melt mantle interaction and melt evolution in the sartohay high al chromite deposits of the dalabute ophiolite nw china the sartohay block of the dalabute ophiolite consists chiefly of mantle harzburgite and lherzolite with minor dunite these rocks host voluminous chromite deposits with lenticular or vein like shapes the podiform chromitites are associated with and cross cut by numerous troctolite dykes chromite in the chromitites has al2o3 23 31 wt tio2 0 29 0 44 wt and cr2o3 contents 45 wt with cr s 100cr cr al 60 typical of high al chromite deposits the host peridotites in sartohay have been texturally and geochemically modified by magmas from which the high al chromitites and mafic dykes formed dunites commonly envelop the podiform chromite bodies and show transitional contacts with the peridotites some of the peridotites and chromitites contain plagioclase that crystallized from impregnated melts the dunite locally grades into troctolite with increasing plagioclase contents as a result of melt impregnation peridotites and dunites show variable ca and al contents and lree enrichment the parental magma of the chromitites was likely tholeiitic in composition derived from partial melting of the asthenospheric mantle in a rising diapir the interaction between this magma and pre existing lithospheric mantle composed of depleted lherzolite would have formed a more silicic tholeiitic magma from which high al chromitites crystallized during this interaction harzburgite and dunite were depleted in modal pyroxene and enriched in some incompatible elements such as al ca and lree due to melt impregnation

primary platinum mineralization in the nizhny tagil and kachkanar ultramafic complexes urals russia a genetic model for pge concentration in chromite rich zones the nizhny tagil uralian alaskan type ultramafic complex is composed of a dunite core with a rim of wehrlite and pyroxenite the dunite unit which has a uniform mineral composition olivine fo 89 8 92 1 avg 0 25 wt cao represents an accumulation of olivine and minor chromite from a mafic magma in an open system magma chamber after solidification of the dunite differentiation began with the formation of the wehrlite olivine fo 77 9 88 8 and pyroxenite corresponding to the closure of the magmatic system two main types of chromitite occurrence are observed in the dunite body small 100 5 cm scattered schlieren of chromite crystals within the ultramafic unit and much larger up to 100 5 m concentrations the chromite composition in both types is similar and uniform cr cr al fe 3 66 02 mg mg fe 2 49 05 and cr cr al fe 3 69 14 mg mg fe 2 49 81 respectively the larger concentrations of chromite include brecciated and net textured ore or massive veins that reflect the dynamic accumulation of chromite crystals in cavities along magma conduits inside the dunite body these cavities are analogous to small magma chambers within the consolidated part of the dunite body and similar to podiform chromitites in ophiolites were created by continuous feeding of magma to the system the platinum group element pge content of the chromite free facies of the dunite is relatively low pt being the only significant pge typically between 0 3 35 ppb but as much as 98 ppb in one sample in contrast both types of chromite occurrence are enriched in pge two pge mineralizing episodes are distinguished an ir ru rh os rich pt pd poor episode in which the platinum group minerals pgm are mainly ir alloys and a pt pd rich episode in which the pgm are mainly pt alloys i e pt 2 fe ni cu ptfeni 2 and tetraferroplatinum and tulameenite like compositions along with rare hollingworthite and an unidentified rhnias mineral the primary pgm have been affected locally by a serpentinization related low temperature alteration that is characterized by addition of copper with alteration to complex pt fe ni cu alloys of varied stoichiometry including tulameenite and platinian copper placer concentrations derived from the nizhny tagil complex have a slightly different assemblage of pgm with abundant isoferroplatinum the presence of gold commonly aggregating the pgm and unusual au sn and au pb alloys the kachkanar complex situated 120 km north of the nizhny tagil complex has essentially the same characteristics apart from a slightly different composition of the ultramafic cumulates suggesting that they were derived from a magma of different composition the pgm distribution also differs in that it contains more abundant isoferroplatinum than other pgm the high pt concentrations are the result of the affinity of platinum minerals for chromite and the mode of chromite deposit formation in the cavities in which chromite crystallized pgm tended to be incorporated in chromite or remained attached to chromite crystals to explain the very high concentrations of chromite associated pgm we define a cr factor which is the ratio of the mass of silicate magma to the mass of chromite in contact with the ascending magma where the cr is high i e a relatively low proportion of chromite in contact with a large amount of magma crystallizing the pt minerals the efficiency of the mechanical collection of pt minerals is at a maximum and the pt concentration i e amount of pt minerals will be high possible variations due to the collection of pge by a sulfide liquid have not been established a model is proposed whereby the observed chromite and associated pt mineralization can result from the normal magmatic evolution of the complex provided that a structural hydrodynamic trap is available to concentrate the chromite

boninitic and tholeiitic dykes in the lewis hills mantle section of the bay of islands ophiolite implications for magmatism adjacent to a fracture zone in a back arc spreading environment a detailed integrated field petrographic and geochemical study of the springers hill area of the bay of islands ophiolite exposed in the lewis hills was undertaken to explain the anomalously high abundance of veins and dykes of chromitite orthopyroxenite and clinopyroxenite and their associated dunites hosted by a refractory harzburgite dunite mixture a geodynamic situation is presented which is constrained by previous studies requiring formation of the springers hill mantle section at a ridge fracture zone intersection and the whole of the bay of islands ophiolite within a back arc spreading environment the veins and dykes formed during magmatism at the ridge fracture zone intersection and along the fracture zone as progressively hotter more fertile richer in clinopyroxene asthenosphere ascended and was channelled up and along the fracture zone wall shallow melting of refractory harzburgite in the presence of subduction derived hydrous fluids produced light rare earth element lree enric

origin of podiform chromitite a new model based on the luobusa ophiolite tibet abstract podiform chromitites have been interpreted as the result of melt rock reaction and related melt mixing in upper mantle sections of ophiolites however the discovery of ultrahigh pressure uhp minerals especially diamond and coesite in many podiform chromitites and host peridotites raises fundamental questions about the validity of this model chromitites in the luobusa ophiolite of tibet range from massive to nodular to disseminated chromite grains in both the chromitites and peridotites have variable but relatively high mgo and are classified as magnesiochromite many magnesiochromite grains in the massive chromitites contain inclusions of forsterite and pyroxene as well as diamonds and other unusual minerals forsterite inclusions have fo numbers of 97 99 and nio contents of 1 11 1 29 wt mg s 100 mg mg fe of clinopyroxene inclusions are 96 98 and those of orthopyroxene are 96 97 x ray studies show that the olivine inclusions have very small unit cells and short cation oxygen bond distances suggesting crystallization at high pressure in contrast magnesiochromite grains in nodular and disseminated chromitites lack pyroxene inclusions and their olivine inclusions have lower fo numbers of 94 96 and lower nio contents of 0 35 0 58 wt in addition magnesiochromite in massive ores has higher fe3 fetotal 0 42 than that in nodular and disseminated ores which have ratios of 0 22 disseminated chromitites also show systematic changes in olivine and magnesiochromite compositions from the dunite envelope to the massive ore indicating melt rock reaction these observations suggest that the formation of podiform chromitites is a multi stage process magnesiochromite grains and perhaps small bodies of chromitite crystallize deep in the mantle under low ambient o2 from partial melts of peridotite uhp minerals and highly magnesian olivine and pyroxene inclusions are trapped in these magnesiochromite grains when oceanic crustal slabs are trapped in suprasubduction zones ssz they are modified by island arc tholeiitic and boninitic magmas which change the magnesiochromite compositions and deposit chromitite ores in melt channels

value of mineral deposits associated with mafic and ultramafic magmatism implications for exploration strategies abstract magmatic ore deposits associated with ultramafic and mafic magmatism modum include nickel sulfide deposits that are locally enriched in copper cobalt platinum group elements and gold a diverse range in platinum group element deposit types oxide and silicate related nickel cobalt laterite deposits formed by weathering of ultramafic bodies and massive and stratiform accumulations of chromite ilmenite and vanadium bearing magnetite magmatic ore deposits associated with ultramafic and mafic rocks span most of earth s history and are well represented on all continents and in all climatic zones and physiographic regions currently these deposits are estimated to account for approximately 7 of the total value of annual global metal and mineral mining they include the world s greatest concentration of metals the bushveld complex which has an estimated total metal endowment value representing past production and current reserves and resources of us 3 6 trillion recent trends in exploration spending for mafic to ultramafic magmatic ore deposits compared to commodity market size and production value indicate a significant under investment in exploration for magmatic chromite ilmenite vanadium and polymetallic disseminated copper nickel platinum group element sulfide deposits historically exploration of mafic to ultramafic magmatic belts has commonly involved a narrow range of deposit models geological environments and exploration methods a value based and more holistic approach to exploration for mafic to ultramafic magmatic ore deposits is proposed that captures a larger range of known mineralization styles settings and commodities a focus on high value deposits providing excellent operating margins and long mine life is recommended as a starting point for new programs however by recognizing the full range of possible mineralization styles and understanding mineral economic thresholds future explorers can expand the search area and methodology and increase the probability of discoveries that create value assuming that commodity prices continue to rise with declining average grades current global resources hosted by mafic to ultramafic magmatic ore deposits are adequate to meet global demand for nickel platinum group elements pge chromium and vanadium for many decades to come despite this the high net smelter return nsr value of many types of modum deposits provides ongoing economic motivation for both greenfields and brownfields exploration in these strategically important deposits furthermore the large range in producer costs for key modum commodities such as nickel copper and pge provides significant opportunities for new projects to displace existing higher cost operations on the industry cost curve the known clustering of high value deposits within discrete magmatic belts e g nickel sulfide and nickel laterite deposits is an additional motivation for exploration investment as it allows exploration to focus early in areas of higher mineral potential additional incentives for modum exploration include the cumulative size and maturity of modum commodities that reduce investment risk and thereby appeal to junior explorers mid cap explorers and developers and major multi national mining companies the economies of scales that were successfully applied to mining of large porphyry copper deposits for many decades could have a similar positive impact on a large number of advanced modum projects especially polymetallic sulfide deposits capable of generating high value concentrates having significant base metal and precious metal contents

