original tectonic setting in both southern baluchestan i e between the makran range and the lut block the fanuj maskutan sequence and northeastern baluchestan i e in the east iranian ranges the tchehel kureh sequence the ophiolitic complexes are well developed the rocks display affinities with mid ocean ridge tholeiites the basaltic rocks are comparable to respectively transitional fanuj maskutan and normal tchehel kureh morb in contrast the sequence along the zagros the sahneh sequence appears to have formed in an island arc setting in iranshar i e in southeastern baluchestan ophiolites are dismembered into a melange which includes cumulate intrusives similar to those from fanuj maskutan and lavas with island arc tholeiitic affinity a similar island arc origin may be inferred for the sabzevar ophiolitic sequence north of the lut block studied by other authors the chemical data thus help to elucidate the development of the jigsaw puzzle of microplates of gondwanian origin that during the mesozoic were intermediate in position between afro arabia india and eurasia and that now constitute the middle east ophiolite obduction in iran occurred during arc basin continent or ocean continent collisions in the late cretaceous early paleogene largely predating the final suturing between arabia and india with eurasia to which the intermediate microplates were already attached which did not take place before the neogene

rare earth element characteristics and k ar ages of the band ziarat ophiolite complex southeastern iran the band ziarat complex of southeastern iran is located on the western boundary c f the jaz murian depression rand is bounded by two ma jar fault systems the principal rock units of this complex are a gabbro sequence that includcs low and high level cumulate gabbros a late intrusive sequence that consists of diorite and plagiogranite and a volcanic sequence that includes diabase dikes and a lesser anaount of basaltic lava mantle rocks are virtually absent because of the presence of the two bounding fault systems but we consider the complex to be an ophiolite in nature rare earth cleislcnt ree whole rock data clearly diffcremtiate the classic ophiolitic lithologies for the crustal rocks in this complex bas4 on the ree data there are two distinct types of basalt presenl at band ziarat i those that formed from an initial basaltic melt with a light rare earth element lree enriched signature similar to intraplate hasalts and ii those that have lree depleted patterns similar to normal mid ocean ridge basalts the data also suggest i that the gabbros are adcumulates and were derived from a source slightly enriched in lree with fractic natioam controlled by removal of clinopyroxene or hornblende and plagioclase and ii that the late intrusive rocks as well as a ma jority of the cliabnse dikes are cogenetic and were derived from thc same lree enriched source k ar ages ranging from 134 4 to 146 5 ma for lowlevel gabbros and from 821 4 to 930 4 ma for high level gabbros were measured on five hornblende and two whole rock samples which suggests that these rocks may have formed early in the cretaceous period

meso tethyan oceanic sutures and their deformation abstract on the basis of comparative stratigraphie and paleontologic analysis supported by some key paleomagnetic data and interpretations it is shown that during the mesozoic and the cenozoic the eurasian continent grew by accretion of microcontinents these microcontinents separated basins with oceanic crust from the main ocean during the late cretaceous and the early cenozoic the collision of the microcontinents with eurasia resulted in the closure of the basins and meso tethyan oceanic sutures originated in the region under consideration from the carpathians to tibet there are two main meso tethyan sutures the carpathian lesser caucasus and the afghan tibet suture the above mentioned main structures also had branches which remained as sutures of small basins the kamennopotock interpontide nain baft basins and others in the west carpathians the carpathian lesser caucasus suture is overlain by a widespread gemeric tatric allochton from the west carpathians the suture passes through the pannonian basin into the vardar ophiolite zone and farther to the ophiolites of the izmir ankara zone being displaced along the north anatolian right lateral strike slip fault the main suture passes from the eastern pontides into the lesser caucasus where it is marked by ophiolites of the amasia area the shirak bozum and zangezur ridges sunsequently it can be observed through the iranian qara dagh mountains to lake urumiyeh and the north anatolian strike slip fault being again shifted along the strike slip fault the carpathian lesser caucasus meso tethyan suture ends in the western zagros near the cenozoic neo tethyan suture the afghan tibet meso tethyan suture is situated in the pamirs in the rushan pshart zone the east prolongation of the suture has been displaced along the pamir karakorum right lateral strike slip fault in tibet west of the pamirs this suture is also displaced along strike slip faults and continues in the farahrud zone in afghanistan and than passes through the zabol baluch and daz murian ophiolite zones and approaches the neo tethyan suture the paleomagnetic data allow us to reconstruct the location of those sutures in the late cretaceous early cenozoic if one compares the present position of the carpathian lesser caucasus and afghan tibet meso tethyan sutures with the late cretaceous early cenozoic reconstructions of these sutures it is possible to trace the inner deformation of the alpine belt according to these data the carpathian lesser caucasus suture moved northwards over 1200 km ahead of the arabian turkish syntaxis with the displacement direction across the fold belt at the border of the dinarides and hellenides the displacement decreases to 400 500 km and its direction is along the fold belt the amplitude of the suture displacement increases in the pannonian region and again decreases towards the eastern alps the displacement of the afghan tibet suture was 2000 km during the alpine deformation a great ensemble of cenozoic nappes folds and structural arcs of the pamir punjab and arabian turkish syntaxes and carpathian loop is the result of lateral shortening of the alpine belt through the collision

brittle tectonic reconstruction of palaeo extension inherited from mesozoic rifting in west zagros kermanshah iran abstract the present day zagros mountain belt of sw iran corresponds to the former arabian passive continental margin of the southern neo tethyan basin which existed since a permian triassic rifting episode and underwent later collisional deformation in mid to late cenozoic time in west zagros the kermanshah radiolarian trough separated from the neo tethyan basin by the narrow continental platform of the bisotun limestone resulted from the triassic jurassic rifting of the passive margin brittle tectonic analyses of syndepositional normal fault slip data have been undertaken to study the extensional history of the passive margin in terms of palaeostress reconstruction dominant east west trends of the inherited rift related normal faults in west zagros indicate an approximately north south trend of extension that prevailed during the entire mesozoic syn to post rift evolution of the radiolarian trough this extension characterizes a stretched continental margin similar to the present day passive margin of the british isles considering the structural pattern of the inherited basement faults as revealed by the present day earthquake focal mechanisms an oblique crustal stretching model is proposed for the rifting process

the closing of tethys and the tectonics of the himalaya recent geological and geophysical data from southern tibet allow refinement of models for the closing of southern neo tethys and formation of the himalaya shelf sediments of the indian passive continental margin which pass northward into deep sea tethyan sediments of the indus tsangpo suture zone were deposited in the late cretaceous an andean type margin with a 2 500 km long trans himalayan kohistan ladakh gangdese granitoid batholith formed parallel to the southern margin of the lhasa block together with extensive andesites rhyolites and ignimbrites lingzizong formation the southern part of the lhasa block was uplifted deformed and eroded between the cenomanian and the eocene in the western himalaya the kohistan island arc became accreted to the northern plate at this time the northern part of the lhasa block was affected by jurassic metamorphism and plutonism associated with the mid jurassic closure of the bangong nujiang suture zone to the north the timing of collision between the two continental plates ca 50 40 ma marking the closing of tethys is shown by 1 the change from marine flysch like to continental molasse like sedimentation in the indus tsangpo suture zone 2 the end of gangdese i type granitoid injection 3 eocene s type anatectic granites and migmatites in the lhasa block and 4 the start of compressional tectonics in the tibetan tethys and indus tsangpo suture zone south facing folds south directed thrusts after the eocene closure of tethys deformation spread southward across the tibetan tethys zone to the high himalaya deep crustal thrusting barrovian metamorphism migmatization and generation of oligocene miocene leucogranites were accompanied by south verging recumbent nappes inverting metamorphic isograds and by south directed intracontinental shear zones associated with the main central thrust continued convergence in the late tertiary resulted in large scale north directed backthrusting along the indus tsangpo suture zone more than 500 km shortening is recorded in the foreland thrust zones of the indian plate south of the suture and 150 km shortening is recorded across the indian shelf zanskar range and the indus suture in ladakh there was also large scale shortening of the karakoram and tibetan microplates north of the suture as much as 1 000 km shortening occurred in tibet the more recent deformation however involved the spreading of this thickened crust and the lateral motion of the tibetan block along major approximately east west trending strike slip fault zones

