deformation history of tectonic melange and its relationship to the underplating process and relative plate motion an example from the deeply buried shimanto belt sw japan map to microscopic scale structural analysis of the hanazono assemblage of the shimanto belt in sw japan an excellent example of a deeply buried accretionary complex indicates a detailed process from underthrusting to underplating the earliest underthursting process is recorded in fabric of melange which has deformed by shear along the decollement characterized by thinning strain due to extensional breakage of blocks of sandstone and exotic materials such as basalt and pelagic sediments of oceanic affinities the deformation mechanism for melange formation for domain i is plastic flow concordant with metamorphic grade which is higher than that of domain ii the deformation mechanism for domain ii is predominantly pressure solution partly with grain scale brittle breakage for the less metamorphosed part of the hanazono assemblage the posterior underplating process recorded is contraction and thickening due to thrust stacking and folding locations of the folds developed beneath thrusts are emphasized because they are a line of evidence of thickening the main deformation mechanism of the second stage is plastic flow for domain i and cataclasis for domain ii the cataclasis in domain ii may be a result of change in strain rate and seems to be related to seismicity at the time of underplating kinematics deduced from fabric analysis of the earliest deformed melange indicates a good consistency with a relative plate motion between the eurasian and oceanic plates that is estimated from a hot spot reference plate circuit model by engebretson et al 1985 while that of the subsequent thrusting is not this fact suggests that strain partitioning in association with oblique subduction of oceanic plate was minimum during subduction another possibility for strain partitioning is the requirement of coupling between plates that are recorded by the deformation mechanisms

comparison of platinum palladium and rhodium distributions in some layered intrusions with special reference to the late differentiates upper zone of the bushveld complex south africa the stillwater fiskenaesset and bushveld complexes are all composed of layered ultramafic and mafic rocks in which the cumulus phases generally appear in a regular order which have similar petrologic and chemical characteristics which are all precambrian in age and which contain platinum group elements that vary widely in their abundances and relative proportions two of the complexes the stillwater and bushveld contain platinum group element enriched horizons not yet found in the fiskenaesset the ratio of pt pt pd in rocks from the stillwater tends to increase from the basal zone to the middle of the ultramafic zone decreases to the beginning of the banded zone and then increases again through the banded zone many fluctuations however are superimposed upon this generalized trend the trends of pt pt pd ratios appear to show a positive correlation with the mg mg fe super 2 ratio in olivine in the fiskenaesset complex the pt pt pd ratio increases rapidly from 0 3 to 0 7 from the lower gabbro and ultramafic units through the leucogabbro units then increases slowly through the anorthosite and upper gabbro unit superimposed on this generalized trend are increasing and decreasing patterns within each unit analyses of platinum palladium and rhodium for 69 rocks from the main and upper zones of the eastern bushveld from the roossenekal area show generally low concentrations of less than 1 ppb pt 0 1 ppb pd and 0 1 ppb rh but larger amounts occur in some magnetite layers and sulfide bearing zones within the main zone up to the pyroxenite marker from the pyroxenite marker to below lower magnetite layer 2 and in the upper zone the pt pt pd ratio decreases with stratigraphic position although with minor fluctuations in these intervals the concentration levels of the platinum group metals appear to be directly related to the amount of sulfide material present and the proportion of intercumulus material variations in pt pt pd ratios in the bushveld and fiskenaesset complexes are presumably related to changes in major mineral compositions as they are in the stillwater complex

the genesis of archaean chromitites from the nuasahi and sukinda massifs in the singhbhum craton india abstract the chromitite deposits of the nuasahi and sukinda massifs are part of layered ultramafic bodies which occur within archaean low grade metamorphic rocks of the iron ore group iog in the singhbhum craton of the indian shield the chromitite seams are interlayered with dunite and associated with orthopyroxenite detailed electron microprobe study reveals very high mg numbers 0 66 0 82 and cr numbers 0 75 0 87 for chromite in massive chromitite from the seams high fo content fo 92 95 for olivine in dunite and high en content en 89 94 for orthopyroxene in orthopyroxenite our study suggests that the original igneous compositions for these minerals are preserved in the respective monomineralic rocks and can be used to evaluate the primary magmatic petrogenesis of the rocks the primitive compositions of the minerals in monomineralic rocks suggest high degree of partial melting of the source mantle during the mesoarchaean the parental magma from which the massive chromitites crystallized was of siliceous high mg basaltic or boninitic compositions similar to the compositions of spatially associated chromite bearing siliceous high mg basalts of the iron ore group the parental magma may have generated due to the interaction of a depleted mantle with a fluid enriched melt possibly derived in response to the dehydration of a subducting slab monomineralic chromitite layers formed either in response to mixing of magmas or due to the suppression of silicate mineral crystallization in response to elevated h 2 o concentrations in the parent magmas the parental magmas that produced chromitites may have intruded into the volcano sedimentary greenstone belts of a supra subduction zone setting

