controls on hydrothermal alteration in a section of fast spreading oceanic crust abstract young oceanic crustal sections that formed at the fast spreading east pacific rise epr were mapped by submersible along steep fault scarps that bound the hess deep rift valley excellent exposures allowed the documentation of the spatial distribution of alteration within the lava sequence sheeted dike complex and uppermost plutonic sequence alteration assemblages record the initiation and subsequent cooling of an axial hydrothermal system s that was periodically perturbed by the injection of sheeted dikes mineralogical and geochemical data indicate that temperatures increased across the lava dike transition 50 180 c and that the grade of metamorphism was heterogeneous within the sheeted dike complex in that temperatures varied laterally from 100 300 c to 600 800 c within the upper dikes and were 400 450 c within the lower sheeted dikes during vigorous hydrothermal circulation at the epr the base of the hydrothermal system was located within or at the top of a sequence of gabbros that is interpreted to be the root zone of the sheeted dike complex the base of the hydrothermal cell did not penetrate into the subjacent plutonic sequence until the section of crust had migrated off axis and the magma lens had solidified the vigor of the hydrothermal system would have waned by this time and hydrothermal fluids migrated into the lower crust along broadly distributed microfracture networks at temperatures between 450 and 650 c although locally temperatures may have reached 800 c the highest temperature alteration assemblages within the sheeted dikes formed in response to the injection of a dike or swarm of dikes that initiated a short lived hydrothermal system the sheeted dikes and plutonic rocks experienced very little retrograde alteration and there is no evidence for a discrete off axis hydrothermal event s incipient alteration conditions in crustal sections that formed at mid ocean ridges with slower rates of spreading than the epr are similar to hess deep although there are differences in the distribution and mineralogical expression of incipient alteration in plutonic sequences the evolution of alteration as a section of crust cools and moves off axis reflects its particular magmatic tectonic history and thus may vary significantly in ridges with all spreading rates

multi channel seismic imaging of a crustal magma chamber along the east pacific rise a reflection observed on multi channel seismic profiles along and across the east pacific rise between 8 50 n and 13 30 n is interpreted to arise from the top of a crustal magma chamber located 1 2 2 4 km below the sea floor the magma chamber is quite narrow 4 6 km wide but can be traced as a nearly continuous feature for tens of kilometres along the rise axis

sr isotopic tracer study of the samail ophiolite oman we have measured rb and sr concentrations and sr isotopic compositions in 41 whole rock samples and 12 mineral separates from units of the samail ophiolite including peridotite gabbro plagiogranite diabase dikes and gabbro and websterite dikes within the metamorphic peridotite ten samples of cumulate gabbro from the wadir kadir section and nine samples from the wadi khafifah section have mean 87sr 86sr ratios and standard deviations of 0 70314 0 00030 and 0 70306 0 00034 respectively the dispersion in sr isotopic composition may reflect real heterogeneities in the magma source region the average sr isotopic composition of cumulate gabbro falls in the range of isotopic compositions of modern midocean ridge basalt the 87sr 86sr ratios of noncumulate gabbro plagiogranite and diabase dikes range from 0 7034 to 0 7047 0 7038 to 0 7046 and 0 7037 to 0 7061 respectively these higher 87sr 86sr ratios are due to alteration of initial magmatic compositions by hydrothermal exchange with seawater mineral separates from dikes that cut harzburgite tectonite have sr isotopic compositions which agree with that of cumulate gabbro these data indicate that the cumulate gabbro and the different dikes were derived from partial melting of source regions that had similar long term histories and chemical compositions

extreme element mobility during transformation of neoarchean ca 2 7ga pillow basalts to a paleoproterozoic ca 1 9ga paleosol schreiber beach ontario canada abstract this study reports petrographic major and trace element and oxygen isotopic data for a subaerial weathering profile at schreiber beach ontario canada the weathering profile developed on neoarchean ca 2 7 ga pillow basalts and is unconformably overlain by the paleoproterozoic ca 1 88 ga gunflint chert and basal conglomerates this stratigraphy suggests that the basalts were uplifted and subaerially weathered prior to deposition of the gunflint formation rocks at schreiber beach have been classified as pillow cores pillow rims hyaloclastites weathered red basalts and weathered brown to green basalts there are gradual textural mineralogical and geochemical transitions from unweathered basalts to intensely weathered hematite bearing basalts with stratigraphic height rocks at schreiber beach underwent seafloor hydrothermal alteration and greenschist facies regional metamorphism in the neoarchean following the subaerial weathering they were buried and re metamorphosed to greenschist facies in the paleoproterozoic given the preservation of primary igneous textures and trace element patterns in pillow cores they are used as a protolith to estimate element mobility during seafloor alteration metamorphism and subaerial weathering chloritized pillow rims and hyaloclastites are characterized by moderate losses or gains of major si ca na fe ti k and trace lile ree hfse and transition metals elements during neoarchean seafloor alteration alteration of these rocks at relatively low temperatures is indicated by enrichment in 18 o 18 o 10 4 to 12 7 relative to fresh basalt 5 5 subaerial weathering resulted in significant textural mineralogical e g hematite precipitation and geochemical variations in metamorphosed basalts causing additional redistribution of most major and trace elements and further enrichment in 18 o the highest 18 o values 16 1 to 17 1 occur at the silica rich base of the weathering profile many elements including lile ree and hfse are strongly enriched 100 or depleted up to 96 relative to their original abundances alkaline metasomatism which is a characteristic of precambrian paleosols is also evident at schreiber beach where extreme enrichment in k 280 to 1000 rb 390 to 660 and cs 1160 to 2560 occurs in the weathering profile

87sr 86sr ratios in hydrothermal waters and deposits from the east pacific rise at 21 n abstract 87 sr 86 sr ratios of three hydrothermal waters collected on the east pacific rise at 21 n define a mixing line between seawater and a hydrothermal end member at 0 7030 which is derived by seawater basalt interaction at ca 350 c and water rock ratio of about 1 5 sr concentrations are not affected in the process while mg uptake from seawater is almost complete up to2 3 of this hydrothermal component is involved in anhydrite precipitation while the sr isotopic ratio in sulfides chalcopyrite sphalerite cannot be distinguished from that of sulfate it is estimated that ca 1 10 10 moles of strontium are yearly cycled in the hydrothermal systems of mid oceanic ridges thereby affecting the 87 sr 86 sr budget of seawater mass balance between river runoff limestone precipitation and ridge basalt alteration suggests that the 87 sr 86 sr ratios of the river runoff are in the range 0 7097 0 7113 and are largely dominated by limestone alteration

subsidence in magma chamber and the development of magmatic foliation in oman ophiolite gabbros abstract in the oman ophiolite the horizon where the melt lens pinched during drifting away from the ocean ridge axis has been identified starting in the root zone of the sheeted dike complex rzsdc located above this horizon 18 sections down to the upper gabbros unit have been mapped in great detail in selected areas of the southern massifs of this ophiolite they are complemented by 133 sites located throughout the entire ophiolite where the transition from the rzsdc to the uppermost foliated gabbros is well exposed altogether half the sites and 11 cross sections display within a few tens of meters beneath the rzsdc a magmatic foliation which is parallel to the overlying sheeted dikes in the other sites and cross sections the gabbro foliation is either flat lying or steep but not parallel to the sheeted dikes compared to the rzsdc isotropic ophitic gabbros where clinopyroxene is interstitial between plagioclase laths in the topmost steeply foliated gabbros clinopyroxene is idiomorphic becoming rapidly granular and tabular down section by recrystallization and peripheral alteration to hornblende moving down from these uppermost gabbros and over one hundred meters the steep foliation becomes stronger and the poorly recovered top gabbros grade into the recrystallized granular gabbros of the gabbro unit these repeated observations indicate to ascribe these gabbros to subsidence of a compacting mush from the floor of the melt lens into the underlying main magma chamber the topmost gabbros beneath rzsdc which were expelled from the melt lens by drifting very soon after settling on the melt lens floor display in plagioclase a spectacular zoning pointing to a fast cooling moving downwards stronger foliation increased compaction and recovery recrystallization are explained by the time spent by the subsiding mush inside the magma chamber increasing by one order of magnitude 50 to 500 yr over this vertical distance this field based study brings compelling field evidence supporting the former models of subsidence which were based on the assumption that the mush that settled onto the floor of the melt lens is sucked downwards during drifting of the crust away from the ridge axis

high temperature alteration of oceanic gabbros by seawater hess deep ocean drilling program leg 147 evidence from oxygen isotopes and elemental fluxes two major stages of high temperature water rock interactions have been identified in gabbros of oceanic layer 3 exposed in the hess deep rift valley at the east pacific rise 2 n 101 w the 154 m of plutonic rocks cored at site 894g show mineralogies and textures that suggest that they represent the roof of an approximate 1 myr magma chamber a late stage of hydrothermal alteration produced mineral assemblages typical of lower amphibolite upper greenschist facies conditions the hydration of the gabbros occurred locally through the development of well crystallized green amphiboles significant chemical fluxes were associated with the fluid flow and precipitation dissolution mechanisms oxygen isotope data suggest an earlier cryptic stage of hydrothermal alteration of the hess deep gabbros at temperatures above 500 c hydration was limited with only minute amphibole lamellae in pyroxenes that trapped cl na and minor k from seawater ca plagioclases remained stable and preserved their magmatic cationic compositions and zoning patterns we propose that this high temperature event lowered the 18o of the gabbros by isotopic exchange with a hydrothermal fluid through solid state oxygen diffusion although it is unclear whether the 18o depletion of hole 894g gabbros is representative of the entire lower oceanic crust it is nevertheless thought to contribute to the oxygen isotope buffering of the oceans during this stage water rock ratios calculated using sr isotopes are underestimated 0 1 0 5 because of insignificant diffusion of this cation in plagioclases better estimates of water rock ratios are provided by oxygen isotopes 0 2 1 which are important in unraveling the cooling history of the oceanic crust