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

the waziristan ophiolite pakistan general geology and chemistry of chromite and associated phases the waziristan complex covering over 500 km 2 was tectonically emplaced during the paleocene to early eocene it consists of ultramafic rocks gabbros sheeted dikes pillow lavas and pelagic sediments a typical ophiolite suite however a complete sequence does not occur in any single locality due to tectonic dismemberment the podiform chromitites consist of aluminochromite cr 2 o 3 49 61 with subordinate chromian chlorite and or chromian serpentine magnetite in cr al fe super 3 ti space the chromites plot in the area of alpine chromites but on other diagrams they overlap the fields of chromites from alpine and stratiform complexes many chromites are zoned with ferrit chromit margins richer in cr cr al and poorer in mg mg fe super 2 than the unaltered cores the chlorites are generally rich in sio 2 and mgo and poor in al 2 o 3 it appears that during greenschist facies metamorphism or alteration diffusion of mg al and some cr from the chromite margins toward the silicate matrix resulted in the development of ferrit chromit and chlorite

collision tectonics in the nw himalayas west himalayan tectonics involve the collision of microplates between the indian and asian plates the kohistan complex consists largely of tightly folded basic volcanics and sediments generated as late jurassic to late cretaceous island arcs these were intruded by post folding mid cretaceous eocene plutonics produced from continued subduction of the indian plate after closure of a suture between kohistan and the karakorum the himalayan structures show major thrust sheets and the kohistan arc is essentially a crustal pop up with southward upright and northward verging structures developed above a thick ductile decoupling zone the indus suture which can be traced for 100 km beneath kohistan on large reentrants this pop up formed by a two stage process closure of the northern suture followed by closure of the southern indus suture granitic rocks of the kohis tan ladakh batholith dated at 100 40 ma post date most of the structures related to the northern suture but were deformed and carried southwards on shear structures related to the indus suture postcollisional deformation carried this kohistan complex on deep decoupling zones over the indian plate on a series of imbricated gneiss sheets the thrusts climbing up section in the movement direction so that in the far s some override their own molasse debris folds above these deep decoupling zones deformed their overlying thrust sheets into large antiforms i e the nanga parbat and hazara syntaxes the nanga parbat syntaxis probably formed due to a shear couple near a branch line where one of the main himalayan thrusts joined the indus suture beneath kohistan crustal delamination to produce the imbricated gneiss sheets could not account for all the displacement of india into asia suggested by palaeomagnetic data there must also have been lateral displacement as demonstrated by the large oblique slip shear zone in the hunza valley n of kohistan

petrology geochemistry and tectonics of the sabzevar ophiolite north central iran abstract the sabzevar ophiolite is a highly dismembered ophiolite complex located along the northern boundary of the central iranian microcontinent cim and is one of the internal iranian group of ophiolites and colored melanges the igneous rocks of this complex consist of peridotites harzburgite dunite and lherzolite serpentinite minor pyroxenite gabbros and a volcanic sequence that exhibits a wide range of composition from basalts and basaltic andesites to rhyodacite dacites rhyolites and basanites sedimentary rocks include a variety of upper triassic to lower cretaceous deep and shallow marine rocks these include pelagic fossiliferous carbonates that are mixed with the pillow basalt and basaltic andesite as interlayers or exotic blocks ranging in size from 10 to 100 m also present are extensive units of radiolarian chert which are interbedded within the basalts and basaltic andesites a combination of petrographic observations and analyses of incompatible trace elements and rare earth elements indicates the presence of at least four different types of extrusive rocks in the sabzevar ophiolite the geochemical data clearly identifies some of the extrusive rocks to have formed from three distinct types of basaltic melts i the group 1 basaltic rocks which formed from an initial melt with n morb like lree depleted chemical signatures and are petrogenetically related to the gabbros ii group 2 basaltic rocks which have e morb chemical signatures and iii group 3 basaltic rocks with lree enriched signatures and incompatible trace element patterns that suggest an island arc affinity the data also suggest that the gabbros and the group 1 basaltic rocks are petrogenetically related and produced by fractionation of plagioclase and to a lesser extent clinopyroxene and hornblende similar to other studied iranian ophiolites e g khoy band e zeyarat dar anar neyriz kermanshah and baft the results from the sabzevar ophiolite demonstrate the presence of relics of mid ocean ridge basalt morb like signatures in the extrusive units of neo tethyan ophiolites and suggests that the fossil oceanic lithosphere preserved in these ophiolites developed at an oceanic spreading center in addition the sabzevar ophiolite contains other volcanic rocks that have non morb like geochemical signatures e g within plate island arc the differences between these geochemical signatures could be a result of a differing upper mantle composition or different degrees of partial melting of the same upper mantle the presence of sizable nb anomalies in the extended rare earth element ree patterns for andesites and the group 3 basalts is characteristic of volcanic arc magmas or possibly are products of the ascending magma that was modified by crustal contamination the presences of the basanites and the silica undersaturated rocks could be attributed to extrusive activity following ophiolite emplacement plate reconstructions suggest that rocks of the sabzevar ophiolite were part of the tethyan oceanic crust that was formed during rifting of a narrow but deep seaway that separated the cim from the eurasian plate the ophiolite was emplaced during northeast dipping subduction i e closure of this segment of the tethyan seaway sabzevar ocean

paleozoic and mesozoic stratigraphy of the peshawar basin pakistan correlations and implications the most complete paleozoic sequence described from pakistan is exposed in bedrock inliers and in ranges fringing the eastern peshawar basin interbedded quartzite and argillite of the precambrian and cambrian tanawal formation is overlain unconformably by the cambrian ambar formation the misri banda quartzite unconformably overlies the ambar and contains ordovician cruziana ichnofossils new conodont discoveries restrict the ages of overlying formations as follows panjpir formation llandoverian to pridolian nowshera formation lochkovian to frasnian and jafar kandao formation kinderhookian to westphalian the karapa greenschist consisting of metamorphosed lava flows separates the jafar kandao from upper triassic carnian marbles of the kashala formation the upper triassic and jurassic nikanai ghar formation forms the top of the section correlatives to the peshawar basin stratigraphy are present locally in the sherwan synclinorium of hazara and in the khyber pass region the sequence contrasts markedly with the paleozoic and mesozoic section exposed south of the khairabad thrust in the attock cherat range this thrust and its northeastern continuation in hazara north of abbottabad thus form the boundary in pakistan between the lesser himalayan and tethyan himalayan sections a function performed by the main central thrust mct in the central himalaya of india and nepal the newly dated carboniferous to triassic horizons provide the first firm age constraints on the protoliths of the high grade swat metasediments the dating of the metasediments has in turn provided age constraints on pre himalayan tectonism and associated intrusions two major tectonic episodes during the late cambrian and carboniferous produced positive areas north of the peshawar basin that provided coarse detritus to the misri banda quartzite and jafar kandao formation

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

manganiferous cherts of the franciscan assemblage i general geology ancient and modern analogues and implications for hydrothermal convection at oceanic spreading centers there are several hundred ophiolitic manganiferous chert deposits primarily of late jurassic to early cretaceous age known within the franciscan assemblage of california the sequences typically consist of one to three massive manganiferous chert lenses containing 30 to 50 percent mn and averaging 1 m in thickness by 15 m subcircular diameter these are separated by an average 2 to 10 m of thin bedded radiolarian cherts and overlie basalts or greenstones both their geology and chemistry indicate that the ore lenses are hydrothermal and may have formed on the flanks of a mid ocean ridge or within a back arc basin it is proposed that the sequences developed as a result of sea floor spreading over a series of deep hydrothermal seawater convection cells paralleling a spreading center and spaced roughly 5 to 10 km apart chemical profiles of mn fe si al cu ni zn co ba ti the rare earth elements and 87 sr 86 sr have been determined through two complete sections these profiles indicate hydrothermal input of mn si cu ni zn and ba and detrital or hydrogenous input of al and co they illustrate the use of ti as a measure of relative detrital sedimentation rates fe is strongly fractionated from mn within the ores fe mn 0 1 and fe mn ratios decrease upward throughout each section suggesting preferential deposition of fe within the sediment and of mn at the seawater interface rare earth element distributions reflect the interaction of sea water and underlying basalts sr isotopic ratios of the ores and basalts demonstrate both strong and moderate seawater influences respectively fluid inclusion analyses on veins of undetermined age show seawater salinity temperatures of roughly 200 degrees c and tentative entrapment pressures corresponding to 1 700 m water depth early and intermediate veins were injected into unconsolidated siliceous sediment producing a characteristic bleached and pseudobrecciated texture an analogy is drawn with the present day field of hydrothermal mounds near the galapagos rift and with ophiolitic complexes of the northern apennines and other localities