mesoproterozoic ophiolitic melange from the se periphery of the indian plate u pb zircon ages and tectonic implications abstract the kanigiri melange within the proterozoic nellore khammam schist belt in southern peninsular india includes ophiolitic fragments that represent the remnants of an oceanic plate the ophiolitic units were accreted along a ne trending suture that juxtaposes the proterozoic eastern ghats granulite belt eggb against the archean nellore schist belt of the dharwar craton the ophiolite components in the kanigiri melange include plagiogranites and gabbros which show mutually intrusive relations indicating their coeval nature we report laser ablation icp ms age data and ree geochemistry of zircons from the gabbro and granite the zircons from both gabbro and granite show high ree contents prominent hree enrichment and a conspicuous negative eu anomaly suggesting a common melt source zircon ree abundances and normalized patterns show little intersample and intrasample variations u pb dating of the zircons reveals prominent mesoproterozoic ages for the plagiogranite with the ca 1 33 ga age of the kanigiri ophiolitic melange offering important clues for arc continent collision during the final stages of amalgamation of the columbia derived fragments within the neoproterozoic supercontinent assembly

chemometrics reveals oil sources in the fangzheng fault depression ne china one hundred and twenty five rock and oil samples collected from the fangzheng fault depression were analyzed by gc ms principal component analysis pca and multidimensional scaling mds based on nine biomarker ratios were used for oil source rock correlation in this region the oils are characterized by a predominance of low to middle molecular weight normal alkanes n c12 n c20 or n c12 n c25 moderate to high pr ph ratios 1 44 5 3 relatively low c27 c29 regular steranes c35 c34 hopanes and gammacerane c31r ratios all of which suggest relatively oxic conditions under fresh water depositional conditions with significant terrigenous organic matter input pca and mds results show that the investigated oils were derived mainly from the xinancun wuyun formation and also illustrate the maturation and depositional conditions of the rocks and oils through an mds plot we show that mds is a reliable multi parameter oil source rock correlation method

palladium platinum rhodium ruthenium and iridium in chromitites from the massif du sud and tiebaghi massif new caledonia massive and disseminated podiform chromitite from 43 mines and other occurrences in the massif du sud and tiebaghi massif both ophiolites in new caledonia contain up to 9 ppb pd up to 45 ppb pt up to 31 ppb rh up to 410 ppb ir and up to 1 130 ppb ru the chromitites are strongly depleted in pt and pd and mildly depleted in ir and ru with respect to an average chondrite composition as are podiform chromitites from oman turkey greece and northern california stratiform chromitites from the stillwater complex montana and the bushveld complex south africa show inverse patterns of enrichment and depletion these patterns suggest that either chromitite from ophiolites represents mantle rocks depleted in pt and pd or that the magmas that formed the ophiolites were depleted in pt and pd or that the process of forming the chromitite involved the concentration of os and ir from a magma the relatively high contents of ir and ru in the new caledonian chromitite suggest the potential for the occurrence of placers that contain concentrations of os ir alloys