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

segmentation at a fossil spreading axis the plutonic sequence of the wadi haymiliyah area haylayn block sumail nappe oman abstract wadi haymiliyah in the central part of the haylayn block offers a splendid continuous section across the crustal sequence of the sumail ophiolite the plutonic sequence of this area presents several specific features of great interest for the understanding of the structure and evolution of this ophiolite the petrological moho and the whole plutonic sequence display a subvertical attitude parallel to the mean dyke complex orientation high temperature folding occurred in the mantle sequence and in the lower crust whereas the upper crust remained unfolded and behaved as a rigid slab during this deformation resulting in a tectonic decoupling between lower and upper crust in the gabbro cumulate sequence a remarkable large scale curvature of the magmatic structures layering and lamination planes whose axis is totally discordant with the regional folds axes is interpreted as a primary magmatic feature indicating the southern ending of a magma chamber the lower crust about 2 km thick consists of a primary layered gabbroic sequence consisting of rhythmic mineral graded layers from several metres to several hundred metres thick syndepositional structures such as oblique stratification microslumps eroding mini channels and small normal faults are widespread the mineralogical composition modal variations and cryptic evolution of the whole layered sequence are described both at the megascale and on the simple cyclic scale the data support a model of crystallization of the layered sequence by open system fractionation at the floor and sides of a periodically replenished steady state magma chamber a unique feature of this cumulate sequence is the ubiquitous presence of orthopyroxene throughout the whole cumulate pile this is very unusual in the sumail crust with the exception of the wadi rajmi section in the fizh block two huge plagioclase free dunite wehrlite laccoliths about 300 m thick and numerous ultramafic sills and mushroom like bodies intrude the base of the layered sequence these ultramafic intrusions exhibit in places an internal layering and dunitic differentiation products they have sharp sinuous and lobate contacts with the gabbros which were still hot and probably not entirely consolidated at the moment of the intrusion the mineral compositions of these intrusions are much more primitive than those of the adjacent gabbros higher in the plutonic section the layered gabbros are overlain by 1 km of fine grained non layered planar laminated gabbronorites characterized by the abundance of pleochroic cumulus hypersthene the disappearance of olivine and the development of fe ti oxides and amphibole as post cumulus phases numerous occurrences of concordant coarse grained layered gabbros occur in this unit which is characterized also by a dense net of highly residual dioritic to tonalitic veins resulting in a huge magmatic breccia in the uppermost few hundred metres where the gabbronorites are brecciated by the acidic rocks hydrothermal veins are also extremely abundant in this unit which coincides with the core of the large scale curvature this unit comprising unusually fractionated rocks is interpreted as resulting from the evolution of the haymiliyah magma chamber towards a closed system fractionation due to the cutting off of source supply our interpretation of both structural and petrological data may be summarized by the following major points 1 1 the haymailiyah magma chamber was located at the southern tip of a spreading cell and was cut off from the magma supply at the end of its evolution either vertically when accretion slowed down or horizontally when it became isolated from the central part of the spreading segment we suggest that orthopyroxene bearing cumulates should be looked for all along the sumail nappe and used as markers of segmentation 2 2 early high temperature compressional deformation took place in both the mantle and crust before the intraoceanic detachment of the ophiolite the development of this compression regime activated filter pressing processes and in particular helped with the extraction of wehrlitic magmas from the upper mantle and their ascent to the uppermost levels of the crust

the petrology of seafloor rodingites insights from geochemical reaction path modeling abstract thermodynamic reaction path models were applied to provide insights into the formation of rodingites at mafic ultramafic boundaries the models were set up to investigate the shifts in fluid rock equilibria as fluids move from peridotite undergoing serpentinization into a gabbroic body phase relations were investigated in the direction of increasing extent of reaction of the serpentinization fluid with gabbro at 200 c 300 c and 400 c phase assemblages typical of rodingite grossular diopside chlorite are predicted to form at 200 c and 300 c but only in areas where the fluid is essentially unaffected by reactions with gabbro i e near the contact with ultramafic rock or adjacent to a fissure filled with serpentinization fluid as the fluid becomes more affected by reactions with the gabbro prehnite or epidote ss replace garnet and tremolite replaces clinopyroxene once the fluid chemistry is completely reset by reactions with gabbro the predicted assemblage is typical of greenschist facies albitic plagioclase actinolite chlorite and epidote ss these transitions predicted in the model are very similar to what is observed in natural rodingites from different settings our model results hence support the hypothesis that rodingites form during serpentinization and only in areas where fluids controlled by serpentinization reactions are present our calculations further indicate that the formation of mineral assemblages and spatial mineralogical variability within rodingites from the ocean floor is mainly driven by steep activity gradients in protons and aqueous silica in pore fluids mass transfer of ca is likely by diffusion of the caoh species which is predicted to show a very steep concentration gradient across a mafic ultramafic boundary rodingites act as ca traps because of the great stability of diopside and garnet at low sio 2 activities our model calculations further predict the formation of diopsidite from gabbroic and clinopyroxenite precursor rocks at 200 c and 300 c suggesting that very high temperatures on the order of 800 c are not required in the formation of diopsidite veins

constraints on the magmatic evolution of the oceanic crust from plagiogranite intrusions in the oman ophiolite we present major and trace element as well as sr nd and hf isotope data on a suite of 87 plutonic rock samples from 27 felsic crustal intrusions in seven blocks of the oman ophiolite the rock compositions of the sample suite including associated more mafic rocks range from 48 to 79 wt sio2 i e from gabbros to tonalites the samples are grouped into a ti rich and relatively light rare earth element lree enriched p1 group ce yb n 0 7 resembling the early v1 lavas and a ti poor and lree depleted p2 group ce yb n 0 7 resembling the late stage v2 lavas based on the geochemical differences and in agreement with previous structural and petrographic models we define phase 1 p1 and phase 2 p2 plutonic rocks felsic magmas in both groups formed by extensive fractional crystallization of olivine clinopyroxene plagioclase apatite and ti magnetite from mafic melts the incompatible element compositions of p1 rocks overlap with those from mid ocean ridges but have higher ba nb and th nb trending towards the p2 rock compositions and indicating an influence of a subducting slab the p2 rocks formed from a more depleted mantle source but show a more pronounced slab signature these rocks also occur in the southern blocks with the exception of the tayin block of the oman ophiolite implying that the entire ophiolite formed above a subducting slab initial nd and hf isotope compositions suggest an indian morb type mantle source for the oman ophiolite magmas isotope compositions and high th nb in some p2 rocks indicate mixing of a melt from subducted sediment into this mantle

early permian a type granites from central inner mongolia north china magmatic tracer of post collisional tectonics and oceanic crustal recycling abstract a type granitoids constitute volumetrically minor but genetically important building blocks of most post archean continental landmasses on earth and their diversified genesis encapsulates key information for ascertaining critical geodynamic controls on continental crustal formation and differentiation this zircon u pb dating and geochemical study documents the early permian peralkaline granites from central inner mongolia of china eastern central asian orogenic belt caob these granites show an a type magmatic affinity with typical enrichment in alkalis ga zr nb and y depletion in sr and p and fractionated ree patterns showing strong negative eu anomalies they have meaningful initial 87 sr 86 sr ratio of 0 70594 e nd t of 4 25 to 6 05 zircon e hf t from 9 8 to 16 0 and 18 o values from 2 41 to 4 72 these elemental and isotopic characters argue for parental magmas from partial melting of a newly trapped charnockitic lower crustal protolith and their subsequent fractional crystallization with an inherent hydrothermally altered oceanic crustal heritage and possible tectonic linkage to post collisional slab breakoff this kind of a type granites could not only serve as an insightful temporal milestone for monitoring the amalgamation between various constituent terranes in the caob but also present a spatial magmatic proxy for marking the maturation stage of juvenile crustal reworking during the final phase of a large orogenic event

fluid rock interactions in the lower oceanic crust thermodynamic models of hydrothermal alteration rocks from the lower oceanic crust show both petrologic and isotopic evidence for extensive alteration by circulating hydrothermal fluids derived from seawater deeply circulating fluids may have a substantial but as yet largely undetermined impact on the thermal regime of spreading centers magmatic processes at mid ocean ridges and elemental fluxes at ridge hydrothermal systems in the present study the consequences of water rock interactions during the penetration of seawater into hot lower oceanic crust were investigated using thermodynamic reaction path models alteration assemblages predicted for reaction of an evolving seawater with olivine gabbro gabbronorite and troctolite during heating from 300 to 900 c closely resemble alteration assemblages observed in rock samples from the lower oceanic crust these assemblages include sodic plagioclase actinolite diopsidic clinopyroxene epidote chlorite prehnite analcime quartz at lower temperatures 300 500 c and plagioclase hornblende diopside chlorite trace magnetite at intermediate temperatures 500 700 c the relative abundances of these alteration products show much greater variation with temperature than with water rock w r ratio indicating that variations in the amounts of hydrated minerals such as amphiboles may not be an accurate indication of the w r ratio experienced by rocks in the lower crust since the same amount of fluid may produce widely differing amounts of amphibole at slightly different temperatures depending on the temperature of interaction some rocks in the lower oceanic crust may have experienced much higher w r ratios than has previously been proposed based on the amount of amphibole present at higher temperatures 750 900 c the models predict that very little alteration occurs before the fluid equilibrates with the rock producing only trace amounts of amphibole as an alteration phase with the remainder of the rock retaining its igneous mineral composition essentially unchanged this result suggests that extensive penetration of rocks by aqueous fluids may occur in the lower ocean crust at temperatures 700 c and leave very little conspicuous petrologic evidence for their presence such cryptic alteration is consistent with isotopic evidence for hydrothermal alteration of oxygen and sr isotopes by seawater in lower ocean crust rock samples which otherwise retain their igneous compositions and textures taken together the model results allow for much more extensive circulation of fluids in the lower oceanic crust and at much higher temperatures than has been proposed by previous studies based on petrologic interpretations