k ar ages of metamorphic rocks at the base of the samail ophiolite oman hornblendes from amphiobolities in the sheet of metamorphic rocks beneath the peridotite member of the samail ophiolite and phyllites farther from the peridotite contact have weighted mean sup 40 ar sup 39 ar total fusion ages of 90 0 3 0 m y and 79 5 3 0 m y respectively the amphibolities represent the first tectonic slice welded to the base of the samail ophiolite after it was detached from the tethyan oceanic crust formation of the amphiobolities occurred no more than 3 to 7 m y after crystallization of plagiogranite in the ophiolite the phyllites represent another tectonic slice of ocean floor sediments welded to the ophiolite as it was transported further from the tethyan spreading axis the k ar ages suggest assuming a half spreading rate of 2 to 5 cm yr that detachment of the samail ophiolite and formation of amphibolite facies rocks occurred no more than 60 to 350 km from spreading center using the same spreading rate one can calculate a minimum half width of 300 to 750 km for the tethyan ocean during the late cretaceous

geochemistry of manganese bearing cherts associated with alpine ophiolites and the hawasina formations in oman abstract based on correlation coefficients and analogies with studies on recent mn enrichments the major and trace elements of mn occurrences associated with ophiolites that have been variously affected by regional alpine metamorphism were attributed to different origins mn zn v ce nd and la are hydrothermal al ti k zr rb ga sc and some fe and pb are detrital and si p u y nb and the remainder of the fe are biogenic preliminary studies of mn occurrences in the wahrah formation of oman reveal a very similar geochemistry as the extensive exposure of this formation does not display any evidence of hydrothermal systems only a hydrothermal supply of mn to seawater from a distant source is envisaged for these and other deposits

age and petrogenesis of na rich felsic rocks in western iran evidence for closure of the southern branch of the neo tethys in the late cretaceous abstract intermediate to felsic volcanic and granitic rocks with high na2o concentrations 5 2 9 1 wt are widely distributed in the harsin area along the zagros thrust zone in western iran most of these rocks are classified as low potassium tholeiite display affinity with oceanic plagiogranite and contain somewhat high na content and low concentrations of k2o al2o3 rb sr and ca thus we prefer to apply the term na rich felsic rocks to this complex u pb dating yielded ages of 94 6 2 7 ma 2 from baddeleyite and 95 0 2 4 ma 2 from zircon grains indicating that the complex crystallized in the late cretaceous based on the mineral compositions the crystallization occurred at low pressures mostly

mineralogy geochemistry and genesis of the ferromanganese ores from hazara area nw himalayas northern pakistan abstract ferromanganese ores occur in hazara area near abbottabad within the hazira formation of the kalachitta margala thrust belt of the nw himalayas of the indo pakistan plate in pakistan the hazira formation of cambrian age is a relatively thin unit up to 150 m thick which consists of either thin ferruginous siltstone with variable amounts of clay shale ferromanganese ores phosphorite and barite or thicker evenly bedded reddish brown siltstone it has a conformable lower contact with the abbottabad formation cambrian and an unconformable upper contact with the samana suk limestone jurassic the ferromanganese ores of the hazara area were studied in terms of their mineral and geochemical composition mineralogically a variety of mn and fe phases have been noticed in kakul galdanian and chura gali ferromanganese ores bixbyite is the principal mn phase while pyrolusite hollandite partridgeite and braunite occur in lesser amounts hematite however is the only fe phase in these ores gangue minerals include carbonates cryptocrystalline quartz iron clay and variable amounts of apatite plumbogummite and barite glauconite occurs in traces textural features suggest remobilization and subsequent precipitation of early diagenetic mn and fe phases along the fractures and interstices of the gangue minerals during later metamorphism chemical analyses show that the mn fe ratio in the ores is highly variable and ranges from 0 46 to 5 25 these ores exhibit a line of descent from lree to hree with a small positive ce anomaly their ree is higher than the hydrothermal mn deposits and lower than the hydrogenous crust the petrochemical characteristics suggest a mixed hydrothermal hydrogenetic source for the ferromanganese ores which have formed in shallow water or continental shelf environment due to the upwelling of anoxic deep seated water

ophiolites as indicators of the geodynamic evolution of the tethyan ocean abstract reviewing the ophiolites of the alpine chain from the western mediterranean to the indian ocean we subdivide them into ten groups of outcrops the geological environment through time the internal composition and history are given for each emphasis is put on the variegated and successive geodynamic processes which can be reconstructed for the different ophiolitic belts since their birth as an ocean like crust to their incorporation through accretion to a continent one can distinguish low versus high tholeiitic partial melting fairly or poorly established ridges simple ridge type crust versus complex ridge ensimatic arc and even more complex type long lived versus short lived oceanic crust stages abrupt large scale pure obduction versus progressive collision related obduction or simple progressive juxtaposition to continent the major events through time in the tethyan ophiolitic belts can be listed as follows spreading ridge activity during the jurassic liguria dinaro hellenides lesser caucasus and the cretaceous peri arabic nain sabzewar with a jump of ridge in between repeated change from ridge to ensimatic arc dinaro hellenic lesser caucasus peri arabic west indian during the latest jurassic and cretaceous upper cretaceous tertiary collision related obductions pontic lesser caucasus liguria squeezing out the elevated portions of these basins lack of oceanic magmatism during the cenozoic but progressive accretion along accretionary wedges of the elevated portions of the remaining mesozoic crust van nain sabzewar zahedan mekran

geochemical characteristics 40ar 39ar ages and original tectonic setting of the band e zeyarat dar anar ophiolite makran accretionary prism s e iran abstract the makran accretionary prism in southeastern iran contains extensive mesozoic zones of melange and large intact ophiolites representing remnants of the tethys oceanic crust that was subducted beneath eurasia to the north of the makran accretionary prism lies the jaz murian depression which is a subduction related back arc basin the band e zeyarat dar anar ophiolite is one of the ophiolite complexes it is located on the west side of the makran accretionary prism and jaz murian depression and is bounded by two major fault systems the principal rock units of this complex are a gabbro sequence which includes low and high level gabbros an extensive sheeted diabase dike sequence late intrusive rocks which consist largely of trondhjemites and diorites and volcanic rocks which are largely pillow basalts interbedded with pelagic sedimentary rocks including radiolarian chert chondrite and primitive mantle normalized incompatible trace element data and age corrected nd pb and sr isotopic data indicate that the band e zeyarat dar anar ophiolite was derived from a midocean ridge basalt like mantle source the isotopic data also reveal that the source for basalts was indian ocean type mantle based on the rare earth element ree data and small isotopic range all the rocks from the band e zeyarat dar anar ophiolite are cogenetic and were derived by fractionation from melts with a composition similar to average e morb fractionation was controlled by the removal of clinopyroxene hornblende and plagioclase three 40ar 39ar plateau ages of 140 7 2 2 142 9 3 5 and 141 7 1 0 ma and five previously published k ar ages ranging from 121 4 to 146 5 ma for the hornblende gabbros suggest that rocks from this ophiolite were formed during the late jurassic early cretaceous plate reconstructions suggest that the rocks of this complex appear to be approximately contemporaneous with the masirah ophiolite which has crystallization age of 150 ma like masirah the rocks from the band e zeyarat dar anar ophiolite complex represent southern tethyan ocean crust that was formed distinctly earlier than crust preserved in the 90 100 ma bitlis zagros ophiolites including the samail ophiolite

geochemistry and origin of mn deposits in the waziristan ophiolite complex north waziristan pakistan the waziristan ophiolite complex is located along the western margin of the indian plate in northwestern pakistan the mn deposits in the saidgi and shuidar areas are part of this ophiolite complex these deposits both banded and massive in nature are hosted by metachert and are generally overlying metavolcanics braunite and cryptocrystalline quartz are the main constituents of the manganese ores in both areas hematite occurs in the shuidar deposits as a minor phase metacherts are microcrystalline aggregates of microcrystalline quartz chalcedony and lesser hematite chemically the studied mn deposits and associated metacherts are very similar to those formed by submarine hydrothermal effusive processes the deposits originated along sea floor spreading centers mid ocean ridges within the neo tethys ocean and were later obducted as part of the waziristan ophiolite complex