proterozoic orogens in southern peninsular india contiguities and complexities abstract the precambrian terranes of southern peninsular india have been central to discussions on the history of formation and breakup of supercontinents of particular interest are the proterozoic high grade metamorphic orogens at the southern and eastern margins of the indian shield skirting the 3 4 ga dharwar craton which not only preserve important records of lower crustal processes and lithospheric geodynamics but also carry imprints of the tectonic framework related to the assembly of the major neoproterozoic supercontinents rodinia and gondwana these proterozoic orogens are described as southern granulite terrane sgt in the southern tip and the eastern ghats mobile belt egmb in the eastern domains of the peninsula the contiguity of these orogens is broken for a distance of 400 km and disappears in the bay of bengal these orogens expose windows of middle to lower crust with well preserved rock records displaying multiple tectonothermal events and multiphase exhumation paths recent studies in these orogens have led to the recognition of discrete crustal blocks or terranes separated by major shear zone systems some of which represent collisional sutures the sgt and egmb carry several important features such as fold thrust tectonics regional granulite facies metamorphism of up to ultrahigh temperature conditions in some cases multiple p t paths development of lithospheric shear zones emplacement of ophiolites presence of alkaline and anorthositic complexes development of crustal scale flower structures transpressional strains and reactivation tectonics a heterogeneous distribution of different metamorphic and magmatic assemblages with distinct spatial and temporal strain variations in shaping the fabric elements in different blocks is identified both egmb and sgt share a common transpressional deformation history during the latest neoproterozoic characterized by the steepening of the initial low angle crustal scale structures leading to a subvertical grain conducive to reactivation tectonics our synthesis of the spatial distribution geometry kinematics and the transpressional strain of the shear zone systems provides insights into the tectono metamorphic history of the proterozoic orogens of southern india and their contiguity and complexities recent understanding of subduction accretion and collisional history along these zones together with a long lived transpressional tectonic regime imply that these orogens witnessed identical tectonic regimes at different times in earth history although the major and common structural architecture was built during the final assembly of the gondwana supercontinent

asymmetrical melange fabrics as possible indicators of the convergent direction of plates a case study from the shimanto belt of the akaishi mountains central japan abstract scaly foliated melanges in the upper maastrichtian lowest paleogene strata occurring in a zone several kilometers wide in the shimanto belt of the akaishi mountains central japan often exhibit composite planar fabrics on a mesoscopic scale with distinct monoclinic symmetry they are characterized by block in matrix fabrics including scaly foliations in mudstone matrices and asymmetric lensoidal and or ellipsoidal clasts of sandstone which were generated by extension parallel to the layers these fabrics are geometrically very similar to those of foliated fault related rocks another remarkable fabric is the imbricated stacks of lensoidal sandstone clasts forming mesoscopic duplexes by layer parallel contraction the co existence of these extensional and contraction planar fabrics suggests that non coaxial deformation had considerably progressed and that strain distribution was heterogeneous during formation of the melange systematic analysis of attitudes of these melange fabrics in many outcrops indicates that the fabrics have a remarkably consistent preferred orientation suggesting that a south trending overthrust shear about 40 70 counterclockwise with respect to the trend of strata prevailed during melange formation this direction is fairly consistent with the relative plate motion deduced from the regional tectonic framework of this age which indicates the oblique subduction of the pacific plate relative to the eastern margin of the eurasian continent thus the melange fabrics with monoclinic symmetry probably reflect the relative motion of consuming plates and may become excellent geological indicators for determining the direction of convergence of plates during melange formation since data on melange fabrics are directly and easily available in the field

petrological and geochemical characterization of proterozoic ophiolitic melange nellore khammam schist belt se india abstract the ophiolitic melange occurs close to a major fault within the metavolcanics of kanigiri area in the western margin of the khammam nellore schist belt andhra pradesh india the unique feature of kanigiri melange is the occurrence of ophiolitic bodies of various sizes most of which display petrological and geochemical characteristics typical of supra subduction zone oceanic crust the ophiolitic melange is composed mainly of blocks of variably deformed and metamorphosed pillow basalts serpentinized ultramafic rocks meta gabbros cherts and pelagic sediments in a pervasive scaly argillite matrix the melange has a tectonic contact with a granite intrusion and numerous thrust faults cut across the melange exposures primary magmatic features such as drainage cavities in pillow basalts magmatic layering in gabbros and clinopyroxene cumulates in ultramafic rocks are well preserved in low strain domains mafic components enclosed in the melange are characterized by low pressure secondary mineral assemblages retrograded from amphibolite down to greenschist facies as commonly observed in subducted and exhumed orogens all amphiboles in the studied fragments are calcic and cluster in the range from winchite actinolite to hornblende amphibole compositions are characterized by al 2 o 3 2 4 13 8 feo 5 13 k 2 o 0 04 0 7 and x mg 0 67 0 77 the metabasalt and wherlite samples exhibit light rare earth element lree enriched patterns and negative high field strength element hfse zr and ti consistent with a suprasubduction zone geodynamic setting a notable feature of both metabasalt and wherlite samples is their pronounced negative ce anomaly and a positive eu anomaly as observed in a number of modern island arc suites as the petrological features and the geochemical characteristics of the remnant fragments we analyzed in this study are reminiscent of their phanerozoic ophiolite counterparts we suggest that these rocks were derived from a proterozoic oceanic plate belonging to a suprasubduction ophiolite assemblage subduction zone geochemical signatures and ophiolite like crustal lithologies in the study region are consistent with the accretion of juvenile oceanic fragments to the pre existing continental blocks through a forearc continent collision