controls on platinum group elemental distributions of podiform chromitites a case study of high cr and high al chromitites from chinese orogenic belts abstract a study of podiform chromite deposits from the asiatic orogenic belt and the qilian qiangling kunlun himalaya tectonic domain provides new insights into the geochemistry of the pges in podiform chromite deposits and the genesis of the deposits themselves the bulk of deposits which occur in mantle peridotites of ophiolites have typical ophiolitic pge patterns that are depleted in pt and pd relative to the average upper mantle and have negatively sloping distributions on mantle normalized diagrams type i high cr chromitites have higher os ir ru and rh contents than type ii high al chromitites although both have similar pd and pt most of the type i and ii chromite deposits have lower pd and pt contents than the upper mantle peridotites in which they occur podiform chromitites are essentially products of melt rock interaction in the upper mantle their cr and pges were contributed by not only the invading magmas but also by the upper mantle host the chromite deposits are in part metasomatic replacement bodies the type i high cr chromitite pge patterns were produced by interaction between s undersaturated boninitic magmas and depleted harzburgites whereas the type ii high al chromitite pge patterns were formed by interaction between initially s saturated tholeiitic magmas and depleted harzburgites the low to very low pd and pt contents of both type i and type ii chromitites require that the mantle assemblage in which the chromite deposits were formed had lost their sulfides and hence pd and pt prior to formation of the chromite deposits in addition no or little pd and pt were deposited by the invading magma which either remained s undersaturated boninite or became morb s undersaturated due to interaction with the s depleted harzburgitic mantle it is suggested that the very low ir os and ru contents of boninites in general might be due to loss of ir during the formation of podiform chromitites it is suggested that podiform chromitites with type i pge patterns were formed in an island arc environment whereas those with type ii pge patterns were formed in a back arc setting

evidence for seafloor spreading in the laxmi basin northeastern arabian sea abstract marine magnetic anomaly data from the laxmi basin for the first time reveal the presence of fairly correlatable nnw trending magnetic lineations these magnetic lineations are symmetric about a central negative magnetic anomaly and the axis of symmetry coincides with a characteristic short wavelength free air gravity low the anomalies are interpreted as representing a two limbed seafloor spreading sequence which can be equated to the a28 a33 interval of the geomagnetic polarity reversal timescale these results suggest that the laxmi basin is underlain by an oceanic crust evidence of seafloor spreading in this basin possibly implies a pre a27 spreading episode in the evolutionary history of the arabian sea

multiple events in the neo tethyan oceanic upper mantle evidence from ru os ir alloys in the luobusa and dongqiao ophiolitic podiform chromitites tibet in situ analysis of major elements platinum group elements and os isotope composition data for 170 grains of ru os ir alloys from podiform chromitites in the luobusa and dongqiao ophiolites tibet provide new constraints on the sources of the alloy grains the os isotope composition of the neo tethyan oceanic upper mantle and the timing of melt rock reactions in the ultramafic peridotites of the host ophiolites most alloy grains are osmiridium and iridosmine based on elemental and isotopic compositions two populations can be identified group i found in both ophiolite bodies has fractionated pge patterns typical of alloys and homogeneous 187 os 188 os 0 12645 4 2s n 145 group ii alloys from the dongqiao ophiolites show smooth pge patterns and 187 os 188 os ranges from 0 12003 to 0 12194 yielding re depletion ages 1 1 ga both the pge patterns and the os isotopic compositions of group ii resemble those of sulfides residual after melting group i alloys represent an early cumulate phase during the crystallization of magmas and are interpreted as derived from an asthenospheric melt in contrast group ii alloys are inferred to derive from the breakdown of residual sulfides from the mantle section of the ophiolite which may represent relics of a sub continental lithospheric mantle the coexistence of the two groups of alloys in dongqiao samples reflects interaction between asthenosphere derived melts and old depleted lithospheric mantle and suggests that the chromitites originated through melt rock reactions the data on the group i alloys support the hypothesis that the re os evolution of the convective oceanic mantle resembles that of enstatite chondrites 2007 elsevier b v all rights reserved

refractory and metallurgical type chromite ores zambales ophiolite luzon philippines the zambales ophiolite is the major source of chromite ore in the philippines the chromitites are concordant cumulates and are associated with distinct chromitite bearing sequences within the mantle peridotites refractory and metallurgical chromite deposits are spatially separated and related to different lithologic associations which crystallized from different parental magmas refractory chromite ores 30 44 wt cr2o3 20 30 wt al2o3 are linked with the peridotite troctolite olivine gabbro lineage two main types were found 1 al rich refractory ores associated with harzburgites and feldspathic periodotites and 2 more cr rich varieties associated with lherzolites metallurgical chromite ores 45 53 wt cr2o3 12 18 wt al2o3 are linked with the peridotite pyroxenite norite lineage two main types were also found 1 cr rich metallurgical ores associated with orthopyroxenites and 2 more al rich varieties related to clinopyroxenites the chemical composition of chromite within the deposits varies depending on the chromite silicate ratios of the ore types and grades continuously into accessory chrome spinels in the wall rock peridotites the geochemistry of accessory chrome spinels in various peridotites and mafic cumulates depends on the mineralogical composition and the stratigraphic position of their host rocks

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

structures in podiform chromite from the maqsad district sumail ophiolite oman about 15 chromite bodies have been recognized in the maqsad area of the oman ophiolite the occurrence in this area of three chromite bodies within the cumulate sequence must be integrated into the classification of cassard et al 1981 which presently explains only those pods lying in the uppermost mantle sequence plastically deformed harzburgites and dunites the occurrence of chromite bodies within the cumulates and the abundance of chromite in the maqsad area are related to the exceptional magmatic activity and the unusual plastic flow pattern particular to this area it was probably a feeding zone along the oceanic spreading center sitting on top of a mantle diapir

iron and magnesium isotopic constraints on the origin of chemical heterogeneity in podiform chromitite from the luobusa ophiolite tibet we present high precision measurements of iron fe and magnesium mg isotopic compositions of olivine orthopyroxene and chromite separates from harzburgites dunites and chromitites in the mantle section of the luobusa ophiolite southern tibet to investigate the origins of podiform chromitite two harzburgites in the zedong ophiolite southern tibet are also reported for comparison the olivine and orthopyroxene in the luobusa and zedong harzburgites have similar fe and mg isotopic compositions with fe 56 values ranging from 0 to 0 083 in olivine from 0 034 to 0 081 in orthopyroxene and mg 26 values ranging from 0 25 parts per thousand to 0 20 parts per thousand in olivine from 0 29 parts per thousand to 0 26 parts per thousand in orthopyroxene respectively the olivines of two dunites from the luobusa display small fe and mg isotopic variations with fe 56 values of 0 014 parts per thousand and 0 116 parts per thousand and mg 26 values of 0 21 parts per thousand and 0 29 parts per thousand all chromites in the luobusa chromitites have lighter fe isotopic compositions than the coexisting olivines with fe 56 values ranging from 0 247 parts per thousand to 0 043 parts per thousand in chromite and from 0 146 parts per thousand to 0 215 parts per thousand in olivine fe 56 chr ol 0 294 to 0 101 parts per thousand the chromite mg 26 values span a significant range from 0 41 parts per thousand to 0 14 parts per thousand large disequilibrium fe and mg isotope fractionation between chromite and olivine as well as positive correlation of chromite fe 56 values with their mgo contents could be attributed to fe mg exchange between chromite and olivine in the disseminated chromitites the higher modal abundances of olivine than chromite would result in a more extensive fe mg exchange whereas chromite in the massive chromitite where olivine is rare could not be affected by this process

near n s paleo extension in the western deccan region india does it link strike slip tectonics with india seychelles rifting this is the first detailed report and analyses of deformation from the w part of the deccan large igneous province dlip maharashtra india this deformation related to the india seychelles rifting during late cretaceous early paleocene was studied and the paleostress tensors were deduced near n s trending shear zones lineaments and faults were already reported without significant detail an e w extension was envisaged by the previous workers to explain the india seychelles rift at 64 ma the direction of extension however does not match with their n s brittle shear zones and also those faults sub vertical ne sw nw se and few n s we report and emphasize in this work slickenside bearing fault planes brittle shear zones and extension fractures in meso scale enabled us to estimate the paleostress tensors directions and relative magnitudes the field study was complemented by remote sensing lineament analyses to map dykes and shear zones dykes emplaced along pre existing n s to ne sw nw se shears fractures this information was used to derive regional paleostress trends a nw se ne sw minimum compressive stress in the oldest kalsubai subgroup and a n s direction for the younger lonavala wai and salsette subgroups were deciphered thus a nw ne to n s extension is put forward that refutes the popular view of e w india seychelles extension paleostress analyses indicate that this is an oblique rifted margin field criteria suggest only ne sw and nw se with some n s strike slip faults brittle shear zones we refer this deformation zone as the western deccan strike slip zone wdsz the observed deformation was matched with offshore tectonics deciphered mainly from faults interpreted on seismic profiles and from magnetic seafloor spreading anomalies these geophysical findings too indicate oblique rifting in this part of the w indian passive margin we argue that the seychelles microcontinent separated from india only after much of the dlip erupted further studies of magma rich passive margins with respect to timing and architecture of deformation and emplacement of volcanics are required

structural history of continental margin sediments beneath the bay of islands ophiolite newfoundland deformed continental margin sediments of the curling group underlie the bay of islands ophiolite in the humber arm allochthon of western newfoundland within the allochthon tectonic slices of sediments are separated by zones of melange the earliest structures in the slices are synsedimentary features produced by soft sediment deformation on the continental slope or rise later west facing asymmetrical f 1 folds without penetrative axial plane cleavage were probably produced during the emplacement of the allochthon in the middle ordovician taconic orogeny associated extensional structures include shear fracture and extension fracture boudins pervasive cataclastic shearing of shale and boudinage of competent beds produced melange between slices shear surfaces and the original bed parallel fissility of the shale both contribute to the anastomosing fabric of the melange matrix a subsequent folding event of probable acadian age produced upright to moderately inclined f 2 folds with axial plane cleavage increasing in intensity eastwards across the allochthon this event refolded the folds thrust slices and melange zones produced during emplacement later gentle cross folds associated with sporadic development of crenulation cleavage produced culminations and depressions on the f 2 fold hinges