biostratigraphic zonation and 40ar 39ar ages for the neotethyan khoy ophiolite of nw iran abstract the khoy ophiolite in northwestern iran represents a piece of oceanic lithosphere formed in the mesozoic neotethys this ophiolite consists from bottom to top of well defined basal metamorphic rocks peridotites gabbros and a variety of upper to lower cretaceous deep marine rocks these include pelagic fossiliferous carbonates and basalts as interlayers or exotic blocks four hornblende separates from gabbros and amphibolites give values from 108 to 111 ma the results from microfossil studies show the presence of late cretaceous e g marginotruncana globotruncana and middle to late eocene e g globorotalia orbulonoides planktonic foraminifera the data suggest that although the initial rifting and crystallization of the oceanic crust of the khoy ophiolite began in the late albian the volcanic activities continued through to the late eocene time another possibility for such variation of lithofacies and biofacies and disturbances in the fossil record may be the result of bioturbation and transportation and rapid changes of depositional conditions during eruption of the basaltic lava flows pelagic sediments alone have been listed and of the foraminiferas the first three are planktonic as would be expected only orbulonoides is a benthic form and aberrant the third possibility is that the khoy ophiolite may represent a classic supra subduction type ophiolite where volcanic members of the different parts of oceanic environment e g morb volcanic arc ocean island are now juxtaposed

the petrochemistry of ophiolite gabbroic complexes a key for the classification of ophiolites into low ti and high ti types ophiolites have been divided into two groups high ti and low ti types these can be discriminated by studying the fractionation trends of both gabbroic complexes this work and lavas and dykes 16 particularly in the tio2 m i diagram the first type typically shows morb like magmas whereas in the second the magma types have a spectrum of composition from mid ocean ridge basalts to island arc tholeiites and boninite like magmas often occur high ti ophiolites are petrologically and geochemically similar to major oceanic and ensialic back arc basin crusts as well as oceanic crust generated during the intermediate and late stage opening of intraoceanic back arc basins parental magmas and fractionation processes of low ti ophiolites fit with an hypothesis of their formation in the early stage of opening of intraoceanic back arc basins

stratigraphic evidence for an early collision between northwest india and asia the collision of india with asia1 had a profound influence on late cretaceous and cenozoic oceanography2 climate3 fauna extinctions4 and the motion of at least some of the earth s litho spheric plates5 as the collision ended a period of rapid indo asian convergence6 a precise knowledge of its timing when the crust of the neo tethys ocean was completely subducted7 at some point along the plate boundary is important for understanding its wider consequences but current estimates of the collision age range from 65 to 38 myr before present6 8 11 here we report the results of extensive biostratigraphic analyses from waziristan and kurram in northwest pakistan which show that accretionary prism and trench strata were first thrust onto the northwest indian passive margin after 66 myr but before 55 5 myr after this time volcanic arc fragments the accretionary prism trench material and imbricates of the north indian slope were raised to shallow water depths and overlapped by upper palaeocene shallow water carbonates and shales12 14 indicative of post collision thrusting in this region finally both the suture and the indian craton were overlapped by continuous unconformable upper lower eocene shallow marine strata demonstrating that suturing was complete by 49 myr

structure and tectonics of the himalaya constraints and implications of geophysical data the himalayan range is a product of continental collision between india and eurasia and for several reasons it is a logical place to examine the physical causes and mechanisms of mountain building the exceptionally high mountains and the great length of the range alone single the himalaya out as a major structural feature the convergence rate between india and eurasia of about 50 mm yr 1 minster jordan 1978 requires rapid rates of slip and the occurrence of four major earthquakes with magnitudes in excess of 8 4 in the last 100 years attests to a high level of seismicity and a rapid rate of active deformation e g richter 1958 seeber armbruster 1981 the exposures of high grade metamorphic rocks in the greater himalaya may suggest large vertical transport of lower crustal rocks e g valdiya 1980a b and the occurrences of young postcollision granites imply recent large perturbations of the temperature structure of the range e g lefort 1981 moreover as sediments derived from the himalaya fill the broad ganga basin south of the range the mountain belt simul taneously seems to override the basin e g g d johnson et al 1979 n m johnson et al 1982 as such the processes occurring now and since the collision began in the early tertiary as well as the present structure of the himalayan belt are probably similar to those at different stages of collision and mountain building in other older orogenic belts such as the appalachians the scottish and scandinavian caledonides the alps and

geochemistry and depositional environment of mn oxide deposits in the tokoro belt northeastern hokkaido japan numerous early cretaceous mn oxide and mn fe deposits are distributed in the tokoro belt northeastern hokkaido japan the mn oxide deposits usually occur in the southwestern part of the belt and within bedded cherts whereas the mn fe deposits are restricted to the northeastern part of the belt and occur between bedded cherts and underlying basaltic pillow lava the mn oxide ores are characterized by low fe mn ratios and the trace element concentrations and rare earth element ree patterns are similar to those of the submarine hydrothermal mn deposits distribution patterns of trace elements co ni cu and zn and ree in the mn oxide ores determine two types of deposits hydrogenous deposits which have relatively high concentrations and submarine hydrothermal deposits which have low concentrations these compositional trends and geologic evidences of the tokoro mn deposits suggest that the fe phases precipitated first closer to their source whereas the mn phases deposited later at a distance from their source some hydrogenous effects on the mn oxide deposits are due to the adsorption of trace elements from seawater trace element concentrations of tokoro mn oxide deposits show that they formed within 40 km from their source

u pb zircon ages field geology and geochemistry of the kermanshah ophiolite iran from continental rifting at 79ma to oceanic core complex at ca 36ma in the southern neo tethys abstract the geodynamic evolution of the zagros mountains of iran remains obscure in particular the time of formation of the zagros ophiolites and the closure of the neo tethys ocean are highly controversial here we present new precise zircon u pb ages that show that the younger part sahneh kamyaran of the kermanshah ophiolite formed at 35 7 0 5 ma and the older part harsin at 79 3 0 9 ma field relations and geochemical evidence show that the younger sahneh kamyaran part is probably a fossil oceanic core complex and the older sahneh part is probably a continental oceanic transition complex both the sahneh kamyaran and sahneh parts were later emplaced into an accretionary complex we conclude and infer that the final closure time of the southern neo tethys ocean was after the late eocene our data and tectonic model have crucial implications for the geodynamic evolution of the zagros region

hydrothermal chert and associated siliceous rocks from the northern pacific their geological significance as indication od ocean ridge activity adachi m yamamoto k and sugisaki r 1986 hydrothermal chert and associated siliceous rocks from the northern pacific their geological significance as indication of ocean ridge activity sediment geol 47 i25 148 analyses of 42 cretaceous rock samples from deep sea drilling project dsdp leg 32 in tile northern pacific show that chert and porcellanite are enriched in fe mn p cu ba mo ni and pb relative to both triassic biogenic cherts of central japan and cretaceous dsdp cherts of leg 62 the iron enrichment up to 6 68ve fe20 as total iron is most prominent in chert immediately overlying basal basah which is the only chert free of radiolarians and veined by megaquartz containing many fluid inclusions chemical features characterized by a low ratio for a1 ai fe mn and ai z tio 2 and a high ratio for total fe2oj tio 2 mno tio 2 and mo tio 2 as well as petrological evidence indicatc that the cherts formed in an intense submarine hydrothermal environment on or near the cretaceous japanese spreading center which separated the kula plate from the pacific plate the spreading rate of the cretaceous japanese spreading center estimated from the chemical composition of the cherts was about 3 7 cm r i the rate is comparable to that inferred from the geomagnetic data

late diagenetic changes in manganese ores of the upper proterozoic penganga group india sedimentary manganese oxide ores occur interstratified with chert and minor calcareous shale and enclosed in limestone in the transgressive upper proterozoic marine sequence of the penganga group andhra pradesh india this sequence shows no evidence of volcanism or hydrothermal activity and is not metamorphosed the manganeseoxide ores show unusual coexistence of the primary sedimentary minerals todorokite and birnessite with their diagenetic conversion products manganite braunite and bixbyite the mineralogical characters and chemical compositions of the important phases as well as their texture are discussed braunite occurs in two generations one earlier or contemporaneous with bixbyite and the other later than bixbyite the different stages of conversion of todorokite and birnessite to braunite and todorokite to manganite are preserved the general observations suggest that bixbyite was also produced by conversion of todorokite birnessite pyrolusite and cryptomelane conventionally considered as diagenetic products of mn oxide hydroxide precursors deposited as primary sediments are here rarely present and then only as supergene derivatives this documentation of diagenetic transformation of primary todorokite and birnessite directly to braunite and bixbyite should be considered while extrapolating the progenitor mineralogy of the metamorphosed manganese oxide ores that abound in the precambrian