chlorine rich amphibole in deep layered gabbros as evidence for brine rock interaction in the lower oceanic crust a case study from the wadi wariyah samail ophiolite sultanate of oman abstract hydrothermal veins and dykelets that cross cut layered olivine gabbros deep in the plutonic section of the samail ophiolite sultanate of oman point towards the occurrence of hydrothermal circulation in the deep oceanic crust and these features record interactions between rock and high temperature seawater derived fluids or brines deep penetration of seawater derived fluids down to 100 m above the moho transition zone and the consequent interactions with the host rock lead to hydrothermal alteration from granulite facies grading down to greenschist facies conditions here we present a study of veins and dykelets formed by hydrothermal interaction cutting layered gabbro in the wadi wariyah using petrographic microanalytical isotopic and structural methods we focus on amphiboles which show a conspicuous compositional variation from high ti magnesiohastingsite and pargasite via magnesiohornblende and edenite to cl rich ferropargasite and hastingsite up to 1 5 a p f u cl and actinolite these minerals record a wide range of formation conditions from magmatic to hydrothermal and reveal a complex history of interactions between rock and hydrothermal fluid or brine in a lower oceanic crustal setting large variations in cl content and cation configurations in amphibole suggest formation in equilibrium with fluids of different salinities at variable fluid rock ratios the presence of subsolidus amphibole extremely enriched in chlorine implies phase separation and brine rock interactions 87sr 86sr values of 0 7031 to 0 7039 and stable 18o isotopic compositions of 4 1 to 5 6 of the different amphibole types suggest a rock dominated environment i e with low fluid rock ratios however the slight departure from mean oman isotope values may indicate there was some influence of seawater in the aforementioned fluid rock interactions our study provides new petrological data for the subsolidus evolution of gabbro hosted amphibole rich veins in the presence of a seawater derived fluid

interaction of highly saline fluid and olivine gabbro experimental simulation of deep hydrothermal processes involving amphibole at the base of the oceanic crust abstract natural occurrences of cl bearing amphibole indicate crystallization in the presence of a highly saline fluid amphibole with varying cl contents 0 1 to 5 wt cl found in lower oceanic crustal rocks demonstrates the activity of saline hydrothermal fluids at depth here we present an experimental study done in cold seal pressure vessels cspv and internally heated pressure vessels ihpv to illustrate the process by which gabbro hosted amphibole rich parageneses evolve in the presence of a hydrothermal saline fluid the starting material used was a natural gabbroic rock containing high ti magnesio hastingsite with the addition of a moderately to highly saline fluid nacl h2o with xnacl 0 02 0 07 and 0 24 6 20 and 50 wt nacl we evaluated a range of conditions from hydrothermal 500 750 c to magmatic 900 c at pressures of 200 mpa and fo2 close to nno fluid rock mass ratios used were 0 2 and 1 in subsolidus hydrothermal experiments and 0 07 in partial melting magmatic experiments new amphibole was formed on rims of the starting high ti magnesio hastingsite with product compositions corresponding to magnesio hastingsite high si magnesio hornblende tschermakite edenite hastingsite and ferro pargasite with varying cl contents up to 0 47 wt cl our results from subsolidus experiments demonstrate a decrease in olivine plagioclase and clinopyroxene in the starting rock and formation of new amphibole with decreased ival and ti with respect to starting amphibole product amphibole does not display any correlation between cl and fe2 ival and k in contrast to natural highly cl rich amphibole suggesting that these correlations hold true only at cl contents higher than those attained in our experiments in addition we found that increasing nacl in the fluid correlates with increased maximum cl contents in amphibole compositional variations found in product amphibole highlight the heterogeneities in fluid infiltration and cl activity that account for the complexity of hydrothermal fluid rock interactions in deep oceanic geological systems

constraints on the accretion of the gabbroic lower oceanic crust from plagioclase lattice preferred orientation in the samail ophiolite abstract oceanic crust represents more than 60 of the earth s surface and despite a large body of knowledge regarding the formation and chemistry of the extrusive upper oceanic crust there still remains significant debate over how the intrusive gabbroic lower oceanic crust is accreted at the ridge axis the two proposed end member models the gabbro glacier and the sheeted sills predict radically different strain accumulation in the lower crust during accretion in order to determine which of these two hypotheses is most applicable to a well studied lower crustal section we present data on plagioclase lattice preferred orientations lpo in the wadi khafifah section of the samail ophiolite we observe no systematic change in the strength of the plagioclase lpo with height above the crust mantle transition no dominant orientation of the plagioclase a axis lineation and no systematic change in the obliquity of the plagioclase lpo with respect to the modal layering and macroscopic foliation evident in outcrop these observations are most consistent with the sheeted sills hypothesis in which gabbros are crystallized in situ and fabrics are dominated by compaction and localized extension rather than by systematically increasing shear strain with increasing depth in a gabbro glacier our data support the hypothesis of macleod and yaouancq 2000 that the rotation of the outcrop scale layering from sub horizontal in the layered gabbros to sub vertical near the sheeted dikes is due to rapid vertical melt migration through upper gabbros close to the axial magma chamber additionally our results support the hypothesis that the majority of extensional strain in fast spreading ridges is accommodated in partially molten regions at the ridge axis whereas in slow and ultra slow ridges large shear strains are accommodated by plastic deformation

modes of crustal accretion and their implications for hydrothermal circulation hydrothermal convection at mid ocean ridges links the ocean s long term chemical evolution to solid earth processes forms hydrothermal ore deposits and sustains the unique chemosynthetic vent fauna yet the depth extent of hydrothermal cooling and the inseparably connected question of how the lower crust accretes remain poorly constrained here based on coupled models of crustal accretion and hydrothermal circulation we provide new insights into which modes of lower crust formation and hydrothermal cooling are thermally viable and most consistent with observations at fast spreading ridges we integrate numerical models with observations of melt lens depth thermal structure and melt fraction models matching all these observations always require a deep crustal scale hydrothermal flow component and less than 50 of the lower crust crystallizing in situ

high temperature seawater circulation throughout crust of oceanic ridges a model derived from the oman ophiolites 1 we describe here the high temperature ht 700 c and very high temperature vht 1000 c recharge and discharge systems which are responsible for ht vht hydrothermal alterations at the oceanic ridge of origin in the crust of oman ophiolites the recharge system is a regular network of parallel microcracks a fraction of a millimeter wide which carries seawater to the walls of the magma chamber where hydrous melting is triggered the discharge system is a network of clinopyroxene pargasite gabbroic dikes issued from the hydrous melting upsection these ht vht gabbroic dikes grade into the well documented low temperature lt 400 500 c hydrothermal veins microcracks gabbroic dikes and veins are preferentially parallel to the diabase sheeted dikes and normal to magmatic lineations in enclosing gabbros thus the fractures of both the recharge and discharge systems are parallel to the paleoridge plane in the brittle field fracturing is induced by tensional stress related to seafloor spreading in the ductile field and up to temperatures of 900 c opening of microcracks is ascribed to the anisotropy of thermal contraction during gabbros cooling gabbro fabrics and anisotropy are such that the induced microcracks are also parallel to the ridge plane thermal contraction between 1200 c temperature of magma chamber and 700 c brittle ductile limit produces a differential strain of 0 1 comparable to that deduced from width and spacing of microcracks in the gabbros simple estimates on activity time fluid velocity and fluid flux in microcracks account for all available data and suggest that in microcracks activity was short lived and episodic

paragenesis of magma chamber internal wall discovered in oman ophiolite gabbros a detailed study in wadi farah reveals a singular point in jebel dihm southern oman ophiolite here we investigated some primitive features of accretion at a fast spreading ridge that has exceptionally been exempt from hydrous alteration while crossing the tbl thermal boundary layer the conjunction of two factors has made this possible in the farah area its location at the point of sharp foliation rotation from the upper to the transitional gabbros and a locally twisted tbl these factors may have favoured the intrusion within the tbl of hydrous wedges transverse to the ridge deeper into the tbl near the active ridge the wedges merge and are covered by veneers of totally fresh gabbros bearing glittering acicular clinopyroxene these gabbros are interpreted as relics of the crystallizing internal wall of the magma chamber that have interacted with hydrous fluids this internal tbl has anisotropic thermal properties and acts as a thermal blanket maintaining high magma chamber temperatures

zabargad peridotite evidence for multistage metasomatism during red sea rifting new analyses of rare earth elements rees and abundances of k rb sr ba th u ta and hf in peridotites from zabargad island red sea indicate that light ree enriched amphibole peridotites in the northern and central bodies have undergone a complex multistage metasomatic history during red sea rifting the metasomatism is partly attributed to the passage of alkalic basalts through the mantle lithosphere which possibly occurred during the early continental rift history of the red sea the metasomatism also results from the interaction of the uppermost lithosphere with hydrous crustal fluids released from thinning continental crust the last metasomatic event is associated with the development of an oceanic hydrothermal system in the red sea after the continental breakup the available data also suggest that the zabargad peridotites represent pieces of pan african lithospheric mantle the northern and central bodies pierced by asthenospheric material the southern body