deformation fabric and microstructures in ophiolitic chromitites and host ultramafics sultanate of oman harzburgite and dunite from the oman ophiolite display a strong olivine fabric and microstructures typical for high temperature mantle deformation the syntectonic magmatically formed chromitites occurring throughout the mantle sequence vary from almost undeformed resembling chromitite from stratiform complexes to highly deformed usually the interstitial silicates in the deformed ore possess a fabric similar to those of the host rocks whereas a weak or poikilitic fabric is observed on relatively undeformed deposits chromite grains are elongated and flattened defining strong foliations and lineations in disseminated ore and pull apart like structures perpendicular to the lineation form in the coarser massive ore no preferred crystallographic orientation of chromite has been detected with the x ray texture goniometer and it is proposed that chromite mainly deforms by passive rotation in the weaker silicate matrix cataclasis and diffusion creep

characteristics and petrogenesis of the pale microgranular masses in ophiolite type chromitites in northern xinjiang abstract massive pale microgranular masses pmms with mafic to ultramafic compositions have been discovered in representative chromite deposits i e the sartohay jingyu tangbale saleinuohai hongguleleng and qingshui deposits and related ophiolite belts based on a detailed study of the occurrence petrology alteration minerals and whole rock geochemistry these pmms can be subdivided into six categories olivine rich vitrobasaltic rocks gabbro diabasic rocks ultramafic rocks tuffaceous pyroclastic rocks skarns rodingites without protolith structure and metasomatites the wall rocks of the pmms are mainly harzburgites subordinate chromitites and lherzolites according to comprehensive analyses of the occurrence wall rock types metasomatic relationships and fabrics the pmms are considered to be xenoliths in ultramafic magma the geochemical characteristics of the associated alteration minerals indicate that altered augite endiopsides and regenerated diopsides grossularite andradites and chrysotiles are dominant in the pmms and the chlorites in the pmms can be divided into four categories brunsvigite pycnochlorites diabantites sheridanite chlorophaeite pycnochlorites and clinochlore penninites the pmms have been mostly or completely subjected to rodingitization and exhibit ca rich and si undersaturated features in terms of major element compositions these characteristics indicate that the pmms were formed by metasomatism during the capture process by peridotites with an import of mafic components dominantly magnesium and an export of felsic components accompanied by serpentinization and chloritization the trace element characteristics of the pmms are obviously influenced by the wall rock type pmms in contact with peridotites generally have higher rare earth element ree and trace element contents than the primitive mantle are characterized by slightly left sloping chondrite normalized ree patterns with weakly positive to negative eu anomalies and exhibit various degrees of sr and k depletions pmms in contact with chromitites have lower ree and trace element contents than the primitive mantle distinctly left sloping chondrite normalized ree patterns with obviously positive eu anomalies and sr and k enrichments the eu sr and k anomalies show an obvious complementary relationship between the peridotites and the captured pmms and metasomatites which is most likely indicative of ca metasomatism by the ultramafic magma the breakdown of clinopyroxene in peridotites liberated ca which accumulated and infused into the metasomatites consequently the results of this study suggest that no less than two different genetic mechanisms might account for the generation of pmms the dominant characteristics of the fabric resulted from the processeses of xenolith formation and metasomatism by ultramafic magma in the early stage whereas the characteristics of the mineral assemblages and geochemical components resulted from rodingitization in the later stage

synthesis of palaeozoic deformational events and terrane accretion in the canadian appalachians plate tectonic theory predicts that most deformation is associated with subduction and terrane accretion with some deformation associated with transform transcurrent movements deformation associated with subduction varies between two end members 1 where the tectonic regime is dominated by subduction of oceanic lithosphere containing small terranes a narrow surface zone of accretionary deformation along the subduction zone starts diachronously on the subducting plate at the trench as material is transferred from the subducting plate to the over riding plate and 2 where continent continent collision is occurring a wide surface zone of accretionary deformation starts synchronously or with limited diachronism palaeozoic deformational events in the canadian appalachians correspond to narrow diachronous events in the ordovician and silurian whereas devonian carboniferous and permian deformational events are widespread and broadly synchronous along the western side of the canadian appalachians the taconian deformational event starts diachronously throughout the ordovician and corresponds to the north north west accretion of the notre dame ascot weedon st victor and various ophiolitic massifs volcanic arc and peri arc terranes over cratonic north america within the eastern half of the central mobile belt the late cambrian early ordovician penobscotian deformational event corresponds to the south easterly accretion of the exploits subzone various volcanic are and peri arc terranes over the gander zone continental rise in the centre of the orogen the late ordovician silurian beothukan deformational event corresponds to the south easterly accretion of the notre dame over the exploits gander subzones along the south eastern side of the central mobile belt the silurian ganderian deformational event corresponds to the north north east sinistral transcurrent accretion of the avalon composite terrane microcontinent over the gander exploits zones along the south eastern half of the orogen the late silurian middle devonian acadian deformation event corresponds to the westerly accretion of the meguma terrane intradeep or continental rise over the avalon composite terrane affecting the entire orogen the late devonian carboniferous and permian acadian alleghanian deformational events correspond to the east west convergence between laurentia and gondwana continent continent collision

solubilities of bone mineral from archaeological sites the recrystallization window abstract bone mineral solubility is an important parameter for understanding the preservation of bones in the archaeological and palaeontological records in this study we have measured the solubility of the carbonated hydroxyl apatite of sub recent and fossil bones as well as synthetic hydroxyl apatite in deionized water and in ph buffered solutions the solutions were open to the atmosphere and the ph values were around neutral measurement conditions that are relevant to bone mineral preservation in nature but that were absent from most previous studies we obtained internally consistent results from both the water and the buffered experiments supporting the notion that we are measuring an inherent property of the mineral phase we found that bone mineral is much more soluble than synthetic hydroxyl apatite we measured the ionic activity products at steady state conditions and we identify a recrystallization window between ph 7 6 and 8 1 which defines the conditions under which bone crystals dissolve and reprecipitate as a more insoluble form of carbonated hydroxyl apatite as these conditions are common in nature most fossil bones will not maintain their original crystals with time we also found that bones that contained small amounts of calcite did not dissolve at all during our experiments these results provide a basis for better understanding the conditions in sediments under which bones are preserved and the relative states of preservation of bone they also have important implications for the selection of the most appropriate bone samples for paleoenvironmental and paleodiet analyses and dating

artifact preservation and post depositional site formation processes in an urban setting a geoarchaeological study of a 19th century neighborhood in detroit michigan usa a geoarchaeological study was carried out to assess levels of artifact deterioration occurring in a historic period urban soil during the 20th century the study site is a former house lot in a park created in 1919 by demolition of a residential community in detroit michigan usa the results show that despite nearly a century of burial in an urban soil impacted heavily by pollution and other anthropogenic activity many 19th century artifacts are remarkably well preserved the observed weathering stability sequence of glass bone mortar plaster paint is consistent with decreasing solubility product values of the corresponding principal mineral constituent glass apatite portlandite gypsum cerrusite even severely weathered 19th century nails and mortar can often be distinguished using optical petrographic and sem edax methods the excellent state of artifact preservation is attributed to a calcareous soil microenvironment and artificial compaction which limited the weathering effects of water and oxygen artifact preservation was further enhanced by burial beneath a thick biomantle created by the casting activity of an invasive species of earthworm however lumbricus terrestris may now pose the greatest threat to artifact preservation because casting and burrowing activities are decreasing bulk density and promoting the diffusion of air and water into the soil early excavation is recommended to recover artifacts in soils impacted by the combined effects of urban pollution and earthworm burrowing anthropogenic microparticles smaller than those normally classified as microartifacts were found to be useful indicators of human occupation

petrological characteristics of the hantaishir ophiolite complex altai region mongolia coexistence of podiform chromitite and boninite abstract petrological and geochemical examination shows that the hantaishir ophiolite complex of the altai region in mongolia contains two large ultramafic massifs the taishir and naran massifs a sheeted dyke complex and a serpentine melange zone containing blocks of chromitite and peridotites the ultramafic massifs consist mainly of harzburgite and dunite with a small amount of podiform chromitite cr cr al atomic ratios of chromian spinels are high in the peridotites and sheeted dykes 0 7 and in the chromitites 0 8 fo contents of olivines in the peridotites span a relatively narrow range 91 8 93 0 most of the sheeted dykes are boninite type high mg andesites the coexistence of podiform chromitite and boninite in the hantaishr ophiolite complex suggests that the ophiolite formed in an intra oceanic subduction zone from a depleted mantle source

formation of chromitites and ferrogabbros in ultramafic and mafic members of the variscan śleza ophiolite sw poland abstract ultramafic and mafic rocks of the variscan leza ophiolite bohemian massif sudetes sw poland contain abundant cr and fe ti mineralizations the occurrence of chromitites is related with serpentinites of refractory harzburgitic protoliths chromite i from chromitites has variable cr and mg numbers cr 0 41 0 58 mg 0 62 0 83 and low tio 2 contents 2 al 2 o 3 and v 2 o 3 contents 0 19 9 82 wt 0 13 1 67 wt and 1 08 1 61 wt respectively it forms xenomorphic grains with trellis or sandwich exsolutions of ilmenite as well as aggregates with individual ilmenite grains the moderate cr of chromite i points to an its origin from a morb type melt the occurrence of hydrous inclusions and nodular textures indicates chromitite formation and alteration under low pressure conditions primary sulfide phlogopite and pargasite inclusions suggest that the parental melt was moderately hydrous and sulphur rich water in the magmatic system originated supposedly from interaction of primary morb melt with hydrothermal fluids and a significant influence of fluids on chromite composition is expressed by its very low ti content metagabbros containing abundant fe ti oxide mineralization may be classified as ferrogabbros high v and ti contents in magnetite indicate a magmatic origin its xenomorphic and intragrain mode of occurrence suggest crystallization between pre existing gabbroic minerals like pyroxene and plagioclase ilmenite exsolutions indicate its growth at the expense of magnetite under oxidizing conditions occurrence of chromitites in harzburgites and ferrogabbros related to isotropic gabbros may suggest various spreading conditions during formation of the mafic and ultramafic members of the leza ophiolite