the himalayan arc large scale continental subduction oroclinal bending and back arc spreading abstract palaeomagnetic results from the himalayan arc and southern tibet compared with simulated apparent polar wander paths for the indian plate show a consistent pattern of rotations of the himalayan arc relative to the indian shield varying gradually from 45 clockwise in the northwestern himalaya to slightly counterclockwise in the lhasa region this pattern is consistent with continental underthrusting of greater india beneath the tibetan plateau since the early miocene over at least 650 km at the longitude of western nepal and oroclinal bending since the latest miocene available palaeomagnetic observations support the steady state model for the formation of the himalayan arc with refinements as follows 1 collision between greater india s northern boundary and southern asia occurred at equatorial latitudes with progressive suturing from palaeocene in the northwestern himalaya until early eocene in the eastern himalaya 2 continuing convergence and indentation of greater india into southern asia over about 2000 km up to the early miocene resulted in southeastward extrusion of indochina and 3 neogene counterclockwise rotational underthrusting of greater india along the main central thrust with pliocene quaternary oroclinal bending of the himalayan arc

genetic diversity of manganese deposition in the terrestrial geological record terrestrial manganese deposits formed by hydrothermal sedimentary and supergene processes ancient analogues of modern oceanic hydrothermal deposits formed in spreading centre and subduction related settings and those deposited from terrestrial hot springs are discussed sedimentary mn oxide deposits formed in shallow water at the margins of stratified oceans above the redoxcline during sea level changes mn carbonate deposits probably formed by diagenesis through mn oxyhydroxide reduction coupled with organic matter oxidation climatic variation and basin water stratification responsible for mn concentration were manifestations of atmospheric coz content prompted by tectonism supergene manganese enrichment in continental weathering profiles was mainly dictated by climate topography and drainage systems manganese deposits of diverse genetic types occur in the terrestrial geological record roy 1981 these were produced by direct hydro thermal activity sedimentary processes and continental weathering although the processes may be interrelated each involves distinct mechanisms that place the deposits into specific genetic types some of these processes are best understood in preset day depositional sites the ancient environments were determined by the intensity and style of tectonism volcanism and hydrothermal activity the composition of the atmosphere and the hydrosphere and the development of the biosphere which all varied with time it is my intention to unfold the total panorama of the environmental evolution during different time envelopes that produced a variety of manganese deposits in the past only major deposits and those providing distinctive evidence of their environment of deposition will be considered here

origin of manganese giants sea level change and anoxic oxic history inversely graded pisolites of manganese oxide in the mid cretaceous manganese giant at groote eylandt australia were deposited during a regional fall in sea level as indicated by their relationships to wave cut benches dissolved manganese was concentrated in relatively anoxic waters of the intracratonic basin during a marine transgression and precipitated as oxides and carbonates during a regression ore bodies ultimately formed by concentration of abundant particulate manganese by a tidal lag mechanism

jurassic rifting at the eurasian tethys margin geochemical and geochronological constraints from granitoids of north makran southeastern iran this study focuses on an east west trending belt of granitic to intermediate intrusions and their volcanic cover in the northern dur kan complex a continental slice outcropping to the north of the exposed makran accretionary wedge in southeastern iran field observations petrographic descriptions trace element and isotope analyses combined with u pb zircon geochronology are presented to determine the time frame of magmatism and tectonic setting during the formation of these rocks results document three magmatic episodes with different melt sources for 1 granites 2 a diorite trondhjemite plagiogranite sequence and 3 diabases and lavas granites dated at 170 175 ma represent crystallized melt with a strong continental isotopic contribution the diorite trondhjemite plagiogranite sequence is 165 153 ma old and derives from a mantle magma source with minor continental contribution east west trending diabase dikes and bodies intruded the granitoids which were eroded and then covered by valanginian 140 133 ma alkaline lavas and sediments alkaline dikes and lavas have a mantle isotopic composition temporal correlation with plutonites of the sanandaj sirjan zone to the northwest defines a narrow nw se striking and nearly 2000 km long belt of jurassic intrusions the increasing mantle influence in the magma sources is explained by thinning of continental lithosphere and related mantle upwelling decompression melting accordingly the formation of the studied igneous rocks is related to the extension of the iranian continental margin which ultimately led to the formation of the tethys related north makran ophiolites

hydrothermal mn deposits of the franciscan assemblage ii isotope and trace element geochemistry and implications for hydrothermal convection at spreading centers abstract major element trace element and sr nd pb and o isotopic data for a franciscan mn deposit suggest an origin by seafloor hydrothermal circulation based on q mode factor analysis the cherts and mn lenses of the blue jay mine formed from a combination of 4 components representing 1 biogenic 1 hydrothermal and 2 detrital sources rb sr u th pb and o isotopic systematics in the mn lenses were affected by input from the hydrothermal circulation of material leached from the underlying basalts nd isotopic compositions in both cherts and mn lenses are identical and within the range measured for pacific ocean water suggesting the ree were not mobilized by hydrothermal activity correlation of 18 o with sio 2 and mno 2 in the mn lenses implies the lenses formed by simple mixing of hydrothermally derived mn oxides with seawater and biogenic silica 18 o of the cherts is both uniform and depleted relative to dsdp jurassic cherts but similar to microquartz bearing cherts of the monterey formation this suggests that diagenetic activity exerted more control on oxygen isotope compositions then hydrothermal alteration or metamorphism finally a well defined rb sr isochron of 158 5myr was obtained for these cherts and opens the possibility of determining absolute radiometric ages for similar cherts throughout the geologic record

iranshahr blueschist subduction of the inner makran oceanic crust the makran accretionary prism in se iran and sw pakistan is one of the most extensive subduction accretions on earth it is characterized by intense folding thrust faulting and dislocation of the cenozoic units that consist of sedimentary igneous and metamorphic rocks rock units forming the northern makran ophiolites are amalgamated as a melange metamorphic rocks including greenschist amphibolite and blueschist resulted from metamorphism of mafic rocks and serpentinites in spite of geodynamic significance of blueschist in this area it has been rarely studied peak metamorphic phases of the northern makran mafic blueschist in the iranshahr area are glaucophane phengite quartz omphacite epidote post peak minerals are chlorite albite and calcic amphibole blueschist facies metasedimentary rocks contain garnet phengite albite and epidote in the matrix and as inclusions in glaucophane the calculated p t pseudosection for a representative metabasic glaucophane schist yields peak pressure and temperature of 11 5 15 kbar at 400 510 c these rocks experienced retrograde metamorphism from blueschist to greenschist facies 350 450 c and 7 8 kbar during exhumation a back arc basin was formed due to northward subduction of neotethys under eurasia lut block exhumation of the high pressure metamorphic rocks in northern makran occurred contemporarily with subduction several reverse faults played an important role in exhumation of the ophiolitic and hp lt rocks the presence of serpentinite shows the possible role of a serpentinite diapir for exhumation of the blueschist a tectonic model is proposed here for metamorphism and exhumation of oceanic crust and accretionary sedimentary rocks of the makran area vast accretion of subducted materials caused southward migration of the shore this article is protected by copyright all rights reserved

braunite mineralogy and paragenesis from the kajlidongri mine madhya pradesh india the kajlidongri manganese deposit madhya pradesh india contains braunite belonging to at least two different parageneses i e a fine grained metamorphic type and a younger coarser grained hydrothermal type microanalyses of braunite from these paragenetic types indicate considerable variations in iron content from zero to 32 7 fe3o3 which the writers consider is due to varying proportions of braunite i and bixbyite modules in polysomatic braunite

sediment sea water interaction at 200 and 300 c 500 bars pressure the role of sediment composition in diagenesis and low grade metamorphism of marine clay experimental interaction of clay rich marine sediments with sea water at 200 and 300 c 500 bars and a water rock mass ratio of 5 resulted in the generation of relatively acid oxidizing conditions the development of acidity is attributed to the formation of smectite whereas the oxidation state of the fluids was dependent on the relative proportions of ferromanganese oxide phase ferrous rich silicate phases and organic carbon an experiment conducted with an oxidized sediment at 300 c thus produced higher aqueous concentrations of mn and associated base metals at a relatively high fo2 10 15 but an organic carbon rich sediment yielded fluids characterized by a lower fo2 10 20 lower concentrations of mn higher fe and exhibited substantial thermal alteration and dissolution of organic components silica concentrations in the experimental fluids ranged from cristobalite to amorphous silica saturation levels in response to the dissolution of amorphous silica comparison of fluid data and mineralogical assemblages from the gulf coast and salton sea regions with experimental results suggests that alteration processes in sedimentary basins can in large part be attributed to water rock interaction observed differences are apparently due to the higher degree of reaction in the geologic environment and to the increased stability of the illite chlorite assemblage at the lower silica activity characteristic of geologic fluids