chemical and thermal constraints on focussed fluid flow in the lower oceanic crust the mechanism of heat extraction from the lower oceanic crust near the ridge axis is poorly constrained despite its importance for understanding both the process of accretion of the plutonic complex and the mass fluxes associated with ridge hydrothermal systems we have investigated the role of zones of focussed fluid flow in the plutonic complex of the oman ophiolite in the near axis cooling of the oceanic crust lineaments identified on aerial photographs that occur at 1 km spacing show evidence for extensive hydrothermal fluid flow through regions 10 to 50 m wide fluid flow is initiated in these regions at 800 c and continues at least into the lower greenschist facies strontium isotope analyses indicate that the fluid flux through these zones is sufficient to transport a metasomatic front from the base of the sheeted dike complex to close to the moho computed minimum fluid fluxes to transport a metasomatic front through the focussed fluid flow zones are 1x108 kgm2 modeling of diffusive exchange of calcium from olivine to clinopyroxene indicates enhanced cooling rates adjacent to the focussed fluid flow zones heat fluxes estimated from the enhanced cooling rates are broadly consistent with the fluid fluxes determined from modeling the sr isotopic composition of samples from the focussed fluid flow zones the combination of independent estimates of the fluid and heat fluxes such as these can provide more rigorous constraints on the thermal history than either approach used in isolation our results show that focussed fluid flow could play a major role in the cooling in the lower oceanic crust significant focussed fluid flow in the lower oceanic crust has important implications for predicting the total mass flux associated with hydrothermal circulation at mid ocean ridges this is because fluids flowing through channels become chemically rock buffered at smaller fluid fluxes than those flowing pervasively through a rock mass thus if focussed fluid flow is an important mechanism of heat loss from the lower oceanic crust the chemical fluxes from ridge hydrothermal systems into the oceans may be smaller than currently thought

the northern samail ophiolite an oxygen isotope microprobe and field study geological petrological and oxygen isotopic data are presented for 228 whole rock and mineral samples collected from a 100 20 km area of the northern samail ophiolite in oman most of these samples are from three detailed profiles through the pillow lavas sheeted dikes and layered gabbros of this laterally heterogeneous fragment of cretaceous oceanic crust down to and across the petrologic moho the profiles encompass a range of petrologic and tectonic styles and each profile exhibits distinctive 18 16 o variations compared to one another and to mid ocean ridge basalts as a result of pervasive seawater hydrothermal interaction that varied in intensity along strike in the ophiolite in general 18 o depletions are observed in the layered gabbros and 18 o enrichments in most of the sheeted dikes and pillow lavas similar to results previously observed in the southern part of the ophiolite ibra area where 18 o depletions within the gabbroic section are quantitatively balanced by 18 o enrichments in the shallower parts of the oceanic crust the wadi hilti profile selected as an example of relatively intact crust differs from ibra in having more uniform and slightly higher 18 o in the gabbros 5 4 to 6 3 as well as in containing more hydrous alteration minerals amphibole epidote chlorite and prehnite the profiles in the wadi kanut shafan and wadi rajmi sections are much more complex and reveal the impact of off axis intrusions and deep crustal shearing plagiogranite wehrlite intrusions in the shafan kanut area superimposed a local hydrothermal aureole on the ophiolite evident in dikes highly depleted in 18 o quartz sulfide veins abundant epidote thullite and chlorite in shallower rocks and low temperature hydrous alteration of deeper gabbroic rocks the latter produced an overall increase in whole rock 18 o 6 2 to 6 9 such late stage intrusions are found throughout the northern half of the samail ophiolite the wadi rajmi area which is a possible fossil transform or propagating rift represents the most complex of the three profiles it also contains the most abundant highly deformed and hydrothermally altered rocks together with the deepest and largest zone of 18 o depletion yet found in any ophiolite locally 18 o 2 0 conduits for large volumes of high temperature hydrothermal fluids were provided by fractures now occupied by low 18 o gabbro pegmatites and low 18 o dikes material balance estimates for the regional samples and from the various transects through the ophiolite give crustal bulk 18 o averages 5 9 to 6 3 that are within sampling error almost identical to the average morb basalt value of about 5 8 if both vertical and lateral crustal heterogeneities are integrated into a three dimensional model this supports and amplifies the conclusion of earlier workers that the 18 o of seawater is buffered and controlled by hydrothermal interaction with oceanic crust as long as the cumulative effects both spatial and temporal of all seafloor magmatic hydrothermal processes are considered the very slight out of balance enrichment of the integrated crustal average 18 o compared to morb may be explained by the ubiquitous mineralogical and isotopic evidence for a late low temperature alteration event in the basal gabbros these effects are prominent in the vicinity of the petrologic moho and may indicate exchange with low temperature aqueous fluids during or after detachment of the obducted slab

iron transformations during low temperature alteration of variably serpentinized rocks from the samail ophiolite oman abstract partially serpentinized peridotites in the samail ophiolite in the sultanate of oman currently undergo low temperature alteration and hydration both at shallow levels with water recently in contact with the atmosphere and at depth with anoxic reducing fluids however it is unclear how changes in the distribution and oxidation state of fe are driving the production of energy rich gases such as hydrogen and methane detected in peridotite catchments we track the fe transformations in a suite of outcrop samples representing a subset of the spectrum of least to most altered end members of the oman peridotites we use microscale mineralogical and geochemical analyses including qemscan raman spectroscopy synchrotron radiation x ray fluorescence xrf mapping and electron microprobe wavelength dispersive spectroscopy the less altered peridotites possess a diversity of fe bearing phases including relict primary minerals e g olivine pyroxene chromite and secondary phases e g serpentine and brucite raman spectroscopy and electron microprobe data si mg fe indicate that much of the serpentine is significantly intergrown with brucite on the sub micron scale these data also indicate that the fe content of the brucite ranges from 10 to 20 wt feo the mineral assemblage of the highly reacted rocks is less diverse dominated by serpentine and carbonate while olivine and brucite are absent magnetite is relatively rare and mainly associated with chromite goethite and hematite both fe iii hydroxides were also identified in the highly altered rocks whole rock chemical analyses reflect these mineralogical differences and show that fe in the partially serpentinized samples is on average more reduced 0 40 0 55 fe3 fetotal than fe in the highly reacted rocks 0 85 0 90 fe3 fetotal we propose that olivine brucite chromite and perhaps serpentine in the less altered peridotites act as reactive phases during low temperature alteration of the oman peridotite the pervasive oxidation of fe ii in the less altered peridotites to fe iii in the most altered peridotites during water rock reaction in the subsurface of the samail ophiolite may produce h2 which will influence the development of microbial energy sources and habitats and carbon cycling and sequestration within the ultra mafic ocean crust

local magnitude discrepancies for near event receivers implications for the u k traffic light scheme local seismic magnitudes provide a practical and efficient scale for the implementation of regulation designed to manage the risk of induced seismicity such as traffic light schemes tls we demonstrate that significant magnitude discrepancies up to a unit higher occur between seismic events recorded on nearby stations 5 km compared with those at greater distances this is due to the influence of sedimentary layers which are generally lower in velocity and more attenuating than the underlying crystalline basement rocks and requires a change in the attenuation term of the m l scale this has a significant impact on the united kingdom s u k hydraulic fracturing tls whose red light is set at m l 0 5 because the nominal detectability of the u k network is m l 2 this scheme will require the deployment of monitoring stations in close proximity to well sites using data collected from mining events near new ollerton nottinghamshire we illustrate the effects that proximity has on travel path velocities and attenuation then perform a damped least squares inversion to determine appropriate constants within the m l scale we show that the attenuation term needs to increase from 0 00183 to 0 0514 and demonstrate that this higher value is representative of a ray path within a slower more attenuating sedimentary layer compared with the continental crust we therefore recommend that the magnitude scale m l log a 1 17log r 0 0514 r 3 0 should be used when local monitoring networks are within 5 km of the event epicenters

roof assimilation at fast spreading ridges an investigation combining geophysical geochemical and field evidence 1 the role of assimilation at the roof of the axial magma chamber amc at fast spreading mid ocean ridges is investigated using field geophysical and geochemical data field observations from ophiolites indicate that roof assimilation is a common process but provide little constraint on the amount of material assimilated arguments based on geophysical data that include the subsidence of the layer 2a 2b boundary across the width of the amc and the depth of large axial summit troughs require at least 50 100 m of assimilation however significant variations in the depth of the amc on the east pacific rise epr over short along axis distances suggest up to 500 m fluctuations in the depth of the amc may be common on short timescales geochemical modeling of the over enrichment of chlorine in epr basalts suggests that 20 of the oceanic crust may go through a cycle of crystallization alteration then assimilation this is far more than can be accounted for by roof subsidence and suggests that fluctuations in the level of the roof must account for the majority of the assimilation assimilation most likely occurs on a number of different timescales ranging from decadal due to perturbations in the thermal regime related to diking to longer period fluctuations controlled by magma supply from the mantle large scale roof assimilation has implications for the accretion of the lower crust due to heat losses associated with assimilation and for the interpretation of the geochemistry of the oceanic crust