compositional variations within and between five sudbury ore deposits recent work on the platinum group element pge content in ni sulfide ores has shown that those associated with komatiite magmas generally have low pt pd ru ir os ratios 13 the levack west and little stobie 1 and 2 deposits conform with this generalization having ratios of 18 6 19 2 and 13 2 respectively hoffman et al 1979 pt and pd are present in roughly equal proportions in these deposits ranging between 1 000 and 3 000 ppm in massive sulfides new data from the strathcona deposit indicate values of close to 400 ppm for both pt and pd approximately one third of the levels at the nearby levack west deposit both strathcona and levack west are strongly zoned with respect to platinum group elements au and base metals cu ni zn pt pd and au increase away from the irruptive hanging wall into the footwall and co rh ru ir and os increase in the reverse direction at strathcona this zoning results in a decrease of the pt pd ru ir os ratio from 132 in the footwall to 2 2 in the hanging wall ore relevant phase equilibria data indicate that cu ni zn pt and pd are likely to become enriched in the residual liquid of a fractionating sulfide melt whereas co rh and ru will concentrate in the early crystallizing monosulfide solid solution the correspondence between the experimental data and natural observations is strong evidence that the zoning is the consequence of the fractionation of a sulfide melt with the residual liquid being expelled progressively into the footwall average data obtained for the falconbridge depost resemble those of the hanging wall at strathcona both in the low pt pd ru ir os ratio of 2 3 and in a low cu ni ratio suggesting that the ore sampled represents an early crystallizing fraction and that a late crystallizing pt rich fraction equivalent to the footwall ore at strathcona and amounting to perhaps twice the present volume of ore was not included in the sampling and may well not be within the present orebody the deposit is located largely within a vertical fault zone it is possible that the sulfide melt was introduced up the fault crystallizing as it moved upward and that the pt rich portion crystallized farthest up the fault and has been removed by erosion however sampling over a 1 600 m vertical interval of the ore zone has failed to reveal any progressive variation in the pt pd ru ir os ratio within the fault zone an alternative and preferred hypothesis is that the deposit was originally zoned as at strathcona with the pt rich portion forming away from the irruptive within the footwall this portion may have been faulted off leaving the early crystallizing portion behind to become remobilized into the fault zone if true the present location of the pt rich portion is intriguing in addition to having lower concentrations of platinum group elements than levack west the strathcona deposit also has lower concentrations of ni and cu these compositional differences can be modeled in terms of a larger amount of sulfide equilibrating with a given amount of the host silicate magma at the former deposit than at the latter sequences of similar magmatic events coupled with a proposed fractionation of olivine can account for many of the compositional variations between different sudbury deposits

precambrian continental crust of india and its evolution the indian precambrian continental crust exhibits a variety of geological features fashioned at different times by different geotectonic processes the bulk of this crust was formed prior to 2600 m y ago and remobilized at least twice between 2600 2000 m y ago early proterozoic mobile belt epmb and 2000 1500 m y ago middle proterozoic mobile belt mpmb three early precambrian nucleii karnataka kn jeypore bastar jbn and singhbhum sn appear to have survived in the craton and are characterized by low grade supracrustals and tonalitic trondhjemite gneisses formed 3800 2600 m y ago the epmb event involved sedimentation amphibolite granulite facies metamorphism and co 2 k metasomatism and produced amphibolite facies rocks and k granites in the north and charnockite and other granulite facies rocks in the south gold sporadically distributed in the supracrustal rocks of the craton was remobilized during the epmb event k granites form a garland around the central dharwar craton sugg