convergence history across zagros iran constraints from collisional and earlier deformation the zagros orogen provides a unique opportunity within the alpine system to evaluate the interplay between a young tertiary collision and earlier subduction obduction processes within the crush zone and the sanandaj sirjan internal zone separating the zagros fold belt from central iran we document several major tectonic events taking place at the end of the cretaceous of the eocene and from the mio pliocene onwards ca 20 15 ma contrary to recent interpretations our data cross sections and description of the overall deformation style strongly suggest that the main zagros thrust mzt is deeply rooted possibly to moho depths and that the suture zone effectively runs along the mzt field observations show that the final resorption of the oceanic domain took place slightly after 35 ma and that collision must have started before ca 23 25 ma in northern zagros the shortening rate across the crush zone since the mid miocene 20 15 ma is estimated at a minimum 3 4 mm year shear movements in the crush zone during the eocene oligocene period and extensional strike slip movements in the internal zones during the late cretaceous point to an oblique setting early in the convergence history a geotectonic scenario for convergence from the time of obduction to the present is finally proposed

a new tectonic scenario for the sanandaj sirjan zone iran abstract recent geochemical studies of volcanic rocks forming part of the ophiolites within the zagros and naien baft orogen indicate that most of them were developed as supra subduction ophiolites in intra oceanic island arc environments intra oceanic island arcs and ophiolites now forming the naien baft zone were emplaced southwestward onto the northeastern margin of the south sanandaj sirjan zone while those now in the high zagros were emplaced southwestward onto the northern margin of arabia thereafter subduction continued on opposite sides of the remnant oceans the floor of neo tethys ocean was subducted at a low angle beneath the entire sanandaj sirjan zone and the floor of the naien baft ocean was subducted beneath the central iranian micro continent the naien baft ocean extended into north west iran only temporarily this failed ocean arm between the urumieh dokhtar magmatic assemblage and the main zagros thrust was filled by thick upper triassic upper jurassic sediments the naien baft ocean finally closed in the paleocene and neo tethys closed in the early to middle eocene after arabia was sutured to iran the urumieh dokhtar magmatic assemblage recorded slab break off in the middle eocene

glauconite formation as a factor in sedimentary manganese deposit genesis concentrations of manganese in the crust are dominated by a single ore type described as the orthoquartzite glauconite clay association or as manganese giants current research suggests that these large accumulations of manganese oxide and or carbonate are associated with relatively rapid marine transgressions and regressions across older basement rocks some of the larger deposits of this type such as nikopol and chiatura ussr and groote eylandt australia occur in petrologically simple terrigenous formations generally sandstones and shales interpreted as paleoshoreline sediments a major problem lies in the very high mn fe ratio of these deposits 30 50 as against that of ordinary sediments 0 02 research on the groote eylandt cenomanian age deposit suggests that the high mn fe of the primary ore 15 20 was associated with manganese enrichment in coastal waters which in part was related to glauconitization iron trapping reactions mineralogical evidence indicates a sequence from a well defined glauconite phase in deeper water off shore sediments with whole rock mn fe of 0 3 1 2 to a glauconite smectite in near shore estuaries and rivers as a minor component of pisolitic manganese oxides of mn fe ranging from 15 to 20 data on these glauconite minerals including x ray diffraction infrared spectroscopy microanalyzes textural occurrences and genesis are presented geologically glauconite is a product of a postoxic environment possibly related to fluctuating water depth during transgression regression seaward from the site of manganese oxide precipitation

manganese and ferromanganese ores from different tectonic settings in the nw himalayas pakistan abstract in pakistan manganese and ferromanganese ores have been reported from the hazara area of north west frontier province waziristan agencies in the federally administered tribal areas and the lasbela khuzdar regions of baluchistan this study is focused on comparison of mineralogy and geochemistry of the continental ferromanganese ores of hazara and the ophiolitic manganese ores of the waziristan area of pakistan in the hazara area ferromanganese ores occur at kakul galdanian and chura gali near abbottabad within the hazira formation of the kalachitta margala thrust belt of the nw himalayas of the indo pakistan plate the cambrian hazira formation is composed of reddish brown ferruginous siltstone with variable amounts of clay shale ferromanganese ores phosphorite and barite in waziristan manganese ores occur at shuidar mohammad khel and saidgi within the waziristan ophiolite complex on the western margin of the indo pakistan plate in nw pakistan these banded and massive ores are hosted by metachert and overlie metavolcanics the ferromanganese ores of the hazara area contain variable amount of bixbyite partridgeite hollandite pyrolusite and braunite bixbyite and partridgeite are the dominant mn bearing phases hematite dominates in fe rich ores gangue minerals are iron rich clay alumino phosphate minerals apatite barite and glauconite are present in variable amounts in both fe rich and mn rich varieties the texture of the ore phases indicates greenschist facies metamorphism the waziristan ores are composed of braunite with minor pyrolusite and hollandite hematite occurs as an additional minor phase in the fe rich ores of the shuidar area the only silicate phase in these ores is cryptocrystalline quartz the chemical composition of the ferromanganese ores in hazara suggests that the mn fe was contributed by both hydrogenous and hydrothermal sources while the manganese ores of waziristan originated only from a hydrothermal source it is suggested that the fe mn ores of the hazara area originated from a mixed hydrothermal hydrogenetic source in shallow water in a ontinental shelf environment due to the transgression and regression of the sea while the mn ores of waziristan were formed at sea floor spreading centers within the neo tethys ocean and were later obducted as part of the waziristan ophiolite complex

late cretaceous infant intra oceanic arc volcanism the central pontides turkey petrogenetic and tectonic implications abstract a tectonic slice of an arc sequence consisting of low grade metavolcanic rocks and overlying metasedimentary succession is exposed in the central pontides north of the izmir ankara erzincan suture separating laurasia from gondwana derived terranes the metavolcanic rocks mainly consist of basaltic andesite andesite and mafic cognate xenolith bearing rhyolite with their pyroclastic equivalents which are interbedded with recrystallized pelagic limestone and chert the metasedimentary succession comprises recrystallized micritic limestone with rare volcanogenic metaclastic rocks and stratigraphically overlies the metavolcanic rocks the geochemistry of the metavolcanic rocks indicates an arc setting evidenced by depletion of hfse ti p and nb and enrichment of fluid mobile lile identical trace and rare earth elements compositions of basaltic andesites andesites and rhyolites suggest that they are cogenetic and derived from a common parental magma the arc sequence crops out between an albian turonian subduction accretionary complex representing the laurasian active margin and an ophiolitic melange absence of continent derived detritus in the arc sequence and its tectonic setting in a wide cretaceous accretionary complex suggest that the kosdag arc was intra oceanic zircons from two metarhyolite samples give late cretaceous 93 8 1 9 and 94 4 1 9 ma u pb ages these ages are the same as the age of the supra subduction ophiolites in western turkey which implies that that the kosdag arc may represent part of the incipient arc formed during the generation of the supra subduction ophiolites the low grade regional metamorphism in the kosdag arc is constrained to 69 9 0 4 ma by 40 ar 39 ar muscovite dating indicating that the arc sequence became part of a wide tethyan cretaceous accretionary complex by the latest cretaceous non collisional cessation of the arc volcanism is possibly associated with southward migration of the magmatism as in the izu bonin mariana arc system

the petrogenesis and tectonic setting of lavas from the baft ophiolitic melange southwest of kerman iran a late cretaceous ophiolite complex in the baft area southwest of kerman iran is characteristic of the central iranian ophiolitic melange belt which wraps around the lut block despite the extensive tectonic disruption of the baft complex most ophiolitic lithologies are present and many original igneous contacts are preserved a lack of cumulate gabbros within the sequence suggests that a large and continuous magma chamber did not exist beneath the baft spreading axis geochemical data confirm the presence of two distinct compositional groups in the mafic lavas 1 tholeiitic basalt and 2 transitional tholeiitic basalt the tholeiitic lavas are similar to typical mid ocean ridge basalt compositions whereas the transitional tholeiites are similar to intraplate basalts the available data suggest that the baft ophiolite complex formed in a small ocean basin possibly at or near a ridge transform intersection emplacement may have occurred as a result of conversion of the transform fault to a subduc