proposal to add anthrostratigraphic and technostratigraphic units to the stratigraphic code for classification of anthropogenic holocene deposits anthropogenic surficial deposits are formed from holocene and older deposits that have been so modified by human activity that their original form is unrecognizable because their characteristics are fundamentally different from those of natural origin the creation of special stratigraphic categories seems justified two new material categories are proposed for the stratigraphic classification of anthropogenic deposits an anthrostratigraphic unit asu is defined as a stratiform or irregularly shaped body of anthropogenic origin in the sedimentary record distinguished and delineated on the basis of lithologic characteristics and or bounding disconformities an anthropogenic origin is indicated if the deposit 1 lies beneath an anthropogenic landform 2 contains artifacts 3 shows evidence of artificial mixing or other human disturbance and or 4 was imported from offsite and is allochthonous the basic asu is the anthrostratum defined as a mostly stratiform body of artificially mixed earth rock

composition micromorphology and distribution of microartifacts in anthropogenic soils detroit michigan usa abstract the composition micromorphology and distribution of microartifacts in anthropogenic soils were studied as part of a project that was evaluating the utility of geophysical surveying techniques for mapping vacant urban land petrographic criteria for the identification and classification of microartifacts mas were generated using a set of 25 different types of reference artifacts of known origin and then tested on a suite of 20 urban topsoils from sites in detroit michigan representing three different land use types residential demolition undemolished residential and industrial petrographic and x ray diffraction analyses showed that reference mas may be classified into five basic compositional types carbonaceous calcareous siliceous ferruginous and miscellaneous reference mas were generally distinguishable using optical microscopy by color luster fracture and microtexture and further differentiable using scanning electron microscopy and energy dispersive x ray spectroscopy mas were found in all of the anthropogenic soils studied but in highly variable proportions 80 coal related wastes were the most common types of mas and included unspent coal ash microspheres microagglomerate cinders and burnt shale probably representing a legacy from the coal burning era 1850 1936 ad these were associated with mas derived from waste building materials brick mortar glass and manufacturing wastes iron making slag coked coal in demolition and industrial site soils respectively urban soils impacted by airborne deposition of fly ash were widespread including conspicuous black 10yr2 1 topsoils at undemolished residential sites located near railroads and areas of heavy industrial activity coal combustion products and iron smelting slag had distinctive compositions that included magnetite bearing aluminosilicate glass these results support our hypothesis that there are systematic relationships between soil geophysical properties type and abundance of microartifacts and land use history hence it seems likely that magnetic susceptibility surveying and other geophysical methods will facilitate the mapping of soils in urbanized areas

formation of podiform chromitites by melt rock interaction in the upper mantle high cr and high al podiform chromitites are derived from melts formed by high and low degrees of partial melting in the upper mantle respectively the close association of the chromitites regardless of compositions with dunites and harzburgites can be explained by interaction between melts and harzburgites or depleted lherzolites in the upper mantle continuous reaction between melt and wallrock modifies the compositions of the magmas towards the precipitation of chromite alone under upper mantle conditions the modified magmas would have higher silica and lower magnesia and would be extracted into the crust or erupted leaving chromite pods in situ with dunite rinds that grade to harzburgites or lherzolites

podiform chromitite in the arc mantle chromitite xenoliths from the takashima alkali basalt southwest japan arc chromitite xenoliths from the takashima alkali basalt in the southwest japan arc are classified into two types type 1 chromitite in thin layers in dunite or wehrlite xenoliths and type 2 chromitite in discrete xenoliths which has an orbicular texture previously documented only from podiform chromitites in ophiolites type 1 may be equivalent to layered chromitites in ophiolitic cumulates and type 2 to podiform chromitites in the transition zone of ophiolites this example of podiform chromitite from the southwest japan arc suggest that these podiform chromitites may exist in the upper mantle beneath an arc where their formation is favored

the emplacement of peridotites and associated oceanic rocks from the lizard complex southwest england upper mantle peridotites and associated oceanic rocks from the lizard complex south west england preserve evidence for a multistage geological history steeply dipping pre emplacement fabrics record high temperature 900 1100 c shearing and exhumation of the mantle peridotites apparently formed during localized ne sw rifting in a pull apart basin setting c 400 390 ma associated oceanic rocks landewednack amphibolites preserve a pre emplacement prograde brown amphibole bearing metamorphic assemblage and steeply dipping fabric thought to have formed as the newly formed oceanic crust was juxtaposed with newly exhumed hot mantle peridotite during ne sw rifting in both the peridotites and landewednack amphibolites steep pre emplacement structures are cross cut by low angle mylonitic fabrics thought to have formed during the initial phases of emplacement of mantle over crustal rocks in a partially intra oceanic setting c 390 375 ma the fabrics in peridotites and amphibolites exhibit retrograde mineral assemblages c 500 800 c with the amphibolites preserving two superimposed assemblages green amphibole titanite and colourless magnesio hornblende respectively that are thought to record progressive down temperature deforma tion during thrusting emplacement related structures in both the basal peridotites and amphibolites consistently dip at low to moderate angles nw with down dip lineations and kinematic indicators showing consistent top to the nw senses of shear syn emplacement magmatism is recorded by intrusions of foliated kennack gneiss anastomosing serpentine filled faults mark many existing low angle contacts between the peridotites and landewednack amphibolites and appear to represent the final lowest temperature 250 c stages of emplacement c 370 ma this study shows that dynamothermal aureoles in ophiolites may preserve evidence for tectonothermal events that pre date thrust emplacement

a comparison of mineralogical techniques and potassium adsorption isotherm analysis for relative dating and correlation of late quaternary soil chronosequences abstract the utility of potassium adsorption isotherm analysis kia for relative dating and correlation of soil chronosequences was assessed by a comparison with some widely used mineralogical techniques the soils studied near detroit michigan are brown 10yr calcareous entisols developed in holocene 4 ka floodplain sediments and inceptisols and alfisols with redder 7 5yr 5yr cambic and argillic b horizons formed on late pleistocene 14 5 12 4 ka fluvial terraces fluvial paleochannels and paleoshorelines beach ridges of proglacial lakes the soils show a positive linear correlation between soil age and clay content depth of carbonate leaching and fe oxide accumulation in contrast the duration of soil formation is too short for variations in heavy mineral frequencies to discriminate soil age reliably scanning electron microscopy shows that chemical weathering stage is indicated by progressive etching and pitting of calcite apatite and amphibole with increasing soil age k isotherms are linear or slightly curvilinear indicating that ion exchange is dominant over specific adsorption a statistically significant linear correlation between k distribution constant and soil age attributed to progressive weathering of mica suggests that kia is useful for relative dating of genetically related soils once a well dated chronosequence is established kia produces results comparable to or better than mineralogical techniques however soil morphology and a clay redness index are more effective than mineralogical and chemical techniques for interchronosequence correlation

systematics of chromitite occurrences in central palawan philippines the chromitite occurrences in the central palawan ophiolite can be classified into four groups based on their chemistry and geological relationships group i chromites from a deep tectonite level of the ophiolite complex show cr cr al values of between 0 78 and 0 90 whereas group ii from a shallower level gives values of 0 64 0 78 and group iii from the immediate gabbro lower contact dunite cumulates and diapirs show values of 0 5 0 64 group iv chromites from the gabbro zone have ratios around 0 38 0 5 larger orebodies type a representing a first stage of partial melting of a primordial mantle show cr fe ratios between 2 5 and 4 5 whilst those of small chromitite bodies with a thin dunite envelope type b lie below 2 5 these small bodies belong to a second stage of magma generation for both types the cr fe ratio increases with the chromite silicate ratio within individual occurrences an additional method which may help to subdivide these chromitites involves their silicate and ore inclusions the platinum group minerals for example occur mainly in chromites of group ii type a the intensive tertiary tectonic activity to which the ophiolite was subjected during and after emplacement disturbs the regular pattern of the ophiolite stratigraphy and its chromitite occurrences

compositions of chromite associated minerals and parental magmas of podiform chromite deposits the role of slab contamination of asthenospheric melts in suprasubduction zone environments abstract podiform chromitites in the mantle sections of ophiolites belong to either high cr metallurgical or high al refractory varieties their highly variable compositions are reflected by different cr s 100cr cr al and cr2o3 and al2o3 contents of the chromite falling in the boninitic and morb fields respectively parental magmas of high cr chromitites have higher sc mn co and ni and lower ti v zn and ga concentrations than morb melts their trace element patterns are similar to those of boninites except for ni and zn in contrast high al chromitites have parental magmas characterized by generally flat morb normalized patterns showing slight enrichments in v mn and co and depletion in ni and zn regardless of their compositions both types of chromitites have chondrite normalized platinum group element pge patterns showing enrichment in ipge and depletion in ppge a variety of platinum group minerals are typically present in both types occurring either as euhedral inclusions or along fractures in chromite grains these minerals have a wide span of re os isotopic compositions reflecting a variety of origins there is a diversity of unusual minerals and mineral inclusions associated with podiform chromitites the presence of these minerals suggest that grains of amphibolite plagioclase amphibole and zircon and eclogite coesite kyanite and garnet were present in the magmas from which chromite crystallized multiphase mineral inclusions demonstrate that podiform chromitites form from hydrous mafic magmas in suprasubduction zone environments ssz we propose a new model in which chromitite formation was involved in intra oceanic subduction zones initiated in closing oceanic basins continued subduction carries oceanic and possibly continental crustal materials to deep levels where they are metamorphosed under greenschist amphibolite and eclogite facies conditions the tearing and breakoff of the subducted slab possibly along the transitional contact between amphibolites and eclogites create a slab window through which the underlying asthenosphere rises and melts to generate cr rich mafic magmas these upward migrating magmas pass through the subduction zone and assimilate the subducted slab as a result of slab contamination these magmas become more siliceous more oxidized and more hydrous rapidly triggering chromite crystallization minute grains of chromite are suspended in the upward moving magmas as they migrate through the overlying metasomatized mantle wedge such chromite bearing magmas eventually deposit chromite in magma conduits in the uppermost mantle close to the moho where the upward flow changes from vertical to subhorizontal and velocity is greatly reduced highly reduced and ultrahigh pressure minerals including diamonds are reported in literature both in podiform chromitites and host peridotites of ophiolites some of these minerals in association with host peridotites may have been brought by the uprising asthenosphere at mid oceanic ridges due to the mantle convection it is also possible that some diamonds may have formed in the subducted slab below about 150 km some minerals of subducted slabs are preserved because they are encapsulated in chromite grains where they are protected from the ssz melts some of these ssz mantle wedges are emplaced on land to become podiform chromitite bearing ophiolites