dependence of ridge axis morphology on magma supply and spreading rate why do some mid ocean ridge axes have as their topographic expression a median valley 1 2 km deep and 15 30 km wide whereas others have an axial high 100 200 m high and 1 2 km wide in general median valleys exist at the axes of ridges spreading at half rates less than 20 25 mm yr 1 whereas axial highs are found at faster spreading ridges1 2 a recent crustal genesis model3 incorporating hydrothermal cooling and crustal accretion by means of a magma lens agrees well with observations of the spreading rate dependence of axial morphology1 2 the depth of the axial magma chamber3 4 and the thickness of the ridge axis lithosphere5 6 but there are several notable exceptions to the correlation between spreading rate and axial morphology such as the reykjanes ridge which spreads at l0mm yr 1 but has an axial high here we show that by extending the model of ref 3 to include variations in crustal thickness we can explain the observed dependence of axial morphology on crustal thickness and spreading rate our results suggest that the ultimate control on axial morphology is the thermal structure at the ridge axis which is a function of both spreading rate and magma supply

rapid high temperature metamorphism of east pacific rise gabbros from hess deep abstract metamorphosed oceanic gabbros provide a record of the cooling history of the lower crust near mid ocean ridges but the temperature range rate and location of subsolidus events are poorly known we combine hornblende plagioclase thermometry statistical analysis and thermal models to estimate precisely the temperature time distance from axis and duration of metamorphism in east pacific rise gabbros from hess deep odp hole 894g metamorphic hornblende and plagioclase which formed during microfracturing and sea water penetration equilibrated at a mean temperature of 716 8 c 90 confidence level comparison of the properties of the observed temperature distribution with those of model events indicates that metamorphism spanned 60 c when combined with thermal models of fast spreading centers this implies that metamorphism was rapid 6000 yr and occurred 1 4 km off axis application of this approach to other gabbros will allow comparison of spatial and temporal characteristics of deformation and fluid flow in the lower oceanic crust as a function of ridge setting

petrogenetic and geodynamic origin of the neoarchean dore lake complex abitibi subprovince superior province canada the neoarchean ca 2728 ma anorthosite bearing dore lake complex in the northeastern abitibi subprovince quebec was emplaced into an association of intra oceanic tholeiitic basalts and gabbros known as the obatogamau formation the obatogamau formation constitutes the lower part of the roy group which is composed of two cycles of tholeiitic to calc alkaline volcanic and volcaniclastic rocks siliciclastic and chemical sedimentary rocks and layered mafic to ultramafic sills in this study we report major and trace element results and nd sr pb and o isotope data for anorthosites leucogabbros gabbros and mafic dykes from the dore lake complex and spatially associated basalts and gabbros of the obatogamau formation to assess their petrogenetic origin and geodynamic setting field and petrographic observations indicate that the dore lake complex and associated volcanic rocks underwent extensive metamorphic alteration under greenschist facies conditions resulting in widespread epidotization 20 40 and chloritization 10 40 of many rock types plagioclase recrystallized mainly to anorthite and albite endmembers erasing intermediate compositions metamorphic alteration also led to the mobilization of many elements e g lile and transition metals and to significant disturbance of the rb sr and u pb isotope systems resulting in 1935 150 and 3326 270 ma errorchron ages respectively the sm nd isotope system was less disturbed yielding an errorchron age of 2624 160 ma on many binary major and trace element diagrams the least altered anorthosites and leucogabbros and the gabbros and mafic dykes of the dore lake complex plot in separate fields signifying the presence of two distinct magma types in the complex the gabbros and mafic dykes in the dore lake complex share the geochemical characteristics of tholeiitic basalts and gabbros in the obatogamau formation suggesting a possible genetic link between the two rock associations initial end 2 6 to 5 0 and 18o 6 1 to 7 9 values for the dore lake complex and gabbros of the obatogamau formation end 2 8 to 4 0 18o 7 3 to 8 0 are consistent with depleted mantle sources all rock types in the dore lake complex and the roy group share the trace element characteristics of modern arc magmas suggesting a suprasubduction zone setting for these two lithological associations on the basis of regional geology and geochemical data we suggest that the dore lake complex and the obatogamau formation represent a dismembered fragment of a suture zone like many phanerozoic ophiolites resulting from closure of a back arc basin between 2703 and 2690 ma

mid ocean ridge sulfide deposits evidence for heat extraction from magma chambers or cracking fronts numerous seafloor observations show that the sizes of high temperature hydrothermal sulfide edifices vary dramatically with spreading rate on fast spreading ridges venting occurs from spindly chimneys which reach heights of about 10 m while on slow spreading ridges vents are frequently located near the tops of large sulfide mounds whose volumes may reach 105 106 m3 we argue that such variations are the result of a profound difference in the nature of hydrothermal heat extraction between fast spreading ridges where spreading occurs predominantly by magmatism and slow spreading ridges where there is a significant component of tectonic extension along fast spreading ridges a steady state axial magma chamber can be insulated by a relatively thick conductive boundary layer because heat extraction is limited by low permeability in such systems episodes of vigorous venting are linked to diking events not only because these introduce a significant heat source into the upper crust but also because the intrusion of a dike is the primary source of increased permeability near the ridge axis over a time scale of the order of a decade mineral deposition tends to clog the reaction and upflow zones and there is a high probability that young sulfide structures will be buried by subsequent eruptions on slow spreading ridges tectonic extension maintains the fluid pathways necessary to support vigorous convection in the waning stages of magmatism hydrothermal circulation driven by a downward migrating cracking front can cool the entire crust leading to the formation of very large sulfide deposits as the depth of circulation increases overburden pressures reduce the permeability and the hydrothermal heat fluxes decrease progressively once hydrothermal fluids penetrate the moho serpentinization clogs the fluid pathways and high temperature venting ceases until it is reactivated by a fresh magmatic intrusion

genesis of andesitic boninitic magmas at mid ocean ridges by melting of hydrated peridotites geochemical evidence from dsdp site 334 gabbronorites abstract the gabbronoritic cumulates drilled at dsdp site 334 mid atlantic ridge off the famous area are neither crystallization products of the associated basalts nor from any morb composition documented along ocean ridges their parent melts are richer in sio2 than morb at a given mgo content as attested by the crystallization sequence starting with an olivine calcic and sub calcic pyroxene assemblages these melts are issued from a source highly depleted in incompatible elements likely residual peridotite left after morb extraction to understand the role of water in the genesis of these lithologies whose occurrence in a mid ocean ridge setting is rather puzzling we performed a geochemical study on clinopyroxene separates following an analytical protocol able to remove the effects of water rock interactions post dating their crystallization accordingly the measured isotopic signatures can be used to trace magma sources we find that site 334 clinopyroxenes depart from the global mantle correlation normal morb values for the 143nd 144nd ratio 0 51307 0 51315 are associated to highly radiogenic 87sr 86sr 0 7034 0 7067 ratios this indicates that the parent melts of site 334 cumulates are issued from a morb source but that seawater contamination occurred at some stage of their genesis the extent of contamination traced by the sr isotopic signature is variable within all cumulates but more developed for gabbronorites sensus stricto suggesting that seawater introduction was a continuous process during all the magmatic evolution of the system from partial melting to fractional crystallization simple masse balance calculations are consistent with a contaminating agent having the characters of a highly hydrated possibly water saturated silica rich melt depleted in almost all incompatible major minor and trace elements relative to morb mixing in various proportions of contaminated melts similar to the parent melts of site 334 cumulates with morb can account for part of the variability in the sr isotopic signature of oceanic basalts among other to the short wavelength isotopic noise superimposed on regional trends we conclude that seawater introduction into residual peridotite at shallow depth beneath mid ocean ridges can lead mantle rocks and their melts to follow complex p t fh2o paths that mimic petrogenetic contexts classically attributed to subduction zone environments like the production of boninitic andesitic magmas

oxygen isotope exchange kinetics of mineral pairs in closed and open systems applications to problems of hydrothermal alteration of igneous rocks and precambrian iron formations the systematics of stable isotope exchange between minerals and fluids are examined in the context of modal mineralogical variations and mass balance considerations both in closed and in open systems on mineral pair 18 o plots samples from terranes that have exchanged with large amounts of fluid typically map out steep positively sloped non equilibrium arrays analytical models are derived to explain these effects these models allow for different exchange rates between the various minerals and the external fluids as well as different fluid fluxes the steep arrays are adequately modelled by calculated isochron lines that involve the whole family of possible exchange trajectories these isochrons have initially steep near vertical positive slopes that rotate toward a 45 equilibrium slope as the exchange process proceeds to completion the actual data point array is thus analogous to the hand of an isotopic clock that measures the duration of the hydrothermal episode the dimensionless ratio of the volumetric fluid flux to the kinetic rate parameter u k determines the shape of each individual exchange trajectory in a fluid buffered system u k 1 the solutions to the equations 1 are independent of the mole fractions of the solid phases 2 correspond to taylor s open system water rock equation and 3 yield straight line isochrons that have slopes that approach 1 f where f is the fraction reacted of the more sluggishly exchanging mineral the isochrons for this simple exchange model are closely congruent with the isochrons calculated for all of the more complex models thereby simplifying the application of theory to actual hydrothermal systems in nature in all of the models an order of magnitude of time in units of kt separates steep non equilibrium arrays e g slope 10 from arrays approaching an equilibrium slope of unity on a diagram because we know the approximate lifetimes of many hydrothermal systems from geologic and heat balance constraints we can utilize the 18 o 16 o data on natural mineral assemblages to calculate the kinetic rate constants k s and the effective diffusion constants d s for mineral h 2o exchange these calculated values k qtz 10 14 k feld 10 13 10 12 agree with experimental determinations of such constants in nature once the driving force or energy source for the external infiltrating fluid phase is removed the disequilibrium mineral pair arrays will either 1 remain frozen in their existing state if the temperatures are low enough or 2 re equilibrate along specific closed system exchange vectors determined solely by the temperature path and the mineral modal proportions thus modal mineralogical information is a particularly important parameter in both the open and closed system scenarios and should in general always be reported in stable isotopic studies of mineral assemblages these concepts are applied to an analysis of 18 o 16 o systematics of gabbros plagioclase clinopyroxene and plagioclase amphibole pairs granitic plutons quartz feldspar pairs and precambrian siliceous iron formations quartz magnetite pairs in all these examples striking regularities are observed on and plots but we point out that plots have many advantages over their equivalent diagrams as the latter are more susceptible to misinterpretation using the equations developed in this study these regularities can be interpreted to give semiquantitative information on the exchange histories of these rocks subsequent to their formation in particular we present a new interpretation indicating that precambrian cherty iron formations have in general undergone a complex fluid exchange history in which the iron oxide magnetite precursor has exchanged much faster with low temperature 400 c fluids than has the relatively inert quartz