underplated units in an accretionary complex melange of the shimanto belt of eastern shikoku southwest japan imbricated thrust stacks composed of melange and oceanic slabs occur in the shimanto belt eastern shikoku sw japan the melanges are divided into two types melange i consists of lenticular sandstone blocks surrounded by scaly shale matrix and melange ii is composed of blocks of oceanic material such as basalts cherts and red shale in a scaly matrix of shale melange ii is observed only near the boundaries between melange i and oceanic slabs the fabric of melange i indicates progressive deformation during underthrusting of originally coherent trench fill turbidites layer parallel extension characterizes early deformation and is divided into two stages an early stage formed during normal faulting and a subseguent stage formed perpendicular to the early extension the early extension is interpreted in terms of riedel shear associated with layer parallel shearing and tectonically induced loading normal to the layering the second extension is due to sticking thrust movement late deformation of melange i is represented by folding in association with layer parallel shearing this appears to be related to compression just before or during underplating of melange i beneath the accretionary prism layer parallel shear extends downward due to strain hardening of sheared sediments and then reaches to the base of the sediments subsequently the shear zone penetrates pelagic sediments and finally oceanic basement at this time melange ii is formed by the shearing an imbricated stack of melanges i ii and oceanic slabs forms duplexes during the underplating process the weak metamorphism of the melange complex in the study area suggests that underplating occurs at shallow depths where trench fill is thin

underplating process from melange formation to duplexing example from the cretaceous shimanto belt kii peninsula southwest japan accretionary complexes are considered to form through off scraping and underplating which results in the lateral and vertical growth of an accretionary prism accretion of the upper part of the oceanic crust and the formation of melange are important components of the accretionary process although the origin of melange is considered controversial in order to better understand the origin of melange and its role in accretion in particular during underplating we have studied the relationship between the formation of melange and duplexing in the miyama assemblage here the melange is composed dominantly of shale basalt chert and sandstone this study stresses the following features 1 the structure of the miyama assemblage appears to be a composite suite of landward and seaward dipping duplexes 2 the melange fabric displays a systematic asymmetry which may have resulted through shearing along the decollement and 3 facing of horses within duplexes changes from north to south coinciding with a change in the sense of shear inferred from the melange fabric which suggests that it formed prior to duplexing

a deep strong reflector in the nankai accretionary wedge from multichannel seismic data implications for underplating and interseismic shear stress release 1 we present the results of structural and stratigraphic interpretations for the multichannel seismic profiles to reveal the structure of the nankai accretionary wedge off shikoku island on the basis of reflection characteristics we identify three major seismic units that correspond to a unit consisting of both oligocene miocene accretionary prism and the sedimentary cover unit a a unit of underthrust miocene pliocene hemipelagic sediments unit b and a unit of subducting pre miocene oceanic crustal rocks unit c according to relative strength of the topmost reflector of unit b the nankai accretionary wedge can be divided into three different structural zones along the cross strike direction apparent decollement reflector zone low amplitude reflector lar zone and deep strong reflector dsr zone despite the velocity uncertainty involved in the depth conversion and therefore thickness estimation error the decollement step down and the possible duplex structures in the lar and dsr zones support that the landward thickness variation of unit b is caused by the structural thickening such as the underplating process structural features and reverse polarity suggest that the dsr as an upper interface of the underplated layer unit b corresponds to a shear plane or a fault zone with high pore fluid pressures we found that most of the dsr was observed in a region which has not suffered from the 1946 coseismic dislocation even though it was locked during the interseismic period the dsr as a shear plane or a fault zone constrains a regional variety of the coseismic dislocation around the updip limit of the nankai seismogenic zone considering all of the dsr characteristics we propose a hypothesis for the dsr s development and role the dsr may play an important role as a possible releaser of the shear stress energy stored at the plate boundary around the updip limit of the nankai seismogenic zone a good correlation between the theoretical vertical displacement and seafloor topographic feature support the possible slip along the dsr as a shear plane

age of the baula pge mineralization orissa india and its implications concerning the singhbhum archaean nucleus abstract two types of platinum group element pge mineralization are identified in the mafic facies of the baula complex intruding the gabbro anorthosite unit of the singhbhum archaean nucleus the first pt dominated is magmatic the pgm are included in the magmatic silicates of a gabbro norite intrusion the bangur gabbro cutting the complex s ultramafic facies and are found preferentially where the gabbro incorporates clasts of dunite and chromitite near the gabbro ultramafics contact the second pd dominated is hydrothermal the pgm are associated with hydrous silicates located in the gabbro matrix of a magmatic breccia in which the blocks are from the baula ultramafic facies the originating fluids for the second type of mineralization were exsolved from the magma and concentrated pge and volatile elements we see no evidence of remobilization or external fluid contamination a structural study has demonstrated that a the breccia was provoked by the intrusion of the upper part of the bangur gabbro and b the breccia matrix corresponds to the bangur gabbro consequently both types of pge mineralization are associated with the same magmatic event a zircon age of 3122 5 ma has been obtained for the main bangur gabbro intrusion and similar ages 3123 7 and 3119 6 ma have been obtained on the pge mineralized gabbro matrix of the breccia this supports the hypothesis of a genetic link between the bangur gabbro with its pt dominated pge and the breccia with its pd dominated pge giving a mean weighted age of 3121 3 ma for the mineralization sm nd data on eight gabbro samples of the baula complex yielded a similar age of 3205 280 ma the dating of these events places them close to the broad scale stabilization of the singhbhum archaean nucleus estimated at 3 1 ga and defines one of the oldest pge mineralizing events ever recorded the bangur gabbro shows many affinities with the gabbro anorthosite unit of the singhbhum archaean nucleus in terms of age of emplacement and isotopic composition it may be therefore that the bangur gabbro actually forms part of the gabbro anorthosite unit and that a fundamental control of its two types of pge mineralization is the presence of ultramafic clasts