the giant chromite deposits at kempirsai urals constraints from trace element pge ree and isotope data the investigation of stable and radiogenic isotopes and of platinum group pge and rare earth elements ree in chromitites and associated ultramafic rocks of the kempirsai massif southern urals gives strong evidence for a multistage formation of giant ophiolitic podiform chromite deposits present in the southeastern part of the massif the kempirsai ophiolite massif is divided by a shear zone into two parts in the northwestern area small bodies of al rich chromite formed from basaltic melts between 420 to 400 ma according to sm nd mineral isochrons of harzburgite pyroxenite websterite and gabbro harzburgites and pyroxenites in this area are enriched in light ree and have nd 400 6 and sr 400 5 chromitites have scattered pge distributions pd ir 0 4 7 0 being partly enriched in pd and pt os 400 of one chromitite is 4 4 the southeastern part of the kempirsai massif well known for its world class deposits of podiform low al magnesiochromite is characterized by harzburgite and dunite enriched in light ree with very low nd 400 4 3 to 17 1 and positive sr 400 10 values chromitites are strongly enriched in ir os and ru and depleted in pd and pt os 400 of three chromitites is uniform and approaches c1 and dmm compositions in veins and pods postdating crystallization of massive chromite pargasitic amphibole formed in equilibrium with fluid inclusion bearing chromite at temperatures close to 1000 c these amphiboles give 40ar 39ar stepwise heating ages of 365 to 385 ma and are characterized by low nd 400 0 6 to 4 6 and general enrichment in ree the cooling ages correspond to a 379 3 1 6 ma rb sr mineral isochron produced from amphibole and phlogopite of a pyroxenite vein in the western part of the massif from these data it is concluded that parts of the kempirsai massif have been pervasively metasomatized by large amounts of fluids and melts derived from a subducted slab composed of oceanic crust and sediments subduction occurred at least 15 35 ma after a melting event that produced a typical ophiolitic sequence in the paleozoic sakmara zone we conclude that large chromite orebodies formed from second stage high mg melts that interacted with depleted mantle and fluids on their way upward in a suprasubduction zone regime and in a fore arc position to the magnitogorsk island arc

unexpectedly high pge chromitite from the deeper mantle section of the northern oman ophiolite and its tectonic implications unusually high platinum group element pge enrichments are reported for the first time in a podiform chromitite of the northern oman ophiolite the chromitite contains 1 5 ppm of total pge being highly enriched in the ipge subgroup ir os and ru and strongly depleted in the ppge subgroup rh pt and pd its platinum group minerals pgms are classified into three types arranged in order of abundance 1 sulphides os rich laurite laurite erlishmanite solid solution and an unnamed ir sulphide 2 alloys os ir alloy and ir rh alloy and 3 sulpharsenides irarsite and hollingworthite the high pge concentrations are observed only in a discordant chromitite deep in the mantle section which has high cr 0 7 spinel with an olivine matrix all the other types of chromitite in the moho transition zone mtz and concordant pods in the deeper mantle section are poor in pges and tend to have spinels with lower cr up to 0 6 this diversity of chromitite types suggests two stages of magmatic activity were responsible for the chromitite genesis in response to a switch of tectonic setting the first is residual from lower degree partial melting of peridotite which produced low cr pge poor chromitites at the moho transition zone and to a lesser extent within the mantle possibly beneath a fast spreading mid ocean ridge the second chromitite forming event involves higher degree partial melting which produced high cr pge rich discordant chromitite in the upper mantle possibly in a supra subduction zone setting

upper crustal podiform chromitite from the northern oman ophiolite as the stratigraphically shallowest chromitite in ophiolite and its implication for cr concentration a new type of podiform chromitite was found at wadi hilti in the northern oman ophiolite it is within a late intrusive dunite body possibly derived from olivine rich crystal mush between the sheeted dike complex and upper gabbro this chromitite forms small 30 cm in thickness pods with irregular to lenticular shapes neither layering nor graded bedding is observed within the pods the chromitite is in the upper crust by far shallower in ophiolite stratigraphy than the other podiform chromitites that have ever been found in the moho transition zone to the upper mantle it is dis tributed along a small felsic to gabbroic melt pool within the dunite body which was formed by melting of gab broic blocks captured by the mush chromian spinel was precipitated due to mixing of two kinds of melt a basaltic interstitial melt from the mush and an evolved possibly felsic melt formed by the melting of gabbro blocks the podiform chromitite reported here is strik ingly similar in petrography and spinel chemistry to the stratiform chromitite from layered intrusions the for mer contains plagioclase and clinopyroxene as matrix silicates instead of olivine as well as includes euhedral and fine spinel with solid mineral inclusions chromian spinel of the upper crustal podiform chromitite from oman has relatively low content of cr2o3 a l 2o3 the cr cr al atomic ratio of around 0 6 and the relatively high tio2 content ranging from 1 to 3 wt we conclude that assimilation of relatively si rich materials crustal rocks or mantle orthopyroxene by olivine spinel saturated melts can explain the genesis of any type of chromitite

mineralogy and environmental geochemistry of historical iron slag hopewell furnace national historic site pennsylvania usa the hopewell furnace national historic site in southeastern pennsylvania which features an fe smelter that was operational in the 18th and 19th centuries is dominated by three slag piles pile 1 slag from the hopewell furnace and pile 2 slag likely from the nearby cornwall furnace were both produced in cold blast charcoal fired smelters in contrast pile 3 slag was produced in an anthracite furnace ore samples from the nearby jones and hopewell mines that fed the smelter are mainly magnetite rich with some sulfides pyrite chalcopyrite sphalerite and accessory silicates quartz garnet feldspar and clay minerals slag piles 1 and 2 are similar mineralogically containing predominantly skeletal and dendritic aluminian diopside and augite skeletal forsteritic olivine glass rounded blebs of metallic fe and exotic quartz olivine is a major phase in all samples from pile 2 whereas it occurs in only a few samples from pile 1 samples of the 2 mm size fraction of surface composite slag material or crushed slag from at depth in piles 1 and 2 are mineralogically similar to the large surface slag fragments from those piles with the addition of phases such as feldspars fe oxides and clay minerals that are either secondary weathering products or entrained from the underlying bedrock pile 3 slag contains mostly skeletal forsteritic olivine and ti bearing aluminian diopside dendritic or fine grained subhedral melilite glass euhedral spinel metallic fe alabandite oldhamite solid solution as well as a sparse ti carbonitride phase the bulk chemistry of the slag is dominated by al2o3 8 5 16 2 wt cao 8 2 26 2 wt mgo 4 2 24 7 wt and sio2 36 4 59 8 wt constituting between 81 and 97 of the mass of the samples piles 1 and 2 are chemically similar pile 1 slag overall contains the highest fe2o3 k2o and mno and the lowest mgo concentrations pile 3 slag is high in al2o3 cao and s and low in fe2o3 k2o and sio2 compared to the other piles in general piles 1 and 2 are chemically similar to each other whereas pile 3 is distinct a conclusion that reflects their mineralogy the similarities and differences among piles in terms of mineralogy and major element chemistry result from the different smelting conditions under which the slag formed and include the fuel source the composition of the ore and flux the type of blast cold versus hot which affects the furnace temperature and other beneficiation methods the three distinct slag piles at hopewell are enriched in numerous trace elements such as as up to 12 mg kg cd up to 0 4 mg kg co up to 31 8 mg kg cu up to 647 mg kg mn up to 0 69 wt pb up to 172 mg kg and zn up to 393 mg kg together with fe 13 9 wt when compared to the average for the continental crust with the 2 mm size fraction commonly containing the highest concentrations enrichments in various elements e g cd co cu pb zn were also found in the ore samples despite these enrichments comparison of bulk chemistry trace element concentrations to the environmental guidelines suggests most elements are likely not problematic with the exception of as co fe and mn leachate tests that simulate weathering indicate fe up to 973 g l and mn up to 133 g l are readily released in potentially harmful concentrations compared to secondary drinking water and some aquatic ecosystem toxicity criteria aluminum and cu although not high in the solid compared to environmental guidelines also exceed relevant criteria in leachate extracts with maximum concentrations of 2700 g l and 17 7 g l respectively in contrast as and co which are significant in the solids are not leached in concentrations that exceed guidelines i e 3 g l or less for both elements the weathering rates of the fe metal and fe oxides which host cu and some fe are likely higher than the silicate glass which hosts the majority of al mn and some fe and the crystalline silicates and spinels affecting which elements and how much are released into the environment and surrounding aquatic ecosystem the mineral assemblages and their chemical composition the bulk sample chemistry and leachability of trace elements are all important components in understanding the potential environmental impacts of the slag piles