hydrothermal alteration in a modern suprasubduction zone the tonga forearc crust an extensive suite of hydrothermally altered basalts gabbros and plagiogranites was recovered from the trench facing slope of the tonga forearc the tectonic setting lithology and geochemistry of these samples make them a unique collection for comparison with suprasubduction zone ssz ophiolites petrography mineral chemistry and geothermometry are used to constrain the metamorphic evolution of ocean crust formed in a modern ssz setting seawater derived hydrothermal fluids first penetrated the lower crust along grain boundaries and microscopic fracture networks at temperatures 800 c as the plutonic sequence cooled amphibole progressively replaced the mafic phases followed by chlorite and epidote below 550 c basalts record peak alteration temperatures up to 773 c however most were altered at lower temperatures typical of mid ocean ridge mor volcanic sequences kelman 1998 epidosites formed by pervasive alteration of basalt and plagiogranite at greenschist facies conditions and at high water rock ratios pervasively altered gabbros and basalts display evidence of late cataclastic deformation and or contain veins that is likely related to a later tectonic event such as trench rollback the range of alteration temperatures and mineral assemblages in basalts and gabbros are similar to those described from both ssz ophiolites and mors however the degree of alteration in basalts and the presence of epidosites in the tonga collection are most similar to alteration characteristics in ssz ophiolites the initiation of high temperature brittle deformation in the absence of ductile deformation suggests that the tonga forearc crust was constructed in a magma rich environment similar to fast spreading mors

petrogenesis of oceanic plagiogranites by partial melting of gabbros an experimental study we performed hydrous partial melting experiments at shallow pressures 0 2 gpa under slightly oxidizing conditions nno oxygen buffer on oceanic cumulate gabbros drilled by odp ocean drilling program cruises to evaluate whether the partial melting of oceanic gabbro can generate sio2 rich melts with compositions typical of oceanic plagiogranites the experimental melts of the low temperature runs broadly overlap those of natural plagiogranites at 940 c the normalized sio2 contents of the experimental melts of all systems range between 60 and 61 wt and at 900 c between 63 and 68 wt these liquids are characterized by low tio2 and feotot contents similar to those of natural plagiogranites from the plutonic section of the oceanic crust but in contrast to fe and ti rich low temperature experimental melts obtained in morb systems at 950 c the 1 500 m long drilled gabbroic section of odp hole 735b legs 118 and 176 at the southwest indian ridge contains numerous small plagiogranitic veins often associated with zones which are characterized by high temperature shearing the compositions of the experimental melts obtained at low temperatures match those of the natural plagiogranitic veins while the compositions of the crystals of low temperature runs correspond to those of minerals from high temperature microscopic veins occurring in the gabbroic section of the hole 735b this suggests that the observed plagiogranitic veins are products of a partial melting process triggered by a water rich fluid phase if the temperature estimations for high temperature shear zones are correct up to 1 000 c and a water rich fluid phase is present the formation of plagiogranites by partial melting of gabbros is probably a widespread phenomenon in the genesis of the ocean crust

burst of high temperature seawater injection throughout accreting oceanic crust a case study in oman ophiolite following the discovery of a high temperature ht 800 c and a very high temperature 1000 c hydrothermal alteration in the crust of the oman ophiolite a systematic structural and petrological study was conducted throughout the entire ophiolite with supporting isotopic geochemistry the published results showed that the crustal gabbros are extensively altered down to moho by a large seawater flux which was channelled through an identified recharge and discharge circuit microcracks constituting the recharge system propagated through the hot gabbros accreting at the ridge and in spite of their submillimetre width provided the conduit for the large volume of seawater necessary for the observed alteration building on these results we show here that these microcracks opened and were active over a time of a few tens of thousands years while the newly accreted gabbros were drifting away from the ridge microcrack activity was highly episodic with bursts of seawater ingression lasting a few days to a few weeks followed by quiescence periods of a few tens of years this model of ht oceanic hydrothermal alteration has several implications concerning fast spreading oceanic ridges

the formation of sio2 rich melts within the deep oceanic crust by hydrous partial melting of gabbros small amounts of felsic evolved plutonic rocks often called oceanic plagiogranites always occur as veins or small stocks within the gabbroic section of the oceanic crust four major models are under debate to explain the formation of these rocks 1 late stage differentiation of a parental morb melt 2 partial melting of gabbroic rocks 3 immiscibility in an evolved tholeiitic liquid and 4 assimilation and partial melting of previously altered dikes recent experimental data in hydrous morb type systems are used to evaluate the petrogenesis of oceanic plagiogranites within the deep oceanic crust experiments show that tio2 is a key parameter for the discrimination between different processes tio2 is relatively low in melts generated by anatexis of gabbros which is a consequence of the low tio2 contents of the protolith due to the depleted nature of typical cumulate gabbros formed in the oceanic crust on the other hand tio2 is relatively high in those melts generated by morb differentiation or liquid immiscibility since the tio2 content of many oceanic plagiogranites is far below that expected in case of a generation by simple morb differentiation or immiscibility these rocks may be regarded as products of anatexis this may indicate that partial melting processes triggered by water rich fluids are more common in the deep oceanic crust than believed up to now at slow spreading ridges seawater may be transported via high temperature shear zones deeply into the crust and thus made available for melting processes

uniformly mantle like δ18o in zircons from oceanic plagiogranites and gabbros lower ocean crust is primarily gabbroic although 1 2 felsic igneous rocks that are referred to collectively as plagiogranites occur locally recent experimental evidence suggests that plagiogranite magmas can form by hydrous partial melting of gabbro triggered by seawater derived fluids and thus they may indicate early high temperature hydrothermal fluid circulation to explore seawater rock interaction prior to and during the genesis of plagiogranite and other late stage magmas oxygen isotope ratios preserved in igneous zircon have been measured by ion microprobe a total of 197 zircons from 43 plagiogranite evolved gabbro and hydrothermally altered fault rock samples have been analyzed samples originate primarily from drill core acquired during ocean drilling program and integrated ocean drilling program operations near the mid atlantic and southwest indian ridges with the exception of rare distinctively luminescent rims all zircons from ocean crust record remarkably uniform 18o with an average value of 5 2 0 5 2sd the average 18o zrc would be in magmatic equilibrium with unaltered morb 18o wr 5 6 5 7 and is consistent with the previously determined value for equilibrium with the mantle the narrow range of measured 18o values is predicted for zircon crystallization from variable parent melt compositions and temperatures in a closed system and provides no indication of any interactions between altered rocks or seawater and the evolved parent melts if plagiogranite forms by hydrous partial melting the uniform mantle like 18o zrc requires melting and zircon crystallization prior to significant amounts of water rock interactions that alter the protolith 18o zircons from ocean crust have been proposed as a tectonic analog for 3 9 ga detrital zircons from the earliest hadean earth by multiple workers however zircons from ocean crust are readily distinguished geochemically from zircons formed in continental crustal environments many of the 3 9 ga zircons have mildly elevated 18o 6 0 7 5 but such values have not been identified in any zircons from the large sample suite examined here the difference in 18o in combination with newly acquired lithium concentrations and published trace element data clearly shows that the 3 9 ga detrital zircons did not originate by processes analogous to those in modern mid ocean ridge settings

late stage magmatic evolution of oceanic gabbros as a result of hydrous partial melting evidence from the ocean drilling program odp leg 153 drilling at the mid atlantic ridge petrographic observations within oceanic gabbros drilled by ocean drilling program odp at leg 153 from the mid atlantic ridge kane fracture zone mark area 23 n in combination with experimental findings suggest that the late stage magmatic evolution of these gabbros is characterized by pervasive hydrous partial melting water saturated melting experiments on a variety of natural gabbros between 900 and 1000 c at 200 mpa produced newly formed minerals forming a characteristic paragenesis consisting of plagioclase orthopyroxene and pargasitic amphibole clinopyroxene the an content of the new plagioclases is uniformly higher than that of the protolith olivine and clinopyroxene primocrysts react to form neoblastic orthopyroxene and pargasitic amphibole these features can also be observed in the gabbros from the mark area here we found zones at plagioclase grain boundaries showing a strong enrichment in an component with an contents up to 20 to 25 mol higher than those of the host plagioclase primary olivines and clinopyroxenes in contact with such zones react to orthopyroxene and pargasitic amphibole these phases rim olivine and clinopyroxene and grow interstitially typical petrographic characteristics of a late stage magmatic phase the observed late stage microstructures in the gabbros thus appear to be the results of partial melting processes triggered by water rich fluids and are not crystallization products of a percolating differentiated late melt here we demonstrate that hydrothermal circulation within the gabbroic layer starts at much higher temperatures 900 1000 c than up to now believed water rich fluids propagate on grain boundaries in a ductile regime causing hydrous partial melting on a large scale a cracks system a prerequisite in current models for enabling hydrothermal circulation is not necessary the observed process has the potential for transfer of heat and mass between the upper and lower oceanic crust provided that the water rich fluids triggering the partial melting process are seawater derived this process may have a significant influence on the cooling of the deep oceanic crust which is in concordance with new thermal models implying that high temperature hydrothermal circulation is regarded to play an important role in transport of heat in the deep oceanic crust copyright 2005 by the american geophysical union