petrology geochemistry and tectonic setting of the khoy ophiolite northwest iran implications for tethyan tectonics abstract the khoy ophiolite in northwestern iran represents a remnant of oceanic lithosphere formed in the mesozoic neo tethys this northwest southeast trending ophiolite complex consists from bottom to top east to west of a well defined basal metamorphic zone peridotites dunite harzburgite and serpentinized peridotite gabbros sheeted dikes pillow and massive lava flows and pelagic sedimentary rocks including radiolarian chert the rocks of the metamorphic zone have an inverse thermal gradient from amphibolite facies to greenschist facies the high grade metamorphic rocks are immediately adjacent to the peridotite and the gabbros and the low grade rocks are in contact with the precambrian kahar formation based on mantle normalized incompatible trace element diagrams there are two distinct types of basalt flows present at the khoy ophiolite 1 massive basalts that have patterns virtually identical to e morb and 2 pillow basalts that have more primitive chemical composition whose trace element patterns plot between e morb and n morb the chondrite normalized ree patterns for the pillow basalts are lree depleted lan smn ave 0 70 similar to patterns for the mean diabase composition for the oman ophiolite and lree depleted basalts of the band e zeyarat ophiolite of southern iran the ree patterns for the massive basalts are similar in general ree abundances to the pillow basalt patterns but they are slightly lree enriched lan smn ave 1 09 and their patterns cross those of the pillow basalts the ree patterns for the gabbros and diorites indicates that the crustal suite rocks were most likely derived by a process of fractional crystallization from a common basaltic melt this basaltic melt was most likely generated by approx 20 25 partial melting of a simple lherzolite source and had ree concentrations of roughly 10 chondrite a comparison between the results from the khoy ophiolite and the data from other iranian ophiolites reveals geochemical evidence to suggest a tectonic link between the khoy ophiolite and the rest of the iranian ophiolites our results suggest that khoy ophiolite is equivalent to the inner group of iranian ophiolites e g nain shahr babak sabzevar tchehel kureh and band e zeyarat and was formed as a result of closure of the northwestern branch of a narrow mesozoic seaway which once surrounded the central iranian microcontinent

the origin of hydrothermal metalliferous sediments associated with the early mesozoic othris and pindos ophiolites mainland greece abstract metalliferous sediments and sulphides are associated with the jurassic othris and pindos ophiolites central and northern greece these ophiolites formed at an early mesozoic spreading ridge within a small pindos ocean basin created by rifting of the northern margin of gondwanaland ubiquitous terrigenous silt was derived from adjacent continental margins mainly the pelagonian microcontinent to the east in the othris area disseminated massive and vein sulphides occur within and above morb type pillow lavas and volcaniclastic sediments sulphide was precipitated in quartz veins within fault controlled stockworks episode with local epidosite is locally very abundant in extrusives immediately underlying thick si fe rich hydrothermal sediments massive sulphides were precipitated as small fault controlled bodies that are directly overlain by fe rich mudstones and fe rich cherts strongly depleted in mn mudstones relatively enriched in hydrothermal fe mn cu zn pb ree and ba were ponded into small fault controlled hollows around the fringes of the sulphide precipitating field elsewhere e g northern othris manganese rich layers in ribbon radiolarites are strongly enriched in mn and trace elements metal enriched deposits are also found within slices of mid ocean ridge type lavas associated with melange avdella melange structurally underlying the pindos ophiolite nw greece sulphides there are restricted to tiny massive deposits overlying pillow lava and sulphide veins within ophiolitic extrusives pillow lavas are overlain by volcanic talus breccias shed from submarine fault scarps overlying metalliferous mudstones are enriched in fe and other hydrothermal constituents probably derived from nearby sulphide precipitating zones and mixed with lithogenous sediments the metalliferous deposits preserve a variety of hydrothermal deposits within an early mesozoic small ocean basin the pindos sediments reflect mixing of ferruginous hydrothermal constituents with terrigenous sediment possibly during the earlier stages of basin opening the othris sulphide and ferruginous oxide sediments record evidence of a small high temperature hydrothermal field formed near the spreading axis when the ocean neared its maximum width manganese rich cherts in the northern othris area reflect widespread accumulation of hydrothermal constituents on the abyssal plain derived from axial high temperature hydrothermal fields and or related to low temperature off axis discharge

review of platinum group element distribution and mineralogy in chromitite ores from southern iran abstract the platinum group element pge potential of four chromite mining districts in southern and south eastern iran was investigated using geochemical and mineralogical methods a total of 20 mines and prospects were sampled in the neyriz ophiolite the abdasht and sikhuran complexes in the esfandagheh district and the faryab district chromitite and dunite were analysed for major trace element and pge geochemistry and the platinum group element mineralogy pgm in all mining districts chromites analysed from mantle tectonite probable transition zone and cumulate zone chromitites are refractory low in tio2 pge concentrations range from 5000 ppb with median values around 200 ppb in all complexes investigated the 187os 188os isotope ratios of chromitite indicate chondritic to slightly suprachondritic initial os mantle normalised pt ir ratios are generally low in the sikhuran complex pods of sulphide bearing chromitite 0 2 1 2 sulphides occur in a dunite zone interpreted as an ultramafic cumulate zone chromites in chromitite are characterised by high cr 75 81 and intermediate mg numbers 45 65 in associated dunite chromites cr 74 80 are more fe rich mg 29 50 chromitite and dunite carry up to 5 2 ppm total pge and are characterised by high normalised pt ir up to 5 pt pd large degrees of partial melting in a supra subduction zone geotectonic setting probably in a two stage melting process produced boninitic melts that crystallised refractory cr mg rich and ti poor chromite in some cases sulphide saturation was achieved during and after chromite formation saturation during chromite crystallisation is evidenced by ni cu rich sulphide droplets preserved in some chromitites sulphur saturation after chromite crystallisation governed the formation of ni rich sulphide liquids that exsolved monosulphide solid solution which later crystallised to form pge bearing pentlandite platinum and pd did not partition into pentlandite but crystallised as pt and pd rich pgm associated with the pentlandite

rare earth element geochemistry of oceanic ferromanganese nodules and associated sediments abstract analyses have been made of ree contents of a well characterized suite of deep sea 4000 m principally todorokite bearing ferromanganese nodules and associated sediments from the pacific ocean ree in nodules and their sediments are closely related nodules with the largest positive ce anomalies are found on sediments with the smallest negative ce anomalies in contrast nodules with the highest contents of other rare earths 3 ree are found on sediments with the lowest 3 ree contents and vice versa 143 nd 144 nd ratios in the nodules 0 51244 point to an original seawater source but an identical ratio for sediments in combination with the ree patterns suggests that diagenetic reactions may transfer elements into the nodules analysis of biogenic phases shows that the direct contribution of plankton and carbonate and siliceous skeletal materials to ree contents of nodules and sediments is negligible inter element relationships and leaching tests suggest that ree contents are controlled by a p rich phase with a ree pattern similar to that for biogenous apatite and an fe rich phase with a pattern the mirror image of that for sea water it is proposed that 3 ree concentrations are controlled by the surface chemistry of these phases during diagenetic reactions which vary with sediment accumulation rate processes which favour the enrichment of transition metals in equatorial pacific nodules favour the depletion of 3 ree in nodules and enrichment of 3 ree in associated sediments in contrast ce appears to be added both to nodules and sediments directly from seawater and is not involved in diagenetic reactions

a ground water mixing model for the origin of the imini manganese deposit cretaceous of morocco the imini deposit just south of the atlas mountains of morocco consists of three beds of manganese oxide ore in a slightly deformed dolomite unit about 10 m thick of cenomanian turonian late cretaceous age the dolomite host is sugary textured massive and permeable dolomite occurs as nearly euhedral rhombs each ore bed is about a meter thick and consists almost entirely of intergrowths of pyrolusite and hollandite family minerals bao up to 11 and pbo up to 28 with a flattened geode structure overlying the ore beds are solution collapse breccias with dolomite clasts janggunite mn fe oxide and dolomite matrix and pyrolusite hollandite calcite cement the ore solution collapse breccia and coarsely recrystallized pure dolomite are coextensive a zone only 400 to 1 000 m wide but 25 km long and elongate east northeast west southwest diagenetic features of the deposit are recrystallized texture of the dolomite flattened geode structure of the ore solution collapse breccia and associated draped laminae of the insoluble residues fossil replacement by mn oxides and zoned chert nodules with inclusions a mineralogic paragenesis shown independently in each of these features from older to younger is 1 precursor carbonates 2 dolomite janggunite and 3 hollandite pyrolusite chert calcite diagenesis probably occurred during the late cretaceous in a mixing zone of fresh and saline ground water as similar diagenetic sequences without manganese deposits are widely reported to have formed in such environments carbon and oxygen isotope compositions of carbonates and oxygen isotope compositions of chert show that an increasing component of fresh meteoric ground water was present as diagenesis proceeded falling sea level allowed the mixing zone to occupy various positions within the carbonate unit and convert the primary calcareous carbonate rocks to dolomite with chert nodules anoxic seawater is known both worldwide and from the local depositional basin atlas gulf in the cenomanian turonian this is believed to be the key to manganese deposition because anoxic conditions greatly enhance manganese solubility two specific versions of the mixing zone model are presented for manganese deposition one in which a primary sedimentary manganese precipitate formed along margins of the anoxic basin and was recrystallized during diagenesis in the mixing zone and the other in which anoxic saline ground water brought manganese into the mixing zone where it precipitated