late pleistocene glaciolacustrine sedimentation and paleogeography of southeastern michigan usa abstract the geomorphic stratigraphic and sedimentological characteristics of glaciolacustrine sediments in the metropolitan detroit michigan area were studied to determine environments of deposition and make paleogeographic reconstructions nine lithofacies were identified and paleoenvironments interpreted based on their morphostratigraphic relationships with relict landforms the sediments studied are found southeast of the defiance and birmingham moraines lying beneath a lowland characterized by a low morainal swell detroit moraine and a series of lacustrine terraces that descend progressively in elevation southeastward the glaciolacustrine sediments were deposited approximately 14 3 12 4 ka bp during the port bruce and port huron glacial phases of late wisconsinan time and are related to proglacial paleolakes maumee arkona whittlesey warren wayne grassmere lundy and rouge the glaciolacustrine section is typically 2 4 m thick and consists of a basal unit of wavy bedded clayey diamicton overlain by a surficial deposit of stratified and cross stratified sand and gravel the basal unit is comprised of subaqueous debris flow deposits that accumulated as subaqueous moraine in paleolake maumee along the retreating front of the huron lobe the surficial deposits of sand and gravel were formed by traction resulting from lacustrine wave activity and fluvial processes in lakebed plain beach ridge and deltaic depositional settings much of the lake margin sand and gravel was derived from clayey diamicton by lacustrine wave action and winnowing and that associated with paleolakes of the port huron phase is largely reworked port bruce sediment paleogeographic reconstructions show that the defiance birmingham and detroit moraines defiance and rochester channels and the rochester delta were deposited penecontemporaneously as paleolake maumee expanded northward across the map area a unique type of wavy bedform is characteristic of clayey diamicton deposited by subaqueous mass flow in the study area that is useful for differentiating sediment 1 deposited by mass flow in subaqueous vs subaerial settings and 2 deposited by subaqueous mass flow vs basal till these bedforms are a useful tool for identifying subglacial meltwater deposits and facilitate the mapping and correlation of glacial sediments based on till sheets the map area provides a continental record of ice sheet dynamics along the southern margin of the laurentide ice sheet during heinrich event h 1 the record reveals rapid glacial retreat 0 8 km yr contemporaneous with the discharge of a large volume of meltwater evidence in the study area for subglacial meltwater is problematic but indications that periglacial conditions persisted in the map area until 12 7 ka bp and extended for 200 km or more south of the ice front suggest that a frozen substrate may have contributed to instability of the lis

structural evolution of a section along the base of the st anthony complex northwest newfoundland the basal portion of the st anthony complex is made up of structural slices 10 30 m thick with unknown amounts of relative movement between them the lowest rocks are variably deformed pillow lavas and related rocks with one pervasive foliation these rocks are overlain by greenschists phyllonites and mylonites with up to three planar structural elements developed at least one of these latter surfaces the earliest had formed before the schists were thrust over the underlying pillow lavas the overthrusting event resulted in deformation of underlying pillow lavas all deformation occurred under lower greenschist facies conditions fold axes in the least deformed rocks have variable orientations and this variation decreases up section until axes are parallel and plunge gently eastwards parallel to the stretching lineation parallelism between fold axes and stretching lineation implies rotation of fold axes during progressive deformation and is consistent with the rocks being part of a high strain z

silicate and base metal sulfide inclusions in chromites from the maqsad area oman ophiolite gulf of oman a model for entrapment abstract al rich chromites from the maqsad area oman ophiolite have been studied to propose a genetic model for silicate and base metal sulfide bms inclusion entrapment samples were collected in both concordant and discordant nodular pods and in a peculiar stratiform ore chromite ore contains olivine clinopyroxene plagioclase as interstitial silicates regardless of the ore type the silicate inclusions are in decreasing order of abundance composed of ti and na rich phlogopite ti pargasite enstatite mg diopside olivine albite and secondary hydrothermal products chromite hosted bms are dominated by pentlandite heazlewoodite millerite and are nearly devoid of fe and cu sulfides the samples which come from concordant ores are very poor in mineral inclusions a consequence of the strain induced grain boundary migration and recrystallization which they suffered during plastic deformation in contrast the nodular and stratiform samples which escaped post magmatic deformation and densification are inclusion rich the inclusions display negative crystal shape following the chromite symmetry if present in nodular samples they may be arranged as coronas which mimic the shape of the host chromite grains all these textural features are considered to be evidence for mineral inclusion entrapment during magmatic precipitation of chromite by reference to a previous dynamic model of chromite ore genesis we consider that the nodules of chromite crystallized from basaltic magma were flowing inside a narrow cavity in which a non turbulent convective system was established we deduce from the presence of several inclusion coronas or patches in the largest nodules and their aabsence in the smallest that the inclusions were trapped preferentially at the point in which the primitive magma flow meets the convective current the peculiar mineralogy of mineral inclusions dominated by hydroxyl sodic phases indicates a complex interaction at the magmatic stage between the chromite precipitating magma and an unrelated aqueous fluid phase this fluid which was sodium and sulfur bearing travelled through the top of the mantle sequence and the basis of cumulate rocks of the ophiolite it could have been injected at certain stages of its evolution in the same conduits as the chromite precipitating magma locally giving rise to volatile enriched differentiated melts the latter have been sealed together with the liquidus phases of magma olivine clinopyroxene orthopyroxene and plagioclase as inclusions during chromite growth and or dissolution olivine and pyroxenes have partly reacted with the trapped volatile rich melt producing phlogopite and pargasite several hundred degrees below trapping temperatures bms have formed through sulfidization reactions with enclosed silicates and or host chromite

the chemistry of chromite from two mafic ultramafic complexes in northern greece abstract chromites have been analysed from vavdos and gomati two mafic ultramafic complexes in the chalkidiki peninsula greece the complexes interpreted as ophiolites generated in small ocean basins or marginal seas have been tectonically incorporated forming allochthonous bodies bounded by thrust faults chromite analyses reveal that the ores are high cr metallurgical type at vavdos but high al refractory type at gomati the origin of the ores and of accessory chromite in the ultramafic rocks is related to extraction of basaltic melt during progressive partial melting of the upper mantle and subsequent fractional crystallisation podiform chromites are an early product of the crystallisation of such a basaltic melt they are intimately associated with a dunite host rock

chondritic osmium isotopic composition of archean ophiolitic mantle north china craton abstract podiform chromitites are diagnostic but rare features of phanerozoic ophiolites and often contain the most pristine textural chemical and isotopic record of convective upper mantle conditions extant during ophiolite genesis ophiolitic podiform chromitites owing to their high os concentrations and low re os ratios provide the best evidence for the os isotopic evolution of oceanic mantle but established records of ophiolitic chromites from bona fide archean ophiolites are still lacking we report re os isotopic compositions of the world s oldest known ophiolitic podiform chromites from the 2 5 billion year old dongwanzi zunhua ophiolite north china this provides the oldest os isotope composition for the convective upper mantle yet obtained from ophiolitic podiform chromitites and reveals a chondritic os isotopic composition of the archean convective upper mantle

on the formation mode of the guleman chromite deposits turkey kefdag and soridag chromite pods occur in upper mantle residual peridotites which consist of harzburgite and dunites the peridotites represent the residual of multistage depleted upper mantle peridotites the chromitite bodies were formed during the uprising of chromium rich picritic melts through the residual upper mantle diapir along the magma conduits chromitite grains were deposited in the caves of the magma conduits under the control of the convection currents

40ar 39ar dating of the emplacement of the muslim bagh ophiolite pakistan abstract the obduction related basal part of the muslim bagh ophiolite baluchistan pakistan and the underlying metamorphic sequence were studied structurally which demonstrated a wsw ene trending thrusting sequence for the initial obduction 40ar 39ar measurements on amphiboles and plagioclase from the subophiolitic metamorphic rocks and on plastically deformed and recrystallized dolerite samples from the base of the sheeted dyke complex give apparent ages between 70 7 5 0 and 65 1 4 1 ma interpreted as cooling ages dating approximately the formation of the plastic deformation and obduction the results indicate that the muslim bagh ophiolite represents a segment of ocean floor from the small and slow spreading ocean branch of the neo tethys located between the indo pakistani and the afro arabian plates the wsw ene oriented obduction of the muslim bagh ophiolite onto the indo pakistani continental margin occurred with the convergence of the neo tethys branch during the late cretaceous and before the tertiary collision of the indo pakistani plate with the eurasian plate

high cr and high al chromitites from the sagua de tanamo district mayari cristal ophiolitic massif eastern cuba constraints on their origin from mineralogy and geochemistry of chromian spinel and platinum group elements article i nfo several chromitite bodies of variable sizes are hosted in dunite harzburgite of the small b3 km mining district of sagua de tanamo in the mayari cristal ophiolitic massif eastern of cuba the chromian spinel in these bodies displays a large range in cr cr cr al atomic ratio from 0 74 to 0 45 in high cr chromitites cr and ti contents increase from harzburgite to dunite to chromitite a roughly opposite trend of variation in cr is observed in high al chromitites although in this case ti distributes randomly the differences in the type of chromian spinel i e high cr or high al in the chromitite coincide with a different behavior of the platinum group elements pge whereas high cr chromitites are rich in pge and contain abundant grains of platinum group minerals pgm high al chromitites are systematically poor in pge and in pgm the calculated melts in equilibrium with chromian spinel of high cr chromitite are island arc thoeliites iat with boninitic affinity whereas those in equilibrium with chromian spinel of high al chromitites are back arc basin basalts babb the formation of high cr chromitites is interpreted as a result of the extensive reaction of harzburgite with migrating island arc tholeiite melts of boninitic affinity melt rock reaction produces boninitic melts with variable composition and porous dunitic channels in which the mixing mingling of melts promotes crystallization of mononomineralic high cr chromian spinel in contrast high al chromitites formed by the mixing mingling of babb melts within conduits not in equilibrium with dunite percolation of primitive babb melts through pre existing dunite dissolved olivine producing melt conduits in which babb melts mixed and formed high al chromian spinel the higher pge and pgm in high cr chromitites is not only a function of the degree of partial melting but is linked to interaction between the migrating melt and mantle harzburgite during the formation of the chromitite the coexistence of both types of chromitites in one small mining district reflects the temporal and or spatial variations of separate melt intrusions emplaced in different subarc mantle domains during the opening of a back arc basin in a supra subduction zone environment