perspectives on the origin of plagiogranite in ophiolites from oxygen isotopes in zircon abstract the formation of oceanic plagiogranite has been attributed primarily to either 1 extreme fractional crystallization of a mantle melt or 2 partial melting of hydrated mafic crust with support for the latter from field evidence and recent melting experiments remelting of hydrothermally altered ocean crust could yield rocks and minerals with diverse primary magmatic 18 o values forming proximal to the magmatic center where crustal growth is occurring to constrain the magmatic 18 o of a wide range of silicic rocks in oceanic crust and evaluate their petrogenesis we characterized the 18 o of zircons in 22 plagiogranite samples tonalite and trondhjemite from 8 different ophiolites and one dacite sampled along the east pacific rise using secondary ion mass spectrometry sims the 18 o values of 202 magmatic zircons from ophiolites range from 3 9 to 5 6 n 244 spots average 4 9 0 6 2sd extending 1 below typical zircon in equilibrium with mantle and from gabbroic massifs along slow spreading mid ocean ridges 4 7 5 9 east pacific rise dacite zircons range from 4 6 to 5 0 n 12 spots plagiogranite from the dike gabbro transition zone of the northern oman ophiolite yield the lowest 18 o zrn with rock average values of 4 3 5 0 the low 18 o values are best explained by remelting of crust altered by hydrothermal fluids with seawater like isotopic compositions at high temperatures possibly due to vertical migration of the boundary between an active magma chamber and a vigorous high temperature hydrothermal system in the overlying crust if the partial melt was assimilated into a fractionating melt lens with morb like 18 o as envisioned for km scale plagiogranite bodies in oman up to 20 contamination by a protolith with 18 o 2 would be required previous oxygen isotope constraints from quartz in oman plagiogranite suggested melting of both high and low 18 o crust had occurred comparison of quartz zircon pairs indicates that quartz has been modified in most samples and we find no evidence for the involvement of high 18 o rocks during plagiogranite formation

deep and high temperature hydrothermal circulation in the oman ophiolite petrological and isotopic evidence a systematic search for evidence of high temperature hydrous alteration within the gabbros of the samail ophiolite oman shows that most of the gabbros have been affected by successive stages of alteration starting above 975 c and ending below 500 c sr and o isotopic analyses provide constraints on the nature and origin of the fluids associated with these alteration events massive gabbros dykes and veins and their associated minerals depart from mid ocean ridge basalt morb source magma signatures 87sr 86sr 0 7032 and depleted 18o 10 a second group of samples including massive gabbros and high temperature dykes and veins is characterized by lower water rock ratios in the range of 3 5 these samples display a high temperature hydrothermal alteration related to seawater ingression the main fluid channels may be sub millimetre sized microcracks with a dominantly vertical attitude which constitute the recharge hydrothermal system whereas the dykes and veins represent the discharge part this model requires that these dykes have been generated by hydration of the crystallizing gabbros via seawater penetration near the internal wall of the magma chamber

hydrothermal fault zones in the lower oceanic crust an example from wadi gideah samail ophiolite oman abstract hydrothermal circulation is a key process for chemical and isotopic exchange between the solid earth and oceans and for the extraction of heat from newly accreted crust at mid ocean ridges however due to a dearth of samples from intact oceanic crust or continuous sample suites from ophiolites there remain major shortcomings in our understanding of hydrothermal circulation in the oceanic crust especially in the lower plutonic crust in particular it remains unknown whether fluid recharge and discharge occurs pervasively or if it is mainly channelled within discrete zones such as faults here we describe a hydrothermally altered fault zone that crops out in the wadi gideah in the layered gabbro section of the samail ophiolite of oman a one metre thick normal fault comprising deformed chlorite epidote fault rock with disseminated chalcopyrite and pyrite offsets gently dipping layered olivine gabbros the chlorite rich fault rocks surround strongly altered clasts of layered gabbro 50 to 80 cm in size layered gabbros in the hanging wall and the footwall are partially altered and abundantly veined by epidote prehnite laumontite and calcite veins in the wall rocks igneous plagioclase an82 2 is partially altered towards more albitic compositions an75 81 and chlorite tremolite partially replaces plagioclase and clinopyroxene clinopyroxene is moderately overgrown by mg hornblende whole rock mass change calculations show that the chlorite rich fault rocks are enriched in fe mn h2o co2 co cu zn ba and u but have lost significant amounts of si ca na cr ni rb sr cs light rare earth elements lree eu and pb gabbro clasts within the fault zone as well as altered rock from the immediate hanging wall show enrichments in na volatiles sr ba and u and depletions of si ti al fe mn mg ca cu zn rb cs lree and pb chlorite thermometry suggests a formation temperature of 300 350 c for the fault rock and based on the si loss and solubility of silica in hydrothermal fluids the intensity of alteration requires a fluid to rock ratio of up to 900 1 strontium isotope whole rock data of the fault rock yield 87sr 86sr ratios of 0 7043 0 7048 which is considerably more radiogenic than fresh layered gabbro from this locality 87sr 86sr 0 7030 0 7033 and similar to black smoker hydrothermal signatures based on epidote measured from epidote veins in the footwall and elsewhere in the ophiolite 87sr 86sr 0 7043 0 7051 altered gabbro clasts within the fault zone show similar values with 87sr 86sr ratios of 0 7045 0 7050 in contrast the hanging and footwall gabbros display values only slightly more radiogenic than fresh layered gabbro the elevated strontium isotope composition of the fault rock and clasts together with the observed secondary mineral assemblages and calculated mass changes strongly supports the intense interaction with seawater derived up welling hydrothermal fluids active during oceanic spreading assuming that such a fault zone is globally representative of faulting in the lower crust an extrapolation of our results from mass change calculations to elemental fluxes shows a significant contribution to the global hydrothermal budgets of si ti fe mn mg ca h2o cu zn sr and cs

cooling rates in the lower crust of the oman ophiolite ca in olivine revisited debate over the mechanism of accretion of the lower oceanic crust has centered around the gabbro glacier and the sheeted sills models the thermal profile of the crust specifically the roles of hydrothermal circulation and cooling rate is a key component in distinguishing between these two models results of this study show no systematic variation of cooling rate with depth in the lower crust of the khafifah section in the wadi tayin massif of the oman ophiolite additionally very high rates recorded near the base of the crust suggest that hydrothermal circulation plays an important role in the removal of heat throughout the crust while the conclusions presented here do not rule out the possibility of accretion by gabbro glacier they do contradict the initial tenet of the gabbro glacier model convective heat loss by hydrothermal circulation can penetrate to moho depths and crystallization is not by necessity constrained to the upper crust on the basis of these results it can be concluded that the sheeted sills model is a thermally viable mechanism for accretion of oceanic lower crust

detrital zircon and provenance analysis of late cretaceous miocene onshore iranian makran strata implications for the tectonic setting a multidisciplinary provenance study including sandstone framework heavy mineral analysis in situ u pb dating of detrital zircon and hf isotopic ratio analysis of dated zircons was undertaken on late cretaceous miocene deep marine turbiditic and deltaic sandstones of makran accretionary wedge se iran to determine their sedimentary provenance and tectonic setting sandstone framework modes reveal both magmatic arc rocks as a source of late cretaceous oligocene detritus and recycling of miocene sandstones heavy mineral assemblages cr spinel and blue amphibole indicate ophiolite and high pressure low temperature metamorphic rocks blueschists as a supplementary provenance in total 2931 laser ablation inductively coupled plasma mass spectrometry icp ms u pb detrital zircon ages on 21 sandstone samples yielded three major age peaks at ca 167 ma 88 7 ma and 48 9 ma also 241 in situ hf isotope analyses of dated zircons provide evidence for dominantly igneous source rocks two main detrital zircon ages are identified 1 abundant middle jurassic grains with hf isotopic compositions of continental crust suggesting a rifting related magmatic provenance and 2 late cretaceous eocene grains with hf isotopic compositions of continental crust and nondepleted mantle suggesting a continental magmatic arc provenance this change in provenance is attributed to the late cretaceous convergence between arabia and eurasia

oxygen isotope evidence for short lived high temperature fluid flow in the lower oceanic crust at fast spreading ridges millimeter scale amphibole veins in the lower oceanic crust record fracture controlled fluid flow at high temperatures but the importance of this fluid flow for the thermal and chemical evolution of the lower oceanic crust is unclear in the section of lower oceanic crust recovered at hess deep from odp hole 894g which formed at the fast spreading east pacific rise these veins are randomly distributed with an average spacing of 1 m we unravel the history of fluid flow through one of these veins by combining in situ o isotope analyses of wall rock plagioclase with major element analyses geothermometry and diffusion modeling thermometry indicates vein sealing by amphibole at 720 c over a narrow temperature interval 20 c in situ oisotope analyses by ion microprobe with a precision of b0 5 reveal zoning of o isotopes in plagioclase adjacent to the vein the zoning profiles can be reproduced using a diffusion model if the duration of o isotope exchange was 100 yr a similar interval of fluid rock exchange is suggested by modeling potassium depletion in plagioclase adjacent to the vein if representative of fracture controlled fluid flow in the lower oceanic crust the limited duration of fluid flow and its occurrence over a narrow temperature interval suggest that high temperature fluid flow in this porosity network does not transport significant heat 2007 elsevier b v all rights reserved

the base of a sheeted dyke complex oman ophiolite implications for magma chambers at oceanic spreading axes the sheeted dyke complex in the maydan syncline area of the oman ophiolite was formed during a single sea floor spreading episode in some areas individual dykes from this sheeted dyke complex have been seen to root in the high level gabbro and others are truncated by high level gabbro diorite and trondhjemite however elsewhere dykes penetrate laterally into the layered part of the plutonic sequence this situation suggests that active magma chambers and zones of dyke injection were discontinuous along the spreading axis at any one time