environments and processes of manganese deposition concentration of manganese in solution and its deposition takes place by redox controlled processes in a variety of modern and ancient geologic and geochemical environments modern mn deposition occurs predominantly in deep sea areas rather than shallow water domains although deep sea sedimentary deposits dominate hydrothermal contribution of mn to the ocean system may be substantial mn deposition from hydrothermal solutions at or near sea floor spreading centers and less commonly in island arc areas is known in addition near and far field dispersion of mn from vent sites is also substantial such distributions are controlled by the flow rate and egress temperature of the solution and the residence time of mn in seawater thus even in sedimentary deposit domains at least partial derivation of mn from a hydrothermal source is possible sedimentary fe mn crusts on older volcanic substrates on seamounts form by hydrogenous deposition of metal concentrated from terrigenous sources in the mid water column oxygen minimum zones thus the presence or absence of volcanic rocks is not a clear indication of whether sedimentary mn deposits particularly in the ancient geologic record are the result of a totally terrigenous or a totally volcanogenic source abyssal fe mn nodules are considered to form from a basin water hydrogenous and or pore water early diagenetic supply of metals but in most cases the extent of supply from either of the sources is unknown the metal incorporation mechanisms of free moving nodules is little understood and it is possible that in most cases both sources contribute to the nodule composition therefore no nodule should be considered as totally hydrogenous or totally early diagenetic based only on its bulk composition the determined growth rate giving only an average value cannot by itself reveal the growth history of the nodules biological participation directly or indirectly controls mn deposition the stratified black sea demonstrates the concentration of mn in solution in an anoxic zone its advection toward the redox interface and its precipitation in an oxygenated condition similar stratified basins are contemplated for ancient mn deposition in shallow water basin margin areas geologic and geochemical signatures indicate that during sea level highstands stratified basins formed in which mn was concentrated in solution in the anoxic part corresponding transgression led to the impingement of the redox interface on the continental shelf and precipitation of mn oxides could take place across the interface during transgression regression cycles offshore in anoxic or dysaerobic conditions mn carbonate could form by early diagenetic reaction of mn super 2 with co 2 or hco 3 super produced by organic carbon oxidation critical mn deposits occurring in transgressive glaciogenic and black shale bearing ancient sequences support this paleoenvironmental model for mn deposition

geology of the onshore makran accretionary wedge synthesis and tectonic interpretation abstract this contribution is a synthesis of studies carried out since 2004 in the makran accretionary wedge exposed in se iran these studies were intended to improve our understanding of the behaviour of accretionary wedges and of the significance of the makran wedge in the frame of tethys tectonics reassessing the stratigraphy and structure of the iranian makran has allowed distinguishing four main litho tectonic units the lithological content of these four units is briefly described to discuss the sedimentary environments which record the general filling of an oceanic basin with clastic sediments derived from the north the petrology of ophiolites exposed in the backstop region to the north of the accretionary wedge itself demonstrates the existence of an oceanic basin formed in the jurassic late cretaceous to the north of continental slivers which had separated from central iran there is petrological evidence for a subduction related magmatic arc during the late cretaceous which sourced much of the clastic material in the cretaceous lower miocene turbidites present throughout the wedge the literature survey shows that the late cretaceous to present evolution of the pakistan part of the makran fold and trust belt is in its major attributes similar to that of the iranian makran these observations and results are discussed in a tectonic model that integrates knowledge on the oman mountains on the other side of the current oman subduction

mixing characteristics and compositional differences in mantle derived melts beneath spreading axes evidence from cyclically layered rocks in the ophiolite of north oman within the ophiolite of north oman interlayered gabbros and peridotites form a unit up to 3 5 km thick between pervasively foliated harzburgites and dunites of mantle origin and isotropic gabbros and plagiogranites spatial and chemical relationships within this sequence require the existence of a large reservoir 20 km wide 3 5 km high of tholeiitic magma that was periodically replenished by mantle derived picritic to tholeiitic melts during the spreading process cyclicity in the appearance of minerals during cooling of the magma is marked by the repetition of stratigraphies in the layered rocks within these cyclic units basal peridotites with magnesian olivines and pyroxenes are abruptly overlain by gabbros with less magnesian mafic phases this abrupt discontinuity is explained by density stratification set up at the base of the chamber whenever a picritic melt influx occurs by virtue of its greater density than tholeiitic magma sparks et al 1979 any picritic magma emplaced into the base of the tholeiitic magma reservoir would spread out along the floor of the chamber with only limited mixing taking place across the liquid liquid interface cooling of this basal layer by contact with the cooler overlying magma would result in the precipitation of olivine chrome spinel pyroxene from the picritic liquid this process would continue until the now more evolved input liquids were incorporated into the tholeiitic magma resevoir the precipitation of a gabbroic layer would then take place until a new magma input occurred to repeat the cycle variable thicknesses of the cyclic units different liquid lines of descent and various entry points within the liquid line of descent represented by the lowermost layered rocks reveal that the input liquids varied in volume primary composition and the extent and nature of prefractionation in the mantle liquids expelled from the chamber as lavas and dykes are shown to be more fractionated than liquids precipitating the layered rocks and are considered to have undergone additional fractionation within a zone of underplated gabbro and plagiogranite beneath the dyke swarm the existence of a large magma reservoir supports earlier interpretations based on age tilton et al 1981 and geochemical data pearce 1980 that the oman ophiolite is an analogue for fast spreading ridges

hydrothermal manganese deposits in island arcs and related to subduction processes a possible model for genesis abstract submarine hydrothermal manganese deposits have recently been recovered from a number of island arc settings such as the tonga kermadec ridge ogasawa arc trench bonin arc mariana arc and bismarck archipelago it has also been shown that large volumes of fluid are released during dewatering of subducting sediments the amount of water subducted globally 1 5 10 15 g yr 1 is about two orders of magnitude less than that advected at ocean spreading centres hydrothermal manganese deposits may be precipitated from these waters the distribution of terrestrial manganese deposits around the circum pacific belt and elsewhere suggests that some of these deposits may be of hydrothermal origin related to subduction processes at present most of these terrestrial deposits are inadequately described to permit the establishment of the precise tectonic setting at the time of deposition of the manganese deposits and therefore an unambiguous assessment of this hypothesis

restoring earth surface processes through landform design a 13 year monitoring of a geomorphic reclamation model for quarries on slopes the application of geomorphic principles to land reclamation after surface mining has been reported in the literature since the mid 1990s mostly from australia canada and the usa this paper discusses the reclamation problems of contour mining and quarries on slopes where steep gradients are prone to both mass movement and water erosion to address these problems simultaneously a geomorphic model for reclaiming surface mined slopes is described called the highwall trench concave slope model it was first applied in the 1995 reclamation of a quarry on a slope la revilla in central spain the geomorphic model does not reproduce the original topography but has two very different sectors and objectives i the highwall trench sector allows the former quarry face to evolve naturally by erosion accommodating fallen debris by means of a trench constructed at the toe of the highwall ii the concave slope base sector mimicking the landforms of the surrounding undisturbed landscape promotes soil formation and the establishment of self sustaining functional ecosystems in the area protected from sedimentation by the trench the model improves upon simple topographic reconstruction because it rebuilds the surficial geology architecture and facilitates re establishment of equilibrium slopes through the management and control of geomorphic processes thirteen years of monitoring of the geomorphic and edaphic evolution of la revilla reclaimed quarry confirms that the area is functioning as intended the highwall is backwasting and material is accumulating at the trench permitting the recovery of soils and vegetation on the concave slope however the trench is filling faster than planned which may lead to run off and sedimentation on the concave slope once the trench is full the lesson learned for other scenarios is that the model works well in a two dimensional scheme but requires a three dimensional drainage management breaking the reclaimed area into several watersheds with stream channels copyright 2010 john wiley sons ltd