chromite and pge geochemistry of the elekdag ophiolite kastamonu northern turkey implications for deep magmatic processes in a supra subduction zone setting abstract the geochemistry of chromium spinel and platinum group elements pge provides significant information about conditions in the mantle during the emplacement and genesis of ophiolitic complexes the elekdag ophiolite is a greenschist to amphibolite facies ophiolite that was emplaced onto the sakarya zone in the central part of the pontides of northern turkey nw se trending elekdag ophiolite hosts small scale chromitite occurrences in serpentinized harzburgite and dunite and have maximum resource of several thousands t cr2o3 the chromitites the elekdag ophiolite can be divided in to high cr and high al types the calculated melt composition of the high cr chromitites is 9 4 13 2 wt al2o3 0 2 0 4 wt tio2 with feo mgo ratios of 0 4 1 9 which are indicative of boninitic compositions however the high al chromitites have 14 7 17 3 wt al2o3 and 0 3 0 8 wt tio2 with feo mgo ratios of between 0 3 and 0 8 which are consistent with morb like tholeiitic affinities the total pge content of the chromitites range between 82 and 1466 ppb and the average pge content of the high cr and high al chromitites are 267 ppb and 195 ppb respectively the chromitites are characterized by low pd ir ratios of 0 02 0 20 that is characteristic of non fractionated chromitites the chondrite normalized pge pattern of the chromitites resemble those of mantle derived cr rich chromitites in ophiolites of the supra subduction zone ssz elsewhere mineral and bulk rock pge data for the elekdag chromitites indicate that high cr chromitites formed from cr rich and al and ti poor boninitic melts most probably in the fore arc setting in the deeper part of the mantle this was succeeded by the mineralization of high al chromitites from a differentiated boninitic melt at shallow depths close to the moho discontinuity

mantle flow at oceanic spreading centers models derived from ophiolites abstract the internal structures of eleven ophiolite complexes have been structurally investigated the attitude of the foliation and the lineation in the peridotites is related to asthenospheric flow in the uppermost mantle at an oceanic spreading center from the orientation of these structures with respect to the paleohorizontal plane two oceanic situations are distinguished the horizontally spreading asthenosphere illustrated by the table mountain massif newfoundland is characterized by 1 a homogeneous regular and sub horizontal plastic flow pattern over large distances 2 b flat paleo mono and absence of large scale folding 3 c rare economical chromite deposits and depleted mantle the diapirically spreading asthenosphere illustrated by the acoje type philippines displays 1 a inhomogeneous and sub vertical plastic flow pattern 2 b deformed paleo moho and large scale folds 3 c important economical chromite deposits and less depleted mantle presumably due to secondary magmatic impregnation this leads to vogt s model of asthenospheric flow beneath an oceanic ridge the direction of which being equated with the dike swarm in this model the horizontally spreading asthenosphere is derived from the diapirically spreading one in the following way the vertical mantle flow at the diapir center diverges horizontally in every direction including that parallel to the ridge at about 50 km from the diapir center the mantle flow becomes progressively perpendicular to the ridge due to the damming effect of transform faults an important economical consequence of this model is that large chromite deposits are expected preferentially in the diapiric type of ophiolite because chromite is preferentially precipitated at the diapir center from pristine magma

cristallisation et deformation des orbicules de chromite les orbicules de chromites structures typiques des corps podiformes de chromite des ophiolites sont decrites en detail elles correspondent a des concretions magmatiques en milieu turbulent soumises en fin de cristallisation a un allongement et un cisaillement contemporains de la deformation plastique de l olivine les autres types de minerai des corps podiformes sont affectes d un plan de pull apart marque par des feuillets lenticulaires et paralleles a remplissage d olivine ulterieurement un reseau de diaclases a remplissage serpentineux se developpera ces observations sont coherentes avec l hypothese de la formation des corps podiformes dans des conduits magmatiques au sein des peridotites mantelliques

hydration of an active shear zone interactions between deformation metasomatism and magmatism the spinel lherzolites from the montferrier southern france oligocene basalts abstract geochemical and textural investigations have been simultaneously performed on spinel lherzolite xenoliths from the oligo miocene alkali basalts of montferrier southern france all the investigated samples have undergone a deformation very particular by intense shearing under high stresses up to 1 75 kbar low temperatures 900 c and strain rates of about 10 18 to 10 15 s 1 mineral chemistry reveals that the montferrier lherzolites are fragments of an undepleted relatively shallow upper mantle level located at a depth of 50 km 15 kbar moreover na and ti enrichment in diopside would reflect a metasomatic event also emphasized by the common occurrence of pargasite in 50 70 of the investigated samples crystallization of this amphibole is attributed to a hydrous infiltration which is related in time and space to the deformation indeed amphibole is preferentially concentrated in strongly deformed zones and in kink band boundaries of orthopyroxene porphyroclasts moreover the grain boundaries were used by the pervasive agent to percolate into the lherzolite significant chemical variations increase in mgo 15 and decrease in al 2 o 3 55 are observed within the range of 7 5 m adjacent to the grain boundary finally sr isotopic data 87 sr 86 sr demonstrate that the amphibole i e the metasomatic agent is genetically related to the host lava of the xenoliths thus the hydrous silicate liquid from which the amphibole has crystallized may be an early percolation of the ascending alkali magma this silicate liquid hydrated the shear zone located at a depth of 50 km induced the hydraulic fracturation of the lherzolite and the magmatic conduit opening subsequently the alkali magma sampled some fragments of this strongly deformed and metasomatized undepleted upper mantle level and brought them to the surface

origin of the xigaze ophiolite yarlung zangbo suture zone southern tibet abstract the xigaze ophiolite tibet displays unusual lithological petrological textural and structural characteristics there are no large masses of cumulate gabbros but dolerite intrusives throughout the whole ophiolite sequence some of which were intruded into already serpentinized peridotites only minor residual harzburgites and dunites in dominantly iherzolitic peridotites equilibrated at low temperatures and pressures and relatively low temperature deformation structures in the uppermost peridotites these features suggest a very low heat flow at the spreading center where the xigaze ophiolite was formed in good agreement with a discontinuous and slowly accreting spreading center origin however this ophiolite does not represent a typical mid oceanic ridge ophiolite rather it was formed in a small basin located at the southern margin of eurasia hence within a preexisting oceanic lithosphere the opening of the nearly n s xigaze paleo ridge resulted from the w e drift of africa relative to eurasia from 180 to 110 ma the n s emplacement onto the continent of the xigaze ophiolite formed 120 to 110 ma ago can be correlated to changes in direction of motion of the african and indian plates a primary intra oceanic thrusting event probably occurring at 110 or 85 ma and the final obduction near 50 ma during the india eurasia collision

petrology and texture of the ultramafic rocks of the xigaze ophiolite tibet constraints for mantle structure beneath slow spreading ridges abstract the xigaze ophiolite southern tibet exhibits an unusual ophiolite sequence characterized by 1 1 the limited thickness less than 3 km of its crustal unit which is mostly composed of dolerite intrusives the gabbros being very scarce 2 2 the scarcity of harzburgites and dunites the ultramafic rocks consisting mostly of cr diopside bearing harzburgites and iherzolites although there are some textural and chemical variations from the basal iherzolites to the upper harzburgite and dunites no sharp discontinuity exists between the peridotite series of the xigaze ophiolite which all belong to the same peridotite suite the peridotites have been equilibrated at relatively low temperatures and pressures 1155 c 0 5 0 6 gpa for the cr diopside bearing harzburgites and 1255 c 1 0 1 1 gpa for the stratigraphically 2 km deeper iherzolites indicative of a low thermal gradient at the xigaze paleo ridge accordingly the peridotites have locally been deformed under low stress 0 2 gpa conditions and at decreasing temperatures from up to 1000 c to 900 800 c as evidenced by the activation of the 010 100 ok1 100 and 001 100 olivine glide systems the equilibrium temperatures and pressures estimated for the peridotites the inferred shape of the melting zone beneath the xigaze paleo ridge the low temperature low stress conditions inferred for the deformation of the peridotites and the pattern of the mantle structure determined on the basis of structural and microstructural data are in good agreement with a slow spreading ridge origin for the xigaze ophiolite

reply to the comment of rollinson and adetunji podiform chromitites do form beneath mid ocean ridges by arai s and miura m

an overview of the pge concentrations in the shetland ophiolite complex extremely anomalous platinum group element pge concentrations are known at one site cliff in the shetland ophiolite complex systematic pge analysis of all the mafic and ultramafic igneous and alteration lithologies in the ophiolite has revealed many lower grade anomalous pge occurrences throughout the cumulate ultramafic sequence and their distribution is described here more sites with pge enrichments as anomalous as those at cliff have not been found at cliff the anomalous values of over 60 ppm pt pd are restricted to a chromite rich sulphide bearing dunite forming a small part of a 200 m dunite lens in mantle harzburgite close to the basal thrust of the ophiolite lower values of up to 6 4 ppm pt pd occur in chromite poor sulphide rich dunites also within this dunite lens the only other chromite rich dunite lens in the cliff area also has detected pt and pd but with low pt pd values of 200 ppb in contrast harzburgites metasediments underlying the basal thrust serpentinites from the basal thrust and internal faults are all barren of pt and pd detection limit 20 ppb in the cliff area therefore detected pge values are restricted to magmatic chromite rich dunite lenses pge analyses of drill core from beneath the disused chromite quarries at cliff are low 227 ppb pt pd indicating that the extremely anomalous pge values are very restricted this supports the idea that pge concentrated magmatically within the dunite lens at cliff may have been remobilized locally to produce extremely anomalous pge values in an alteration zone only a few metres in diameter elsewhere in the ophiolite anomalous pt pd values of up to 4 ppm occur in chromite rich sulphide bearing dunite within the dunite unit which stratigraphically overlies the harzburgite pt pd concentrations of up to 1 ppm are present in the pyroxenites and wehrlites of the upper part of the ultramafic sequences high level wehrlites and pyroxenites within the gabbro unit representing late ultramafic differentiates contain lower levels of pge of up to 310 ppb pt pd the gabbros all have less than 20 ppb pt and pd this distribution indicates that the pge concentrations occur in specific primary magmatic lithologies and pathfinder analysis shows that they are associated with ni cu au and cr pge have not been found above detection limits in fault zones away from primary pge mineralization sites and it is concluded that pge are not significantly hydrothermally reconcentrated at cliff remobilization may have occurred over short distances of only a few metres at most