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

formation of the neoarchean bad vermilion lake anorthosite complex and spatially associated granitic rocks at a convergent plate margin superior province western ontario canada abstract the bad vermilion lake anorthosite complex henceforth the bvla complex in western ontario is one of the well exposed anorthosite bearing archean layered intrusions in the superior province canada this study presents new whole rock major and trace element data for the various units of the complex oxygen isotope data for the anorthosite and major and trace element data for the spatially associated granitic rocks intruding the bvla complex to constrain their petrogenetic and geodynamic origin zircons from granitic rocks have yielded a 207pb 206pb age of 2716 18 ma constraining the minimum intrusion age of the complex despite deformation and greenschist facies metamorphism primary igneous textures are locally well preserved in the bvla complex its whole rock major and trace elemental compositions and the oxygen isotopic systematics appear not to have been substantially modified by deformation and metamorphism mantle like oxygen isotope signatures and major and trace element compositions are inconsistent with significant crustal contamination of the bvla complex during its emplacement the existence of primary calcic igneous plagioclase coherent negative nb anomalies nb nb 0 08 0 88 and geochemical similarities between gabbros from the bvla complex and gabbros from cenozoic arcs collectively suggest an intra oceanic subduction zone geodynamic setting for the complex near flat ree patterns in the various units of the bvla complex suggest that they were derived from melting of a shallow source beneath a subarc mantle wedge trends in immobile major e g mgo and trace e g zr element data indicate that the mineralogical composition of the complex can be explained by fractional crystallization and accumulation of olivine orthopyroxene clinopyroxene plagioclase and possibly amphibole compositionally the bordering granitic rocks are a2 type and strongly enriched in th and ree 100 times chondrite and depleted of ba sr eu and ti we suggest that they formed in a post collisional extensional tectonic regime following emplacement of the bvla complex in an oceanic arc

abiotic hydrogen h2 sources and sinks near the mid ocean ridge mor with implications for the subseafloor biosphere free hydrogen h2 is a basal energy source underlying chemosynthetic activity within igneous ocean crust in an attempt to systematically account for all h2 within young oceanic lithosphere

origin of rodingite forming fluids constrained by calcium and strontium isotope ratios in the leka ophiolite complex abstract alteration of oceanic lithosphere plays an important role in the global element cycling due to its dominant abundance on the earth s surface the formation of rodingites a ca rich lithology that occurs intimately associated with ca depleted serpentinite complexes is a widespread process during strong alteration of ocean floor lithologies and may represent a key process that influences the global ca cycle stable ca and radiogenic sr isotope compositions of rodingites and associated lithologies from the leka ophiolite complex norway preserve mantle signatures and thus provide evidence that the ca and sr sources for rodingitisation are dominantly mantle derived with no detectable seawater input the difference of 0 7 in 44 40ca of different phases found in the rocks is overall consistent with inter mineral fractionation at high temperatures related to the cao bonding environment but there are also indications for isotopic disequilibria between non cogenetic mineral phases while the water required for rodingite formation may originate from seawater the dissolved ca and sr involved in the mineral reactions during rodingitisation plg ca gros clz al can be balanced at a local scale being derived from the serpentinisation of mantle peridotites cpx1 h2o al srp chlorite ca cpx2 consequently rodingites do not act as a sink for oceanic ca instead they are a sink for ca released during serpentinisation of the oceanic lithospheric mantle

hydrothermal activity in the lau back arc basin sulfides and water chemistry the submersible nautile completed 22 dives during the nautilau cruise r v nadir april 17 may 10 1989 for a detailed investigation of the southern lau basin near tonga the objective of the scientific team from france germany and tonga was to understand the process of sea floor ore formation associated with hydrothermal circulation along the valu fa back arc ridge behind the tonga kermadec trench the four diving areas between lat 21 25 s and 22 40 s in water 2000 m deep were selected on the basis of results from cruises of the r v jeancharcot and r v sonne the nadir cruise provided proof of hydrothermal activity in all four areas over more than 100 km as indicated by the widespread occurrence of hydrothermal deposits and by heat flow conductivity and temperature measurements near the sea bottom the most spectacular findings were high temperature white and black smokers and associated fauna and ore deposits hydrothermal water chemistry and sulfide composition data presented here indicate that this hydrothermal field is very different from the hydrothermal fields in oceanic ridges this difference is seen in the water chemistry of the hydrothermal fluid ph 2 and high metal content and the chemical composition of sulfides enrichment in ba as and pb

seawater strontium and sr ca variability in the atlantic and pacific oceans abstract seawater sr and sr ca exhibit spatial gradients of 2 3 globally with the deep ocean more enriched relative to the surface in latitudinal transects the highest surface values are found at high latitudes and associated with areas of upwelling a pronounced upper ocean vertical sr gradient is attributable to the production of celestite skeletons by surface dwelling acantharia coupled to a shallow dissolution cycle the upper ocean residence time of sr with respect to celestite cycling is much shorter than its global oceanic residence time although the magnitude of seawater sr ca variability is relatively small it is significant with respect to high precision paleoceanographic applications sr ca gradients in the contemporary ocean also complicates evaluating quaternary changes in seawater sr ca that may have resulted from other processes such as aragonite recrystallization during sea level low stands

near solidus evolution of oceanic gabbros insights from amphibole geochemistry the near solidus evolution of plutonic rocks formed at slow spreading ridges is investigated using the major and trace element compositions of amphiboles in a suite of gabbros from the mid atlantic ridge these new data allow unambiguous geochemical discrimination between amphiboles of magmatic and hydrothermal origin in turn this allows the gabbro solidus to be constrained to 860 30 c using amphibole plagioclase thermometry this is consistent with temperatures from associated secondary clinopyroxene magmatic amphibole which can be identified in almost all samples formed during metasomatism of a low porosity crystal mush by an evolved hydrous silicate melt these amphiboles are characterised by high f nb and f cl and low cl contents the amphibole forming reaction involved melt plagioclase and clinopyroxene amphibole blebs with a geochemically magmatic signature are found enclosed in the cores of some primitive clinopyroxene crystals there is no evidence for a seawater component in the magmatic amphibole as would be expected if high temperature seawater ingress leads to flux melting as has recently been suggested however the ingress of seawater derived fluids did occur at temperatures within error of the gabbro solidus forming amphibole in veins and replacing igneous phases these amphiboles are characterised by high cl b and cl f and low nb f and nb la the fluids involved in the formation of these amphiboles had compositions unlike seawater or hydrothermal vent fluids

metallogenic epochs in the inner zone of southwest japan abstract the principal metallogenic epochs in the inner zone of southwestern japan are classified by considering mainly radiometric and microfossil age data as follows 1 early cambrian epoch characterized by podiform type chromite deposits hosted in ophiolite complexes these are the earliest formed mineral deposits in the japanese islands and probably formed in the shallow mantle of island arc settings 2 carboniferous to permian epoch characterized by besshi type base metal sulfide deposits mainly associated with submarine lavas and tuffs of basaltic composition including their metamorphosed equivalents and by bedded cupriferous pyrite deposits associated with submarine acidic tuffs both types of deposits would have been formed in a back arc basin environment geochemically influenced by an island arc 3 middle triassic to jurassic epoch characterized by most of the bedded manganese deposits associated with bedded cherts including their metamorphosed equivalents which would have formed by hot spring activity in an ocean basin environment with a limited current circulation the manganese deposits and their host rocks are the result of seafloor accretion 4 late cretaceous to paleogene epoch characterized by extensive continental arc magmatism volcanic and plutonic resulting in formation of various mineral deposit types related mainly to ilmenite series late cretaceous or magnetite series paleogene granitic rocks related to the former are polymetallic skarns and veins containing cu sn and w also related to this metallogenic activity are stratabound hydrothermal clay deposits of kaolin pyrophyllite and sericite associated with pre ilmenite series granitoids subaerial felsic pyroclastic rocks molybdenite bearing veins and replacement type sericite deposits are characteristics of the magnetite series terranes with local skarns and vein occurrences as with ilmenite series granitoids occurring in southwestern japan eastward younging is also recognized in these mineral deposits regardless of ore type 5 neogene epoch characterized by intense submarine volcanism of acidic composition and associated kuroko deposits in the green tuff region a major tectonic division and a major metallogenic province of the neogene tertiary of japan these mineral deposits would be related to japan sea opening in the non green tuff region important mineralization events are represented by sedimentary uranium deposits and veins characterized by au sb or hg occurrences 6 plio pleistocene epoch characterized by epithermal au ag and au ag cu veins associated with subaerial andesitic to dacitic volcanic rocks of plio pleistocene age volcanism of this and following epochs is characteristic of an island arc setting and 7 quaternary epoch characterized by volcanic sulfur deposits and minor iron sand deposits