integrated geological and geophysical studies for delineation of laterite covered chromiferous ultramafic bodies around bhuban southwestern part of sukinda ultramafic complex odisha abstract chromite and platinum group of elements pge mineralization in the eastern indian shield region is found in the ultramafic rocks distributed in the baula nuasahi and sukinda massifs intrusive into the iron ore group iog of rocks in addition to this recent geological studies have also reported the occurrences of chromiferous ultramafic bodies within the granulite tracts of eastern ghat group around asurabandha and maulabhanja areas in the southwestern part of sukinda massif in the present study detailed gravity and magnetic surveys 1510 stations were conducted over a laterite covered area of 0 16 km2 at bhuban asurabandha to locate the anomalous zones sources the geometry of the chromite bearing host rocks and its depth continuity in the subsurface four distinct coincident positive gravity anomalies up to 0 18 to 0 36 mgal and residual magnetic anomalies 200 to 350 nt with the ne sw strike indicate the presence of high density mafic ultramafic bodies under the laterite cover the results of 3 d euler deconvolution and two dimensional compact gravity inversion models in this region further suggest the presence of high density material 3100 3600 kg m3 at a depth of 20 40 m in the subsurface exploratory drill hole data in the area confirms the subsurface occurrence of thin chromiferous serpentinite as lensoidal and sandwiched bodies within the granulite rocks of eastern ghat group petrological studies of the drill core samples reveal the occurrences of disseminated and thin banded chromite within the serpentinites which is identical to the sukinda ultramafics in terms of its mineralogy and alteration on the other hand the surrounding mafic granulite rocks contain occasional magnetite and sulfides within the granulite rocks hence the chromiferous ultramafic bodies within the granulite rocks can be considered as the tectonically detached portions of the sukinda ultramafic rocks during the regional thrusting

deformation style of slickenlines on melange foliations and change in deformation mechanisms along subduction interface example from the cretaceous shimanto belt shikoku japan abstract accretionary complexes record the histories of changes in physical properties of sediments from unlithified sediments to lithified rocks through the deformation processes along subduction interface the trench sediment suffered various deformation of particulate flow pressure solution deformation and cataclastic faultings from ductile to brittle regime during accretion in subduction zone tectonic melange is a characteristic rock in on land accretionary complexes the dominant deformation mechanism of tectonic melange formation is pressure solution on the basis of microscopic observation however brittle slickenlines are also commonly observed on melange foliations at the outcrop scale although the slickenlines as a brittle failure is common on the surface of the pressure solution foliation the relationship of their kinetic are still uncertain detailed observations of slickenlines suggest that they are formed by reactivation of the melange foliations which indicates that the slickenlines are developed after formation of block in matrix texture characterized in melange in addition melange foliations are cut by faults related to underplating of oceanic materials therefore formation of slickenlines occur before underplating in a relatively deep portion along subduction interface on the basis of p t conditions reported from other parts of the cretaceous shimanto belt the melange formation and underplating is inferred to have occurred around the seismic front or within the seismogenic zone the change in deformation mechanisms from pressure solution to brittle failure may be the first change in physical properties from plastic to brittle around seismic front