metallogeny of the chrome ores of the xerolivado skoumtsa mine vourinos ophiolite greece implications on the genesis of ipge bearing high cr chromitites within a heterogeneously depleted mantle section abstract chrome ore deposits comprise less than 1 of the volume of a pervasively serpentinized dunite body 3 5 km 3 that constitutes the xerolivado skoumtsa mine of the vourinos ophiolite in northwestern greece ores have been examined to aid determination of their geological mode of occurrence their mineralogy and mineral chemistry cr spinel and olivine and platinum group element pge geochemistry the ore bodies are highly deformed and of the schlieren type where cr spinel rich and serpentine after olivine rich layers 1 20 cm thick alternate the cr spinel displays a limited range in cr 100 cr cr al atomic ratio 77 85 and tio 2 content 0 05 0 22 wt the mg 100 mg mg fe 2 of cr spinel shows a wider variation 52 74 indicating that subsolidus mg sp fe 2 ol re equilibration occurred remnant fresh olivine in the serpentine rich layers is typical forsterite fo 100 mg mg fe 2 93 95 the cr rich chromitite ores have total pge and au abundances that range between 74 3 and 205 ppb they show a general enrichment in ir and ru over ppge rh pt and pd resulting mostly in a negatively sloping c1 chondrite normalized pge profile with a quite strong ru peak field observations indicate that the schlieren banding of the deformed ore bodies is a tectonometamorphic feature acquired within ductile deformation of the ophiolitic slab geochemical calculations demonstrate that the parental melts of the xerolivado skoumtsa high cr chromitites were boninitic their high ru content and variation in cr sp values indicate a polygenetic origin from geochemically similar but spatially distinct melt inputs these melts originated within a hydrated mantle wedge beneath a forearc basin within an evolving lithospheric slab we emphasize that the pge budget of the xerolivado skoumtsa chromitites was not inherited from interactions of the migrating melts with their parental harzburgites but it is an intrinsic feature of the deep seated and heterogeneously depleted mantle source of these magmas

origin of podiform chromite deposits by magma mingling abstract i have oversaturated the immiscible system sio2 al2o3 feo k2o with fecr2o4 to study the fractionation of chromite between conjugate siliceous and fayalitic melts although both melts are oversaturated with chromite chromite only nucleates in the fayalite rich melt where the crystal melt interfacial energy sl is lowest in less than three days at 1150 c all excess chromium in the bulk composition is concentrated as cumulus chromite in the fayalitic melt pools producing nodular cumulate textures as observed in podiform chromite ore when these results are applied to the lithospheric mantle podiform chromite deposits may form when droplets of olivine normative high pressure low viscosity melt mingle with siliceous low pressure more viscous melt in shallow magma conduits it is inferred from the experimental analogue that chromite only nucleates in the melt where sl is lowest which in the mantle would be the high pressure melt the low pressure melt only serves as the diffusive chromium reservoir the richest chromite ores form when the volume ratio of the melt nucleating chromite is small relative to the ambient melt

the nature and rheology of the cordilleran upper mantle of british columbia inferences from peridotite xenoliths abstract peridotite xenoliths in alkalic basaltic rocks from eight localities within the british columbian cordillera include metamorphic tectonites metacumulates and cumulates chemical analyses of these show that the xenoliths encompass the range of both depleted and primitive undepleted mantle application of two pyroxene thermobarometric techniques yield geotherms characteristic of normal oceanic mantle and diapiric mantle wherein upper mantle stratigraphic profiles are either homogeneously stratified or heterogeneously mixed respectively stress estimates as a function of depth are obtained for peridotite xenoliths from each of the localities by applying recrystallised grain size paleopiezometry and pyroxene thermobarometry stress and temperatures obtained are combined with a representative creep flow law to calculate strain rate and viscosity profiles that appear to be physically reasonable these profiles are juxtaposed to present a dynamic model of a mantle diapir convecting through a stratified mantle section and these two different types of mantle stratigraphic profile are related to current large scale tectonic theories of the cordillera

chromite pods in a lherzolite massif collo algeria evidence of oceanic type mantle rocks along the west mediterranean alpine belt abstract the high temperature lherzolite massifs of ronda southern spain and beni bousera northern morocco are ascribed by various authors to sub continental mantle diapirs emplaced within a granulite metamorphic aureole in continental rifts along transcurrent lithospheric faults the collo plagioclase lherzolite massif with associated kinzigite and related anatectic rocks belongs to the same type of geodynamic context about 17 chromitite bodies are found in the collo lherzolite massif their structural textural and geochemical characteristics are those of chromitite pods that are typical of ophiolite complexes i e of an oceanic lithosphere the collo massif is an atypical high temperature lherzolite massif displaying both continental and oceanic characteristics it could correspond to a mantle intrusion in a transition zone from continental to oceanic rift type

tectonic structures related to thrusting of ophiolitic complexes the white hills peridotite newfoundland the white hills peridotite and its underlying metamorphic rocks represent the basal portion of a partly eroded ophiolitic complex at the contact with the metamorphic rocks the peridotites display low temperature high stress mylonitic textures which grade upwards to fine grained porphyroclastic textures indicative of higher temperatures and lower stresses petrological and structural evidence indicates that the mylonitic peridotites and underlying metamorphic rocks have been deformed during the oceanic thrusting of the lithosphere the analysis of the main tectonic structures observed in the peridotites shows that the complex has been thrust northwestward along a southeast dipping thrust plane the data are compared with those obtained from the bay of islands ophiolite complex

the chromite deposits of the troodos complex cyprus evidence for the role of a fluid phase accompanying chromite formation the troodos ophiolite of cyprus hosts chromite deposits both as podiform bodies within the basal harzburgite and as lesser layers and schlieren within the overlying dunites and clinopyroxene dunites of the metacumulate sequence all chromitite types have been studied from both field and mineralogical aspects with particular emphasis on the chemistry of the ubiquitous silicate inclusions within chromite grains two distinct inclusion assemblages have been documented chromites from podiform bodies contain abundant hornblende serpentine ortho and clinopyroxene and lesser olivine while those from the metacumulates host pargasite albite serpentine and rare pyroxenes this implies that no single silicate inclusion assemblage in chromite can be considered typically ophiolitic the presence of an alkalirich fluid phase during chromite crystallization as evidenced by the unusual mineralogy of these inclusions has already been widely discussed in the literature however the marked stratigraphic control of inclusion types in chromite as reported here enables more complete modelling of the evolution of this phase to be made

high pressure phase transitions of natural chromitite from tibetan ophiolites abstract the podiform chromitites in the tibetan ophiolites have been thought to have experienced metamorphism corresponding to depths at the top of the mantle transition zone mtz however little is known about the phase relations of natural podiform chromitite in the mtz as well as the implications for processes related to formation of the ultra high pressure uhp chromitite an experimental study is presented to constrain the phase relations of natural chromitite at pressures and temperatures of 11 18 gpa and 1400 1600 c the experimental results show that the chromitite transforms into phase assemblages composed of chromite eskolaite garnet for pressures at the base of the upper mantle and chromite eskolaite garnet mc phase at the top of the mtz chromitite is replaced by a cati2o4 structured phase ct phase eskolaite garnet in the middle of the mtz the mc phase has an average composition of mg fe 3 cr al 2 2x 4 3y mg fe xsix y o6 x 0 2 y 0 3 with a cubic space group of fd3m it is suggested that the mc phase is a new silicon bearingmg and cr rich phase with idealized stoichiometry of mg3cr2o6 the observation of the mc phase in a natural rock system is helpful for understanding the incorporation of mg and cr minerals in the mtz and helps to constrain the related geological processes that controlled the formation of the tibetan chromitites

spinel compositions of mantle hosted chromitite from the eastern anatolian ophiolite body turkey implications for deep and shallow magmatic processes abstract the eastern anatolian accretionary complex includes several ophiolitic megablocks and or tectonic slivers within a 150 180 km long east west trending complex formed during the late cretaceous tertiary in eastern turkey the alabayir mollatopuz yukaribalcikli and mehmetalan ophiolites are megablocks or tectonic slivers containing locally massive nodular or schlieren banded chromitite layers these podiform chromitites formed in mantle sequences and are classified as high cr chromitites cr 0 63 0 88 mg 0 50 0 64 0 01 0 5 wt tio 2 5 7 18 8 wt al 2 o 3 calculated parental melt compositions of these chromitites indicate boninite magma characteristics 8 2 13 4 wt al 2 o 3 0 64 1 50 feo mgo the pge patterns also support the view that they crystallized from a boninitic melt the total pge contents of chromitites vary between 79 and 390 ppb pd ir ratios of chromitites vary from 0 07 to 0 28 and are consistent with an ipge fractionated nature the examined chromitites were divided into two groups in terms of their mineral chemistry group i alabayir mollatopuz yukaribalcikli and mehmetalan i chromitites group ii mehmetalan ii chromitites mineral chemistry of these chromitites and their parental melt composition indicate that group i chromitites were probably formed at shallow mantle depths and that group ii chromitites formed in deeper parts of the mantle mineral chemistry of these eastern anatolia high cr chromitites indicate that they formed not only in the deeper mantle but also in shallower parts of the moho transition zone