noble metals in thetford mines ophiolites quebec canada part i distribution of gold iridium platinum and palladium in the ultramafic and gabbroic rocks the gold iridium palladium and platinum contents of ophiolites from the thetford mines area quebec were determined by radiochemical neutron activation analysis the main lithologies studied are the tectonite peridotites harzburgites of the lower structural unit and the ultramafic mafic plutonic members of the overlying cumulates average noble metal contents of the main rock types are au ppb ir ppb pt ppb pd ppb tectonite peridotitesharzburgite 1 5 3 2 10 0 3 8dunite lenses in harzburgite 0 27 3 6 2 1 0 29cumulate rocksolivine chromitite 0 28 30 0 1 5 0 33dunite 1 3 2 4 28 0 21 0pyroxenite 1 4 0 24 17 0 29 0gabbro 1 7 0 017 4 5 3 0the harzburgites have chondritelike noble metal abundance patterns and relatively uniform noble metal contents both features are compatible with an origin for the harzburgite as a residue from extensive partial melting of mantle peridotite the cumulates are highly variable in noble metal content iridium is enriched in early olivine chromitite and progressively depleted in later rocks palladium increases from early to late ultramafic cumulates and decreases in gabbros whereas pt shows no systematic trend in the cumulates gold increases very slightly from early to late cumulates the noble metal trends in cumulate rocks result mainly from fractional crystallization but other processes including sulfur saturation of magma and adcumulus growth may enrich rocks in noble metals sulfur rich late dunites and early pyroxenites probably have the best mineralization potential of the cumulate rocks there is little preferential partition of noble metals into any ferromagnesian silicate but the platinum group elements particularly ir are strongly associated with chromite acid leaching experiments suggest that in harzburgites ir is concentrated along chromite silicate grain boundaries whereas in cumulus chromitites ir occurs partly within chromite grains it is suggested that ir was originally incorporated into the chromite structure during a high temperature magmatic stage and in the case of the harzburgites has diffused to grain boundaries in response to cooling and to recrystallization induced in part by tectonic stress serpentinization probably occurred in both oceanic and continental environments cumulates suffered intense serpentinization but little noble metal mobility resulted harzburgites were serpentinized in both oceanic and continental environments and the latter process probably caused some mobilization of au in conjunction with formation of asbestos veins

characterization of the sukinda and nausahi ultramafic complexes orissa india by platinum group element geochemistry abstract samples of 20 chromitite 14 ultramafic and mafic rock and 9 laterite and soil samples from the precambrian sukinda and nausahi ultramafic complexes orissa india were analyzed for platinum group elements pge the maximum concentrations are palladium 13 parts per billion ppb platinum 120 ppb rhodium 21 ppb iridium 210 ppb and ruthenium 630 ppb comparison of chondrite normalized ratios of pge for the chromitite samples of lower proterozoic to archean age with similar data from paleozoic and mesozoic ophiolite complexes strongly implies that these complexes represent precambrian analogs of ophiolite complexes this finding is consistent with the geology and petrology of the indian complexes and suggests that plate tectonic and ocean basin developement models probably apply to some parts of precambrian shield areas

platinum group elements in rocks from the voikar syninsky ophiolite complex polar urals u s s r analyses of platinum group elements pge in rocks collected from the voikar syninsky ophiolite in the polar urals suggest that the distribution and geochemistry of pge in this paleozoic ophiolite are similar to those in mesozoic ophiolites from elsewhere chondrite normalized pge patterns for chromitite the tectonite unit and ultramafic and mafic cumulate unit have negative slopes these results are similar to those found for chromitites from other ophiolites stratiform chromities show positive slopes if the magmas that form both types of chromitite originate from similar mantle source material with respect to pge content the processes involved must be quite different however the distinct chondrite normalized pge patterns may reflect differing source materials

the late proterozoic ophiolite of sol hamed ne sudan abstract the sol hamed complex a sequence from ultramafics through gabbros thin sheeted dykes to pillow lavas is an ophiolite it was obducted tilted nearly to vertical eroded and covered unconformably by the nafirdeib series this is a volcaniclastic series dated at 712 58 ma the lower part of which includes conglomerates with ophiolitic clasts olistostromes one with large oolitic limestone slabs felsitic and dacitic tuffs turbidites and black shales with andesites above some of the structures in the ophiolite are attributed to sub oceanic deformation deformation after the deposition of the nafirdeib series produced folds and cleavage trending about ne sw and more locally a cleavage trending nw in a shear zone where the ophiolite was thrust northeastwards over the nafirdeib series the ophiolite and the nafirdeib series were intruded by the batholithic granite dated elsewhere at ca 669 ma the ophiolite is thought to occur on a suture with the nafirdeib series representing part of an island arc sequence